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.