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.
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ABSTRACT: Vasopressin is the main hormone that regulates water conservation in mammals and one of its major targets is the principal cells in the renal collecting duct. Vasopressin increases the apical water permeability of principal cells, mediated by apical accumulation of aquaporin-2 (AQP2), a water channel protein, thus facilitating water reabsorption by the kidney. The mechanisms underlying the accumulation of AQP2 in response to vasopressin include vesicular trafficking from intracellular storage vesicles expressing AQP2 within several tens of minutes (short-term regulation) and protein expression of AQP2 over a period of hours to days (long-term regulation). This chapter reviews vasopressin signaling in the kidney, focusing on the molecular mechanisms of short- and long-term regulations of AQP2 expression. © 2015 Elsevier Inc. All rights reserved.Vitamins & Hormones 01/2015; 98:307-37. DOI:10.1016/bs.vh.2014.12.008 · 1.78 Impact Factor
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ABSTRACT: P2Y12 receptor (P2Y12-R) signaling is mediated through Gi, ultimately reducing cellular cAMP levels. Because cAMP is a central modulator of arginine vasopressin (AVP)-induced water transport in the renal collecting duct (CD), we hypothesized that if expressed in the CD, P2Y12-R may play a role in renal handling of water in health and in nephrogenic diabetes insipidus. We found P2Y12-R mRNA expression in rat kidney, and immunolocalized its protein and aquaporin-2 (AQP2) in CD principal cells. Administration of clopidogrel bisulfate, an irreversible inhibitor of P2Y12-R, significantly increased urine concentration and AQP2 protein in the kidneys of Sprague-Dawley rats. Notably, clopidogrel did not alter urine concentration in Brattleboro rats that lack AVP. Clopidogrel administration also significantly ameliorated lithium-induced polyuria, improved urine concentrating ability and AQP2 protein abundance, and reversed the lithium-induced increase in free-water excretion, without decreasing blood or kidney tissue lithium levels. Clopidogrel administration also augmented the lithium-induced increase in urinary AVP excretion and suppressed the lithium-induced increase in urinary nitrates/nitrites (nitric oxide production) and 8-isoprostane (oxidative stress). Furthermore, selective blockade of P2Y12-R by the reversible antagonist PSB-0739 in primary cultures of rat inner medullary CD cells potentiated the expression of AQP2 and AQP3 mRNA, and cAMP production induced by dDAVP (desmopressin). In conclusion, pharmacologic blockade of renal P2Y12-R increases urinary concentrating ability by augmenting the effect of AVP on the kidney and ameliorates lithium-induced NDI by potentiating the action of AVP on the CD. This strategy may offer a novel and effective therapy for lithium-induced NDI. Copyright © 2015 by the American Society of Nephrology.Journal of the American Society of Nephrology 04/2015; DOI:10.1681/ASN.2014010118 · 9.47 Impact Factor
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ABSTRACT: Helium ion scanning microscopy is a novel imaging technology with the potential to provide sub-nanometer resolution images of uncoated biological tissues. So far, however, it has been used mainly in materials science applications. Here, we took advantage of helium ion microscopy to explore the epithelium of the rat kidney with unsurpassed image quality and detail. In addition, we evaluated different tissue preparation methods for their ability to preserve tissue architecture. We found that high contrast, high resolution imaging of the renal tubule surface is possible with a relatively simple processing procedure that consists of transcardial perfusion with aldehyde fixatives, vibratome tissue sectioning, tissue dehydration with graded methanol solutions and careful critical point drying. Coupled with the helium ion system, fine details such as membrane texture and membranous nanoprojections on the glomerular podocytes were visualized, and pores within the filtration slit diaphragm could be seen in much greater detail than in previous scanning EM studies. In the collecting duct, the extensive and striking apical microplicae of the intercalated cells were imaged without the shrunken or distorted appearance that is typical with conventional sample processing and scanning electron microscopy. Membrane depressions visible on principal cells suggest possible endo- or exocytotic events, and central cilia on these cells were imaged with remarkable preservation and clarity. We also demonstrate the use of colloidal gold probes for highlighting specific cell-surface proteins and find that 15 nm gold labels are practical and easily distinguishable, indicating that external labels of various sizes can be used to detect multiple targets in the same tissue. We conclude that this technology represents a technical breakthrough in imaging the topographical ultrastructure of animal tissues. Its use in future studies should allow the study of fine cellular details and provide significant advances in our understanding of cell surface structures and membrane organization.PLoS ONE 03/2013; 8(3):e57051. DOI:10.1371/journal.pone.0057051 · 3.53 Impact Factor