Identification and Characterization of Osteocyte-Selective Proteins

Osteocytes are dendritic cells within the mineralized bone matrix that comprise 90- 95% of all bone cells. Therefore they are the most abundant bone cell in the body. Little is known regarding the function of osteocytes in bone, therefore it was hypothesized that the identification and gene deletion of osteocyte selective proteins could provide information regarding osteocyte function. A proteomics approach was used to discover proteins more highly expressed in osteocytes than osteoblasts and to identify proteins regulated by mechanical loading in osteocytes. Proteins were identified in an osteocyte-like cell line, MLO-Y4 and verified in vivo in loaded and unloaded bone. Proteins with greater expression in osteocytes included those that could potentially play a role in dendrite formation through actin cytoskeleton regulation, protection against hypoxia, and with potential function in glycolysis and in facilitating protein folding. Proteins involved in glycolysis, mRNA processing and regulation of the actin cytoskeleton were elevated in response to shear stress.

In vivo targeted deletion of an osteocyte-selective membrane glycoprotein, E11/gp38, thought to play a role in dendrite formation, was performed. A murine model containing floxed alleles of the E11 gene was created, followed by crossing of E11 fix/fix mice with transgenic mice expressing Cre recombinase driven by the osteocalcin promoter. The E11 flx/flx mice (i.e. in the absence of the Cre transgene) were hypomorphic with reduced E11 expression in lung, kidney and bone. This resulted in either normal sized animals with average survival or in animals with defective kidney and lymphatic function. Therefore for the conditional knockout mice, only those with normal size and kidney function were used. Immunohistochemistry confirmed removal of E11/gp38 protein in cKO bone. The adult conditional knockout mice showed slightly higher body weight and bone mineral density, most likely due to a significant increase in the number and volume of trabecular bone (two fold) with no change in cortical bone compared to littermate controls. A decrease in the number of canaliculi was observed, suggesting that osteocytes in trabecular bone may regulate bone remodeling by a different mechanism than osteocytes in cortical bone.

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Year

Entry

1990

Kufahl RH, Saha S. A theoretical model for stress-generated fluid flow in the canaliculi-lacunae network in bone tissue. J Biomech. 1990;23(2):171-180.

1983

Sudo H, Kodama H-A, Amagai Y, Yamamoto S, Kasai S. In vitro differentiation and calcification in a new clonal osteogenic cell line derived from newborn mouse calvaria. J Cell Biol. January 1983;96(1):191-198.

2009

Li X, Ominsky MS, Warmington KS, Morony S, Gong J, Cao J, Gao Y, Shalhoub V, Tipton B, Haldankar R, Chen Q, Winters A, Boone T, Geng Z, Niu Q-T, Ke HZ, Kostenuik PJ, Simonet WS, Lacey DL, Paszty C. Sclerostin antibody treatment increases bone formation, bone mass, and bone strength in a rat model of postmenopausal osteoporosis. J Bone Miner Res. April 2009;24(4):578-588.

1992

van der Plas A, Nijweide PJ. Isolation and purification of osteocytes. J Bone Miner Res. April 1992;7(4):389-396.

2004

van Bezooijen RL, Roelen BAJ, Visser A, van der Wee-Pals L, de Wilt E, Karperien M, Hamersma H, Papapoulos SE, ten Dijke P, Löwik CWGM. Sclerostin is an osteocyte-expressed negative regulator of bone formation, but not a classical BMP antagonist. J Exp Med. March 15, 2004;199(6):805-814.