Deaf diggers
Mutant proteins of hair cells made the diggers almost deaf
Anna Muravyeva, N+1
Biologists have explained why naked diggers are very hard of hearing. There is no sound amplification in their ears due to mutations in the proteins that bind the hairs of the auditory receptors. These mutations were supported by natural selection, which may indicate their benefits.
This is stated in a study published in the journal Current Biology (Pyott et al., Functional, Morphological, and Evolutionary Characterization of Hearing in Subterranean, Eusocial African Mole-Rats).
One of the most important elements of the vertebrate ear is the cochlea, the spiral bone canal. Inside it there is a sound-receiving apparatus, which consists of receptor hair and support cells. At the top of each hair cell there is a bundle of stereocilia (hairs), which are connected to each other by protein strands. When the sound vibration reaches the stereocilia, they deviate from their original position together, the strands between them stretch, and channels open on the membrane of the hairs. Through them, positively charged calcium ions enter the cell, it depolarizes and transmits a signal to the neurons of the auditory nerve.
The inner and outer hair cells of the snail. Drawings from an article in Current Biology.
Hair cells are divided into two groups according to their location in the cochlea and functions – internal and external. The outer ones are connected in bundles (largely due to the connection of stereocilia with neighbors) and act as a cochlear amplifier – they are able to selectively amplify quiet sounds and thereby increase the sensitivity of mammalian hearing. When the sound wave reaches the outer hair cells, they begin to oscillate synchronously, lengthening, and this amplifies the sound going deep into the cochlea.
Despite the wide repertoire of sounds made by naked diggers Heterocephalus glaber and their cubs, they hear very weakly. The same is true for other closely related species. To find out why this is happening, researchers from The University Medical Center of Groningen, led by Sonja Pyott, conducted several series of experiments on digger rodents – the naked digger and the Damarian sandpiper Fukomys damarensis. Since hearing disorders can occur at different levels of the sensory system, scientists have checked the condition of the bundles of external hair cells of the diggers, and the stereocilia of these cells, and the conductive pathways of the auditory system.
Biologists measured the strength of the response of brain stem neurons to sounds of different frequencies in rodents, as well as otoacoustic emission – vibrations that occur when the effect of a cochlear amplifier is manifested. To investigate the structure of the outer hair cells, biologists used scanning electron microscopy.
The authors also analyzed the structure of the prestin protein gene responsible for the elongation of these receptors. Mutations in this gene could be the cause of the diggers' hearing loss. It is also known that mutations in some proteins that bind human hair cells into bundles are associated with deafness. The researchers assumed that such mutations also exist in diggers, and checked the amino acid composition of the corresponding proteins Heterocephalus glaber and Fukomys damarensis and compared it with that of other mammals.
Photos of bundles of hair cells of naked and Damar navvies in different parts of the snail.
It turned out that the naked and Damar navvies have a much higher threshold of auditory sensitivity than mice. The bundles of external hair cells in the diggers were absent in some parts of the snail, where they are in other species, and many of the existing ones were organized differently than in mice. The angles between such bundles in naked diggers ranged from -30.5 to 34.5 degrees, and in Damarian ones – from -39 to 37 degrees. In house mice that hear well, similar angles range from -6.5 to 3.5 degrees.
At the same time, the outer hair cells of the diggers, apparently, retained the ability to lengthen: scientists have not identified significant mutations in the prestin gene. But they found 78 such mutations in 12 proteins binding hair cells (for example, ADGRV1, CDH23, CEACAM16). Most of them were unique to one of the two species, but some were present in both diggers. This may indicate the directed evolution of these molecules.
The researchers compared all the nucleotide substitutions in the genes of these proteins and found that for five of them the proportion of non-synonymous substitutions is higher than the proportion of synonymous ones. This means that mutations that change the structure of ADGRV1 and others considered in the work are more common in the genomes of diggers than those that leave proteins intact. Comparison of the proportions of substitutions is used to study the evolution of proteins: if there are significantly more non-synonymous mutations, it can be argued that they were subjected to positive selection and allowed their carriers to survive and reproduce more successfully. Based on these data, scientists suggest that deafness could be useful to diggers.
Recently it was found out that in naked diggers, the enzyme hexokinase binds to the membranes of mitochondria in much larger quantities than in mice, in which, in adulthood and old age, most of the molecules of this substance are in a free state in the cytoplasm. In conjunction with mitochondria, hexokinase reduces the amount of free radicals formed during respiration and protects the cell from oxidative stress – and thus, probably, from aging.
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