Purinergic inhibition of Na⁺,K⁺,Cl⁻ cotransport in C11-MDCK cells: Role of stress-activated protein kinases

Hypotonic Stress-induced Down-regulation of Claudin-1 and -2 Mediated by Dephosphorylation and Clathrin-dependent Endocytosis in Renal Tubular Epithelial Cells

Hypotonic stress decreased claudin-1 and -2 expression levels in renal tubular epithelial HK-2 and Madin-Darby canine kidney cells. Here, we examined the regulatory mechanism involved in this decrease. The hypotonicity-induced decrease in claudin expression was inhibited by the following: SB202190, a p38 MAPK inhibitor, but not by U0126, a MEK inhibitor; Go6983, a protein kinase C inhibitor; or SP600125, a Jun N-terminal protein kinase inhibitor. Hypotonic stress increased transepithelial electrical resistance, which was inhibited by SB202190. The mRNA expression level of claudin-1 was decreased by hypotonic stress but that of claudin-2 was not. Hypotonic stress decreased the protein stability of claudin-1 and -2. The hypotonicity-induced decrease in claudin expression was inhibited by the following: chloroquine, a lysosome inhibitor; dynasore and monodansylcadaverine, clathrin-dependent endocytosis inhibitors; and siRNA against clathrin heavy chain. Claudin-1 and -2 were mainly distributed in the cytosol and tight junctions (TJs) in the chloroquine- and monodansylcadaverine-treated cells, respectively. Hypotonic stress decreased the phosphorylation levels of claudin-1 and -2, which were inhibited by the protein phosphatase inhibitors okadaic acid and cantharidin. Dephosphorylated mutants of claudin-1 and -2 were mainly distributed in the cytosol, which disappeared in response to hypotonic stress. In contrast, mimicking phosphorylation mutants were distributed in the TJs, which were not decreased by hypotonic stress. We suggest that hypotonic stress induces dephosphorylation, clathrin-dependent endocytosis, and degradation of claudin-1 and -2 in lysosomes, resulting in disruption of the TJ barrier in renal tubular epithelial cells.

Purinergic signalling in the kidney in health and disease

The involvement of purinergic signalling in kidney physiology and pathophysiology is rapidly gaining recognition and this is a comprehensive review of early and recent publications in the field. Purinergic signalling involvement is described in several important intrarenal regulatory mechanisms, including tuboglomerular feedback, the autoregulatory response of the glomerular and extraglomerular microcirculation and the control of renin release. Furthermore, purinergic signalling influences water and electrolyte transport in all segments of the renal tubule. Reports about purine- and pyrimidine-mediated actions in diseases of the kidney, including polycystic kidney disease, nephritis, diabetes, hypertension and nephrotoxicant injury are covered and possible purinergic therapeutic strategies discussed.

Formation of cysts by principal-like MDCK cells depends on the synergy of cAMP- and ATP-mediated fluid secretion

It has been suggested that more than 70% of the renal cysts in patients with autosomal dominant polycystic kidney disease (ADPKD) arise from the collecting duct and that within this segment cysts originate almost exclusively from principal rather than intercalated cells. The mechanisms for this predisposition of principal cells have so far remained elusive. We, therefore, used Madin-Darby canine kidney (MDCK) subclones resembling principal cells and alpha-intercalated cells in a three-dimensional in vitro model to determine differences in cystogenesis and cyst growth, including the response to cyclic adenosine monophosphate (cAMP) elevation and the dependence on ATP signaling. We found that in vitro cysts developed only from principal-like but not from intercalated-like MDCK cell clones. This specificity could be verified in mixed MDCK cultures enriched for principal- or intercalated-like cells. In vitro cyst growth upon elevation of intracellular cAMP was mainly driven by fluid secretion, rather than increased cell proliferation. The cAMP-dependent fluid secretion was found to depend on extracellular adenosine-5'-triphosphate (ATP) and to act synergistically with purinergic signaling, as the use of the ATP scavenger apyrase, as well as the P2 receptor inhibitor suramin, reduced cAMP-driven fluid secretion, while increasing extracellular ATP potentiated cAMP-mediated cyst growth. In conclusion, we provide in vitro evidence for the ability of principal rather than intercalated cells to form cysts, based on a synergism of cAMP and ATP signaling in enhancing apical fluid secretion.

Intercalated cell BK-alpha/beta4 channels modulate sodium and potassium handling during potassium adaptation

The large-conductance, calcium-activated potassium (BK) channels help eliminate potassium in mammals consuming potassium-rich diets. In the distal nephron, principal cells contain BK-alpha/beta1 channels and intercalated cells contain BK-alpha/beta4 channels. We studied whether BK-beta4-deficient mice (Kcnmb4(-/-)) have altered renal sodium and potassium clearances compared with wild-type mice when fed a regular or potassium-rich diet for ten days. We did not detect differences in urinary flow or fractional excretions of potassium (FE(K)) or sodium (FE(Na)) between Kcnmb4-deficient and wild-type mice fed a regular diet. However, a potassium-rich diet led to >4-fold increases in urinary flows for both groups of mice, although Kcnmb4-deficient mice exhibited less urinary flow, higher plasma potassium concentration, more fluid retention, and significantly lower FE(K) and FE(Na) than wild-type mice despite similar plasma aldosterone levels. Immunohistochemical analysis revealed increased basolateral Na-K-ATPase in principal cells of all potassium-adapted mice, but expression of Na-K-ATPase in intercalated cells was >10-fold lower. The size of intercalated cells reduced and luminal volume increased among potassium-adapted wild-type but not Kcnmb4-deficient mice. Paradoxically, this led to increased urinary fluid velocity in potassium-adapted Kcnmb4-deficient mice compared with wild-type mice. Taken together, these data suggest that BK-alpha/beta4 channels in intercalated cells reduce cell size, increasing luminal volume to accommodate higher distal flow rates during potassium adaptation. These changes streamline flow across the principal cells, producing gradients more favorable for potassium secretion and less favorable for sodium reabsorption.

Oxygen-dependent ion transport in erythrocytes

The present contribution reviews current knowledge of apparently oxygen-dependent ion transport in erythrocytes and presents modern hypotheses on their regulatory mechanisms and physiological roles. In addition to molecular oxygen as such, reactive oxygen species, nitric oxide, carbon monoxide, regional variations of cellular ATP and hydrogen sulphide may play a role in the regulation of transport, provided that they are affected by oxygen tension. It appears that the transporter molecules themselves do not have direct oxygen sensors. Thus, the oxygen level must be sensed elsewhere, and the effect transduced to the transporter. The possible pathways involved in the regulation of transport, including haemoglobin as a sensor, and phosphorylation/dephosphorylation reactions both in the transporter and its upstream effectors, are discussed.