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Saturday, November 30th, 2013

Yuan, Q., Song, Y., Yang, C. H., Jan, L. Y. and Jan, Y. N. (2013). Female contact modulates male aggression via a sexually dimorphic GABAergic circuit in Drosophila. Nat Neurosci. PubMed ID: 24241395
Summary: Intraspecific male-male aggression, which is important for sexual selection, is regulated by environment, experience and internal states through largely undefined molecular and cellular mechanisms. To understand the basic neural pathway underlying the modulation of this innate behavior, a behavioral assay was establised in Drosophila melanogaster, and the relationship between sexual experience and aggression was investigated. In the presence of mating partners, adult male flies exhibited elevated levels of aggression, which was largely suppressed by prior exposure to females via a sexually dimorphic neural mechanism. The suppression involved the ability of male flies to detect females by contact chemosensation through the pheromone-sensing ion channel ppk29 and was mediated by male-specific GABAergic neurons acting on the GABAA receptor RDL in target cells. Silencing or activating this circuit led to dis-inhibition or elimination of sex-related aggression, respectively. It is proposed that the GABAergic inhibition represents a critical cellular mechanism that enables prior experience to modulate aggression.

Smith, S. L., Smith, I. T., Branco, T. and Hausser, M. (2013). Dendritic spikes enhance stimulus selectivity in cortical neurons in vivo. Nature 503: 115-120. PubMed ID: 24162850
Summary: Neuronal dendrites are electrically excitable: they can generate regenerative events such as dendritic spikes in response to sufficiently strong synaptic input. Although such events have been observed in many neuronal types, it is not well understood how active dendrites contribute to the tuning of neuronal output in vivo. This study show that dendritic spikes increase the selectivity of neuronal responses to the orientation of a visual stimulus (orientation tuning). Direct patch-clamp recordings were performed from the dendrites of pyramidal neurons in the primary visual cortex of lightly anaesthetized and awake mice, during sensory processing. Visual stimulation triggered regenerative local dendritic spikes that were distinct from back-propagating action potentials. These events were orientation tuned and were suppressed by either hyperpolarization of membrane potential or intracellular blockade of NMDA receptors (see Drosophila NMDA receptors). Both of these manipulations also decreased the selectivity of subthreshold orientation tuning measured at the soma, thus linking dendritic regenerative events to somatic orientation tuning. Together, these results suggest that dendritic spikes that are triggered by visual input contribute to a fundamental cortical computation: enhancing orientation selectivity in the visual cortex. Thus, dendritic excitability is an essential component of behaviourally relevant computations in neurons.

Bloodgood, B. L., Sharma, N., Browne, H. A., Trepman, A. Z. and Greenberg, M. E. (2013). The activity-dependent transcription factor NPAS4 regulates domain-specific inhibition. Nature 503: 121-125. PubMed ID: 24201284
Summary: A heterogeneous population of inhibitory neurons controls the flow of information through a neural circuit. Inhibitory synapses that form on pyramidal neuron dendrites modulate the summation of excitatory synaptic potentials and prevent the generation of dendritic calcium spikes. Precisely timed somatic inhibition limits both the number of action potentials and the time window during which firing can occur. The activity-dependent transcription factor NPAS4 regulates inhibitory synapse number and function in cell culture, but how this transcription factor affects the inhibitory inputs that form on distinct domains of a neuron in vivo was unclear. This study shows that in the mouse hippocampus behaviourally driven expression of NPAS4 coordinates the redistribution of inhibitory synapses made onto a CA1 pyramidal neuron, simultaneously increasing inhibitory synapse number on the cell body while decreasing the number of inhibitory synapses on the apical dendrites. This rearrangement of inhibition is mediated in part by the NPAS4 target gene brain derived neurotrophic factor (Bdnf), which specifically regulates somatic, and not dendritic, inhibition. These findings indicate that sensory stimuli, by inducing NPAS4 and its target genes, differentially control spatial features of neuronal inhibition in a way that restricts the output of the neuron while creating a dendritic environment that is permissive for plasticity.

Friday, November 29th

Sasamura, T., Matsuno, K. and Fortini, M. E. (2013). Disruption of Drosophila melanogaster Lipid Metabolism Genes Causes Tissue Overgrowth Associated with Altered Developmental Signaling. PLoS Genet 9: e1003917. PubMed ID: 24244188
Summary: Developmental patterning requires the precise interplay of numerous intercellular signaling pathways to ensure that cells are properly specified during tissue formation and organogenesis. The spatiotemporal function of many developmental pathways is strongly influenced by the biosynthesis and intracellular trafficking of signaling components. Receptors and ligands must be trafficked to the cell surface where they interact, and their subsequent endocytic internalization and endosomal trafficking is critical for both signal propagation and its down-modulation. In a forward genetic screen for mutations that alter intracellular Notch receptor trafficking in Drosophila melanogaster, mutants were recovered that disrupt genes encoding serine palmitoyltransferase and Acetyl-CoA Carboxylase (ACC). Both mutants cause Notch, Wingless, the Epidermal Growth Factor Receptor (EFGR), and Patched to accumulate abnormally in endosomal compartments. In mosaic animals, mutant tissues exhibit an unusual non-cell-autonomous effect whereby mutant cells are functionally rescued by secreted activities emanating from adjacent wildtype tissue. Strikingly, both mutants display prominent tissue overgrowth phenotypes that are partially attributable to altered Notch and Wnt signaling. This analysis of the mutants demonstrates genetic links between abnormal lipid metabolism, perturbations in developmental signaling, and aberrant cell proliferation.

Penmatsa, A., Wang, K. H. and Gouaux, E. (2013). X-ray structure of dopamine transporter elucidates antidepressant mechanism. Nature 503: 85-90. PubMed ID: 24037379
Summary: Antidepressants targeting Na+)/Cl--coupled neurotransmitter uptake define a key therapeutic strategy to treat clinical depression and neuropathic pain. However, identifying the molecular interactions that underlie the pharmacological activity of these transport inhibitors, and thus the mechanism by which the inhibitors lead to increased synaptic neurotransmitter levels, has proven elusive. This study presents the crystal structure of the Drosophila melanogaster Dopamine transporter at 3.0 Å resolution bound to the tricyclic antidepressant nortriptyline. The transporter is locked in an outward-open conformation with nortriptyline wedged between transmembrane helices 1, 3, 6 and 8, blocking the transporter from binding substrate and from isomerizing to an inward-facing conformation. Although the overall structure of the dopamine transporter is similar to that of its prokaryotic relative LeuT, there are multiple distinctions, including a kink in transmembrane helix 12 halfway across the membrane bilayer, a latch-like carboxy-terminal helix that caps the cytoplasmic gate, and a cholesterol molecule wedged within a groove formed by transmembrane helices 1a, 5 and 7. Taken together, the dopamine transporter structure reveals the molecular basis for antidepressant action on sodium-coupled neurotransmitter symporters and elucidates critical elements of eukaryotic transporter structure and modulation by lipids, thus expanding our understanding of the mechanism and regulation of neurotransmitter uptake at chemical synapses.

Morillo Prado, J. R., Srinivasan, S. and Fuller, M. T. (2013). The Histone Variant His2Av is Required for Adult Stem Cell Maintenance in the Drosophila Testis. PLoS Genet 9: e1003903. PubMed ID: 24244183
Summary: Many tissues are sustained by adult stem cells, which replace lost cells by differentiation and maintain their own population through self-renewal. The mechanisms through which adult stem cells maintain their identity are thus important for tissue homeostasis and repair throughout life. This study shows that a histone variant, His2Av, is required cell autonomously for maintenance of germline and cyst stem cells in the Drosophila testis. The ATP-dependent chromatin-remodeling factor Domino is also required in this tissue for adult stem cell maintenance possibly by regulating the incorporation of His2Av into chromatin. Interestingly, although expression of His2Av was ubiquitous, its function was dispensable for germline and cyst cell differentiation, suggesting a specific role for this non-canonical histone in maintaining the stem cell state in these lineages.

Thursday, November 28th

Dang, I., et al. (2013). Inhibitory signalling to the Arp2/3 complex steers cell migration. Nature 503: 281-284. PubMed ID: 24132237
Summary: Cell migration requires the generation of branched actin networks that power the protrusion of the plasma membrane in lamellipodia. The actin-related proteins 2 and 3 (Arp2/3) complex is the molecular machine that nucleates these branched actin networks. This machine is activated at the leading edge of migrating cells by Wiskott-Aldrich syndrome protein (WASP)-family verprolin-homologous protein (WAVE, also known as SCAR - see Drosophila SCAR). The WAVE complex is itself directly activated by the small GTPase Rac (see Drosophila Rac1), which induces lamellipodia. However, how cells regulate the directionality of migration is poorly understood. This study identified a new protein, Arpin, that inhibits the Arp2/3 complex in vitro, and has shown that Rac signalling recruits and activates Arpin at the lamellipodial tip, like WAVE. Consistently, after depletion of the inhibitory Arpin, lamellipodia protrude faster and cells migrate faster. A major role of this inhibitory circuit, however, is to control directional persistence of migration. Indeed, Arpin depletion in both mammalian cells and Dictyostelium discoideum amoeba resulted in straighter trajectories, whereas Arpin microinjection in fish keratocytes, one of the most persistent systems of cell migration, induced these cells to turn. The coexistence of the Rac-Arpin-Arp2/3 inhibitory circuit with the Rac-WAVE-Arp2/3 activatory circuit can account for this conserved role of Arpin in steering cell migration.

Vanderheyden, W. M., Gerstner, J. R., Tanenhaus, A., Yin, J. C. and Shaw, P. J. (2013). ERK Phosphorylation Regulates Sleep and Plasticity in Drosophila. PLoS One 8: e81554. PubMed ID: 24244744
Summary: Given the relationship between sleep and plasticity, this study examined the role of Extracellular signal-regulated kinase (ERK, Rolled in Drosophila) in regulating baseline sleep, and modulating the response to waking experience. Both sleep deprivation and social enrichment increase ERK phosphorylation in wild-type flies. The effects of both sleep deprivation and social enrichment on structural plasticity in the within the Pigment Dispersing Factor (PDF)-expressing ventral lateral neurons (LNvs) can be recapitulated by expressing an active version of ERK (UAS-ERKSEM) pan-neuronally in the adult fly using GeneSwitch (Gsw) Gsw-elav-GAL4. Conversely, disrupting ERK reduces sleep and prevents both the behavioral and structural plasticity normally induced by social enrichment. Finally, using transgenic flies carrying a cAMP response Element (CRE)-luciferase reporter it was shown that activating ERK enhances CRE-Luc activity while disrupting ERK reduces it. These data suggest that ERK phosphorylation is an important mediator in transducing waking experience into sleep.

Schorderet, P., Lonfat, N., Darbellay, F., Tschopp, P., Gitto, S., Soshnikova, N. and Duboule, D. (2013). A Genetic Approach to the Recruitment of PRC2 at the HoxD Locus. PLoS Genet 9: e1003951. PubMed ID: 24244202
Summary: Polycomb group (PcG) proteins are essential for the repression of key factors during early development. In Drosophila, the polycomb repressive complexes (PRC) associate with defined polycomb response DNA elements (PREs). In mammals, however, the mechanisms underlying polycomb recruitment at targeted loci are poorly understood. This study used an in vivo approach to identify DNA sequences of importance for the proper recruitment of polycomb proteins at the HoxD locus (see Drosophila Antennapedia and Bithorax complexes). Various genomic re-arrangements of the gene cluster do not strongly affect PRC2 recruitment, and relatively small polycomb interacting sequences appear necessary and sufficient to confer polycomb recognition and targeting to ectopic loci. In addition, a high GC content, while not sufficient to recruit PRC2, may help its local spreading. PRC2 recruitment over Hox gene clusters in embryonic stem cells is important for their subsequent coordinated transcriptional activation during development. It is concluded that a range of low affinity sequences synergize to recruit PRCs over the gene cluster, which makes this process very robust and resistant to genetic perturbations

Wednesday, November 27

Zhou, Q., Zhang, T., Jemc, J. C., Chen, Y., Chen, R., Rebay, I. and Pignoni, F. (2013). Onset of atonal expression in Drosophila retinal progenitors involves redundant and synergistic contributions of Ey/Pax6 and So bindings sites within two distant enhancers. Dev Biol. PubMed ID: 24247006
Summary: Proneural transcription factors drive the generation of specialized neurons during nervous system development, and their dynamic expression pattern is critical to their function. The activation of the proneural gene atonal (ato) in the Drosophila eye disc epithelium represents a critical step in the transition from retinal progenitor cell to developing photoreceptor neuron. This study shows that the onset of ato transcription depends on two distant enhancers that function differently in subsets of retinal progenitor cells. A detailed analysis of the crosstalk between these enhancers identifies a critical role for three binding sites for the Retinal Determination factors Eyeless (Ey) and Sine oculis (So). The study shows how these sites interact to induce ato expression in distinct regions of the eye field and confirms the sites are occupied by endogenous Ey and So proteins in vivo. This study suggests that Ey and So operate differently through the same 3' cis-regulatory sites in distinct populations of retinal progenitors.

Gerhart-Hines, Z., et al. (2013). The nuclear receptor Rev-erbalpha controls circadian thermogenic plasticity. Nature 503: 410-413. PubMed ID: 24162845
Summary: Circadian oscillation of body temperature is a basic, evolutionarily conserved feature of mammalian biology. In addition, homeostatic pathways allow organisms to protect their core temperatures in response to cold exposure. However, the mechanism responsible for coordinating daily body temperature rhythm and adaptability to environmental challenges is unknown. This study shows that the nuclear receptor Rev-erbα (also known as Nr1d1; Drosophila homolog Eip78C), a powerful transcriptional repressor, links circadian and thermogenic networks through the regulation of brown adipose tissue (BAT) function. Mice exposed to cold fare considerably better at 5:00AM (Zeitgeber time 22) when Rev-erbα is barely expressed than at 5:00PM (Zeitgeber time 10) when Rev-erbα is abundant. Deletion of Rev-erbα markedly improves cold tolerance at 5:00 PM, indicating that overcoming Rev-erbα-dependent repression is a fundamental feature of the thermogenic response to cold. Physiological induction of uncoupling protein 1 (Ucp1) by cold temperatures is preceded by rapid downregulation of Rev-erbα in BAT. Rev-erbα represses Ucp1 in a brown-adipose-cell-autonomous manner and BAT Ucp1 levels are high in Rev-erbaα-null mice, even at thermoneutrality. Genetic loss of Rev-erbα also abolishes normal rhythms of body temperature and BAT activity. Thus, Rev-erbalpha acts as a thermogenic focal point required for establishing and maintaining body temperature rhythm in a manner that is adaptable to environmental demands.

Fica, S. M., Tuttle, N., Novak, T., Li, N. S., Lu, J., Koodathingal, P., Dai, Q., Staley, J. P. and Piccirilli, J. A. (2013). RNA catalyses nuclear pre-mRNA splicing. Nature 503: 229-234. PubMed ID: 24196718
Summary: In nuclear pre-messenger RNA splicing, introns are excised by the spliceosome, a dynamic machine composed of both proteins and small nuclear RNAs (snRNAs). Over thirty years ago, after the discovery of self-splicing group II intron RNAs, the snRNAs were proposed to catalyse splicing. However, no definitive evidence for a role of either RNA or protein in catalysis by the spliceosome has been reported so far. By using metal rescue strategies in spliceosomes from budding yeast, this study shows that the U6 snRNA catalyses both of the two splicing reactions by positioning divalent metals that stabilize the leaving groups during each reaction. Notably, all of the U6 catalytic metal ligands that were identified correspond to the ligands observed to position catalytic, divalent metals in crystal structures of a group II intron RNA. These findings indicate that group II introns and the spliceosome share common catalytic mechanisms and probably common evolutionary origins. These results demonstrate that RNA mediates catalysis within the spliceosome.

Tuesday, November 26th

Komori, H., Xiao, Q., McCartney, B. M. and Lee, C. Y. (2013). Brain tumor specifies intermediate progenitor cell identity by attenuating beta-catenin/Armadillo activity. Development. PubMed ID: 24257623
Summary: During asymmetric stem cell division, both the daughter stem cell and the presumptive intermediate progenitor cell inherit cytoplasm from their parental stem cell. Thus, proper specification of intermediate progenitor cell identity requires an efficient mechanism to rapidly extinguish the activity of self-renewal factors, but the mechanisms remain unknown in most stem cell lineages. During asymmetric division of a type II neural stem cell (neuroblast) in the Drosophila larval brain, the Brain tumor (Brat) protein segregates unequally into the immature intermediate neural progenitor (INP), where it specifies INP identity by attenuating the function of the self-renewal factor Klumpfuss (Klu), but the mechanisms are not understood. This study reports that Brat specifies INP identity through its N-terminal B-boxes via a novel mechanism that is independent of asymmetric protein segregation. Brat-mediated specification of INP identity is critically dependent on the function of the Wnt destruction complex, which attenuates the activity of β-catenin/Armadillo (Arm) in immature INPs. Aberrantly increasing Arm activity in immature INPs further exacerbates the defects in the specification of INP identity and enhances the supernumerary neuroblast mutant phenotype in brat mutant brains. By contrast, reducing Arm activity in immature INPs suppresses supernumerary neuroblast formation in brat mutant brains. Finally, reducing Arm activity also strongly suppresses supernumerary neuroblasts induced by overexpression of klu. Thus, the Brat-dependent mechanism extinguishes the function of the self-renewal factor Klu in the presumptive intermediate progenitor cell by attenuating Arm activity, balancing stem cell maintenance and progenitor cell specification.

Lee, M. T., Bonneau, A. R., Takacs, C. M., Bazzini, A. A., DiVito, K. R., Fleming, E. S. and Giraldez, A. J. (2013). Nanog, Pou5f1 and SoxB1 activate zygotic gene expression during the maternal-to-zygotic transition. Nature 503: 360-364. PubMed ID: 24056933
Summary: After fertilization, maternal factors direct development and trigger zygotic genome activation (ZGA) at the maternal-to-zygotic transition (MZT). In zebrafish, ZGA is required for gastrulation and clearance of maternal messenger RNAs, which is in part regulated by the conserved microRNA miR-430. However, the factors that activate the zygotic program in vertebrates are unknown. this study shows that Nanog, Pou5f1 (also called Oct4) and SoxB1 (Drosophila homolog - Dichaete) regulate zygotic gene activation in zebrafish. Several hundred genes directly activated by maternal factors were identified, constituting the first wave of zygotic transcription. Ribosome profiling revealed that nanog, sox19b and pou5f1 are the most highly translated transcription factors pre-MZT. Combined loss of these factors resulted in developmental arrest before gastrulation and a failure to activate >75% of zygotic genes, including miR-430. These results demonstrate that maternal Nanog, Pou5f1 and SoxB1 are required to initiate the zygotic developmental program and induce clearance of the maternal program by activating miR-430 expression.

Unhavaithaya, Y. and Orr-Weaver, T. L. (2013). Centromere proteins CENP-C and CAL1 functionally interact in meiosis for centromere clustering, pairing, and chromosome segregation. Proc Natl Acad Sci U S A. PubMed ID: 24248385
Summary: Meiotic chromosome segregation involves pairing and segregation of homologous chromosomes in the first division and segregation of sister chromatids in the second division. Although it is known that the centromere and kinetochore are responsible for chromosome movement in meiosis as in mitosis, potential specialized meiotic functions are being uncovered. Centromere pairing early in meiosis I, even between nonhomologous chromosomes, and clustering of centromeres can promote proper homolog associations in meiosis I in yeast, plants, and Drosophila. It was not known, however, whether centromere proteins are required for this clustering. This study exploited Drosophila mutants for the centromere proteins centromere protein-C (CENP-C) and chromosome alignment 1 (CAL1) to demonstrate that a functional centromere is needed for centromere clustering and pairing. The cenp-C and cal1 mutations result in C-terminal truncations, removing the domains through which these two proteins interact. The mutants show striking genetic interactions, failing to complement as double heterozygotes, resulting in disrupted centromere clustering and meiotic nondisjunction. The cluster of meiotic centromeres localizes to the nucleolus, and this association requires centromere function. In Drosophila, synaptonemal complex (SC) formation can initiate from the centromere, and the SC is retained at the centromere after it disassembles from the chromosome arms. Although functional CENP-C and CAL1 are dispensable for assembly of the SC, they are required for subsequent retention of the SC at the centromere. These results show that integral centromere proteins are required for nuclear position and intercentromere associations in meiosis.

Monday, November 25th

Schuster, A. T., Sarvepalli, K., Murphy, E. A. and Longworth, M. S. (2013). Condensin II Subunit dCAP-D3 Restricts Retrotransposon Mobilization in Drosophila Somatic Cells. PLoS Genet 9: e1003879. PubMed ID: 24204294
Summary: Retrotransposon sequences are positioned throughout the genome of almost every eukaryote that has been sequenced. As mobilization of these elements can have detrimental effects on the transcriptional regulation and stability of an organism's genome, most organisms have evolved mechanisms to repress their movement.This study has identified a novel role for the Drosophila melanogaster Condensin II subunit, dCAP-D3 in preventing the mobilization of retrotransposons located in somatic cell euchromatin. dCAP-D3 regulates transcription of euchromatic gene clusters which contain or are proximal to retrotransposon sequence. ChIP experiments demonstrate that dCAP-D3 binds to these loci and is important for maintaining a repressed chromatin structure within the boundaries of the retrotransposon and for repressing retrotransposon transcription. dCAP-D3 prevents accumulation of double stranded DNA breaks within retrotransposon sequence, and decreased dCAP-D3 levels leads to a precise loss of retrotransposon sequence at some dCAP-D3 regulated gene clusters and a gain of sequence elsewhere in the genome. Homologous chromosomes exhibit high levels of pairing in Drosophila somatic cells, and FISH analyses demonstrate that retrotransposon-containing euchromatic loci are regions which are actually less paired than euchromatic regions devoid of retrotransposon sequences. Decreased dCAP-D3 expression increases pairing of homologous retrotransposon-containing loci in tissue culture cells. It is proposed that the combined effects of dCAP-D3 deficiency on double strand break levels, chromatin structure, transcription and pairing at retrotransposon-containing loci may lead to 1) higher levels of homologous recombination between repeats flanking retrotransposons in dCAP-D3 deficient cells and 2) increased retrotransposition. These findings identify a novel role for the anti-pairing activities of dCAP-D3/Condensin II and uncover a new way in which dCAP-D3/Condensin II influences local chromatin structure to help maintain genome stability.

Murakami, K., Elmlund, H., Kalisman, N., Bushnell, D. A., Adams, C. M., Azubel, M., Elmlund, D., Levi-Kalisman, Y., Liu, X., Gibbons, B. J., Levitt, M. and Kornberg, R. D. (2013). Architecture of an RNA polymerase II transcription pre-initiation complex. Science 342: 1238724. Abstract
Summary: The protein density and arrangement of subunits of a complete, 32-protein, RNA polymerase II (pol II) transcription pre-initiation complex (PIC) were determined by means of cryogenic electron microscopy and a combination of chemical cross-linking and mass spectrometry. The PIC showed a marked division in two parts, one containing all the general transcription factors (GTFs) and the other pol II. Promoter DNA was associated only with the GTFs, suspended above the pol II cleft and not in contact with pol II. This structural principle of the PIC underlies its conversion to a transcriptionally active state; the PIC is poised for the formation of a transcription bubble and descent of the DNA into the pol II cleft.

Sia, G. M., Clem, R. L. and Huganir, R. L. (2013). The Human Language-Associated Gene SRPX2 Regulates Synapse Formation and Vocalization in Mice. Science. PubMed ID: 24179158
Summary: Synapse formation in the developing brain depends on the coordinated activity of synaptogenic proteins, some which have been implicated in a number of neurodevelopmental disorders. This study shows that the sushi repeat-containing domain protein X-linked 2 (SRPX2) gene encodes a protein that promotes synaptogenesis in the cerebral cortex. In humans, SRPX2 is an epilepsy- and language-associated gene that is a target of the foxhead box protein P2 (FoxP2; Drosophila homolog, Forkhead box P) transcription factor. It was also shown that FoxP2 modulates synapse formation through regulating SRPX2 levels, and that SRPX2 reduction impairs development of ultrasonic vocalization in mice. These results suggest FoxP2 modulates the development of neural circuits through regulating synaptogenesis and that SRPX2 is a synaptogenic factor that plays a role in the pathogenesis of language disorders.

Sunday, November 24th

Ayeni, J. O., Varadarajan, R., Mukherjee, O., Stuart, D. T., Sprenger, F., Srayko, M. and Campbell, S. D. (2013). Dual Phosphorylation of Cdk1 Coordinates Cell Proliferation with Key Developmental Processes in Drosophila. . Genetics. PubMed ID: 24214341
Summary: Eukaryotic organisms use conserved checkpoint mechanisms that regulate Cdk1 by inhibitory phosphorylation to prevent mitosis from interfering with DNA replication or repair. In metazoans, this checkpoint mechanism is also used for coordinating mitosis with dynamic developmental processes. Inhibitory phosphorylation of Cdk1 is catalyzed by Wee1 kinases that phosphorylate tyrosine 15 (Y15) and dual-specificity Myt1 kinases found only in metazoans that phosphorylate Y15 and the adjacent threonine (T14) residue. Despite partially redundant roles in Cdk1 inhibitory phosphorylation, Wee1 and Myt1 serve specialized developmental functions that are not well understood. Wild type and phospho-acceptor mutant Cdk1 proteins were expressed to investigate how biochemical differences in Cdk1 inhibitory phosphorylation influence Drosophila imaginal development. Phosphorylation of Cdk1 on Y15 appeared to be crucial for developmental and DNA damage-induced G2 phase checkpoint arrest, consistent with other evidence that Myt1 is the major Y15-directed Cdk1 inhibitory kinase at this stage of development. Expression of non-inhibitable Cdk1 also caused chromosome defects in larval neuroblasts that were not observed with Cdk1(Y15F) mutant proteins that were phosphorylated on T14, implicating Myt1 in a novel mechanism promoting genome stability. Collectively, these results suggest that dual inhibitory phosphorylation of Cdk1 by Myt1 serves at least two functions during development. Phosphorylation of Y15 is essential for the pre-mitotic checkpoint mechanism, whereas T14 phosphorylation facilitates accumulation of dually inhibited Cdk1-Cyclin B complexes that can be rapidly activated once checkpoint-arrested G2 phase cells are ready for mitosis.

Ciurciu, A., Duncalf, L., Jonchere, V., Lansdale, N., Vasieva, O., Glenday, P., Rudenko, A., Vissi, E., Cobbe, N., Alphey, L. and Bennett, D. (2013). PNUTS/PP1 Regulates RNAPII-Mediated Gene Expression and Is Necessary for Developmental Growth. PLoS Genet 9: e1003885. PubMed ID: 24204300
Summary: In multicellular organisms, tight regulation of gene expression ensures appropriate tissue and organismal growth throughout development. Reversible phosphorylation of the RNA Polymerase II (RNAPII) C-terminal domain (CTD) is critical for the regulation of gene expression states, but how phosphorylation is actively modified in a developmental context remains poorly understood. Protein phosphatase 1 (PP1) is one of several enzymes that has been reported to dephosphorylate the RNAPII CTD. However, PP1's contribution to transcriptional regulation during animal development and the mechanisms by which its activity is targeted to RNAPII have not been fully elucidated. This study shows that the Drosophila orthologue of the PP1 Nuclear Targeting Subunit (PNUTS) is essential for organismal development and is cell autonomously required for growth of developing tissues. The function of dPNUTS in tissue development depends on its binding to PP1 (see Flap wing), which is targeted by dPNUTS to RNAPII at many active sites of transcription on chromosomes. Loss of dPNUTS function or specific disruption of its ability to bind PP1 results in hyperphosphorylation of the RNAPII CTD in whole animal extracts and on chromosomes. Consistent with dPNUTS being a global transcriptional regulator, it was found that loss of dPNUTS function affects the expression of the majority of genes in developing 1st instar larvae, including those that promote proliferative growth. Together, these findings shed light on the in vivo role of the PNUTS-PP1 holoenzyme and its contribution to the control of gene expression during early Drosophila development.

Saturday, November 23rd

Ellison, C. E. and Bachtrog, D. (2013). . Dosage compensation via transposable element mediated rewiring of a regulatory network. Science 342: 846-850. PubMed ID: 24233721
Summary: Transposable elements (TEs) may contribute to evolutionary innovations through the rewiring of networks by supplying ready-to-use cis regulatory elements. Genes on the Drosophila X chromosome are coordinately regulated by the male specific lethal (MSL) complex (see Male-specific lethal 2) to achieve dosage compensation in males. This study shows that the acquisition of dozens of MSL binding sites on evolutionarily new X chromosomes, in Drosophila miranda, was facilitated by the independent co-option of a mutant helitron TE that attracts the MSL complex (TE domestication). The recently formed neo-X recruits helitrons that provide dozens of functional, but suboptimal, MSL binding sites, whereas the older XR chromosome has ceased acquisition and appears to have fine-tuned the binding affinities of more ancient elements for the MSL complex. Thus, TE-mediated rewiring of regulatory networks through domestication and amplification may be followed by fine-tuning of the cis-regulatory element supplied by the TE and erosion of nonfunctional regions.

Sun, L. O., Jiang, Z., Rivlin-Etzion, M., Hand, R., Brady, C. M., Matsuoka, R. L., Yau, K. W., Feller, M. B. and Kolodkin, A. L. (2013). On and off retinal circuit assembly by divergent molecular mechanisms. Science 342: 1241974. Abstract
Summary: Direction-selective responses to motion can be to the onset (On) or cessation (Off) of illumination. This study shows that the transmembrane protein semaphorin 6A (see Drosophila Semaphorin-2) and its receptor plexin A2 (see Drosophila Plexin A) are critical for achieving radially symmetric arborization of On starburst amacrine cell (SAC) dendrites and normal SAC stratification in the mouse retina. Plexin A2 is expressed in both On and Off SACs; however, semaphorin 6A is expressed in On SACs. Specific On-Off bistratified direction-selective ganglion cells in semaphorin 6A-/- mutants exhibit decreased tuning of On directional motion responses. These results correlate the elaboration of symmetric SAC dendritic morphology and asymmetric responses to motion, shedding light on the development of visual pathways that use the same cell types for divergent outputs.

Friday, November 22nd

Owusu-Ansah, E., Song, W. and Perrimon, N. (2013). Muscle Mitohormesis Promotes Longevity via Systemic Repression of Insulin Signaling. Cell 155: 699-712.
Summary: Mitochondrial dysfunction is usually associated with aging. To systematically characterize the compensatory stress signaling cascades triggered in response to muscle mitochondrial perturbation, this study analyzed a Drosophila model of muscle mitochondrial injury. Mild muscle mitochondrial distress was found to preserve mitochondrial function, impede the age-dependent deterioration of muscle function and architecture, and prolong lifespan. Strikingly, this effect is mediated by at least two prolongevity compensatory signaling modules: one involving a muscle-restricted redox-dependent induction of genes that regulate the mitochondrial unfolded protein response (UPRmt), e.g., Heat shock protein cognate 5, and another involving the transcriptional induction of the Drosophila ortholog of insulin-like growth factor-binding protein 7 (Ecdysone-inducible gene L2), which systemically antagonizes insulin signaling and facilitates mitophagy. Given that several secreted IGF-binding proteins (IGFBPs) exist in mammals, this work raises the possibility that muscle mitochondrial injury in humans may similarly result in the secretion of IGFBPs, with important ramifications for diseases associated with aberrant insulin signaling.

Kasahara, A., Cipolat, S., Chen, Y., Dorn, G. W., 2nd and Scorrano, L. (2013). Mitochondrial fusion directs cardiomyocyte differentiation via calcineurin and Notch signaling. Science 342: 734-737. PubMed ID: 24091702
Summary: Mitochondrial morphology is crucial for tissue homeostasis, but its role in cell differentiation is unclear. This study found that mitochondrial fusion is required for proper cardiomyocyte development. Ablation of mitochondrial fusion proteins Mitofusin 1 and 2 in the embryonic mouse heart, or gene-trapping of Mitofusin 2 or Optic atrophy 1 in mouse embryonic stem cells (ESCs), arrested mouse heart development and impaired differentiation of ESCs into cardiomyocytes. Gene expression profiling revealed decreased levels of transforming growth factor-beta/bone morphogenetic protein and of transcription factors serum response factor, GATA4, and myocyte enhancer factor 2 linked to increased Ca(2+)-dependent calcineurin activity and Notch1 signaling that impaired ESC differentiation. Orchestration of cardiomyocyte differentiation by mitochondrial morphology reveals how mitochondria, Ca(2+), and calcineurin interact to regulate Notch1 signaling.

Thursday, November 21st

Wapinski, O. L., et al. (2013). Hierarchical mechanisms for direct reprogramming of fibroblasts to neurons. Cell 155: 621-635. PubMed ID: 24243019
Summary: Direct lineage reprogramming is a promising approach for human disease modeling and regenerative medicine, with poorly understood mechanisms. This study revealed a hierarchical mechanism in the direct conversion of fibroblasts into induced neuronal (iN) cells mediated by the transcription factors Ascl1 (see Drosophila Achaete), Brn2 (see Drosophila Vvl/Drifter), and Myt1l. Ascl1 acts as an 'on-target' pioneer factor by immediately occupying most cognate genomic sites in fibroblasts. In contrast, Brn2 and Myt1l do not access fibroblast chromatin productively on their own; instead, Ascl1 recruits Brn2 to Ascl1 sites genome wide. A unique trivalent chromatin signature in the host cells predicts the permissiveness for Ascl1 pioneering activity among different cell types. Finally, this study identified Zfp238 as a key Ascl1 target gene that can partially substitute for Ascl1 during iN cell reprogramming. Thus, a precise match between pioneer factors and the chromatin context at key target genes is determinative for transdifferentiation to neurons and likely other cell types
Ramos, A. I. and Barolo, S. (2013). Low-affinity transcription factor binding sites shape morphogen responses and enhancer evolution. Philos Trans R Soc Lond B Biol Sci 368: 20130018. PubMed ID: 24218631
Summary: In the era of functional genomics, the role of transcription factor (TF)-DNA binding affinity is of increasing interest: for example, it has recently been proposed that low-affinity genomic binding events, though frequent, are functionally irrelevant. This study investigated the role of binding site affinity in the transcriptional interpretation of Hedgehog (Hh) morphogen gradients. It is noted that enhancers of several Hh-responsive Drosophila genes have low predicted affinity for Cubitus interruptus (Ci), the Gli family TF that transduces Hh signalling in the fly. Contrary to an initial hypothesis, improving the affinity of Ci/Gli sites in enhancers of dpp, wingless and stripe, by transplanting optimal sites from the patched gene, did not result in ectopic responses to Hh signalling. Instead, it was found that these enhancers require low-affinity binding sites for normal activation in regions of relatively low signalling. When Ci/Gli sites in these enhancers were altered to improve their binding affinity, patterning defects were observed in the transcriptional response that are consistent with a switch from Ci-mediated activation to Ci-mediated repression. Synthetic transgenic reporters containing isolated Ci/Gli sites confirmed this finding in imaginal discs. It is proposed that the requirement for gene activation by Ci in the regions of low-to-moderate Hh signalling results in evolutionary pressure favouring weak binding sites in enhancers of certain Hh target genes.

Wednesday, November 20th

Krstic, D., Boll, W. and Noll, M. (2013). Influence of the white locus on the courtship behavior of Drosophila males. PLoS One 8: e77904. PubMed ID: 24205022
Summary: Since its discovery by Morgan, the Drosophila white gene has become one of the most intensely studied genes and has been widely used as a genetic marker. Earlier reports that over- and misexpression of White protein in Drosophila males leads to male-male courtship implicated white in courtship control. While previous studies suggested that it is the mislocalization of White protein within cells that causes the courtship phenotype, this study has demonstrated that also the lack of extra-retinal White can cause very similar behavioral changes. Moreover, evidence is provided that the lack of White function increases the sexual arousal of males in general, of which the enhanced male-male courtship might be an indirect effect. It was further shown that white mutant flies are not only optomotor blind but also dazzled by the over-flow of light in daylight. Implications of these findings for the proper interpretation of behavioral studies with white mutant flies are discussed.

Crickmore, M. A. and Vosshall, L. B. (2013). Opposing dopaminergic and GABAergic neurons control the duration and persistence of copulation in Drosophila. Cell 155: 881-893. PubMed ID: 24209625
Summary: Behavioral persistence is a major factor in determining when and under which circumstances animals will terminate their current activity and transition into more profitable, appropriate, or urgent behavior. This study shows that, for the first 5 min of copulation in Drosophila, stressful stimuli do not interrupt mating, whereas 10 min later, even minor perturbations are sufficient to terminate copulation. This decline in persistence occurs as the probability of successful mating increases and is promoted by approximately eight sexually dimorphic, GABAergic interneurons of the male abdominal ganglion. When these interneurons were silenced, persistence increased and males copulated far longer than required for successful mating. When these interneurons were stimulated, persistence decreased and copulations were shortened. In contrast, dopaminergic neurons of the ventral nerve cord promote copulation persistence and extend copulation duration. Thus, copulation duration in Drosophila is a product of gradually declining persistence controlled by opposing neuronal populations using conserved neurotransmission systems.

Tuesday, November 19th

Lin, C. and Katanaev, V. L. (2013). Kermit interacts with galphao, vang, and motor proteins in Drosophila planar cell polarity. PLoS One 8: e76885. PubMed ID: 24204696
Summary: In addition to the ubiquitous apical-basal polarity, epithelial cells are often polarized within the plane of the tissue - the phenomenon known as planar cell polarity (PCP). In Drosophila, manifestations of PCP are visible in the eye, wing, and cuticle. Several components of the PCP signaling have been characterized in flies and vertebrates, including the heterotrimeric Go protein. However, Go signaling partners in PCP remain largely unknown. Using a genetic screen Kermit, previously implicated in G protein and PCP signaling, was unvcovered as a novel binding partner of Go. Through pull-down and genetic interaction studies, it was found that Kermit interacts with Go and another PCP component Vang, known to undergo intracellular relocalization during PCP establishment. It was further demonstrated that the activity of Kermit in PCP differentially relies on the motor proteins: the microtubule-based dynein and kinesin motors and the actin-based myosin VI. The results place Kermit as a potential transducer of Go, linking Vang with motor proteins for its delivery to dedicated cellular compartments during PCP establishment.

Fanucchi, S., Shibayama, Y., Burd, S., Weinberg, M. S. and Mhlanga, M. M. (2013). Chromosomal Contact Permits Transcription between Coregulated Genes. Cell 155: 606-620
Summary: Transcription of coregulated genes occurs in the context of long-range chromosomal contacts that form multigene complexes. Such contacts and transcription are lost in knockout studies of transcription factors and structural chromatin proteins. To ask whether chromosomal contacts are required for cotranscription in multigene complexes, a strategy was devised using TALENs to cleave and disrupt gene loops in a well-characterized multigene complex. Monitoring this disruption using RNA FISH and immunofluorescence microscopy revealed that perturbing the site of contact had a direct effect on transcription of other interacting genes. Unexpectedly, this effect on cotranscription was hierarchical, with dominant and subordinate members of the multigene complex engaged in both intra- and interchromosomal contact. This observation reveals the profound influence of these chromosomal contacts on the transcription of coregulated genes in a multigene complex.

Monday, November 18th

Evans, D. S. and Cline, T. W. (2013). Drosophila switch gene Sex-lethal can bypass its switch-gene target transformer to regulate aspects of female behavior. Proc Natl Acad Sci U S A. PubMed ID: 24191002
Summary: The switch gene Sex-lethal (Sxl) was thought to elicit all aspects of Drosophila female somatic differentiation other than size dimorphism by controlling only the switch gene transformer (tra). This study shows instead that Sxl controls an aspect of female sexual behavior by acting on a target other than or in addition to tra. The existence of this unknown Sxl target was inferred from the observation that a constitutively feminizing tra transgene that restores fertility to tra- females failed to restore fertility to Sxl-mutant females that were adult viable but functionally tra-. The sterility of these mutant females was caused by an ovulation failure. Because tra expression is not sufficient to render these Sxl-mutant females fertile, this pathway is referred to as the tra-insufficient feminization (TIF) branch of the sex-determination regulatory pathway. Using a transgene that conditionally expresses two Sxl feminizing isoforms, it was found that the TIF branch is required developmentally for neurons that also sex-specifically express fruitless, a tra gene target controlling sexual behavior. Thus, in a subset of fruitless neurons, targets of the TIF and tra pathways appear to collaborate to control ovulation. In most insects, Sxl has no sex-specific functions, and tra, rather than Sxl, is both the target of the primary sex signal and the gene that maintains the female developmental commitment via positive autoregulation. The TIF pathway may represent an ancestral female-specific function acquired by Sxl in an early evolutionary step toward its becoming the regulator of tra in Drosophila.

Placais, P. Y., Trannoy, S., Friedrich, A. B., Tanimoto, H. and Preat, T. (2013). Two Pairs of Mushroom Body Efferent Neurons Are Required for Appetitive Long-Term Memory Retrieval in Drosophila. Cell Rep. PubMed ID: 24209748
Summary: One of the challenges facing memory research is to combine network- and cellular-level descriptions of memory encoding. In this context, Drosophila offers the opportunity to decipher, down to single-cell resolution, memory-relevant circuits in connection with the mushroom bodies (MBs), prominent structures for olfactory learning and memory. Although the MB-afferent circuits involved in appetitive learning were recently described, the circuits underlying appetitive memory retrieval remain unknown. This study has identified two pairs of cholinergic neurons efferent from the MB alpha vertical lobes, named MB-V3, that are necessary for the retrieval of appetitive long-term memory (LTM). Furthermore, LTM retrieval was correlated to an enhanced response to the rewarded odor in these neurons. Strikingly, though, silencing the MB-V3 neurons did not affect short-term memory (STM) retrieval. This finding supports a scheme of parallel appetitive STM and LTM processing.

Sunday, November 17th

Berke, B., Wittnam, J., McNeill, E., Van Vactor, D. L. and Keshishian, H. (2013). Retrograde BMP Signaling at the Synapse: A Permissive Signal for Synapse Maturation and Activity-Dependent Plasticity. J Neurosci 33: 17937-17950. PubMed ID: 24198381
Summary: At the Drosophila neuromuscular junction (NMJ), the loss of retrograde, trans-synaptic BMP signaling causes motoneuron terminals to have fewer synaptic boutons, whereas increased neuronal activity results in a larger synapse with more boutons. This study shows that an early and transient BMP signal is necessary and sufficient for NMJ growth as well as for activity-dependent synaptic plasticity. This early critical period was revealed by the temporally controlled suppression of Mad, the SMAD1 transcriptional regulator. Similar results were found by genetic rescue tests involving the BMP4/5/6 ligand Glass bottom boat (Gbb) in muscle, and alternatively the type II BMP receptor Wishful Thinking (Wit) in the motoneuron. These observations support a model where the muscle signals back to the innervating motoneuron's nucleus to activate presynaptic programs necessary for synaptic growth and activity-dependent plasticity. Molecular genetic gain- and loss-of-function studies show that genes involved in NMJ growth and plasticity, including the adenylyl cyclase Rutabaga, the Ig-CAM Fasciclin II, the transcription factor AP-1 (Fos/Jun), and the adhesion protein Neurexin, all depend critically on the canonical BMP pathway for their effects. By contrast, elevated expression of Lar, a receptor protein tyrosine phosphatase found to be necessary for activity-dependent plasticity, rescued the phenotypes associated with the loss of Mad signaling. Synaptic structure and function develop using genetically separable, BMP-dependent mechanisms. Although synaptic growth depended on Lar and the early, transient BMP signal, the maturation of neurotransmitter release was independent of Lar and required later, ongoing BMP signaling.

Karpowicz, P., Zhang, Y., Hogenesch, J. B., Emery, P. and Perrimon, N. (2013). The circadian clock gates the intestinal stem cell regenerative state. Cell Rep 3: 996-1004. PubMed ID: 23583176
Summary: The intestine has evolved under constant environmental stresses, because an animal may ingest harmful pathogens or chemicals at any time during its lifespan. Following damage, intestinal stem cells (ISCs) regenerate the intestine by proliferating to replace dying cells. ISCs from diverse animals are remarkably similar, and the Wnt, Notch, and Hippo signaling pathways, important regulators of mammalian ISCs, are conserved from flies to humans. Unexpectedly, this study identified the transcription factor Period, a component of the circadian clock, as critical for regeneration, which itself follows a circadian rhythm. Hundreds of transcripts were found that are regulated by the clock during intestinal regeneration, including components of stress response and regeneration pathways. Disruption of clock components leads to arrhythmic ISC divisions, revealing their underappreciated role in the healing process.

Saturday, November 16th

Wan, P., Wang, D., Luo, J., Chu, D., Wang, H., Zhang, L. and Chen, J. (2013). Guidance receptor promotes the asymmetric distribution of exocyst and recycling endosome during collective cell migration. Development. PubMed ID: 24198275
Summary: During collective migration, guidance receptors signal downstream to result in a polarized distribution of molecules, including cytoskeletal regulators and guidance receptors themselves, in response to an extracellular gradient of chemotactic factors. However, the underlying mechanism of asymmetry generation in the context of the migration of a group of cells is not well understood. Using border cells in the Drosophila ovary as a model system for collective migration, this study found that the receptor tyrosine kinase (RTK) PDGF/VEGF receptor (PVR) is required for a polarized distribution of recycling endosome and exocyst in the leading cells of the border cell cluster. Interestingly, PVR signaled through the small GTPase Rac to positively affect the levels of Rab11-labeled recycling endosomes, probably in an F-actin-dependent manner. Conversely, the exocyst complex component Sec3 was required for the asymmetric localization of RTK activity and F-actin, similar to that previously reported for the function of Rab11. Together, these results suggested a positive-feedback loop in border cells, in which RTKs such as PVR act to induce a higher level of vesicle recycling and tethering activity in the leading cells, which in turn enables RTK activity to be distributed in a more polarized fashion at the front. Evidence is also provided that E-cadherin, the major adhesion molecule for border cell migration, is a specific cargo in the Rab11-labeled recycling endosomes and that Sec3 is required for the delivery of the E-cadherin-containing vesicles to the membrane.

Losick, V. P., Fox, D. T. and Spradling, A. C. (2013). Polyploidization and Cell Fusion Contribute to Wound Healing in the Adult Drosophila Epithelium. Curr. Biol. 23(22): 2224-32. PubMed ID: 24184101
Summary: Reestablishing epithelial integrity and biosynthetic capacity is critically important following tissue damage. The adult Drosophila abdominal epithelium provides an attractive new system to address how postmitotic diploid cells contribute to repair. Puncture wounds to the adult Drosophila epidermis close initially by forming a melanized scab. Epithelial cells near the wound site fuse to form a giant syncytium, which sends lamellae under the scab to re-epithelialize the damaged site. Other large cells arise more peripherally by initiating endocycles and becoming polyploid, or by cell fusion. Rac GTPase activity is needed for syncytium formation, while the Hippo signaling effector Yorkie modulates both polyploidization and cell fusion. Large cell formation is functionally important because when both polyploidization and fusion are blocked, wounds do not re-epithelialize. These observations indicate that cell mass lost upon wounding can be replaced by polyploidization instead of mitotic proliferation. It is proposed that large cells generated by polyploidization or cell fusion are essential because they are better able than diploid cells to mechanically stabilize wounds, especially those containing permanent acellular structures, such as scar tissue.

Friday, November 15th

Mani, S. R., Megosh, H. and Lin, H. (2013). PIWI proteins are essential for early Drosophila embryogenesis. Dev Biol. PubMed ID: 24184635
Summary: Piwi proteins, a subfamily of the Argonaute/Piwi protein family, have been implicated in transcriptional and posttranscriptional gene regulation and transposon silencing mediated by small non-coding RNAs, especially piRNAs. Although these proteins are known to be required for germline development, their somatic function remains elusive. This study examined the maternal function of all three PIWI proteins in Drosophila; Piwi, Aubergine (Aub) and Argonaute3 (Ago3) during early embryogenesis. In syncytial embryos, Piwi displays an embryonic stage-dependent localization pattern. Piwi is localized in the cytoplasm during mitotic cycles 1-10. Between cycles 11 and 14, Piwi remains in the cytoplasm during mitosis but moves into the somatic nucleus during interphase. Beyond cycle 14, it stays in the nucleus. Aub and Ago3 are diffusely cytoplasmic from cycle 1 to 14. Embryos maternally depleted of any one of the three PIWI proteins display severe mitotic defects, including abnormal chromosome and nuclear morphology, cell cycle arrest, asynchronous nuclear division and aberrant nuclear migration. Furthermore, all three PIWI proteins are required for the assembly of mitotic machinery and progression through mitosis. Embryos depleted of maternal PIWI proteins also exhibit chromatin organization abnormalities. These observations indicate that maternal Piwi, Aub and Ago3 play a critical role in the maintenance of chromatin structure and cell cycle progression during early embryogenesis, with compromised chromatin integrity as a possible cause of the observed mitotic defects. This study demonstrates the essential function of PIWI proteins in the first phase of somatic development.

Truong Quang, B. A., Mani, M., Markova, O., Lecuit, T. and Lenne, P. F. (2013). Principles of E-Cadherin supramolecular organization in vivo.. Curr Biol. PubMed ID: 24184100
Summary: E-cadherin plays a pivotal role in tissue morphogenesis by forming clusters that support intercellular adhesion and transmit tension. What controls E-cadherin mesoscopic organization in clusters is unclear. This study used 3D superresolution quantitative microscopy in Drosophila embryos to characterize the size distribution of E-cadherin nanometric clusters. The cluster size follows power-law distributions over three orders of magnitude with exponential decay at large cluster sizes (> 100 molecules). By exploring the predictions of a general theoretical framework including cluster fusion and fission events and recycling of E-cadherin, two distinct active mechanisms setting the cluster-size distribution were identified. Dynamin-dependent endocytosis targets large clusters only, thereby imposing a cutoff size. Moreover, interactions between E-cadherin clusters and actin filaments control the fission in a size-dependent manner. It is concluded that E-cadherin clustering depends on key cortical regulators, which provide tunable and local control over E-cadherin organization. The data provide the foundation for a quantitative understanding of how E-cadherin distribution affects adhesion and might regulate force transmission in vivo.

Thursday, November 14th

Ferrari, F., Plachetka, A., Alekseyenko, A. A., Jung, Y. L., Ozsolak, F., Kharchenko, P. V., Park, P. J. and Kuroda, M. I. (2013). 'Jump Start and Gain' Model for Dosage Compensation in Drosophila Based on Direct Sequencing of Nascent Transcripts. Cell Rep. PubMed ID: 24183666
Summary: Dosage compensation in Drosophila is mediated by the MSL complex (see Male-specific lethal 2), which increases male X-linked gene expression approximately 2-fold. The MSL complex preferentially binds the bodies of active genes on the male X, depositing H4K16ac with a 3' bias. Two models have been proposed for the influence of the MSL complex on transcription: one based on promoter recruitment of RNA polymerase II (Pol II), and a second featuring enhanced transcriptional elongation. This study utilize nascent RNA sequencing to document dosage compensation during transcriptional elongation. Also, comparison was made of X and autosomes from published data on paused and elongating polymerase in order to assess the role of Pol II recruitment. The results support a model for differentially regulated elongation, starting with release from 5' pausing and increasing through X-linked gene bodies. The results highlight facilitated transcriptional elongation as a key mechanism for the coordinated regulation of a diverse set of genes.

Shirangi, T. R., Stern, D. L. and Truman, J. W. (2013). Motor Control of Drosophila Courtship Song. Cell Rep. PubMed ID: 24183665
Summary: Many animals utilize acoustic signals-or songs-to attract mates. During courtship (see Genes involved in courtship behavior), Drosophila melanogaster males vibrate a wing to produce trains of pulses and extended tone, called pulse and sine song, respectively. Courtship songs in the genus Drosophila are exceedingly diverse, and different song features appear to have evolved independently of each other. How the nervous system allows such diversity to evolve is not understood. This study identified a wing muscle in D. melanogaster (hg1) that is uniquely male-enlarged. The hg1 motoneuron and the sexually dimorphic development of the hg1 muscle, whose dimorphic character is regulated by doublesex but not male specific fruitless, are required specifically for the sine component of the male song. In contrast, the motoneuron innervating a sexually monomorphic wing muscle, ps1, is required specifically for a feature of pulse song. Thus, individual wing motor pathways can control separate aspects of courtship song and may provide a 'modular' anatomical substrate for the evolution of diverse songs.

Wednesday, November 13th

Klebba, J. E., Buster, D. W., Nguyen, A. L., Swatkoski, S., Gucek, M., Rusan, N. M. and Rogers, G. C. (2013). Polo-like Kinase 4 autodestructs by generating Its Slimb-binding phosphodegron. Curr. Biol. PubMed ID: 24184097
Summary: Polo-like kinase 4 (Plk4) is a conserved master regulator of centriole assembly. Previous study has found that Drosophila Plk4 protein levels are actively suppressed during interphase. Degradation of interphase Plk4 prevents centriole overduplication and is mediated by the ubiquitin-ligase complex SCFSlimb/betaTrCP. Since Plk4 stability depends on its activity, the consequences were examined of inactivating Plk4 or perturbing its phosphorylation state within its Slimb-recognition motif (SRM). Plk3 was shown to be directly responsible for extensively autophosphorylating and for generating its Slimb-binding phosphodegron (the residues that direct the starting place of degradation). Phosphorylatable residues within this regulatory region were systematically mutated to determine their impact on Plk4 protein levels and centriole duplication when expressed in S2 cells. Notably, autophosphorylation of a single residue (Ser293) within the SRM is critical for Slimb binding and ubiquitination. These data also demonstrate that autophosphorylation of numerous residues flanking S293 collectively contribute to establishing a high-affinity binding site for SCFSlimb. Taken together, these findings suggest that Plk4 directly generates its own phosphodegron and can do so without the assistance of an additional kinase(s).

Yan, B., Memar, N., Gallinger, J. and Conradt, B. (2013). Coordination of Cell Proliferation and Cell Fate Determination by CES-1 Snail. PLoS Genet 9: e1003884. PubMed ID: 24204299
Summary: The coordination of cell proliferation and cell fate determination is critical during development but the mechanisms through which this is accomplished are unclear. This study presents evidence that the Snail-related transcription factor CES-1 of Caenorhabditis elegans coordinates these processes in a specific cell lineage. CES-1 can cause loss of cell polarity in the NSM neuroblast. By repressing the transcription of the BH3-only (Bcl-2 antagonist) gene egl-1, CES-1 can also suppress apoptosis in the daughters of the NSM neuroblasts. CES-1 also affects cell cycle progression in this lineage. Specifically, CES-1 was found to repress the transcription of the cdc-25.2 gene, which encodes a Cdc25-like phosphatase (see Drosophila String), thereby enhancing the block in NSM neuroblast division caused by the partial loss of cya-1, which encodes Cyclin A. These results indicate that CDC-25.2 and CYA-1 control specific cell divisions and that the over-expression of the ces-1 gene leads to incorrect regulation of this functional 'module'. Finally, evidence is provided that cochaperone dnj-11 MIDA1 not only regulates CES-1 activity in the context of cell polarity and apoptosis but also in the context of cell cycle progression. In mammals, the over-expression of Snail-related genes has been implicated in tumorigenesis. The current findings support the notion that the oncogenic potential of Snail-related transcription factors lies in their capability to, simultaneously, affect cell cycle progression, cell polarity and apoptosis and, hence, the coordination of cell proliferation and cell fate determination.

Tuesday, November 12th

Bilen, J., Atallah, J., Azanchi, R., Levine, J. D. and Riddiford, L. M. (2013). Regulation of onset of female mating and sex pheromone production by juvenile hormone in Drosophila melanogaster. Proc Natl Acad Sci U S A 110: 18321-18326. PubMed ID: 24145432
Summary: Juvenile hormone (JH; see Drosophila Ring gland) coordinates timing of female reproductive maturation in most insects. In Drosophila, JH plays roles in both mating and egg maturation. However, very little is known about the molecular pathways associated with mating. Behavioral analysis of females genetically lacking the corpora allata, the glands that produce JH, showed that they were courted less by males and mated later than control females. Application of the JH mimic, methoprene, to the allatectomized females just after eclosion rescued both the male courtship and the mating delay. Studies of the null mutants of the JH receptors, Methoprene tolerant (Met) and germ cell-expressed (gce), showed that lack of Met in Met27 females delayed the onset of mating, whereas lack of Gce had little effect. The Met27 females were shown to be more attractive but less behaviorally receptive to copulation attempts. The behavioral but not the attractiveness phenotype was rescued by the Met genomic transgene. Analysis of the female cuticular hydrocarbon profiles showed that corpora allata ablation caused a delay in production of the major female-specific sex pheromones (the 7,11-C27 and -C29 dienes) and a change in the cuticular hydrocarbon blend. In the Met27 null mutant, by 48 h, the major C27 diene was greatly increased relative to wild type. In contrast, the gce2.5k null mutant females were courted similarly to control females despite changes in certain cuticular hydrocarbons. These findings indicate that JH acts primarily via Met to modulate the timing of onset of female sex pheromone production and mating.

Nikonenko, I., Nikonenko, A., Mendez, P., Michurina, T. V., Enikolopov, G. and Muller, D. (2013). Nitric oxide mediates local activity-dependent excitatory synapse development. Proc Natl Acad Sci U S A 110: E4142-4151. PubMed ID: 24127602
Summary: Learning related paradigms play an important role in shaping the development and specificity of synaptic networks, notably by regulating mechanisms of spine growth and pruning. The molecular events underlying these synaptic rearrangements remain poorly understood. This study, carried out in mice, identify NO signaling as a key mediator of activity-dependent excitatory synapse development. Chronic blockade of NO production (see Drosophila Nos) interferes with the development of hippocampal and cortical excitatory spine synapses. The effect results from a selective loss of activity-mediated spine growth mechanisms and is associated with morphological and functional alterations of remaining synapses. These effects of NO are mediated by a cGMP cascade and can be reproduced or prevented by postsynaptic expression of vasodilator-stimulated phosphoprotein phospho-mimetic or phospho-resistant mutants. In vivo analyses show that absence of NO prevents the increase in excitatory synapse density induced by environmental enrichment and interferes with the formation of local clusters of excitatory synapses. It is concluded that NO plays an important role in regulating the development of excitatory synapses by promoting local activity-dependent spine-growth mechanisms.

Monday, November 11th

Ishimoto, H., Wang, Z., Rao, Y., Wu, C. F. and Kitamoto, T. (2013). Novel Role for Ecdysone in Drosophila Conditioned Behavior: Linking GPCR-Mediated Non-canonical Steroid Action to cAMP Signaling in the Adult Brain. PLoS Genet 9: e1003843. PubMed ID: 24130506
Summary: DopEcR, a G-protein coupled receptor for ecdysteroids, is involved in activity- and experience-dependent plasticity of the adult central nervous system. Remarkably, a courtship memory defect in rutabaga (Ca(2+)/calmodulin-responsive adenylate cyclase) mutants is rescued by DopEcR overexpression or acute 20E feeding, whereas a memory defect in dunce (cAMP-specific phosphodiestrase) mutants is counteracted when a loss-of-function DopEcR mutation is introduced. A memory defect caused by suppressing dopamine synthesis is also restored through enhanced DopEcR-mediated ecdysone signaling, and rescue and phenocopy experiments revealed that the mushroom body (MB)-a brain region central to learning and memory in Drosophila-is critical for the DopEcR-dependent processing of courtship memory. Consistent with this finding, acute 20E feeding induced a rapid, DopEcR-dependent increase in cAMP levels in the MB. The multidisciplinary approach demonstrates that DopEcR mediates the non-canonical actions of 20E and rapidly modulates adult conditioned behavior through cAMP signaling, which is universally important for neural plasticity. This study provides novel insights into non-genomic actions of steroids, and opens a new avenue for genetic investigation into an underappreciated mechanism critical to behavioral control by steroids.

Fujioka, M., Sun, G. and Jaynes, J. B. (2013). The Drosophila eve Insulator Homie Promotes eve Expression and Protects the Adjacent Gene from Repression by Polycomb Spreading. PLoS Genet 9: e1003883. PubMed ID: 24204298
Summary: The Drosophila even skipped (eve) gene has a Polycomb-group response element (PRE) at one end, flanked by an insulator, an arrangement also seen in other genes. This study show that this insulator has three major functions. It blocks the spreading of the eve Pc-silenced region, preventing repression of the adjacent gene, TER94. It prevents activation of TER94 by eve regulatory DNA. It also facilitates normal eve expression. When the insulator Homie is deleted in the context of a large transgene that mimics both eve and TER94 regulation, TER94 is repressed. This repression depends on the eve PRE. Ubiquitous TER94 expression is 'replaced' by expression in an eve pattern when Homie is deleted, and this effect is reversed when the PRE is also removed. Repression of TER94 is attributable to spreading of the eve Pc-silenced domain into the TER94 locus, accompanied by an increase in histone H3 trimethylation at lysine 27. Other PREs can functionally replace the eve PRE, and other insulators can block PRE-dependent repression in this context. The full activity of the eve promoter is also dependent on Homie, and other insulators can promote normal eve enhancer-promoter communication. These data suggest that this is not due to preventing promoter competition, but is likely the result of the insulator organizing a chromosomal conformation favorable to normal enhancer-promoter interactions. Thus, insulator activities in a native context include enhancer blocking and enhancer-promoter facilitation, as well as preventing the spread of repressive chromatin.

Sunday, November 10th

Rujano, M. A., Sanchez-Pulido, L., Pennetier, C., le Dez, G. and Basto, R. (2013). The microcephaly protein Asp regulates neuroepithelium morphogenesis by controlling the spatial distribution of myosin II. Nat Cell Biol 15: 1294-1306. PubMed ID: 24142104
Summary: Mutations in ASPM are the most frequent cause of microcephaly, a disorder characterized by reduced brain size at birth. ASPM is recognized as a major regulator of brain size, yet its role during neural development remains poorly understood. Moreover, the role of ASPM proteins in invertebrate brain morphogenesis has never been investigated. This study characterized the function of the Drosophila ASPM orthologue, abnormal spindle (Asp), and found that asp mutants present severe defects in brain size and neuroepithelium morphogenesis. Size reduction depends on the mitotic function of Asp, whereas regulation of tissue shape depends on an uncharacterized function. Asp interacts with myosin II regulating its polarized distribution along the apico-basal axis. In the absence of Asp, mislocalization of myosin II results in interkinetic nuclear migration and tissue architecture defects. It is proposed that Asp regulates neuroepithelium morphogenesis through myosin-II-mediated structural and mechanical processes to maintain force balance and tissue cohesiveness.

Ma, T., Wang, C., Wang, L., Zhou, X., Tian, M., Zhang, Q., Zhang, Y., Li, J., Liu, Z., Cai, Y., Liu, F., You, Y., Chen, C., Campbell, K., Song, H., Ma, L., Rubenstein, J. L. and Yang, Z. (2013). . Subcortical origins of human and monkey neocortical interneurons. Nat Neurosci 16: 1588-1597. PubMed ID: 24097041
Summary: Cortical GABAergic inhibitory interneurons have crucial roles in the development and function of the cerebral cortex. In rodents, nearly all neocortical interneurons are generated from the subcortical ganglionic eminences. In humans and nonhuman primates, however, the developmental origin of neocortical GABAergic interneurons remains unclear. This study shows that the expression patterns of several key transcription factors in the developing primate telencephalon are very similar to those in rodents, delineating the three main subcortical progenitor domains (the medial, lateral and caudal ganglionic eminences) and the interneurons tangentially migrating from them. On the basis of the continuity of Sox6, COUP-TFII (Drosophila homolog: Seven up) and Sp8 transcription factor expression and evidence from cell migration and cell fate analyses, it is proposed that the majority of primate neocortical GABAergic interneurons originate from ganglionic eminences of the ventral telencephalon. These findings reveal that the mammalian neocortex shares basic rules for interneuron development, substantially reshaping understanding of the origin and classification of primate neocortical interneurons.

Saturday November 9th

Lu, A. and Pfeffer, S. R. (2013). Golgi-associated RhoBTB3 targets Cyclin E for ubiquitylation and promotes cell cycle progression. J Cell Biol 203: 233-250. PubMed ID: 24145166
Summary: Cyclin E regulates the cell cycle transition from G1 to S phase and is degraded before entry into G2 phase. This study shows that in mice RhoBTB3, a Golgi-associated, Rho-related ATPase, regulates the S/G2 transition of the cell cycle by targeting Cyclin E for ubiquitylation. Depletion of RhoBTB3 arrested cells in S phase, triggered Golgi fragmentation, and elevated Cyclin E levels. On the Golgi, RhoBTB3 bound Cyclin E as part of a Cullin3 (CUL3)-dependent RING-E3 ubiquitin ligase complex comprised of RhoBTB3, CUL3 (see Drosophila Cullin-3), and RBX1. Golgi association of this complex is required for its ability to catalyze Cyclin E ubiquitylation and allow normal cell cycle progression. These experiments reveal a novel role for a Ras superfamily member in catalyzing Cyclin E turnover during S phase, as well as an unexpected, essential role for the Golgi as a ubiquitylation platform for cell cycle control.

Corsetti, E. and Azpiazu, N. (2013). Functional dissection of the splice variants of the Drosophila gene homothorax (hth). Dev Biol 384: 72-82. PubMed ID: 24075905
Summary: TALE-homeodomain family member Homothorax interacts with a second TALE-homeodomain protein Extradenticle to facilitate Exd entrance to the nucleus. The many different functions described for Hth rely on the complexity of the locus, from which six different isoforms arise. The isoforms can be grouped into full-length and short versions, which contain either one or the two conserved domains of the protein (homeodomain and Exd-interacting domain). This study used molecular and genetic tools to analyze the levels of expression, the distribution and the function of the isoforms during embryonic development. The results clearly show that the isoforms display distinct levels of expression and are differentially distributed in the embryo. This detailed study also shows that during normal embryonic development not all the Hth isoforms translocate Exd into the nucleus, suggesting that both the proteins can also function separately. The full-length Hth protein activates transcription of exd, augmenting the levels of exd mRNA in the cell. The higher levels of Exd protein in those cells facilitate its entrance to the nucleus. This work demonstrates that hth is a complex gene that should not be considered as a functional unit. The roles of the different isoforms probably rely on their distinct protein domains and conformations and, at the end, on interactions with particular partners.

Friday, November 8th

Yamben, I. F., Rachel, R. A., Shatadal, S., Copeland, N. G., Jenkins, N. A., Warming, S. and Griep, A. E. (2013). Scrib is required for epithelial cell identity and prevents epithelial to mesenchymal transition in the mouse. Dev Biol 384: 41-52. PubMed ID: 24095903
Summary: This study asked if the mouse homolog of Drosophila Scribbled is required for establishment and/or maintenance of epithelial identity in vivo. To do so, Scrib was conditionally deleted in the head ectoderm tissue that gives rise to both the ocular lens and the corneal epithelium. Deletion of Scrib in the lens resulted in a change in epithelial cell shape from cuboidal to flattened and elongated. Early in the process, the cell adhesion protein, E-cadherin (Drosophila homolog: Shotgun), and apical polarity protein, ZO-1 (Drosophila homolog: Discs large), were downregulated and the myofibroblast protein, αSMA, was upregulated, suggesting epithelial-mesenchymal transition (EMT) was occurring in the Scrib deficient lenses. Correlating temporally with the upregulation of alphaSMA, Smad3 and Smad4, TGFβ signaling intermediates, accumulated in the nucleus and Snail (see Drosophila Snail), a TGFβ target and transcriptional repressor of the gene encoding E-cadherin, was upregulated. Pax6, a lens epithelial transcription factor required to maintain lens epithelial cell identity also was downregulated. Loss of Scrib in the corneal epithelium also led to molecular changes consistent with EMT, suggesting that the effect of Scrib deficiency was not unique to the lens. Together, these data indicate that mammalian Scrib is required to maintain epithelial identity and that loss of Scrib can culminate in EMT, mediated, at least in part, through TGFβ signaling.

Svoboda, D. S., Paquin, A., Park, D. S. and Slack, R. S. (2013). Pocket proteins pRb and p107 are required for cortical lamination independent of apoptosis. Dev Biol 384: 101-113. PubMed ID: 24056077
Summary: Pocket proteins (pRb, p107 and p130, homologs of Drosophila Retinoblastoma-family protein) are well studied in their role of regulating cell cycle progression. Increasing evidence suggests that these proteins also control early differentiation and even later stages of cell maturation, such as migration. However, pocket proteins also regulate apoptosis, and many of the developmental defects in knock out models have been attributed to increased cell death. This study eliminated ectopic apoptosis in the developing brain through the deletion of Bax, and showed that pocket proteins are required for radial migration independent of their role in cell death regulation. Following loss of pRb and p107, a population of cortical neurons fails to pass through the intermediate zone into the cortical plate. Importantly, these neurons are born at the appropriate time and this migration defect cannot be rescued by eliminating ectopic cell death. In addition, it was shown that pRb and p107 regulate radial migration through a cell autonomous mechanism since pRb/p107 deficient neurons fail to migrate to the correct cortical layer within a wild type brain. These results define a novel role of pocket proteins in regulating cortical lamination through a cell autonomous mechanism independent of their role in apoptosis.

Thursday, November 7th

DONE Webber, J. L., Zhang, J., Mitchell-Dick, A. and Rebay, I. (2013). 3D chromatin interactions organize Yan chromatin occupancy and repression at the even-skipped locus. Genes Dev 27: 2293-2298. PubMed ID: 24186975
Summary: Long-range integration of transcriptional inputs is critical for gene expression, yet the mechanisms remain poorly understood. This study investigated the molecular determinants that confer fidelity to expression of the heart identity gene even-skipped (eve). Targeted deletion of regions bound by the repressor Yan defined two novel enhancers that contribute repressive inputs to stabilize tissue-specific output from a third enhancer. Deletion of any individual enhancer reduced Yan occupancy at the other elements, impacting eve expression, cell fate specification, and cardiac function. These long-range interactions may be stabilized by three-dimensional chromatin contacts that were detected between the elements. This work provides a new paradigm for chromatin-level integration of general repressive inputs with specific patterning information to achieve robust gene expression.
Alfieri, C., Gambetta, M. C., Matos, R., Glatt, S., Sehr, P., Fraterman, S., Wilm, M., Muller, J. and Muller, C. W. (2013). Structural basis for targeting the chromatin repressor Sfmbt to Polycomb response elements. Genes Dev 27: 2367-2379. PubMed ID: 24186981
Summary: Polycomb group (PcG) protein complexes repress developmental regulator genes by modifying their chromatin. How different PcG proteins assemble into complexes and are recruited to their target genes is poorly understood. This study reports the crystal structure of the core of the Drosophila PcG protein complex Pleiohomeotic (Pho)-repressive complex (PhoRC), which contains the Polycomb response element (PRE)-binding protein Pho and Scm-related gene containing four mbt domains (Sfmbt). The spacer region of Pho, separated from the DNA-binding domain by a long flexible linker, forms a tight complex with the four malignant brain tumor (4MBT) domain of Sfmbt. The highly conserved spacer region of the human Pho ortholog YY1 binds three of the four human 4MBT domain proteins in an analogous manner but with lower affinity. Structure-guided mutations that disrupt the interaction between Pho and Sfmbt abolish formation of a ternary Sfmbt:Pho:DNA complex in vitro and repression of developmental regulator genes in Drosophila. PRE tethering of Sfmbt by Pho is therefore essential for Polycomb repression in Drosophila. The results support a model where DNA tethering of Sfmbt by Pho and multivalent interactions of Sfmbt with histone modifications and other PcG proteins create a hub for PcG protein complex assembly at PREs.

Wednesday, November 6th

DONE Lopez, A. L., 3rd, Chen, J., Joo, H. J., Drake, M., Shidate, M., Kseib, C. and Arur, S. (2013). DAF-2 and ERK Couple Nutrient Availability to Meiotic Progression during Caenorhabditis elegans Oogenesis. Dev Cell 27: 227-240. PubMed ID: 24120884
Summary: Coupling the production of mature gametes and fertilized zygotes to favorable nutritional conditions improves reproductive success. In invertebrates, the proliferation of female germline stem cells is regulated by nutritional status. However, in mammals, the number of female germline stem cells is set early in development, with oocytes progressing through meiosis later in life. Mechanisms that couple later steps of oogenesis to environmental conditions remain largely undefined. This study shows that, in the presence of food, the DAF-2 insulin-like receptor (see Drosophila Insulin-like receptor) signals through the RAS-ERK pathway to drive meiotic prophase I progression and oogenesis; in the absence of food, the resultant inactivation of insulin-like signaling leads to downregulation of the RAS-ERK pathway, and oogenesis is stalled. Thus, the insulin-like signaling pathway couples nutrient sensing to meiotic I progression and oocyte production in C. elegans, ensuring that oocytes are only produced under conditions favorable for the survival of the resulting zygotes.

Xin, T., Xuan, T., Tan, J., Li, M., Zhao, G. and Li, M. (2013). The Drosophila putative histone acetyltransferase Enok maintains female germline stem cells through regulating Bruno and the niche. Dev Biol 384: 1-12. PubMed ID: 24120347
Summary: Maintenance of adult stem cells is largely dependent on the balance between their self-renewal and differentiation. The Drosophila ovarian germline stem cells (GSCs) provide a powerful in vivo system for studying stem cell fate regulation. This study finds that Enoki mushroom (Enok), a Drosophila putative MYST family histone acetyltransferase controls GSC maintenance in the ovary at multiple levels. Removal or knockdown of Enok in the germline causes a GSC maintenance defect. Further studies show that the cell-autonomous role of Enok in maintaining GSCs is not dependent on the BMP/Bam pathway. Interestingly, molecular studies reveal an ectopic expression of Bruno, an RNA binding protein, in the GSCs and their differentiating daughter cells elicited by the germline Enok deficiency. Misexpression of Bruno in GSCs and their immediate descendants results in a GSC loss that can be exacerbated by incorporating one copy of enok mutant allele. These data suggest a role for Bruno in Enok-controlled GSC maintenance. In addition, it was observed that Enok is required for maintaining GSCs non-autonomously. Compromised expression of enok in the niche cells impairs the niche maintenance and BMP signal output, thereby causing defective GSC maintenance. This is the first demonstration that the niche size control requires an epigenetic mechanism, in that involves a histone-modifying agent. Taken together, studies in this paper provide new insights into the GSC fate regulation.

Tuesday, November 5th

Dalton, R. P., Lyons, D. B. and Lomvardas, S. (2013). Co-opting the unfolded protein response to elicit olfactory receptor feedback. Cell 155: 321-332. PubMed ID: 24120133
Summary: Olfactory receptor (OR) expression in mammals requires the transcriptional activation of 1 out of 1,000s of OR alleles and a feedback signal that preserves this transcriptional choice. The mechanism by which olfactory sensory neurons (OSNs) detect ORs to signal to the nucleus remains elusive. This study shows that OR proteins generate this feedback by activating the unfolded protein response (UPR). OR expression induces Perk-mediated phosphorylation of the translation initiation factor eif2α causing selective translation of activating transcription factor 5 (ATF5, a member of the ATF/CREB family of transcription factors). ATF5 induces the transcription of adenylyl cyclase 3 (Adcy3; Drosophila homolog Rutabaga), which relieves the UPR. These data provide a role for the UPR in defining neuronal identity and cell fate commitment and support a two-step model for the feedback signal: (1) OR protein, as a stress stimulus, alters the translational landscape of the OSN and induces Adcy3 expression; (2), Adcy3 relieves that stress, restores global translation, and makes OR choice permanent.

Hiramatsu, R., Matsuoka, T., Kimura-Yoshida, C., Han, S. W., Mochida, K., Adachi, T., Takayama, S. and Matsuo, I. (2013). External mechanical cues trigger the establishment of the anterior-posterior axis in early mouse embryos. Dev Cell 27: 134. PubMed ID: 24176640
Summary: Mouse anterior-posterior axis polarization is preceded by formation of the distal visceral endoderm (DVE) by unknown mechanisms. This study shows by in vitro culturing of embryos immediately after implantation in microfabricated cavities that the external mechanical cues exerted on the embryo are crucial for DVE formation, as well as the elongated egg cylinder shape, without affecting embryo-intrinsic transcriptional programs except those involving DVE-specific genes. This implies that these developmental events immediately after implantation are not simply embryo-autonomous processes but require extrinsic factors from maternal tissues. Moreover, the mechanical forces induce a breach of the basement membrane barrier at the distal portion locally, and thereby the transmigrated epiblast cells emerge as the DVE cells. Thus, it is proposed that external mechanical forces exerted by the interaction between embryo and maternal uterine tissues directly control the location of DVE formation at the distal tip and consequently establish the mammalian primary body axis.

Monday, November 4th

Morante, J., Vallejo, D. M., Desplan, C. and Dominguez, M. (2013). Conserved miR-8/miR-200 Defines a Glial Niche that Controls Neuroepithelial Expansion and Neuroblast Transition. Dev Cell. PubMed ID: 24139822
Summary: Neuroepithelial cell proliferation must be carefully balanced with the transition to neuroblast (neural stem cell) to control neurogenesis. This study shows that loss of the Drosophila microRNA mir-8 (the homolog of vertebrate miR-200 family) results in both excess proliferation and ectopic neuroblast transition. Unexpectedly, mir-8 is expressed in a subpopulation of optic-lobe-associated cortex glia that extend processes that ensheath the neuroepithelium, suggesting that glia cells communicate with the neuroepithelium. Evidence is provided that miR-8-positive glia express Spitz, a transforming growth factor alpha (TGF-alpha)-like ligand that triggers epidermal growth factor receptor (EGFR) activation to promote neuroepithelial proliferation and neuroblast formation. Further, these experiments suggest that miR-8 ensures both a correct glial architecture and the spatiotemporal control of Spitz protein synthesis via direct binding to Spitz 3' UTR. Together, these results establish glial-derived cues as key regulatory elements in the control of neuroepithelial cell proliferation and the neuroblast transition.

Jepson, J., Sheldon, A., Shahidullah, M., Fei, H., Koh, K. and Levitan, I. B. (2013). Cell-specific fine-tuning of neuronal excitability by differential expression of modulator protein isoforms. J Neurosci 33: 16767-16777. PubMed ID: 24133277
Summary: Slob (Slowpoke-binding protein) modulates the Drosophila Slowpoke calcium-activated potassium channel. Slob deletion or RNAi knockdown decreases excitability of neurosecretory pars intercerebralis (PI) neurons in the adult Drosophila brain. In contrast, this study found that Slob deletion/knockdown enhances neurotransmitter release from motor neurons at the fly larval neuromuscular junction, suggesting an increase in excitability. Because two prominent Slob isoforms, SLOB57 and SLOB71, modulate Slowpoke channels in opposite directions in vitro, whether divergent expression patterns of these two isoforms might underlie the differential modulation of excitability in PI and motor neurons was investigated. Strikingly different modes of regulatory control by the slob57 and slob71 promoters were found. The slob71, but not slob57, promoter contains binding sites for the Hunchback and Mirror transcriptional repressors. Furthermore, several core promoter elements that are absent in the slob57 promoter coordinately drive robust expression of a luciferase vector by the slob71 promoter in vitro. In addition, the expression patterns of the slob57 and slob71 promoters was investigated in vivo and clear spatiotemporal differences were found in promoter activity. Slob57 is expressed prominently in adult PI neurons, whereas larval motor neurons exclusively express Slob71. In contrast, at the larval neuromuscular junction, Slob57 expression appears to be restricted mainly to a subset of glial cells. These results illustrate how the use of alternative transcriptional start sites within an ion channel modulator locus coupled with functionally relevant alternative splicing can be used to fine-tune neuronal excitability in a cell-specific manner.

Sunday, November 3rd

Scheunemann, L., Skroblin, P., Hundsrucker, C., Klussmann, E., Efetova, M. and Schwarzel, M. (2013). AKAPS Act in a Two-Step Mechanism of Memory Acquisition. J Neurosci 33: 17422-17428. PubMed ID: 24174675
Summary: Defining the molecular and neuronal basis of associative memories is based upon behavioral preparations that yield high performance due to selection of salient stimuli, strong reinforcement, and repeated conditioning trials. One of those preparations is the Drosophila aversive olfactory conditioning procedure where animals initiate multiple memory components after experience of a single cycle training procedure. This study explored the analysis of acquisition dynamics as a means to define memory components and revealed strong correlations between particular chronologies of shock impact and number experienced during the associative training situation and subsequent performance of conditioned avoidance. Analyzing acquisition dynamics in Drosophila memory mutants revealed that rutabaga (rut)-dependent cAMP signals couple in a divergent fashion for support of different memory components. In case of anesthesia-sensitive memory (ASM) this study identified a characteristic two-step mechanism that links rut-AC1 to A-kinase anchoring proteins (AKAP)-sequestered protein kinase A at the level of Kenyon cells, a recognized center of olfactory learning within the fly brain. It is proposed that integration of rut-derived cAMP signals at level of AKAPs might serve as counting register that accounts for the two-step mechanism of ASM acquisition.

Cameron, S., Chang, W. T., Chen, Y., Zhou, Y., Taran, S. and Rao, Y. (2013). Visual circuit assembly requires fine tuning of the novel Ig transmembrane protein Borderless. J Neurosci 33: 17413-17421. PubMed ID: 24174674
Summary: Establishment of synaptic connections in the neuropils of the developing nervous system requires the coordination of specific neurite-neurite interactions (i.e., axon-axon, dendrite-dendrite and axon-dendrite interactions). The molecular mechanisms underlying coordination of neurite-neurite interactions for circuit assembly are incompletely understood. In this report, a novel Ig superfamily transmembrane protein that was named Borderless (Bdl) was identified as a novel regulator of neurite-neurite interactions in Drosophila. Bdl induces homotypic cell-cell adhesion in vitro and mediates neurite-neurite interactions in the developing visual system. Bdl interacts physically and genetically with the Ig transmembrane protein Turtle, which is expressed in R7 axons and target neurons in the optic lobe and functions as a key regulator of axonal tiling. The results also show that the receptor tyrosine phosphatase leukocyte common antigen-related protein (LAR) negatively regulates Bdl to control synaptic-layer selection. It is proposed that precise regulation of Bdl action coordinates neurite-neurite interactions for circuit formation in Drosophila.

Saturday, November 2nd

Sia GM, Clem RL, Huganir RL. The Human Language-Associated Gene SRPX2 Regulates Synapse Formation and Vocalization in Mice. Science 342(6161): 987-91. PubMed ID: 24179158
Summary: Synapse formation in the developing brain depends on the coordinated activity of synaptogenic proteins, some which have been implicated in a number of neurodevelopmental disorders. This study shows that the sushi repeat-containing domain protein X-linked 2 (SRPX2; Drosophila homolog, Hikaru Genki) gene encodes a protein that promotes synaptogenesis in the cerebral cortex. In humans, SRPX2 is an epilepsy- and language-associated gene that is a target of the foxhead box protein P2 (FoxP2; Drosophila homolog, Forkhead) transcription factor. It was also shown that FoxP2 modulates synapse formation through regulating SRPX2 levels, and that SRPX2 reduction impairs development of ultrasonic vocalization in mice. The results suggest FoxP2 modulates the development of neural circuits through regulating synaptogenesis and that SRPX2 is a synaptogenic factor that plays a role in the pathogenesis of language disorders.

Sun, L. Y., et al. (2013). Growth hormone-releasing hormone disruption extends lifespan and regulates response to caloric restriction in mice. . Elife. e01098. PubMed ID: 24175087
Summary: This study examined the impact of targeted disruption of growth hormone-releasing hormone (GHRH) in mice on longevity and the putative mechanisms of delayed aging. GHRH knockout mice are remarkably long-lived, exhibiting major shifts in the expression of genes related to xenobiotic detoxification, stress resistance, and insulin signaling. These mutant mice also have increased adiponectin levels and alterations in glucose homeostasis consistent with the removal of the counter-insulin effects of growth hormone. While these effects overlap with those of caloric restriction, it was shown that the effects of caloric restriction (CR) and the GHRH mutation are additive, with lifespan of GHRH-KO mutants further increased by CR. It is concluded that GHRH-KO mice feature perturbations in a network of signaling pathways related to stress resistance, metabolic control and inflammation, and therefore provide a new model that can be used to explore links between GHRH repression, downregulation of the somatotropic axis, and extended longevity.

Friday, November 1st

Gruntman, E. and Turner, G. C. (2013). Integration of the olfactory code across dendritic claws of single mushroom body neurons.. Nat Neurosci. PubMed ID: 24141312
Summary: In the olfactory system, sensory inputs are arranged in different glomerular channels, which respond in combinatorial ensembles to the various chemical features of an odor. This study investigated where and how this combinatorial code is read out deeper in the brain. The unique morphology was exploited of neurons in the Drosophila mushroom body, which receive input on large dendritic claws. Imaging odor responses of these dendritic claws revealed that input channels with distinct odor tuning converge on individual mushroom body neurons. How these inputs interact to drive the cell to spike threshold was determined using intracellular recordings to examine mushroom body responses to optogenetically controlled input. The results provide an elegant explanation for the characteristic selectivity of mushroom body neurons: these cells receive different types of input and require those inputs to be coactive to spike. These results establish the mushroom body as an important site of integration in the fly olfactory system.

Baraban, M., Anselme, I., Schneider-Maunoury, S. and Giudicelli, F. (2013). Zebrafish embryonic neurons transport messenger RNA to axons and growth cones in vivo. J Neurosci 33: 15726-15734. PubMed ID: 24089481
Summary: Although mRNA was once thought to be excluded from the axonal compartment, the existence of protein synthesis in growing or regenerating axons in culture is now generally accepted. However, its extent and functional importance remain a subject of intense investigation. Furthermore, unambiguous evidence of mRNA axonal transport and local translation in vivo, in the context of a whole developing organism is still lacking. This study provides direct evidence of the presence of mRNAs of the tubb5, nefma, and stmnb2 genes in several types of axons in the developing zebrafish (Danio rerio) embryo, with frequent accumulation at the growth cone. Axonal localization of mRNA is a specific property of a subset of genes, as mRNAs of the huc and neurod genes, abundantly expressed in neurons, were not found in axons. Aa reporter system was set up in which the 3' untranslated region (UTR) of candidate mRNA, fused to a fluorescent protein coding sequence, was expressed in isolated neurons of the zebrafish embryo. Using this reporter, a motif was identified in the 3'UTR of tubb5 mRNA necessary and sufficient for axonal localization. This work thus establishes the zebrafish as a model system to study axonal transport in a whole developing vertebrate organism, provides an experimental frame to assay this transport in vivo and to study its mechanisms, and identifies a new zipcode involved in axonal mRNA localization.

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