Thyroid hormone-dependent regulation of metabolism and heart regeneration

While adult zebrafish and newborn mice possess a robust capacity to regenerate their hearts, this ability is generally lost in adult mammals. The logic behind the diversity of cardiac regenerative capacity across the animal kingdom is not well understood. We have recently reported that animal metabolism is inversely correlated to the abundance of mononucleated diploid cardiomyocytes in the heart, which retain proliferative and regenerative potential. Thyroid hormones are classical regulators of animal metabolism, mitochondrial function, and thermogenesis and a growing body of scientific evidence demonstrates that these hormonal regulators also have direct effects on cardiomyocyte proliferation and maturation. We propose that thyroid hormones dually control animal metabolism and cardiac regenerative potential through distinct mechanisms, which may represent an evolutionary tradeoff for the acquisition of endothermy and loss of heart regenerative capacity. In this review, we describe the effects of thyroid hormones on animal metabolism and cardiomyocyte regeneration, and highlight recent reports linking the loss of mammalian cardiac regenerative capacity to metabolic shifts occurring after birth.

Improving Cardiac Reprogramming for Heart Regeneration in Translational Medicine

Direct reprogramming of fibroblasts into CM-like cells has emerged as an attractive strategy to generate induced CMs (iCMs) in heart regeneration. However, low conversion rate, poor purity, and the lack of precise conversion of iCMs are still present as significant challenges. In this review, we summarize the recent development in understanding the molecular mechanisms of cardiac reprogramming with various strategies to achieve more efficient iCMs. reprogramming. Specifically, we focus on the identified critical roles of transcriptional regulation, epigenetic modification, signaling pathways from the cellular microenvironment, and cell cycling regulation in cardiac reprogramming. We also discuss the progress in delivery system optimization and cardiac reprogramming in human cells related to preclinical applications. We anticipate that this will translate cardiac reprogramming-based heart therapy into clinical applications. In addition to optimizing the cardiogenesis related transcriptional regulation and signaling pathways, an important strategy is to modulate the pathological microenvironment associated with heart injury, including inflammation, pro-fibrotic signaling pathways, and the mechanical properties of the damaged myocardium. We are optimistic that cardiac reprogramming will provide a powerful therapy in heart regenerative medicine.

Blood flow modeling reveals improved collateral artery performance during the regenerative period in mammalian hearts

Collateral arteries are a vessel subtype that bridges two artery branches, forming a natural bypass that can deliver blood flow downstream of an occlusion. These bridges in the human heart are associated with better outcomes during coronary artery disease. We recently found that their rapid development in neonates supports heart regeneration, while the non-regenerative adult heart displays slow and minimal collateralization. Thus, inducing robust collateral artery networks could serve as viable treatment for cardiac ischemia, but reaching this goal requires more knowledge on their developmental mechanisms and functional capabilities. Here, we use whole-organ imaging and 3D computational fluid dynamics (CFD) modeling to identify the spatial architecture of and predict blood flow through collaterals in neonate and adult hearts. We found that neonate collaterals are more numerous, larger in diameter, and, even when similar in size/number, are predicted to more effectively re-perfuse an occluded coronary network when compared to adults. CFD analysis revealed that collaterals perform better in neonates because of decreased differential pressures along their coronary artery tree. Furthermore, testing of various collateral configurations indicated that larger, more proximal collaterals are more beneficial than many smaller ones, identifying a target architecture for therapeutic interventions. Morphometric analysis revealed how the coronary artery network expands during postnatal growth. Vessel diameters do not scale with cardiac muscle growth. Instead, the coronary tree expands solely by adding additional branches of a set length, a burst of which occurs during murine puberty. Finally, we compared mouse structural and functional data to human hearts. Surprisingly, fetal human hearts possessed a very large number of small, but mature, smooth muscle cell covered collaterals while angiogram data indicated adult patients with chronic coronary occlusions contained at least two. Comparing size ratios with modeled mouse data suggested low re-perfusion capabilities of the embryonic collaterals but higher functional benefits of those in diseased adults. Our unique interdisciplinary approach allowed us to quantify the functional significance of collateral arteries during heart regeneration and repair–a critical step towards realizing their therapeutic potential.

Oxytocin promotes epicardial cell activation and heart regeneration after cardiac injury

Cardiovascular disease (CVD) is one of the leading causes of mortality worldwide, and frequently leads to massive heart injury and the loss of billions of cardiac muscle cells and associated vasculature. Critical work in the last two decades demonstrated that these lost cells can be partially regenerated by the epicardium, the outermost mesothelial layer of the heart, in a process that highly recapitulates its role in heart development. Upon cardiac injury, mature epicardial cells activate and undergo an epithelial-mesenchymal transition (EMT) to form epicardial-derived progenitor cells (EpiPCs), multipotent progenitors that can differentiate into several important cardiac lineages, including cardiomyocytes and vascular cells. In mammals, this process alone is insufficient for significant regeneration, but it may be possible to prime it by administering specific reprogramming factors, leading to enhanced EpiPC function. Here, we show that oxytocin (OXT), a hypothalamic neuroendocrine peptide, induces epicardial cell proliferation, EMT, and migration in a mature-like model of human induced pluripotent stem cell (hiPSC)-derived epicardial cells. In addition, we demonstrate that OXT is released from the brain into the bloodstream after cardiac cryoinjury in zebrafish, eliciting significant epicardial activation and promoting heart regeneration. Oxytocin signaling is also critical for proper epicardium and myocardium development in zebrafish embryos. The above processes are significantly impaired when OXT signaling is inhibited chemically and genetically through RNA interference. Mechanistically, RNA sequencing analyses suggest that the transforming growth factor beta (TGF-β) pathway is the primary mediator of OXT-induced epicardial activation. Our research reveals for the first time a primarily brain-controlled mechanism that induces cellular reprogramming and regeneration of the injured heart, a finding that could yield significant translational advances for the treatment of CVD.

From Hair to Heart: Hair-Follicle-Associated Pluripotent (HAP) Stem Cells Can Simultaneously Differentiate Into Mature Cardiomyocytes and Atrial Myocytes

Hair-follicle-associated pluripotent (HAP) stem cells express nestin, and are located in the bulge area of hair follicles and can differentiate to numerous types of cells. In the present study, we demonstrate that rat HAP stem cells simultaneously differentiated to mature cardiomyocytes and atrial myocytes. The addition of isoproterenol, activin A, bone morphogenetic protein 4 (BMP 4), basic fibroblast growth factor (bFGF), and cyclosporin A (CSA), induced simultaneously differentiation of HAP stem cells to c-kit-positive cardiomyocytes and MLC-2a-expressing atrial myocytes. The results of the present study suggest that HAP stem cells differentiating to cardiomyocytes and atrial myocytes have future clinical potential for heart regeneration.

Regenerative and non-regenerative transcriptional states of the human epicardium: from foetus to adult and back again

Epicardial activation appears to be required for cardiac regeneration. Although reverting quiescent adult epicardium to an active neonatal or foetal state will likely represent a key therapeutic approach for human cardiac regeneration, the exact molecular differences between human adult and foetal epicardium are not understood. We used single-cell RNA sequencing to compare epicardial cells from both foetal and adult hearts. We found two foetal epicardial cell types, mesothelial and fibroblast-like, with only the mesothelial population present in adults. We also identified foetal-specific epicardial genes associated with regeneration and angiogenesis, and found that adult epicardium may be primed for immune and inflammatory responses. We predict that restoring the foetal epicardial state in human hearts would increase adult angiogenic potential. Finally, we demonstrated that human embryonic stem-cell derived epicardium is a valid model for the foetal epicardium and for investigating epicardial-mediated cardiac regeneration in humans. Our study defines regenerative programs in human foetal epicardium that are absent in the adult, brings human context to animal studies, and provides a roadmap for directing the epicardium in human heart regeneration.