Fifty years in the vineyard of transplantation: looking back. Transplant Proc. 2011;43:2853-2859. [PMID: 21996173 DOI: 10.1016/j.transproceed.2011.08.058]

Contemporary perspective on endogenous myocardial regeneration

Considering the complex nature of the adult heart, it is no wonder that innate regenerative processes, while maintaining adequate cardiac function, fall short in myocardial jeopardy. In spite of these enchaining limitations, cardiac rejuvenation occurs as well as restricted regeneration. In this review, the background as well as potential mechanisms of endogenous myocardial regeneration are summarized. We present and analyze the available evidence in three subsequent steps. First, we examine the experimental research data that provide insights into the mechanisms and origins of the replicating cardiac myocytes, including cell populations referred to as cardiac progenitor cells (i.e., c-kit+ cells). Second, we describe the role of clinical settings such as acute or chronic myocardial ischemia, as initiators of pathways of endogenous myocardial regeneration. Third, the hitherto conducted clinical studies that examined different approaches of initiating endogenous myocardial regeneration in failing human hearts are analyzed. In conclusion, we present the evidence in support of the notion that regaining cardiac function beyond cellular replacement of dysfunctional myocardium via initiation of innate regenerative pathways could create a new perspective and a paradigm change in heart failure therapeutics. Reinitiating cardiac morphogenesis by reintroducing developmental pathways in the adult failing heart might provide a feasible way of tissue regeneration. Based on our hypothesis “embryonic recall”, we present first supporting evidence on regenerative impulses in the myocardium, as induced by developmental processes.

The cardiac stem cell compartment is indispensable for myocardial cell homeostasis, repair and regeneration in the adult

Resident cardiac stem cells in embryonic, neonatal and adult mammalian heart have been identified by different membrane markers and transcription factors. However, despite a flurry of publications no consensus has been reached on the identity and actual regenerative effects of the adult cardiac stem cells. Intensive research on the adult mammalian heart's capacity for self-renewal of its muscle cell mass has led to a consensus that new cardiomyocytes (CMs) are indeed formed throughout adult mammalian life albeit at a disputed frequency. The physiological significance of this renewal, the origin of the new CMs, and the rate of adult CM turnover are still highly debated. Myocyte replacement, particularly after injury, was originally attributed to differentiation of a stem cell compartment. More recently, it has been reported that CMs are mainly replaced by the division of pre-existing post-mitotic CMs. These latter results, if confirmed, would shift the target of regenerative therapy toward boosting mature CM cell-cycle re-entry. Despite this controversy, it is documented that the adult endogenous c-kit(pos) cardiac stem cells (c-kit(pos) eCSCs) participate in adaptations to myocardial stress, and, when transplanted into the myocardium, regenerate most cardiomyocytes and microvasculature lost in an infarct. Nevertheless, the in situ myogenic potential of adult c-kit(pos) cardiac cells has been questioned. To revisit the regenerative potential of c-kit(pos) eCSCs, we have recently employed experimental protocols of severe diffuse myocardial damage in combination with several genetic murine models and cell transplantation approaches showing that eCSCs are necessary and sufficient for CM regeneration, leading to complete cellular, anatomical, and functional myocardial recovery. Here we will review the available data on adult eCSC biology and their regenerative potential placing it in the context of the different claimed mechanisms of CM replacement. These data are in agreement with and have reinforced our view that most CMs are replaced by de novo CM formation through the activation, myogenic commitment and specification of the eCSC cohort.

Cyclophilin D extramitochondrial signaling controls cell cycle progression and chemokine-directed cell motility

Mitochondria control bioenergetics and cell fate decisions, but how they influence nuclear gene expression is understood poorly. Here, we show that deletion or reduction in the levels of cyclophilin D (CypD, also called Ppif), a mitochondrial matrix peptidyl prolyl isomerase and apoptosis regulator, results in increased cell proliferation and enhanced cell migration and invasion. These responses are associated with extensive transcriptional changes, modulation of a chemokine/chemokine receptor gene signature, and activation of the pleiotropic inflammatory mediator, STAT3. In the absence of CypD, active STAT3 enhances cell proliferation via accelerated entry into S-phase and stimulates autocrine/paracrine cell motility through Cxcl12-Cxcr4-directed chemotaxis. Therefore, CypD directs mitochondria-to-nuclei inflammatory gene expression in normal and tumor cells. This pathway may contribute to malignant traits under conditions of CypD modulation.

Optimizing cardiac repair and regeneration through activation of the endogenous cardiac stem cell compartment

Given the aging of the Western World and declining death rates due to acute coronary syndromes, the increasing trends in the magnitude and morbidity of heart failure (HF) are predicted to continue for the foreseeable future. It is imperative to develop effective therapies for the amelioration and prevention of HF. The search for the best cell type to be used in clinical protocols of cardiac regeneration is still on. That the adult mammalian heart harbors endogenous, multipotent cardiac stem/progenitor cells (eCSCs) and that cardiomyocytes are replaced throughout adulthood represent a paradigm shift in cardiovascular biology. The presence of eCSCs supports the view that the heart can repair itself if the eCSCs can be properly stimulated. Pending a better understanding of eCSC biology, it should be possible to replace autologous cell transplantation-based myocardial regeneration protocols with an "off-the-shelf," readily available, and effective regenerative/reparative therapy based on activation of the eCSCs in situ.