Nikola Radović

Localizations of Na + - d -glucose cotransporters SGLT1 and SGLT2 in human kidney and of SGLT1 in human small intestine, liver, lung, and heart

Novel affinity-purified antibodies against human SGLT1 (hSGLT1) and SGLT2 (hSGLT2) were used to localize hSGLT2 in human kidney and hSGLT1 in human kidney, small intestine, liver, lung, and heart. The renal locations of both transporters largely resembled those in rats and mice; hSGLT2 and SGLT1 were localized to the brush border membrane (BBM) of proximal tubule S1/S2 and S3 segments, respectively. Different to rodents, the renal expression of hSGLT1 was absent in thick ascending limb of Henle (TALH) and macula densa, and the expression of both hSGLTs was sex-independent. In small intestinal enterocytes, hSGLT1 was localized to the BBM and subapical vesicles. Performing double labeling with glucagon-like peptide 1 (GLP-1) or glucose-dependent insulinotropic peptide (GIP), hSGLT1 was localized to GLP-1-secreting L cells and GIP-secreting K cells as has been shown in mice. In liver, hSGLT1 was localized to biliary duct cells as has been shown in rats. In lung, hSGLT1 was localized to alveolar epithelial type 2 cells and to bronchiolar Clara cells. Expression of hSGLT1 in Clara cells was verified by double labeling with the Clara cell secretory protein CC10. Double labeling of human heart with aquaporin 1 immunolocalized the hSGLT1 protein in heart capillaries rather than in previously assumed myocyte sarcolemma. The newly identified locations of hSGLT1 implicate several extra renal functions of this transporter, such as fluid absorption in the lung, energy supply to Clara cells, regulation of enteroendocrine cells secretion, and release of glucose from heart capillaries. These functions may be blocked by reversible SGLT1 inhibitors which are under development.

Distribution of organic anion transporters NaDC3 and OAT1-3 along the human nephron

The initial step in renal secretion of organic anions (OAs) is mediated by transporters in the basolateral membrane (BLM). Contributors to this process are primary active Na(+)-K(+)-ATPase (EC 3.6.3.9), secondary active Na(+)-dicarboxylate cotransporter 3 (NaDC3/SLC13A3), and tertiary active OA transporters (OATs) OAT1/SLC22A6, OAT2/SLC22A7, and OAT3/SLC22A8. In human kidneys, we analyzed the localization of these transporters by immunochemical methods in tissue cryosections and isolated membranes. The specificity of antibodies was validated with human embryonic kidney-293 cells stably transfected with functional OATs. Na(+)-K(+)-ATPase was immunolocalized to the BLM along the entire human nephron. NaDC3-related immunostaining was detected in the BLM of proximal tubules and in the BLM and/or luminal membrane of principal cells in connecting segments and collecting ducts. The thin and thick ascending limbs, macula densa, and distal tubules exhibited no reactivity with the anti-NaDC3 antibody. OAT1-OAT3-related immunostaining in human kidneys was detected only in the BLM of cortical proximal tubules; all three OATs were stained more intensely in S1/S2 segments compared with S3 segment in medullary rays, whereas the S3 segment in the outer stripe remained unstained. Expression of NaDC3, OAT1, OAT2, and OAT3 proteins exhibited considerable interindividual variability in both male and female kidneys, and sex differences in their expression could not be detected. Our experiments provide a side-by-side comparison of basolateral transporters cooperating in renal OA secretion in the human kidney.