Kainic acid activates transient expression of tenascin-C in the adult rat hippocampus

The expression of tenascin-C in neural stem/progenitor cells is stimulated by the growth factors EGF and FGF-2, but not by TGFβ1

Neural stem/progenitor cells (NSPCs) rely on internal and external cues determining their lineage decisions during brain development. The progenitor cells of the embryonic mammalian forebrain reside in the ventricular and subventricular zones of the lateral ventricles, where they proliferate, generate neurons and glial cells, and respond to external cues like growth factors. The extracellular matrix (ECM) surrounds NSPCs and influences the cell fate by providing mechanical scaffold, trophic support, and instructive signals. The ECM molecule tenascin-C (Tnc) is expressed in the proliferative zones of the developing forebrain and involved in the proliferation and maturation of NSPCs. Here, we analyzed the regulation of the Tnc gene expression by NSPCs cultivated under the influence of different growth factors. We observed that the epidermal growth factor (EGF) and the fibroblast growth factor (FGF)-2 strongly increased the expression of Tnc, whereas the transforming growth factor (TGF)β 1 had no effect on Tnc gene expression, in contrast to previous findings in cell cultures of neural and non-neural origin. The stimulation of the Tnc gene expression induced by EGF or FGF-2 was reversible and seen in constantly treated as well as short term stimulated NSPC cultures. The activation depended on the presence of the respective receptors, which was slightly different in cortical and striatal NSPC cultures. Our results confirm the influence of extracellular stimuli regulating the expression of factors that form a niche for NSPCs during embryonic forebrain development.

Site-specific HNK-1 epitope on alternatively spliced fibronectin type-III repeats in tenascin-C promotes neurite outgrowth of hippocampal neurons through contactin-1

The human natural killer-1 (HNK-1) carbohydrate epitope, composed of a unique sulfated trisaccharide (HSO₃–3GlcAβ1–3Galβ1–4GlcNAc-R), is highly expressed during brain development and regulates higher brain function. However, it remains unclear which glycoprotein carries the HNK-1 epitope in the embryonic brain and the functional role it plays. Here, we showed that one of the major HNK-1 carrier proteins in the embryonic brain is tenascin-C (TNC), an extracellular matrix protein that regulates neurite outgrowth by interacting with the GPI-anchored protein contactin-1 (CNTN). Because the alternatively spliced fibronectin type-III (FNIII) repeats in TNC give rise to many isoforms and affect neuronal function, we evaluated neurite outgrowth of primary hippocampal neurons on purified recombinant FNIII repeats with or without the HNK-1 epitope as a substrate. We found that the presence of the HNK-1 epitope on the C domain of TNC promoted neurite outgrowth, and that this signal was mediated by CNTN, which is an HNK-1-expressing neuronal receptor. The neurite-promoting activity of the HNK-1 epitope on TNC required neuronal HNK-1 expression, which was defective in neurons lacking the glucuronyltransferases GlcAT-P and GlcAT-S. These results suggest that the HNK-1 epitope is a key modifier of TNC and CNTN in the regulation of embryonic brain development.

Role of TGF-β signaling pathway on Tenascin C protein upregulation in a pilocarpine seizure model

Seizures have been shown to upregulate the expression of numerous extracellular matrix molecules. Tenascin C (TNC) is an extracellular matrix protein involved in several physiological roles and in pathological conditions. Though TNC upregulation has been described after excitotoxins injection, to date there is no research work on the signal transduction pathway(s) participating in TNC protein overproduction. The aim of this study was to evaluate the role of TGF-β signaling pathway on TNC upregulation. In this study, we used male rats, which were injected with saline or pilocarpine to induce status epilepticus (SE) and killed 24h, 3 and 7 days after pilocarpine administration. For evaluating biochemical changes, we measured protein content of TNC, TGF-β1 and phospho-Smad2/3 for localization of TNC in coronal brain hippocampus at 24h, 3 and 7 days after pilocarpine-caused SE. We found a significant increase of TNC protein content in hippocampal homogenates after 1, 3, and 7 days of pilocarpine-caused SE, together with an enhancement of TNC immunoreactivity in several hippocampal layers and the dentate gyrus field where more dramatic changes occurred. We also observed a significant enhancement of protein content of both the TGF-β1 and the critical downstream transduction effector phospho-Smad2/3 throughout the chronic exposure. Interestingly, animals injected with SB-431542, a TGF-β-type I receptor inhibitor, decreased TNC content in cytosolic fraction and diminished phospho-Smad2/3 content in both cytoplasmic and nuclear fraction compared with pilocarpine vehicle-injected. These findings suggest the participation of TGF-β signaling pathway on upregulation of TNC which in turn support the idea that misregulation of this signaling pathway produces changes that may contribute to disease.

Influence of the extracellular matrix on endogenous and transplanted stem cells after brain damage

The limited regeneration capacity of the adult central nervous system (CNS) requires strategies to improve recovery of patients. In this context, the interaction of endogenous as well as transplanted stem cells with their environment is crucial. An understanding of the molecular mechanisms could help to improve regeneration by targeted manipulation. In the course of reactive gliosis, astrocytes upregulate Glial fibrillary acidic protein (GFAP) and start, in many cases, to proliferate. Beside GFAP, subpopulations of these astroglial cells coexpress neural progenitor markers like Nestin. Although cells express these markers, the proportion of cells that eventually give rise to neurons is limited in many cases in vivo compared to the situation in vitro. In the first section, we present the characteristics of endogenous progenitor-like cells and discuss the differences in their neurogenic potential in vitro and in vivo. As the environment plays an important role for survival, proliferation, migration, and other processes, the second section of the review describes changes in the extracellular matrix (ECM), a complex network that contains numerous signaling molecules. It appears that signals in the damaged CNS lead to an activation and de-differentiation of astrocytes, but do not effectively promote neuronal differentiation of these cells. Factors that influence stem cells during development are upregulated in the damaged brain as part of an environment resembling a stem cell niche. We give a general description of the ECM composition, with focus on stem cell-associated factors like the glycoprotein Tenascin-C (TN-C). Stem cell transplantation is considered as potential treatment strategy. Interaction of transplanted stem cells with the host environment is critical for the outcome of stem cell-based therapies. Possible mechanisms involving the ECM by which transplanted stem cells might improve recovery are discussed in the last section.

Astrocytes as a source for extracellular matrix molecules and cytokines

Research of the past 25 years has shown that astrocytes do more than participating and building up the blood-brain barrier and detoxify the active synapse by reuptake of neurotransmitters and ions. Indeed, astrocytes express neurotransmitter receptors and, as a consequence, respond to stimuli. Within the tripartite synapse, the astrocytes owe more and more importance. Besides the functional aspects the differentiation of astrocytes has gained a more intensive focus. Deeper knowledge of the differentiation processes during development of the central nervous system might help explaining and even help treating neurological diseases like Alzheimer’s disease, Amyotrophic lateral sclerosis, Parkinsons disease, and psychiatric disorders in which astrocytes have been shown to play a role. Specific differentiation of neural stem cells toward the astroglial lineage is performed as a multi-step process. Astrocytes and oligodendrocytes develop from a multipotent stem cell that prior to this has produced primarily neuronal precursor cells. This switch toward the more astroglial differentiation is regulated by a change in receptor composition on the cell surface and responsiveness to Fibroblast growth factor and Epidermal growth factor (EGF). The glial precursor cell is driven into the astroglial direction by signaling molecules like Ciliary neurotrophic factor, Bone Morphogenetic Proteins, and EGF. However, the early astrocytes influence their environment not only by releasing and responding to diverse soluble factors but also express a wide range of extracellular matrix (ECM) molecules, in particular proteoglycans of the lectican family and tenascins. Lately these ECM molecules have been shown to participate in glial development. In this regard, especially the matrix protein Tenascin C (Tnc) proved to be an important regulator of astrocyte precursor cell proliferation and migration during spinal cord development. Nevertheless, ECM molecules expressed by reactive astrocytes are also known to act mostly in an inhibitory fashion under pathophysiological conditions. Thus, we further summarize resent data concerning the role of chondroitin sulfate proteoglycans and Tnc under pathological conditions.

Integrins as receptor targets for neurological disorders

This review focuses on the neurobiology of integrins, pathophysiological roles of integrins in neuroplasticity and nervous system disorders, and therapeutic implications of integrins as potential drug targets and possible delivery pathways. Neuroplasticity is a central phenomenon in many neurological conditions such as seizures, trauma, and traumatic brain injury. During the course of many brain diseases, in addition to intracellular compartment changes, alterations in non-cell compartments such as extracellular matrix (ECM) are recognized as an essential process in forming and reorganizing neural connections. Integrins are heterodimeric transmembrane receptors that mediate cell-ECM and cell-cell adhesion events. Although the mechanisms of neuroplasticity remain unclear, it has been suggested that integrins undergo plasticity including clustering through interactions with ECM proteins, modulating ion channels, intracellular Ca(2+) and protein kinase signaling, and reorganization of cytoskeletal filaments. As cell surface receptors, integrins are central to the pathophysiology of many brain diseases, such as epilepsy, and are potential targets for the development of new drugs for neurological disorders.

The extracellular matrix glycoprotein tenascin-C is beneficial for spinal cord regeneration

Tenascin-C (TNC), a major component of the extracellular matrix, is strongly upregulated after injuries of the central nervous system (CNS) but its role in tissue repair is not understood. Both regeneration promoting and inhibiting roles of TNC have been proposed considering its abilities to both support and restrict neurite outgrowth in vitro. Here, we show that spontaneous recovery of locomotor functions after spinal cord injury is impaired in adult TNC-deficient (TNC(-/-)) mice in comparison to wild-type (TNC(+/+)) mice. The impaired recovery was associated with attenuated excitability of the plantar Hoffmann reflex (H-reflex), reduced glutamatergic input, reduced sprouting of monaminergic axons in the lumbar spinal cord and enhanced post-traumatic degeneration of corticospinal axons. The degeneration of corticospinal axons in TNC(-/-) mice was normalized to TNC(+/+) levels by application of the alternatively spliced TNC fibronectin type III homologous domain D (fnD). Finally, overexpression of TNC-fnD via adeno-associated virus in wild-type mice improved locomotor recovery, increased monaminergic axons sprouting, and reduced lesion scar volume after spinal cord injury. The functional efficacy of the viral-mediated TNC indicates a potentially useful approach for treatment of spinal cord injury.

Analysis of combinatorial variability reveals selective accumulation of the fibronectin type III domains B and D of tenascin-C in injured brain

Tenascin-C (Tnc) is a multimodular extracellular matrix glycoprotein that is markedly upregulated in CNS injuries where it is primarily secreted by reactive astrocytes. Different Tnc isoforms can be generated by the insertion of variable combinations of one to seven (in rats) alternatively spliced distinct fibronectin type III (FnIII) domains to the smallest variant. Each spliced FnIII repeat mediates specific actions on neurite outgrowth, neuron migration or adhesion. Hence, different Tnc isoforms might differentially influence CNS repair. We explored the expression pattern of Tnc variants after cortical lesions and after treatment of astrocytes with various cytokines. Using RT-PCR, we observed a strong upregulation of Tnc transcripts containing the spliced FnIII domains B or D in injured tissue at 2-4 days post-lesion (dpl). Looking at specific combinations, we showed a dramatic increase of Tnc isoforms harboring the neurite outgrowth-promoting BD repeat with both the B and D domains being adjacent to each other. Isoforms containing only the axon growth-stimulating spliced domain D were also dramatically enhanced after injury. Injury-induced increase of Tnc proteins comprising the domain D was confirmed by Western Blotting and immunostaining of cortical lesions. In contrast, the FnIII modules C and AD1 were weakly modulated after injury. The growth cone repulsive A1A2A4 domains were poorly expressed in normal and injured tissue but the smallest isoform, which is also repellant, was highly expressed after injury. Expression of the shortest Tnc isoform and of variants containing B, D or BD, was strongly upregulated in cultured astrocytes after TGFbeta1 treatment, suggesting that TGFbeta1 could mediate, at least in part, the injury-induced upregulation of these isoforms. We identified complex injury-induced differential regulations of Tnc isoforms that may well influence axonal regeneration and repair processes in the damaged CNS.

Zebrafish as a model for studying genetic aspects of epilepsy

Despite a long tradition of using rats and mice to model epilepsy, several aspects of rodent biology limit their use in large-scale genetic and therapeutic drug screening programs. Neuroscientists interested in vertebrate development and diseases have recently turned to zebrafish (Danio rerio) to overcome these limitations. Zebrafish can be studied at all stages of development and several methods are available for the manipulation of genes in zebrafish. In addition, developing zebrafish larvae can efficiently equilibrate drugs placed in the bathing medium. Taking advantage of these features and adapting electrophysiological recording methods to an agar-immobilized zebrafish preparation, we describe here our efforts to model seizure disorders in zebrafish. We also describe the initial results of a large-scale mutagenesis screen to identify gene mutation(s) that confer seizure resistance. Although the adaptation of zebrafish to epilepsy research is in its early stages, these studies highlight the rapid progress that can be made using this simple vertebrate species.

Genetic ablation of tenascin-C expression leads to abnormal hippocampal CA1 structure and electrical activity in vivo

Despite evidence that the extracellular matrix glycoprotein tenascin-C (TNC) is implicated in brain development and plasticity, its roles in the intact adult brain are unknown. Here we report that spontaneous local field potential (LFP) activity in freely moving adult TNC-deficient mice is abnormal. The power of cortical and hippocampal theta and gamma oscillations was enhanced in comparison to wild-type mice. The alteration in hippocampal gamma rhythm was subfield specific, such that CA1 gamma was accentuated while dentate gyrus gamma was normal. Similar to LFP, synaptic transmission and plasticity at perforant path synapses in the dentate gyrus were unaffected by the mutation. Morphological analyses revealed a subfield-specific reduction in the CA1 volume and a reduction in the numbers of somatostatin-positive interneurons in the hippocampus as potential structural substrates of the observed functional aberrations. These findings indicate a role for tenascin-C in structural organization of the CA1 hippocampal subfield and in shaping neural activity.

The extracellular matrix protein SC1/hevin localizes to excitatory synapses following status epilepticus in the rat lithium-pilocarpine seizure model

The epileptic brain is characterized by increased susceptibility to neuronal hyperexcitability. The rat lithium-pilocarpine model, which mimics many features of temporal lobe epilepsy, has been used to study processes leading to the development of recurrent seizures. After a prolonged seizure episode, termed status epilepticus (SE), neural changes occur during a period known as epileptogenesis and include neuronal cell death, reactive gliosis, axonal sprouting, and synaptogenesis. Extracellular matrix adhesion molecules are important regulators of synaptogenesis and axonal sprouting resulting from SE. SC1, also known as hevin, is an antiadhesive extracellular matrix molecule that localizes to synapses in the mammalian brain. In this study, the distribution of SC1 protein in neurons following SE was examined using the lithium-pilocarpine model. SC1 protein levels in neuronal cell bodies showed a transient decrease at 1 day post-SE, which coincided with an increase of SC1 in the synapse-rich neuropil that was identified with the synaptic marker synaptophysin. Immunoelectron microscopy confirmed the decrease of SC1 signal in neurons at 1 day post-SE and showed that SC1 remained localized to postsynaptic elements throughout the seizure time course. Increased colocalization of SC1 was detected with the excitatory synaptic markers vesicular glutamate transporter 1 (VGLUT1), AMPA receptor subunit GluR1, and N-methyl-D-aspartate receptor subunit NR1, but not with the inhibitory synaptic markers vesicular gamma-aminobutyric acid (GABA) transporter (VGAT) and GABA(A) receptor subunit beta2 (GABA(A) beta2), which could reflect enhanced association of SC1 with excitatory synapses. These findings suggest that SC1 may be involved in synaptic modifications underlying epileptogenesis.

Retarded kindling progression in mice deficient in the extracellular matrix glycoprotein tenascin-R

PURPOSE

We investigated the role of the extracellular matrix glycoprotein tenascin-R (TNR) in formation of a hyperexcitable network in the kindling model of epilepsy. The idea that TNR may be important for this process was suggested by previous studies showing that deficiency in TNR leads to abnormalities in synaptic plasticity, perisomatic GABAergic inhibition and more astrocytes in the hippocampus of adult mice.

METHODS

Constitutively TNR deficient (TNR-/-) mice and their wild-type littermates received repeated electrical stimulation in the amygdala over several days until they developed fully kindled generalized seizures at which time their brains were studied immunohistochemically.

RESULTS

In TNR-/- mice, kindling progression was retarded compared with wild-type littermate controls. Morphological analysis of the mice used for the kindling studies revealed that, independently of genotype, numbers of parvalbumin-positive interneurons in the dentate gyrus correlated positively with afterdischarge threshold alterations in kindled mice. The kindling-induced increase in the number of S100 expressing astrocytes in the dentate gyrus was enhanced by TNR deficiency and correlated negatively with the kindling rate.

DISCUSSION

Our data support the view that TNR promotes formation of a hyperexcitable network during kindling and suggest that an increase in S100-expressing astrocytes may contribute to retarded epileptogenesis in TNR-/- mice.

Tenascin C in stem cell niches: redundant, permissive or instructive?

The stem cell niche provides the specialized environment that is able to sustain the lifelong maintenance of stem cells in their discrete locations within organs. The niche is usually composed of several different cell types and a specialized extracellular matrix consisting of many different constituents. Additionally, a variety of growth factors are secreted into the extracellular space and contribute to the functional organization of the niche. Here, I will concentrate on the multimodular extracellular matrix glycoprotein tenascin C (Tnc) and discuss it as an exemplary molecule that is present in several stem cell niches. In spite of its intuitively suggestive presence, it has been difficult to provide functional evidence for the importance of Tnc in the context of stem cells. In the nervous system, the careful analysis of Tnc-deficient mice has revealed that the developmental program neural stem cell pass-through is delayed due to changes in growth factor responsiveness. To gain further insight, we have employed the gene trap technology in neural stem cells to identify potential Tnc target genes. This approach has surfaced 2 interesting candidates that may contribute to a better understanding of the signal(s) elicited by Tnc in neural stem/progenitor cells in the niche.

Lesion-Induced Axonal Sprouting in the Central Nervous System

Injury or neuronal death often come about as a result of brain disorders. Inasmuch as the damaged nerve cells are interconnected via projections to other regions of the brain, such lesions lead to axonal loss in distal target areas. The central nervous system responds to deafferentation by means of plastic remodeling processes, in particular by inducing outgrowth of new axon collaterals from surviving neurons (collateral sprouting). These sprouting processes result in a partial reinnervation, new circuitry, and functional changes within the deafferented brain regions. Lesioning of the entorhinal cortex is an established model system for studying the phenomenon of axonal sprouting. Using this model system, it could be shown that the sprouting process respects the pre-existing lamination pattern of the deafferented fascia dentata, i. e., it is layer-specific. A variety of different molecules are involved in regulating this reorganization process (extracellular matrix molecules, cell adhesion molecules, transcription factors, neurotrophic factors, growth-associated proteins). It is proposed here that molecules of the extracellular matrix define the boundaries of the laminae following entorhinal lesioning and in so doing limit the sprouting process to the deafferented zone. To illustrate the role of axonal sprouting in disease processes, special attention is given to its significance for neurodegenerative disorders, particularly Alzheimer's disease (AD), and temporal lobe epilepsy. Finally, we discuss both the beneficial as well as disadvantageous functional implications of axonal sprouting for the injured organism in question.

Molecular signaling mechanisms underlying epileptogenesis

Epilepsy, a disorder of recurrent seizures, is a common and frequently devastating neurological condition. Available therapy is only symptomatic and often ineffective. Understanding epileptogenesis, the process by which a normal brain becomes epileptic, may help identify molecular targets for drugs that could prevent epilepsy. A number of acquired and genetic causes of this disorder have been identified, and various in vivo and in vitro models of epileptogenesis have been established. Here, we review current insights into the molecular signaling mechanisms underlying epileptogenesis, focusing on limbic epileptogenesis. Study of different models reveals that activation of various receptors on the surface of neurons can promote epileptogenesis; these receptors include ionotropic and metabotropic glutamate receptors as well as the TrkB neurotrophin receptor. These receptors are all found in the membrane of a discrete signaling domain within a particular type of cortical neuron--the dendritic spine of principal neurons. Activation of any of these receptors results in an increase Ca2+ concentration within the spine. Various Ca2+-regulated enzymes found in spines have been implicated in epileptogenesis; these include the nonreceptor protein tyrosine kinases Src and Fyn and a serine-threonine kinase [Ca2+-calmodulin-dependent protein kinase II (CaMKII)] and phosphatase (calcineurin). Cross-talk between astrocytes and neurons promotes increased dendritic Ca2+ and synchronous firing of neurons, a hallmark of epileptiform activity. The hypothesis is proposed that limbic epilepsy is a maladaptive consequence of homeostatic responses to increases of Ca2+ concentration within dendritic spines induced by abnormal neuronal activity.

Enhanced novelty-induced activity, reduced anxiety, delayed resynchronization to daylight reversal and weaker muscle strength in tenascin-C-deficient mice

Tenascin-C (TNC) is an extracellular matrix protein with multiple and important functions during development and in the adult. We here present a study on the behaviour of TNC-deficient (knockout, KO) mice. Longitudinal experiments including tests for circadian activity, exploration, state and trait anxiety, motor coordination and cognition were performed. KO mice showed increased reactivity to explore a novel environment and decreased anxiety. Spontaneous circadian activity was unaffected, but KO mice showed delayed resynchronization to daylight reversal. TNC deficiency caused weaker muscle strength, whereas gait, coordination and motor learning were unaltered. Short- and long-term memory in the fear conditioning task and working memory in the spontaneous alternation test were normal in KO mice. KO mice showed impaired memory recall in the step-down, but not in the step-through, passive avoidance task. Ethological observation of mice behaviour and principal component analyses indicated that the higher novelty- and stress-induced active responses of KO mice account for their poorer performance in passive avoidance tasks, whereas cognitive abilities are unaltered. The present study extends and corrects previous results, and is an example of how an ethological approach allows a precise description and interpretation of the behavioural alterations of mutant mice.

Potential roles for hyaluronan and CD44 in kainic acid-induced mossy fiber sprouting in organotypic hippocampal slice cultures

The most well-documented synaptic rearrangement associated with temporal lobe epilepsy is mossy fiber sprouting (MFS). MFS is a pronounced expansion of granule cell mossy fiber axons into the inner dentate molecular layer. The recurrent excitatory network formed by MFS is hypothesized to play a critical role in epileptogenesis, which is the transformation of the normal brain into one that is prone to recurrent spontaneous seizures. While many studies have focused on the functional consequences of MFS, relatively few have investigated the molecular mechanisms underlying the increased propensity of mossy fibers to invade the inner molecular layer. We hypothesized that changes in two components of the extracellular matrix, hyaluronan and its primary receptor, CD44, contribute to MFS. Hyaluronan contributes to laminar-specificity in the hippocampus and increases in hyaluronan and CD44 are associated with temporal lobe epilepsy. We tested our hypothesis in an in vitro model of MFS using a combination of histological and biochemical approaches. Application of kainic acid (KA) to organotypic hippocampal slice cultures induced robust MFS into the inner dentate molecular layer compared with vehicle-treated controls. Degradation of hyaluronan with hyaluronidase significantly reduced but did not eliminate KA-induced MFS, suggesting that hyaluronan played a permissive role in MFS, but that loss of hyaluronan signaling alone was not sufficient to block mossy fiber reorganization. Comparison of CD44 expression with MFS revealed that when CD44 expression in the molecular layers was high, MFS was minimal and when CD44 expression/function was reduced following KA treatment or with function blocking antibodies, MFS was increased. The time course of KA-induced reductions in CD44 expression was identical to the temporal progression of KA-induced MFS reported previously in hippocampal slice cultures, suggesting that reduced CD44 expression may help promote MFS. Understanding the molecular mechanisms underlying MFS may lead to therapeutic interventions that limit epileptogenesis.

Involvement of tissue inhibition of metalloproteinases-1 in learning and memory in mice

Tissue inhibitor of metalloproteinases (TIMP-1) is one of the four-member family (TIMPs-1-4) of multifunctional proteins that inhibit matrix metalloproteinases (MMPs). Its expression in the hippocampus is neuronal-activity-dependent and dramatically induced by stimuli leading to long-term potentiation (LTP), suggesting that TIMP-1 is a candidate plasticity protein potentially involved in learning and memory processes. We tested this hypothesis in a hippocampus-dependent task using the new olfactory tubing maze, with mice carrying a null mutation for TIMP-1 (TIMP-1 KO) and mice overexpressing TIMP-1 (TIMP-1 (tg)). The TIMP-1 KO mice were significantly impaired in making correct odor-reward associations when compared with their respective wild type (WT) littermates, while TIMP-1 overexpressing mice performed better than their WT controls. Both genetically modified mice learned the paradigm and the timing of the task, like their respective WTs, and no olfactory dysfunctioning was observed. These data suggest that TIMP-1 is involved in learning and memory processes related to the hippocampus, and support the hypothesis that the MMP/TIMP ratio, and hence MMP activity, modulates neuronal plasticity in normal learning and memory processes, while altered proteolytic activity could impair cognitive functions.

Regional and cellular distribution of the extracellular matrix protein tenascin-C in the chick forebrain and its role in neonatal learning

The juvenile brain's pronounced synaptic plasticity in response to early experience and learning events is related to the fact that the genetically pre-programmed molecular machinery mediating neuronal development and synapse formation, is activated throughout postnatal brain development and thereby can be recruited for learning and long-term memory formation. In situ hybridization and immunocytochemistry experiments revealed that tenascin-C, one candidate molecule which we suspect to be involved in neonatal learning, is expressed in the forebrain of domestic chicks around the sensitive period during which auditory filial imprinting takes place. The involvement of tenascin-C in this juvenile learning task was tested by injections of monoclonal antibodies directed to distinct domains of the tenascin-C molecule into the avian prefrontal cortex analog, the medio-rostral nidopallium/mesopallium (formerly termed medio-rostral neostriatum/hyperstriatum ventrale), a forebrain area which has been shown to be critically involved in auditory filial imprinting. Injections of monoclonal antibody Tn 68, which is directed against a cell-binding domain of the tenascin-C molecule, strongly reduced the imprinting rate, as opposed to injections of the monoclonal antibody Tn 578, which binds to a domain involved in neurite outgrowth. Double labeling immunohistochemistry revealed that tenascin-C is associated with neurons which express the Ca(2+)-binding protein parvalbumin, and displays a staining pattern highly reminiscent of perineuronal nets of the extracellular matrix. These results indicate that a distinct domain of tenascin-C is functionally involved in the juvenile learning process of filial imprinting and further suggest a critical role of a specific neuronal subpopulation.

Differential upregulation of extracellular matrix molecules associated with the appearance of granule cell dispersion and mossy fiber sprouting during epileptogenesis in a murine model of temporal lobe epilepsy

We have investigated changes in the extracellular matrix of the hippocampus associated with the early progression of epileptogenesis in a murine model of temporal lobe epilepsy using immunohistochemistry. In the first week following intrahippocampal injection of the glutamate agonist, domoate, there is a latent period at the end of which begins a sequential upregulation of extracellular matrix (ECM) molecules in the granule cell layer of the dentate gyrus, beginning with neurocan and tenascin-C. This expression precedes the characteristic dispersion of the granule cell layer which is evident at 14 days post-injection when the first recurrent seizures can be recorded. At this stage, an upregulation of the chondroitin sulfate proteoglycan, phosphacan, the DSD-1 chondroitin sulfate motif, and the HNK-1 oligosaccharide are also observed. The expression of these molecules is localized differentially in the epileptogenic dentate gyrus, especially in the sprouting molecular layer, where a strong upregulation of phosphacan, tenascin-C, and HNK-1 is observed but there is no expression of the proteoglycan, neurocan, nor of the DSD-1 chondroitin sulfate motif. Hence, it appears that granule cell layer dispersion is accompanied by a general increase in the ECM, while mossy fiber sprouting in the molecular layer is associated with a more restricted repertoire. In contrast to these changes, the expression of the ECM glycoproteins, laminin and fibronectin, both of which are frequently implicated in tissue remodelling events, showed no changes associated with either granule cell dispersion or mossy fiber sprouting, indicating that the epileptogenic plasticity of the hippocampus is accompanied by ECM interactions that are characteristic of the CNS.

Mice deficient for the extracellular matrix glycoprotein tenascin-r show physiological and structural hallmarks of increased hippocampal excitability, but no increased susceptibility to seizures in the pilocarpine model of epilepsy

Recognition molecules provide important cues for neuronal survival, axonal fasciculation, axonal pathfinding, synaptogenesis, synaptic plasticity, and regeneration. Our previous studies revealed a link between perisomatic inhibition and the extracellular matrix glycoprotein tenascin-R (TN-R). Therefore, we here studied neuronal excitability and epileptic susceptibility in mice constitutively deficient in TN-R. In vitro analysis of populational spikes in hippocampal slices of TN-R-deficient mice revealed a significant increase in multiple spikes in the CA1 region, as compared with wild-type mice. This difference between genotypes was only partially reduced after blockade of GABA(A) receptors with picrotoxin, indicating a deficit in GABAergic inhibition and an increase in intrinsic excitability of CA1 pyramidal cells in TN-R-deficient mice. Using a battery of immunohistochemical markers and histological stainings, we were able to identify two abnormalities in the hippocampus of TN-R-deficient mice possibly related to increased excitability: the high number of glial fibrillary acidic protein-positive astrocytes and low number of calretinin-positive interneurons in the CA1 and CA3 regions. In order to test whether the revealed abnormalities give rise to increased susceptibility to seizures in TN-R-deficient mice, we used the pilocarpine model of epilepsy. No genotype-specific differences were found with regard to the time-course of pilocarpine-induced and spontaneous seizures, neuronal cell loss, aberrant sprouting and distribution of synaptic and inhibitory interneuron markers. However, pilocarpine-induced astrogliosis and reduction in calretinin-positive interneurons were less pronounced in TN-R mutants, thereby resulting in an occlusion of effects induced by TN-R deficiency and pilocarpine. Thus, TN-R-deficient mutants show several electrophysiological and morphological hallmarks of increased neuronal excitability, which, however, do not give rise to more accelerated or severe epileptogenesis in the pilocarpine model of epilepsy.

Integrins, synaptic plasticity and epileptogenesis

A number of processes are thought to contribute to the development of epilepsy including enduring increases in excitatory synaptic transmission, changes in GABAergic inhibition, neuronal cell death and the development of aberrant innervation patterns in part arising from reactive axonal growth. Recent findings indicate that adhesion chemistries and, most particularly, activities of integrin class adhesion receptors play roles in each of these processes and thereby are likely to contribute significantly to the cell biology underlying epileptogenesis. As reviewed in this chapter, studies of long-term potentiation have shown that integrins are important for stabilizing activity-induced increases in synaptic strength and excitability. Other work has demonstrated that seizures, and in some instances subseizure neuronal activity, modulate the expression of integrins and their matrix ligands and the activities of proteases which regulate them both. These same adhesion proteins and proteases play critical roles in axonal growth and synaptogenesis including processes induced by seizure in adult brain. Together, these findings indicate that seizures activate integrin signaling and induce a turnover in adhesive contacts and that both processes contribute to lasting changes in circuit and synaptic function underlying epileptogenesis.

The role of ECM molecules in activity-dependent synaptic development and plasticity

Growth and guidance of neurites (axons and dendrites) during development is the prerequisite for the establishment of functional neural networks in the adult organism. In the adult, mechanisms similar to those used during development may regulate plastic changes that underlie important nervous system functions, such as memory and learning. There is now ever-increasing evidence that extracellular matrix (ECM)-associated factors are critically involved in the formation of neuronal connections during development, and their plastic changes in the adult. Here, we review the current literature on the role of ECM components in activity-dependent synaptic development and plasticity, with the major focus on the thrombospondin type I repeat (TSR) domain-containing proteins. We propose that ECM components may modulate neuronal development and plasticity by: 1) regulating cellular motility and morphology, thus contributing to structural alterations that are associated with the expression of synaptic plasticity, 2) coordinating transsynaptic signaling during plasticity via their cell surface receptors, and 3) defining the physical parameters of the extracellular space, thereby regulating diffusion of soluble signaling molecules in the extracellular space (ECS).

Region and lamina-specific distribution of extracellular matrix proteoglycans, hyaluronan and tenascin-R in the mouse hippocampal formation

The extracellular matrix is known to show region-specific characteristics in the adult brain. Our comparative cytochemical study is focused on the laminar organisation of major extracellular matrix constituents in the murine hippocampal formation, including the regions CA1, CA2 and CA3 of the hippocampus proper, the dentate gyrus, the subiculum and the presubiculum. Components related to chondroitin sulphate proteoglycans were detected by N-acetylgalactosamine-binding Wisteria floribunda agglutinin, colloidal iron staining, and antibodies to different proteoglycan domains, including the Cat-301 and Cat-315 epitopes of aggrecan, as well as neurocan, brevican and phosphacan. The distribution patterns of these components were correlated with the patterns revealed for hyaluronan and the brain-specific extracellular matrix glycoprotein, tenascin-R, known to be ligands of extracellular matrix proteoglycans. Lectin binding clearly labelled perineuronal nets of the extracellular matrix around interneurons, which were preferentially located within or near the principal cell layers in all regions. In the hippocampus proper, the CA2 subfield showed an intense labelling of the neuropil around pyramidal cell bodies and the neuropil zones in the strata oriens and radiatum. These patterns were also seen after immunoreaction for chondroitin proteoglycan domains, brevican and phosphacan, as well as after detection of hyaluronan and tenascin-R. Characteristic laminar and intralaminar patterns were additionally expressed in the neuropil in all regions. In the dentate gyrus, the staining intensity for brevican, phosphacan and tenascin-R was predominant in the middle molecular layer, and for Cat-315 in the inner molecular layer, whereas immunoreactivity for neurocan increased within the outer molecular layer towards the hippocampal fissure. Our findings indicate that proteoglycans, hyaluronan and tenascin-R show differential patterns of co-expression in the individual regions and laminae of the hippocampal formation. The inhomogeneous composition of these major components suggests that the extracellular matrix is specifically adapted to the functional domains of intrahippocampal connections and afferent fibre systems.

Tenascin-N: characterization of a novel member of the tenascin family that mediates neurite repulsion from hippocampal explants

Tenascin-N, a novel member of the tenascin family, was identified and shown to encode characteristic structural motifs of a cysteine-rich stretch, 3.5 epidermal growth factor-like repeats, 12 fibronectin type III homologous domains, and a fibrinogen-like domain. The third fibronectin type III homologous domain is altered by RNA splicing. Characterization of the expression of tenascin-N by in situ hybridization analysis assigned transcripts to many types of neurons in the central nervous system, to the medullary region in the kidney, and to resident macrophages of the T-cell zone in the splenic white pulp. By immunohistochemistry, tenascin-N expression is detectable in all brain regions, with a characteristic staining pattern in the hippocampus demarcating the CA3 region. Recombinantly expressed protein fragments of the alternatively spliced isoforms were presented in choice assays on patterned substrates to neurites and migrating neurons from hippocampal CA3 region explant cultures. The smaller splice variant inhibited neurite outgrowth or cell migration, whereas the longer splice form did not inhibit these functions. These observations suggest that the novel tenascin family member mediates specific repulsive properties on neurites and neurons by generating splice isoforms.

Impairment of L-type Ca2+ channel-dependent forms of hippocampal synaptic plasticity in mice deficient in the extracellular matrix glycoprotein tenascin-C

The extracellular matrix glycoprotein tenascin-C (TN-C) has been suggested to play important functional roles during neural development, axonal regeneration, and synaptic plasticity. We generated a constitutively TN-C-deficient mouse mutant from embryonic stem cells with a floxed tn-C allele, representing a standard for future analysis of conditionally targeted mice. The gross morphology of the CNS was not detectably affected, including no evidence for perturbed nerve cell migration, abnormal oligodendrocyte distribution, or defective myelination. Despite the apparent normal histology of the hippocampus and normal performance in the water maze, theta-burst stimulation (TBS) of Schaffer collaterals elicited reduced long-term potentiation (LTP) in the CA1 region of TN-C-deficient mutants, as compared with wild-type littermates. However, high-frequency stimulation evoked normal LTP not only in CA1, but also at mossy fiber-CA3 and medial and lateral perforant path-granule cell synapses in the dentate gyrus. Low-frequency stimulation failed to induce long-term depression in the CA1 region of TN-C-deficient animals. Recordings of TBS-induced LTP in the presence of nifedipine, an antagonist of L-type voltage-dependent Ca2+ channels (VDCCs), did not affect LTP in TN-C-deficient mice, but reduced LTP in wild-type mice to the levels seen in mutants. Furthermore, chemical induction of a L-type VDCC-dependent LTP in the CA1 region by application of the K+ channel blocker tetraethylammonium resulted in impaired LTP in TN-C mutants. Thus, reduction in L-type VDCC-mediated signaling appears to mediate the deficits in certain forms of synaptic plasticity in constitutively TN-C-deficient mice.

Fibronectin domains of extracellular matrix molecule tenascin-C modulate hippocampal learning and synaptic plasticity

The extracellular matrix molecule tenascin-C (TN-C) has been shown to be involved in hippocampal synaptic plasticity in vitro. Here, we describe a deficit in hippocampus-dependent contextual memory in TN-C-deficient mice using the step-down avoidance paradigm. We further show that a fragment of TN-C containing the fibronectin type-III repeats 6-8 (FN6-8), but not a fragment containing repeats 3-5, bound to pyramidal and granule cell somata in the hippocampal formation of C57BL/6J mice and repelled axons of pyramidal neurons when presented as a border in vitro. Injection of the FN6-8 fragment into the hippocampus inhibited retention of memory in the step-down paradigm and reduced levels of long-term potentiation in the CA1 region of the hippocampus. In summary, our data show that TN-C is involved in hippocampus-dependent contextual memory and synaptic plasticity and identify the FN6-8 domain as one of molecular determinants mediating these functions.

Kainate-induced genes in the hippocampus: lessons from expression patterns

Kainate, the analog of the excitatory amino acid L-glutamate, upon binding to non-NMDA glutamate receptors, causes depolarization of neurons followed by severe status epilepticus, neurodegeneration, plasticity and gliosis. These events are best observed in hippocampus, the limbic structure implicated in learning and long-term memory formation. Neurons in all hippocampal structures undergo hyper-activation, however, whereas the cells in the CA subfields degenerate within 2--3 days following the application of kainate, the granule cells of the dentate gyrus are resistant to any form of neurodegeneration and even initiate new synaptic contacts. These physiological and histological changes are modulated by short-term and long-term alterations in gene expression. Perhaps close examination of the changing spatio-temporal patterns of mRNAs of various genes may help in generating a clearer picture of the molecular events leading to complex cognitive functions.

Loss of cortical and thalamic neuronal tenascin-C expression in a transgenic mouse expressing exon 1 of the human Huntington disease gene

A transgenic mouse containing the first exon of the human Huntington's disease (HD) gene has revealed a variety of behavioral and pathophysiological anomalies reminiscent of certain aspects of human Huntington's disease (HD). The present study has found that expression of the extracellular matrix glycoprotein tenascin-C appears to be unaffected in astroglial cells in wild-type and R6/2 transgenic mice that express the mutant huntingtin protein but that it is conspicuously absent in two neuronal populations within the cerebral cortex and thalamus of the R6/2 mice. Loss of tenascin-C expression begins between the fourth and eighth postnatal weeks, coincidental with the onset of abnormal behavioral phenotype and the appearance of intranuclear inclusion bodies and neuropil aggregates. By 12 weeks, R6/2 mice exhibit a complete absence of tenascin-C neuronal immunolabeling, a disappearance of cRNA probe-positive neurons across discrete cytoarchitectonic regions of the dorsal thalamus (e.g., the ventromedial, parafascicular, lateral posterior, and posterior thalamic groups) and frontal cortex, and an accompanying thalamic astrogliosis. The loss of neuronal tenascin-C expression includes structures that are known to send converging excitatory axonal projections to the caudate-putamen, the structure that is most at risk for neurodegeneration in HD. Altered neuronal expression of tenascin-C in R6/2 mice implicates altered transcriptional activities of the mutant huntingtin protein. The abnormal biochemistry and possibly abnormal activity of thalamostriate and corticostriate projection neurons may also affect abnormal neuronal activities in their primary connectional target, the neostriatum, which is severely compromised in HD.

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.

Nervous system proteoglycans as modulators of neurite outgrowth

The proteoglycans are multifunctional macromolecules composed of a core polypeptide and a variable number of glycosaminoglycan chains. The structural diversity and complexities of proteoglycan expression in the developing and adult Nervous System underlies the variety of biological functions that these molecules fulfill. Thus, in the Nervous System, proteoglycans regulate the structural organisation of the extracellular matrix, modulate growth factor activities and cellular adhesive and motility events, such as cell migration and axon outgrowth. This review summarises the evidences indicating that proteoglycans have an important role as modulators of neurite outgrowth and neuronal polarity. Special emphasis will be placed on those studies that have shown that proteoglycans of certain subtypes inhibit neurite extension either during the development and/or the regeneration of the vertebrate Central Nervous System.

Entorhinal cortex lesion in adult rats induces the expression of the neuronal chondroitin sulfate proteoglycan neurocan in reactive astrocytes

The chondroitin sulfate proteoglycan neurocan is a major component of brain extracellular matrix during development. Neurocan is primarily synthesized by neurons and has the ability to interact with cell adhesion molecules involved in the regulation of cell migration and axonal growth. Within the first weeks postnatally, neurocan expression is strongly downregulated. To test whether neurocan is reexpressed in areas of axonal growth (sprouting) after brain injury, the time course of neurocan expression was analyzed in the denervated fascia dentata of the rat after entorhinal cortex lesion (12 hr; 1, 2, 4, and 10 d; 2 and 4 weeks; and 6 months after lesion). In the denervated zone, immunohistochemistry revealed neurocan-positive astrocytes by 2 d after lesion and a diffuse labeling of the extracellular matrix at all later time points. Electron microscopy confirmed the deposition of neurocan in the extracellular matrix compartment. In situ hybridization demonstrated a strong upregulation of neurocan mRNA within the denervated outer molecular layer 1 and 4 d after lesion. The combination of in situ hybridization with immunohistochemistry for glial fibrillary acidic protein demonstrated that the neurocan mRNA-expressing cells are astrocytes. These data demonstrate that neurocan is reexpressed in the injured brain. In contrast to the situation during development, astrocytes, but not neurons, express neurocan and enrich the extracellular matrix with this molecule. Similar to the situation during development, neurocan is expressed in an area of active axon growth, and it is suggested that neurocan acts to maintain the boundaries of the denervated fascia dentata after entorhinal cortex lesion.

Serotonin depletion in the adult rat produces differential changes in highly polysialylated form of neural cell adhesion molecule and tenascin-C immunoreactivity

Levels of immunoreactivity for highly polysialylated neural cell adhesion molecule (PSA-NCAM), NCAM, and tenascin-C (TN-C), were examined in the basal ganglia regions and hypothalamic nuclei of adult rats after serotonergic (5-HT) lesions induced by 5,7-dihydroxytryptamine injections in the dorsal and medial raphe nuclei. Decreases in the density of serotonin fibers were associated with no changes in NCAM and general decreases in PSA-NCAM staining, the time-course of changes being selective for each region. Taken that the confocal analysis indicated that serotonin neurons do not express PSA-NCAM and that similar decreases in PSA-NCAM staining were observed after inhibition of 5-HT synthesis induced by parachlorophenylalanine administration, these results suggest that 5-HT may reduce adhesion by acting on PSA-NCAM expression in its environment, and thus facilitate plasticity in adult brain. Two months after the neurotoxin lesions, a normalization of PSA-NCAM staining was associated with a partial restoration in 5-HT fiber density in the nucleus accumbens and the supraoptic nucleus, suggesting that PSA-NCAM may facilitate sprouting of 5-HT fibers. Since a similar normalization was also detected in the suprachiasmatic nucleus, which remained deprived of serotonin fibers, negative factors are likely to be involved in regeneration processes. Indeed, increases in glial fibrillary acidic protein (GFAP) followed by increases in TN-C were observed in these areas, suggesting that the secretion of TN-C by astrocytes may have negative consequences on the sprouting of 5-HT fibers. Finally, the lack of changes in striatal PSA-NCAM or TN-C staining observed after selective lesions of the dopaminergic pathway induced by intranigral injections of 6-hydroxydopamine indicates that 5-HT has a selective and critical role in adult brain plasticity.

Seizure induced synthesis of fibronectin is rapid and age dependent: implications for long-term potentiation and sprouting

Extracellular matrix proteins are induced by activity in adult brain but the time course of these responses, and hence the possibility of their involvement in use-dependent synaptic plasticity, is not known. To evaluate this issue, the influence of seizures on fibronectin expression was evaluated in the adult and developing hippocampus. In adult rats, kainic acid-induced seizures increased fibronectin mRNA and immunoreactivity (ir) by about 1 h after the first behavioral seizure. In situ hybridization analysis indicated that fibronectin mRNA was increased in broadly distributed glial cells as well as within discrete neuronal populations that normally express this transcript. Western blots demonstrated that increased fibronectin-ir was evident in both soluble and non-soluble fractions at the same time point. Immunocytochemical colocalization confirmed that fibronectin-ir was indeed elevated in broadly distributed glial fibrillary acidic protein-ir astroglia. Seizures had no detectable effect on fibronectin-ir in the hippocampus of nine day old rats. Together with previous results, the above findings suggest that intense physiological activity triggers a 'matrix response' (i.e., release proteases, activate integrins, secrete matrix proteins) that is sufficiently rapid to participate in the consolidation of long-term potentiation (LTP). The absence of such reactions in the immature hippocampus is in accord with the hypothesis that matrix proteins generated by mature astroglia impose temporal and spatial limitations on axonal remodeling.

Molecular mechanisms that underlie structural and functional changes at the postsynaptic membrane during synaptic plasticity

The synaptic plasticity that is addressed in this review follows neurodegeneration in the brain and thus has both structural as well as functional components. The model of neurodegeneration that has been selected is the kainic acid lesioned hippocampus. Degeneration of the CA3 pyramidal cells results in a loss of the Schaffer collateral afferents innervating the CA1 pyramidal cells. This is followed by a period of structural plasticity where new synapses are formed. These are associated with changes in the numbers and shapes of spines as well as changes in the morphometry of the dendrites. It is suggested that this synaptogenesis is responsible for an increase in the ratio of NMDA to AMPA receptors mediating excitatory synaptic transmission at these synapses. Changes in the temporal and spatial properties of these synapses resulted in an altered balance between LTP and LTD. These properties together with a reduction in the inhibitory drive increased the excitability of the surviving CA1 pyramidal cells which in turn triggered epileptiform bursting activity. In this review we discuss the insights that may be gained from studies of the underlying molecular machinery. Developments in one of the collections of the cogs in this machinery has been summarized through recent studies characterizing the roles of neural recognition molecules in synaptic plasticity in the adult nervous systems of vertebrates and invertebrates. Such investigations of neural cell adhesion molecules, cadherins and amyloid precursor protein have shown the involvement of these molecules on the morphogenetic level of synaptic changes, on the one hand, and signal transduction effects, on the other. Further complex cogs are found in the forms of the low-density lipoprotein receptor (LDL-R) family of genes and their ligands play pivotal roles in the brain development and in regulating the growth and remodelling of neurones. Evidence is discussed for their role in the maintenance of cognitive function as well as Alzheimer's. The molecular mechanisms responsible for the clustering and maintenance of transmitter receptors at postsynaptic sites are the final cogs in the machinery that we have reviewed. Postsynaptic densities (PSD) from excitatory synapses have yielded many cytoskeletal proteins including actin, spectrin, tubulin, microtubule-associated proteins and calcium/calmodulin-dependent protein kinase II. Isolated PSDs have also been shown to be enriched in AMPA, kainate and NMDA receptors. However, recently, a new family of proteins, the MAGUKs (for membrane-associated guanylate kinase) has emerged. The role of these proteins in clustering different NMDA receptor subunits is discussed. The MAGUK proteins are also thought to play a role in synaptic plasticity mediated by nitric oxide (NO). Both NMDA and non-NMDA receptors are highly clustered at excitatory postsynaptic sites in cortical and hippocampal neurones but have revealed differences in their choice of molecular components. Both GABAA and glycine (Gly) receptors mediate synaptic inhibition in the brain and spinal cord. Whilst little is known about how GABAA receptors are localized in the postsynaptic membrane, considerable progress has been made towards the elucidation of the molecular mechanisms underlying the formation of Gly receptors. It has been shown that the peripheral membrane protein gephyrin plays a pivotal role in the formation of Gly receptor clusters most likely by anchoring the receptor to the subsynaptic cytoskeleton. Evidence for the distribution as well as function of gephyrin and Gly receptors is discussed. Postsynaptic membrane specializations are complex molecular machinery subserving a multitude of functions in the proper communication between neurones. Despite the fact that only a few key players have been identified it will be a fascinating to watch the story as to how they contribute to structural and functional plasticity unfold.

A neurite outgrowth-inhibitory proteoglycan expressed during development is similar to that isolated from adult brain after isomorphic injury

The expression of proteoglycans (PGs) in the mammalian central nervous system (CNS) appears to be strictly regulated both during development and after damage to the mammalian CNS. Recently, we have isolated from membranes of injured adult brain a neurite outgrowth-inhibitory proteoglycan (IMP), the activity of which could be specifically counteracted by a monoclonal antibody (mAB) against the PG. We described in this report the characterization of perinatal membrane proteoglycan (PMP), a heparan-sulfate/chondroitin-sulfate-containing PG expressed during brain development. Its maximal expression was observed around postnatal day 3, decreasing strongly in normal adult tissue. This PG was purified and characterized using mABs generated against IMP. The comparison of PMP and IMP properties indicates that the two PGs are highly related and share expression patterns, biochemical characteristics, and the ability to inhibit neurite initiation in culture. However, IMP and PMP displayed a distinct effect on neurite elongation, which may be explained by their differences in glycosilation pattern. The data presented in this report support the idea that proteoglycans expressed during CNS development are re-expressed following injury.

mRNA coding for voltage-gated sodium channel beta2 subunit in rat central nervous system: cellular distribution and changes following kainate-induced seizures

The cellular distribution of sodium channel beta2 subunit mRNA was examined in the central nervous system from adult Wistar rats using a non-radioactive in situ hybridization method with digoxigenin-labeled cRNA probes. The expression of the subunit was strong in cerebral and cerebellar cortex, in medulla oblongata and in the spinal cord whereas heterogeneous in hippocampus. The distribution was evaluated in hippocampus and cerebral cortex from 1 to 72 h after kainate injection and compared to control rats using densitometric analysis. In these areas, a transient increase was seen 1 h after the drug administration, followed, in the hippocampus, by a significant decrease. These variations differ from those we previously reported for alpha subunits and might play a role in cellular excitability changes occurring in the course of seizures.

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.

Neural recognition molecules and synaptic plasticity

Recent studies of neural recognition molecules have revealed similarities between their functions during ontogenetic development and in neural plasticity in the adult. Observations both at the cellular level in vitro and at the behavioural level in vivo suggest that altered recognition molecule expression can lead to changes in synaptic efficacy, and alterations in synaptic function in turn evoke changes in recognition molecule expression. These changes can manifest themselves as morphological alterations and modulations of the synapse's signal transduction machinery.