The role of miRNA regulation in fetal cardiomyocytes, cardiac maturation and the risk of heart disease in adults

Abstract

Myocardial infarction is a primary contributor towards the global burden of cardiovascular disease. Rather than repairing the existing damage of myocardial infarction, current treatments only address the symptoms of the disease and reducing the risk of a secondary infarction. Cardiac regenerative capacity is dependent on cardiomyocyte proliferation, which concludes soon after birth in humans and precocial species such as sheep. Human fetal cardiac tissue has some ability to repair following tissue damage, whereas a fully matured human heart has minimal capacity for cellular regeneration. This is in contrast to neonatal mice and adult zebrafish hearts, which retain the ability to undergo cardiomyocyte proliferation and can regenerate cardiac tissue after birth. In mice and zebrafish models, microRNAs (miRNAs) have been implicated in the regulation of genes involved in cardiac cell cycle progression and regeneration. However, the significance of miRNA regulation in cardiomyocyte proliferation for humans and other large mammals, where the timing of heart development in relation to birth is similar, remains unclear. miRNAs may be valuable targets for therapies that promote cardiac repair after injury. Therefore, elucidating the role of specific miRNAs in large animals, where heart development closely resembles that of humans, remains vitally important for identifying therapeutic targets that may be translated into clinical practice focused on tissue repair.

Keywords: epigenetics; fetal development; heart attack; heart disease; miRNA; programming; regeneration.

Figure 1. Chronic heart disease timeline

(1) Hypertension and MI account for 75% of the total risk for chronic heart failure. (2) Using a combination of risk factors an absolute 5‐ or 10‐year individual risk for chronic heart disease can be calculated. While those patients at high risk can be readily identified using these criteria, the majority of MIs occur in a much larger pool of people who have intermediate risk with only one major risk factor (Krum et al. 2011). Most of the patients at intermediate risk do not experience a cardiac event until they are at an older age, and therefore lifestyle modifications as early as possible are important in reducing cardiovascular disease risk. (3) Cardiac remodelling occurs during chronic heart failure. (4) Treatments utilized and effectiveness varies from patient to patient, often requiring multiple treatments before stabilization of chronic heart failure. Some clinically stabilized patients will return to an unstable chronic heart failure state (red arrow). (5) Quality of life and survival rates for severe chronic heart failure are similar to the most common forms of cancer, with a case‐fatality rate of 75% over 5 years overall. CHF, chronic heart failure; MI, myocardial infarction. Adapted from Krum et al. (2011).

Figure 2. The transition of cardiomyocyte proliferation (green) to quiescence and hypertrophy (red) across gestation in zebrafish rodents, sheep and humans

Guinea pigs have not been included because the timing of the transition is not fully known. Adapted from Morrison et al. (2015).

Figure 3. Generalized comparison of oxygen availability across gestation in different species and the timing of cardiomyocyte quiescence

Quiescence in human and sheep cardiomyocytes starts to occur before birth when fetal $P_{\mathrm{a}{\mathrm{O}}_2}$ is still low. This transition occurs after birth in mice and rats. The much slower rise of neonatal $P_{\mathrm{a}{\mathrm{O}}_2}$ in these rodent species may reflect their underdeveloped lungs during the first two weeks of life. Zebrafish remain in a relatively hypoxic environment throughout development and after birth and retain the capacity for cardiomyocyte proliferation throughout their lifespan (Bensley et al. 2010; Jopling et al. 2010; Porrello et al. 2011b ; Mollova et al. 2013).

Figure 4. Proliferation of fetal cardiomyocytes is regulated by multiple signalling pathways that stimulate or inhibit cyclins and cytokinesis

Promotion of cell cycle progression is indicated by blue arrows and inhibition is indicated by red arrows. AKT, protein kinase B; Ang‐II, angiontensin II; AT‐R, angiotensin receptor; CDK, cyclin dependent kinase; ERK, extracellular signal‐related kinase; FGF1, fibroblast growth factor; FGFR, fibroblast growth factor receptor; G₀, gap zero phase (quiescent); G₁, cell cycle first gap phase; G₂, second gap phase; GSK‐3β, glycogen synthase kinase‐3β; IGF, insulin‐like growth factor; IGF‐1R, insulin‐like growth factor‐1 receptor; M, mitosis; NRG1, neuregulin‐1; PI3K, phosphoinositide‐3 kinase; S, DNA synthesis phase. Adapted from Botting et al. (2012).

Figure 5. Regulation of cardiomyocyte proliferation through signalling of FGF1, NRG1 and IGF1

Inhibition of the MAPK p38 in the presence of FGF1 or the activation of NRG1 signalling stimulates cardiomyocytes to re‐enter the cell cycle by activating PI3K causing DNA synthesis and subsequent cytokinesis. ERBB2, erb‐b2 receptor tyrosine kinase 2; ERBB4, erb‐b2 receptor tyrosine kinase 4; FGF1, fibroblast growth factor 1; FGFR, fibroblast growth factor receptor; IGF1, insulin‐like growth factor 1; IGF‐1R, insulin‐like growth factor 1 receptor; IGF2, insulin‐like growth factor 2; NRG1, neuregulin 1.

Figure 6. Regulation of cardiomyocyte proliferation via β‐catenin signalling

Activation of the Hippo pathway involves SAV, mammalian STE20‐like protein kinase 1 (MST1), MST2, MOB, and large tumour suppressor 1 and 2 (LATS1/2) and results in the phosphorylation and inactivation of the transcriptional co‐activator YAP impeding cardiomyocyte proliferation. Unphosphorylated YAP activates the WNT signalling pathway by interacting with β‐catenin, and consequently represses the Hippo kinases LATS2 and MST1/2 or the upstream scaffold protein SAV promotes cardiomyocyte proliferation. Suppression of YAP impedes cardiomyocyte proliferation, whereas overexpression of YAP stimulates cardiomyocyte proliferation via the triggering of pro‐growth signalling pathways such as WNT and insulin‐like growth factor 1 (IGF1). Axin, APC and GSK3β are components of the complex that phosphorylates β‐catenin to stimulate its degradation. Dashed line shows that additional molecules are involved. APC, adenomatosis polyposis coli; AXIN, axis inhibition protein; DVL, dishevelled; GPCR, G protein‐coupled receptor; GSK3β, glycogen synthase kinase 3β; IGF1R, IGF1 receptor; MOB, Mps one binder; SAV, Salvador; TCF‐LEF, T cell factor ‐ lymphoid enhancer‐binding factor; (P) represents phosphorylation. Adapted from Xin et al. (2013).

Figure 7. miRNAs can negatively (red) or positively (green) regulate cardiomyocyte proliferation

The miR‐15 family, including miR‐195 and miR‐133, inhibit the cell cycle and therefore cardiomyocyte proliferation by suppressing genes that stimulate cell cycle progression such as cyclin‐dependent kinases, checkpoint kinase 1 and fibroblast growth factor receptor. miR‐652, miR‐25, miR‐208a and the miR‐34 family act to suppress cardioprotective genes following myocardial infarction. miR‐590‐3p, miR‐199a‐3p, and miR‐17‐92 and miR‐302‐367 clusters stimulate cardiomyocyte proliferation by suppressing the expression of genes that inhibit cell proliferation such as HOMER1, HOPX and CLIC5 and members of the Hippo pathway (Montgomery et al. 2011; Porrello et al. 2011a ; Bernardo et al. 2012, 2014; Eulalio et al. 2012; Chen et al. 2013; Wahlquist et al. 2014; Tian et al. 2015).