Barrier Found To Nerve Regeneration

EGF Receptor Mediates Inhibitory Signals from Two Pathways

Scientists have long dreamed of prompting adult neurons of the central nervous system to regenerate. But these cells have the deck stacked against them in several ways. Molecules from the myelin sheath surrounding their axons actively discourage growth. After injury, nearby astrocytes form a dense scar to block them. Even the signals that once guided axons as they formed during development now seem to prevent regeneration. And most neurons also have lost the internal factors that enabled them to stretch their axons out in the first place—even if allowed to, they wouldn’t grow.

Image courtesy of Zhigang He

Regrowth. After injury, the optic nerves of mice treated with an EGF receptor inhibitor (bottom) were able to grow farther than controls (top).

Regeneration seems like a daunting task with all of these circumstances conspiring against it. Still, most researchers in the field believe it will be possible to remove the brakes on growth in the CNS if they can identify the restraints. An encouraging study in the Oct. 7 Science from the lab of Zhigang He, HMS assistant professor of neurology at Children’s Hospital Boston, uncovers a surprising new player on the side of inhibition—the well-known epidermal growth factor (EGF) receptor. The molecule appears to mediate the inhibitory signals of both myelin and proteoglycans from the glial scar—a convergence of pathways in a field that has become increasingly complex.

Releasing the Myelin Brake
He’s lab has been working for several years to identify and characterize the substances in myelin that inhibit regeneration. “Our approach is to first study the inhibitory mediators in the environment and see if we can find pharmacological or genetic means to block inhibition,” he said. His team began screening hundreds of compounds to identify molecules capable of reversing the action of myelin. They placed cultured neurons on a myelin mixture and measured the elaboration of neurites, axonlike projections from cultured cells. Among the most potent substances that induced sprouting were molecules that block the EGF receptor.

To further study the receptor’s role, they used an engineered virus to introduce a mutant form of the EGF receptor into neurons that blocks the naturally occurring receptor. These cells were also able to grow neurites more robustly. “The normal way people consider EGF is as a pathway leading to cell proliferation,” He said. In fact, EGF receptor inhibitors are currently used to treat cancer, so it was surprising to see the same class of molecule promoting growth.

Myelin contains three primary inhibitory molecules, all of which signal through a receptor complex that contains the Nogo-66 receptor. In this study, He’s team found that myelin inhibitors activate the EGF receptor in cultured neurons, and this activation could be blocked by disrupting the function of this receptor complex. How the two membrane receptors interact is unknown, but He believes the Nogo-66 receptor complex somehow causes calcium to rush into the cell, which stimulates the EGF receptor.

Cultured neurons surrounded by proteoglycans, inhibitory signals from the glial scar, also sprouted neurites if the EGF receptor was blocked. Clifford Woolf, the Richard J. Kitz professor of anesthesia research at Massachusetts General Hospital, who was not involved in the study, said that one of its interesting findings is “a convergence of quite different inhibitory components of the environment.” As new inhibitory molecules have been identified, scientists have worried that each one must be targeted separately. That proteoglycans and myelin inhibitors both work through such common signals is encouraging.

A Common Target
He’s team worked with the lab of Dong Feng Chen, HMS assistant professor of ophthalmology at Schepens Eye Research Institute, to see whether blocking the EGF receptor could help regenerate optic nerves in mice. After crushing the optic nerves of adult mice, the researchers surrounded the injury site with a gelfoam soaked in a solution containing an EGF receptor inhibitor. After receiving the treatment every three days for two weeks, animals treated with the inhibitor had a ninefold increase in regenerating axons versus controls, with the axons stretching about a quarter of a millimeter in length beyond the injury site.

Photo by Steve Gilbert By targeting a single pathway, (left to right) Glenn Yiu, Jong Bae Park, Vuk Koprovica, and Zhigang He could remove two sets of barriers that keep neurons from regenerating.

While the regeneration was significant, the study also clearly showed that blocking myelin inhibition alone will not promote dramatic regeneration. The researchers estimate that only about .5 percent of the retinal axons grew at all, a figure that He believes represents the limit of the cells’ intrinsic potential. “A few years ago, the whole field believed that myelin was the whole picture,” He said. But this study shows that more factors are at play. “Removing the barrier is not sufficient,” Woolf agreed. The vast majority of neurons of the central nervous system seem to lose the ability to grow axons rapidly in adulthood. The intrinsic program of the cells and perhaps other outside factors may need to be overcome to promote more growth.

But with molecules that can block myelin inhibition now in hand, He hopes to clarify what role myelin plays versus other factors. His lab is collaborating with Marc Tessier-Lavigne at Genentech to begin testing the effect of the company’s EGF receptor inhibitor Tarceva, which is currently used as a treatment for lung cancer.

Meanwhile, other researchers are tackling the additional barriers one by one. Both Chen and Woolf have studied intrinsic factors in the cell that keep adult cells from growing. Other labs have begun to tease apart the complex components of the glial scar. Tessier-Lavigne’s group is looking at how external cues that guide wandering axons during development may also play a role in regeneration in the adult.

With so many molecular players thwarting growth, there is no easy fix. But He remains optimistic, pointing out that while models of regeneration focus on the extreme cases of optic nerve and spinal cord injury, the science of promoting axon growth could benefit treatment for stroke and less severe injuries. And breaching even a small distance in an injured spinal cord may mean the difference between complete paralysis of the lower body and regaining control over the bladder and other functions. According to He, “You may only need one or two millimeters to keep people alive in some cases.”

—Courtney Humphries

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