The Na+/K+ ATPase is an electrogenic transmembrane ion transport protein that localizes to the plasma membrane in mammals. Its primary function is to maintain the sodium ion and potassium ion electrochemical gradients across the plasma membrane; it does so by utilizing ATP molecules as an energy source. Three major polypeptides make up the Na+/K+ ATPase—the alpha, beta, and FXYD subunits. The alpha subunit is primarily responsible for the catalytic properties of the protein. Furthermore, the alpha subunit has four isoforms, named alpha1-alpha 4 (encoded by ATP1A1-ATP1A4), with the difference between the isoforms being mainly on the surface of the N domain [1] as well as the cell types each isoform localize to. Varying the combination of subunits provides for different kinetic properties and affinities to the ions which the pump is responsible for transporting. Alpha 1 is ubiquitous at the cellular level; alpha 2 is expressed mainly in skeletal muscle and glial cell types; alpha 3 is found predominately in neuronal projections [5] and dendritic spines [6], and our research shows that alpha 3 also localizes to the sperm head in humans; and alpha 4 was previously thought to localize solely in mammal spermatozoa. Since alpha 4 localizes solely to spermatozoa in mice and rats, studies have proposed targeting alpha 4 to design a male contraceptive [2-4]. A study on an Italian family with familial hemiplegic migraine (FHM), however, concluded via exome sequencing that the affected individuals had a heterozygous ATP1A4 mutation [7]. This finding is significant because the exome sequencing indicated that there were no mutations found in the commonly associated genes for FHM, including ATP1A3 and ATP1A2. These studies suggest that ATP1A4 is also expressed in the human brain. Our study aims to further characterize the localization of the alpha 3 and alpha 4 subunits. RT-PCR confirms the mRNA expression of human alpha 3 in testis and alpha 4 isoforms in the brain. Additionally, immunohistochemistry and immunoblotting confirm the presence of these isoforms in human testis and brain. Although the physiological significance of our findings is still yet to be determined, our findings suggest that humans evolved different functions of the alpha isoform subunits compared to other species in the brain and testis/sperm. This study furthermore cautions the design of inhibitor molecules targeting the Na+/K+ ATPase alpha4 subunit, for male contraception as it may result in deleterious consequences in the human brain.
Author(s): Polina Eskin, Cameron C. Gardner
Advisor(s): Paul F. James, Biology
You must be logged in to post a comment.