|A Test of Nerves|
|Tracking the elusive touch receptor across three genomes yields a gene required for sensing light touch|
Edward R. Winstead
November 17, 2000
Poking worms with eyelashes has paid off. In the 1980s, the poking and brushing of mutant soil worms led to the identification of genes required for the sense of touch. In the mid-1990s, researchers at the University of Iowa used the worm DNA sequences to identify a related human gene. This led them to the mouse version of the gene. They developed mice that lack the gene, and in 1998 the mice were shipped to colleagues in Germany for testing. Now, the collaborators report that the gene, BNC1, is needed for sensing light touch. It is the first evidence to link a gene in mammals to a touch receptor. All, ultimately, because of the worms.
"This is exciting because it is the first demonstration that a single gene implicated in touch in one organism actually has an effect in a higher organism," says Monica Driscoll, of Rutgers University in New Jersey. Her laboratory investigates the genetics of touch and aging in the worm, C. elegans. "The result validates the strategy of starting with C. elegans," she adds.
The architect of that strategy is Martin Chalfie, of Columbia University in New York. He co-authored a 1981 paper on molecular mechanisms of touch in C. elegans and led mutagenesis screens for sensory genes. In a series of experiments, researchers attached eyelashes to toothpicks and prodded and brushed the sides of worms to identify touch mutants. Over the next decade or so, Chalfie, Driscoll and others isolated the 12 worm genes required for sensing touch and pinpointed the six worm touch receptors.
Touch receptors in C. elegans consist of ion channel proteins. Ion channels are pores in the membranes of nerve cells through which sodium flows. The mechanical opening and closing of channels leads to the conversion of a stimulus, such as a poke, into electrical impulses. Many in the field hypothesize that touch receptors in mammals are complexes of ion channel proteins, but the proteins themselves are elusive.
"People were never able to clone these channels because they are dispersed within the skin," says Driscoll. "One receptor can feel things across a relatively broad area." Touch receptors are found in places that are generally inaccessible to researchers. The BNC1 gene, for example, is expressed below the skin in the endings of nerve fibers that surround mouse hair follicles.
"Molecular research on touch has lagged behind other senses because the ion channel proteins are rare and buried," says Margaret P. Price, of the University of Iowa. "That's why it was so important to identify them in C. elegans."
In 1996, Price and two colleagues used the worm touch receptors to discover a previously unidentified human gene, Brain Sodium Channel 1, or BNC1. The sequence turned up in a database of partial gene sequences called expressed sequence tags, or ESTs. The gene had the look of a touch receptor, but of course its function was unknown. So Price and Michael J. Welsh, a Howard Hughes Medical Institute investigator at the University of Iowa, disrupted BNC1 in the mouse to see what happened.
Nothing happened. "For a year, the mice seemed perfectly normal," says Price. Throughout their lives, in fact, the animals show no signs of deficiencies in sense of touch. But it's hard to know what a mouse is feeling. There are no good behavioral tests in this area, and because touch receptors may consist of multiple sub-channels, the ideal test is one that picks up subtle changes in touch sensation.
Price found the ideal test in Germany. Gary Lewin and colleagues at the Max Delbrück Center for Molecular Medicine, in Berlin, had developed technology to quantify the sensitivity of individual nerve cells, or neurons, in the mouse. Like many investigators, Lewin was searching for mammalian touch receptors using the worm sequences.
"I contacted them nearly two years ago to see if they were making a BNC1 mouse," recalls Lewin. "We started bringing the animals over to Berlin about 18 months ago." A paper describing the research appeared in a recent issue of Nature.
Testing dozens of BNC1 mice and controls takes time. The test involves removing a nerve from the leg of the mouse, then recording the electrical activity of individual neurons in response to varying levels of stimuli. The loss of sensitivity in BNC1 mice was a matter of degree. "Neurons without the ion channel," Lewin explains, "could hardly distinguish between a small amount of stimulus and a large amount of stimulus." The researchers concluded that BNC1 is required for the normal functioning of certain touch receptors.
"We don't prove that BNC1 is a receptor," says Michael Welsh. "We speculate that it is involved in light touch." The exact role of BNC1 channel, he says, is likely to vary depending upon what type of cell it's in and the proteins to which it binds: "There are many types of touch sensationa caress, a pinch, a splinterand there are many types of receptors."
"The current thinking is that the touch complex is going to be huge, so you need to have a lot of proteins," says Margaret Price. Having one component of the channel complex is a start. Still to be identified are proteins outside the cell to which the ion channels are tethered. According to models of touch receptors, tethering creates the tension needed for mechanical actions of the channel in response to pressure or movement.
Another aspect of the story is the reduction but not the elimination of touch sensation in the BNC1 mice. All feeling was not lost. Price says that in a diverse family of sensory channel proteins there may be redundancy and the ability to compensate for a defect. Monica Driscoll agrees. "There is probably a big family of these, and you have constellations of sub-units in different receptors," she says. "So it may be that if you lose one you don't lose the entire function."
Sensory signaling is not the whole story for ion channel proteins. Various insults make ion channels stay open, which can cause a cell to swell up and die. The role of C. elegans ion channels in neurodegenerative disease is a focus of Driscoll's laboratory at Rutgers. The Iowa researchers believe that BNC1 will be turn out to be quite interesting. The gene is expressed throughout the nervous system and in the eye and the tongue, for instance. "We are testing a number of domains in the mouse," says Price. "The animals look like normal mice, but we think there will be major differences down the road."
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