Italian scientists have discovered that the Golgi ribbon, a cellular structure previously thought to be unique to vertebrates, also exists in other animal groups, including mollusks, earthworms, and sea urchins. The related study was published on March 1st in Cell Reports.
For a long time, the function of the Golgi ribbon has been a mystery, but its presence across different animal lineages suggests that its function is not as vertebrate-specific as previously believed. The research team also found that Golgi ribbons form at specific times during embryonic development, indicating that they may play a role in cell differentiation.
"The Golgi ribbon is a very ancient structure, much more widespread than previously imagined," said Francesco Ferraro, the lead author of the paper and a cell biologist at the Anton Dohrn Zoological Station in Naples. "It's still unclear why animals need this structure, but it has been around for over 600 million years and remains so prevalent, indicating it has important functions."
Breaking the Consensus
Golgi complexes are widespread among eukaryotes, but their overall organization varies. These organelles consist of stacks of membrane sacs that process and package proteins for transport to other parts of the cell or for export. Single-celled eukaryotes often have only one Golgi stack, while plants, fungi, and some animals have multiple Golgi stacks dispersed throughout the cytoplasm, while other animals have multiple Golgi stacks that are connected to form a ribbon-like structure.
Currently, the consensus among cell biologists is that the Golgi ribbon is unique to vertebrates. However, when Ferraro's team observed ribbon-like Golgi bodies in sea urchin embryos, they began to investigate how widespread Golgi ribbons are in non-vertebrate animals. To do this, researchers examined Golgi structures representing various animal taxa and closely related representatives of single-celled eukaryotes.
They found that Golgi ribbons are present in both vertebrates and invertebrates, including mollusks, annelids (such as earthworms), and echinoderms (such as starfish and sea urchins). However, Golgi ribbons are absent in arthropods or nematodes or more primitive animal taxa such as sponges and cnidarians.
Based on its scattered but widespread distribution, researchers believe that the Golgi ribbon evolved only once: about 600 million years ago, in the ancestors of all bilaterians and cnidarians, including all animals except sponges, cnidarians, and placozoans, but subsequently disappeared in some bilaterian lineages, including nematodes and arthropods.
Ferraro said, "The appearance of the Golgi ribbon predates the evolution and diversification of vertebrate lineages, which means it did not evolve to have a specific role in vertebrate cell physiology."
Function Puzzle Awaits Solution
In mammalian cells, when multiple Golgi stacks are linked together by molecular tethers, a Golgi ribbon is formed. Molecular tethers include the GRASP protein, which connects adjacent Golgi stacks, and Golgi matrix proteins that serve as anchor points for GRASP proteins.
Researchers learned from previous studies that ribbon formation requires GRASP protein binding to either of two Golgi matrix proteins. Using the AlphaFold2 model, they inferred that these anchor points may have evolved at different times: analysis predicted that one anchor point evolved in the common ancestor of all multicellular animals, while the other appears to have evolved in the common ancestor of cnidarians (jellyfish, corals, and hydras) and bilaterians.
However, the function of the Golgi ribbon remains a mystery, but researchers suspect it is involved in cell differentiation during embryonic development. By analyzing Golgi dynamics in developing sea urchins, sea squirts, and zebrafish, they found that Golgi stacks remain separate during the initial rounds of cell division but come together to form a centralized Golgi ribbon at specific moments during embryogenesis.
"The formation of these Golgi ribbons occurs before gastrulation, a stage of embryonic development where the fate of cells to become different tissues and body structures is determined," Ferraro said. "Based on this, we propose that the Golgi ribbon may play a role in embryonic development and differentiation, which was not previously considered."
Ferraro said researchers are now "testing this idea that the Golgi ribbon is actually related to cell differentiation." Understanding the function of the Golgi ribbon could have significant medical implications, as Golgi ribbon breakdown is known to contribute to neurodegenerative diseases, cancer, and some viral infections. "Golgi ribbon breakdown is implicated in many human diseases, so if we understand what its function is, we can better understand the causes of these diseases," Ferraro said.
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