Recombinant Mouse Adiponectin receptor protein 2 (Adipor2)

What is the basic structure of AdipoR2 and how does it differ from typical membrane receptors?

AdipoR2 is a 386-amino acid protein containing seven transmembrane domains. Unlike typical G protein-coupled receptors (GPCRs), AdipoR2 has an inverted membrane topology with the N-terminus facing the cytoplasm and the C-terminus oriented extracellularly . This unique orientation is functionally significant, as it affects how AdipoR2 interacts with intracellular signaling molecules rather than G proteins. The protein has a molecular weight of approximately 44 kDa and functions as a receptor for full-length adiponectin, mediating its metabolic effects primarily in the liver.

What are the key differences between AdipoR1 and AdipoR2?

Despite structural similarities, AdipoR1 and AdipoR2 have distinct tissue distribution patterns and downstream signaling pathways:

These differences highlight the complementary roles these receptors play in mediating adiponectin's metabolic effects across different tissues .

What expression systems are most effective for producing functional recombinant mouse AdipoR2?

Multiple expression systems can be used to produce recombinant mouse AdipoR2, each with distinct advantages:

• E. coli expression: Provides high yield but often results in inclusion bodies requiring refolding. This system has been successfully used to produce full-length mouse AdipoR2 (1-386aa) with N-terminal His tags .

• Mammalian cell expression (HEK-293): Offers proper protein folding and post-translational modifications, resulting in higher-quality protein with improved functionality. HEK-293 cells are particularly valuable for expressing full-length AdipoR2 protein with native conformation .

• Insect cell expression: Provides a balance between yield and proper folding for membrane proteins.

For functional studies requiring properly folded protein, mammalian expression systems are generally preferred despite lower yields, as they maintain the native structure necessary for ligand binding studies .

What methods can be used to verify recombinant AdipoR2 functionality?

Several biophysical and biochemical approaches can verify the functionality of recombinant AdipoR2:

• Ligand binding assays:

  • Surface Plasmon Resonance (SPR): SPR analysis of ADIPOR2 interaction with compounds like SCM-198 can determine binding affinity (Kd ≈ 1.99 μM) .

  • Microscale Thermophoresis (MST): Provides binding affinity measurements in solution (Kd ≈ 2.308 μM for AdipoR2-SCM-198 interaction) .

• Thermal stability assessment:

  • Differential Scanning Fluorimetry (DSF): Measures thermal transition midpoints (Tm) of recombinant AdipoR2 in the presence/absence of ligands. Functional AdipoR2 shows a shift in Tm from 62.8°C to 68.6°C when bound to ligands like SCM-198 .

• Pull-down assays:

  • Biotinylated ligands can be used to affinity-purify functional AdipoR2 from either recombinant preparations or tissue extracts .

• Downstream signaling activation:

  • Measuring PPAR-α activation or fatty acid oxidation in cells expressing recombinant AdipoR2 after stimulation with adiponectin.

These complementary approaches ensure that recombinant AdipoR2 maintains not only its structural integrity but also its functional capacity to bind ligands and initiate downstream signaling.

How does AdipoR2 deficiency impact metabolic phenotypes in mouse models?

AdipoR2 deficiency in mouse models reveals complex metabolic phenotypes with important implications for metabolic disease research:

• Improved insulin sensitivity initially: AdipoR2 knockout mice show reduced high-fat diet-induced insulin resistance, suggesting a protective effect against certain aspects of metabolic syndrome .

• Reduced dyslipidemia: AdipoR2 deletion diminishes high-fat diet-induced dyslipidemia .

• Paradoxical glucose homeostasis deterioration: Despite initial improvements in insulin sensitivity, prolonged high-fat feeding in AdipoR2-deficient mice leads to deterioration of glucose homeostasis due to failure of pancreatic β-cells to adequately compensate for the moderate insulin resistance .

• Altered membrane phospholipid composition: AdipoR2 knockdown increases the rigidifying effect of palmitic acid on cell membranes and causes an excess of saturated fatty acids among phosphatidylcholines (PCs) and phosphatidylethanolamines (PEs) .

These findings suggest that AdipoR2 defects may represent a mechanism underlying the etiology of certain subgroups of type 2 diabetic patients who eventually develop overt diabetes despite initial metabolic advantages .

What molecular mechanisms explain AdipoR2's role in membrane lipid homeostasis?

AdipoR2 plays a critical role in membrane lipid homeostasis through several mechanisms:

• Prevention of palmitic acid-induced membrane rigidification: FRAP (Fluorescence Recovery After Photobleaching) analysis demonstrates that AdipoR2 knockdown greatly increases the rigidifying effect of palmitic acid (PA) on cell membranes. When AdipoR2 is knocked down in HEK293 cells treated with PA, membrane fluidity is significantly reduced .

• Regulation of membrane phospholipid composition: Lipidomics analysis reveals that AdipoR2 knockdown causes:

  • Dramatic increase in saturated fatty acids (SFAs) among phosphatidylcholines (PCs)

  • Higher levels of SFAs in phosphatidylethanolamines (PEs)

  • Increased saturated fatty acid content in triacylglycerols (TAGs)

• Morphological cellular changes: HEK293 cells with AdipoR2 knockdown display altered morphology when exposed to palmitic acid, including the appearance of numerous circular structures in BODIPY-labeled cells .

These findings suggest that AdipoR2 functions as a protective factor against palmitic acid-induced membrane rigidification, which may contribute to its role in maintaining insulin sensitivity and proper cellular function.

How does adiponectin regulate its own receptor expression?

Adiponectin regulates its own receptor expression through a sophisticated feedback mechanism:

• Negative feedback on AdipoR2: Transgenic mice with moderate expression of exogenous adiponectin targeted to adipose tissue show a reduction in AdipoR2 mRNA levels and protein content in fat depots, along with decreased circulating adiponectin in adult mice .

• Cell culture confirmation: Recombinant adiponectin added to 3T3-F442A adipocytes causes a decrease in AdipoR2 mRNA levels but does not affect AdipoR1 expression .

• Inverse regulation: In contrast, AdipoR2 (but not AdipoR1) is specifically upregulated in fat tissue of adiponectin knockout mice (ApN-/-) .

This regulatory feedback loop demonstrates how adiponectin downregulates its own production and the expression of AdipoR2, potentially as a homeostatic mechanism to prevent excessive signaling. The feedback affects multiple metabolic regulators, as reduced AdipoR2 expression in fat tissue is associated with diminished expression of uncoupling protein 2 (involved in energy dissipation) and higher expression of fatty acid synthase and TNFα .

What specific binding sites on AdipoR2 are critical for ligand interactions?

Recent research has identified specific binding sites on AdipoR2 that mediate its interaction with ligands:

• R335 residue: Molecular dynamics simulations and experimental studies have identified R335 as a critical binding residue in AdipoR2. This residue forms guanidinium pairing with compounds like SCM-198, contributing significantly to binding affinity .

• R335A mutation effects: SPR binding studies show that the R335A mutation in AdipoR2 significantly reduces binding affinity for ligands like SCM-198, confirming the importance of this residue for ligand interactions .

• Transmembrane pocket: The seven transmembrane domains of AdipoR2 form a barrel-like structure that creates a hydrophobic pocket, which accommodates ligands and contributes to AdipoR2's function in lipid metabolism.

These structural insights are valuable for developing selective AdipoR2 modulators with therapeutic potential and understanding how different ligands might activate distinct signaling pathways downstream of the receptor.

How can recombinant AdipoR2 be used to screen for novel therapeutic compounds?

Recombinant AdipoR2 protein provides an invaluable tool for screening and developing novel therapeutic compounds:

• Binding assays:

  • Surface Plasmon Resonance (SPR) allows real-time monitoring of binding kinetics between AdipoR2 and potential ligands

  • Microscale Thermophoresis (MST) can detect binding interactions in solution

  • Differential Scanning Fluorimetry (DSF) identifies compounds that stabilize AdipoR2 structure

• Structure-activity relationship studies:

  • Comparing binding affinities of structural analogs (e.g., SCM-198 vs. Sinapine) helps identify critical molecular features for AdipoR2 interaction

  • Mutational analysis of key residues (e.g., R335A) confirms binding determinants

• Therapeutic applications:

  • Identification of compounds like SCM-198 that selectively target the AdipoR2-CaM-CaMKII-NOS3 signaling axis has led to extended treatment windows for acute liver failure in mice (from 3 to 24 hours)

  • Development of structural analogs with improved pharmacokinetic properties

• Compound validation pipeline:

  • Initial screening using purified recombinant AdipoR2

  • Secondary validation in cellular models expressing AdipoR2

  • Tertiary testing in animal models of metabolic disease

This pipeline has successfully identified compounds that modulate AdipoR2 signaling with therapeutic potential for metabolic disorders, liver diseases, and other conditions where adiponectin signaling plays a role.

How do polymorphisms in ADIPOR2 relate to adiponectin levels and obesity phenotypes?

Genetic variations in the ADIPOR2 gene have significant associations with adiponectin levels and obesity-related phenotypes:

• Race-specific associations: Studies examining single nucleotide polymorphisms (SNPs) in ADIPOR2 in relation to serum adiponectin levels and body mass index (BMI) have found different associations between black and white participants, highlighting the importance of including diverse populations in genetic studies .

• Methodology for association studies:

  • Adiponectin measurement in serum by immunoassay using LINCOplex kit (Luminex® xMAP™ Technology)

  • BMI calculation from height and weight measurements

  • Genotyping of SNPs in ADIPOR2

  • Race-stratified linear regression models adjusted for age and percentage African ancestry

• Clinical relevance: Polymorphisms in ADIPOR2 may explain part of the variability in adiponectin levels and obesity prevalence between different ethnic groups, providing insights into the genetic basis of metabolic disorders.

Understanding these genetic associations could help identify individuals at higher risk of metabolic diseases and potentially guide personalized prevention or treatment strategies.

What is the prognostic value of ADIPOR2 expression in cancer research?

ADIPOR2 expression has emerging value as a prognostic marker in cancer research:

• Pan-cancer analysis approaches:

  • Univariate Cox analysis using data from TCGA, GETx, CCLE, and PCAWG databases

  • Survival Map modules and Kaplan-Meier plots to evaluate the relationship between ADIPOR2 expression and patient outcomes

• Distinct roles from ADIPOR1:

  • Despite their functional similarities in metabolic regulation, ADIPOR1 and ADIPOR2 show distinct prognostic patterns across different cancer types

  • These differences suggest tissue-specific functions that may be relevant to cancer progression

• Potential mechanisms:

  • ADIPOR2's role in regulating fatty acid oxidation and membrane lipid composition may affect cancer cell metabolism

  • Adiponectin signaling through ADIPOR2 may influence inflammatory pathways relevant to tumor microenvironment

This research direction highlights the expanding significance of adiponectin receptors beyond metabolic disorders into cancer biology, suggesting potential for ADIPOR2-targeted therapies or its use as a biomarker in specific cancer contexts.

What are the latest antibody-based methods for detecting and quantifying AdipoR2?

Several advanced antibody-based methods are available for detecting and quantifying AdipoR2 in research settings:

• Western blot analysis:

  • Rabbit polyclonal antibodies against AdipoR2 can detect the protein in various tissues including human placenta, rat liver, and cell lines such as HeLa and A549

  • The observed molecular weight is typically 40-45 kDa

  • Recommended antibody dilutions range from 1:500 to 1:2000

• Immunohistochemistry (IHC):

  • Paraffin-embedded tissues can be stained for AdipoR2 using specific antibodies

  • AdipoR2 expression has been detected in multiple rat tissues including liver, testis, skeletal muscle, heart, kidney, and stomach

  • Typical working concentrations for IHC are around 20 μg/ml followed by DAB staining

• Immunoprecipitation:

  • AdipoR2 can be immunoprecipitated from tissue lysates using 0.5-4.0 μg of antibody for 1.0-3.0 mg of total protein

  • This technique is valuable for studying protein-protein interactions involving AdipoR2

• Immunofluorescence:

  • Allows visualization of AdipoR2 cellular localization

  • Particularly useful for studying membrane distribution and internalization dynamics

These methods provide complementary approaches for studying AdipoR2 expression, localization, and interactions in both basic research and disease-focused investigations.

How can advanced lipidomics approaches enhance our understanding of AdipoR2 function?

Advanced lipidomics approaches have revealed critical insights into AdipoR2 function:

• Comprehensive lipid profiling:

  • Lipidomics analysis of cells with AdipoR2 knockdown reveals dramatic increases in saturated fatty acids among phosphatidylcholines (PCs), phosphatidylethanolamines (PEs), and triacylglycerols (TAGs)

  • These changes are particularly pronounced when cells are exposed to palmitic acid

• Membrane fluidity assessment:

  • FRAP (Fluorescence Recovery After Photobleaching) analysis complements lipidomics data by demonstrating functional consequences of altered lipid composition

  • AdipoR2 knockdown greatly increases the rigidifying effect of palmitic acid on cell membranes

• Methodological approaches:

  • Cells are typically cultivated in serum-free media containing different fatty acids (PA, OA, or PA+OA)

  • Lipid extraction followed by mass spectrometry analysis

  • Statistical comparison between siRNA-treated cells and non-target siRNA controls under the same cultivation conditions

• Functional correlations:

  • Changes in membrane phospholipid composition can be correlated with alterations in insulin signaling

  • Lipidomics data help explain mechanistically how AdipoR2 contributes to insulin sensitivity

These advanced lipidomics approaches provide molecular-level insights into how AdipoR2 regulates membrane lipid composition and cellular responses to fatty acids, contributing significantly to our understanding of its role in metabolic health and disease.

What are promising strategies for developing selective AdipoR2 modulators?

Several promising strategies are emerging for developing selective AdipoR2 modulators:

• Structure-guided drug design:

  • Targeting specific binding residues like R335 that form guanidinium pairing with ligands

  • Exploiting structural differences between AdipoR1 and AdipoR2 to achieve selectivity

• Identification of natural compounds:

  • SCM-198, derived from a traditional Chinese medicine herb, selectively targets the AdipoR2-CaM-CaMKII-NOS3 axis

  • Comparing structural analogs (e.g., SCM-198 vs. Sinapine) to identify critical molecular features

• Peptide-based approaches:

  • Development of peptide fragments derived from adiponectin that selectively activate AdipoR2

  • Engineering peptides to target specific binding domains

• Biophysical screening methods:

  • Using SPR, MST, and DSF to screen compound libraries for selective AdipoR2 binders

  • SPR and MST have successfully identified compounds with Kd values in the low micromolar range (1.99-2.308 μM)

• Therapeutic applications:

  • Compounds that extend treatment windows for acute liver failure

  • Modulators that improve metabolic parameters without adversely affecting glucose homeostasis

  • Context-specific activation of AdipoR2 signaling pathways

These approaches hold promise for developing selective AdipoR2 modulators with therapeutic potential for metabolic disorders, liver diseases, and other conditions where adiponectin signaling plays a role.

How might single-cell technologies advance our understanding of AdipoR2 biology?

Single-cell technologies offer unprecedented opportunities to advance AdipoR2 research:

• Single-cell RNA sequencing (scRNA-seq):

  • Reveals cell-type specific expression patterns of AdipoR2 in heterogeneous tissues like liver

  • Identifies co-expression patterns with other metabolic regulators

  • Detects changes in AdipoR2 expression during disease progression or in response to treatments

• Single-cell proteomics:

  • Maps AdipoR2 protein levels and post-translational modifications at single-cell resolution

  • Correlates AdipoR2 expression with cell state and metabolic phenotypes

• Spatial transcriptomics:

  • Preserves spatial context while measuring AdipoR2 expression

  • Particularly valuable for understanding zonation of AdipoR2 expression in liver lobules

• CRISPR screening at single-cell level:

  • Identifies genes that regulate AdipoR2 expression or function

  • Discovers synthetic lethal interactions with AdipoR2 modulation

• Live-cell imaging techniques:

  • Tracks AdipoR2 trafficking and dynamics in real-time

  • Visualizes signaling events downstream of AdipoR2 activation

These technologies promise to resolve current contradictions in AdipoR2 biology by accounting for cellular heterogeneity, providing context-specific insights into receptor function, and revealing cell-type specific responses to AdipoR2 modulation that may be obscured in bulk tissue analyses.