Drosophila neuroblasts: a model for stem cell biology

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.

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.

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.

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.

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.

Drosophila Adducin facilitates phase separation and function of a conserved spindle orientation complex

Asymmetric cell division (ACD) allows stem cells to generate differentiating progeny while simultaneously maintaining their own pluripotent state. ACD involves coupling mitotic spindle orientation with cortical polarity cues to direct unequal segregation of cell fate determinants. In Drosophila neural stem cells (neuroblasts; NBs), spindles orient along an apical-basal polarity axis through a conserved complex of Partner of Inscuteable (Pins; human LGN) and Mushroom body defect (Mud; human NuMA). While many details of its function are well known, the molecular mechanics that drive assembly of the cortical Pins/Mud complex remain unclear, particularly with respect to the mutually exclusive Pins complex formed with the apical scaffold protein Inscuteable (Insc). Here we identify Hu li tai shao (Hts; human Adducin) as a direct Mud-binding protein, using an aldolase fold within its head domain (HtsHEAD) to bind a short Mud coiled-coil domain (MudCC) that is adjacent to the Pins-binding domain (MudPBD). Hts is expressed throughout the larval central brain and apically polarizes in mitotic NBs where it is required for Mud-dependent spindle orientation. In vitro analyses reveal that Pins undergoes liquid-liquid phase separation with Mud, but not with Insc, suggesting a potential molecular basis for differential assembly mechanics between these two competing apical protein complexes. Furthermore, we find that Hts binds an intact Pins/Mud complex, reduces the concentration threshold for its phase separation, and alters the liquid-like property of the resulting phase separated droplets. Domain mapping and mutational analyses implicate critical roles for both multivalent interactions (via MudCC oligomerization) and protein disorder (via an intrinsically disordered region in Hts; HtsIDR) in phase separation of the Hts/Mud/Pins complex. Our study identifies a new component of the spindle positioning machinery in NBs and suggests that phase separation of specific protein complexes might regulate ordered assembly within the apical domain to ensure proper signaling output.

Neural stem cell metabolism revisited: a critical role for mitochondria

Metabolism has emerged as a key regulator of stem cell behavior. Mitochondria are crucial metabolic organelles that are important for differentiated cells, yet considered less so for stem cells. However, recent studies have shown that mitochondria influence stem cell maintenance and fate decisions, inviting a revised look at this topic. In this review, we cover the current literature addressing the role of mitochondrial metabolism in mouse and human neural stem cells (NSCs) in the embryonic and adult brain. We summarize how mitochondria are implicated in fate regulation and how substrate oxidation affects NSC quiescence. We further explore single-cell RNA sequencing (scRNA-seq) data for metabolic signatures of adult NSCs, highlight emerging technologies reporting on metabolic signatures, and discuss mitochondrial metabolism in other stem cells.

Polarized SCAR and the Arp2/3 complex regulate apical cortical remodeling in asymmetrically dividing neuroblasts

Although the formin-nucleated actomyosin cortex has been shown to drive the changes in cell shape that accompany animal cell division in both symmetric and asymmetric cell divisions, the mitotic role of cortical Arp2/3-nucleated actin networks remain unclear. Here using asymmetrically dividing Drosophila neural stem cells as a model system, we identify a pool of membrane protrusions that form at the apical cortex of neuroblasts as they enter mitosis. Strikingly, these apically localized protrusions are enriched in SCAR, and depend on SCAR and Arp2/3 complexes for their formation. Because compromising SCAR or the Arp2/3 complex delays the apical clearance of Myosin II at the onset of anaphase and induces cortical instability at cytokinesis, these data point to a role for an apical branched actin filament network in fine-tuning the actomyosin cortex to enable the precise control of cell shape changes during an asymmetric cell division.

Deterministic and probabilistic fate decisions co-exist in a single retinal lineage

Correct nervous system development depends on the timely differentiation of progenitor cells into neurons. While the output of progenitor differentiation is well investigated at the population and clonal level, how stereotypic or variable fate decisions are during development is still more elusive. To fill this gap, we here follow the fate outcome of single neurogenic progenitors in the zebrafish retina over time using live imaging. We find that neurogenic progenitor divisions produce two daughter cells, one of deterministic and one of probabilistic fate. Interference with the deterministic branch of the lineage affects lineage progression. In contrast, interference with fate probabilities of the probabilistic branch results in a broader range of fate possibilities than in wild-type and involves the production of any neuronal cell type even at non-canonical developmental stages. Combining the interference data with stochastic modelling of fate probabilities revealed that a simple gene regulatory network is able to predict the observed fate decision probabilities during wild-type development. These findings unveil unexpected lineage flexibility that could ensure robust development of the retina and other tissues.

A STIM dependent dopamine-neuropeptide axis maintains the larval drive to feed and grow in Drosophila

Appropriate nutritional intake is essential for organismal survival. In holometabolous insects such as Drosophila melanogaster, the quality and quantity of food ingested as larvae determines adult size and fecundity. Here we have identified a subset of dopaminergic neurons (THD') that maintain the larval motivation to feed. Dopamine release from these neurons requires the ER Ca2+ sensor STIM. Larvae with loss of STIM stop feeding and growing, whereas expression of STIM in THD' neurons rescues feeding, growth and viability of STIM null mutants to a significant extent. Moreover STIM is essential for maintaining excitability and release of dopamine from THD' neurons. Optogenetic stimulation of THD' neurons activated neuropeptidergic cells, including median neuro secretory cells that secrete insulin-like peptides. Loss of STIM in THD' cells alters the developmental profile of specific insulin-like peptides including ilp3. Loss of ilp3 partially rescues STIM null mutants and inappropriate expression of ilp3 in larvae affects development and growth. In summary we have identified a novel STIM-dependent function of dopamine neurons that modulates developmental changes in larval feeding behaviour and growth.

Systematic analysis of YFP traps reveals common mRNA/protein discordance in neural tissues

While post-transcriptional control is thought to be required at the periphery of neurons and glia, its extent is unclear. Here, we investigate systematically the spatial distribution and expression of mRNA at single molecule sensitivity and their corresponding proteins of 200 YFP trap lines across the intact Drosophila nervous system. 97.5% of the genes studied showed discordance between the distribution of mRNA and the proteins they encode in at least one region of the nervous system. These data suggest that post-transcriptional regulation is very common, helping to explain the complexity of the nervous system. We also discovered that 68.5% of these genes have transcripts present at the periphery of neurons, with 9.5% at the glial periphery. Peripheral transcripts include many potential new regulators of neurons, glia, and their interactions. Our approach is applicable to most genes and tissues and includes powerful novel data annotation and visualization tools for post-transcriptional regulation.

Consumption of a polarized membrane reservoir drives asymmetric membrane expansion during the unequal divisions of neural stem cells

The asymmetric divisions of Drosophila neural stem cells (NSCs) produce unequally sized siblings, with most volume directed into the sibling that retains the NSC fate. Sibling size asymmetry results from the preferential expansion of the NSC sibling surface during division. Here, we show that a polarized membrane reservoir constructed by the NSC in early mitosis provides the source for expansion. The reservoir is formed from membrane domains that contain folds and microvilli that become polarized by apically directed cortical flows of actomyosin early in mitosis. When furrow ingression begins and internal pressure increases, the stores of membrane within the apical reservoir are rapidly consumed. Expansion is substantially diminished in NSCs that lack a reservoir, and membrane expansion equalizes when the reservoir is not polarized. Our results suggest that the cortical flows that remodel the plasma membrane during asymmetric cell division function to satisfy the dynamic surface area requirements of unequally dividing cells.

The RNA-binding protein, Imp specifies olfactory navigation circuitry and behavior in Drosophila

Complex behaviors depend on the precise developmental specification of neuronal circuits, but the relationship between genetic prograssms for neural development, circuit structure, and behavioral output is often unclear. The central complex (CX) is a conserved sensory-motor integration center in insects that governs many higher order behaviors and largely derives from a small number of Type II neural stem cells. Here, we show that Imp, a conserved IGF-II mRNA-binding protein expressed in Type II neural stem cells, specifies components of CX olfactory navigation circuitry. We show: (1) that multiple components of olfactory navigation circuitry arise from Type II neural stem cells and manipulating Imp expression in Type II neural stem cells 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. (2) Imp regulates the specification of Tachykinin expressing ventral fan-shaped body input neurons. (3) Imp in Type II neural stem cells alters the morphology of the CX neuropil structures. (4) Loss of Imp in Type II neural stem cells abolishes upwind orientation to attractive odor while leaving locomotion and odor-evoked regulation of movement intact. Taken together, our work establishes that a single temporally expressed gene can regulate the expression of a complex behavior through the developmental specification of multiple circuit components and provides a first step towards a developmental dissection of the CX and its roles in behavior.

Formation of the brain by stem cell divisions of large neuroblasts in Oikopleura dioica, a simple chordate

Stem cell division contributes to the generation of various cell types during animal development, especially a diverse pool of neural cells in the nervous system. One example is reiterated unequal stem cell divisions, in which a large stem cell undergoes a series of oriented unequal divisions to produce a chain of small daughter cells that differentiate. We show that reiterated unequal stem cell divisions are involved in the formation of the brain in simple chordate appendicularians (larvaceans). Two large neuroblasts in the anterior and middle of the brain-forming region of hatched larvae were observed. They produced at least 30 neural cells out of 96 total brain cells before completion of brain formation at 10 hours after fertilization by reiterated unequal stem cell divisions. The daughter cells of the anterior neuroblast were postmitotic, and the number was at least 19. The neuroblast produced small daughter neural cells posteriorly every 20 min. The neural cells first moved toward the dorsal side, turned in the anterior direction, aligned in a single line according to their birth order, and showed collective movement to accumulate in the anterior part of the brain. The anterior neuroblast originated from the right-anterior blastomeres of the eight-cell embryos and the right a222 blastomere of the 64-cell embryo. The posterior neuroblast also showed reiterated unequal stem cell divisions, and generated at least 11 neural cells. Sequential unequal stem cell divisions without stem cell growth have been observed in protostomes, such as insects and annelids. The results provide the first examples of this kind of stem cell division during brain formation in non-vertebrate deuterostomes.

Astrocyte development—More questions than answers

The past 15–20 years has seen a remarkable shift in our understanding of astrocyte contributions to central nervous system (CNS) function. Astrocytes have emerged from the shadows of neuroscience and are now recognized as key elements in a broad array of CNS functions. Astrocytes comprise a substantial fraction of cells in the human CNS. Nevertheless, fundamental questions surrounding their basic biology remain poorly understood. While recent studies have revealed a diversity of essential roles in CNS function, from synapse formation and function to blood brain barrier maintenance, fundamental mechanisms of astrocyte development, including their expansion, migration, and maturation, remain to be elucidated. The coincident development of astrocytes and synapses highlights the need to better understand astrocyte development and will facilitate novel strategies for addressing neurodevelopmental and neurological dysfunction. In this review, we provide an overview of the current understanding of astrocyte development, focusing primarily on mammalian astrocytes and highlight outstanding questions that remain to be addressed. We also include an overview of Drosophila glial development, emphasizing astrocyte-like glia given their close anatomical and functional association with synapses. Drosophila offer an array of sophisticated molecular genetic tools and they remain a powerful model for elucidating fundamental cellular and molecular mechanisms governing astrocyte development. Understanding the parallels and distinctions between astrocyte development in Drosophila and vertebrates will enable investigators to leverage the strengths of each model system to gain new insights into astrocyte function.

Energy metabolic pathways in neuronal development and function

Neuronal development and function are known to be among the most energy-demanding functions of the body. Constant energetic support is therefore crucial at all stages of a neuron's life. The two main adenosine triphosphate (ATP)-producing pathways in cells are glycolysis and oxidative phosphorylation. Glycolysis has a relatively low yield but provides fast ATP and enables the metabolic versatility needed in dividing neuronal stem cells. Oxidative phosphorylation, on the other hand, is highly efficient and therefore thought to provide most or all ATP in differentiated neurons. However, it has recently become clear that due to their distinct properties, both pathways are required to fully satisfy neuronal energy demands during development and function. Here, we provide an overview of how glycolysis and oxidative phosphorylation are used in neurons during development and function.

Simultaneous activation of Tor and suppression of ribosome biogenesis by TRIM-NHL proteins promotes terminal differentiation

Tissue development and homeostasis depend on the balance between growth and terminal differentiation, but the mechanisms coordinating these processes remain elusive. Accumulating evidence indicates that ribosome biogenesis (RiBi) and protein synthesis, two cellular processes sustaining growth, are tightly regulated and yet can be uncoupled during stem cell differentiation. Using the Drosophila adult female germline stem cell and larval neuroblast systems, we show that Mei-P26 and Brat, two Drosophila TRIM-NHL paralogs, are responsible for uncoupling RiBi and protein synthesis during differentiation. In differentiating cells, Mei-P26 and Brat activate the target of rapamycin (Tor) kinase to promote translation, while concomitantly repressing RiBi. Depletion of Mei-P26 or Brat results in defective terminal differentiation, which can be rescued by ectopic activation of Tor together with suppression of RiBi. Our results indicate that uncoupling RiBi and translation activities by TRIM-NHL activity creates the conditions required for terminal differentiation.

Apical-basal polarity precisely determines intestinal stem cell number by regulating Prospero threshold

Apical-basal polarity and cell-fate determinants are crucial for the cell fate and control of stem cell numbers. However, their interplay leading to a precise stem cell number remains unclear. Drosophila pupal intestinal stem cells (pISCs) asymmetrically divide, generating one apical ISC progenitor and one basal Prospero (Pros)+ enteroendocrine mother cell (EMC), followed by symmetric divisions of each daughter before adulthood, providing an ideal system to investigate the outcomes of polarity loss. Using lineage tracing and ex vivo live imaging, we identify an interlocked polarity regulation network precisely determining ISC number: Bazooka inhibits Pros accumulation by activating Notch signaling to maintain stem cell fate in pISC apical daughters. A threshold of Pros promotes differentiation to EMCs and avoids ISC-like cell fate, and over-threshold of Pros inhibits miranda expression to ensure symmetric divisions in pISC basal daughters. Our work suggests that a polarity-dependent threshold of a differentiation factor precisely controls stem cell number.

Modulation of Notch Signaling at Early Stages of Differentiation of Human Induced Pluripotent Stem Cells to Dopaminergic Neurons

Elaboration of protocols for differentiation of human pluripotent stem cells to dopamine neurons is an important issue for development of cell replacement therapy for Parkinson's disease. A number of protocols have been already developed; however, their efficiency and specificity still can be improved. Investigating the role of signaling cascades, important for neurogenesis, can help to solve this problem and to provide a deeper understanding of their role in neuronal development. Notch signaling plays an essential role in development and maintenance of the central nervous system after birth. In our study, we analyzed the effect of Notch activation and inhibition at the early stages of differentiation of human induced pluripotent stem cells to dopaminergic neurons. We found that, during the first seven days of differentiation, the cells were not sensitive to the Notch inhibition. On the contrary, activation of Notch signaling during the same time period led to significant changes and was associated with an increase in expression of genes, specific for caudal parts of the brain, a decrease of expression of genes, specific for forebrain, as well as a decrease of expression of genes, important for the formation of axons and dendrites and microtubule stabilizing proteins.

The neurodevelopmental transcriptome of the Drosophila melanogaster microcephaly gene abnormal spindle reveals a role for temporal transcription factors and the immune system in regulating brain size

The coordination of cellular behaviors during neurodevelopment is critical for determining the form, function, and size of the central nervous system. Mutations in the vertebrate Abnormal Spindle-Like, Microcephaly Associated (ASPM) gene and its Drosophila melanogaster ortholog abnormal spindle (asp) lead to microcephaly, 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 (MCPH) and extend our findings into the functional realm in an attempt to identify the key cellular mechanisms responsible for Asp-dependent brain growth and development. We identify multiple transcriptomic signatures, including new patterns of co-expressed 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 MCPH 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 MCPH phenotype, further highlighting an intrinsic Asp-dependent neurogenesis promotion mechanism that is independent of cell death. Collectively, our results suggest that the etiology of asp MCPH is complex and that a comprehensive view of the cellular basis of the disorder requires an understanding of how multiple pathway inputs collectively determine the microcephaly phenotype.

dMyc-dependent upregulation of CD98 amino acid transporters is required for Drosophila brain tumor growth

Tumor cells have an increased demand for nutrients to sustain their growth, but how these increased metabolic needs are ensured or how this influences tumor formation and progression remains unclear. To unravel tumor metabolic dependencies, particularly from extracellular metabolites, we have analyzed the role of plasma membrane metabolic transporters in Drosophila brain tumors. Using a well-established neural stem cell-derived tumor model, caused by brat knockdown, we have found that 13 plasma membrane metabolic transporters, including amino acid, carbohydrate and monocarboxylate transporters, are upregulated in tumors and are required for tumor growth. We identified CD98hc and several of the light chains with which it can form heterodimeric amino acid transporters, as crucial players in brat RNAi (brat IR) tumor progression. Knockdown of these components of CD98 heterodimers caused a dramatic reduction in tumor growth. Our data also reveal that the oncogene dMyc is required and sufficient for the upregulation of CD98 transporter subunits in these tumors. Furthermore, tumor-upregulated dmyc and CD98 transporters orchestrate the overactivation of the growth-promoting signaling pathway TOR, forming a core growth regulatory network to support brat IR tumor progression. Our findings highlight the important link between oncogenes, metabolism, and signaling pathways in the regulation of tumor growth and allow for a better understanding of the mechanisms necessary for tumor progression.

Identification of a GABAergic neuroblast lineage modulating sweet and bitter taste sensitivity

In Drosophila melanogaster, processing of gustatory information and controlling feeding behavior are executed by neural circuits located in the subesophageal zone (SEZ) of the brain.1 Gustatory receptor neurons (GRNs) project their axons in the primary gustatory center (PGC), which is located in the SEZ.1,2,3,4 To address the function of the PGC, we need detailed information about the different classes of gustatory interneurons that frame the PGC. In this work, we screened large collections of driver lines for SEZ interneuron-specific labeling and subsequently used candidate lines to access the SEZ neuroblast lineages. We converted 130 Gal4 lines to LexA drivers and carried out functional screening using calcium imaging. We found one neuroblast lineage, TRdm, whose neurons responded to both sweet and bitter tastants, and formed green fluorescent protein (GFP) reconstitution across synaptic partners (GRASP)-positive synapses with sweet sensory neurons. TRdm neurons express the inhibitory transmitter GABA, and silencing these neurons increases appetitive feeding behavior. These results demonstrate that TRdm generates a class of inhibitory local neurons that control taste sensitivity in Drosophila.

Mitochondrial respiration and dynamics of in vivo neural stem cells

Neural stem cells (NSCs) in the developing and adult brain undergo many different transitions, tightly regulated by extrinsic and intrinsic factors. While the role of signalling pathways and transcription factors is well established, recent evidence has also highlighted mitochondria as central players in NSC behaviour and fate decisions. Many aspects of cellular metabolism and mitochondrial biology change during NSC transitions, interact with signalling pathways and affect the activity of chromatin-modifying enzymes. In this Spotlight, we explore recent in vivo findings, primarily from Drosophila and mammalian model systems, about the role that mitochondrial respiration and morphology play in NSC development and function.

Illuminati: a form of gene expression plasticity in Drosophila neural stem cells

While testing for genome instability in Drosophila as reported by unscheduled upregulation of UAS-GFP in cells that co-express GAL80 and GAL4, we noticed that, as expected, background levels were low in most developing tissues. However, GFP-positive clones were frequent in the larval brain. Most of these clones originated from central brain neural stem cells. Using imaging-based approaches and genome sequencing, we show that these unscheduled clones do not result from chromosome loss or mutations in GAL80. We have named this phenomenon 'Illuminati'. Illuminati is strongly enhanced in brat tumors and is also sensitive to environmental conditions such as food content and temperature. Illuminati is suppressed by Su(var)2-10, but it is not significantly affected by several modifiers of position effect variegation or Gal4::UAS variegation. We conclude that Illuminati identifies a previously unknown type of functional instability that may have important implications in development and disease.

An interplay between cellular growth and atypical fusion defines morphogenesis of a modular glial niche in Drosophila

Neural stem cells (NSCs) live in an intricate cellular microenvironment supporting their activity, the niche. Whilst shape and function are inseparable, the morphogenetic aspects of niche development are poorly understood. Here, we use the formation of a glial niche to investigate acquisition of architectural complexity. Cortex glia (CG) in Drosophila regulate neurogenesis and build a reticular structure around NSCs. We first show that individual CG cells grow tremendously to ensheath several NSC lineages, employing elaborate proliferative mechanisms which convert these cells into syncytia rich in cytoplasmic bridges. CG syncytia further undergo homotypic cell–cell fusion, using defined cell surface receptors and actin regulators. Cellular exchange is however dynamic in space and time. This atypical cell fusion remodels cellular borders, restructuring the CG syncytia. Ultimately, combined growth and fusion builds the multi-level architecture of the niche, and creates a modular, spatial partition of the NSC population. Our findings provide insights into how a niche forms and organises while developing intimate contacts with a stem cell population.

Little is known of how the architectural complexity of the neural stem cell niche is achieved. Rujano et al. show that the morphogenesis of a glial niche in Drosophila involves complex proliferative strategies and atypical cell–cell fusion.

Motor defects in a Drosophila model for spinal muscular atrophy result from SMN depletion during early neurogenesis

Spinal muscular atrophy (SMA) is the most common autosomal recessive neurodegenerative disease, and is characterised by spinal motor neuron loss, impaired motor function and, often, premature death. Mutations and deletions in the widely expressed survival motor neuron 1 (SMN1) gene cause SMA; however, the mechanisms underlying the selectivity of motor neuron degeneration are not well understood. Although SMA is degenerative in nature, SMN function during embryonic and early postnatal development appears to be essential for motor neuron survival in animal models and humans. Notwithstanding, how developmental defects contribute to the subversion of postnatal and adult motor function remains elusive. Here, in a Drosophila SMA model, we show that neurodevelopmental defects precede gross locomotor dysfunction in larvae. Furthermore, to specifically address the relevance of SMN during neurogenesis and in neurogenic cell types, we show that SMN knockdown using neuroblast-specific and pan-neuronal drivers, but not differentiated neuron or glial cell drivers, impairs adult motor function. Using targeted knockdown, we further restricted SMN manipulation in neuroblasts to a defined time window. Our aim was to express specifically in the neuronal progenitor cell types that have not formed synapses, and thus a time that precedes neuromuscular junction formation and maturation. By restoring SMN levels in these distinct neuronal population, we partially rescue the larval locomotor defects of Smn mutants. Finally, combinatorial SMN knockdown in immature and mature neurons synergistically enhances the locomotor and survival phenotypes. Our in-vivo study is the first to directly rescue the motor defects of an SMA model by expressing Smn in an identifiable population of Drosophila neuroblasts and developing neurons, highlighting that neuronal sensitivity to SMN loss may arise before synapse establishment and nerve cell maturation.

Spinal muscular atrophy (SMA) is the most common genetic cause of infant mortality and leads to the degeneration of the nerves that control muscle function. Loss-of-function mutations in the widely expressed survival motor neuron 1 (SMN1) gene cause SMA, but how low levels of SMN protein cause the neuronal dysfunction is not known. Although SMA is a disease of nerve degeneration, SMN function during nerve cell development may be important, particularly in severe forms of SMA. Nevertheless, how the defects during development and throughout early life contribute to the disease is not well understood. We have previously demonstrated that SMN protein becomes enriched in neuroblasts, which are the cells that divide to produce neurons. In the present study, motor defects observed in our fly model for SMA could be rescued by restoring SMN in neuroblasts alone. In addition, we show that knocking down SMN in healthy flies within the same cell type causes impaired motor function. The present study shows that the manipulation of SMN in a developmentally important cell type can cause motor defects, indicating that a period of abnormal neurodevelopment may contribute to SMA.

Stochastic fluctuations promote ordered pattern formation of cells in the Notch-Delta signaling pathway

The Notch-Delta signaling pathway mediates cell differentiation implicated in many regulatory processes including spatiotemporal patterning in tissues by promoting alternate cell fates between neighboring cells. At the multicellular level, this "lateral inhibition” principle leads to checkerboard patterns with alternation of Sender and Receiver cells. While it is well known that stochasticity modulates cell fate specification, little is known about how stochastic fluctuations at the cellular level propagate during multicell pattern formation. Here, we model stochastic fluctuations in the Notch-Delta pathway in the presence of two different noise types–shot and white–for a multicell system. Our results show that intermediate fluctuations reduce disorder and guide the multicell lattice toward checkerboard-like patterns. By further analyzing cell fate transition events, we demonstrate that intermediate noise amplitudes provide enough perturbation to facilitate “proofreading” of disordered patterns and cause cells to switch to the correct ordered state (Sender surrounded by Receivers, and vice versa). Conversely, high noise can override environmental signals coming from neighboring cells and lead to switching between ordered and disordered patterns. Therefore, in analogy with spin glass systems, intermediate noise levels allow the multicell Notch system to escape frustrated patterns and relax towards the lower energy checkerboard pattern while at large noise levels the system is unable to find this ordered base of attraction.

The Notch pathway is involved in many biological processes and is known to form precise spatial patterns alternating Sender and Receiver cell states. Quantifying the implications of stochastic fluctuations provided insight that patterns formed in Notch-mediated pathways must follow a predetermined path towards checkerboard or exist in a noisy environment which promotes order through error correction.

We model Notch pattern formation stochastically and analyze the spatiotemporal dynamics. Our results show multicellular systems equilibrate towards ordered systems, but mistakes in the initial lattice propagate causing the systems to relax into frustrated systems. Only through existing in a noisy environment are the systems able to relax into the checkerboard pattern. Analyzing the temporal dynamics confirms, in environments with intermediate noise, the “incorrect” cells (Sender in a Sender environment, and vice versa) can be flipped to the correct state (Sender in a Receiver environment, and vice versa). Comparing with the spin glass energy landscape, we suggest the multicellular model follows a rugged landscape to form patterns with stochastic fluctuations required to enforce order throughout the system.

Regulation of developmental hierarchy in Drosophila neural stem cell tumors by COMPASS and Polycomb complexes

COMPASS and Polycomb complexes are antagonistic chromatin complexes that are frequently inactivated in cancers, but how these events affect the cellular hierarchy, composition, and growth of tumors is unclear. These characteristics can be systematically investigated in Drosophila neuroblast tumors in which cooption of temporal patterning induces a developmental hierarchy that confers cancer stem cell (CSC) properties to a subset of neuroblasts retaining an early larval temporal identity. Here, using single-cell transcriptomics, we reveal that the trithorax/MLL1/2-COMPASS-like complex guides the developmental trajectory at the top of the tumor hierarchy. Consequently, trithorax knockdown drives larval-to-embryonic temporal reversion and the marked expansion of CSCs that remain locked in a spectrum of early temporal states. Unexpectedly, this phenotype is amplified by concomitant inactivation of Polycomb repressive complex 2 genes, unleashing tumor growth. This study illustrates how inactivation of specific COMPASS and Polycomb complexes cooperates to impair tumor hierarchies, inducing CSC plasticity, heterogeneity, and expansion.

ASC proneural factors are necessary for chromatin remodeling during neuroectodermal to neuroblast fate transition to ensure the timely initiation of the neural stem cell program

Background

In both Drosophila and mammals, the achaete-scute (ASC/ASCL) proneural bHLH transcription factors are expressed in the developing central and peripheral nervous systems, where they function during specification and maintenance of the neural stem cells in opposition to Notch signaling. In addition to their role in nervous system development, ASC transcription factors are oncogenic and exhibit chromatin reprogramming activity; however, the impact of ASC on chromatin dynamics during neural stem cell generation remains elusive. Here, we investigate the chromatin changes accompanying neural commitment using an integrative genetics and genomics methodology.

Results

We found that ASC factors bind equally strongly to two distinct classes of cis-regulatory elements: open regions remodeled earlier during maternal to zygotic transition by Zelda and less accessible, Zelda-independent regions. Both classes of cis-elements exhibit enhanced chromatin accessibility during neural specification and correlate with transcriptional regulation of genes involved in a variety of biological processes necessary for neuroblast function/homeostasis. We identified an ASC-Notch regulated TF network that includes likely prime regulators of neuroblast function. Using a cohort of ASC target genes, we report that ASC null neuroblasts are defectively specified, remaining initially stalled, unable to divide, and lacking expression of many proneural targets. When mutant neuroblasts eventually start proliferating, they produce compromised progeny. Reporter lines driven by proneural-bound enhancers display ASC dependency, suggesting that the partial neuroblast identity seen in the absence of ASC genes is likely driven by other, proneural-independent, cis-elements. Neuroblast impairment and the late differentiation defects of ASC mutants are corrected by ectodermal induction of individual ASC genes but not by individual members of the TF network downstream of ASC. However, in wild-type embryos, the induction of individual members of this network induces CNS hyperplasia, suggesting that they synergize with the activating function of ASC to consolidate the chromatin dynamics that promote neural specification.

Conclusions

We demonstrate that ASC proneural transcription factors are indispensable for the timely initiation of the neural stem cell program at the chromatin level by regulating a large number of enhancers in the vicinity of neural genes. This early chromatin remodeling is crucial for both neuroblast homeostasis as well as future progeny fidelity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01300-8.