Membrane Disorders and Diseases

Overview of Membrane Pathologies - Membrane Mayhem

Cell membrane dysfunction, or "Membrane Mayhem," underlies diverse diseases. Key mechanisms:

• Genetic: Mutations affecting membrane proteins.
  • Channelopathies (e.g., Cystic Fibrosis - CFTR defect).
  • Transportopathies (e.g., Cystinuria - amino acid transporter defect).
  • Receptor defects (e.g., Familial Hypercholesterolemia - LDLR).
  • Structural defects (e.g., Hereditary Spherocytosis - spectrin/ankyrin).
• Autoimmune: Antibodies target membrane components (e.g., Myasthenia Gravis - AChR).
• Toxin-induced: Pathogen toxins or drugs disrupt membrane (e.g., Diphtheria toxin).
• Other: Oxidative stress, lipid peroxidation, nutritional deficiencies (e.g., Vitamin E).

⭐ In Cystic Fibrosis, mutations in the CFTR gene lead to defective chloride ion transport, causing thick, sticky mucus in lungs and pancreas.

Channelopathies & Transporter Defects - Gated Grief & Cargo Chaos

Dysfunctional ion channels or transporters cause inherited/acquired diseases.

• Channelopathies (Gated Grief): Defective ion channels.

  • Cystic Fibrosis (CF): AR. CFTR (Cl⁻ channel) defect. 📌 Chloride Flow Trouble Results. Affects lungs, pancreas.

    ⭐ Most common CF mutation: ΔF508 in CFTR (misfolding, degradation).

  • Long QT Syndromes (LQTS): K⁺/Na⁺ channel gene defects. Prolonged QT, risk of Torsades de Pointes, sudden death.

• Transporter Defects (Cargo Chaos): Impaired solute carriers.

  • Glucose-Galactose Malabsorption: AR. SGLT1 (Na⁺-glucose cotransporter) defect. Severe neonatal diarrhea.
  • Hartnup Disease: AR. Neutral amino acid transporter (SLC6A19) defect. Pellagra-like symptoms (dermatitis, diarrhea, ataxia); ↓ tryptophan absorption → ↓ niacin.

Receptor & Structural Protein Disorders - Signal Sickness & Framework Flaws

• Receptor Disorders (Signal Sickness):

  • Familial Hypercholesterolemia (FH):
    • Defect: LDL Receptor (LDLR); AD.
    • Patho: ↓Hepatic LDL uptake → ↑Plasma LDL/Cholesterol → Atherosclerosis, tendon xanthomas.
  • Myasthenia Gravis (MG):
    • Defect: Autoantibodies vs. postsynaptic ACh Receptors (AChR) at NMJ.
    • Patho: ↓ACh binding → Fatigable muscle weakness (ocular, bulbar).
    • ⭐ > ~75% of MG patients show thymic abnormalities (hyperplasia/thymoma).

• Structural Protein Disorders (Framework Flaws):

  • Hereditary Spherocytosis (HS):
    • Defect: Ankyrin, Spectrin; AD common.
    • Patho: RBC membrane instability → Spherocytes → Splenic sequestration, hemolysis, ↑MCHC, ↑Osmotic Fragility.
  • Duchenne Muscular Dystrophy (DMD):
    • Defect: Dystrophin (DMD gene); X-linked Recessive.
    • Patho: Absent dystrophin → Muscle fiber necrosis, progressive proximal weakness, Gower's sign, calf pseudohypertrophy, ↑CK. 📌 Duchenne = Dystrophin Deficient.

Membrane‑Targeting Toxins & Infections - Pathogen Pillage

Pathogens strategically exploit host cell membranes for entry, replication, nutrient acquisition, and immune evasion.

• Bacterial Toxins:
  • Pore-Forming Toxins: Disrupt membrane integrity.
    • Streptolysin O, Pneumolysin: Cholesterol-dependent cytolysins.
    • Staph. aureus α-toxin: Forms β-barrel pores.
  • Enzymatic (A-B) Toxins: B-subunit binds; A-subunit (active) enters.
    • Diphtheria Toxin: ADP-ribosylates $eEF-2$ $\rightarrow$ halts protein synthesis.
    • Cholera Toxin: ADP-ribosylates $G_s\alpha$ $\rightarrow$ $\uparrow\text{cAMP}$.
• Viral Membrane Interactions:
  • Enveloped Viruses (HIV, Influenza): Membrane fusion via viral glycoproteins.
  • Non-enveloped Viruses: Utilize endocytosis or direct membrane penetration.

⭐ Cholera toxin's A subunit ADP-ribosylates the Gs alpha subunit of G-protein, causing persistent cAMP elevation and severe secretory diarrhea.

High‑Yield Points - ⚡ Biggest Takeaways

• Cystic Fibrosis: CFTR gene mutation (common ΔF508), defective Cl- channel transport, ↑ sweat Cl-.
• Hereditary Spherocytosis: Spectrin/Ankyrin defects impair RBC membrane; spherocytes, ↑ osmotic fragility.
• PNH: Acquired PIGA mutation causes GPI anchor deficiency (loss of CD55/CD59), intravascular hemolysis.
• Familial Hypercholesterolemia: LDL receptor gene defect, significantly ↑ plasma LDL-C, xanthomas, early CAD.
• Renal Tubulopathies: Gitelman (NCC defect) and Bartter (NKCC2 defect) syndromes cause hypokalemia, metabolic alkalosis.