An intersectional gene regulatory strategy defines subclass diversity of C. elegans motor neurons

The Iroquois (Iro/Irx) homeobox genes are conserved Hox targets involved in motor neuron development

The Iroquois (Iro/Irx) homeobox genes encode transcription factors with fundamental roles in animal development. Despite their link to various congenital conditions in humans, our understanding of Iro/Irx gene expression, function, and regulation remains incomplete. Here, we conducted a systematic expression analysis of all six mouse Irx genes in the embryonic spinal cord. We found five Irx genes (Irx1, Irx2, Irx3, Irx5, and Irx6) to be confined mostly to ventral spinal domains, offering new molecular markers for specific groups of post-mitotic motor neurons (MNs). Further, we engineered Irx2, Irx5, and Irx6 mouse mutants and uncovered essential but distinct roles for Irx2 and Irx6 in MN development. Last, we found that the highly conserved regulators of MN development across species, the HOX proteins, directly control Irx gene expression both in mouse and C. elegans MNs, critically expanding the repertoire of HOX target genes in the developing nervous system. Altogether, our study provides important insights into Iro/Irx expression and function in the developing spinal cord, and uncovers an ancient gene regulatory relationship between HOX and Iro/Irx genes.

A molecular atlas of adult C. elegans motor neurons reveals ancient diversity delineated by conserved transcription factor codes

Motor neurons (MNs) constitute an ancient cell type targeted by multiple adult-onset diseases. It is therefore important to define the molecular makeup of adult MNs in animal models and extract organizing principles. Here, we generate a comprehensive molecular atlas of adult Caenorhabditis elegans MNs and a searchable database. Single-cell RNA sequencing of 13,200 cells reveals that ventral nerve cord MNs cluster into 29 molecularly distinct subclasses. Extending C. elegans Neuronal Gene Expression Map and Network (CeNGEN) findings, all MN subclasses are delineated by distinct expression codes of either neuropeptide or transcription factor gene families. Strikingly, combinatorial codes of homeodomain transcription factor genes succinctly delineate adult MN diversity in both C. elegans and mice. Further, molecularly defined MN subclasses in C. elegans display distinct patterns of connectivity. Hence, our study couples the connectivity map of the C. elegans motor circuit with a molecular atlas of its constituent MNs and uncovers organizing principles and conserved molecular codes of adult MN diversity.

Maintenance of neuronal identity in C. elegans and beyond: Lessons from transcription and chromatin factors

Neurons are remarkably long-lived, non-dividing cells that must maintain their functional features (e.g., electrical properties, chemical signaling) for extended periods of time - decades in humans. How neurons accomplish this incredible feat is poorly understood. Here, we review recent advances, primarily in the nematode C. elegans, that have enhanced our understanding of the molecular mechanisms that enable post-mitotic neurons to maintain their functionality across different life stages. We begin with "terminal selectors" - transcription factors necessary for the establishment and maintenance of neuronal identity. We highlight new findings on five terminal selectors (CHE-1 [Glass], UNC-3 [Collier/Ebf1-4], LIN-39 [Scr/Dfd/Hox4-5], UNC-86 [Acj6/Brn3a-c], AST-1 [Etv1/ER81]) from different transcription factor families (ZNF, COE, HOX, POU, ETS). We compare the functions of these factors in specific neuron types of C. elegans with the actions of their orthologs in other invertebrate (D. melanogaster) and vertebrate (M. musculus) systems, highlighting remarkable functional conservation. Finally, we reflect on recent findings implicating chromatin-modifying proteins, such as histone methyltransferases and Polycomb proteins, in the control of neuronal terminal identity. Altogether, these new studies on transcription factors and chromatin modifiers not only shed light on the fundamental problem of neuronal identity maintenance, but also outline mechanistic principles of gene regulation that may operate in other long-lived, post-mitotic cell types.

Hox gene functions in the C. elegans nervous system: From early patterning to maintenance of neuronal identity

The nervous system emerges from a series of genetic programs that generate a remarkable array of neuronal cell types. Each cell type must acquire a distinct anatomical position, morphology, and function, enabling the generation of specialized circuits that drive animal behavior. How are these diverse cell types and circuits patterned along the anterior-posterior (A-P) axis of the animal body? Hox genes encode transcription factors that regulate cell fate and patterning events along the A-P axis of the nervous system. While most of our understanding of Hox-mediated control of neuronal development stems from studies in segmented animals like flies, mice, and zebrafish, important new themes are emerging from work in a non-segmented animal: the nematode Caenorhabditis elegans. Studies in C. elegans support the idea that Hox genes are needed continuously and across different life stages in the nervous system; they are not only required in dividing progenitor cells, but also in post-mitotic neurons during development and adult life. In C. elegans embryos and young larvae, Hox genes control progenitor cell specification, cell survival, and neuronal migration, consistent with their neural patterning roles in other animals. In late larvae and adults, C. elegans Hox genes control neuron type-specific identity features critical for neuronal function, thereby extending the Hox functional repertoire beyond early patterning. Here, we provide a comprehensive review of Hox studies in the C. elegans nervous system. To relate to readers outside the C. elegans community, we highlight conserved roles of Hox genes in patterning the nervous system of invertebrate and vertebrate animals. We end by calling attention to new functions in adult post-mitotic neurons for these paradigmatic regulators of cell fate.

Nuclear factor Y is a pervasive regulator of neuronal gene expression

Nervous system function relies on the establishment of complex gene expression programs that provide neuron-type-specific and core pan-neuronal features. These complementary regulatory paradigms are controlled by terminal selector and parallel-acting transcription factors (TFs), respectively. Here, we identify the nuclear factor Y (NF-Y) TF as a pervasive direct and indirect regulator of both neuron-type-specific and pan-neuronal gene expression. Mapping global NF-Y targets reveals direct binding to the cis-regulatory regions of pan-neuronal genes and terminal selector TFs. We show that NFYA-1 controls pan-neuronal gene expression directly through binding to CCAAT boxes in target gene promoters and indirectly by regulating the expression of terminal selector TFs. Further, we find that NFYA-1 regulation of neuronal gene expression is important for neuronal activity and motor function. Thus, our research sheds light on how global neuronal gene expression programs are buffered through direct and indirect regulatory mechanisms.

A molecular atlas of adult C. elegans motor neurons reveals ancient diversity delineated by conserved transcription factor codes

Motor neurons (MNs) constitute an ancient cell type targeted by multiple adult-onset diseases. It is therefore important to define the molecular makeup of adult MNs in animal models and extract organizing principles. Here, we generated a comprehensive molecular atlas of adult Caenorhabditis elegans MNs and a searchable database (http://celegans.spinalcordatlas.org). Single-cell RNA-sequencing of 13,200 cells revealed that ventral nerve cord MNs cluster into 29 molecularly distinct subclasses. All subclasses are delineated by unique expression codes of either neuropeptide or transcription factor gene families. Strikingly, we found that combinatorial codes of homeodomain transcription factor genes define adult MN diversity both in C. elegans and mice. Further, molecularly defined MN subclasses in C. elegans display distinct patterns of connectivity. Hence, our study couples the connectivity map of the C. elegans motor circuit with a molecular atlas of its constituent MNs, and uncovers organizing principles and conserved molecular codes of adult MN diversity.

Cell context-dependent CFI-1/ARID3 functions control neuronal terminal differentiation

AT-rich interaction domain 3 (ARID3) transcription factors are expressed in the nervous system, but their mechanisms of action are largely unknown. Here, we provide, in vivo, a genome-wide binding map for CFI-1, the sole C. elegans ARID3 ortholog. We identify 6,396 protein-coding genes as putative direct targets of CFI-1, most of which encode neuronal terminal differentiation markers. In head sensory neurons, CFI-1 directly activates multiple terminal differentiation genes, thereby acting as a terminal selector. In motor neurons, however, CFI-1 acts as a direct repressor, continuously antagonizing three transcriptional activators. By focusing on the glr-4/GRIK4 glutamate receptor locus, we identify proximal CFI-1 binding sites and histone methyltransferase activity as necessary for glr-4 repression. Rescue assays reveal functional redundancy between core and extended DNA-binding ARID domains and a strict requirement for REKLES, the ARID3 oligomerization domain. Altogether, this study uncovers cell-context-dependent mechanisms through which a single ARID3 protein controls the terminal differentiation of distinct neuron types.

Transcriptome age of individual cell types in Caenorhabditis elegans

The phylotranscriptomic analysis of development in several species revealed the expression of older and more conserved genes in midembryonic stages and younger and more divergent genes in early and late embryonic stages, which supported the hourglass mode of development. However, previous work only studied the transcriptome age of whole embryos or embryonic sublineages, leaving the cellular basis of the hourglass pattern and the variation of transcriptome ages among cell types unexplored. By analyzing both bulk and single-cell transcriptomic data, we studied the transcriptome age of the nematode Caenorhabditis elegans throughout development. Using the bulk RNA-seq data, we identified the morphogenesis phase in midembryonic development as the phylotypic stage with the oldest transcriptome and confirmed the results using whole-embryo transcriptome assembled from single-cell RNA-seq data. The variation in transcriptome ages among individual cell types remained small in early and midembryonic development and grew bigger in late embryonic and larval stages as cells and tissues differentiate. Lineages that give rise to certain tissues (e.g., hypodermis and some neurons) but not all recapitulated the hourglass pattern across development at the single-cell transcriptome level. Further analysis of the variation in transcriptome ages among the 128 neuron types in C. elegans nervous system found that a group of chemosensory neurons and their downstream interneurons expressed very young transcriptomes and may contribute to adaptation in recent evolution. Finally, the variation in transcriptome age among the neuron types, as well as the age of their cell fate regulators, led us to hypothesize the evolutionary history of some neuron types.

Cell-specific effects of the sole C. elegans Daughterless/E protein homolog, HLH-2, on nervous system development

Are there common mechanisms of neurogenesis used throughout an entire nervous system? We explored to what extent canonical proneural class I/II bHLH complexes are responsible for neurogenesis throughout the entire Caenorhabditis elegans nervous system. Distinct, lineage-specific proneural class II bHLH factors are generally thought to operate via interaction with a common, class I bHLH subunit, encoded by Daughterless in flies, the E proteins in vertebrates and HLH-2 in C. elegans. To eliminate function of all proneuronal class I/II bHLH complexes, we therefore genetically removed maternal and zygotic hlh-2 gene activity. We observed broad effects on neurogenesis, but still detected normal neurogenesis in many distinct neuron-producing lineages of the central and peripheral nervous system. Moreover, we found that hlh-2 selectively affects some aspects of neuron differentiation while leaving others unaffected. Although our studies confirm the function of proneuronal class I/II bHLH complexes in many different lineages throughout a nervous system, we conclude that their function is not universal, but rather restricted by lineage, cell type and components of differentiation programs affected.

Coordinated control of neuronal differentiation and wiring by sustained transcription factors

The large diversity of cell types in nervous systems presents a challenge in identifying the genetic mechanisms that encode it. Here, we report that nearly 200 distinct neurons in the Drosophila visual system can each be defined by unique combinations of on average 10 continuously expressed transcription factors. We show that targeted modifications of this terminal selector code induce predictable conversions of neuronal fates that appear morphologically and transcriptionally complete. Cis-regulatory analysis of open chromatin links one of these genes to an upstream patterning factor that specifies neuronal fates in stem cells. Experimentally validated network models describe the synergistic regulation of downstream effectors by terminal selectors and ecdysone signaling during brain wiring. Our results provide a generalizable framework of how specific fates are implemented in postmitotic neurons.

Unconventional Myosins from Caenorhabditis elegans as a Probe to Study Human Orthologues

Unconventional myosins are a superfamily of actin-based motor proteins that perform a number of roles in fundamental cellular processes, including (but not limited to) intracellular trafficking, cell motility, endocytosis, exocytosis and cytokinesis. 40 myosins genes have been identified in humans, which belong to different 12 classes based on their domain structure and organisation. These genes are widely expressed in different tissues, and mutations leading to loss of function are associated with a wide variety of pathologies while over-expression often results in cancer. Caenorhabditis elegans (C. elegans) is a small, free-living, non-parasitic nematode. ~38% of the genome of C. elegans has predicted orthologues in the human genome, making it a valuable tool to study the function of human counterparts and human diseases. To date, 8 unconventional myosin genes have been identified in the nematode, from 6 different classes with high homology to human paralogues. The hum-1 and hum-5 (heavy chain of an unconventional myosin) genes encode myosin of class I, hum-2 of class V, hum-3 and hum-8 of class VI, hum-6 of class VII and hum-7 of class IX. The hum-4 gene encodes a high molecular mass myosin (307 kDa) that is one of the most highly divergent myosins and is a member of class XII. Mutations in many of the human orthologues are lethal, indicating their essential properties. However, a functional characterisation for many of these genes in C. elegans has not yet been performed. This article reviews the current knowledge of unconventional myosin genes in C. elegans and explores the potential use of the nematode to study the function and regulation of myosin motors to provide valuable insights into their role in diseases.

Rapid assessment of the temporal function and phenotypic reversibility of neurodevelopmental disorder risk genes in Caenorhabditis elegans

Recent studies have indicated that some phenotypes caused by decreased function of select neurodevelopmental disorder (NDD) risk genes can be reversed by restoring gene function in adulthood. However, few of the hundreds of risk genes have been assessed for adult phenotypic reversibility. We developed a strategy to rapidly assess the temporal requirements and phenotypic reversibility of NDD risk gene orthologs using a conditional protein degradation system and machine-vision phenotypic profiling in Caenorhabditis elegans. We measured how degrading and re-expressing orthologs of EBF3, BRN3A and DYNC1H1 at multiple periods throughout development affect 30 morphological, locomotor, sensory and learning phenotypes. We found that phenotypic reversibility was possible for each gene studied. However, the temporal requirements of gene function and degree of rescue varied by gene and phenotype. This work highlights the critical need to assess multiple windows of degradation and re-expression and a large number of phenotypes to understand the many roles a gene can have across the lifespan. This work also demonstrates the benefits of using a high-throughput model system to prioritize NDD risk genes for re-expression studies in other organisms.

Summary: We identified diverse temporal functional windows and phenotypic reversibility profiles for three neurodevelopmental disorder risk genes: EBF3•unc-3, BRN3A•unc-86 and DYNC1H1•dhc-1. Re-expression of certain genes could rescue multiple phenotypes later in life, prioritizing them for study.

Hox proteins interact to pattern neuronal subtypes in Caenorhabditis elegans males

Hox transcription factors are conserved regulators of neuronal subtype specification on the anteroposterior axis in animals, with disruption of Hox gene expression leading to homeotic transformations of neuronal identities. We have taken advantage of an unusual mutation in the Caenorhabditis elegans Hox gene lin-39, lin-39(ccc16), which transforms neuronal fates in the C. elegans male ventral nerve cord in a manner that depends on a second Hox gene, mab-5. We have performed a genetic analysis centered around this homeotic allele of lin-39 in conjunction with reporters for neuronal target genes and protein interaction assays to explore how LIN-39 and MAB-5 exert both flexibility and specificity in target regulation. We identify cis-regulatory modules in neuronal reporters that are both region-specific and Hox-responsive. Using these reporters of neuronal subtype, we also find that the lin-39(ccc16) mutation disrupts neuronal fates specifically in the region where lin-39 and mab-5 are coexpressed, and that the protein encoded by lin-39(ccc16) is active only in the absence of mab-5. Moreover, the fates of neurons typical to the region of lin-39-mab-5 coexpression depend on both Hox genes. Our genetic analysis, along with evidence from Bimolecular Fluorescence Complementation protein interaction assays, supports a model in which LIN-39 and MAB-5 act at an array of cis-regulatory modules to cooperatively activate and to individually activate or repress neuronal gene expression, resulting in regionally specific neuronal fates.

Control of spinal motor neuron terminal differentiation through sustained Hoxc8 gene activity

Spinal motor neurons (MNs) constitute cellular substrates for several movement disorders. Although their early development has received much attention, how spinal MNs become and remain terminally differentiated is poorly understood. Here, we determined the transcriptome of mouse MNs located at the brachial domain of the spinal cord at embryonic and postnatal stages. We identified novel transcription factors (TFs) and terminal differentiation genes (e.g. ion channels, neurotransmitter receptors, adhesion molecules) with continuous expression in MNs. Interestingly, genes encoding homeodomain TFs (e.g. HOX, LIM), previously implicated in early MN development, continue to be expressed postnatally, suggesting later functions. To test this idea, we inactivated Hoxc8 at successive stages of mouse MN development and observed motor deficits. Our in vivo findings suggest that Hoxc8 is not only required to establish, but also maintain expression of several MN terminal differentiation markers. Data from in vitro generated MNs indicate Hoxc8 acts directly and is sufficient to induce expression of terminal differentiation genes. Our findings dovetail recent observations in Caenorhabditis elegans MNs, pointing toward an evolutionarily conserved role for Hox in neuronal terminal differentiation.

Joint actions of diverse transcription factor families establish neuron-type identities and promote enhancer selectivity

To systematically investigate the complexity of neuron specification regulatory networks, we performed an RNA interference (RNAi) screen against all 875 transcription factors (TFs) encoded in Caenorhabditis elegans genome and searched for defects in nine different neuron types of the monoaminergic (MA) superclass and two cholinergic motoneurons. We identified 91 TF candidates to be required for correct generation of these neuron types, of which 28 were confirmed by mutant analysis. We found that correct reporter expression in each individual neuron type requires at least nine different TFs. Individual neuron types do not usually share TFs involved in their specification but share a common pattern of TFs belonging to the five most common TF families: homeodomain (HD), basic helix loop helix (bHLH), zinc finger (ZF), basic leucine zipper domain (bZIP), and nuclear hormone receptors (NHR). HD TF members are overrepresented, supporting a key role for this family in the establishment of neuronal identities. These five TF families are also prevalent when considering mutant alleles with previously reported neuronal phenotypes in C. elegans, Drosophila, and mouse. In addition, we studied terminal differentiation complexity focusing on the dopaminergic terminal regulatory program. We found two HD TFs (UNC-62 and VAB-3) that work together with known dopaminergic terminal selectors (AST-1, CEH-43, CEH-20). Combined TF binding sites for these five TFs constitute a cis-regulatory signature enriched in the regulatory regions of dopaminergic effector genes. Our results provide new insights on neuron-type regulatory programs in C. elegans that could help better understand neuron specification and evolution of neuron types.

Emerging Roles for Hox Proteins in the Last Steps of Neuronal Development in Worms, Flies, and Mice

A remarkable diversity of cell types characterizes every animal nervous system. Previous studies provided important insights into how neurons commit to a particular fate, migrate to the right place and form precise axodendritic patterns. However, the mechanisms controlling later steps of neuronal development remain poorly understood. Hox proteins represent a conserved family of homeodomain transcription factors with well-established roles in anterior-posterior (A-P) patterning and the early steps of nervous system development, including progenitor cell specification, neuronal migration, cell survival, axon guidance and dendrite morphogenesis. This review highlights recent studies in Caenorhabditis elegans, Drosophila melanogaster and mice that suggest new roles for Hox proteins in processes occurring during later steps of neuronal development, such as synapse formation and acquisition of neuronal terminal identity features (e.g., expression of ion channels, neurotransmitter receptors, and neuropeptides). Moreover, we focus on exciting findings suggesting Hox proteins are required to maintain synaptic structures and neuronal terminal identity during post-embryonic life. Altogether, these studies, in three model systems, support the hypothesis that certain Hox proteins are continuously required, from early development throughout post-embryonic life, to build and maintain a functional nervous system, significantly expanding their functional repertoire beyond the control of early A-P patterning.

Nervous system-wide analysis of Hox regulation of terminal neuronal fate specification in Caenorhabditis elegans

Hox genes encode evolutionarily conserved transcription factors that specify regional identities along the anterior-posterior (A-P) axis. Although some Hox genes are known to regulate the differentiation of certain neurons, to what extent Hox genes are involved in the terminal specification of the entire nervous system is unclear. Here, we systematically mapped the expression of all six Hox genes in C. elegans nervous system and found Hox expression in 97 (32%) of the 302 neurons in adult hermaphrodites. Our results are generally consistent with previous high-throughput expression mapping and single-cell transcriptomic studies. Detailed analysis of the fate markers for these neurons revealed that Hox genes regulate the differentiation of 29 (25%) of the 118 classes of C. elegans neurons. Hox genes not only regulate the specification of terminal neuronal fates through multiple mechanisms but also control subtype diversification along the A-P axis. The widespread involvement of Hox genes in neuronal differentiation indicates their roles in establishing complex nervous systems.

The nervous system contains an extraordinary array of neuron types. How this neuronal diversity arises during development and what genes regulate the differentiation of each neuron type are among the major questions of neurobiology. Hox genes are a set of transcription factors highly conserved in the animal kingdom and are involved in setting up the body plan in the embryos. Hox genes are known to regulate the differentiation of some neurons, but their contribution to the overall development of a nervous system is unclear. In this study, we analyzed the activity of the Hox genes in the differentiation of the 302 neurons of the C. elegans nervous system in its entirety. We found that the six Hox genes are expressed in 32% of all neurons and five Hox genes regulate the differentiation of 25% of all neuron types through multiple mechanisms and act at various stages of cellular development. Thus, our results suggest that a small number of Hox genes could control the development of a significant portion of the nervous system. Given the conserved functions of Hox genes across species, we suspect that the increasing number of Hox genes may have allowed increased complexity in the nervous system in animal evolution.

Visualizing the organization and differentiation of the male-specific nervous system of C. elegans

Sex differences in the brain are prevalent throughout the animal kingdom and particularly well appreciated in the nematode Caenorhabditis elegans, where male animals contain a little-studied set of 93 male-specific neurons. To make these neurons amenable for future study, we describe here how a multicolor reporter transgene, NeuroPAL, is capable of visualizing the distinct identities of all male-specific neurons. We used NeuroPAL to visualize and characterize a number of features of the male-specific nervous system. We provide several proofs of concept for using NeuroPAL to identify the sites of expression of gfp-tagged reporter genes and for cellular fate analysis by analyzing the effect of removal of several developmental patterning genes on neuronal identity acquisition. We use NeuroPAL and its intrinsic cohort of more than 40 distinct differentiation markers to show that, even though male-specific neurons are generated throughout all four larval stages, they execute their terminal differentiation program in a coordinated manner in the fourth larval stage. This coordinated wave of differentiation, which we call 'just-in-time' differentiation, couples neuronal maturation programs with the appearance of sexual organs.

Homeobox genes and the specification of neuronal identity

The enormous diversity of cell types that characterizes any animal nervous system is defined by neuron-type-specific gene batteries that endow cells with distinct anatomical and functional properties. To understand how such cellular diversity is genetically specified, one needs to understand the gene regulatory programmes that control the expression of cell-type-specific gene batteries. The small nervous system of the nematode Caenorhabditis elegans has been comprehensively mapped at the cellular and molecular levels, which has enabled extensive, nervous system-wide explorations into whether there are common underlying mechanisms that specify neuronal cell-type diversity. One principle that emerged from these studies is that transcription factors termed 'terminal selectors' coordinate the expression of individual members of neuron-type-specific gene batteries, thereby assigning unique identities to individual neuron types. Systematic mutant analyses and recent nervous system-wide expression analyses have revealed that one transcription factor family, the homeobox gene family, is broadly used throughout the entire C. elegans nervous system to specify neuronal identity as terminal selectors. I propose that the preponderance of homeobox genes in neuronal identity control is a reflection of an evolutionary trajectory in which an ancestral neuron type was specified by one or more ancestral homeobox genes, and that this functional linkage then duplicated and diversified to generate distinct cell types in an evolving nervous system.

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.

The Conserved ASCL1/MASH-1 Ortholog HLH-3 Specifies Sex-Specific Ventral Cord Motor Neuron Fate in Caenorhabditis elegans

Neural specification is regulated by one or many transcription factors that control expression of effector genes that mediate function and determine neuronal type. Here we identify a novel role for one conserved proneural factor, the bHLH protein HLH-3, implicated in the specification of sex-specific ventral cord motor neurons in C. elegans. Proneural genes act in early stages of neurogenesis in early progenitors, but here, we demonstrate a later role for hlh-3. First, we document that differentiation of the ventral cord type C motor neuron class (VC) within their neuron class, is dynamic in time and space. Expression of VC class-specific and subclass-specific identity genes is distinct through development and is dependent on the VC position along the A-P axis and their proximity to the vulva. Our characterization of the expression of VC class and VC subclass-specific differentiation markers in the absence of hlh-3 function reveals that VC fate specification, differentiation, and morphology requires hlh-3 function. Finally, we conclude that hlh-3 cell-autonomously specifies VC cell fate.

Establishment and maintenance of motor neuron identity via temporal modularity in terminal selector function

Terminal selectors are transcription factors (TFs) that establish during development and maintain throughout life post-mitotic neuronal identity. We previously showed that UNC-3/Ebf, the terminal selector of C. elegans cholinergic motor neurons (MNs), acts indirectly to prevent alternative neuronal identities (Feng et al., 2020). Here, we globally identify the direct targets of UNC-3. Unexpectedly, we find that the suite of UNC-3 targets in MNs is modified across different life stages, revealing ‘temporal modularity’ in terminal selector function. In all larval and adult stages examined, UNC-3 is required for continuous expression of various protein classes (e.g. receptors, transporters) critical for MN function. However, only in late larvae and adults, UNC-3 is required to maintain expression of MN-specific TFs. Minimal disruption of UNC-3’s temporal modularity via genome engineering affects locomotion. Another C. elegans terminal selector (UNC-30/Pitx) also exhibits temporal modularity, supporting the potential generality of this mechanism for the control of neuronal identity.

Unique homeobox codes delineate all the neuron classes of C. elegans

It is presently not known whether neuronal cell type diversity, defined by cell type-specific anatomical, biophysical, functional and molecular signatures, can be reduced to relatively simple molecular descriptors of neuronal identity ¹. Examination of the expression of all conserved homeodomain proteins encoded by the Caenorhabditis elegans genome ² reveals that the complete set of 118 C. elegans neuron classes can be described individually by unique combinations of homeodomain protein expression, thereby providing the simplest currently known descriptor of neuronal diversity. Computational as well as genetic loss of function analyses corroborate that homeodomain proteins not only provide unique descriptors of neuron type, but also play a critical role specifying neuronal identity. We speculate that the pervasive employment of homeobox genes in defining unique neuronal identities reflects the evolutionary history of neuronal cell-type specification.

A terminal selector prevents a Hox transcriptional switch to safeguard motor neuron identity throughout life

To become and remain functional, individual neuron types must select during development and maintain throughout life their distinct terminal identity features, such as expression of specific neurotransmitter receptors, ion channels and neuropeptides. Here, we report a molecular mechanism that enables cholinergic motor neurons (MNs) in the C. elegans ventral nerve cord to select and maintain their unique terminal identity. This mechanism relies on the dual function of the conserved terminal selector UNC-3 (Collier/Ebf). UNC-3 synergizes with LIN-39 (Scr/Dfd/Hox4-5) to directly co-activate multiple terminal identity traits specific to cholinergic MNs, but also antagonizes LIN-39’s ability to activate terminal features of alternative neuronal identities. Loss of unc-3 causes a switch in the transcriptional targets of LIN-39, thereby alternative, not cholinergic MN-specific, terminal features become activated and locomotion defects occur. The strategy of a terminal selector preventing a transcriptional switch may constitute a general principle for safeguarding neuronal identity throughout life.

Transcriptional mechanisms of motor neuron development in vertebrates and invertebrates

Across phylogeny, motor neurons (MNs) represent a single but often remarkably diverse neuronal class composed of a multitude of subtypes required for vital behaviors, such as eating and locomotion. Over the past decades, seminal studies in multiple model organisms have advanced our molecular understanding of the early steps of MN development, such as progenitor specification and acquisition of MN subtype identity, by revealing key roles for several evolutionarily conserved transcription factors. However, very little is known about the molecular strategies that allow distinct MN subtypes to maintain their identity- and function-defining features during the late steps of development and postnatal life. Here, we provide an overview of invertebrate and vertebrate studies on transcription factor-based strategies that control early and late steps of MN development, aiming to highlight evolutionarily conserved gene regulatory principles necessary for establishment and maintenance of neuronal identity.

Neuronal identity control by terminal selectors in worms, flies, and chordates

How do post-mitotic neurons acquire and maintain their terminal identity? Genetic mutant analysis in the nematode Caenorhabditis elegans has revealed common molecular programs that control neuronal identity. Neuron type-specific combinations of transcription factors, called terminal selectors, act as master regulatory factors to initiate and maintain terminal identity programs through direct regulation of neuron type-specific effector genes. We will provide here an update on recent studies that solidify the terminal selector concept in worms, flies and chordates. We will also describe how the terminal selector concept has been expanded by recent work in C. elegans to explain neuronal subtype diversification and plasticity of neuronal identity.

An ancient role for collier/Olf/Ebf (COE)-type transcription factors in axial motor neuron development

BACKGROUND

Mammalian motor circuits display remarkable cellular diversity with hundreds of motor neuron (MN) subtypes innervating hundreds of different muscles. Extensive research on limb muscle-innervating MNs has begun to elucidate the genetic programs that control animal locomotion. In striking contrast, the molecular mechanisms underlying the development of axial muscle-innervating MNs, which control breathing and spinal alignment, are poorly studied.

METHODS

Our previous studies indicated that the function of the Collier/Olf/Ebf (COE) family of transcription factors (TFs) in axial MN development may be conserved from nematodes to simple chordates. Here, we examine the expression pattern of all four mouse COE family members (mEbf1-mEbf4) in spinal MNs and employ genetic approaches in both nematodes and mice to investigate their function in axial MN development.

RESULTS

We report that mEbf1 and mEbf2 are expressed in distinct MN clusters (termed "columns") that innervate different axial muscles. Mouse Ebf1 is expressed in MNs of the hypaxial motor column (HMC), which is necessary for breathing, while mEbf2 is expressed in MNs of the medial motor column (MMC) that control spinal alignment. Our characterization of Ebf2 knock-out mice uncovered a requirement for Ebf2 in the differentiation program of a subset of MMC MNs and revealed for the first time molecular diversity within MMC neurons. Intriguingly, transgenic expression of mEbf1 or mEbf2 can rescue axial MN differentiation and locomotory defects in nematodes (Caenorhabditis elegans) lacking unc-3, the sole C. elegans ortholog of the COE family, suggesting functional conservation among mEbf1, mEbf2 and nematode UNC-3.

CONCLUSIONS

These findings support the hypothesis that genetic programs controlling axial MN development are deeply conserved across species, and further advance our understanding of such programs by revealing an essential role for Ebf2 in mouse axial MNs. Because human mutations in COE orthologs lead to neurodevelopmental disorders characterized by motor developmental delay, our findings may advance our understanding of these human conditions.

Transcription factor mechanisms guiding motor neuron differentiation and diversification

The embryonic generation of motor neurons is a complex process involving progenitor patterning, fate specification, differentiation, and maturation. Throughout this progression, the differential expression of transcription factors has served as our road map for the eventual cell fate of nascent motor neurons. Recent findings from in vivo and in vitro models of motor neuron development have expanded our understanding of how transcription factors govern motor neuron identity and their individual regulatory mechanisms. With the advent of next generation sequencing approaches, researchers now have unprecedented access to the gene regulatory dynamics involved in motor neuron development and are uncovering new connections linking neurodevelopment and neurodegenerative disease.

A transcription factor collective defines the HSN serotonergic neuron regulatory landscape

Cell differentiation is controlled by individual transcription factors (TFs) that together activate a selection of enhancers in specific cell types. How these combinations of TFs identify and activate their target sequences remains poorly understood. Here, we identify the cis-regulatory transcriptional code that controls the differentiation of serotonergic HSN neurons in Caenorhabditis elegans. Activation of the HSN transcriptome is directly orchestrated by a collective of six TFs. Binding site clusters for this TF collective form a regulatory signature that is sufficient for de novo identification of HSN neuron functional enhancers. Among C. elegans neurons, the HSN transcriptome most closely resembles that of mouse serotonergic neurons. Mouse orthologs of the HSN TF collective also regulate serotonergic differentiation and can functionally substitute for their worm counterparts which suggests deep homology. Our results identify rules governing the regulatory landscape of a critically important neuronal type in two species separated by over 700 million years.