Unusual Blob-Like Creature Offers Clues to the Origins of Neurons

An unassuming sea creature, resembling a blobby pancake, may harbor the enigmatic secret behind the origin of neurons. Placozoans, constituting one of the five major animal branches alongside bilaterians (encompassing everything from worms to humans), cnidarians (including corals and medusas), sponges, and ctenophores (comb jellies), stand out as the most rudimentary of them all. These millimeter-sized cell clusters lack organs or body structures, propelling themselves through water with the aid of cilia (tiny hair-like structures), nourishing themselves by engulfing particles, and reproducing through a simple budding process.

Placozoans diverged from their animal relatives roughly 800 million years ago, and their existence is limited to only a few known species. However, recent research has unveiled a surprising revelation – these unassuming organisms may possess clues to the eventual evolution of the nervous system. Astonishingly, placozoans contain cells that exhibit striking similarities to neurons, despite their significantly lower complexity.

Xavier Grau-Bové, a study author and postdoctoral researcher at the Centre for Genomic Regulation in Barcelona, describes the findings as fitting into the notion that neurons are a highly intricate cell type that evolved gradually. He suggests that placozoans may retain remnants of an ancestral neuron with a different function, dating back to when they diverged from other animal lineages.

To delve into this mystery, Grau-Bové and his colleagues undertook a comprehensive genetic investigation of all cell types within placozoans. As he notes, the cell biology of these organisms has remained largely unexplored until now, with researchers essentially starting from scratch. The team identified nine primary cell types and numerous intermediate ones, with one subset known as peptidergic cells holding particular fascination. These cells play a role in placozoans’ movements by releasing short amino acid chains called peptides. The stimulation of these cells with various peptides triggers changes in placozoans’ shape and motion, causing them to flatten, undulate, or crinkle up.

What makes peptidergic cells especially intriguing is their striking resemblance to neurons found in the nervous systems of more complex animals like humans. They possess the proteins responsible for constructing the “pre-synaptic scaffold,” a structure vital for neuronal communication. While peptidergic cells lack synapses, the junctions where neurons release chemicals to transmit signals, they do feature similar protein complexes to those found in neurons, enabling the accumulation and release of chemicals.

The precise function of this scaffold in placozoans remains a mystery. Grau-Bové admits, “We do not yet know exactly what this scaffold is doing in these organisms; we just know that it is being expressed there.” Nonetheless, the team’s research reveals that peptidergic cells develop in a manner analogous to neurons, and they engage in cell-to-cell communication using neuropeptides – amino acid chains employed by neurons in their own messaging systems.

The origin of neurons remains a contentious subject among biologists. Some animals, like sponges, lack neurons altogether, while comb jellies possess neurons with distinct characteristics compared to other animals. In contrast, cnidarians and bilaterians exhibit more similarities in their nervous systems. It remains unclear whether the common ancestor of these diverse animal groups possessed a nervous system that was subsequently lost in certain lineages, such as sponges, or if nervous systems evolved independently in multiple lineages after their divergence.

Grau-Bové suggests that further research on ctenophores and their peculiar nervous systems will be essential to unravel this question. Nevertheless, the new findings propose a gradual evolution of neurons from a simpler cell specialized in communication and messaging. These groundbreaking results were published on September 19th in the journal Cell.

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