Alpha-Synuclein Accumulation and Its Phosphorylation in the Enteric Nervous System of Patients Without Neurodegeneration: An Explorative Study

A review of the gastrointestinal, olfactory, and skin abnormalities in patients with Parkinson's disease

Parkinson's disease (PD) is a complex neurodegenerative motor disorder caused by the loss of dopaminergic neurons in the substantia nigra pars compacta. The substantia nigra is neither the first nor the only brain region affected by PD. Recent and old studies have shown that PD does not only affect the CNS; in fact, autonomic innervation in the GIT, skin, and olfactory system was found to be affected by α-synuclein pathology outside the CNS, affecting patients' quality of life. In the gastrointestinal system, dysphagia, constipation, and bacterial overgrowth in the small intestine are common in patients with PD. In addition, several skin conditions were reported in PD, including seborrheic dermatitis, rosacea, melanoma, and others. Finally, olfactory system dysfunction, such as reduced touch sensation and smell, was associated with motor abnormalities. Further high-quality studies are needed to develop reliable tests that could help in the early diagnosis of PD.

How should we be using biomarkers in trials of disease modification in Parkinson's disease?

The recent validation of the α-synuclein seed amplification assay as a biomarker with high sensitivity and specificity for the diagnosis of Parkinson's disease has formed the backbone for a proposed staging system for incorporation in Parkinson's disease clinical studies and trials. The routine use of this biomarker should greatly aid in the accuracy of diagnosis during recruitment of Parkinson's disease patients into trials (as distinct from patients with non-Parkinson's disease parkinsonism or non-Parkinson's disease tremors). There remain, however, further challenges in the pursuit of biomarkers for clinical trials of disease modifying agents in Parkinson's disease, namely: optimizing the distinction between different α-synucleinopathies; the selection of subgroups most likely to benefit from a candidate disease modifying agent; a sensitive means of confirming target engagement; and the early prediction of longer-term clinical benefit. For example, levels of CSF proteins such as the lysosomal enzyme β-glucocerebrosidase may assist in prognostication or allow enrichment of appropriate patients into disease modifying trials of agents with this enzyme as the target; the presence of coexisting Alzheimer's disease-like pathology (detectable through CSF levels of amyloid-β42 and tau) can predict subsequent cognitive decline; imaging techniques such as free-water or neuromelanin MRI may objectively track decline in Parkinson's disease even in its later stages. The exploitation of additional biomarkers to the α-synuclein seed amplification assay will, therefore, greatly add to our ability to plan trials and assess the disease modifying properties of interventions. The choice of which biomarker(s) to use in the context of disease modifying clinical trials will depend on the intervention, the stage (at risk, premotor, motor, complex) of the population recruited and the aims of the trial. The progress already made lends hope that panels of fluid biomarkers in tandem with structural or functional imaging may provide sensitive and objective methods of confirming that an intervention is modifying a key pathophysiological process of Parkinson's disease. However, correlation with clinical progression does not necessarily equate to causation, and the ongoing validation of quantitative biomarkers will depend on insightful clinical-genetic-pathophysiological comparisons incorporating longitudinal biomarker changes from those at genetic risk with evidence of onset of the pathophysiology and those at each stage of manifest clinical Parkinson's disease.

Distinct molecular mechanisms contribute to the reduction of melanoma growth and tumor pain after systemic and local depletion of alpha-Synuclein in mice

Epidemiological studies show a coincidence between Parkinson's disease (PD) and malignant melanoma. It has been suggested that this relationship is due, at least in part, to modulation of alpha-Synuclein (αSyn/Snca). αSyn oligomers accumulate in PD, which triggers typical PD symptoms, and in malignant melanoma, which increases the proliferation of tumor cells. In addition, αSyn contributes to non-motor symptoms of PD, including pain. In this study, we investigated the role of αSyn in melanoma growth and melanoma-induced pain in a mouse model using systemic and local depletion of αSyn. B16BL6 wild-type as well as αSyn knock-down melanoma cells were inoculated into the paws of αSyn knock-out mice and wild-type mice, respectively. Tumor growth and tumor-induced pain hypersensitivity were assessed over a period of 21 days. Molecular mechanisms were analyzed by RT-PCR and Western Blot in tumors, spinal cord, and sciatic nerve. Our results indicate that both global and local ablation of Snca contribute to reduced tumor growth and to a reduction of tumor-induced mechanical allodynia, though mechanisms contributing to these effects differ. While injection of wild-type cells in Snca knock-out mice strongly increased the immune response in the tumor, local Snca knock-down decreased autophagy mechanisms and the inflammatory reaction in the tumor. In conclusion, a knockdown of αSyn might constitute a promising approach to inhibiting the progression of melanoma and reducing tumor-induced pain.

The Role of Immune Dysfunction in Parkinson's Disease Development

Recent research has unveiled intriguing insights suggesting that the body's immune system may be implicated in Parkinson's disease (PD) development. Studies have observed disparities in pro-inflammatory and anti-inflammatory markers between PD patients and healthy individuals. This finding underscores the potential influence of immune system dysfunction in the genesis of this condition. A dysfunctional immune system can serve as a primary catalyst for systemic inflammation in the body, which may contribute to the emergence of various brain disorders. The identification of several genes associated with PD, as well as their connection to neuroinflammation, raises the likelihood of disease susceptibility. Moreover, advancing age and mitochondrial dysfunction can weaken the immune system, potentially implicating them in the onset of the disease, particularly among older individuals. Compromised integrity of the blood-brain barrier could facilitate the immune system's access to brain tissue. This exposure may lead to encounters with native antigens or infections, potentially triggering an autoimmune response. Furthermore, there is mounting evidence supporting the notion that gut dysbiosis might represent an initial trigger for brain inflammation, ultimately promoting neurodegeneration. In this comprehensive review, we will delve into the numerous hypotheses surrounding the role of both innate and adaptive immunity in PD.

Nanowired delivery of antibodies to tau and neuronal nitric oxide synthase together with cerebrolysin attenuates traumatic brain injury induced exacerbation of brain pathology in Parkinson's disease

Concussive head injury (CHI) is one of the major risk factors for developing Parkinson's disease in later life of military personnel affecting lifetime functional and cognitive disturbances. Till date no suitable therapies are available to attenuate CHI or PD induced brain pathology. Thus, further exploration of novel therapeutic agents are highly warranted using nanomedicine in enhancing the quality of life of veterans or service members of US military. Since PD or CHI induces oxidative stress and perturbs neurotrophic factors regulation associated with phosphorylated tau (p-tau) deposition, a possibility exists that nanodelivery of agents that could enhance neurotrophic factors balance and attenuate oxidative stress could be neuroprotective in nature. In this review, nanowired delivery of cerebrolysin-a balanced composition of several neurotrophic factors and active peptide fragments together with monoclonal antibodies to neuronal nitric oxide synthase (nNOS) with p-tau antibodies was examined in PD following CHI in model experiments. Our results suggest that combined administration of nanowired antibodies to nNOS and p-tau together with cerebrolysin significantly attenuated CHI induced exacerbation of PD brain pathology. This combined treatment also has beneficial effects in CHI or PD alone, not reported earlier.

Targeting α-synuclein post-translational modifications in Parkinson's disease

Parkinson's disease (PD) is a neurodegenerative disease characterized by the progressive loss of dopaminergic neurons in the nigrostriatal pathway. Although the exact mechanisms underlying PD are still not completely understood, it is well accepted that α-synuclein plays key pathophysiological roles as the main constituent of the cytoplasmic inclusions known as Lewy bodies. Several post-translational modifications (PTMs), such as the best-known phosphorylation, target α-synuclein and are thus implicated in its physiological and pathological functions. In this review, we present (1) an overview of the pathophysiological roles of α-synuclein, (2) a descriptive analysis of α-synuclein PTMs, including phosphorylation, ubiquitination, SUMOylation, acetylation, glycation, truncation, and O-GlcNAcylation, as well as (3) a brief summary on α-synuclein PTMs as potential biomarkers for PD. A better understanding of α-synuclein PTMs is of paramount importance for elucidating the mechanisms underlying PD and can thus be expected to improve early detection and monitoring disease progression, as well as identify promising new therapeutic targets.

Age-dependent Microglial Disease Phenotype Results in Functional Decline in Gut Macrophages

BACKGROUND AND AIMS

Muscularis macrophages (MMs) are tissue-resident macrophages in the gut muscularis externa which play a supportive role to the enteric nervous system. We have previously shown that age-dependent MM alterations drive low-grade enteric nervous system inflammation, resulting in neuronal loss and disruption of gut motility. The current studies were designed to identify the MM genetic signature involved in these changes, with particular emphasis on comparison to genes in microglia, the central nervous system macrophage population involved in age-dependent cognitive decline.

METHODS

Young (3 months) and old (16-24 months) C57BL/6 mice and human tissue were studied. Immune cells from mouse small intestine, colon, and spinal cord and human colon were dissociated, immunophenotyped by flow cytometry, and examined for gene expression by single-cell RNA sequencing and quantitative real-time PCR. Phagocytosis was assessed by in vivo injections of pHrodo beads (Invitrogen). Macrophage counts were performed by immunostaining of muscularis whole mounts.

RESULTS

MMs from young and old mice express homeostatic microglial genes, including Gpr34, C1qc, Trem2, and P2ry12. An MM subpopulation that becomes more abundant with age assumes a geriatric state (GS) phenotype characterized by increased expression of disease-associated microglia genes including Cd9, Clec7a, Itgax (CD11c), Bhlhe40, Lgals3, IL-1β, and Trem2 and diminished phagocytic activity. Acquisition of the GS phenotype is associated with clearance of α-synuclein aggregates. Human MMs demonstrate a similar age-dependent acquisition of the GS phenotype associated with intracellular α-synuclein accumulation.

CONCLUSION

MMs demonstrate age-dependent genetic changes that mirror the microglial disease-associated microglia phenotype and result in functional decline.

Can kynurenine pathway be considered as a next-generation therapeutic target for Parkinson's disease? An update information

By far, no revolutionary breakthrough in the treatment of Parkinson's disease (PD) was found. It is indeed a knotty problem to select a satisfactory strategy for treating some patients with advanced stage PD. Development of novel therapeutic targets against PD has been an urgent task faced by global PD researchers. Targets in the tryptophan-kynurenine pathway (KP) were then considered. Metabolites in the KP are liposoluble. Some neurotoxic metabolites, including 3-hydroxykynurenine and its downstream 3-hydroxyanthranilic acid and quinolinic acid, are mainly produced peripherally. They can easily cross the blood-brain barrier (BBB) and exert their neurotoxic effects in the central neuron system (CNS), which is considered as a potential pathophysiological mechanism of neurodegenerative diseases. Hence, agents against the targets in the KP have two characteristics: (1) being independent from the dopaminergic system and (2) being seldom affected by the BBB. Inspiringly, one agent, namely, the inhibitor of indoleamine 2,3-dioxygenase 1, has been currently reported to present satisfactory efficacy comparable to levodopa, implying that the KP might be a potential novel target for PD. This review collected and summarized the updated information regarding the association of the KP with PD, which is helpful for understanding the clinical value of the KP in the PD scenario.

Exploration of the α-syn/T199678/miR-519-3p/KLF9 pathway in a PD-related α-syn pathology

BACKGROUND

Kruppel-like factor 9 (KLF9) plays a key role as an inducer of cellular oxidative stress in the modulation of cell death and in oxidant-dependent tissue injury. Our previous study indicated that lncRNA-T199678 (T199678) affected the expression of KLF9 in an α-synuclein (α-syn) induced cellular model. However, the roles of interactions among α-syn, T199678, KLF9 and related microRNAs (miRNAs) in the Parkinson's disease (PD)-related α-syn pathology are unclear and were therefore investigated in this study.

METHODS

An α-syn-injected mouse model and an α-syn exposed SY-SH5Y cellular model were used in this study. We confirmed the utility of these established models with morphological and behavioral methods. We checked how expression of T199678 and KLF9 were affected by α-syn and demonstrated their interaction by fluorescence in situ hybridization (FISH) staining and western blots. We analyzed expression in ROS+ cells by immunohistochemistry. Finally, we obtained seven miRNAs through bioinformatic analysis simultaneously affected by T199678 and α-syn and verified these with RT-PCR.

RESULTS

We found that expression of KLF9 was regulated by T199678, whereas expression of T199678 was not affected by KLF9 in the α-syn exposed SY-SH5Y cells. These findings suggest that KLF9 is the downstream gene regulated by T199678, whereas miR-519-3p may play a contributing role. We also confirmed that α-syn injection upregulated the expression of ROS, which could be downregulated by upregulation of T199678, indicating an anti-oxidative role of T199678 in the α-syn-related mechanisms.

CONCLUSIONS

Our results indicate the existence of a potential α-syn/T199678/miR-519-3p /KLF9 pathway in PD-related α-syn pathology. This pathway might explain oxidative stress processes in α-syn-related mechanisms, which requires further verification.

Alpha-Synuclein Aggregation Pathway in Parkinson's Disease: Current Status and Novel Therapeutic Approaches

Following Alzheimer’s, Parkinson’s disease (PD) is the second-most common neurodegenerative disorder, sharing an unclear pathophysiology, a multifactorial profile, and massive social costs worldwide. Despite this, no disease-modifying therapy is available. PD is tightly associated with α-synuclein (α-Syn) deposits, which become organised into insoluble, amyloid fibrils. As a typical intrinsically disordered protein, α-Syn adopts a monomeric, random coil conformation in an aqueous solution, while its interaction with lipid membranes drives the transition of the molecule part into an α-helical structure. The central unstructured region of α-Syn is involved in fibril formation by converting to well-defined, β-sheet rich secondary structures. Presently, most therapeutic strategies against PD are focused on designing small molecules, peptides, and peptidomimetics that can directly target α-Syn and its aggregation pathway. Other approaches include gene silencing, cell transplantation, stimulation of intracellular clearance with autophagy promoters, and degradation pathways based on immunotherapy of amyloid fibrils. In the present review, we sum marise the current advances related to α-Syn aggregation/neurotoxicity. These findings present a valuable arsenal for the further development of efficient, nontoxic, and non-invasive therapeutic protocols for disease-modifying therapy that tackles disease onset and progression in the future.

Mass Spectrometry for Neurobiomarker Discovery: The Relevance of Post-Translational Modifications

Neurodegenerative diseases are incurable, heterogeneous, and age-dependent disorders that challenge modern medicine. A deeper understanding of the pathogenesis underlying neurodegenerative diseases is necessary to solve the unmet need for new diagnostic biomarkers and disease-modifying therapy and reduce these diseases’ burden. Specifically, post-translational modifications (PTMs) play a significant role in neurodegeneration. Due to its proximity to the brain parenchyma, cerebrospinal fluid (CSF) has long been used as an indirect way to measure changes in the brain. Mass spectrometry (MS) analysis in neurodegenerative diseases focusing on PTMs and in the context of biomarker discovery has improved and opened venues for analyzing more complex matrices such as brain tissue and blood. Notably, phosphorylated tau protein, truncated α-synuclein, APP and TDP-43, and many other modifications were extensively characterized by MS. Great potential is underlying specific pathological PTM-signatures for clinical application. This review focuses on PTM-modified proteins involved in neurodegenerative diseases and highlights the most important and recent breakthroughs in MS-based biomarker discovery.

Hydrogen Sulfide Produced by Gut Bacteria May Induce Parkinson's Disease

Several bacterial species can generate hydrogen sulfide (H2S). Study evidence favors the view that the microbiome of the colon harbors increased amounts of H2S producing bacteria in Parkinson's disease. Additionally, H2S can easily penetrate cell membranes and enter the cell interior. In the cells, excessive amounts of H2S can potentially release cytochrome c protein from the mitochondria, increase the iron content of the cytosolic iron pool, and increase the amount of reactive oxygen species. These events can lead to the formation of alpha-synuclein oligomers and fibrils in cells containing the alpha-synuclein protein. In addition, bacterially produced H2S can interfere with the body urate metabolism and affect the blood erythrocytes and lymphocytes. Gut bacteria responsible for increased H2S production, especially the mucus-associated species of the bacterial genera belonging to the Desulfovibrionaceae and Enterobacteriaceae families, are likely play a role in the pathogenesis of Parkinson's disease. Special attention should be devoted to changes not only in the colonic but also in the duodenal microbiome composition with regard to the pathogenesis of Parkinson's disease. Influenza infections may increase the risk of Parkinson's disease by causing the overgrowth of H2S-producing bacteria both in the colon and duodenum.

Update on alpha-synuclein-based biomarker approaches in the skin, submandibular gland, gastrointestinal tract, and biofluids

PURPOSE OF REVIEW

There is a need for objective diagnostic and prognostic biomarkers in Parkinson's disease (PD), partly given the expected increase in clinical trials aimed at demonstrating a disease-modifying effect in early disease. Alpha-synuclein (α-syn) plays a decisive role in the pathogenesis of PD. Here, we review recent publications exploring established and novel methodologies to detect α-syn species in tissues and biofluids.

RECENT FINDINGS

Using immunohistochemistry (IHC), recent studies have focused on the detection of phosphorylated α-syn (p-α-syn) in cutaneous nerve fibers, reporting varying sensitivity and high specificity for the diagnosis of PD. A predilection for p-α-syn depositions in cutaneous autonomic nerve fibers has emerged, possibly contrasting with other synucleinopathies.Novel studies utilizing the seeding propensity of pathological α-syn have generated encouraging results with regard to diagnostic performance in both tissues and biofluids including skin, submandibular gland, and cerebrospinal fluid.

SUMMARY

Detection of neuronal p-α-syn in skin punch biopsies remains a promising minimally invasive diagnostic tool in PD. Seeding assays have emerged as a new method with its diagnostic potential warranting replication in further studies from various tissues and biofluids. Longitudinal studies employing both IHC and seeding assays are needed to identify possible biomarkers of disease progression.