The hallmarks of cell-cell fusion

TFEB controls syncytiotrophoblast formation and hormone production in placenta

TFEB, a bHLH-leucine zipper transcription factor belonging to the MiT/TFE family, globally modulates cell metabolism by regulating autophagy and lysosomal functions. Remarkably, loss of TFEB in mice causes embryonic lethality due to severe defects in placentation associated with aberrant vascularization and resulting hypoxia. However, the molecular mechanism underlying this phenotype has remained elusive. By integrating in vivo analyses with multi-omics approaches and functional assays, we have uncovered an unprecedented function for TFEB in promoting the formation of a functional syncytiotrophoblast in the placenta. Our findings demonstrate that constitutive loss of TFEB in knock-out mice is associated with defective formation of the syncytiotrophoblast layer. Indeed, using in vitro models of syncytialization, we demonstrated that TFEB translocates into the nucleus during syncytiotrophoblast formation and binds to the promoters of crucial placental genes, including genes encoding fusogenic proteins (Syncytin-1 and Syncytin-2) and enzymes involved in steroidogenic pathways, such as CYP19A1, the rate-limiting enzyme for the synthesis of 17β-Estradiol (E2). Conversely, TFEB depletion impairs both syncytial fusion and endocrine properties of syncytiotrophoblast, as demonstrated by a significant decrease in the secretion of placental hormones and E2 production. Notably, restoration of TFEB expression resets syncytiotrophoblast identity. Our findings identify that TFEB controls placental development and function by orchestrating both the transcriptional program underlying trophoblast fusion and the acquisition of endocrine function, which are crucial for the bioenergetic requirements of embryonic development.

Novel requirements for HAP2/GCS1-mediated gamete fusion in Tetrahymena

The ancestral gamete fusion protein, HAP2/GCS1, plays an essential role in fertilization in a broad range of taxa. To identify factors that may regulate HAP2/GCS1 activity, we screened mutants of the ciliate Tetrahymena thermophila for behaviors that mimic Δhap2/gcs1 knockout phenotypes in this species. Using this approach, we identified two new genes, GFU1 and GFU2, whose products are necessary for membrane pore formation following mating type recognition and adherence. GFU2 is predicted to be a single-pass transmembrane protein, while GFU1, though lacking obvious transmembrane domains, has the potential to interact directly with membrane phospholipids in the cytoplasm. Like Tetrahymena HAP2/GCS1, expression of GFU1 is required in both cells of a mating pair for efficient fusion to occur. To explain these bilateral requirements, we propose a model that invokes cooperativity between the fusion machinery on apposed membranes of mating cells and accounts for successful fertilization in Tetrahymena's multiple mating type system.

MTM1-mediated production of phosphatidylinositol 5-phosphate fuels the formation of podosome-like protrusions regulating myoblast fusion

Myogenesis is a multistep process that requires a spatiotemporal regulation of cell events resulting finally in myoblast fusion into multinucleated myotubes. Most major insights into the mechanisms underlying fusion seem to be conserved from insects to mammals and include the formation of podosome-like protrusions (PLPs) that exert a driving force toward the founder cell. However, the machinery that governs this process remains poorly understood. In this study, we demonstrate that MTM1 is the main enzyme responsible for the production of phosphatidylinositol 5-phosphate, which in turn fuels PI5P 4-kinase α to produce a minor and functional pool of phosphatidylinositol 4,5-bisphosphate that concentrates in PLPs containing the scaffolding protein Tks5, Dynamin-2, and the fusogenic protein Myomaker. Collectively, our data reveal a functional crosstalk between a PI-phosphatase and a PI-kinase in the regulation of PLP formation.

How Much Do You Fuse? A Comparison of Cell Fusion Assays in a Breast Cancer Model

Cell fusion is a biological process that is crucial for the development and homeostasis of different tissues, but it is also pathophysiologically associated with tumor progression and malignancy. The investigation of cell fusion processes is difficult because there is no standardized marker. Many studies therefore use different systems to observe and quantify cell fusion in vitro and in vivo. The comparability of the results must be critically questioned, because both the experimental procedure and the assays differ between studies. The comparability of the fluorescence-based fluorescence double reporter (FDR) and dual split protein (DSP) assay was investigated as part of this study, in which general conditions were kept largely constant. In order to be able to induce both a high and a low cell fusion rate, M13SV1 breast epithelial cells were modified with regard to the expression level of the fusogenic protein Syncytin-1 and its receptor ASCT2 and were co-cultivated for 72 h with different breast cancer cell lines. A high number of fused cells was found in co-cultures with Syncytin-1-overexpressing M13SV1 cells, but differences between the assays were also observed. This shows that the quantification of cell fusion events in particular is highly dependent on the assay selected, but the influence of fusogenic proteins can be visualized very well.

Cell fusion: Inter-organ tissue communication promotes a union between myoblasts

Repeated rounds of fusion between apposing myoblasts allow muscles to become multinucleated. New research finds that myoblasts undergoing fusion in the Drosophila embryo respond to hormone signaling from a nearby tissue, resulting in the activation of a myoblast-specific gene necessary for the fusion process.

Infection kinetics, syncytia formation, and inflammatory biomarkers as predictive indicators for the pathogenicity of SARS-CoV-2 Variants of Concern in Calu-3 cells

The ongoing COVID-19 pandemic has led to the emergence of new SARS-CoV-2 variants as a result of continued host-virus interaction and viral genome mutations. These variants have been associated with varying levels of transmissibility and disease severity. We investigated the phenotypic profiles of six SARS-CoV-2 variants (WT, D614G, Alpha, Beta, Delta, and Omicron) in Calu-3 cells, a human lung epithelial cell line. In our model demonstrated that all variants, except for Omicron, had higher efficiency in virus entry compared to the wild-type. The Delta variant had the greatest phenotypic advantage in terms of early infection kinetics and marked syncytia formation, which could facilitate cell-to-cell spreading, while the Omicron variant displayed slower replication and fewer syncytia formation. We also identified the Delta variant as the strongest inducer of inflammatory biomarkers, including pro-inflammatory cytokines/chemokines (IP-10/CXCL10, TNF-α, and IL-6), anti-inflammatory cytokine (IL-1RA), and growth factors (FGF-2 and VEGF-A), while these inflammatory mediators were not significantly elevated with Omicron infection. These findings are consistent with the observations that there was a generally more pronounced inflammatory response and angiogenesis activity within the lungs of COVID-19 patients as well as more severe symptoms and higher mortality rate during the Delta wave, as compared to less severe symptoms and lower mortality observed during the current Omicron wave in Thailand. Our findings suggest that early infectivity kinetics, enhanced syncytia formation, and specific inflammatory mediator production may serve as predictive indicators for the virulence potential of future SARS-CoV-2 variants.

Myomerger Induces Membrane Hemifusion and Regulates Fusion Pore Expansion

Membrane fusion is a crucial mechanism in a wide variety of important events in cell biology from viral infection to exocytosis. However, despite many efforts and much progress, cell-cell fusion has remained elusive to our understanding. Along the life of the fusion pore, large conformational changes take place from the initial lipid bilayer bending, passing through the hemifusion intermediates, and ending with the formation of the first nascent fusion pore. In this sense, computer simulations are an ideal technique for describing such complex lipid remodeling at the molecular level. In this work, we studied the role played by the muscle-specific membrane protein Myomerger during the formation of the fusion pore. We have conducted μs length atomistic and coarse-grained molecular dynamics, together with free-energy calculations using ad hoc collective variables. Our results show that Myomerger favors the hemifusion diaphragm-stalk transition, reduces the nucleation-expansion energy difference, and promotes the formation of nonenlarging fusion pores.

Cell fusion upregulates PD-L1 expression for evasion from immunosurveillance

MSCs (mesenchymal stem cells), responsible for tissue repair, rarely undergo cell fusion with somatic cells. Here, we show that ~5% of bladder cancer cells (UMUC-3) fuses with bone marrow-derived MSC (BM-MSC) in co-culture and maintains high tumorigenicity. In eleven fusion cell clones that have been established, Mb-scale deletions carried by the bladder cancer cells are mostly absent in the fusion cells, but copy number gains contributed by the cancer cells have stayed. Fusion cells exhibit increased populations of mitotic cells with 3-polar spindles, indicative of genomic instability. They grow faster in vitro and exhibit higher colony formation in anchorage-independent growth assay in soft agar than the parent UMUC-3 does. Fusion cells develop tumors, after 4 weeks of time lag, as efficiently as the parent UMUC-3 does in xenograft experiments. 264 genes are identified whose expression is specifically altered in the fusion cells. Many of them are interferon-stimulated genes (ISG), but are activated in a manner independent of interferon. Among them, we show that PD-L1 is induced in fusion cells, and its knockout decreases tumorigenesis in a xenograft model. PD-L1 is induced in a manner independent of STAT1 known to regulate PD-L1 expression, but is regulated by histone modification, and is likely to inhibit phagocytosis by PD1-expressing macrophages, thus protecting cancer cells from immunological attacks. The fusion cells overexpress multiple cytokines including CCL2 that cause tumor progression by converting infiltrating macrophages to tumor-associated-macrophage (TAM). The results present mechanisms of how cell fusion promotes tumorigenesis, revealing a novel link between cell fusion and PD-L1, and underscore the efficacy of cancer immunotherapy.

Somatic Cell Fusion in Host Defense and Adaptation

Evidence of fusion of somatic cells has been noted in health and in disease for more than a century. The most obvious but uncertain hallmark has been the presence of multiple nuclei in cells. Although multinucleated cells are found in normal and diseased tissues, the benefit or harm of such cells can be difficult to elucidate. Still more difficult however is the identification of mononuclear cells previously formed by fusion of somatic cells with one or more nuclei disposed. The later process can introduce mutations that promote viral diversification, cancer, and tissue senescence. Less obvious the potential benefits of cell fusion. Recent work in cell biology, immunology, and genomic analysis however makes it possible to postulate benefits and potentially arrive at novel therapeutic agents and approaches that replicate or enhance these benefits.

Mechanisms of Cell Fusion in Cancer

Cell-cell fusion is a normal physiological mechanism that requires a well-orchestrated regulation of intracellular and extracellular factors. Dysregulation of this process could lead to diseases such as osteoporosis, malformation of muscles, difficulties in pregnancy, and cancer. Extensive literature demonstrates that fusion occurs between cancer cells and other cell types to potentially promote cancer progression and metastasis. However, the mechanisms governing this process in cancer initiation, promotion, and progression are less well-studied. Fusogens involved in normal physiological processes such as syncytins and associated factors such as phosphatidylserine and annexins have been observed to be critical in cancer cell fusion as well. Some of the extracellular factors associated with cancer cell fusion include chronic inflammation and inflammatory cytokines, hypoxia, and viral infection. The interaction between these extracellular factors and cell's intrinsic factors potentially modulates actin dynamics to drive the fusion of cancer cells. In this review, we have discussed the different mechanisms that have been identified or postulated to drive cancer cell fusion.

Cell Fusion and Syncytia Formation in Cancer

The natural phenomenon of cell-cell fusion does not only take place in physiological processes, such as placentation, myogenesis, or osteoclastogenesis, but also in pathophysiological processes, such as cancer. More than a century ago postulated, today the hypothesis that the fusion of cancer cells with normal cells leads to the formation of cancer hybrid cells with altered properties is in scientific consensus. Some studies that have investigated the mechanisms and conditions for the fusion of cancer cells with other cells, as well as studies that have characterized the resulting cancer hybrid cells, are presented in this review. Hypoxia and the cytokine TNFα, for example, have been found to promote cell fusion. In addition, it has been found that both the protein Syncytin-1, which normally plays a role in placentation, and phosphatidylserine signaling on the cell membrane are involved in the fusion of cancer cells with other cells. In human cancer, cancer hybrid cells were detected not only in the primary tumor, but also in the circulation of patients as so-called circulating hybrid cells, where they often correlated with a worse outcome. Although some data are available, the questions of how and especially why cancer cells fuse with other cells are still not fully answered.

Altered Phenotypes of Breast Epithelial × Breast Cancer Hybrids after ZEB1 Knock-Out

ZEB1 plays a pivotal role in epithelial-to-mesenchymal transition (EMT), (cancer) cell stemness and cancer therapy resistance. The M13HS tumor hybrids, which were derived from spontaneous fusion events between the M13SV1-EGFP-Neo breast epithelial cells and HS578T-Hyg breast cancer cells, express ZEB1 and exhibit prospective cancer stem cell properties. To explore a possible correlation between the ZEB1 and stemness/ EMT-related properties in M13HS tumor hybrids, ZEB1 was knocked-out by CRISPR/Cas9. Colony formation, mammosphere formation, cell migration, invasion assays, flow cytometry and Western blot analyses were performed for the characterization of ZEB1 knock-out cells. The ZEB1 knock-out in M13HS tumor cells was not correlated with the down-regulation of the EMT-related markers N-CADHERIN (CDH2) and VIMENTIN and up-regulation of miR-200c-3p. Nonetheless, both the colony formation and mammosphere formation capacities of the M13HS ZEB1 knock-out cells were markedly reduced. Interestingly, the M13HS-2 ZEB1-KO cells harbored a markedly higher fraction of ALDH1-positive cells. The Transwell/ Boyden chamber migration assay data indicated a reduced migratory activity of the M13HS ZEB1-knock-out tumor hybrids, whereas in scratch/ wound-healing assays only the M13SH-8 ZEB1-knock-out cells possessed a reduced locomotory activity. Similarly, only the M13HS-8 ZEB1-knock-out tumor hybrids showed a reduced invasion capacity. Although the ZEB1 knock-out resulted in only moderate phenotypic changes, our data support the role of ZEB1 in EMT and stemness.

COVID-19-Related Age Profiles for SARS-CoV-2 Variants in England and Wales and States of the USA (2020 to 2022): Impact on All-Cause Mortality

Since 2020, COVID-19 has caused serious mortality around the world. Given the ambiguity in establishing COVID-19 as the direct cause of death, we first investigate the effects of age and sex on all-cause mortality during 2020 and 2021 in England and Wales. Since infectious agents have their own unique age profile for death, we use a 9-year time series and several different methods to adjust single-year-of-age deaths in England and Wales during 2019 (the pre-COVID-19 base year) to a pathogen-neutral single-year-of-age baseline. This adjusted base year is then used to confirm the widely reported higher deaths in males for most ages above 43 in both 2020 and 2021. During 2020 (+COVID-19 but no vaccination), both male and female population-adjusted deaths significantly increased above age 35. A significant reduction in all-cause mortality among both males and females aged 75+ could be demonstrated in 2021 during the widespread COVID-19 vaccination period; however, deaths below age 75 progressively increased. This finding arises from a mix of vaccination coverage and year-of-age profiles of deaths for the different SARS-CoV-2 variants. In addition, specific effects of age around puberty were demonstrated, where females had higher deaths than males. There is evidence that year-of-birth cohorts may also be involved, indicating that immune priming to specific pathogen outbreaks in the past may have led to lower deaths for some birth cohorts. To specifically identify the age profile for the COVID-19 variants from 2020 to 2023, we employ the proportion of total deaths at each age that are potentially due to or 'with' COVID-19. The original Wuhan strain and the Alpha variant show somewhat limited divergence in the age profile, with the Alpha variant shifting to a moderately higher proportion of deaths below age 84. The Delta variant specifically targeted individuals below age 65. The Omicron variants showed a significantly lower proportion of overall mortality, with a markedly higher relative proportion of deaths above age 65, steeply increasing with age to a maximum around 100 years of age. A similar age profile for the variants can be seen in the age-banded deaths in US states, although they are slightly obscured by using age bands rather than single years of age. However, the US data shows that higher male deaths are greatly dependent on age and the COVID variant. Deaths assessed to be 'due to' COVID-19 (as opposed to 'involving' COVID-19) in England and Wales were especially overestimated in 2021 relative to the change in all-cause mortality. This arose as a by-product of an increase in COVID-19 testing capacity in late 2020. Potential structure-function mechanisms for the age-specificity of SARS-CoV-2 variants are discussed, along with potential roles for small noncoding RNAs (miRNAs). Using data from England, it is possible to show that the unvaccinated do indeed have a unique age profile for death from each variant and that vaccination alters the shape of the age profile in a manner dependent on age, sex, and the variant. The question is posed as to whether vaccines based on different variants carry a specific age profile.

Acquisition and Exaptation of Endogenous Retroviruses in Mammalian Placenta

Endogenous retroviruses (ERVs) are retrovirus-like sequences that were previously integrated into the host genome. Although most ERVs are inactivated by mutations, deletions, or epigenetic regulation, some remain transcriptionally active and impact host physiology. Several ERV-encoded proteins, such as Syncytins and Suppressyn, contribute to placenta acquisition, a crucial adaptation in mammals that protects the fetus from external threats and other risks while enabling the maternal supply of oxygen, nutrients, and antibodies. In primates, Syncytin-1 and Syncytin-2 facilitate cell-cell fusion for placental formation. Suppressyn is the first ERV-derived protein that inhibits cell fusion by binding to ASCT2, the receptor for Syncytin-1. Furthermore, Syncytin-2 likely inserted into the genome of the common ancestor of Anthropoidea, whereas Syncytin-1 and Suppressyn likely inserted into the ancestor of catarrhines; however, they were inactivated in some lineages, suggesting that multiple exaptation events had occurred. This review discusses the role of ERV-encoded proteins, particularly Syncytins and Suppressyn, in placental development and function, focusing on the integration of ERVs into the host genome and their contribution to the genetic mechanisms underlying placentogenesis. This review provides valuable insights into the molecular and genetic aspects of placentation, potentially shedding light on broader evolutionary and physiological processes in mammals.

Why do certain cancer cells alter functionality and fuse?

Cancer cell fusion represents a rare event. However, the surviving cancer hybrid cells after a post-hybrid selection process (PHSP) can overgrow other cancer cells by exhibiting a proliferation advantage and/or expression of cancer stem-like properties. Addition of new tumor properties during hetero-fusion of cancer cells e.g. with mesenchymal stroma-/stem-like cells (MSC) contribute to enhanced tumor plasticity via acquisition of new/altered functionalities. This provides new avenues for tumor development and metastatic behavior. Consequently, the present review article will also address the question as to whether cancer cell fusion represents a general and possibly evolutionary-conserved program or rather a random process?

Antioxidant and Antiaging Activity of Fermented Coix Seed Polysaccharides on Caenorhabditis elegans

Aging is closely related to many diseases and is a long-term challenge that humans face. The oxidative damage caused by the imbalance of free radicals is an important factor in aging. In this study, we investigate the antioxidant and antiaging activities of fermented coix seed polysaccharides (FCSPs) via in vitro and in vivo experiments. The FCSPs were extracted by fermenting coix seed with Saccharomyces cerevisiae for 48 h and utilizing water-extracted coix seed polysaccharides (WCSPs) as a control. Their antiaging activity and mechanism were evaluated based on the antiaging model organism Caenorhabditis elegans (C. elegans). The results showed that the molecular weight of the FCSPs extracted by fermentation was smaller than that of the WCSPs, making them more easily absorbed and utilized. At a concentration of 5 g/L, the FCSPs' capacity to scavenge the DPPH·, ABTS⁺·, OH·, and O₂^-· radicals was greater than the WCSPs' capacity by 10.09%, 14.40%, 49.93%, and 12.86%, respectively. Moreover, C. elegans treated with FCSPs exhibited higher antioxidant enzyme activities and a lower accumulation of malonaldehyde. By inhibiting the expression of the pro-aging genes daf-2 and age-1, and upregulating the expression of the antiaging genes daf-16, sod-3, skn-1, and gcs-1 in the insulin/insulin-like growth factor-1 (IIS) signaling pathway, the FCSPs could effectively enhance stress tolerance and delay C. elegans aging. The lifespan of C. elegans in the FCSPs group was 5.91% higher than that of the WCSPs group. In conclusion, FCSPs exert better antioxidant and antiaging effects than WCSPs, which can act as a potential functional ingredient or supplement in food.

Ultrastructural study confirms the formation of single and heterotypic syncytial cells in bronchoalveolar fluids of COVID-19 patients

BACKGROUND

SARS-CoV-2 was reported to induce cell fusions to form multinuclear syncytia that might facilitate viral replication, dissemination, immune evasion, and inflammatory responses. In this study, we have reported the types of cells involved in syncytia formation at different stages of COVID-19 disease through electron microscopy.

METHODS

Bronchoalveolar fluids from the mild (n = 8, SpO2 > 95%, no hypoxia, within 2-8 days of infection), moderate (n = 8, SpO2 90% to ≤ 93% on room air, respiratory rate ≥ 24/min, breathlessness, within 9-16 days of infection), and severe (n = 8, SpO2 < 90%, respiratory rate > 30/min, external oxygen support, after 17th days of infection) COVID-19 patients were examined by PAP (cell type identification), immunofluorescence (for the level of viral infection), scanning (SEM), and transmission (TEM) electron microscopy to identify the syncytia.

RESULTS

Immunofluorescence studies (S protein-specific antibodies) from each syncytium indicate a very high infection level. We could not find any syncytial cells in mildly infected patients. However, identical (neutrophils or type 2 pneumocytes) and heterotypic (neutrophils-monocytes) plasma membrane initial fusion (indicating initiation of fusion) was observed under TEM in moderately infected patients. Fully matured large-size (20-100 μm) syncytial cells were found in severe acute respiratory distress syndrome (ARDS-like) patients of neutrophils, monocytes, and macrophage origin under SEM.

CONCLUSIONS

This ultrastructural study on the syncytial cells from COVID-19 patients sheds light on the disease's stages and types of cells involved in the syncytia formations. Syncytia formation was first induced in type II pneumocytes by homotypic fusion and later with haematopoetic cells (monocyte and neutrophils) by heterotypic fusion in the moderate stage (9-16 days) of the disease. Matured syncytia were reported in the late phase of the disease and formed large giant cells of 20 to 100 μm.

Expression of Myomaker and Myomerger in myofibers causes muscle pathology

BACKGROUND

Skeletal muscle development and regeneration depend on cellular fusion of myogenic progenitors to generate multinucleated myofibers. These progenitors utilize two muscle-specific fusogens, Myomaker and Myomerger, which function by remodeling cell membranes to fuse to each other or to existing myofibers. Myomaker and Myomerger expression is restricted to differentiating progenitor cells as they are not detected in adult myofibers. However, Myomaker remains expressed in myofibers from mice with muscular dystrophy. Ablation of Myomaker from dystrophic myofibers results in reduced membrane damage, leading to a model where persistent fusogen expression in myofibers, in contrast to myoblasts, is harmful.

METHODS

Dox-inducible transgenic mice were developed to ectopically express Myomaker or Myomerger in the myofiber compartment of skeletal muscle. We quantified indices of myofiber membrane damage, such as serum creatine kinase and IgM+ myofibers, and assessed general muscle histology, including central nucleation, myofiber size, and fibrosis.

RESULTS

Myomaker or Myomerger expression in myofibers independently caused membrane damage at acute time points. This damage led to muscle pathology, manifesting with centrally nucleated myofibers and muscle atrophy. Dual expression of both Myomaker and Myomerger in myofibers exacerbated several aspects of muscle pathology compared to expression of either fusogen by itself.

CONCLUSIONS

These data reveal that while myofibers can tolerate some level of Myomaker and Myomerger, expression of a single fusogen above a threshold or co-expression of both fusogens is damaging to myofibers. These results explain the paradigm that their expression in myofibers can have deleterious consequences in muscle pathologies and highlight the need for their highly restricted expression during myogenesis and fusion.

First Isolation and Genome Sequence Analysis of West Nile Virus in Mosquitoes in Brazil

West Nile virus is a flavivirus transmitted by mosquitoes, mainly of the genus Culex. In Brazil, serological studies have already indicated the circulation of the virus since 2003, with the first human case detected in 2014. The objective of the present paper is to report the first isolation of WNV in a Culex (Melanoconion) mosquito. Arthropods were collected by protected human attraction and CDC light bait, and taxonomically identified and analyzed by viral isolation, complement fixation and genomic sequencing tests. WNV was isolated from samples of Culex (Melanoconion) mosquitoes, and the sequencing analysis demonstrated that the isolated strain belonged to lineage 1a. The finding of the present study presents the first evidence of the isolation and genome sequencing of WNV in arthropods in Brazil.

Transcriptomic Differences Underlying the Activin-A Induced Large Osteoclast Formation in Both Healthy Control and Fibrodysplasia Ossificans Progressiva Osteoclasts

Fibrodysplasia Ossificans Progressiva (FOP) is a very rare genetic disease characterized by progressive heterotopic ossification (HO) of soft tissues, leading to immobility and premature death. FOP is caused by a mutation in the Activin receptor Type 1 (ACVR1) gene, resulting in altered responsiveness to Activin-A. We recently revealed that Activin-A induces fewer, but larger and more active, osteoclasts regardless of the presence of the mutated ACVR1 receptor. The underlying mechanism of Activin-A-induced changes in osteoclastogenesis at the gene expression level remains unknown. Transcriptomic changes induced by Activin-A during osteoclast formation from healthy controls and patient-derived CD14-positive monocytes were studied using RNA sequencing. CD14-positive monocytes from six FOP patients and six age- and sex-matched healthy controls were differentiated into osteoclasts in the absence or presence of Activin-A. RNA samples were isolated after 14 days of culturing and analyzed by RNA sequencing. Non-supervised principal component analysis (PCA) showed that samples from the same culture conditions (e.g., without or with Activin-A) tended to cluster, indicating that the variability induced by Activin-A treatment was larger than the variability between the control and FOP samples. RNA sequencing analysis revealed 1480 differentially expressed genes induced by Activin-A in healthy control and FOP osteoclasts with p(adj) < 0.01 and a Log2 fold change of ≥±2. Pathway and gene ontology enrichment analysis revealed several significantly enriched pathways for genes upregulated by Activin-A that could be linked to the differentiation or function of osteoclasts, cell fusion or inflammation. Our data showed that Activin-A has a substantial effect on gene expression during osteoclast formation and that this effect occurred regardless of the presence of the mutated ACVR1 receptor causing FOP.

Intrinsic signalling factors associated with cancer cell-cell fusion

Cellular fusion e.g. between cancer cells and normal cells represents a stepwise process that is tightly regulated. During a pre-hybrid preparation program somatic cells and/or cancer cells are promoted to a pro-fusogenic state as a prerequisite to prepare a fusion process. A pro-fusogenic state requires significant changes including restructure of the cytoskeleton, e.g., by the formation of F-actin. Moreover, distinct plasma membrane lipids such as phosphatidylserine play an important role during cell fusion. In addition, the expression of distinct fusogenic factors such as syncytins and corresponding receptors are of fundamental importance to enable cellular mergers. Subsequent hybrid formation and fusion are followed by a post-hybrid selection process. Fusion among normal cells is important and often required during organismal development. Cancer cells fusion appears more rarely and is associated with the generation of new cancer hybrid cell populations. These cancer hybrid cells contribute to an elevated tumour plasticity by altered metastatic behaviour, changes in therapeutic and apoptotic responses, and even in the formation of cancer stem/ initiating cells. While many parts within this multi-step cascade are still poorly understood, this review article predominantly focusses on the intracellular necessities for fusion among cancer cells or with other cell populations of the tumour microenvironment. Video Abstract.

Mammalian fertilization: Does sperm IZUMO1 mediate fusion as well as adhesion?

The molecular mechanism of sperm-egg fusion is a long-standing mystery in reproduction. Brukman and colleagues (2022. J. Cell Biol.https://doi.org/10.1083/jcb.202207147) now provide evidence that the sperm surface protein IZUMO1, which is essential for mammalian fertilization, can induce membrane fusion in cultured cells.

A focus on yeast mating: From pheromone signaling to cell-cell fusion

Cells live in a chemical environment and are able to orient towards chemical cues. Unicellular haploid fungal cells communicate by secreting pheromones to reproduce sexually. In the yeast models Saccharomyces cerevisiae and Schizosaccharomyces pombe, pheromonal communication activates similar pathways composed of cognate G-protein-coupled receptors and downstream small GTPase Cdc42 and MAP kinase cascades. Local pheromone release and sensing, at a mobile surface polarity patch, underlie spatial gradient interpretation to form pairs between two cells of distinct mating types. Concentration of secretion at the point of cell-cell contact then leads to local cell wall digestion for cell fusion, forming a diploid zygote that prevents further fusion attempts. A number of asymmetries between mating types may promote efficiency of the system. In this review, we present our current knowledge of pheromone signaling in the two model yeasts, with an emphasis on how cells decode the pheromone signal spatially and ultimately fuse together. Though overall pathway architectures are similar in the two species, their large evolutionary distance allows to explore how conceptually similar solutions to a general biological problem can arise from divergent molecular components.

Extracellular Events Involved in Cancer Cell-Cell Fusion

Fusion among different cell populations represents a rare process that is mediated by both intrinsic and extracellular events. Cellular hybrid formation is relayed by orchestrating tightly regulated signaling pathways that can involve both normal and neoplastic cells. Certain important cell merger processes are often required during distinct organismal and tissue development, including placenta and skeletal muscle. In a neoplastic environment, however, cancer cell fusion can generate new cancer hybrid cells. Following survival during a subsequent post-hybrid selection process (PHSP), the new cancer hybrid cells express different tumorigenic properties. These can include elevated proliferative capacity, increased metastatic potential, resistance to certain therapeutic compounds, and formation of cancer stem-like cells, all of which characterize significantly enhanced tumor plasticity. However, many parts within this multi-step cascade are still poorly understood. Aside from intrinsic factors, cell fusion is particularly affected by extracellular conditions, including an inflammatory microenvironment, viruses, pH and ionic stress, hypoxia, and exosome signaling. Accordingly, the present review article will primarily highlight the influence of extracellular events that contribute to cell fusion in normal and tumorigenic tissues.

Eukaryotic fertilization and gamete fusion at a glance

In sexually reproducing organisms, the genetic information is transmitted from one generation to the next via the merger of male and female gametes. Gamete fusion is a two-step process involving membrane recognition and apposition through ligand-receptor interactions and lipid mixing mediated by fusion proteins. HAP2 (also known as GCS1) is a bona fide gamete fusogen in flowering plants and protists. In vertebrates, a multitude of surface proteins have been demonstrated to be pivotal for sperm-egg fusion, yet none of them exhibit typical fusogenic features. In this Cell Science at a Glance article and the accompanying poster, we summarize recent advances in the mechanistic understanding of gamete fusion in eukaryotes, with a particular focus on mammalian species.

The origins and formation of bone-resorbing osteoclasts

Osteoclasts (OCLs) are hematopoietic cells whose physiological function is to degrade bone. OCLs are key players in the processes that determine and maintain the mass, shape, and physical properties of bone. OCLs adhere to bone tightly and degrade its matrix by secreting protons and proteases onto the underlying surface. The combination of low pH and proteases degrades the mineral and protein components of the matrix and forms a resorption pit; the degraded material is internalized by the cell and then secreted into the circulation. Insufficient or excessive activity of OCLs can lead to significant changes in bone and either cause or exacerbate symptoms of diseases, as in osteoporosis, osteopetrosis, and cancer-induced bone lysis. OCLs are derived from monocyte-macrophage precursor cells whose origins are in two distinct embryonic cell lineages - erythromyeloid progenitor cells of the yolk sac, and hematopoietic stem cells. OCLs are formed in a multi-stage process that is induced by the cytokines M-CSF and RANKL, during which the cells differentiate, fuse to form multi-nucleated cells, and then differentiate further to become mature, bone-resorbing OCLs. Recent studies indicate that OCLs can undergo fission in vivo to generate smaller cells, called "osteomorphs", that can be "re-cycled" by fusing with other cells to form new OCLs. In this review we describe OCLs and discuss their cellular origins and the cellular and molecular events that drive osteoclastogenesis.

Detailed Molecular Interactions between Respiratory Syncytial Virus Fusion Protein and the TLR4/MD-2 Complex In Silico

Molecular interactions between respiratory syncytial virus (RSV) fusion protein (F protein) and the cellular receptor Toll-like receptor 4 (TLR4) and myeloid differentiation factor-2 (MD-2) protein complex are unknown. Thus, to reveal the detailed molecular interactions between them, in silico analyses were performed using various bioinformatics techniques. The present simulation data showed that the neutralizing antibody (NT-Ab) binding sites in both prefusion and postfusion proteins at sites II and IV were involved in the interactions between them and the TLR4 molecule. Moreover, the binding affinity between postfusion proteins and the TLR4/MD-2 complex was higher than that between prefusion proteins and the TLR4/MD-2 complex. This increased binding affinity due to conformational changes in the F protein may be able to form syncytium in RSV-infected cells. These results may contribute to better understand the infectivity and pathogenicity (syncytium formation) of RSV.

Malaria parasites utilize two essential plasma membrane fusogens for gamete fertilization

Cell fusion of female and male gametes is the climax of sexual reproduction. In many organisms, the Hapless 2 (HAP2) family of proteins play a critical role in gamete fusion. We find that Plasmodium falciparum, the causative agent of human malaria, expresses two HAP2 proteins: PfHAP2 and PfHAP2p. These proteins are present in stage V gametocytes and localize throughout the flagellum of male gametes. Gene deletion analysis and genetic crosses show that PfHAP2 and PfHAP2p individually are essential for male fertility and thereby, parasite transmission to the mosquito. Using a cell fusion assay, we demonstrate that PfHAP2 and PfHAP2p are both authentic plasma membrane fusogens. Our results establish nonredundant essential roles for PfHAP2 and PfHAP2p in mediating gamete fusion in Plasmodium and suggest avenues in the design of novel strategies to prevent malaria parasite transmission from humans to mosquitoes.

Longevity-promoting properties of ginger extract in Caenorhabditis elegans via the insulin/IGF-1 signaling pathway

Ginger is a traditional medicinal and edible plant with multiple health-promoting properties. Nevertheless, the effects and potential mechanism of ginger on antiaging remain unknown. The aim of this study was to comprehend the antiaging effects and potential mechanism of ginger in Caenorhabditis elegans (C. elegans). The current findings showed that the lifespan of C. elegans was prolonged by 23.16% with the supplementation of 60 μg mL-1 ginger extract (GE), and the extension of lifespan was mainly attributed to the major bioactive compounds in GE, 6-, 8-, 10-gingerol and 6-, 8-, 10-shogaol. Subsequently, GE promoted healthy aging by improving nematode movement and attenuating lipofuscin accumulation, and enhanced stress tolerance by up-regulating the expression of stress-related genes and activating DAF-16 and SKN-1. Moreover, lifespan assays of relative mutants revealed that GE mediated extension of lifespan via the insulin/IGF-1 signaling (IIS) pathway. In summary, GE endowed nematodes (C. elegans) with longevity and stress resistance in an IIS pathway dependent manner.

Uncovering an ancestral green ménage à trois: Contributions of Chlamydomonas to the discovery of a broadly conserved triad of plant fertilization proteins

During sexual reproduction in the unicellular green alga Chlamydomonas, gametes undergo the conserved cellular events that define fertilization across the tree of life. After initial ciliary adhesion, plus and minus gametes attach to each other at plasma membrane sites specialized for fusion, their bilayers merge, and cell coalescence into a quadri-ciliated cell signals for nuclear fusion. Recent findings show that these conserved cellular events are driven by 3 conserved protein families, FUS1/GEX2, HAP2/GCS1, and KAR5/GEX1. New results also show that species-specific recognition in Chlamydomonas activates the ancestral, viral-like fusogen HAP2 to drive fusion; that the conserved nuclear envelope fusion protein KAR5/GEX1 is also essential for nuclear fusion in Arabidopsis; and that heterodimerization of BELL-KNOX proteins signals for nuclear fusion in Chlamydomonas through early diverging land plants. This review outlines how Chlamydomonas's Janus-like position in evolution along with the ease of working with its gametes have revealed broadly conserved mechanisms.

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.

Little is known of how the architectural complexity of the neural stem cell niche is achieved. Rujano et al. show that the morphogenesis of a glial niche in Drosophila involves complex proliferative strategies and atypical cell–cell fusion.

Implications of Polyploidy and Ploidy Alterations in Hepatocytes in Liver Injuries and Cancers

Polyploidy, a condition in which more than two sets of chromosomes are present in a cell, is a characteristic feature of hepatocytes. A significant number of hepatocytes physiologically undergo polyploidization at a young age. Polyploidization of hepatocytes is enhanced with age and in a diseased liver. It is worth noting that polyploid hepatocytes can proliferate, in marked contrast to other types of polyploid cells, such as megakaryocytes and cardiac myocytes. Polyploid hepatocytes divide to maintain normal liver homeostasis and play a role in the regeneration of the damaged liver. Furthermore, polyploid hepatocytes have been shown to dynamically reduce ploidy during liver regeneration. Although it is still unclear why hepatocytes undergo polyploidization, accumulating evidence has revealed that alterations in the ploidy in hepatocytes are involved in the pathophysiology of liver cirrhosis and carcinogenesis. This review discusses the significance of hepatocyte ploidy in physiological liver function, liver injury, and liver cancer.

Plasma membrane protrusions mediate host cell-cell fusion induced by Burkholderia thailandensis

Cell-cell fusion is important for biological processes including fertilization, development, immunity, and microbial pathogenesis. Bacteria in the pseudomallei group of the Burkholderia species, including B. thailandensis, spread between host cells by inducing cell-cell fusion. Previous work showed that B. thailandensis-induced cell-cell fusion requires intracellular bacterial motility and a bacterial protein secretion apparatus called the type VI secretion system-5 (T6SS-5), including the T6SS-5 protein VgrG5. However, the cellular-level mechanism of and T6SS-5 proteins important for bacteria-induced cell-cell fusion remained incompletely described. Using live-cell imaging, we found bacteria used actin-based motility to push on the host cell plasma membrane to form plasma membrane protrusions that extended into neighboring cells. Then, membrane fusion occurred within membrane protrusions either proximal to the bacterium at the tip or elsewhere within protrusions. Expression of VgrG5 by bacteria within membrane protrusions was required to promote cell-cell fusion. Furthermore, a second predicted T6SS-5 protein, TagD5, was also required for cell-cell fusion. In the absence of VgrG5 or TagD5, bacteria in plasma membrane protrusions were engulfed into neighboring cells. Our results suggest that the T6SS-5 effectors VgrG5 and TagD5 are secreted within membrane protrusions and act locally to promote membrane fusion.

Impaired activity of the fusogenic micropeptide Myomixer causes myopathy resembling Carey-Fineman-Ziter syndrome

Skeletal muscle fibers contain hundreds of nuclei, which increase the overall transcriptional activity of the tissue and perform specialized functions. Multinucleation occurs through myoblast fusion, mediated by the muscle fusogens Myomaker (MYMK) and Myomixer (MYMX). We describe a human pedigree harboring a recessive truncating variant of the MYMX gene that eliminates an evolutionarily conserved extracellular hydrophobic domain of MYMX, thereby impairing fusogenic activity. Homozygosity of this human variant resulted in a spectrum of abnormalities that mimicked the clinical presentation of Carey-Fineman-Ziter syndrome (CFZS), caused by hypomorphic MYMK variants. Myoblasts generated from patient-derived induced pluripotent stem cells displayed defective fusion, and mice bearing the human MYMX variant died perinatally due to muscle abnormalities. In vitro assays showed that the human MYMX variant conferred minimal cell-cell fusogenicity, which could be restored with CRISPR/Cas9-mediated base editing, thus providing therapeutic potential for this disorder. Our findings identify MYMX as a recessive, monogenic human disease gene involved in CFZS, and provide new insights into the contribution of myoblast fusion to neuromuscular diseases.

Generation of Cancer Stem/Initiating Cells by Cell-Cell Fusion

CS/ICs have raised great expectations in cancer research and therapy, as eradication of this key cancer cell type is expected to lead to a complete cure. Unfortunately, the biology of CS/ICs is rather complex, since no common CS/IC marker has yet been identified. Certain surface markers or ALDH1 expression can be used for detection, but some studies indicated that cancer cells exhibit a certain plasticity, so CS/ICs can also arise from non-CS/ICs. Another problem is intratumoral heterogeneity, from which it can be inferred that different CS/IC subclones must be present in the tumor. Cell–cell fusion between cancer cells and normal cells, such as macrophages and stem cells, has been associated with the generation of tumor hybrids that can exhibit novel properties, such as an enhanced metastatic capacity and even CS/IC properties. Moreover, cell–cell fusion is a complex process in which parental chromosomes are mixed and randomly distributed among daughter cells, resulting in multiple, unique tumor hybrids. These, if they have CS/IC properties, may contribute to the heterogeneity of the CS/IC pool. In this review, we will discuss whether cell–cell fusion could also lead to the origin of different CS/ICs that may expand the overall CS/IC pool in a primary tumor.

Inducible knockout of syncytin-a leads to poor placental glucose transport in mice

INTRODUCTION

Cell-cell fusion of cytotrophoblasts into the syncytiotrophoblast layer is a key process in placental development. Syncytin, an endogenous retroviral envelope protein, is expressed in placental trophoblasts and specifically mediates syncytiotrophoblast layer formation. Syncytin deficiency has been observed in fetal growth-restricted placentas. Abnormal fetal growth, especially fetal growth restriction, is associated with the decreased expression of glucose transporters. Here, we aimed to determine the role of syncytin in fetal growth restriction in placental glucose transport capacity.

METHODS

To better explore the function of syncytin in fetal growth-restricted placenta, we generated an inducible knockout mouse model of syncytin-a gene. The expression levels of glucose transporters in BeWo cells were measured before and after HERV-W knockdown.

RESULTS

Syncytin-A disruption was associated with significant abnormalities in placental and fetal development in mice. Syncytin-A destruction causes extensive abnormalities in the maternal-fetal exchange structures in the labyrinth, including an extremely reduced number and dramatically irregular distribution of fetal vessels. Moreover, glucose transporter 1, glucose transporters 3, and connexin 26 expression levels decreased after E14.5. Consistently, low glucose transporter 1, glucose transporter 3, and connexin 26 levels were observed in HERV-W-silenced BeWo cells.

DISCUSSION

Syncytin-A is crucial for both syncytiotrophoblast layer development and morphogenesis, suggesting that syncytin-A disruption leads to fetal growth restriction associated with abnormalities in the maternal-fetal exchange barrier and decreased glucose transport.

Regulation of the myoblast fusion reaction for muscle development, regeneration, and adaptations

Fusion of plasma membranes is essential for skeletal muscle development, regeneration, exercise-induced adaptations, and results in a cell that contains hundreds to thousands of nuclei within a shared cytoplasm. The differentiation process in myocytes culminates in their fusion to form a new myofiber or fusion to an existing myofiber thereby contributing more synthetic material to the syncytium. The choice for two cells to fuse and become one could be a dangerous event if the two cells are not committed to an allied function. Thus, fusion events are highly regulated with positive and negative factors to fine-tune the process, and requires muscle-specific fusogens (Myomaker and Myomerger) as well as general cellular machinery to achieve the union of membranes. While a unified vertebrate myoblast fusion pathway is not yet established, recent discoveries should make this pursuit attainable. Not only does myocyte fusion impact the normal biology of skeletal muscle, but new evidence indicates dysregulation of the process impacts pathologies of skeletal muscle. Here, I will highlight the molecular players and biochemical mechanisms that drive fusion events in muscle, and discuss how this key myogenic process impacts skeletal muscle diseases.

Involvement of cellular protrusions in gamete interactions

Gamete fusion is of considerable importance in reproductive events, as it determines the gamete pairs or chromosomes that the next generation will inherit. To preserve species specificity with an appropriate karyotype, the fusion between gametes requires regulatory mechanisms to ensure limited fusion competency. In many organisms, gamete surfaces are not smooth, but present constitutive or transient cellular protrusions suggested to be involved in gamete fusion. However, the molecular mechanisms and the factors essential for the membrane-membrane fusion process and cellular protrusion involvement have remained unclear. Recent advances in the identification and functional analysis of the essential factors for gamete interaction have revealed the molecular mechanisms underlying their activity regulation and dynamics. In homogametic fertilization, dynamic regulation of the fusion core machinery on cellular protrusions was precisely uncovered. In heterogametic fertilization, oocyte fusion competency was suggested to correlate with the compartmentalization of the fusion essential factor and protrusion formation. These findings shed light on the significance of cellular protrusions in gamete fusion as a physically and functionally specialized site for cellular fusion. In this review, we consider the developments in gamete interaction research in various species with different fertilization modes, highlighting the commonalities in the relationship between gamete fusion and cellular protrusions.

Cell Fusion-Related Proteins and Signaling Pathways, and Their Roles in the Development and Progression of Cancer

Cell fusion is involved in many physiological and pathological processes, including gamete binding, and cancer development. The basic processes of cell fusion include membrane fusion, cytoplasmic mixing, and nuclear fusion. Cell fusion is regulated by different proteins and signaling pathways. Syncytin-1, syncytin-2, glial cell missing 1, galectin-1 and other proteins (annexins, myomaker, myomerger etc.) involved in cell fusion via the cyclic adenosine-dependent protein kinase A, mitogen-activated protein kinase, wingless/integrase-1, and c-Jun N-terminal kinase signaling pathways. In the progression of malignant tumors, cell fusion is essential during the organ-specific metastasis, epithelial-mesenchymal transformation, the formation of cancer stem cells (CSCs), cancer angiogenesis and cancer immunity. In addition, diploid cells can be induced to form polyploid giant cancer cells (PGCCs) via cell fusion under many kinds of stimuli, including cobalt chloride, chemotherapy, radiotherapy, and traditional Chinese medicine. PGCCs have CSC-like properties, and the daughter cells derived from PGCCs have a mesenchymal phenotype and exhibit strong migration, invasion, and proliferation abilities. Therefore, exploring the molecular mechanisms of cell fusion can enable us better understand the development of malignant tumors. In this review, the basic process of cell fusion and its significance in cancer is discussed.

Fusexins, HAP2/GCS1 and Evolution of Gamete Fusion

Gamete fusion is the climax of fertilization in all sexually reproductive organisms, from unicellular fungi to humans. Similarly to other cell-cell fusion events, gamete fusion is mediated by specialized proteins, named fusogens, that overcome the energetic barriers during this process. In recent years, HAPLESS 2/GENERATIVE CELL-SPECIFIC 1 (HAP2/GCS1) was identified as the fusogen mediating sperm-egg fusion in flowering plants and protists, being both essential and sufficient for the membrane merger in some species. The identification of HAP2/GCS1 in invertebrates, opens the possibility that a similar fusogen may be used in vertebrate fertilization. HAP2/GCS1 proteins share a similar structure with two distinct families of exoplasmic fusogens: the somatic Fusion Family (FF) proteins discovered in nematodes, and class II viral glycoproteins (e.g., rubella and dengue viruses). Altogether, these fusogens form the Fusexin superfamily. While some attributes are shared among fusexins, for example the overall structure and the possibility of assembly into trimers, some other characteristics seem to be specific, such as the presence or not of hydrophobic loops or helices at the distal tip of the protein. Intriguingly, HAP2/GCS1 or other fusexins have neither been identified in vertebrates nor in fungi, raising the question of whether these genes were lost during evolution and were replaced by other fusion machinery or a significant divergence makes their identification difficult. Here, we discuss the biology of HAP2/GCS1, its involvement in gamete fusion and the structural, mechanistic and evolutionary relationships with other fusexins.

Cell-Cell Mating Interactions: Overview and Potential of Single-Cell Force Spectroscopy

It is an understatement that mating and DNA transfer are key events for living organisms. Among the traits needed to facilitate mating, cell adhesion between gametes is a universal requirement. Thus, there should be specific properties for the adhesion proteins involved in mating. Biochemical and biophysical studies have revealed structural information about mating adhesins, as well as their specificities and affinities, leading to some ideas about these specialized adhesion proteins. Recently, single-cell force spectroscopy (SCFS) has added important findings. In SCFS, mating cells are brought into contact in an atomic force microscope (AFM), and the adhesive forces are monitored through the course of mating. The results have shown some remarkable characteristics of mating adhesins and add knowledge about the design and evolution of mating adhesins.

Drosophila melanogaster: A Model System to Study Distinct Genetic Programs in Myoblast Fusion

Muscle fibers are multinucleated cells that arise during embryogenesis through the fusion of mononucleated myoblasts. Myoblast fusion is a lifelong process that is crucial for the growth and regeneration of muscles. Understanding the molecular mechanism of myoblast fusion may open the way for novel therapies in muscle wasting and weakness. Recent reports in Drosophila and mammals have provided new mechanistic insights into myoblast fusion. In Drosophila, muscle formation occurs twice: during embryogenesis and metamorphosis. A fundamental feature is the formation of a cell–cell communication structure that brings the apposing membranes into close proximity and recruits possible fusogenic proteins. However, genetic studies suggest that myoblast fusion in Drosophila is not a uniform process. The complexity of the players involved in myoblast fusion can be modulated depending on the type of muscle that is formed. In this review, we introduce the different types of multinucleated muscles that form during Drosophila development and provide an overview in advances that have been made to understand the mechanism of myoblast fusion. Finally, we will discuss conceptual frameworks in cell–cell fusion in Drosophila and mammals.

TMEM8C-mediated fusion is regionalized and regulated by NOTCH signalling during foetal myogenesis

The location and regulation of fusion events within skeletal muscles during development remain unknown. Using the fusion marker myomaker (Mymk), named TMEM8C in chicken, as a readout of fusion, we identified a co-segregation of TMEM8C-positive cells and MYOG-positive cells in single-cell RNA-sequencing datasets of limbs from chicken embryos. We found that TMEM8C transcripts, MYOG transcripts and the fusion-competent MYOG-positive cells were preferentially regionalized in central regions of foetal muscles. We also identified a similar regionalization for the gene encoding the NOTCH ligand JAG2 along with an absence of NOTCH activity in TMEM8C+ fusion-competent myocytes. NOTCH function in myoblast fusion had not been addressed so far. We analysed the consequences of NOTCH inhibition for TMEM8C expression and myoblast fusion during foetal myogenesis in chicken embryos. NOTCH inhibition increased myoblast fusion and TMEM8C expression and released the transcriptional repressor HEYL from the TMEM8C regulatory regions. These results identify a regionalization of TMEM8C-dependent fusion and a molecular mechanism underlying the fusion-inhibiting effect of NOTCH in foetal myogenesis. The modulation of NOTCH activity in the fusion zone could regulate the flux of fusion events.

Sperm IZUMO1 Is Required for Binding Preceding Fusion With Oolemma in Mice and Rats

Fertilization occurs as the culmination of multi-step complex processes. First, mammalian spermatozoa undergo the acrosome reaction to become fusion-competent. Then, the acrosome-reacted spermatozoa penetrate the zona pellucida and adhere to and finally fuse with the egg plasma membrane. IZUMO1 is the first sperm protein proven to be essential for sperm-egg fusion in mammals, as Izumo1 knockout mouse spermatozoa adhere to but fail to fuse with the oolemma. However, the IZUMO1 function in other species remains largely unknown. Here, we generated Izumo1 knockout rats by CRISPR/Cas9 and found the male rats were infertile. Unlike in mice, Izumo1 knockout rat spermatozoa failed to bind to the oolemma. Further investigation revealed that the acrosome-intact sperm binding conceals a decreased number of the acrosome-reacted sperm bound to the oolemma in Izumo1 knockout mice. Of note, we could not see any apparent defects in the binding of the acrosome-reacted sperm to the oolemma in the mice lacking recently found fusion-indispensable genes, Fimp, Sof1, Spaca6, or Tmem95. Collectively, our data suggest that IZUMO1 is required for the sperm-oolemma binding prior to fusion at least in rat.

ERK1/2 inhibition promotes robust myotube growth via CaMKII activation resulting in myoblast-to-myotube fusion

Myoblast fusion is essential for muscle development and regeneration. Yet, it remains poorly understood how mononucleated myoblasts fuse with preexisting fibers. We demonstrate that ERK1/2 inhibition (ERKi) induces robust differentiation and fusion of primary mouse myoblasts through a linear pathway involving RXR, ryanodine receptors, and calcium-dependent activation of CaMKII in nascent myotubes. CaMKII activation results in myotube growth via fusion with mononucleated myoblasts at a fusogenic synapse. Mechanistically, CaMKII interacts with and regulates MYMK and Rac1, and CaMKIIδ/γ knockout mice exhibit smaller regenerated myofibers following injury. In addition, the expression of a dominant negative CaMKII inhibits the formation of large multinucleated myotubes. Finally, we demonstrate the evolutionary conservation of the pathway in chicken myoblasts. We conclude that ERK1/2 represses a signaling cascade leading to CaMKII-mediated fusion of myoblasts to myotubes, providing an attractive target for the cultivated meat industry and regenerative medicine.

Major Evolutionary Transitions and the Roles of Facilitation and Information in Ecosystem Transformations

A small number of extraordinary “Major Evolutionary Transitions” (METs) have attracted attention among biologists. They comprise novel forms of individuality and information, and are defined in relation to organismal complexity, irrespective of broader ecosystem-level effects. This divorce between evolutionary and ecological consequences qualifies unicellular eukaryotes, for example, as a MET although they alone failed to significantly alter ecosystems. Additionally, this definition excludes revolutionary innovations not fitting into either MET type (e.g., photosynthesis). We recombine evolution with ecology to explore how and why entire ecosystems were newly created or radically altered – as Major System Transitions (MSTs). In doing so, we highlight important morphological adaptations that spread through populations because of their immediate, direct-fitness advantages for individuals. These are Major Competitive Transitions, or MCTs. We argue that often multiple METs and MCTs must be present to produce MSTs. For example, sexually-reproducing, multicellular eukaryotes (METs) with anisogamy and exoskeletons (MCTs) significantly altered ecosystems during the Cambrian. Therefore, we introduce the concepts of Facilitating Evolutionary Transitions (FETs) and Catalysts as key events or agents that are insufficient themselves to set a MST into motion, but are essential parts of synergies that do. We further elucidate the role of information in MSTs as transitions across five levels: (I) Encoded; (II) Epigenomic; (III) Learned; (IV) Inscribed; and (V) Dark Information. The latter is ‘authored’ by abiotic entities rather than biological organisms. Level IV has arguably allowed humans to produce a MST, and V perhaps makes us a FET for a future transition that melds biotic and abiotic life into one entity. Understanding the interactive processes involved in past major transitions will illuminate both current events and the surprising possibilities that abiotically-created information may produce.

Cell fusion as a link between the SARS-CoV-2 spike protein, COVID-19 complications, and vaccine side effects

A distinctive feature of the SARS-CoV-2 spike protein is its ability to efficiently fuse cells, thus producing syncytia found in COVID-19 patients. This commentary proposes how this ability enables spike to cause COVID-19 complications as well as side effects of COVID-19 vaccines, and suggests how these effects can be prevented.

Cell-Cell Fusion Mediated by Viruses and HERV-Derived Fusogens in Cancer Initiation and Progression

Cell fusion is a well-known, but still scarcely understood biological phenomenon, which might play a role in cancer initiation, progression and formation of metastases. Although the merging of two (cancer) cells appears simple, the entire process is highly complex, energy-dependent and tightly regulated. Among cell fusion-inducing and -regulating factors, so-called fusogens have been identified as a specific type of proteins that are indispensable for overcoming fusion-associated energetic barriers and final merging of plasma membranes. About 8% of the human genome is of retroviral origin and some well-known fusogens, such as syncytin-1, are expressed by human (cancer) cells. Likewise, enveloped viruses can enable and facilitate cell fusion due to evolutionarily optimized fusogens, and are also capable to induce bi- and multinucleation underlining their fusion capacity. Moreover, multinucleated giant cancer cells have been found in tumors derived from oncogenic viruses. Accordingly, a potential correlation between viruses and fusogens of human endogenous retroviral origin in cancer cell fusion will be summarized in this review.