Neuronal cell fate diversification controlled by sub-temporal action of Kruppel

The Hunchback transcription factor determines interneuron molecular identity, morphology, and presynapse targeting in the Drosophila NB5-2 lineage

Interneuron diversity within the central nervous system (CNS) is essential for proper circuit assembly. Functional interneurons must integrate multiple features, including combinatorial transcription factor (TF) expression, axon/dendrite morphology, and connectivity to properly specify interneuronal identity. Yet, how these different interneuron properties are coordinately regulated remains unclear. Here we used the Drosophila neural progenitor, NB5-2, known to generate late-born interneurons in a proprioceptive circuit, to determine if the early-born temporal transcription factor (TTF), Hunchback (Hb), specifies early-born interneuron identity, including molecular profile, axon/dendrite morphology, presynapse targeting, and behavior. We found that prolonged Hb expression in NB5-2 increases the number of neurons expressing early-born TFs (Nervy, Nkx6, and Dbx) at the expense of late-born TFs (Runt and Zfh2); thus, Hb is sufficient to promote interneuron molecular identity. Hb is also sufficient to transform late-born neuronal morphology to early-born neuronal morphology. Furthermore, prolonged Hb promotes the relocation of late-born neuronal presynapses to early-born neuronal presynapse neuropil locations, consistent with a change in interneuron connectivity. Finally, we found that prolonged Hb expression led to defects in proprioceptive behavior, consistent with a failure to properly specify late-born interneurons in the proprioceptive circuit. We conclude that the Hb TTF is sufficient to specify multiple aspects of early-born interneuron identity, as well as disrupt late-born proprioceptive neuron function.

The Hunchback temporal transcription factor determines interneuron molecular identity, morphology, and presynapse targeting in the Drosophila NB5-2 lineage

Interneuron diversity within the central nervous system (CNS) is essential for proper circuit assembly. Functional interneurons must integrate multiple features, including combinatorial transcription factor (TF) expression, axon/dendrite morphology, and connectivity to properly specify interneuronal identity. Yet, how these different interneuron properties are coordinately regulated remains unclear. Here we used the Drosophila neural progenitor, NB5-2, known to generate late-born interneurons in a proprioceptive circuit, to determine if the early-born temporal transcription factor (TTF), Hunchback (Hb), specifies early-born interneuron identity, including molecular profile, axon/dendrite morphology, and presynapse targeting. We found that prolonged Hb expression in NB5-2 increases the number of neurons expressing early-born TFs (Nervy, Nkx6, and Dbx) at the expense of late-born TFs (Runt and Zfh2); thus, Hb is sufficient to promote interneuron molecular identity. Hb is also sufficient to transform late-born neuronal morphology to early-born neuronal morphology. Furthermore, prolonged Hb promotes the relocation of late-born neuronal presynapses to early-born neuronal presynapse neuropil locations, consistent with a change in interneuron connectivity. Finally, we found that prolonged Hb expression led to defects in proprioceptive behavior, consistent with a failure to properly specify late-born interneurons in the proprioceptive circuit. We conclude that the Hb TTF is sufficient to specify multiple aspects of early-born interneuron identity, as well as disrupt late-born proprioceptive neuron function.

Loss of CASZ1 tumor suppressor linked to oncogenic subversion of neuroblastoma core regulatory circuitry

The neural crest lineage regulatory transcription factors (TFs) form a core regulatory circuitry (CRC) in neuroblastoma (NB) to specify a noradrenergic tumor phenotype. Oncogenic subversion of CRC TFs is well documented, but the role of loss of tumor suppressors plays remains unclear. Zinc-finger TF CASZ1 is a chromosome 1p36 (chr1p36) tumor suppressor. Single-cell RNA sequencing data analyses indicate that CASZ1 is highly expressed in developing chromaffin cells coincident with an expression of NB CRC TFs. In NB tumor cells, the CASZ1 tumor suppressor is silenced while CRC components are highly expressed. We find the NB CRC component HAND2 directly represses CASZ1 expression. ChIP-seq and transcriptomic analyses reveal that restoration of CASZ1 upregulates noradrenergic neuronal genes and represses expression of CRC components by remodeling enhancer activity. Our study identifies that the restored CASZ1 forms a negative feedback regulatory circuit with the established NB CRC to induce noradrenergic neuronal differentiation of NB.

Selective role of the DNA helicase Mcm5 in BMP retrograde signaling during Drosophila neuronal differentiation

The MCM2-7 complex is a highly conserved hetero-hexameric protein complex, critical for DNA unwinding at the replicative fork during DNA replication. Overexpression or mutation in MCM2-7 genes is linked to and may drive several cancer types in humans. In mice, mutations in MCM2-7 genes result in growth retardation and mortality. All six MCM2-7 genes are also expressed in the developing mouse CNS, but their role in the CNS is not clear. Here, we use the central nervous system (CNS) of Drosophila melanogaster to begin addressing the role of the MCM complex during development, focusing on the specification of a well-studied neuropeptide expressing neuron: the Tv4/FMRFa neuron. In a search for genes involved in the specification of the Tv4/FMRFa neuron we identified Mcm5 and find that it plays a highly specific role in the specification of the Tv4/FMRFa neuron. We find that other components of the MCM2-7 complex phenocopies Mcm5, indicating that the role of Mcm5 in neuronal subtype specification involves the MCM2-7 complex. Surprisingly, we find no evidence of reduced progenitor proliferation, and instead find that Mcm5 is required for the expression of the type I BMP receptor Tkv, which is critical for the FMRFa expression. These results suggest that the MCM2-7 complex may play roles during CNS development outside of its well-established role during DNA replication.

The MCM2-7 complex plays a critical role in the DNA replication allowing cells to progress throughout the cell cycle and divide. Overexpression or mutation in MCM2-7 genes is linked to and may drive several cancer types in humans. While MCM2-7 complex is widely expressed in the central nervous system (CNS) during development, its role is not yet clear. Here, we use the CNS of Drosophila melanogaster to address the role of the MCM complex, focusing on the specification of a well-studied neuropeptide expressing neuron: the Tv4/FMRFa neuron. We identified that Mcm5 plays a highly specific role in the specification of this neuron, and it involves other components of the MCM2-7 complex. Despite the described importance of this complex on DNA replication, we find no evidence of reduced progenitor proliferation, and instead we find that Mcm5 is required for the expression of the type I BMP receptor Tkv, which is critical for the specification of the Tv4/FMRFa neuron. These results suggest that the MCM2-7 complex may play roles during CNS development outside of its well-established role during DNA replication.

Integrated Patterning Programs During Drosophila Development Generate the Diversity of Neurons and Control Their Mature Properties

During the approximately 5 days of Drosophila neurogenesis (late embryogenesis to the beginning of pupation), a limited number of neural stem cells produce approximately 200,000 neurons comprising hundreds of cell types. To build a functional nervous system, neuronal types need to be produced in the proper places, appropriate numbers, and correct times. We discuss how neural stem cells (neuroblasts) obtain so-called area codes for their positions in the nervous system (spatial patterning) and how they keep time to sequentially produce neurons with unique fates (temporal patterning). We focus on specific examples that demonstrate how a relatively simple patterning system (Notch) can be used reiteratively to generate different neuronal types. We also speculate on how different modes of temporal patterning that operate over short versus long time periods might be linked. We end by discussing how specification programs are integrated and lead to the terminal features of different neuronal types.

CASZ1 induces skeletal muscle and rhabdomyosarcoma differentiation through a feed-forward loop with MYOD and MYOG

Embryonal rhabdomyosarcoma (ERMS) is a childhood cancer that expresses myogenic master regulatory factor MYOD but fails to differentiate. Here, we show that the zinc finger transcription factor CASZ1 up-regulates MYOD signature genes and induces skeletal muscle differentiation in normal myoblasts and ERMS. The oncogenic activation of the RAS-MEK pathway suppresses CASZ1 expression in ERMS. ChIP-seq, ATAC-seq and RNA-seq experiments reveal that CASZ1 directly up-regulates skeletal muscle genes and represses non-muscle genes through affecting regional epigenetic modifications, chromatin accessibility and super-enhancer establishment. Next generation sequencing of primary RMS tumors identified a single nucleotide variant in the CASZ1 coding region that potentially contributes to ERMS tumorigenesis. Taken together, loss of CASZ1 activity, due to RAS-MEK signaling or genetic alteration, impairs ERMS differentiation, contributing to RMS tumorigenesis.

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.

Branching gene regulatory network dictating different aspects of a neuronal cell identity

The nervous system displays a daunting cellular diversity. Neuronal sub-types differ from each other in several aspects, including their neurotransmitter expression and axon projection. These aspects can converge, but can also diverge, such that neurons expressing the same neurotransmitter may project axons to different targets. It is not well understood how regulatory programs converge/diverge to associate/dissociate different cell fate features. Studies of the Drosophila Tv1 neurons have identified a regulatory cascade; ladybird early -> collier -> apterous/eyes absent -> dimmed, which specifies Tv1 neurotransmitter expression. Here, we conduct genetic and transcriptome analysis to address how other aspects of Tv1 cell fate is governed. We find that an initiator terminal selector gene triggers a feedforward loop which branches into different subroutines, each of which establishes different features of this one unique neuronal cell fate.

Temporal control of Drosophila central nervous system development

A complex nervous system requires precise numbers of various neuronal types produced with exquisite spatiotemporal control. This striking diversity is generated by a limited number of neural stem cells (NSC), where spatial and temporal patterning intersect. Drosophila is a genetically tractable model system that has significant advantages for studying stem cell biology and neuronal fate specification. Here we review the latest findings in the rich literature of temporal patterning of neuronal identity in the Drosophila central nervous system. Rapidly changing consecutive transcription factors expressed in NSCs specify short series of neurons with considerable differences. More slowly progressing changes are orchestrated by NSC intrinsic temporal factor gradients which integrate extrinsic signals to coordinate nervous system and organismal development.

Specification of Drosophila neuropeptidergic neurons by the splicing component brr2

During embryonic development, a number of genetic cues act to generate neuronal diversity. While intrinsic transcriptional cascades are well-known to control neuronal sub-type cell fate, the target cells can also provide critical input to specific neuronal cell fates. Such signals, denoted retrograde signals, are known to provide critical survival cues for neurons, but have also been found to trigger terminal differentiation of neurons. One salient example of such target-derived instructive signals pertains to the specification of the Drosophila FMRFamide neuropeptide neurons, the Tv4 neurons of the ventral nerve cord. Tv4 neurons receive a BMP signal from their target cells, which acts as the final trigger to activate the FMRFa gene. A recent FMRFa-eGFP genetic screen identified several genes involved in Tv4 specification, two of which encode components of the U5 subunit of the spliceosome: brr2 (l(3)72Ab) and Prp8. In this study, we focus on the role of RNA processing during target-derived signaling. We found that brr2 and Prp8 play crucial roles in controlling the expression of the FMRFa neuropeptide specifically in six neurons of the VNC (Tv4 neurons). Detailed analysis of brr2 revealed that this control is executed by two independent mechanisms, both of which are required for the activation of the BMP retrograde signaling pathway in Tv4 neurons: (1) Proper axonal pathfinding to the target tissue in order to receive the BMP ligand. (2) Proper RNA splicing of two genes in the BMP pathway: the thickveins (tkv) gene, encoding a BMP receptor subunit, and the Medea gene, encoding a co-Smad. These results reveal involvement of specific RNA processing in diversifying neuronal identity within the central nervous system.

The nervous system displays daunting cellular diversity, largely generated through complex regulatory input operating on stem cells and their neural lineages during development. Most of the reported mechanisms acting to generate neural diversity pertain to transcriptional regulation. In contrast, little is known regarding the post-transcriptional mechanisms involved. Here, we use a specific group of neurons, Apterous neurons, in the ventral nerve cord of Drosophila melanogaster as our model, to analyze the function of two essential components of the spliceosome; Brr2 and Prp8. Apterous neurons require a BMP retrograde signal for terminal differentiation, and we find that brr2 and Prp8 play crucial roles during this process. brr2 is critical for two independent events; axon pathfinding and BMP signaling, both of which are required for the activation of the retrograde signaling pathway necessary for Apterous neurons. These results identify a post-transcriptional mechanism as key for specifying neuronal identity, by ensuring the execution of a retrograde signal.

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.

Playing Well with Others: Extrinsic Cues Regulate Neural Progenitor Temporal Identity to Generate Neuronal Diversity

During neurogenesis, vertebrate and Drosophila progenitors change over time as they generate a diverse population of neurons and glia. Vertebrate neural progenitors have long been known to use both progenitor-intrinsic and progenitor-extrinsic cues to regulate temporal patterning. In contrast, virtually all temporal patterning mechanisms discovered in Drosophila neural progenitors (neuroblasts) involve progenitor-intrinsic temporal transcription factor cascades. Recent results, however, have revealed several extrinsic pathways that regulate Drosophila neuroblast temporal patterning: nutritional cues regulate the timing of neuroblast proliferation/quiescence and a steroid hormone cue that is required for temporal transcription factor expression. Here, we discuss newly discovered extrinsic cues regulating neural progenitor temporal identity in Drosophila, highlight conserved mechanisms, and raise open questions for the future.

Neuronal cell fate specification by the molecular convergence of different spatio-temporal cues on a common initiator terminal selector gene

The extensive genetic regulatory flows underlying specification of different neuronal subtypes are not well understood at the molecular level. The Nplp1 neuropeptide neurons in the developing Drosophila nerve cord belong to two sub-classes; Tv1 and dAp neurons, generated by two distinct progenitors. Nplp1 neurons are specified by spatial cues; the Hox homeotic network and GATA factor grn, and temporal cues; the hb -> Kr -> Pdm -> cas -> grh temporal cascade. These spatio-temporal cues combine into two distinct codes; one for Tv1 and one for dAp neurons that activate a common terminal selector feedforward cascade of col -> ap/eya -> dimm -> Nplp1. Here, we molecularly decode the specification of Nplp1 neurons, and find that the cis-regulatory organization of col functions as an integratory node for the different spatio-temporal combinatorial codes. These findings may provide a logical framework for addressing spatio-temporal control of neuronal sub-type specification in other systems.

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.

Anterior-Posterior Gradient in Neural Stem and Daughter Cell Proliferation Governed by Spatial and Temporal Hox Control

A readily evident feature of animal central nervous systems (CNSs), apparent in all vertebrates and many invertebrates alike, is its "wedge-like" appearance, with more cells generated in anterior than posterior regions. This wedge could conceivably be established by an antero-posterior (A-P) gradient in the number of neural progenitor cells, their proliferation behaviors, and/or programmed cell death (PCD). However, the contribution of each of these mechanisms, and the underlying genetic programs, are not well understood. Building upon recent progress in the Drosophila melanogaster (Drosophila) ventral nerve cord (VNC), we address these issues in a comprehensive manner. We find that, although PCD plays a role in controlling cell numbers along the A-P axis, the main driver of the wedge is a gradient of daughter proliferation, with divisions directly generating neurons (type 0) being more prevalent posteriorly and dividing daughters (type I) more prevalent anteriorly. In addition, neural progenitor (NB) cell-cycle exit occurs earlier posteriorly. The gradient of type I > 0 daughter proliferation switch and NB exit combine to generate radically different average lineage sizes along the A-P axis, differing by more than 3-fold in cell number. We find that the Hox homeotic genes, expressed in overlapping A-P gradients and with a late temporal onset in NBs, trigger the type I > 0 daughter proliferation switch and NB exit. Given the highly evolutionarily conserved expression of overlapping Hox homeotic genes in the CNS, our results point to a common mechanism for generating the CNS wedge.

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.

Re-utilization of a transcription factor

The temporal transcription factor Krüppel has a dual role in the development of neurons.