Regulation of Skeletal Development by Sphingomyelin Phosphodiesterase 3

长期以来，鞘脂被认为是惰性结构分子。然而，随着鞘脂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，这种观点在最近几年发生了根本性的变化。神经鞘磷脂磷酸二酯酶3(SMPD3；又称中性神经鞘磷脂酶2)是一种新的骨骼发育调节因子，最近发展的时空缺乏或过度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是一种细胞膜结合的酶，它能裂解鞘磷脂生成磷胆碱和神经酰胺，这两种代谢物是多种代谢途径的中间产物。Smpd3高表达于两种主要的骨骼细胞类型：软骨细胞和成骨细胞。此外，在神经元中也检测到Smpd3的强烈表达。Smpd3基因功能隐性突变的丧失会导致生长板软骨和骨骼矿化不良。我们报告了这些突变小鼠肥大软骨细胞正常凋亡的延迟。总体而言，这些异常会影响所有软骨内骨骼的发育。 最初有人提出，大脑中的SMPD3活动通过下丘脑的继电器来调节骨骼发育。然而，使用转基因方法，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。尽管在不同的组织中还表达了其他的鞘磷脂酶，但SMPD3似乎在骨骼组织的发育和功能中扮演着不同的角色。目前，SMPD3在骨骼组织中的作用机制还很不清楚，因为对SMPD3产生的两种代谢物的细胞特异性作用的彻底分析仍然缺乏。在细胞外小泡中检测到SMPD3的存在，这些小泡被认为为初始矿物成核提供了受保护的环境。考虑到抑制鞘磷脂酶活性阻止外体释放，SMPD3可能通过细胞外小泡帮助启动基质矿化。我们建议使用最近报道的几种转基因和基因敲除小鼠模型、细胞培养系统和包括微型计算机断层扫描、组织形态计量学、生物化学和电子显微镜在内的各种分析技术来研究这些可能性。 拟议的计划将产生新的知识，通过一种新的调节器来调节成骨细胞的功能，这可能对组织工程有未来的意义，例如开发用于骨组织再生的改进的生物材料。

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.