Post-endocytic sorting of Plexin-D1 controls signal transduction and development of axonal and vascular circuits

Repulsive Sema3E-Plexin-D1 signaling coordinates both axonal extension and steering via activating an autoregulatory factor, Mtss1

Axon guidance molecules are critical for neuronal pathfinding because they regulate directionality and growth pace during nervous system development. However, the molecular mechanisms coordinating proper axonal extension and turning are poorly understood. Here, metastasis suppressor 1 (Mtss1), a membrane protrusion protein, ensured axonal extension while sensitizing axons to the Semaphorin 3E (Sema3E)-Plexin-D1 repulsive cue. Sema3E-Plexin-D1 signaling enhanced Mtss1 expression in projecting striatonigral neurons. Mtss1 localized to the neurite axonal side and regulated neurite outgrowth in cultured neurons. Mtss1 also aided Plexin-D1 trafficking to the growth cone, where it signaled a repulsive cue to Sema3E. Mtss1 ablation reduced neurite extension and growth cone collapse in cultured neurons. Mtss1-knockout mice exhibited fewer striatonigral projections and irregular axonal routes, and these defects were recapitulated in Plxnd1- or Sema3e-knockout mice. These findings demonstrate that repulsive axon guidance activates an exquisite autoregulatory program coordinating both axonal extension and steering during neuronal pathfinding.

Invasion of glioma cells through confined space requires membrane tension regulation and mechano-electrical coupling via Plexin-B2

Glioblastoma (GBM) is a malignant brain tumor with uncontrolled invasive growth. Here, we demonstrate how GBM cells usurp guidance receptor Plexin-B2 to gain biomechanical plasticity for polarized migration through confined space. Using live-cell imaging to track GBM cells negotiating microchannels, we reveal active endocytosis at cell front and filamentous actin assembly at rear to propel GBM cells through constrictions. These two processes are interconnected and governed by Plexin-B2 that orchestrates cortical actin and membrane tension, shown by biomechanical assays. Molecular dynamics simulations predict that balanced membrane and actin tension are required for optimal migratory velocity and consistency. Furthermore, Plexin-B2 mechanosensitive function requires a bendable extracellular ring structure and affects membrane internalization, permeability, phospholipid composition, as well as inner membrane surface charge. Together, our studies unveil a key element of membrane tension and mechanoelectrical coupling via Plexin-B2 that enables GBM cells to adapt to physical constraints and achieve polarized confined migration.

A role for pathogenic autoantibodies in small fiber neuropathy?

The immune system has a role in neuropathic pain which includes autoimmune mechanisms (e.g., autoantibodies). Clinical studies have identified a number of conditions where neuropathic pain is common and that are associated with autoantibodies targeting antigens within the nervous system. Interestingly sensory symptoms can be relieved with immunotherapies or plasma exchange, suggesting that pain in these patients is antibody-mediated. Recent preclinical studies have directly addressed this. For example, passive transfer of CASPR2 autoantibodies from patients cause increased pain sensitivity and enhanced sensory neuron excitability in mice confirming pathogenicity and demonstrating that patient autoantibodies are a mechanism to cause neuropathic pain. Small fiber neuropathy (SFN) exclusively affects small sensory fibers (typically nociceptors) and is characterized by severe neuropathic pain. Known causes include diabetes, B12 deficiency and rare variants in sodium channel genes, although around 50% of cases are idiopathic. SFN is associated with autoimmune conditions such as Sjorgen's syndrome, Sarcoidosis and Celiac disease and immunotherapy in the form of Intravenous immunoglobulin (IVIG) has proved an effective treatment. Autoantibodies have been identified and, in some cases, passive transfer of SFN patient IgG in mice can recapitulate neuropathic pain-like behavior. Here we will discuss clinical and preclinical data relating to the idea that pathogenic autoantibodies contribute to SNF. We discuss putative pathogenic antibodies, cellular targets and the molecular mechanisms by which they cause sensory neuron damage and the development of neuropathic pain. Finally, we will comment on future directions which may provide further insights into the mechanisms underlying SFN in patients.

PlexinD1 signaling controls domain-specific dendritic development in newborn neurons in the postnatal olfactory bulb

Newborn neurons show immature bipolar morphology and continue to migrate toward their destinations. After the termination of migration, newborn neurons undergo spatially controlled dendrite formation and change into a complex morphology. The mechanisms of dendritic development of newborn neurons have not been fully understood. Here, we show that in the postnatal olfactory bulb (OB), the Sema3E-PlexinD1 signaling, which maintains bipolar morphology of newborn neurons, also regulates their dendritic development after the termination of migration in a dendritic domain-specific manner. Genetic ablation of Sema3E or PlexinD1 enhanced dendritic branching in the proximal domain of the apical dendrites of OB newborn granule cells, whereas PlexinD1 overexpression suppressed it in a Rho binding domain (RBD)-dependent manner. Furthermore, RhoJ, a small GTPase that directly binds to PlexinD1RBD in vascular endothelial cells, is expressed in migrating and differentiating newborn granule cells in the OB and is also involved in the suppression of proximal branching of their apical dendrites. These results suggest that the Sema3E-PlexinD1-RhoJ axis regulates domain-specific dendrite formation of newborn neurons in the postnatal OB.

Plexin D1 negatively regulates zebrafish lymphatic development

Lymphangiogenesis is a dynamic process that involves the directed migration of lymphatic endothelial cells (LECs) to form lymphatic vessels. The molecular mechanisms that underpin lymphatic vessel patterning are not fully elucidated and, to date, no global regulator of lymphatic vessel guidance is known. In this study, we identify the transmembrane cell signalling receptor Plexin D1 (Plxnd1) as a negative regulator of both lymphatic vessel guidance and lymphangiogenesis in zebrafish. plxnd1 is expressed in developing lymphatics and is required for the guidance of both the trunk and facial lymphatic networks. Loss of plxnd1 is associated with misguided intersegmental lymphatic vessel growth and aberrant facial lymphatic branches. Lymphatic guidance in the trunk is mediated, at least in part, by the Plxnd1 ligands, Semaphorin 3AA and Semaphorin 3C. Finally, we show that Plxnd1 normally antagonises Vegfr/Erk signalling to ensure the correct number of facial LECs and that loss of plxnd1 results in facial lymphatic hyperplasia. As a global negative regulator of lymphatic vessel development, the Sema/Plxnd1 signalling pathway is a potential therapeutic target for treating diseases associated with dysregulated lymphatic growth.

Formation of the Mouse Internal Capsule and Cerebral Peduncle: A Pioneering Role for Striatonigral Axons as Revealed in Isl1 Conditional Mutants

The projection neurons of the striatum, the principal nucleus of the basal ganglia, belong to one of the following two major pathways: the striatopallidal (indirect) pathway or the striatonigral (direct) pathway. Striatonigral axons project long distances and encounter ascending tracts (thalamocortical) while coursing alongside descending tracts (corticofugal) as they extend through the internal capsule and cerebral peduncle. These observations suggest that striatal circuitry may help to guide their trajectories. To investigate the developmental contributions of striatonigral axons to internal capsule formation, we have made use of Sox8-EGFP (striatal direct pathway) and Fezf2-TdTomato (corticofugal pathway) BAC transgenic reporter mice in combination with immunohistochemical markers to trace these axonal pathways throughout development. We show that striatonigral axons pioneer the internal capsule and cerebral peduncle and are temporally and spatially well positioned to provide guidance for corticofugal and thalamocortical axons. Using Isl1 conditional knock-out (cKO) mice, which exhibit disrupted striatonigral axon outgrowth, we observe both corticofugal and thalamocortical axon defects with either ventral forebrain- or telencephalon-specific Isl1 inactivation, despite Isl1 not being expressed in either cortical or thalamic projection neurons. Striatonigral axon defects can thus disrupt internal capsule formation. Our genome-wide transcriptomic analysis in Isl1 cKOs reveals changes in gene expression relevant to cell adhesion, growth cone dynamics, and extracellular matrix composition, suggesting potential mechanisms by which the striatonigral pathway exerts this guidance role. Together, our data support a novel pioneering role for the striatal direct pathway in the correct assembly of the ascending and descending axon tracts within the internal capsule and cerebral peduncle.SIGNIFICANCE STATEMENT The basal ganglia are a group of subcortical nuclei with established roles in the coordination of voluntary motor programs, aspects of cognition, and the selection of appropriate social behaviors. Hence, disruptions in basal ganglia connectivity have been implicated in the motor, cognitive, and social dysfunction characterizing common neurodevelopmental disorders such as attention-deficit/hyperactivity disorder, autism spectrum disorder, obsessive-compulsive disorder, and tic disorder. Here, we identified a novel role for the striatonigral (direct) pathway in pioneering the internal capsule and cerebral peduncle, and in guiding axons extending to and from the cortex. Our findings suggest that the abnormal development of basal ganglia circuits can drive secondary internal capsule defects and thereby may contribute to the pathology of these disorders.

Tubular microdomains of Rab7-positive endosomes retrieve TrkA, a mechanism disrupted in Charcot-Marie-Tooth disease 2B

Axonal survival and growth requires signalling from tropomyosin receptor kinases (Trks). To transmit their signals, receptor-ligand complexes are endocytosed and undergo retrograde trafficking to the soma, where downstream signalling occurs. Vesicles transporting neurotrophic receptors to the soma are reported to be Rab7-positive late endosomes and/or multivesicular bodies (MVBs), where receptors localize within so-called intraluminal vesicles (herein Rab7 corresponds to Rab7A unless specified otherwise). Therefore, one challenging question is how downstream signalling is possible given the insulating properties of intraluminal vesicles. In this study, we report that Rab7-positive endosomes and MVBs retrieve TrkA (also known as NTRK1) through tubular microdomains. Interestingly, this phenotype is absent for the EGF receptor. Furthermore, we found that endophilinA1, endophilinA2 and endophilinA3, together with WASH1 (also known as WASHC1), are involved in the tubulation process. In Charcot-Marie-Tooth disease 2B (CMT2B), a neuropathy of the peripheral nervous system, this tubulating mechanism is disrupted. In addition, the ability to tubulate correlates with the phosphorylation levels of TrkA as well as with neurite length in neuronal cultures from dorsal root ganglia. In all, we report a new retrieval mechanism of late Rab7-positive endosomes, which enables TrkA signalling and sheds new light onto how neurotrophic signalling is disrupted in CMT2B. This article has an associated First Person interview with the first author of the paper.

Role of PLEXIND1/TGFβ Signaling Axis in Pancreatic Ductal Adenocarcinoma Progression Correlates with the Mutational Status of KRAS

Simple Summary

Pancreatic cancer is among the most lethal cancers. The expression of PLEXIND1, a receptor, is upregulated in many cancers (including pancreatic cancer). Traditionally, PLEXIND1 is known to be involved in neuron development and mediate semaphorin signaling. However, its role and signaling in cancer is not fully understood. In our study, we present a new mechanism through which PLEXIND1 mediates its roles in cancer. For the first time, we demonstrate that it can function as a transforming growth factor beta coreceptor and modulate SMAD3 signaling. Around 90% of pancreatic cancer patients have mutant KRAS. Our work suggests that PLEXIND1 functions differently in pancreatic cancer cell lines, and the difference correlates with KRAS mutational status. Additionally, we demonstrate a novel peptide based therapeutic approach to target PLEXIND1 in cancer cells. Our work is valuable to both neuroscience and cancer fields, as it demonstrates an association between two previously unrelated signaling pathways.

Abstract

PLEXIND1 is upregulated in several cancers, including pancreatic ductal adenocarcinoma (PDAC). It is an established mediator of semaphorin signaling, and neuropilins are its known coreceptors. Herein, we report data to support the proposal that PLEXIND1 acts as a transforming growth factor beta (TGFβ) coreceptor, modulating cell growth through SMAD3 signaling. Our findings demonstrate that PLEXIND1 plays a pro-tumorigenic role in PDAC cells with oncogenic KRAS (KRAS^mut). We show in KRAS^mut PDAC cell lines (PANC-1, AsPC-1,4535) PLEXIND1 downregulation results in decreased cell viability (in vitro) and reduced tumor growth (in vivo). Conversely, PLEXIND1 acts as a tumor suppressor in the PDAC cell line (BxPC-3) with wild-type KRAS (KRAS^wt), as its reduced expression results in higher cell viability (in-vitro) and tumor growth (in vivo). Additionally, we demonstrate that PLEXIND1-mediated interactions can be selectively disrupted using a peptide based on its C-terminal sequence (a PDZ domain-binding motif), an outcome that may possess significant therapeutic implications. To our knowledge, this is the first report showing that (1) PLEXIND1 acts as a TGFβ coreceptor and mediates SMAD3 signaling, and (2) differential roles of PLEXIND1 in PDAC cell lines correlate with KRAS^mut and KRAS^wt status.

Architecture of the Sema3A/PlexinA4/Neuropilin tripartite complex

Secreted class 3 semaphorins (Sema3s) form tripartite complexes with the plexin receptor and neuropilin coreceptor, which are both transmembrane proteins that together mediate semaphorin signal for neuronal axon guidance and other processes. Despite extensive investigations, the overall architecture of and the molecular interactions in the Sema3/plexin/neuropilin complex are incompletely understood. Here we present the cryo-EM structure of a near intact extracellular region complex of Sema3A, PlexinA4 and Neuropilin 1 (Nrp1) at 3.7 Å resolution. The structure shows a large symmetric 2:2:2 assembly in which each subunit makes multiple interactions with others. The two PlexinA4 molecules in the complex do not interact directly, but their membrane proximal regions are close to each other and poised to promote the formation of the intracellular active dimer for signaling. The structure reveals a previously unknown interface between the a2b1b2 module in Nrp1 and the Sema domain of Sema3A. This interaction places the a2b1b2 module at the top of the complex, far away from the plasma membrane where the transmembrane regions of Nrp1 and PlexinA4 embed. As a result, the region following the a2b1b2 module in Nrp1 must span a large distance to allow the connection to the transmembrane region, suggesting an essential role for the long non-conserved linkers and the MAM domain in neuropilin in the semaphorin/plexin/neuropilin complex.

Insights into the regulatory role of Plexin D1 signalling in cardiovascular development and diseases

Plexin D1 (PLXND1), which was previously thought to mediate semaphorin signalling, belongs to the Plexin family of transmembrane proteins. PLXND1 cooperates mostly with the coreceptor neuropilin and participates in many aspects of axonal guidance. PLXND1 can also act as both a tumour promoter and a tumour suppressor. Emerging evidence suggests that mutations in PLXND1 or Semaphorin 3E, the canonical ligand of PLXND1, can lead to serious cardiovascular diseases, such as congenital heart defects, CHARGE syndrome and systemic sclerosis. Upon ligand binding, PLXND1 can act as a GTPase‐activating protein (GAP) and modulate integrin‐mediated cell adhesion, cytoskeletal dynamics and cell migration. These effects may play regulatory roles in the development of the cardiovascular system and disease. The cardiovascular effects of PLXND1 signalling have gradually been elucidated. PLXND1 was recently shown to detect physical forces and translate them into intracellular biochemical signals in the context of atherosclerosis. Therefore, the role of PLXND1 in cardiovascular development and diseases is gaining research interest because of its potential as a biomarker and therapeutic target. In this review, we describe the cardiac effects, vascular effects and possible molecular mechanisms of PLXND1 signalling.

New insights into the molecular mechanisms of axon guidance receptor regulation and signaling

As the nervous system develops, newly differentiated neurons need to extend their axons toward their synaptic targets to form functional neural circuits. During this highly dynamic process of axon pathfinding, guidance receptors expressed at the tips of motile axons interact with soluble guidance cues or membrane tethered molecules present in the environment to be either attracted toward or repelled away from the source of these cues. As competing cues are often present at the same location and during the same developmental period, guidance receptors need to be both spatially and temporally regulated in order for the navigating axons to make appropriate guidance decisions. This regulation is exerted by a diverse array of molecular mechanisms that have come into focus over the past several decades and these mechanisms ensure that the correct complement of surface receptors is present on the growth cone, a fan-shaped expansion at the tip of the axon. This dynamic, highly motile structure is defined by a lamellipodial network lining the periphery of the growth cone interspersed with finger-like filopodial projections that serve to explore the surrounding environment. Once axon guidance receptors are deployed at the right place and time at the growth cone surface, they respond to their respective ligands by initiating a complex set of signaling events that serve to rearrange the growth cone membrane and the actin and microtubule cytoskeleton to affect axon growth and guidance. In this review, we highlight recent advances that shed light on the rich complexity of mechanisms that regulate axon guidance receptor distribution, activation and downstream signaling.

New Molecular Players in the Development of Callosal Projections

Cortical development in humans is a long and ongoing process that continuously modifies the neural circuitry into adolescence. This is well represented by the dynamic maturation of the corpus callosum, the largest white matter tract in the brain. Callosal projection neurons whose long-range axons form the main component of the corpus callosum are evolved relatively recently with a substantial, disproportionate increase in numbers in humans. Though the anatomy of the corpus callosum and cellular processes in its development have been intensively studied by experts in a variety of fields over several decades, the whole picture of its development, in particular, the molecular controls over the development of callosal projections, still has many missing pieces. This review highlights the most recent progress on the understanding of corpus callosum formation with a special emphasis on the novel molecular players in the development of axonal projections in the corpus callosum.

M-Ras is Muscle-Ras, Moderate-Ras, Mineral-Ras, Migration-Ras, and Many More-Ras

The Ras family of small GTPases comprises about 36 members in humans. M-Ras is related to classical Ras with regard to its regulators and effectors, but solely constitutes a subfamily among the Ras family members. Although classical Ras strongly binds Raf and highly activates the ERK pathway, M-Ras less strongly binds Raf and moderately but sustainedly activates the ERK pathway to induce neuronal differentiation. M-Ras also possesses specific effectors, including RapGEFs and the PP1 complex Shoc2-PP1c, which dephosphorylates Raf to activate the ERK pathway. M-Ras is highly expressed in the brain and plays essential roles in dendrite formation during neurogenesis, in contrast to the axon formation by R-Ras. M-Ras is also highly expressed in the bone and induces osteoblastic differentiation and transdifferentiation accompanied by calcification. Moreover, M-Ras elicits epithelial-mesenchymal transition-mediated collective and single cell migration through the PP1 complex-mediated ERK pathway activation. Activating missense mutations in the MRAS gene have been detected in Noonan syndrome, one of the RASopathies, and MRAS gene amplification occurs in several cancers. Furthermore, several SNPs in the MRAS gene are associated with coronary artery disease, obesity, and dyslipidemia. Therefore, M-Ras carries out a variety of cellular, physiological, and pathological functions. Further investigations may reveal more functions of M-Ras.

Axon guidance receptors: Endocytosis, trafficking and downstream signaling from endosomes

During the development of the nervous system, axons extend through complex environments. Growth cones at the axon tip allow axons to find and innervate their appropriate targets and form functional synapses. Axon pathfinding requires axons to respond to guidance signals and these cues need to be detected by specialized receptors followed by intracellular signal integration and translation. Several downstream signaling pathways have been identified for axon guidance receptors and it has become evident that these pathways are often initiated from intracellular vesicles called endosomes. Endosomes allow receptors to traffic intracellularly, re-locating receptors from one cellular region to another. The localization of axon guidance receptors to endosomal compartments is crucial for their function, signaling output and expression levels. For example, active receptors within endosomes can recruit downstream proteins to the endosomal membrane and facilitate signaling. Also, endosomal trafficking can re-locate receptors back to the plasma membrane to allow re-activation or mediate downregulation of receptor signaling via degradation. Accumulating evidence suggests that axon guidance receptors do not follow a pre-set default trafficking route but may change their localization within endosomes. This re-routing appears to be spatially and temporally regulated, either by expression of adaptor proteins or co-receptors. These findings shed light on how signaling in axon guidance is regulated and diversified - a mechanism which explains how a limited set of guidance cues can help to establish billions of neuronal connections. In this review, we summarize and discuss our current knowledge of axon guidance receptor trafficking and provide directions for future research.

RhoJ integrates attractive and repulsive cues in directional migration of endothelial cells

During angiogenesis, VEGF acts as an attractive cue for endothelial cells (ECs), while Sema3E mediates repulsive cues. Here, we show that the small GTPase RhoJ integrates these opposing signals in directional EC migration. In the GTP-bound state, RhoJ interacts with the cytoplasmic domain of PlexinD1. Upon Sema3E stimulation, RhoJ released from PlexinD1 induces cell contraction. PlexinD1-bound RhoJ further facilitates Sema3E-induced PlexinD1-VEGFR2 association, VEGFR2 transphosphorylation at Y1214, and p38 MAPK activation, leading to reverse EC migration. Upon VEGF stimulation, RhoJ is required for the formation of the holoreceptor complex comprising VEGFR2, PlexinD1, and neuropilin-1, thereby preventing degradation of internalized VEGFR2, prolonging downstream signal transductions via PLCγ, Erk, and Akt, and promoting forward EC migration. After conversion to the GDP-bound state, RhoJ shifts from PlexinD1 to VEGFR2, which then terminates the VEGFR2 signals. RhoJ deficiency in ECs efficiently suppressed aberrant angiogenesis in ischemic retina. These findings suggest that distinct Rho GTPases may act as context-dependent integrators of chemotactic cues in directional cell migration and may serve as candidate therapeutic targets to manipulate cell motility in disease or tissue regeneration.

The endosomal sorting adaptor HD-PTP is required for ephrin-B:EphB signalling in cellular collapse and spinal motor axon guidance

The signalling output of many transmembrane receptors that mediate cell-cell communication is restricted by the endosomal sorting complex required for transport (ESCRT), but the impact of this machinery on Eph tyrosine kinase receptor function is unknown. We identified the ESCRT-associated adaptor protein HD-PTP as part of an EphB2 proximity-dependent biotin identification (BioID) interactome, and confirmed this association using co-immunoprecipitation. HD-PTP loss attenuates the ephrin-B2:EphB2 signalling-induced collapse of cultured cells and axonal growth cones, and results in aberrant guidance of chick spinal motor neuron axons in vivo. HD-PTP depletion abrogates ephrin-B2-induced EphB2 clustering, and EphB2 and Src family kinase activation. HD-PTP loss also accelerates ligand-induced EphB2 degradation, contrasting the effects of HD-PTP loss on the relay of signals from other cell surface receptors. Our results link Eph function to the ESCRT machinery and demonstrate a role for HD-PTP in the earliest steps of ephrin-B:EphB signalling, as well as in obstructing premature receptor depletion.

Divergent roles of Plexin D1 in cancer

Plexin D1 belongs to a family of transmembrane proteins called plexins. It was characterized as a receptor for semaphorins and is known to be essential for axonal guidance and vascular patterning. Mutations in Plexin D1 have been implicated in pathologic conditions such as truncus arteriosus and Möbius syndrome. Emerging data show that expression of Plexin D1 is deregulated in several cancers; it can support tumor development by aiding in tumor metastasis and EMT; and conversely, it can act as a dependence receptor and stimulate cell death in the absence of its canonical ligand, semaphorin 3E. The role of Plexin D1 in tumor development and progression is thereby garnering research interest for its potential as a biomarker and as a therapeutic target. In this review, we describe its discovery, structure, mutations, role(s) in cancer, and therapeutic potential.

Sensory Axon Growth Requires Spatiotemporal Integration of CaSR and TrkB Signaling

Neural circuit development involves the coordinated growth and guidance of axons. During this process, axons encounter many different cues, but how these cues are integrated and translated into growth is poorly understood. In this study, we report that receptor signaling does not follow a linear path but changes dependent on developmental stage and coreceptors involved. Using developing chicken embryos of both sexes, our data show that calcium-sensing receptor (CaSR), a G-protein-coupled receptor important for regulating calcium homeostasis, regulates neurite growth in two distinct ways. First, when signaling in isolation, CaSR promotes growth through the PI3-kinase-Akt pathway. At later developmental stages, CaSR enhances tropomyosin receptor kinase B (TrkB)/BDNF-mediated neurite growth. This enhancement is facilitated through a switch in the signaling cascade downstream of CaSR (i.e., from the PI3-kinase-Akt pathway to activation of GSK3α Tyr279). TrkB and CaSR colocalize within late endosomes, cotraffic and coactivate GSK3, which serves as a shared signaling node for both receptors. Our study provides evidence that two unrelated receptors can integrate their individual signaling cascades toward a nonadditive effect and thus control neurite growth during development.SIGNIFICANCE STATEMENT This work highlights the effect of receptor coactivation and signal integration in a developmental setting. During embryonic development, neurites grow toward their targets guided by cues in the extracellular environment. These cues are sensed by receptors at the surface that trigger intracellular signaling events modulating the cytoskeleton. Emerging evidence suggests that the effects of guidance cues are diversified, therefore expanding the number of responses. Here, we show that two unrelated receptors can change the downstream signaling cascade and regulate neuronal growth through a shared signaling node. In addition to unraveling a novel signaling pathway in neurite growth, this research stresses the importance of receptor coactivation and signal integration during development of the nervous system.

Multigenic truncation of the semaphorin-plexin pathway by a germline chromothriptic rearrangement associated with Moebius syndrome

Moebius syndrome (MBS) is a congenital disorder caused by paralysis of the facial and abducens nerves. Although a number of candidate genes have been suspected, so far only mutations in PLXND1 and REV3L are confirmed to cause MBS. Here, we fine mapped the breakpoints of a complex chromosomal rearrangement (CCR) 46,XY,t(7;8;11;13) in a patient with MBS, which revealed 41 clustered breakpoints with typical hallmarks of chromothripsis. Among 12 truncated protein-coding genes, SEMA3A is known to bind to the MBS-associated PLXND1. Intriguingly, the CCR also truncated PIK3CG, which in silico interacts with REVL3 encoded by the other known MBS-gene REV3L, and with the SEMA3A/PLXND1 complex via FLT1. Additional studies of other complex rearrangements may reveal whether the multiple breakpoints in germline chromothripsis may predispose to complex multigenic disorders.

GIPC proteins negatively modulate Plexind1 signaling during vascular development

Semaphorins (SEMAs) and their Plexin (PLXN) receptors are central regulators of metazoan cellular communication. SEMA-PLXND1 signaling plays important roles in cardiovascular, nervous, and immune system development, and cancer biology. However, little is known about the molecular mechanisms that modulate SEMA-PLXND1 signaling. As PLXND1 associates with GIPC family endocytic adaptors, we evaluated the requirement for the molecular determinants of their association and PLXND1's vascular role. Zebrafish that endogenously express a Plxnd1 receptor with a predicted impairment in GIPC binding exhibit low penetrance angiogenesis deficits and antiangiogenic drug hypersensitivity. Moreover, gipc mutant fish show angiogenic impairments that are ameliorated by reducing Plxnd1 signaling. Finally, GIPC depletion potentiates SEMA-PLXND1 signaling in cultured endothelial cells. These findings expand the vascular roles of GIPCs beyond those of the Vascular Endothelial Growth Factor (VEGF)-dependent, proangiogenic GIPC1-Neuropilin 1 complex, recasting GIPCs as negative modulators of antiangiogenic PLXND1 signaling and suggest that PLXND1 trafficking shapes vascular development.

Deficiency in the secreted protein Semaphorin3d causes abnormal parathyroid development in mice

Primary hyperparathyroidism (PHPT) is a common endocrinopathy characterized by hypercalcemia and elevated levels of parathyroid hormone. The primary cause of PHPT is a benign overgrowth of parathyroid tissue causing excessive secretion of parathyroid hormone. However, the molecular etiology of PHPT is incompletely defined. Here, we demonstrate that semaphorin3d (Sema3d), a secreted glycoprotein, is expressed in the developing parathyroid gland in mice. We also observed that genetic deletion of Sema3d leads to parathyroid hyperplasia, causing PHPT. In vivo and in vitro experiments using histology, immunohistochemistry, biochemical, RT-qPCR, and immunoblotting assays revealed that Sema3d inhibits parathyroid cell proliferation by decreasing the epidermal growth factor receptor (EGFR)/Erb-B2 receptor tyrosine kinase (ERBB) signaling pathway. We further demonstrate that EGFR signaling is elevated in Sema3d ^-/- parathyroid glands and that pharmacological inhibition of EGFR signaling can partially rescue the parathyroid hyperplasia phenotype. We propose that because Sema3d is a secreted protein, it may be possible to use recombinant Sema3d or derived peptides to inhibit parathyroid cell proliferation causing hyperplasia and hyperparathyroidism. Collectively, these findings identify Sema3d as a negative regulator of parathyroid growth.

Origin and evolution of plexins, semaphorins, and Met receptor tyrosine kinases

The transition from unicellular to multicellular organisms poses the question as to when genes that regulate cell-cell interactions emerged during evolution. The receptor and ligand pairing of plexins and semaphorins regulates cellular interactions in a wide range of developmental and physiological contexts. We surveyed here genomes of unicellular eukaryotes and of non-bilaterian and bilaterian Metazoa and performed phylogenetic analyses to gain insight into the evolution of plexin and semaphorin families. Remarkably, we detected plexins and semaphorins in unicellular choanoflagellates, indicating their evolutionary origin in a common ancestor of Choanoflagellida and Metazoa. The plexin domain structure is conserved throughout all clades; in contrast, semaphorins are structurally diverse. Choanoflagellate semaphorins are transmembrane proteins with multiple fibronectin type III domains following the N-terminal Sema domain (termed Sema-FN). Other previously not yet described semaphorin classes include semaphorins of Ctenophora with tandem immunoglobulin domains (Sema-IG) and secreted semaphorins of Echinoderamata (Sema-SP, Sema-SI). Our study also identified Met receptor tyrosine kinases (RTKs), which carry a truncated plexin extracellular domain, in several bilaterian clades, indicating evolutionary origin in a common ancestor of Bilateria. In addition, a novel type of Met-like RTK with a complete plexin extracellular domain was detected in Lophotrochozoa and Echinodermata (termed Met-LP RTK). Our findings are consistent with an ancient function of plexins and semaphorins in regulating cytoskeletal dynamics and cell adhesion that predates their role as axon guidance molecules.

Keeping up with advances in axon guidance

Twenty-five years after the discovery of the first chemotropic molecules for growing axons, what are the new findings? This review describes the latest progress made in our understanding of the molecular control of axonal guidance in the vertebrate nervous system. Special focus will be given to new molecular players, their source and location in vivo, and the role of membrane/receptor trafficking and RNA-based mechanisms in axon guidance cue signalling.

Regulation of thymocyte trafficking by Tagap, a GAP domain protein linked to human autoimmunity

Multiple autoimmune pathologies are associated with single-nucleotide polymorphisms of the human gene TAGAP, which encodes TAGAP, a guanosine triphosphatase (GTPase)-activating protein. We showed in mice that Tagap-mediated signaling by the sema3E/plexin-D1 ligand-receptor complex attenuates thymocytes' adhesion to the cortex through their β₁-containing integrins. By promoting thymocyte detachment within the cortex of the thymus, Tagap-mediated signaling enabled their translocation to the medulla, which is required for continued thymic selection. Tagap physically interacted with the cytoplasmic domain of plexin-D1 and directly stimulated the activity and signaling of the GTPase RhoA. In addition, Tagap indirectly mediated the activation of Cdc42 in response to the binding of sema3E to plexin-D1. Both RhoA and Cdc42 are key mediators of cytoskeletal and integrin dynamics in thymocytes. Knockdown of Tagap in mice suppressed the sema3E- and plexin-D1-mediated release of thymocytes that adhered within the cortex through β₁-containing integrins. This suppression led to the impaired translocation of thymocytes from the cortex to the medulla and resulted in the formation of ectopic medullary structures within the thymic cortex. Our results suggest that TAGAP variation modulates the risk of autoimmunity by altering thymocyte migration during thymic selection.

Functional dissection of astrocyte-secreted proteins: Implications in brain health and diseases

Astrocytes, which are homeostatic cells of the central nervous system (CNS), display remarkable heterogeneity in their morphology and function. Besides their physical and metabolic support to neurons, astrocytes modulate the blood-brain barrier, regulate CNS synaptogenesis, guide axon pathfinding, maintain brain homeostasis, affect neuronal development and plasticity, and contribute to diverse neuropathologies via secreted proteins. The identification of astrocytic proteome and secretome profiles has provided new insights into the maintenance of neuronal health and survival, the pathogenesis of brain injury, and neurodegeneration. Recent advances in proteomics research have provided an excellent catalog of astrocyte-secreted proteins. This review categorizes astrocyte-secreted proteins and discusses evidence that astrocytes play a crucial role in neuronal activity and brain function. An in-depth understanding of astrocyte-secreted proteins and their pathways is pivotal for the development of novel strategies for restoring brain homeostasis, limiting brain injury/inflammation, counteracting neurodegeneration, and obtaining functional recovery.

Structure analyses reveal a regulated oligomerization mechanism of the PlexinD1/GIPC/myosin VI complex

The GIPC family adaptor proteins mediate endocytosis by tethering cargo proteins to the myosin VI motor. The structural mechanisms for the GIPC/cargo and GIPC/myosin VI interactions remained unclear. PlexinD1, a transmembrane receptor that regulates neuronal and cardiovascular development, is a cargo of GIPCs. GIPC-mediated endocytic trafficking regulates PlexinD1 signaling. Here, we unravel the mechanisms of the interactions among PlexinD1, GIPCs and myosin VI by a series of crystal structures of these proteins in apo or bound states. GIPC1 forms a domain-swapped dimer in an autoinhibited conformation that hinders binding of both PlexinD1 and myosin VI. PlexinD1 binding to GIPC1 releases the autoinhibition, promoting its interaction with myosin VI. GIPCs and myosin VI interact through two distinct interfaces and form an open-ended alternating array. Our data support that this alternating array underlies the oligomerization of the GIPC/Myosin VI complexes in solution and cells.

DOI:http://dx.doi.org/10.7554/eLife.27322.001