LOCALIZATION AND QUANTIFICATION OF ENDOPLASMIC-RETICULUM CA2-ATPASE ISOFORM TRANSCRIPTS()

The Ca2+-adenosinetriphosphatase pump of the sarcoplasmic or endoplasm ic reticulum (SERCA) plays a critical role in Ca2+ signaling and homeo stasis in all cells and is encoded by a family of homologous and alter natively spliced genes. To understand more clearly the role the differ ent isoforms play in cell physiology, we have undertaken a quantitativ e and qualitative assessment of the tissue distribution of transcripts encoding each SERCA isoform. SERCA1 expression is restricted to fast- twitch striated muscles, SERCA2a to cardiac and slow-twitch striated m uscles, whereas SERCA2b is ubiquitously expressed. SERCA3 is expressed most abundantly in large and small intestine, thymus, and cerebellum and at lower levels in spleen, lymph node, and lung. In situ hybridiza tion analyses revealed SERCA3 transcripts in cells of the intestinal c rypt, the thymic cortex, and Purkinje cells in cerebellum. In addition , SERCA3 was expressed abundantly in isolated rat spleen lymphocytes, in various murine lymphoid cell lines, and in primary cultured microva scular endothelial cells. This analysis demonstrates that SERCA3 is ex pressed selectively in cells in which Ca2+ signaling plays a critical and sensitive role in regulating physiological processes.