Mouse GLUT9: evidences for a urate uniporter

GLUT9 (SLC2A9) is a newly described urate transporter whose function, characteristics, and localization have just started to be elucidated. Some transport properties of human GLUT9 have been studied in the Xenopus laevisoocyte expression system, but the type of transport (uniport, coupled transport system, stoichiometry … .) is still largely unknown. We used the same experimental system to characterize in more detail the transport properties of mouse GLUT9, its sensitivity to several uricosuric drugs, and the specificities of two splice variants, mGLUT9a and mGLUT9b. [14C]urate uptake measurements show that both splice variants are high-capacity urate transporters and have a Kmof ∼650 μM. The well-known uricosuric agents benzbromarone (500 μM) and losartan (1 mM) inhibit GLUT9-mediated urate uptake by 90 and 50%, respectively. Surprisingly, phloretin, a glucose-transporter blocker, inhibits [14C]urate uptake by ∼50% at 1 mM. Electrophysiological measurements suggest that urate transport by mouse GLUT9 is electrogenic and voltage dependent, but independent of the Na+and Cl−transmembrane gradients. Taken together, our results suggest that GLUT9 works as a urate (anion) uniporter. Finally, we show by RT-PCR performed on RNA from mouse kidney microdissected tubules that GLUT9a is expressed at low levels in proximal tubules, while GLUT9b is specifically expressed in distal convoluted and connecting tubules. Expression of mouse GLUT9 in the kidney differs from that of human GLUT9, which could account for species differences in urate handling.