Heart failure is characterized by a debilitating decline in cardiac function1, and recent clinical trial results indicate that improving the contractility of heart muscle cells by boosting intracellular calcium handling might be an effective therapy2,3. microRNAs (miRs) are dysregulated with heart failure4,5 but whether they control contractility or constitute therapeutic targets remain speculative. Using high throughput, functional screening of the human microRNAome, we identified miRs that suppress intracellular calcium handling in heart muscle by interacting with mRNA encoding the sarcoplasmic reticulum calcium uptake pump SERCA2a. Of 875 miRs tested, miR-25 potently delayed calcium uptake kinetics in cardiomyocytes in vitro and was upregulated in heart failure, both in mice and humans. Whereas AAV9-mediated overexpression of miR-25 in vivo resulted in a significant loss of contractile function, injection of an antisense oligonucleotide (antagomiR) against miR-25 dramatically halted established heart failure in a mouse model, improving cardiac function and survival relative to a control antagomiR. These data reveal that increased expression of endogenous miR-25 contributes to declining cardiac function during heart failure and suggests that it might be targeted therapeutically to restore function.

To date, there is no suitable in vitro model to study human adult cardiac cell biology. Although embryonic stem cells are able to differentiate into cardiomyocytes in vitro, the efficiency of this process is very low. Other methods to differentiate progenitor cells into beating cardiomyocytes rely on coculturing with rat neonatal cardiomyocytes, making it difficult to study human cardiomyocyte differentiation and (patho)physiology. Here we have developed a method for efficient isolation and expansion of human cardiomyocyte progenitor cells (CMPCs) from cardiac surgical waste or alternatively from fetal heart tissue. Furthermore, we provide a detailed in vitro protocol for efficient differentiation of CMPCs into cardiomyocytes with great efficiency (80-90% of differentiation). Once CMPCs are rapidly dividing ( approximately 1 month after isolation), differentiation can be achieved in 3-4 weeks.

Extracellular vesicles (EVs)—particularly exosomes and microvesicles (MVs)—are attracting considerable interest in the cardiovascular field as the wide range of their functions is recognized. These capabilities include transporting regulatory molecules including different RNA species, lipids, and proteins through the extracellular space including blood and delivering these cargos to recipient cells to modify cellular activity. EVs powerfully stimulate angiogenesis, and can protect the heart against myocardial infarction. They also appear to mediate some of the paracrine effects of cells, and have therefore been proposed as a potential alternative to cell-based regenerative therapies. Moreover, EVs of different sources may be useful biomarkers of cardiovascular disease identities. However, the methods used for the detection and isolation of EVs have several limitations and vary widely between studies, leading to uncertainties regarding the exact population of EVs studied and how to interpret the data. The number of publications in the exosome and MV field has been increasing exponentially in recent years and, therefore, in this ESC Working Group Position Paper, the overall objective is to provide a set of recommendations for the analysis and translational application of EVs focussing on the diagnosis and therapy of the ischaemic heart. This should help to ensure that the data from emerging studies are robust and repeatable, and optimize the pathway towards the diagnostic and therapeutic use of EVs in clinical studies for patient benefit.