Wednesday, January 02, 2008

Regenerating Nerves

A polymer studded with chemicals that resemble a common neurotransmitter can stimulate the growth of neurons.

By Eric Bland

Nursing nerves: Christiane Gumera and Yadong Wang of Georgia Tech (top) look at stained neurons grown on top of a polymer laced with chemical structures that resemble the neurotransmitter acetylcholine. The polymer stimulates the growth of the neurons’ information-carrying projections, known as neurites (below). Credit: Christiane Gumera

The human body can be amazingly resilient: wounds heal, bones mend, ligaments grow back together. But recovery from nerve damage is far less reliable. In the latest issue of Advanced Materials, researchers Christiane Gumera and Yadong Wang from the Georgia Institute of Technology announced that they have triggered the regrowth of nerve cells using a polymer coated with chemical structures that resemble acetylcholine, a common neurotransmitter. The research, which is the first to combine a neurotransmitter and a polymer, could one day lead to treatments for neurodegenerative diseases and spinal-cord injuries.

"Lots of people have done biopolymer work," says Christine Schmidt, a biomedical engineer at the University of Texas at Austin. "But this demonstrates that a polymer with a neurotransmitter can be used to guide growth in the nervous system."

Previous research has identified several agents that can stimulate the regrowth of nerve cells, or neurons, a protein called laminin foremost among them. But laminin is water soluble and dissolves quickly in the water-based environment of the body. The Georgia Tech researchers' material worked as well as laminin, but because it is water insoluble, it is more likely to stay in place if inserted into a patient's body, and it could stimulate the growth of nerve cells for weeks instead of days.

Acetylcholine, the neurotransmitter that the researchers concentrated on, is one of the most commonly occurring. Like all neurotransmitters, it carries, amplifies, and modulates signals sent between neurons. For years it has been known that acetylcholine stimulates the growth of a neuron's information-carrying projections, or neurites: the lone, long transmitter called the axon, and the many short receivers called dendrites. But the body's enzymes break apart acetylcholine less than a second after detecting it, since too much acetylcholine is toxic and inhibits neurites.

To give their acetylcholine-like chemical a controlled, long-term effect, the researchers attached it to a material that enzymes can't break apart: a flexible and biodegradable polyester. Wang notes that other chemical messengers can be attached to the polymer backbone, and that the whole assemblage breaks down after several weeks.

Wang and Gumera placed a small piece of rat nerve and its surrounding tissue on the polymer and measured nerve length over the next four to six days. The neurites grew steadily for six days, then began to slow down. The longest neurite produced was more than five millimeters long, with a maximum growth rate of 0.7 millimeters a day.