A proneural gene controls C. elegans neuroblast asymmetric division and migration

Neurogenesis in Caenorhabditis elegans

Animals rely on their nervous systems to process sensory inputs, integrate these with internal signals, and produce behavioral outputs. This is enabled by the highly specialized morphologies and functions of neurons. Neuronal cells share multiple structural and physiological features, but they also come in a large diversity of types or classes that give the nervous system its broad range of functions and plasticity. This diversity, first recognized over a century ago, spurred classification efforts based on morphology, function, and molecular criteria. Caenorhabditis elegans, with its precisely mapped nervous system at the anatomical level, an extensive molecular description of most of its neurons, and its genetic amenability, has been a prime model for understanding how neurons develop and diversify at a mechanistic level. Here, we review the gene regulatory mechanisms driving neurogenesis and the diversification of neuron classes and subclasses in C. elegans. We discuss our current understanding of the specification of neuronal progenitors and their differentiation in terms of the transcription factors involved and ensuing changes in gene expression and chromatin landscape. The central theme that has emerged is that the identity of a neuron is defined by modules of gene batteries that are under control of parallel yet interconnected regulatory mechanisms. We focus on how, to achieve these terminal identities, cells integrate information along their developmental lineages. Moreover, we discuss how neurons are diversified postembryonically in a time-, genetic sex-, and activity-dependent manner. Finally, we discuss how the understanding of neuronal development can provide insights into the evolution of neuronal diversity.

NMY-2, TOE-2 and PIG-1 regulate Caenorhabditis elegans asymmetric cell divisions

Asymmetric cell division is an important mechanism that generates cellular diversity during development. Not only do asymmetric cell divisions produce daughter cells of different fates, but many can also produce daughters of different sizes, which we refer to as Daughter Cell Size Asymmetry (DCSA). In Caenorhabditis elegans, apoptotic cells are frequently produced by asymmetric divisions that exhibit DCSA, where the smaller daughter dies. We focus here on the divisions of the Q.a and Q.p neuroblasts, which produce larger surviving cells and smaller apoptotic cells and divide with opposite polarity using both distinct and overlapping mechanisms. Several proteins regulate DCSA in these divisions. Previous studies showed that the PIG-1/MELK and TOE-2 proteins regulate DCSA in both the Q.a and Q.p divisions, and the non-muscle myosin NMY-2 regulates DCSA in the Q.a division but not the Q.p division. In this study, we examined endogenously tagged NMY-2, TOE-2, and PIG-1 reporters and characterized their distribution at the cortex during the Q.a and Q.p divisions. In both divisions, TOE-2 localized toward the side of the dividing cell that produced the smaller daughter, whereas PIG-1 localized toward the side that produced the larger daughter. As previously reported, NMY-2 localized to the side of Q.a that produced the smaller daughter and did not localize asymmetrically in Q.p. We used temperature-sensitive nmy-2 mutants to determine the role of nmy-2 in these divisions and were surprised to find that these mutants only displayed DCSA defects in the Q.p division. We generated double mutant combinations between the nmy-2 mutations and mutations in toe-2 and pig-1. Because previous studies indicate that DCSA defects result in the transformation of cells fated to die into their sister cells, the finding that the nmy-2 mutations did not significantly alter the Q.a and Q.p DCSA defects of toe-2 and pig-1 mutants but did alter the number of daughter cells produced by Q.a and Q.p suggests that nmy-2 plays a role in specifying the fates of the Q.a and Q.p that is independent of its role in DCSA.

Control of successive unequal cell divisions by neural cell fate regulators determines embryonic neuroblast cell size

Asymmetric cell divisions often generate daughter cells of unequal size in addition to different fates. In some contexts, daughter cell size asymmetry is thought to be a key input to specific binary cell fate decisions. An alternative possibility is that unequal division is a mechanism by which a variety of cells of different sizes are generated during embryonic development. We show here that two unequal cell divisions precede neuroblast formation in the C lineage of Caenorhabditis elegans. The equalisation of these divisions in a pig-1/MELK mutant background has little effect on neuroblast specification. Instead, we demonstrate that let-19/MDT13 is a regulator of the proneural basic helix-loop-helix transcription factor hlh-14/ASCL1 and find that both are required to concomitantly regulate the acquisition of neuroblast identity and neuroblast cell size. Thus, embryonic neuroblast cell size in this lineage is progressively regulated in parallel with identity by key neural cell fate regulators. We propose that key cell fate determinants have a previously unappreciated function in regulating unequal cleavage, and therefore cell size, of the progenitor cells whose daughter cell fates they then go on to specify.

Mechanisms of lineage specification in Caenorhabditis elegans

The studies of cell fate and lineage specification are fundamental to our understanding of the development of multicellular organisms. Caenorhabditis elegans has been one of the premiere systems for studying cell fate specification mechanisms at single cell resolution, due to its transparent nature, the invariant cell lineage, and fixed number of somatic cells. We discuss the general themes and regulatory mechanisms that have emerged from these studies, with a focus on somatic lineages and cell fates. We next review the key factors and pathways that regulate the specification of discrete cells and lineages during embryogenesis and postembryonic development; we focus on transcription factors and include numerous lineage diagrams that depict the expression of key factors that specify embryonic founder cells and postembryonic blast cells, and the diverse somatic cell fates they generate. We end by discussing some future perspectives in cell and lineage specification.

Evolutionary origin of the neural tube in basal deuterostomes

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

Piecemeal regulation of convergent neuronal lineages by bHLH transcription factors in Caenorhabditis elegans

Cells of the same type can be generated by distinct cellular lineages that originate in different parts of the developing embryo ('lineage convergence'). Several Caenorhabditis elegans neuron classes composed of left/right or radially symmetric class members display such lineage convergence. We show here that the C. elegans Atonal homolog lin-32 is differentially expressed in neuronal lineages that give rise to left/right or radially symmetric class members. Loss of lin-32 results in the selective loss of the expression of pan-neuronal markers and terminal selector-type transcription factors that confer neuron class-specific features. Another basic helix-loop-helix (bHLH) gene, the Achaete-Scute homolog hlh-14, is expressed in a mirror image pattern relative to lin-32 and is required to induce neuronal identity and terminal selector expression on the contralateral side of the animal. These findings demonstrate that distinct lineage histories converge via different bHLH factors at the level of induction of terminal selector identity determinants, which thus serve as integrators of distinct lineage histories. We also describe neuron-to-neuron identity transformations in lin-32 mutants, which we propose to also be the result of misregulation of terminal selector gene expression.

Regulation of Gliogenesis by lin-32/Atoh1 in Caenorhabditis elegans

The regulation of gliogenesis is a fundamental process for nervous system development, as the appropriate glial number and identity is required for a functional nervous system. To investigate the molecular mechanisms involved in gliogenesis, we used C. elegans as a model and identified the function of the proneural gene lin-32/Atoh1 in gliogenesis. We found that lin-32 functions during embryonic development to negatively regulate the number of AMsh glia. The ectopic AMsh cells at least partially arise from cells originally fated to become CEPsh glia, suggesting that lin-32 is involved in the specification of specific glial subtypes. Moreover, we show that lin-32 acts in parallel with cnd-1/ NeuroD1 and ngn-1/ Neurog1 in negatively regulating an AMsh glia fate. Furthermore, expression of murine Atoh1 fully rescues lin-32 mutant phenotypes, suggesting lin-32/Atoh1 may have a conserved role in glial specification.

Comparison of differentiation gene batteries for migratory mechanosensory neurons across bilaterians

In embryos of distantly related bilaterian phyla, their lateral neural borders give rise to the peripheral nervous system elements, including various mechanosensory cells derived from migratory precursors, such as hair cells and dorsal root ganglion (DRG) neurons in vertebrates, bipolar tail neuron (BTN) in Ciona, chordotonal organ in Drosophila, and AVM/PVM in Caenorhabditis elegans. Developmental genetics studies had revealed a couple of transcription factors (TFs) regulating differentiation of mechanosensory cells shared by vertebrates and arthropods. However, unbiased systematic profiling of regulators is needed to demonstrate conservation of differentiation gene batteries for mechanosensory cells across bilaterians. At first, we observed that in both C. elegans Q neuroblasts and Drosophila lateral neuroectoderm, conserved NPB specifier Msx/vab-15 regulates Atoh1/lin-32, supporting the homology of mechanosensory neuron development in lateral neural border lineage of Ecdysozia. So we used C. elegans as a protostomia model. Single-cell resolution expression profiling of TFs and genetic analysis revealed a differentiation gene battery (Atonh1/lin-32, Drg11/alr-1, Gfi1/pag-3, Lhx5/mec-3, and Pou4/unc-86) for AVM/PVM mechanosensory neurons. The worm-gene battery significantly overlaps with both that of placode-derived Atonh1/lin-32-dependent hair cells and that of NPB-derived Neurogenin-dependent DRG neurons in vertebrates, supporting the homology of molecular mechanisms underlying the differentiation of neural border-derived mechanosensory cells between protostome and deuterostome. At last, Ciona BTN, the homolog of vertebrate DRG, also expresses Atonh1/lin-32, further supporting the homology notion and indicating a common origin of hair cells and DRG in vertebrate lineage.

The planar cell polarity protein VANG-1/Vangl negatively regulates Wnt/β-catenin signaling through a Dvl dependent mechanism

Van Gogh-like (Vangl) and Prickle (Pk) are core components of the non-canonical Wnt planar cell polarity pathway that controls epithelial polarity and cell migration. Studies in vertebrate model systems have suggested that Vangl and Pk may also inhibit signaling through the canonical Wnt/β-catenin pathway, but the functional significance of this potential cross-talk is unclear. In the nematode C. elegans, the Q neuroblasts and their descendants migrate in opposite directions along the anteroposterior body axis. The direction of these migrations is specified by Wnt signaling, with activation of canonical Wnt signaling driving posterior migration, and non-canonical Wnt signaling anterior migration. Here, we show that the Vangl ortholog VANG-1 influences the Wnt signaling response of the Q neuroblasts by negatively regulating canonical Wnt signaling. This inhibitory activity depends on a carboxy-terminal PDZ binding motif in VANG-1 and the Dishevelled ortholog MIG-5, but is independent of the Pk ortholog PRKL-1. Moreover, using Vangl1 and Vangl2 double mutant cells, we show that a similar mechanism acts in mammalian cells. We conclude that cross-talk between VANG-1/Vangl and the canonical Wnt pathway is an evolutionarily conserved mechanism that ensures robust specification of Wnt signaling responses.

Wnt proteins are signaling molecules with a wide range of functions in embryonic development and the maintenance of adult tissues. Wnt proteins can trigger several different signaling pathways that are grouped in β-catenin dependent (canonical) and independent (non-canonical) signaling mechanisms. Here, we have investigated cross-talk between these different Wnt signaling pathways. We show that VANG-1/Vangl, a component of the non-canonical planar cell polarity pathway, negatively regulates canonical Wnt signaling. We propose that this cross-talk mechanism ensures that Wnt stimulated cells always activate the proper downstream signaling response.

Asymmetric development of the nervous system

The human nervous system consists of seemingly symmetric left and right halves. However, closer observation of the brain reveals anatomical and functional lateralization. Defects in brain asymmetry correlate with several neurological disorders, yet our understanding of the mechanisms used to establish lateralization in the human central nervous system is extremely limited. Here, we review left-right asymmetries within the nervous system of humans and several model organisms, including rodents, Zebrafish, chickens, Xenopus, Drosophila, and the nematode Caenorhabditis elegans. Comparing and contrasting mechanisms used to develop left-right asymmetry in the nervous system can provide insight into how the human brain is lateralized. Developmental Dynamics 247:124-137, 2018. © 2017 Wiley Periodicals, Inc.

LIN-32/Atonal Controls Oxygen Sensing Neuron Development in Caenorhabditis elegans

Development of complex nervous systems requires precisely controlled neurogenesis. The generation and specification of neurons occur through the transcriptional and post-transcriptional control of complex regulatory networks. In vertebrates and invertebrates, the proneural basic-helix-loop-helix (bHLH) family of transcription factors has multiple functions in neurogenesis. Here, we identified the LIN-32/Atonal bHLH transcription factor as a key regulator of URXL/R oxygen-sensing neuron development in Caenorhabditis elegans. When LIN-32/Atonal expression is lost, the expression of URX specification and terminal differentiation genes is abrogated. As such, lin-32 mutant animals are unable to respond to increases in environmental oxygen. The URX neurons are generated from a branch of the cell lineage that also produces the CEPDL/R and URADL/R neurons. We found development of these neurons is also defective, suggesting that LIN-32/Atonal regulates neuronal development of the entire lineage. Finally, our results show that aspects of URX neuronal fate are partially restored in lin-32 mutant animals when the apoptosis pathway is inhibited. This suggests that, as in other organisms, LIN-32/Atonal regulates neuronal apoptosis.

Conserved gene regulatory module specifies lateral neural borders across bilaterians

The lateral neural plate border (NPB), the neural part of the vertebrate neural border, is composed of central nervous system (CNS) progenitors and peripheral nervous system (PNS) progenitors. In invertebrates, PNS progenitors are also juxtaposed to the lateral boundary of the CNS. Whether there are conserved molecular mechanisms determining vertebrate and invertebrate lateral neural borders remains unclear. Using single-cell-resolution gene-expression profiling and genetic analysis, we present evidence that orthologs of the NPB specification module specify the invertebrate lateral neural border, which is composed of CNS and PNS progenitors. First, like in vertebrates, the conserved neuroectoderm lateral border specifier Msx/vab-15 specifies lateral neuroblasts in Caenorhabditis elegans Second, orthologs of the vertebrate NPB specification module (Msx/vab-15, Pax3/7/pax-3, and Zic/ref-2) are significantly enriched in worm lateral neuroblasts. In addition, like in other bilaterians, the expression domain of Msx/vab-15 is more lateral than those of Pax3/7/pax-3 and Zic/ref-2 in C. elegans Third, we show that Msx/vab-15 regulates the development of mechanosensory neurons derived from lateral neural progenitors in multiple invertebrate species, including C. elegans, Drosophila melanogaster, and Ciona intestinalis We also identify a novel lateral neural border specifier, ZNF703/tlp-1, which functions synergistically with Msx/vab-15 in both C. elegans and Xenopus laevis These data suggest a common origin of the molecular mechanism specifying lateral neural borders across bilaterians.

A bHLH Code for Sexually Dimorphic Form and Function of the C. elegans Somatic Gonad

How sexually dimorphic gonads are generated is a fundamental question at the interface of developmental and evolutionary biology [1-3]. In C. elegans, sexual dimorphism in gonad form and function largely originates in different apportionment of roles to three regulatory cells of the somatic gonad primordium in young larvae. Their essential roles include leading gonad arm outgrowth, serving as the germline niche, connecting to epithelial openings, and organizing reproductive organ development. The development and function of the regulatory cells in both sexes requires the basic-helix-loop-helix (bHLH) transcription factor HLH-2, the sole ortholog of the E proteins mammalian E2A and Drosophila Daughterless [4-8], yet how they adopt different fates to execute their different roles has been unknown. Here, we show that each regulatory cell expresses a distinct complement of bHLH-encoding genes-and therefore distinct HLH-2:bHLH dimers-and formulate a "bHLH code" hypothesis for regulatory cell identity. We support this hypothesis by showing that the bHLH gene complement is both necessary and sufficient to confer particular regulatory cell fates. Strikingly, prospective regulatory cells can be directly reprogrammed into other regulatory cell types simply by loss or ectopic expression of bHLH genes, and male-to-female and female-to-male transformations indicate that the code is instructive for sexual dimorphism. The bHLH code appears to be embedded in a bow-tie regulatory architecture [9, 10], wherein sexual, positional, temporal, and lineage inputs connect through bHLH genes to diverse outputs for terminal features and provides a plausible mechanism for the evolutionary plasticity of gonad form seen in nematodes [11-15].