Retroviral retention activates a Syk-dependent HemITAM in human tetherin

Abstract

Tetherin (BST2/CD317) restricts the release of enveloped viral particles from infected cells. Coupled to this virion retention, hominid tetherins induce proinflammatory gene expression via activating NF-κB. We investigated the events initiating this tetherin-induced signaling and show that physical retention of retroviral particles induces the phosphorylation of conserved tyrosine residues in the cytoplasmic tails of tetherin dimers. This phosphorylation induces the recruitment of spleen tyrosine kinase (Syk), which is required for downstream NF-κB activation, indicating that the tetherin cytoplasmic tail resembles the hemi-immunoreceptor tyrosine-based activation motifs (hemITAMs) found in C-type lectin pattern recognition receptors. Retroviral-induced tetherin signaling is coupled to the cortical actin cytoskeleton via the Rac-GAP-containing protein RICH2 (ARHGAP44), and a naturally occurring tetherin polymorphism with reduced RICH2 binding exhibits decreased phosphorylation and NF-κB activation. Thus, upon virion retention, this linkage to the actin cytoskeleton likely triggers tetherin phosphorylation and subsequent signal transduction to induce an antiviral state.

Graphical abstract

Figure 1

Tyrosine Phosphorylation of Transiently Expressed Human Tetherin (A) Schematic representation of tetherin including the amino acid sequence of the cytoplasmic tail from human tetherin, with the conserved tyrosines highlighted in gray. (B) Fold activation of a firefly luciferase NF-κB reporter in 293 cells transiently transfected with tetherin, and mutants thereof, compared to control YFP vector (top). Tetherin expression levels were confirmed by western blot (bottom). (C) Lysates of 293 cells transiently expressing tetherin or the indicated mutant were immunoprecipitated with an anti-pY antibody. Deglycosylated precipitates were analyzed by western blot using an anti-tetherin antibody. (D) As in (C), but lysates were immunoprecipitated with an anti-tetherin antibody and blotted with an anti-pY antibody. (E) Fold activation of NF-κB reporter in 293 cells transiently transfected with fixed amounts of NF-κB-luc reporter and wild-type tetherin plus increasing amounts of tetherin tyrosine mutants (or wild-type as internal control). Total DNA amounts transfected were kept constant by the addition of pCR3.1 YFP. Fold changes relative to YFP control. (F) Lysates from (E) were immunoprecipitated with anti-pY and visualized by western blot. All error bars are ± SEM of three independent experiments. See also Figure S1.

Figure 2

Retroviral Particle Retention Induces Tyrosine Phosphorylation of Human Tetherin (A) Anti-pY immunoprecipitation of cell lysates from 293/tetherin cells transfected with wild-type and ΔVpu HIV-1 proviruses or MLV provirus and derivatives (MLVΔPY and MLVΔPY/p6). Immunoprecipitates were deglycosylated, and tetherin was detected by western blot. (B) Phosphorylated proteins were separated from lysates of 293/tetherin cells transfected with YFP control, ΔVpu HIV-1, or MLV proviruses using a Phosphoprotein Purification Column. Tetherin was detected by western blot of the flow-through or eluted fractions. (C) 293/tetherin, single tyrosine (Y6A, Y8A), or double-tyrosine mutants (Y6,8A) assayed for the ability to induce NF-κB-luc reporter activation upon transfection with MLV, MLVΔPY, and MLVΔPY/p6 (top). MLV infectivity was determined by titration of supernatants on 293T cells and analyzed by flow cytometry for GFP expression 48 hr later (bottom). Error bars are ± SEM of three independent experiments. (D and E) The same cells were transfected with MLV provirus or late domain mutant MLV ΔPY, and 24 hr later cells were lysed for immunoprecipitation with anti-pY (D) or anti-tetherin (E) antibodies. Precipitates were deglycosylated and analyzed by western blot as before. See also Figure S2.

Figure 3

Restriction of Tetherin-Sensitive HIV-1 Mutant Release from CD4+ T Cells Induces Tetherin Phosphorylation (A) Purified CD4+ T cells from two donors were infected with HIV-1 wild-type or ΔVpu virus at an moi of 3. At 48 hr later, infectious virus release was determined by infection of HeLa-TZMbl reporter cell lines and western blot of cell lysates and supernatants for HIV-1 p24-CA. (B) Total RNA from the cells infected in (A) was analyzed for cxcl10, il6, and ifnb mRNA levels relative to gapdh by qRT-PCR. (C) Parallel CD4+ T cell cultures were infected as in (A). Cell lysates from these cultures were immunoprecipitated with an anti-pY antibody, deglycosylated, and tetherin analyzed by western blot. (D) U87MG cells were treated overnight with supernatants from CD4+ T cells infected as in (A) and then challenged with VSV-G pseudotyped HIV-1 WT virus (moi of 0.5) in a one-round viral replication assay. Infectious virus release was determined 48 hr later as in (A).

Figure 4

Tetherin Phosphorylation Is Sensitive to Inhibitors of Src-Family Kinases and Requires the Recruitment of Syk (A and B) Fold activation of NF-κB-luc reporter in 293/tetherin cells transfected with HIV-1 ΔVpu or MLV proviruses in the presence of increasing amounts of the inhibitors Genistein (A) or PP2 (B). NF-κB fold changes relative to cells transfected with control YFP and not treated with inhibitors. (C) Immunoprecipitation of cell lysates from (B) with an anti-pY antibody. Precipitates from 293/tetherin cells transfected with MLV provirus were deglycosylated and analyzed as previously described. (D) 293/tetherin cells were transfected twice over 72 hr with siRNAs directed against Syk (siSyk_1 and _2) or a nontargeting control (siNeg). 293 tetherin cells were transfected with the NF-κB reporter plasmid and the indicated viral plasmid. Fold increases in luciferase expression are plotted relative to YFP control. All error bars represent ± SEM of three experiments. ^∗p > 0.05, ^∗∗p > 0.01, and ^∗∗∗p > 0.001 as determined by paired two-tailed t test. (E) Western blots of anti-pY immunoprecipitates from (D). (F) Lysates of 293 stably expressing wild-type tetherin or tyrosine mutants transfected with MLV provirus were immunoprecipitated with anti-tetherin antibody. Pull-downs were analyzed by western blot for tetherin, pY, Syk, and its active phosphorylated form (pSyk). (G and H) Phosphorylated proteins were separated from MLV-transfected lysates of 293/tetherin and 293/tetherin-Y6,8A (G) or 293/tetherin cells depleted of Syk by siRNA (H),using a Phosphoprotein Purification Column. Eluate fractions were analyzed by western blot for the indicated protein. (I) Purified CD4+ T cells were infected with HIV-1 WT or ΔVpu viruses (moi of 3) in the presence or absence of the inhibitors PP2 (20 μM), Piceatannol (10 μM), or BAY61-3606 (2.5 μM). At 48 hr after infection, cell lysates were immunoprecipitated with an anti-pY antibody, and pull-downs were deglycosylated and analyzed by western blot for tetherin, pSyk, and pZAP70. (J) Cxcl10, il6, and ifnb mRNA levels in parallel cultures infected and treated as in (I). Data are represented as ± SEM. See also Figure S3.

Figure 5

Species Specificity of Tetherin’s Signaling Activity Is Linked to Syk Recruitment and Tyrosine Phosphorylation (A) NF-κB-luc reporter activation in 293 cells transfected with increasing amounts of human, chimpanzee, gorilla, AGM, and rhesus tetherins. Alignment of cytoplasmic tails of primate tetherins is presented in Figure S4A. Error bars represent ± SEM of three experiments. (B) Anti-pY immunoprecipitation of lysates from the 50 ng input of (A) deglycosylated and western blotted for tetherin. (C) As in (B), but cell lysates were precipitated with anti-tetherin antibodies, and precipitates were analyzed for phosphorylated tetherin (anti-pY), Syk, or pSyk. See also Figure S4.

Figure 6

Tetherin-Mediated Signaling Is Impaired by Cytochalasin D Treatment and RICH2 Depletion (A) Fold activation of NF-κB-luc reporter in 293/tetherin cells transfected with MLV provirus, wild-type, ΔVpu HIV-1 proviruses, or MAVS and treated for 6–8 hr with increasing amounts of the actin polymerization inhibitor cytochalasin D. (B) HIV-1 infectious viral particle release from supernatants in (A) was determined on HeLa-TZM cells. (C) 293/tetherin cells were transfected with MLV provirus in the presence of increasing amounts of cytochalasin D inhibitor. Lysates were immunoprecipitated with an anti-tetherin antibody and analyzed by western blot for anti-pY, Syk, and pSyk. (D) NF-κB reporter activation in RICH2 siRNA-depleted 293/tetherin cells transfected with wild-type HIV-1, ΔVpu HIV-1, or MLV proviruses. Knockdown of RICH2 was confirmed by western blot (inset). Error bars represent ± SEM of three experiments. ^∗p > 0.05, ^∗∗p > 0.01, and ^∗∗∗p > 0.001 as determined by paired two-tailed t test. (E and F) Infectious HIV release (E) and precipitation of phospho-tetherin by anti pY from MLV transfected cells (F) were analyzed as in previous figures. See also Figure S5.

Figure 7

RICH2/Tetherin Interactions Are Required for Virion-Induced Signaling (A) 293 cells were cotransfected with an N-terminally HA-tagged human RICH2 and tetherin from humans, chimpanzee, gorilla, rhesus, or AGM. Lysates were immunoprecipitated with an anti-tetherin antibody, and precipitates were blotted for HA and tetherin. (B) Fold activation of a NF-κB-luc reporter in 293 cells expressing tetherin, or the mutants Y6,8A and R19H, transfected with HIV-1 WT or ΔVpu proviruses relative to YFP control. Error bars represent ± SEM of three experiments. (C) The above stable cells lines were transfected with MLV provirus and lysed 24 hr later for immunoprecipitation with a anti-pY (left) or anti-tetherin (right) antibodies. Precipitates were deglycosylated and analyzed by western blot as before. (D) 293 cells were cotransfected with HA-RICH2 and wild-type tetherin or the mutants Y6,8A, Y6A, Y8A, or R19H. Lysates were immunoprecipitated with an anti-tetherin antibody and analyzed as in (A). (E) Model for Syk-dependent tetherin-mediated signaling as discussed in the Discussion. See also Figure S6.