47BOVINE OOCYTE CYTOPLASM SUPPORTS NUCLEAR REMODELING BUT NOT REPROGRAMMING OF MURINE FIBROBLASTS

Abstract

Nuclear transfer (NT) is used to elucidate fundamental biological functions such as cell differentiation reversibility and interactions between the cytoplasm and nucleus. In the present study, we compared nuclear and microtubule dynamics in bovine oocytes following NT of bovine and murine fibroblast cells. To clarify the nuclear reprogramming procedures, we additionally examined the expression of development-related genes (Octamer-binding transcription factors, Oct-4; E-cadherin, E-cad) and housekeeping genes (Heat shock protein 70.1, Hsp; Bos taurus apoptosis regulator box-a, Bax; Glucose transporter 1, Glut-1) in bovine embryos that had received nuclei from bovine and murine fibroblast cells. Bovine oocytes were matured in vitro and enucleated after 22h. The oocytes reconstructed with mouse embryonic fibroblast cells or bovine somatic fibroblast cells were cultured in CR1aa media. While the embryos that received nuclei from bovine fibroblast cells developed into blastocysts, those that received nuclei from murine fibroblasts did not develop beyond the 8-cell stage. Similar nuclear and microtubule dynamics were observed in oocytes reconstructed with murine and bovine fibroblast cells. A small microtubule aster-containing γ-tubulin spot was observed in association with decondensed chromatin following NT of mouse fibroblasts. Within 1h of fusion of enucleated, non-activated cytoplasm, most mouse fibroblast nuclei were transformed to premature chromosome condensation (PCC). Randomly arrayed microtubules were tightly associated with PCC and formed meiotic-like microtubular spindles in all cases. Condensed chromosomes were divided into two or three chromatin masses and developed into multiple pronuclear-like structures. Microtubule asters were observed near the pronuclear-like structures during apposition in the cytoplasm. Two poles of the γ-tubulin spot evident at the mitotic metaphase stage are involved in the formation of the astral microtubule spindle for initial mitosis. A number of housekeeping mouse genes (hsp70, bax and glt-1) were abnormally expressed in embryos that had received nuclei from mouse fibroblast cells. However, development-related genes, such as Oct-4 and E-cad, were not expressed. The results collectively suggest that the bovine oocyte cytoplasm supports nuclear remodeling, but not reprogramming of mouse fibroblast cells. Table 1 Relative abundance of mRNA expression (mean±SEM) in mouse and xenonuclear-transferred (X-NT) embryos