Regulation of luminal acidification by the V-ATPase

Abstract

Specialized cells in the body express high levels of V-ATPase in their plasma membrane and respond to hormonal and nonhormonal cues to regulate extracellular acidification. Mutations in or loss of some V-ATPase subunits cause several disorders, including renal distal tubular acidosis and male infertility. This review focuses on the regulation of V-ATPase-dependent luminal acidification in renal intercalated cells and epididymal clear cells, which are key players in these physiological processes.

FIGURE 1.

V-ATPase subunit composition and organization Subunits A, B, C, D, E, F, G, and H form the V₁ cytosolic domain, and subunits a, c, d, and e form the V₀ transmembrane domain. ATP catalytic sites are located at the B/A subunit interfaces. Protons are transported between subunit a and the proteolipid c-ring. The V-ATPase rotor is formed by subunits D, F, and d and the c-ring. Three EG complexes together with subunits C and H form a stator, which is connected to the AB hexamer and the large transmembrane subunit a. The function of subunit e is still unknown. Image is based on data from Refs. and 88.

FIGURE 2.

Localization of the V-ATPase B1 and a4 subunit isoforms in epididymal clear cells The distal region (cauda) of rat epididymis was immunofluorescently labeled with antibodies against a4 (A and D: green) and B1 (B and E: red). C and F: merged images from A and B, and D and E, respectively. B1 and a4 colocalize in the apical membrane and subapical vesicles in clear cells exclusively. Nuclei and sperm are labeled in blue with DAPI. Bars = 50 μm (A, B, C) and 5 μm (D, E, F).

FIGURE 3.

Expression of the V-ATPase in kidney intercalated cells A: the renal inner stripe of the outer medulla was immunofluorescently labeled for the V-ATPase B1 subunit (red) and AQP2 (green). The V-ATPase and AQP2 are located apically in intercalated cells and principal cells, respectively. B and C: immunogold electron microscopy labeling of the apical pole of two intercalated cells for the V-ATPase A subunit. B: an activated IC with numerous apical microvilli that contain a high density of V-ATPase-associated gold particles. C: a resting IC with fewer and less developed microvilli. Most V-ATPase-gold particles are located in subapical tubulo-vesicles.

FIGURE 4.

Immunofluorescence labeling of CCs from the cauda epididymis Immunofluorescence labeling of CCs from the cauda epididymis after in vivo luminal perfusion with a control solution adjusted to the resting pH of 6.6 (A) or with an activating solution containing bicarbonate and the cAMP permeant analog cpt-cAMP (B). Horseradish peroxidase (HRP) was added to both solutions to label endosomes (red). Epididymis sections were labeled for the V-ATPase B1 subunit (green). Arrows indicate the frontier between the apical cytoplasm and the base of microvilli. Short microvilli labeled for the V-ATPase are detected in the resting CC shown in A. The yellow color in A shows colocalization of V-ATPase in HRP-containing endosomes located in the subapical pole of clear cells. In the activated CC shown in B, the V-ATPase is mainly located in longer and more numerous microvilli (green), and it is absent from the apical endosomes that contain the internalized HRP (red). Scale bars = 5 μm. Figure reproduced from Ref. with permission from the American Society of Andrology.