Neurodegenerative diseases Flashcards
NDD epidemiology
Dementia 5% 65y/o -> 30% 85y/o
Alzheimer’s -> £17Bn per year in UK (20% health budget)
PD 1% 65y/o -> 5% 85y/o
Types of study
Post-mortem brain tissue -> protein, RNA
TG and KO mice -> neuropharmacology/ electrophysiology
Neuronal tissue culture/ IPSCs
Koychev et al 2017
Extracellular A-beta plaques correlate poorly with cognitive decline
- PET scan for A-beta
- Compared MMSE scores
APP in EOAD
- Downs syndrome associated with AD (APP gene mapped to chromosome 21)
- Some cases of EOAD linked to chromosome 21 in early 90s linkage pedigree studies
- First mutation found in 1991 -> now 25 APP mutations
- APP duplications -> EOAD and haemorrhage vascular dementia (2006)
APP
- Large TM protein of unknown function
- Processed by sequential proteinase cleavage - 3 secretases. Cleavage by alpha secretes precludes A-beta formation. A-beta produced by beta and gamma secretes cleavage -> transported into cerebral vasculature then cleared in blood.
- 40 and 42 amino acid version of A-beta
- Dominant mutations: clustered around cleavage sites, increase A-beta production (esp 42 AA) and increase fibrillation
- Gene dosage: Downs, APP duplications -> increase A-beta peptide production
- APP mutations account for a minority of familial EOAD
Gamma secretase complex
- 4 main proteins: PS1/PS2, APH1, PEN2, Nicastrin
- 160 presenilin mutations known (151 of which are in PS1)
Amlyoid cascade hypothesis
- A-beta in both EOAD and LOAD
- Mechanism by which EOAD and LOAD lead to plaques and NFTs are different
- Key concept is imbalance between production and clearance
- A-beta initiates a cascade of events resulting in plaques and tangles, neuronal loss and dementia
Rapoport et al 2002
- Primary hippocampal neuron cultures made from WT and tau KO mice
- Apply A-beta peptide -> WT neutrons die; tau KO survive
=> fibrillar A-beta is toxic to neurons and tau is required for toxicity
*authors used concentrations of A-beta much higher than in normal disease
Vargas-Caballero et al 2011
- Tau requires for inhibition of LTP by A-beta
- WT vs MAPT-/- mous hippocampe slices
- LTP decreased in WT with addition of A-beta42 but not tau KO
- Addition of GSK3 inhibitor blocked A-beta mediated LTP inhibition in WT mice => tau phosphorylation by GSK3 is required for the process
Jansen et al 2019
GWAS 72,000 AD vs 380,000 controls
- 28 gene loci
- including APOE4, Clusterin, PICALM, CR1
- no presenilins or APP
PD pathology
Lewy bodies - aggregated protein inclusions in sporadic and familial PD, which stain for ubiquitin and a-syn
a-syn is a protein highly enriched in presynaptic terminals; highly expressed throughout brain; point mutations in SNCA -> fPD
SNCA mutations
- 3 mutations known
- SNCA duplications and triplications on chromosome 4
- GWAS with sporadic PD
Why are DA neurons preferentially vulnerable?
- V large, unmyelinated neurons with high metabolic demand
- Vulnerability is region-specific: lose A9 mesostriatal neurons (calbindin D28K -ve, GIRK2 +ve) but preserve A10 mesolimbic neurons (calbindin D28K +ve, GIRK2 -ve)
- DA is highly cytotoxic. Taken up by DAT, sequestered in vesicles by VMAT2 => susceptible SNc has high DAT, low VMAT2; resistant VTA low DAT, high VMAT2
LRRK2
- 7% fPD, 0.5-3% sPD
- G2019S common in Europe (3% sPD, 6% fPD) and 40% North African Arabs
- Dominant disease with low penetrance (G2019S ~30% penetrance)
Glucocerebrosidase (GBA)
- strong susceptibility factor
- encodes lysosomal enzyme (GCase) deficient in Gauchers
- homozygous mutations -> Gauchers; heterozygous -> increased risk in PD (observation from clinic that relatives of Gauchers patients had PD)
- GBA-associated PD has earlier onset and more cognitive changes
Hypothesis: GOF due to mutations in GBA/GCase -> ER stress;
LOF GBA function, similar to Gauchers
Recessive mutations PD
- Parkin, PTEN induced kinase 1 (PINK1), oncogene DJ1
- rare; Parkin -> ~50% familial juvenile PD
- LOF
- onset before 40y/o, L-dopa responsive
- roles in ubiquitination, mitochondrial function and oxidative stress
Nalls et al 2019
- Meta-analysis of GWAS studies, including 56,000 cases and 1.4million controls.
- 90 loci including SNCA, MAPT, LRRK2, GBA
- modest risk (up to 1.4x)
- genes correlate with Mendelian forms better than for Alzheimer’s
Mouse models
- longitudinal study, with limitations of post-mortem tissue
- mice only live 2 years so not really suitable to model late stage disease
- TG GOF
- KO LOF
Mapt-/-
- compensation by MAP1A and MAP1B
- Mapt-/-MAP1B-/- is lethal
- Mapt-/- shows deficiency in maturation of hippocampal neurons in culture vs WT
- has been used to show that tau is necessary for toxic effects of A-beta in Alzheimer’s for example
Snca-/-
- subtle dysregulation of DA signalling (paired pulse depression and decreased striata DA at 18-24 months)
- compensation by beta and gamma synuclein
Senior et al 2008
- a/y-syn double KO show increased DA release in striatum
Lewis et al 2000
Mapt TG mouse model
- exon 10+ 4R P301L; exon 2 and 3 -ve
- widespread tau neuronal expression, hindlimb weakness and paralysis
- 48% loss of motor neurons in spinal cord
- authors stained NFTs from spinal cord and dentate nucleus with congo red, Gallayas, Bielshowsky, AT8 Ab
- spinal cord pathology and hindlimb paralysis are not features of pure tauopathies in humans
Denk & Caffrey
Tau has complex genetic structure (splicing) so rather than just adding the cDNA to the mouse model (exon lined up), should include introns too.
- authors did this using bacterial artificial chromosomes
- allowed mice to express all 6 isoforms of tau
