Initiation and propagation of α-synuclein aggregation in the nervous system

Abstract

The two main pathological hallmarks of Parkinson's disease are loss of dopamine neurons in the substantia nigra pars compacta and proteinaceous amyloid fibrils composed mostly of α-synuclein, called Lewy pathology. Levodopa to enhance dopaminergic transmission remains one of the most effective treatment for alleviating the motor symptoms of Parkinson's disease (Olanow, Mov Disord 34:812-815, 2019). In addition, deep brain stimulation (Bronstein et al., Arch Neurol 68:165, 2011) to modulate basal ganglia circuit activity successfully alleviates some motor symptoms. MRI guided focused ultrasound in the subthalamic nucleus is a promising therapeutic strategy as well (Martinez-Fernandez et al., Lancet Neurol 17:54-63, 2018). However, to date, there exists no treatment that stops the progression of this disease. The findings that α-synuclein can be released from neurons and inherited through interconnected neural networks opened the door for discovering novel treatment strategies to prevent the formation and spread of Lewy pathology with the goal of halting PD in its tracks. This hypothesis is based on discoveries that pathologic aggregates of α-synuclein induce the endogenous α-synuclein protein to adopt a similar pathologic conformation, and is thus self-propagating. Phase I clinical trials are currently ongoing to test treatments such as immunotherapy to prevent the neuron to neuron spread of extracellular aggregates. Although tremendous progress has been made in understanding how Lewy pathology forms and spreads throughout the brain, cell intrinsic factors also play a critical role in the formation of pathologic α-synuclein, such as mechanisms that increase endogenous α-synuclein levels, selective expression profiles in distinct neuron subtypes, mutations and altered function of proteins involved in α-synuclein synthesis and degradation, and oxidative stress. Strategies that prevent the formation of pathologic α-synuclein should consider extracellular release and propagation, as well as neuron intrinsic mechanisms.

Keywords: Amyloid; Fibril; Lewy body; Neurodegeneration; Oligomer; Parkinson’s disease; α-Synuclein.

Fig. 1

Human α-synuclein fibrils were labeled with Alexa-488. To visualize fibrils at the cell surface, primary hippocampal neurons were incubated at 4 °C for 30 min with Alexa-488 labeled fibrils, rinsed, and images were captured. To visualize internalized Alexa-488 labeled fibrils and oligomers, primary hippocampal neurons were pre-incubated with the fibrils or oligomers for 30 min at 4 °C and then incubated for 15 min at 37 °C. The neurons were rinsed and incubated with trypan blue to quench extracellular Alexa-488 label and to visualize intracellular fibrils and oligomers. When trypan blue binds to proteins on the cell surface, it fluoresces at 560 nm, which is shown in the images as magenta. The right panel shows quantitation of internalization of unsonicated, long fibrils (F-L), sonicated 50 nm fibrils (F-s) and oligomers (O). From Froula JM, Castellana-Cruz M, Anabtawi NM, Camino JD, Chen SW, Thrasher DR, Freire J, Yazdi AA, Fleming S, Dobson CM, et al.: Defining alpha-synuclein species responsible for Parkinson’s disease phenotypes in mice. The Journal of biological chemistry 2019, 294:10392–10,406

Fig. 2

Immunofluorescence was performed in primary hippocampal neurons using antibodies to α-synuclein and either vGLUT1 to identify glutamatergic presynaptic terminals or vGAT to identify GABAergic presynaptic terminals. Colocalization of α-synuclein with vGLUT1 is visualized as yellow in the merged image. Image from: Froula JM, Henderson BW, Gonzalez JC, Vaden JH, McLean JW, Wu Y, et al. alpha-Synuclein fibril-induced paradoxical structural and functional defects in hippocampal neurons. Acta Neuropathol Commun. 2018;6(1):35