What's hot today
Friday, January 31st, 2014
Hamada-Kawaguchi, N., Nore, B. F., Kuwada, Y., Smith, C. I. and Yamamoto, D. (2014). Btk29A promotes Wnt4 signaling in the niche to terminate germ cell proliferation in Drosophila. Science 343: 294-297. PubMed ID: 24436419
Btk29A is the Drosophila ortholog of the mammalian Bruton's tyrosine kinase (Btk), mutations of which in humans cause a heritable immunodeficiency disease. Btk29A mutations effect germ cell proliferation by stabilizing the proliferating cystoblast fate, leading to an ovarian tumor. This phenotype can be rescued by overexpression of wild-type Btk29A and phenocopied by the interference of Wnt4-beta-catenin (Armadillo) signaling or its putative downstream nuclear protein Piwi in somatic escort cells. Btk29A and mammalian Btk directly phosphorylate tyrosine residues of β-catenin, leading to the up-regulation of its transcriptional activity. Thus, this study identified a transcriptional switch involving the kinase Btk29A/Btk and its phosphorylation target, β-catenin, which functions downstream of Wnt4 in escort cells to terminate Drosophila germ cell proliferation through up-regulation of piwi expression. This signaling mechanism likely represents a versatile developmental switch.
Seth, M., Shirayama, M., Gu, W., Ishidate, T., Conte, D., Jr. and Mello, C. C. (2013). The C. elegans CSR-1 Argonaute Pathway Counteracts Epigenetic Silencing to Promote Germline Gene ExpressionE. Dev Cell. PubMed ID: 24360782
Organisms can develop adaptive sequence-specific immunity by reexpressing pathogen-specific small RNAs that guide gene silencing. For example, the C. elegans PIWI-Argonaute/piwi-interacting RNA (piRNA) pathway recruits RNA-dependent RNA polymerase (RdRP) to foreign sequences to amplify a transgenerational small-RNA-induced epigenetic silencing signal (termed RNAe). This study provides evidence that, in addition to an adaptive memory of silenced sequences, C. elegans can also develop an opposing adaptive memory of expressed/self-mRNAs. This mechanism, which can prevent or reverse RNAe, is referred to as RNA-induced epigenetic gene activation (RNAa). CSR-1 (see Drosophila Argonaute 3), which engages RdRP-amplified small RNAs complementary to germline-expressed mRNAs, is required for RNAa. A transgene with RNAa activity also exhibits accumulation of cognate CSR-1 small RNAs. These findings suggest that C. elegans adaptively acquires and maintains a transgenerational CSR-1 memory that recognizes and protects self-mRNAs, allowing piRNAs to recognize foreign sequences innately, without the need for prior exposure.
Fischer, S. E., Pan, Q., Breen, P. C., Qi, Y., Shi, Z., Zhang, C. and Ruvkun, G. (2013).. Multiple small RNA pathways regulate the silencing of repeated and foreign genes in C. elegans. Genes Dev 27: 2678-2695. PubMed ID: 24352423
Gene segments from other organisms, such as viruses, are detected as foreign and targeted for silencing by RNAi pathways. A deep-sequencing map of the small RNA response to repeated transgenes introduced to Caenorhabditis elegans revealed that specific segments are targeted by siRNAs. Silencing of the foreign gene segments depends on an antiviral response that involves changes in active and silent chromatin modifications and altered levels of antisense siRNAs. Distinct Argonaute proteins target foreign genes for silencing or protection against silencing. This study used a repeated transgene in a genome-wide screen to identify gene disruptions that enhance silencing of foreign genetic elements and identified 69 genes. These genes cluster in four groups based on overlapping sets of coexpressed genes, including a group of germline-expressed genes that are likely coregulated by the E2F transcription factor. Many of the gene inactivations enhance exogenous RNAi. About half of the 69 genes have roles in endogenous RNAi pathways that regulate diverse processes, including silencing of duplicated genes and transposons and chromosome segregation. Of these newly identified genes, several are required for siRNA biogenesis or stability in the oocyte-specific ERGO-1 pathway, including eri-12, encoding an interactor of the RNAi-defective protein RDE-10, and ntl-9/CNOT9, one of several CCR4/NOT complex genes that were identified. The conserved ARF-like small GTPase ARL-8 is required specifically for primary siRNA biogenesis or stability in the sperm-specific ALG-3/4 endogenous RNAi pathway.
Lu, Z., Guan, X., Schmidt, C. A. and Matera, A. G. (2014). RIP-seq analysis of eukaryotic Sm proteins identifies three major categories of Sm-containing ribonucleoproteins. Genome Biol 15: R7. PubMed ID: 24393626
Sm proteins (see Sm proteins specify germ cell fate by facilitating oskar mRNA localization) are multimeric RNA-binding factors, found in all three domains of life. Eukaryotic Sm proteins, together with their associated RNAs, form small ribonucleoprotein (RNP) complexes important in multiple aspects of gene regulation. Comprehensive knowledge of the RNA components of Sm RNPs is critical for understanding their functions. This study developed a multi-targeting RNA-immunoprecipitation sequencing (RIP-seq) strategy to reliably identify Sm-associated RNAs from Drosophila ovaries and cultured human cells. Using this method, three major categories of Sm-associated transcripts were discovered: small nuclear (sn)RNAs, small Cajal body (sca)RNAs and mRNAs. Additional RIP-PCR analysis showed both ubiquitous and tissue-specific interactions. Evidence is provided that the mRNA-Sm interactions are mediated by snRNPs, and that one of the mechanisms of interaction is via base pairing. Moreover, the Sm-associated mRNAs are mature, indicating a splicing-independent function for Sm RNPs. This study represents the first comprehensive analysis of eukaryotic Sm-containing RNPs, and provides a basis for additional functional analyses of Sm proteins and their associated snRNPs outside of the context of pre-mRNA splicing. These findings expand the repertoire of eukaryotic Sm-containing RNPs and suggest new functions for snRNPs in mRNA metabolism.
Thursday, January 30th
Boisclair Lachance, J. F., Pelaez, N., Cassidy, J. J., Webber, J. L., Rebay, I. and Carthew, R. W. (2014). A comparative study of Pointed and Yan expression reveals new complexity to the transcriptional networks downstream of receptor tyrosine kinase signaling. Dev Biol 385: 263-278. PubMed ID: 24240101
The biochemical regulatory network downstream of receptor tyrosine kinase (RTK) signaling is controlled by two opposing ETS family members: the transcriptional activator Pointed (Pnt) and the transcriptional repressor Yan. A bistable switch model has been invoked to explain how pathway activation can drive differentiation by shifting the system from a high-Yan/low-Pnt activity state to a low-Yan/high-Pnt activity state. Although the model explains yan and pnt loss-of-function phenotypes in several different cell types, how Yan and Pointed protein expression dynamics contribute to these and other developmental transitions remains poorly understood. Toward this goal this study used a functional GFP-tagged Pnt transgene (Pnt-GFP) to perform a comparative study of Yan and Pnt protein expression throughout Drosophila development. Consistent with the prevailing model of the Pnt-Yan network, numerous instances were found where Pnt-GFP and Yan adopt a mutually exclusive pattern of expression. However, many examples were also found of co-expression. While some co-expression occurred in cells where RTK signaling is presumed low, other co-expression occurred in cells with high RTK signaling. The instances of co-expressed Yan and Pnt-GFP in tissues with high RTK signaling cannot be explained by the current model, and thus they provide important contexts for future investigation of how context-specific differences in RTK signaling, network topology, or responsiveness to other signaling inputs, affect the transcriptional response.
Guarner, A., Manjon, C., Edwards, K., Steller, H., Suzanne, M. and Sanchez-Herrero, E. (2014). The zinc finger homeodomain-2 gene of Drosophila controls Notch targets and regulates apoptosis in the tarsal segments. Dev Biol 385: 350-365. PubMed ID: 24144920
The development of the Drosophila leg is a good model to study processes of pattern formation, cell death and segmentation. Such processes require the coordinate activity of different genes and signaling pathways that progressively subdivide the leg territory into smaller domains. One of the main pathways needed for leg development is the Notch pathway, required for determining the proximo-distal axis of the leg and for the formation of the joints that separate different leg segments. The mechanisms required to coordinate such events are largely unknown. This study describes that the zinc finger homeodomain-2 (zfh-2) gene is highly expressed in cells that will form the leg joints and needed to establish a correct size and pattern in the distal leg. There is an early requirement of zfh-2 to establish the correct proximo-distal axis, but zfh-2 is also needed at late third instar to form the joint between the fourth and fifth tarsal segments. The expression of zfh-2 requires Notch activity but zfh-2 is necessary, in turn, to activate Notch targets such as Enhancer of split and big brain. zfh-2 is controlled by the Drosophila activator protein 2 gene and regulates the late expression of tarsal-less. In the absence of zfh-2 many cells ectopically express the pro-apoptotic gene head involution defective, activate caspase-3/Ice and are positive for acridine orange, indicating they undergo apoptosis. These results demonstrate the key role of zfh-2 in the control of cell death and Notch signaling during leg development.
Rogers, W. A., Grover, S., Stringer, S. J., Parks, J., Rebeiz, M. and Williams, T. M. (2014). A survey of the trans-regulatory landscape for Drosophila melanogaster abdominal pigmentation. Dev Biol 385: 417-432. PubMed ID: 24269556
Trait development results from the collaboration of genes interconnected in hierarchical networks that control which genes are activated during the progression of development. While networks are understood to change over developmental time, the alterations that occur over evolutionary times are much less clear. A multitude of transcription factors and a far greater number of linkages between transcription factors and cis-regulatory elements (CREs) have been found to structure well-characterized networks, but the best understood networks control traits that are deeply conserved. Fruit fly abdominal pigmentation may represent an optimal setting to study network evolution, as this trait diversified over short evolutionary time spans. However, the current understanding of the underlying network includes a small set of transcription factor genes. This study greatly expand this network through an RNAi-screen of 558 transcription factors. 28 genes were identified, including previously implicated abd-A, Abd-B, bab1, bab2, dsx, exd, hth, and jing, as well as 20 novel factors with uncharacterized roles in pigmentation development. These include genes which promote pigmentation, suppress pigmentation, and some that have either male- or female-limited effects. Many of these transcription factors control the reciprocal expression of two key pigmentation enzymes, whereas a subset controls the expression of key factors in a female-specific circuit. This study found the pupal Abd-A expression pattern was conserved between species with divergent pigmentation, indicating diversity resulted from changes to other loci. Collectively, these results reveal a greater complexity of the pigmentation network, presenting numerous opportunities to map transcription factor-CRE interactions that structure trait development and numerous candidate loci to investigate as potential targets of evolution.
Wednesday, January 29th
Daou, N., Lecolle, S., Lefebvre, S., Gaspera, B. D., Charbonnier, F., Chanoine, C. and Armand, A. S. (2013). A new role for the calcineurin/NFAT pathway in neonatal myosin heavy chain expression via the NFATc2/MyoD complex during mouse myogenesis. Development 140: 4914-4925. PubMed ID: 24301466
The calcineurin/NFAT (nuclear factor of activated T-cells, see Drosophila NFAT homolog) signaling pathway is involved in the modulation of the adult muscle fiber type, but its role in the establishment of the muscle phenotype remains elusive. This study shows that the NFAT member NFATc2 cooperates with the basic helix-loop-helix transcription factor MyoD (see Drosophila Nautilus) to induce the expression of a specific myosin heavy chain (MHC) isoform (see Drosophila Zipper), the neonatal one, during embryogenesis. This cooperation is crucial, as Myod/Nfatc2 double-null mice die at birth, with a dramatic reduction of the major neonatal MHC isoform normally expressed at birth in skeletal muscles, such as limb and intercostal muscles, whereas its expression is unaffected in myofibers mutated for either factor alone. Using gel shift and chromatin immunoprecipitation assays, NFATc2 was found bound to the neonatal Mhc gene, whereas NFATc1 and NFATc3 would preferentially bind the embryonic Mhc gene. Evidence is provided that MyoD synergistically cooperates with NFATc2 at the neonatal Mhc promoter. Altogether, these findings demonstrate that the calcineurin/NFAT pathway plays a new role in establishing the early muscle fiber type in immature myofibers during embryogenesis.
Chen, F. and Krasnow, M. A. (2014). Progenitor outgrowth from the niche in Drosophila trachea is guided by FGF from decaying branches. Science 343: 186-189. PubMed ID: 24408434
Although there has been progress identifying adult stem and progenitor cells and the signals that control their proliferation and differentiation, little is known about the substrates and signals that guide them out of their niche. By examining Drosophila tracheal outgrowth during metamorphosis, this study shows that progenitors follow a stereotyped path out of the niche, tracking along a subset of tracheal branches destined for destruction. The embryonic tracheal inducer branchless FGF (fibroblast growth factor) is expressed dynamically just ahead of progenitor outgrowth in decaying branches. Knockdown of branchless abrogates progenitor outgrowth, whereas misexpression redirects it. Thus, reactivation of an embryonic tracheal inducer in decaying branches directs outgrowth of progenitors that replace them. This explains how the structure of a newly generated tissue is coordinated with that of the old.
Weavers, H. and Skaer, H. (2013). Tip cells act as dynamic cellular anchors in the morphogenesis of looped renal tubules in Drosophila. Dev Cell 27: 331-344. PubMed ID: 24229645
Tissue morphogenesis involves both the sculpting of tissue shape and the positioning of tissues relative to one another in the body. Using the renal tubules of Drosophila, this study shows that a specific distal tubule cell regulates both tissue architecture and position in the body cavity. Focusing on the anterior tubules, it was demonstrated that tip cells make transient contacts with alary muscles at abdominal segment boundaries, moving progressively forward as convergent extension movements lengthen the tubule. Tip cell anchorage antagonizes forward-directed, Dpp/TGF-beta-guided tubule elongation, thereby ensuring the looped morphology characteristic of renal tubules from worms to humans. Distinctive tip cell exploratory behavior, adhesion, and basement membrane clearing underlie target recognition and dynamic interactions. Defects in these features obliterate tip cell anchorage, producing misshapen and misplaced tubules with impaired physiological function.
Tuesday, January 28th
Figard, L., Xu, H., Garcia, H. G., Golding, I. and Sokac, A. M. (2013). The Plasma Membrane Flattens Out to Fuel Cell-Surface Growth during Drosophila Cellularization. Dev Cell 27(6): 648-55. PubMed ID: 24316147
Cell-shape change demands cell-surface growth, but how growth is fueled and choreographed is still debated. This study used cellularization, the first complete cytokinetic event in Drosophila embryos, to show that cleavage furrow ingression is kinetically coupled to the loss of surface microvilli. Furrow kinetics were modulated with RNAi against the Rho1-GTPase regulator slow as molasses (slam), and furrow ingression controls the rate of microvillar depletion. Finally, the microvillar membrane was directly tracked, and it was seen to move along the cell surface and into ingressing furrows, independent of endocytosis. Together, these results demonstrate that the kinetics of the ingressing furrow regulate the utilization of a microvillar membrane reservoir. Because membranes of the furrow and microvilli are contiguous, it is suggested that ingression drives unfolding of the microvilli and incorporation of microvillar membrane into the furrow. It is concluded that plasma membrane folding/unfolding can contribute to the cell-shape changes that promote embryonic morphogenesis.
Estacio-Gomez, A. and Diaz-Benjumea, F. J. (2013). Roles of Hox genes in the patterning of the central nervous system of Drosophila. Fly (Austin) 8. PubMed ID: 24406332
One of the key aspects of functional nervous systems is the restriction of particular neural subtypes to specific regions, which permits the establishment of differential segment-specific neuromuscular networks. Although Hox genes play a major role in shaping the anterior-posterior body axis during animal development, understanding of how they act in individual cells to determine particular traits at precise developmental stages is rudimentary. This study has used the abdominal leucokinergic neurons (ABLKs) to address this issue. These neurons are generated during both embryonic and postembryonic neurogenesis by the same progenitor neuroblast, and are designated embryonic and postembryonic ABLKs, respectively. The genes of the Bithorax-Complex, Ultrabithorax (Ubx) and abdominal-A (abd-A) are redundantly required to specify the embryonic ABLKs. Moreover, the segment-specific pattern of the postembryonic ABLKs, which are restricted to the most anterior abdominal segments, is controlled by the absence of Abdominal-B (Abd-B), which was found to be able to repress the expression of the neuropeptide leucokinin. This and other examples of how Hox genes generate diversity within the central nervous system of Drosophila is discussed.
Shi, Q., Han, Y. and Jiang, J. (2014). Suppressor of fused impedes Ci/Gli nuclear import by opposing Trn/Kapbeta2 in Hedgehog signaling. J Cell Sci. PubMed ID: 24413177
The Hedgehog (Hh) family of secreted proteins governs a myriad of key developmental processes by regulating Ci/Gli transcription factors at multiple levels including nuclear/cytoplasmic shuttling. This study investigated the mechanism underlying the regulation of Ci/Gli subcellular localization by identifying and characterizing a novel nuclear localization sequence (NLS) in the N-terminal conserved domain of Ci/Gli that matches the PY-NLS consensus. This study demonstrates that the PY-NLS functions in parallel with a previously identified bipartite NLS to promote nucleaphenotyper localization and activity of full-length Ci. Transportin (Trn), a Drosophila homolog of Kapbeta2, is responsible for PY-NLS-mediated nuclear localization of Ci. Furthermore, it was shown that the tumor suppressor and conserved Hh pathway component Suppressor of fused (Sufu) opposes Trn-mediated Ci nuclear import by masking its PY-NLS. Finally, evidence is provided that Gli proteins also contain a functional PY-NLS and that mammal Sufu employs a similar mechanism to regulate Gli nuclear translocation. This study not only provides a mechanistic insight into how Sufu regulates Ci/Gli subcellular localization and Hh signaling but also reveals a role of Trn/Kapbeta2 in developmental regulation.
Monday, January 27th
Miyamoto, T. and Amrein, H. (2013). Diverse roles for the fructose sensor Gr43a. Fly (Austin) 8. PubMed ID: 24406333
The detection of nutrients, both in food and within the body, is crucial for the regulation of feeding behavior, growth and metabolism. While the molecular basis for sensing food chemicals by the taste system has been firmly linked to specific taste receptors, relatively little is known about the molecular nature of the sensors that monitor nutrients internally. Recent reports of taste receptors expressed in other organ systems, foremost in the gastrointestinal tract of mammals and insects, has led to the proposition that some taste receptors may also be used as sensors of internal nutrients. Indeed, direct evidence has been provided that the Drosophila gustatory receptor 43a (Gr43a) plays a critical role in sensing internal fructose levels in the fly brain. In addition to the brain and the taste system, Gr43a is also expressed in neurons of the proventricular ganglion and the uterus. This paper discusses the multiple potential roles of Gr43a in the fly. Evidence is provided that its activation in the brain is likely mediated by the neuropeptide Corazonin. Finally, it is posited that Gr43a may represent only a precedent for other taste receptors that sense internal nutrients, not only in flies but, quite possibly, in other animals, including mammals.
van Breugel, F. and Dickinson, M. H. (2014). Plume-Tracking Behavior of Flying Drosophila Emerges from a Set of Distinct Sensory-Motor Reflexes. Curr Biol. PubMed ID: 24440395
For a fruit fly, locating fermenting fruit where it can feed, find mates, and lay eggs is an essential and difficult task requiring the integration of olfactory and visual cues. An approach has been developed to correlate flies' free-flight behavior with their olfactory experience under different wind and visual conditions, yielding new insight into plume tracking based on over 70 hr of data. To localize an odor source, flies exhibit three iterative, independent, reflex-driven behaviors, which remain constant through repeated encounters of the same stimulus: (1) 190 ± 75 ms after encountering a plume, flies increase their flight speed and turn upwind, using visual cues to determine wind direction. Due to this substantial response delay, flies pass through the plume shortly after entering it. (2) 450 ± 165 ms after losing the plume, flies initiate a series of vertical and horizontal casts, using visual cues to maintain a crosswind heading. (3) After sensing an attractive odor, flies exhibit an enhanced attraction to small visual features, which increases their probability of finding the plume's source. Due to plume structure and sensory-motor delays, a fly's olfactory experience during foraging flights consists of short bursts of odor stimulation. As a consequence, delays in the onset of crosswind casting and the increased attraction to visual features are necessary behavioral components for efficiently locating an odor source. These results provide a quantitative behavioral background for elucidating the neural basis of plume tracking using genetic and physiological approaches.
von Philipsborn, A. C., Jorchel, S., Tirian, L., Demir, E., Morita, T., Stern, D. L. and Dickson, B. J. (2014). Cellular and Behavioral Functions of fruitless Isoforms in Drosophila Courtship. Curr Biol. PubMed ID: 24440391
Male-specific products of the fruitless (fru) gene control the development and function of neuronal circuits that underlie male-specific behaviors in Drosophila, including courtship. Alternative splicing generates at least three distinct Fru isoforms, each containing a different zinc-finger domain. This study examined the expression and function of each of these isoforms. Most fru+ cells were shown to express all three isoforms, yet each isoform has a distinct function in the elaboration of sexually dimorphic circuitry and behavior. The strongest impairment in courtship behavior is observed in fruC mutants, which fail to copulate, lack sine song, and do not generate courtship song in the absence of visual stimuli. Cellular dimorphisms in the fru circuit are dependent on FruC rather than other single Fru isoforms. Removal of FruC from the neuronal classes vAB3 or aSP4 leads to cell-autonomous feminization of arborizations and loss of courtship in the dark. These data map specific aspects of courtship behavior to the level of single fru isoforms and fru+ cell types - an important step toward elucidating the chain of causality from gene to circuit to behavior.
Pan, Y. and Baker, B. S. (2014). Genetic identification and separation of innate and experience-dependent courtship behaviors in Drosophila. Cell 156: 236-248. PubMed ID: 24439379
Wild-type D. melanogaster males innately possess the ability to perform a multistep courtship ritual to conspecific females. The potential for this behavior is specified by the male-specific products of the fruitless (fruM) gene; males without fruM do not court females when held in isolation. Such fruM null males acquire the potential for courtship when grouped with other flies; they apparently learn to court flies with which they were grouped, irrespective of sex or species and retain this behavior for at least a week. The male-specific product of the doublesex gene dsxM is necessary and sufficient for the acquisition of the potential for such experience-dependent courtship. These results reveal a process that builds, via dsxM and social experience, the potential for a more flexible sexual behavior, which could be evolutionarily conserved as dsx-related genes that function in sexual development are found throughout the animal kingdom.
Sunday, January 26th
Kondo, T., Isono, K., Kondo, K., Endo, T. A., Itohara, S., Vidal, M. and Koseki, H. (2014). Polycomb potentiates meis2 activation in midbrain by mediating interaction of the promoter with a tissue-specific enhancer. ev Cell 28: 94-101. PubMed ID: 24374176Summary:
Polycomb-group (PcG) proteins mediate repression of developmental regulators in a reversible manner, contributing to their spatiotemporally regulated expression. However, it is poorly understood how PcG-repressed genes are activated by developmental cues. This study used the mouse Meis2 gene (Drosophila homolog: Homofhorax) as a model to identify a role of a tissue-specific enhancer in removing PcG from the promoter. Meis2 repression in early development depends on binding of RING1B (see Drosophila Sex combs extra), an essential E3 component of PcG, to its promoter, coupled with its association with another RING1B-binding site (RBS) at the 3' end of the Meis2 gene. During early midbrain development, a midbrain-specific enhancer (MBE) transiently associates with the promoter-RBS, forming a promoter-MBE-RBS tripartite interaction in a RING1-dependent manner. Subsequently, RING1B-bound RBS dissociates from the tripartite complex, leaving promoter-MBE engagement to activate Meis2 expression. This study therefore demonstrates that PcG and/or related factors play a role in Meis2 activation by regulating the topological transition of cis-regulatory elements.
Son, J., Shen, S. S., Margueron, R. and Reinberg, D. (2013). . Nucleosome-binding activities within JARID2 and EZH1 regulate the function of PRC2 on chromatin. Genes Dev 27: 2663-2677. PubMed ID: 24352422
Polycomb-repressive complex 2 (PRC2) comprises specific members of the Polycomb group of epigenetic modulators. PRC2 catalyzes methylation of histone H3 at Lys 27 (H3K27me3) through its Enhancer of zeste (Ezh) constituent, of which there are two mammalian homologs: Ezh1 and Ezh2. Several ancillary factors, including Jarid2 (Drosophila homolog: Jumonji, AT rich interactive domain 2), modulate PRC2 function, with Jarid2 facilitating its recruitment to target genes. Jarid2, like Ezh2, is present in poorly differentiated and actively dividing cells, while Ezh1 associates with PRC2 in all cells, including resting cells. Jarid2 was found to exhibit nucleosome-binding activity that contributes to PRC2 stimulation. Moreover, such nucleosome-binding activity is exhibited by PRC2 comprising Ezh1 (PRC2-Ezh1), in contrast to PRC2-Ezh2. The presence of Ezh1 helps to maintain PRC2 occupancy on its target genes in myoblasts where Jarid2 is not expressed. These findings lead to a model in which PRC2-Ezh2 is important for the de novo establishment of H3K27me3 in dividing cells, whereas PRC2-Ezh1 is required for its maintenance in resting cells.
Shi, J., et al. (2013). Role of SWI/SNF in acute leukemia maintenance and enhancer-mediated Myc regulation. Genes Dev 27: 2648-2662. PubMed ID: 24285714
Cancer cells frequently depend on chromatin regulatory activities to maintain a malignant phenotype. This study shows that leukemia cells require the mammalian SWI/SNF chromatin remodeling complex for their survival and aberrant self-renewal potential. While Brg1 (see Drosophila Brahma), an ATPase subunit of SWI/SNF, is known to suppress tumor formation in several cell types, this study found that leukemia cells instead rely on Brg1 to support their oncogenic transcriptional program, which includes Myc (see Drosophila Myc) as one of its key targets. To account for this context-specific function, a cluster of lineage-specific enhancers located 1.7 Mb downstream from Myc was identified that are occupied by SWI/SNF as well as the BET protein Brd4 (Drosophila homolog female sterile (1) homeotic). Brg1 is required at these distal elements to maintain transcription factor occupancy and for long-range chromatin looping interactions with the Myc promoter. Notably, these distal Myc enhancers coincide with a region that is focally amplified in approximately 3% of acute myeloid leukemias. Together, these findings define a leukemia maintenance function for SWI/SNF that is linked to enhancer-mediated gene regulation, providing general insights into how cancer cells exploit transcriptional coactivators to maintain oncogenic gene expression programs.
Saturday, January 25th
Domanitskaya, E., Anllo, L. and Schupbach, T. (2013). Phantom, a cytochrome P450 enzyme essential for ecdysone biosynthesis, plays a critical role in the control of border cell migration in Drosophila. Dev Biol. PubMed ID: 24373956
The border cells of Drosophila are a model system for coordinated cell migration. Ecdysone signaling has been shown to act as the timing signal to initiate the migration process. This study found that mutations in phantom (phm), encoding an enzyme in the ecdysone biosynthesis pathway, block border cell migration when the entire follicular epithelium of an egg chamber is mutant, even when the associated germline cells (nurse cells and oocyte) are wildtype. Conversely, mutant germline cells survive and do not affect border cell migration, as long as the surrounding follicle cells are wildtype. Interestingly, even small patches of wildtype follicle cells in a mosaic epithelium are sufficient to allow the production of above-threshold levels of ecdysone to promote border cell migration. The same phenotype is observed with mutations in shade (shd), encoding the last enzyme in the pathway that converts ecdysone to the active 20-hydroxyecdysone. Administration of high 20-hydroxyecdysone titers in the medium can also rescue the border cell migration phenotype in cultured egg chambers with an entirely phm mutant follicular epithelium. These results indicate that in normal oogenesis, the follicle cell epithelium of each individual egg chamber must supply sufficient ecdysone precursors, leading ultimately to high enough levels of mature 20-hydroxyecdysone to the border cells to initiate their migration. Neither the germline, nor the neighboring egg chambers, nor the surrounding hemolymph appear to provide threshold amounts of 20-hydroxyecdysone to do so. This 'egg chamber autonomous' ecdysone synthesis constitutes a useful way to regulate the individual maturation of the asynchronous egg chambers present in the Drosophila ovary.
Sitnik, J. L., Francis, C., Hens, K., Huybrechts, R., Wolfner, M. F. and Callaerts, P. (2014). Neprilysins: An Evolutionarily Conserved Family of Metalloproteases That Play Important Roles in Reproduction in Drosophila. Genetics. PubMed ID: 24395329
Members of the M13 class of metalloproteases have been implicated in diseases and in reproductive fitness. Nevertheless, their physiological role remains poorly understood. To obtain a tractable model to analyze this protein family's function, the gene family was characterized in Drosophila melanogaster and focused on reproductive phenotypes. The D. melanogaster genome contains 24 M13 class protease homologs, some of which are orthologs of human proteases, including Neprilysin. Many are expressed in the reproductive tracts of either sex. Using RNAi the five Nep genes most closely related to vertebrate Neprilysin, Nep1-5, were individually targeted to investigate their roles in reproduction. A reduction in Nep1, Nep2, or Nep4 expression in females reduced egg-laying. Nep1 and Nep2 are required in the CNS and the spermathecae for wild-type fecundity. Females that are null for Nep2 also show defects as hosts of sperm competition as well as an increased rate of depletion for stored sperm. Furthermore, eggs laid by Nep2 mutant females are fertilized normally, but arrest early in embryonic development. In the male, only Nep1 was required to induce normal patterns of female egg-laying. Reduction in the expression of Nep2-5 in the male did not cause any dramatic effects on reproductive fitness, which suggests that these genes are either nonessential for male fertility or perform redundant functions. These results suggest that, consistent with the functions of neprilysins in mammals, these proteins are also required for reproduction in Drosophila, opening up this model system for further functional analysis of this protein class and their substrates.
Tan, J., Oh, K., Burgess, J., Hipfner, D. R. and Brill, J. A. (2014). PI4KIIIalpha is required for cortical integrity and cell polarity during Drosophila oogenesis. J Cell Sci. PubMed ID: 24413170
Phosphoinositides regulate myriad cellular processes, acting as potent signaling molecules in conserved signaling pathways and as organelle gatekeepers that recruit effector proteins to membranes. Phosphoinositide-generating enzymes have been studied extensively in yeast and cultured cells, yet their roles in animal development are not well understood. This study analyzed Drosophila melanogaster phosphatidylinositol (PI) 4-kinase IIIalpha (PI4KIIIalpha) during oogenesis. PI4KIIIalpha was found to be required for production of plasma membrane PI4P and PI(4,5)P2 and is crucial for actin organization, membrane trafficking and cell polarity. Female germ cells mutant for PI4KIIIalpha exhibit defects in cortical integrity associated with failure to recruit the cytoskeletal-membrane crosslinker Moesin and the exocyst subunit Sec5. These effects reflect a unique requirement for PI4KIIIalpha, as egg chambers from flies mutant for either of the other Drosophila PI4Ks, fwd or PI4KII, show Golgi but not plasma membrane phenotypes. Thus, PI4KIIIalpha is a critical regulator of a functionally distinct pool of PI4P that is essential for PI(4,5)P2-dependent processes in Drosophila development.
Friday, January 24th
Abreu-Blanco, M. T., Verboon, J. M. and Parkhurst, S. M. (2013). Coordination of Rho Family GTPase Activities to Orchestrate Cytoskeleton Responses during Cell Wound Repair. Curr Biol. PubMed ID: 24388847
Cells heal disruptions in their plasma membrane using a sophisticated, efficient, and conserved response involving the formation of a membrane plug and assembly of an actomyosin ring. This paper describes how Rho family GTPases modulate the cytoskeleton machinery during single cell wound repair in the genetically amenable Drosophila embryo model. Rho, Rac, and Cdc42 were found to rapidly accumulate around the wound and segregate into dynamic, partially overlapping zones. Genetic and pharmacological assays show that each GTPase makes specific contributions to the repair process. Rho1 is necessary for myosin II activation, leading to its association with actin. Rho1, along with Cdc42, is necessary for actin filament formation and subsequent actomyosin ring stabilization. Rac is necessary for actin mobilization toward the wound. These GTPase contributions are subject to crosstalk among the GTPases themselves and with the cytoskeleton. Rho1 GTPase was found to use several downstream effectors, including Diaphanous, Rok, and Pkn, simultaneously to mediate its functions. These results reveal that the three Rho GTPases are necessary to control and coordinate actin and myosin dynamics during single-cell wound repair in the Drosophila embryo. Wounding triggers the formation of arrays of Rho GTPases that act as signaling centers that modulate the cytoskeleton. In turn, coordinated crosstalk among the Rho GTPases themselves, as well as with the cytoskeleton, is required for assembly/disassembly and translocation of the actomyosin ring. The cell wound repair response is an example of how specific pathways can be activated locally in response to the cell's needs.
Bouissou, A., Verollet, C., de Forges, H., Haren, L., Bellafche, Y., Perez, F., Merdes, A. and Raynaud-Messina, B. (2014). γ-Tubulin Ring Complexes and EB1 play antagonistic roles in microtubule dynamics and spindle positioning. EMBO J 33: 114-128. PubMed ID: 24421324
γ-Tubulin is critical for microtubule (MT) assembly and organization. In metazoa, this protein acts in multiprotein complexes called γ-Tubulin Ring Complexes (γ-TuRCs). While the subunits that constitute γ-Tubulin Small Complexes (γ-TuSCs), the core of the MT nucleation machinery, are essential, mutation of γ-TuRC-specific proteins in Drosophila causes sterility and morphological abnormalities via hitherto unidentified mechanisms. This study demonstrates a role of γ-TuRCs in controlling spindle orientation independent of MT nucleation activity, both in cultured cells and in vivo, and examines a potential function for γ-TuRCs on astral MTs. γ-TuRCs locate along the length of astral MTs, and depletion of γ-TuRC-specific proteins increases MT dynamics and causes the plus-end tracking protein EB1 to redistribute along MTs. Moreover, suppression of MT dynamics through drug treatment or EB1 down-regulation rescues spindle orientation defects induced by γ-TuRC depletion. Therefore, a role ia preposed for γ-TuRCs in regulating spindle positioning by controlling the stability of astral MTs.
Mendes Maia, T., Gogendeau, D., Pennetier, C., Janke, C. and Basto, R. (2014). Bug22 influences cilium morphology and the post-translational modification of ciliary microtubules. Biol Open. PubMed ID: 24414207
Cilia and flagella are organelles essential for motility and sensing of environmental stimuli. Depending on the cell type, cilia acquire a defined set of functions and, accordingly, are built with an appropriate length and molecular composition. Several ciliary proteins display a high degree of conservation throughout evolution and mutations in ciliary genes are associated with various diseases such as ciliopathies and infertility. This study describes the role of the highly conserved ciliary protein, Bug22 (CG5343), in Drosophila. Previous studies in unicellular organisms have shown that Bug22 is required for proper cilia function, but its exact role in ciliogenesis has not been investigated yet. Null Bug22 mutant flies display cilia-associated phenotypes and nervous system defects. Furthermore, sperm differentiation is blocked at the individualization stage, due to impaired migration of the individualization machinery. Tubulin post-translational modifications (PTMs) such as polyglycylation, polyglutamylation or acetylation, are determinants of microtubule (MT) functions and stability in centrioles, cilia and neurons. Defects were found in the timely incorporation of polyglycylation in sperm axonemal MTs of Bug22 mutants. In addition, it was found that depletion of human Bug22 in RPE1 cells resulted in the appearance of longer cilia and reduced axonemal polyglutamylation. This work identifies Bug22 as a protein that plays a conserved role in the regulation of PTMs of the ciliary axoneme.
Thursday, January 23rd
Bao, S. (2014). Notch controls cell adhesion in the Drosophila eye. PLoS Genet 10: e1004087. PubMed ID: 24415957
Sporadic evidence suggests Notch is involved in cell adhesion. However, the underlying mechanism is unknown. This study has investigated an epithelial remodeling process in the Drosophila eye in which two primary pigment cells (PPCs) with a characteristic 'kidney' shape enwrap and eventually isolate a group of cone cells from inter-ommatidial cells (IOCs). This paper shows that in the developing Drosophila eye the ligand Delta was transcribed in cone cells and Notch was activated in the adjacent PPC precursors. In the absence of Notch, emerging PPCs failed to enwrap cone cells, and hibris (hbs) and sns, two genes coding for adhesion molecules of the Nephrin group that mediate preferential adhesion, were not transcribed in PPC precursors. Conversely, activation of Notch in single IOCs led to ectopic expression of hbs and sns. By contrast, in a single IOC that normally transcribes rst, a gene coding for an adhesion molecule of the Neph1 group that binds Hbs and Sns, activation of Notch led to a loss of rst transcription. In addition, in a Notch mutant where two emerging PPCs failed to enwrap cone cells, expression of hbs in PPC precursors restored the ability of these cells to surround cone cells. Further, expression of hbs or rst in a single rst- or hbs-expressing cell, respectively, led to removal of the counterpart from the membrane within the same cell through cis-interaction and forced expression of Rst in all hbs-expressing PPCs strongly disrupted the remodeling process. Finally, a loss of both hbs and sns in single PPC precursors led to constriction of the apical surface that compromised the 'kidney' shape of PPCs. Taken together, these results indicate that cone cells utilize Notch signaling to instruct neighboring PPC precursors to surround them and Notch controls the remodeling process by differentially regulating four adhesion genes.
Banreti, A., Hudry, B., Sass, M., Saurin, A. J. and Graba, Y. (2013). Hox Proteins Mediate Developmental and Environmental Control of Autophagy. Dev Cell. PubMed ID: 24389064
Hox genes encode evolutionarily conserved transcription factors, providing positional information used for differential morphogenesis along the anteroposterior axis. This study shows that Drosophila Hox proteins are potent repressors of the autophagic process. In inhibiting autophagy, Hox proteins display no apparent paralog specificity and do not provide positional information. Instead, they impose temporality on developmental autophagy and act as effectors of environmental signals in starvation-induced autophagy. Further characterization establishes that temporality is controlled by Pontin, a facultative component of the Brahma chromatin remodeling complex, and that Hox proteins impact on autophagy by repressing the expression of core components of the autophagy machinery. Finally, the potential of central and posterior mouse Hox proteins to inhibit autophagy in Drosophila and in vertebrate COS-7 cells indicates that regulation of autophagy is an evolutionary conserved feature of Hox proteins.
Su, C. Y., Kemp, H. A. and Moens, C. B. (2014). Cerebellar development in the absence of Gbx function in zebrafish. Dev Biol 386: 181-190. PubMed ID: 24183937
The midbrain-hindbrain boundary (MHB) is a well-known organizing center during vertebrate brain development. The MHB forms at the expression boundary of Otx2 and Gbx2 (homologs of Drosophila Orthodenticle and Unplugged respectively), mutually repressive homeodomain transcription factors expressed in the midbrain/forebrain and anterior hindbrain, respectively. The genetic hierarchy of gene expression at the MHB is complex, involving multiple positive and negative feedback loops that result in the establishment of non-overlapping domains of Wnt1 (see Drosophila Wingless) and Fgf8 (see Drosophila pyramus and thisbe) on either side of the boundary and the consequent specification of the cerebellum. The cerebellum derives from the dorsal part of the anterior-most hindbrain segment, rhombomere 1 (r1), which undergoes a distinctive morphogenesis to give rise to the cerebellar primordium within which the various cerebellar neuron types are specified. Previous studies in the mouse have shown that Gbx2 is essential for cerebellar development. Using zebrafish mutants this study shows that in the zebrafish gbx1 and gbx2 are required redundantly for morphogenesis of the cerebellar primordium and subsequent cerebellar differentiation, but that this requirement is alleviated by knocking down Otx. Expression of fgf8, wnt1 and the entire MHB genetic program is progressively lost in gbx1-;gbx2- double mutants but is rescued by Otx knock-down. This rescue of the MHB genetic program depends on rescued Fgf signaling, however the rescue of cerebellar primordium morphogenesis is independent of both Gbx and Fgf. Based on these findings a revised model is proprosed for the role of Gbx in cerebellar development.
Wednesday, January 22nd
Li, S., Wang, C., Sandanaraj, E., Aw, S. S., Koe, C. T., Wong, J. J., Yu, F., Ang, B. T., Tang, C. and Wang, H. (2014). The SCFSlimb E3 ligase complex regulates asymmetric division to inhibit neuroblast overgrowth. EMBO Rep. PubMed ID: 24413555
Drosophila larval brain neuroblasts divide asymmetrically to balance between self-renewal and differentiation. This study demonstrates that the SCFSlimb E3 ubiquitin ligase complex, which is composed of Cul1, SkpA, Roc1a and the F-box protein Supernumerary limbs (Slimb), inhibits ectopic neuroblast formation and regulates asymmetric division of neuroblasts. Hyperactivation of Akt leads to similar neuroblast overgrowth and defects in asymmetric division. Slimb associates with Akt in a protein complex, and SCFSlimb acts through SAK and Akt to inhibit neuroblast overgrowth. Moreover, Beta-transducin repeat containing, the human ortholog of Slimb, is frequently deleted in highly aggressive gliomas, suggesting a conserved tumor suppressor-like function.
Han, C., Song, Y., Xiao, H., Wang, D., Franc, N. C., Jan, L. Y. and Jan, Y. N. (2014). Epidermal Cells Are the Primary Phagocytes in the Fragmentation and Clearance of Degenerating Dendrites in Drosophila. Neuron. PubMed ID: 24412417
During developmental remodeling, neurites destined for pruning often degenerate on-site. Physical injury also induces degeneration of neurites distal to the injury site. Prompt clearance of degenerating neurites is important for maintaining tissue homeostasis and preventing inflammatory responses. This study shows that in both dendrite pruning and dendrite injury of Drosophila sensory neurons, epidermal cells rather than hemocytes are the primary phagocytes in clearing degenerating dendrites. Epidermal cells act via Draper-mediated recognition to facilitate dendrite degeneration and to engulf and degrade degenerating dendrites. Using multiple dendritic membrane markers to trace phagocytosis, it was shown that two members of the CD36 family, croquemort (crq) and debris buster (dsb), act at distinct stages of phagosome maturation for dendrite clearance. These findings reveals the physiological importance of coordination between neurons and their surrounding epidermis, for both dendrite fragmentation and clearance.
Stone, M. C., Albertson, R. M., Chen, L. and Rolls, M. M. (2014). Dendrite Injury Triggers DLK-Independent Regeneration. Cell Rep. PubMed ID: 24412365
Axon injury triggers regeneration through activation of a conserved kinase cascade, which includes the dual leucine zipper kinase (DLK: Wallenda). Although dendrites are damaged during stroke, traumatic brain injury, and seizure, it is not known whether mature neurons monitor dendrite injury and initiate regeneration. This study probed the response to dendrite damage using model Drosophila neurons. Two larval neuron types regrew dendrites in distinct ways after all dendrites were removed. Dendrite regeneration was also triggered by injury in adults. Next, whether dendrite injury was initiated with the same machinery as axon injury was tested. Surprisingly, DLK, JNK, and fos were dispensable for dendrite regeneration. Moreover, this MAP kinase pathway was not activated by injury to dendrites. Thus, neurons respond to dendrite damage and initiate regeneration without using the conserved DLK cascade that triggers axon regeneration.
Tuesday, January 21st
Mishra, A. K., Tsachaki, M., Rister, J., Ng, J., Celik, A. and Sprecher, S. G. (2013). Binary cell fate decisions and fate transformation in the Drosophila larval eye. PLoS Genet 9: e1004027. PubMed ID: 24385925
The functionality of sensory neurons is defined by the expression of specific sensory receptor genes. During the development of the Drosophila larval eye, photoreceptor neurons (PRs) make a binary choice to express either the blue-sensitive Rhodopsin 5 (Rh5) or the green-sensitive Rhodopsin 6 (Rh6). Later during metamorphosis, ecdysone signaling induces a cell fate and sensory receptor switch: Rh5-PRs are re-programmed to express Rh6 and become the eyelet, a small group of extraretinal PRs involved in circadian entrainment. However, the genetic and molecular mechanisms of how the binary cell fate decisions are made and switched remain poorly understood. This study shows that interplay of two transcription factors Senseless (Sens) and homeodomain transcription factor Hazy [PvuII-PstI homology 13, Pph13] control cell fate decisions, terminal differentiation of the larval eye and its transformation into eyelet. During initial differentiation, a pulse of Sens expression in primary precursors regulates their differentiation into Rh5-PRs and repression of an alternative Rh6-cell fate. Later, during the transformation of the larval eye into the adult eyelet, Sens serves as an anti-apoptotic factor in Rh5-PRs, which helps in promoting survival of Rh5-PRs during metamorphosis and is subsequently required for Rh6 expression. Comparably, during PR differentiation Hazy functions in initiation and maintenance of rhodopsin expression. Hazy represses Sens specifically in the Rh6-PRs, allowing them to die during metamorphosis. These findings show that the same transcription factors regulate diverse aspects of larval and adult PR development at different stages and in a context-dependent manner.
Bovetti, S., Bonzano, S., Garzotto, D., Giannelli, S. G., Iannielli, A., Armentano, M., Studer, M. and De Marchis, S. (2013). COUP-TFI controls activity-dependent tyrosine hydroxylase expression in adult dopaminergic olfactory bulb interneurons. Development 140: 4850-4859. PubMed ID: 24227652Summary:
COUP-TFI (Drosophila homolog: Seven up) is an orphan nuclear receptor acting as a strong transcriptional regulator in different aspects of forebrain embryonic development. In this study, COUP-TFI expression and function were investigated in the mouse olfactory bulb (OB), a highly plastic telencephalic region in which continuous integration of newly generated inhibitory interneurons occurs throughout life. OB interneurons belong to different populations that originate from distinct progenitor lineages. This study shows that COUP-TFI is highly expressed in tyrosine hydroxylase (TH)-positive dopaminergic interneurons in the adult OB glomerular layer (GL). Odour deprivation, which is known to downregulate TH expression in the OB, also downregulates COUP-TFI in dopaminergic cells, indicating a possible correlation between TH- and COUP-TFI-activity-dependent action. Moreover, conditional inactivation of COUP-TFI in the EMX1 lineage results in a significant reduction of both TH and ZIF268 expression in the GL. Finally, lentiviral vector-mediated COUP-TFI deletion in adult-generated interneurons confirmed that COUP-TFI acts cell-autonomously in the control of TH and ZIF268 expression. These data indicate that COUP-TFI regulates TH expression in OB cells through an activity-dependent mechanism involving ZIF268 induction and strongly argue for a maintenance rather than establishment function of COUP-TFI in dopaminergic commitment. This study reveals a previously unknown role for COUP-TFI in the adult brain as a key regulator in the control of sensory-dependent plasticity in olfactory dopaminergic neurons.
Giachino, C., Barz, M., Tchorz, J. S., Tome, M., Gassmann, M., Bischofberger, J., Bettler, B. and Taylor, V. (2014). GABA suppresses neurogenesis in the adult hippocampus through GABAB receptors. Development 141: 83-90. PubMed ID: 24284211
Adult neurogenesis is tightly regulated through the interaction of neural stem/progenitor cells (NSCs) with their niche. Neurotransmitters, including GABA activation of GABAA receptor ion channels (Drosophila homolog: Resistant to dieldrin), are important niche signals. This study shows that adult mouse hippocampal NSCs and their progeny express metabotropic GABAB receptors (see Metabotropic GABA-B receptors). Pharmacological inhibition of GABAB receptors stimulated NSC proliferation and genetic deletion of GABAB1 receptor subunits increased NSC proliferation and differentiation of neuroblasts in vivo. Cell-specific conditional deletion of GABAB receptors supports a cell-autonomous role in newly generated cells. These data indicate that signaling through GABAB receptors is an inhibitor of adult neurogenesis.
Monday, January 20th
Steinecke, A., Gampe, C., Zimmer, G., Rudolph, J. and Bolz, J. (2014). EphA/ephrin A reverse signaling promotes the migration of cortical interneurons from the medial ganglionic eminence. Development 141: 460-471. PubMed ID: 24381199
Inhibitory interneurons control the flow of information and synchronization in the cerebral cortex at the circuit level. During embryonic development, multiple subtypes of cortical interneurons are generated in different regions of the ventral telencephalon, such as the medial and caudal ganglionic eminence (MGE and CGE), as well as the preoptic area (POA). These neurons then migrate over long distances towards their cortical target areas. Diverse families of diffusible and cell-bound signaling molecules, including the Eph/ephrin (see Drosophila Ephrin and Ephrin receptor tyrosine kinase) system, regulate and orchestrate interneuron migration. Ephrin A3 and A5, for instance, are expressed at the borders of the pathway of MGE-derived interneurons and prevent these cells from entering inappropriate regions via EphA4 forward signaling. This study found that MGE-derived interneurons, in addition to EphA4, also express ephrin A and B ligands, suggesting Eph/ephrin forward and reverse signaling in the same cell. In vitro and in vivo approaches showed that EphA4-induced reverse signaling in MGE-derived interneurons promotes their migration and that this effect is mediated by ephrin A2 ligands. In EphA4 mutant mice, as well as after ephrin A2 knockdown using in utero electroporation, delayed interneuron migration was found at embryonic stages. Thus, besides functions in guiding MGE-derived interneurons to the cortex through forward signaling, this study describes a novel role of the ephrins in driving these neurons to their target via reverse signaling,
Agoston, Z., Heine, P., Brill, M. S., Grebbin, B. M., Hau, A. C., Kallenborn-Gerhardt, W., Schramm, J., Gotz, M. and Schulte, D. (2014). Meis2 is a Pax6 co-factor in neurogenesis and dopaminergic periglomerular fate specification in the adult olfactory bulb. Development 141: 28-38. PubMed ID: 24284204
Meis homeodomain transcription factors (see Drosophila Homothorax) control cell proliferation, cell fate specification and differentiation in development and disease. Previous studies have largely focused on Meis contribution to the development of non-neuronal tissues. By contrast, Meis function in the brain is not well understood. Here, we provide evidence for a dual role of the Meis family protein Meis2 in adult olfactory bulb (OB) neurogenesis. Meis2 is strongly expressed in neuroblasts of the subventricular zone (SVZ) and rostral migratory stream (RMS) and in some of the OB interneurons that are continuously replaced during adult life. Targeted manipulations with retroviral vectors expressing function-blocking forms or with small interfering RNAs demonstrated that Meis activity is cell-autonomously required for the acquisition of a general neuronal fate by SVZ-derived progenitors in vivo and in vitro. Additionally, Meis2 activity in the RMS is important for the generation of dopaminergic periglomerular neurons in the OB. Chromatin immunoprecipitation identified doublecortin and tyrosine hydroxylase as direct Meis targets in newly generated neurons and the OB, respectively. Furthermore, biochemical analyses revealed a previously unrecognized complex of Meis2 with Pax6 (see Drosophila Eyeless) and Dlx2 (see Drosophila Distal-less), two transcription factors involved in OB neurogenesis. The full pro-neurogenic activity of Pax6 in SVZ derived neural stem and progenitor cells requires the presence of Meis. Collectively, these results show that Meis2 cooperates with Pax6 in generic neurogenesis and dopaminergic fate specification in the adult SVZ-OB system.
Dyer, C., Blanc, E., Hanisch, A., Roehl, H., Otto, G. W., Yu, T., Basson, M. A. and Knight, R. (2014). A bi-modal function of Wnt signalling directs an FGF activity gradient to spatially regulate neuronal differentiation in the midbrain. Development 141: 63-72. PubMed ID: 24284206
FGFs (see Drosophila Branchless) and Wnts (see Drosophila Wingless) are important morphogens during midbrain development, but their importance and potential interactions during neurogenesis are poorly understood. This study employed a combination of genetic and pharmacological manipulations in zebrafish to show that during neurogenesis FGF activity occurs as a gradient along the anterior-posterior axis of the dorsal midbrain and directs spatially dynamic expression of the Hairy gene her5 (see Drosophila Hairy). As FGF activity diminishes during development, Her5 is lost and differentiation of neuronal progenitors occurs in an anterior-posterior manner. Mathematical models were generated to explain how Wnt and FGFs direct the spatial differentiation of neurons in the midbrain through Wnt regulation of FGF signalling. These models suggested that a negative-feedback loop controlled by Wnt is crucial for regulating FGF activity. Sprouty genes (see Drosophila Sprouty) were tested as mediators of this regulatory loop using conditional mouse knockouts and pharmacological manipulations in zebrafish. These reveal that Sprouty genes direct the positioning of early midbrain neurons and are Wnt responsive in the midbrain. A model is proposed in which Wnt regulates FGF activity at the isthmus by driving both FGF and Sprouty gene expression. This controls a dynamic, posteriorly retracting expression of her5 that directs neuronal differentiation in a precise spatiotemporal manner in the midbrain.
Sunday, January 19th
Alexandre, C., Baena-Lopez, A. and Vincent, J. P. (2014). Patterning and growth control by membrane-tethered Wingless. Nature 505: 180-185. PubMed ID: 24390349
Wnts are evolutionarily conserved secreted signalling proteins that, in various developmental contexts, spread from their site of synthesis to form a gradient and activate target-gene expression at a distance. However, the requirement for Wnts to spread has never been directly tested. This study used genome engineering to replace the endogenous wingless gene, which encodes the main Drosophila Wnt, with one that expresses a membrane-tethered form of the protein. Surprisingly, the resulting flies were viable and produced normally patterned appendages of nearly the right size, albeit with a delay. In the prospective wing, prolonged wingless transcription followed by memory of earlier signalling allows persistent expression of relevant target genes. It is suggested therefore that the spread of Wingless is dispensable for patterning and growth even though it probably contributes to increasing cell proliferation.
Steinfeld, J., Steinfeld, I., Coronato, N., Hampel, M. L., Layer, P. G., Araki, M. and Vogel-Hopker, A. (2013). RPE specification in the chick is mediated by surface ectoderm-derived BMP and Wnt signalling. Development 140: 4959-4969. PubMed ID: 24227655
The retinal pigment epithelium (RPE) is indispensable for vertebrate eye development and vision. In the classical model of optic vesicle patterning, the surface ectoderm produces fibroblast growth factors (FGFs) that specify the neural retina (NR) distally, whereas TGFbeta family members released from the proximal mesenchyme are involved in RPE specification. However, it was previously proposed that bone morphogenetic proteins (BMPs; see Drosophila Decapentaplegic) released from the surface ectoderm are essential for RPE specification in chick. This study now shows that the BMP- and Wnt-expressing surface ectoderm is required for RPE specification. Wnt signalling (see Drosophila Wingless) from the overlying surface ectoderm is involved in restricting BMP-mediated RPE specification to the dorsal optic vesicle. Wnt2b is expressed in the dorsal surface ectoderm and subsequently in dorsal optic vesicle cells. Activation of Wnt signalling by implanting Wnt3a-soaked beads or inhibiting GSK3beta (see Drosophila Shaggy) at optic vesicle stages inhibits NR development and converts the entire optic vesicle into RPE. Surface ectoderm removal at early optic vesicle stages or inhibition of Wnt, but not Wnt/beta-catenin, signalling prevents pigmentation and downregulates the RPE regulatory gene Mitf (see Drosophila Mitf). Activation of BMP or Wnt signalling can replace the surface ectoderm to rescue MITF expression and optic cup formation. Evidence is provided that BMPs and Wnts cooperate via a GSK3beta-dependent but beta-catenin-independent pathway at the level of pSmad (see Drosophila Pangolin) to ensure RPE specification in dorsal optic vesicle cells. A new dorsoventral model of optic vesicle patterning is proposed, whereby initially surface ectoderm-derived Wnt signalling directs dorsal optic vesicle cells to develop into RPE through a stabilising effect of BMP signalling.
Saturday, January 18th
Spahn, P., Huelsmann, S., Rehorn, K. P., Mischke, S., Mayer, M., Casali, A. and Reuter, R. (2013). Multiple regulatory safeguards confine the expression of the GATA factor serpent to the hemocyte primordium within the Drosophila mesoderm. Dev Biol. PubMed ID: 24360907
Serpent (srp) encodes a GATA-factor that controls various aspects of embryogenesis in Drosophila, such as fatbody development, gut differentiation and hematopoiesis. During hematopoiesis, srp expression is required in the embryonic head mesoderm and the larval lymph gland, the two known hematopoietic tissues of Drosophila, to obtain mature hemocytes. srp expression in the hemocyte primordium is known to depend on snail and buttonhead, but the regulatory complexity that defines the primordium has not been addressed yet. This study found that srp is sufficient to transform trunk mesoderm into hemocytes. Two disjoint cis-regulatory modules were identified that direct the early expression in the hemocyte primordium and the late expression in mature hemocytes and lymph gland, respectively. During embryonic hematopoiesis, a combination of snail, buttonhead, empty spiracles and even-skipped confines the mesodermal srp expression to the head region. This restriction to the head mesoderm is crucial as ectopic srp in mesodermal precursors interferes with the development of mesodermal derivates and promotes hemocytes and fatbody development. Thus, several genes work in a combined fashion to restrain early srp expression to the head mesoderm in order to prevent expansion of the hemocyte primordium.
Riddle, M. R., Weintraub, A., Nguyen, K. C., Hall, D. H. and Rothman, J. H. (2013). Transdifferentiation and remodeling of post-embryonic C. elegans cells by a single transcription factor. Development 140: 4844-4849. PubMed ID: 24257624
Terminally differentiated post-mitotic cells are generally considered irreversibly developmentally locked, i.e. incapable of being reprogrammed in vivo into entirely different cell types. Brief expression of a single transcription factor, the ELT-7 GATA factor (homolog of Drosophila Serpent), can convert the identity of fully differentiated, highly specialized non-endodermal cells of the pharynx into fully differentiated intestinal cells in intact larvae and adult Caenorhabditis elegans. Stable expression of intestine-specific molecular markers parallels loss of markers for the original differentiated pharynx state; hence, there is no apparent requirement for a dedifferentiated intermediate during the transdifferentiation process. Based on high-resolution morphological characteristics, the transdifferentiated cells become remodeled to resemble typical intestinal cells at the level of both the cell surface and internal organelles. Thus, post-mitotic cells, though terminally differentiated, remain plastic to transdifferentiation across germ layer lineage boundaries and can be remodeled to adopt the characteristics of a new cell identity without removal of inhibitory factors. These findings establish a simple model to investigate how cell context influences forced transdifferentiation of mature cells.
Atallah, J., Vurens, G., Mavong, S., Mutti, A., Hoang, D. and Kopp, A. (2013). Sex-specific repression of dachshund is required for Drosophila sex comb development. Dev Biol 386(2): 440-7. PubMed ID: 24361261
The origin of new morphological structures requires the establishment of new genetic regulatory circuits to control their development, from initial specification to terminal differentiation. The upstream regulatory genes are usually the first to be identified, while the mechanisms that translate novel regulatory information into phenotypic diversity often remain obscure. In particular, elaborate sex-specific structures that have evolved in many animal lineages are inevitably controlled by sex-determining genes, but the genetic basis of sexually dimorphic cell differentiation is rarely understood. This report examines the role of dachshund (dac), a gene with a deeply conserved function in sensory organ and appendage development, in the sex comb, a recently evolved male-specific structure found in some Drosophila species. dac is shown to acts during metamorphosis to restrict sex comb development to the appropriate leg region. Localized repression of dac by the sex determination pathway is necessary for male-specific morphogenesis of sex comb bristles. This pupal function of dac is separate from its earlier role in leg patterning, and Dac at this stage is not dependent on the pupal expression of Distalless (Dll), the main regulator of dac during the larval period. Dll acts in the epithelial cells surrounding the sex comb during pupal development to promote sex comb rotation, a complex cellular process driven by coordinated cell rearrangement. These results show that genes with well-conserved developmental functions can be re-used at later stages in development to regulate more recently evolved traits. This mode of gene co-option may be an important driver of evolutionary innovations.
Friday, January 17th
Yokoyama, H., Nakos, K., Santarella-Mellwig, R., Rybina, S., Krijgsveld, J., Koffa, M. D. and Mattaj, I. W. (2013). CHD4 Is a RanGTP-Dependent MAP that Stabilizes Microtubules and Regulates Bipolar Spindle Formation. Curr Biol 23: 2443-2451,
PubMed ID: 24268414
Summary: Production of the GTP-bound form of the Ran GTPase (RanGTP; Drosophila Ran) around chromosomes induces spindle assembly by activating nuclear localization signal (NLS)-containing proteins. Several NLS proteins have been identified as spindle assembly factors, but the complexity of the process led to search for additional proteins with distinct roles in spindle assembly. This study identified a chromatin-remodeling ATPase, CHD4 (54% identical to Drosophila Mi2), as a RanGTP-dependent microtubule (MT)-associated protein (MAP). MT binding occurs via the region containing an NLS and chromatin-binding domains. In Xenopus egg extracts and cultured cells, CHD4 largely dissociates from mitotic chromosomes and partially localizes to the spindle. Immunodepletion of CHD4 from egg extracts significantly reduces the quantity of MTs produced around chromatin and prevents spindle assembly. CHD4 RNAi in both HeLa and Drosophila S2 cells induces defects in spindle assembly and chromosome alignment in early mitosis, leading to chromosome missegregation. Further analysis in egg extracts and in HeLa cells reveals that CHD4 is a RanGTP-dependent MT stabilizer. Moreover, the CHD4-containing NuRD complex promotes organization of MTs into bipolar spindles in egg extracts. Importantly, this function of CHD4 is independent of chromatin remodeling. These results uncover a new role for CHD4 as a MAP required for MT stabilization and involved in generating spindle bipolarity.
Joyce, E. F., Apostolopoulos, N., Beliveau, B. J. and Wu, C. T. (2013). Germline Progenitors Escape the Widespread Phenomenon of Homolog Pairing during Drosophila Development. PLoS Genet 9: e1004013. PubMed ID: 24385920
Homolog pairing, which plays a critical role in meiosis, poses a potential risk if it occurs in inappropriate tissues or between nonallelic sites, as it can lead to changes in gene expression, chromosome entanglements, and loss-of-heterozygosity due to mitotic recombination. This is particularly true in Drosophila, which supports organismal-wide pairing throughout development. Discovered over a century ago, such extensive pairing has led to the perception that germline pairing in the adult gonad is an extension of the pairing established during embryogenesis and, therefore, differs from the mechanism utilized in most species to initiate pairing specifically in the germline. This study shows that, contrary to long-standing assumptions, Drosophila meiotic pairing in the gonad is not an extension of pairing established during embryogenesis. Instead, it was found that homologous chromosomes are unpaired in primordial germ cells from the moment the germline can be distinguished from the soma in the embryo and remain unpaired even in the germline stem cells of the adult gonad. It was further established that pairing originates immediately after the stem cell stage. This pairing occurs well before the initiation of meiosis and, strikingly, continues through the several mitotic divisions preceding meiosis. These discoveries indicate that the spatial organization of the Drosophila genome differs between the germline and the soma from the earliest moments of development and thus argue that homolog pairing in the germline is an active process as versus a passive continuation of pairing established during embryogenesis.
Thursday, January 16th
Kohl, J., Ostrovsky, A. D., Frechter, S. and Jefferis, G. S. (2013). A bidirectional circuit switch reroutes pheromone signals in male and female brains. Cell 155: 1610-1623. PubMed ID: 24360281
The Drosophila pheromone cis-vaccenyl acetate (cVA) elicits different behaviors in males and females. First- and second-order olfactory neurons show identical pheromone responses, suggesting that sex genes differentially wire circuits deeper in the brain. Using in vivo whole-cell electrophysiology, this study has shown that two clusters of third-order olfactory neurons have dimorphic pheromone responses. One cluster responds in females; the other responds in males. These clusters are present in both sexes and share a common input pathway, but sex-specific wiring reroutes pheromone information. Regulating dendritic position, the Fruitless transcription factor both connects the male-responsive cluster and disconnects the female-responsive cluster from pheromone input. Selective masculinization of third-order neurons transforms their morphology and pheromone responses, demonstrating that circuits can be functionally rewired by the cell-autonomous action of a switch gene. This bidirectional switch, analogous to an electrical changeover switch, provides a simple circuit logic to activate different behaviors in males and females.
Lee, Y. (2013). Contribution of Drosophila TRPA1-Expressing Neurons to Circadian Locomotor Activity Patterns. PLoS One 8: e85189. PubMed ID: 24367706
In both vertebrates and invertebrates, Transient Receptor Potential (TRP) channels are expressed in sensory neurons and mediate environmental stimuli such as light, sound, temperature, and taste. Some of these channels, however, are expressed only in the brain and their functions remain incompletely understood. Using the GAL4/UAS binary system with a line in which the GAL4 had been knocked into the trpA1 locus in Drosophila, new insights have been recently been reported into TRPA1 localization and function, including its expression in approximately 15% of all circadian neurons. TRPA1 is expressed in lateral posterior neurons (LPNs), which are known to be highly sensitive to entrainment by temperature cycles. This study used the bacterial sodium channel, NaChBac, to examine the effects of altering the electrical properties of trpA1 neurons on circadian rhythms. The results indicate that circadian activity of the flies in the morning, daytime, and evening is affected in a temperature-dependent manner following TRPA1 neuronal activation. Remarkably, TRPA1 neuron activation in flies kept at 18 degrees C impact the morning peak of circadian activity even though TRPA1 is not expressed in morning cells. Taken together, these results suggest that the activation of TRPA1-expressing neurons may differentially coordinate light/dark circadian entrainment, depending on the temperature.
Kulalert, W. and Kim, D. H. (2013). The Unfolded Protein Response in a Pair of Sensory Neurons Promotes Entry of C. elegans into Dauer Diapause. Curr Biol 23: 2540-2545. PubMed ID: 24316205
In response to unfavorable environmental conditions such as starvation, crowding, and elevated temperature, Caenorhabditis elegans larvae enter an alternative developmental stage known as dauer, which is characterized by adaptive changes in stress resistance and metabolism. The genetic dissection of the molecular mechanisms of the C. elegans dauer developmental decision has defined evolutionarily conserved signaling pathways of organismal neuroendocrine physiology. This study has identified a mechanism by which a dominant mutation in a neuronal insulin gene, daf-28sa191 (see Insulin receptor pathway in C. elegans), causes constitutive entry into dauer diapause. Expression of the mutant DAF-28 insulin peptide results in endoplasmic reticulum (ER) stress in the ASI pair of chemosensory neurons. The neuronal ER stress does not compromise cellular survival but activates PEK-1, the C. elegans ortholog of the mammalian eIF2alpha kinase PERK, which in turn phosphorylates Ser49 of eIF2alpha (see Drosophila eIF2alpha), specifically in the ASI neuron pair, to promote entry into dauer diapause. These data establish a novel role for ER stress and the unfolded protein response (UPR) in promoting entry into dauer diapause and suggest that, in addition to cell-autonomous activities in the maintenance of ER homeostasis, the UPR may act in a non-cell-autonomous manner to promote organismal adaptation to stress during larval development.
Wednesday, January 15th
Di Giovannantonio, L. G., Di Salvio, M., Omodei, D., Prakash, N., Wurst, W., Pierani, A., Acampora, D. and Simeone, A. (2014). Otx2 cell-autonomously determines dorsal mesencephalon versus cerebellum fate independently of isthmic organizing activity. Development 141: 377-388. PubMed ID: 24335253
During embryonic development, the rostral neuroectoderm is regionalized into broad areas that are subsequently subdivided into progenitor compartments with specialized identity and fate. These events are controlled by signals emitted by organizing centers and interpreted by target progenitors, which activate superimposing waves of intrinsic factors restricting their identity and fate. The transcription factor Otx2 (Drosophila homolog: Orthodenticle) plays a crucial role in mesencephalic development by positioning the midbrain-hindbrain boundary (MHB) and its organizing activity. This study investigated whether Otx2 is cell-autonomously required to control identity and fate of dorsal mesencephalic progenitors. With this aim, Otx2 was inactivated in the Pax7+ (Drosophila homolog: Paired) dorsal mesencephalic domain, previously named m1, without affecting MHB integrity. The Pax7+ m1 domain can be further subdivided into a dorsal Zic1+ m1a and a ventral Zic1- m1b sub-domain (Zic1 Drosophila homolog is Odd-paired). Loss of Otx2 in the m1a (Pax7+ Zic1+) sub-domain impairs the identity and fate of progenitors, which undergo a full switch into a coordinated cerebellum differentiation program. By contrast, in the m1b sub-domain (Pax7+ Zic1-) Otx2 is prevalently required for post-mitotic transition of mesencephalic GABAergic precursors. Moreover, genetic cell fate, BrdU cell labeling and Otx2 conditional inactivation experiments indicate that in Otx2 mutants all ectopic cerebellar cell types, including external granule cell layer (EGL) precursors, originate from the m1a progenitor sub-domain and that reprogramming of mesencephalic precursors into EGL or cerebellar GABAergic progenitors depends on temporal sensitivity to Otx2 ablation. Together, these findings indicate that Otx2 intrinsically controls different aspects of dorsal mesencephalic neurogenesis. In this context, Otx2 is cell-autonomously required in the m1a sub-domain to suppress cerebellar fate and promote mesencephalic differentiation independently of the MHB organizing activity.
Green, M. J., Myat, A. M., Emmenegger, B. A., Wechsler-Reya, R. J., Wilson, L. J. and Wingate, R. J. (2014). Independently specified Atoh1 domains define novel developmental compartments in rhombomere 1. Development 141: 389-398. PubMed ID: 24381197
The rhombic lip gives rise to neuronal populations that contribute to cerebellar, proprioceptive and interoceptive networks. Cell production depends on the expression of the basic helix-loop-helix (bHLH) transcription factor Atoh1. In rhombomere 1, Atoh1-positive cells (Atoh1 Drosophila homolog is Atonal) give rise to both cerebellar neurons and extra-cerebellar nuclei in ventral hindbrain. The origin of this cellular diversity has previously been attributed to temporal signals rather than spatial patterning. This study shows that in both chick and mouse the cerebellar Atoh1 precursor pool is partitioned into initially cryptic spatial domains that reflect the activity of two different organisers: an isthmic Atoh1 domain, which gives rise to isthmic nuclei, and the rhombic lip, which generates deep cerebellar nuclei and granule cells. A combination of in vitro explant culture, genetic fate mapping and gene overexpression and knockdown was used to explore the role of isthmic signalling in patterning these domains. An FGF-dependent isthmic Atoh1 domain was shown to be the origin of distinct populations of Lhx9-positive neurons in the extra-cerebellar isthmic nuclei. In the cerebellum, ectopic FGF induces proliferation while blockade reduces the length of the cerebellar rhombic lip. FGF signalling is not required for the specification of cerebellar cell types from the rhombic lip and its upregulation inhibits their production. This suggests that although the isthmus regulates the size of the cerebellar anlage, the downregulation of isthmic FGF signals is required for induction of rhombic lip-derived cerebellar neurons.
Zhang, F., Bhattacharya, A., Nelson, J. C., Abe, N., Gordon, P., Lloret-Fernandez, C., Maicas, M., Flames, N., Mann, R. S., Colon-Ramos, D. A. and Hobert, O. (2014). The LIM and POU homeobox genes ttx-3 and unc-86 act as terminal selectors in distinct cholinergic and serotonergic neuron types. Development 141: 422-435. PubMed ID: 24353061
Transcription factors that drive neuron type-specific terminal differentiation programs in the developing nervous system are often expressed in several distinct neuronal cell types, but to what extent they have similar or distinct activities in individual neuronal cell types is generally not well explored. This problem was investigated using, as a starting point, the C. elegans LIM homeodomain transcription factor ttx-3 (Drosophila homolog: Apterous), which acts as a terminal selector to drive the terminal differentiation program of the cholinergic AIY interneuron class. Using a panel of different terminal differentiation markers, including neurotransmitter synthesizing enzymes, neurotransmitter receptors and neuropeptides, it was shown that ttx-3 also controls the terminal differentiation program of two additional, distinct neuron types, namely the cholinergic AIA interneurons and the serotonergic NSM neurons. The type of differentiation program that is controlled by ttx-3 in different neuron types is specified by a distinct set of collaborating transcription factors. One of the collaborating transcription factors is the POU homeobox gene unc-86 (Drosophila homolog Pdm-1), which collaborates with ttx-3 to determine the identity of the serotonergic NSM neurons. unc-86 in turn operates independently of ttx-3 in the anterior ganglion where it collaborates with the ARID-type transcription factor cfi-1 to determine the cholinergic identity of the IL2 sensory and URA motor neurons. In conclusion, transcription factors operate as terminal selectors in distinct combinations in different neuron types, defining neuron type-specific identity features (Zhang, 2014).
Tuesday, January 14th
Cassidy, J. J., Jha, A. R., Posadas, D. M., Giri, R., Venken, K. J., Ji, J., Jiang, H., Bellen, H. J., White, K. P. and Carthew, R. W. (2013). miR-9a Minimizes the Phenotypic Impact of Genomic Diversity by Buffering a Transcription Factor. Cell 155: 1556-1567. PubMed ID: 24360277
Gene expression has to withstand stochastic, environmental, and genomic perturbations. For example, in the latter case, 0.5%-1% of the human genome is typically variable between any two unrelated individuals. Such diversity might create problematic variability in the activity of gene regulatory networks and, ultimately, in cell behaviors. Using multigenerational selection experiments, this study found that for the Drosophila proneural network, the effect of genomic diversity is dampened by miR-9a-mediated regulation of senseless expression. Reducing miR-9a regulation of the Senseless transcription factor frees the genomic landscape to exert greater phenotypic influence. Whole-genome sequencing identified genomic loci that potentially exert such effects. A larger set of sequence variants, including variants within proneural network genes, exhibits these characteristics when miR-9a concentration is reduced. These findings reveal that microRNA-target interactions may be a key mechanism by which the impact of genomic diversity on cell behavior is dampened.
Heraud-Farlow, J. E., et al. (2013). Staufen2 Regulates Neuronal Target RNAs. Cell Rep 5: 1511-1518. PubMed ID: 24360961
RNA-binding proteins play crucial roles in directing RNA translation to neuronal synapses. Staufen2 (Stau2) has been implicated in both dendritic RNA localization and synaptic plasticity in mammalian neurons. This study reports the identification of functionally relevant Stau2 target mRNAs in neurons. The majority of Stau2-copurifying mRNAs expressed in the hippocampus are present in neuronal processes, further implicating Stau2 in dendritic mRNA regulation. Stau2 targets are enriched for secondary structures similar to those identified in the 3' UTRs of Drosophila Staufen targets. Next, it was shown that Stau2 regulates steady-state levels of many neuronal RNAs and that its targets are predominantly downregulated in Stau2-deficient neurons. Detailed analysis confirms that Stau2 stabilizes the expression of one synaptic signaling component, the regulator of G protein signaling 4 (Rgs4) mRNA, via its 3' UTR. This study defines the global impact of Stau2 on mRNAs in neurons, revealing a role in stabilization of the levels of synaptic targets.
Minakhina, S., Changela, N. and Steward, R. (2014). Zfrp8/PDCD2 is required in ovarian stem cells and interacts with the piRNA pathway machinery. Development 141: 259-268. PubMed ID: 24381196
The maintenance of stem cells is central to generating diverse cell populations in many tissues throughout the life of an animal. Elucidating the mechanisms involved in how stem cells are formed and maintained is crucial to understanding both normal developmental processes and the growth of many cancers. Previously, studies have shown that Zfrp8/PDCD2 is essential for the maintenance of Drosophila hematopoietic stem cells. This study shows that Zfrp8/PDCD2 is also required in both germline and follicle stem cells in the Drosophila ovary. Expression of human PDCD2 fully rescues the Zfrp8 phenotype, underlining the functional conservation of Zfrp8/PDCD2. The piRNA pathway is essential in early oogenesis, and this study found that nuclear localization of Zfrp8 in germline stem cells and their offspring is regulated by some piRNA pathway genes. Zfrp8 forms a complex with the piRNA pathway protein Maelstrom and controls the accumulation of Maelstrom in the nuage. Furthermore, Zfrp8 regulates the activity of specific transposable elements also controlled by Maelstrom and Piwi. These
results suggest that Zfrp8/PDCD2 is not an integral member of the piRNA pathway, but has an overlapping function, possibly competing with Maelstrom and Piwi.
Monday, January 13th
Lecona, E., Rojas, L. A., Bonasio, R., Johnston, A., Fernandez-Capetillo, O. and Reinberg, D. (2013). Polycomb Protein SCML2 Regulates the Cell Cycle by Binding and Modulating CDK/CYCLIN/p21 Complexes. PLoS Biol 11: e1001737. PubMed ID: 24358021
Polycomb group (PcG) proteins are transcriptional repressors of genes involved in development and differentiation, and also maintain repression of key genes involved in the cell cycle, indirectly regulating cell proliferation. The human SCML2 gene, a mammalian homologue of the Drosophila PcG protein SCM, encodes two protein isoforms: SCML2A that is bound to chromatin and SCML2B that is predominantly nucleoplasmic. SCML2B was purified and found to form a stable complex with CDK/CYCLIN/p21 and p27, enhancing the inhibitory effect of p21/p27 (see Drosophila Dacapo). SCML2B participates in the G1/S checkpoint by stabilizing p21 and favoring its interaction with CDK2/CYCE, resulting in decreased kinase activity and inhibited progression through G1. In turn, CDK/CYCLIN complexes phosphorylate SCML2, and the interaction of SCML2B with CDK2 is regulated through the cell cycle. These findings highlight a direct crosstalk between the Polycomb system of cellular memory and the cell-cycle machinery in mammals.
Mardaryev, A. N., et al. (2014). p63 and Brg1 control developmentally regulated higher-order chromatin remodelling at the epidermal differentiation complex locus in epidermal progenitor cells. Development 141: 101-111. PubMed ID: 24346698
Chromatin structural states and their remodelling, including higher-order chromatin folding and three-dimensional (3D) genome organisation, play an important role in the control of gene expression. The role of 3D genome organisation in the control and execution of lineage-specific transcription programmes during the development and differentiation of multipotent stem cells into specialised cell types remains poorly understood. This study shows that substantial remodelling of the higher-order chromatin structure of the epidermal differentiation complex (EDC), a keratinocyte lineage-specific gene locus on mouse chromosome 3, occurs during epidermal morphogenesis. During epidermal development, the locus relocates away from the nuclear periphery towards the nuclear interior into a compartment enriched in SC35-positive nuclear speckles. Relocation of the EDC locus occurs prior to the full activation of EDC genes involved in controlling terminal keratinocyte differentiation and is a lineage-specific, developmentally regulated event controlled by transcription factor p63 (see Drosophila p53), a master regulator of epidermal development. It was also shown that, in epidermal progenitor cells, p63 directly regulates the expression of the ATP-dependent chromatin remodeller Brg1 (see Drosophila Brahma), which binds to distinct domains within the EDC and is required for relocation of the EDC towards the nuclear interior. Furthermore, Brg1 also regulates gene expression within the EDC locus during epidermal morphogenesis. Thus, p63 and its direct target Brg1 play an essential role in remodelling the higher-order chromatin structure of the EDC and in the specific positioning of this locus within the landscape of the 3D nuclear space, as required for the efficient expression of EDC genes in epidermal progenitor cells during skin development.
Li, H. Y., Grifone, R., Saquet, A., Carron, C. and Shi, D. L. (2013). The Xenopus homologue of Down syndrome critical region protein 6 drives dorsoanterior gene expression and embryonic axis formation by antagonising polycomb group proteins. Development 140: 4903-4913. PubMed ID: 24301465
Mesoderm and embryonic axis formation in vertebrates is mediated by maternal and zygotic factors that activate the expression of target genes. Transcriptional derepression plays an important role in the regulation of expression in different contexts; however, its involvement and possible mechanism in mesoderm and embryonic axis formation are largely unknown. This study demonstrates that XDSCR6, a Xenopus homologue of human Down syndrome critical region protein 6 (DSCR6, or RIPPLY3), regulates mesoderm and embryonic axis formation through derepression of polycomb group (PcG) proteins. Xdscr6 maternal mRNA is enriched in the endoderm of the early gastrula and potently triggers the formation of dorsal mesoderm and neural tissues in ectoderm explants; it also dorsalises ventral mesoderm during gastrulation and induces a secondary embryonic axis. A WRPW motif, which is present in all DSCR6 homologues, is necessary and sufficient for the dorsal mesoderm- and axis-inducing activity. Knockdown of Xdscr6 inhibits dorsal mesoderm gene expression and results in head deficiency. XDSCR6 physically interacts with PcG proteins through the WRPW motif, preventing the formation of PcG bodies and antagonising their repressor activity in embryonic axis formation. By chromatin immunoprecipitation, it was demonstrated that XDSCR6 releases PcG proteins from chromatin and allows dorsal mesoderm gene transcription. These studies suggest that XDSCR6 might function to sequester PcG proteins and identify a novel derepression mechanism implicated in embryonic induction and axis formation.
Sunday, December 12th
Morishita, J., Kang, M. J., Fidelin, K. and Ryoo, H. D. (2013). CDK7 Regulates the Mitochondrial Localization of a Tail-Anchored Proapoptotic Protein, Hid. Cell Rep 5: 14888. PubMed ID: 24360962
The mitochondrial outer membrane is a major site of apoptosis regulation across phyla. Human and C. elegans Bcl-2 family proteins and Drosophila Hid require the C-terminal tail-anchored (TA) sequence in order to insert into the mitochondrial membrane, but it remains unclear whether cytosolic proteins actively regulate the mitochondrial localization of these proteins. This study reports that the cdk7 complex regulates the mitochondrial localization of Hid and its ability to induce apoptosis. cdk7 was identified through an in vivo RNAi screen of genes required for cell death. Although CDK7 is best known for its role in transcription and cell-cycle progression, a hypomorphic cdk7 mutant suppresses apoptosis without impairing these other known functions. In this cdk7 mutant background, Hid fails to localize to the mitochondria and fails to bind to recombinant inhibitors of apoptosis (IAPs). These findings indicate that apoptosis is promoted by a newly identified function of CDK7, which couples the mitochondrial localization and IAP binding of Hid.
Yang, C. S., Sinenko, S. A., Thomenius, M. J., Robeson, A. C., Freel, C. D., Horn, S. R. and Kornbluth, S. (2013). The deubiquitinating enzyme DUBAI stabilizes DIAP1 to suppress Drosophila apoptosis. Cell Death Differ. PubMed ID: 24362437
Deubiquitinating enzymes (DUBs) counteract ubiquitin ligases to modulate the ubiquitination and stability of target signaling molecules. In Drosophila, the ubiquitin-proteasome system has a key role in the regulation of apoptosis, most notably, by controlling the abundance of the central apoptotic regulator DIAP1. Although the mechanism underlying DIAP1 ubiquitination has been extensively studied, the precise role of DUB(s) in controlling DIAP1 activity has not been fully investigated. This study reports the identification of a DIAP1-directed DUB using two complementary approaches. First, a panel of putative Drosophila DUBs was expressed in S2 cells to determine whether DIAP1 could be stabilized, despite treatment with death-inducing stimuli that would induce DIAP1 degradation. In addition, RNAi fly lines were used to detect modifiers of DIAP1 antagonist-induced cell death in the developing eye. Together, these approaches identified a previously uncharacterized protein encoded by CG8830, which was named DeUBiquitinating-Apoptotic-Inhibitor (DUBAI), as a novel DUB capable of preserving DIAP1 to dampen Drosophila apoptosis. DUBAI interacts with DIAP1 in S2 cells, and the putative active site of its DUB domain (C367) is required to rescue DIAP1 levels following apoptotic stimuli. DUBAI, therefore, represents a novel locus of apoptotic regulation in Drosophila, antagonizing cell death signals that would otherwise result in DIAP1 degradation.
Tasdemir-Yilmaz, O. E. and Freeman, M. R. (2013). Astrocytes engage unique molecular programs to engulf pruned neuronal debris from distinct subsets of neurons. Genes Dev. 28(1): 20-33. PubMed ID: 24361692
Precise neural circuit assembly is achieved by initial overproduction of neurons and synapses, followed by refinement through elimination of exuberant neurons and synapses. Glial cells are the primary cells responsible for clearing neuronal debris, but the cellular and molecular basis of glial pruning is poorly defined. This study shows that Drosophila larval astrocytes transform into phagocytes through activation of a cell-autonomous, steroid-dependent program at the initiation of metamorphosis and are the primary phagocytic cell type in the pupal neuropil. The developmental elimination was examined of two neuron populations - mushroom body (MB) gamma neurons and vCrz+ neurons (expressing Corazonin [Crz] neuropeptide in the ventral nerve cord [VNC]) - where only neurites are pruned or entire cells are eliminated, respectively. MB gamma axons were found to be engulfed by astrocytes using the Draper and Crk/Mbc/dCed-12 signaling pathways in a partially redundant manner. In contrast, while elimination of vCrz+ cell bodies requires Draper, elimination of vCrz+ neurites is mediated by Crk/Mbc/dCed-12 but not Draper. Intriguingly, it was also found that elimination of Draper delayed vCrz+ neurite degeneration, suggesting that glia promote neurite destruction through engulfment signaling. This study identifies a novel role for astrocytes in the clearance of synaptic and neuronal debris and for Crk/Mbc/dCed-12 as a new glial pathway mediating pruning and reveals, unexpectedly, that the engulfment signaling pathways engaged by glia depend on whether neuronal debris was generated through cell death or local pruning.
Saturday, January 11th
Sanchez-Higueras, C., Sotillos, S. and Castelli-Gair Hombria, J. (2013). Common Origin of Insect Trachea and Endocrine Organs from a Segmentally Repeated Precursor. Curr Biol. 24(1):76-81. PubMed ID: 24332544
Segmented organisms have serially repeated structures that become specialized in some segments. The Drosophila corpora allata, prothoracic glands, and trachea are shown to have a homologous origin and can convert into each other. The tracheal epithelial tubes develop from ten trunk placodes, and homologous ectodermal cells in the maxilla and labium form the corpora allata and the prothoracic glands. The early endocrine and trachea gene networks are similar, with STAT and Hox genes inducing their activation. The initial invagination of the trachea and the endocrine primordia is identical, but activation of Snail in the glands induces an epithelial-mesenchymal transition (EMT), after which the corpora allata and prothoracic gland primordia coalesce and migrate dorsally, joining the corpora cardiaca to form the ring gland. It is proposed that the arthropod ectodermal endocrine glands and respiratory organs arose through an extreme process of divergent evolution from a metameric repeated structure.
Acharya, S., Laupsien, P., Wenzl, C., Yan, S. and Grosshans, J. (2013). Function and dynamics of slam in furrow formation in early Drosophila embryo. Dev Biol. PubMed ID: 24368071
The Drosophila embryo undergoes a developmental transition, cellularization, in the blastoderm stage switching from syncytial to cellular development. The cleavage furrow, which encloses nuclei into cells, is a prominent morphological feature of this transition. It is not clear how the pattern of the furrow array is defined and how zygotic genes trigger the formation and invagination of interphase furrows. A key to these questions is provided by the gene slow as molasses (slam), which has been previously implicated in controlling furrow invagination. This study investigated the null phenotype of slam, the dynamics of Slam protein, and its control by the recycling endosome. slam was found to be essential for furrow invagination during cellularisation and together with nullo, for specification of the furrow. During cellularisation, Slam marks first the furrow, which is derived from the metaphase furrow of the previous mitosis. Slightly later, Slam accumulates at new furrows between daughter cells early in interphase. Slam is stably associated with the furrow canal except for the onset of cellularisation as revealed by FRAP experiments. Restriction of Slam to the furrow canal and Slam mobility during cellularisation is controlled by the recycling endosome and centrosomes. A three step model is proposed. The retracting metaphase furrow leaves an initial mark. This mark and the border between corresponding daughter nuclei are refined by vesicular transport away from pericentrosomal recycling endosome towards the margins of the somatic buds. Following the onset of zygotic gene expression, Slam and Nullo together stabilise this mark and Slam triggers invagination of the cleavage furrow.
Roth-Johnson, E. A., Vizcarra, C. L., Bois, J. S. and Quinlan, M. E. (2013). Interaction between microtubules and the Drosophila formin Cappuccino and its effect on actin assembly. J Biol Chem. PubMed ID: 24362037
Formin family actin nucleators are potential coordinators of the actin and microtubule cytoskeletons, as they can both nucleate actin filaments and bind microtubules in vitro. To gain a more detailed mechanistic understanding of formin-microtubule interactions and formin-mediated actin-microtubule crosstalk, microtubule binding by Cappuccino (Capu), a formin involved in regulating actin and microtubule organization was studied during Drosophila oogenesis. Two distinct domains were found within Capu; FH2 and tail, work together to promote high-affinity microtubule binding. The tail domain appears to bind microtubules through non-specific charge-based interactions. In contrast, distinct residues within the FH2 domain are important for microtubule binding. The first visualization is reported of a formin polymerizing actin filaments in the presence of microtubules. Interestingly, microtubules are potent inhibitors of Capu's actin nucleation activity but appear to have little effect on Capu once it is bound to the barbed end of an elongating filament. Because Capu does not simultaneously bind microtubules and assemble actin filaments in vitro, its actin assembly and microtubule binding activities likely require spatial and/or temporal regulation within the Drosophila oocyte.
Friday, January 10th
Campbell, D. S. and Okamoto, H. (2013). Local caspase activation interacts with Slit-Robo signaling to restrict axonal arborization. Local caspase activation interacts with Slit-Robo signaling to restrict axonal arborization. J Cell Biol 203: 657-672. PubMed ID: 24385488
In addition to being critical for apoptosis, components of the apoptotic pathway, such as caspases, are involved in other physiological processes in many types of cells, including neurons. However, very little is known about their role in dynamic, nonphysically destructive processes, such as axonal arborization and synaptogenesis (see Drosophila Axonogenesis). This study shows that caspases are locally active in vivo at the branch points of young, dynamic retinal ganglion cell axonal arbors but not in the cell body or in stable mature arbors. Caspase activation, dependent on Caspase-3, Caspase-9 (see Drosophila Death related ced-3), and p38 mitogen-activated protein kinase (MAPK; see Drosophila p38b), rapidly increased at branch points corresponding with branch tip addition. Time-lapse imaging revealed that knockdown of Caspase-3 and Caspase-9 led to more stable arbors and presynaptic sites. Genetic analysis showed that Caspase-3, Caspase-9, and p38 MAPK interacted with Slit1a-Robo2 (see Drosophila Slit) signaling, suggesting that localized activation of caspases lie downstream of a ligand receptor system, acting as key promoters of axonal branch tip and synaptic dynamics to restrict arbor growth in vivo in the central nervous system.
Gu, T., Zhao, T. and Hewes, R. S. (2013). Insulin signaling regulates neurite growth during metamorphic neuronal remodeling. Biol Open. PubMed ID: 24357229
Although the growth capacity of mature neurons is often limited, some neurons can shift through largely unknown mechanisms from stable maintenance growth to dynamic, organizational growth (e.g. to repair injury, or during development transitions). During insect metamorphosis, many terminally differentiated larval neurons undergo extensive remodeling, involving elimination of larval neurites and outgrowth and elaboration of adult-specific projections. This study shows in the fruit fly that a metamorphosis-specific increase in insulin signaling promotes neuronal growth and axon branching after prolonged stability during the larval stages. FOXO, a negative effector in the insulin signaling pathway, blocks metamorphic growth of peptidergic neurons that secrete the neuropeptides CCAP and bursicon. RNA interference and CCAP/bursicon cell-targeted expression of dominant-negative constructs for other components of the insulin signaling pathway (InR, Pi3K92E, Akt1, S6K) also partially suppresses the growth of the CCAP/bursicon neuron somata and neurite arbor. In contrast, expression of wild-type or constitutively active forms of InR, Pi3K92E, Akt1, Rheb, and TOR, as well as RNA interference for negative regulators of insulin signaling (PTEN, FOXO), stimulate overgrowth. Interestingly, InR displays little effect on larval CCAP/bursicon neuron growth, in contrast to its strong effects during metamorphosis. Manipulations of insulin signaling in many other peptidergic neurons revealed generalized growth stimulation during metamorphosis, but not during larval development. These findings reveal a fundamental shift in growth control mechanisms when mature, differentiated neurons enter a new phase of organizational growth. Moreover, they highlight strong evolutionarily conservation of insulin signaling in neuronal growth regulation.
Inagaki, H. K., Jung, Y., Hoopfer, E. D., Wong, A. M., Mishra, N., Lin, J. Y., Tsien, R. Y. and Anderson, D. J. (2013). Optogenetic control of Drosophila using a red-shifted channelrhodopsin reveals experience-dependent influences on courtship. Nat Methods. PubMed ID: 24363022
Optogenetics allows the manipulation of neural activity in freely moving animals with millisecond precision, but its application in Drosophila has been limited. This study shows that a recently described red activatable channelrhodopsin (ReaChR) permits control of complex behavior in freely moving adult flies, at wavelengths that are not thought to interfere with normal visual function. This tool affords the opportunity to control neural activity over a broad dynamic range of stimulation intensities. Using time-resolved activation, this study shows that the neural control of male courtship song can be separated into (1) probabilistic, persistent and (2) deterministic, command-like components. The former, but not the latter, neurons are subject to functional modulation by social experience, which supports the idea that they constitute a locus of state-dependent influence. This separation is not evident using thermogenetic tools, a result underscoring the importance of temporally precise control of neuronal activation in the functional dissection of neural circuits in Drosophila.
Thursday, January 9th
Zhang, Z., Feng, J., Pan, C., Lv, X., Wu, W., Zhou, Z., Liu, F., Zhang, L. and Zhao, Y. (2013). Atrophin-Rpd3 complex represses Hedgehog signaling by acting as a corepressor of CiR. J Cell Biol 203: 575-583. PubMed ID: 24385484
The evolutionarily conserved Hedgehog (Hh) signaling pathway is transduced by the Cubitus interruptus (Ci)/Gli family of transcription factors that exist in two distinct repressor (CiR/GliR) and activator (CiA/GliA) forms. Aberrant activation of Hh signaling is associated with various human cancers, but the mechanism through which CiRGliR properly represses target gene expression is poorly understood. This study used Drosophila and zebrafish models to define a repressor function of Atrophin (Atro) in Hh signaling. Atro directly binds to Ci through its C terminus. The N terminus of Atro interacts with a histone deacetylase, Rpd3, to recruit it to a Ci-binding site at the decapentaplegic (dpp) locus and reduce dpp transcription through histone acetylation regulation. The repressor function of Atro in Hh signaling is dependent on Ci. Furthermore, Rerea, a homologue of Atro in zebrafish, represses the expression of Hh-responsive genes. It is proposed that the Atro-Rpd3 complex plays a conserved role to function as a CiR corepressor.
Folker, E. S., Schulman, V. K. and Baylies, M. K. (2013). Translocating myonuclei have distinct leading and lagging edges that require Kinesin and Dynein. Development 141(2): 355-66. PubMed ID: 24335254
Nuclei are precisely positioned within all cells, and mispositioned nuclei are a hallmark of many muscle diseases. Myonuclear positioning is dependent on Kinesin and Dynein, but interactions between these motor proteins and their mechanisms of action are unclear. This study found that in developing Drosophila muscles, Dynein and Kinesin work together to move nuclei in a single direction by two separate mechanisms that are spatially segregated. First, the two motors work together in a sequential pathway that acts from the cell cortex at the muscle poles. This mechanism requires Kinesin-dependent localization of Dynein to cell cortex near the muscle pole. From this location Dynein can pull microtubule minus-ends and the attached myonuclei toward the muscle pole. Second, the motors exert forces directly on individual nuclei independently of the cortical pathway. However, the activities of the two motors on the nucleus are polarized relative to the direction of myonuclear translocation: Kinesin acts at the leading edge of the nucleus, whereas Dynein acts at the lagging edge of the nucleus. Consistent with the activities of Kinesin and Dynein being polarized on the nucleus, nuclei rarely change direction, and those that do, reorient to maintain the same leading edge. Conversely, nuclei in both Kinesin and Dynein mutant embryos change direction more often and do not maintain the same leading edge when changing directions. These data implicate Kinesin and Dynein in two distinct and independently regulated mechanisms of moving myonuclei, which together maximize the ability of myonuclei to achieve their proper localizations within the constraints imposed by embryonic development.
Tutor, A. S., Prieto-Sanchez, S. and Ruiz-Gomez, M. (2013). Src64B phosphorylates Dumbfounded and regulates slit diaphragm dynamics: Drosophila as a model to study nephropathies. Development 141(2): 367-76. PubMed ID: 24335255
Drosophila nephrocytes are functionally homologous to vertebrate kidney podocytes. Both share the presence of slit diaphragms that function as molecular filters during the process of blood and haemolymph ultrafiltration. The protein components of the slit diaphragm are likewise conserved between flies and humans, but the mechanisms that regulate slit diaphragm dynamics in response to injury or nutritional changes are still poorly characterised. This study shows that Dumbfounded/Neph1, a key diaphragm constituent, is a target of the Src kinase Src64B. Loss of Src64B activity leads to a reduction in the number of diaphragms, and this effect is in part mediated by loss of Dumbfounded/Neph1 tyrosine phosphorylation. The phosphorylation of Duf by Src64B, in turn, regulates Duf association with the actin regulator Dock. Diaphragm damage induced by administration of the drug puromycin aminonucleoside (PAN model) directly associates with Src64B hyperactivation, suggesting that diaphragm stability is controlled by Src-dependent phosphorylation of diaphragm components. These findings indicate that the balance between diaphragm damage and repair is controlled by Src-dependent phosphorylation of diaphragm components, and point to Src family kinases as novel targets for the development of pharmacological therapies for the treatment of kidney diseases that affect the function of the glomerular filtration barrier.
Wednesday, January 8th
Liang, X., Madrid, J., Gartner, R., Verbavatz, J. M., Schiklenk, C., Wilsch-Brauninger, M., Bogdanova, A., Stenger, F., Voigt, A. and Howard, J. (2013). A NOMPC-dependent membrane-microtubule connector is a candidate for the gating spring in fly mechanoreceptors. Curr Biol 23: 755-763. PubMed ID: 23583554
Mechanoreceptors contain compliant elements, termed 'gating springs,' that transfer macroscopic stimuli impinging on the cells into microscopic stimuli that open the mechanosensitive channels. Evidence for gating springs comes from mechanical experiments; they have not been identified molecularly or ultrastructurally. This study shows that the filamentous structures that connect the plasma membrane to the microtubules are compliant structural elements in the mechanoreceptive organelle of fly campaniform receptors. These filaments colocalize with the ankyrin-repeat domain of the transient receptor potential (TRP) channel NOMPC. In addition, they resemble the purified ankyrin-repeat domain in size and shape. Most importantly, these filaments are nearly absent in nompC mutants and can be rescued by the nompC gene. Finally, mechanical modeling suggests that the filaments provide most of the compliance in the distal tip of the cell, thought to be the site of mechanotransduction. These results provide strong evidence that the ankyrin-repeat domains of NOMPC structurally contribute to the membrane-microtubule connecting filaments. These filaments, as the most compliant element in the distal tip, are therefore good candidates for the gating springs.
Shahidullah, M., Le Marchand, S. J., Fei, H., Zhang, J., Pandey, U. B., Dalva, M. B., Pasinelli, P. and Levitan, I. B. (2013). Defects in Synapse Structure and Function Precede Motor Neuron Degeneration in Drosophila Models of FUS-Related ALS. J Neurosci 33: 19590-19598. PubMed ID: 24336723
Amyotrophic lateral sclerosis (ALS) is an adult-onset neurodegenerative disease that leads invariably to fatal paralysis associated with motor neuron degeneration and muscular atrophy. One gene associated with ALS encodes the DNA/RNA-binding protein Fused in Sarcoma (FUS). There now exist two Drosophila models of ALS. In one, human FUS with ALS-causing mutations is expressed in fly motor neurons; in the other, the gene cabeza (caz), the fly homolog of FUS, is ablated. These FUS-ALS flies exhibit larval locomotor defects indicative of neuromuscular dysfunction and early death. The locus and site of initiation of this neuromuscular dysfunction remain unclear. This study shows that in FUS-ALS flies, motor neuron cell bodies fire action potentials that propagate along the axon and voltage-dependent inward and outward currents in the cell bodies are indistinguishable in wild-type and FUS-ALS motor neurons. In marked contrast, the amplitude of synaptic currents evoked in the postsynaptic muscle cell is decreased by >80% in FUS-ALS larvae. Furthermore, the frequency but not unitary amplitude of spontaneous miniature synaptic currents is decreased dramatically in FUS-ALS flies, consistent with a change in quantal content but not quantal size. Although standard confocal microscopic analysis of the larval neuromuscular junction reveals no gross abnormalities, superresolution stimulated emission depletion (STED) microscopy demonstrates that the presynaptic active zone protein Bruchpilot is aberrantly organized in FUS-ALS larvae. The results are consistent with the idea that defects in presynaptic terminal structure and function precede, and may contribute to, the later motor neuron degeneration that is characteristic of ALS.
Walls, S. M., Jr., Attle, S. J., Brulte, G. B., Walls, M. L., Finley, K. D., Chatfield, D. A., Herr, D. R. and Harris, G. L. (2013). Identification of Sphingolipid Metabolites That Induce Obesity via Misregulation of Appetite, Caloric Intake and Fat Storage in Drosophila. PLoS Genet 9: e1003970. PubMed ID: 24339790
Obesity is defined by excessive lipid accumulation. However, the active mechanistic roles that lipids play in its progression are not understood. Accumulation of ceramide, the metabolic hub of sphingolipid metabolism, has been associated with metabolic syndrome and obesity in humans and model systems. This study used Drosophila genetic manipulations to cause accumulation or depletion of ceramide and sphingosine-1-phosphate (S1P) intermediates. Sphingolipidomic profiles were characterized across mutants for various sphingolipid metabolic genes using liquid chromatography electrospray ionization tandem mass spectroscopy. Biochemical assays and microscopy were used to assess classic hallmarks of obesity including elevated fat stores, increased body weight, resistance to starvation induced death, increased adiposity, and fat cell hypertrophy. Multiple behavioral assays were used to assess appetite, caloric intake, meal size and meal frequency. Additionally,DNA microarrays were used to profile differential gene expression between these flies, which mapped to changes in lipid metabolic pathways. The results show that accumulation of ceramides is sufficient to induce obesity phenotypes by two distinct mechanisms: 1) Dihydroceramide (C14:0) and ceramide diene (C14:2) accumulation lowered fat store mobilization by reducing adipokinetic hormone- producing cell functionality and 2) Modulating the S1P: ceramide (C14:1) ratio suppressed postprandial satiety via the hindgut-specific neuropeptide like receptor dNepYr, resulting in caloric intake-dependent obesity.
Tuesday, January 7th
McNamara, R. P., McCann, J. L., Gudipaty, S. A. and D'Orso, I. (2013). Transcription Factors Mediate the Enzymatic Disassembly of Promoter-Bound 7SK snRNP to Locally Recruit P-TEFb for Transcription Elongation. Cell Rep 5: 1256-1268. PubMed ID: 24316072
The transition from transcription initiation into elongation is controlled by transcription factors, which recruit positive transcription elongation factor b (P-TEFb) to promoters to phosphorylate RNA polymerase II. A fraction of P-TEFb is recruited as part of the inhibitory 7SK small nuclear ribonucleoprotein particle (snRNP), which inactivates the kinase and prevents elongation. However, it is unclear how P-TEFb is captured from the promoter-bound 7SK snRNP to activate elongation. This study describes a mechanism by which transcription factors mediate the enzymatic release of P-TEFb (Drosophila Cyclin T/CDK9) from the 7SK snRNP at promoters to trigger activation in a gene-specific manner. Tat recruits PPM1G/PP2Cγ to locally disassemble P-TEFb from the 7SK snRNP at the HIV promoter via dephosphorylation of the kinase T loop. Similar to Tat, nuclear factor (NF)-κB recruits PPM1G in a stimulus-dependent manner to activate elongation at inflammatory-responsive genes. Recruitment of PPM1G to promoter-assembled 7SK snRNP provides a paradigm for rapid gene activation through transcriptional pause release.
Nishihara, T., Zekri, L., Braun, J. E. and Izaurralde, E. (2013). miRISC recruits decapping factors to miRNA targets to enhance their degradation. Nucleic Acids Res 41: 8692-8705. PubMed ID: 23863838
MicroRNA (miRNA)-induced silencing complexes (miRISCs) repress translation and promote degradation of miRNA targets. Target degradation occurs through the 5'-to-3' messenger RNA (mRNA) decay pathway, wherein, after shortening of the mRNA poly(A) tail, the removal of the 5' cap structure by decapping triggers irreversible decay of the mRNA body. This study demonstrates that miRISC enhances the association of the decapping activators DCP1, Me31B and HPat with deadenylated miRNA targets that accumulate when decapping is blocked. DCP1 and Me31B recruitment by miRISC occurs before the completion of deadenylation. Remarkably, miRISC recruits DCP1, Me31B and HPat to engineered miRNA targets transcribed by RNA polymerase III, which lack a cap structure, a protein-coding region and a poly(A) tail. Furthermore, miRISC can trigger decapping and the subsequent degradation of mRNA targets independently of ongoing deadenylation. Thus, miRISC increases the local concentration of the decapping machinery on miRNA targets to facilitate decapping and irreversibly shut down their translation.
Tisdale, S., Lotti, F., Saieva, L., Van Meerbeke, J. P., Crawford, T. O., Sumner, C. J., Mentis, G. Z. and Pellizzoni, L. (2013). SMN Is Essential for the Biogenesis of U7 Small Nuclear Ribonucleoprotein and 3'-End Formation of Histone mRNAs. Cell Rep 5: 1187-1195. PubMed ID: 24332368
Spinal muscular atrophy (SMA) is a neurodegenerative disease caused by a deficiency in the survival motor neuron (SMN; see Drosophila SMN) protein. SMN mediates the assembly of spliceosomal small nuclear ribonucleoproteins (snRNPs) and possibly other RNPs. This study investigated SMN requirement for the biogenesis and function of U7-an snRNP specialized in the 3'-end formation of replication-dependent histone mRNAs that normally are not polyadenylated. SMN deficiency impairs U7 snRNP assembly and decreases U7 levels in mammalian cells. The SMN-dependent U7 reduction affects endonucleolytic cleavage of histone mRNAs leading to abnormal accumulation of 3'-extended and polyadenylated transcripts followed by downstream changes in histone gene expression. Importantly, SMN deficiency induces defects of histone mRNA 3'-end formation in both SMA mice and human patients. These findings demonstrate that SMN is essential for U7 biogenesis and histone mRNA processing in vivo and identify an additional RNA pathway disrupted in SMA.
Monday, January 6th
Pi, H. J., Hangya, B., Kvitsiani, D., Sanders, J. I., Huang, Z. J. and Kepecs, A. (2013). Cortical interneurons that specialize in disinhibitory control. Nature 503: 521-524. PubMed ID: 24097352
In the mammalian cerebral cortex the diversity of interneuronal subtypes underlies a division of labour subserving distinct modes of inhibitory control. A unique mode of inhibitory control may be provided by inhibitory neurons that specifically suppress the firing of other inhibitory neurons. Such disinhibition could lead to the selective amplification of local processing and serve the important computational functions of gating and gain modulation. Although several interneuron populations are known to target other interneurons to varying degrees, little is known about interneurons specializing in disinhibition and their in vivo function. This study shows that a class of interneurons that express vasoactive intestinal polypeptide (VIP) mediates disinhibitory control in multiple areas of neocortex and is recruited by reinforcement signals. By combining optogenetic activation with single-cell recordings, the functional role of VIP interneurons was examined in awake mice, and the underlying circuit mechanisms was investigated in vitro in auditory and medial prefrontal cortices. A basic disinhibitory circuit module was identified in which activation of VIP interneurons transiently suppresses primarily somatostatin- and a fraction of parvalbumin-expressing inhibitory interneurons that specialize in the control of the input and output of principal cells, respectively. During the performance of an auditory discrimination task, reinforcement signals (reward and punishment) strongly and uniformly activated VIP neurons in auditory cortex, and in turn VIP recruitment increased the gain of a functional subpopulation of principal neurons. These results reveal a specific cell type and microcircuit underlying disinhibitory control in cortex and demonstrate that it is activated under specific behavioural conditions.
Alkhori, L., Ost, A. and Alenius, M. (2013). The corepressor Atrophin specifies odorant receptor expression in Drosophila. FASEB J. PubMed ID: 24334704
In both insects and vertebrates, each olfactory sensory neuron (OSN) expresses one odorant receptor (OR) from a large genomic repertoire. How a receptor is specified is a tantalizing question addressing fundamental aspects of cell differentiation. This study demonstrates that the corepressor Atrophin (Atro) segregates OR gene expression between OSN classes in Drosophila. Knockdown of Atro results in either loss or gain of a broad set of ORs. Each OR phenotypic group correlated with one of two opposing Notch fates, Notch responding, Nba (Non), and nonresponding, Nab (Noff) OSNs. The data show that Atro segregates ORs expressed in the Nba OSN classes and helps establish the Nab fate during OSN development. Consistent with a role in recruiting histone deacetylates, immunohistochemistry revealed that Atro regulates global histone 3 acetylation (H3ac) in OSNs and requires Hdac3 to segregate OR gene expression. It was further found that Nba OSN classes exhibit variable but higher H3ac levels than the Nab OSNs. Together, these data suggest that Atro determines the level of H3ac, which ensures correct OR gene expression within the Nba OSNs. A mechanism is proposed by which a single corepressor can specify a large number of neuron classes.
Fisek, M. and Wilson, R. I. (2013). Stereotyped connectivity and computations in higher-order olfactory neurons. Nat Neurosci. PubMed ID: 24362761
In the first brain relay of the olfactory system, odors are encoded by combinations of glomeruli, but it is not known how glomerular signals are ultimately integrated. In Drosophila melanogaster, the majority of glomerular projections target the lateral horn, involved in processing inate responses. Lateral horn neurons (LHNs) receive input from sparse and stereotyped combinations of glomeruli that are coactivated by odors, and certain combinations of glomeruli are over-represented. One morphological LHN type is broadly tuned and sums input from multiple glomeruli. These neurons have a broader dynamic range than their individual glomerular inputs do. By contrast, a second morphological type is narrowly tuned and receives prominent odor-selective inhibition through both direct and indirect pathways. This wiring scheme confers increased selectivity. The biased stereotyped connectivity of the lateral horn contrasts with the probabilistic wiring of the mushroom body, reflecting the distinct roles of these regions in innate as compared to learned behaviors.
Sunday, January 5th
Greer, E. R., Chao, A. T. and Bejsovec, A. (2013). Pebble/ECT2 RhoGEF negatively regulates the Wingless/Wnt signaling pathway. Development 140: 4937-4946. PubMed ID: 24198276
Wingless (Wg)/Wnt signaling is essential for patterning invertebrate and vertebrate embryos, and inappropriate Wnt activity is associated with a variety of human cancers. Despite intensive study, Wnt pathway mechanisms are not fully understood. A new mechanism has been discovered for regulating the Wnt pathway: activity of a Rho guanine nucleotide exchange factor (GEF) encoded by pebble (pbl) in Drosophila and ECT2 in humans. This RhoGEF has an essential role in cytokinesis, but also plays an unexpected, conserved role in inhibiting Wg/Wnt activity. Loss and gain of pbl function in Drosophila embryos cause pattern defects that indicate altered Wg activity. Both Pbl and ECT2 repress Wg/Wnt target gene expression in cultured Drosophila and human cells. The GEF activity is required for Wnt regulation, whereas other protein domains important for cytokinesis are not. Unlike most negative regulators of Wnt activity, Pbl/ECT2 functions downstream of Armadillo (Arm)/beta-catenin stabilization. These results indicate GTPase regulation at a novel point in Wg/Wnt signal transduction, and provide new insight into the categorization of ECT2 as a human proto-oncogene.
Maurya, A. K., Ben, J., Zhao, Z., Lee, R. T., Niah, W., Ng, A. S., Iyu, A., Yu, W., Elworthy, S., van Eeden, F. J. and Ingham, P. W. (2013). Positive and negative regulation of gli activity by kif7 in the zebrafish embryo. PLoS Genet 9: e1003955. PubMed ID: 24339784
Loss of function mutations of Kif7, the vertebrate orthologue of the Drosophila Hh pathway component Costal2, cause defects in the limbs and neural tubes of mice, attributable to ectopic expression of Hh target genes. While this implies a functional conservation of Cos2 and Kif7 between flies and vertebrates, the association of Kif7 with the primary cilium, an organelle absent from most Drosophila cells, suggests their mechanisms of action may have diverged. Using mutant alleles induced by Zinc Finger Nuclease-mediated targeted mutagenesis, this study shows that in zebrafish, Kif7 acts principally to suppress the activity of the Gli1 transcription factor (see Drosophila Cubitus interruptus). Notably, endogenous Kif7 protein accumulates not only in the primary cilium, as previously observed in mammalian cells, but also in cytoplasmic puncta that disperse in response to Hh pathway activation. Moreover, Drosophila Costal2 can substitute for Kif7, suggesting a conserved mode of action of the two proteins. Kif7 interacts with both Gli1 and Gli2a, suggest that it functions to sequester Gli proteins in the cytoplasm, in a manner analogous to the regulation of Ci by Cos2 in Drosophila. Zebrafish Kif7 potentiates Gli2a activity by promoting its dissociation from the Suppressor of Fused (Sufu) protein and mediates a Smo dependent modification of the full length form of Gli2a. Surprisingly, the function of Kif7 in the zebrafish embryo appears restricted principally to mesodermal derivatives, its inactivation having little effect on neural tube patterning, even when Sufu protein levels are depleted. Remarkably, zebrafish lacking all Kif7 function are viable, in contrast to the peri-natal lethality of mouse kif7 mutants but similar to some Acrocallosal or Joubert syndrome patients who are homozygous for loss of function KIF7 alleles.
Kang, Y. and Bashirullah, A. (2013). A steroid-controlled global switch in sensitivity to apoptosis during Drosophila development. Dev Biol. PubMed ID: 24333635
Precise control over activation of the apoptotic machinery is critical for development, tissue homeostasis and disease. In Drosophila, the decision to trigger apoptosis-whether in response to developmental cues or to DNA damage-converges on transcription of inhibitor of apoptosis protein (IAP) antagonists Reaper, Hid and Grim. This study describes a parallel process that regulates the sensitivity to, rather than the execution of, apoptosis. This process establishes developmental windows that are permissive or restrictive for triggering apoptosis, where the status of cells determines their capacity to die. One switch in the sensitivity to apoptotic triggers, from restrictive to permissive, was characterized that occurs during third-instar larval (L3) development. Early L3 animals are highly resistant to induction of apoptosis by expression of IAP-antagonists, DNA-damaging agents and even knockdown of the IAP diap1. This resistance to apoptosis, however, is lost in wandering L3 animals after acquiring a heightened sensitivity to apoptotic triggers. This switch in sensitivity to death activators is mediated by a change in mechanisms available for activating endogenous caspases, from an apoptosome-independent to an apoptosome-dependent pathway. This switch in apoptotic pathways is regulated in a cell-autonomous manner by the steroid hormone ecdysone, through changes in expression of critical pro-, but not anti-, apoptotic genes. This steroid-controlled switch defines a novel, physiologically-regulated, mechanism for controlling sensitivity to apoptosis and provides new insights into the control of apoptosis during development.
Saturday, January 4th
Vecsey, C. G., Pirez, N. and Griffith, L. C. (2013). The Drosophila Neuropeptides PDF and sNPF Have Opposing Electrophysiological and Molecular Effects on Central Neurons. J Neurophysiol. PubMed ID: 24353297
Neuropeptides have widespread effects on behavior, but how these molecules alter the activity of their target cells is poorly understood. A new model system was employed in Drosophila melanogaster to assess the electrophysiological and molecular effects of neuropeptides, recording in situ from larval motor neurons which transgenically express a receptor of choice. Focus was placed on two neuropeptides, pigment-dispersing factor (PDF) and small neuropeptide F (sNPF), which play important roles in sleep/rhythms and feeding/metabolism. PDF treatment depolarized motor neurons expressing the PDF receptor (PDFR), increasing excitability. sNPF treatment had the opposite effect, hyperpolarizing neurons expressing the sNPF receptor (sNPFR). Live optical imaging using a genetically encoded FRET-based sensor for cyclic AMP (cAMP) showed that PDF induced a large increase in cAMP, whereas sNPF caused a small but significant decrease in cAMP. Co-expression of pertussis toxin or RNAi interference to disrupt the G-protein Galphao blocked the electrophysiological responses to sNPF, showing that sNPFR acts via Galphao signaling. Using a fluorescent sensor for intracellular calcium, it was observed that sNPF-induced hyperpolarization blocked spontaneous waves of activity propagating along the ventral nerve cord, demonstrating that the electrical effects of sNPF can cause profound changes in natural network activity in the brain. This new model system provides a platform for mechanistic analysis of how neuropeptides can affect target cells at the electrical and molecular level, allowing for predictions of how they regulate brain circuits that control behaviors such as sleep and feeding.
Javaid, S., et al. (2013). Dynamic Chromatin Modification Sustains Epithelial-Mesenchymal Transition following Inducible Expression of Snail-1. Cell Rep 5: 1679-1689. PubMed ID: 24360956
Epithelial-mesenchymal transition (EMT) is thought to contribute to cancer metastasis, but its underlying mechanisms are not well understood. To define early steps in this cellular transformation, this study analyzed human mammary epithelial cells with tightly regulated expression of Snail-1 (see Drosophila Snail), a master regulator of EMT. After Snail-1 induction, epithelial markers were repressed within 6 hr, and mesenchymal genes were induced at 24 hr. Snail-1 binding to its target promoters was transient (6–48 hr) despite continued protein expression, and it was followed by both transient and long-lasting chromatin changes. Pharmacological inhibition of selected histone acetylation and demethylation pathways suppressed the induction as well as the maintenance of Snail-1-mediated EMT. Thus, EMT involves an epigenetic switch that may be prevented or reversed with the use of small-molecule inhibitors of chromatin modifiers.
Stankic, M., Pavlovic, S., Chin, Y., Brogi, E., Padua, D., Norton, L., Massague, J. and Benezra, R. (2013). TGF-β-Id1 Signaling Opposes Twist1 and Promotes Metastatic Colonization via a Mesenchymal-to-Epithelial Transition. Cell Rep 5: 1228-1242. PubMed ID: 24332369
ID genes (see Drosophila Extra macrochaetae) are required for breast cancer colonization of the lungs, but the mechanism remains poorly understood. This study shows that Id1 expression induces a stem-like phenotype in breast cancer cells while retaining epithelial properties, contrary to the notion that cancer stem-like properties are inextricably linked to the mesenchymal state. During metastatic colonization, Id1 induces a mesenchymal-to-epithelial transition (MET), specifically in cells whose mesenchymal state is dependent on the Id1 target protein Twist1 (see Drosophila Twist), but not at the primary site, where this state is controlled by the zinc finger protein Snail1. Knockdown of Id expression in metastasizing cells prevents MET and dramatically reduces lung colonization. Furthermore, Id1 is induced by transforming growth factor (TGF)-β only in cells that have first undergone epithelial-to-mesenchymal transition (EMT), demonstrating that EMT is a prerequisite for subsequent Id1-induced MET during lung colonization. Collectively, these studies underscore the importance of Id-mediated phenotypic switching during distinct stages of breast cancer metastasis.
Friday, January 3rd
Breig, O., Bras, S., Soria, N. M., Osman, D., Heidenreich, O., Haenlin, M. and Waltzer, L. (2013). Pontin is a critical regulator for AML1-ETO-induced leukemia. Leukemia. PubMed ID: 24342949
The oncogenic fusion protein AML1-ETO, also known as RUNX1-RUNX1T1 (see Drosophila Runt) is generated by the t(8;21)(q22;q22) translocation, one of the most frequent chromosomal rearrangements in acute myeloid leukemia (AML). Identifying the genes that cooperate with or are required for the oncogenic activity of this chimeric transcription factor remains a major challenge. Previous studies showed that Drosophila provides a genuine model to study how AML1-ETO promotes leukemia. Using an in vivo RNAi screen for suppressors of AML1-ETO activity, pontin/RUVBL1 was identified as a gene required for AML1-ETO-induced lethality and blood cell proliferation in Drosophila. PONTIN inhibition strongly impairs the growth of human t(8;21)+ or AML1-ETO-expressing leukemic blood cells. Interestingly, AML1-ETO promotes the transcription of PONTIN. Moreover transcriptome analysis in Kasumi-1 cells revealed a strong correlation between PONTIN and AML1-ETO gene signatures and demonstrated that PONTIN chiefly regulated the expression of genes implicated in cell cycle progression. Concordantly, PONTIN depletion inhibited leukemic self-renewal and caused cell cycle arrest. All together these data suggest that the up-regulation of PONTIN by AML1-ETO participate in the oncogenic growth of t(8;21) cells.
Sarkar, S., Carroll, B., Buganim, Y., Maetzel, D., Ng, A. H., Cassady, J. P., Cohen, M. A., Chakraborty, S., Wang, H., Spooner, E., Ploegh, H., Gsponer, J., Korolchuk, V. I. and Jaenisch, R. (2013). Impaired autophagy in the lipid-storage disorder niemann-pick type c1 disease. Cell Rep 5: 1302-1315. PubMed ID: 24290752
Autophagy dysfunction has been implicated in misfolded protein accumulation and cellular toxicity in several diseases. Whether alterations in autophagy also contribute to the pathology of lipid-storage disorders is not clear. This study shows defective autophagy in Niemann-Pick type C1 (NPC1) disease associated with cholesterol accumulation, where the maturation of autophagosomes is impaired because of defective amphisome formation caused by failure in SNARE machinery, whereas the lysosomal proteolytic function remains unaffected. Expression of functional NPC1 protein (see Drosophila Niemann-Pick Type C-1a) rescues this defect. Inhibition of autophagy also causes cholesterol accumulation. Compromised autophagy was seen in disease-affected organs of Npc1 mutant mice. Of potential therapeutic relevance is that HP-β-cyclodextrin, which is used for cholesterol-depletion treatment, impedes autophagy, whereas stimulating autophagy restores its function independent of amphisome formation. These data suggest that a low dose of HP-β-cyclodextrin that does not perturb autophagy, coupled with an autophagy inducer, may provide a rational treatment strategy for NPC1 disease.
Li, J., Chen, X., Ding, X., Cheng, Y., Zhao, B., Lai, Z. C., Al Hezaimi, K., Hakem, R., Guan, K. L. and Wang, C. Y. (2013). LATS2 suppresses oncogenic Wnt signaling by disrupting beta-Catenin/BCL9 Interaction. Cell Rep 5: 1650-1663. PubMed ID: 24360964
Abnormal activation of Wnt/β-catenin-mediated transcription is associated with a variety of human cancers. This study reports that LATS2 (see Drosophila Lats/Warts) inhibits oncogenic Wnt/β-catenin-mediated transcription by disrupting the β-catenin/BCL9 (see Drosophila Legless) interaction. LATS2 directly interacts with β-catenin (see Drosophila Armadillo) and is present on Wnt target gene promoters. Mechanistically, LATS2 inhibits the interaction between BCL9 and β-catenin and subsequent recruitment of BCL9, independent of LATS2 kinase activity. LATS2 is downregulated and inversely correlated with the levels of Wnt target genes in human colorectal cancers. Moreover, nocodazole, an antimicrotubule drug, potently induces LATS2 to suppress tumor growth in vivo by targeting β-catenin/BCL9. These results suggest that LATS2 is not only a key tumor suppressor in human cancer but may also be an important target for anticancer therapy.
Thursday, January 2nd
Chen, D., Li, P. W., Goldstein, B. A., Cai, W., Thomas, E. L., Chen, F., Hubbard, A. E., Melov, S. and Kapahi, P. (2013). Germline Signaling Mediates the Synergistically Prolonged Longevity Produced by Double Mutations in daf-2 and rsks-1 in C. elegans. Cell Rep 5: 1600-1610. PubMed ID: 24332851
Inhibition of DAF-2 (insulin-like growth factor 1 [IGF-1] receptor; see Drosophila Insulin-like receptor) or RSKS-1 (S6K; see Drosophila S6k), key molecules in the insulin/IGF-1 signaling (IIS) and target of rapamycin (TOR) pathways, respectively, extend lifespan in Caenorhabditis elegans. However, it has not been clear how and in which tissues they interact with each other to modulate longevity. This study demonstrates that a combination of mutations in daf-2 and rsks-1 produces a nearly 5-fold increase in longevity that is much greater than the sum of single mutations. This synergistic lifespan extension requires positive feedback regulation of DAF-16 (FOXO; see Drosophila Foxo) via the AMP-activated protein kinase (AMPK) complex. Furthermore, germline was identified as the key tissue for this synergistic longevity. Moreover, germline-specific inhibition of rsks-1 activates DAF-16 in the intestine. Together, these findings highlight the importance of the germline in the significantly increased longevity produced by daf-2 rsks-1, which has important implications for interactions between the two major conserved longevity pathways in more complex organisms.
Molnar, C. and de Celis, J. F. (2013). Tay Bridge Is a Negative Regulator of EGFR Signalling and Interacts with Erk and Mkp3 in the Drosophila melanogaster Wing. PLoS Genet 9: e1003982. PubMed ID: 24348264
The regulation of Extracellular regulated kinase (Erk) activity is a key aspect of signalling by pathways activated by extracellular ligands acting through tyrosine kinase transmembrane receptors. Proteins with kinase activity that phosphorylate and activate Erk, as well as different phosphatases that inactivate Erk by de-phosphorylation, participate in this process. The state of Erk phosphorylation affects not only its activity, but also its subcellular localization, defining the repertoire of Erk target proteins, and consequently, the cellular response to Erk. This work characterises Tay bridge as a novel component of the EGFR/Erk signalling pathway. Tay bridge is a large nuclear protein with a domain of homology with human AUTS2, and was previously identified due to the neuronal phenotypes displayed by loss-of-function mutations. This study shows that Tay bridge antagonizes EGFR signalling in the Drosophila melanogaster wing disc and other tissues, and that the protein interacts with both Erk and Mkp3. It is suggested that Tay bridge constitutes a novel element involved in the regulation of Erk activity, acting as a nuclear docking for Erk that retains this protein in an inactive form in the nucleus.
Udayar, V., et al. (2013). A Paired RNAi and RabGAP Overexpression Screen Identifies Rab11 as a Regulator of β-Amyloid Production. Cell Rep 5: 1536-1551. PubMed ID: 24373285
Alzheimer's disease (AD) is characterized by cerebral deposition of β-amyloid (Aβ) peptides, which are generated from amyloid precursor protein (APP; see Drosophila Appl) by β- and γ-secretases (see Drosophila Presenilin, a component of the γ-secretase). APP and the secretases are membrane associated, but whether membrane trafficking controls Aβ levels is unclear. An RNAi screen of all human Rab-GTPases, which regulate membrane trafficking, was performed, complemented with a Rab-GTPase-activating protein screen, and a road map is presented of the membrane-trafficking events regulating Aβ production. Rab11 (see Drosophila Rab11) and Rab3 (see Drosophila Rab3) were identified as key players. Although retromers and retromer-associated proteins control APP recycling, Rab11 controlled β-secretase endosomal recycling to the plasma membrane and thus affected Aβ production. Exome sequencing revealed a significant genetic association of Rab11A with late-onset AD, and network analysis identified Rab11A and Rab11B as components of the late-onset AD risk network, suggesting a causal link between Rab11 and AD. These results reveal trafficking pathways that regulate Aβ levels and show how systems biology approaches can unravel the molecular complexity underlying AD.
Wednesday, January 1st
Meireles-Filho, A. C., Bardet, A. F., Yanez-Cuna, J. O., Stampfel, G. and Stark, A. (2013). cis-Regulatory Requirements for Tissue-Specific Programs of the Circadian Clock. Curr Biol. PubMed ID: 24332542
Broadly expressed transcriptions factors (TFs) control tissue-specific programs of gene expression through interactions with local TF networks. A prime example is the circadian clock: although the conserved TFs CLOCK (CLK) and CYCLE (CYC) control a transcriptional circuit throughout animal bodies, rhythms in behavior and physiology are generated tissue specifically. Yet, how CLK and CYC determine tissue-specific clock programs has remained unclear. This study used a functional genomics approach to determine the cis-regulatory requirements for clock specificity. First CLK and CYC genome-wide binding targets in heads and bodies were determined by ChIP-seq, and they were shown to have distinct DNA targets in the two tissue contexts. Computational dissection of CLK/CYC context-specific binding sites reveals sequence motifs for putative partner factors, which are predictive for individual binding sites. Among them, it was shown that the opa and GATA motifs, differentially enriched in head and body binding sites respectively, can be bound by OPA and SERPENT (SRP). They act synergistically with CLK/CYC in the Drosophila feedback loop, suggesting that they help to determine their direct targets and therefore orchestrate tissue-specific clock outputs. In addition, using in vivo transgenic assays, it was validated that GATA motifs are required for proper tissue-specific gene expression in the adult fat body, midgut, and Malpighian tubules, revealing a cis-regulatory signature for enhancers of the peripheral circadian clock. These results reveal how universal clock circuits can regulate tissue-specific rhythms and, more generally, provide insights into the mechanism by which universal TFs can be modulated to drive tissue-specific programs of gene expression.
Eckel-Mahan, K. L., Patel, V. R., de Mateo, S., Orozco-Solis, R., Ceglia, N. J., Sahar, S., Dilag-Penilla, S. A., Dyar, K. A., Baldi, P. and Sassone-Corsi, P. (2013). Reprogramming of the circadian clock by nutritional challenge. Cell 155: 1464-1478. PubMed ID: 24360271
Circadian rhythms and cellular metabolism are intimately linked. This study reveals that a high-fat diet (HFD) generates a profound reorganization of specific metabolic pathways, leading to widespread remodeling of the liver clock. Strikingly, in addition to disrupting the normal circadian cycle, HFD causes an unexpectedly large-scale genesis of de novo oscillating transcripts, resulting in reorganization of the coordinated oscillations between coherent transcripts and metabolites. The mechanisms underlying this reprogramming involve both the impairment of CLOCK:BMAL1 chromatin recruitment and a pronounced cyclic activation of surrogate pathways through the transcriptional regulator PPARγ. Finally, it was demonstrated that the specifically of the nutritional challenge, and not the development of obesity, that causes the reprogramming of the clock and that the effects of the diet on the clock are reversible.
Leyton, V., Goles, N. I., Fuenzalida-Uribe, N. and Campusano, J. M. (2013). Octopamine and Dopamine differentially modulate the nicotine-induced calcium response in Drosophila Mushroom Body Kenyon Cells. Neurosci Lett. PubMed ID: 24334164
In Drosophila associative olfactory learning, an odor, the conditioned stimulus (CS), is paired to an unconditioned stimulus (US). The CS and US information arrive at the Mushroom Bodies (MB), a Drosophila brain region that processes the information to generate new memories. It has been shown that olfactory information is conveyed through cholinergic inputs that activate nicotinic acetylcholine receptors (nAChRs) in the MB, while the US is coded by biogenic amine (BA) systems that innervate the MB. In this regard, the MB acts as a coincidence detector. A better understanding of the properties of the responses gated by nicotinic and BA receptors are required to get insights on the cellular and molecular mechanisms responsible for memory formation. In recent years, information has become available on the properties of the responses induced by nAChR activation in Kenyon Cells (KCs), the main neuronal MB population. However, very little information exists on the responses induced by aminergic systems in fly MB. This study evaluated some of the properties of the calcium responses gated by Dopamine (DA) and Octopamine (Oct) in identified KCs in culture. Exposure to BAs induces a fast but rather modest increase in intracellular calcium levels in cultured KCs. The responses to Oct and DA are fully blocked by a VGCC blocker, while they are differentially modulated by cAMP. Moreover, co-application of BAs and nicotine has different effects on intracellular calcium levels: while DA and nicotine effects are additive, Oct and nicotine induce a synergistic increase in calcium levels. These results suggest that a differential modulation of nicotine-induced calcium increase by DA and Oct could contribute to the events leading to learning and memory in flies.
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