Nerve Growth Factor
Nerve growth factor (NGF) is a polypeptide, a molecule composed of several amino acid units that has a protein-like behavior, but is not as complex as a protein in structure. NGF increases the growth of nerve cells, especially those in the peripheral nervous system, and directs the growth and orientation of nerve cell axons (processes which carry impulses away from the nerve cell body to adjoining dendrites). NGF is one of many growth factors found to be essential in cell division (mitosis), and has been isolated from a variety of cells such as mouse salivary glands and developing nerve tissue. The behavior of NGF resembles that of polypeptide hormones such as insulin.
NGF has been studied most frequently in the development of the nervous system of embryos though surgical and chemical manipulation. Transplanting extra legs onto the backs of tadpoles has been found to cause the outgrowth of nerve fibers from the central nervous system to the leg. The chemical substance that directs the movement of the axons seems to react with receptors on the membrane of cells and causes a chain of metabolic events that stimulates the growth of the axons in a certain direction. Scientists have found a molecule that guides the growth of nerve cell axons in the visual retina of chick embryos. Injection of NGF into young mice or chickens causes the nerve cell bodies in sympathetic and sensory ganglia to enlarge, and seems to be essential for cell division (mitosis) in sympathetic nerve cells. If an antibody against NGF is injected into young mice, no sympathetic nervous system develops.
The study of NGF has applications in the treatment of injuries to the spinal cord, prevention of cognitive decline, and the treatment of certain brain diseases such as Alzheimer's. In experiments, animals that were deliberately injured to stimulate spinal injury victims were treated with collagen containing skin cells genetically modified to produce NGF. It was found that the experimental group of animals showed significant axon formation. The control animals did not show any signs of axon formation. This research could lead to the treatment of stroke and accident victims where there has been significant nervous system damage. Infusing NGF into the brains of aged rats with memory and learning impairments was found to increase their memory. The infused NGF seemed to prevent brain nerve cell death, and to stimulate the development of damaged neurons, which secrete acetylcholine, a brain chemical associated with memory. Researchers are also trying to find a way to introduce NGF into the brains of Alzheimer disease patients, in an attempt to retard memory loss.
See also Neuron.