Insulin increases glomerular filtration barrier permeability through PKGI alpha-dependent mobilization of BKCa channels in cultured rat podocytes

Podocytes are highly specialized cells that wrap around glomerular capillaries and comprise a key component of the glomerular filtration barrier. They are uniquely sensitive to insulin; like skeletal muscle and fat cells, they exhibit insulin-stimulated glucose uptake and express glucose transporters. Podocyte insulin signaling is mediated by protein kinase G type I (PKGI), and it leads to changes in glomerular permeability to albumin. Here, we investigated whether large-conductance Ca2+ -activated K+ channels (BKCa) were involved in insulin-mediated, PKGI alpha-dependent filtration barrier permeability. Insulin-induced glomerular permeability was measured in glomeruli isolated from Wistar rats. Transepithelial albumin flux was measured in cultured rat podocyte monolayers. Expression of BKCa subunits was detected by RT-PCR. BKCa, PKGI alpha, and upstream protein expression were examined in podocytes with Western blotting and immunofluorescence. The BKCa-PKGI alpha interaction was assessed with co-immunoprecipitation. RT-PCR showed that primary cultured rat podocytes expressed mRNAs that encoded the pore-forming a subunit and four accessory 13 subunits of BKCa. The BKCa inhibitor, iberiotoxin (ibTX), abolished insulin-dependent glomerular albumin permeability and PKGI-dependent transepithelial albumin flux. Insulin-evoked albumin permeability across podocyte monolayers was also blocked with BKCa siRNA. Moreover, ibTX blocked insulin-induced disruption of the actin cytoskeleton and changes in the phosphorylation of PKG target proteins, MYPT1 and RhoA. These results indicated that insulin increased filtration barrier permeability through mobilization of BKCa channels via PKGI in cultured rat podocytes. This molecular mechanism may explain podocyte injury and proteinuria in diabetes. (C) 2015 Elsevier B.V. All rights reserved.