The steady state level of phosphorylation within a cell is determined by the combined action of kinases and phosphatases. The kinases and phosphatases form a major portion of intercellular signaling mechanism and execute a myriad of cellular processes and physiological activities. Phospho-protein phosphatase 1 (PP1) isoforms belong to the family of protein serine/threonine phosphatases. The role of PP1 has been implicated in various eukaryotic process including metabolism, cell cycle regulation, apoptosis, etc. In mammals three genes, namely Ppp1ca, Ppp1cb, and Ppp1cc, encode four PP1 isoforms, PP1alpha, PP1beta, PP1gamma1 and PP1gamma2. The isoforms PP1gamma1 and PP1gamma2 result from differential splicing of Ppp1cc gene. The PP1 isoforms are virtually identical (approximately 90% identity) except at their extreme C-termini. While PP1gamma1, PP1alpha and PP1beta are ubiquitous, PP1gamma2 is largely restricted to adult testis. Elimination of Ppp1cc gene that eliminates both splice variants results in male infertility due to lack of spermatogenesis. Females appear normal suggesting that the PP1alpha and PP1beta can substitute for the PP1gamma isoforms in all tissues except testis. Interestingly, loss of PP1alpha seems to have no apparent phenotype. Importantly, elimination of PP1beta by gene-trap and insertion- deletion method leads to embryonic lethality both in drosophila and mouse. These data suggest that of the four PP1 isoforms only PP1beta is essential and irreplaceable for embryonic development. In this work we have characterized the expression of the PP1 isoforms at various stages of mouse embryonic development. Northern blot analysis was performed on total RNA collected from embryonic stages E8.5-E16.5 to detect the levels of expression for each isoforms. We could clearly detect all the four isoforms in all the embryonic stages examined including embryos and yolk sac. Placental expression of all isoforms could also be detected as early as E9.5. PP1beta emerged to be the most abundantly expressed isoform in the embryonic heart, followed by PP1alpha and low level of PP1gamma1 and PP1gamma2 respectively. PP1alpha expression was high in lungs of E14.5 and E16.5 embryos, suggesting a role for the isoform in lung development. Protein levels of the four isoforms were confirmed by immuno-histochemistry (IHC) of paraffin-embedded embryos. Specificity of antibodies used in IHC was confirmed by western blot analysis using purified proteins. In conclusion, we for the first time determined the expression profile of the PP1 isoforms in developing mouse embryos and extra-embryonic tissues and organs. Our data suggest an essential role for PP1beta in the developing heart. However, determination of the exact stage of the developing embryo where PP1beta is irreplaceable will emerge from studies with conditional knockout mice.

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