Caenorhabditis elegans flamingo cadherin fmi-1 regulates GABAergic neuronal development

Essential Role of Latrophilin-1 Adhesion GPCR Nanoclusters in Inhibitory Synapses

Latrophilin-1 (Lphn1, aka CIRL1 and CL1; gene symbol Adgrl1) is an adhesion GPCR that has been implicated in excitatory synaptic transmission as a candidate receptor for α-latrotoxin. Here we analyzed conditional knock-in/knock-out mice for Lphn1 that contain an extracellular myc epitope tag. Mice of both sexes were used in all experiments. Surprisingly, we found that Lphn1 is localized in cultured neurons to synaptic nanoclusters that are present in both excitatory and inhibitory synapses. Conditional deletion of Lphn1 in cultured neurons failed to elicit a detectable impairment in excitatory synapses but produced a decrease in inhibitory synapse numbers and synaptic transmission that was most pronounced for synapses close to the neuronal soma. No changes in axonal or dendritic outgrowth or branching were observed. Our data indicate that Lphn1 is among the few postsynaptic adhesion molecules that are present in both excitatory and inhibitory synapses and that Lphn1 by itself is not essential for excitatory synaptic transmission but is required for some inhibitory synaptic connections.

Experience-dependent, sexually dimorphic synaptic connectivity defined by sex-specific cadherin expression

We describe here the molecular mechanisms by which juvenile experience defines patterns of sexually dimorphic synaptic connectivity in the adult nervous system of the nematode C. elegans. We show that starvation of juvenile males disrupts serotonin-dependent activation of the CREB transcription factor in a nociceptive sensory neuron, PHB. CREB acts through a cascade of transcription factors to control expression of an atypical cadherin protein, FMI-1/Flamingo. During postembryonic development, FMI-1/Flamingo has the capacity to promote and maintain synaptic connectivity of the PHB nociceptive sensory to a command interneuron, AVA, in both sexes, but the serotonin transcriptional regulatory cassette antagonizes FMI-1/Flamingo expression in males, thereby establishing sexually dimorphic connectivity between PHB and AVA. A critical regulatory node in this process is the CREB-target LIN-29, a Zn finger transcription factor which integrates four different layers of information - sexual specificity, past feeding status, time and cell-type specificity. Our findings provide the mechanistic details of how an early juvenile experience defines sexually dimorphic synaptic connectivity.

The adhesion GPCR and PCP component flamingo (FMI-1) alters body size and regulates the composition of the extracellular matrix

The extracellular matrix (ECM) is a network of macromolecules that presents a vital scaffold for cells and enables multiple ways of cellular communication. Thus, it is essential for many physiological processes such as development, tissue morphogenesis, homeostasis, the shape and partially the size of the body and its organs. To ensure these, the composition of the ECM is tissue-specific and highly dynamic. ECM homeostasis is therefore tightly controlled by several mechanisms. Here, we show that FMI-1, the homolog of the Adhesion GPCR Flamingo/CELSR/ADGRC in the nematode Caenorhabditis elegans, modulates the composition of the ECM by controlling the production both of ECM molecules such as collagens and also of ECM modifying enzymes. Thereby, FMI-1 affects the morphology and functionality of the nematode´s cuticle, which is mainly composed of ECM, and also modulates the body size. Mechanistic analyses highlight the fact that FMI-1 exerts its function from neurons non-cell autonomously (trans) solely via its extracellular N terminus. Our data support a model, by which the activity of the receptor, which has a well-described role in the planar cell polarity (PCP) pathway, involves the PCP molecule VANG-1, but seems to be independent of the DBL-1/BMP pathway.

Dissecting the genetic landscape of GPCR signaling through phenotypic profiling in C. elegans

G protein-coupled receptors (GPCRs) mediate responses to various extracellular and intracellular cues. However, the large number of GPCR genes and their substantial functional redundancy make it challenging to systematically dissect GPCR functions in vivo. Here, we employ a CRISPR/Cas9-based approach, disrupting 1654 GPCR-encoding genes in 284 strains and mutating 152 neuropeptide-encoding genes in 38 strains in C. elegans. These two mutant libraries enable effective deorphanization of chemoreceptors, and characterization of receptors for neuropeptides in various cellular processes. Mutating a set of closely related GPCRs in a single strain permits the assignment of functions to GPCRs with functional redundancy. Our analyses identify a neuropeptide that interacts with three receptors in hypoxia-evoked locomotory responses, unveil a collection of regulators in pathogen-induced immune responses, and define receptors for the volatile food-related odorants. These results establish our GPCR and neuropeptide mutant libraries as valuable resources for the C. elegans community to expedite studies of GPCR signaling in multiple contexts.

The adhesion GPCRs CELSR1-3 and LPHN3 engage G proteins via distinct activation mechanisms

Adhesion G protein-coupled receptors (aGPCRs) are a large GPCR class that direct diverse fundamental biological processes. One prominent mechanism for aGPCR agonism involves autoproteolytic cleavage, which generates an activating, membrane-proximal tethered agonist (TA). How universal this mechanism is for all aGPCRs is unclear. Here, we investigate G protein induction principles of aGPCRs using mammalian latrophilin 3 (LPHN3) and cadherin EGF LAG-repeat 7-transmembrane receptors 1-3 (CELSR1-3), members of two aGPCR families conserved from invertebrates to vertebrates. LPHNs and CELSRs mediate fundamental aspects of brain development, yet CELSR signaling mechanisms are unknown. We find that CELSR1 and CELSR3 are cleavage deficient, while CELSR2 is efficiently cleaved. Despite differential autoproteolysis, CELSR1-3 all engage GαS, and CELSR1 or CELSR3 TA point mutants retain GαS coupling activity. CELSR2 autoproteolysis enhances GαS coupling, yet acute TA exposure alone is insufficient. These studies support that aGPCRs signal via multiple paradigms and provide insights into CELSR biological function.

The adhesion GPCRs CELSR1-3 and LPHN3 engage G proteins via distinct activation mechanisms

Adhesion GPCRs (aGPCRs) are a large GPCR class that direct diverse fundamental biological processes. One prominent mechanism for aGPCR agonism involves autoproteolytic cleavage, which generates an activating, membrane-proximal tethered agonist (TA). How universal this mechanism is for all aGPCRs is unclear. Here, we investigate G protein induction principles of aGPCRs using mammalian LPHN3 and CELSR1-3, members of two aGPCR families conserved from invertebrates to vertebrates. LPHNs and CELSRs mediate fundamental aspects of brain development, yet CELSR signaling mechanisms are unknown. We found that CELSR1 and CELSR3 are cleavage-deficient, while CELSR2 is efficiently cleaved. Despite differential autoproteolysis, CELSR1-3 all engage GαS, and CELSR1 or CELSR3 TA point mutants retain GαS coupling activity. CELSR2 autoproteolysis enhances GαS coupling, yet acute TA exposure alone is insufficient. These studies support that aGPCRs signal via multiple paradigms and provide insights into CELSR biological function.

Function of cell adhesion molecules in differentiation of ray sensory neurons in C. elegans

For proper functioning of the nervous system, it is crucial that neurons find their appropriate partners and build the correct neural connection patterns. Although cell adhesion molecules (CAMs) have been studied for many years as essential players in neural connections, we have yet to unravel the code by which CAMs encode synaptic specificity. We analyzed the effects of mutations in CAM genes on the morphology and synapses of a set of sensory neurons in the Caenorhabditis elegans male tail. B-type ray sensory neurons express 10 genes encoding CAMs. We examined the effect on axon trajectory and localization of pre-synaptic components in viable mutants of nine of these. We found axon trajectory defects in mutants of UNC-40/DCC, SAX-3/ROBO, and FMI-1/Flamingo/Celsr1. None of the mutations caused loss of pre-synaptic components in axons, and in several the level even appeared to increase, suggesting possible accumulation of pre-synapses. B-type sensory neurons fasciculate with a second type of ray sensory neuron, the A-type, in axon commissures. We found a CAM expressed in A-type functions additively with a CAM expressed in B-type in axon guidance, and lack of a CAM expressed in B-type affected A-type axon guidance. Overall, single and multiple mutants of CAM genes had limited effects on ray neuron trajectories and accumulation of synaptic components.

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.

Age-dependent accumulation of tau aggregation in Caenorhabditis elegans

Aging is the primary risk factor for Alzheimer's disease (AD) and related disorders (ADRDs). Tau aggregation is a hallmark of AD and other tauopathies. Even in normal aging, tau aggregation is found in brains, but in disease states, significantly more aggregated tau is present in brain regions demonstrating synaptic degeneration and neuronal loss. It is unclear how tau aggregation and aging interact to give rise to the phenotypes observed in disease states. Most AD/ADRD animal models have focused on late stages, after significant tau aggregation has occurred. There are fewer where we can observe the early aggregation events and progression during aging. In an attempt to address this gap, we created C. elegans models expressing a GFP-tagged version of the human tau protein. Here we examined how tau-gfp behaved during aging, comparing wild-type tau (hTau40), a disease-associated mutation (P301S), and an aggregation-prone variant (3PO). We measured age-dependent changes in GFP intensity and correlated those changes to normal aging in the nematode. We found differences in tau stability and accumulation depending on the tau variant expressed. hTau40GFP and P301SGFP were localized to axons and cell bodies, while 3POGFP was more concentrated within cell bodies. Expression of 3POGFP resulted in decreased lifespan and variations in locomotor rate, consistent with a pathological effect. Finally, we found that the human tau interacted genetically with the C. elegans ortholog of human tau, ptl-1, where the loss of ptl-1 significantly accelerated the time to death in animals expressing 3PO.

Celsr family genes are dynamically expressed in embryonic and juvenile zebrafish

The Cadherin EGF LAG seven-pass G-type receptor (Celsr) family belongs to the adhesion G-protein coupled receptor superfamily. In most vertebrates, the Celsr family has three members (CELSR1-3), whereas zebrafish display four paralogues (celsr1a, 1b, 2, 3). Although studies have shown the importance of the Celsr family in planar cell polarity, axonal guidance, and dendritic growth, the molecular mechanisms of the Celsr family regulating these cellular processes in vertebrates remain elusive. Zebrafish is an experimentally more amenable model to study vertebrate development, as zebrafish embryos develop externally, optically transparent, remain alive with malformed organs, and zebrafish is genetically similar to humans. Understanding the detailed expression pattern is the first step of exploring the functional mechanisms of the genes involved in development. Thus, we report the spatiotemporal expression pattern of Celsr family members in zebrafish nervous tissues. Our analysis shows that celsr1b and celsr2 are expressed maternally. In embryos, celsr1a, celsr1b, and celsr2 are expressed in the neural progenitors, and celsr3 is expressed in all five primary neural clusters of the brain and mantle layer of the spinal cord. In juvenile zebrafish, celsr1a, celsr1b, and celsr2 are presumably expressed in the neural progenitor enriched regions of the CNS. Therefore, the expression pattern of zebrafish Celsr family members is reminiscent of patterns described in other vertebrates or mammalian speciate. This indicates the conserved role of Celsr family genes in nervous system development and suggests zebrafish as an excellent model to explore the cellular and molecular mechanisms of Celsr family genes in vertebrate neurogenesis. This article is protected by copyright. All rights reserved.

The Immunoglobulin Superfamily Members syg-2 and syg-1 Regulate Neurite Development in C. elegans

Neurons form elaborate networks by guiding axons and dendrites to appropriate destinations. Neurites require information about the relative body axes during the initial projection from the cell body, and failure to receive or interpret those cues correctly can result in outgrowth errors. We identified a mutation in the Ig superfamily member syg-2 in a screen for animals with anterior/posterior (A/P) axon guidance defects. We found that syg-2 and its cognate Ig family member syg-1 appear to function in a linear genetic pathway to control the outgrowth of GABAergic axons. We determined that this pathway works in parallel to Wnt signaling. Specifically, mutations in syg-2 or syg-1 selectively affected the embryonically derived Dorsal D-type (DD) GABAergic neurons. We found no evidence that these mutations affected the Ventral D-type neurons (VD) that form later, during the first larval stage. In addition, mutations in syg-1 or syg-2 could result in the DD neurons forming multiple processes, becoming bipolar, rather than the expected pseudounipolar morphology. Given SYG-2′s essential function in synaptogenesis of the hermaphrodite-specific neurons (HSNs), we also examined DD neuron synapses in syg-2 mutants. We found syg-2 mutants had a decreased number of synapses formed, but synaptic morphology was largely normal. These results provide further evidence that the GABAergic motorneurons use multiple guidance pathways during development.

Mutations in the Spliceosome Component prp-6 and Overexpression of cdh-5 Suppress Axon Guidance Defects of cdh-4 Mutants in Caenorhabditis elegans

During nervous system development, axons must navigate to specific target areas. In Caenorhabditis elegans, the cadherin CDH-4 is required for ventral nerve cord axonal navigation, and dorsal nerve cord fasciculation. How CDH-4 mediates axon navigation and fasciculation is currently unknown. To identify genes acting together with cdh-4, we isolated mutants suppressing the axon guidance defects of cdh-4 mutants. These suppressors showed partial suppression of axonal defects in the dorsal and ventral nerve cords seen in cdh-4 mutants. We identified one suppressor gene, prp-6, which encodes a component of the spliceosome. Complete loss-of-function alleles of prp-6 are lethal, suggesting that the mutation isolated in our suppressor screen is a partial loss-of-function allele. A previous study found that RNAi-induced suppression of prp-6 leads to changes in the expression of several 100 genes including the cadherin cdh-5. We found that overexpression of cdh-5 mimics the suppression seen in prp-6 mutants, suggesting that CDH-5 can partially compensate for the loss of CDH-4.

Tissue polarity and PCP protein function: C. elegans as an emerging model

Polarity is the basis for the generation of cell diversity, as well as the organization, morphogenesis, and functioning of tissues. Studies in Caenorhabditis elegans have provided much insight into PAR-protein mediated polarity; however, the molecules and mechanisms critical for cell polarization within the plane of epithelia have been identified in other systems. Tissue polarity in C. elegans is organized by Wnt-signaling with some resemblance to the Wnt/planar cell polarity (PCP) pathway, but lacking core PCP protein functions. Nonetheless, recent studies revealed that conserved PCP proteins regulate directed cell migratory events in C. elegans, such as convergent extension movements and neurite formation and guidance. Here, we discuss the latest insights and use of C. elegans as a PCP model.

Ordered arrangement of dendrites within a C. elegans sensory nerve bundle

Biological systems are organized into well-ordered structures and can evolve new patterns when perturbed. To identify principles underlying biological order, we turned to C. elegans for its simple anatomy and powerful genetics. We developed a method to quantify the arrangement of three dendrites in the main sensory nerve bundle, and found that they exhibit a stereotyped arrangement throughout larval growth. Dendrite order does not require prominent features including sensory cilia and glial junctions. In contrast, loss of the cell adhesion molecule (CAM) CDH-4/Fat-like cadherin causes dendrites to be ordered randomly, despite remaining bundled. Loss of the CAMs PTP-3/LAR or SAX-7/L1CAM causes dendrites to adopt an altered order, which becomes increasingly random as animals grow. Misexpression of SAX-7 leads to subtle but reproducible changes in dendrite order. Our results suggest that combinations of CAMs allow dendrites to self-organize into a stereotyped arrangement and can produce altered patterns when perturbed.

Cancer Cell Mechanics: Adhesion G Protein-coupled Receptors in Action?

In mammals, numerous organ systems are equipped with adhesion G protein-coupled receptors (aGPCRs) to shape cellular processes including migration, adhesion, polarity and guidance. All of these cell biological aspects are closely associated with tumor cell biology. Consistently, aberrant expression or malfunction of aGPCRs has been associated with dysplasia and tumorigenesis. Mounting evidence indicates that cancer cells comprise viscoelastic properties that are different from that of their non-tumorigenic counterparts, a feature that is believed to contribute to the increased motility and invasiveness of metastatic cancer cells. This is particularly interesting in light of the recent identification of the mechanosensitive facility of aGPCRs. aGPCRs are signified by large extracellular domains (ECDs) with adhesive properties, which promote the engagement with insoluble ligands. This configuration may enable reliable force transmission to the ECDs and may constitute a molecular switch, vital for mechano-dependent aGPCR signaling. The investigation of aGPCR function in mechanosensation is still in its infancy and has been largely restricted to physiological contexts. It remains to be elucidated if and how aGPCR function affects the mechanoregulation of tumor cells, how this may shape the mechanical signature and ultimately determines the pathological features of a cancer cell. This article aims to view known aGPCR functions from a biomechanical perspective and to delineate how this might impinge on the mechanobiology of cancer cells.

Seven pass Cadherins CELSR1-3

Cadherin EGF LAG seven-pass G-type receptors 1, 2 and 3 (CELSR1-3) form a family of three atypical cadherins with multiple functions in epithelia and in the nervous system. During the past decade, evidence has accumulated for a key role of CELSR1 in epithelial planar cell polarity (PCP), and for CELSR2 and CELSR3 in ciliogenesis and neural development, especially neuron migration and axon guidance in the central, peripheral and enteric nervous systems. Phenotypes in mutant mice indicate that CELSR proteins work in concert with FZD3 and FZD6, but several questions remain. Apart from PCP signaling pathways implicating CELSR1 that begin to be unraveled, little is known about other signals generated by CELSR2 and CELSR3. A crucial question concerns the putative ligands that trigger signaling, in particular what is the role of WNT factors. Another critical issue is the identification of novel intracellular pathways and effectors that relay and transmit signals in receptive cells? Answers to those questions should further our understanding of the role of those important molecules not only in development but also in regeneration and disease.

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.

Excitatory neurons sculpt GABAergic neuronal connectivity in the C. elegans motor circuit

Establishing and maintaining the appropriate number of GABA synapses is key for balancing excitation and inhibition in the nervous system, though we have only a limited understanding of the mechanisms controlling GABA circuit connectivity. Here, we show that disrupting cholinergic innervation of GABAergic neurons in the C. elegans motor circuit alters GABAergic neuron synaptic connectivity. These changes are accompanied by reduced frequency and increased amplitude of GABAergic synaptic events. Acute genetic disruption in early development, during the integration of post-embryonic-born GABAergic neurons into the circuit, produces irreversible effects on GABAergic synaptic connectivity that mimic those produced by chronic manipulations. In contrast, acute genetic disruption of cholinergic signaling in the adult circuit does not reproduce these effects. Our findings reveal that GABAergic signaling is regulated by cholinergic neuronal activity, probably through distinct mechanisms in the developing and mature nervous system.

Evidence for opposing roles of Celsr3 and Vangl2 in glutamatergic synapse formation

The signaling mechanisms that choreograph the assembly of the highly asymmetric pre- and postsynaptic structures are still poorly defined. Using synaptosome fractionation, immunostaining, and coimmunoprecipitation, we found that Celsr3 and Vangl2, core components of the planar cell polarity (PCP) pathway, are localized at developing glutamatergic synapses and interact with key synaptic proteins. Pyramidal neurons from the hippocampus of Celsr3 knockout mice exhibit loss of ∼50% of glutamatergic synapses, but not inhibitory synapses, in culture. Wnts are known regulators of synapse formation, and our data reveal that Wnt5a inhibits glutamatergic synapses formed via Celsr3. To avoid affecting earlier developmental processes, such as axon guidance, we conditionally knocked out Celsr3 in the hippocampus 1 week after birth. The CA1 neurons that lost Celsr3 also showed a loss of ∼50% of glutamatergic synapses in vivo without affecting the inhibitory synapses assessed by miniature excitatory postsynaptic current (mEPSC) and electron microscopy. These animals displayed deficits in hippocampus-dependent behaviors in adulthood, including spatial learning and memory and fear conditioning. In contrast to Celsr3 conditional knockouts, we found that the conditional knockout of Vangl2 in the hippocampus 1 week after birth led to a large increase in synaptic density, as evaluated by mEPSC frequency and spine density. PCP signaling is mediated by multiple core components with antagonizing functions. Our results document the opposing roles of Celsr3 and Vangl2 in glutamatergic synapse formation.

The Effect of In Vitro Cultivation on the Transcriptome of Adult Brugia malayi

BACKGROUND

Filarial nematodes cause serious and debilitating infections in human populations of tropical countries, contributing to an entrenched cycle of poverty. Only one human filarial parasite, Brugia malayi, can be maintained in rodents in the laboratory setting. It has been a widely used model organism in experiments that employ culture systems, the impact of which on the worms is unknown.

METHODOLOGY/PRINCIPAL FINDINGS

Using Illumina RNA sequencing, we characterized changes in gene expression upon in vitro maintenance of adult B. malayi female worms at four time points: immediately upon removal from the host, immediately after receipt following shipment, and after 48 h and 5 days in liquid culture media. The dramatic environmental change and the 24 h time lapse between removal from the host and establishment in culture caused a globally dysregulated gene expression profile. We found a maximum of 562 differentially expressed genes based on pairwise comparison between time points. After an initial shock upon removal from the host and shipping, a few stress fingerprints remained after 48 h in culture and until the experiment was stopped. This was best illustrated by a strong and persistent up-regulation of several genes encoding cuticle collagens, as well as serpins.

CONCLUSIONS/SIGNIFICANCE

These findings suggest that B. malayi can be maintained in culture as a valid system for pharmacological and biological studies, at least for several days after removal from the host and adaptation to the new environment. However, genes encoding several stress indicators remained dysregulated until the experiment was stopped.

International Union of Basic and Clinical Pharmacology. XCIV. Adhesion G protein-coupled receptors

The Adhesion family forms a large branch of the pharmacologically important superfamily of G protein-coupled receptors (GPCRs). As Adhesion GPCRs increasingly receive attention from a wide spectrum of biomedical fields, the Adhesion GPCR Consortium, together with the International Union of Basic and Clinical Pharmacology Committee on Receptor Nomenclature and Drug Classification, proposes a unified nomenclature for Adhesion GPCRs. The new names have ADGR as common dominator followed by a letter and a number to denote each subfamily and subtype, respectively. The new names, with old and alternative names within parentheses, are: ADGRA1 (GPR123), ADGRA2 (GPR124), ADGRA3 (GPR125), ADGRB1 (BAI1), ADGRB2 (BAI2), ADGRB3 (BAI3), ADGRC1 (CELSR1), ADGRC2 (CELSR2), ADGRC3 (CELSR3), ADGRD1 (GPR133), ADGRD2 (GPR144), ADGRE1 (EMR1, F4/80), ADGRE2 (EMR2), ADGRE3 (EMR3), ADGRE4 (EMR4), ADGRE5 (CD97), ADGRF1 (GPR110), ADGRF2 (GPR111), ADGRF3 (GPR113), ADGRF4 (GPR115), ADGRF5 (GPR116, Ig-Hepta), ADGRG1 (GPR56), ADGRG2 (GPR64, HE6), ADGRG3 (GPR97), ADGRG4 (GPR112), ADGRG5 (GPR114), ADGRG6 (GPR126), ADGRG7 (GPR128), ADGRL1 (latrophilin-1, CIRL-1, CL1), ADGRL2 (latrophilin-2, CIRL-2, CL2), ADGRL3 (latrophilin-3, CIRL-3, CL3), ADGRL4 (ELTD1, ETL), and ADGRV1 (VLGR1, GPR98). This review covers all major biologic aspects of Adhesion GPCRs, including evolutionary origins, interaction partners, signaling, expression, physiologic functions, and therapeutic potential.

Cadherin-13, a risk gene for ADHD and comorbid disorders, impacts GABAergic function in hippocampus and cognition

Cadherin-13 (CDH13), a unique glycosylphosphatidylinositol-anchored member of the cadherin family of cell adhesion molecules, has been identified as a risk gene for attention-deficit/hyperactivity disorder (ADHD) and various comorbid neurodevelopmental and psychiatric conditions, including depression, substance abuse, autism spectrum disorder and violent behavior, while the mechanism whereby CDH13 dysfunction influences pathogenesis of neuropsychiatric disorders remains elusive. Here we explored the potential role of CDH13 in the inhibitory modulation of brain activity by investigating synaptic function of GABAergic interneurons. Cellular and subcellular distribution of CDH13 was analyzed in the murine hippocampus and a mouse model with a targeted inactivation of Cdh13 was generated to evaluate how CDH13 modulates synaptic activity of hippocampal interneurons and behavioral domains related to psychopathologic (endo)phenotypes. We show that CDH13 expression in the cornu ammonis (CA) region of the hippocampus is confined to distinct classes of interneurons. Specifically, CDH13 is expressed by numerous parvalbumin and somatostatin-expressing interneurons located in the stratum oriens, where it localizes to both the soma and the presynaptic compartment. Cdh13(-/-) mice show an increase in basal inhibitory, but not excitatory, synaptic transmission in CA1 pyramidal neurons. Associated with these alterations in hippocampal function, Cdh13(-/-) mice display deficits in learning and memory. Taken together, our results indicate that CDH13 is a negative regulator of inhibitory synapses in the hippocampus, and provide insights into how CDH13 dysfunction may contribute to the excitatory/inhibitory imbalance observed in neurodevelopmental disorders, such as ADHD and autism.

New functions and signaling mechanisms for the class of adhesion G protein-coupled receptors

The class of adhesion G protein-coupled receptors (aGPCRs), with 33 human homologs, is the second largest family of GPCRs. In addition to a seven-transmembrane α-helix-a structural feature of all GPCRs-the class of aGPCRs is characterized by the presence of a large N-terminal extracellular region. In addition, all aGPCRs but one (GPR123) contain a GPCR autoproteolysis-inducing (GAIN) domain that mediates autoproteolytic cleavage at the GPCR autoproteolysis site motif to generate N- and a C-terminal fragments (NTF and CTF, respectively) during protein maturation. Subsequently, the NTF and CTF are associated noncovalently as a heterodimer at the plasma membrane. While the biological function of the GAIN domain-mediated autocleavage is not fully understood, mounting evidence suggests that the NTF and CTF possess distinct biological activities in addition to their function as a receptor unit. We discuss recent advances in understanding the biological functions, signaling mechanisms, and disease associations of the aGPCRs.

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.

Tethering membrane fusion: common and different players in myoblasts and at the synapse

Drosophila

Membrane fusion is essential for the communication of membrane-defined compartments, development of multicellular organisms and tissue homeostasis. Although membrane fusion has been studied extensively, still little is known about the molecular mechanisms. Especially the intercellular fusion of cells during development and tissue homeostasis is poorly understood. Somatic muscle formation in Drosophila depends on the intercellular fusion of myoblasts. In this process, myoblasts recognize each other and adhere, thereby triggering a protein machinery that leads to electron-dense plaques, vesicles and F-actin formation at apposing membranes. Two models of how local membrane stress is achieved to induce the merging of the myoblast membranes have been proposed: the electron-dense vesicles transport and release a fusogen and F-actin bends the plasma membrane. In this review, we highlight cell-adhesion molecules and intracellular proteins known to be involved in myoblast fusion. The cell-adhesion proteins also mediate the recognition and adhesion of other cell types, such as neurons that communicate with each other via special intercellular junctions, termed chemical synapses. At these synapses, neurotransmitters are released through the intracellular fusion of synaptic vesicles with the plasma membrane. As the targeting of electron-dense vesicles in myoblasts shares some similarities with the targeting of synaptic vesicle fusion, we compare molecules required for synaptic vesicle fusion to recently identified molecules involved in myoblast fusion.

The fat-like cadherin CDH-4 acts cell-non-autonomously in anterior-posterior neuroblast migration

Directed migration of neurons is critical in the normal and pathological development of the brain and central nervous system. In Caenorhabditis elegans, the bilateral Q neuroblasts, QR on the right and QL on the left, migrate anteriorly and posteriorly, respectively. Initial protrusion and migration of the Q neuroblasts is autonomously controlled by the transmembrane proteins UNC-40/DCC, PTP-3/LAR, and MIG-21. As QL migrates posteriorly, it encounters and EGL-20/Wnt signal that induces MAB-5/Hox expression that drives QL descendant posterior migration. QR migrates anteriorly away from EGL-20/Wnt and does not activate MAB-5/Hox, resulting in anterior QR descendant migration. A forward genetic screen for new mutations affecting initial Q migrations identified alleles of cdh-4, which caused defects in both QL and QR directional migration similar to unc-40, ptp-3, and mig-21. Previous studies showed that in QL, PTP-3/LAR and MIG-21 act in a pathway in parallel to UNC-40/DCC to drive posterior QL migration. Here we show genetic evidence that CDH-4 acts in the PTP-3/MIG-21 pathway in parallel to UNC-40/DCC to direct posterior QL migration. In QR, the PTP-3/MIG-21 and UNC-40/DCC pathways mutually inhibit each other, allowing anterior QR migration. We report here that CDH-4 acts in both the PTP-3/MIG-21 and UNC-40/DCC pathways in mutual inhibition in QR, and that CDH-4 acts cell-non-autonomously. Interaction of CDH-4 with UNC-40/DCC in QR but not QL represents an inherent left-right asymmetry in the Q cells, the nature of which is not understood. We conclude that CDH-4 might act as a permissive signal for each Q neuroblast to respond differently to anterior-posterior guidance information based upon inherent left-right asymmetries in the Q neuroblasts.

Extension of the Caenorhabditis elegans Pharyngeal M1 neuron axon is regulated by multiple mechanisms

The guidance of axons to their correct targets is a critical step in development. The C. elegans pharynx presents an attractive system to study neuronal pathfinding in the context of a developing organ. The worm pharynx contains relatively few cells and cell types, but each cell has a known lineage and stereotyped developmental patterns. We found that extension of the M1 pharyngeal axon, which spans the entire length of the pharynx, occurs in two distinct phases. The first proximal phase does not require genes that function in axon extension (unc-34, unc-51, unc-115, and unc-119), whereas the second distal phase does use these genes and is guided in part by the adjacent g1P gland cell projection. unc-34, unc-51, and unc-115 had incompletely penetrant defects and appeared to act in conjunction with the g1P cell for distal outgrowth. Only unc-119 showed fully penetrant defects for the distal phase. Mutations affecting classical neuronal guidance cues (Netrin, Semaphorin, Slit/Robo, Ephrin) or adhesion molecules (cadherin, IgCAM) had, at best, weak effects on the M1 axon. None of the mutations we tested affected the proximal phase of M1 elongation. In a forward genetic screen, we isolated nine mutations in five genes, three of which are novel, showing defects in M1, including axon overextension, truncation, or ectopic branching. One of these mutations appeared to affect the generation or differentiation of the M1 neuron. We conclude that M1 axon extension is a robust process that is not completely dependent on any single guidance mechanism.

The EBAX-type Cullin-RING E3 ligase and Hsp90 guard the protein quality of the SAX-3/Robo receptor in developing neurons

Although protein quality control (PQC) is generally perceived as important for the development of the nervous system, the specific mechanisms of neuronal PQC have remained poorly understood. Here, we report that C. elegans Elongin BC-binding axon regulator (EBAX-1), a conserved BC-box protein, regulates axon guidance through PQC of the SAX-3/Robo receptor. EBAX-1 buffers guidance errors against temperature variations. As a substrate-recognition subunit in the Elongin BC-containing Cullin-RING ubiquitin ligase (CRL), EBAX-1 also binds to DAF-21, a cytosolic Hsp90 chaperone. The EBAX-type CRL and DAF-21 collaboratively regulate SAX-3-mediated axon pathfinding. Biochemical and imaging assays indicate that EBAX-1 specifically recognizes misfolded SAX-3 and promotes its degradation in vitro and in vivo. Importantly, vertebrate EBAX also shows substrate preference toward aberrant Robo3 implicated in horizontal gaze palsy with progressive scoliosis (HGPPS). Together, our findings demonstrate a triage PQC mechanism mediated by the EBAX-type CRL and DAF-21/Hsp90 that maintains the accuracy of neuronal wiring.

Shaping the nervous system: role of the core planar cell polarity genes

Planar cell polarity (PCP) is complementary to the intrinsic polarization of single cells and refers to the global coordination of cell behaviour in the plane of a tissue and, by extension, to the signalling pathways that control it. PCP is most evident in cell sheets, and research into PCP was for years confined to studies in Drosophila melanogaster. However, PCP has more recently emerged as an important phenomenon in vertebrates, in which it regulates various developmental processes and is associated with multiple disorders. In particular, core PCP genes are crucial for the development and function of the nervous system. They are involved in neural tube closure, ependymal polarity, neuronal migration, dendritic growth and axon guidance.

C. elegans fmi-1/flamingo and Wnt pathway components interact genetically to control the anteroposterior neurite growth of the VD GABAergic neurons

During development, multiple environmental cues, e.g., growth factors, cell adhesion molecules, etc., interact to influence the pattern of outgrowth of axons and dendrites in a cell-specific fashion. As a result, individual neurons may receive similar signals, but make unique choices, leading to distinct wiring within the nervous system. C. elegans has been useful in identifying molecular cues that influence neuronal development, as well as the downstream mechanisms that allow individual neurons to make cell-specific responses. Recently, we described a role for the conserved cadherin domain-containing protein, FMI-1/flamingo, in multiple stages of neural development in C. elegans. During the initial phase of neurite outgrowth, FMI-1 seems to have a relatively cell-specific effect on the VD neurons to promote the initial neurite formed to grow toward the anterior. In this capacity, FMI-1 appears to work coordinately with at least two Wnt ligands, EGL-20 and LIN-44, and multiple downstream Wnt signaling components (including LIN-17/Frizzled, DSH-1/Disheveled, and BAR-1/β-catenin). Here I will discuss some of the ideas we considered about how FMI-1 could affect neurons as they acquire their morphology during development.

C. elegans fmi-1/flamingo and Wnt pathway components interact genetically to control the anteroposterior neurite growth of the VD GABAergic neurons

Directed axonal growth is essential to establish neuronal networks. During the early development of the VD neurons, an anterior neurite that will become the VD axon extends along the anteroposterior (A/P) axis in the ventral nerve cord (VNC) in Caenorhabditis elegans. Little is known about the cellular and molecular mechanisms that are important for correct neurite growth in the VNC. In fmi-1/flamingo mutant animals, we observed that some postembryonically born VD neurons had a posterior neurite instead of a normal anterior neurite, which caused aberrant VD commissure patterning along the A/P axis. In addition, VD anterior neurites had underextension defects in the VNC in fmi-1 animals, whereas VD commissure growth along the dorsoventral (D/V) axis occurred normally in these animals, suggesting that fmi-1 is important for neurite growth along the A/P axis but not the D/V axis. We also uncovered unknown details of the early development of the VD neurons, indicating that the neurite defects arose during their early development. Interestingly, though fmi-1 is present at this time in the VNC, we did not observe FMI-1 in the VD neurons themselves, suggesting that fmi-1 might be working in a cell non-autonomous fashion. Furthermore, fmi-1 appears to be working in a novel pathway, independently from the planar cell polarity pathway and in parallel to lin-17/frizzled and dsh-1/dishevelled, to determine the direction of neurite growth. Our findings indicate that redundant developmental pathways regulate neurite growth in the VNC in C. elegans.

Cadherins and their partners in the nematode worm Caenorhabditis elegans

The extreme simplicity of Caenorhabditis elegans makes it an ideal system to study the basic principles of cadherin function at the level of single cells within the physiologically relevant context of a developing animal. The genetic tractability of C. elegans also means that components of cadherin complexes can be identified through genetic modifier screens, allowing a comprehensive in vivo characterization of the macromolecular assemblies involved in cadherin function during tissue formation and maintenance in C. elegans. This work shows that a single cadherin system, the classical cadherin-catenin complex, is essential for diverse morphogenetic events during embryogenesis through its interactions with a range of mostly conserved proteins that act to modulate its function. The role of other members of the cadherin family in C. elegans, including members of the Fat-like, Flamingo/CELSR and calsyntenin families is less well characterized, but they have clear roles in neuronal development and function.

Cadherin complexity: recent insights into cadherin superfamily function in C. elegans

Cadherin superfamily proteins mediate cell-cell adhesion during development. The C. elegans embryo is a powerful system for analyzing how cadherins function in highly stereotyped morphogenetic events. In the embryo, the classical cadherin HMR-1 acts along with the Rac pathway and SAX-7/L1CAM during gastrulation. As adherens junctions mature, PAR complex proteins differentially regulate cadherin complex localization, and SRGP-1/Slit/Robo GAP aids adhesion by promoting membrane bending. Once adherens junctions form, actin is linked to the cell surface via HMP-1/α-catenin, whose actin binding activity is regulated in novel ways. FMI-1/Flamingo and CDH-4/Fat-like regulate axonal morphology of both pioneer and follower neurons. C. elegans thus continues to be useful for uncovering precise functions for cadherin superfamily proteins and their associates in a simple metazoan.