Recombinant Human C-C chemokine receptor type 5 (CCR5)

What is the genetic and structural basis of CCR5?

CCR5 is a G protein-coupled receptor belonging to the beta chemokine receptors family of integral membrane proteins. The human CCR5 gene is located on the short (p) arm at position 21 on chromosome 3 . Structurally, CCR5's C-terminal region is enriched in serines and threonines that provide phosphorylation sites for G-protein coupled receptor kinases . The protein contains distinct transmembrane domains, with specific regions crucial for ligand binding and receptor activation.

For researchers studying CCR5 structure-function relationships, it's important to note that amino acid modifications of CCR5 have significant consequences for both HIV infection and ligand binding affinity . The receptor contains key residues like M287 (position 7.43), Y108 (position 3.32), and E283 (position 7.39) that are critical for signaling functions, as mutations in these positions can reduce activation by approximately 40-70% without affecting ligand binding affinity .

What are the primary ligands for CCR5 and their binding mechanisms?

CCR5's cognate ligands include several chemokines:

• CCL3 (also known as MIP-1α)

• CCL4 (also known as MIP-1β)

• CCL3L1

• CCL5 (also known as RANTES)

The binding mechanism involves a complex interplay between the N-terminal domain of chemokines and specific receptor domains. Research has shown that CCR5 activation by chemokines like CCL5 involves interactions between the aspartic acid residue at positions 5 or 6 of the chemokine and the K26 residue (position 1.28) of CCR5 . This interaction stabilizes the extended hinge structure of the chemokine, which is crucial for receptor activation.

Experimental evidence indicates that mutations in the chemokine structure affect both signaling efficiency and binding affinity. For example, D5A and D5K mutations in [6P4]CCL5 decrease both maximum activation (Emax) by 30% and 60% respectively, while increasing EC50 approximately 5- to 10-fold .

How is CCR5 expressed in different cell types and what controls its regulation?

CCR5 is predominantly expressed on:

• T cells

• Macrophages

• Dendritic cells

• Eosinophils

• Microglia

• Specific subpopulations of breast or prostate cancer cells

Notably, CCR5 expression is selectively induced during cancer transformation and is not expressed in normal breast or prostate epithelial cells . Approximately 50% of human breast cancers express CCR5, primarily in triple-negative breast cancer subtypes .

The regulation of CCR5 expression is influenced by cell activation status. Studies have shown that the percentage of CD4+CCR5+ T-cells is higher (13.2%) in HIV-infected individuals compared to uninfected individuals (6.2%) . The activation state of CD4+ cells, as measured by HLA-DR expression, positively correlates with CCR5 expression levels .

What are the most effective methods for expressing and purifying recombinant human CCR5?

For producing functional recombinant human CCR5, researchers typically employ mammalian expression systems rather than bacterial ones due to the need for proper post-translational modifications.

Methodological Approach:

• Expression System Selection:

  • Human embryonic kidney (HEK) cells and Chinese hamster ovary (CHO) cells have been demonstrated as effective systems for functional CCR5 expression

  • These cell types support proper protein folding and post-translational modifications

• Vector Design Considerations:

  • Include affinity tags (e.g., His-tag, FLAG-tag) for purification

  • Consider including fluorescent protein fusions for trafficking studies

  • Codon optimization for mammalian expression

• Purification Strategy:

  • Detergent solubilization (typically with mild detergents like DDM or LMNG)

  • Affinity chromatography using engineered tags

  • Size exclusion chromatography for final purification

• Functional Validation:

  • Calcium flux activation assays in transfected cells to assess receptor function

  • Binding assays with labeled chemokines to confirm ligand interaction

  • Conformational antibody binding to verify proper folding

How can researchers effectively study CCR5 activation and signaling in experimental settings?

Several methodological approaches are available for investigating CCR5 activation and downstream signaling:

• Calcium Flux Assays:

  • Widely used for assessing functional responses of CCR5 to ligands

  • HEK and CHO cells expressing CCR5 can be used to measure calcium mobilization upon stimulation

  • Enables determination of key pharmacological parameters (EC50 and Emax values)

• Mutational Analysis:

  • Site-directed mutagenesis of key CCR5 residues (e.g., M287A, Y108A, E283A) to assess their role in signaling

  • Mutations in these positions reduce signaling without affecting chemokine affinity

• Chemokine Variant Testing:

  • Modifications to chemokine structure (e.g., D5A and D5K mutations in [6P4]CCL5) affect both signaling efficiency and binding affinity

  • Truncation studies show that variants lacking the first few N-terminal residues (CCL5 3-68, CCL5 4-68) have reduced receptor affinity and diminished functional responses

• G-Protein Coupling Assessment:

  • BRET (Bioluminescence Resonance Energy Transfer) assays to measure G-protein dissociation

  • Analysis of downstream signaling using phospho-specific antibodies against key signaling molecules

What are the optimal approaches for studying CCR5 genetic variants and their functional consequences?

Research into CCR5 genetic variants, particularly the Δ32 mutation, requires specific methodological considerations:

• Variant Identification Strategy:

  • Extract CCR5 missense and loss-of-function variants from genomic databases including:

    • gnomADv2.1

    • COSMIC

    • Bravo for TOPmed variants

    • ClinVar

    • Geno2MP

• Functional Prediction Analysis:

  • Assess variants using multiple prediction tools:

    • PolyPhen2

    • Provean

    • SIFT

    • Align-GVGD

  • Calculate impact scores incorporating functional prediction, codon selection, conservation, and allele frequency data

• Experimental Validation Methods:

  • Site-directed mutagenesis to recreate variants

  • Cell-based functional assays to assess:

    • Receptor expression levels

    • Ligand binding affinity

    • Calcium signaling capacity

    • Chemotactic responses

    • HIV co-receptor function

• Population Analysis Considerations:

  • Different populations have varying frequencies of CCR5 variants

  • The Δ32 mutation appears to have arisen once in the last millennium and increased to approximately 10% allele frequency in European populations due to strong selective pressure

How does CCR5 function as a co-receptor for HIV-1 and what are the implications for therapeutic development?

CCR5 serves as a primary co-receptor for macrophage-tropic (M-tropic) strains of HIV-1, facilitating viral entry into host cells. Understanding this mechanism has significant implications for therapeutic development.

Mechanism of HIV Co-Receptor Function:

CCR5 functions as a co-receptor alongside CD4 for HIV-1 entry. The virus initially binds to CD4, inducing conformational changes in the viral envelope protein that enable subsequent interaction with CCR5, ultimately leading to membrane fusion and viral entry.

The discovery that homozygous carriers of the CCR5-Δ32 mutation display near-complete resistance to HIV infection, regardless of exposure, provided a critical insight into the essential role of CCR5 in HIV pathogenesis . This finding has directly influenced therapeutic strategies targeting CCR5.

Therapeutic Approaches Targeting CCR5:

• Small Molecule CCR5 Antagonists:

  • Prevent HIV binding to CCR5 by inducing conformational changes in the receptor

  • Current challenges include optimizing pharmacokinetics and minimizing side effects

• Gene Editing Approaches:

  • The case of an HIV-positive individual who received bone marrow transplant from a donor homozygous for CCR5-Δ32 and subsequently became HIV-negative demonstrates proof-of-concept

  • CRISPR-Cas9 and other gene editing tools are being investigated to recreate CCR5-Δ32 phenotype in patients

• Experimental Considerations for Researchers:

  • Assessment of off-target effects remains critical given CCR5's roles in inflammation and immune function

  • Long-term monitoring for unexpected consequences is essential as CCR5 has functions beyond HIV co-receptor activity

What role does CCR5 play in cancer biology and what research methods best elucidate these functions?

Recent research has revealed important roles for CCR5 in cancer biology, particularly in breast and prostate cancers:

CCR5 Expression in Cancer:

• Expression is selectively induced during cancer transformation

• Not expressed in normal breast or prostate epithelial cells

• Approximately 50% of human breast cancers express CCR5, primarily in triple-negative breast cancer

Functional Roles in Cancer:

• CCR5 inhibitors block migration and metastasis of CCR5-expressing cancer cells

• CCR5 is expressed in a subset of cancer cells with cancer stem cell characteristics

• These cells drive therapy resistance

• CCR5 inhibitors enhance the efficacy of current chemotherapy by increasing cell death

Recommended Research Methodologies:

• Expression Analysis:

  • Immunohistochemistry and flow cytometry to quantify CCR5 expression in tumor tissues and cancer cell lines

  • Single-cell RNA sequencing to identify CCR5-expressing subpopulations

• Functional Assays:

  • Migration and invasion assays with CCR5 inhibitors

  • Chemotaxis assays toward CCR5 ligands

  • Cancer stem cell marker co-expression analysis

• Therapeutic Response Assessment:

  • Combination studies with CCR5 inhibitors and standard chemotherapeutics

  • Patient-derived xenograft models to evaluate CCR5 targeting in vivo

How do CCR5 polymorphisms, particularly Δ32, affect immune function beyond HIV resistance?

The CCR5-Δ32 mutation has implications beyond HIV resistance, affecting broader immune function and disease outcomes:

Immune System Effects:

• CCR5 enhances T-cell co-stimulation and cytokine release from CD4+ T-cells

• CCR5 ligands augment T-cell activation responses and enhance production of antigen-specific T-cells

• During inflammation, CCR5 expression is up-regulated in CD8+ cells, facilitating migration to sites of CD4+ T-cell and dendritic cell interactions

Disease Associations Beyond HIV:

• CCR5 has been implicated in regulating Staphylococcus aureus infection

• It plays a role in stroke recovery

• It affects outcomes in fatal graft complications

Research Approaches for Investigating Δ32 Effects:

• Immunophenotyping:

  • Compare immune cell subsets and activation markers between wild-type, heterozygous, and homozygous Δ32 individuals

  • Analyze cytokine production profiles in response to various stimuli

• Infection Models:

  • Challenge models using pathogens beyond HIV

  • Assess differences in pathogen clearance and inflammatory responses

• Population-Based Studies:

  • Investigate associations between CCR5 genotype and outcomes in various inflammatory and infectious diseases

  • Apply principles of population genetics to understand selective pressures that influenced Δ32 frequency

What are the critical structural domains of CCR5 for ligand binding and receptor activation?

Understanding the structure-function relationship of CCR5 is crucial for rational drug design and therapeutic development. Several critical domains have been identified:

Key Structural Elements:

• N-Terminal Domain:

  • Important for initial chemokine recognition

  • Interacts with the core domain of chemokines

• Transmembrane Helices:

  • TM1 contains K26 (position 1.28), which forms a salt bridge with aspartic acid residues in chemokine ligands

  • TM3 contains Y108 (position 3.32), critical for signaling

  • TM6 and TM7 undergo conformational changes during receptor activation

  • TM7 contains M287 (position 7.43) and E283 (position 7.39), both crucial for signaling functions

• Extracellular Loops:

  • Form the chemokine recognition site 2 (CRS2)

  • Critical for selective binding of different chemokines

• C-Terminal Region:

  • Enriched in serines and threonines that provide phosphorylation sites for G-protein coupled receptor kinases

  • Important for receptor desensitization and internalization

Activation Mechanism:
The binding of chemokines to CCR5 induces conformational changes connecting the receptor activation pathways primarily through TM7 and TM6. The W248 residue (position 6.48) lies at the center of these conformational changes, playing a crucial role in signal transduction .

How do specific amino acid modifications affect CCR5 function and ligand interactions?

Amino acid modifications of CCR5 have significant impacts on receptor function and ligand interactions:

Impact of Receptor Mutations:

Data derived from calcium flux activation assays in HEK and CHO cells expressing CCR5

Impact of Ligand Modifications:

Data based on functional and binding studies with CCL5 variants

These structure-function studies provide critical insights for researchers designing CCR5-targeted therapeutics or investigating natural receptor variants.

What are the evolutionary implications of CCR5 structure conservation across species?

The evolutionary analysis of CCR5 provides important insights into protein function and the origins of variants like Δ32:

Evolutionary Conservation:

• CCR5 belongs to the CC chemokine receptor family, which shows varying degrees of homology

• Homology within this family may provide compensatory mechanisms for some CCR5 functions in individuals carrying Δ32 or other variants

• The extent of functional compensation by other family members remains incompletely understood

Δ32 Mutation Origin and Selective Pressure:

• Molecular evolutionary theory suggests that the CCR5-Δ32 mutation occurred only once in the last millennium

• Strong selective pressure drove its frequency to approximately 10% in European populations relatively recently

• Several hypotheses exist regarding the nature of this selective pressure, including protection against historical pandemics

Research Approaches for Evolutionary Studies:

• Comparative Genomics:

  • Cross-species comparison of CCR5 sequence and structure

  • Analysis of functional conservation versus divergence

• Population Genetics:

  • Analysis of CCR5 allele frequencies across global populations

  • Assessment of signatures of selection in genomic regions surrounding CCR5

• Functional Redundancy Assessment:

  • Evaluate compensation by other chemokine receptors in CCR5-deficient models

  • Compare phenotypes across species with varying CCR5 structures

How can researchers investigate CCR5's role in neurological conditions and stroke recovery?

Recent findings have implicated CCR5 in neurological processes, particularly in stroke recovery:

Current Understanding:

• CCR5 has been identified as a factor affecting recovery from stroke

• The receptor is expressed on microglia, the resident immune cells of the brain

• CCR5 may influence neuroinflammatory processes that impact neurological outcomes

Suggested Research Approaches:

• Animal Models:

  • Compare stroke outcomes in CCR5-deficient versus wild-type animals

  • Assess post-stroke inflammatory responses and neural repair mechanisms

  • Test CCR5 antagonists as potential treatments to promote recovery

• Molecular Mechanisms:

  • Investigate CCR5-mediated signaling in microglia and its impact on neuroinflammation

  • Explore interactions between CCR5+ immune cells and neural progenitors during recovery

  • Analyze gene expression changes in CCR5+ cells in neural tissues after injury

• Clinical Correlations:

  • Examine associations between CCR5 genotypes and stroke recovery outcomes in patients

  • Consider CCR5 expression as a biomarker for recovery potential

  • Design pilot studies of CCR5 antagonists in stroke recovery

What methodologies are most effective for investigating CCR5 as a therapeutic target in inflammatory diseases?

Beyond HIV and cancer, CCR5 represents a potential therapeutic target in various inflammatory conditions:

Target Validation Approaches:

• Disease-Specific Animal Models:

  • Evaluate CCR5 knockout or antagonist effects in models of:

    • Autoimmune diseases

    • Inflammatory bowel disease

    • Rheumatoid arthritis

    • Transplant rejection

• Ex Vivo Human Tissue Studies:

  • Analyze CCR5 expression in affected tissues from patients

  • Test effects of CCR5 inhibition on inflammatory markers in patient-derived samples

  • Compare responses between different patient subgroups

• Multiomics Integration:

  • Combine transcriptomics, proteomics, and metabolomics to map CCR5 signaling networks

  • Identify disease-specific alterations in CCR5 pathways

  • Discover potential combination therapy approaches

• Translational Biomarkers:

  • Develop assays to predict and monitor response to CCR5-targeted therapies

  • Identify patient subgroups most likely to benefit from CCR5 modulation