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Nerve cells are the main component of nervous tissue in all animals except sponges and platydiscs. Scientists believe that the ability to generate electrical signals first appeared 700 million years ago, and in the latest study, scientists found that peptidergic cells that originated in animals 800 million years ago may be the precursors of nerve cells by conducting a "cell census" of platydiscs.

Wen| Wen Jing

A nerve cell is a specialized cell that has some structures common to all other eukaryotic cells, such as the cell body, nucleus, smooth rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. In addition, nerve cells have some unique structures, such as dendrites and single axons. Neurons communicate with other cells via synapses, usually using trace amounts of chemical neurotransmitters to transmit electrical signals from presynaptic neurons to target cells through synaptic gaps.

The diversity and origin of nerve cells has been one of the central questions in developmental neurobiology. A recent paper published in Cell suggests that a variety of peptidergic cells in platydiscs may be precursors to nerve cells. These peptidergic cells acquired new genetic modules capable of producing rapid electrical signals, and the ability to produce electrical signals was a key step in the evolution of the nervous system.

Most animals are bilaterally symmetrical, except sponges, Loropetalum, Platydisca, and Cnidaria. Among them, flat disc animals are multicellular animals with no tissues and organs, and no nervous system. They are similar in shape, size and movement mode to amoebas. Therefore, they are also called multicellular amoebas. Platydisca is considered the simplest animal on earth.

In the new study, the scientists analyzed four species of platydisca, using cutting-edge high-throughput chromatin analysis and single-cell genomics techniques combined with advanced computational methods to map the cells of platydisca. In the new map, the scientists mapped all cell types of the four platychid species based on gene expression patterns, what each type did, and how the cells worked together.

Previous studies have shown that the thousands of cells that make up Platydiscus can be divided into nine main types according to morphology, but the new map shows that in addition to the nine main cell types, there are many "intermediate types" of cells that maintain the balance between different cell types in growth, division and differentiation.

It is worth noting that there are special cells that are neither of the nine main types nor of intermediate types. These cells secrete small fragments of peptides, which the researchers call peptidergic cells. In the new study, the scientists identified 14 different peptidergic cells.

Genetic analysis and microscopic screening have shown that these peptidergic cells behave like nerve cells and can coordinate flat-disc animal behavior by releasing peptide signals. The communication between flat-disc animal cells uses a special set of proteins: GPCRs (G protein-coupled receptors), which detect signaling molecules outside the cell and initiate a series of reactions inside the cell. Neuropeptides expressed by peptidergic cells can act as external signals to activate GPCR.

In addition, scientists have found that peptidergic cells and nerve cells share similarities in other ways. One is that the differentiation process of these cells, including the developmental signals received, is very similar to the formation process of newborn neurons.

Second, peptidergic cells express sets of genes, or gene modules, necessary for the pre-synaptic structure of neurons. Synapse is the information transmission node of neuron network, pre-synapse constitutes the information output terminal, post-synapse is the information input terminal. Plagidisca cells lack postsynaptic components, but peptidergic cells have begun to take shape presynaptically.

Does the nervous system develop in diversity? Previous studies have shown that modern nerve cells originated 650 million years ago in ctenoid jellyfish (a bilaterally symmetrical animal like platydisca). "Ctenophore jellyfish have neural networks that have key differences and similarities to our own neural networks. "said Xavier Grau-Bové, one of the authors of the article.

The similarities between nerve cells and peptidergic cells also surprised scientists. "Plagidiscal peptidergic cells have many similarities to primitive nerve cells, although they are not identical. We seem to be seeing an intermediate process of evolution over long distances. "Sebastián R., one of the authors of the article. Najle said.

Scientists speculate that the "ancestors" of nerve cells may have been very similar to the peptidergic cells that secrete peptides in today's platychetes. They used neuropeptides to communicate, and later had new gene modules to create post-synaptic structures, forming axons and dendrites, and equipped with ion channels that can produce electrical signals quickly. Finally, the current nerve cells were born-laying the foundation for the brain that humans are proud of.

"Did nerve cells undergo one evolutionary process and then differentiate, or did they undergo more than one evolutionary process in parallel? Or is it that the nervous systems of different animals have different origins? These are outstanding issues that remain unresolved. Xavier Grau-Bové said.

References:

https://doi.org/10.1016/j.cell.2023.08.027

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