In bone, osteoblast/osteoclast interactions appear to be mediated indirectly in part by components of the extracellular matrix. To control formation, induction, localization, and activation of osteoclasts, osteoblasts may transfer signals/messages via macromolecules they secrete and deposit in the extracellular matrix or osteoid. Non-collagenous glycoproteins, ie., osteopontin, bone sialoprotein, bone acidic glycoprotein-75, decorin, and biglycan, synthesized by osteoblasts and secreted into forming bone matrix contain 0-linked and/or N-linked oligosaccharide chains. However, the significance of these post-translational modifications to the development and function of osteoclasts has not been fully explored. Our hypothesis is that carbohydrate receptors (lectins or carbohydrate recognition domains) play a role in the development and function of osteoclasts. We will address the following basic question.
AIM 1. Do multinucleated osteoclasts express carbohydrate receptors in vivo in young rat bone and in the marrow ablation model of accelerated bone turnover? The marrow ablation model permits an analysis of different predominant stages of bone turnover, ie., osteoclast rich phase at 8- 10 days, and yields induced bone amenable to isolation of RNA or sectioning. Considering a shared stem cell origin of osteoclasts with macrophage/ monocyte cells, which express carbohydrate receptors used in endocytosis or adhesion to matrices, and the availability of cloned receptor cDNAs, our approach will be to first determine whether osteoclasts co-express known macrophage/Kupffer cell lectins through use of Northern blotting and in situ hybridization. Despite data from the mid 1980's to the contrary, osteoclasts and macrophage-like cells have been shown to share a number of common antigens and a wealth of data on macrophage/Kupffer cell/leukocyte carbohydrate receptors has appeared in the intervening period. However, if Northern blots with induced bone are negative for these cloned lectins, a second approach will be taken to demonstrate carbohydrate receptors in/on osteoclasts in tissue sections through direct binding and competition with carbohydrate ligands. These alternate approaches should lead to an answer to the question posed in Aim 1. Clinical Relevance: Skeletal and alveolar bone mass represents the net sum of dynamic resorptive and formative processes which continue throughout life in humans. All individuals experience osteopenia with increasing age. When bone mass reaches a critical threshold, fractures occur with little trauma (osteoporosis). Osteoporosis is a major health problem in the U.S. where it is estimated that 20 million people are currently affected and 1.3 million fractures are attributable to it each year. While supported by limited research, the scientific literature suggests that skeletal osteopenia may be accompanied by accelerated loss of craniofacial bone; in recognition of this fact, NlDR and NIAMSD have initiated a Research Program in Oral Bone Loss and Osteoporosis. This proposal is a new investigation of carbohydrate receptor expression by osteoclasts in young and induced bone tissue. If present, carbohydrate receptors could mediate the localization, attachment, fusion, and resorptive function of osteoclasts on bone. Osteoclast-matrix recognition mechanisms are a prime target for therapeutic intervention.
| Gorski, Jeff P; Wang, Aimin; Lovitch, Dinah et al. (2004) Extracellular bone acidic glycoprotein-75 defines condensed mesenchyme regions to be mineralized and localizes with bone sialoprotein during intramembranous bone formation. J Biol Chem 279:25455-63 |
| Midura, Ronald J; Wang, Aimin; Lovitch, Dinah et al. (2004) Bone acidic glycoprotein-75 delineates the extracellular sites of future bone sialoprotein accumulation and apatite nucleation in osteoblastic cultures. J Biol Chem 279:25464-73 |
| Gorski, J P; Apone, S; Shaffer, K A et al. (2000) Hypercalcemia during the osteogenic phase after rat marrow ablation coincides with increased bone resorption assessed by the NTx marker. Bone 27:103-10 |
| Magnuson, S K; Booth, R; Porter, S et al. (1997) Bilateral tibial marrow ablation in rats induces a rapid hypercalcemia arising from extratibial bone resorption inhibitable by methylprednisolone or deflazacort. J Bone Miner Res 12:200-9 |
