Cytoplasmic sharing through apical membrane remodeling

Cell-cell fusion: To lose one life and begin another

As life extended into eukaryota, a great host of strategies emerged in the pursuit of cellular life. Some cells have been successful in solitude, some moved into cooperatives (i.e., multicellular organisms), but one additional strategy emerged. Throughout eukaryotes, many of the diverse multicellular cooperatives took life in partnership one step further. These cells came together and lost their singularity in the expanse of syncytial life. Recently in our search for this elusive "how", we discovered the intriguing peculiarity of a nuclear, RNA-binding protein living a second life as a fusion manager at the surface of developing osteoclasts, ushering them into syncytia 1. It is from here that we will develop several thoughts about the advantages of multinucleated cells and discuss how these fusing cells pass through several hallmarks of cell death. We will propose that cell fusion shares much with cell death because cell fusion is a death of sorts for the cells that undergo it - a death of the life that was and the beginning of new life in a community without borders.

Integrating the Study of Polyploidy Across Organisms, Tissues, and Disease

Polyploidy is a cellular state containing more than two complete chromosome sets. It has largely been studied as a discrete phenomenon in either organismal, tissue, or disease contexts. Increasingly, however, investigation of polyploidy across disciplines is coalescing around common principles. For example, the recent Polyploidy Across the Tree of Life meeting considered the contribution of polyploidy both in organismal evolution over millions of years and in tumorigenesis across much shorter timescales. Here, we build on this newfound integration with a unified discussion of polyploidy in organisms, cells, and disease. We highlight how common polyploidy is at multiple biological scales, thus eliminating the outdated mindset of its specialization. Additionally, we discuss rules that are likely common to all instances of polyploidy. With increasing appreciation that polyploidy is pervasive in nature and displays fascinating commonalities across diverse contexts, inquiry related to this important topic is rapidly becoming unified.

Epithelial mechanics are maintained by inhibiting cell fusion with age in Drosophila

A characteristic of normal aging and age-related diseases is the remodeling of the cellular organization of a tissue through polyploid cell growth. Polyploidy arises from an increase in nuclear ploidy or the number of nuclei per cell. However, it is not known whether age-induced polyploidy is an adaption to stressors or a precursor to degeneration. Here, we find that abdominal epithelium of the adult fruit fly becomes polyploid with age through generation of multinucleated cells by cell fusion. Inhibition of fusion does not improve the lifespan of the fly, but does enhance its biomechanical fitness, a measure of the healthspan of the animal. Remarkably, Drosophila can maintain their epithelial tension and abdominal movements with age when cell fusion is inhibited. Epithelial cell fusion also appears to be dependent on a mechanical cue, as knockdown of Rho kinase, E-cadherin or α-catenin is sufficient to induce multinucleation in young animals. Interestingly, mutations in α-catenin in mice result in retina pigment epithelial multinucleation associated with macular disease. Therefore, we have discovered that polyploid cells arise by cell fusion and contribute to the decline in the biomechanical fitness of the animal with age.

Using Mosaic Cell Labeling to Visualize Polyploid Cells in the Drosophila Brain

Traditional methods used to study endoreplication have limitations when used to identify rare events of polyploidization in complex, densely-packed tissues. Here, we describe a method to identify and visualize polyploid cells in situ using an existing mosaic, multicolor labeling technique named "CoinFLP" (Bosch et al., Development 142(3):597-606, 2015). CoinFLP allows easy visualization of polyploid cells in situ and can be combined with other techniques such as immunofluorescence for cell-type-specific labeling and flow cytometry to perform quantifications and can also be used for genetic manipulations. Further, by modifying the time of labeling, this technique can also be used to distinguish events of cell fusion from endocycle (Nandakumar et al., eLife 25:9, 2020)-allowing one to infer the method of polyploidization.

An interplay between cellular growth and atypical fusion defines morphogenesis of a modular glial niche in Drosophila

Neural stem cells (NSCs) live in an intricate cellular microenvironment supporting their activity, the niche. Whilst shape and function are inseparable, the morphogenetic aspects of niche development are poorly understood. Here, we use the formation of a glial niche to investigate acquisition of architectural complexity. Cortex glia (CG) in Drosophila regulate neurogenesis and build a reticular structure around NSCs. We first show that individual CG cells grow tremendously to ensheath several NSC lineages, employing elaborate proliferative mechanisms which convert these cells into syncytia rich in cytoplasmic bridges. CG syncytia further undergo homotypic cell-cell fusion, using defined cell surface receptors and actin regulators. Cellular exchange is however dynamic in space and time. This atypical cell fusion remodels cellular borders, restructuring the CG syncytia. Ultimately, combined growth and fusion builds the multi-level architecture of the niche, and creates a modular, spatial partition of the NSC population. Our findings provide insights into how a niche forms and organises while developing intimate contacts with a stem cell population.

Conserved function of Drosophila Fancd2 monoubiquitination in response to double-strand DNA breaks

Fanconi anemia genes play key roles in metazoan DNA damage responses, and human FA mutations cause numerous disease phenotypes. In human cells, activating monoubiquitination of the Fanconi anemia protein Fancd2 occurs following diverse DNA damage stimuli. Monoubiquitinated Fancd2 forms nuclear foci to recruit additional repair factors. Fancd2 animal models to date have focused on molecular nulls or whole gene knockdown, leaving the specific in vivo role of monoubiquitination unclear. Using a point mutant in a conserved residue, we recently linked Drosophila Fancd2 monoubiquitination to a mitosis-specific DNA double-strand break response. In this context, we used CRISPR/Cas9 to generate the first animal model of an endogenous mutation in the conserved monoubiquitination site (fancd2K595R). Here, we expand upon our characterization of fancd2K595R. We also introduce and characterize additional Drosophila tools to study fancd2, including new mutant alleles and GFP-tagged rescue transgenes. Using these new reagents, we show the impact of Drosophila Fancd2 on organismal and cell viability, as well as on repair protein localization, in the presence or absence of double-strand breaks. These findings expand our understanding of Fanconi anemia gene function in vivo and provide useful reagents for DNA repair research.

Persistent DNA damage signaling and DNA polymerase theta promote broken chromosome segregation

Cycling cells must respond to DNA double-strand breaks (DSBs) to avoid genome instability. Missegregation of chromosomes with DSBs during mitosis results in micronuclei, aberrant structures linked to disease. How cells respond to DSBs during mitosis is incompletely understood. We previously showed that Drosophilamelanogaster papillar cells lack DSB checkpoints (as observed in many cancer cells). Here, we show that papillar cells still recruit early acting repair machinery (Mre11 and RPA3) and the Fanconi anemia (FA) protein Fancd2 to DSBs. These proteins persist as foci on DSBs as cells enter mitosis. Repair foci are resolved in a stepwise manner during mitosis. DSB repair kinetics depends on both monoubiquitination of Fancd2 and the alternative end-joining protein DNA polymerase θ. Disruption of either or both of these factors causes micronuclei after DNA damage, which disrupts intestinal organogenesis. This study reveals a mechanism for how cells with inactive DSB checkpoints can respond to DNA damage that persists into mitosis.

Polyploidy in Tissue Repair and Regeneration

Polyploidy is defined as a cell with three or more whole genome sets and enables cell growth across the kingdoms of life. Studies in model organisms have revealed that polyploid cell growth can be required for optimal tissue repair and regeneration. In mammals, polyploid cell growth contributes to repair of many tissues, including the liver, heart, kidney, bladder, and eye, and similar strategies have been identified in Drosophila and zebrafish tissues. This review discusses the heterogeneity and versatility of polyploidy in tissue repair and regeneration. Polyploidy has been shown to restore tissue mass and maintain organ size as well as protect against oncogenic insults and genotoxic stress. Polyploid cells can also serve as a reservoir for new diploid cells in regeneration. The numerous mechanisms to generate polyploid cells provide an unlimited resource for tissues to exploit to undergo repair or regeneration.

Polyploidy in development and tumor models in Drosophila

Polyploidy, a cell status defined as more than two sets of genomic DNA, is a conserved strategy across species that can increase cell size and biosynthetic production, but the functional aspects of polyploidy are nuanced and vary across cell types. Throughout Drosophila developmental stages (embryo, larva, pupa and adult), polyploid cells are present in numerous organs and help orchestrate development while contributing to normal growth, well-being and homeostasis of the organism. Conversely, increasing evidence has shown that polyploid cells are prevalent in Drosophila tumors and play important roles in tumor growth and invasiveness. Here, we summarize the genes and pathways involved in polyploidy during normal and tumorigenic development, the mechanisms underlying polyploidization, and the functional aspects of polyploidy in development, homeostasis and tumorigenesis in the Drosophila model.

Communal living: the role of polyploidy and syncytia in tissue biology

Multicellular organisms are composed of tissues with diverse cell sizes. Whether a tissue primarily consists of numerous, small cells as opposed to fewer, large cells can impact tissue development and function. The addition of nuclear genome copies within a common cytoplasm is a recurring strategy to manipulate cellular size within a tissue. Cells with more than two genomes can exist transiently, such as in developing germlines or embryos, or can be part of mature somatic tissues. Such nuclear collectives span multiple levels of organization, from mononuclear or binuclear polyploid cells to highly multinucleate structures known as syncytia. Here, we review the diversity of polyploid and syncytial tissues found throughout nature. We summarize current literature concerning tissue construction through syncytia and/or polyploidy and speculate why one or both strategies are advantageous.