Transcallosal Projections Require Glycoprotein M6-Dependent Neurite Growth and Guidance

Non-synonymous single nucleotide polymorphisms (nsSNPs) within the extracellular domains of the GPM6A gene impair hippocampal neuron development

Psychiatric disorders are complex pathologies influenced by both environmental and genetic factors, ultimately leading to synaptic plasticity dysfunction. Altered expression levels of neuronal glycoprotein GPM6a or polymorphisms within the GPM6A gene are associated with neuropsychiatric disorders like schizophrenia, depression, and claustrophobia. This protein promotes neurite outgrowth, filopodia formation, dendritic spine, and synapse maintenance in vitro. Although strong evidence suggests that its extracellular domains (ECs) are responsible for its function, the molecular mechanisms linking GPM6a to the onset of such diseases remain unknown. To gain knowledge of these mechanisms, we characterized new non-synonymous polymorphisms (nsSNPs) within the ECs of GPM6a. We identified six nsSNPs (T71P, T76I, M154V, F156Y, R163Q, and T210N) that impair GPM6a-induced plasticity in neuronal cultures without affecting GPM6a expression, folding, and localization to the cell membrane. However, we observed that some of these modified GPM6a's distribution at the cell membrane. Additionally, one of the nsSNPs exhibited alterations in GPM6a oligomerization, highlighting the importance of this amino acid in establishing homophilic cis interactions. Furthermore, we observed that the ability of GPM6a's extracellular domains to interact and induce cell aggregation was significantly decreased in several of the nsSNP variants studied here. Altogether, these results provide new insights into the key residues within GPM6a's extracellular regions that are crucial for its self-association, which is essential for promoting neuronal morphogenesis. Besides, these findings highlight the importance of reverse genetics approaches to gain knowledge on GPM6a's mechanisms of action and the genetic susceptibility of certain GPM6A variants.

Mechanisms of GPM6A in Malignant Tumors

BACKGROUND

Glycoprotein M6A (GPM6A) encodes a transmembrane protein, expressing in large quantities on the cell surface of central nervous system (CNS) neurons. GPM6A acts importantly in neurodevelopment by modulating neuronal differentiation, migration, axon growth, synaptogenesis, and spine formation, but its role in malignancy remains controversial and requires further research. This article reviewed the mechanisms of GPM6A in colorectal cancer, liver cancer, lung cancer, glioblastoma, and other malignant tumors, and made a "one-stop" summary of the relevant mechanisms.

RECENT FINDINGS

Researches have indicated that GPM6A is related to malignant tumors. It affects epithelial-mesenchymal transition and induces the formation of filopodia, participating in the adhesion, migration, and metastasis of cancer cells. Its role in malignant tumors remains controversial, however. On the one hand, GPM6A may have carcinogenic properties and is related to poor prognosis of malignant tumors. It is highly expressed in lymphoblastic leukemia and is a potential oncogene. It also shows carcinogenic properties in colorectal cancer, glioblastoma, gonadotroph adenomas and so on. On the other hand, the expression of GPM6A decreases in lung adenocarcinoma, liver cancer, thyroid cancer, and so forth as the tumor progresses, and it can inhibit the progression of malignant tumors by inhibiting some signaling pathways, suggesting that it may be a tumor suppressor gene.

CONCLUSION

Carcinogenic or tumor suppressive? Although the biological function of GPM6A in the development of malignant tumors is still unclear, according to the current research progress, it is still expected to become an effective molecular marker for predicting tumor occurrence, metastasis and prognosis, as well as a new target for diagnosis and treatment.

Zdhhc1- and Zdhhc2-mediated Gpm6a palmitoylation is essential for maintenance of mammary stem cell activity

Adult mammary stem cells (aMaSCs) are vital to tissue expansion and remodeling during the process of postnatal mammary development. The protein C receptor (Procr) is one of the well-identified surface markers of multipotent aMaSCs. However, an understanding of the regulatory mechanisms governing Procr's protein stability remains incomplete. In this study, we identified Glycoprotein m6a (Gpm6a) as a critical protein for aMaSC activity modulation by using the Gpm6a knockout mouse model. Interestingly, we determined that Gpm6a depletion results in a reduction of Procr protein stability. Mechanistically, Gpm6a regulates Procr protein stability by mediating the formation of lipid rafts, a process requiring Zdhhc1 and Zdhhc2 to palmitate Gpm6a at Cys17,18 and Cys246 sites. Our findings highlight an important mechanism involving Zdhhc1- and Zdhhc2-mediated Gpm6a palmitoylation for the regulation of Procr stability, aMaSC activity, and postnatal mammary development.

AAV-mediated Gpm6b expression supports hair cell reprogramming

Irreversible damage to hair cells (HCs) in the cochlea leads to hearing loss. Cochlear supporting cells (SCs) in the murine cochlea have the potential to differentiate into HCs. Neuron membrane glycoprotein M6B (Gpm6b) as a four-transmembrane protein is a potential regulator of HC regeneration according to our previous research. In this study, we found that AAV-ie-mediated Gpm6b overexpression promoted SC-derived organoid expansion. Enhanced Gpm6b prevented the normal decrease in SC plasticity as the cochlea develops by supporting cells re-entry cell cycle and facilitating the SC-to-HC transformation. Also, overexpression of Gpm6b in the organ of Corti through the round window membrane injection facilitated the trans-differentiation of Lgr5+ SCs into HCs. In conclusion, our results suggest that Gpm6b overexpression promotes HC regeneration and highlights a promising target for hearing repair using the inner ear stem cells combined with AAV.

CRISPR/Cas9-mediated deletion of a GA-repeat in human GPM6B leads to disruption of neural cell differentiation from NT2 cells

The human neuron-specific gene, GPM6B (Glycoprotein membrane 6B), is considered a key gene in neural cell functionality. This gene contains an exceptionally long and strictly monomorphic short tandem repeat (STR) of 9-repeats, (GA)9. STRs in regulatory regions, may impact on the expression of nearby genes. We used CRISPR-based tool to delete this GA-repeat in NT2 cells, and analyzed the consequence of this deletion on GPM6B expression. Subsequently, the edited cells were induced to differentiate into neural cells, using retinoic acid (RA) treatment. Deletion of the GA-repeat significantly decreased the expression of GPM6B at the RNA (p < 0.05) and protein (40%) levels. Compared to the control cells, the edited cells showed dramatic decrease of the astrocyte and neural cell markers, including GFAP (0.77-fold), TUBB3 (0.57-fold), and MAP2 (0.2-fold). Subsequent sorting of the edited cells showed an increased number of NES (p < 0.01), but a decreased number of GFAP (p < 0.001), TUBB3 (p < 0.05), and MAP2 (p < 0.01), compared to the control cells. In conclusion, CRISPR/Cas9-mediated deletion of a GA-repeat in human GPM6B, led to decreased expression of this gene, which in turn, disrupted differentiation of NT2 cells into neural cells.

An appraisal of emerging therapeutic targets for multiple sclerosis derived from current preclinical models

INTRODUCTION

Multiple sclerosis (MS) is a chronic inflammatory, demyelinating, and neurodegenerative condition affecting the central nervous system (CNS). Although therapeutic approaches have become available over the last 20 years that markedly slow the progression of disease, there is no cure for MS. Furthermore, the capacity to repair existing CNS damage caused by MS remains very limited.

AREAS COVERED

Several animal models are widely used in MS research to identify potential druggable targets for new treatment of MS. In this review, we look at targets identified since 2019 in studies using these models, and their potential for effecting a cure for MS.

EXPERT OPINION

Refinement of therapeutic strategies targeting key molecules involved in the activation of immune cells, cytokine, and chemokine signaling, and the polarization of the immune response have dominated recent publications. While some progress has been made in identifying effective targets to combat chronic demyelination and neurodegeneration, much more work is required. Progress is largely limited by the gaps in knowledge of how the immune system and the nervous system interact in MS and its animal models, and whether the numerous targets present in both systems respond in the same way in each system to the same therapeutic manipulation.

Endogenous Glycoprotein GPM6a Is Involved in Neurite Outgrowth in Rat Dorsal Root Ganglion Neurons

The peripheral nervous system (PNS) has a unique ability for self-repair. Dorsal root ganglion (DRG) neurons regulate the expression of different molecules, such as neurotrophins and their receptors, to promote axon regeneration after injury. However, the molecular players driving axonal regrowth need to be better defined. The membrane glycoprotein GPM6a has been described to contribute to neuronal development and structural plasticity in central-nervous-system neurons. Recent evidence indicates that GPM6a interacts with molecules from the PNS, although its role in DRG neurons remains unknown. Here, we characterized the expression of GPM6a in embryonic and adult DRGs by combining analysis of public RNA-seq datasets with immunochemical approaches utilizing cultures of rat DRG explants and dissociated neuronal cells. M6a was detected on the cell surfaces of DRG neurons throughout development. Moreover, GPM6a was required for DRG neurite elongation in vitro. In summary, we provide evidence on GPM6a being present in DRG neurons for the first time. Data from our functional experiments support the idea that GPM6a could contribute to axon regeneration in the PNS.

Molecular Mechanisms Involved in the Regulation of Neurodevelopment by miR-124

miR-124 is a miRNA predominantly expressed in the nervous system and accounts for more than a quarter of the total miRNAs in the brain. It regulates neurogenesis, neuronal differentiation, neuronal maturation, and synapse formation and is the most important miRNA in the brain. Furthermore, emerging evidence has suggested miR-124 may be associated with the pathogenesis of various neurodevelopmental and neuropsychiatric disorders. Here, we provide an overview of the role of miR-124 in neurodevelopment and the underling mechanisms, and finally, we prospect the significance of miR-124 research to the field of neuroscience.

The retinal pigmentation pathway in human albinism: Not so black and white

Albinism is a pigment disorder affecting eye, skin and/or hair. Patients usually have decreased melanin in affected tissues and suffer from severe visual abnormalities, including foveal hypoplasia and chiasmal misrouting. Combining our data with those of the literature, we propose a single functional genetic retinal signalling pathway that includes all 22 currently known human albinism disease genes. We hypothesise that defects affecting the genesis or function of different intra-cellular organelles, including melanosomes, cause syndromic forms of albinism (Hermansky-Pudlak (HPS) and Chediak-Higashi syndrome (CHS)). We put forward that specific melanosome impairments cause different forms of oculocutaneous albinism (OCA1-8). Further, we incorporate GPR143 that has been implicated in ocular albinism (OA1), characterised by a phenotype limited to the eye. Finally, we include the SLC38A8-associated disorder FHONDA that causes an even more restricted "albinism-related" ocular phenotype with foveal hypoplasia and chiasmal misrouting but without pigmentation defects. We propose the following retinal pigmentation pathway, with increasingly specific genetic and cellular defects causing an increasingly specific ocular phenotype: (HPS1-11/CHS: syndromic forms of albinism)-(OCA1-8: OCA)-(GPR143: OA1)-(SLC38A8: FHONDA). Beyond disease genes involvement, we also evaluate a range of (candidate) regulatory and signalling mechanisms affecting the activity of the pathway in retinal development, retinal pigmentation and albinism. We further suggest that the proposed pigmentation pathway is also involved in other retinal disorders, such as age-related macular degeneration. The hypotheses put forward in this report provide a framework for further systematic studies in albinism and melanin pigmentation disorders.

Genome-wide translation control analysis of developing human neurons

During neuronal differentiation, neuroprogenitor cells become polarized, change shape, extend axons, and form complex dendritic trees. While growing, axons are guided by molecular cues to their final destination, where they establish synaptic connections with other neuronal cells. Several layers of regulation are integrated to control neuronal development properly. Although control of mRNA translation plays an essential role in mammalian gene expression, how it contributes temporarily to the modulation of later stages of neuronal differentiation remains poorly understood. Here, we investigated how translation control affects pathways and processes essential for neuronal maturation, using H9-derived human neuro progenitor cells differentiated into neurons as a model. Through Ribosome Profiling (Riboseq) combined with RNA sequencing (RNAseq) analysis, we found that translation control regulates the expression of critical hub genes. Fundamental synaptic vesicle secretion genes belonging to SNARE complex, Rab family members, and vesicle acidification ATPases are strongly translationally regulated in developing neurons. Translational control also participates in neuronal metabolism modulation, particularly affecting genes involved in the TCA cycle and glutamate synthesis/catabolism. Importantly, we found translation regulation of several critical genes with fundamental roles regulating actin and microtubule cytoskeleton pathways, critical to neurite generation, spine formation, axon guidance, and circuit formation. Our results show that translational control dynamically integrates important signals in neurons, regulating several aspects of its development and biology.

Proteomic and functional analyses of the periodic membrane skeleton in neurons

Actin, spectrin, and associated molecules form a membrane-associated periodic skeleton (MPS) in neurons. The molecular composition and functions of the MPS remain incompletely understood. Here, using co-immunoprecipitation and mass spectrometry, we identified hundreds of potential candidate MPS-interacting proteins that span diverse functional categories. We examined representative proteins in several of these categories using super-resolution imaging, including previously unknown MPS structural components, as well as motor proteins, cell adhesion molecules, ion channels, and signaling proteins, and observed periodic distributions characteristic of the MPS along the neurites for ~20 proteins. Genetic perturbations of the MPS and its interacting proteins further suggested functional roles of the MPS in axon-axon and axon-dendrite interactions and in axon diameter regulation, and implicated the involvement of MPS interactions with cell adhesion molecules and non-muscle myosin in these roles. These results provide insights into the interactome of the MPS and suggest previously unknown functions of the MPS in neurons.

A mutant allele of glycoprotein M6-B (Gpm6b) facilitates behavioral flexibility but increases delay discounting

The neuronal membrane glycoprotein M6B (Gpm6b) gene encodes a membrane glycoprotein that belongs to the proteolipid protein family, and is enriched in neurons, oligodendrocytes, and subset of astrocytes in the central nervous system. GPM6B is thought to play a role in neuronal differentiation, myelination, and inactivation of the serotonin transporter via internalization. Recent human genome-wide association studies (GWAS) have implicated membrane glycoproteins (both GPM6B and GPM6A) in the regulation of traits relevant to psychiatric disorders, including neuroticism, depressed affect, and delay discounting. Mouse studies have implicated Gpm6b in sensorimotor gating and regulation of serotonergic signaling. We used CRISPR to create a mutant Glycoprotein M6B (Gpm6b) allele on a C57BL/6J mouse background. Because Gpm6b is located on the X chromosome, we focused on male Gpm6b mutant mice and their wild-type littermates (WT) in two behavioral tests that measured aspects of impulsive or flexible decision-making. We found that Gpm6b deletion caused deficits in a delay discounting task. In contrast, reward sensitivity was enhanced thereby facilitating behavioral flexibility and improving performance in the probabilistic reversal learning task. Taken together these data further delineate the role of Gpm6b in decision making behaviors that are relevant to multiple psychiatric disorders.

Identification by proximity labeling of novel lipidic and proteinaceous potential partners of the dopamine transporter

Dopamine (DA) transporters (DATs) are regulated by trafficking and modulatory processes that probably rely on stable and transient interactions with neighboring proteins and lipids. Using proximity-dependent biotin identification (BioID), we found novel potential partners for DAT, including several membrane proteins, such as the transmembrane chaperone 4F2hc, the proteolipid M6a and a potential membrane receptor for progesterone (PGRMC2). We also detected two cytoplasmic proteins: a component of the Cullin1-dependent ubiquitination machinery termed F-box/LRR-repeat protein 2 (FBXL2), and the enzyme inositol 5-phosphatase 2 (SHIP2). Immunoprecipitation (IP) and immunofluorescence studies confirmed either a physical association or a close spatial proximity between these proteins and DAT. M6a, SHIP2 and the Cullin1 system were shown to increase DAT activity in coexpression experiments, suggesting a functional role for their association. Deeper analysis revealed that M6a, which is enriched in neuronal protrusions (filopodia or dendritic spines), colocalized with DAT in these structures. In addition, the product of SHIP2 enzymatic activity (phosphatidylinositol 3,4-bisphosphate [PI(3,4)P2]) was tightly associated with DAT, as shown by co-IP and by colocalization of mCherry-DAT with a specific biosensor for this phospholipid. PI(3,4)P2 strongly stimulated transport activity in electrophysiological recordings, and conversely, inhibition of SHIP2 reduced DA uptake in several experimental systems including striatal synaptosomes and the dopaminergic cell line SH-SY5Y. In summary, here we report several potential new partners for DAT and a novel regulatory lipid, which may represent new pharmacological targets for DAT, a pivotal protein in dopaminergic function of the brain.

Neuronal Glycoprotein M6a: An Emerging Molecule in Chemical Synapse Formation and Dysfunction

The cellular and molecular mechanisms underlying neuropsychiatric and neurodevelopmental disorders show that most of them can be categorized as synaptopathies-or damage of synaptic function and plasticity. Synaptic formation and maintenance are orchestrated by protein complexes that are in turn regulated in space and time during neuronal development allowing synaptic plasticity. However, the exact mechanisms by which these processes are managed remain unknown. Large-scale genomic and proteomic projects led to the discovery of new molecules and their associated variants as disease risk factors. Neuronal glycoprotein M6a, encoded by the GPM6A gene is emerging as one of these molecules. M6a has been involved in neuron development and synapse formation and plasticity, and was also recently proposed as a gene-target in various neuropsychiatric disorders where it could also be used as a biomarker. In this review, we provide an overview of the structure and molecular mechanisms by which glycoprotein M6a participates in synapse formation and maintenance. We also review evidence collected from patients carrying mutations in the GPM6A gene; animal models, and in vitro studies that together emphasize the relevance of M6a, particularly in synapses and in neurological conditions.

Proteomics Profiling of Neuron-Derived Small Extracellular Vesicles from Human Plasma: Enabling Single-Subject Analysis

Small extracellular vesicles have been intensively studied as a source of biomarkers in neurodegenerative disorders. The possibility to isolate neuron-derived small extracellular vesicles (NDsEV) from blood represents a potential window into brain pathological processes. To date, the absence of sensitive NDsEV isolation and full proteome characterization methods has meant their protein content has been underexplored, particularly for individual patients. Here, we report a rapid method based on an immunoplate covalently coated with mouse monoclonal anti-L1CAM antibody for the isolation and the proteome characterization of plasma-NDsEV from individual Parkinson’s disease (PD) patients. We isolated round-shaped vesicles with morphological characteristics consistent with exosomes. On average, 349 ± 38 protein groups were identified by liquid chromatography–tandem mass spectrometry (LC-MS/MS) analysis, 20 of which are annotated in the Human Protein Atlas as being highly expressed in the brain, and 213 were shared with a reference NDsEV dataset obtained from cultured human neurons. Moreover, this approach enabled the identification of 23 proteins belonging to the Parkinson disease KEGG pathway, as well as proteins previously reported as PD circulating biomarkers.

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.

GPM6B Inhibit PCa Proliferation by Blocking Prostate Cancer Cell Serotonin Absorptive Capacity

Prostate cancer is currently one of the most common fatal tumor types in men. Although multiple treatments can alleviate some cases, advanced prostate cancer, especially CRPC, still has a very poor prognosis. Therefore, early detection and diagnosis of prostate cancer have a very important role in the prognosis of patients. Glycoprotein M6B (GPM6B) is a transmembrane protein that belongs to the proteolipid protein family. GPM6B has been proved and can be used as a biomarker for gynecological malignancies and breast carcinoma. However, there are no studies that explored the functions of GPM6B in PCa. We explored differentially expressed genes in prostate cancer by analyzing TCGA data and found GPM6B downregulated in PCa tissues compared to that in normal prostate tissues. The GPM6B expression in PCa patient's tumor tissues was significantly related to clinical stage, T classification, lymph node metastasis, and distant metastasis, but not significantly related to age and Gleason score. Also, patients with highGPM6B expression had a better prognosis. The overexpression of GPM6B in prostate cancer cells could inhibit cell proliferation. Serotonin treatment could enhance the proliferation of PCa cell lines; moreover, fluoxetine could reverse this result. In conclusion, we identified GPM6B as a tumor suppressor in PCa. In mechanism, it can regulate the uptaking of serotonin and inhibit the growth of prostate cancer. These results suggested the potential function of GPM6B as a diagnostic marker of PCa and provided clues for the development of new treatment targets for PCa.

Identification of Potential Interacting Proteins With the Extracellular Loops of the Neuronal Glycoprotein M6a by TMT/MS

Nowadays, great efforts are made to gain insight into the molecular mechanisms that underlie structural neuronal plasticity. Moreover, the identification of signaling pathways involved in the development of psychiatric disorders aids the screening of possible therapeutic targets. Genetic variations or alterations in GPM6A expression are linked to neurological disorders such as schizophrenia, depression, and Alzheimer's disease. GPM6A encodes the neuronal surface glycoprotein M6a that promotes filopodia/spine, dendrite, and synapse formation by unknown mechanisms. A substantial body of evidence suggests that the extracellular loops of M6a command its function. However, the proteins that associate with them and that modulate neuronal plasticity have not been determined yet. To address this question, we generated a chimera protein that only contains the extracellular loops of M6a and performed a co-immunoprecipitation with rat hippocampus samples followed by TMT/MS. Here, we report 72 proteins, which are good candidates to interact with M6a's extracellular loops and modify its function. Gene ontology (GO) analysis showed that 63% of the potential M6a's interactor proteins belong to the category "synapse," at both sides of the synaptic cleft, "neuron projections" (51%) and "presynapse" (49%). In this sense, we showed that endogenous M6a interacts with piccolo, synaptic vesicle protein 2B, and synapsin 1 in mature cultured hippocampal neurons. Interestingly, about 28% of the proteins left were related to the "myelin sheath" annotation, suggesting that M6a could interact with proteins at the surface of oligodendrocytes. Indeed, we demonstrated the (cis and trans) interaction between M6a and proteolipid protein (PLP) in neuroblastoma N2a cells. Finally, the 72 proteins were subjected to disease-associated genes and variants screening by DisGeNET. Apart from the diseases that have already been associated with M6a, most of the proteins are also involved in "autistic disorder," "epilepsy," and "seizures" increasing the spectrum of disorders in which M6a could play a role. Data are available via ProteomeXchange with identifier PXD017347.

Anti-Myelin Proteolipid Protein Peptide Monoclonal Antibodies Recognize Cell Surface Proteins on Developing Neurons and Inhibit Their Differentiation

Using a panel of monoclonal antibodies (mAbs) to myelin proteolipid protein (PLP) peptides, we found that in addition to CNS myelin, mAbs to external face but not cytoplasmic face epitopes immunostained neurons in immature human CNS tissues and in adult hippocampal dentate gyrus and olfactory bulbs, that is neural stem cell niches (NSCN). To explore the pathobiological significance of these observations, we assessed the mAb effects on neurodifferentiation in vitro. The mAbs to PLP 50-69 (IgG1κ and IgG2aκ), and 178-191 and 200-219 (both IgG1κ) immunostained live cell surfaces and inhibited neurite outgrowth of E18 rat hippocampal precursor cells and of PC12 cells, which do not express PLP. Proteins immunoprecipitated from PC12 cell extracts and captured by mAb-coated magnetic beads were identified by GeLC-MS/MS. Each neurite outgrowth-inhibiting mAb captured a distinct set of neurodifferentiation molecules including sequence-similar M6 proteins and other unrelated membrane and extracellular matrix proteins, for example integrins, Eph receptors, NCAM-1, and protocadherins. These molecules are expressed in adult human NSCN and are implicated in the pathogenesis of many chronic CNS disease processes. Thus, diverse anti-PLP epitope autoantibodies may inhibit neuronal precursor cell differentiation via multispecific recognition of cell surface molecules thereby potentially impeding endogenous neuroregeneration in NSCN and in vivo differentiation of exogenous neural stem cells.

Expression pattern of Nav2 in the murine CNS with development

Neuron navigator 2 (NAV2, RAINB1, POMFIL2, HELAD1, unc53H2) is essential for nervous system development. In the present study the spatial distribution of Nav2 transcript in mouse CNS during embryonic, postnatal and adult life is examined. Because multiple NAV2 proteins are predicted based on alternate promoter usage and RNA splicing, in situ hybridization was performed using probes designed to the 5' and 3' ends of the Nav2 transcript, and PCR products using primer sets spanning the length of the mRNA were also examined by real time PCR (qPCR). These studies support full-length Nav2 transcript as the predominant form in the wild-type mouse CNS. The developing cortex, hippocampus, thalamus, olfactory bulb, and granule cells (GC) within the cerebellum show the highest expression, with a similar staining pattern using either the 5'Nav2 or 3'Nav2 probe. Nav2 is expressed in GC precursors migrating over the cerebellar primordium as well as in the postmitotic premigratory cells of the external granule cell layer (EGL). It is expressed in the cornu ammonis (CA) and dentate gyrus (DG) throughout hippocampal development. In situ hybridization was combined with immunohistochemistry for Ki67, CTIP2 and Nissl staining to follow Nav2 transcript location during cortical development, where it is observed in neuroepithelial cells exiting the germinal compartments, as well as later in the cortical plate (CP) and developing cortical layers. The highest levels of Nav2 in all brain regions studied are observed in late gestation and early postnatal life which coincides with times when neurons are migrating and differentiating. A hypomorphic mouse that lacks the full-length transcript but expresses shorter transcript shows little staining in the CNS with either probe set except at the base of the cerebellum, where a shorter Nav2 transcript is detected. Using dual fluorescent probe in situ hybridization studies, these cells are identified as oligodendrocytes and are detected using both Olig1 and the 3'Nav2 probe. The identification of full-length Nav2 as the primary transcript in numerous brain regions suggests NAV2 could play a role in CNS development beyond that of its well-established role in the cerebellum.

Multi-Omic Analyses of Growth Cones at Different Developmental Stages Provides Insight into Pathways in Adult Neuroregeneration

Growth cones (GCs) are structures associated with growing neurons. GC membrane expansion, which necessitates protein-lipid interactions, is critical to axonal elongation in development and in adult neuritogenesis. We present a multi-omic analysis that integrates proteomics and lipidomics data for the identification of GC pathways, cell phenotypes, and lipid-protein interactions, with an analytic platform to facilitate the visualization of these data. We combine lipidomic data from GC and adult axonal regeneration following optic nerve crush. Our results reveal significant molecular variability in GCs across developmental ages that aligns with the upregulation and downregulation of lipid metabolic processes and correlates with distinct changes in the lipid composition of GC plasmalemma. We find that these processes also define the transition into a growth-permissive state in the adult central nervous system. The insight derived from these analyses will aid in promoting adult regeneration and functional innervation in devastating neurodegenerative diseases.

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Simultaneous proteomics and lipidomics analyses of developmental growth cones

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Combined multi-omics analyses of regenerating optic nerves and growth cones

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Integrating protein-protein with protein-lipid interactions in growth cones

A Novel Birthdate-Labeling Method Reveals Segregated Parallel Projections of Mitral and External Tufted Cells in the Main Olfactory System

A fundamental strategy in sensory coding is parallel processing, whereby unique, distinct features of sensation are computed and projected to the central target in the form of submodal maps. It remains unclear, however, whether such parallel processing strategy is employed in the main olfactory system, which codes the complex hierarchical odor and behavioral scenes. A potential scheme is that distinct subsets of projection neurons in the olfactory bulb (OB) form parallel projections to the targets. Taking advantage of the observation that the distinct projection neurons develop at different times, we developed a Cre-loxP-based method that allows for birthdate-specific labeling of cell bodies and their axon projections in mice. This birthdate tag analysis revealed that the mitral cells (MCs) born in an early developmental stage and the external tufted cells (TCs) born a few days later form segregated parallel projections. Specifically, the latter subset converges the axons onto only two small specific targets, one of which, located at the anterolateral edge of the olfactory tubercle (OT), excludes widespread MC projections. This target is made up of neurons that express dopamine D1 but not D2 receptor and corresponds to the most anterolateral isolation of the CAP compartments (aiCAP) that were defined previously. This finding of segregated projections suggests that olfactory sensing does indeed involve parallel processing of functionally distinct submodalities. Importantly, the birthdate tag method used here may pave the way for deciphering the functional meaning of these individual projection pathways in the future.

In vivo epigenetic editing of Sema6a promoter reverses transcallosal dysconnectivity caused by C11orf46/Arl14ep risk gene

Many neuropsychiatric risk genes contribute to epigenetic regulation but little is known about specific chromatin-associated mechanisms governing the formation of neuronal connectivity. Here we show that transcallosal connectivity is critically dependent on C11orf46, a nuclear protein encoded in the chromosome 11p13 WAGR risk locus. C11orf46 haploinsufficiency was associated with hypoplasia of the corpus callosum. C11orf46 knockdown disrupted transcallosal projections and was rescued by wild type C11orf46 but not the C11orf46^R236H mutant associated with intellectual disability. Multiple genes encoding key regulators of axonal development, including Sema6a, were hyperexpressed in C11orf46-knockdown neurons. RNA-guided epigenetic editing of Sema6a gene promoters via a dCas9-SunTag system with C11orf46 binding normalized SEMA6A expression and rescued transcallosal dysconnectivity via repressive chromatin remodeling by the SETDB1 repressor complex. Our study demonstrates that interhemispheric communication is sensitive to locus-specific remodeling of neuronal chromatin, revealing the therapeutic potential for shaping the brain's connectome via gene-targeted designer activators and repressor proteins.

Molecular basis of the functions of the mammalian neuronal growth cone revealed using new methods

The neuronal growth cone is a highly motile, specialized structure for extending neuronal processes. This structure is essential for nerve growth, axon pathfinding, and accurate synaptogenesis. Growth cones are important not only during development but also for plasticity-dependent synaptogenesis and neuronal circuit rearrangement following neural injury in the mature brain. However, the molecular details of mammalian growth cone function are poorly understood. This review examines molecular findings on the function of the growth cone as a result of the introduction of novel methods such superresolution microscopy and (phospho)proteomics. These results increase the scope of our understating of the molecular mechanisms of growth cone behavior in the mammalian brain.

Glycoprotein M6B Interacts with TβRI to Activate TGF-β-Smad2/3 Signaling and Promote Smooth Muscle Cell Differentiation

Smooth muscle cells (SMCs), which form the walls of blood vessels, play an important role in vascular development and the pathogenic process of vascular remodeling. However, the molecular mechanisms governing SMC differentiation remain poorly understood. Glycoprotein M6B (GPM6B) is a four-transmembrane protein that belongs to the proteolipid protein family and is widely expressed in neurons, oligodendrocytes, and astrocytes. Previous studies have revealed that GPM6B plays a role in neuronal differentiation, myelination, and osteoblast differentiation. In the present study, we found that the GPM6B gene and protein expression levels were significantly upregulated during transforming growth factor-β1 (TGF-β1)-induced SMC differentiation. The knockdown of GPM6B resulted in the downregulation of SMC-specific marker expression and repressed the activation of Smad2/3 signaling. Moreover, GPM6B regulates SMC Differentiation by Controlling TGF-β-Smad2/3 Signaling. Furthermore, we demonstrated that similar to p-Smad2/3, GPM6B was profoundly expressed and coexpressed with SMC differentiation markers in embryonic SMCs. Moreover, GPM6B can regulate the tightness between TβRI, TβRII, or Smad2/3 by directly binding to TβRI to activate Smad2/3 signaling during SMC differentiation, and activation of TGF-β-Smad2/3 signaling also facilitate the expression of GPM6B. Taken together, these findings demonstrate that GPM6B plays a crucial role in SMC differentiation and regulates SMC differentiation through the activation of TGF-β-Smad2/3 signaling via direct interactions with TβRI. This finding indicates that GPM6B is a potential target for deriving SMCs from stem cells in cardiovascular regenerative medicine. Stem Cells 2018 Stem Cells 2019;37:190-201.

Alanine Scanning Mutagenesis of the C-Terminal Cytosolic End of Gpm6a Identifies Key Residues Essential for the Formation of Filopodia

Neuronal membrane glycoprotein M6a (Gpm6a) is a protein with four transmembrane regions and the N- and the C-ends facing the cytosol. It functions in processes of neuronal development, outgrowth of neurites, and formation of filopodia, spines, and synapsis. Molecular mechanisms by which Gpm6a acts in these processes are not fully comprehended. Structural similarities of Gpm6a with tetraspanins led us to hypothesize that, similarly to tetraspanins, the cytoplasmic tails function as connections with cytoskeletal and/or signaling proteins. Here, we demonstrate that the C- but not the N-terminal cytosolic end of Gpm6a is required for the formation of filopodia by Gpm6a in cultured neurons from rat hippocampus and in neuroblastoma cells N2a. Further immunofluorescence microcopy and flow cytometry analysis show that deletion of neither the N- nor the C-terminal intracellular domains interferes with the recognition of Gpm6a by the function-blocking antibody directed against the extracellular part of Gpm6a. Expression levels of both truncation mutants were not affected but we observed decrease in the amount of both truncated proteins on cell surface suggesting that the incapacity of the Gpm6a lacking C-terminus to induce filopodium formation is not due to the lower amount of Gpm6a on cell surface. Following colocalization assays shows that deletion of the C- but not the N-terminus diminishes the association of Gpm6a with clathrin implying involvement of clathrin-mediated trafficking events. Next, using comprehensive alanine scanning mutagenesis of the C-terminus we identify K250, K255, and E258 as the key residues for the formation of filopodia by Gpm6a. Substitution of these charged residues with alanine also diminishes the amount of Gpm6a on cell surface and in case of K255 and E258 leads to the lower amount of total expressed protein. Subsequent bioinformatic analysis of Gpm6a amino acid sequence reveals that highly conserved and functional residues cluster preferentially within the C- and not within the N-terminus and that K250, K255, and E258 are predicted as part of sorting signals of transmembrane proteins. Altogether, our results provide evidence that filopodium outgrowth induced by Gpm6a requires functionally critical residues within the C-terminal cytoplasmic tail.

Glial M6B stabilizes the axonal membrane at peripheral nodes of Ranvier

Glycoprotein M6B and the closely related proteolipid protein regulate oligodendrocyte myelination in the central nervous system, but their role in the peripheral nervous system is less clear. Here we report that M6B is located at nodes of Ranvier in peripheral nerves where it stabilizes the nodal axolemma. We show that M6B is co-localized and associates with gliomedin at Schwann cell microvilli that are attached to the nodes. Developmental analysis of sciatic nerves, as well as of myelinating Schwann cells/dorsal root ganglion neurons cultures, revealed that M6B is already present at heminodes, which are considered the precursors of mature nodes of Ranvier. However, in contrast to gliomedin, which accumulates at heminodes with or prior to Na+ channels, we often detected Na+ channel clusters at heminodes without any associated M6B, indicating that it is not required for initial channel clustering. Consistently, nodal cell adhesion molecules (NF186, NrCAM), ion channels (Nav1.2 and Kv7.2), cytoskeletal proteins (AnkG and βIV spectrin), and microvilli components (pERM, syndecan3, gliomedin), are all present at both heminodes and mature nodes of Ranvier in Gpm6b null mice. Using transmission electron microscopy, we show that the absence of M6B results in progressive appearance of nodal protrusions of the nodal axolemma, that are often accompanied by the presence of enlarged mitochondria. Our results reveal that M6B is a Schwann cell microvilli component that preserves the structural integrity of peripheral nodes of Ranvier.

The integrated landscape of causal genes and pathways in schizophrenia

Genome-wide association studies (GWAS) have identified more than 100 loci that show robust association with schizophrenia risk. However, due to the complexity of linkage disequilibrium and gene regulatory, it is challenging to pinpoint the causal genes at the risk loci and translate the genetic findings from GWAS into disease mechanism and clinical treatment. Here we systematically predicted the plausible candidate causal genes for schizophrenia at genome-wide level. We utilized different approaches and strategies to predict causal genes for schizophrenia, including Sherlock, SMR, DAPPLE, Prix Fixe, NetWAS, and DEPICT. By integrating the results from different prediction approaches, we identified six top candidates that represent promising causal genes for schizophrenia, including CNTN4, GATAD2A, GPM6A, MMP16, PSMA4, and TCF4. Besides, we also identified 35 additional high-confidence causal genes for schizophrenia. The identified causal genes showed distinct spatio-temporal expression patterns in developing and adult human brain. Cell-type-specific expression analysis indicated that the expression level of the predicted causal genes was significantly higher in neurons compared with oligodendrocytes and microglia (P < 0.05). We found that synaptic transmission-related genes were significantly enriched among the identified causal genes (P < 0.05), providing further support for the dysregulation of synaptic transmission in schizophrenia. Finally, we showed that the top six causal genes are dysregulated in schizophrenia cases compared with controls and knockdown of these genes impaired the proliferation of neuronal cells. Our study depicts the landscape of plausible schizophrenia causal genes for the first time. Further genetic and functional validation of these genes will provide mechanistic insights into schizophrenia pathogenesis and may facilitate to provide potential targets for future therapeutics and diagnostics.

hnRNP R and its main interactor, the noncoding RNA 7SK, coregulate the axonal transcriptome of motoneurons

Neurons are highly polarized cells. RNA-binding proteins contribute to this polarization by generating diverse subcellular transcriptomes. The RNA-binding protein hnRNP R is essential for axon growth in motoneurons. This study reports the RNA interactome for hnRNP R. The main interacting RNA of hnRNP R was the noncoding RNA 7SK. Depletion of 7SK from primary motoneurons disturbed axon growth. This effect was dependent on the interaction of 7SK with hnRNP R. Both hnRNP R and 7SK localize to axons. Loss of 7SK led to a similar depletion of axonal transcripts as loss of hnRNP R. Our data suggest that 7SK, in addition to its role in transcriptional regulation, acts in concert with hnRNP R to sort specific transcripts into axons.

Disturbed RNA processing and subcellular transport contribute to the pathomechanisms of motoneuron diseases such as amyotrophic lateral sclerosis and spinal muscular atrophy. RNA-binding proteins are involved in these processes, but the mechanisms by which they regulate the subcellular diversity of transcriptomes, particularly in axons, are not understood. Heterogeneous nuclear ribonucleoprotein R (hnRNP R) interacts with several proteins involved in motoneuron diseases. It is located in axons of developing motoneurons, and its depletion causes defects in axon growth. Here, we used individual nucleotide-resolution cross-linking and immunoprecipitation (iCLIP) to determine the RNA interactome of hnRNP R in motoneurons. We identified ∼3,500 RNA targets, predominantly with functions in synaptic transmission and axon guidance. Among the RNA targets identified by iCLIP, the noncoding RNA 7SK was the top interactor of hnRNP R. We detected 7SK in the nucleus and also in the cytosol of motoneurons. In axons, 7SK localized in close proximity to hnRNP R, and depletion of hnRNP R reduced axonal 7SK. Furthermore, suppression of 7SK led to defective axon growth that was accompanied by axonal transcriptome alterations similar to those caused by hnRNP R depletion. Using a series of 7SK-deletion mutants, we show that the function of 7SK in axon elongation depends on its interaction with hnRNP R but not with the PTEF-B complex involved in transcriptional regulation. These results propose a role for 7SK as an essential interactor of hnRNP R to regulate its function in axon maintenance.

WNT/β-Catenin Pathway and Epigenetic Mechanisms Regulate the Pitt-Hopkins Syndrome and Schizophrenia Risk Gene

Genetic variation within the transcription factor TCF4 locus can cause the intellectual disability and developmental disorder Pitt-Hopkins syndrome (PTHS), whereas single-nucleotide polymorphisms within noncoding regions are associated with schizophrenia. These genetic findings position TCF4 as a link between transcription and cognition; however, the neurobiology of TCF4 remains poorly understood. Here, we quantitated multiple distinct TCF4 transcript levels in human induced pluripotent stem cell-derived neural progenitors and differentiated neurons, and PTHS patient fibroblasts. We identify two classes of pharmacological treatments that regulate TCF4 expression: WNT pathway activation and inhibition of class I histone deacetylases. In PTHS fibroblasts, both of these perturbations upregulate a subset of TCF4 transcripts. Finally, using chromatin immunoprecipitation sequencing in conjunction with genome-wide transcriptome analysis, we identified TCF4 target genes that may mediate the effect of TCF4 loss on neuroplasticity. Our studies identify new pharmacological assays, tools, and targets for the development of therapeutics for cognitive disorders.

miR-124 and miR-9 mediated downregulation of HDAC5 promotes neurite development through activating MEF2C-GPM6A pathway

The class IIa histone deacetylases (HDACs) play important roles in the central nervous system during diverse biological processes such as synaptic plasticity, axon regeneration, cell apoptosis, and neural differentiation. Although it is known that HDAC5 regulates neuronal differentiation, neither the physiological function nor the regulation of HDAC5 in neuronal differentiation is clear. Here, we identify HDAC5 as an inhibitor of neurite elongation and show that HDAC5 is regulated by the brain enriched microRNA miR-124 and miR-9. We discover that HDAC5 inhibits neurite extension both in differentiated P19 cells and primary neurons. We also show that the neuronal membrane glycoprotein GPM6A (M6a) is a direct target gene of HDAC5 regulated transcriptional factor MEF2C. HDAC5 inhibits neurite elongation, acting at least partially via a MEF2C/M6a signaling pathway. We also confirmed the miR-124/miR-9 regulated HDAC5-MEF2C-M6a pathway regulates neurite development in primary neurons. Thus, HDAC5 emerges as a cellular conductor of MEF2C and M6a activity and is regulated by miR-124 and miR-9 to control neurite development.

Extracellular Signals Induce Glycoprotein M6a Clustering of Lipid Rafts and Associated Signaling Molecules

Lipid raft domains, where sphingolipids and cholesterol are enriched, concentrate signaling molecules. To examine how signaling protein complexes are clustered in rafts, we focused on the functions of glycoprotein M6a (GPM6a), which is expressed at a high concentration in developing mouse neurons. Using imaging of lipid rafts, we found that GPM6a congregated in rafts in a GPM6a palmitoylation-dependent manner, thereby contributing to lipid raft clustering. In addition, we found that signaling proteins downstream of GPM6a, such as Rufy3, Rap2, and Tiam2/STEF, accumulated in lipid rafts in a GPM6a-dependent manner and were essential for laminin-dependent polarity during neurite formation in neuronal development. In utero RNAi targeting of GPM6a resulted in abnormally polarized neurons with multiple neurites. These results demonstrate that GPM6a induces the clustering of lipid rafts, which supports the raft aggregation of its associated downstream molecules for acceleration of neuronal polarity determination. Therefore, GPM6a acts as a signal transducer that responds to extracellular signals.SIGNIFICANCE STATEMENT Lipid raft domains, where sphingolipids and cholesterol are enriched, concentrate signaling molecules. We focused on glycoprotein M6a (GPM6a), which is expressed at a high concentration in developing neurons. Using imaging of lipid rafts, we found that GPM6a congregated in rafts in a palmitoylation-dependent manner, thereby contributing to lipid raft clustering. In addition, we found that signaling proteins downstream of GPM6a accumulated in lipid rafts in a GPM6a-dependent manner and were essential for laminin-dependent polarity during neurite formation. In utero RNAi targeting of GPM6a resulted in abnormally polarized neurons with multiple neurites. These results demonstrate that GPM6a induces the clustering of lipid rafts, which supports the raft aggregation of its associated downstream molecules for acceleration of polarity determination. Therefore, GPM6a acts as a signal transducer that responds to extracellular signals.

The transcriptome of mouse central nervous system myelin

Rapid nerve conduction in the CNS is facilitated by insulation of axons with myelin, a specialized oligodendroglial compartment distant from the cell body. Myelin is turned over and adapted throughout life; however, the molecular and cellular basis of myelin dynamics remains elusive. Here we performed a comprehensive transcriptome analysis (RNA-seq) of myelin biochemically purified from mouse brains at various ages and find a surprisingly large pool of transcripts enriched in myelin. Further computational analysis showed that the myelin transcriptome is closely related to the myelin proteome but clearly distinct from the transcriptomes of oligodendrocytes and brain tissues, suggesting a highly selective incorporation of mRNAs into the myelin compartment. The mRNA-pool in myelin displays maturation-dependent dynamic changes of composition, abundance, and functional associations; however ageing-dependent changes after 6 months were minor. We suggest that this transcript pool enables myelin turnover and the local adaptation of individual pre-existing myelin sheaths.

Mislocalization of syntaxin-1 and impaired neurite growth observed in a human iPSC model for STXBP1-related epileptic encephalopathy

Syntaxin-binding protein 1 (STXBP1) is essential for synaptic vesicle exocytosis. Mutations of its encoding gene, STXBP1, are among the most frequent genetic causes of epileptic encephalopathies. However, the precise pathophysiology of STXBP1 haploinsufficiency has not been elucidated. Using patient-derived induced pluripotent stem cells (iPSCs), we aimed to establish a neuronal model for STXBP1 haploinsufficiency and determine the pathophysiologic basis for STXBP1 encephalopathy. We generated iPSC lines from a patient with Ohtahara syndrome (OS) harboring a heterozygous nonsense mutation of STXBP1 (c.1099C>T; p.R367X) and performed neuronal differentiation. Both STXBP1 messenger RNA (mRNA) and STXBP1 protein expression levels of OS-derived neurons were approximately 50% lower than that of control-derived neurons, suggesting that OS-derived neurons are a suitable model for elucidating the pathophysiology of STXBP1 haploinsufficiency. Through Western blot and immunocytochemistry assays, we found that OS-derived neurons show reduced levels and mislocalization of syntaxin-1, a component of soluble N-ethylmaleimide-sensitive factor attachment receptor (SNARE) proteins. In addition, OS-derived neurons have impaired neurite outgrowth. In conclusion, this model enables us to investigate the neurobiology of STXBP1 encephalopathy throughout the stages of neurodevelopment. Reduced expression of STXBP1 leads to changes in the expression and localization of syntaxin-1 that may contribute to the devastating phenotype of STXBP1 encephalopathy.

Neuronal filopodium formation induced by the membrane glycoprotein M6a (Gpm6a) is facilitated by coronin-1a, Rac1, and p21-activated kinase 1 (Pak1)

Stress-responsive neuronal membrane glycoprotein M6a (Gpm6a) functions in neurite extension, filopodium and spine formation and synaptogenesis. The mechanisms of Gpm6a action in these processes are incompletely understood. Previously, we identified the actin regulator coronin-1a (Coro1a) as a putative Gpm6a interacting partner. Here, we used co-immunoprecipitation assays with the anti-Coro1a antibody to show that Coro1a associates with Gpm6a in rat hippocampal neurons. By immunofluorescence microscopy, we demonstrated that in hippocampal neurons Coro1a localizes in F-actin-enriched regions and some of Coro1a spots co-localize with Gpm6a labeling. Notably, the over-expression of a dominant-negative form of Coro1a as well as its down-regulation by siRNA interfered with Gpm6a-induced filopodium formation. Coro1a is known to regulate the plasma membrane translocation and activation of small GTPase Rac1. We show that Coro1a co-immunoprecipitates with Rac1 together with Gpm6a. Pharmacological inhibition of Rac1 resulted in a significant decrease in filopodium formation by Gpm6a. The same was observed upon the co-expression of Gpm6a with the inactive GDP-bound form of Rac1. In this case, the elevated membrane recruitment of GDP-bound Rac1 was detected as well. Moreover, the kinase activity of the p21-activated kinase 1 (Pak1), a main downstream effector of Rac1 that acts downstream of Coro1a, was required for Gpm6a-induced filopodium formation. Taken together, our results provide evidence that a signaling pathway including Coro1a, Rac1, and Pak1 facilitates Gpm6a-induced filopodium formation. Formation of filopodia by membrane glycoprotein M6a (Gpm6a) requires actin regulator coronin-1a (Coro1a), known to regulate plasma membrane localization and activation of Rac1 and its downstream effector Pak1. Coro1a associates with Gpm6a. Blockage of Coro1a, Rac1, or Pak1 interferes with Gpm6a-induced filopodium formation. Moreover, Gpm6a facilitates Rac1 membrane recruitment. Altogether, a mechanistic insight into the process of Gpm6a-induced neuronal filopodium formation is provided.