Glycosylated Matrix Proteins

Connective tissues have many features in common, including resisting and dissipating mechanical load and providing shape, as well as acting to provide barriers regulating water flow and diffusibility of macromolecules. In this chapter we have chosen to focus on cartilage and bone, covering many aspects of connective tissues. We give special focus to the noncollagenous proteins in their matrices

Ultrastructural distribution of osteoadherin in rat bone shows a pattern similar to that of bone sialoprotein

Osteoadherin (OSAD) is a keratan sulfate proteoglycan recently isolated from bovine and rat bone. Based on results obtained from in vitro experiments, the protein was shown to bind osteoblasts via the integrin receptor alpha(v)beta(3). Due to OSAD's capacity to bind hydroxyapatite crystals, a role for the protein in the mineralization process has also been suggested. To test these hypotheses in an in vivo model, the ultrastructural localization of OSAD in bone, tibial (metaphyses and diaphyses). and calvarial samples from normal 10 to 12-day-old rats were examined by immunohistochemical techniques at the ultrastructural level. In addition to the qualitative studies, quantitative measurements of OSAD marker density were performed in relevant compartments. Immunolabeling for OSAD was located to the mineralized bone matrix, with highest concentration of marker at the border between bone and cartilage remnants in the metaphyseal trabeculi. Intracellular labeling was low and no systemic accumulation of OSAD markers was observed at the cell-matrix interface. The observed distribution pattern of OSAD is strikingly similar to that of bone sialoprotein (BSP), confirmed by double labeling. The results of the current study support a role for OSAD in the mineralization process. In this process BSP is assumed to be a nucleator of hydroxyapatite crystals, and OSAD could work in concert with BSP to regulate nucleation. However, the mechanisms involved remain to be elucidated

Altered osteoclast development and function in osteopontin deficient mice.

The role of osteopontin in bone resorption was elucidated by studies of mice with knock out of the osteopontin gene generated by a different approach compared to previous models. Thus, a targeting vector with the promoter region as well as exons 1, 2, and 3 of the osteopontin gene was replaced by a loxP-flanked Neo-TK cassette, and this cassette was eliminated through transient expression of Cre recombinase. The recombined ES cells were used to create mice lacking expression of the osteopontin gene. Tissues from these mice were subjected structural and molecular analyses including morphometry and proteomics. The bone of the null mice contained no osteopontin but showed no significant alterations with regard to other bone proteins. The bone volume was normal in young null animals but in the lower metaphysis, the volume and number of osteoclasts were increased. Notably, the volume and length of the osteoclast ruffled border was several folds lower, indicating a lower resorptive capacity. The null mice did not develop the bone loss characteristic for osteoporosis demonstrated in old wild-type female animals. This quantitative study demonstrates a bone phenotype in the osteopontin null mice of all ages. The data provides further evidence for a role of osteopontin in osteoclast activity. (c) 2007 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res