Epigenetic rejuvenation

Abstract

Induced pluripotent stem (iPS) cells have provided a rational means of obtaining histo-compatible tissues for 'patient-specific' regenerative therapies (Hanna et al. 2010; Yamanaka & Blau 2010). Despite the obvious potential of iPS cell-based therapies, there are certain problems that must be overcome before these therapies can become safe and routine (Ohi et al. 2011; Pera 2011). As an alternative, we have recently explored the possibility of using 'epigenetic rejuvenation', where the specialized functions of an old cell are rejuvenated in the absence of any change in its differentiated state (Singh & Zacouto 2010). The mechanism(s) that underpin 'epigenetic rejuvenation' are unknown and here we discuss model systems, using key epigenetic modifiers, which might shed light on the processes involved. Epigenetic rejuvenation has advantages over iPS cell techniques that are currently being pursued. First, the genetic and epigenetic abnormalities that arise through the cycle of dedifferentiation of somatic cells to iPS cells followed by redifferentiation of iPS cells into the desired cell type are avoided (Gore et al. 2011; Hussein et al. 2011; Pera 2011): epigenetic rejuvenation does not require passage through the de-/redifferentiation cycle. Second, because the aim of epigenetic rejuvenation is to ensure that the differentiated cell type retains its specialized function it makes redundant the question of transcriptional memory that is inimical to iPS cell-based therapies (Ohi et al. 2011). Third, to produce unrelated cell types using the iPS technology takes a long time, around three weeks, whereas epigenetic rejuvenation of old cells will take only a matter of days. Epigenetic rejuvenation provides the most safe, rapid and cheap route to successful regenerative medicine.

Figure 1

Epigenetic rejuvenation bypasses the ES/iPS cell stage. Box 1 describes the characteristics of an old senescent cell. Senescent cells possess chromatin damage, telomere attrition, morphological changes in shape, dysfunction in DNA repair mechanisms, oxidative damage, increased levels of the cell cycle inhibitors p21 and p53, increased levels of the epigenetic modifiers heterochromatin protein 1 (HP1β) and mH2A (variant histone macro H2A), presence of SAHF (senescent-associated heterochromatin foci) (depicted as large ‘dots’ in the old cell), and expression of β-galactosidase activity. The horizontal arrow depicts the dedifferentiation pathway from the senescent cell to the ES/iPS cell stage after somatic cell nuclear transfer (SCNT) or introduction of the ‘reprogramming factors’. Epigenetic rejuvenation of old cells (diagonal arrow) can be achieved by ‘conditioning’ of old cells within the oocyte cytoplasm via SCNT as explained in the text. For simplicity, we have not shown somatic cell transfer into the germinal vesicle (GV) of frog oocytes but GV system does represent another model system for the study of ‘epigenetic rejuvenation’, as detailed in the text. The vertical arrow downwards depicts the redifferentiation of the ES/iPS cells into a young differentiated cell type, which has reversed or repaired all the senescence characteristics of the old cell (see box 2). The young cell has lost its SAHF.

Figure 2

iPS system for the study of epigenetic rejuvenation. In (A) is shown the relationship of epigenetic instability (the y-axis) after introduction of the ‘reprogramming factors’ to the developmental potential of cells (the x-axis). After introduction of the reprogramming factors, cells set out on the path toward iPS cells and pass, after 4–7 days a zone of ‘epigenetic instability’ where the fate of the cells undergoing reprogramming is pliant. Using defined media, the ‘unstable’ cells can be forced down different developmental pathways giving rise to other cell linages (Efe et al. 2011). Of interest is the fact that if the expression of the reprogramming factors is silenced in the ‘zone of epigenetic instability’, the cells return back to being fibroblasts (Nagy & Nagy 2010). In (B), reprogramming factors have been introduced into an old senescent fibroblast (with SAHF represented by ‘dots’ in the nucleus) that has a low potential to age (y-axis) as it is already old. It then sets out on the path toward becoming an iPS cell. As it does so its epigenetic instability increases (x-axis) and passes through the ‘zone of epigenetic instability’. On reaching the ‘zone of epigenetic instability’, reprogramming factor expression is silenced and the cells return back to the fibroblast ‘ground-state’. In the diagram given, the fibroblast produced after passage through the ‘zone of epigenetic instability’ is a young fibroblast (with no SAHF) with a high potential to age. This is to be tested.