Role of mir-302 microRNA family in stem cell pluripotency and renewal

Abstract

Recent research in human embryonic stem (hES) cells has shown a highly promising potential in transplantation therapy. Nevertheless, it is very difficult to maintain the purity and pluripotency of these hES cells. In a fertilized egg, maternal materials naturally maintain stem cell renewal and totipotency before the 128-cell stage of embryonic development. Mouse oocytes lacking Dicer, a conserved ribonuclease required for microRNA (miRNA) biogenesis, arrest in the division phase of meiosis I, indicating that miRNAs play a critical role in oogenesis. We have observed that mir-302 familial microRNAs (mir-302s) were expressed at extremely high levels in mouse oocytes and human embryonic stem cells, and gradually decreased after cell differentiation. Therefore, we proposed that the mir-302 family is one of the key maternal materials essential for maintenance and renewal of the hES cell pluripotency. For this, we developed a Pol-II-based intronic miRNA expression system to transgenically express the mir-302s in several human epidermal and cancerous cell lines. Surprisingly, these mir-302s-transfected cells, namely miRNA-induced pluripotent stem (mirPS) cells, were shown to not only express all sorts of hES markers, such as Oct3/4, SSEA-3, SSEA-4, Sox2 and Nanog, but also have a highly demethylated genome similar to the reprogrammed genome of a fertilized egg. Microarray analyses further revealed that genome-wide gene expression patterns of these mirPS cells shared over 86% similarity with those of hES H1 and H9 cell lines. With certain molecular guidance ex vivo, these mirPS cells could differentiate into distinct tissue cell types, such as neuron-, chondrocyte-, fibroblast- and spermatogonia-like primordial cells. Based on these findings, we suggest that the function of mir-302s is able to not only maintain the hES cell renewal and pluripotency but also to reprogram differentiated cells into a hES-like pluripotent state, which may provide insights into areas of opportunity for therapeutic intervention.