Expression and mapping of protein phosphatase 2A alpha in the developing rat heart.

Protein phosphatase 2A (PP2A) is a second messenger involved in cell cycle regulation, cell transformation, and cell fate determination. We previously identified a gene encoding the alpha catalytic subunit of PP2A in the embryonic rat heart, but its role in cardiac morphogenesis was unknown. In this study, we examined the developmental expression of PP2A alpha mRNA and protein in the heart using Northern and Western analysis, in situ hybridization, and immumohistochemical staining. We found two major PP2A alpha transcripts in the rat heart (1.8 and 2.4 kb), at all stages examined. By Western blotting, PP2A alpha protein levels were twice as high in the embryonic rat heart compared with the adult. In situ hybridization on embryonic d 12 showed that PP2A alpha mRNA was expressed in the heart, brain, tail, and limb buds. Cardiac PP2A alpha expression was regionally restricted to the atrium, ventricle, and truncus arteriosus. PP2A alpha expression did not extend into the more distal aortic sac or aortic arches. Cross-sectional hybridization revealed PP2A alpha mRNA in the epicardium, pericardium, and endothelium. Later in development, mRNA expression was also detected at high levels in mesenchymal cells populating the endocardial cushions and in myocardium. At term, PP2A alpha was highly expressed in endothelial cells, but not in the underlying myocardium. PP2A alpha protein had a similar distribution at all embryonic stages examined. These results show that there is transcriptional, translational, and cell-specific regulation of PP2A alpha during heart development. We speculate on the role of PP2A alpha-mediated dephosphorylation in cardiac morphogenesis and suggest a number of possible molecular targets.