UCSF DIABETES, ENDOCRINOLOGY & METABOLISM TRAINING PROGRAM FACULTY RESEARCH SUMMARIES

Work in my laboratory focuses on mechanisms of hormone signaling, particularly as it relates to aldosterone-regulated ion transport in the kidney. Aldosterone is the principal regulator of salt and volume homeostasis in mammals, and as such, defects in its cellular actions, or in the gene products it regulates, are major causes of both hypertension and salt-wasting disorders. The receptor for aldosterone, the mineralocorticoid receptor (MR), is a hormone-activated transcription factor. We have focused on understanding how it regulates gene transcription and on how the regulated gene products in turn control cellular functions, particularly ion transport. Two major themes underlie our work.

The Role of Sgk Kinases in Controlling Ion Transport in Epithelia

We identified Sgk1 in a subtractive library by searching for aldosterone early response genes. Sgk1 is a serine-threonine kinase whose gene transcription is stimulated by aldosterone and whose activity is controlled by PI3-kinase. Work from our lab and others have confirmed the physiological relevance of Sgk1, which at present remains the only aldosterone target gene with a proven physiological role in epithelial ion transport. We have shown that Sgk1 stimulates sodium transport by regulating membrane trafficking of the epithelial sodium channel (ENaC). Current efforts focus on the underlying mechanisms. In particular, our recent collaborative work has shown that Sgk1 acts to inhibit a ubiquitin ligase, Nedd4-2, which itself triggers ENaC internalization and blocks its trafficking to the plasma membrane. We have found that phosphorylation of Nedd4-2 recruits a chaperone protein that inhibits Nedd4-2. Furthermore, we have found that Sgk1 acts as an integrator of aldosterone and insulin regulation of Na + transport. Sgk1 is targeted to the appropriate subcellular location by a combination of protein-protein and protein-lipid interactions. These latter observations provide targets for identification of novel treatments for hypertension. We recently knocked out one of the Sgk isoforms (Sgk3) in mice and are characterizing its phenotype.

Basic Mechanisms of MR Regulation of Gene Transcription

MR is a hormone-activated transcription factor that can both stimulate or repress gene transcription. We are interested in the mechanistic basis of receptor function as a transcription factor, particularly determinants of its specificity. Synergistic regulation of gene transcription is an important determinant of receptor specificity. We have identified a synergy control circuit, involving the DNA-binding domain and a short motif in the receptor N-terminus. Binding to DNA not only brings MR into the vicinity of regulated genes, but also exerts regulatory effects of its own on the receptor. We identified an important determinant of this signal detection system, which informs the receptor when it is bound to various types of DNA sites (response elements). Binding as a dimer to a palindromic response element engages the synergy control function, which induces SUMO-1 (small ubiquitin-like modifier) modification of a regulatory sequence (synergy control motif) in the receptor N-terminal domain. We are currently examining how SUMO-1 ligases (which are similar in function to ubiquitin ligases) modify the synergy control motif. We are also examining how MR specifically regulates transcription of the Sgk1 gene.

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