Merkel cell polyomavirus (MCPyV) is a small, nonenveloped tumor virus associated with an aggressive form of skin cancer, Merkel cell carcinoma (MCC). MCPyV infections are highly prevalent in the human population, with MCPyV virions being continuously shed from human skin. However, the precise host cell tropism(s) of MCPyV remains unclear: MCPyV is able to replicate within a subset of dermal fibroblasts, but MCPyV DNA has also been detected in a variety of other tissues. However, MCPyV appears different from other polyomaviruses, as it requires sulfated polysaccharides, such as heparan sulfates and/or chondroitin sulfates, for initial attachment. Like other polyomaviruses, MCPyV engages sialic acid as a (co)receptor. To explore the infectious entry process of MCPyV, we analyzed the cell biological determinants of MCPyV entry into A549 cells, a highly transducible lung carcinoma cell line, in comparison to well-studied simian virus 40 and a number of other viruses. Our results indicate that MCPyV enters cells via caveolar/lipid raft-mediated endocytosis but not macropinocytosis, clathrin-mediated endocytosis, or glycosphingolipid-enriched carriers. The viruses were internalized in small endocytic pits that led the virus to endosomes and from there to the endoplasmic reticulum (ER). Similar to other polyomaviruses, trafficking required microtubular transport, acidification of endosomes, and a functional redox environment. To our surprise, the virus was found to acquire a membrane envelope within endosomes, a phenomenon not reported for other viruses. Only minor amounts of viruses reached the ER, while the majority was retained in endosomal compartments, suggesting that endosome-to-ER trafficking is a bottleneck during infectious entry. IMPORTANCE MCPyV is the first polyomavirus directly implicated in the development of an aggressive human cancer, Merkel cell carcinoma (MCC). Although MCPyV is constantly shed from healthy skin, the MCC incidence increases among aging and immunocompromised individuals. To date, the events connecting initial MCPyV infection and subsequent transformation still remain elusive. MCPyV differs from other known polyomaviruses concerning its cell tropism, entry receptor requirements, and infection kinetics. In this study, we examined the cellular requirements for endocytic entry as well as the subcellular localization of incoming virus particles. A thorough understanding of the determinants of the infectious entry pathway and the specific biological niche will benefit prevention of virus-derived cancers such as MCC.
27Merkel Cell Polyomavirus (MCPyV) is a small, non-enveloped tumor virus associated 28 with an aggressive form of skin cancer, the Merkel cell carcinoma (MCC). MCPyV 29 infections are highly prevalent in the human population with MCPyV virions being 30 continuously shed from human skin. However, the precise host cell tropism(s) of 31MCPyV remains unclear: MCPyV is able to replicate within a subset of dermal 32fibroblasts, but MCPyV DNA has also been detected in a variety of other tissues. 33However, MCPyV appears different from other polyomaviruses as it requires sulfated 34 polysaccharides such as heparan sulfates and/or chondroitin sulfates for initial 35 attachment. Like other polyomaviruses, MCPyV engages sialic acid as a (co-36 )receptor. To explore the infectious entry process of MCPyV, we analyzed the cell 37 biological determinants of MCPyV entry into A549 cells, a highly transducible lung 38 carcinoma cell line, in comparison to well-studied simian virus 40 and a number of 39 other viruses. Our results indicate that MCPyV enters cells via caveolar/lipid raft-40 mediated endocytosis but not macropinocytosis, clathrin-mediated endocytosis or 41 glycosphingolipid-enriched carriers. The viruses internalized in small endocytic pits 42 that led the virus to endosomes and from there to the endoplasmic reticulum (ER). 43Similar to other polyomaviruses, trafficking required microtubular transport, 44 acidification of endosomes, and a functional redox environment. To our surprise, the 45 virus was found to acquire a membrane envelope within endosomes, a phenomenon 46 not reported for other viruses. Only minor amounts of viruses reached the ER, while 47 the majority was retained in endosomal compartments suggesting that endosome-to-48 ER trafficking is a bottleneck during infectious entry. 49 50 51 52 3 Importance 53 MCPyV is the first polyomavirus directly implicated in the development of an 54 aggressive human cancer, the Merkel Cell Carcinoma (MCC). Although MCPyV is 55 constantly shed from healthy skin, MCC incidence increases among aging and 56 immunocompromised individuals. To date, the events connecting initial MCPyV 57 infection and subsequent transformation still remain elusive. MCPyV differs from 58 other known polyomaviruses concerning its cell tropism, entry receptor requirements, 59 and infection kinetics. In this study, we examined the cellular requirements for 60 endocytic entry as well as the subcellular localization of incoming virus particles. A 61 thorough understanding of the determinants of the infectious entry pathway and the 62 specific biological niche will benefit prevention of virus-derived cancers such as 63 MCC. 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 Polyomaviruses (PyV) are small, non-enveloped dsDNA viruses with a diameter of 80 45-50 nm. The icosahedral (T=7) capids consist of 72 homopentameric capsomers of 81 the major capid protein VP1 with minor capsid proteins VP2/VP3 located within a 82 cavity underneath the VP1 pentamers. The PyV capsid harbors a chromatinized, 83 circu...
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