Rationale Myocardial infarction (MI) is a leading cause of death in developed nations, and there remains a need for cardiac therapeutic systems that mitigate tissue damage and. Cardiac progenitor cells (CPCs) and other stem cell types are attractive candidates for treatment of MI; however, the benefit of these cells may be due to paracrine effects. Objective We tested the hypothesis that CPCs secrete pro-regenerative exosomes in response to hypoxic conditions. Methods and Results The angiogenic and anti-fibrotic potential of secreted exosomes on cardiac endothelial cells and cardiac fibroblasts were assessed. We found that CPC exosomes secreted in response to hypoxia enhanced tube formation of endothelial cells and decreased pro-fibrotic gene expression in TGF-β stimulated fibroblasts, indicating that these exosomes possess therapeutic potential. Microarray analysis of exosomes secreted by hypoxic CPCs identified eleven miRNAs that were upregulated compared to exosomes secreted by CPCs grown under normoxic conditions. Principle component analysis was performed to identify miRNAs that were co-regulated in response to distinct exosome generating conditions. To investigate the cue-signal-response relationships of these miRNA clusters with a physiological outcome of tube formation or fibrotic gene expression, partial least squares regression analysis was applied. The importance of each up- or downregulated miRNA on physiological outcomes was determined. Finally, to validate the model we delivered exosomes following ischemia-reperfusion injury. Exosomes from hypoxic CPCs improved cardiac function and reduced fibrosis. Conclusions These data provide a foundation for subsequent research of the use of exosomal miRNA and systems biology as therapeutic strategies for the damaged heart.

Inositol 1,4,5-trisphosphate (IP(3)) receptor (IP(3)R)-dependent Ca(2+) signaling exerts positive inotropic, but also arrhythmogenic, effects on excitation-contraction coupling (ECC) in the atrial myocardium. The role of IP(3)R-dependent sarcoplasmic reticulum (SR) Ca(2+) release in ECC in the ventricular myocardium remains controversial. Here we investigated the role of this signaling pathway during ECC in isolated rabbit ventricular myocytes. Immunoblotting of proteins from ventricular myocytes showed expression of both type 2 and type 3 IP(3)R at levels approximately 3.5-fold less than in atrial myocytes. In permeabilized myocytes, direct application of IP(3) (10 microM) produced a transient 21% increase in the frequency of Ca(2+) sparks (P < 0.05). This increase was accompanied by a 13% decrease in spark amplitude (P < 0.05) and a 7% decrease in SR Ca(2+) load (P < 0.05) and was inhibited by IP(3)R antagonists 2-aminoethoxydiphenylborate (2-APB; 20 microM) and heparin (0.5 mg/ml). In intact myocytes endothelin-1 (100 nM) was used to stimulate IP(3) production and caused a 38% (P < 0.05) increase in the amplitude of action potential-induced (0.5 Hz, field stimulation) Ca(2+) transients. This effect was abolished by the IP(3)R antagonist 2-APB (2 microM) or by using adenoviral expression of an IP(3) affinity trap that buffers cellular IP(3). Together, these data suggest that in rabbit ventricular myocytes IP(3)R-dependent Ca(2+) release has positive inotropic effects on ECC by facilitating Ca(2+) release through ryanodine receptor clusters.

Rationale Studies have demonstrated that exosomes can repair cardiac tissue post myocardial infarction (MI) and recapitulate the benefits of cellular therapy. Objective We evaluated the role of donor age and hypoxia of human pediatric cardiac progenitor cell (CPC)-derived exosomes, in a rat model of ischemia reperfusion (IR) injury. Methods and Results Human CPCs from the right atrial appendages from children of different ages undergoing cardiac surgery for congenital heart defects were isolated and cultured under hypoxic or normoxic conditions. Exosomes were isolated from the culture-conditioned media and delivered to athymic rats following IR injury. Echocardiography at day-3 post-MI suggested statistically improved function in neonatal hypoxic and neonatal normoxic groups compared to saline-treated controls. At 28 days post-MI exosomes derived from neonatal normoxia, neonatal hypoxia, infant hypoxia, and child hypoxia significantly improved cardiac function compared to saline-treated controls. Staining showed decreased fibrosis and improved angiogenesis in hypoxic groups compared to controls. Finally, using sequencing data, a computational model was generated to link microRNA levels to specific outcomes. Conclusion CPC exosomes derived from neonates improved cardiac function independent of culture oxygen levels, while CPC-exosomes from older children were not reparative unless subjected to hypoxic conditions. Cardiac functional improvements were associated with increased angiogenesis, reduced fibrosis and improved hypertrophy resulting in improved cardiac function; however, mechanisms for normoxic neonatal CPC exosomes improved function independent of those mechanisms. This is the first study of its kind demonstrating that donor age and oxygen content in the microenvironment significantly alter the efficacy of human CPC-derived exosomes.

Shkryl VM, Maxwell JT, Domeier TL, Blatter LA. Refractoriness of sarcoplasmic reticulum Ca 2ϩ release determines Ca 2ϩ alternans in atrial myocytes.