Hypoxic stress upregulates Kir2.1 expression by a pathway including hypoxic-inducible factor-1α and dynamin2 in brain capillary endothelial cells

Brain capillary endothelial cells (BCECs) play a central role in maintenance of blood-brain barrier (BBB) function and, therefore, are essential for central nervous system homeostasis and integrity. Although brain ischemia damages BCECs and causes disruption of BBB, the related influence of hypoxia on BCECs is not well understood. Hypoxic stress can upregulate functional expression of specific K+ currents in endothelial cells, e.g., K(ir)2.1 channels without any alterations in the mRNA level, in t-BBEC117, a cell line derived from bovine BCECs. The hyperpolarization of membrane potential due to K(ir)2.1 channel upregulation significantly facilitates cell proliferation. In the present study. the mechanisms underlying the hypoxia-induced K(ir)2.1 upregulation was examined. We emphasize the involvement of dynamin2, a protein known to be involved in a number of surface expression pathways. Hypoxic culture upregulated dynamin2 expression in t-BBEC117 cells. The inhibition of dynamin2 by Dynasore canceled hypoxia-induced upregulation of K(ir)2.1 currents by reducing surface expression. On the contrary, K(ir)2.1 currents and proteins in t-BBEC117 cultured under normoxia were increased by overexpression of dynamin2, but not by dominant-negative dynamin2. Molecular imaging based on bimolecular fluorescence complementation, double-immunostaining, and coimmunoprecipitation assays revealed that dynamin2 can directly bind to the K(ir)2.1 channel. Moreover, hypoxic culture downregulated hypoxic-inducible factor-1 alpha (HIF-1 alpha) expression. Knockdown of HIF-1 alpha increased dynamin2 expression in t-BBEC117 cells, in both normoxic and hypoxic culture conditions. In summary, our results demonstrated that hypoxia downregulates HIF-1 alpha. increases dynamin2 expression, and facilitates K(ir)2.1 surface expression. resulting in hyperpolarization of membrane potential and subsequent increase in Ca2+ influx in BCECs.