1. Mitochondria in the etiology and pathogenesis of Parkinson's disease
PHM Fall 2015
Coordinator: Dr. Jeffrey Henderson
Instructor: Dr. David Hampson
Mitochondria in the etiology and pathogenesis of Parkinson's disease
By: Vargha Eslami Amirabadi, Hui En Sally Chen, Jonathan Nhan, Woo Sik Yoon
Oct. 13, 2015

2. Parkinson’s Disease (PD)
Destruction of dopaminergic neurons in substantia nigra pars compacta (SNc) and striatum
Lewy bodies
Unknown cause
Sporadic (~90%)
Familial (~10%)
No available cure for the disease
PD is the 2nd most common neurological disorder that affects more than 1% of individuals over the age of 55. About new cases of PD are diagnosed each year. This neurodegenerative disease progressively worsens with time. The pathological trademark of this disease is the destruction of dopaminergic neurons in the midbrain, specifically the substantia nigra par compacta and the intraneuronal formation of proteinaceous inclusions called Lewy bodies.
The molecular pathogenesis of PD is still not understood. Two forms of PD exist: Sporadic (unknown/idiopathic) and familial (genetic).
There is currently no cure and the etiology of the disease remains uncertain.

3. ~90%
~10%
PD is found in two forms – sporadic (spontaneous/unknown) and familial (genetic). It is suggested that PD occurs from a mix of sporadic and genetic factors rather than just either one.
It is important to keep MPTP, PINK1 and parkin in mind for later.
MPTP

4. Why is dopamine so important?
Destruction of DA neurons
Resting tremors
Parkinson’s Disease
Dopamine is a neurotransmitter that is involved in movement. In reflection of PD, dopaminergic neurons are destroyed. And because of this it causes: 1.) Akinesia (Loss of movement) and Bradykinesia (Slow movement) 2.) Catelepsy 3.) Mood disorders 4.) Resting tremors
Akinesia, Bradykinesia
Mood disorders
Catalepsy

5. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)
Amphiphilic heroine analogue
Evidence linking mitochondrial dysfunction to PD
Extensively used in animal models
Evidence linking mitochondrial dysfunction to PD arose in the 1980s
group of drug addicts in California were rushed to the emergency room with a severe bradykinetic and rigid syndrome. it was discovered that this syndrome was induced by the self-administration of street batches of a synthetic meperidine analogue whose synthesis had been heavily contaminated by a by-product, MPTP. In the period of a few days following the administration of MPTP, these patients exhibited a severe and irreversible akinetic rigid syndrome. Was PD like and acute.
Animal models like mice, non human primates and zebrafish

6. MPTP Mechanism
MPTP = 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine
NOS = nitric oxide synthase
neuronal NOS (nNOS)
Inducible NOS (iNOS)
After its systemic administration, MPTP crosses the blood–brain barrier. Once entering glial cells/astrocytes, MPTP is converted to MPDP+ by Monoamine oxidase B (MAO-B ) within non-dopaminergic (DA) cells, and then to MPP+ by an unknown mechanism. Thereafter, MPP+ is released, again by an unknown mechanism, in the extracellular space. From there, MPP+ is taken up by the dopamine transporter (DAT) and enters into DA neurons.
Within DA, MPP+ inhibits complex I in the ETC, resulting in ATP deficit and increased ‘leakage’ of ROS, like superoxide (O2) from the respiratory chain. Superoxide remains in the cell in which it is produced. On the other hand, nitric oxide (NO), which is produced by nNOS and iNOS outside dopaminergic neurons, is membrane-permeable and can diffuse into neighboring neurons. If the neighboring cell has elevated levels of superoxide, then there is an increased probability of superoxide reacting with NO to form peroxynitrite (ONOO), which can damage lipids, proteins, and DNA. Damaged DNA stimulates PARS (facilitates DNA repair, using NAD as substrate) activity, which further depletes ATP stores. On the other hand, MPP+ may induce the release of cytochrome C from the mitochondria to the cytosol where it initiates the intrinsic apoptosis pathway.
This discovery lead to the hypothesis that mitochondrial dysfunction probably plays a role in PD.
Przedborski, S and Vila, M., Clinical Neuroscience Research 1, (Dec 2001)

7. PINK1 PTEN-induced kinase 1 Encoded by PARK6 gene on chromosome 1
Autosomal recessive
Studies show early onset of PD caused by mutation at PARK6 locus
Detects initial mitochondria dysfunction, then signals Parkin to eventually remove damaged mitochondria
1) PTEN= phosphate and tensin homolog
3) Similar to sporadic PD symptoms
4) Detecting mechanism: when normal degradation of PINK1 in mitochondria is bypassed causing over accumulation of PINK1 on outer membrane

8. Parkin Encoded by PARK2 gene on chromosome 6
Mutation in Parkin is the most common cause of autosomal-recessive PD
Parkin is a ubiquitin E3 ligase  neuroprotective function
Parkin tags (ubiquinates) damaged mitochondria for mitophagy

9. Narendra, D. P. , and Youle, R. J. Antioxidants & Redox Signalling

10. Narendra, D. P. , and Youle, R. J. Antioxidants & Redox Signalling

11. Pickrell, A.M., and Youle, R.J. Neuron. 85,257-73 (2015)
PINK1’s responsibility:
In normal state, PINK1 gets imported through the TOM complex of the outer mitochondrial membrane and into the TIM complex of the inner membrane. Once inside, mitochondrial processing peptidase (MPP) cleaves PINK1 in the hydrophobic domain. This then causes the spanning of the inner mitochondrial membrane via the rhomboid protease called presenilin-associated rhomboid-like (PARL) protein.
Parkin’s responsibility:
When mitochondria is damaged (Diagram explanation):
Through its detecting mechanism, the overaccumulation of PINK1 on the outer mitochondrial membrane (OMM) signals Parkin gene when the mitochondria becomes dysfunctional. PINK1 phosphorylates ubiquitin and Parkin in order to activate Parkin E3 ligase activity. Parkin’s then continues to ubiquitinate the substrates from the outer membrane leading to 2 processes: autophagasome recruitment leading to removal of damaged mitochondria via autophagy (mitophagy) and ubiquitin proteosome degradation. Eventually the mitochondrion becomes an autophagosome that gets final delivery to a lysosome degradation. Overall, both genes work together and are responsible for monitoring and regulating functional mitochondria.
Pickrell, A.M., and Youle, R.J. Neuron. 85, (2015)

12. Current Drug Therapies
Dopamine precursors
Levodopa (L-DOPA) in combination with Carbidopa
Dopamine Agonists
Ex: Pramipexole (Mirapex)
MAO-B Inhibitors
Ex: Rasagiline (Azilect)
Catechol-O-methyltransferase (COMT) inhibitors
Ex: Entacapone (Comptan®)
Anticholinergics
EX: Benztropine (Cogentin)
As we know, there is no cure for PD, however there are a number of drugs provide relief from the symptoms.
Most therapies treat the motor symptoms of PD.
The most common drug therapy of PD is the dopamine precursor, levodopa or L-DOPA
Dopamine agonistsact to mimic the effects of dopamine in the brain
MAO-B Inhibitorsact to inhibit the breakdown of dopamine in the brain by inhibiting the enzyme monoamine oxidase B.
Catechol-O-methyltransferase (COMT) inhibitorslike carbidopa, they inhibit the breakdown of levodopa to dopamine. Helps to prolong dose of levodopa

13. Dopamine synthesis
-This diagram depicts dopamine synthesis
-L-DOPA crosses the blood-brain barrier and is converted to dopamine by dopa decarboxylase, increasing -dopamine availability in the brain
-Oral administration of L-DOPA leads to significant metabolism (converting it to dopamine) in the GI tract which leads to adverse effects. Dopamine is unable to cross the BBB
-To deal with this L-DOPA is almost always used in combination with carbidopa, which is a dopa decarboxylase inhibitor. Carbidopa prevents conversion of L-DOPA to dopamine before reaching the brain. -After reaching the brain, carbidopa can't cross the BBB.
Youdim et al. Nature Reviews Neuroscience 7, 295–309 (April 2006)

14. Mitochondria-targeted antioxiadants (MTAs)
Lipophilic cation allows targeting to mitochondria
-Another approach being explored in the treatment of PD is targeting the reactive oxygen species (ROS) produced by dysfunctional mitochondria.
-This is done by conjugating a lipophilic cation (commonly TPP) to an antioxidant moiety
MitoQ is an example of a MTA where there is evidence that it accumulates in the mitochondria.
-Most recently published clinical trials have shown MitoQ doesn't reduced clinical progression over the course of a year.
-It has been suggested that upon diagnosis of PD, most of the dopaminergic neurons have already been destroyed, thus its not effective to reduce progression of PD
-More studies are required to understand therapeutic effects.

15. Summary
PD caused by destruction of dopaminergic neurons in midbrain (SNc + striatum)
Sporadic (spontaneous/unknown) and familial (genetic) causes, but suggested to be mix of both factors
MPTP induces acute PD symptoms by ROS and activation of the apoptosis cascade
MPTP linked role of mitochondrial dysfunction to PD
Under normal conditions, PINK1 and parkin dispose of dysfunctional mitochondrial by ubiquitination leading to autophagy (mitophagy)
Suggested that mutations in either gene lead to dysfunctional control system and survival of dysfunctional mitochondria = increase ROS and destruction of dopaminergic neurons
PD treatments mostly alleviate motor symptoms without slowing progression
L-DOPA (crosses BBB) with Dopadecarboxylase inhibitors (prevent conversion of L-DOPA to dopamine) most common and effective treatment
MTA - Lipophillic cation (ie. TPP) + Antioxidant (ie. MitoQ10), being explored as possible therapy to target mitochondria

16. References
Jin, H., Kanthasamy, A., Ghosh , A., Anantharama, V., Kalyanaraman , B., and Kanthasamy, (2014) A.G. Mitochondria-targeted antioxidants for treatment of Parkinson's disease: Preclinical and clinical outcomes. Biochimica et Biophysica Acta
Narendra, D.P., and Youle, R.J. (2011) Targeting Mitochondrial Dysfunction: Role for PINK1 and Parkin in Mitochondrial Quality Control. Antioxidants & Redox Signalling. 14,
Pickrell, A.M., and Youle, R.J. (2015) The Roles of PINK1, Parkin, and Mitochondrial Fidelity in Parkinson’s Disease. Neuron. 85,257-73
Przedborski, S., and Miquel, V. (2001) MPTP: a review of its mechanisms of neurotoxicity. Clinical Neuroscience research. 1,
Sanchez-Padilla, J., Guzman, J.N., Ilijic, E., Kondapalli, J., Galtieri, D.J., Yang, B., Schieber, S., Oertel, Q., Wokosin, D., Schumacker, P.T., and Surmeier, D.J. (2014) Mitochondrial oxidant stress in locus coeruleus is regulated by activity and nitric oxide synthase. Nature neuroscience. 17,