In this month's Proceedings of the National Academy of Sciences, scientists at Johns Hopkins and the National Institutes of Health describe an important piece in the puzzle of what can go wrong in nerve-damaging disorders such as multiple sclerosis and Guillian-Barre syndrome.
They've verified a previously suspected molecular bridge between nerve cells and their surroundings which, when broken, causes nerves to deteriorate in a way apparently identical to a variety of neurodegenerative diseases. The research adds new insight on nerve disease as well as a new focus for research on stimulating nerve cell regrowth in the brain and spinal cord.
Although the studies were performed in mice, "the human versions of the molecules involved are essentially identical in structure and type," says Ronald Schnaar, Ph.D., who led the research team; "we have no reason to believe the process in people would be significantly different."
In MS and other as-yet "unsolved" nervous system diseases, the spotlight has been on myelin, the insulating material that sheaths nerve cells in layers like a burrito wrap. In the disorders, the myelin sloughs off the surface of the nerve cell and beneath it, the axon -- the elongated part of the cell that relays messages -- disintegrates.
The new research focuses on a natural linkage -- or a "molecular handshake" as one researcher calls it -- between the myelin and the axon it insulates. The handshake isn't structurally important, the researchers say; it's not what holds the myelin in place, for example. But, as in such linkages that occur in immune cells, it apparently trips a series of reactions on either side that are necessary for normal nerve behavior.
The Hopkins/NIH researchers focused on one half of the handshake, on molecules called gangliosides that extend from the surface of the nerve cell membrane. Gangliosides are a family of complex carbohydrate and lipid-based molecules and a hallmark of nerve cell membranes. Using knock-out gene techniques, the scientists created mice with simpler-than-normal gangliosides by eliminating a gene that affects molecule length. This action blocked attachment of a protein-attractive portion at one end of the ganglioside.
A microscopic study of brain, spinal cord and peripheral nerves in the knock-out mice showed a deterioration whose onset came as the mice approached early middle-age. "We could immediately see something amiss in these nerve cells," says Kazim Sheikh, who did the neuropathology studies, "and the damage is quite similar in chronic human neuropathies." Mouse behavior also reflected the deterioration: mice were weaker, had trouble moving and lost key reflexes.
"We had predicted that altered gangliosides would make such a difference," says Schnaar. The reason stems, he says, from disruption of the linkage between the gangliosides in the nerve cell membrane and a second molecule in its myelin sheath. That molecule, myelin-associated glycoprotein (MAG), is known to influence nerve cell stability. Mice with the gene for MAG knocked out also suffer a loss of myelin and deteriorating nerves.
Schnaar's team knew MAG and gangliosides are capable of binding together; they found such bonds in earlier cell culture work that mixed complex gangliosides with cells engineered to carry MAG on their surfaces.
The present follow-up animal study shows both the existence and the importance of the ganglioside half of the "molecular handshake." Animals without complex gangliosides also suffer nerve deterioration. "The damage so resembles what you get when you knock out MAG that it suggests the two are normally partners," says Schnaar.
A second function of the handshake may be to regulate nerve regeneration. "The main reason nerves don't regenerate in the brain or spinal cord is because myelin inhibits nerve regrowth there." MAG, which is found in high levels on myelin in the central nervous system, is thought to be an important inhibitor of nerve regrowth. After an injury, Schnaar adds, regrowth-blocking, myelin-containing debris gets cleaned up quickly by the immune system in peripheral nerves. The cleanup is slower in the brain and spinal cord than in the peripheral nerves.
"Knowing about this handshake' means we might be able to design something that could block MAG's inhibitory action. Now we have another handle that can help us understand inhibition," says Schnaar. Therapies of the future, based on regrowth of existing nerves, may mean combining nerve stimulators, such as nerve growth factors, with agents that remove blocks to growth.
Gangliosides are also a new focus for nerve disease research for another reason, Schnaar says. In one form of Guillian-Barre syndrome, a crippling disease with a probable auto-immune tie, patients are known to have had a previous infection with the diarrhea-causing bacteria Campylobacter. Other work by a research team led by John W. Griffin, M.D., one of the collaborators, describes how patients make antibodies against Campylobacter, specifically against ganglioside-like molecules in its outer coat. The patients also make antibodies against their own nerve cell gangliosides, which Griffin says, begins a process that leads to crippling.
Other researchers in the study were Thomas O. Crawford, M.D., and John W. Griffin, M.D., and Ji Sun of Hopkins, and Yujing Liu, Hiromichi Kawai and Richard L. Proia of NIH.
The research was funded by grants from the National Multiple Sclerosis Society, the Paralyzed Veterans of America Spinal Cord Research Foundation, the National Science Foundation and NIH.
Related Web Sites: The Schnaar lab Web site contains photographs showing nerve devastation following knockout of the complex ganglioside gene and other useful images: http://www.bs.jhmi.edu/pharmacology/schnaarlab/welcome.html
The Web site of the National Multiple Sclerosis Society gives useful background on the science of the disorder: http://www.nmss.org.
The study was published in the Proceedings of the National Academy of Sciences, vol 96, June 22, 1999.
Journal
Proceedings of the National Academy of Sciences