Insulin Promotes Neuronal Survival via the Alternatively Spliced Protein Kinase CδII Isoform

Insulin signaling pathways in the brain regulate food uptake and memory and learning. Insulin and protein kinase C (PKC) pathways are integrated and function closely together. PKC activation in the brain is essential for learning and neuronal repair. Intranasal delivery of insulin to the central nervous system (CNS) has been shown to improve memory, reduce cerebral atrophy, and reverse neurodegeneration. However, the neuronal molecular mechanisms of these effects have not been studied in depth. PKC delta plays a central role in cell survival. Its splice variants, PKC delta I and PKC delta II, are switches that determine cell survival and fate. PKC delta I promotes apoptosis, whereas PKC delta II promotes survival. Here, we demonstrate that insulin promotes alternative splicing of PKC delta II isoform in HT22 cells. The expression of PKC delta I splice variant remains unchanged. Insulin increases PKC delta II alternative splicing via the PI3K pathway. We further demonstrate that Akt kinase mediates phosphorylation of the splicing factor SC35 to promote PKC delta II alternative splicing. Using overexpression and knockdown assays, we demonstrate that insulin increases expression of Bcl2 and bcl-xL via PKC delta II. We demonstrate increased cell proliferation and increased BrdU incorporation in insulin-treated cells as well as in HT22 cells overexpressing PKC delta II. Finally, we demonstrate in vivo that intranasal insulin promotes cognitive function in mice with concomitant increases in PKC delta II expression in the hippocampus. This is the first report of insulin, generally considered a growth or metabolic hormone, regulating the alternative isoform expression of a key signaling kinase in neuronal cells such that it results in increased neuronal survival.