Insights into neural stem cell biology from flies

Escargot a Snail superfamily member and its multiple roles in Drosophila melanogaster development

The Snail superfamily of transcription factors plays a crucial role in metazoan development; one of the most important vertebrate members of this family is Snai1 which is orthologous to the Drosophila melanogaster esg gene. This review offers a comprehensive examination of the roles of the esg gene in Drosophila development, covering its expression pattern and downstream targets, and draws parallels between the vertebrate Snai1 family proteins on controlling the epithelial-to-mesenchymal transition and esg. This gene regulates stemness, ploidy, and pluripontency. esg is expressed in various tissues during development, including the gut, imaginal discs, and neuroblasts. The functions of the esg include the suppression of differentiation in intestinal stem cells and the preservation of diploidy in imaginal cells. In the nervous system development, esg expression also inhibits neuroblast differentiation, thus regulating the number of neurons and the moment in development of neuronal differentiation. Loss of esg function results in diverse developmental defects, including defects in intestinal stem cell maintenance and differentiation, and alters imaginal disc and nervous system development. Expression levels of esg also play a role in regulating longevity and metabolism in adult stages. This review provides an overview of the current understanding of esg's developmental role, emphasizing cellular and tissue effects that arise from its loss of function. The insights gained may contribute to a better understanding of evolutionary conserved developmental mechanisms and certain metabolic diseases.

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.

Orb2 enables rare-codon-enriched mRNA expression during Drosophila neuron differentiation

Regulation of codon optimality is an increasingly appreciated layer of cell- and tissue-specific protein expression control. Here, we use codon-modified reporters to show that differentiation of Drosophila neural stem cells into neurons enables protein expression from rare-codon-enriched genes. From a candidate screen, we identify the cytoplasmic polyadenylation element binding (CPEB) protein Orb2 as a positive regulator of rare-codon-dependent expression in neurons. Using RNA sequencing, we reveal that Orb2-upregulated mRNAs in the brain with abundant Orb2 binding sites have a rare-codon bias. From these Orb2-regulated mRNAs, we demonstrate that rare-codon enrichment is important for expression control and social behavior function of the metabotropic glutamate receptor (mGluR). Our findings reveal a molecular mechanism by which neural stem cell differentiation shifts genetic code regulation to enable critical mRNA and protein expression.

Evolutionary origin of the neural tube in basal deuterostomes

The central nervous system (CNS) of chordates, including humans, develops as a hollow tube with ciliated walls containing cerebrospinal fluid. However, most of the animals inhabiting our planet do not use this design and rather build their centralized brains from non-epithelialized condensations of neurons called ganglia, with no traces of epithelialized tubes or liquid-containing cavities. The evolutionary origin of tube-type CNSs stays enigmatic, especially as non-epithelialized ganglionic-type nervous systems dominate the animal kingdom. Here, I discuss recent findings relevant to understanding the potential homologies and scenarios of the origin, histology and anatomy of the chordate neural tube. The nerve cords of other deuterostomes might relate to the chordate neural tube at histological, developmental and cellular levels, including the presence of radial glia, layered stratification, retained epithelial features, morphogenesis via folding and formation of a lumen filled with liquid. Recent findings inspire a new view of hypothetical evolutionary scenarios explaining the tubular epithelialized structure of the CNS. One such idea suggests that early neural tubes were key for improved directional olfaction, which was facilitated by the liquid-containing internal cavity. The later separation of the olfactory portion of the tube led to the formation of the independent olfactory and posterior tubular CNS systems in vertebrates. According to an alternative hypothesis, the thick basiepithelial nerve cords could provide deuterostome ancestors with additional biomechanical support, which later improved by turning the basiepithelial cord into a tube filled with liquid - a hydraulic skeleton.

Stem cell homeostasis regulated by hierarchy and neutral competition

Tissue stem cells maintain themselves through self-renewal while constantly supplying differentiating cells. Two distinct models have been proposed as mechanisms of stem cell homeostasis. According to the classical model, there is hierarchy among stem cells, and master stem cells produce stem cells by asymmetric division; whereas, according to the recent model, stem cells are equipotent and neutrally compete. However, the mechanism remains controversial in several tissues and species. Here, we developed a mathematical model linking the two models, named the hierarchical neutral competition (hNC) model. Our theoretical analysis showed that the combination of the hierarchy and neutral competition exhibited bursts in clonal expansion, which was consistent with experimental data of rhesus macaque hematopoiesis. Furthermore, the scaling law in clone size distribution, considered a unique characteristic of the recent model, was satisfied even in the hNC model. Based on the findings above, we proposed the criterion for distinguishing the three models based on experiments.

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.

Non-autonomous regulation of neurogenesis by extrinsic cues: a Drosophila perspective

The formation of a functional circuitry in the central nervous system (CNS) requires the correct number and subtypes of neural cells. In the developing brain, neural stem cells (NSCs) self-renew while giving rise to progenitors that in turn generate differentiated progeny. As such, the size and the diversity of cells that make up the functional CNS depend on the proliferative properties of NSCs. In the fruit fly Drosophila, where the process of neurogenesis has been extensively investigated, extrinsic factors such as the microenvironment of NSCs, nutrients, oxygen levels and systemic signals have been identified as regulators of NSC proliferation. Here, we review decades of work that explores how extrinsic signals non-autonomously regulate key NSC characteristics such as quiescence, proliferation and termination in the fly.

The Fly Blood-Brain Barrier Fights Against Nutritional Stress

In the wild, animals face different challenges including multiple events of food scarcity. How they overcome these conditions is essential for survival. Thus, adaptation mechanisms evolved to allow the development and survival of an organism during nutrient restriction periods. Given the high energy demand of the nervous system, the molecular mechanisms of adaptation to malnutrition are of great relevance to fuel the brain. The blood-brain barrier (BBB) is the interface between the central nervous system (CNS) and the circulatory system. The BBB mediates the transport of macromolecules in and out of the CNS, and therefore, it can buffer changes in nutrient availability. In this review, we collect the current evidence using the fruit fly, Drosophila melanogaster, as a model of the role of the BBB in the adaptation to starvation. We discuss the role of the Drosophila BBB during nutrient deprivation as a potential sensor for circulating nutrients, and transient nutrient storage as a regulator of the CNS neurogenic niche.

Neurogenesis in the adult Drosophila brain

Neurodegenerative diseases such as Alzheimer's and Parkinson's currently affect ∼25 million people worldwide (Erkkinen et al. 2018). The global incidence of traumatic brain injury (TBI) is estimated at ∼70 million/year (Dewan et al. 2018). Both neurodegenerative diseases and TBI remain without effective treatments. We are utilizing adult Drosophila melanogaster to investigate the mechanisms of brain regeneration with the long term goal of identifying targets for neural regenerative therapies. We specifically focused on neurogenesis, i.e. the generation of new cells, as opposed to the regrowth of specific subcellular structures such as axons. Like mammals, Drosophila have few proliferating cells in the adult brain. Nonetheless, within 24 hours of a Penetrating Traumatic Brain Injury (PTBI) to the central brain, there is a significant increase in the number of proliferating cells. We subsequently detect both new glia and new neurons and the formation of new axon tracts that target appropriate brain regions. Glial cells divide rapidly upon injury to give rise to new glial cells. Other cells near the injury site upregulate neural progenitor genes including asense and deadpan and later give rise to the new neurons. Locomotor abnormalities observed after PTBI are reversed within two weeks of injury, supporting the idea that there is functional recovery. Together, these data indicate that adult Drosophila brains are capable of neuronal repair. We anticipate that this paradigm will facilitate the dissection of the mechanisms of neural regeneration and that these processes will be relevant to human brain repair.

The RNA-Binding Protein SBR (Dm NXF1) Is Required for the Constitution of Medulla Boundaries in Drosophila melanogaster Optic Lobes

Drosophila melanogaster sbr (small bristles) is an orthologue of the Nxf1 (nuclear export factor 1) genes in different Opisthokonta. The known function of Nxf1 genes is the export of various mRNAs from the nucleus to the cytoplasm. The cytoplasmic localization of the SBR protein indicates that the nuclear export function is not the only function of this gene in Drosophila. RNA-binding protein SBR enriches the nucleus and cytoplasm of specific neurons and glial cells. In sbr12 mutant males, the disturbance of medulla boundaries correlates with the defects of photoreceptor axons pathfinding, axon bundle individualization, and developmental neurodegeneration. RNA-binding protein SBR participates in processes allowing axons to reach and identify their targets.

Role of nuclear-cytoplasmic protein localization during Drosophila neuroblast development

Nuclear-cytoplasmic localization is an efficient way to regulate transcription factors and chromatin remodelers. Altering the location of existing protein pools also facilitates a more rapid response to changes in cell activity or extracellular signals. There are several examples of proteins that are regulated by nucleo-cytoplasmic shuttling, which are required for Drosophila neuroblast development. Disruption of the localization of homologs of these proteins has also been linked to several neurodegenerative disorders in humans. Drosophila has been used extensively to model the neurodegenerative disorders caused by aberrant nucleo-cytoplasmic localization. Here, we focus on the role of alternative nucleo-cytoplasmic protein localization in regulating proliferation and cell fate decisions in the Drosophila neuroblast and in neurodegenerative disorders. We also explore the analogous role of RNA binding proteins and mRNA localization in the context of regulation of nucleo-cytoplasmic localization during neural development and a role in neurodegenerative disorders.

Nucleolar stress in Drosophila neuroblasts, a model for human ribosomopathies

Different stem cells or progenitor cells display variable threshold requirements for functional ribosomes. This is particularly true for several human ribosomopathies in which select embryonic neural crest cells or adult bone marrow stem cells, but not others, show lethality due to failures in ribosome biogenesis or function (now known as nucleolar stress). To determine if various Drosophila neuroblasts display differential sensitivities to nucleolar stress, we used CRISPR-Cas9 to disrupt the Nopp140 gene that encodes two splice variant ribosome biogenesis factors (RBFs). Disruption of Nopp140 induced nucleolar stress that arrested larvae in the second instar stage. While the majority of larval neuroblasts arrested development, the mushroom body (MB) neuroblasts continued to proliferate as shown by their maintenance of deadpan, a neuroblast-specific transcription factor, and by their continued EdU incorporation. MB neuroblasts in wild-type larvae appeared to contain more fibrillarin and Nopp140 in their nucleoli as compared to other neuroblasts, indicating that MB neuroblasts stockpile RBFs as they proliferate in late embryogenesis while other neuroblasts normally enter quiescence. A greater abundance of Nopp140 encoded by maternal transcripts in Nopp140-/- MB neuroblasts of 1----2-day-old larvae likely rendered these cells more resilient to nucleolar stress.

Single cell transcriptome atlas of the Drosophila larval brain

Cell diversity of the brain and how it is affected by starvation, remains largely unknown. Here, we introduce a single cell transcriptome atlas of the entire Drosophila first instar larval brain. We first assigned cell-type identity based on known marker genes, distinguishing five major groups: neural progenitors, differentiated neurons, glia, undifferentiated neurons and non-neural cells. All major classes were further subdivided into multiple subtypes, revealing biological features of various cell-types. We further assessed transcriptional changes in response to starvation at the single-cell level. While after starvation the composition of the brain remains unaffected, transcriptional profile of several cell clusters changed. Intriguingly, different cell-types show very distinct responses to starvation, suggesting the presence of cell-specific programs for nutrition availability. Establishing a single-cell transcriptome atlas of the larval brain provides a powerful tool to explore cell diversity and assess genetic profiles from developmental, functional and behavioral perspectives.

Nutrient restriction causes reversible G2 arrest in Xenopus neural progenitors

Nutrient status affects brain development; however, the effects of nutrient availability on neural progenitor cell proliferation in vivo are poorly understood. Without food, Xenopus laevis tadpoles enter a period of stasis during which neural progenitor proliferation is drastically reduced, but resumes when food becomes available. Here, we investigate how neural progenitors halt cell division in response to nutrient restriction and subsequently re-enter the cell cycle upon feeding. We demonstrate that nutrient restriction causes neural progenitors to arrest in G2 of the cell cycle with increased DNA content, and that nutrient availability triggers progenitors to re-enter the cell cycle at M phase. Initiation of the nutrient restriction-induced G2 arrest is rapamycin insensitive, but cell cycle re-entry requires mTOR. Finally, we show that activation of insulin receptor signaling is sufficient to increase neural progenitor cell proliferation in the absence of food. A G2 arrest mechanism provides an adaptive strategy to control brain development in response to nutrient availability by triggering a synchronous burst of cell proliferation when nutrients become available. This may be a general cellular mechanism that allows developmental flexibility during times of limited resources.

Expression of Mammalian BM88/CEND1 in Drosophila Affects Nervous System Development by Interfering with Precursor Cell Formation

We used Drosophila melanogaster as an experimental model to express mouse and pig BM88/CEND1 (cell cycle exit and neuronal differentiation 1) in order to investigate its potential functional effects on Drosophila neurogenesis. BM88/CEND1 is a neuron-specific protein whose function is implicated in triggering cells to exit from the cell cycle and differentiate towards a neuronal phenotype. Transgenic flies expressing either mouse or pig BM88/CEND1 in the nervous system had severe neuronal phenotypes with variable expressivity at various stages of embryonic development. In early embryonic stage 10, BM88/CEND1 expression led to an increase in the neural-specific antigenicity of neuroectoderm at the expense of precursor cells [neuroblasts (Nbs) and ganglion mother cells (GMCs)] including the defective formation and differentiation of the MP2 precursors, whereas at later stages (12-15), protein accumulation induced gross morphological defects primarily in the CNS accompanied by a reduction of Nb and GMC markers. Furthermore, the neuronal precursor cells of embryos expressing BM88/CEND1 failed to carry out proper cell-cycle progression as revealed by the disorganized expression patterns of specific cell-cycle markers. BM88/CEND1 accumulation in the Drosophila eye affected normal eye disc development by disrupting the ommatidia. Finally, we demonstrated that expression of BM88/CEND1 modified/reduced the levels of activated MAP kinase indicating a functional effect of BM88/CEND1 on the MAPK signaling pathway. Our findings suggest that the expression of mammalian BM88/CEND1 in Drosophila exerts specific functional effects associated with neuronal precursor cell formation during embryonic neurogenesis and proper eye disc development. This study also validates the use of Drosophila as a powerful model system in which to investigate gene function and the underlying molecular mechanisms.

Histidine is selectively required for the growth of Myc-dependent dedifferentiation tumours in the Drosophila CNS

Rewired metabolism of glutamine in cancer has been well documented, but less is known about other amino acids such as histidine. Here, we use Drosophila cancer models to show that decreasing the concentration of histidine in the diet strongly inhibits the growth of mutant clones induced by loss of Nerfin‐1 or gain of Notch activity. In contrast, changes in dietary histidine have much less effect on the growth of wildtype neural stem cells and Prospero neural tumours. The reliance of tumours on dietary histidine and also on histidine decarboxylase (Hdc) depends upon their growth requirement for Myc. We demonstrate that Myc overexpression in nerfin‐1 tumours is sufficient to switch their mode of growth from histidine/Hdc sensitive to resistant. This study suggests that perturbations in histidine metabolism selectively target neural tumours that grow via a dedifferentiation process involving large cell size increases driven by Myc.

Drosophila nucleostemin 3 is required to maintain larval neuroblast proliferation

Stem cells must maintain proliferation during tissue development, repair and homeostasis, yet avoid tumor formation. In Drosophila, neural stem cells (neuroblasts) maintain proliferation during embryonic and larval development and terminate cell cycle during metamorphosis. An important question for understanding how tissues are generated and maintained is: what regulates stem cell proliferation versus differentiation? We performed a genetic screen which identified nucleostemin 3 (ns3) as a gene required to maintain neuroblast proliferation. ns3 is evolutionarily conserved with yeast and human Lsg1, which encode putative GTPases and are essential for organism growth and viability. We found NS3 is cytoplasmic and it is required to retain the cell cycle repressor Prospero in neuroblast cytoplasm via a Ran-independent pathway. NS3 is also required for proper neuroblast cell polarity and asymmetric cell division. Structure-function analysis further shows that the GTP-binding domain and acidic domain are required for NS3 function in neuroblast proliferation. We conclude NS3 has novel roles in regulating neuroblast cell polarity and proliferation.

aaquetzalli is required for epithelial cell polarity and neural tissue formation in Drosophila

Morphogenetic movements during embryogenesis require dynamic changes in epithelial cell polarity and cytoskeletal reorganization. Such changes involve, among others, rearrangements of cell-cell contacts and protein traffic. In Drosophila melanogaster, neuroblast delamination during early neurogenesis is a well-characterized process requiring a polarized neuroepithelium, regulated by the Notch signaling pathway. Maintenance of epithelial cell polarity ensues proper Notch pathway activation during neurogenesis. We characterize here aaquetzalli (aqz), a gene whose mutations affect cell polarity and nervous system specification. The aqz locus encodes a protein that harbors a domain with significant homology to a proline-rich conserved domain of nuclear receptor co-activators. aqz expression occurs at all stages of the fly life cycle, and is dynamic. aqz mutants are lethal, showing a disruption of cell polarity during embryonic ventral neuroepithelium differentiation resulting in loss of epithelial integrity and mislocalization of membrane proteins (shown by mislocalization of Crumbs, DE-Cadherin, and Delta). As a consequence, aqz mutant embryos with compromised apical-basal cell polarity develop spotty changes of neuronal and epithelial numbers of cells.

Regulation of neuroblast proliferation by surface glia in the Drosophila larval brain

Despite the importance of precisely regulating stem cell division, the molecular basis for this control is still elusive. Here, we show that surface glia in the developing Drosophila brain play essential roles in regulating the proliferation of neural stem cells, neuroblasts (NBs). We found that two classes of extracellular factors, Dally-like (Dlp), a heparan sulfate proteoglycan, and Glass bottom boat (Gbb), a BMP homologue, are required for proper NB proliferation. Interestingly, Dlp expressed in perineural glia (PG), the most outer layer of the surface glia, is responsible for NB proliferation. Consistent with this finding, functional ablation of PG using a dominant-negative form of dynamin showed that PG has an instructive role in regulating NB proliferation. Gbb acts not only as an autocrine proliferation factor in NBs but also as a paracrine survival signal in the PG. We propose that bidirectional communication between NBs and glia through TGF-β signaling influences mutual development of these two cell types. We also discuss the possibility that PG and NBs communicate via direct membrane contact or transcytotic transport of membrane components. Thus, our study shows that the surface glia acts not only as a simple structural insulator but also a dynamic regulator of brain development.

Systemic and local cues drive neural stem cell niche remodelling during neurogenesis in Drosophila

Successful neurogenesis requires adequate proliferation of neural stem cells (NSCs) and their progeny, followed by neuronal differentiation, maturation and survival. NSCs inhabit a complex cellular microenvironment, the niche, which influences their behaviour. To ensure sustained neurogenesis, niche cells must respond to extrinsic, environmental changes whilst fulfilling the intrinsic requirements of the neurogenic program and adapting their roles accordingly. However, very little is known about how different niche cells adjust their properties to such inputs. Here, we show that nutritional and NSC-derived signals induce the remodelling of Drosophila cortex glia, adapting this glial niche to the evolving needs of NSCs. First, nutrition-induced activation of PI3K/Akt drives the cortex glia to expand their membrane processes. Second, when NSCs emerge from quiescence to resume proliferation, they signal to glia to promote membrane remodelling and the formation of a bespoke structure around each NSC lineage. The remodelled glial niche is essential for newborn neuron survival.

Control of nerve cord formation by Engrailed and Gooseberry-Neuro: A multi-step, coordinated process

One way to better understand the molecular mechanisms involved in the construction of a nervous system is to identify the downstream effectors of major regulatory proteins. We previously showed that Engrailed (EN) and Gooseberry-Neuro (GsbN) transcription factors act in partnership to drive the formation of posterior commissures in the central nervous system of Drosophila. In this report, we identified genes regulated by both EN and GsbN through chromatin immunoprecipitation ("ChIP on chip") and transcriptome experiments, combined to a genetic screen relied to the gene dose titration method. The genomic-scale approaches allowed us to define 175 potential targets of EN-GsbN regulation. We chose a subset of these genes to examine ventral nerve cord (VNC) defects and found that half of the mutated targets show clear VNC phenotypes when doubly heterozygous with en or gsbn mutations, or when homozygous. This strategy revealed new groups of genes never described for their implication in the construction of the nerve cord. Their identification suggests that, to construct the nerve cord, EN-GsbN may act at three levels, in: (i) sequential control of the attractive-repulsive signaling that ensures contralateral projection of the commissural axons, (ii) temporal control of the translation of some mRNAs, (iii) regulation of the capability of glial cells to act as commissural guideposts for developing axons. These results illustrate how an early, coordinated transcriptional control may orchestrate the various mechanisms involved in the formation of stereotyped neuronal networks. They also validate the overall strategy to identify genes that play crucial role in axonal pathfinding.

The Notch pathway in the annelid Platynereis: insights into chaetogenesis and neurogenesis processes

Notch is a key signalling pathway playing multiple and varied functions during development. Notch regulates the selection of cells with a neurogenic fate and maintains a pool of yet uncommitted precursors through lateral inhibition, both in insects and in vertebrates. Here, we explore the functions of Notch in the annelid Platynereis dumerilii (Lophotrochozoa). Conserved components of the pathway are identified and a scenario for their evolution in metazoans is proposed. Unexpectedly, neither Notch nor its ligands are expressed in the neurogenic epithelia of the larva at the time when massive neurogenesis begins. Using chemical inhibitors and neural markers, we demonstrate that Notch plays no major role in the general neurogenesis of larvae. Instead, we find Notch components expressed in nascent chaetal sacs, the organs that produce the annelid bristles. Impairing Notch signalling induces defects in chaetal sac formation, abnormalities in chaetae producing cells and a change of identity of chaeta growth accessory cells. This is the first bilaterian species in which the early neurogenesis processes appear to occur without a major involvement of the Notch pathway. Instead, Notch is co-opted to pattern annelid-specific organs, likely through a lateral inhibition process. These features reinforce the view that Notch signalling has been recruited multiple times in evolution due to its remarkable ‘toolkit’ nature.

Drosophila embryonic type II neuroblasts: origin, temporal patterning, and contribution to the adult central complex

Drosophila neuroblasts are an excellent model for investigating how neuronal diversity is generated. Most brain neuroblasts generate a series of ganglion mother cells (GMCs) that each make two neurons (type I lineage), but 16 brain neuroblasts generate a series of intermediate neural progenitors (INPs) that each produce 4-6 GMCs and 8-12 neurons (type II lineage). Thus, type II lineages are similar to primate cortical lineages, and may serve as models for understanding cortical expansion. Yet the origin of type II neuroblasts remains mysterious: do they form in the embryo or larva? If they form in the embryo, do their progeny populate the adult central complex, as do the larval type II neuroblast progeny? Here, we present molecular and clonal data showing that all type II neuroblasts form in the embryo, produce INPs and express known temporal transcription factors. Embryonic type II neuroblasts and INPs undergo quiescence, and produce embryonic-born progeny that contribute to the adult central complex. Our results provide a foundation for investigating the development of the central complex, and tools for characterizing early-born neurons in central complex function.

Identification and functional characterization of muscle satellite cells in Drosophila

Work on genetic model systems such as Drosophila and mouse has shown that the fundamental mechanisms of myogenesis are remarkably similar in vertebrates and invertebrates. Strikingly, however, satellite cells, the adult muscle stem cells that are essential for the regeneration of damaged muscles in vertebrates, have not been reported in invertebrates. In this study, we show that lineal descendants of muscle stem cells are present in adult muscle of Drosophila as small, unfused cells observed at the surface and in close proximity to the mature muscle fibers. Normally quiescent, following muscle fiber injury, we show that these cells express Zfh1 and engage in Notch-Delta-dependent proliferative activity and generate lineal descendant populations, which fuse with the injured muscle fiber. In view of strikingly similar morphological and functional features, we consider these novel cells to be the Drosophila equivalent of vertebrate muscle satellite cells.

Interaction of Spoonbill with Prospero in Drosophila: Implications in neuroblast development

Identification of Spoon as a suppressor of SCA8 associated neurodegeneration provides us a hint about its role in neuronal development and maintenance. However, a detailed molecular characterization of spoon has not yet been reported. Here, we describe spatial expression pattern of Spoon during Drosophila development. Quantitative real time-PCR and fluorescent RNA-RNA in situ hybridization indicate that Spoon is expressed at relatively high levels in larval brain and photoreceptors of eye-antennal discs. Immunostaining reveals that Spoon is subcellularly localized in the cytoplasm and is also membrane bound. Strong expression is also seen in adult ovary and testes. Spoon on immunostaining exhibits unique pattern of expression in larval brain. We observed that Spoon in the neuroblasts colocalizes with Prospero, a transcription factor regulating genes involved in neuroblast self-renewal or cell-cycle control. Co-immunoprecipitation suggests that Spoon and Prospero reside in the same protein complex. Using Drosophila model of SCA8 RNA neuropathy we have also shown that loss of Prospero hinders the suppression of SCA8 associated neurodegeneration by Spoonbill, suggesting Prospero and Spoon might genetically interact and function together. Our study presents Spoon as a novel interacting partner of Prospero and this might be critical in determining the polarized localization of cell fate determinants.

Adult Muscle Formation Requires Drosophila Moleskin for Proliferation of Wing Disc-Associated Muscle Precursors

Adult muscle precursor (AMP) cells located in the notum of the larval wing disc undergo rapid amplification and eventual fusion to generate the Drosophila melanogaster indirect flight muscles (IFMs). Here we find that loss of Moleskin (Msk) function in these wing disc-associated myoblasts reduces the overall AMP pool size, resulting in the absence of IFM formation. This myoblast loss is due to a decrease in the AMP proliferative capacity and is independent of cell death. In contrast, disruption of Msk during pupal myoblast proliferation does not alter the AMP number, suggesting that Msk is specifically required for larval AMP proliferation. It has been previously shown that Wingless (Wg) signaling maintains expression of the Vestigial (Vg) transcription factor in proliferating myoblasts. However, other factors that influence Wg-mediated myoblast proliferation are largely unknown. Here we examine the interactions between Msk and the Wg pathway in regulation of the AMP pool size. We find that a myoblast-specific reduction of Msk results in the absence of Vg expression and a complete loss of the Wg pathway readout β-catenin/Armadillo (Arm). Moreover, msk RNA interference knockdown abolishes expression of the Wg target Ladybird (Lbe) in leg disc myoblasts. Collectively, our results provide strong evidence that Msk acts through the Wg signaling pathway to control myoblast pool size and muscle formation by regulating Arm stability or nuclear transport.

Size Matters: How C. elegans Asymmetric Divisions Regulate Apoptosis

Apoptosis is a form of programmed cell death used by metazoans to eliminate abnormal cells, control cell number, and shape the development of organs. The use of the nematode Caenorhabditis elegans as a model for the study of apoptosis has led to important insights into how cells die and how their corpses are removed. Eighty percent of these apoptotic cell deaths occur during nervous system development and in daughters of neuroblasts that divide asymmetrically. Pioneering work defined a conserved apoptosis pathway that is initiated in C. elegans by the BH3-only protein EGL-1 and that leads to the activation of the caspase CED-3. While the execution of the apoptotic fate is well understood, much less is known about the mechanisms that specify the apoptotic fate of particular cells. In some cells fated to die, this regulation occurs at the level of the egl-1 gene transcription, and investigators have identified several lineage-specific transcription factors that both positively and negatively regulate egl-1. In this review, we focus on a second set of molecules that appear to influence apoptosis by controlling the position of the cleavage plane in divisions that produce apoptotic cells.

Drosophila melanogaster Neuroblasts: A Model for Asymmetric Stem Cell Divisions

Asymmetric cell division (ACD) is a fundamental mechanism to generate cell diversity, giving rise to daughter cells with different developmental potentials. ACD is manifested in the asymmetric segregation of proteins or mRNAs, when the two daughter cells differ in size or are endowed with different potentials to differentiate into a particular cell type (Horvitz and Herskowitz, Cell 68:237-255, 1992). Drosophila neuroblasts, the neural stem cells of the developing fly brain, are an ideal system to study ACD since this system encompasses all of these characteristics. Neuroblasts are intrinsically polarized cells, utilizing polarity cues to orient the mitotic spindle, segregate cell fate determinants asymmetrically, and regulate spindle geometry and physical asymmetry. The neuroblast system has contributed significantly to the elucidation of the basic molecular mechanisms underlying ACD. Recent findings also highlight its usefulness to study basic aspects of stem cell biology and tumor formation. In this review, we will focus on what has been learned about the basic mechanisms underlying ACD in fly neuroblasts.

Isolation, Characterization, and Establishment of Spontaneously Immortalized Cell Line HRPE-2S With Stem Cell Properties

The retinal pigment epithelium is a monolayer of highly specialized pigmented cells located between the neural retina and the Bruch's membrane of the choroid. RPE cells play a crucial role in the maintenance and function of the underlying photoreceptors. This study introduces a spontaneously arising human retinal pigment epithelial cell line, HRPE-2S, which was isolated from primary RPE cell culture of 2 days old male donor. We characterized morphology and functional properties of the new cell line. The immortalized cell line was maintained in culture for more than 70 passages and 240 divisions. The average doubling time of the cells was approximately 22 h and got freezed at 26th passage. The cell line expressed RPE-specific markers RPE65 and cell junction protein ZO1 as an epithelial cell marker. It also expressed CHX10, PAX6, Nestin, SOX2 as stem and retinal progenitor cell markers. Ki67 as a marker of cell proliferation was expressed in all HRPE-2S cells. It represented typical epithelial cobblestone morphology and did not phenotypically change through several passages. Stem cell-like aggregations (neurospheres) were observed in SEM microscopy. The cells represented high mitotic index. They could be viable under hypoxic conditions and serum deprivation. According to functional studies, the cell line exhibited stem cell-like behaviors with particular emphasis on its self-renewal capacity. LDH isoenzymes expression pattern confirmed the same cellular source for both of the HRPE-2S cells and primary RPE cells. Characteristics of HRPE-2S cells promise it as an in vitro model for RPE stem cell-based researches. J. Cell. Physiol. 232: 2626-2640, 2017. © 2016 Wiley Periodicals, Inc.

Programmed cell death acts at different stages of Drosophila neurodevelopment to shape the central nervous system

Nervous system development is a process that integrates cell proliferation, differentiation, and programmed cell death (PCD). PCD is an evolutionary conserved mechanism and a fundamental developmental process by which the final cell number in a nervous system is established. In vertebrates and invertebrates, PCD can be determined intrinsically by cell lineage and age, as well as extrinsically by nutritional, metabolic, and hormonal states. Drosophila has been an instrumental model for understanding how this mechanism is regulated. We review the role of PCD in Drosophila central nervous system development from neural progenitors to neurons, its molecular mechanism and function, how it is regulated and implemented, and how it ultimately shapes the fly central nervous system from the embryo to the adult. Finally, we discuss ideas that emerged while integrating this information.

Glia-derived neurons are required for sex-specific learning in C. elegans

Sex differences in behaviour extend to cognitive-like processes such as learning, but the underlying dimorphisms in neural circuit development and organization that generate these behavioural differences are largely unknown. Here we define at the single-cell level-from development, through neural circuit connectivity, to function-the neural basis of a sex-specific learning in the nematode Caenorhabditis elegans. We show that sexual conditioning, a form of associative learning, requires a pair of male-specific interneurons whose progenitors are fully differentiated glia. These neurons are generated during sexual maturation and incorporated into pre-exisiting sex-shared circuits to couple chemotactic responses to reproductive priorities. Our findings reveal a general role for glia as neural progenitors across metazoan taxa and demonstrate that the addition of sex-specific neuron types to brain circuits during sexual maturation is an important mechanism for the generation of sexually dimorphic plasticity in learning.

The insect central complex as model for heterochronic brain development-background, concepts, and tools

The adult insect brain is composed of neuropils present in most taxa. However, the relative size, shape, and developmental timing differ between species. This diversity of adult insect brain morphology has been extensively described while the genetic mechanisms of brain development are studied predominantly in Drosophila melanogaster. However, it has remained enigmatic what cellular and genetic mechanisms underlie the evolution of neuropil diversity or heterochronic development. In this perspective paper, we propose a novel approach to study these questions. We suggest using genome editing to mark homologous neural cells in the fly D. melanogaster, the beetle Tribolium castaneum, and the Mediterranean field cricket Gryllus bimaculatus to investigate developmental differences leading to brain diversification. One interesting aspect is the heterochrony observed in central complex development. Ancestrally, the central complex is formed during embryogenesis (as in Gryllus) but in Drosophila, it arises during late larval and metamorphic stages. In Tribolium, it forms partially during embryogenesis. Finally, we present tools for brain research in Tribolium including 3D reconstruction and immunohistochemistry data of first instar brains and the generation of transgenic brain imaging lines. Further, we characterize reporter lines labeling the mushroom bodies and reflecting the expression of the neuroblast marker gene Tc-asense, respectively.

From the Eye to the Brain: Development of the Drosophila Visual System

How stem cells produce the huge diversity of neurons that form the visual system, and how these cells are assembled in neural circuits are a critical question in developmental neurobiology. Investigations in Drosophila have led to the discovery of several basic principles of neural patterning. In this chapter, we provide an overview of the field by describing the development of the Drosophila visual system, from the embryo to the adult and from the gross anatomy to the cellular level. We then explore the general molecular mechanisms identified that might apply to other neural structures in flies or in vertebrates. Finally, we discuss the major challenges that remain to be addressed in the field.

Epigenetic modifications in human fragile X pluripotent stem cells; Implications in fragile X syndrome modeling

Patients with fragile X syndrome (FXS) exhibit moderate to severe intellectual disabilities. In addition, one-third of FXS patients show characteristics of autism spectrum disorder. FXS is caused by a trinucleotide repeat expansion, which leads to silencing of the fragile X mental retardation (FMR1) gene. The absence of the FMR1 gene product, FMRP, is the reason for the disease symptoms. It has been suggested that repeat instability and transcription of the FMR1 gene occur during early embryonic development, while after cell differentiation repeats become stable and the FMR1 gene is silent. Epigenetic marks, such as DNA methylation, are associated with gene silencing and repeat stability at the FMR1 locus. However, the mechanisms leading to gene silencing and repeat expansion are still ambiguous, because studies at the human genomic locus were limited until now. The FXS pluripotent stem cells, recently derived from FXS adult cells and FXS blastocysts, are new useful tools to examine these mechanisms at the human endogenous FMR1 locus. This review summarizes the epigenetic features and experimental studies of FXS human embryonic and FXS induced pluripotent stem cells, generated so far. This article is part of a Special Issue entitled SI: Exploiting human neurons.

Molecular mechanisms regulating the defects in fragile X syndrome neurons derived from human pluripotent stem cells

Fragile X syndrome (FXS) is caused by the absence of the fragile X mental retardation protein (FMRP). We have previously generated FXS-induced pluripotent stem cells (iPSCs) from patients' fibroblasts. In this study, we aimed at unraveling the molecular phenotype of the disease. Our data revealed aberrant regulation of neural differentiation and axon guidance genes in FXS-derived neurons, which are regulated by the RE-1 silencing transcription factor (REST). Moreover, we found REST to be elevated in FXS-derived neurons. As FMRP is involved in the microRNA (miRNA) pathway, we employed miRNA-array analyses and uncovered several miRNAs dysregulated in FXS-derived neurons. We found hsa-mir-382 to be downregulated in FXS-derived neurons, and introduction of mimic-mir-382 into these neurons was sufficient to repress REST and upregulate its axon guidance target genes. Our data link FMRP and REST through the miRNA pathway and show a new aspect in the development of FXS.

Transient nuclear Prospero induces neural progenitor quiescence

Stem cells can self-renew, differentiate, or enter quiescence. Understanding how stem cells switch between these states is highly relevant for stem cell-based therapeutics. Drosophila neural progenitors (neuroblasts) have been an excellent model for studying self-renewal and differentiation, but quiescence remains poorly understood. In this study, we show that when neuroblasts enter quiescence, the differentiation factor Prospero is transiently detected in the neuroblast nucleus, followed by the establishment of a unique molecular profile lacking most progenitor and differentiation markers. The pulse of low level nuclear Prospero precedes entry into neuroblast quiescence even when the timing of quiescence is advanced or delayed by changing temporal identity factors. Furthermore, loss of Prospero prevents entry into quiescence, whereas a pulse of low level nuclear Prospero can drive proliferating larval neuroblasts into quiescence. We propose that Prospero levels distinguish three progenitor fates: absent for self-renewal, low for quiescence, and high for differentiation.

DOI:http://dx.doi.org/10.7554/eLife.03363.001

Stem cells provide tissues in the body with a continuing source of new cells, both when the tissues are first developing and when they are growing or repairing in adulthood. A stem cell can divide to create either another stem cell, or a cell that will mature into one of many different cell types.

Neuroblasts are a type of brain stem cell and can divide to create two new cells: another neuroblast that will continue to replicate itself and a cell called a ganglion mother cell that will go on to produce two mature cells for the nervous system. Moreover, when a neuroblast divides, it splits unequally, so that certain molecules end up predominantly in the ganglion mother cell—including a protein called Prospero. Once partitioned into the ganglion mother cell, the Prospero protein enters the nucleus, where it switches off ‘stem cell genes’ and switches on ‘neuron genes’ so the ganglion mother cell can form the mature neurons of the brain. Thus, neuroblasts must keep Prospero out of the nucleus to maintain stem cell properties, whereas ganglion mother cells must move Prospero into the nucleus to form neurons.

Now, Lai and Doe discover a new way that the Prospero protein is used to control stem cell biology. Neuroblasts, like all stem cells, can enter periods where they go dormant or quiescent—that is, they temporarily stop generating ganglion mother cells. By analyzing which proteins are present in neuroblasts during this transition to quiescence, Lai and Doe discovered that the Prospero protein was briefly detected, at low levels, in the nucleus of the neuroblast just before it became dormant.

To see whether this ‘low-level pulse’ of nuclear Prospero is linked to the cell entering a dormant state, Lai and Doe investigated two types of mutant fly in which neuroblasts become dormant either earlier or later than in normal flies. A low-level pulse of nuclear Prospero still precisely matched the start of the dormant state in both mutants. When the Prospero protein was removed altogether, the neuroblasts failed to become dormant, and instead continued dividing.

Lai and Doe propose that different levels of Prospero distinguish three different fates for neuroblasts. Neuroblasts self-replicate when Prospero is kept out of the nucleus, become dormant when exposed to low level nuclear Prospero, and produce the mature cells of the brain when nuclear Prospero levels are high. Exactly how the intermediate levels of nuclear Prospero trigger the dormant state remains a question for future work, as is the question of whether the related mammalian protein, called Prox1, has a similar function.

Understanding how stem cells switch between cell division and quiescence is important for developing effective stem cell-based therapies. It could also help us understand cancer, as cancer cells go through similar periods of inactivity, during which they do not respond to many anti-tumor drugs.

DOI:http://dx.doi.org/10.7554/eLife.03363.002

The RanGEF Bj1 promotes prospero nuclear export and neuroblast self-renewal

Drosophila larval neuroblasts are a model system for studying stem cell self-renewal and differentiation. Here, we report a novel role for the Drosophila gene Bj1 in promoting larval neuroblast self-renewal. Bj1 is the guanine-nucleotide exchange factor for Ran GTPase, which regulates nuclear import/export. Bj1 transcripts are highly enriched in larval brain neuroblasts (in both central brain and optic lobe), while Bj1 protein is detected in both neuroblasts and their neuronal progeny. Loss of Bj1 using both mutants or RNAi causes a progressive loss of larval neuroblasts, showing that Bj1 is required to maintain neuroblast numbers. Loss of Bj1 does not result in neuroblast apoptosis, but rather leads to abnormal nuclear accumulation of the differentiation factor Prospero, and premature neuroblast differentiation. We conclude that the Bj1 RanGEF promotes Prospero nuclear export and neuroblast self-renewal.

Identification of a new stem cell population that generates Drosophila flight muscles

How myoblast populations are regulated for the formation of muscles of different sizes is an essentially unanswered question. The large flight muscles of Drosophila develop from adult muscle progenitor (AMP) cells set-aside embryonically. The thoracic segments are all allotted the same small AMP number, while those associated with the wing-disc proliferate extensively to give rise to over 2500 myoblasts. An initial amplification occurs through symmetric divisions and is followed by a switch to asymmetric divisions in which the AMPs self-renew and generate post-mitotic myoblasts. Notch signaling controls the initial amplification of AMPs, while the switch to asymmetric division additionally requires Wingless, which regulates Numb expression in the AMP lineage. In both cases, the epidermal tissue of the wing imaginal disc acts as a niche expressing the ligands Serrate and Wingless. The disc-associated AMPs are a novel muscle stem cell population that orchestrates the early phases of adult flight muscle development.

Insights into brain development and disease from neurogenetic analyses in Drosophila melanogaster

Groundbreaking work by Obaid Siddiqi has contributed to the powerful genetic toolkit that is now available for studying the nervous system of Drosophila. Studies carried out in this powerful neurogenetic model system during the last decade now provide insight into the molecular mechanisms that operate in neural stem cells during normal brain development and during abnormal brain tumorigenesis. These studies also provide strong support for the notion that conserved molecular genetic programs act in brain development and disease in insects and mammals including humans.

Optix defines a neuroepithelial compartment in the optic lobe of the Drosophila brain

BACKGROUND

During early brain development, the organisation of neural progenitors into a neuroepithelial sheet maintains tissue integrity during growth. Neuroepithelial cohesion and patterning is essential for orderly proliferation and neural fate specification. Neuroepithelia are regionalised by the expression of transcription factors and signalling molecules, resulting in the formation of distinct developmental, and ultimately functional, domains.

RESULTS

We have discovered that the Six3/6 family orthologue Optix is an essential regulator of neuroepithelial maintenance and patterning in the Drosophila brain. Six3 and Six6 are required for mammalian eye and forebrain development, and mutations in humans are associated with severe eye and brain malformation. In Drosophila, Optix is expressed in a sharply defined region of the larval optic lobe, and its expression is reciprocal to that of the transcription factor Vsx1. Optix gain- and loss-of-function affects neuroepithelial adhesion, integrity and polarity. We find restricted cell lineage boundaries that correspond to transcription factor expression domains.

CONCLUSION

We propose that the optic lobe is compartmentalised by expression of Optix and Vsx1. Our findings provide insight into the spatial patterning of a complex region of the brain, and suggest an evolutionarily conserved principle of visual system development.

Spindle orientation: what if it goes wrong?

The angle of cell division is critical in at least two contexts. It can determine cell fate, as it does in developing neural tissue. It can also dictate tissue architecture, as it does in many epithelia. One way to ensure the correct angle of cell division is through controlled orientation of the spindle at metaphase. What happens when that control is lost? Ongoing work suggests that the consequence of metaphase spindle misorientation may be significant, but multiple mechanisms exist to protect the cell and the tissue. We speculate that one such mechanism involves a recently identified anaphase activity for two of the key players at metaphase: NuMA (Mud, LIN-5) and dynein.

Dedifferentiation of neurons precedes tumor formation in Lola mutants

The ability to reprogram differentiated cells into a pluripotent state has revealed that the differentiated state is plastic and reversible. It is evident, therefore, that mechanisms must be in place to maintain cells in a differentiated state. Transcription factors that specify neuronal characteristics have been well studied, but less is known about the mechanisms that prevent neurons from dedifferentiating to a multipotent, stem cell-like state. Here, we identify Lola as a transcription factor that is required to maintain neurons in a differentiated state. We show that Lola represses neural stem cell genes and cell-cycle genes in postmitotic neurons. In lola mutants, neurons dedifferentiate, turn on neural stem cell genes, and begin to divide, forming tumors. Thus, neurons rather than stem cells or intermediate progenitors are the tumor-initiating cells in lola mutants.

Nutritional regulation of stem and progenitor cells in Drosophila

Stem cells and their progenitors are maintained within a microenvironment, termed the niche, through local cell-cell communication. Systemic signals originating outside the niche also affect stem cell and progenitor behavior. This review summarizes studies that pertain to nutritional effects on stem and progenitor cell maintenance and proliferation in Drosophila. Multiple tissue types are discussed that utilize the insulin-related signaling pathway to convey nutritional information either directly to these progenitors or via other cell types within the niche. The concept of systemic control of these cell types is not limited to Drosophila and may be functional in vertebrate systems, including mammals.