The bare bones
Perhaps the biggest shift in how bone is perceived is in its function as an endocrine organ. Bone used to be thought of as a tissue that responded only to a couple of hormones, such as the parathyroid hormone sent out by the parathyroid glands and estrogen made by the ovaries. But findings in the past two decades have given indications that the bone doesn’t just passively take orders from other organs: It makes its own hormones to modulate mineral metabolism and energy expenditure.
In 2002, President Bush proclaimed the decade to be the National Bone and Joint Decade. The Bone and Joint Decade recently renewed its mandate for another 10 years to 2020. Every October 12–20, the Bone and Joint Decade and US Bone and Joint Initiative recognize the week as their National Awareness Week to inform the public about musculoskeletal disorders.
Rheumatoid arthritis causes inflammation of the joints. It occurs when the immune system mistakenly attacks the healthy joint tissue. While many antiarthritic drugs suppress inflammation, they offer poor or no protection against bone damage. Therefore, a drug for both symptoms is being pursued. Read about a recent related study.
The role of bone in mineral metabolism came as a surprise less than 15 years ago when a hormone called fibroblast growth factor 23 was discovered. FGF23 “has potent effects on the proximal tubule of the kidney to regulate the reabsorption of phosphate,” says Kronenberg. “It was interesting and surprising when it was first realized that the major source of FGF23 was the osteocyte.” That osteocytes signaled to the kidneys when the body needed to hold onto phosphate alerted researchers to that fact that bone actively manages mineral metabolism.
The connection between bone and energy expenditure was first proposed by the group of Gerard Karsenty at Columbia University. His group used genetic approaches to show that leptin, the hormone released from fat tissue to regulate appetite and metabolism, inhibited bone formation through the nervous system. The work tied together appetite, energy metabolism and bone remodeling.
Osteocalcin was another surprise in the energy-expenditure picture. The protein has been cited in the literature for more than 40 years and is used as a marker for osteoblast activity. But “we didn’t know what osteocalcin did,” says Thomas Clemens at Johns Hopkins University.