Viral entry mechanisms: human papillomavirus and a long journey from extracellular matrix to the nucleus

Abstract

Papillomaviruses are epitheliotropic non-enveloped double stranded DNA viruses, whose replication is strictly dependent on the terminally differentiating tissue of the epidermis. They induce self-limiting benign tumors of skin and mucosa, which may progress to malignancy like cervical carcinoma. Prior to entry into basal cells, virions attach to heparan sulfate moieties of the basement membrane. This triggers conformational changes, which affect both capsid proteins, L1 and L2, and which are a prerequisite for interaction with the elusive uptake receptor. These processes are very slow resulting in an uptake halftime of up to 14 h. This review summarizes recent advances in our understanding of cell surface events, internalization and subsequent intracellular trafficking of Papillomaviruses. Introductory remarksPapillomaviruses (PV) are epitheliotropic nonenveloped small DNA viruses with icosahedral symmetry. Their strict dependence on terminally differentiating keratinocytes for completion of the replication cycle initially made the study of entry processes difficult with regard to two aspects. First, it was impossible to produce virions until the development of organotypic raft cultures based on keratinocytes harboring human papillomavirus (HPV) genomes [1]. Since these culture systems produced only very limited amounts of virions they provided only a partial relief. The limitation was partially overcome by the use of DNA-free virus-like particles and later by pseudovirions harboring marker plasmids, which were generated using heterologous expression systems [2][3][4]. The observation that codon optimization of capsid genes yielded high level expression of capsid proteins [5,6] and the development of packaging cell lines harboring high copy numbers of packaging plasmids finally allowed large scale productions of pseudovirions [7] as well as quasivirions [8]. This advance further facilitated the investigation of early events of PV infection. Second, until very recently [9], it was not possible to infect either organotypic raft cultures or primary keratinocytes in vitro unless pseudovirions had been activated (see below) [10]. The reason for this deficiency is unknown but suggests that taking primary keratinocytes into culture induces enough changes to make them refractory to HPV infection. Therefore, researchers have had to rely on established cell lines, the most commonly used of which is the HaCaT cell line, to study PV binding and uptake. However, the recent development of an in vivo mouse model by the Schiller group will allow for the testing of observations made in vitro. In this review we will focus on the entry of HPV type 16 (HPV16) and closely related viruses, which are the main cause of various cancers including cervical carcinoma. In vitro data backed by recent in vivo studies suggest an elaborate sequence of cell surface events Capsid structureTo fully appreciate viral entry strategies we have to discuss their surface structure. The outer shell of PV is composed of 360 molecules of the major ...

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