The role of endogenous low molecular weight mineralization inhibitors in spine health - Abstract In vertebrates, calcium and phosphate form a metastable solution, from which they precipitate at specific nucleation sites to form hydroxyapatite. Under physiological conditions, mineralization is restricted to skeleton and teeth, despite the abundant presence of nucleation sites in soft connective tissues. The pathological deposition of hydroxyapatite outside the boundaries of the skeleton, is known as ectopic mineralization, a condition that affects millions of people around the globe. Ectopic mineralization is linked to a wide variety of clinical conditions such as trauma, aging, cancer, diabetes, and autoimmune diseases, all major causes of significant morbidity and mortality. Related to the skeleton, an understudied research area is the pathological mineralization of spinal joints, and especially the tissues of the intervertebral disc (IVD). This is remarkable considering the high prevalence of chronic low back and neck pain and associated IVD degeneration and spinal pathologies and their dramatic socio-economic consequences. The major risk factor associated with back pain is the degenerative status of the IVD and ectopic mineralization significantly contributes to this pathology. Current information indicates that tissues are protected against uncontrolled mineralization by two classes of complementary systems, proteins and low molecular weight (LMW) compounds (< 1000 Da). The first group includes proteins like fetuin-A, osteopontin and Matrix Gla Protein whereas the best-known example of the second class is inorganic pyrophosphate (PPi). Noteworthy, the role that citrate and phosphocitrate (PC), two other endogenous metabolites that inhibit mineralization, play in the prevention of ectopic mineralization has received surprisingly little attention. Moreover, how the extracellular levels of these endogenous LMW compounds are regulated is incompletely understood. One major goal of this proposal is understanding the mechanisms by which extracellular PPi together with citrate and PC regulate the mineralization of spinal and other soft tissues. These studies will build on our previous work in which we identified the membrane proteins ABCC6 and ANK as crucial factors in extracellular PPi homeostasis. We propose to delineate how these proteins collaborate to prevent ectopic mineralization of IVD and other tissues. In addition, we recently discovered that ANK regulates extracellular levels of citrate. We now plan to determine if extracellular citrate, which tightly binds calcium, protects the IVD against dystrophic mineralization. Finally, we propose to elucidate the mechanisms underlying synthesis and cellular extrusion of PC, an understudied but very potent endogenous mineralization inhibitor. We expect that the proposed studies will provide mechanistic insight into how extracellular homeostasis of PPi, citrate and PC is maintained, and how these metabolites prevent dystrophic mineralization of the IVD. Moreover, we will determine if these endogenous LMW mineralization inhibitors can be used therapeutically, addressing the unmet clinical need of effective treatments for ectopic mineralization in the spine, a condition that affects millions of individuals worldwide.
