Taking a Bite Out of Amyloid: Mechanistic Insights into α-Synuclein Degradation by Cathepsin L

Cathepsin L prevents the accumulation of alpha-synuclein fibrils in the cell

The deposition of α-synuclein (α-Syn) fibrils in neuronal cells has been implicated as a causative factor in Parkinson's disease (PD) and dementia with Lewy Bodies (DLB). α-Syn can be degraded by autophagy, proteasome, and chaperone-mediated autophagy, and previous studies have suggested the potency of certain cathepsins, lysosomal proteases, for α-Syn degradation. However, no studies have comprehensively evaluated all cathepsins. Here, we evaluated the efficacy of all 15 cathepsins using a cell model of α-Syn fibril propagation and found that overexpression of cathepsin L (CTSL) was the most effective in preventing the accumulation of α-Syn aggregates. CTSL-mediated degradation of α-Syn aggregates was dependent on the autophagy machinery, and CTSL itself promoted autophagy flux. Interestingly, CTSL was effective in autophagic degradation of wild-type (WT) α-Syn, but not in the case of A53T and E46K missense mutations, which are causative for familial PD. These results suggest that CTSL is a potential therapeutic strategy for sporadic PD pathology in WT α-Syn.

Cellular processing of α-synuclein fibrils results in distinct physiological C-terminal truncations with a major cleavage site at residue Glu 114

α-synuclein (αS) is an abundant, neuronal protein that assembles into fibrillar pathological inclusions in a spectrum of neurodegenerative diseases that include Lewy body diseases (LBD) and Multiple System Atrophy (MSA). The cellular and regional distributions of pathological inclusions vary widely between different synucleinopathies contributing to the spectrum of clinical presentations. Extensive cleavage within the carboxy (C)-terminal region of αS is associated with inclusion formation, although the events leading to these modifications and the implications for pathobiology are of ongoing study. αS preformed fibrils can induce prion-like spread of αS pathology in both in vitro and animal models of disease. Using C truncation-specific antibodies, we demonstrated here that prion-like cellular uptake and processing of αS preformed fibrils resulted in two major cleavages at residues 103 and 114. A third cleavage product (122 αS) accumulated upon application of lysosomal protease inhibitors. In vitro, both 1-103 and 1-114 αS polymerized rapidly and extensively in isolation and in the presence of full-length αS. 1-103 αS also demonstrated more extensive aggregation when expressed in cultured cells. Furthermore, we used novel antibodies to αS cleaved at residue Glu114, to assess x-114 αS pathology in postmortem brain tissue from patients with LBD and MSA, as well as three different transgenic αS mouse models of prion-like induction. The distribution of x-114 αS pathology was distinct from that of overall αS pathology. These studies reveal the cellular formation and behavior of αS C-truncated at residues 114 and 103 as well as the disease dependent distribution of x-114 αS pathology.

Reciprocal effects of alpha-synuclein aggregation and lysosomal homeostasis in synucleinopathy models

BACKGROUND

Lysosomal dysfunction has been implicated in a number of neurodegenerative diseases such as Parkinson's disease (PD). Various molecular, clinical and genetic studies have highlighted a central role of lysosomal pathways and proteins in the pathogenesis of PD. Within PD pathology the synaptic protein alpha-synuclein (αSyn) converts from a soluble monomer to oligomeric structures and insoluble amyloid fibrils. The aim of this study was to unravel the effect of αSyn aggregates on lysosomal turnover, particularly focusing on lysosomal homeostasis and cathepsins. Since these enzymes have been shown to be directly involved in the lysosomal degradation of αSyn, impairment of their enzymatic capacity has extensive consequences.

METHODS

We used patient-derived induced pluripotent stem cells and a transgenic mouse model of PD to examine the effect of intracellular αSyn conformers on cell homeostasis and lysosomal function in dopaminergic (DA) neurons by biochemical analyses.

RESULTS

We found impaired lysosomal trafficking of cathepsins in patient-derived DA neurons and mouse models with αSyn aggregation, resulting in reduced proteolytic activity of cathepsins in the lysosome. Using a farnesyltransferase inhibitor, which boosts hydrolase transport via activation of the SNARE protein ykt6, we enhanced the maturation and proteolytic activity of cathepsins and thereby decreased αSyn protein levels.

CONCLUSIONS

Our findings demonstrate a strong interplay between αSyn aggregation pathways and function of lysosomal cathepsins. It appears that αSyn directly interferes with the enzymatic function of cathepsins, which might lead to a vicious cycle of impaired αSyn degradation. Lysosomal trafficking of cathepsin D (CTSD), CTSL and CTSB is disrupted when alpha-synuclein (αSyn) is aggregated. This results in a decreased proteolytic activity of cathepsins, which directly mediate αSyn clearance. Boosting the transport of the cathepsins to the lysosome increases their activity and thus contributes to efficient αSyn degradation.

SARS-COV-2 infection and Parkinson's disease: Possible links and perspectives

Parkinson's disease (PD) is a neurodegenerative disorder characterized by the progressive loss of dopaminergic neurons in the substantia nigra. The hallmarks are the presence of Lewy bodies composed mainly of aggregated α-synuclein and immune activation and inflammation in the brain. The neurotropism of SARS-CoV-2 with induction of cytokine storm and neuroinflammation can contribute to the development of PD. Interestingly, overexpression of α-synuclein in PD patients may limit SARS-CoV-2 neuroinvasion and degeneration of dopaminergic neurons; however, on the other hand, this virus can speed up the α-synuclein aggregation. The review aims to discuss the potential link between COVID-19 and the risk of PD, highlighting the need for further studies to authenticate the potential association. We have also overviewed the influence of SARS-CoV-2 infection on the PD course and management. In this context, we presented the prospects for controlling the COVID-19 pandemic and related PD cases that, beyond global vaccination and novel anti-SARS-CoV-2 agents, may include the development of graphene-based nanoscale platforms offering antiviral and anti-amyloid strategies against PD.

Cathepsin L-containing exosomes from α-synuclein-activated microglia induce neurotoxicity through the P2X7 receptor

Uncontrolled microglial activation is pivotal to the pathogenesis of Parkinson's disease (PD), which can secrete Cathepsin L (CTSL) to affect the survival of neurons in the PD patients; however, the precise mechanism has yet to be determined. We demonstrated for the first time that CTSL was mostly released by exosomes derived from α-Syn-activated microglia, resulting in neuronal damage and death. The elevation of CTSL activity was blocked by GW4869, suggesting a critical role for exosomes in mediating CTSL release. Furthermore, the P2X7R/PI3K/AKT signalling pathway was identified as the underlying molecular mechanism since specific antagonists of this signalling pathway, P2X7R knockdown and exosome release inhibitors significantly reduced the injury to cultured mouse cortical neurons. Our study suggests that increased extracellular release of CTSL from α-Syn-activated microglia through exosomes amplifies and aggravates of the neurotoxic effect of microglia, implying that CTSL may be involved in a fresh mechanism of PD pathogenesis, and serve as a potential biomarker and a target for PD drug development.

The role of lysosomal cathepsins in neurodegeneration: Mechanistic insights, diagnostic potential and therapeutic approaches

Lysosomes are ubiquitous organelles with a fundamental role in maintaining cellular homeostasis by mediating degradation and recycling processes. Cathepsins are the most abundant lysosomal hydrolyses and are responsible for the bulk degradation of various substrates. A correct autophagic function is essential for neuronal survival, as most neurons are post-mitotic and thus susceptible to accumulate cellular components. Increasing evidence suggests a crucial role of the lysosome in neurodegeneration as a key regulator of aggregation-prone and disease-associated proteins, such as α-synuclein, β-amyloid and huntingtin. Particularly, alterations in lysosomal cathepsins CTSD, CTSB and CTSL can contribute to the pathogenesis of neurodegenerative diseases as seen for neuronal ceroid lipofuscinosis, synucleinopathies (Parkinson's disease, Dementia with Lewy Body and Multiple System Atrophy) as well as Alzheimer's and Huntington's disease. In this review, we provide an overview of recent evidence implicating CTSD, CTSB and CTSL in neurodegeneration, with a special focus on the role of these enzymes in α-synuclein metabolism. In addition, we summarize the potential role of lysosomal cathepsins as clinical biomarkers in neurodegenerative diseases and discuss potential therapeutic approaches by targeting lysosomal function.

The Role of Extracellular Matrix Components in the Spreading of Pathological Protein Aggregates

Several neurodegenerative diseases are characterized by the accumulation of aggregated misfolded proteins. These pathological agents have been suggested to propagate in the brain via mechanisms similar to that observed for the prion protein, where a misfolded variant is transferred from an affected brain region to a healthy one, thereby inducing the misfolding and/or aggregation of correctly folded copies. This process has been characterized for several proteins, such as α-synuclein, tau, amyloid beta (Aβ) and less extensively for huntingtin and TDP-43. α-synuclein, tau, TDP-43 and huntingtin are intracellular proteins, and their aggregates are located in the cytosol or nucleus of neurons. They have been shown to spread between cells and this event occurs, at least partially, via secretion of these protein aggregates in the extracellular space followed by re-uptake. Conversely, Aβ aggregates are found mainly extracellularly, and their spreading occurs in the extracellular space between brain regions. Due to the inherent nature of their spreading modalities, these proteins are exposed to components of the extracellular matrix (ECM), including glycans, proteases and core matrix proteins. These ECM components can interact with or process pathological misfolded proteins, potentially changing their properties and thus regulating their spreading capabilities. Here, we present an overview of the documented roles of ECM components in the spreading of pathological protein aggregates in neurodegenerative diseases with the objective of identifying the current gaps in knowledge and stimulating further research in the field. This could potentially lead to the identification of druggable targets to slow down the spreading and/or progression of these pathologies.

Comparative proteomic profiling in Chinese shrimp Fenneropenaeus chinensis under low pH stress

Lower pH gives rise to a harmful stress to crustacean. Here, we analyzed the proteomic response of Fenneropenaeus chinensis from control pH (pH value 8.2) and low pH (pH value 6.5) - treated groups by employing absolute quantitation-based quantitative proteomic (iTRAQ) analysis. Among the identified proteins, a total of 76 proteins differed in their abundance levels, including 45 upregulated and 31 downregulated proteins. The up-regulation of proteins like citrate synthase, cytochrome c oxidase, V-type proton ATPase, glyceraldehyde-3-phosphate dehydrogenase and fructose 1,6-bisphosphate-aldolase as well as the enrichment of the DEPs in multiple metabolic processes and pathways illustrated that increased energy and substrates metabolism was essential for F. chinensis to counteract low pH stress. Ion transporting related proteins, such as Na⁺/K⁺/2Cl^- cotransporter and calmodulin, participated in the homeostatic maintenance of pH in F. chinensis. There were significant downregulation expressions of lectin, lipopolysaccharide- and beta-1,3-glucan binding protein, chitinase, cathepsin L and beta-glucuronidase, which indicating the immune dysfunction of F. chinensis when exposure to low pH condition. These findings can extend our understanding on the defensive mechanisms of the low pH stress and accelerate the breeding process of low pH tolerance in F. chinensis.

Targeted proteolytic products of τ and α-synuclein in neurodegeneration

CNS pathological inclusions comprising τ or α-synuclein (αSyn) define a spectrum of neurodegenerative diseases, and these can often present concurrently in the same individuals. The aggregation of both proteins is clearly associated with neurodegeneration and the deleterious properties of each protein is further supported by mutations in each gene (MAPT and SNCA, respectively) resulting in disease. The initiating events in most sporadic neurodegenerative diseases are still unclear but growing evidence suggests that the aberrant proteolytic cleavage of τ and αSyn results in products that can be toxic and/or initiate aggregation that can further spread by a prion-like mechanism. The accumulation of some of these cleavage products can further potentiate the progression of protein aggregation transmission and lead to their accumulation in peripheral biofluids such as cerebrospinal fluid (CSF) and blood. The future development of new tools to detect specific τ and αSyn abnormal cleavage products in peripheral biofluids could be useful biomarkers and better understand of the role of unique proteolytic activities could yield therapeutic interventions.

Proteolysis of Amyloid β by Lysosomal Enzymes as a Function of Fibril Morphology

Aggregation of amyloid-β (Aβ) into extracellular plaques is a well-known hallmark of Alzheimer's disease (AD). Similarly, autophagic vacuoles, autophagosomes, and other residual bodies within dystrophic neurites, though more difficult to detect, are characteristic features of AD. To explore the potential intersection between these observations, we conducted experiments to assess whether Aβ fibril formation disrupts proteolysis by lysosomal enzymes. Fibrils constituted by either Aβ 1-40 or Aβ 1-42 were grown under both neutral and acidic pH. The extent of proteolysis by individual cathepsins (L, D, B, and H) was monitored by both thioflavin T fluorescence and liquid chromatography combined with mass spectrometry. The results show that all Aβ fibril morphologies are resistant to cathepsin digestion, with significant amounts of the undigested material remaining for samples grown in either neutral or acidic pH. Further analysis revealed that the neutral-grown fibrils are proteolytically resistant throughout the sequence, while the acid-grown fibrils prevented digestion primarily in the C-terminal portion of the sequence. Fibrils grown from Aβ 1-42 are generally more resistant to degradation compared to Aβ 1-40. Overall, the results indicate that Aβ fibrils formed in the neutral pH environments found in intracellular or extracellular spaces may pose the greatest difficulty for complete digestion by the lysosome, particularly when the fibrils are comprised of Aβ 1-42.

Current Evidence for a Bidirectional Loop Between the Lysosome and Alpha-Synuclein Proteoforms

Cumulative evidence collected in recent decades suggests that lysosomal dysfunction contributes to neurodegenerative diseases, especially if amyloid proteins are involved. Among these, alpha-synuclein (aSyn) that progressively accumulates and aggregates in Lewy bodies is undisputedly a main culprit in Parkinson disease (PD) pathogenesis. Lysosomal dysfunction is evident in brains of PD patients, and mutations in lysosomal enzymes are a major risk factor of PD. At first glance, the role of protein-degrading lysosomes in a disease with pathological protein accumulation seems obvious and should guide the development of straightforward and rational therapeutic targets. However, our review demonstrates that the story is more complicated for aSyn. The protein can possess diverse posttranslational modifications, aggregate formations, and truncations, all of which contribute to a growing known set of proteoforms. These interfere directly or indirectly with lysosome function, reducing their own degradation, and thereby accelerating the protein aggregation and disease process. Conversely, unbalanced lysosomal enzymatic processes can produce truncated aSyn proteoforms that may be more toxic and prone to aggregation. This highlights the possibility of enhancing lysosomal function as a treatment for PD, if it can be confirmed that this approach effectively reduces harmful aSyn proteoforms and does not produce novel, toxic proteoforms.

Cathepsin K is a potent disaggregase of α-synuclein fibrils

The intracellular accumulation of α-synuclein (α-syn) amyloid fibrils is a hallmark of Parkinson's disease. Because lysosomes are responsible for degrading aggregated species, enhancing lysosomal function could alleviate the overburden of α-syn. Previously, we showed that cysteine cathepsins (Cts) is the main class of lysosomal proteases that degrade α-syn, and in particular, CtsL was found to be capable of digesting α-syn fibrils. Here, we report that CtsK is a more potent protease for degrading α-syn amyloids. Using peptide mapping by liquid chromatography with mass spectrometry, critical cleavage sites involved in destabilizing fibril structure are identified. CtsK is only able to devour the internal regions after the removal of both N- and C-termini, indicating their protective role of the amyloid core from proteolytic attack. Our results suggest that if overexpressed in lysosomes, CtsK has the potential to ameliorate α-syn pathology.

The emerging role of α-synuclein truncation in aggregation and disease

α-Synuclein (αsyn) is an abundant brain neuronal protein that can misfold and polymerize to form toxic fibrils coalescing into pathologic inclusions in neurodegenerative diseases, including Parkinson's disease, Lewy body dementia, and multiple system atrophy. These fibrils may induce further αsyn misfolding and propagation of pathologic fibrils in a prion-like process. It is unclear why αsyn initially misfolds, but a growing body of literature suggests a critical role of partial proteolytic processing resulting in various truncations of the highly charged and flexible carboxyl-terminal region. This review aims to 1) summarize recent evidence that disease-specific proteolytic truncations of αsyn occur in Parkinson's disease, Lewy body dementia, and multiple system atrophy and animal disease models; 2) provide mechanistic insights on how truncation of the amino and carboxyl regions of αsyn may modulate the propensity of αsyn to pathologically misfold; 3) compare experiments evaluating the prion-like properties of truncated forms of αsyn in various models with implications for disease progression; 4) assess uniquely toxic properties imparted to αsyn upon truncation; and 5) discuss pathways through which truncated αsyn forms and therapies targeted to interrupt them. Cumulatively, it is evident that truncation of αsyn, particularly carboxyl truncation that can be augmented by dysfunctional proteostasis, dramatically potentiates the propensity of αsyn to pathologically misfold into uniquely toxic fibrils with modulated prion-like seeding activity. Therapeutic strategies and experimental paradigms should operate under the assumption that truncation of αsyn is likely occurring in both initial and progressive disease stages, and preventing truncation may be an effective preventative strategy against pathologic inclusion formation.

Glucocerebrosidase as a therapeutic target for Parkinson's disease

Introduction: The association between Gaucher disease, caused by the inherited deficiency of glucocerebrosidase, and Parkinson's disease was first recognized in the clinic, noting that patients with Gaucher disease and their carrier relatives had an increased incidence of Parkinson's disease. Currently, mutations in glucocerebrosidase (GBA1) are the most common genetic risk factor for Parkinson's disease and dementia with Lewy bodies, with an inverse relationship between glucocerebrosidase and α-synuclein, a key factor in Parkinson pathogenesis. The hypothesis that therapeutic enhancement of brain glucocerebrosidase levels might reduce the aggregation, accumulation or spread of α-synuclein has spurred great interest in glucocerebrosidase as a novel therapeutic target.Area covered: This article explores the potential molecular mechanisms underlying the association between GBA1 mutations and Parkinson's disease and outlines therapeutic strategies to increase brain glucocerebrosidase, including gene therapy, targeted delivery of recombinant glucocerebrosidase to the brain, small-molecule chaperones to rescue mutant glucocerebrosidase, and small-molecule modulators to activate wild-type glucocerebrosidase.Expert opinion: Although an improved understanding of the mechanistic basis for GBA1-associated parkinsonism is essential, enhancing levels of brain glucocerebrosidase may have wide therapeutic implications. While gene therapy may ultimately be effective, less expensive and invasive small-molecule non-inhibitory chaperones or activators could significantly impact the disease course.

Identification of a potential tissue-specific biomarker cathepsin L-like gene from the planarian Dugesia japonica: Molecular cloning, characterization, and expression in response to heavy metal exposure

Heavy metal pollution is a global health issue affecting people worldwide, and the exploration of sensitive biomarkers to assess the toxicity of heavy metals is an important work for researchers. Cathepsin L, role as a tissue-specific biomarker to assess the biological effects of environmental pollutants, has not received much attention. In this work, the full-length cDNA of cathepsin L gene from the planarian Dugesia japonica (designated DjCatL) was cloned by rapid amplification of cDNA ends (RACE) technique. The cDNA sequence of DjCatL is 1161 bp, which encodes a protein of 346 amino acids with a molecular weight of 39.03 kDa. Sequence analysis revealed that DjCatL contains highly conserved ERF/WNIN, GNFD, and GCXGG motifs, which are the features of the cathepsin L protein family. Whole-mount in situ hybridization (WISH) results revealed that the transcripts of DjCatL are specifically distributed in the intestinal system, suggesting that this gene is related to food digestion in planarians. Both quantitative polymerase chain reaction (qPCR) and WISH results revealed that the transcriptional levels of DjCatL are inhibited significantly by heavy metal (Cd²⁺, Hg²⁺, and Cu²⁺) exposure in a dose-dependent manner. Therefore, we proposed that cathepsin L can be used as a tissue-specific biomarker to assess the heavy metal pollution in the aquatic environment.

Structural Insights into α-Synuclein Fibril Polymorphism: Effects of Parkinson's Disease-Related C-Terminal Truncations

Lewy bodies, hallmarks of Parkinson's disease, contain C-terminally truncated (ΔC) α-synuclein (α-syn). Here, we report fibril structures of three N-terminally acetylated (Ac) α-syn constructs, Ac1-140, Ac1-122, and Ac1-103, solved by cryoelectron microscopy. Both ΔC-α-syn variants exhibited faster aggregation kinetics, and Ac1-103 fibrils efficiently seeded the full-length protein, highlighting their importance in pathogenesis. Interestingly, fibril helical twists increased upon the removal of C-terminal residues and can be propagated through cross-seeding. Compared to that of Ac1-140, increased electron densities were seen in the N-terminus of Ac1-103, whereas the C-terminus of Ac1-122 appeared more structured. In accord, the respective termini of ΔC-α-syn exhibited increased protease resistance. Despite similar amyloid core residues, distinctive features were seen for both Ac1-122 and Ac1-103. Particularly, Ac1-103 has the tightest packed core with an additional turn, likely attributable to conformational changes in the N-terminal region. These molecular differences offer insights into the effect of C-terminal truncations on α-syn fibril polymorphism.

C-terminal α-synuclein truncations are linked to cysteine cathepsin activity in Parkinson's disease

A pathological feature of Parkinson's disease (PD) is Lewy bodies (LBs) composed of α-synuclein (α-syn) amyloid fibrils. α-Syn is a 140 amino acids-long protein, but truncated α-syn is enriched in LBs. The proteolytic processes that generate these truncations are not well-understood. On the basis of our previous work, we propose that these truncations could originate from lysosomal activity attributable to cysteine cathepsins (Cts). Here, using a transgenic SNCA ^A53T mouse model, overexpressing the PD-associated α-syn variant A53T, we compared levels of α-syn species in purified brain lysosomes from nonsymptomatic mice with those in age-matched symptomatic mice. In the symptomatic mice, antibody epitope mapping revealed enrichment of C-terminal truncations, resulting from CtsB, CtsL, and asparagine endopeptidase. We did not observe changes in individual cathepsin activities, suggesting that the increased levels of C-terminal α-syn truncations are because of the burden of aggregated α-syn. Using LC-MS and purified α-syn, we identified C-terminal truncations corresponding to amino acids 1-122 and 1-90 from the SNCA ^A53T lysosomes. Feeding rat dopaminergic N27 cells with exogenous α-syn fibrils confirmed that these fragments originate from incomplete fibril degradation in lysosomes. We mimicked these events in situ by asparagine endopeptidase degradation of α-syn fibrils. Importantly, the resulting C-terminally truncated fibrils acted as superior seeds in stimulating α-syn aggregation compared with that of the full-length fibrils. These results unequivocally show that C-terminal α-syn truncations in LBs are linked to Cts activities, promote amyloid formation, and contribute to PD pathogenesis.

Interplay between α-synuclein amyloid formation and membrane structure

Amyloid formation is a pathological hallmark of many neurodegenerative diseases, including Alzheimer's, Parkinson's, and Huntington's. While it is unknown how these disorders are initiated, in vitro and cellular experiments confirm the importance of membranes. Ubiquitous in vivo, membranes induce conformational changes in amyloidogenic proteins and in some cases, facilitate aggregation. Reciprocally, perturbations in the bilayer structure can be induced by amyloid formation. Here, we review studies in the last 10 years describing α-synuclein (α-syn) and its interactions with membranes, detailing the roles of anionic and zwitterionic lipids in aggregation, and their contribution to Parkinson's disease. We summarize the impact of α-syn - comparing monomeric, oligomeric, and fibrillar forms - on membrane structure, and the effect of membrane remodeling on amyloid formation. Finally, perspective on future studies investigating the interplay between α-syn aggregation and membranes is discussed. This article is part of a Special Issue entitled: Amyloids.

pH-Dependent fibril maturation of a Pmel17 repeat domain isoform revealed by tryptophan fluorescence

The pre-melanosomal protein (Pmel17) aggregates within melanosomes to form functional amyloid fibrils that facilitate melanin polymerization. The repeat domain (RPT) of Pmel17 fibrillates under strict acidic melanosomal pH. Alternative splicing results in a shortened repeat domain (sRPT), which also forms amyloid fibrils. Here, we explored the effects of pH and protein concentration on sRPT aggregation by monitoring the intrinsic fluorescence of the sole tryptophan at position 381 (³⁸¹W). ³⁸¹W emission properties revealed changes of local environment polarity for sRPT fibrils formed at different pH. At pH 4, fibrils formed rapidly with no lag phase. A high ³⁸¹W intensity was observed with a slight blue shift (10 nm). These fibrils underwent further structural rearrangements at intermediate pH (5-6), mirroring that of melanosome maturation, which initiates at pH 4 and increases to near neutral pH. In contrast, typical sigmoidal kinetics were observed at pH 6 with slower rates and ³⁸¹W exhibited quenched emission. Interestingly, biphasic kinetics were observed at pH 5 in a protein concentration-dependent manner. A large ³⁸¹W blue shift (23 nm) was measured, indicating a more hydrophobic environment for fibrils made at pH 5. Consistent with ³⁸¹W fluorescence, Raman spectroscopy revealed molecular level perturbations in sRPT fibrils that were not evident from circular dichroism, transmission electron microscopy, or limited proteolysis analysis. Finally, sRPT fibrils did not form at pH ≥7 and preformed fibrils rapidly disaggregated under these solution conditions. Collectively, this work yields mechanistic insights into pH-dependent sRPT aggregation in the context of melanosome maturation.

Effects of phosphatidylcholine membrane fluidity on the conformation and aggregation of N-terminally acetylated α-synuclein

Membrane association of α-synuclein (α-syn), a neuronal protein associated with Parkinson's disease (PD), is involved in α-syn function and pathology. Most previous studies on α-syn-membrane interactions have not used the physiologically relevant N-terminally acetylated (N-acetyl) α-syn form nor the most naturally abundant cellular lipid, i.e. phosphatidylcholine (PC). Here, we report on how PC membrane fluidity affects the conformation and aggregation propensity of N-acetyl α-syn. It is well established that upon membrane binding, α-syn adopts an α-helical structure. Using CD spectroscopy, we show that N-acetyl α-syn transitions from α-helical to disordered at the lipid melting temperature (T_m ). We found that this fluidity sensing is a robust characteristic, unaffected by acyl chain length (T_m = 34-55 °C) and preserved in its homologs β- and γ-syn. Interestingly, both N-acetyl α-syn membrane binding and amyloid formation trended with lipid order (1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) > 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC)/sphingomyelin/cholesterol (2:2:1) ≥ DOPC), with gel-phase vesicles shortening aggregation kinetics and promoting fibril formation compared to fluid membranes. Furthermore, we found that acetylation enhances binding to PC micelles and small unilamellar vesicles with high curvature (r ∼16-20 nm) and that DPPC binding is reduced in the presence of cholesterol. These results confirmed that the exposure of hydrocarbon chains (i.e. packing defects) is essential for binding to zwitterionic gel membranes. Collectively, our in vitro results suggest that N-acetyl α-syn localizes to highly curved, ordered membranes inside a cell. We propose that age-related changes in membrane fluidity can promote the formation of amyloid fibrils, insoluble materials associated with PD.

Stimulation of α-synuclein amyloid formation by phosphatidylglycerol micellar tubules

α-Synuclein (α-Syn) is a presynaptic protein that is accumulated in its amyloid form in the brains of Parkinson's patients. Although its biological function remains unclear, α-syn has been suggested to bind to synaptic vesicles and facilitate neurotransmitter release. Recently, studies have found that α-syn induces membrane tubulation, highlighting a potential mechanism for α-syn to stabilize highly curved membrane structures which could have both functional and dysfunctional consequences. To understand how membrane remodeling by α-syn affects amyloid formation, we have studied the α-syn aggregation process in the presence of phosphatidylglycerol (PG) micellar tubules, which were the first reported example of membrane tubulation by α-syn. Aggregation kinetics, β-sheet content, and macroscopic protein-lipid structures were observed by Thioflavin T fluorescence, circular dichroism spectroscopy and transmission electron microscopy, respectively. Collectively, the presence of PG micellar tubules formed at a stochiometric (L/P = 1) ratio was found to stimulate α-syn fibril formation. Moreover, transmission electron microscopy and solid-state nuclear magnetic resonance spectroscopy revealed the co-assembly of PG and α-syn into fibril structures. However, isolated micellar tubules do not form fibrils by themselves, suggesting an important role of free α-syn monomers during amyloid formation. In contrast, fibrils did not form in the presence of excess PG lipids (≥L/P = 50), where most of the α-syn molecules are in a membrane-bound α-helical form. Our results provide new mechanistic insights into how membrane tubules modulate α-syn amyloid formation and support a pivotal role of protein-lipid interaction in the dysfunction of α-syn.

Structural features of α-synuclein amyloid fibrils revealed by Raman spectroscopy

Parkinson's disease (PD) is associated with the formation of α-synuclein amyloid fibrils. Elucidating the role of these β-sheet-rich fibrils in disease progression is crucial; however, collecting detailed structural information on amyloids is inherently difficult because of their insoluble, non-crystalline, and polymorphic nature. Here, we show that Raman spectroscopy is a facile technique for characterizing structural features of α-synuclein fibrils. Combining Raman spectroscopy with aggregation kinetics and transmission electron microscopy, we examined the effects of pH and ionic strength as well as four PD-related mutations (A30P, E46K, G51D, and A53T) on α-synuclein fibrils. Raman spectral differences were observed in the amide-I, amide-III, and fingerprint regions, indicating that secondary structure and tertiary contacts are influenced by pH and to a lesser extent by NaCl. Faster aggregation times appear to facilitate unique fibril structure as determined by the highly reproducible amide-I band widths, linking aggregation propensity and fibril polymorphism. Importantly, Raman spectroscopy revealed molecular-level perturbations of fibril conformation by the PD-related mutations that are not apparent through transmission electron microscopy or limited proteolysis. The amide-III band was found to be particularly sensitive, with G51D exhibiting the most distinctive features, followed by A53T and E46K. Relating to a cellular environment, our data would suggest that fibril polymorphs can be formed in different cellular compartments and potentially result in distinct phenotypes. Our work sets a foundation toward future cellular Raman studies of amyloids.

Fluorescence spectroscopy reveals N-terminal order in fibrillar forms of α-synuclein

The neuronal protein α-synuclein (αS) plays a key role in Parkinson's disease, forming inclusions termed Lewy bodies and Lewy neurites. Recent improvements in cryo-electron diffraction and solid state NMR (ssNMR) have led to the elucidation of the structures of peptides derived from the αS fibril core and full-length human αS in fibrils. Despite the valuable insight offered by these methods, there are still several questions about the structures' relevance to pathological aggregates. Herein, we present fluorescence data collected in vitro under the conditions which fibrils are typically assembled. Our data suggest that, in solution, fibrils are largely structured as observed by ssNMR. However, we observe significant disparities in the αS N-terminus as compared to ssNMR data, which provide insight on its important role in αS aggregation and fibril structure.