Regulation of Skeletal Development by Sphingomyelin Phosphodiesterase 3

长期以来，鞘脂被认为是一种惰性结构分子。然而，近年来这种观点发生了根本性的变化，因为鞘脂现在被认为是调节细胞生长、死亡、分化和应激反应的重要代谢物。鞘磷脂磷酸二酯酶3 （SMPD3，也称为中性鞘磷脂酶2）作为骨骼发育的一种新型调节剂的鉴定，以及最近开发的在时空方式上缺乏或过表达该酶的动物模型，为进一步了解导致软骨内骨发育的细胞事件创造了令人兴奋的机会。****** SMPD3是一种细胞膜结合酶，可裂解鞘磷脂生成磷脂胆碱和神经酰胺，这两种代谢物作为多种代谢途径的中间体。Smpd3在两种主要的骨骼细胞类型中高度表达：软骨细胞和成骨细胞。此外，在神经元中也检测到强烈的Smpd3表达。在fro/fro小鼠中，Smpd3的功能隐性突变缺失导致生长板软骨和骨的矿化不良。我们报道了这些突变小鼠中肥大软骨细胞正常凋亡的延迟。总的来说，这些异常影响所有软骨内骨的发育。******最初提出大脑中的SMPD3活动通过下丘脑中继调节骨骼发育。然而，利用转基因方法，我们的团队最近证明了SMPD3在成骨细胞和软骨细胞中的局部作用是正常骨骼发育所必需的。尽管在各种组织中也有其他鞘磷脂酶表达，但SMPD3似乎在骨组织发育和功能中发挥着独特的作用。目前，SMPD3在骨组织中的作用机制在很大程度上是未知的，因为对SMPD3产生的两种代谢物的细胞特异性作用的深入分析仍然缺失。在细胞外囊泡中检测到SMPD3的存在，这被认为为初始矿物成核提供了保护环境。考虑到鞘磷脂酶活性的抑制可以阻止外泌体的释放，SMPD3可能通过细胞外囊泡帮助启动基质矿化。我们建议使用几种新报道的转基因和基因敲除小鼠模型、细胞培养系统和各种分析技术，包括微计算机断层扫描、组织形态计量学、生物化学和电子显微镜来研究这些可能性。******该计划将通过一种新型调节剂产生关于成骨细胞功能调节的新知识，这可能对组织工程（如用于骨骼组织再生的改良生物材料的开发）具有未来的意义。

For a long time, sphingolipids were considered as inert structural molecules. This view, however, has radically changed in recent years as sphingolipids are now recognized as important metabolites regulating cell growth, death, differentiation and stress responses. The identification of sphingomyelin phosphodiesterase 3 (SMPD3; also known as neutral sphingomyelinase 2) as a novel regulator of skeletal development and the recently developed animal models lacking or overexpressing this enzyme in a spatiotemporal manner have created exciting opportunities to further understand the cellular events leading to endochondral bone development. ****** SMPD3, a cell membrane-bound enzyme, cleaves sphingomyelin to generate phosphocholine and ceramide, two metabolites which act as intermediates for multiple metabolic pathways. Smpd3 is highly expressed by two major skeletal cell types: chondrocytes and osteoblasts. Also, strong Smpd3 expression has been detected in neurons. A loss of function recessive mutation in Smpd3 in fro/fro mice leads to poor mineralization of both growth plate cartilage and bone. We reported a delay in normal apoptosis of hypertrophic chondrocytes in these mutant mice. Collectively, these anomalies affect the development of all endochondral bones. ****** It was initially proposed that SMPD3 activity in the brain regulates skeletal development through a hypothalamic relay. However, using a transgenic approach our group has recently demonstrated that the local actions of SMPD3 in both osteoblasts and chondrocytes are required for a normal skeletal development. Although there are other sphingomyelinases expressed in various tissues, SMPD3 appear to play distinct roles in skeletal tissue development and function. At present the mechanism of action of SMPD3 in the skeletal tissue is largely unknown as a thorough analysis of the cell-specific roles of two metabolites generated by SMPD3 is still missing. The presence of SMPD3 has been detected in the extracellular vesicles, which are thought to provide the protected environment for initial mineral nucleation. Considering that inhibition of sphingomyelinase activity prevents exosome release, it is possible that SMPD3 helps the initiation of matrix mineralization via extracellular vesicles. We propose to investigate these possibilities using several newly reported transgenic and knockout mouse models, cell culture systems and a variety of analytical techniques including microcomputed tomography, histomorphometry, biochemistry and electron microscopy.****** The proposed program will generate new knowledge on the regulation of osteoblast function by a novel regulator, which may have future implications on tissue engineering such as development of improved biomaterials for skeletal tissue regeneration.