🧠🧬Patho of Neurodegeneration

🧠🧬 Pathogenesis of Neurodegeneration – High-Yield Overview

Definition:
Progressive dysfunction and death of neurons due to interacting molecular pathways, leading to cognitive, motor, or behavioral deficits. Despite different clinical phenotypes, most neurodegenerative diseases share common mechanisms.

🔹 1️⃣ Protein Misfolding & Aggregation (Proteostasis Failure)

• Misfolded proteins → oligomers → accumulation → overwhelm clearance systems
• Clearance failure: ubiquitin–proteasome system, autophagy–lysosome pathway
• Disease examples:
  • 🧠 Alzheimer’s: Aβ plaques, tau tangles
  • 🟣 Parkinson’s: α-synuclein (Lewy bodies)
  • 🧩 ALS/FTD: TDP-43, FUS
  • 🟡 Huntington’s: mutant huntingtin (polyQ repeats)
• Toxic effects: synaptic disruption, ER stress, apoptosis, prion-like spread

🔹 2️⃣ Mitochondrial Dysfunction

• ↓ ATP → synaptic failure
• ↑ Reactive oxygen species (ROS)
• Fission–fusion imbalance
• Release of pro-apoptotic factors
• Diseases: Parkinson’s (complex I, PINK1/parkin), ALS, Alzheimer’s, mitochondrial encephalopathies

🔹 3️⃣ Oxidative Stress & Free Radical Injury

• ROS damage DNA, proteins, lipids
• Sources: mitochondria, microglia, iron accumulation, environmental toxins
• Consequences: lipid peroxidation, DNA damage, protein oxidation → misfolding

🔹 4️⃣ Excitotoxicity (Glutamate-Mediated)

• Excess NMDA/AMPA receptor activation → ↑ intracellular Ca²⁺
• Mitochondrial failure, ROS, calpain activation → cytoskeletal breakdown
• Diseases: ALS, Huntington’s, acute insults (stroke, hypoxia)

🔹 5️⃣ Impaired Axonal Transport

• Microtubules & motor proteins (kinesin/dynein) fail
• Consequences: synaptic starvation, toxic cargo accumulation, “dying-back” neuropathy
• Examples: ALS (TDP-43), Tauopathies

🔹 6️⃣ Neuroinflammation (Glial-Mediated)

• Chronic microglia & astrocyte activation → IL-1β, TNF-α, IL-6, complement activation
• Reactive astrocytes (A1 phenotype) become neurotoxic
• Diseases: Alzheimer’s (TREM2 risk), Parkinson’s, ALS

🔹 7️⃣ Synaptic Dysfunction

• Early mechanism preceding neuronal death
• Mechanisms:
  • Aβ oligomers block NMDA/AMPA signaling
  • α-synuclein disrupts vesicle release
  • Prion-like spreading disrupts networks
• Clinical implication: early cognitive/motor changes before structural atrophy

🔹 8️⃣ Genetic Vulnerability

• Mutations disrupt cellular pathways:
  • APP, PSEN1/2 → early-onset Alzheimer’s
  • LRRK2, PARKIN, PINK1, GBA → Parkinson’s
  • C9orf72 → ALS/FTD
  • HTT expansion → Huntington’s
• Mechanisms: gain of toxic function or loss of normal function

🔹 9️⃣ Impaired Autophagy–Lysosome Pathway

• Defective autophagosome formation, lysosomal enzyme deficiency, impaired mitophagy
• Diseases: Alzheimer’s, Parkinson’s, lysosomal storage disorders, C9orf72 ALS

🔹 🔟 Prion-like Propagation

• Misfolded proteins template misfolding in neighboring cells
• Agents: Aβ, tau, α-synuclein, huntingtin, TDP-43
• Explains progressive spread (e.g., Braak staging in AD/Parkinson’s)

🔹 Integrated Model

1. Misfolded proteins accumulate
2. Mitochondrial failure → energy crisis
3. Axonal transport collapses
4. Synapses weaken → cognitive/motor deficits
5. Chronic inflammation accelerates death
6. Neurons undergo apoptosis/necrosis

💡 High-Yield Clinical Pearls

• Neurodegeneration begins with synaptic dysfunction, not cell death
• Protein aggregation + impaired autophagy = central mechanism
• Mitochondria & microglia = key therapeutic targets
• Prion-like spread explains topographical progression
• Genetics → pathway; environment → disease rate