Singh et al. reply

Replying to: H. F. Jørgensen, Z.-F. Chen, M. Merkenschlager & A. G. Fisher Nature 457, 10.1038/nature07783 ; N. J. Buckley, R. Johnson, Y.-M. Sun & L. W. Stanton Nature 457, 10.1038/nature07784 (2009) In contrast to the comments made by Jørgensen et al. 1 and Buckley et al. 2, our experiments showed that REST maintains the self-renewal and pluripotency of mouse embryonic stem cells (mESCs)3. Two recent papers support our work: ref. 4 indicated that REST is indeed in the network that regulates ESC self-renewal and pluripotency and ref. 5 showed that mESCs with lower REST levels derived from a mouse model of Down’s syndrome have decreased levels of self-renewal markers and a higher propensity towards differentiation, even when cultured in the presence of LIF. We note that Buckley and Stanton also recently concluded that REST is part of the Oct4–Sox2–Nanog regulatory network and has “a key role in the maintenance of the ESC phenotype”6. We proposed that REST represses a set of microRNAs that potentially target self-renewal genes. At least one of them, miR-21, represses self-renewal, probably by destabilizing the messenger RNAs of Sox2 and/or Nanog (not Tbx3 or c-Myc as suggested by Buckley et al.2). In this model, changes in the cellular environment that counter this function of REST or stimulate the mRNA levels of Sox2 or Nanog could minimize the effect of REST. For this reason, in our study we used mESCs with a low passage number, and we cultured them without feeder cells to avoid possible contributions of the feeder cells or an adaptive response to high passage.