The discovery of human pluripotent stem cells (hPSCs), including both human embryonic stem cells and human-induced pluripotent stem cells, has opened up novel paths for a wide range of scientific studies. The capability to direct the differentiation of hPSCs into functional cardiomyocytes has provided a platform for regenerative medicine, development, tissue engineering, disease modeling, and drug toxicity testing. Despite exciting progress, achieving the optimal benefits has been hampered by the immature nature of these cardiomyocytes. Cardiac maturation has long been studied in vivo using animal models; however, finding ways to mature hPSC cardiomyocytes is only in its initial stages. In this review, we discuss progress in promoting the maturation of the hPSC cardiomyocytes, in the context of our current knowledge of developmental cardiac maturation and in relation to in vitro model systems such as rodent ventricular myocytes. Promising approaches that have begun to be examined in hPSC cardiomyocytes include long-term culturing, 3-dimensional tissue engineering, mechanical loading, electric stimulation, modulation of substrate stiffness, and treatment with neurohormonal factors. Future studies will benefit from the combinatorial use of different approaches that more closely mimic nature's diverse cues, which may result in broader changes in structure, function, and therapeutic applicability.

Engineering Adolescence Maturation of Human Pluripotent Stem Cell–Derived Cardiomyocytes

The adult mammalian heart has limited potential for regeneration. Thus, after injury, cardiomyocytes are permanently lost, and contractility is diminished. In contrast, the neonatal heart can regenerate owing to sustained cardiomyocyte proliferation. Identification of critical regulators of cardiomyocyte proliferation and quiescence represents an important step toward potential regenerative therapies. Yes-associated protein (Yap), a transcriptional cofactor in the Hippo signaling pathway, promotes proliferation of embryonic cardiomyocytes by activating the insulin-like growth factor and Wnt signaling pathways. Here we report that mice bearing mutant alleles of Yap and its paralog WW domain containing transcription regulator 1 (Taz) exhibit gene dosage-dependent cardiac phenotypes, suggesting redundant roles of these Hippo pathway effectors in establishing proper myocyte number and maintaining cardiac function. Cardiac-specific deletion of Yap impedes neonatal heart regeneration, resulting in a default fibrotic response. Conversely, forced expression of a constitutively active form of Yap in the adult heart stimulates cardiac regeneration and improves contractility after myocardial infarction. The regenerative activity of Yap is correlated with its activation of embryonic and proliferative gene programs in cardiomyocytes. These findings identify Yap as an important regulator of cardiac regeneration and provide an experimental entry point to enhance this process.

Hippo pathway effector Yap promotes cardiac regeneration

Ischemic cell death during a myocardial infarction leads to a multiphase reparative response in which the damaged tissue is replaced with a fibrotic scar produced by fibroblasts and myofibroblasts. This also induces geometrical, biomechanical, and biochemical changes in the uninjured ventricular wall eliciting a reactive remodeling process that includes interstitial and perivascular fibrosis. Although the initial reparative fibrosis is crucial for preventing rupture of the ventricular wall, an exaggerated fibrotic response and reactive fibrosis outside the injured area are detrimental as they lead to progressive impairment of cardiac function and eventually to heart failure. In this review, we summarize current knowledge of the mechanisms of both reparative and reactive cardiac fibrosis in response to myocardial infarction, discuss the potential of inducing cardiac regeneration through direct reprogramming of fibroblasts and myofibroblasts into cardiomyocytes, and review the currently available and potential future therapeutic strategies to inhibit cardiac fibrosis. Graphical abstract Reparative response following a myocardial infarction. Hypoxia-induced cardiomyocyte death leads to the activation of myofibroblasts and a reparative fibrotic response in the injured area. Right top In adult mammals, the fibrotic scar formed at the infarcted area is permanent and promotes reactive fibrosis in the uninjured myocardium. Right bottom In teleost fish and newts and in embryonic and neonatal mammals, the initial formation of a fibrotic scar is followed by regeneration of the cardiac muscle tissue. Induction of post-infarction cardiac regeneration in adult mammals is currently the target of intensive research and drug discovery attempts.

/pdf/cardiac-fibrosis-in-myocardial-infarction-from-repair-and-2k1stwz9tr.pdf

Cardiac fibrosis in myocardial infarction—from repair and remodeling to regeneration

https://www.cell.com/cell-stem-cell/pdf/S1934-5909(14)00004-6.pdf

Direct Reprogramming of Human Fibroblasts to Functional and Expandable Hepatocytes

As the adult mammalian heart has limited potential for regeneration and repair, the loss of cardiomyocytes during injury and disease can result in heart failure and death. The cellular processes and regulatory mechanisms involved in heart growth and development can be exploited to repair the injured adult heart through 'reawakening' pathways that are active during embryogenesis. Heart function has been restored in rodents by reprogramming non-myocytes into cardiomyocytes, by expressing transcription factors (GATA4, HAND2, myocyte-specific enhancer factor 2C (MEF2C) and T-box 5 (TBX5)) and microRNAs (miR-1, miR-133, miR-208 and miR-499) that control cardiomyocyte identity. Stimulating cardiomyocyte dedifferentiation and proliferation by activating mitotic signalling pathways involved in embryonic heart growth represents a complementary approach for heart regeneration and repair. Recent advances in understanding the mechanistic basis of heart development offer exciting opportunities for effective therapies for heart failure.

/pdf/mending-broken-hearts-cardiac-development-as-a-basis-for-4qfc3lfdu0.pdf

Mending broken hearts: cardiac development as a basis for adult heart regeneration and repair

Reprogramming of mouse fibroblasts toward a myocardial cell fate by forced expression of cardiac transcription factors or microRNAs has recently been demonstrated The potential clinical applicability of these findings is based on the minimal regenerative potential of the adult human heart and the limited availability of human heart tissue An initial but mandatory step toward clinical application of this approach is to establish conditions for conversion of adult human fibroblasts to a cardiac phenotype Toward this goal, we sought to determine the optimal combination of factors necessary and sufficient for direct myocardial reprogramming of human fibroblasts Here we show that four human cardiac transcription factors, including GATA binding protein 4, Hand2, T-box5, and myocardin, and two microRNAs, miR-1 and miR-133, activated cardiac marker expression in neonatal and adult human fibroblasts After maintenance in culture for 4–11 wk, human fibroblasts reprogrammed with these proteins and microRNAs displayed sarcomere-like structures and calcium transients, and a small subset of such cells exhibited spontaneous contractility These phenotypic changes were accompanied by expression of a broad range of cardiac genes and suppression of nonmyocyte genes These findings indicate that human fibroblasts can be reprogrammed to cardiac-like myocytes by forced expression of cardiac transcription factors with muscle-specific microRNAs and represent a step toward possible therapeutic application of this reprogramming approach

https://www.pnas.org/content/pnas/110/14/5588.full.pdf

Reprogramming of human fibroblasts toward a cardiac fate

Background Aspirin is widely administered to prevent cardiovascular disease (CVD). However, appropriate use of aspirin depends on patient understanding of its risks, benefits, and indications, especially where aspirin is available over the counter (OTC). Methods and Results We did a survey of patient‐reported 10‐year cardiovascular risk; aspirin therapy status; form of aspirin access (OTC versus prescription); and knowledge of the risks, benefits, and role of aspirin in CVD prevention. Consecutive adults aged ≥50 years with ≥1 cardiovascular risk factor attending outpatient clinics in America and Europe were recruited. We also systematically reviewed national policies regulating access to low‐dose aspirin for CVD prevention. At each site, 150 responses were obtained (300 total). Mean±SD age was 65±10 years, 40% were women, and 41% were secondary prevention patients. More than half of the participants at both sites did not know (1) their own level of 10‐year CVD risk, (2) the expected magnitude of reduction in CVD risk with aspirin, or (3) aspirin’s bleeding risks. Only 62% of all participants reported that aspirin was routinely indicated for secondary prevention, whereas 47% believed it was routinely indicated for primary prevention (P=0.048). In America, 83.5% participants obtained aspirin OTC compared with 2.5% in Europe (P<0.001). Finally, our review of European national policies found only 2 countries where low‐dose aspirin was available OTC. Conclusions Many patients have poor insight into their objectively calculated 10‐year cardiovascular risk and do not know the risks, benefits, and role of aspirin in CVD prevention. Aspirin is mainly obtained OTC in America in contrast to Europe, where most countries restrict access to low‐dose aspirin.

https://www.ahajournals.org/doi/pdf/10.1161/JAHA.121.023995

Contextualizing National Policies Regulating Access to Low‐Dose Aspirin in America and Europe Using the Full Report of a Transatlantic Patient Survey of Aspirin in Preventive Cardiology

Left atrial appendage occlusion (LAAO) is effective in preventing thromboembolism. Risk stratification tools could help identify patients at risk for early mortality after LAAO. In this study, we validated and recalibrated a clinical risk score (CRS) to predict risk of all-cause mortality after LAAO. This study used data from patients who underwent LAAO in a single-center, tertiary hospital. A previously developed CRS using 5 variables (age, body mass index [BMI], diabetes, heart failure, and estimated glomerular filtration rate) was applied to each patient to assess risk of all-cause mortality at 1 and 2 years. The CRS was recalibrated to the present study cohort and compared with established atrial fibrillation-specific (CHA2DS2-VASc and HAS-BLED) and generalized (Walter index) risk scores. Cox proportional hazard models were used to assess the risk of mortality and discrimination was assessed by Harrel C-index. Among 223 patients, the 1- and 2-year mortality rates were 6.7% and 11.2%, respectively. With the original CRS, only low BMI (<23 kg/m2) was a significant predictor of all-cause mortality (hazard ratio [HR] [95% CI] 2.76 [1.03 to 7.35]; p = 0.04). With recalibration, BMI <29 kg/m2 and estimated glomerular filtration rate <60 ml/min/1.73 m2 were significantly associated with an increased risk of death (HR [95% CI] 3.24 [1.29 to 8.13] and 2.48 [1.07 to 5.74], respectively), with a trend toward significance noted for history of heart failure (HR [95% CI] 2.13 [0.97 to 4.67], p = 0.06). Recalibration improved the discriminative ability of the CRS from 0.65 to 0.70 and significantly outperformed established risk scores (CHA2DS2-VASc = 0.58, HAS-BLED = 0.55, Walter index = 0.62). In this single-center, observational study, the recalibrated CRS accurately risk stratified patients who underwent LAAO and significantly outperformed established atrial fibrillation-specific and generalized risk scores. In conclusion, clinical risk scores should be considered as an adjunct to standard of care when evaluating a patient's candidacy for LAAO. 

Risk Stratification in Patients Who Underwent Percutaneous Left Atrial Appendage Occlusion.

Electrocardiographic P-wave Parameters and Lifetime Atrial Fibrillation Risk: A Multi-Cohort Study.

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Reprogramming of human fibroblasts toward a cardiac fate

Contextualizing National Policies Regulating Access to Low‐Dose Aspirin in America and Europe Using the Full Report of a Transatlantic Patient Survey of Aspirin in Preventive Cardiology

Risk Stratification in Patients Who Underwent Percutaneous Left Atrial Appendage Occlusion.

Electrocardiographic P-wave Parameters and Lifetime Atrial Fibrillation Risk: A Multi-Cohort Study.