Characterization of mineral deposits in matrix gla protein-deficient mice, a model of vascular matrix calcification

Abstract

Event Abstract Back to Event Characterization of mineral deposits in matrix gla protein-deficient mice, a model of vascular matrix calcification Ophélie Gourgas1, Juliana Marulanda2, Peng Zhang1, Monzur Murshed2, 3, 4 and Marta Cerruti1 1 McGill University, Mining and Materials Engineering, Canada 2 McGill University, Faculty of Dentistry, Canada 3 McGill University, Department of Medicine, Canada 4 McGill University, Genetic sUnit, Shriners Hospital for Children, Canada Introduction: Vascular calcification is a pathological condition that significantly increases the morbidity in patients suffering from chronic kidney disease, and cardiovascular complications. Recent results showed this to be a complex and regulated process[1]. We recently showed that vascular calcification in the medial layer of the arteries is mainly regulated by two extracellular proteins, matrix Gla protein (MGP) and elastin (ELN)[2], but their mechanism of action is still unknown. One hypothesis is that MGP deficiency leads to an unmasking of ELN, which can provide a scaffold for mineral nucleation[3]. This study aims at shedding some light on this process by characterizing the mineral deposits found in the aortas of mice models genetically modified so that they do not express MGP, and their arteries easily mineralize[3]. We compare the minerals formed in these models with those found in bone, and relate their distribution and type to the location in the artery and mouse age. These results are crucial to develop a treatment aimed at stopping or even reverting vascular calcification. Materials and Methods: MGP-deficient mice were generated as described in[3]. Autopsy samples from the thoracic and the abdominal aortas from 1, 2, 3, and 5 week-old mice were characterized by X-ray photoelectron spectroscopy (XPS) (Thermo Scientific K Alpha), Raman spectroscopy (Bruker Senterra), Fourier transform infrared (FTIR) spectroscopy (Bruker Tensor), and Ca-edge bulk X-Ray absorption fine edge spectroscopy (XAFS) at the Canadian Light Source. Results and Discussions: Both Raman Spectroscopy and XAFS (Figure 1A) show that the main component of minerals found in MGP-deficient mice aortas is hydroxyapatite (HA). Amorphous calcium phosphate (ACP) and octacalcium phosphate (OCP) are also present. The amount of HA increases with the age of the mice, while the amounts of ACP and OCP decrease, indicating the conversion of ACP and OCP to HA (Figure 1B). XPS and Raman show that the minerals are not distributed uniformly within the arteries. While there is some correlation with the amount of ELN, larger variations are found on a microscopic scale (Figure 2A). All techniques indicate that matrix mineralization is an age-dependent process; for example, the crystallinity of the apatitic phases evaluated by Raman increase with the age of the mouse (Figure 2B), reaching values close to those found in bone when the mice are 5 week-old. Conclusion: These results show for the first time that the maturation of minerals in vascular calcification is similar to that found in bone, with ACP and OCP as precursors of HA[4]. This suggests that by inhibiting the formation of the early mineral precursors and preventing their transformation into more crystalline phases it may be possible to prevent or reduce vascular calcification. Figure 1. A) Representative Ca K-edge XAFS spectra of calcified aortas of MGP-deficient mice at various ages, and reference HA; B) Percentage of the different calcium phosphate phases in calcified aortas from MGP-deficient mice at different ages determined by linear combination fitting of the XAFS spectra. Figure 2. A) Distribution of Ca/P ratios determined by XPS on a section of calcified aorta from 5 week-old MGP-deficient mouse; B) Raman spectra of calcified aortas from mice at different ages and bone. The Full Width at Half Maximum (FWHM) of the ν1 phosphate at 960 cm-1, which is inversely proportional to the crystallinity of the minerals.