Experimental fracture healing in Fas Mutant Mice (B6.MRL/FasLPR/J)
Date
2007
DOI
Authors
Al-Sebaei, Maisa O.
Version
OA Version
Citation
Abstract
Introduction: The term "osteoimmunology" is a new concept that has received much attention in the recent years. It refers to the relationship between the immune system and skeletal system. lt is based on observations that bone destruction is caused by an abnormaI activation of the immune system in rheumatoid arthritis, and that mice lacking immunomoduIatory molecules often exhibit an unexpected bone phenotype. Fas, an important memberoOf the TNF superfamily and death receptor provides apoptosis for many key cell types which participate in normaI bone remodeling as well as the endochondral healing process. Fas is a critical receptor in regulating the life cycle of osteoclasts. Osteoclastogenesis has been implicated in many disease processes such as osteoporosis.
Studies on fracture healing in TNFR1 and TNRR2 knock-out mice have shown that chondrocyte apoptosis through TNF[alpha] is an essential step in endochondraI bone formation (Gerstenfeld et al, 2003). Furthermore; TNF[alpha] was demonstrated to upregulate Fas expression (Cho et al, 2003). Since both receptors mediate apoptosis of essential cells involved in the fracture healing process (chondrocytes and osteoclasts), the functional inter-relationship between the TNF[alpha] receptors and Fas must be defined.
Materials and Methods: Experimental closed femur fractures were generated in 8-10 week oId male B6.MRL/Fas[lpr]/J mice and their control, C57BL/B6 mice. These mice exhibit Fas receptor mutation and are a well-established modeI for systemic lupus erythematosus (SLE). The mice were sacrificed and bones were harvested for both strains on days 0, 3, 5, 10, 14, 21, 28 for molecular analysis by RPA and RT-PCR, on days 21 and 35 for microcomputed tomography and mechanical testing and on days 14 and 21 for histomorphometry.
Results: Calluses in the B6.MRL/Fas[lpr]/J showed a lower ratio of bone volume to total callus volume than in the controIs on day 35 (P[less than]0.05). Trabecular number and thickness on day 35 were lower in the B6.MRL/Fas[lpr]/J calluses (P=0.001 and p=0.024) whiIe trabecular spacing was higher (P=0.0089). The fractured control femurs sustained a higher maximum torque than the B6.MRL/Fas[lpr]/J strain (P=0.005). Histomorphometric analysis demonstrated a higher TRAP-positive cell count per area of bone and cartilage in the day 21 B6.MRL/Fas[lpr]/J sections as well as a higher total calIus area in days 14 and 21. The extracellular matrix RPA demonstrated that there was no difference in cartilage marker expression (type ll and X collagen). An over-expression of ECM proteins (OPN, BSP, OC and type I collagen) was observed in the B6.MRL/Fas[lpr]/J fractures at later time-points during secondary bone formation (d14 - 28). Osteoclastic markers and key osteoclastic regulators which were assayed by RT-PCR showed a trend toward increased osteoclastogenesis.
Conclusion: In conclusion, our data shows that the B6.MRL/Fas[lpr]/J bones are weaker on mechanical testing and exhibit less bone volume fraction. The trabeculae are thinner, Iess in number and more porous. The number of TRAP-positive cells per area of bone and cartilage was greater in the B6.MRL/Fas[lpr]/J day 21 histological sections and this corresponded to a higher ratio of RANKL/OPG at days 21 and 28. An examination of initial endochondral bone formation requires the coupling between bone formation and resorption through a balance between osteoblasts and osteoclasts. From these data, we determined that there is a higher osteoclast activity in the B6.MRL/Fas[lpr]/J yet more expression of extracellular matrix proteins at later stages in the fracture repair. We conclude that there is a state of high turnover osteopenia occurring in the B6.MRL/Fas[lpr]/J mouse.
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Thesis (D.Sc.D.)--Boston University, Goldman School of Dental Medicine, 2007 (Oral and Maxillofacial Surgery).
Includes bibliography: leaves 111-130.
Thesis (D.Sc.D.)--Boston University, Goldman School of Dental Medicine, 2007 (Oral and Maxillofacial Surgery).
Includes bibliography: leaves 111-130.
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This work is protected by copyright. Downloading is restricted to the BU community. If you are the author of this work and would like to make it publicly available, please contact open-help@bu.edu.