HIV-1 triggers WAVE2 phosphorylation in primary CD4 T cells and macrophages, mediating Arp2/3-dependent nuclear migration

From Entry to the Nucleus: How Retroviruses Commute

Once inside host cells, retroviruses generate a double-stranded DNA copy of their RNA genomes via reverse transcription inside a viral core, and this viral DNA is subsequently integrated into the genome of the host cell. Before integration can occur, the core must cross the cell cortex, be transported through the cytoplasm, and enter the nucleus. Retroviruses have evolved different mechanisms to accomplish this journey. This review examines the various mechanisms retroviruses, especially HIV-1, have evolved to commute throughout the cell. Retroviruses cross the cell cortex while modulating actin dynamics and use microtubules as roads while connecting with microtubule-associated proteins and motors to reach the nucleus. Although a clearer picture exists for HIV-1 compared with other retroviruses, there is still much to learn about how retroviruses accomplish their commute.

Characterization of a CXCR4 antagonist TIQ-15 with dual tropic HIV entry inhibition properties

The chemokine co-receptors CXCR4 and CCR5 mediate HIV entry and signal transduction necessary for viral infection. However, to date only the CCR5 antagonist maraviroc is approved for treating HIV-1 infection. Given that approximately 50% of late-stage HIV patients also develop CXCR4-tropic virus, clinical anti-HIV CXCR4 antagonists are needed. Here, we describe a novel allosteric CXCR4 antagonist TIQ-15 which inhibits CXCR4-tropic HIV-1 infection of primary and transformed CD4 T cells. TIQ-15 blocks HIV entry with an IC50 of 13 nM. TIQ-15 also inhibits SDF-1α/CXCR4-mediated cAMP production, cofilin activation, and chemotactic signaling. In addition, TIQ-15 induces CXCR4 receptor internalization without affecting the levels of the CD4 receptor, suggesting that TIQ-15 may act through a novel allosteric site on CXCR4 for blocking HIV entry. Furthermore, TIQ-15 did not inhibit VSV-G pseudotyped HIV-1 infection, demonstrating its specificity in blocking CXCR4-tropic virus entry, but not CXCR4-independent endocytosis or post-entry steps. When tested against a panel of clinical isolates, TIQ-15 showed potent inhibition against CXCR4-tropic and dual-tropic viruses, and moderate inhibition against CCR5-tropic isolates. This observation was followed by a co-dosing study with maraviroc, and TIQ-15 demonstrated synergistic activity. In summary, here we describe a novel HIV-1 entry inhibitor, TIQ-15, which potently inhibits CXCR4-tropic viruses while possessing low-level synergistic activities against CCR5-tropic viruses. TIQ-15 could potentially be co-dosed with the CCR5 inhibitor maraviroc to block viruses of mixed tropisms.

HIV-1 Mediated Cortical Actin Disruption Mirrors ARP2/3 Defects Found in Primary T Cell Immunodeficiencies

During cell movement, cortical actin balances mechanical and osmotic forces to maintain cell function while providing the scaffold for cell shape. Migrating CD4+ T cells have a polarized structure with a leading edge containing dynamic branched and linear F-actin structures that bridge intracellular components to surface adhesion molecules. These actin structures are complemented with a microtubular network beaded with membrane bound organelles in the trailing uropod. Disruption of actin structures leads to dysregulated migration and changes in morphology of affected cells. In HIV-1 infection, CD4+ T cells have dysregulated movement. However, the precise mechanisms by which HIV-1 affects CD4+ T cell movement are unknown. Here, we show that HIV-1 infection of primary CD4+ T cells causes at least four progressive morphological differences as a result of virally induced cortical cytoskeleton disruption, shown by ultrastructural and time lapse imaging. Infection with a ΔNef virus partially abrogated the dysfunctional phenotype in infected cells and partially restored a wild-type shape. The pathological morphologies after HIV-1 infection phenocopy leukocytes which contain genetic determinants of specific T cell Inborn Errors of Immunity (IEI) or Primary Immunodeficiencies (PID) that affect the actin cytoskeleton. To identify potential actin regulatory pathways that may be linked to the morphological deformities, uninfected CD4+ T cell morphology was characterized following addition of small molecule chemical inhibitors. The ARP2/3 inhibitor CK-666 recapitulated three of the four abnormal morphologies we observed in HIV-1 infected cells. Restoring ARP2/3 function and cortical actin integrity in people living with HIV-1 infection is a new avenue of investigation to eradicate HIV-1 infected cells from the body.

The chemokine receptor CCR5: multi-faceted hook for HIV-1

Chemokines are cytokines whose primary role is cellular activation and stimulation of leukocyte migration. They perform their various functions by interacting with G protein-coupled cell surface receptors (GPCRs) and are involved in the regulation of many biological processes such as apoptosis, proliferation, angiogenesis, hematopoiesis or organogenesis. They contribute to the maintenance of the homeostasis of lymphocytes and coordinate the function of the immune system. However, chemokines and their receptors are sometimes hijacked by some pathogens to infect the host organism. For a given chemokine receptor, there is a wide structural, organizational and conformational diversity. In this review, we describe the evidence for structural variety reported for the chemokine receptor CCR5, how this variability can be exploited by HIV-1 to infect its target cells and what therapeutic solutions are currently being developed to overcome this problem.

Cell-to-cell transmission of HIV-1 from provirus-activated cells to resting naïve and memory human primary CD4 T cells is highly efficient and requires CD4 and F-actin but not chemokine receptors

Latently infected cells harboring replication-competent proviruses represent a major barrier to HIV-1 cure. One major effort to purge these cells has focused on developing the "shock and kill" approach for forcing provirus reactivation to induce cell killing by viral cytopathic effects, host immune responses, or both. We conducted kinetic and mechanistic studies of HIV-1 protein expression, virion production, and cell-to-cell virus transmission during provirus reactivation. Provirus-activated ACH-2 cells stimulated with romidepsin (RMD) or PMA produced Nef early, and then Env and Gag in parallel with the appearance of virions. Env on the surface of provirus-activated cells and cellular F-actin were critical in the formation of virological synapses to mediate cell-to-cell transmission of HIV-1 from provirus-activated cells to uninfected cells. This HIV-1 cell-to-cell transmission was substantially more efficient than transmission seen via cell-free virus spread and required F-actin remodeling and CD4, but not chemokine receptors. Resting human primary CD4⁺ T cells including naïve and memory subpopulations and, especially the memory CD4⁺ T cells, were highly susceptible to HIV-1 infection via cell-to-cell transmission. Cell-to-cell transmission of HIV-1 from provirus-activated cells was profoundly decreased by protease inhibitors (PIs) and neutralizing antibodies (nAbs) that recognize the CD4-binding site (CD4bs) such as VRC01, but not by reverse transcriptase (RT) inhibitor Emtricitabine (FTC). Therefore, our results suggest that PIs with potent blocking abilities should be used in clinical application of the "shock and kill" approach, most likely in combination with CD4bs nAbs, to prevent new HIV-1 infections.

WASP family proteins: Molecular mechanisms and implications in human disease

Proteins of the Wiskott-Aldrich syndrome protein (WASP) family play a central role in regulating actin cytoskeletal dynamics in a wide range of cellular processes. Genetic mutations or misregulation of these proteins are tightly associated with many diseases. The WASP-family proteins act by transmitting various upstream signals to their conserved WH2-Central-Acidic (WCA) peptide sequence at the C-terminus, which in turn binds to the Arp2/3 complex to stimulate the formation of branched actin networks at membranes. Despite this common feature, the regulatory mechanisms and cellular functions of distinct WASP-family proteins are very different. Here, we summarize and clarify our current understanding of WASP-family proteins and how disruption of their functions is related to human disease.

The Actin Regulators Involved in the Function and Related Diseases of Lymphocytes

Actin is an important cytoskeletal protein involved in signal transduction, cell structure and motility. Actin regulators include actin-monomer-binding proteins, Wiskott-Aldrich syndrome (WAS) family of proteins, nucleation proteins, actin filament polymerases and severing proteins. This group of proteins regulate the dynamic changes in actin assembly/disassembly, thus playing an important role in cell motility, intracellular transport, cell division and other basic cellular activities. Lymphocytes are important components of the human immune system, consisting of T-lymphocytes (T cells), B-lymphocytes (B cells) and natural killer cells (NK cells). Lymphocytes are indispensable for both innate and adaptive immunity and cannot function normally without various actin regulators. In this review, we first briefly introduce the structure and fundamental functions of a variety of well-known and newly discovered actin regulators, then we highlight the role of actin regulators in T cell, B cell and NK cell, and finally provide a landscape of various diseases associated with them. This review provides new directions in exploring actin regulators and promotes more precise and effective treatments for related diseases.

Prestimulation of CD2 confers resistance to HIV-1 latent infection in blood resting CD4 T cells

HIV-1 infects blood CD4 T cells through the use of CD4 and CXCR4 or CCR5 receptors, which can be targeted through blocking viral binding to CD4/CXCR4/CCR5 or virus-cell fusion. Here we describe a novel mechanism by which HIV-1 nuclear entry can also be blocked through targeting a non-entry receptor, CD2. Cluster of differentiation 2 (CD2) is an adhesion molecule highly expressed on human blood CD4, particularly, memory CD4 T cells. We found that CD2 ligation with its cell-free ligand LFA-3 or anti-CD2 antibodies rendered blood resting CD4 T cells highly resistant to HIV-1 infection. We further demonstrate that mechanistically, CD2 binding initiates competitive signaling leading to cofilin activation and localized actin polymerization around CD2, which spatially inhibits HIV-1-initiated local actin polymerization needed for viral nuclear migration. Our study identifies CD2 as a novel target to block HIV-1 infection of blood resting T cells.

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CD2 is highly expressed on human blood CD4 T cells, particularly memory T cells

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Prestimulation of CD2 rendered resting T cells highly resistant to HIV infection

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CD2 signaling activates cofilin and actin polymerization blocking HIV nuclear entry

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CD2 may serve as a novel target to inhibit HIV-1 infection of blood resting T cells

How Physical Factors Coordinate Virus Infection: A Perspective From Mechanobiology

Pandemics caused by viruses have threatened lives of thousands of people. Understanding the complicated process of viral infection provides significantly directive implication to epidemic prevention and control. Viral infection is a complex and diverse process, and substantial studies have been complemented in exploring the biochemical and molecular interactions between viruses and hosts. However, the physical microenvironment where infections implement is often less considered, and the role of mechanobiology in viral infection remains elusive. Mechanobiology focuses on sensation, transduction, and response to intracellular and extracellular physical factors by tissues, cells, and extracellular matrix. The intracellular cytoskeleton and mechanosensors have been proven to be extensively involved in the virus life cycle. Furthermore, innovative methods based on micro- and nanofabrication techniques are being utilized to control and modulate the physical and chemical cell microenvironment, and to explore how extracellular factors including stiffness, forces, and topography regulate viral infection. Our current review covers how physical factors in the microenvironment coordinate viral infection. Moreover, we will discuss how this knowledge can be harnessed in future research on cross-fields of mechanobiology and virology.

IP-10 Promotes Latent HIV Infection in Resting Memory CD4+ T Cells via LIMK-Cofilin Pathway

A major barrier to HIV eradication is the persistence of viral reservoirs. Resting CD4⁺ T cells are thought to be one of the major viral reservoirs, However, the underlying mechanism regulating HIV infection and the establishment of viral reservoir in T cells remain poorly understood. We have investigated the role of IP-10 in the establishment of HIV reservoirs in CD4⁺ T cells, and found that in HIV-infected individuals, plasma IP-10 was elevated, and positively correlated with HIV viral load and viral reservoir size. In addition, we found that binding of IP-10 to CXCR3 enhanced HIV latent infection of resting CD4⁺ T cells in vitro. Mechanistically, IP-10 stimulation promoted cofilin activity and actin dynamics, facilitating HIV entry and DNA integration. Moreover, treatment of resting CD4⁺ T cells with a LIM kinase inhibitor R10015 blocked cofilin phosphorylation and abrogated IP-10-mediated enhancement of HIV latent infection. These results suggest that IP-10 is a critical factor involved in HIV latent infection, and that therapeutic targeting of IP-10 may be a potential strategy for inhibiting HIV latent infection.

Co-receptor signaling in the pathogenesis of neuroHIV

The HIV co-receptors, CCR5 and CXCR4, are necessary for HIV entry into target cells, interacting with the HIV envelope protein, gp120, to initiate several signaling cascades thought to be important to the entry process. Co-receptor signaling may also promote the development of neuroHIV by contributing to both persistent neuroinflammation and indirect neurotoxicity. But despite the critical importance of CXCR4 and CCR5 signaling to HIV pathogenesis, there is only one therapeutic (the CCR5 inhibitor Maraviroc) that targets these receptors. Moreover, our understanding of co-receptor signaling in the specific context of neuroHIV is relatively poor. Research into co-receptor signaling has largely stalled in the past decade, possibly owing to the complexity of the signaling cascades and functions mediated by these receptors. Examining the many signaling pathways triggered by co-receptor activation has been challenging due to the lack of specific molecular tools targeting many of the proteins involved in these pathways and the wide array of model systems used across these experiments. Studies examining the impact of co-receptor signaling on HIV neuropathogenesis often show activation of multiple overlapping pathways by similar stimuli, leading to contradictory data on the effects of co-receptor activation. To address this, we will broadly review HIV infection and neuropathogenesis, examine different co-receptor mediated signaling pathways and functions, then discuss the HIV mediated signaling and the differences between activation induced by HIV and cognate ligands. We will assess the specific effects of co-receptor activation on neuropathogenesis, focusing on neuroinflammation. We will also explore how the use of substances of abuse, which are highly prevalent in people living with HIV, can exacerbate the neuropathogenic effects of co-receptor signaling. Finally, we will discuss the current state of therapeutics targeting co-receptors, highlighting challenges the field has faced and areas in which research into co-receptor signaling would yield the most therapeutic benefit in the context of HIV infection. This discussion will provide a comprehensive overview of what is known and what remains to be explored in regard to co-receptor signaling and HIV infection, and will emphasize the potential value of HIV co-receptors as a target for future therapeutic development.

Relationships Between HIV-Mediated Chemokine Coreceptor Signaling, Cofilin Hyperactivation, Viral Tropism Switch and HIV-Mediated CD4 Depletion

HIV infection causes CD4 depletion and immune deficiency. The virus infects CD4 T cells through binding to CD4 and one of the chemokine coreceptors, CXCR4 (X4) or CCR5 (R5). It has also been known that HIV tropism switch, from R5 to X4, is associated with rapid CD4 depletion, suggesting a key role of viral factors in driving CD4 depletion. However, the virological driver for HIV-mediated CD4 depletion has not been fully elucidated. We hypothesized that HIV-mediated chemokine coreceptor signaling, particularly chronic signaling through CXCR4, plays a major role in CD4 dysfunction and depletion; we also hypothesized that there is an R5X4 signaling (R5X4sig) viral subspecies, evolving from the natural replication course of R5-utilizing viruses, that is responsible for CD4 T cell depletion in R5 virus infection. To gain traction for our hypothesis, in this review, we discuss a recent finding from Cui and co-authors who described the rapid tropism switch and high pathogenicity of an HIV-1 R5 virus, CRF01_AE. We speculate that CRF01_AE may be the hypothetical R5X4sig viral species that is rapidly evolving towards the X4 phenotype. We also attempt to discuss the intricate relationships between HIV-mediated chemokine coreceptor signaling, viral tropism switch and HIV-mediated CD4 depletion, in hopes of providing a deeper understanding of HIV pathogenesis in blood CD4 T cells.

Identification of A functional region in Bombyx mori nucleopolyhedrovirus VP39 that is essential for nuclear actin polymerization

Nuclear actin polymerization plays an indispensable role in the nuclear assembly of baculovirus nucleocapsid, but the underlying viral infection-mediated mechanism remains unclear. VP39 is the major protein in baculovirus capsid, which builds the skeleton of the capsid tubular structure. VP39 is suggested in previous studies to interact with cellular actin and mediate actin polymerization. However, it is unclear about the role of VP39 in mediating nuclear actin polymerization. Results in this study indicated that vp39 deletion abolished nuclear actin polymerization, which was recovered after vp39 repair, revealing the essential part of VP39 in nuclear actin polymerization. Furthermore, a series of mutants with vp39 deletions were constructed to analyze the important region responsible for nuclear actin polymerization. In addition, intracellular localization analysis demonstrated that the amino acids 192-286 in VP39 C-terminal are responsible for nuclear actin polymerization.

Quantum dot conjugated saporin activates microglia and induces selective substantia nigra degeneration

Parkinson's disease (PD) is characterized by profound microglial driven inflammatory processes and the loss of dopamine neurons of the substantia nigra (SNc). Both microglia and dopamine neurons that are affected in the SNc are particularly vulnerable to environmental toxicants and finding more selective ways of targeting these cell types is of importance. Quantum dots (QDs) might be a useful vehicle for selectively delivering toxicants to microglia and owing to their fluorescent capability, they can be microscopically tracked within the cell. Accordingly, we assessed the impact of QDs alone and QDs conjugated to the ribosomal toxin, saporin, upon SNc microglia and dopamine neurons. We found that intra-SNc infused QDs selectively entered microglia and induced morphological changes consistent with an activated state. QDs conjugated to saporin also caused a significant loss of dopamine neurons and motor coordination (on a rotarod test) deficits, along with an increase in the inflammatory microglial actin regulatory factors, WAVE2. These data suggest that QDs might be a viable route for toxicant delivery and also has an added advantage of being fluorescently visible. Ultimately, we found SNc neurons to be exceptionally vulnerable to QD-saporin and suggest that this could be a novel targeted approach to model PD-like inflammatory pathology.

All-Round Manipulation of the Actin Cytoskeleton by HIV

While significant progress has been made in terms of human immunodeficiency virus (HIV) therapy, treatment does not represent a cure and remains inaccessible to many people living with HIV. Continued mechanistic research into the viral life cycle and its intersection with many aspects of cellular biology are not only fundamental in the continued fight against HIV, but also provide many key observations of the workings of our immune system. Decades of HIV research have testified to the integral role of the actin cytoskeleton in both establishing and spreading the infection. Here, we review how the virus uses different strategies to manipulate cellular actin networks and increase the efficiency of various stages of its life cycle. While some HIV proteins seem able to bind to actin filaments directly, subversion of the cytoskeleton occurs indirectly by exploiting the power of actin regulatory proteins, which are corrupted at multiple levels. Furthermore, this manipulation is not restricted to a discrete class of proteins, but rather extends throughout all layers of the cytoskeleton. We discuss prominent examples of actin regulators that are exploited, neutralized or hijacked by the virus, and address how their coordinated deregulation can lead to changes in cellular behavior that promote viral spreading.

Flow Cytometry-Based Quantification of HIV-Induced T Cell Chemotactic Response

The human immunodeficiency virus (HIV) infects blood CD4 T cells through binding to CD4 and the chemokine co-receptor, CXCR4 or CCR5. This viral binding to CXCR4 or CCR5 also triggers the activation of a variety of signaling molecules such as LIMK/cofilin and WAVE2/Arp2/3 to promote actin dynamics, which are necessary for viral nuclear migration and the latent infection of blood resting CD4 T cells. In this chapter, we describe the methods for quantification of HIV-induced actin polymerization and cofilin phosphorylation in human T cells using flow cytometry.

Chemokines and Chemokine Receptors: Accomplices for Human Immunodeficiency Virus Infection and Latency

Chemokines are small chemotactic cytokines that are involved in the regulation of immune cell migration. Multiple functional properties of chemokines, such as pro-inflammation, immune regulation, and promotion of cell growth, angiogenesis, and apoptosis, have been identified in many pathological and physiological contexts. Human immunodeficiency virus (HIV) infection is characterized by persistent inflammation and immune activation during both acute and chronic phases, and the "cytokine storm" is one of the hallmarks of HIV infection. Along with immune activation after HIV infection, an extensive range of chemokines and other cytokines are elevated, thereby generating the so-called "cytokine storm." In this review, the effects of the upregulated chemokines and chemokine receptors on the processes of HIV infection are discussed. The objective of this review was to focus on the main chemokines and chemokine receptors that have been found to be associated with HIV infection and latency. Elevated chemokines and chemokine receptors have been shown to play important roles in the HIV life cycle, disease progression, and HIV reservoir establishment. Thus, targeting these chemokines and receptors and the other proteins of related signaling pathways might provide novel therapeutic strategies, and the evidence indicates a promising future regarding the development of a functional cure for HIV.

Impairment of CCR6+ and CXCR3+ Th Cell Migration in HIV-1 Infection Is Rescued by Modulating Actin Polymerization

CD4+ T cell repopulation of the gut is rarely achieved in HIV-1-infected individuals who are receiving clinically effective antiretroviral therapy. Alterations in the integrity of the mucosal barrier have been indicated as a cause for chronic immune activation and disease progression. In this study, we present evidence that persistent immune activation causes impairment of lymphocytes to respond to chemotactic stimuli, thus preventing their trafficking from the blood stream to peripheral organs. CCR6+ and CXCR3+ Th cells accumulate in the blood of aviremic HIV-1-infected patients on long-term antiretroviral therapy, and their frequency in the circulation positively correlates to levels of soluble CD14 in plasma, a marker of chronic immune activation. Th cells show an impaired response to chemotactic stimuli both in humans and in the pathogenic model of SIV infection, and this defect is due to hyperactivation of cofilin and inefficient actin polymerization. Taking advantage of a murine model of chronic immune activation, we demonstrate that cytoskeleton remodeling, induced by okadaic acid, restores lymphocyte migration in response to chemokines, both in vitro and in vivo. This study calls for novel pharmacological approaches in those pathological conditions characterized by persistent immune activation and loss of trafficking of T cell subsets to niches that sustain their maturation and activities.

Autographa californica Multiple Nucleopolyhedrovirus Ac34 Protein Retains Cellular Actin-Related Protein 2/3 Complex in the Nucleus by Subversion of CRM1-Dependent Nuclear Export

Actin, nucleation-promoting factors (NPFs), and the actin-related protein 2/3 complex (Arp2/3) are key elements of the cellular actin polymerization machinery. With nuclear actin polymerization implicated in ever-expanding biological processes and the discovery of the nuclear import mechanisms of actin and NPFs, determining Arp2/3 nucleo-cytoplasmic shuttling mechanism is important for understanding the function of nuclear actin. A unique feature of alphabaculovirus infection of insect cells is the robust nuclear accumulation of Arp2/3, which induces actin polymerization in the nucleus to assist in virus replication. We found that Ac34, a viral late gene product encoded by the alphabaculovirus Autographa californica multiple nucleopolyhedrovirus (AcMNPV), is involved in Arp2/3 nuclear accumulation during virus infection. Further assays revealed that the subcellular distribution of Arp2/3 under steady-state conditions is controlled by chromosomal maintenance 1 (CRM1)-dependent nuclear export. Upon AcMNPV infection, Ac34 inhibits CRM1 pathway and leads to Arp2/3 retention in the nucleus.

CDC42 Use in Viral Cell Entry Processes by RNA Viruses

The cellular actin cytoskeleton presents a barrier that must be overcome by many viruses, and it has become increasingly apparent many viral species have developed a diverse repertoire of mechanisms to hijack cellular actin-regulating signalling pathways as part of their cell entry processes. The Rho family GTPase Cdc42 is appreciated as a key moderator of cellular actin dynamics, and the development of specific Cdc42-inhibiting agents has given us an unprecedented ability to investigate its individual role in signalling pathways. However, investigative use of said agents, and the subsequent characterisation of the role Cdc42 plays in viral entry processes has been lacking. Here, we describe the current literature on the role of Cdc42 in human immunodeficiency virus (HIV)-1 cell entry, which represents the most investigated instance of Cdc42 function in viral cell entry processes, and also review evidence of Cdc42 use in other RNA virus cell entries, demonstrating prime areas for more extensive research using similar techniques.

Involvement of the Rac1-IRSp53-Wave2-Arp2/3 Signaling Pathway in HIV-1 Gag Particle Release in CD4 T Cells

During HIV-1 assembly, the Gag viral proteins are targeted and assemble at the inner leaflet of the cell plasma membrane. This process could modulate the cortical actin cytoskeleton, located underneath the plasma membrane, since actin dynamics are able to promote localized membrane reorganization. In addition, activated small Rho GTPases are known for regulating actin dynamics and membrane remodeling. Therefore, the modulation of such Rho GTPase activity and of F-actin by the Gag protein during virus particle formation was considered. Here, we studied the implication of the main Rac1, Cdc42, and RhoA small GTPases, and some of their effectors, in this process. The effect of small interfering RNA (siRNA)-mediated Rho GTPases and silencing of their effectors on Gag localization, Gag membrane attachment, and virus-like particle production was analyzed by immunofluorescence coupled to confocal microscopy, membrane flotation assays, and immunoblot assays, respectively. In parallel, the effect of Gag expression on the Rac1 activation level was monitored by G-LISA, and the intracellular F-actin content in T cells was monitored by flow cytometry and fluorescence microscopy. Our results revealed the involvement of activated Rac1 and of the IRSp53-Wave2-Arp2/3 signaling pathway in HIV-1 Gag membrane localization and particle release in T cells as well as a role for actin branching and polymerization, and this was solely dependent on the Gag viral protein. In conclusion, our results highlight a new role for the Rac1-IRSp53-Wave2-Arp2/3 signaling pathway in the late steps of HIV-1 replication in CD4 T lymphocytes.

IMPORTANCE

During HIV-1 assembly, the Gag proteins are targeted and assembled at the inner leaflet of the host cell plasma membrane. Gag interacts with specific membrane phospholipids that can also modulate the regulation of cortical actin cytoskeleton dynamics. Actin dynamics can promote localized membrane reorganization and thus can be involved in facilitating Gag assembly and particle formation. Activated small Rho GTPases and effectors are regulators of actin dynamics and membrane remodeling. We thus studied the effects of the Rac1, Cdc42, and RhoA GTPases and their specific effectors on HIV-1 Gag membrane localization and viral particle release in T cells. Our results show that activated Rac1 and the IRSp53-Wave2-Arp2/3 signaling pathway are involved in Gag plasma membrane localization and viral particle production. This work uncovers a role for cortical actin through the activation of Rac1 and the IRSp53/Wave2 signaling pathway in HIV-1 particle formation in CD4 T lymphocytes.

Pathogenic microbes manipulate cofilin activity to subvert actin cytoskeleton

Actin-depolymerizing factor (ADF)/cofilin proteins are key players in controlling the temporal and spatial extent of actin dynamics, which is crucial for mediating host-pathogen interactions. Pathogenic microbes have evolved molecular mechanisms to manipulate cofilin activity to subvert the actin cytoskeletal system in host cells, promoting their internalization into the target cells, modifying the replication niche and facilitating their intracellular and intercellular dissemination. The study of how these pathogens exploit cofilin pathways is crucial for understanding infectious disease and providing potential targets for drug therapies.

WASp family verprolin-homologous protein-2 (WAVE2) and Wiskott-Aldrich syndrome protein (WASp) engage in distinct downstream signaling interactions at the T cell antigen receptor site

T cell antigen receptor (TCR) engagement has been shown to activate pathways leading to actin cytoskeletal polymerization and reorganization, which are essential for lymphocyte activation and function. Several actin regulatory proteins were implicated in regulating the actin machinery, such as members of the Wiskott-Aldrich syndrome protein (WASp) family. These include WASp and the WASp family verprolin-homologous protein-2 (WAVE2). Although WASp and WAVE2 share several structural features, the precise regulatory mechanisms and potential redundancy between them have not been fully characterized. Specifically, unlike WASp, the dynamic molecular interactions that regulate WAVE2 recruitment to the cell membrane and specifically to the TCR signaling complex are largely unknown. Here, we identify the molecular mechanism that controls the recruitment of WAVE2 in comparison with WASp. Using fluorescence resonance energy transfer (FRET) and novel triple-color FRET (3FRET) technology, we demonstrate how WAVE2 signaling complexes are dynamically regulated during lymphocyte activation in vivo. We show that, similar to WASp, WAVE2 recruitment to the TCR site depends on protein-tyrosine kinase, ZAP-70, and the adaptors LAT, SLP-76, and Nck. However, in contrast to WASp, WAVE2 leaves this signaling complex and migrates peripherally together with vinculin to the membrane leading edge. Our experiments demonstrate that WASp and WAVE2 differ in their dynamics and their associated proteins. Thus, this study reveals the differential mechanisms regulating the function of these cytoskeletal proteins.

Quantitative analysis of the processes and signaling events involved in early HIV-1 infection of T cells

Lymphocyte invasion by HIV-1 is a complex, highly regulated process involving many different types of molecules that is prompted by the virus's association with viral receptors located at the cell-surface membrane that culminates in the formation of a fusion pore through which the virus enters the cell. A great deal of work has been done to identify the key actors in the process and determine the regulatory interactions; however, there have been no reports to date of attempts being made to fully understand the system dynamics through a systemic, quantitative modeling approach. In this paper, we introduce a dynamic mathematical model that integrates the available information on the molecular events involved in lymphocyte invasion. Our model shows that moesin activation is induced by virus signaling, while filamin-A is mobilized by the receptor capping. Actin disaggregation from the cap is facilitated by cofilin. Cofilin is inactivated by HIV-1 signaling in activated lymphocytes, while in resting lymphocytes another signal is required to activate cofilin in the later stages in order to accelerate the decay of the aggregated actin as a restriction factor for the viral entry. Furthermore, stopping the activation signaling of moesin is sufficient to liberate the actin filaments from the cap. The model also shows the positive effect of gelsolin on actin capping by means of the nucleation effect. These findings allow us to propose novel approaches in the search for new therapeutic strategies. In particular, gelsolin inhibition is seen as a promising target for preventing HIV-1 entry into lymphocytes, due to its role in facilitating the capping needed for the invasion. Also it is shown that HIV-1 should overcome the cortical actin barrier during early infection and predicts the different susceptibility of CD4+ T cells to be infected in terms of actin cytoskeleton dynamics driven by associated cellular factors.

Viral exploitation of actin: force-generation and scaffolding functions in viral infection

As a fundamental component of the host cellular cytoskeleton, actin is routinely engaged by infecting viruses. Furthermore, viruses from diverse groups, and infecting diverse hosts, have convergently evolved an array of mechanisms for manipulating the actin cytoskeleton for efficacious infection. An ongoing chorus of research now indicates that the actin cytoskeleton is critical for viral replication at many stages of the viral life cycle, including binding, entry, nuclear localization, genomic transcription and reverse transcription, assembly, and egress/dissemination. Specifically, viruses subvert the force-generating and macromolecular scaffolding properties of the actin cytoskeleton to propel viral surfing, internalization, and migration within the cell. Additionally, viruses utilize the actin cytoskeleton to support and organize assembly sites, and eject budding virions for cell-to-cell transmission. It is the purpose of this review to provide an overview of current research, focusing on the various mechanisms and themes of virus-mediated actin modulation described therein.