Purinergic receptors in multiple sclerosis pathogenesis.

Multiple sclerosis (MS) is a chronic inflammatory demyelinating disease of the central nervous system, characterized by the presence of focal lesions in white and grey matter with peripheral immune cells infiltration. Purinergic receptors control immune cell function as well as neuronal and oligodendroglial survival, and the activation of astrocytes and microglia, the endogenous brain immune cells. In particular, ionotropic purinergic receptors P2X4 and P2X7 and metabotropic receptor P2Y12 are differently expressed along the disease and their activation or blockage modifies the course of texperimental autoimmune encephalomyelitis (EAE), the dominant animal model of MS. In this review, we will summarize emerging evidence of the role of these three receptor types as potential MS biomarkers and therapeutic targets.

Computer simulation of the ionic atmosphere around Z-DNA.

We describe a coarse-grained model for Z-DNA that mimics the DNA shape with a relatively small number of repulsive interaction sites. In addition, negative charges are placed at the phosphate positions. The ionic atmosphere around this grooved Z-DNA model is then investigated with Monte Carlo simulation. Cylindrically averaged concentration profiles as well as the spatial distribution of ions have been calculated. The results are compared to those for other DNA models differing in the repulsive core. This allows the examination of the effect of the DNA shape in the ionic distribution. It is seen that the penetrability of the ions to the DNA groove plays an important role in the ionic distribution. The results are also compared with those reported for B-DNA. In both conformers the ions are structured in alternating layers of positive and negative charge. In Z-DNA the layers are more or less concentric to the molecular axis. Besides, no coions enter into the single groove of this conformer. On the contrary, the alternating layers of B-DNA are also structured along the axial coordinate with some coions penetrating into the major groove. In both cases we have found five preferred locations of the counterions and two for the coions. The concentration of counterions reaches its absolute maximum at the narrow Z-DNA groove and at the minor groove of B-DNA, the value of the maximum being higher in the Z conformer.

CXCR4-activated astrocyte glutamate release via TNFalpha: amplification by microglia triggers neurotoxicity.

Astrocytes actively participate in synaptic integration by releasing transmitter (glutamate) via a calcium-regulated, exocytosis-like process. Here we show that this process follows activation of the receptor CXCR4 by the chemokine stromal cell-derived factor 1 (SDF-1). An extraordinary feature of the ensuing signaling cascade is the rapid extracellular release of tumor necrosis factor-alpha (TNFalpha). Autocrine/paracrine TNFalpha-dependent signaling leading to prostaglandin (PG) formation not only controls glutamate release and astrocyte communication, but also causes their derangement when activated microglia cooperate to dramatically enhance release of the cytokine in response to CXCR4 stimulation. We demonstrate that altered glial communication has direct neuropathological consequences and that agents interfering with CXCR4-dependent astrocyte-microglia signaling prevent neuronal apoptosis induced by the HIV-1 coat glycoprotein, gp120IIIB. Our results identify a new pathway for glia-glia and glia-neuron communication that is relevant to both normal brain function and neurodegenerative diseases.

The link between excitotoxic oligodendroglial death and demyelinating diseases.

Oligodendrocytes, the myelinating cells of CNS axons, are highly vulnerable to excitotoxic signals mediated by glutamate receptors of the AMPA and kainate classes. Receptors in these cells are commonly activated by glutamate that is released from axons and glial cells. In addition, oligodendrocytes contribute to the control of extracellular glutamate levels by means of their own transporters. However, acute and chronic alterations in glutamate homeostasis can result in overactivation of AMPA and kainate receptors and subsequent excitotoxic oligodendroglial death. Furthermore, demyelinating lesions caused by excitotoxins can be similar to those observed in multiple sclerosis. This, together with the effect of AMPA and kainate receptor antagonists in ameliorating the neurological score of animals with experimental autoimmune encephalomyelitis (an animal model of multiple sclerosis), indicates that oligodendrocyte excitotoxicity could be involved in the pathogenesis of demyelinating disorders.

Altered expression of the glutamate transporter EAAC1 in neurons and immature oligodendrocytes after transient forebrain ischemia.

Glutamate uptake is reduced during ischemia because of perturbations of ionic gradients across neuronal and glial membranes. Using immunohistochemical and Western blot analyses, the authors examined the expression of the glutamate transporters EAAC1, GLAST, and GLT-1 in the rat hippocampus and cerebral cortex 8 hours and 1 to 28 days after transient forebrain ischemia. Densitometric analysis of immunoblots of CA1 homogenates showed a moderate increase in EAAC1 protein levels early after the insult. Consistently, it was observed that EAAC1 immunostaining in CA1 pyramidal neurons was more intense after 8 hours and 1 day of reperfusion and reduced at later postischemia stages. A similar transient increase of EAAC1 immunolabeling was detected in layer V pyramidal neurons of the cerebral cortex. In addition, the authors observed that EAAC1 also was located in oligodendroglial progenitor cells in subcortical white matter. The number of EAAC1-labeled cells in this region was increased after 3 and 28 days of reperfusion. Finally, changes in GLAST and GLT-1 expression were not observed in the CA1 region after ischemia using immunohistochemical study or immunoblotting. Enhanced expression of EAAC1 may be an adaptive response to increased levels of extracellular glutamate during ischemia.

Expression of glutamate transporters in rat optic nerve oligodendrocytes.

To investigate the role of glutamate transport in non-synaptic glia, we characterized the expression of three major glutamate transporters (EAAC1, GLAST and GLT-1) in rat optic nerve in situ using reverse transcription-polymerase chain reaction in combination with Western blot and immunochemistry with specific antibodies. GLAST was localized to interfascicular oligodendrocytes, whereas a subpopulation of cells, probably immature oligodendrocyte cells, expressed EAAC1. In contrast, astrocytes, expressed only GLT-1, consistent with the idea that this is the major glutamate transporter in this cell type. In addition, we observed that glutamine synthetase, a key enzyme in glutamate metabolism, was localized in oligodendrocytes in situ. To examine the properties of these glutamate transporters, we conducted uptake experiments in glial cultures. The kinetics of sodium-dependent glutamate uptake in cultured oligodendrocytes from the perinatal rat optic nerve were markedly different from those observed in type-1 astrocytes from the newborn rat cerebral cortex, with higher affinity and lower Vmax. In both cell types, glutamate transport was inhibited by L-trans-pyrrolidine-2,4-dicarboxylate (t-PDC). In contrast, dihydrokainate exhibited significantly more uptake inhibition in oligodendrocytes than in type-1 astrocytes. These results provide evidence for the expression of functional sodium-dependent glutamate transporters in optic nerve oligodendrocytes, and suggest that this cell type may play a role in the glutamate-glutamine cycle.

Expression of glutamate transporters in the adult bovine corpus callosum.

The presence and distribution of the glutamate transporters, EAAC1, GLAST and GLT-1, were examined in the adult bovine corpus callosum by means of Western blotting and immunohistochemistry. We found GLAST to be expressed in oligodendrocytes and in some astrocytes. GLT-1 was located throughout the processes of fibrous astrocytes. In addition, EAAC1 was present in a small population of cells that are probably adult oligodendrocyte progenitor cells.

On how altered glutamate homeostasis may contribute to demyelinating diseases of the CNS.

Glial cells communicate reciprocally with neurons in multiple ways, both in synaptic and non-synaptic regions of the central nervous system. In the latter, neuron to glial and glial to glial signals can be mediated by neurotransmitters. Here, we review the presence and some of the functional properties of glutamate transporters and receptors in oligodendrocytes. In addition, we present data illustrating that alterations in glutamate homeostasis can be excitotoxic to oligodendroglia and that the tissue lesions caused by overactivation of glutamate receptors resemble those observed in demyelinating diseases such as multiple sclerosis. Overall, this information indicates that aberrant glutamate signaling may contribute to the development of some white matter pathologies.