A study published in the journal Cell sheds new light on the evolution of neurons, focusing on placozoans, a millimeter-sized sea creature.
Researcher of the Center for Genomic Regulation in Barcelona Find evidence that specialized secretory cells found in these ancient and unique creatures may have given rise to neurons in more complex animals.
Placozoa are tiny animals, about the size of a large grain of sand, They feed on algae and microbes that live on the surface of rocks and other substrates in warm, shallow seas. These blob and pancake shaped creatures are so simple that they live without any body parts or organs.
These animals, which are believed to have first appeared on Earth about 800 million years ago, They belong to the five major animal lineages, along with Ctenophora (jellyfish), Porifera (sponges), Cnidaria (corals, sea anemones and jellyfish) and Bilateria (all other animals).
Sea creatures coordinate their behavior using peptidic cells, a special type of cell that releases small peptides that can control the animal’s movement or feeding. Driven by curiosity about the origins of these cells, the study authors used a range of molecular techniques and computational models to understand how placozoan cell types evolved. and reconstruct how our ancient ancestors might have been and functioned.
The researchers first mapped all the different types of placozoan cells and noted their properties in four different species. Each cell type has a specific function that comes from specific sets of genes. The maps, or “cell atlases,” allowed researchers to map groups or “modules” of these genes. They then created a map of the DNA regulatory regions that control these genetic modules, revealing a clear picture of what each cell does and how they work together. Finally, they performed comparisons between species to reconstruct the evolution of cell types.
The research showed that the nine major cell types of placozoans appear to be connected by many “intermediate” cell types that switch from one type to another. The cells grow and divide, maintaining the delicate balance of cell types that the animal needs to move and eat. The researchers also found fourteen different types of peptidic cells, but they were different from all other cells. and they showed no intermediate types or signs of growth or division.
Surprisingly, peptidic cells had many similarities to neurons, a cell type that only appeared many millions of years later in more advanced animals such as bilateria (animals with bilateral symmetry). Cross-species analyzes revealed that these similarities occur only in placozoans and are not present in other animals that branch early. such as sponges or comb jellies (ctenophores).
The similarities between peptide cells and neurons were threefold. First, the researchers discovered that these placozoan cells differ from a population of epithelial progenitor cells by developmental signals similar to neurogenesis, the process by which new neurons are formed in cnidarians and bilateria.
Second, they discovered that peptidic cells have many genetic modules necessary to build the part of a neuron that can send a message (the presynaptic framework). However, these cells are far from being a true neuron. because they lack the components for receiving a neuronal message (postsynaptic) or the components required to transmit electrical signals.
Finally, the authors used deep learning techniques to show that placozoan cell types communicate with each other through a system within the cells in which certain proteins called GPCRs (G protein-coupled receptors) detect external signals and produce a variety of Trigger cell reactions. These external signals are mediated by neuropeptides, chemical messengers used by neurons in many different physiological processes.
