The origin of hearing in humans is linked to the sense of touch of sea anemones

⇧ [VIDÉO] You might also like this partner content (after ad)

Behind its apparent simplicity — a cylinder surmounted by tentacles — the sea anemone conceals great complexity. It is notably closer to us than we might think, its genome being very similar to ours. These similarities make the sea anemone an ideal model for studying the animal genome and understanding the interactions that occur between genes. Recently, an international team of researchers discovered a touch-related developmental gene in the tentacles of sea anemones. This gene is already known to be involved in the development of hearing in humans. This discovery reveals a genetic link between the two species, testifying to a common ancestor and the evolutionary history of hearing in humans.

In 2007, surprisingly, a team of American researchers discovered that the genome of the anemone, which belongs to the same category as corals and jellyfish, the first divergent branch of metazoans, more closely resembles that of humans and others. vertebrates than those of classic laboratory models, such as Drosophila and the nematode worm. The latter would have lost a number of genes from common ancestors, which the anemone and the vertebrates would have kept.

The anemone genome, which is more similar, provides a good reference for comparison with the human genome, with the aim of discovering the genes of our common ancestor and their organization on the chromosomes. Heather Marlow, specialist in developmental biology in the Genomics and Epigenomics of Vertebrate Development Unit at the Institut Pasteur, explains: “ When the sea anemone genome was sequenced in 2007, it was found to be very similar to that of humans, both in the number of genes, with around 20,000 genes, and in the organization . These similarities make the sea anemone an ideal model for studying the animal genome and understanding the interactions between genes. “.

Furthermore, the sea anemone occupies a strategic position in the tree of life. The evolutionary branch of the cnidarians – to which the anemones belong – separated from that of the bilaterians, in other words from most other animals, including humans, more than 600 million years ago. Heather Marlow summarizes: “ The anemone can therefore also help us to understand the origin and evolution of the multiple cell types that make up the bodies and organs of animals, and in particular their nervous system. “. In 2018, the same team highlighted a highly complex nervous and sensory system, with nearly thirty different types of neurons – peptidergic, glutamatergic or even insulinergic.

In this context, an international team led by biologist Ethan Ozment from the University of Arkansas recently published an article in the journal eLifereporting the discovery of a developmental gene linked to touch in the tentacles of sea anemones, known to also be linked to hearing in humans.

Sensory cells with a common origin

One of the most fundamental sensory cell types that emerged during animal evolution is the mechanosensory cell. It is a specialized sensory epithelial cell that transforms mechanical stimuli — for example, water vibrations, pressure on the skin, stretching, etc. — in internal signals. These signals are then communicated, usually via the nervous system, to effector cells—for example, muscle cells—to elicit behavioral and/or physiological responses from the body. We are talking about a mechanoreceptor.

Despite this place in animal phylogeny, the earliest evolutionary histories of mechanoreceptor development remain enigmatic. We know that the classic mechano-sensory cell type with a dedicated sensory-neuronal function, i.e. producing a nerve impulse when a deformation of adjacent tissue occurs, is the hair cell. Moreover, in humans and other vertebrates, the sensory receptors of the auditory system are equipped with them. These cells have bundles of finger-like organelles, called stereocilia, which detect mechanical stimuli, that is, the vibrations we hear as sound.

As mentioned above, sea anemones are a more relevant model of study in researching the history of human evolution, because features shared by bilateral animals and cnidarians were likely present in our last common ancestor. Indeed, these sea anemones belong to the group of Cnidaria, sister group of Bilateria including vertebrates. These two groups diverged from their last common ancestor who lived around 748 to 604 million years ago. In addition, sea anemones also possess hair cells, with morphological and functional characteristics parallel to those of the mechanosensory cells of other animal lineages. Unfortunately, no study has looked at the genes essential for the development of these cnidarian hair cells, which could tell us about our evolutionary history.

In order to clarify these questions, the researchers of the present study based themselves on previous work, which revealed the existence of a particular gene, the POU-IV gene. The latter is shared by all existing groups of animals, with the exception of Ctenophora, indicating an early emergence in animal evolution. Its involvement in the development of ciliated cells, in mammals, is attested by experiments in mice. The latter, if they are deprived of the POU-IV gene, are deaf. Nevertheless, its role in the sensory development of the sea anemone, and its evolution through animal phylogeny, remained unknown.

A gene involved in hearing and touch

In order to understand what the POU-IV gene was doing in the starry sea anemone (Nematostella vectensis), the team turned it off using the CRISPR-Cas9 gene-editing tool. To do this, the researchers injected a mixture containing the Cas9 protein into fertilized sea anemone eggs to knock out the gene, and studied the developing embryos, as well as cultured mutated anemones.

They then discovered that deleting the gene led to abnormal development of sprawling hair cells. Indeed, mutant anemones exhibited aberrant sprawling hair cell growth and no sensitivity to touch compared to wild-type control anemones. In other words, without the POU-IV gene, they couldn’t sense physical stimuli through their hair cells.

Behavior of wild-type (F2 POU-IV +/+, A, B) and mutant (F2 POU-IV -/-, C, D) anemones in response to tactile stimuli from their oral tentacles. Animals before (A, C) and after (B, D) tentacle touch are shown. Tactile stimuli to the tentacles cause tentacle retraction in the wild-type individual, at the arrowheads. © E. Ozment et al., 2022

Additionally, the mutant anemones strongly repressed a gene extremely similar to the polycystin 1 gene, which is required for normal fluid flow sensing by vertebrate kidney cells. A sensation of fluid flow could be a beneficial characteristic for aquatic organisms, even though sea anemones do not have kidneys.

Taken together, the results show that POU-IV may have played a role in the evolution of mechanoreceptor cells in the single ancestor of cnidarians and bilaterians. The researchers say in a statement: This study is exciting because it has not only opened up a new field of research into how mechanosensation develops and functions in a sea anemone, but it also informs us that the building blocks of our sense of hearing have d evolutionary roots dating back hundreds of millions of years to the Precambrian. The earliness of the role of POU-IV in mechanoreceptor differentiation in animal evolution remains unresolved and requires comparative data on placozoans and sponges. “.

The results of this work open up a whole new field of research on the development and functions of mechanoreception in cnidarians. Furthermore, the discovery indicates that the evolution of our hearing has a very ancient history. It will be necessary to use information from other phyla, with older divergence sites, in order to trace the history of the gene even further.

Ultimately, the researchers plan to study the mechanism by which POU-IV activates distinct sets of genes across cnidocytes and hair cells to shed light on how POU-IV may have contributed to the evolution of the new type. mechanosensory cell Cnidaria.

Source: eLife

Leave a Comment