Nuclear reprogramming: the strategy used in normal development is also used in somatic cell nuclear transfer and parthenogenesis

Abstract

Somatic cell nuclear transfer (SCNT) and parthenogenesis are alternative forms of reproduction and development, building new life cycles on differentiated somatic cell nuclei and duplicated maternal chromatin, respectively. In the preceding paper (Sun F, et al., Cell Res 2007; 17:117-134.), we showed that an "erase-and-rebuild" strategy is used in normal development to transform the maternal gene expression profile to a zygotic one. Here, we investigate if the same strategy also applies to SCNT and parthenogenesis. The relationship between chromatin and chromatin factors (CFs) during SCNT and parthenogenesis was examined using immunochemical and GFP-fusion protein assays. Results from these studies indicated that soon after nuclear transfer, a majority of CFs dissociated from somatic nuclei and were redistributed to the cytoplasm of the egg. The erasure process in oogenesis is recaptured during the initial phase in SCNT. Most CFs entered pseudo-pronuclei shortly after their formation. In parthenogenesis, all parthenogenotes underwent normal oogenesis, and thus had removed most CFs from chromosomes before the initiation of development. The CFs were subsequently re-associated with female pronuclei in time and sequence similar to that in fertilized embryos. Based on these data, we conclude that the "erase-and-rebuild" process observed in normal development also occurs in SCNT and in parthenogenesis, albeit in altered fashions. The process is responsible for transcription reprogramming in these procedures. The "erase" process in SCNT is compressed and the efficiency is compromised, which likely contribute to the developmental defects often observed in nuclear transfer (nt) embryos. Furthermore, results from this study indicated that the cytoplasm of an egg contains most, if not all, essential components for assembling the zygotic program and can assemble them onto appropriate diploid chromatin of distinct origins.

Mechanisms of nuclear reprogramming by eggs and oocytes: a deterministic process?

“…This change is reminiscent of what happens to sperm chromatin at fertilization. Chromatin decondensation seems to be a hallmark of reprogramming, as it is observed following nuclear transfer to eggs 17 and oocytes 18 , in cell fusion experiments 19,20 , and in somatic nuclei exposed to egg extract 21 .…”

Recipient cell nuclear factors are required for reprogramming by nuclear transfer

“…In interphase, transcription factors localize to the nucleus, where they are in intimate association with the chromatin, regulating gene expression. When cells enter into mitosis, gene expression becomes repressed and these proteins are dispersed throughout the cytoplasm, allowing them to be equally inherited by each daughter cell (Egli et al, 2008;Gao et al, 2007;Gottesfeld and Forbes, 1997;Martinez-Balbas et al, 1995;Sun et al, 2007). We propose that it is the sum of these transcriptional regulators in a particular cell that define its cell-type-specific gene expression pattern and the transcriptional program that is engaged after its use as a recipient cell in nuclear transfer.…”

Oocyte-associated transcription factors in reprogramming after somatic cell nuclear transfer: a review

“…In mice, transcription arrest occurs at GV stage and is maintained until zygotic genome activation takes place approximately 9-10 h after fertilization [79][80][81]. Between cessation and resumption of transcription, the maternal transcription profile is reprogrammed to that observed in the embryos [82,83]. The mechanisms underlying transcription-reprogramming still warrant further investigation.…”

“…Three hypotheses have been proposed to explain transcription silencing during embryogenesis: (1) transcription before the mid-blastula transition is prevented by rapid cell cycling as shown in Xenopus during early development; (2) the presence of inhibitory factors in eggs represses transcription; and (3) a deficiency in, or absence of, critical transcriptional factors leads to transcriptional silencing [82,83]. It has been reported in mice that dynamic changes in transcriptional activity occurred when a nonsurrounding nucleolus changes to a surrounding nucleolus configuration [86,87].…”