microRNAs (miRNAs) are small non-coding RNA molecules (21-23 nucleotides) that have been identified as important conserved regulators of gene expression. In animals miRNAs usually bind to a complementary region within the 3’UTR of target mRNA molecules resulting either in target degradation o translational repression. miRNAs have been associated to a variety of diseases, cancer and the development of the nervous system, among other processes. Related to regeneration, miRNAs have been involved in liver and cardiac regeneration.
Now a recent paper from the laboratory of Jamie Ian Morrison has characterized the role of miRNAs during heart regeneration in newts (http://www.ncbi.nlm.nih.gov/pubmed/24055866). Contrary to mammals, newts (similarly to zebrafish) are capable of regenerating their heart. The authors divide newt heart regeneration into three main stages: early wounding response (7-14 dpi), hyperplasia (21-30 dpi) and reverse remodelling (45-60 dpi). They performed a microarray screen to identify miRNAs differentially expressed during heart regeneration. Initially, they identified 37 miRNAs that showed a significant expression pattern between 7 and 21 dpi. Then they decided to concentrate in miR-128 that showed a very high expression at a time-point when cardiac hyperplasia is elevated. In situ hybridizations experiment showed that in uninjured hearts miR-128 was expressed at very low levels in sporadic cardiomyocytes lining the epicardial border. In contrast, at 21 dpi there was a significant increase of miR-128 expression around the regenerative zone, not only in cardiomyocytes but also in non-cardiomyocytes. This peak of expression of miR-128 coincides with the peak of hyperplasia during heart regeneration.
In other models miR-128 has been described as a tumour suppressor gene but it has not been related to heart biology. Using an antagomir (miRNAs inhibitors that prevent the binding of miRNAs to their targets) knockdown strategy the authors found first a significant increase in the proliferation of non-cardiomyocyte cells in the regenerating heart, whereas no clear effect on the proliferation of cardiomyocytes was observed. These results suggest that during heart regeneration miR-128 would act as an inhibitor of proliferation of non-cardiomoycyte cells. Another consequence of miR-128 inhibition was that as regeneration proceeds there is a persistent presence of extracellular matrix and fibrin tissue within the regeneration zone. This excessive fibrin and collagen matrix deposition after miR-128 inhibition (in clear contrast to the little scarring observed in control regenerating hearts), resulted in a delayed regeneration. Therefore, miR-128 appears to have an important role regulation extracellular matrix deposition during heart regeneration.
Finally, the authors followed a bioinformatics approach to identify putative targets of miR-128 and found the transcription factor islet1 as a potential gene to be regulated by miR-128. These finding agree with previous results of the same laboratory that had identified islet1 as a factor differentially expressed during newt heart regeneration. Here, the authors report that, compared to uninjured hearts, the expression of islet1 significantly decreases at 21 dpi when the expression of miR-128 is maximum. Importantly, the expression of islet1 increased by a 60-fold when miR-128 was inhibited. As islet1 has been described as a chordate cardiac progenitor cell marker these results would fit with a model in which miR-128 would act, by inhibiting islet1, as a negative regulator of progenitor cell activity and would promote cell differentiation during heart regeneration.
In summary, this is the first report of miRNAs being involved in heart regeneration in newts and provide new actors and tools that can help to understand in the near future how these animals regenerate their heart.