Scientists explore the evolution of neurons

Neurons, the specialized cells of the nervous system, are probably the most complex type of cell to have ever evolved. In humans, these cells are able to process and transmit vast amounts of information. But how these complicated cells first appeared remains a long-standing debate.

Today Japanese scientists have revealed the style of messengers — molecules that carry signals from one cell to another — that likely functioned in the most ancestral nervous system.

The study, published Aug. 8 in Character Ecology and Evolution, also revealed key similarities between the nervous systems of two early divergent animal lineages – the lineage of jellyfish and anemones (also called cnidarians) and that of comb jellies (ctenophores), reviving an earlier hypothesis that the neurons have evolved only once.

Despite their supposed simplicity, very little is known about the nervous system of ancient animals. Of the four animal lineages that branched before the appearance of more complex animals, only the comb jellies (the first ancient lineage to diverge) and the cnidarians (the last ancient lineage to diverge) are known to possess neurons . But the uniqueness of the nervous system of comb jellies compared to that seen in cnidarians and more complex animals, and the absence of neurons in the two lineages that diverged between the two, have led some scientists to speculate. hypothesis that neurons have evolved twice.

But Professor Watanabe, who heads the Evolutionary Neurobiology Unit at the Okinawa Institute of Science and Technology (OIST) , is unconvinced.

“Indeed, comb jellies lack many of the neural proteins that we see in more evolved animal lines,” he said. he declares. “But for me, a lack of these proteins is not sufficient evidence for two independent neural origins.”

In his study, Professor Watanabe focused on a ancient and diverse group of neural messengers. Called neuropeptides, these short peptide chains are first synthesized in neurons as one long peptide chain, before being cleaved by digestive enzymes into many short peptides. They are the primary form of messenger found in cnidarians and also play a role in neural communication in humans and other complex animals.

However, previous research that attempted to find similar neuropeptides in comb jellies failed. The main problem, Professor Watanabe explained, is that short mature peptides are only encoded by short DNA sequences and mutate frequently in these ancient lineages, making DNA comparisons too difficult. While artificial intelligence has identified potential peptides, these have yet to be validated experimentally.

So Professor Watanabe’s research team approached the problem from a new angle. They extracted peptides from sponges, cnidarians and comb jellies and used mass spectrometry to search for short peptides. The team was able to find 28 short peptides in cnidaria and comb jellies and determine their amino acid sequences.

Now knowing their structures, the researchers visualized the short peptides under a fluorescent microscope, allowing them to see in which cells they were produced in both cnidarians and comb jellies.

In the comb jellies, they found that a cell fate expressing neuropeptides resembled classic neurons, with thin projections called neurites extending out of the cell.

But the short peptides were also produced in a second type of cell lacking neurites. Researchers suspect it could be an early version of neuroendocrine cells – cells that receive signals from neurons and then release signals, like hormones, to other organs in the body.

The researchers also compared the genes expressed in cnidarian and comb neurons. They found that in addition to having short neuropeptides in common, the two neurons also expressed a similar array of other proteins essential for neuronal function.

“We know already that the neurons expressing cnidarian peptides are homologous to those observed in more complex animals. Now the comb jelly neurons were also found to have a similar ‘genetic signature’, suggesting that these neurons share the same evolutionary origin,” Professor Watanabe said. “In other words, it is highly likely that neurons evolved only once.”

This means, Professor Watanabe added, that neurons expressing peptides are probably the most ancestral form, with chemical neurotransmitters appearing later. For Professor Watanabe, these findings bring exciting new questions to the forefront of his research. neurons expressing the peptides? And why did the ancestral animal need to evolve neurons? Now that we have a clearer idea of ​​what the first neurons looked like, the search for their original function can begin.”

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