Parkinson’s disease, a neurodegenerative disorder that affects more than 6 million people worldwide, is caused by the buildup of alpha-synuclein proteins in the brain.
The biological function of alpha-synuclein is still not well understood, but because of its role in neurodegenerative diseases, researchers are actively studying this protein to understand the mechanisms of the disease and to look for new treatment strategies.
A new study from Elizabeth Rhoades and postdoc Melissa Birol found that when alpha-synuclein binds to extracellular glycoproteins, proteins with added sugar molecules, it can be taken up by neurons more easily.
The paper also identified a specific presynaptic protein, neurexin 1β, as a key regulator in this process and a potential therapeutic target.
Their findings were published in the journal PLOS Biology.
In one possible model for the pathology of Parkinson’s disease, bundles of alpha-synuclein proteins, known as aggregates, form inside a neuron.
This then leads to cell death and the release of alpha-synuclein protein clusters that are taken up by other neurons.
Since neurodegenerative diseases have typical progression patterns, knowing how alpha-synuclein moves between neurons in the brain helps researchers understand disease propagation.
Previous work from the Rhoades lab implicated the presence of a glycan binding site on alpha-synuclein.
This finding, combined with Birol’s experience in analyzing protein-membrane interactions, led to this study of how alpha-synuclein interacts with cell membranes.
Birol was able to enzymatically remove specific glycans from the cell surface to see how their presence or absence would change how alpha-synuclein was taken up by neurons.
The study found that when glycans were removed, the amount of alpha-synuclein clusters taken up by cells was greatly reduced.
And by analyzing giant plasma membrane vesicles, synthetic membranes derived from components of real cells that have the same protein and lipid composition, Birol was also able to see the detailed physical interactions between alpha-synuclein and glycans.
“There’s a structural basis for the alpha-synuclein binding to the glycan, and when the glycans are removed, it changes the nature of the interaction of alpha-synuclein with the cell membrane,” explains Rhoades.
This research focused on the acetylated form of alpha-synuclein proteins, which is present in both healthy and diseased neurons and is less frequently studied.
They found that the acetylated form was more effective at forming clusters of proteins inside neurons and was required for interactions with glycans.
“No one’s really stressed the importance of these acetylated versions,” Birol says. “Generally, we need take a step back in trying to understand how this protein may be propagating between cells, and I think glycans could be an aspect.”
Rhoades and Birol say that the most unexpected finding was the discovery of neurexin 1β as a potential partner in how alpha-synuclein is taken up by neurons.
They hope that future research on this presynaptic protein could provide insights into new treatment strategies for Parkinson’s and other neurodegenerative diseases.
In the near term, Rhoades and her group hope to obtain higher-resolution structural information of alpha-synuclein proteins bound to glycans.
They also hope that this study will inspire future research on alpha-synuclein acetylation and the role of glycans in the progression of the disease and will provide an impetus to look at previously unstudied protein modifications that might be connected to Parkinson’s disease.
“Some cells spontaneously internalize these [alpha-synuclein] proteins and some do not. It has generally been assumed that there are alpha-synuclein specific receptors on the cells that do internalize aggregates.
That may or may not be true, but [our study] suggests that it’s not just the protein receptors but the glycans that are also important,” says Rhoades.
Parkinson’s disease (PD) is the second most common neurodegenerative disorder after Alzheimer’s disease (AD) (Kalia and Lang, 2015).
The pathological hallmarks of PD are intracellular proteinaceous inclusions called Lewy bodies (LB) and Lewy neurites (LN) that are predominantly formed of misfolded and aggregated forms of the presynaptic protein α-Synuclein (α-Syn), and the loss of dopaminergic (DA) neurons in the substantia nigra (SN) (Spillantini et al., 1997; Lang and Lozano, 1998a,b).
Loss of DA neurons in the SN leads to marked decrease of dopamine levels in synaptic terminals of the dorsal striatum (Figures 1A,B), ultimately leading to a loss of the nigrostriatal pathway (Cheng et al., 2010).
The reduction of striatal dopamine triggers a range of motor symptoms including bradykinesia, uncontrollable tremor at rest, postural impairment, and rigidity which together characterize PD as a movement disorder.
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.
The onset of PD, however, is considered to commence at least 20 years prior to detectable motor abnormalities, when a variety of non-motor symptoms can be observed (Hawkes et al., 2010; Kalia and Lang, 2015; Mahlknecht et al., 2015).
This period is referred to as the prodromal phase where patients experience a range of non-motor symptoms including constipation, olfactory dysfunction (hyposmia), sleep disturbance, obesity and depression (Figure (Figure1C;1C; Hawkes et al., 2010; Kalia and Lang, 2015; Mahlknecht et al., 2015).
During the prodromal phase of PD and PD-related disorders, which precedes degenerative cell loss, the expression levels of a range of proteins involved in synaptic transmission are altered in the prefrontal and cingulate cortex, and SN (Dijkstra et al., 2015; Bereczki et al., 2016; Table Table1),1), suggesting that both non-motor and motor symptoms are caused by impaired synaptic communication.
This data indicates that neurodegeneration in PD is a dying back-like phenomenon which starts at synaptic terminals in the striatum and progresses along the nigrostriatal pathway, ultimately affecting homeostasis and survival of DA cell bodies in the SN (Hornykiewicz, 1998; Calo et al., 2016; Caminiti et al., 2017).
It is due to these early-onset synaptic alterations observed prior to DA neuron loss, PD has also been classified as a synaptopathy (Brose et al., 2010; Schirinzi et al., 2016).
The majority of PD cases are sporadic with unknown cause. However, familial cases with autosomal dominant or recessive Mendelian inheritance have revealed fundamental insights into pathogenic mechanisms underlying PD (Keane et al., 2011; Massano and Bhatia, 2012; Kalia and Lang, 2015).
The most commonly identified genetic mutations linked to heritable PD were found in the genes SNCA and LRRK2, responsible for an autosomal-dominant forms of PD, and in Parkin, PINK1, DJ-1, and ATP13A2 which account for PD with autosomal recessive mode of inheritance (Klein and Westenberger, 2012; Ferreira and Massano, 2017).
PD-related mutations in PINK1 and Parkin and the functional interaction of the two proteins led to the identification of mitochondrial dysfunction as one of the major pathogenic pathways underlying PD (reviewed in Exner et al., 2012; Pickrell and Youle, 2015). Additionally, PD-related mutations in Glucocerebrosidase (GBA) , SCARB2 and TP13A2 established lysosomal storage dysfunction as a second pathogenic pathway that also contributes to PD etiology (reviewed in Hardy, 2010; Gan-Or et al., 2015).
However, among the identified PD-related genes, SNCA encoding the presynaptic protein α-Syn remains the most potent culprit underlying PD
. Accumulated α-Syn is the main component of LB, and together with genome-wide association studies it has been shown to have central pathogenic role in both familial and sporadic PD (Satake et al., 2009; Simón-Sánchez et al., 2009).
Yet despite recent progress, the pathogenic mechanisms underlying α-Syn related PD are only starting to emerge.
Here we provide a focussed review emphasizing current knowledge on synaptic function and the various mechanisms of α-Syn induced synaptopathy, and its role in the early stages of PD progression.
Where does PD pathology start?
Several neurodegenerative diseases exhibit early impairment of synaptic function (Bae and Kim, 2017).
This often occurs concomitantly with the manifestation of cognitive symptoms, with a neuronal degeneration emerging at later stages of disease (Milnerwood and Raymond, 2010; Schulz-Schaeffer, 2010; Picconi et al., 2012).
Thus, synaptic dysfunction is considered to be the first step followed by active deconstruction of axons and loss of neuronal connectivity, eventually leading to the death of neuronal perikarya (Figure (Figure3;3; Scott et al., 2010; Lu et al., 2014; Morales et al., 2015; Schulz-Schaeffer, 2015; Calo et al., 2016; Grosch et al., 2016; Tagliaferro and Burke, 2016; Bae and Kim, 2017; Fang et al., 2017; Kouroupi et al., 2017; Roy, 2017).
This succession of events suggests that neural death in PD is initiated at synaptic terminals and progresses proximally toward neural cell bodies in a dying back-like manner.
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.
In line with this hypothesis, almost 20 years ago Hornykiewicz suggested that formation of PD starts by affecting axons in the dorsal striatum prior to degeneration of DA neurons in the SN (Hornykiewicz, 1998). However, this hypothesis only recently gained strong support by a range of pathological and molecular data.
The first evidence that axons were affected in PD came from the pioneer study performed by Braak et al. (1999).
They were able to demonstrate that extensive and very thin α-Syn inclusions were not only present in LB inclusions at the neuron soma but also present in axonal processes (Braak et al., 1999).
More recent studies found α-Syn localized within axonal dystrophic neurites in the striatum of patients with Alzheimer’s disease (AD) and PD (Duda et al., 2002), as well as in multiple system atrophy (MSA) and dementia with Lewy body (DLB) (Galvin et al., 1999).
The localization of α-Syn was further studied through the development of a technique called paraffin-embedded tissue (Kramer and Schulz-Schaeffer, 2007).
This method allowed for the detection of abundant α-Syn micro-aggregates in the neuropil rather than in the cell body of DLB patient brains.
Furthermore, Kramer and Schulz-Schaeffer showed the abundance of synaptic α-Syn micro-aggregates exceeded the amount of α-Syn aggregates in LB or LN by one to two orders of magnitude.
Their filtration technique revealed 50–92% of α-Syn micro-aggregates were found entrapped within the presynaptic terminal. This result correlated with the striking downregulation of presynaptic proteins like syntaxin and synaptophysin, and postsynaptic proteins such as PSD95 and drebrin (Kramer and Schulz-Schaeffer, 2007).
This new mechanistic insight exploring synaptic deficits as a starting point in α-Syn related pathology is in agreement with previous findings, where α-Syn aggregates were observed in axon terminals preceding the formation of LB in DLB and correlating with cognitive impairment (Marui et al., 2002).
Moreover, similar to DLB, PD patient data demonstrates that these individuals experience first locomotor symptoms once 50–60% of DA striatal terminals have already been lost, while the loss of DA neurons in the SN is believed to be only around 30% (reviewed in Burke and O’Malley, 2013).
These observations have been independently confirmed by positron emission tomography; PD patients in early stages of the disease show extensive axonal damage and loss of nigrostriatal pathway connectivity (Caminiti et al., 2017).
This data suggests that α-Syn pathology is abundant in presynaptic terminals and axons, in line with the observation that its normal localization is predominantly in presynaptic terminals (Figure (Figure3).3).
Yet little is known how early accumulation of toxic α-Syn species impairs synaptic homeostasis and function, ultimately leading to DA neurodegeneration in PD.
More information: Melissa Birol et al, Identification of N-linked glycans as specific mediators of neuronal uptake of acetylated α-Synuclein, PLOS Biology (2019). DOI: 10.1371/journal.pbio.3000318.
ournal information: PLoS Biology
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