Background Directed monocyte migration, cell-cell contacts and fusion to multinucleated cells are necessary events in the process of osteoclast differentiation. Since transient receptor potential canonical channel 1 (TRPC1) has a pivotal role in these specific processes in other cell types, we investigated the role of TRPC1 in osteoclastogenesis and bone remodeling.
Objectives To analyse the effect of TRPC1 knockout on in-vitro osteoclast differentiation and to study the skeletal phenotype of TRPC1-/- mice under physiological conditions as well as using an animal model of postmenopausal osteoporosis.
Methods For all in-vitro experiments, bone marrow macrophages were isolated from TRPC1-/- mice and corresponding wild type (WT) controls and were cultured in the presence of macrophage colony-stimulating factor and receptor activator of nuclear factor kappa-B ligand. Osteoclast-like cells were characterized by staining for tartrate-resistant acid phosphatase (TRAP). Using quantitative real-time PCR mRNA levels of TRPC1 and nuclear factor of activated T-cells, cytoplasmic, calcineurin-dependent 1 (NFATC1) were analysed. Time-lapse videomicroscopy was used to study osteoclast migration and fusion. The intracellular Ca2+ concentration of osteoclasts was measured ratiometrically with the fluorescent Ca2+ dye Fura-2. The skeletal phenotype of 16-week old mice was investigated by μCT-analyses of trabecular bone in the lumbar spine. Ovariectomy was performed on 12-week old sex- and aged-matched littermates as a model for human postmenopausal bone loss.
Results PCR analyses revealed that TRPC1 is hardly expressed in monocytes and pre-osteoclasts, but gets upregulated during osteoclast-differentiation. In osteoclast formation assays, the loss of TRPC1 is associated with a significantly impaired osteoclast differentiation. As seen in μCT analysis, TRPC1-/- mice showed no differences in bone phenotype compared to WT mice under physiological conditions. However, in the ovariectomy model of estrogen-deficiency mediated bone loss, TRPC1-/- mice exhibited a reduced loss of trabecular bone volume (-28,9% in WT compared to -13,1% in TRPC1-/-) and bone tissue density (-3,2% in WT compared to +0,9% in TRPC1-/-) in LW5. Analysing the underlying signaling pathways, we found no differences in Ca2+-oscillations and store-operated calcium entries and no differences in NFATC1 expression. In time-lapse microscopy, however, we observed a reduced capacity of TRPC1-/- osteoclast precursors to migrate and to fuse, due to a reduced velocity, translocation and a reduced increase of cell area over time.
Conclusions These results provide the first evidence that TRPC1 is necessary for migration and fusion of osteoclasts and that the loss of TRPC1 while not affecting physiological bone turnover has a clear effect on accelerated bone loss as seen in estrogen deficiency. Thus, our data implicate that TRPC1 may represent a potential target for treating rapid osteoporotic bone loss.
Disclosure of Interest None Declared