John (Jack) Kent, College of Natural Science

UT Austin neurobiologist Dr. George D. Bittner and his colleagues have used the new technique to rejoin severed nerves from mammalian central and peripheral nervous systems, suggesting the technique ultimately may be applicable to human patients.

Nerve axons are extensions of nerve cells that transmit electrical signals over long distances in the body. When axons are cut or crushed, their electrical signals can no longer be transmitted. In mammals, including humans, damaged axons in the central nervous system (the brain and spinal cord) usually cannot regenerate.

Hence, at present, the functions once controlled by those axons cannot be restored. Axons severed in the peripheral nervous system (PNS) -- the system that connects the central nervous system to organs and muscles -- may regenerate, but very slowly, about 1/25th of an inch per day. "For example, if you cut PNS axons in your upper leg, those motor axons that normally contact muscles in your foot would take several years to regenerate from your upper leg to your foot," Bittner explained. "Often, those severed axons do not ever regenerate properly."

Annually, several hundred thousand CNS or PNS injuries occur in the United States. There is no technique now in use for humans or other mammals that has been shown to repair severed CNS axons reliably, or to speed up the repair of severed PNS axons. In the Journal of Neuroscience paper, Bittner and his collaborators describe a technique to rejoin the cut or crushed ends of severed CNS or PNS axons that allows the repaired axons to resume electrical signaling through the lesion site within seconds to minutes after the cut ends are rejoined.

For mammalian nerves, these scientists first apply a calcium-free solution of polyethylene glycol (PEG) for one to two minutes to the cut ends of severed axons. PEG, a high-molecular-weight industrial polymer, is routinely used in molecular biology to fuse the cell membranes of adjacent cells. Once the cut axon ends are fused, the PEG solution is washed off and replaced with calcium-containing solutions that mimic the salt composition of mammalian body fluids. The scientists find that many of the once-severed axons can again transmit electrical impulses through the lesion site within two to 30 minutes.

Since the mechanical strength of the PEG-rejoined axons is weak, they are sealed with a biological adhesive (a PEG hydrogel) developed by one of Bittner's collaborators, Dr. Jeffery Hubbell, now at the Swiss Federal Institute at Zurich, Switzerland. This PEG hydrogel binds very tightly to the severed axons and prevents the rejoined axons from pulling apart once the animal recovers from anesthesia. The PEG hydrogel technique has so far been used in earthworms, but Bittner and his collaborators have shown that the hydrogel is nontoxic to nerve cells in intact rats, opening the way for use of the technique in mammals.

Bittner discovered the PEG fusion repair procedure some years ago while he was studying the repair of axons taken from crayfish and earthworms. Nerve axon regeneration is more common in these invertebrates than in mammals. In the current paper, Bittner and his collaborators show for the first time that PEG solutions can be used to rejoin CNS and PNS axons removed from the sciatic nerves and spinal cords of mammals (rats).

They also show that the PEG solutions combined with the PEG hydrogel can be used to maintain rejoined axons for many weeks in earthworms after the animals recover from anesthesia. They currently are testing techniques to use PEG solutions and PEG hydrogel to repair CNS and PNS axons in rats. The procedures are not difficult to learn and have been mastered within several months by undergraduates working in BittnerĖs laboratory. This PEG fusion technique uses no substances that might prevent its use on humans.

Since they now have successfully used the PEG fusion technique to rejoin the severed halves of CNS and PNS axons in crayfish, earthworms, rats, rabbits and guinea pigs, the scientists suggest that that this new approach can almost certainly be used to rapidly rejoin cut or crushed axons in humans.

To aid this effort, Bittner and his colleagues already have published papers showing how the severed ends of mammalian axons can be kept alive for at least several days after the lesion. An ability to keep severed axons alive would give surgeons a longer time to rejoin those axons with PEG solutions. Bittner also suggests this approach might be combined with strategies currently being developed by other scientists to obtain even better results.