GPCRs constitute the largest family of membrane proteins in mammals, comprising over 800 members [1], [2]. These receptors play essential roles in regulating diverse physiological processes by converting extracellular stimuli—such as neurotransmitters, hormones, and sensory stimuli—into intracellular signaling events. Through this mechanism, GPCRs contribute to a wide range of fundamental biological functions, including vision, taste, immune modulation, cardiovascular homeostasis, etc. [3], [4], [5], [6]. Dysregulation of GPCR signaling is implicated in numerous diseases such as cancer, metabolic disorders, and neurological conditions, highlighting their central roles in both normal physiology and disease pathogenesis [7], [8]. These have made GPCRs major targets in therapeutic drug development, with over 30% of marketed drugs targeting these receptors [9], [10]. Understanding of the molecular mechanisms underlying GPCR pharmacology and signal transduction is essential for rational design of therapeutics.

The human GPCRs are primarily classified into five subfamilies based on sequence homology, structural characteristics, and functional properties, including class A (rhodopsin-like receptors), class B (secretin/adhesion receptors), class C (metabotropic glutamate receptors), class F (frizzled/smoothened receptors) and class T (taste2 receptors) [11], [12]. Despite a conserved structural architecture characterized by seven transmembrane helices connected by three extracellular loops (ECL1-3) and three intracellular loops (ICL1-3), different GPCR families exhibit distinct structural features and ligand binding preferences [13], [14], [15]. These structural differences contribute to the functional diversity of GPCRs, allowing them to mediate a wide range of physiological processes. Class A receptors constitute the largest subfamily, capable of binding a diverse range of ligands, including aminergic, peptide, protein, lipid, melatonin, nucleotide, steroid, alicarboxylic acid, and odorants [1], [16], [17]. Class B GPCRs are characterized by a unique extracellular N-terminal domain and are subdivided into class B1 (secretin receptors) and class B2 (adhesion receptors). Among the class B1 receptors, the secretin receptors such as glucagon-like peptide 1 receptor (GLP-1R), gastric inhibitory polypeptide receptor (GIPR), and glucagon receptor (GCGR) have emerged as notable due to their significant roles in obesity and diabetes [18], [19]. Class B2 receptors are primarily involved in cell adhesion and are largely considered orphan receptors, as their endogenous ligands remain unidentified [20], [21]. Class C GPCRs exert their functions by forming constitutive homodimers or heterodimers, which are essential for relaying the agonist-induced signaling from the extracellular domain of the receptor to its transmembrane domain [22], [23], [24]. Class F GPCRs include frizzled receptors (FZD) and smoothened receptors (SMO), which are primarily involved in the Wnt and Hedgehog signaling pathways [25]. Class T GPCRs are classified separately due to their specific involvement in taste perception, and their role in specialized gustatory signaling pathways [5]. These differential functional and structural properties add diversity to the physiological and pathological mechanisms of the GPCR superfamily.

GPCRs undergo complex and dynamic signal transduction processes, which further increase complexity of the functional modulation of these receptors. In human cells, two major classes of transducer proteins mediate these signals: G proteins and arrestins. G proteins are classified into four major families—Gs, Gi/o, Gq, and G12/13—each of which plays a distinct role in regulating cellular responses through different downstream signaling pathways [26], [27]. In the resting state, G proteins exist as a heterotrimer composed of Gα, Gβ, and Gγ subunits, with Gα bound to guanosine diphosphate (GDP). Upon agonist binding, the receptor undergoes a conformational change, which triggers G protein coupling and activates the associated G protein by promoting the exchange of GDP for guanosine triphosphate (GTP) in the Gα subunit. Once G protein is activated, the Gα subunit dissociates from the Gβγ dimer. Both the Gα and Gβγ subunits can then initiate downstream signaling events, such as the activation of adenylyl cyclase (AC), phospholipase C (PLC), and the production of cyclic AMP (cAMP). To terminate G protein signaling, GPCRs undergo desensitization through phosphorylation by G protein-coupled receptor kinases (GRKs). The phosphorylated receptors then bind arrestins, which prevent further G protein activation and promote receptor internalization through endocytosis. Beyond this, arrestins also function as signaling molecules, activating pathways such as mitogen-activated protein (MAP) kinases (e.g., ERK1/2), JNK, p38, and RhoA. There are seven GRK subtypes (GRK1-7) and four arrestin isoforms (two visual, arrestin-1 and arrestin-4, and two non-visual, β-arrestin-1 and β-arrestin-2), each likely associated with unique regulatory mechanisms [28], [29]. With the recent rapid advancements in cryo-electron microscopy (cryo-EM), a wealth of molecular information regarding GPCR signal transduction has been uncovered [30], [31]. However, investigation of the diverse roles of the transducers in regulating various physiological processes of GPCRs still remains challenging.

To systematically study the signal transduction and modulation mechanisms of GPCRs, which are key for understanding their physiological and pathological functions and developing novel therapeutic strategies, our laboratory, together with collaborators, has established an integrated GPCR research platform by combining structural biology, molecular dynamics, cellular analyses, and drug discovery. Using this platform, we have made great efforts to elucidate how the diverse array of ligands recognize and regulate the GPCRs and how different types of receptors exert their functions through differential modulation pathways. Our findings not only provide valuable insights into GPCR signaling landscape, but also offer new opportunities for drug discovery. The GPCR structures solved by our group are summarized in Table 1.