Up-regulation of the extracellular matrix glycoprotein tenascin-R during axonal reorganization and astrogliosis in the adult rat hippocampus

Treadmill exercise improves hippocampal neural plasticity and relieves cognitive deficits in a mouse model of epilepsy

Epilepsy frequently leads to cognitive dysfunction and approaches to treatment remain limited. Although regular exercise effectively improves learning and memory functions across multiple neurological diseases, its application in patients with epilepsy remains controversial. Here, we adopted a 14-day treadmill-exercise paradigm in a pilocarpine injection-induced mouse model of epilepsy. Cognitive assays confirmed the improvement of object and spatial memory after endurance training, and electrophysiological studies revealed the maintenance of hippocampal plasticity as a result of physical exercise. Investigations of the mechanisms underlying this effect revealed that exercise protected parvalbumin interneurons, probably via the suppression of neuroinflammation and improved integrity of blood-brain barrier. In summary, this work identified a previously unknown mechanism through which exercise improves cognitive rehabilitation in epilepsy.

Distribution and classification of the extracellular matrix in the olfactory bulb

Extracellular matrix (ECM) became an important player over the last few decades when studying the plasticity and regeneration of the central nervous system. In spite of the established role of ECM in these processes throughout the central nervous system (CNS), only few papers were published on the ECM of the olfactory system, which shows a lifelong plasticity, synaptic remodeling and postnatal neurogenesis. In the present study, we have described the localization and organization of major ECM molecules, the hyaluronan, the lecticans, tenascin-R and HAPLN1 link protein in the olfactory bulb (OB) of the rat. We detected all of these molecules in the OB showing differences in the molecular composition, staining intensity, and organization of ECM between the layers and in some cases within a single layer. One of the striking features of ECM staining pattern in the OB was that the reactions are shown dominantly in the neuropil, the PNNs were found rarely and they exhibited thin or diffuse appearance Similar organization was shown in human and mice samples. As the PNN limits the neural plasticity, its rare appearance may be related to the high degree of plasticity in the OB.

AMPA Receptor Antagonist NBQX Decreased Seizures by Normalization of Perineuronal Nets

Epilepsy is a serious brain disorder with diverse seizure types and epileptic syndromes. AMPA receptor antagonist 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzoquinoxaline-2,3-dione (NBQX) attenuates spontaneous recurrent seizures in rats. However, the anti-epileptic effect of NBQX in chronic epilepsy model is poorly understood. Perineuronal nets (PNNs), specialized extracellular matrix structures, surround parvalbumin-positive inhibitory interneurons, and play a critical role in neuronal cell development and synaptic plasticity. Here, we focused on the potential involvement of PNNs in the treatment of epilepsy by NBQX. Rats were intraperitoneally (i.p.) injected with pentylenetetrazole (PTZ, 50 mg/kg) for 28 consecutive days to establish chronic epilepsy models. Subsequently, NBQX (20 mg/kg, i.p.) was injected for 3 days for the observation of behavioral measurements of epilepsy. The Wisteria floribundi agglutinin (WFA)-labeled PNNs were measured by immunohistochemical staining to evaluate the PNNs. The levels of three components of PNNs such as tenascin-R, aggrecan and neurocan were assayed by Western blot assay. The results showed that there are reduction of PNNs and decrease of tenascin-R, aggrecan and neurocan in the medial prefrontal cortex (mPFC) in the rats injected with PTZ. However, NBQX treatment normalized PNNs, tenascin-R, aggrecan and neurocan levels. NBQX was sufficient to decrease seizures through increasing the latency to seizures, decrease the duration of seizure onset, and reduce the scores for the severity of seizures. Furthermore, the degradation of mPFC PNNs by chondroitinase ABC (ChABC) exacerbated seizures in PTZ-treated rats. Finally, the anti-epileptic effect of NBQX was reversed by pretreatment with ChABC into mPFC. These findings revealed that PNNs degradation in mPFC is involved in the pathophysiology of epilepsy and enhancement of PNNs may be effective for the treatment of epilepsy.

A potential role for glia-derived extracellular matrix remodeling in postinjury epilepsy

Head trauma and vascular injuries are known risk factors for acquired epilepsy. The sequence of events that lead from the initial injury to the development of epilepsy involves complex plastic changes and circuit rewiring. In-depth, comprehensive understanding of the epileptogenic process is critical for the identification of disease-modifying targets. Here we review the complex interactions of cellular and extracellular components that may promote epileptogenesis, with an emphasis on the role of astrocytes. Emerging evidence demonstrates that astrocytes promptly respond to brain damage and play a critical role in the development of postinjury epilepsy. Astrocytes have been shown to regulate extracellular matrix (ECM) remodeling, which can affect plasticity and stability of synapses and, in turn, contribute to the epileptogenic process. From these separate lines of evidence, we present a hypothesis suggesting a possible role for astrocyte-regulated remodeling of ECM and perineuronal nets, a specialized ECM structure around fast-spiking inhibitory interneurons, in the development and progression of posttraumatic epilepsies. © 2016 Wiley Periodicals, Inc.

Chondroitin sulfate proteoglycan tenascin-R regulates glutamate uptake by adult brain astrocytes

In our previous study, the CS-56 antibody, which recognizes a chondroitin sulfate moiety, labeled a subset of adult brain astrocytes, yielding a patchy extracellular matrix pattern. To explore the molecular nature of CS-56-labeled glycoproteins, we purified glycoproteins of the adult mouse cerebral cortex using a combination of anion-exchange, charge-transfer, and size-exclusion chromatographies. One of the purified proteins was identified as tenascin-R (TNR) by mass spectrometric analysis. When we compared TNR mRNA expression patterns with the distribution patterns of CS-56-positive cells, TNR mRNA was detected in CS-56-positive astrocytes. To examine the functions of TNR in astrocytes, we first confirmed that cultured astrocytes also expressed TNR protein. TNR knockdown by siRNA expression significantly reduced glutamate uptake in cultured astrocytes. Furthermore, expression of mRNA and protein of excitatory amino acid transporter 1 (GLAST), which is a major component of astrocytic glutamate transporters, was reduced by TNR knockdown. Our results suggest that TNR is expressed in a subset of astrocytes and contributes to glutamate homeostasis by regulating astrocytic GLAST expression.

Identifying SNP targeted pathways in partial epilepsies with genome-wide association study data

PURPOSE

In a recent genome-wide association study for partial epilepsies in the European population, a common genetic variation has been reported to affect partial epilepsy only modestly. However, in complex diseases such as partial epilepsy, multiple factors (e.g. single nucleotide polymorphisms, microRNAs, metabolic and epigenetic factors) may target different sets of genes in the same pathway, affecting its function and thus causing the disease development. In this regard, we hypothesize that the pathways are critical for elucidating the mechanisms underlying partial epilepsy.

METHODS

Previously we had developed a novel methodology with the aim of identifying the disease-related pathways. We had combined evidence of genetic association with current knowledge of (i) biochemical pathways, (ii) protein-protein interaction networks, and (iii) the functional information of selected single nucleotide polymorphisms. In our present study, we apply this methodology to a data set on partial epilepsy, including 3445 cases and 6935 controls of European ancestry.

RESULTS

We have identified 30 overrepresented pathways with corrected p-values smaller than 10(-12). These pathways include complement and coagulation cascades, cell cycle, focal adhesion, extra cellular matrix-receptor interaction, JAK-STAT signaling pathway, MAPK signaling pathway, proteasome, ribosome, calcium signaling and regulation of actin cytoskeleton pathways. Most of these pathways have growing scientific support in the literature as being associated with partial epilepsy. We also demonstrate that different factors affect distinct parts of the pathways, as shown here on complement and coagulation cascades pathway with a comparison of gene expression vs. genome-wide association study.

CONCLUSIONS

Traditional studies on genome-wide association have not revealed strong associations in epilepsies, since these single nucleotide polymorphisms are not shared by most of the patients. Our results suggest that it is more effective to incorporate the functional effect of a single nucleotide polymorphism on the gene product, protein-protein interaction networks and functional enrichment tools into genome-wide association studies. These can then be used to determine leading molecular pathways, which cannot be detected through traditional analyses. We hope that this type of analysis brings the research community one step closer to unraveling the complex genetic structure of epilepsies.

Extracellular matrix in plasticity and epileptogenesis

Extracellular matrix (ECM) in the brain is composed of molecules synthesized and secreted by neurons and glial cells in a cell-type-specific and activity-dependent manner. During development, ECM plays crucial roles in proliferation, migration and differentiation of neural cells. In the mature brain, ECM undergoes a slow turnover and supports multiple physiological processes, while restraining structural plasticity. In the first part of this review, we discuss the contribution of ECM molecules to different forms of plasticity, including developmental plasticity in the cortex, long-term potentiation and depression in the hippocampus, homeostatic scaling of synaptic transmission and metaplasticity. In the second part, we focus on pathological changes associated with epileptogenic mutations in ECM-related molecules or caused by seizure-induced remodeling of ECM. The available data suggest that ECM components regulating physiological plasticity are also engaged in different aspects of epileptogenesis, such as dysregulation of excitatory and inhibitory neurotransmission, sprouting of mossy fibers, granule cell dispersion and gliosis. At the end, we discuss combinatorial approaches that might be used to counteract seizure-induced dysregulation of both ECM molecules and extracellular proteases. By restraining ECM modification and preserving the status quo in the brain, these treatments might prove to be valid therapeutic interventions to antagonize the progression of 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.

Metallothioneins and zinc dysregulation contribute to neurodevelopmental damage in a model of perinatal viral infection

Neonatal Borna disease (NBD) virus infection in the Lewis rat results in life-long viral persistence and causes behavioral and neurodevelopmental abnormalities. A hallmark of the disorder is progressive loss of cerebellar Purkinje and dentate gyrus granule cells. Findings of increased brain metallothionein-I and -II (MT-I/-II) mRNA expression in cDNA microarray experiments led us to investigate MT isoforms and their relationship to brain zinc metabolism, cellular toxicity, and neurodevelopmental abnormalities in this model. Real-time PCR confirmed marked induction of MT-I/-II mRNA expression in the brains of NBD rats (40.5-fold increase in cerebellum, p<0.0001; 6.8-fold increase in hippocampus, p=0.003; and 9.5-fold increase in striatum, p=0.0012), whereas a trend toward decreased MT-III mRNA was found in hippocampus (1.25-fold decrease, p=0.0841). Double label immunofluorescence revealed prominent MT-I/-II expression in astrocytes throughout the brain; MT-III protein was decreased in granule cell neurons and increased in astrocytes, with differential subcellular distribution from cytoplasmic to nuclear compartments in NBD rat hippocampus. Modified Timm staining of hippocampus revealed reduced zinc in mossy fiber projections to the hilus and CA3, accumulation of zinc in glial cells and degenerating granule cell somata, and robust mossy fiber sprouting into the inner molecular layer of the dentate gyrus. Zinc Transporter 3 (ZnT-3) mRNA expression was decreased in hippocampus (2.3-fold decrease, p= 0.0065); staining for its correlate protein was reduced in hippocampal mossy fibers. Furthermore, 2 molecules implicated in axonal pathfinding and mossy fiber sprouting, the extracellular matrix glycoprotein, tenascin-R (TN-R), and the hyaluronan receptor CD44, were increased in NBD hippocampal neuropil. Abnormal zinc metabolism and mechanisms of neuroplasticity may contribute to the pathogenesis of disease in this model, raising more general implications for neurodevelopmental damage following viral infections in early life.

Functional properties and oxidative modulation of A-type K currents in hippocampal granule cells of control and chronically epileptic rats

A-type K+ channels are crucial determinants of neuronal firing. For example, reducing the amplitude of A-type currents (I(A)) increases seizure susceptibility. We have therefore examined the functional and molecular properties of I(A) in dentate granule neurons following pilocarpine-induced status epilepticus (SE). We found that the levels of various A-type channel subunit mRNAs are unaltered following SE. Furthermore, current density and biophysical properties of I(A) recorded in outside-out and cell-attached patches from dentate granule cells are not modified by SE. However, I(A) in both control and epileptic rats was powerfully regulated by the cellular redox state. I(A) was recorded in outside-out patches with the recording pipette containing either reduced (GSH) or oxidized (GSSG) glutathione. In both control and epileptic rats, the presence of GSSG caused a similar, marked acceleration of recovery from inactivation. Additionally, GSSG produced a small but significant reduction of I(A) amplitudes only in control rats. The inactivation time course of I(A) during depolarizing voltage steps was not modified by GSH or GSSG. Cell-attached recordings, in which the intracellular milieu is conserved, revealed a slow time course of recovery more comparable to that with GSH. In summary, epileptic activity does not produce chronic changes in the molecular and functional properties of the somatic I(A) of dentate granule cells. However, I(A) is powerfully modulated by oxidation in both control and epileptic rats. This finding suggests that the availability of I(A) may be strongly regulated by changes in the GSH/GSSG ratio occurring during prolonged seizure activity or hypoxia.

Loss of perineuronal net N-acetylgalactosamine in Alzheimer's disease

The perineuronal net (PN), a specialised region of extracellular matrix, is interposed between the neuronal cell surface and astrocytic processes. It is involved in the buffering of ions, in the development, stabilisation and remodelling of synapses and in the regulating the neuronal microenvironment particularly around the parvalbumin-positive GABAergic neurons. We have investigated the relative preservation of Wisteria floribunda agglutinin (WFA)-positive PNs and parvalbumin-positive neurons in Alzheimer's disease (AD), and the relationship of WFA-positive PNs to parenchymal tau, amyloid beta-peptide (Abeta) and MHC class II antigen (a marker of activated microglia), in paraffin sections of 100 cases with AD and 45 controls. The density of PNs that could be labelled with WFA, which binds to the N-acetylgalactosamine (GalNAc) residues of chondroitin sulphate proteoglycans, was reduced by about 2/3 in AD (P<0.001). In contrast, the density of parvalbumin-positive neurons did not differ significantly between AD and controls. Combined fluorescence imaging showed granular disintegration of WFA labelling around some parvalbumin-positive neurons. There was no significant difference in the amount of phosphorylated tau, Abeta or MHC class II antigen in areas with and without WFA-positive PNs. In AD, there is marked loss of PN GalNAc that is not topographically related to neurofibrillary pathology, parenchymal Abeta load or activated microglia. Although the parvalbumin-positive neurons themselves are relatively spared, the loss of PN GalNAc, which maintains a polyanionic microenvironment around neurons, is likely to impair the function of these inhibitory interneurons. This could in turn lead to increased activity of the glutamatergic and other neurons onto which they synapse.