PD: Mechanisms of Neurodegeneration

Question 1

Why do DA neurons degenerate?

Answer 1

• oxidative stress
• mitochondrial dysfunction
• ubiquitin-proteasome dysfunction
• neuroinflammation

Question 2

Describe ubiquitin-proteasome pathway

Answer 2

Polyubiquitin chain conjugated to substrate to tag it for degradation by

PD causing mutation -> disrupted ubiquitination -> decreased degradation of damaged proteins -> protein aggregate -> cell death

Question 3

Genetic causes of defects in ubiquitin-proteasome pathway?

Answer 3

PARKIN gene (associated w/ E3 ligase)

NB: E1, E2, E3 are enzymes involved in ubiquitination

Question 4

Mechanism of ROS generation in PD?

Answer 4

DA and its metabolites have tendency to form ROS

SNpc is rich in iron, redox cycles of iron generate ROS

Mitochondrial dysfunctions uncouple redox reactions -> ROS

Question 5

Genetic causes of ROS generation in PD?

Answer 5

Deficiency in antioxidant molecules (glutathione) -> ROS generation

Question 6

Mechanisms of mitochondrial dysfunction in PD?

Answer 6

a-synuclein aggregates can inhibit complex 1->mitchond. dysfunction
OR
ROS -> dysfunction
OR
DJ1 mut-> Mitochondrial membrane dysfunction -> dysfunction

Mitochondrial dysfunction -> Cyt C released -> apoptosis

Question 7

Mechanism of neuroinflammation in PD?

Answer 7

[6-OHDA, MPTP] -> direct microglial activation and indirect (LPS) neurotoxins

Question 8

Overview of midbrain DA neurons?

Answer 8

nigrostriatal: DA neurons in SNpc innervate the dorsal striatum
mesocorticolimbic: VTA neurons innervate nucleus accumbens and olfactory tubercle (via mesolimbic and mesocortical projections)

Question 9

Overview of theories of SNpc DA neurons susceptibility?

Answer 9

• environmental toxins lead to specific degeneration of SNpc neurons
• transcriptional profile may confer inherent invulnerability of SNpc DA neurons
• VTA neurons are more resilient than SNpc neurons

Question 10

How can environmental toxins lead to specific degeneration of SNpc neurons?

Answer 10

MPTP-induced Parkinsonism discovered in heroin users]- MPTP inhibits complex 1

uptake of MPP+ is specific for SNpc neurons, concentrated in mitochondria

NB: not true PD as you are artificially killing DA neurons to produce a similar state

Question 11

Neurodevelopment of DA neurons?

Answer 11

Neural stem progenitor cell -> mDA progenitor -> mDA immature neuron -> mDA mature neuron

Question 12

Main features of DA neuronal transcriptional profile

Answer 12

• Defined as midbrain neurons
• Define by DA neurons

Question 13

Overview of genetic influence of transcriptional profile of DA neurons?

Answer 13

Regional identity: Otx2 (SHH, Wnt1, engrailed)

Specification: LMX1a (FGFB, SHH)

Survival: Engrailed, Nurr 1

Question 14

Nurr 1 role (and experimental evidence)?

Answer 14

Regulates NT indentity of neuron

Nurr-1 deficiency -> DA neurons can’t produce DA

Question 15

FoxA2 role (and experimental evidence)?

Answer 15

Involved in late-stage DA neuron develeopment

Heterozygotes FoxA2 (+/-) mice -> unilateral DA neuron degeneration

Question 16

Engrailed experimental evidence?

Answer 16

Engrailed 1/2

Engrailed KO -> total loss of DA neurons (dose-dependent)

Heterozygote -> cell lost by P90

Question 17

Compare resilience of VTA and SNpc neurons (and experimental evidence)?

Answer 17

Otx2 upregulated in VTA > SNpc

Otx2 overexpression in SNpc -> neuroprotective

Overexpression of pro-survival genes (upreg in VTA but downreg in SNpc) -> neuroprotective

Question 18

Overview of genes involved in PD?

Answer 18

• SNCA (codes for alpha-synuclein)]- first PD gene found
• Parkin]- most common genetic cause; autosomal recessive PD
• PINK-1
• DJ1

Question 19

SNCA: wild type vs mutated (+experiment)?

Answer 19

WT: pre-synaptic vesicle trafficking?

mut: proteasome inhibition, complex-1 inhibition, autophagy inhibition, form LBs

SNCA KO mice -> resistnance to MPTP-induced DA activity

Question 20

Parkin: wild type vs mutated (+experiment)?

Answer 20

WT: E3 ligase adds ubiquitin and aids degradation

mut: Parkin KO -> unable to remove/break down damaged proteins + organelles

Question 21

PINK-1: wild type vs mutated (+experiment)?

Answer 21

WT: marks mitochondria (removed by Parkin)

Parkin overexpression -> rescues PINK1 KO

Question 22

DJ1: wild type vs mutated (+experiment)?

Answer 22

WT: inhibits aggregation of alpha-syn (via its chaperone activity), antioxidant, modulate mitoch. membrane potential

DJ1 KO -> disrupted mit. membrane potential -> marked by PINK-1 to be removed by Parkin for degradation

Question 23

Causes of Sporadic PD and Familial PD?

Answer 23

SPORADIC PD:

• genetic risk factors (Tau, LRRK2, alpha-syn)
• environmental factors (ageing, toxins e.g. MPTP)

FAMILIAL PD:

• autosomal dominant ( LRRK-2, alpha-syn)
• autosomal recessive (Parkin, PINK-1, DJ-1)