RESEARCH ARTICLE Studies of OC-STAMP in Osteoclast Fusion: A New Knockout Mouse Model, Rescue of Cell Fusion, and Transmembrane Topology

The fusion of monocyte/macrophage lineage cells into fully active, multinucleated, bone re-sorbing osteoclasts is a complex cell biological phenomenon that utilizes specialized proteins. OC-STAMP, a multi-pass transmembrane protein, has been shown to be required for pre-os-teoclast fusion and for optimal bone resorption activity. A previously reported knockout mouse model had only mononuclear osteoclasts with markedly reduced resorption activity in vitro, but with paradoxically normal skeletal micro-CT parameters. To further explore this and relat-ed questions, we usedmouse ES cells carrying a gene trap allele to generate a secondOC-STAMP null mouse strain. Bone histology showed overall normal bone form with large num-bers of TRAP-positive, mononuclear osteoclasts. Micro-CT parameters were not significantly different between knockout and wild typemice at 2 or 6 weeks old. At 6 weeks, metaphyseal TRAP-positive areas were lower and mean size of the areas were smaller in knockout femora, but bone turnover markers in serum were normal. Bone marrow mononuclear cells became TRAP-positive when cultured with CSF-1 and RANKL, but they did not fuse. Expression levels of other osteoclast markers, such as cathepsin K, carbonic anhydrase II, and NFATc1, wer