Nuclear-cytoplasmic "tug of war" during cloning: effects of somatic cell nuclei on culture medium preferences of preimplantation cloned mouse embryos.

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Abstract
Cloning by somatic cell nuclear transfer is critically dependent upon early events that occur immediately after nuclear transfer, and possibly additional events that occur in the cleaving embryo. Embryo culture conditions have not been optimized for cloned embryos, and the effects of culture conditions on these early events and the successful initiation of clonal development have not been examined. To evaluate the possible effect of culture conditions on early cloned embryo development, we have compared a number of different culture media, either singly or in sequential combinations, for their ability to support preimplantation development of clones produced using cumulus cell nuclei. We find that glucose is beneficial during the 1-cell stage when CZB medium is employed. We also find that potassium simplex optimized medium (KSOM), which is optimized to support efficient early cleavage divisions in mouse embryos, does not support development during the 1-cell or 2-cell stages in the cloned embryos as well as other media. Glucose-supplemented CZB medium (CZB-G) supports initial development to the 2-cell stage very well, but does not support later cleavage stages as well as Whittten medium or KSOM. Culturing cloned embryos either entirely in Whitten medium or initially in Whittens medium and then changing to KSOM at the late 4-cell/early 8-cell stage produces consistent production of blastocysts at a greater frequency than using CZB-G medium alone. The combination of Whitten medium followed by KSOM resulted in an increased number of cells per blastocyst. Because normal embryos do not require glucose during the early cleavage stages and develop efficiently in all of the media employed, these results reveal unusual culture medium requirements that are indicative of altered physiology and metabolism in the cloned embryos. The relevance of this to understanding the kinetics and mechanisms of nuclear reprogramming and to the eventual improvement of the overall success in cloning is discussed.