Mechanisms of α-Synuclein Induced Synaptopathy in Parkinson's Disease

Abstract

Parkinson's disease (PD) is characterized by intracellular inclusions of aggregated and misfolded α-Synuclein (α-Syn), and the loss of dopaminergic (DA) neurons in the brain. The resulting motor abnormalities mark the progression of PD, while non-motor symptoms can already be identified during early, prodromal stages of disease. Recent studies provide evidence that during this early prodromal phase, synaptic and axonal abnormalities occur before the degenerative loss of neuronal cell bodies. These early phenotypes can be attributed to synaptic accumulation of toxic α-Syn. Under physiological conditions, α-Syn functions in its native conformation as a soluble monomer. However, PD patient brains are characterized by intracellular inclusions of insoluble fibrils. Yet, oligomers and protofibrils of α-Syn have been identified to be the most toxic species, with their accumulation at presynaptic terminals affecting several steps of neurotransmitter release. First, high levels of α-Syn alter the size of synaptic vesicle pools and impair their trafficking. Second, α-Syn overexpression can either misregulate or redistribute proteins of the presynaptic SNARE complex. This leads to deficient tethering, docking, priming and fusion of synaptic vesicles at the active zone (AZ). Third, α-Syn inclusions are found within the presynaptic AZ, accompanied by a decrease in AZ protein levels. Furthermore, α-Syn overexpression reduces the endocytic retrieval of synaptic vesicle membranes during vesicle recycling. These presynaptic alterations mediated by accumulation of α-Syn, together impair neurotransmitter exocytosis and neuronal communication. Although α-Syn is expressed throughout the brain and enriched at presynaptic terminals, DA neurons are the most vulnerable in PD, likely because α-Syn directly regulates dopamine levels. Indeed, evidence suggests that α-Syn is a negative modulator of dopamine by inhibiting enzymes responsible for its synthesis. In addition, α-Syn is able to interact with and reduce the activity of VMAT2 and DAT. The resulting dysregulation of dopamine levels directly contributes to the formation of toxic α-Syn oligomers. Together these data suggest a vicious cycle of accumulating α-Syn and deregulated dopamine that triggers synaptic dysfunction and impaired neuronal communication, ultimately causing synaptopathy and progressive neurodegeneration in Parkinson's disease.

Keywords: Parkinson's disease; SNARE complex; active zone; dopamine; neurodegeneration; synapse; synaptopathy; α-synuclein.

Figure 1

Midbrain dopaminergic neurons are specifically vulnerable in Parkinson's disease. (A) The predominant symptoms of Parkinson's disease (PD) are caused by loss of dopaminergic (DA) neurons in the substantia nigra (SN). According to the dying-back hypothesis, the degeneration of DA neurons is preceded by dysfunction and in turn degeneration of the nigrostriatal pathway, which innervates the caudate nucleus and the putamen that together form the striatum. (B) Compared to healthy controls (left), nigrostriatal degeneration results in the depletion and ultimate loss of the neurotransmitter dopamine on synaptic terminals of striatal neurons (right). (C) The resulting motor symptoms, among others, are usually diagnosed when approximately 30–60% of striatal DA neurons are already lost. However, PD patients can experience non-motors symptoms 20 years before the onset of motor abnormalities in the so-called prodromal phase; these include olfactory dysfunction, sleep disturbances and depression.

Figure 2

The presynaptic protein α-Synuclein is a pathological seed for Parkinson's disease formation. (A) α-Synuclein (α-Syn) is a small soluble cytoplasmic protein of 140 amino acid encoded by the SNCA gene; its main protein domains comprise an N-terminal amphipathic region, a non-amyloid-β component (NAC) domain and a C-terminal acidic tail. Several dominant inherited missense mutations have been identified in the amphipathic region causing early-onset PD, whereas the NAC domain has been implicated in α-Syn aggregation. (B) Under normal physiological conditions, α-Syn monomers function in a dynamic equilibrium between a soluble and a membrane-bound state. Under cellular stress and in disease-relation conditions, α-Syn monomers can interact leading to oligomers that buffer the formation of protofibrils, ultimately causing the formation of amyloid-β sheet fibrils that aggregate into Lewy bodies (LB).

Figure 3

α-Syn accumulation in presynaptic terminals causes synaptopathy ultimately leading to dying back-like neurodegeneration. (A) Under physiological conditions, α-Syn functions as monomers at the presynaptic terminals in synaptic transmission. (B) Formation of toxic α-Syn species, such as oligomers and fibrils, have been shown to play a pivotal role in PD pathogenesis. These toxic species accumulate at the presynaptic terminal, leading to altered levels of proteins involved in synaptic transmission, ultimately causing synaptic dysfunction. (C) As a result of toxic α-Syn accumulation, affected synapses will undergo a process of active deconstruction leading to loss of neuronal connections and subsequent death of the neuronal perikarya.

Figure 4

The presynaptic exo-endocytotic cycle regulating neurotransmitter release is specifically affected in α-Syn-related synaptopathies. Upon an incoming action potential, calcium (Ca²⁺) channels become permeable to Ca²⁺ entry in the presynaptic terminal. This activates a molecular machinery including the SNARE complex proteins that recruit synaptic vesicles (SV) from the proximal resting and recycling (RP) pools via trafficking and tethering to form the readily releasable pool (RRP). After docking and priming, RRP vesicles undergo SNARE-mediated membrane fusion at the active zone (AZ, shaded area), ultimately leading to neurotransmitter (NT) release into the synaptic cleft. After exocytosis, the SV membrane is retrieved to the presynaptic terminal via endocytosis, to be filled with NT (NT uptake) and re-enter the exo-endocytotic cycle, thereby granting the neuron its ability to sustain high firing rates. PSD95, postsynaptic density protein-95.

Figure 5

Onset of α-Syn-mediated synaptopathies is likely related to impaired presynaptic active zone function and defective neurotransmitter release. (A) In healthy subjects, the presynaptic terminal of a neuron comprises a functional exo-endocytotic cycling machinery, including vesicle pools, SNARE complex proteins and the active zone (AZ) which is formed of a dense network of proteins called AZ matrix. The AZ matrix is the site of action for synaptic vesicle tethering, docking and membrane fusion for neurotransmitter release which is mediated by SNARE complex proteins including VAMP-2, SNAP-25, and Syntaxin-1. This process is directed and co-chaperoned by CSP and α-Syn toward the AZ, a scaffold that includes ELKS/CAST proteins, the Drosophila homolog of Bruchpilot (BRP). BRP mutants have been shown to cause dramatic reduction of nerve-evoked transmission. Maintenance of BRP within the AZ matrix scaffold is dependent on direct interaction with NMNAT which protects BRP against ubiquitin (Ub)-mediated degradation and activity-induced neurodegeneration. (B) In PD patients, presynaptic neuronal terminals accumulate toxic oligomers and protofibrils of α-Syn that interfere with the exo-endocytotic cycling machinery, including AZ proteins NMNAT and BRP/ELKS/CAST (enlarged in C). (C) Disease-related downregulation of NMNAT leads to ubiquitination and degradation of BRP/ELKS/CAST, resulting in the gradual decrease and eventual dissolution of the AZ matrix, ultimately causing impaired synaptic transmission and α-Syn-mediated synaptopathy. However, it is currently unknown (doted red lines) whether toxic forms of α-Syn directly or indirectly interfere with NMNAT and/or BRP/ELKS/CAST, thereby causing the AZ to lose its physiological function.