Italian scientists have discovered that the Golgi ribbon, a cellular organelle structure previously thought to be exclusive to vertebrates, is also present in other animal phyla, including mollusks, earthworms, and sea urchins. The relevant research was published on March 1 in the journal "Cell Reports."
The function of the Golgi ribbon has long been a mystery, but its presence across different animal lineages suggests that its function is not exclusive to vertebrates as previously believed. The research team also found that the Golgi ribbon forms at specific points during embryonic development, indicating a potential role in cell differentiation.
"The Golgi ribbon is a very ancient structure, and its presence is 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 is still unclear why animals need this structure, but it has been around for over 600 million years, and it remains so prevalent, indicating its significant function."
Challenging the Consensus
Golgi complexes are widespread in eukaryotic organisms, but their overall organization varies. These organelles consist of stacks of membrane sacs that process and package proteins for transport within the cell or for expulsion. Single-celled eukaryotes often have a single Golgi stack, while plants, fungi, and some animals have multiple Golgi stacks scattered throughout the cytoplasm, forming 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 a ribbon-like Golgi structure in sea urchin embryos, they began investigating how common the Golgi ribbon is in non-vertebrate animals. To do this, researchers examined various animal phyla and closely related representatives of single-celled eukaryotes for Golgi structures.
They found that the Golgi ribbon is present in both vertebrates and invertebrates, including mollusks, annelids (such as earthworms), and echinoderms (such as starfish and sea urchins). However, the Golgi ribbon is absent in arthropods or nematodes and more primitive animal phyla such as sponges and cnidarians.
Based on its scattered but widespread distribution, researchers believe that the Golgi ribbon evolved only once, approximately 600 million years ago, in the common ancestor of all bilaterians (animals with bilateral symmetry), excluding sponges, cnidarians, and placozoans. Subsequently, it disappeared in some bilaterian lineages, including nematodes and arthropods.
Ferraro stated, "The appearance of the Golgi ribbon predates the evolution and diversification of vertebrate lineages, suggesting that it did not evolve for a specific physiological role in vertebrates."
Unraveling the Mystery of Function
In mammalian cells, the Golgi ribbon forms when multiple Golgi bodies connect through molecular chains. The molecular chains include the GRASP protein, which links adjacent Golgi bodies, and Golgi body proteins serving as anchor points for the GRASP protein.
Previous studies indicated that the ribbon's formation requires the GRASP protein binding to either of the two Golgi body proteins. Using the AlphaFold2 model, researchers inferred that these anchor points likely evolved at different times: 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 (such as jellyfish, corals, and hydroids) and bilaterians.
However, the function of the Golgi ribbon remains a mystery, but researchers suspect its involvement in cell differentiation during embryonic development. By analyzing the Golgi dynamics in developing sea urchins, ascidians, and zebrafish, researchers observed that the Golgi bodies remained separate during the initial cell divisions but connected at specific moments during embryonic development to form a concentrated Golgi ribbon.
"The formation of these Golgi ribbons occurs before the formation of the primitive gut, a developmental stage where the embryo determines the development of different tissues and body structures," Ferraro explained. "Based on this, we propose that the Golgi ribbon may play a role in embryonic development and differentiation, which was not previously considered."
Ferraro mentioned that researchers are currently "testing this idea that the Golgi ribbon is actually involved in 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.
For more information, refer to the original paper.
