Mice deficient for tenascin-R display alterations of the extracellular matrix and decreased axonal conduction velocities in the CNS

Regulation of Na+ channel distribution in the nervous system

An important aspect of Na+ channel regulation is their distribution on neuronal membranes within the nervous system. The complexity of this process is brought by the molecular diversity of Na+ channels and differential regulation of their distribution. In addition, Na+ channel localization is a highly dynamic process depending on the status of the cell in vitro, and (patho)physiological condition of the organism in vivo. Nonetheless, the pharmacological manipulation of Na+ channel distribution should be possible and will hopefully bring safer and more-potent medicines in the future.

Developmental expression of the novel voltage-gated sodium channel auxiliary subunit beta3, in rat CNS

1. We have compared the mRNA distribution of sodium channel alpha subunits known to be expressed during development with the known auxiliary subunits Nabeta1.1 and Nabeta2.1 and the novel, recently cloned subunit, beta3. 2. In situ hybridisation studies demonstrated high levels of Nav1.2, Nav1.3, Nav1.6 and beta3 mRNA at embryonic stages whilst Nabeta1.1 and Nabeta2.1 mRNA was absent throughout this period. 3. Nabeta1.1 and Nabeta2.1 expression occurred after postnatal day 3 (P3), increasing steadily in most brain regions until adulthood. beta3 expression differentially decreased after P3 in certain areas but remained high in the hippocampus and striatum. 4. Emulsion-dipped slides showed co-localisation of beta3 with Nav1.3 mRNA in areas of the CNS suggesting that these subunits may be capable of functional interaction. 5. Co-expression in Xenopus oocytes revealed that beta3 could modify the properties of Nav1.3; beta3 changed the equilibrium of Nav1.3 between the fast and slow gating modes and caused a negative shift in the voltage dependence of activation and inactivation. 6. In conclusion, beta3 is shown to be the predominant beta subunit expressed during development and is capable of modulating the kinetic properties of the embryonic Nav1.3 subunit. These findings provide new information regarding the nature and properties of voltage-gated sodium channels during development.

Intrahippocampal administration of an antibody against the HNK-1 carbohydrate impairs memory consolidation in an inhibitory learning task in mice

Many cell adhesion molecules express the HNK-1 carbohydrate involved in formation and functioning of synapses. To assess its role in learning, we injected the monoclonal HNK-1 antibody or nonimmune IgG into the hippocampus of C57BL/6J mice 1 h after training in a step-down avoidance task. In animals treated with the HNK-1 antibody, latencies of step down in a recall session 48 h after injection did not change compared to training values and were significantly shorter versus IgG-treated controls, which acquired the task normally. Similar differences between the two treatments were also observed after a stronger training protocol in a step-down avoidance paradigm. The HNK-1 antibody was effective only when injected 1 h, but not 48 h after training, thus affecting memory consolidation but not memory recall itself. The HNK-1 antibody impaired memory also in tenascin-R knock-out mice, indicating that extracellular matrix molecule tenascin-R, one of the carriers of the HNK-1epitope in the hippocampus, does not mediate the function of the HNK-1 carbohydrate in this task. Our observations show that the HNK-1 carbohydrate is critically involved in memory consolidation in hippocampus-dependent learning in mammals.

Brevican in the developing hippocampal fimbria: differential expression in myelinating oligodendrocytes and adult astrocytes suggests a dual role for brevican in central nervous system fiber tract development

Brevican is one of the most abundant extracellular matrix proteoglycans in the mammalian brain. We have previously shown that brevican produced by gray matter astrocytes constitutes a major component of perineuronal extracellular matrix in the adult brain. In this paper, we investigate the expression of brevican in the postnatal hippocampal fimbria to explore the role of the proteoglycan in central nervous system fiber tract development. We demonstrate that brevican is expressed by both oligodendrocytes and white matter astrocytes in the fimbria, but the expression of brevican in these two glial cell types is differently regulated during development. At P14, brevican immunoreactivity was observed throughout the fimbria, with particularly strong immunoreactivity in the developing interfascicular glial rows. In situ hybridization showed that oligodendrocytes in the glial rows strongly express brevican during the second and third postnatal weeks. Expression in oligodendrocytes was then down-regulated after P21. In the adult fimbria, no brevican expression was observed in oligodendrocytes. The time window of brevican expression coincides with the phase in which immature oligodendrocytes actively extend membrane processes and enwrap axon fibers. In contrast, the expression in astrocytes started around P21 as oligodendrocytes began to down-regulate the expression. In the adult fimbria, brevican expression was restricted to astrocytes. In situ hybridization with isoform-specific probes and RNase protection assays showed that the authentic, secreted form of brevican, not the glycosylphosphatidylinositol-anchored variant, is the predominant species expressed in the developing fimbria. Our results suggest that brevican plays a dual role in developing and adult fiber tracts.

The structure and function of tenascins in the nervous system

The tenascins are a family of large extracellular matrix glycoproteins that comprise five known members. Three of these, tenascin-C (TN-C) tenascin-R (TN-R) and tenascin-Y (TN-Y) are expressed in specific patterns during nervous system development and are down-regulated after maturation. The expression of TN-C, the best studied member of the family, persists in restricted areas of the nervous system that exhibit neuronal plasticity and is reexpressed after lesion. Numerous studies in vitro suggest specific roles for tenascins in the nervous system involving precursor cell migration, axon growth and guidance. TN-C has been shown to occur in a large number of isoform variants generated by combinatorial variation of alternatively spliced fibronectin type III (FNIII) repeats. This finding indicates that TN-C might specify neural microenvironments, a hypothesis supported by recent analysis of TN-C knockout animals, which has begun to reveal subtle nervous system dysfunctions.

Sodium channel beta subunits: anything but auxiliary

Voltage-gated sodium channels are glycoprotein complexes responsible for initiation and propagation of action potentials in excitable cells such as central and peripheral neurons, cardiac and skeletal muscle myocytes, and neuroendocrine cells. Mammalian sodium channels are heterotrimers, composed of a central, pore-forming alpha subunit and two auxiliary beta subunits. The alpha subunits form a gene family with at least 10 members. Mutations in alpha subunit genes have been linked to paroxysmal disorders such as epilepsy, long QT syndrome, and hyperkalemic periodic paralysis in humans, and motor endplate disease and cerebellar ataxia in mice. Three genes encode sodium channel beta subunits with at least one alternative splice product. A mutation in the beta 1 subunit gene has been linked to generalized epilepsy with febrile seizures plus type 1 (GEFS + 1) in a human family with this disease. Sodium channel beta subunits are multifunctional. They modulate channel gating and regulate the level of channel expression at the plasma membrane. More recently, they have been shown to function as cell adhesion molecules in terms of interaction with extracellular matrix, regulation of cell migration, cellular aggregation, and interaction with the cytoskeleton. Structure-function studies have resulted in the preliminary assignment of functional domains in the beta 1 subunit. A sodium channel signaling complex is proposed that involves beta subunits as channel modulators as well as cell adhesion molecules, other cell adhesion molecules such as neurofascin and contactin, RPTP beta, and extracellular matrix molecules such as tenascin.

Reduced perisomatic inhibition, increased excitatory transmission, and impaired long-term potentiation in mice deficient for the extracellular matrix glycoprotein tenascin-R

The role of the extracellular matrix molecule tenascin-R (TN-R) in regulation of synaptic transmission and plasticity in the CA1 region of the hippocampus was studied using mice deficient in expression of this molecule. The mutant mice showed normal NMDA-receptor-mediated currents but an impaired NMDA-receptor-dependent form of long-term potentiation (LTP) as compared to wild-type littermates. Reduced LTP in mutants was accompanied by increased basal excitatory synaptic transmission in synapses formed on CA1 pyramidal neurons. A possible mechanism for increased excitatory synaptic transmission in mutants could involve modulation of inhibition, since TN-R and its associated carbohydrate HNK-1 decorate perisomatic interneurons. Indeed, the amplitudes of unitary perisomatic inhibitory currents were smaller in mutants compared to wild-type mice. Thus, our data show that a deficit in TN-R results in reduction of perisomatic inhibition and, as a consequence, in an increase of excitatory synaptic transmission in CA1 to the levels close to saturation, impeding further expression of LTP.

Postnatal development of perineuronal nets in wild-type mice and in a mutant deficient in tenascin-R

The extracellular matrix glycoprotein tenascin-R (TN-R), colocalizing with hyaluronan, phosphacan, and aggregating chondroitin sulphate proteoglycans in the white and grey matter, is accumulated in perineuronal nets that surround different types of neurons in many brain regions. To characterize the role of TN-R in the formation of perineuronal nets, we studied their postnatal development in wild-type mice and in a TN-R knock-out mutant by using the lectin Wisteria floribunda agglutinin and an antibody to nonspecified chondroitin sulphate proteoglycans as established cytochemical markers. We detected the matrix components TN-R, hyaluronan, phosphacan, neurocan, and brevican in the perineuronal nets of cortical and subcortical regions. In wild-type mice, lectin-stained, immature perineuronal nets were first seen on postnatal day 4 in the brainstem and on day 14 in the cerebral cortex. The staining intensity of these nets for TN-R, hyaluronan, phosphacan, neurocan, and brevican was extremely weak or not distinguishable from that of the surrounding neuropil. However, all markers showed an increase in staining intensity of perineuronal nets reaching maximal levels between postnatal days 21 and 40. In TN-R-deficient animals, the perineuronal nets tended to show a granular component within their lattice-like structure at early stages of development. Additionally, the staining intensity in perineuronal nets was reduced for brevican, extremely low for hyaluronan and neurocan, and virtually no immunoreactivity was detectable for phosphacan. The granular configuration of perineuronal nets became more predominant with advancing age of the mutant animals, indicating the continued abnormal aggregation of chondroitin sulphate proteoglycans complexed with hyaluronan. As shown by electron microscopy in the cerebral cortex, the disruption of perineuronal nets was not accompanied by apparent changes in the synaptic structure on net-bearing neurons. The regional distribution patterns and the temporal course of development of perineuronal nets were not obviously changed in the mutant. We conclude that the lack of TN-R initially and continuously disturbs the molecular scaffolding of extracellular matrix components in perineuronal nets. This may interfere with the development of the specific micromilieu of the ensheathed neurons and adjacent glial cells and may also permanently change their functional properties.

Differential expression of tenascin-C, tenascin-R, tenascin/J1, and tenascin-X in spinal cord scar tissue and in the olfactory system

The members of the tenascin family are involved in a number of developmental processes, mainly by their ability to regulate cell adhesion. We have here studied the distribution of mRNAs for tenascin-X, -C, and -R and the closely related molecule tenascin/J1 in the olfactory system and spinal cord. The olfactory bulb and nasal mucosa were studied during late embryonic and early postnatal development as well as in the adult. The spinal cord was studied during late embryonic development and after mechanical lesions. In the normal rat, the spinal cord and olfactory bulb displayed similar patterns of tenascin expression. Tenascin-C, tenascin-R, and tenascin/J1 were all expressed in the olfactory bulb and spinal cord during development, while tenascin/J1 was the only extensively expressed tenascin molecule in the adult. In both regions tenascin/J1 was expressed in both nonneuronal and neuronal cells. After a spinal cord lesion, mRNAs for tenascin-C, -X, -R, and/J1 were all upregulated and had their own specific spatial and temporal expression patterns. Thus, even if axonal outgrowth occurs to some extent both in the adult rat primary olfactory system and in spinal cord scar tissue after lesion, the tenascin expression patterns in these two situations are totally different.

Contactin-associated protein (Caspr) and contactin form a complex that is targeted to the paranodal junctions during myelination

Specialized paranodal junctions form between the axon and the closely apposed paranodal loops of myelinating glia. They are interposed between sodium channels at the nodes of Ranvier and potassium channels in the juxtaparanodal regions; their precise function and molecular composition have been elusive. We previously reported that Caspr (contactin-associated protein) is a major axonal constituent of these junctions (Einheber et al., 1997). We now report that contactin colocalizes and forms a cis complex with Caspr in the paranodes and juxtamesaxon. These proteins coextract and coprecipitate from neurons, myelinating cultures, and myelin preparations enriched in junctional markers; they fractionate on sucrose gradients as a high-molecular-weight complex, suggesting that other proteins may also be associated with this complex. Neurons express two contactin isoforms that differ in their extent of glycosylation: a lower-molecular-weight phosphatidylinositol phospholipase C (PI-PLC)-resistant form is associated specifically with Caspr in the paranodes, whereas a higher-molecular-weight form of contactin, not associated with Caspr, is present in central nodes of Ranvier. These results suggest that the targeting of contactin to different axonal domains may be determined, in part, via its association with Caspr. Treatment of myelinating cocultures of Schwann cells and neurons with RPTPbeta-Fc, a soluble construct containing the carbonic anhydrase domain of the receptor protein tyrosine phosphatase beta (RPTPbeta), a potential glial receptor for contactin, blocks the localization of the Caspr/contactin complex to the paranodes. These results strongly suggest that a preformed complex of Caspr and contactin is targeted to the paranodal junctions via extracellular interactions with myelinating glia.

Molecular domains of myelinated axons

Myelinated axons are organized into specific domains as the result of interactions with glial cells. Recently, distinct protein complexes of cell adhesion molecules, Na(+) channels and ankyrin G at the nodes, Caspr and contactin in the paranodes, and K(+) channels and Caspr2 in the juxtaparanodal region have been identified, and new insights into the role of the paranodal junctions in the organization of these domains have emerged.

Both cell-surface carbohydrates and protein tyrosine phosphatase are involved in the differentiation of astrocytes in vitro

Astrocytes are important in the development and maintenance of functions of the CNS, acting in cooperation with neurons and other glial cells. The glycans on astrocyte membrane are believed to play important roles in cell-cell communication. Plant lectins are useful probes, because the lectins can bind to certain cell surface receptors and elicit cellular responses that are normally activated by endogenous ligands for those receptors. In the present study, we investigated the effect of Datura stramonium agglutinin (DSA) on astrocytes and characterized several molecular events. The addition of DSA to a culture of flat, polygonal, immature astrocytes derived from the neonatal rat cerebellum caused the cells to become stellate in shape, similar to astrocytes observed in vivo, concomitant with an increase in expression of astrocyte-specific intermediate filament (glial fibrillary acidic protein [GFAP]) and inhibition of proliferation. These results indicate that DSA binds to astrocytes and triggers differentiation. We also found a decrease in the extent of tyrosine-phosphorylation of a 38-kDa protein. To elucidate the molecular events during astrocyte differentiation, we examined the effects of various signal transduction inhibitors on the transformation from the polygonal to stellate shape (stellation). Interestingly, only tyrosine phosphatase inhibitors, orthovanadate and phenylarsine oxide, showed an inhibitory effect. Our results suggest that DSA induced astrocyte differentiation acts via tyrosine dephosphorylation.

The extracellular matrix molecule tenascin-R and its HNK-1 carbohydrate modulate perisomatic inhibition and long-term potentiation in the CA1 region of the hippocampus

Perisomatic inhibition of pyramidal cells regulates efferent signalling from the hippocampus. The striking presence of HNK-1, a carbohydrate expressed by neural adhesion molecules, on perisomatic interneurons and around somata of CA1 pyramidal neurons led us to apply monoclonal HNK-1 antibodies to acute murine hippocampal slices. Injection of these antibodies decreased GABAA receptor-mediated perisomatic inhibitory postsynaptic currents (pIPSCs) but did not affect dendritic IPSCs or excitatory postsynaptic currents. The decrease in the mean amplitude of evoked pIPSCs by HNK-1 antibodies was accompanied by an increase in the coefficient of variation of pIPSC amplitude, number of failures and changes in frequency but not amplitude of miniature IPSCs, suggesting that HNK-1 antibodies reduced efficacy of evoked GABA release. HNK-1 antibodies did not affect pIPSCs in knock-out mice deficient in the extracellular matrix molecule tenascin-R which carries the HNK-1 carbohydrate as analysed by immunoblotting in synaptosomal fractions prepared from the CA1 region of the hippocampus. For control, HNK-1 antibody was applied to acute sections of mice deficient in the neural cell adhesion molecule NCAM, another potential carrier of HNK-1, and resulted in decrease of pIPSCs as observed in wild-type mice. Reduction in perisomatic inhibition is expected to promote induction of long-term potentiation (LTP) by increasing the level of depolarization during theta-burst stimulation. Indeed, LTP was increased by HNK-1 antibody applied before stimulation. Moreover, LTP was reduced by an HNK-1 peptide mimic, but not control peptide. These results provide first evidence that tenascin-R and its associated HNK-1 carbohydrate modulate perisomatic inhibition and synaptic plasticity in the hippocampus.

The yin and yang of tenascin-R in CNS development and pathology

An important biological consequence of the initial interactions between the cell surface and its extracellular environment is the diversity of cellular responses ranging from overt repulsion or avoidance reaction to stable adhesion or final positioning. It is now evident that positive and negative guiding mechanisms are equally relevant to normal pattern formation during development and decisive for the outcome of a regenerative process. In this context, the present review summarizes the knowledge about the extracellular matrix glycoprotein tenascin-R, a member of the tenascin gene family. In contrast to all other known family members, tenascin-R is exclusively expressed in the central nervous system of vertebrates by oligodendrocytes and neuronal subsets at later developmental stages and in adulthood. We focus on the glycoprotein's structure, tissue distribution and functional implications in the molecular control of axon targeting, neural cell adhesion, migration and differentiation during nervous system morphogenesis and pathology.

Distribution of voltage-gated sodium channel alpha-subunit and beta-subunit mRNAs in human hippocampal formation, cortex, and cerebellum

The distribution of mRNAs encoding voltage-gated sodium channel alpha subunits (I, II, III, and VI) and beta subunits (beta1 and beta2) was studied in selected regions of the human brain by Northern blot and in situ hybridisation experiments. Northern blot analysis showed that all regions studied exhibited heterogenous expression of sodium channel transcripts. In situ hybridisation experiments confirmed these findings and revealed a predominantly neuronal distribution. In the parahippocampal gyrus, subtypes II and VI and the beta-subunit mRNAs exhibited robust expression in the granule cells of the dentate gyrus and pyramidal cell layer of the hippocampus. Subtypes I and III showed moderate expression in granule cells and low expression in the pyramidal cell layer. Distinct expression patterns were also observed in the cortical layers of the middle frontal gyrus and in the entorhinal cortex. In particular, all subtypes exhibited higher levels of expression in cortical layers III, V, and VI compared with layers I and II. All subtypes were expressed in the granular layer of the cerebellum, whereas specific expression of subtypes I, VI, beta1, and beta2 mRNAs was observed in Purkinje cells. Subtypes I, VI, and beta1 mRNAs were expressed, at varying levels, in the pyramidal cells of the deep cerebellar nuclei. These data indicate that, as in rat, human brain sodium channel mRNAs have a distinct regional distribution, with individual cell types expressing different compliments of sodium channels. The differential distribution of sodium channel subtypes suggest that they have distinct roles that are likely to be of paramount importance in maintaining the functional heterogeneity of central nervous system neurons.

The tenascin family of ECM glycoproteins: structure, function, and regulation during embryonic development and tissue remodeling

The determination of animal form depends on the coordination of events that lead to the morphological patterning of cells. This epigenetic view of development suggests that embryonic structures arise as a consequence of environmental influences acting on the properties of cells, rather than an unfolding of a completely genetically specified and preexisting invisible pattern. Specialized cells of developing multicellular organisms are surrounded by a complex extracellular matrix (ECM), comprised largely of different collagens, proteoglycans, and glycoproteins. This ECM is a substrate for tissue morphogenesis, lends support and flexibility to mature tissues, and acts as an epigenetic informational entity in the sense that it transduces and integrates intracellular signals via distinct cell surface receptors. Consequently, ECM-receptor interactions have a profound influence on major cellular programs including growth, differentiation, migration, and survival. In contrast to many other ECM proteins, the tenascin (TN) family of glycoproteins (TN-C, TN-R, TN-W, TN-X, and TN-Y) display highly restricted and dynamic patterns of expression in the embryo, particularly during neural development, skeletogenesis, and vasculogenesis. These molecules are reexpressed in the adult during normal processes such as wound healing, nerve regeneration, and tissue involution, and in pathological states including vascular disease, tumorigenesis, and metastasis. In concert with a multitude of associated ECM proteins and cell surface receptors that include members of the integrin family, TN proteins impart contrary cellular functions, depending on their mode of presentation (i.e., soluble or substrate-bound) and the cell types and differentiation states of the target tissues. Expression of tenascins is regulated by a variety of growth factors, cytokines, vasoactive peptides, ECM proteins, and biomechanical factors. The signals generated by these factors converge on particular combinations of cis-regulatory elements within the recently identified TN gene promoters via specific transcriptional activators or repressors. Additional complexity in regulating TN gene expression is achieved through alternative splicing, resulting in variants of TN polypeptides that exhibit different combinations of functional protein domains. In this review, we discuss some of the recent advances in TN biology that provide insights into the complex way in which the ECM is regulated and how it functions to regulate tissue morphogenesis and gene expression.

Ion channel sequestration in central nervous system axons

Na+ and K+ channel localization and clustering are essential for proper electrical signal generation and transmission in CNS myelinated nerve fibres. In particular, Na+ channels are clustered at high density at nodes of Ranvier, and Shaker-type K+ channels are sequestered in juxtaparanodal zones, just beyond the paranodal axoglial junctions. The mechanisms of channel localization at nodes of Ranvier in the CNS during development in both normal and hypomyelinating mutant animals are discussed and reviewed. As myelination proceeds, Na+ channels are initially found in broad zones within gaps between neighbouring oligodendroglial processes, and then are condensed into focal clusters. This process appears to depend on the formation of axoglial junctions. K+ channels are first detected in juxtaparanodal zones, and in mutant mice lacking normal axoglial junctions, these channels fail to cluster. In these mice, despite the presence of numerous oligodendrocytes, Na+ channel clusters are rare, and when present, are highly irregular. A number of molecules have recently been described that are candidates for a role in the neuron-glial interactions driving ion channel clustering. This paper reviews the cellular and molecular events responsible for formation of the mature node of Ranvier in the CNS.

Morphology of perineuronal nets in tenascin-R and parvalbumin single and double knockout mice

Recently identified chondroitin sulphate proteoglycans in perineuronal nets include neurocan and phosphacan. However, the function and assembly of these components has yet to be resolved. In this study we show morphological alteration in Wisteria floribunda labelled nets around cortical interneurones both in tenascin-R knockout and tenascin-R/parvalbumin double knockout mice. This alteration reflects the loss of phosphacan and neurocan from cortical nets in mice deficient in tenascin-R. No effect on the membrane related cytoskeleton, as revealed by ankyrin(R), was observed in any of the mice. These results on mice lacking tenascin-R substantiate previously reported in vitro interactions between tenascin-R and phosphacan and neurocan.

A sodium channel signaling complex: modulation by associated receptor protein tyrosine phosphatase beta

Voltage-gated sodium channels in brain neurons were found to associate with receptor protein tyrosine phosphatase beta (RPTPbeta) and its catalytically inactive, secreted isoform phosphacan, and this interaction was regulated during development. Both the extracellular domain and the intracellular catalytic domain of RPTPbeta interacted with sodium channels. Sodium channels were tyrosine phosphorylated and were modulated by the associated catalytic domains of RPTPbeta. Dephosphorylation slowed sodium channel inactivation, positively shifted its voltage dependence, and increased whole-cell sodium current. Our results define a sodium channel signaling complex containing RPTPbeta, which acts to regulate sodium channel modulation by tyrosine phosphorylation.

Tenascin-R inhibits regrowth of optic fibers in vitro and persists in the optic nerve of mice after injury

Tenascin-R, an extracellular matrix constituent expressed by oligodendrocytes and some neuronal cell types, may contribute to the inhibition of axonal regeneration in the adult central nervous system. Here we show that outgrowth of embryonic and adult retinal ganglion cell axons from mouse retinal explants is significantly reduced on homogeneous substrates of tenascin-R or a bacterially expressed tenascin-R fragment comprising the epidermal growth factor-like repeats (EGF-L). When both molecules are presented as a sharp substrate border, regrowing adult axons do not cross into the tenascin-R or EGF-L containing territory. All in vitro experiments were done in the presence of laminin, which strongly promotes growth of embryonic and adult retinal axons, suggesting that tenascin-R and EGF-L actively inhibit axonal growth. Contrary to the disappearance of tenascin-R from the regenerating optic nerve of salamanders (Becker et al., J Neurosci 19:813-827, 1999), the molecule remains present in the lesioned optic nerve of adult mice at levels similar to those in unlesioned control nerves for at least 63 days post-lesion (the latest time point investigated), as shown by immunoblot analysis and immunohistochemistry. In situ hybridization analysis revealed an increase in the number of cells expressing tenascin-R mRNA in the lesioned nerve. We conclude that, regardless of the developmental stage, growth of retinal ganglion cell axons is inhibited by tenascin-R and we suggest that the continued expression of the protein after an optic nerve crush may contribute to the failure of adult retinal ganglion cells to regenerate their axons in vivo.

Molecular interactions of neural chondroitin sulfate proteoglycans in the brain development

Aggrecan family proteoglycans, phosphacan/RPTPzeta/beta, and neuroglycan C (NGC) are the major classes of chondroitin sulfate proteoglycan in the developing mammalian brain. A multidomain is a common structural feature of these proteoglycans which can interact with various molecules including growth factors, cell adhesion molecules, and extracellular matrix molecules. Individual proteoglycans are distributed in the developing brain in a distinct temporal and spatial pattern, suggesting that they are involved in distinct phases of the brain development through multiple molecular interactions. This review mainly summarizes recent studies on the involvement of these three classes of proteoglycan in cell-cell and cell-substratum interactions during the brain development. Their expressions and proposed functional roles in injured brains are also mentioned. In addition, this review briefly covers potential functions of other neural chondroitin sulfate proteoglycans such as decorin, testican, NG2 proteoglycan, and amyloid precursor protein (APP) in developing and injured brains.

Multiple functions of the myelin-associated glycoprotein MAG (siglec-4a) in formation and maintenance of myelin

The myelin-associated glycoprotein, a minor component of myelin in the central and peripheral nervous system, has been implicated in the formation and maintenance of myelin. Although the analysis of MAG null mutants confirms this view, the phenotype of this mutant is surprisingly subtle. In the CNS of MAG-deficient mice, initiation of myelination, formation of morphologically intact myelin sheaths and to a minor extent, integrity of myelin is affected. In the PNS, in comparison, only maintenance of myelin is impaired. Recently, the large isoform of MAG has been identified as the functionally important isoform in the CNS, whereas the small MAG isoform is sufficient to maintain the integrity of myelinated fibers in the PNS. Remarkably, none of the different defects in the MAG mutant is consistently associated with each myelinated fiber. These observations suggest that other molecules performing similar functions as MAG might compensate, at least partially, for the absence of MAG in the null mutant.

Extracellular matrix-associated protein Sc1 is not essential for mouse development

Sc1 is an extracellular matrix-associated protein whose function is unknown. During early embryonic development, Sc1 is widely expressed, and from embryonic day 12 (E12), Sc1 is expressed primarily in the developing nervous system. This switch in Sc1 expression at E12 suggests an importance for nervous-system development. To gain insight into Sc1 function, we used gene targeting to inactivate mouse Sc1. The Sc1-null mice showed no obvious deficits in any organs. These mice were born at the expected ratios, were fertile, and had no obvious histological abnormalities, and their long-term survival did not differ from littermate controls. Therefore, the function of Sc1 during development is not critical or, in its absence, is subserved by another protein.

Tenascin-R is a functional modulator of sodium channel beta subunits

Voltage-gated sodium channels isolated from mammalian brain are composed of alpha, beta1, and beta2 subunits. The alpha subunit forms the ion conducting pore of the channel, whereas the beta1 and beta2 subunits modulate channel function, as well as channel plasma membrane expression levels. beta1 and beta2 each contain a single, extracellular Ig-like domain with structural similarity to the neural cell adhesion molecule (CAM), myelin Po. beta2 contains strong amino acid homology to the third Ig domain and to the juxtamembrane region of F3/contactin. Many CAMs of the Ig superfamily have been shown to interact with extracellular matrix molecules. We hypothesized that beta2 may interact with tenascin-R (TN-R), an extracellular matrix molecule that is secreted by oligodendrocytes during myelination and that binds F3-contactin. We show here that cells expressing sodium channel beta1 or beta2 subunits are functionally modulated by TN-R. Transfected cells stably expressing beta1 or beta2 subunits initially recognized and then were repelled from TN-R substrates. The cysteine-rich amino-terminal domain of TN-R expressed as a recombinant peptide, termed EGF-L, appears to be responsible for the repellent effect on beta subunit-expressing cells. The epidermal growth factor-like repeats and fibronectin-like repeats 6-8 are most effective in the initial adhesion of beta subunit-expressing cells. Application of EGF-L to alphaIIAbeta1beta2 channels expressed in Xenopus oocytes potentiated expressed sodium currents without significantly altering current time course or the voltage dependence of current activation or inactivation. Thus, sodium channel beta subunits appear to function as CAMs, and TN-R may be an important regulator of sodium channel localization and function in neurons.