Ultrastructural Analysis of Vascular Calcifications in Uremia

Abstract

Accelerated intimal and medial calcification and sclerosis accompany the increased cardiovascular mortality of dialysis patients, but the pathomechanisms initiating microcalcifications of the media are largely unknown. In this study, we systematically investigated the ultrastructural properties of medial calcifications from patients with uremia. We collected iliac artery segments from 30 dialysis patients before kidney transplantation and studied them by radiography, microcomputed tomography, light microscopy, and transmission electron microscopy including electron energy loss spectrometry, energy dispersive spectroscopy, and electron diffraction. In addition, we performed synchrotron x-ray analyses and immunogold labeling to detect inhibitors of calcification. Von Kossa staining revealed calcification of 53% of the arteries. The diameter of these microcalcifications ranged from 20 to 500 nm, with a core-shell structure consisting of up to three layers (subshells). Many of the calcifications consisted of 2-to 10-nm nanocrystals and showed a hydroxyapatite and whitlockite crystalline structure and mineral phase. Immunogold labeling of calcification foci revealed the calcification inhibitors fetuin-A, osteopontin, and matrix gla protein. These observations suggest that uremic microcalcifications originate from nanocrystals, are chemically diverse, and intimately associate with proteinaceous inhibitors of calcification. Furthermore, considering the core-shell structure of the calcifications, apoptotic bodies or matrix vesicles may serve as a calcification nidus.

Serum Fetuin-A Levels Are Associated with Vascular Calcifications and Predict Cardiovascular Events in Renal Transplant Recipients

“…These findings are in line with ultrastructural analyses performed on iliac artery segments of dialysis patients, showing the close spatial relationship between fetuin-A and vascular microcalcifications (42). Distinct pathophysiology mechanisms operate in different vascular beds (14,42). The fact that low serum fetuin-A is a predictor of aortic-but not coronary-calcification in this cohort must be pointed out.…”

“…Correlations between low fetuin-A levels and vascular or valvular calcifications have been shown in smaller cohorts of ESRD patients after renal replacement therapy (26,40,41). These findings are in line with ultrastructural analyses performed on iliac artery segments of dialysis patients, showing the close spatial relationship between fetuin-A and vascular microcalcifications (42). Distinct pathophysiology mechanisms operate in different vascular beds (14,42).…”

“…These differences may reflect the fact that coronary calcifications occur generally in the intima, whereas calcifications in noncoronary arteries such as the aorta can involve both the intima and the media layers of the artery, thus reflecting different pathogenic processes (43). Of note, Schlieper et al (42) observed an association beween microcalcification of the media and inhibitors of calcification like fetuin-A. Thus, different pathophysiology mechanisms operating in different vascular beds probably account for the association between fetuin-A levels and aortic calcifications but not coronary calcifications.…”

CKD and Sudden Cardiac Death

“…In the setting of ESRD, vascular remodeling is even more pronounced because calciumphosphate deposition may further exacerbate vascular integrity. 56,61 Elevated phosphate concentrations and an increase in the calcium-phosphate product contribute to myocardial and vessel calcification as well as plaque instability, and increase SCD risk by 20%-30%. 62 Structural changes can alter the electrophysiologic properties of the myocardium.…”

A materials science vision of extracellular matrix mineralization

“…Unveiling the full mechanism of pathological cardiovascular calcification is thus an active are of research and many questions remain on the origin of the mineral and its nucleation and growth. It is important to note that the cardiovascular calcification comprises a different variety of calcium phosphate to bone (whitlockite versus hydroxyapatite 85 ) and cardiovascular mineralization is thus quite different from bone proper. Nonetheless, a postulated driving force for cardiovascular calcification at the local level is provided by osteoblasts-like cells, which can transdifferentiate from local pericytes and smooth muscle cells, or can differentiate from circulating pluripotent cells 74 .…”