Chemical Messenger Controls Bone Growth In Embryos, Study Finds

“This signaling molecule puts the brakes on the cartilage growth that determines the length of bones,” says David M. Ornitz, Ph.D., professor of molecular biology and pharmacology, who led the study. First author on the paper was Zhonghao Liu, a graduate student at the School of Medicine.

The messenger, known as fibroblast growth factor 18 (FGF 18), also appears to regulate the hardening, or ossification, of bone. “This suggests that FGF 18 coordinates the process by which bones lengthen with the process by which they harden,” Ornitz says. “That came as a surprise to us.”

He believes the study may lead to a better understanding of congenital and genetic diseases that cause bone malformation, and perhaps of cancer and bone diseases such as osteoporosis. Ornitz and his research team engineered mouse embryos that lacked genes for FGF 18. These embryos showed increased growth activity in the bones. For example, in embryos that lacked FGF 18, the growth regions, or growth plates, of the femur were 37 to 60 percent broader than those in normal mouse embryos.

This and other changes seen in the embryos lacking FGF 18 mimic the condition of embryos that lack a receptor molecule known as fibroblast growth factor (FGF) receptor 3. This led Ornitz and his colleagues to conclude that FGF 18 is the messenger molecule for FGF receptor 3.

Receptors are molecules on the surface of cells that trigger some change in the cell when activated by molecules such as hormones or growth factors. Scientists have known for 10 years that FGF receptors are important for skeletal development. For example, mutations in FGF receptor 3 cause achondroplasia, the most common form of dwarfism in humans. But they did not know which of the 22 known fibroblast growth factors activate the receptors associated with developing bone.

But while embryos that lacked FGF 18 were very similar to embryos that lacked FGF receptor 3, there also was an important difference: The bones of mice lacking FGF 18 didn’t begin to harden as soon as they should have.

“Ossification was delayed by about two days, which is a long time for mouse embryos,” says Ornitz. “That suggests that FGF 18 also influences a second receptor, probably FGF receptor 1 or 2.” Ornitz and his colleagues now are working to identify that second receptor and will soon begin studying how FGF 18 is regulated.

Ornitz’ findings may have future clinical applications. Mutations in FGF receptors have been linked to several bone diseases. Mutations in FGF receptor 3, for example, cause several kinds of dwarfism, and mutations in FGF receptors 1 and 2 cause craniosynostosis syndromes, in which the cranial bones fuse prematurely causing deformities of the skull.

“If we understand the relationship between FGF 18 and its receptors,” says Ornitz, “perhaps someday we can prevent some of the pathology that occurs in diseases such as achondroplasia and craniosynostosis syndromes.”

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Reference:

Liu Z, Xu J, Colvin JS, Ornitz DM. Coordination of chondrogenesis and osteogenesis by fibroblast growth factor 18. Genes and Development, 16 (7), 859-869, April 1, 2002.

This research was supported by grants from the National Institute of Child Health and Human Development, the American Heart Association and Zymogenetics Inc.