To identify growth factors deemed important for the survival, differentiation, and/or maturation of human oligodendrocytes, we initiated cDNA microarray analysis of O4 +-immunopanned oligodendrocytes from second trimester human fetal spinal cord, a time of active ongoing myelination. Further, studies on oligodendrocyte development in rodents have shown the presence of an oligodendrocyte type 2 astrocyte bipotential progenitor cell that yields oligodendrocytes ( Raff et al., 1983), but it is questionable as to whether this progenitor cell exists in human oligodendrocyte development. Cultured human oligodendrocytes exhibit poor viability relative to rat oligodendrocytes, which survive in culture for 3 d before undergoing programmed cell death ( Barres et al., 1992 Trapp et al., 1997). The signaling pathways and molecular events required for human oligodendrocyte survival and maturation have not been elucidated fully. Thus Gas6 sustains human fetal oligodendrocyte viability by receptor activation and downstream signaling via the PI3-kinase/Akt pathway. PI3-kinase inhibitors blocked the anti-apoptotic effect of rhGas6, whereas a MEK/ERK inhibitor had no effect. A significant decrease in CNP +/TUNEL + oligodendrocytes was observed when recombinant human Gas6 (rhGas6) was administered to oligodendrocytes plated on poly- l-lysine, supporting a role for Gas6 signaling in oligodendrocyte survival during a period of active myelination in human fetal spinal cord development. The effect was abolished in the presence of Axl-Fc but remained unchanged in the presence of the irrelevant receptor fusion molecule TrkA-Fc. Also, a twofold increase in CNP + and MBP + oligodendrocytes was observed when they were plated on the Gas6-secreting cells. CNP + oligodendrocytes on Gas6-secreting 3T3 cells had more primary processes and arborizations than those plated solely on 3T3 cells. To test this hypothesis, we grew enriched human oligodendrocytes for 6 d on a monolayer of NIH3T3 cells stably expressing Gas6. We hypothesized that Gas6 is a survival factor for oligodendrocytes and receptor activation signals downstream to the phosphatidylinositol 3 (PI3)-kinase/Akt pathway to increase cell survival in the absence of cell proliferation. In humans the sole known ligand for the Axl/Rse/Mer kinases is growth arrest-specific gene 6 (Gas6), which in the CNS is secreted by neurons and endothelial cells. Microarray analysis revealed that transcripts for the Axl and Mer receptor tyrosine kinases are expressed at high levels in O4 +-immunopanned oligodendrocytes isolated from second trimester human fetal spinal cord.
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