Association of Nef with p21-activated kinase 2 is dispensable for efficient human immunodeficiency virus type 1 replication and cytopathicity in ex vivo-infected human lymphoid tissue

HIV-1 Balances the Fitness Costs and Benefits of Disrupting the Host Cell Actin Cytoskeleton Early after Mucosal Transmission

HIV-1 primarily infects T lymphocytes and uses these motile cells as migratory vehicles for effective dissemination in the host. Paradoxically, the virus at the same time disrupts multiple cellular processes underlying lymphocyte motility, seemingly counterproductive to rapid systemic infection. Here we show by intravital microscopy in humanized mice that perturbation of the actin cytoskeleton via the lentiviral protein Nef, and not changes to chemokine receptor expression or function, is the dominant cause of dysregulated infected T cell motility in lymphoid tissue by preventing stable cellular polarization required for fast migration. Accordingly, disrupting the Nef hydrophobic patch that facilitates actin cytoskeletal perturbation initially accelerates systemic viral dissemination after female genital transmission. However, the same feature of Nef was subsequently critical for viral persistence in immune-competent hosts. Therefore, a highly conserved activity of lentiviral Nef proteins has dual effects and imposes both fitness costs and benefits on the virus at different stages of infection.

HIV-1 Nef Disrupts CD4+ T Lymphocyte Polarity, Extravasation, and Homing to Lymph Nodes via Its Nef-Associated Kinase Complex Interface

HIV-1 Nef is a multifunctional protein that optimizes virus spread and promotes immune evasion of infected cells to accelerate disease progression in AIDS patients. As one of its activities, Nef reduces the motility of infected CD4⁺ T lymphocytes in confined space. In vivo, Nef restricts T lymphocyte homing to lymph nodes as it reduces the ability for extravasation at the diapedesis step. Effects of Nef on T lymphocyte motility are typically mediated by its ability to reduce actin remodeling. However, interference with diapedesis does not depend on residues in Nef required for inhibition of host cell actin dynamics. In search for an alternative mechanism by which Nef could alter T lymphocyte extravasation, we noted that the viral protein interferes with the polarization of primary human CD4⁺ T lymphocytes upon infection with HIV-1. Expression of Nef alone is sufficient to disrupt T cell polarization, and this effect is conserved among lentiviral Nef proteins. Nef acts by arresting the oscillation of CD4⁺ T cells between polarized and nonpolarized morphologies. Mapping studies identified the binding site for the Nef-associated kinase complex (NAKC) as critical determinant of this Nef activity and a NAKC-binding-deficient Nef variant fails to impair CD4⁺ T lymphocyte extravasation and homing to lymph nodes. These results thus imply the disruption of T lymphocyte polarity via its NAKC binding site as a novel mechanism by which lentiviral Nef proteins alter T lymphocyte migration in vivo.

The interaction between HIV-1 Nef and adaptor protein-2 reduces Nef-mediated CD4+ T cell apoptosis

Acquired Immune Deficiency Syndrome is characterized by a decline in CD4⁺ T cells. Here, we elucidated the mechanism underlying apoptosis in Human Immunodeficiency Virus-1 (HIV-1) infection by examining host apoptotic pathways hijacked by the HIV-1 Nef protein in the CD4⁺ T-cell line Sup-T1. Using a panel of Nef mutants unable to bind specific host proteins we uncovered that Nef generates pro- and anti-apoptotic signals. Apoptosis increased upon mutating the motifs involved in the interaction of Nef:AP-1 (Nef_M20A or Nef_{EEEE62-65AAAA}) or Nef:AP-2 (Nef_LL164/165AA), implying these interactions limit Nef-mediated apoptosis. In contrast, disrupting the Nef:PAK2 interaction motifs (Nef_H89A or Nef_F191A) reduced apoptosis. To validate further, apoptosis was measured after short-hairpin RNA knock-down of AP-1, AP-2 and PAK2. AP-2α depletion enhanced apoptosis, demonstrating that disrupting the Nef:AP-2α interaction limits Nef-mediated apoptosis. Collectively, we describe a mechanism by which HIV-1 regulates cell survival and demonstrate the consequence of interfering with Nef:host protein interactions.

Function of ubiquitin (Ub) specific protease 15 (USP15) in HIV-1 replication and viral protein degradation

HIV-1 Nef is necessary and may be sufficient for HIV-1-associated AIDS pathogenicity, in that knockout of Nef alone can protect HIV-infected patients from AIDS. We therefore investigated the feasibility of physical knockout of Nef, using the host ubiquitin proteasome system in HIV-1-infected cells. Our co-immunoprecipitation analysis demonstrated that Nef interacted with ubiquitin specific protease 15 (USP15), and that USP15, which is known to stabilize cellular proteins, degraded Nef. Nef could also cause decay of USP15, although Nef-mediated degradation of USP15 was weaker than USP15-mediated Nef degradation. Direct interaction between Nef and USP15 was essential for the observed reciprocal decay of the proteins. Further, USP15 degraded not only Nef but also HIV-1 structural protein, Gag, thereby substantially inhibiting HIV-1 replication. However, Gag did not degrade USP15, indicating that the Nef and USP15 complex, in distinction to other viral proteins, play an integral role in coordinating viral protein degradation and hence HIV-1 replication. Moreover, Nef and USP15 globally suppressed ubiquitylation of cellular proteins, indicating that these proteins are major determinants for the stability of cellular as well as viral proteins. Taken together, these data indicate that Nef and USP15 are vital in regulating degradation of viral and cellular proteins and thus HIV-1 replication, and specific degradation of viral, not cellular proteins, by USP15 points to USP15 as a candidate therapeutic agent to combat AIDS by eliminating viral proteins from the infected cells via USP15-mediated proteosomal degradation.

Optimized Replicating Renilla Luciferase Reporter HIV-1 Utilizing Novel Internal Ribosome Entry Site Elements for Native Nef Expression and Function

We previously developed replication-competent reporter HIV-1 (referred to herein as LucR.T2A reporter viruses), utilizing a "ribosome skipping" T2A peptide strategy to link Renilla luciferase (LucR) with Nef expression. The demonstrated utility for HIV-1 vaccine and transmission study applications included measurement of neutralizing antibody (NAb) activity in vaccine sera, improved cell-mediated virus inhibition assays, such as T cell-mediated virus inhibition and antibody-dependent cell-mediated cytotoxicity (ADCC) assays, and humanized mouse models. Herein, we extend our prior work and introduce reporter virus technology for applications that require fully functional Nef. We demonstrate that in CD4(+) T cells productively infected with LucR.T2A reporter viruses, T2A peptide-driven Nef expression and function, such as down-regulation of surface CD4 and MHC-I, were impaired. We overcame this limitation of LucR.T2A reporter viruses and achieved physiological Nef expression and function by engineering novel LucR reporter HIV-1 comprising 11 different internal ribosome entry site (IRES) elements chosen for size and relative activity. A range of Nef expression was observed in 293T cells transfected with the different LucR.IRES reporter virus constructs. Iteratively, we identified IRES reporter genomes that expressed Nef closest to physiological levels and produced virus with infectivity, titers, and replication kinetics similar to nonreporter viruses. Our results demonstrated that LucR reporter activity was stable over multiple replication cycles in peripheral blood mononuclear cells (PBMCs). Furthermore, we analyzed Nef functionality, i.e., down-modulation of MHC-I and CD4, following infection of T cell lines and PBMCs. Unlike LucR.T2A reporter virus, one of the redesigned LucR.IRES reporter viruses [containing the modified encephalomyocarditis virus (EMCV) 6ATR IRES element, "6ATRi"] demonstrated Nef expression and function similar to parental "nonreporter" virus. In a previously validated (nef-independent) T cell-based NAb neutralization assay, LucR.6ATRi reporter virus performed indistinguishably from LucR.T2A reporter virus. In summary, reporter viruses comprising the "6ATRi" element promise to augment HIV-1 vaccine and transmission research approaches requiring a sensitive reporter readout combined with wild-type Nef function.

Hemopoietic cell kinase (Hck) and p21-activated kinase 2 (PAK2) are involved in the down-regulation of CD1a lipid antigen presentation by HIV-1 Nef in dendritic cells

Dendritic cells (DCs) play a major role in in vivo pathogenesis of HIV-1 infection. Therefore, DCs may provide a promising strategy to control and eventually overcome the fatal infection. Especially, immature DCs express all CD1s, the non-MHC lipid antigen -presenting molecules, and HIV-1 Nef down-regulates CD1 expression besides MHC. Moreover, CD1d-restricted CD4(+) NKT cells are infected by HIV-1, reducing the number of these cells in HIV-1-infected individuals. To understand the exact role of DCs and CD1-mediated immune response during HIV-1 infection, Nef down-regulation of CD1a-restricted lipid/glycolipid Ag presentation in iDCs was analyzed. We demonstrated the involvement of the association of Nef with hemopoietic cell kinase (Hck) and p21-activated kinase 2 (PAK2), and that Hck, which is expressed strongly in iDCs, augmented this mutual interaction. Hck might be another therapeutic target to preserve the function of HIV-1 infected DCs, which are potential reservoirs of HIV-1 even after antiretroviral therapy.

Association with PAK2 Enables Functional Interactions of Lentiviral Nef Proteins with the Exocyst Complex

Human immunodeficiency virus type 1 (HIV-1) Nef enhances virus replication and contributes to immune evasion in vivo, but the underlying molecular mechanisms remain incompletely defined. Nef interferes with host cell actin dynamics to restrict T lymphocyte responses to chemokine stimulation and T cell receptor engagement. This relies on the assembly of a labile multiprotein complex including the host kinase PAK2 that Nef usurps to phosphorylate and inactivate the actin-severing factor cofilin. Components of the exocyst complex (EXOC), an octameric protein complex involved in vesicular transport and actin remodeling, were recently reported to interact with Nef via the same molecular surface that mediates PAK2 association. Exploring the functional relevance of EXOC in Nef-PAK2 complex assembly/function, we found Nef-EXOC interactions to be specifically mediated by the PAK2 interface of Nef, to occur in infected human T lymphocytes, and to be conserved among lentiviral Nef proteins. In turn, EXOC was dispensable for direct downstream effector functions of Nef-associated PAK2. Surprisingly, PAK2 was essential for Nef-EXOC association, which required a functional Rac1/Cdc42 binding site but not the catalytic activity of PAK2. EXOC was dispensable for Nef functions in vesicular transport but critical for inhibition of actin remodeling and proximal signaling upon T cell receptor engagement. Thus, Nef exploits PAK2 in a stepwise mechanism in which its kinase activity cooperates with an adaptor function for EXOC to inhibit host cell actin dynamics.

IMPORTANCE

Human immunodeficiency virus type 1 (HIV-1) Nef contributes to AIDS pathogenesis, but the underlying molecular mechanisms remain incompletely understood. An important aspect of Nef function is to facilitate virus replication by disrupting T lymphocyte actin dynamics in response to stimulation via its association with the host cell kinase PAK2. We report here that the molecular surface of Nef for PAK2 association also mediates interaction of Nef with EXOC and establish that PAK2 provides an essential adaptor function for the subsequent formation of Nef-EXOC complexes. PAK2 and EXOC specifically cooperate in the inhibition of actin dynamics and proximal signaling induced by T cell receptor engagement by Nef. These results establish EXOC as a functionally relevant Nef interaction partner, emphasize the suitability of the PAK2 interaction surface for future therapeutic interference with Nef function, and show that such strategies need to target activity-independent PAK2 functions.

Viremic long-term nonprogressive HIV-1 infection is not associated with abnormalities in known Nef functions

Background

A small minority of HIV-1-infected individuals show low levels of immune activation and do not develop immunodeficiency despite high viral loads. Since the accessory viral Nef protein modulates T cell activation and plays a key role in the pathogenesis of AIDS, we investigated whether specific properties of Nef may be associated with this highly unusual clinical outcome of HIV-1 infection.

Findings

Comprehensive functional analyses of sequential HIV-1 strains from three viremic long-term non-progressors (VNP) showed that they encode full-length Nef proteins that are capable of modulating CD4, CD28, CD8ß, MHC-I and CD74 cell surface expression. Similar to Nef proteins from HIV-1-infected individuals with progressive infection (P-Nefs) and unlike Nefs from simian immunodeficiency viruses (SIVs) that do not cause chronic immune activation and disease in their natural simian hosts, VNP-Nefs were generally unable to down-modulate TCR-CD3 cell surface expression to block T cell activation and apoptosis. On average, VNP-Nefs suppressed NF-AT activation less effectively than P-Nefs and were slightly less active in enhancing NF-κB activity. Finally, we found that VNP-Nefs increased virion infectivity and enhanced HIV-1 replication and cytopathicity in primary human cells and in ex vivo infected lymphoid tissues.

Conclusions

Our results show that nef alleles from VNPs and progressors of HIV-1 infection show only modest differences in established functions. Thus, the lack of chronic immune activation and disease progression in HIV-1-infected VNPs is apparently not associated with unusual functional properties of the accessory viral Nef protein.

Primate lentiviral Nef proteins deregulate T-cell development by multiple mechanisms

Background

A nef gene is present in all primate lentiviral genomes and is important for high viral loads and progression to AIDS in human or experimental macaque hosts of HIV or SIV, respectively. In these hosts, infection of the thymus results in a decreased output of naive T cells that may contribute to the development of immunodeficiency. We have previously shown that HIV-1 subtype B Nef proteins can block human T-cell development. However, the underlying mechanism(s) and the conservation of this Nef function between different groups of HIV and SIV remained to be determined.

Results

We investigated whether reduction of thymic output is a conserved function of highly divergent lentiviral Nef proteins including those from both types of human immunodeficiency viruses (HIV-1 and HIV-2), their direct simian counterparts (SIVcpz, SIVgor and SIVsmm, respectively), and some additional SIV strains. We found that expression of most of these nef alleles in thymocyte progenitors impaired T-cell development and reduced thymic output. For HIV-1 Nef, binding to active p21 protein (Cdc42/Rac)-activated kinase (PAK2) was a major determinant of this function. In contrast, selective disruption of PAK2 binding did not eliminate the effect on T-cell development of SIVmac239 Nef, as was shown by expressing mutants in a newly discovered PAK2 activating structural motif (PASM) constituted by residues I117, H121, T218 and Y221, as well as previously described mutants. Rather, down-modulation of cell surface CD3 was sufficient for reduced thymic output by SIVmac Nef, while other functions of SIV Nefs contributed.

Conclusions

Our results indicate that primate lentiviral Nef proteins impair development of thymocyte precursors into T cells in multiple ways. The interaction of HIV-1 Nef with active PAK2 by HIV-1 seem to be most detrimental, and downregulation of CD3 by HIV-2 and most SIV Nef proteins sufficient for reduced thymic output. Since the reduction of thymic output by Nef is a conserved property of divergent lentiviruses, it is likely to be relevant for peripheral T-cell depletion in poorly adapted primate lentiviral infections.

HIV-1 Nef interferes with T-lymphocyte circulation through confined environments in vivo

HIV-1 negative factor (Nef) elevates virus replication and contributes to immune evasion in vivo. As one of its established in vitro activities, Nef interferes with T-lymphocyte chemotaxis by reducing host cell actin dynamics. To explore Nef's influence on in vivo recirculation of T lymphocytes, we assessed lymph-node homing of Nef-expressing primary murine lymphocytes and found a drastic impairment in homing to peripheral lymph nodes. Intravital imaging and 3D immunofluorescence reconstruction of lymph nodes revealed that Nef potently impaired T-lymphocyte extravasation through high endothelial venules and reduced subsequent parenchymal motility. Ex vivo analyses of transendothelial migration revealed that Nef disrupted T-lymphocyte polarization and interfered with diapedesis and migration in the narrow subendothelial space. Consistently, Nef specifically affected T-lymphocyte motility modes used in dense environments that pose high physical barriers to migration. Mechanistically, inhibition of lymph node homing, subendothelial migration and cell polarization, but not diapedesis, depended on Nef's ability to inhibit host cell actin remodeling. Nef-mediated interference with in vivo recirculation of T lymphocytes may compromise T-cell help and thus represents an important mechanism for its function as a HIV pathogenicity factor.

HIV-1 Nef limits communication between linker of activated T cells and SLP-76 to reduce formation of SLP-76-signaling microclusters following TCR stimulation

Signal initiation by engagement of the TCR triggers actin rearrangements, receptor clustering, and dynamic organization of signaling complexes to elicit and sustain downstream signaling. Nef, a pathogenicity factor of HIV, disrupts early TCR signaling in target T cells. To define the mechanism underlying this Nef-mediated signal disruption, we employed quantitative single-cell microscopy following surface-mediated TCR stimulation that allows for dynamic visualization of distinct signaling complexes as microclusters (MCs). Despite marked inhibition of actin remodeling and cell spreading, the induction of MCs containing TCR-CD3 or ZAP70 was not affected significantly by Nef. However, Nef potently inhibited the subsequent formation of MCs positive for the signaling adaptor Src homology-2 domain-containing leukocyte protein of 76 kDa (SLP-76) to reduce MC density in Nef-expressing and HIV-1-infected T cells. Further analyses suggested that Nef prevents formation of SLP-76 MCs at the level of the upstream adaptor protein, linker of activated T cells (LAT), that couples ZAP70 to SLP-76. Nef did not disrupt pre-existing MCs positive for LAT. However, the presence of the viral protein prevented de novo recruitment of active LAT into MCs due to retargeting of LAT to an intracellular compartment. These modulations in MC formation and composition depended on Nef's ability to simultaneously disrupt both actin remodeling and subcellular localization of TCR-proximal machinery. Nef thus employs a dual mechanism to disturb early TCR signaling by limiting the communication between LAT and SLP-76 and preventing the dynamic formation of SLP-76-signaling MCs.

The Nef-infectivity enigma: mechanisms of enhanced lentiviral infection

The Nef protein is an essential factor for lentiviral pathogenesis in humans and other simians. Despite a multitude of functions attributed to this protein, the exact role of Nef in disease progression remains unclear. One of its most intriguing functions is the ability of Nef to enhance the infectivity of viral particles. In this review we will discuss current insights in the mechanism of this well-known, yet poorly understood Nef effect. We will elaborate on effects of Nef, on both virion biogenesis and the early stage of the cellular infection, that might be involved in infectivity enhancement. In addition, we provide an overview of different HIV-1 Nef domains important for optimal infectivity and briefly discuss some possible sources of the frequent discrepancies in the field. Hereby we aim to contribute to a better understanding of this highly conserved and therapeutically attractive Nef function.

The identification of a small molecule compound that reduces HIV-1 Nef-mediated viral infectivity enhancement

Nef is a multifunctional HIV-1 protein that accelerates progression to AIDS, and enhances the infectivity of progeny viruses through a mechanism that is not yet understood. Here, we show that the small molecule compound 2c reduces Nef-mediated viral infectivity enhancement. When added to viral producer cells, 2c did not affect the efficiency of viral production itself. However, the infectivity of the viruses produced in the presence of 2c was significantly lower than that of control viruses. Importantly, an inhibitory effect was observed with Nef(+) wild-type viruses, but not with viruses produced in the absence of Nef or in the presence of proline-rich PxxP motif-disrupted Nef, both of which displayed significantly reduced intrinsic infectivity. Meanwhile, the overexpression of the SH3 domain of the tyrosine kinase Hck, which binds to a PxxP motif in Nef, also reduced viral infectivity. Importantly, 2c inhibited Hck SH3-Nef binding, which was more marked when Nef was pre-incubated with 2c prior to its incubation with Hck, indicating that both Hck SH3 and 2c directly bind to Nef and that their binding sites overlap. These results imply that both 2c and the Hck SH3 domain inhibit the interaction of Nef with an unidentified host protein and thereby reduce Nef-mediated infectivity enhancement. The first inhibitory compound 2c is therefore a valuable chemical probe for revealing the underlying molecular mechanism by which Nef enhances the infectivity of HIV-1.

Nef-mediated enhancement of cellular activation and human immunodeficiency virus type 1 replication in primary T cells is dependent on association with p21-activated kinase 2

Background

The HIV-1 accessory protein Nef is an important determinant of lentiviral pathogenicity that contributes to disease progression by enhancing viral replication and other poorly understood mechanisms. Nef mediates diverse functions including downmodulation of cell surface CD4 and MHC Class I, enhancement of viral infectivity, and enhancement of T cell activation. Nef interacts with a multiprotein signaling complex that includes Src family kinases, Vav1, CDC42, and activated PAK2 (p21-activated kinase 2). Although previous studies have attempted to identify a biological role for the Nef-PAK2 signaling complex, the importance of this complex and its constituent proteins in Nef function remains unclear.

Results

Here, we show that Nef mutants defective for PAK2-association, but functional for CD4 and MHC Class I downmodulation and infectivity enhancement, are also defective for the ability to enhance viral replication in primary T cells that are infected and subsequently activated by sub-maximal stimuli (1 μg/ml PHA-P). In contrast, these Nef mutants had little or no effect on HIV-1 replication in T cells activated by stronger stimuli (2 μg/ml PHA-P or anti-CD3/CD28-coated beads). Viruses bearing wild-type Nefs, but not Nef mutants defective for PAK2 association, enhanced NFAT and IL2 receptor promoter activity in Jurkat cells. Moreover, expression of wild-type Nefs, but not mutant Nefs defective for PAK2 association, was sufficient to enhance responsiveness of primary CD4 and CD8 T cells to activating stimuli in Nef-expressing and bystander cells. siRNA knockdown of PAK2 in Jurkat cells reduced NFAT activation induced by anti-CD3/CD28 stimulation both in the presence and absence of Nef, and expression of a PAK2 dominant mutant inhibited Nef-mediated enhancement of CD25 expression.

Conclusion

Nef-mediated enhancement of cellular activation and viral replication in primary T cells is dependent on PAK2 and on the strength of the activating stimuli, and correlates with the ability of Nef to associate with PAK2. PAK2 is likely to play a role in Nef-mediated enhancement of viral replication and immune activation in vivo.

How HIV takes advantage of the cytoskeleton in entry and replication

The host cell cytoskeleton plays a key role in the life cycle of viral pathogens whose propagation depends on mandatory intracellular steps. Accordingly, also the human immunodeficiency virus type 1 (HIV-1) has evolved strategies to exploit and modulate in particular the actin cytoskeleton for its purposes. This review will recapitulate recent findings on how HIV-1 hijacks the cytoskeleton to facilitate entry into, transport within and egress from host cells as well as to commandeer communication of infected with uninfected bystander cells.

Mechanisms of HIV-1 Nef function and intracellular signaling

Advances in the last several years have enhanced mechanistic understanding of Nef-induced CD4 and MHCI downregulation and have suggested a new paradigm for analyzing Nef function. In both of these cases, Nef acts by forming ternary complexes with significant contributions to stability imparted by non-canonical interactions. The mutational analyses and binding assays that have led to these conclusions are discussed. The recent progress has been dependent on conservative mutations and multi-protein binding assays. The poorly understood Nef functions of p21 activated protein kinase (PAK2) activation, enhancement of virion infectivity, and inhibition of immunoglobulin class switching are also likely to involve ternary complexes and non-canonical interactions. Hence, investigation of these latter Nef functions should benefit from a similar approach. Six historically used alanine substitutions for determining structure-function relationships of Nef are discussed. These are M20A, E62A/E63A/E64A/E65A (AAAA), P72A/P75A (AXXA), R106A, L164A/L165A, and D174A/D175A. Investigations of less-disruptive mutations in place of AAAA and AXXA have led to different interpretations of mechanism. Two recent examples of this alternate approach, F191I for studying PAK2 activation and D123E for the critical residue D123 are discussed. The implications of the new findings and the resulting new paradigm for Nef structure-function are discussed with respect to creating a map of Nef functions on the protein surface. We report the results of a PPI-Pred analysis for protein-protein interfaces. There are three predicted patches produced by the analysis which describe regions consistent with the currently known mutational analyses of Nef function.

Inhibition of the Nef regulatory protein of HIV-1 by a single-domain antibody

The Nef protein of HIV-1 is important for AIDS pathogenesis, but it is not targeted by current antiviral strategies. Here, we describe a single-domain antibody (sdAb) that binds to HIV-1 Nef with a high affinity (K(d) = 2 × 10(-9)M) and inhibited critical biologic activities of Nef both in vitro and in vivo. First, it interfered with the CD4 down-regulation activity of a broad panel of nef alleles through inhibition of the Nef effects on CD4 internalization from the cell surface. Second, it was able to interfere with the association of Nef with the cellular p21-activated kinase 2 as well as with the resulting inhibitory effect of Nef on actin remodeling. Third, it counteracted the Nef-dependent enhancement of virion infectivity and inhibited the positive effect of Nef on virus replication in peripheral blood mononuclear cells. Fourth, anti-Nef sdAb rescued Nef-mediated thymic CD4(+) T-cell maturation defects and peripheral CD4(+) T-cell activation in the CD4C/HIV-1(Nef) transgenic mouse model. Because all these Nef functions have been implicated in Nef effects on pathogenesis, this anti-Nef sdAb may represent an efficient tool to elucidate the molecular functions of Nef in the virus life cycle and could now help to develop new strategies for the control of AIDS.

PAK1 as a therapeutic target

IMPORTANCE OF THE FIELD

P21-activated kinases (PAKs) are involved in multiple signal transduction pathways in mammalian cells. PAKs, and PAK1 in particular, play a role in such disorders as cancer, mental retardation and allergy. Cell motility, survival and proliferation, the organization and function of cytoskeleton and extracellular matrix, transcription and translation are among the processes affected by PAK1.

AREAS COVERED IN THIS REVIEW

We discuss the mechanisms that control PAK1 activity, its involvement in physiological and pathophysiological processes, the benefits and the drawbacks of the current tools to regulate PAK1 activity, the evidence that suggests PAK1 as a therapeutic target and the likely directions of future research.

WHAT THE READER WILL GAIN

The reader will gain a better knowledge and understanding of the areas described above.

TAKE HOME MESSAGE

PAK1 is a promising therapeutic target in cancer and allergen-induced disorders. Its suitability as a target in vascular, neurological and infectious diseases remains ambiguous. Further advancement of this field requires progress on such issues as the development of specific and clinically acceptable inhibitors, the choice between targeting one or multiple PAK isoforms, elucidation of the individual roles of PAK1 targets and the mechanisms that may circumvent inhibition of PAK1.

Human immunodeficiency virus (HIV) type-1, HIV-2 and simian immunodeficiency virus Nef proteins

The genomes of all retroviruses encode the Gag Pol and Env structural proteins. Human and simian lentiviruses have acquired non-structural proteins among which Nef plays a major role in the evolution of viral infection towards an immunodeficiency syndrome. Indeed, in the absence of a functional nef gene, primate lentiviruses are far less pathogenic than their wild type counterparts. The multiple protein-protein interactions in which Nef is involved all contribute to explain the role played by Nef in HIV- and SIV-associated disease progression. This review summarizes common and distinct features among Nef proteins and how they contribute to increasing HIV and SIV fitness towards their respective hosts.

Lentiviral Nef proteins utilize PAK2-mediated deregulation of cofilin as a general strategy to interfere with actin remodeling

Nef is an accessory protein and pathogenicity factor of human immunodeficiency virus (HIV) and simian immunodeficiency virus (SIV) which elevates virus replication in vivo. We recently described for HIV type 1(SF2) (HIV-1(SF2)) the potent interference of Nef with T-lymphocyte chemotaxis via its association with the cellular kinase PAK2. Mechanistic analysis revealed that this interaction results in deregulation of the actin-severing factor cofilin and thus blocks the chemokine-mediated actin remodeling required for cell motility. However, the efficiency of PAK2 association is highly variable among Nef proteins from different lentiviruses, prompting us to evaluate the conservation of this actin-remodeling/cofilin-deregulating mechanism. Based on the analysis of a total of 17 HIV-1, HIV-2, and SIV Nef proteins, we report here that inhibition of chemokine-induced actin remodeling as well as inactivation of cofilin are strongly conserved activities of lentiviral Nef proteins. Of note, even for Nef variants that display only marginal PAK2 association in vitro, these activities require the integrity of a PAK2 recruitment motif and the presence of endogenous PAK2. Thus, reduced in vitro affinity to PAK2 does not indicate limited functionality of Nef-PAK2 complexes in intact HIV-1 host cells. These results establish hijacking of PAK2 for deregulation of cofilin and inhibition of triggered actin remodeling as a highly conserved function of lentiviral Nef proteins, supporting the notion that PAK2 association may be critical for Nef's activity in vivo.

An emerging role for p21-activated kinases (Paks) in viral infections

p21-activated protein kinases (Paks) are cytosolic serine/threonine protein kinases that act as effectors for small (p21) GTPases of the Cdc42 and Rac families. It has long been established that Paks play a major role in a host of vital cellular functions such as proliferation, survival and motility, and abnormal Pak function is associated with a number of human diseases. Here, we discuss emerging evidence that these enzymes also play a major role in the entry, replication and spread of many important pathogenic human viruses, including HIV. Careful assessment of the potential role of Paks in antiviral immunity will be pivotal to evaluate thoroughly the potential of agents that inhibit Pak as a new class of anti-viral therapeutics.

Role of HIV-1 Nef protein for virus replication in vitro

The Nef protein of primate lentiviruses (simian and human immunodeficiency viruses; SIV/HIVs) appears to be multi-functional and plays a pivotal role in viral persistence and pathogenesis in vivo. Of its numerous functions reported to date, the ability to enhance virion infectivity in indicator cell lines and to augment viral replication in peripheral blood mononuclear cells (PBMCs) and lymphocytes (PBLs) is very well conserved among various SIV/HIVs. This review summarizes and organizes current knowledge of HIV-1 Nef with respect to this particularly virological activity for understanding the basis of its in vivo function.

Determinants in HIV-1 Nef for enhancement of virus replication and depletion of CD4+ T lymphocytes in human lymphoid tissue ex vivo

Background

HIV-1 Nef critically contributes to AIDS in part by augmenting virus titers in infected individuals. Analyzing which of Nef's activities contribute to HIV pathogenesis has been hampered by the lack of a cell culture model in which Nef exerts pronounced effects on HIV replication. The human lymphoid aggregate culture (HLAC) from tonsil maintains the cell populations and cytokine milieu found in vivo, supports a productive infection without exogenous stimulation, and Nef contributes to efficient HIV-1 replication as well as CD4⁺ T cell depletion in this experimental ex vivo-model.

Results

To identify determinants in Nef that mediate these activities, we infected HLAC with a panel of isogenic HIV-1_NL4-3 strains that encode for well-characterized mutants of HIV-1_SF2 Nef. Determination of HIV-1 replication revealed that enhancement of the virus spread by Nef is governed by a complex set of protein interaction surfaces. In contrast, increased CD4⁺ T lymphocyte depletion depended on only two protein interaction surfaces in Nef that mediate either downregulation of cell surface CD4 or interaction with the NAKC signalosome. Consistently, in HLAC from 9 out of 14 donors, Nef enhanced CD4⁺ T cell depletion in the absence of a significant effect on virus replication. Moreover, our results suggest that this Nef-dependent enhancement in depletion occurred predominately in uninfected bystander CD4⁺ T cells.

Conclusion

Our findings suggest that Nef facilitates depletion of CD4⁺ T lymphocytes in HIV-1-infected lymphoid tissue ex vivo by increasing the pool of productively infected cells and by sensitizing bystander cells for killing. This ability might contribute to Nef's pathogenic potential in vivo.

Implications of Nef: host cell interactions in viral persistence and progression to AIDS

The HIV and SIV Nef accessory proteins are potent enhancers of viral persistence and accelerate progression to AIDS in HIV-1-infected patients and non-human primate models. Although relatively small (27-35 kD), Nef can interact with a multitude of cellular factors and induce complex changes in trafficking, signal transduction, and gene expression that together converge to promote viral replication and immune evasion. In particular, Nef recruits several immunologically relevant cellular receptors to the endocytic machinery to reduce the recognition and elimination of virally infected cells by the host immune system, while simultaneously interacting with various kinases to promote T cell activation and viral replication. This review provides an overview on selected Nef interactions with host cell proteins, and discusses their possible relevance for viral spread and pathogenicity.

Human immunodeficiency virus type 1 Nef incorporation into virions does not increase infectivity

The viral protein Nef contributes to the optimal infectivity of human and simian immunodeficiency viruses. The requirement for Nef during viral biogenesis particles suggests that Nef might play a role in this process. Alternatively, because Nef is incorporated into viruses, it might play a role when progeny virions reach target cells. We challenged these hypotheses by manipulating the amounts of Nef incorporated in viruses while keeping its expression level constant in producer cells. This was achieved by forcing the incorporation of Nef into viral particles by fusing a Vpr sequence to the C-terminal end of Nef. A cleavage site for the viral protease was introduced between Nef and Vpr to allow the release of Nef fragments from the fusion protein during virus maturation. We show that the resulting Nef-CS-Vpr fusion partially retains the ability of Nef to downregulate cell surface CD4 and that high amounts of Nef-CS-Vpr are incorporated into viral particles compared with what is seen for wild-type Nef. The fusion protein is processed during virion maturation and releases Nef fragments similar to those found in viruses produced in the presence of wild-type Nef. Unlike viruses produced in the presence of wild-type Nef, viruses produced in the presence of Nef-CS-Vpr do not have an increase in infectivity and are as poorly infectious as viruses produced in the absence of Nef. These findings demonstrate that the presence of Nef in viral particles is not sufficient to increase human immunodeficiency virus type 1 infectivity and suggest that Nef plays a role during the biogenesis of viral particles.

HIV-1 Nef: at the crossroads

The development of anti-virals has blunted the AIDS epidemic in the Western world but globally the epidemic has not been curtailed. Standard vaccines have not worked, and attenuated vaccines are not being developed because of safety concerns. Interest in attenuated vaccines has centered on isolated cases of patients infected with HIV-1 containing a deleted nef gene. Nef is a multifunctional accessory protein that is necessary for full HIV-1 virulence. Unfortunately, some patients infected with the nef-deleted virus eventually lose their CD4⁺ T cells to levels indicating progression to AIDS.

This renders the possibility of an attenuated HIV-1 based solely on a deleted nef remote. In this review we discuss the knowledge gained both from the study of these patients and from in vitro investigations of Nef function to assess the possibility of developing new anti-HIV-1 drugs based on Nef. Specifically, we consider CD4 downregulation, major histocompatibility complex I downregulation, Pak2 activation, and enhancement of virion infectivity. We also consider the recent proposal that simian immunodeficiency viruses are non-pathogenic in their hosts because they have Nefs that downregulate CD3, but HIV-1 is pathogenic because its Nef fails to downregulate CD3. The possibility of incorporating the CD3 downregulation function into HIV-1 Nef as a therapeutic option is also considered. Finally, we conclude that inhibiting the CD4 downregulation function is the most promising Nef-targeted approach for developing a new anti-viral as a contribution to combating AIDS.