Hypertension affects more than 25% of the adult population and is a major risk factor for coronary artery disease, stroke, and chronic kidney disease. Previous studies have revealed that interactions between genetic and environmental factors play a critical role in the development of elevated blood pressure, and a large body of evidence implicates the inappropriate retention of sodium by the kidney in the pathophysiology of hypertension. However, the regulatory steps that govern and modulate sodium reabsorption by the kidney are not fully understood. Within the kidney, the epithelial sodium channel (ENaC) is expressed in the aldosterone- responsive distal portions of the nephron, where it serves as the rate-limiting step in the reabsorption of filtered sodium, hence playing a crucial role in the regulation of sodium balance, extracellular fluid volume, and blood pressure. Results have emerged that document a critical role for serine protease-mediated cleavage of ENaC subunits in ENaC activation, and recent studies suggest that plasmin is a key protease in this process. This proposal is based on our proteomics-based discovery of a new protein, the plasminogen receptor, Plg-RKT, which markedly enhances the activation of the zymogen plasminogen to plasmin, as well as concentrates and localizes the proteolytic activity of plasmin at specific site on the cell surface. We have observed prominent (indeed striking) expression of Plg-RKT in renal epithelial cells, particularly in the distal portions of the nephron, and in confocal imaging studies have noted prominent co-localization of Plg-RKT with ENaC, and with the urokinase receptor (uPAR), another key component of the plasminogen activation pathway. Our preliminary data suggest the presence of a local, renal distal nephron plasminogen activation system, which includes all major components of the plasminogen activation pathway, including plasminogen activator [urokinase (uPA)], the urokinase receptor (uPAR), the zymogen plasminogen, and of high importance, the specific plasminogen receptor Plg-RKT, for local amplication and concentration of cell surface plasminogen activation. In other recent results, we have demonstrated a key role for Plg-RKT and the local plasminogen activation system in ENaC proteolytic processing, and have shown that genetic ablation of various components of the plasminogen activation system (including plasminogen, urokinase, and Plg-RKT) results in substantial alterations in sodium homeostasis in vivo. The overall objective of this proposal is to test the hypothesis that Plg-RKT plays a crucial role in ENaC activation and sodium retention in salt-sensitive hypertension. We have generated a novel Plg- RKT deficient mouse model and will use this to comprehensively investigate the role of Plg-RKT in sodium retention in vivo in models of salt-sensitive hypertension, including models of mineralocorticoid excess, obesity/metabolic syndrome, and Liddle's syndrome. In addition, we will examine the regulation of Plg-RKT expression (including transcriptional activation and protein expression) and trafficking in renal epithelial cells, and determine the extent to which Plg-RKT expression is regulated by hormonal agents that are active in the distal nephron and that function to regulate sodium balance, including aldosterone and vasopressin. Thus, we will evaluate the role of Plg-RKT as a key mediator of hormone-stimulated sodium reabsorption in the distal nephron. This project seeks to provide new information on sodium reabsorption by the kidney in hypertension, which will lead to improved management and novel therapeutic targets for this highly prevalent disorder, as well as for other diseases characterized by increased sodium reabsorption.
These studies seek to understand novel regulatory mechanisms that modulate sodium handling by the kidney. Abnormal (excessive) sodium reabsorption by the kidney is a major factor in the development of essential hypertension, a disease which occurs with high frequency in the adult population, and is a major risk factor for coronary artery disease, stroke, and chronic kidney disease. Other diseases in which renal sodium reabsorption is abnormal include obesity and major edema-forming diseases such as nephrotic syndrome, congestive heart failure, and cirrhosis, which are leading causes of death and disability. This work thus represents a unique opportunity within the VA Healthcare System, to provide better understanding of sodium reabsorption by the kidney, which will lead to improved management and novel therapeutic targets for these major and highly prevalent disorders.