Reprogramming and epigenesis

The fact that the nucleus of a differentiated somatic cell can be reprogrammed in order to sustain embryonic development is now well established. Experiments of somatic cell nuclear transfer (cloning) have proved that a foreign nucleus introduced into an enucleated oocyte can give rise to physiologically normal offsprings, with a normal lifespan. Such evidence of genome expression plasticity is also observed experimentally with heterokaryons, created by the fusion or the nuclear transfer between two somatic cells, where differentiated nuclei are able to express genes characteristic of the host cell. However, the epigenetic mechanisms that permit nuclear plasticity remain poorly understood. In this paper we present the main evidences showing important modifications of the large scale organisation of chromosomal domains and of the DNA methylation pattern upon nuclear transfer and during the first cleavages. These modifications of epigenetic marks, brought by an intimate contact between the chromatin and the recipient oocyte cytoplasmic factors, appear essential for further development. They are established over the first cell cycles of development. The onset of embryonic genome activation and the first cellular differentiation events that occur over the implantation period are two additional check-points of reprogramming that appear to be also highly dependent on epigenetic alterations. Beyond those stages, defective placental functions might be directly responsible for the fetal and postnatal physiopathologies frequently observed in cloned animals. No direct link between preimplantation reprogramming defaults, placental dysfunctions and low development to term has been established yet. The epigenetics studies which are now used to characterise loci specific and probably genotype dependent alterations in cloned animals of different species will provide invaluable help to define the role of epigenesis in the achievement of a developmental program.     

Reprogramming in nuclear transfer

Reprogramming a nucleus by transferring it to oocyte cytoplasm triggers epigenetic changes that eventually lead to the restoration of a totipotent state and the birth of a viable animal. A simplified way of studying this complex process is to study cell hybrids. These studies suggest that the pluripotent character of one nucleus is dominant over that of the other nucleus. The development of nuclear transfer embryos shows that there are several restriction points and that the extra-embryonic lineages may be the primary source of death. Successful reprogramming depends on the balance between epigenetic modifications and the regulative properties of development.     

Reprogramming cell fates in the mammary microenvironment

The capacity of any portion of the murine mammary gland to produce a complete functional mammary outgrowth upon transplantation to an epithelium-divested fat pad is unaffected by the age or reproductive history of the donor. Likewise, through serial transplantations, no loss of potency is detected when compared to similar transplantations of the youngest mammary tissue tested. This demonstrates that stem cell activity is maintained intact throughout the lifetime of the animal despite aging and the repeated expansion and depletion of the mammary epithelium through multiple rounds of pregnancy, lactation and involution. These facts support the contention that mammary stem cells reside in protected tissue locales (niches), where their reproductive potency remains essentially unchanged through life. Disruption of the tissue, to produce dispersed cells results in the desecration of the protection afforded by the “niche” and leads to a reduced capacity of dispersed epithelial cells (in terms of the number transplanted) to recapitulate complete functional mammary structures. Our studies demonstrate that during the reformation of mammary stem cell niches by dispersed epithelial cells in the context of the intact epithelium-free mammary stroma, non-mammary cells may be sequestered and reprogrammed to perform mammary epithelial cell functions including those ascribed to mammary stem/progenitor cells.     

Reprogramming a broken heart

Fibrosis resulting from cardiac injury presents a major challenge to restoring heart function after myocardial infarction. Two recent papers in Nature report successful in vivo reprogramming of fibroblasts to cardiomyocytes in injured mouse hearts (Qian et al., 2012; Song et al., 2012), resulting in improved cardiac function and reduced scar formation.     

Reprogramming committed B lineage cells

It is generally assumed that once a cell commits to a certain lineage it no longer can change its fate. A new study in this issue of Cell provides compelling evidence that committed mature B lineage cells can be reprogrammed to become macrophages.