Cell Surface Receptors and GPCR Signaling

Major Classes of Cell Surface Receptors

• Ion-channel-coupled receptors (ionotropic receptors)
• G-protein-coupled receptors (GPCRs, metabotropic receptors)
• Enzyme-coupled receptors

G-protein-Coupled Receptor (GPCR) Signaling

Key Components

1. G-protein Activation
2. Downstream of GPCRs:
   • cAMP → Protein Kinase A (PKA)
   • Phospholipase C (PLC) → Ca²⁺ → Ca²⁺/Calmodulin Kinase (CaMK)
3. Sensory Transduction
4. GPCR Turn-off by Negative Feedback

Structure of GPCRs

• Composition: Seven transmembrane α helices linked by loops
• Function: Ligand binding induces a change in conformation, activating trimeric G proteins
• Role as a GEF: GPCRs act as Guanine nucleotide Exchange Factors (GEF) for trimeric G-proteins

Trimeric G Proteins as Signal Relays

• Structure: Composed of three subunits (Gα, Gβ, Gγ)
• Gα: Largest subunit, can bind GDP or GTP
• Activation Process:
  1. Ligand binds to GPCR, altering its conformation
  2. GPCR releases GDP bound to G-protein
  3. Gα binds GTP and detaches from the complex
  4. Gα or Gβγ initiates signaling

Effectors and Their Functions Downstream of Trimeric G-Proteins

Cyclic AMP (cAMP) Pathway

• Source: cAMP is synthesized from ATP by adenylyl cyclase
• Activation by Gsα:
  • Inactive until activated by Gsα
  • GTP-Gsα strikingly activates adenylyl cyclase
• Target: PKA
  • cAMP binds to PKA's regulatory subunit, releasing inhibition from the catalytic subunit, activating PKA

Hormonal Responses Mediated by cAMP (See Table 15-1)

• Thyroid gland: TSH stimulates thyroid hormone synthesis
• Adrenal cortex: ACTH stimulates cortisol secretion
• Ovary: LH stimulates progesterone secretion
• Muscle and Liver: Adrenaline induces glycogen breakdown

Short-lived Activation of G Protein Signaling

• Quick Response: G proteins are active for a short time
• Deactivation Process:
  • Inactive Gsα halts cAMP production
  • cAMP degrades via phosphodiesterase
  • PKA activates phosphodiesterase, restoring cAMP levels

Phosphoinositide (PIP₂) and Calcium Signaling

• IP3: Functions as a second messenger produced by PLC from PIP₂
• Ca²⁺ Release Mechanism: IP3 binds to receptors in the ER, releasing Ca²⁺ into the cytosol
• DAG: Works alongside IP3 to activate protein kinase C (PKC)

Calcium in Signaling

• Calcium Gradients: High extracellular and ER Ca²⁺ concentration vs. low cytosolic concentration
• Oscillations: Vary in frequency and amplitude, encoding cellular signals
• Calciums' Actions: Rapid increases activate various Ca²⁺ responsive proteins

Feedback Mechanisms in Calcium Signal Generation

1. IP3 opens Ca²⁺ release channels on the ER
2. Ca²⁺ activates more Ca²⁺ release channels (CICR) → Positive feedback
3. High extracellular Ca²⁺ shuts down channels → Negative feedback
4. Ca²⁺ pumps remove excess, resetting the signal
5. New Ca²⁺ oscillations can initiate again

Activation of Calcium/Calmodulin-dependent Kinases (CaM Kinases)

• Mechanism: Ca²⁺ binding activates CaM which in turn activates kinases
• Autophosphorylation: CaM Kinase II maintains activity even after Ca²⁺ has decreased
• Role in Frequency Responses:
  • At lower frequencies, activity resets between pulses
  • At higher frequencies, cumulative activation can occur

Sensory Transduction via GPCRs

• Olfaction:
  • Odorants activate GPCRs, leading through cAMP to Na⁺ influx
• Vision:
  • Light activates rhodopsin GPCR leading to cGMP breakdown and hyperpolarization of photoreceptors

Summary of GPCR Signaling Principles

1. Understand GPCR signaling pathways:
   • Adenylate cyclase → cAMP → PKA
   • PLC → IP3 & DAG → PKC
2. Importance of oscillation frequencies in calcium signaling and phospho-regulation via CaM Kinase