Deficiency of sphingomyelin synthase 1 but not sphingomyelin synthase 2 reduces bone formation due to impaired osteoblast differentiation

Background There are two isoforms of sphingomyelin synthase (SMS): SMS1 and SMS2. SMS1 is located in the Golgi apparatus only while SMS2 is located in both the plasma membrane and the Golgi apparatus. SMS1 and SMS2 act similarly to generate sphingomyelin (SM). We have undertaken the experiments reported here on SMS and osteoblast differentiation in order to better understand the role SMS plays in skeletal development. Methods We analyzed the phenotype of a conditional knockout mouse, which was generated by mating a Sp7 promoter-driven Cre-expressing mouse with an SMS1-floxed SMS2-deficient mouse (Sp7-Cre;SMS1(f/f);SMS2(-/-) mouse). Results When we compared Sp7-Cre;SMS1(f/f);SMS2(-/-) mice with C57BL/6, SMS2-deficient mice (SMS1(f/f);SMS2(-/-)) and SP7-Cre positive control mice (Sp7-Cre, Sp7-Cre;SMS1(+/+);SMS2(+/-) and Sp7-Cre;SMS1(+/+);SMS2(-/-)), we found that although cartilage formation is normal, Sp7-Cre;SMS1(f/f);SMS2(-/-) mice showed reduced trabecular and cortical bone mass, had lower bone mineral density, and had a slower mineral apposition rate than control mice. Next, we have used a tamoxifen-inducible knockout system in vitro to show that SMS1 plays an important role in osteoblast differentiation. We cultured osteoblasts derived from ERT2-Cre;SMS1(f/f)SMS2(-/-) mice. We observed impaired differentiation of these cells in response to Smad1/5/8 and p38 that were induced by bone morphogenic protein 2 (BMP2). However, Erk1/2 phosphorylation was unaffected by inactivation of SMS1. Conclusions These findings provide the first genetic evidence that SMS1 plays a role in bone development by regulating osteoblast development in cooperation with BMP2 signaling. Thus, SMS1 acts as an endogenous signaling component necessary for bone formation.