Bone detectives uncover new clues in understanding skeletal homeostasis
Feb 25, 2015
Searching for water in a labyrinth of underground caves in Mexico’s Yucután Peninsula, the teenage girl slipped and plummeted 100 feet to her death. Over 12,000 years later, divers found her almost complete skeleton in a deep, water-filled cave. Her discovery sparked great excitement. Naia, as she was named, is one of the oldest complete human skeletons ever found. Her bones are yielding answers to the question: Who were the first Americans?
Such discoveries of ancient, preserved skeletons can give the impression that while all else in our bodies is in a constant state of flux, our bones remain unchanged. However, the reality is that as long as their owners are still breathing, skeletons are in a continuous state of renewal. Old bone tissue is digested by cells called osteoclasts, and then the osteoblasts get busy, laying down new bone tissue to fill the gaps and cracks. The end result is that most of the average adult skeleton is replaced every decade.
But it is only in the past ten years that scientists have begun to truly understand how this delicate balance of skeletal remodelling is orchestrated. Instrumental in the body’s development, the WNT signaling pathway has also now emerged as the key regulator in bone homeostasis.
Postdoctoral researcher Ming-Kang Chang is the latest in a line of researchers at the Novartis Institutes for BioMedical Research (NIBR) to uncover new details about the pathway. Chang and her colleagues have focused their research efforts on sclerostin, a protein that puts the brakes on bone formation by inhibiting WNT signaling.
“We still have a lot to learn about the WNT pathway, but what we know for sure is that sclerostin is the main antagonist in bone,” says Chang. It blocks the growth of new bone by binding to LRP5 and LRP6, disabling these co-receptors on the surface of the cell. With sclerostin pressing on them, LRP5 and LRP6 can no longer signal along the WNT pathway to trigger bone formation.
“If you can figure out how to control sclerostin’s action, you could potentially modulate bone growth,” says Chang. “So we went on the hunt.”
Novartis researcher Olivier Leupin was the first to identify a way that nature controls this particular set of bone formation brakes. By studying people suffering from rare diseases of bone overgrowth, he uncovered something very surprising. Contrary to what one would assume, some people with the extreme bone overgrowth phenotype did not lack sclerostin. Instead, they had mutations in another receptor in the WNT pathway—LRP4.
Leupin went on to uncover LRP4’s role as the interaction partner for sclerostin, publishing the findings in The Journal of Biological Chemistry in 2011. He found that LRP4 introduces sclerostin to LRP5 and LRP6. Take LRP4 away and presumably sclerostin can’t bind to them and so block the signaling pathway. Take LRP4 away and presumably you could trigger new bone growth?
The bones of mice lacking the Lrp4 gene are thicker than normal. One way that researchers measure bone thickness is through three-dimensional micro-computed tomography. They generate images such as this one of the porous bone compartment of a lumbar vertebra.
Credit: Novartis Institutes for BioMedical Research
To answer that question, the baton was then passed to postdoc Ina Kramer and then to Chang. They generated mouse lines deficient in the LRP4 gene and also blocked its action with anti-LRP4 antibodies. The results were striking.
“You learn to expect the unexpected in scientific research,” says Chang. “But, the only real surprise here was how conclusive the results were. We grew bone! By deleting and blocking LRP4 we were able to achieve bone gain in mouse models in vivo for the very first time.”
The findings were published in the Proceedings of the National Academy of Sciences in December 2014. Novartis is not taking LRP4 forward as a potential therapeutic target because the company has opted out of osteoporosis drug development. However, as Michaela Kneissel, head of Musculoskeletal Disease at NIBR, explains, the focus is on helping the wider scientific and medical community understand bone homeostasis.
“It is the potential for these types of discoveries that persuaded me to change from my original career studying prehistoric skeletal remains,” says Kneissel, who’s enthralled with the mechanisms of bone renewal.
LRP4 might represent a therapeutic target for others to take forward and develop for any bone disorder where the WNT pathway is dysregulated. Examples of such disorders include osteoporosis and rare genetic diseases where bone mass is too low or too high.
“To accelerate the field, we’re publishing our findings and sharing genetic models and tools,” says Kneissel.
New research sheds light on a molecular signaling pathway that could hold the key to restoring bone strength.