Altered expression of neuronal cell adhesion molecules induced by nerve injury and repair

MyoD and myogenin protein expression in skeletal muscles of senile rats

We analyzed the level of protein expression of two myogenic regulatory factors (MRFs), MyoD and myogenin, in senile skeletal muscles and determined the cellular source of their production in young adult (4 months old), old (24, 26, and 28 months old), and senile (32 months old) male rats. Immunoblotting demonstrated levels of myogenin approximately 3.2, approximately 4.0, and approximately 5.5 times higher in gastrocnemius muscles of 24-, 26-, and 32-month-old animals, respectively, than in those of young adult rats. Anti-MyoD antibody recognized two major areas of immunoreactivity in Western blots: a single MyoD-specific band (approximately 43-45 kDa) and a double (or triple) MyoD-like band (approximately 55-65 kDa). Whereas the level of MyoD-specific protein in the 43- to 45-kDa band remained relatively unchanged during aging compared with that of young adult rats, the total level of MyoD-like immunoreactivity within the 55- to 65-kDa bands was approximately 3.4, approximately 4.7, approximately 9.1, and approximately 11.7 times higher in muscles of 24-, 26-, 28-, and 32-month-old rats, respectively. The pattern of MRF protein expression in intact senile muscles was similar to that recorded in young adult denervated muscles. Ultrastructural analysis of extensor digitorum longus muscle from senile rats showed that, occasionally, the area of the nerve-muscle junction was partially or completely devoid of axons, and satellite cells with the features of activated cells were found on the surface of living fibers. Immunohistochemistry detected accumulated MyoD and myogenin proteins in the nuclei of both fibers and satellite cells in 32-month-old muscles. We suggest that the up-regulated production of MyoD and myogenin proteins in the nuclei of both fibers and satellite cells could account for the high level of MRF expression in muscles of senile rats.

Replicate high-density rat genome oligonucleotide microarrays reveal hundreds of regulated genes in the dorsal root ganglion after peripheral nerve injury

BACKGROUND

Rat oligonucleotide microarrays were used to detect changes in gene expression in the dorsal root ganglion (DRG) 3 days following sciatic nerve transection (axotomy). Two comparisons were made using two sets of triplicate microarrays, naïve versus naïve and naïve versus axotomy.

RESULTS

Microarray variability was assessed using the naïve versus naïve comparison. These results support use of a P < 0.05 significance threshold for detecting regulated genes, despite the large number of hypothesis tests required. For the naïve versus axotomy comparison, a 2-fold cut off alone led to an estimated error rate of 16%; combining a >1.5-fold expression change and P < 0.05 significance reduced the estimated error to 5%. The 2-fold cut off identified 178 genes while the combined >1.5-fold and P < 0.05 criteria generated 240 putatively regulated genes, which we have listed. Many of these have not been described as regulated in the DRG by axotomy. Northern blot, quantitative slot blots and in situ hybridization verified the expression of 24 transcripts. These data showed an 83% concordance rate with the arrays; most mismatches represent genes with low expression levels reflecting limits of array sensitivity. A significant correlation was found between actual mRNA differences and relative changes between microarrays (r2 = 0.8567). Temporal patterns of individual genes regulation varied.

CONCLUSIONS

We identify parameters for microarray analysis which reduce error while identifying many putatively regulated genes. Functional classification of these genes suggest reorganization of cell structural components, activation of genes expressed by immune and inflammatory cells and down-regulation of genes involved in neurotransmission.

Analysis of genes induced in peripheral nerve after axotomy using cDNA microarrays

One of the most striking features of neurons in the mature peripheral nervous system is their ability to survive and to regenerate their axons following axonal injury. To perform a comprehensive survey of the molecular mechanisms that underlie peripheral nerve regeneration, we analyzed a cDNA library derived from the distal stumps of post-injured sciatic nerve which was enriched in non-myelinating Schwann cells using cDNA microarrays. The number of up- and down-regulated genes in the transected sciatic nerve was 370 and 157, respectively, of the 9596 spotted genes. In the up-regulated group, the number of known genes was 216 and the number of expressed sequence tag (EST) sequences was 154. In the down-regulated group, the number of known genes was 103 and that of EST sequences was 54. We obtained several genes that were previously reported to be involved in regeneration of the injured neurons, such as cathepsin D, ninjurin 1, tenascin C, and co-receptor for glial cell line-derived neurotrophic factor family of trophic factors. In addition to unknown genes, there seemed to be a lot of annotated genes whose role in nerve regeneration remains unknown.

Immunohistological Localization of Tenascin in the Developing and Lesioned Adult Mouse Optic Nerve

To gain insight into the morphogenetic functions of the recognition molecule tenascin in the central nervous system, we have studied its localization in the developing and lesioned adult mouse optic nerve using light and electron microscopic immunocytochemistry. Since tenascin is a secreted molecule, we have analysed the tenascin-synthesizing cells in tissue sections of retinae and optic nerves by in situ hybridization. A weak and homogeneous tenascin immunoreactivity was detectable in the developing retinal nerve fibre layer and optic nerve of 14-day-old mouse embryos, the earliest developmental age investigated. In the optic nerve of neonatal and 1-week-old animals, a high number of tenascin messenger RNA (mRNA)-containing cells were present, and antibodies to tenascin labelled the surfaces of astrocytes and unmyelinated retinal ganglion cell axons. With increasing age, expression of tenascin in the optic nerve was down-regulated at the mRNA and protein levels. At the fourth postnatal week, blood vessels in the optic nerve and collagen fibrils in the vicinity of meningeal fibroblast-like cells still showed significant immunoreactivity, but the optic nerve tissue proper no longer did so. In adult animals, tenascin was no longer detectable in association with blood vessels located in the myelinated part of the optic nerve, and meninges were only weakly immunoreactive. Also, tenascin mRNA-containing cells were no longer detectable in the myelinated part of the adult mouse optic nerve and few labelled cells were found in the meninges. In the retina, ganglion cells contained no detectable levels of tenascin mRNA at any of the developmental ages analysed. No significant up-regulation of tenascin expression was seen in the nerve tissue proper of transected proximal (i.e. retinal) and distal (i.e. cranial) optic nerve stumps of adult mice during the first 4 weeks after lesioning, the time period studied. However, collagen fibrils associated with meningeal fibroblast-like cells and located near the lesion site became strongly tenascin-immunoreactive 2 days after lesioning. Also, some blood vessels at the lesion site became immunoreactive. We conclude that tenascin in the optic nerve is synthesized by glial cells and not by retinal ganglion cells. The detectability of tenascin at embryonic ages suggests that it may mediate neurite growth in vivo. The absence of a strong, lesion-induced up-regulation of tenascin expression in the regeneration-prohibitive mouse optic nerve contrasts with the lesion-induced pronounced up-regulation in the regeneration-permissive peripheral nervous system, and may indicate a functional involvement of tenascin in regenerative processes. The high tenascin positivity of collagen fibrils at early postnatal ages and after lesioning suggests that tenascin expression may be correlated with mitotic activity of the associated meningeal fibroblast-like cells. Finally, tenascin may be involved in the process of vascularization, since the molecule is associated with blood vessels in developing and adult lesioned, but not intact adult, optic nerves.

L1 Cell Adhesion Molecule is Expressed by Noradrenergic but not Adrenergic Chromaffin Cells: A Possible Major Role for L1 in Adrenal Medullary Design

The adrenal medulla of higher animals is constituted of homotypic groups of chromaffin cells secreting either adrenalin or noradrenalin. Since not all chromaffin cells are individually innervated by fibres of the splanchnic nerve, this tissue characteristic is crucial to the physiological function of the gland. In an attempt to analyse differences between these chromaffin cell types which might underlie the establishment of this tissue pattern, we examined the expression of the adhesion molecule L1 in this gland by immunocytochemistry at the optical and ultrastructural levels in rats. L1, an adhesion molecule abundant in the central nervous system, was found to be present in the adrenal medulla of adults; it was strongly expressed on innervating axons and their surrounding Schwann cells and also on a subpopulation of chromaffin cells. The nature of these chromaffin cells was examined by immunocytochemistry using antibodies against the catecholamine-synthesizing enzyme phenylethanolamine N-methyltransferase (PNMT), which are capable of distinguishing between adrenergic and noradrenergic cells. Immunofluorescence labelling of sequential frozen sections demonstrated that chromaffin cells which express L1 do not express PNMT; conversely, L1 was not detected in any chromaffin cells expressing PNMT. Ultrastructural immunocytochemistry confirmed the existence of two non-overlapping populations of chromaffin cells. It is concluded that, in the adrenal medulla, noradrenergic but not adrenergic chromaffin cells express this adhesion molecule. These data, together with our previous observations that all chromaffin cells express the neural cell adhesion molecule, NCAM, suggest that L1, in cooperation with NCAM, could be responsible for the association of noradrenergic cells in the form of homotypic aggregates segregated from groups of adrenergic cells within the adrenal medulla.

Adult Rat Dorsal Root Ganglion Neurons Extend Neurites on Predegenerated But Not on Normal Peripheral Nerves In Vitro

The abilities of embryonic and adult rat sensory neurons to regenerate were compared when cultured on cryostat sections of normal and lesioned sciatic nerve tissues. Differences in neurite growth, visualized by GAP-43 immunolabelling, were most pronounced on substrata consisting of longitudinal sections of normal versus predegenerated sciatic nerve. Adult dorsal root ganglion (DRG) neurons grew only on the lesioned nerves. Neurites extended along these sections in a characteristically longitudinal orientation, and this growth was not dependent on nerve growth factor. Embryonic DRG neurons extended neurites on sections from both types of nerves. These results highlight important differences in the regenerative abilities of embryonic and adult DRG neurons when grown on physiologically appropriate substrata.

The L2/HNK-1 Carbohydrate Epitope is Involved in the Preferential Outgrowth of Motor Neurons on Ventral Roots and Motor Nerves

Based on the observation that in adult mice the carbohydrate epitope L2/HNK-1 is detectable on Schwann cells in ventral spinal roots, but only scarcely in dorsal roots (Martini et al., Dev. Biol., 129, 330 - 338, 1988), the possibility was investigated that the carbohydrate is involved in the outgrowth of regenerating motor neuron axons on peripheral nerve substrates expressing the epitope. To monitor whether the L2 carbohydrate remains present during the time periods in which regenerating axons penetrate the denervated distal nerve stumps, the expression of L2 in motor and sensory branches of the femoral nerve was investigated in normal animals and after a crush lesion. During the first two postoperative weeks, L2 immunoreactivity remained high in the myelinating Schwann cells of the motor branch, whereas L2 immunoreactivity was virtually absent in the sensory branch. In a first experimental approach, cryosections of ventral and dorsal spinal roots and of motor and sensory nerves of adult rats and mice were used as substrates for neurite outgrowth. Neurites of motor neurons from chicken embryos were approximately 35% longer after 30 h of maintenance on ventral roots than on dorsal roots. Neurites from sensory neurons had the same length on dorsal as on ventral motors and were as long as neurites from motor neurons grown on dorsal roots. L2 antibodies reduced neurite outgrowth of motor neurons on ventral roots but not on dorsal roots. Neurite outgrowth of sensory neurons on both roots was not altered by the antibodies. Neurite outgrowth of motor neurons on a mixture of the extracellular matrix glycoprotein laminin and the L2 carbohydrate-carrying glycolipid was significantly higher than on the laminin substrate mixture with GD1b ganglioside or sulphatide. L2 antibodies reduced neurite outgrowth of motor neurons by 50% on the L2 glycolipid, but not on GD1b or sulphatide. These observations indicate that the L2 carbohydrate promotes neurite outgrowth of motor neurons in vitro and may thus contribute to the preferential reinnervation of motor nerves by regenerating motor axons in vivo.

Contribution of the neural cell adhesion molecule to neuronal and synaptic plasticity

The neural cell adhesion molecule (NCAM) and its polysialylated form PSA-NCAM contribute to many aspects of the development and plasticity of the central nervous system. This includes mechanisms of cell differentiation and migration, neurite outgrowth, establishment of specific patterns of synaptic connections, synaptic plasticity and long-term potentiation. How NCAM and PSA-NCAM contribute to regulate all these different mechanisms remains essentially unknown. Adhesive properties appear to be important, but recent studies also point to possible interactions between NCAM and PSA-NCAM with intracellular signalling cascades that are essential to biological functions. Some of these mechanisms are discussed and a hypothesis is proposed based on the existence of cross-talk between these molecules and signalling pathways mediated by growth factors.

N-cadherin mediates axon-aligned process growth and cell-cell interaction in rat Schwann cells

The molecular mechanisms underlying the contact behavior of Schwann cells (SCs) and SC-axon association are poorly understood. SC-SC and SC-axon interactions were studied using purified adult rat SCs and cocultures of SCs with embryonic dorsal root ganglion neurons. After contact of SCs with axons, SCs start to extend processes in alignment with axons. This unique alignment was quantitated using a new assay. SC-axon alignment and SC-SC band formation were disrupted in medium containing low extracellular calcium, indicating the involvement of calcium-dependent adhesion molecules. N-cadherin expression was strong in developing rat sciatic nerves but weak in adult sciatic nerves. In purified adult-derived rat SCs, N-cadherin expression was increased by mitogens (neuregulins) and decreased by high cell density. High-resolution confocal images show intense N-cadherin signals in SC process tips. Subcellular N-cadherin was accumulated in bands at intercellular junctions between SCs and was clustered at axon-SC contact sites. Blocking antibodies (rabbit and guinea pig IgG directed against the first extracellular domain of N-cadherin) and cyclic pentapeptides (including the HAV motif) were used to perturb N-cadherin function. All blocking agents reduced the number of N-cadherin-positive SC-SC junctions and perturbed axon-aligned growth of SC processes. Averaging over all N-cadherin-perturbation experiments, in controls 67-86% of SCs exhibited axon-aligned process growth, whereas in treated cultures only 41% of the SCs aligned with axons. These results are evidence that in mammals N-cadherin is important for formation of SC-SC junctions and SC process growth in alignment with axons.

Expression of neural cell adhesion molecule (NCAM) in the peripheral nerve fibers demonstrated by postembedding immunogold method on ultrathin sections. A preliminary study

The neural cell adhesion molecule, NCAM, is thought to be necessary for the interaction between neurons, axons and glial cells, axons and target structures, and for myelination. The NCAM localization in the peripheral nerves has been less well studied. The data have shown significant differences. In this study, the distribution of the NCAM immunoreactivity in the sciatic nerves of 10- and 15-day-old Wistar rats was examined on ultrathin sections by the immunogold postembedding method. The sections were immunotested with a polyclonal antibody (Santa Cruz Biotechnology) that recognizes rat NCAM. The antibody was visualized with donkey anti-goat IgG, conjugated to 6 nm colloidal gold. NCAM immuno-like activity was found on axoplasmic profiles of myelinated and nonmyelinated axons, at non-assembled myelin lamellae, and at axo-glial junctions of the paranodes. The compact myelin and the Schwann cell cytoplasm were immunonegative. The presence of some NCAM immunoactivity on the level of the noncompact myelin membranes, deprived of the major myelin protein Po, might be associated with the mechanism of their maintenance.

Effect of allogeneic Schwann cell transplantation on peripheral nerve regeneration

Transplantation of allogeneic Schwann cells (SC) would make it feasible to reconstruct immediately peripheral nerve defects, compared to using autologous SC; however, this treatment modality has not been adequately evaluated. The aim of this study was to characterize and compare the effects of allogeneic versus syngeneic SC transplantation following peripheral nerve injury. Polyhydroxybutyrate conduits were used to bridge a 10-mm gap in the rat sciatic nerve. The conduits were filled with alginate hydrogel with or without cultured allogeneic or syngeneic genetically labeled SC, without the use of immunosuppressive therapy, and examined after 2, 3, and 6 weeks with 5-bromo-4-chloro-3-indoyl-beta-D-galactosidase chemical staining and immunohistochemistry to quantify SC migration into the conduit, axonal regeneration, the state of SC differentiation, and the expression of major histocompatibility complexes (MHC) I and II, as well as to quantify macrophage and B- and T-lymphocyte infiltration. Allogeneic SC were rejected by 6 weeks, whereas syngeneic SC could still be identified. Allogeneic and syngeneic SC equally enhanced the axonal regeneration distance but the quantity of axons was greater using syngeneic SC. The ingrowth of SC into the conduits containing allogeneic SC was similar to that observed in the presence of syngeneic SC, indicating the absence of deleterious immune response. SC continued to express phenotypic markers of nonmyelination and these were highest in conduits with allogeneic SC. Expression of MHC I and II was higher in the conduits with allogeneic SC at 3 weeks and without significant difference in the number of macrophages and lymphocytes, except at 6 weeks, when there was a larger number of lymphocytes using syngeneic SC. In conclusion, allogeneic SC enhanced axonal regeneration distance and did not induce a deleterious immune response. In a clinical setting the immediate availability of allogeneic SC for transplantation may compensate for the better outcome achieved by the use of autologous SC that require a longer preparation time in culture.

A decline in glial cell-line-derived neurotrophic factor expression is associated with impaired regeneration after long-term Schwann cell denervation

In the peripheral nervous system, regeneration of motor and sensory axons into chronically denervated distal nerve segments is impaired compared to regeneration into acutely denervated nerves. In order to find possible causes for this phenomenon we examined the changes in the expression pattern of the glial cell-line-derived neurotrophic factor (GDNF) family of growth factors and their receptors in chronically denervated rat sciatic nerves as a function of time with or without regeneration. Among the GDNF family of growth factors, only GDNF mRNA expression was rapidly upregulated in Schwann cells as early as 48 h after denervation. This upregulation peaked at 1 week and then declined to minimal levels by 6 months of denervation. The changes in the protein expression paralleled the changes in the expression of the GDNF mRNA. The mRNAs for receptors GFRalpha-1 and GFRalpha-2 were upregulated only after maximal GDNF upregulation and remained elevated as late as 6 months. There were no significant changes in the expression of GFRalpha-3 or the tyrosine kinase coreceptor, RET. When we examined the expression of GDNF in a delayed regeneration paradigm, there was no upregulation in the distal chronically denervated tibial nerve even when the freshly axotomized peroneal branch of the sciatic nerve was sutured to the distal tibial nerve. This study suggests that one of the reasons for impaired regeneration into chronically denervated peripheral nerves may be the inability of Schwann cells to maintain important trophic support for both motor and sensory neurons.

A specific force deficit exists in skeletal muscle after partial denervation

Skeletal muscle demonstrates a specific force deficit after repair of injured peripheral nerves, microneurovascular muscle transfer, and normal aging. Because atrophy cannot account for deficits in specific force, other, unknown, mechanisms are responsible for the resulting muscle contractile dysfunction under these circumstances. We tested the hypothesis that a subpopulation of denervated fibers is partially or completely responsible for the specific force deficit after partial denervation of the rat extensor digitorum longus muscle (EDL). Adult Fisher rats underwent either sham exposure or partial transection of 80% of the cross-sectional area of the left deep peroneal nerve. After a 2-week recovery period, maximum isometric force (F(0)) was measured in situ and maximum specific force (sF(0)) was calculated for EDL from both control (n = 8) and partial denervation (n = 7) groups. Innervated fiber cross-sectional area (CSA(inn)) was measured directly from whole EDL cross sections after immunohistochemical labeling for neural cell adhesion molecule (NCAM), a marker of muscle fiber denervation. A corrected specific force value (sF(0-inn)) was calculated by normalizing F(0) to CSA(inn). Partial skeletal muscle denervation resulted in significant reductions in muscle mass, F(0), and sF(0). The percentage of muscle fibers expressing NCAM in the extrajunctional sarcolemma increased from 1.0 +/- 0.8% in control to 49 +/- 15% in partially denervated EDL muscles. A 62.7% deficit in EDL specific force was observed after partial denervation. Denervated muscle fibers accounted for 59.3% of this deficit, but sF(0-inn) still differed significantly between control and partially denervated muscles, with a 25.5% difference between groups. In partially denervated muscles, the specific force deficit is partially but not fully explained by a subpopulation of noncontractile, denervated fibers.

The role of neural cell adhesion molecules in plasticity and repair

Repair and functional recovery after brain injury critically depends on structural and functional plasticity of preserved neuronal networks. A striking feature of brain structures where tissue reorganization and plasticity occur is a strong expression of the polysialylated neural cell adhesion molecule (PSA-NCAM). An important role of this molecule in various aspects of neuronal and synaptic plasticity has been revealed by many studies. Recently, a new mechanism has been elucidated whereby PSA-NCAM may contribute to signalling mediated by the neurotrophic factor BDNF, thereby sensitizing neurons to this growth factor. This mechanism was shown to be important for activity-induced synaptic plasticity and for the survival and differentiation of cortical neurons. A cross-talk between these molecules may, thus, reveal a key factor for properties of structural plasticity and in particular could mediate the activity-dependent aspects of synaptic network remodeling. Animal models have been developed to assess the role of these molecules in functional recovery after lesions.

Soluble forms of NCAM and F3 neuronal cell adhesion molecules promote Schwann cell migration: identification of protein tyrosine phosphatases zeta/beta as the putative F3 receptors on Schwann cells

Neural cell adhesion molecule (NCAM) and F3 are both axonal adhesion molecules which display homophilic (NCAM) or heterophilic (NCAM, F3) binding activities and participate in bidirectional exchange of information between neurones and glial cells. Engineered Fc chimeric molecules are fusion proteins that contain the extracellular part of NCAM or F3 and the Fc region of human IgG1. Here, we investigated the effect of NCAM-Fc and F3-Fc chimeras on Schwann cell (SC) migration. Binding sites were identified at the surface of cultured SCs by chimera coated fluorospheres. The functional effect of NCAM-Fc and F3-Fc binding was studied in two different SC migration models. In the first, migration is monitored at specific time intervals inside a 1-mm gap produced in a monolayer culture of SCs. In the second, SCs from a dorsal root ganglion explant migrate on a sciatic nerve cryosection. In both systems addition of the chimeras significantly increased the extent of SC migration and this effect could be prevented by the corresponding anti-NCAM or anti-F3 blocking antibodies. Furthermore, antiproteoglycan-type protein tyrosine phosphatase zeta/beta (RPTPzeta/beta) antibodies identified the presence of RPTPzeta/beta on SCs and prevented the enhancing effect of soluble F3 on SC motility by 95%. The F3-Fc coated Sepharose beads precipitated RPTPzeta/beta from SC lysates. Altogether these data point to RPTPzeta/beta is the putative F3 receptor on SCs. These results identify F3 and NCAM receptors on SC as potential mediators of signalling occurring between axons and glial cells during peripheral nerve development and regeneration.

Specific force deficit in skeletal muscles of old rats is partially explained by the existence of denervated muscle fibers

We tested the hypothesis that denervated muscle fibers account for part of the specific force (sF(o)) deficit observed in muscles from old adult (OA) mammals. Whole muscle force (F(o)) was quantified for extensor digitorum longus (EDL) muscles of OA and young adult (YA) rats. EDL muscle sF(o) was calculated by dividing F(o) by either total muscle fiber cross-sectional area (CSA) or by innervated fiber CSA. Innervated fiber CSA was estimated from EDL muscle cross sections labeled for neural cell adhesion molecules, whose presence is a marker for muscle fiber denervation. EDL muscles from OA rats contained significantly more denervated fibers than muscles from YA rats (5.6% vs 1.1% of total CSA). When compared with YA muscle, OA muscle demonstrated deficits of 34.1% for F(o), 28.3% for sF(o), and 24.9% for sF(o) calculated by using innervated CSA as the denominator. Denervated muscle fibers accounted for 11.3% of the specific force difference between normal YA and OA skeletal muscle. Other mechanisms in addition to denervation account for the majority of the sF(o) deficit with aging.

Immunohistochemical localization of the neural cell adhesion molecule in the rat sciatic nerve

The neural cell adhesion molecule (NCAM) has been widely studied in the early embryonal development of the nervous system. The data about NCAM distribution in the peripheral nerves during postnatal life are scant and some controversial. In the present study, the NCAM localization in the sciatic nerves of 15-day-old Wistar rats has been studied. Semi-thin sections of the nerves were immunotested with a polyclonal antibody (Santa Cruz Biotechnology) that recognizes rat NCAM. The antibody was visualized with donkey anti-goat IgG, conjugated to 12 nm colloidal gold, and silver amplification. In the myelinated nerve fibres, the immunoreactivity was associated with the axons, mainly with their plasma membrane, which was unstained in the nodes of Ranvier. The myelin sheaths and the myelinating Schwann cells were negative. The extracellular matrix and the bundles of non-myelinated nerve fibres were immunopositive.

NF-kappaB activation by N-CAM and cytokines in astrocytes is regulated by multiple protein kinases and redox modulation

Interaction of the neural cell adhesion molecule (N-CAM) with astrocytes activates a transcription factor, NF-kappaB, that mediates inflammatory responses after neural injury. Here we describe intracellular signaling events that link N-CAM binding to NF-kappaB-mediated transcription. Addition of the third immunoglobulin domain of N-CAM (Ig III), which mimics the activity of intact N-CAM, or of cytokines (interleukin-1beta or tumor necrosis factor-alpha), increased transcription from an NF-kappaB-responsive luciferase reporter gene construct that had been transiently transfected into neonatal rat forebrain astrocytes. NF-kappaB activity induced by Ig III or cytokines was decreased by inhibition of nonreceptor protein tyrosine kinases (PTKs), phospholipase C, protein kinase C (PKC), calcium/calmodulin-dependent protein kinase II (CaMKII), or oxidative stress. Inhibition of PKC blocked nuclear translocation of NF-kappaB protein while binding of NF-kappaB to DNA was decreased by modulation of redox homeostasis. In contrast, inhibition of CaMKII and nonreceptor PTKs altered neither nuclear translocation nor DNA binding, suggesting that these kinases affect NF-kappaB transactivation. A number of agents that inhibit NF-kappaB activation in other cell types did not affect activation in astrocytes. These findings suggest that activation of NF-kappaB by N-CAM and cytokines in astrocytes involves multiple signals that differentially affect NF-kappaB nuclear translocation, DNA binding, and transactivation.

Cellular signaling by neural cell adhesion molecules of the immunoglobulin superfamily

Neural cell adhesion molecules (CAMs) of the immunoglobulin superfamily nucleate and maintain groups of cells at key sites during early development and in the adult. In addition to their adhesive properties, binding of CAMs can affect intracellular signaling. Their ability to influence developmental events, including cell migration, proliferation, and differentiation can therefore result both from their adhesive as well as their signaling properties. This review focuses on the two CAMs for which the most information is known, the neural CAM, N-CAM, and L1. N-CAM was the first CAM to be characterized and, therefore, has been studied extensively. The binding of N-CAM to cells leads to a number of signaling events, some of which result in changes in gene expression. Interest in L1 derives from the fact that mutations in its gene lead to human genetic diseases including mental retardation. Much is known about modifications of the L1 cytoplasmic domain and its interaction with cytoskeletal molecules. The study of CAM signaling mechanisms has been assay-dependent rather than molecule-dependent, with particular emphasis on assays of neurite outgrowth and gene expression, an emphasis that is maintained throughout the review. The signals generated following CAM binding that lead to alterations in cell morphology and gene expression have been linked directly in only a few cases. We also review information on other CAMs, giving special consideration to those that are anchored in the membrane by a phospholipid anchor. These proteins, including a form of N-CAM, are presumed to be localized in lipid rafts, membrane substructures that include distinctive subsets of cytoplasmic signaling molecules such as members of the src-family of nonreceptor protein tyrosine kinases. In the end, these studies may reveal that what CAMs do after they bind cells together may have as profound consequences for the cells as the adhesive interactions themselves. This area will therefore remain a rich ground for future studies.

Neural cell adhesion molecule (NCAM) and myoblast fusion

Considerable evidence points to an involvement of neural cell adhesion molecule (NCAM) in myoblast fusion. Changes in the level of NCAM expression, isoform specificity, and localization in muscle cells and tissues correspond to key morphogenetic events during muscle differentiation and repair. Furthermore, anti-NCAM antibodies have been shown by others to reduce the rate of myoblast fusion, whereas overexpression of NCAM cDNAs increases the rate of myoblast fusion compared to controls. In this study we have used a novel fusion assay based on intracistronic complementation of lacZ, in combination with fluorescent X-gal histochemistry and immunocytochemistry to assess levels of NCAM expression in individual muscle cells. Our results indicate that a substantial proportion of newly fused myoblasts have NCAM expression levels unchanged from the levels of the surrounding unfused population suggesting that increased expression of NCAM is not required for wild-type myoblasts to fuse. Moreover, pure populations of primary myoblasts isolated from mice homozygous null for NCAM and therefore lacking the molecule, when placed in differentiation medium, consistently fused to form contractile myotubes with kinetics equivalent to wild-type primary myoblasts. We conclude that the increase in expression of NCAM, although typically observed during myogenesis, is not essential to myoblast fusion to form myotubes.

The neural cell adhesion molecule in synaptic plasticity and ageing

By mediating cell adhesion and signal transduction, the neural cell adhesion molecule (NCAM) regulates neurite outgrowth, fasciculation and target recognition in the developing nervous system. In addition, a number of studies suggest an important role for the NCAM in regeneration and learning in the adult nervous system. NCAM-deficient mice are impaired in spatial learning. Moreover, by interfering with normal NCAM function by intracranial injections of NCAM-antibodies, long-term potentiation (LTP) in rat hippocampal slices and learning in rats and chicks have been inhibited. In the vertebrate nervous system, NCAM is the dominant carrier of polysialic acid (PSA), an unusual carbohydrate consisting of long homopolymers of sialic acid. The PSA-NCAM expression decreases markedly during development. However, an upregulation of polysialic acid (PSA) in restricted brain areas including the hippocampus has been observed following learning. Moreover, enzymatic removal of PSA results in impaired LTP and learning. In muscle, the PSA-NCAM expression is upregulated following denervation. This response is weakened in aging rats. The expression of NCAM and PSA have been shown to be regulated by neuronal activity suggesting that the NCAM may promote structural remodelling in an activity dependent manner associated with learning and regeneration.

Differential expression of two adhesion molecules of the immunoglobulin superfamily, F3 and polysialylated NCAM, in hypothalamic magnocellular neurones capable of plasticity

The adult hypothalamo-neurohypophysial system undergoes activity-dependent, reversible morphological changes which result in reduced astrocytic coverage of its neurones and an increase in their synaptic contacts. Our recent observations show that neurones and glia of the hypothalamo-neurohypophysial system continue to express 'embryonic' molecular features which may underlie their capacity to undergo such plasticity. These include expression of cell surface molecules like the glycosyl phosphatidyl inositol (GPI)-linked glycoprotein F3, which intervenes in axonal outgrowth, and the polysialylated isoform of the neural cell adhesion molecule (PSA-NCAM), which reduces cell adhesion and promotes dynamic cell interactions. F3 is colocalised with vasopressin and oxytocin hormones in neurosecretory granules and follows an activity-dependent, regulated pathway for surface expression on neurohypophysial axons. In contrast, PSA-NCAM appears to follow a constitutive pathway, independent of the activity of the hypothalamo-neurohypophysial system, for expression on axonal and glial surfaces, in the hypothalamic magnocellular nuclei and in the neurohypophysis. The role of F3 remains to be determined but in view of its presumptive functions during development, we propose that it promotes remodelling of neurosecretory terminals. On the other hand, we provide direct evidence that surface expression of PSA on NCAM is essential to morphological plasticity since its specific enzymatic degradation in vivo inhibited the neuronal-glial and synaptic changes normally induced by stimulation of secretion from the hypothalamo-neurohypophysial system.

Cell surface expression of polysialic acid on NCAM is a prerequisite for activity-dependent morphological neuronal and glial plasticity

Polysialic acid (PSA) on the extracellular domain of the neural cell adhesion molecule (NCAM) reduces cell adhesion and is considered an important regulator of cell surface interactions. The hypothalamo-neurohypophysial system (HNS), whose glia, neurons, and synapses undergo striking, reversible morphological changes in response to physiological stimulation, expresses high levels of PSA-NCAM throughout life. Light and electron microscopic immunocytochemistry in normal rats and rats in which cell transport was blocked with colchicine showed that PSA-NCAM is expressed in both HNS neurons and glia, particularly at the level of astrocytic processes that envelop neuronal profiles and can undergo remodeling. Moreover, we confirmed that the overall levels of PSA-NCAM were not greatly altered by stimulation (lactation and chronic salt ingestion). Nevertheless, PSA is essential to morphological plasticity. Using comparative ultrastructural analysis, we found that, after specific enzymatic removal of PSA from NCAM by microinjection of endoneuraminidase close to the hypothalamic magnocellular nuclei in vivo, there was no apparent withdrawal of astrocytic processes nor any increase in synaptic contacts normally induced by lactation and dehydration. Our observations demonstrate, therefore, that expression of PSA on cell surfaces in the adult HNS is indispensable to its capacity for activity-dependent morphological neuronal-glial and synaptic plasticity. The carbohydrate PSA on NCAM can thus be considered a necessary permissive factor to allow neuronal and glial remodeling to occur whenever the proper inductive stimulus intervenes.

Viability of cultured nerve grafts: An assessment of proliferation of Schwann cells and fibroblasts

Previous studies demonstrated that the viability of nerve grafts had a positive effect on nerve regeneration, while the cold storage of nerve grafts obtained few viable cells at the later stage. The purpose of this study was to examine the cellular activities of Schwann cells and fibroblasts in cultured nerve grafts prior to transplantation. 2.5-cm long sciatic nerve grafts were harvested from 75 male Lewis rats. Two different media were utilized to culture the nerve grafts up to 3 weeks: Dulbecco's modified eagle medium (DMEM) only or DMEM supplemented with 2 microM forskolin and 10 microg/ml pituitary exact (mitogen medium for Schwann cells). In vivo predegenerated and normal nerve grafts were used as positive and negative controls, respectively. We employed a 5-bromo-2'-deoxyuridine (BrdU) incorporation method to evaluate the proliferating cells in the cultured nerve grafts. S-100 and vimentin immunostaining were used to estimate the presence of Schwann cells and fibroblasts in all nerve grafts at different intervals. The results showed that the proliferating cells increased progressively under culture conditions. The proliferating cells distributed evenly in small fascicles (average diameter 251 +/- 71.5 microm), whereas they appeared mainly in the margin of large fascicles (average diameter 624 +/- 87.3 microm). The mitogen medium stimulated Schwann cell multiplication more significantly in comparison with DMEM after 3 days of culture (P < 0.01), however, there were fewer fibroblasts present in the mitogen medium than in DMEM after 2 days of culture (P < 0.01). It is suggested that the viability of nerve grafts can be preserved under culture conditions. Furthermore, the cellular activity of the Schwann cells and fibroblasts in nerve grafts can be manipulated in in vitro Wallerian degeneration.

Cell surface expression of polysialic acid on NCAM is a prerequisite for activity-dependent morphological neuronal and glial plasticity

Polysialic acid (PSA) on the extracellular domain of the neural cell adhesion molecule (NCAM) reduces cell adhesion and is considered an important regulator of cell surface interactions. The hypothalamo-neurohypophysial system (HNS), whose glia, neurons, and synapses undergo striking, reversible morphological changes in response to physiological stimulation, expresses high levels of PSA-NCAM throughout life. Light and electron microscopic immunocytochemistry in normal rats and rats in which cell transport was blocked with colchicine showed that PSA-NCAM is expressed in both HNS neurons and glia, particularly at the level of astrocytic processes that envelop neuronal profiles and can undergo remodeling. Moreover, we confirmed that the overall levels of PSA-NCAM were not greatly altered by stimulation (lactation and chronic salt ingestion). Nevertheless, PSA is essential to morphological plasticity. Using comparative ultrastructural analysis, we found that, after specific enzymatic removal of PSA from NCAM by microinjection of endoneuraminidase close to the hypothalamic magnocellular nuclei in vivo, there was no apparent withdrawal of astrocytic processes nor any increase in synaptic contacts normally induced by lactation and dehydration. Our observations demonstrate, therefore, that expression of PSA on cell surfaces in the adult HNS is indispensable to its capacity for activity-dependent morphological neuronal-glial and synaptic plasticity. The carbohydrate PSA on NCAM can thus be considered a necessary permissive factor to allow neuronal and glial remodeling to occur whenever the proper inductive stimulus intervenes.

The monoclonal antibody 23E9 defines a novel developmentally-regulated Schwann cell surface antigen

The present study describes the identification and partial characterization of a novel Schwann cell surface molecule by means of a monoclonal antibody (23E9). The 23E9 antigen was found in association with Schwann cells of the peripheral nerve but not with sensory neurons and satellite cells of the dorsal root ganglion. The expression of the antigen in the sciatic nerve starts after birth, is high around postnatal day 8 and becomes down-regulated towards the adult stage. This suggests that it may be involved in the induction of myelin formation. On Western blots, the antibody identified two major bands of approximately 27 and 42 kDa. Treatment of cultured Schwann cells with forskolin, an agent known to mimic neuronal contact in vitro, stimulated the up-regulation of the antigen. This implies that the expression of 23E9 is induced and maintained by axon-derived signals in vivo. Comparison of the presented data with the literature suggests that we have identified a novel cell surface molecule not previously characterized in the context of Schwann cell biology. To clarify the molecular identity of the antigen and define its physiological relevance, the antibody will be used in future studies for immunoprecipitation and functional in vitro assays.

Developmental methylmercury administration alters cerebellar PSA-NCAM expression and Golgi sialyltransferase activity

Brain dysmorphogenesis and persistent psychomotor disturbances are hallmarks of developmental methylmercury (MeHg) exposure, but the molecular mechanisms underlying these effects are poorly understood. Targets of developmental MeHg exposure include neural cell adhesion molecules (NCAMs), sialoglycoconjugate molecules whose proper temporal and spatial expression is important at all stages of neurodevelopment and especially during synaptic structuring. To investigate the effects of MeHg on the temporal expression of NCAM during development, rat pups were dosed with 7.0 mg/kg MeHgCl (s.c.) on alternate days from postnatal days (PNDs) 3-13 and killed on PNDs 15, 30 and 60. Brain MeHg concentrations were determined in a subset of litters injected with CH(3)203Hg. Expression of NCAM180 protein and of NCAM180 polysialylation was examined in whole cerebellum homogenates, cerebellar synaptosomes and isolated cerebellar growth cones by Western blotting and immunocytochemical staining. NCAM sialyltransferase activity was assayed in preparations of purified Golgi apparatus from the cerebelli of rats treated in vivo, or following in vitro incubation with 0, 1, 2.5, or 7.5 microM MeHg for 2 h. At PND15, no change in NCAM180 protein expression was observed in any cerebellar preparations, but decreased polysialylation of NCAM180 was observed in cerebellar whole homogenates, synaptosomes and isolated growth cones. At PND30, both NCAM180 protein expression and NCAM180 polysialylation were elevated in whole homogenate preparations but not in synaptosomes. NCAM180 expression in MeHg-treated rats was similar to controls at PND60, 47 days after the last methylmercury administration. In vivo studies of cerebellar Golgi sialyltransferase activity revealed significant reductions in PND15 MeHg-treated rats as compared to controls, but no changes in sialyltransferase activity in PND30 and PND60 animals. In vitro experiments revealed decreasing sensitivity of cerebellar sialyltransferases to MeHg as the developmental age of the rat increased. Toxic perturbation of the developmentally-regulated expression of polysialylated NCAM during brain formation may disturb the stereotypic formation of neuronal contacts and could contribute to the behavioral and morphological disturbances observed following MeHg poisoning.

Effects of axotomy on the expression and ultrastructural localization of N-cadherin and neural cell adhesion molecule in the quail ciliary ganglion: an in vivo model of neuroplasticity

Postganglionic nerve crush of the avian ciliary ganglion induces detachment of preganglionic terminals from the soma of the injured ciliary neurons, followed by reattachment at about the same time that the postganglionic axons regenerate to their targets. In order to determine the role played by cell adhesion molecules in this response, we have studied injury-induced changes in the amount and distribution of N-cadherin and neural cell adhesion molecule, together with modifications in the expression of their messenger RNAs. Both N-cadherin and neural cell adhesion molecule immunoreactivities associated with postsynaptic specializations decreased between one and three days following postganglionic nerve crush, preceding the detachment of the preganglionic boutons. Immunoreactivities subsequently increased between 13 and 20 days, in parallel with restoration of synaptic contacts on the ganglion cells and the progressive reinnervation of the peripheral targets. In contrast to the rapid decrease in immunoreactivity, the messenger RNA levels of N-cadherin and neural cell adhesion molecule both increased after crush, and remained elevated throughout the 20-day period of the experiment. These results are consistent with roles for N-cadherin and neural cell adhesion molecule in the maintenance of synaptic contacts. The rapid regulation of these proteins in injury-induced synaptic plasticity occurs at the post-transcriptional level, whereas longer term regulation associated with the re-establishment of synapses may be promoted by the increased levels of gene expression.

Heparan sulphates upregulate regeneration of transected sciatic nerves of adult guinea-pigs

The increased content of soluble glycosaminoglycan-containing forms in sciatic nerves during recovery from crush injury [Shum & Chau (1996) J. Neurosci. Res., 46, 465] suggests that the glycosaminoglycans modulate the environment for post-traumatic tissue remodelling and axonal regrowth. To test this, defined amounts of soluble heparan sulphates from bovine kidney or guinea-pig nerve were introduced into the regenerating environment via silicone conduits that bridged 8-mm gaps of transected sciatic nerves of adult guinea-pigs. Controls were bridged using the phosphate-buffered saline (PBS) vehicle or a chondroition sulphate preparation from whale cartilage. After timed periods of recovery, the animals were assessed for electromyographic signals at the target gastrocnemius muscle to determine the conduction velocity across the bridged nerve. Sections of the bridge were also histologically examined for nerve fibres. Transected sciatic nerves bridged with heparan sulphates or chondroitin sulphate showed earlier stimulated myelination of axons (week 5-6) than PBS-bridged nerves (week 9). Initial electromyographic indication of reconnection with the target was at week 9 post-transection. In the course of 20 weeks, transected sections of the bridge indicated similar numbers of unmyelinated axons irrespective of bridge material, but distinctly higher numbers of myelinated axons in heparan sulphate-bridged nerves than either PBS- or chondroitin sulphate-bridged nerves. At the end of the same period, heparan sulphate-bridged nerves resumed normal conduction velocities, but both PBS- and chondroitin sulphate-bridged nerves remained at 50% of that of the intact contralateral nerves. These results are the first to demonstrate that supplementation of soluble heparan sulphate to the fluid regenerative neural environment can restore functional, axonal reconnection of the severed nerve with the target muscle.

Inactivity-induced remodeling of neuromuscular junctions in rat diaphragmatic muscle

We hypothesized that inactivity-induced remodeling of neuromuscular junctions (NMJs) depends on fiber type and the match between muscle fiber and motoneuron (MN) activities. Two inactivity models were studied in rat diaphragmatic muscle: spinal hemisection at C2 (SH), where both diaphragmatic muscle fibers and phrenic MNs were inactive, and tetrodotoxin (TTX) nerve blockade, where only muscle fibers were inactive. After 2 weeks of inactivity, there was increased number of pre- and postsynaptic branches (fragmentation) of NMJs at type IIx/b fibers in both models. In addition, planar NMJ areas at type IIx/b fibers in the SH model were enlarged. In contrast, NMJs at type I and IIa fibers were unaffected in both SH and TTX models. Functionally, neuromuscular transmission in diaphragmatic muscle fibers improved in the SH model, but worsened in the TTX model, compared to controls. These results suggest that NMJ remodeling depends on the level of MN activity. The relative preservation of NMJs at type I and IIa fibers suggests a potential for recovery from diaphragmatic paralysis in the clinical setting, at least for respiratory behaviors.

Polysialylated neural cell adhesion molecule serum levels in normal children

Serum concentrations of polysialylated neural cell adhesion molecule (PSA-NCAM), a developmentally regulated form of the NCAM, have been recently described to be elevated in children with rhabdomyosarcoma and neuroblastoma, proving PSA-NCAM to be a tumor marker of diagnostic relevance to these malignancies. The present investigation was undertaken to define age-dependent reference intervals in normal children. Serum concentrations of polysialylated NCAM were determined in 366 children aged newborn to 17 y and in 18 adult patients by an immunoluminescence assay using the polysialic acid-specific MAb 735. Serum levels in newborn children were 51.7 kU/L (mean +/- 12.0 kU/L SD), whereas in adult patients they were 9.9 kU/L (mean +/- 3.5 kU/l SD). Assigning the patients to 14 different age groups, a gradual decay of PSA-NCAM serum concentrations was observed, and therefore, mean levels and empirical interpolated percentiles were determined for every age group. Applying specially fitted logistic functions, two different sigmoid graphs were obtained describing the age-dependent decrease of serum PSA-NCAM during the neonatal period and during childhood. The age at which the levels reach half the initial value was located at 3.1 d (mean +/- 2 d SE) and 14 y (mean +/- 1 y SE), respectively. There was no difference between male and female individuals. Repeated measurements revealed variations below 10%. For the first time, our study describes serum levels of PSA-NCAM in children of different age and their gradual decay until adulthood.

Intrinsic and extrinsic factors in muscle aging

The regenerative potential of skeletal muscle, and overall muscle mass, decline with age. This regenerative potential may be influenced by autocrine growth factors intrinsic to the muscle itself. Extrinsic host factors that may influence muscle regeneration include hormones, growth factors secreted in a paracrine manner by accessory cells, innervation, and antioxidant mechanisms. Unaccustomed exercise, which involves mechanical overload of myofibers, provides a convenient method for studying muscle regeneration in both humans and animal models. An inflammatory response ensues in which distinctive populations of macrophages infiltrate the affected tissue: some of these macrophages are involved in phagocytosis of damaged fibers; other macrophages arriving at later times may deliver growth factors or cytokines that promote regeneration. These include fibroblast growth factor and insulin-like growth factor, which are important regulators of muscle precursor cell growth and differentiation, as well as nerve growth factor, which is essential for maintenance or reestablishment of neuronal contact. Other cytokines, including interleukin-1, tumor necrosis factor, interleukin-15, and ciliary neurotrophic factor, have a strong influence on the balance between muscle protein synthesis and breakdown. The functional activity of invading macrophages can be influenced by age, by factors in myofibers and extracellular matrix, and can be influenced systemically by the antioxidant status of the host.

The expression of receptor tyrosine phosphatases is responsive to sciatic nerve crush

Given the importance of phosphotyrosine signaling in growth cone dynamics, we have examined the embryonic and adult expression of receptor-like protein tyrosine phosphatases in sensory neurons and studied their responsiveness to nerve lesions in young adult animals. The phosphatases LAR, PTPsigma, and PTPalpha are expressed in most neurons of E14 and E18 rat embryo dorsal root ganglia, while BEM-1 is expressed in a more restricted subset of these neurons. These phosphatases continue to be expressed in young adult animals, suggesting that they have roles in mature as well as in developing dorsal root ganglia neurons. After an experimental sciatic nerve crush, the expression of the phosphatase genes was significantly and differentially altered in these neurons. PTPsigma mRNA was increased by 50% after 3 days, while LAR and PTPalpha expression dropped by 50 and 20%, respectively. BEM-1 mRNA levels were unaltered. These data show that mRNA levels of specific tyrosine phosphatase genes are highly responsive to nerve damage and may be reset to a new and potentially optimal pattern of expression more conducive for nerve regeneration. We propose that tyrosine phosphatases are not only involved in primary axonogenesis but can also now be implicated in the molecular control of adult nerve repair.

Polysialic acid at the cell surface: biophysics in service of cell interactions and tissue plasticity

Polysialic acid (PSA) is a long polymer of negatively-charged sialic acid associated with the neural cell adhesion molecule. PSA serves as a potent negative regulator of cell interactions via its unusual biophysical properties. During development the abundant and regulated expression of this carbohydrate is closely correlated with axon pathfinding and targeting, and with certain aspects of muscle formation. Its level can also be modulated by synaptic activity. PSA expression is more restricted in the neonatal and adult brain, being primarily associated with regions capable of morphological or physiological changes. Studies on the function of PSA studies suggest that its primary role is to promote developmentally-controlled and activity-dependent plasticity in cell interactions and thereby facilitate changes in the structure and function of the nervous system. The presence of PSA on a variety of metastatic tumor lines has also attracted the attention of oncologists, and its late appearance in evolution raises interesting questions about the phylogeny of complex tissue formation.

Readiness of zebrafish brain neurons to regenerate a spinal axon correlates with differential expression of specific cell recognition molecules

We analyzed changes in the expression of mRNAs for the axonal growth-promoting cell recognition molecules L1.1, L1.2, and neural cell adhesion molecule (NCAM) after a rostral (proximal) or caudal (distal) spinal cord transection in adult zebrafish. One class of cerebrospinal projection nuclei (represented by the nucleus of the medial longitudinal fascicle, the intermediate reticular formation, and the magnocellular octaval nucleus) showed a robust regenerative response after both types of lesions as determined by retrograde tracing and/or in situ hybridization for GAP-43. A second class (represented by the nucleus ruber, the nucleus of the lateral lemniscus, and the tangential nucleus) showed a regenerative response only after proximal lesion. After distal lesion, upregulation of L1.1 and L1.2 mRNAs, but not NCAM mRNA expression, was observed in the first class of nuclei. The second class of nuclei did not show any changes in their mRNA expression after distal lesion. After proximal lesion, both classes of brain nuclei upregulated L1.1 mRNA expression (L1.2 and NCAM were not tested after proximal lesion). In the glial environment distal to the spinal lesion, labeling for L1.2 mRNA but not L1.1 or NCAM mRNAs was increased. These results, combined with findings in the lesioned retinotectal system of zebrafish (Bernharnhardt et al., 1996), indicate that the neuron-intrinsic regulation of cell recognition molecules after axotomy depends on the cell type as well as on the proximity of the lesion to the neuronal soma. Glial reactions differ for different regions of the CNS.

Readiness of zebrafish brain neurons to regenerate a spinal axon correlates with differential expression of specific cell recognition molecules

We analyzed changes in the expression of mRNAs for the axonal growth-promoting cell recognition molecules L1.1, L1.2, and neural cell adhesion molecule (NCAM) after a rostral (proximal) or caudal (distal) spinal cord transection in adult zebrafish. One class of cerebrospinal projection nuclei (represented by the nucleus of the medial longitudinal fascicle, the intermediate reticular formation, and the magnocellular octaval nucleus) showed a robust regenerative response after both types of lesions as determined by retrograde tracing and/or in situ hybridization for GAP-43. A second class (represented by the nucleus ruber, the nucleus of the lateral lemniscus, and the tangential nucleus) showed a regenerative response only after proximal lesion. After distal lesion, upregulation of L1.1 and L1.2 mRNAs, but not NCAM mRNA expression, was observed in the first class of nuclei. The second class of nuclei did not show any changes in their mRNA expression after distal lesion. After proximal lesion, both classes of brain nuclei upregulated L1.1 mRNA expression (L1.2 and NCAM were not tested after proximal lesion). In the glial environment distal to the spinal lesion, labeling for L1.2 mRNA but not L1.1 or NCAM mRNAs was increased. These results, combined with findings in the lesioned retinotectal system of zebrafish (Bernharnhardt et al., 1996), indicate that the neuron-intrinsic regulation of cell recognition molecules after axotomy depends on the cell type as well as on the proximity of the lesion to the neuronal soma. Glial reactions differ for different regions of the CNS.

Regenerative capacity of retinal ganglion cells in mammals

Retinal ganglion cells (RGCs) and their projections in the optic nerve offer a convenient model to study survival and regeneration of mammalian central nervous system (CNS) nerve cells following injury. Possible factors affecting the death of RGCs following axotomy and various approaches to rescue the axotomized RGCs are discussed. In addition, two main strategies currently used to enhance axonal regeneration of damaged RGCs are described. The first focuses on overcoming the unfavorable extrinsic CNS environment and the second concentrates on upregulating the intrinsic growth potential of RGCs. Thus, the failure or success of RGC axonal regrowth after injury depends on the complicated interplay between the extrinsic and intrinsic factors.

Neural cell adhesion molecule, neuron-specific enolase and calcitonin gene-related peptide immunoreactivity in hamster taste buds after chorda tympani/lingual nerve denervation

Hamster fungiform papilla taste buds persist in an atrophic form following sensory denervation. While atrophic and innervated taste buds are morphologically similar, it is not known whether their gemmal cells have similar molecular characteristics. Three neurochemicals, neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide have been implicated in trophic phenomena, synaptogenesis and cell recognition in neurons and sensory neuroepithelia. The present study uses immunocytochemical localization of these molecular markers to characterize normal and denervated fungiform taste buds following unilateral chorda tympani/lingual nerve denervation in hamsters. In normal taste buds, immunoreactivity to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide was present in a group of cells located centrally in the bud as well as in fungiform nerve fibres and endings. After denervation, gemmal cell immunoreactivity to all three markers was reduced and often confined to a single or a few bud cell(s). Also, fibre staining was absent except for sparse calcitonin gene-related peptide-immunoreactive fibres associated with blood vessels and within the fungiform papillae. These remaining fibres may be autonomic or somatomotor in origin. These results indicate that sensory denervation of hamster taste buds reduces, but does not wholly eliminate the immunoreactivity of surviving gemmal cells to neural cell adhesion molecule, neuron-specific enolase, and calcitonin gene-related peptide. While the number of taste bud cells expressing the markers appears to be nerve-dependent, immunoreactivity in sensory-denervated bud cells of hamster may reflect the influence of local tissue factors.

Increased flexibility and selectivity in spatial learning of transgenic mice ectopically expressing the neural cell adhesion molecule L1 in astrocytes

The expression of the neural cell adhesion molecule L1 is altered by neuronal activity and promotes neurite outgrowth in vitro. To study the effects of L1 on learning and synaptic plasticity, transgenic mice have been created which express L1 ectopically in glial fibrillary acidic protein (GFAP) expressing astrocytes. Ninety mice, including GFAP-L1-transgenic mice from two genetic backgrounds and their littermates, were tested for swimming navigation learning in the Morris water maze according to a standardized protocol. While learning the position of an invisible target platform and also relearning its position after relocation, GFAP-L1-transgenic mice spent a greater fraction of their swim time in the target quadrant. Moreover, they showed a more rapid improvement of escape performance during the first day of training. Factor analysis revealed that this difference in swimming pattern could not be explained by non-cognitive factors. Factor analysis also revealed that, during a probe trial, the GFAP-L1-transgenic mice spent comparatively less time in the old target quadrant than predicted by the increased searching they had shown during acquisition learning. Hence, ectopic expression of L1 by astrocytes in mice appears to be linked to a factor which increases behavioural flexibility and selectivity while learning and relearning, but concomitantly may lead to a relative reduction of spatial retention.

Enhancing nerve regeneration across a silicone tube conduit by using interposed short-segment nerve grafts

Isolated nerve segments may inherently contain all of the necessary factors required to support regeneration within a silicone tube conduit placed across a nerve gap. Thirty-six adult Lewis rats each weighing approximately 250 g were randomized into three groups. A sciatic nerve gap (13-15 mm in length) was bridged by an empty silicone tube (Group I), a silicone tube containing a short 2-mm interposed nerve segment (Group II), or a nerve autograft (Group III). At 16 weeks postoperatively, no regeneration was observed through the empty silicone tube. In contrast, regeneration across the silicone tube containing the isolated nerve segment was equivalent to that noted through nerve autografts as assessed by histological, electrophysiological, and functional criteria. Thus, an interposed nerve segment will extend the length of successful nerve regeneration through a silicone tube conduit.

Predegenerated nerve allografts versus fresh nerve allografts in nerve repair

This study reevaluated the possibility of using predegenerated nerves as donor nerve allografts for nerve repair and compared the results of functional recovery to those obtained after standard, fresh nerve allograft repair. Twenty donor rats underwent a ligature/ section of the left sciatic nerve 4 weeks before nerve graft harvesting. Forty recipient rats underwent severing of the left sciatic nerve leaving a 15-mm gap between the nerve stumps. Graft repair was undertaken using either the predegenerated left sciatic nerve of the 20 donor rats (predegenerated group, 20 recipient rats) or the normal right sciatic nerve of the 20 donor rats (fresh group, 20 recipient rats). Recovery of function was assessed by gait analysis, electrophysiologic testing and histologic studies. Walking tracks measurements at 2 and 3 months, electromyography parameters at 2 and 3 months, peroperative nerve conduction velocity and nerve action potential amplitude measurements at 3 months, as well as assessments of myelinated nerve fiber density and surface of myelination showed that fresh and predegenerated nerve grafts induced a comparable return of function although there was some trend in higher electrophysiologic values in the predegenerated group. The only slight but significant difference was a larger mean nerve fiber diameter in the nerve segment distal to a predegenerated nerve graft compared to a fresh nerve graft. Although our study does not show a dramatic long-term advantage for predegenerated nerve grafts compared to fresh nerve grafts, their use as prosthetic material is encouraging.

Expression of cell adhesion molecules in normal nerves, chronic axonal neuropathies and Schwann cell tumors

Cell adhesion molecules (CAMs) play a role in the normal development and regeneration of tissues as well as in the biological behaviour of tumors. We studied the immunohistochemical expression of various CAMs, such as neural cell adhesion molecule (NCAM), its polysialylated isoform (PSA-NCAM), epithelial (E-) cadherin, and beta1 integrins (alpha2beta1, alpha5beta1, alpha6beta1) in a series of frozen specimens of 10 normal nerves, 5 axonal neuropathies, 26 benign Schwannomas and 2 malignant peripheral nerve sheath tumors (MNST). NCAM was expressed by non-myelinating Schwann cells from normal nerves and overexpressed by Schwann cells from patients with chronic axonal neuropathies and Schwannomas. The expression was lower in MNST. Expression of PSA-NCAM was heterogeneously displayed by Schwann cells from the various tissues studied. Anti E-cadherin immunoreactivity was present in myelin sheath in normal nerves and axonopathies. It was expressed in some Schwannomas especially in vestibular Schwannomas. Integrins VLA alpha2 and VLA alpha6 were widely expressed by Schwann cells from normal nerves, axonal neuropathies and Schwannomas but their expression was low in MNST. VLA alpha5 was not expressed by Schwann cells from normal nerve and Schwannomas but present in chronic axonal neuropathies and MNST. In addition VLA alpha6 was strongly expressed by perineurial cells. These data show that CAMs have a characteristic pattern of expression in normal nerve. Also, some CAMs are always expressed by Schwann cells but the expression of others differs in normal nerves versus axonopathies or tumors, suggesting a role of the microcellular environment in the regulation of CAM expression. Schwannomas have different pattern of expression than MNST.

Identification and characterization of differentiation-dependent Schwann cell surface antigens by novel monoclonal antibodies: introduction of a marker common to the non-myelin-forming phenotype

In an attempt to identify and characterize novel Schwann cell surface molecules with putative functions during development, maintenance, and regeneration of the peripheral nervous system (PNS), we have produced monoclonal antibodies against viable neonatal rat Schwann cells. Using a sensitive live cell ELISA protocol, three monoclonal antibodies reactive with cultured Schwann cells, designated 27B10, 26F2, and 27C7 were isolated. The 27B10 and 26F2 antibodies specifically labelled forskolin-stimulated secondary Schwann cells in vitro as determined by live cell ELISA implying that the expression of the antigens in situ is regulated by axonal contact. The observation that the antigens seemed to be associated with both Schwann cell phenotypes clearly discriminated them from the well characterized myelin proteins as well as from molecules known to be confined to the non-myelin-forming phenotype. Interestingly, both antigens were found to be concentrated at the nodes of Ranvier. Further studies therefore have to show whether the identified antigens share structural or functional homology with adhesion or channel molecules, which display a similar distribution. Following transection of the adult sciatic nerve, the 26F2 antigen was rapidly down-regulated in the distal nerve stump. The 27C7 antibody reacted with an 80 kDa cell surface molecule common to non-myelin-forming Schwann cells. No differences in expression of the antigen between forskolin-treated and untreated Schwann cells in vitro were found, suggesting that the antigen is expressed independently from axonal contact. Two weeks after nerve transection in the absence of myelinating Schwann cells, the antigen was associated with S-100-positive Schwann cells of the distal nerve stump. The antigen was found to be expressed also by non-neuronal tissues, the level of the protein declined towards the adult stage. Comparison of the 27C7 antigen with previously described marker molecules suggests that we have identified a novel Schwann cell surface antigen of the non-myelin-forming phenotype.

A binding site for Pax proteins regulates expression of the gene for the neural cell adhesion molecule in the embryonic spinal cord

The neural cell adhesion molecule (N-CAM) mediates cell-cell interactions and is expressed in characteristic spatiotemporal patterns during development. In previous studies of factors that control N-CAM gene expression, we identified a binding site for the paired domain of Pax proteins (designated PBS) in the mouse N-CAM promoter. In this study, we demonstrate that a transcription factor known to be important for development of the central nervous system, Pax-6, binds to the N-CAM PBS and show that the PBS can influence N-CAM expression in vivo. Pax-6, produced in COS-1 cells, bound to the PBS through two half-sites, PBS-1 and PBS-2; mutations in both of these sites completely disrupted binding. Moreover, nuclear extracts from embryonic day (E) 11.5 mouse embryos bound to the PBS, and this binding was inhibited by antibodies to Pax-6. To determine the role of the PBS in vivo, we generated transgenic mice with N-CAM promoter/lacZ gene constructs containing either a wild-type or a mutated PBS. Mutations in PBS-1 and PBS-2 decreased the extent of beta-galactosidase expression in the mantle layer of the spinal cord limiting it to ventral regions at E11.5. At E14.5, these mutations eliminated most of the expression that was seen in the wild-type spinal cord. Taken together with our previous observations that the PBS binds multiple Pax proteins, the data indicate that such binding contributes to the regulation of N-CAM gene expression during neural development.