EVIDENCE FOR A ROLE OF PROTEIN PHOSPHATASE-1 AND PHOSPHATASE-2A DURING EARLY NEPHROGENESIS

Although most transcriptional events appear to be modulated by reversi ble protein phosphorylation, little is known about the role of this re gulatory system during the development of mammalian organs. Here we ha ve studied the serine/threonine protein phosphatases (PP) 1 and 2A in the early embryonic rat kidney with regard to expression and effects o n growth and differentiation. All isoforms of PP 1 and PP-2A were ubiq uitously expressed in 15-day embryonic (E15) kidneys (in situ hybridiz ation studies). In contrast, mRNA for inhibitor-1 (I-1), an endogenous inhibitor of PP-I, was detected only in undifferentiated stem cells i n the outer cortical area. I-1 is a novel marker for these cells. The abundance of the PP-1 protein, confirmed with immunoblotting, was high in the embryonic kidney. In organ culture of E13 kidneys, okadaic aci d (OA), an exogenous inhibitor of PP-1 and PP-2A, dose-dependently inh ibited growth and nephron formation (apparent half-maximal effect at 6 nM). OA 10 nM had little effect on the growth of cultured E15 kidneys , whereas nephron formation was disturbed and morphological evidence o f apoptosis was seen. In summary, this study points towards important roles for protein phosphatases 1 and/or 2A in regulation of mitogenic activity in the early embryonic kidney.