Date Published: December 19, 2013
Publisher: Public Library of Science
Author(s): Tatiana Gianni, Stefano Salvioli, Liudmila S. Chesnokova, Lindsey M. Hutt-Fletcher, Gabriella Campadelli-Fiume, Rozanne M. Sandri-Goldin.
Herpes simplex virus (HSV) – and herpesviruses in general – encode for a multipartite entry/fusion apparatus. In HSV it consists of the HSV-specific glycoprotein D (gD), and three additional glycoproteins, gH/gL and gB, conserved across the Herpesviridae family and responsible for the execution of fusion. According to the current model, upon receptor binding, gD propagates the activation to gH/gL and to gB in a cascade fashion. Questions remain about how the cascade of activation is controlled and how it is synchronized with virion endocytosis, to avoid premature activation and exhaustion of the glycoproteins. We considered the possibility that such control might be carried out by as yet unknown receptors. Indeed, receptors for HSV gB, but not for gH/gL, have been described. In other members of the Herpesviridae family, such as Epstein-Barr virus, integrin receptors bind gH/gL and trigger conformational changes in the glycoproteins. We report that αvβ6- and αvβ8-integrins serve as receptors for HSV entry into experimental models of keratinocytes and other epithelial and neuronal cells. Evidence rests on loss of function experiments, in which integrins were blocked by antibodies or silenced, and gain of function experiments in which αvβ6-integrin was expressed in integrin-negative cells. αvβ6- and αvβ8-integrins acted independently and are thus interchangeable. Both bind gH/gL with high affinity. The interaction profoundly affects the route of HSV entry and directs the virus to acidic endosomes. In the case of αvβ8, but not αvβ6-integrin, the portal of entry is located at lipid microdomains and requires dynamin 2. Thus, a major role of αvβ6- or αvβ8-integrin in HSV infection appears to be to function as gH/gL receptors and to promote virus endocytosis. We propose that placing the gH/gL activation under the integrin trigger point enables HSV to synchronize virion endocytosis with the cascade of glycoprotein activation that culminates in execution of fusion.
The glycoproteins of enveloped virions fulfill three major functions to enable virus entry into target cells; the attachment of virions to cells, a step that partly determines the type of cells that the virus targets, hence the viral tropism; the triggering of fusion, i.e. the activation of the fusion machinery, and the execution of fusion. For a number of viruses, a fourth event occurs between these steps, virion internalization by endocytosis, or macropinocytosis. The domains responsible for all these activities are often localized in one or two glycoproteins; this is the case for example for ortho-, paramyxo- and retroviruses. Virion glycoproteins can be considered ready-to-use machines that need to undergo a transition in conformation from the metastable fusion-inactive to the fusion-active form, in order to induce the merging of the two membranes – that of the virion and that of cell – so that lipids are mixed and fusion is executed . A fundamental aspect of the process is that the steps are sequentially ordered and coordinated, to ensure that the glycoprotein transition takes place only after the virus has attached to the cells. Indeed, a premature activation would irreversibly exhaust the fusogenic potential of the virion glycoproteins, and lead to failure to infect. A key question is therefore how the timing of glycoprotein transition and activation is controlled. Essentially, there are two strategies. Either the glycoprotein transition is dependent on the glycoprotein encounter with the cognate cellular receptor, or on the low pH of the endosomal compartment. These levels of control guarantee that the virion fusion machinery is only active after the virus has attached to cells, or, for those viruses which undergo internalization, after they have been endocytosed and the endosomal pH has been lowered. According to this view, two major functions of cellular receptors are determination of viral tropism and triggering of fusion.
The epithelial αv group of integrins, also named as RGD receptors, includes αvβ3-, αvβ5-, αvβ6-, αvβ8-integrins, in addition to the αIIbβ3. We report that: