Potential of Cellular and Animal Models Based on a Prion-Like Propagation of α-Synuclein for Assessing Antiparkinson Agents

Experimental animal models of Parkinson's disease: A transition from assessing symptomatology to α-synuclein targeted disease modification

With the understanding that α-synuclein plays a major role in the pathogenesis of Parkinson's disease (PD), novel animal models have been developed for conducting preclinical research in screening novel disease modifying therapies. Advancements in research techniques in α-synuclein targeted disease modification have utilised methods such as viral mediated expression of human α-synuclein, as well as the inoculation of pathogenic α-synuclein species from Lewy Bodies of PD patients, for accurately modelling progressive self-propagating neurodegeneration. In applying these cutting-edge research tools with sophisticated trial designs in preclinical drug trials, a useful platform has emerged for developing candidate agents with disease modifying actions, promising a greater chance of success for clinical translation. In this article, we describe the transition of well-established animal models of PD symptomatology to newly developed models of PD pathogenesis, with specific focus on methods of viral-mediated and inoculation of pathogenic α-synuclein, that aim to aid scientific translation of neuroprotective strategies.

A sensitive assay reveals structural requirements for α-synuclein fibril growth

The accumulation of α-synuclein (α-syn) fibrils in neuronal inclusions is the defining pathological process in Parkinson's disease (PD). A pathogenic role for α-syn fibril accumulation is supported by the identification of dominantly inherited α-syn (SNCA) gene mutations in rare cases of familial PD. Fibril formation involves a spontaneous nucleation event in which soluble α-syn monomers associate to form seeds, followed by fibril growth during which monomeric α-syn molecules sequentially associate with existing seeds. To better investigate this process, we developed sensitive assays that use the fluorescein arsenical dye FlAsH (fluorescein arsenical hairpin binder) to detect soluble oligomers and mature fibrils formed from recombinant α-syn protein containing an N-terminal bicysteine tag (C2-α-syn). Using seed growth by monomer association (SeGMA) assays to measure fibril growth over 3 h in the presence of C2-α-syn monomer, we observed that some familial PD-associated α-syn mutations (i.e. H50Q and A53T) greatly increased growth rates, whereas others (E46K, A30P, and G51D) decreased growth rates. Experiments with wild-type seeds extended by mutant monomer and vice versa revealed that single-amino acid differences between seed and monomer proteins consistently decreased growth rates. These results demonstrate that α-syn monomer association during fibril growth is a highly ordered process that can be disrupted by misalignment of individual amino acids and that only a subset of familial-PD mutations causes fibril accumulation through increased fibril growth rates. The SeGMA assays reported herein can be utilized to further elucidate structural requirements of α-syn fibril growth and to identify growth inhibitors as a potential therapeutic approach in PD.

TDP-43 is intercellularly transmitted across axon terminals

Transactive response DNA-binding protein 43 kD (TDP-43) is an aggregation-prone prion-like domain-containing protein and component of pathological intracellular aggregates found in most amyotrophic lateral sclerosis (ALS) patients. TDP-43 oligomers have been postulated to be released and subsequently nucleate TDP-43 oligomerization in recipient cells, which might be the molecular correlate of the systematic symptom spreading observed during ALS progression. We developed a novel protein complementation assay allowing quantification of TDP-43 oligomers in living cells. We demonstrate the exchange of TDP-43 between cell somata and the presence of TDP-43 oligomers in microvesicles/exosomes and show that microvesicular TDP-43 is preferentially taken up by recipient cells where it exerts higher toxicity than free TDP-43. Moreover, studies using microfluidic neuronal cultures suggest both anterograde and retrograde trans-synaptic spreading of TDP-43. Finally, we demonstrate TDP-43 oligomer seeding by TDP-43-containing material derived from both cultured cells and ALS patient brain lysate. Thus, using an innovative detection technique, we provide evidence for preferentially microvesicular uptake as well as both soma-to-soma "horizontal" and bidirectional "vertical" synaptic intercellular transmission and prion-like seeding of TDP-43.

Alpha-synuclein propagation: New insights from animal models

Aggregation of alpha-synuclein is implicated in several neurodegenerative diseases collectively termed synucleinopathies. Emerging evidence strongly implicates cell-to-cell transmission of misfolded alpha-synuclein as a common pathogenetic mechanism in synucleinopathies. The impact of alpha-synuclein pathology on neuronal dysfunction and behavioral impairments is being explored in animal models. This review provides an update on how research in animal models supports the concept that misfolded alpha-synuclein spreads from cell to cell and describes how findings in animal models might relate to the disease process in humans. Finally, we discuss the current underlying molecular and cellular mechanisms and future therapeutic strategies targeting alpha-synuclein propagation.

Alpha-synuclein spreading in Parkinson's disease

Formation and accumulation of misfolded protein aggregates are a central hallmark of several neurodegenerative diseases. In Parkinson’s disease (PD), the aggregation-prone protein alpha-synuclein (α-syn) is the culprit. In the past few years, another piece of the puzzle has been added with data suggesting that α-syn may self-propagate, thereby contributing to the progression and extension of PD. Of particular importance, it was the seminal observation of Lewy bodies (LB), a histopathological signature of PD, in grafted fetal dopaminergic neurons in the striatum of PD patients. Consequently, these findings were a conceptual breakthrough, generating the “host to graft transmission” hypothesis, also called the “prion-like hypothesis.” Several in vitro and in vivo studies suggest that α-syn can undergo a toxic templated conformational change, spread from cell to cell and from region to region, and initiate the formation of “LB–like aggregates,” contributing to the PD pathogenesis. Here, we will review and discuss the current knowledge for such a putative mechanism on the prion-like nature of α-syn, and discuss about the proper use of the term prion-like.