Molecular determinants of the sensory and motor neuron-derived factor insertion into plasma membrane

How membrane dysfunction influences neuronal survival pathways in sphingolipid storage disorders

Sphingolipidoses are a class of inherited diseases that result from the toxic accumulation of undigested sphingolipids in lysosomes and other cellular membranes. Sphingolipids are particularly enriched in cells of the nervous system, and their excessive accumulation during disease has a significant impact on the nervous system. Neuronal dysfunction followed by neurological compromise is a common feature in many of these diseases; however, the underlying mechanisms that cause vulnerability of neurons are not fully understood. The plasma membrane plays a critical role in regulating cellular survival pathways, and its dysfunction has been implicated in neuronal failure in various adult-onset neuropathies. In the context of sphingolipidoses, we hypothesize that gradual accumulation of undigested lipids in plasma membranes causes local disruptions in lipid raft domains, leading to deregulation of multiple signaling pathways important for neuronal survival and function. We propose that defects in downstream signaling as a result of membrane dysfunction are common mechanisms underlying neuronal vulnerability in sphingolipid storage disorders with neurological compromise. © 2016 Wiley Periodicals, Inc.

Neuregulin/ErbB Signaling in Developmental Myelin Formation and Nerve Repair

Myelin is essential for rapid and accurate conduction of electrical impulses by axons in the central and peripheral nervous system (PNS). Myelin is formed in the early postnatal period, and developmental myelination in the PNS depends on axonal signals provided by Nrg1/ErbB receptors. In addition, Nrg1 is required for effective nerve repair and remyelination in adulthood. We discuss here similarities and differences in Nrg1/ErbB functions in developmental myelination and remyelination after nerve injury.

Neuregulin 1 role in Schwann cell regulation and potential applications to promote peripheral nerve regeneration

Neuregulin 1 (NRG1) is a multifunctional and versatile protein: its numerous isoforms can signal in a paracrine, autocrine, or juxtacrine manner, playing a fundamental role during the development of the peripheral nervous system and during the process of nerve repair, suggesting that the treatment with NRG1 could improve functional outcome following injury. Accordingly, the use of NRG1 in vivo has already yielded encouraging results. The aim of this review is to focus on the role played by the different NRG1 isoforms during peripheral nerve regeneration and remyelination and to identify good candidates to be used for the development of tissue engineered medical devices delivering NRG1, with the objective of promoting better nerve repair.

L1CAM binds ErbB receptors through Ig-like domains coupling cell adhesion and neuregulin signalling

During nervous system development different cell-to-cell communication mechanisms operate in parallel guiding migrating neurons and growing axons to generate complex arrays of neural circuits. How such a system works in coordination is not well understood. Cross-regulatory interactions between different signalling pathways and redundancy between them can increase precision and fidelity of guidance systems. Immunoglobulin superfamily proteins of the NCAM and L1 families couple specific substrate recognition and cell adhesion with the activation of receptor tyrosine kinases. Thus it has been shown that L1CAM-mediated cell adhesion promotes the activation of the EGFR (erbB1) from Drosophila to humans. Here we explore the specificity of the molecular interaction between L1CAM and the erbB receptor family. We show that L1CAM binds physically erbB receptors in both heterologous systems and the mammalian developing brain. Different Ig-like domains located in the extracellular part of L1CAM can support this interaction. Interestingly, binding of L1CAM to erbB enhances its response to neuregulins. During development this may synergize with the activation of erbB receptors through L1CAM homophilic interactions, conferring diffusible neuregulins specificity for cells or axons that interact with the substrate through L1CAM.

Bace1 processing of NRG1 type III produces a myelin-inducing signal but is not essential for the stimulation of myelination

Myelin sheath thickness is precisely adjusted to axon caliber, and in the peripheral nervous system, neuregulin 1 (NRG1) type III is a key regulator of this process. It has been proposed that the protease BACE1 activates NRG1 dependent myelination. Here, we characterize the predicted product of BACE1-mediated NRG1 type III processing in transgenic mice. Neuronal overexpression of a NRG1 type III-variant, designed to mimic prior cleavage in the juxtamembrane stalk region, induces hypermyelination in vivo and is sufficient to restore myelination of NRG1 type III-deficient neurons. This observation implies that the NRG1 cytoplasmic domain is dispensable and that processed NRG1 type III is sufficient for all steps of myelination. Surprisingly, transgenic neuronal overexpression of full-length NRG1 type III promotes hypermyelination also in BACE1 null mutant mice. Moreover, NRG1 processing is impaired but not abolished in BACE1 null mutants. Thus, BACE1 is not essential for the activation of NRG1 type III to promote myelination. Taken together, these findings suggest that multiple neuronal proteases collectively regulate NRG1 processing. © 2011 Wiley Periodicals, Inc.

Neuronal Neuregulin 1 type III directs Schwann cell migration

During peripheral nerve development, each segment of a myelinated axon is matched with a single Schwann cell. Tight regulation of Schwann cell movement, proliferation and differentiation is essential to ensure that these glial cells properly associate with axons. ErbB receptors are required for Schwann cell migration, but the operative ligand and its mechanism of action have remained unknown. We demonstrate that zebrafish Neuregulin 1 (Nrg1) type III, which signals through ErbB receptors, controls Schwann cell migration in addition to its previously known roles in proliferation and myelination. Chimera analyses indicate that ErbB receptors are required in all migrating Schwann cells, and that Nrg1 type III is required in neurons for migration. Surprisingly, expression of the ligand in a few axons is sufficient to induce migration along a chimeric nerve constituted largely of nrg1 type III mutant axons. These studies also reveal a mechanism that allows Schwann cells to fasciculate axons regardless of nrg1 type III expression. Time-lapse imaging of transgenic embryos demonstrated that misexpression of human NRG1 type III results in ectopic Schwann cell migration, allowing them to aberrantly enter the central nervous system. These results demonstrate that Nrg1 type III is an essential signal that controls Schwann cell migration to ensure that these glia are present in the correct numbers and positions in developing nerves.

Sustained axon-glial signaling induces Schwann cell hyperproliferation, Remak bundle myelination, and tumorigenesis

Type III neuregulins exposed on axon surfaces control myelination of the peripheral nervous system. It has been shown, for example, that threshold levels of type III beta1a neuregulin dictate not only the myelination fate of axons but also myelin thickness. Here we show that another neuregulin isoform, type III-beta3, plays a distinct role in myelination. Neuronal overexpression of this isoform in mice stimulates Schwann cell proliferation and dramatically enlarges peripheral nerves and ganglia-which come to resemble plexiform neurofibromas-but have no effect on myelin thickness. The nerves display other neurofibroma-like properties, such as abundant collagen fibrils and abundant dissociated Schwann cells that in some cases produce big tumors. Moreover, the organization of Remak bundles is dramatically altered; the small-caliber axons of each bundle are no longer segregated from one another within the cytoplasm of a nonmyelinating Schwann cell but instead are close packed and the whole bundle wrapped as a single unit, frequently by a compact myelin sheath. Because Schwann cell hyperproliferation and Remak bundle degeneration are early hallmarks of type I neurofibromatosis, we suggest that sustained activation of the neuregulin pathway in Remak bundles can contribute to neurofibroma development.

Transcriptional control of cholesterol biosynthesis in Schwann cells by axonal neuregulin 1

A characteristic feature of many vertebrate axons is their wrapping by a lamellar stack of glially derived membranes known as the myelin sheath. Myelin is a cholesterol-rich membrane that allows for rapid saltatory nerve impulse conduction. Axonal neuregulins instruct glial cells on when and how much myelin they should produce. However, how neuregulin regulates myelin sheath development and thickness is unknown. Here we show that neuregulin receptors are activated by drops in plasma membrane cholesterol, suggesting that they can sense sterol levels. In Schwann cells neuregulin-1 increases the transcription of the 3-hydroxy-3-methylglutarylcoenzyme A reductase, the rate-limiting enzyme for cholesterol biosynthesis. Neuregulin activity is mediated by the phosphatidylinositol 3-kinase pathway and a cAMP-response element located on the reductase promoter. We propose that by activating neuregulin receptors, neurons exploit a cholesterol homeostatic mechanism forcing Schwann cells to produce new membranes for the myelin sheath. We also show that a strong phylogenetic correlation exists between myelination and cholesterol biosynthesis, and we propose that the absence of the sterol branch of the mevalonate pathway in invertebrates precluded the myelination of their nervous system.

Transmembrane Mutations to FcγRIIA Alter Its Association with Lipid Rafts: Implications for Receptor Signaling

Many immunoreceptors have been reported to associate with lipid rafts upon ligand binding. The way in which this association is regulated is still obscure. We investigated the roles for various domains of the human immunoreceptor FcgammaRIIA in regulating its association with lipid rafts by determining the resistance of unligated, or ligated and cross-linked, receptors to solubilization by the nonionic detergent Triton X-100, when expressed in RBL-2H3 cells. Deletion of the cytoplasmic domain, or destruction of the cytoplasmic palmitoylation site, had no effect on the association of the receptor with lipid rafts. A transmembrane mutant, A224S, lost the ability to associate with lipid rafts upon receptor cross-linking, whereas transmembrane mutants VA231-2MM and VVAL234-7GISF showed constitutive lipid raft association. Wild-type (WT) FcgammaRIIA and all transmembrane mutants activated Syk, regardless of their association with lipid rafts. WT FcgammaRIIA and mutants that associated with lipid rafts efficiently activated NF-kappaB, in an ERK-dependent manner. In contrast, WT FcgammaRIIA and the A224S mutant both presented efficient phagocytosis, while VA231-2MM and VVAL234-7GISF mutants presented lower phagocytosis, suggesting that phagocytosis may proceed independently of lipid raft association. These data identify the transmembrane domain of FcgammaRIIA as responsible for regulating its inducible association with lipid rafts and suggest that FcgammaRIIA-mediated responses, like NF-kappaB activation or phagocytosis, can be modulated by lipid raft association of the ligated receptor.

Neuregulins: Versatile growth and differentiation factors in nervous system development and human disease

The neuregulins are a family of growth and differentiation factors with a wide range of functions in the nervous system. The power and diversity of the neuregulin signaling system comes in part from a large number of alternatively-spliced forms of the NRG1 gene that can produce both soluble and membrane-bound forms. The soluble forms of neuregulin are unique from other factors in that they have a structurally distinct heparin-binding domain that targets and potentiates its actions. In addition, a finely tuned, bidirectional mechanism regulates when and where neuregulin is released from neurons in response to neurotrophic factors produced by both neuronal targets and supporting glial cells. Together, this produces a balanced intercellular signaling system that can be localized to distinct regions for both normal development and maintenance of the mature nervous system. Recent evidence suggests that neuregulin signaling plays important roles in many neurological disorders including multiple sclerosis, traumatic brain and spinal cord injury, peripheral neuropathy, and schizophrenia. Here, we review the basic biology of neuregulins and relate this to research suggesting their involvement with and potential therapeutic uses for neurological disorders.

Characterization of a neural-specific splicing form of the human neuregulin 3 gene involved in oligodendrocyte survival

Neuregulins are a family of genes involved in key aspects of neural biology. Neuregulins 1, 2 and 3 (NRG1, NRG2 and NRG3) are expressed in the mammalian nervous system. It is well established that NRG1, with fifteen different splicing forms, is central for brain development and function. However, the biological relevance of NRG2 and NRG3 remains elusive. Here, we report the identification of a new isoform of NRG3 that is specifically expressed in the human embryonic central nervous system. Sequence alignment with the human genome suggests that this transcript is produced by alternative promoter usage. The encoded polypeptide is a type-I-glycosylated plasma membrane protein, which is shed into the extracellular space where it activates erbB4, a pivotal receptor for brain development. In addition, we show that the protein has a signal sequence that is cleaved after membrane insertion. Proteasome inhibition with Lactacystin enhances the expression of the protein, whereas impairment of ubiquitylation in the conditional mutant cell line ts20 protects the protein from degradation. These observations imply that the ubiquitin/proteasome pathway regulates biogenesis of the protein. We also show that recombinant neuregulin 3 acts as an oligodendrocyte survival factor by activating the phosphoinositide 3-kinase signalling pathway. Therefore, we report a new post-translationally regulated isoform of neuregulin 3 expressed in the developing human central nervous system with a role in oligodendrocyte survival.

Neuregulin-1 type III determines the ensheathment fate of axons

The signals that determine whether axons are ensheathed or myelinated by Schwann cells have long been elusive. We now report that threshold levels of neuregulin-1 (NRG1) type III on axons determine their ensheathment fate. Ensheathed axons express low levels whereas myelinated fibers express high levels of NRG1 type III. Sensory neurons from NRG1 type III deficient mice are poorly ensheathed and fail to myelinate; lentiviral-mediated expression of NRG1 type III rescues these defects. Expression also converts the normally unmyelinated axons of sympathetic neurons to myelination. Nerve fibers of mice haploinsufficient for NRG1 type III are disproportionately unmyelinated, aberrantly ensheathed, and hypomyelinated, with reduced conduction velocities. Type III is the sole NRG1 isoform retained at the axon surface and activates PI 3-kinase, which is required for Schwann cell myelination. These results indicate that levels of NRG1 type III, independent of axon diameter, provide a key instructive signal that determines the ensheathment fate of axons.

Roles for neuregulins in human cancer

The human epidermal growth factor (EGF) receptor (HER) family of receptor tyrosine kinases has frequently been implicated in cancer. Apart from overexpression or mutation of these receptors, also the aberrant autocrine or paracrine activation of HERs by EGF-like ligands may be important in cancer progression. Neuregulins constitute a family of EGF-like ligands that bind to HER3 or HER4, preferably forming heterodimers with the orphan receptor HER2. Mesenchymal neuregulin typically serves as a pro-survival and pro-differentiation signal for adjacent epithelia. Disruption of the balance between proliferation and differentiation, because of autocrine production by the epithelial cells, increased sensitivity to paracrine signals or disruption of the spatial organization, may lead to constitutive receptor activation, in the absence of receptor overexpression. Consequently, the analysis of ligand expression and/or activated receptors in tumor samples may broaden the group of patients that can benefit from targeted therapies.

Identification of a tetramerization domain in the C terminus of the vanilloid receptor

TRPV1 (transient receptor potential vanilloid receptor subtype 1) is a member of the TRP channel family gated by vanilloids, protons, and heat. Structurally, TRPV1 appears to be a tetramer formed by the assembly of four identical subunits around a central aqueous pore. The molecular determinants that govern its subunit oligomerization remain elusive. Here, we report the identification of a segment comprising 684Glu-721Arg (referred to as the TRP-like domain) in the C terminus of TRPV1 as an association domain (AD) of the protein. Purified recombinant C terminus of TRPV1 (TRPV1-C) formed discrete and stable multimers in vitro. Yeast two-hybrid and pull-down assays showed that self-association of the TRPV1-C is blocked when segment 684Glu-721Arg is deleted. Biochemical and immunological analysis indicate that removal of the AD from full-length TRPV1 monomers blocks the formation of stable heteromeric assemblies with wild-type TRPV1 subunits. Deletion of the AD in a poreless TRPV1 subunit suppressed its robust dominant-negative phenotype. Together, these findings are consistent with the tenet that the TRP-like domain in TRPV1 is a molecular determinant of the tetramerization of receptor subunits into functional channels. Our observations suggest that the homologous TRP domain in the TRP protein family may function as a general, evolutionary conserved AD involved in subunit multimerization.

Oligomerization of the Sensory and Motor Neuron-derived Factor Prevents Protein O-Glycosylation

The sensory and motor neuron-derived factor (SMDF) is a neuregulin that promotes Schwann cell proliferation and differentiation. Hence, understanding axon myelination is important to unveil the mechanisms involved in SMDF biogenesis, membrane delivery, and compartmentalization. SMDF is a type II membrane protein expressed as two distinct polypeptides of approximately 40 and 83 kDa. Whether the 83-kDa polypeptide results from posttranslational modifications of the protein monomers or protein dimerization remains unknown. Here we have addressed this question and shown that the 83-kDa polypeptide is an O-glycosylated form of the protein. Deletion of the N-terminal domain fully abrogates the SMDF O-glycosylation, indicating that incorporation of O-glycans occurs in the intracellular domain of the protein. Notably, O-glycosylated forms are excluded from partitioning into lipid raft microdomains. In addition, we found that heterologously expressed SMDF monomers interact in intact living cells as evidenced from fluorescence resonance energy transfer of cyan fluorescent protein/yellow fluorescent protein.SMDF fusion proteins. A stepwise deletion approach demonstrated that SMDF self-association is primarily determined by its transmembrane segment. Notably, biochemical analysis revealed that SMDF multimers are exclusively composed of the 40-kDa polypeptide. Collectively, these findings indicate that the 40-kDa form corresponds to unmodified SMDF, which may be present as multimers, whereas the 83-kDa polypeptide is a monomeric O-glycosylated form of the protein. Furthermore, our observations imply a role for oligomerization as a potential modulator of the distribution in membrane domains and O-glycosylation of the protein.

Regulated exocytosis contributes to protein kinase C potentiation of vanilloid receptor activity

The vanilloid receptor-1 (TRPV1) plays a key role in the perception of peripheral thermal and inflammatory pain. TRPV1 expression and channel activity are notably up-regulated by proalgesic agents. The transduction pathways involved in TRPV1 sensitization are still elusive. We have used a yeast two-hybrid screen to identify proteins that associate with the N terminus of TRPV1. We report that two vesicular proteins, Snapin and synaptotagmin IX (Syt IX), strongly interact in vitro and in vivo with the TRPV1 N-terminal domain. In primary dorsal root ganglion neurons, TRPV1 co-distributes in vesicles with Syt IX and the vesicular protein synaptobrevin. Neither Snapin nor Syt IX affected channel function, but they notably inhibited protein kinase C (PKC)-induced potentiation of TRPV1 channel activity with a potency that rivaled the blockade evoked by botulinum neurotoxin A, a potent blocker of neuronal exocytosis. Noteworthily, we found that PKC activation induced a rapid delivery of functional TRPV1 channels to the plasma membrane. Botulinum neurotoxin A blocked the TRPV1 membrane translocation induced by PKC that was activated with a phorbol ester or the metabotropic glutamate receptor mGluR5. Therefore, our results indicate that PKC signaling promotes at least in part the SNARE-dependent exocytosis of TRPV1 to the cell surface. Taken together, these findings imply that activity-dependent delivery of channels to the neuronal surface may contribute to the buildup and maintenance of thermal inflammatory hyperalgesia in peripheral nociceptor terminals.

Small peptides patterned after the N-terminus domain of SNAP25 inhibit SNARE complex assembly and regulated exocytosis

Synthetic peptides patterned after the C-terminus of synaptosomal associated protein of 25 kDa (SNAP25) efficiently abrogate regulated exocytosis. In contrast, the use of SNAP25 N-terminal-derived peptides to modulate SNAP receptors (SNARE) complex assembly and neurosecretion has not been explored. Here, we show that the N-terminus of SNAP25, specially the segment that encompasses 22Ala-44Ile, is essential for the formation of the SNARE complex. Peptides patterned after this protein domain are potent inhibitors of SNARE complex formation. The inhibitory activity correlated with their propensity to adopt an alpha-helical secondary structure. These peptides abrogated SNARE complex formation only when added previous to the onset of aggregate assembly. Analysis of the mechanism of action revealed that these peptides disrupted the binary complex formed by SNAP25 and syntaxin. The identified peptides inhibited Ca2+-dependent exocytosis from detergent-permeabilized excitable cells. Noteworthy, these amino acid sequences markedly protected intact hippocampal neurones against hypoglycaemia-induced, glutamate-mediated excitotoxicity with a potency that rivalled that displayed by botulinum neurotoxins. Our findings indicate that peptides patterned after the N-terminus of SNAP25 are potent inhibitors of SNARE complex formation and neuronal exocytosis. Because of their activity in intact neurones, these cell permeable peptides may be hits for antispasmodic and analgesic drug development.

Neuregulin-2 is developmentally regulated and targeted to dendrites of central neurons

Neuregulin-1 (NRG-1) regulates numerous aspects of neural development and synaptic plasticity; the functions of NRG-2 and NRG-3 are presently unknown. As a first step toward understanding how NRGs contribute to distinct aspects of neural development and function, we characterized their regional and subcellular expression patterns in developing brain. The expression of NRG-1-3 mRNAs was compared postnatally (P0, P7, adult) by using in situ hybridization. NRG-1 expression is highest at birth, whereas NRG-2 mRNA levels increase with development; expression of both genes is restricted to distinct brain regions. In contrast, NRG-3 transcripts are abundant in most brain regions throughout development. NRG-2 antibodies were generated to analyze protein processing, expression, and subcellular distribution. As with NRG-1, the transmembrane NRG-2 proprotein is proteolytically processed in transfected HEK 293 cells and in neural tissues, and its ectodomain is exposed and accumulates on the neuron surface. Despite the structural similarities between NRG-1 and NRG-2, we unexpectedly found that NRG-2 colocalizes with MAP2 in proximal primary dendrites of hippocampal neurons in culture and in vivo, although it is not detectable in axons or in axon terminals. These findings were confirmed with NRG-2 ectodomain antisera and epitope-tagged recombinant protein. In cerebellum, NRG-2 colocalizes with calbindin in proximal dendrites and soma of Purkinje cells. In contrast, NRG-1 is highly expressed in axons of dissociated hippocampal neurons, as well as in somas and dendrites. The distinct temporal, regional, and subcellular expression of NRG-2 suggests its unique and nonredundant role in neural function.

Back signaling by the Nrg-1 intracellular domain

Transmembrane isoforms of neuregulin-1 (Nrg-1), ligands for erbB receptors, include an extracellular domain with an EGF-like sequence and a highly conserved intracellular domain (ICD) of unknown function. In this paper, we demonstrate that transmembrane isoforms of Nrg-1 are bidirectional signaling molecules in neurons. The stimuli for Nrg-1 back signaling include binding of erbB receptor dimers to the extracellular domain of Nrg-1 and neuronal depolarization. These stimuli elicit proteolytic release and translocation of the ICD of Nrg-1 to the nucleus. Once in the nucleus, the Nrg-1 ICD represses expression of several regulators of apoptosis, resulting in decreased neuronal cell death in vitro. Thus, regulated proteolytic processing of Nrg-1 results in retrograde signaling that appears to mediate contact and activity-dependent survival of Nrg-1-expressing neurons.

GAP43 stimulates inositol trisphosphate-mediated calcium release in response to hypotonicity

The identification of osmo/mechanosensory proteins in mammalian sensory neurons is still elusive. We have used an expression cloning approach to screen a human dorsal root ganglion cDNA library to look for proteins that respond to hypotonicity by raising the intracellular Ca(2+) concentration ([Ca(2+)](i)). We report the unexpected identification of GAP43 (also known as neuromodulin or B50), a membrane-anchored neuronal protein implicated in axonal growth and synaptic plasticity, as an osmosensory protein that augments [Ca(2+)](i) in response to hypotonicity. Palmitoylation of GAP43 plays an important role in the protein osmosensitivity. Depletion of intracellular stores or inhibition of phospholipase C (PLC) activity abrogates hypotonicity-evoked, GAP43-mediated [Ca(2+)](i) elevations. Notably, hypotonicity promoted the selective association of GAP43 with the PLC-delta(1) isoform, and a concomitant increase in inositol-1,4,5-trisphosphate (IP(3)) formation. Collectively, these findings indicate that hypo-osmotic activation of GAP43 induces Ca(2+) release from IP(3)-sensitive intracellular stores. The osmosensitivity of GAP43 furnishes a mechanistic framework that links axon elongation with phospho inositide metabolism, spontaneous triggering of cytosolic Ca(2+) transients and the regulation of actin dynamics and motility at the growth cone in response to temporal and local mechanical forces.

Lipid rafts and the control of neurotrophic factor signaling in the nervous system: variations on a theme

Lipid rafts are specialized, liquid-ordered subdomains of the plasma membrane. Through their ability to promote specific compartmentalization of lipids and membrane proteins, lipid rafts have emerged as membrane platforms specialized for signal transduction. In recent years, signaling by neurotrophic factors and their receptors has been shown to depend upon the integrity and function of lipid rafts and associated components. It has also been shown that these microdomains play critical roles in selective axon-dendritic sorting and the proteolytic processing of several neurotrophic ligands and receptors in neuronal cells. The available evidence supports an important role for lipid rafts in the initiation, propagation and maintenance of signal transduction events triggered by different neurotrophic factors and their receptors in the nervous system.