Animal Models of Parkinson's Disease Flashcards
State the primary quartet of symptoms in PD
Bradykinesia, resting tremor, rigidity, loss of postural reflexes
State some symptoms of autonomic dysfunction
Constipation, postural hypotension, urinary frequency and urgency, impotence, increased sweating
State some symptoms of PD other than the primary quartet
Depression, pain, taste disturbances, anosmia, cognitive decline and dementia, hallucinations, anxiety, confusion, sleep disturbances, autonomic dysfunction
Do animals naturally develop PD?
No
How many more nigrostriatal connections are there in the human brain than the mouse brain?
150,000
Why might human nigrostriatal connections be more vulnerable than mouse migrostriatal connections?
Many more connections, greater ATP requirement, more mitochondrial activity, more alpha-synuclein mediated neurotransmission
Name the 2 main types of animal model
Toxin-treated, genetically-altered
Name 3 toxin-treated animal models of PD
6-hydroxydopamine (6-OHDA), MPTP, lactacysteine
Summarise the features of the 6-hydroxydopamine model
Mitochondrial inhibition, selective neuronal loss, iron accumulation causing oxidative stress. No altered proteins
Summarise the features of the MPTP model
Mitochondrial inhibition, selective neuronal loss, iron accumulation causing oxidative stress. May cause altered protein formation and deposition if co-administered with probenecid
Summarise the features of the lactacystin model
Selective neuronal loss, iron accumulation causing oxidative stress, altered protein formation and deposition
Name 4 toxins known to cause parkinsonism in humans
MPTP, manganese, carbon disulphide, cycad seeds
Describe the features of manganese-induced parkinsonism
Fixed gaze, bradykinesia, postural difficulties, rigidity, dystonia
Describe how 6-OHDA is administered to rodent models
Stereotactic injection into the medial forebrain (loss of cells over 7-9 days) or striatum (slower loss) unilaterally into anaesthetised animals
Why is 6-OHDA not used to create a bilateral lesion?
Bilaterally lesioned animals have difficulty eating
Why is 6-OHDA not injected directly into the substantia nigra?
The dopaminergic neuron loss occurs too rapidly to adequately mimic PD
State the 4 ways in which animal models must be valid
Construct validity, face validity, aetiological validity, predictive validity
What is 6-OHDA?
An unstable oxidative by-product of dopamine
Name a way of assessing motor asymmetry in the 6-OHDA rodent model
Amphetamine-induced rotational assessment
Name 2 animals which MPTP can be used in
Non-human primates, C57black mouse
How does MPTP cause parkinsonism?
It inhibits complex I and causes oxidative stress, resulting in mitochondrial inhibition
Name the compound co-administered with MPTP to cause protein inclusion formation and state why it is required
Probenecid, to prevent clearance of the active molecule MPP+
Why are dopaminergic neurons particularly susceptible to MPP+?
They have high energy requirements due to being long and poorly myelinated, and they contain neuromelanin which accumulates MPP+
Describe the effect of MPTP on non-human primates
After 3 doses, they develop a persistent parkinsonian behavioural syndrome which an akinetic, flexed posture, increased limb rigidity, tremor, clumsiness, and freezing episodes
Why does MPTP not affect most animals?
It and its active form MPP+ are rapidly removed from the brain - it only persists if there are high levels of neuromelanin
State the pathological correlate of MPTP-induced parkinsonian behaviour
Persistent and severe depletion of dopamine in the caudate and putamen due to dopaminergic cell loss in the substantia nigra. The adjacent ventral tegmental area is not affected
Can MPTP-induced behavioural changes be reversed?
Yes, with L-DOPA or dopamine agonists
What are proteasomes?
Part of the cellular machinery that degrades altered proteins
Why is the proteasome inhibitor PSI no longer used?
It did not produce reproducible results
Name a proteasome inhibitor model
Lactacysteine
Describe how lacatcystin is administered and its effects
A single admission by stereotactic surgery to the substantia nigra produces microglial activation, nigral cell loss, protein inclusions, and chronic progression
Name the 3 PD-causing gene mutations identified in alpha-synuclein
A53T, A30P, E46K
What is the most commonly affected gene in familail PD?
LRRK2
Describe a Drosophila model of PD
Drosophilae flies overexpressing alpha-synuclein develop selective neuronal cell death, dense, Lewy body-like structures, and a reduction in climbing ability during flight
Describe the effect of wild-type alpha-synuclein overexpression on mice
These transgenic mice develop non-fibrillar neuronal inclusions, but without any cell loss or other neuropathological features of PD
Do mice expressing a single alpha-synuclein gene mutation have any pathological features?
No
Describe a transgenic mouse model of PD
Transgenic mice expressing the A30P/A53T double mutant form of alpha-synuclein show an age-dependent decline in substantia nigra cell number and a reduction in motor function, but no protein inclusions
State 2 disadvantages of the transgenic mouse model of PD
1) Expensive to produce
2) Must be maintained for a long time to see age-related neurodegeneration
3) No human with PD has ever had an alpha-synuclein double mutation
Describe a more effective way of genetically altering mice to express parkinsonism than transgenics
Transfection with a viral vector
Describe a viral transfection mouse model of PD
The rAAV-alpha-synuclein model produces a loss of dopaminergic striatal terminals and cell bodies with progressive motor behaviour deficits
Describe the effect of injecting pre-formed alpha-synuclein fibrils into the mouse striatum
The fibrils are taken up by neurons and result in the formation of inclusions resembling Lewy bodies and neurites. The fibrils cause native alpha-synuclein to misfold, with inclusions spreading throughout the brain. However, this model produces little dopaminergic neuron loss and no behavioural deficits
Describe the effect of LRRK2 mutations in vitro
They produce apoptotic cell death in neuroblastoma cells and mouse cortical neurons
Describe the effect of LRRK2 mutations on mice
Synaptic dopamine dysfunction occurs with some movement dysfunction, but no neuronal loss
What is the MitoPark mouse?
A Tfam knockout mouse with reduced mitochondrial DNA expression and mitochondrial respiratory chain deficiency in dopaminergic neurons, leading to adult-onset striatal neuronal loss with inclusions and progressive motor impairment
Describe the theory behind the MitoPark mouse
Tfam - mitochondrial transcription factor A - directly regulates mitochondrial DNA transcription, which encodes 12 respiratory chain subunits. Mitochondrial DNA mutations increase with age, and PD patients have more mitochondrial DNA mutations in dopaminergic neurons than age-matched controls
Name 2 drugs which entered human clinical trials for PD based on mouse models
Deferiprone (iron chelator) and pioglitzaone (anti-inflammatory and blood glucose regulator)
Describe the results of human clinical trials for deferiprone for PD
It decreased the level of iron in the brains of PD patients with a corresponding improvement in motor symptoms
State a disadvantage of deferiprone
It causes neutropenia in 1% of patients, which can progress to agranulocytosis, so necessitates weekly haematological monitoring
Describe the result of human clinical trials of pioglitazone for PD
It failed at phase II - but epidemiological studies indicate that glitazone-treated individuals have a 28% lower incidence of PD, so it is possible that it is neuroprotective but once symptom-onset has started it is too late for it to be beneficial
State at least 4 obstacles to neuroprotection
Incomplete understanding of disease pathogenesis, lack of accurate animal models, challenges in clinical trials, need for validated biomarkers, insensitive end-points and outcome measures, may be applying treatments too late
