Na⁺-d-glucose cotransporter in the kidney of Leucoraja erinacea: molecular identification and intrarenal distribution

Abstract

Studies on membrane vesicles from the kidney of Leucoraja erinacea suggested the sole presence of a sodium-D-glucose cotransporter type 1 involved in renal D-glucose reabsorption. For molecular characterization of this transport system, an mRNA library was screened with primers directed against conserved regions of human sglt1. A cDNA was cloned whose nucleotide and derived amino acid sequence revealed high homology to sodium glucose cotransporter 1 (SGLT1). Xenopus laevis oocytes injected with the respective cRNA showed sodium-dependent high-affinity uptake of D-glucose. Many positions considered functionally essential for sodium glucose cotransporter 1 (SGLT1) are also found in the skate protein. High conservation preferentially in transmembrane helices and small linking loops suggests early appearance and continued preservation of these regions. Larger loops, especially loop 13, which is associated with phlorizin binding, were more variable, as is the interaction with the specific inhibitor in various species. To study the intrarenal distribution of the transporter, a skate SGLT1-specific antibody was generated. In cryosections of skate kidney, various nephron segments could be differentiated by lectin staining. Immunoreaction with the antibody was observed in the proximal tubule segments PIa and PIIa, the early distal tubule, and the collecting tubule. Thus Leucoraja, in contrast to the mammalian kidney, employs only SGLT1 to reabsorb d-glucose in the early, as well as in the late segments of the proximal tubule and probably also in the late distal tubule (LDT). Thereby, it differs also partly from the kidney of the close relative Squalus acanthias, which uses SGLT2 in more distal proximal tubule segments but shows also expression in the later nephron parts.

Three Surface Subdomains Form the Vestibule of the Na+/Glucose Cotransporter SGLT1

“…Therefore, minor differences in the functional structure of different SGLT1 isoforms, particularly in the surface loops, might be expected. Such diversity is also evident in recent studies concerning the evolution of SGLT (1). Irrespective what the exact partners are, such an intramolecular disulfide bridge would bring loop 6 -7, loop 8 -9, and loop 13-14 closer together and create the critical conditions for phlorizin and D-glucose binding.…”

Tubular localization and expressional dynamics of aquaporins in the kidney of seawater-challenged Atlantic salmon

“…There is evidence of SO 4 2− coupled Na + absorption in proximal tubules of FW-acclimated eels (Anguilla japonica) and SO 4 2− secretion in SW-acclimated eels (Nakada et al 2005;Watanabe and Takei 2011). The Na + -glucose cotransporter (SGLT), which absorbs glucose from the primary filtrate in higher vertebrates, has been localized in the apical membrane of epithelia in proximal and distal tubules of the little skate kidney (Leucoraja erinacea) (Althoff et al 2007) supporting the idea that some Na + may be reabsorbed across the proximal tubules of teleost and elasmobranch fish. The distal tubule of the fish nephron acts as a "diluting" segment, where NaCl is reabsorbed.…”

Rhesus Glycoprotein P2 (Rhp2) Is a Novel Member of the Rh Family of Ammonia Transporters Highly Expressed in Shark Kidney

“…In situ hybridization and immunohistochemistry showed that Rhp2 was basolaterally localized in the renal tubule cells of the second and fourth loops in the sinus zone. Although several ion transporters have been localized to a specific segment of renal tubules in elasmobranch fish (21,(41)(42)(43)(44)(45), the functional significance of ammonia transport in the second and fourth loops is poorly understood. From its cellular and subcellular location, we assumed that Rhp2 facilitates ammonia transport across the basolateral membranes of renal tubule cells, but concerning the direction of the transport, we cannot conclude whether it mediates ammonia reabsorption or secretion in vivo.…”