Resistance to diabetes and hypertension in the Fxyd2 knockout mouse: role of AT2R

Diabetes and hypertension have long been known to be interrelated, but much remains to be understood about the metabolic links. Elevated angiotensin II produces hypertension and insulin resistance, and often precedes the development of Type 2 diabetes (T2D). Angiotensin blockers (ACE inhibitors and receptor antagonists) have been found to be beneficial in prevention of T2D in multiple clinical trials. The classic angiotensin type 1 receptor, AT1R, increases hypertension and metabolic phenotype, but the homologous type 2 AT2R receptor, which acts via different G proteins, opposes them. AT2R has been reported to be highly expressed in islets, and to be protective in streptozotocin challenge. Normal, healthy renal adaptation to excess salt includes a decrease in the functional prominence AT1R and an increase in the influence of AT2R, specifically in the proximal tubule. We have obtained surprising evidence that Fxyd2knockout mice are in a highly desirable state: resistant to salt- or angiotensin II-induced hypertension and to beta cell loss in streptozotocin challenge. FXYD2 is a regulator of Na,K-ATPase ion pumping, but Na,K-ATPase also has signaling roles and has been reported to associate with and regulate angiotensin receptors. Our hypothesis is that in the knockout, there is baseline activation the angiotensin type 2 receptor (AT2R), part of the angiotensin counter-regulatory pathway, and it is responsible for the “super mouse” phenotype in both tissues, islets and kidney. Islets and kidney, incidentally, have the highest levels of FXYD2 in the body, and possibly of AT2R. Preliminary qPCR data show a large increase in AT2R mRNA in the knockout islets. The aims will be to investigate the expression and regulatory status of the two angiotensin receptors in beta cell and proximal tubule tissue from WT and Fxyd2 knockout mice. Systems-level studies will entail infusion of WT and Fxyd2 knockout mice with angiotensin receptor-specific agonists and antagonists; the effect on phenotype will test the hypothesis. The physiological and signaling responses of isolated islets and renal slices in vitro will be studied with the same agonists and antagonists of AT1R and AT2R. Finally, immunoprecipitation will be used to investigate whether there is a direct association of Na,K-ATPase with either Ang II receptor in islets and kidney, and the influence of FXYD2. The work is expected to be predictive of mouse physiology, and to test the potential of FXYD2 as a therapeutic target.