Following severe myocardial assault, such as myocardial infarction (MI), cardiomyocytes perish and are never replaced. The permanent loss of these contractile cells leads to deteriorated heart function and ultimately heart failure. Hence, there has been intensive interest in uncovering the molecular aetiology that control myocyte proliferative capacity to augment adult cardiomyocyte turnover for heart regeneration. We recently showed that transgenic expression of the pro-proliferative transcription factor Myc and positive transcription elongation factor (P-TEFb) complex elements, namely Cyclin T1, drives substantial adult cardiomyocyte proliferation. However, the sustained whole heart expression leads to functional decline and death over 4 days.
In this current study, we hypothesised that careful titration of the dose and transient timing of Myc and Cyclin T1 may be required to sustain cardiomyocyte proliferation and cardiac regeneration in vivo. Human cardiac tissues derived from pluripotent stem cells were exposed to modified mRNA (modRNA) containing Myc and Cyclin T1 sequences. The proteomic response was monitored at 1, 4, 18 and 48hrs post-transfection to identify the pathways stimulated through ectopic expression of these mitogens. Whilst the protein abundance of Myc and CyclinT1 is transient and depleted within 48hrs, the downstream proliferative and associated pathways are activated at the later time points. Cell counts and microscopy confirmed a cardiomyocyte cell growth signature at 48hrs. Using a mouse MI model, we then tested the regenerative capacity of transient Myc and CyclinT1 through intramyocardial modRNA injection following traumatic injury. Alike the stem cell model, transient expression was observed, with DNA synthesis and mitosis assays confirming proliferative effects within 24hrs. At 28 days post-MI, a demonstrable improvement to cardiac function can be observed by increased ejection fraction and left ventricle size due to Myc-CyclinT1 expression compared to control mice.
Collectively, these findings indicate a single transient dose of Myc-CyclinT1 is sufficient to induce cardiomyocyte proliferation and combats deleterious effects of MI. The clinical potential for modRNAs has recently been demonstrated where combination with refined modRNA delivery platforms for heart-specific targeting enable the potential for Myc-CyclinT1 modRNA as an effective therapeutic.