Signal Transduction

• Medical Physiology (Boron & Boulpaep), p. 73-84
• Robbins & Cotran Pathologic Basis of Disease, 10e, p. 33-34
• Harrison's Principles of Internal Medicine, 22e, p. 3696

1. Modes of Intercellular Communication

2. Classes of Extracellular Signaling Molecules

1. Amines - epinephrine, dopamine, serotonin
2. Peptides & proteins - insulin, angiotensin II, growth factors
3. Steroids - aldosterone, estrogens, retinoic acid
4. Other small molecules - amino acids, nucleotides, Ca²⁺, nitric oxide (NO)

3. Receptor Classes and Their Mechanisms

3.1 Ligand-Gated Ion Channels

• Hybrid receptor-channel proteins. Ligand binding causes rapid conformational change, opening an ion pore.
• Produces an electrical signal (change in membrane potential).
• Examples: nicotinic ACh receptor (Na⁺/K⁺), GABA-A receptor (Cl⁻), NMDA receptor (Ca²⁺/Na⁺).
• Speed: milliseconds - fastest signaling modality.

3.2 G Protein-Coupled Receptors (GPCRs)

• Over 1,500 GPCRs identified; the largest receptor superfamily.
• Characterized by 7 transmembrane helices (serpentine/heptahelical receptors).
• Work indirectly through a heterotrimeric GTP-binding protein (Gα, Gβ, Gγ subunits).

1. Ligand binds → GPCR changes conformation
2. GPCR associates with G protein (GDP-bound, inactive)
3. GDP exchanged for GTP → Gα dissociates from Gβγ
4. Active Gα (or Gβγ) modulates downstream effectors
5. Intrinsic GTPase activity of Gα hydrolyzes GTP → GDP → system returns to rest

3.3 Catalytic (Enzyme-Linked) Receptors

Receptor Tyrosine Kinases (RTKs)

• Examples: insulin receptor, EGF receptor, PDGF receptor, VEGF receptor.
• Ligand binding induces receptor dimerization → trans-autophosphorylation of tyrosine residues → recruitment of SH2 domain-containing adaptor proteins.
• Key downstream cascades:
  • RAS/MAPK pathway: Ras (small GTPase) → Raf → MEK → ERK → transcription factors (proliferation, differentiation)
  • PI3K/Akt pathway: PI3K phosphorylates PIP₂ → PIP₃ → recruits PDK1 and Akt → promotes cell survival, growth, metabolism
  • JAK-STAT pathway: used by many cytokine receptors (not classical RTKs but same logic)

Non-receptor Tyrosine Kinases

• Receptors with no intrinsic catalytic activity (immune receptors, integrins) recruit cytoplasmic kinases.
• Src-family kinases (c-Src, Lck, Fyn) are the prototype - contain SH2 and SH3 domains that mediate protein-protein interactions.
• SH2 domains bind phosphotyrosine-containing sequences; SH3 domains bind proline-rich sequences.

Receptor Serine/Threonine Kinases

• Examples: TGF-β receptors (type I and II).
• Phosphorylate SMAD proteins → nuclear translocation → gene regulation.

Receptor Guanylyl Cyclases

• Ligand binding (e.g., atrial natriuretic peptide [ANP]) activates the intracellular guanylyl cyclase domain → converts GTP to cGMP → activates PKG.

3.4 Nuclear (Intracellular) Receptors

• Ligands: steroid hormones (cortisol, aldosterone, sex steroids), thyroid hormones, vitamin D, retinoids.
• Receptors reside in cytosol or nucleus; ligand binding triggers conformational change, dimerization, and binding to hormone response elements (HREs) in DNA.
• Can activate or repress gene transcription.
• Slowest signaling (hours) - but produces long-lasting changes in gene expression.

3.5 Receptors That Undergo Regulated Cleavage (RIP)

• Examples: Notch receptor, APP (amyloid precursor protein).
• Ligand binding triggers proteolytic cleavage (by metalloproteases and then γ-secretase) → release of an intracellular fragment → nuclear translocation → transcription.

4. Second Messengers

cAMP (cyclic adenosine monophosphate)

• Synthesized by adenylyl cyclase from ATP; degraded by phosphodiesterases (PDEs).
• Primary effector: Protein Kinase A (PKA) - phosphorylates serine/threonine residues on target proteins.
• PKA also phosphorylates the transcription factor CREB (cAMP response element-binding protein) → gene expression changes.
• Physiological roles: glycogen breakdown (liver), lipolysis (adipose), cardiac inotropy (heart), gluconeogenesis.

cGMP (cyclic guanosine monophosphate)

• Synthesized by guanylyl cyclase (membrane-bound or soluble forms; the soluble form is activated by nitric oxide, NO).
• Primary effectors: PKG, cyclic nucleotide-gated ion channels, PDEs.
• Roles: smooth muscle relaxation (vasodilation), visual phototransduction, platelet inhibition.

IP₃ / DAG (Phosphoinositide pathway)

• Phospholipase C (PLC) cleaves PIP₂ into two second messengers:
  • IP₃ (inositol-1,4,5-trisphosphate): water-soluble, diffuses to ER → opens IP₃-gated Ca²⁺ channels → cytoplasmic Ca²⁺ spike
  • DAG (diacylglycerol): lipid-soluble, remains in membrane → activates Protein Kinase C (PKC)
• PKC phosphorylates many substrates → gene expression, cell growth, secretion.

Ca²⁺

• A universal second messenger. Cytosolic [Ca²⁺] is kept very low (~100 nM at rest vs. 1 mM extracellular) by pumps (SERCA, PMCA) and exchangers.
• Released from ER via IP₃ receptors or ryanodine receptors (RyRs), or enters from outside via voltage-gated or receptor-operated channels.
• Acts through calmodulin (CaM) → Ca²⁺-CaM complex activates CaM kinases (CaMKII), calcineurin (phosphatase), and other enzymes.

5. Signal Amplification and Cascade Logic

• Each step can be modulated by inhibitors or activators, conferring specificity.
• The same second messenger (e.g., cAMP) can produce different responses in different cell types depending on which PKA substrates and effectors are present.
• Example of diversity: ACh stimulates skeletal muscle contraction (via nicotinic receptors), inhibits cardiac muscle contraction (via muscarinic M2 → Gαi → ↓cAMP), and stimulates glandular secretion (via muscarinic M3 → Gαq → IP₃/Ca²⁺).
• Crosstalk: signaling cascades are not linear silos - they intersect extensively. PKA can phosphorylate components of the MAPK pathway; Ca²⁺-CaM kinases can modulate cAMP signaling; and PI3K products influence GTPase activity.

6. Regulation and Termination of Signals

7. Clinical Relevance

Summary Diagram

Extracellular ligand
        ↓ (binds)
    GPCR (7-TM)
        ↓ (activates)
    G protein (Gα-GTP released)
       /          \
   Gαs             Gαq
     ↓               ↓
Adenylyl           PLC-β
cyclase              ↓
   ↓         IP₃ + DAG
 cAMP           /       \
   ↓          Ca²⁺     PKC
  PKA            ↓        ↓
   ↓         CaM-KII   Phosphorylation
  CREB →                  cascades
Gene expression