BMP2 and atrial fibrillation: It has been shown that the BMP-2 receptor BMPRII was expressed predominantly in the AF associated with severe disc degeneration in our previous study [21], and the BMP-2 uptake at this site (where fibroblast-like cells reside) typically leads to osteophyte formation, because it has been shown in several studies that exogenous BMP-2 induced the BMPRII expression of ligamentous fibroblasts and, in turn, initiated endochondral ossification [47,48].