Mechanisms and relevance of ENaC regulation by EGF and Rac1

Medical College Of Wisconsin, Milwaukee WI

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Abstract

DESCRIPTION (provided by applicant): Long term control of blood pressure involves Na+ homeostasis through the precise regulation of the Epithelial Na+ Channel (ENaC) in the aldosterone-sensitive distal nephron (ASDN). ENaC dysfunction is causative for disturbances in total body Na+ levels associated with abnormal regulation of blood volume, blood pressure, and lung fluid balance. We provide preliminary evidence that epidermal growth factor (EGF) can serve as an ENaC ligand and hypothesize that members of the EGF-family and Rac1 modulate ENaC-mediated Na+ transport in the ASDN and participate in the development of salt-sensitive hypertension. EGF and its related EGF-family members bind to ErbB receptors and act as signaling factors responsible for renal development, physiology and pathophysiology. Under physiological conditions, ErbB receptors play an important role in the regulation of renal hemodynamics and electrolyte handling by the kidney, while in different pathophysiological states ErbB activation may mediate either beneficial or detrimental effects on the kidney. Stimulation of ErbB receptors activates an intracellular cascade involving small GTPases, particularly Rac1. Small G proteins and their regulatory proteins contribute to the pathology of renal and cardiovascular diseases. Our preliminary results, including electrophysiological experiments using isolated, split open ASDN, demonstrate that ENaC is regulated by EGF and Rac1, possibly through a convergent mechanism. Dahl salt-sensitive (SS) rats used in these studies develop severe hypertension on high-salt diet. We provide preliminary data indicating that ENaC contributes to the development of hypertension in the SS rat strain. Furthermore, our preliminary data reveal that EGF concentration is reduced in the SS rats, which we propose would enhance ENaC activity, sodium retention and hypertension. Building upon this preliminary data and our previously published findings, the specific objective of this proposal is to determine whether EGF acting through Rac1 is important for physiologic control of renal sodium handling through regulation of ENaC and define the precise mechanisms of EGF- and Rac1-mediated changes in ENaC activity. A combination of electrophysiological, immunohistochemical, biochemical, microscopy and chronic studies in vivo and in vitro will be used in this proposal to provide mechanistic insights on how ENaC is regulated by member of the EGF-family and Rac1 and how changes in this pathway contributes to salt induced hypertension in SS rats. These studies will address three Specific Aims: 1) To identify and quantify the role of EGF and related growth factors in the physiological regulation of ENaC activity in the ASDN and establish the role of this pathway in the development of salt-sensitive hypertension; 2) To establish the physiological role of RhoGDI and Rac1 in regulation of ENaC and determine the role of Rac1 in mediating EGF effects on ENaC; and 3) To define the cellular and molecular mechanism by which Rac1 modulates ENaC activity: do WAVEs convey Rac1 regulation to ENaC?