An organotypic model of the bone remodelling process

Context of the research:
Strong, healthy bones are necessary for an active life. Bone is a dynamic, living organ full of specialist cells and these cells work to continually produce and repair bone tissue, in fact 10% of all bone within the adult human body is replaced each year. This bone remodelling process first requires the removal of old bone and then its replacement with new bone. The amount of bone removed must equal the amount replaced, if there is an imbalance between removal and formation the result is bone fragility and disability. Disrupted bone remodelling is the cause of many musculoskeletal health problems including osteoporosis, arthritis and failure of fracture healing and is a huge contributor to disability worldwide, especially in ageing populations; clearly there is a pressing need to develop treatments that restore bone strength for patients.

Bone remodelling requires coordination between 3 key cell-types: OsteoCLASTs, osteoBLASTs and osteoCYTEs. Osteoclasts form by the fusion of 8-20 white blood cells into giant cells that pump out enzymes to destroy old, damaged or excess bone. Osteoblast cells then make and mineralise new bone tissue. Osteocytes are the longest-lived cell in the body, living up to 25 years. These cells are entombed within the bone tissue and, although they can’t move, each osteocyte cell has up to 250 long microscopic extensions that allow the cell to reach out and touch other osteocytes as well as osteoblasts and osteoclasts on the bone surface. Osteocytes sense damage, load and strain on the bone tissue and they communicate this information to osteoblasts and osteoclasts to control the repair and remodelling of the bone.

Aims and Objectives:
Although osteocytes represent approximately 90% of all cells in bone and are responsible for controlling the activity of the other bone cells, they are notoriously difficult to study in the laboratory. Once released from their entrapment within the bone and put into laboratory culture they very rapidly (within a few days) lose their distinctive shape and stop behaving like osteocytes. Because of this, almost all researchers who study bone remodelling use animal experiments, usually mouse or rat models of fracture repair or osteoporosis.

We have overcome this major hurdle by developing a 3-dimensional culture method that supports osteocyte survival for up to 1 year in culture. The osteocytes are embedded within a fibrin tissue scaffold that is held between two calcium phosphate anchors. We have shown that the osteocytes look and behave like osteocytes do in a living bone. We now propose to use this technology to develop a new test-tube assay of bone remodelling, aiming to replace much of our requirement for animal experiments. To do this we will add osteoblasts and osteoclasts to our osteocyte 3D cultures and work out the perfect conditions for these cells to flourish. We will test whether the cells behave and interact with each other as they do in a real bone and whether they respond to drug treatments that are known to work well in patients. Once we have established this we will miniaturise our assay to make it suitable for use as a screening tool to test new candidate drugs.

Potential Applications and Benefits:
Existing animal studies of bone remodelling are expensive, time-consuming and cause pain and harm to the animals involved. Existing test-tube studies are inadequate for studying the process as they do not include the main cell type (the osteocyte) that is found in bone. We anticipate that our new test-tube method will reduce the numbers of animals required for these kinds of experiments by allowing much of the research to be performed in the laboratory and not in animals. Not only would this reduce the number of animals used by researchers, it would also speed up the drug-discovery process by allowing rapid screening of candidate drugs that will work for patients.