title: Site-specific mutations in HIV-1 gp41 reveal a correlation between HIV-1-mediated bystander apoptosis and fusion/hemifusion.
abstract: The loss of CD4(+) T cells in HIV-1 infections is hypothesized to be caused by apoptosis of bystander cells mediated by cell surface-expressed HIV-1 Env glycoprotein. However, the mechanism by which Env mediates this process remains controversial. Specifically, the role of HIV-1 gp120 binding to CD4 and CXCR4 versus the fusion process mediated by gp41 remains unresolved. Env-induced apoptosis in bystander cells has been shown to be gp41-dependent and correlates with the redistribution of membrane lipids between Env-expressing cells and target cells (hemifusion). Using a rational mutagenesis approach aimed at targeting Env function via the gp41 subunit, we examined the role of HIV gp41 in bystander apoptosis. A mutation in the fusion domain of gp41 (V513E) resulted in a fusion-defective Env that failed to induce apoptosis. A mutation in the gp41 N-terminal helix (G547D) reduced cell fusion capacity and apoptosis; conversely, an Env mutant with a deletion of the gp41 cytoplasmic tail (Ct Del) enhanced both cell-to-cell fusion and apoptosis. Most significantly, an Env mutant containing a substitution in the loop region of gp41 (D589L) mediated transfer of lipids (hemifusion) to bystander cells but was defective in cell-to-cell and to a lesser degree virus-to-cell fusion. This mutant was still able to induce apoptosis in bystander cells. Hence, we have provided the first direct evidence that gp41-mediated hemifusion is both required and sufficient for induction of apoptosis in bystander cells. These results may help to explain the mechanism of HIV-1 Env-induced T cell depletion.
authors.affiliation: Membrane Structure and Function Section, Center for Cancer Research Nanobiology Program, NCI, National Institutes of Health, Frederick, Maryland 21702, USA.
authors.forename: Himanshu
authors.initials: H
authors.lastname: Garg
authors.forename: Anjali
authors.initials: A
authors.lastname: Joshi
authors.forename: Eric O
authors.initials: EO
authors.lastname: Freed
authors.forename: Robert
authors.initials: R
authors.lastname: Blumenthal
mesh_terms: D017209:Apoptosis / Q000187:drug effects; D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors / Q000494:pharmacology; D015700:HIV Envelope Protein gp41 / Q000235:genetics / Q000502:physiology*; D015497:HIV-1 / Q000502:physiology*; D006367:HeLa Cells; D006801:Humans; D008561:Membrane Fusion / Q000502:physiology*; D016297:Mutagenesis, Site-Directed; D019888:Nelfinavir / Q000494:pharmacology; D018894:Reverse Transcriptase Inhibitors / Q000494:pharmacology; D014779:Virus Replication
chemical_list: D061945:Caspase Inhibitors; D015853:Cysteine Proteinase Inhibitors; D015700:HIV Envelope Protein gp41; D018894:Reverse Transcriptase Inhibitors; D019888:Nelfinavir