Abstract
The strength of bone depends on bone quantity and quality. Osteocalcin (Ocn) is the most abundant noncollagenous protein in bone and is produced by osteoblasts. It has been previously claimed that Ocn inhibits bone formation and also functions as a hormone to regulate insulin secretion in the pancreas, testosterone synthesis in the testes, and muscle mass. We generated Ocn-deficient (Ocn-/-) mice by deleting Bglap and Bglap2. Analysis of Ocn-/-mice revealed that Ocn is not involved in the regulation of bone quantity, glucose metabolism, testosterone synthesis, or muscle mass. The orientation degree of collagen fibrils and size of biological apatite (BAp) crystallites in the c-axis were normal in the Ocn-/-bone. However, the crystallographic orientation of the BAp c-axis, which is normally parallel to collagen fibrils, was severely disrupted, resulting in reduced bone strength. These results demonstrate that Ocn is required for bone quality and strength by adjusting the alignment of BAp crystallites parallel to collagen fibrils; but it does not function as a hormone.
Highlights
The strength of bone is dependent on its quantity and quality
Earlier studies by other investigators have suggested that Ocn decreases the quantity of bone by decreasing bone formation; and in addition it works as a hormone to regulate glucose metabolism, testosterone synthesis, and muscle mass in distant tissues
We show that glucose metabolism, testosterone synthesis, and muscle mass are normal in the Ocn-deficient mice
Summary
The strength of bone is dependent on its quantity and quality. Bone quantity is determined by both bone volume and bone mineral density (BMD), which is the amount of mineral per volume unit. Several parameters, including architecture and geometry, bone turnover, cortical porosity, damage, mineralization, and the properties of minerals, collagen, and non-collagenous proteins, have been proposed as the determinants of bone quality [1]. Non-collagenous proteins play significant roles in the structural organization of bone and influence its mechanical properties [2]. Ocn is synthesized by osteoblasts, and is the most abundant non-collagenous protein in bone. The three glutamic acid residues of the protein are carboxylated. Carboxylated Ocn has been implicated in mineralization, inhibition of hydroxyapatite growth, and chemotactic activity of osteoclast precursors [3]
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