Regulation and mutation of Na,K-ATPase subunits: metabolism and beta cell hyperplasia
This laboratory investigates the structure, function, regulation, and biological roles of one of the most abundant and physiologically important membrane proteins in animal cells: the Na,K-ATPase. The Na,K-ATPase catalyzes active ion transport and establishes the Na+ and K+ gradients that are harnessed for many physiological processes. The enzyme has multiple isoforms of each of its subunits, all expressed in a tissue-specific manner and subject to change in response to physiological demand.
We have two distinct BADERC-relevant projects.
- Mutations in a Na,K-ATPase subunit expressed in the brain (ATP1A3) produce neurological disease. In two different lines of mutant mice, this manifests as observable motor symptoms: hypotonia, myoclonus, tremor, and dystonia, and also as an elevated arousal system and elevated sensitivity to certain behavioral drugs. The mice have reduced body weight, and we are investigating whether this is due to hyperactivity. One line is a knockout with viable heterozygotes; the other line is a knock-in of one of the mutations that causes neurological disease in humans. A third mouse line with similar but even stronger motor symptoms, with mutation in a different gene, exhibits a striking hypermetabolic phenotype with greatly reduced fat mass and normal lean mass. This was done in collaboration with Dr. Joseph Avruch’s lab, and who provided much valuable expertise. Our current hypothesis is that among the neurological consequences of Na,K-ATPase mutation is elevated sympathetic output, causing upregulation of metabolism. This we plan to test with the help of testing resources provided by BADERC.
- There is a family of small membrane proteins that regulate the Na,K-ATPase. We discovered two new members of this family (the FXYD proteins, named after a sequence motif Phe-X-Tyr-Asp). These are proving to be highly specialized modulatory proteins with complex patterns of regulation at the level of expression, post-translational modification, and participation in signaling complexes. We have knockout mice for two of the FXYD proteins, both of which are involved in metabolism or diabetes.
The one that we study most intensively, FXYD2, is expressed abundantly in the kidney. However, the knockout mouse suffers from high mortality that can be traced instead to altered pancreatic islet function and beta cell hyperplasia. The mouse symptoms led us to discover its expression in pancreatic beta cells. It was independently claimed to be a specific beta cell biomarker by others. In this project we have collaborated with another BADERC-associated lab, that of Dr. Joel Habener. The mice exhibit hyperinsulinemia without insulin resistance. They have greatly enhanced glucose tolerance. Pregnancy is a period of great plasticity in beta cells. Mice bulk up their islets in parallel with body mass, and then rapidly dispose of beta cells after parturition. We have found that the hyperplasia of pregnancy is accompanied by downregulation of FXYD2. This is a physiological correlate of the pathological phenomenon we first uncovered. We have also established FXYD2 as a regulator of cell proliferation in cultured cells using cell transfectants and siRNA methodology in combination with genotoxic stress protocols. The current research focus is on the Akt pathway that controls beta cell proliferation, and the mechanism whereby the presence or absence of FXYD2 regulates it. Because FXYD2 has a very restricted body distribution, and because the knockout mice have well-adapted renal physiology, we propose that FXYD2 is a suitable protein to target in strategies to support beta cell proliferation in either early stage T1D or in T2D.
The other FXYD protein we work on, phospholemman or FXYD1, is known to be phosphorylated in response to insulin, and is a link in the insulin-induced activation of Na,K-ATPase that is responsible for the uptake of dietary K+. Our recent research on FXYD1 mice has been on its protective role in oxidative modification of cardiac and vascular tissue, and its role in the response of the kidney to angiotensin and vasopressin. Our key findings have been that at the cellular level, FXYD expression is dynamic and downregulation of one invariably results in compensatory upregulation of another. We and others have shown that each FXYD has a different effect on pump function and cellular physiology.
1.Arystarkhova, E, Donnet, C, Munoz-Matta, A, Specht, SC, and Sweadner, KJ. Multiplicity of expression of FXYD proteins in mammalian cells: dynamic exchange of phospholemman and gamma subunit in response to stress. Am.J.Physiol. 2007, 292, C1179-C1191.
2. Bibert, S, Liu, CC, Figtree, GA, Garcia, A, Hamilton, EJ, Marassi, FM, Sweadner, KJ, Cornelius, F, Geering, K, Rasmussen, HH. FXYD proteins reverse inhibition of the Na+-K+ pump mediated by glutathionylation of its β1 subunit. J Biol. Chem.. 2011, 286, 18,562-18,572. PMCID 3093901.
3. Sweadner, KJ, Pascoa, JL, Salazar, CA, Arystarkhova, E. Post-transcriptional control of Na,K-ATPase activity and cell growth by a splice variant of FXYD2 protein with modified mRNA. J Biol. Chem. 2011, 286, 18,290-18,300. PMCID 21460224.
4. K.J. Sweadner, Y.B. Liu, L.J. Ozelius, A. Brashear. A new mutant mouse with symptoms of dystonia. Movement Disorders 27(Suppl. 1): S367, 2012.
5. Arystarkhova, E, Liu, YB, Salazar,C, Stanojevic, V, Clifford, RJ, Kaplan, JH, Kidder, GM, Sweadner, KJ. Hyperplasia of pancreatic beta cells and improved glucose tolerance in mice deficient in the FXYD2 subunit of Na,K-ATPase. J. Biol. Chem. 2013, 288: 7077-7085. PMC3591617.