Interactions of CNS cells lead to the establishment of complex neural systems. Specifically, oligodendrocytes form myelin sheaths around axons that enable rapid electrical conduction of impulses. Recent evidence has emerged that oligodendrocytes may also release trophic factors promoting neuronal survival. We therefore studied the effects of factors released from cells of the oligodendrocyte lineage on neuronal survival and also on the morphology of neurons. Neurons derived from rat embryonic cortices were cultured and exposed to media conditioned by oligodendrocyte precursor cells (OPCs) or differentiated oligodendrocytes. In line with previous studies, exposure of OPC and oligodendrocyte-conditioned media (OCM) increased survival, a phosphatidylinositol 3'-kinase (PI3kinase)/Akt-dependent phenomenon. In addition, exposure of neurons to OCM but not OPC conditioned media resulted in increased axonal length per neuron, as detected by antibodies to phosphorylated neurofilaments. OCM exposure resulted in activation of the MAPkinase/extracellular signal-regulated kinase pathway, inhibition of which significantly reduced oligodendrocyte-mediated enhancement of axonal length but, unlike PI3kinase inhibition, had no effect on neuronal survival. Furthermore, we identify glial cell line-derived neurotrophic factor (GDNF) production by differentiated oligodendrocytes and provide evidence that implicates GDNF in OCM-mediated axonal effects, independent of its effect on neuronal survival. Therefore, we have shown that factors released by OPCs and oligodendrocytes induce the activation of distinct intracellular pathways within neurons, which have different functional effects on the cell.
Microglia exist under physiological conditions in a resting state but become activated after neuronal injury. Recent studies have highlighted the reciprocal role of neurons in controlling both the number and activity of microglia. In this study, microglia derived from newborn rat cortices were cultured and activated by interferon-␥ (IFN␥) treatment, then exposed to recombinant Sema3A or conditioned medium derived from stressed embryonic cortical neurons. We found that activation of microglia by IFN␥ induced differential upregulation of the semaphorin receptors Plexin-A1 and Neuropilin-1. This result was confirmed by Northern blotting, reverse transcription-PCR, and Western blotting. Furthermore, recombinant Sema3A induced apoptosis of microglia when added to the in vitro culture, and a similar result was obtained on activated microglia when Sema3A was produced by stressed neurons. Using an in vivo model of microglia activation by striatal injection of lipopolysaccharide demonstrated a corresponding upregulation of Plexin-A1 and Neuropilin-1 in activated microglia and enhanced production of Sema3A by stressed adult neurons. These results suggest a novel semaphorin-mediated mechanism of neuroprotection whereby stressed neurons can protect themselves from further damage by activated microglia.
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