Antigen retrieval reveals widespread basolateral expression of syntaxin 3 in renal epithelia

Soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins play a key role in docking and fusion of intracellular transport vesicles and may regulate apical and basolateral membrane protein delivery in epithelial cells. In a previous study, syntaxin 3 (a target SNARE) protein was detectable in the kidney only in intercalated cells. We now report a more widespread distribution of syntaxin 3 in a variety of renal epithelial cells after antigen retrieval. Sections of rat kidney were treated with SDS and incubated with antisyntaxin 3 antibodies. Strong basolateral membrane staining was seen in descending and ascending thin limbs of Henle, thick ascending limbs of Henle, the macula densa, distal and connecting tubules, and all cells of the collecting duct including A- and B-intercalated cells. The papillary surface epithelium and the transitional epithelium of the ureter were also stained, but proximal tubules were negative. Western blotting revealed a strong signal at 37 kDa in all regions, and the antigen was restricted to membrane fractions. SDS treatment was not necessary to reveal syntaxin 3 in intercalated cells. These data show that syntaxin 3 might be involved in basolateral trafficking pathways in most renal epithelial cell types. The exclusive basolateral location of syntaxin 3 in situ, however, contrasts with the apical location of this SNARE protein in some kidney epithelial cells in culture.