Actin-regulated Siglec-1 nanoclustering influences HIV-1 capture and virus-containing compartment formation in dendritic cells
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Siglec-1 (CD169), a plasma membrane-associated sialic acid-binding lectin, has been implicated in the capture of HIV and other viruses by dendritic cells and macrophages. However, the molecular details of how HIV particles are captured by Siglec-1 are poorly understood. In this paper, the authors use advanced imaging methods to analyse the cell surface distribution of Siglec-1 on immature and mature dendritic cells to study the regulation of Siglec-1 distribution by actin and regulators of actin polymerization and to understand how virus-Siglec-1 engagement leads to virus sequestration within so-called virus containing compartments. These types of analyses have only recently become feasible with the implementation of super-resolution imaging and as yet few virus-host cell systems have been examined in detail. Thus, this study has relevance for researchers studying the engagement of HIV and many other viruses with cells, as well as researchers interested in the mechanisms regulating receptor distribution and function on cells.
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Abstract
The immunoglobulin-like lectin receptor CD169 (Siglec-1) mediates the capture of HIV-1 by activated dendritic cells (DCs) through binding to sialylated ligands. These interactions result in a more efficient virus capture as compared to resting DCs, although the underlying mechanisms are poorly understood. Using a combination of super-resolution microscopy, single-particle tracking and biochemical perturbations we studied the nanoscale organization of Siglec-1 on activated DCs and its impact on viral capture and its trafficking to a single viral-containing compartment. We found that activation of DCs leads to Siglec-1 basal nanoclustering at specific plasma membrane regions where receptor diffusion is constrained by Rho-ROCK activation and formin-dependent actin polymerization. Using liposomes with varying ganglioside concentrations, we further demonstrate that Siglec-1 nanoclustering enhances the receptor avidity to limiting concentrations of gangliosides carrying sialic ligands. Binding to either HIV-1 particles or ganglioside-bearing liposomes lead to enhanced Siglec-1 nanoclustering and global actin rearrangements characterized by a drop in RhoA activity, facilitating the final accumulation of viral particles in a single sac-like compartment. Overall, our work provides new insights on the role of the actin machinery of activated DCs in regulating the formation of basal Siglec-1 nanoclustering, being decisive for the capture and actin-dependent trafficking of HIV-1 into the virus-containing compartment.
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eLife assessment
Siglec-1 (CD169), a plasma membrane-associated sialic acid-binding lectin, has been implicated in the capture of HIV and other viruses by dendritic cells and macrophages. However, the molecular details of how HIV particles are captured by Siglec-1 are poorly understood. In this paper, the authors use advanced imaging methods to analyse the cell surface distribution of Siglec-1 on immature and mature dendritic cells to study the regulation of Siglec-1 distribution by actin and regulators of actin polymerization and to understand how virus-Siglec-1 engagement leads to virus sequestration within so-called virus containing compartments. These types of analyses have only recently become feasible with the implementation of super-resolution imaging and as yet few virus-host cell systems have been examined in detail. Thus, this …
eLife assessment
Siglec-1 (CD169), a plasma membrane-associated sialic acid-binding lectin, has been implicated in the capture of HIV and other viruses by dendritic cells and macrophages. However, the molecular details of how HIV particles are captured by Siglec-1 are poorly understood. In this paper, the authors use advanced imaging methods to analyse the cell surface distribution of Siglec-1 on immature and mature dendritic cells to study the regulation of Siglec-1 distribution by actin and regulators of actin polymerization and to understand how virus-Siglec-1 engagement leads to virus sequestration within so-called virus containing compartments. These types of analyses have only recently become feasible with the implementation of super-resolution imaging and as yet few virus-host cell systems have been examined in detail. Thus, this study has relevance for researchers studying the engagement of HIV and many other viruses with cells, as well as researchers interested in the mechanisms regulating receptor distribution and function on cells.
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Reviewer #1 (Public Review):
Gutiérrez-Martínez et al. present a detailed analysis of Siglec-1 nano-distribution on the surface of dendritic cells (DCs) and the role of Siglec-1 in HIV-1 interactions with DCs.
DCs have been proposed as key cellular intermediates in the transmission of HIV and other viruses. Not only can these cells be crucial for the presentation of virus-derived antigens, but, in tissue culture at least, mature DCs (mDC) have been observed to sequester HIV particles into compartments (virus-containing compartment [VCC]) from which the virus can be subsequently transmitted to CD4+ve T cells through cell-cell contacts often termed virological synapses. This so-called trans-infection mechanism is believed to be important in establishing HIV infection and transmission of the virus to immunological tissues. Although there …
Reviewer #1 (Public Review):
Gutiérrez-Martínez et al. present a detailed analysis of Siglec-1 nano-distribution on the surface of dendritic cells (DCs) and the role of Siglec-1 in HIV-1 interactions with DCs.
DCs have been proposed as key cellular intermediates in the transmission of HIV and other viruses. Not only can these cells be crucial for the presentation of virus-derived antigens, but, in tissue culture at least, mature DCs (mDC) have been observed to sequester HIV particles into compartments (virus-containing compartment [VCC]) from which the virus can be subsequently transmitted to CD4+ve T cells through cell-cell contacts often termed virological synapses. This so-called trans-infection mechanism is believed to be important in establishing HIV infection and transmission of the virus to immunological tissues. Although there is considerable evidence for this process, the molecular details of how HIV particles are captured by DCs and transferred to VCC are poorly understood. In recent years Siglec-1 (CD169), a plasma membrane-associated sialic acid-binding lectin expressed on monocytic cells has been implicated in the capture of HIV and other viruses. In this paper, the authors have used super-resolution and other imaging methods to perform a detailed quantitative analysis of the cell surface distribution of Siglec-1 on immature and mature DCs, the relationship between this distribution with actin and regulators of actin polymerization, and then how this impacts on the capture of HIV particles and their association with VCCs.
The principal findings, which for the most part are well supported by the data, suggest that small clusters of Siglec-1, which are restricted in their mobility by formin-associated actin, provide platforms with increased avidity for binding virus particles or large unilamellar vesicles through sialic-acid containing gangliosides. In mDCs at least this binding appears to induce the sequestration of bound particles into VCC-like structures. This is a topical and detailed study that addresses important questions of how viral engagement with cell surface receptors leads to events crucial for viral infection and, potentially, pathogenesis. These types of analyses have only recently become feasible with the implementation of super-resolution imaging and few virus-host cell systems have been examined in detail. Thus, this study has relevance not only to HIV but potentially to many other viruses.
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Reviewer #2 (Public Review):
The authors first characterize Siglec-1 clustering on immature and mature DCs and observe that clustering increases in mature DCs. Concomitantly with clustering, the mobility of Siglec-1 reduced. At the cell periphery of mDCs, Siglec-1 was enriched in actin-rich areas. A role for actin, specifically for the formin-nucleated actin was supported using inhibitors. Concomitantly the clustering of Siglec-1 was reduced. The localization of Siglec-1 to actin-rich filopodia was dependent on formin activation and RhoA, ROCK-mediated ERM phosphorylation. With respect to consequences for the binding of HIV particles, forming, and Rho-dependent Siglec-1 nanoclustering, enhanced binding of virus particles indicating that clustering of Siglec-1 provides for better docking sites. On the ligand side, high amounts of GM1 …
Reviewer #2 (Public Review):
The authors first characterize Siglec-1 clustering on immature and mature DCs and observe that clustering increases in mature DCs. Concomitantly with clustering, the mobility of Siglec-1 reduced. At the cell periphery of mDCs, Siglec-1 was enriched in actin-rich areas. A role for actin, specifically for the formin-nucleated actin was supported using inhibitors. Concomitantly the clustering of Siglec-1 was reduced. The localization of Siglec-1 to actin-rich filopodia was dependent on formin activation and RhoA, ROCK-mediated ERM phosphorylation. With respect to consequences for the binding of HIV particles, forming, and Rho-dependent Siglec-1 nanoclustering, enhanced binding of virus particles indicating that clustering of Siglec-1 provides for better docking sites. On the ligand side, high amounts of GM1 lipids (4%) were needed for liposomes to be captured by Siglec-1, reinforcing the idea of docking sites. Consistent with the important role of actin in the process, time course studies of virus binding to mDCs revealed dramatic changes in the plasma membrane architecture including the emergence of membrane ruffles, shrinkage of the basal membrane, and constriction of the cell membrane where VLPs accumulate on route to the formation of the virus-containing compartment. Overall, the strength of this report is its comprehensive nature, detailed and quantitative imaging analysis, and confirmation of the importance of Siglec-1 clustering (receptor) with liposomes containing the ligand GM1.
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Reviewer #3 (Public Review):
In this manuscript, the authors address the mechanism of concentration of HIV-1 particles following interaction with Siglec-1 and define important differences in this process between immature DCs and mature DCs. The methods are largely derived from imaging that is followed by quantitation of nanoclustering of Siglec-1, distance from the center of the cell, and the effects of inhibitors of actin and RhoA pathways. The quantitative imaging approach is a strength and appears quite carefully done. Another strength is the new findings regarding the role of the formin-dependent actin cytoskeletal rearrangements and RhoA activation on clustering and polarization leading to the formation of the virus-containing compartment (VCC). The results are convincing that mature DCs demonstrate more nanoclustering and that …
Reviewer #3 (Public Review):
In this manuscript, the authors address the mechanism of concentration of HIV-1 particles following interaction with Siglec-1 and define important differences in this process between immature DCs and mature DCs. The methods are largely derived from imaging that is followed by quantitation of nanoclustering of Siglec-1, distance from the center of the cell, and the effects of inhibitors of actin and RhoA pathways. The quantitative imaging approach is a strength and appears quite carefully done. Another strength is the new findings regarding the role of the formin-dependent actin cytoskeletal rearrangements and RhoA activation on clustering and polarization leading to the formation of the virus-containing compartment (VCC). The results are convincing that mature DCs demonstrate more nanoclustering and that formins and RhoA are important in the clustering that occurs of viruses or virus-like particles following capture by Siglec-1. This information should be valuable to the field.
The weaknesses are not in the methods and major conclusions themselves, but there are a number of aspects of the study that could be strengthened. The definition of a VCC here is simply a spot of Siglec-1 that has coalesced with VLPs. A more complete study would include typical VCC markers such as CD81, CD9, and others and would extend the findings to prove that the mechanism invoked actually elicits VCC formation, as opposed to clustering of Siglec-1 and VLPs along the surface of the cell. This study does not establish the mechanism of membrane invagination or tubule formation that occurs with VCC formation, so perhaps it is really describing the initial, surface-related steps of VCC formation but not subsequent internalization events required to form the deeper, vacuole-like VCC.
Nevertheless, this study provides new insights into the initial steps of VCC formation and is provocative regarding how this can be achieved by Siglec-1 in the absence of the need for a cytoplasmic tail. The formin-dependence of VCC formation will be of interest in future studies of HIV uptake and trans-infection events mediated by dendritic cells and macrophages. Some of the findings can be directly translated to the biological context of how VCCs form in HIV-infected macrophages. These will all likely be of substantial interest to those working on HIV and other viruses that are captured by Siglec-1.
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