New Osteoporosis Tx Could Avoid Infection, Cancer Risk

Working with a protein known as RANKL (receptor activator of nuclear factor kappa-B ligand), scientists at Washington University School of Medicine in St. Louis have created a new way to develop treatments for osteoporosis and autoimmune diseases. The key differentiator is that this treatment could avoid the risk of infection and cancer posed by some current medications.

According to the September 24, 2014 news release, co-senior authors Steven Teitelbaum, M.D., and Daved Fremont, Ph.D., showed that copies of RANKL naturally come together in clusters of three. To send a signal, which triggers the development of bone-dismantling cells, each copy of RANKL in the cluster has to bind to a copy of a receptor, which passes on the signal. They mutated individual copies of the RANKL protein to make them better or worse at hooking up with the receptor. They then joined together two copies of the protein that were very good at attracting and holding onto the receptor with a third copy that did not bind to the receptor.

First author Julia Warren, an M.D./Ph.D. student, told OTW, “When thinking about manipulating the region of RANKL that interacts with its receptor RANK to either enhance or abolish binding, we were met by a challenge shared across the tumor-necrosis factor superfamily that is in part a result of the homotrimeric nature of the protein. Specifically, the binding cleft that accommodates RANK receptor is actually formed by the interface between two RANKL monomers. Therefore, to individually alter each of the three RANK binding sites formed on a homotrimer of RANKL, we were limited by the requirement that our mutations all be on one side of the binding cleft. Further, we were hoping to identify mutations that limited the ability of RANKL to bind to its endogenous decoy receptor OPG (osteoprotegerin).”

“Luckily, we knew that this was a property we desired at all interfaces and therefore did not face precisely the same limitations. This unique difficulty was only overcome by using a screening approach called yeast surface display to randomly select for mutations in RANKL in combination with rational design of mutants based on established structural data. Ultimately, we identified a series of mutations that allowed us to selectively block or enhance RANKL binding to RANK while simultaneously preventing binding the decoy receptor OPG. When inserted in strategic combinations into our single-chain RANKL protein, we were able to individually manipulate each of the three receptor binding clefts to develop a highly potent inhibitor of RANKL/RANK signaling.”

Asked about their future work, Warren commented, “Within the context of RANKL, we would like to assess our novel inhibitor of RANKL/RANK signaling in other in vivo models of human disease where osteoclast formation/activation plays a critical role. Examples include breast cancer metastatic to bone and rheumatoid arthritis. Additionally, we envision generalizing this strategy of protein engineering to other members of the tumor necrosis factor superfamily which could have an impact on a wide range of human diseases.”