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

Goichi Matsumoto,Makoto Taniguchi,Ken Watanabe,Chieko Hashizume,Toshiro Okazaki

doi:10.1186/s10020-019-0123-0

Abstract

BackgroundThere 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.MethodsWe 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;SMS1f/f;SMS2−/− mouse).ResultsWhen we compared Sp7-Cre;SMS1f/f;SMS2−/− mice with C57BL/6, SMS2-deficient mice (SMS1f/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;SMS1f/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;SMS1f/fSMS2−/− 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.ConclusionsThese 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.

Highlights

Skeletal homeostasis requires a precise balance between bone-forming osteoblasts and bone-resorbing osteoclasts
E17.5 and E15.5 Sp7Cre;SMS1f/f;SMS2−/− mice were smaller than littermate controls, but no significant patterning changes were observed in their bone or cartilage
Mineralization of calvarial bone was delayed in Sp7-Cre;SMS1f/f;SMS2−/− mice, but no skeletal defects were observed in SMS1f/f; SMS2−/− and Sp7-Cre mice

Summary

Introduction

Skeletal homeostasis requires a precise balance between bone-forming osteoblasts and bone-resorbing osteoclasts. Osteoblasts differentiate and produce the bone matrix during skeletal development (Ducy et al 2000). SMS1 is located in the Golgi apparatus and is responsible for bulk production of SM, whereas SMS2 is located in both the plasma membrane and Golgi apparatus (Jeckel et al 1990; Villani et al 2008; Yeang et al 2008). Homozygous SMS1-mutant mice exhibit moderate neonatal lethality, reduced body weight, loss of fat-tissue mass, and growth deterioration, all of which suggest that they have metabolic abnormalities (Yano et al 2011). SMS1 is located in the Golgi apparatus only while SMS2 is located in both the plasma membrane and the Golgi apparatus. We have undertaken the experiments reported here on SMS and osteoblast differentiation in order to better understand the role SMS plays in skeletal development

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