Excitatory amino acid regulation of astrocyte proteoglycans

In vitro analysis of mechanisms underlying age-dependent failure of axon regeneration

Severed axons of the inferior colliculus (IC) commissure can regenerate across a lesion in organotypic cultures from postnatal day (P) 6 gerbils, but this regenerative capacity is lost by P12 (Hafidi et al. [ 1995] J Neurosci 15:1298-1307, [1999] J Neurobiol 41:267-280). In the present study, we examined the mechanisms underlying this age-dependent failure of axons to regenerate. In P6-P12 heterochronic cultures, the P12 axons failed to cross the lesion site and project to the contralateral P6 IC lobe. In contrast, axons originating from the P6 lobe could regenerate through the lesion and invade the contralateral P12 IC lobe. To determine whether this age-dependent change in regenerative capacity can develop in organotypic cultures, IC slices with an intact commissure were obtained from P6 animals, grown in vitro for 6 days, and then lesioned at the commissure. In these slices, axon regeneration failure was similar to that observed in normal P12 tissue. Several in vitro treatments enhanced axon regeneration: removal of the entire midline region, inhibition of protein synthesis at the lesion site, and exposure to ABC chondroitinase. Furthermore, when the injured commissural axons were provided with a carpet of C6-R cells (a radial glia-like cell line), significantly more axons projected to the contralateral lobe of the IC. Taken together, these results suggest that the maturation of nonneuronal cells within the lesion site lead to failed axon regeneration in mature animals, and show that ameliorative strategies can be evaluated in vitro.

Glycosaminoglycan levels and proteoglycan expression are altered in the hippocampus of patients with mesial temporal lobe epilepsy

Extracellular matrix proteoglycans (PGs) and glycosaminoglycans (GAGs) play a crucial role in cell differentiation and synaptogenesis by modulating neurite outgrowth. The chondroitin sulfate (CS)-rich PG, the receptor protein tyrosine phosphatase zeta/beta (RPTP zeta/beta), has been related to neural morphogenesis and axon guidance. Hippocampal sclerosis is the most frequent pathologic finding in patients with intractable mesial temporal lobe epilepsy (MTLE), which is associated with neuron loss, reactive gliosis, and mossy fiber sprouting. In the present study, we investigated the concentration of CS, heparan sulfate (HS) and hyaluronic acid (HA) in the hippocampus and temporal neocortex as well as RPTP zeta/beta expression in the hippocampus of patients with MTLE. Compared to autopsy control tissue, epileptic hippocampi showed a significantly increased concentration of CS (224%; p=0.0109) and HA (146%; p=0.039). HS was instead similar to control values. No differences were found in the concentration of CS, HS, or HA in the temporal neocortex of epileptic patients when compared to control values. In contrast, RPTP zeta/beta immunoreactivity was induced in astrocytes of the inner molecular layer of the dentate gyrus of the sclerotic hippocampus. Because matrix compounds have been associated with tissue injury and repair, the present findings suggest that changes in PGs and GAGs might be related to damage-induced gliosis and neuronal reorganization in the hippocampus of MTLE patients.

Transient expression of juvenile-type neurocan by reactive astrocytes in adult rat brains injured by kainate-induced seizures as well as surgical incision

Neurocan is one of the major chondroitin sulfate proteoglycans expressed in nervous tissues. The expression of neurocan is developmentally regulated, and full-length neurocan is detected in juvenile brains but not in adult brains. In the present study, we demonstrated by western blot analysis that full-length neurocan transiently appeared in adult rat hippocampus when it was lesioned by kainate-induced seizures. Immunohistochemical studies showed that neurocan was detected mainly around the CA1 region although the seizure resulted in neuronal cell degeneration in both the CA1 and CA3 regions of the hippocampus. Double-labeling for neurocan mRNA and glial fibrillary acidic protein demonstrated that many reactive astrocytes expressed neurocan mRNA. The re-expression of full-length neurocan was also observed in the surgically injured adult rat brain. In contrast, the expression of other nervous tissue chondroitin sulfate proteoglycans, such as phosphacan and neuroglycan C, was not intensified but rather was either reduced in the kainate-lesioned hippocampus or in the surgically injured cerebral cortex. These observations indicate that induction of neurocan expression by reactive astrocytes is a common phenomenon in the repair process of adult brain injury, and therefore, it can be postulated that juvenile-type neurocan plays some roles in brain repair.

Extracellular matrix components are altered in the hippocampus, cortex, and cerebrospinal fluid of patients with mesial temporal lobe epilepsy

PURPOSE

This work studied the profile of glycosaminoglycans (GAGs) in the hippocampus, cortex, and cerebrospinal fluid of patients with temporal lobe epilepsy (TLE).

METHODS

The GAGs were analyzed by agarose gel electrophoresis, enzymatic degradation, and enzyme-linked immunosorbent assay (ELISA).

RESULTS

The hippocampus of TLE patients showed increased levels of chondroitin sulfate and hyaluronic acid against normal levels of these GAGs in the neocortex and cerebrospinal fluid (CSF).

CONCLUSIONS

These results suggest that these matrix components could be involved in the pathophysiology of TLE.

Activation of fibres in rat sciatic nerve alters phosphorylation state of connexin-43 at astrocytic gap junctions in spinal cord: evidence for junction regulation by neuronal-glial interactions

Intercellular communication via gap junction channels composed of connexin-43 is known to be regulated by phosphorylation of this protein. We investigated whether connexin-43 at astrocytic gap junctions is similarly regulated in response to neural activation. The effect of peripheral nerve stimulation on connexin-43 phosphorylation state in the spinal cord of rats was examined with a monoclonal antibody (designated 13-8300) shown previously to recognize selectively a dephosphorylated form of connexin-43. Immunolabelling with 13-8300 was absent in the lumbar spinal cord in control animals, but was induced in the dorsal horn ipsilateral to sciatic nerve electrical stimulation for 15min or 1h at a frequency of 1 or 100Hz. Immunorecognition of connexin-43 by a polyclonal anti-connexin-43 antibody, shown previously to undergo epitope masking under various conditions, was reduced in the dorsal horn on the stimulated side. These responses were abolished by local anaesthetic or tetrodotoxin application proximal to the site of nerve stimulation. Selective electrical stimulation of A-fibres or activation of cutaneous C-fibres by capsaicin evoked labelling with 13-8300 in deep and superficial laminae of the dorsal horn, respectively. Nerve stimulation increased the number of 13-8300-positive astrocytic gap junctions, as well as the levels of dephosphorylated connexin-43 in the dorsal horn on the stimulated side. Sciatic nerve transection produced results similar to those seen after C-fibre activation with capsaicin.Thus, peripheral nerve stimulation evokes astrocytic connexin-43 dephosphorylation in the spinal cord dorsal horn, suggesting that gap junctional coupling between astrocytes in vivo is subject to regulation by neuronal-glial interactions following neural activation.

Selective alterations of glycosaminoglycans synthesis and proteoglycan expression in rat cortex and hippocampus in pilocarpine-induced epilepsy

Proteoglycans and glycosaminoglycans are elements of matrix. In the nervous system, glycosaminoglycans modulate neurite outgrowth and are co-receptors for growth factors playing a crucial role in cell differentiation and synaptogenesis. The receptor of protein tyrosine phosphatase beta (RPTPbeta) is a chondroitin sulphate proteoglycan which plays an important role in neural morphogenesis and axon guidance mechanisms. Pilocarpine-treated rats present status epilepticus, which is followed by a seizure-free period (silent), by a period of spontaneous recurrent seizures (chronic), and the hippocampus of these animals exhibits cell loss and mossy fiber sprouting. Thus, the synthesis of heparan sulphate and chondroitin sulphate and the time course of RPTPbeta immunoreactivity were studied in the hippocampus and cerebral cortex during these phases of pilocarpine-induced epilepsy. The results showed decreased synthesis of heparan sulphate during the acute phase and an increased synthesis of chondroitin sulphate during the silent period in the cortex and hippocampus. In control rats RPTPbeta immunoreactivity was detected only in glial cells. After 6 h of status epilepticus the RPTPbeta immunoreactivity was no longer detectable in the glial cells in both tissues and intense staining became evident in the matrix, surrounding CA3 and dentate gyrus and piriform cortex neurones. In the silent and chronic periods RPTPbeta immunoreactivity was mainly detected in neuronal somata and fibers of neurones of hippocampus and cortex. These changes show a selective variation of synthesis and expression of glycosaminoglycans and RPTPbeta in relation to epilepsy suggesting a molecular interplay between glia and neurones during seizures.

VEGF mRNA and its receptor flt-1 are expressed in reactive astrocytes following neural grafting and tumor cell implantation in the adult CNS

Significant angiogenesis occurs only after injury in the adult mammalian brain; capillaries proliferate and astrocytes are activated by presently unresolved cellular mechanisms. Because of the intimate relationship between astrocytes and brain capillaries we examined the expression of the specific endothelial mitogen vascular endothelial growth factor (VEGF) in reactive astrocytes following CNS trauma models: neural grafting, stab wounds, and glioma implantation. In situ hybridization was combined with GFAP immunohistochemistry to delineate VEGF mRNA expression in reactive astrocytes. In addition, VEGF and its receptor flt-1 protein expression were detected immunohistochemically. In all three models we found unexpectedly that only reactive astrocytes, not endothelium, expressed the VEGF receptor flt-1, VEGF mRNA, and VEGF protein in a spatiotemporal manner, suggesting that activated astroglia may have a direct role in the induction of angiogenesis or permeability in mature brain. In addition, secreted VEGF may play a part in astroglial signalling by the induction of its own receptor in reactive astroglia following injury. These findings may have significant implications with regard to growth and reparative mechanisms of the adult cerebrovasculature.

Immunorecognition, ultrastructure and phosphorylation status of astrocytic gap junctions and connexin43 in rat brain after cerebral focal ischaemia

Gap junctions between astrocytes support a functional syncytium that is thought to play an important role in neural homeostasis. In order to investigate regulation of this syncytium and of connexin43 (Cx43), a principal astrocytic gap junction protein, we determined the sequelae of gap junction and Cx43 disposition in a rat cerebral focal ischaemia model with various ischaemia/reperfusion times using sequence-specific anti-Cx43 antibodies (designated 13-8300, 18A, 16A and 71-0700) that exhibit differential recognition of Cx43, perhaps reflecting functional aspects of gap junctions. Antibody 13-8300 specifically detects only an unphosphorylated form of Cx43 in both Western blots and tissue sections. In hypothalamus after brief (15 min) ischaemic injury, Cx43 at intact gap junctions undergoes dephosphorylation, accompanied by reduced epitope recognition by antibodies 16A and 71-0700. Tissue examined 24 h after reperfusion showed that these effects were reversible. Astrocytic gap junction internalization occurring 1 h after ischaemia was accompanied by decreased immunodetection with 13-8300. At this time, gap junctions were absent in the ischaemic core, coinciding with a loss of Cx43 recognition with 18A and 13-8300, but elevated labelling of internalized Cx43 with 16A and 71-0700. Unphosphorylated Cx43 persisted at intact gap junctions confined to a thin corridor at the ischaemic penumbra which contained presumptive apoptotic cell profiles. Similar results were obtained in ischaemic striatum and cerebral cortex, though with a delayed time course that depended on the severity of the ischaemic insult. These results demonstrate that astrocytic Cx43 epitope masking, dephosphorylation and cellular redistribution occur after ischaemic brain injury, proceed as a temporally and spatially ordered sequence of events and culminate in differential patterns of Cx43 modification and sequestration at the lesion centre and periphery. These observations suggest an attempt by astrocytes in the vicinity of injury to remodel the junctional syncytium according to altered tissue homeostatic requirements.

Effects of neuronal proteoglycans on activity-dependent growth responses of fetal hippocampal neurons

Excitatory amino-acid (EAA) neurotransmitters act as molecular signals influencing the structure of neurons during development. However, the signal transduction and effector mechanisms responsible for these effects have yet to be fully elucidated. We have previously provided evidence that EAA agonists induce the synthesis and release of proteoglycans (PGs) with neurite-promoting activity from fetal hippocampal neurons. In the present studies exposure of fetal hippocampal neurons to glutamate (100 microM) for 5 min resulted in increases in the neuron-specific growth-associated genes T alpha 1 alpha-tubulin (T alpha 1), microtubule-associated protein-2 (MAP-2) and growth-associated protein-43 (GAP-43). mRNA levels peaked at between 8 and 12 h following exposure as determined by competitive reverse transcription polymerase chain reaction (RT-PCR). Increases in neurite growth as measured by axonal length, the total length of dendrites, the number of branches per axon, the total length of branches per axon and the total neurite length were also observed 48 h after glutamate exposure. The increase in T alpha 1, MAP-2 and GAP-43 mRNA levels following glutamate exposure was mediated via both N-methyl-D-aspartate and metabotropic receptor activation. Heparin, which inhibits the neurite growth-promoting effects of PGs in vitro, and heparitinase, which catalyzes the cleavage of heparan sulphate, also inhibited the glutamate-dependent induction of T alpha 1, MAP-2 and GAP-43 mRNA expression and neurite growth when added to culture medium following glutamate exposure. Chondroitin sulphate and chondroitinase AC had no effects on the mRNA levels tested or on neurite growth. Therefore, these studies suggest that neuronal PGs regulated by activation of EAA receptors mediate neuronal growth responses.