Upregulation of Cellular Palmitoylation Mitigates α-Synuclein Accumulation and Neurotoxicity

Therapeutic Implications and Regulations of Protein Post-translational Modifications in Parkinsons Disease

Parkinsons disease (PD) is a neurodegenerative disorder characterized by dopaminergic neuron loss and alpha-synuclein aggregation. This comprehensive review examines the intricate role of post-translational modifications (PTMs) in PD pathogenesis, focusing on DNA methylation, histone modifications, phosphorylation, SUMOylation, and ubiquitination. Targeted PTM modulation, particularly in key proteins like Parkin, DJ1, and PINK1, emerges as a promising therapeutic strategy for mitigating dopaminergic degeneration in PD. Dysregulated PTMs significantly contribute to the accumulation of toxic protein aggregates and dopaminergic neuronal dysfunction observed in PD. Targeting PTMs, including epigenetic strategies, addressing aberrant phosphorylation events, and modulating SUMOylation processes, provides potential avenues for intervention. The ubiquitin-proteasome system, governed by enzymes like Parkin and Nedd4, offers potential targets for clearing misfolded proteins and developing disease-modifying interventions. Compounds like ginkgolic acid, SUMO E1 enzyme inhibitors, and natural compounds like Indole-3-carbinol illustrate the feasibility of modulating PTMs for therapeutic purposes in PD. This review underscores the therapeutic potential of PTM-targeted interventions in modulating PD-related pathways, emphasizing the need for further research in this promising area of Parkinsons disease therapeutics.

Let's get fat: emergence of S-acylation as a therapeutic target in Huntington disease

Protein mislocalization is a key initial step in neurodegeneration, regardless of etiology, and has been linked to changes in the dynamic addition of saturated fatty acids to proteins, a process known as S-acylation. With the advent of new techniques to study S-acylation and the recent discovery of new enzymes that facilitate protein deacylation, novel small molecules are emerging as potential new therapeutic treatments. Huntington disease (HD) is a devastating, fatal neurodegenerative disease characterized by motor, cognitive, and psychiatric deficits caused by a CAG repeat expansion in the HTT gene. The protein that is mutated in HD, huntingtin, is less S-acylated which is associated with mutant HTT aggregation and cytotoxicity. Recent exciting findings indicate that restoring S-acylation in HD models using small molecule inhibitors of the deacylation enzymes is protective. Herein, we set out to describe the known roles of S-acylation in HD and how it can be targeted for therapeutic design.

A stem cell-based assay platform demonstrates alpha-synuclein dependent synaptic dysfunction in patient-derived cortical neurons

Alpha-synuclein (αS)-rich Lewy bodies and neurites in the cerebral cortex correlate with the presence of dementia in Parkinson disease (PD) and Dementia with Lewy bodies (DLB), but whether αS influences synaptic vesicle dynamics in human cortical neurons is unknown. Using a new iPSC-based assay platform for measuring synaptic vesicle cycling, we found that in human cortical glutamatergic neurons, increased αS from either transgenic expression or triplication of the endogenous locus in patient-derived neurons reduced synaptic vesicle cycling under both stimulated and spontaneous conditions. Thus, using a robust, easily adopted assay platform, we show for the first time αS-induced synaptic dysfunction in human cortical neurons, a key cellular substrate for PD dementia and DLB.

Protein lipidation in health and disease: molecular basis, physiological function and pathological implication

Posttranslational modifications increase the complexity and functional diversity of proteins in response to complex external stimuli and internal changes. Among these, protein lipidations which refer to lipid attachment to proteins are prominent, which primarily encompassing five types including S-palmitoylation, N-myristoylation, S-prenylation, glycosylphosphatidylinositol (GPI) anchor and cholesterylation. Lipid attachment to proteins plays an essential role in the regulation of protein trafficking, localisation, stability, conformation, interactions and signal transduction by enhancing hydrophobicity. Accumulating evidence from genetic, structural, and biomedical studies has consistently shown that protein lipidation is pivotal in the regulation of broad physiological functions and is inextricably linked to a variety of diseases. Decades of dedicated research have driven the development of a wide range of drugs targeting protein lipidation, and several agents have been developed and tested in preclinical and clinical studies, some of which, such as asciminib and lonafarnib are FDA-approved for therapeutic use, indicating that targeting protein lipidations represents a promising therapeutic strategy. Here, we comprehensively review the known regulatory enzymes and catalytic mechanisms of various protein lipidation types, outline the impact of protein lipidations on physiology and disease, and highlight potential therapeutic targets and clinical research progress, aiming to provide a comprehensive reference for future protein lipidation research.

Increased palmitoylation improves estrogen receptor alpha-dependent hippocampal synaptic deficits in a mouse model of synucleinopathy

Parkinson's disease (PD) is characterized by conversion of soluble α-synuclein (αS) into intraneuronal aggregates and degeneration of neurons and neuronal processes. Indications that women with early-stage PD display milder neurodegenerative features suggest that female sex partially protects against αS pathology. We previously reported that female sex and estradiol improved αS homeostasis and PD-like phenotypes in E46K-amplified (3K) αS mice. Here, we aimed to further dissect mechanisms that drive this sex dimorphism early in disease. We observed that synaptic abnormalities were delayed in females and improved by estradiol, mediated by local estrogen receptor alpha (ERα). Aberrant ERα distribution in 3K compared to wild-type mice was paired with its decreased palmitoylation. Treatment with ML348, a de-palmitoylation inhibitor, increased ERα availability and soluble αS homeostasis, ameliorating synaptic plasticity and cognitive and motor phenotypes. Our finding that sex differences in early-disease αS-induced synaptic impairment in 3KL mice are in part mediated by palmitoylated ERα may have functional and pathogenic implications for clinical PD.

α-synuclein promotes neuronal dysfunction and death by disrupting the binding of ankyrin to ß-spectrin

α-synuclein plays a key role in the pathogenesis of Parkinson's disease and related disorders, but critical interacting partners and molecular mechanisms mediating neurotoxicity are incompletely understood. We show that α-synuclein binds directly to ß-spectrin. Using males and females in a Drosophila model of α-synuclein-related disorders we demonstrate that ß-spectrin is critical for α-synuclein neurotoxicity. Further, the ankyrin binding domain of ß-spectrin is required for α-synuclein binding and neurotoxicity. A key plasma membrane target of ankyrin, Na+/K+ ATPase, is mislocalized when human α-synuclein is expressed in Drosophila. Accordingly, membrane potential is depolarized in α-synuclein transgenic fly brains. We examine the same pathway in human neurons and find that Parkinson's disease patient-derived neurons with a triplication of the α-synuclein locus show disruption of the spectrin cytoskeleton, mislocalization of ankyrin and Na+/K+ ATPase, and membrane potential depolarization. Our findings define a specific molecular mechanism by which elevated levels of α-synuclein in Parkinson's disease and related α-synucleinopathies leads to neuronal dysfunction and death.

α-Synuclein Promotes Neuronal Dysfunction and Death by Disrupting the Binding of Ankyrin to β-Spectrin

α-Synuclein plays a key role in the pathogenesis of Parkinson's disease and related disorders, but critical interacting partners and molecular mechanisms mediating neurotoxicity are incompletely understood. We show that α-synuclein binds directly to β-spectrin. Using males and females in a Drosophila model of α-synuclein-related disorders, we demonstrate that β-spectrin is critical for α-synuclein neurotoxicity. Further, the ankyrin binding domain of β-spectrin is required for α-synuclein binding and neurotoxicity. A key plasma membrane target of ankyrin, Na+/K+ ATPase, is mislocalized when human α-synuclein is expressed in Drosophila Accordingly, membrane potential is depolarized in α-synuclein transgenic fly brains. We examine the same pathway in human neurons and find that Parkinson's disease patient-derived neurons with a triplication of the α-synuclein locus show disruption of the spectrin cytoskeleton, mislocalization of ankyrin and Na+/K+ ATPase, and membrane potential depolarization. Our findings define a specific molecular mechanism by which elevated levels of α-synuclein in Parkinson's disease and related α-synucleinopathies lead to neuronal dysfunction and death.SIGNIFICANCE STATEMENT The small synaptic vesicle associate protein α-synuclein plays a critical role in the pathogenesis of Parkinson's disease and related disorders, but the disease-relevant binding partners of α-synuclein and proximate pathways critical for neurotoxicity require further definition. We show that α-synuclein binds directly to β-spectrin, a key cytoskeletal protein required for localization of plasma membrane proteins and maintenance of neuronal viability. Binding of α-synuclein to β-spectrin alters the organization of the spectrin-ankyrin complex, which is critical for localization and function of integral membrane proteins, including Na+/K+ ATPase. These finding outline a previously undescribed mechanism of α-synuclein neurotoxicity and thus suggest potential new therapeutic approaches in Parkinson's disease and related disorders.

Palmitoylation of the Parkinson's disease-associated protein synaptotagmin-11 links its turnover to α-synuclein homeostasis

Synaptotagmin-11 (Syt11) is a vesicle-trafficking protein that is linked genetically to Parkinson's disease (PD). Likewise, the protein α-synuclein regulates vesicle trafficking, and its abnormal aggregation in neurons is the defining cytopathology of PD. Because of their functional similarities in the same disease context, we investigated whether the two proteins were connected. We found that Syt11 was palmitoylated in mouse and human brain tissue and in cultured cortical neurons and that this modification to Syt11 disrupted α-synuclein homeostasis in neurons. Palmitoylation of two cysteines adjacent to the transmembrane domain, Cys39 and Cys40, localized Syt11 to digitonin-insoluble portions of intracellular membranes and protected it from degradation by the endolysosomal system. In neurons, palmitoylation of Syt11 increased its abundance and enhanced the binding of α-synuclein to intracellular membranes. As a result, the abundance of the physiologic tetrameric form of α-synuclein was decreased, and that of its aggregation-prone monomeric form was increased. These effects were replicated by overexpression of wild-type Syt11 but not a palmitoylation-deficient mutant. These findings suggest that palmitoylation-mediated increases in Syt11 amounts may promote pathological α-synuclein aggregation in PD.

Unconventional secretion of α-synuclein mediated by palmitoylated DNAJC5 oligomers

Alpha-synuclein (α-syn), a major component of Lewy bodies found in Parkinson's disease (PD) patients, has been found exported outside of cells and may mediate its toxicity via cell-to-cell transmission. Here, we reconstituted soluble, monomeric α-syn secretion by the expression of DnaJ homolog subfamily C member 5 (DNAJC5) in HEK293T cells. DNAJC5 undergoes palmitoylation and anchors on the membrane. Palmitoylation is essential for DNAJC5-induced α-syn secretion, and the secretion is not limited by substrate size or unfolding. Cytosolic α-syn is actively translocated and sequestered in an endosomal membrane compartment in a DNAJC5-dependent manner. Reduction of α-syn secretion caused by a palmitoylation-deficient mutation in DNAJC5 can be reversed by a membrane-targeting peptide fusion-induced oligomerization of DNAJC5. The secretion of endogenous α-syn mediated by DNAJC5 is also found in a human neuroblastoma cell line, SH-SY5Y, differentiated into neurons in the presence of retinoic acid, and in human-induced pluripotent stem cell-derived midbrain dopamine neurons. We propose that DNAJC5 forms a palmitoylated oligomer to accommodate and export α-syn.

Altered Cortical Palmitoylation Induces Widespread Molecular Disturbances in Parkinson's Disease

The relationship between Parkinson's disease (PD), the second-most common neurodegenerative disease after Alzheimer's disease, and palmitoylation, a post-translational lipid modification, is not well understood. In this study, to better understand the role of protein palmitoylation in PD and the pathways altered in this disease, we analyzed the differential palmitoyl proteome (palmitome) in the cerebral cortex of PD patients compared to controls (n = 4 per group). Data-mining of the cortical palmitome from PD patients and controls allowed us to: (i) detect a set of 150 proteins with altered palmitoylation in PD subjects in comparison with controls; (ii) describe the biological pathways and targets predicted to be altered by these palmitoylation changes; and (iii) depict the overlap between the differential palmitome identified in our study with protein interactomes of the PD-linked proteins α-synuclein, LRRK2, DJ-1, PINK1, GBA and UCHL1. In summary, we partially characterized the altered palmitome in the cortex of PD patients, which is predicted to impact cytoskeleton, mitochondrial and fibrinogen functions, as well as cell survival. Our study suggests that protein palmitoylation could have a role in the pathophysiology of PD, and that comprehensive palmitoyl-proteomics offers a powerful approach for elucidating novel cellular pathways modulated in this neurodegenerative disease.

Induced pluripotent stem cells: a tool for modeling Parkinson's disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative disorder. Among its pathologies, progressive loss of dopaminergic (DA) neurons in the substantia nigra is characteristic and contributes to many of the most severe symptoms of PD. Recent advances in induced pluripotent stem cell (iPSC) technology have made it possible to generate patient-derived DA neuronal cell culture and organoid models of PD. These models have contributed to understanding disease mechanisms and the identification of novel targets and therapeutic candidates. Still needed are better ways to model the age-related aspects of PD, as well as a deeper understanding of the interactions among disease-modifying genes and between genetic and environmental contributions to the etiology and progression of PD.

Deciphering therapeutic options for neurodegenerative diseases: insights from SIRT1

Silent information regulator 1 (SIRT1) is a nicotinamide adenine dinucleotide (NAD +)-dependent protein deacetylase that exerts biological effects through nucleoplasmic transfer. Recent studies have highlighted that SIRT1 deacetylates protein substrates to exert its neuroprotective effects, including decreased oxidative stress and inflammatory, increases autophagy, increases levels of nerve growth factors (correlated with behavioral changes), and maintains neural integrity (affects neuronal development and function) in aging or neurological disorder. In this review, we highlight the molecular mechanisms underlying the protective role of SIRT1 in modulating neurodegeneration, focusing on protein homeostasis, aging-related signaling pathways, neurogenesis, and synaptic plasticity. Meanwhile, the potential of targeting SIRT1 to block the occurrence and progression of neurodegenerative diseases is also discussed. Taken together, this review provides an up-to-date evaluation of our current understanding of the neuroprotective mechanisms of SIRT1 and also be involved in the potential therapeutic opportunities of AD and related neurodegenerative diseases.

Non-clinical Pharmacology of YTX-7739: a Clinical Stage Stearoyl-CoA Desaturase Inhibitor Being Developed for Parkinson's Disease

Stearoyl-CoA desaturase (SCD) is a potential therapeutic target for Parkinson’s and related neurodegenerative diseases. SCD inhibition ameliorates neuronal toxicity caused by aberrant α-synuclein, a lipid-binding protein implicated in Parkinson’s disease. Its inhibition depletes monounsaturated fatty acids, which may modulate α-synuclein conformations and membrane interactions. Herein, we characterize the pharmacokinetic and pharmacodynamic properties of YTX-7739, a clinical-stage SCD inhibitor. Administration of YTX-7739 to rats and monkeys for 15 days caused a dose-dependent increase in YTX-7739 concentrations that were well-tolerated and associated with concentration-dependent reductions in the fatty acid desaturation index (FADI), the ratio of monounsaturated to saturated fatty acids. An approximate 50% maximal reduction in the carbon-16 desaturation index was observed in the brain, with comparable responses in the plasma and skin. A study with a diet supplemented in SCD products indicates that changes in brain C16 desaturation were due to local SCD inhibition, rather than to changes in systemic fatty acids that reach the brain. Assessment of pharmacodynamic response onset and reversibility kinetics indicated that approximately 7 days of dosing were required to achieve maximal responses, which persisted for at least 2 days after cessation of dosing. YTX-7739 thus achieved sufficient concentrations in the brain to inhibit SCD and produce pharmacodynamic responses that were well-tolerated in rats and monkeys. These results provide a framework for evaluating YTX-7739 pharmacology clinically as a disease-modifying therapy to treat synucleinopathies.

A sticky situation: regulation and function of protein palmitoylation with a spotlight on the axon and axon initial segment

In neurons, the axon and axon initial segment (AIS) are critical structures for action potential initiation and propagation. Their formation and function rely on tight compartmentalisation, a process where specific proteins are trafficked to and retained at distinct subcellular locations. One mechanism which regulates protein trafficking and association with lipid membranes is the modification of protein cysteine residues with the 16-carbon palmitic acid, known as S-acylation or palmitoylation. Palmitoylation, akin to phosphorylation, is reversible, with palmitate cycling being mediated by substrate-specific enzymes. Palmitoylation is well-known to be highly prevalent among neuronal proteins and is well studied in the context of the synapse. Comparatively, how palmitoylation regulates trafficking and clustering of axonal and AIS proteins remains less understood. This review provides an overview of the current understanding of the biochemical regulation of palmitoylation, its involvement in various neurological diseases, and the most up-to-date perspective on axonal palmitoylation. Through a palmitoylation analysis of the AIS proteome, we also report that an overwhelming proportion of AIS proteins are likely palmitoylated. Overall, our review and analysis confirm a central role for palmitoylation in the formation and function of the axon and AIS and provide a resource for further exploration of palmitoylation-dependent protein targeting to and function at the AIS.

Lipid-induced S-palmitoylation as a Vital Regulator of Cell Signaling and Disease Development

Lipid metabolites are emerging as pivotal regulators of protein function and cell signaling. The availability of intracellular fatty acid is tightly regulated by glycolipid metabolism and may affect human body through many biological mechanisms. Recent studies have demonstrated palmitate, either from exogenous fatty acid uptake or de novo fatty acid synthesis, may serve as the substrate for protein palmitoylation and regulate protein function via palmitoylation. Palmitoylation, the most-studied protein lipidation, encompasses the reversible covalent attachment of palmitate moieties to protein cysteine residues. It controls various cellular physiological processes and alters protein stability, conformation, localization, membrane association and interaction with other effectors. Dysregulation of palmitoylation has been implicated in a plethora of diseases, such as metabolic syndrome, cancers, neurological disorders and infections. Accordingly, it could be one of the molecular mechanisms underlying the impact of palmitate metabolite on cellular homeostasis and human diseases. Herein, we explore the relationship between lipid metabolites and the regulation of protein function through palmitoylation. We review the current progress made on the putative role of palmitate in altering the palmitoylation of key proteins and thus contributing to the pathogenesis of various diseases, among which we focus on metabolic disorders, cancers, inflammation and infections, neurodegenerative diseases. We also highlight the opportunities and new therapeutics to target palmitoylation in disease development.

Parkinson's disease risk genes act in glia to control neuronal α-synuclein toxicity

Idiopathic Parkinson's disease is the second most common neurodegenerative disease and is estimated to be approximately 30% heritable. Genome wide association studies have revealed numerous loci associated with risk of development of Parkinson's disease. The majority of genes identified in these studies are expressed in glia at either similar or greater levels than their expression in neurons, suggesting that glia may play a role in Parkinson's disease pathogenesis. The role of individual glial risk genes in Parkinson's disease development or progression is unknown, however. We hypothesized that some Parkinson's disease risk genes exert their effects through glia. We developed a Drosophila model of α-synucleinopathy in which we can independently manipulate gene expression in neurons and glia. Human wild type α-synuclein is expressed in all neurons, and these flies develop the hallmarks of Parkinson's disease, including motor impairment, death of dopaminergic and other neurons, and α-synuclein aggregation. In these flies, we performed a candidate genetic screen, using RNAi to knockdown 14 well-validated Parkinson's disease risk genes in glia and measuring the effect on locomotion in order to identify glial modifiers of the α-synuclein phenotype. We identified 4 modifiers: aux, Lrrk, Ric, and Vps13, orthologs of the human genes GAK, LRRK2, RIT2, and VPS13C, respectively. Knockdown of each gene exacerbated neurodegeneration as measured by total and dopaminergic neuron loss. Knockdown of each modifier also increased α-synuclein oligomerization. These results suggest that some Parkinson's disease risk genes exert their effects in glia and that glia can influence neuronal α-synuclein proteostasis in a non-cell-autonomous fashion. Further, this study provides proof of concept that our novel Drosophila α-synucleinopathy model can be used to study glial modifier genes, paving the way for future large unbiased screens to identify novel glial risk factors that contribute to PD risk and progression.

Modulation of the Interactions Between α-Synuclein and Lipid Membranes by Post-translational Modifications

Parkinson's disease is characterised by the presence in brain tissue of aberrant inclusions known as Lewy bodies and Lewy neurites, which are deposits composed by α-synuclein and a variety of other cellular components, including in particular lipid membranes. The dysregulation of the balance between lipid homeostasis and α-synuclein homeostasis is therefore likely to be closely involved in the onset and progression of Parkinson's disease and related synucleinopathies. As our understanding of this balance is increasing, we describe recent advances in the characterisation of the role of post-translational modifications in modulating the interactions of α-synuclein with lipid membranes. We then discuss the impact of these advances on the development of novel diagnostic and therapeutic tools for synucleinopathies.

Triple-Knockout, Synuclein-Free Mice Display Compromised Lipid Pattern

Recent studies have implicated synucleins in several reactions during the biosynthesis of lipids and fatty acids in addition to their recognised role in membrane lipid binding and synaptic functions. These are among aspects of decreased synuclein functions that are still poorly acknowledged especially in regard to pathogenesis in Parkinson’s disease. Here, we aimed to add to existing knowledge of synuclein deficiency (i.e., the lack of all three family members), with respect to changes in fatty acids and lipids in plasma, liver, and two brain regions in triple synuclein-knockout (TKO) mice. We describe changes of long-chain polyunsaturated fatty acids (LCPUFA) and palmitic acid in liver and plasma, reduced triacylglycerol (TAG) accumulation in liver and non-esterified fatty acids in plasma of synuclein free mice. In midbrain, we observed counterbalanced changes in the relative concentrations of phosphatidylcholine (PC) and cerebrosides (CER). We also recorded a notable reduction in ethanolamine plasmalogens in the midbrain of synuclein free mice, which is an important finding since the abnormal ether lipid metabolism usually associated with neurological disorders. In summary, our data demonstrates that synuclein deficiency results in alterations of the PUFA synthesis, storage lipid accumulation in the liver, and the reduction of plasmalogens and CER, those polar lipids which are principal compounds of lipid rafts in many tissues. An ablation of all three synuclein family members causes more profound changes in lipid metabolism than changes previously shown to be associated with γ-synuclein deficiency alone. Possible mechanisms by which synuclein deficiency may govern the reported modifications of lipid metabolism in TKO mice are proposed and discussed.

Increasing brain palmitoylation rescues behavior and neuropathology in Huntington disease mice

Huntington disease (HD) damages the corticostriatal circuitry in large part by impairing transport of brain-derived neurotrophic factor (BDNF). We hypothesized that improving vesicular transport of BDNF could slow or prevent disease progression. We therefore performed selective proteomic analysis of vesicles transported within corticostriatal projecting neurons followed by in silico screening and identified palmitoylation as a pathway that could restore defective huntingtin-dependent trafficking. Using a synchronized trafficking assay and an HD network-on-a-chip, we found that increasing brain palmitoylation via ML348, which inhibits the palmitate-removing enzyme acyl-protein thioesterase 1 (APT1), restores axonal transport, synapse homeostasis, and survival signaling to wild-type levels without toxicity. In human HD induced pluripotent stem cell-derived cortical neurons, ML348 increased BDNF trafficking. In HD knock-in mice, it efficiently crossed the blood-brain barrier to restore palmitoylation levels and reverse neuropathology, locomotor deficits, and anxio-depressive behaviors. APT1 and its inhibitor ML348 thus hold therapeutic interest for HD.