No abstract

Bone is a complex material with a hierarchical multi-scale organization from the molecule to the organ scale. The genetic bone disease, osteogenesis imperfecta, is primarily caused by mutations in the collagen type I genes, resulting in bone fragility. Because the basis of the disease is molecular with ramifications at the whole bone level, it provides a platform for investigating the relationship between structure, composition, and mechanics throughout the hierarchy. Prior studies have individually shown that OI leads to: 1. increased bone mineralization, 2. decreased elastic modulus, and 3. smaller apatite crystal size. However, these have not been studied together and the mechanism for how mineral structure influences tissue mechanics has not been identified. This lack of understanding inhibits the development of more accurate models and therapies. To address this research gap, we used a mouse model of the disease (oim) to measure these outcomes together in order to propose an underlying mechanism for the changes in properties. Our main finding was that despite increased mineralization, oim bones have lower stiffness that may result from the poorly organized mineral matrix with significantly smaller, highly packed and disoriented apatite crystals. Using a composite framework, we interpret the lower oim bone matrix elasticity observed as the result of a change in the aspect ratio of apatite crystals and a disruption of the crystal connectivity.

Osteogenesis imperfecta (OI or brittle bone disease) is a disorder of connective tissues caused by mutations in the collagen genes. We previously showed that intrauterine transplantation of human blood fetal stem/stromal cells in OI mice (oim) resulted in a significant reduction of bone fracture. This work examines the cellular mechanisms and mechanical bone modifications underlying these therapeutic effects, particularly examining the direct effects of donor collagen expression on bone material properties. In this study, we found an 84% reduction in femoral fractures in transplanted oim mice. Fetal blood stem/stromal cells engrafted in bones, differentiated into mature osteoblasts, expressed osteocalcin, and produced COL1a2 protein, which is absent in oim mice. The presence of normal collagen decreased hydroxyproline content in bones, altered the apatite crystal structure, increased the bone matrix stiffness, and reduced bone brittleness. In conclusion, expression of normal collagen from mature osteoblast of donor origin significantly decreased bone brittleness by improving the mechanical integrity of the bone at the molecular, tissue, and whole bone levels.

Stem cells have considerable potential to repair damaged organs and tissues. We previously showed that prenatal transplantation of human first trimester fetal blood mesenchymal stem cells (hfMSCs) in a mouse model of osteogenesis imperfecta (oim mice) led to a phenotypic improvement, with a marked decrease in fracture rate. Donor cells differentiated into mature osteoblasts, producing bone proteins and minerals, including collagen type I␣2, which is absent in nontransplanted mice. This led to modifications of the bone matrix and subsequent decrease of bone brittleness, indicating that grafted cells directly contribute to improvement of bone mechanical properties. Nevertheless, the therapeutic effect was incomplete, attributing to the limited level of engraftment in bone. In this study, we show that although migration of hfMSCs to bone and bone marrow is CXCR4-SDF1 (SDF1 is stromal-derived factor) dependent, only a small number of cells present CXCR4 on the cell surface despite high levels of internal CXCR4. Priming with SDF1, however, upregulates CXCR4 to increase the CXCR4 ؉ cell fraction, improving chemotaxis in vitro and enhancing engraftment in vivo at least threefold in both oim and wild-type bone and bone marrow. Higher engraftment in oim bones was associated with decreased bone brittleness. This strategy represents a step to improve the therapeutic benefits of fetal cell therapy toward being curative. STEM CELLS TRANSLATIONAL MED-ICINE 2012;1:70 -78

Osteogenesis imperfecta (OI) is characterized by extremely brittle bone. Currently, bisphosphonate drugs allow a decrease of fracture by inhibiting bone resorption and increasing bone mass but with possible long term side effects. Whole body mechanical vibrations (WBV) treatment may offer a promising route to stimulate bone formation in OI patients as it has exhibited health benefits on both muscle and bone mass in human and animal models. The present study has investigated the effects of WBV (45 Hz, 0.3 g, 15 minutes/days, 5 days/week) in young OI (oim) and wild type female mice from 3 to 8 weeks of age. Vibration therapy resulted in a significant increase in the cortical bone area and cortical thickness in the femur and tibia diaphysis of both vibrated oim and wild type mice compared to sham controls. Trabecular bone was not affected by vibration in the wild type mice; vibrated oim mice, however, exhibited significantly higher trabecular bone volume fraction in the proximal tibia. Femoral stiffness and yield load in three point bending were greater in the vibrated wild type mice than in sham controls, most likely attributed to the increase in femur cortical cross sectional area observed in the μCT morphology analyses. The vibrated oim mice showed a trend toward improved mechanical properties, but bending data had large standard deviations and there was no significant difference between vibrated and non-vibrated oim mice. No significant difference of the bone apposition was observed in the tibial metaphyseal trabecular bone for both the oim and wild type vibrated mice by histomorphometry analyses of calcein labels. At the mid diaphysis, the cortical bone apposition was not significantly influenced by the WBV treatment in both the endosteum and periosteum of the oim vibrated mice while a significant change is observed in the endosteum of the vibrated wild type mice. As only a weak impact in bone apposition between the vibrated and sham groups is observed in the histological sections, it is possible that WBV reduced bone resorption, resulting in a relative increase in cortical thickness.Whole body vibration appears as a potential effective and innocuous means for increasing bone formation and strength, which is particularly attractive for treating the growing skeleton of children suffering from brittle bone disease or low bone density pathologies without the long term disadvantages of current pharmacological therapies.