Myocyte deficiency as a target in the treatment of cardiomyopathy

Hypoxic Stress Decreases c-Myc Protein Stability in Cardiac Progenitor Cells Inducing Quiescence and Compromising Their Proliferative and Vasculogenic Potential

Cardiac progenitor cells (CPCs) have been shown to promote cardiac regeneration and improve heart function. However, evidence suggests that their regenerative capacity may be limited in conditions of severe hypoxia. Elucidating the mechanisms involved in CPC protection against hypoxic stress is essential to maximize their cardioprotective and therapeutic potential. We investigated the effects of hypoxic stress on CPCs and found significant reduction in proliferation and impairment of vasculogenesis, which were associated with induction of quiescence, as indicated by accumulation of cells in the G0-phase of the cell cycle and growth recovery when cells were returned to normoxia. Induction of quiescence was associated with a decrease in the expression of c-Myc through mechanisms involving protein degradation and upregulation of p21. Inhibition of c-Myc mimicked the effects of severe hypoxia on CPC proliferation, also triggering quiescence. Surprisingly, these effects did not involve changes in p21 expression, indicating that other hypoxia-activated factors may induce p21 in CPCs. Our results suggest that hypoxic stress compromises CPC function by inducing quiescence in part through downregulation of c-Myc. In addition, we found that c-Myc is required to preserve CPC growth, suggesting that modulation of pathways downstream of it may re-activate CPC regenerative potential under ischemic conditions.

Lessons learned from the Pediatric Cardiomyopathy Registry (PCMR) Study Group

Cardiomyopathy is a rare disorder of the heart muscle, affecting 1.13 cases per 100,000 children, from birth to 18 years of age. Cardiomyopathy is the leading cause of heart transplantation in children over the age of 1. The Pediatric Cardiomyopathy Registry funded in 1994 by the National Heart, Lung, and Blood Institute was established to examine the epidemiology of the disease in children below 18 years of age. More than 3500 children across the United States and Canada have been enrolled in the Pediatric Cardiomyopathy Registry, which has followed-up these patients until death, heart transplantation, or loss to follow-up. The Pediatric Cardiomyopathy Registry has provided the most in-depth illustration of this disease regarding its aetiology, clinical course, associated risk factors, and patient outcomes. Data from the registry have helped in guiding the clinical management of cardiomyopathy in children under 18 years of age; however, questions still remain regarding the most clinically effective diagnostic and treatment approaches for these patients. Future directions of the registry include the use of next-generation whole-exome sequencing and cardiac biomarkers to identify aetiology-specific treatments and improve diagnostic strategies. This article provides a brief synopsis of the work carried out by the Pediatric Cardiomyopathy Registry since its inception, including the current knowledge on the aetiologies, outcomes, and treatments of cardiomyopathy in children.

Heterogeneity in SDF-1 expression defines the vasculogenic potential of adult cardiac progenitor cells

Rationale

The adult myocardium has been reported to harbor several classes of multipotent progenitor cells (CPCs) with tri-lineage differentiation potential. It is not clear whether c-kit+CPCs represent a uniform precursor population or a more complex mixture of cell types.

Objective

To characterize and understand vasculogenic heterogeneity within c-kit+presumptive cardiac progenitor cell populations.

Methods and Results

c-kit+, sca-1+ CPCs obtained from adult mouse left ventricle expressed stem cell-associated genes, including Oct-4 and Myc, and were self-renewing, pluripotent and clonogenic. Detailed single cell clonal analysis of 17 clones revealed that most (14/17) exhibited trilineage differentiation potential. However, striking morphological differences were observed among clones that were heritable and stable in long-term culture. 3 major groups were identified: round (7/17), flat or spindle-shaped (5/17) and stellate (5/17). Stellate morphology was predictive of vasculogenic differentiation in Matrigel. Genome-wide expression studies and bioinformatic analysis revealed clonally stable, heritable differences in stromal cell-derived factor-1 (SDF-1) expression that correlated strongly with stellate morphology and vasculogenic capacity. Endogenous SDF-1 production contributed directly to vasculogenic differentiation: both shRNA-mediated knockdown of SDF-1 and AMD3100, an antagonist of the SDF-1 receptor CXC chemokine Receptor-4 (CXCR4), reduced tube-forming capacity, while exogenous SDF-1 induced tube formation by 2 non-vasculogenic clones. CPCs producing SDF-1 were able to vascularize Matrigel dermal implants in vivo, while CPCs with low SDF-1 production were not.

Conclusions

Clonogenic c-kit+, sca-1+ CPCs are heterogeneous in morphology, gene expression patterns and differentiation potential. Clone-specific levels of SDF-1 expression both predict and promote development of a vasculogenic phenotype via a previously unreported autocrine mechanism.

The pediatric cardiomyopathy registry and heart failure: key results from the first 15 years

Cardiomyopathy is a serious disorder of the heart muscle and, although rare, is a common cause of heart failure in children and the most common cause for heart transplantation in children older than 1 year of age. Funded by the National Heart Lung and Blood Institute since 1994, the Pediatric Cardiomyopathy Registry (PCMR) has followed more than 3500 North American children with cardiomyopathy. Early analyses determined estimates for the incidence of pediatric cardiomyopathy (1.13 cases per 100,000 children per year), risk factors for cardiomyopathy (age <1 year, male sex, black race, and living in New England as opposed to the central southwestern states), the prevalence of heart failure at diagnosis (6%-84% depending on cause), and 10-year survival (29%-94% depending on cause). More recent analyses explored cause-specific functional status, survival and transplant outcomes, and risk factors in greater detail. For many topics these analyses are based on the largest and best-documented samples of children with disease such as the muscular dystrophies, mitochondrial disorders, and Noonan syndrome. Data from the PCMR continue to provide valuable information that guides clinical management and the use of life-saving therapies, such as cardiac transplantation and approaches to treating heart failure, and prepares children, their families, and their caregivers to deal with this serious condition.