Protein degradation, molecular chaperones, and osteogenic proteins in bone

• Main Author: Behnam, Keyvan
• Format: Dissertation
• Language: English
• Published: ProQuest Dissertations & Theses 01-01-2003
• Subjects: Anatomy & physiology; Animals; Biochemistry; Molecular biology; Physiology
• ISBN: 9780496827534; 0496827537

Bone is the only tissue that is remodeled throughout adult life. As the major load bearing skeletal element, bone is constantly subjected to unusual physical forces. Gravitational forces result in mechanical deformation of the tissue which in turn leads to the flow of intersitial fluids through the canaliculi which are found throughout the matrix. As such it would be expected that mechanisms for protecting bone from mechanical damage would have evolved. This dissertation is an attempt to characterize the molecules in bone matrix and osteoblasts that protect the tissue from mechanical damage and regulate protein turnover. In the initial part of this work we identify the molecular chaperone αB-crystallin in bone and osteoblast like cells and show that it can affect the activity of the ubiquitin-proteasome pathway (UPP), the major nonlysosomal intracellular regulatory protease system of the cell. Mechanical stress is shown to upregulate both the activity of the UPP and the expression of valosin-containing protein, a molecule involved in targeting proteins destined for degradation to the proteasome complex. We also identify the major molecular chaperones that are upregualted in MC3T3-E1 osteoblast-like cells as a result of culture medium flow, a simple model of mechanical stress. Finally, we show that noncollagenous protein extracts of bone matrix contain an 18.5 kDa form of secreted phosphoprotein 24 (SPP2), a molecule with sequence similarity to chick cystatin and bovine fetuin. We demonstrate that while this form of SPP2 does not bind to the cysteine protease papain, a cyclized peptide identical in sequence to a presumed disulfide bonded domain of SPP2 interacts with and enhances the osteogenic activity of bone morphogenetic protein 2. We also show that native SPP2 has many of the characteristics of the original bone morphogenetic protein molecule described by Marshall R. Urist and his colleagues at the UCLA Bone Research Laboratory.