# Lecture 7: Cell Signaling I: G-Protein Coupled Receptors
## Cell Biology
---
## Learning Objectives
By the end of this lecture, students will be able to:
1. Describe the general principles of cell signaling and types of signaling molecules
2. Explain the structure and activation mechanism of GPCRs
3. Describe the cAMP/PKA and IP3/DAG/PKC signaling pathways in detail
4. Explain how G-protein signaling is terminated
5. Discuss clinical examples of GPCR-related diseases and pharmacology
---
## Lecture Content
### I. General Principles of Cell Signaling
Cell communication is essential for the coordinated function of multicellular organisms. Signaling can be classified by the distance over which it operates. **Endocrine** signaling involves hormones that travel through the bloodstream to act on distant target cells, as exemplified by insulin and epinephrine. **Paracrine** signaling involves molecules that act on nearby cells, such as growth factors and neurotransmitters. In **autocrine** signaling, a cell responds to signals it produces itself, a phenomenon commonly seen in cancer cells that produce their own growth factors. **Juxtacrine (contact-dependent)** signaling requires direct cell-cell contact, as in Notch-Delta signaling. **Synaptic** signaling is a specialized form of paracrine communication at neuronal synapses.
Every signaling pathway follows a common logic. A **signal (ligand)**, the extracellular first messenger, is detected by a **receptor**, which may be on the cell surface or intracellular. The signal is then relayed and amplified through **signal transduction** cascades involving **second messengers** such as cAMP, Ca2+, IP3, DAG, and cGMP. **Effector** proteins carry out the final cellular response, which may include changes in gene expression, metabolism, cell shape, or division.
Two important principles govern signaling. **Signal amplification** occurs because one receptor can activate many downstream molecules, each of which can generate many second messenger molecules, creating a cascade effect. **Signal specificity** is determined by which receptors and intracellular signaling proteins are expressed in a given cell type, explaining how different cells can respond differently to the same signal.
### II. G-Protein Coupled Receptors (GPCRs) — Structure
GPCRs constitute the largest family of cell-surface receptors, with approximately 800 members in the human genome. They are also the most important drug targets, with 30 to 40 percent of all approved drugs acting through GPCRs. Structurally, all GPCRs share seven transmembrane alpha-helices (hence their alternative name, 7TM or serpentine receptors), with an extracellular N-terminus and an intracellular C-terminus. The extracellular loops and transmembrane domains form the ligand-binding pocket, while the intracellular loops (particularly ICL3) and C-terminus interact with G-proteins.
GPCRs bind an astonishing diversity of ligands. Beta-adrenergic receptors respond to epinephrine and norepinephrine. Muscarinic acetylcholine receptors bind acetylcholine. Opioid receptors are activated by endorphins and pharmacological opioids like morphine. Rhodopsin detects light. The approximately 400 types of olfactory receptors detect odorant molecules. Chemokine receptors, including CXCR4 (a co-receptor for HIV), bind chemokine signaling molecules. The 2012 Nobel Prize in Chemistry was awarded to Robert Lefkowitz and Brian Kobilka for their studies of GPCRs, including Kobilka's determination of the crystal structure of the beta-2 adrenergic receptor.
### III. Heterotrimeric G-Proteins and GPCR Activation Cycle
**Heterotrimeric G-proteins** consist of three subunits: G-alpha, which binds and hydrolyzes GTP and determines which downstream pathway is activated; and G-beta and G-gamma, which form a stable dimer and also have independent signaling roles.
The G-protein activation cycle proceeds in a series of defined steps. In the **inactive state**, G-alpha is bound to GDP and associated with the G-beta-gamma dimer as a complete trimer, and the receptor is unoccupied. When a ligand binds the receptor, the receptor undergoes a conformational change and acts as a GEF (guanine nucleotide exchange factor), promoting the exchange of GDP for GTP on G-alpha. GTP-bound G-alpha then dissociates from G-beta-gamma, and both species can activate downstream effectors. G-alpha-GTP stimulates or inhibits target enzymes or channels. The signal is terminated when the intrinsic GTPase activity of G-alpha hydrolyzes GTP back to GDP. RGS proteins (Regulators of G-protein Signaling) act as GAPs to accelerate this hydrolysis. G-alpha-GDP then reassociates with G-beta-gamma, restoring the inactive trimer.
The identity of the G-alpha subunit determines the downstream signaling pathway. **G-alpha-s** (stimulatory) activates adenylyl cyclase, increasing cAMP levels. **G-alpha-i** (inhibitory) inhibits adenylyl cyclase, decreasing cAMP. **G-alpha-q** activates phospholipase C-beta (PLC-beta). **G-alpha-12/13** activates Rho GTPases to drive cytoskeletal changes.
The GPCR activation cycle. Panel A: Inactive state — GPCR with no ligand, heterotrimeric G-protein (alpha-GDP, beta, gamma) associated with receptor cytoplasmic face. Panel B: Ligand binding causes receptor conformational change, promoting GDP-to-GTP exchange on G-alpha. Panel C: Active state — G-alpha-GTP and G-beta-gamma dissociate and each activate downstream effectors (adenylyl cyclase or PLC shown). Panel D: Inactivation — G-alpha hydrolyzes GTP to GDP (accelerated by RGS proteins), reassociates with G-beta-gamma, returning to inactive trimer. Circular arrow diagram emphasizing the cyclical nature.
### IV. The cAMP / PKA Pathway
**Adenylyl cyclase** is an integral membrane enzyme with 12 transmembrane domains and catalytic domains on the cytoplasmic face. Activated by G-alpha-s and inhibited by G-alpha-i, it converts ATP into **cAMP (cyclic AMP)**, one of the most important second messengers in cell biology. cAMP was discovered by Earl Sutherland (Nobel Prize, 1971). It is rapidly produced upon receptor stimulation and rapidly degraded by phosphodiesterases (PDEs) to 5'-AMP. Caffeine and theophylline prolong cAMP signaling by inhibiting PDEs.
The primary effector of cAMP is **Protein Kinase A (PKA)**. In its inactive form, PKA is a tetramer of two regulatory (R) and two catalytic (C) subunits. When cAMP binds the R subunits (two molecules of cAMP per R subunit), the active C subunits are released and proceed to phosphorylate serine and threonine residues on a variety of target proteins. Key PKA targets include glycogen phosphorylase kinase (which activates glycogen breakdown), CREB (the cAMP Response Element Binding protein, a transcription factor that upon phosphorylation binds CRE elements in gene promoters to activate transcription), and hormone-sensitive lipase (which promotes triglyceride breakdown).
A classic illustration of this pathway is **epinephrine signaling in the liver**. Epinephrine binds the beta-adrenergic receptor (a GPCR), which activates G-alpha-s. G-alpha-s stimulates adenylyl cyclase to produce cAMP, which activates PKA. PKA then phosphorylates phosphorylase kinase, which in turn activates glycogen phosphorylase, leading to glycogen breakdown and glucose release into the blood.
### V. The IP3 / DAG / Ca2+ / PKC Pathway
When G-alpha-q is activated, it stimulates **phospholipase C-beta (PLC-beta)**, which cleaves the membrane phospholipid PIP2 (phosphatidylinositol 4,5-bisphosphate) into two second messengers. **IP3** (inositol 1,4,5-trisphosphate) is water-soluble and diffuses through the cytoplasm, while **DAG** (diacylglycerol) is lipid-soluble and remains in the membrane.
IP3 binds to IP3 receptors on the ER membrane, which are ligand-gated Ca2+ channels. This triggers the release of Ca2+ from the ER lumen into the cytosol, causing cytosolic Ca2+ to rise from its resting level of approximately 100 nM to about 1 micromolar. The released Ca2+ binds **calmodulin (CaM)**, a small protein with four Ca2+-binding EF-hand motifs. The Ca2+/CaM complex activates several important downstream targets, including CaM-kinase II (CaMKII, involved in learning, memory, and muscle contraction), myosin light chain kinase (MLCK, which drives smooth muscle contraction), and calcineurin (a phosphatase critical for T-cell activation and the target of the immunosuppressant cyclosporin A).
Meanwhile, DAG together with Ca2+ activates **Protein Kinase C (PKC)** at the membrane. PKC phosphorylates serine and threonine residues on a diverse array of substrates. Phorbol esters such as PMA/TPA are tumor promoters that mimic DAG and act as potent PKC activators. Calcium homeostasis is restored when SERCA pumps return Ca2+ to the ER and PMCA pumps and Na+/Ca2+ exchangers remove Ca2+ from the cell.
The PLC-beta signaling pathway. Panel A: Activated GPCR stimulates G-alpha-q, which activates PLC-beta at the membrane. PLC-beta cleaves PIP2 into IP3 and DAG. Panel B: IP3 diffuses to the ER, binds IP3 receptors, and triggers Ca2+ release into the cytosol. Ca2+ binds calmodulin, forming the Ca2+/CaM complex that activates CaMKII and other targets. Panel C: DAG remains in the membrane and, together with Ca2+, recruits and activates PKC. PKC phosphorylates downstream substrates. Arrows show signal termination: IP3 dephosphorylation, Ca2+ re-uptake by SERCA, and DAG metabolism.
### VI. Termination and Regulation of GPCR Signaling
Precise termination of signaling is as important as activation. **Receptor desensitization** begins when GRKs (G-protein-coupled receptor kinases) phosphorylate the activated receptor on its C-terminus. The phosphorylated receptor then recruits **beta-arrestins**, which serve multiple functions: they block G-protein coupling (desensitization), recruit clathrin and AP-2 to promote receptor internalization via endocytosis, and can themselves serve as signaling scaffolds (beta-arrestin-mediated signaling). Internalized receptors may be recycled back to the cell surface or degraded in lysosomes (a process termed downregulation).
Signaling is also terminated at the **G-protein level** through the intrinsic GTPase activity of G-alpha, which is accelerated by RGS proteins acting as GAPs. **Second messengers** are rapidly removed: phosphodiesterases degrade cAMP and cGMP, phosphatases dephosphorylate IP3, and Ca2+ is pumped back into the ER or out of the cell.
The clinical significance of GPCR signaling is well illustrated by several examples. **Cholera toxin** ADP-ribosylates G-alpha-s, locking it in the GTP-bound active state, resulting in constitutive adenylyl cyclase activation, massive cAMP accumulation in intestinal cells, Cl- and water secretion, and severe diarrhea. **Pertussis toxin** ADP-ribosylates G-alpha-i, preventing its activation and thereby elevating cAMP levels. **McCune-Albright syndrome** results from a somatic activating mutation in G-alpha-s. Beta-blockers such as propranolol block beta-adrenergic receptors to treat hypertension and anxiety. Opioid agonists like morphine activate opioid GPCRs to achieve pain relief.
---