RAINing on the RANK Parade
Authors: Davenport R. John 
Published: August 2003

SAN ANTONIO, TEXAS
--Like do-it-yourselfers stripping varnish from old furniture, cells called osteoclasts chew up tired bone so that it can be reenergized with fresh material. Overactive osteoclasts, however, can weaken the skeleton. A newly identified protein helps temper an osteoclast-activating signal, according to new research presented here 23 September 2002 at the 24th Annual Meeting of the American Society for Bone and Mineral Research. Defects in the quencher might underlie osteoporosis, which results when osteoclasts overeat.

When a protein called RANKL binds to a receptor on bone marrow cells, the receptor--named RANK--prods the cells to become bone-busters. The cells fuse to form behemoths that contain multiple nuclei and produce proteins that dissolve mineralized tissue. Scientists had previously identified a set of proteins that bind to the part of RANK that lies inside the cell and help transmit its signal to the nucleus. Poblenz and colleagues wanted to identify other proteins that interact with the receptor and modify its signal strength.
To find new RANK partners, the researchers searched through a collection, or library, of genes likely to be activated in bone marrow. They created yeast that produce RANK; in addition, each cell contained a gene for a different protein from the library. The investigators engineered the yeast so that a given cell would survive only if RANK attached to the second introduced protein. By sequencing the library gene carried by yeast that persisted, they were able to pick out proteins that stuck to RANK. The team identified two such proteins. One was TRAF2, known to glom onto RANK. Its presence suggested that the procedure worked.
The researchers turned to the second conspirator, whose sequence didn't resemble that of any known protein. They engineered osteoclast precursor cells to produce the mystery protein. When exposed to RANKL, fewer of these cells formed multinucleated osteoclasts than did controls. And when the team quenched rather than boosted production of the protein, more cells morphed into osteoclasts than in control experiments. Because the protein appears to shut off the RANK signal, the team named it RAIN, for RANK-associated inhibitor. Additional experiments revealed that RAIN prevents precursor cells from fusing into multinucleated monsters, but it doesn't interfere with other aspects of osteoclast biology: Even when flooded with RAIN, cells produced some molecules that characterize functioning osteoclasts.

"It's brand-new," says molecular biologist Dwight Towler of Washington University in St. Louis, Missouri. "It's interesting that [RAIN] targets the multinucleation step," a process whose regulation isn't well understood but that is crucial for turning bone marrow cells into osteoclasts. The team also found that RANK signaling turns on the
RAIN
gene, so Towler suggests that RANK could constitute a feedback mechanism that squelches osteoclast function. He also proposes that RAIN might help explain the bone-damaging Paget disease, which is associated with infection by particular viruses: "Wouldn't it be interesting if these viruses somehow muck up RAIN?" Considerable work remains to uncover RAIN's machinations, but the finding might reveal how organisms strip away tarnished bone without turning the skeleton into a pile of rubble.

--R. John Davenport

A. Poblenz, K. Du, B. Lamothe, N. K. Shevde, J. W. Pike, B. G. Darnay, RAIN, a novel intracellular molecule that associates with RANK and negatively regulates osteoclastogenesis. American Society for Bone and Mineral Research, 24th Annual Meeting, 20 to 24 September 2002, San Antonio, Texas.
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