Herpes simplex virus Membrane Fusion

Abstract

Herpes simplex virus mediates multiple distinct fusion events during infection. HSV entry is initiated by fusion of the viral envelope with either the limiting membrane of a host cell endocytic compartment or the plasma membrane. In the infected cell during viral assembly, immature, enveloped HSV particles in the perinuclear space fuse with the outer nuclear membrane in a process termed de-envelopment. A cell infected with some strains of HSV with defined mutations spread to neighboring cells by a fusion event called syncytium formation. Two experimental methods, the transient cell-cell fusion approach and fusion from without, are useful surrogate assays of HSV fusion. These five fusion processes are considered in terms of their requirements, mechanism, and regulation. The execution and modulation of these events require distinct yet often overlapping sets of viral proteins and host cell factors. The core machinery of HSV gB, gD, and the heterodimer gH/gL is required for most if not all of the HSV fusion mechanisms.

Fig. 2.1

Types of HSV-1 membrane fusion. (a) Virus-cell fusion during entry. HSV entry proceeds by either an endocytosis mechanism (left) or by direct penetration at the plasma membrane. There is fusion of the viral membrane (green) with either the cell endosomal membrane (em; orange) or the host plasma membrane (pm; orange), respectively, (b) Fusion of primary enveloped HSV-1 with the outer nuclear membrane. The primary envelope of HSV is derived from the inner nuclear membrane (inm) of the infected cell. The membrane of primary enveloped virions (blue) fuses with the outer nuclear membrane (onm; black). Shown spanning the inm and onm is a nuclear pore complex (npc), which is too narrow to allow passage of HSV particles, (c) Syncytium formation. The surface of a cell infected with a syncytial strain of HSV-1 (green) fuses directly with a neighboring uninfected cell (orange). (d) Fusion from without. The envelope of an FFWO strain of HSV triggers cell fusion in the absence of de novo protein synthesis. (e) Transfected cell-cell fusion. A cell transiently expressing HSV glycoproteins (green) fuses with a permissive target cell (orange)

Fig. 2.2

Fusion from without induced by HSV. (a) Uninfected Vero cells. (b) HSV-1 strain ANG path added to Vero cells for 3 h in the presence of cycloheximide

Fig. 2.3

Transfected cell-cell fusion. Effector CHO-K1 cells were transfected with plasmids for HSV-1 gB, gD, gH, gL, and GFP (a–c) or gD, gH, gL, and GFP (d–f). Target CHO-nectin-1 cells were labeled with CMAC CellTracker Blue. Effector and target cells were mixed for 6 h. Arrow in (a), (b), and (c) indicates at least one target and one effector cell that have fused. Arrows in (d), (e), and (f) indicate effector cells that have not fused with a target cell