Lincoln Haiby
Unit Assessment
November 18, 2018
Proteins and Cellular Components Associated with Parkinson’s Disease
Parkinson’s disease is the second most common neurodegenerative disease and affects approximately 1%-2% of the population.1 There is no cure for Parkinson’s, but there are several proteins that have been identified to play a part in the disease when they are dysfunctional. The characteristic motor skill degradation is caused in part by loss of dopaminergic neurons in the substantia nigra, which is where the discussions of these proteins will be focused.1
One gene mutation that is one of the identified causes of Parkinson’s disease is the gene SNCA, which encodes for α-synuclein, a small protein whose normal function is not very well understood. Researchers do know that it is found mainly at a presynaptic location of many neurons, and specifically at the nerve terminal, rather than in the cell body or dendrites.2 Aggregations of α-synuclein oligomers into fibrils are one of the most common characteristics of Parkinson’s disease, these are one of the main constituents of Lewy Bodies. These oligomers contain both a very lipophilic element, which helps α-synuclein associate with the negatively charged phospholipid membranes in a promiscuous manner, and a structured region that is able to insert into lipid bilayers in order to weaken or break the membrane.3 When these aggregates weaken the mitochondrial membrane, the elements of the electron transport chain are damaged or inhibited, and ROS begin to build up. This leads to oxidative damage in dopaminergic neurons, as well as loss of function due to their high dependence on the mitochondria to meet their energy demands for membrane excitation.4 One theorized mechanism by which α-synuclein binds to membranes and facilitates vesicle trafficking is through forming a cross-bridge between vesicles and plasma membranes. This is mediated with VAMP2, a vesicle SNARE protein responsible for vesicular fusion with the plasma membrane.
Dopamine itself is also susceptible to oxidation,6 so a higher concentration of ROS in dopaminergic neurons could be leading to an increase in the amount of oxidized dopamine, which is not in the active/correct form to perform its many cellular roles. This decreases the amount of dopamine being released from the synaptic terminals of the neurons. When there is less dopamine being released, there will be less activation of other neurons by dopamine, and due to the brain’s plasticity, more dopaminergic neurons will die since the brain senses that dopamine activity is decreased overall.
The protein Parkin is an E3 ubiquitin ligase.1 The N-terminus has a ubiquitin-like domain, and the carboxy-terminus contains a ubiquitin ligase domain. The normal function of Parkin involves degrading and trafficking of proteins, such as PICK1, synphilin-1, and Pael-R .7 When Parkin ubiquitinates proteins, they are sent to a proteasome for degradation. Parkin works closely with PINK1, a serine/threonine kinase, in a pathway that involves mitochondrial regulation. Essentially, PINK1 is responsible for detecting specific mitochondria that are dysfunctional and accumulating on them, then recruiting Parkin to the dysfunctional mitochondria; PINK1 does this though direct phosphorylation of Parkin on the ubiquitin-like domain at Ser65.1 Parkin polyubiquitinates several proteins on the mitochondria, which initiates mitophagy and marks the entire mitochondria for degradation and vesicular trafficking by a lysosome.7 One possible mechanism of Parkin is the recruitment of p62, an autophagy receptor, when it forms K63-linked chains.1 Since Parkin is a type of mitochondrial quality assurance specialist, it is important for dopamine handling. When Parkin ubiquitinates a protein, it is eventually engulfed into a vesicle and sent off to a lysosome. The cell notices this and initiates turnover of the degraded protein or organelle. Mitochondria are the organelles that provide most of the energy every cell requires, but they can break down over time. Without a mechanism to get rid of old, dysfunctional mitochondria and form new ones, the cell is in trouble. Microtubules are very important to both the Parkin and PINK1 mechanisms; microtubules help regulate cellular trafficking, and Parkin ubiquitinates misfolded α- and β-tubulin in order to degrade dysfunctional microtubules. The binding domains of Parkin that are most involved with this are RING0 (also known as the linker domain), RING1, IBR, and RING2.8 When Parkin itself is dysfunctional, this ubiquitination does not occur and cellular trafficking using microtubules suffers due to accumulation of misfolded microtubules.8 This affects many aspects of cellular and mitochondrial homeostasis, including a build-up of ROS and other damaged cellular components. It is also thought that mitochondrial trafficking is reliant upon functional mitochondria due to the observation that PINK1 is cleaved into ∆N-PINK1 on active mitochondria and then degraded, but this does not happen on dysfunctional mitochondria.8
More on PINK1, this protein plays a role in protecting cells against oxidative stress and apoptosis9 through phosphorylating TRAP1 in the mitochondrial intermembrane space; the phosphorylated TRAP1 then inhibits cytochrome c (in the ETC) from releasing, which protects the mitochondria from undergoing stress-induced apoptosis.9 A more recent discovery in the function of PINK1 demonstrated that PINK1 regulated calcium leaving the mitochondria, but when there is a deficiency of PINK1, ROS production is stimulated due to the calcium buildup in the mitochondria, which can lead to apoptosis.9 In Parkinson’s disease, these proteins are mutated, typically a loss-of-function mutation, and lead to the production of reactive oxygen species (ROS) when defective mitochondria are no longer degraded using lysosomes.
Mutations of leucine-rich repeat kinase 2 (LRRK2) are the most common cause of familial Parkinson’s disease.11 LRRK2 has two different enzymatic functions: a GTPase domain and a serine/threonine kinase domain.11 The kinase domain activates a MAPK pathway that leads to several different results including cellular proliferation, differentiation, and survival.12 One specific set of substrates of LRRK2 are Rab GTPases: LRRK2 phosphorylates the Rab proteins at a conserved threonine site in the switch II region, a location highly involved in switching the protein from active to inactive, and mutations of LRRK2 which inhibit the phosphorylation of Rab proteins causes dopaminergic neuron degeneration.13 LRRK2 and Rab proteins can function to rescue α-synuclein toxicity in the cell, they do this by overpowering the effects of α-synuclein when Rab1 is overexpressed.14 This pathway involves transportation from the ER to the Golgi, and Rab is potentially responsible for tethering and docking transport vesicles to the Golgi. Overexpression of LRRK2 and Rab proteins can overcome the inhibition of vesicle trafficking from the ER to Golgi when it is blocked by aggregations of α-synuclein.14
DJ-1 is a redox sensitive protein that signals oxidative issues to the mitochondria, and possibly begins a signaling cascade for multiple oxidative defense mechanisms for the mitochondria.6 When DJ-1 has a loss-of-function mutation, the defense mechanisms are impaired. This is particularly important in dopaminergic neurons, which are under high and sustained oxidative stress. If the mitochondria cannot overcome the damage from ROS, apoptosis will occur more often.
The vesicular monoamine transporter type-2 (VMAT2) is a dopamine transporter that is located on membranes of vesicles. Its function is to transport monoamines, such as dopamine, from the cytosol into synaptic vesicles; it does this through its activity as an H+-ATPase antiporter, using the electrochemical gradient of synaptic vesicles to package dopamine.15 An increase in VMAT causes increases in vesicular capacity for dopamine, dopamine vesicle volume, and dopamine levels in basal tissue.15 Dopamine in the cytosol is toxic to neurons, so increasing the amount of VMAT, and thus the amount of dopamine that is packaged into vesicles, will reduce the risk of neurotoxicity.15
Knowing some of the mechanisms that are not functioning correctly in Parkinson’s disease is a start, but it seems that there is a long way to go before a way to slow or halt disease progression is found. The specific mechanisms of the proteins discussed must be elucidated in order to make further progress.