Direct reprogramming of mouse fibroblasts into induced pluripotent stem cells (iPS)
was achieved recently by overexpression of four transcription factors encoded by retroviral
vectors. Most of the virus vectors, however, may cause insertional mutagenesis in
the host genome and may also induce tumor formation. Therefore, it is very important
to discover novel and safer, non-viral reprogramming methods. Here we describe the
reprogramming of somatic cells into iPS cells by a novel protein-based technique.
Engineered Oct4, Sox2 and Klf4 transcription factors carrying an N-terminal Flag-tag
and a C-terminal polyarginine tail were synthesized by a recently described mammalian
artificial chromosome expression system (ACEs). This system is suitable for the high-level
production of recombinant proteins in mammalian tissue culture cells. Recombinant
proteins produced in this system contain all the post-translational modifications
essential for the stability and the authentic function of the proteins. The engineered
Oct4, Sox2 and Klf4 proteins efficiently induced the reprogramming of mouse embryonic
fibroblasts by means of protein transduction. This novel method allows for the generation
of iPS cells, which may be suitable for therapeutic applications in the future.
