CELLFUSION by EU Research

A new perspective on cell fusion

Fission yeast gametes during sexual reproduction. Purple marks sites of polarization for location of the actin focus. Green is expressed in one of the two gamete types.

Cell fusion is critical to sexual reproduction and the ongoing development of an organism, yet many questions remain around the molecular basis of the process. Researchers at the University of Lausanne are combining several different techniques to gain deeper insights in this area, as Professor Sophie Martin explains.

Cell fusion A model organism called Schizosaccharomyces pombe, a species of yeast, is being used to investigate this process in great detail. This organism has attributes that make it ideal for observing cellular processes. “It’s a single-

celled organism, so it’s easy to image, because you have immediate access to the cells, which are not buried deep in a tissue,” she outlines. This model system has proved to be very useful in the past, for instance to unravel the controls of cell division. Professor Martin and her colleagues are using it now to gain insights into the cell fusion process. “It’s relatively easy to modify the genome or to mark endogenous proteins with a fluorescent tag,

Professor Martin and her research group previously observed that the actin cytoskeleton becomes very concentrated at the point of cell-to-cell contact during the preparation for cell fusion. “This reorganization of the actin permits the transport of cargoes to a very restricted location at the zone of cell contact. That, in turn, allows the secretion of enzymes that digest the cell wall, so that the plasma

During sexual reproduction two gametes – in mammals it would be an oocyte and sperm – fuse together. Once they’ve fused, the development of the new organism starts, from the fertilised oocyte, also called a zygote. It’s important that this fusion does not happen repeatedly. What changes in the zygote to inform it that it has fused successfully, and that it may look towards development? so that we can image the fusion process with minimal disruption,” she says. “This allows us to visualize the fusion process, take movies, and investigate how cells are progressively reorganized over time.” This work builds on earlier research in cell biology, in particular the observation that some level of cellular re-organisation takes place at the time that cells fuse together. Through the use of live-cell microscopy,

membranes can come into contact and eventually fuse,” she outlines. Typically there is one point of contact between fusing cells, which allows the exchange of materials. “There are some examples where cells may fuse in several locations, or make several little holes. But typically there’s only one point of contact,” says Professor Martin. A number of different approaches are being applied in the project, including optogenetics

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process, a topic that fascinates Professor Martin. “How do cells know when to digest their cell wall? If this happened before the gametes are in contact, they would burst,” she explains. Significant progress has been achieved in this respect over the course of the project. “We’ve found that the pheromone signal is emitted and perceived at the site of polarisation. As cells grow closer together, the proximity of the signal and thus its strength increase, and this serves to stabilize the actin focus. So only when cells are close will they focus their actin cytoskeleton and digest their cell wall,” she explains. “This illustrates interesting links between cell signaling and the cytoskeleton and shows the importance of the spatial dimension of signaling.”

Cell fate change after fusion Professor Martin and her colleagues have also gained important results on what happens after cell fusion. Once two gametes have fused, the newly-formed zygote should now differentiate and avoid fusing with another gamete. “We’ve found that this is mediated by a change in the transcriptional capacity of the cell,” she continues. “Before they fuse, the two gametes each carry half of a factor that then re-combines after fusion and forms one entity that very rapidly initiates gene expression.” The initiation of zygotic gene expression essentially informs the zygote that the fusion process has been completed. This prevents a second fusion event, which would result in excessive genomic content, and moves the zygote to the next stage in development. While the project’s research has been conducted using Schizosaccharomyces pombe as a model system, Professor Martin hopes that their results will hold broader relevance beyond this specific fungal species. “We work on proteins and cellular processes that are very conserved through evolution, so they’re not present only in yeast. We hope that a number of our results will be valid not only in the yeast that we’re studying, but more broadly,” she says.

CellFusion Cell-Cell fusion in fertilization and developmental biology: a cellular and molecular dissection Project Objectives

The objectives of CellFusion are to depict at the molecular level the process through which two cells fuse together during sexual reproduction. We aim to understand how the cells communicate, assemble a fusion structure, fuse and terminate the process.

Project Funding

ERC Consolidator Grant - Cellular and Developmental Biology.

Contact Details

Professor Sophie Martin, Ph.D Department of Fundamental Microbiology University of Lausanne Biophore Building CH-1015 Lausanne Switzerland T: +41 21 692 3931 E: Sophie.Martin@unil.ch W: http://wp.unil.ch/martinlab Merlini L, Khalili B, Bendezú FO, Hurwitz D, Vincenzetti V, Vavylonis D, Martin SG. Local Pheromone Release from Dynamic Polarity Sites Underlies Cell-Cell Pairing during Yeast Mating. Curr Biol. 2016 Apr 25;26(8):1117-25. Dudin O, Merlini L, Martin SG. Spatial focalization of pheromone/MAPK signaling triggers commitment to cellcell fusion. Genes Dev. 2016 Oct 1;30(19):2226-2239. Merlini L, Khalili B, Dudin O, Michon L, Vincenzetti V, Martin SG. Inhibition of Ras activity coordinates cell fusion with cell-cell contact during yeast mating. J Cell Biol. 2018 Apr 2;217(4):1467-1483. Vještica A, Merlini L, Nkosi PJ, Martin SG. Gamete fusion triggers bipartite transcription factor assembly to block re-fertilization. Nature. 2018 Aug;560(7718):397-400.

Professor Sophie Martin, Ph.D

The process of cell fusion is integral to sexual reproduction and the ongoing development of an organism, yet there are still significant gaps in our understanding of it. One of the biggest open questions is that of how two previously separate cells, each surrounded by a plasma membrane, come together to form a single unit. “This is still largely a mystery in many organisms,” says Sophie Martin, a Professor in the Department of Microbiology at the University of Lausanne. This question is at the core of Professor Martin’s work as the Principal Investigator of the Cell Fusion project. “There are also other questions about communication between the cells, and about what happens just after fusion,” she continues. “During sexual reproduction two gametes – in mammals it would be an oocyte and sperm – fuse together. Once they’ve fused, the development of the new organism starts, from the fertilised oocyte, also called a zygote. It’s important that this fusion does not happen repeatedly. What changes in the zygote to inform it that it has fused successfully, and that it may look towards development?”

and biochemical methods, with the wider aim of gaining new insights into the cell fusion process. In the case of yeast cells, cell fusion takes place during sexual reproduction, which can be triggered by starving the cells. “We can induce the process by simply starving the cells – typically we withdraw nitrogen,” explains Professor Martin. When the two mating types of yeast are present in a cell population in these circumstances, Professor Martin says they will then start to communicate. “That communication is based on chemical perception; the cells secrete pheromones, which will diffuse in the medium. The cells can sense these pheromones through receptors which they express on their cell surface and differentiate into gametes,” she continues. “The pheromones are short peptides which are secreted outside the cell. They’re detected by the partner cell, by receptors that are part of a very large family, called G-protein coupled receptors.” Once the cells have detected these pheromones, they then face the challenge of precisely locating the pheromone source before fusion can take place, because the growth machinery needs to be accurately positioned for the cell to grow in the direction of a partner cell. “This is highly dependent on a protein called Cdc42, which is part of a family of enzymes called small GTPases,” says Professor Martin. Cdc42 has been found to be an important factor in cell polarisation in pretty much all of the organisms in which it has been investigated, now Professor Martin is looking at its role in Schizosaccharomyces pombe. “The aim is to try and understand how Cdc42 becomes active in the right direction, on the right position at the periphery of the cell – the one that faces the partner cell,” she outlines. Once Cdc42 is active at the right place, the cell grows towards its partner and both cells eventually come into contact. “Only then should the cells focus their actin cytoskeleton and digest their protective cell walls to start the fusion process,” says Professor Martin. The question then arises of how the cells coordinate the progression of the fusion

Professor Sophie Martin, Ph.D is a Full Professor in the Department of Microbiology at the University of Lausanne. She previously held research positions in Europe and America, including post-doctoral training at Columbia University in New York, where she studied cell polarization and the cytoskeleton in the fission yeast.

Fission yeast gametes (green and purple) normally fuse only once during fertilization. In abnormal mutant situations, a second cell fusion event can be observed.