Distribution of Na+, K(+)-ATPase alpha-subunit isoforms in rat pituitary

Parallel detection of Na,K-ATPase alpha subunit isoforms by pan-specific monoclonal mAb 9A7

While emphasis has been placed upon those proteins which either mediate or respond to the rapid influx of calcium following depolarization, there has been little emphasis upon those proteins which aid in the reequilibration of the membrane potential. In an effort to identify presynaptic membrane proteins implicated in neurosecretion, monoclonal antibodies were screened against proteins which cosegregated with neuronal voltage-dependent calcium channels (VDCC) following immunoprecipitation. One monoclonal antibody (mAb 9A7) identified a 110-kDa protein. Micropeptide sequencing of (i) the mAb 9A7 immunoaffinity purified antigen and (ii) the 110-kDa protein present in the neuronal (N-type) VDCC preparation (McEnery et al., 1991, Proc. Natl. Acad. Sci. 88, 11095-11099) indicated identity with the alpha subunit(s) of the Na,K-ATPase. Further characterization by Western blotting, immunochemical localization, and immunoaffinity purification indicated that mAb 9A7 not only recognized the alpha3 isoform which is predominant in neuronal tissues but also identified the alpha1 and alpha2 isoforms. mAb 9A7 exhibited a wide cross-species reactivity and recognized human, rat, and mouse alpha subunit isoforms at an internal epitope. The pan-specificity of mAb 9A7 and the differential mobility of the alpha1 isoform relative to the alpha2 and alpha3 permitted parallel detection of multiple alpha isoforms. Western blot analysis of undifferentiated rat pheochromocytoma cell line (PC12) and human neuroblastoma (IMR32) cells indicated coexpression of the alpha1 and alpha3 isozymes. Upon differentiation of IMR32 cells by dibutrylyl-cAMP, a substantial increase in the alpha3 relative to the alpha1 isoform was observed. While the enrichment of total Na,K-ATPase may reflect the increased demand for ATP-dependent ion transport as IMR32 cells become more excitable, the specific increase in the alpha3 isoform suggests a unique role of this isoform during IMR32 cell differentiation.

Immunolocalization of GLUT1 and GLUT3 glucose transporters in primary cultured neurons and glia

Immunofluorescence analysis was used to study the cellular localization of glucose transporters 1 and 3 (GLUT1 and GLUT3) in primary rat neuronal and glial cultures. In primary cultured cerebellar granule neurons and cortical neurons, GLUT3 was detected in a pattern consistent with a generalized cell surface distribution. GLUT3 distribution corresponded most closely with the neural cell adhesion molecule (NCAM), and showed overlapping but distinct distributions compared to synaptophysin, microtubule-associated protein 2 (MAP2), neurofilament protein, and growth-associated protein (GAP43). Culture of neurons in the presence of glia did not alter the cellular localization of GLUT3. GLUT1 was detectable in primary cerebellar granule neurons both at the cell surface and in the cytoplasm, and appeared decreased in neurons cocultured with glia. GLUT1, but not GLUT3, was detected in glial fibrillary acidic protein (GFAP)-positive astrocytes present in mixed neuronal-glial cultures derived from cerebellum and cerebral cortex, as well as in cortical astrocyte cultures. GLUT1, but not GLUT3, was also detected in microglia and oligodendrocytes present in these cultures. This study indicates a generalized cell surface expression of the glucose transporters expressed in neurons and glia, rather than selective targeting to different cellular domains or subcellular locations.

Activity-dependent regulation of Na+, K(+)-ATPase alpha isoform mRNA expression in vivo

To investigate the functional role of the different Na+, K(+)-ATPase alpha (catalytic) subunit isoforms in neuronal cells, we used quantitative in situ hybridization with riboprobes specific for alpha 1, alpha 2, and alpha 3 isoforms to measure the level of alpha isoform-specific expression in the neuroendocrine cells of the supraoptic (SON) and paraventricular (PVN) nuclei of rat hypothalamus. A prolonged increase in electrical activity of these cells, achieved by 5 days of salt treatment, increased the amount of alpha 1 isoform mRNA in the SON and PVN by 50%. Levels of alpha 1 mRNA in other brain regions and levels of alpha 2 and alpha 3 mRNAs were not affected by salt treatment. We conclude that the alpha 1 isoform Na+, K(+)-ATPase may be specifically adapted to pump out Na+, which enters the cells through voltage-gated channels during neuronal depolarization.