Changes in the miRNA-mRNA Regulatory Network Precede Motor Symptoms in a Mouse Model of Multiple System Atrophy: Clinical Implications

Transcriptional dysregulation in the cerebellum triggered by oligodendroglial α-synucleinopathy: insights from a transgenic mouse into the early disease mechanisms of MSA

Multiple system atrophy (MSA) is a fatal neurodegenerative disorder characterized by abnormal accumulation of α-synuclein, progressive neuronal loss, motor impairment and widespread pathological changes, which include significant involvement of the cerebellum. To understand the early molecular mechanisms that might underlie α-synuclein-triggered MSA cerebellar pathology, we performed RNA sequencing (RNA-Seq) of cerebellar samples from a well-established model of MSA. RNA-Seq and differential gene expression analysis was conducted in the PLP-αSyn model of MSA. Cerebellum from two and 12-month-old MSA and wildtype mice were used. Gene ontology (GO) and KEGG enrichment analyses of the differentially expressed genes (DEGs) were performed to explore processes involved in MSA-like disease progression. The overlap between transcriptional changes in MSA and those associated with aging was also evaluated. RNA-Seq analysis demonstrated significant transcriptional dysregulation in cerebellum from MSA mice, even at early stages. GO and KEGG analyses of DEGs point to a potential role of synaptic dysfunction, cellular signaling dysregulation and inflammation in the cerebellar pathology of MSA mice. In addition, those changes exacerbate with disease progression. Additionally, our analysis of aging in both control and PLP-αSyn mice showed that age-related transcriptional changes in mid-aged controls seem to be present in young MSA mice. Thus, MSA-like pathology might lead to an acceleration of aging-related mechanisms. Our findings demonstrate significant cerebellar transcriptional dysregulation triggered by oligodendroglial α-synucleinopathy in PLP-αSyn mice, revealing pathways that might be critical for the early cerebellar pathology of MSA, and that may serve as potential molecular targets for therapeutic interventions in this devastating disorder.

Transcriptomic insights into multiple system atrophy from a PLP-α-synuclein transgenic mouse model

Multiple system atrophy (MSA) is a rare, neurodegenerative disorder with rapid motor and non-motor symptom progression. MSA is characterized by protein aggregations of α-synuclein found in the cytoplasm of oligodendrocytes. Despite this pathological hallmark, there is still little known about the cause of this disease, resulting in poor treatment options and quality of life post-diagnosis. In this study, we investigated differentially expressed genes (DEGs) via RNA-sequencing of brain samples from a validated PLP-α-synuclein transgenic mouse model, identifying a total of 40 DEGs in the PLP group compared to wild-type (WT), with top detected genes being Gm15446, Mcm6, Aldh7a1 and Gm3435. We observed a significant enrichment of immune pathways and endothelial cell genes among the upregulated genes, whereas downregulated genes were significantly enriched for oligodendrocyte and neuronal genes. We then calculated possible overlap of these DEGs with previously profiled human MSA RNA, resulting in the identification of significant downregulation of the Tsr2 gene. Identifying key gene expression profiles specific to MSA patients is crucial to further understanding the cause, and possible prevention, of this rapidly progressive neurodegenerative disorder.

Inflammation in multiple system atrophy

Misfolding protein aggregation inside or outside cells is the major pathological hallmark of several neurodegenerative diseases. Among proteinopathies are neurodegenerative diseases with atypical Parkinsonism and an accumulation of insoluble fibrillary alpha-synuclein (synucleinopathies) or hyperphosphorylated tau protein fragments (tauopathies). As there are no therapies available to slow or halt the progression of these disea ses, targeting the inflammatory process is a promising approach. The inflammatory biomarkers could also help in the differential diagnosis of Parkinsonian syndromes. Here, we review inflammation's role in multiple systems atrophy pathogenesis, diagnosis, and treatment.

Multiple system atrophy: at the crossroads of cellular, molecular and genetic mechanisms

Multiple system atrophy (MSA) is a rare oligodendroglial α-synucleinopathy characterized by neurodegeneration in striatonigral and olivopontocerebellar regions and autonomic brain centres. It causes complex cumulative motor and non-motor disability with fast progression and effective therapy is currently lacking. The difficulties in the diagnosis and treatment of MSA are largely related to the incomplete understanding of the pathogenesis of the disease. The MSA pathogenic landscape is complex, and converging findings from genetic and neuropathological studies as well as studies in experimental models of MSA have indicated the involvement of genetic and epigenetic changes; α-synuclein misfolding, aggregation and spreading; and α-synuclein strain specificity. These studies also indicate the involvement of myelin and iron dyshomeostasis, neuroinflammation, mitochondrial dysfunction and other cell-specific aspects that are relevant to the fast progression of MSA. In this Review, we discuss these findings and emphasize the implications of the complexity of the multifactorial pathogenic cascade for future translational research and its impact on biomarker discovery and treatment target definitions.

microRNA and circRNA in Parkinson's Disease and atypical parkinsonian syndromes

Multiple System Atrophy (MSA) and Progressive Supranuclear Palsy (PSP) are atypical parkinsonian syndromes (APS) with various clinical phenotypes and considerable clinical overlap with idiopathic Parkinson's disease (iPD). This disease heterogeneity makes ante-mortem diagnosis extremely challenging with up to 24% of patients misdiagnosed. Because diagnosis is predominantly clinical, there is great interest in identifying biomarkers for early diagnosis and differentiation of the different types of parkinsonism. Compared to protein biomarkers, microRNAs (miRNAs) and circularRNAs (circRNAs) are stable tissue-specific molecules that can be accurately measured by reverse transcription-quantitative polymerase chain reaction (RT-qPCR). This chapter critically reviews miRNAs and circRNAs as diagnostic biomarkers and therapeutics to differentiate atypical parkinsonian disorders and their role in disease pathogenesis.

A Mouse Model of Multiple System Atrophy: Bench to Bedside

Multiple system atrophy (MSA) is a rare neurodegenerative disorder with unclear etiology, currently difficult and delayed diagnosis, and rapid progression, leading to disability and lethality within 6 to 9 years after symptom onset. The neuropathology of MSA classifies the disease in the group of a-synucleinopathies together with Parkinson's disease and other Lewy body disorders, but features specific oligodendroglial inclusions, which are pathognomonic for MSA. MSA has no efficient therapy to date. Development of experimental models is crucial to elucidate the disease mechanisms in progression and to provide a tool for preclinical screening of putative therapies for MSA. In vitro and in vivo models, based on selective neurotoxicity, a-synuclein oligodendroglial overexpression, and strain-specific propagation of a-synuclein fibrils, have been developed, reflecting various facets of MSA pathology. Over the years, the continuous exchange from bench to bedside and backward has been crucial for the advancing of MSA modelling, elucidating MSA pathogenic pathways, and understanding the existing translational gap to successful clinical trials in MSA. The review discusses specifically advantages and limitations of the PLP-a-syn mouse model of MSA, which recapitulates motor and non-motor features of the human disease with underlying striatonigral degeneration, degeneration of autonomic centers, and sensitized olivopontocerebellar system, strikingly mirroring human MSA pathology.

MicroRNA as Candidate Biomarkers in Atypical Parkinsonian Syndromes: Systematic Literature Review

Background and Objectives: Multiple system atrophy (MSA) and progressive supranuclear palsy (PSP) are rare atypical parkinsonian syndromes, characterized by motor and cognitive symptoms. Their clinical diagnosis is challenging because there are no established biomarkers. Dysregulation of microRNAs (miRNAs/miRs) has been reported to serve an important role in neurodegenerative diseases. However, the miRNA profiles of MSA and PSP patients are rarely reported. The aim of this study was to critically review the role of miRNAs as diagnostic biomarkers to differentiate these atypical parkinsonian disorders and their role in disease pathogenesis. Materials and Methods: A systematic literature search of PubMed was conducted up to February 2022 according the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Results: A total of 15 studies were analyzed. Three studies have shown that miR-9-3p, miR-19a, miR-19b, and miR-24 are potential biomarkers for MSA. In two studies, miR-132 was downregulated, whereas miR-147a and miR-518e were upregulated in the brain tissue of PSP patients. Conclusions: The potential of miRNA is still uncertain as a potential differential diagnostic marker to identify these disorders. Pre-analytical and analytical factors of included studies were important limitations to justify the introduction of miRNAs into clinical practice.

Serum miR-96-5P and miR-339-5P Are Potential Biomarkers for Multiple System Atrophy and Parkinson's Disease

Parkinson's disease (PD) and Multiple System Atrophy (MSA) are progressive neurodegenerative diseases with overlap of symptoms in early stages of disease. No reliable biomarker exists and the diagnosis is mainly based on clinical features. Several studies suggest that miRNAs are involved in PD and MSA pathogenesis. Our goal was to study two serum circulating microRNAs (miR-96-5p and miR-339-5p) as novel biomarkers for the differential diagnosis between PD and MSA. Serum samples were obtained from 51 PD patients, 52 MSA patients and 56 healthy controls (HC). We measured levels of miRNAs using quantitative PCR and compared the levels of miR-96-5p and miR-339-5p among PD, MSA and HC groups using a one-way analysis of variance. Correlations between miRNA expression and clinical data were calculated using Pearson's rho test. We used the miRTarBase to detect miRNA targets and STRING to evaluate co-expression relationship among target genes. MiR-96-5p was significantly increased in MSA patients compared with HC (Fold change (fc): 3.6; p = 0.0001) while it was decreased in PD patients compared with HC (Fold change: 4; p = 0.0002). Higher miR-96-5P levels were directly related to longer disease duration in MSA patients. We observed a significant increase of miR-339-5p in MSA patients compared with PD patients (fc: 2.5; p = 0.00013). miR-339-5p was increased in MSA patients compared with HC (fc: 2.4; p = 0.002). We identified 32 target genes of miR-96-5p and miR-339-5p, some of which are involved in neurodegenerative diseases. The study of those miRNAs could be useful to identify non-invasive biomarkers for early differential diagnosis between PD and MSA.

From Synaptic Protein to Prion: The Long and Controversial Journey of α-Synuclein

Since its discovery 30 years ago, α-synuclein (α-syn) has been one of the most studied proteins in the field of neuroscience. Dozens of groups worldwide have tried to reveal not only its role in the CNS but also in other organs. α-syn has been linked to several processes essential in brain homeostasis such as neurotransmitter release, synaptic function, and plasticity. However, despite the efforts made in this direction, the main function of α-syn is still unknown. Moreover, α-syn became a protein of interest for neurologists and neuroscientists when mutations in its gene were found associated with Parkinson's disease (PD) and even more when α-syn protein deposits were observed in the brain of PD, dementia with Lewy bodies (DLB), and multiple system atrophy (MSA) patients. At present, the abnormal accumulation of α-syn constitutes one of the pathological hallmarks of these disorders, also referred to as α-synucleinopathies, and it is used for post-mortem diagnostic criteria. Whether α-syn aggregation is cause or consequence of the pathogenic events underlying α-synucleinopathies remains unclear and under discussion. Recently, different in vitro and in vivo studies have shown the ability of pathogenic α-syn to spread between cells, not only within the CNS but also from peripheral locations such as the gut, salivary glands, and through the olfactory network into the CNS, inducing abnormal misfolding of endogenous α-syn and leading to neurodegeneration and motor and cognitive impairment in animal models. Thus, it has been suggested that α-syn should be considered a prion protein. Here we present an update of what we know about α-syn function, aggregation and spreading, and its role in neurodegeneration. We also discuss the rationale and findings supporting the hypothetical prion nature of α-syn, its weaknesses, and future perspectives for research and the development of disease-modifying therapies.

MicroRNA: A Key Player for the Interplay of Circadian Rhythm Abnormalities, Sleep Disorders and Neurodegenerative Diseases

Circadian rhythms are endogenous 24-h oscillators that regulate the sleep/wake cycles and the timing of biological systems to optimize physiology and behavior for the environmental day/night cycles. The systems are basically generated by transcription-translation feedback loops combined with post-transcriptional and post-translational modification. Recently, evidence is emerging that additional non-coding RNA-based mechanisms are also required to maintain proper clock function. MicroRNA is an especially important factor that plays critical roles in regulating circadian rhythm as well as many other physiological functions. Circadian misalignment not only disturbs the sleep/wake cycle and rhythmic physiological activity but also contributes to the development of various diseases, such as sleep disorders and neurodegenerative diseases. The patient with neurodegenerative diseases often experiences profound disruptions in their circadian rhythms and/or sleep/wake cycles. In addition, a growing body of recent evidence implicates sleep disorders as an early symptom of neurodegenerative diseases, and also suggests that abnormalities in the circadian system lead to the onset and expression of neurodegenerative diseases. The genetic mutations which cause the pathogenesis of familial neurodegenerative diseases have been well studied; however, with the exception of Huntington's disease, the majority of neurodegenerative diseases are sporadic. Interestingly, the dysfunction of microRNA is increasingly recognized as a cause of sporadic neurodegenerative diseases through the deregulated genes related to the pathogenesis of neurodegenerative disease, some of which are the causative genes of familial neurodegenerative diseases. Here we review the interplay of circadian rhythm disruption, sleep disorders and neurodegenerative disease, and its relation to microRNA, a key regulator of cellular processes.

Elevated Percentage of CD3+ T-Cells and CD4+/CD8+ Ratios in Multiple System Atrophy Patients

α-synuclein is involved in the pathogenesis of multiple system atrophy (MSA) and can be regulated by peripheral immune activation (PIA). We aimed to clarify the correlations between PIA and the prevalence of MSA and to analyze the role of PIA in the progression of the disease. A total of 321 patients with probable MSA and 321 age- and gender-matched healthy controls were included in this study. Lymphocyte subsets, including CD3⁺, CD4⁺, and CD8⁺ cells, and the levels of immunoglobulins IgG, IgM, and IgA were evaluated. The proportions of CD3⁺ and CD4⁺ T-lymphocytes were significantly increased in MSA patients compared with those of normal controls. In addition, the ratio of CD4⁺ to CD8⁺ cells was significantly increased in male MSA patients and IgG concentrations were decreased in female MSA patients. Furthermore, the concentrations of IgM in female MSA patients were dynamically different at various disease stages and gradually decreased from the early stage until the end stage of the disease (p = 0.029). Other detected immunological indexes were not significantly different during the entire disease course. In this study, high proportions of CD3⁺ and CD4⁺ T-lymphocytes and decreased IgG levels were associated with an increased risk for MSA in a Chinese patient population. In addition, PIA may be involved in the progression of MSA.

Role of circular RNAs in brain development and CNS diseases

In mammals, many classes of noncoding RNAs (ncRNAs) are expressed at a much higher level in the brain than in other organs. Recent studies have identified a new class of ncRNAs called circular RNAs (circRNAs), which are produced by back-splicing and fusion of either exons, introns, or both exon-intron into covalently closed loops. The circRNAs are also highly enriched in the brain and increase continuously from the embryonic to the adult stage. Although the functional significance and mechanism of action of circRNAs are still being actively explored, they are thought to regulate the transcription of their host genes and sequestration of miRNAs and RNA binding proteins. Some circRNAs are also shown to have translation potential to form peptides. The expression and abundance of circRNAs seem to be spatiotemporally maintained in a normal brain. Altered expression of circRNAs is also thought to mediate several disorders, including brain-tumor growth, and acute and chronic neurodegenerative disorders by affecting mechanisms such as angiogenesis, neuronal plasticity, autophagy, apoptosis, and inflammation. This review discusses the involvement of various circRNAs in brain development and CNS diseases. A better understanding of the circRNA function will help to develop novel therapeutic strategies to treat CNS complications.

Differential role of triggering receptors expressed on myeloid cells 2 R47H in 3 neurodegenerative diseases based on a systematic review and meta-analysis

BACKGROUND

Recent studies have suggested that the potential functional polymorphism R47H in triggering receptors expressed on myeloid cells 2 (TREM2) is associated with several neurodegenerative diseases, however, the results remain inconclusive. This meta-analysis aimed to investigate the association between TREM2 R47H and the risk for 3 typical neurodegenerative diseases: Alzheimer disease (AD), Parkinson disease (PD), and amyotrophic lateral sclerosis (ALS).

METHODS

A literature review was carried out using PubMed, Medline, and Embase. Data analysis was conducted using Stata 15.0 software. The pooled odds ratio (ORs) and 95% confidence interval (CIs) were calculated.

RESULTS

A total of 35 articles were identified as eligible: 22 on AD, 3 on ALS, 7 on PD, 2 on AD and ALS, and 1 on ALS and PD. The AD set included 23,092 cases and 30,920 controls, the ALS set included 7391 cases and 12,442 controls, and the PD set included 8498 patients and 9161 controls. We found that R47H was associated with an increased risk of AD in the total pooled population (P < .001, OR = 4.02, 95% CI = 3.15-5.13). However, this significant difference existed for Caucasian people (OR = 4.16, 95% CI = 3.24-5.33) but not for Asian or African people. Moreover, we did not find any significant differences in minor allele frequency distribution between the PD and control groups or between the ALS and control groups, not only for the total pooled population but also for the subgroups of different ethnicities.

CONCLUSION

Our study suggested that R47H in the TREM2 gene leads to an increased risk for developing AD, but not for ALS and PD, which adds evidence to the notion that diverse pathogenesis may be involved in different neurogenerative diseases.

MicroRNAs Dysregulation and Metabolism in Multiple System Atrophy

Multiple system atrophy (MSA) is an adult onset, fatal disease, characterized by an accumulation of alpha-synuclein (α-syn) in oligodendroglial cells. MicroRNAs (miRNAs) are small non-coding RNAs involved in post-translational regulation and several biological processes. Disruption of miRNA-related pathways in the central nervous system (CNS) plays an important role in the pathogenesis of neurodegenerative diseases, including MSA. While the exact mechanisms underlying miRNAs in the pathogenesis of MSA remain unclear, it is known that miRNAs can repress the translation of messenger RNAs (mRNAs) that regulate the following pathogenesis associated with MSA: autophagy, neuroinflammation, α-syn accumulation, synaptic transmission, oxidative stress, and apoptosis. In this review, the metabolism of miRNAs and their functional roles in the pathogenesis of MSA are discussed, thereby highlighting miRNAs as potential new biomarkers for the diagnosis of MSA and in increasing our understanding of the disease process.

Glial α-synuclein promotes neurodegeneration characterized by a distinct transcriptional program in vivo

α-Synucleinopathies are neurodegenerative diseases that are characterized pathologically by α-synuclein inclusions in neurons and glia. The pathologic contribution of glial α-synuclein in these diseases is not well understood. Glial α-synuclein may be of particular importance in multiple system atrophy (MSA), which is defined pathologically by glial cytoplasmic α-synuclein inclusions. We have previously described Drosophila models of neuronal α-synucleinopathy, which recapitulate key features of the human disorders. We have now expanded our model to express human α-synuclein in glia. We demonstrate that expression of α-synuclein in glia alone results in α-synuclein aggregation, death of dopaminergic neurons, impaired locomotor function, and autonomic dysfunction. Furthermore, co-expression of α-synuclein in both neurons and glia worsens these phenotypes as compared to expression of α-synuclein in neurons alone. We identify unique transcriptomic signatures induced by glial as opposed to neuronal α-synuclein. These results suggest that glial α-synuclein may contribute to the burden of pathology in the α-synucleinopathies through a cell type-specific transcriptional program. This new Drosophila model system enables further mechanistic studies dissecting the contribution of glial and neuronal α-synuclein in vivo, potentially shedding light on mechanisms of disease that are especially relevant in MSA but also the α-synucleinopathies more broadly.

Alterations in Striatal microRNA-mRNA Networks Contribute to Neuroinflammation in Multiple System Atrophy

Multiple systems atrophy (MSA) is a rare neurodegenerative disorder characterized by the accumulation of α-synuclein in glial cells and neurodegeneration in the striatum, substantia nigra, and cerebellum. Aberrant miRNA regulation has been associated with neurodegeneration, including alterations of specific miRNAs in brain tissue, serum, and cerebrospinal fluid from MSA patients. Still, a causal link between deregulation of miRNA networks and pathological changes in the transcriptome remains elusive. We profiled ~ 800 miRNAs in the striatum of MSA patients in comparison to healthy individuals to identify specific miRNAs altered in MSA. In addition, we performed a parallel screening of 700 transcripts associated with neurodegeneration to determine the impact of miRNA deregulation on the transcriptome. We identified 60 miRNAs with abnormal levels in MSA brains that are involved in extracellular matrix receptor interactions, prion disease, inflammation, ubiquitin-mediated proteolysis, and addiction pathways. Using the correlation between miRNA expression and the abundance of their known targets, miR-124-3p, miR-19a-3p, miR-27b-3p, and miR-29c-3p were identified as key regulators altered in MSA, mainly contributing to neuroinflammation. Finally, our study also uncovered a potential link between Alzheimer's disease (AD) and MSA pathologies that involves miRNAs and deregulation of BACE1. Our results provide a comprehensive appraisal of miRNA alterations in MSA and their effect on the striatal transcriptome, supporting that aberrant miRNA expression is highly correlated with changes in gene transcription associated with MSA neuropathology, in particular those driving inflammation, disrupting myelination, and potentially impacting α-synuclein accumulation via deregulation of autophagy and prion mechanisms.

MicroRNAs to differentiate Parkinsonian disorders: Advances in biomarkers and therapeutics

Parkinsonian disorders are a set of progressive neurodegenerative movement disorders characterized by rigidity, tremor, bradykinesia, postural instability and their distinction has significant implications in terms of management and prognosis. Parkinson's disease (PD) is the most common among them. Its clinical diagnosis is challenging and, it can be misdiagnosed in the early stages. Multiple system atrophy and progressive supranuclear palsy are the close mimickers in early stages, due to overlapping clinical features. MicroRNAs are a class of stable non-coding small RNA molecules implicated in post-transcriptional gene regulation. Current studies propose that miRNAs play an essential role in the pathobiology of multiple neurodegenerative disorders including Parkinsonism, and they seem to be one of the reasonably available methods to aid in the differential diagnosis between PD and related disorders. MicroRNA-based diagnostic biomarkers and therapeutics are a powerful tool to understand and explore the function of the pathogenic gene/s, their mechanism in the disease pathobiology, and to validate drug targets. In this review, we emphasize on the recent developments in the usage of miRNAs as diagnostic biomarkers to identify PD and to differentiate it from atypical parkinsonian conditions, their role in disease pathogenesis, and their possible utility in the therapy of these disorders.

Multiple system atrophy: experimental models and reality

Multiple system atrophy (MSA) is a rapidly progressing fatal synucleinopathy of the aging population characterized by parkinsonism, dysautonomia, and in some cases ataxia. Unlike other synucleinopathies, in this disorder the synaptic protein, α-synuclein (α-syn), predominantly accumulates in oligodendroglial cells (and to some extent in neurons), leading to maturation defects of oligodendrocytes, demyelination, and neurodegeneration. The mechanisms through which α-syn deposits occur in oligodendrocytes and neurons in MSA are not completely clear. While some studies suggest that α-syn might transfer from neurons to glial cells, others propose that α-syn might be aberrantly overexpressed by oligodendroglial cells. A number of in vivo models have been developed, including transgenic mice overexpressing α-syn under oligodendroglial promoters (e.g.: MBP, PLP, and CNP). Other models have been recently developed either by injecting synthetic α-syn fibrils or brain homogenates from patients with MSA into wild-type mice or by using viral vectors expressing α-syn under the MBP promoter in rats and non-human primates. Each of these models reproduces some of the neuropathological and functional aspects of MSA; however, none of them fully replicate the spectrum of MSA. Understanding better the mechanisms of how α-syn accumulates in oligodendrocytes and neurons will help in developing better models that recapitulate various pathogenic aspects of MSA in combination with translatable biomarkers of early stages of the disease that are necessary to devise disease-modifying therapeutics for MSA.

microRNAs in Parkinson's Disease: From Pathogenesis to Novel Diagnostic and Therapeutic Approaches

Parkinson’s disease (PD) is the most prevalent central nervous system (CNS) movement disorder and the second most common neurodegenerative disease overall. PD is characterized by the progressive loss of dopaminergic (DAergic) neurons in the substantia nigra pars compacta (SNpc) within the midbrain, accumulation of alpha-synuclein (α-SYN) in Lewy bodies and neurites and excessive neuroinflammation. The neurodegenerative processes typically begin decades before the appearance of clinical symptoms. Therefore, the diagnosis is achievable only when the majority of the relevant DAergic neurons have already died and for that reason available treatments are only palliative at best. The causes and mechanism(s) of this devastating disease are ill-defined but complex interactions between genetic susceptibility and environmental factors are considered major contributors to the etiology of PD. In addition to the role of classical gene mutations in PD, the importance of regulatory elements modulating gene expression has been increasingly recognized. One example is the critical role played by microRNAs (miRNAs) in the development and homeostasis of distinct populations of neurons within the CNS and, in particular, in the context of PD. Recent reports demonstrate how distinct miRNAs are involved in the regulation of PD genes, whereas profiling approaches are unveiling variations in the abundance of certain miRNAs possibly relevant either to the onset or to the progression of the disease. In this review, we provide an overview of the miRNAs recently found to be implicated in PD etiology, with particular focus on their potential relevance as PD biomarkers, as well as their possible use in PD targeted therapy.

Serum microRNA expression profiling in patients with multiple system atrophy

Multiple system atrophy (MSA) is a sporadic neurodegenerative disease that is pathologically characterized by α-synuclein positive glial cytoplasmic inclusions in oligodendrocytes. The clinical diagnosis of MSA is often challenging as there are no established biomarkers and diagnoses are now based on clinical findings alone. At present, the etiology and pathogenesis of MSA are unclear. It has been reported that dysregulation of microRNA (miRNA/miR) serves an important role in neurodegenerative disorders including Alzheimer's disease, Parkinson's disease and amyotrophic lateral sclerosis. The miRNA profile of patients with MSA remains to be established. The present study investigated the serum miRNA expression level of 10 patients with MSA, using microarray chips including 668 miRNAs. It was identified that 50 miRNAs were significantly upregulated and 17 miRNAs were significantly downregulated in the serum of the patients with MSA. The most upregulated miRNA was miR-16, which may induce the accumulation of α-synuclein. The target genes of some miRNAs upregulated in MSA (including miR-17, 20a, 24, 25, 30d and 451) were associated with autophagy-associated molecules. The present study concluded that the expression pattern of miRNAs may be a clinical biomarker for MSA and targeting these miRNAs may provide a novel treatment for MSA.

Exosomes, an Unmasked Culprit in Neurodegenerative Diseases

Exosomes are extracellular nanovesicles (30–100 nm) generated from endosomal membranes and known to be released by all cell lineages of the Central Nervous System (CNS). They constitute important vesicles for the secretion and transport of multilevel information, including signaling, toxic, and regulatory molecules. Initially thought to have a function merely in waste disposal, the involvement of exosomes in neuronal development, maintenance, and regeneration through its paracrine and endocrine signaling functions has drawn particular attention in recent years. These vesicles, being involved in the clearance and cell-to-cell spreading of toxic molecules, have been naturally implicated in aging, and in several neurodegenerative diseases associated with pathological conversion of proteins, as well as in the transport of other disease-associated molecules, such as nucleic acids or pro-inflammatory cytokines. Our understanding of such unique form of communication may provide not only answers about (patho)physiological processes in the brain, but can also offer means to exploit these vesicles as vehicles for the delivery of biologically relevant molecules or as tools to monitor brain diseases in a non-invasive way. A promising field in expansion, the study of exosomes and related extracellular vesicles has just commenced to unveil their potential as therapeutic tools for brain disorders as well as biomarkers of disease state.

Role of microRNAs in the Regulation of α-Synuclein Expression: A Systematic Review

Growing evidence suggests that increased levels of α-synuclein might contribute to the pathogenesis of Parkinson’s disease (PD) and therefore, it is crucial to understand the mechanisms underlying α-synuclein expression. Recently, microRNAs (miRNAs) have emerged as key regulators of gene expression involved in several diseases such as PD and other neurodegenerative disorders. A systematic literature search was performed here to identify microRNAs that directly or indirectly impact in α-synuclein expression/accumulation and describe its mechanism of action. A total of 27 studies were incorporated in the review article showing evidences that six microRNAs directly bind and regulate α-synuclein expression while several miRNAs impact on α-synuclein expression indirectly by targeting other genes. In turn, α-synuclein overexpression also impacts miRNAs expression, indicating the complex network between miRNAs and α-synuclein. From the current knowledge on the central role of α-synuclein in PD pathogenesis/progression, miRNAs are likely to play a crucial role at different stages of PD and might potentially be considered as new PD therapeutic approaches.

Investigating miRNA-mRNA regulatory networks using crosslinking immunoprecipitation methods for biomarker and target discovery in cancer

MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the post-transcriptional level. Recently, different experimental approaches, such as RNA Sequencing, crosslinking immunoprecipitation (CLIP) methods and its variations, together with computational approaches have been developed to elucidate the miRNA-mRNA targetome. Areas covered: This report focuses on comparing the different experimental and computational approaches, describing their advantages and disadvantages and providing several examples of preclinical (in vitro and in vivo) and clinical studies that have identified miRNA target genes in various tumour types, including breast, ovary, colorectal and pancreas. Expert commentary: The combination of CLIP methods with bioinformatic analyses is essential to better predict miRNA-mRNA interactions and associate their specific pathways within the extensive regulatory network. Nevertheless, further studies are needed to overcome the difficulties these methods have, in order to find a gold standard method that identifies, without any bias, the regulatory association between miRNAs and their target mRNAs.

Multiple system atrophy: pathogenic mechanisms and biomarkers

Multiple system atrophy (MSA) is a unique proteinopathy that differs from other α-synucleinopathies since the pathological process resulting from accumulation of aberrant α-synuclein (αSyn) involves the oligodendroglia rather than neurons, although both pathologies affect multiple parts of the brain, spinal cord, autonomic and peripheral nervous system. Both the etiology and pathogenesis of MSA are unknown, although animal models have provided insight into the basic molecular changes of this disorder. Accumulation of aberrant αSyn in oligodendroglial cells and preceded by relocation of p25α protein from myelin to oligodendroglia results in the formation of insoluble glial cytoplasmic inclusions that cause cell dysfunction and demise. These changes are associated with proteasomal, mitochondrial and lipid transport dysfunction, oxidative stress, reduced trophic transport, neuroinflammation and other noxious factors. Their complex interaction induces dysfunction of the oligodendroglial-myelin-axon-neuron complex, resulting in the system-specific pattern of neurodegeneration characterizing MSA as a synucleinopathy with oligodendroglio-neuronopathy. Propagation of modified toxic αSyn species from neurons to oligodendroglia by "prion-like" transfer and its spreading associated with neuronal pathways result in a multi-system involvement. No reliable biomarkers are currently available for the clinical diagnosis and prognosis of MSA. Multidisciplinary research to elucidate the genetic and molecular background of the deleterious cycle of noxious processes, to develop reliable diagnostic biomarkers and to deliver targets for effective treatment of this hitherto incurable disorder is urgently needed.