Parkinson’s Disease: Misfolding and a Parkin Problem

Parkinson's disease (PD) is a progressive neurological disorder characterized primarily by motor symptoms that include rigidity, hypokinesia and tremor. PD is the second most common neurodegenerative disease and affects about 2-3% of the population 65 years of age and older. Located in the midbrain, this neurological condition particularly affects the substantia nigra pars compacta (SNc) region that produces dopamine. The etiology of the disease is not completely understood; however, inherited risk factors and environmental toxins are considered to be likely causes.

Protein misfolding and oxidative stress

PD is associated with brain lesions known as Lewy bodies, which contain α-synuclein (α-Syn) as the major component. Mutations in the α-Syn gene have been linked to certain familial forms of PD. It has been suggested that mutations in α-Syn (Ala53 to Thr53 and Ala30 to Pro30) may cause a conformational change that renders α-Syn more prone to self-aggregation and deposition in Lewy bodies. Expression of mutant α-Syn in dopaminergic neurons has been shown to impair synaptic vesicle formation, increase cytoplasmic levels of dopamine, and elevate superoxide radicals, which lead to oxidative stress and misfolding of α-Syn. Interestingly, expression of mutant α-Syn (G209A) has been shown to specifically enhance dopamine induced oxidative damage in cell models.

Dysfunctional mitochondrial oxidative metabolism has also been linked to the pathogenesis of PD. Excess reactive oxygen species and higher levels of iron have been detected in the SNc region in PD patients. Current interest in the involvement of mitochondrial dysfunction in PD stems from studies of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-induced PD. MPP+ (1-methyl-4-phenylpyridinium), a metabolite of MPTP, is selectively taken up by dopaminergic terminals and concentrated in neuronal mitochondria in the substantia nigra. Increased MPP+ inhibits complex I of the electron transport chain, leading to reduction in ATP production, increases in free radical formation and lipid peroxidation.

Disruptions in protein degradation and removal

Although much is known about α-Syn aggregation, little information is available on its degradation. Neurosin (kallikrein-6), a serine protease that degrades α-Syn, has been shown to co-localize with Lewy bodies. In vitro studies have shown that neurosin prevents α-Syn polymerization by reducing the amount of monomer and by generating fragmented α-Syn. Under cellular stress, neurosin is released from mitochondria to the cytosol, which results in an increase in degraded α-Syn species.  An autosomal dominant mutation in PD causes a significant decrease in Ubiquitin C-Terminal Hydrolase L1 (UCH-L1) activity, which leads to a reduction in the quantity of available ubiquitin for ubiquitination of misfolded α-Syn and other toxic proteins. UCH-L1 functions as a deubiquitinating enzyme and is responsible for hydrolyzing bonds between ubiquitins to produce monomeric ubiquitin molecules.

Parkin and ubiquitination

Mutations in Parkin, a ubiquitin E3 ligase that can monoubiquitinate and polyubiquitinate proteins to regulate a variety of cellular processes, is associated with familial and sporadic PD.  When over-expressed in a variety of stressful conditions, Parkin serves as a multipurpose protective agent. For instance, Parkin overexpression has been shown to prevent mitochondrial swelling in PC-12 cells treated with ceramide, subjected to serum withdrawal, and can also attenuate kainic acid toxicity in neurons. Mutations in Parkin that result in PD may range from single base pair substitutions, small deletions and splice site mutations, or deletions that span hundreds of thousands of nucleotides. Although various mutations in the Parkin gene are reported in an early autosomal-recessive form of PD, these mutations do not generate Lewy bodies.

Parkin substrates and mediators of mitochondrial health

Well-known substrates of Parkin include the Parkin-associated endothelin-receptor-like receptor (Pael-R), cell-division-control-related protein 1 (CDCrel-1), and Synphilin-1. Synphlilin-1 is a highly conserved presynaptic protein that interacts with α-Syn via its C-terminus and is found in Lewy bodies. Unfolding of Pael-R makes it insoluble and allows it to accumulate in the endoplasmic reticulum. Ubiquitination of Pael-R by Parkin leads to its degradation in the proteasome; however, failure to ubiquitinate leads to neuronal death.  

Interestingly, PTEN-induced putative kinase 1 (PINK1) and Parkin are also shown to work together to govern mitochondrial quality control. PINK1 that accumulates on the outer mitochondrial membrane of damaged mitochondria activates Parkin’s E3 ubiquitin ligase activity. Activated Parkin recruits additional Parkin to translocate from the cytosol to the dysfunctional mitochondrion, which then ubiquitinates outer mitochondrial membrane proteins to trigger selective mitophagy. The damage-sensing mechanism of PINK1 stems from its rapid and constitutive degradation in healthy mitochondria and is inhibited with damaged mitochondria. Mutated PINK1 and Parkin-induced mitophagy may be a critical factor in the pathogenesis of PD and synaptic dysfunction resulting from the aggregation of misfolded proteins.

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