Laboratory of Molecular Neurophysiology

Structure, function, and biological roles of Na,K-ATPase isoforms in excitable tissues.

The active transport of Na+ and K+ is carried out by an ATP-hydrolyzing enzyme, the Na,K-ATPase. This laboratory found that there are multiple isoforms of the enzyme expressed in the nervous system and heart. Our research concerns the control of isoform expression; the functional properties of the different isoforms; and the molecular structure of the protein itself.

Na,K-ATPase isoform expression is tissue- and cell type-specific, but is regulated both developmentally and in response to environmental factors. We have recently found that the -2 isoform is expressed in glia only when complex glial phenotypes are expressed in culture, and in skeletal muscle myocytes only when they differentiate to myotubes. Transfection of subunit isoforms in cells in culture gives useful information about their functions, particularly for glia, which show some unusual characteristics suggesting a specialized role for the pump in K+ clearance.

Efforts to find straightforward kinetic differences between isoforms have been controversial, and we now think it likely that isoform-specific regulation by second messengers is the key to understanding their physiological significance. Protein kinase-mediated regulation is being studied in purified enzyme preparations and in cells in culture.

To investigate how the protein is folded and how its conformational changes result in transport of ions, we are mapping the epitopes of monoclonal antibodies, using phage epitope library selection and DNA sequencing. This, combined with protein chemistry, is a useful tool for determining the enzyme's 3-dimensional structure.

Links: Representative Publications:

• Urayama, O, Shutt, H, Sweadner, KJ. Identification of three isozyme proteins of the catalytic subunit of the Na,K-ATPase in rat brain. J. Biol. Chem. 1989; 264: 8271-8280.
• Gloor, S, Antonicek, H, Sweadner, KJ, Pagliusi, S, Frank, R, Moos, M, and Schachner, M. The adhesion molecule on glia (AMOG) is a homologue of the beta subunit of the Na,K-ATPase. J. Cell Biol. 1990; 110: 165-174.
• McGrail, KM, Phillips, JM, and Sweadner, KJ. Immunofluorescent localization of three Na,K-ATPase isozymes in the rat central nervous system: Both neurons and glia can express more than one Na,K-ATPase. J. Neurosci. 1991; 11: 381-391.
• Mohraz, M, Arystarkhova, E, and Sweadner, KJ. Immunoelectron microscopy of epitopes on Na,K-ATPase catalytic subunit. Implications for the transmembrane organization of the C-terminal domain. J. Biol. Chem. 1994; 269: 2929-2936.
• Sweadner, KJ. Na,K-ATPase isoforms in glial cells. in Neuroglial Cells, H. Kettenman and B. Ransom, Eds., Oxford University Press, 1994, in press.