"Our findings are important for understanding how bone forms and breaks down, and how those processes are disrupted in diseases like JPD," says F. Patrick Ross, Ph.D., research professor of pathology and immunology at the School of Medicine.
Ross led the study, which appears online in Nature Medicine on Aug. 5 and will be published in the September issue of the journal. The first authors were Sunao Takeshita, Ph.D., and Noriyuki Namba, Ph.D., both post-doctoral fellows in Ross' laboratory.
JPD, also known as hereditary hyperphosphatasia or hyperostosis corticalis deformans juvenilis, is a painful skeletal disease characterized by abnormally fast formation and breakdown of bone throughout the body that leads to debilitating fractures and deformities beginning soon after birth.
In healthy individuals, there is a careful balance between the number of osteoblasts (cells that create bone) and osteoclasts (cells that break down bone). But research suggests that people with JPD have more osteoclasts and that these cells are larger than normal, which creates a dangerous imbalance in bone turnover.
Researchers in Vancouver recently engineered a strain of mice lacking the gene for SHIP. The mice have abnormally high numbers of macrophages, a type of immune cell. Because macrophages can develop into osteoclasts, the Washington University team hypothesized that the mice lacking SHIP may eventually develop symptoms similar to JPD.
They were right. The mice had twice as many osteoclasts as normal mice, and the cells were much larger than normal, with about 100 nuclei. Since each macrophage has just one nucleus, the researchers conclude that each enlarged osteoclast represents about 100 fused cells. In other words, they looked exactly like osteoclasts from a person with JPD.
When the team examined cell samples in petri dishes, macrophages from mice lacking SHIP not only rapidly developed more osteoclasts than normal, the osteoclasts also lived longer. Moreover, they broke down bone much faster than normal osteoclasts.
The researchers also determined how a deficiency in SHIP leads to an excess number of enlarged osteoclasts. In addition to having too many nuclei, the cells also were hypersensitive to two proteins - macrophage-colony stimulating factor (M-CSF) and receptor activator of nuclear factor-kB ligand (RANKL) - which stimulate macrophages to become osteoclasts. Researchers believe that SHIP normally dampens the message sent from M-CSF and RANKL, keeping those signals at a reasonable level. This current study supports that theory: Without SHIP, macrophages became hypersensitive to M-CSF and RANKL. As a result, too many macrophages developed into osteoclasts, and they did so at a dangerously fast pace.
With too many enlarged osteoclasts, the mice had shorter, less thick bones, lost about 22 percent of their bone-mineral density and were far more susceptible to bone fractures, all hallmarks of JPD.
"These findings provide valuable insights into the molecular basis for how bone is degraded, a process important for several serious bone diseases," says Ross. "We hope the results also may lead to effective new treatments for such diseases in the future."
Takeshita S, Namba N, Zhao JJ, Jiang Y, Genant HK, Silva MJ, Brodt MD, Helgason CD, Kalesnikoff J, Rauh MJ, Humphries RK, Krystal G, Teitelbaum SL, Ross FP. SHIP-deficient mice are severely osteoporotic due to increased numbers of hyper-resorptive osteoclasts. Nature Medicine, 8(9), September 2002. Funding from the National Institutes of Health supported this research.