Abstract 107: The Mevalonate Pathway Controls Cardiomyocyte Proliferation

Abstract

Induction of cardiomyocyte proliferation is emerging as a promising strategy for heart regeneration. Using a recently developed human cardiac organoid system, we have recently discovered a small molecule (Compound 6.28) that inhibits both GSK3 and MST1 and drives robust proliferation (Mills et al., PNAS, 2017). However, chronic stimulation with this compound also causes a decline in contractile function, which would prevent its use as a therapeutic. We have now performed quantitative proteomics of 50 individual organoids under different conditions to de-convolute the effects of GSK3 and MST1 on the pro-proliferative response. GSK3 inhibition was found to be responsible for a cell cycle network whereas MST1 inhibition activated the mevalonate pathway; the detrimental functional effects were specifically associated with inhibition of GSK3. An additional screen of 105 compounds was performed to identify new compounds that could activate proliferation in human cardiac organoids without detrimental effects on functionality. We identified a compound targeting p38, which activated a similar cell cycle network to GSK3, and another compound targeting TGFBR/ACVR/BMPR, which activated the mevalonate pathway similar to MST1 inhibition. A strong synergistic pro-proliferative response was observed when both of these compounds were combined, without any detrimental effects on cardiac function. The pro-proliferative effect could be blocked by inhibition of β-catenin using ICRT14 or inhibition of the mevalonate pathway using a statin. In addition, we provide RNA-seq data revealing large scale transcriptional changes in the mevalonate pathway during heart maturation in vivo and demonstrate that blocking the mevalonate pathway in immature cardiomyocytes reduces proliferation. By re-supplementing different metabolites in the mevalonate pathway, a specific function for the prenylation branch of this metabolic pathway for proliferation was identified and direct inhibition of a downstream kinase activated by Rho prenylation, ROCK, also lead to a reduction in proliferation. Our results reveal a novel role for the mevalonate pathway in cardiomyocyte proliferation and suggest that inhibition of this pathway occurs during postnatal heart maturation.