Recombinant Mouse C-C chemokine receptor 1-like protein 1 (Ccr1l1)

What is Ccr1l1 and how does it relate to other chemokine receptors?

Ccr1l1 (C-C chemokine receptor 1-like protein 1) is a Rodentia-specific G protein-coupled receptor that shows high homology to Ccr1 and Ccr3, particularly Ccr1, from which it appears to have evolved directly. Phylogenetic analyses place Ccr1l1 in the CC chemokine receptor subfamily, sharing a common ancestor with Ccr1 and Ccr3 .

Structurally, Ccr1l1 preserves key features of chemokine receptors, including the canonical DRYLAIV sequence of the DRY motif (with most rodent Ccr1l1 orthologs encoding a DRYLAVV sequence), which is critical for G-protein coupling . This sequence is also found in human CCR8 and CCR5, suggesting potential functional similarities despite the absence of a direct human ortholog.

Unlike broadly conserved chemokine receptors, Ccr1l1 is restricted to rodent species, with orthologs identified in at least ten rodent species showing amino acid identities >78% . This restricted distribution suggests a specialized function that may have evolved to address rodent-specific immune challenges.

What is known about Ccr1l1 expression patterns in rodents?

Ccr1l1 exhibits a highly selective expression pattern, predominantly in eosinophils . This cell-specific expression suggests a potential role in eosinophil-mediated immune responses, though knockout studies have revealed normal eosinophil phenotypes, development, and chemokine responsiveness in naïve Ccr1l1 knockout mice under standard laboratory conditions .

The restricted expression in eosinophils contrasts with Ccr1, which is expressed on multiple immune cell types including monocytes, macrophages, neutrophils, and certain lymphocyte subsets . This differential expression pattern may indicate a specialized function in allergic responses or parasite immunity, areas where eosinophils play crucial roles.

For researchers investigating Ccr1l1 expression, it is advisable to employ complementary approaches:

• Single-cell RNA sequencing to identify potentially rare cell populations expressing this receptor

• Flow cytometry with specific antibodies to confirm protein expression at the cellular level

• Tissue-specific expression analysis under various inflammatory conditions to identify contexts where expression might be upregulated

What structural features define Ccr1l1 as a functional GPCR?

Ccr1l1 maintains the core structural architecture characteristic of the chemokine receptor subfamily of GPCRs. Experimental evidence confirms that Ccr1l1 can be expressed on the plasma membrane with the correct topological orientation (extracellular N-terminus and intracellular C-terminus) , which is essential for GPCR function.

Key structural features preserved in Ccr1l1 include:

• Seven-transmembrane domain architecture, confirmed by structural modeling

• The highly conserved DRY motif (DRYLAIV/DRYLAVV sequence) at the intracellular end of transmembrane helix III

• Two conserved extracellular disulfide bridges (TMVII-N-terminus and TMIII-ECL2)

• The β-hairpin fold typically observed in the second extracellular loop (ECL2) of chemokine receptors

These structural elements collectively suggest that Ccr1l1 maintains the capacity to function as a genuine chemokine receptor, potentially coupling to G proteins to initiate downstream signaling cascades, despite the current lack of identified natural ligands.

What methodologies have been employed to investigate Ccr1l1 function?

Researchers have utilized multiple complementary approaches to investigate Ccr1l1 function, though definitive functional characterization remains elusive. Table 1 summarizes key methodologies and their findings:

Table 1: Methodologies for Studying Ccr1l1 Function

Despite systematic screening of 37 available mouse chemokines and two viral chemokines, no natural ligand has been identified for Ccr1l1 . This suggests either that its ligand was not among those tested or that it may have unique activation requirements or context-dependent function.

For researchers seeking to further investigate Ccr1l1 function, complementing these established methodologies with newer approaches is recommended:

• CRISPR-Cas9 genome editing for more precise genetic manipulation

• Phosphoproteomics to identify potential downstream signaling targets

• In vivo disease models challenging Ccr1l1-knockout mice with various immunological stimuli

• Single-cell analysis to identify contexts where Ccr1l1 might be functionally important

How do Ccr1l1 knockout models differ from wild-type in immune response assays?

Studies with Ccr1l1 knockout mice have yielded surprising results. Under basal conditions in naïve mice, knockout of Ccr1l1 did not result in observable phenotypic changes :

• Eosinophil phenotypes appeared normal

• Eosinophil development proceeded without apparent defects

• Responsiveness to chemokines remained intact

This lack of obvious phenotype under standard laboratory conditions suggests several possibilities:

• Functional redundancy with other chemokine receptors compensating for Ccr1l1 deficiency

• Context-specific functions that become apparent only under particular immune challenges

• Subtle phenotypes requiring more sensitive detection methods

To uncover potential functions of Ccr1l1, researchers should consider challenging knockout models with various immune stimuli specifically relevant to eosinophil biology:

• Allergic inflammation models (e.g., ovalbumin-induced airway inflammation)

• Helminth infection models where eosinophils play key roles

• Diverse pathogen challenges including bacterial, viral, or fungal infections

• Autoimmune disease models to examine potential roles in pathological immune responses

A comprehensive immunophenotyping approach combining flow cytometry, cytokine/chemokine profiling, histopathology, and in vivo imaging would provide the most complete assessment of any subtle differences between wild-type and Ccr1l1 knockout mice.

What signaling pathways might be activated downstream of Ccr1l1?

While definitive signaling pathways for Ccr1l1 have not been experimentally confirmed due to the lack of identified natural ligands, its structural similarities to other chemokine receptors, particularly Ccr1, allow for informed predictions:

Table 2: Predicted Signaling Pathways for Ccr1l1

The presence of the conserved DRY motif strongly suggests Ccr1l1 can couple to G proteins . Based on related chemokine receptors, Ccr1l1 likely couples primarily to Gαi/o proteins, potentially leading to inhibition of adenylyl cyclase and activation of downstream pathways through released Gβγ subunits.

To experimentally validate these potential signaling pathways, researchers should employ:

• Phosphoproteomic analysis of cells expressing Ccr1l1 following stimulation

• Western blotting for phosphorylated signaling intermediates

• BRET/FRET-based biosensors to detect protein-protein interactions

• Genetic approaches (siRNA, CRISPR) to confirm signaling component requirements

How can researchers determine potential natural ligands for Ccr1l1?

Identifying natural ligands for orphan receptors like Ccr1l1 requires a systematic, multi-faceted approach:

Table 3: Strategies for Identifying Ccr1l1 Ligands

Given that Ccr1l1 is selectively expressed in eosinophils , researchers should particularly focus on contexts where eosinophils are activated, such as allergic inflammation or parasite infections, as these may provide clues to natural ligands.

MIP-1gamma (CCL9/10), which is known to signal through CCR1 , represents an interesting starting point despite initial negative results, as structural variants or context-dependent interactions might still occur with Ccr1l1.

What are the differences between Ccr1 and Ccr1l1 in terms of ligand binding specificity?

Despite high sequence similarity, Ccr1 and Ccr1l1 appear to have distinct ligand binding profiles:

Table 4: Comparison of Ccr1 and Ccr1l1 Ligand Binding Properties

The surprising finding that Ccr1l1 does not respond to any of the tested chemokines, including Ccr1 ligands , suggests that despite evolutionary relatedness, Ccr1l1 has evolved distinct ligand recognition properties. These differences likely result from key amino acid variations in:

• The N-terminal domain, critical for initial chemokine recognition

• The extracellular loops, particularly ECL2, which form part of the binding pocket

• The transmembrane helices that shape the ligand binding cavity

To elucidate the structural basis for these differences, researchers should consider:

• Creating chimeric receptors swapping domains between Ccr1 and Ccr1l1

• Site-directed mutagenesis targeting specific amino acid differences

• Comparative structural modeling and molecular dynamics simulations

• Direct binding studies with purified receptor components