Nuclear Reprogramming and Phenotype in Cloned Embryos

点击翻译按钮获取中文摘要

DESCRIPTION (provided by applicant): The oocyte is uniquely endowed with the molecular machinery to convert gamete genomes to an embryonic genome. The oocyte is also endowed with a rich supply of maternal mRNA (MmRNA) to sustain the embryo until its genome is formed and becomes activated, and these MmRNAs are recruited for translation in a carefully orchestrated, stage-specific manner. Somatic cell nuclear transfer provides an innovative tool for exploring ooplasm-nucleus interactions, to gain insight into the early processes that initiate each new life, such as nuclear reprogramming and MmRNA regulation. During the previous award period we discovered that the temporal pattern of MmRNA degradation is disrupted in cloned embryos, revealing a novel role for the nucleus in controlling MmRNA recruitment and degradation. This defect is associated with a reduced total protein synthesis rate and over-expression of EIF4EBP1, which inhibits translation. In our first Aim, we will characterize the stage-specific expression and phosphorylation of EIF4EBP1 and EIF4E. Furthermore, we will manipulate these activities in cloned and fertilized embryos in order to dissect their contributions to MmRNA regulation during development, thereby gaining insight into how the nucleus controls MmRNA translation. We also discovered that >800 genes are over-expressed in cloned 2-cell embryos. The largest category of over-expressed genes encodes transcription factors (TFs) active in the donor cell genome, along with entire networks of their downstream target genes. We hypothesize that persistent expression of the donor cell repertoire of TFs creates a "ripple effect", directing aberrant expression of many genes, and creating the many aberrant characteristics of clones. We also hypothesize that transcription state in the donor nucleus anticipates gene expression after SCNT. In Aim 2, we will manipulate the expression and activities of two TFs (KLF4, CBX4) in cloned embryos and monitor effects on gene expression in order to test this hypothesis. We will also manipulate TF expression in donor cells and in normal embryos to determine the degree to which transcription state in the donor nucleus anticipates expression in the cloned embryo. This will provide new insight into parameters governing nuclear reprogramming, as well as a quantitative measure of actually reprogramming success for specific gene networks. We also observe that two of the four genes that can be used to convert somatic cells to induced pluripotent stem cells (iPS cells) in vitro are mis-expressed in clones compared to fertilized embryos; Klf4 is over-expressed in cloned embryos, while Sox2 is under-expressed. Another of these genes, Myc, is normally expressed at a low level that may be inadequate for reprogramming. In Aim 3 we will determine whether the genes employed to make iPS cells in vitro can also be employed to improve cloned embryo phenotype. Aims 2 and 3 will collectively, for the first time, address mechanisms and limitations of reprogramming of specific genes, advancing our understanding of "limited reprogramming" from an abstract concept to an understanding of specific molecular pathways that are affected. The results of manipulations of fertilized embryos in comparison to cloned embryos will also provide new insight into how networks of coordinately expressed genes are controlled by ontogenetic processes.