Regulation of neuroblast competence in Drosophila

Dan forms condensates in neuroblasts and regulates nuclear architecture and progenitor competence in vivo

Genome organization is thought to underlie cell type specific gene expression, yet how it is regulated in progenitors to produce cellular diversity is unknown. In Drosophila, a developmentally-timed genome reorganization in neural progenitors terminates competence to produce early-born neurons. These events require downregulation of Distal antenna (Dan), part of the conserved pipsqueak DNA-binding superfamily. Here we find that Dan forms liquid-like condensates with high protein mobility, and whose size and subnuclear distribution are balanced with its DNA-binding. Further, we identify a LARKS domain, a structural motif associated with condensate-forming proteins. Deleting just 13 amino acids from LARKS abrogates Dan's ability to retain the early-born neural fate gene, hunchback, in the neuroblast nuclear interior and maintain competence in vivo. Conversely, domain-swapping with LARKS from known phase-separating proteins rescues Dan's effects on competence. Together, we provide in vivo evidence for condensate formation and the regulation of progenitor nuclear architecture underlying neuronal diversification.

Seven-up acts in neuroblasts to specify adult central complex neuron identity and initiate neuroblast decommissioning

An unanswered question in neurobiology is how are diverse neuron cell types generated from a small number of neural stem cells? In the Drosophila larval central brain, there are eight bilateral Type 2 neuroblast (T2NB) lineages that express a suite of early temporal factors followed by a different set of late temporal factors and generate the majority of the central complex (CX) neurons. The early-to-late switch is triggered by the orphan nuclear hormone receptor Seven-up (Svp), yet little is known about how this Svp-dependent switch is involved in specifying CX neuron identities. Here, we: (1) birth date the CX neurons P-EN and P-FN (early and late, respectively); (2) show that Svp is transiently expressed in all early T2NBs; and (3) show that loss of Svp expands the population of early born P-EN neurons at the expense of late born P-FN neurons. Furthermore, in the absence of Svp, T2NBs fail decommissioning and abnormally extend their lineage into week-old adults. We conclude that Svp is required to specify CX neuron identity, as well as to initiate T2NB decommissioning.

Seven-up acts in neuroblasts to specify adult central complex neuron identity and initiate neuroblast decommissioning

An open question in neurobiology is how diverse neuron cell types are generated from a small number of neural stem cells. In the Drosophila larval central brain, there are eight bilateral Type 2 neuroblast (T2NB) lineages that express a suite of early temporal factors followed by a different set of late temporal factors and generate the majority of the central complex (CX) neurons. The early-to-late switch is triggered by the orphan nuclear hormone receptor Seven-up (Svp), yet little is known about this Svp-dependent switch in specifying CX neuron identities. Here, we (i) birthdate the CX neurons P-EN and P-FN (early and late, respectively); (ii) show that Svp is transiently expressed in all early T2NBs; and (iii) show that loss of Svp expands the population of early born P-EN neurons at the expense of late born P-FN neurons. Furthermore, in the absence of Svp, T2NBs fail decommissioning and abnormally extend their lineage into week-old adults. We conclude that Svp is required to specify CX neuron identity, as well as to initiate T2NB decommissioning.

From temporal patterning to neuronal connectivity in Drosophila type I neuroblast lineages

The development of the central nervous system (CNS) in flies and mammals requires the production of distinct neurons in different locations and times. Here we review progress on how Drosophila stem cells (neuroblasts; NBs) generate distinct neurons over time. There are two types of NBs: type I and type II NBs (defined below); here we focus on type I NBs; type II NBs are reviewed elsewhere in this issue. Type I NBs generate neural diversity via the cascading expression of specific temporal transcription factors (TTFs). TTFs are sequentially expressed in neuroblasts and required for the identity of neurons born during each TTF expression window. In this way TTFs specify the "temporal identity" or birth-order dependent identity of neurons. Recent studies have shown that TTF expression in neuroblasts alter the identity of their progeny, including directing motor neurons to form proper connectivity to the proper muscle targets, independent of their birth-order. Similarly, optic lobe (OL) type I NBs express a series of TTFs that promote proper neuron morphology and targeting to the four OL neuropils. Together, these studies demonstrate how temporal identity is crucial in promoting proper circuit assembly within the Drosophila CNS. In addition, TTF orthologs in mouse are good candidates for specifying neuron types in the neocortex and retina. In this review we highlight the recent advances in understanding the role of TTFs in CNS circuit assembly in Drosophila and reflect on the conservation of these mechanisms in mammalian CNS development.

Specification of the Drosophila Orcokinin A neurons by combinatorial coding

The central nervous system contains a daunting number of different cell types. Understanding how each cell acquires its fate remains a major challenge for neurobiology. The developing embryonic ventral nerve cord (VNC) of Drosophila melanogaster has been a powerful model system for unraveling the basic principles of cell fate specification. This pertains specifically to neuropeptide neurons, which typically are stereotypically generated in discrete subsets, allowing for unambiguous single-cell resolution in different genetic contexts. Here, we study the specification of the OrcoA-LA neurons, characterized by the expression of the neuropeptide Orcokinin A and located laterally in the A1-A5 abdominal segments of the VNC. We identified the progenitor neuroblast (NB; NB5-3) and the temporal window (castor/grainyhead) that generate the OrcoA-LA neurons. We also describe the role of the Ubx, abd-A, and Abd-B Hox genes in the segment-specific generation of these neurons. Additionally, our results indicate that the OrcoA-LA neurons are "Notch Off" cells, and neither programmed cell death nor the BMP pathway appears to be involved in their specification. Finally, we performed a targeted genetic screen of 485 genes known to be expressed in the CNS and identified nab, vg, and tsh as crucial determinists for OrcoA-LA neurons. This work provides a new neuropeptidergic model that will allow for addressing new questions related to neuronal specification mechanisms in the future.

Ikaros family proteins redundantly regulate temporal patterning in the developing mouse retina

Temporal identity factors regulate competence of neural progenitors to generate specific cell types in a time-dependent manner, but how they operate remains poorly defined. In the developing mouse retina, the Ikaros zinc-finger transcription factor Ikzf1 regulates production of early-born cell types, except cone photoreceptors. In this study we show that, during early stages of retinal development, another Ikaros family protein, Ikzf4, functions redundantly with Ikzf1 to regulate cone photoreceptor production. Using CUT&RUN and functional assays, we show that Ikzf4 binds and represses genes involved in late-born rod photoreceptor specification, hence favoring cone production. At late stages, when Ikzf1 is no longer expressed in progenitors, we show that Ikzf4 re-localizes to target genes involved in gliogenesis and is required for Müller glia production. We report that Ikzf4 regulates Notch signaling genes and is sufficient to activate the Hes1 promoter through two Ikzf GGAA-binding motifs, suggesting a mechanism by which Ikzf4 may influence gliogenesis. These results uncover a combinatorial role for Ikaros family members during nervous system development and provide mechanistic insights on how they temporally regulate cell fate output.

Developmental origin of the Pair1 descending interneuron

Pair1 is part of a Drosophila larval locomotor circuit that promotes backward locomotion by inhibiting forward locomotion. We hypothesize that lineage related neurons may function in neuronal circuits together. Testing this hypothesis requires knowing the progenitor of each neuron within this locomotor circuit, and here we focus exclusively on Pair1. During Drosophila melanogaster embryogenesis, unique neuroblasts form by inheriting the spatial transcription factors (TFs) expressed in their birth location within the neuroectoderm. We examine the Pair1 neurons using immunofluorescence to determine which neuroblast the Pair1s derive from. Our results show that Pair1 is derived from gnathal neuroblast 5-3 which expresses Gooseberry (Gsb) and Intermediate neuroblasts defective (Ind). When Gsb or Ind were overexpressed in the Pair1 lineage, extra neurons formed with similar Pair1 morphology.

Early embryonic development of Johnston's organ in the antenna of the desert locust Schistocerca gregaria

Johnston's organ has been shown to act as an antennal auditory organ across a spectrum of insect species. In the hemimetabolous desert locust Schistocerca gregaria, Johnston's organ must be functional on hatching and so develops in the pedicellar segment of the antenna during embryogenesis. Here, we employ the epithelial cell marker Lachesin to identify the pedicellar domain of the early embryonic antenna and then triple-label against Lachesin, the mitosis marker phosphohistone-3, and neuron-specific horseradish peroxidase to reveal the sense-organ precursors for Johnston's organ and their lineages. Beginning with a single progenitor at approximately a third of embryogenesis, additional precursors subsequently appear in both the ventral and dorsal pedicellar domains, each generating a lineage or clone. Lineage locations are remarkably conserved across preparations and ages, consistent with the epithelium possessing an underlying topographic coordinate system that determines the cellular organization of Johnston's organ. By mid-embryogenesis, twelve lineages are arranged circumferentially in the pedicel as in the adult structure. Each sense-organ precursor is associated with a smaller mitotically active cell from which the neuronal complement of each clone may derive. Neuron numbers within a clone increase in discrete steps with age and are invariant between clones and across preparations of a given age. At mid-embryogenesis, each clone comprises five cells consolidated into a tightly bound cartridge. A long scolopale extends apically from each cartridge to an insertion point in the epithelium, and bundled axons project basally toward the brain. Comparative data suggest mechanisms that might also regulate the developmental program of Johnston's organ in the locust.

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.

Finding the potency in planarians

Recent developments in the field of planarian stem cell research are discussed, an active and accessible stem cell system that can generate any cell type of the planarian body, to address the question of pluripotency among neoblasts.

The Drosophila homologue of CTIP1 (Bcl11a) and CTIP2 (Bcl11b) regulates neural stem cell temporal patterning

In the developing nervous system, neural stem cells (NSCs) use temporal patterning to generate a wide variety of different neuronal subtypes. In Drosophila, the temporal transcription factors, Hunchback, Kruppel, Pdm and Castor, are sequentially expressed by NSCs to regulate temporal identity during neurogenesis. Here, we identify a new temporal transcription factor that regulates the transition from the Pdm to Castor temporal windows. This factor, which we call Chronophage (or 'time-eater'), is homologous to mammalian CTIP1 (Bcl11a) and CTIP2 (Bcl11b). We show that Chronophage binds upstream of the castor gene and regulates its expression. Consistent with Chronophage promoting a temporal switch, chronophage mutants generate an excess of Pdm-specified neurons and are delayed in generating neurons associated with the Castor temporal window. In addition to promoting the Pdm to Castor transition, Chronophage also represses the production of neurons generated during the earlier Hunchback and Kruppel temporal windows. Genetic interactions with Hunchback and Kruppel indicate that Chronophage regulates NSC competence to generate Hunchback- and Kruppel-specified neurons. Taken together, our results suggest that Chronophage has a conserved role in temporal patterning and neuronal subtype specification.

Transcriptional profiling from whole embryos to single neuroblast lineages in Drosophila

Embryonic development results in the production of distinct tissue types, and different cell types within each tissue. A major goal of developmental biology is to uncover the "parts list" of cell types that comprise each organ. Here we perform single cell RNA sequencing (scRNA-seq) of the Drosophila embryo to identify the genes that characterize different cell and tissue types during development. We assay three different timepoints, revealing a coordinated change in gene expression within each tissue. Interestingly, we find that the elav and Mhc genes, whose protein products are widely used as markers for neurons and muscles, respectively, show broad pan-embryonic expression, indicating the importance of post-transcriptional regulation. We next focus on the central nervous system (CNS), where we identify genes whose expression is enriched at each stage of neuronal differentiation: from neural progenitors, called neuroblasts, to their immediate progeny ganglion mother cells (GMCs), followed by new-born neurons, young neurons, and the most mature neurons. Finally, we ask whether the clonal progeny of a single neuroblast (NB7-1) share a similar transcriptional identity. Surprisingly, we find that clonal identity does not lead to transcriptional clustering, showing that neurons within a lineage are diverse, and that neurons with a similar transcriptional profile (e.g. motor neurons, glia) are distributed among multiple neuroblast lineages. Although each lineage consists of diverse progeny, we were able to identify a previously uncharacterized gene, Fer3, as an excellent marker for the NB7-1 lineage. Within the NB7-1 lineage, neurons which share a temporal identity (e.g. Hunchback, Kruppel, Pdm, and Castor temporal transcription factors in the NB7-1 lineage) have shared transcriptional features, allowing for the identification of candidate novel temporal factors or targets of the temporal transcription factors. In conclusion, we have characterized the embryonic transcriptome for all major tissue types and for three stages of development, as well as the first transcriptomic analysis of a single, identified neuroblast lineage, finding a lineage-enriched transcription factor.

NanoDam identifies Homeobrain (ARX) and Scarecrow (NKX2.1) as conserved temporal factors in the Drosophila central brain and visual system

Temporal patterning of neural progenitors is an evolutionarily conserved strategy for generating neuronal diversity. Type II neural stem cells in the Drosophila central brain produce transit-amplifying intermediate neural progenitors (INPs) that exhibit temporal patterning. However, the known temporal factors cannot account for the neuronal diversity in the adult brain. To search for missing factors, we developed NanoDam, which enables rapid genome-wide profiling of endogenously tagged proteins in vivo with a single genetic cross. Mapping the targets of known temporal transcription factors with NanoDam revealed that Homeobrain and Scarecrow (ARX and NKX2.1 orthologs) are also temporal factors. We show that Homeobrain and Scarecrow define middle-aged and late INP temporal windows and play a role in cellular longevity. Strikingly, Homeobrain and Scarecrow have conserved functions as temporal factors in the developing visual system. NanoDam enables rapid cell-type-specific genome-wide profiling with temporal resolution and is easily adapted for use in higher organisms.

Hunchback prevents notch-induced apoptosis in the serotonergic lineage of Drosophila Melanogaster

The serotonergic lineage (NB7-3) in the Drosophila ventral nerve cord produces six cells during neurogenesis. Four of the cells differentiate into neurons: EW1, EW2, EW3 and GW. The other two cells undergo apoptosis. This simple lineage provides an opportunity to examine genes that are required to induce or repress apoptosis during cell specification. Previous studies have shown that Notch signaling induces apoptosis within the NB7-3 lineage. The three EW neurons are protected from Notch-induced apoptosis by asymmetric distribution of Numb protein, an inhibitor of Notch signaling. In a numb¹ mutant EW2 and EW3 undergo apoptosis. The EW1 and GW neurons survive even in a numb¹ mutant background suggesting that these cells are protected from Notch-induced apoptosis by some factor other than Numb. The EW1 and GW neurons are mitotic sister cells, and uniquely express the transcription factor Hunchback. We present evidence that Hunchback prevents apoptosis in the NB7-3 lineage during normal CNS development and can rescue the two apoptotic cells in the lineage when it is ectopically expressed. We show that hunchback overexpression produces ectopic cells that express markers similar to the EW2 neuron and changes the expression pattern of the EW3 neuron to a EW2 neuron In addition we show that hunchback overexpression can override apoptosis that is genetically induced by the pro-apoptotic genes grim and hid.

A complete temporal transcription factor series in the fly visual system

The brain consists of thousands of neuronal types that are generated by stem cells producing different neuronal types as they age. In Drosophila, this temporal patterning is driven by the successive expression of temporal transcription factors (tTFs)1-6. Here we used single-cell mRNA sequencing to identify the complete series of tTFs that specify most Drosophila optic lobe neurons. We verify that tTFs regulate the progression of the series by activating the next tTF(s) and repressing the previous one(s), and also identify more complex mechanisms of regulation. Moreover, we establish the temporal window of origin and birth order of each neuronal type in the medulla and provide evidence that these tTFs are sufficient to explain the generation of all of the neuronal diversity in this brain region. Finally, we describe the first steps of neuronal differentiation and show that these steps are conserved in humans. We find that terminal differentiation genes, such as neurotransmitter-related genes, are present as transcripts, but not as proteins, in immature larval neurons. This comprehensive analysis of a temporal series of tTFs in the optic lobe offers mechanistic insights into how tTF series are regulated, and how they can lead to the generation of a complete set of neurons.

Enhancer of trithorax/polycomb, Corto, regulates timing of hunchback gene relocation and competence in Drosophila neuroblasts

Background

Neural progenitors produce diverse cells in a stereotyped birth order, but can specify each cell type for only a limited duration. In the Drosophila embryo, neuroblasts (neural progenitors) specify multiple, distinct neurons by sequentially expressing a series of temporal identity transcription factors with each division. Hunchback (Hb), the first of the series, specifies early-born neuronal identity. Neuroblast competence to generate early-born neurons is terminated when the hb gene relocates to the neuroblast nuclear lamina, rendering it refractory to activation in descendent neurons. Mechanisms and trans-acting factors underlying this process are poorly understood. Here we identify Corto, an enhancer of Trithorax/Polycomb (ETP) protein, as a new regulator of neuroblast competence.

Methods

We used the GAL4/UAS system to drive persistent misexpression of Hb in neuroblast 7–1 (NB7-1), a model lineage for which the early competence window has been well characterized, to examine the role of Corto in neuroblast competence. We used immuno-DNA Fluorescence in situ hybridization (DNA FISH) in whole embryos to track the position of the hb gene locus specifically in neuroblasts across developmental time, comparing corto mutants to control embryos. Finally, we used immunostaining in whole embryos to examine Corto’s role in repression of Hb and a known target gene, Abdominal B (Abd-B).

Results

We found that in corto mutants, the hb gene relocation to the neuroblast nuclear lamina is delayed and the early competence window is extended. The delay in gene relocation occurs after hb transcription is already terminated in the neuroblast and is not due to prolonged transcriptional activity. Further, we find that Corto genetically interacts with Posterior Sex Combs (Psc), a core subunit of polycomb group complex 1 (PRC1), to terminate early competence. Loss of Corto does not result in derepression of Hb or its Hox target, Abd-B, specifically in neuroblasts.

Conclusions

These results show that in neuroblasts, Corto genetically interacts with PRC1 to regulate timing of nuclear architecture reorganization and support the model that distinct mechanisms of silencing are implemented in a step-wise fashion during development to regulate cell fate gene expression in neuronal progeny.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13064-022-00159-3.

Sequential addition of neuronal stem cell temporal cohorts generates a feed-forward circuit in the Drosophila larval nerve cord

How circuits self-assemble starting from neuronal stem cells is a fundamental question in developmental neurobiology. Here, we addressed how neurons from different stem cell lineages wire with each other to form a specific circuit motif. In Drosophila larvae, we combined developmental genetics (twin-spot mosaic analysis with a repressible cell marker, multi-color flip out, permanent labeling) with circuit analysis (calcium imaging, connectomics, network science). For many lineages, neuronal progeny are organized into subunits called temporal cohorts. Temporal cohorts are subsets of neurons born within a tight time window that have shared circuit-level function. We find sharp transitions in patterns of input connectivity at temporal cohort boundaries. In addition, we identify a feed-forward circuit that encodes the onset of vibration stimuli. This feed-forward circuit is assembled by preferential connectivity between temporal cohorts from different lineages. Connectivity does not follow the often-cited early-to-early, late-to-late model. Instead, the circuit is formed by sequential addition of temporal cohorts from different lineages, with circuit output neurons born before circuit input neurons. Further, we generate new tools for the fly community. Our data raise the possibility that sequential addition of neurons (with outputs oldest and inputs youngest) could be one fundamental strategy for assembling feed-forward circuits.

Discrete cis-acting element regulates developmentally timed gene-lamina relocation and neural progenitor competence in vivo

The nuclear lamina is typically associated with transcriptional silencing, and peripheral relocation of genes highly correlates with repression. However, the DNA sequences and proteins regulating gene-lamina interactions are largely unknown. Exploiting the developmentally timed hunchback gene movement to the lamina in Drosophila neuroblasts, we identified a 250 bp intronic element (IE) both necessary and sufficient for relocation. The IE can target a reporter transgene to the lamina and silence it. Endogenously, however, hunchback is already repressed prior to relocation. Instead, IE-mediated relocation confers a heritably silenced gene state refractory to activation in descendent neurons, which terminates neuroblast competence to specify early-born identity. Surprisingly, we found that the Polycomb group chromatin factors bind the IE and are required for lamina relocation, revealing a nuclear architectural role distinct from their well-known function in transcriptional repression. Together, our results uncover in vivo mechanisms underlying neuroblast competence and lamina association in heritable gene silencing.

In Drosophila neuroblasts, relocation of the hunchback gene locus to the nuclear lamina confers heritable silencing in daughter neurons. Lucas et al. identify a genomic element necessary and sufficient for hunchback gene movement in vivo. Polycomb proteins target this element for lamina relocation, thereby regulating competence, but not hunchback expression.

Sustained expression of unc-4 homeobox gene and unc-37/Groucho in postmitotic neurons specifies the spatial organization of the cholinergic synapses in C. elegans

Neuronal cell fate determinants establish the identities of neurons by controlling gene expression to regulate neuronal morphology and synaptic connectivity. However, it is not understood if neuronal cell fate determinants have postmitotic functions in synapse pattern formation. Here we identify a novel role for UNC-4 homeobox protein and its corepressor UNC-37/Groucho, in tiled synaptic patterning of the cholinergic motor neurons in Caenorhabditis elegans. We show that unc-4 is not required during neurogenesis but is required in the postmitotic neurons for proper synapse patterning. In contrast, unc-37 is required in both developing and postmitotic neurons. The synaptic tiling defects of unc-4 mutants are suppressed by bar-1/β-catenin mutation, which positively regulates the expression of ceh-12/HB9. Ectopic ceh-12 expression partly underlies the synaptic tiling defects of unc-4 and unc-37 mutants. Our results reveal a novel postmitotic role of neuronal cell fate determinants in synapse pattern formation through inhibiting the canonical Wnt signaling pathway.

Zfh-2 facilitates Notch-induced apoptosis in the CNS and appendages of Drosophila melanogaster

Apoptosis is a fundamental remodeling process for most tissues during development. In this manuscript we examine a pro-apoptotic function for the Drosophila DNA binding protein Zfh-2 during development of the central nervous system (CNS) and appendages. In the CNS we find that a loss-of-function zfh-2 allele gives an overall reduction of apoptotic cells in the CNS, and an altered pattern of expression for the axonal markers 22C10 and FasII. This same loss-of-function zfh-2 allele causes specific cells in the NB7-3 lineage of the CNS that would normally undergo apoptosis to be inappropriately maintained, whereas a gain-of-function zfh-2 allele has the opposite effect, resulting in a loss of normal NB 7-3 progeny. We also demonstrate that Zfh-2 and Hunchback reciprocally repress each other's gene expression which limits apoptosis to later born progeny of the NB7-3 lineage. Apoptosis is also required for proper segmentation of the fly appendages. We find that Zfh-2 co-localizes with apoptotic cells in the folds of the imaginal discs and presumptive cuticular joints. A reduction of Zfh-2 levels with RNAi inhibits expression of the pro-apoptotic gene reaper, and produces abnormal joints in the leg, antenna and haltere. Apoptosis has previously been shown to be activated by Notch signaling in both the NB7-3 CNS lineage and the appendage joints. Our results indicate that Zfh-2 facilitates Notch-induced apoptosis in these structures.

Development of motor circuits: From neuronal stem cells and neuronal diversity to motor circuit assembly

In this review, we discuss motor circuit assembly starting from neuronal stem cells. Until recently, studies of neuronal stem cells focused on how a relatively small pool of stem cells could give rise to a large diversity of different neuronal identities. Historically, neuronal identity has been assayed in embryos by gene expression, gross anatomical features, neurotransmitter expression, and physiological properties. However, these definitions of identity are largely unlinked to mature functional neuronal features relevant to motor circuits. Such mature neuronal features include presynaptic and postsynaptic partnerships, dendrite morphologies, as well as neuronal firing patterns and roles in behavior. This review focuses on recent work that links the specification of neuronal molecular identity in neuronal stem cells to mature, circuit-relevant identity specification. Specifically, these studies begin to address the question: to what extent are the decisions that occur during motor circuit assembly controlled by the same genetic information that generates diverse embryonic neuronal diversity? Much of the research addressing this question has been conducted using the Drosophila larval motor system. Here, we focus largely on Drosophila motor circuits and we point out parallels to other systems. And we highlight outstanding questions in the field. The main concepts addressed in this review are: (1) the description of temporal cohorts-novel units of developmental organization that link neuronal stem cell lineages to motor circuit configuration and (2) the discovery that temporal transcription factors expressed in neuronal stem cells control aspects of circuit assembly by controlling the size of temporal cohorts and influencing synaptic partner choice.

Sequential activation of Notch and Grainyhead gives apoptotic competence to Abdominal-B expressing larval neuroblasts in Drosophila Central nervous system

Neural circuitry for mating and reproduction resides within the terminal segments of central nervous system (CNS) which express Hox paralogous group 9–13 (in vertebrates) or Abdominal-B (Abd-B) in Drosophila. Terminal neuroblasts (NBs) in A8-A10 segments of Drosophila larval CNS are subdivided into two groups based on expression of transcription factor Doublesex (Dsx). While the sex specific fate of Dsx-positive NBs is well investigated, the fate of Dsx-negative NBs is not known so far. Our studies with Dsx-negative NBs suggests that these cells, like their abdominal counterparts (in A3-A7 segments) use Hox, Grainyhead (Grh) and Notch to undergo cell death during larval development. This cell death also happens by transcriptionally activating RHG family of apoptotic genes through a common apoptotic enhancer in early to mid L3 stages. However, unlike abdominal NBs (in A3-A7 segments) which use increasing levels of resident Hox factor Abdominal-A (Abd-A) as an apoptosis trigger, Dsx-negative NBs (in A8-A10 segments) keep the levels of resident Hox factor Abd-B constant. These cells instead utilize increasing levels of the temporal transcription factor Grh and a rise in Notch activity to gain apoptotic competence. Biochemical and in vivo analysis suggest that Abdominal-A and Grh binding motifs in the common apoptotic enhancer also function as Abdominal-B and Grh binding motifs and maintains the enhancer activity in A8-A10 NBs. Finally, the deletion of this enhancer by the CRISPR-Cas9 method blocks the apoptosis of Dsx-negative NBs. These results highlight the fact that Hox dependent NB apoptosis in abdominal and terminal regions utilizes common molecular players (Hox, Grh and Notch), but seems to have evolved different molecular strategies to pattern CNS.

Two major characteristic features of bilaterian organisms are the head to tail axis and a complex central nervous system. The Hox family of transcription factors, which are expressed segmentally along the head to tail axis, plays a critical role in determining both of these features. One of the ways by which Hox factors do this is by mediating differential programmed cell death of the neural stem cells along the head to tail axis of the developing central nervous system, thereby regulating the numerical diversity of the neurons generated along this axis. Our study with a subpopulation of neural stem cells in the most terminal region of the Drosophila larval central nervous system highlights that region-specific Hox-dependent cell death of neural stem cells in abdominal and terminal regions utilizes common molecular players (Hox, Grh and Notch), but seems to have evolved different molecular strategies to pattern the developing central nervous system.

A novel temporal identity window generates alternating Eve+/Nkx6+ motor neuron subtypes in a single progenitor lineage

BACKGROUND

Spatial patterning specifies neural progenitor identity, with further diversity generated by temporal patterning within individual progenitor lineages. In vertebrates, these mechanisms generate "cardinal classes" of neurons that share a transcription factor identity and common morphology. In Drosophila, two cardinal classes are Even-skipped (Eve)+ motor neurons projecting to dorsal longitudinal muscles, and Nkx6+ motor neurons projecting to ventral oblique muscles. Cross-repressive interactions prevent stable double-positive motor neurons. The Drosophila neuroblast 7-1 (NB7-1) lineage uses a temporal transcription factor cascade to generate five distinct Eve+ motor neurons; the origin and development of Nkx6+ motor neurons remains unclear.

METHODS

We use a neuroblast specific Gal4 line, sparse labelling and molecular markers to identify an Nkx6+ VO motor neuron produced by the NB7-1 lineage. We use lineage analysis to birth-date the VO motor neuron to the Kr+ Pdm+ neuroblast temporal identity window. We use gain- and loss-of-function strategies to test the role of Kr+ Pdm+ temporal identity and the Nkx6 transcription factor in specifying VO neuron identity.

RESULTS

Lineage analysis identifies an Nkx6+ neuron born from the Kr+ Pdm+ temporal identity window in the NB7-1 lineage, resulting in alternation of cardinal motor neuron subtypes within this lineage (Eve>Nkx6 > Eve). Co-overexpression of Kr/Pdm generates ectopic VO motor neurons within the NB7-1 lineage - the first evidence that this TTF combination specifies neuronal identity. Moreover, the Kr/Pdm combination promotes Nkx6 expression, which itself is necessary and sufficient for motor neuron targeting to ventral oblique muscles, thereby revealing a molecular specification pathway from temporal patterning to cardinal transcription factor expression to motor neuron target selection.

CONCLUSIONS

We show that one neuroblast lineage generates interleaved cardinal motor neurons fates; that the Kr/Pdm TTFs form a novel temporal identity window that promotes expression of Nkx6; and that the Kr/Pdm > Nkx6 pathway is necessary and sufficient to promote VO motor neuron targeting to the correct ventral muscle group.

Stochasticity and determinism in cell fate decisions

During development, cells need to make decisions about their fate in order to ensure that the correct numbers and types of cells are established at the correct time and place in the embryo. Such cell fate decisions are often classified as deterministic or stochastic. However, although these terms are clearly defined in a mathematical sense, they are sometimes used ambiguously in biological contexts. Here, we provide some suggestions on how to clarify the definitions and usage of the terms stochastic and deterministic in biological experiments. We discuss the frameworks within which such clear definitions make sense and highlight when certain ambiguity prevails. As an example, we examine how these terms are used in studies of neuronal cell fate decisions and point out areas in which definitions and interpretations have changed and matured over time. We hope that this Review will provide some clarification and inspire discussion on the use of terminology in relation to fate decisions.

Tep1 Regulates Yki Activity in Neural Stem Cells in Drosophila Glioma Model

Glioblastoma Multiforme (GBM) is the most common form of malignant brain tumor with poor prognosis. Amplification of Epidermal Growth Factor Receptor (EGFR), and mutations leading to activation of Phosphatidyl-Inositol-3 Kinase (PI3K) pathway are commonly associated with GBM. Using a previously published Drosophila glioma model generated by coactivation of PI3K and EGFR pathways [by downregulation of Pten and overexpression of oncogenic Ras] in glial cells, we showed that the Drosophila Tep1 gene (ortholog of human CD109) regulates Yki (the Drosophila ortholog of human YAP/TAZ) via an evolutionarily conserved mechanism. Oncogenic signaling by the YAP/TAZ pathway occurs in cells that acquire CD109 expression in response to the inflammatory environment induced by radiation in clinically relevant models. Further, downregulation of Tep1 caused a reduction in Yki activity and reduced glioma growth. A key function of Yki in larval CNS is stem cell renewal and formation of neuroblasts. Other reports suggest different upstream regulators of Yki activity in the optic lobe versus the central brain regions of the larval CNS. We hypothesized that Tep1 interacts with the Hippo pathway effector Yki to regulate neuroblast numbers. We tested if Tep1 acts through Yki to affect glioma growth, and if in normal cells Tep1 affects neuroblast number and proliferation. Our data suggests that Tep1 affects Yki mediated stem cell renewal in glioma, as reduction of Tep significantly decreases the number of neuroblasts in glioma. Thus, we identify Tep1-Yki interaction in the larval CNS that plays a key role in glioma growth and progression.

Neuronal upregulation of Prospero protein is driven by alternative mRNA polyadenylation and Syncrip-mediated mRNA stabilisation

During Drosophila and vertebrate brain development, the conserved transcription factor Prospero/Prox1 is an important regulator of the transition between proliferation and differentiation. Prospero level is low in neural stem cells and their immediate progeny, but is upregulated in larval neurons and it is unknown how this process is controlled. Here, we use single molecule fluorescent in situ hybridisation to show that larval neurons selectively transcribe a long prospero mRNA isoform containing a 15 kb 3' untranslated region, which is bound in the brain by the conserved RNA-binding protein Syncrip/hnRNPQ. Syncrip binding increases the stability of the long prospero mRNA isoform, which allows an upregulation of Prospero protein production. Adult flies selectively lacking the long prospero isoform show abnormal behaviour that could result from impaired locomotor or neurological activity. Our findings highlight a regulatory strategy involving alternative polyadenylation followed by differential post-transcriptional regulation.This article has an associated First Person interview with the first author of the paper.

Foxn4 is a temporal identity factor conferring mid/late-early retinal competence and involved in retinal synaptogenesis

During development, neural progenitors change their competence states over time to sequentially generate different types of neurons and glia. Several cascades of temporal transcription factors (tTFs) have been discovered in Drosophila to control the temporal identity of neuroblasts, but the temporal regulation mechanism is poorly understood in vertebrates. Mammalian retinal progenitor cells (RPCs) give rise to several types of neuronal and glial cells following a sequential yet overlapping temporal order. Here, by temporal cluster analysis, RNA-sequencing analysis, and loss-of-function and gain-of-function studies, we show that the Fox domain TF Foxn4 functions as a tTF during retinogenesis to confer RPCs with the competence to generate the mid/late-early cell types: amacrine, horizontal, cone, and rod cells, while suppressing the competence of generating the immediate-early cell type: retinal ganglion cells (RGCs). In early embryonic retinas, Foxn4 inactivation causes down-regulation of photoreceptor marker genes and decreased photoreceptor generation but increased RGC production, whereas its overexpression has the opposite effect. Just as in Drosophila, Foxn4 appears to positively regulate its downstream tTF Casz1 while negatively regulating its upstream tTF Ikzf1. Moreover, retina-specific ablation of Foxn4 reveals that it may be indirectly involved in the synaptogenesis, establishment of laminar structure, visual signal transmission, and long-term maintenance of the retina. Together, our data provide evidence that Foxn4 acts as a tTF to bias RPCs toward the mid/late-early cell fates and identify a missing member of the tTF cascade that controls RPC temporal identities to ensure the generation of proper neuronal diversity in the retina.

Pou2f1 and Pou2f2 cooperate to control the timing of cone photoreceptor production in the developing mouse retina

Multipotent retinal progenitor cells (RPCs) generate various cell types in a precise chronological order, but how exactly cone photoreceptor production is restricted to early stages remains unclear. Here, we show that the POU-homeodomain factors Pou2f1/Pou2f2, the homologs of Drosophila temporal identity factors nub/pdm2, regulate the timely production of cones in mice. Forcing sustained expression of Pou2f1 or Pou2f2 in RPCs expands the period of cone production, whereas misexpression in late-stage RPCs triggers ectopic cone production at the expense of late-born fates. Mechanistically, we report that Pou2f1 induces Pou2f2 expression, which binds to a POU motif in the promoter of the rod-inducing factor Nrl to repress its expression. Conversely, conditional inactivation of Pou2f2 in RPCs increases Nrl expression and reduces cone production. Finally, we provide evidence that Pou2f1 is part of a cross-regulatory cascade with the other temporal identity factors Ikzf1 and Casz1. These results uncover Pou2f1/2 as regulators of the temporal window for cone genesis and, given their widespread expression in the nervous system, raise the possibility of a general role in temporal patterning.

Temporal transcription factors determine circuit membership by permanently altering motor neuron-to-muscle synaptic partnerships

How circuit wiring is specified is a key question in developmental neurobiology. Previously, using the Drosophila motor system as a model, we found the classic temporal transcription factor Hunchback acts in NB7-1 neuronal stem cells to control the number of NB7-1 neuronal progeny form functional synapses on dorsal muscles (Meng et al., 2019). However, it is unknown to what extent control of motor neuron-to-muscle synaptic partnerships is a general feature of temporal transcription factors. Here, we perform additional temporal transcription factor manipulations-prolonging expression of Hunchback in NB3-1, as well as precociously expressing Pdm and Castor in NB7-1. We use confocal microscopy, calcium imaging, and electrophysiology to show that in every manipulation there are permanent alterations in neuromuscular synaptic partnerships. Our data show temporal transcription factors, as a group of molecules, are potent determinants of synaptic partner choice and therefore ultimately control circuit membership.

Drosophila Embryonic CNS Development: Neurogenesis, Gliogenesis, Cell Fate, and Differentiation

The Drosophila embryonic central nervous system (CNS) is a complex organ consisting of ∼15,000 neurons and glia that is generated in ∼1 day of development. For the past 40 years, Drosophila developmental neuroscientists have described each step of CNS development in precise molecular genetic detail. This has led to an understanding of how an intricate nervous system emerges from a single cell. These studies have also provided important, new concepts in developmental biology, and provided an essential model for understanding similar processes in other organisms. In this article, the key genes that guide Drosophila CNS development and how they function is reviewed. Features of CNS development covered in this review are neurogenesis, gliogenesis, cell fate specification, and differentiation.

How prolonged expression of Hunchback, a temporal transcription factor, re-wires locomotor circuits

How circuits assemble starting from stem cells is a fundamental question in developmental neurobiology. We test the hypothesis that, in neuronal stem cells, temporal transcription factors predictably control neuronal terminal features and circuit assembly. Using the Drosophila motor system, we manipulate expression of the classic temporal transcription factor Hunchback (Hb) specifically in the NB7-1 stem cell, which produces U motor neurons (MNs), and then we monitor dendrite morphology and neuromuscular synaptic partnerships. We find that prolonged expression of Hb leads to transient specification of U MN identity, and that embryonic molecular markers do not accurately predict U MN terminal features. Nonetheless, our data show Hb acts as a potent regulator of neuromuscular wiring decisions. These data introduce important refinements to current models, show that molecular information acts early in neurogenesis as a switch to control motor circuit wiring, and provide novel insight into the relationship between stem cell and circuit.

Opposing Gradients of MicroRNA Expression Temporally Pattern Layer Formation in the Developing Neocortex

The precisely timed generation of different neuronal types is a hallmark of development from invertebrates to vertebrates. In the developing mammalian neocortex, neural stem cells change competence over time to sequentially produce six layers of functionally distinct neurons. Here, we report that microRNAs (miRNAs) are dispensable for stem-cell self-renewal and neuron production but essential for timing neocortical layer formation and specifying laminar fates. Specifically, as neurogenesis progresses, stem cells reduce miR-128 expression and miR-9 activity but steadily increase let-7 expression, whereas neurons initially maintain the differences in miRNA expression present at birth. Moreover, miR-128, miR-9, and let-7 are functionally distinct; capable of specifying neurons for layer VI and layer V and layers IV, III, and II, respectively; and transiently altering their relative levels of expression can modulate stem-cell competence in a neurogenic-stage-specific manner to shift neuron production between earlier-born and later-born fates, partly by temporally regulating a neurogenesis program involving Hmga2.

Principles of progenitor temporal patterning in the developing invertebrate and vertebrate nervous system

During the development of the central nervous system, progenitors successively generate distinct types of neurons which assemble into the circuits that underlie our ability to interact with the environment. Spatial and temporal patterning mechanisms are partially evolutionarily conserved processes that allow generation of neuronal diversity from a limited set of progenitors. Here, we review examples of temporal patterning in neuronal progenitors in the Drosophila ventral nerve cord and in the mammalian cerebral cortex. We discuss cell-autonomous mechanisms and environmental influences on the temporal transitions of neuronal progenitors. Identifying the principles controlling the temporal specification of progenitors across species, as highlighted here, may help understand the evolutionary constraints over brain circuit design and function.

The Hunchback temporal transcription factor determines motor neuron axon and dendrite targeting in Drosophila

The generation of neuronal diversity is essential for circuit formation and behavior. Morphological differences in sequentially born neurons could be due to intrinsic molecular identity specified by temporal transcription factors (henceforth called intrinsic temporal identity) or due to changing extrinsic cues. Here, we have used the Drosophila NB7-1 lineage to address this issue. NB7-1 generates the U1-U5 motor neurons sequentially; each has a distinct intrinsic temporal identity due to inheritance of different temporal transcription factors at its time of birth. We show that the U1-U5 neurons project axons sequentially, followed by sequential dendrite extension. We misexpressed the earliest temporal transcription factor, Hunchback, to create 'ectopic' U1 neurons with an early intrinsic temporal identity but later birth-order. These ectopic U1 neurons have axon muscle targeting and dendrite neuropil targeting that are consistent with U1 intrinsic temporal identity, rather than with their time of birth or differentiation. We conclude that intrinsic temporal identity plays a major role in establishing both motor axon muscle targeting and dendritic arbor targeting, which are required for proper motor circuit development.

Neuroblast-specific open chromatin allows the temporal transcription factor, Hunchback, to bind neuroblast-specific loci

Spatial and temporal cues are required to specify neuronal diversity, but how these cues are integrated in neural progenitors remains unknown. Drosophila progenitors (neuroblasts) are a good model: they are individually identifiable with relevant spatial and temporal transcription factors known. Here we test whether spatial/temporal factors act independently or sequentially in neuroblasts. We used Targeted DamID to identify genomic binding sites of the Hunchback temporal factor in two neuroblasts (NB5-6 and NB7-4) that make different progeny. Hunchback targets were different in each neuroblast, ruling out the independent specification model. Moreover, each neuroblast had distinct open chromatin domains, which correlated with differential Hb-bound loci in each neuroblast. Importantly, the Gsb/Pax3 spatial factor, expressed in NB5-6 but not NB7-4, had genomic binding sites correlated with open chromatin in NB5-6, but not NB7-4. Our data support a model in which early-acting spatial factors like Gsb establish neuroblast-specific open chromatin domains, leading to neuroblast-specific temporal factor binding and the production of different neurons in each neuroblast lineage.

The human brain is considered to be the most complicated object in the universe, but it only takes a handful of stem cells to make one. The process depends on two types of information: signals separated across space and time. Spatial cues tell a stem cell what type of cell it is going to be, while temporal cues work as molecular clocks to generate a sequence of different neurons over time. Together, these cues generate the large array of cell types in the nervous system.

Each stem cell occupies its own space in the developing body and receives its own spatial cues, but they all follow the same timeline. For example, proteins called transcription factors act as molecular clocks and interact with specific genes, telling the cell when to turn them on or off. The same series of transcription factors operates in different stem cells, but they have different effects. So far, it has been unclear whether spatial and temporal signals work independently or sequentially to generate new cell types.

To find out, Sen et al. studied two distinct, developing stem cells in fruit flies, which receive different spatial signals. Transcription factors only work if they are able to get to their target genes. Cells can open or close access to different genes by changing the structure of the chromatin wrapping that surrounds the genes. In the experiments, a marker was used to reveal the areas of open chromatin in each of the cells. Another marker was used to track the transcription factors. The results showed that the areas of open chromatin varied between stem cells. Moreover, although both cells used the same transcription factor called Hunchback, it targeted different genes in each stem cell. This was due to changes in the chromatin wrapping: Hunchback only acted in areas where the chromatin was open. This suggests that the spatial cues first sculpt the chromatin, making some genes easier to get to than others. Then, the same transcription factors go to the accessible gene, which will differ from one stem cell to another.

These findings help us to understand how different types of brain cells develop, which may also aid us in finding a way how to engineer specific cell types. If we could turn stem cells into different types of brain cells, it might help us to treat brain diseases. This may involve giving the right spatial signal before starting the temporal cues.

Dysregulation of axogenesis in the antennal nervous system of the embryonic grasshopper Schistocerca gregaria

The antennal nervous system of the grasshopper Schistocerca gregaria features two parallel axon tracts each established early in embryogenesis by discrete pairs of pioneer neurons located at the antennal tip and whose growth cones contact so-called base pioneers en route to the brain. Here we present two antennal phenotypes in which a stereotypic dysregulation of axogenesis in a given tract is observed when only the base pioneer associated with that pathway is missing, consistent with a role for this cell type in guided axogenesis. Dysregulation involves defasciculation and aberrant navigation by pioneer axons resulting in a missing or depleted primordial antennal nerve to the brain. The dysregulated phenotypes reveal that axogenesis in each pathway is regulated independently. Previously unseen discrepancies in the navigational decisions made by pioneer neurons which derive sequentially from the same mother cell demonstrate that these progeny have separate identities. Possible mechanisms for the dysregulated phenotypes are considered.

Temporal Patterning in the Drosophila CNS

A small pool of neural progenitors generates the vast diversity of cell types in the CNS. Spatial patterning specifies progenitor identity, followed by temporal patterning within progenitor lineages to expand neural diversity. Recent work has shown that in Drosophila, all neural progenitors (neuroblasts) sequentially express temporal transcription factors (TTFs) that generate molecular and cellular diversity. Embryonic neuroblasts use a lineage-intrinsic cascade of five TTFs that switch nearly every neuroblast cell division; larval optic lobe neuroblasts also use a rapid cascade of five TTFs, but the factors are completely different. In contrast, larval central brain neuroblasts undergo a major molecular transition midway through larval life, and this transition is regulated by a lineage-extrinsic cue (ecdysone hormone signaling). Overall, every neuroblast lineage uses a TTF cascade to generate diversity, illustrating the widespread importance of temporal patterning.

Temporal Progression of Retinal Progenitor Cell Identity: Implications in Cell Replacement Therapies

Retinal degenerative diseases, which lead to the death of rod and cone photoreceptor cells, are the leading cause of inherited vision loss worldwide. Induced pluripotent or embryonic stem cells (iPSCs/ESCs) have been proposed as a possible source of new photoreceptors to restore vision in these conditions. The proof of concept studies carried out in mouse models of retinal degeneration over the past decade have highlighted several limitations for cell replacement in the retina, such as the low efficiency of cone photoreceptor production from stem cell cultures and the poor integration of grafted cells in the host retina. Current protocols to generate photoreceptors from stem cells are largely based on the use of extracellular factors. Although these factors are essential to induce the retinal progenitor cell (RPC) fate from iPSCs/ESCs, developmental studies have shown that RPCs alter fate output as a function of time (i.e., their temporal identity) to generate the seven major classes of retinal cell types, rather than spatial position. Surprisingly, current stem cell differentiation protocols largely ignore the intrinsic temporal identity of dividing RPCs, which we argue likely explains the low efficiency of cone production in such cultures. In this article, we briefly review the mechanisms regulating temporal identity in RPCs and discuss how they could be exploited to improve cone photoreceptor production for cell replacement therapies.

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.

An intrinsic mechanism controls reactivation of neural stem cells by spindle matrix proteins

The switch between quiescence and proliferation is central for neurogenesis and its alteration is linked to neurodevelopmental disorders such as microcephaly. However, intrinsic mechanisms that reactivate Drosophila larval neural stem cells (NSCs) to exit from quiescence are not well established. Here we show that the spindle matrix complex containing Chromator (Chro) functions as a key intrinsic regulator of NSC reactivation downstream of extrinsic insulin/insulin-like growth factor signalling. Chro also prevents NSCs from re-entering quiescence at later stages. NSC-specific in vivo profiling has identified many downstream targets of Chro, including a temporal transcription factor Grainy head (Grh) and a neural stem cell quiescence-inducing factor Prospero (Pros). We show that spindle matrix proteins promote the expression of Grh and repress that of Pros in NSCs to govern their reactivation. Our data demonstrate that nuclear Chro critically regulates gene expression in NSCs at the transition from quiescence to proliferation.

The spindle matrix proteins, including Chro, are known to regulate mitotic spindle assembly in the cytoplasm. Here the authors show that in Drosophila larval brain, Chro promotes neural stem cell (NSC) reactivation and prevents activated NSCs from entering quiescence, and that Chro carries out such a role by regulating the expression of key transcription factors in the nucleus.

Opportunities lost and gained: Changes in progenitor competence during nervous system development

During development of the central nervous system, a small pool of stem cells and progenitors generate the vast neural diversity required for neural circuit formation and behavior. Neural stem and progenitor cells often generate different progeny in response to the same signaling cue (e.g. Notch or Hedgehog), including no response at all. How does stem cell competence to respond to signaling cues change over time? Recently, epigenetics particularly chromatin remodeling - has emerged as a powerful mechanism to control stem cell competence. Here we review recent Drosophila and vertebrate literature describing the effect of epigenetic changes on neural stem cell competence.

Steroid hormone induction of temporal gene expression in Drosophila brain neuroblasts generates neuronal and glial diversity

An important question in neuroscience is how stem cells generate neuronal diversity. During Drosophila embryonic development, neural stem cells (neuroblasts) sequentially express transcription factors that generate neuronal diversity; regulation of the embryonic temporal transcription factor cascade is lineage-intrinsic. In contrast, larval neuroblasts generate longer ~50 division lineages, and currently only one mid-larval molecular transition is known: Chinmo/Imp/Lin-28+ neuroblasts transition to Syncrip+ neuroblasts. Here we show that the hormone ecdysone is required to down-regulate Chinmo/Imp and activate Syncrip, plus two late neuroblast factors, Broad and E93. We show that Seven-up triggers Chinmo/Imp to Syncrip/Broad/E93 transition by inducing expression of the Ecdysone receptor in mid-larval neuroblasts, rendering them competent to respond to the systemic hormone ecdysone. Importantly, late temporal gene expression is essential for proper neuronal and glial cell type specification. This is the first example of hormonal regulation of temporal factor expression in Drosophila embryonic or larval neural progenitors.

The Hunchback temporal transcription factor establishes, but is not required to maintain, early-born neuronal identity

Background

Drosophila and mammalian neural progenitors typically generate a diverse family of neurons in a stereotyped order. Neuronal diversity can be generated by the sequential expression of temporal transcription factors. In Drosophila, neural progenitors (neuroblasts) sequentially express the temporal transcription factors Hunchback (Hb), Kruppel, Pdm, and Castor. Hb is necessary and sufficient to specify early-born neuronal identity in multiple lineages, and is maintained in the post-mitotic neurons produced during each neuroblast expression window. Surprisingly, nothing is currently known about whether Hb acts in neuroblasts or post-mitotic neurons (or both) to specify first-born neuronal identity.

Methods

Here we selectively remove Hb from post-mitotic neurons, and assay the well-characterized NB7-1 and NB1-1 lineages for defects in neuronal identity and function.

Results

We find that loss of Hb from embryonic and larval post-mitotic neurons does not affect neuronal identity. Furthermore, removing Hb from post-mitotic neurons throughout the entire CNS has no effect on larval locomotor velocity, a sensitive assay for motor neuron and pre-motor neuron function.

Conclusions

We conclude that Hb functions in progenitors (neuroblasts/GMCs) to establish heritable neuronal identity that is maintained by a Hb-independent mechanism.

We suggest that Hb acts in neuroblasts to establish an epigenetic state that is permanently maintained in early-born neurons.

Electronic supplementary material

The online version of this article (doi:10.1186/s13064-017-0078-1) contains supplementary material, which is available to authorized users.

Development of the Neurochemical Architecture of the Central Complex

The central complex represents one of the most conspicuous neuroarchitectures to be found in the insect brain and regulates a wide repertoire of behaviors including locomotion, stridulation, spatial orientation and spatial memory. In this review article, we show that in the grasshopper, a model insect system, the intricate wiring of the fan-shaped body (FB) begins early in embryogenesis when axons from the first progeny of four protocerebral stem cells (called W, X, Y, Z, respectively) in each brain hemisphere establish a set of tracts to the primary commissural system. Decussation of subsets of commissural neurons at stereotypic locations across the brain midline then establishes a columnar neuroarchitecture in the FB which is completed during embryogenesis. Examination of the expression patterns of various neurochemicals in the central complex including neuropeptides, a neurotransmitter and the gas nitric oxide (NO), show that these appear progressively and in a substance-specific manner during embryogenesis. Each neuroactive substance is expressed by neurons located at stereotypic locations in a given central complex lineage, confirming that the stem cells are biochemically multipotent. The organization of axons expressing the various neurochemicals within the central complex is topologically related to the location, and hence birthdate, of the neurons within the lineages. The neurochemical expression patterns within the FB are layered, and so reflect the temporal topology present in the lineages. This principle relates the neuroanatomical to the neurochemical architecture of the central complex and so may provide insights into the development of adaptive behaviors.

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.

Lineage mapping identifies molecular and architectural similarities between the larval and adult Drosophila central nervous system

Neurogenesis in Drosophila occurs in two phases, embryonic and post-embryonic, in which the same set of neuroblasts give rise to the distinct larval and adult nervous systems, respectively. Here, we identified the embryonic neuroblast origin of the adult neuronal lineages in the ventral nervous system via lineage-specific GAL4 lines and molecular markers. Our lineage mapping revealed that neurons born late in the embryonic phase show axonal morphology and transcription factor profiles that are similar to the neurons born post-embryonically from the same neuroblast. Moreover, we identified three thorax-specific neuroblasts not previously characterized and show that HOX genes confine them to the thoracic segments. Two of these, NB2-3 and NB3-4, generate leg motor neurons. The other neuroblast is novel and appears to have arisen recently during insect evolution. Our findings provide a comprehensive view of neurogenesis and show how proliferation of individual neuroblasts is dictated by temporal and spatial cues.

DOI:http://dx.doi.org/10.7554/eLife.13399.001

Fruit flies undergo a process called metamorphosis in which they change from a maggot or larva into an adult fly. These two life stages look and behave differently and appear to have strikingly different nervous systems. The relationship between the two nervous systems has been most extensively studied in the ventral nerve cord (which is the equivalent to the spinal cord in humans). Although the ventral nerve cords of a larva and an adult fly look quite different, they are generated by the same set of stem cells known as neuroblasts. This is made possible because the neuroblasts proliferate in two separate phases: the first phase occurs in the embryo to generate the neurons of the larval nervous system, and the second phase occurs in the larva to generate neurons for the adult’s nervous system.

Now, Lacin and Truman have paired each of the neurons in the adult fruit fly’s nerve cord with their corresponding neurons in the nerve cords of fruit fly larvae. This involved identifying the original neuroblasts that gave rise to each of the neurons in both larval and adult fruit flies. The results suggest that most neurons that arise from a given neuroblast produce a similar set of molecules and extend similar nerve fibers, even though they work in two different nervous systems. Since neuroblasts in non-metamorphosing insects proliferate continuously, these findings also suggest that, when metamorphosis evolved, a pause was introduced to create the two separate phases of proliferation without a big effect on the types of neurons generated.

Lacin and Truman then went on to discover three neuroblasts that appear to be unique to the middle (or thoracic) segments of a fruit fly. The experiments reveal that the presence of these neuroblasts depended on specific genes that control the development of animal body plans. Two of these neuroblasts generate the so-called motor neurons that control the movement of a fly’s legs. Flies only have legs on their thoracic segments, so this indicates that the development of new neurons is coordinated with the development of the body plan at the stem cell level. The third neuroblast generates neurons that connect with the leg motor neurons, and Lacin and Truman propose that this neuroblast arose from a copy of a neighboring stem cell. The resulting extra neurons may have enabled finer control over the leg movements required for activities such as walking and grooming.

Following on from this work, it is now possible to investigate how molecular events that occur from the embryonic to the adult stages of a fruit fly’s life control the formation and function of its nervous system.

DOI:http://dx.doi.org/10.7554/eLife.13399.002

Encoding and decoding time in neural development

The development of a multicellular organism involves time-dependent changes in molecular and cellular states; therefore 'time' is an indispensable mathematical parameter of ontogenesis. Regardless of their inextricable relationship, there is a limited number of events for which the output of developmental phenomena primarily uses temporal cues that are generated through multilevel interactions between molecules, cells, and tissues. In this review, we focus on neural stem cells, which serve as a faithful decoder of temporal cues to transmit biological information and generate specific output in the developing nervous system. We further explore the identity of the temporal information that is encoded in neural development, and how this information is decoded into various cellular fate decisions.