Heart regeneration: 20 years of progress and renewed optimism

Abstract

Cardiovascular disease is a leading cause of death worldwide, and thus there remains great interest in regenerative approaches to treat heart failure. In the past 20 years, the field of heart regeneration has entered a renaissance period with remarkable progress in the understanding of endogenous heart regeneration, stem cell differentiation for exogenous cell therapy, and cell-delivery methods. In this review, we highlight how this new understanding can lead to viable strategies for human therapy. For the near term, drugs, electrical and mechanical devices, and heart transplantation will remain mainstays of cardiac therapies, but eventually regenerative therapies based on fundamental regenerative biology may offer more permanent solutions for patients with heart failure.

Keywords: cardiomyocyte proliferation; heart regeneration; reprogramming; stem cells; tissue engineering.

Figure 1.. Overview of heart regeneration approaches.

Approaches to regenerate myocardium include renewal of pre-existing cardiomyocytes by stimulating de-differentiation and proliferation of existing, mature cardiomyocytes, trans-differentiation of non-cardiomyocytes into cardiomyocytes such as with gene therapy methods, and delivery of stem cell-derived cardiomyocytes either as an injectable system or as a tissue engineered patch.

Figure 2.. Mechanisms regulating the cardiomyocyte cell cycle.

Proliferation of existing cardiomyocytes requires de-differentiation and sarcomere disassembly followed by re-entry into the cell cycle. Growth factors such as bone morphogenic protein (BMP), transforming growth factor (TGF), or follistatin like-1 protein (FSTL1), or proliferative miRNAs can lead to stimulation of pro-proliferative developmental pathways such as YAP, NOTCH, and Wnt/B-catenin and/or affect expression of transcription factors promoting cardiomyocyte proliferation. In contrast, transcription factors p53 or MEIS1 lead to upregulation of cell cycle inhibitors, and Hippo pathway activation leads to inhibition of the pro-proliferative YAP pathway. miRNAs can also inhibit cardiomyocyte proliferation by inhibiting sarcomere disassembly and/or promoting cell cycle arrest. Pro-proliferative factors shown in blue; inhibitory factors shown in orange.

Figure 3.. Mechanisms of cardiac regenerative potential loss in postnatal mammals.

The switch from anaerobic to aerobic respiration after birth leads to increased production of reactive oxygen species and induction of a DNA damage response pathway, which leads to cell cycle arrest and cardiomyocyte maturation. Similarly, transition from placental to enteral nutrition, with breastmilk containing higher levels of fatty acids, leads to mitochondrial maturation which also releases reactive oxygen species during oxidative phosphorylation. The DNA damage response pathway leads to histone modifications and subsequent shifts in gene expression from early to late cardiomyocyte genes, which promote cardiomyocyte cell cycle arrest and maturation. Maturation of the immune system, due to changes in the gut microbiome and pathogen exposure after birth, shifts the post-injury inflammatory response toward fibrosis and reduced angiogenic potential. Increased metabolic rates are required to maintain temperature outside of the womb, which is regulated by thyroid hormone and beta-adrenergic signaling and simultaneously leads to cell cycle arrest by downregulation of Ect2. Establishment of the circadian cycle after birth also acts via the sympathetic nervous system to upregulate Per1/2, which promote cell cycle arrest in cardiomyocytes.

Figure 4.. Adult stem cells can have paracrine effects but do not become new cardiomyocytes.

Bone marrow derived cells have been studied in multiple clinical trials to test whether they can promote cardiomyocyte maturation. While there is some evidence of clinical improvement, these findings do not appear to be due to replacement with new cardiomyocytes but rather through paracrine effects leading to enhanced angiogenesis.

Figure 5.. Challenges to clinical translation of myocyte therapy approaches.

Cell therapy with pluripotent stem cell-derived cardiomyocytes can lead to ventricular arrhythmias, thought to be due to delivery of immature cardiomyocytes that continue to exhibit automaticity, or spontaneous beating. Combination of non-cardiomyocytes into a tissue engineered structure can promote maturation of cardiomyocytes. Allogeneic stem cells have risk of immune rejection, thus strategies to eliminate human leukocyte antigens may permit cell survival without immunosuppression. Methods to promote direct reprogramming of non-cardiomyocytes into cardiomyocytes within the heart are inefficient and long-term safety concerns such as uncontrolled transdifferentiation or tumor formation are unclear. Use of transient delivery systems with improved efficiency may make this approach more clinically feasible.