Fig. 8. α-La treatment suppresses ectopic osteoclast formation in fibrous dysplasia explants.
a Illustration of ex vivo bone marrow culture system based on a tetracycline-inducible model of FD. b TRAP staining of bone marrow explants from a homozygous GαsR201C mouse (FD) and a wild-type littermate (WT). Explants were either cultured with M-CSF alone, or M-CSF and Doxy. c ELISA quantification of mRANKL produced in FD vs WT cultures following Doxy treatment. (WT n = 2; FD n = 4) (P = 0.0005). d Representative images of FD explants activated by Doxy treated with 6 μg/ml isotype control or α-La mAb. e Quantification of the number of fusions producing osteoclast syncytia with ≥3 nuclei. (n = 4) (P = 0.005) f Quantification of the number of osteoclast syncytia with ≥3 nuclei from d. (n = 4) (P = 0.02) In b and d, arrowheads denote multinucleated osteoclasts and arrows denote fibrous cell masses developed after Doxy addition and characteristic for FD. Statistical significance was assessed via one-tailed unpaired t-test in c or paired, t-tests in e and f. Data are presented as mean values + /- SEM. g An illustration of changes in the steady-state level and cellular localization of La protein in the process of osteoclast formation. La (green) carries out its canonical, ancient function in the nuclei of all eukaryotic cells as an essential RNA-binding protein. We propose that La has an additional, specialized function in the formation of multinucleated osteoclasts. First, La dissipates as circulating monocytes become osteoclast precursor cells. When osteoclast commitment is initiated by RANKL, La returns but is quickly cleaved by proteases and shuttled to the surface of osteoclasts. At the surface of fusing osteoclasts, La plays a novel role as a membrane fusion manager. When osteoclasts arrive at the “right size” for their biological function, surface La dissipates and is replaced by canonical, non-cleaved La that returns to the nuclei of mature osteoclasts. Source data are provided as a Source Data file.