MO054VASCULAR CALCIFICATION IMPAIRS BONE MINERALIZATION
Authors: Mace Maria Lerche , Gravesen Eva , Nordholm Anders , Egstrand Soeren , Morevati Marya , Behets Geert , D'Haese Patrick , Olgaard Klaus , Lewin Ewa 
Published: June 2020

Abstract

Background and Aims
Several studies report an association between low BMD and vascular calcification (VC) in the general population and in CKD patients. Development of VC in CKD is a cell regulated process where the vascular smooth muscle cell alters phenotype to a bone-like secretory cell. Our group has previously demonstrated the expression of Wnt inhibitors in VC and so we ask if the presence of VC affects bone metabolism.

Method
A novel model of isogenic aorta transplantation (ATx) was developed and used in the study. Severe uremic vascular calcifications were induced in inbred Dark Aguti (DA) rats by 5/6 nephrectomy, high phosphate diet and calcitriol treatment. After 14 weeks the calcified abdominal aorta of the uremic rat was transplanted into a normal DA rat (uremic ATx, n=16). A normal aorta was transplanted into a normal DA rat as sham group (normal ATx, n=9) and age-matched normal rats were used as one more control group (control, n=6). Rats were sacrificed 4 weeks after ATx and plasma biochemistry, bone and vessels were analyzed. Data are presented as mean±SEM or median [range]. Statistical significance *p&lt;0.5,**p&lt;0.01,*** p&lt;0.001.

Results
The uremic donor rat suffered from severe kidney disease with disturbed mineral balance and its aorta had a high calcium content of 15.7±0.8 µg Ca/mg vs. none in the normal aorta. Uremic ATx, normal ATx and control rats had same plasma levels of creatinine, Ca2+, phosphate, PTH, FGF23 and sclerostin. The uremic ATx had significant lower bone mineral density (BMD) compared to normal ATx and control rats (1576±5 vs. 1592±5* & 1613±6* mg/cc). The impact on bone mineralization was also detected in the bone histomorphometry analysis, where the uremic ATx had less osteoid compared to normal ATx (median 3% [0,3%;8%] vs. 5% [2%;8%]*). Moreover, the uremic ATx had fewer osteoblasts and more osteoclasts. The effect on bone was supported by substantial gene analysis of several genes related to bone remodeling, mineralization and Wnt signaling. The uremic ATx rats had significant changes in mRNA levels of several genes in bone compared to normal ATX and control rats such as increased alkaline phosphatase (3.26±0.29 vs. 1.56±0.32*** & 0.86±0.12***), decreased osteoblast marker osteocalcin (0.54±0.06 vs. 0.92±0.14* & 1.19±0.06***) and increased osteoclast marker cathepsin K (2.36±0.24 vs. 1.19±0.23** & 1.01±0.07***). In addition, we found upregulation of the bone mineralization inhibitors osteopontin (1.46±0.18 vs. 0.69±0.17** & 0.49±0.06**) and progressive ankylosis protein homolog (8.10±0.60 vs. 3.21±0.74** & 2.39±0.48**) as well as collagen I (9.30 [2.73;14.81] vs. 2.30 [0.31;19.33]* & 2.78 [1.63;7.52]***). The inhibitor of bone formation sclerostin was significant increased (2.90 [1.54;13,29] vs. 1,23 [0.32;3.20]** & 0.93 [0.39;2,19]**) along with a slight downregulation of bone stimulator BMP2 (3.18±0.15 vs. 4.06±0.30* & 3.98±0.44*. Finally, the Wnt signaling pathway was affected by upregulation of β-catenin (3.15±0.25 vs. 1.28±0.26*** & 1.03±0.13***) and the downstream gene Snail1 (3.73±0.33 vs. 1.54±0.45*** & 1.04±0.12***).

Conclusion
These novel findings indicate the existence of a tissue crosstalk between vessels and bone. The presence of vascular calcification lowers BMD, decreases the amount of osteoid and affects several pathways in bone.