Up-regulation of the betaine/GABA transporter BGT1 by JAK2

Abstract

Janus-activated kinase-2 JAK2 is activated by hyperosmotic shock and modifies the activity of several Na+ coupled transporters. Carriers up-regulated by osmotic shock include the Na+ coupled osmolyte transporter BGT1 (betaine/GABA transporter 1), which accomplishes the concentrative cellular uptake of γ-amino-butyric acid (GABA). The present study thus explored whether JAK2 participates in the regulation of BGT1 activity. To this end, cRNA encoding BGT1 was injected into Xenopus oocytes with or without cRNA encoding wild type JAK2, constitutively active V617FJAK2 or inactive K882EJAK2, and electrogenic GABA transport determined by dual electrode voltage clamp. In oocytes injected with cRNA encoding BGT1 but not in oocytes injected with water or with cRNA encoding JAK2 alone, the addition of 1mM GABA to the extracellular fluid generated an inward current (IBGT). In BGT1 expressing oocytes IBGT was significantly increased by coexpression of JAK2 or V617FJAK2, but not by coexpression of K882EJAK2. According to kinetic analysis coexpression of JAK2 increased the maximal IBGT without significantly modifying the concentration required for halfmaximal IBGT (KM). In oocytes expressing BGT1 and V617FJAK2 IBGT was gradually decreased by JAK2 inhibitor AG490 (40μM). The decline of IBGT following disruption of carrier insertion with brefeldin A (5μM) was similar in the absence and presence of the JAK2 inhibitor AG490 (40μM). In conclusion, JAK2 is a novel regulator of the GABA transporter BGT1. The kinase up-regulates the carrier presumably by enhancing the insertion of carrier protein into the cell membrane.