A Local Effect on Bone Mass

Since his surprising discovery in 2001 that the Wnt pathway, specifically a gene called LRP5, is a key regulator of bone mass, Matt Warman has been steadily building evidence to flesh out the picture. Other researchers have added support to that finding, but none has demonstrated where in the body LRP5 manifests its impact.

Now Warman and collaborators have shown that the gene sends its signals through osteocytes, a type of mature bone cell. And this signaling builds bone locally. When Lrp5 expression is boosted in osteocytes in the limb of a mouse, bone mass increases specifically in that region.

In an inherited human disorder characterized by excessive bone mass, a mutation in LRP5 exists in every cell of the body. Warman’s team engineered a mouse model with the same mutation, but they included a switch to control when and in what cells it was turned on. With this precise control, they could study the effect of the mutation in different cell types, in animals of different ages, and in cells at different stages of differentiation.

The results indicated that activating the altered gene in the mouse osteocytes was as effective at increasing bone mass as inheriting the active gene in every cell. In addition, when Lrp5 was inactivated in osteocytes, the mice had lower bone mass than their wild-type littermates. Published in Nature Medicine in June 2011, the work suggests that bone mass is regulated by Lrp5 signaling in mature cells and that those signals must be reaching immature cells in the bone marrow and on the outer, or periosteal, surface of the bone.

“The excitement of this discovery is that to increase bone mass, we might not need to search for ‘undecided cells’ hanging out in the bone marrow to tickle,” says Warman. “We think LRP5, activated in mature bone cells, is sending the message to undecided cells to produce new bone.” This activation can happen as a response to load-bearing stress, for example. That conclusion, he points out, supports Wolff’s law—a long-held principle stating that mechanical stress causes bone growth. Bone gets that message and alters growth to accommodate.

To test whether Lrp5 activated in one area might have far-reaching effects elsewhere in the body, the researchers selectively activated the engineered gene in the limbs of the mice, and not, for example, in their spines. When the team tested both areas, they found that bone mass increased only in the limbs, indicating that the effect of Lrp5 is local rather than systemic. That LRP5 functions locally in bone suggests that drugs that can increase LRP5 signaling may be able to “trick cells into thinking they are experiencing extra load, so they send signals that cause nearby cells to produce more bone,” says Warman.

The researchers also investigated conflicting claims, from the Columbia University Medical Center lab of Gerard Karsenty, that LRP5 influences bone mass indirectly, through its involvement in the production of serotonin in the gut. Working with their own mouse models, Warman’s group tried activating the high bone mass Lrp5 mutation and inactivating normal Lrp5 in serotonin-producing cells in the mouse gut. Neither action affected bone mass in the animals. In addition, bone mass did not appear to be affected by decreasing levels of Tph1, an enzyme that is key to serotonin production in the intestine.

The reasons for the discrepancy between the two groups’ findings aren’t clear. Both groups acknowledge the possibility of technical explanations, though Warman remains skeptical that the principal role of Lrp5 on bone mass is via the regulation of serotonin production in the gut. “Work from several labs demonstrates that Lrp5 functions as a co-receptor for Wnt signaling, that enhancing Wnt signaling increases bone mass, and that Lrp5 mutations that cause high bone mass reduce the ability of inhibitory proteins highly expressed in bone, such as Sost and Dkk1, to dampen Lrp5 signaling,” says Warman. To enable other investigators to independently test and extend his group’s findings, Warman has deposited his mice at the Jackson Laboratory—an animal resource center that facilitates the distribution of mice to research labs.

Karsenty says he thinks the answer will come from pharmacological studies specifically targeting each of the pathways proposed.

Moving forward, Warman’s group is testing whether increasing Lrp5 activity affects bone mass and strength in older mice as well as it does in younger mice and whether increasing Lrp5 activity will improve bone properties in mouse models of human skeletal fragility syndromes. If so, the answer could lead to the development of LRP5-targeted treatments for osteoporosis and for genetic diseases such as osteogenesis imperfecta in humans.

-- Mary Beth Gardiner
HHMI Bulletin, May 2011

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