Polyclonal antibodies against NCAM and tenascin delay endplate reinnervation

The dynamic TRPV2 ion channel proximity proteome reveals functional links of calcium flux with cellular adhesion factors NCAM and L1CAM in neurite outgrowth

TRPV2 voltage-insensitive, calcium-permeable ion channels play important roles in cancer progression, immune response, and neuronal development. Despite TRPV2's physiological impact, underlying endogenous proteins mediating TRPV2 responses and affected signaling pathways remain elusive. Using quantitative peroxidase-catalyzed (APEX2) proximity proteomics we uncover dynamic changes in the TRPV2-proximal proteome and identify calcium signaling and cell adhesion factors recruited to the molecular channel neighborhood in response to activation. Quantitative TRPV2 proximity proteomics further revealed activation-induced enrichment of protein clusters with biological functions in neural and cellular projection. We demonstrate a functional connection between TRPV2 and the neural immunoglobulin cell adhesion molecules NCAM and L1CAM. NCAM and L1CAM stimulation robustly induces TRPV2 [Ca2+]I flux in neuronal PC12 cells and this TRPV2-specific [Ca2+]I flux requires activation of the protein kinase PKCα. TRPV2 expression directly impacts neurite lengths that are modulated by NCAM or L1CAM stimulation. Hence, TRPV2's calcium signaling plays a previously undescribed, yet vital role in cell adhesion, and TRPV2 calcium flux and neurite development are intricately linked via NCAM and L1CAM cell adhesion proteins.

Effects of Repeated Eyelid Injections with Botulinum Toxin A on Innervation of Treated Muscles in Patients with Blepharospasm

PURPOSE

To assess changes in innervation and muscle morphology after repeated botulinum toxin A injections in subjects with benign essential blepharospasm.

METHODS

Surgical waste specimens were processed for histologic examination of nerve fibers, neuromuscular junctions, fiber size, and central nucleation and compared to age matched controls and to two subjects with blepharospasm that had not received botulinum toxin A injections.

RESULTS

There was a significant increase in amount of nerve fibers and numbers of neuromuscular junctions in the orbicularis oculi muscles from subjects with blepharospasm treated repetitively with botulinum toxin A. In addition there was a significant decrease in mean muscle fiber cross-sectional area and an increase in central nucleation. The specimens from the subjects with only blepharospasm had the same density of nerves but had intermediate levels of neuromuscular junctions.

CONCLUSIONS

These data suggest that repeated injections of botulinum toxin A has an effect on nerve and neuromuscular junction numbers, which are partly mirrored in orbicularis oculi muscle from subjects with blepharospasm only. These studies suggest the potential for modulating these changes in order to extend the duration of effectiveness of botulinum toxin.

Putative Inhibitory Extracellular Matrix Molecules Do Not Prevent Dorsal Root Regeneration into Fetal Spinal Cord Transplants

We examined the distribution of several extracellular matrix molecules (ECM) and their relationship to regenerating axons in embryonic day 14 spinal cord transplants 1 to 12 weeks after transplantation into adult rats. We used immunocytochemical tech niques to label chondroitin sulfate proteoglycans (CSPGs) and tenascin-C in adjacent sections. Synthesis of these molecules by astrocytes is thought to be one mechanism by which astrocytes inhibit regeneration in the central nervous system (CNS); glial fibrillary acidic protein antibody was used to label astrocytes and examine their rela tionship to both the ECM molecules and regenerating calcitonin gene-related pep tide (CORP)-contammg dorsal roots. We also compared the expression and distribu tion of these five markers in transplants with normal spinal cord development.

The Injured and Regenerating Nervous System

Understanding restricted functional recovery and designing efficient treatments to alleviate dysfunction after injury of the nervous system remain major challenges in neuroscience and medicine. Numerous molecules of potential significance in neural repair have been identified in vitro, but only few of these have proved to be of major importance in vivo up to now. Among the molecules involved in regeneration are several members of the immunoglobulin superfamily, most notably the neural cell adhesion molecules L1, its close homologue CHL1, and NCAM and, in particular, its polysialic acid glycan moiety. Sufficient evidence is now available to justify the statement that these molecules are major players not only in nervous system development but also in the adult during neural repair and synaptic plasticity. Importantly, insights into the functions of these molecules in promoting or inhibiting functional recovery have allowed the design and assessment of therapeutic approaches in animal models of central nervous system injury that could prove to be applicable in clinical settings.

A novel motion analysis approach reveals late recovery in C57BL/6 mice and deficits in NCAM-deficient mice after sciatic nerve crush

Assessment of motor abilities after sciatic nerve injury in rodents, in particular mice, relies exclusively on walking track (footprint) analysis despite known limitations of this method. Using principles employed recently for video-based motion analyses after femoral nerve and spinal cord injuries, we have designed and report here a novel approach for functional assessments after sciatic nerve lesions in mice. Functional deficits are estimated by angle and distance measurements on single video frames recorded during beam-walking and inclined ladder climbing. Analyses of adult C57BL/6J mice after crush of the sciatic, tibial, or peroneal nerve allowed the identification of six numerical parameters, detecting impairments of the plantar flexion of the foot and the toe spread. Some of these parameters, as well as footprint functional indices, revealed severe impairment after crush injury of the sciatic or tibial, but not the peroneal nerve, and complete recovery within 3 weeks after lesion. Other novel estimates, however, showed that complete recovery is reached as late as 2-3 months after sciatic nerve crush. These measures detected both tibial and peroneal dysfunction. In contrast to the complete restoration of function in wild-type mice (100%), our new parameters, in contrast to the sciatic functional index, showed incomplete recovery (85%) 90 days after sciatic nerve crush in mice deficient in the neural cell adhesion molecule (NCAM). We conclude that the novel video-based approach is more precise, sensitive, and versatile than established tests, allowing objective numerical assessment of different motor functions in a sciatic nerve injury paradigm in mice.

Early detection of denervated muscle fibers in hindlimb muscles after sciatic nerve transection in wild type mice and in the G93A mouse model of amyotrophic lateral sclerosis

The cell adhesion molecule N-CAM is localized to the adult neuromuscular junction but is also expressed in the extrajunctional membrane of denervated muscles concurrent with extrajunctional acetylcholine receptors. Here we used N-CAM immunohistochemistry to determine whether we could detect early denervation in hindlimb muscles of the G93A transgenic mouse model of amyotrophic lateral sclerosis (ALS). In denervated wild type mouse muscles, N-CAM immunoreactivity on the sarcolemma of all fiber types and within the sarcoplasm of only type IIA fibers was detected at day 2: approximately 30% of the muscle fibers in cross-section were fully circumscribed by N-CAM immunoreactivity and approximately 25% of fibers were incompletely circumscribed. The proportion of the latter fibers remained constant over the next 8 days as the proportions of the former fibers increased exponentially. Thereafter, fully circumscribed muscle fibers increased to a maximum by 30 days with a concomitant fall in the incompletely circumscribed fibers. Hence, early muscle denervation was detected by the incomplete circumscription of fiber membranes by N-CAM immunoreactivity with full circumscription and intracellular localization indicating more long-term denervation. In the G93A transgenic mouse, rapid denervation of fast-twitch muscles was readily detected by a corresponding proportion of muscle fibers in cross-section with positive N-CAM immunoreactivity. The proportions of incompletely and completely circumscribed muscle fibers corresponded well with the rate of decline in intact motor units and reduced muscle contractile forces. Progressively more fully circumscribed muscle fibers became evident with age. We conclude that the N-CAM immunoreactivity on muscle fiber membranes in muscle cross-sections provides a sensitive means of detecting early muscle fiber denervation.

Distinct patterns of motor nerve terminal sprouting induced by ciliary neurotrophic factor vs. botulinum toxin

Both diffusible and surface-bound molecules are thought to induce sprouting of motor nerve terminals in response to paralysis. Here we report that the sprouting induced by ciliary neurotrophic factor (CNTF) is qualitatively different from the sprouting induced by botulinum toxin (BoTX). We show first that subcutaneous application of CNTF to levator auris longus muscles of adult mice evokes sprouting from nearly all nerve terminals. Surprisingly, however, most terminal sprouts remain within the boundaries of the endplate region and rarely grow extrasynaptically even if CNTF is administered chronically. In contrast, terminal sprouts induced by BoTX extend vigorously along the extrasynaptic muscle surface. The different patterns of sprout elongation are attributable in part to different patterns of initiation: whereas CNTF-induced sprouts emerge randomly from the surface of terminal branches, BoTX-induced sprouts emerge exclusively along the perimeter of terminal branches in direct apposition to muscle fiber membranes. Combined treatment with CNTF and BoTX produces exceptionally robust extraterminal sprouting with little if any intrasynaptic growth of terminal sprouts. We interpret these results as showing that paralysis induces sprouting primarily by muscle-associated, surface-bound molecules rather than by diffusible factors. Our findings may be useful in defining the physiological role of the numerous candidate sprouting-inducers and in promoting compensatory sprouting after nerve injury for therapeutic benefit.

Opposite impacts of tenascin-C and tenascin-R deficiency in mice on the functional outcome of facial nerve repair

The glycoproteins tenascin-C (TNC) and tenascin-R (TNR) are extracellular matrix proteins involved in the development, plasticity and repair of the nervous system. Altered expression patterns after nerve lesions in adult animals have suggested that these molecules influence axonal regeneration. To test this hypothesis, we investigated adult mice constitutively deficient in the expression of TNC, TNR or both, using the facial nerve injury paradigm. Quantitative analysis of vibrissal movements prior to nerve transection and repair (facial-facial anastomosis) did not reveal genotype-specific differences, and thus impacts of the mutations on motor function in intact animals. Two months after nerve repair, recovery of vibrissal whisking was poor in wild-type mice, a typical finding after facial-facial anastomosis in rodents. Differential effects of the mutations on whisking were found: recovery of function was worse in TNC-deficient and better in TNR null mice compared with wild-type littermates. In double-knockout animals, vibrissal performance was insufficient, but to a lesser extent compared with TNC null mutant mice. Retrograde labelling of motoneurons in the same animals showed that similar numbers of motoneurons had reinnervated the whisker pads in all experimental groups precluding varying extents of motoneuron death and/or axon regeneration failures as causes for the different outcomes of nerve repair. Our results provide strong evidence that TNC promotes and TNR impedes recovery after nerve lesion. These findings are of particular interest with regard to the scanty knowledge about factors determining success of regeneration in the peripheral nervous system of mammals.

Neurite outgrowth by the alternatively spliced region of human tenascin-C is mediated by neuronal alpha7beta1 integrin

The region of tenascin-C containing only alternately spliced fibronectin type-III repeat D (fnD) increases neurite outgrowth by itself and also as part of tenascin-C. We previously localized the active site within fnD to an eight amino acid sequence unique to tenascin-C, VFDNFVLK, and showed that the amino acids FD and FV are required for activity. The purpose of this study was to identify the neuronal receptor that interacts with VFDNFVLK and to investigate the hypothesis that FD and FV are important for receptor binding. Function-blocking antibodies against both alpha7 and beta1 integrin subunits were found to abolish VFDNFVLK-mediated process extension from cerebellar granule neurons. VFDNFVLK but not its mutant, VSPNGSLK, induced clustering of neuronal beta1 integrin immunoreactivity. This strongly implicates FD and FV as important structural elements for receptor activation. Moreover, biochemical experiments revealed an association of the alpha7beta1 integrin with tenascin-C peptides containing the VFDNFVLK sequence but not with peptides with alterations in FD and/or FV. These findings are the first to provide evidence that the alpha7beta1 integrin mediates a response to tenascin-C and the first to demonstrate a functional role for the alpha7beta1 integrin receptor in CNS neurons.

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.

The spatiotemporal relationship among Schwann cells, axons and postsynaptic acetylcholine receptor regions during muscle reinnervation in aged rats

To morphologically define the aging-related features during muscle reinnervation the spatiotemporal relationships among the major components of the neuromuscular junctions (NMJs) were investigated. A total of 64 rats, 30 adults (4 months old) and 34 aged adults (24 months old), were used. Between 1 and 12 weeks after sciatic nerve-crushing injury, cryosections of skeletal muscle were single or double labeled for S100, a marker of Schwann cells (SCs), for protein gene product 9.5, a neuronal marker, and for alpha-bungarotoxin (alpha-BT), a marker of the acetylcholine receptor site (AChR site), and then observed by confocal laser microscopy. The most obvious age changes were noted: (1) the regenerating SCs and axons were delayed in their arrival at the NMJ, (2) the dimensions of terminal SCs and AChR sites displayed a drastic and long-lasting drop (for terminal SCs, during 1-8 weeks; for AChR sites, during 1-12 weeks); (3) the degree of spatial overlap between AChR sites and terminal SCs was markedly low until 8 weeks post-crush; (4) damage and poor formation in the SCs, terminal axons and AChR sites, together with poor process extension from the terminal SC or terminal axon, were pronounced; (5) persistent aberrant changes, such as multiple innervation and terminal axon sprouting, together with poorly formed collateral innervation, nerve bundles, and NMJs, more frequently occurred in the later reinnervation period. Thus, with aging, regeneration is impaired during the period in which regenerating SC strands and axons extend into NMJs and the subsequent establishment of nerve-muscle contact is in progress. A complex set of morphological abnormalities between or among the TSCs, terminal axons, and AChR sites may be important in slowing of regeneration and reinnervation in aged motor endplates.

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.

Chondroitin sulfates affect the formation of the segmental motor nerves in zebrafish embryos

Chondroitin sulfates have been implicated in the promotion and in the inhibition of axon growth. In the zebrafish embryo, chondroitin sulfates are present at the interface of the somites and the notochord where spinal motor axons extend ventrally to establish the midsegmental ventral motor nerves. Injection of chondroitinase ABC prior to motor axon outgrowth effectively removed all chondroitin sulfate immunoreactivity and induced abnormal axonal outgrowth in many (39%) of the ventral motor nerves. The most common abnormality was the formation of side branches, approximately half of which extended posteriorly, the others anteriorly. The effect was specific to the removal of chondroitin sulfates, since injections of vehicle solution or of heparinase III did not affect the ventral motor nerves. Electron microscopic examination demonstrated that the injections caused no damage to spinal cord, somite, and notochord. This suggests that chondroitin sulfates normally constrain the outgrowth of the ventral motor nerves. Consistent with this hypothesis, injections of soluble chondroitin sulfates, either as a mixture or individually, led to truncated or missing ventral motor nerves. Truncations were most frequent after injection of chondroitin sulfate-B (up to 23%) while chondroitin sulfate-A had a lesser, and chondroitin sulfate-C no apparent, effect.

Ectopic expression of NCAM in skeletal muscle of transgenic mice results in terminal sprouting at the neuromuscular junction and altered structure but not function

The neuromuscular system provides an excellent model for the analysis of molecular interactions involved in the development and plasticity of synaptic contacts. The neural cell adhesion molecule (NCAM) is believed to be involved in the development and plasticity of the neuromuscular junction, in particular the axonal sprouting response observed in paralyzed and denervated muscle. In order to explore the role of myofiber NCAM in modulating the differentiation of motor neurons, we generated transgenic mice expressing a GPI-anchored NCAM isoform that is normally found in developing and denervated muscle, under the control of a skeletal muscle-specific promoter. This results in the constitutive expression of NCAM at postnatal ages, a time when the endogenous mouse NCAM is absent from the myofiber. We found that a significant number of neuromuscular junctions in adult transgenic animals displayed terminal sprouting (>20%) reminiscent of that elicited in response to cessation of neuromuscular activity. Additionally, a significant increase in the size and complexity of neuromuscular synapses as a result of extensive intraterminal sprouting was detected. Electrophysiological studies, however, revealed no significant alterations of neuromuscular transmission at this highly efficient synapse. Sprouting in response to paralysis or following nerve crush was also significantly enhanced in transgenic animals. These results suggest that in this ectopic expression model NCAM can directly modulate synaptic structure and motor neuron-muscle interactions. The results contrast with knockout experiments of the NCAM gene, where very limited changes in the neuromuscular system were observed.

The tenascin-C knockout revisited

In the past seven years, two groups have independently produced tenascin-C-knockout mice. These mice are born alive and, originally, were described as showing no abnormalities. More recent studies, many involving pathological intervention, have shown that tenascin-C-knockout mice have several defects. The mice exhibit abnormal behaviour, as well as abnormalities in brain chemistry. They also show defects in structure and repair of neuromuscular junctions, in the ability to recover from snake-venom-induced glomerulonephritis and in chemically induced dermatitis. Healing of skin wounds is morphologically normal, but the mice exhibit defects in healing after suture injury of corneas. In both skin and corneal wounds, fibronectin expression is abnormally low in tenascin-C-knockout mice. Finally, in vitro studies indicate that haemopoietic activity is defective in bone marrow from these mice. When examined together, these studies provide evidence for precise functions for tenascin-C, as well as an explanation for why the sequence of tenascin-C is so highly phylogenetically conserved.

Non-myelin-forming perisynaptic schwann cells express protein zero and myelin-associated glycoprotein

Perisynaptic Schwann cells (PSCs) envelop axonal terminals and are physiologically distinct from the nearby myelinating Schwann cells (MSCs), which surround the same innervating motor axons. PSCs have special functions at the neuromuscular synapse, where they detect and can modulate neurotransmitter release. Although PSCs are similar to non-myelinating Schwann cells in that they do not form multiple myelin wrappings around nerve terminals, they do wrap around single nerve terminals. These differences, as well as others, lead us to question whether PSCs are truly of Schwann cell origin. We thus characterized the expression of molecules, classically associated with myelin and Schwann cells, in PSCs at the frog neuromuscular junction. We wondered whether PSCs express the Schwann cell marker protein zero (P(0)) and whether their lack of myelination was related to an absence of myelin-associated glycoprotein (MAG), a protein found in myelinating cells that is considered important in myelination. Instead, we found that PSCs express both P(0) and MAG, and other myelinating glial markers such as galactocerebroside and 2',3'-cyclic nucleotide 3'-phosphodiesterase. In denervated preparations, P(0) and MAG expression persisted, including at newly formed PSC extensions. Because PSCs do not myelinate, it is clear that expression of these proteins alone is not sufficient for myelin formation. It is possible that factors present at synapses may prevent myelination, while P(0) and MAG may mediate adhesion between nerve terminals and the surrounding PSCs. The results indicate that PSCs are of Schwann cell origin.

Immunolocalization of tenascin-C in human type II fiber atrophy

Tenascin-C is a multifunctional extracellular matrix glycoprotein with stimulatory and anti-adhesive or inhibitory properties for axon growth. Its location and discontinuous expression are restricted in innervated muscle tissues. Tenascin-C accumulated interstitially among human denervated muscle fibers and close to normal-sized fibers. To expand our knowledge of the expression of tenascin-C in human neuromuscular disorders, we investigated immunohistologically 20 human muscle specimens with type II myofiber atrophy of children and adults. Tenascin-C immunoreactivity in adult type II atrophy was frequent, and accumulation in children was sparse and weak. In both groups, tenascin-C immunoreactivity was found: 1. Interstitially around normal-sized type II muscle fibers. 2. Around atrophic type II muscle fibers. 3. Around small-caliber myofibers with centrally located nuclei. These results indicate that tenascin-C immunoreactivity: (1) is detectable around early denervated and reinnervated muscle fibers and, therefore, (2) may reflect in part the molecularly ongoing process of denervation and reinnervation in human type II fiber atrophy.

Tenascin-C modulates tumor stroma and monocyte/macrophage recruitment but not tumor growth or metastasis in a mouse strain with spontaneous mammary cancer

The local growth of tumors and their ability to metastasize are crucially dependent on their interactions with the surrounding extracellular matrix. Tenascin-C (TNC) is an extracellular matrix protein which is highly expressed during development, tissue repair and cancer. Despite the high levels of TNC in the stroma of primary and metastatic tumors, the function of TNC is not known. In the present study we have crossed TNC-null mice with a mouse strain where both female and male mice spontaneously develop mammary tumors followed by metastatic disease in the lungs. We report that the absence of TNC had no effect on the temporal occurrence of mammary tumors and their metastatic dissemination in lungs. Furthermore, the number and size of tumors, the number and size of metastatic foci in the lungs, the proliferation rate and apoptosis of tumor cells and tumor angiogenesis were not altered in the absence of TNC. Histological examination revealed that the tumor organisation, however, was modulated by TNC. In the presence of TNC both primary as well as metastatic tumors were organised in large tumor cell nests surrounded by thick layers of extracellular matrix proteins. In the absence of TNC these tumor cell nests were smaller but still separated from each other by extracellular matrix proteins. In addition, the TNC-null stromal compartment contained significantly more monocytes/macrophages than tumor stroma from TNC wild-type mice. Using in vitro coculture experiments we show that TNC-null tumor cells were still able to activate the TNC gene in fibroblasts which express low basal levels of TNC. Altogether these data indicate that TNC has a very limited role during the spontaneous development and growth of mamary tumors and their metastasis to the lungs.

Association of adhesive macromolecules with terminal sprouts at the neuromuscular junction after botulinum treatment

Small quantities of botulinum toxin (BTX) are useful in the treatment of certain movement disorders, such as laryngeal spasmodic dysphonia, blepharospasm, and cervical dystonia. However, the corrective paralytic effects of BTX are only temporary, in part because of the formation of remodeled neuromuscular junctions. Here, we questioned whether various factors within and near the neuromuscular junction could contribute to the remodeling seen after BTX treatment. BTX was injected subcutaneously in the region of the levator auris longus muscle. At 1-week intervals, levator auris longus muscles were removed and examined histochemically. As previously described, BTX treatment results in a progressive elongation of end plates. The neural cell adhesion molecule was not associated with the elongated end plates but was associated with the BTX-induced nerve sprouts after long intervals (3 to 4 weeks). Similarly, after BTX, laminin-1 (composed of alpha 1, beta 1, and gamma 1 chains) reactivity was associated with the nerve sprouts, but not with the end plates. Laminin beta 2 reactivity at the end plate dispersed somewhat within 1 week but remained diffusely associated with the elongating end plates for up to 5 weeks. Together these results suggest that neural cell adhesion molecule and laminins may participate in the sprouting observed after BTX treatment and that alterations in laminin beta 2 expression may participate in initial loss of contacts.

Accumulation of F-spondin in injured peripheral nerve promotes the outgrowth of sensory axons

F-spondin, an extracellular matrix protein, is present in peripheral nerve during embryonic development, but its amount diminishes by birth. Axotomy of adult rat sciatic nerve, however, causes a massive upregulation of both F-spondin mRNA and protein distal to the lesion. F-spondin in the distal stump of axotomized nerve promotes neurite outgrowth of sensory neurons, as revealed by protein neutralization with F-spondin-specific antibodies. Thus, F-spondin is likely to play a role in promoting axonal regeneration after nerve injury.

The expression of tenascin-C along the lamprey olfactory pathway during embryonic development and following axotomy-induced replacement of the olfactory receptor neurons

Extracellular guidance molecules affect the pathway of growing axons by both attractive and repulsive interactions. Tenascin-C, a glycoprotein of the extracellular matrix, is localized along developing axonal pathways where it may function by repulsion, restricting axons within specific boundaries. The lamprey olfactory pathway offers an advantageous model for studying the role of extracellular matrix proteins in axon guidance because the entire pathway is readily seen in horizontal sections and because lesioning the olfactory nerve will induce the system into a new phase of coordinated neurogenesis and axon outgrowth. Although tenascin-C expression was absent during embryonic development, olfactory nerve fascicles contained tenascin-C-immunoreactivity (IR) during the larval stage. During retrograde degeneration, the fascicles lost tenascin-C-IR. Diffuse unfasciculated axonal processes extending from the olfactory epithelium did not express tenascin-C-IR; however, acetylated tubulin and GAP-43-IR was present, indicating axonal outgrowth. When the newly extended axons of olfactory receptor neurons converged to form fascicles, tenascin-C-IR was evident within the fascicular boundaries. The absence of tenascin-C expression when axonal process were short and diffuse, and its return when axons coalesced within fascicles, supports the view that tenascin-C functions as a boundary molecule in the olfactory pathway.

Recovery of corneal nerve morphology following laser in situ keratomileusis

Morphological changes in the corneal nerves after laser in situ keratomileusis (LASIK) were investigated and the changes were compared with those observed after creation of the keratectomy flap without subsequent photoablation. After creating the hinged flap, a multizone excimer laser photoablation (myopic correction from 6.00 to 6.66 D; diameter 6 mm) was performed on 27 rabbit corneas. Seven of these 27 rabbits received an automated keratectomy without laser photoablation on the fellow eye. A histochemical acetylcholinesterase reaction was used to demonstrate the changes in the morphology of the corneal nerves 3 days, 2.5 and 5 months after the operations. In all specimens the deepest stromal nerve bundles showed normal morphology. Cut nerve trunks were found at the wound margins and at the level of the flap interphase in the stromal bed. At 3 days, both epithelial and basal epithelial/subepithelial nerves were found at the hinge of the flap but the rest of the flap showed a major loss of epithelial, basal epithelial/subepithelial and superficial stromal nerves. A few new regenerating thin nerve fibers were found to emerge from the cut stromal nerve trunks. They appeared to pass the wound margin into the flap area below the epithelium. At 2.5 and 5 months an increasing number of regenerating nerve leashes were observed to emerge from the cut stromal nerve trunks. They appeared to send anastomosing fibers among the neighboring stromal nerves. By this time the epithelial, basal epithelial/subepithelial and anterior stromal innervation had gained an almost normal nerve density and architecture. In the corneas with the flap only, the epithelial innervation was slightly better spared in the center of the flap, and the stromal changes were somewhat less prominent compared with the LASIK corneas.

Organization and reorganization of neuromuscular junctions in mice lacking neural cell adhesion molecule, tenascin-C, or fibroblast growth factor-5

Many proteins have been hypothesized to mediate intercellular interactions that regulate the formation, maturation, and maintenance of the skeletal neuromuscular junction. Three of the best characterized of these are a membrane-associated adhesion molecule, neural cell adhesion molecule (N-CAM), an extracellular matrix component, tenascin-C, and a soluble growth factor, fibroblast growth factor-5 (FGF-5). To assess the roles of these molecules in synaptogenesis in vivo, we examined neuromuscular junctions in homozygous mutant mice lacking N-CAM, tenascin-C, FGF-5, or both N-CAM and tenascin-C. End plates were 14% smaller in N-CAM-deficient mice than in controls, and formation of junctional folds was delayed in this mutant. In all other respects tested, however, the structure and molecular architecture of neuromuscular junctions were normal in all three single mutants and in the double mutant. We also tested the abilities of damaged motor axons to reinnervate mutant muscle after axotomy and of intact motor axons to sprout after partial denervation. Again, no significant differences among genotypes were observed. Together, these results demonstrate that N-CAM, tenascin-C, and FGF-5 are dispensable for major aspects of synaptic development and regeneration.

Up-regulation of astrocyte-derived tenascin-C correlates with neurite outgrowth in the rat dentate gyrus after unilateral entorhinal cortex lesion

The extracellular matrix protein tenascin-C has been implicated in the regulation of axonal growth. Using unilateral entorhinal cortex lesions, which induce a massive sprouting response in the denervated outer molecular layer of the rat fascia dentata, the role of tenascin-C for axonal growth was investigated in vivo. Monoclonal antibodies against the neurite outgrowth and anti-adhesive domains of the molecule were employed. Immunostaining was increased throughout the denervated outer molecular layer by day 2, reached a maximum around day 10, and was back to control levels by four weeks post lesion. Growth cone deflecting as well as neurite outgrowth promoting isoforms of tenascin-C were up-regulated after the lesion. Using electron microscopy, single intensely tenascin-C immunoreactive cells were identified as reactive astrocytes that phagocytose degenerated terminals. In situ hybridization histochemistry for tenascin-C messenger RNA revealed numerous cellular profiles in the denervated outer molecular layer of the ipsilateral and contralateral dentate gyrus two days post lesion. Tenascin-C messenger RNA-positive cells in the outer molecular layer were identified as astrocytes using double-labelling for tenascin-C messenger RNA and glial fibrillary acidic protein immunohistochemistry. Thus, a tenascin-C-rich substrate is present in the outer molecular layer during the time of sprouting and a sharp boundary is formed against the inner molecular layer. This pattern may contribute to the layer-specific sprouting response of surviving afferents after entorhinal lesion. Neurite outgrowth may be promoted within the denervated zone, whereas axons trying to grow into the denervated outer molecular layer, for example from the inner molecular layer, would be deflected by a tenascin-C-rich barrier.

Chondroitin sulfate proteoglycan and tenascin in the wounded adult mouse neostriatum in vitro: dopamine neuron attachment and process outgrowth

Extracellular matrix (ECM) molecules, including chondroitin-4 or chondroitin-6 sulfate proteoglycans (CSPGs) and tenascin, are upregulated in and around wounds and transplants to the adult CNS. In the present study, striatal wounds from adult mice were used in a novel in vitro paradigm to assess the effects of these wound-associated molecules on embryonic dopamine cell attachment and neurite outgrowth. Light and electron microscopic immunocytochemistry studies have shown that astroglial scar constituents persist in cultured explants for at least 1 week in vitro, and despite the loss of neurons from adult striatal explants, there is a retention of certain structural features suggesting that the wound explant-neuron coplant is a viable model for analysis of graft-scar interactions. Explants from the wounded striatum taken at different times after a penetrating injury in vivo were used as substrates for embryonic ventral mesencephalon neurons that were plated on their surfaces. Dopamine cell attachment is increased significantly in relation to the expression of both CSPG and tenascin. The increase in neuronal attachment in this paradigm, however, is accompanied by a postlesion survival time-dependent significant decrease in neuritic growth from these cells. In vitro ECM antibody treatment suggests that CSPG may be responsible for heightened dopamine cell attachment and that tenascin simultaneously may support cell attachment while inhibiting neurite growth. The present study offers a new approach for the in vitro analysis of cell and molecular interactions after brain injury and brain grafting, in essence acting as a nigrostriatal transplant-in-a-dish.

Tenascin-C expression correlates with macrophage invasion in Duchenne muscular dystrophy and in myositis

Tenascin-C (TN-C) is an extracellular matrix protein expressed during development in several tissues, but restricted to only a few areas in normal adult tissues. By immunizing mice with human fetal myoblasts we generated a monoclonal antibody to TN-C and mapped the epitope to the aminoterminal end containing EGF-like repeats. Using this antibody we detected by immunohistochemistry TN-C in the epimysium and perimysium of human fetal muscles, as well as in nonfibrillar deposits in myoblast cultures. In situ hybridization did not reveal any signal within human fetal muscle groups, suggesting that non-muscle cells synthesize the majority of the tenascin that localizes in and around human fetal muscle. Immunohistochemical analysis of muscle biopsies from Duchenne/Becker muscular dystrophy and myositis patients revealed that TN-C is expressed in skeletal muscle. Although the patterns of TN-C immunoreactivity were quite different in the two disease entities, the endomysial TN-C reactivity in both DMD/BMD and in myositis invariably correlated with the presence of macrophages.

Characteristic expression of cell adhesion molecules in adhalin deficiency

We have reported the reduction of the B1 subunit of laminin and that of heparan sulfate proteoglycan (HSPG) in two Japanese patients with adhalin deficiency. We here investigated immunohistochemically the expression of cell adhesion molecules, including intercellular adhesion molecule-1 (ICAM-1), neural cell adhesion molecule (NCAM), and CD44 (HCAM), in four Japanese patients with adhalin deficiency, compared to other types of muscular dystrophy. We found that NCAM was upregulated in a fair number of muscle fibers, regardless of the type of muscular dystrophy. ICAM-1 was detected on the rare muscle cell membrane in all patients. CD44 was barely detected on the muscle cell membrane in adhalin deficiency, in contrast to the strong expression of CD44 which was observed in other types of muscular dystrophy. These findings suggest that a different degenerative or regenerative process is involved in adhalin deficiency compared to other types of muscular dystrophy.

Skin wounds and severed nerves heal normally in mice lacking tenascin-C

A large number of functions have been demonstrated for tenascin-C by antibody perturbation assays and in vitro cell culture experiments. However, these results contrast sharply with the lack of any apparent phenotype in mice with a genetic deletion of tenascin-C. A possible explanation for the lack of phenotype would be expression of some altered but functional tenascin-C in the mutant. We report the generation of an independent tenascin-C null mouse and conclude that the original tenascin-C knockout, which is genetically very similar to ours, is also a true null. As found previously, the absence of tenascin-C has no influence on development, adulthood, life span, and fecundity. We have studied in detail two models of wound healing. After axotomy, the regeneration of the sciatic nerve is not altered without tenascin-C. During healing of cutaneous wounds, deposition of collagen I, fibulin-2, and nidogen is identical in mutant and wild-type mice. In contrast. fibronectin appears diminished in wounds of tenascin-C-deficient mice. However, the lack of tenascin-C together with the reduced amount of fibronectin has no influence on the quality of the healing process.

Glial cell interactions with tenascin-C: adhesion and repulsion to different tenascin-C domains is cell type related

The multimodular glycoprotein tenascin-C is transiently expressed, predominantly by glial cells, during the development of the central and peripheral nervous systems. This extracellular matrix glycoprotein is involved in the control of cell adhesion, neuron migration and neurite outgrowth. Distinct functional properties for neuronal cell types have been attributed to separate tenascin-C domains using antibody perturbation studies and in vitro experiments on tenascin-C fragments. In order to study potential roles of tenascin-C for glial cell biology, a library of recombinant tenascin-C domains was used in a bioassay in vitro. Embryonic day 14 astrocytes, various astroglial-derived cell lines (C6, A7 and Neu7) and oligodendroglial-derived cell types (Oli-neu and G26-20) were examined in an adhesion assay and compared to the neuroblastoma cell line N2A. A binding site for most cell types, except for A7 and N2A, could be assigned to the first three fibronectin type III domains. Repulsive properties could be mapped to three different sites the epidermal growth factor-like repeats, fibronectin type III repeats 4 and 5 and to the alternatively spliced region of the molecule. The responses to these repulsive sites varied according to the cell type. These data are consistent with the interpretation that different cell types express distinct sets of tenascin-C receptors which might regulate cellular responses via distinct second messenger pathways.

Contractile activity regulates isoform expression and polysialylation of NCAM in cultured myotubes: involvement of Ca2+ and protein kinase C

Muscle development involves a series of complex cell-cell interactions that are mediated, at least in part, by several different cell adhesion molecules. Previous work from this lab showed that the different isoforms of NCAM and its level of polysialylation are developmentally regulated during chick myogenesis in vivo and that this regulation is important for normal muscle development. Using developing chick secondary myotubes grown in culture, we show here that both the polysialylation of NCAM and the developmental switch in isoform expression are regulated by activity and that Ca2+ entry through voltage-gated channels and the subsequent activation of protein kinase C are required for the developmental changes in NCAM isoform synthesis. Specifically, PSA expression was shown to be developmentally regulated with high expression being temporally correlated with the onset of spontaneous contractile activity. Furthermore, blocking contractile activity caused a decrease in PSA expression, while increasing activity with electrical stimulation resulted in its up-regulation. Immunoblot and metabolic labeling studies indicated that dividing myoblasts synthesize primarily 145-kD NCAM, newly formed, spontaneously contracting myotubes synthesize 130-, 145-, and 155-kD NCAM isoforms, while older, more mature myotubes primarily synthesize the glycosylphosphatidylinositol-anchored 130-kD isoform which, in contrast to the other three isoforms, had a high rate of turnover. This developmental switch in NCAM isoform expression could be inhibited with Ca2+ channel blockers and inhibitors of protein kinase C. Taken together, these results suggest that Ca2+ ions and protein kinase C are involved in a second messenger cascade coupling membrane depolarization with transcriptional factors that regulate NCAM isoform synthesis and polysialylation.

Tenascin-C contains distinct adhesive, anti-adhesive, and neurite outgrowth promoting sites for neurons

The glia-derived extracellular matrix glycoprotein tenascin-C (TN-C) is transiently expressed in the developing CNS and may mediate neuron-glia interactions. Perturbation experiments with specific monoclonal antibodies suggested that TN-C functions for neural cells are encoded by distinct sites of the glycoprotein (Faissner, A., A. Scholze, and B. Götz. 1994. Tenascin glycoproteins in developing neural tissues--only decoration? Persp. Dev. Neurobiol. 2:53-66). To characterize these further, bacterially expressed recombinant domains were generated and used for functional studies. Several short-term-binding sites for mouse CNS neurons could be assigned to the fibronectin type III (FNIII) domains. Of these, the alternatively spliced insert TNfnA1,2,4,B,D supported initial attachment for both embryonic day 18 (E18) rat and postnatal day 6 (P6) mouse neurons. Only TNfn1-3 supported binding and growth of P6 mouse cerebellar neurons after 24 h, whereas attachment to the other domains proved reversible and resulted in cell detachment or aggregation. In choice assays on patterned substrates, repulsive properties could be attributed to the EGF-type repeats TNegf, and to TNfnA1,2,4. Finally, neurite outgrowth promoting properties for E18 rat hippocampal neurons and P0 mouse DRG explants could be assigned to TNfnB,D, TNfnD,6, and TNfn6. The epitope of mAb J1/tn2 which abolishes the neurite outgrowth inducing effect of intact TN-C could be allocated to TNfnD. These observations suggest that TN-C harbors distinct cell-binding, repulsive, and neurite outgrowth promoting sites for neurons. Furthermore, the properties of isoform-specific TN-C domains suggest functional significance of the alternative splicing of TN-C glycoproteins.

Molecular basis of interactions between regenerating adult rat thalamic axons and Schwann cells in peripheral nerve grafts. II. Tenascin-C

Tenascin-C is a developmentally regulated extracellular matrix component. There is evidence that it may be involved in axon growth and regeneration in peripheral nerves. We have used in situ hybridization and immunocytochemistry to investigate the association of tenascin-C with central nervous system axons regenerating through a peripheral nerve autograft inserted into the thalamus of adult rats. Between 3 days and 4 weeks after implantation, tenascin-C immunoreactivity was increased in the grafts, first at the graft/brain interface, then in the endoneurium of the graft, and finally within the Schwann cell columns of the graft. By electron microscopy, reaction product was present around collagen fibrils and basal laminae in the endoneurium, but the heaviest deposits were found at the surface of regenerating thalamic axons within Schwann cell columns. Schwann cell surfaces were not associated with tenascin-C reaction product except where they faced the tenascin-rich basal lamina or were immediately opposite axons surrounded by tenascin-C. By 8 weeks after graft implantation tenascin-C in the endoneurium and around axons of the graft was decreased. In the brain parenchyma around the proximal part of the graft, axonal sprouts associated with tenascin-C could not be identified earlier than 2 weeks after grafting and were sparse at this stage. Larger numbers of such axons were present at 8-13 weeks after grafting and were located predominantly where the glia limitans between brain and graft appeared to be incomplete, suggesting that the tenascin-C may have penetrated the brain parenchyma from the graft. By in situ hybridization, cells expressing tenascin-C mRNA (probably Schwann cells) appeared first at the brain/graft interface 3 days after grafting and thereafter were mainly located within the grafts. Lightly labelled cells containing tenascin-C mRNA (probably glial cells) were scattered in the thalamic parenchyma both ipsilateral and contralateral to the graft and a few heavily labelled cells were located very close to the tip of the graft. These results show that regenerating adult thalamic axons, unlike regenerating peripheral axons, become intimately associated with peripheral nerve graft-derived tenascin-C, suggesting that they express a tenascin-C receptor, as many neurons do during development, and that tenascin-C derived from Schwann cells may play a role in the regenerative growth of such axons through the grafts.

Differential distributions of HNK-1 and tenascin immunoreactivity during innervation of myotomal muscle in Xenopus

The HNK-1 antibody labels developing neurons and their processes in Xenopus [40,43]. HNK-1 recognizes a carbohydrate bound to subsets of a number of cell and extracellular matrix (ECM) adhesion molecules, including those of the tenascin/cytotactin family. Both HNK-1 and Tenascin (TN) antigens have been implicated in neural development. Here we describe and compare expression patterns of the two antigens during development of the myotomal neuromuscular system. HNK-1 marks peripheral motor axon processes, including their fine branches at neuromuscular junctions (NMJs), in a pattern that persists throughout development. NMJs of myotomal muscle are located on either side of the intermyotomal spaces (IMS). From the beginning, TN-immunoreactivity (TN-IR) appears predominantly at the IMS, but not on peripheral nerves. Expression of both antigens is graded along the anteroposterior axis, with tenascin appearing first. At the ultrastructural level, HNK1-immunoreactivity appears on motor axons and at the neuromuscular junction on pre- and postsynaptic membranes. In contrast, TN-IR appears ubiquitously in the ECM of the intermyotomal junction, but not in association with axons or synaptic components. The respective distributions of the two antigens during myotomal innervation indicate that tenascin does not bear the HNK-1 antigen in Xenopus embryos and larvae, but that both antigens occupy positions consistent with roles in the establishment of NMJs at the myotomes.

Tenascin-C expression by neurons and glial cells in the rat spinal cord: changes during postnatal development and after dorsal root or sciatic nerve injury

We have used in situ hybridization with a digoxigenin-labelled probe for tenascin-C mRNA and immunocytochemistry with antibodies against tenascin-C, glial fibrillary acidic protein, OX-42 and the 200 kDa neurofilament protein to study the expression, distribution and cellular relationships of tenascin-C mRNA and protein in the developing (postnatal) and adult spinal cord of rat, and the effects thereon of dorsal root, ventral root and sciatic nerve injuries. The most interesting finding was that on postnatal day 7 (P7), P14 and in the adult, but not on P0 or P3, a group of neurons in the lumbar ventral horn expressed the tenascin-C mRNA gene. They represented about 5% of ventral horn neurons in the adult and were among the smaller such neurons. Since 40-60% of such cells were lost at P13 following sciatic nerve crush on P0, some were almost certainly motor neurons. In addition, we found that at P0 and P3, mRNA-containing glial cells were widespread in grey and white matter but sparse in the developing dorsal columns; tenascin-C immunofluorescence showed a similar distribution. By P7 there were fewer mRNA-containing cells in the ventral horns and in the area of the dorsal columns containing the developing corticospinal tract where immunofluorescence was also weak. At P14 there were no glial-like mRNA-containing cells in the grey matter; such cells were confined to the periphery of the lateral and ventral white columns but were present throughout the dorsal columns where tenascin-C immunofluorescence was also strong. No glial-like mRNA-containing cells were present in the adult lumbar spinal cord and tenascin-C immunofluorescence was confirmed to irregular patches in the ventral horn, especially around immunonegative cell bodies of small neurons, a zone around the central canal, and a thin zone adjacent to the glia limitans. Thus the expression of tenascin-C is differentially developmentally regulated in the grey matter and in different parts of the white matter. Three days after injury of dorsal roots L4-6, many cells containing tenascin-C mRNA, some identified as glial fibrillary acidic protein-positive astrocytes, were present in the ipsilateral dorsal column, but were rare after longer survivals. Immunoreactivity, however, was elevated in the ipsilateral dorsal column at 3 days, remained high for several months and disappeared at 6.5 months. Dorsal root injury had no effect on tenascin-C mRNA or protein in the grey matter. Sciatic nerve or ventral root injury had no effect on these molecules in any part of the spinal cord.

Boundaries and inhibitory molecules in developing neural tissues

Numerous studies of the past decade have illuminated the importance of intercellular adhesion events for neural pattern formation. It has been documented that members of the Ig and cadherin gene superfamilies, that glycoproteins and, probably to some extent, proteoglycans of the extracellular matrix play a role in this context. Recent observations suggest that, in addition to adhesive interactions, repulsive and/or inhibitory phenoma are also of importance in regulating neural pattern formation. Several molecules are under study which are considered possible mediators of inhibitory interactions in the nervous system. The hypothesis has been advanced that some of these might be partially responsible for restrictive, boundary-like properties ascribed to glial cells in developing and regenerating tissues. The current review summarizes these studies and focusses on molecular aspects of boundary and compartmentation phenomena.