Background: Among mitogen-activated protein kinase (MAPK) family members, extracellular signal-regulated kinase (ERK) promotes proliferation or differentiation, whereas c-Jun N-terminal kinase (JNK) and p38 MAPK (p38) are thought to inhibit cell growth and induce apoptosis. MAPK phosphatase-1 (MKP-1) inactivates and modulates MAPKs. During renal development, large scale proliferation and apoptosis occur. We investigated the temporal and spatial expression patterns of MAPKs and MKP-1 in rat kidney during development. Methods: Western blot analysis and immunohistochemistry were performed in the developing and mature kidney of the rat. Results: The expression of ERK, p38, and MKP-1 were high in developing kidney. On the other hand, JNK was abundantly expressed in adult kidney. Active forms of ERK, p38, and JNK correlated with the protein expression levels. Immunohistochemical studies revealed that ERK was strongly expressed by blastema cells, mesenchymal cells, and ureteric bud tips in nephrogenic zone of embryonic kidney. In neonatal kidney, ERK was more abundant in the deep cortex and the medulla corresponding to tubule maturation, p38 and MKP-1 were detected uniformly in mesenchymal cells, mesenchymal cells, and ureteric bud epithelia of fetal kidney without an obvious correlation with the occurrence of apoptosis, JNK was expressed by tubular cells and podocytes of adult kidney. Conclusions: ERK, p38, and MKP-1 are strongly expressed in developing kidney, and JNK is detected predominantly in adult kidney. Both the temporal and spatial expression of ERK coincides with the maturation of the kidney.
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