Q MARINE BIOLOGY SOUND SENSE F 26


Scientists have fi nally found out how fi sh identify the direction of water fl ow – a discovery that could help cure deafness in humans


ish sense water motion the same way humans sense sound, according to new research carried out at the Case Western Reserve University School of Medicine in Cleveland, Ohio, USA. The researchers say that they


have discovered a gene, also found in humans, which helps zebrafi sh convert water motion into electrical impulses which are sent to the brain for perception. While this shared gene allows a zebrafi sh to sense water fl ow direction across its body, it also helps cells inside the human ear to sense a broad range of sounds. The newly discovered gene encodes a protein


in hair cells – the type of cells that receive sound inside the human ear. Mutations in the gene can result in deafness in humans. In addition to hair cells inside their ears,


zebrafi sh also have hair cells along the entire length of their bodies to help them sense water fl ow. The hair cells on the surface of the zebrafi sh’s bodies face in varying directions to sense the direction of water moving around them. The new study has, however, now found that


the hair cells facing in different directions are not actually identical, as previously thought. Instead, each group of hairs makes use of the deafness gene in a slightly different way.


“We found that detection of water fl ow from


the front of the fi sh is more dependent on the zebrafi sh gene tmc2b than water fl ow from the back of the fi sh,” explains Dr Brian McDermott, associate professor of otolaryngology at Case Western, who led the research. “Water fl owing from the front of the fi sh is


routine of course, because it accompanies forward swimming. But water coming from the rear could mean a predator is in pursuit. Zebrafi sh therefore use different molecular mechanisms to distinguish the actual water fl ow direction.” The study, recently published in Nature Communications, delves into mechanotransduction – the way in which hair cells sense mechanical sound waves, or in this case, water waves, and then convert them into brain signals. McDermott’s team discovered that the hair cells on the zebrafi sh’s skin use different mechanotransduction genes – such as tmc2b – depending on their orientation. The study identifi es tmc2b as being central


to mechanotransduction. The tmc2b gene is required for hair cells to transmit signals to the brain, which could eventually help to explain its role in genetic deafness. “Our fi ndings are directly connected to human


hearing,” explains McDermott. “We studied a zebrafi sh gene that is analogous to a human gene


that causes deafness, and here we are able to demonstrate that the defect originates in the process of mechanotransduction.” The researchers used their fi ndings to create


a ‘mechanosensory map’ of zebrafi sh hair cells. The map shows how a hair cell’s location and


orientation relates to its ability to sense water motion. It also shows how different hair cells require, or can function independently of, the deafness gene tmc2b. It’s hoped that the map could inform future studies related to human hair cell mechanotransduction, and identify the causes of genetic deafness. “Zebrafi sh hair cells are particularly


accessible for experimentation, unlike hair cells of the ear of mammals, so they offer a special advantage,” says McDermott. “You can therefore study the development and functioning of intact hair cells to a higher level in fi sh than those of the mammalian ear. “Differences in hair cells help fi sh sense


water fl ow patterns – and may also help humans sense different sounds. “In mammals, hair cells are the sensory cells


of the ear. Our fi ndings suggest that in mammals, including humans, there may be molecular differences between hair cells that allow us to hear the wonderful range of sounds that we enjoy.”


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