Grants and Contributions:

Grant or Award spanning more than one fiscal year. (2017-2018 to 2022-2023)

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.