Nuclear reprogramming in zygotes

Abstract

Nuclear reprogramming, the conversion of the epigenome of a differentiated cell to one that is similar to the undifferentiated embryonic state, can be facilitated by several methods, such as nuclear transfer, cell fusion, use of embryonic stem cell extracts, and more recently, by the introduction of exogenous transcription factors. Amongst these various strategies, somatic cell nuclear transfer (SCNT) is, by far, the most effective method of nuclear reprogramming. The majority of SCNT studies have been carried out using enucleated mature oocytes, as reprogramming is efficient and can be completed within hours following the introduction of the somatic cell nuclei into the recipient oocyte. Fertilized eggs, on the other hand, were regarded as poor recipients for nuclear transfer, as previous studies showed that embryonic blastomeres transferred into enucleated zygotes were unable to develop to blastocysts. However, more recent studies have demonstrated that the method of enucleation and the cell cycle phase of the embryos can impact the success of somatic cell reprogramming when zygotes were used as nuclear recipients. It is, therefore, timely to revisit and further explore the nuclear reprogramming capacity of zygotes as recipients for SCNT. Assessment of the various factors that influence the reprogramming capacity of zygotes in SCNT also provide hints of the mechanistic nature of nuclear reprogramming. KEY WORDS: nuclear transfer, reprogramming, zygote, embryo, SCNTDuring nuclear reprogramming, the cell-type specific epigenetic program of a differentiated cell is erased and the somatic genome re-acquires the potential to give rise to other cell types. In somatic cell nuclear transfer (SCNT), differentiated donor cells or nuclei are introduced into the cytoplasm of enucleated recipient cells to induce de-differentiation of the somatic genome to an embryonic state. Successful nuclear reprogramming will reset the transferred genome to totipotency. Thus, complete reprogramming of somatic genome via SCNT into embryos would result in normal embryonic development, giving rise to cloned animals that are genetically identical to the nuclear donor. The first mammal cloned from adult cell nuclei, Dolly the sheep, was successfully generated by SCNT in which nuclei of terminally differentiated mammary gland cells were transferred into enucleated mature oocytes and reprogrammed to a totipotent state (Wilmut et al., 1997). Since this breakthrough work, many other mammals such as mice (Wakayama et al., 1999), cattle (Kato et al., 1998), pigs (Polejaeva et al., 2000, cats (Shin et al., 2002), rabbits (Chesne et al., 2002;Li et al., 2006a), dogs (Lee et al., 2005, rats (Zhou et al., 2003), buffalos (Shi et al., 2007), ferrets (Li et al., 2006b), and camels (Wani et al., 2010) have been cloned by SCNT into Int. J. Dev. Biol. 54: 1631-1640 (2010) Abbreviations used in this paper: ECNT, embryonic cell nuclear transfer; ES, embryonic stem; GV, germinal vesicle; ICM, inner cell mass; MII, metaphase II; SCNT, somatic ce...

The acetyllysine reader BRD3R promotes human nuclear reprogramming and regulates mitosis

“…Our model provides a molecular explanation to previous observations that only mitotic cells have sufficient reprogramming power to enable cloning of animals 13 15 16 , and that donor nuclei also have mitotic advantage in reprogramming 17 . During mitosis, cell-type-specific transcription factors dissociate from chromatin 45 and the unloaded chromatin provides an opportunity for reprogramming factors to access the target loci and therefore facilitate reprogramming 46 47 . Mitosis plays a paramount role in reprogramming, but mitotic advantage of reprogramming does not exclude the reported possibility that some reprogramming factors exist within the nuclei of the interphase recipient cells 18 19 .…”

Early Aberrations in Chromatin Dynamics in Embryos Produced UnderIn VitroConditions

“…In the last decade, several studies aimed on discovering the molecular mechanisms contributing to the functional reprogramming of the genome. The majority of the studies focused on epigenetic modifiers and processes enabling activation and/ or silencing of developmentally important genes (Bourc'his and Voinnet 2010;Corry et al, 2009;Lorthongpanich et al, 2010). However, changes in selected epigenetic marks may not necessarily represent the ultimate marker of reprogramming.…”

LINEs of evidence: noncanonical DNA replication as an epigenetic determinant

“…The model implies that the carried-over FL-L1 transcripts delivered by gametes and ORF2p are indispensable to set up the initial 3D genome architecture and replication timing program through noncanonical L1 DNA replication. Because the metaphase II oocyte, the common recipient used for somatic cell nuclear transfer [159], contains nuclear factors in its cytoplasm, FL-L1 transcripts might be available in the ooplasm if not bound to chromatin. ORF2p is present in the epididymal spermatozoa [27].…”