Functional interaction between herpes simplex virus type 2 gD and HVEM transiently dampens local chemokine production after murine mucosal infection.

Herpes virus entry mediator (HVEM) is one of two principal receptors mediating herpes simplex virus (HSV) entry into murine and human cells. It functions naturally as an immune signaling co-receptor, and may participate in enhancing or repressing immune responses depending on the natural ligand used. To investigate whether engagement of HVEM by HSV affects the in vivo response to HSV infection, we generated recombinants of HSV-2(333) that expressed wild-type gD (HSV-2/gD) or mutant gD able to bind to nectin-1 (the other principal entry receptor) but not HVEM. Replication kinetics and yields of the recombinant strains on Vero cells were indistinguishable from those of wild-type HSV-2(333). After intravaginal inoculation with mutant or wild-type virus, adult female C57BL/6 mice developed vaginal lesions and mortality in similar proportions, and mucosal viral titers were similar or lower for mutant strains at different times. Relative to HSV-2/gD, percentages of HSV-specific CD8(+) T-cells were similar or only slightly reduced after infection with the mutant strain HSV-2/gD-Δ7-15, in all tissues up to 9 days after infection. Levels of HSV-specific CD4(+) T-cells five days after infection also did not differ after infection with either strain. Levels of the cytokine IL-6 and of the chemokines CXCL9, CXCL10, and CCL4 were significantly lower in vaginal washes one day after infection with HSV-2/gD compared with HSV-2/gD-Δ7-15. We conclude that the interaction of HSV gD with HVEM may alter early innate events in the murine immune response to infection, without significantly affecting acute mortality, morbidity, or initial T-cell responses after lethal challenge.

Hydrogen tunneling in adenosylcobalamin-dependent glutamate mutase: evidence from intrinsic kinetic isotope effects measured by intramolecular competition.

Hydrogen atom transfer reactions between the substrate and coenzyme are key mechanistic features of all adenosylcobalamin-dependent enzymes. For one of these enzymes, glutamate mutase, we have investigated whether hydrogen tunneling makes a significant contribution to the mechanism by examining the temperature dependence of the deuterium kinetic isotope effect associated with the transfer of a hydrogen atom from methylaspartate to the coenzyme. To do this, we designed a novel intramolecular competition experiment that allowed us to measure the intrinsic kinetic isotope effect, even though hydrogen transfer may not be rate-determining. From the Arrhenius plot of the kinetic isotope effect, the ratio of the pre-exponential factors (A(H)/A(D)) was 0.17 +/- 0.04 and the isotope effect on the activation energy [DeltaE(a(D-H))] was 1.94 +/- 0.13 kcal/mol. The results imply that a significant degree of hydrogen tunneling occurs in glutamate mutase, even though the intrinsic kinetic isotope effects are well within the semiclassical limit and are much smaller than those measured for other AdoCbl enzymes and model reactions for which hydrogen tunneling has been implicated.

The virological synapse facilitates herpes simplex virus entry into T cells.

The virological synapse (VS) is a specialized molecular structure that facilitates the transfer of certain lymphotropic viruses into uninfected T cells. However, the role of the VS in the transfer of nonlymphotropic viruses into T cells is unknown. Herpes simplex virus (HSV) has been shown in vitro to infect T cells and modulate T-cell receptor function, thereby suppressing T-cell antiviral function. However, whether such infection of T cells occurs in vivo is unknown. Here, we examined whether T-cell infection could be observed in human HSV disease and investigated the mechanism of HSV entry into T cells. We found that HSV-infected T cells were readily detectable during human disease, suggesting that infection and modulation of T-cell function plays a role in human immunopathology. HSV infection of both CD4(+) and CD8(+) T cells occurred much more efficiently via direct cell-to-cell spread from infected fibroblasts than by cell-free virus. Activation of T cells increased their permissivity to HSV infection. Cell-to-cell spread to T cells did not require HSV glycoproteins E and I (gE and gI), which are critical for cell-to-cell spread between epithelial cells. Transfer of HSV to T cells required gD, and the four known entry receptors appear to be contributing to viral entry, with a dominant role for the herpesvirus entry mediator and nectin-1. VS-like structures enriched in activated lymphocyte function-associated antigen 1 (LFA-1) were observed at the point of contact between HSV-infected fibroblasts and T cells. Consistent with spread occurring via the VS, transfer of HSV was increased by activation of LFA-1, and cell-to-cell spread could be inhibited by antibodies to LFA-1 or gD. Taken together, these results constitute the first demonstration of VS-dependent cell-to-cell spread for a predominantly nonlymphotropic virus. Furthermore, they support an important role for infection and immunomodulation of T cells in clinical human disease. Targeting of the VS might allow selective immunopotentiation during infections with HSV or other nonlymphotropic viruses.

Alternative entry receptors for herpes simplex virus and their roles in disease.

Either herpesvirus entry mediator (HVEM, TNFRSF14) or nectin-1 (PVRL1) is sufficient for herpes simplex virus (HSV) infection of cultured cells. The contribution of individual receptors to infection in vivo and to disease is less clear. To assess this, Tnfrsf14(-/-) and/or Pvrl1(-/-) mice were challenged intravaginally with HSV-2. Infection of the vaginal epithelium occurred in the absence of either HVEM or nectin-1 but was virtually undetectable when both receptors were absent, indicating that either HVEM or nectin-1 was necessary. Absence of nectin-1 (but not HVEM) reduced efficiency of infection of the vaginal epithelium and viral spread to the nervous system, attenuating neurological disease and preventing external lesion development. While nectin-1 proved not to be essential for infection of the nervous system, it is required for the full manifestations of disease. This study illustrates the value of mutant mice for understanding receptor contributions to disease caused by a human virus.

Synthesis of Mono- and Di-Deuterated (2S, 3S)-3-Methylaspartic Acids to Facilitate Measurement of Intrinsic Kinetic Isotope Effects in Enzymes.

Kinetic isotope effects provide a powerful method to investigate the mechanisms of enzyme-catalyzed reactions, but often other slow steps in the reaction such as substrate binding or product release suppress the isotopically sensitive step. For reactions at methyl groups, this limitation may be overcome by measuring the isotope effect by an intra-molecular competition experiment. This requires the synthesis of substrates containing regio-specifically mono- or dideuterated methyl groups. To facilitate mechanistic investigations of the adenosylcobalamin-dependent enzyme, glutamate mutase we have developed a synthesis of mono- and di-deuterated (2S, 3S)-3-methylaspartic acids. Key intermediates are the correspondingly labeled mesaconic acids and their dimethyl esters that potentially provide starting materials for a variety of isotopically labeled molecules.

Reaction of adenosylcobalamin-dependent glutamate mutase with 2-thiolglutarate.

We have investigated the reaction of glutamate mutase with the glutamate analogue, 2-thiolglutarate. In the standard assay, 2-thiolglutarate behaves as a competitive inhibitor with a Ki of 0.05 mM. However, rather than simply binding inertly at the active site, 2-thiolglutarate elicits cobalt-carbon bond homolysis and the formation of 5'-deoxyadenosine. The enzyme exhibits a complicated EPR spectrum in the presence of 2-thiolglutarate that is markedly different from any previously observed with the enzyme. The spectrum was simulated well by assuming that it arises from electron-electron spin coupling between a thioglycolyl radical and low-spin Co2+ in cob(II)alamin. Analysis of the zero-field splitting parameters obtained from the simulations places the organic radical approximately 10 A from the cobalt and at a tilt angle of approximately 70 degrees to the normal of the corrin ring. This orientation is in good agreement with that expected from the crystal structure of glutamate mutase complexed with the substrate. 2-Thiolglutarate appears to react in a manner analogous to that of glutamate by first forming a thiolglutaryl radical at C-4 that then undergoes fragmentation to produce acrylate and the sulfur-stabilized thioglycolyl radical. The thioglycolyl radical accumulates on the enzyme, suggesting it is too stable to undergo further steps in the mechanism at a detectable rate.

Inhibition of TCR signaling by herpes simplex virus.

T lymphocytes are an essential component of the immune response against HSV infection. We previously reported that T cells became functionally impaired or inactivated after contacting HSV-infected fibroblasts. In our current study, we investigate the mechanisms of inactivation. We report that HSV-infected fibroblasts or HSV alone can inactivate T cells by profoundly inhibiting TCR signal transduction. Inactivation requires HSV penetration into T cells but not de novo transcription or translation. In HSV-inactivated T cells stimulated through the TCR, phosphorylation of Zap70 occurs normally. However, TCR signaling is inhibited at linker for activation of T cells (LAT) and at steps distal to LAT in the TCR signal cascade including inhibition of calcium flux and inhibition of multiple MAPK. Inactivation of T cells by HSV leads to the reduced phosphorylation of LAT at tyrosine residues critical for TCR signal propagation. Treatment of T cells with tyrosine phosphatase inhibitors attenuates inactivation by HSV, and stimulus with a mitogen that bypasses LAT phosphorylation overcomes inactivation. Our findings elucidate a potentially novel method of viral immune evasion that could be exploited to better manage HSV infection, aid in vaccine design, or allow targeted manipulation of T cell function.

Different receptors binding to distinct interfaces on herpes simplex virus gD can trigger events leading to cell fusion and viral entry.

One of the herpes simplex virus envelope glycoproteins, designated gD, is the principal determinant of cell recognition for viral entry. Other viral glycoproteins, gB, gH and gL, cooperate with gD to mediate the membrane fusion that is required for viral entry and cell fusion. Membrane fusion is triggered by the binding of gD to one of its receptors. These receptors belong to three different classes of cell surface molecules. This review summarizes recent findings on the structure and function of gD. The results presented indicate that gD may assume more than one conformation, one in the absence of receptor, another when gD is bound to the herpesvirus entry mediator, a member of the TNF receptor family, and a third when gD is bound to nectin-1, a cell adhesion molecule in the immunoglobulin superfamily. Finally, information and ideas are presented about a membrane-proximal region of gD that is required for membrane fusion, but not for receptor binding, and that may have a role in activating the fusogenic activity of gB, gH and gL.

Evolutionarily divergent herpesviruses modulate T cell activation by targeting the herpesvirus entry mediator cosignaling pathway.

The herpesvirus entry mediator (HVEM), a member of the TNF receptor (TNFR) superfamily, can act as a molecular switch that modulates T cell activation by propagating positive signals from the TNF-related ligand LIGHT (TNFR superfamily 14), or inhibitory signals through the Ig superfamily member B and T lymphocyte attenuator (BTLA). Competitive binding analysis and mutagenesis reveals a unique BTLA binding site centered on a critical lysine residue in cysteine-rich domain 1 of HVEM. The BTLA binding site on HVEM overlaps with the binding site for the herpes simplex virus 1 envelope glycoprotein D, but is distinct from where LIGHT binds, yet glycoprotein D inhibits the binding of both ligands, potentially nullifying the pathway. The binding site on HVEM for BTLA is conserved in the orphan TNFR, UL144, present in human CMV. UL144 binds BTLA, but not LIGHT, and inhibits T cell proliferation, selectively mimicking the inhibitory cosignaling function of HVEM. The demonstration that distinct herpesviruses target the HVEM-BTLA cosignaling pathway suggests the importance of this pathway in regulating T cell activation during host defenses.

Random mutagenesis of the gene encoding a viral ligand for multiple cell entry receptors to obtain viral mutants altered for receptor usage.

Herpes simplex virus type 1 (HSV-1) can enter cells expressing any one of multiple entry receptors, including the herpesvirus entry mediator (HVEM), nectin-1, and sites in heparan sulfate generated by specific 3-O-sulfotransferases. The viral ligand for these receptors is glycoprotein D (gD). To define structural requirements for functional interactions of gD with its receptors and to obtain viral mutants altered for receptor usage, we generated a library of HSV-1 mutants with random mutations in the gD gene. Viral isolates selected on a monkey cell line (Vero) were screened for the loss of ability to infect cells expressing each of the HSV-1 receptors. The 10 HSV-1 mutants obtained had 12 mutations in gD, affecting 11 amino acids. All mutations reduced or abrogated viral entry through HVEM and 3-O-sulfated heparan sulfate, indicating that similar features of gD are critical for functional interactions with both these receptors. None of the mutations reduced viral entry through nectin-1, whereas a subset of the mutations conferred ability to use nectin-2 as an entry receptor. These and other results show that features of gD, including conformation of the N terminus, critical for functional interactions with HVEM/3-O-sulfated heparan sulfate, differ from those critical for interactions with nectin-1.

Mutations in herpes simplex virus glycoprotein D that prevent cell entry via nectins and alter cell tropism.

Glycoprotein D (gD) determines which cells can be infected by herpes simplex virus (HSV) by binding to one of the several cell surface receptors that can mediate HSV entry or cell fusion. These receptors include the herpesvirus entry mediator (HVEM), nectin-1, nectin-2, and sites in heparan sulfate generated by specific 3-O-sulfotransferases. The objective of the present study was to identify residues in gD that are critical for physical and functional interactions with nectin-1 and nectin-2. We found that double or triple amino acid substitutions at positions 215, 222, and 223 in gD caused marked reduction in gD binding to nectin-1 and a corresponding inability to function in cell fusion or entry of HSV via nectin-1 or nectin-2. These substitutions either enhanced or did not significantly inhibit functional interactions with HVEM and modified heparan sulfate. These and other results demonstrate that different domains of gD, with some overlap, are critical for functional interactions with each class of entry receptor. Viral entry assays, using gD mutants described here and previously, revealed that nectins are the principal entry receptors for selected human cell lines of neuronal and epithelial origin, whereas HVEM or nectins could be used to mediate entry into a T lymphocyte line. Because T cells and fibroblasts can be infected via HVEM, HSV strains carrying gD mutations that prevent entry via nectins may establish transient infections in humans, but perhaps not latent infections of neurons, and are therefore candidates for development of safe live virus vaccines and vaccine vectors.

Mutations in the N termini of herpes simplex virus type 1 and 2 gDs alter functional interactions with the entry/fusion receptors HVEM, nectin-2, and 3-O-sulfated heparan sulfate but not with nectin-1.

Multiple cell surface molecules (herpesvirus entry mediator [HVEM], nectin-1, nectin-2, and 3-O-sulfated heparan sulfate) can serve as entry receptors for herpes simplex virus type 1 (HSV-1) or HSV-2 and also as receptors for virus-induced cell fusion. Viral glycoprotein D (gD) is the ligand for these receptors. A previous study showed that HVEM makes contact with HSV-1 gD at regions within amino acids 7 to 15 and 24 to 32 at the N terminus of gD. In the present study, amino acid substitutions and deletions were introduced into the N termini of HSV-1 and HSV-2 gDs to determine the effects on interactions with all of the known human and mouse entry/fusion receptors, including mouse HVEM, for which data on HSV entry or cell fusion were not previously reported. A cell fusion assay was used to assess functional activity of the gD mutants with each entry/fusion receptor. Soluble gD:Fc hybrids carrying each mutation were tested for the ability to bind to cells expressing the entry/fusion receptors. We found that deletions overlapping either or both of the HVEM contact regions, in either HSV-1 or HSV-2 gD, severely reduced cell fusion and binding activity with all of the human and mouse receptors except nectin-1. Amino acid substitutions described previously for HSV-1 (L25P, Q27P, and Q27R) were individually introduced into HSV-2 gD and, for both serotypes, were found to be without effect on cell fusion and the binding activity for nectin-1. Each of these three substitutions in HSV-1 gD enhanced fusion with cells expressing human nectin-2 (ordinarily low for wild-type HSV-1 gD), but the same substitutions in HSV-2 gD were without effect on the already high level of cell fusion observed with the wild-type protein. The Q27P or Q27R substitution in either HSV-1 and HSV-2 gD, but not the L25P substitution, significantly reduced cell fusion and binding activity for both human and mouse HVEM. Each of the three substitutions in HSV-1 gD, as well as the deletions mentioned above, reduced fusion with cells bearing 3-O-sulfated heparan sulfate. Thus, the N terminus of HSV-1 or HSV-2 gD is not necessary for functional interactions with nectin-1 but is necessary for all of the other receptors tested here. The sequence of the N terminus determines whether nectin-2 or 3-O-sulfated heparan sulfate, as well as HVEM, can serve as entry/fusion receptors.

Rapid transport of neural intermediate filament protein.

Peripherin is a neural intermediate filament protein that is expressed in peripheral and enteric neurons, as well as in PC12 cells. A determination of the motile properties of peripherin has been undertaken in PC12 cells during different stages of neurite outgrowth. The results reveal that non-filamentous, non-membrane bound peripherin particles and short peripherin intermediate filaments, termed 'squiggles', are transported at high speed throughout PC12 cell bodies, neurites and growth cones. These movements are bi-directional, and the majority require microtubules along with their associated molecular motors, conventional kinesin and cytoplasmic dynein. Our data demonstrate that peripherin particles and squiggles can move as components of a rapid transport system capable of delivering cytoskeletal subunits to the most distal regions of neurites over relatively short time periods.

Disruption of adherens junctions liberates nectin-1 to serve as receptor for herpes simplex virus and pseudorabies virus entry.

Nectin-1, a cell adhesion molecule belonging to the immunoglobulin superfamily, can bind to virion glycoprotein D (gD) to mediate entry of herpes simplex viruses (HSV) and pseudorabies virus (PRV). Nectin-1 colocalizes with E-cadherin at adherens junctions in epithelial cells. The disruption of cell junctions can result in the redistribution of nectin-1. To determine whether disruption of junctions by calcium depletion influenced the susceptibility of epithelial cells to viral entry, Madin-Darby canine kidney cells expressing endogenous nectin-1 or transfected human nectin-1 were tested for the ability to bind soluble forms of viral gD and to be infected by HSV and PRV, before and after calcium depletion. Confocal microscopy revealed that binding of HSV and PRV gD was localized to adherens junctions in cells maintained in normal medium but was distributed, along with nectin-1, over the entire cell surface after calcium depletion. Both the binding of gD and the fraction of cells that could be infected by HSV-1 and PRV were enhanced by calcium depletion. Taken together, these results provide evidence that nectin-1 confined to adherens junctions in epithelial cells is not very accessible to virus, whereas dissociation of cell junctions releases nectin-1 to serve more efficiently as an entry receptor.