A study on the function of Sphingomyelin Synthases

骨细胞被认为在骨代谢中起着关键作用，尽管关于其特定产物和生理功能的信息有限。为了分离在骨细胞中表达的基因，我们使用了从富含骨细胞的骨组织中提取RNA的消减抑制聚合酶链式反应方法。用这种方法分离的A基因编码一种与磷脂代谢有关的新酶，它被发现是一种鞘磷脂合成酶(SMS2：Huitema，et.等(2004)；Yamaoka等人。Al.(2004))。为了检测Sms2在体内的表达，将LacZ基因导入小鼠Sms2基因座，以监测Sms2的表达。在E13.5，X-Gal染色仅在骨骼元素中检测到该表达，尤其是在富含成骨细胞的区域。该基因不仅在成骨细胞中表达，而且在E16.5胚胎的骨细胞中也有表达。SMS2在长骨的部分分化软骨细胞中有表达，但在未钙化的肋骨或关节软骨中未见表达。在成人中，虽然在非骨骼组织中检测到该基因的表达，但在骨细胞中明显表达。有报道称，在基质矿化过程中起关键作用的基质小泡中检测到了鞘磷脂酶的活性，并且在基质矿化过程中，鞘磷脂发生了水解性反应。此外，携带SmpdS突变的小鼠表现出骨骼畸形和钙化延迟。SmpdS是一种编码中性鞘磷脂酶的基因。事实上，它促使我们假设神经鞘磷脂可能对矿化有一些抑制作用。事实上，当Sms2基因在成骨细胞中过表达时，矿化明显受到抑制。因此，Sms2可能在成骨细胞的终末分化和/或骨中基质的矿化中发挥作用。

Osteocytes are thought to play crucial roles in bone metabolism, although limited information has been available on their specific products and physiological function. To isolate genes expressed in osteocytes, we employed subtractive suppression PCR method using RNA from osteocyte-enriched bone fraction. A genes isolated by the method encodes a novel enzyme involved in phospholipid metabolism, which has been found as a sphingomyelin synthase(SMS2 : Huitema, et. al.(2004) ; Yamaoka, et. al.(2004)). To determine the expression of Sms2 in vivo, LacZ gene was introduced into mouse Sms2 locus to monitor the expression of Sms2. At E13.5, the expression, as monitored by X-gal staining, was detected only in skeletal elements, especially in the areas enriched in osteoblasts. The expression was observed not only in osteoblasts but also in osteocytes of E16.5 embryo. Sms2 was expressed in some differentiated chondrocytes of long bones, but not detected in rib or articular cartilages which remain uncalcified. In adult, although the expression was detected in non-skeletal tissues, it is markedly expressed in osteocytes. It has been reported that sphingomyelinase activity was detected in matrix vesicles, which play a pivotal role in matrix mineralization, and that sphingomyelin was hydrolyzed upon mineralization. Furthermore, mice carrying mutations in SmpdS, a gene encoding neutral sphingomyelinase, exhibited skeletal malformation and delayed calcification. Indeed, it prompted us to hypothesize that sphingomyelin may have some inhibitory effects on mineralization. In fact, when Sms2 gene was overexpressed in osteoblasts, mineralization was significantly suppressed. Thus, Sms2 may play a role in terminal differentiation of osteoblast and/or matrix mineralization in bone.