Zooming in on the genesis of atherosclerotic plaque microcalcifications

Abstract

Epidemiological evidence conclusively demonstrates that calcium burden is a significant predictor of cardiovascular morbidity and mortality; however, the underlying mechanisms remain largely unknown. These observations have challenged the previously held notion that calcification serves to stabilize the atherosclerotic plaque. Recent studies have shown that microcalcifications that form within the fibrous cap of the plaques lead to the accrual of plaque-destabilizing mechanical stress. Given the association between calcification morphology and cardiovascular outcomes, it is important to understand the mechanisms leading to calcific mineral deposition and growth from the earliest stages. We highlight the open questions in the field of cardiovascular calcification and include a review of the proposed mechanisms involved in extracellular vesicle-mediated mineral deposition.

Figure 1. High resolution micro‐computed tomography (CT) of explanted human coronary artery

The yellow arrow indicates a large calcification, and the box highlights spotty microcalcifications. (Adapted from Kelly‐Arnold et al. 2013.)

Figure 2. Schematic illustration of the mechanism of atherosclerotic plaque stabilization by a large calcification

The lipid pool/necrotic core of a non‐calcified plaque is deformable, which allows for high tissue strain in the fibrous cap during systole. Large, dense calcifications help counteract the resulting stress by limiting the degree of fibrous cap deformation that occurs under systolic pressure. (From Ruiz et al. 2015.)

Figure 3. Microcalcifications within the fibrous cap

Microcalcifications serve as foci for high levels of local stress within the fibrous cap. Finite element analysis of the local stress levels surrounding microcalcifications within a fibrous cap. The gradient from blue to red indicates a transition from low to high mechanical stress. (Adapted from Vengrenyuk et al. 2006; Ruiz et al. 2015.)

Figure 4. Proposed mechanism of vesicle‐mediated mineral formation

Top, cell‐derived vesicles are preloaded with a nucleational core (NC), composed of annexins A2, A5 and A6, as well as a complex of PS, Ca²⁺ and P_i. Calcium and phosphate ions are accumulated within the vesicle lumen via multiple routes: intravesicular P_i cleavage from PChol and PEA via PHOSPHO1; P_i cleavage from ATP in the extravesicular space via TNAP and NPP1/3, and import via Pit1/2; and calcium import via annexins A2, A5 and A6. These ions are patterned as calcium phosphate mineral at the NC. Bottom, over time, mineral crystals propagate through the vesicle membrane, where their continued growth is regulated by the ratio of PP_i, an inhibitor of crystal growth, to its derivative P_i. Note that the hypothesis that annexins form transmembrane ion channels, as depicted in this figure, is controversial (given that annexins are believed to primarily function as peripheral membrane proteins), thus meriting further investigation.

Figure 5. Electron micrographs of mineral formation on EV inner membrane (A) and outer membrane (B)

(Adapted from Aikawa et al. 2007.)