UT‐A3: Is it an Apical or Basolateral Membrane Urea Transporter?

Abstract

Urea transport by the inner medullary collecting duct (IMCD) is critically important to the urine concentrating mechanism. Transepithelial urea transport requires urea transporters in both apical and basolateral membranes of the IMCD. It is generally agreed that urea moves across the apical membrane by UT-A1, but the identity of the basolateral urea transporter is controversial. One published account localizes UT-A3 at the basolateral membrane in mouse IMCD providing a mechanism for transepithelial urea transport. This study used immunohistochemistry (IHC) with an antibody to the mouse UT-A3 C-terminus. Another report shows UT-A3 localized to the apical membrane in rat IMCD by immunofluorescence (IF) with an antibody to the rat UT-A3 C-terminus. If UT-A3 is not in the basolateral membrane, then the basolateral urea transporter remains unidentified. The goal of this study: directly compare UT-A3 localization in rat and mouse. UT-A3 was identified by western blot using the mouse UT-A3 antibody, and verified using an N-terminal UT-A1 antibody that sees both UT-A1 and UT-A3. IHC analysis revealed that UT-A3, as well as UT-A1, is most abundant in IM tip. Co-immunoprecipitation did not reveal a UT-A1/UT-A3 interaction. Both IHC and IF analysis of perfused rat and mouse kidneys showed UT-A3 was predominately expressed at the apical plasma membrane. We conclude that UT-A3 primarily localizes to the apical membrane in both rat and mouse IMCD. The mechanistic advantage of UT-A3 in the apical membrane, in addition to UT-A1, remains to be solved. The identity of the basolateral urea transporter remains unknown. Support: NIH RO1 DK62081