Probing novel GPCR interactions using a combination of FRET and TIRF

Detecting and measuring of GPCR signaling - comparison of human induced pluripotent stem cells and immortal cell lines

G protein-coupled receptors (GPCRs) are a large family of transmembrane proteins that play a major role in many physiological processes, and thus GPCR-targeted drug development has been widely promoted. Although research findings generated in immortal cell lines have contributed to the advancement of the GPCR field, the homogenous genetic backgrounds, and the overexpression of GPCRs in these cell lines make it difficult to correlate the results with clinical patients. Human induced pluripotent stem cells (hiPSCs) have the potential to overcome these limitations, because they contain patient specific genetic information and can differentiate into numerous cell types. To detect GPCRs in hiPSCs, highly selective labeling and sensitive imaging techniques are required. This review summarizes existing resonance energy transfer and protein complementation assay technologies, as well as existing and new labeling methods. The difficulties of extending existing detection methods to hiPSCs are discussed, as well as the potential of hiPSCs to expand GPCR research towards personalized medicine.

Phosphatidylinositol 4,5-bisphosphate (PIP2) regulates KCNQ3 K+ channels by interacting with four cytoplasmic channel domains

Phosphatidylinositol 4,5-bisphosphate (PIP₂) in the plasma membrane regulates the function of many ion channels, including M-type (potassium voltage-gated channel subfamily Q member (KCNQ), K_v7) K⁺ channels; however, the molecular mechanisms involved remain unclear. To this end, we here focused on the KCNQ3 subtype that has the highest apparent affinity for PIP₂ and performed extensive mutagenesis in regions suggested to be involved in PIP₂ interactions among the KCNQ family. Using perforated patch-clamp recordings of heterologously transfected tissue culture cells, total internal reflection fluorescence microscopy, and the zebrafish (Danio rerio) voltage-sensitive phosphatase to deplete PIP₂ as a probe, we found that PIP₂ regulates KCNQ3 channels through four different domains: 1) the A-B helix linker that we previously identified as important for both KCNQ2 and KCNQ3, 2) the junction between S6 and the A helix, 3) the S2-S3 linker, and 4) the S4-S5 linker. We also found that the apparent strength of PIP₂ interactions within any of these domains was not coupled to the voltage dependence of channel activation. Extensive homology modeling and docking simulations with the WT or mutant KCNQ3 channels and PIP₂ were consistent with the experimental data. Our results indicate that PIP₂ modulates KCNQ3 channel function by interacting synergistically with a minimum of four cytoplasmic domains.

Toward fluorescent probes for G-protein-coupled receptors (GPCRs)

G-protein-coupled receptors (GPCRs), a superfamily of cell-surface receptors that are the targets of about 40% of prescription drugs on the market, can sense numerous critical extracellular signals. Recent breakthroughs in structural biology, especially in holo-form X-ray crystal structures, have contributed to our understanding of GPCR signaling. However, actions of GPCRs at the cellular and molecular level, interactions between GPCRs, and the role of protein dynamics in receptor activities still remain controversial. To overcome these dilemmas, fluorescent probes of GPCRs have been employed, which have advantages of in vivo safety and real-time monitoring. Various probes that depend on specific mechanisms and/or technologies have been used to study GPCRs. The present review focuses on surveying the design and applications of fluorescent probes for GPCRs that are derived from small molecules or using protein-labeling techniques, as well as discussing some design strategies for new probes.

Genetically induced dysfunctions of Kir2.1 channels: implications for short QT3 syndrome and autism-epilepsy phenotype

Short QT3 syndrome (SQT3S) is a cardiac disorder characterized by a high risk of mortality and associated with mutations in Kir2.1 (KCNJ2) channels. The molecular mechanisms leading to channel dysfunction, cardiac rhythm disturbances and neurodevelopmental disorders, potentially associated with SQT3S, remain incompletely understood. Here, we report on monozygotic twins displaying a short QT interval on electrocardiogram recordings and autism-epilepsy phenotype. Genetic screening identified a novel KCNJ2 variant in Kir2.1 that (i) enhanced the channel's surface expression and stability at the plasma membrane, (ii) reduced protein ubiquitylation and degradation, (iii) altered protein compartmentalization in lipid rafts by targeting more channels to cholesterol-poor domains and (iv) reduced interactions with caveolin 2. Importantly, our study reveals novel physiological mechanisms concerning wild-type Kir2.1 channel processing by the cell, such as binding to both caveolin 1 and 2, protein degradation through the ubiquitin-proteasome pathway; in addition, it uncovers a potential multifunctional site that controls Kir2.1 surface expression, protein half-life and partitioning to lipid rafts. The reported mechanisms emerge as crucial also for proper astrocyte function, suggesting the need for a neuropsychiatric evaluation in patients with SQT3S and offering new opportunities for disease management.

Uniform total internal reflection fluorescence illumination enables live cell fluorescence resonance energy transfer microscopy

Fluorescence resonance energy transfer (FRET) microscopy is a powerful technique to quantify dynamic protein-protein interactions in live cells. Total internal reflection fluorescence (TIRF) microscopy can selectively excite molecules within about 150 nm of the glass-cell interface. Recently, these two approaches were combined to enable high-resolution FRET imaging on the adherent surface of living cells. Here, we show that interference fringing of the coherent laser excitation used in TIRF creates lateral heterogeneities that impair quantitative TIRF-FRET measurements. We overcome this limitation by using a two-dimensional scan head to rotate laser beams for donor and acceptor excitation around the back focal plane of a high numerical aperture objective. By setting different radii for the circles traced out by each laser in the back focal plane, the penetration depth was corrected for different wavelengths. These modifications quell spatial variations in illumination and permit calibration for quantitative TIRF-FRET microscopy. The capability of TIRF-FRET was demonstrated by imaging assembled cyan and yellow fluorescent protein-tagged HIV-Gag molecules in single virions on the surfaces of living cells. These interactions are shown to be distinct from crowding of HIV-Gag in lipid rafts.

Fluorescence resonance energy transfer-based technologies in the study of protein-protein interactions at the cell surface

Understanding of the molecular mechanisms of protein-protein interactions (PPIs) at the cell surface of living cells is fundamental to comprehend the functional meaning of a large number of cellular processes. Here we discuss how new methodological strategies derived from non-invasive fluorescence-based approaches (i.e. fluorescence resonance energy transfer, FRET) have been successfully developed to characterize plasma membrane PPIs. Importantly, these technologies alone - or in concert with complementary methods (i.e. SNAP-tag/TR-FRET, TIRF/FRET) - can become extremely powerful approaches for visualizing cell surface PPIs, even between more than two proteins and also in native tissues. Interestingly, these methods would also be relevant in drug discovery in order to develop new high-throughput screening approaches or to identify new therapeutic targets. Accordingly, herein we provide a thorough assessment on all biotechnological aspects, including strengths and weaknesses, of these fluorescence-based methodologies when applied in the study of PPIs occurring at the cell surface of living cells.

Contributions of fluorescence techniques to understanding G protein-coupled receptor dimerisation

G protein-coupled receptors (GPCRs) are the largest class of eukaryotic cell-surface receptors and, over the last decade, it has become clear that they are capable of dimerisation. Whilst many biochemical and biophysical approaches have been used to study dimerisation, fluorescence techniques, including Förster resonance energy transfer and single molecule fluorescence, have been key players. Here we review recent contributions of fluorescence techniques to investigate GPCR dimers, including dimerisation in cell membranes and native tissues, the effect of ligand binding on dimerisation and the kinetics of dimer formation and dissociation. The challenges of studying multicomponent membrane protein systems have led to the development and refinement of many fluorescence assays, allowing the functional consequences of receptor dimerisation to be investigated and individual protein molecules to be imaged in the membranes of living cells. It is likely that the fluorescence techniques described here will be of use for investigating many other multicomponent membrane protein systems.