Neurons, the specialized cells of the nervous system, are probably the most complex cell type ever to evolve. In humans, these cells are capable of processing and transmitting vast amounts of information. But how such complex cells first appeared remains a long-standing debate.
Now scientists in Japan have revealed the type of messenger – molecules that carry signals from one cell to another – that likely functioned in the earliest nervous system.
The study, published on August 8 in Natural Ecology and Evolution, also reveals key similarities between the nervous systems of two early diverging animal lineages—the lineage of jellyfish and anemones (also called cnidarians) and that of scallops (ctenophores), reviving an earlier hypothesis that neurons evolved only once.
Despite their supposed simplicity, very little is known about the nervous systems of ancient animals. Of the four animal lineages that diverged before the emergence of more complex animals, only comb jellies (the first ancient lineage to diverge) and cnidarians (the last ancient lineage to diverge) are known to possess neurons. But the uniqueness of the comb’s nervous system compared to that seen in cnidarians and more complex animals, and the lack of neurons in the two diverging lineages, has led some scientists to suggest that neurons evolved twice.
But Professor Watanabe, who heads the Department of Evolutionary Neurobiology at the Okinawa Institute of Science and Technology (OIST), remains unconvinced.
“Comb jellies actually don’t have many of the neuronal proteins that we see in more evolved animal lineages,” he said. “But to me, the absence of these proteins is not sufficient evidence for the origin of two independent neurons.”
In his research, Prof. Watanabe focused on an ancient and diverse group of neural messengers. Called neuropeptides, these short peptide chains are first synthesized in neurons as a long peptide chain before being cleaved by digestive enzymes into very short peptides. They are the main form of messenger found in cnidarians and also play a role in neural communication in humans and other complex animals.
However, previous studies trying to find similar neuropeptides in jelly have been unsuccessful. The main problem, Prof. Watanabe explained, is that the mature short peptides are only encoded by short DNA sequences and mutate frequently in these ancient lineages, making DNA comparisons too difficult. Although artificial intelligence has identified potential peptides, they have not yet been experimentally validated.
So Prof. Watanabe’s research team approached the problem from a new direction. They extracted peptides from fungi, cnidaria and scallops and used mass spectrometry to search for short peptides. The team was able to find 28 short peptides in the cnidarians and scallops and determine their amino acid sequences.
Now knowing their structures, the researchers visualized the short peptides under a fluorescence microscope, allowing them to see in which cells they were produced in both cnidarians and scallops.
They found that in comb jellies, one type of neuropeptide-expressing cell resembled classical neurons, with thin protrusions called neurites extending from the cell.
But short peptides are also produced in a second type of cell that lacks neurites. The researchers suspect that these may be an early version of neuroendocrine cells—cells that receive signals from neurons and then release signals, such as hormones, to other organs in the body.
The researchers also compared what genes were expressed in cnidarian and comb neurons. They found that, in addition to having some short neuropeptides in common, both neurons express a similar set of other proteins that are essential for neuronal function.
“We already know that the peptide-expressing neurons of cnidarians are homologous to those seen in more complex animals. It has now been found that comb jelly neurons also have a similar ‘genetic signature’, suggesting that these neurons share the same evolutionary origin,” Prof. Watanabe said. “In other words, it’s most likely that neurons only evolved once.”
This means, Prof. Watanabe added, that peptide-expressing neurons are probably the most primitive form, with chemical neurotransmitters arising later. For Prof. Watanabe, these findings bring new, exciting questions to the fore in his research.
“If this is true, I’m most interested in knowing – where did the peptide-expressing neurons come from? And why did the ancestral animal have to evolve neurons? Now that we have a clearer picture of what the earliest neurons looked like, the study of their original function can begin.”
Scientists identify our most distant animal relatives
Eisuke Hayakawa, Mass spectrometry of short peptides reveals common features of metazoan peptidergic neurons, Natural ecology and evolution (2022). DOI: 10.1038/s41559-022-01835-7. www.nature.com/articles/s41559-022-01835-7
Courtesy of Okinawa Institute of Science and Technology
Quote: Inside the Comb Jellies’ Brain: Scientists Explore Neuron Evolution (2022, August 8) Retrieved August 8, 2022, from https://phys.org/news/2022-08-brain-jellies-scientists-explore -evolution.html
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