FLIM-FRET-Based Structural Characterization of a Class-A GPCR Dimer in the Cell Membrane

Dimerisation of the VIP receptor VIPR2 is essential to its binding VIP and Gαi proteins, and to its functions in breast cancer cells

BACKGROUND AND PURPOSE

Several G protein-coupled receptors (GPCRs) are known to homodimerise. Dimeric GPCRs may have different properties from their monomers, but the molecular basis and functional significance of GPCR dimerisation remain largely unknown. We recently found that signalling by the vasoactive intestinal peptide receptor, VIPR2, regulates breast cancer cell migration and proliferation. However, it is unclear whether VIPR2 monomers directly interact with each other and what function the dimeric receptor has. Here, we showed that VIPR2 dimerises and investigated their role in breast cancer progression.

EXPERIMENTAL APPROACH

Dimerisation of VIPR2 was assessed by fluorescence resonance energy transfer (FRET) analysis and a pull-down assay. Breast cancer progression was analysed by orthotopic growth and metastasis of human breast cancers into proper axillary and subiliac lymph-nodes in mice.

KEY RESULTS

VIPR2 monomers directly interacted with each other through transmembrane domains (TM)3-4. FRET analysis revealed that VIPR2 moved further apart in cells expressing TM3-4-peptides, suggesting that TM3-4 prevents VIPR2 dimerisation. Breast cancer cells stably expressing TM3-4 region exhibited suppressed tumour growth and lymph-node metastasis. Furthermore, ligand-receptor binding assays revealed that VIP-FITC bound to cells dose-dependently, and VIPR2 de-dimerisation by TM3-4 expression decreased VIP's affinity to cells. Additionally, TM3-4 expression decreased Gαi-VIPR2 interactions.

CONCLUSION AND IMPLICATIONS

Dimeric VIPR2 forms the minimal functional unit that effectively promotes growth and metastasis of breast cancer. Therefore, dimeric VIPR2 is a potential therapeutic target for breast cancer, and TM3-4-peptides are potential anti-cancer drug candidates to suppress cancer progression.

ChIP-seq and structural analyses delineating the regulatory mechanism of master regulator EsrB in Edwardsiella piscicida

As a response regulator of the EsrA-EsrB two-component system, EsrB is conserved in Hafniaceae and plays a crucial role in virulence and pathogenicity. EsrB possesses DNA binding abilities, enabling it to regulate the transcription of virulence genes to confront different stresses and achieve systematic infections. Here, ChIP-seq analysis of EsrB in Dulbecco's Modified Eagle's Medium (DMEM) (mimicking in vivo environments) revealed that EsrB preferred to bind to virulence-associated promoters with a distinct 7'-4-7'' pseudopalindromic DNA motif and interact with metabolic-related promoters with a high AT DNA motif. The crystal structure of the C-terminal of EsrB (EsrBC) was solved at 2.20-Å resolution. Specifically, Lys181 enabled the DNA-binding affinity of EsrB and promoted the in vitro and in vivo pathogenicity of Edwardsiella piscicida. Moreover, EsrB directly regulated the expression of genes associated with basal metabolism, including iron and tricarboxylic acid (TCA) cycles. Furthermore, EsrB enhanced iron transport capability and the enzyme activity of succinate dehydrogenase and pyruvate dehydrogenase in DMEM. Collectively, our structural and ChIP-seq analysis provides valuable insights into the DNA binding mechanism of EsrB which will facilitate our understanding of EsrB coordinating virulence and basal metabolism gene expression.

IMPORTANCE

As a crucial virulence regulator, EsrB possesses a LuxR-like superfamily domain at the C-terminal, which is conserved within the canonical NarL family regulators. Due to its critically important role in virulence and pathogenicity in fish hosts, the DNA binding ability has been believed to allow EsrB to regulate genes associated with the invasion process of host cells and basal metabolism in response to environmental stimuli. The lack of EsrB's crystal structure has been a major obstacle in understanding the molecular mechanisms of EsrB-DNA interaction which choreographs EsrB-mediated pathogenic behavior. Here, we conducted ChIP-seq and solved the crystal structure of the C-terminal of EsrB (EsrBC) at 2.20-Å resolution, which revealed that EsrB preferred to bind to virulence-associated promoters with a distinct 7'-4-7' pseudopalindromic DNA motif and interacted with metabolic-related promoters with a high AT DNA motif in Dulbecco's Modified Eagle's Medium (DMEM) (mimicking in vivo environments). Our results facilitate a detailed understanding of EsrB's regulatory role in Edwardsiella piscicida pathogenesis and expand our knowledge of virulence regulators in the family Hafniaceae.

Mutation in the Kinase Domain Alters the VEGFR2 Membrane Dynamics

BACKGROUND

Recently, the substitution R1051Q in VEGFR2 has been described as a cancer-associated "gain of function" mutation. VEGFR2R1051Q phosphorylation is ligand-independent and enhances the activation of intracellular pathways and cell growth both in vitro and in vivo. In cancer, this mutation is found in heterozygosity, suggesting that an interaction between VEGFR2R1051Q and VEGFR2WT may occur and could explain, at least in part, how VEGFR2R1051Q acts to promote VEGFR2 signaling. Despite this, the biochemical/biophysical mechanism of the activation of VEGFR2R1051Q remains poorly understood. On these bases, the aim of our study is to address how VEGFR2R1051Q influences the biophysical behavior (dimerization and membrane dynamics) of the co-expressed VEGFR2WT.

METHODS

We employed quantitative FLIM/FRET and FRAP imaging techniques using CHO cells co-transfected with the two forms of VEGFR2 to mimic heterozygosity.

RESULTS

Membrane protein biotinylation reveals that VEGFR2WT is more exposed on the cell membrane with respect to VEGFR2R1051Q. The imaging analyses show the ability of VEGFR2WT to form heterodimers with VEGFR2R1051Q and this interaction alters its membrane dynamics. Indeed, when the co-expression of VEGFR2WT/VEGFR2R1051Q occurs, VEGFR2WT shows reduced lateral motility and a minor pool of mobile fraction.

CONCLUSIONS

This study demonstrates that active VEGFR2R1051Q can affect the membrane behavior of the VEGFR2WT.

Structures of human γδ T cell receptor-CD3 complex

Gamma delta (γδ) T cells, a unique T cell subgroup, are crucial in various immune responses and immunopathology1-3. The γδ T cell receptor (TCR), which is generated by γδ T cells, recognizes a diverse range of antigens independently of the major histocompatibility complex2. The γδ TCR associates with CD3 subunits, initiating T cell activation and holding great potential in immunotherapy4. Here we report the structures of two prototypical human Vγ9Vδ2 and Vγ5Vδ1 TCR-CD3 complexes5,6, revealing two distinct assembly mechanisms that depend on Vγ usage. The Vγ9Vδ2 TCR-CD3 complex is monomeric, with considerable conformational flexibility in the TCRγ-TCRδ extracellular domain and connecting peptides. The length of the connecting peptides regulates the ligand association and T cell activation. A cholesterol-like molecule wedges into the transmembrane region, exerting an inhibitory role in TCR signalling. The Vγ5Vδ1 TCR-CD3 complex displays a dimeric architecture, whereby two protomers nestle back to back through the Vγ5 domains of the TCR extracellular domains. Our biochemical and biophysical assays further corroborate the dimeric structure. Importantly, the dimeric form of the Vγ5Vδ1 TCR is essential for T cell activation. These findings reveal organizing principles of the γδ TCR-CD3 complex, providing insights into the unique properties of γδ TCR and facilitating immunotherapeutic interventions.

Membrane Heteroreceptor Complexes as Second-Order Protein Modulators: A Novel Integrative Mechanism through Allosteric Receptor-Receptor Interactions

Bioluminescence and fluorescence resonance energy transfer (BRET and FRET) together with the proximity ligation method revealed the existence of G-protein-coupled receptors, Ionotropic and Receptor tyrosine kinase heterocomplexes, e.g., A2AR-D2R, GABAA-D5R, and FGFR1-5-HT1AR heterocomplexes. Molecular integration takes place through allosteric receptor-receptor interactions in heteroreceptor complexes of synaptic and extra-synaptic regions. It involves the modulation of receptor protomer recognition, signaling and trafficking, as well as the modulation of behavioral responses. Allosteric receptor-receptor interactions in hetero-complexes give rise to concepts like meta-modulation and protein modulation. The introduction of receptor-receptor interactions was the origin of the concept of meta-modulation provided by Katz and Edwards in 1999, which stood for the fine-tuning or modulation of nerve cell transmission. In 2000-2010, Ribeiro and Sebastiao, based on a series of papers, provided strong support for their view that adenosine can meta-modulate (fine-tune) synaptic transmission through adenosine receptors. However, another term should also be considered: protein modulation, which is the key feature of allosteric receptor-receptor interactions leading to learning and consolidation by novel adapter proteins to memory. Finally, it must be underlined that allosteric receptor-receptor interactions and their involvement both in brain disease and its treatment are of high interest. Their pathophysiological relevance has been obtained, especially for major depressive disorder, cocaine use disorder, and Parkinson's disease.

Xplor-NIH: Better parameters and protocols for NMR protein structure determination

The present work describes an update to the protein covalent geometry and atomic radii parameters in the Xplor-NIH biomolecular structure determination package. In combination with an improved treatment of selected non-bonded interactions between atoms three bonds apart, such as those involving methyl hydrogens, and a previously developed term that affects the system's gyration volume, the new parameters are tested using structure calculations on 30 proteins with restraints derived from nuclear magnetic resonance data. Using modern structure validation criteria, including several formally adopted by the Protein Data Bank, and a clear measure of structural accuracy, the results show superior performance relative to previous Xplor-NIH implementations. Additionally, the Xplor-NIH structures compare favorably against originally determined NMR models.

Progress, Challenges and Opportunities of NMR and XL-MS for Cellular Structural Biology

The validity of protein structures and interactions, whether determined under ideal laboratory conditions or predicted by AI tools such as Alphafold2, to precisely reflect those found in living cells remains to be examined. Moreover, understanding the changes in protein structures and interactions in response to stimuli within living cells, under both normal and disease conditions, is key to grasping proteins' functionality and cellular processes. Nevertheless, achieving high-resolution identification of these protein structures and interactions within living cells presents a technical challenge. In this Perspective, we summarize the recent advancements in in-cell nuclear magnetic resonance (NMR) and in vivo cross-linking mass spectrometry (XL-MS) for studying protein structures and interactions within a cellular context. Additionally, we discuss the challenges, opportunities, and potential benefits of integrating in-cell NMR and in vivo XL-MS in future research to offer an exhaustive approach to studying proteins in their natural habitat.

Microscopy and spectroscopy approaches to study GPCR structure and function

The GPCR signalling cascade is a key pathway responsible for the signal transduction of a multitude of physical and chemical stimuli, including light, odorants, neurotransmitters and hormones. Understanding the structural and functional properties of the GPCR cascade requires direct observation of signalling processes in high spatial and temporal resolution, with minimal perturbation to endogenous systems. Optical microscopy and spectroscopy techniques are uniquely suited to this purpose because they excel at multiple spatial and temporal scales and can be used in living objects. Here, we review recent developments in microscopy and spectroscopy technologies which enable new insights into GPCR signalling. We focus on advanced techniques with high spatial and temporal resolution, single-molecule methods, labelling strategies and approaches suitable for endogenous systems and large living objects. This review aims to assist researchers in choosing appropriate microscopy and spectroscopy approaches for a variety of applications in the study of cellular signalling.

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.

Class A and C GPCR Dimers in Neurodegenerative Diseases

Neurodegenerative diseases affect over 30 million people worldwide with an ascending trend. Most individuals suffering from these irreversible brain damages belong to the elderly population, with onset between 50 and 60 years. Although the pathophysiology of such diseases is partially known, it remains unclear upon which point a disease turns degenerative. Moreover, current therapeutics can treat some of the symptoms but often have severe side effects and become less effective in long-term treatment. For many neurodegenerative diseases, the involvement of G proteincoupled receptors (GPCRs), which are key players of neuronal transmission and plasticity, has become clearer and holds great promise in elucidating their biological mechanism. With this review, we introduce and summarize class A and class C GPCRs, known to form heterodimers or oligomers to increase their signalling repertoire. Additionally, the examples discussed here were shown to display relevant alterations in brain signalling and had already been associated with the pathophysiology of certain neurodegenerative diseases. Lastly, we classified the heterodimers into two categories of crosstalk, positive or negative, for which there is known evidence.

Lipid-Protein Interactions in Plasma Membrane Organization and Function

Lipid-protein interactions in cells are involved in various biological processes, including metabolism, trafficking, signaling, host-pathogen interactions, and transmembrane transport. At the plasma membrane, lipid-protein interactions play major roles in membrane organization and function. Several membrane proteins have motifs for specific lipid binding, which modulate protein conformation and consequent function. In addition to such specific lipid-protein interactions, protein function can be regulated by the dynamic, collective behavior of lipids in membranes. Emerging analytical, biochemical, and computational technologies allow us to study the influence of specific lipid-protein interactions, as well as the collective behavior of membranes on protein function. In this article, we review the recent literature on lipid-protein interactions with a specific focus on the current state-of-the-art technologies that enable novel insights into these interactions.

Structural Characterization of Receptor-Receptor Interactions in the Allosteric Modulation of G Protein-Coupled Receptor (GPCR) Dimers

G protein-coupled receptor (GPCR) oligomerization, while contentious, continues to attract the attention of researchers. Numerous experimental investigations have validated the presence of GPCR dimers, and the relevance of dimerization in the effectuation of physiological functions intensifies the attractiveness of this concept as a potential therapeutic target. GPCRs, as a single entity, have been the main source of scrutiny for drug design objectives for multiple diseases such as cancer, inflammation, cardiac, and respiratory diseases. The existence of dimers broadens the research scope of GPCR functions, revealing new signaling pathways that can be targeted for disease pathogenesis that have not previously been reported when GPCRs were only viewed in their monomeric form. This review will highlight several aspects of GPCR dimerization, which include a summary of the structural elucidation of the allosteric modulation of class C GPCR activation offered through recent solutions to the three-dimensional, full-length structures of metabotropic glutamate receptor and γ-aminobutyric acid B receptor as well as the role of dimerization in the modification of GPCR function and allostery. With the growing influence of computational methods in the study of GPCRs, we will also be reviewing recent computational tools that have been utilized to map protein-protein interactions (PPI).

A Guide to Fluorescence Lifetime Microscopy and Förster's Resonance Energy Transfer in Neuroscience

Fluorescence lifetime microscopy (FLIM) and Förster's resonance energy transfer (FRET) are advanced optical tools that neuroscientists can employ to interrogate the structure and function of complex biological systems in vitro and in vivo using light. In neurobiology they are primarily used to study protein-protein interactions, to study conformational changes in protein complexes, and to monitor genetically encoded FRET-based biosensors. These methods are ideally suited to optically monitor changes in neurons that are triggered optogenetically. Utilization of this technique by neuroscientists has been limited, since a broad understanding of FLIM and FRET requires familiarity with the interactions of light and matter on a quantum mechanical level, and because the ultra-fast instrumentation used to measure fluorescent lifetimes and resonance energy transfer are more at home in a physics lab than in a biology lab. In this overview, we aim to help neuroscientists overcome these obstacles and thus feel more comfortable with the FLIM-FRET method. Our goal is to aid researchers in the neuroscience community to achieve a better understanding of the fundamentals of FLIM-FRET and encourage them to fully leverage its powerful ability as a research tool. Published 2020. U.S. Government.