Recombinant Mouse G-protein coupled receptor 15 (Gpr15)

What is GPR15 and what are its basic structural characteristics?

GPR15, also known as Bob, is a seven-transmembrane G protein-coupled receptor primarily expressed in CD4+ T cells and alveolar macrophages. It possesses a structure resembling CC chemokine receptors but with unique structural peculiarities. Mouse GPR15 shares approximately 76% amino acid sequence identity with human GPR15, while human GPR15 shares 96-100% amino acid sequence identity with chimpanzee, macaque, and rhesus GPR15, indicating its evolutionary conservation .

What cellular populations express GPR15?

GPR15 is predominantly expressed in specific immune cell populations. Flow cytometry analysis has revealed that approximately 2-3% of living cells in lymph nodes and spleen express GPR15 under naïve conditions. During inflammatory responses (such as in EBA models), GPR15+ cells in lymph nodes increase, with a significant proportion expressing CD3. Further characterization shows GPR15 is mostly expressed on CD8+ T cells rather than CD4+ T cells. Importantly, there is no co-expression between GPR15 and the B cell marker CD19 or neutrophil markers Ly-6G/CD11b .

What is known about the GPR15 ligand (GPR15L)?

GPR15L is the cognate ligand of GPR15. Two significant peptide fragments have been extensively studied: GPR15L(25-81) and GPR15L(71-81). Research indicates that GPR15L is markedly upregulated in inflamed tissues, such as in antibody-mediated skin inflammation models. The GPR15-GPR15L axis plays crucial roles in immune cell recruitment and potentially exerts inhibitory or chemo-repulsive effects on the migration of certain cell populations, including γδ T cells .

What mouse models are available for studying GPR15 function?

Several genetically modified mouse models have been developed for studying GPR15:

• B6;129P2-Gpr15tm1.1Litt/J mouse strain: In this model, the Gpr15 gene is replaced by a sequence encoding green fluorescent protein (GFP), allowing visualization of GPR15-expressing cells through GFP detection. Homozygous mice (Gpr15-/- or Gpr15Gfp/Gfp) are GPR15-deficient .

• AOM-DSS CAC model: This chemically-induced colon cancer model is used to study the role of GPR15 in colorectal cancer development. Comparing Gpr15-Het (heterozygous) and Gpr15-KO (knockout) mice in this model reveals the tumor-suppressive functions of GPR15 .

• MC38-CRC model: This transplantable colon cancer model allows investigation of GPR15's role in tumor growth and potential therapeutic applications of GPR15L .

What experimental techniques are used to study GPR15 signaling?

Several sophisticated techniques have been established to study GPR15 signaling pathways:

• BRET-based assays: Bioluminescence resonance energy transfer assays using Venus(155-239)-Gβ1, Venus(1-155)-Gγ2 and masGRK3ct-Nluc components to measure G protein coupling and activation. This technique enables real-time monitoring of GPR15 signaling with high sensitivity .

• HTRF-based cAMP and IP1 assays: Homogeneous time-resolved FRET assays to evaluate downstream signaling pathways (Gi/o, Gs and Gq/11) activated by GPR15 .

• Flow cytometry analysis: To identify and quantify GPR15-expressing cell populations in various tissues, particularly in lymphoid organs .

• RT-PCR: For measuring GPR15 and GPR15L mRNA expression levels in different tissues under normal and pathological conditions .

How can GPR15 activity be experimentally modulated?

GPR15 activity can be modulated through several experimental approaches:

• Genetic manipulation: Using knockout mice (Gpr15-/-) to study the consequences of GPR15 deficiency .

• Ligand administration: Intratumoral injection of GPR15L peptides (GPR15L(25-81) or GPR15L(71-81)) to examine therapeutic effects. In Gpr15-Het mice with established tumors, GPR15L administration results in significant tumor shrinkage, whereas no effect is observed in Gpr15-KO mice, confirming the specificity of GPR15-GPR15L interaction .

• Cell transfection models: HEK293A cells can be transfected with GPR15 receptor DNA, along with other components for signaling studies, providing a controlled system to examine receptor pharmacology .

What is the role of GPR15 in colorectal cancer development?

GPR15 demonstrates a novel tumor-suppressive function in colorectal cancer:

• Expression pattern: GPR15 expression and GPR15+ T cells are significantly reduced in human colon cancer tumors compared to surgical tumor margins (STM) .

• Functional consequences of deficiency: In AOM-DSS mouse models of colitis-associated cancer (CAC), GPR15 deficiency results in:

  • Increased tumor burden (more and larger tumors)

  • Higher morbidity and mortality

  • Reduced T cell infiltration (especially CD8+ T cells) in the tumor microenvironment

  • Increased populations of pro-inflammatory immune cells (IL-17+CD4+ and IL-17+CD8+ T cells)

• Immune environment alteration: GPR15 appears to regulate the recruitment of T cells with tumor-suppressive function to the colon, creating an unfavorable environment for tumor growth .

What is the therapeutic potential of targeting the GPR15-GPR15L axis in cancer?

Research provides proof-of-concept for therapeutic targeting of the GPR15-GPR15L axis in colorectal cancer:

• Tumor reduction: Intratumoral administration of GPR15L in established MC38-CRC mouse models resulted in dramatic shrinkage in tumor size in Gpr15-Het mice but not in Gpr15-KO mice, confirming the specificity of the effect .

• Immune environment modulation: GPR15L treatment leads to:

  • Significant expansion of CD45+ T cells, CD4+ and CD8+ T cells

  • Increased IFNγ+ and TNFα+ T cell subsets

  • Increased frequency of TNFα+ NK cells, IL-12+IFNγ+, and TNFα+ NKT cells

  • Enhanced TNFα+ DCs and macrophages

• Mechanism: GPR15-GPR15L axis appears to impair tumor growth by facilitating the infiltration of cytolytic T cells into the tumor microenvironment .

How does GPR15 influence the tumor immune microenvironment?

GPR15 significantly shapes the tumor immune microenvironment through several mechanisms:

These alterations collectively create a tumor-favoring immune signature in Gpr15-KO mice, highlighting the importance of GPR15-mediated immune mechanisms in preventing colon cancer development .

Which G protein pathways are activated by GPR15?

GPR15 signaling has been characterized across multiple G protein pathways:

• G protein coupling: GPR15 has been evaluated for coupling to various G protein families including Gi/o, Gs, Gq/11, and G13 .

• Experimental approach: BRET-based assays allow real-time measurement of G protein coupling using a system with Venus(155-239)-Gβ1, Venus(1-155)-Gγ2, and masGRK3ct-Nluc components .

• Downstream signaling: HTRF-based cAMP and IP1 assays have been used to assess the activation of Gi/o (inhibition of cAMP), Gs (stimulation of cAMP), and Gq/11 (IP1 production) pathways following GPR15 stimulation with its ligand .

The specific G protein coupling profile of GPR15 provides insights into its functional consequences in different cellular contexts.

How do different GPR15L peptide fragments affect signaling?

Different peptide fragments of GPR15L demonstrate varying potencies and potentially different signaling properties:

• GPR15L(25-81): This longer peptide fragment has been extensively studied and shows robust activation of GPR15 signaling cascades .

• GPR15L(71-81): This shorter C-terminal fragment also activates GPR15 but may exhibit different potency and efficacy compared to the longer fragment .

• Kinetic differences: Comparative kinetic analyses have been performed between these peptides using statistics (Student's t-test) to determine significant differences in their signaling properties .

Understanding these differences is crucial for developing therapeutic approaches targeting the GPR15-GPR15L axis.

What is the role of GPR15 in antibody-mediated inflammation?

GPR15 serves as a counter-regulator of neutrophilic, antibody-mediated cutaneous inflammation:

• Protective effect: Gpr15-/- mice subjected to the antibody transfer BP-like EBA model exhibited markedly aggravated disease compared to wild-type littermate controls, indicating a protective role of GPR15 .

• Ligand induction: This aggravation of skin inflammation was associated with pronounced induction of GPR15L and increased accumulation of γδ T cells in the dermis .

• Mechanism hypothesis: GPR15L/GPR15 may exert inhibitory or chemo-repulsive effects on the migration of certain cell populations, including γδ T cells, thus counteracting their recruitment into the dermis during inflammation .

How does GPR15 expression change during inflammatory responses?

GPR15 expression demonstrates tissue-specific and condition-dependent regulation:

• Lymphoid tissue expression: Under naïve conditions, approximately 2-3% of living cells in lymph nodes and spleen express GPR15. This percentage increases slightly in the lymph nodes during EBA (antibody-mediated inflammation) .

• Cell type shifts: While under naïve conditions GPR15+ cells are mainly CD3-CD19-, during EBA a significant proportion of GPR15+ cells express CD3, predominantly on CD8+ cells .

• Skin expression: Paradoxically, GPR15 mRNA levels are reduced in inflamed skin despite upregulation of its ligand, GPR15L. This may reflect a regulatory mechanism where GPR15L/GPR15 exerts inhibitory effects on the migration of GPR15+ cells to the skin .

What are the unresolved questions about GPR15 biology?

Several critical aspects of GPR15 biology remain to be fully elucidated:

What are promising therapeutic applications of GPR15 research?

Based on current evidence, several therapeutic applications show promise:

• Cancer immunotherapy: Development of GPR15L-based therapies to enhance anti-tumor immune responses in colorectal cancer, potentially in combination with existing immunotherapies .

• Inflammatory disorders: Targeting the GPR15-GPR15L axis to modulate inflammatory responses in conditions like antibody-mediated skin inflammation .

• HIV infection: Given GPR15's role as a co-receptor for some isolates of HIV-1, HIV-2, and SIV, therapeutic approaches targeting this interaction could potentially inhibit viral entry .

What methodological advances would enhance GPR15 research?

Several methodological advances could significantly advance GPR15 research:

• Cell-specific knockout models: Development of conditional knockout mice with cell-type-specific deletion of GPR15 to dissect its role in different immune populations.

• Advanced imaging techniques: Implementation of intravital microscopy to visualize GPR15+ cell trafficking in real-time during inflammation or tumor development.

• Single-cell analysis: Application of single-cell RNA sequencing and proteomics to comprehensively characterize GPR15+ cells and their functional states across different tissues and disease contexts.

• Humanized models: Development of humanized mouse models expressing human GPR15 to better translate findings to human disease.