Fish electroreceptors development related to inner ear hair cells, study finds

Clare Baker's group reveals how the electroreceptor cells in fish and the inner ear hair cells in other vertebrates are closely related in new paper published on eLife

Hearing and balance depend on sensory ‘hair cells’ located inside the fluid-filled chambers of the inner ear. Sound waves or head movements displace the fluid, bending the tiny ‘hair bundles’ at the surface of the hair cells. The bending triggers the hair cells to release ‘neurotransmitter’ chemicals that stimulate electrical impulses in the nerve cells connecting them to the brain. In fishes and amphibians, mechanosensory hair cells are also found in tiny sense organs arranged in lines over the head and body, which detect local water movement. This “touch at a distance” sense is useful for detecting obstacles, predators, and prey. Some fishes and amphibians, such as sharks, sturgeons, and salamanders, also have ‘electrosensory organs’ containing ‘electroreceptor cells’, which are triggered by the weak electric fields that surround animals in water. Electroreception is primarily used for hunting: this is how a shark can detect a flounder buried in the sand.

 "We already knew that both electrosensory and mechanosensory organs develop from the same patches of thickened embryonic skin on the head" says Clare Baker, corresponding author of the new study. "We wanted to know what genes are used in the embryo to make electroreceptor cells that respond to electric fields, rather than hair cells that respond to water movement".

To identify genes that are active in developing electrosensory organs, the teams studied embryos of the Mississippi paddlefish (related to sturgeons and bred for caviar in the USA), which has more electrosensory organs than any other species. Their analysis showed that electroreceptors are very closely related to hair cells: many genes essential for human hearing are also active in paddlefish electroreceptors. The tean identified some differences, including a gene whose activity might work like a railroad switch, sending a cell down the electroreceptor track instead of the hair cell track, and genes likely to be important for responding to electric fields.

"Our next step is to test whether our candidate ‘railroad switch’ gene really does send cells down the electroreceptor track, and, if so, precisely how it does this" says Baker. "Studying which genes make hair cells and electroreceptors different may give us novel insights into how to make or regenerate mature, functioning hair cells in the inner ear and potential help in understanding deafness in the long run".

The work was funded by the BBSRC, the Leverhulme Trust, the Fisheries Society of the British Isles (FSBI), and the NSF.

Reference: Insights into electrosensory organ development, physiology and evolutionfrom a lateral line-enriched transcriptome, eLife 2017;10.7554/eLife.24197