Introduction
Parkinson’s disease is a devastating neurological condition affecting millions of people worldwide. Despite decades of research, current treatments are purely symptomatic and there remains no preventative or curative therapy. Thus, there is an unmet clinical need for a disease-modifying treatment capable of slowing or inhibiting the course of the disease for patients suffering from Parkinson’s disease. There are many issues impeding the development of a neuroprotective treatment, but one key factors has been consistently identified, is the lack of clinically-relevant animal models to test treatment efficacy (Beal, 2010).
Presently, many of the established animal models of Parkinson’s disease rely on the administration of a toxin either systemically or directly into the brain. Two of the earliest and most notable Parkinson’s disease models are induced by 6-hydroxydopamine (6-OHDA) or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). These are the most commonly used for drug discovery and research because of their rapid development, simplicity, reproducibility and cost effectiveness (Beal, 2010, Bezard and Przedborski, 2011, Trigo-Damas et al., 2018). While certainly these models have been invaluable in Parkinson’s disease research, they are not without their limitations, primarily that toxin-based models lack key neuropathological changes relevant to the disease state, such as α-synuclein pathology, and that they are usually not progressive in nature.
A few years after a major breakthrough in understanding Parkinson’s pathology - where the α-synuclein protein was identified as the main component of Lewy bodies and implicated in the disease pathogenesis (Spillantini et al., 1998) - the viral vector α-synuclein overexpression models of Parkinson’s disease were first introduced (Lo Bianco et al., 2002, Kirik et al., 2002). Deniz Kirik and colleagues utilised adeno-associated viral (AAV) vectors injected into the substantia nigra of rats to overexpress either wild-type or A53T mutated human α-synuclein (Kirik et al., 2002). As a consequence of the protein overexpression, they observed a significant loss, albeit highly variable, of dopaminergic neurons in the nigrostriatal pathway was capable of producing motor defects when exceeding 50–60% dopaminergic degeneration. They also found α-synuclein positive inclusions and dystrophic neurites, similar to those observed in patients with Parkinson’s disease. This model has been employed in other animals such as mice (St Martin et al., 2007) and primates (Kirik et al., 2003) with comparable success.
Although this model holds many advantages, has a strong etiological background, and is capable of mimicking a progressive disease state in aged animals, it has its limitations. Primarily, the model suffers from a highly variable and inconsistent disease manifestation, as well as a pathology that is slow to develop (Van der Perren et al., 2015). Many factors can be attributed to the disparity between results such as vector, serotype, promotor and titre (Volpicelli-Daley et al., 2016, Van der Perren et al., 2011, Van der Perren et al., 2015). However, these variations do not explain the discrepancies of neurodegeneration observed within singular studies (Kirik et al., 2002). Therefore, the considerable drawbacks restricting this model signify that enhanced viral α-synuclein-based Parkinson’s disease models are necessary.
One approach to enhancing the viral α-synuclein model may be to sequentially inject AAV-α-synuclein and the novel α-synuclein aggregator, FN075, into the nigrostriatal pathway. FN075 is a peptidomimetic small molecule that has been shown to accelerate α-synuclein aggregation, and the fibrils formed by this process are structurally similar to those formed by naturally occurring aggregation (Horvath et al., 2012, Cegelski et al., 2009). In mice, FN075 caused impairments in motor function and nigrostriatal degeneration 6 months after a single intracerebral injection (Chermenina et al., 2015), while in rats, sequential exposure to the viral mimetic, Poly I:C, followed by FN075, exacerbated the neurodegeneration, neuroinflammation and motor impairment caused by the inflammagen (Olsen et al., 2019).
We recently demonstrated the potential of this molecule for enhancing viral overexpression models by sequentially injecting the rat substantial nigra with AAV-α-synuclein followed 4 weeks later by FN075. FN075 significantly increased AAV-mediated pathological α-synuclein protein levels with a significant increase in protein phosphorylated at serine 129 (pS129-α-synuclein; (Oueslati, 2016, Xu et al., 2015)) in rat substantia nigra (Kelly et al., 2021). However, despite this enhanced α-synuclein pathology this did not enhance nigrostriatal degeneration or motor dysfunction. Following this, we hypothesised that administration of FN075 into the nigrostriatal terminals in the striatum rather than into the cell bodies in the substantia nigra might be a more promising approach. This hypothesis was primarily based on the fact that α-synuclein is strongly enriched in synaptic terminals (Burre, 2015, Sharma and Burre, 2023, Burre et al., 2018) and is thought to contribute to the terminal degeneration and striatal deafferentation associated with the disease (Murphy and McKernan, 2022, Wong et al., 2019, Tofaris, 2022, Tagliaferro and Burke, 2016). Thus, in this study, we sought to determine if administration of this α-synuclein aggregator into the striatum of rats already overexpressing α-synuclein (induced by AAV vectors) would drive further α-synuclein pathology, and lead to nigrostriatal neurodegeneration and the precipitation of motor dysfunction.