Mineralization osteoblast transport

Bone is a porous tissue composite material containing a fluid phase, a calcified bone mineral, hydroxyapatite (HA), and organic components (mainly, collagen type). The variety of cellular and noncellular components consist of approximately 69% organic and 22% inorganic material and 9% water. The principal constiments of bone tissue are calcium (Ca ), phosphate (PO ), and hydroxyl (OH ) ions and calcium carbonate. There are smaller quantities of sodium, magnesium, and fluoride. The major compound, HA, has the formula Caio(P04)g(OH)2 in its unit cell. The porosity of bone includes membrane-lined capillary blood vessels, which function to transport nutrients and ions in bone, canaliculi, and the lacunae occupied in vivo by bone cells (osteoblasts), and the micropores present in the matrix. 

The mechanism of action of the vitamin D metabolites remains under active investigation. However, calcitriol is well established as the most potent agent with respect to stimulation of intestinal calcium and phosphate transport and bone resorption. Calcitriol appears to act on the intestine both by induction of new protein synthesis (eg, calcium-binding protein and TRPV6, an intestinal calcium channel) and by modulation of calcium flux across the brush border and basolateral membranes by a means that does not require new protein synthesis. The molecular action of calcitriol on bone has received less attention. However, like PTH, calcitriol can induce RANK ligand in osteoblasts and proteins such as osteocalcin, which may regulate the mineralization process. The metabolites 25(OH)D and 24,25(OH)2D are far less... 

The only essential components of the mineral deposition mechanism that are fairly certain at this time relate to phosphate. Even for phosphate, alternative mechanisms are proposed, which are not mutually exclusive but probably function in parallel, in the regulation of different aspects of skeletal calcium transport, and to some extent provide redundancy that allows many mineral transport disorders to be survivable. Alkaline phosphatase activity is essential to produce phosphate. Its major substrate is pyrophosphate. In the absence of the alkaline phosphatase, normally highly expressed as an ectoenzyme by osteoblasts, there is little matrix mineralization... 

Fluoroaluminate complexes can mimic the action of many hormones, neurotransmitters, and growth factors. G-protein-mediated cell responses are key steps in neurotransmission and intercellular signaling in the brain [20], and TFA acts as an active stimulatory species [21]. Exposure of osteoblasts to TFA results in a marked potentiation of intracellular orthophosphate transport, alluding to the anion s ability to increase bone mineralization [22]. Brief exposure to aluminum fluoride complexes induces prolonged enhancement of synaptic transmission [23] and can potentially affect the activity of many other ion channels and enzymes in the kidney [24]. Rapid and dynamic changes of the cytoskeletal actin network are of vital importance to the motility of many cells, and TFA induction effects a pronounced and sustained... 

Laboratory findings in rickets and osteomalacia include an increased serum ALP, with other alterations in bone and mineral metabolism dependent on the cause and severity of the disorder. ALP is usually increased because of the increased osteoblastic activity associated with producing unmineralized osteoid. Calcium may be low-normal or low in vitamin D deficiency depending on the severity of the disease. Phosphate may be normal or low, but falls with the development of secondary hyperparathyroidism. The serum calcium and PTH concentrations are usually normal in renal tubular defects of phosphate transport. Vitamin D nutrition may be assessed by the determination of serum 25(OH)D. Renal phosphate defects can be best assessed by determination of the renal phosphate threshold. 

Mineralization is the precipitation of calcium phosphate, but biochemical mediation of this process is not fully understood. In this chapter, the chemistry underlying mineralization (Sect. 1) and the structures ofbones and teeth (Sect. 2) are described. Osteoblasts secrete osteoid matrix and matrix vesicles that transport type I collagen and calcium phosphate, respectively, to the matrix where they will mineralize. Secreted matrix vesicles take up calcium and phosphate until they burst and release the calcium phosphate, which then redissolves and remineralizes around the type I collagen (Sect. 3). Glycoproteins involved in correctly modeling bone and dentin, and the role of osteocalcin in limiting excessive bone growth is then discussed (Sect. 4). There follows a detailed description of enamel (E) mineralization and of the major proteins involved (Sect. 5) followed by two summaries the difference between enamel and bone mineralization, and the vitamins required for mineralization (Sect. 6). 

Skeletal tissue mineralization (bone formation) is initiated by osteoblasts, which secrete the osteoid matrix (Fig. 9.4). They express type I procollagen in secretory vesicles together with matrix vesicles that pinch off from the membrane. The matrix vesicles are pushed away from the cell surface, possibly by the flow of fluid containing calcium and phosphate ions that are also transported through the cell from the extracellular fluid on the outer surface. Collagen fibers develop further away from the cell surface than from fibroblasts. 

Fig.9.8 Removal of pyrophosphate is necessary for precipitation. Pyrophosphate (PPi) inhibits the precipitation of calcium phosphate. In the bone matrix, PC-1 (red) is the major producer of PPi from nucleotide triphosphates (NTPs, thick arrow on left) and ANK is a minor producer by transporting it from the cytosol of osteoblasts. TNAP (green) causes mineralization by its phosphatase activity converting PPi to two molecules of Pi. TNAP also generates Pi directly from NTPs and PPi, but most Pi and most Ca2+ are derived directly from the diet (thick arrow on right) (Slightly modified from Fig. 4 in Hessle L et al. (2002) Tissue-nonspecific alkaline phosphatase and plasma cell membrane glycoprotein-1 are central antagonistic regulators of bone mineralization. Proceedings of the National Academy of Sciences 99 9445-9449. Copyright (2002) National Academy of Sciences, U.S.A)...

Vitamin D is a fat soluble vitamin related to cholesterol. In the skin, sunlight spontaneously oxidizes cholesterol to 7-dehydrocholesterol. 7-Dehydrocholesterol spontaneously isomerizes to cholecalciferol (vitamin D3), which is oxidized in the liver to 25-hydroxy cholecalciferol and, under the influence of PTH in the kidney, to 1,25-dihy-droxy cholecalciferol (calcitriol), the active form of vitamin D. Vitamin D induces the expression of calcium ion transport proteins (calbindins) in intestinal epithelium, osteoclasts, and osteoblasts. Calbindins and transient receptor potential channels (TRPV) are responsible for the uptake of calcium from the diet. In children, the absence of sunlight provokes a deficiency of vitamin D, causing an absence of calbindins and inadequate blood calcium levels. Osteoid tissue cannot calcify, causing skeletal deformities (rickets). In the elderly, there is a loss of intestinal TRPV receptors and decreased calbindin expression by vitamin D. In both cases, the resultant low blood calcium levels cause poor mineralization during bone remodeling (osteomalacia). Rickets is the childhood expression of osteomalacia. Osteoclast activity is normal but the bone does not properly mineralize. In osteoporosis, the bone is properly mineralized but osteoclasts are overly active. 

Osteocytes are mature cells derived from the osteoblasts implanted in mineralized bone matrix. They have a minor contribution to new bone formation compared to osteoblasts. Osteocytes are arranged around the central lumen of an osteon and between lamellae (Fig. 1). Osteocytes have an interconnecting three-dimensional (3D) network. They are linked with adjacent osteocytes through small channels called canaliculi. Osteocytes are responsive to physiological stress and strain signals in bone tissue and also help to balance osteoblastic and osteoclastic activity to deposit new bone and to dissolve old bone, respectively. Additionally, they act as transporting agents of minerals between bone and blood. 

Dixon, S. J. Wilson, J. X. 1992. Adaptive regulation of ascorbate transport in osteoblastic cells. J. Bone Miner. Res. 7 675-681. 

---