Age-dependent alpha-synuclein aggregation and Lewy body formation in Parkinson’s disease

Parkinson’s disease (PD) is a common degenerative disorder that is becoming increasingly prevalent because of the global aging population. The exact cause of the disorder is unknown; however, recent studies have suggested that multiple factors may contribute to its pathogenesis. PD is characterized by a movement disorder that primarily affects motor control; pathologically, the disease is marked by the presence of Lewy bodies (LBs) in the brain. LBs were first described by Friederich Lewy as intraneuronal protein inclusion bodies observed in PD patients. The mechanisms of LB formation have been the focus of much PD research, as understanding the mechanisms of LB formation could provide potential treatments for PD. PD is mostly sporadic, meaning that the majority of cases arise randomly in the population. The cause of the disease is believed to involve multiple factors such as genetics, the environment, and personal lifestyle. However, more exclusively hereditary cases have been found in cohorts of familial PD (FPD) patients. Mapping of the genetic mutations in these patients has been a major focus of PD research, as the identification of gene mutations linked to FPD could provide crucial insights into the mechanism of PD pathogenesis. The first gene mutation associated with FPD was alpha-synuclein (α-syn), the major component of LBs (Polymeropoulos et al., 1997; Singleton et al., 2003). Both pathological mutations of α-syn, such as the A53T single nucleotide polymorphism, as well as its triplication, have been linked to the development of FPD (Polymeropoulos et al., 1997; Singleton et al., 2003). With the findings that α-syn is strongly implicated in genetic predisposition to PD and is also the major component of LBs, intense study has focused on determining the role of α-syn in both physiological and pathological brain conditions. α-Syn has been shown to be highly expressed in young zebra finches when they are learning to sing, suggesting that it has a role in the development of neuronal plasticity and learning (Polymeropoulos et al., 1997). Further molecular studies have shown that α-syn is involved in synaptic function related to the SNARE complex and the modulation of vesicle release in synaptic terminals (Burre et al., 2010). In terms of its role in the pathogenesis of PD, studies have shown that α-syn oligomerization, fibrilization, and aggregation may lead to the demise of dopaminergic (DA) neurons in PD. Both FPD-linked α-syn mutations and increased expression of α-syn have been shown to modulate and facilitate the processes of oligomerization, fibrilization, and aggregation of α-syn. These α-syn fibrils or aggregates may increase oxidative stress and cause mitochondrial and synaptic dysfunction, which could contribute to neurodegeneration in PD. Interestingly, reports have also found that α-syn fibrils might transmit intercellularly and contribute to the formation of LBs and neurodegeneration (Mahul-Mellier et al., 2020). If mutations or overexpression of α-syn contribute to the pathogenesis of PD, proper degradation of α-syn would be a crucial mechanism to prevent the accumulation of α-syn in neurons. Early studies into this process found that α-syn was degraded by the ubiquitin and proteasomal system (UPS); however, later studies have shown that α-syn can also be degraded through the autophagy–lysosomal pathway (ALP) (Cuervo et al., 2004; Oueslati et al., 2013). In some studies, degradation of α-syn was found to be dependent on the phosphorylation of α-syn at S129 through the ALP (Oueslati et al., 2013). These results suggest a close relationship between the UPS and the ALP in the metabolism of α-syn, and that dysfunction of these pathways might contribute to PD. Identifying the mechanisms by which the UPS and the ALP might work together to modulate α-syn degradation in PD pathogenesis is of significant interest. The mechanisms of oligomerization, fibrilization, and aggregation of α-syn that might lead to the formation of LBs and contribute to PD pathogenesis have been intensively studied in the past two decades. An early study showed that free α-syn had no defined structure and formed an alpha-helix upon binding to lipid vesicles (Mehra et al., 2019). Recent studies have employed stochastic optical reconstruction microscopy and cryogenic electron microscopy to determine the structure of α-syn aggregates (Yang et al., 2022; Vovard et al., 2024). The results from these studies suggest that α-syn aggregates in LBs are formed with defined structures and may involve unidentified cofactors (Yang et al., 2022; Vovard et al., 2024). Therefore, the formation of LBs from α-syn may involve a more complicated mechanism than first hypothesized. For instance, in a study that used a neuronal seed-based model of α-syn aggregation to determine how LB-like structures could be formed in neurons, it was found that a complex process was involved (Mahul-Mellier et al., 2020) that included the initial formation of α-syn filaments, followed by post-translational modification, the recruitment of organelles such as the autophagosome and mitochondria, and the formation of LB-like aggregates in neurons (Mahul-Mellier et al., 2020). In our previous study, we established that the expression of a modified, aggregation-prone mutant of wild-type (WT) human α-syn, designated WT-CL1 α-syn, was more toxic than the WT gene in DA neurons of an adenoviral mouse model of PD (Wan and Chung, 2012). These results suggest that WT and WT-CL1 α-syn could be used to study the process of α-syn aggregation toxicity in the pathogenesis of PD. In a more recent follow-up study, we developed a conditional mouse α-syn transgenic model of PD that selectively expressed WT or WT-CL1 α-syn in different neurons (Li et al., 2024). We first tested this model by expressing WT or WT-CL1 α-syn in DA neurons. We then monitored age-dependent α-syn aggregate formation specifically in DA neurons associated with behavioral deficits and biochemical changes (Li et al., 2024). As expected, we found that there was an age-dependent association of α-syn accumulation with behavioral dysfunction, biochemical changes, and DA neuronal degeneration (Li et al., 2024). These results support the notion that the expression of α-syn alone, specifically in DA neurons, could induce α-syn aggregation linked to behavioral and biochemical dysfunction. One important finding from our recent study was that the accumulation of small α-syn aggregates in DA neurons developed into bigger and perinuclear localized aggregates in an age-dependent manner (Li et al., 2024). These results agree with the hypothesis that α-syn aggregate accumulation is localized to a cellular compartment, which minimizes its interference with normal cellular processes. With a decreased ability to remove these aggregates in aging, increased accumulation and formation of perinuclear α-syn LB-like aggregates would be expected, as was observed in our study. In this study, in addition to the accumulation of α-syn LB-like aggregates that were positive for ubiquitin, we also observed increases in the autophagic markers sequestosome-1 (also known as p62) and microtubule-associated protein 1A/1B-light chain 3 in α-syn-expressing DA neurons in aged mice (Li et al., 2024). This increase in autophagic markers suggests that the accumulation of α-syn aggregates may trigger the activation of the autophagy pathway to clear these aggregates. Hence, these results support the idea that with aging, increased accumulation of α-syn aggregates could overwhelm the UPS and the ALP’s ability to clear α-syn aggregates, which might contribute to the formation of LBs (Figure 1). Again, these results consolidate the notion of a multifactorial pathogenic mechanism in PD, such as the abnormal accumulation of α-syn, the dysfunction of the UPS and the ALP, and aging.Figure 1: Age-dependent formation of α-syn aggregates and LBs in PD.α-Syn fibrils and aggregates slowly accumulate in the cell body. Along with aging and lysosomal dysfunction in the clearance of α-syn fibrils and aggregates, LBs form and contribute to the degeneration of DA neurons. Created with BioRender.com. ALP: Autophagy–lysosomal pathway; α-syn: α-synuclein; DA: dopaminergic; LBs: Lewy bodies; LC3: microtubule-associated protein 1A/1B-light chain 3; PD: Parkinson’s disease; p62: sequestosome-1.Knowing that the formation of LBs is an age-dependent process with a complex mechanism, one interesting question is how this process might be modulated to develop therapeutic approaches in PD. Success in this regard might depend on reducing α-syn accumulation while enhancing its metabolism through the UPS and the ALP. Another unanswered question is why DA neurons are vulnerable to α-syn-induced toxicity in PD. For instance, LBs have been recognized as a pathological hallmark of dementia with LBs, although the clinical and pathological characteristics of PD and dementia with LBs remain distinct (Mensikova et al., 2022). These disorders suggest that toxicity-induced α-syn oligomers or aggregates might differently affect different types of neurons (Mensikova et al., 2022). Our newly developed conditional mouse α-syn transgenic model will enable us to address this question. Studying how α-syn aggregation induces dysfunction in different neurons will provide insights into the role of protein aggregation in different types of neurodegenerative disorders. With our model, we can test this by expressing α-syn in different neuron types and measuring whether it responds to α-syn-induced neurotoxicity differently. This approach will help to answer the following questions: (1) does this expression of α-syn affect the process of its aggregation in different neurons? and (2) does the formation of LB-like aggregates and their structure vary in different neurons? These answers may help to address the question of why DA neurons are vulnerable to α-syn aggregation and LB formation in PD. This work was supported by the Innovation and Technology Commission (ITCPD/17-9) and the Hong Kong Research Grants Council, China (GRF16104517) (to KKKC). C-Editors: Zhao M, Liu WJ, Qiu Y; T-Editor: Jia Y