Compositions and methods are provided for modulating cell-cell fusion by using agents that modulate expression, activity, or function of intermediate-conductance calcium-activated potassium (SK4) channel. In some embodiments, the compositions and methods of the invention provide for inhibition of multi-nucleated osteoclastogenesis and cell-cell fusion, especially cell fusion involving macrophages. In such embodiments, the compositions can comprise inhibitory nucleic acids, monoclonal antibodies or small molecule inhibitors of the SK4 channels and find use in preventing and/or treating various diseases or disorders including bone loss, autoimmune and inflammatory diseases or disorders, implant and transplant rejection, and cancer metastasis. In other embodiments, the compositions and methods of the invention provide for activation of cell-cell fusion. Also provided are methods to screen for SK4 channel modulators (inhibitors or activators) that modulate cell-cell fusion, particularly macrophage cell fusion.