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(definition)–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 biochem