SNCA was the first genetic mutation causally associated with autosomal-dominant PD (Polymeropoulos et al., 1997). SNCA mutations are rare in the general population, with five presumably causal point mutations now discovered (Table 1). Duplications and triplications of the entire gene have also been previously reported in familial cases of PD (Singleton et al., 2003). Through these genetic studies it was discovered that overexpression of α-synuclein could produce aggregates and potentially lead to Lewy pathology (Singleton et al., 2003). SNCA variants demonstrate high penetrance, with 85% of people with the most common A53T variant manifesting the disease (Klein and Westenberger, 2012). Five missense mutations associated with PD (Table 1) reside in the amino-terminal domain, demonstrating importance within the region for pathogenicity of the protein (Polymeropoulos et al., 1997; Krüger et al., 1998; Zarranz et al., 2003; Proukakis et al., 2013; Pasanen et al., 2014; Kiely et al., 2015). These single nucleotide variants tend to form the stable β sheets that skew the protein toward irreversible aggregation and exacerbate the formation of toxic oligomers and fibrils (Stefanis, 2012; Flagmeier et al., 2016).

In 2011, a missense mutation in vacuolar protein sorting homolog 35 (VPS35) was linked to autosomal dominant PD (Zimprich et al., 2011). VPS35 is a vital portion of the retromer complex that mediates transport of cargo from the endosomal system to the trans-Golgi network (Zimprich et al., 2011). VPS35 is part of the cargo recognition portion of the retromer (Zimprich et al., 2011). The retromer complex is responsible for delivery of lysosomal enzymes, such as cathepsin D, to lysosomes (Miura et al., 2014). Cathepsin D is a soluble aspartic endopeptidase involved in the protein degradation in the strongly acidic milieu of lysosomes (Kollmann et al., 2013). Cathepsin D is also known as one specific enzyme that can breakdown α-synuclein (Sevlever et al., 2008). Therefore, dysfunction to the transport of cathepsin D via mutations in the retromer complex can lead to an alteration in degradation of α-synuclein (Sevlever et al., 2008). When VPS35 was silenced in Drosophilla, intracellular α-synuclein accumulated in the late endolysosomal compartments, indicating the retromer complex plays a critical role in α-synuclein catabolism (Miura et al., 2014).

Mutations in leucine-rich repeat kinase 2 (LRRK2) are the most frequent causes of both autosomal-dominant and sporadic PD with more than 50 different missense mutations reported and at least 16 have been associated with pathogenicity (Cilia et al., 2014; Deng et al., 2018). LRRK2 encodes a complex multi-domain 2527 amino acid cytoplasmic protein, LRRK2 (Deng et al., 2018). G2019S is the most common and studied variant and accounts for as many as 40% of cases in certain geographic denominations, such as the North African Berber population (Lesage and Brice, 2009; Cilia et al., 2014; El Haj et al., 2017). LRRK2 and α-synuclein have been seen to physically interact and can directly regulate the function and/or activity of the other (O’Hara et al., 2020). α-synuclein is either present natively in the cytosol or vesicular structures of neurons that is pathogenic when phosphorylated (O’Hara et al., 2020). LRRK2 is a serine-threonine kinase that was thought to directly phosphorylate α-synuclein in neurons which can result in aggregation and Lewy body formation (Zimprich et al., 2004; O’Hara et al., 2020). In vitro studies have since demonstrated that the G2019S mutant can induce an indirect kinase-dependent increase in levels of phosphorylated α-synuclein, demonstrated to be a pathogenic posttranslational modification to the protein (Guerreiro et al., 2012). Other studies have further sought to investigate the relationship between α-synuclein and LRRK2 and suggest that LRRK2 could regulate the cell-to-cell transmission of α-synuclein and interrupt autolysosomal degradation of α-synuclein (O’Hara et al., 2020).

Lysosomes are vital organelles involved in synucleinopathies and are the main degradative component of the cell (Ferreira and Gahl, 2017). Defects in lysosomal function can result in lysosomal storage disorders and often characterize progressive neurodegenerative diseases, such as PD (Ferreira and Gahl, 2017). The most common lysosomal storage disorder is Gaucher disease which results from the loss of function of lysosomal enzyme β-glucocerebrosidase (GCase) (Ferreira and Gahl, 2017). Individuals that manifest Gaucher disease as well as carriers of heterozygous mutations in GBA are at increased risk of developing PD due to the dysfunctional lysosomes (Ferreira and Gahl, 2017). Dysfunction of GCase in the lysosomes can increase α-synuclein accumulation directly, causing excess of misfolded proteins within the cell (Bae et al., 2014). Functional loss of GCase causes the accumulation of glucocerebroside, which directly influences aggregation of α-synuclein as it can stabilize the oligomeric intermediates that are thought to be the most toxic form of α-synuclein (Bae et al., 2014, 2015; Mazzulli et al., 2016). Therefore, this creates a positive feedback loop of α-synuclein and β-GCase that leads to accumulation of α-synuclein (Mazzulli et al., 2016).

Adenosine triphosphate cation transporting 13A2 (ATP13A2) encodes an integral decamembrane spanning ATPase located in the lysosomal membrane that functions as a late endo-/lysosomal polyamine transporter (van Veen et al., 2020). Familial ATP13A2 mutations were demonstrated to enhance α-synuclein aggregation and promote cell death (Si et al., 2021). Research indicates that ATP13A2 and α-synuclein interact in the endolysosomal system and that ATP13A2 directly affects α-synuclein homeostasis (Fonseca et al., 2015). Loss of ATP13A2 impairs lysosomal membrane integrity and induces α-synuclein mutlimerisation at the ER membrane which then can further the pathology of α-synuclein (Fonseca et al., 2015). Further, ATP13A2 deficiency in neuroblastoma cells leads to lysosomal dysfunction and reduces the function of the endolysosomal trafficking of α-synuclein (Si et al., 2021).

These genetic studies illustrate the importance of the homeostasis of vesicular trafficking pathways as dysregulation is highly correlated with the progression of PD. Furthermore, variants of proteins involved in the endolysosomal pathway are emerging as important for their involvement in PD.

The endolysosomal pathway is the main route for uptake, processing and clearance of cargoes collected from the extracellular space (Vidyadhara et al., 2019). It comprises a highly dynamic network of vesicular structures that facilitates nutritional intake, autophagy and maintenance of cellular homeostasis (Vidyadhara et al., 2019). The major endocytic route for internalization of many cargoes and the primary route of uptake in neurons is clathrin-mediated endocytosis (Robinson, 1994; Ford et al., 2002; Delenclos et al., 2017). This specific process is initiated when endocytic coat proteins from the cytosol cluster on the inner leaflet of the plasma membrane (Robinson, 1994; Ford et al., 2002; Delenclos et al., 2017). The assembling coat promotes positive membrane curvature and transforms into a clathrin-coated pit (Robinson, 1994; Ford et al., 2002; Vidyadhara et al., 2019). Upon entering the cytoplasm, primary endocytic vesicles undergo homotypic fusion and become early/sorting endosomes (Delenclos et al., 2017). Cargoes can then be recycled to the plasma membrane, sent to the trans-Golgi network or sorted to the lysosome for degradation (Robinson, 1994; Ford et al., 2002; Delenclos et al., 2017). Increasing evidence suggests that the endolysosomal system is the key pathway affected in PD (Kett and Dauer, 2016). The role of the endolysosomal system in PD has been elucidated through genetic studies including genome wide association studies which identified a large portion of disease-associated risk genes belonging to the endolysosomal system (Nalls et al., 2019). DNAJC6 encodes for auxilin a major pre-synaptic endocytic protein that is a chaperone and plays an important role in clathrin uncoating (Köroğlu et al., 2013). Mutations in DNAJC6 have been linked to PD, suggesting a vital role for endocytic processing in pathogenesis of disease. Transmembrane protein 106B (TMEM106B) encodes for TMEM106B, a lysosomal membrane protein which is highly expressed in CNS neurons and is localized to late endolysosomal compartments (Brady et al., 2013; Feng et al., 2021). TMEM106B plays a vital role in lysosomal function, where overexpression demonstrated lysosomal enlargement, oxidative stress, and subsequent cell death in HeLa cells (Lang et al., 2012; Brady et al., 2013; Suzuki and Matsuoka, 2016; Feng et al., 2021). In recent years, TMEM106B has been linked to many neurological disorders and genome wide association studies has recently found that TMEM106B may play a role in PD, demonstrating a vital role for endolysosomal processing in PD (Tropea et al., 2019).

Autophagy is a key component of the endolysosomal system and is an intracellular protein clearance pathway to maintain cellular homeostasis (Demirsoy et al., 2017). Lysosomes are organelles that contain proteolytic enzymes that play a role in proteostasis and are the primary destination for a large portion of endolysosomal system cargo (Demirsoy et al., 2017). Lysosomal dysfunction can perturb cellular homeostasis (Bento et al., 2016). Mutations in ATP13A2 has shown impaired lysosomal acidification leading to dysfunctional protein degradation and autophagosome clearance (Bento et al., 2016). One of the most well-defined forms of monogenic PD results from point mutations in the LRRK2 mutant and it has been discovered to have a role in lysosomal homeostasis and autophagy (Kett and Dauer, 2016; Kuwahara and Iwatsubo, 2020). Phosphorylation by LRRK2 is essential for a subset of guanosine triphosphatases (GTPases) that function to regulate endosomal trafficking (Kett and Dauer, 2016; Kuwahara and Iwatsubo, 2020). Mutations in LRRK2 significantly affect Ras-associated binding proteins (Rab) and are strongly related to PD (Kett and Dauer, 2016; Kuwahara and Iwatsubo, 2020). Protein uptake, trafficking and clearance is therefore a vital pathway to understand and investigate in PD pathogenesis and is a key target for therapeutic intervention (Vidyadhara et al., 2019).

New treatment options could be aimed at earlier interventions targeting cellular dysfunction prior to neuron loss. Protein trafficking is one of the overarching cellular dysfunction mechanisms in PD (Beyer et al., 2009). A common theme in recent PD research is identifying ways to prevent or alter progression of PD through α-synuclein manipulation. Therefore, α-synuclein aggregation has been widely targeted for treatment of PD with varying success.

Monomers of α-synuclein are typically characterized as 14 kDa, soluble and intrinsically disordered as these species do not have a stable conformational state (Maroteaux and Scheller, 1991; Kahle, 2007; Ferreon et al., 2009; Stefanis, 2012). Monomers are believed to be the predominant conformational species existing in neuronal cytosol (Ferreon et al., 2009; Alam et al., 2019). Monomeric α-synuclein is dynamic and complex and exists in a variety of conformational states, hence why it is considered natively unfolded (Ferreon et al., 2009). α-synuclein conformational plasticity is limited by its functional aspects of lipid binding and membrane curvature for vesicle fusion (Frimpong et al., 2010). Each conformation α-synuclein can adopt depends on interactions by hydrogen bonds, pH and solvent conditions (Alam et al., 2019).

There are an extremely large variety of early, prefibrillar α-synuclein species called oligomers that differ in molecular weight and structure (Fauvet et al., 2012; Lashuel et al., 2013; Pieri et al., 2016; Ghosh et al., 2017). Whether oligomers or fibrils are the most toxic species of α-synuclein is still debated widely in literature. Oligomers have the ability to spread and can potentiate into fibrils at any part of the cell, making them pathogenic (Winner et al., 2011; Fauvet et al., 2012; Ghosh et al., 2017). Overexpression of α-synuclein oligomers can lead to cellular toxicity in vitro (Winner et al., 2011). Oligomeric species can impair the autophagy lysosomal pathway and ubiquitin proteasome system protein degradation system, disrupt lipid bilayer and increase the influx of calcium ions to increase aggregation, cause cytoskeletal alternations, damage mitochondrial and endoplasmic reticulum membranes and increase ROS production (Danzer et al., 2007; Parihar et al., 2009; Colla et al., 2012).

There are two main oligomeric subsets: on-fibrillar assembly pathway and off-fibrillar assembly pathway (Alam et al., 2019). However, this is complicated as both types of oligomers present in many shapes and sizes and ensues differences in reactivity and toxicity (Winner et al., 2011). On-pathway fibrillar oligomers are species that directly form or take part in fibril formation (Alam et al., 2019). Oligomers that exhibit a degree of stability and do not tend toward fibrillization are off-fibrillar species (Alam et al., 2019). These are stable species that can be formed in the early stages of fibrillation (starting with monomers) that do not eventuate into fibrils. Off-pathway oligomers can be formed by addition of small molecules, such as epigallocatechin gallate (EGCG) that inhibit fibrillization but not oligomerization (Alam et al., 2019).

The fibrillar form of α-synuclein is most commonly associated with PD as it is located within LBs and inside the neuronal cytoplasm (Spillantini et al., 1998; Volpicelli-Daley et al., 2011; Stefanis, 2012; Ghosh et al., 2017; Gómez-Benito et al., 2020). Both toxic and non-toxic α-synuclein fibrillar species have been reported; these have varying effects on neuron survival (Alam et al., 2019). The fibrils that have been proposed to contribute to neurodegeneration are those that seed assembly of soluble α-synuclein species to form high molecular weight aggregates and form an imbalance in cellular proteostasis (Volpicelli-Daley et al., 2011; Alam et al., 2019). Fibrillar species are believed to be toxic due to their propagation, amplification, and seeding properties (Danzer et al., 2012). This is the clearest distinction of function between the two-toxic species. On-pathway oligomers have been shown to possess seeding activities but not to the length and pathogenicity of fibrils (Alam et al., 2019).

Kinetics of α-synuclein fibril formation demonstrate a lag phase (nucleation phase), an exponential phase (elongation) and a plateau phase that demonstrates fibrils formation (Arosio et al., 2015; Iljina et al., 2016). This occurs over 7 days of monomers shaking at 37°C (Li et al., 2018). Furthermore, fibrillization of α-synuclein is irreversible, which is a key determinant against reversible and soluble oligomeric species (Tuttle et al., 2016). Nuclear magnetic resonance spectroscopy has deduced a “Greek key” conformation where each α-synuclein subunit within the fibril has a β-sheet conformation with hydrogen bonding between adjacent α-synuclein subunits spaced evenly apart (Tuttle et al., 2016). The central β-sheet core is located primarily within the NAC region (Tuttle et al., 2016). N- and C- termini display flexible, random coil that is poorly reserved in the structure (Tuttle et al., 2016). Electron microscopy has shown typical fibrils range from 10 nm in diameter, comparable to the PD brain (Meade et al., 2019; Yang et al., 2022). Approximately 25% of extracted α-synuclein filaments from the cingulate cortex of post-mortem PD brains contained helical twists which allowed for structural determination. α-synuclein filaments were consistently single protofilaments, whereby the ordered core of the filament was termed a Lewy fold (Yang et al., 2022). A Lewy fold is formed by 31–100 residues which arrange as 9 β-strands that layer within the highly ordered core region (Yang et al., 2022). The Lewy fold differs from other synucleinopathies (such as multiple system atrophy), however, remained consistent across Lewy body diseases (Yang et al., 2022). This finding was consistent with differences in α-synuclein seeding amplification between PD and multiple systems atrophy, suggesting the importance of filament conformation is vital for pathological seeding (Yamasaki et al., 2019; Yang et al., 2022).

α-synuclein uptake has been widely researched in recent years as it is vital to understand this mechanism for prion spreading pathology and how this process can be inhibited for therapeutic outcomes. All forms of α-synuclein can be efficiently internalized without the need of transfection reagents (Konno et al., 2012; Delenclos et al., 2017; Masaracchia et al., 2018; Rodriguez et al., 2018; Gribaudo et al., 2019). Uptake is cell-type dependent, as astrocytes more readily internalize fibrils than primary neurons (Rodriguez et al., 2018). Within the endocytic pathway, there are several specific- and non-specific routes of uptake and this is the primary pathway that is studied in α-synuclein pathology (Delenclos et al., 2017; Rodriguez et al., 2018).

A commonly used non-invasive inhibitor of endocytosis is low temperature and this can reduce uptake of fibrils in a wide range of non- and neuronal cell lines (Lee et al., 2008). The same effect is seen with dynamin mutants and dynamin inhibitors that block dynamin mediated endocytosis (Lee et al., 2008). Fibrils over 50 nm do not appear to be taken up and there is an upper limit to the uptake pathway (i.e., saturation of the endocytic route) (Rodriguez et al., 2018).

α-synuclein monomers are reported to passively diffuse in an out of membranes (Lee et al., 2008; Steiner et al., 2018). Due to the small size of the protein and propensity to interact with lipid membranes, it can facilitate passive diffusion in some cell types (Lee et al., 2008). In SH-SY5Y cells, the monomeric protein was internalized and either cleared or exocytosed in approximately 2 min (Lee et al., 2008). This process has been suggested in cases where exogenous, monomeric α-synuclein can enter the cell regardless of the application of endocytic inhibitors such as low temperature or dynamin inhibition (Lee et al., 2008; Steiner et al., 2011).

The processing of α-synuclein in the cell is a vital determinant of its toxicity and pathogenesis (Flavin et al., 2017). Studies in induced pluripotent stem cells differentiated into dopaminergic neurons found that α-synuclein was trafficked a considerable distance in the cell, including through the soma and cellular projections and that fusion of ruptured vesicles due to α-synuclein aggregation led to the formation of large cytoplasmic inclusions (Flavin et al., 2017).

Following internalization, α-synuclein fibrils are trafficked to late endosomal compartments and into lysosomes (Figure 3; Vogiatzi et al., 2008; Dilsizoglu Senol et al., 2021). Lysosomal inhibition has been shown to cause accumulation of α-synuclein in early and late endosomes, suggesting an important role for the lysosomal pathway in clearance of α-synuclein (Vogiatzi et al., 2008; Cunningham and Moore, 2020). Furthermore, extracellular aggregated α-synuclein alters lysosomal morphology and function (Cunningham and Moore, 2020).

The autophagolysosomal pathway comprises several catabolic processes that converge at the lysosome and are three main processes: Macroautophagy, chaperone-mediated autophagy, and microautophagy (Konno et al., 2012; Parzych and Klionsky, 2014). Macroautophagy begins with the engulfment of proteins in autophagosomes and targeted to lysosomes for degradation. The majority of large protein aggregates are degraded via macroautophagy (Parzych and Klionsky, 2014). Inhibitors of macroautophagy have seen a build-up of α-synuclein monomeric and fibrillar species in vitro (Konno et al., 2012). This suggests a role for macroautophagy in the degradation of many conformations of α-synuclein (Konno et al., 2012). Chaperone mediated autophagy involves the selective targeting of substrates to the lysosome via Hsc70 that recognize cargo with the KFERQ-motif (Kaushik and Cuervo, 2018). Chaperone mediated autophagy is involved in the processing of α-synuclein as in vitro studies have shown α-synuclein contains the motif and directly interacts with LAMP2a and Hsc70 (Kaushik and Cuervo, 2018). Mutations to the KFERQ-motif abolished lysosomal degradation of α-synuclein (Wildburger et al., 2020). Several acidic proteases are believed to breakdown α-synuclein in lysosomes, such as cathepsins and glucocerebrosidase (Wildburger et al., 2020). Microautophagy is the direct uptake of cytoplasm into lysosomes, which can be in bulk or selective fashion (Lee et al., 2013). It is unclear in research as to whether α-synuclein is cleared via microautophagy, where macroautophagy and chaperone-mediated autophagy appear to be the primary degradation pathways of pathogenic α-synuclein (Lee et al., 2013; Fonseca et al., 2015).

Oligomeric and fibrillar species of α-synuclein can spread cell-to-cell, evade protein degradation systems and deposit within the cytoplasm and other organelles in the cell (such as the mitochondria) (Fonseca et al., 2015). Here, these toxic species can seed endogenous protein to form larger aggregates leading to cellular toxicity and eventual death (Fonseca et al., 2015). Intracellular α-synuclein does not aggregate under healthy conditions as it is limited by the number of active nucleation sites within the cytoplasm (Wood et al., 1999). Exogenously added α-synuclein fibrils can alter this by acting as seeds for aggregation by presenting nucleation sites (Luk et al., 2009). Further, increased concentrations of α-synuclein also enhances the kinetics of fibrillization, as determined by genetic studies (Conway et al., 1998). Fibrillar species act as exogenous seeds, however, monomers do not seed the formation of aggregates in neuronal and non-neuronal cell models and in vivo (Luk et al., 2009). Oligomeric species are also believed to have the capacity to template aggregation, however, some studies dispute this as intracellular aggregation does not occur upon exposure to soluble species of α-synuclein (Waxman and Giasson, 2009). Inhibition/reduction of the seeding properties of α-synuclein has been used for drug screening in situ as this is a vital pathogenic pathway for the protein (Ardah et al., 2020).

α-synuclein can be secreted through tunneling nanotubules (TnTs) to neighboring cells providing a direct path for the spreading of pathology (Abounit et al., 2016). TnTs are actin-based membrane channels that directly connect cells (Abounit et al., 2016). Mono- and co-culture experiments have been used to directly visualize TnTs using microscopy (Abounit et al., 2016; Dieriks et al., 2017; Valdinocci et al., 2021). TnT transfer has been demonstrated in a variety of cellular models and α-synuclein can be involved with lysosomal and mitochondrial membranes within the TNTs (Abounit et al., 2016; Valdinocci et al., 2021). Studies have found that the spread of α-synuclein through TnTs is not cell type specific and has been demonstrated in both neuronal and non-neuronal models (Abounit et al., 2016; Valdinocci et al., 2021).

Intercellular exchange of mitochondria has been previously defined previously for respiration rescue of neighboring cells (Wang et al., 2019). A recent study has found that aggregated α-synuclein is associated with mitochondria within TnTs in KCl-treated SH-SY5Y cells (Valdinocci et al., 2021). Utilizing fluorescence microscopy, TnT structures were found to contain α-synuclein bound TOM-20 positive mitochondria (Valdinocci et al., 2021). The same study demonstrated similar results for human astrocytes in monoculture and co-culture (Valdinocci et al., 2021). The evasion of cellular degradation systems by α-synuclein via TnT structures can therefore occur within lysosomes or mitochondrion which facilitates cell-to-cell spread of the protein (Abounit et al., 2016; Valdinocci et al., 2021).

A cellular secretory mechanism known as exocytosis utilizes the release of cargo in the form of 30–150 nm vesicles known as exosomes (Johnstone et al., 1987; Doyle and Wang, 2019). Exosomes are formed from multivesicular bodies that can fuse with the plasma membrane and release the contents to the extracellular environment (Johnstone et al., 1987; Doyle and Wang, 2019). One proposed pathway of α-synuclein exocytosis involves secretory autophagosomes that are in the process of α-synuclein degradation fusing with multi vesicular bodies to form amphisomes (Danzer et al., 2012; Rodriguez et al., 2018). These secretory vesicles can fuse with the plasma membrane and release α-synuclein into the extracellular space (Danzer et al., 2012; Rodriguez et al., 2018). In vitro studies have shown that α-synuclein oligomers can utilize exosomes and are then preferentially endocytosed by neighboring cells (Danzer et al., 2012). However, this has still not been well defined in literature as to the precise mechanism that α-synuclein uses exosomes for release.

Use of the clathrin-mediated endocytosis blocker dynasore is widely used to inhibit uptake in cellular models (Oh et al., 2016). In donor-acceptor co-culture methods that demonstrate the transfer of α-synuclein from donor cells to acceptor cells; transmission was infrequent with dynasore treatment (Oh et al., 2016; Masaracchia et al., 2018). As clathrin-mediated endocytosis is a widely used process majority of mammalian cells it is not feasible to completely inhibit the pathway to reduce α-synuclein pathology (Konno et al., 2012). Therefore, researchers sourced a pharmacological intervention that disrupts dynamin GTPases but does not completely abolish endocytosis (Konno et al., 2012). With the administration of sertraline, an antidepressant, cells significantly decreased the internalization and translocation of α-synuclein (Konno et al., 2012). As this is a widely used drug, the side effects can be well managed and is previously clinically tested (Konno et al., 2012).

LAG3 is a cell-surface protein that functions as an immune receptor directly involved in the endocytosis of extracellular fibrillar α-synuclein (Mao et al., 2016). LAG3-directed antibodies and silencing of the gene can significantly reduce α-synuclein aggregate transmission and toxicity as quantified by levels of phosphorylated protein at serine 129 (Mao et al., 2016). In vivo, knock down of LAG3 also decreased PFF-induced death of dopaminergic neurons and associated behavioral and motor deficits (Mao et al., 2016). Pharmaceutical agents are being developed to find LAG3 blockers that do not induce cytotoxicity (Mao et al., 2016).

Another mode of endocytosis for α-synuclein is via binding to HSPGs (Holmes et al., 2013). Known inhibitors of these glycoproteins are soluble heparin, heparinase, and chloral hydrate (Holmes et al., 2013; Ihse et al., 2017; Fields et al., 2019). In vitro, administration of these compounds interferes with HSPGs and decreases uptake of pathogenic α-synuclein (Holmes et al., 2013; Fields et al., 2019). However, blocking this pathway can interfere with other vital cellular processes and continues to be investigated for toxicity and reproducibility (Fields et al., 2019).

The role of the autophagolysosomal pathway in PD is well developed and researchers have sought to enhance clearance of misfolded α-synuclein (Oh et al., 2017). The mammalian target of rapamycin (mTOR) is a conserved serine/threonine protein kinase that plays a role in the process and termination of autophagy (Kim and Guan, 2015). Rapamycin is a direct inhibitor of mTOR and a potent inducer of autophagy (Kim and Guan, 2015). mTOR expression levels increase in overexpression models of α-synuclein (Ramalingam et al., 2019). Rapamycin can reverse the increased mTOR activity induced by α-synuclein and assist in clearance of α-synuclein (Ramalingam et al., 2019). Rapamycin was shown to directly reduce α-synuclein accumulation in PC-12 cell model by inhibiting the termination of autophagy (Sarkar et al., 2005). Rapamycin has also been used in vivo where it improved the motor function in A53T transgenic mice (Bai et al., 2015). However, rapamycin has been seen to cause immunosuppression and is therefore not an ideal drug candidate.

Disaccharide trehalose induces autophagy and has displayed therapeutic effects in vitro and in vivo models of neurodegeneration (Hoffmann et al., 2019; Pupyshev et al., 2019). Trehalose has the properties of a chaperone and can induce chaperone-mediated autophagy independent of mTOR autophagy (Pupyshev et al., 2019). Trehalose enhances lysosomal function and biogenesis resulting in clearance of α-synuclein (Hoffmann et al., 2019). Trehalose is controversial as may not protect cultured neurons against toxicity and therefore has not been widely accepted (Lee et al., 2018).

Pharmacological manipulation of the chaperone-mediated autophagy pathway using AR7, a retinoic acid receptor alpha antagonist in mutant fibroblasts restores lysosomal function (Ho et al., 2020). Attenuated progressive accumulation of α-synuclein oligomers (Ho et al., 2020). However, the compound is unstable and could limit medical use. Therefore, chemists have sought to synthesize derivatives of AR7 that have shown strong aggregation inhibition in situ (Ho et al., 2020).

In vivo and in vitro models have shown that overexpression of Rab1, Rab3a, Rab7, and Rab8a have provided substantial rescue of α-synuclein-induced pathogenesis (Gitler et al., 2008; Dinter et al., 2016). These studies together suggest an important role of the trafficking process of α-synuclein, whereby overexpressing proteins in this pathway could reduce pathology (Gitler et al., 2008; Dinter et al., 2016). However, overexpression of Rab proteins are also seen in cancers and other neurodegenerative diseases, such as Alzheimer’s disease, debating this as a therapeutic option for PD (Li, 2011; Jordan et al., 2022).

At least 5 α-synuclein targeted programs in phase 1 or 2 of clinical testing. However, due to a lack of biomarkers, this can lead to uninformed results (Brundin et al., 2017). It is still unclear what molecular species of α-synuclein is best to target, as this could provide different pathogenicity between patients (Brundin et al., 2017).