Background: Vascular calcification has been widely recognized as a significant contributor to cardiovascular risk in patients with chronic kidney disease. Recent evidence suggests that BMP-7 decreases the vascular calcification observed in uraemic rats, while BMP-2 could also be participating in this process. Gremlin, a bone morphogenetic protein antagonist, has been detected in rat aortic vascular smooth muscle cells (VSMCs), and since the role of the VSMCs into vascular calcification in uraemia is considered critical in this process, we hypothesized that gremlin could be participating in its pathogenesis. With this aim, we studied its expression in aorta from uraemic rats with calcitriol-induced vascular calcification and in 16-vessel biopsies of uraemic patients undergoing kidney transplantation.
Methods: Gremlin was detected by in situ hybridization (ISH) and immunohistochemistry (IMH). BMP-7, BMP-2 and BMP-2 receptor (BMPR2) were detected by IMH. Vascular calcification was assessed by the von Kossa staining method. Sham-operated and 5/6 nephrectomized rats (NFX) (1.2%P) were treated with vehicle or calcitriol (80 ng/kg, intraperitoneally every other day). Rats were killed after 4 weeks of treatment, and abdominal aorta was dissected for assessment of gremlin expression and vascular calcification. Epigastric arteries were obtained from dialysis patients during kidney transplantation procedure. Arteries from kidney donors were also studied.
Results: NFX rats developed a mild vascular calcification, whereas NFX-calcitriol rats developed a severe vascular and tissue calcification. A marked overexpression of gremlin was observed in the vascular media of aorta from NFX-calcitriol rats as compared with NFX and sham-calcitriol groups (4.8 +/- 1.3 versus 0.59 +/- 0.17 versus 0.19 +/- 0.07 percentage/mm(2), P < 0.01), and correlated with the BMP-2 and BMPR2 expression. Sham rats showed minimal or null gremlin expression. BMP-7 was not found in sham or calcified arteries. In human studies, we observed strong expression of gremlin mRNA and protein in the media layer of vessels from uraemic patients as compared with those from normal humans (staining score 3.72 +/- 0.95 versus 0.91 +/- 0.08 percentage/mm(2), P < 0.05).
Conclusion: We observed a marked gremlin overexpression in the media layer of vessels in uraemic rats and patients in association with vascular calcification and BMP-2 expression. We postulate that gremlin may play a role in the vascular calcification process in uraemia, and its interaction with BMP-7 or BMP-2 remains to be elucidated.