Drosophila neuroblasts: a model for stem cell biology

Shaping epithelial tissues by stem cell mechanics in development and cancer

Adult stem cells balance self-renewal and differentiation to build, maintain and repair tissues. The role of signalling pathways and transcriptional networks in controlling stem cell function has been extensively studied, but there is increasing appreciation that mechanical forces also have a crucial regulatory role. Mechanical forces, signalling pathways and transcriptional networks must be coordinated across diverse length and timescales to maintain tissue homeostasis and function. Such coordination between stem cells and neighbouring cells dictates when cells divide, migrate and differentiate. Recent advances in measuring and manipulating the mechanical forces that act upon and are produced by stem cells are providing new insights into development and disease. In this Review, we discuss the mechanical forces involved when epithelial stem cells construct their microenvironment and what happens in cancer when stem cell niche mechanics are disrupted or dysregulated. As the skin has evolved to withstand the harsh mechanical pressures from the outside environment, we often use the stem cells of mammalian skin epithelium as a paradigm for adult stem cells shaping their surrounding tissues.

Arsenic impairs Drosophila neural stem cell mitotic progression and sleep behavior in a tauopathy model

Despite established exposure limits, arsenic remains the most significant environmental risk factor detrimental to human health and is associated with carcinogenesis and neurotoxicity. Arsenic compromises neurodevelopment, and it is associated with peripheral neuropathy in adults. Exposure to heavy metals, such as arsenic, may also increase the risk of neurodegenerative disorders. Nevertheless, the molecular mechanisms underlying arsenic-induced neurotoxicity remain poorly understood. Elucidating how arsenic contributes to neurotoxicity may mitigate some of the risks associated with chronic sublethal exposure and inform future interventions. In this study, we examine the effects of arsenic exposure on Drosophila larval neurodevelopment and adult neurologic function. Consistent with prior work, we identify significant developmental delays and heightened mortality in response to arsenic. Within the developing larval brain, we identify a dose-dependent increase in brain volume. This aberrant brain growth is coupled with impaired mitotic progression of the neural stem cells (NSCs), progenitors of the neurons and glia of the central nervous system. Live imaging of cycling NSCs reveals significant delays in cell cycle progression upon arsenic treatment, leading to genomic instability. In adults, chronic arsenic exposure reduces neurologic function, such as locomotion. Finally, we show arsenic selectively impairs circadian rhythms in a humanized tauopathy model. These findings inform mechanisms of arsenic neurotoxicity and reveal sex-specific and genetic vulnerabilities to sublethal exposure.

Differences in size and number of embryonic type II neuroblast lineages correlate with divergent timing of central complex development between beetle and fly

The insect brain and the timing of its development underwent evolutionary adaptations. However, little is known about the underlying developmental processes. The central complex of the brain is an excellent model to understand neural development and divergence. It is produced in large parts by type II neuroblasts, which produce intermediate progenitors, another type of cycling precursor, to increase their neural progeny. Type II neuroblasts lineages are believed to be conserved among insects, but little is known on their molecular characteristics in insects other than flies. Tribolium castaneum has emerged as a model for brain development and evolution. However, type II neuroblasts have so far not been studied in this beetle. We created a fluorescent enhancer trap marking expression of Tc-fez/earmuff, a key marker for intermediate progenitors. Using combinatorial labeling of further markers, including Tc-pointed, we characterized embryonic type II neuroblast lineages. Intriguingly, we found nine lineages per hemisphere in the Tribolium embryo while Drosophila produces only eight per brain hemisphere. These embryonic lineages are significantly larger in Tribolium than they are in Drosophila and contain more intermediate progenitors. Finally, we mapped these lineages to the domains of head patterning genes. Notably, Tc-otd is absent from all type II neuroblasts and intermediate progenitors, whereas Tc-six3 marks an anterior subset of the type II lineages. Tc-six4 specifically marks the territory where anterior-medial type II neuroblasts differentiate. In conclusion, we identified a conserved pattern of gene expression in holometabolan central complex forming type II neuroblast lineages, and conserved head patterning genes emerged as new candidates for conferring spatial identity to individual lineages. The higher number and greater lineage size of the embryonic type II neuroblasts in the beetle correlate with a previously described embryonic phase of central complex formation. These findings stipulate further research on the link between stem cell activity and temporal and structural differences in central complex development.

The Critical Balance Between Quiescence and Reactivation of Neural Stem Cells

Neural stem cells (NSC) are multipotent, self-renewing cells that give rise to all neural cell types within the central nervous system. During adulthood, most NSCs exist in a quiescent state which can be reactivated in response to metabolic and signalling changes, allowing for long-term continuous neurogenesis and response to injury. Ensuring a critical balance between quiescence and reactivation is required to maintain the limited NSC reservoir and neural replenishment throughout lifetime. The precise mechanisms and signalling pathways behind this balance are at the focus of current research. In this review, we highlight and discuss recent studies using Drosophila, mammalian and zebrafish models contributing to the understanding of molecular mechanisms underlying quiescence and reactivation of NSCs.

Asymmetry of centrosomes in Drosophila neural stem cells requires protein phosphatase 4

Asymmetric cell division is used by stem cells to create diverse cell types while self-renewing the stem cell population. Biased segregation of molecularly distinct centrosomes could provide a mechanism to maintain stem cell fate, induce cell differentiation or both. However, the molecular mechanisms generating molecular and functional asymmetric centrosomes remain incompletely understood. Here, we show that in asymmetrically dividing fly neural stem cells, protein phosphatase 4 (Pp4) is necessary for correct centrosome asymmetry establishment during mitosis, and microtubule organizing center (MTOC) maintenance in interphase. Using in vivo live-cell imaging, we show that while wild-type neural stem cells always maintain one active MTOC, Pp4 mutant neuroblasts contain two inactive centrioles in interphase. Furthermore, centrosomes of Pp4 mutant neural stem cells mature in mitosis but fail to correctly transfer the centriolar protein Centrobin (Cnb) from the mother to the daughter centriole. Using superresolution imaging, we find that phosphomimetic Centrobin fails to accurately relocalize in mitosis. We propose that Pp4 regulates the timely relocalization of Cnb in mitosis to establish two molecularly distinct centrosomes. In addition, Pp4 is also necessary to maintain MTOC activity in interphase, ensuring biased centrosome segregation. Mechanistically, Pp4 could regulate centrosome asymmetry by dephosphorylating both Cnb and gamma-Tubulin.

Developmental Genetic and Molecular Analysis of Drosophila Central Complex Lineages

Complex behaviors are mediated by a diverse class of neurons and glia produced during development. Both neural stem cell-intrinsic and -extrinsic temporal cues regulate the appropriate number, molecular identity, and circuit assembly of neurons. The Drosophila central complex (CX) is a higher-order brain structure regulating various behaviors, including sensory-motor integration, celestial navigation, and sleep. Most neurons and glia in the adult CX are formed during larval development by 16 Type II neural stem cells (NSCs). Unlike Type I NSCs, which directly give rise to the ganglion mother cells (GMCs), Type II NSCs give rise to multiple intermediate neural progenitors (INPs), and each INP in turn generates multiple GMCs, hence fostering the generation of longer and more diverse lineages. This makes Type II NSCs a suitable model to unravel the molecular mechanisms regulating neural diversity in more complex lineages. In this review, we elaborate on the classification and identification of NSCs based on the types of division adopted and the molecular markers expressed in each type. In the end, we discuss genetic methods for lineage analysis and birthdating. We also explain the temporal expression of stem cell factors and genetic techniques to study how stem cell factors may regulate neural fate specification.

In vivo AGO-APP identifies a module of microRNAs cooperatively preserving neural progenitors

MicroRNAs are essential regulators of gene expression. Their function is particularly important during neurogenesis, when the production of large numbers of neurons from a limited number of neural stem cells depends on the precise control of determination, proliferation and differentiation. However, microRNAs can target many mRNAs and vice-versa, raising the question of how specificity is achieved to elicit a precise regulatory response. Here we introduce in vivo AGO-APP, a novel approach to purify Argonaute-bound, and therefore active microRNAs from specific cell types. Using AGO-APP in the larval Drosophila central nervous system, we identify a module of microRNAs predicted to redundantly target all iconic genes known to control the transition from neuroblasts to neurons. While microRNA overexpression generally validated predictions, knockdown of individual microRNAs did not induce detectable phenotypes. In contrast, neuroblasts were induced to differentiate precociously when several microRNAs were knocked down simultaneously. Our data supports the concept that at physiological expression levels, the cooperative action of miRNAs allows efficient targeting of entire gene networks.

A space for time. Exploring temporal regulation of plant development across spatial scales

Plants continuously undergo change during their life cycle, experiencing dramatic phase transitions altering plant form, and regulating the assignment and progression of cell fates. The relative timing of developmental events is tightly controlled and involves integration of environmental, spatial, and relative age-related signals and actors. While plant phase transitions have been studied extensively and many of their regulators have been described, less is known about temporal regulation on a smaller, cell-level scale. Here, using examples from both plant and animal systems, we outline time-dependent changes. Looking at systemic scale changes, we discuss the timing of germination, juvenile-to-adult transition, flowering, and senescence, together with regeneration timing. Switching to temporal regulation on a cellular level, we discuss several instances from the animal field in which temporal control has been examined extensively at this scale. Then, we switch back to plants and summarize examples where plant cell-level changes are temporally regulated. As time cannot easily be separated from signaling derived from the environment and tissue context, we next discuss factors that have been implicated in controlling the timing of developmental events, reviewing temperature, photoperiod, nutrient availability, as well as tissue context and mechanical cues on the cellular scale. Afterwards, we provide an overview of mechanisms that have been shown or implicated in the temporal control of development, considering metabolism, division control, mobile signals, epigenetic regulation, and the action of transcription factors. Lastly, we look at remaining questions for the future study of developmental timing in plants and how recent technical advancement can enable these efforts.

Latest News from the "Guardian": p53 Directly Activates Asymmetric Stem Cell Division Regulators

Since its discovery in 1979, the human tumor suppressor gene TP53-also known as the "guardian of the genome"-has been the subject of intense research. Mutated in most human cancers, TP53 has traditionally been considered a key fighter against stress factors by trans-activating a network of target genes that promote cell cycle arrest, DNA repair, or apoptosis. Intriguingly, over the past years, novel non-canonical functions of p53 in unstressed cells have also emerged, including the mode of stem cell division regulation. However, the mechanisms by which p53 modulates these novel functions remain incompletely understood. In a recent work, we found that Drosophila p53 controls asymmetric stem cell division (ASCD) in neural stem cells by transcriptionally activating core ASCD regulators, such as the conserved cell-fate determinants Numb and Brat (NUMB and TRIM3/TRIM2/TRIM32 in humans, respectively). In this short communication, we comment on this new finding, the mild phenotypes associated with Drosophila p53 mutants in this context, as well as novel avenues for future research.

The TRIM-NHL RNA-binding protein MEI-P26 modulates the size of Drosophila Type I neuroblast lineages

The Drosophila TRIM-NHL RNA-binding protein (RBP), MEI-P26, has previously been shown to suppress tumor formation in the germline. Here we show that, in the Drosophila larval central brain, cell-type-specific expression of MEI-P26 plays a vital role in regulating neural development. MEI-P26 and another TRIM-NHL RBP, Brain tumor (BRAT), have distinct expression patterns in Type I neuroblast (NB) lineages: While both proteins are expressed in NBs, BRAT is expressed in ganglion mother cells (GMCs) but not neurons, whereas MEI-P26 is expressed in neurons but not GMCs. Knockdown of MEI-P26 leads to re-expression of the stem cell marker Deadpan (DPN) and over-production of neurons. In contrast, ectopically expressed MEI-P26 reduces NB lineage size by repressing division of GMCs, resulting in reduced neuron production. We show that MEI-P26 positively regulates expression of Prospero (PROS), a transcription factor that is known to repress cell cycle-related genes. Ectopic expression of PROS phenocopies ectopic expression of MEI-P26. In both cases, Cyclin B (CYCB) expression is downregulated. Importantly, knockdown of PROS in the context of ectopic MEI-P26 rescues the neural lineage. Based on these results, we conclude that MEI-P26 functions to prevent over-production of neurons by promoting production of PROS which, in turn, downregulates cell division.

Embryonic BPF exposure induces neurodevelopmental and neurobehavioral toxicity by affecting neural stem cell proliferation in Drosophila

BPF is a ubiquitous environmental chemical that has been shown to affect neurodevelopmental toxicity from animals to humans. Whether BPF exposure affects neural stem cell proliferation and differentiation is unknown. Here, we utilized a method of permeabilization of Drosophila embryos to analyze the effects of exposure to 0.5 mM, 1 mM, and 2 mM BPF on the proliferation and differentiation of neural stem cells. Our results showed that BPF exposure reduced the number of neuroblasts and intermediate neural progenitors during the embryonic stage, which caused the neuron/glial cell ratio to be out of balance, with a decrease in the number of neurons and an increase in the number of glial cells. BPF exposure caused neurotoxicity by reducing the activities of the antioxidant enzymes CAT and SOD, the downregulation of the transcriptional levels of oxidative stress-related genes, which triggered oxidative damage. As a result, embryonic BPF exposure affected the development of the neuromuscular junctions (NMJs) by reducing the number of axon branches and synaptic buttons, decreasing the number of peristaltic contractions, and reducing larval locomotion. In conclusion, our results demonstrate that embryonic BPF exposure disrupts neural stem cell proliferation, causing neurodevelopmental toxicity and abnormal larval behavior.

Imp/IGF2BP and Syp/SYNCRIP temporal RNA interactomes uncover combinatorial networks of regulators of Drosophila brain development

Temporal patterning of neural progenitors is an evolutionarily conserved mechanism generating neural diversity. In Drosophila, postembryonic neurogenesis requires the RNA binding proteins (RBPs) Imp/IGF2BP and Syp/SYNCRIP. However, how they coachieve their function is not well understood. Here, we elucidate the in vivo temporal RNA interactome landscapes of Imp and Syp during larval brain development. Imp and Syp bind a highly overlapping set of conserved mRNAs encoding proteins involved in neurodevelopment. We identify transcripts differentially occupied by Imp/Syp over time, featuring a network of known and previously unknown candidate temporal regulators that are post-transcriptionally regulated by Imp/Syp. Furthermore, the physical and coevolutionary relationships between Imp and Syp binding sites reveal a combinatorial, rather than competitive, mode of molecular interplay. Our study establishes an in vivo framework for dissecting the temporal coregulation of RBP networks as well as providing a resource for understanding neural fate specification.

The Drosophila adult midgut progenitor cells arise from asymmetric divisions of neuroblast-like cells

The Drosophila adult midgut progenitor cells (AMPs) give rise to all cells in the adult midgut epithelium, including the intestinal stem cells (ISCs). While they share many characteristics with the ISCs, it remains unclear how they are generated in the early embryo. Here, we show that they arise from a population of endoderm cells, which exhibit multiple similarities with Drosophila neuroblasts. These cells, which we have termed endoblasts, are patterned by homothorax (Hth) and undergo asymmetric divisions using the same molecular machinery as neuroblasts. We also show that the conservation of this molecular machinery extends to the generation of the enteroendocrine lineages. Parallels have previously been drawn between the pupal ISCs and larval neuroblasts. Our results suggest that these commonalities exist from the earliest stages of specification of progenitor cells of the intestinal and nervous systems and may represent an ancestral pathway for multipotent progenitor cell specification.

The transmembrane protein Syndecan is required for stem cell survival and maintenance of their nuclear properties

Tissue maintenance is underpinned by resident stem cells whose activity is modulated by microenvironmental cues. Using Drosophila as a simple model to identify regulators of stem cell behaviour and survival in vivo, we have identified novel connections between the conserved transmembrane proteoglycan Syndecan, nuclear properties and stem cell function. In the Drosophila midgut, Syndecan depletion in intestinal stem cells results in their loss from the tissue, impairing tissue renewal. At the cellular level, Syndecan depletion alters cell and nuclear shape, and causes nuclear lamina invaginations and DNA damage. In a second tissue, the developing Drosophila brain, live imaging revealed that Syndecan depletion in neural stem cells results in nuclear envelope remodelling defects which arise upon cell division. Our findings reveal a new role for Syndecan in the maintenance of nuclear properties in diverse stem cell types.

Beyond the heat shock pathway: Heat stress responses in Drosophila development

Heat stress has broad effects on an organism and is an inevitable part of life. Embryos face a particular challenge when faced with heat stress - the intricate molecular processes that pattern the embryo can all be affected by heat, and the embryo lacks some of the strategies that adults can use to manage or avoid heat stress. We use Drosophila melanogaster as a model, as insects are capable of developing normally under a wide range of temperatures and are exposed to daily temperature swings as they develop. Research has focused on the heat shock pathway and the transcription of heat shock proteins as the main response to heat and heat damage. This review explores embryonic heat responses beyond the heat shock pathway. We examine the effects of heat from a biochemical standpoint, as well as highlighting other mechanisms of heat stress regulation, such as miRNA activity or other signaling pathways. We discuss how different elements of the heat stress response must be coordinated across the embryo to enable development under a wide range of temperatures. Studying heat stress in Drosophila melanogaster can be a powerful lens into how developmental systems ensure robustness to environmental factors.

Developmental and tissue-specific roles of mammalian centrosomes

Centrosomes are dominant microtubule organizing centers in animal cells with a pair of centrioles at their core. They template cilia during interphase and help organize the mitotic spindle for a more efficient cell division. Here, we review the roles of centrosomes in the early developing mouse and during organ formation. Mammalian cells respond to centrosome loss-of-function by activating the mitotic surveillance pathway, a timing mechanism that, when a defined mitotic duration is exceeded, leads to p53-dependent cell death in the descendants. Mouse embryos without centrioles are highly susceptible to this pathway and undergo embryonic arrest at mid-gestation. The complete loss of the centriolar core results in earlier and more severe phenotypes than that of other centrosomal proteins. Finally, different developing tissues possess varying thresholds and mount graded responses to the loss of centrioles that go beyond the germ layer of origin.

TBP bookmarks and preserves neural stem cell fate memory by orchestrating local chromatin architecture

Mitotic bookmarking has been posited as an important strategy for cells to faithfully propagate their fate memory through cell generations. However, the physiological significance and regulatory mechanisms of mitotic bookmarking in neural development remain unexplored. Here, we identified TATA-binding protein (TBP) as a crucial mitotic bookmarker for preserving the fate memory of Drosophila neural stem cells (NSCs). Phosphorylation by the super elongation complex (SEC) is important for TBP to retain as discrete foci at mitotic chromosomes of NSCs to effectively transmit their fate memory. TBP depletion leads to drastic NSC loss, whereas TBP overexpression enhances the ability of SEC to induce neural progenitor dedifferentiation and tumorigenesis. Importantly, TBP achieves its mitotic retention through recruiting the chromatin remodeler EP400, which in turn increases local chromatin accessibility via depositing H2A.Z. Thus, local chromatin remodeling ensures mitotic bookmarking, which may represent a general principle underlying the preservation of cell fate memory.

Protein phosphatase 4 is required for centrosome asymmetry in fly neural stem cells

Asymmetric cell division is used by stem cells to create diverse cell types while self-renewing the stem cell population. Biased segregation of molecularly distinct centrosomes could provide a mechanism to maintain stem cell fate, induce cell differentiation or both. However, the molecular mechanisms generating molecular and functional asymmetric centrosomes remain incompletely understood. Here, we show that in asymmetrically dividing fly neural stem cells, Protein phosphatase 4 (Pp4) is necessary for correct centrosome asymmetry establishment during mitosis, and microtubule organizing center (MTOC) maintenance in interphase. Using in-vivo live cell imaging we show that while wild type neural stem cells always maintain one active MTOC, Pp4 mutant neuroblasts contain two inactive centrioles in interphase. Furthermore, centrosomes of Pp4 mutant neural stem cells mature in mitosis but fail to correctly transfer the centriolar protein Centrobin (Cnb) from the mother to the daughter centriole. Using superresolution imaging, we find that phosphomimetic Centrobin fails to accurately relocalize in mitosis. We propose that Pp4 regulates the timely relocalization of Cnb in mitosis to establish two molecularly distinct centrosomes. In addition, Pp4 is also necessary to maintain MTOC activity in interphase, ensuring biased centrosome segregation. Mechanistically, Pp4 could regulate centrosome asymmetry by dephosphorylating both Cnb and gamma-Tubulin.

SIGNIFICANCE STATEMENT

Asymmetric centrosome segregation occurs in stem cells and has been linked with cell fate decisions. Protein phosphatase 4 (Pp4), a conserved Serine/Threonine phosphatase, regulates centrosome asymmetry in Drosophila neural stem cells by acting upon gamma tubulin and Centrobin. Pp4 regulates centrosome asymmetry establishment in mitosis and interphase, necessary for biased centrosome segregation.

A novel adipose loss-of-function mutant in Drosophila

To identify genes required for brain growth, we took an RNAi knockdown reverse genetic approach in Drosophila. One potential candidate isolated from this effort is the anti-lipogenic gene adipose (adp). Adp has an established role in the negative regulation of lipogenesis in the fat body of the fly and adipose tissue in mammals. While fat is key to proper development in general, adp has not been investigated during brain development. Here, we found that RNAi knockdown of adp in neuronal stem cells and neurons results in reduced brain lobe volume and sought to replicate this with a mutant fly. We generated a novel adp mutant that acts as a loss-of-function mutant based on buoyancy assay results. We found that despite a change in fat content in the body overall and a decrease in the number of larger (>5 µm) brain lipid droplets, there was no change in the brain lobe volume of mutant larvae. Overall, our work describes a novel adp mutant that can functionally replace the long-standing adp60 mutant and shows that the adp gene has no obvious involvement in brain growth.

BPAP induces autism-like behavior by affecting the expression of neurodevelopmental genes in Drosophila melanogaster

Bisphenol AP (BPAP), an environmental endocrine disruptor, may cause neurodevelopmental disorders affecting human health. Studies have shown that BPAP impacts hormone synthesis and metabolism, causes social behavior abnormalities, and induces anxiety-like behavioral impairments in mice. However, evidence for the neurobehavioral effects of BPAP is still lacking. Here, we examined the toxic effects of BPAP on neurodevelopment using a Drosophila model. We assessed the role of BPAP exposure in autism-like behavior and explored the underlying mechanisms. Our findings indicated that BPAP exposure reduced pupation and eclosion rates and delayed growth in Drosophila. Furthermore, BPAP exposure caused autism-like behaviors, characterized by increased grooming times and aberrant social interactions, along with abnormalities in locomotor activity, as well as learning and memory ability. Mechanistically, we found that BPAP decreases the number of neuroblasts (NBs) and mature intermediate neural progenitors (INPs) in the 3rd larval brain, impairing axon guidance in the mushroom body of the adult Drosophila brain. Additionally, our transcriptome analysis revealed that BPAP exposure alters the expression of neurodevelopment-related genes (Nplp3, sand, lush, and orco) and affects the estrogen signaling pathway (Hsp70Ab, Hsp70Bc, Hsp70Ba, and Hsp70Bb). These changes potentially explain the BPAP-induced autism-like behavior in Drosophila.

Neuronal Progenitors Suffer Genotoxic Stress in the Drosophila Clock Mutant per0

The physiological role and the molecular architecture of the circadian clock in fully developed organisms are well established. Yet, we have a limited understanding of the function of the clock during ontogenesis. We have used a null mutant (per0) of the clock gene period (per) in Drosophila melanogaster to ask whether PER may play a role during normal brain development. In third-instar larvae, we have observed that the absence of functional per results in increased genotoxic stress compared to wild-type controls. We have detected increased double-strand DNA breaks in the central nervous system and chromosome aberrations in dividing neuronal precursor cells. We have demonstrated that reactive oxygen species (ROS) are causal to the genotoxic effect and that expression of PER in glia is necessary and sufficient to suppress such a phenotype. Finally, we have shown that the absence of PER may result in less condensed chromatin, which contributes to DNA damage.

Drosophila p53 tumor suppressor directly activates conserved asymmetric stem cell division regulators

p53 is the most mutated tumor suppressor gene in human cancers. Besides p53 classical functions inducing cell-cycle arrest and apoptosis in stressed cells, additional p53 non-canonical roles in unstressed cells have emerged over the past years, including the mode of stem cell division regulation. However, the mechanisms by which p53 impacts on this process remain elusive. Here, we show that Drosophila p53 controls asymmetric stem cell division (ASCD), a key process in development, cancer and adult tissue homeostasis, by transcriptionally activating Numb, Brat, and Traf4 ASCD regulators. p53 knockout caused failures in their localization in dividing neural stem cells, as well as a significant decrease in their expression levels. Moreover, p53 directly bound numb, brat, and Traf4 regulatory regions. Remarkably, human and mice genes related to Drosophila brat (TRIM32) and Traf4 (TRAF4) were recently identified in a meta-analysis of transcriptomic and ChIP-seq datasets as predicted conserved p53 targets.

Stem cell-specific ecdysone signaling regulates the development of dorsal fan-shaped body neurons and sleep homeostasis

Complex behaviors arise from neural circuits that assemble from diverse cell types. Sleep is a conserved behavior essential for survival, yet little is known about how the nervous system generates neuron types of a sleep-wake circuit. Here, we focus on the specification of Drosophila 23E10-labeled dorsal fan-shaped body (dFB) long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex. We use lineage analysis and genetic birth dating to identify two bilateral type II neural stem cells (NSCs) that generate 23E10 dFB neurons. We show that adult 23E10 dFB neurons express ecdysone-induced protein 93 (E93) and that loss of ecdysone signaling or E93 in type II NSCs results in their misspecification. Finally, we show that E93 knockdown in type II NSCs impairs adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate the neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.

Loss of function in Drosophila transcription factor Dif delays brain development in larvae resulting in aging adult brain

Drosophila NF-κB transcription factor Dif has been well known for its function in innate immunity, and recent study also reveals its role in neuronal cells. However, the underlying mechanisms of Dif in the brain remain elusive. In this study, we aim to investigate the function of Dif in Drosophila brain development and how Dif regulates structure and plasticity of the brain to affect aging and behaviors. Based on the analysis of differentially expressed genes, we identified key genes associated with cell division, development and aging in the brain of Dif1 loss of function mutant. In Dif1 larvae, we found that the metamorphosis and brain development were delayed, and cell division was decreased. In Dif1 adults, the number of neuron cells was reduced in the brain, the lifespan and locomotor activity were decreased, protein markers associated with aging-related neurodegenerative diseases in the brain were altered in abundance or activity. Our results indicated that Dif plays a crucial role in brain plasticity and neurogenesis, dysfunction of Dif delays larval brain development and impacts proliferation of neuronal cells, resulting in aging adult brain by regulating expression of key genes in multiple signaling pathways involved in cell division, neurogenesis and aging.

Membrane oscillations driven by Arp2/3 constrict the intercellular bridge during neural stem cell divisions

After the first furrowing step of animal cell division, the nascent sibling cells remain connected by a thin intercellular bridge (ICB). In isolated cells nascent siblings migrate away from each other to generate tension and constrict the ICB, but less is known about how cells complete cytokinesis when constrained within tissues. We examined the ICBs formed by Drosophila larval brain neural stem cell (NSC) asymmetric divisions and find that they rely on constriction focused at the central midbody region rather than the flanking arms of isolated cell ICBs. Super-resolution, full volume imaging revealed unexpected oscillatory waves in plasma membrane sheets surrounding the ICB pore during its formation and constriction. We find that these membrane dynamics are driven by Arp2/3-dependent branched actin networks. Inhibition of Arp2/3 complex activity blocks membrane oscillations and prevents ICB formation and constriction. Our results identify a previously unrecognized role for localized membrane oscillations in ICB function when cells cannot generate tension through migration.

SUMOylation of Warts kinase promotes neural stem cell reactivation

A delicate balance between neural stem cell (NSC) quiescence and proliferation is important for adult neurogenesis and homeostasis. Small ubiquitin-related modifier (SUMO)-dependent post-translational modifications cause rapid and reversible changes in protein functions. However, the role of the SUMO pathway during NSC reactivation and brain development is not established. Here, we show that the key components of the SUMO pathway play an important role in NSC reactivation and brain development in Drosophila. Depletion of SUMO/Smt3 or SUMO conjugating enzyme Ubc9 results in notable defects in NSC reactivation and brain development, while their overexpression leads to premature NSC reactivation. Smt3 protein levels increase with NSC reactivation, which is promoted by the Ser/Thr kinase Akt. Warts/Lats, the core protein kinase of the Hippo pathway, can undergo SUMO- and Ubc9-dependent SUMOylation at Lys766. This modification attenuates Wts phosphorylation by Hippo, leading to the inhibition of the Hippo pathway, and consequently, initiation of NSC reactivation. Moreover, inhibiting Hippo pathway effectively restores the NSC reactivation defects induced by SUMO pathway inhibition. Overall, our study uncovered an important role for the SUMO-Hippo pathway during Drosophila NSC reactivation and brain development.

The FAcilitates Chromatin Transcription complex regulates the ratio of glycolysis to oxidative phosphorylation in neural stem cells

Defects in the FAcilitates Chromatin Transcription (FACT) complex, a histone chaperone composed of SSRP1 and SUPT16H, are implicated in intellectual disability. Here, we reveal that the FACT complex promotes glycolysis and sustains the correct cell fate of neural stem cells/neuroblasts in the Drosophila 3rd instar larval central brain. We show that the FACT complex binds to the promoter region of the estrogen-related receptor (ERR) gene and positively regulates ERR expression. ERR is known to act as an aerobic glycolytic switch by upregulating the enzymes required for glycolysis. Dysfunction of the FACT complex leads to the downregulation of ERR transcription, resulting in a decreased ratio of glycolysis to oxidative phosphorylation (G/O) in neuroblasts. Consequently, neuroblasts exhibit smaller cell sizes, lower proliferation potential, and altered cell fates. Overexpression of ERR or suppression of mitochondrial oxidative phosphorylation in neuroblasts increases the relative G/O ratio and rescues defective phenotypes caused by dysfunction of the FACT complex. Thus, the G/O ratio, mediated by the FACT complex, plays a crucial role in neuroblast cell fate maintenance. Our study may shed light on the mechanism by which mutations in the FACT complex lead to intellectual disability in humans.

Arsenic impairs Drosophila neural stem cell mitotic progression and sleep behavior in a tauopathy model

Despite established exposure limits, arsenic remains the most significant environmental risk factor detrimental to human health and is associated with carcinogenesis and neurotoxicity. Arsenic compromises neurodevelopment, and it is associated with peripheral neuropathy in adults. Exposure to heavy metals, such as arsenic, may also increase the risk of neurodegenerative disorders. Nevertheless, the molecular mechanisms underlying arsenic-induced neurotoxicity remain poorly understood. Elucidating how arsenic contributes to neurotoxicity may mitigate some of the risks associated with chronic sublethal exposure and inform future interventions. In this study, we examine the effects of arsenic exposure on Drosophila larval neurodevelopment and adult neurologic function. Consistent with prior work, we identify significant developmental delays and heightened mortality in response to arsenic. Within the developing larval brain, we identify a dose-dependent increase in brain volume. This aberrant brain growth is coupled with impaired mitotic progression of the neural stem cells (NSCs), progenitors of the neurons and glia of the central nervous system. Live imaging of cycling NSCs reveals significant delays in cell cycle progression upon arsenic treatment, leading to genomic instability. In adults, chronic arsenic exposure reduces neurologic function, such as locomotion. Finally, we show arsenic selectively impairs circadian rhythms in a humanized tauopathy model. These findings inform mechanisms of arsenic neurotoxicity and reveal sex-specific and genetic vulnerabilities to sublethal exposure.

Genotoxic effects of sub-lethal doses of nicotine and acetamiprid in neuroblasts of Drosophila melanogaster and Drosophila suzukii

Neonicotinoids form a class of insecticides that are chemically related to nicotine and are widely used in crop protection. They have adverse effects on the neuronal nicotinic acetylcholine receptors (nAChRs). One of the neonicotinoids approved for control of the invasive pest Drosophila suzukii is acetamiprid. Despite concerns regarding its genotoxicity and data indicating the presence of small amounts of this substance in fruits intended for consumption, effects of its low doses on nerve cells are yet to be investigated. To determine whether the neurotoxic effects are species-specific and vary depending on the insecticide present in diet, multigenerational cultures of Drosophila melanogaster and D. suzukii were prepared, in this study, in media supplemented with different concentrations (below the LC50) of acetamiprid and nicotine. Acetamiprid, analogous to nicotine, caused damage to the DNA of neuroblasts in both species, at sublethal concentrations, along with a decrease in mobility, which remained at a similar level over subsequent generations. D. suzukii was found to be more sensitive to nicotine and acetamiprid, due to which the genotoxic effects were stronger even at lower doses of toxins. The results collectively indicated that even low concentrations of acetamiprid affect the stem cells of developing fly brain, and that long-term response to the tested insecticides is species-specific.

The Biological Roles of microRNAs in Drosophila Development

Drosophila is a well-established insect model system for studying various physiological phenomena and developmental processes, with a focus on gene regulation. Drosophila development is controlled by programmed regulatory mechanisms specific to individual tissues. When key developmental processes are shared among various insects, the associated regulatory networks are believed to be conserved across insects. Thus, studies of developmental regulation in Drosophila have substantially contributed to our understanding of insect development. Over the past two decades, studies on microRNAs (miRNAs) in Drosophila have revealed their crucial regulatory roles in various developmental processes. This review focuses on the biological roles of miRNAs in specific tissues and processes associated with Drosophila development. Additionally, as a future direction, we discuss sequencing technologies that can analyze the interactions between miRNAs and their target genes, with the aim of enhancing miRNA studies in Drosophila development.

Multiple isoforms of the Activin-like receptor baboon differentially regulate proliferation and conversion behaviors of neuroblasts and neuroepithelial cells in the Drosophila larval brain

In Drosophila coordinated proliferation of two neural stem cells, neuroblasts (NB) and neuroepithelial (NE) cells, is pivotal for proper larval brain growth that ultimately determines the final size and performance of an adult brain. The larval brain growth displays two phases based on behaviors of NB and NEs: the first one in early larval stages, influenced by nutritional status and the second one in the last larval stage, promoted by ecdysone signaling after critical weight checkpoint. Mutations of the baboon (babo) gene that produces three isoforms (BaboA-C), all acting as type-I receptors of Activin-type transforming growth factor β (TGF-β) signaling, cause a small brain phenotype due to severely reduced proliferation of the neural stem cells. In this study we show that loss of babo function severely affects proliferation of NBs and NEs as well as conversion of NEs from both phases. By analyzing babo-null and newly generated isoform-specific mutants by CRISPR mutagenesis as well as isoform-specific RNAi knockdowns in a cell- and stage-specific manner, our data support differential contributions of the isoforms for these cellular events with BaboA playing the major role. Stage-specific expression of EcR-B1 in the brain is also regulated primarily by BaboA along with function of the other isoforms. Blocking EcR function in both neural stem cells results in a small brain phenotype that is more severe than baboA-knockdown alone. In summary, our study proposes that the Babo-mediated signaling promotes proper behaviors of the neural stem cells in both phases and achieves this by acting upstream of EcR-B1 expression in the second phase.

3D reconstruction of the mouse cochlea from scRNA-seq data suggests morphogen-based principles in apex-to-base specification

In the mammalian auditory system, frequency discrimination depends on numerous morphological and physiological properties of the organ of Corti, which gradually change along the apex-to-base (tonotopic) axis of the organ. For example, the basilar membrane stiffness changes tonotopically, thus affecting the tuning properties of individual hair cells. At the molecular level, those frequency-specific characteristics are mirrored by gene expression gradients; however, the molecular mechanisms controlling tonotopic gene expression in the mouse cochlea remain elusive. Through analyzing single-cell RNA sequencing (scRNA-seq) data from E12.5 and E14.5 time points, we predicted that morphogens, rather than a cell division-associated mechanism, confer spatial identity in the extending cochlea. Subsequently, we reconstructed the developing cochlea in 3D space from scRNA-seq data to investigate the molecular pathways mediating positional information. The retinoic acid (RA) and hedgehog pathways were found to form opposing apex-to-base gradients, and functional interrogation using mouse cochlear explants suggested that both pathways jointly specify the longitudinal axis.

Atypical Protein Kinase C Promotes its own Asymmetric Localisation by Phosphorylating Cdc42 in the C. elegans zygote

Atypical protein kinase C (aPKC) is a major regulator of cell polarity. Acting in conjunction with Par6, Par3 and the small GTPase Cdc42, aPKC becomes asymmetrically localised and drives the polarisation of cells. aPKC activity is crucial for its own asymmetric localisation, suggesting a hitherto unknown feedback mechanism contributing to polarisation. Here we show in the C. elegans zygote that the feedback relies on aPKC phosphorylation of Cdc42 at serine 71. The turnover of CDC-42 phosphorylation ensures optimal aPKC asymmetry and activity throughout polarisation by tuning Par6/aPKC association with Par3 and Cdc42. Moreover, turnover of Cdc42 phosphorylation regulates actomyosin cortex dynamics that are known to drive aPKC asymmetry. Given the widespread role of aPKC and Cdc42 in cell polarity, this form of self-regulation of aPKC may be vital for the robust control of polarisation in many cell types.

Two distinct waves of transcriptome and translatome changes drive Drosophila germline stem cell differentiation

The tight control of fate transitions during stem cell differentiation is essential for proper tissue development and maintenance. However, the challenges in studying sparsely distributed adult stem cells in a systematic manner have hindered efforts to identify how the multilayered regulation of gene expression programs orchestrates stem cell differentiation in vivo. Here, we synchronised Drosophila female germline stem cell (GSC) differentiation in vivo to perform in-depth transcriptome and translatome analyses at high temporal resolution. This characterisation revealed widespread and dynamic changes in mRNA level, promoter usage, exon inclusion, and translation efficiency. Transient expression of the master regulator, Bam, drives a first wave of expression changes, primarily modifying the cell cycle program. Surprisingly, as Bam levels recede, differentiating cells return to a remarkably stem cell-like transcription and translation program, with a few crucial changes feeding into a second phase driving terminal differentiation to form the oocyte. Altogether, these findings reveal that rather than a unidirectional accumulation of changes, the in vivo differentiation of stem cells relies on distinctly regulated and developmentally sequential waves.

Molecular Mechanisms of Drosophila Hematopoiesis

As a model organism, the fruit fly (Drosophila melanogaster) has assumed a leading position in modern biological research. The Drosophila genetic system has a number of advantages making it a key model in investigating the molecular mechanisms of metazoan developmental processes. Over the past two decades, significant progress has been made in understanding the molecular mechanisms regulating Drosophila hematopoiesis. This review discusses the major advances in investigating the molecular mechanisms involved in maintaining the population of multipotent progenitor cells and their differentiation into mature hemocytes in the hematopoietic organ of the Drosophila larva. The use of the Drosophila hematopoietic organ as a model system for hematopoiesis has allowed to characterize the complex interactions between signaling pathways and transcription factors in regulating the maintenance and differentiation of progenitor cells through the signals from the hematopoietic niche, autocrine and paracrine signals, and the signals emanated by differentiated cells.

Condensin-mediated restriction of retrotransposable elements facilitates brain development in Drosophila melanogaster

Neural stem and progenitor cell (NSPC) maintenance is essential for ensuring that organisms are born with proper brain volumes and head sizes. Microcephaly is a disorder in which babies are born with significantly smaller head sizes and cortical volumes. Mutations in subunits of the DNA organizing complex condensin have been identified in microcephaly patients. However, the molecular mechanisms by which condensin insufficiency causes microcephaly remain elusive. We previously identified conserved roles for condensins in repression of retrotransposable elements (RTEs). Here, we show that condensin subunit knockdown in NSPCs of the Drosophila larval central brain increases RTE expression and mobility which causes cell death, and significantly decreases adult head sizes and brain volumes. These findings suggest that unrestricted RTE expression and activity may lead to improper brain development in condensin insufficient organisms, and lay the foundation for future exploration of causative roles for RTEs in other microcephaly models.

Kin17 regulates proper cortical localization of Miranda in Drosophila neuroblasts by regulating Flfl expression

During asymmetric division of Drosophila larval neuroblasts, the fate determinant Prospero (Pros) and its adaptor Miranda (Mira) are segregated to the basal cortex through atypical protein kinase C (aPKC) phosphorylation of Mira and displacement from the apical cortex, but Mira localization after aPKC phosphorylation is not well understood. We identify Kin17, a DNA replication and repair protein, as a regulator of Mira localization during asymmetric cell division. Loss of Kin17 leads to aberrant localization of Mira and Pros to the centrosome, cytoplasm, and nucleus. We provide evidence to show that the mislocalization of Mira and Pros is likely due to reduced expression of Falafel (Flfl), a component of protein phosphatase 4 (PP4), and defects in dephosphorylation of serine-96 of Mira. Our work reveals that Mira is likely dephosphorylated by PP4 at the centrosome to ensure proper basal localization of Mira after aPKC phosphorylation and that Kin17 regulates PP4 activity by regulating Flfl expression.

The Drosophila neuroblast polarity cycle at a glance

Drosophila neural stem cells, or neuroblasts, rapidly proliferate during embryonic and larval development to populate the central nervous system. Neuroblasts divide asymmetrically to create cellular diversity, with each division producing one sibling cell that retains the neuroblast fate and another that differentiates into glia or neurons. This asymmetric outcome is mediated by the transient polarization of numerous factors to the cell cortex during mitosis. The powerful genetics and outstanding imaging tractability of the neuroblast make it an excellent model system for studying the mechanisms of cell polarity. This Cell Science at a Glance article and the accompanying poster explore the phases of the neuroblast polarity cycle and the regulatory circuits that control them. We discuss the key features of the cycle - the targeted recruitment of proteins to specific regions of the plasma membrane and multiple phases of highly dynamic actomyosin-dependent cortical flows that pattern both protein distribution and membrane structure.

Delta-dependent Notch activation closes the early neuroblast temporal program to promote lineage progression and neurogenesis termination in Drosophila

Neuroblasts in Drosophila divide asymmetrically, sequentially expressing a series of intrinsic factors to generate a diversity of neuron types. These intrinsic factors known as temporal factors dictate timing of neuroblast transitions in response to steroid hormone signaling and specify early versus late temporal fates in neuroblast neuron progeny. After completing their temporal programs, neuroblasts differentiate or die, finalizing both neuron number and type within each neuroblast lineage. From a screen aimed at identifying genes required to terminate neuroblast divisions, we identified Notch and Notch pathway components. When Notch is knocked down, neuroblasts maintain early temporal factor expression longer, delay late temporal factor expression, and continue dividing into adulthood. We find that Delta, expressed in cortex glia, neuroblasts, and after division, their GMC progeny, regulates neuroblast Notch activity. We also find that Delta in neuroblasts is expressed high early, low late, and is controlled by the intrinsic temporal program: early factor Imp promotes Delta, late factors Syp/E93 reduce Delta. Thus, in addition to systemic steroid hormone cues, forward lineage progression is controlled by local cell-cell signaling between neuroblasts and their cortex glia/GMC neighbors: Delta transactivates Notch in neuroblasts bringing the early temporal program and early temporal factor expression to a close.

Understanding Developmental Cell Death Using Drosophila as a Model System

Cell death plays an essential function in organismal development, wellbeing, and ageing. Many types of cell deaths have been described in the past 30 years. Among these, apoptosis remains the most conserved type of cell death in metazoans and the most common mechanism for deleting unwanted cells. Other types of cell deaths that often play roles in specific contexts or upon pathological insults can be classed under variant forms of cell death and programmed necrosis. Studies in Drosophila have contributed significantly to the understanding and regulation of apoptosis pathways. In addition to this, Drosophila has also served as an essential model to study the genetic basis of autophagy-dependent cell death (ADCD) and other relatively rare types of context-dependent cell deaths. Here, we summarise what is known about apoptosis, ADCD, and other context-specific variant cell death pathways in Drosophila, with a focus on developmental cell death.

The conserved RNA-binding protein Imp is required for the specification and function of olfactory navigation circuitry in Drosophila

Complex behaviors depend on the precise developmental specification of neuronal circuits, but the relationship between genetic programs for neural development, circuit structure, and behavioral output is often unclear. The central complex (CX) is a conserved sensory-motor integration center in insects, which governs many higher-order behaviors and largely derives from a small number of type II neural stem cells (NSCs). Here, we show that Imp, a conserved IGF-II mRNA-binding protein expressed in type II NSCs, plays a role in specifying essential components of CX olfactory navigation circuitry. We show the following: (1) that multiple components of olfactory navigation circuitry arise from type II NSCs. (2) Manipulating Imp expression in type II NSCs alters the number and morphology of many of these circuit elements, with the most potent effects on neurons targeting the ventral layers of the fan-shaped body (FB). (3) Imp regulates the specification of Tachykinin-expressing ventral FB input neurons. (4) Imp is required in type II NSCs for establishing proper morphology of the CX neuropil structures. (5) Loss of Imp in type II NSCs abolishes upwind orientation to attractive odor while leaving locomotion and odor-evoked regulation of movement intact. Taken together, our findings establish that a temporally expressed gene can regulate the expression of a complex behavior by developmentally regulating the specification of multiple circuit components and provides a first step toward a developmental dissection of the CX and its roles in behavior.

Ribogenesis boosts controlled by HEATR1-MYC interplay promote transition into brain tumour growth

Cell commitment to tumourigenesis and the onset of uncontrolled growth are critical determinants in cancer development but the early events directing tumour initiating cell (TIC) fate remain unclear. We reveal a single-cell transcriptome profile of brain TICs transitioning into tumour growth using the brain tumour (brat) neural stem cell-based Drosophila model. Prominent changes in metabolic and proteostasis-associated processes including ribogenesis are identified. Increased ribogenesis is a known cell adaptation in established tumours. Here we propose that brain TICs boost ribogenesis prior to tumour growth. In brat-deficient TICs, we show that this dramatic change is mediated by upregulated HEAT-Repeat Containing 1 (HEATR1) to promote ribosomal RNA generation, TIC enlargement and onset of overgrowth. High HEATR1 expression correlates with poor glioma patient survival and patient-derived glioblastoma stem cells rely on HEATR1 for enhanced ribogenesis and tumourigenic potential. Finally, we show that HEATR1 binds the master growth regulator MYC, promotes its nucleolar localisation and appears required for MYC-driven ribogenesis, suggesting a mechanism co-opted in ribogenesis reprogramming during early brain TIC development.

Spt5 interacts genetically with Myc and is limiting for brain tumor growth in Drosophila

The transcription factor SPT5 physically interacts with MYC oncoproteins and is essential for efficient transcriptional activation of MYC targets in cultured cells. Here, we use Drosophila to address the relevance of this interaction in a living organism. Spt5 displays moderate synergy with Myc in fast proliferating young imaginal disc cells. During later development, Spt5-knockdown has no detectable consequences on its own, but strongly enhances eye defects caused by Myc overexpression. Similarly, Spt5-knockdown in larval type 2 neuroblasts has only mild effects on brain development and survival of control flies, but dramatically shrinks the volumes of experimentally induced neuroblast tumors and significantly extends the lifespan of tumor-bearing animals. This beneficial effect is still observed when Spt5 is knocked down systemically and after tumor initiation, highlighting SPT5 as a potential drug target in human oncology.

Imaging and Analysis of Drosophila Neural Stem Cell Asymmetric Division

Cell division is a conserved process among eukaryotes. It is designed to segregate chromosomes into future daughter cells and involves a complex rearrangement of the cytoskeleton, including microtubules and actin filaments. An additional level of complexity is present in asymmetric dividing stem cells because cytoskeleton elements are also regulated by polarity cues. The neural stem cell system of the fruit fly represents a simple model to dissect the mechanisms that control cytoskeleton reorganization during asymmetric division. In this chapter, we propose to describe protocols that allow accurate analysis of microtubule reorganization during cell division in this model.

Mutations in abnormal spindle disrupt temporal transcription factor expression and trigger immune responses in the Drosophila brain

The coordination of cellular behaviors during neurodevelopment is critical for determining the form, function, and size of the central nervous system (CNS). Mutations in the vertebrate Abnormal Spindle-Like, Microcephaly Associated (ASPM) gene and its Drosophila melanogaster ortholog abnormal spindle (asp) lead to microcephaly (MCPH), a reduction in overall brain size whose etiology remains poorly defined. Here, we provide the neurodevelopmental transcriptional landscape for a Drosophila model for autosomal recessive primary microcephaly-5 (MCPH5) and extend our findings into the functional realm to identify the key cellular mechanisms responsible for Asp-dependent brain growth and development. We identify multiple transcriptomic signatures, including new patterns of coexpressed genes in the developing CNS. Defects in optic lobe neurogenesis were detected in larval brains through downregulation of temporal transcription factors (tTFs) and Notch signaling targets, which correlated with a significant reduction in brain size and total cell numbers during the neurogenic window of development. We also found inflammation as a hallmark of asp mutant brains, detectable throughout every stage of CNS development, which also contributes to the brain size phenotype. Finally, we show that apoptosis is not a primary driver of the asp mutant brain phenotypes, further highlighting an intrinsic Asp-dependent neurogenesis promotion mechanism that is independent of cell death. Collectively, our results suggest that the etiology of the asp mutant brain phenotype is complex and that a comprehensive view of the cellular basis of the disorder requires an understanding of how multiple pathway inputs collectively determine tissue size and architecture.

Homemade: building the structure of the neurogenic niche

Neural stem/progenitor cells live in an intricate cellular environment, the neurogenic niche, which supports their function and enables neurogenesis. The niche is made of a diversity of cell types, including neurons, glia and the vasculature, which are able to signal to and are structurally organised around neural stem/progenitor cells. While the focus has been on how individual cell types signal to and influence the behaviour of neural stem/progenitor cells, very little is actually known on how the niche is assembled during development from multiple cellular origins, and on the role of the resulting topology on these cells. This review proposes to draw a state-of-the art picture of this emerging field of research, with the aim to expose our knowledge on niche architecture and formation from different animal models (mouse, zebrafish and fruit fly). We will span its multiple aspects, from the existence and importance of local, adhesive interactions to the potential emergence of larger-scale topological properties through the careful assembly of diverse cellular and acellular components.

A modified method to analyse cell proliferation using EdU labelling in large insect brains

The study of neurogenesis is critical to understanding of the evolution of nervous systems. Within invertebrates, this process has been extensively studied in Drosophila melanogaster, which is the predominant model thanks to the availability of advanced genetic tools. However, insect nervous systems are extremely diverse, and by studying a range of taxa we can gain additional information about how nervous systems and their development evolve. One example of the high diversity of insect nervous system diversity is provided by the mushroom bodies. Mushroom bodies have critical roles in learning and memory and vary dramatically across species in relative size and the type(s) of sensory information they process. Heliconiini butterflies provide a useful snapshot of this diversity within a closely related clade. Within Heliconiini, the genus Heliconius contains species where mushroom bodies are 3-4 times larger than other closely related genera, relative to the rest of the brain. This variation in size is largely explained by increases in the number of Kenyon cells, the intrinsic neurons which form the mushroom body. Hence, variation in mushroom body size is the product of changes in cell proliferation during Kenyon cell neurogenesis. Studying this variation requires adapting labelling techniques for use in less commonly studied organisms, as methods developed for common laboratory insects often do not work. Here, we present a modified protocol for EdU staining to examine neurogenesis in large-brained insects, using Heliconiini butterflies as our primary case, but also demonstrating applicability to cockroaches, another large-brained insect.

Cooperative regulation of C1-domain membrane recruitment polarizes atypical protein kinase C

Recruitment of the Par complex protein atypical protein kinase C (aPKC) to a specific membrane domain is a key step in the polarization of animal cells. While numerous proteins and phospholipids interact with aPKC, how these interactions cooperate to control its membrane recruitment has been unknown. Here, we identify aPKC's C1 domain as a phospholipid interaction module that targets aPKC to the membrane of Drosophila neural stem cells (NSCs). The isolated C1 binds the NSC membrane in an unpolarized manner during interphase and mitosis and is uniquely sufficient among aPKC domains for targeting. Other domains, including the catalytic module and those that bind the upstream regulators Par-6 and Bazooka, restrict C1's membrane targeting activity-spatially and temporally-to the apical NSC membrane during mitosis. Our results suggest that aPKC polarity results from cooperative activation of autoinhibited C1-mediated membrane binding activity.

Stem cell-specific ecdysone signaling regulates the development and function of a Drosophila sleep homeostat

Complex behaviors arise from neural circuits that are assembled from diverse cell types. Sleep is a conserved and essential behavior, yet little is known regarding how the nervous system generates neuron types of the sleep-wake circuit. Here, we focus on the specification of Drosophila sleep-promoting neurons-long-field tangential input neurons that project to the dorsal layers of the fan-shaped body neuropil in the central complex (CX). We use lineage analysis and genetic birth dating to identify two bilateral Type II neural stem cells that generate these dorsal fan-shaped body (dFB) neurons. We show that adult dFB neurons express Ecdysone-induced protein E93, and loss of Ecdysone signaling or E93 in Type II NSCs results in the misspecification of the adult dFB neurons. Finally, we show that E93 knockdown in Type II NSCs affects adult sleep behavior. Our results provide insight into how extrinsic hormonal signaling acts on NSCs to generate neuronal diversity required for adult sleep behavior. These findings suggest that some adult sleep disorders might derive from defects in stem cell-specific temporal neurodevelopmental programs.

A novel proneural function of Asense is integrated with the sequential actions of Delta-Notch, L'sc and Su(H) to promote the neuroepithelial to neuroblast transition

In order for neural progenitors (NPs) to generate distinct populations of neurons at the right time and place during CNS development, they must switch from undergoing purely proliferative, self-renewing divisions to neurogenic, asymmetric divisions in a tightly regulated manner. In the developing Drosophila optic lobe, neuroepithelial (NE) cells of the outer proliferation center (OPC) are progressively transformed into neurogenic NPs called neuroblasts (NBs) in a medial to lateral proneural wave. The cells undergoing this transition express Lethal of Scute (L'sc), a proneural transcription factor (TF) of the Acheate Scute Complex (AS-C). Here we show that there is also a peak of expression of Asense (Ase), another AS-C TF, in the cells neighboring those with transient L'sc expression. These peak of Ase cells help to identify a new transitional stage as they have lost NE markers and L'sc, they receive a strong Notch signal and barely exhibit NB markers. This expression of Ase is necessary and sufficient to promote the NE to NB transition in a more robust and rapid manner than that of l'sc gain of function or Notch loss of function. Thus, to our knowledge, these data provide the first direct evidence of a proneural role for Ase in CNS neurogenesis. Strikingly, we found that strong Delta-Notch signaling at the lateral border of the NE triggers l'sc expression, which in turn induces ase expression in the adjacent cells through the activation of Delta-Notch signaling. These results reveal two novel non-conventional actions of Notch signaling in driving the expression of proneural factors, in contrast to the repression that Notch signaling exerts on them during classical lateral inhibition. Finally, Suppressor of Hairless (Su(H)), which seems to be upregulated late in the transitioning cells and in NBs, represses l'sc and ase, ensuring their expression is transient. Thus, our data identify a key proneural role of Ase that is integrated with the sequential activities of Delta-Notch signaling, L'sc, and Su(H), driving the progressive transformation of NE cells into NBs.