USC University of Southern California

Herman Ostrow School of Dentistry of USC

Amelogenin’s Interaction with Signaling Pathway Bosses Around Bone Marrow Stem Cells

In a study released June 8 in the journal Biomaterials, Ostrow School of Dentistry Center for Craniofacial Molecular Biology researchers Xin Wen, Yan Zhou and Malcolm Snead discuss how one form of a protein important in forming tooth enamel can also tip the balance of mesenchymal stem cells within bone marrow into the formation of bone or fat tissue. This work was in collaboration with Ormond MacDougald and William Cawthorn at University of Michigan and Samuel Stupp at Northwestern University.

The amelogenin protein, the most abundant protein in the complex matrix that makes up tooth enamel, is also expressed elsewhere in the body in other variations, or splicing isoforms. One of these alternative isoforms, Leucine-Rich Amelogenin Peptide (LRAP), is known to have an osteogenic effect – it promotes the creation of brand new bone, Zhou said.

For the study, “The influence of Leucine-rich amelogenin peptide on MSC fate by inducing Wnt10b expression,” the researchers used a stromal cell line ST2 and primary bone marrow mesenchymal stem cells (BMMSC), which have the ability to differentiate into either bone cells or fat cells. When cells were treated with synthetic LRAP, the balance of differentiation was shifted toward bone, Snead said. However, when the cells were treated simultaneously with sFRP-1, a small protein known to inhibit the Wnt signaling pathway, the balance was tipped toward fat.

This finding helped the research team zero in on the underlying mechanism by which LRAP stimulates the formation of bone at the expense of fat  – the Wnt signaling pathway, which is known to affect BMMSC fate. After using small interfering RNA (siRNA) to knock down a single protein in the pathway, Wnt10b, the bias toward the formation of bone diminished. Thus, it was Wnt10b that LRAP had been influencing in order to tip the balance in favor of bone formation, Zhou said.

“This is part of a larger story,” Snead said. “A large potential for regeneration rests in the induction of Wnt proteins. It portends a lot of
capacity for regenerative medicine.”

Zhou agreed, citing the need to develop more effective treatments for osteoporosis, a disease characterized by low bone density and high marrow adiposity.

“The data is very promising,” he said. “[LRAP] is a small, naturally occurring peptide. With further studies in animal models, we anticipate that LRAP possesses the potential to harvest the therapeutic value of Wnt signaling pathways without direct application of Wnt proteins, thereby mitigating the adverse effect on cells of direct Wnt protein exposure. LRAP is much smaller than Wnt proteins, giving LRAP a distinct advantage as a small molecule for delivery as a therapeutic agent.”