Molecular mechanisms underlying neurotrophin-3 function in oligodendrocytes

Oligodendrocytes are the cells that make the myelin membrane in the central nervous system, allowing for the efficient conduction of nerve impulses. Demyelinating diseases such as multiple sclerosis (MS) are characterized by myelin and oligodendrocyte loss. Therefore, understanding the mechanisms that regulate oligodendrocyte survival should provide important clues in the design of strategies to stimulate remyelination.; We have demonstrated that neurotrophin-3 (NT-3) treatment inhibits caspase-3 activation and DNA fragmentation, protecting the oligodendrocyte progenitors from apoptotic cell death induced by growth factor deprivation. This effect of NT-3 is accompanied by an increase in the levels of the anti-apoptotic protein Bcl-2. The Bcl-2 gene promoter contains a binding site for CREB, a transcription factor which we found to be phosphorylated and activated by treatment of the cells with NT-3. By performing promoter driven studies as well as electrophoretic mobility shift assays, we demonstrated that the NT-3 dependent increase in Bcl-2 expression could be attributed to a direct regulation of the Bcl-2 gene by CREB.; Interestingly, the present results also show that CREB phosphorylation in response to NT-3 involves sphingosine kinase 1 (SphK1), an enzyme that catalyzes sphingosine phosphorylation generating the bioactive lipid metabolite sphingosine-1-phosphate (S1P). Moreover, S1P by itself is able to stimulate CREB phosphorylation. By using immunocytochemistry and confocal microscopy together with analysis of enzymatic activity in different cell subfractions, we demonstrated that NT-3 treatment elicits intracellular translocation of SphK1 and results in increased Sphk1 activity in the plasma membrane of the oligodendrocytes. By down-regulating SphK1 expression using SphK1-sequence specific small interfering RNA, we showed that the capacity of NT-3 to stimulate the survival of the oligodendrocyte progenitors is dependent on the expression of SphK1. Altogether, these results suggest that SphK1 plays a crucial role in the stimulation of oligodendrocyte progenitor survival by NT-3 and demonstrate a functional link between NT-3 and S1P signaling, adding to the complexity of mechanisms that modulate neurotrophin function and oligodendrocyte development.