Re-routing GPR56 signalling using Gα12/13 G protein chimeras.

Adhesion G protein-coupled receptors (aGPCRs) constitute the second largest subclass of the GPCR superfamily. Although canonical GPCRs are explored pharmacologically as drug targets, no clinically approved drugs target the aGPCR family so far. The aGPCR GPR56/ADGRG1 stands out as an especially promising target, given its direct link to the monogenetic disease bilateral frontoparietal polymicrogyria and implications in cancers. Key to understanding GPCR pharmacology has been mapping out intracellular signalling activity. Detection of GPCR signalling in the Gαs /Gαi /Gαq G protein pathways is feasible with second messenger detection systems. However, in the case of Gα12/13 -coupled receptors, like GPR56, signalling detection is more challenging due to the lack of direct second messenger generation. To overcome this challenge, we engineered a Gαq chimera to translate Gα12/13 signalling. We show the ability of the chimeric GαΔ6q12myr and GαΔ6q13myr to translate basal Gα12/13 signalling of GPR56 to a Gαq readout in transcription factor luciferase reporter systems and show that the established peptide ligands (P7 and P19) function to enhance this signal. We further demonstrate the ability to directly influence the generation of second messengers in inositol-3-phosphate assays. In the future, these chimeric G proteins could facilitate basic functional studies, drug screenings and deorphanization of other aGPCRs.

Single-molecule FRET imaging of GPCR dimers in living cells.

Class C G protein-coupled receptors (GPCRs) are known to form stable homodimers or heterodimers critical for function, but the oligomeric status of class A and B receptors, which constitute >90% of all GPCRs, remains hotly debated. Single-molecule fluorescence resonance energy transfer (smFRET) is a powerful approach with the potential to reveal valuable insights into GPCR organization but has rarely been used in living cells to study protein systems. Here, we report generally applicable methods for using smFRET to detect and track transmembrane proteins diffusing within the plasma membrane of mammalian cells. We leverage this in-cell smFRET approach to show agonist-induced structural dynamics within individual metabotropic glutamate receptor dimers. We apply these methods to representative class A, B and C receptors, finding evidence for receptor monomers, density-dependent dimers and constitutive dimers, respectively.

Novel Fluorescent Ligands Enable Single-Molecule Localization Microscopy of the Dopamine Transporter.

The dopamine transporter (DAT) is critical for spatiotemporal control of dopaminergic neurotransmission and is the target for therapeutic agents, including ADHD medications, and abused substances, such as cocaine. Here, we develop new fluorescently labeled ligands that bind DAT with high affinity and enable single-molecule detection of the transporter. The cocaine analogue MFZ2-12 (1) was conjugated to novel rhodamine-based Janelia Fluorophores (JF549 and JF646). High affinity binding of the resulting ligands to DAT was demonstrated by potent inhibition of [3H]dopamine uptake in DAT transfected CAD cells and by competition radioligand binding experiments on rat striatal membranes. Visualization of binding was substantiated by confocal or TIRF microscopy revealing selective binding of the analogues to DAT transfected CAD cells. Single particle tracking experiments were performed with JF549-conjugated DG3-80 (3) and JF646-conjugated DG4-91 (4) on DAT transfected CAD cells enabling quantification and categorization of the dynamic behavior of DAT into four distinct motion classes (immobile, confined, Brownian, and directed). Finally, we show that the ligands can be used in direct stochastic optical reconstruction microscopy (dSTORM) experiments permitting further analyses of DAT distribution on the nanoscale. In summary, these novel fluorescent ligands are promising new tools for studying DAT localization and regulation with single-molecule resolution.

G12/13 is activated by acute tethered agonist exposure in the adhesion GPCR ADGRL3.

The adhesion G-protein-coupled receptor (GPCR) latrophilin 3 (ADGRL3) has been associated with increased risk of attention deficit hyperactivity disorder (ADHD) and substance use in human genetic studies. Knockdown in multiple species leads to hyperlocomotion and altered dopamine signaling. Thus, ADGRL3 is a potential target for treatment of neuropsychiatric disorders that involve dopamine dysfunction, but its basic signaling properties are poorly understood. Identification of adhesion GPCR signaling partners has been limited by a lack of tools to acutely activate these receptors in living cells. Here, we design a novel acute activation strategy to characterize ADGRL3 signaling by engineering a receptor construct in which we could trigger acute activation enzymatically. Using this assay, we found that ADGRL3 signals through G12/G13 and Gq, with G12/13 the most robustly activated. Gα12/13 is a new player in ADGRL3 biology, opening up unexplored roles for ADGRL3 in the brain. Our methodological advancements should be broadly useful in adhesion GPCR research.

Agonist-induced formation of unproductive receptor-G12 complexes.

G proteins are activated when they associate with G protein-coupled receptors (GPCRs), often in response to agonist-mediated receptor activation. It is generally thought that agonist-induced receptor-G protein association necessarily promotes G protein activation and, conversely, that activated GPCRs do not interact with G proteins that they do not activate. Here we show that GPCRs can form agonist-dependent complexes with G proteins that they do not activate. Using cell-based bioluminescence resonance energy transfer (BRET) and luminescence assays we find that vasopressin V2 receptors (V2R) associate with both Gs and G12 heterotrimers when stimulated with the agonist arginine vasopressin (AVP). However, unlike V2R-Gs complexes, V2R-G12 complexes are not destabilized by guanine nucleotides and do not promote G12 activation. Activating V2R does not lead to signaling responses downstream of G12 activation, but instead inhibits basal G12-mediated signaling, presumably by sequestering G12 heterotrimers. Overexpressing G12 inhibits G protein receptor kinase (GRK) and arrestin recruitment to V2R and receptor internalization. Formyl peptide (FPR1 and FPR2) and Smoothened (Smo) receptors also form complexes with G12 that are insensitive to nucleotides, suggesting that unproductive GPCR-G12 complexes are not unique to V2R. These results indicate that agonist-dependent receptor-G protein association does not always lead to G protein activation and may in fact inhibit G protein activation.

Publisher Correction: G12/13 is activated by acute tethered agonist exposure in the adhesion GPCR ADGRL3.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Single Proteoliposome High-Content Analysis Reveals Differences in the Homo-Oligomerization of GPCRs.

G-protein-coupled receptors (GPCRs) control vital cellular signaling pathways. GPCR oligomerization is proposed to increase signaling diversity. However, many reports have arrived at disparate conclusions regarding the existence, stability, and stoichiometry of GPCR oligomers, partly because of cellular complexity and ensemble averaging of intrareconstitution heterogeneities that complicate the interpretation of oligomerization data. To overcome these limitations, we exploited fluorescence-microscopy-based high-content analysis of single proteoliposomes. This allowed multidimensional quantification of intrinsic monomer-monomer interactions of three class A GPCRs (ß2-adrenergic receptor, cannabinoid receptor type 1, and opsin). Using a billion-fold less protein than conventional assays, we quantified oligomer stoichiometries, association constants, and the influence of two ligands and membrane curvature on oligomerization, revealing key similarities and differences for three GPCRs with decidedly different physiological functions. The assays introduced here will assist with the quantitative experimental observation of oligomerization for transmembrane proteins in general.

Single-molecule analysis of ligand efficacy in ß2AR-G-protein activation.

G-protein-coupled receptor (GPCR)-mediated signal transduction is central to human physiology and disease intervention, yet the molecular mechanisms responsible for ligand-dependent signalling responses remain poorly understood. In class A GPCRs, receptor activation and G-protein coupling entail outward movements of transmembrane helix 6 (TM6). Here, using single-molecule fluorescence resonance energy transfer imaging, we examine TM6 movements in the ß2 adrenergic receptor (ß2AR) upon exposure to orthosteric ligands with different efficacies, in the absence and presence of the Gs heterotrimer. We show that partial and full agonists differentially affect TM6 motions to regulate the rate at which GDP-bound ß2AR-Gs complexes are formed and the efficiency of nucleotide exchange leading to Gs activation. These data also reveal transient nucleotide-bound ß2AR-Gs species that are distinct from known structures, and provide single-molecule perspectives on the allosteric link between ligand- and nucleotide-binding pockets that shed new light on the G-protein activation mechanism.

Electronic tuning of self-healing fluorophores for live-cell and single-molecule imaging.

Bright, long-lasting organic fluorophores enable a broad range of imaging applications. "Self-healing" fluorophores, in which intra-molecularly linked protective agents quench photo-induced reactive species, exhibit both enhanced photostability and biological compatibility. However, the self-healing strategy has yet to achieve its predicted potential, particularly in the presence of ambient oxygen where live-cell imaging studies must often be performed. To identify key bottlenecks in this technology that can be used to guide further engineering developments, we synthesized a series of Cy5 derivatives linked to the protective agent cyclooctatetraene (COT) and examined the photophysical mechanisms curtailing their performance. The data obtained reveal that the photostability of self-healing fluorophores is limited by reactivity of the COT protective agent. The addition of electron withdrawing substituents to COT reduced its susceptibility to reactions with molecular oxygen and the fluorophore to which it is attached and increased its capacity to participate in triplet energy transfer. Exploiting these insights, we designed and synthesized a suite of modified COT-fluorophores spanning the visible spectrum that exhibited markedly increased intra-molecular photostabilization. Under ambient oxygen conditions, the photostability of Cy3 and Cy5 fluorophore derivatives increased by 3- and 9-fold in vitro and by 2- and 6-fold in living cells, respectively. We further show that this approach can improve a silicon rhodamine fluorophore. These findings offer a clear strategy for achieving the full potential of the self-healing strategy and its application to the gamut of fluorophore species commonly used for biomedical imaging.

CALM regulates clathrin-coated vesicle size and maturation by directly sensing and driving membrane curvature.

The size of endocytic clathrin-coated vesicles (CCVs) is remarkably uniform, suggesting that it is optimized to achieve the appropriate levels of cargo and lipid internalization. The three most abundant proteins in mammalian endocytic CCVs are clathrin and the two cargo-selecting, clathrin adaptors, CALM and AP2. Here we demonstrate that depletion of CALM causes a substantial increase in the ratio of "open" clathrin-coated pits (CCPs) to "necked"/"closed" CCVs and a doubling of CCP/CCV diameter, whereas AP2 depletion has opposite effects. Depletion of either adaptor, however, significantly inhibits endocytosis of transferrin and epidermal growth factor. The phenotypic effects of CALM depletion can be rescued by re-expression of wild-type CALM, but not with CALM that lacks a functional N-terminal, membrane-inserting, curvature-sensing/driving amphipathic helix, the existence and properties of which are demonstrated. CALM is thus a major factor in controlling CCV size and maturation and hence in determining the rates of endocytic cargo uptake.

Nanoscale high-content analysis using compositional heterogeneities of single proteoliposomes.

Proteoliposome reconstitution is a standard method to stabilize purified transmembrane proteins in membranes for structural and functional assays. Here we quantified intrareconstitution heterogeneities in single proteoliposomes using fluorescence microscopy. Our results suggest that compositional heterogeneities can severely skew ensemble-average proteoliposome measurements but also enable ultraminiaturized high-content screens. We took advantage of this screening capability to map the oligomerization energy of the ß2-adrenergic receptor using ∼10(9)-fold less protein than conventional assays.