The Role of α-Synuclein in the Regulation of Serotonin System: Physiological and Pathological Features

Dysregulation of serotonergic neurotransmission in Parkinson disease: A key duet

Parkinson's disease (PD) is the most common movement disorder, affecting approximately 1% of the general population over 65 years of age. PD is commonly associated with the development of motor and non-motor symptoms. Non-motor symptoms arise decades earlier than motor symptoms due to the degeneration of GABAergic, serotonergic, and other neurons involved in autonomic regulation. However, motor symptoms in PD are developed due to degeneration of the dopaminergic neurons in the substantia nigra pars compacta (SNpc) of midbrain. The PD neuropathology is related to the progressive loss of the dopaminergic neurons in the SNpc of midbrain. Particularly, dysfunction of serotonergic system is implicated in the development of non-motor symptoms such as sleep disorders, cognitive dysfunction, depression and anxiety. In addition, dysfunction of serotonergic neurons which affects the dopaminergic neurons in the SNpc leads to the development of motor symptoms. Moreover, dysfunction of serotonergic neurons is associated with the development of L-dopamine (L-DOPA)-induced dyskinesia. Consistently, administration of serotonin (5-HT) receptor agonist attenuates the development of L-DOPA-induced dyskinesia. These findings emphasized the possible role of serotonergic system in PD. However, the underlying mechanisms that mediate the latent effect of 5-HT in PD are not completely elucidated. Therefore, this mini-review aims to discuss the exact role of 5-HT in PD, and how the 5-HT modulators affect PD neuropathology.

Cognitive impairment in Parkinson's disease and other parkinsonian syndromes

In this narrative review, we address mild cognitive impairment, a frequent complication of Parkinson's disease (PD) and atypical parkinsonian disorders (APDs). Recent diagnostic criteria have blurred the lines between PD and dementia with Lewy bodies (DLB), particularly in the cognitive domain. Additionally, atypical parkinsonian syndromes like progressive supranuclear palsy (PSP) and corticobasal degeneration (CBD) often present with significant cognitive decline. Even multiple system atrophy (MSA) can be associated with cognitive impairment in some cases. Several biomarkers, including imaging techniques, such brain magnetic resonance imaging (MRI) and fluorodeoxyglucose positron emission tomography (FDG-PET), as well as pathological proteins either of the cerebrospinal fluid (CSF), such as Tau, amyloid beta, and synuclein, or of the serum, such as neurofilament light chain (Nfl) are more and more often utilized in the early differential diagnosis of APDs. The complex interplay between these conditions and the evolving understanding of their underlying pathologies highlight the need for further research to refine diagnostic criteria, possibly incorporate the new findings from the biomarker's field into the diagnostic criteria and develop targeted therapeutic strategies.

Impaired unfolded protein response, BDNF and synuclein markers in postmortem dorsolateral prefrontal cortex and caudate nucleus of patients with depression and Parkinson's disease

Major depressive disorder (MDD) is characterized by significant impairment in social, emotional, and cognitive functioning. Its precise pathophysiology remains poorly understood. Alterations in protein homeostasis and some misfolded proteins have been identified within the brains of patients diagnosed with neuropsychiatric disorders. In contrast to neurodegenerative processes such as Parkinson's disease (PD), where the accumulation of aggregated α-synuclein (α-Syn) protein is a primary cause of significant neuronal loss, altered proteostasis in MDD may result in loss-of-function effects by modifying synaptic neuroplasticity. Moreover, aberrant activation of endoplasmic reticulum (ER) pathways may intensify the pathological alterations due to altered proteostasis. In this study, dorsolateral prefrontal cortex (dlPFC) and caudate nucleus from MDD patients and non-psychiatric controls were used. Postmortem samples of same brain areas from PD patients (Braak 2-3 and 5-6) and controls were also included. Protein levels of ER and unfolded protein response (UPR), synucleins (α-, β- and γ-Syn), and brain-derived neurotrophic factor (BDNF) were measured by Western-Blot. Phospho-eIF2α/eIF2α ratio was increased in the dlPFC and caudate nucleus of MDD and PD patients compared to their respective controls. Brain area-dependent changes in BiP and GRP94 levels were also found. We further detected accumulation of immature BDNF precursors and opposite changes in α- and β-Syn levels in the dlPFC of MDD and PD patients compared to controls. Our findings suggest that alterations in proteostasis contribute to the pathophysiology of MDD, as previously described in PD. A deeper understanding of the pathways involved will identify other candidate proteins and new targets with therapeutic potential.

Genetic and neurochemical profiles underlying cortical morphometric vulnerability to Parkinson's disease

BACKGROUND

Increasing evidence has documented cortical involvement at all stages of PD. The local vulnerabilities within certain brain regions in PD have been previously demonstrated, whereas its underlying genetic and neurochemical factors remain unclear. This study aims to investigate the spatial spectrum of cortical atrophy in Parkinson's disease (PD) and link these variances in gray matter properties and curvature respectively to putative molecular pathways and neurotransmitter factors.

METHODS

We recruited 141 clinically diagnosed PD patients and 70 healthy controls. Cortical morphological abnormalities of PD were obtained by intergroup comparisons in gray matter properties metrics and curvature measurements. Then we performed gene-category enrichment and spatial correlation analyses to evaluate the specific correspondence between cortical alteration in PD and genetic expression from the Allen Human Brain Atlas and normative neurotransmitter atlases from Neuromaps.

RESULTS

We found decreased gray matter properties in temporal, somatomotor, cingulate and occipital cortices, decreased curvature measures in occipital, temporal and orbitofrontal cortices, and increased curvature measures in somatomotor, prefrontal and posterior parietal cortices for PD patients. The related genes were enriched for the glucose metabolism, mitochondrial function, and post-translational histone modifications processes. In addition, the serotonin and norepinephrine transporter devoted more to gray matter properties alterations while the dopamine, gamma-aminobutyric acid receptors, and norepinephrine transporter were strong contributors of curvature abnormalities in PD.

CONCLUSIONS

Collectively, the present study offered interpretation of cortical morphological alterations and the cortical pathogenic theory in PD from genetic and neurochemical perspectives, which inspire further research on new pharmacotherapeutic approaches.

A review of symptom, pathogenesis and treatment characteristics of the elderly with chronic insomnia

One third of a person's life is in a state of sleep, and good sleep quality is one of the indicators of health. Long-term insomnia has a great impact on patients' quality of life, physical and mental state. Especially in the elderly, long-term insomnia will induce a variety of chronic diseases, seriously affecting the quality of life. Because of the characteristics of the physiological structure of the elderly, insomnia in the elderly has its unique characteristics different from that of the young. This article reviews the characteristics of chronic insomnia in the elderly in terms of symptom, pathogenesis and treatment. By elaborating the characteristics of senile chronic insomnia, we hope to provide ideas for clinical treatment of senile chronic insomnia.

Revolutionizing our understanding of Parkinson's disease: Dr. Heinz Reichmann's pioneering research and future research direction

Millions of individuals around the world are afflicted with Parkinson's disease (PD), a prevalent and incapacitating neurodegenerative disorder. Dr. Reichmann, a distinguished professor and neurologist, has made substantial advancements in the domain of PD research, encompassing both fundamental scientific investigations and practical applications. His research has illuminated the etiology and treatment of PD, as well as the function of energy metabolism and premotor symptoms. As a precursor to a number of neurotransmitters and neuromodulators that are implicated in the pathophysiology of PD, he has also investigated the application of tryptophan (Trp) derivatives in the disease. His principal findings and insights are summarized and synthesized in this narrative review article, which also emphasizes the challenges and implications for future PD research. This narrative review aims to identify and analyze the key contributions of Reichmann to the field of PD research, with the ultimate goal of informing future research directions in the domain. By examining Reichmann's work, the study seeks to provide a comprehensive understanding of his major contributions and how they can be applied to advance the diagnosis and treatment of PD. This paper also explores the potential intersection of Reichmann's findings with emerging avenues, such as the investigation of Trp and its metabolites, particularly kynurenines, which could lead to new insights and potential therapeutic strategies for managing neurodegenerative disorders like PD.

Posttranslational Modifications of -Synuclein, Their Therapeutic Potential, and Crosstalk in Health and Neurodegenerative Diseases

α-Synuclein (α-Syn) aggregation in Lewy bodies and Lewy neurites has emerged as a key pathogenetic feature in Parkinson's disease, dementia with Lewy bodies, and multiple system atrophy. Various factors, including posttranslational modifications (PTMs), can influence the propensity of α-Syn to misfold and aggregate. PTMs are biochemical modifications of a protein that occur during or after translation and are typically mediated by enzymes. PTMs modulate several characteristics of proteins including their structure, activity, localization, and stability. α-Syn undergoes various posttranslational modifications, including phosphorylation, ubiquitination, SUMOylation, acetylation, glycation, O-GlcNAcylation, nitration, oxidation, polyamination, arginylation, and truncation. Different PTMs of a protein can physically interact with one another or work together to influence a particular physiological or pathological feature in a process known as PTMs crosstalk. The development of detection techniques for the cooccurrence of PTMs in recent years has uncovered previously unappreciated mechanisms of their crosstalk. This has led to the emergence of evidence supporting an association between α-Syn PTMs crosstalk and synucleinopathies. In this review, we provide a comprehensive evaluation of α-Syn PTMs, their impact on misfolding and pathogenicity, the pharmacological means of targeting them, and their potential as biomarkers of disease. We also highlight the importance of the crosstalk between these PTMs in α-Syn function and aggregation. Insight into these PTMS and the complexities of their crosstalk can improve our understanding of the pathogenesis of synucleinopathies and identify novel targets of therapeutic potential. SIGNIFICANCE STATEMENT: α-Synuclein is a key pathogenic protein in Parkinson's disease and other synucleinopathies, making it a leading therapeutic target for disease modification. Multiple posttranslational modifications occur at various sites in α-Synuclein and alter its biophysical and pathological properties, some interacting with one another to add to the complexity of the pathogenicity of this protein. This review details these modifications, their implications in disease, and potential therapeutic opportunities.

Upregulation of γ-synuclein in the prefrontal cortex and hippocampus following dopamine depletion: A study using the striatal 6-hydroxydopamine hemiparkinsonian rat model

Synucleins, including α-synuclein (α-syn), β-syn, and γ-syn, have been implicated in various synucleinopathies, notably Parkinson's disease (PD), which has generated increased interest in understanding their roles. Although α-syn and β-syn have contrasting neuropathological consequences, the precise role of γ-syn remains unclear. This study validated non-motor symptoms, specifically anxiety-like behavior, along with the degradation of dopaminergic (DAergic) neurons in the nigrostriatal system and DAergic neurites in the prefrontal cortex and hippocampus of rats infused with striatal 6-hydroxydopamine (6-OHDA). Our study further investigated the alterations in γ-syn expression levels in the prefrontal cortices and hippocampi of these 6-OHDA-treated rats, aiming to establish foundational insights into the neuropathophysiology of DA depletion, a central feature of PD. Our findings revealed a significant increase in the expression of γ-syn mRNA and protein in these brain regions, in contrast to unaltered α- and β-syn expression levels. This suggests a distinct role of γ-syn within the neurobiological milieu under conditions of DA deficiency. Overall, our data shed light on the neurobiological changes observed in the hemiparkinsonian rat model induced with 6-OHDA, underscoring the potential significance of γ-syn in PD pathology.

New insights into the effects of serotonin on Parkinson's disease and depression through its role in the gastrointestinal tract

Neuropsychiatric and neurodegenerative disorders are frequently associated with gastrointestinal (GI) co-pathologies. Although the central and enteric nervous systems (CNS and ENS, respectively) have been studied separately, there is increasing interest in factors that may contribute to conditions affecting both systems. There is compelling evidence that serotonin (5-HT) may play an important role in several gut-brain disorders. It is well known that 5-HT is essential for the development and functioning of the CNS. However, most of the body's 5-HT is produced in the GI tract. A deeper understanding of the specific effects of enteric 5-HT on gut-brain disorders may provide the basis for the development of new therapeutic targets. This review summarizes current data focusing on the important role of 5-HT in ENS development and motility, with particular emphasis on novel aspects of 5-HT signaling in conditions where CNS and ENS comorbidities are common, such as Parkinson's disease and depressive disorders.

ER stress in mouse serotonin neurons triggers a depressive phenotype alleviated by ketamine targeting eIF2α signaling

Depression is a devastating mood disorder that causes significant disability worldwide. Current knowledge of its pathophysiology remains modest and clear biological markers are lacking. Emerging evidence from human and animal models reveals persistent alterations in endoplasmic reticulum (ER) homeostasis, suggesting that ER stress-related signaling pathways may be targets for prevention and treatment. However, the neurobiological basis linking the pathways involved in depression-related ER stress remains unknown. Here, we report that an induced model of ER stress in mouse serotonin (5-HT) neurons is associated with reduced Egr1-dependent 5-HT cellular activity and 5-HT neurotransmission, resulting in neuroplasticity deficits in forebrain regions and a depressive-like phenotype. Ketamine administration engages downstream eIF2α signaling to trigger rapid neuroplasticity events that rescue the depressive-like effects. Collectively, these data identify ER stress in 5-HT neurons as a cellular pathway involved in the pathophysiology of depression and show that eIF2α is critical in eliciting ketamine's fast antidepressant effects.

MUFA synthesis and stearoyl-CoA desaturase as a new pharmacological target for modulation of lipid and alpha-synuclein interaction against Parkinson's disease synucleinopathy

Protein pathology spreading within the nervous system, accompanies neurodegeneration and a spectrum of motor and cognitive dysfunctions. Currently available therapies against Parkinson's disease and other synucleinopathies are mostly symptomatic and fail to slow the disease progression in the long term. Modification of α-synuclein (αS) aggregation and toxicity of its pathogenic forms is one of the main goals in neuroprotective approach. Since the discovery of lipid component of Lewy bodies, fatty acids became a crucial, yet little explored target for research. MUFAs (monounsaturated fatty acids) are substrates for lipids, such as phospholipids, triglycerides and cholesteryl esters. They regulate membrane fluidity, take part in signal transduction, cellular differentiation and other fundamental processes. αS and MUFA interactions are essential for Lewy body pathology. αS increases levels of MUFAs, mainly oleic acid, which in turn can enhance αS toxicity and aggregation. Thus, reduction of MUFAs synthesis by inhibition of stearoyl-CoA desaturase (SCD) activity could be the new way to prevent aggravation of αS pathology. Due to the limited distribution in peripheral tissues, SCD5 is a potential target in novel therapies and therefore could be an important starting point in search for disease-modifying neuroprotective therapy. Here we summarize facts about physiology and pathology of αS, explain recently discovered lipid-αS interactions, review SCD function and involved mechanisms, present available SCD inhibitors and discuss their pharmacological potential in disease management. Modulation of MUFA synthesis, decreasing αS and lipid toxicity is clearly essential, but unexplored avenue in pharmacotherapy of Parkinson's disease and synucleinopathies.

Investigation of the causal association between Parkinson's disease and autoimmune disorders: a bidirectional Mendelian randomization study

Background

To date, an increasing number of epidemiological evidence has pointed to potential relationships between Parkinson's disease (PD) and various autoimmune diseases (AIDs), however, no definitive conclusions has been drawn about whether PD is causally related to AIDs risk.

Methods

By employing summary statistics from the latest and most extensive genome-wide association studies (GWAS), we performed a bidirectional two-sample Mendelian randomization (MR) analysis to investigate the causal associations between PD and a variety of 17 AIDs, encompassing multiple sclerosis, neuromyelitis optica spectrum disorder, myasthenia gravis, asthma, inflammatory bowel disease, Crohn's disease, ulcerative colitis, irritable bowel syndrome, celiac disease, primary biliary cirrhosis, primary sclerosing cholangitis, type 1 diabetes, ankylosing spondylitis, rheumatoid arthritis, systemic lupus erythematosus, psoriasis and vitiligo. Inverse-variance weighted (IVW) was adopted as the main statistical approach to obtain the causal estimates of PD on different AIDs, supplemented by a series of complementary analyses (weighted median, MR Egger regression, and MR-PRESSO) for further strengthening the robustness of results.

Results

Our MR findings suggested that genetically predicted higher liability to PD was causally associated with a decreased risk of irritable bowel syndrome (OR = 0.98; 95% CI: 0.96-0.99; P = 0.032). On the contrary, IVW analysis showed a potential positive correlation between genetically determined PD and the incidence of type 1 diabetes (OR = 1.10; 95%CI: 1.02-1.19; P = 0.010). Subsequent MR tests ended up in similar results, confirming our findings were reliable. Additionally, in the reverse MR analyses, we did not identify any evidence to support the causal relationship of genetic predisposition to AIDs with PD susceptibility.

Conclusion

In general, a bifunctional role that PD exerted on the risk of developing AIDs was detected in our studies, both protecting against irritable bowel syndrome occurrence and raising the incidence of type 1 diabetes. Future studies, including population-based observational studies and molecular experiments in vitro and in vivo, are warranted to validate the results of our MR analyses and refine the underlying pathological mechanisms involved in PD-AIDs associations.

Serotonin transporter density in isolated rapid eye movement sleep behavioral disorder

Background/objective

The serotoninergic nervous system is known to play a role in the maintenance of rapid eye movement (REM) sleep. Serotoninergic projections are known to be vulnerable in synucleinopathies. To date, positron emission tomography (PET) studies using serotonin-specific tracers have not been reported in isolated REM sleep behavior disorder (iRBD).

Methods

We conducted a cross-sectional imaging study using serotonin transporter (SERT) 11C-3-amino-4-(2-dimethylaminomethyl-phenylsulfaryl)-benzonitrile (DASB) PET to identify differences in serotonin system integrity between 11 participants with iRBD and 16 older healthy controls.

Results

Participants with iRBD showed lower DASB distribution volume ratios (DVRs) in the total neocortical mantle [1.13 (SD: 0.07) vs. 1.19 (SD: 0.06); t = 2.33, p = 0.028)], putamen [2.07 (SD: 0.19) vs. 2.25 (SD: 0.18); t = 2.55, p = 0.017], and insula [1.26 (SD: 0.11) vs. 1.39 (SD: 0.09); t = 3.58, p = 0.001]. Paradoxical increases relative to controls were seen in cerebellar hemispheres [0.98 (SD: 0.04) vs. 0.95 (SD: 0.02); t = 2.93, p = 0.007)]. No intergroup differences were seen in caudate, substantia nigra, or other brainstem regions with the exception of the dorsal mesencephalic raphe [3.08 (SD: 0.53) vs. 3.47 (SD: 0.48); t = 2.00, p = 0.056] that showed a non-significant trend toward lower values in iRBD.

Conclusions

Insular, neocortical, and striatal serotoninergic terminal loss may be common in prodromal synucleinopathies before the onset of parkinsonism or dementia. Given our small sample size, these results should be interpreted as hypothesis-generating/exploratory in nature.

Effect of Transcranial Direct Current Stimulation (tDCS) on Depression in Parkinson's Disease-A Narrative Review

INTRODUCTION

Depression is the most prevalent comorbid neuropsychiatric condition in individuals with Parkinson's disease (PD), and its underlying mechanisms are not yet fully understood. Current treatment methods are characterised by moderate effectiveness and possible side effects, prompting the search for new non-invasive and safe treatment methods.

METHODS

This narrative review explores the use of transcranial direct current stimulation (tDCS) in the treatment of depression in PD, based on neuropsychological measures. Searches were conducted in the PubMed/Medline, Research Gate, and Cochrane databases.

RESULTS

Nine relevant studies were identified, where depression scores served as either primary or secondary outcomes. Stimulation protocols displayed heterogeneity, especially concerning choice of stimulation site. Patient samples were also heterogeneous. The majority of the studies incorporated anodal stimulation targeting the left dorsolateral prefrontal cortex (DLPFC). The results revealed a reduction in depression scores among PD patients following tDCS. Potential mechanisms through which tDCS may alleviate depression in PD were discussed and recommendations for future research were made.

CONCLUSIONS

Preliminary evidence suggests that tDCS applied anodally to the left DLPFC reduces depression scores in people with PD; however, due to the heterogeneity of the studies analysed, the use of tDCS in this field should be approached with caution and warrants further validation and confirmation.

From Synaptic Physiology to Synaptic Pathology: The Enigma of α-Synuclein

Alpha-synuclein (α-syn) has gained significant attention due to its involvement in neurodegenerative diseases, particularly Parkinson's disease. However, its normal function in the human brain is equally fascinating. The α-syn protein is highly dynamic and can adapt to various conformational stages, which differ in their interaction with synaptic elements, their propensity to drive pathological aggregation, and their toxicity. This review will delve into the multifaceted role of α-syn in different types of synapses, shedding light on contributions to neurotransmission and overall brain function. We describe the physiological role of α-syn at central synapses, including the bidirectional interaction between α-syn and neurotransmitter systems.

Advancements in Genetic and Biochemical Insights: Unraveling the Etiopathogenesis of Neurodegeneration in Parkinson's Disease

Parkinson's disease (PD) is the second most prevalent neurodegenerative movement disorder worldwide, which is primarily characterized by motor impairments. Even though multiple hypotheses have been proposed over the decades that explain the pathogenesis of PD, presently, there are no cures or promising preventive therapies for PD. This could be attributed to the intricate pathophysiology of PD and the poorly understood molecular mechanism. To address these challenges comprehensively, a thorough disease model is imperative for a nuanced understanding of PD's underlying pathogenic mechanisms. This review offers a detailed analysis of the current state of knowledge regarding the molecular mechanisms underlying the pathogenesis of PD, with a particular emphasis on the roles played by gene-based factors in the disease's development and progression. This study includes an extensive discussion of the proteins and mutations of primary genes that are linked to PD, including α-synuclein, GBA1, LRRK2, VPS35, PINK1, DJ-1, and Parkin. Further, this review explores plausible mechanisms for DAergic neural loss, non-motor and non-dopaminergic pathologies, and the risk factors associated with PD. The present study will encourage the related research fields to understand better and analyze the current status of the biochemical mechanisms of PD, which might contribute to the design and development of efficacious and safe treatment strategies for PD in future endeavors.

Neuroplasticity: Pathophysiology and Role in Major Depressive Disorder

Neuroplasticity is characterized by the brain's ability to change its activity in response to extrinsic and intrinsic factors and is thought to be the mechanism behind all brain functions. Neuroplasticity causes structural and functional changes on a molecular level, specifically the growth of different regions in the brain and changes in synaptic and post-synaptic activities. The four types of neuroplasticity are homologous area adaption, compensatory masquerade, cross-modal reassignment, and map expansion. All of these help the brain work around injuries or new information inputs. In addition to baseline physical functions, neuroplasticity is thought to be the basis of emotional and mental regulations and the impairment of it can cause various mental illnesses. Concurrently, these mental illnesses further the damage of synaptic plasticity in the brain. Major depressive disorder (MDD) is one of the most common mental illnesses. It is affected by and accelerates the impairment of neuroplasticity. It is characterized by a chronically depressed state of mind that can impact the patient's daily life, including work life and interests. This review will focus on highlighting the physiological aspects of the disease and the role of neuroplasticity in the pathogenesis and pathology of the disorder. Moreover, the role of monoamine regulation and ketamine uptake will be discussed in terms of their antidepressant effects on the outcomes of MDD.

Exploring the interplay between metabolomics and genetics in Parkinson's disease: Insights from ongoing research and future avenues

Parkinson's disease (PD) is a widespread neurodegenerative disorder, whose complex aetiology remains under construction. While rare variants have been associated with the monogenic PD form, most PD cases are influenced by multiple genetic and environmental aspects. Nonetheless, the pathophysiological pathways and molecular networks involved in monogenic/idiopathic PD overlap, and genetic variants are decisive in elucidating the convergent underlying mechanisms of PD. In this scenario, metabolomics has furnished a dynamic and systematic picture of the synergy between the genetic background and environmental influences that impact PD, making it a valuable tool for investigating PD-related metabolic dysfunctions. In this review, we performed a brief overview of metabolomics current research in PD, focusing on significant metabolic alterations observed in idiopathic PD from different biofluids and strata and exploring how they relate to genetic factors associated with monogenic PD. Dysregulated amino acid metabolism, lipid metabolism, and oxidative stress are the critical metabolic pathways implicated in both genetic and idiopathic PD. By merging metabolomics and genetics data, it is possible to distinguish metabolic signatures of specific genetic backgrounds and to pinpoint subgroups of PD patients who could derive personalized therapeutic benefits. This approach holds great promise for advancing PD research and developing innovative, cost-effective treatments.

Editorial of the Special Issue: Recent Advances in Understanding of the Role of Synuclein Family Members in Health and Disease

Extensive studies of α-synuclein function and dysfunction revealed its involvement in multiple normal and aberrant molecular processes and, consequently, numerous and diverse effects on the neuronal cell biology [...].