Recombinant Human Atypical chemokine receptor 3 (ACKR3)

What is Recombinant Human ACKR3 and how does it differ from classical chemokine receptors?

Recombinant Human Atypical Chemokine Receptor 3 (ACKR3/CXCR7) belongs to a small subfamily of receptors (ACKR1-4) with distinctive functional characteristics compared to classical chemokine receptors. The primary distinguishing feature of ACKR3 is its inability to trigger G protein-dependent signaling pathways in response to ligand binding . Instead, ACKR3 exclusively recruits arrestins when activated by its ligands, making it functionally distinct from classical chemokine receptors .

ACKR3 functions primarily as a scavenger receptor that captures, internalizes, and degrades chemokines, thereby regulating their availability for signaling through classical chemokine receptors . This scavenging activity is critical for modulating chemokine gradients and fine-tuning chemokine-dependent processes including cell migration and inflammatory responses.

Another distinguishing characteristic of ACKR3 is its ability to bind ligands from different chemokine families (both CC and CXC chemokines), which appears to be a common feature among atypical chemokine receptors . Perhaps most surprisingly, ACKR3 has been shown to bind and respond to non-chemokine ligands such as opioid peptides, revealing an unexpected intersection between chemokine and opioid signaling systems .

For researchers working with recombinant ACKR3, recognizing these fundamental differences is essential when designing experiments and interpreting results, particularly when comparing ACKR3 function to classical chemokine receptors.

What are the primary ligands of ACKR3 and what binding affinities do they exhibit?

ACKR3 has a remarkably diverse ligand profile that includes both chemokines and non-chemokine ligands. The primary chemokine ligands include:

• CXCL12 (SDF-1), which also binds to the classical receptor CXCR4

• CXCL11 (I-TAC), which also binds to CXCR3

• vCCL2/vMIP-II, a viral broad-spectrum chemokine antagonist encoded by the sarcoma-associated herpesvirus (HHV-8)

• Macrophage migration-inhibitory factor (MIF), a pseudo-chemokine that contributes to inflammatory responses

Research has revealed that ACKR3 also interacts with numerous endogenous opioid peptides, including:

• Dynorphin A and derivatives (dynorphin A 1-13, big dynorphin)

• BAM22 and adrenorphin

• Nociceptin and nociceptin 1-13-amide

These ligands bind to ACKR3 with high affinity, as demonstrated in competitive binding and functional assays. The following table shows binding and activation parameters for wild-type ACKR3 and a cysteine mutant:

Mutational analysis has revealed that CXCL11 and CXCL12 interact differently with ACKR3. CXCL11 binding depends primarily on the ACKR3 N-terminus and certain extracellular loop (ECL) positions for primary binding, while ECL residues mediate secondary binding and arrestin recruitment. In contrast, CXCL12 binding requires specific key residues including Asp-179 .

For researchers, understanding these specific interactions is crucial when designing experiments to study ACKR3 function or when developing potential therapeutic modulators targeting this receptor.

Where is ACKR3 expressed in human tissues and what are the implications for experimental design?

ACKR3 exhibits a specific expression pattern across various tissues and cell types in humans, with important implications for experimental design. Based on research findings, ACKR3 is expressed in:

• Central Nervous System (CNS): ACKR3 is abundantly expressed in numerous regions of the CNS, with expression patterns overlapping with classical opioid receptors . This co-expression suggests a potential regulatory role in opioid signaling.

• Adrenal glands: Significant expression has been detected in adrenal tissues, indicating possible involvement in hormone regulation .

• Vascular system: ACKR3 is expressed on endothelial cells, where it plays roles in angiogenesis and vascular development .

• Immune cells: Various immune cell populations express ACKR3, suggesting its involvement in immune regulation and inflammatory responses .

• Platelets: Expression in platelets has been linked to modulation of cell survival and thrombus formation .

• Tumors: ACKR3 is frequently upregulated in various cancer types, particularly in tumor vasculature, suggesting potential roles in tumor progression and metastasis .

The expression pattern of ACKR3 provides important insights into its physiological functions and should guide experimental design. For researchers, several methodological considerations arise from this expression profile:

• For in vivo studies, tissue-specific expression patterns should inform the choice of experimental models and analysis of tissue-specific effects.

• For cell culture experiments, researchers should verify ACKR3 expression in their chosen cell types, as expression levels can vary significantly.

• When studying complex tissues with multiple cell types, cell-type specific analyses may be necessary to distinguish ACKR3 functions in different populations.

• Co-expression with classical chemokine receptors or opioid receptors should be considered when interpreting functional outcomes, as ACKR3 may modulate signaling through these receptors.

These considerations become particularly important when designing experiments to study ACKR3 function in physiological or pathological contexts.

How does ACKR3 function as a chemokine and opioid peptide scavenger?

ACKR3 functions as an efficient scavenger through a distinctive mechanism that differs from classical receptor signaling. The scavenging process involves several well-characterized steps:

• Ligand binding: ACKR3 binds to its chemokine ligands (CXCL12, CXCL11) or opioid peptides with high affinity, as demonstrated in competitive binding assays .

• Arrestin recruitment: Upon ligand binding, ACKR3 exclusively recruits β-arrestins (both β-arrestin-1 and β-arrestin-2) without activating G protein-dependent signaling pathways . This selective recruitment is a defining characteristic of atypical chemokine receptors.

• Receptor internalization: The recruitment of arrestins leads to receptor internalization through clathrin-dependent endocytosis, forming endosomal vesicles containing the receptor-ligand complex.

• Ligand degradation: After internalization, the bound ligands are directed to lysosomes where they are degraded, effectively removing them from the extracellular environment. This process constitutes the actual "scavenging" function.

• Receptor recycling: ACKR3 can constitutively cycle between the plasma membrane and intracellular compartments, allowing for continuous scavenging activity even in the absence of ligands .

The functional significance of this scavenging activity is profound. By regulating the local availability of chemokines, ACKR3 shapes chemokine gradients that guide cell migration and other chemokine-dependent processes. This is particularly important for CXCL12, which signals through CXCR4 to regulate critical processes such as stem cell homing, leukocyte trafficking, and organogenesis .

Similarly, by scavenging opioid peptides, ACKR3 regulates their availability for signaling through classical opioid receptors, thus modulating endogenous opioid signaling in the central nervous system and potentially influencing pain perception and analgesic responses .

For experimental approaches, researchers can assess ACKR3 scavenging function by measuring the disappearance of labeled ligands from culture media, the accumulation of ligands inside cells expressing ACKR3, or changes in extracellular ligand concentrations in the presence versus absence of ACKR3 function.

What experimental techniques are most effective for studying ACKR3-ligand interactions?

Multiple complementary techniques have been developed to study ACKR3-ligand interactions, addressing the unique challenges posed by atypical chemokine receptors that do not signal through G proteins. To effectively characterize ACKR3 function, researchers should consider these methodological approaches:

• Radioligand binding assays: These assays use radiolabeled ligands (such as 125I-CXCL12) to measure direct binding to ACKR3, allowing for determination of binding affinity (IC50 values) and competition studies . This technique provides quantitative data on ligand binding but requires careful optimization due to the high constitutive internalization activity of ACKR3.

• Arrestin recruitment assays: Since ACKR3 exclusively signals through arrestin pathways, arrestin recruitment serves as the primary functional readout. Several approaches have proven effective:

  • Nanoluciferase complementation assays that detect protein-protein interactions between ACKR3 and β-arrestins

  • BRET (Bioluminescence Resonance Energy Transfer)-based assays to monitor arrestin recruitment kinetics and potency

  • These assays allow for determination of EC50 values for different ligands in activating ACKR3

• Receptor trafficking assays: Techniques that monitor receptor internalization and recycling provide valuable insights into ACKR3 function:

  • Flow cytometry to measure surface receptor levels

  • Confocal microscopy with fluorescently-tagged receptors to visualize trafficking

  • ELISA-based approaches to quantify surface receptor expression

• Chemokine scavenging assays: Methods to assess the functional ability of ACKR3 to remove chemokines from the extracellular environment:

  • Measurement of chemokine degradation over time using ELISA

  • Chemokine uptake assays using fluorescently labeled chemokines

  • In vivo assessment of chemokine levels in ACKR3 knockout versus wild-type animals

• Mutational analysis: Systematic mutagenesis of ACKR3 residues identifies key determinants of ligand binding and receptor function. Comprehensive mutational analysis of ACKR3 using 30 substitution mutants has successfully elucidated different binding modes for chemokines .

When studying novel ligands or investigating structure-function relationships, a combination of binding and functional assays is necessary to fully characterize interactions with ACKR3. The choice of techniques should be guided by the specific research question, available resources, and required sensitivity of detection.

How do mutations in ACKR3 affect ligand binding and functional activity?

Mutational analysis has provided critical insights into the structure-function relationships of ACKR3 and its interactions with different ligands. Research using 30 substitution mutants has revealed distinct binding modes for different ACKR3 ligands and identified key residues essential for receptor function .

Key findings from mutational studies include:

• N-terminal residues: Acidic residues in the N-terminus (D2, D7, E10, D16, D25, and D30) play differential roles in ligand binding. Mutations of these residues have revealed their importance particularly for CXCL11 binding, while CXCL12 binding is less dependent on these N-terminal acidic residues .

• Tyrosine residues: Potential sulfation sites such as Y8 and Y45 have been examined through Y8F and Y45F mutations. Tyrosine sulfation is a common post-translational modification in chemokine receptors that often enhances ligand binding affinity .

• Glycosylation sites: Mutations at potential N-glycosylation sites (N13A, N22A, and N39A) and O-glycosylation sites (S23A/S24A) have revealed the role of glycosylation in ACKR3 function and proper folding .

• Extracellular loop (ECL) residues: Charged residues in the extracellular loops are particularly important for ACKR3 function:

  • CXCL11 depends on ECL positions for secondary binding and arrestin recruitment potency

  • CXCL12 binding required specific key residues such as Asp-179

• Conserved cysteines: Mutation of conserved cysteines (C21S/C26S) that form disulfide bonds affects receptor conformation and ligand binding, as shown in binding and functional assays .

These mutational studies demonstrate that CXCL11 and CXCL12 interact with ACKR3 through different binding modes, which has important implications for the development of selective ligands targeting ACKR3. The functional consequences of mutations can be assessed through multiple parameters:

• Binding affinity (IC50 in competition binding assays)

• Activation potency (EC50 for arrestin recruitment)

• Maximum response (efficacy for arrestin recruitment)

• Receptor expression levels

• Internalization and recycling kinetics

For researchers conducting structure-function studies, these findings highlight the importance of considering both binding affinity and functional activity when characterizing ACKR3 mutants, as mutations can differentially affect these parameters and reveal distinct aspects of receptor function.

What is the role of ACKR3 in opioid peptide signaling and how can this be studied?

The discovery that ACKR3 interacts with opioid peptides has revealed a novel regulatory mechanism in opioid signaling pathways. Research has demonstrated that ACKR3 functions as a broad-spectrum scavenger of endogenous opioid peptides, regulating their availability for signaling through classical opioid receptors .

Key aspects of ACKR3's role in opioid peptide signaling include:

• Ligand specificity: ACKR3 is activated by a diverse array of endogenous opioid peptides from different families:

  • Enkephalin family peptides

  • Dynorphin family (dynorphin A, dynorphin A 1-13, big dynorphin)

  • Nociceptin family (nociceptin, nociceptin 1-13-amide)

  • Other peptides like BAM22 and adrenorphin

• Activation mechanism: Opioid peptides induce β-arrestin recruitment to ACKR3, similar to chemokine ligands, but without activating G protein signaling. This was demonstrated through comprehensive screening of 58 opioid peptides for their ability to induce β-arrestin-2 recruitment to ACKR3 .

• Binding affinity and potency: Many opioid peptides activate ACKR3 at concentrations comparable to those required for classical opioid receptor activation, suggesting physiological relevance:

  • Dynorphin A, dynorphin A 1-13, big dynorphin, BAM22, and adrenorphin activate ACKR3 at low concentrations

  • Higher concentrations of dynorphin B, nociceptin, or nociceptin 1-13-amide are needed for ACKR3 activation

• Specificity among chemokine receptors: The ability to bind opioid peptides appears to be unique to ACKR3 among the chemokine receptor family. Testing of all 21 other chemokine receptors showed no significant arrestin recruitment in response to ACKR3-binding opioid peptides .

Methodological approaches to study this function include:

• Arrestin recruitment assays using purified opioid peptides to assess activation potency and efficacy

• Competition binding assays to determine binding affinity of opioid peptides to ACKR3

• Opioid peptide scavenging assays to measure degradation of labeled peptides

• In vivo studies examining endogenous opioid levels in tissues from ACKR3 knockout versus wild-type animals

• Behavioral studies in pain models with ACKR3 modulators to assess functional consequences

This role in opioid signaling has significant therapeutic implications, particularly for pain management. Targeting ACKR3 to block its opioid peptide scavenging function has been proposed as a new approach to develop safer pain medications with fewer side effects, which is critically needed for treating chronic pain .

How can researchers distinguish between ACKR3-mediated and classical chemokine receptor-mediated effects?

Distinguishing between effects mediated by ACKR3 and those mediated by classical chemokine receptors presents significant challenges, particularly because some ligands (such as CXCL12 and CXCL11) bind to both ACKR3 and classical receptors (CXCR4 and CXCR3, respectively). Researchers have developed several methodological approaches to address this challenge:

• Signaling pathway analysis:

  • ACKR3 exclusively recruits arrestins without G protein activation, while classical receptors activate both pathways

  • G protein activation assays (cAMP or calcium flux measurements) alongside arrestin recruitment assays can distinguish receptor contributions

  • Effects that persist in the presence of G protein inhibitors (such as pertussis toxin) but are abolished by arrestin knockdown suggest ACKR3 involvement

• Selective inhibitors and blocking antibodies:

  • Receptor-selective antagonists: AMD3100 for CXCR4, specific antagonists for CXCR3

  • ACKR3-selective small molecules or antibodies can block ACKR3 function specifically

  • Combinations of selective inhibitors can help delineate the contribution of each receptor to observed effects

• Genetic approaches:

  • CRISPR/Cas9-mediated knockout or siRNA-mediated knockdown of individual receptors

  • Receptor-specific knockout animal models

  • Reconstitution experiments in cells lacking specific receptors

  • These approaches allow for assessment of phenotypes in the absence of individual receptors

• Scavenging activity assessment:

  • Monitoring chemokine levels in the presence and absence of ACKR3

  • Chemokine degradation assays specific to ACKR3's scavenging function

  • These approaches focus on ACKR3's unique role in chemokine removal rather than direct signaling

• Context-dependent analysis:

  • Examining effects in tissues or cell types with differential expression of ACKR3 versus classical receptors

  • Temporal analysis, as ACKR3-mediated scavenging may have delayed effects compared to direct signaling

For researchers investigating systems where multiple chemokine receptors are expressed, combining these approaches provides the most robust strategy for distinguishing ACKR3-mediated effects from those mediated by classical chemokine receptors. This distinction is crucial for accurately interpreting experimental results and for developing receptor-selective therapeutic strategies.

What structural determinants govern ACKR3 binding to different ligands?

Understanding the structural basis of ACKR3 interactions with different ligands is essential for rational drug design and for elucidating molecular mechanisms of ligand specificity. Comprehensive mutational analyses have revealed distinct binding modes for different ligands, highlighting the complex structural determinants involved .

Key structural elements that determine ACKR3-ligand interactions include:

• N-terminal domain: The N-terminus of ACKR3 contains multiple acidic residues and potential post-translational modification sites that contribute to ligand binding:

  • Acidic residues (D2, D7, E10, D16, D25, D30) play differential roles in binding different chemokines

  • CXCL11 binding is more dependent on the N-terminal domain compared to CXCL12

  • Opioid peptide binding may involve N-terminal interactions, consistent with the presence of positively charged residues in these peptides

• Extracellular loops (ECLs): Charged residues in the ECLs form critical interaction points with ligands:

  • ECL residues are particularly important for secondary binding interactions with CXCL11

  • Specific residues like Asp-179 are critical for CXCL12 binding

  • The shallow binding pocket formed by the ECLs accommodates different ligands with varying specificities

• Post-translational modifications:

  • Potential tyrosine sulfation sites (Y8, Y45) influence binding affinity

  • N-glycosylation sites (N13, N22, N39) and O-glycosylation sites (S23/S24) modulate receptor conformation and ligand accessibility

• Disulfide bonds: Conserved cysteines that form disulfide bonds (such as C21-C26) maintain the structural integrity of the receptor and proper positioning of binding elements, as demonstrated by mutations affecting these residues .

• Ligand structural features: The ability of ACKR3 to bind both chemokines and opioid peptides suggests common structural motifs or electrostatic characteristics among these ligands:

  • Opioid peptides share sequence homologies including the F/YGGFL/M motif at their N termini and positively charged residues throughout the sequence

  • The presence of positively charged residues in both chemokines and opioid peptides may facilitate interaction with negatively charged receptor domains

These structural insights help explain ACKR3's unusual ability to bind chemokines from different families (CXCL12, CXCL11, vCCL2) and structurally distinct opioid peptides, a promiscuity that appears to be characteristic of atypical chemokine receptors .

For researchers conducting structure-function studies or developing selective ACKR3 modulators, these findings highlight critical regions and residues that determine ligand specificity and receptor activation.

What novel pairings and ligands for ACKR3 have recently been discovered?

Recent research has significantly expanded our understanding of ACKR3's ligand repertoire beyond its initially described chemokine ligands (CXCL12 and CXCL11). These discoveries reveal ACKR3 as a remarkably promiscuous receptor that interacts with diverse molecular classes, with important implications for its physiological functions and therapeutic targeting.

Key novel pairings and ligands include:

• Opioid peptides: A comprehensive screen of 58 opioid peptides revealed that ACKR3 is activated by numerous endogenous opioid peptides from different families:

  • Dynorphin A, dynorphin A 1-13, big dynorphin

  • BAM22, BAM18, Peptide E, adrenorphin

  • Nociceptin, nociceptin 1-13-amide

  • These peptides induce arrestin recruitment to ACKR3 with potencies comparable to their activity at classical opioid receptors

• Viral chemokines: The broad-spectrum antagonist CC chemokine vMIP-II/vCCL2 encoded by the sarcoma-associated herpesvirus (HHV-8) was identified as a ligand for ACKR3. This represents the first CC chemokine ligand for ACKR3 and highlights the receptor's ability to interact with chemokines from different structural families .

• Macrophage migration-inhibitory factor (MIF): This inflammatory cytokine that functions as a chemoattractant was shown to bind ACKR3, promoting receptor internalization and contributing to cell signaling and B-cell chemotaxis. MIF-induced ACKR3 signaling in platelets modulates cell survival and thrombus formation .

• PAMP-12: Recently identified as an ACKR3 ligand, adding to the growing list of non-chemokine peptides that interact with this receptor .

These findings establish ACKR3 as a unique intersection point between chemokine signaling, opioid systems, and inflammatory responses. The cross-family selectivity observed with ACKR3 (binding both CC and CXC chemokines) appears to be a common characteristic of atypical chemokine receptors that is not observed among classical chemokine receptors .

For researchers, these discoveries open new avenues for investigating ACKR3 function in diverse physiological contexts and for developing targeted therapeutics. They also necessitate a broader experimental approach when studying ACKR3, considering its potential interactions with multiple signaling systems beyond the classical chemokine network.

How can ACKR3 be targeted therapeutically in cancer and chronic pain research?

ACKR3 has emerged as a promising therapeutic target for both cancer and chronic pain management, with distinct targeting strategies being developed for each condition. The therapeutic potential is based on ACKR3's unique dual role in chemokine scavenging and opioid peptide regulation .

For cancer therapy:

• Targeting ACKR3 in tumor microenvironment:

  • ACKR3 is frequently upregulated in various cancers, particularly in tumor vasculature

  • ACKR3 scavenges chemokines like CXCL9 and CXCL10 that are important for recruiting cytotoxic T cells and NK cells into tumors

  • Inhibiting ACKR3 could increase local chemokine levels, potentially converting "cold" tumors to "hot" tumors more responsive to immunotherapy

• Therapeutic approaches being developed:

  • Small molecule antagonists that block ACKR3 scavenging function

  • Monoclonal antibodies targeting ACKR3

  • Modified chemokines that bind ACKR3 without being degraded

  • Combined approaches targeting both ACKR3 and CXCR4 to fully modulate the CXCL12 axis

• Methodological considerations for researchers:

  • Assessment of ACKR3 expression in tumor samples

  • Evaluation of chemokine levels in tumor microenvironment before and after ACKR3 targeting

  • Monitoring immune cell infiltration as a readout of therapeutic efficacy

  • Combination with established immunotherapies such as checkpoint inhibitors

For chronic pain management:

• Targeting ACKR3's opioid peptide scavenging function:

  • ACKR3 regulates the availability of endogenous opioid peptides by scavenging them

  • Blocking this function could increase local concentrations of endogenous opioids

  • This approach might enhance endogenous pain control mechanisms without the side effects associated with exogenous opioids

• Therapeutic strategies under investigation:

  • Development of selective ACKR3 antagonists that specifically block opioid peptide binding

  • Small molecules that modulate ACKR3 trafficking to reduce its scavenging efficiency

  • Biased ligands that preferentially affect specific aspects of ACKR3 function

• Research methodologies:

  • Pain behavior assessment in animal models with genetic or pharmacological modulation of ACKR3

  • Measurement of local opioid peptide levels in pain-relevant tissues

  • Evaluation of analgesic efficacy without classical opioid side effects

  • Investigation of potential synergistic effects with low-dose classical opioids

Common challenges for therapeutic development include achieving selectivity for ACKR3 over related receptors, developing tissue-specific targeting strategies, designing functionally selective ligands, and identifying reliable biomarkers to monitor therapeutic efficacy in clinical settings.

For researchers in both cancer and pain fields, understanding ACKR3's complex biology and developing appropriate assays to evaluate potential therapeutic candidates remains a critical challenge and opportunity for innovative drug development .

What are the prospects for ACKR5 (GPR182) and other potential new members of the atypical chemokine receptor family?

The atypical chemokine receptor family may be expanding beyond the currently established members (ACKR1-4), with several candidates under investigation. Most notably, GPR182 has been recently proposed as ACKR5, with evidence supporting its classification as an atypical chemokine receptor.

The emerging role of GPR182 as ACKR5:

• Ligand profile: GPR182 has been identified as a broad-spectrum atypical chemokine receptor with scavenging activity towards multiple chemokines:

  • CXCL9, CXCL10, CXCL12, and CXCL13 have been identified as ligands

  • This represents the only scavenger receptors identified so far for chemokines like CXCL9 and CXCL13

• Functional characteristics:

  • Like other ACKRs, GPR182 shows no detectable ligand-induced G protein signaling

  • It exhibits high levels of basal cycling activity and β-arrestin interactions

  • It functions as a chemokine scavenger, consistent with the defining characteristic of ACKRs

• Current status: While evidence supports its classification as an ACKR, official inclusion in the ACKR family by the IUPHAR remains pending. Further validation of ligand specificity is needed, as this aspect shows some inconsistency between studies .

Other potential ACKR family members under investigation:

• CXCR3B: The extended isoform of CXCR3 has recently been proposed to display attributes of ACKRs, suggesting functional diversity within established receptor subtypes .

• CCRL2 and PITPNM3: These receptors await validation with regard to chemokine binding and direct regulatory functions. Additional studies are needed to determine whether they share common functional properties with established atypical chemokine receptors .

Methodological challenges in identifying new ACKRs:

• In the absence of G protein signaling, traditional receptor activation assays are ineffective

• High basal activity of some candidates complicates functional characterization

• Confirmation of scavenging activity requires multiple complementary approaches

Recent technological and scientific advances have facilitated the identification of new ACKR candidates and ligand pairings:

• Development of sensitive arrestin recruitment assays

• Improved understanding of receptor trafficking and localization

• Availability of recombinant chemokines for screening

• Advanced genetic tools for receptor knockout and knockdown studies

For researchers interested in this evolving field, a combination of experimental approaches is recommended to characterize potential new ACKRs, including receptor sequence comparison, expression profiling, binding competition studies, and in vivo validation using knockout models to examine changes in chemokine plasma concentrations .