Depletion of the cell-cycle inhibitor p27 Xic1 impairs neuronal differentiation and increases the number of ElrC + progenitor cells in Xenopus tropicalis

Abstract

The Xenopus p27 Xic1 gene encodes a cyclin dependent kinase (CDK) inhibitor of the Cip/Kip family. We have previously shown that p27 Xic1 is expressed in the cells of the neural plate as they become post-mitotic (Development 127 (2000) 1303). To investigate whether p27 Xic1 is necessary for cell cycle exit and/or neuronal differentiation, we used antisense morpholino oligos (MO) to knockdown the protein levels in vivo. For such knockdown studies, Xenopus tropicalis is a better model system than Xenopus laevis, since it has a diploid genome. Indeed, while X. laevis has two p27 Xic1 paralogs, p27 Xic1 and p28 Kix1 , we have found only one ortholog in X. tropicalis, equidistant from the X. laevis genes. The X. tropicalis p27 Xic1 was expressed in a similar pattern to the X. laevis gene. Depletion of p27 Xic1 in X. tropicalis caused an increase in proliferation and a suppression of the neuronal differentiation marker, N-tubulin. At the same time, we found an increase in the expression of ElrC, a marker of cells as they undergo a transition from proliferation to differentiation. We conclude that p27 Xic1 is necessary for cells to exit the cell cycle and differentiate; in its absence, cells accumulate in a progenitor state. The expression of p27 Xic1 in the embryo is regionalised but the transcriptional regulation of p27 Xic1 is not well understood. We report the isolation of a p27 Xic1 genomic clone and we identify a 5′ region capable of driving reporter gene expression specifically in the neural tube and the eye.