Sidney Omelon

Control of Vertebrate Skeletal Mineralization by Polyphosphates

Background Skeletons are formed in a wide variety of shapes, sizes, and compositions of organic and mineral components. Many invertebrate skeletons are constructed from carbonate or silicate minerals, whereas vertebrate skeletons are instead composed of a calcium phosphate mineral known as apatite. No one yet knows why the dynamic vertebrate skeleton, which is continually rebuilt, repaired, and resorbed during growth and normal remodeling, is composed of apatite. Nor is the control of bone and calcifying cartilage mineralization well understood, though it is thought to be associated with phosphate-cleaving proteins. Researchers have assumed that skeletal mineralization is also associated with non-crystalline, calcium- and phosphate-containing electron-dense granules that have been detected in vertebrate skeletal tissue prepared under non-aqueous conditions. Again, however, the role of these granules remains poorly understood. Here, we review bone and growth plate mineralization before showing that polymers of phosphate ions (polyphosphates: (PO3 −)n) are co-located with mineralizing cartilage and resorbing bone. We propose that the electron-dense granules contain polyphosphates, and explain how these polyphosphates may play an important role in apatite biomineralization. Principal Findings/Methodology The enzymatic formation (condensation) and destruction (hydrolytic degradation) of polyphosphates offers a simple mechanism for enzymatic control of phosphate accumulation and the relative saturation of apatite. Under circumstances in which apatite mineral formation is undesirable, such as within cartilage tissue or during bone resorption, the production of polyphosphates reduces the free orthophosphate (PO4 3−) concentration while permitting the accumulation of a high total PO4 3− concentration. Sequestering calcium into amorphous calcium polyphosphate complexes can reduce the concentration of free calcium. The resulting reduction of both free PO4 3− and free calcium lowers the relative apatite saturation, preventing formation of apatite crystals. Identified in situ within resorbing bone and mineralizing cartilage by the fluorescent reporter DAPI (4′,6-diamidino-2-phenylindole), polyphosphate formation prevents apatite crystal precipitation while accumulating high local concentrations of total calcium and phosphate. When mineralization is required, tissue non-specific alkaline phosphatase, an enzyme associated with skeletal and cartilage mineralization, cleaves orthophosphates from polyphosphates. The hydrolytic degradation of polyphosphates in the calcium-polyphosphate complex increases orthophosphate and calcium concentrations and thereby favors apatite mineral formation. The correlation of alkaline phosphatase with this process may be explained by the destruction of polyphosphates in calcifying cartilage and areas of bone formation. Conclusions/Significance We hypothesize that polyphosphate formation and hydrolytic degradation constitute a simple mechanism for phosphate accumulation and enzymatic control of biological apatite saturation. This enzymatic control of calcified tissue mineralization may have permitted the development of a phosphate-based, mineralized endoskeleton that can be continually remodeled.

Graduate Journal: Working for balance

NATURE | VOL 428 | 4 MARCH 2004 | www.nature.com/nature We all know those dark days of graduate work. A three-week experiment has failed. You dropped your perfect gel. The questions after your presentation made you feel as if you had the brains of a hamster. You think of leaving and opening a fast-food franchise. I have learned the importance of including something else in my life to keep me from disintegrating after a dark day of science. This something else must wipe all thoughts of the day of disaster from my mind. Justifying the time spent on extracurricular pursuits to one’s supervisor can be daunting, but some labs incorporate these pursuits into everyday life (see Nature 427, 268–269; 2004). Rowing indirectly brought me to my graduate life, and it still propels me through days of data despair. If I fail to focus on balancing the boat, I am certain to become a soggy scientist. Biological chemistry graduate student Josh Finkelstein hunts for harmony with his band of rock’n’roll chemists, and physics postdoc Etienne Boaknin lost and found his balance on the judo mat. The time and energy invested in our hobbies clears our minds, gives us something else to have nightmares about, and provides another subject for conversation in job interviews. ■ Sidney Omelon is a PhD student in bone biomaterials at the Samuel Lunenfeld Research Institute, Mount Sinai Hospital, Toronto, Canada. GRADUATE JOURNAL