In my opinion, for what it’s worth, we will probably see direct reprogramming take a prominent place in regenerative medicine in the future. It will not be in the near future, but as direct reprogramming becomes better understood and more feasible, it will probably become a central part of the discussion of regenerative medical strategies.
Direct reprogramming, which is also known as lineage conversion, uses cell type-specific transcription factors to convert a mature, adult cell into a different type of mature, adult cell. The cell does not pass through a pluripotent intermediate, and becomes a wholly different type of cell.
Of course, forcing the expression of lineage-specific transcription factors in cells requires that they be treated with recombinant viruses or other such tools. These genetic manipulations present problems for regenerative medicine, since such viruses can cause mutations or cause the introduced genes to be constantly activated, both of which can cause cells to die to grow uncontrollably. Genetically engineering cells needs to be done in a “kinder and gentler” way (to quote George HW Bush).
To that end Dennis Clegg and his colleagues from the Center for Stem Cell Biology and Engineering at UC Santa Barbara have used specially designed proteins to directly cultured retinal pigmented epithelial cells to neurons.
Newly discovered C-end rule (CendR) cell- and tissue-penetrating peptides have a arginine-rich sequence at the end of proteins that allows them to bind particular cell receptors and be internalized into the cell. These CendR peptides bind to the NRP-1 protein and are internalized. Several laboratories have used CendR peptides to increase the efficacy of anti-cancer drugs in experimental cases (see Alberici L, et al (2013) Cancer Res 73:804–812; Sugahara KN, et al. (2010) Science 328:1031–1035; Sugahara KN, et al. (2009) Cancer Cell 16:510–520; and Roth L,, et al. (2012) Oncogene 31:3754–3763).
By tacking a CendR peptide to the end of the Sox2 protein, Clegg and others were able to convert retinal pigmented epithelial (RPEs) cells to neurons. The Sox2 protein is highly expressed in neural progenitor cells. Other studies have shown that Sox2 can reprogram mouse and human fibroblasts to neural stem cells (Ring KL, et al. (2012) Cell Stem Cell 11:100–109). Thus, Sox2 should do the trick.
Making cultured RPE cells from embryonic stem cells is relatively easy to do. Therefore, Clegg and his coworkers made cultured RPEs and then treated them with viruses that expressed Sox2. The cultured RPEs showed conversion to neurons and the expression of neuron-specific genes.
Since they had established that Sox2 could convert RPEs to neurons, they tried recombinant Sox2 protein with the CendR peptide RPARPAR at the end of the protein. After 60 days in culture, the cells expressed a host of neuron-specific genes, and were capable of taking up a dye that only active neurons can take up (FM1-43).
The efficiency for this experiment was lousy (0.3%) as opposed to the efficiency for the use of recombinant viruses (11%). Nevertheless, this experiment shows that it is possible to directly reprogram cells without using recombinant viruses.