Basement membranes during development of human nerve: Schwann cells and perineurial cells display marked changes in their expression profiles for laminin subunits and beta 1 and beta 4 integrins

Case report: Intraneural perineurioma in dogs: a case series and brief literature review

Intraneural perineurioma is an exceptionally rare neoplasm in animals. This case study comprises a series of three cases and a brief literature review focusing on canine intraneural perineurioma. The pathological and immunohistochemical findings are documented, revealing that canine intraneural perineurioma frequently affects adult dogs aged between 3 and 10 years old, with a male predominance. Clinical signs associated with intraneural perineurioma in dogs include spinal pain, lameness, and paresis, resulting from the involvement of spinal nerve roots of the pelvic limbs, brachial plexus, or distal part of the median nerve. Most neoplasms had characteristic pseudo-onion bulb patterns on histopathology. Neoplastic perineurial cells, in most cases, expressed laminin and claudin-1, and NF200 consistently highlighted the central axon. While the immunohistochemical (IHC) profile of intraneural perineurioma in veterinary medicine remains incompletely characterized, the available IHC data from all reported cases suggest that a combination of laminin and claudin-1 immunomarkers, along with distinctive histological features, can assist in establishing a definitive diagnosis of intraneural perineurioma.

Autism and Williams syndrome: Dissimilar socio-cognitive profiles with similar patterns of abnormal gene expression in the blood

LAY ABSTRACT

Autism spectrum disorders and Williams syndrome are complex cognitive conditions exhibiting quite opposite features in the social domain: whereas people with autism spectrum disorders are mostly hyposocial, subjects with Williams syndrome are usually reported as hypersocial. At the same time, autism spectrum disorders and Williams syndrome share some common underlying behavioral and cognitive deficits. It is not clear, however, which genes account for the attested differences (and similarities) in the socio-cognitive domain. In this article, we adopted a comparative molecular approach and looked for genes that might be differentially (or similarly) regulated in the blood of people with these conditions. We found a significant overlap between genes dysregulated in the blood of patients compared to neurotypical controls, with most of them being upregulated or, in some cases, downregulated. Still, genes with similar expression trends can exhibit quantitative differences between conditions, with most of them being more dysregulated in Williams syndrome than in autism spectrum disorders. Differentially expressed genes are involved in aspects of brain development and function (particularly dendritogenesis) and are expressed in brain areas (particularly the cerebellum, the thalamus, and the striatum) of relevance for the autism spectrum disorder and the Williams syndrome etiopathogenesis. Overall, these genes emerge as promising candidates for the similarities and differences between the autism spectrum disorder and the Williams syndrome socio-cognitive profiles.

Rare variant analysis in multiply affected families, association studies and functional analysis suggest a role for the ITGΒ4 gene in schizophrenia and bipolar disorder

Recent results imply that rare variants contribute to the risk of schizophrenia. Exome sequence data from the UK10K project was used to identify three rare, amino acid changing variants in the ITGB4 gene which segregated with schizophrenia in two families: rs750367954, rs147480547 and rs145976111. Association analysis was carried out in the exome-sequenced Swedish schizophrenia study and in UCL schizophrenia and bipolar cases and controls genotyped for these variants. A gene-wise weighted burden test was performed on a trio sample of schizophrenia cases and their parents. rs750367954 was seen in two Swedish cases and in no controls. The other two variants were commoner in cases than controls in both Swedish and UCL cohort samples and an overall burden test was significant at p=0.0000031. The variants were not observed in the trio sample but ITGB4 was most highly ranked out of 14,960 autosomal genes in a gene-wise weighted burden test. The effect of rs147480547 and rs145976111 was studied in human neuroblastoma SH-SY5Y cells. Cells transfected with both variants had increased proliferation at both 24 and 48h (p=0.013 and p=0.05 respectively) compared to those with wild-type ITGB4. Taken together, these results suggest that rare variants in ITGB4 which affect function may contribute to the aetiology of schizophrenia and bipolar disorder.

Type IV collagen is an activating ligand for the adhesion G protein-coupled receptor GPR126

GPR126 is an orphan heterotrimeric guanine nucleotide-binding protein (G protein)-coupled receptor (GPCR) that is essential for the development of diverse organs. We found that type IV collagen, a major constituent of the basement membrane, binds to Gpr126 and activates its signaling function. Type IV collagen stimulated the production of cyclic adenosine monophosphate in rodent Schwann cells, which require Gpr126 activity to differentiate, and in human embryonic kidney (HEK) 293 cells expressing exogenous Gpr126. Type IV collagen specifically bound to the extracellular amino-terminal region of Gpr126 containing the CUB (complement, Uegf, Bmp1) and pentraxin domains. Gpr126 derivatives lacking the entire amino-terminal region were constitutively active, suggesting that this region inhibits signaling and that ligand binding relieves this inhibition to stimulate receptor activity. A new zebrafish mutation that truncates Gpr126 after the CUB and pentraxin domains disrupted development of peripheral nerves and the inner ear. Thus, our findings identify type IV collagen as an activating ligand for GPR126, define its mechanism of activation, and highlight a previously unrecognized signaling function of type IV collagen in basement membranes.

Barriers of the peripheral nerve

This review introduces the traditionally defined anatomic compartments of the peripheral nerves based on light and electron microscopic topography and then explores the cellular and the most recent molecular basis of the different barrier functions operative in peripheral nerves. We also elucidate where, and how, the homeostasis of the normal human peripheral nerve is controlled in situ and how claudin-containing tight junctions contribute to the barriers of peripheral nerve. Also, the human timeline of the development of the barriers of the peripheral nerve is depicted. Finally, potential future therapeutic modalities interfering with the barriers of the peripheral nerve are discussed.

Regulation of the basement membrane by epithelia generated forces

Tumor metastasis involves a progressive loss of tissue architecture and dissolution of structural boundaries between the epithelium and connective tissue. The basement membrane (BM), a specialized network of extracellular matrix proteins forms a barrier that physically restricts pre-invasive lesions such that they remain as local insults. The BM is not a static structure, but one that is constantly regenerated and remodeled in the adult organism. Matrix organization also regulates cell function. Thus alterations in the balance of synthesis, remodeling and proteolytic degradation of the extracellular matrix proteins may contribute to a loss of structural integrity. However, the de novo assembly and maintenance of the complex structural properties of in vivo basement membranes remain elusive. Here, this paper highlights the current understanding on the structural properties and the establishment of the BM, and discusses the potential role of self-generated forces in adult tissue remodeling and the maintenance of the BM as a malignancy suppressor.

HLA class I: an unexpected role in integrin β4 signaling in endothelial cells

The production of anti-donor antibodies to HLA class I and class II antigens following transplantation is associated with development of transplant vasculopathy and graft loss. Antibodies against HLA class I (HLA-I) molecules are thought to contribute to transplant vasculopathy by triggering signals that elicit the activation and proliferation of endothelial cells. The proximal molecular events that regulate HLA-I dependent signal transduction are not well understood. We demonstrated a mutual dependency between HLA-I and integrin β4 to stimulate signal transduction and cell proliferation. Similarly, we found that integrin β4-mediated cell migration was dependent upon its interactions with HLA-I molecules. Since integrin β4 has been implicated in angiogenesis and tumor formation, associations between integrin β4 and HLA-I may play an important role in cancer. Further characterization of interactions between HLA-I and integrin β4 may lead to the development of therapeutic strategies for the treatment and prevention of chronic allograft rejection and cancer.

Tissue distribution of the laminin beta1 and beta2 chain during embryonic and fetal human development

Laminins are the major glycoproteins present in all basement membranes. Previously, we showed that perlecan is present during human development. Although an overview of mRNA-expression of the laminin β1 and β2 chains in various developing fetal organs is already available, a systematic localization of the laminin β1 and β2 chains on the protein level during embryonic and fetal human development is missing. Therefore, we studied the immunohistochemical expression and tissue distribution of the laminin β1 and β2 chains in various developing embryonic and fetal human organs between gestational weeks 8 and 12. The laminin β1 chain was ubiquitously expressed in the basement membrane zones of the brain, ganglia, blood vessels, liver, kidney, skin, pancreas, intestine, heart and skeletal system. Furthermore, the laminin β2 chain was present in the basement membrane zones of the brain, ganglia, skin, heart and skeletal system. The findings of this study support and expand upon the theory that these two laminin chains are important during human development.

Tight junction proteins in human Schwann cell autotypic junctions

Tight junctions (TJs) form physical barriers in various tissues and regulate paracellular transport of ions, water, and molecules. Myelinating Schwann cells form highly organized structures, including compact myelin, nodes of Ranvier, paranodal regions, Schmidt-Lanterman incisures, periaxonal cytoplasmic collars, and mesaxons. Autotypic TJs are formed in non-compacted myelin compartments between adjacent membrane lamellae of the same Schwann cell. Using indirect immunofluorescence and RT-PCR, we analyzed the expression of adherens junction (E-cadherin) and TJ [claudins, zonula occludens (ZO)-1, occludin] components in human peripheral nerve endoneurium, showing clear differences with published rodent profiles. Adult nerve paranodal regions contained E-cadherin, claudin-1, claudin-2, and ZO-1. Schmidt-Lanterman incisures contained E-cadherin, claudin-1, claudin-2, claudin-3, claudin-5, ZO-1, and occludin. Mesaxons contained E-cadherin, claudin-1, claudin-2, claudin-3, ZO-1, and occludin. None of the proteins studied were associated with nodal inter-Schwann cell junctions. Fetal nerve expression of claudin-1, claudin-3, ZO-1, and occludin was predominantly punctate, with a mesaxonal labeling pattern, but paranodal (ZO-1, claudin-3) and Schmidt-Lanterman incisure (claudins-1 and -3) expression profiles typical of compact myelin were visible by gestational week 37. The clear differences observed between human and published rodent nerve profiles emphasize the importance of human studies when translating the results of animal models to human diseases.

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.

Integrin beta 4 in neural cells

Integrin beta 4, one of the heterodimeric receptors, is expressed predominantly on epithelial cells. It is concentrated at the basement membrane zone, where it localizes to specialized adhesion structures called hemidesmosomes. In addition to its adhesive functions, novel insights have emerged regarding the specific roles of integrin beta 4 in their attachment to extracellular matrix and in their signal transduction pathways within the central nervous system (CNS) and peripheral nervous system in the past few years. It has been reported that integrin beta 4 is expressed in several kinds of neural cells including astrocyte, Schwann cells, neurons, and neural stem cells. In the mean while, it is expressed by some Schwann cells in the peripheral nervous system and mediated the Mycobacterium leprae invade the peripheral nervous system to reach the Schwann cells. This review highlights recent progress in the function and regulation of integrin beta 4 in neural cells.

Knockdown of integrin beta4 in primary cultured mouse neurons blocks survival and induces apoptosis by elevating NADPH oxidase activity and reactive oxygen species level

Recently, the specific roles of integrin beta4 in the signaling networks that drive pathological angiogenesis and tumor progression have been revealed. Our previous study showed that integrin beta4 might be involved in neuron survival signal transduction. To further our study on the role of integrin beta4 in the survival and apoptosis of primary cultured mouse neurons, we inhibited the expression of integrin beta4 by its specific small interfering RNA. Viability of the cells remarkably declined, and neurons underwent apoptosis with down-regulation of integrin beta4. Next, we investigated the effect of siRNA-mediated down-regulation of integrin beta4 on the level of intracellular reactive oxygen species and the activities of NADPH oxidase and superoxide dismutase. The level of reactive oxygen species in the neurons was elevated significantly, the activities of manganese-dependent superoxide dismutase and copper/zinc-dependent superoxide dismutase were not altered, but the activity of NADPH oxidase was increased. Furthermore, inhibition of NADPH oxidase by its specific inhibitor dibenziodolium chloride attenuated the neuronal death induced by integrin beta4 knockdown. The data suggest that integrin beta4 is a key factor in neuron survival and apoptosis and indicate that this integrin subunit might perform its action through regulating NADPH oxidase and the level of reactive oxygen species in neuronal survival and apoptosis.

α7β1 integrin is a receptor for laminin-2 on Schwann cells

The Schwann cell basal lamina acts as an organizer of peripheral nerve tissue and influences many aspects of cell behavior during development and regeneration. A principal component of the Schwann cell basal lamina is laminin-2. This study was undertaken to identify Schwann cell receptors for laminin-2. We found that among several Schwann cell integrins that can potentially interact with laminin-2, only alpha7beta1 bound to laminin-2-Sepharose. Dystroglycan, a non-integrin Schwann cell receptor for laminin-2 identified previously, was also found to bind to laminin-2-Sepharose. Antibody to the alpha7 integrin subunit partially inhibited Schwann cell adhesion to laminin-2. Small interfering RNA-mediated suppression of either alpha7 integrin or dystroglycan expression decreased adhesion and spreading of Schwann cells on laminin-2, whereas knocking down both proteins together inhibited adhesion and spreading on laminin-2 almost completely. alpha7 integrin and dystroglycan both colocalized with laminin-2 containing basal lamina tubes in differentiating neuron-Schwann cell cocultures. The alpha7beta1 integrin also coprecipitates with focal adhesion kinase in differentiating cocultures. These findings strongly suggest that alpha7beta1 integrin is a Schwann cell receptor for laminin-2 that provides transmembrane linkage between the Schwann cell basal lamina and cytoskeleton.

The neurobiology of Schwann cells

This selective review of Schwann cell biology focuses on questions relating to the origins, development and differentiation of Schwann cells and the signals that control these processes. The importance of neuregulins and their receptors in controlling Schwann cell precursor survival and generation of Schwann cells, and the role of these molecules in Schwann cell biology is addressed. The reciprocal signalling between peripheral glial cells and neurons in development and adult life revealed in recent years is highlighted, and the profound change in survival regulation from neuron-dependent Schwann cell precursors to adult Schwann cells that depend on autocrine survival signals is discussed. Besides providing neuronal and autocrine signals, Schwann cells signal to mesenchymal cells and influence the development of the connective tissue sheaths of peripheral nerves. The importance of Desert Hedgehog in this process is described. The control of gene expression during Schwann cell development and differentiation by transcription factors is reviewed. Knockout of Oct-6 and Krox-20 leads to delay or absence of myelination, and these results are related to morphological or physiological observations on knockout or mutation of myelin-related genes. Finally, the relationship between selected extracellular matrix components, integrins and the cytoskeleton is explored and related to disease.

Pathological adhesion of primary human schwannoma cells is dependent on altered expression of integrins

Mutations in the tumor suppressor gene coding for merlin cause Neurofibromatosis type 2 (NF2), all spontaneous schwannomas, and a majority of meningiomas. Merlin links transmembrane proteins to the cytoskeleton. Accordingly, primary human schwannoma cells lacking merlin show an increased number of lamellipodia and filopodia as well as increased cell spreading. We show enhanced adhesion in primary human schwannoma cells and present evidence that this is dependent on the integrin chains alpha6beta1 and alpha6beta4. We further demonstrate that the integrin chains beta1 and beta4 are upregulated in schwannomas using different complementary methods, and report higher expression of these integrins per schwannoma cell by fluorescence assisted cell sorting (FACS). Finally we report clustering of the integrin chains alpha6, beta1, and beta4 on schwannoma cells. Our findings fit well into recent data on the role of merlin in signaling cascades connected to integrins and help explain pathological ensheathment of extracellular matrix or pseudomesaxon formation which is a hallmark of schwannoma histopathology.

Tight junction proteins ZO-1, occludin, and claudins in developing and adult human perineurium

In peripheral nerves, groups of Schwann cell-axon units are isolated from the adjacent tissues by the perineurium, which creates a diffusion barrier responsible for the maintenance of endoneurial homeostasis. The perineurium is formed by concentric layers of overlapping, polygonal perineurial cells that form tight junctions at their interdigitating cell borders. In this study, employing indirect immunofluorescence and immunoelectron microscopy, we demonstrate that claudin-1 and -3, ZO-1, and occludin, but not claudin-2, -4, and -5, are expressed in the perineurium of adult human peripheral nerve. We also describe the expression of occludin, ZO-1, claudin-1, -3, and -5 in the developing human perineurium, showing that the expressions of claudin-1 and -3, ZO-1, and occludin follow similar spatial developmental expression patterns but follow different timetables in achieving their respective adult distributions. Specifically, claudin-1 is already largely restricted to perineurium-derived structures at 11 fetal weeks, whereas claudin-3 and occludin are weakly expressed in the perineurial structures at this age and acquire a well-defined perineurial distribution only between 22 and 35 fetal weeks. ZO-1 appears to acquire its mature profile even later during the third trimester. The results of the present and previous studies show that the perineurial diffusion barrier matures relatively late during human peripheral nerve development.

Laminin assembles into separate basement membrane and fibrillar matrices in Schwann cells

Laminins are important for Schwann cell basement membrane assembly and axonal function. In this study, we found that exogenous laminin-1, like neuromuscular laminins-2/4, formed two distinct extracellular matrices on Schwann cell surfaces, each facilitated by laminin polymerization. Assembly of one, a densely-distributed reticular matrix, was accompanied by a redistribution of cell-surface dystroglycan and cytoskeletal utrophin into matrix-receptor-cytoskeletal complexes. The other, a fibrillar matrix,accumulated in separate zones associated with pre-existing β1-integrin arrays. The laminin-1 fragment E3 (LG-modules 4-5), which binds dystroglycan and heparin, inhibited reticular-matrix formation. By contrast,β1-integrin blocking antibody (Ha2/5) prevented fibrillar assembly. Ultrastructural analysis revealed that laminin treatment induced the formation of a linear electron-dense extracellular matrix (lamina densa)separated from plasma membrane by a narrow lucent zone (lamina lucida). This structure was considerably reduced with non-polymerizing laminin, fully blocked by E3, and unaffected by Ha2/5. Although it formed in the absence of type IV collagen, it was nonetheless able to incorporate this collagen. Finally, cell competency to bind laminin and form a basement membrane was passage-dependent. We postulate that laminin induces the assembly of a basement membrane on competent cell surfaces probably mediated by anchorage through LG 4-5. Upon binding, laminin interacts with dystroglycan,mobilizes utrophin, and assembles a `nascent' basement membrane, independent of integrin, that is completed by incorporation of type IV collagen. However,the fibrillar β1-integrin dependent matrix is unlikely to be precursor to basement membrane.

Nonmuscular involvement in merosin-negative congenital muscular dystrophy

The spectrum of nonmuscular involvement in six children with merosin-negative congenital muscular dystrophy is described. In all children, biochemical, neuroradiologic, cardiac, and neurophysiologic studies were performed. Cerebral structures that were myelinated at gestation, including internal capsule, corpus callosum, brainstem, and cerebellar white matter, demonstrated no abnormalities, whereas the periventricular and subcortical white matter, which were myelinated in the first postnatal year, demonstrated signs of leukoencephalopathy. Cerebrospinal fluid analysis revealed an elevated albumin cerebrospinal fluid to serum ratio in the younger children. Electroencephalogram results were abnormal in the two elder children. One child suffered from congestive cardiomyopathy. The increase in nerve conduction velocity in these children over the years lagged behind those of healthy patients, pointing to a demyelinating neuropathy. We conclude that in merosin-negative congenital muscular dystrophy patients, nonmuscular involvement includes the central and peripheral nervous system and the heart. The pattern of myelination of the brain and nerve conduction slowing suggests a myelination arrest. Merosin deficiency can give rise to a congestive cardiomyopathy, which is of no clinical relevance in the majority of children.

Regulation of laminin beta2 chain gene expression in human cancer cell lines

The laminin beta2 chain is a basement membrane component expressed in a tissue- and developmental stage-specific manner. In this report we have examined the transcriptional and post-transcriptional regulation of the human laminin beta2 chain in human tumor cell lines. Both the A204 rhabdomyosarcoma and clone A colon carcinoma cells express the laminin beta2 chain mRNA, but only the A204 cells secrete laminin heterotrimers containing the beta2 chain. Segments of the beta2 chain gene promoter region were cloned into luciferase reporter vectors, and their ability to stimulate transcription was tested by transient transfection. Sequences downstream of the transcription start site between nucleotides +91 and +120 were found to be essential for luciferase activity in the two cell lines. Additional positive regulatory regions were present further upstream, between nucleotides -164 to -667 and between nucleotides -667 to -1724. Genomic DNA at the 3' end of the gene also appeared to have enhancer activity, as a 1.1-kb fragment located downstream of the last exon stimulated the luciferase activity of the nucleotides -667/+297 promoter segment approximately threefold. Alternative splicing of the first intron of the human laminin beta2 chain gene generates two isoforms of the 5' untranslated region of the beta2 chain mRNA. The translational efficiencies of the two laminin beta2 chain leaders did not differ significantly, when assayed by polysome profile analysis of endogenous clone A cell beta2 chain mRNA, transient transfection of chimeric beta2 chain leader/luciferase expression plasmids in clone A cells, and translation of in vitro synthesized RNAs in rabbit reticulocyte lysates.

Role of integrins in the peripheral nervous system

Integrins, a subgroup of adhesion receptors, are transmembrane glycoproteins that mediate interactions between cytoplasm and the extracellular environment. These interactions influence, among others, events such as cell migration, proliferation, and differentiation. Differential expression of integrins is developmentally regulated in the peripheral nervous system (PNS) and is associated with crucial events in both physiological and pathological processes. Preliminary studies suggest that integrin expression influences neural crest cell migration, axonal outgrowth, and Schwann cell differentiation. Similarly, the abnormal expression of integrins or their ligands, is associated with degenerative, inflammatory, and malignant disorders of the PNS. Finally, integrins participate in the complex interactions that promote repair of the PNS. A better comprehension of the role of integrins in the PNS, their protein interactions and transducing signals is being achieved by selected biochemical and genetic experiments. Here we review a large bias of evidence suggesting the key functions for integrins in the PNS.

Expression of laminins and their integrin receptors in different conditions of synovial membrane and synovial membrane-like interface tissue

OBJECTIVE

To demonstrate the expression of laminins (Lns) and their integrin (Int) receptors in different synovial samples and synovial membrane-like interface tissues from well fixed and aseptically loosened total hip replacement (THR), and the potential role of Ln-Int interaction in the production of collagenases and cytokines.

METHODS

Immunohistochemical staining was done to detect the distribution of EHS Ln, Ln alpha2, alpha3, alpha5, beta1, beta2 chains and Int alpha1, alpha2, alpha3, alpha6, beta1, beta4 subunits in different samples. Double immunofluorescence labelling was used to find colocalisation of Int alpha6 subunit and collagenase-1/collagenase-3/TNFalpha/IL6.

RESULTS

General Ln immunoreactivity was detected in all specimens. Ln alpha5, beta1 and beta2, but not alpha2 and alpha3 chains were seen in the synovial lining and the basement membrane of blood vessels with the intensity/extent of labelling in the following rank order: rheumatoid arthritis (RA) loosened prostheses, osteoarthritis, well fixed prostheses, traumatic knees. Among Int subunits, staining for beta1 was usually the strongest, followed by staining for Int alpha6, alpha1, alpha3, and alpha2 subunits, with the same rank order for overall expression of Lns. Int beta4 subunit was not detectable in most of the specimens. Double labelling focused on Int alpha6 subunit disclosed its frequent colocalisation with collagenases 1 and 3 and with tumour necrosis factor alpha and interleukin 6 in synovial lining.

CONCLUSION

Synovial lining contains Ln-10, Ln-11, and Int alpha6beta1 and alpha1beta1 receptors. In aseptic loosening of THR, interface tissue has a similar Ln subtype and Int receptor composition as RA synovium, which confirms its "lining-like" phenotype. Synovial lining does not contain Ln-5 (alpha3beta3gamma2) or Int alpha6beta4, which are components of epithelial hemidesmosomes. The expression of Lns and their Int receptors is upregulated in inflammation. The close spatial relation between Ln and its Int receptors in synovial lining cells containing proteinases and cytokines suggests a potential role in joint destruction and prosthetic loosening.

Differential expression of gap junction proteins connexin26, 32, and 43 in normal and crush-injured rat sciatic nerves. Close relationship between connexin43 and occludin in the perineurium

We immunohistochemically and morphometrically examined the expression of gap junction protein connexin (Cx) in normal and crush-injured rat sciatic nerves using confocal laser scanning microscopy. Cx26 was localized in the perineurium and Cx43 was present in the perineurium and the epineurium, whereas Cx32 was confined to the paranodal regions of the nodes of Ranvier. Double labeling for connexins and laminin revealed that Cx43 was localized in multiple layers of the perineurium, whereas Cx26 was confined to the innermost layer. Double labeling for connexins and a tight junction protein, occludin, showed that occludin frequently coexisted with Cx43 but existed separately from Cx26 in the perineurium. After crush injury, the pattern of normal Cx32 expression was initially lost but recovered, whereas Cx43 rapidly appeared in the endoneurium and its expression was subsequently attenuated. Although crush injury produced no apparent alteration in Cx43 and occludin in the perineurium, a rapid increase and a subsequent decrease in the frequency of Cx26-positive spots during nerve regeneration were shown by morphometric analysis. These results indicate that Cx26, Cx32, and Cx43 are expressed differently in various types of cells in peripheral nerves and that their expressions are differentially regulated after injury. The expression of connexins and occludin in the perineurium suggests that perineurial cells are not uniform in type and that the regulation of gap junctions and tight junctions is closely related in the perineurium.

Basement membranes in development

The aim of this review is to introduce the reader to the main ECM constituents and to some of their roles in development. The main functions of the ECM during embryogenesis are the production, promotion, and regulation of normal tissue structure. Among the ECM components, LMs have been the most extensively studied in relation to embryo-genesis. Skin and skeletal muscle disorders have been shown to be caused by LM alterations. Additional experiments, e.g., with knockout mice, will help enormously to elucidate the functional significance of many ECM constituents and their involvement in development and disease.

Expression of integrins in experimental autoimmune neuritis and Guillain-Barré syndrome

Integrins are a subclass of adhesion molecules that mediate cell-cell and cell-extracellular matrix interactions. Integrins influence transendothelial migration of lymphocytes and monocytes and are suitable targets for experimental immunotherapy. They are critically involved in the pathogenesis of autoimmune neuritis and abnormally expressed in human neuropathies. Also, the role of integrins in myelination, neurite outgrowth, and nerve regeneration suggests that they could be involved in the recovery phase of immune-mediated neuropathies. We investigated by immunohistochemistry the expression of a number of integrin subunits during the course of experimental autoimmune neuritis (EAN). Results were compared with the human immune neuropathy Guillain-Barre syndrome (GBS) and extended in vitro. Inflammation and demyelination in both EAN and GBS induced the down-regulation of beta4 integrin in Schwann cells (SCs), whereas loss of alpha2 was noted only in EAN. When axonal loss was present, SCs displayed alpha5 integrin, in both EAN and GBS. In vitro, basal lamina and inflammatory cytokines modulated the expression of beta4 in SCs, but they did not influence alpha2 and alpha5 expression. Finally, integrins were differentially expressed in blood vessels during EAN. In conclusion, the spatiotemporal changes in integrin expression may be used to characterize, stage, and better understand the pathogenesis and evolution of inflammation during GBS and EAN. This may help to establish useful, novel therapy for immune-mediated neuropathies.

Adhesion molecules and inherited diseases of the human nervous system

Mutations in the human genes for the adhesion molecules Po, L1, and merosin cause severe abnormalities in nervous system development. Po and merosin are required for normal myelination in the nervous system, and L1 is essential for development of major axon pathways such as the corticospinal tract and corpus callosum. While mutations that lead to a loss of the adhesive function of these molecules produce severe phenotypes, mutations that disrupt intracellular signals or intracellular interactions are also deleterious. Geneticists have found that more than one clinical syndrome can be caused by mutations in each of these adhesion molecules, confirming that these proteins are multifunctional. This review focuses on identifying common mechanisms by which mutations in adhesion molecules alter neural development.

Plasticity in adult and ageing sympathetic neurons

The nature of neural plasticity and the factors that influence it vary throughout life. Adult neurons undergo extensive and continual adaptation in response to demands that are quite different from those of early development. We review the main influences on the survival, growth and neurotransmitter expression in adult and ageing sympathetic neurons, comparing these influences to those at work in early development. This "developmental" approach is proposed because, despite the contrasting needs of different phases of development, each phase has a profound influence on the mechanisms of plasticity available to its successors. Interactions between neurons and their targets, whether effector cells or other neurons, are vital to all of these aspects of neural plasticity. Sympathetic neurons require access to target-derived diffusible neurotrophic factors such as NGF, NT3 and GDNF, as well as to bound elements of the extracellular matrix such as laminin. These factors probably influence plasticity throughout life. In adult life, and even in old age, sympathetic neurons are relatively resistant to cell death. However, they continue to require target-derived diffusible and bound factors for their maintenance, growth and neurotransmitter expression. Failure to maintain appropriate neuronal function in old age, for example in the breakdown of homeostasis, may result partly from a disturbance of the dynamic, trophic relationship between neurons and their targets. However, there is no clear evidence that this is due to a failure of targets to synthesize neurotrophic factors. On the neural side of the equation, altered responsiveness of sympathetic neurons to neurotrophic factors suggests that expression of the trk and p75 neurotrophin receptors contributes to neuronal survival, maintenance and growth in adulthood and old age. Altered receptor expression may therefore underlie the selective vulnerability of some sympathetic neurons in old age. The role of neural connectivity and activity in the regulation of synthesis of target-derived factors, as well as in neurotransmitter dynamics, is reviewed.

Distribution and function of laminins in the neuromuscular system of developing, adult, and mutant mice

Laminins, heterotrimers of alpha, beta, and gamma chains, are prominent constituents of basal laminae (BLs) throughout the body. Previous studies have shown that laminins affect both myogenesis and synaptogenesis in skeletal muscle. Here we have studied the distribution of the 10 known laminin chains in muscle and peripheral nerve, and assayed the ability of several heterotrimers to affect the outgrowth of motor axons. We show that cultured muscle cells express four different alpha chains (alpha1, alpha2, alpha4, and alpha5), and that developing muscles incorporate all four into BLs. The portion of the muscle's BL that occupies the synaptic cleft contains at least three alpha chains and two beta chains, but each is regulated differently. Initially, the alpha2, alpha4, alpha5, and beta1 chains are present both extrasynaptically and synaptically, whereas beta2 is restricted to synaptic BL from its first appearance. As development proceeds, alpha2 remains broadly distributed, whereas alpha4 and alpha5 are lost from extrasynaptic BL and beta1 from synaptic BL. In adults, alpha4 is restricted to primary synaptic clefts whereas alpha5 is present in both primary and secondary clefts. Thus, adult extrasynaptic BL is rich in laminin 2 (alpha2beta1gamma1), and synaptic BL contains laminins 4 (alpha2beta2gamma1), 9 (alpha4beta2gamma1), and 11 (alpha5beta2gamma1). Likewise, in cultured muscle cells, alpha2 and beta1 are broadly distributed but alpha5 and beta2 are concentrated at acetylcholine receptor-rich "hot spots," even in the absence of nerves. The endoneurial and perineurial BLs of peripheral nerve also contain distinct laminin chains: alpha2, beta1, gamma1, and alpha4, alpha5, beta2, gamma1, respectively. Mutation of the laminin alpha2 or beta2 genes in mice not only leads to loss of the respective chains in both nerve and muscle, but also to coordinate loss and compensatory upregulation of other chains. Notably, loss of beta2 from synaptic BL in beta2(-/-) "knockout" mice is accompanied by loss of alpha5, and decreased levels of alpha2 in dystrophic alpha2(dy/dy) mice are accompanied by compensatory retention of alpha4. Finally, we show that motor axons respond in distinct ways to different laminin heterotrimers: they grow freely between laminin 1 (alpha1beta1gamma1) and laminin 2, fail to cross from laminin 4 to laminin 1, and stop upon contacting laminin 11. The ability of laminin 11 to serve as a stop signal for growing axons explains, in part, axonal behaviors observed at developing and regenerating synapses in vivo.

Characterization of a 30-kDa peripheral nerve glycoprotein that binds laminin and heparin

We have shown previously that a bovine peripheral nerve protein with a molecular mass of about 30 kDa binds laminin in blot overlay assay. In this paper, we have characterized this 30-kDa laminin-binding protein (LBP30). LBP30 was extracted from the crude bovine peripheral nerve membranes at pH 12 or by 0.5 M NaCl but not by 2% Triton X-100. LBP30 bound to heparin-Sepharose in the presence of 0.5 M NaCl. The results of lectin staining indicated that LBP30 contained both terminally sialylated and nonsialylated Ser/Thr-linked oligosaccharides. LBP30 bound laminin-2 as well as laminin-1 but not fibronectin or collagen type IV. When immobilized LBP30 was incubated with the crude peripheral nerve membrane extracts, all of the endogenous peripheral nerve laminin chain isoforms, the alpha1, alpha2, beta1, beta2, and gamma1 chains, were detected bound to LBP30. The binding of LBP30 to laminin was inhibited by heparin, heparan sulfate, dextran sulfate, or NaCl but was not affected significantly by chondroitin sulfate, dextran, or EDTA. Although LBP30 bound to laminin-1 denatured with SDS in a nonreducing condition, the binding was reduced drastically when laminin-1 was denatured with SDS in a reducing condition, suggesting that the binding of LBP30 is somewhat dependent on the high order structure of laminin-1. Immunohistochemical analysis demonstrated the broad distribution of LBP30 in the perineurium and endoneurium of bovine peripheral nerve. These results indicate that LBP30 is a laminin- and heparin-binding glycoprotein localized in the perineurium and endoneurium of bovine peripheral nerve.

Responses of mature and aged sympathetic neurons to laminin and NGF: an in vitro study

Whilst the potent effects of NGF and laminin on developing neurons are well documented, relatively little is known about the effects of, or altered availability of or altered responsiveness to, these substances on the growth of adult neurons. We have therefore examined this question using explant cultures of sympathetic neurons from the superior cervical ganglion (SCG) of mature and aged rats. Explants were grown on substrata containing different doses of laminin, either with or without added NGF in culture medium containing FCS. Individually, laminin and NGF had relatively small effects on neurite outgrowth and length, which tended to be reduced in old neurons. In contrast, laminin in the presence of exogenous NGF exerted a powerful effect on nerve growth which was substantially greater than the sum of the effects of the individual factors. This synergy was evident in all experimental groups and was greatest in old explants at high doses of laminin, where growth was comparable to that of mature neurons. The dose-response curve of old neurons to laminin in the presence of added NGF indicated reduced responsiveness. These results suggest that variations in the availability of laminin and/or exogenous NGF, together with altered patterns of neuronal responsiveness, may contribute to impaired neuronal plasticity in old age.

Division of labor of Schwann cell integrins during migration on peripheral nerve extracellular matrix ligands

Myelination of the peripheral nervous system (PNS) requires the migration of Schwann cells during both development and regeneration. We have characterized the expression pattern of Schwann cell integrins and analyzed their role in migration on different ECM substrates known to be present within the PNS. We found that Schwann cells in cell culture express four beta1 integrins, alpha1 beta1, alpha2 beta1, alpha6 beta1, and another unidentified beta1 integrin, as well as two alpha v integrins, alpha v beta3 and alpha v beta8. Using the Varani migration assay, we found that laminin-1, laminin-2 (merosin), and fibronectin increased Schwann cell migration, while vitronectin and collagen did not increase migration compared to an uncoated plastic substrate. Schwann cell migration on laminin-1 and laminin-2 (merosin) was blocked by antibodies against beta1 integrins, but not affected by RGD peptides or antibodies against beta3 integrins. In contrast, migration on fibronectin was unaffected by antibodies against beta1 and beta3 integrins but was blocked by RGD peptides. This in vitro study shows that there is a division of labor of Schwann cell integrins in the regulation of migration on peripheral nerve ECM components; beta1 integrins mediate migration on laminin-1 and laminin-2 (merosin), while alpha v integrins mediate migration on fibronectin. Taken together, these results suggest that multiple interactions between Schwann cell integrins and ECM within the PNS will contribute to Schwann cell migration during myelination of the PNS.

Neural targeting of Mycobacterium leprae mediated by the G domain of the laminin-alpha2 chain

We report that the molecular basis of the neural tropism of Mycobacterium leprae is attributable to the specific binding of M. leprae to the laminin-alpha2 (LN-alpha2) chain on Schwann cell-axon units. Using recombinant fragments of LN-alpha2 (rLN-alpha2), the M. leprae-binding site was localized to the G domain. rLN-alpha2G mediated M. leprae binding to cell lines and to sciatic nerves of dystrophic dy/dy mice lacking LN-alpha2, but expressing laminin receptors. Anti-beta4 integrin antibody attenuated rLN-alpha2G-mediated M. leprae adherence, suggesting that M. leprae interacts with cells by binding to beta4 integrin via an LN-alpha2G bridge. Our results indicate a novel role for the G domain of LN-2 in infection and reveal a model in which a host-derived bridging molecule determines nerve tropism of a pathogen.

Perineurium talin immunoreactivity decreases in diabetic neuropathy

We studied the immunolocalization of Dp116 (a 116 kDa protein product of the dystrophin gene), vinculin, talin, vimentin, desmin, spectrin and titin in the sural nerve biopsies of 25 patients with peripheral neuropathies of different origin. 4 patients presented with HMSN type 1, 4 with HMSN type 2, 2 with HNPP, 4 with CIDP, 5 with chronic axonal neuropathy of unknown origin, 3 with vasculitic neuropathy, 3 with diabetic neuropathy. Expression and localization of Dp116, vinculin, vimentin, desmin, spectrin and titin did not differ from normal control cases. Spectrin and titin immunoreactivities were absent and desmin was occasionally found in few epineurial vessels. A thin rim of Dp116 binding surrounded the outermost layer of myelin sheaths. Perineurium and epineurial vessels stained deeply for vinculin. Vimentin immunoreactivity was seen in all endoneurial, perineurial and epineurial cells. Immunoreactivity for talin was normally found at endoneurial and epineurial vessel walls, perineurial cells and epineurial fibroblasts in all the sural nerves except diabetic nerves. In the latter, whereas talin binding was normal in the vessel walls and epineurial fibroblasts, it was markedly reduced in the perineurium. On immunoblot, two bands at 235 and 190 kDa were found in the sural nerves with the antibody anti-talin, and both were reduced only in the patients with diabetic neuropathy. We postulate that decreased perineurium talin in diabetic polyneuropathy may be related to the known alterations of the tight junctions of the perineurial cells, which have been proposed to be a contributory factor to impaired permeability barrier properties.

The functions of laminins: lessons from in vivo studies

This series of three short reviews is an attempt to summarize our current knowledge of the in vivo tests of hypotheses of laminin functions. The structures of the laminins have been thoroughly reviewed recently (P. Ekblom and R. Timpl, in press), and I will not attempt to repeat this information here. Instead, I will focus on the recent evidence gathered from gene knock out experiments in mice and from naturally occurring human and mouse gene mutations. The most obvious lesson from the above studies--other than demonstrating the importance of laminins in general--is that the structural diversity of the laminin family members makes highly specialized functions possible. While all laminins may share many functional properties, the individual chains are involved in interactions which cannot be substituted for by other laminins or by other basement membrane components. While this concept is not new, it is very satisfying to see its validity so dramatically confirmed. It is therefore predictable that additional gene ablation experiments using other known and yet undescribed laminin genes will be equally interesting and informative. To me, one of the most striking lessons from these studies is how strongly the induced mouse mutations mimic human disease. With all the concerns with genetic background differences and species specific effects, manipulation of the laminin genes appears to be a particularly good first approach to identifying the causes of human disease. There is an abundant literature accumulated from biochemical and, more recently, molecular structural analyses, and from in vitro systems, suggesting a role of laminins contributing directly to the stability of the basement membrane. There is an equally vast literature supporting an indirect role in mediating cellular behavior, through interactions with various receptors. It is interesting that the in vivo studies summarized above support both activities. In the case of laminin 5 mutations, the phenotypic consequence appears to be due primarily to the loss of an important structural link between the epithelial cytokeratins and the dermal anchoring fibrils. The ultrastructure of the epithelium appears normal, as does the architecture of the papillary dermis. Only the anchoring complex itself is aberrant. The absence of laminin 5 appears not to compromise the development or viability of the epidermis. The basement membrane appears normal-other than the anchoring complex itself. The pathology observed in the newborn is believed to be due to the frictional trauma of birth, with the expectation that the function of the fetal skin is normal in utero. The Herlitz epidermolysis bullosa phenotype is obvious immediately at birth, and it does not progress postnatally beyond the extent to which the affected individual experiences additional frictional trauma or secondary consequences such as infection or fluid loss. Since laminin 5 is only one of a series of structural links within the anchoring complex, one would predict that a loss of any of these links would result in the same phenotype. Current evidence supports this view, as the absence of integrin alpha 6 beta 4 (Vidal et al., 1995; Dowling et al., 1996; Georges-Labouesse et al., 1996; van der Neut et al., 1996) or of collagen VII (A. M. Christiano and J. Uitto, in press) also results in dramatic neonatal dermal-epidermal fragility. The differences in phenotype, such as the pyloric atresia in the case of loss of integrin alpha 6 beta 4, are presumably due to additional functions of the integrin in other tissues or in other developmental processes. Therefore, the laminin 5 mutations may be unique, in that the in vivo studies suggest that the primary role of the molecule is in the elaboration and stability of the anchoring complex, but not in the basement membrane itself. Of course, since the in vivo phenotype reflects only losses that cannot be compensated, this interpretation may be much too narrow. (ABSTRACT TRUNCATED)

Beta 4 integrin and other Schwann cell markers in axonal neuropathy

Schwann cell gene expression is dynamically regulated after peripheral nerve injury and during regeneration. We hypothesized that the changes in protein expression described after rat peripheral nerve injury could be used to identify single Schwann cell-axon units in human axonal neuropathy. Therefore, we performed immuno-fluorescence staining on sections of injured rat sciatic nerves compared with sections of neuropathic human sural nerves. We chose the markers beta 4 integrin, P0 glycoprotein, and glial fibrillary acidic protein (GFAP) to characterize Schwann cells, and neurofilament-heavy (NF-H) to recognize axons. Normal rat or human myelin-forming units demonstrated a sharp ring of beta 4 staining at their outer surface, P0 staining in the myelin sheath, and NF-H staining in the axon. Acutely denervated rat units transited from broken rings of beta 4 and P0 staining, to diffuse beta 4 and absent P0 and NF-H staining. Chronically denervated rat Schwann cells re-expressed beta 4 more highly, but in a diffuse, non-polarized pattern. In contrast, regenerating units re-expressed beta 4, P0, and NF-H; beta 4 staining was polarized to the outer surface of Schwann cells. Finally, GFAP staining increased progressively after injury and decreased during regeneration in the distal nerve stump. In neuropathic human sural nerves, we identified units exhibiting each of these beta 4, P0, and NF-H staining patterns; the proportion of each pattern correlated best with the extent and chronicity of axonal injury. Thus, synchronous injury of rat sciatic nerve predicts patterns of Schwann cell marker expression in human axonal neuropathy. In addition, the unique changes in the polarity of beta 4 integrin expression, in combination with changes in P0 and NF-H expression, may distinguish normal from denervated or reinnervated myelin-forming Schwann cells in human sural nerve biopsies.

Extracellular matrix upregulates synthesis of glucosylceramide-based glycosphingolipids in primary Schwann cells

The formation of basement membrane around Schwann cells that are in contact with axons is necessary for Schwann cell differentiation and myelin formation in the peripheral nervous system. However, primary Schwann cells grown on basement membrane in the absence of neuronal influence show increased proliferation rather than differentiation, which implies that the signals generated by Schwann cell-basement membrane interactions are multipotential. We examined the effect of matrigel, an exogenous basement membrane preparation, and other extracellular matrix growth surfaces on primary Schwann cells to determine if the resulting interactions play a role in the control of glycosphingolipid synthesis. Isolated primary Schwann cells grown on a thin layer of matrigel rapidly adhered to the surface and exhibited a greater degree of cell spreading when compared to cells grown on the nonspecific substrate polylysine. Labeling of the cells with [3H]galactose between 24 and 48 hr after plating revealed that the incorporation of [3H]galactose into glucosylceramide-based glycosphingolipids increased from 1.5-3-fold on matrigel in comparison to cells grown on polylysine. The major labeled glycolipids under both conditions were GM3 ganglioside and two neutral glycolipids that comigrated with GbOse4Cer (GalNAc beta 1-3Gal alpha 1-4Gal beta 1-1Cer) and GbOse5Cer (GalNAc alpha 1-3Gal-NAc beta 1-3Gal alpha 1-4Gal beta 1-4Glc beta 1-1Cer) standards. There was little or no increase in the incorporation of [3H]leucine, [3H]galactose, or [3H]glucosamine into proteins or [3H]palmitic acid into phospholipids, free ceramides, or sphingomyelin, suggesting that the matrigel-induced increase in the synthesis of the glycolipids was selective. In the absence of serum, there was little or no difference in the levels of glycolipid labeling between cells grown on the two substrata, demonstrating that serum factors were required for matrigel to have this effect. When cells were grown on surfaces coated with individual extracellular matrix components, those cells grown on laminin and collagen IV showed an increase in glycolipid labeling similar to that produced by matrigel, while labeling increased to a lesser degree for the other components tested. Thus, the signals generated by interactions between Schwann cells and basement membrane, particularly the laminin and collagen IV constituents, contribute to the regulation of glycolipid synthesis which in turn may affect cell morphology and proliferation.

Structural organization of the human and mouse laminin beta2 chain genes, and alternative splicing at the 5' end of the human transcript

We have determined the structural organization of the human and mouse genes that encode the laminin beta2 chain (s-laminin), an essential component of the basement membranes of the neuromuscular synapse and the kidney glomerulus. The human and mouse genes have a nearly identical exon-intron organization and are the smallest laminin chain genes characterized to date, due to the unusually small size of their introns. The laminin beta2 chain genes of both species consist of 33 exons that span </=12 kilobase pairs of genomic DNA. The exon-intron pattern of the laminin beta2 chain gene is also highly similar to that of the human genes encoding the homologous laminin beta1 and beta3 chains. The putative promoter regions of the human and mouse laminin beta2 chain genes have features characteristic of the promoters of genes that have a limited tissue expression. Considerable conservation of the intron sequences of the mouse and human genes was observed. The first intron of the human gene, located 1 base pair upstream of the translation start codon, contains a non-consensus 5' splice site. This intron was shown to be inefficiently spliced in humans, suggesting that post-transcriptional mechanisms may be involved in the regulation of laminin beta2 chain gene expression.

Peripheral nerve regeneration

Peripheral nerve regeneration comprises the formation of axonal sprouts, their outgrowth as regenerating axons and the reinnervation of original targets. This review focuses on the morphological features of axonal sprouts at the node of Ranvier and their subsequent outgrowth guided by Schwann cells or by Schwann cell basal laminae. Adhesion molecules such as N-CAM, L1 and N-cadherin are involved in the axon-to-axon and axon-to-Schwann cell attachment, and it is suggested that integrins such as alpha 1 beta 1 and alpha 6 beta 1 mediate the attachment between axons and Schwann cell basal laminae. The presence of synaptic vesicle-associated proteins such as synaptophysin, synaptotagmin and synapsin I in the growth cones of regenerating axons indicates the possibility that exocytotic fusion of vesicles with the surface axolemma supplies the membranous components for the extension of regenerating axons. Almost all the subtypes of protein kinase C have been localized in growth cones both in vivo and in vitro. Protein kinase C and GAP-43 are implicated to be involved in at least some part of the adhesion of growth cones to the substrate and their growth activity. The significance of tyrosine kinase in growth cones is emphasized. Tyrosine kinase plays an important role in intracellular signal transduction of the growth of regenerating axons mediated by both nerve trophic factors and adhesion molecules. Growth factors such as NGF, BDNF, CNTF and bFGF are also discussed mainly in terms of the influence of Schwann cells on regenerating axons.

The integrin alpha 6 beta 4 and the biology of carcinoma

The integrin family of adhesion receptors plays a major role in epithelial organization and function. Moreover, the altered expression and function of specific integrins most likely contributes significantly to carcinoma progression. The integrin alpha 6 beta 4, the focus of this review, is a receptor for several members of the laminin family and is preferentially expressed at the basal surface of most epithelia, where it contributes to basement membrane interactions. Mounting evidence suggests that the alpha 6 beta 4 integrin plays a key role in carcinoma cell biology. Several histopathological studies have established a correlation between alpha 6 beta 4 integrin expression and tumor progression. The importance of alpha 6 beta 4 expression in tumors in underscored by the findings that invading fronts of several carcinomas are enriched in the expression of alpha 6 beta 4 integrin ligands, such as laminin-1 and laminin-5. The participation of the alpha 6 beta 4 integrin in invasion is supported further by in vitro functional studies using carcinoma cells that have been transfected with the beta 4 cDNA. The mechanisms by which alpha 6 beta 4 contributes to tumor progression are probably related to its mechanical and signaling properties and are currently under intense study.

Molecular cloning of a novel laminin chain, alpha 5, and widespread expression in adult mouse tissues

We have identified a fifth member of the alpha subfamily of vertebrate laminin chains. Sequence analysis revealed a close relationship of alpha 5 to the only known Drosophila alpha chain, suggesting that the ancestral alpha gene was more similar to alpha 5 than to alpha 1-4. Analysis of RNA expression showed that alpha 5 is widely expressed in adult tissues, with highest levels in lung, heart, and kidney. Our results suggest that alpha 5 may be a major laminin chain of adult basal laminae.

Immuno-electron-microscopic localization of types III pN-collagen and IV collagen, laminin and tenascin in developing and adult human spleen

The distribution of the extracellular matrix proteins types III pN-collagen and IV collagen, laminin and tenascin was investigated in fetal, infant, and adult human spleens by using immuno-electron microscopy. The presence of type III pN-collagen was assessed by using an antibody against the aminoterminal propeptide of type III procollagen. All the proteins other than type III pN-collagen were found in reticular fibers throughout development. In the white pulp of the fetus aged 16 gestational weeks, only an occasional type III pN-collagen-containing fibril was present, although type III pN-collagen was abundant in the reticular fibers of the red pulp. Conversely, in adults, most of the reticular fibers of the white pulp, but not of the red pulp, were immunoreactive for type III pN-collagen. Ring fibers, the basement membranes of venous sinuses, were well developed in both infant and adult spleens. The first signs of their formation could be seen as a discontinuous basement membrane, which was immunoreactive for type IV collagen, laminin, and tenascin in the fetus aged 20 gestational weeks. Intracytoplasmic immunoreactivity for all the proteins studied was visible in the mesenchymal cells of the fetus aged 16 gestational weeks and in the reticular cells of the older fetuses, which also showed labeling for type IV collagen and laminin in the endothelial cells. The results suggest that proteins of the extracellular matrix are produced by these stationary cells.

Composition in situ and in vitro of vascular smooth muscle laminin in the rat

Vascular smooth muscle cells are surrounded by a basal lamina containing an array of macromolecules: included among these are the laminins, a family of oligomeric glycoproteins composed of subunits encoded by different genes. In this study, we have used monoclonal antibodies to several of these subunits, including S-laminin, laminin B2, and laminin B1, to study these proteins in tail artery, superior mesenteric artery, and aorta of rats. In situ, immunostaining for the B2 and S chains was present in the basal lamina, between the smooth muscle cells, throughout the tunica media. In contrast, B1 chain immunostaining was concentrated around cells in the inner media. To investigate whether smooth muscle cells can produce S-laminin, laminin B2, and laminin B1, smooth muscle cells from the superior mesenteric artery were grown in culture and laminin subunit expression determined. In early culture (4 days), immunostaining showed abundant laminin B2 and less B1 synthesis and incorporation into the matrix. Staining for S-laminin was even less intense than for B1 and was localized to areas where cells were densely packed. The same pattern of S-laminin immunostaining was seen during early culture in cells grown on fibronectin, type IV collagen, or gelatin. Immunoblotting detected S-laminin in the conditioned medium from early cultured cells. In later culture (12 days), S-laminin incorporation into the matrix increased markedly compared to incorporation at 4 days. At this time, cells are much more densely packed and multilayered with extensive matrix accumulation. Cyclical stretching of cells in vitro did not increase immunostaining for S-laminin. Together these data show that S-laminin is a component of the arterial media in situ and that in vitro S-laminin is synthesized by smooth muscle cells. Increased incorporation of S-laminin into the matrix in later culture correlates with the presence of a more extensive matrix, suggesting that matrix organization may be critical to S-laminin incorporation.

Cryo-immunogold ultrastructural localization of laminin in adult rat peripheral nerve

Elucidation of the functional roles of the extracellular matrix component laminin in adult peripheral nerve has been hindered by differing accounts of its ultrastructural localization. This is the first report applying the advantages of the cryo-immunogold technique to laminin localization in peripheral nerve. Laminin labelling was found over the basal lamina and possibly over the immediately subjacent Schwann cell plasma membrane, but specific labelling appeared to be absent from other membranes (including those of non-myelinated axon/Schwann cell clusters) and from endoneurial collagen fibrils. It would appear that the functional roles played by laminin in normal adult peripheral nerve are likely to be mediated via its localization in the basal lamina, rather than through a more widespread distribution within the endoneurium.

Collagen IV alpha 3, alpha 4, and alpha 5 chains in rodent basal laminae: sequence, distribution, association with laminins, and developmental switches

Collagen IV is a major component of vertebrate basal laminae (BLs). Studies in humans have revealed a family of genes encoding alpha 1-alpha 6 collagen IV chains and implicated alpha 3-alpha 6 in disease processes (Goodpasture and Alport syndromes and diffuse leiomyomatosis). To extend studies of these components to an experimentally accessible animal, we cloned cDNAs encoding partial collagen alpha 3, alpha 4, and alpha 5(IV) chains from the mouse. Ribonuclease protection assays showed that all three genes were expressed at highest levels in kidney and lung; alpha 5(IV) was also expressed at high levels in heart. We then made antibodies specific for each collagen IV chain. Immunohistochemical studies of several tissues revealed many combinations of collagen IV chains; however, alpha 3 and alpha 4 (IV) were always coexpressed, and only appeared in BLs that were alpha 5(IV) positive. The alpha 3-alpha 5(IV) chains were frequently but not exclusively associated with the S (beta 2) chain of laminin, as were the alpha 1, 2 (IV) collagen chains with laminin B1 (beta 1). An analysis of developing rat kidney BLs showed that newly formed (S-shaped) nephrons harbored collagen alpha 1 and alpha 2(IV) and laminin B1; maturing (capillary loop stage) BLs contained collagen alpha 1-alpha 5(IV) and laminin B1 and S-laminin; and mature glomerular BLs contained mainly collagen alpha 3-alpha 5(IV) and S-laminin. Thus, collagen alpha 1 and alpha 2(IV) and laminin B1 appear to be fetal components of the glomerular BL, and there is a developmental switch to collagen alpha 3-alpha 5(IV) and S-laminin expression.

Anti-beta 1 integrin antibody inhibits Schwann cell myelination

Schwann cells (SCs) co-cultured with sensory neurons require ascorbate supplementation for basal lamina assembly and differentiation into myelinating cells. The ascorbate requirement can be bypassed by adding a purified basal lamina component, laminin, to SC/neuron co-cultures. We have examined the role of laminin receptors, namely, the beta 1 subfamily of integrins, in the process of myelination. We demonstrate by immunostaining or immunoprecipitation that undifferentiated SCs in contact with axons express large amounts of the beta 1 subunit in association with the alpha 1 or alpha 6 subunit. In co-cultures of myelinating SCs, alpha 1 beta 1 is no longer present, alpha 6 beta 1 is still present but at reduced levels, and alpha 6 beta 4 is expressed at much higher levels than in co-cultures of undifferentiated SCs. Immunogold labelling at the electron microscope level suggested that beta 1 integrins are randomly distributed on undifferentiated SCs, become localized to the SC surface contacting basal lamina in differentiating SCs before the onset of myelination, and are not detected on myelinating SCs. Fab fragments of beta 1 function-blocking antibody block both attachment of isolated SCs to laminin and formation of myelin sheaths by SCs co-cultured with neurons in ascorbate-supplemented medium. SCs unable to myelinate in the presence of the anti-beta 1 antibody assemble patchy basal lamina that is only loosely attached to the cell surface and in some cases appears to be detaching from the membrane. In contrast, an alpha 1 beta 1 function-blocking antibody only partially blocks attachment of isolated SCs to laminin but has no inhibitory effect on SC myelination. These results are consistent with the hypothesis that a member of the beta 1 subfamily of integrins other than alpha 1 beta 1 binds laminin present in basal lamina to the SC surface and transduces signals that are critical for initiation of SC differentiation into a myelinating cell.

Dystroglycan is a binding protein of laminin and merosin in peripheral nerve

alpha-Dystroglycan, a 156 kDa dystrophin-associated glycoprotein, binds laminin in skeletal muscle. Here we demonstrate that alpha-dystroglycan is a binding protein of laminin (A/B1/B2) and merosin (M/B1/B2) in peripheral nerve. Immunocytochemical analysis demonstrates the localization of alpha-dystroglycan and merosin surrounding myelin sheath of peripheral nerve fibers. Biochemical analysis demonstrates that the 120 kDa peripheral nerve alpha-dystroglycan binds merosin as well as laminin. The binding of laminin and merosin is Ca2+ dependent and is inhibited by NaCl and heparin. Recently, merosin was shown to be deficient in the peripheral nerve of dy mice which have defects in myelination. The interaction between alpha-dystroglycan and merosin may play a role in the regulation of Schwann cell myelination and/or maintenance of myelin sheath.

Beta 4 integrin expression in myelinating Schwann cells is polarized, developmentally regulated and axonally dependent

In developing and regenerating peripheral nerve, Schwann cells interact with axons and extracellular matrix in order to ensheath and myelinate axons. Both of these interactions are likely to be mediated by adhesion molecules, including integrins, which mediate cell-cell and cell-extracellular matrix interactions. Recently, the beta 4 integrin subunit was reported to be expressed by Schwann cells in peripheral nerve. We have examined the expression of beta 4, beta 1 and their common heterodimeric partner, the alpha 6 integrin subunit, in developing and regenerating rat peripheral nerve. beta 4 and alpha 6 are enriched in peripheral nerve and they co-localize at the abaxonal surface of myelinating Schwann cells, opposite the Schwann cell basal lamina, which contains possible ligands of alpha 6 beta 4. In contrast, beta 4 and alpha 6 are expressed in a different pattern in non-myelinating Schwann cells. The level of beta 4, but not alpha 6 or beta 1 mRNAs, increases progressively in developing nerves, reaching a peak in adult nerves well after the peak of the myelin-specific mRNAs. After axotomy, the expression of beta 4 mRNA and protein, but not alpha 6 or beta 1 mRNAs, fall rapidly but subsequently are reinduced by regenerating axons. Similarly, in cultured Schwann cells, the expression of beta 4 mRNA, but not alpha 6 mRNA, is significantly modulated by forskolin, a drug that elevates cAMP and mimics some of the effects of axonal contact. beta 4 integrin expression in Schwann cells, therefore, is regulated by Schwann cell-axon interactions, which are known to be critical in determining the Schwann cell phenotype. Furthermore, the polarized expression of alpha 6 beta 4 to the abaxonal surface of myelinating Schwann cells suggests that alpha 6 beta 4 may mediate in part the morphological changes required of Schwann cells in the process of myelination in the peripheral nervous system.

Expression of the integrin alpha 6 beta 4 in peripheral nerves: localization in Schwann and perineural cells and different variants of the beta 4 subunit

Integrin alpha 6 beta 4 is expressed in human peripheral nerves, but not in the central nervous system. This integrin heterodimer has previously been found in perineural fibroblast-like cells and in Schwann cells (SCs), which both assemble a basement membrane but do not form hemidesmosomes. We show here that in SCs, which had formed a myelin sheath, alpha 6 beta 4 was enriched in the proximity of the nucleus, at Ranvier paranodal areas and at Schmitt-Lanterman clefts; alpha 6 beta 4 was also found at the grooved interface between small axons and non-myelinating SCs. Immunoprecipitation of human peripheral nerves, in combination with Western blotting showed that beta 4 is associated with the alpha 6A subunit. Northern blot analysis of human peripheral nerves showed a single beta 4 transcript of 6 kb. Using the reverse transcriptase polymerase chain reaction, we detected two mRNA species, one for the most common (−70, -53) form of beta 4 and the other encoding the (+53) variant of beta 4. Cultured SCs were devoid of alpha 6 beta 4 but expressed alpha 6 beta 1, indicating that SCs lose beta 4 expression when contact with neurons is lost. Thus, resting SCs in contact with axons express alpha 6A in combination with beta 4, irrespective of myelin formation. We suggest that alpha 6 beta 4 expressed in SCs plays a role in peripheral neurogenesis.

Axonal regulation of Schwann cell integrin expression suggests a role for alpha 6 beta 4 in myelination

Ensheathment and myelination of axons by Schwann cells in the peripheral nervous system requires contact with a basal lamina. The molecular mechanism(s) by which the basal lamina promotes myelination is not known but is likely to reflect the activity of integrins expressed by Schwann cells. To initiate studies on the role of integrins during myelination, we characterized the expression of two integrin subunits, beta 1 and beta 4, in an in vitro myelination system and compared their expression to that of the glial adhesion molecule, the myelin-associated glycoprotein (MAG). In the absence of neurons, Schwann cells express significant levels of beta 1 but virtually no beta 4 or MAG. When Schwann cells are cocultured with dorsal root ganglia neurons under conditions promoting myelination, expression of beta 4 and MAG increased dramatically in myelinating cells, whereas beta 1 levels remained essentially unchanged. (In general agreement with these findings, during peripheral nerve development in vivo, beta 4 levels also increase during the period of myelination in sharp contrast to beta 1 levels which show a striking decrease.) In cocultures of neurons and Schwann cells, beta 4 and MAG appear to colocalize in nascent myelin sheaths but have distinct distributions in mature sheaths, with beta 4 concentrated in the outer plasma membrane of the Schwann cell and MAG localized to the inner (periaxonal) membrane. Surprisingly, beta 4 is also present at high levels with MAG in Schmidt-Lanterman incisures. Immunoprecipitation studies demonstrated that primary Schwann cells express beta 1 in association with the alpha 1 and alpha 6 subunits, while myelinating Schwann cells express alpha 6 beta 4 and possibly alpha 1 beta 1. beta 4 is also downregulated during Wallerian degeneration in vitro, indicating that its expression requires continuous Schwann cell contact with the axon. These results indicate that axonal contact induces the expression of beta 4 during Schwann cell myelination and suggest that alpha 6 beta 4 is an important mediator of the interactions of myelinating Schwann cells with the basal lamina.