pH regulation in early endosomes and interferon-inducible transmembrane proteins control avian retrovirus fusion

Abstract

Edited by Thomas SöllnerEnveloped viruses infect host cells by fusing their membranes with those of the host cell, a process mediated by viral glycoproteins upon binding to cognate host receptors or entering into acidic intracellular compartments. Whereas the effect of receptor density on viral infection has been well studied, the role of cell type-specific factors/processes, such as pH regulation, has not been characterized in sufficient detail. Here, we examined the effects of cell-extrinsic factors (buffer environment) and cell-intrinsic factors (interferon-inducible transmembrane proteins, IFITMs), on the pH regulation in early endosomes and on the efficiency of acid-dependent fusion of the avian sarcoma and leukosis virus (ASLV), with endosomes. First, we found that a modest elevation of external pH can raise the pH in early endosomes in a cell type-dependent manner and thereby delay the acid-induced fusion of endocytosed ASLV. Second, we observed a cell type-dependent delay between the low pH-dependent and temperature-dependent steps of viral fusion, consistent with the delayed enlargement of the fusion pore. Third, ectopic expression of IFITMs, known to potently block influenza virus fusion with late compartments, was found to only partially inhibit ASLV fusion with early endosomes. Interestingly, IFITM expression promoted virus uptake and the acidification of endosomal compartments, resulting in an accelerated fusion rate when driven by the glycosylphosphatidylinositol-anchored, but not by the transmembrane isoform of the ASLV receptor. Collectively, these results highlight the role of cell-extrinsic and cell-intrinsic factors in regulating the efficiency and kinetics of virus entry and fusion with target cells.Infectivity of enveloped viruses often varies depending on the cell type, even for cells expressing comparable levels of cognate receptors. This is largely due to the varied efficiency of multiple steps of entry leading to productive infection. Whereas multiple host factors are involved in late post-fusion steps of virus entry (1-9), the effects of intrinsic, cell type-dependent factors and extrinsic factors on viral fusion are poorly characterized. After the initial interaction of viruses with cellular receptors or attachment factors, low endosomal pH is required to trigger fusion-inducing conformational changes in most viral proteins (reviewed in Refs. 10 and 11). The low pH requirement of virus entry could also stem from the need for optimal milieu (e.g. endosomal protease activity) for priming the viral glycoproteins for the fusion reaction (10). It is thus likely that cell type-dependent regulation of endosomal pH modulates the efficiency and kinetics of virus fusion. To date, however, only a few studies have directly examined the link between the pH in virus-carrying endosomes and the efficiency/kinetics of subsequent viral fusion (12-15).Endosome-resident lipids and proteins have been implicated in the completion of virus fusion and/or the nucleocapsid release into the cytoplasm (8, 16 -22)...

Identification of Residues Controlling Restriction versus Enhancing Activities of IFITM Proteins on Entry of Human Coronaviruses

“…In addition, the sensitivity of IAVs to IFITM3 appears to depend on the pH value at which the viral hemagglutinin (HA) undergoes a conformational transition and mediates membrane fusion (54). More interestingly, IFITM expression promotes the uptake of avian sarcoma leukosis virus (ASLV) and the acidification of endosomal compartments, resulting in accelerated membrane fusion when driven by the glycosylphosphatidylinositol-anchored, but not by the transmembrane, isoform of the ASLV receptor (55). These recent findings clearly highlight the fact that multiple viral and host cellular components regulate IFITM activity in the fusion of viral envelope and endosomal membranes.…”

Virus Entry: Looking Back and Moving Forward

“…An incoming virus particle is therefore exposed to a continuously changing environment, and it has to respond to it properly. When the trigger for membrane fusion and penetration involves low pH, the threshold pH is the main factor defining the location and timing of the penetration event [56][57][58]. Rhabdo-and alphaviruses undergo acid-activated fusion in early endosomes where the pH is about 6.2.…”

Interference with the production of infectious viral particles and bimodal inhibition of replication are broadly conserved antiviral properties of IFITMs

“…The first and most studied mechanism through which IFITMs interfere with viral replication takes place in target cells. In this setting, IFITMs sequester incoming virion particles in endosomes by preventing viral-to-cellular membrane fusion, a phenomenon that first impedes the access of the virus to the cell cytoplasm and subsequently leads to its degradation [ 6 , 8 , 9 , 12 , 14 , 16 , 20 , 23 – 25 , 34 – 39 ]. It is interesting to note that this mechanism of inhibition, albeit with efficacies that depend on both the IFITM and the virus considered, targets viruses for which the passage through the acidic pH of endosomes is mandatory to trigger viral-to-cellular membranes fusion, as well as those for which this passage is not obligatory, as HIV-1 [ 23 , 40 , 41 , 42 ].…”