Calcification of soft tissues occurs in the cardiovascular system with age, one of the processes that causes arterial stiffening and other pathogenic conditions such as aortic stenosis. Considered at a very high level, this happens because a fraction of cells in the blood vessel walls malfunction and begin to act in ways more appropriate to a bone environment, laying down deposits of minerals. The causes of this malfunction are incompletely understood, but evidence suggests that the presence of senescent cells and their inflammatory signaling is an important cause.
In this open access paper, researchers investigate cellular signals carried via exosomes in the context of vascular calcification. Exosomes are a class of extracellular vesicle, small membrane-bound packages of molecules that carry a sizable fraction of the signaling traffic between cells. In recent years, scientists have been paying a lot more attention to these packages, as they appear to carry most of the signals that are important in, for example, the beneficial effects of stem cell transplants. They are also probably a sizable part of the harmful signals produced by senescent cells. While the authors don't mention cellular senescence here, it is interesting to speculate on the overlap between this research and what is being discovered of the role of vesicles in senescent signaling.
Vascular calcification (VC) is caused by hydroxyapatite deposition in the intimal and medial layers of the vascular wall, leading to severe cardiovascular events in patients. Importantly, exosomes have been demonstrated to be involved in VC recently. Exosomes have up-regulated secretion from vascular smooth muscle cells (VSMCs) in vivo after pro-calcifying stimulation and become "calcifying" exosomes to induce VC. Calcium binds with phosphate to form hydroxyapatite nodes on the inner and outside of "calcifying" exosomes membranes, which further initializes mineral deposition. Although these studies did reveal that exosomes participated in the calcification procession through promoting mineral deposition sites formation, they did not discuss exosomes functioning as mediators for RNAs transportation, which is vital for exosome function.
Exosomes are secreted by diverse cells to mediate cell-to-cell communications. However, how exosomes regulate VC has been only preliminarily explored. It is found that exosomes with diverse origins mainly mediate microRNAs (miRs) transporting to VSMCs in coronary artery calcification. A bioinformatics analysis revealed that cultured in osteogenic medium, mesenchymal stem cells secreted exosomes with alterations of miRs, comparing with normal culturing. Such alterations were suggested to accelerate calcification in other mesenchymal stem cells to modulate osteogenic phenotype transition. Thus, it implies that besides heterogeneous mineral deposition inside vessel wall, exosomes can also promote VC by transporting messages among cells.