Control of Vertebrate Skeletal Mineralization by Polyphosphates

Abstract

BackgroundSkeletons 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/MethodologyThe 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/SignificanceWe hypothesize that polyphosphate formation and hydrolytic degradation constitute a simple mechanism for phosphate accumulation and enzymatic control of biologica...

Transient precursor amorphous phases in biomineralization.In the footsteps of Heinz A. Lowenstam

“…An elemental analysis of the mice vesicles shows that the Ca/P ratio is below 1, implying that the disordered phase is rich in phosphate and could possibly be a polyphosphate, as has been suggested by Omelon et al [53] based on biochemical evidence and in situ identification within resorbing bone and mineralizing cartilage. We do not know how the mineral in the intracellular vesicles is transported to the extracellular space.…”

The role of intracellular calcium phosphate in osteoblast-mediated bone apatite formation

“…Nucleation theory has been used to explain bone apatite formation because extracellular fluid is sufficiently saturated with respect to calcium and phosphate [perhaps stored as calciumpolyphosphate complexes (30)] to allow for mineral formation (31). However, recent insights into the role of amorphous mineral precursors in bone mineralization (3), and our and other's (12) observations of intracellular calcium and phosphorus-containing vesicles, suggests that active transport of mineral from the intrato the extracellular space may play a role in bone apatite formation.…”

Intracellular Phosphate Dynamics in Muscle Measured by Magnetic Resonance Spectroscopy during Hemodialysis

“…13 We suggested that polyP could be involved in this increase of Pi; polyP is an energy-rich polymer consisting of up to several hundreds of phosphate residues. 14,15 There are only a few studies concerning the occurrence of polyP in animals; they are widely distributed and can be localized in the cell in a flexible, oriented, and relatively stable form, but also used in response to a wide variety of metabolic needs as a source of energy. These findings could have important consequences in clinical practice.…”