Same Process Discovered To Both Form Skeleton and Protect it For Life

Findings Suggests Related New Treatment Approach for Osteoporosis

March 12, 2008

Mesenchymal Stems Can Become Bone-forming Cells

A protein signaling pathway recently discovered to guide the formation of the skeleton in the fetus also keeps bones strong through adult life, according to two papers published recently in the journal Nature Medicine. Furthermore, the same mechanism may be at the heart of osteoporosis, where too little bone is made over time.

Human cells must be able to send signals that switch life processes on and off as they form the fetus, and later, to maintain the integrity of adult tissue. Notch proteins have been recognized for many years as part of signaling cascades that drive the development of the fetal brain, nerves and blood vessels. What had remained a mystery was whether Notch has any role in bone formation and health in adults.

The current results demonstrate for the first time in live, adult animals that genetic changes made to increase Notch signaling specifically in bone-making cells (osteoblasts) resulted in thickened, abnormal bone. Conversely, eliminating notch resulted over the long term in the weaker bones seen in osteoporosis. The studies confirm that Notch plays a role in bone development, and suggest it also maintains bone strength with aging.

”The findings are important because without Notch signaling in osteoblasts, the cells that build bone, you get inadequate new bone formation along with aggressive bone destruction by bone-degrading cells, both typical of osteoporosis,” said Brendan Boyce, M.D. professor of Pathology at the University of Rochester Medical Center, and a study author. “These studies suggest that well timed manipulations this process may represent a new way to fight a major bone disease.”

Brendan Lee, M.D., Ph.D., associate professor of Molecular and Human Genetics at the Baylor College of Medicine led the first study along with Boyce. The second study was led by Matthew Hilton, Ph.D., now assistant professor of Orthopaedics and Rehabilitation at the Medical Center, with Fanxin Long, Ph.D., principle investigator of this study. Long is assistant professor of Medicine at Washington University, where, until recently, Hilton was a post-doctoral fellow. They also partnered with the Endocrine Unit at Massachusetts General Hospital. Both studies, supported by grants from the National Institutes of Health, were published online on Feb. 24 and in hard copy on March 6.

Study Details

Having been around since early in evolution, Notch proteins are named for notches in the wings of the flies in which Notch-related genes were discovered. Such protein receptors span a cell’s outer membrane, enabling external biochemical messages to penetrate cells.

Part of the receptor is exposed to the cell’s outside and designed to react with a specific signaling molecule (ligand). When a ligand docks into the receptor, like a ship coming into port, it changes the shape of the dock such that it sets off chain reactions inside the cell. When a ligand binds to Notch in particular, part of the protein, the notch intracellular domain (NICD), breaks away inside of the cell, travels to the cell’s nucleus and influences gene expression there. Gene expression is the process whereby genetic instructions encoded in genes are converted into protein workhorses that make up the body’s structures and carry its signals. In the current case, the NICD signal was shown to influence the decision made by stem cells in bone marrow about whether or not to become bone-making cells.

As we develop in the womb, successive generations of stem cells specialize (differentiate), with each group able to differentiate into fewer and fewer cell types. Many tissues maintain pools of stem cells into adulthood in case replacement cells are needed for healing or maintenance. Among theses are mesenchymal stem cells, which reside in adult bone marrow and can differentiate into, among other things, bone-making cells called osteoblasts. Osteoblasts are one of two cell types, which coupled together, enable bone to continuously recycle itself and stay strong. Where osteoblasts make new bone, osteoclasts “eat” aging bone to make way for new bone in a careful balance with osteoblasts.

The current study suggests for the first time that Notch signaling influences the process by which stem cells “decide” whether to become bone-making osteoblasts. The data also argue that Notch regulates the process by which osteoblasts signal to osteoclasts, regulating their ability to eat bone.

In recent years, bone biologists have constructed a theoretical model that they believe represents the stages involved in the differentiation of mesenchymal cells into mature osteoblasts. The model holds that intermediates exist between stem cell and mature osteoblast and that signaling processes, including Notch, control the transition from one to the next. In the first step, mesenchymal stem cells commit to the osteoblast pathway or lineage. Once that decision is made, they become, in distinct stages, osteoblast precursors then immature osteoblasts and then mature osteoblasts. Notch signaling has different roles at each stage, inhibiting some transitions while encouraging others, researchers found. In short, Notch “escorts” the stem cells through the process until they form a pool of immature osteoblasts, then maintains that pool until the body calls for more bone-making cells.

In both studies, researchers manipulated the osteoblast differentiation process at different stages in study mice. The Long-Hilton study used genetic tools to shut down Notch early in the differentiation process, at the point where the signals would have enabled mesenchymal stem cells to commit to the osteoblast pathway. By interfering early, the Hilton team demonstrated that such signaling normally maintains a pool of mesenchymal stem cells, inhibiting their ability to differentiate into osteoblasts. Thus, shutting down Notch led to an initial increase in osteoblast differentiation and related bone formation. The initial spurt of bone making, however, was followed by decreased production by osteoblasts of osteoprotegerin, a protein that protects the skeleton by inhibiting osteoclast formation. In the absence of Notch, osteoclast formation increased over time, resulting in long-term, age-related bone loss.

While the Hilton team only shut down Notch signaling, the Lee-Boyce study used genetic engineering to greatly increase it in one set of experiments and to shut it down in another. Both sets of changes were made later in the differentiation process than in the Hilton study, near the last step in the pathway where immature osteoblasts mature into active bone-making cells. In the Boyce study, mice genetically engineered to have more Notch signaling in osteoblast precursors had a striking increase in the number of immature osteoblasts, which led to abnormally thickened bones. When they shut down notch signaling, the mice lost bone like those in the Hilton experiments.

“One theory is that Notch signaling normally maintains the mesenchymal stem cell pool in our bone marrow and inhibits their differentiation into osteoblasts,” said Hilton. “So for some patients with oteoporosis, we may be able to briefly inhibit Notch signaling to allow more mesenchymal stem cells to differentiate into osteoblasts, creating a larger pool of bone-building cells. The obstacle to this strategy – that generating more bone building cells inevitably activates more bone degrading cells – could be overcome by combining a transient Notch inhibitor with any of several already approved osteoporosis treatments that inhibit osteoclast formation or activity.“

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