doi:10.1155/2012/541014 Research Article Temporal Requirements of cMyc Protein for Reprogramming Mouse Fibroblasts

License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Exogenous expression of Oct4, Sox2, Klf4, and cMyc forces mammalian somatic cells to adopt molecular and phenotypic char-acteristics of embryonic stem cells, commencing with the required suppression of lineage-associated genes (e.g., Thy1 in mouse). Although omitting cMyc from the reprogramming cocktail minimizes risks of uncontrolled proliferation, its exclusion results in fold reductions in reprogramming efficiency. Thus, the feasibility of substituting cMyc transgene with (non-integrative) recom-binant “pTAT-mcMyc ” protein delivery was assessed, without compromising reprogramming efficiency or the pluripotent phe-notype. Purification and delivery of semisoluble/particulate pTAT-mcMyc maintained Oct4-GFP+ colony formation (i.e., repro-gramming efficiency) whilst supporting pluripotency by various criteria. Differential repression of Thy1 by pTAT-mcMyc±Oct4, Sox2, and Klf4 (OSK) suggested differential (and non-additive) mechanisms of repression. Extending these findings, attempts to enhance reprogramming efficiency through a staggered approach (prerepression of Thy1) failed to improve reprogramming efficiency. We consider protein delivery a useful tool to decipher temporal/molecular events characterizing somatic cell reprogramming. 1