Doublecortin mutation leads to persistent defects in the Golgi apparatus and mitochondria in adult hippocampal pyramidal cells

Human doublecortin (DCX) mutations are associated with severe brain malformations leading to aberrant neuron positioning (heterotopia), intellectual disability and epilepsy. DCX is a microtubule-associated protein which plays a key role during neurodevelopment in neuronal migration and differentiation. Dcx knockout (KO) mice show disorganized hippocampal pyramidal neurons. The CA2/CA3 pyramidal cell layer is present as two abnormal layers and disorganized CA3 KO pyramidal neurons are also more excitable than wild-type (WT) cells. To further identify abnormalities, we characterized Dcx KO hippocampal neurons at subcellular, molecular and ultrastructural levels. Severe defects were observed in mitochondria, affecting number and distribution. Also, the Golgi apparatus was visibly abnormal, increased in volume and abnormally organized. Transcriptome analyses from laser microdissected hippocampal tissue at postnatal day 60 (P60) highlighted organelle abnormalities. Ultrastructural studies of CA3 cells performed in P60 (young adult) and > 9 months (mature) tissue showed that organelle defects are persistent throughout life. Locomotor activity and fear memory of young and mature adults were also abnormal: Dcx KO mice consistently performed less well than WT littermates, with defects becoming more severe with age. Thus, we show that disruption of a neurodevelopmentally-regulated gene can lead to permanent organelle anomalies contributing to abnormal adult behavior.

[Axon and Schwann cells... so far away, so close]

Myelination was a major step in the evolution of the nervous system. Appearing first in jaw fish, myelination allows the fast and secure propagation of action potentials at a low energetic cost, and without exaggerated increase in axonal diameter. In the peripheral nervous system of mammals, myelination results from the tight interactions between Schwann cells and axons, leading to the formation of highly differentiated domains along the axon. The molecular determinants of these interactions are starting to be well identified. Their understanding provides a precise framework to interpret the defects, which occur in pathological circumstances. This review summarizes the present state of knowledge concerning axoglial interactions in peripheral nerves.

Abnormal reinnervation of skeletal muscle in a tenascin-C-deficient mouse

The possible involvement of tenascin-C in the reinnervation of a skeletal muscle was investigated in the tenascin-C-deficient mouse (T-/-) produced by Saga et al. (1992; Genes Dev 6:1821-1831). The pattern of reinnervation, observed after denervation of the triangularis sterni muscle, differs in T-/- and wild-type muscles in several traits. Axonal growth and stability of terminal arbors are impaired in the T-/- muscle: Some axons in mutant muscles grow beyond their original targets and reinnervate other synaptic sites, which may become dually innervated. In contrast to wild type, polyinnervation increases with time after denervation in T-/- muscles and is still present 7 months after nerve crush. The expression of a tenascin-C mRNA product disappears between 1 and 2 months after nerve crush. Of interest is that this transcriptional regulation in T-/- muscles occurs when major alterations in the morphology of regenerating endings become obvious. These observations strongly implicate tenascin-C in the formation, maturation, and stabilization of the neuromuscular junction.

Modulation of HSP25 expression during anterior horn motor neuron degeneration in the paralysé mouse mutant

The paralysé spontaneous mutation in mice involves degeneration and death of anterior horn motor neurons. Mutant mice are not viable past postnatal day 16. At present, the mechanisms involved in motor neuron death are unknown. Here, we investigate the expression of the small heat shock protein Hsp25, in the spinal cord of paralysé at two different stages during postnatal development, i.e., day 11 and day 14. Western blot analysis reveals that the level of Hsp25 was strikingly different in paralysé as compared to control littermates. Hsp25 expression level in paralysé at day 11 was much lower than in control mice. At day 14, an opposite pattern was observed. Such pattern seems to be restricted to spinal cord, since level of Hsp25 in other tissues (lung, brain, liver, and heart) was quite similar. Immunofluorescence examination of the lumbar spinal cord sections reveals that in control mice, Hsp25 was expressed at high level in motor neurons located in the ventral horn at both day 11 and day 14. By contrast, in paralysé mice, Hsp25 staining within the motor neurons was barely detectable except as a spot in the nucleolus (day 11). At the end stage of the disease (day 14), not only was Hsp25 staining even less intense in motor neurons, but also a strong Hsp25 staining was observed in reactive astrocytes within the gray matter. Taken together, these data suggest that Hsp25 expression is differently modulated in neuronal and glial cells during neurodegenerative processes leading to motor neuron death.

Peptide secretion in the cutaneous glands of South American tree frog Phyllomedusa bicolor: an ultrastructural study

The development of the dermal glands of the arboreal frog Phyllomedusa bicolor was investigated by immunocytochemistry and electron microscopy. The 3 types of glands (mucous, lipid and serous) differed in size and secretory activity. The mucous and serous glands were apparent in the tadpole skin, whereas the lipid glands developed later in ontogenesis. The peptide antibiotics dermaseptins and the D-amino acid-containing peptide opioids dermorphins and deltorphins are abundant in the skin secretions of P. bicolor. Although these peptides differ in their structure and activity they are derived from precursors that have very similar preproregions. We used an antibody to the common preproregion of preprodermaseptins and preprodeltorphins and immunofluorescence analysis to show that only the serous glands are specifically involved in the biosynthesis and secretion of dermaseptins and deltorphins. Scanning and transmission electron microscopy revealed that the serous glands of P bicolor have morphological features, especially the secretory granules, which differ from those of the glands in Xenopus laevis skin.

Myoneurin, a novel member of the BTB/POZ-zinc finger family highly expressed in human muscle

Initially characterized as Drosophila developmental regulators, the BTB/POZ and zinc finger proteins (BTB/POZ-ZF) constitute a growing family of proteins with gene expression regulatory functions since they have been shown to be involved in both transcriptional activation and repression of various genes in a broad range of species, including mammals. Here we report the cloning of a novel human transcript, coding for a 68-kDa deduced BTB/POZ-ZF protein. This molecule, called myoneurin on the basis of its prevalent expression in the neuromuscular system, contains an amino-terminal BTB/POZ domain and eight tandemly repeated zinc-finger motifs of the C(2)H(2) type. The murine myoneurin, identified in the mouse embryo, is highly homologous to the human protein.

Regulated expression of the proteoglycan SPOCK in the neuromuscular system

SPOCK is prevalent in developing synaptic fields of the central nervous system (Charbonnier et al., 2000. Mech. Dev. 90, 317-321). The expression of SPOCK during neuromuscular junction (NMJ) formation was compared to agrin and acetylcholine receptor (AChR) distribution. SPOCK is detected within the myogenic masses during the early steps of embryonic development, and distributed in the cytoplasm of myotubes before coclustering with AChRs. In the adult, SPOCK is present in axons and is highly expressed by Schwann cells. SPOCK altered expression pattern after nerve lesioning, or cholinergic transmission blockade, strongly indicate that its cellular distribution at the NMJ depends on innervation.

Nuclear targeting defect of SMN lacking the C-terminus in a mouse model of spinal muscular atrophy

Deletion of the murine survival of motor neuron gene (SMN) exon 7, the most frequent mutation found in spinal muscular atrophy (SMA) patients, directed to neurons but not to skeletal muscle, enabled generation of a mouse model of SMA providing evidence that motor neurons are the primary target of the gene defect. Moreover, the mutated SMN protein (SMNDeltaC15) is dramatically reduced in the motor neuron nuclei and causes a lack of gems associated with large aggregates of coilin, a coiled-body-specific protein. These results identify the lack of the nuclear targeting of SMN as the biochemical defect in SMA.

New perspectives in multiple sclerosis: retroviral involvement and glial cell death

Retroviral involvement in the pathogenic cascade in multiple sclerosis (MS) and a cytotoxic activity with narrow specificity towards glial cells have been recently considered as credible working hypotheses to explain some of the complex pathophysiological and neuropathological features of MS. The partial characterization of exogenous retroviral sequences, thought to be associated with MS, has led us to the identification of new human endogenous retroviruses closely related to the extracellular multiple sclerosis associated retrovirus (MSRV). These endogenous retroviruses (HERV-TcR and HERV-7q) have the potential to be transcribed into RNA and proteins. Interestingly, the env domain of HERV-7q could code for a 59.8 kDa secreted glycoprotein (called enverin) with an immunoregulatory region. The presence in various MS biological fluids of a cytotoxic activity able to induce programmed cell death for oligodendrocytes and astrocytes suggests the possibility of a demyelination phenomenon as part of direct glial cell damage. Moreover, both retroviral expression and cytotoxic factor production have been evidenced in MS monocyte/macrophage cultures and MS cerebrospinal fluid. It is now crucial to better characterize the endo/exo retroviruses possibly involved in MS and their pathogenic potential, and to identify the contributing factor(s) to the gliotoxicity found in the MS cerebrospinal fluid or serum, as well as to elucidate the mechanism of induction of the observed programmed glial cell death.

Expression of the proteoglycan SPOCK during mouse embryo development

SPOCK is a modular proteoglycan, with homology with proteins involved in cell adhesion processes and neurogenesis. We have previously shown that SPOCK transcripts predominate in the adult mouse brain. Here, we report its expression during mouse embryonic development by in situ hybridization, and immunocytochemistry. SPOCK is actively expressed at the onset of neurogenesis during periods of neuron migration and axonal outgrowth. At a later developmental stage, its expression is particularly prevalent within developing synaptic fields. In the peripheral nervous system, SPOCK expression is also developmentally regulated particularly in dorsal root ganglion neurons.

Overexpression of the stathmin gene in a subset of human breast cancer

Stathmin is a highly conserved cytosolic phosphoprotein that destabilizes microtubules. Stathmin, which has been proposed as a relay protein integrating diverse cell signalling pathways, acts in vitro as a tubulin-sequestering protein, and its activity is dramatically reduced by phosphorylation. Interestingly, stathmin expression and phosphorylation are regulated during the control of cell growth and differentiation, and there is much evidence suggesting that in vivo stathmin plays a role in the control of microtubule dynamics during mitosis. Stathmin may thus be considered as one of the key regulators of cell division. We examined 50 human primary breast tumours for stathmin mRNA and protein expression and screened for abnormalities in the chromosome region harbouring the stathmin gene. Overexpression of stathmin was found in 15 tumours (30%). At the present stage, no clear correlation emerged between stathmin expression and several prognosis markers. Interestingly, perfect matching was observed between stathmin mRNA overexpression, protein overexpression and strong staining for stathmin on paraffin-embedded tumour sections when specimens were available. Furthermore, a tentative link between loss of heterozygosity (LOH) in the 1p32-1pter region and stathmin overexpression was observed. Our results suggest that stathmin might play a role in breast carcinogenesis and that stathmin-overexpressing tumours may represent a new subtype of breast cancer.

The peripheral nerve and the neuromuscular junction are affected in the tenascin-C-deficient mouse

A thorough examination of the structure and plasticity of the neuromuscular system was performed in tenascin-C mutant mice deficient in tenascin-C. The study of the peripheral nerve revealed a number of abnormal features. In the motor nerve, numerous unmyelinated and myelinated fibers with degraded myelin were present. Schwann cell processes often enclosed degenerative terminals. Transgene (beta-galactosidase) expression analyzed at the ultrastructural level was found to be unequally distributed in the mutant's neuromuscular tissues. At the NMJ, preterminal disorganization was prevalent. Some axon terminals exhibited abnormal overgrowth. A surprising lack of beta-galactosidase expression at some cellular sites known to possess tenascin-C in wild type mice correlated best with marked changes in the cytoarchitecture of the peripheral nerve and NMJ. In some other -but not all- cellular sites which normally express the molecule, immunofluorescence analysis suggested the presence of significant but low levels of tenascin-C-like immunoreactivity together with beta-galactosidase expression. Messenger RNA detection by RT-PCR confirmed the presence of low amounts of tenascin-C mRNA in skeletal muscle suggesting that the mice deficient in tenascin-C are not complete knock-outs of this gene, but low-expression mutants. Following in vivo injections of botulinum type-A toxin, we observed a greatly reduced sprouting response of the motor nerves in tenascin-C mutant mice. We also observed that N-CAM and beta-catenin were overexpressed in the mutant. Our results suggest that tenascin-C is involved both in stabilization and in plasticity of the NMJ.

Distinct location and prevalence of alpha-, beta-catenins and gamma-catenin/plakoglobin in developing and denervated skeletal muscle

We studied the distribution of alpha-catenin, beta-catenin and gamma-catenin/plakoglobin in developing, adult and denervated mouse skeletal muscle. During primary myogenesis, all three catenins present a subsarcolemmal distribution within primary myotubes. During secondary myogenesis they accumulate at myotube-myotube contacts. In contrast to the other catenins, gamma-catenin is strongly expressed in the sarcoplasm. In adult muscle, all three catenins are localized on the presynaptic elements of the neuromuscular junction. In denervated muscles, alpha- and beta-catenins are upregulated like N- and M-cadherin, while the levels of gamma-catenin/plakoglobin remain unchanged. The developmental changes in localization and regulation of alpha- and beta-catenins in muscle compared to gamma-catenin/plakoglobin are suggestive of a privileged association of alpha- and beta-catenins with N- and M-cadherins, while gamma-catenin/plakoglobin appears to be expressed quite independently and must assume a different role during myogenesis.

A gliotoxic factor and multiple sclerosis

The pathogenesis of multiple sclerosis (MS) is unknown. Searching for possible toxic factors, it was found that 3-day exposure to heat-treated cerebrospinal fluid (CSF) from MS patients caused apoptotic death of astrocytes and oligodendrocytes, but not fibroblasts, myoblasts, Schwann cells, endothelial cells and neurons, in vitro. CSFs from other inflammatory or non-inflammatory neurological diseases showed no toxicity. Exposure of these glial cells to partially purified MS CSF produced DNA fragmentation, apoptotic bodies, chromatin condensation, cell shrinkage, and changes in the levels of known cytokines. A cytotoxic factor, called gliotoxin, was characterized chromatographically as a stable 17-kDa glycoprotein. Since this protein is highly cytotoxic for astrocytes and oligodendrocytes, it may represent an initial pathogenic factor, leading to the neuropathological features of MS, such as blood-brain barrier involvement and demyelination.

Localized deposition of M-cadherin in the glomeruli of the granular layer during the postnatal development of mouse cerebellum

M-cadherin is a Ca2+-dependent cell adhesion molecule of the cadherin family, initially localized at the areas of contact between myotubes during myogenesis, but also detected in the peripheral nerve and at the adult neuromuscular junction. In this study, searching for the expression of M-cadherin in the adult mouse brain, we observed a restricted expression of M-cadherin in one of the three layers of the cerebellar cortex: the granular layer. M-cadherin was accumulated in structures rich in synapses and other intercellular junctions where mossy fibers connect granule cell dendrites, the glomeruli. This molecule was not expressed in the cerebellum during the first steps of postnatal cerebellar neurogenesis: granule cell proliferation and migration and Purkinje cell alignment. M-cadherin expression was first detected at postnatal day (P) 11, after the establishment of the synaptic connections between mossy fibers and granule cell dendrites. It then accumulated in glomeruli during their phase of maturation which is characterized by the formation of puncta adherentia between granule cell dendrites. M-cadherin was undetectable in the cerebella of the weaver and staggerer mutants, lacking granule cells, and therefore mature glomeruli and puncta adherentia. Furthermore, other components classically associated with intercellular junctions, i.e., alpha-caterin, beta-catenin and actin filaments, closely paralleled M-cadherin appearance and colocalized with M-cadherin in the mature glomeruli. M-cadherin, which appears as a molecular marker of glomerulus maturation, might be implicated in the formation, and be the ligand, of adherens junctions encountered in this structure.

M-cadherin distribution in the mouse adult neuromuscular system suggests a role in muscle innervation

M-cadherin belongs to the Ca(2+)-dependent cadherin family of cell adhesion molecules and was first isolated from a mouse muscle cell line cDNA library. It is specifically expressed in muscle tissue during development and is supposed to play an important role in secondary myogenesis. In the present study the expression of M-cadherin mRNA and protein and its localization were investigated in adult mouse skeletal muscle and peripheral nerve. The mRNA was abundant in embryonic legs from embryonic day (E)14 to E18. It remained expressed in new-born and adult muscles. In the adult muscle M-cadherin immunoreactivity was only detected at the neuromuscular junction, associated with perijunctional mononucleated cells and on intramuscular nerves. Peripheral nerves were also M-cadherin-positive. The molecule was found at the surface of myelinated nerve fibres where it was concentrated at the node of Ranvier. When a nerve was crushed and allowed to regenerate, M-cadherin was over-expressed at the site of nerve injury and in the distal stump. M-cadherin was also upregulated on the sarcolemma of denervated muscle fibres. Taken together, these observations point toward a much wider tissue distribution of M-cadherin than previously thought. M-cadherin might be involved not only in specific steps of myogenesis but also in some aspects of synaptogenesis, axon/Schwann cell interactions and node of Ranvier structural maintenance.

[Gliotoxic factor and multiple sclerosis]

Multiple sclerosis in a disease of the central nervous system characterized by perivascular and periventricular lesions of the myelin and immune cell infiltrates and increased permeability of the blood-brain barrier. We have found a cytotoxic factor of the cerebrospinal fluid (CSF) specific for multiple sclerosis patients which has 2 main characteristic effects in vitro on primary or immortalized astrocyte cultures: (1) disruption of the gliofilament network of the cells; and (2) apoptotic cell death induction. Moreover, in vivo, intraventricular injections of minute amounts of partially purified gliotoxic factor in adult rats have striking effects on both the morphology and general organization of astrocytes in the entire brain and the permeability characteristics of the blood brain barrier, which becomes leaky to immunoglobulins. These pathological effects are strongly similar to some of the neuropathological findings reported during the course of MS--They suggest an entirely new hypothesis to explain the active stage of the disease: the presence of a new factor of unknown extrinsic (viral) or intrinsic (cellular) origin, able to disorganize the glial cytoskeleton and glial cell differentiation. This factor is then able to provoke glial cell death. Such glial cell death may result in both demyelination and increased blood brain barrier permeability. Both in vitro and in vivo studies strongly support the idea that this gliotoxic factor plays a central role in the pathogenesis of MS, making its full identification a critical theme for MS research.

Expression and localization of RAP1 proteins during myogenic differentiation

The RAP1 subfamily of small GTPases has been involved in various differentiation programs. In skeletal muscle, several lines of evidence suggest that various small GTPases could be implicated in muscle development. This raised the question of whether the RAP1 proteins (RAP1A and/or RAP1B) could be involved in myogenesis. In the present study, we report on the regulation of RAP1 transcripts and proteins during myogenic differentiation. Northern blot analysis performed with differentiated and undifferentiated C2 myogenic cells pointed out that both genes undergo specific regulation during myogenesis in vitro since differentiation of C2 cells was accompanied by a down-regulation of RAP1B gene transcription and continuous expression of the RAP1A mRNA. In addition, immunofluorescence experiments revealed the accumulation of the RAP1 proteins in differentiated C2 cells and in primary culture of mouse myotubes. Investigation of the intracellular location of RAP1 proteins in undifferentiated and differentiated C2 cells showed that the proteins were associated with the late endocytic compartments. To verify that the build-up of RAP1 proteins had a relevance for developmental mechanisms in vivo, we studied their expression and localization at different stages of skeletal muscle development. We found that RAP1 proteins accumulated in specialized muscle cell domains undergoing important modifications during early and late myogenesis: these were the neuromuscular and myotendinous junctions, respectively. Altogether, our data indicate that RAP1 proteins are regulated during myogenic differentiation.

Identification and molecular characterization of Unc-33-like phosphoprotein (Ulip), a putative mammalian homolog of the axonal guidance-associated unc-33 gene product

The control of neuritic extension and guidance is critical for the development, maturation, and regeneration of functional neuronal circuits. We identified a neuronal 64-85 kDa phosphoprotein, the expression of which in mouse brain is regulated during development, reaching a peak at approximately 5 d postnatal, when maturation of neurons and synaptic connections is highly active. The amino acid sequence of the mouse protein deduced from its cloned cDNA reveals similarities with that of the neuritic outgrowth- and guidance-related product of the unc-33 gene in Caenorhabditis elegans. The regulation of its phosphorylation in response to nerve growth factor, as well as its localization in neurites and growth cones and at the neuromuscular junction, further indicates that Ulip (for Unc-33-like phosphoprotein) is not only a structural but likely is also a functional mammalian homolog of Unc-33, potentially involved in the control of neuritic outgrowth and axonal guidance.

Targeting widespread sites of damage in dystrophic muscle: engrafted macrophages as potential shuttles

Inherited muscle diseases are characterized by widespread muscle damage in the body. This limits the clinical relevance of cell or gene therapy based upon direct injections into muscles. One way to circumvent this obstacle would be to use circulating cells, capable of homing naturally to the sites of lesion, to deliver therapeutic substances. Certain muscular dystrophies present successive cycles of degeneration-regeneration. These sporadic necrotic lesions trigger local inflammations with subsequent infiltration of blood-borne mononuclear cells. We have, therefore, tested the possibility that homing monocytes and macrophages could be appropriate shuttles for delivering a therapeutic agent to disseminated pathogenic sites, their targeting being triggered by the pathogeny itself. First, fluorescently labeled immortalized monocytes were intravenously injected into mice which had previously undergone freeze-damaging of individual muscles. In agreement with our hypothesis, intense labelling was observed in the muscle, specifically in damaged regions. Second, the technique was adapted to meet the needs of chronic diseases with characteristic continuous, widespread degeneration of muscle fibers, by creating a reservoir of genetically engineered monocytes, via bone marrow transplantation. Mdx mice received bone marrow from transgenic mice expressing the lacZ reporter gene, under the control of the vimentin promoter, which is active in monocytes and macrophages. Histological and molecular analyses demonstrated the homing of engineered macrophages at the sites of muscle damage, for periods as long as 2 months. Bone marrow progenitor cells, appropriately engineered to elicit the synthesis, in macrophages, of therapeutically relevant substances, may be of clinical value in various pathologies involving an inflammatory phase.

M-cadherin localization in developing adult and regenerating mouse skeletal muscle: possible involvement in secondary myogenesis

In this work, we investigated the distribution of the Ca(2+)-dependent cell adhesion molecule, M-cadherin, in mouse limb muscle during normal development and regeneration. Using two unrelated anti-M-cadherin peptide antibodies, we found scarce M-cadherin immunostaining during primary myogenesis (E12-E14) with no accumulation at areas of cell-cell contact. In contrast, the staining sharply increased in intensity at E16, remained high during secondary myogenesis (E16-P0) but disappeared soon after birth. During secondary myogenesis, M-cadherin was specifically accumulated at the characteristic sites of insertion of secondary myotubes in neighbouring primary myotubes. M-cadherin was also accumulated at the areas of contact between fusing secondary myoblasts and myotubes in vitro. In the adult normal and regenerating muscle, we did not detect M-cadherin accumulations at the surface of myofibres. All together, these observations suggest that M-cadherin is specifically involved in secondary myogenesis.

[Cell adhesion and development of skeletal muscle]

Cell adhesion is a cell autonomous property of pluricellular organisms at the basis of tissues and organs formation. Thus, adhesive processes must be considered as key features of the development of skeletal muscle as well as of other tissues. We present here the actual knowledge on cell adhesion molecules in skeletal muscle morphogenesis. The spatio-temporal expression patterns of N-CAM, N-cadherin, M-cadherin, VLA-4 and VCAM-1 during chicken and mouse myogenesis suggest that these cell adhesion molecules are differentially involved in myoblast-myoblast, myoblast-myotube and myotube-myotube interactions. These molecules link myogenic cells before they are separated by their basal laminae. They can potentially induce preferential cell adhesion and sorting-out as it has been described by Holtfreter. This differential adhesion may lead either to myoblast fusion or to preferential association between primary and secondary myotubes.

N-cadherin expression in developing, adult and denervated chicken neuromuscular system: accumulations at both the neuromuscular junction and the node of Ranvier

N-cadherin, a member of the Ca(2+)-dependent cell adhesion molecule family plays essential roles in morphogenesis and histogenesis. N-cadherin has been shown in vitro to promote myoblast fusion and neurite outgrowth. We report here the cellular localization of N-cadherin during development and regeneration of the chick neuromuscular system. N-cadherin was uniformly expressed along the surface of myoblasts and myotubes of E6 limb muscles. Later, as synaptogenesis and secondary myogenesis proceeded, N-cadherin expression was down-regulated and restricted to some large-diameter fibres, then to the areas of contact between few myofibres and subsequently disappeared by embryonic day 17, suggesting that this cadherin may be implicated predominantly in fusion of primary myoblasts and, at lower degree, of secondary myoblasts. The presence of N-cadherin in muscle during the period of nerve trunk ingrowth and its down-regulation after synaptogenesis suggests that this molecule might be implicated in both processes. N-cadherin became accumulated at the neuromuscular junction only a few days after the first synaptic contacts were established and remained at the adult neuromuscular junction, suggesting a role of this molecule in the stabilization of the mature neuromuscular junction. In sciatic nerve, the level of N-cadherin expression remained unchanged from hatching to adult life. N-cadherin was widely distributed on the surface of myelinated fibres and on myelinating Schwann cells: in addition, it was concentrated at the node of Ranvier. At the ultrastructural level, the molecule was detected inside, at the surface and in the basal lamina of Schwann cells and also associated with endoneurial collagen. These observations suggest a role of N-cadherin in the structuring and stabilization of the myelin sheaths. After nerve injury, N-cadherin continued to be expressed by proliferating Schwann cells in the distal stump providing a substratum for regenerating axons. N-cadherin reappeared at the surface of denervated muscle fibres without disappearing from the former synaptic sites. It was detected not only in the sarcoplasm and on sarcolemma of denervated muscle fibres, but also in the basal lamina and in the extracellular matrix. The reexpression of N-cadherin at the surface of denervated muscle fibres suggests a role for this molecule in muscle reinnervation. The presence of N-cadherin in basal lamina and its association with collagen fibres raise questions about the release of N-cadherin in the extracellular space and the existence of a putative heterophilic ligand for N-cadherin.

N-cadherin and N-CAM-mediated adhesion in development and regeneration of skeletal muscle

In this review, the experimental evidence supporting the fact that the cell adhesion molecules N-CAM and N-cadherin are involved in myogenesis has been surveyed. In order to give access to the function of these molecules, a strategy of in vivo localization and in vitro perturbation of their adhesive function by interfering antibodies and peptides was applied. Both molecules are expressed at the surface of myogenic cells during myogenesis in vivo and in vitro. The blockade of the N-CAM adhesion function leads to a mild reduction of the rate of myoblast fusion, while the inhibition of the N-cadherin function induces a drastic inhibition of fusion suggesting that N-cadherin-mediated adhesion is a critical step in the process of myoblast fusion. Both molecules are re-expressed during muscle regeneration suggesting that adult myogenesis is under the control of the same adhesive systems as embryonic and foetal myogenesis.

Class II MHC antigens in normal human skeletal muscle

Class II MHC antigen expression has been investigated in muscle tissue and cultured cells from normal human skeletal muscle by light and electron immunocytochemistry. In muscle tissue, these antigens were detected in satellite cells, interstitial cells, and blood vessels. In cultures, muscle cells were stained with a pan-reactive anti-HLA class II antibody and with isotypes specific for DP, DQ, and DR. The staining was present on mononucleated cells and persisted on myotubes; it was stronger for DR and DQ isotypes than for DP. At the subcellular level, staining was located not only at the cell surface, but also next to the endoplasmic reticulum and in the cytosol. Thus, myosatellite cells and aneurally cultured cells from human normal skeletal muscle express class II MHC antigens. Moreover, the myotube staining and the presence of gold particles inside the cells suggested synthesis of these antigens after myoblast fusion.