Role of post-translational modifications on the alpha-synuclein aggregation-related pathogenesis of Parkinson’s disease

Tailored pharmacotherapy monitoring in Parkinson's disease and Schizophrenia using a rapid and sensitive α-Synuclein assay

BACKGROUND

While Parkinson's disease is a low dopamine neurodegenerative disorder, Schizophrenia is considered a high dopamine psychiatric disorder. Pharmacological interventions that are directed to normalize dopamine concentrations in the mid-brain for an extended duration lead to unintended consequences. Parkinson's disease patients experience psychosis, and Schizophrenia patients develop extra-pyramidal symptoms due to dopamine levels overshooting their physiological range. An objective monitoring technique is therefore required for better therapeutic efficacy in these two neurological diseases.

METHODS

A rapid and sensitive assay for α-Synuclein based on magnetic enrichment and enzymatic fluorescent signal generation was developed. This assay has been benchmarked with conventional ELISA and validated in 53 CSF and 36 serum samples.

RESULTS

Developed assay from an experimental perspective has a sensitivity of less than 10 pg/mL; requires a turnaround time of 45 mins; and uses 2 µL of CSF/serum fluid samples to quantify alpha synuclein. From a utility perspective, the assay showed (a) a two-fold linearity across clinical phenotypes of Parkinson's disease, neurological controls, and schizophrenia patients; (b) variation between the naïve and treated patients; (c) correlation with severity of the disease. From a diagnostic perspective, the serum-based assay had a 100 % specificity and a minimum of 67 % sensitivity in differentiating naïve patients from treated patients; the CSF/serum-based assays had a minimum of 91 % specificity and a minimum of 85 % sensitivity in differentiating patients from neurological controls.

CONCLUSIONS

The developed assay can be used to quantify alpha-synuclein in serum and CSF samples, thereby setting a translational platform for diagnosis, prognosis, and monitoring pharmacotherapy patients with Parkinson's disease and Schizophrenia.

Plasma Metabolites as Mediators Between Gut Microbiota and Parkinson's Disease: Insights from Mendelian Randomization

Recent evidence supports the causal role of both plasma metabolites and gut microbiota (GM) in Parkinson's disease (PD). However, it remains unclear whether GM are responsible for causing PD through plasma metabolites. Here, we used Mendelian randomization (MR) to investigate the intrinsic causal relationships among GM, plasma metabolites, and PD. Summary statistics were derived from a GWAS of 1400 metabolites (N = 8299), GM (N = 18,340), and PD (Ncase = 33,674 and Ncontrol = 449,056). We used two-step/mediation MR (TSMR) to study the mediating effect of plasma metabolites on the association between GM and the risk of developing PD. We detected 54 genetic traits that were causally associated with PD development. According to the TSMR analysis, ceramide had a mediating effect on the relationship between the genus Clostridium sensu stricto 1 and the risk of developing PD (15.35% mediation; 95% CI = 1.29-32.75%). 7-Alpha-hydroxy-3-oxo-4-cholestenoate had a mediating effect on the relationship between the genus Eubacterium xylanophilum group and the risk of developing PD (11.04% mediation; 95% CI = 0.11-27.07%). In the present study, we used MR analysis to investigate the connections among GM, plasma metabolites, and PD. This comprehensive investigation offers insights into the pathogenic mechanisms of PD and the roles of the intestinal microbiota and metabolites in this disease.

Alpha-synuclein pathology and Parkinson's disease-related olfactory dysfunctions: an update on preclinical models and therapeutic approaches

Olfactory dysfunction (OD) is considered one of the early signs of Parkinson's disease (PD), affecting over 90% of PD patients. OD often appears several years before the onset of motor symptoms and is therefore considered an early biomarker of PD. Recent studies have shown that COVID-19 infection might lead to worsening of symptoms and acceleration of disease progression in neurodegenerative disorders, where OD is a common symptom to both. Hence, it is essential to accurately monitor olfactory fitness in clinical settings using any of the currently available olfactory function tests. Even after a quarter of a century of the discovery of α-synuclein (α-syn) pathogenesis in PD, many aspects related to the α-syn pathogenesis in OD remain unknown. Currently, there is no definitive cure for PD; the disease management options include dopaminergic medications, deep brain stimulations, stem cells, and immunotherapy. Generating reliable PD animal models is critical for understanding the molecular pathways and neural circuits affected by disease conditions. This might contribute to the development and validation of new therapeutic approaches. This review discusses the known mechanisms of α-syn aggregated forms causing neuronal death, the recent developments in the PD preclinical models with ODs, and the treatment strategies employed.

The interplay of iron, oxidative stress, and α-synuclein in Parkinson's disease progression

The irreversible degeneration of dopamine neurons induced by α-synuclein (α-syn) aggregation in the substantia nigra is the central pathological feature of Parkinson's disease (PD). Neuroimaging and pathological autopsy studies consistently confirm significant iron accumulation in the brain of PD patients, suggesting a critical role for iron in disease progression. Current research has established that iron overload induces ferroptosis in dopaminergic neurons, evidence indicates that the impact of iron on PD pathology extends beyond ferroptosis. Iron also plays a regulatory role in modulating α-syn, affecting its aggregation, spatial conformation, post-translational modifications, and mRNA stability. Iron-induced α-syn aggregation can contribute to dopaminergic neurodegeneration through additional mechanisms, potentially creating a feedback loop in which α-syn further enhances iron accumulation, thus perpetuating a vicious cycle of neurotoxicity. Given α-syn's intrinsically disordered structure, targeting iron metabolism presents a promising therapeutic strategy for PD. Therefore, the development of iron chelators, alone or in combination with other therapeutic drugs, may offer a beneficial approach to alleviating PD symptoms and slowing disease progression.

Protein aggregation identified in olfactory neuronal cells is associated with cognitive impairments in a subset of living schizophrenia patients

Schizophrenia is a heterogeneous disorder, and likely results from multiple pathophysiological mechanisms. Protein aggregation, resulting from disruption of protein homeostasis (proteostasis), has been implicated in many diseases, including cancer, cardiac and pulmonary diseases, muscle diseases, and neurodegenerative disorders, but is a relatively new pathophysiological hypothesis for schizophrenia. Genetic findings implicate proteostasis in schizophrenia, and individual proteins associated with the disorder may undergo aggregation. While there is some evidence of associations between genetic variants and protein aggregation, the extent to which genetic variations influence protein aggregation remains unknown. We have previously reported increased protein insolubility and increased ubiquitination of the insoluble protein fraction, two markers of protein aggregation, in human postmortem brains from a subset of patients with schizophrenia. In the present study, we investigate whether protein aggregation is observed in an independent model system, olfactory neuronal cells derived from living patients with schizophrenia, and examine the relationship between aggregation and patient clinical and cognitive status. We demonstrate that, as in postmortem brain, olfactory neurons from a subset of patients with schizophrenia exhibit protein aggregation, identified by increased protein insolubility and ubiquitination of the insoluble protein fraction, and by ubiquitin positive protein aggregates. Patients with protein aggregation exhibit more severe cognitive deficits than those without aggregation, as revealed by between-group comparisons and correlational analyses. Understanding the mechanisms of the aggregation process, the factors that differentiate individuals who develop aggregates from those who do not, and the relationship between aggregation and cell function, has important implications for the pathophysiology of schizophrenia, and may provide insight into disease heterogeneity and novel therapeutic targets.

Exploring Acoustic Detection of α-Synuclein Fibrils

Over the past decades, the incidence of Parkinson's disease (PD) cases has doubled in industrialized countries. While patients over 70 years old still represent more than half of the cases, the disease is increasingly affecting younger individuals. Environmental factors have been implicated, such as the effects of certain pesticides or chemicals on neurons, such as rotenone or 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Researchers have also demonstrated the influence of genetic mutations in younger patients. A-synuclein is a protein encoded by the SNCA gene, known to undergo various mutations in hereditary cases of PD. These mutations alter the composition and spatial arrangements of α-synuclein. The proteins, originally of linear shape, aggregate during the progression of PD, forming fibrillary structures that propagate through brain tissues. Among the physical therapies investigated for treating α-synuclein aggregation, ultrasonic waves, capable of altering protein and cell behaviors, have recently been used to disrupt α-synuclein fibrils within tissues in cellular and animal models, with the hope of developing treatments based on ultrasound properties. However, detecting fibrils typically requires invasive and non-biocompatible chemical compounds or cumbersome machinery. In this study, our acoustic experimental setup allowed us to investigate the response of α-synuclein to ultrasound perturbations. By capturing the transmitted wave across proteins over a frequency range 10 kHz to 10 MHz, no ultrasound signature indicating the presence of proteins was observed.Significance Statement: The results report there is no ultrasound signature of the presence of α-synuclein fibrils, from 10 kHz to 10 MHz.

Factors responsible for alpha-Synuclein aggregation

Aggregation of α-Synuclein (α-Syn) is the hallmark of the pathophysiology of Parkinson's disease. Apart from aggregates, α-Syn can exist in multiple abnormal forms such as oligomers, protofibrils, fibrils amorphous aggregates etc. These forms initiate aggressive, selective and progressive neuronal atrophy through various modes such as mitochondrial dysfunction, lysosomal malfunction, and disruption of calcium homeostasis in various α-Syn-related neurodegenerative disorders. Structurally α-Syn is divided into three domains: N-terminal region made by amino acids1-67 (amphipathic, lysine-rich and interacts with acidic lipid membranes), Non-amyloid-β component (NAC) region made by amino acids 67-95 (hydrophobic region, central to α-syn aggregation) and C-terminal region made by amino acids 96-140 (acidic and proline-rich region responsible for interaction with other proteins). α-Syn follows the pattern of a typical intrinsically disordered protein and lacks a proper folded conformation and exist majorly in a random coil form, though on lipid binding the protein assumes an α-helical structure. The central random coil region of α-Syn is involved in fibril formation transforming into β-sheet rich secondary structures which is a characteristic of amyloids. This chapter entails an elaborate explanation of factors influencing the structure, function and aggregation of α-Syn. Major factors being abnormally high physiological expression of the protein, mutations, posttranslational modifications and also interactions with small molecules such as osmolytes in the cellular milieu. Studying the factors responsible for misfolding and aggregation of α-Syn along with the mechanism involved is crucial to understanding their implications in Parkinson's disease, and will yield valuable insights into disease mechanisms, potential therapeutic strategies.

Liquid-liquid phase separation and conformational strains of α-Synuclein: implications for Parkinson's disease pathogenesis

Parkinson's disease (PD) and other synucleinopathies are characterized by the aggregation and deposition of alpha-synuclein (α-syn) in brain cells, forming insoluble inclusions such as Lewy bodies (LBs) and Lewy neurites (LNs). The aggregation of α-syn is a complex process involving the structural conversion from its native random coil to well-defined secondary structures rich in β-sheets, forming amyloid-like fibrils. Evidence suggests that intermediate species of α-syn aggregates formed during this conversion are responsible for cell death. However, the molecular events involved in α-syn aggregation and its relationship with disease onset and progression remain not fully elucidated. Additionally, the clinical and pathological heterogeneity observed in various synucleinopathies has been highlighted. Liquid-liquid phase separation (LLPS) and condensate formation have been proposed as alternative mechanisms that could underpin α-syn pathology and contribute to the heterogeneity seen in synucleinopathies. This review focuses on the role of the cellular environment in α-syn conformational rearrangement, which may lead to pathology and the existence of different α-syn conformational strains with varying toxicity patterns. The discussion will include cellular stress, abnormal LLPS formation, and the potential role of LLPS in α-syn pathology.

The potential of phosphorylated α-synuclein as a biomarker for the diagnosis and monitoring of multiple system atrophy

INTRODUCTION

Multiple system atrophy (MSA) is a rapidly progressive neurodegenerative disorder characterized by the presence of glial cytoplasmic inclusions (GCIs) containing aggregated α-synuclein (α-Syn). Accurate diagnosis and monitoring of MSA present significant challenges, which can lead to potential misdiagnosis and inappropriate treatment. Biomarkers play a crucial role in improving the accuracy of MSA diagnosis, and phosphorylated α-synuclein (p-syn) has emerged as a promising biomarker for aiding in diagnosis and disease monitoring.

METHODS

A literature search was conducted on PubMed, Scopus, and Google Scholar using specific keywords and MeSH terms without imposing a time limit. Inclusion criteria comprised various study designs including experimental studies, case-control studies, and cohort studies published only in English, while conference abstracts and unpublished sources were excluded.

RESULTS

Increased levels of p-syn have been observed in various samples from MSA patients, such as red blood cells, cerebrospinal fluid, oral mucosal cells, skin, and colon biopsies, highlighting their diagnostic potential. The α-Syn RT-QuIC assay has shown sensitivity in diagnosing MSA and tracking its progression. Meta-analyses and multicenter investigations have confirmed the diagnostic value of p-syn in cerebrospinal fluid, demonstrating high specificity and sensitivity in distinguishing MSA from other neurodegenerative diseases. Moreover, combining p-syn with other biomarkers has further improved the diagnostic accuracy of MSA.

CONCLUSION

The p-syn stands out as a promising biomarker for MSA. It is found in oligodendrocytes and shows a correlation with disease severity and progression. However, further research and validation studies are necessary to establish p-syn as a reliable biomarker for MSA. If proven, p-syn could significantly contribute to early diagnosis, disease monitoring, and assessing treatment response.

In silico analysis of alpha-synuclein protein variants and posttranslational modifications related to Parkinson's disease

Parkinson's disease (PD) is among the most prevalent neurodegenerative disorders, affecting over 10 million people worldwide. The protein encoded by the SNCA gene, alpha-synuclein (ASYN), is the major component of Lewy body (LB) aggregates, a histopathological hallmark of PD. Mutations and posttranslational modifications (PTMs) in ASYN are known to influence protein aggregation and LB formation, possibly playing a crucial role in PD pathogenesis. In this work, we applied computational methods to characterize the effects of missense mutations and PTMs on the structure and function of ASYN. Missense mutations in ASYN were compiled from the literature/databases and underwent a comprehensive predictive analysis. Phosphorylation and SUMOylation sites of ASYN were retrieved from databases and predicted by algorithms. ConSurf was used to estimate the evolutionary conservation of ASYN amino acids. Molecular dynamics (MD) simulations of ASYN wild-type and variants A30G, A30P, A53T, and G51D were performed using the GROMACS package. Seventy-seven missense mutations in ASYN were compiled. Although most mutations were not predicted to affect ASYN stability, aggregation propensity, amyloid formation, and chaperone binding, the analyzed mutations received relatively high rates of deleterious predictions and predominantly occurred at evolutionarily conserved sites within the protein. Moreover, our predictive analyses suggested that the following mutations may be possibly harmful to ASYN and, consequently, potential targets for future investigation: K6N, T22I, K34E, G36R, G36S, V37F, L38P, G41D, and K102E. The MD analyses pointed to remarkable flexibility and essential dynamics alterations at nearly all domains of the studied variants, which could lead to impaired contact between NAC and the C-terminal domain triggering protein aggregation. These alterations may have functional implications for ASYN and provide important insight into the molecular mechanism of PD, supporting the design of future biomedical research and improvements in existing therapies for the disease.

Alpha Synuclein: Neurodegeneration and Inflammation

Alpha-Synuclein (α-Syn) is one of the most important molecules involved in the pathogenesis of Parkinson's disease and related disorders, synucleinopathies, but also in several other neurodegenerative disorders with a more elusive role. This review analyzes the activities of α-Syn, in different conformational states, monomeric, oligomeric and fibrils, in relation to neuronal dysfunction. The neuronal damage induced by α-Syn in various conformers will be analyzed in relation to its capacity to spread the intracellular aggregation seeds with a prion-like mechanism. In view of the prominent role of inflammation in virtually all neurodegenerative disorders, the activity of α-Syn will also be illustrated considering its influence on glial reactivity. We and others have described the interaction between general inflammation and cerebral dysfunctional activity of α-Syn. Differences in microglia and astrocyte activation have also been observed when in vivo the presence of α-Syn oligomers has been combined with a lasting peripheral inflammatory effect. The reactivity of microglia was amplified, while astrocytes were damaged by the double stimulus, opening new perspectives for the control of inflammation in synucleinopathies. Starting from our studies in experimental models, we extended the perspective to find useful pointers to orient future research and potential therapeutic strategies in neurodegenerative disorders.

Disease-relevant β2-microglobulin variants share a common amyloid fold

β2-microglobulin (β2m) and its truncated variant ΔΝ6 are co-deposited in amyloid fibrils in the joints, causing the disorder dialysis-related amyloidosis (DRA). Point mutations of β2m result in diseases with distinct pathologies. β2m-D76N causes a rare systemic amyloidosis with protein deposited in the viscera in the absence of renal failure, whilst β2m-V27M is associated with renal failure, with amyloid deposits forming predominantly in the tongue. Here we use cryoEM to determine the structures of fibrils formed from these variants under identical conditions in vitro. We show that each fibril sample is polymorphic, with diversity arising from a 'lego-like' assembly of a common amyloid building block. These results suggest a 'many sequences, one amyloid fold' paradigm in contrast with the recently reported 'one sequence, many amyloid folds' behaviour of intrinsically disordered proteins such as tau and Aβ.

The couple of netrin-1/α-Synuclein regulates the survival of dopaminergic neurons via α-Synuclein disaggregation

The abnormal accumulation and aggregation of the misfolded α-synuclein protein is the neuropathological hallmark of all α-synucleinopathies, including Parkinson's disease. The secreted proteins known as netrins (netrin-1, netrin-3, and netrin-4) are related to laminin and have a role in the molecular pathway for axon guidance and cell survival. Interestingly, only netrin-1 is significantly expressed in the substantia nigra (SN) of healthy adult brains and its expression inversely correlates with that of α-synuclein, which prompted us to look into the role of α-synuclein and netrin-1 molecular interaction in the future of dopaminergic neurons. Here, we showed that netrin-1 and α-synuclein directly interacted in pre-formed fibrils (PFFs) generation test, real time binding assay, and co-immunoprecipitation with neurotoxin treated cell lysates. Netrin-1 deficiency appeared to activate the dopaminergic neuronal cell death signal pathway via α-synuclein aggregation and hyperphosphorylation of α-synuclein S129. Taken together, netrin-1 can be a promising therapeutic molecule in Parkinson's disease. [BMB Reports 2023; 56(2): 126-131].

Interlinked role of ASN, TDP-43 and Miro1 with parkinopathy: Focus on targeted approach against neuropathy in parkinsonism

Parkinsonism is a complex neurodegenerative disease that is difficult to differentiate because of its idiopathic and unknown origins. The hereditary parkinsonism known as autosomal recessive-juvenile parkinsonism (AR-JP) is marked by tremors, dyskinesias, dystonic characteristics, and manifestations that improve sleep but do not include dementia. This was caused by deletions and point mutations in PARK2 (chromosome 6q25.2-27). Diminished or unusual sensations (paresthesias), loss of neuron strength both in the CNS and peripheral nerves, and lack of motor coordination are the hallmarks of neuropathy in parkinsonism. The incidence of parkinsonism during oxidative stress and ageing is associated with parkinopathy. Parkinopathy is hypothesized to be triggered by mutation of the parkin (PRKN) gene and loss of normal physiological functions of PRKN proteins, which triggers their pathogenic aggregation due to conformational changes. Two important genes that control mitochondrial health are PRKN and phosphatase and tensin homologue deleted on chromosome 10-induced putative kinase 1 (PINK1). Overexpression of TAR DNA-binding protein-43 (TDP-43) increases the aggregation of insoluble PRKN proteins in OMM. Foreign α-synuclein (ASN) promotes parkinopathy via S-nitrosylation and hence has a neurotoxic effect on dopaminergic nerves. Miro1 (Miro GTPase1), a member of the RAS superfamily, is expressed in nerve cells. Due to PINK1/PRKN and Miro1's functional relationship, an excess of mitochondrial calcium culminates in the destruction of dopaminergic neurons. An interlinked understanding of TDP-43, PINK1/PRKN, ASN, and Miro1 signalling in the communication among astrocytes, microglia, neurons, and immune cells within the brain explored the pathway of neuronal death and shed light on novel strategies for the diagnosis and treatment of parkinsonism.

Synucleins: New Data on Misfolding, Aggregation and Role in Diseases

The synucleins are a family of natively unfolded (or intrinsically unstructured) proteins consisting of α-, β-, and γ-synuclein involved in neurodegenerative diseases and cancer. The current number of publications on synucleins has exceeded 16.000. They remain the subject of constant interest for over 35 years. Two reasons explain this unchanging attention: synuclein's association with several severe human diseases and the lack of understanding of the functional roles under normal physiological conditions. We analyzed recent publications to look at the main trends and developments in synuclein research and discuss possible future directions. Traditional areas of peak research interest which still remain high among last year's publications are comparative studies of structural features as well as functional research on of three members of the synuclein family. Another popular research topic in the area is a mechanism of α-synuclein accumulation, aggregation, and fibrillation. Exciting fast-growing area of recent research is α-synuclein and epigenetics. We do not present here a broad and comprehensive review of all directions of studies but summarize only the most significant recent findings relevant to these topics and outline potential future directions.

Identification of Kv4.2 protein complex and modifications by tandem affinity purification-mass spectrometry in primary neurons

Proteins usually form complexes to fulfill variable physiological functions. In neurons, communication relies on synapses where receptors, channels, and anchoring proteins form complexes to precisely control signal transduction, synaptic integration, and action potential firing. Although there are many published protocols to isolate protein complexes in cell lines, isolation in neurons has not been well established. Here we introduce a method that combines lentiviral protein expression with tandem affinity purification followed by mass-spectrometry (TAP-MS) to identify protein complexes in neurons. This protocol can also be used to identify post-translational modifications (PTMs) of synaptic proteins. We used the A-type voltage-gated K⁺ channel subunit Kv4.2 as the target protein. Kv4.2 is highly expressed in the hippocampus where it contributes to learning and memory through its regulation of neuronal excitability and synaptic plasticity. We tagged Kv4.2 with the calmodulin-binding-peptide (CBP) and streptavidin-binding-peptide (SBP) at its C-terminus and expressed it in neurons via lentivirus. Kv4.2 was purified by two-step TAP and samples were analyzed by MS. MS identified two prominently known Kv4.2 interacting proteins [dipeptidyl peptidase like (DPPs) and Kv channel-interacting proteins (KChIPs)] in addition to novel synaptic proteins including glutamate receptors, a calcium channel, and anchoring proteins. Co-immunoprecipitation and colocalization experiments validated the association of Kv4.2 with glutamate receptors. In addition to protein complex identification, we used TAP-MS to identify Kv4.2 phosphorylation sites. Several known and unknown phosphorylation sites were identified. These findings provide a novel path to identify protein-protein interactions and PTMs in neurons and shed light on mechanisms of neuronal signaling potentially involved in the pathology of neurological diseases.

Integration of O-GlcNAc into Stress Response Pathways

The modification of nuclear, mitochondrial, and cytosolic proteins by O-linked βN-acetylglucosamine (O-GlcNAc) has emerged as a dynamic and essential post-translational modification of mammalian proteins. O-GlcNAc is cycled on and off over 5000 proteins in response to diverse stimuli impacting protein function and, in turn, epigenetics and transcription, translation and proteostasis, metabolism, cell structure, and signal transduction. Environmental and physiological injury lead to complex changes in O-GlcNAcylation that impact cell and tissue survival in models of heat shock, osmotic stress, oxidative stress, and hypoxia/reoxygenation injury, as well as ischemic reperfusion injury. Numerous mechanisms that appear to underpin O-GlcNAc-mediated survival include changes in chaperone levels, impacts on the unfolded protein response and integrated stress response, improvements in mitochondrial function, and reduced protein aggregation. Here, we discuss the points at which O-GlcNAc is integrated into the cellular stress response, focusing on the roles it plays in the cardiovascular system and in neurodegeneration.