In amphibians, early steps for a successful regenerative response imply the de-differentiation of differentiated cell types and their re-entry into the cell cycle. A good example are newt myotubes that upon serum stimulation are induced to reprogram by dedifferentiating and re-entering the cell cycle, this last being dependent on the phosphorylation of Rb (retinoblastoma) and the downregulation of p53 activity. A recent paper from the laboratory of Maximina Yun and Jeremy Brockes analyses the role of ERK (extracellular signal-regulated kinase) signalling during newt myotubes reprogramming and how it may differ in muscle cells from regeneration-incompetent animals (http://www.ncbi.nlm.nih.gov/pubmed/25068118).
The first thing they saw was that serum stimulation of myotubes triggered a fast activation of ERK signalling that was sustained for up to 48 h. In addition to ERK, other MAPK pathways, such as JNK and p38, were also activated although at a much lower level. Then, the authors analysed whether the activation of those pathways was required for the re-entry to the cell cycle. By using different specific inhibitors of ERK, JNK and p38 alongside with serum stimulation, they found a differential disruption of Rb phosphorylation and cell cycle re-entry. The highest impairment was seen after ERK inhibition, which suggests that the activation of this pathway is critical for cell-cycle re-entry. The inhibition of ERK even at 24 h post serum stimulation impaired Rb phosphorylation suggesting that a sustained ERK activity is required for reprogramming.
Previous studies have shown that a sustained ERK activity results in the downregulation of Gadd45, a p53 target. Moreover, this same group has recently shown that the downregulation of p53 is a necessary step for newt myotube cell cycle re-entry. Here, a series of experiments combining ERK inhibition with p53 stabilization or inhibition suggests that the action of ERK signalling on cell cycle re-entry is mediated, at least in part, by downregulating p53 activity. This is further supported by the fact that ERK inhibition abrogated the downregulation of Gadd45 induced by serum stimulation. Next, the authors sought to determine whether ERK activity was also necessary to promote cell dedifferentiation in addition to cell cycle re-entry. To do this, they analysed the expression of Sox6, a muscle-specific gene. Upon serum stimulation the expression of this gene was downregulated, however, ERK inhibition abrogated this downregulation. Moreover, they also studied the effects of ERK inhibition on epigenetic changes that occurred in myotubes upon serum stimulation. The levels of expression of the repressive histone mark dimethyl H3K9 decreases upon serum stimulation. However, this decrease is abrogated by ERK inhibition. Taken into account that in other models it has been show that the demethylation of H3K9 is required for cell cycle progression and the expression of pluripotency-associated genes, the authors suggest here that ERK dependent-H3K9 demethylation in newt myotubes may provide a favorable environment for their reprogramming.
Finally, the authors compared these changes in ERK activity in newt myotubes with the response to serum stimulation of mouse myotubes. Upon serum stimulation, ERK was transiently activated in mouse myotubes at 1 h post induction, but then the levels went down to baseline after 3 h. In addition, no changes in the repression marker dimethyl H3K9 were observed. These results suggest that the extent of ERK signalling could underlie differences in the regenerative capabilities shown by salamander and mammalian cells.
In summary, the authors propose here that a sustained activation of ERK signalling leads to the downregulation of p53 activity, which would facilitate cell cycle re-entry through Rb phosphorylation as well as alterations in the gene expression landscape facilitating also cell dedifferentiation. Future experiment should try to determine the upstream tyrosine kinase receptor that activates ERK as well as the serum component responsible of such activation, and subsequent reprogramming of newt myotubes.