Rab-GAP TBC1D4 (AS160) is dispensable for the renal control of sodium and water homeostasis but regulates GLUT4 in mouse kidney

Abstract

The Rab GTPase-activating protein TBC1D4 (AS160) controls trafficking of the glucose transporter GLUT4 in adipocytes and skeletal muscle cells. TBC1D4 is also highly abundant in the renal distal tubule, although its role in this tubule is so far unknown. In vitro studies suggest that it is involved in the regulation of renal transporters and channels such as the epithelial sodium channel (ENaC), aquaporin-2 (AQP2), and the Na(+)-K(+)-ATPase. To assess the physiological role of TBC1D4 in the kidney, wild-type (TBC1D4(+/+)) and TBC1D4-deficient (TBC1D4(-/-)) mice were studied. Unexpectedly, neither under standard nor under challenging conditions (low Na(+)/high K(+), water restriction) did TBC1D4(-/-) mice show any difference in urinary Na(+) and K(+) excretion, urine osmolarity, plasma ion and aldosterone levels, and blood pressure compared with TBC1D4(+/+) mice. Also, immunoblotting did not reveal any change in the abundance of major renal sodium- and water-transporting proteins [Na-K-2Cl cotransporter (NKCC2) NKCC2, NaCl cotransporter (NCC), ENaC, AQP2, and the Na(+)-K(+)-ATPase]. However, the abundance of GLUT4, which colocalizes with TBC1D4 along the distal nephron of TBC1D4(+/+) mice, was lower in whole kidney lysates of TBC1D4(-/-) mice than in TBC1D4(+/+) mice. Likewise, primary thick ascending limb (TAL) cells isolated from TBC1D4(-/-) mice showed an increased basal glucose uptake and an abrogated insulin response compared with TAL cells from TBC1D4(+/+) mice. Thus, TBC1D4 is dispensable for the regulation of renal Na(+) and water transport, but may play a role for GLUT4-mediated basolateral glucose uptake in distal tubules. The latter may contribute to the known anaerobic glycolytic capacity of distal tubules during renal ischemia.