MicroRNA extracellular vesicle stowaways in cell-cell communication and organ crosstalk

Extracellular vesicles (EVs) function as mediators of cell-cell communication¹ and organ crosstalk², but a complete understanding of the mechanistic roles this critical communication process plays in cardiovascular and associated diseases is lacking. In normal conditions EV crosstalk is important for proper cell and organ function, but in diseased conditions EV cargos can be altered in a way that induces disease pathology. Multiple cargos have been observed in EVs including RNAs such as microRNA, DNA, proteins, and lipids, which can be encased within EVs or associated with EV membranes. Given this critical function, identifying the communication roles of EVs in normal physiology and disease-driving mechanisms holds great therapeutic promise. In this issue of Ateriosclerosis, Thrombosis, and Vascular Biology Chang et al.³ propose EV transport as a major pathway in which miR-92a is transported from endothelial cells to macrophages that in turn promotes development of atherosclerosis. This study identifies a key mechanism of how miR-92a cell-cell communication occurs in atherosclerotic vasculature and raises the potential of EV-associated miR-92a as a disease biomarker and therapeutic target.

Inhibition of miR-92a suppresses endothelial dysfunction and atherosclerosis in low-density lipoprotein deficient mice⁴, and miR-92a positively correlates with atherosclerosis in humans⁵. Krüppel-like factor 4 (KLF4) regulates macrophage inflammation⁶ and is an established target of miR-92a that is known to be produced in endothelial cells⁴. Together these prior studies raised the possibility that macrophage-induced inflammation is promoted by endothelial derived miR-92a in a KLF4-dependent manner in atherosclerosis, but if and how that mechanism occurs had not been established. Chang et al.³ provide evidence for this KLF4 mechanistic pathway occurring via EV cell-cell communication. Using a combination of overexpression, labeling, and knockdown experiments the authors convincingly demonstrate this EV-mediated transfer of pro-inflammatory microRNA in vitro. Using co-culture of endothelial cells and macrophages under proinflammatory stimuli the authors found that mature miR-92a was increased in both endothelial cells and macrophages. An increase in the miR-92a precursor form was only observed in endothelial cells suggesting that these cells transfer miR-92a to macrophages. Increased miR-92a was observed within endothelial cell EVs in inflammatory conditions. Treatment of macrophages with inflammation stimulated endothelial cell EVs led to increased inflammation that was suppressed by knocking down miR-92a in endothelial cells, suggesting a major route of miR-92a transfer to macrophages is likely occurring via EVs. Supporting the possibility of EV-trafficked miR-92a in vivo, the authors observed increased miR-92a in EV-enriched fractions in the blood of Paigen diet fed mice. MiR-92a trafficking is not restricted to EV transport, and has been found to also occur via lipoproteins⁷, as such further work demonstrating the contribution of EV-derived miR-92a to disease progression is needed. This study raises the distinct possibility that EV delivered microRNA may be more prone to or used to exert pro-inflammatory action than other means of miR-92a delivery in cell-cell communication, but this needs to be clearly demonstrated. It will also be of interest to test the possibility of EV miR-92a as a vascular inflammation and dysfunctional endothelial biomarker in humans, but this requires implementation of very stringent and verified methodology to clearly distinguish between EV-associated and non-EV associated miR-92a in patient blood.

EVs are produced in virtually all cells and tissues. Beyond endothelial cells and macrophages many other cell types including smooth muscle cells and T cells are known to be involved in mediating vascular dysfunction via EVs^8,9. A comprehensive understanding of the various cell-cell communication events occurring via EVs is needed in vascular disease. A quantitative multi-omics assessment of EVs addressing questions of cell origin and EV content in diseased human vasculature would provide novel mechanistic insight and be a required first step for targeting pathology driven by EV cell-cell communication. Vascular changes correlate with diseases and dysfunction occurring in multiple organs including heart¹⁰, bone¹¹, liver¹², renal¹⁰, brain¹³, lung⁹, and intestine¹⁴ among others. Mechanistic studies examining EVs and EV tissue crosstalk between distinct tissues and/or vascular cells and various organs is an exciting and emerging area of biomedical exploration. Chang et al.³ expand our understanding of EV trafficked microRNA in cell-cell communication in atherosclerosis context. Expanding these cell-cell communication EV findings to the next level involving organ crosstalk could help generate mechanistic understanding of how cardiovascular disease associates with numerous distinct diseases in multiple organs (Figure 1).

Figure 1: Extracellular vesicles (EVs) are mediators of cell-cell communication and organ crosstalk in vascular and associated diseases.

Endothelial cells, immune cells, and other vascular cells release EVs. Conditions such as inflammation induce alterations in EV cargos including microRNAs, proteins, and lipids utilizing this cell-cell communication process to promote disease pathogenesis. EVs also function in organ crosstalk and may contribute to the association of vascular disease with diseases in other organs in a similar manner.