Animals, from worms and sponges to jellyfish and whales, contain anywhere from a few thousand to tens of trillions of nearly genetically identical cells. Depending on the organism, these cells arrange themselves into a variety of tissues and organs, such as a gut, muscles, and sensory systems. While not all animals have each of these tissues, they do all have one tissue, the germline, that produces sperm or eggs to propagate the species.
Scientists don't completely understand how this kind of multicellularity evolved in animals. Cell-cell adhesion, or the ability for individual cells to stick to each other, certainly plays a role, but scientists already know that the proteins that serve these functions evolved in single-celled organisms, well before animal life emerged.
Now, research from the University of Chicago provides a new view into key innovations that allowed modern, multicellular animals to emerge. By analyzing the proteins predicted from the genomes of many animals (and close relatives to the animal kingdom), researchers found that animals evolved a more sophisticated mechanism for cell division that also contributes to developing multicellular tissues and the germline.
"This work strongly suggests that one of the early steps in the evolution of animals was the formation of the germline through the ability of cells to stay connected by incomplete cytokinesis," said Michael Glotzer, PhD, Professor of Molecular Genetics and Cell Biology at UChicago and author of the new study. "The evolution of these three proteins allowed both multicellularity and the ability to form a germline: two of the key features of animals."
Positioning the dividing line
Cell division, or cytokinesis, is the process by which a cell divides into two distinct daughter cells. Many of the proteins involved with cytokinesis are ancient, present long before the first Metazoa arose about 800 million years ago.
Glotzer has been studying animal cell division for several decades, focusing on how cells determine where to divide. In animal cells, a structure called the mitotic spindle segregates the chromosomes before the cells divide; it also dictates the position where cell division occurs. Glotzer and his team homed in on a set of three proteins—Kif23, Cyk4, and Ect2—that bind to each other and the spindle, and which are directly involved in establishing the division plane. Close relatives of these proteins had only been found previously in animals.
Two of these proteins, Kif23 and Cyk4, form a stable protein complex called centralspindlin that Glotzer and his colleagues discovered more than 20 years ago. Not only does centralspindlin contribute to division plane positioning, but it also generates a bridge between the two incipient daughter cells.
The cells that make up non-germline tissues and organs are called somatic cells, which are not passed on to the next generation. Germline cells are special because they can become any cell type. During the development of sperm and eggs, these cells also recombine the chromosomes they inherited from their parents, generating genetic diversity. While centralspindlin-dependent bridges are generally severed in somatic cells, the germlines of most animals have cells that remain connected by stable bridges.
Tracking down the proteins
Given the recent explosion in genome sequence data now available for a wide range of animals, Glotzer first wanted to determine if the two proteins that make up the centralspindlin complex, as well as Ect2, the regulatory protein that binds to it, were present and well conserved in all animals. During his analysis for this study, which was published in Current Biology, he found that all branches of animals have all three of these proteins.
Studies of these proteins in species commonly used in the lab discovered conserved sequence motifs that are linked to their known functions. Using Google DeepMind's AlphaFold AI platform (developed by UChicago alum and recent Nobel Laureate John Jumper), he was able to predict the interactions among these different proteins and found that every interaction is likely conserved across all animals. This suggests that these proteins were all in place at the beginning of the animal kingdom more than 800 million years ago and have not undergone any dramatic changes since that time.
Next, Glotzer wondered whether any related proteins could be found in single-celled organisms. He identified somewhat related proteins in choanoflagellates, the group of single-celled creatures most closely related to animals. Alphafold predicted that some of them can form a complex somewhat like centralspindlin. Though related, these complexes are clearly distinct from centralspindlin, and they lack the sequences that allow Ect2 to bind to the structure. Remarkably, some choanoflagellate species that have this complex can form colonies via incomplete cytokinesis too.
"Pre-metazoan cells have mechanisms of dividing and separating, probably with some themes and variations. Then this protein complex allowed cells to stop at the stage just before separation," Glotzer said. "Maybe multicellular life evolved because of a genetic change that prevented cells from fully separating."
"A mutation that disrupted the assembly of centralspindlin is what allowed my colleagues and me to find these proteins in the first place, more than 25 years ago," he continued. "And it appears that the evolution of this exact same region contributed to the evolution of animal life on the planet, which is mind blowing."
The study, "A key role for centralspindlin and Ect2 in the development of multicellularity and the emergence of Metazoa" was supported by the National Institutes of Health.
