Ca2+-Dependent Regulation of NFATc1 via KCa3.1 in Inflammatory Osteoclastogenesis

In inflammatory arthritis, the dysregulation of osteoclast activity by proinflammatory cytokines, including TNF, interferes with bone remodeling during inflammation through Ca 2+ -dependent mechanisms causing pathological bone loss. Ca 2+ -dependent CREB/c-fos activation via Ca 2+ -calmodulin kinase IV (CaMKIV) induces transcriptional regulation of osteoclast-specific genes via NFATc1, which facilitate bone resorption. In leukocytes, Ca 2+ regulation of NFAT-dependent gene expression oftentimes involves the activity of the Ca 2+ -activated K + channel KCa3.1. In this study, we evaluate KCa3.1 as a modulator of Ca 2+ -induced NFAT-dependent osteoclast differentiation in inflammatory bone loss. Microarray analysis of receptor activator of NF-κB ligand (RANKL)-activated murine bone marrow macrophage (BMM) cultures revealed unique upregulation of KCa3.1 during osteoclastogenesis. The expression of KCa3.1 in vivo was confirmed by immunofluorescence staining on multinucleated cells at the bone surface of inflamed mouse joints. Experiments on in vitro BMM cultures revealed that KCa3.1 −/− and TRAM-34 treatment significantly reduced the expression of osteoclast-specific genes ( p p 2+ imaging and Western blot analysis showed that TRAM-34 pretreatment decreased transient RANKL-induced Ca 2+ amplitudes in BMMs by ∼50% ( p −/− reduced RANKL+/−TNF-stimulated phosphorylation of CREB and expression of c-fos in BMMs ( p p 2+ -dependent NFATc1 expression via CaMKIV/CREB during inflammatory osteoclastogenesis in the presence of TNF, corroborating its role as a target candidate for the treatment of bone erosion in inflammatory arthritis.