Animal Models of Parkinson's Disease

Question 1

State the primary quartet of symptoms in PD

Answer 1

Bradykinesia, resting tremor, rigidity, loss of postural reflexes

Question 2

State some symptoms of autonomic dysfunction

Answer 2

Constipation, postural hypotension, urinary frequency and urgency, impotence, increased sweating

Question 3

State some symptoms of PD other than the primary quartet

Answer 3

Depression, pain, taste disturbances, anosmia, cognitive decline and dementia, hallucinations, anxiety, confusion, sleep disturbances, autonomic dysfunction

Question 4

Do animals naturally develop PD?

Answer 4

No

Question 5

How many more nigrostriatal connections are there in the human brain than the mouse brain?

Answer 5

150,000

Question 6

Why might human nigrostriatal connections be more vulnerable than mouse migrostriatal connections?

Answer 6

Many more connections, greater ATP requirement, more mitochondrial activity, more alpha-synuclein mediated neurotransmission

Question 7

Name the 2 main types of animal model

Answer 7

Toxin-treated, genetically-altered

Question 8

Name 3 toxin-treated animal models of PD

Answer 8

6-hydroxydopamine (6-OHDA), MPTP, lactacysteine

Question 9

Summarise the features of the 6-hydroxydopamine model

Answer 9

Mitochondrial inhibition, selective neuronal loss, iron accumulation causing oxidative stress. No altered proteins

Question 10

Summarise the features of the MPTP model

Answer 10

Mitochondrial inhibition, selective neuronal loss, iron accumulation causing oxidative stress. May cause altered protein formation and deposition if co-administered with probenecid

Question 11

Summarise the features of the lactacystin model

Answer 11

Selective neuronal loss, iron accumulation causing oxidative stress, altered protein formation and deposition

Question 12

Name 4 toxins known to cause parkinsonism in humans

Answer 12

MPTP, manganese, carbon disulphide, cycad seeds

Question 13

Describe the features of manganese-induced parkinsonism

Answer 13

Fixed gaze, bradykinesia, postural difficulties, rigidity, dystonia

Question 14

Describe how 6-OHDA is administered to rodent models

Answer 14

Stereotactic injection into the medial forebrain (loss of cells over 7-9 days) or striatum (slower loss) unilaterally into anaesthetised animals

Question 15

Why is 6-OHDA not used to create a bilateral lesion?

Answer 15

Bilaterally lesioned animals have difficulty eating

Question 16

Why is 6-OHDA not injected directly into the substantia nigra?

Answer 16

The dopaminergic neuron loss occurs too rapidly to adequately mimic PD

Question 17

State the 4 ways in which animal models must be valid

Answer 17

Construct validity, face validity, aetiological validity, predictive validity

Question 18

What is 6-OHDA?

Answer 18

An unstable oxidative by-product of dopamine

Question 19

Name a way of assessing motor asymmetry in the 6-OHDA rodent model

Answer 19

Amphetamine-induced rotational assessment

Question 20

Name 2 animals which MPTP can be used in

Answer 20

Non-human primates, C57black mouse

Question 21

How does MPTP cause parkinsonism?

Answer 21

It inhibits complex I and causes oxidative stress, resulting in mitochondrial inhibition

Question 22

Name the compound co-administered with MPTP to cause protein inclusion formation and state why it is required

Answer 22

Probenecid, to prevent clearance of the active molecule MPP+

Question 23

Why are dopaminergic neurons particularly susceptible to MPP+?

Answer 23

They have high energy requirements due to being long and poorly myelinated, and they contain neuromelanin which accumulates MPP+

Question 24

Describe the effect of MPTP on non-human primates

Answer 24

After 3 doses, they develop a persistent parkinsonian behavioural syndrome which an akinetic, flexed posture, increased limb rigidity, tremor, clumsiness, and freezing episodes

Question 25

Why does MPTP not affect most animals?

Answer 25

It and its active form MPP+ are rapidly removed from the brain - it only persists if there are high levels of neuromelanin

Question 26

State the pathological correlate of MPTP-induced parkinsonian behaviour

Answer 26

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

Question 27

Can MPTP-induced behavioural changes be reversed?

Answer 27

Yes, with L-DOPA or dopamine agonists

Question 28

What are proteasomes?

Answer 28

Part of the cellular machinery that degrades altered proteins

Question 29

Why is the proteasome inhibitor PSI no longer used?

Answer 29

It did not produce reproducible results

Question 30

Name a proteasome inhibitor model

Answer 30

Lactacysteine

Question 31

Describe how lacatcystin is administered and its effects

Answer 31

A single admission by stereotactic surgery to the substantia nigra produces microglial activation, nigral cell loss, protein inclusions, and chronic progression

Question 32

Name the 3 PD-causing gene mutations identified in alpha-synuclein

Answer 32

A53T, A30P, E46K

Question 33

What is the most commonly affected gene in familail PD?

Answer 33

LRRK2

Question 34

Describe a Drosophila model of PD

Answer 34

Drosophilae flies overexpressing alpha-synuclein develop selective neuronal cell death, dense, Lewy body-like structures, and a reduction in climbing ability during flight

Question 35

Describe the effect of wild-type alpha-synuclein overexpression on mice

Answer 35

These transgenic mice develop non-fibrillar neuronal inclusions, but without any cell loss or other neuropathological features of PD

Question 36

Do mice expressing a single alpha-synuclein gene mutation have any pathological features?

Answer 36

No

Question 37

Describe a transgenic mouse model of PD

Answer 37

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

Question 38

State 2 disadvantages of the transgenic mouse model of PD

Answer 38

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

Question 39

Describe a more effective way of genetically altering mice to express parkinsonism than transgenics

Answer 39

Transfection with a viral vector

Question 40

Describe a viral transfection mouse model of PD

Answer 40

The rAAV-alpha-synuclein model produces a loss of dopaminergic striatal terminals and cell bodies with progressive motor behaviour deficits

Question 41

Describe the effect of injecting pre-formed alpha-synuclein fibrils into the mouse striatum

Answer 41

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

Question 42

Describe the effect of LRRK2 mutations in vitro

Answer 42

They produce apoptotic cell death in neuroblastoma cells and mouse cortical neurons

Question 43

Describe the effect of LRRK2 mutations on mice

Answer 43

Synaptic dopamine dysfunction occurs with some movement dysfunction, but no neuronal loss

Question 44

What is the MitoPark mouse?

Answer 44

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

Question 45

Describe the theory behind the MitoPark mouse

Answer 45

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

Question 46

Name 2 drugs which entered human clinical trials for PD based on mouse models

Answer 46

Deferiprone (iron chelator) and pioglitzaone (anti-inflammatory and blood glucose regulator)

Question 47

Describe the results of human clinical trials for deferiprone for PD

Answer 47

It decreased the level of iron in the brains of PD patients with a corresponding improvement in motor symptoms

Question 48

State a disadvantage of deferiprone

Answer 48

It causes neutropenia in 1% of patients, which can progress to agranulocytosis, so necessitates weekly haematological monitoring

Question 49

Describe the result of human clinical trials of pioglitazone for PD

Answer 49

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

Question 50

State at least 4 obstacles to neuroprotection

Answer 50

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