Way to grow

日期:2017-08-15 05:37:08 作者:万扃 阅读:

By Jonathan Knight A CHEMICAL switch discovered by researchers in California could explain how embryonic nerve cells grow in the right direction. The finding might eventually lead to a way of regenerating damaged nerves, so reactivating paralysed limbs. When a nerve cell grows, the long, thin extensions known as axons, which act as the electrical wiring of the nervous system, have to find their proper destinations. They do this by following chemical cues. The tip of the axon, known as the growth cone, will turn towards cells that secrete a substance called neurotrophin and away from cells that release another chemical called semaphorin. To reach its ultimate target, the axon must grow past a series of “guide post” cells that emit the signalling chemicals. “Attraction has to change into repulsion,” says Mu-ming Poo, a neurobiologist from the University of California at San Diego. The first hint of an explanation for how this might occur came last year, when Poo and colleagues found that the common biological messenger molecules known as cyclic nucleotides could change the attraction of neurotrophin into repulsion. Now they have completed the story by showing that cyclic nucleotides can make substances that normally repel growing axons attract them instead. The researchers started with a dish that contained growing frog neurons, and added small quantities of semaphorin. As expected, the nerve fibres grew away from the source of the chemical. But when they added the cyclic nucleotide cGMP, the axons instantly began growing toward the semaphorin, like a plant growing towards sunlight (Science, vol 281, p 1515). Biochemists already know that cells produce cyclic nucleotides in response to specific environmental cues. Poo says this could be the switch that allows nerves in developing embryos to follow guide posts towards target tissues several centimetres away. On reaching a guide post, the neuron might receive a signal switching the production of cyclic nucleotide on or off. This team’s discoveries could lead to a way of regrowing damaged nerves. The fatty myelin sheath that surrounds nerves secretes chemicals that normally repel growing frog neurons. Scientists believe that this is what prevents damaged nerves from regenerating. But the researchers found that this repulsion turned to attraction when they added a cyclic nucleotide called cAMP to a dish containing growing frog neurons and a source of the myelin chemical. Adding cyclic nucleotides might also allow damaged neurons to regenerate, Poo says. “It’s very neat,” says Jonathan Raper, a biologist at the University of Pennsylvania who was one of the first people to describe semaphorin. Using cyclic nucleotides for nerve therapy is speculative but possible,