By David N. Leff
If a spotted salamander (Ambystoma maculata) loses an eye or a leg, it promptly grows a new one. And if a human (Homo sapiens) forfeits up to three-fourths of his or her liver, that organ can sometimes regenerate to full size and function.
Nerves are a different story.
Once neurons in the central nervous system (CNS) of the brain die, they can never make a comeback. But motor neuron axons, which actuate muscles beyond the brain, can and do regenerate after injury or loss.
Neuroscientists the world over are striving to solve this dichotomy and somehow endow the CNS neurons with the ability to regenerate.
Neuroscientist Jerry Silver, at Case Western Reserve University, in Cleveland, has spent the last 12 years pursuing the hypothesis "that a certain family of molecules, the proteoglycans, are very critical in the phenomenon of regeneration failure in the adult CNS in vertebrates."
Proteoglycans are mucopolysaccharides complexed to chains of proteins. Silver has found that they are "potently inhibitory molecules."
He added: "We are the first ones to find them in the CNS and define their role in glial scarring. We think they are potent inhibitors of nerve regeneration."
Glial scars on brain cells, Silver explained, "are formed of large neuroglial cells, which get a little bit bigger and fatter around the injury. Their molecular part consists of those proteoglycans that we discovered in our lab."
Silver is senior author of an article in today's Nature, dated Dec. 18, 1997, titled: "Regeneration of adult axons in white matter tracts of the central nervous system."
"Our paper suggests for the very first time," he told BioWorld Today, "that once the nerve fibers are past the area of glial scar, there's an enormous potential for regeneration on the adult CNS. That we never expected."
To make this point, the article's first author, Stephen Davies, devised a high-precision transplantation technique for creating glial scarring in the stereotactically targeted brain cells of rats.
"He created very small micropipettes, with 70-micron bores, highly sharpened at the end," Silver recounted. "A commercial 'picospritzer' pump then pulse-injected adult rat dorsal root ganglia neurons into the CNS pathway. Davies made the injection so gently that it did not create scar tissue."
The result: "When we injected cells with this kind of trauma condition, they regenerated their axons at great speed, with high efficiency over long distances. I never thought this would work."
It worked in 35 of the 41 animals they injected, a success rate of 83 percent.
"For every 100 cells injected," Silver observed, "70 could send out nerve fibers. So the efficiency is about 70 percent. And of those, 80 percent could continue on into the gray matter on the opposite side of the brain, about 8 millimeters away.
"The speed at which the cells were regenerating their axons," he continued, "was a remarkable 1 to 2 millimeters a day."
British Scientist Reports Protein In Pancreas Promotes Motor Neuron Axonal Growth
The cells in the Islets of Langerhans that churn out insulin on demand owe their development in part to a little-known molecule in the pancreas called Reg-1.
The flip face of Reg-1 is Reg-2, formerly known as the pancreatic secreted protein.
"The crosstalk between that pancreatic area and the neuronal regeneration we are working on is quite extensive," observed neuroscientist Frederick Livesey, now at Trinity College, Dublin, Ireland. "Those islet cells are called neuroendocrine cells," he pointed out. "They have a lot of similarities with neurons, particularly sensory neurons."
Livesey is first author of a paper in last week's Nature, dated Dec. 11, 1997. It bears the title: "A Schwann cell mitogen accompanying regeneration of motor neurons."
"The fact that Schwann cells, which sheath neurons in insulating myelin, are necessary for motor neuron regeneration was spotted nearly 100 years ago," Livesey observed. "What we found," he told BioWorld Today, "is that the Reg-2 protein is a signal from motor neurons to Schwann cells to divide, and get on with their job."
Motor Neuron Studies Just The Beginning
When he and his co-authors inhibited that cell's action by blocking the Reg-2 signaling with a polyclonal antibody, "we could knock regeneration of motor neurons down to a third of their normal rate.
"Central nervous system neurons don't regenerate in adult mammals," Livesey recalled; "motor neurons do. They are actually CNS cells, but their processes grow out into the periphery of the body. Thus, they're the only CNS neurons that can regenerate," he pointed out.
"The fundamental biology of how they regenerate," he went on, "is core information we must have before we can understand how to make other types of neurons regenerate." So the co-authors turned to nervus sciaticus.
The sciatic nerve runs from hip to toe. In rats, Livesey said, "this peripheral motor neuron is relatively thin, like rather thick cotton thread. It's large and visible; you can manipulate it."
To model their in vivo exploration of Reg-2's role in regeneration, they crushed the sciatic nerves of rats. This injury has the effect of stimulating regeneration, and Reg-2 secretion. "Then," Livesey recounted, "we took a very fine microsyringe and injected the antibody to the Reg-2 into the nerve."
After waiting three days, the team "simply took out the nerve, and measured the length of the axons that were growing past the point at which we'd blocked the Reg-2. We found a very specific inhibition of the growth of that axon, which would normally contain Reg-2."
One such clinical application of this finding, he suggested, might be to enhance current efforts to graft pieces of peripheral nerves to bridge damaged tracts of CNS nerves. "That's possibly a good use of Reg-2," he speculated, "as an adjunct to such bridging grafts." (See BioWorld Today, July 29, 1996, p. 1.)
Another potential avenue for the protein, which he is now pursuing, "relates to what Reg-2 might be doing when motor neurons themselves are damaged, as in ALS — amyotrophic lateral sclerosis." *