Recombinant Bovine G protein-coupled receptor 161 (GPR161)

What is the molecular structure of GPR161 and how does it differ between species?

GPR161 is an orphan G protein-coupled receptor encoded by six exons, with a 529 amino acid sequence and a calculated molecular mass of approximately 58.5-59 kDa in mammals. The receptor contains a seven-transmembrane domain structure typical of GPCRs, with an important C-terminal tail that mediates downstream signaling. Western blot analysis of cell lysates transfected with tagged GPR161 typically reveals two major isoforms: one at the predicted size (~58-59 kDa) and a larger band at ~70 kDa representing post-translationally modified forms . Sequence homology is highly conserved across mammals (human, mouse, bovine), especially in transmembrane domains and intracellular loops involved in G protein coupling.

What are the main signaling pathways regulated by GPR161?

GPR161 functions primarily as a negative regulator of the Sonic Hedgehog (Shh) signaling pathway. It exhibits constitutive activity that couples to Gαs, activating adenylyl cyclase and increasing cAMP levels, which subsequently activates protein kinase A (PKA) . The activated PKA phosphorylates Gli2/3 transcription factors, promoting their proteolytic conversion into repressor forms (Gli-R) that inhibit Shh target gene expression . When Shh signaling is activated, GPR161 is removed from primary cilia, allowing Gli activator forms to accumulate and induce target gene expression. Additionally, GPR161 has been implicated in mTORC1 pathway activation in cancer contexts .

How is GPR161 trafficking regulated in primary cilia?

GPR161 trafficking follows a two-step regulatory process:

• Ciliary Entry: Requires the IFT-A complex and TULP3, which coordinate GPR161 transport into primary cilia .

• Ciliary Exit: Upon Shh pathway activation, GPR161 undergoes:

  • β-arrestin recruitment to the signaling-competent receptor, facilitated by GPCR kinase 2 (Grk2)

  • Smoothened accumulation in cilia, which enhances Gpr161-β-arrestin binding

  • Clathrin-mediated endocytosis outside the ciliary compartment

  • BBSome-mediated removal from primary cilia

The Fuz protein, an effector of planar cell polarity signaling, interacts with the N-terminal region of GPR161 and regulates its ciliary trafficking via β-arrestin2 .

What are the optimal conditions for recombinant GPR161 expression in bacterial systems?

Recombinant expression of GPR161 in bacterial systems requires careful optimization due to potential toxicity issues. Based on experimental data:

The recombinant protein can be expressed effectively using pET32 vectors that incorporate fusion tags to improve solubility and reduce toxicity. Expression is detectable within 2 hours after IPTG induction, with optimal yield typically observed after 6 hours . For transmembrane domain-containing fragments, lower temperatures (18-26°C) and moderate inducer concentrations help maintain proper folding.

How can one optimize purification of recombinant GPR161 while maintaining its functional properties?

Purification of recombinant GPR161 requires specialized approaches to maintain structural integrity:

• Solubilization: After cell lysis, the protein remains in the soluble fraction when expressed with appropriate solubility tags .

• Affinity Chromatography: Purification using Ni²⁺-Sepharose columns for His-tagged recombinant GPR161 yields high initial protein concentrations.

• Refolding Strategy: Optimal refolding is achieved by spatial separation of protein molecules through column pores during purification, preventing aggregation .

• Validation: Confirm identity using:

  • SDS-PAGE analysis (expected size of ~49 kDa for the recombinant fragment)

  • Mass spectrometry analysis against SwissProt database

  • Western blot using specific antibodies

For functional studies, consider incorporating detergent micelles or nanodiscs to maintain the native conformation of transmembrane regions.

What domains of GPR161 are essential for its interaction with other proteins?

GPR161 contains several critical interaction domains:

• C-terminal Tail (377-529 aa):

  • Contains the primary β-arrestin binding region (specifically 377-401 aa)

  • Deletion of this region (Gpr161FullCT-del) abrogates β-arrestin2 recruitment

  • Contains nine putative S/T phosphorylation sites important for receptor-mediated endocytosis

  • Acts as a scaffold for GPCR-interacting proteins (GIPs)

• C-terminal PKA-binding Domain:

  • The L465P mutation in the C-terminus disrupts interaction with PKA regulatory subunit I (PKA-RI)

  • This interaction is essential for Hedgehog pathway repression in primary cilia

• N-terminal Region:

  • Mediates interaction with Fuz protein, as demonstrated by co-immunoprecipitation studies with N-terminal deletion mutants

The vacuolated lens (vl) mouse mutation results in truncation of the receptor at residue 386, deleting 143 (of 203) amino acids of the C-terminal tail, which significantly impairs receptor function .

How does GPR161 interact with β-arrestins and what methods can detect these transient interactions?

GPR161 demonstrates constitutive activity-dependent interactions with β-arrestins that can be challenging to detect due to their transient nature:

• Detection Methods:

  • Traditional co-immunoprecipitation assays often fail to capture this interaction

  • Intermolecular bioluminescence resonance energy transfer (BRET) ratiometric assays in live cells provide quantitative measurement of these transient interactions

• Interaction Characteristics:

  • GPR161 shows preferential binding to β-arrestin2 over β-arrestin1

  • The signaling-deficient mutant (V158E) demonstrates significantly reduced β-arrestin recruitment

  • Saturating BRET signals indicate specific interaction between wild-type GPR161 and β-arrestin2

• Functional Significance:

  • β-arrestin recruitment is enhanced by Smoothened activation during Shh signaling

  • In β-arrestin double-knockout MEFs, GPR161 fails to exit cilia upon Shh pathway activation, resulting in approximately 60% decrease in Gli1 upregulation

How does GPR161 achieve its dual functions in Gs-mediated signaling and PKA anchoring?

GPR161 employs two distinct mechanisms to regulate Sonic Hedgehog signaling:

• G Protein Signaling:

  • GPR161 constitutively couples to Gαs, activating adenylyl cyclase and increasing cAMP levels

  • This activity depends on the active conformation of the receptor, as demonstrated by the signaling-deficient V158E mutant

  • Increased cAMP activates PKA, leading to phosphorylation of Gli transcription factors

• PKA Anchoring:

  • GPR161 directly binds to the regulatory subunit of PKA (PKA-RI) via its C-terminal domain

  • This anchoring function positions PKA in proximity to its Gli substrates within primary cilia

  • The L465P mutation in GPR161's C-terminus disrupts this interaction without affecting G protein coupling

Interestingly, experimental evidence shows that while PKA anchoring is essential for Hedgehog pathway repression, the G protein signaling activity is dispensable, as demonstrated by GPR161-AAA7.52,7.56,8.51 mutants that disrupt G protein coupling but maintain PKA binding and Hedgehog inhibition .

What are the molecular mechanisms by which Smoothened activation leads to GPR161 removal from cilia?

The process of GPR161 removal from cilia upon Smoothened activation involves several coordinated steps:

• Smoothened Ciliary Accumulation:

  • Upon Shh pathway activation, Smoothened accumulates in primary cilia

  • This occurs independently of β-arrestins, as demonstrated in β-arrestin double-knockout MEFs

• Enhanced GPR161-β-arrestin Interaction:

  • Activated Smoothened promotes increased binding between GPR161 and β-arrestin, particularly β-arrestin2

  • This interaction requires GPCR kinase 2 (Grk2), which likely phosphorylates the C-terminal tail of GPR161

• Clathrin-mediated Endocytosis:

  • Following β-arrestin recruitment, GPR161 undergoes clathrin-mediated endocytosis outside the ciliary compartment

  • The BBSome complex also mediates GPR161 removal from cilia

• Recycling Endosome Internalization:

  • After removal from cilia, GPR161 is internalized into recycling endosomes

  • This prevents its inhibitory activity and allows activation of Shh signaling cascade

What is the developmental expression pattern of GPR161 and how does it correlate with its function?

GPR161 shows a highly specific spatiotemporal expression pattern during embryonic development:

This expression pattern correlates with the phenotypes observed in vacuolated lens (vl) mouse mutants, which display neural tube defects due to abnormal neural fold fusion and congenital cataracts due to disrupted lens development . The restricted expression in differentiating lens fiber cells but absence from proliferating anterior lens epithelium suggests a specific role in cell differentiation rather than proliferation in lens development .

How does GPR161 dysregulation affect embryonic development and what molecular pathways are involved?

Disruption of GPR161 function leads to severe developmental abnormalities through dysregulation of Sonic Hedgehog signaling:

• Neural Tube Defects:

  • GPR161 mutants show normal elevation and bending of the neural plate but exhibit abnormal apposition and fusion

  • Cellular protrusions from the apical neural folds display abnormal ultrastructural morphology

  • This results from inappropriate activation of Shh signaling in lateral neural folds

• Lens Abnormalities:

  • Lens defects become apparent after E14.5, affecting lens fiber differentiation

  • Disruption of ocular environment signaling that normally coordinates lens development

• Molecular Mechanism:

  • Loss of GPR161 function leads to decreased cAMP levels and reduced PKA activity

  • This prevents Gli3 processing into its repressor form (Gli3-R)

  • Results in inappropriate activation of Shh target genes

  • The severity of phenotypes can be modified by genetic background, as demonstrated by quantitative trait loci (QTL) analysis

GPR161 knockout in mice is embryonically lethal, with embryos displaying severe limb, facial, and nervous system defects consistent with hyperactive Hedgehog signaling .

How is GPR161 implicated in cancer pathogenesis and what mechanisms are involved?

GPR161 has significant associations with cancer development through multiple mechanisms:

• Triple-Negative Breast Cancer (TNBC):

  • GPR161 is upregulated in TNBC, serving as a prognostic biomarker

  • Promotes cancer cell proliferation by activating the mTORC1 pathway

  • Regulates migration and invasion by disrupting E-cadherin localization

  • Functions in an IQGAP1-dependent manner to induce proliferation and migration

• Medulloblastoma:

  • GPR161 mutations have been identified in medulloblastoma patients

  • Loss of GPR161 function enhances Shh pathway activity, a known driver of medulloblastoma

• Mechanism of Action in Cancer:

  • Altered GPR161 expression or function disrupts normal Hedgehog pathway regulation

  • Can function as either an oncogene (when overexpressed) or tumor suppressor (when mutated), depending on cancer context

  • May serve as a target for developing cancer diagnostics and therapeutics

What human developmental disorders are associated with GPR161 mutations and what are their molecular bases?

GPR161 mutations are associated with several congenital developmental disorders:

• Neural Tube Defects (NTDs):

  • GPR161 mutations lead to spina bifida and other NTDs

  • Molecular basis: Disrupted Shh pathway regulation causes abnormal neural tube closure

• Pituitary Stalk Interruption Syndrome:

  • Characterized by an absent or thin pituitary stalk, hypoplastic anterior pituitary, and ectopic posterior pituitary

  • Results from abnormal development of pituitary structures during embryogenesis

• Congenital Cataracts:

  • Similar to the vacuolated lens (vl) phenotype in mice

  • Molecular basis: Disrupted lens fiber cell differentiation due to altered signaling

• Other Developmental Anomalies:

  • Limb and facial defects observed in mouse models likely have human counterparts

  • These defects stem from inappropriate activation of Hedgehog signaling during embryonic development

The severity and penetrance of these disorders can be modified by genetic background factors, as demonstrated by the identification of Foxe3 as a genetic modifier that interacts with Gpr161 to regulate lens development in mice .

What are the most effective methods to study GPR161 localization and trafficking in primary cilia?

Studying GPR161 localization and trafficking in primary cilia requires specialized techniques:

• Immunofluorescence Microscopy:

  • Use validated antibodies (such as 13398-1-AP or 29328-1-AP) with appropriate ciliary markers

  • Recommended dilutions: 1:200-1:800 for immunofluorescence applications

  • Cell models: ARPE-19, hTERT-RPE1, MDCK, or C2C12 cells which form primary cilia

• Live Cell Imaging:

  • Fluorescently tagged GPR161 constructs to monitor real-time trafficking

  • Photoactivatable or photoconvertible fusion proteins to track specific protein populations

• Quantitative Analysis Methods:

  • Measure ciliary intensity of GPR161 under various conditions

  • Calculate the ratio of ciliary to cytoplasmic fluorescence intensity

  • Assess colocalization with other ciliary proteins using Pearson's correlation coefficient

• Biochemical Trafficking Assays:

  • Cell surface biotinylation to measure membrane-localized receptors

  • Subcellular fractionation to isolate ciliary membranes

  • Bioluminescence resonance energy transfer (BRET) assays to detect protein-protein interactions during trafficking

What genetic approaches can be used to investigate GPR161 function in development and disease models?

Advanced genetic approaches for studying GPR161 function include:

• CRISPR/Cas9 Genome Editing:

  • Generate complete knockouts or specific point mutations

  • Create knock-in models with fluorescent tags or domain modifications

  • Develop conditional knockout systems using floxed alleles and tissue-specific Cre expression

• Genetic Interaction Studies:

  • Quantitative trait loci (QTL) analysis to identify genetic modifiers, as demonstrated with Foxe3 for the cataract phenotype

  • Cross GPR161 mutants with other signaling pathway mutants to assess epistatic relationships

• Rescue Experiments:

  • Test functionality of different GPR161 domains through rescue of mutant phenotypes

  • Structure-function analysis using targeted mutations:

    • V158E mutation to disrupt signaling activity

    • C-terminal truncations to affect β-arrestin binding

    • L465P mutation to specifically disrupt PKA binding

• Spatiotemporal Control Systems:

  • Inducible expression systems to control timing of GPR161 manipulation

  • Tissue-specific promoters to restrict genetic modifications

  • Optogenetic or chemogenetic tools to achieve precise temporal control of GPR161 function