p56Lck anchors CD4 to distinct microdomains on microvilli

Unique roles of co-receptor-bound LCK in helper and cytotoxic T cells

The kinase LCK and CD4/CD8 co-receptors are crucial components of the T cell antigen receptor (TCR) signaling machinery, leading to key T cell fate decisions. Despite decades of research, the roles of CD4-LCK and CD8-LCK interactions in TCR triggering in vivo remain unknown. In this study, we created animal models expressing endogenous levels of modified LCK to resolve whether and how co-receptor-bound LCK drives TCR signaling. We demonstrated that the role of LCK depends on the co-receptor to which it is bound. The CD8-bound LCK is largely dispensable for antiviral and antitumor activity of cytotoxic T cells in mice; however, it facilitates CD8⁺ T cell responses to suboptimal antigens in a kinase-dependent manner. By contrast, the CD4-bound LCK is required for efficient development and function of helper T cells via a kinase-independent stabilization of surface CD4. Overall, our findings reveal the role of co-receptor-bound LCK in T cell biology, show that CD4- and CD8-bound LCK drive T cell development and effector immune responses using qualitatively different mechanisms and identify the co-receptor-LCK interactions as promising targets for immunomodulation.

ERM-Dependent Assembly of T Cell Receptor Signaling and Co-stimulatory Molecules on Microvilli prior to Activation

T cell surfaces are covered with microvilli, actin-rich and flexible protrusions. We use super-resolution microscopy to show that ≥90% of T cell receptor (TCR) complex molecules TCRαβ and TCRζ, as well as the co-receptor CD4 (cluster of differentiation 4) and the co-stimulatory molecule CD2, reside on microvilli of resting human T cells. Furthermore, TCR proximal signaling molecules involved in the initial stages of the immune response, including the protein tyrosine kinase Lck (lymphocyte-specific protein tyrosine kinase) and the key adaptor LAT (linker for activation of T cells), are also enriched on microvilli. Notably, phosphorylated proteins of the ERM (ezrin, radixin, and moesin) family colocalize with TCRαβ as well as with actin filaments, implying a role for one or more ERMs in linking the TCR complex to the actin cytoskeleton within microvilli. Our results establish microvilli as key signaling hubs, in which the TCR complex and its proximal signaling molecules and adaptors are preassembled prior to activation in an ERM-dependent manner, facilitating initial antigen sensing.

Surfing on Membrane Waves: Microvilli, Curved Membranes, and Immune Signaling

Microvilli are finger-like membrane protrusions, supported by the actin cytoskeleton, and found on almost all cell types. A growing body of evidence suggests that the dynamic lymphocyte microvilli, with their highly curved membranes, play an important role in signal transduction leading to immune responses. Nevertheless, challenges in modulating local membrane curvature and monitoring the high dynamicity of microvilli hampered the investigation of the curvature-generation mechanism and its functional consequences in signaling. These technical barriers have been partially overcome by recent advancements in adapted super-resolution microscopy. Here, we review the up-to-date progress in understanding the mechanisms and functional consequences of microvillus formation in T cell signaling. We discuss how the deformation of local membranes could potentially affect the organization of signaling proteins and their biochemical activities. We propose that curved membranes, together with the underlying cytoskeleton, shape microvilli into a unique compartment that sense and process signals leading to lymphocyte activation.

A biophysical perspective on receptor-mediated virus entry with a focus on HIV

As part of their entry and infection strategy, viruses interact with specific receptor molecules expressed on the surface of target cells. The efficiency and kinetics of the virus-receptor interactions required for a virus to productively infect a cell is determined by the biophysical properties of the receptors, which are in turn influenced by the receptors' plasma membrane (PM) environments. Currently, little is known about the biophysical properties of these receptor molecules or their engagement during virus binding and entry. Here we review virus-receptor interactions focusing on the human immunodeficiency virus type 1 (HIV), the etiological agent of acquired immunodeficiency syndrome (AIDS), as a model system. HIV is one of the best characterised enveloped viruses, with the identity, roles and structure of the key molecules required for infection well established. We review current knowledge of receptor-mediated HIV entry, addressing the properties of the HIV cell-surface receptors, the techniques used to measure these properties, and the macromolecular interactions and events required for virus entry. We discuss some of the key biophysical principles underlying receptor-mediated virus entry and attempt to interpret the available data in the context of biophysical mechanisms. We also highlight crucial outstanding questions and consider how new tools might be applied to advance understanding of the biophysical properties of viral receptors and the dynamic events leading to virus entry.

Temporary Reduction of Membrane CD4 with the Antioxidant MnTBAP Is Sufficient to Prevent Immune Responses Induced by Gene Transfer

Unexpectedly, the synthetic antioxidant MnTBAP was found to cause a rapid and reversible downregulation of CD4 on T cells in vitro and in vivo. This effect resulted from the internalization of membrane CD4 T cell molecules into clathrin-coated pits and involved disruption of the CD4/p56^Lck complex. The CD4 deprivation induced by MnTBAP had functional consequences on CD4-dependent infectious processes or immunological responses as shown in various models, including gene therapy. In cultured human T cells, MnTBAP-induced downregulation of CD4 functionally suppressed gp120- mediated lentiviral transduction in a model relevant for HIV infection. The injection of MnTBAP in mice reduced membrane CD4 on lymphocytes in vivo within 5 days of treatment, preventing OVA peptide T cell immunization while allowing subsequent immunization once treatment was stopped. In a mouse gene therapy model, MnTBAP treatment at the time of adenovirus-associated virus (AAV) vector administration, successfully controlled the induction of anti-transgene and anti-capsid immune responses mediated by CD4⁺ T cells, enabling the redosing mice with the same vector. These functional data provide new avenues to develop alternative therapeutic immunomodulatory strategies based on temporary regulation of CD4. These could be particularly useful for AAV gene therapy in which novel strategies for redosing are needed.

Dual Role of CD4 in Peripheral T Lymphocytes

The interaction of T-cell receptors (TCRs) with self- and non-self-peptides in the major histocompatibility complex (MHC) stimulates crucial signaling events, which in turn can activate T lymphocytes. A variety of accessory molecules further modulate T-cell signaling. Of these, the CD4 and CD8 coreceptors make the most critical contributions to T cell sensitivity in vivo. Whereas, CD4 function in T cell development is well-characterized, its role in peripheral T cells remains incompletely understood. It was originally suggested that CD4 stabilizes weak interactions between TCRs and peptides in the MHC and delivers Lck kinases to that complex. The results of numerous experiments support the latter role, indicating that the CD4-Lck complex accelerates TCR-triggered signaling and controls the availability of the kinase for TCR in the absence of the ligand. On the other hand, extremely low affinity of CD4 for MHC rules out its ability to stabilize the receptor-ligand complex. In this review, we summarize the current knowledge on CD4 in T cells, with a special emphasis on the spatio-temporal organization of early signaling events and the relevance for CD4 function. We further highlight the capacity of CD4 to interact with the MHC in the absence of TCR. It drives the adhesion of T cells to the cells that express the MHC. This process is facilitated by the CD4 accumulation in the tips of microvilli on the surface of unstimulated T cells. Based on these observations, we suggest an alternative model of CD4 role in T-cell activation.

Identification of candidate genes that may contribute to the metastasis of prostate cancer by bioinformatics analysis

To screen for marker genes associated with to the metastasis of prostate cancer (PCa), in silico analysis of the Gene Expression Omnibus dataset GSE27616, which included 4 metastatic and 5 localized PCa tissue samples, was performed. Differentially expressed genes (DEGs) were identified. Their potential functions were identified by Gene Ontology and Kyoto Encyclopedia of Gene Genomes pathway enrichment analyses. Furthermore, protein-protein interaction (PPI) networks for DEGs were constructed using Cytoscape. Module analysis of the PPI networks was performed with Cluster ONE. A total of 561 DEGs were screened, including 208 upregulated and 353 downregulated genes. Proliferating cell nuclear antigen (PCNA) and cluster of differentiation 4 (CD4) exhibited the highest degrees of connectivity in the PPI networks for up- and down-regulated DEGs, respectively. The DEGs in module A, including CD58, 2, 4 and major histocompatibility complex, class II DP-β1 were enriched in 'cell adhesion molecules'. Anaphase promoting complex subunit 4, cell division cycle 20 and cell division cycle 16 in module B were primarily enriched in 'cell cycle'. The DEGs, including CD4, PCNA and baculoviral IAP repeat containing 5, may have critical roles in PCa metastasis and could thus be used as novel biomarker candidates for metastatic PCa. However, further studies are required to verify these results.

Interdependence of free zinc changes and protein complex assembly – insights into zinc signal regulation

Small and local changes in cellular free zinc concentration affect protein assembly.

Low level of Lck kinase in Th2 cells limits expression of CD4 co-receptor and S73 phosphorylation of transcription factor c-Jun

The Src-family tyrosine kinase Lck is an enzyme associated with the CD4 and CD8 co-receptors and promoting signaling through the T cell receptor (TCR) complex. The levels of Lck expression and activity change during the development and differentiation of T cells. Here we show that Lck expression is higher in Th1 cells as compared to Th2 cells. Ectopic overexpression of Lck in Th2 cells results in increased expression of CD4 co-receptor and enhanced S73 phosphorylation of transcription factor c-Jun. Our findings indicate that TCR-mediated signaling in Th2 cells may be directly attenuated by Lck protein expression level.

Multiple CCR5 conformations on the cell surface are used differentially by human immunodeficiency viruses resistant or sensitive to CCR5 inhibitors

Resistance to small-molecule CCR5 inhibitors arises when HIV-1 variants acquire the ability to use inhibitor-bound CCR5 while still recognizing free CCR5. Two isolates, CC101.19 and D1/85.16, became resistant via four substitutions in the gp120 V3 region and three in the gp41 fusion peptide (FP), respectively. The binding characteristics of a panel of monoclonal antibodies (MAbs) imply that several antigenic forms of CCR5 are expressed at different levels on the surfaces of U87-CD4-CCR5 cells and primary CD4(+) T cells, in a cell-type-dependent manner. CCR5 binding and HIV-1 infection inhibition experiments suggest that the two CCR5 inhibitor-resistant viruses altered their interactions with CCR5 in different ways. As a result, both mutants became generally more sensitive to inhibition by CCR5 MAbs, and the FP mutant is specifically sensitive to a MAb that stains discrete cell surface clusters of CCR5 that may correspond to lipid rafts. We conclude that some MAbs detect different antigenic forms of CCR5 and that inhibitor-sensitive and -resistant viruses can use these CCR5 forms differently for entry in the presence or absence of CCR5 inhibitors.

The transmembrane domains of L-selectin and CD44 regulate receptor cell surface positioning and leukocyte adhesion under flow

During inflammation and immune surveillance, initial contacts (tethering) between free-flowing leukocytes and the endothelium are vitally dependent on the presentation of the adhesion receptor L-selectin on leukocyte microvilli. Determinants that regulate receptor targeting to microvilli are, however, largely elusive. Therefore, we systematically swapped the extracellular (EC), transmembrane (TM), and intracellular (IC) domains of L-selectin and CD44, a hyaluronan receptor expressed on the cell body and excluded from microvilli. Electron microscopy of transfected human myeloid K562 cells showed that the highly conserved TM domains are responsible for surface positioning. The TM segment of L-selectin forced chimeric molecules to microvilli, and the CD44 TM domain evoked expression on the cell body, whereas the IC and EC domains hardly influenced surface localization. Transfectants with microvillus-based chimeras showed a significantly higher adhesion rate under flow but not under static conditions compared with cells with cell body-expressed receptors. Substitution of the IC domain of L-selectin caused diminished tethering but no change in surface distribution, indicating that both microvillus positioning and cytoskeletal anchoring contribute to leukocyte tethering. These findings demonstrate that TM domains of L-selectin and CD44 play a crucial role in cell adhesion under flow by targeting receptors to microvilli or the cell body, respectively.

NADPH oxidase 1 deficiency alters caveolin phosphorylation and angiotensin II-receptor localization in vascular smooth muscle

The superoxide-generating NADPH oxidase NOX1 is thought to be involved in signaling by the angiotensin II-receptor AT1R. However, underlying signaling steps are poorly understood. In this study, we investigated the effect of AngII on aortic smooth muscle from wild-type and NOX1-deficient mice. NOX1-deficient cells showed decreased basal ROS generation and did not produce ROS in response to AngII. Unexpectedly, AngII-dependent Ca(2+) signaling was markedly decreased in NOX1-deficient cells. Immunostaining demonstrated that AT1R was localized on the plasma membrane in wild-type, but intracellularly in NOX1-deficient cells. Immunohistochemistry and immunoblotting showed a decreased expression of AT1R in the aorta of NOX1-deficient mice. To investigate the basis of the abnormal AT1R targeting, we studied caveolin expression and phosphorylation. The amounts of total caveolin and of caveolae were not different in NOX1-deficient mice, but a marked decrease occurred in the phosphorylated form of caveolin. Exogenous H(2)O(2) or transfection of a NOX1 plasmid restored AngII responses in NOX1-deficient cells. Based on these findings, we propose that NOX1-derived reactive oxygen species regulate cell-surface expression of AT1R through mechanisms including caveolin phosphorylation. The lack cell-surface AT1R expression in smooth muscle could be involved in the decreased blood pressure in NOX1-deficient mice.

Restoring the association of the T cell receptor with CD8 reverses anergy in human tumor-infiltrating lymphocytes

For several days after antigenic stimulation, human cytolytic T lymphocyte (CTL) clones exhibit a decrease in their effector activity and in their binding to human leukocyte antigen (HLA)-peptide tetramers. We observed that, when in this state, CTLs lose the colocalization of the T cell receptor (TCR) and CD8. Effector function and TCR-CD8 colocalization were restored with galectin disaccharide ligands, suggesting that the binding of TCR to galectin plays a role in the distancing of TCR from CD8. These findings appear to be applicable in vivo, as TCR was observed to be distant from CD8 on human tumor-infiltrating lymphocytes, which were anergic. These lymphocytes recovered effector functions and TCR-CD8 colocalization after ex vivo treatment with galectin disaccharide ligands. The separation of TCR and CD8 molecules could be one major mechanism of anergy in tumors and other chronic stimulation conditions.

CD4 interacts constitutively with multiple CCR5 at the plasma membrane of living cells. A fluorescence recovery after photobleaching at variable radii approach

The entry of human immunodeficiency virus into target cells requires successive interactions of the viral envelope glycoprotein gp120 with CD4 and the chemokine receptors CCR5 or CXCR4. We previously demonstrated, by Förster resonance energy transfer experiments, the constitutive association of CD4 and CCR5 at the surface of living cells. We therefore speculated that this interaction may correlate with compartmentalization of CD4 and CCR5 within the plasma membrane. Here, we characterize the lateral distribution, the dynamics, and the stoichiometry of these receptors in living cells stably expressing CD4 and/or CCR5 by means of fluorescence recovery after photobleaching at variable radii experiments. We found that (i) these receptors expressed alone are confined into 1-microm-sized domains, (ii) CD4-CCR5 associations occur outside and inside smaller domains, and (iii) these interactions involve multiple CCR5 molecules per CD4.

Sphingomyelinase restricts the lateral diffusion of CD4 and inhibits human immunodeficiency virus fusion

Previously, we reported that treatment of cells with sphingomyelinase inhibits human immunodeficiency virus type 1 (HIV-1) entry. Here, we determined by measuring fluorescence recovery after photobleaching that the lateral diffusion of CD4 decreased 4-fold following sphingomyelinase treatment, while the effective diffusion rate of CCR5 remained unchanged. Notably, sphingomyelinase treatment of cells did not influence gp120 binding, HIV-1 attachment, or fluid-phase and receptor-mediated endocytosis. Furthermore, sphingomyelinase treatment did not affect the membrane disposition of the HIV receptor proteins CD4, CXCR4, and CCR5, as determined by Triton X-100 extraction. Restriction of CD4 diffusion by antibody cross-linking also inhibited HIV infection. We therefore interpret the decrease in CD4 lateral mobility following sphingomyelinase treatment in terms of clustering of CD4 molecules. Examination of fusion intermediates indicated that sphingomyelinase treatment inhibited HIV at a step in the fusion process after CD4 engagement. Maximal inhibition of fusion was observed following short coculture times and with target cells that express low levels of CD4. As HIV entry into cells requires the sequential engagement of viral envelope protein with CD4 and coreceptor, we propose that sphingomyelinase inhibits HIV infection by inducing CD4 clustering that prevents coreceptor engagement and HIV fusion.

Proteomics analysis of insulin secretory granules

Insulin secretory granules (ISGs) are cytoplasmic organelles of pancreatic beta-cells. They are responsible for the storage and secretion of insulin. To date, only about 30 different proteins have been clearly described to be associated with these organelles. However, data from two-dimensional gel electrophoresis analyses suggested that almost 150 different polypeptides might be present within ISGs. The elucidation of the identity and function of the ISG proteins by proteomics strategies would be of considerable help to further understand some of the underlying mechanisms implicated in ISG biogenesis and trafficking. Furthermore it should give the bases to the comprehension of impaired insulin secretion observed during diabetes. A proteomics analysis of an enriched insulin granule fraction from the rat insulin-secreting cell line INS-1E was performed. The efficacy of the fractionation procedure was assessed by Western blot and electron microscopy. Proteins of the ISG fraction were separated by SDS-PAGE, excised from consecutive gel slices, and tryptically digested. Peptides were analyzed by nano-LC-ESI-MS/MS. This strategy identified 130 different proteins that were classified into four structural groups including intravesicular proteins, membrane proteins, novel proteins, and other proteins. Confocal microscopy analysis demonstrated the association of Rab37 and VAMP8 with ISGs in INS-1E cells. In conclusion, the present study identified 130 proteins from which 110 are new proteins associated with ISGs. The elucidation of their role will further help in the understanding of the mechanisms governing impaired insulin secretion during diabetes.

Association of herpesvirus saimiri tip with lipid raft is essential for downregulation of T-cell receptor and CD4 coreceptor

Lipid rafts are membrane microdomains that are proposed to function as platforms for both receptor signaling and trafficking. Our previous studies have demonstrated that Tip of herpesvirus saimiri (HVS), which is a T-lymphotropic tumor virus, is constitutively targeted to lipid rafts and interacts with cellular Lck tyrosine kinase and p80 WD repeat-containing endosomal protein. Through the interactions with Lck and p80, HVS Tip modulates diverse T-cell functions, which leads to the downregulation of T-cell receptor (TCR) and CD4 coreceptor surface expression, the inhibition of TCR signal transduction, and the activation of STAT3 transcription factor. In this study, we investigated the functional significance of Tip association with lipid rafts. We found that Tip expression remarkably increased lipid raft fractions in human T cells by enhancing the recruitment of lipid raft-resident proteins. Genetic analysis showed that the carboxyl-terminal transmembrane, but not p80 and Lck interaction, of Tip was required for the lipid raft localization and that lipid raft localization of Tip was necessary for the efficient downregulation of TCR and CD4 surface expression. Correlated with this, treatment with Filipin III, a lipid raft-disrupting agent, effectively reversed the downregulation of CD3 and CD4 surface expression induced by Tip. On the other hand, Tip mutants that were no longer present in lipid rafts were still capable of inhibiting TCR signaling and activating STAT3 transcription factor activity as efficiently as wild-type (wt) Tip. These results indicate that the association of Tip with lipid rafts is essential for the downregulation of TCR and CD4 surface expression but not for the inhibition of TCR signal transduction and the activation of STAT3 transcription factor. These results also suggest that the signaling and targeting activities of HVS Tip rely on functionally and genetically separable mechanisms, which may independently modulate T-cell function for viral persistence or pathogenesis.

Lipid rafts in T cell receptor signalling

The molecular events and the protein components that are involved in signalling by the T cell receptor (TCR) for antigen have been extensively studied. Activation of signalling cascades following TCR stimulation depends on the phosphorylation of the receptor by the tyrosine kinase Lck, which localizes to the cytoplasmic face of the plasma membrane by virtue of its post-translational modification. However, the precise order of events during TCR phosphorylation at the plasma membrane, remains to be defined. A current theory that describes early signalling events incorporates the function of lipid rafts, microdomains at the plasma membrane with distinct lipid and protein composition. Lipid rafts have been implicated in diverse biological functions in mammalian cells. In T cells, molecules with a key role in TCR signalling, including Lck, localize to these domains. Importantly, mutant versions of these proteins which fail to localise to raft domains were unable to support signalling by the TCR. Biochemical studies using purified detergent-resistant membranes (DRM) and confocal microscopy have suggested that upon stimulation, the TCR and Lck-containing lipid rafts may come into proximity allowing phosphorylation of the receptor. Further, there are data suggesting that phosphorylation of the TCR could depend on a transient increase in Lck activity that takes place within lipid rafts to initiate signalling. Current results and a model of how lipid rafts may regulate TCR signalling are discussed.

Dynamic tradeoffs in the raft-mediated entry of human immunodeficiency virus type 1 into cells

To initiate an infection human immunodeficiency virus type 1 (HIV-1) particles must first bind to receptors on the surface of their host cells, a process that eventually leads to fusion of viral and cellular membranes and release of the viral genome into the cytoplasm. Understanding the molecular mechanisms of these processes may enable the development of new anti-HIV strategies. Disagreement currently prevails on the role in virus entry of microdomains within the cellular plasma membrane known as lipid rafts. Experiments have suggested that lipid rafts, in their interactions with cellular receptors and viral particles, either promote or have minimal effect on viral entry. Here we develop a dynamic model for HIV-1 entry that enables us to identify and quantitatively assess tradeoffs that can arise from the clustering of receptors in rafts. Specifically, receptor clustering can be detrimental to the initiation of viral infection by reducing the probability that a virus particle finds its primary receptor, CD4. However, receptor clustering can also enable a virus particle, once bound, to rapidly form multivalent interactions with receptors and co-receptors that are required for virus-cell membrane fusion. We show how the resolution of such tradeoffs hinges on the level and spatial distribution of receptors and co-receptors on the cell surface, and we discuss implications of these effects for the design of therapeutics that inhibit HIV-1 entry.

Distinct roles for multiple Src family kinases at fertilization

Egg activation at fertilization requires the release of Ca2+ from the endoplasmic reticulum of the egg. Recent evidence indicates that Src family kinases (SFKs) function in the signaling pathway that initiates this Ca2+ release in the eggs of many deuterostomes. We have identified three SFKs expressed in starfish (Asterina miniata) eggs, designated AmSFK1, AmSFK2 and AmSFK3. Antibodies made against the unique domains of each AmSFK protein revealed that all three are expressed in eggs and localized primarily to the membrane fraction. Both AmSFK1 and AmSFK3 (but not AmSFK2) are necessary for egg activation, as determined by injection of starfish oocytes with dominant-interfering Src homology 2 (SH2) domains, which specifically delay and reduce the initial release of Ca2+ at fertilization. AmSFK3 exhibits a very rapid and transient kinase activity in response to fertilization, peaking at 30 seconds post sperm addition. AmSFK1 kinase activity also increases transiently at fertilization, but peaks later, at 2 minutes. These results indicate that there are multiple SFKs present in starfish eggs with distinct, perhaps sequential, signaling roles.

Lipid rafts and their roles in T-cell activation

Lipid rafts are defined as detergent-resistant membrane microdomains of specific lipid and protein composition. They are involved in many aspects of cell biology, including T-cell activation and immunoreceptor signaling. This review discusses current controversies around lipid rafts and summarizes recent developments in the area.

Regulation of T-cell receptor signalling by membrane microdomains

There is now considerable evidence suggesting that the plasma membrane of mammalian cells is compartmentalized by functional lipid raft microdomains. These structures are assemblies of specialized lipids and proteins and have been implicated in diverse biological functions. Analysis of their protein content using proteomics and other methods revealed enrichment of signalling proteins, suggesting a role for these domains in intracellular signalling. In T lymphocytes, structure/function experiments and complementary pharmacological studies have shown that raft microdomains control the localization and function of proteins which are components of signalling pathways regulated by the T-cell antigen receptor (TCR). Based on these studies, a model for TCR phosphorylation in lipid rafts is presented. However, despite substantial progress in the field, critical questions remain. For example, it is unclear if membrane rafts represent a homogeneous population and if their structure is modified upon TCR stimulation. In the future, proteomics and the parallel development of complementary analytical methods will undoubtedly contribute in further delineating the role of lipid rafts in signal transduction mechanisms.

Mobility of the human immunodeficiency virus (HIV) receptor CD4 and coreceptor CCR5 in living cells: implications for HIV fusion and entry events

The sequence of events leading to human immunodeficiency virus fusion and entry likely involves the recruitment of multiple receptor and coreceptor proteins to a specific complex by the viral envelope. Using fluorescence recovery after photobleaching technology, we find that both CD4 and CCR5 are mobile in the cell membrane. Interestingly, our findings also suggest that the seven-span transmembrane coreceptor is significantly more mobile than CD4 and requires membrane cholesterol for mobility.

HIV-1 cell to cell transfer across an Env-induced, actin-dependent synapse

Direct cell–cell transfer is an efficient mechanism of viral dissemination within an infected host, and human immunodeficiency virus 1 (HIV-1) can exploit this mode of spread. Receptor recognition by HIV-1 occurs via interactions between the viral surface envelope glycoprotein (Env), gp120, and CD4 and a chemokine receptor, CCR5 or CXCR4. Here, we demonstrate that the binding of CXCR4-using HIV-1–infected effector T cells to primary CD4⁺/CXCR4⁺ target T cells results in rapid recruitment to the interface of CD4, CXCR4, talin, and lymphocyte function–associated antigen 1 on the target cell, and of Env and Gag on the effector cell. Recruitment of these membrane molecules into polarized clusters was dependent on Env engagement of CD4 and CXCR4 and required remodelling of the actin cytoskeleton. Transfer of Gag from effector to target cell was observed by 1 h after conjugate formation, was independent of cell–cell fusion, and was probably mediated by directed virion fusion with the target cell. We propose that receptor engagement by Env directs the rapid, actin-dependent recruitment of HIV receptors and adhesion molecules to the interface, resulting in a stable adhesive junction across which HIV infects the target cell.

Sulfated polymannuroguluronate, a novel anti-acquired immune deficiency syndrome (AIDS) drug candidate, targeting CD4 in lymphocytes

Sulfated polymannuroguluronate (SPMG), a marine sulfated polysaccharide, has entered the Phase II clinical trial in China as the first anti-acquired immune deficiency syndrome (AIDS) drug candidate obtained from marine organisms. To determine the binding site(s) (receptors) of SPMG in lymphocytes mediating its anti-AIDS activities, fluorescein-5-isothiocyanate (FITC)-labeled SPMG was used to investigate SPMG binding to lymphocytes. Flow cytometry (FCM) and fluorescence microscopy analysis showed that the SPMG binds to lymphocytes in a rapid, specific, reversible, and saturable fashion. Several SPMG binding proteins were purified by affinity chromatography from lymphocyte membrane preparations. Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) and Western blotting analysis revealed that a 55kDa lymphocyte membrane protein is CD4. To characterize the SPMG and CD4 interaction, inhibition assay and surface plasmon resonance (SPR) assay were carried out. SPMG bound to CD4 in a multivalent fashion with specificity. The binding of SPMG to human lymphocyte CD4 was competitively inhibited by human soluble CD4 (hsCD4). Likewise, the binding between hsCD4 and immobilized SPMG was blocked by excess free SPMG. These results indicate that CD4 is one of the specific SPMG binding sites (receptors) in lymphocytes. The interaction between SPMG and CD4 may provide a mechanistic explanation of the immunopotentiating and anti-AIDS activities of SPMG in human immunodeficiency virus (HIV) infected individuals.

Human immunodeficiency virus type 1 Nef associates with lipid rafts to downmodulate cell surface CD4 and class I major histocompatibility complex expression and to increase viral infectivity

Lipid rafts are membrane microdomains that are functionally distinct from other membrane regions. We have shown that 10% of human immunodeficiency virus type 1 (HIV-1) Nef expressed in SupT1 cells is present in lipid rafts and that this represents virtually all of the membrane-associated Nef. To determine whether raft targeting, rather than simply membrane localization, has functional significance, we created a Nef fusion protein (LAT-Nef) containing the N-terminal 35 amino acids from LAT, a protein that is exclusively localized to rafts. Greater than 90% of the LAT-Nef protein was found in the raft fraction. In contrast, a mutated form, lacking two cysteine palmitoylation sites, showed less than 5% raft localization. Both proteins were equally expressed and targeted nearly exclusively to membranes. The LAT-Nef protein was more efficient than its nonraft mutant counterpart at downmodulating both cell surface CD4 and class I major histocompatibility complex (MHC) expression, as well as in enhancing first-round infectivity and being incorporated into virus particles. This demonstrates that targeting of Nef to lipid rafts is mechanistically important for all of these functions. Compared to wild-type Nef, LAT-Nef downmodulated class I MHC nearly as effectively as the wild-type Nef protein, but was only about 60% as effective for CD4 downmodulation and 30% as effective for infectivity enhancement. Since the LAT-Nef protein was found entirely in rafts while the wild-type Nef protein was distributed 10% in rafts and 90% in the soluble fraction, our results suggest that class I MHC downmodulation by Nef may be performed exclusively by raft-bound Nef. In contrast, CD4 downmodulation and infectivity enhancement may require a non-membrane-bound Nef component as well as the membrane-bound form.

Multiple contributing roles for NOS2 in LPS-induced acute airway inflammation in mice

Acute lung inflammation and injury were induced by intranasal instillation of lipopolysaccharide (LPS) in normal and type 2 nitric oxide synthase (NOS2)-deficient (NOS2-/-) C57BL/6 mice. LPS-induced increases in extravasated airway neutrophils and in lung lavage fluid of TNF-alpha and macrophage inflammatory protein-2 were markedly lower in NOS2-/- than in wild-type mice, indicating that NOS2-derived nitric oxide (NO.) participates in inflammatory cytokine production and neutrophil recruitment. Instillation of LPS also increased total lung lavage protein and induced matrix metalloproteinase-9 and mucin 5AC, as indexes of lung epithelial injury and/or mucus hyperplasia, and increased tyrosine nitration of lung lavage proteins, a marker of oxidative injury. All these responses were less pronounced in NOS2-/- than in wild-type mice. Inhibition of NOS activity also suppressed production of TNF-alpha and macrophage inflammatory protein-2 by LPS-stimulated mouse alveolar MH-S macrophages, and this was restored by NO. donors, illustrating involvement of NO. in macrophage cytokine signaling. Oligonucleotide microarray (GeneChip) analysis of global lung gene expression revealed that LPS inhalation induced a range of transcripts encoding proinflammatory cytokines and chemokines, stress-inducible factors, and other extracellular factors and suppressed mRNAs encoding certain cytoskeletal proteins and signaling proteins, responses that were generally attenuated in NOS2-/- mice. Comparison of both mouse strains revealed altered expression of several cytoskeletal proteins, cell surface proteins, and signaling proteins in NOS2-/- mice, changes that may partly explain the reduced responsiveness to LPS. Collectively, our results suggest that NOS2 participates in the acute inflammatory response to LPS by multiple mechanisms: involvement in proinflammatory cytokine signaling and alteration of the expression of various genes that affect inflammatory-immune responses to LPS.

CD4 receptor localized to non-raft membrane microdomains supports HIV-1 entry. Identification of a novel raft localization marker in CD4

Despite the preferential localization of CD4 to lipid rafts, the significance and role of these microdomains in HIV-1 entry is still controversial. The possibility that CD4, when localized to non-raft domains, might be able to support virus entry cannot be excluded. Because disintegration of rafts by extraction of cellular cholesterol with methyl-beta-cyclodextrin suffers from various adverse effects, we investigated molecular determinants controlling raft localization of the CD4 receptor. Extensive mutagenesis of the receptor showed that a raft-localizing marker, consisting of a short sequence of positively charged amino acid residues, RHRRR, was present in the membrane-proximal cytoplasmic domain of CD4. Substitution of the RHRRR sequence with alanine residues abolished raft localization of the CD4 mutant, RA5, as determined biochemically using solubilization in nonionic detergents and by confocal microscopy. The possible inhibitory effect of the introduced mutations on the adjacent CVRC palmitoylation site was ruled out because wild type (wt) CD4 and RA5, but not a palmitoylation-deficient mutant, were efficiently palmitoylated. Nonetheless, the RA5 mutant supported productive virus entry to levels equivalent to that of wild type (wt) CD4. Sucrose gradient analysis of Triton X-100 virus lysates showed that Gag and envelope gp120 proteins accumulated in low buoyant, high-density fractions. This pattern was changed after virus incubation with cells. Whereas Gag proteins localized to lipid rafts in cells expressing wt CD4 and RA5, gp120 accumulated in rafts in cells expressing wt CD4 but not RA5. We propose that raft localization of CD4 is not required for virus entry, however, post-binding fusion/entry steps may require lipid raft assembly.

Distinct roles of cellular Lck and p80 proteins in herpesvirus saimiri Tip function on lipid rafts

Lipid rafts are proposed to function as platforms for both receptor signaling and trafficking. Following interaction with antigenic peptides, the T-cell receptor (TCR) rapidly translocates to lipid rafts, where it transmits signals and subsequently undergoes endocytosis. The Tip protein of herpesvirus saimiri (HVS), which is a T-lymphotropic tumor virus, interacts with cellular Lck tyrosine kinase and p80, a WD domain-containing endosomal protein. Interaction of Tip with p80 induces enlarged vesicles and recruits Lck and TCR complex into these vesicles for trafficking. We report here that Tip is constitutively present in lipid rafts and that Tip interaction with p80 but not with Lck is necessary for its efficient localization in lipid rafts. The Tip-Lck interaction was required for recruitment of the TCR complex to lipid rafts, and the Tip-p80 interaction was critical for the aggregation and internalization of lipid rafts. These results suggest the potential mechanism for Tip-mediated TCR downregulation: Tip interacts with Lck to recruit TCR complex to lipid rafts, and it subsequently interacts with p80 to initiate the aggregation and internalization of the lipid raft domain and thereby downregulate the TCR complex. Thus, the signaling and targeting functions of HVS Tip rely on two functionally and genetically separable mechanisms that independently target cellular Lck tyrosine kinase and p80 endosomal protein.

The HIV Env-mediated fusion reaction

The current general model of HIV viral entry involves the binding of the trimeric viral envelope glycoprotein gp120/gp41 to cell surface receptor CD4 and chemokine co-receptor CXCR4 or CCR5, which triggers conformational changes in the envelope proteins. Gp120 then dissociates from gp41, allowing for the fusion peptide to be inserted into the target membrane and the pre-hairpin configuration of the ectodomain to form. The C-terminal heptad repeat region and the leucine/isoleucine zipper region then form the thermostable six-helix coiled-coil, which drives the membrane merger and eventual fusion. This model needs updating, as there has been a wealth of data produced in the last few years concerning HIV entry, including target cell dependencies, fusion kinetic data, and conformational intermediates. A more complete model must include the involvement of membrane microdomains, actin polymerization, glycosphingolipids, and possibly CD4 and chemokine signaling in entry. In addition, kinetic experiments involving the addition of fusion inhibitors have revealed some of the rate-limiting steps in this process, adding a temporal component to the model. A review of these data that may require an updated version of the original model is presented here.

Localization of CD4 and CCR5 in living cells

The events preceding human immunodeficiency virus fusion and entry are influenced by the concentration and distribution of receptor and coreceptor molecules on the cell surface. However, the extent to which these proteins colocalize with one another in the cell membrane remains unclear. Using high-resolution deconvolution fluorescent microscopy of living cells, we found that both CD4 and CCR5 accumulate in protruding membrane structures containing actin and ezrin. Although CD4 and CCR5 extensively colocalize in these structures, they do not exist in a stable complex.

The roles of membrane microdomains (rafts) in T cell activation

Detergent-resistant membrane microdomains enriched in sphingolipids, cholesterol and glycosylphosphatidylinositol-anchored proteins play essential roles in T cell receptor (TCR) signaling. These 'membrane rafts' accumulate several cytoplasmic lipid-modified molecules, including Src-family kinases, coreceptors CD4 and CD8 and transmembrane adapters LAT and PAG/Cbp, essential for either initiation or amplification of the signaling process, while most other abundant transmembrane proteins are excluded from these structures. TCRs in various T cell subpopulations may differ in their use of membrane rafts. Membrane rafts also seem to be involved in many other aspects of T cell biology, such as functioning of cytokine and chemokine receptors, adhesion molecules, antigen presentation, establishing cell polarity or interaction with important pathogens. Although the concept of membrane rafts explains several diverse biological phenomena, many basic issues, such as composition, size and heterogeneity, under native conditions, as well as the dynamics of their interactions with TCRs and other immunoreceptors, remain unclear, partially because of technical problems.

HIV-1 entry into T-cells is not dependent on CD4 and CCR5 localization to sphingolipid-enriched, detergent-resistant, raft membrane domains

The contribution of raft domains to human immunodeficiency virus (HIV) 1 entry was assessed. In particular, we asked whether the CD4 and CCR5 HIV-1 receptors need to associate with sphingolipid-enriched, detergent-resistant membrane domains (rafts) to allow viral entry into primary and T-cell lines. Based on Triton X-100 solubilization and confocal microscopy, CD4 was shown to distribute partially to rafts. In contrast, CCR5 did not associate with rafts and localized in nonraft plasma membrane domains. HIV-1-receptor partitioning remained unchanged upon viral adsorption, suggesting that viral entry probably takes place outside rafts. To directly investigate this possibility, we targeted CD4 to nonraft domains of the membrane by preventing CD4 palmitoylation and interaction with p56(lck). Directed mutagenesis of both targeting signals significantly prevented association of CD4 with rafts, but did not suppress the HIV-1 receptor function of CD4. Collectively, these results strongly suggest that the presence of HIV-1 receptors in rafts is not required for viral infection. We show, however, that depleting plasma membrane cholesterol inhibits HIV-1 entry. We therefore propose that cholesterol modulates the HIV-1 entry process independently of its ability to promote raft formation.

Role of two dileucine-like motifs in insulin receptor anchoring to microvilli

In the absence of ligand, the insulin receptor is maintained on microvilli on the cell surface. A dileucine motif (LL(986-987)) is necessary but not sufficient for this anchoring, which also required the presence of additional sequence(s) downstream of position 1000. The aim of the present study was to identify this (these) additional sequence(s). First, exons 16 or 17 were fused to the extracellular and transmembrane domains of complement receptor 1 and stably expressed in Chinese hamster ovary cells. Results obtained indicate that exon 17 is sufficient for anchoring to microvilli. Second, analysis of insulin receptor mutants truncated within exon 17 demonstrated that whereas receptors truncated at position 1000 showed no preferential association with microvilli, receptors truncated at position 1012 displayed a level of association identical to that of the full-length insulin receptor. Third, mutation of a diisoleucine motif (II(1006-1007)) present within this 12-amino acid stretch abrogated the preferential association of the receptor with microvilli. These results indicate that the domain required for association of insulin receptor with microvilli is contained within the region encoded by exon 17 and that, within this sequence, two dileucine-like motifs (LL(986-987) and II(1006-1007)) play a crucial role.

Role of cholesterol in human immunodeficiency virus type 1 envelope protein-mediated fusion with host cells

In this study we examined the effects of target membrane cholesterol depletion and cytoskeletal changes on human immunodeficiency virus type 1 (HIV-1) Env-mediated membrane fusion by dye redistribution assays. We found that treatment of peripheral blood lymphocytes (PBL) with methyl-beta-cyclodextrin (MbetaCD) or cytochalasin reduced their susceptibility to membrane fusion with cells expressing HIV-1 Env that utilize CXCR4 or CCR5. However, treatment of human osteosarcoma (HOS) cells expressing high levels of CD4 and coreceptors with these agents did not affect their susceptibility to HIV-1 Env-mediated membrane fusion. Removal of cholesterol inhibited stromal cell-derived factor-1alpha- and macrophage inflammatory protein 1beta-induced chemotaxis of both PBL and HOS cells expressing CD4 and coreceptors. The fusion activity as well as the chemotactic activity of PBL was recovered by adding back cholesterol to these cells. Confocal laser scanning microscopy analysis indicated that treatment of lymphocytes with MbetaCD reduced the colocalization of CD4 or of CXCR4 with actin presumably in microvilli. These findings indicate that, although cholesterol is not required for HIV-1 Env-mediated membrane fusion per se, its depletion from cells with relatively low coreceptor densities reduces the capacity of HIV-1 Env to engage coreceptor clusters required to trigger fusion. Furthermore, our results suggest that coreceptor clustering may occur in microvilli that are supported by actin polymerization.