Wnt-Ror signaling to SIA and SIB neurons directs anterior axon guidance and nerve ring placement in C. elegans

Snake Venom-Inspired Novel Peptides Protect Caenorhabditis elegans against Paraquat-Induced Parkinson's Pathology

The in vivo protective mechanisms of two low-molecular-mass (∼1.4 kDa) novel custom peptides (CPs) against paraquat-induced neurodegenerative dysfunction in the Caenorhabditis elegans model were deciphered. CPs prevented the paraquat from binding to the nerve ring adjacent to the pharynx in C. elegans (wild-type) by stable and high-affinity binding to the tyrosine-protein kinase receptor CAM-1, resulting in significant inhibition of paraquat-induced toxicity by reducing the production of reactive oxygen species, mitochondrial membrane depolarization, and chemosensory dysfunction. The CPs inhibited paraquat-induced dopaminergic neuron degeneration and alpha-synuclein protein expression, the hallmarks of Parkinson's disease, in transgenic BZ555 and NL5901 strains of C. elegans. Transcriptomic, functional proteomics, and quantitative reverse transcription-polymerase chain reaction analyses show that CPs prevented the increased expression of the genes involved in the skn-1 downstream pathway, thereby restoring paraquat-mediated oxidative stress, apoptosis, and neuronal damage in C. elegans. The ability of CPs to repair paraquat-induced damage was demonstrated by a network of gene expression profiles, illustrating the molecular relationships between the regulatory proteins.

Nematicidal effects of silencing arginine kinase in the pine wood nematode, Bursaphelenchus xylophilus, determined using a dsRNA-like siRNA assembly

The pine wood nematode Bursaphelenchus xylophilus is a highly invasive species responsible for the widespread pine wilt disease. Double-stranded RNA (dsRNA) biopesticides represent a novel strategy for controlling plant-parasitic nematodes. The B. xylophilus arginine kinase (BxAK) features a conserved ATP-binding domain and exhibits nematode-specific divergence in the phylogenetic tree. Notably, whole-mount in situ hybridization signals are evident in the nematode head and middle sections, particularly in the juvenile stage before sex differentiation. In this study, we developed a novel dsRNA-like small interfering RNA (siRNA) assembly that specifically targets BxAK and presents highly nematicidal effects. The RNA interference (RNAi) efficiency achieved a 95.9 % reduction in second-stage juveniles. In bioassays, the median lethal concentrations of this siRNA assembly against B. xylophilus were 168.5 ng/μl for juveniles and 603.8 ng/μl for adults within 48 h. Moreover, transcriptomic results revealed significantly downregulated expression levels of genes related to metabolism and development, suggesting that the mode of action of BxAK silencing is related to disruptions in energy homeostasis and juvenile development. In conclusion, BxAK is a molecular target for controlling B. xylophilus, and our siRNA assembly significantly enhances RNAi efficiency and lowers the lethal concentration required, making it a promising candidate for future biocontrol applications.

Cytoplasmic ribosomes hitchhike on mitochondria to dendrites

Neurons rely on local protein synthesis to rapidly modify the proteome of neurites distant from the cell body. A prerequisite for local protein synthesis is the presence of ribosomes in the neurite, but the mechanisms of ribosome transport in neurons remain poorly defined. Here, we find that ribosomes hitchhike on mitochondria for their delivery to the dendrite of a sensory neuron in C. elegans. Ribosomes co-transport with dendritic mitochondria, and their association requires the atypical Rho GTPase MIRO-1. Disrupting mitochondrial transport prevents ribosomes from reaching the dendrite, whereas ectopic re-localization of mitochondria results in a concomitant re-localization of ribosomes, demonstrating that mitochondria are required and sufficient for instructing ribosome distribution in dendrites. Endolysosomal organelles that are involved in mRNA transport and translation can associate with mitochondria and ribosomes but do not play a significant role in ribosome transport. These results reveal a mechanism for dendritic ribosome delivery, which is a critical upstream requirement for local protein synthesis.

Wnt signaling pathway in spinal cord injury: from mechanisms to potential applications

Spinal cord injury (SCI) denotes damage to both the structure and function of the spinal cord, primarily manifesting as sensory and motor deficits caused by disruptions in neural transmission pathways, potentially culminating in irreversible paralysis. Its pathophysiological processes are complex, with numerous molecules and signaling pathways intricately involved. Notably, the pronounced upregulation of the Wnt signaling pathway post-SCI holds promise for neural regeneration and repair. Activation of the Wnt pathway plays a crucial role in neuronal differentiation, axonal regeneration, local neuroinflammatory responses, and cell apoptosis, highlighting its potential as a therapeutic target for treating SCI. However, excessive activation of the Wnt pathway can also lead to negative effects, highlighting the need for further investigation into its applicability and significance in SCI. This paper provides an overview of the latest research advancements in the Wnt signaling pathway in SCI, summarizing the recent progress in treatment strategies associated with the Wnt pathway and analyzing their advantages and disadvantages. Additionally, we offer insights into the clinical application of the Wnt signaling pathway in SCI, along with prospective avenues for future research direction.

LET-381/FoxF and its target UNC-30/Pitx2 specify and maintain the molecular identity of C. elegans mesodermal glia that regulate motor behavior

While most glial cell types in the central nervous system (CNS) arise from neuroectodermal progenitors, some, like microglia, are mesodermally derived. To understand mesodermal glia development and function, we investigated C. elegans GLR glia, which envelop the brain neuropil and separate it from the circulatory system cavity. Transcriptome analysis shows that GLR glia combine astrocytic and endothelial characteristics, which are relegated to separate cell types in vertebrates. Combined fate acquisition is orchestrated by LET-381/FoxF, a fate-specification/maintenance transcription factor also expressed in glia and endothelia of other animals. Among LET-381/FoxF targets, the UNC-30/Pitx2 transcription factor controls GLR glia morphology and represses alternative mesodermal fates. LET-381 and UNC-30 co-expression in naive cells is sufficient for GLR glia gene expression. GLR glia inactivation by ablation or let-381 mutation disrupts locomotory behavior and promotes salt-induced paralysis, suggesting brain-neuropil activity dysregulation. Our studies uncover mechanisms of mesodermal glia development and show that like neuronal differentiation, glia differentiation requires autoregulatory terminal selector genes that define and maintain the glial fate.

LET-381/FoxF and UNC-30/Pitx2 control the development of C. elegans mesodermal glia that regulate motor behavior

While most CNS glia arise from neuroectodermal progenitors, some, like microglia, are mesodermally derived. To understand mesodermal glia development and function, we investigated C. elegans GLR glia, which ensheath the brain neuropil and separate it from the circulatory-system cavity. Transcriptome analysis suggests GLR glia merge astrocytic and endothelial characteristics relegated to separate cell types in vertebrates. Combined fate acquisition is orchestrated by LET-381/FoxF, a fate-specification/maintenance transcription factor expressed in glia and endothelia of other animals. Among LET-381/FoxF targets, UNC-30/Pitx2 transcription factor controls GLR glia morphology and represses alternative mesodermal fates. LET-381 and UNC-30 co-expression in naïve cells is sufficient for GLR glia gene expression. GLR glia inactivation by ablation or let-381 mutation disrupts locomotory behavior and induces salt hypersensitivity, suggesting brain-neuropil activity dysregulation. Our studies uncover mechanisms of mesodermal glia development and show that like neurons, glia differentiation requires autoregulatory terminal selector genes that define and maintain the glial fate.

Wnt signaling and contact-mediated repulsion shape sensory dendritic fields

The complete and non-redundant coverage of sensory tissues by neighboring neurons enables effective detection of stimuli in the environment. How the neurites of adjacent neurons establish their boundaries to achieve this completeness in coverage remains incompletely understood. Here, we use distinct fluorescent reporters to study two neighboring sensory neurons with complex dendritic arbors, FLP and PVD, in C. elegans . We quantify the sizes of their dendritic fields, and identify CWN-2/Wnt and LIN-17/Frizzled as a ligand and receptor that regulate the relative dendritic field sizes of these two neurons. Loss of either cwn-2 or lin-17 results in complementary changes in the size of the dendritic fields of both neurons; the FLP arbor expands, while that of PVD shrinks. Using an endogenous knock-in mNeonGreen-CWN-2/Wnt, we find that CWN-2/Wnt is localized along the path of growing FLP dendrites. Dynamic imaging shows a significant braking of FLP dendrite growth upon CWN-2/Wnt contact. We find that LIN-17/Frizzled functions cell-autonomously in FLP to limit dendritic field size and propose that PVD fills the space left by FLP through contact-induced retraction. Our results reveal that interactions of dendrites with adjacent dendrites and with environmental cues both shape the boundaries of neighboring dendritic fields.

Highlights

▫ Secreted Wnt CWN-2 and cell-autonomous activity of neuronal LIN-17/Frizzled receptors restrict FLP dendritic field sizes▫ Endogenously tagged CWN-2/Wnt is punctate and visible in the same plane of growing FLP dendrites▫ Growth of developing FLP dendrites is inhibited upon contact with extracellular CWN-2/Wnt and with PVD dendrites.

The regulatory landscape of neurite development in Caenorhabditis elegans

Neuronal communication requires precise connectivity of neurite projections (axons and dendrites). Developing neurites express cell-surface receptors that interpret extracellular cues to enable correct guidance toward, and connection with, target cells. Spatiotemporal regulation of neurite guidance molecule expression by transcription factors (TFs) is critical for nervous system development and function. Here, we review how neurite development is regulated by TFs in the Caenorhabditis elegans nervous system. By collecting publicly available transcriptome and ChIP-sequencing data, we reveal gene expression dynamics during neurite development, providing insight into transcriptional mechanisms governing construction of the nervous system architecture.

Cross-modality synthesis of EM time series and live fluorescence imaging

Analyses across imaging modalities allow the integration of complementary spatiotemporal information about brain development, structure, and function. However, systematic atlasing across modalities is limited by challenges to effective image alignment. We combine highly spatially resolved electron microscopy (EM) and highly temporally resolved time-lapse fluorescence microscopy (FM) to examine the emergence of a complex nervous system in Caenorhabditis elegans embryogenesis. We generate an EM time series at four classic developmental stages and create a landmark-based co-optimization algorithm for cross-modality image alignment, which handles developmental heterochrony among datasets to achieve accurate single-cell level alignment. Synthesis based on the EM series and time-lapse FM series carrying different cell-specific markers reveals critical dynamic behaviors across scales of identifiable individual cells in the emergence of the primary neuropil, the nerve ring, as well as a major sensory organ, the amphid. Our study paves the way for systematic cross-modality data synthesis in C. elegans and demonstrates a powerful approach that may be applied broadly.

A perspective on C. elegans neurodevelopment: from early visionaries to a booming neuroscience research

The formation of the nervous system and its striking complexity is a remarkable feat of development. C. elegans served as a unique model to dissect the molecular events in neurodevelopment, from its early visionaries to the current booming neuroscience community. Soon after being introduced as a model, C. elegans was mapped at the level of genes, cells, and synapses, providing the first metazoan with a complete cell lineage, sequenced genome, and connectome. Here, I summarize mechanisms underlying C. elegans neurodevelopment, from the generation and diversification of neural components to their navigation and connectivity. I point out recent noteworthy findings in the fields of glia biology, sex dimorphism and plasticity in neurodevelopment, highlighting how current research connects back to the pioneering studies by Brenner, Sulston and colleagues. Multifaceted investigations in model organisms, connecting genes to cell function and behavior, expand our mechanistic understanding of neurodevelopment while allowing us to formulate emerging questions for future discoveries.

Wnt5a in cancer-associated fibroblasts promotes colorectal cancer progression

Cancer-associated fibroblasts (CAFs) are a major component of the tumor microenvironment and have been shown to promote cancer aggressiveness. In our previous study, analysis of expression profiles obtained from paired CAFs and normal fibroblasts from colorectal cancer (CRC) tissue revealed that gene sets related to the Wnt signaling pathway were highly enriched in colorectal CAFs. Furthermore, among the components of the β-catenin-independent Wnt pathway, Wnt5a was highly expressed in CAFs. Since Wnt5a is considered to be a regulator of CRC progression in CAFs, we performed immunohistochemical analysis on Wnt5a in 171 patients who underwent surgery for CRC. Positive staining for Wnt5a was often found in cancer stroma, particularly in fibromatous areas, although the immunoreactivity for Wnt5a was weak in cancer cells. Wnt5a status in CAFs was significantly associated with tumor size, depth of invasion, lymphatic and vascular invasion, lymph node metastasis, TNM stage, and recurrence. Subsequent in vitro analyses using human recombinant Wnt5a protein revealed that cancer cell proliferation and migration were significantly increased by stimulation with Wnt5a. Our findings suggest that Wnt5a-derived CAFs play a crucial role in CRC progression and have potential as a target of anti-cancer therapies.

Structural and developmental principles of neuropil assembly in C. elegans

Neuropil is a fundamental form of tissue organization within the brain¹, in which densely packed neurons synaptically interconnect into precise circuit architecture^2,3. However, the structural and developmental principles that govern this nanoscale precision remain largely unknown^4,5. Here we use an iterative data coarse-graining algorithm termed 'diffusion condensation'⁶ to identify nested circuit structures within the Caenorhabditis elegans neuropil, which is known as the nerve ring. We show that the nerve ring neuropil is largely organized into four strata that are composed of related behavioural circuits. The stratified architecture of the neuropil is a geometrical representation of the functional segregation of sensory information and motor outputs, with specific sensory organs and muscle quadrants mapping onto particular neuropil strata. We identify groups of neurons with unique morphologies that integrate information across strata and that create neural structures that cage the strata within the nerve ring. We use high resolution light-sheet microscopy^7,8 coupled with lineage-tracing and cell-tracking algorithms^9,10 to resolve the developmental sequence and reveal principles of cell position, migration and outgrowth that guide stratified neuropil organization. Our results uncover conserved structural design principles that underlie the architecture and function of the nerve ring neuropil, and reveal a temporal progression of outgrowth-based on pioneer neurons-that guides the hierarchical development of the layered neuropil. Our findings provide a systematic blueprint for using structural and developmental approaches to understand neuropil organization within the brain.

Cadherin preserves cohesion across involuting tissues during C. elegans neurulation

The internalization of the central nervous system, termed neurulation in vertebrates, is a critical step in embryogenesis. Open questions remain regarding how force propels coordinated tissue movement during the process, and little is known as to how internalization happens in invertebrates. We show that in C. elegans morphogenesis, apical constriction in the retracting pharynx drives involution of the adjacent neuroectoderm. HMR-1/cadherin mediates this process via inter-tissue attachment, as well as cohesion within the neuroectoderm. Our results demonstrate that HMR-1 is capable of mediating embryo-wide reorganization driven by a centrally located force generator, and indicate a non-canonical use of cadherin on the basal side of an epithelium that may apply to vertebrate neurulation. Additionally, we highlight shared morphology and gene expression in tissues driving involution, which suggests that neuroectoderm involution in C. elegans is potentially homologous with vertebrate neurulation and thus may help elucidate the evolutionary origin of the brain.

ngn-1/neurogenin Activates Transcription of Multiple Terminal Selector Transcription Factors in the Caenorhabditis elegans Nervous System

Proper nervous system development is required for an organism's survival and function. Defects in neurogenesis have been linked to neurodevelopmental disorders such as schizophrenia and autism. Understanding the gene regulatory networks that orchestrate neural development, specifically cascades of proneural transcription factors, can better elucidate which genes are most important during early neurogenesis. Neurogenins are a family of deeply conserved factors shown to be both necessary and sufficient for the development of neural subtypes. However, the immediate downstream targets of neurogenin are not well characterized. The objective of this study was to further elucidate the role of ngn-1/neurogenin in nervous system development and to identify its downstream transcriptional targets, using the nematode Caenorhabditis elegans as a model for this work. We found that ngn-1 is required for axon outgrowth, nerve ring architecture, and neuronal cell fate specification. We also showed that ngn-1 may have roles in neuroblast migration and epithelial integrity during embryonic development. Using RNA sequencing and comparative transcriptome analysis, we identified eight transcription factors (hlh-34/NPAS1, unc-42/PROP1, ceh-17/PHOX2A, lim-4/LHX6, fax-1/NR2E3, lin-11/LHX1, tlp-1/ZNF503, and nhr-23/RORB) whose transcription is activated, either directly or indirectly, by ngn-1 Our results show that ngn-1 has a role in transcribing known terminal regulators that establish and maintain cell fate of differentiated neural subtypes and confirms that ngn-1 functions as a proneural transcription factor in C. elegans neurogenesis.

Wnt ligands regulate the asymmetric divisions of neuronal progenitors in C. elegans embryos

Wnt/β-catenin signalling has been implicated in the terminal asymmetric divisions of neuronal progenitors in vertebrates and invertebrates. However, the role of Wnt ligands in this process remains poorly characterized. Here, we used the terminal divisions of the embryonic neuronal progenitors in C. elegans to characterize the role of Wnt ligands during this process, focusing on a lineage that produces the cholinergic interneuron AIY. We observed that, during interphase, the neuronal progenitor is elongated along the anteroposterior axis, then divides along its major axis, generating an anterior and a posterior daughter with different fates. Using time-controlled perturbations, we show that three Wnt ligands, which are transcribed at higher levels at the posterior of the embryo, regulate the orientation of the neuronal progenitor and its asymmetric division. We also identify a role for a Wnt receptor (MOM-5) and a cortical transducer APC (APR-1), which are, respectively, enriched at the posterior and anterior poles of the neuronal progenitor. Our study establishes a role for Wnt ligands in the regulation of the shape and terminal asymmetric divisions of neuronal progenitors, and identifies downstream components.

Intestinal Epithelial Wnt Signaling Mediates Acetylcholine-Triggered Host Defense against Infection

Regulated antimicrobial peptide expression in the intestinal epithelium is key to defense against infection and to microbiota homeostasis. Understanding the mechanisms that regulate such expression is necessary for understanding immune homeostasis and inflammatory disease and for developing safe and effective therapies. We used Caenorhabditis elegans in a preclinical approach to discover mechanisms of antimicrobial gene expression control in the intestinal epithelium. We found an unexpected role for the cholinergic nervous system. Infection-induced acetylcholine release from neurons stimulated muscarinic signaling in the epithelium, driving downstream induction of Wnt expression in the same tissue. Wnt induction activated the epithelial canonical Wnt pathway, resulting in the expression of C-type lectin and lysozyme genes that enhanced host defense. Furthermore, the muscarinic and Wnt pathways are linked by conserved transcription factors. These results reveal a tight connection between the nervous system and the intestinal epithelium, with important implications for host defense, immune homeostasis, and cancer.

RPM-1 and DLK-1 regulate pioneer axon outgrowth by controlling Wnt signaling

Axons must correctly reach their targets for proper nervous system function, although we do not fully understand the underlying mechanism, particularly for the first 'pioneer' axons. In C. elegans, AVG is the first neuron to extend an axon along the ventral midline, and this pioneer axon facilitates the proper extension and guidance of follower axons that comprise the ventral nerve cord. Here, we show that the ubiquitin ligase RPM-1 prevents the overgrowth of the AVG axon by repressing the activity of the DLK-1/p38 MAPK pathway. Unlike in damaged neurons, where this pathway activates CEBP-1, we find that RPM-1 and the DLK-1 pathway instead regulate the response to extracellular Wnt cues in developing AVG axons. The Wnt LIN-44 promotes the posterior growth of the AVG axon. In the absence of RPM-1 activity, AVG becomes responsive to a different Wnt, EGL-20, through a mechanism that appears to be independent of canonical Fz-type receptors. Our results suggest that RPM-1 and the DLK-1 pathway regulate axon guidance and growth by preventing Wnt signaling crosstalk.

Wnts Promote Synaptic Assembly Through T-Cell Specific Transcription Factors in Caenorhabditis elegans

Synapses are specialized neuronal connections essential for neuronal function. Defects in synaptic assembly or maintenance usually lead to various neurological disorders. Synaptic assembly is regulated by secreted molecules such as Wnts. Wnts are a large family of conserved glycosylated signaling molecules involved in many aspects of neural development and maintenance. However, the molecular mechanisms by which Wnts regulate synaptic assembly remain elusive due to the large number of ligands/receptors, the diversity of signaling cascades and the complexity of the nervous system. In this study, through genetic manipulation, we uncover that C. elegans Wnt-2 (CWN-2) is required for synaptic development. The CWN-2 signal is required during both embryonic and postembryonic development, in the nervous system and intestine, for promoting synaptic assembly. Furthermore, we provide genetic evidence for CWN-2 promoting synaptogenesis through the Frizzled receptor (FZD) CFZ-2, the Dishevelled (DVL) DSH-2, the β-catenin SYS-1 and the only T-cell specific transcription factor POP-1/TCF. Importantly, it is the first time to report the requirement of a TCF for presynaptic assembly. These findings expand our understanding of the synaptogenic mechanisms and may provide therapeutic insights into Wnt-related neurological disorders.

Automated C. elegans embryo alignments reveal brain neuropil position invariance despite lax cell body placement

The Caenorhabditis elegans cell lineage is nearly invariant. Whether this stereotyped cell-division pattern promotes reproducibility in cell shapes/positions is not generally known, as manual spatiotemporal cell-shape/position alignments are labor-intensive, and fully-automated methods are not described. Here, we report automated algorithms for spatiotemporal alignments of C. elegans embryos from pre-morphogenesis to motor-activity initiation. We use sparsely-labeled green-fluorescent nuclei and a pan-nuclear red-fluorescent reporter to register consecutive imaging time points and compare embryos. Using our method, we monitor early assembly of the nerve-ring (NR) brain neuropil. While NR pioneer neurons exhibit reproducible growth kinetics and axon positions, cell-body placements are variable. Thus, pioneer-neuron axon locations, but not cell-body positions, are under selection. We also show that anterior NR displacement in cam-1/ROR Wnt-receptor mutants is not an early NR assembly defect. Our results demonstrate the utility of automated spatiotemporal alignments of C. elegans embryos, and uncover key principles guiding nervous-system development in this animal.

Robo and Ror function in a common receptor complex to regulate Wnt-mediated neurite outgrowth in Caenorhabditis elegans

Growing axons are exposed to various guidance cues en route to their targets, but the mechanisms that govern the response of growth cones to combinations of signals remain largely elusive. Here, we found that the sole Robo receptor, SAX-3, in Caenorhabditis elegans functions as a coreceptor for Wnt/CWN-2 molecules. SAX-3 binds to Wnt/CWN-2 and facilitates the membrane recruitment of CWN-2. SAX-3 forms a complex with the Ror/CAM-1 receptor and its downstream effector Dsh/DSH-1, promoting signal transduction from Wnt to Dsh. sax-3 functions in Wnt-responsive cells and the SAX-3 receptor is restricted to the side of the cell from which the neurite is extended. DSH-1 has a similar asymmetric distribution, which is disrupted by sax-3 mutation. Taking these results together, we propose that Robo receptor can function as a Wnt coreceptor to regulate Wnt-mediated biological processes in vivo.

Glia initiate brain assembly through noncanonical Chimaerin-Furin axon guidance in C. elegans

Brain assembly is hypothesized to begin when pioneer axons extend over non-neuronal cells, forming tracts guiding follower axons. Yet pioneer-neuron identities, their guidance substrates, and their interactions are not well understood. Here, using time-lapse embryonic imaging, genetics, protein-interaction, and functional studies, we uncover the early events of C. elegans brain assembly. We demonstrate that C. elegans glia are key for assembly initiation, guiding pioneer and follower axons using distinct signals. Pioneer sublateral neurons, with unique growth properties, anatomy, and innervation, cooperate with glia to mediate follower-axon guidance. We further identify a Chimaerin (CHIN-1)- Furin (KPC-1) double-mutant that severely disrupts assembly. CHIN-1 and KPC-1 function noncanonically, in glia and pioneer neurons, for guidance-cue trafficking. We exploit this bottleneck to define roles for glial Netrin and Semaphorin in pioneer- and follower-axon guidance, respectively, and for glial and pioneer-neuron Flamingo (CELSR) in follower-axon navigation. Taken together, our studies reveal previously undescribed glial roles in pioneer-axon guidance, suggesting conserved principles of brain assembly.

The ENU-3 protein family members function in the Wnt pathway parallel to UNC-6/Netrin to promote motor neuron axon outgrowth in C. elegans

The axons of the DA and DB classes of motor neurons fail to reach the dorsal cord in the absence of the guidance cue UNC-6/Netrin or its receptor UNC-5 in C. elegans. However, the axonal processes usually exit their cell bodies in the ventral cord in the absence of both molecules. Strains lacking functional versions of UNC-6 or UNC-5 have a low level of DA and DB motor neuron axon outgrowth defects. We found that mutations in the genes for all six of the ENU-3 proteins function to enhance the outgrowth defects of the DA and DB axons in strains lacking either UNC-6 or UNC-5. A mutation in the gene for the MIG-14/Wntless protein also enhances defects in a strain lacking either UNC-5 or UNC-6, suggesting that the ENU-3 and Wnt pathways function parallel to the Netrin pathway in directing motor neuron axon outgrowth. Our evidence suggests that the ENU-3 proteins are novel members of the Wnt pathway in nematodes. Five of the six members of the ENU-3 family are predicted to be single-pass trans-membrane proteins. The expression pattern of ENU-3.1 was consistent with plasma membrane localization. One family member, ENU-3.6, lacks the predicted signal peptide and the membrane-spanning domain. In HeLa cells ENU-3.6 had a cytoplasmic localization and caused actin dependent processes to appear. We conclude that the ENU-3 family proteins function in a pathway parallel to the UNC-6/Netrin pathway for motor neuron axon outgrowth, most likely in the Wnt pathway.

Diverse roles for the ror-family receptor tyrosine kinases in neurons and glial cells during development and repair of the nervous system

The Ror-family of receptor tyrosine kinases (RTKs) are involved critically in tissue genesis and organogenesis during development. In mammals, Ror1 and Ror2, members of the Ror-family RTKs, have been shown to mediate cell polarity, migration, proliferation, and differentiation through the activation of noncanonical Wnt signaling by acting as receptors or co-receptors for Wnt5a. Nematodes bearing mutations within the cam-1 gene, encoding a Ror2 ortholog, exhibit defects in various developmental processes of the nervous system, including neuronal cell migration, polarization, axonal extension, and synaptic transmission. In mice, Ror2 and/or Ror1 are also shown to play roles in regulating neurite extension, synapse formation, and synaptic transmission of hippocampal neurons, indicating that the Ror-family RTKs have evolutionarily conserved functions at least in part in neurons during development. Furthermore, Ror2 and/or Ror1 are expressed in neural stem/progenitor cells of the developing brain and in astrocytes of the adult brain after injury, and they play important roles in regulating cell proliferation under these different contexts. In this article, we overview recent advances in our understanding of the roles of the Ror-family RTKs in the development and repair of the nervous system and discuss their potential for therapeutic targets to neurodegenerative diseases. Developmental Dynamics 247:24-32, 2018. © 2017 Wiley Periodicals, Inc.

Regulation of WNT Signaling at the Neuromuscular Junction by the Immunoglobulin Superfamily Protein RIG-3 in Caenorhabditis elegans

Perturbations in synaptic function could affect the normal behavior of an animal, making it important to understand the regulatory mechanisms of synaptic signaling. Previous work has shown that in Caenorhabditis elegans an immunoglobulin superfamily protein, RIG-3, functions in presynaptic neurons to maintain normal acetylcholine receptor levels at the neuromuscular junction (NMJ). In this study, we elucidate the molecular and functional mechanism of RIG-3. We demonstrate by genetic and BiFC (Bi-molecular Fluorescence Complementation) assays that presynaptic RIG-3 functions by directly interacting with the immunoglobulin domain of the nonconventional Wnt receptor, ROR receptor tyrosine kinase (RTK), CAM-1, which functions in postsynaptic body-wall muscles. This interaction in turn inhibits Wnt/LIN-44 signaling through the ROR/CAM-1 receptor, and allows for maintenance of normal acetylcholine receptor, AChR/ACR-16, levels at the neuromuscular synapse. Further, this work reveals that RIG-3 and ROR/CAM-1 function through the β-catenin/HMP-2 at the NMJ. Taken together, our results demonstrate that RIG-3 functions as an inhibitory molecule of the Wnt/LIN-44 signaling pathway through the RTK, CAM-1.

PCP and SAX-3/Robo Pathways Cooperate to Regulate Convergent Extension-Based Nerve Cord Assembly in C. elegans

Formation and resolution of multicellular rosettes can drive convergent extension (CE) type cell rearrangements during tissue morphogenesis. Rosette dynamics are regulated by both planar cell polarity (PCP)-dependent and -independent pathways. Here we show that CE is involved in ventral nerve cord (VNC) assembly in Caenorhabditis elegans. We show that a VANG-1/Van Gogh and PRKL-1/Prickle containing PCP pathway and a Slit-independent SAX-3/Robo pathway cooperate to regulate, via rosette intermediaries, the intercalation of post-mitotic neuronal cell bodies during VNC formation. We show that VANG-1 and SAX-3 are localized to contracting edges and rosette foci and act to specify edge contraction during rosette formation and to mediate timely rosette resolution. Simultaneous loss of both pathways severely curtails CE resulting in a shortened, anteriorly displaced distribution of VNC neurons at hatching. Our results establish rosette-based CE as an evolutionarily conserved mechanism of nerve cord morphogenesis and reveal a role for SAX-3/Robo in this process.

Analysis of the NK2 homeobox gene ceh-24 reveals sublateral motor neuron control of left-right turning during sleep

Sleep is a behavior that is found in all animals that have a nervous system and that have been studied carefully. In Caenorhabditis elegans larvae, sleep is associated with a turning behavior, called flipping, in which animals rotate 180° about their longitudinal axis. However, the molecular and neural substrates of this enigmatic behavior are not known. Here, we identified the conserved NK-2 homeobox gene ceh-24 to be crucially required for flipping. ceh-24 is required for the formation of processes and for cholinergic function of sublateral motor neurons, which separately innervate the four body muscle quadrants. Knockdown of cholinergic function in a subset of these sublateral neurons, the SIAs, abolishes flipping. The SIAs depolarize during flipping and their optogenetic activation induces flipping in a fraction of events. Thus, we identified the sublateral SIA neurons to control the three-dimensional movements of flipping. These neurons may also control other types of motion.

DOI:http://dx.doi.org/10.7554/eLife.24846.001

Although sleeping individuals do not move voluntarily, they are not completely immobile. Both people and animals regularly change position in their sleep, but it is not known why these movements occur or what regulates them.

One of the simplest animals known to require sleep is the nematode worm Caenorhabditis elegans, which is often used by researchers to study the molecular basis of behavior. In common with more complex animals, worms go to sleep lying on either their left or right side and then switch periodically between the two. This “flipping” behavior is typically not seen outside of sleep.

By screening worms with mutations in different genes, Schwarz and Bringmann identified one mutant that does not flip during sleep. The mutant lacked a gene called ceh-24, which is normally active in a set of four neurons known as SIAs. These are a type of motor neuron; that is, neurons that control the contraction of muscles.

The body wall muscles of C. elegans run along the length of its body and are organized into “quadrants” that each cover a quarter of the worm. Schwarz and Bringmann show that unlike other C. elegans motor neurons, SIA neurons control each quadrant separately. By activating specific SIA neurons the worms can contract the muscles on each side of the body independently, and thereby flip from one side to the other.

Further investigation revealed that the SIA motor neurons can also control other types of complex movement. Additional experiments are now needed to determine how the neurons support these behaviors. Another challenge will be to work out the purpose of posture changes during sleep for C. elegans and other animals.

DOI:http://dx.doi.org/10.7554/eLife.24846.002

The Caenorhabditis elegans matrix non-peptidase MNP-1 is required for neuronal cell migration and interacts with the Ror receptor tyrosine kinase CAM-1

Directed cell migration is critical for metazoan development. During Caenorhabditis elegans development many neuronal, muscle and other cell types migrate. Multiple classes of proteins have been implicated in cell migration including secreted guidance cues, receptors for guidance cues and intracellular proteins that respond to cues to polarize cells and produce the forces that move them. In addition, cell surface and secreted proteases have been identified that may clear the migratory route and process guidance cues. We report here that mnp-1 is required for neuronal cell and growth cone migrations. MNP-1 is expressed by migrating cells and functions cell autonomously for cell migrations. We also find a genetic interaction between mnp-1 and cam-1, which encodes a Ror receptor tyrosine kinase required for some of the same cell migrations.

The Genetics of Axon Guidance and Axon Regeneration in Caenorhabditis elegans

The correct wiring of neuronal circuits depends on outgrowth and guidance of neuronal processes during development. In the past two decades, great progress has been made in understanding the molecular basis of axon outgrowth and guidance. Genetic analysis in Caenorhabditis elegans has played a key role in elucidating conserved pathways regulating axon guidance, including Netrin signaling, the slit Slit/Robo pathway, Wnt signaling, and others. Axon guidance factors were first identified by screens for mutations affecting animal behavior, and by direct visual screens for axon guidance defects. Genetic analysis of these pathways has revealed the complex and combinatorial nature of guidance cues, and has delineated how cues guide growth cones via receptor activity and cytoskeletal rearrangement. Several axon guidance pathways also affect directed migrations of non-neuronal cells in C. elegans, with implications for normal and pathological cell migrations in situations such as tumor metastasis. The small number of neurons and highly stereotyped axonal architecture of the C. elegans nervous system allow analysis of axon guidance at the level of single identified axons, and permit in vivo tests of prevailing models of axon guidance. C. elegans axons also have a robust capacity to undergo regenerative regrowth after precise laser injury (axotomy). Although such axon regrowth shares some similarities with developmental axon outgrowth, screens for regrowth mutants have revealed regeneration-specific pathways and factors that were not identified in developmental screens. Several areas remain poorly understood, including how major axon tracts are formed in the embryo, and the function of axon regeneration in the natural environment.

ROR-Family Receptor Tyrosine Kinases

ROR-family receptor tyrosine kinases form a small subfamily of receptor tyrosine kinases (RTKs), characterized by a conserved, unique domain architecture. ROR RTKs are evolutionary conserved throughout the animal kingdom and act as alternative receptors and coreceptors of WNT ligands. The intracellular signaling cascades activated downstream of ROR receptors are diverse, including but not limited to ROR-Frizzled-mediated activation of planar cell polarity signaling, RTK-like signaling, and antagonistic regulation of WNT/β-Catenin signaling. In line with their diverse repertoire of signaling functions, ROR receptors are involved in the regulation of multiple processes in embryonic development such as development of the axial and paraxial mesoderm, the nervous system and the neural crest, the axial and appendicular skeleton, and the kidney. In humans, mutations in the ROR2 gene cause two distinct developmental syndromes, recessive Robinow syndrome (RRS; MIM 268310) and dominant brachydactyly type B1 (BDB1; MIM 113000). In Robinow syndrome patients and animal models, the development of multiple organs is affected, whereas BDB1 results only in shortening of the distal phalanges of fingers and toes, reflecting the diversity of functions and signaling activities of ROR-family RTKs. In this chapter, we give an overview on ROR receptor structure and function. We discuss their signaling functions and role in vertebrate embryonic development with a focus on those developmental processes that are affected by mutations in the ROR2 gene in human patients.

A map of terminal regulators of neuronal identity in Caenorhabditis elegans

Our present day understanding of nervous system development is an amalgam of insights gained from studying different aspects and stages of nervous system development in a variety of invertebrate and vertebrate model systems, with each model system making its own distinctive set of contributions. One aspect of nervous system development that has been among the most extensively studied in the nematode Caenorhabditis elegans is the nature of the gene regulatory programs that specify hardwired, terminal cellular identities. I first summarize a number of maps (anatomical, functional, and molecular) that describe the terminal identity of individual neurons in the C. elegans nervous system. I then provide a comprehensive summary of regulatory factors that specify terminal identities in the nervous system, synthesizing these past studies into a regulatory map of cellular identities in the C. elegans nervous system. This map shows that for three quarters of all neurons in the C. elegans nervous system, regulatory factors that control terminal identity features are known. In-depth studies of specific neuron types have revealed that regulatory factors rarely act alone, but rather act cooperatively in neuron-type specific combinations. In most cases examined so far, distinct, biochemically unlinked terminal identity features are coregulated via cooperatively acting transcription factors, termed terminal selectors, but there are also cases in which distinct identity features are controlled in a piecemeal fashion by independent regulatory inputs. The regulatory map also illustrates that identity-defining transcription factors are reemployed in distinct combinations in different neuron types. However, the same transcription factor can drive terminal differentiation in neurons that are unrelated by lineage, unrelated by function, connectivity and neurotransmitter deployment. Lastly, the regulatory map illustrates the preponderance of homeodomain transcription factors in the control of terminal identities, suggesting that these factors have ancient, phylogenetically conserved roles in controlling terminal neuronal differentiation in the nervous system. WIREs Dev Biol 2016, 5:474-498. doi: 10.1002/wdev.233 For further resources related to this article, please visit the WIREs website.

Morphogenesis of the C. elegans Intestine Involves Axon Guidance Genes

Genetic and molecular studies have provided considerable insight into how various tissue progenitors are specified in early embryogenesis, but much less is known about how those progenitors create three-dimensional tissues and organs. The C. elegans intestine provides a simple system for studying how a single progenitor, the E blastomere, builds an epithelial tube of 20 cells. As the E descendants divide, they form a primordium that transitions between different shapes over time. We used cell contours, traced from confocal optical z-stacks, to build a 3D graphic reconstruction of intestine development. The reconstruction revealed several new aspects of morphogenesis that extend and clarify previous observations. The first 8 E descendants form a plane of four right cells and four left cells; the plane arises through oriented cell divisions and VANG-1/Van Gogh-dependent repositioning of any non-planar cells. LIN-12/Notch signaling affects the left cells in the E8 primordium, and initiates later asymmetry in cell packing. The next few stages involve cell repositioning and intercalation events that shuttle cells to their final positions, like shifting blocks in a Rubik’s cube. Repositioning involves breaking and replacing specific adhesive contacts, and some of these events involve EFN-4/Ephrin, MAB-20/semaphorin-2a, and SAX-3/Robo. Once cells in the primordium align along a common axis and in the correct order, cells at the anterior end rotate clockwise around the axis of the intestine. The anterior rotation appears to align segments of the developing lumen into a continuous structure, and requires the secreted ligand UNC-6/netrin, the receptor UNC-40/DCC, and an interacting protein called MADD-2. Previous studies showed that rotation requires a second round of LIN-12/Notch signaling in cells on the right side of the primordium, and we show that MADD-2-GFP appears to be downregulated in those cells.

This report uses the intestine of the nematode C. elegans as a model system to address how progenitor cells form a three-dimensional organ. The fully formed intestine is a cylindrical tube of only 20 epithelial cells, and all of these cells are descendants of a single cell, the E blastomere. The E descendants form a primordium that changes shape over time as different E descendants divide and move. Cells in the primordium must continually adhere to each other during these movements to maintain the integrity of the primordium. Here, we generated a 3D graphic reconstruction of the developing intestine in order to analyze these events. We found that the cell movements are highly reproducible, suggesting that they are programmed by asymmetric gene expression in the primordium. In particular, we found that the conserved receptor LIN-12/Notch appears to modulate left-right adhesion in the primordium, leading to the asymmetric packing of cells. One of the most remarkable events in intestinal morphogenesis is the circumferential rotation of a subset of cells. We found that rotation appears to have a role in aligning the developing lumen of the intestine, and involves a conserved, UNC-6/netrin signaling pathway that is best known for its roles in the guided growth of neurons.

The Caenorhabditis elegans Ephrin EFN-4 Functions Non-cell Autonomously with Heparan Sulfate Proteoglycans to Promote Axon Outgrowth and Branching

The Eph receptors and their cognate ephrin ligands play key roles in many aspects of nervous system development. These interactions typically occur within an individual tissue type, serving either to guide axons to their terminal targets or to define boundaries between the rhombomeres of the hindbrain. We have identified a novel role for the Caenorhabditis elegans ephrin EFN-4 in promoting primary neurite outgrowth in AIY interneurons and D-class motor neurons. Rescue experiments reveal that EFN-4 functions non-cell autonomously in the epidermis to promote primary neurite outgrowth. We also find that EFN-4 plays a role in promoting ectopic axon branching in a C. elegans model of X-linked Kallmann syndrome. In this context, EFN-4 functions non-cell autonomously in the body-wall muscle and in parallel with HS modification genes and HSPG core proteins. This is the first report of an epidermal ephrin providing a developmental cue to the nervous system.

The C-terminal binding protein (CTBP-1) regulates dorsal SMD axonal morphology in Caenorhabditis elegans

C-terminal binding proteins (CtBPs) are transcriptional co-repressors which cooperate with a variety of transcription factors to repress gene expression. Caenorhabditis elegans CTBP-1 expression has been observed in the nervous system and hypodermis. In C. elegans, CTBP-1 regulates several processes including Acute Functional Tolerance to ethanol and functions in the nervous system to modulate both lifespan and expression of a lipase gene called lips-7. Incorrect structure and/or function of the nervous system can lead to behavioral changes. Here, we demonstrate reduced exploration behavior in ctbp-1 mutants. Our examination of a subset of neurons involved in regulating locomotion revealed that the axonal morphology of dorsal SMD (SMDD) neurons is altered in ctbp-1 mutants at the fourth larval (L4) stage. Expressing CTBP-1 under the control of the endogenous ctbp-1 promoter rescued both the exploration behavior phenotype and defective SMDD axon structure in ctbp-1 mutants at the L4 stage. Interestingly, the pre-synaptic marker RAB-3 was found to localize to the mispositioned portion of SMDD axons in a ctbp-1 mutant. Further analysis of SMDD axonal morphology at days 1, 3 and 5 of adulthood revealed that the number of ctbp-1 mutants showing an SMDD axonal morphology defect increases in early adulthood and the observed defect appears to be qualitatively more severe. CTBP-1 is prominently expressed in the nervous system with weak expression detected in the hypodermis. Surprisingly, solely expressing CTBP-1a in the nervous system or hypodermis did not restore correct SMDD axonal structure in a ctbp-1 mutant. Our results demonstrate a role for CTBP-1 in exploration behavior and the regulation of SMDD axonal morphology in C. elegans.

The Evolutionarily Conserved LIM Homeodomain Protein LIM-4/LHX6 Specifies the Terminal Identity of a Cholinergic and Peptidergic C. elegans Sensory/Inter/Motor Neuron-Type

The expression of specific transcription factors determines the differentiated features of postmitotic neurons. However, the mechanism by which specific molecules determine neuronal cell fate and the extent to which the functions of transcription factors are conserved in evolution are not fully understood. In C. elegans, the cholinergic and peptidergic SMB sensory/inter/motor neurons innervate muscle quadrants in the head and control the amplitude of sinusoidal movement. Here we show that the LIM homeobox protein LIM-4 determines neuronal characteristics of the SMB neurons. In lim-4 mutant animals, expression of terminal differentiation genes, such as the cholinergic gene battery and the flp-12 neuropeptide gene, is completely abolished and thus the function of the SMB neurons is compromised. LIM-4 activity promotes SMB identity by directly regulating the expression of the SMB marker genes via a distinct cis-regulatory motif. Two human LIM-4 orthologs, LHX6 and LHX8, functionally substitute for LIM-4 in C. elegans. Furthermore, C. elegans LIM-4 or human LHX6 can induce cholinergic and peptidergic characteristics in the human neuronal cell lines. Our results indicate that the evolutionarily conserved LIM-4/LHX6 homeodomain proteins function in generation of precise neuronal subtypes.

Autonomous and nonautonomous regulation of Wnt-mediated neuronal polarity by the C. elegans Ror kinase CAM-1

Wnts are a conserved family of secreted glycoproteins that regulate various developmental processes in metazoans. Three of the five Caenorhabditis elegans Wnts, CWN-1, CWN-2 and EGL-20, and the sole Wnt receptor of the Ror kinase family, CAM-1, are known to regulate the anterior polarization of the mechanosensory neuron ALM. Here we show that CAM-1 and the Frizzled receptor MOM-5 act in parallel pathways to control ALM polarity. We also show that CAM-1 has two functions in this process: an autonomous signaling function that promotes anterior polarization and a nonautonomous Wnt-antagonistic function that inhibits anterior polarization. These antagonistic activities can account for the weak ALM phenotypes displayed by cam-1 mutants. Our observations suggest that CAM-1 could function as a Wnt receptor in many developmental processes, but the analysis of cam-1 mutants may fail to reveal CAM-1's role as a receptor in these processes because of its Wnt-antagonistic activity. In this model, loss of CAM-1 results in increased levels of Wnts that act through other Wnt receptors, masking CAM-1's autonomous role as a Wnt receptor.

TMC-1 attenuates C. elegans development and sexual behaviour in a chemically defined food environment

Although diet affects growth and behaviour, the adaptive mechanisms that coordinate these processes in non-optimal food sources are unclear. Here we show that the C. elegans tmc-1 channel, which is homologous to the mammalian tmc deafness genes, attenuates development and inhibits sexual behaviour in non-optimal food, the synthetic CeMM medium. In CeMM medium, signalling from the pharyngeal MC neurons and body wall muscles slows larval development. However, in the non-standard diet, mutation in tmc-1 accelerates development, by impairing the excitability of these cells. The tmc-1 larva can immediately generate ATP when fed CeMM, and their fast development requires insulin signalling. Our findings suggest that the tmc-1 channel indirectly affects metabolism in wild-type animals. In addition to regulating the development, we show that mutating tmc-1 can relax diet-induced inhibition of male sexual behaviour, thus indicating that a single regulator can be genetically modified to promote growth rate and reproductive success in new environments.

Wnt-signaling and planar cell polarity genes regulate axon guidance along the anteroposterior axis in C. elegans

During the development of the nervous system, neurons encounter signals that inform their outgrowth and polarization. Understanding how these signals combinatorially function to pattern the nervous system is of considerable interest to developmental neurobiologists. The Wnt ligands and their receptors have been well characterized in polarizing cells during asymmetric cell division. The planar cell polarity (PCP) pathway is also critical for cell polarization in the plane of an epithelium. The core set of PCP genes include members of the conserved Wnt-signaling pathway, such as Frizzled and Disheveled, but also the cadherin-domain protein Flamingo. In Drosophila, the Fat and Dachsous cadherins also function in PCP, but in parallel to the core PCP components. C. elegans also have two Fat-like and one Dachsous-like cadherins, at least one of which, cdh-4, contributes to neural development. In C. elegans Wnt ligands and the conserved PCP genes have been shown to regulate a number of different events, including embryonic cell polarity, vulval morphogenesis, and cell migration. As is also observed in vertebrates, the Wnt and PCP genes appear to function to primarily provide information about the anterior to posterior axis of development. Here, we review the recent work describing how mutations in the Wnt and core PCP genes affect axon guidance and synaptogenesis in C. elegans.

The Nesprin family member ANC-1 regulates synapse formation and axon termination by functioning in a pathway with RPM-1 and β-Catenin

Mutations in Nesprin-1 and 2 (also called Syne-1 and 2) are associated with numerous diseases including autism, cerebellar ataxia, cancer, and Emery-Dreifuss muscular dystrophy. Nesprin-1 and 2 have conserved orthologs in flies and worms called MSP-300 and abnormal nuclear Anchorage 1 (ANC-1), respectively. The Nesprin protein family mediates nuclear and organelle anchorage and positioning. In the nervous system, the only known function of Nesprin-1 and 2 is in regulation of neurogenesis and neural migration. It remains unclear if Nesprin-1 and 2 regulate other functions in neurons. Using a proteomic approach in C. elegans, we have found that ANC-1 binds to the Regulator of Presynaptic Morphology 1 (RPM-1). RPM-1 is part of a conserved family of signaling molecules called Pam/Highwire/RPM-1 (PHR) proteins that are important regulators of neuronal development. We have found that ANC-1, like RPM-1, regulates axon termination and synapse formation. Our genetic analysis indicates that ANC-1 functions via the β-catenin BAR-1, and the ANC-1/BAR-1 pathway functions cell autonomously, downstream of RPM-1 to regulate neuronal development. Further, ANC-1 binding to the nucleus is required for its function in axon termination and synapse formation. We identify variable roles for four different Wnts (LIN-44, EGL-20, CWN-1 and CWN-2) that function through BAR-1 to regulate axon termination. Our study highlights an emerging, broad role for ANC-1 in neuronal development, and unveils a new and unexpected mechanism by which RPM-1 functions.

The molecular mechanisms that underpin synapse formation and axon termination are central to forming a functional, fully connected nervous system. The PHR proteins are important regulators of neuronal development that function in axon outgrowth and termination, as well as synapse formation. Here we describe the discovery of a novel, conserved pathway that is positively regulated by the C. elegans PHR protein, RPM-1. This pathway is composed of RPM-1, ANC-1 (a Nesprin family protein), and BAR-1 (a canonical β-catenin). Nesprins, such as ANC-1, regulate nuclear anchorage and positioning in multinuclear cells. We now show that in neurons, ANC-1 regulates neuronal development by positively regulating BAR-1. Thus, Nesprins are multi-functional proteins that act through β-catenin to regulate neuronal development, and link the nucleus to the actin cytoskeleton in order to mediate nuclear anchorage and positioning in multi-nuclear cells.

The role of Ryk and Ror receptor tyrosine kinases in Wnt signal transduction

Receptor tyrosine kinases of the Ryk and Ror families were initially classified as orphan receptors because their ligands were unknown. They are now known to contain functional extracellular Wnt-binding domains and are implicated in Wnt-signal transduction in multiple species. Although their signaling mechanisms still remain to be resolved in detail, both Ryk and Ror control important developmental processes in different tissues. However, whereas many other Wnt-signaling responses affect cell proliferation and differentiation, Ryk and Ror are mostly associated with controlling processes that rely on the polarized migration of cells. Here we discuss what is currently known about the involvement of this exciting class of receptors in development and disease.

The conserved transmembrane RING finger protein PLR-1 downregulates Wnt signaling by reducing Frizzled, Ror and Ryk cell-surface levels in C. elegans

Wnts control a wide range of essential developmental processes, including cell fate specification, axon guidance and anteroposterior neuronal polarization. We identified a conserved transmembrane RING finger protein, PLR-1, that governs the response to Wnts by lowering cell-surface levels of the Frizzled family of Wnt receptors in Caenorhabditis elegans. Loss of PLR-1 activity in the neuron AVG causes its anteroposterior polarity to be symmetric or reversed because signaling by the Wnts CWN-1 and CWN-2 are inappropriately activated, whereas ectopic PLR-1 expression blocks Wnt signaling and target gene expression. Frizzleds are enriched at the cell surface; however, when PLR-1 and Frizzled are co-expressed, Frizzled is not detected at the surface but instead is colocalized with PLR-1 in endosomes. The Frizzled cysteine-rich domain (CRD) and invariant second intracellular loop lysine are crucial for PLR-1 downregulation. The PLR-1 RING finger and protease-associated (PA) domain are essential for activity. In a Frizzled-dependent manner, PLR-1 reduces surface levels of the Wnt receptors CAM-1/Ror and LIN-18/Ryk. PLR-1 is a homolog of the mammalian transmembrane E3 ubiquitin ligases RNF43 and ZNRF3, which control Frizzled surface levels in an R-spondin-sensitive manner. We propose that PLR-1 downregulates Wnt receptor surface levels via lysine ubiquitylation of Frizzled to coordinate spatial and temporal responses to Wnts during neuronal development.

Wnt signaling through the Ror receptor in the nervous system

The receptor tyrosine kinase-like orphan receptor (Ror) proteins are conserved tyrosine kinase receptors that play roles in a variety of cellular processes that pattern tissues and organs during vertebrate and invertebrate development. Ror signaling is required for skeleton and neuronal development and modulates cell migration, cell polarity, and convergent extension. Ror has also been implicated in two human skeletal disorders, brachydactyly type B and Robinow syndrome. Rors are widely expressed during metazoan development including domains in the nervous system. Here, we review recent progress in understanding the roles of the Ror receptors in neuronal migration, axonal pruning, axon guidance, and synaptic plasticity. The processes by which Ror signaling execute these diverse roles are still largely unknown, but they likely converge on cytoskeletal remodeling. In multiple species, Rors have been shown to act as Wnt receptors signaling via novel non-canonical Wnt pathways mediated in some tissues by the adapter protein disheveled and the non-receptor tyrosine kinase Src. Rors can either activate or repress Wnt target expression depending on the cellular context and can also modulate signal transduction by sequestering Wnt ligands away from their signaling receptors. Future challenges include the identification of signaling components of the Ror pathways and bettering our understanding of the roles of these pleiotropic receptors in patterning the nervous system.

Neuronal target identification requires AHA-1-mediated fine-tuning of Wnt signaling in C. elegans

Electrical synaptic transmission through gap junctions is a vital mode of intercellular communication in the nervous system. The mechanism by which reciprocal target cells find each other during the formation of gap junctions, however, is poorly understood. Here we show that gap junctions are formed between BDU interneurons and PLM mechanoreceptors in C. elegans and the connectivity of BDU with PLM is influenced by Wnt signaling. We further identified two PAS-bHLH family transcription factors, AHA-1 and AHR-1, which function cell-autonomously within BDU and PLM to facilitate the target identification process. aha-1 and ahr-1 act genetically upstream of cam-1. CAM-1, a membrane-bound receptor tyrosine kinase, is present on both BDU and PLM cells and likely serves as a Wnt antagonist. By binding to a cis-regulatory element in the cam-1 promoter, AHA-1 enhances cam-1 transcription. Our study reveals a Wnt-dependent fine-tuning mechanism that is crucial for mutual target cell identification during the formation of gap junction connections.

The establishment of functional neuronal circuits requires that different neurons respond selectively to guidance molecules at particular times and in specific locations. In the target region, where cells connect, the same guidance molecules steer the growth of neurites from both the neuron and its target cell. The spatial, temporal, and cell-type-specific regulation of neuronal connection needs to be tightly regulated and precisely coordinated within the neuron and its target cell to achieve effective connection. In this study, we found that the precise connectivity of the BDU interneuron and the PLM mechanoreceptor in the nematode worm Caenorhabditis elegans is influenced by Wnt signaling. BDU-PLM contact also depends on the transcription factor AHA-1, which functions within both BDU and PLM cells to enhance transcription of the gene encoding the trans-membrane receptor CAM-1. CAM-1 is present on BDU and PLM and likely serves as a Wnt antagonist, thus linking transcriptional regulation by AHA-1 to modulation of Wnt signaling. Therefore, our study reveals a locally confined, cell type-specific and cell-autonomous mechanism that mediates mutual target identification.

Nicotinic acetylcholine receptors: a comparison of the nAChRs of Caenorhabditis elegans and parasitic nematodes

Nicotinic acetylcholine receptors (nAChRs) play a key role in the normal physiology of nematodes and provide an established target site for anthelmintics. The free-living nematode, Caenorhabditis elegans, has a large number of nAChR subunit genes in its genome and so provides an experimental model for testing novel anthelmintics which act at these sites. However, many parasitic nematodes lack specific genes present in C. elegans, and so care is required in extrapolating from studies using C. elegans to the situation in other nematodes. In this review the properties of C. elegans nAChRs are reviewed and compared to those of parasitic nematodes. This forms the basis for a discussion of the possible subunit composition of nAChRs from different species of parasitic nematodes. Currently our knowledge on this is largely based on studies using heterologous expression and pharmacological analysis of receptor subunits in Xenopus laevis oocytes. It is concluded that more information is required regarding the subunit composition and pharmacology of endogenous nAChRs in parasitic nematodes.

Neurons refine the Caenorhabditis elegans body plan by directing axial patterning by Wnts

Metazoans display remarkable conservation of gene families, including growth factors, yet somehow these genes are used in different ways to generate tremendous morphological diversity. While variations in the magnitude and spatio-temporal aspects of signaling by a growth factor can generate different body patterns, how these signaling variations are organized and coordinated during development is unclear. Basic body plans are organized by the end of gastrulation and are refined as limbs, organs, and nervous systems co-develop. Despite their proximity to developing tissues, neurons are primarily thought to act after development, on behavior. Here, we show that in Caenorhabditis elegans, the axonal projections of neurons regulate tissue progenitor responses to Wnts so that certain organs develop with the correct morphology at the right axial positions. We find that foreshortening of the posteriorly directed axons of the two canal-associated neurons (CANs) disrupts mid-body vulval morphology, and produces ectopic vulval tissue in the posterior epidermis, in a Wnt-dependent manner. We also provide evidence that suggests that the posterior CAN axons modulate the location and strength of Wnt signaling along the anterior-posterior axis by employing a Ror family Wnt receptor to bind posteriorly derived Wnts, and hence, refine their distributions. Surprisingly, despite high levels of Ror expression in many other cells, these cells cannot substitute for the CAN axons in patterning the epidermis, nor can cells expressing a secreted Wnt inhibitor, SFRP-1. Thus, unmyelinated axon tracts are critical for patterning the C. elegans body. Our findings suggest that the evolution of neurons not only improved metazoans by increasing behavioral complexity, but also by expanding the diversity of developmental patterns generated by growth factors such as Wnts.

β-catenin-dependent Wnt signaling in C. elegans: teaching an old dog a new trick

Wnt signaling is an evolutionarily ancient pathway used to regulate many events during metazoan development. Genetic results from Caenorhabditis elegans more than a dozen years ago suggested that Wnt signaling in this nematode worm might be different than in vertebrates and Drosophila: the worm had a small number of Wnts, too many β-catenins, and some Wnt pathway components functioned in an opposite manner than in other species. Work over the ensuing years has clarified that C. elegans does possess a canonical Wnt/β-catenin signaling pathway similar to that in other metazoans, but that the majority of Wnt signaling in this species may proceed via a variant Wnt/β-catenin signaling pathway that uses some new components (mitogen-activated protein kinase signaling enzymes), and in which some conserved pathway components (β-catenin, T-cell factor [TCF]) are used in new and interesting ways. This review summarizes our current understanding of the canonical and novel TCF/β-catenin-dependent signaling pathways in C. elegans.

Wnt signaling regulates acetylcholine receptor translocation and synaptic plasticity in the adult nervous system

The adult nervous system is plastic, allowing us to learn, remember, and forget. Experience-dependent plasticity occurs at synapses--the specialized points of contact between neurons where signaling occurs. However, the mechanisms that regulate the strength of synaptic signaling are not well understood. Here, we define a Wnt-signaling pathway that modifies synaptic strength in the adult nervous system by regulating the translocation of one class of acetylcholine receptors (AChRs) to synapses. In Caenorhabditis elegans, we show that mutations in CWN-2 (Wnt ligand), LIN-17 (Frizzled), CAM-1 (Ror receptor tyrosine kinase), or the downstream effector DSH-1 (disheveled) result in similar subsynaptic accumulations of ACR-16/α7 AChRs, a consequent reduction in synaptic current, and predictable behavioral defects. Photoconversion experiments revealed defective translocation of ACR-16/α7 to synapses in Wnt-signaling mutants. Using optogenetic nerve stimulation, we demonstrate activity-dependent synaptic plasticity and its dependence on ACR-16/α7 translocation mediated by Wnt signaling via LIN-17/CAM-1 heteromeric receptors.

The tangled web of non-canonical Wnt signalling in neural migration

In all multicellular animals, successful embryogenesis is dependent on the ability of cells to detect the status of the local environment and respond appropriately. The nature of the extracellular environment is communicated to the intracellular compartment by ligand/receptor interactions at the cell surface. The Wnt canonical and non-canonical signalling pathways are found in the most primitive metazoans, and they play an essential role in the most fundamental developmental processes in all multicellular organisms. Vertebrates have expanded the number of Wnts and Frizzled receptors and have additionally evolved novel Wnt receptor families (Ryk, Ror). The multiplicity of potential interactions between Wnts, their receptors and downstream effectors has exponentially increased the complexity of the signal transduction network. Signalling through each of the Wnt pathways, as well as crosstalk between them, plays a critical role in the establishment of the complex architecture of the vertebrate central nervous system. In this review, we explore the signalling networks triggered by non-canonical Wnt/receptor interactions, focussing on the emerging roles of the non-conventional Wnt receptors Ryk and Ror. We describe the role of these pathways in neural tube formation and axon guidance where Wnt signalling controls tissue polarity, coordinated cell migration and axon guidance via remodelling of the cytoskeleton.

Ror-family receptor tyrosine kinases regulate maintenance of neural progenitor cells in the developing neocortex

The Ror-family of receptor tyrosine kinases (RTKs), Ror1 and Ror2, have been shown to play crucial roles in the developmental morphogenesis by acting as receptors or co-receptors to mediate Wnt5a-induced signaling. Although Ror1, Ror2, and Wnt5a are expressed in the developing brain, little is known about their roles in the neural development. Here we show that Ror1, Ror2, and their ligand Wnt5a are highly expressed in neocortical neural progenitor cells (NPCs). siRNA-mediated suppression of Ror1, Ror2, or Wnt5a in cultured NPCs isolated from embryonic neocortex results in the reduction of βIII-tubulin-positive neurons that are produced from NPCs possibly through the generation of T-box brain 2 (Tbr2)-positive intermediate progenitors. BrdU-labeling experiments further reveal that proportion of proliferative and neurogenic NPCs, that are positive for neural progenitor cell marker (Pax6), but negative for glial cell marker (glial fibrillary acidic protein; GFAP), is reduced within a few days in culture following knockdown of these molecules, suggesting that Ror1, Ror2, and Wnt5a regulate neurogenesis through the maintenance of NPCs. Moreover, we show that Dishevelled2 (Dvl2) is involved in Wnt5a–Ror1 and Wnt5a–Ror2 signalings in NPCs, and that suppressed expression of Dvl2 indeed reduces the proportion of proliferative and neurogenic NPCs. Interestingly, suppressed or forced expression of either Ror1 or Ror2 in NPCs in the developing neocortex results in their precocious or delayed differentiation into neurons, respectively. Collectively, these results indicate that Wnt5a–Ror1 and Wnt5a–Ror2 signalings play roles in maintaining proliferative and neurogenic NPCs during neurogenesis of the developing neocortex.