Exosomes and cardioprotection
In the case of AMI, reperfusion of an occluded coronary vessel can lead to reperfusion injury, which adds to the initial injury caused by the abrupt occlusion. Therefore, the reducing reperfusion injury is crucial for improving the long-term evolution of AMI survivors.
The acute cardioprotective effects of exosomes were demonstrated in 2010, following the observation that exosomes injected into mice suffering a 30 min ischaemia led to a significant reduction of infarct size within 20 h. However, exosomes have been demonstrated to play a significant cardioprotective role in models of continuous ischaemia without reperfusion. In a study by Zhao et al, after ligation of the left anterior descending artery in rats, injection of exosomes was associated with a significant improvement in systolic function at 4 wk, concomitant with a significant reduction in cardiac fibrosis and apoptosis.
Plasma exosomes originating from various cells demonstrated significant cardioprotective effects in the post-AMI period, reducing infarct size after intravenous administration. At the same time, the release of cytoprotective HSP70 and HSP90 from exosomes has been identified in mouse cardiomyocytes. HSP70 present at the surface of plasma exosomes stimulates the activation of several cardioprotective pathways.
The effects of exosomes on ischaemic hearts can be mediated through various types of receptor cells. In macrophages and other cells, exosomes are involved in immunosuppression mechanisms. Alternately, they stimulate angiogenesis at the level of endothelial cells, inhibition of fibrosis at the level of fibroblasts, and cardioprotection at the level of cardiomyocytes.
Post-myocardial infarction release of exosomes containing cardiac-specific miRNA is essential to ensure an adequate level of cardioprotection, as cardiac-specific miRNA exhibits significant protective effects: miRNA-133 has anti-apoptotic and anti-fibrotic effects; miRNA-1 has a specific anti-oxidant role; and miRNA-499 has anti-apoptotic properties. In another study, microRNA analysis of CPC-derived exosomes indicated the presence of increased levels of miR-210, miR-132 and miR-146a-3p in a myocardial infarction model, inducing a sustained anti-apoptotic and pro-angiogenic response.
Hypoxic exosomes contain higher amounts of pro-angiogenic miRNAs, showing a more pronounced angiogenic potential. Interestingly, exosomes from the pericardial fluid during the post-infarction period also exhibited cardioprotective effects by decreasing apoptosis and enhancing arteriogenesis.
An interesting finding was the role of exercise in further increasing the number of circulating exosomes in healthy individuals but not in patients with CAD. At the same time, cardiomyocyte-derived exosomes from exercised mice expressed higher levels of miR-29b and miR-455 compared to sedentary ones, and these miRNAs had the capacity to downregulate matrix-metalloprotease 9 and reduce cardiac fibrosis. Thus, we can conclude that in the post-MI period, cardiac cells release exosomes with augmented cardioprotective effects to promote myocardial regeneration.
Exosomes as myocardial regenerative tools
The efforts to regenerate myocardium via injecting various types of SCs into the myocardium or into the infarct-affected coronary arteries did not lead to significant evidence of their potential to generate new myocardium. However, the benefits of SCs have been attributed to their paracrine effects, which could be mediated by exosomes[64,72]. Following the observation that the SCs remain at the site of injection release factors mediating this paracrine effect, exosomes have been proposed as important potential paracrine mediators for myocardial regeneration. Given their carrier capacity, exosomes exhibit the potential for delivering biologics containing proteins or small interfering RNA (siRNA). Experimental studies have demonstrated that engineered CD34+ SCs were able to excrete manipulated exosomes containing a proangiogenic factor, which was delivered to infarcted mouse myocardium and led to decreased infarct size, increased angiogenesis and improved long-term regeneration[11,73].
In AMI, myocardial tissue is exposed to increased ischaemic stress signals. As a result, cardiomyocytes respond by increasing the secretion of exosomes, which has been identified in different amounts in peri-infarcted areas and in healthy myocardium. Exosomes released by the damaged myocardium transfer proteins and miRNAs that send ischaemic signals to distant tissues or organs, such as bone marrow (BM), and stimulate the production of SC from the BM. In turn, BM releases SCs and exosomes that travel back to the ischaemic myocardium to stimulate the repair process and trigger myocardial regeneration. Injured myocardium exhibits a multitude of responses to injury, including necrosis, inflammation, apoptosis, remodelling and fibrosis. Paracrine effects of exosomes released by non-injured myocardium from peri-infarcted areas can reprogram cardiomyocytes and rescue the peri-infarcted region from these deleterious mechanisms. This is mediated by the specific transfer of RNAs, peptides and small molecules.
Several preclinical studies have demonstrated the beneficial role of SC-derived exosomes in the repair of ischaemic tissues and myocardial regeneration[27,75-77]. Therefore, exosomes can represent a new line of cell-free therapy for myocardial regeneration in AMI. However, their translation into clinical application is still far away.
Arslan et al showed that exosome treatment in the post-MI period enhanced myocardial viability and reduced adverse ventricular remodelling by decreasing oxidative stress and activating the PI3K/Akt pathway. Lai et al also demonstrated that the administration of MSC-derived exosomes significantly reduced infarct size in mice. Intramyocardial injection of CSC-derived exosomes in mice undergoing ischaemia-reperfusion injury led to a 53% reduction in cardiomyocyte-related apoptosis. Furthermore, Barile et al found that intramyocardial injection of CSC-derived exosomes reduced the amount of scar tissue, increased vessel density via angiogenic effects and significantly decreased apoptosis of cardiomyocytes.
In a study on acute myocardial ischaemic injury, Luo et al demonstrated that exosomes derived from adipose-derived stem cells (ADSCs) overexpressing miR-126 decreased myocardial injury by reducing the expression of inflammation factors. This suggests that ADSC-derived exosomes can also protect myocardial cells from apoptosis, inflammation and fibrosis, thus preventing myocardial damage and favouring angiogenesis and myocardial repair. These findings were demonstrated in both in vitro and in vivo environments; thus, the administration of miR-126–enriched exosome treatment may serve as a potential therapeutic alternative where SC therapy fails to reduce myocardial injury or promote the regeneration process after myocardial infarction.
Exosomes may also play a role in vascular regeneration. Endothelial cells, monocytes and vascular smooth muscle cells also possess the ability to secrete exosomes, which stimulate and mediate angiogenesis, vascular healing, and remodelling by promoting cell migration, adhesion, and proliferation. Experimental studies also suggest that due to their autocrine and paracrine effects, exosomes are implicated in the modulation of physiological processes such as thrombus formation by binding coagulation factors. In contrast to the protective effects, vascular smooth muscle cell exosomes can play a detrimental role in vascular calcification and atherogenesis. These findings open the way for therapeutic approaches targeting inhibition of exosome secretion, thus preventing excessive coagulation and vascular calcification. Inhibiting exosome secretion may be extremely challenging considering the fine line between the physiological role of exosomes in healing processes and the harmful effect in pathological conditions.
It is interesting to note that different cells release exosomes that can exhibit a dual role in CVD: On the one hand, a protective role, especially with respect to their cardioprotective properties, and on the other hand, a destructive role, with respect to their role in mediating inflammatory responses.