Partial reprogramming involves exposing cells to the Yamanaka factors capable of turning somatic cells into induced pluripotent stem cells, but not for so long as to result in that transformation. The initial stage of reprogramming, prior to transformation into stem cells, in which epigenetic marks are reset to a youthful configuration, is the desirable outcome. This results in rejuvenation of cell function, as protein production and the operation of cellular processes return to that of youth. This cannot repair DNA damage, and will probably help little with problems relating to persistent metabolic waste in long-lived cells, but the evidence from animal studies suggests that partial reprogramming can be beneficial enough to form the basis for a true rejuvenation therapy.
Partial reprogramming is a relatively new line of research and development, through very well funded of late. Numerous biotech companies are working towards the production of therapies based upon partial reprogramming techniques. These are first generation efforts, however, and the research community will inevitably improve upon the protocols of partial reprogramming, even as the first commercial efforts move towards the clinic. Today's research materials are an example of the sort of work taking place in this part of the field: attempts to optimize partial reprogramming in cell culture, alongside better measurements of the degree to which benefits to cell function are produced.
The full process of stem cell reprogramming takes around 50 days using four key molecules called the Yamanaka factors. The new method, called 'maturation phase transient reprogramming', exposes cells to Yamanaka factors for just 13 days. At this point, age-related changes are removed and the cells have temporarily lost their identity. The partly reprogrammed cells were given time to grow under normal conditions, to observe whether their specific skin cell function returned. Genome analysis showed that cells had regained markers characteristic of skin cells (fibroblasts), and this was confirmed by observing collagen production in the reprogrammed cells.
Researchers looked at multiple measures of cellular age. The first is the epigenetic clock, where chemical tags present throughout the genome indicate age. The second is the transcriptome, all the gene readouts produced by the cell. By these two measures, the reprogrammed cells matched the profile of cells that were 30 years younger compared to reference data sets. "Our results represent a big step forward in our understanding of cell reprogramming. We have proved that cells can be rejuvenated without losing their function and that rejuvenation looks to restore some function to old cells. The fact that we also saw a reverse of ageing indicators in genes associated with diseases is particularly promising for the future of this work."
Somatic cell reprogramming, the process of converting somatic cells to induced pluripotent stem cells (iPSCs), can reverse age-associated changes. However, during iPSC reprogramming, somatic cell identity is lost, and can be difficult to reacquire as re-differentiated iPSCs often resemble foetal rather than mature adult cells. Recent work has demonstrated that the epigenome is already rejuvenated by the maturation phase of reprogramming, which suggests full iPSC reprogramming is not required to reverse ageing of somatic cells. Here we have developed the first 'maturation phase transient reprogramming' (MPTR) method, where reprogramming factors are expressed until this rejuvenation point followed by withdrawal of their induction.
Using dermal fibroblasts from middle age donors, we found that cells temporarily lose and then reacquire their fibroblast identity during MPTR, possibly as a result of epigenetic memory at enhancers and/or persistent expression of some fibroblast genes. Excitingly, our method substantially rejuvenated multiple cellular attributes including the transcriptome, which was rejuvenated by around 30 years as measured by a novel transcriptome clock. The epigenome, including H3K9me3 histone methylation levels and the DNA methylation ageing clock, was rejuvenated to a similar extent.
The magnitude of rejuvenation instigated by MTPR appears substantially greater than that achieved in previous transient reprogramming protocols. In addition, MPTR fibroblasts produced youthful levels of collagen proteins, and showed partial functional rejuvenation of their migration speed. Finally, our work suggests that more extensive reprogramming does not necessarily result in greater rejuvenation but instead that optimal time windows exist for rejuvenating the transcriptome and the epigenome. Overall, we demonstrate that it is possible to separate rejuvenation from complete pluripotency reprogramming, which should facilitate the discovery of novel anti-ageing genes and therapies.