Hypertension and Impaired Glycine Handling in Mice Lacking the Orphan Transporter XT2

ABSTRACT A family of orphan transporters has been discovered that are structurally related to the Na+-Cl−-dependent neurotransmitter transporters, including the dopamine transporter. One member of this family, the mouse XT2 gene, is predominantly expressed in the kidney and has 95% homology to rat ROSIT (renal osmotic stress-induced Na+-Cl− organic solute cotransporter). To study the physiological functions of this transporter, we generated XT2-knockout mice by gene targeting. XT2−/− mice develop and survive normally with no apparent abnormalities. To attempt to identify potential substrates for XT2, we screened urine from XT2-knockout mice by high-pressure liquid chromatography and mass spectrometry and found significantly elevated concentrations of glycine. To study glycine handling, XT2+/+ and XT2−/− mice were injected with radiolabeled glycine, and urine samples were collected to monitor glycine excretion. After 2 h, XT2−/− mice were found to excrete almost twice as much glycine as the XT2+/+ controls (P = 0.03). To determine whether the absence of the XT2 transporter affected sodium and fluid homeostasis, we measured systolic blood pressure by computerized tail-cuff manometry. Systolic blood pressure was significantly higher in XT2−/− mice (127 ± 3 mmHg) than in wild-type controls (114 ± 2 mmHg; P < 0.001). This difference in systolic blood pressure was maintained on high and low salt feeding. To examine whether the alteration in blood pressure and the defect in glycine handling were related, we measured systolic blood pressure in the XT2−/− mice during dietary glycine supplementation. Glycine loading caused systolic blood pressure to fall in the XT2−/− mice from 127 ± 3 to 115 ± 3 mmHg (P < 0.001), a level virtually identical to that of the wild-type controls. These data suggest that the XT2 orphan transporter is involved in glycine reabsorption and that the absence of this transporter is sufficient to cause hypertension.