Synaptic sabotage: How Tau and α-Synuclein undermine synaptic health

In the two most common neurodegenerative diseases, Alzheimer’s disease (AD) and Parkinson’s disease (PD), the characteristic symptoms, such as cognitive decline and motor impairment, occur only after a substantial number of neurons have already died during the asymptomatic prodromal phase (Schapira et al., 2017; McDade et al., 2021). In the prodromal phase, which precedes disease onset by decades, synaptic dysfunction ensues, followed by synaptic loss, and ultimately neurodegeneration (Cheng et al., 2010; Janezic et al., 2013; Fang et al., 2024; Holmes et al., 2024; Pech et al., 2024; Garcia-Reitböck et al., 2010; Lleó et al., 2019; D’Amelio et al., 2011). Therefore, AD and PD could be considered synaptopathies (Lepeta et al., 2016; Selkoe, 2002).

The temporal sequence of these events may form a causative chain, where initial presynaptic problems lead to the destabilization and loss of synaptic connections, disrupting neuronal circuits, and ultimately contributing to neuron death. Conceptually this may bear similarity to mechanisms of synaptic plasticity and synapse elimination observed in neurodevelopment and neurodevelopmental disorders (Fig. 1 A) (Yaron and Schuldiner, 2016; Taoufik et al., 2018). In an α-Synuclein (α-Syn) overexpression mouse model, loss of presynaptic terminals was observed in the hippocampus (Lim et al., 2011). Subsequently switching off the transgene reversed both the α-Syn pathology and regenerated synapses. This exciting finding suggests that there is a window in which α-Syn–mediated synapse loss is reversible. Additionally, there are several studies indicating that protecting synapses in the context of neurodegeneration models can be beneficial. Kauwe et al. (2024) demonstrated that expressing a C-terminal fragment of Kidney/BRAin (KIBRA)—a protein associated with learning and memory that is reduced in AD brains—could rescue synaptic defects, although not synaptic loss, in rat primary neurons overexpressing human TauP301L and in TauKQ^high mice by enhancing synaptic plasticity. Additionally, memory defects were rescued when the C-terminal KIBRA fragment was expressed in the hippocampus of this mouse model. Similarly, an antibody targeting the negative synaptic contact regulator FAM19A5 restored synapses and improved cognitive function by blocking a synapse elimination pathway in AD mouse models for Tau and amyloid beta (Aβ) (Kim et al., 2024, Preprint). Moreover, reducing levels of the synaptic vesicle (SV) protein Synaptogyrin-3 (Syngr3), the Tau interaction partner on SVs, prevented synapse loss and memory impairment in TauP301S (PS19) mice by blocking the excessive pathological interaction of Tau with SVs (Largo-Barrientos et al., 2021). Together, these studies demonstrate that countering synaptic dysfunction and loss through various mechanisms can prevent memory impairment in AD rodent models, highlighting that this is a promising approach for future therapeutic strategies.

It is well established that Tau and α-Syn—the proteins responsible for the hallmark aggregates in AD and PD, respectively—share molecular characteristics, including prion-like behavior (Jucker and Walker, 2013). Emerging research indicates that these proteins may affect common molecular pathways during the early stages of synaptic dysfunction in neurodegenerative diseases.

Here, we discuss how Tau and α-Syn interfere with shared synaptic processes essential for normal synaptic function. Both proteins have been found to disrupt key presynaptic pathways, including impairing SV dynamics, reducing synaptic protein quality control (PQC), and hampering synaptic signaling pathways (Table 1). This direct interference with local synaptic functions suggests an immediate role in synaptic degeneration. Moreover, we briefly discuss how Tau and α-Syn can also indirectly impair synapses by affecting mitochondrial function (recently reviewed by Thorne and Tumbarello [2022] and Calabresi et al. [2023]).

There are no therapies available yet that can correct the synaptic dysfunction caused by Tau or α-Syn during the prodromal phase, but preventing synaptic loss or promoting synapse regeneration at this stage could be a promising strategy for future therapies to slow down or even counteract neurodegeneration. We highlight why interfering with these processes is detrimental to synapses and discuss the molecular mechanisms in each of these pathways that are disrupted by either protein to propose potential angles for new interventions.

The 14 kDa α-Syn protein is encoded by the SNCA gene, and neurons are among the cell types with the highest expression level (Jakes et al., 1994; Uéda et al., 1993). α-Syn is predominantly enriched at presynapses where it interacts with SVs and regulates neurotransmitter release (Sharma and Burré, 2023; Maroteaux et al., 1988; Burré et al., 2010). Disruption of these functions likely interferes with the synaptic machinery. In neurons, minor amounts of α-Syn were also found in other organelles, such as mitochondria, the nucleus, the endoplasmic reticulum (ER), and the Golgi apparatus, but a comprehensive understanding of its physiological functions outside the presynapse is still lacking (Calabresi et al., 2023; Li et al., 2007; Tompkins et al., 2003; Guardia-Laguarta et al., 2014; Yu et al., 2007).

The α-Syn protein consists of three functionally distinct regions, which are depicted in Fig. 1 B. The N-terminus contains amphipathic repeats that mediate membrane binding (Vamvaca et al., 2009). The non-amyloid-β component (NAC) domain is required for aggregation (Giasson et al., 2001; Uéda et al., 1993). The negatively charged C-terminus can bind Ca²⁺, which regulates its interaction with lipid membranes (Lautenschläger et al., 2018; Nielsen et al., 2001). α-Syn is an intrinsically disordered protein (IDP), but it adopts a partially folded conformation through interaction with lipid membranes (Snead and Eliezer, 2019; Theillet et al., 2016).

α-Syn forms intracellular aggregates, which are the hallmarks of synucleinopathies (Goedert et al., 2017; Spillantini et al., 1997). Like prions, α-Syn monomers first misfold and assemble into oligomeric species that recruit an increasing number of monomers by templated misfolding to form large amyloid structures (Jucker and Walker, 2013). However, the underlying mechanisms inducing the aggregation are not yet fully understood. In sporadic and familial forms of PD as well as in dementia with Lewy bodies (DLB) and pure autonomic failure, α-Syn forms Lewy body inclusions in neurons, whereas in multiple system atrophy (MSA), α-Syn aggregates are found in oligodendrocytes (Calabresi et al., 2023; Spillantini et al., 1997; Kosaka et al., 1976; Vanderhaeghen et al., 1970; Papp et al., 1989). Several missense mutations in α-Syn as well as gene duplication and triplication cause familial PD, linking α-Syn to disease etiology (Goedert et al., 2017; Blauwendraat et al., 2020).

The presynaptic enrichment of α-Syn could explain why synapses are among the first structures to exhibit functional defects in synucleinopathies (Fig. 1 C). The sequestration of α-Syn molecules into aggregates might lead to synaptic impairment by depleting the pool of functional α-Syn molecules at the synapse (Volpicelli-Daley et al., 2011; Chen et al., 2022). Additionally, aberrant interactions between α-Syn and the presynaptic proteome have been described (Cuervo et al., 2004; Xu et al., 2016). These could impair synapse function through a gain of function.

In the central nervous system, Tau exists in six isoforms (ranging from 352 to 441 amino acid [aa] residues), stemming from mRNA alternative splicing of the MAPT gene transcript (Xia et al., 2021) (Fig. 1 D). Despite Tau being an IDP, it exhibits a propensity to form local secondary structures, particularly β-strands in the microtubuli-binding region (MTBR) and polyproline II helices in the proline-rich region (PRR) which are involved in the formation of filaments and fibrils (Scheres et al., 2020; Fitzpatrick et al., 2017; Lövestam et al., 2022, 2024; Sadqi et al., 2002).

Tau plays a pivotal role in stabilizing microtubule bundles and acts to safeguard them from depolymerization by reducing tubulin dissociation at both ends (Best et al., 2019; Venkatramani and Panda, 2019). Tau has been shown to induce microtubule formation in axons and synapses in in vitro and in silico settings and in mouse brain slices (Devred et al., 2004; Hervy and Bicout, 2019; Hori et al., 2022; Kutter et al., 2016; McMillan et al., 2023). Tau binds to microtubules and undergoes stringent regulation by various factors, including post-translational modifications (PTMs), likely to maintain the precise dynamics of the system, but also to regulate the levels of free, unbound Tau, which is prone to aggregation (Lindwall and Cole, 1984; Mandelkow et al., 1995; Ramkumar et al., 2018). Tau PTMs include phosphorylation, glycosylation, O-linked N-acetylglucosamine glycosylation, acetylation, methylation, oxidation, nitrification, ubiquitination, SUMOylation, and truncations, all of which play a role in pathology (Yang et al., 2023). While there is extensive research into these modifications, the exact functional effects of these PTMs are not fully understood. Hyperphosphorylation of Tau is strongly associated with the development of neurofibrillary tangles in tauopathies (Meng et al., 2022). Phosphorylation of Tau disrupts its binding to microtubules, leading to an increase in unbound Tau that is prone to mislocalization of the protein into different compartments, including the synapse (Chakraborty et al., 2023, Preprint) (Fig. 1 C). Pathogenic Tau mutations also impair microtubule-binding affinity (Barbier et al., 2019; Xia et al., 2023; Cario et al., 2022), suggesting this may be a defining feature of Tau-induced disease.

Tau assemblies exhibit several prion-like characteristics which are amplified by inducing the aggregation of native Tau proteins, with pathology spreading between neurons (Jucker and Walker, 2013). Despite their pivotal role in this prion-like transmission, the mechanism by which exogenous Tau assemblies interact with and are internalized by unaffected neurons in human brains remains unclear.

Interestingly, co-pathology of Tau and α-Syn aggregates is found in over 50% of AD brains (Hamilton, 2000). Tau pathology was also observed in PD brains, where it may be one of the earliest contributors to neurodegeneration (Pan et al., 2021; Zhang et al., 2023; Chu et al., 2024). In addition, as will be discussed, it has been shown that the two proteins can directly interact. α-Syn fibrils significantly enhance the clustering, synaptic localization, and spreading of fibrillar Tau resulting in the disruption of neuronal homeostasis (Shrivastava et al., 2019; Bassil et al., 2021). Furthermore, Tau enhances α-Syn aggregation and toxicity (Badiola et al., 2011). Hence, Tau and α-Syn have synergistic effects in neurodegeneration and may aggravate disease.

Numerous interaction partners of both native and aggregated α-Syn and Tau have been identified, and their physiological relevance is being investigated. However, to fully understand the role of α-Syn and Tau in early synaptic dysfunction, it is essential to investigate their local, presynaptic interactomes under native and disease conditions.

Since α-Syn is predominantly a presynaptic protein, conventional interaction methods, such as co-immunoprecipitation (co-IP), which do not specifically target the presynaptic α-Syn pool, have still mostly identified presynaptic proteins as primary interaction partners, as expected (Table 2). Consequently, most interactomes are enriched with synaptic proteins, such as Synapsin and EndophilinA (EndoA) (Parra-Rivas et al., 2023). Additionally, even for cytoplasmic interactors not highly concentrated in synapses, such as leucine-rich repeat kinase 2 (LRRK2) (Guerreiro et al., 2013), it is reasonable to assume they primarily interact with α-Syn at the synapse due to the predominantly presynaptic localization of α-Syn under endogenous conditions. Furthermore, α-Syn co-localizes with various endolysosomal proteins such as lysosome-associated membrane proteins (LAMPs) and the Rab GTPases Rab5a, Rab7, and Rab11a (Teixeira et al., 2021; Hasegawa et al., 2011). In addition, mitochondrial, ER, autophagy, and ubiquitin-proteasome system (UPS) proteins were identified in in vitro assays with pathogenic α-Syn (Leitão et al., 2021).

Interestingly, although Tau is primarily axonal in its native state, its localization shifts toward the synapse in disease conditions, and many presynaptic interaction partners, such as Syngr3 and Syntaxin have been identified for Tau (Table 2) (Tracy et al., 2022; McInnes et al., 2018). Moreover, frontotemporal disease (FTD)–associated Tau mutations or Tau phosphorylation (p-Tau) have been shown to alter the Tau interactome. In neurons expressing the FTD-associated V337M or P301L mutations, interactions with SV proteins increased, and p-Tau showed enhanced interactions with proteins involved in degradation pathways, such as the proteasome and the autophagosome–lysosome system, and mitochondrial proteins, such as Drp1, VDAC1, and electron transport chain components (Drummond et al., 2020; Manczak and Reddy, 2012a, 2012b). Additionally, studies across various models, from yeast to mice and postmortem brain tissue, consistently identify overlapping sets of interaction partners for monomeric α-Syn and Tau as well as for the aggregated forms of both proteins; however, aggregation may alter the abundance of certain interactors (Italia et al., 2022; Leitão et al., 2021; Ferrari et al., 2020). These interaction partners are typically synaptic and membrane-associated proteins, RNA-binding proteins, and the PQC machinery, which is crucial for maintaining presynaptic function (Drummond et al., 2020; Popova et al., 2021; Shrivastava et al., 2019). These findings underscore the importance of understanding the presynaptic roles of Tau in both health and disease.

A study using human embryonic stem cell–derived neurons directly compared the interactomes of wild type (WT) and disease-variant biotin identification (BioID)–tagged α-Syn and Tau following seeding with their respective recombinant fibrils (Griffin et al., 2023). The authors identified an overlapping interaction network for both proteins, suggesting they may converge on similar pathways. Common Gene Ontology terms included microtubule cytoskeleton, Wnt signaling, RNA granules, and proteins associated with synapses and membranes. Fibril transduction remodeled the interactomes of both α-Syn and Tau, leading to the loss of native partners and the formation of new interactions, such as with the synaptic plasma membrane protein Na⁺-K⁺-ATPase, ultimately contributing to glutamate-induced neurotoxicity (Kahle et al., 2015; Shrivastava et al., 2015, 2019). In addition to the binding of Tau and α-Syn to the membranes of organelles and the plasma membrane, it will be interesting to investigate whether these membranes promote Tau and α-Syn aggregation and secretion linked to increased neuronal activity. For example, exosomes are proposed to be involved in Tau and α-Syn secretion (Fan et al., 2019; Fowler et al., 2024; Howitt et al., 2021, Preprint; Miyoshi et al., 2021; Polanco et al., 2021; Zhao et al., 2023; Camilleri et al., 2020; Rose et al., 2024; Meier et al., 2015). These findings further suggest that Tau and α-Syn may impair synaptic function through shared pathways.

Proximity labeling techniques, which allow the detection of transient or less stable interactions, have recently been applied to refine the interactomes of α-Syn and Tau in human induced pluripotent stem cell (iPSC)–derived neurons (Chung et al., 2017; Tracy et al., 2022).

As expected, proteins involved in the SV cycle and membrane trafficking were among the top interactors for both α-Syn and Tau (Table 2). Additionally, Tau interactors included mitochondrial, proteasomal, and autophagy-related proteins. These datasets further included cytosolic proteins involved in cytoskeletal functions, RNA binding, and Tau and α-Syn. Key findings included that Tau interacts with presynaptic vesicle proteins in an activity-dependent manner and more specifically with SV proteins regulating exocytosis. Importantly, many interaction hits identified in these APEX studies have previously been linked to PD or AD, emphasizing the physiological relevance of these approaches. Many proteins associated with PD risk or progression, such as LRRK2 (Guerreiro et al., 2013), Parkin (Norris et al., 2015), DJ-1 (Kumar et al., 2019), Tau (Torres-Garcia et al., 2022), and Synphilin-1 (Bernal-Conde et al., 2020), have been experimentally validated as α-Syn interactors. Similarly, presynaptic Tau interaction partners, such as Bin1 (Amphiphysin-2) (Lasorsa et al., 2018; Sottejeau et al., 2015; Dräger et al., 2017), α-Syn (Yan et al., 2020; Twohig and Nielsen, 2019), and LRRK2 (Zhao et al., 2011), are associated with increased risk for AD.

This list of interaction partners is not exhaustive but demonstrates that Tau and α-Syn engage with overlapping presynaptic pathways. This positions α-Syn and Tau as central players in the converging presynaptic dysfunction observed in both PD and AD (van der Gaag et al., 2024).

The observed similarities between α-Syn and Tau suggest they may be involved in shared disease mechanisms, possibly raising the potential for a common therapeutic approach targeting both proteins. Notably, different proximity labeling biotin ligases generally have a labeling radius of 10–35 nm (Mair and Bergmann, 2022), which can also label nearby non-interacting proteins, potentially leading to false positives. Therefore, putative shared interactors between Tau and α-Syn require validation through independent assays, such as co-IPs.

Current interactome studies, however, lack subcellular resolution. As discussed above, synaptic proteins have been identified as interaction partners in cell-wide approaches; however, interactions involving low-abundance synaptic proteins may go undetected in such setups. Additionally, determining whether an interaction with a cytosolic protein occurs broadly within the cell or is specific to subcellular regions, such as the synapse, cell body, or axon is essential. For example, several RNA-binding proteins have been identified as interactors of Tau and α-Syn (Chung et al., 2017). The exact subcellular locations of these interactions remain unknown; potentially indicating nuclear functions of Tau and α-Syn or suggesting roles in local mRNA translation within axons and synapses.

To address this, more advanced study designs that integrate proximity labeling techniques with synaptosome isolation and mass spectrometry could help capture synapse-specific interactions. Identifying these local interactors may reveal novel drug targets to address synaptic dysfunction without affecting other cellular functions.

Functional SV cycling is essential for neurotransmitter release, vesicle recycling, synaptic strength regulation, homeostasis maintenance, and rapid response to stimuli, all of which are fundamental for proper neuronal function and communication (Ivanova and Cousin, 2022; Bonnycastle et al., 2021).

The most investigated function of α-Syn is its involvement in the SV cycle (reviewed by Lautenschläger et al. [2017]). α-Syn is thought to control the homeostasis of the SV pool by affecting SV clustering, trafficking, and docking at the active zone, and to impact neurotransmitter release via regulating fusion with the plasma membrane (Fig. 2 A). Mechanistically, α-Syn exerts its functions both by interacting with SV proteins, such as the soluble N-ethylmaleimide attachment protein receptor (SNARE) vesicle-associated membrane protein 2 (VAMP2/Synaptobrevin-2), and by directly binding to the SV lipid membranes (Burré et al., 2010; Fusco et al., 2016). α-Syn chaperones the assembly of the SNARE complex, coordinating the fusion of SVs and the plasma membrane (Burré et al., 2010). However, the exact mode of action of α-Syn in SV exocytosis is not yet entirely clear (Lautenschläger et al., 2017; Wang et al., 2014; Diao et al., 2013). α-Syn seems to be a negative regulator of SV exocytosis; however, there is also evidence that α-Syn could facilitate exocytosis (Nemani et al., 2010; Larsen et al., 2006; Chandra et al., 2005; Lautenschläger et al., 2017). Employing comparable models to evaluate α-Syn’s specific impact on exocytosis—independent of its other roles in the SV cycle—is essential to address this issue. While α-Syn knockout mice show only mild and inconsistent phenotypes (Abeliovich et al., 2000; Cabin et al., 2002), triple knockout animals, which lack all three members of the synuclein protein family, revealed increased neurotransmitter release, suggesting an inhibitory function for α-Syn (Greten-Harrison et al., 2010; Anwar et al., 2011). This is in line with other studies demonstrating that α-Syn clusters and immobilizes SVs (Diao et al., 2013; Wang et al., 2014) (Fig. 2 B). Similarly, the injection of recombinant α-Syn phosphorylated at the disease-relevant amino acid S129 into lamprey neurons rearranged the SV cluster, resulting in fewer SVs at the synapse, impaired SV cycling, and synaptic activity defects (Wallace et al., 2024). Additionally, there is evidence suggesting α-Syn affects SV endocytosis (Busch et al., 2014; Vargas et al., 2014; Xu et al., 2016).

Since different α-Syn models have yielded conflicting results regarding the net effect of α-Syn on neurotransmission, more physiological approaches that abstain from α-Syn overexpression will be needed to gain further insight.

Like α-Syn, Tau also appears to functionally impact synaptic functions: when Tau detaches from the microtubuli, soluble Tau appears to migrate toward compartments where Tau levels are otherwise strictly controlled, and this includes the presynaptic terminal (Fig. 1 C) (McInnes et al., 2018; Zhou et al., 2017).

The precise roles of α-Syn and Tau at the presynapse remain incompletely understood. SV cycling requires substantial amounts of ATP (Rangaraju et al., 2014; Li and Sheng, 2021), but since α-Syn and Tau disrupt mitochondrial function, ATP production may be impaired, significantly impacting the SV cycle (Choi et al., 2022; Paillusson et al., 2017; Lurette et al., 2023; Liu et al., 2023; Pérez et al., 2018). Interestingly, recent research suggests that presynaptic mitochondria in cortical pyramidal neurons may primarily consume and not produce ATP as complex V of the electron transport chain functions in reverse in these mitochondria (Hirabayashi et al., 2024, Preprint). To maintain SV cycling, glycolysis may serve as an alternate ATP source. Proteomic analysis of cortical synaptosomes shows enrichment in glycolytic proteins (van Oostrum et al., 2023), which are also known to localize at synapses (Jang et al., 2016) and associate with SV membranes (Ikemoto et al., 2003; Hinckelmann et al., 2016), potentially providing a local ATP supply for the SV cycle. Interestingly, the glycolytic pathway is downregulated in α-Syn and Tau mutant neurons, further suggesting that ATP levels may indeed be low in α-Syn and Tau mutant presynapses (Glasauer et al., 2022; Melnikova et al., 2020; Dai et al., 2023; Zhang et al., 2021). Additionally, smooth ER is present in many presynaptic terminals (Özkan et al., 2021; Bezprozvanny and Kavalali, 2020), where it contributes locally to synthesizing presynaptic proteins that support processes essential for SV cycling (Deng et al., 2021; Perrone-Capano et al., 2021). Given that α-Syn and Tau disrupt ER function and affect mRNA translation through ribosome binding (Evans et al., 2019, 2021; Meier et al., 2015; Papanikolopoulou et al., 2019), they may also impair the SV cycle by this mechanism; however, further research is required to confirm this at the presynaptic terminal.

Effective protein turnover is also necessary to sustain a functional pool of presynaptic vesicle proteins for neurotransmission (Uytterhoeven et al., 2011, 2015), which also becomes disrupted by α-Syn and Tau (discussed in the section: “α-Syn and Tau disrupt presynaptic PQC”). As indicated in the section “Synapse-specific interactomes of α-Syn and Tau,” there is ample evidence of Tau interacting with presynaptic proteins. The interaction of Tau with the SV protein Syngr3 has been linked to a reduction in the mobility of SVs and a decrease in neurotransmission during intense stimulation paradigms both in fruit flies and in mouse and rat neurons (McInnes et al., 2018; Zhou et al., 2017) (Fig. 2, A and B). Decreasing the levels of Syngr3, which consequently reduces the interaction between Tau and SVs, rescues age-related Tau mutant phenotypes, including memory and cognition deficits, deficits in long-term potentiation (LTP), and synapse degeneration in the PS19 mouse model (Largo-Barrientos et al., 2021) and SV clustering, neurotransmission, and neurodegeneration in pathogenic Tau expressing fly models (McInnes et al., 2018). Given the proposed role of Syngr3 in inhibiting synaptic autophagy (Hernandez-Diaz et al., 2023, Preprint), the loss of Tau association with SV upon Syngr3 depletion could coincide with increased clearance of Tau. These studies suggest that the interaction of Tau with SV is an important and defining aspect of Tau-induced synaptic dysfunction and neurodegeneration.

The N-terminus of Tau has a crucial role in its interaction with SV. Peptides directed against the Tau N-terminus disrupt its interaction with SV and correct Tau-mediated presynaptic defects (McInnes et al., 2018; Zhou et al., 2017), while 11 amino acids (18–28) specifically regulate interactions with proteins that control synaptic function (Stefanoska et al., 2018). This 11-aa sequence, which is unique to the N-terminus of primates, may explain the vulnerability of humans to Tau in mediating neurodegenerative disorders compared with other mammalian species. Together, these findings suggest that the N-terminus of Tau influences native interactions, affecting distinct cellular and molecular targets of synapse function.

While mice that express pathogenic forms of Tau display reduced LTP and reduced neurotransmission (Decker et al., 2015; Largo-Barrientos et al., 2021; McInnes et al., 2018; Zhou et al., 2017), there is a significant increase in network electrophysiological activity in Tau disease animal models (Shimojo et al., 2019; Gomez-Murcia et al., 2020), and in induced human neuron cultures derived from tauopathy patients (Sohn et al., 2019; Iovino et al., 2015). While the precise mechanisms by which aberrant Tau leads to neuronal hypoactivity at the single cell level and to hyperexcitability in neuronal networks remains unclear but it is believed that hyperexcitability may contribute to neuronal dysfunction at the onset of AD (Hall et al., 2015; Decker and Mandelkow, 2019).

Preserving or restoring normal synaptic transmission at early stages of AD and PD could be a promising strategy to prevent synaptic loss and impaired neuronal circuits. As discussed earlier, the primary focus for α-Syn should be to clarify its overall role in regulating SV dynamics in the human brain. This understanding will enable the development of therapeutics that help maintain α-Syn’s function in this context. In the case of Tau, targeted disruption of the aberrant Tau-SV interaction, such as by reducing Syngr3 levels, has shown promising results in a mouse model (Largo-Barrientos et al., 2021).

Ca²⁺ is a crucial signaling factor. Persistent and elevated Ca²⁺ levels play a significant role in the progression of neurodegeneration observed in synucleinopathies and tauopathies (Webber et al., 2023; Imamura et al., 2016; Zaichick et al., 2017). Mitochondria, lysosomes, and the ER regulate presynaptic Ca²⁺ levels by buffering and release (Fig. 2 A) (Devine and Kittler, 2018). In addition, interactions between mitochondria, ER, and lysosomes—whether through direct contact sites or various signaling pathways—are essential for overall Ca²⁺ regulation at presynaptic terminals (Calì et al., 2012). Disruption of Ca²⁺ homeostasis can harm neurons through several pathways, such as impaired neuronal transmission and a decline in PQC due to lysosomal and mitochondrial dysfunction and protein misfolding stress within the ER (Mustaly-Kalimi et al., 2022; Lee et al., 2022; Lim et al., 2023; Britti et al., 2020). While increased Ca²⁺ levels may drive α-Syn and Tau pathology (Datta et al., 2021; Nath et al., 2011), there is growing evidence that α-Syn and Tau themselves can disrupt Ca²⁺ homeostasis by impairing lysosomal, mitochondrial, and ER functions (Fig. 2, A and B) (Kovacs et al., 2021; Webber et al., 2023; Britti et al., 2020; Calì et al., 2012), creating a vicious cycle.

Presynaptic cytosolic Ca²⁺ plays a critical role in modulating the interaction of α-Syn with SVs. Increased Ca²⁺ levels promote α-Syn binding to lipid membranes, which alters SV pool homeostasis (Lautenschläger et al., 2018). At the same time, Ca²⁺ facilitates α-Syn liquid–liquid phase separation (LLPS); however, as shown by Huang et al. (2022), this was observed under non-physiological, high Ca²⁺ concentrations. To derive biologically relevant insights from future LLPS studies, experiments should be conducted under physiological Ca²⁺ conditions. In addition, Ca²⁺ induces α-Syn aggregation (Huang et al., 2022; Nath et al., 2011; Follett et al., 2013) and triggers α-Syn-release from neurons (Emmanouilidou et al., 2010). Interestingly, α-Syn can directly affect intracellular Ca²⁺ homeostasis through aberrant interactions with membranes and transporters (Butler et al., 2015; Reynolds et al., 2011; Liu et al., 2021). α-Syn may oligomerize on mitochondrial membranes, which leads to membrane permeabilization and cytosolic Ca²⁺ influx (Choi et al., 2022). In addition, α-Syn controls mitochondrial Ca²⁺ uptake and pathogenic overexpression of α-Syn disrupts Ca²⁺ exchange between the ER and mitochondria (Thorne and Tumbarello, 2022; Ramezani et al., 2023). Extracellularly applied α-Syn monomers and oligomers increased the cytosolic Ca²⁺ concentration in various cell culture systems (Angelova et al., 2016). Consistently, deregulated Ca²⁺ homeostasis correlated with the appearance of α-Syn oligomers in induced dopaminergic neurons harboring SNCA mutations (Virdi et al., 2022) before cell stress pathways became affected. A different line of research demonstrated that α-Syn aggregates bind the sarco/ER Ca²⁺-ATPase, resulting in reduced intracellular Ca²⁺, which could explain the initial drop in intracellular Ca²⁺ during early stages of PD before Ca²⁺ levels are elevated at later phases of the disease process (Betzer et al., 2018). Taken together, these studies reveal that α-Syn plays a dual role in Ca²⁺ homeostasis: while α-Syn-mediated SV release is regulated by Ca²⁺, α-Syn can disturb the intracellular Ca²⁺ balance. This disruption may interfere with its own function and impact other critical processes, such as mitochondrial activity, which is essential for synaptic function, potentially creating a vicious cycle.

Striking similarities have been observed between the effects of α-Syn and Tau on Ca²⁺ homeostasis. Like α-Syn, Tau phosphorylation and aggregation are facilitated by Ca²⁺ in vitro (Moreira et al., 2019; Cao et al., 2019). A recent study in non-human primates shows Ca²⁺-dependent phosphorylation of Tau (Datta et al., 2021). Furthermore, Ca²⁺ affects Tau LLPS; however, additional research is needed to determine its physiological significance (Yabuki et al., 2024).

Tau also disrupts Ca²⁺ homeostasis through various mechanisms. Aberrant forms of Tau, such as caspase-3–cleaved Tau, interact with and sequester Sorcin, a Ca²⁺-binding protein, leading to defective Ca²⁺ homeostasis (Kim et al., 2016). In iPSC-derived human neurons, mutant Tau inhibited Ca²⁺efflux from mitochondria, resulting in increased vulnerability to Ca²⁺-induced cell death (Britti et al., 2020). Furthermore, in an AD mouse model, increased Ca²⁺ levels were also seen in mitochondria (Calvo-Rodriguez et al., 2020). In addition, in human AD brain and aged Drosophila brains expressing human pathogenic Tau, mitochondrial Ca²⁺ transport genes are deregulated compared with control brains according to bulk and single-cell RNA sequencing and microarray analyses (De Jager et al., 2018; Wang et al., 2018; Praschberger et al., 2023). AD-related Tau phosphorylation at serine 262/356 promotes the activation of Ca²⁺/calmodulin-dependent protein kinase II, a central molecular organizer of synaptic plasticity (Oka et al., 2017). Moreover, Tau aggregates can integrate into the cell membrane, modifying ion currents and activating voltage-gated Ca²⁺ channels resulting in an influx of Ca²⁺ ions that trigger reactive oxygen species production contributing to toxicity (Esteras et al., 2021). In addition, in presymptomatic AD mice, increased ryanodine receptor-evoked Ca²⁺ release from ER resulted in increased spontaneous vesicle release and altered release probability of SVs, likely as a compensatory mechanism of the increased cytosolic Ca²⁺ early in the disease (Chakroborty et al., 2012b). The expression of 4R human Tau in rat hippocampal neurons enhances the entry of Ca²⁺ through its interaction with L-type channels (Stan et al., 2022). Consequently, increased Ca²⁺ entry following hyperpolarization in hippocampal neurons inhibits action potential firing ultimately contributing to the development of cognitive deficits (Moore et al., 2023). In addition, pathological Tau increases TCP2 activity, a Ca²⁺ channel expressed on endolysosomal membranes, resulting in aberrant lysosomal Ca²⁺ and pH levels (Tong et al., 2022). Defective lysosomes impact the PQC machinery and the turnover of Tau resulting in Tau accumulation and aggregation (see section “α-Syn and Tau promote proteostasis collapse”). Finally, genes encoding Ca²⁺ homeostasis-regulating proteins were among the top differentially expressed genes in those neurons most vulnerable to the expression of pathogenic Tau in flies; these Ca²⁺-regulating genes are also strong modifiers of Tau-induced dysfunction (Praschberger et al., 2023). This suggests that Tau toxicity converges on Ca²⁺ homeostasis and this defective pathway creates a positive feedback loop further enhancing the pathology, similar to α-Syn.

Clinically approved drugs targeting Ca²⁺ dysregulation have demonstrated limited effectiveness in AD and none have received approval for PD (Calvo-Rodriguez et al., 2020; Alam et al., 2022). These drugs lack specificity, and currently, no therapies have been developed to prevent disruptions in Ca²⁺ homeostasis in Tau- or α-Syn–related diseases. A recent study by Princen et al. (2024) might break this inertia. This work identified a Septin-mediated Ca²⁺ entry pathway which was found to be excessively activated by pathological Tau. The authors also present a highly promising compound that restored Ca²⁺ homeostasis, resulting in improved synaptic function and the rescue of cognitive defects in AD mouse models. Another promising example is the use of allosteric ryanodine receptor modulators, which help to lower abnormal ER Ca²⁺ responses and reduce p-Tau, thereby rescuing Tau-induced pre- and postsynaptic deficits. Although further research is necessary to evaluate their effects on neurodegeneration and safety, these findings support the potential of targeting Ca²⁺ homeostasis as a mechanism of action for a new class of drugs (Oulè et al., 2012; Chakroborty et al., 2012a; Hiess et al., 2022).

In recent years, many intrinsically disordered region-containing proteins, including Tau and α-Syn, have been associated with neurodegeneration. Both were also shown to undergo LLPS (Alberti and Dormann, 2019; Chakraborty and Zweckstetter, 2023). LLPS promotes the local enrichment of biomolecules, such as proteins and RNAs, into membraneless condensates, driven by low-affinity multivalent interactions between these molecules (Shin and Brangwynne, 2017). Interestingly, initial data point toward a role of phase-separated Tau and α-Syn in regulating SV mobility, potentially initiating disease etiology. Exciting work showed that the SV pool is organized as a phase-separated compartment and that the synaptic protein Synapsin is the main organizer of SV condensates (Milovanovic et al., 2018; Hoffmann et al., 2023). Interestingly, α-Syn is recruited into these Synapsin-organized SV droplets both in complex in vitro systems and in cell culture models, while α-Syn alone is not capable of driving the phase separation of SVs under physiological conditions in vitro (Hoffmann et al., 2021). However, when component concentrations exceed physiological levels, α-Syn alone can independently draw liposomes into condensates in vitro (Hardenberg et al., 2021). These data suggest a regulatory function of α-Syn within the phase-separated SV cluster, where it might help to maintain the condensate by modulating its LLPS properties, which should be investigated in future research (Fig. 2 C). This adds another regulatory level to how α-Syn modulates SV dynamics in addition to its role in chaperoning SNAREs during SV exocytosis. Moreover, VAMP2 promotes α-Syn LLPS formation in vitro and in cells (Wang et al., 2024; Agarwal et al., 2024). In contrast to the Synapsin–α-Syn condensates that cluster together with SVs, the VAMP2–α-Syn condensates recruit a range of vesicles, including autophagosomes, and could thus serve a complementary role at the synapse.

While these studies are descriptive, we still do not know whether the presence of α-Syn in the SV cluster is beneficial or detrimental in the disease context. Therefore, it will be important to elucidate what the functional consequences of a-Syn’s presence in Synapsin-SV clusters are in vivo. Some forms of familial PD are caused by SNCA gene multiplications or mutations that increase α-Syn levels (Chartier-Harlin et al., 2004; Singleton et al., 2003; Blauwendraat et al., 2020). Such an imbalance could increase the α-Syn abundance in the phase-separated SV cluster disrupting the stoichiometry within the condensate. This could possibly reduce SV mobility and thus contribute to the early synaptic dysfunction seen in the disease (Brodin et al., 2022). Moreover, the local increase of α-Syn concentration within the SV condensate might facilitate α-Syn aggregation thereby initiating downstream events. An alternative consideration is that retaining α-Syn in the SV cluster could prevent its aggregation as the molecules in the cluster remain in a liquid phase, and spreading may be also diminished, given that less α-Syn may interact with the membrane or may be secreted via exosomes (Brodin et al., 2022). Employing in vivo models for synaptic dysfunction and neurotoxicity will be crucial to answering these questions.

Similar mechanisms apply to Tau, which is also an IDP that undergoes LLPS in vitro. Tau mutations such as P301L, P301S, and A152T have been shown to promote Tau droplet assembly, and this process can be modulated by PTMs (Hernández-Vega et al., 2017; Ainani et al., 2023; Kanaan et al., 2020). Additionally, Tau droplets have been observed in Tau-transfected neurons (Wegmann et al., 2018), and several studies have shown that Tau invades the SV pool (McInnes et al., 2018; Longfield et al., 2023; Zhou et al., 2017; Largo-Barrientos et al., 2021). Recently, single-molecule super-resolution microscopy revealed that Tau undergoes LLPS at the presynapse where it forms transient nanoclusters (Longfield et al., 2023). The density and quantity of these clusters are finely tuned by synaptic activity, and these condensates play a pivotal role in selectively regulating the mobility of recycling SVs. Like α-Syn, Tau also directly interacts with lipid membranes (McInnes et al., 2018; Katsinelos et al., 2018). Together, these findings suggest that Tau might modify the organization of the SV pool by interfering with their LLPS behavior. However, experimental data on whether Tau affects the Synapsin-SV cluster or also other vesicle pools in a manner similar to α-Syn are still lacking. Additionally, it has been shown that Tau and α-Syn can co-phase separate in vitro (Gracia et al., 2022; Siegert et al., 2021), adding further complexity and potentially contributing to our understanding of Tau and α-Syn co-pathologies. Further research is needed to determine if this has any physiological role or disease relevance in humans.

Hence, there may be opportunities in the development of drugs that modify the properties of phase-separated compartments (Alberti and Dormann, 2019; Wheeler, 2020). It would be exciting to test whether the phase-separated SV pool could be targeted using LLPS modifiers and whether this would prevent synaptic decline. However, a much deeper understanding of the different proteins involved in LLPS of the SV cluster along with their mechanisms of action in both health and disease is essential if we want to identify specific candidates for drug development aimed at restoring or maintaining native mobility within the SV condensate.

Synapses rely on local PQC mechanisms to maintain their protein homeostasis (proteostasis) due to their distance from the cell body and the constant challenges posed by synaptic activity. Disruption of this delicate balance is detrimental to synaptic function. It is becoming increasingly evident that both Tau and α-Syn interfere with the proteostasis machinery, leading to its collapse and resulting in synaptic dysfunction.

The proteostasis network contains various components involved in protein synthesis, folding, and degradation: the translation machinery, chaperones, the UPS, and autophagy pathways (Klaips et al., 2018). Multiple autophagy types exist and three are discussed in this review (Fig. 3): macroautophagy (commonly termed autophagy), endosomal microautophagy (eMI), and chaperone-mediated autophagy (CMA) (Kaushik and Cuervo, 2018). In (macro)autophagy, a double membrane forms an autophagosome around the cargo, which then fuses with a lysosome for degradation. Autophagy can either be a bulk or a selective process, targeting specific organelles such as mitochondria (mitophagy). During eMI, the late endosomal membrane forms small vesicles to engulf cytosolic cargo, either in bulk or recruited by HSC70 and co-chaperones, which is selective for cargo containing KFERQ-like motifs. In CMA, HSC70 and co-chaperones, such HSP40s, also sequester KFERQ-like motif-containing substrates and facilitate their translocation into the lysosome through the membrane protein LAMP2A (Kaushik and Cuervo, 2018). The proteostasis network components can compensate for each other if one arm is impaired (Klaips et al., 2018).

Autophagy is particularly important for the presynapse because it couples synaptic activity with local protein turnover (Decet and Verstreken, 2021; Hernandez et al., 2012; Hill et al., 2019; Kuijpers et al., 2021; Soukup et al., 2016; Bademosi et al., 2023). Many presynaptic proteins play a role in regulating autophagy and endolysosomal protein turnover (Gundelfinger et al., 2022). Quantitative mass spectrometry analysis of LC3-positive autophagic vacuoles purified from the mouse forebrain identified numerous presynaptic substrates, suggesting that many presynaptic proteins are degraded by autophagy. Additionally, this study indicated that beyond macroautophagy, synaptic proteins may utilize alternative routes to the lysosome by regulating their proteostasis (Kallergi et al., 2023). Interestingly, some of these substrates were previously also reported as targets of the proteasome (Hakim et al., 2016; Bingol and Schuman, 2005).

Recent research identified a local regulatory hub for synaptic autophagy that includes EndoA, Synaptojanin-1 (SYNJ1), and LRRK2, all of which are associated with familial and sporadic forms of PD (Bademosi et al., 2023; Soukup et al., 2016). Additionally, in rat primary cortical neurons, most synaptic proteins are primarily degraded through autophagy rather than the UPS (Hakim et al., 2016). Nearly 60% of the presynaptic proteins possess one or multiple KFERQ-like motifs (Uytterhoeven et al., 2015), and CMA has been shown to play an essential role in preventing proteostasis collapse in neurons in vivo (Bourdenx et al., 2021). This may explain why impaired local proteostasis is harmful to synaptic function and why synaptic dysfunction is an early defect observed in neurodegenerative diseases.

α-Syn and Tau, in their monomeric, oligomeric, and aggregated forms, are degraded by various PQC pathways to different degrees preventing their accumulation and proteotoxicity (Fig. 3).

α-Syn was initially identified as a UPS substrate in vitro, in cell culture, and in mouse models (Bennett et al., 1999; Ebrahimi-Fakhari et al., 2011). However, proteomic analysis of cultured rat neurons demonstrated that α-Syn as well as many other synaptic proteins are mostly degraded via autophagic pathways, and their levels are not significantly affected by UPS inhibition (Hakim et al., 2016). UPS-mediated degradation does thus not appear to be the primary turnover mechanism of α-Syn under native conditions. More recent studies suggest that the UPS may help in clearing phosphorylated and aggregated species (Stefanis et al., 2019; Pantazopoulou et al., 2021).

Interestingly, Tau and α-Syn contain at least one KFERQ-like motif. In line with this, both monomeric and disease-associated oligomeric α-Syn species can be handled by the various autophagic pathways (reviewed in Stefanis et al. [2019]), with CMA being the main degradation route for α-SynWT (Cuervo et al., 2004). Moreover, deregulated autophagic lysosomal degradation has been linked to increased α-Syn aggregation in both PD models and patient samples (Yu et al., 2009; Dehay et al., 2010).

Like α-Syn, Tau is turned over by both UPS and autophagy pathways. However, in contrast to α-Syn, UPS is the primary route for the degradation of soluble Tau fractions while macroautophagy appears to mainly deal with the clearance of pathological aggregates in neurons (Dikic and Elazar, 2018). Recent work shows that in addition to the UPS route, soluble Tau monomers and oligomers can also be cleared by CMA and eMI (Caballero et al., 2018; Uytterhoeven et al., 2018; Vaz-Silva et al., 2018).

A growing body of research demonstrates that both α-Syn and Tau actively interfere with the PQC machinery by targeting its various components, thereby ultimately hindering their own turnover and that of other critical substrates (Fig. 3). This suggests that α-Syn and Tau may contribute to synaptic dysfunction by disrupting local proteostasis, pointing toward entirely new treatment opportunities.

Both recombinant monomeric and aggregated α-Syn inhibit proteasome function thereby impairing the degradation of UPS-relevant substrates in vitro and in dopamine-expressing cell lines, such as SH-SY5Y and PC12 cells, while not affecting non-dopaminergic U2OS ps 2042 [Ubi(G76V)-GFP] cells (Snyder et al., 2003; Suzuki et al., 2020; Zondler et al., 2017). Similarly, in an α-SynA53T mouse model, UPS function was impaired in nigral dopaminergic neurons (McKinnon et al., 2020). While these studies indicate a potential specificity of this pathway for dopaminergic neurons, which may in part explain their selective vulnerability in PD, the underlying molecular mechanism remains unknown. Additional in vivo studies across dopaminergic and non-dopaminergic cell types will help to answer this question. Moreover, further research is needed to clarify the contribution of the UPS to α-Syn turnover in healthy and disease conditions.

In many α-Syn overexpression models, autophagic degradation is compromised due to α-Syn interfering with multiple steps of the autophagy–lysosome pathway. For instance, overexpression of α-SynWT disrupted the initiation of autophagosome biogenesis (Winslow et al., 2010), while α-SynA30P blocked autophagosome formation (Lei et al., 2019). α-SynWT has also been shown to inhibit the SNARE-mediated fusion of autophagosomes with lysosomes (Tang et al., 2021). Additionally, α-Syn impaired the retrograde transport of endosomes (Volpicelli-Daley et al., 2014). α-Syn also directly reduced lysosomal function by disturbing lysosomal Ca²⁺ homeostasis and raising their pH (Nascimento et al., 2020). Furthermore, α-Syn aggregates can disrupt the lysosomal membrane integrity, causing dysfunction and leakage of lysosomal content into the cytoplasm (Freeman et al., 2013; Sandhof et al., 2020).

In contrast, other studies have shown that α-Syn can increase autophagic flux, likely as a compensatory mechanism to clear toxic species. Both postmortem brain samples and an α-SynA53T mouse model exhibited autophagy induction (Yu et al., 2009). In rat primary neurons, the effect of α-Syn on autophagic flux depended on the specific α-Syn variant expressed (Koch et al., 2015). In this study, overexpression of both the α-SynWT and A53T variants led to an increase in autophagic flux, while the α-SynA30P variant showed no significant effect. The authors speculate that α-SynA30P may inhibit autophagophore formation potentially due to its reduced membrane-binding capacity. Consistent with this, iPSC-derived neurons from patients with SNCA gene triplication also showed increased autophagic flux (Oliveira et al., 2015). Additional studies are needed to resolve these inconsistent findings across the α-Syn field, which may stem from differences in α-Syn variants used, α-Syn overexpression models vs. models relying on endogenous levels, or the specific disease stage (developmental age of the model system) being investigated. Clarifying the effect of α-Syn on autophagic flux is crucial as it would determine the treatment strategies that are pursued.

Disease-associated α-Syn variants have also been shown to interfere with other autophagy pathways. CMA is hindered by mutant and dopamine-modified α-Syn as they bind to LAMP2A and block their own translocation into the lysosome, thereby preventing their own degradation as well as the turnover of other substrates (Cuervo et al., 2004; Martinez-Vicente et al., 2008; Xilouri et al., 2009). Moreover, α-Syn impairs mitophagy, compounding its detrimental effects on mitochondrial transport and function. This disruption may ultimately compromise ATP supply at synapses, which are compartments with high energy demand (reviewed in Thorne and Tumbarello [2022]).

Given the synaptic localization of α-Syn, it is conceivable that the protein potentially targets autophagy in the synapse more than at other sites in the neuron. Proteomics of synaptosomes from α-Syn knockout and overexpression mouse models showed an inverse correlation between α-Syn and EndoA protein levels (Westphal and Chandra, 2013). In mouse brain lysates, α-Syn phosphorylated on the disease-related residue S129 bound EndoA, suggesting this interaction is tightly controlled (Parra-Rivas et al., 2023). Important future questions are whether higher α-Syn levels indeed affect EndoA during early PD stages and by what mechanism α-Syn regulates EndoA levels. This is especially interesting because decreased EndoA function affects SV endocytosis (Verstreken et al., 2002; Schuske et al., 2003; Milosevic et al., 2011). However, specific mutations that block the LRRK2-dependent phosphorylation of EndoA or mutations in the flexible loop of EndoA, including a pathogenic mutation that causes Parkinsonism, prevent synaptic autophagy, while leaving SV endocytosis intact (Bademosi et al., 2023; Soukup et al., 2016; Soukup and Verstreken, 2017; Arranz et al., 2015; Matta et al., 2012). These data indicate that EndoA’s contribution to PD etiology is likely driven by its role in synaptic autophagy rather than its involvement in SV endocytosis.

In tauopathies, Tau homeostasis is disturbed, leading to abnormal posttranslational modifications, reduced microtubule affinity, and accumulation in aggregates which affect the capacity of the UPS and autophagy (Tai et al., 2012; Esteves et al., 2019; Hamano and Endo, 2022). The buildup of hyperphosphorylated Tau oligomers at presynaptic sites is linked to a dysfunctional UPS (Tai et al., 2012). Phosphorylated or pathogenic Tau fails to stabilize microtubules, inhibiting the movement and fusion of autophagosomes with lysosomes. This leads to the accumulation of Tau-containing autophagosomes and neurodegeneration (Farfel-Becker et al., 2019; Balabanian et al., 2022). Like α-Syn, accumulating evidence places Tau interference at various stages of autophagy, including autophagy initiation (Li et al., 2022), autophagosome formation (Dakkak et al., 2022, Preprint), and autophagosome–lysosome fusion (Feng et al., 2020). In vitro and in vivo studies have demonstrated that pathological Tau species induce lysosomal morphological abnormalities and dysfunction (Feng et al., 2020; Piovesana et al., 2023).

Further studies suggest that pathogenic forms of Tau, such as the P301L mutation, inhibit its degradation through all three autophagy pathways (Caballero et al., 2018). Additionally, acetylated Tau becomes resistant to CMA-mediated degradation, leading to its accumulation and propagation. Blocking Tau acetylation rescues these defects and Tau-mediated neurodegeneration, and cognitive defects are mitigated (Caballero et al., 2021).

In conclusion, both Tau and α-Syn disrupt protein degradation pathways, affecting their own turnover and that of the local proteome. This could derail presynaptic PQC and create an aggregation-prone cytosol, potentially contributing to synaptic dysfunction through various mechanisms (Nachman and Verstreken, 2022). This model also suggests that maintaining α-Syn and Tau homeostasis would be critical for proper protein turnover and synaptic function. Several studies have shown that enhancing autophagy, e.g., with drugs such as rapamycin, can reduce the aggregation of Tau and α-Syn and improve neuronal function (Silva et al., 2020; Ozcelik et al., 2013; Wang et al., 2019; Xu et al., 2022; Gao et al., 2019). Whether these treatment approaches can prevent the synaptic dysfunction related to Tau and α-Syn in humans is an exciting question for future research. However, any strategy must carefully balance protein turnover. Recent findings, including our own, have demonstrated that both elevated and impaired autophagic turnover at the presynapse are linked to neurodegeneration (Bademosi et al., 2023; Kilic et al., 2024, Preprint). More specific interventions could reduce the risks of side effects, e.g., targeting CMA, which has been shown to prevent neuronal dysfunction (Wang and Lu, 2022; Bourdenx et al., 2021). Alternatively, stimulating eMI might be an option as its activation reduced presynaptic Tau levels and rescued neurodegeneration in a Tau fly model expressing human pathogenic TauP301L (Uytterhoeven et al., 2018). Hence, further work will be needed to carefully titrate the balance of macroautophagy and the other branches of the PQC.

Current research suggests that both α-Syn and Tau could contribute to or elicit the early (and later) steps of various neurodegenerative diseases by interfering with multiple synaptic pathways that ultimately disturb SV cycling and the local PQC. However, the physiological relevance of this impairment in the human brain at the early stages of the disease is still unknown. Since the synaptic defects appear before the onset of symptoms (Pelucchi et al., 2022), it is inherently challenging to study these effects in humans. Thus, future studies should elucidate the pathological relevance of Tau and α-Syn for early synaptic decline in in vivo models tailored to these early phenotypes. Moreover, it will be essential to determine which of the affected pathways are critical for synaptic demise.

Additionally, a direct comparison between the effects of Tau and α-Syn at the presynapse within the same experimental setups and model systems is essential, but so far only a few studies have used a design that would allow for direct comparisons (Praschberger et al., 2023; Griffin et al., 2023). There is also a gap in our knowledge as not all molecular mechanisms deregulated by one of the proteins have been investigated in the context of the other, e.g., the effect of α-Syn on various steps of SV cycling has been analyzed in depth, while data on Tau in this context is scarcer. Additionally, the two proteins probably act synergistically to cause synaptic decline as co-pathologies are often observed (Hamilton, 2000; Spires-Jones et al., 2017).

Another important consideration is that the presynaptic effects of Tau and α-Syn have mainly been investigated using overexpression models, with sometimes conflicting results. In recent years, the field has increasingly turned toward knock-in models (Pech et al., 2024; Kaempf et al., 2024, Preprint; Catterson et al., 2024; Hashimoto et al., 2019; Saito et al., 2019; Caputo et al., 2020) and iPSC-derived human neurons. Although iPSCs hold inherent variability and are confounded by specific and limited genetic backgrounds, recent advancements in reproducibility have been achieved through standardized differentiation protocols, such as the Ngn2-induced neuronal differentiation and the introduction of pathogenic mutations in standardized control cell lines (like KOLF2) (Fernandopulle et al., 2018). With these improvements, numerous disease models are now being generated which will enhance comparability across studies (Pantazis et al., 2022). It will be interesting to see which previous findings will be recapitulated in these human and endogenous expression models.

Tauopathies and synucleinopathies each cause neurodegeneration in disease-specific brain areas, which reflects the selective vulnerability of certain cell types toward Tau or α-Syn toxicity, respectively (Balusu et al., 2023). Cell types that heavily rely on synaptic pathways affected by pathological α-Syn or Tau may thus be more vulnerable than others. Moreover, it is evident that the specific molecular environment in a given neuron, which is defined by both its cellular identity as well as its cell state, is an important element to consider when investigating synapse and neuronal demise mediated by Tau and α-Syn. However, such fine-grained analysis became only recently available with the advancement of single-cell technologies.

Although Tau and α-Syn have different physiological functions in neurons under native conditions, they share several pathogenic mechanisms leading to similar neurotoxic effects (Table 1). Besides their biochemical and neuropathological similarities, Tau and α-Syn converge onto several presynaptic pathways impairing synaptic function that could be at play during prodromal disease stages. While α-Syn natively resides in the presynapse (Jakes et al., 1994), Tau increasingly invades synaptic terminals under disease conditions (McInnes et al., 2018; Zhou et al., 2017), where they interrupt essential pathways for synaptic function, ultimately accounting for synaptic demise.

Particularly, SV mobility, Ca²⁺ homeostasis, and synaptic autophagy present promising targets to correct deregulated presynaptic processes. Intervening at the early stages of synaptic dysfunction could enable synaptic regeneration and prevent synapse loss and subsequent neuronal death. By targeting the shared synaptic pathways of Tau and α-Syn, a universal therapeutic approach could potentially treat both tauopathies and synucleinopathies. Starting such treatment during the earliest phases of the disease would be most beneficial before overt damage to the brain has occurred.

We used OpenAI’s GPT-4o and 3.5 tools to improve the clarity and readability of the text. Afterward, we reviewed and edited the optimized text. We take responsibility for the content.

The authors thank all members of the Verstreken laboratory for helpful discussions. The authors apologize to researchers whose work could not be discussed due to space constraints. The figures were created using https://BioRender.com.

This work was supported by Vlaams Instituut voor Biotechnologie, Fonds voor Wetenschappelijk Onderzoek (FWO) Vlaanderen, European Research Council advanced grant, the Chan Zuckerberg Initiative, a Methusalem grant of the Flemish Government, Opening the Future (Leuven University fund), Aligning Science Across Parkinson’s Collaborative Research Network, KU Leuven Parkinson Fund, SAO-FRA (Belgian Alzheimer fund), Rainwater Charitable Foundation, Cure Alzheimer Fund to P. Verstreken. E. Nachman is supported by an FWO postdoc fellowship (1282123N).

Author contributions: V. Uytterhoeven: Conceptualization, Investigation, Project administration, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing, P. Verstreken: Conceptualization, Funding acquisition, Project administration, Supervision, Writing - original draft, Writing - review & editing, E. Nachman: Conceptualization, Funding acquisition, Investigation, Project administration, Supervision, Validation, Visualization, Writing - original draft, Writing - review & editing.