Viral infection

Abstract

Viruses have developed different survival strategies in host cells by crossing cell-membrane compartments, during different steps of their viral life cycle. In fact, the non-regenerative viral membrane of enveloped viruses needs to encounter the dynamic cell-host membrane, during early steps of the infection process, in which both membranes fuse, either at cell-surface or in an endocytic compartment, to promote viral entry and infection. Once inside the cell, many viruses accomplish their replication process through exploiting or modulating membrane traffic, and generating specialized compartments to assure viral replication, viral budding and spreading, which also serve to evade the immune responses against the pathogen. In this review, we have attempted to present some data that highlight the importance of membrane dynamics during viral entry and replicative processes, in order to understand how viruses use and move through different complex and dynamic cell-membrane structures and how they use them to persist.

Acknowledgments

This work and A.V.F. are supported by SAF2008-01729 (MICINN, Spain), European Regional Development Fund (ERDF), 24661/07 and 24-0740-09 (Fundacion Investigacion y Prevencion del SIDA en España), and ProID20100020 (Agencia Canaria de Investigación, Innovación y Soc. Información; Gobierno de Canarias) grants. The HIVACAT Program, the FIS project PI08/1306 and the Spanish AIDS network (RD06/0006). J.B.G., L.G.E. and L. de A.R. are supported by FIPSE-24-0740-09, SAF2008-01729 and ProID20100020 associated fellowships, respectively. J.D.M. is supported by the Ramón y Cajal program (R&C-2010-06256, MICINN). J.B. is supported by the ISCIII and Health Department (Generalitat de Catalunya). The authors declare that they have no conflicting financial interests. We specially thank to Prof. Manuel Feria for critical reading of the manuscript and for his continuous and generous support. We apologize for all research works and reviews that we have not reported or considered in this minireview, where we have tried to avoid omissions by our unpremeditated unknowledge.

Figures and Tables

Figure 1 Membrane dynamics processes in host cells that are involved in the life cycle of viral infection. The schematic representation of a cell and the different intracellular membrane-compartments, constitutive or putative sites formed by viruses to accomplish their infection processes, are shown. The non-regenerative viral membrane of enveloped viruses and the dynamics cell-host membranes play an important role during early infection process, since these two opposed membranes need to fuse, either at cell-surface or in an endocytic route (clathrin-, caveolae- or pinocytosis-dependent), to promote viral entry and infection. Endocytosis is initiated at the plasma membrane and progress through early and late endosomes, where some viruses replicate and are recycled back to the plasma membrane or transported to lysosomes to complete the life cycle. In the case of HIV-1, this enveloped virus requires Arf6-membrane dynamics to efficiently fuse with plasma membrane and promote entry and infection of CD4⁺ T lymphocytes (Asterisk scheme and Fig. 2). The non-endocytic route followed by HIV-1 during early infection is decisive to establish viral latent infection. Once inside the cell, many viruses accomplish their replication process through exploiting or modulating membrane traffic, and generating specialized compartments to assure viral survival, such as Viral Factories (VF), multivesicular bodies (MVB), double-membrane compartments, budding on plasma membrane and exosomes (it is conceivable that some viruses may actually be released as exosomes ). These membrane structures, cell-constitutive or arranged by the different viral proteins, are required for viral-gene replication, morphogenesis, export, viral maturation and release from cell-surface, and also serve to evade the immune responses against viral genomes. Viral proteins could enter the secretory pathway by co-translational translocation into the endoplasmic reticulum (ER; only a part of the perinuclear ER is shown), to be further transported from the ER to the Golgi complex in vesicles and in a coatomer protein complex (COP) II-dependent manner. Viral complexes formed inside MVBs, in communication with vesicles, mitochondria, Golgi cisternae and ER-membranes, could be transported through the Golgi network to the plasma membrane to be released as viral particles. Viral budding of enveloped viruses is mainly under the control of the activity of ESCRT-III complexes that are recruited to the site of viral release by ESCRT-I or Alix proteins that interacts with matrix viral proteins located on cell-surface.

Figure 2 Arf6-membrane dynamics regulates efficient HIV-1 infection. HIV-1 requires Arf6-coordinated membrane dynamics to efficiently fuse with plasma membrane and promote entry and infection of CD4⁺ T lymphocytes. In fact, movement of PIP2-associated membrane structures, driven by the Arf6-GTP/GDP cycle activity on plasma membrane from a sorting and recycling endosomal compartment, assures the regeneration of cell-surface membrane by coordinating the turnover of these PIP₂-associated vesicles. This membrane traffic has synergy with the key first HIV-1/receptors interactions to promote pore fusion formation, between the non-regenerative HIV-1 viral membrane and the dynamic cell-surface, thereby favouring efficient virus-cell fusion, entry and infection (scheme corresponding to early fusion and entry steps of the HIV-1 infection process, also indicated in Fig. 1 by an asterisk). The alteration of the Arf6-GTP/GDP cycle, by GDP-bound or GTP-bound mutants provokes an accumulation of Arf6/PIP₂-membrane structures on the plasma membrane. Specific Arf6 silencing also inhibits HIV-1-envelope-induced membrane fusion, entry and infection of T lymphocytes and permissive cells, regardless of viral tropism.

Figure 3 Membrane dynamics at the virological synapse. At the virological synapse (VS), some viruses either attach to the plasma membrane or surf along the filopodia and finally bind to specific receptors on the target cell. Viruses can also directly fuse with the plasma membrane, as in the case of HIV-1. Cell-to-cell transfer of HIV-1 takes place in an Arf6-membrane dynamics-dependent manner, which hijack endocytic pathways, including clathrin-dependent, caveolin-dependent or both independent pathways for viral internalization. The VS represents an efficient environment for viral budding, where the membrane of the infected cell is polarized towards the synaptic junction by the movement for of vesicles or MVBs coordinated by the translocation of the microtubule organizing centre (MTOC). This scaffolding allows for a subsequent viral infection and spread, that favours viral fusion and entry, viral endocytosis and viral protein/gene transfer from the infected to the close non-infected cell. Besides, long membrane nanotubes may also be formed between neighboring cells, which promote viral protein traffic and also HIV surfing and infection, from infected cell to non-infected cell. Other membrane dynamics events involved or occurred during viral infection and spreading are Trogocytosis and exosomal transport. Trogocytosis of cell-surface patches, containing CD4/HIV-1-bound molecules, occurs from non-infected to infected cells in a gp120/CD4-dependent manner. Exosomes are membrane vesicles, formed from MVB that could account for viral infection and spreading within membrane structures that are protected from immune responses.