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.

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.

Epicardial Transplantation of Adipose Mesenchymal Stromal Cell Sheets Promotes Epicardial Activation and Stimulates Angiogenesis in Myocardial Infarction (Experimental Study)

Aim: to evaluate the impact of tissue-engineered structures (TES) transplantation based on mesenchymal stromal cell (MSC) sheets in myocardial infarction on the activation of the epicardial cell pool and vascularization of the damaged zone.Materials and methods. Mesenchymal stromal cells were obtained from samples of subcutaneous fat of Wistar rats and C57Bl/6 mice. Tissue engineering structures were obtained by culturing cell sheets on thermosensitive plates (Nunc Dishes with UpCell Surface). Transplantation of TESs was performed after myocardial infarction modeling in rats by ligation of the anterior descending coronary artery. Transplant cells and damaged zones were assessed using immunofluorescent staining of myocardial cryosections. The impact of MSC secretion products on the migration activity of epicardial cells in vitro was evaluated using the explant culture method.Results. MSCs in TESs after transplantation remain viable and induce activation of the epicardial cell pool and local increase of the damaged zone vascularization. The in vitro experiments showed that the conditioned environment of MSCs stimulates the migratory activity of epicardial cells and initiates the formation of activated Wt1/POD1 precursor cells.Conclusion. TES transplantation on the basis of MSC sheets seems to be a promising approach for effective delivery of viable cells into myocardium to activate the epicardial cellular niche and reparative angiogenesis.

The multiple functions of the proepicardial/epicardial cell lineage in heart development

The epicardium is the outer cell layer of the vertebrate heart. In recent years, both the embryonic and adult epicardium have revealed unsuspected peculiarities and functions, which are essential for cardiac development. In this chapter we review the current literature on the epicardium, and describe its evolutionary origin, the mechanisms leading to the induction of its extracardiac progenitor tissue, the proepicardium, and the way in which the proepicardium is transferred to the heart to form the epicardium. We also describe the epicardial epithelial–mesenchymal transition from which mesenchymal cells originate, and the developmental fate of these cells, which contribute to the vascular, interstitial, valvular, and adipose tissue. Finally, we review the molecular interactions established between the epicardium and the myocardium, which are key for myocardial development and can also play a role in cardiac homeostasis. This chapter highlights how the epicardium has become a major protagonist in cardiac biology.