Signalling mechanisms of bone loss in weightlessness: The evidence thus far
Bone loss in microgravity is the second most important risk to space missions.
Exposure to the microgravity environment of space causes astronauts to lose calcium from bones. This loss occurs because the absence of Earth's gravity disrupts the process of bone maintenance in its major function of supporting body weight.
Exposure to the microgravity environment of space causes men and women of all ages to lose up to 1% of their bone mass per month due to disuse atrophy, a condition similar to osteoporosis. It is not yet clear whether losses in bone mass will continue as long as a person remains in the microgravity environment or level off in time.
The mystery, for the moment, is what signals permit bone tissue to adapt to a weightless or an Earth (1 g) environment. Researchers do not yet know whether the biomechanical stimuli that are changed by microgravity directly affect osteoblast and osteoclast function or if other physiological factors such as hormone levels or poor nutrition contribute to bone loss. NASA investigators are studying gravity-sensing systems in individual bone cells by flying cultures of these cells on the Space Shuttle and observing how they function. Discoveries made in the course of space biomedical research on bone are already contributing to a better understanding of osteoporosis and the treatment of bone mass loss on Earth as well as in space. The single most important contribution that NASA research has made to the understanding of bone deterioration in osteoporosis is heightened awareness of the importance of gravity, activity, and biomechanics - that is, the mechanical basis of biological activity - in bone remodeling.
Mechanical forces - the action of energy on matter - appear to coordinate bone shaping processes. The standard theory of bone remodeling states the body translates mechanical force into biochemical signals that drive the basic processes of bone formation and resorption. Aging, especially in post-menopausal women, and exposure to microgravity uncouple bone resorption and formation. When this uncoupling occurs, formation lags behind resorption, and the result is bone loss.
Researchers are not yet certain whether bone resorption speeds up or the bone formation slows down, though recent experimentation in space indicates that microgravity might somehow affect both processes. Progress in developing methods of preventing or treating disuse atrophy and osteoporosis depends on better understanding the mechanisms that cause the problem. Determining how the body translates mechanical loading (physical stress or force) into the signals that control bone structure may reveal how aging, inactivity, and space flight uncouple bone formation and resorption. Only in the absence of gravity can we determine the influence of weight and stress on bone dynamics.
By studying what mechanisms translates mechanical stress on bones into biochemical signals that stimulate bone formation and resorption, space life scientists may be able to determine how to maintain bone mass. Researchers do not yet know exactly what type and amount of exercise, hormones, or drugs might prevent bone loss or promote bone formation. However, some combination of sex hormones growth hormones, and exercise seems to be the key to preventing bone mass loss associated with chronological aging and post-menopausal hormone changes on Earth.
