αvβ6- and αvβ8-Integrins Serve As Interchangeable Receptors for HSV gH/gL to Promote Endocytosis and Activation of Membrane Fusion

Abstract

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.

Dissociation of HSV gL from gH by αvβ6- or αvβ8-integrin promotes gH activation and virus entry

“…1A shows that both WT-αvβ6-integrin and αvβ6N1 chimera increased HSV entry, suggesting that the integrin-mediated increase in infection is independent of type of C-tail and most likely mediated by αvβ6-integrin ectodomain. This integrin portion binds gH/gL, as seen also by surface plasmon resonance (20).…”

“…αvβ6-and αvβ8-integrin play two distinct roles in HSV infection of human cells: they enable HSV-1 entry by an unknown mechanism; and they promote virus endocytosis into acidic endosomes (20). To differentiate between these two functions and define the integrin domains where they map, we generated the β6N1 chimera, in which the transmembrane and C-tail portions of β6-integrin were replaced with those of nectin1.…”

“…They become infectable upon transfection with a gD receptor, nectin1 or HVEM (29). J cells express endogenous integrins in limited amounts; overexpression of human αvβ6-or αvβ8-integrin in nectin1-positive J cells increases the extent of infection (20).…”

“…Recently, we discovered that αvβ6-and αvβ8-integrins serve as interchangeable receptors for HSV gH/gL. They play two distinct roles in infection (20). They enable virus entry, as inferred by inhibition of infection following integrin depletion by siRNAs or exposure of cells to anti-integrin antibodies.…”

“…They enable virus entry, as inferred by inhibition of infection following integrin depletion by siRNAs or exposure of cells to anti-integrin antibodies. Second, they promote HSV endocytosis into acidic endosomes (20); the latter function is nonessential because the virus may enter some cells also by fusion with plasma membranes or with neutral endosomes (21)(22)(23). Remarkably, the use of integrins as receptors is a common feature among herpesviruses.…”