What's hot today
Thursday, October 31st, 2013
Nachtergaele, S., Whalen, D. M., Mydock, L. K., Zhao, Z., Malinauskas, T., Krishnan, K., Ingham, P. W., Covey, D. F., Siebold, C. and Rohatgi, R. (2013). Structure and function of the Smoothened extracellular domain in vertebrate Hedgehog signaling. Elife 2: e01340. PubMed ID: 24171105
The Hedgehog (Hh) signal is transduced across the membrane by the heptahelical protein Smoothened (Smo), a developmental regulator, oncoprotein and drug target in oncology. This study presents the 2.3 Å crystal structure of the extracellular cysteine rich domain (CRD) of vertebrate Smo and show that it binds to oxysterols, endogenous lipids that activate Hh signaling. The oxysterol-binding groove in the Smo CRD is analogous to that used by Frizzled 8 to bind to the palmitoleyl group of Wnt ligands and to similar pockets used by other Frizzled-like CRDs to bind hydrophobic ligands. The CRD is required for signaling in response to native Hh ligands, showing that it is an important regulatory module for Smo activation. Indeed, targeting of the Smo CRD by oxysterol-inspired small molecules can block signaling by all known classes of Hh activators and by clinically relevant Smo mutants.
Sahai-Hernandez, P. and Nystul, T. G. (2013). A dynamic population of stromal cells contributes to the follicle stem cell niche in the Drosophila ovary. Development. PubMed ID: 24131631
Epithelial stem cells are maintained within niches that promote self-renewal by providing signals that specify the stem cell fate. In the Drosophila ovary, epithelial follicle stem cells (FSCs) reside in niches at the anterior tip of the tissue and support continuous growth of the ovarian follicle epithelium. This study demonstrates that a neighboring dynamic population of stromal cells, called escort cells, are FSC niche cells. Escort cells produce both Wingless and Hedgehog ligands for the FSC lineage, and that Wingless signaling is specific for the FSC niche whereas Hedgehog signaling is active in both FSCs and daughter cells. In addition, it was shown that multiple escort cells simultaneously encapsulate germ cell cysts and contact FSCs. Thus, FSCs are maintained in a dynamic niche by a non-dedicated population of niche cells.
Wednesday, October 30
Yun, M. H., Gates, P. B. and Brockes, J. P. (2013). Regulation of p53 is critical for vertebrate limb regeneration. Proc Natl Acad Sci U S A 110: 17392-17397. PubMed ID: 24101460
Extensive regeneration of the vertebrate body plan is found in salamander and fish species. In these organisms, regeneration takes place through reprogramming of differentiated cells, proliferation, and subsequent redifferentiation of adult tissues. Such plasticity is rarely found in adult mammalian tissues, and this has been proposed as the basis of their inability to regenerate complex structures. Despite their importance, the mechanisms underlying the regulation of the differentiated state during regeneration remain unclear. This study analyzed the role of the tumor-suppressor p53 (see Drosophila p53 ) during salamander limb regeneration. The activity of p53 initially decreases and then returns to baseline. Its down-regulation is required for formation of the blastema, and its up-regulation is necessary for the redifferentiation phase. Importantly, it was shown that a decrease in the level of p53 activity is critical for cell cycle reentry of postmitotic, differentiated cells, whereas an increase is required for muscle differentiation. In addition, a potential mechanism was uncovered for the regulation of p53 during limb regeneration, based on its competitive inhibition by DeltaNp73. These results suggest that the regulation of p53 activity is a pivotal mechanism that controls the plasticity of the differentiated state during regeneration.
Shang, Y., Donelson, N. C., Vecsey, C. G., Guo, F., Rosbash, M. and Griffith, L. C. (2013). Short neuropeptide f is a sleep-promoting inhibitory modulator. Neuron 80: 171-183. PubMed ID: 24094110
To advance the understanding of sleep regulation, a screen was performed in Drosophila for sleep-promoting cells, and neurons expressing neuropeptide Y-like short neuropeptide F (sNPF) were identifed. Sleep induction by sNPF meets all relevant criteria. Rebound sleep following sleep deprivation is reduced by activation of sNPF neurons, and flies experience negative sleep rebound upon cessation of sNPF neuronal stimulation, indicating that sNPF provides an important signal to the sleep homeostat. Only a subset of sNPF-expressing neurons, which includes the small ventrolateral clock neurons, is sleep promoting. Their release of sNPF increases sleep consolidation in part by suppressing the activity of wake-promoting large ventrolateral clock neurons, and suppression of neuronal firing may be the general response to sNPF receptor activation. sNPF acutely increases sleep without altering feeding behavior, which it affects only on a much longer time scale. The profound effect of sNPF on sleep indicates that it is an important sleep-promoting molecule.
Tuesday, October 29th
Raghuram, N., Strickfaden, H., McDonald, D., Williams, K., Fang, H., Mizzen, C., Hayes, J. J., Th'ng, J. and Hendzel, M. J. (2013). Pin1 promotes histone H1 dephosphorylation and stabilizes its binding to chromatin. J Cell Biol 203: 57-71. PubMed ID: 24100296
Histone H1 plays a crucial role in stabilizing higher order chromatin structure. Transcriptional activation, DNA replication, and chromosome condensation all require changes in chromatin structure and are correlated with the phosphorylation of histone H1. This study describes a novel interaction between Pin1, a phosphorylation-specific prolyl isomerase, and phosphorylated histone H1. A sub-stoichiometric amount of Pin1 stimulates the dephosphorylation of H1 in vitro and modulates the structure of the C-terminal domain of H1 in a phosphorylation-dependent manner. Depletion of Pin1 destabilizes H1 binding to chromatin only when Pin1 binding sites on H1 were present. Pin1 recruitment and localized histone H1 phosphorylation are associated with transcriptional activation independent of RNA polymerase II. This study thus has identified a novel form of histone H1 regulation through phosphorylation-dependent proline isomerization, which has consequences on overall H1 phosphorylation levels and the stability of H1 binding to chromatin.
Deryusheva, S. and Gall, J. G. (2013). Novel small Cajal-body-specific RNAs identified in Drosophila: probing guide RNA function. RNA. PubMed ID: 24149844
The Cajal body is a site for the modification of small nuclear RNAs (snRNAs) and snoRNAs, as well as for the assembly and trafficking of ribonuclear proteins. The snRNAs are modified post-transcriptionally by introduction of pseudouridines and 2'-O-methyl modifications, which are mediated by box H/ACA and box C/D guide RNAs, respectively. Because of their concentration in the nuclear Cajal body (CB), these guide RNAs are known as small CB-specific (sca) RNAs. In the cell, scaRNAs are associated with the WD-repeat protein WDR79. This study used coimmunoprecipitation with WDR79 to recover seven new scaRNAs from Drosophila cell lysates. Concentration of these new scaRNAs in the CB was demonstrated by in situ hybridization, and it was verified experimentally that they can modify their putative target RNAs. Surprisingly, one of the new scaRNAs targets U6 snRNA, whose modification is generally assumed to occur in the nucleolus, not in the CB. Two other scaRNAs have dual guide functions, one for an snRNA and one for 28S rRNA. Again, the modification of 28S rRNA is assumed to take place in the nucleolus. These findings suggest that canonical scaRNAs may have functions in addition to their established role in modifying U1, U2, U4, and U5 snRNAs. The likelihood is discussed that processing by scaRNAs is not limited to the CB.
Monday, October 28th
Wajapeyee, N., Malonia, S. K., Palakurthy, R. K. and Green, M. R. (2013). Oncogenic RAS directs silencing of tumor suppressor genes through ordered recruitment of transcriptional repressors. Genes Dev 27: 2221-2226. PubMed ID: 24105743
Previous studies have identified 28 cofactors through which a RAS oncoprotein (see Drosophila Ras oncogene at 85D) directs transcriptional silencing of proapoptotic TNF family member Fas and other tumor suppressor genes (TSGs). This study performed RNAi-based epistasis experiments and found that RAS-directed silencing occurs through a highly ordered pathway that is initiated by binding of ZFP354B, a sequence-specific DNA-binding protein, and culminates in recruitment of the DNA methyltransferase DNMT1. RNAi and pharmacological inhibition experiments reveal that silencing requires continuous function of RAS and its cofactors and can be rapidly reversed, which may have therapeutic implications for reactivation of silenced TSGs in RAS-positive cancers.
Chen, P., et al. (2013). H3.3 actively marks enhancers and primes gene transcription via opening higher-ordered chromatin. Genes Dev 27: 2109-2124. PubMed ID: 24065740
The histone variants H3.3 and H2A.Z (see Drosophila Histone H3.3A) and H2A.Z) have recently emerged as two of the most important features in transcriptional regulation, the molecular mechanism of which still remains poorly understood. This study investigated the regulation of H3.3 and H2A.Z on chromatin dynamics during transcriptional activation. This in vitro investigation showed that H2A.Z promotes chromatin compaction and represses transcriptional activity. Surprisingly, with only four to five amino acid differences from the canonical H3, H3.3 greatly impairs higher-ordered chromatin folding and promotes gene activation, although it has no significant effect on the stability of mononucleosomes. It was further demonstrated that H3.3 actively marks enhancers and determines the transcriptional potential of retinoid acid (RA)-regulated genes via creating an open chromatin signature that enables the binding of RAR/RXR. Additionally, the H3.3-dependent recruitment of H2A.Z on promoter regions resulted in compaction of chromatin to poise transcription, while RA induction results in the incorporation of H3.3 on promoter regions to activate transcription via counteracting H2A.Z-mediated chromatin compaction. These results provide key insights into the mechanism of how histone variants H3.3 and H2A.Z function together to regulate gene transcription via the modulation of chromatin dynamics over the enhancer and promoter regions.
Sunday, October 27th
Attanasio, C., et al. (2013). Fine tuning of craniofacial morphology by distant-acting enhancers. Science 342: 1241006. PubMed ID: 24159046
The shape of the human face and skull is largely genetically determined. However, the genomic basis of craniofacial morphology is incompletely understood and hypothesized to involve protein-coding genes, as well as gene regulatory sequences. This study used a combination of epigenomic profiling, in vivo characterization of candidate enhancer sequences in transgenic mice, and targeted deletion experiments to examine the role of distant-acting enhancers in craniofacial development. Complex regulatory landscapes were identified consisting of enhancers that drive spatially complex developmental expression patterns. Analysis of mouse lines in which individual craniofacial enhancers had been deleted revealed significant alterations of craniofacial shape, demonstrating the functional importance of enhancers in defining face and skull morphology. These results demonstrate that enhancers are involved in craniofacial development and suggest that enhancer sequence variation contributes to the diversity of human facial morphology.
Yeh, C., Li, A., Chuang, J. Z., Saito, M., Caceres, A. and Sung, C. H. (2013). IGF-1 activates a cilium-localized noncanonical Gbetagamma signaling pathway that regulates cell-cycle progression. Dev Cell 26: 358-368. PubMed ID: 23954591
Primary cilia undergo cell-cycle-dependent assembly and disassembly. Emerging data suggest that ciliary resorption is a checkpoint for S phase reentry and that the activation of a phosphorylated cytoplasmic dynein light chain [phospho(T94)Tctex-1] couples these two events. However, the environmental cues and molecular mechanisms that trigger these processes remain unknown. This study shows that insulin-like growth-1 (IGF-1; see Drosophila Insulin-related peptide) accelerates G1-S progression by causing cilia to resorb. The mitogenic signals of IGF-1 are predominantly transduced through IGF-1 receptor (IGF-1R; Drosophila homolog, Insulin-like receptor) on the cilia of fibroblasts and epithelial cells. At the base of the cilium, phosphorylated IGF-1R activates an AGS3-regulated Gβγ signaling pathway that subsequently recruits phospho(T94)Tctex-1 to the transition zone. Perturbing any component of this pathway in cortical progenitors induces premature neuronal differentiation at the expense of proliferation. These data suggest that during corticogenesis, a cilium-transduced, noncanonical IGF-1R-Gβγ-phospho(T94)Tctex-1 signaling pathway promotes the proliferation of neural progenitors through modulation of ciliary resorption and G1 length.
Saturday, October 26th
Bischoff, M., Gradilla, A. C., Seijo, I., Andres, G., Rodriguez-Navas, C., Gonzalez-Mendez, L. and Guerrero, I. (2013). Cytonemes are required for the establishment of a normal Hedgehog morphogen gradient in Drosophila epithelia. Nat Cell Biol. PubMed ID: 24121526
Hedgehog (Hh) signalling is important in development, stem cell biology and disease. In a variety of tissues, Hh acts as a morphogen to regulate growth and cell fate specification. Several hypotheses have been proposed to explain morphogen movement, one of which is transport along filopodia-like protrusions called cytonemes. This study analysed the mechanism underlying Hh movement in the wing disc and the abdominal epidermis of Drosophila melanogaster. In both epithelia, cells were shown to generate cytonemes in regions of Hh signalling. These protrusions are actin-based and span several cell diameters. Various Hh signalling components localize to cytonemes, as well as to punctate structures that move along cytonemes and are probably exovesicles. In vivo imaging was used show that cytonemes are dynamic structures and that Hh gradient establishment correlates with cytoneme formation in space and time. Indeed, mutant conditions that affect cytoneme formation reduce both cytoneme length and Hh gradient length. The results suggest that cytoneme-mediated Hh transport is the mechanistic basis for Hh gradient formation
Fujita, N., Nagata, Y., Nishiuchi, T., Sato, M., Iwami, M. and Kiya, T. (2013). Visualization of Neural Activity in Insect Brains Using a Conserved Immediate Early Gene, Hr38. Curr Biol. PubMed ID: 24120640
Many insects exhibit stereotypic instinctive behavior, but the underlying neural mechanisms are not well understood due to difficulties in detecting brain activity in freely moving animals. Immediate early genes (IEGs), such as c-fos, whose expression is transiently and rapidly upregulated upon neural activity, are powerful tools for detecting behavior-related neural activity in vertebrates. In insects, however, this powerful approach has not been realized because no conserved IEGs have been identified. This study identified Hr38 as a novel IEG that is transiently expressed in the male silkmoth Bombyx mori by female odor stimulation. Using Hr38 expression as an indicator of neural activity, comprehensive activity patterns of the silkmoth brain were mapped in response to female sex pheromones. Hr38 could also be used as a neural activity marker in the fly Drosophila melanogaster. Using Hr38, a neural activity map of the fly brain was mapped that partially overlaps with fruitless (fru)-expressing neurons in response to female stimulation. These findings indicate that Hr38 is a novel and conserved insect neural activity marker gene that will be useful for a wide variety of neuroethologic studies
Friday, October 25th
Yang, X., Mao, F., Lv, X., Zhang, Z., Fu, L., Lu, Y., Wu, W., Zhou, Z., Zhang, L. and Zhao, Y. (2013). Drosophila Vps36 regulates Smo trafficking in Hedgehog signaling. J Cell Sci 126: 4230-4238. PubMed ID: 23843610
The hedgehog (Hh) signaling pathway plays a important role in metazoan development by controlling pattern formation. Malfunction of the Hh signaling pathway leads to numerous serious human diseases, including congenital disorders and cancers. The seven-transmembrane domain protein Smoothened (Smo) is a key transducer of the Hh signaling pathway, and mediates the graded Hh signal across the cell plasma membrane, thereby inducing the proper expression of downstream genes. Smo accumulation on the cell plasma membrane is regulated by its C-tail phosphorylation and the graded Hh signal. The inhibitory mechanism for Smo membrane accumulation in the absence of Hh, however, is still largely unknown. This study reports that Vps36 of the ESCRT-II complex regulates Smo trafficking between the cytosol and plasma membrane by specifically recognizing the ubiquitin signal on Smo in the absence of Hh. Furthermore, in the absence of Hh, Smo is ubiquitylated on its cytoplasmic part, including its internal loops and C-tail. Taken together, these data suggest that the ESCRT-II complex, especially Vps36, has a special role in controlling Hh signaling by targeting the membrane protein Smo for its trafficking in the absence of Hh, thereby regulating Hh signaling activity.
Sutcliffe, B., Forero, M. G., Zhu, B., Robinson, I. M. and Hidalgo, A. (2013). Neuron-Type Specific Functions of DNT1, DNT2 and Spz at the Drosophila Neuromuscular Junction. PLoS One 8: e75902. PubMed ID: 24124519
Retrograde growth factors regulating synaptic plasticity at the neuromuscular junction (NMJ) in Drosophila have long been predicted but their discovery has been scarce. In vertebrates, such retrograde factors produced by the muscle include GDNF and the neurotrophins (NT: NGF, BDNF, NT3 and NT4). The NT family of proteins in Drosophila consists of DNT1, DNT2 and Spätzle (Spz), with sequence, structural and functional conservation relative to mammalian NTs. This study investigated the functions of Drosophila NTs (DNTs) at the larval NMJ. All three DNTs are expressed in larval body wall muscles, targets for motor-neurons. Over-expression of DNTs in neurons, or the activated form of the Spz receptor, Toll, in neurons only, rescued the semi-lethality of spz and DNT1;DNT2 double mutants, indicating retrograde functions in neurons. In spz mutants, DNT1;DNT2 double mutants, and upon over-expression of the DNTs, NMJ size and bouton number increased. Boutons were morphologically abnormal. Mutations in spz and DNT1,DNT2 resulted in decreased number of active zones per bouton and decreased active zone density per terminal. Alterations in DNT function induced ghost boutons and synaptic debris. Frequency and amplitude of spontaneous events were reduced in spz2 mutants suggesting defective neurotransmission. These data indicate that DNTs are produced in muscle and are required in neurons for synaptogenesis. Most likely, alterations in DNT function and synapse formation induce NMJ plasticity leading to homeostatic adjustments that increase terminal size restoring overall synaptic transmission.
Thursday, October 24th
Fuse, N., Yu, F. and Hirose, S. (2013). Gprk2 adjusts Fog signaling to organize cell movements in Drosophila gastrulation. Development 140: 4246-4255. PubMed ID: 24026125
Gastrulation of Drosophila melanogaster proceeds through sequential cell movements: ventral mesodermal (VM) cells are induced by secreted Fog protein to constrict their apical surfaces to form the ventral furrow, and subsequently lateral mesodermal (LM) cells involute toward the furrow. How these cell movements are organized remains elusive. This study observed that LM cells extend apical protrusions and then undergo accelerated involution movement, confirming that VM and LM cells display distinct cell morphologies and movements. In a mutant for the GPCR kinase Gprk2, apical constriction expands to all mesodermal cells and the involution movement is abolished. In addition, the mesodermal cells halt apical constriction prematurely in accordance with the aberrant accumulation of Myosin II. Epistasis analyses revealed that the Gprk2 mutant phenotypes are dependent on the fog gene. Overexpression of Gprk2 suppresses the effects of excess Cta, a downstream component of Fog signaling. Based on these findings, it is proposed that Gprk2 attenuates and tunes Fog-Cta signaling to prevent apical constriction in LM cells and to support appropriate apical constriction in VM cells. Thus, the two distinct cell movements in mesoderm invagination are not predetermined, but rather are organized by the adjustment of cell signaling.
Tan, F. E., Vladar, E. K., Ma, L., Fuentealba, L. C., Hoh, R., Espinoza, F. H., Axelrod, J. D., Alvarez-Buylla, A., Stearns, T., Kintner, C. and Krasnow, M. A. (2013). Myb promotes centriole amplification and later steps of the multiciliogenesis program. Development 140: 4277-4286. PubMed ID: 24048590Summary:
The transcriptional control of primary cilium formation and ciliary motility are beginning to be understood, but little is known about the transcriptional programs that control cilium number and other structural and functional specializations. One of the most intriguing ciliary specializations occurs in multiciliated cells (MCCs), which amplify their centrioles to nucleate hundreds of cilia per cell, instead of the usual monocilium. This study reports that the transcription factor MYB (see Drosophila Myb oncogene-like), which promotes S phase and drives cycling of a variety of progenitor cells, is expressed in postmitotic epithelial cells of the mouse airways and ependyma destined to become MCCs. MYB is expressed early in multiciliogenesis, as progenitors exit the cell cycle and amplify their centrioles, then switches off as MCCs mature. Conditional inactivation of Myb in the developing airways blocks or delays centriole amplification and expression of FOXJ1, a transcription factor that controls centriole docking and ciliary motility, and airways fail to become fully ciliated. Evidence is provided that MYB acts in a conserved pathway downstream of Notch signaling and multicilin, a protein related to the S-phase regulator geminin, and upstream of FOXJ1. MYB can activate endogenous Foxj1 expression and stimulate a cotransfected Foxj1 reporter in heterologous cells, and it can drive the complete multiciliogenesis program in Xenopus embryonic epidermis. It is concluded that MYB has an early, crucial and conserved role in multiciliogenesis, and it is proposed that MYB promotes a novel S-like phase in which centriole amplification occurs uncoupled from DNA synthesis, and then drives later steps of multiciliogenesis through induction of Foxj1.
Wednesday, October 23rd
Targoff, K. L., Colombo, S., George, V., Schell, T., Kim, S. H., Solnica-Krezel, L. and Yelon, D. (2013). Nkx genes are essential for maintenance of ventricular identity. Development 140: 4203-4213. PubMed ID: 24026123
Establishment of specific characteristics of each embryonic cardiac chamber is crucial for development of a fully functional adult heart. Despite the importance of defining and maintaining unique features in ventricular and atrial cardiomyocytes, the regulatory mechanisms guiding these processes are poorly understood. This study shows that the homeodomain transcription factors Nkx2.5 and Nkx2.7 (homologs of Drosophila Tinman) are necessary to sustain ventricular chamber attributes through repression of atrial chamber identity. Mutation of nkx2.5 in zebrafish yields embryos with diminutive ventricular and bulbous atrial chambers. Removal of nkx2.7 function from nkx2.5 mutants exacerbates the loss of ventricular cells and the gain of atrial cells. Moreover, in these Nkx-deficient embryos, expression of vmhc, a ventricular gene, fades, whereas expression of amhc, an atrial gene, expands. Cell-labeling experiments suggest that ventricular cardiomyocytes can transform into atrial cardiomyocytes in the absence of Nkx gene function. Through suggestion of transdifferentiation from ventricular to atrial fate, these data reveal a pivotal role for Nkx genes in maintaining ventricular identity and highlight remarkable plasticity in differentiated myocardium.
Poulton, J. S., Mu, F. W., Roberts, D. M. and Peifer, M. (2013). APC2 and Axin promote mitotic fidelity by facilitating centrosome separation and cytoskeletal regulation. Development 140: 4226-4236. PubMed ID: 24026117
To ensure the accurate transmission of genetic material, chromosome segregation must occur with extremely high fidelity. Segregation errors lead to chromosomal instability (CIN), with deleterious consequences. Mutations in the tumor suppressor adenomatous polyposis coli (APC) initiate most colon cancers and have also been suggested to promote disease progression through increased CIN, but the mechanistic role of APC in preventing CIN remains controversial. Using fly embryos as a model, this study investigated the role of APC proteins in CIN. The findings suggest that APC2 loss leads to increased rates of chromosome segregation error. This occurs through a cascade of events beginning with incomplete centrosome separation leading to failure to inhibit formation of ectopic cleavage furrows, which result in mitotic defects and DNA damage. Several hypotheses related to the mechanism of action of APC2 were tested, revealing that APC2 functions at the embryonic cortex with several protein partners, including Axin, to promote mitotic fidelity. These in vivo data demonstrate that APC2 protects genome stability by modulating mitotic fidelity through regulation of the cytoskeleton.
Tuesday, October 22nd
Cavodeassi, F., Ivanovitch, K. and Wilson, S. W. (2013). Eph/Ephrin signalling maintains eye field segregation from adjacent neural plate territories during forebrain morphogenesis. Development 140: 4193-4202. PubMed ID: 24026122
During forebrain morphogenesis, there is extensive reorganisation of the cells destined to form the eyes, telencephalon and diencephalon. Little is known about the molecular mechanisms that regulate region-specific behaviours and that maintain the coherence of cell populations undergoing specific morphogenetic processes. his study shows that the activity of the Eph/Ephrin signalling pathway maintains segregation between the prospective eyes and adjacent regions of the anterior neural plate during the early stages of forebrain morphogenesis in zebrafish. Several Ephrins (see Drosophia Ephrin) and Ephs (see Drosophila Eph receptor tyrosine kinase) are expressed in complementary domains in the prospective forebrain and combinatorial abrogation of their activity results in incomplete segregation of the eyes and telencephalon and in defective evagination of the optic vesicles. Conversely, expression of exogenous Ephs or Ephrins in regions of the prospective forebrain where they are not usually expressed changes the adhesion properties of the cells, resulting in segregation to the wrong domain without changing their regional fate. The failure of eye morphogenesis in rx3 mutants (see Drosophila Retinal homeobox) is accompanied by a loss of complementary expression of Ephs and Ephrins, suggesting that this pathway is activated downstream of the regional fate specification machinery to establish boundaries between domains undergoing different programmes of morphogenesis.
Cheung, L. S., Simakov, D. S., Fuchs, A., Pyrowolakis, G. and Shvartsman, S. Y. (2013). Dynamic model for the coordination of two enhancers of broad by EGFR signaling. Proc Natl Acad Sci U S A. PubMed ID: 24127599
Although it is widely appreciated that a typical developmental control gene is regulated by multiple enhancers, coordination of enhancer activities remains poorly understood. This study proposes a mechanism for such coordination in Drosophila oogenesis, when the expression of the transcription factor Broad (BR) evolves from a uniform to a two-domain pattern that prefigures the formation of two respiratory eggshell appendages. This change reflects sequential activities of two enhancers of the br gene, early and late, both of which are controlled by the epidermal growth factor receptor (EGFR) pathway. The late enhancer controls br in the appendage-producing cells, but the function of the early enhancer remained unclear. This study found that the early enhancer is essential for the activity of the late enhancer and induction of eggshell appendages. This requirement can be explained by a mechanism whereby the BR protein produced by the early enhancer protects the late enhancer from EGFR-dependent repression. This complex mechanism is illustrated using a computational model that correctly predicts the wild-type dynamics of BR expression and its response to genetic perturbations.
Monday, October 21st
Reichert, S., Randall, R. A. and Hill, C. S. (2013). A BMP regulatory network controls ectodermal cell fate decisions at the neural plate border. Development 140: 4435-4444. PubMed ID: 24089471
During ectodermal patterning the neural crest and preplacodal ectoderm are specified in adjacent domains at the neural plate border. BMP signalling is required for specification of both tissues, but how it is spatially and temporally regulated to achieve this is not understood. This study shows that at the beginning of neurulation in zebrafish, the ventral-to-dorsal gradient of BMP activity evolves into two distinct domains at the neural plate border: one coinciding with the neural crest and the other abutting the epidermis. In between is a region devoid of BMP activity, which is specified as the preplacodal ectoderm. The ligands required for these domains of BMP activity have been identified. The BMP-interacting protein Crossveinless 2 (see Drosophila Cv-2) is expressed in the BMP activity domains and is under the control of BMP signalling. Crossveinless 2 functions at this time in a positive-feedback loop to locally enhance BMP activity, and it was shown that it is required for neural crest fate. It was further demonstrated that the Distal-less transcription factors Dlx3b and Dlx4b, which are expressed in the preplacodal ectoderm, are required for the expression of a cell-autonomous BMP inhibitor, Bambi-b, which can explain the specific absence of BMP activity in the preplacodal ectoderm. It is proposed that high BMP activity observed ventrally as generated by bmp2b/4/7a ligand expression and reinforced by Cvl2. BMP activity is inhibited dorsally by expression of the diffusible antagonists Chordin and Noggin. Prospective preplacodal ectoderm and neural crest are formed at intermediate levels of BMP signalling. Taken together, our data define a BMP regulatory network that controls cell fate decisions at the neural plate border
Gancz, D. and Gilboa, L. (2013). Insulin and Target of rapamycin signaling orchestrate the development of ovarian niche-stem cell units in Drosophila. Development 140: 4145-4154. PubMed ID: 24026119
Tissue-specific stem cells and their niches are organized into functional units that respond to external cues in order to maintain organ homeostasis. Insulin and Target of rapamycin (Tor) signaling mediate external cues that control adult niches and stem cells. Whether these pathways play a role in the establishment of niche-stem cell units during organogenesis has been little explored. This study shows that during larval development both Insulin-like receptor (InR) and Tor participate in the establishment of ovarian niches and germline stem cells (GSCs) in Drosophila. Tor and InR are required cell-autonomously for the proliferation of precursors for both somatic niches and GSCs. These pathways also promote the formation of terminal filaments (part of the somatic niche). Significantly, InR, but not Tor, signaling non-autonomously promotes primordial germ cell (PGC) differentiation. Somatic attenuation of the pathway retards PGC differentiation, whereas its activation results in their precocious differentiation. It was also shown that InR-mediated PGC differentiation is independent of somatic ecdysone signaling, but that further differentiation into cysts requires an ecdysone input. These results demonstrate that Tor and InR signaling actively participate in the formation of ovarian niches and stem cells by affecting both cell numbers and differentiation. The dual influence of Tor and InR on both somatic cells and PGCs ensures that these two cell populations develop coordinately. This work further identifies a novel step in the regulation of germ cell differentiation by demonstrating that following bag of marbles expression, cyst formation requires an additional hormonal input
Sunday, October 20th
Mavromatakis, Y. E. and Tomlinson, A. (2013).. Switching cell fates in the developing Drosophila eye. Development 140(21): 4353-61 PubMed ID: 24067351
The developing Drosophila ommatidium is characterized by two distinct waves of pattern formation. In the first wave, a precluster of five cells is formed by a complex cellular interaction mechanism. In the second wave, cells are systematically recruited to the cluster and directed to their fates by developmental cues presented by differentiating precluster cells. These developmental cues are mediated through the receptor tyrosine kinase (RTK) and Notch (N) signaling pathways and their combined activities are crucial in specifying cell type. The transcription factor Lozenge (Lz) is expressed exclusively in second wave cells. In this study Lz was ectopically supplied to precluster cells, and the various RTK/N codes that specify each of three second wave cell fates were concomitantly supplied. This protocol reproduced molecular markers of each of the second wave cell types in first wave precluster cells. Three inferences were drawn; 1) it was confirmed that Lz provides key intrinsic information to second wave cells, and this can now be combined with the RTK/N signaling to provide a cell fate specification code that entails both extrinsic and intrinsic information. 2) the reproduction of each second wave cell type in the precluster confirms the accuracy of the RTK/N signaling code, and 3) RTK/N signaling and Lz need only be presented to the cells for a short period of time in order to specify their fate (Mavromatakis, 2013).
Getahun, M. N., Olsson, S. B., Lavista-Llanos, S., Hansson, B. S. and Wicher, D. (2013). Insect odorant response sensitivity is tuned by metabotropically autoregulated olfactory receptors. PLoS One 8: e58889. PubMed ID: 23554952
Insects possess one of the most exquisitely sensitive olfactory systems in the animal kingdom, consisting of three different types of chemosensory receptors: ionotropic glutamate-like receptors (IRs; see for example Ionotropic receptor 84a), gustatory receptors (GRs) and odorant receptors (ORs). Both insect ORs and IRs are ligand-gated ion channels, but ORs possess a unique configuration composed of an odorant-specific protein OrX and a ubiquitous coreceptor (Orco). In addition, these two ionotropic receptors confer different tuning properties for the neurons in which they are expressed. Unlike IRs, neurons expressing ORs are more sensitive and can also be sensitized by sub-threshold concentrations of stimuli. What is the mechanistic basis for these differences in tuning? This study shows that intrinsic regulation of Orco enhances neuronal response to odorants and sensitizes the ORs. It was also demonstrated that inhibition of metabotropic regulation prevents receptor sensitization. These results indicate that Orco-mediated regulation of OR sensitivity provides tunable ionotropic receptors capable of detecting odors over a wider range of concentrations, providing broadened sensitivity over IRs themselves.
Saturday, October 19th
Sadanandappa, M. K., Redondo, B. B., Michels, B., Rodrigues, V., Gerber, B., Vijayraghavan, K., Buchner, E. and Ramaswami, M. (2013). Synapsin Function in GABA-ergic Interneurons Is Required for Short-Term Olfactory Habituation. J Neurosci 33: 16576-16585. PubMed ID: 24133261
In Drosophila, short-term (STH) and long-term habituation (LTH) of olfactory avoidance behavior are believed to arise from the selective potentiation of GABAergic synapses between multiglomerular local circuit interneurons (LNs) and projection neurons in the antennal lobe. However, the underlying mechanisms remain poorly understood. This study shows that synapsin (syn) function is necessary for STH and that syn97-null mutant defects in STH can be rescued by syn+ cDNA expression solely in the LN1 subset of GABAergic local interneurons. As Synapsin is a synaptic vesicle-clustering phosphoprotein, these observations identify a presynaptic mechanism for STH as well as the inhibitory interneurons in which this mechanism is deployed. Viochemical analyses using a phospho-synapsin-specific antiserum show that Synapsin is a target of Ca2+ calmodulin-dependent kinase II (CaMKII) phosphorylation in vivo. Additional behavioral and genetic observations demonstrate that CaMKII function is necessary in LNs for STH. Together, these data support a model in which CaMKII-mediated synapsin phosphorylation in LNs induces synaptic vesicle mobilization and thereby presynaptic facilitation of GABA release that underlies olfactory STH. Finally, the striking observation that LTH occurs normally in syn97 mutants indicates that signaling pathways for STH and LTH diverge upstream of Synapsin function in GABAergic interneurons.
Berghoff, E. G., Clark, M. F., Chen, S., Cajigas, I., Leib, D. E. and Kohtz, J. D. (2013). Evf2 (Dlx6as) lncRNA regulates ultraconserved enhancer methylation and the differential transcriptional control of adjacent genes. Development 140: 4407-4416. PubMed ID: 24089468
Several lines of evidence suggest that long non-coding RNA (lncRNA)-dependent mechanisms regulate transcription and CpG DNA methylation. Whereas CpG island methylation has been studied in detail, the significance of enhancer DNA methylation and its relationship with lncRNAs is relatively unexplored. Previous experiments proposed that the ultraconserved lncRNA Evf2 represses transcription through Dlx6 antisense (Dlx6as) transcription and methyl-CpG binding protein MECP2) recruitment to the Dlx5/6 ultraconserved DNA regulatory enhancer (Dlx5/6ei) in embryonic day 13.5 medial ganglionic eminence (E13.5 MGE). In this study, genetic epistasis experiments show that MECP2 transcriptional repression of Evf2 and Dlx5, but not Dlx6, occurs through antagonism of DLX1/2 in E13.5 MGE. Analysis of E13.5 MGE from mice lacking Evf2 and of partially rescued Evf2 transgenic mice shows that Evf2 prevents site-specific CpG DNA methylation of Dlx5/6ei in trans, without altering Dlx5/6 expression. Dlx1/2 loss increases CpG DNA methylation, whereas Mecp2 loss does not affect Dlx5/6ei methylation. Based on these studies, a model is proposed in which Evf2 inhibits enhancer DNA methylation, effectively modulating competition between the DLX1/2 activator and MECP2 repressor. Evf2 antisense transcription and Evf2-dependent balanced recruitment of activator and repressor proteins enables differential transcriptional control of adjacent genes with shared DNA regulatory elements.
Friday, October 18th
Saiz, N., Grabarek, J. B., Sabherwal, N., Papalopulu, N. and Plusa, B. (2013). Atypical protein kinase C couples cell sorting with primitive endoderm maturation in the mouse blastocyst. Development 140: 4311-4322. PubMed ID: 24067354
During mouse pre-implantation development, extra-embryonic primitive endoderm (PrE) and pluripotent epiblast precursors are specified in the inner cell mass (ICM) of the early blastocyst in a 'salt and pepper' manner, and are subsequently sorted into two distinct layers. Positional cues provided by the blastocyst cavity are thought to be instrumental for cell sorting; however, the sequence of events and the mechanisms that control this segregation remain unknown. This study shows that atypical protein kinase C (aPKC; see Drosophila aPKC), a protein associated with apicobasal polarity, is specifically enriched in PrE precursors in the ICM prior to cell sorting and prior to overt signs of cell polarisation. aPKC adopts a polarised localisation in PrE cells only after they reach the blastocyst cavity and form a mature epithelium, in a process that is dependent on FGF signalling. To assess the role of aPKC in PrE formation, its activity was disrupted using either chemical inhibition or RNAi knockdown. Inhibition of aPKC from the mid blastocyst stage not only prevents sorting of PrE precursors into a polarised monolayer but concomitantly affects the maturation of PrE precursors. These results suggest that the processes of PrE and epiblast segregation and cell fate progression are interdependent, and place aPKC as a central player in the segregation of epiblast and PrE progenitors in the mouse blastocyst.
Moshkin, Y. M., Doyen, C. M., Kan, T. W., Chalkley, G. E., Sap, K., Bezstarosti, K., Demmers, J. A., Ozgur, Z., van Ijcken, W. F. and Verrijzer, C. P. (2013). Histone Chaperone NAP1 Mediates Sister Chromatid Resolution by Counteracting Protein Phosphatase 2A. PLoS Genet 9: e1003719. PubMed ID: 24086141Summary:
Chromosome duplication and transmission into daughter cells requires the precisely orchestrated binding and release of cohesin (see Drosophila Rad 21). This study found that the Drosophila histone chaperone NAP1 is required for cohesin release and sister chromatid resolution during mitosis. Genome-wide surveys revealed that NAP1 and cohesin co-localize at multiple genomic loci. Proteomic and biochemical analysis established that NAP1 associates with the full cohesin complex, but it also forms a separate complex with the cohesin subunit stromalin (SA). NAP1 binding to cohesin is cell-cycle regulated and increases during G2/M phase. This causes the dissociation of protein phosphatase 2A (PP2A) from cohesin, increased phosphorylation of SA and cohesin removal in early mitosis. PP2A depletion led to a loss of centromeric cohesion. The distinct mitotic phenotypes caused by the loss of either PP2A or NAP1, were both rescued by their concomitant depletion. In is concluded that the balanced antagonism between NAP1 and PP2A controls cohesin dissociation during mitosis.
Thursday, October 17th
Shaw, J. L. and Chang, K. T. (2013). Nebula/DSCR1 Upregulation Delays Neurodegeneration and Protects against APP-Induced Axonal Transport Defects by Restoring Calcineurin and GSK-3beta Signaling. PLoS Genet 9: e1003792. PubMed ID: 24086147
This study explored the functional interaction between Down syndrome critical region 1 protein (DSCR1) and the amyloid precursor protein (APP), which is known to cause AD when duplicated or upregulated in DS. The Drosophila homolog of DSCR1, Nebula (FlyBase name: Sarah), delays neurodegeneration and ameliorates axonal transport defects caused by APP overexpression. Live-imaging reveals that Nebula facilitates the transport of synaptic proteins and mitochondria affected by APP upregulation. Furthermore, it was shown that Nebula upregulation protects against axonal transport defects by restoring calcineurin and GSK-3beta signaling altered by APP overexpression, thereby preserving cargo-motor interactions. As impaired transport of essential organelles caused by APP perturbation is thought to be an underlying cause of synaptic failure and neurodegeneration in AD, these findings imply that correcting calcineurin and GSK-3beta signaling can prevent APP-induced pathologies. The data further suggest that upregulation of Nebula/DSCR1 is neuroprotective in the presence of APP upregulation and provides evidence for calcineurin inhibition as a novel target for therapeutic intervention in preventing axonal transport impairments associated with AD.
Lin, S., Marin, E. C., Yang, C. P., Kao, C. F., Apenteng, B. A., Huang, Y., O'Connor, M. B., Truman, J. W. and Lee, T. (2013). Extremes of Lineage Plasticity in the Drosophila Brain. Curr Biol 23: 1908-1913. PubMed ID: 24055154
An often-overlooked aspect of neural plasticity is the plasticity of neuronal composition, in which the numbers of neurons of particular classes are altered in response to environment and experience. The larval brain of Drosophila features several well-characterized lineages in which a single neuroblast gives rise to multiple neuronal classes in a stereotyped sequence during development. This study found that in the intrinsic mushroom body neuron lineage, the numbers for each class are highly plastic, depending on the timing of temporal fate transitions and the rate of neuroblast proliferation. For example, mushroom body neuroblast cycling can continue under starvation conditions, uncoupled from temporal fate transitions that depend on extrinsic cues reflecting organismal growth and development. In contrast, the proliferation rates of antennal lobe lineages are closely associated with organismal development, and their temporal fate changes appear to be cell cycle-dependent, such that the same numbers and types of uniglomerular projection neurons innervate the antennal lobe following various perturbations. It is proposed that this surprising difference in plasticity for these brain lineages is adaptive, given their respective roles as parallel processors versus discrete carriers of olfactory information.
Wednesday, October 16th
Wang, H. and Sieburth, D. (2013).. PKA Controls Calcium Influx into Motor Neurons during a Rhythmic Behavior. PLoS Genet 9: e1003831. PubMed ID: 24086161
Cyclic adenosine monophosphate (cAMP) has been implicated in the execution of diverse rhythmic behaviors, but how cAMP functions in neurons to generate behavioral outputs remains unclear. During the defecation motor program in C. elegans, a peptide released from the pacemaker (the intestine) rhythmically excites the GABAergic neurons that control enteric muscle contractions by activating a G protein-coupled receptor (GPCR) signaling pathway that is dependent on cAMP. This study shows that the C. elegans PKA catalytic subunit, KIN-1 (see Drosophila cAMP-dependent protein kinase 1), is the sole cAMP target in this pathway and that PKA is essential for enteric muscle contractions. Genetic analysis using cell-specific expression of dominant negative or constitutively active PKA transgenes reveals that knockdown of PKA activity in the GABAergic neurons blocks enteric muscle contractions, whereas constitutive PKA activation restores enteric muscle contractions to mutants defective in the peptidergic signaling pathway. Using real-time, in vivo calcium imaging, this study found that PKA activity in the GABAergic neurons is essential for the generation of synaptic calcium transients that drive GABA release. In addition, constitutively active PKA increases the duration of calcium transients and causes ectopic calcium transients that can trigger out-of-phase enteric muscle contractions. Finally, it was shown that the voltage-gated calcium channels UNC-2 and EGL-19, but not CCA-1 function downstream of PKA to promote enteric muscle contractions and rhythmic calcium influx in the GABAergic neurons. Thus, these results suggest that PKA activates neurons during a rhythmic behavior by promoting presynaptic calcium influx through specific voltage-gated calcium channels.
Weir, P. T., Schnell, B. and Dickinson, M. H. (2013). Central complex neurons exhibit behaviorally gated responses to visual motion in Drosophila. J Neurophysiol. PubMed ID: 24108792
Sensory systems provide abundant information about the environment surrounding an animal, but only a small fraction of that information is relevant for any given task. One example of this requirement for context-dependent filtering of a sensory stream is the role that optic flow plays in guiding locomotion. Flying animals, which do not have access to a direct measure of ground speed, rely on optic flow to regulate their forward velocity. This observation suggests that progressive optic flow, the pattern of front-to-back motion on the retina created by forward motion, should be especially salient to an animal while it is in flight, but less important while it is standing still. This study recorded the activity of cells in the central complex of Drosophila during quiescence and tethered flight using both calcium imaging and whole-cell patch-clamp techniques. A genetically identified set of neurons was identified in the fan-shaped body that are unresponsive to visual motion while the animal is quiescent. During flight their baseline activity increases and they respond to front-to-back motion with changes relative to this baseline. The results provide an example of how nervous systems selectively respond to complex sensory stimuli depending on the current behavioral state of the animal.
Tuesday, October 15th
Zeron-Medina, J., et al. (2013). A Polymorphic p53 Response Element in KIT Ligand Influences Cancer Risk and Has Undergone Natural Selection
The ability of p53 (see Drosophila p53) to regulate transcription is crucial for tumor suppression and implies that inherited polymorphisms in functional p53-binding sites could influence cancer. This study has identified a polymorphic p53 responsive element and has demonstrate its influence on cancer risk using genome-wide data sets of cancer susceptibility loci, genetic variation, p53 occupancy, and p53-binding sites. A single-nucleotide polymorphism (SNP) was uncovered in a functional p53-binding site and its influence on the ability of p53 to bind to and regulate transcription of the KITLG gene was establised. The Kit ligand, also known as Stem cell factor, is a cytokine that plays an important role in hematopoiesis, spermatogenesis, and melanogenesis. The SNP resides in KITLG and associates with one of the largest risks identified among cancer genome-wide association studies. This study established that the SNP has undergone positive selection throughout evolution, signifying a selective benefit, but goes on to show that similar SNPs are rare in the genome due to negative selection, indicating that polymorphisms in p53-binding sites are primarily detrimental to humans.
Garbe, D. S., Fang, Y., Zheng, X., Sowcik, M., Anjum, R., Gygi, S. P. and Sehgal, A. (2013). Cooperative Interaction between Phosphorylation Sites on PERIOD Maintains Circadian Period in Drosophila. PLoS Genet 9: e1003749. PubMed ID: 24086144
Circadian rhythms in Drosophila rely on cyclic regulation of the period (per) and timeless (tim) clock genes. The molecular cycle requires rhythmic phosphorylation of PER and TIM proteins, which is mediated by several kinases and phosphatases such as Protein Phosphatase-2A (PP2A) and the multi-isoformed Protein Phosphatase-1 (PP1) (see Flap wing and Protein phosphatase 1 at 87B, which was implicated in a photoperiod response in a previous study). This study used mass spectrometry to identify 35 'phospho-occupied' serine/threonine residues within PER, 24 of which are specifically regulated by PP1/PP2A. per transgenes were generated carrying phosphorylation site mutations and rescue of the per01 arrhythmic phenotype was tested. Surprisingly, most transgenes restore wild type rhythms despite carrying mutations in several phosphorylation sites. One particular transgene, in which T610 and S613 are mutated to alanine, restores daily rhythmicity, but dramatically lengthens the period to ~30 hrs. Interestingly, the single S613A mutation extends the period by 2-3 hours, while the single T610A mutation has a minimal effect, suggesting these phospho-residues cooperate to control period length. Conservation of S613 from flies to humans suggests that it possesses a critical clock function, and mutational analysis of residues surrounding T610/S613 implicates the entire region in determining circadian period. A mutation at a previously identified site, S596, is largely epistatic to S613A, suggesting that S613 negatively regulates phosphorylation at S596. Together these data establish functional significance for a new domain of PER, demonstrate that cooperativity between phosphorylation sites maintains PER function, and support a model in which specific phosphorylated regions regulate others to control circadian period.
Monday, October 14th
Dong, X., Liu, O. W., Howell, A. S. and Shen, K. (2013). An Extracellular Adhesion Molecule Complex Patterns Dendritic Branching and Morphogenesis. Cell 155: 296-307
Robust dendrite morphogenesis is a critical step in the development of reproducible neural circuits. However, little is known about the extracellular cues that pattern complex dendrite morphologies. In the model nematode C. elegans, the sensory neuron PVD establishes stereotypical, highly branched dendrite morphology. This study reports the identification of a tripartite ligand-receptor complex of membrane adhesion molecules that is both necessary and sufficient to instruct spatially restricted growth and branching of PVD dendrites. The ligand complex SAX-7/L1CAM (Drosophila homolog: Fasciclin 2) and the novel protein MNR-1 function at defined locations in the surrounding hypodermal tissue, whereas leucine-rich repeat protein DMA-1 (Drosophila homolog: Reduced ocelli) acts as the cognate receptor on PVD. Mutations in this complex lead to dramatic defects in the formation, stabilization, and organization of the dendritic arbor. Ectopic expression of SAX-7 and MNR-1 generates a predictable, unnaturally patterned dendritic tree in a DMA-1-dependent manner. Both in vivo and in vitro experiments indicate that all three molecules are needed for interaction.
Salzberg, Y, et al. (2013). Skin-Derived Cues Control Arborization of Sensory Dendrites in Caenorhabditis elegans. Cell Volume 155: 308-320Summary:
Sensory dendrites depend on cues from their environment to pattern their growth and direct them toward their correct target tissues. Yet, little is known about dendrite-substrate interactions during dendrite morphogenesis. This study describes MNR-1/menorin, which is part of the conserved Fam151 family of proteins and is expressed in the skin to control the elaboration of 'menorah'-like dendrites of mechanosensory neurons in Caenorhabditis elegans. Evidence is provided that MNR-1 acts as a contact-dependent or short-range cue in concert with the neural cell adhesion molecule SAX-7/L1CAM (Drosophila homolog: Fasciclin 2) in the skin and through the neuronal leucine-rich repeat transmembrane receptor DMA-1 (Drosophila homolog: Reduced ocelli) on sensory dendrites. These data describe an unknown pathway that provides spatial information from the skin substrate to pattern sensory dendrite development nonautonomously.
Paridaen, J. T. M. L., Wilsch-Bräuninger, M. and Huttner, W. B. (2013). Asymmetric Inheritance of Centrosome-Associated Primary Cilium Membrane Directs Ciliogenesis after Cell Division. Cell 155: 333-344
Primary cilia are key sensory organelles that are thought to be disassembled prior to mitosis. Inheritance of the mother centriole, which nucleates the primary cilium, in relation to asymmetric daughter cell behavior has previously been studied. However, the fate of the ciliary membrane upon cell division is unknown. This study followed the ciliary membrane in dividing embryonic neocortical stem cells and cultured cells. Ciliary membrane attached to the mother centriole was endocytosed at mitosis onset, persisted through mitosis at one spindle pole, and was asymmetrically inherited by one daughter cell, which retained stem cell character. This daughter re-established a primary cilium harboring an activated signal transducer earlier than the noninheriting daughter. Centrosomal association of ciliary membrane in dividing neural stem cells decreased at late neurogenesis when these cells differentiate. These data imply that centrosome-associated ciliary membrane acts as a determinant for the temporal-spatial control of ciliogenesis.
Sunday, October 13th
Clark, R. I., Tan, S. W., Pean, C. B., Roostalu, U., Vivancos, V., Bronda, K., Pilatova, M., Fu, J., Walker, D. W., Berdeaux, R., Geissmann, F. and Dionne, M. S. (2013). MEF2 Is an In Vivo Immune-Metabolic Switch. Cell. PubMed ID: 24075010
Infections disturb metabolic homeostasis in many contexts, but the underlying connections are not completely understood. To address this, paired genetic and computational screens were used in Drosophila to identify transcriptional regulators of immunity and pathology and their associated target genes and physiologies. Mef2 was found to be required in the fat body for anabolic function and the immune response. Using genetic and biochemical approaches, this study found that MEF2 is phosphorylated at a conserved site in healthy flies and promotes expression of lipogenic and glycogenic enzymes. Upon infection, this phosphorylation is lost, and the activity of MEF2 changes - MEF2 now associates with the TATA binding protein to bind a distinct TATA box sequence and promote antimicrobial peptide expression. The loss of phosphorylated MEF2 contributes to loss of anabolic enzyme expression in Gram-negative bacterial infection. MEF2 is thus a critical transcriptional switch in the adult fat body between metabolism and immunity.
Bandura, J. L., Jiang, H., Nickerson, D. W. and Edgar, B. A. (2013). The Molecular Chaperone Hsp90 Is Required for Cell Cycle Exit in Drosophila. PLoS Genet 9: e1003835. PubMed ID: 24086162
The coordination of cell proliferation and differentiation is crucial for proper development. In particular, robust mechanisms exist to ensure that cells permanently exit the cell cycle upon terminal differentiation, and these include restraining the activities of both the E2F/DP transcription factor and Cyclin/Cdk kinases. A genetic screen in Drosophila was designed to identify genes required for cell cycle exit. This screen utilized a reporter that is highly E2F-responsive and results in a darker red eye color when crossed into genetic backgrounds that delay cell cycle exit. Mutation of Hsp83, the Drosophila homolog of mammalian Hsp90, results in increased E2F-dependent transcription and ectopic cell proliferation in pupal tissues at a time when neighboring wild-type cells are postmitotic. Further, these Hsp83 mutant cells have increased Cyclin/Cdk activity and accumulate proteins normally targeted for proteolysis by the anaphase-promoting complex/cyclosome (APC/C), suggesting that APC/C function is inhibited. Indeed, reducing the gene dosage of an inhibitor of Cdh1/Fzr, an activating subunit of the APC/C that is required for timely cell cycle exit, can genetically suppress the Hsp83 cell cycle exit phenotype. Based on these data, it is proposed that Cdh1/Fzr is a client protein of Hsp83. The results reveal that Hsp83 plays a heretofore unappreciated role in promoting APC/C function during cell cycle exit and suggest a mechanism by which Hsp90 inhibition could promote genomic instability and carcinogenesis.
Saturday, October 12th
Seelig, J. D. and Jayaraman, V. (2013). Feature detection and orientation tuning in the Drosophila central complex. Nature. PubMed ID: 24107996
Many animals, including insects, are known to use visual landmarks to orient in their environment. In Drosophila, behavioural genetics studies have identified a higher brain structure called the central complex as being required for the fly's innate responses to vertical visual features and its short- and long-term memory for visual patterns. But whether and how neurons of the fly central complex represent visual features are unknown. This study used two-photon calcium imaging in head-fixed walking and flying flies to probe visuomotor responses of ring neurons - a class of central complex neurons that have been implicated in spatial memory. Dendrites of ring neurons were shown to be visually responsive and arranged retinotopically. Ring neuron receptive fields comprise both excitatory and inhibitory subfields, resembling those of simple cells in the mammalian primary visual cortex. Ring neurons show strong and, in some cases, direction-selective orientation tuning. Visual responses were diminished during flight, but, in contrast with the hypothesized role of the central complex in the control of locomotion, not modulated during walking. These results indicate that ring neurons represent behaviourally relevant visual features in the fly's environment, enabling downstream central complex circuits to produce appropriate motor commands. More broadly, this study opens the door to mechanistic investigations of circuit computations underlying visually guided action selection in the Drosophila central complex.
Spencer, S. L., Cappell, S. D., Tsai, F. C., Overton, K. W., Wang, C. L. and Meyer, T. (2013). The Proliferation-Quiescence Decision Is Controlled by a Bifurcation in CDK2 Activity at Mitotic Exit. Cell. PubMed ID: 24075009
Tissue homeostasis in metazoans is regulated by transitions of cells between quiescence and proliferation. The hallmark of proliferating populations is progression through the cell cycle, which is driven by cyclin-dependent kinase (CDK) activity. This study introduced a live-cell sensor for CDK2 activity (see Drosophila Cdk1) and unexpectedly found that proliferating cells bifurcate into two populations as they exit mitosis. Many cells immediately commit to the next cell cycle by building up CDK2 activity from an intermediate level, while other cells lack CDK2 activity and enter a transient state of quiescence. This bifurcation is directly controlled by the CDK inhibitor p21 (see Drosophila Dacapo) and is regulated by mitogens during a restriction window at the end of the previous cell cycle. Thus, cells decide at the end of mitosis to either start the next cell cycle by immediately building up CDK2 activity or to enter a transient G0-like state by suppressing CDK2 activity.
Friday, October 11th
Jukam, D., Xie, B., Rister, J., Terrell, D., Charlton-Perkins, M., Pistillo, D., Gebelein, B., Desplan, C. and Cook, T. (2013). Opposite Feedbacks in the Hippo Pathway for Growth Control and Neural Fate. Science. PubMed ID: 23989952
Signaling pathways are reused for multiple purposes in plant and animal development. The
Hippo pathway in mammals and Drosophila coordinates proliferation and apoptosis via the coactivator and oncogene, YAP/Yorkie (Yki), which is homeostatically regulated through negative feedback. In the Drosophila eye, cross-repression between the Hippo pathway kinase, LATS/Warts (Wts), and growth regulator, Melted, generates mutually exclusive photoreceptor subtypes. This study show that this all-or-nothing neuronal differentiation results from Hippo pathway positive feedback: Yki both represses its negative regulator, warts, and promotes its positive regulator, melted. This postmitotic Hippo network behavior relies on a tissue-restricted transcription factor network-including a conserved Otx/Orthodenticle-Nrl/Traffic Jam feedforward module-that allows Warts-Yki-Melted to operate as a bistable switch. Altering feedback architecture provides an efficient mechanism to co-opt conserved signaling networks for diverse purposes in development and evolution.
Lone, I. N., Shukla, M. S., Charles Richard, J. L., Peshev, Z. Y., Dimitrov, S. and Angelov, D. (2013).. Binding of NF-κB to Nucleosomes: Effect of Translational Positioning, Nucleosome Remodeling and Linker Histone H1. PLoS Genet 9: e1003830. PubMed ID: 24086160
Summary: NF-κB (see Drosophila Dorsal) is a key transcription factor regulating the expression of inflammatory responsive genes. How NF-κB binds to naked DNA templates is well documented, but how it interacts with chromatin is far from being clear. This study used a combination of UV laser footprinting, hydroxyl footprinting and electrophoretic mobility shift assay to investigate the binding of NF-κB to nucleosomal templates. NF-κB p50 homodimer is able to bind to its recognition sequence, when it is localized at the edge of the core particle, but not when the recognition sequence is at the interior of the nucleosome. Remodeling of the nucleosome by the chromatin remodeling machine RSC was not sufficient to allow binding of NF-κB to its recognition sequence located in the vicinity of the nucleosome dyad, but RSC-induced histone octamer sliding allowed clearly detectable binding of NF-κB with the slid particle. Importantly, nucleosome dilution-driven removal of H2A-H2B dimer led to complete accessibility of the site located close to the dyad to NF-κB. Finally, it was found that NF-κB was able to displace histone H1 (see Drosophila Histone H1) and prevent its binding to nucleosome. These data provide important insight on the role of chromatin structure in the regulation of transcription of NF-κB dependent genes.
Thursday, October 10th
Hon, G. C., Rajagopal, N., Shen, Y., McCleary, D. F., Yue, F., Dang, M. D. and Ren, B. (2013).. Epigenetic memory at embryonic enhancers identified in DNA methylation maps from adult mouse tissues. Nat Genet 45: 1198-1206. PubMed ID: 23995138
Summary: Mammalian development requires cytosine methylation, a heritable epigenetic mark of cellular memory believed to maintain a cell's unique gene expression pattern. However, it remains unclear how dynamic DNA methylation relates to cell type-specific gene expression and animal development. It this study, by mapping base-resolution methylomes in 17 adult mouse tissues at shallow coverage, 302,864 tissue-specific differentially methylated regions (tsDMRs) were identified, and it was estimated that >:6.7% of the mouse genome is variably methylated. Supporting a prominent role for DNA methylation in gene regulation, most tsDMRs occur at distal cis-regulatory elements. Unexpectedly, some tsDMRs mark enhancers that are dormant in adult tissues but active in embryonic development. These 'vestigial' enhancers are hypomethylated and lack active histone modifications in adult tissues but nevertheless exhibit activity during embryonic development. These results provide new insights into the role of DNA methylation at tissue-specific enhancers and suggest that epigenetic memory of embryonic development may be retained in adult tissues.
He, H. and Noll, M. (2013). Differential and redundant functions of gooseberry and gooseberry neuro in the central nervous system and segmentation of the Drosophila embryo. Dev Biol 382: 209-223. PubMed ID: 23886579
The gooseberry locus of Drosophila consists of two homologous Pax genes, gooseberry neuro (gsbn) and gooseberry (gsb). Homologous recombination was obtained of null mutants of either gene as well as a deficiency inactivating only gsbn and gsb. This analysis shows that (1) gsbn null mutants are subviable while all surviving males and most females are sterile; (2) gsb and gsbn share overlapping functions in segmentation and the CNS, in which gsbn largely, but not completely depends on the transcriptional activation by the product of gsb; (3) as a consequence, in the absence of gsbn, gsb becomes haploinsufficient for its function in the CNS, and gsbn-/-gsb-/+ mutants die as larvae. Such mutants display defects in the proper specification of the SNa branch of the segmental nerve, which appears intact in gsbn-/- mutants. Analysis of gsbn-/-gsb-/+ clones originating from NB5-4 suggests that together the genes specify the SNa fate and concomitantly repress the SNc fate in this lineage and that their products activate BarH1 transcription. Specification of the SNa fate by Gsb and Gsbn occurs mainly at the NB and GMC stage. However, the SNa mutant phenotype can be rescued by providing Gsbn as late as at the postmitotic stage. A model is proposed how selection for both genes occurred after their duplication during evolution.
Wednesday, October 9th
Mirzoyan, Z. and Pandur, P. (2013). The Iroquois Complex Is Required in the Dorsal Mesoderm to Ensure Normal Heart Development in Drosophila. PLoS One 8: e76498. PubMed ID: 24086746
Loss of the whole Iroquois complex, as well as loss of either ara/caup or mirr only, affect heart development in Drosophila. The data indicate that the GATA factor Pannier requires the presence of Iro-C to function in cardiogenesis. A detailed expression pattern analysis of the members of the Iro-C revealed the presence of a possibly novel subpopulation of Even-skipped expressing pericardial cells and seven pairs of heart-associated cells that have not been described before. Taken together, this work introduces Iro-C as a new set of transcription factors that are required for normal development of the heart. As the members of the Iro-C may function, at least partly, as competence factors in the dorsal mesoderm, these results are fundamental for future studies aiming to decipher the regulatory interactions between factors that determine different cell fates in the dorsal mesoderm.
Babaoglan, A. B., Housden, B. E., Furriols, M. and Bray, S. J. (2013). Deadpan Contributes to the Robustness of the Notch Response. PLoS One 8: e75632. PubMed ID: 24086596
This study has identified several Notch responsive enhancers in the bHLH hairy and Enhancer of split (Espl) family gene dpn, demonstrating its direct regulation by Notch in a range of contexts including the Drosophila wing and eye. dpn expression largely overlaps that of several Espl genes and the combined knock-down leads to more severe phenotypes than either alone. In addition, Dpn contributes to the establishment of Cut expression at the wing dorsal-ventral (D/V) boundary; in its absence Cut expression is delayed. Furthermore, over-expression of Dpn inhibits expression from Espl gene enhancers, but not vice versa, suggesting that dpn contributes to a feed-back mechanism that limits Espl gene expression following Notch activation. Thus the combined actions of dpn and Espl appear to provide a mechanism that confers an initial rapid output from Notch activity which becomes self-limited via feedback between the targets.
Tuesday, October 8th
Upadhyai, P. and Campbell, G. (2013). Brinker possesses multiple mechanisms for repression because its primary co-repressor, Groucho, may be unavailable in some cell types. Development 140: 4256-4265. PubMed ID: 24086079Summary:
Transcriptional repressors function primarily by recruiting co-repressors, which are accessory proteins that antagonize transcription by modifying chromatin structure. Although a repressor could function by recruiting just a single co-repressor, many can recruit more than one, with Drosophila Brinker (Brk) recruiting the co-repressors CtBP and Groucho (Gro), in addition to possessing a third repression domain, 3R. Previous studies indicated that Gro is sufficient for Brk to repress targets in the wing, questioning why it should need to recruit CtBP, a short-range co-repressor, when Gro is known to be able to function over longer distances. To resolve this, genomic engineering was used to generate a series of brk mutants that are unable to recruit Gro, CtBP and/or have 3R deleted. These reveal that although the recruitment of Gro is necessary and can be sufficient for Brk to make an almost morphologically wild-type fly, it is insufficient during oogenesis, where Brk must utilize CtBP and 3R to pattern the egg shell appropriately. Gro insufficiency during oogenesis can be explained by its downregulation in Brk-expressing cells through phosphorylation downstream of EGFR signaling.
Lerit, D. A. and Rusan, N. M. (2013). PLP inhibits the activity of interphase centrosomes to ensure their proper segregation in stem cells. J Cell Biol 202: 1013-1022. PubMed ID: 24081489
Centrosomes determine the mitotic axis of asymmetrically dividing stem cells. Several studies have shown that the centrosomes of the Drosophila central brain neural stem cells are themselves asymmetric, organizing varying levels of pericentriolar material and microtubules. This asymmetry produces one active and one inactive centrosome during interphase. We identify pericentrin-like protein (PLP) as a negative regulator of centrosome maturation and activity. PLP is enriched on the inactive interphase centrosome, where it blocks recruitment of the master regulator of centrosome maturation, Polo kinase. Furthermore, this study found that ectopic Centrobin expression influences PLP levels on the basal centrosome, suggesting it may normally function to regulate PLP. Finally, it is concluded that, although asymmetric centrosome maturation is not required for asymmetric cell division, it is required for proper centrosome segregation to the two daughter cells.
Monday, October 7th
Hu, D., Gao, X., Morgan, M. A., Herz, H. M., Smith, E. R. and Shilatifard, A. (2013). The MLL3/MLL4 branch of the COMPASS family is a major H3K4 monomethylase at enhancers. Mol Cell Biol. PubMed ID: 24081332
Histone H3 lysine 4 (H3K4) can be mono-, di-, and trimethylated by members of the COMPASS (COMplex of Proteins ASsociated with Set1) family from yeast to human. Monomethylation of histone H3K4 (H3K4me1) is relatively more enriched at metazoan enhancer regions compared to trimethylated histone H3K4 (H3K4me3), which are found at transcription start sites in all eukaryotes. There are six COMPASS family members in mammals, two of which, MLL3 and MLL4, are most closely related to Drosophila Trr, implicated in regulation of enhancer activity. This study found that MLL4 is preferentially found at enhancer regions. The widely used HCT116 cancer cell line contains inactivating mutations in the MLL3 gene. Using HCT116 cells in which MLL4 has also been knocked out, it was demonstrated that MLL3 and MLL4 are major regulators of H3K4me1 in these cells, with the greatest loss of monomethylation at enhancer regions. A redundant role between Mll3 and Mll4 was found in enhancer H3K4 monomethylation in mouse embryonic fibroblast (MEF) cells. These findings suggest that mammalian MLL3/MLL4 function in the regulation of enhancer activity and enhancer-promoter communication during gene expression and that mutations of MLL3 and MLL4 found in cancer could exert their properties through enhancer malfunction.
Manier, M. K., Lupold, S., Belote, J. M., Starmer, W. T., Berben, K. S., Ala-Honkola, O., Collins, W. F. and Pitnick, S. (2013. Postcopulatory Sexual Selection Generates Speciation Phenotypes in Drosophila. Curr Biol. PubMed ID: 24076241
Identifying traits that reproductively isolate species, and the selective forces underlying their divergence, is a central goal of evolutionary biology and speciation research. There is growing recognition that postcopulatory sexual selection, which can drive rapid diversification of interacting ejaculate and female reproductive tract traits that mediate sperm competition, may be an engine of speciation. Conspecific sperm precedence (CSP) is a taxonomically widespread form of reproductive isolation. To test the hypothesis that postcopulatory sexual selection can generate reproductive isolation, GFP or RFP were expressed in sperm heads of recently diverged sister species, Drosophila simulans and D. mauritiana, to enable detailed resolution of species-specific sperm precedence mechanisms. Four distinct mechanisms of CSP were resolved in this study. These comprise interactions between multiple sex-specific traits, including two independent mechanisms by which females exert sophisticated control over sperm fate to favor the conspecific male. These results confirm that reproductive isolation can quickly arise from diversifying (allopatric) postcopulatory sexual selection. This experimental approach to 'speciation phenotypes' illustrates how knowledge of sperm precedence mechanisms can be used to predict the mechanisms and extent of reproductive isolation between populations and species.
Sunday, October 6th
Leinwand, S. G. and Chalasani, S. H. (2013). Neuropeptide signaling remodels chemosensory circuit composition in Caenorhabditis elegans. Nat Neurosci 16: 14667. PubMed ID: 24013594
Neural circuits detect environmental changes and drive behavior. The routes of information flow through dense neural networks are dynamic, but the mechanisms underlying this circuit flexibility are poorly understood. This study describes a sensory context-dependent and neuropeptide-regulated switch in the composition of a C. elegans salt sensory circuit. The primary salt detectors, ASE sensory neurons, used BLI-4 endoprotease-dependent cleavage to release the insulin-like peptide INS-6 (see Drosophila Insulin-related peptided) in response to large, but not small, changes in external salt stimuli. Insulins, signaling through the insulin receptor DAF-2 (see Drosophila Insulin-like receptor), functionally switched the AWC olfactory sensory neuron into an interneuron in the salt circuit. Worms with disrupted insulin signaling had deficits in salt attraction, suggesting that peptidergic signaling potentiates responses to high salt stimuli, which may promote ion homeostasis. These results indicate that sensory context and neuropeptide signaling modify neural networks and suggest general mechanisms for generating flexible behavioral outputs by modulating neural circuit composition.
Isono, K., Endo, T. A., Ku, M., Yamada, D., Suzuki, R., Sharif, J., Ishikura, T., Toyoda, T., Bernstein, B. E. and Koseki, H. (2013). SAM Domain Polymerization Links Subnuclear Clustering of PRC1 to Gene Silencing. Dev Cell 26: 565-577. PubMed ID: 24091011
The Polycomb-group (PcG) repressive complex-1 (PRC1) forms microscopically visible clusters in nuclei; however, the impact of this cluster formation on transcriptional regulation and the underlying mechanisms that regulate this process remain obscure. This study reports that the sterile alpha motif (SAM) domain of a PRC1 core component Phc2 (Drosophila homolog; Polyhomeotic) plays an essential role for PRC1 clustering through head-to-tail macromolecular polymerization, which is associated with stable target binding of PRC1/PRC2 and robust gene silencing activity. A role is proposed for SAM domain polymerization in this repression by two distinct mechanisms: first, through capturing and/or retaining PRC1 at the PcG targets, and second, by strengthening the interactions between PRC1 and PRC2 to stabilize transcriptional repression. These findings reveal a regulatory mechanism mediated by SAM domain polymerization for PcG-mediated repression of developmental loci that enables a robust yet reversible gene repression program during development.
Saturday, October 5th
Hayama, T., Noguchi, J., Watanabe, S., Takahashi, N., Hayashi-Takagi, A., Ellis-Davies, G. C., Matsuzaki, M. and Kasai, H. (2013). GABA promotes the competitive selection of dendritic spines by controlling local Ca2+ signaling. Nat Neurosci 16: 1409-1416. PubMed ID: 23974706
Activity-dependent competition of synapses plays a key role in neural organization and is often promoted by GABA; however, its cellular bases are poorly understood. Excitatory synapses of cortical pyramidal neurons are formed on small protrusions known as dendritic spines, which exhibit structural plasticity. This study used two-color uncaging of glutamate and GABA in rat hippocampal CA1 pyramidal neurons and found that spine shrinkage and elimination were markedly promoted by the activation of GABAA receptors (Drosophila homolog: Rdl) shortly before action potentials. GABAergic inhibition suppressed bulk increases in cytosolic Ca2+ concentrations, whereas it preserved the Ca2+ nanodomains generated by NMDA-type receptors (see Drosophila Nmdar1 and Nmdar2), both of which were necessary for spine shrinkage. Unlike spine enlargement, spine shrinkage spread to neighboring spines (<15 μm) and competed with their enlargement, and this process involved the actin-depolymerizing factor ADF/cofilin (see Drosophila Twinstar). Thus, GABAergic inhibition directly suppresses local dendritic Ca2+ transients and strongly promotes the competitive selection of dendritic spines.
Toth, A. B., Terauchi, A., Zhang, L. Y., Johnson-Venkatesh, E. M., Larsen, D. J., Sutton, M. A. and Umemori, H. (2013). Synapse maturation by activity-dependent ectodomain shedding of SIRPα. Nat Neurosci 16: 1417-1425. PubMed ID: 24036914
Formation of appropriate synaptic connections is critical for proper functioning of the brain. After initial synaptic differentiation, active synapses are stabilized by neural activity-dependent signals to establish functional synaptic connections. However, the molecular mechanisms underlying activity-dependent synapse maturation remain to be elucidated. This study shows that activity-dependent ectodomain shedding of signal regulatory protein-α (SIRPα) mediates presynaptic maturation. Two target-derived molecules, fibroblast growth factor 22 and SIRPα, sequentially organize the glutamatergic presynaptic terminals during the initial synaptic differentiation and synapse maturation stages, respectively, in the mouse hippocampus. SIRPα drives presynaptic maturation in an activity-dependent fashion. Remarkably, neural activity cleaves the extracellular domain of SIRPα, and the shed ectodomain in turn promotes the maturation of the presynaptic terminal. SIRPα-dependent synapse maturation has an impact on synaptic function and plasticity. Thus, ectodomain shedding of SIRPα is an activity-dependent trans-synaptic mechanism for the maturation of functional synapses.
Friday, October 4th
Moshkin, Y. M., Doyen, C. M., Kan, T. W., Chalkley, G. E., Sap, K., Bezstarosti, K., Demmers, J. A., Ozgur, Z., van Ijcken, W. F. and Verrijzer, C. P. (2013). Histone Chaperone NAP1 Mediates Sister Chromatid Resolution by Counteracting Protein Phosphatase 2A. PLoS Genet 9: e1003719. PubMed ID: 24086141
Chromosome duplication and transmission into daughter cells requires the precisely orchestrated binding and release of cohesin (see Drosophila Nipped B). This study found that the Drosophila histone chaperone NAP1 is required for cohesin release and sister chromatid resolution during mitosis. Genome-wide surveys revealed that NAP1 and cohesin co-localize at multiple genomic loci. Proteomic and biochemical analysis established that NAP1 associates with the full cohesin complex, but it also forms a separate complex with the cohesin subunit stromalin (SA). NAP1 binding to cohesin is cell-cycle regulated and increases during G2/M phase. This causes the dissociation of protein phosphatase 2A (PP2A) from cohesin, increased phosphorylation of SA and cohesin removal in early mitosis. PP2A depletion leads to a loss of centromeric cohesion. The distinct mitotic phenotypes caused by the loss of either PP2A or NAP1 are both rescued by their concomitant depletion. It is concluded that the balanced antagonism between NAP1 and PP2A controls cohesin dissociation during mitosis.
Kotani, T., Yasuda, K., Ota, R. and Yamashita, M. (2013). Cyclin B1 mRNA translation is temporally controlled through formation and disassembly of RNA granules. J Cell Biol 202: 1041-1055. PubMed ID: 24062337
Temporal control of messenger RNA (mRNA) translation is an important mechanism for regulating cellular, neuronal, and developmental processes. However, mechanisms that coordinate timing of translational activation remain largely unresolved. Full-grown oocytes arrest meiosis at prophase I and deposit dormant mRNAs. Of these, translational control of cyclin B1 mRNA (see Drosophila Cyclin B) in response to maturation-inducing hormone is important for normal progression of oocyte maturation, through which oocytes acquire fertility. This study found that dormant cyclin B1 mRNA forms granules in the cytoplasm of zebrafish and mouse oocytes. Real-time imaging of translation revealed that the granules disassemble at the time of translational activation during maturation. Formation of cyclin B1 RNA granules requires binding of the mRNA to Pumilio1 (see Drosophila Pumilio) protein and depends on actin filaments. Disruption of cyclin B1 RNA granules accelerated the timing of their translational activation after induction of maturation, whereas stabilization hindered translational activation. Thus, these results suggest that RNA granule formation is critical for the regulation of timing of translational activation.
Thursday, October 3rd
Kim, T., Vidal, G. S., Djurisic, M., William, C. M., Birnbaum, M. E., Garcia, K. C., Hyman, B. T. and Shatz, C. J. (2013). Human LilrB2 is a beta-amyloid receptor and its murine homolog PirB regulates synaptic plasticity in an Alzheimer's model. Science 341: 1399-1404. PubMed ID: 24052308
Soluble β-amyloid (Aβ: see Drosophila β amyloid protein precursor-like) oligomers impair synaptic plasticity and cause synaptic loss associated with Alzheimer's disease (AD). Murine PirB (paired immunoglobulin-like receptor B) and its human ortholog LilrB2 (leukocyte immunoglobulin-like receptor B2), present in human brain, are receptors for Aβ oligomers, with nanomolar affinity. The first two extracellular immunoglobulin (Ig) domains of PirB and LilrB2 mediate this interaction, leading to enhanced cofilin signaling, also seen in human AD brains. In mice, the deleterious effect of Aβ oligomers on hippocampal long-term potentiation required PirB, and in a transgenic model of AD, PirB not only contributed to memory deficits present in adult mice, but also mediated loss of synaptic plasticity in juvenile visual cortex. These findings imply that LilrB2 contributes to human AD neuropathology and suggest therapeutic uses of blocking LilrB2 function.
Nguyen, K. D., Fentress, S. J., Qiu, Y., Yun, K., Cox, J. S. and Chawla, A. (2013). Circadian gene Bmal1 regulates diurnal oscillations of Ly6C(hi) inflammatory monocytes. Science 341: 1483-1488. PubMed ID: 23970558
Circadian clocks have evolved to regulate physiologic and behavioral rhythms in anticipation of changes in the environment. Although the molecular clock is present in innate immune cells, its role in monocyte homeostasis remains unknown. This study report that Ly6Chi inflammatory monocytes exhibit diurnal variation, which controls their trafficking to sites of inflammation. This cyclic pattern of trafficking confers protection against Listeria monocytogenes and is regulated by the repressive activity of the circadian gene Bmal1 (Drosophila homolog: Clock). Accordingly, myeloid cell-specific deletion of Bmal1 induces expression of monocyte-attracting chemokines and disrupts rhythmic cycling of Ly6C(hi) monocytes, predisposing mice to development of pathologies associated with acute and chronic inflammation. These findings have unveiled a critical role for BMAL1 in controlling the diurnal rhythms in Ly6C(hi) monocyte numbers.
Wednesday, October 2nd
Banaszynski, L. A., Wen, D., Dewell, S., Whitcomb, S. J., Lin, M., Diaz, N., Elsasser, S. J., Chapgier, A., Goldberg, A. D., Canaani, E., Rafii, S., Zheng, D. and Allis, C. D. (2013). Hira-dependent Histone H3.3 deposition facilitates PRC2 recruitment at developmental loci in ES cells. Cell 155: 107-120. PubMed ID: 24074864
Polycomb repressive complex 2 (PRC2) regulates gene expression during lineage specification through trimethylation of lysine 27 on histone H3 (see Drosophila Histone H3). In Drosophila, polycomb binding sites are dynamic chromatin regions enriched with the histone variant H3.3. This study shows that, in mouse embryonic stem cells (ESCs), H3.3 is required for proper establishment of H3K27me3 at the promoters of developmentally regulated genes. Upon H3.3 depletion, these promoters show reduced nucleosome turnover measured by deposition of de novo synthesized histones and reduced PRC2 occupancy. Further, this study shows H3.3-dependent interaction of PRC2 with the histone chaperone, Hira (see Drosophila Hira), and that Hira localization to chromatin requires H3.3. The data demonstrate the importance of H3.3 in maintaining a chromatin landscape in ESCs that is important for proper gene regulation during differentiation. Moreover, these findings support the emerging notion that H3.3 has multiple functions in distinct genomic locations that are not always correlated with an 'active' chromatin state.
Wairkar, Y. P., Trivedi, D., Natarajan, R., Barnes, K., Dolores, L. and Cho, P. (2013). CK2-alpha regulates the transcription of BRP in Drosophila. Dev Biol. PubMed ID: 24080510
Although signaling pathways that regulate the synaptic plasticity from the postsynaptic compartments are well defined, the pathways that control these changes presynaptically are poorly described. In a genetic screen for synapse development in Drosophila, this study found that mutations in CK2α lead to an increase in the levels of Bruchpilot (Brp), a scaffolding protein associated with the active zones. Using a combination of genetic and biochemical approaches, this study found that the increase in Brp in ck2α mutants is largely due to an increase in the transcription of brp. Interestingly, the transcripts of other active zone proteins that are important for function of active zones were also increased, while the transcripts from some other synaptic proteins were unchanged. Thus, these data suggest that CK2α might be important in regulating synaptic plasticity by modulating the transcription of Brp. Hence, it is proposed that CK2α is a novel regulator of the active zone protein, Brp, in Drosophila.
Tuesday, October 1st
Costa, A., Pazman, C., Sinsimer, K. S., Wong, L. C., McLeod, I., Yates, J., 3rd, Haynes, S. and Schedl, P. (2013). Rasputin functions as a positive regulator of orb in Drosophila oogenesis. PLoS One 8: e72864. PubMed ID: 24069162
The determination of cell fate and the establishment of polarity axes during Drosophila oogenesis depend upon pathways that localize mRNAs within the egg chamber and control their on-site translation. One factor that plays a central role in regulating on-site translation of mRNAs is Orb. Orb is a founding member of the conserved CPEB family of RNA-binding proteins, which proteins bind to target sequences in 3' UTRs and regulate mRNA translation by modulating poly(A) tail length. Rasputin (Rin), the Drosophila homologue of Ras-GAP SH3 Binding Protein (G3BP), associates with Orb in a messenger ribonucleoprotein (mRNP) complex. Rin is an evolutionarily conserved RNA-binding protein believed to function as a link between Ras signaling and RNA metabolism. This study shows that Orb and Rin form a complex in the female germline. rin functions as a positive regulator in the orb autoregulatory pathway by increasing Orb protein expression. Several canonical stress granule proteins are associated with the Orb-Rin complex suggesting that a conserved mRNP complex regulates localized translation during oogenesis in Drosophila.
Dutriaux, A., Godart, A., Brachet, A. and Silber, J. (2013). The insulin receptor is required for the development of the Drosophila peripheral nervous system. PLoS One 8: e71857. PubMed ID: 24069139Summary:
The Insulin Receptor (InR) in Drosophila is required for growth; it is expressed in the central and embryonic nervous system and modulates the time of differentiation of the eye photoreceptor without altering cell fate. This study shows that the InR is required for the formation of the peripheral nervous system during larval development and more particularly for the formation of sensory organ precursors (SOPs) on the fly notum and scutellum. Misexpression of the InR induces the development of an abnormal number of macrochaetes that are Drosophila mechanoreceptors. The data suggest that InR regulates the neural genes ac, sc and sens. The FOXO transcription factor which is localized in the cytoplasm upon insulin uptake, displays strong genetic interaction with the InR and is involved in Ac regulation. The genetic interactions between the epidermal growth factor receptor (EGFR), Ras and InR/FOXO suggest that these proteins cooperate to induce neural gene expression. Moreover, InR/FOXO is probably involved in the lateral inhibition process, since genetic interactions with Notch are highly significant. These results show that the InR can alter cell fate, independently of its function in cell growth and proliferation.
Home page: The Interactive Fly© 2013 Thomas B. Brody, Ph.D.
The Interactive Fly resides on the
Society for Developmental Biology's Web server.