Rescue of HIV-1 receptor function through cooperation between different forms of the CCR5 chemokine receptor

Chemokine receptor oligomerization and allostery

Oligomerization of chemokine receptors has been reported to influence many aspects of receptor function through allosteric communication between receptor protomers. Allosteric interactions within chemokine receptor hetero-oligomers have been shown to cause negative cooperativity in the binding of chemokines and to inhibit receptor activation in the case of some receptor pairs. Other receptor pairs can cause enhanced signaling and even activate entirely new, hetero-oligomer-specific signaling complexes and responses downstream of receptor activation. Many mechanisms contribute to these effects including direct allosteric coupling between the receptors, G protein-mediated allostery, G protein stealing, ligand sequestration, and recruitment of new intracellular proteins by exposing unique binding interfaces on the oligomerized receptors. These effects present both challenges as well as exciting opportunities for drug discovery. One of the most difficult challenges will involve determining if and when hetero-oligomers versus homomeric receptors are involved in specific disease states.

Common principles and intermediates of viral protein-mediated fusion: the HIV-1 paradigm

Enveloped viruses encode specialized fusion proteins which promote the merger of viral and cell membranes, permitting the cytosolic release of the viral cores. Understanding the molecular details of this process is essential for antiviral strategies. Recent structural studies revealed a stunning diversity of viral fusion proteins in their native state. In spite of this diversity, the post-fusion structures of these proteins share a common trimeric hairpin motif in which the amino- and carboxy-terminal hydrophobic domains are positioned at the same end of a rod-shaped molecule. The converging hairpin motif, along with biochemical and functional data, implies that disparate viral proteins promote membrane merger via a universal "cast-and-fold" mechanism. According to this model, fusion proteins first anchor themselves to the target membrane through their hydrophobic segments and then fold back, bringing the viral and cellular membranes together and forcing their merger. However, the pathways of protein refolding and the mechanism by which this refolding is coupled to membrane rearrangements are still not understood. The availability of specific inhibitors targeting distinct steps of HIV-1 entry permitted the identification of key conformational states of its envelope glycoprotein en route to fusion. These studies provided functional evidence for the direct engagement of the target membrane by HIV-1 envelope glycoprotein prior to fusion and revealed the role of partially folded pre-hairpin conformations in promoting the pore formation.

An allosteric rheostat in HIV-1 gp120 reduces CCR5 stoichiometry required for membrane fusion and overcomes diverse entry limitations

Binding of the human immunodeficiency virus (HIV-1) envelope glycoprotein gp120 to the CCR5 co-receptor reduces constraints on the metastable transmembrane subunit gp41, thereby enabling gp41 refolding, fusion of viral and cellular membranes, and infection. We previously isolated adapted HIV-1(JRCSF) variants that more efficiently use mutant CCR5s, including CCR5(Delta18) lacking the important tyrosine sulfate-containing amino terminus. Effects of mutant CCR5 concentrations on HIV-1 infectivities were highly cooperative, implying that several may be required. However, because wild-type CCR5 efficiently mediates infections at trace concentrations that were difficult to measure accurately, analyses of its cooperativity were not feasible. New HIV-1(JRCSF) variants efficiently use CCR5(HHMH), a chimera containing murine extracellular loop 2. The adapted virus induces large syncytia in cells containing either wild-type or mutant CCR5s and has multiple gp120 mutations that occurred independently in CCR5(Delta18)-adapted virus. Accordingly, these variants interchangeably use CCR5(HHMH) or CCR5(Delta18). Additional analyses strongly support a novel energetic model for allosteric proteins, implying that the adaptive mutations reduce quaternary constraints holding gp41, thus lowering the activation energy barrier for membrane fusion without affecting bonds to specific CCR5 sites. In accordance with this mechanism, highly adapted HIV-1s require only one associated CCR5(HHMH), whereas poorly adapted viruses require several. However, because they are allosteric ensembles, complexes with additional co-receptors fuse more rapidly and efficiently than minimal ones. Similarly, wild-type HIV-1(JRCSF) is highly adapted to wild-type CCR5 and minimally requires one. The adaptive mutations cause resistances to diverse entry inhibitors and cluster appropriately in the gp120 trimer interface overlying gp41. We conclude that membrane fusion complexes are allosteric machines with an ensemble of compositions, and that HIV-1 adapts to entry limitations by gp120 mutations that reduce its allosteric hold on gp41. These results provide an important foundation for understanding the mechanisms that control membrane fusion and HIV-1's facile adaptability.

Identification of a postendocytic sorting sequence in CCR5

The chemokine receptor 5 (CCR5), a member of the G protein-coupled receptor family (GPCR), is used by human immunodeficiency virus type 1 (HIV-1) with a R5 tropism as an entry receptor in addition to CD4. It is a key target for an antiviral action aiming at inhibiting the HIV-1 entry process. Only few data are available today regarding the mechanism involved in the intracellular trafficking process of CCR5. Understanding how CCR5 cell surface expression is regulated is particularly important with regard to HIV-1 entry inhibition. We set out to investigate whether CCR5 molecular determinants were involved in the postendocytic recycling and degradative pathways. We constructed progressive deletion mutants of the C-terminal domain of CCR5 that we stably expressed in HEK293 cells. All of the deletion mutants were expressed at the cell surface and were functional HIV-1 receptors. The deletion mutants were internalized after stimulation, but they lost their ability to recycle to the plasma membrane. They were rerouted toward a lysosomal degradative pathway. We identified here a sequence of four amino acids, present at the extreme C terminus of CCR5, that is necessary for the recycling of the internalized receptor, independently of its phosphorylation. A detailed analysis of this sequence indicated that the four amino acids acted as a postsynaptic density 95/discs-large/zona occludens (PDZ) interacting sequence. These results show that the CCR5 cytoplasmic domain bears a sequence similar to the "recycling signals" previously identified in other GPCRs. Drugs able to disrupt the recycling pathway of CCR5 may constitute promising tools for therapeutic treatment.

Variants of CCR5, which are permissive for HIV-1 infection, show distinct functional responses to CCL3, CCL4 and CCL5

CCR5 is one of the primary coreceptors for Env-mediated fusion between cells and human immunodeficiency virus type 1 (HIV-1). Analyses of CCR5 variants in cohorts of HIV-1 high-risk individuals led to the identification of multiple single amino-acid substitutions, which may have functional consequences. This study focused on eight naturally occurring allelic variants located between amino-acid residues 60 and 334 of CCR5. All studied allelic variants were highly expressed on the cell surface of HEK-293 cells and permissive for HIV-1 infection. Variant G301V showed 3.5-fold increase in 50% effective concentration (EC(50)) for CCL4 (MIP 1beta) in a competitive binding assay. There was also a significant reduction in CCL5 (RANTES) EC(50) for the R223Q, A335V and Y339F variants. The most unexpected functional abnormality was exhibited by the R60S variant that exhibited a loss of ligand-induced desensitization in chemotaxis assays, but showed normal CCL4 and CCL5 binding avidity. This mutation is located in the first intracellular loop, a domain that has not previously been shown to be involved in receptor desensitization. In conclusion, our results support earlier studies showing that these naturally occurring point mutations do not limit HIV-1 infection, and indicated that single amino-acid changes can have unexpected functional consequences.

G protein-coupled receptor kinases promote phosphorylation and beta-arrestin-mediated internalization of CCR5 homo- and hetero-oligomers

Expression levels of the chemokine receptor, CC chemokine receptor 5 (CCR5), at the cell surface determine cell susceptibility to HIV entry and infection. Cellular activation by CCR5 itself, but also by unrelated receptors leads to cross-phosphorylation and cross-internalization of CCR5. This study addresses the underlying molecular mechanisms of homologous and heterologous CCR5 regulation. As shown by bioluminescence resonance energy transfer experiments, CCR5 formed constitutive homo- as well as heterooligomeric complexes together with C5aR but not with the unrelated AT(1a)R in living cells. Stimulation with CCL5 of RBL cells, which co-expressed CCR5 together with an N-terminally truncated CCR5-DeltaNT mutant, resulted in both protein kinase C (PKC)- and G protein-coupled receptor (GPCR) kinase (GRK)-mediated cross-phosphorylation of the mutant unligated receptor, as determined by phosphosite-specific monoclonal antibody. Similarly, both PKC and GRK cross-phosphorylated CCR5 in a heterologous manner after C5a stimulation of RBL-CCR5/C5aR cells, whereas AT(1a)R stimulation resulted only in classical PKC-mediated CCR5 phosphorylation. Co-expression of CCR5-DeltaNT together with a phosphorylation-deficient CCR5 mutant that neither binds beta-arrestin nor undergoes internalization partially restored the CCL5-induced association of beta-arrestin with the homo-oligomeric receptor complex and augmented cellular uptake of (125)I-CCL5. Co-expression of C5aR, but not of AT(1a)R, promoted CCR5 co-internalization upon agonist stimulation by a mechanism independent of CCR5 phosphorylation. Co-internalization of phosphorylated CCR5 was also observed in C5a-stimulated macrophages. Finally, co-expression of a constitutively internalized C5aR-US28(CT) mutant led to intracellular accumulation of CCR5 in the absence of ligand stimulation. These results show that GRKs and beta-arrestin are involved in heterologous receptor regulation by cross-phosphorylating and co-internalizing unligated receptors within homo- or hetero-oligomeric protein complexes.

Restricted Variable Residues in the C-terminal Segment of HIV-1 V3 Loop Regulate the Molecular Anatomy of CCR5 Utilization

The V3 loop of the HIV-1 envelope glycoprotein (Env) is the major determinant for coreceptor utilization, but the structural basis for this specificity remains to be defined. By characterizing a set of naturally occurring R5 Env variants, we demonstrate that Asp324 in the conserved IIGDIR motif of the V3 loop (CTRPN(300)NNTRKSIHIGP(311)GRAFYTTGEIIGD(324)IRQAHC) C-terminal segment regulates the molecular anatomy of CCR5 utilization. Whereas gp120 subunits with Asp or Asn at position 324 were fusogenic with coreceptor chimeras containing either the N-terminal domain or the body of CCR5, substitution of charged (Glu, Lys) or small hydrophobic (Gly, Ala) residues resulted in complete loss of fusogenic activity with the N terminus and markedly reduced utilization of the body of CCR5, although their ability to use wild-type CCR5 was unchanged. This phenotypic conversion was confirmed in both gain and loss of function experiments using Env from multiple subtypes. Alignment of sequences of R5 V3 loops (n=599) from the HIV database revealed that the mutation of Asp324 in the conserved IIGDIR motif is restricted to Asn324, with proportions of 71.5% and 28%, respectively. Infection of primary CD4(+)T cells demonstrated that Env bearing Asp324 was less sensitive to RANTES, suggesting that Asp or Asn in this position may be crucial for viral fitness. The CD4-dependent gp120 binding to CCR5 was decreased when Asp324 was replaced with a charged or hydrophobic residue, but unchanged when replaced with Asn. Molecular modeling analyses predicted that Asp/Asn324 forms a critical H-bond with Asn300. These findings indicate that Asp or Asn at position 324 of the V3 stem stabilizes the conformation of V3 loop and hence influences the intensities of interaction between CD4-activated gp120 and CCR5 which results in viral entry.

Chemokine receptor CCR5: insights into structure, function, and regulation

CC chemokine receptor 5 (CCR5) is a seven-transmembrane, G protein-coupled receptor (GPCR) which regulates trafficking and effector functions of memory/effector T-lymphocytes, macrophages, and immature dendritic cells. It also serves as the main coreceptor for the entry of R5 strains of human immunodeficiency virus (HIV-1, HIV-2). Chemokine binding to CCR5 leads to cellular activation through pertussis toxin-sensitive heterotrimeric G proteins as well as G protein-independent signalling pathways. Like many other GPCR, CCR5 is regulated by agonist-dependent processes which involve G protein coupled receptor kinase (GRK)-dependent phosphorylation, beta-arrestin-mediated desensitization and internalization. This review discusses recent advances in the elucidation of the structure and function of CCR5, as well as the complex mechanisms that regulate CCR5 signalling and cell surface expression.

Genetic and phenotypic analyses of human immunodeficiency virus type 1 escape from a small-molecule CCR5 inhibitor

We have described previously the generation of an escape variant of human immunodeficiency virus type 1 (HIV-1), under the selection pressure of AD101, a small molecule inhibitor that binds the CCR5 coreceptor (A. Trkola, S. E. Kuhmann, J. M. Strizki, E. Maxwell, T. Ketas, T. Morgan, P. Pugach, S. X. L. Wojcik, J. Tagat, A. Palani, S. Shapiro, J. W. Clader, S. McCombie, G. R. Reyes, B. M. Baroudy, and J. P. Moore, Proc. Natl. Acad. Sci. USA 99:395-400, 2002). The escape mutant, CC101.19, continued to use CCR5 for entry, but it was at least 20,000-fold more resistant to AD101 than the parental virus, CC1/85. We have now cloned the env genes from the the parental and escape mutant isolates and made chimeric infectious molecular clones that fully recapitulate the phenotypes of the corresponding isolates. Sequence analysis of the evolution of the escape mutants suggested that the most relevant changes were likely to be in the V3 loop of the gp120 glycoprotein. We therefore made a series of mutant viruses and found that full AD101 resistance was conferred by four amino acid changes in V3. Each change individually caused partial resistance when they were introduced into the V3 loop of a CC1/85 clone, but their impact was dependent on the gp120 context in which they were made. We assume that these amino acid changes alter how the HIV-1 Env complex interacts with CCR5. Perhaps unexpectedly, given the complete dependence of the escape mutant on CCR5 for entry, monomeric gp120 proteins expressed from clones of the fully resistant isolate failed to bind to CCR5 on the surface of L1.2-CCR5 cells under conditions where gp120 proteins from the parental virus and a partially AD101-resistant virus bound strongly. Hence, the full impact of the V3 substitutions may only be apparent at the level of the native Env complex.

Experimental evidence for lack of homodimerization of the G protein-coupled human N-formyl peptide receptor

A large number of G protein-coupled receptors have been shown to form homodimers based on a number of different techniques such as receptor coimmunoprecipitation, cross-linking, and fluorescence resonance energy transfer. In addition, functional assays of cells coexpressing a mutant receptor with a wild-type receptor have shown receptor phenotypes that can best be explained through dimerization. We asked whether the human neutrophil N-formyl peptide receptor (FPR) forms dimers in Chinese hamster ovary cells by coexpressing wild-type FPR with one of two mutants: D71A, which is uncoupled from G protein, and N297A, which has a defect in receptor phosphorylation and endocytosis. Experiments measuring chemotaxis, ligand-induced release of intracellular calcium, and p42/44 mitogen-activated protein kinase activation did not show an inhibitory effect of the coexpressed FPR D71A mutant. Coexpressed wild-type receptor was efficiently internalized, but failed to correct the endocytosis defects of the D71A and the N297A mutants. To explore the possibility that the mutations themselves prevented dimerization, we examined the coimmunoprecipitation of differentially epitope-tagged FPR. Immunoprecipitation of hemagglutinin-tagged FPR failed to coimmunoprecipitate coexpressed c-myc-tagged FPR and vice versa. Together, these data suggest that, unlike many other G protein-coupled receptors, FPR does not form homodimers.