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Optic Nerve Regrown in Mice
Study Removes Internal and External Barriers to Full-length Nerve Cell Regeneration
The nervous system of lower vertebrates can regenerate after an injury, as can the peripheral nervous system of mammals. But the mammalian central nervous system has lost the ability to regrow. For humans, this means that the relative inflexibility of our CNS enables us to store memories and maintain complex functions, but it leaves us susceptible to injuries from which we can never recover.
Right target, wrong side. Up to two weeks after birth, mice genetically engineered by Dong Feng Chen and her colleagues were able to regenerate the optic nerve into the brain (left) after the nerve was injured. Though the pattern of regeneration mimicked normal development (right), the axons did not cross over to the opposite side of the brain. (Image courtesy of Dong Feng Chen)
Researchers have been hunting for ways to help the mammalian central nervous system regain the properties of self-renewal. In the March Journal of Cell Science, a team led by Dong Feng Chen reports that it has engineered mice to fully regrow damaged optic nerves for the first time. Yet this mile-stone is still only a step in what has proved to be a tortuous exploration.
The Double Bind
When the nervous system is first forming, neurons stretch their axons out
over long distances to form major nerves. But shortly after birth, the
neurons that form the brain, spinal cord, and optic nerve lose this ability
and can
only make small changes in response to external stimuli. Chen, HMS assistant
professor of ophthalmology at Schepens Eye Research Institute, said that
there seem to be two forces at work: mature neurons switch off genes
that enabled them to regrow in their younger days, and the neurons’ environment
changes after development, encouraging the cells to keep still.
Chen’s group first focused on one of the intrinsic factors that is lost in mature neurons—Bcl-2, a protein that inhibits cell death. Her lab had previously shown that Bcl-2 is a determining factor in axon growth in the optic nerve. Expression levels of the protein are high during early development, but drop off after the neurons lose their ability to regenerate. But when other scientists engineered mice that overexpress Bcl-2, their optic nerves could not regrow after an injury made five days after birth. Chen wanted to find out if the protein could at least promote regeneration when the environment of the axons is permissive for growth. Her team showed that if the injury was made just three days after birth, the axons of the engineered mice would indeed grow back.
Bcl-2, it seemed, could at least give neurons the intrinsic properties they needed to grow. The next step was to try to change the environment. When nerves in the adult CNS are damaged, nearby astrocytes form a dense tissue at the injury site, called a glial scar. To see whether removing this physical barrier could release the brakes on nerve growth, research associate Kin-Sang Cho first applied an astrocyte-specific toxin to the target areas in the brains of adult mice and found that killing the local astrocytes allowed optic nerves to grow 20 times more robustly than with Bcl-2 overexpression alone. The researchers then created mice that not only overexpressed Bcl-2, but also lacked two cytoskeleton proteins that help astrocytes cluster into dense scars. With this combination of factors, the optic nerves were restored after injury, growing rapidly and deeply into the brain in just a few days, mimicking the pace of normal development.
Kin-Sang Cho (left), Dong Feng Chen, and colleagues overcame two genetic hurdles, allowing mice to regenerate optic nerves after injury. (Photo by Steve Gilbert)
Such robust regeneration is a first in the field, though Chen’s team still needs to determine whether the impressive growth of the nerve actually allows the mice to see. In addition, the regenerative ability of the mice was only sustained up to two weeks of age.
An Obstacle Course
Chen said the study shows there are two “doors” that stand
in the way of optic nerve growth in mature neurons: the inner door created
by
intrinsic properties of a cell and the outer door created by external
constraints. “If
you want regeneration to occur, you have to open both doors, so you
need two keys,” she said. Her work has demonstrated that Bcl-2 is
a key to removing the intrinsic barriers. “The second door looks
like it has two parts,” she said.
Chen focused on one of these parts—the glial scar formed by astrocytes. But other researchers have focused on the properties of the myelin sheath that surrounds axons. Zhigang He, HMS assistant professor of neurology at Children’s Hospital Boston, has studied how inhibitory proteins in myelin bind to receptors on neurons to keep them from growing. Last year the lab of Larry Benowitz, HMS associate professor of neurosurgery at Children’s, teamed with He to partially regenerate an optic nerve by blocking myelin inhibitors while also stimulating nerve growth by inducing an eye injury in rats (See Research Briefs, Focus, March 19, 2004.)
| “It’s possible that if we manipulate both factors, the myelin and the glial scar, the regeneration would be even more robust.” |
Chen agrees. “By eliminating one of the inhibitory factors from the environment, we can make the environment more permissive than the normal adult environment,” she said. “It’s possible that if we manipulate both factors, the myelin and the glial scar, the regeneration would be even more robust.”
Once these factors are determined, the next step would be to look for ways to unlock these doors without manipulating the genome, either by designing small molecules to target these proteins or through gene therapy. But that is not the only hurdle: although the mice grew healthy optic nerves into the brain, the nerves grew to the same side of the brain rather than crossing into the opposite side as they normally do. Chen speculates that another signal may be required to properly direct the optic nerve. She notes that reptiles have the same problem when their optic nerve is severed—unlike amphibians, whose optic nerve readily regenerates to the proper target. “Maybe we’re only up to the reptile stage,” she said.