Immunohistochemical analysis of Musashi-1 expression during retinal regeneration of adult newt.

The adult newt retinal regeneration is an ideal model for studying retinal regeneration by transdifferentiation of the retinal pigment epithelium (RPE) cells. Accumulated evidence suggests that the RNA-binding protein Musashi-1 (Msi1) is expressed in mature photoreceptors and RPE cells as well as in retinal stem/progenitor cells, being essential for vision. We have been investigating whether Msi1 is also essential for retinal regeneration. In the last paper [K. Susaki, J. Kaneko, Y. Yamano, K. Nakamura, W. Inami, T. Yoshikawa, Y. Ozawa, S. Shibata, O. Matsuzaki, H. Okano, C. Chiba, Musashi-1, an RNA-binding protein, is indispensable for survival of photoreceptors. Exp. Eye Res. (in press)], we showed that the expression profiles of Msi1 transcripts and protein isoforms change during retinal regeneration. In the current report, we show by immunohistochemistry that Msi1 is expressed in transdifferentiating cells or cells of regenerating retinal tissues. Upon retinectomy, Msi1 protein, which is expressed in the nuclei of intact (stage E-0) RPE cells, changed its subcellular localization, being expressed in both the nucleus and cytoplasm of the RPE-derived stem-like cells at stage E-1. As the retinal rudiment/regenerating retina (rR) and renewing RPE (rRPE) are specified from the stem-like cell population (stage E-2), Msi1 expression was maintained or up-regulated in the rR, while down-regulated in the rRPE. During further retinal regeneration, Msi1 expression was decreased in association with cell differentiation, except for photoreceptors and RPE cells whose Msi1 expression increased as they differentiate. Thus, Msi1 is likely to be regulated at various cellular events during retinal regeneration, implying that Msi1 may have multi-functions in retinal regeneration. All together, it is probable that Msi1 is one of the essential factors that need to be regulated in retinal regeneration.