Anti-beta 1 integrin antibody inhibits Schwann cell myelination

Investigation of the effects of laminin present in the basal lamina of the peripheral nervous system on axon regeneration and remyelination using the nerve acellular scaffold

This study aimed to investigate the effects of laminin (LN) located in the basal lamina, which are important components of the peripheral nervous system-extracellular matrix, on axon regeneration and remyelination. Nerve acellular scaffolds (NASs) (S-untreated) were prepared using the acellular technique. The active component LN in the NASs was blocked (S-LN^- ) or upregulated (S-LN⁺ ); S-LN+ contained seven times more LN than did the S-untreated group. The adhesion capacity of Schwann cells (SCs) to the three types of NAS (S-untreated, S-LN^- , and S-LN⁺ ) was assessed in vitro. Our results showed that the adhesion of SCs to the NASs was significantly reduced in the S-LN^- group, whereas no difference was observed between the S-LN⁺ and S-untreated groups. The pretreated NASs were used to repair nerves in a nerve injury mouse model with the animals divided into four groups (S-LN^- group, S-untreated group, S-LN⁺ group, and autograft group). Two weeks after surgery, although there was no difference in the S-LN^- group, S-untreated group and S-LN⁺ group, the newly formed basal lamina in the S-LN^- group were significantly lower than those in the other two groups. Four weeks after surgery, the S-LN⁺ group had higher numbers of newly generated axons and their calibers, more myelinated fibers, thicker myelin sheaths, increased myelin basic protein expression, and improved recovery of neural function compared to those of the S-LN^- and S-untreated groups, but all of these parameters were significantly worse than those of the autograft group. Downregulation of the LN level in the NAS leads to a reduction in all of the above parameters.

■ REVIEW : Gene Expression in Nerve Regeneration

Injury of peripheral nerve in mammals leads to a complex but stereotypical pattern of histological events that comprise a highly reproducible sequence of degenerative reactions (Wallerian degeneration) succeeded by regenerative responses. These reactions are based on a corresponding sequence of cellular and mo lecular interactions that, in turn, reflect the differential expression of specific genes with functions in nerve degeneration and repair. We report on more than 60 genes and their products that show a specific pattern of regulation following peripheral nerve lesion. The group of regulated genes encoding, e.g., transcription factors, growth factors and their receptors, cytokines, neuropeptides, myelin proteins and lipid carriers, and cytoskeletal proteins as well as extracellular matrix and cell adhesion molecules. We describe and compare the distinct time-courses and cellular origin of expression and further discuss established or putative mo lecular interrelationships and functions with respect to the contribution of these genes/gene products to the molecular regeneration program of the PNS. NEUROSCIENTIST 3:112-122, 1997

Myelination and node of Ranvier formation on sensory neurons in a defined in vitro system

One of the most important developmental modifications of the nervous system is Schwann cell myelination of axons. Schwann cells ensheath axons to create myelin segments to provide protection to the axon as well as increase the conduction of action potentials. In vitro neuronal systems provide a unique modality to study a variety of factors influencing myelination as well as diseases associated with myelin sheath degradation. This work details the development of a patterned in vitro myelinating dorsal root ganglion culture. This defined system utilized a serum-free medium in combination with a patterned substrate, utilizing the cytophobic and cytophilic molecules (poly)ethylene glycol (PEG) and N-1[3 (trimethoxysilyl) propyl] diethylenetriamine (DETA), respectively. Directional outgrowth of the neurites and subsequent myelination was controlled by surface modifications, and conformity to the pattern was measured over the duration of the experiments. The myelinated segments and nodal proteins were visualized and quantified using confocal microscopy. This tissue-engineered system provides a highly controlled, reproducible model for studying Schwann cell interactions with sensory neurons, as well as the myelination process, and its effect on neuronal plasticity and peripheral nerve regeneration. It is also compatible for use in bio-hybrid constructs to reproduce the stretch reflex arc on a chip because the media combination used is the same that we have used previously for motoneurons, muscle, and for neuromuscular junction (NMJ) formation. This work could have application for the study of demyelinating diseases such as diabetes induced peripheral neuropathy and could rapidly translate to a role in the discovery of drugs promoting enhanced peripheral nervous system (PNS) remyelination.

Identification of novel cell-adhesion molecules in peripheral nerves using a signal-sequence trap

The development and maintenance of myelinated nerves in the PNS requires constant and reciprocal communication between Schwann cells and their associated axons. However, little is known about the nature of the cell-surface molecules that mediate axon-glial interactions at the onset of myelination and during maintenance of the myelin sheath in the adult. Based on the rationale that such molecules contain a signal sequence in order to be presented on the cell surface, we have employed a eukaryotic-based, signal-sequence-trap approach to identify novel secreted and membrane-bound molecules that are expressed in myelinating and non-myelinating Schwann cells. Using cDNA libraries derived from dbcAMP-stimulated primary Schwann cells and 3-day-old rat sciatic nerve mRNAs, we generated an extensive list of novel molecules expressed in myelinating nerves in the PNS. Many of the identified proteins are cell-adhesion molecules (CAMs) and extracellular matrix (ECM) components, most of which have not been described previously in Schwann cells. In addition, we have identified several signaling receptors, growth and differentiation factors, ecto-enzymes and proteins that are associated with the endoplasmic reticulum and the Golgi network. We further examined the expression of several of the novel molecules in Schwann cells in culture and in rat sciatic nerve by primer-specific, real-time PCR and in situ hybridization. Our results indicate that myelinating Schwann cells express a battery of novel CAMs that might mediate their interactions with the underlying axons.

Extracellular matrix molecules enhance the neurotrophic effect of Schwann cell-like differentiated adipose-derived stem cells and increase cell survival under stress conditions

Since the first reports of induction of adipose-derived stem cells (ASC) into neuronal and glial cell phenotypes, expectations have increased regarding their use in tissue engineering applications for nerve repair. Cell adhesion to extracellular matrix (ECM) is a basic feature of survival, differentiation, and migration of Schwann cells (SC) during nerve regeneration, and fibronectin and laminin are two key molecules of this process. Interaction between ECM and SC-like differentiated ASC (dASC) could potentially improve the neurotrophic potential of the stem cells. We have investigated the effect of ECM molecules on SC-like dASC in terms of proliferation, adhesion, and cell viability. Fibronectin and laminin did not affect the proliferation of dASC when compared with cell adherent tissue culture plastic, but significantly improved viability and cell attachment when dASC were exposed to apoptotic conditions. To assess the influence of the ECM molecules on dASC neurotrophic activity, dASC were seeded onto ECM-coated culture inserts suspended above dorsal root ganglia (DRG) sensory neurons. Neurite outgrowth of DRG neurons was enhanced when dASC were seeded on fibronectin and laminin when compared with controls. When DRG neurons and dASC were in direct contact on the various surfaces there was significantly enhanced neurite outgrowth and coculture with laminin-conditioned dASC produced the longest neurites. Compared with primary SCs, dASC grown on laminin produced similar levels of neurite outgrowth in the culture insert experiments but neurite length was shorter in the direct contact groups. Anti β1 integrin blocking antibody could inhibit baseline and dASC evoked neurite elongation but had no effect on outgrowth mediated by laminin-conditioned dASC. ECM molecules had no effect on the levels of nerve growth factor and brain-derived neurotrophic factor secretion from dASC. The results of the study suggest that ECM molecules can significantly improve the potential of dASC for nerve regeneration.

The actin-severing protein cofilin is downstream of neuregulin signaling and is essential for Schwann cell myelination

Myelination is a complex process requiring coordination of directional motility and an increase in glial cell size to generate a multilamellar myelin sheath. Regulation of actin dynamics during myelination is poorly understood. However, it is known that myelin thickness is related to the abundance of neuregulin-1 (NRG1) expressed on the axon surface. Here we identify cofilin1, an actin depolymerizing and severing protein, as a downstream target of NRG1 signaling in rat Schwann cells (SCs). In isolated SCs, NRG1 promotes dephosphorylation of cofilin1 and its upstream regulators, LIM kinase (LIMK) and Slingshot-1 phosphatase (SSH1), leading to cofilin1 activation and recruitment to the leading edge of the plasma membrane. These changes are associated with rapid membrane expansion yielding a 35-50% increase in SC size within 30 min. Cofilin1-deficient SCs increase phosphorylation of ErbB2, ERK, focal adhesion kinase, and paxillin in response to NRG1, but fail to increase in size possibly due to stabilization of unusually long focal adhesions. Cofilin1-deficient SCs cocultured with sensory neurons do not myelinate. Ultrastructural analysis reveals that they unsuccessfully segregate or engage axons and form only patchy basal lamina. After 48 h of coculturing with neurons, cofilin1-deficient SCs do not align or elongate on axons and often form adhesions with the underlying substrate. This study identifies cofilin1 and its upstream regulators, LIMK and SSH1, as end targets of a NRG1 signaling pathway and demonstrates that cofilin1 is necessary for dynamic changes in the cytoskeleton needed for axon engagement and myelination by SCs.

Realizing the maximum potential of Schwann cells to promote recovery from spinal cord injury

Transplantation of Schwann cells (SCs) has been extensively investigated as a therapeutic intervention in rodent models of spinal cord injury (SCI). Here we review both strengths and weaknesses of this approach and discuss additional strategies for maximizing the potential of SCs to repair the injured spinal cord. With no additional treatments, SCs were consistently shown to provide a bridge across the lesion site, supporting the ingrowth of sensory and propriospinal axons, to myelinate axons and to decrease the size of cavities formed after injury. Supraspinal axons did not, however, grow onto the bridge, axons failed to traverse the caudal SC-host cord interface and transplanted SC survival was poor. More recent studies have shown that the potential of SC transplantation as a therapeutic approach can be strongly enhanced by combining additional strategies . For example, combining SC transplantation with elevation of cAMP levels resulted in growth of brainstem axons into the SC graft and caudal to the lesion and in significant improvements in locomotion. Axon growth (and functional improvement) have been increased by strategies to raise neurotrophin levels, either by injection or by genetic modification of the SCs before transplantation. A major problem in maximizing SC potential in injured cord has been in achieving good integration of the transplanted cells with the adjacent cord parenchyma. Several previous studies suggested an ability of SCs to migrate extensively in CNS tissue when astroctyes were absent and to myelinate CNS axons. Furthermore, in some cases involving very limited injury, SCs migrated and integrated well even in the presence of host astrocytes. Consistent with these observations, treatments with an enzyme, chondroitinase, to modify the SC-astrocyte interface surrounding the graft, have shown much promise. Very new studies have shown that SCs derived from SC precursors show a higher ability to survive, integrate well with host tissue and support brainstem axon growth into and beyond the graft, confirming the innate promise of SCs in spinal cord repair. We review one clinical trial already underway in Iran testing SC transplantation in patients with SCI. Finally, we briefly describe a protocol, adaptable to the principles of good manufacturing practice, for generating large numbers of human SCs. Overall, the available evidence suggests that SCs, especially when used in combination with other treatments, offer one of the best hopes we have today of devising an effective treatment for spinal cord repair.

Integrins as receptor targets for neurological disorders

This review focuses on the neurobiology of integrins, pathophysiological roles of integrins in neuroplasticity and nervous system disorders, and therapeutic implications of integrins as potential drug targets and possible delivery pathways. Neuroplasticity is a central phenomenon in many neurological conditions such as seizures, trauma, and traumatic brain injury. During the course of many brain diseases, in addition to intracellular compartment changes, alterations in non-cell compartments such as extracellular matrix (ECM) are recognized as an essential process in forming and reorganizing neural connections. Integrins are heterodimeric transmembrane receptors that mediate cell-ECM and cell-cell adhesion events. Although the mechanisms of neuroplasticity remain unclear, it has been suggested that integrins undergo plasticity including clustering through interactions with ECM proteins, modulating ion channels, intracellular Ca(2+) and protein kinase signaling, and reorganization of cytoskeletal filaments. As cell surface receptors, integrins are central to the pathophysiology of many brain diseases, such as epilepsy, and are potential targets for the development of new drugs for neurological disorders.

Integrins are necessary for the development and maintenance of the glial layers in the Drosophila peripheral nerve

Peripheral nerve development involves multiple classes of glia that cooperate to form overlapping glial layers paired with the deposition of a surrounding extracellular matrix (ECM). The formation of this tubular structure protects the ensheathed axons from physical and pathogenic damage and from changes in the ionic environment. Integrins, a major family of ECM receptors, play a number of roles in the development of myelinating Schwann cells, one class of glia ensheathing the peripheral nerves of vertebrates. However, the identity and the role of the integrin complexes utilized by the other classes of peripheral nerve glia have not been determined in any animal. Here, we show that, in the peripheral nerves of Drosophila melanogaster, two integrin complexes (αPS2βPS and αPS3βPS) are expressed in the different glial layers and form adhesion complexes with integrin-linked kinase and Talin. Knockdown of the common beta subunit (βPS) using inducible RNAi in all glial cells results in lethality and glial defects. Analysis of integrin complex function in specific glial layers showed that loss of βPS in the outermost layer (the perineurial glia) results in a failure to wrap the nerve, a phenotype similar to that of Matrix metalloproteinase 2-mediated degradation of the ECM. Knockdown of βPS integrin in the innermost wrapping glia causes a loss of glial processes around axons. Together, our data suggest that integrins are employed in different glial layers to mediate the development and maintenance of the protective glial sheath in Drosophila peripheral nerves.

The molecular biology and novel treatments of vestibular schwannomas

Vestibular schwannomas are histopathologically benign tumors arising from the Schwann cell sheath surrounding the vestibular branch of cranial nerve VIII and are related to the NF2 gene and its product merlin. Merlin acts as a tumor suppressor and as a mediator of contact inhibition. Thus, deficiencies in both NF2 genes lead to vestibular schwannoma development. Recently, there have been major advances in our knowledge of the molecular biology of vestibular schwannomas as well as the development of novel therapies for its treatment. In this article the authors comprehensively review the recent advances in the molecular biology and characterization of vestibular schwannomas as well as the development of modern treatments for vestibular schwannoma. For instance, merlin is involved with a number of receptors including the CD44 receptor, EGFR, and signaling pathways, such as the Ras/raf pathway and the canonical Wnt pathway. Recently, merlin was also shown to interact in the nucleus with E3 ubiquitin ligase CRL4(DCAF1). A greater understanding of the molecular mechanisms behind vestibular schwannoma tumorigenesis has begun to yield novel therapies. Some authors have shown that Avastin induces regression of progressive schwannomas by over 40% and improves hearing. An inhibitor of VEGF synthesis, PTC299, is currently in Phase II trials as a potential agent to treat vestibular schwannoma. Furthermore, in vitro studies have shown that trastuzumab (an ERBB2 inhibitor) reduces vestibular schwannoma cell proliferation. With further research it may be possible to significantly reduce morbidity and mortality rates by decreasing tumor burden, tumor volume, hearing loss, and cranial nerve deficits seen in vestibular schwannomas.

Biological role of dystroglycan in Schwann cell function and its implications in peripheral nervous system diseases

Dystroglycan is a central component of the dystrophin-glycoprotein complex (DGC) that links extracellular matrix with cytoskeleton, expressed in a variety of fetal and adult tissues. Dystroglycan plays diverse roles in development and homeostasis including basement membrane formation, epithelial morphogenesis, membrane stability, cell polarization, and cell migration. In this paper, we will focus on biological role of dystroglycan in Schwann cell function, especially myelination. First, we review the molecular architecture of DGC in Schwann cell abaxonal membrane. Then, we will review the loss-of-function studies using targeted mutagenesis, which have revealed biological functions of each component of DGC in Schwann cells. Based on these findings, roles of dystroglycan in Schwann cell function, in myelination in particular, and its implications in diseases will be discussed in detail. Finally, in view of the fact that understanding the role of dystroglycan in Schwann cells is just beginning, future perspectives will be discussed.

Disruption of laminin in the peripheral nervous system impedes nonmyelinating Schwann cell development and impairs nociceptive sensory function

The mechanisms controlling the differentiation of immature Schwann cells (SCs) into nonmyelinating SCs is not known. Laminins are extracellular matrix proteins critical for myelinating SC differentiation, but their roles in nonmyelinating SC development have not been established. Here, we show that the peripheral nerves of mutant mice with laminin-deficient SCs do not form Remak bundles, which consist of a single nonmyelinating SC interacting with multiple unmyelinated axons. These mutant nerves show aberrant L1 and neural cell adhesion molecule (N-CAM) expression pattern during development. The homophilic and heterophilic interactions of N-CAM are also impaired in the mutant nerves. Other molecular markers for nonmyelinating SCs, including Egr-1, glial fibrillary acidic protein, and AN2/NG2, are all absent in adult mutant nerves. Analysis of expression of SC lineage markers demonstrates that nonmyelinating SCs do not develop in mutant nerves. Additionally, mutant mice are insensitive to heat stimuli and show a decreased number of C-fiber sensory neurons, indicating reduced nociceptive sensory function. These results show that laminin participates in nonmyelinating SC development and Remak bundle formation and suggest a possible role for laminin deficiency in peripheral sensory neuropathies.

Regulation of Schwann cell function by the extracellular matrix

Laminins and collagens are extracellular matrix proteins that play essential roles in peripheral nervous system development. Laminin signals regulate Schwann cell proliferation and survival as well as actin cytoskeleton dynamics, which are essential steps for radial sorting and myelination of peripheral axons by Schwann cells. Collagen and their receptors promote Schwann cell adhesion, spreading, and myelination as well as neurite outgrowth. In this article, we will review the recent advances in the studies of laminin and collagen function in Schwann cell development.

Neuregulin and laminin stimulate phosphorylation of the NF2 tumor suppressor in Schwann cells by distinct protein kinase A and p21-activated kinase-dependent pathways

Mutations in the neurofibromatosis type 2 (NF2) gene cause formation of schwannomas and other tumors in the nervous system. The NF2 protein, Schwannomin/Merlin, is a cytoskeleton-associated tumor suppressor regulated by phosphorylation at serine 518 (S518). Unphosphorylated Schwannomin restricts cell proliferation in part by inhibiting Rac- and p21-activated kinase (Pak). In a negative-feedback loop, Pak phosphorylates Schwannomin inactivating its ability to inhibit Pak. Little is known about receptor mechanisms that promote Pak activity and Schwannomin phosphorylation. Here we demonstrate in primary Schwann cells (SCs) that Schwannomin is rapidly phosphorylated on S518 by Pak following laminin-1 binding to beta1 integrin, and by protein kinase A following neuregulin-1beta (NRG1beta) binding to ErbB2/ErbB3 receptors. These receptors, together with phosphorylated Schwannomin, P-Pak, Cdc42 and paxillin are enriched at the distal tips of SC processes, and can be isolated as a complex using beta1 integrin antibody. Dual stimulation with laminin-1 and NRG1beta does not synergistically increase Schwannomin phosphorylation because ErbB2 kinase partially antagonizes integrin-dependent activation of Pak. These results identify two parallel, but interactive pathways that inactivate the tumor suppressor activity of Schwannomin to allow proliferation of subconfluent SCs. Moreover, they identify ErbB2, ErbB3 and beta1 integrins as potential therapeutic targets for NF2.

Beta1 integrin activates Rac1 in Schwann cells to generate radial lamellae during axonal sorting and myelination

Myelin is a multispiraled extension of glial membrane that surrounds axons. How glia extend a surface many-fold larger than their body is poorly understood. Schwann cells are peripheral glia and insert radial cytoplasmic extensions into bundles of axons to sort, ensheath, and myelinate them. Laminins and β1 integrins are required for axonal sorting, but the downstream signals are largely unknown. We show that Schwann cells devoid of β1 integrin migrate to and elongate on axons but cannot extend radial lamellae of cytoplasm, similar to cells with low Rac1 activation. Accordingly, active Rac1 is decreased in β1 integrin–null nerves, inhibiting Rac1 activity decreases radial lamellae in Schwann cells, and ablating Rac1 in Schwann cells of transgenic mice delays axonal sorting and impairs myelination. Finally, expressing active Rac1 in β1 integrin–null nerves improves sorting. Thus, increased activation of Rac1 by β1 integrins allows Schwann cells to switch from migration/elongation to the extension of radial membranes required for axonal sorting and myelination.

Temporal and spatial regulation of alpha6 integrin expression during the development of the cochlear-vestibular ganglion

The neurons of the cochlear-vestibular ganglion (CVG) that innervate the sensory hair cells of the inner ear are derived from the otic epithelium early in development. Neuroblasts detach from neighboring cells, migrate into the mesenchyme where they coalesce to form the ganglion complex, then send processes back into the epithelium. Cell migration and neuronal process formation involve changes in cellular interactions with other cells and proteins in the extracellular matrix that are orchestrated by cell surface-expressed adhesion molecules, including the integrins. I studied the expression pattern of the alpha6 integrin subunit during the early development of the CVG using immunohistochemistry and reverse-transcriptase polymerase chain reaction (RT-PCR) in murine tissue sections, otocyst, and ganglion explants. At embryonic day (E)10.5 alpha6 integrin was expressed in the otic epithelium but not in migrating neuroblasts. Importantly, the loss of alpha6 was associated with exit from the epithelium, not neuronal determination, revealing differentiation cues acutely associated with the cellular environment. Markers of glial and neuronal phenotype showed that alpha6-expressing cells present in the CVG at this stage were glia of neural crest origin. By E12.5 alpha6 expression in the ganglion increased alongside the elaboration of neuronal processes. Immunohistochemistry applied to otocyst cultures in the absence of glia revealed that neuronal processes remained alpha6-negative at this developmental stage and confirmed that alpha6 was expressed by closely apposed glia. The spatiotemporal modulation of alpha6 expression suggests changing roles for this integrin during the early development of inner ear innervation.

Nidogen is a prosurvival and promigratory factor for adult Schwann cells

Schwann cells provide a favorable microenvironment for successful regeneration of the injured peripheral nerve. Even though the roles of extracellular matrix proteins in the Schwann cell physiology have long been studied, the precise function of nidogen, a ubiquitous component of the basal lamina, in Schwann cells is unknown. In this study, we show that the protein and mRNA messages for nidogens are up-regulated in the sciatic nerve after sciatic nerve transection. We demonstrate that recombinant nidogen-1 increased the process formation of Schwann cells cultured from adult rat sciatic nerves and that nidogen-1 prevented Schwann cells from serum-deprivation-induced death. In addition, nidogen-1 promoted spontaneous migration of Schwann cells in two-independent migration assays. The Schwann cell responses to the recombinant nidogen-1 were specific because the nidogen-binding ectodomain of tumor endothelial marker 7 inhibited the nidogen responses without affecting Schwann cell response to laminin. Finally, we found that beta1 subunit-containing integrins play a key role in the nidogen-induced process formation, survival, and migration of Schwann cells. Altogether, these results indicate that nidogen has a prosurvival and promigratory activity on Schwann cells in the peripheral nerve.

Role of integrins in peripheral nerves and hereditary neuropathies

Interactions between Schwann cells and extracellular matrix on one surface, and axons on the other, are required for correct myelination in the developing peripheral nervous system. Integrins are transmembrane proteins that mediate the former in association with other surface receptors. This review focuses on the role that integrins play in the development of the peripheral nervous system, and in inherited human peripheral neuropathies. Here we describe recent findings on integrin signaling to different intracellular pathways, focusing on cell adhesion, migration, and polarization. Then we use information derived from recent experiments of targeted mutagenesis in mice to show that, consistent with temporally regulated expression, different integrins serve multiple roles in developing nerve.

FAK is required for axonal sorting by Schwann cells

Signaling by laminins and axonal neuregulin has been implicated in regulating axon sorting by myelin-forming Schwann cells. However, the signal transduction mechanisms are unknown. Focal adhesion kinase (FAK) has been linked to α6β1 integrin and ErbB receptor signaling, and we show that myelination by Schwann cells lacking FAK is severely impaired. Mutant Schwann cells could interdigitate between axon bundles, indicating that FAK signaling was not required for process extension. However, Schwann cell FAK was required to stimulate cell proliferation, suggesting that amyelination was caused by insufficient Schwann cells. ErbB2 receptor and AKT were robustly phosphorylated in mutant Schwann cells, indicating that neuregulin signaling from axons was unimpaired. These findings demonstrate the vital relationship between axon defasciculation and Schwann cell number and show the importance of FAK in regulating cell proliferation in the developing nervous system.

N-WASP regulates extension of filopodia and processes by oligodendrocyte progenitors, oligodendrocytes, and Schwann cells—implications for axon ensheathment at myelination

The molecular mechanisms used by oligodendrocyte precursor cells (OPCs), oligodendrocytes (OLs), and Schwann cells (SCs) to advance processes for motility in the developing nervous system and to ensheath axons at myelination are currently not well defined. Here we demonstrate that OPCs, OLs, and SCs express the major proteins involved in actin polymerization-driven protrusion; these key proteins including F-actin, the Arp2/3 complex, neural-Wiskott Aldrich Syndrome protein (N-WASP) and WAVE proteins, and the RhoGTPases Rac and Cdc42 are present at the leading edges of processes being extended by OPCs, OLs, and SCs. We reveal by real-time PCR that OLs and SCs have different dominant WAVE isoforms. Inhibition of the WASP/WAVE protein, N-WASP, with wiskostatin that prevents activation of the Arp2/3 complex, blocks process extension by OPCs and SCs. Inhibition of N-WASP also causes OPC and SC process retraction, which is preceded by retraction of filopodia. This implicates filopodia in OPC and SC process stability and also of N-WASP in OPC and SC process dynamics. We also demonstrate that p34 (a component of the Arp2/3 complex), WASP/WAVE proteins, actin, alpha-tubulin, Rac, Cdc42, vinculin, and focal adhesion kinase are detected in water-shocked myelin purified from brain. Inhibition of N-WASP with wiskostatin decreases the number of axons undergoing initial ensheathment in intact optic nerve samples and reduces the Po content of dorsal root ganglia:SC co-cultures. Our findings indicate that OPCs, OLs, and SCs extend processes using actin polymerization-driven protrusion dependent on N-WASP. We hypothesize that inner mesaxons of OLs and SCs use the same mechanism to ensheath axons at myelination.

Phosphorylation of the NF2 tumor suppressor in Schwann cells is mediated by Cdc42-Pak and requires paxillin binding

Mutations in the Neurofibromatosis type 2 tumor suppressor gene that encodes Schwannomin causes formation of benign schwannomas. Schwannoma cells lose their characteristic bipolar shape and become rounded with excessive ruffling membranes. Schwannomin is phosphorylated at serine 518 (S518) by p21 activated kinase (Pak). Unphosphorylated schwannomin is associated with growth inhibition but little is known about the function of the phosphorylated form, or the molecular events leading to its phosphorylation. Here, we report in SCs that schwannomin S518 phosphorylation requires binding to paxillin and targeting to the plasma membrane. Phospho-S518-schwannomin is enriched in the peripheral-most aspects of membrane specializations where paxillin, activated Pak, Cdc42 but not Rac are highly expressed. Schwannomin and Pak phosphorylation levels are not reduced in response to lowering Rac-GTP levels with NSC23766. Expression of schwannomin S518A/D-GFP variants each distinctively altered Schwann cell shape and polarity. These results are consistent with tight spatial regulation of S518 phosphorylation at the plasma membrane in a paxillin and Cdc42-Pak dependent manner that leads to local reorganization of the SC cytoskeleton.

Basal lamina: Schwann cells wrap to the rhythm of space-time

Schwann cells form myelin in the peripheral nervous system. All Schwann cells are surrounded by a basal lamina. Extracellular matrix molecules in the basal lamina, such as laminin, regulate key aspects of Schwann cell development including the formation, architecture and function of myelin. Recent genetic and cell biological experiments suggest that Schwann cells regulate the basal lamina and its receptors in both time and space, resulting in differential functions. These findings have important implications for diseases resulting from laminin dysfunction, such as congenital muscular dystrophy 1A.

Disrupted Schwann cell-axon interactions in peripheral nerves of mice with altered L1-integrin interactions

The cell adhesion molecule L1 is important for peripheral nerve development. Mice lacking the 6th Ig domain of L1 (L1-6D mice) lose L1 homophilic binding and RGD dependent LI-integrin binding [Itoh,K., Cheng, L., Kamei, Y., Fushiki, S., Kamiguchi, H., Gutwein, P.,Stoeck, A., Arnold, B., Altevogt, P., Lemmon, V., 2004. Brain development in mice lacking Li-L homophilic adhesion. J. Cell Biol.165, 145-154]. We examined the ultrastructure of sciatic nerves from L1-6D at postnatal day 7 and 8 weeks. Unmyelinated axons frequently detached at the edge of Schwann cells, and naked axons were observed. Myelin was thinner in L1-6D and abnormal, multiple axons wrapped in a single myelin sheath were routinely observed. Previous work has shown that L1 on axons interacts with a heterophilic binding partner on Schwann cells to facilitate normal peripheral nerve formation. Taken together, it is likely that L1 on axons binds integrins on Schwann cells, resulting in interactions between axons and Schwann cells that are essential for ensheathment and myelination.

Schwann cell-specific ablation of laminin gamma1 causes apoptosis and prevents proliferation

To investigate the function of laminin in peripheral nerve development, we specifically disrupted the laminin gamma1 gene in Schwann cells. Disruption of laminin gamma1 gene expression resulted in depletion of all other laminin chains known to be expressed in Schwann cells. Schwann cells lacking laminin do not extend processes required for initiating axonal sorting and mediating axon-Schwann cell interaction. They fail to downregulate Oct-6 and arrest at the premyelinating stage. The impaired axon-Schwann cell interaction prevents phosphorylation of beta-neuregulin-1 receptors and results in decreased cell proliferation. Postnatally, laminin-null Schwann cells exhibit reduced phosphatidylinositol 3 (PI3)-kinase activity and activation of caspase cascades, leading to apoptosis. Injection of a laminin peptide into mutant sciatic nerves partially restores PI3-kinase activity and reduces apoptotic signals. These results demonstrate the following: (1) that laminin initiates axonal sorting and mediates axon-Schwann cell interactions required for Schwann cell proliferation and differentiation, and (2) that laminin provides a PI3-kinase/Akt-mediated Schwann cell survival signal.

Glypican-1 and alpha4(V) collagen are required for Schwann cell myelination

Schwann cell myelination requires interactions with the extracellular matrix (ECM) mediated by cell surface receptors. Previously, we identified a type V collagen family member, alpha4(V) collagen, which is expressed by Schwann cells during peripheral nerve differentiation. This collagen binds with high affinity to heparan sulfate through a unique binding motif in the noncollagenous N-terminal domain (NTD). The principal alpha4(V) collagen-binding protein on the Schwann cell surface is the heparan sulfate proteoglycan glypican-1. We investigated the role of alpha4(V) collagen and glypican-1 in Schwann cell terminal differentiation in cultures of Schwann cells and dorsal root ganglion neurons. Small interfering RNA-mediated suppression of glypican-1 expression decreased binding of alpha4(V)-NTD to Schwann cells, adhesion and spreading of Schwann cells on alpha4(V)-NTD, and incorporation of alpha4(V) collagen into Schwann cell ECM. In cocultures, alpha4(V) collagen coassembles with laminin on the surface of polarized Schwann cells to form tube-like ECM structures that are sites of myelination. Suppression of glypican-1 or alpha4(V) collagen expression significantly inhibited myelination. These results demonstrate an important role for these proteins in peripheral nerve terminal differentiation.

P0-Cre Transgenic Mice for Inactivation of Adhesion Molecules in Schwann Cells

Normal peripheral nerve myelination depends on Schwann cell-basal lamina interactions. An important component of Schwann cell basal lamina is laminin-predominantly laminins 2 and 4. Mutations in the alpha 2 chain common to these two isoforms are associated with dysmyelination in mouse (dy) and man (congenital muscular dystrophy). Thus, laminin 2 and 4 receptors are also likely to be important for myelin formation. Several laminin 2/4 receptors are detected at the basal lamina surface of myelin-forming Schwann cells, namely, α6β4 and α6β1 integrins and dystroglycan. The evidence linking these receptors to myelination is suggestive, but not conclusive. Genetic studies have not yet confirmed a role for these molecules in myelin formation. Natural or targeted inactivation of α6, β4, and β1 integrins and of dystroglycan have profound effects on other tissues causing embryonic or perinatal death before myelination. Therefore, to conditionally inactivate these receptors specifically in myelin-forming Schwann cells, we have constructed and initially characterized a P₀-Cre transgene that activates Cre-mediated recombination of loxP-containing genes in peripheral nerve.

Human diseases reveal novel roles for neural laminins

Extracellular matrix molecules such as laminins have a central role in regulating cell behaviour. However, our understanding of their functions in the mammalian nervous system is incomplete. It is important to establish these functions, both for an understanding of normal development and to devise strategies to enhance repair. Here, we review how insights gained from human diseases caused by genetic mutations in laminins or their receptors have revealed significant and sometimes unexpected roles for laminins in neural stem cells, migrating neurons and myelinating glia, in both the PNS and CNS.

Collagen XVII and BPAG1 expression in the retina: evidence for an anchoring complex in the central nervous system

The ectoderm gives rise not only to the skin but also to the entire CNS. This common embryonic lineage suggests that some molecular isoforms might serve analogous functions in both tissues. Indeed, not only are laminins important components of dermal adhesion mechanisms, but they also regulate some aspects of synaptic development in both the CNS and the PNS. In the skin, laminins are part of a hemidesmosome complex essential for basal keratinocyte adhesion that includes collagen XVII (BP180) and BPAG1 (dystonin/BP230). Here, we show that CNS neurons also express collagen XVII and BPAG1 and that these molecules are expressed in the adult and developing retina. In the retina, isoforms of collagen XVII and BPAG1 are colocalized with laminins at photoreceptor synapses and around photoreceptor outer segments; both molecules are expressed by rods, whereas cones express collagen XVII but not BPAG1. Moreover, biochemical data demonstrate that collagen XVII complexes with retinal laminins. We propose that collagen XVII and BPAG1 isoforms may help to anchor elements of the rod photoreceptor cytomatrix to the extracellular matrix.

Laminins and their receptors in Schwann cells and hereditary neuropathies

This review focuses on the influence of laminins, mediated through laminin receptors present on Schwann cells, on peripheral nerve development and pathology. Laminins influence multiple aspects of cell differentiation and tissue morphogenesis, including cell survival, proliferation, cytoskeletal rearrangements, and polarity. Peripheral nerves are no exception, as shown by the discovery that defective laminin signals contribute to the pathogenesis of diverse neuropathies such as merosin-deficient congenital muscular dystrophy and Charcot-Marie-Tooth 4F, neurofibromatosis, and leprosy. In the last 5 years, advanced molecular and cell biological techniques and conditional mutagenesis in mice began revealing the role of different laminins and receptors in developing nerves. In this way, we are starting to explain morphological and pathological observations beginning at the start of the last century. Here, we review these recent advances and show how the roles of laminins and their receptors are surprisingly varied in both time and place.

Integrin-growth factor interactions as regulators of oligodendroglial development and function

Central nervous system (CNS) development requires mechanisms for the regulation of cell number. Although growth factors are essential determinants of the proliferation and apoptosis that determine final numbers, the long-range nature of signals from diffusible growth factors makes them insufficient for the provision of the precise and localized signals required. Integration of integrin and growth factor receptor signaling in controlling cell behavior has been an important theme of research over the past several years. The focus of this review is on the mechanisms by which integrin-growth factor interactions regulate the development of oligodendrocytes and provide a mechanism for controlling, both in space and in time, oligodendrocyte numbers in the developing CNS.

Coordinate control of axon defasciculation and myelination by laminin-2 and -8

Schwann cells form basal laminae (BLs) containing laminin-2 (Ln-2; heterotrimer α2β1γ1) and Ln-8 (α4β1γ1). Loss of Ln-2 in humans and mice carrying α2-chain mutations prevents developing Schwann cells from fully defasciculating axons, resulting in partial amyelination. The principal pathogenic mechanism is thought to derive from structural defects in Schwann cell BLs, which Ln-2 scaffolds. However, we found loss of Ln-8 caused partial amyelination in mice without affecting BL structure or Ln-2 levels. Combined Ln-2/Ln-8 deficiency caused nearly complete amyelination, revealing Ln-2 and -8 together have a dominant role in defasciculation, and that Ln-8 promotes myelination without BLs. Transgenic Ln-10 (α5β1γ1) expression also promoted myelination without BL formation. Rather than BL structure, we found Ln-2 and -8 were specifically required for the increased perinatal Schwann cell proliferation that attends myelination. Purified Ln-2 and -8 directly enhanced in vitro Schwann cell proliferation in collaboration with autocrine factors, suggesting Lns control the onset of myelination by modulating responses to mitogens in vivo.

Phosphodiesterase-I alpha/autotaxin controls cytoskeletal organization and FAK phosphorylation during myelination

Myelination within the central nervous system (CNS) involves substantial morphogenesis of oligodendrocytes requiring plastic changes in oligodendrocyte-extracellular matrix (ECM) interactions, that is, adhesion. Our previous studies indicated that a regulator of such adhesive plasticity is oligodendrocyte-released phosphodiesterase-I alpha/autotaxin (PD-I alpha/ATX). We report here, that PD-I alpha/ATX's adhesion antagonism is mediated by a protein fragment different from the one that stimulates tumor cell motility. Furthermore, PD-I alpha/ATX's adhesion-antagonizing fragment causes a reorganized distribution of the focal adhesion components vinculin and paxillin and an integrin-dependent reduction in focal adhesion kinase (FAK) phosphorylation at tyrosine residue 925 (pFAK-925). In vivo, a similar reduction in pFAK-925 occurs at the onset of myelination when PD-I alpha/ATX expression is significantly upregulated. Most importantly, it can also be induced by the application of exogenous PD-I alpha/ATX. Our data, therefore, suggest that PD-I alpha/ATX participates in the regulation of myelination via a novel signaling pathway leading to changes in integrin-dependent focal adhesion assembly and consequently oligodendrocyte-ECM interactions.

Conditional β1-integrin gene deletion in neural crest cells causes severe developmental alterations of the peripheral nervous system

Integrins are transmembrane receptors that are known to interact with the extracellular matrix and to be required for migration, proliferation,differentiation and apoptosis. We have generated mice with a neural crest cell-specific deletion of the β1-integrin gene to analyse the role ofβ1-integrins in neural crest cell migration and differentiation. This targeted mutation caused death within a month of birth. The loss ofβ1-integrins from the embryo delayed the migration of Schwann cells along axons and induced multiple defects in spinal nerve arborisation and morphology. There was an almost complete absence of Schwann cells and sensory axon segregation and defective maturation in neuromuscular synaptogenesis. Thus, β1-integrins are important for the control of embryonic and postnatal peripheral nervous system development.

S1P and LPA trigger Schwann cell actin changes and migration

The processes by which a Schwann cell (SC) migrates towards, wraps around and, in some cases, myelinates an axon are incompletely understood. The complex morphological rearrangements involved in these events require fundamental changes in the actin cytoskeleton. Sphingosine 1-phosphate (S1P) and lysophosphatidic acid (LPA) are two modulators of the actin cytoskeleton, and receptors for these signalling lipids are expressed on SCs at the time of differentiation. Previous work has revealed a role for LPA in SC survival, morphology and differentiation, but the effects of S1P have received less attention. Here we show that S1P and LPA both cause major rearrangements to the actin cytoskeleton in primary rat SCs and the SCL4.1/F7 rat SC line. S1P and LPA caused formation of lamellipodia and a circular geodesic actin network. We also show that S1P and LPA increased cell migration. The small GTPases RhoA and Rac1 were both activated by S1P/LPA treatment, but the actin rearrangements were dependent on Rac1 and not RhoA. These effects of S1P/LPA could be mimicked by SCL4.1/F7 cell-conditioned medium, which was found to contain S1P. Reduction in cellular synthesis of S1P by adding the sphingosine kinase inhibitor dimethyl sphingosine during medium conditioning reduced the ability of conditioned medium to cause actin rearrangements. These results support a role for S1P as an autocrine signal regulating the actin cytoskeleton during Schwann cell development.

Laminin gamma1 is critical for Schwann cell differentiation, axon myelination, and regeneration in the peripheral nerve

Laminins are heterotrimeric extracellular matrix proteins that regulate cell viability and function. Laminin-2, composed of alpha2, beta1, and gamma1 chains, is a major matrix component of the peripheral nervous system (PNS). To investigate the role of laminin in the PNS, we used the Cre-loxP system to disrupt the laminin gamma1 gene in Schwann cells. These mice have dramatically reduced expression of laminin gamma1 in Schwann cells, which results in a similar reduction in laminin alpha2 and beta1 chains. These mice exhibit motor defects which lead to hind leg paralysis and tremor. During development, Schwann cells that lack laminin gamma1 were present in peripheral nerves, and proliferated and underwent apoptosis similar to control mice. However, they were unable to differentiate and synthesize myelin proteins, and therefore unable to sort and myelinate axons. In mutant mice, after sciatic nerve crush, the axons showed impaired regeneration. These experiments demonstrate that laminin is an essential component for axon myelination and regeneration in the PNS.

Inhibition of p38 mitogen-activated protein kinase interferes with cell shape changes and gene expression associated with Schwann cell myelination

In the present study we demonstrate that p38, a member of the mitogen-activated protein kinase (MAPK) family, is essential for ascorbate- and laminin-induced myelination in Schwann cell-dorsal root ganglion neuron cocultures. The inhibitory effect of the specific p38 blockers, PD 169316 and SB 203580, on ascorbate-induced myelination was exerted during the early stages (1-2 days) of ascorbate treatment. Inhibition of p38 was further shown to prevent the alignment of Schwann cells along axons in laminin-treated cocultures. The addition of laminin to Schwann cell-dorsal root ganglion neuron cocultures stimulated phosphorylation of p38, thereby demonstrating a link between laminin-induced myelination and p38 activation. Similarly, the small heat shock protein, Hsp27, which is phosphorylated by MAPKAPK2, a downstream substrate of p38, was phosphorylated in response to the addition of laminin to the cocultures. The p38 inhibitors did not affect the proliferation or survival of Schwann cells in the cocultures as assessed by BrdU incorporation and total cell counts. However, p38 inhibition interfered with an early stage in myelination, thereby preventing ascorbate-induced increases in the levels of mRNAs encoding MBP, MAG, and P(0) and reducing laminin deposition. These results indicate that activation of p38 by a signaling pathway(s) involving laminin and appropriate integrin receptor(s) is required for the alignment of Schwann cells with axons that precedes myelination.

Expression of laminin receptors in schwann cell differentiation: evidence for distinct roles

Schwann cells require laminin-2 throughout nerve development, because mutations in the alpha2 chain in dystrophic mice interfere with sorting of axons before birth and formation of myelin internodes after birth. Mature Schwann cells express several laminin receptors, but their expression and roles in development are poorly understood. Therefore, we correlated the onset of myelination in nerve and synchronized myelinating cultures to the appearance of integrins and dystroglycan in Schwann cells. Only alpha6beta1 integrin is expressed before birth, whereas dystroglycan and alpha6beta4 integrin appear perinatally, just before myelination. Although dystroglycan is immediately polarized to the outer surface of Schwann cells, alpha6beta4 appears polarized only after myelination. We showed previously that Schwann cells lacking beta1 integrin do not relate properly to axons before birth. Here we show that the absence of beta1 before birth is not compensated by other laminin receptors, whereas coexpression of both dystroglycan and beta4 integrin is likely required for beta1-null Schwann cells to myelinate after birth. Finally, both beta1-null and dystrophic nerves contain bundles of unsorted axons, but they are predominant in different regions: in spinal roots in dystrophic mice and in nerves in beta1-null mice. We show that differential compensation by laminin-1, but not laminin receptors may partially explain this. These data suggest that the action of laminin is mediated by beta1 integrins during axonal sorting and by dystroglycan, alpha6beta1, and alpha6beta4 integrins during myelination.

Phosphodiesterase-Ialpha/autotaxin: a counteradhesive protein expressed by oligodendrocytes during onset of myelination

The initial stages of central nervous system (CNS) myelination require complex interactions of oligodendrocytes with their surrounding extracellular environment. In the present study, we demonstrate that commencing with active myelination oligodendrocytes express phosphodiesterase-Ialpha/autotaxin [PD-Ialpha/ATX (NPP-2)] as a non-membrane-associated extracellular factor. As such a component of the extracellular environment, PD-Ialpha/ATX has the ability to antagonize the adhesive interactions between oligodendroglial cells and known extracellular matrix (ECM) molecules present in the developing CNS. This counteradhesion requires intracellular signaling through heterotrimeric G proteins on fibronectin substrates and thus represents an active cellular response. Similar counteradhesive effects in other systems have been attributed to the activity of matricellular proteins, which support intermediate stages of cell adhesion thought to facilitate cellular locomotion and remodeling. Thus, the release of PD-Ialpha/ATX may be critically involved in the regulation of the initial stages of myelination, i.e., oligodendrocyte remodeling, via modulation of oligodendrocyte-ECM interactions in a matricellular fashion.

Schwann cells synthesize alpha7beta1 integrin which is dispensable for peripheral nerve development and myelination

Defects in laminins or laminin receptors are responsible for various neuromuscular disorders, including peripheral neuropathies. Interactions between Schwann cells and their basal lamina are fundamental to peripheral nerve development and successful myelination. Selected laminins are expressed in the endoneurium, and their receptors are developmentally regulated during peripheral nerve formation. Loss-of-function mutations have confirmed the importance and the role of some of these molecules. Here we show for the first time that another laminin receptor, alpha7beta1 integrin, previously described only in neurons, is also expressed in Schwann cells. The expression of alpha7 appears postnatally, such that alpha7beta1 is the last laminin receptor expressed by differentiating Schwann cells. Genetic inactivation of the alpha7 subunit in mice does not affect peripheral nerve formation or the expression of other laminin receptors. Of note, alpha7beta1 is not necessary for basal lamina formation and myelination. Nonetheless, these data taken together with the previous demonstration of impaired axonal regrowth in alpha7-null mice suggest a possible Schwann cell-autonomous role for alpha7 in nerve regeneration.

Formation of a beta1 integrin signaling complex in Schwann cells is independent of rho

Schwann cell adhesion to basal lamina is essential for peripheral nerve development. beta(1) integrin receptors for extracellular matrix cooperate with other receptors to transmit signals that coordinate cell cycle progression and initiation of differentiation, including myelin-specific gene expression. In Schwann cell/sensory neuron cocultures, beta(1) integrins complex with focal adhesion kinase (FAK), fyn kinase, paxillin, and schwannomin in response to basal lamina adhesion. To study the assembly of this signaling complex in Schwann cells (SCs), we induced beta(1) integrin clustering on suspended cells using an immobilized antibody and recovered a complex containing beta(1) integrin, FAK, paxillin, and schwannomin. In adherent subconfluent cells, the proteins colocalized to filopodia, ruffling membranes and focal contacts. We assessed the role of rhoGTPase in the process of integrin complex assembly by introducing C3 transferase (C3T), a rho inhibitor, into the cells. Although C3T caused dose-dependent morphological abnormalities, FAK, paxillin, and schwannomin were able to coimmunoprecipitate with beta(1) integrin. Additionally, colocalization of FAK, paxillin, and schwannomin with beta(1) integrin in filopodia and small focal contacts remained unchanged. We conclude that SCs do not require active rho to recruit signaling and structural proteins to beta(1) integrins clustered at the plasma membrane. Rho is required to establish large focal adhesions and to spread and stabilize plasma membrane extensions.

Association of Caspr/paranodin with tumour suppressor schwannomin/merlin and beta1 integrin in the central nervous system

Caspr/paranodin is an essential neuronal component of paranodal axoglial junctions, associated with contactin/F3. Its short intracellular domain contains a conserved motif (GNP motif) capable of binding protein 4.1 domains [FERM domains (four point one, ezrin, radixin, moesin)]. Schwannomin/merlin is a tumour suppressor expressed in many cell types, including in neurons, the function and partners of which are still poorly characterized. We show that the FERM domain of schwannomin binds to the paranodin GNP motif in glutathione S-transferase (GST)-pull down assays and in transfected COS-7 cells. The two proteins co-immunoprecipitated in brain extracts. In addition, paranodin and schwannomin were associated with integrin beta1 in transfected cells and in brain homogenates. The presence of paranodin increased the association between integrin beta1 and schwannomin or its N-terminal domain, suggesting that the interactions between these proteins are interdependent. In jimpy mutant mice, which display a severe dysmyelination with deficient paranodal junctions, the interactions between paranodin, schwannomin and integrin beta1 were profoundly altered. Our results show that schwannomin and integrin beta1 can be associated with paranodin in the central nervous system. Since integrin beta1 and schwannomin do not appear to be enriched in paranodes they may be quantitatively minor partners of paranodin in these regions and/or be associated with paranodin at other locations.

Laminin chains in rat and human peripheral nerve: distribution and regulation during development and after axonal injury

During nerve growth, axons are dependent upon contact with matrix components, such as laminins, for elongation, guidance, and trophic support. Semiquantitative in situ hybridization histochemistry and immunohistochemistry (IHC) were used to identify laminin chains in normal peripheral nerves, during postnatal development, after sciatic nerve transection (SNT), and after sciatic nerve crush (SNC). Laminin alpha2, alpha4, beta1, beta2, and gamma1 chain mRNAs were all expressed at high levels in newborn rat sciatic nerves with declining levels during later developmental stages. At the adult stage, no laminin chain mRNA was detectable. Of interest, the mRNA levels for alpha4 chain declined faster than those for alpha2. After SNT, laminin alpha2, alpha4, beta1, and gamma1 mRNA levels were up-regulated at the site of the injury, with the most profound reaction in the proximal nerve stump. Laminin alpha2 and alpha4 chains differed in that the mRNA levels of alpha4 were up-regulated earlier and declined quicker, whereas alpha2 had a later onset, with high levels remaining even after 6 weeks. After SNC, there was an initial up-regulation of the same laminin chain mRNAs as after SNT in the nerve, however, less intense, and at 6 weeks after SNC, all laminin mRNA levels studied had returned to normal. IHC of adult human normal and transected peripheral nerves stained positive for laminin alpha2, alpha4, beta1, and gamma1 chains in close relation to neurofilament labeled axons. Laminin alpha3, alpha4, alpha5, beta1, beta2, and gamma1 chains were found in blood vessel-like structures and alpha3, alpha4, alpha5, beta2, and gamma1 in the perineurium. These results and a previously published description of integrin regulation in spinal motoneurons suggest that both laminin-2 (alpha2beta1gamma1) and laminin-8 (alpha4beta1gamma1) are important for the postnatal nerve development and axonal regeneration after injury and that laminin-8 may have important functions especially early postnatally and early after adult nerve lesion.

Matrix metalloproteinase mediated degradation of basement membrane proteins in Trembler J neuropathy nerves

A single point mutation in peripheral myelin protein 22 (pmp22) of the Trembler-J (TrJ) mouse models the human peripheral neuropathy, Charcot-Marie-Tooth disease type 1 A (CMT1A). An unexplored aspect of this disease is the gradual remodeling of the extracellular matrix in affected nerves. To elucidate the mechanism responsible for these changes, the levels of the extracellular matrix molecules laminin, collagen IV, and fibronectin were determined. In TrJ nerves, laminin is modestly increased while full-length forms of collagen IV and fibronectin are decreased. Matrix metalloproteinases (MMPs) are known to degrade multiple matrix molecules; therefore, nerves were assayed for MMP-2 and MMP-9 proteins. In neuropathy nerves, elevated levels of MMP-2 and MMP-9 were detected on western blots, and gelatin zymography confirmed the up-regulation of gelatinalytic activity in affected samples. Immunostaining studies revealed an increase in the numbers of MMP-2- and MMP-9-expressing cells in TrJ nerves. Cell type-specific immunolabeling showed that infiltrating macrophages are a significant source of both MMP-2 and MMP-9. Finally, the degradation of exogenous collagen IV by TrJ nerve lysates was prevented with a specific MMP inhibitor. Together these observations suggest that infiltration by MMP-expressing macrophages contributes to the remodeling of the TrJ nerve matrix.

Cellular transformation by a FERM domain mutant of the Nf2 tumor suppressor gene

Mutations in the Nf2 tumor suppressor gene lead to tumor formation in humans and mice and cellular overproliferation phenotypes in Drosophila. The Nf2 encoded protein, merlin, shares close sequence similarity in its amino terminus to members of the band 4.1 family of membrane-cytoskeletal linkers. Similarities between merlin and this family suggest a role for merlin in regulating cytoskeletal function. However, the mechanism of the tumor suppressing activity of merlin is not yet understood. Mutational analysis of Nf2 in flies has led to the identification of a dominant-negative allele, which harbors mutations in the amino terminus of the protein. Here, we report that expression of a murine analog of this amino-terminal mutant of Nf2 leads to complete transformation of NIH3T3 fibroblasts in culture. Cells that express this Nf2 mutant allele display disruptions of the actin cytoskeleton, lack of contact inhibition of growth, and anchorage-independent growth. Finally, fibroblasts that express this mutant Nf2 allele form tumors when injected into nude mice.

Paxillin binds schwannomin and regulates its density-dependent localization and effect on cell morphology

Neurofibromatosis type 2 is an autosomal dominant disorder characterized by tumors, predominantly schwannomas, in the nervous system. It is caused by mutations in the gene NF2, encoding the growth regulator schwannomin (also known as merlin). Mutations occur throughout the 17-exon gene, with most resulting in protein truncation and undetectable amounts of schwannomin protein. Pathogenic mutations that result in production of defective schwannomin include in-frame deletions of exon 2 and three independent missense mutations within this same exon. Mice with conditional deletion of exon 2 in Schwann cells develop schwannomas, which confirms the crucial nature of exon 2 for growth control. Here we report that the molecular adaptor paxillin binds directly to schwannomin at residues 50-70, which are encoded by exon 2. This interaction mediates the membrane localization of schwannomin to the plasma membrane, where it associates with beta 1 integrin and erbB2. It defines a pathogenic mechanism for the development of NF2 in humans with mutations in exon 2 of NF2.

Fibrin inhibits peripheral nerve remyelination by regulating Schwann cell differentiation

Remyelination is a critical step for functional nerve regeneration. Here we show that fibrin deposition in the peripheral nervous system after injury is a key regulator of remyelination. After sciatic nerve crush, fibrin is deposited and its clearance correlates with remyelination. Fibrin induces phosphorylation of ERK1/2 and production of p75 NGF low-affinity receptor in Schwann cells and maintains them in a nonmyelinating state, suppresses fibronectin production, and prevents synthesis of myelin proteins. In mice depleted of fibrin(ogen), remyelination of myelinated axons is accelerated due to the faster transition of the Schwann cells to a myelinating state. Regulation of fibrin clearance and/or deposition could be a key regulatory mechanism for Schwann differentiation after nerve damage.