New insights into the dynamic regulation of water and acid-base balance by renal epithelial cells

Abstract

Maintaining tight control over body fluid and acid-base homeostasis is essential for human health and is a major function of the kidney. The collecting duct is a mosaic of two cell populations that are highly specialized to perform these two distinct processes. The antidiuretic hormone vasopressin (VP) and its receptor, the V2R, play a central role in regulating the urinary concentrating mechanism by stimulating accumulation of the aquaporin 2 (AQP2) water channel in the apical membrane of collecting duct principal cells. This increases epithelial water permeability and allows osmotic water reabsorption to occur. An understanding of the basic cell biology/physiology of AQP2 regulation and trafficking has informed the development of new potential treatments for diseases such as nephrogenic diabetes insipidus, in which the VP/V2R/AQP2 signaling axis is defective. Tubule acidification due to the activation of intercalated cells is also critical to organ function, and defects lead to several pathological conditions in humans. Therefore, it is important to understand how these "professional" proton-secreting cells respond to environmental and cellular cues. Using epididymal proton-secreting cells as a model system, we identified the soluble adenylate cyclase (sAC) as a sensor that detects luminal bicarbonate and activates the vacuolar proton-pumping ATPase (V-ATPase) via cAMP to regulate tubular pH. Renal intercalated cells also express sAC and respond to cAMP by increasing proton secretion, supporting the hypothesis that sAC could function as a luminal sensor in renal tubules to regulate acid-base balance. This review summarizes recent advances in our understanding of these fundamental processes.

Fig. 1.

Medullary collecting duct from rat kidney, triple immunostained to show apical aquaporin 2 (AQP2; green) and basolateral AQP4 (red) in principal cells (PC), and apical vacuolar H⁺-ATPase (V-ATPase) (blue) in A-type intercalated cells (IC). Principal cells are responsible for vasopressin (VP)-sensitive water reabsorption from the lumen while intercalated cells are involved in acid secretion.

Fig. 2.

Diagram of signaling pathways in a principal cell that have been exploited to bypass the VP/VP type 2 receptor (V2R) pathway, and induce apical membrane accumulation of AQP2. The three pathways shown are 1) calcitonin-receptor signaling via cAMP to induce AQP2 phosphorylation via PKA, and membrane accumulation of AQP2; 2) the cGMP-mediated pathway that is activated by phosphodiesterase (PDE5) inhibition using sildenafil (Viagra); AQP2 phosphorylation in this case occurs via PKG; and 3) statin-mediated actin cytoskeleton depolymerization that results in AQP2 membrane accumulation. By inactivating RhoA, simvastatin depolymerizes actin and inhibits AQP2 endocytosis, leading to plasma membrane accumulation. AC, adenylyl cyclase.

Fig. 3.

Comparison of proton-pumping clear cells in the male reproductive tract (in this case the vas deferens) and intercalated cells in a kidney collecting duct (inset) shown at the same magnification. The diameter of the tubule lumen is considerably greater in the vas deferens (and the adjacent cauda epididymis) than in the collecting duct (170 μm vs. 10 μm), allowing easier access to the luminal surface. Proton- secreting cells in the vas deferens are labeled with antibodies against the V-ATPase (green), and have endocytosed Texas-red dextran that was infused directly into the tubule lumen, producing a yellow color at the apical pole where overlap occurs. Adjacent principal cells also endocytose the dextran but to a lesser degree. In the inset, two collecting ducts from the medullary inner stripe of a rat kidney contain intercalated cells that are basolaterally stained to show the AE1 Cl⁻/HCO₃⁻ exchanger (yellow/orange). The rat was infused via the jugular vein with FITC-dextran (10,000 MW), which enters the tubule lumen and is avidly internalized at the apical pole of the type A intercalated cells (A-IC; green). Adjacent principal cells show some, but less, uptake of the dextran. Surrounding thick ascending limbs of Henle show spots of perinuclear AE1 staining that corresponds to the Golgi area. Insights into the regulation of proton secretion using the male reproductive tract have provided important information that can also be extended to kidney intercalated cell function.

Fig. 4.

A: section of kidney from a B1 subunit enhanced green fluorescent protein (B1-EGFP) mouse expressing EGFP in intercalated cells, driven by the promoter of the B1-subunit of the V-ATPase. B: higher-magnification detail in which the mosaic arrangement of intercalated cells (arrows - green) and principal cells can be easily appreciated. Cells from these mice can be isolated by FACS and used for structural and functional analysis (see Fig. 5). Bar, 10 μm. [From Miller et al. (81).]

Fig. 5.

Composite plate showing a single EGFP-positive intercalated cell affixed to a substrate, treated with 8-(4-chlorophenylthio)-cAMP (1 mM), and followed for 60 min by spinning disk confocal microscopy. Over time, the initial smooth surface of the cell extends numerous, fingerlike microvilli from the apical pole that in some cases measure several microns in length. A: 0 min. B: 12 min. C: 24 min. D: 36 min. E: 48 min. F: 60 min. This cAMP effect is part of the response that allows these cells to increase proton secretion by increasing their surface area and the number of V-ATPase pumps at the plasma membrane. Bar, 10 μm. [From Paunescu et al. (97).]