Defects in Signal Transduction and Disease

Overview: Signal Defects - Wires Crossed Badly

• Cellular miscommunication: Errors in signaling pathways underpin many diseases.
• Common failure points:
  • Receptors: Mutations (gain/loss of function), altered expression. E.g., Laron dwarfism (GH receptor defect).
  • Ligands: Deficient or excessive production.
  • G-Proteins:
    • Constitutively active $G_{s\alpha}$ (Cholera toxin).
    • Inhibited $G_{i\alpha}$ (Pertussis toxin).
  • Enzymes: Dysfunctional kinases (e.g., BCR-ABL in CML) or phosphatases.
  • Second Messengers: Imbalanced levels (e.g., cAMP, $IP_3/DAG$, $Ca^{2+}$).
  • Downstream Proteins: Altered expression or function.
• Disease Manifestations: Cancer, diabetes, endocrine disorders, cardiovascular disease.

⭐ Cholera toxin ADP-ribosylates the $G_{s\alpha}$ subunit, locking it in an active state. This leads to persistent adenylyl cyclase activation, ↑cAMP, massive $Cl^-$ secretion into the gut lumen, and life-threatening watery diarrhea.

Receptor & G-Protein Issues - Faulty Locks & Keys

• Cellular communication relies on precise ligand-receptor ("key-lock") interactions and G-protein signaling. Defects disrupt these, causing various diseases.

• GPCR Pathway Defects:

  • Gαs (stimulatory) issues:
    • Cholera: Toxin ADP-ribosylates Gαs → constitutive activation → $↑\text{cAMP}$ → severe diarrhea.
    • McCune-Albright Syndrome: Somatic GNAS mutation → Gαs overactivity.
  • Gαi (inhibitory) issues:
    • Pertussis: Toxin ADP-ribosylates Gαi → prevents Gαi from inhibiting adenylyl cyclase → $↑\text{cAMP}$.
  • Receptor issues:
    • Familial Hypocalciuric Hypercalcemia (FHH): CaSR (Calcium-Sensing Receptor) inactivating mutations.
    • Retinitis Pigmentosa: Rhodopsin (GPCR) mutations.

• Receptor Tyrosine Kinase (RTK) Defects:

  • Insulin Resistance (Type 2 DM): ↓Insulin receptor sensitivity/number or post-receptor defects.
  • Cancers: Overexpression/mutation of growth factor receptors (e.g., EGFR, HER2).

• Ligand-gated Ion Channel Defects:

  • Myasthenia Gravis: Autoantibodies block/destroy nAChRs at NMJ → muscle weakness.
  • Cystic Fibrosis: CFTR gene mutation (Cl⁻ channel) → defective ion transport.

⭐ > In Myasthenia Gravis, antibodies target nicotinic acetylcholine receptors (nAChRs), impairing neuromuscular transmission and causing fluctuating muscle weakness.

Kinases & Cancer Links - Growth Gone Wild

Kinases (phosphorylate) & phosphatases (dephosphorylate) balance cell signals. Imbalance fuels cancer.

• Oncogenic Kinases: Drive uncontrolled cell proliferation & survival.
  • Receptor Tyrosine Kinases (RTKs): e.g., EGFR, HER2. Overexpression/mutations common.
  • Ras-MAPK Pathway:
    • Ras mutations (pancreatic, colorectal cancers) → active Ras.
    • ↑Raf → ↑MEK → ↑ERK signaling → cell proliferation.
  • PI3K/Akt/mTOR Pathway:
    • Activated by RTKs or PIK3CA mutations.
    • Promotes cell growth, survival.
    • mTOR activation → protein synthesis.
  • Non-receptor Tyrosine Kinases:
    • BCR-ABL (CML) from t(9;22). 📌 Philadelphia Chromosome.
• Tumor Suppressor Phosphatases:
  • PTEN: Dephosphorylates $PIP_3$. Loss → sustained PI3K/Akt signaling (endometrial, prostate cancers).

⭐ BCR-ABL fusion protein, from t(9;22) (Philadelphia chromosome), is a constitutively active tyrosine kinase in CML, targeted by Imatinib.

High‑Yield Points - ⚡ Biggest Takeaways

• Cholera toxin ADP-ribosylates Gsα, ↑cAMP, causing massive secretory diarrhea.
• Pertussis toxin ADP-ribosylates Giα, ↑cAMP, leading to whooping cough.
• Insulin resistance in Type 2 DM involves defective insulin receptor or post-receptor signaling.
• Oncogenes often encode constitutively active RTKs (e.g., HER2) or transducers (e.g., Ras).
• Grave's disease: Stimulatory autoantibodies against TSH receptors cause hyperthyroidism.
• Pseudohypoparathyroidism Type 1A: GNAS1 mutation impairs Gsα, causing PTH resistance.