6.1 G protein-coupled receptors

G protein-coupled receptors (GPCRs) are vital membrane proteins involved in various physiological processes. They share a common structure with seven transmembrane segments and are classified into five main families based on their sequence and function.

GPCRs transduce signals through G protein-dependent and independent pathways, with biased signaling offering potential for targeted therapies. Their activation and regulation involve complex mechanisms, making them crucial targets for drug development in numerous diseases.

Structure of GPCRs

• G protein-coupled receptors (GPCRs) are the largest family of membrane proteins in the human genome, playing crucial roles in various physiological processes and serving as major drug targets
• GPCRs share a common structural architecture consisting of seven transmembrane α-helices connected by alternating intracellular and extracellular loops
• The structure of GPCRs can be divided into three main domains: the extracellular domain, the transmembrane domain, and the intracellular domain

Extracellular domain

Top images from around the web for Extracellular domain

• 

  Frontiers | Metabolic Functions of G Protein-Coupled Receptors and β-Arrestin-Mediated Signaling ... View original

  Is this image relevant?

• 

  Frontiers | G Protein-Coupled Receptors in Taste Physiology and Pharmacology View original

  Is this image relevant?

• 

  Frontiers | G Protein-Coupled Receptors in Taste Physiology and Pharmacology View original

  Is this image relevant?

• 

  Frontiers | Metabolic Functions of G Protein-Coupled Receptors and β-Arrestin-Mediated Signaling ... View original

  Is this image relevant?

• 

  Frontiers | G Protein-Coupled Receptors in Taste Physiology and Pharmacology View original

  Is this image relevant?

1 of 3

Top images from around the web for Extracellular domain

• 

  Frontiers | Metabolic Functions of G Protein-Coupled Receptors and β-Arrestin-Mediated Signaling ... View original

  Is this image relevant?

• 

  Frontiers | G Protein-Coupled Receptors in Taste Physiology and Pharmacology View original

  Is this image relevant?

• 

  Frontiers | G Protein-Coupled Receptors in Taste Physiology and Pharmacology View original

  Is this image relevant?

• 

  Frontiers | Metabolic Functions of G Protein-Coupled Receptors and β-Arrestin-Mediated Signaling ... View original

  Is this image relevant?

• 

  Frontiers | G Protein-Coupled Receptors in Taste Physiology and Pharmacology View original

  Is this image relevant?

1 of 3

• Responsible for ligand recognition and binding
• Varies in size and composition among different GPCR families
• Can include N-terminal domain, extracellular loops, and ligand-binding pockets
• Plays a key role in determining ligand specificity and selectivity

Transmembrane domain

• Consists of seven α-helical transmembrane segments (TM1-TM7) arranged in a bundle
• Forms the core of the receptor and is involved in signal transduction
• Contains highly conserved residues and motifs essential for receptor function (DRY motif, NPxxY motif)
• Undergoes conformational changes upon ligand binding, leading to receptor activation

Intracellular domain

• Comprises the intracellular loops and the C-terminal tail
• Interacts with G proteins and other intracellular signaling partners
• Contains sites for post-translational modifications (phosphorylation, palmitoylation) that regulate receptor function
• Plays a crucial role in coupling the receptor to downstream signaling pathways

GPCR families

• GPCRs are classified into five main families based on sequence homology and functional similarity: Rhodopsin-like receptors, Secretin receptor family, Glutamate receptor family, Adhesion receptor family, and Frizzled/Taste2 receptor family
• Each family has distinct structural features, ligand preferences, and signaling properties

Rhodopsin-like receptors

• Largest and most diverse GPCR family, including receptors for light, odorants, neurotransmitters, and hormones
• Characterized by a short N-terminal domain and a conserved disulfide bridge between extracellular loops
• Examples include adrenergic receptors, dopamine receptors, and opioid receptors

Secretin receptor family

• Characterized by a large N-terminal extracellular domain involved in ligand binding
• Receptors for peptide hormones and neuropeptides (secretin, glucagon, vasoactive intestinal peptide)
• Play important roles in regulating endocrine and neuroendocrine functions

Glutamate receptor family

• Includes metabotropic glutamate receptors (mGluRs), GABAB receptors, and taste receptors
• Characterized by a large extracellular domain composed of two lobes (Venus flytrap domain) involved in ligand binding
• Play crucial roles in synaptic transmission and modulation

Adhesion receptor family

• Characterized by long N-terminal domains containing adhesion motifs (EGF-like repeats, mucin-like domains)
• Involved in cell-cell and cell-matrix interactions, as well as signaling
• Examples include latrophilins, brain angiogenesis inhibitors, and GPR56

Frizzled/Taste2 receptor family

• Includes receptors for Wnt proteins (Frizzled) and bitter taste compounds (Taste2)
• Characterized by a large N-terminal cysteine-rich domain (CRD) involved in ligand binding
• Play key roles in embryonic development, cell polarity, and taste perception

GPCR signaling pathways

• GPCRs transduce extracellular signals into intracellular responses through various signaling pathways, which can be broadly classified as G protein-dependent, G protein-independent, and biased signaling
• The diversity of GPCR signaling allows for precise regulation of cellular functions and provides opportunities for targeted therapeutic interventions

G protein-dependent signaling

• Canonical GPCR signaling pathway involving heterotrimeric G proteins (Gα, Gβ, Gγ)
• Activated GPCRs act as guanine nucleotide exchange factors (GEFs) for Gα subunits, promoting GDP-GTP exchange
• GTP-bound Gα and Gβγ dimers dissociate and activate downstream effectors (adenylyl cyclase, phospholipase C, ion channels)
• Leads to the generation of second messengers (cAMP, IP3, DAG) and the regulation of cellular processes

G protein-independent signaling

• GPCRs can signal through G protein-independent mechanisms, often involving β-arrestins
• β-arrestins are multifunctional adaptor proteins that can scaffold signaling complexes and activate kinase cascades (MAPK, Akt, Src)
• G protein-independent signaling can regulate cell proliferation, migration, and survival
• Provides additional layers of complexity and specificity in GPCR signaling

Biased signaling

• Also known as functional selectivity or ligand-directed signaling
• Phenomenon where different ligands can stabilize distinct active conformations of a GPCR, leading to the preferential activation of specific signaling pathways
• Biased ligands can selectively engage G protein-dependent or G protein-independent pathways, or favor one G protein subtype over others
• Offers the potential for developing more targeted and safer therapeutics with reduced side effects

GPCR activation and regulation

• The activation and regulation of GPCRs involve a complex interplay of ligand binding, conformational changes, and interactions with intracellular partners
• Understanding the mechanisms of GPCR activation and regulation is crucial for designing effective therapeutic strategies targeting these receptors

Ligand binding and activation

• GPCRs can be activated by a wide range of ligands, including neurotransmitters, hormones, odorants, and light
• Ligand binding occurs at specific sites within the extracellular domain and transmembrane domain
• Agonist binding stabilizes active receptor conformations, while antagonists prevent receptor activation
• Ligand efficacy and potency determine the extent and duration of receptor activation

Conformational changes

• Ligand binding induces conformational changes in the receptor, particularly in the transmembrane domain
• Active receptor conformations are characterized by the outward movement of TM6 and the rearrangement of conserved motifs (DRY, NPxxY)
• Conformational changes expose intracellular binding sites for G proteins and other signaling partners
• Different ligands can stabilize distinct active conformations, leading to biased signaling

Desensitization and internalization

• GPCRs undergo desensitization to prevent prolonged or excessive signaling
• Desensitization involves the phosphorylation of the receptor by G protein-coupled receptor kinases (GRKs) and the binding of β-arrestins
• β-arrestins sterically hinder G protein coupling and promote receptor internalization through clathrin-mediated endocytosis
• Internalized receptors can be recycled back to the cell surface or targeted for degradation, regulating receptor availability and signaling

Role of GPCRs in physiology

• GPCRs play essential roles in various physiological processes, including neurotransmission, cardiovascular function, endocrine and metabolic regulation, and immune system modulation
• Dysregulation of GPCR signaling is associated with numerous pathological conditions, making these receptors attractive targets for therapeutic interventions

Neurotransmission and neuromodulation

• GPCRs mediate the actions of many neurotransmitters (dopamine, serotonin, glutamate, GABA) and neuromodulators (opioids, cannabinoids)
• Regulate synaptic transmission, neuronal excitability, and plasticity
• Involved in higher brain functions such as cognition, emotion, and reward processing
• Dysregulation of GPCR signaling in the brain is implicated in neurological and psychiatric disorders (Parkinson's disease, schizophrenia, addiction)

Cardiovascular function

• GPCRs regulate various aspects of cardiovascular physiology, including heart rate, contractility, and blood pressure
• Adrenergic receptors mediate the effects of catecholamines on the heart and vasculature
• Angiotensin receptors are involved in the regulation of blood pressure and fluid homeostasis
• Targeting cardiovascular GPCRs is a common strategy for treating hypertension, heart failure, and other cardiovascular diseases

Endocrine and metabolic regulation

• GPCRs are essential for the regulation of endocrine glands and metabolic processes
• Receptors for hormones (insulin, glucagon, thyroid-stimulating hormone) and metabolites (fatty acids, glucose) are involved in energy homeostasis
• GPCRs in the hypothalamus and pituitary gland regulate the production and secretion of various hormones
• Targeting metabolic GPCRs holds promise for the treatment of obesity, diabetes, and other metabolic disorders

Immune system modulation

• GPCRs play important roles in the regulation of immune responses and inflammation
• Chemokine receptors mediate the migration and activation of immune cells
• Receptors for lipid mediators (prostaglandins, leukotrienes) are involved in the resolution of inflammation
• GPCR signaling modulates the production of cytokines and other immune mediators
• Targeting immune-related GPCRs may provide new therapeutic strategies for inflammatory and autoimmune diseases

GPCRs as drug targets

• GPCRs are the targets of over 30% of currently marketed drugs, making them the largest class of druggable targets
• Drugs targeting GPCRs include agonists, antagonists, allosteric modulators, and biased ligands
• The diversity of GPCR ligands and signaling pathways offers numerous opportunities for developing novel therapeutics with improved efficacy and safety profiles

Agonists and antagonists

• Agonists are drugs that activate GPCRs by mimicking the effects of endogenous ligands
• Antagonists block the action of endogenous ligands or agonists, preventing receptor activation
• Examples of GPCR agonists include beta-2 adrenergic receptor agonists (salbutamol) for asthma and mu-opioid receptor agonists (morphine) for pain relief
• Examples of GPCR antagonists include beta blockers (propranolol) for hypertension and dopamine receptor antagonists (haloperidol) for schizophrenia

Allosteric modulators

• Allosteric modulators bind to sites distinct from the orthosteric ligand-binding site and modulate receptor function
• Positive allosteric modulators (PAMs) enhance the affinity or efficacy of orthosteric ligands, while negative allosteric modulators (NAMs) reduce their effects
• Allosteric modulators offer the potential for greater selectivity and reduced side effects compared to orthosteric ligands
• Examples include benzodiazepines (PAMs of GABAA receptors) for anxiety and cinacalcet (PAM of calcium-sensing receptor) for hyperparathyroidism

Biased ligands

• Biased ligands selectively activate specific signaling pathways downstream of a GPCR
• By favoring therapeutically relevant pathways and avoiding those associated with side effects, biased ligands may offer improved safety and efficacy profiles
• Examples include the biased mu-opioid receptor agonist oliceridine, which exhibits reduced respiratory depression compared to morphine
• Developing biased ligands requires a thorough understanding of the signaling pathways and functional outcomes associated with a given GPCR

Orphan GPCRs

• Orphan GPCRs are receptors for which the endogenous ligands have not been identified
• Represent untapped potential for drug discovery, as they may be involved in important physiological processes and disease states
• Deorphanization strategies involve screening libraries of compounds to identify activating ligands and characterizing the receptor's function and signaling properties
• Successful deorphanization examples include the identification of orexin receptors as targets for narcolepsy and the discovery of the GPR40 receptor as a target for type 2 diabetes

GPCR-targeted drug discovery

• Drug discovery efforts targeting GPCRs employ a range of approaches, including high-throughput screening, structure-based drug design, and computational modeling
• Advances in GPCR structural biology and computational tools have greatly facilitated the rational design of novel GPCR ligands with improved properties

High-throughput screening

• Involves screening large libraries of compounds against a GPCR target to identify hits with desired activity
• Can be performed using cell-based assays (measuring second messenger levels or reporter gene expression) or biochemical assays (measuring ligand binding or G protein activation)
• Hits identified through HTS are further optimized through medicinal chemistry to improve potency, selectivity, and drug-like properties
• Examples of drugs discovered through HTS include the CCR5 antagonist maraviroc for HIV and the orexin receptor antagonist suvorexant for insomnia

Structure-based drug design

• Utilizes high-resolution structural information of GPCRs to guide the rational design of ligands
• X-ray crystallography and cryo-electron microscopy have provided invaluable insights into GPCR structure and ligand-binding modes
• Structure-based approaches include virtual screening, de novo design, and fragment-based drug discovery
• Successful examples include the design of the adenosine A2A receptor antagonist istradefylline for Parkinson's disease and the optimization of the angiotensin II receptor antagonist azilsartan for hypertension

Computational modeling and virtual screening

• Computational methods complement experimental approaches in GPCR drug discovery
• Homology modeling and molecular dynamics simulations can predict GPCR structures and dynamics in the absence of experimental data
• Virtual screening involves docking large libraries of compounds into GPCR structures to identify potential ligands
• Machine learning and AI-based approaches are increasingly being applied to GPCR ligand discovery and optimization
• Examples include the discovery of novel chemotypes for the 5-HT2C receptor using a combination of virtual screening and machine learning

Challenges in GPCR drug development

• Despite the success of GPCR-targeted drugs, several challenges remain in the development of new therapeutics
• These challenges include achieving selectivity and specificity, optimizing ligand efficacy and potency, and minimizing drug safety issues and side effects

Selectivity and specificity

• GPCRs often share significant sequence and structural homology, making it difficult to achieve selectivity for a specific receptor subtype
• Off-target activity can lead to unwanted side effects and limit the therapeutic window of a drug
• Strategies to improve selectivity include targeting allosteric sites, exploiting subtype-specific ligand-binding modes, and developing biased ligands
• Careful optimization of ligand structure and rigorous profiling against related receptors are essential for achieving selectivity

Ligand efficacy and potency

• Achieving the desired level of efficacy and potency is crucial for the success of a GPCR-targeted drug
• Factors influencing efficacy and potency include ligand-binding affinity, receptor reserve, and signaling efficiency
• Structure-activity relationship (SAR) studies and iterative medicinal chemistry optimization are employed to improve these properties
• Balancing efficacy and potency with other drug-like properties (solubility, permeability, stability) is a key challenge in GPCR drug development

Drug safety and side effects

• GPCR-targeted drugs can exhibit a range of side effects due to the widespread expression and diverse functions of these receptors
• Common side effects include cardiovascular, gastrointestinal, and CNS-related adverse events
• Strategies to mitigate side effects include developing subtype-selective ligands, targeting peripheral rather than central receptors, and exploiting biased signaling
• Rigorous safety assessment and monitoring are essential throughout the drug development process

Current and future perspectives

• The field of GPCR drug discovery continues to evolve, driven by advances in structural biology, signaling biology, and computational methods
• Novel therapeutic approaches and emerging technologies hold promise for addressing unmet medical needs and improving patient outcomes

Novel GPCR-targeted therapies

• Allosteric modulators and biased ligands represent promising avenues for developing safer and more effective GPCR-targeted drugs
• Combination therapies targeting multiple GPCRs or combining GPCR ligands with drugs acting on other targets may offer synergistic benefits
• GPCR-targeted antibodies and aptamers provide alternative modalities for modulating receptor function
• Gene therapy approaches, such as CRISPR-based editing or RNA interference, may enable the modulation of GPCR expression and function in a tissue-specific manner

Emerging technologies in GPCR research

• Cryo-electron microscopy is revolutionizing the field of GPCR structural biology, enabling the determination of high-resolution structures in complex with ligands and signaling partners
• Single-molecule imaging techniques, such as fluorescence resonance energy transfer (FRET) and total internal reflection fluorescence (TIRF) microscopy, provide insights into GPCR dynamics and signaling at the molecular level
• Optogenetic and chemogenetic tools allow for the precise spatiotemporal control of GPCR sign