Among the chemokine family, fractalkine shows unusual properties: it exists as a membrane‐bound and soluble protein, and both fractalkine and its receptor CX3CR1 are expressed predominantly in the central nervous system. In rat cell culture models, the chemokine fractalkine was expressed in neurons and microglia, but not in astrocytes and its receptor exclusively localized to microglial cells, where its expression was downregulated by treatment with the bacterial endotoxin (LPS). In microglial cultures, LPS (10 ng/ml) induced a marked increase in the release of the proinflammatory cytokine tumor necrosis factor‐α (TNF‐α). The effects of LPS on TNF‐α secretion were partially blocked (30%) by fractalkine and the effects of fractalkine were reversed by a polyclonal anti‐fractalkine antibody. When microglial‐associated fractalkine was neutralized by anti‐fractalkine antibody, the LPS response was increased by 80%, suggesting tonic activation of microglial fractalkine receptors by endogenous fractalkine. The effects of the antibody were antagonized by the addition of fractalkine. LPS‐activated microglia were neurotoxic when added to neuronal hippocampal culture, producing 20% neuronal death, as measured by NeuN‐positive cell counting. An anti‐fractalkine antibody produced neurotoxic effects of similar magnitude in this co‐culture system and also markedly potentiated the neurotoxic effects of LPS‐activated microglia (40% neuronal death). These results suggest that endogenous fractalkine might act tonically as an anti‐inflammatory chemokine in cerebral tissue through its ability to control and suppress certain aspects of microglial activation. These data may have relevance to degenerative conditions such as multiple sclerosis, in which cerebral inflammatory processes may be activated. GLIA 29:305–315, 2000. © 2000 Wiley‐Liss, Inc.

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Much evidence suggests that apoptosis plays a crucial role in cell population homeostasis that depends on the expression of various genes implicated in the control of cell life and death. The sensitivity of human neuroblastoma cells SK-N-SH to undergo apoptosis induced by thapsigargin was examined. SK-N-SH were previously differentiated into neuronal cells by treatments with retinoic acid (RA), 4b-phorbol 12-myristate 13-acetate (PMA) which increases protein kinase C (PKC) activity, and staurosporine which decreases PKC activity. Neuronal differentiation was evaluated by g-enolase, microtubule associated protein 2 (MAP2) and synaptophysin immunocytochemistry. The sensitivity of the cells to thapsigargin-induced apoptosis was evaluated by cell viability and nuclear fragmentation (Hoechst 33258) and compared with pro-(Bcl-2, Bcl-x L ) and anti-apoptotic (Bax, Bak) protein expression of the Bcl-2 family.Cells treated with RA and PMA were more resistant to apoptosis than controls. Conversely, the cells treated with staurosporine were more susceptible to apoptosis. In parallel with morphological modifications, the expression of inhibitors and activators of apoptosis was directly dependent upon the differentiating agent used. Bcl-2 expression was strongly increased by PMA and drastically decreased by staurosporine as was Bcl-x L expression. Bax and Bak expression were not significantly modified. These results demonstrate that drugs that modulate PKC activity may induce a modification of Bcl-2 expression as well as resistance to the apoptotic process. Furthermore, the expression of Bcl-2 was reduced by toxin B from Clostridium difficile and, to a lesser extent, by wortmannin suggesting a role of small G-protein RhoA and PtdIns3 kinase in the control of Bcl-2 expression. Our data demonstrate a relationship between the continuous activation of PKC, the expression of Bcl-2 protein family and the resistance of differentiated SK-N-SH to apoptosis.

The factors that determine whether remyelination fails or succeeds in multiple sclerosis remain unknown. By grafting lymphocytes from patients into demyelinated lesions in mice, El Behi, Sanson et al. show that lymphocytes differ in their ability to induce remyelination. Unravelling the basis of this heterogeneity reveals prerequisites for efficient myelin repair.