Bone is living growing tissue and is remodelled constantly throughout life. The process involves two major types of bone cells: osteoclasts, which break down old or worn bone and osteoblasts, which make new bone. If the amount of new bone equals the amount being dissolved, bones remain strong but, if resorption exceeds new bone formation, the bones become weak and fracture more easily, a condition known as osteoporosis. The balance tips in favour of bone loss at about age 30 and, for women, bone loss is fastest in the first few years after menopause.
It had been thought that the skeleton controlled bone growth, but scientists at Columbia University Medical Center have shown that bone growth is controlled by the neurotransmitter, serotonin, which is produced mainly in the gut. The study had initially focussed on two rare human diseases, one that causes drastic bone loss and one that causes extremely high bone mass, which are both caused by a mutation in a gene called Lrp5. The team found that Lrp5 regulates synthesis of serotonin in the gut and that, by turning on or off production of serotonin in the gut, they could control bone formation. Serotonin signals to cells in the skeleton to slow production of new bone and, by turning off the intestine’s release of serotonin, the team was able to prevent osteoporosis in mice undergoing menopause.
Most osteoporosis drugs prevent the breakdown of old bone but do not increase the formation of new bone and reducing serotonin production in the gut could provide a novel way to treat osteoporosis. In the United States alone, 10 million people have osteoporosis and 34 million have low bone density.
The study is published in full in the November 26 issue of the journal Cell.