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    Topic review

    Cells Fusion

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    Submitted by: Thomas Dittmar


    The biological phenomenon of cell fusion remains a mystery. Even though it is mandatory for several physiological and pathopyhsiological processes considerably less is still known how the merging of two (and more) cells is regulated. Cells are not fusogenic per se. They first have to be converted into a pro-fusogenic state and have to re-enter to a non-fusogenic state after hybridisation. Likewise, different cell fusion mechanisms have been developed during evolution depending on different proteins and different membrane merging strategies. This entry gives a brief overview about those molecules and conditions that direct cell fusion.

    1. Introduction

    Although different physiological processes, such as fertilization, placentation, myogenesis, osteoclastogenesis, and tissue regeneration, depend on cell fusion, the mechanism by which two (or more) cells hybridize is still not well understood [1][2][3][4]. On the one hand, cell fusion is a tightly regulated process that can be subdivided into five steps: i) priming, ii) chemotaxis, iii) adhesion, iv) fusion, and v) postfusion [5].

    2. How Do Cells Fuse with Each Other? 

    Cells are not fusogenic per se, so they have to adopt a pro-fusogenic state first in order to fuse with other cells (“priming”). Subsequently, they have to get in close contact with each other (“chemotaxis” and “adhesion”) before they can merge plasma membranes (“fusion”). Finally, they have to return to a non-fusogenic state after the fusion process (“post-fusion”). Several proteins, such as chemokines, cytokines, proteases, adhesion molecules, transmembrane proteins or proteins that are mandatory for actin remodeling, have been identified so far that mediate distinct steps in this cell fusion cascade. However, it remains to be elucidated how cytokines, such as interleukin-4 (IL-4) or receptor activator of NF-kB ligand (RANKL), or proteases, such as matrix metallopeptidase 9 (MMP-9), are exactly involved in the process of cell fusion (for review see [1][2][5]).

    In addition, different cell fusion mechanisms have been developed during evolution. For instance, the fusion of trophoblasts to syncytiotrophoblasts is chiefly regulated by Syncytin-1 and -2, which are transmembrane proteins of retroviral origin [6][7]. Syncytin-1 and -2 are still the best characterized cell fusion mediating proteins in humans, and they might also be involved in human osteoclast fusion [8] and in the fusion of cancer cells with endothelial cells [9][10] or mesenchymal stem cells [11]. In contrast, fusion of myoblasts to multinucleated myofibers depends on remodeling of the actin cytoskeleton and formation of podosome-like structures, which penetrate the target cell, thereby causing the merging of plasma membranes [12][13]. Likewise, several proteins, such as MMP-9, E-Cadherin, Syncytin-1, CD200, dendrocyte expressed seven transmembrane protein (DC-STAMP), osteoclast stimulatory transmembrane protein (OC-STAMP), CD44, and P2X7, have been identified that play a role in macrophage fusion (for review see: [1][2][4]). It is also known that the expression of these proteins is induced by cytokines, such as IL-4 and RANKL [14][15][16][17][18], suggesting that these factors are likely involved in the transition of macrophages from a non-fusogenic to a pro-fusogenic state. Nonetheless, the detailed process of macrophage fusion remains unclear.

    Numerous studies further showed that the frequency of cell fusion events was increased upon acute tissue damage or chronic inflammation [19][20][21][22][10][11][23][24][25][26], which is plausible with regard to efficient BMSC-based and cell fusion-mediated tissue regeneration. BMSCs not only have to be converted into a pro-fusogenic state for subsequent hybridization with target cells but also have to be recruited to the site of tissue damage. In this context, it has been shown that the pro-inflammatory cytokine tumor necrosis factor-α (TNF-α) might also be a mediator of cell fusion. Osteoclastogenesis [17][25][26], as well as the fusion of cancer cells with endothelial cells [10][27], mesenchymal stem cells [11], or breast epithelial cells [23][24][28] can be induced by TNF-α. Some data revealed that TNF-α could mediate fusion due to induction of MMP-9 expression [24][26], which plays a role in osteoclastogenesis and giant cell formation [26][29]. Hence, it might be assumed that TNF-α could be involved in cell fusion due to induction of pro-fusogenic proteins and/or in an overall conversion of cells into a pro-fusogenic state.

    In addition to inflammation-induced BMSC-based cell fusion events, two studies revealed that cell fusion events could also occur in the absence of tissue damage and inflammation [21][30]. Using a parabiotic model (a green fluorescent protein (GFP) mouse and a ROSA/β-gal mouse were surgically joined), administration of an anti-inflammatory drug cocktail was found to promote cell fusion-derived GFP/β-Gal positive cells, which were found in approximately 5% of the intestinal crypts of ROSA/β-Gal mice [21]. Likewise, noninflammation-related fusion events were found with a frequency of approximately 0.03 to 0.21% in the murine hematopoietic system [30]. Interestingly, examination of donor and host autosomal reporter genes (hCD46, mX, CD45.2, GFP, mY, and CD45.1) revealed independent segregation of alleles in more than half of the fusion products, and a loss of parental markers was even observed in some cells [30]. However, despite these genetic changes, neither lineage restriction nor malignant conversion of hematopoietic cells was observed [30]. Whether this indicates that hematopoietic cells might be more tolerant to limited chromosomal sequence gains than other cells [30] remains to be elucidated.

    Both viruses and exomes have also been associated with cell fusion [31][32][33][34]. In vitro and in vivo studies demonstrated that enveloped and non-enveloped viruses could cause cell fusion (so-called fusogenic viruses), thereby giving rise to bi- and multinucleated heterokaryons (a detailed overview of fusogenic viruses is found here [35]). Enveloped viruses, such as HIV, influenza virus or herpesvirus, fuse with the plasma membrane of host cells (for review see [34]) and could cause cell hybridization by acting as bridging particles. For instance, hybridomas derived from plasma cells and myeloma cells were initially generated by using inactivated Sendai virus as a fusogen [36], which has the ability to induce bi- and multinucleated cell formation in vitro and in vivo [37]. Likewise, virus-infected cells could also fuse with other cells due to the expression of viral-derived fusogenic proteins. The nonenveloped fusogenic avian and Nelson Bay reoviruses could induce cell fusion via the expression of so-called fusion-associated small transmembrane (FAST) proteins that are localized in the plasma membrane of infected cells [38]. Binucleated cell formation by fusion was also induced by the human papillomavirus 16 oncogene E5 [39].

    Exosomes are a type of extracellular vesicle with a diameter of less than 100 nm, and they originate from the invagination of the lipid bilayer of multivesicular bodies in cells (for review see [40][41]). They typically contain tetraspanins (CD9, CD63, CD81, and CD82), heat shock proteins (HSC20, HSP60, HSP70, and HSP909), MHC-I and MHC-II, cell adhesion molecules (P-Selectin, αβ-integrins and annexins), and significant amounts of mRNA, miRNA, and lncRNA (for review see [40][41]). Exosomes play a crucial role in intercellular communication and the regulation of different physiological and pathophysiological conditions, whereby their payload could be delivered to target cells by endocytosis, phagocytosis or membrane fusion [41]. Duelli and colleagues demonstrated that exosomes isolated from virus-infected cells contained viral proteins and exhibited fusogenic properties, suggesting a possible role in cell fusion [31]. Miyado et al. further showed that exosomes might be involved in cell fusion by showing that sperm-egg fusion is mediated by vesicles containing CD9 that are released from the egg and interact with sperm [42]. Because CD9 is a major component of exosomes, the authors concluded that this type of extracellular vesicle was released to mediate fertilization [42].

    In brief, cell fusion is a tightly regulated but not yet fully understood process. Inflammation can induce cell fusion, which would be necessary for rapid and efficient BMSC-based tissue regeneration. However, cell fusion could also occur spontaneously after being triggered by viruses and/or exosomes.

    The entry is from 10.3390/ijms21051811


    1. Pablo Aguilar; Mary K. Baylies; Andre Fleissner; Laura Helming; Naokazu Inoue; Benjamin Podbilewicz; Hongmei Wang; Melissa Wong; Genetic basis of cell-cell fusion mechanisms.. Trends in Genetics 2013, 29, 427-37, 10.1016/j.tig.2013.01.011.
    2. Laura Helming; Siamon Gordon; Molecular mediators of macrophage fusion. Trends in Cell Biology 2009, 19, 514-522, 10.1016/j.tcb.2009.07.005.
    3. Matias Hernandez; Benjamin Podbilewicz; The hallmarks of cell-cell fusion. Development 2017, 144, 4481-4495, 10.1242/dev.155523.
    4. Lena Willkomm; Wilhelm Bloch; State of the Art in Cell–Cell Fusion. Bioinformatics for Cancer Immunotherapy 2015, 1313, 1-19, 10.1007/978-1-4939-2703-6_1.
    5. Xiaofeng Zhou; Jeffrey Platt; Molecular and Cellular Mechanisms of Mammalian Cell Fusion. Advances in Experimental Medicine and Biology 2011, 713, 33-64, 10.1007/978-94-007-0763-4_4.
    6. Huppertz, B.; Gauster,M.; Trophoblast fusion. Adv. Exp. Med. Biol 2011, 713, 81-95, .
    7. Sha Mi; Xinhua Lee; Xiang-Ping Li; Geertruida M. Veldman; Heather Finnerty; Lisa Racie; Edward LaVallie; Xiang-Yang Tang; Philippe Edouard; Steve Howes; et al. Syncytin is a captive retroviral envelope protein involved in human placental morphogenesis. Nature 2000, 403, 785-789, 10.1038/35001608.
    8. Kent Soe; T.L. Andersen; Anne-Sofie Hobolt-Pedersen; Bolette Bjerregaard; Lars-Inge Larsson; Jean-Marie Delaisse; Involvement of human endogenous retroviral syncytin-1 in human osteoclast fusion. Bone 2011, 48, 837-846, 10.1016/j.bone.2010.11.011.
    9. B. Bjerregaard; S. Holck; I. J. Christensen; L. -I. Larsson; Syncytin is involved in breast cancer-endothelial cell fusions. Cellular and Molecular Life Sciences 2006, 63, 1906-1911, 10.1007/s00018-006-6201-9.
    10. Ting-Lin Yan; Meng Wang; Zhi Xu; Chun-Ming Huang; Xiao-Cheng Zhou; Er-Hui Jiang; Xiao-Ping Zhao; Yong Song; Kai Song; Zhe Shao; et al. Up-regulation of syncytin-1 contributes to TNF-α-enhanced fusion between OSCC and HUVECs partly via Wnt/β-catenin-dependent pathway. Scientific Reports 2017, 7, 40983, 10.1038/srep40983.
    11. Catharina Melzer; Juliane Von Der Ohe; Ralf Hass; In Vitro Fusion of Normal and Neoplastic Breast Epithelial Cells with Human Mesenchymal Stroma/Stem Cells Partially Involves Tumor Necrosis Factor Receptor Signaling. STEM CELLS 2018, 36, 977-989, 10.1002/stem.2819.
    12. Susan M. Abmayr; Grace K. Pavlath; Myoblast fusion: lessons from flies and mice.. Development 2012, 139, 641-56, 10.1242/dev.068353.
    13. Adriana Simionescu; Grace K. Pavlath; Molecular Mechanisms of Myoblast Fusion Across Species. Advances in Experimental Medicine and Biology 2011, 713, 113-135, 10.1007/978-94-007-0763-4_8.
    14. Samir M. Abdelmagid; Gregory R. Sondag; Fouad M. Moussa; Joyce Y. Belcher; Bing Yu; Hilary Stinnett; Kimberly Novak; Thomas Mbimba; Matthew Khol; Kurt Hankenson; et al. Mutation in Osteoactivin Promotes Receptor Activator of NFκB Ligand (RANKL)-mediated Osteoclast Differentiation and Survival but Inhibits Osteoclast Function. Journal of Biological Chemistry 2015, 290, 20128-20146, 10.1074/jbc.m114.624270.
    15. Kofi A. Mensah; Christopher T. Ritchlin; Edward M. Schwarz; RANKL induces heterogeneous DC-STAMPloand DC-STAMPhiosteoclast precursors of which the DC-STAMPloprecursors are the master fusogens. Journal of Cellular Physiology 2010, 223, 76-83, 10.1002/jcp.22012.
    16. Jose L. Moreno; Irina Mikhailenko; Mehrdad M. Tondravi; Achsah D. Keegan; IL-4 promotes the formation of multinucleated giant cells from macrophage precursors by a STAT6-dependent, homotypic mechanism: contribution of E-cadherin. Journal of Leukocyte Biology 2007, 82, 1542-1553, 10.1189/jlb.0107058.
    17. Maria Papadaki; Vagelis Rinotas; Foteini Violitzi; Trias Thireou; George Panayotou; M. Samiotaki; Eleni Douni; New Insights for RANKL as a Proinflammatory Modulator in Modeled Inflammatory Arthritis. Frontiers in Immunology 2019, 10, 97, 10.3389/fimmu.2019.00097.
    18. Minjun Yu; Xiulan Qi; Jose L. Moreno; Donna L. Farber; Achsah D. Keegan; NF-κB signaling participates in both RANKL- and IL-4-induced macrophage fusion: receptor cross-talk leads to alterations in NF-κB pathways.. The Journal of Immunology 2011, 187, 1797-806, 10.4049/jimmunol.1002628.
    19. Xin Wang; Holger Willenbring; Yassmine Akkari; Yumi Torimaru; Mark Foster; Muhsen Al-Dhalimy; Eric Lagasse; Milton Finegold; Susan Olson; Markus Grompe; et al. Cell fusion is the principal source of bone-marrow-derived hepatocytes. Nature 2003, 422, 897-901, 10.1038/nature01531.
    20. Stuart J. Forbes; Myelomonocytic cells are sufficient for therapeutic cell fusion in the liver.. Journal of Hepatology 2005, 42, 285-6, 10.1016/j.jhep.2004.10.027.
    21. Paige S. Davies; Anne E. Powell; John R. Swain; Melissa H. Wong; Inflammation and Proliferation Act Together to Mediate Intestinal Cell Fusion. PLOS ONE 2009, 4, e6530, 10.1371/journal.pone.0006530.
    22. Jens M. Nygren; Karina Liuba; Martin Breitbach; Simon R. W. Stott; Lina Thorén; Wilhelm Roell; Caroline Geisen; Philipp Sasse; Deniz Kirik; Anders Björklund; et al. Myeloid and lymphoid contribution to non-haematopoietic lineages through irradiation-induced heterotypic cell fusion. Nature 2008, 10, 584-592, 10.1038/ncb1721.
    23. Julian Weiler; Thomas Dittmar; Minocycline impairs TNF-α-induced cell fusion of M13SV1-Cre cells with MDA-MB-435-pFDR1 cells by suppressing NF-κB transcriptional activity and its induction of target-gene expression of fusion-relevant factors.. Cell Communication and Signaling 2019, 17, 71, 10.1186/s12964-019-0384-9.
    24. Julian Weiler; Marieke Mohr; Kurt S. Zänker; Thomas Dittmar; Matrix metalloproteinase-9 (MMP9) is involved in the TNF-α-induced fusion of human M13SV1-Cre breast epithelial cells and human MDA-MB-435-pFDR1 cancer cells.. Cell Communication and Signaling 2018, 16, 14, 10.1186/s12964-018-0226-1.
    25. Hitoshi Hotokezaka; Eiko Sakai; Naoya Ohara; Yuka Hotokezaka; Carmen Gonzales; Ken-Ichiro Matsuo; Yuji Fujimura; Noriaki Yoshida; Koji Nakayama; Carmen Karadeniz; et al. Molecular analysis of RANKL-independent cell fusion of osteoclast-like cells induced by TNF-α, lipopolysaccharide, or peptidoglycan. Journal of Cellular Biochemistry 2007, 101, 122-134, 10.1002/jcb.21167.
    26. Eleni A. Skokos; Antonios Charokopos; Khadija Khan; Jackie Wanjala; Themis Kyriakides; Lack of TNF-α–Induced MMP-9 Production and Abnormal E-Cadherin Redistribution Associated with Compromised Fusion in MCP-1–Null Macrophages. The American Journal of Pathology 2011, 178, 2311-2321, 10.1016/j.ajpath.2011.01.045.
    27. Kai Song; Fei Zhu; Han-Zhong Zhang; Zheng-Jun Shang; Tumor necrosis factor-α enhanced fusions between oral squamous cell carcinoma cells and endothelial cells via VCAM-1/VLA-4 pathway. Experimental Cell Research 2012, 318, 1707-1715, 10.1016/j.yexcr.2012.05.022.
    28. Marieke Mohr; Songül Tosun; Wolfgang H. Arnold; Frank Edenhofer; Kurt S. Zänker; Thomas Dittmar; Quantification of cell fusion events human breast cancer cells and breast epithelial cells using a Cre-LoxP-based double fluorescence reporter system. Cellular and Molecular Life Sciences 2015, 72, 3769-3782, 10.1007/s00018-015-1910-6.
    29. Susan MacLauchlan; Eleni A. Skokos; Norman Meznarich; Dana H. Zhu; Sana Raoof; J. Michael Shipley; Robert M. Senior; Paul Bornstein; Themis Kyriakides; Macrophage fusion, giant cell formation, and the foreign body response require matrix metalloproteinase 9.. Journal of Leukocyte Biology 2009, 85, 617-26, 10.1189/jlb.1008588.
    30. Amy M. Skinner; Markus Grompe; Peter Kurre; Intra-hematopoietic cell fusion as a source of somatic variation in the hematopoietic system. Journal of Cell Science 2012, 125, 2837-2843, 10.1242/jcs.100123.
    31. Dominik M. Duelli; Stephen Hearn; Michael P. Myers; Yuri Lazebnik; A primate virus generates transformed human cells by fusion. The Journal of Cell Biology 2005, 171, 493-503, 10.1083/jcb.200507069.
    32. Okada, Yoshio; Sendai virus-induced cell fusion. Methods Enzymology 1993, 221, 18, .
    33. Michel Record; Intercellular communication by exosomes in placenta. Placenta 2014, 35, A4, 10.1016/j.placenta.2014.06.015.
    34. Benjamin Podbilewicz; Virus and Cell Fusion Mechanisms. Annual Review of Cell and Developmental Biology 2014, 30, 111-139, 10.1146/annurev-cellbio-101512-122422.
    35. Dominik Duelli; Yuri Lazebnik; Cell-to-cell fusion as a link between viruses and cancer. Nature Reviews Cancer 2007, 7, 968-976, 10.1038/nrc2272.
    36. G. Kohler; C. Milstein; G. K; Continuous cultures of fused cells secreting antibody of predefined specificity. Nature 1975, 256, 495-497, 10.1038/256495a0.
    37. Joanna Rawling; Olga Cano; Minique Garcin; Daniel Kolakofsky; José A. Melero; Recombinant Sendai Viruses Expressing Fusion Proteins with Two Furin Cleavage Sites Mimic the Syncytial and Receptor-Independent Infection Properties of Respiratory Syncytial Virus▿. Journal of Virology 2011, 85, 2771-2780, 10.1128/JVI.02065-10.
    38. Maya Shmulevitz; Roy Duncan; A new class of fusion-associated small transmembrane (FAST) proteins encoded by the non-enveloped fusogenic reoviruses. The EMBO Journal 2000, 19, 902-912, 10.1093/emboj/19.5.902.
    39. Lulin Hu; Kendra Plafker; Valeriya Vorozhko; Rosemary E. Zuna; Marie H. Hanigan; Gary Gorbsky; Scott M. Plafker; Peter C. Angeletti; Brian P. Ceresa; Human papillomavirus 16 E5 induces bi-nucleated cell formation by cell–cell fusion. Virology 2008, 384, 125-34, 10.1016/j.virol.2008.10.011.
    40. Saheli Samanta; Sheeja Rajasingh; Nicholas Drosos; Zhigang Zhou; Buddhadeb Dawn; Johnson Rajasingh; Exosomes: new molecular targets of diseases. Acta Pharmacologica Sinica 2017, 39, 501-513, 10.1038/aps.2017.162.
    41. Laura Mulcahy; Ryan Charles Pink; David Raul Francisco Carter; Routes and mechanisms of extracellular vesicle uptake. Journal of Extracellular Vesicles 2014, 3, 1093, 10.3402/jev.v3.24641.
    42. Kenji Miyado; Keiichi Yoshida; Kazuo Yamagata; Keiichi Sakakibara; Masaru Okabe; Xiaobiao Wang; Kiyoko Miyamoto; Hidenori Akutsu; Takahiko Kondo; Yuji Takahashi; et al. The fusing ability of sperm is bestowed by CD9-containing vesicles released from eggs in mice.. Proceedings of the National Academy of Sciences 2008, 105, 12921-6, 10.1073/pnas.0710608105.