Ganglioside GT1b is a putative host cell receptor for the Merkel cell polyomavirus

Merkel cell polyomavirus and two previously unknown polyomaviruses are chronically shed from human skin

Mounting evidence indicates that Merkel cell polyomavirus (MCV), a circular double-stranded DNA virus, is a causal factor underlying a highly lethal form of skin cancer known as Merkel cell carcinoma. To explore the possibility that MCV and other polyomaviruses commonly inhabit healthy human skin, we developed an improved rolling circle amplification (RCA) technique to isolate circular DNA viral genomes from human skin swabs. Complete MCV genomes were recovered from 40% of healthy adult volunteers tested, providing full-length, apparently wild-type cloned MCV genomes. RCA analysis also identified two previously unknown polyomavirus species that we name human polyomavirus-6 (HPyV6) and HPyV7. Biochemical experiments show that polyomavirus DNA is shed from the skin in the form of assembled virions. A pilot serological study indicates that infection or coinfection with these three skin-tropic polyomaviruses is very common. Thus, at least three polyomavirus species are constituents of the human skin microbiome.

Lipids and proteins act in opposing manners to regulate polyomavirus infection

How receptors control virus infection is poorly understood. Polyomavirus (Py) binds to the sialic acid-galactose moiety on receptors to gain entry into host cells and cause infection. We previously demonstrated that the sialic acid-galactose-containing glycolipids called gangliosides GD1a and GT1b promote Py infection, in part, by sorting the virus from the endolysosomes to the endoplasmic reticulum (ER), a critical infection route. Whether these glycolipids act as Py entry receptors, however, is not clear. Additionally, as the majority of glycoproteins also harbor terminal sialic acid-galactose residues, their roles in Py infection are also not well established. Using a ganglioside-deficient cell line, we show that GD1a is the functional entry receptor for Py. GD1a binds to Py on the plasma membrane, and the receptor-virus complex is internalized and transported to the late endosomes and then the ER to initiate infection. In contrast, our findings indicate that glycoproteins act as decoy receptors, restricting the ER transport and infection of Py. Thus, glycolipids and glycoproteins, two major constituents of the plasma membrane, execute opposing functions in regulating infection by a defined virus.

Update on human polyomaviruses and cancer

Over 50 years of polyomavirus research has produced a wealth of insights into not only general biologic processes in mammalian cells, but also, how conditions can be altered and signaling systems tweaked to produce transformation phenotypes. In the past few years three new members (KIV, WUV, and MCV) have joined two previously known (JCV and BKV) human polyomaviruses. In this review, we present updated information on general virologic features of these polyomaviruses in their natural host, concentrating on the association of MCV with human Merkel cell carcinoma. We further present a discussion on advances made in SV40 as the prototypic model, which has and will continue to inform our understanding about viruses and cancer.

Glycosphingolipids as receptors for non-enveloped viruses

Glycosphingolipids are ubiquitous molecules composed of a lipid and a carbohydrate moiety. Their main functions are as antigen/toxin receptors, in cell adhesion/recognition processes, or initiation/modulation of signal transduction pathways. Microbes take advantage of the different carbohydrate structures displayed on a specific cell surface for attachment during infection. For some viruses, such as the polyomaviruses, binding to gangliosides determines the internalization pathway into cells. For others, the interaction between microbe and carbohydrate can be a critical determinant for host susceptibility. In this review, we summarize the role of glycosphingolipids as receptors for members of the non-enveloped calici-, rota-, polyoma- and parvovirus families.

Structural evaluation of new human polyomaviruses provides clues to pathobiology

In the past three years, remarkable discoveries have added three new human polyomaviruses (KI virus (KIV), WU virus (WUV) and Merkel cell virus (MCV)) to a class that previously had only two disease-causing members (BK virus (BKV) and JC virus (JCV)) identified. Two monkey polyomaviruses, simian virus (SV)40 and B-cell lymphotropic polyomavirus (LPV) are also present in humans. KIV and WUV lack the agnoprotein coding sequence and regulatory micro (mi)RNA clusters of BKV, JCV and SV40. MCV lacks the agnoprotein sequence but generates miRNAs. KIV, WUV and MCV are all widespread in humans. Although they have distinctive tissue tropisms, all these viruses are probably acquired in childhood. Of these viruses, only MCV has thus far been strongly linked to cancer. Marshalled evidence from diverse sources implicates MCV as an etiological agent of Merkel cell carcinoma. This review compares the structural features of the new and previously known polyomaviruses, with the aim of identifying approaches to molecular pathology.

Cellular entry of polyomaviruses

Polyomaviruses (Pys) are nonenveloped DNA tumor viruses that include the murine polyomavirus (mPy), simian virus 40 (SV40), and the human BK, JC, KI, WU, and Merkel Cell viruses. To cause infection, Pys must enter host cells and navigate through various intracellular compartments, where they undergo sequential conformational changes enabling them to uncoat and deliver the DNA genome into the nucleus. The ensuing transcription and replication of the genome leads to lytic infection or cell transformation. In recent years, a more coherent understanding of how Pys are transported from the plasma membrane to the nucleus is starting to emerge. This review will focus on the decisive steps of Py entry, including engagement of the host cell receptor, targeting to the endoplasmic reticulum (ER), penetration across the ER membrane, nuclear entry, and genome release. Strikingly, a number of these steps resemble the intoxication pathway of the AB(5) bacterial toxins. Thus, as Pys and bacterial toxins hijack similar cellular machineries during infection, a general principle appears to guide their entry into host cells.