Stem Cells Using HSPG Uptake of Recombinant Transcription Factors

Stem Cells from Adult Cells using Transcription Factor Genes
Stem cells have been produced from adult cells using transformation with genes for transcription factors. The problem with this approach was that the embryonic transcription factors had a tendency to promote cancer-like proliferation. What was needed was a temporary push toward embryonic gene expression by temporarily introducing a dose of embryonic transcription factors to dominate gene expression long enough to convert adult, differentiated cells into pluripotent stem cells.

Transcription Factors Synthesized by Recombinant Bacteria
The technical solution was tested and successful results were announced in a prior to publication paper in the journal Cell Stem Cell. Four transcription factors successfully used in prior experiments to induce stem cell transformation were synthesized using recombinant bacteria. The problem was getting the proteins into skin cells that were already growing in cell culture.

Protein Uptake via Triplets of Basic Amino Acids (Heparin-Binding Domains, NLS)
Transcription factors bind to DNA via basic amino acids and many of those basic amino acids are parts of the nuclear localization signals (NLS, quartet or two neighboring pairs of basic amino acids) that bind to importin and transport transcription factors from the cytoplasm to the nucleus.

HSPG Circulation Should Take in Transcription Factors
By inspection, I have demonstrated that proteins observed to be taken up by cells, without specific receptors, e.g. HIV-TAT, lactoferrin, heparanase, allergens, autoantigens, have triplets (or neighboring pairs) or basic amino acids, and this uptake is inhibited by heparin. One would expect that transcription factors would be naturally taken into cells by HSPG circulation. Just adding recombinant transcription factors to cultured skin cells should transform them into stem cells. I don’t believe that this was tested. Instead, more powerful heparin-binding domains were added.

Poly Arginine was used for Uptake of Transcription Factors
The investigators ensured a high efficiency of uptake by adding potent poly arginine sequences to the ends of the transcription factors and synthesized them in recombinant bacteria. The recombinant, arg-tailed transcription factors were taken up by the cultured skin cells and changed the pattern of gene expression in the skin cells. The cultured cells reverted to embryonic patterns of gene expression of pluripotent stem cells. The recombinant proteins were eventually metabolized, but the stem cells had been stably transformed.

reference:
Zhou, H. et al., Generation of Induced Pluripotent Stem Cells Using Recombinant Proteins, Cell Stem Cell (2009), ahead of publication 04.005