The collecting duct of the kidney has two major cell types: principal cells which transport water, salt, and potassium, and intercalated cells (ICs) that mediate acid-base transport. There are two functionally distinct subtypes: A-ICs secrete protons via an apical H+ATPase and basolateral Cl-/HCO3- exchanger (kidney AE1), while B-ICs secrete bicarbonate via a basolateral H+ATPase and apical Cl-/HCO3- exchanger (pendrin), thereby enabling the collecting duct to adapt to acid-base disturbances. Metabolic acidosis converts the collecting duct from net HCO3- secretion to HCO3- absorption (H+ secretion). This reversal of polarity of HCO3- flux is associated with major remodeling of the B-ICs and depends on secretion of the large protein hensin into the extracellular matrix surrounding these cells;we have recently shown that galectin-3 facilitates hensin polymerization and is up-regulated during acidosis. The long term goal of this project is to determine how hensin mediates this response to metabolic acidosis. We will make use of a mouse in which hensin expression is selectively deleted from collecting duct cells, which results in a loss of kAE1 expression and the development of distal renal tubular acidosis (dRTA), generated by the Al-Awqati lab.
In Aim 1 we will examine how hensin regulates H+/HCO3- transporter expression by performing transcriptome analysis of medullary cell populations from wild-type and hensin-depleted mice. Validation will be confirmed by RT-qPCR and immunolabeling, and EMSA and luciferase reporter gene assays in an immortalized IC cell line. A proteomics study of hensin- and integrin a6-associated proteins in clone C with and without hensin knockdown will be used to identify molecules in the hensin signaling pathway.
Aim 2 will examine hensin-dependent mechanisms that mediate adaptation to acidosis and alkalosis, utilizing physiologic and immunolabeling studies in mice with hensin or galectin-3 deficiency. We will test whether hensin deficiency affects pendrin regulation and should we find that galectin-3 deficient mice have an incomplete dRTA, we will explore the mechanisms by which galectin-3 facilitates hensin polymerization.
Aim 3 will determine the role of the SDF-1/CXCR4 pathway as an early response in the adaptation to acidosis. SDF-1 is up-regulated in kidneys from acidotic mice, and we are finding that blockade of the CXCR4 receptor prevents adaptation of CCDs to in vitro acidosis. The results of these studies are likely to change our understanding of acid-base physiology and elicit new mechanisms for heretofore unexplained causes of distal renal tubular acidosis.
Acidosis in the blood can occur when the kidney cannot remove acids that accumulate from metabolism, growth, and a high protein diet. Left untreated acidosis may cause cardiovascular problems, bone disease, excessive muscle breakdown, growth failure in children, and a variety of hormonal problems;it is associated with much higher mortality in patients with chronic kidney disease. Our studies will help us understand how the kidney adapts to remove excess acid and will assist physicians in the management of this disorder.
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