The Nature of Functional Features of Different Classes of G-Protein-Coupled Receptors

Integrating molecular and cellular components of endothelial shear stress mechanotransduction

The lining of blood vessels is constantly exposed to mechanical forces exerted by blood flow against the endothelium. Endothelial cells detect these tangential forces (i.e., shear stress), initiating a host of intracellular signaling cascades that regulate vascular physiology. Thus, vascular health is tethered to the endothelial cells' capacity to transduce shear stress. Indeed, the mechanotransduction of shear stress underlies a variety of cardiovascular benefits, including some of those associated with increased physical activity. However, endothelial mechanotransduction is impaired in aging and disease states such as obesity and type 2 diabetes, precipitating the development of vascular disease. Understanding endothelial mechanotransduction of shear stress, and the molecular and cellular mechanisms by which this process becomes defective, is critical for the identification and development of novel therapeutic targets against cardiovascular disease. In this review, we detail the primary mechanosensitive structures that have been implicated in detecting shear stress, including junctional proteins such as platelet endothelial cell adhesion molecule-1 (PECAM-1), the extracellular glycocalyx and its components, and ion channels such as piezo1. We delineate which molecules are truly mechanosensitive and which may simply be indispensable for the downstream transmission of force. Furthermore, we discuss how these mechanosensors interact with other cellular structures, such as the cytoskeleton and membrane lipid rafts, which are implicated in translating shear forces to biochemical signals. Based on findings to date, we also seek to integrate these cellular and molecular mechanisms with a view of deciphering endothelial mechanotransduction of shear stress, a tenet of vascular physiology.

Exploring the Activation Mechanism of the GPR183 Receptor

The G protein-coupled receptors (GPCRs) play a pivotal role in numerous biological processes as crucial cell membrane receptors. However, the dynamic mechanisms underlying the activation of GPR183, a specific GPCR, remain largely elusive. To address this, we employed computational simulation techniques to elucidate the activation process and key events associated with GPR183, including conformational changes from inactive to active state, binding interactions with the Gi protein complex, and GDP release. Our findings demonstrate that the association between GPR183 and the Gi protein involves the formation of receptor-specific conformations, the gradual proximity of the Gi protein to the binding pocket, and fine adjustments of the protein conformation, ultimately leading to a stable GPR183-Gi complex characterized by a high energy barrier. The presence of Gi protein partially promotes GPR183 activation, which is consistent with the observation of GPCR constitutive activity test experiments, thus illustrating the reliability of our calculations. Moreover, our study suggests the existence of a stable partially activated state preceding complete activation, providing novel avenues for future investigations. In addition, the relevance of GPR183 for various diseases, such as colitis, the response of eosinophils to Mycobacterium tuberculosis infection, antiviral properties, and pulmonary inflammation, has been emphasized, underscoring its therapeutic potential. Consequently, understanding the activation process of GPR183 through molecular dynamic simulations offers valuable kinetic insights that can aid in the development of targeted therapies.

Exploring the activation mechanism of metabotropic glutamate receptor 2

Homomeric dimerization of metabotropic glutamate receptors (mGlus) is essential for the modulation of their functions and represents a promising avenue for the development of novel therapeutic approaches to address central nervous system diseases. Yet, the scarcity of detailed molecular and energetic data on mGlu2 impedes our in-depth comprehension of their activation process. Here, we employ computational simulation methods to elucidate the activation process and key events associated with the mGlu2, including a detailed analysis of its conformational transitions, the binding of agonists, Gi protein coupling, and the guanosine diphosphate (GDP) release. Our results demonstrate that the activation of mGlu2 is a stepwise process and several energy barriers need to be overcome. Moreover, we also identify the rate-determining step of the mGlu2's transition from the agonist-bound state to its active state. From the perspective of free-energy analysis, we find that the conformational dynamics of mGlu2's subunit follow coupled rather than discrete, independent actions. Asymmetric dimerization is critical for receptor activation. Our calculation results are consistent with the observation of cross-linking and fluorescent-labeled blot experiments, thus illustrating the reliability of our calculations. Besides, we also identify potential key residues in the Gi protein binding position on mGlu2, mGlu2 dimer's TM6-TM6 interface, and Gi α5 helix by the change of energy barriers after mutation. The implications of our findings could lead to a more comprehensive grasp of class C G protein-coupled receptor activation.

Molecular features of the ligand-free GLP-1R, GCGR and GIPR in complex with Gs proteins

Class B1 G protein-coupled receptors (GPCRs) are important regulators of many physiological functions such as glucose homeostasis, which is mainly mediated by three peptide hormones, i.e., glucagon-like peptide-1 (GLP-1), glucagon (GCG), and glucose-dependent insulinotropic polypeptide (GIP). They trigger a cascade of signaling events leading to the formation of an active agonist-receptor-G protein complex. However, intracellular signal transducers can also activate the receptor independent of extracellular stimuli, suggesting an intrinsic role of G proteins in this process. Here, we report cryo-electron microscopy structures of the human GLP-1 receptor (GLP-1R), GCG receptor (GCGR), and GIP receptor (GIPR) in complex with Gs proteins without the presence of cognate ligands. These ligand-free complexes share a similar intracellular architecture to those bound by endogenous peptides, in which, the Gs protein alone directly opens the intracellular binding cavity and rewires the extracellular orthosteric pocket to stabilize the receptor in a state unseen before. While the peptide-binding site is partially occupied by the inward folded transmembrane helix 6 (TM6)-extracellular loop 3 (ECL3) juncture of GIPR or a segment of GCGR ECL2, the extracellular portion of GLP-1R adopts a conformation close to the active state. Our findings offer valuable insights into the distinct activation mechanisms of these three important receptors. It is possible that in the absence of a ligand, the intracellular half of transmembrane domain is mobilized with the help of Gs protein, which in turn rearranges the extracellular half to form a transitional conformation, facilitating the entry of the peptide N-terminus.