17Neuro - Parkinson's Disease

Question 1

Who was the first person to describe Parkinson’s Disease?

Answer 1

James Parkinson in 1817.

Question 2

What is Parkinson’s disease?

Answer 2

A movement disorder characterised by:
Bradykinesia (slowness), akinesia (absence of movement), muscle stiffness, tremor of limbs = typically in hands, postural instability
Progressive degeneration of CNS, increasing incidence with age, 1% of those over 55

Question 3

What forms of Parkinson’s disease exist?

Answer 3

Sporadic - 90% (mostly idiopathic - unknown causes)
Drug-induced (rare - from the 1980s)
Familial (10-15%)

Question 4

What are the main pathological features of Parkinson’s disease?

Answer 4

Loss of pigmented neurons (neuromelanin granules) of substantia nigra = black substance
Appearance of intracellular accumulations of insoluble proteins
LEWY BODIES, in the substantia nigra, which contain ubiquitin, α-synuclein and neurofilament proteins

Question 5

Describe some of the neurochemistry involved in Parkinson’s disease.

Answer 5

Reserpine which lowers monoamine levels, results in Parkinsonism = Used to treat hypertension
L-dopa alleviated reserpine-induced symptoms = A precursor of dopamine
Dopamine levels reduced in basal ganglia of Parkinsonism patients
Associated with loss of 80% of substantia nigra neurons = to produce symptoms

Question 6

How is the substantia nigra linked to movement disorders?

Answer 6

Substantia nigra neurons release dopamine in basal ganglia (which regulates movement)
Movement problems result of dopamine deficit
Neurons of substantia nigra have long processed which reach the basal ganglia = regulation of movement disregulated in Parkinson’s disease

Question 7

What is the substantia nigra pars compacta?

Answer 7

SNpc = Substantia Nigra pars compacta = portion of the substantia nigra formed by dopaminergic nerves = loss of substantia nigra neurons decreases capacity of the nigrostriatal pathway

Question 8

How can PET be used in the diagnosis of Parkinson’s disease?

Answer 8

Positron Emission Tomography showing caudate and putamen
Dopaminergic nerves release dopamine but will also reuptake dopamine via a reuptake transporter mechanism to control [neurotransmitter] in synapse
A radioactive ligand can be used to visualise dopamine reuptake

Question 9

How is dopamine metabolism in the substantia nigra linked to neurodegeneration in Parkinson’s disease?

Answer 9

Neurons of substantia nigra produce and release dopamine
Degradation of dopamine yields H2O2 and superoxide ion
H2O2 could react with Fe2+ to yield •HO
Death of SN neurons could lead to increased dopamine turnover in surviving neurons and increased ROS
Feed forward = once cells start to die, the remaining cells die at an accelerated rate
Cells of the substantia nigra are at particular risk due to their role in dopamine release

Question 10

How is dopamine broken down?

Answer 10

Dopamine can be broken down enzymatically or non-enzymatically.
The enzymatic route sees dopamine plus water being converted to 3,4-dihydroxyphenylacetaldehyde, hydrogen peroxide and ammonia by the enzyme MAO B.
The non-enzymatic route sees dopamine and oxygen being converted to semiquinone, superoxide and a proton.
Dopamine can also combine with superoxide and 2 protons to form semiquinone and hydrogen peroxide.

Question 11

Why can dopamine be broken down by MAO B?

Answer 11

Dopamine is a monoamine it can therefore be degraded by Monoamine oxidase B (MAO B)

Question 12

What is a hallmark of most cases of Parkinson’s disease?

Answer 12

Hallmark of most cases of Parkinson’s Disease is protein inclusion bodies in the cytoplasm (Lewy bodies) and nerve fibres (Lewy neurites)
Contain α-synuclein, which may be nitrated (free-radical damage) and abnormally glycosylated
Ubiquitin and components of ubiquitin-proteasome system and hsp70 chaperone = may be a failed attempt to remove
Lewy bodies may be aggresomes = cytoprotective inclusions which slowly deliver misfolded proteins to autophagy (autophagy = lysosomal system for degradation)

Question 13

What caused drug-induced Parkinsonism in the 1970s?

Answer 13

In 1978, Californian drug addict synthesised “false heroin” to evade drug laws
Users developed symptoms associated with Parkinson’s Disease
By-product of synthesis is a pyridine, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)
MPTP metabolised to toxic MPP+ (1-methyl-4-phenylpyridinium) by astrocytes
MPP+ taken up by dopamine reuptake system (DAT) which may provide selectivity for dopaminergic neurons
MPP+ binds non-covalently to ubiquinone of NADH-dehydrogenase complex
Blocks electron transport due to free radicals formed from MPP+ (?)
Blocked electron transport may release electrons which will reduce oxygen to superoxide
Loss of mitochondrial membrane potential may lead to apoptosis - release of cytochrome c
Toxicity of MPP+ in vitro depends on nitric oxide synthase (NOS) activity - possible reactive nitrogen species (RNS) involvement
Mitochondrial damage = ROS/RNS = apoptosis

Question 14

How are free radicals linked to Parkinson’s disease?

Answer 14

Tissue from sporadic Parkinson’s disease exhibit lower activity of mitochondrial complex I
Lower levels of glutathione (GSH) in the substantia nigra = suggesting free radical exposure as glutathione is an anti-oxidant
Increased levels of iron = Fenton reaction
With no concomitant increase in transferrin
Fe2+ levels high in the neuromelanin in the substantia nigra (normal brain)
High ROS, Fe2+ leads to •OH

Question 15

What mutations have been identified in familial Parkinson’s disease?

Answer 15

Familial cases of Parkinson’s disease identified with a mutation in α-synuclein (PARK1)
A53T, A30P, G46K autosomal dominant
Toxic gain of function?
Gene duplication and triplication causes Parkinson’s disease
Gene dosage effect? = something intrinsic of alpha-synuclein
Mutant synuclein forms pleated-sheet like structure and aggregates
Non-amyloid component (NAC) domain promotes aggregation
Non-PD “synucleinopathies” e.g. Diffuse Lewy Body Disease = affects different neurons = cerebral cortex = leads to cognition problems = form of dementia

Question 16

What feature of alpha-synuclein makes it prone to aggregation?

Answer 16

IDP = intrinsically disordered protein

Structure of protein changes readily can easily form toxic aggregates

Question 17

Describe some of the main features of alpha-synuclein.

Answer 17

Largely unstructured (Intrinsically Disordered Protein, IDP) forms amino-terminal α-helices on binding acidic phospholipids 
Binds neurotransmitter vesicles (dopamine containing), possible chaperone
Possible modulator of VMAT2 - dopamine transporter 
VMAT2 takes up cytosolic dopamine into vesicles

Question 18

How is alpha-synuclein linked to fibril formation?

Answer 18

α-synuclein readily forms β-sheet-like structure (normal structure is more alpha-helical) leading to fibril formation resulting from:
Mutations associated with Parkinson’s disease (e.g. A53T)
Gene duplication - over expression
Free radical damage e.g. tyrosine nitration
Synuclein aggregation impairs ubiquitin-proteasome system = leads to synuclein accumulation and more oligomerization/fibrils
Reduction in UPS activity leads to Lewy Body formation (aggresome?)

Question 19

How is alpha-synuclein linked to Parkinson’s disease pathology?

Answer 19

α-synuclein (soluble) oligomers:
Form Ca2+ pores in the cell membrane = free radical production = toxic effects
Bind to mitochondria = electron leakage, ROS formation
Disrupt vesicle trafficking/dopamine transport = increase in dopamine metabolism = ROS
Disrupt the cytoskeleton = Large aggegates
May start in the gut and travel up nerves to the brain, and pass from cell to cell - prion-like = Protein conformation transmitted

Question 20

A mutation in which gene leads to autosomal recessive juvenile onset Parkinson’s disease?

Answer 20

An Autosomal Recessive - Juvenile Parkinsonism (AR-JP) results from mutations in the Parkin (PARK2) on chromosome 6
AR-JP onset in early 30s
Loss of cells in substantia nigra, tyrosine hydroxylase (catecholamine biosynthesis) activity low

Question 21

How is ARJP different to other forms of Parkinson’s disease?

Answer 21

No inclusion bodies (LB) detectable with antibodies to α-synuclein or ubiquitin - Parkin required?

Question 22

What is Parkin? Describe its main features.

Answer 22

Parkin
One of largest human genes, over 1Mb, 12 exons coding for 465-residue 52kDa protein
Cytosolic E3 ubiquitin-ligase
N-terminal 76-amino acids similar to ubiquitin
Central unique “Parkin” domain
C-terminal RING finger - IBR (In Between Ring finger domain) - RING finger
C-terminal portion (PAUL) similar to “ariadne” ubiquitin ligases
Abundant in substantia nigra
Multi-domain protein

Question 23

How are mutations in PINK1 and DJ-1 linked to familial Parkinson’s disease?

Answer 23

Mutations in PINK1 (PARK6) and DJ-1 (PARK7) lead to autosomal early onset PD
PINK1 (PTEN): mitochondrial kinase involved in stress response - mitochondrial fission?
DJ-1: involved in stress response, translocates to mitochondria following oxidative stress - promotes mitochondrial fission?

Question 24

How are PINK1 and Parkin related to mitophagy?

Answer 24

PINK1 associates with mitochondria when mitochondria de-polarised
Otherwise, cleaved and fragment in cytosol associated with Parkin = doesn’t occur when damaged
DJ-1 recruited to outer mitochondrial membrane when mitochondria
PINK1, DJ-1 and Parkin form (soluble) complex
Defective mitochondria accumulate in Parkin, Pink1 Drosophila mutants

Uncoupled mitochondria:
Identified by Pink1, ubiquitylation by Parkin (K63)?, mitochondrion degraded by autophagy (mitophagy) and OMM by proteasome
Parkin/PINK1/DJ-1 (?) necessary for elimination of damaged mitochondria?

Loss of function leads to accumulation of damaged mitochondria triggering apoptosis?
Formation of phagopore in cytoplasm surrounds mitochondria = degraded

Question 25

What is the link between damaged mitochondria and Parkinson’s disease?

Answer 25

Dopaminergic neurons have higher levels of mtDNA damage -DA metabolism and ROS?
α-synuclein aggregates bind to the OMM and may increase damage mitochondria
Failure of mitophagy leads to increased levels of damaged mitochondria
Mutant synuclein, Parkin, Pink1, DJ-1 increase damaged mitochondria
Increased levels of damaged mitochondria results in increased calcium concentration, ROS and pro-apoptotic factors

Question 26

How is UCH-L1 related to Parkinson’s disease?

Answer 26

Mutations in PARK5/UCH-L1 can cause familial Parkinson’s disease.
German families harbour a I93M mutation in UCH-L1
UCH-L1 is abundant in the brain - 2% of protein in brain = cleaves poly-ubiquitin and multi-ubiquitin chains
50% reduction of activity in mutant
Inhibition of UPS? Synuclein accumulation?

Question 27

How is PARK8/LRRK-2 related to Parkinson’s disease?

Answer 27

Mutations in PARK8/LRRK-2 can cause familial Parkinson’s disease.
Mutations in leucine repeat rich kinase 2 (LRRK-2) largest cause of FPD = Also polymorphisms in the gene
G2019S 5-6% of FPD
6 of 20 known mutations pathogenic
Also mutations and polymorphisms in LRRK2 associated with risk of “sporadic” late onset Parkinson’s disease
Major cause of “sporadic” Parkinson’s?
Highly conserved
Multiple functions

Question 28

How is the ubiquitin system linked to Parkinson’s disease?

Answer 28

Mutations (e.g. α-synuclein), toxins, free radicals produce abnormal proteins and overwhelm the UPS
α-synuclein readily aggregates
Inhibits proteasome
Lewy bodies produced
Parkin required for Lewy Body formation
Protein aggregates may lead to substantia nigra (SN) cell death
Increased DA metabolism, more ROS

Question 29

What are suspected to be some of the causes of sporadic Parkinson’s disease?

Answer 29

Genetic susceptibility - e.g. LRRK2, GBA, glutathione-S-transferase mutations/polymorphisms
Suggested environmental toxins may be responsible
Pesticides e.g. Rotenone, Paraquat, Cyberquat (=MPP+) may damage mitochondria and increase ROS
Age-related decline in defense against toxins, free radicals, abnormal proteins
Dopamine metabolism puts the substantia nigra at risk

Question 30

What is the main treatment for Parkinson’s disease?

Answer 30

Levodopa (L-dopa) is the main treatment for Parkinson’s disease.
Intermediate in dopamine synthesis used as a therapy to increase dopamine levels
Decarboxylated to dopamine
Absorbed by transporter in the duodenum and jejunum for large neutral amino acids
Metabolized in liver by aromatic amino acid decarboxylase into dopamine
Only 1% reaches brain as L-dopa
Administered with carbidopa or benserazide to inhibit decarboxylase outside the brain and deprenyl (selegilene) to inhibit monoamine oxidase B (MAO B)

Question 31

What are some of the other treatments for Parkinson’s disease?

Answer 31

Inhibitor of catechol-O-methyltransferase (COMT)
Tolcapone (Tasmar), Opicapone
COMT converts levodopa (L-dopa) to 3-O-methyldopa
MAO B inhibitors
Deprenyl (selelgiline), Safinamide
Rasagiline (Azilect) - also appears to protect mitochondria = activates Bcl-2 & PKC = anti-apoptotic
Dopamine receptor agonists
Bromocriptine, pramipexole (Mirapex), ropiniriole (Requip)

Question 32

What are some of the newer treatments for Parkinson’s disease?

Answer 32

Stem cell transplants
Gene therapy - to boost neurotransmitter levels
Growth factors - GDNF = protect neurons
Squalamine - steroid derivative from dogfish shark liver, appears to prevent synuclein aggregation (cell culture, nematodes). Oral dose could act on gut? = Gut involvement = prophylaxis
Gut flora management - may influence synuclein aggregation?
Nilotinib - inhibits c-Abl kinase (cancer) which inhibits Parkin and promotes synuclein aggregation
Anti-synuclein vaccines = Prevent alpha-synuclein conformational change