Introduction: In disease and in ageing, cardiac fibrosis and impairment of calcium cycling mechanisms causes contractile dysfunction, ultimately leading to heart failure. Cardiac fibroblasts, which account for 20% of the non-myocyte component of the heart, accumulate in disease, and may be important in driving these changes. Exosomes, a subset of microvesicles, are intercellular mediators and mediate cardiomyocyte hypertrophy in rats. Their role in human fibroblast-cardiomyocyte interactions has yet to be identified.
Hypothesis: Fibroblast-derived exosomes modulate cardiomyocyte calcium cycling.
Methods: Cardiac fibroblasts isolated from 2 dilated cardiomyopathy patients were incubated for 48 hours in exosome-depleted fibroblast media. Extracellular vesicles and soluble contaminating proteins were efficiently separated, as demonstrated by microBCA assay for quantified protein (Fig 1A). A Nanocyte then estimated particle size (Fig 1C). Protein weight in extracellular vesicle fractions was strongly correlated with particle concentration (Fig 1D). Human induced pluripotent stem cell-cardiomyocytes (hiPSC-CMs) were treated with 5μg exosomes from human fibroblasts for 24 hours (n = number of hiPSC-CMs measured).
Results: Calcium transients were measured in hiPSC-CMs stimulated at 1 Hz, using Fluo-4 AM. Exosome-treatment reduced time to peak of the calcium transient (Fig 1F, p<0.001), but did not change the amplitude (Fig 1E), or time to 50% (Fig 1G) and 80% decay when compared to exosome depleted media alone. This suggests faster calcium entry in the cells. Rate of calcium transient decay and SERCA function were similarly unchanged, as measured with rapid application of caffeine (not shown).
Conclusions: Exosomes from human cardiac fibroblasts modulate cardiomyocyte calcium cycling by abbreviating cytoplasmic calcium transient duration and present a modality by which human fibroblast-cardiomyocyte interactions can occur.