Reprogramming of gene expression following nuclear transfer to the Xenopus oocyte.

Transplantation of Xenopus laevis cell nucleus to enucleated Xenopus egg leads to the generation of cloned animal. This exemplifies the process of nuclear reprogramming by which the nucleus of a specialized cell is reset to an embryonic state from which it can generate all the cells of an organism. Using the precursor of the egg, the oocyte, it is also possible to reprogram somatic cell. The advantage of this approach is the direct reprogramming of gene expression in the absence of cell division. Using this strategy it is possible to investigate the mechanism leading to transcriptional reprogramming of somatic nuclei. By combining real time monitoring of chromatin protein exchange and gene expression analysis, we have observed that a simultaneous loss of somatic H1 linker histone and incorporation of the oocyte-specific linker histone B4 precede transcriptional reprogramming. The loss of H1 is not required for gene reprogramming. We have demonstrated both by antibody injection experiments and by dominant negative interference that the incorporation of B4 linker histone is required for pluripotency gene reactivation during nuclear reprogramming. We suggest that the binding of oocyte specific B4 linker histone to chromatin is a key primary event in the reprogramming of somatic nuclei transplanted to amphibian oocytes.