BONE BIOLOGY LABORATORY
Dr. Marc D. Grynpas (Ph.D., London)
MAJOR RESEARCH ACTIVITIES: Both major functions of the skeletal system (ion homeostasis and mechanical support) are dependent on the chemical nature, size, shape and orientation of the mineral component. It is also dependent on the interaction between the mineral and the organic matrix because soluble, mineral bound and collagen bound proteins have different effects on the fabric of bone. Because of the rates at which calcified tissues are turned over, there are populations of mineral particles of different ages and properties in every sample. Therefore, the changes in chemical and structural characteristics of the mineral component and its interaction with the organic matrix during formation, maturation and resorption need to be understood to interpret changes in the quality of the bone fabric at a molecular and tissue level to distinguish between normal and pathological bone loss. The structure and chemistry of bone mineral and matrix We are interested in the basic structure and chemistry of mineralized tissue, with a particular emphasis on bone. As bone mineral changes with age, location and rate of turnover, we have to distinguish between local areas which have just mineralized and areas which contain mature bone crystals. By establishing mineralization profiles which separate newly formed bone from progressively maturing bone, we can analyze each fraction separately to determine crystal size, mineral chemistry and crystal packing in the bone matrix. By studying the mineral matrix interaction of the extracellular matrix of bone, we are trying to understand the mechanisms of mineral deposition, maturation and resorption. We have also analyzed the changes in the distribution of soluble, mineral bound and collagen bound proteins of bone with age and in osteoporosis to understand the relation between the mineral and the organic matrix of calcified tissues. We have shown that bone mineral does not contain amorphous calcium phosphate even at the earliest embryonic stage of mineralization but was made of poorly crystalline apatite with many lattice substitutions and a changing composition with age and maturation. We are in the process of following bone mineral crystals in the chick from the beginning of mineralization (8 days embryo) to old age (70 weeks old chicken), using x-ray diffraction, neutron activation analysis and infra-red spectroscopy. Changes in the quality of bone with age and disease Mineralization profiles are a much more sensitive index of tissue aging than chronological age. In normal physiological conditions, there are shifts in mineralization towards lower densities when turnover is high and towards higher densities with aging. We are studying the correlation between these changes and the lowering of bone mechanical properties which results in fractures. In the case of postmenopausal osteoporosis (high turnover), where we have found a shift to lower mineralization. We are investigating the changes in cancellous bone architecture and material properties as the mechanism leading to vertebral fractures. In the case of age- related osteoporosis, we have shown that there is a non linear increase in mineralization with age in humans together with a decrease in macroscopic density and an increase in porosity. It has also been shown that microcracks accumulate with age in cortical bone. We are therefore investigating the hypothesis that increased mineralization leads to accumulation of microcracks leading to fatigue damages and to cortical bone fractures. The long term effect of drugs and growth factors on bone quality We are studying the long term effect of drugs like bisphosphonates, which are specific to bone due to their binding to bone crystals and which inhibit bone resorption and therefore decrease bone turnover, on bone mineralization, architecture and quality because the lack of resorption caused by these drugs, coupled with their extremely long half-life, may lead in the long term to fatigue damages. We have shown that second generation bisphosphonates such as pamidronate protect the skeleton and increase mechanical strength in certain conditions. We are now studying third generation bisphosphonates such as zoledronate which is the most potent of the existing bisphosphonates. We have also explored the use of insulin-like growth factor 1(IGF-1) as an anabolic agent for bone and have shown a differential effect on mechanical properties in cancellous versus cortical bone. We are also investigating the interactions between bone, metallic implants, inflammation, and bisphosphonates to help prevent the loosening of hip and knee prosthesis. Effects of trace elements on bone It is possible to alter the mineralization and chemistry of the mineral of bone by the use of trace elements such as fluoride and strontium and therefore change their solubilities and other material properties. In the case of fluoride, we have shown that a biphasic effect can be observed where at low dose, bone mass and bone strength are increased, while at high doses, further increases in bone mass induce bone disorganization and a decrease in bone strength. This may be due to excessive shifts towards higher mineralization which prevents normal bone turnover leading to tissue disorganization. Similar effects are seen with strontium which increases bone mass at low doses but can induces a mineralization defect similar to osteomalacia at high doses. Other trace elements which affect mineralization such as aluminium and vanadium are also under investigation. Genetic diseases of the skeleton and transgenic animal models We are studying the effects of inborn collagen defects on the mineralization and bone fragility in osteogenesis imperfecta where we find a profound decrease in mineral content. We are exploring the abnormality of mineral packing which underlie bone fragility in this disease. We are also investigating a mouse model where only half the type I collagen is produced and another mouse model where the genes encoding some non-collagenous proteins have been deleted, to try and understand the downstream effect of these proteins on mineral size, packing and chemistry. Animal models of osteopenia We have used rodents to study the effects of estrogens and androgens on the skeleton and compared the results with remodelling species such as primates (macaques) which have a skeleton very similar to humans. We have shown substantial differences between the two models especially in the cortical bone where ovariectomy induces profound changes in primates not seen in rodents. We have developed models of immobilization-induced osteoporosis in the dog and of inflammation-induced arthritis in the rabbit. We are exploring the hypothesis that if bone resorption is prevented by anti-resorptive drugs, the cartilage can also be protected in inflammatory arthritis. Finally, we are in the process of developing new animal models of osteopenia in the minipig. Models of osteoarthritis We are comparing human osteoarthritis (OA) with spontaneous OA in free-ranging rhesus monkeys from the Caribbean Primate Research Centre and with induced OA in the dog to try and understand the mechanisms of OA development. We have shown that in human OA the subchondral bone is thicker but undermineralized which may be due to mineralization changes which induce cartilage destruction. We have demonstrated that OA in the rhesus monkeys is similar to OA in humans both at the histological and chemical level. Finally, we are exploring changes in mineralization in human rheumatoid arthritis. Space induced bone loss We are studying space-induced bone loss in rats and monkeys flown on Cosmos satellites and Spacelab Shuttle missions. We have shown that in rats flown both on Shuttle mission and on Biocosmos satellite the mineralization of bone is different in space and that this is not only due to unloading of the skeleton but also due to lack of gravity. We are now exploring the idea that cancellous bone formed in space has a different architecture than bone formed on earth. Pathological mineralization In collaboration with a group in the USA, who has developed the first rat model of kidney stone, we are looking at the mechanisms of their formation and at the regulation of pH balance in the body by the bone mineral carbonate from the skeleton. We are trying to relate the information from the animal model to human stone formation to devise strategies to prevent their formation. Development of new bioresorbable materials for orthopaedic applications In collaboration with Drs. Pilliar and Kandel at the University of Toronto, we are developing new materials for use as bone replacement and other tissue engineering applications. We are working on calcium-polyphosphates as biodegradable implants with specific interest in the relation between tissue ingrowth and material degradation both in vitro and in vivo. |
