How Well Do Rodent Models of Parkinson's Disease Recapitulate Early Non-Motor Phenotypes? A Systematic Review

Early detection of Parkinson's disease: Retinal functional impairments as potential biomarkers

BACKGROUND

Parkinson's disease is typically diagnosed after substantial neurodegeneration despite early non-motor symptoms manifesting decades earlier. These changes offer a promising avenue for diagnostic exploration, especially within the eye, which has been proposed as a "window to the brain."

OBJECTIVE

The aim was to identify biomarkers by validating the use of electroretinography, a non-invasive technique, to detect early retinal function anomalies reflecting central dysfunction.

METHODS

Homozygous M83 transgenic mice (n = 10 males, 11 females), overexpressing human A53T α-synuclein, underwent behavioral tests and electroretinography measurements. Histological evaluation was performed at four months to analyze synucleinopathies and neurodegeneration. Electroretinography was also conducted with idiopathic PD patients (mean age 63.35 ± 7.73; disease duration 4.15 ± 2.06; H&Y score 2.07 ± 0.59; n = 12 males, 8 females) and healthy age-matched controls (mean age 61.65 ± 8.39; n = 9 males, 11 females).

RESULTS

Rodent electroretinography revealed reduced photopic b-wave, PhNR b-wave, and PhNR-wave amplitudes at two and four months, particularly in females, indicating bipolar and retinal ganglion cell impairment. Based on retinal histological assessment, these changes might arise from α-synuclein pathology occurring in outer retinal layers. Likewise, the scotopic b-wave and PhNR waveform were similarly impaired in female participants with Parkinson's disease. The scotopic oscillatory potentials isolated further identified an attenuated amacrine cell output in females.

CONCLUSIONS

Findings from both mice and human cohorts indicate that retinal functional impairments can be detected early in the progression of Parkinson's disease, particularly among females. These tools show promise in facilitating early diagnosis, disease monitoring, therapeutic intervention, and ultimately enhancing patient outcomes.

Assessment of gastrointestinal function and enteric nervous system changes over time in the A53T mouse model of Parkinson's disease

Gastrointestinal (GI) dysfunctions, including constipation and delayed stomach emptying, are prevalent and debilitating non-motor symptoms of Parkinson's disease (PD). These symptoms have been associated with damage in the enteric nervous system (ENS) and the accumulation of pathogenic alpha-synuclein (α-Syn) within the GI tract. While motor deficits and dopaminergic neuron loss in the central nervous system (CNS) of the A53T mouse model are well-characterised, the temporal relationship between GI dysfunction, ENS pathology, and motor symptoms remains unclear. This study aimed to investigate functional alterations in the GI tract at the early stages of the disease, before the appearance of motor deficits, both in vivo and ex vivo. Early colonic motility deficits observed in A53T mice, measured via bead expulsion, preceded motor impairments emerged at 36 weeks. Although whole-gut transit remained unchanged, reduced faecal output was concurrent with marked colonic dysmotility at 36 weeks. Despite a lack of significant neuronal loss, a greater number of enteric neurons in A53T mice showed signs of neuronal hypertrophy and increased nuclear translocation of HuC/D proteins indicative of neuronal stress at 12 and 36 weeks. Calcium imaging revealed differential enteric neuron activity, characterised by exaggerated calcium transients at 12 weeks that normalized by 36 weeks. Furthermore, a reduction in enteric glial populations was observed as early as 12 weeks in both the ileum and colon of A53T mice. These findings provide compelling evidence that ENS pathology, including neuronal stress, disrupted calcium signalling, and glial cell loss, precedes the onset of motor symptoms and may contribute to early GI dysfunction in PD.

Longitudinal multi-omics in alpha-synuclein Drosophila model discriminates disease- from age-associated pathologies in Parkinson's disease

Parkinson's disease (PD) starts decades before symptoms appear, usually in the later decades of life, when age-related changes are occurring. To identify molecular changes early in the disease course and distinguish PD pathologies from aging, we generated Drosophila expressing alpha-synuclein (αSyn) in neurons and performed longitudinal bulk transcriptomics and proteomics on brains at six time points across the lifespan and compared the data to healthy control flies as well as human post-mortem brain datasets. We found that translational and energy metabolism pathways were downregulated in αSyn flies at the earliest timepoints; comparison with the aged control flies suggests that elevated αSyn accelerates changes associated with normal aging. Unexpectedly, single-cell analysis at a mid-disease stage revealed that neurons upregulate protein synthesis and nonsense-mediated decay, while glia drive their overall downregulation. Longitudinal multi-omics approaches in animal models can thus help elucidate the molecular cascades underlying neurodegeneration vs. aging and co-pathologies.

NOTCH2NLC GGC intermediate repeat with serine induces hypermyelination and early Parkinson's disease-like phenotypes in mice

BACKGROUND

The expansion of GGC repeats (typically exceeding 60 repeats) in the 5' untranslated region (UTR) of the NOTCH2NLC gene (N2C) is linked to N2C-related repeat expansion disorders (NREDs), such as neuronal intranuclear inclusion disease (NIID), frontotemporal dementia (FTD), essential tremor (ET), and Parkinson's disease (PD). These disorders share common clinical manifestations, including parkinsonism, dementia, seizures, and muscle weakness. Intermediate repeat sizes ranging from 40 to 60 GGC repeats, particularly those with AGC-encoded serine insertions, have been reported to be associated with PD; however, the functional implications of these intermediate repeats with serine insertion remain unexplored.

METHODS

Here, we utilized cellular models harbouring different sizes of N2C variant 2 (N2C2) GGC repeat expansion and CRISPR-Cas9 engineered transgenic mouse models carrying N2C2 GGC intermediate repeats with and without serine insertion to elucidate the underlying pathophysiology associated with N2C intermediate repeat with serine insertion in NREDs.

RESULTS

Our findings revealed that the N2C2 GGC intermediate repeat with serine insertion (32G13S) led to mitochondrial dysfunction and cell death in vitro. The neurotoxicity was influenced by the length of the repeat and was exacerbated by the presence of the serine insertion. In 12-month-old transgenic mice, 32G13S intensified intranuclear aggregation and exhibited early PD-like characteristics, including the formation of α-synuclein fibers in the midbrain and the loss of tyrosine hydroxylase (TH)-positive neurons in both the cortex and striatum. Additionally, 32G13S induced neuronal hyperexcitability and caused locomotor behavioural impairments. Transcriptomic analysis of the mouse cortex indicated dysregulation in calcium signaling and MAPK signaling pathways, both of which are critical for mitochondrial function. Notably, genes associated with myelin sheath components, including MBP and MOG, were dysregulated in the 32G13S mouse. Further investigations using immunostaining and transmission electron microscopy revealed that the N2C intermediate repeat with serine induced mitochondrial dysfunction-related hypermyelination in the cortex.

CONCLUSIONS

Our in vitro and in vivo investigations provide the first evidence that the N2C-GGC intermediate repeat with serine promotes intranuclear aggregation of N2C, leading to mitochondrial dysfunction-associated hypermyelination and neuronal hyperexcitability. These changes contribute to motor deficits in early PD-like neurodegeneration in NREDs.

Templating of Monomeric Alpha-Synuclein Induces Inflammation and SNpc Dopamine Neuron Death in a Genetic Mouse Model of Synucleinopathy

While the etiology of most cases of Parkinson's disease (PD) are idiopathic, it has been estimated that 5-10% of PD arise from known genetic mutations. The first mutations described that leads to the development of an autosomal dominant form of PD are in the SNCA gene that codes for the protein alpha-synuclein (α-syn). α-syn is an abundant presynaptic protein that is natively disordered and whose function is still unclear. In PD, α-syn misfolds into multimeric b-pleated sheets that aggregate in neurons (Lewy Bodies/neurites) and spread throughout the neuraxis in a pattern that aligns with disease progression. Here, using IHC, HC, HPLC, and cytokine analysis, we examined the sequelae of intraparenchymal brain seeding of pre-formed fibrils (PFFs) and monomeric α-syn in C57BL/6J (WT) and A53T SNCA mutant mice. We found that injection of PFFs, but not monomeric α-syn, into the striatum of C57BL/6J mice induced spread of aggregated α-syn, loss of SNpc DA neurons and increased neuroinflammation. However, in A53T SNCA mice, we found that both PFFs and monomeric α-syn induced this pathology. This suggests that the conformation changes in α-syn seen in the A53T strain can recruit wild-type α-syn to a pathological misfolded conformation which may provide a mechanism for the induction of PD in humans with SNCA duplication/triplication.

Aging accelerates locomotor decline in PINK1 knockout rats in association with decreased nigral, but not striatal, dopamine and tyrosine hydroxylase expression

Parkinson's disease (PD) rodent models provide insight into the relationship between nigrostriatal dopamine (DA) signaling and locomotor function. Although toxin-based rat models produce frank nigrostriatal neuron loss and eventual motor decline characteristic of PD, the rapid nature of neuronal loss may not adequately translate premotor traits, such as cognitive decline. Unfortunately, rodent genetic PD models, like the Pink1 knockout (KO) rat, often fail to replicate the differential severity of striatal DA and tyrosine hydroxylase (TH) loss, and a bradykinetic phenotype, reminiscent of human PD. To elucidate this inconsistency, we evaluated aging as a progression factor in the timing of motor and non-motor cognitive impairments. Male PINK1 KO and age-matched wild type (WT) rats were evaluated in a longitudinal study from 3 to 16 months old in one cohort, and in a cross-sectional study of young adult (6-7 months) and aged (18-19 months) in another cohort. Young adult PINK1 KO rats exhibited hyperkinetic behavior associated with elevated DA and TH in the substantia nigra (SN), which decreased therein, but not striatum, in the aged KO rats. Additionally, norepinephrine levels decreased in aged KO rats in the prefrontal cortex (PFC), paired with a higher DA levels in young and aged KO. Although a younger age of onset characterizes familial forms of PD, our results underscore the critical need to consider age-related factors. Moreover, the results indicate that compensatory mechanisms may exist to preserve locomotor function, evidenced by increased DA in the SN early in the lifespan, in response to deficient PINK1 function, which declines with aging and the onset of motor decline.

PINK1 knockout rats show premotor cognitive deficits measured through a complex maze

Cognitive decline in Parkinson's disease (PD) is a critical premotor sign that may occur in approximately 40% of PD patients up to 10 years prior to clinical recognition and diagnosis. Delineating the mechanisms and specific behavioral signs of cognitive decline associated with PD prior to motor impairment is a critical unmet need. Rodent PD models that have an impairment in a cognitive phenotype for a time period sufficiently long enough prior to motor decline can be useful to establish viable candidate mechanisms. Arguably, the methods used to evaluate cognitive decline in rodent models should emulate methods used in the assessment of humans to optimize translation. Premotor cognitive decline in human PD can potentially be examined in the genetically altered PINK1-/- rat model, which exhibits a protracted onset of motor decline in most studies. To increase translation to cognitive assessment in human PD, we used a modified non-water multiple T-maze, which assesses attention, cognitive flexibility, and working memory similarly to the Trail Making Test (TMT) in humans. Similar to the deficiencies revealed in TMT test outcomes in human PD, 4-month-old PINK1-/- rats made more errors and took longer to complete the maze, despite a hyperkinetic phenotype, compared to wild-type rats. Thus, we have identified a potential methodological tool with cross-species translation to evaluate executive functioning in an established PD rat model.

Aging hastens locomotor decline in PINK1 knockout rats in association with decreased nigral, but not striatal, dopamine and tyrosine hydroxylase expression

Parkinson's disease (PD) rodent models provide insight into the relationship between nigrostriatal dopamine (DA) signaling and locomotor function. Although toxin-based rat models produce frank nigrostriatal neuron loss and eventual motor decline characteristic of PD, the rapid nature of neuronal loss may not adequately translate premotor traits, such as cognitive decline. Unfortunately, rodent genetic PD models, like the Pink1 knockout (KO) rat, often fail to replicate the differential severity of striatal DA and tyrosine hydroxylase (TH) loss, and a bradykinetic phenotype, reminiscent of human PD. To elucidate this inconsistency, we evaluated aging as a progression factor in the timing of motor and non-motor cognitive impairments. Male PINK1 KO and age-matched wild type (WT) rats were evaluated in a longitudinal study from 3 to 16 months old in one cohort, and in a cross-sectional study of young adult (6-7 months) and aged (18-19 months) in another cohort. Young adult PINK1 KO rats exhibited hyperkinetic behavior associated with elevated DA and TH in the substantia nigra (SN), which decreased therein, but not striatum, in the aged KO rats. Additionally, norepinephrine levels decreased in aged KO rats in the prefrontal cortex (PFC), paired with a higher DA content in young and aged KO. Although a younger age of onset characterizes familial forms of PD, our results underscore the critical need to consider age-related factors. Moreover, the results indicate that compensatory mechanisms may exist to preserve locomotor function, evidenced by increased DA in the SN early in the lifespan, in response to deficient PINK1 function, which declines with aging and the onset of motor impairment.

PINK1 knockout rats show premotor cognitive deficits measured through a complex maze

Cognitive decline in Parkinson's disease (PD) emerges up to 10 years before clinical recognition. Neurobiological mechanisms underlying premotor cognitive impairment in PD can potentially be examined in the PINK1 -/- rat, which exhibits a protracted motor onset. To enhance translation to human PD cognitive assessments, we tested a modified multiple T-maze, which measures cognitive flexibility similarly to the Trail-Making Test in humans. Like human PD outcomes, PINK1 -/- rats made more errors and took longer to complete the maze than wild types. Thus, we have identified a potential tool for assessing cross-species translation of cognitive functioning in an established PD animal model.

Investigating the potential effects of α-synuclein aggregation on susceptibility to chronic stress in a mouse Parkinson's disease model

BACKGROUND

Parkinson's disease (PD) is a motor disorder characterized by the degeneration of dopaminergic neurons, putatively due to the accumulation of α-synuclein (α-syn) in Lewy bodies (LBs) in Substantia Nigra. PD is also associated with the formation of LBs in brain areas responsible for emotional and cognitive regulation such as the amygdala and prefrontal cortex, and concurrent depression prevalence in PD patients. The exact link between dopaminergic cell loss, α-syn aggregation, depression, and stress, a major depression risk factor, is unclear. Therefore, we aimed to explore the interplay between sensitivity to chronic stress and α-syn aggregation.

METHODS

Bilateral injections of α-syn preformed fibrils (PFFs) into the striatum of C57Bl/6 J mice were used to induce α-syn aggregation. Three months after injections, animals were exposed to chronic social defeat stress.

RESULTS

α-syn aggregation did not affect stress susceptibility but independently caused increased locomotor activity in the open field test, reduced anxiety in the light-dark box test, and increased active time in the tail suspension test. Ex vivo analysis revealed modest dopaminergic neuron loss in the substantia nigra and reduced dopaminergic innervation in the dorsal striatum in PFFs injected groups. α-Syn aggregates were prominent in the amygdala, prefrontal cortex, and substantia nigra, with minimal α-syn aggregation in the raphe nuclei and locus coeruleus.

CONCLUSIONS

Progressive bilateral α-syn aggregation might lead to compensatory activity increase and alterations in emotionally regulated behavior, without affecting stress susceptibility. Understanding how α-syn aggregation and degeneration in specific brain structures contribute to depression and anxiety in PD patients requires further investigation.

Aging, Parkinson's Disease, and Models: What Are the Challenges?

Parkinson's disease (PD) is a chronic, neurodegenerative condition characterized by motor symptoms such as bradykinesia, rigidity, and tremor, alongside multiple nonmotor symptoms. The appearance of motor symptoms is linked to progressive dopaminergic neuron loss within the substantia nigra. PD incidence increases sharply with age, suggesting a strong association between mechanisms driving biological aging and the development and progression of PD. However, the role of aging in the pathogenesis of PD remains understudied. Numerous models of PD, including cell models, toxin-induced models, and genetic models in rodents and nonhuman primates (NHPs), reproduce different aspects of PD, but preclinical studies of PD rarely incorporate age as a factor. Studies using patient neurons derived from stem cells via reprogramming methods retain some aging features, but their characterization, particularly of aging markers and reproducibility of neuron type, is suboptimal. Investigation of age-related changes in PD using animal models indicates an association, but this is likely in conjunction with other disease drivers. The biggest barrier to drawing firm conclusions is that each model lacks full characterization and appropriate time-course assessments. There is a need to systematically investigate whether aging increases the susceptibility of mouse, rat, and NHP models to develop PD and understand the role of cell models. We propose that a significant investment in time and resources, together with the coordination and sharing of resources, knowledge, and data, is required to accelerate progress in understanding the role of biological aging in PD development and improve the reliability of models to test interventions.

Retinal alpha-synuclein accumulation correlates with retinal dysfunction and structural thinning in the A53T mouse model of Parkinson's disease

Abnormal alpha-synuclein (α-SYN) protein deposition has long been recognized as one of the pathological hallmarks of Parkinson's disease's (PD). This study considers the potential utility of PD retinal biomarkers by investigating retinal changes in a well characterized PD model of α-SYN overexpression and how these correspond to the presence of retinal α-SYN. Transgenic A53T homozygous (HOM) mice overexpressing human α-SYN and wildtype (WT) control littermates were assessed at 4, 6, and 14  months of age (male and female, n = 15-29 per group). In vivo retinal function (electroretinography, ERG) and structure (optical coherence tomography, OCT) were recorded, and retinal immunohistochemistry and western blot assays were performed to examine retinal α-SYN and tyrosine hydroxylase. Compared to WT controls, A53T mice exhibited reduced light-adapted (cone photoreceptor and bipolar cell amplitude, p < 0.0001) ERG responses and outer retinal thinning (outer plexiform layer, outer nuclear layer, p < 0.0001) which correlated with elevated levels of α-SYN. These retinal signatures provide a high throughput means to study α-SYN induced neurodegeneration and may be useful in vivo endpoints for PD drug discovery.