Neurodegenerative disease Study Guide

📖 Core Concepts Neurodegeneration: progressive loss of neurons (structure + function) at molecular, cellular, circuit, and systemic levels. Proteopathy: diseases driven by aggregation of mis‑folded proteins (e.g., Aβ, α‑synuclein, tau, huntingtin, prions). Key cellular stressors: oxidative stress, inflammation, mitochondrial dysfunction, DNA damage, impaired protein degradation, and disrupted axonal transport. Programmed cell death: Apoptosis (Type I) – mitochondrial cytochrome c release → caspase‑9 → caspase‑3. Autophagic cell death (Type II) – autophagosome formation → lysosomal degradation. Cytoplasmic cell death (Type III) – non‑apoptotic toxic pathways (e.g., trophotoxicity). Transglutaminases: enzymes that cross‑link disease‑specific proteins, creating highly resistant aggregates. Major disease hallmarks: Alzheimer’s – amyloid plaques (Aβ 39‑43 aa) & neurofibrillary tangles (hyper‑phospho‑tau). Parkinson’s – loss of nigral dopaminergic neurons, Lewy bodies (α‑synuclein/ubiquitin). Huntington’s – CAG repeat → poly‑glutamine huntingtin, striatal medium‑spiny neuron loss. Multiple Sclerosis – autoimmune demyelination (myelin basic protein, MOG, PLP). ALS – upper + lower motor neuron loss; SOD1, TDP‑43, FUS mutations. CJD – prion protein misfolding, rapid dementia. --- 📌 Must Remember Aging = strongest risk factor; drives mitochondrial DNA mutations, ROS, DNA‑damage accumulation. CAG repeat expansion → poly‑glutamine tract → toxic gain‑of‑function (Huntington’s & spinocerebellar ataxias). Beta‑secretase (BACE1) cleavage of APP → amyloidogenic Aβ; alpha‑secretase cleavage is non‑amyloidogenic. Ubiquitin‑proteasome struggles with poly‑Q & α‑synuclein → toxic fragments; macroautophagy clears large aggregates. Alpha‑synuclein can form membrane pores → ion dysregulation. Transglutaminase cross‑linking stabilizes aggregates in AD, PD, HD. Key genetic mutations: PD – SNCA, LRRK2, GBA, MAPT. ALS – SOD1, TARDBP (TDP‑43), FUS, C9orf72 repeat. Clinical cardinal signs: PD – bradykinesia, rigidity, resting tremor, postural instability. HD – chorea, psychiatric changes, cognitive decline. MS – relapsing‑remitting or progressive demyelination, sensory/motor deficits. --- 🔄 Key Processes Amyloidogenic processing of APP APP → (β‑secretase) → C99 fragment → (γ‑secretase) → Aβ (39‑43 aa) → extracellular plaques. Lewy body formation (PD) α‑synuclein misfolds → ubiquitin‑tagged → aggregates → Lewy bodies → neuronal toxicity. CAG repeat transcription/translation Expanded CAG → poly‑Q huntingtin → misfolding → inclusion bodies → axonal transport blockade & BDNF trafficking failure. Ubiquitin‑proteasome degradation Poly‑ubiquitin chain → 26S proteasome → peptide fragments; impaired with bulky/irregular proteins → accumulation. Macroautophagy Initiation → double‑membrane phagophore → autophagosome → fuse with lysosome → degrade aggregates. Mitochondrial‑mediated apoptosis Stress → Bax/Bak → outer membrane permeabilization → cytochrome c release → caspase‑9 → caspase‑3 → cell death. Transglutaminase cross‑linking Enzyme forms ε‑(γ‑glutamyl)‑lysine bonds between substrate proteins → insoluble, protease‑resistant aggregates. --- 🔍 Key Comparisons Alzheimer’s vs. Parkinson’s AD: extracellular Aβ plaques + intracellular tau tangles; cortical atrophy. PD: intracellular α‑synuclein Lewy bodies; loss of nigral dopaminergic neurons. Proteasome vs. Autophagy Proteasome: degrades short, soluble, ubiquitinated proteins; limited with large aggregates. Autophagy: handles long‑lived proteins, organelles, and large aggregates. Intrinsic vs. Extrinsic apoptosis Intrinsic: mitochondria‑derived (cytochrome c). Extrinsic: death‑receptor signaling (Fas/TNF‑α). (Only intrinsic detailed in outline.) Genetic vs. Sporadic forms Genetic: clear mutation (CAG repeat, SOD1, SNCA). Sporadic: multifactorial (age, oxidative stress, environmental). --- ⚠️ Common Misunderstandings “All neurodegenerative diseases are caused by the same protein.” – Each disease has a distinct pathogenic protein, though pathways (oxidative stress, proteostasis) overlap. “Beta‑secretase inhibition cures Alzheimer’s.” – Inhibition reduces Aβ production but trial failures show dosing, target validation, and disease stage are critical. “Lewy bodies are only found in Parkinson’s.” – Also present in dementia with Lewy bodies & multiple system atrophy. “Axonal transport defects are a consequence, not a cause.” – Transport failure can precede overt neuronal loss and drive pathology (e.g., mutant huntingtin blocking BDNF transport). --- 🧠 Mental Models / Intuition “Protein quality‑control traffic jam” – Imagine a city’s waste‑removal system: proteasome = street sweepers (fast, small debris); autophagy = garbage trucks (big loads). When the trucks break down, trash piles up → toxic city (neuron). “Mitochondria as power plants with fire alarms” – ROS = smoke; if the alarm (DNA repair, antioxidant defenses) fails, the plant self‑destructs via apoptosis. “CAG repeat as a rubber band that gets too long” – Longer repeat = tighter coil → more likely to snap (misfold) and pull other proteins into the knot (aggregation). --- 🚩 Exceptions & Edge Cases Olfactory dysfunction: common early PD sign but its diagnostic specificity is debated. Gut microbiome influence: emerging evidence, not yet definitive for diagnosis or therapy. Transglutaminase activity: elevated in AD, PD, HD, but inhibition strategies are still experimental. Alpha‑synuclein membrane pores: demonstrated in vitro; in vivo relevance still under investigation. --- 📍 When to Use Which Diagnostic focus: Motor signs + resting tremor → prioritize PD work‑up (DAT imaging). Rapid dementia + myoclonus → consider CJD (EEG, prion detection). Chorea + family history → test CAG repeat for HD. Therapeutic targeting: Amyloid‑centric trials → apply beta‑secretase inhibitors only if patient is early‑stage AD with confirmed amyloid burden. Enhance autophagy – consider in diseases with large aggregates (PD, HD, AD) where proteasome is overloaded. Immunotherapy – best suited for diseases with extracellular targets (Aβ, tau). --- 👀 Patterns to Recognize “Protein + location” pattern: Aβ → extracellular plaques (cortex). Tau → intracellular tangles (neuronal soma). α‑synuclein → Lewy bodies (neuronal processes). Huntingtin → nuclear/ cytoplasmic inclusions (striatal neurons). “Age + oxidative stress” → progressive DNA damage → common downstream apoptosis across diseases. “Genetic repeat length ↔ earlier onset” – seen in HD and other CAG‑repeat disorders. --- 🗂️ Exam Traps Trap: “All neurodegenerative diseases are reversible with antioxidants.” – Reality: oxidative stress contributes but no cure; antioxidants alone have not shown disease modification. Trap: “Lewy bodies are only intracellular.” – They may also contain extracellular α‑synuclein aggregates, especially in dementia with Lewy bodies. Trap: “Beta‑secretase is the only enzyme to target in AD.” – Alpha‑secretase modulation and downstream clearance (immunotherapy) are also critical. Trap: “ALS affects only lower motor neurons.” – Both upper and lower motor neurons are lost; upper signs include spasticity and hyperreflexia. Trap: “CAG repeat expansions are only in Huntington’s disease.” – Also cause several spinocerebellar ataxias and other poly‑Q disorders. ---