Oligo-complex formation rescues the constitutively endocytosed mAVPR2 mutant R137H.

<p>The constitutively β-arrestin bound R137H mutant of mAVPR2 is largely endocytosed. With co-expression of mSCTR, however, an increase in net BRET signal suggests rescue of the R137H mutant to the cell surface by heterocomplex formation. The mutations of A89P or Q174R, which lead to improper folded, non-surface reaching receptors, cannot be rescued by the formation of heteromer. Significance level was calculated against the mAVPR1b control. The data were mean±SEM from three to five independent experiments in triplicate. ***, P<0.001.</p

Surface to intracellular fluorescence ratio for cell expressing WT/mutant AVPR2 with or without SCTR co-expression.

<p>The presence of SCTR increased the amount of fluorescence on the cell surface in cell expressing R137H AVPR2 tagged with YFP, indicating a rescue of the mutant receptor. The data were mean±SEM from three independent experiments from 5–6 ROIs per sample.</p

Comparable transcript levels of mSCTR and mAVPR2 in CHOK1 cells transiently transfected with combinations of mSCTR and mAVPR2 by quantitative real-time PCR.

<p>The total receptor gene transcript levels were compared to the internal house-keeping control GAPDH by the 2 ΔΔ ct method. There is no significant difference among the groups. mSCTR/pcDNA3.1, 1ug mSCTR plasmid with 1ug pcDNA 3.1 empty vector; mSCTR/mAVPR2, 1ug receptor plasmid each; pcDNA3.1, 2ug pcDNA3.1; mAVPR2/pcDNA3.1, 1ug mAVPR2 plasmid with 1ug pcDNA 3.1. Data are presented as means ± SEM from three independent experiments in duplicate. ns, not significant.</p

Surface expression of mAVPR1a, mAVPR1b, mAVPR2 and mSCTR are similar.

<p>Shown are representative images of CHOK1 cells expressing mAVPR1a/mAVPR1b/mAVPR2 or mSCTR constructs. Surface to intracellular fluorescence ratios were similar for these four types of cells. The data were mean±SEM from three independent experiments with 5–6 ROIs per sample. Scale bar, 10μM.</p

mSCTR specifically oligomerizes with mAVPR2, and mAVPR1a, but not mAVPR1b.

<p>Shown are the net BRET ratios for CHO-K1 cells expressing a combination of mSCTR-Rlu donor and mAVPR-YFP acceptor constructs. Saturable curves from BRET assays were obtained for mAVPR2 and mAVPR1a, but not for mAVPR1b. The data were mean±SEM from three to five independent experiments in triplicate. ***, P<0.001. **, P<0.01. *, P<0.05.</p

Rescue of R137H mutant by SCTR as reflected by reduced affinity to β-arrestin.

<p>The affinity between WT or mutant AVPR2 with β-arrestin were determined by BRET, using AVRP2 tagged with Rlu and β-arrestin tagged with YFP. A) In the native state, the R137H mutant shows significantly higher affinity to β-arrestin than the WT AVPR2. While the two other mutants, A89P and Q174R, showed slightly higher BRET than the WT receptor, but the increase was not significant. Upon co-expression of SCTR, β-arrestin affinity of R137H was significantly reduced compared to the scenario when no SCTR was present. B) BRET was measured at 10 min after addition of 1μM Vp to stimulate receptor internalization. WT AVPR2 showed increase affinity to β-arrestin, but not the R137H mutant without SCTR co-expression. With SCTR, R137H demonstrated increased β-arrestin binding, suggesting functional rescue of the receptor. Data are presented as means±SEM from three independent experiments in duplicate. ***, P<0.001, **, P<0.01.</p

Rescue of the constitutively endocytosed R137H mAVPR2 mutant upon co-expression of mSCTR.

<p>Representative confocal images indicating the cellular location of mAVPR2-YFP receptors. The WT mAVPR2 is evenly distributed in the cell surface regardless of mSCTR co-expression. The R137H mutant is predominantly located in the intracellular endocytotic vesicles. Vesicular retention is not observed when mSCTR is co-transfected. The A89P and Q174R mutants cannot be rescued by mSCTR co-expression and remain intracellular. Scale bar, 10μM.</p

Signaling modification as a specific consequence of SCTR-AVPR2 oligomerization.

<p>Percentage changes in maximum cellular cAMP levels when cells were expressing a combination of mSCTR and/or mAVPR constructs, comparing to cAMP responses in cells bearing only the native receptor to the ligand. A) SCT stimulated a marked shift in E_max and potency in mSCTR in the presence of mAVPR2. No significant changes were observed when the non-interacting mAVPR1b replaced mAVPR2. B) Similarly, Vp stimulated a notable reduction in E_max and potency in mAVPR2 in the presence of mSCTR. The data were mean±SEM from three to five independent experiments in triplicate. ***, P<0.001. **, P<0.01. *, P<0.05. 10pM to 1μM SCT (panel C) or Vp (panel D) were treated to cells with mSCTR and/or mAVPR2. Calcium response curves are presented as percentages of maximal changes in RFU of cells expressing mSCTR or mAVPR2 only and stimulated with 1μM peptide. Except for cells with mSCTR only, a marked increase in cellular calcium response can only be seen at 1μM peptide concentration. Data were obtained from three individual experiments with 5–6 ROIs per dose.</p

Spirohexene-Tetrazine Ligation Enables Bioorthogonal Labeling of Class B G Protein-Coupled Receptors in Live Cells

A new bioorthogonal reactant pair, spiro[2.3]­hex-1-ene (Sph) and 3,6-di­(2-pyridyl)-<i>s</i>-tetrazine (DpTz), for the strain-promoted inverse electron-demand Diels–Alder cycloaddition, that is, tetrazine ligation, is reported. As compared to the previously reported strained alkenes such as <i>trans</i>-cyclooctene (TCO) and 1,3-disubstituted cyclopropene, Sph exhibits balanced reactivity and stability in tetrazine ligation with the protein substrates. A lysine derivative of Sph, SphK, was site-selectively incorporated into the extracellular loop regions (ECLs) of GCGR and GLP-1R, two members of class B G protein-coupled receptors (GPCRs) in mammalian cells with the incorporation efficiency dependent on the location. Subsequent bioorthogonal reactions with the fluorophore-conjugated DpTz reagents afforded the fluorescently labeled GCGR and GLP-1R ECL mutants with labeling yield as high as 68%. A multitude of functional assays were performed with these GPCR mutants, including ligand binding, ligand-induced receptor internalization, and ligand-stimulated intracellular cAMP accumulation. Several positions in the ECL3s of GCGR and GLP-1R were identified that tolerate SphK mutagenesis and subsequent bioorthogonal labeling. The generation of functional, fluorescently labeled ECL3 mutants of GCGR and GLP-1R should allow biophysical studies of conformation dynamics of this important class of GPCRs in their native environment in live cells

Spirohexene-Tetrazine Ligation Enables Bioorthogonal Labeling of Class B G Protein-Coupled Receptors in Live Cells

A new bioorthogonal reactant pair, spiro[2.3]­hex-1-ene (Sph) and 3,6-di­(2-pyridyl)-<i>s</i>-tetrazine (DpTz), for the strain-promoted inverse electron-demand Diels–Alder cycloaddition, that is, tetrazine ligation, is reported. As compared to the previously reported strained alkenes such as <i>trans</i>-cyclooctene (TCO) and 1,3-disubstituted cyclopropene, Sph exhibits balanced reactivity and stability in tetrazine ligation with the protein substrates. A lysine derivative of Sph, SphK, was site-selectively incorporated into the extracellular loop regions (ECLs) of GCGR and GLP-1R, two members of class B G protein-coupled receptors (GPCRs) in mammalian cells with the incorporation efficiency dependent on the location. Subsequent bioorthogonal reactions with the fluorophore-conjugated DpTz reagents afforded the fluorescently labeled GCGR and GLP-1R ECL mutants with labeling yield as high as 68%. A multitude of functional assays were performed with these GPCR mutants, including ligand binding, ligand-induced receptor internalization, and ligand-stimulated intracellular cAMP accumulation. Several positions in the ECL3s of GCGR and GLP-1R were identified that tolerate SphK mutagenesis and subsequent bioorthogonal labeling. The generation of functional, fluorescently labeled ECL3 mutants of GCGR and GLP-1R should allow biophysical studies of conformation dynamics of this important class of GPCRs in their native environment in live cells