Osteoclast-stimulating factor interacts with the spinal muscular atrophy gene product to stimulate osteoclast formation

We have recently identified and cloned an intracellular peptide termed oste oclast-stimulating factor (OSF) that increases osteoclast (OCL) formation a nd bone resorption through a cellular signal transduction cascade, possibly through its interaction with c-Src or related family members. To further i dentify participants in the OSF signaling cascade, we used yeast two-hybrid screening with Saccharomyces cerevisiae, and we found that the 40-kDa spin al muscular atrophy disease-determining gene product, survival motor neuron (SMN), interacts with the OSF-Src homology 3 domain. Reverse transcription -polymerase chain reaction analysis of SMN mRNA expression in cells of the OCL lineage demonstrates that expression of the exon 7 splice variant of SM N is restricted to mature OCLs, whereas the unspliced transcript was expres sed in OCL precursors as well as mature OCLs. Treatment of murine bone marr ow cultures with conditioned media (5% (v/v)) from 293 cells transiently ex pressing the SMN cDNA significantly increased OCL formation, compared with treatment with conditioned media from mock-transfected cells. Furthermore, OCL-stimulatory activity by OSF or SMN was abolished by antisense construct s to SMN or OSF, respectively. These data confirm the participation of SMN in the OSF-enhanced expression of an OCL stimulator. OSF-SMN interaction ma y provide more insights into novel cellular signaling mechanisms that may p lay an important role in congenital bone fractures associated with type I s pinal muscular atrophy disease.