Recombinant Human Probable G-protein coupled receptor 22 (GPR22)
Description
Introduction to GPR22
Probable G-protein coupled receptor 22 (GPR22) is a seven-transmembrane protein belonging to the class A G-protein coupled receptor (GPCR) family. In humans, it is encoded by the GPR22 gene located on chromosome 7q22.3 . As an orphan receptor, its endogenous ligand remains unidentified, presenting both challenges and opportunities for research exploration . GPR22 exhibits high sequence conservation across species, with human GPR22 sharing 97% amino acid sequence identity with its mouse counterpart .
The receptor has gained significant attention due to its restricted tissue expression pattern and emerging roles in cardiovascular protection, making recombinant versions of this protein valuable tools for advancing our understanding of its biological functions and therapeutic potential .
Expression Pattern and Tissue Distribution
GPR22 exhibits a highly restricted expression pattern that has significant implications for its physiological functions. mRNA analysis has revealed remarkably abundant and selective expression primarily in the brain and heart of humans and rodents .
In the heart, GPR22 mRNA is detected in all chambers at levels comparable to the β1-adrenergic receptor as determined by quantitative PCR analysis . Immunohistochemical studies have confirmed GPR22 protein expression specifically in cardiac myocytes and coronary arteries in rat heart tissue . This cardiac-specific expression pattern suggests a specialized role in cardiovascular physiology.
Additionally, GPR22 expression has been detected in:
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Human glioblastoma cell line A172, as demonstrated by flow cytometry
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Zebrafish embryos, particularly in regions involved in left-right axis determination
Interestingly, myocardial mRNA expression of GPR22 is dramatically reduced following aortic banding in mice, suggesting that its expression is dynamically regulated in response to hemodynamic stress . Similarly, ischemic stress induced by coronary artery ligation or cobalt chloride treatment significantly reduces GPR22 expression in cardiac tissue and cardiomyocyte cell lines .
Signaling Mechanisms
GPR22 exhibits constitutive activity and couples primarily to the inhibitory G proteins (Gi/Go) . When studied in transfected HEK-293 cells, GPR22 activation leads to the inhibition of adenylyl cyclase, resulting in decreased cyclic AMP production . No constitutive coupling to Gs or Gq protein families has been observed .
The receptor's signaling mechanisms have been implicated in several downstream pathways:
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PI3K-Akt signaling pathway: GPR22 overexpression increases activation of this pathway, which is associated with cardioprotective effects
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Bcl-2 regulation: GPR22 influences myocardial levels of the anti-apoptotic protein Bcl-2
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Ciliogenesis: Evidence suggests GPR22 involvement in cilia length and function regulation, particularly in zebrafish embryos
The constitutive activity of GPR22 in the absence of a known ligand represents an interesting feature of this receptor. Like many orphan GPCRs, understanding its binding kinetics and potential endogenous ligands remains a significant challenge in the field .
Physiological Functions and Disease Associations
Research using knockout and overexpression models has revealed multiple physiological functions of GPR22:
The protective role of GPR22 in cardiac tissue has been particularly well-documented. In mouse models with acute myocardial infarction induced by left coronary artery ligation, cardiomyocyte-specific GPR22 overexpression significantly improved cardiac outcomes . This protective effect appears to be mediated through upregulation of Akt signaling and increased Bcl-2 levels, which protect cardiomyocytes from ischemia-induced apoptosis .
Interestingly, genome-wide association studies have also identified a susceptibility locus for knee osteoarthritis on chromosome 7q22, the region containing the GPR22 gene, suggesting potential involvement in joint biology and disease .
Recombinant Production Methods
Recombinant human GPR22 proteins have been successfully produced using various expression systems, each offering distinct advantages for different research applications:
The E. coli-expressed recombinant GPR22 with N-terminal His-tag is often supplied as a lyophilized powder requiring reconstitution . The recommended storage conditions typically involve -20°C to -80°C, with aliquoting necessary to avoid repeated freeze-thaw cycles .
For wheat germ-derived recombinant GPR22, the protein is supplied in liquid form with a storage buffer of 25 mM Tris-HCl (pH 8.0) containing 2% glycerol . This preparation is particularly suitable for antibody production, functional studies, and compound screening .
Research Applications
Recombinant human GPR22 serves numerous research applications essential for understanding its biology and therapeutic potential:
Antibody Development and Validation
Recombinant GPR22 proteins serve as immunogens for antibody production and as positive controls for validating antibody specificity . Monoclonal antibodies against human GPR22, such as clone 455108, have been developed and are available with various conjugations including DyLight 594 for intracellular staining and flow cytometry applications .
Functional Studies
Recombinant GPR22 enables investigations into receptor signaling, protein-protein interactions, and structure-function relationships . These studies have been instrumental in elucidating the constitutive Gi/Go coupling of GPR22 and its downstream effects on adenylyl cyclase inhibition .
Compound Screening
The availability of purified recombinant GPR22 facilitates high-throughput screening for potential ligands and modulators . This approach is particularly valuable for orphan receptors like GPR22, where identifying endogenous ligands remains a significant challenge .
Flow Cytometry Applications
Anti-GPR22 antibodies developed using recombinant proteins enable detection of GPR22 expression in various cell types. Flow cytometry studies have successfully demonstrated GPR22 expression in the A172 human glioblastoma cell line using this approach .
Current Research and Future Directions
Recent studies have significantly advanced our understanding of GPR22 biology and potential therapeutic applications:
Cardiovascular Research
A 2023 study demonstrated that cardiomyocyte-specific GPR22 overexpression ameliorates cardiac injury in mice with acute myocardial infarction . This research revealed that:
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Ischemic stress reduces GPR22 expression in cardiac tissue
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GPR22 overexpression increases myocardial Bcl-2 levels
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GPR22 activates the cardioprotective PI3K-Akt signaling pathway
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GPR22 may represent a novel therapeutic target for myocardial ischemia
These findings build upon earlier work showing that GPR22-deficient mice display increased susceptibility to functional decompensation following aortic banding, despite having normal heart structure and function under baseline conditions .
Developmental Biology
Research in zebrafish models has revealed GPR22's role in regulating cilia length and function, with implications for left-right axis determination during embryonic development . Morpholino knockdown or overexpression of gpr22 led to defective left-right axis formation, indicating its importance in developmental processes .
Future Research Directions
Key areas for future investigation include:
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Identification of endogenous ligands for GPR22
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Elucidation of complete signaling networks downstream of GPR22 activation
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Development of selective GPR22 modulators as potential therapeutics
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Further characterization of GPR22's role in osteoarthritis and other potential disease associations
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Exploration of GPR22 as a biomarker for cardiovascular stress or disease progression
Product Specs
Note: We prioritize shipping the format we have in stock. However, if you have specific format requirements, please indicate them when placing your order, and we will prepare the product according to your request.
Note: Our proteins are shipped with standard blue ice packs by default. If you require dry ice shipping, please inform us in advance, as additional fees will apply.
Generally, the shelf life of liquid form is 6 months at -20°C/-80°C. The shelf life of lyophilized form is 12 months at -20°C/-80°C.
Target Background
- Demonstrates GRR22 expression in the human heart. PMID: 18539757
Q&A
What is GPR22 and what is its expression pattern in human tissues?
GPR22 is an orphan G-protein coupled receptor with a highly restricted tissue expression pattern. Studies have demonstrated remarkably abundant and selective expression in the brain and heart of humans and rodents . Within the heart, GPR22 mRNA is expressed in all chambers at levels comparable to the β1-adrenergic receptor as assessed by Taqman PCR . Immunohistochemical analysis has confirmed GPR22 protein expression specifically in cardiac myocytes and coronary arteries in rat hearts . This distinct expression pattern suggests specialized functions in cardiovascular and neurological systems.
What signaling pathways does GPR22 utilize?
GPR22 has been shown to couple constitutively to Gi/Go proteins, resulting in the inhibition of adenyl cyclase . In studies using synthetic GPR22 (sGPR22) with enhanced expression, researchers demonstrated that cells stably overexpressing sGPR22 showed significantly reduced isoproterenol-stimulated cAMP accumulation, indicating constitutive Gi/Go coupling . This inhibition could be reversed with pertussis toxin treatment, further confirming the Gi/Go coupling mechanism . Importantly, no constitutive coupling to Gs or Gq was observed in these experimental systems . The receptor's ability to constitutively signal through Gi/Go pathways suggests it may play a role in modulating other stimulatory signaling cascades in cardiomyocytes.
Why does wild-type GPR22 show poor expression in heterologous systems?
The wild-type GPR22 nucleotide sequence is unusually A/T rich (64%), significantly higher than the typical ~40% A/T content in most human genes . Analysis revealed that these A/T-rich regions likely affect mRNA stability, resulting in rapid degradation of GPR22 mRNA in heterologous transfected cell systems . When researchers transfected wild-type GPR22 into HEK-293 or COS7 cells, they observed weak mRNA expression requiring 48-72 hours of exposure for detection, and HA-tagged GPR22 protein was not readily detectable by immunocytochemistry . This understanding of GPR22's unusual nucleotide composition provides important insights for researchers attempting to express the receptor in experimental systems.
How can GPR22 expression be optimized in experimental systems?
To overcome the poor expression of wild-type GPR22, researchers have successfully developed a "synthetic" version of GPR22 (sGPR22) with enhanced mRNA stability . This approach involved:
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Converting A/T-rich regions to G/C-rich regions without modifying the wild-type amino acid sequence
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Reversing the A/T-to-G/C ratio from 64% A/T content to only 41% A/T content
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Engineering the cDNA using a segmental approach with annealed fragments
This methodological approach dramatically increased mRNA levels and protein expression in transfected cells. For instance, when COS7 cells were transfected with sGPR22, mRNA was easily detected after 30 minutes of exposure compared to 48-72 hours with wild-type GPR22 . Cell surface expression of the receptor protein was confirmed by immunocytochemical staining and flow cytometry . This synthetic construct approach can serve as a template for researchers working with other poorly expressed GPCRs with similar nucleotide composition issues.
What antibodies and detection methods are recommended for studying GPR22?
For immunohistochemical detection of GPR22, researchers have successfully used polyclonal antibodies directed against the intracellular C-terminus of GPR22 . One specific example is an antibody raised against the peptide sequence NH2-VEADPLPNNAVIHNSWIDPKRN-COOH . Commercially available antibodies include rabbit polyclonal antibodies reactive to human and mouse GPR22 suitable for applications including ELISA, Western blot, and immunofluorescence/immunocytochemistry .
When performing immunohistochemistry on heart tissues, the following protocol has been successfully employed:
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Cryosectioning snap-frozen heart tissues at 8 μm thickness
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Formalin fixation followed by heat-induced epitope retrieval via microwave irradiation
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Detection using the Vectastain ABC System
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Controls including rabbit universal IgG control primary antibody to determine nonspecific staining
For optimal results in Western blot applications, dilutions of 1/500 - 1/5000 are recommended, while immunofluorescence/immunocytochemistry applications work best at dilutions of 1/50 - 1/200 .
What functional assays can be used to measure GPR22 signaling?
Several functional assays have been validated to measure GPR22 signaling activity:
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cAMP accumulation assays: Since GPR22 couples to Gi/Go, measuring inhibition of isoproterenol-stimulated cAMP accumulation is an effective approach. This can be reversed with pertussis toxin to confirm specificity .
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[35S]GTPγS binding assays: Increased [35S]GTPγS binding to membranes derived from GPR22-expressing cells indicates G-protein coupling and can be used to confirm Gi/Go activity .
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Inositol phosphate accumulation: Though GPR22 does not couple to Gq, this assay serves as a negative control when comparing to known Gq-coupled receptors .
These functional assays provide complementary approaches to characterize GPR22 signaling in different experimental contexts.
What is the role of GPR22 in cardiac response to hemodynamic stress?
GPR22 appears to play a protective role in the heart's response to hemodynamic stress, particularly in the transition from compensated hypertrophy to decompensated heart failure . Several lines of evidence support this:
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GPR22 mRNA expression is dramatically reduced following aortic banding in mice, suggesting regulation in response to increased afterload .
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GPR22 knockout mice (GPR22−/−) display increased susceptibility to functional decompensation following aortic banding, despite showing normal cardiac structure and function under basal conditions .
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While both wild-type and GPR22−/− mice showed similar degrees of hypertrophy in response to transverse aortic constriction (TAC), GPR22−/− mice exhibited significantly decreased fractional shortening (%FS) and velocity of circumferential fiber shortening (VCF), indicating compromised contractile function .
How does GPR22 contribute to developmental processes?
GPR22 plays a critical role in left-right (LR) axis formation during development, as demonstrated in zebrafish embryo studies . Specifically:
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Both morpholino knockdown and overexpression of gpr22 led to defective LR patterning, including randomization of left-specific lateral plate mesodermal genes (lefty1, lefty2, southpaw, and pitx2a), resulting in randomized cardiac looping .
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Inactivation of gpr22 in the Kupffer's vesicle (KV) alone was sufficient to generate the phenotype, indicating that GPR22 primarily regulates LR asymmetry through this structure .
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Analysis of KV cilia by immunofluorescence and transmission electron microscopy revealed that gpr22 manipulation resulted in changes to cilia length and structure .
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GPR22 does not appear to act upstream of the two cilia master regulators, Foxj1a and Rfx2, suggesting it functions through a different mechanistic pathway .
These findings establish GPR22 as a novel player in ciliogenesis and developmental patterning, expanding our understanding of its functions beyond cardiac stress response.
What are the implications of GPR22's constitutive activity for drug discovery?
The constitutive Gi/Go coupling activity of GPR22 presents unique opportunities and challenges for drug discovery. As an orphan GPCR with no identified endogenous ligand, GPR22 represents a potential therapeutic target that could be modulated by:
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Inverse agonists: Compounds that could reduce the constitutive Gi/Go signaling of GPR22, potentially enhancing contractility in heart failure scenarios where increased inotropy is desired.
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Positive allosteric modulators: Molecules that could enhance the constitutive activity of GPR22, potentially providing cardioprotection in conditions of excessive β-adrenergic stimulation.
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Expression modulators: Approaches to regulate GPR22 expression levels, given the observation that GPR22 mRNA expression is dramatically altered in response to cardiac stress .
Given GPR22's restricted tissue expression pattern with high levels in heart and brain, targeted modulation could potentially provide tissue-specific effects with reduced systemic side effects compared to receptors with broader expression patterns .
What controls should be included when studying GPR22 function?
When investigating GPR22 function in research settings, the following controls are recommended:
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Antibody specificity controls:
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Signaling pathway controls:
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Expression system controls:
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Genetic model controls:
How can researchers interpret GPR22 alterations in disease models?
When interpreting changes in GPR22 expression or function in disease models, researchers should consider the following factors:
What are the key experimental challenges when working with GPR22?
Researchers working with GPR22 face several experimental challenges:
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Expression difficulties: Wild-type GPR22's unusual A/T-rich sequence (64%) leads to poor expression in heterologous systems, requiring either extended detection times or the use of synthetic constructs with optimized nucleotide composition .
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Orphan receptor status: The lack of a known endogenous ligand complicates functional studies and requires reliance on constitutive activity or genetic approaches rather than ligand-based activation .
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Signal detection sensitivity: GPR22's constitutive Gi/Go coupling may produce subtle effects in some assay systems, requiring sensitive detection methods and appropriate positive controls .
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Physiological relevance of overexpression: While overexpression systems facilitate detection and analysis, they may not accurately reflect the signaling dynamics of endogenous GPR22 expression levels .
Understanding these challenges helps researchers design appropriate experimental approaches and interpret results in the context of GPR22's unique properties.
