An immunoreceptor tyrosine-based inhibition motif in varicella-zoster virus glycoprotein B regulates cell fusion and skin pathogenesis

Abstract

Herpesvirus entry functions of the conserved glycoproteins gB and gH-gL have been delineated, but their role in regulating cell-cell fusion is poorly understood. Varicella-zoster virus (VZV) infection provides a valuable model for investigating cell-cell fusion because of the importance of this process for pathogenesis in human skin and sensory ganglia. The present study identifies a canonical immunoreceptor tyrosine-based inhibition motif (ITIM) in the gB cytoplasmic domain (gBcyt) and demonstrates that the gBcyt is a tyrosine kinase substrate. Orbitrap mass spectrometry confirmed that Y881, central to the ITIM, is phosphorylated. To determine whether the gBcyt ITIM regulates gB/gH-gL-induced cell-cell fusion in vitro, tyrosine residues Y881 and Y920 in the gBcyt were substituted with phenylalanine separately or together. Recombinant viruses with these substitutions were generated to establish their effects on syncytia formation in replication in vitro and in the human skin xenograft model of VZV pathogenesis. The Y881F substitution caused significantly increased cell-cell fusion despite reduced cell-surface gB. Importantly, the Y881F or Y881/920F substitutions in VZV caused aggressive syncytia formation, reducing cell-cell spread. These in vitro effects of aggressive syncytia formation translated to severely impaired skin infection in vivo. In contrast, the Y920F substitution did not affect virus replication in vitro or in vivo. These observations suggest that gB modulates cell-cell fusion via an ITIM-mediated Y881 phosphorylation-dependent mechanism, supporting a unique concept that intracellular signaling through this gBcyt motif regulates VZV syncytia formation and is essential for skin pathogenesis.fusogenicity | mutagenesis | polykaryocyte | virulence T he alphaherpesvirus varicella-zoster virus (VZV) is a human pathogen that spreads from mucosal epithelial sites of initial infection to skin via a T cell-associated viremia (1), causing varicella (chicken pox). Viremia and cutaneous infection enable transfer of VZV to sensory nerve ganglia and establishment of latency in neurons (2). Zoster (shingles) is caused by VZV reactivation from latently infected neurons and can lead to the debilitating condition of postherpetic neuralgia. Live attenuated VZV vaccines are effective against varicella and zoster but are not recommended for immunocompromised patients.Enveloped viruses from several families, including the Herpesviridae, require fusion with cellular membranes for virion entry, and in some cases induce syncytia through cell-cell fusion (2-5). Little is known about the functional role of syncytia during pathogenesis. VZV is a valuable model pathogen for investigating this process because natural infection of the human host involves formation of multinucleated polykaryocytes in skin and fusion of neurons and satellite cells in sensory ganglia (2, 6). In addition, VZV produces syncytia during replication in vitro and triggers fusion between differentiated cells in human skin and dorsal root ganglion xenograf...

Global Mapping of O-Glycosylation of Varicella Zoster Virus, Human Cytomegalovirus, and Epstein-Barr Virus

“…Four envelope glycoproteins are essential for VZV replication in vitro: gB, gH, gL, and gE, of which gB and the gH-gL complex is thought to mediate both virion-cell and cell-cell fusion (41)(42)(43)(44). Two of the fusion complex proteins were found to be O-glycosylated, with seven and eight sites identified in gB and gH, respectively (Table 1).…”

The Glycoprotein B Cytoplasmic Domain Lysine Cluster Is Critical for Varicella-Zoster Virus Cell-Cell Fusion Regulation and Infection

“…After translation, gB is exported from the endoplasmic reticulum (ER), processed in the Golgi apparatus, trafficked to the cell surface, endocytosed, and then returned to the trans-Golgi network (TGN) for insertion into the viral envelope (12,19,20). The protein forms a trimer, with each 931-amino-acid (aa) monomer having an ectodomain, transmembrane domain, and a 124-aa cytoplasmic domain (21,22). VZV gB is presumed to be the fusogen based on structural homology modeling with the solved crystal structure of HSV-1 gB (22,23).…”

“…The protein forms a trimer, with each 931-amino-acid (aa) monomer having an ectodomain, transmembrane domain, and a 124-aa cytoplasmic domain (21,22). VZV gB is presumed to be the fusogen based on structural homology modeling with the solved crystal structure of HSV-1 gB (22,23). The glycoprotein has also been shown to associate with a proposed entry receptor, myelin-associated glycoprotein (MAG), via attached sialic acids to facilitate fusion and entry (24).…”

“…Cell-cell fusion mediated by VZV gB/gH-gL has previously been shown to be regulated by a functional immunoreceptor tyrosine-based inhibition motif (ITIM) in the gB cytoplasmic domain (gBcyt) (22). ITIM-containing proteins have various functions, including negatively regulating immune cell responses, promoting cell proliferation, and blocking apoptosis (25).…”

“…The truncation of the gBcyt preserved the ITIM sequence but removed the YXX⌽ motif, 920 YSRV 923 , which has been shown to have a role in gB trafficking and processing (27). This motif was unlikely to be responsible for the exaggerated syncytium formation, because disrupting the YXX⌽ motif with a Y920F substitution reduces rather than increases gB/gH-gL-mediated cell-cell fusion (22). This suggests the presence of an additional fusion regulatory element in the terminal 36 amino acids of the gBcyt.…”