MicroRNA (miRNA) Signaling in the Human CNS in Sporadic Alzheimer's Disease (AD)-Novel and Unique Pathological Features

MicroRNA (miRNA) as a biomarker for diagnosis, prognosis, and therapeutics molecules in neurodegenerative disease

Neurodegenerative diseases that include Alzheimer's disease (AD), amyotrophic lateral sclerosis (ALS), Parkinson's disease (PD), Huntington's disease (HD), and multiple sclerosis (MS) that arise due to numerous causes like protein accumulation and autoimmunity characterized by neurologic depletion which lead to incapacity in normal physiological function such as thinking and movement in these patients. Glial cells perform an important role in protective neuronal function; in the case of neuroinflammation, glial cell dysfunction can promote the development of neurodegenerative diseases. miRNA that participates in gene regulation and plays a vital role in many biological processes in the body; in the central nervous system (CNS), it can play an essential part in neural maturation and differentiation. In neurodegenerative diseases, miRNA dysregulation occurs, enhancing the development of these diseases. In this review, we discuss neurodegenerative disease (Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), and multiple sclerosis (MS)) and how miRNA is preserved as a diagnostic biomarker or therapeutic agent in these disorders. Finally, we highlight miRNA as therapy.

Sex-associated microRNAs potentially implicated in sporadic Alzheimer's disease (sAD)

BACKGROUND

The onset and pathology of sporadic Alzheimer's disease (sAD) seem to be affected by both sex and genetic mechanisms. Evidence supports that the high prevalence of sAD in women, worldwide, may be attributed to an interplay among aging, sex, and lifestyle, influenced by genetics, metabolic changes, and hormones. Interestingly, epigenetic mechanisms such as microRNAs (miRNAs), known as master regulators of gene expression, may contribute to this observed sexual dimorphism in sAD.

OBJECTIVES

To investigate the potential impact of sex-associated miRNAs on processes manifesting sAD pathology, as described by the Tau-driven Adverse Outcome Pathway (AOP) leading to memory loss.

METHODS

Using publicly available human miRNA datasets, sex-biased miRNAs, defined as differentially expressed by sex in tissues possibly affected by sAD pathology, were collected. In addition, sex hormone-related miRNAs were also retrieved from the literature. The compiled sex-biased and sex hormone-related miRNAs were further plugged into the dysregulated processes of the Tau-driven AOP for memory loss.

RESULTS

Several miRNAs, previously identified as sex-associated, were implicated in dysregulated processes associated with the manifestation of sAD pathology. Importantly, the described pathology processes were not confined to a particular sex. A mechanistic-based approach utilizing miRNAs was adopted in order to elucidate the link between sex and biological processes potentially involved in the development of memory loss.

CONCLUSIONS

The identification of sex-associated miRNAs involved in the early processes manifesting memory loss may shed light to the complex molecular mechanisms underlying sAD pathogenesis in a sex-specific manner.

MicroRNAs in tear fluids predict underlying molecular changes associated with Alzheimer's disease

Extracellular circulating microRNAs (miRNAs) have been discussed as potential biomarkers for Alzheimer's disease (AD) diagnosis. As the retina is a part of the CNS, we hypothesize that miRNAs expression levels in the brain, particularly neocortex-hippocampus, eye tissues, and tear fluids are similar at different stages of AD progression. Ten miRNA candidates were systematically investigated in transgenic APP-PS1 mice, noncarrier siblings, and C57BL/6J wild-type controls at young and old ages. Relative expression levels of tested miRNAs revealed a similar pattern in both APP-PS1 mice and noncarrier siblings when compared with age- and sex-matched wild-type controls. However, the differences seen in expression levels between APP-PS1 mice and noncarrier siblings could possibly have resulted from underlying molecular etiology of AD. Importantly, miRNAs associated with amyloid beta (Aβ) production (-101a, -15a, and -342) and proinflammation (-125b, -146a, and -34a) showed significant up-regulations in the tear fluids with disease progression, as tracked by cortical Aβ load and reactive astrogliosis. Overall, for the first time, the translational potential of up-regulated tear fluid miRNAs associated with AD pathogenesis was comprehensively demonstrated.

MicroRNA (miRNA) Complexity in Alzheimer's Disease (AD)

AD is a complex, progressive, age-related neurodegenerative disorder representing the most common cause of senile dementia and neurological dysfunction in our elderly domestic population. The widely observed heterogeneity of AD is a reflection of the complexity of the AD process itself and the altered molecular-genetic mechanisms operating in the diseased human brain and CNS. One of the key players in this complex regulation of gene expression in human pathological neurobiology are microRNAs (miRNAs) that, through their actions, shape the transcriptome of brain cells that normally associate with very high rates of genetic activity, gene transcription and messenger RNA (mRNA) generation. The analysis of miRNA populations and the characterization of their abundance, speciation and complexity can further provide valuable clues to our molecular-genetic understanding of the AD process, especially in the sporadic forms of this common brain disorder. Current in-depth analyses of high-quality AD and age- and gender-matched control brain tissues are providing pathophysiological miRNA-based signatures of AD that can serve as a basis for expanding our mechanistic understanding of this disorder and the future design of miRNA- and related RNA-based therapeutics. This focused review will consolidate the findings from multiple laboratories as to which are the most abundant miRNA species, both free and exosome-bound in the human brain and CNS, which miRNA species appear to be the most prominently affected by the AD process and review recent developments and advancements in our understanding of the complexity of miRNA signaling in the hippocampal CA1 region of AD-affected brains.

MicroRNAs and MAPKs: Evidence of These Molecular Interactions in Alzheimer's Disease

Alzheimer's disease (AD) is a neurodegenerative disorder known to be the leading cause of dementia worldwide. Many microRNAs (miRNAs) were found deregulated in the brain or blood of AD patients, suggesting a possible key role in different stages of neurodegeneration. In particular, mitogen-activated protein kinases (MAPK) signaling can be impaired by miRNA dysregulation during AD. Indeed, the aberrant MAPK pathway may facilitate the development of amyloid-beta (Aβ) and Tau pathology, oxidative stress, neuroinflammation, and brain cell death. The aim of this review was to describe the molecular interactions between miRNAs and MAPKs during AD pathogenesis by selecting evidence from experimental AD models. Publications ranging from 2010 to 2023 were considered, based on PubMed and Web of Science databases. According to obtained data, several miRNA deregulations may regulate MAPK signaling in different stages of AD and conversely. Moreover, overexpressing or silencing miRNAs involved in MAPK regulation was seen to improve cognitive deficits in AD animal models. In particular, miR-132 is of particular interest due to its neuroprotective functions by inhibiting Aβ and Tau depositions, as well as oxidative stress, through ERK/MAPK1 signaling modulation. However, further investigations are required to confirm and implement these promising results.

Endogenous miRNA-Based Innate-Immunity against SARS-CoV-2 Invasion of the Brain

The severe acute respiratory syndrome Coronavirus-2 (SARS-CoV-2), the causative agent of COVID-19, possesses an unusually large positive-sense, single-stranded viral RNA (ssvRNA) genome of about ~29,903 nucleotides (nt). In many respects, this ssvRNA resembles a very large, polycistronic messenger RNA (mRNA) possessing a 5'-methyl cap (m7GpppN), a 3'- and 5'-untranslated region (3'-UTR, 5'-UTR), and a poly-adenylated (poly-A+) tail. As such, the SARS-CoV-2 ssvRNA is susceptible to targeting by small non-coding RNA (sncRNA) and/or microRNA (miRNA), as well as neutralization and/or inhibition of its infectivity via the human body's natural complement of about ~2650 miRNA species. Depending on host cell and tissue type, in silico analysis, RNA sequencing, and molecular-genetic investigations indicate that, remarkably, almost every single human miRNA has the potential to interact with the primary sequence of SARS-CoV-2 ssvRNA. Individual human variation in host miRNA abundance, speciation, and complexity among different human populations and additional variability in the cell and tissue distribution of the SARS-CoV-2 angiotensin converting enzyme-2 (ACE2) receptor (ACE2R) appear to further contribute to the molecular-genetic basis for the wide variation in individual host cell and tissue susceptibility to COVID-19 infection. In this paper, we review recently described aspects of the miRNA and ssvRNA ribonucleotide sequence structure in this highly evolved miRNA-ssvRNA recognition and signaling system and, for the first time, report the most abundant miRNAs in the control superior temporal lobe neocortex (STLN), an anatomical area involved in cognition and targeted by both SARS-CoV-2 invasion and Alzheimer's disease (AD). We further evaluate important factors involving the neurotropic nature of SARS-CoV-2 and miRNAs and ACE2R distribution in the STLN that modulate significant functional deficits in the brain and CNS associated with SARS-CoV-2 infection and COVID-19's long-term neurological effects.

Non-Coding RNAs in the Regulation of Hippocampal Neurogenesis and Potential Treatment Targets for Related Disorders

Non-coding RNAs (ncRNAs), including miRNAs, lncRNAs, circRNAs, and piRNAs, do not encode proteins. Nonetheless, they have critical roles in a variety of cellular activities-such as development, neurogenesis, degeneration, and the response to injury to the nervous system-via protein translation, RNA splicing, gene activation, silencing, modifications, and editing; thus, they may serve as potential targets for disease treatment. The activity of adult neural stem cells (NSCs) in the subgranular zone of the hippocampal dentate gyrus critically influences hippocampal function, including learning, memory, and emotion. ncRNAs have been shown to be involved in the regulation of hippocampal neurogenesis, including proliferation, differentiation, and migration of NSCs and synapse formation. The interaction among ncRNAs is complex and diverse and has become a major topic within the life science. This review outlines advances in research on the roles of ncRNAs in modulating NSC bioactivity in the hippocampus and discusses their potential applications in the treatment of illnesses affecting the hippocampus.

Lipopolysaccharides (LPSs) as Potent Neurotoxic Glycolipids in Alzheimer's Disease (AD)

Lipopolysaccharides (LPSs) are microbiome-derived glycolipids that are among the most potent pro-inflammatory neurotoxins known. In Homo sapiens, the major sources of LPSs are gastrointestinal (GI)-tract-resident facultative anaerobic Gram-negative bacilli, including Bacteroides fragilis and Escherichia coli. LPSs have been abundantly detected in aged human brain by multiple independent research investigators, and an increased abundance of LPSs around and within Alzheimer's disease (AD)-affected neurons has been found. Microbiome-generated LPSs and other endotoxins cross GI-tract biophysiological barriers into the systemic circulation and across the blood-brain barrier into the brain, a pathological process that increases during aging and in vascular disorders, including 'leaky gut syndrome'. Further evidence indicates that LPSs up-regulate pro-inflammatory transcription factor complex NF-kB (p50/p65) and subsequently a set of NF-kB-sensitive microRNAs, including miRNA-30b, miRNA-34a, miRNA-146a and miRNA-155. These up-regulated miRNAs in turn down-regulate a family of neurodegeneration-associated messenger RNA (mRNA) targets, including the mRNA encoding the neuron-specific neurofilament light (NF-L) chain protein. While NF-L has been reported to be up-regulated in peripheral biofluids in AD and other progressive and lethal pro-inflammatory neurodegenerative disorders, NF-L is significantly down-regulated within neocortical neurons, and this may account for neuronal atrophy, loss of axonal caliber and alterations in neuronal cell shape, modified synaptic architecture and network deficits in neuronal signaling capacity. This paper reviews and reveals the most current findings on the neurotoxic aspects of LPSs and how these pro-inflammatory glycolipids contribute to the biological mechanism of progressive, age-related and ultimately lethal neurodegenerative disorders. This recently discovered gut-microbiota-derived LPS-NF-kB-miRNA-30b-NF-L pathological signaling network: (i) underscores a direct positive pathological link between the LPSs of GI-tract microbes and the inflammatory neuropathology, disordered cytoskeleton, and disrupted synaptic-signaling of the AD brain and stressed human brain cells in primary culture; and (ii) is the first example of a microbiome-derived neurotoxic glycolipid having significant detrimental miRNA-mediated actions on the expression of NF-L, an abundant filamentous protein known to be important in the maintenance of neuronal and synaptic homeostasis.

Traumatic Brain Injury Leads to Alterations in Contusional Cortical miRNAs Involved in Dementia

There is compelling evidence that head injury is a significant environmental risk factor for Alzheimer's disease (AD) and that a history of traumatic brain injury (TBI) accelerates the onset of AD. Amyloid-β plaques and tau aggregates have been observed in the post-mortem brains of TBI patients; however, the mechanisms leading to AD neuropathology in TBI are still unknown. In this study, we hypothesized that focal TBI induces changes in miRNA expression in and around affected areas, resulting in the altered expression of genes involved in neurodegeneration and AD pathology. For this purpose, we performed a miRNA array in extracts from rats subjected to experimental TBI, using the controlled cortical impact (CCI) model. In and around the contusion, we observed alterations of miRNAs associated with dementia/AD, compared to the contralateral side. Specifically, the expression of miR-9 was significantly upregulated, while miR-29b, miR-34a, miR-106b, miR-181a and miR-107 were downregulated. Via qPCR, we confirmed these results in an additional group of injured rats when compared to naïve animals. Interestingly, the changes in those miRNAs were concomitant with alterations in the gene expression of mRNAs involved in amyloid generation and tau pathology, such as β-APP cleaving enzyme (BACE1) and Glycogen synthase-3-β (GSK3β). In addition increased levels of neuroinflammatory markers (TNF-α), glial activation, neuronal loss, and tau phosphorylation were observed in pericontusional areas. Therefore, our results suggest that the secondary injury cascade in TBI affects miRNAs regulating the expression of genes involved in AD dementia.

Sporadic Alzheimer's Disease- and Neurotoxicity-Related microRNAs Affecting Key Events of Tau-Driven Adverse Outcome Pathway Toward Memory Loss

BACKGROUND

A complex network of aging-related homeostatic pathways that are sensitive to further deterioration in the presence of genetic, systemic, and environmental risk factors, and lifestyle, is implicated in the pathogenesis of progressive neurodegenerative diseases, such as sporadic (late-onset) Alzheimer's disease (sAD).

OBJECTIVE

Since sAD pathology and neurotoxicity share microRNAs (miRs) regulating common as well as overlapping pathological processes, environmental neurotoxic compounds are hypothesized to exert a risk for sAD initiation and progression.

METHODS

Literature search for miRs associated with human sAD and environmental neurotoxic compounds was conducted. Functional miR analysis using PathDip was performed to create miR-target interaction networks.

RESULTS

The identified miRs were successfully linked to the hypothetical starting point and key events of the earlier proposed tau-driven adverse outcome pathway toward memory loss. Functional miR analysis confirmed most of the findings retrieved from literature and revealed some interesting findings. The analysis identified 40 miRs involved in both sAD and neurotoxicity that dysregulated processes governing the plausible adverse outcome pathway for memory loss.

CONCLUSION

Creating miR-target interaction networks related to pathological processes involved in sAD initiation and progression, and environmental chemical-induced neurotoxicity, respectively, provided overlapping miR-target interaction networks. This overlap offered an opportunity to create an alternative picture of the mechanisms underlying sAD initiation and early progression. Looking at initiation and progression of sAD from this new angle may open for new biomarkers and novel drug targets for sAD before the appearance of the first clinical symptoms.

The emerging role of noncoding RNAs in neuroinflammation: Implications in pathogenesis and therapeutic approaches

Noncoding RNAs (ncRNAs) are important regulators of gene expression in different cell processes. Due to their ability in monitoring neural development genes, these transcripts confer neurons with the potential to exert broad control over the expression of genes for performing neurobiological functions. Although the change of ncRNA expression in different neurodegenerative diseases has been reviewed elsewhere, only recent evidence drove our attention to unravel the involvement of these molecules in neuroinflammation within these devastating disorders. Remarkably, the interactions between ncRNAs and inflammatory pathways are not fully recognized. Therefore, this review has focused on the interplay between diverse inflammatory pathways and the related ncRNAs, including microRNAs, long noncoding RNAs, and competing endogenous RNAs in Alzheimer's disease, Parkinson's diseases, amyotrophic lateral sclerosis, epilepsy, multiple sclerosis, Huntington's disease, and prion diseases. Providing novel insights in the field of combining biomarkers is a critical step for using them as diagnostic tools and therapeutic targets in clinical settings.

Physical exercise promotes brain remodeling by regulating epigenetics, neuroplasticity and neurotrophins

Exercise has been shown to have beneficial effects on brain functions in humans and animals. Exercise can improve memory and learning in age-related neurodegenerative diseases. In animal models, physical exercise regulates epigenetics, promotes synaptic plasticity and hippocampal neurogenesis, regulates the expression levels of neurotrophic factors, and improves cognitive function. Therefore, exercise is very important for brain rehabilitation and remodeling. The purpose of this review is to explore the mechanisms by which exercise exerts positive effects on brain function. This knowledge implies that physical exercise can be used as a non-drug therapy for neurological diseases.

MiR-10a, 27a, 34b/c, and 300 Polymorphisms are Associated with Ischemic Stroke Susceptibility and Post-Stroke Mortality

A recent study of the ischemic stroke described the roles played by miRNAs in the downregulation of specific cell-cycle gene expression and it is thought to require the development of biomarkers for the prognostic of ischemic stroke. Here, we hypothesized that four miRNA polymorphisms (miR-10a, miR-27a, miR-34b/c, and miR-300) may affect stroke susceptibility and mortality. Blood samples were collected from 530 patients and 403 controls. Genetic polymorphisms were detected by polymerase chain reaction (PCR)-restriction fragment length polymorphism analysis and real-time PCR. We found that the miR-300 rs12894467 TC genotype and the dominant model (AOR: 2.069, p-value: 0.017; AOR: 1.931, p-value: 0.027) were significantly associated with an increased risk for the ischemic stroke subtype. In Cox proportional hazard regression models, the miR-10a rs3809783 A>T and miR-34b/c rs4938723 T>C polymorphisms were associated with the mortality rates among ischemic stroke patients. We found that a miR-300 polymorphism was associated with increased ischemic stroke susceptibility among the Korean population. Additionally, polymorphisms in miR-10a and miR-34b/c were associated with the increased or decreased mortality of ischemic stroke patients. This study marks the first report of an association between ischemic stroke and miRNA polymorphisms (miR-10aA>T, miR-27aT>C, miR-34b/cT>C, and miR-300T>C) in the Korean population.

Expression of miR-28-3p in patients with Alzheimer's disease before and after treatment and its clinical value

Expression of miR-28-3p in patients with Alzheimer's disease (AD) before and after treatment and clinical value of miR-28-3p were determined. There were three groups: 68 AD patients treated with donepezil combined with basic therapy in The People's Hospital of Shouguang collected as an AD group, 70 patients with mild cognitive impairment (MCI) as an MCI group, and 75 healthy people as a normal group. Serum miR-28-3p was detected by qRT-PCR. The Montreal cognitive assessment scale (MoCA), mini mental state examination scale (MMSE), activities of daily living scale (ADL) and homocysteine (Hcy) were adopted to assess patients before and after treatment. miR-28-3p in normal group was significantly lower than that in other two groups, and miR-28-3p in MCI group was significantly lower than that in AD group (P<0.001). miR-28-3p correlated with the course and severity of patients. miR-28-3p in AD group after treatment was significantly lower than that before treatment (P<0.001). ADL and Hcy of AD patients after treatment were significantly lower than before treatment (P<0.05), and MMSE and MoCA after treatment were significantly higher than before treatment (P<0.05). Before and after treatment, miR-28-3p was significantly positively correlated with ADL score and Hcy level, but negatively correlated with MMSE score and MoCA score. Analysis of the working characteristic curve of the patients indicated that miR-28-3p can be used for diagnosis of AD patients. Donepezil therapy may reduce miR-28-3p level to alleviate the symptoms of AD patients, and miR-28-3p level can be used as an early diagnosis and prognosis evaluation of AD patients.

The molecular effect of a polymorphic microRNA binding site of Wolfram syndrome 1 gene in dogs

Background

Although the molecular function of wolframin remains unclear, the lack of this protein is known to cause stress in the endoplasmic reticulum. Some variants in the Wolfram Syndrome 1 gene (WFS1) were associated with various neuropsychiatric disorders in humans, such as aggressiveness, impulsivity and anxiety.

Results

Here we present an in silico study predicting a single nucleotide polymorphism (rs852850348) in the canine WFS1 gene which was verified by direct sequencing and was genotyped by a PCR-based technique. We found that the rs852850348 polymorphism is located in a putative microRNA (cfa-miR-8834a and cfa-miR-1838) binding site. Therefore, the molecular effect of allelic variants was studied in a luciferase reporter system that allowed assessing gene expression. We demonstrated that the variant reduced the activity of the reporter protein expression in an allele-specific manner. Additionally, we performed a behavioral experiment and investigated the association with this locus to different performance in this test. Association was found between food possessivity and the studied WFS1 gene polymorphism in the Border collie breed.

Conclusions

Based on our findings, the rs852850348 locus might contribute to the genetic risk of possessivity behavior of dogs in at least one breed and might influence the regulation of wolframin expression.

Vesicular Transport of Encapsulated microRNA between Glial and Neuronal Cells

Exosomes (EXs) and extracellular microvesicles (EMVs) represent a diverse assortment of plasma membrane-derived nanovesicles, 30–1000 nm in diameter, released by all cell lineages of the central nervous system (CNS). They are examples of a very active and dynamic form of extracellular communication and the conveyance of biological information transfer essential to maintain homeostatic neurological functions and contain complex molecular cargoes representative of the cytoplasm of their cells of origin. These molecular cargoes include various mixtures of proteins, lipids, proteolipids, cytokines, chemokines, carbohydrates, microRNAs (miRNA) and messenger RNAs (mRNA) and other components, including end-stage neurotoxic and pathogenic metabolic products, such as amyloid beta (Aβ) peptides. Brain microglia, for example, respond to both acute CNS injuries and degenerative diseases with complex reactions via the induction of a pro-inflammatory phenotype, and secrete EXs and EMVs enriched in selective pathogenic microRNAs (miRNAs) such as miRNA-34a, miRNA-125b, miRNA-146a, miRNA-155, and others that are known to promote neuro-inflammation, induce complement activation, disrupt innate–immune signaling and deregulate the expression of neuron-specific phosphoproteins involved in neurotropism and synaptic signaling. This communication will review our current understanding of the trafficking of miRNA-containing EXs and EMVs from astrocytes and “activated pro-inflammatory” microglia to target neurons in neurodegenerative diseases with an emphasis on Alzheimer’s disease wherever possible.

Transcriptomics in Alzheimer's Disease: Aspects and Challenges

Alzheimer's disease (AD) is the most common cause of dementia. Although the heritability of AD is high, the knowledge of the disease-associated genes, their expression, and their disease-related pathways remain limited. Hence, finding the association between gene dysfunctions and pathological mechanisms, such as neuronal transports, APP processing, calcium homeostasis, and impairment in mitochondria, should be crucial. Emerging studies have revealed that changes in gene expression and gene regulation may have a strong impact on neurodegeneration. The mRNA-transcription factor interactions, non-coding RNAs, alternative splicing, or copy number variants could also play a role in disease onset. These facts suggest that understanding the impact of transcriptomes in AD may improve the disease diagnosis and also the therapies. In this review, we highlight recent transcriptome investigations in multifactorial AD, with emphasis on the insights emerging at their interface.

Future avenues for Alzheimer's disease detection and therapy: liquid biopsy, intracellular signaling modulation, systems pharmacology drug discovery

When Alzheimer's disease (AD) disease-modifying therapies will be available, global healthcare systems will be challenged by a large-scale demand for clinical and biological screening. Validation and qualification of globally accessible, minimally-invasive, and time-, cost-saving blood-based biomarkers need to be advanced. Novel pathophysiological mechanisms (and related candidate biomarkers) - including neuroinflammation pathways (TREM2 and YKL-40), axonal degeneration (neurofilament light chain protein), synaptic dysfunction (neurogranin, synaptotagmin, α-synuclein, and SNAP-25) - may be integrated into an expanding pathophysiological and biomarker matrix and, ultimately, integrated into a comprehensive blood-based liquid biopsy, aligned with the evolving ATN + classification system and the precision medicine paradigm. Liquid biopsy-based diagnostic and therapeutic algorithms are increasingly employed in Oncology disease-modifying therapies and medical practice, showing an enormous potential for AD and other brain diseases as well. For AD and other neurodegenerative diseases, newly identified aberrant molecular pathways have been identified as suitable therapeutic targets and are currently investigated by academia/industry-led R&D programs, including the nerve-growth factor pathway in basal forebrain cholinergic neurons, the sigma1 receptor, and the GTPases of the Rho family. Evidence for a clinical long-term effect on cognitive function and brain health span of cholinergic compounds, drug candidates for repositioning programs, and non-pharmacological multidomain interventions (nutrition, cognitive training, and physical activity) is developing as well. Ultimately, novel pharmacological paradigms, such as quantitative systems pharmacology-based integrative/explorative approaches, are gaining momentum to optimize drug discovery and accomplish effective pathway-based strategies for precision medicine. This article is part of the special issue on 'The Quest for Disease-Modifying Therapies for Neurodegenerative Disorders'.

microRNA-9-5p alleviates blood-brain barrier damage and neuroinflammation after traumatic brain injury

The level of microRNA‐9‐5p (miRNA‐9‐5p) in brain tissues is significantly changed after traumatic brain injury (TBI). However, the effect of miRNA‐9‐5p for brain function in TBI has not been elucidated. In this study, a controlled cortical impact model was used to induce TBI in Sprague–Dawley rats, and an oxygen glucose deprivation model was used to mimic the pathological state in vitro. Brain microvascular endothelial cells (BMECs) and astrocytes were extracted from immature Sprague–Dawley rats and cocultured to reconstruct blood–brain barrier (BBB) in vitro. The results show that the level of miRNA‐9‐5p was significantly increased in brain tissues after TBI, and up‐regulation of miRNA9‐5p contributed to the recovery of neurological function. Up‐regulation of miRNA‐9‐5p with miRNA agomir may significantly alleviate apoptosis, neuroinflammation, and BBB damage in rats after TBI. Moreover, a dual luciferase reporter assay confirmed that miRNA‐9‐5p is a post‐transcriptional modulator of Ptch‐1. In in vitro experiments, the results confirmed that up‐regulation of miRNA‐9‐5p with miRNA mimic alleviates cellular apoptosis, inflammatory response, and BBB damage mainly by inhibiting Ptch‐1. In addition, we found that the activation of Hedgehog pathway was accompanied by inhibition of NF‐κB/MMP‐9 pathway in the BMECs treated with miRNA‐9‐5p mimic. Taken together, these results indicate that up‐regulation of miRNA‐9‐5p alleviates BBB damage and neuroinflammatory responses by activating the Hedgehog pathway and inhibiting NF‐κB/MMP‐9 pathway, which promotes the recovery of neurological function after TBI.

Down syndrome and Alzheimer's disease: common molecular traits beyond the amyloid precursor protein

Alzheimer’s disease (AD) is the most prevalent type of dementia. Down syndrome (DS) is the leading genetic risk factor for Early-Onset AD, prematurely presenting the classic pathological features of the brain with AD. Augmented gene dosage, including the APP gene, could partially cause this predisposition. Recent works have revealed that alterations in chromosome location due to the extra Chromosome 21, as well as epigenetic modifications, could promote changes in gene expression other than those from Chromosome 21. As a result, similar pathological features and cellular dysfunctions in DS and AD, including impaired autophagy, lysosomal activity, and mitochondrial dysfunction, could be controlled beyond APP overexpression. In this review, we highlight some recent data regarding the origin of the shared features between DS and AD and explore the mechanisms concerning cognitive deficiencies in DS associated with dementia, which could shed some light into the search for new therapeutic targets for AD treatment.

Influence and mechanism of miR-99a suppressing development of colorectal cancer (CRC) with diabetes mellitus (DM)

OBJECTIVE

This study aimed to identify the changes of miRNAs in colorectal cancer (CRC) complicated with diabetes mellitus (DM) (CRC + DM) tissues and their potential effects.

METHODS

The changes of miRNAs in CRC + DM tissues were determined by miRNA microarray. The expression levels of miR-99a in 40 clinical specimens and 6 CRC cell lines were determined by qRT-PCR. The capacity for miR-99a to induce cell proliferation and invasion was examined with miR-99a-overexpressing HCT-116 cells. The relative mTOR mRNA and protein levels were determined by qRT-PCR and Western blotting, respectively, in HCT-116 cells transfected with miR-99a. The dual luciferase assay was performed to confirm the direct regulation of miR-99a on mTOR 3'-UTR. The HCT-116 cells were treated with 100 mg/L advanced glycation end products (AGEs); then, the mTOR expression levels were determined by qRT-PCR, Western blotting, and immunohistochemistry.

RESULTS

Seventeen miRNAs were found to be differentially expressed among normal tissue, CRC tissue, and CRC with DM tissue, including 15 upregulated and 2 downregulated with fold changs of more than 2 times. qRT-PCR confirmed that miR-99a was downregulated in CRC and CRC + DM tissues. In addition, miR-99a overexpression remarkably impaired CRC cell proliferation and metastasis, and negatively regulated mTOR signaling through direct binding to the 3'-UTR of mTOR. AGEs could suppress miR-99a and stimulate mTOR signaling in CRC cells. Increased mTOR was also identified in CRC with DM tissues.

CONCLUSION

Our findings indicate that miR-99a is a potential marker and therapeutic target of CRC complicated with DM, and that AGEs impair miR-99a-overactivated mTOR signaling in CRC with DM patients, which promotes CRC development.

Down Syndrome, Ageing and Epigenetics

During the past decades, life expectancy of subjects with Down syndrome (DS) has greatly improved, but age-specific mortality rates are still important and DS subjects are characterized by an acceleration of the ageing process, which affects particularly the immune and central nervous systems. In this chapter, we will first review the characteristics of the ageing phenomenon in brain and in immune system in DS and we will then discuss the biological hallmarks of ageing in this specific population. Finally, we will also consider in detail the knowledge on epigenetics in DS, particularly DNA methylation.

The long-non-coding RNA NEAT1 is a novel target for Alzheimer's disease progression via miR-124/BACE1 axis

OBJECTIVES

Long-non-coding RNAs (lncRNAs) have been involved in central nervous system recently. A number of studies have reported that lncRNA NEAT1 exerts critical roles in neurodegenerative disorder. Beta-site amyloid precursor protein cleaving enzyme 1 (BACE1) has been reported to exert function in the accumulation of amyloid-β (Aβ). Moreover, BACE1 acts as a target of miR-124 in the progression of AD. So far, the biological role and underlying mechanisms of NEAT1 and miR-124 in AD remains elusive.

METHODS

The relative NEAT1 and miR-124 expression was examined by qRT-PCR in the tissues and cells line of AD. Cell apoptosis was examined by FACS. Luciferase reporter assay was performed to verify that miR-124 is a direct target of NEAT1, and BACE1 is a downstream target of miR-124. qRT-PCR and western blot analysis were also performed to determinate the BACE1 and the phosphorylation of tau protein.

RESULTS

NEAT1 was notably up-regulated and miR-124 was remarkably down-regulated in AD mouse model. Knockdown of NEAT1 or overexpression of miR-124 showed the protective effects on cellular AD model induced by Aβ. Moreover, miR-124 expression could be up- and down-regulated by suppression or overexpression of NEAT1, respectively. In addition, the expression of BACE1 was the potential functional target of miR-124. These findings suggested that NEAT1 might play a vital role in the development of AD by regulating miR-124/BACE1 axis.

DISCUSSION

The present study showed that NEAT1 worked as a regulating factor to promote the development of AD via modulating miR-124/BACE1 axis, which might be considered as a novel target in AD treatment.

MicroRNAs in Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive and devastating neurodegenerative disorder. It is the leading cause of dementia in the world’s rapidly growing aging population. The characteristics of AD are memory loss and cognitive impairment, meaning patients cannot carry out their daily activities independently. The increase of AD cases poses heavy burdens on families, society and the economy. Despite frequent efforts being made to research the etiology of AD, the causes of AD remain unknown, and no curative treatments are available yet. The pathological hallmarks of AD are amyloid plaques and neurofibrillary tangles in the brain. MicroRNAs are endogenous ∼22 nucleotides non-coding RNAs that could regulate gene expression at a post-transcriptional level by transcript degradation or translation repression. MicroRNAs are involved in many biological processes and diseases, particularly multifactorial diseases, providing an excellent tool with which to research the mechanisms of these diseases. AD is a multifactorial disorder, and accumulating evidence shows that microRNAs play a critical role in the pathogenesis of AD. In this review, we will highlight the effect of microRNAs in different pathological processes throughout AD progression.

Role of microRNA (miRNA) and Viroids in Lethal Diseases of Plants and Animals. Potential Contribution to Human Neurodegenerative Disorders

Both plants and animals have adopted a common strategy of using ~18-25-nucleotide small non-coding RNAs (sncRNAs), known as microRNAs (miRNAs), to transmit DNA-based epigenetic information. miRNAs (i) shape the total transcriptional output of individual cells; (ii) regulate and fine-tune gene expression profiles of cell clusters, and (iii) modulate cell phenotype in response to environmental stimuli and stressors. These miRNAs, the smallest known carriers of gene-encoded post-transcriptional regulatory information, not only regulate cellular function in healthy cells but also act as important mediators in the development of plant and animal diseases. Plants possess their own specific miRNAs; at least 32 plant species have been found to carry infectious sncRNAs called viroids, whose mechanisms of generation and functions are strikingly similar to those of miRNAs. This review highlights recent remarkable and sometimes controversial findings in miRNA signaling in plants and animals. Special attention is given to the intriguing possibility that dietary miRNAs and/or sncRNAs can function as mobile epigenetic and/or evolutionary linkers between different species and contribute to both intra- and interkingdom signaling. Wherever possible, emphasis has been placed on the relevance of these miRNAs to the development of human neurodegenerative diseases, such as Alzheimer's disease. Based on the current available data, we suggest that such xeno-miRNAs may (i) contribute to the beneficial properties of medicinal plants, (ii) contribute to the negative properties of disease-causing or poisonous plants, and (iii) provide cross-species communication between kingdoms of living organisms involving multiple epigenetic and/or potentially pathogenic mechanisms associated with the onset and pathogenesis of various diseases.

Comparative study of microRNA profiling in one Chinese Family with PSEN1 G378E mutation

MicroRNAs are not widely studied in familial Alzheimer's disease cases, whether the microRNA profilings in familial Alzheimer's disease patients are similar to the sporadic AD patients is not known. This study aims to investigate the differential expression of microRNAs (miRNAs) associated with early-onset familial Alzheimer's disease (EO-FAD) in a Chinese family. We performed the gene mutation analysis in a family clinically diagnosed of EO-FAD. Micro-arrays were used to profile the miRNAs in cerebrospinal fluid of 2 affected members, 2 unaffected carriers and 2 mutation negative controls. The clinical presentation confirmed the EO-FAD diagnosis, and a recurrent mutation of the PSEN1 p.G378E was found in the family. The result showed that in the miRNAs expression profile, a total of 166 miRNAs were up-regulated and 3 miRNAs were down-regulated in the affected individuals compared with mutation negative individuals. But after Benjamini Hochberg FDR correction, only 25 miRNAs were significantly up-regulated and no miRNA was down-regulated, the levels of miR-30a-5p, miR-4758-3p and let-7a-3p were elevated significantly. Compared with mutation negative controls, 21 miRNAs were up-regulated and 18 microRNAs were down-regulated in the unaffected mutation carriers, after Benjamini Hochberg FDR correction, miR-345-5p was up-regulated and miR-4795-3p was down-regulated in the unaffected mutation carriers. And there was no difference between the affected members and unaffected mutation carriers. GO database showed that the top biological processes affected by the predicted target genes are nucleic acid binding transcription factor activity and transcription factor activity (sequence-specific DNA binding) (GO:0001071 and GO:0003700). The result of KEGG pathways showed 64 pathways were implicated in the regulatory network. The present study identified the miRNA profiling of Chinese siblings with G378E mutation in the PSEN1. Compared with mutation negative controls, the levels of 25 miRNAs including miR-30a-5p, miR-4758-3p and let-7a-3p were elevated significantly in the affected members, miR-345-5p was up-regulated and miR-4795-3p was down-regulated in the unaffected mutation carriers. Our study showed the microRNA profilings in the cases of a EO-FAD family with PSEN1 p.G378E mutation, but because of the individuals in the family was small, the results in other types of EO-FAD still need further studied.

Micro RNA as a potential blood-based epigenetic biomarker for Alzheimer's disease

As the prevalence of Alzheimer's disease (AD) increases, the search for a definitive, easy to access diagnostic biomarker has become increasingly important. Micro RNA (miRNA), involved in the epigenetic regulation of protein synthesis, is a biological mark which varies in association with a number of disease states, possibly including AD. Here we comprehensively review methods and findings from 26 studies comparing the measurement of miRNA in blood between AD cases and controls. Thirteen of these studies used receiver operator characteristic (ROC) analysis to determine the diagnostic accuracy of identified miRNA to predict AD, and three studies did this with a machine learning approach. Of 8098 individually measured miRNAs, 23 that were differentially expressed between AD cases and controls were found to be significant in two or more studies. Only six of these were consistent in their direction of expression between studies (miR-107, miR-125b, miR-146a, miR-181c, miR-29b, and miR-342), and they were all shown to be down regulated in individuals with AD compared to controls. Of these directionally concordant miRNAs, the strongest evidence was for miR-107 which has also been shown in previous studies to be involved in the dysregulation of proteins involved in aspects of AD pathology, as well as being consistently downregulated in studies of AD brains. We conclude that imperative to the discovery of reliable and replicable miRNA biomarkers of AD, standardised methods of measurements, appropriate statistical analysis, utilization of large datasets with machine learning approaches, and comprehensive reporting of findings is urgently needed.

Regulation of MicroRNAs-Mediated Autophagic Flux: A New Regulatory Avenue for Neurodegenerative Diseases With Focus on Prion Diseases

Prion diseases are fatal neurological disorders affecting various mammalian species including humans. Lack of proper diagnostic tools and non-availability of therapeutic remedies are hindering the control strategies for prion diseases. MicroRNAs (miRNAs) are abundant endogenous short non-coding essential RNA molecules that negatively regulate the target genes after transcription. Several biological processes depend on miRNAs, and altered profiles of these miRNAs are potential biomarkers for various neurodegenerative diseases, including prion diseases. Autophagic flux degrades the misfolded prion proteins to reduce chronic endoplasmic reticulum stress and enhance cell survival. Recent evidence suggests that specific miRNAs target and regulate the autophagic mechanism, which is critical for alleviating cellular stress. miRNAs-mediated regulation of these specific proteins involved in the autophagy represents a new target with highly significant therapeutic prospects. Here, we will briefly describe the biology of miRNAs, the use of miRNAs as potential biomarkers with their credibility, the regulatory mechanism of miRNAs in major neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and prion diseases, degradation pathways for aggregated prion proteins, the role of autophagy in prion diseases. Finally, we will discuss the miRNAs-modulated autophagic flux in neurodegenerative diseases and employ them as potential therapeutic intervention strategy in prion diseases.

MicroRNA Expression Levels Are Altered in the Cerebrospinal Fluid of Patients with Young-Onset Alzheimer's Disease

Clinical diagnosis of Alzheimer’s disease (AD) prior to the age of 65 years is classified as young-onset (YOAD), whereas diagnosis after the age of 65 years is considered late-onset (LOAD). Although rare autosomal mutations more commonly associate with YOAD, most YOAD and LOAD cases are sporadic. YOAD and LOAD share amyloid and tau pathology, but many YOAD patients show increased disease severity and rate of progression. The current study examined the microRNA (miRNA) expression profile from exosomes isolated from the cerebrospinal fluid (CSF) of YOAD patients with biomarker-confirmed AD. Results uncovered miR-16-5p, miR-125b-5p, miR-451a, and miR-605-5p as differentially expressed in the CSF-derived exosomes of YOAD patients when compared with healthy controls (HC). In a cohort of LOAD patients, miR-125b-5p, miR-451a, and miR-605-5p were similarly altered in expression, but miR-16-5p showed similar expression to control. Analysis of the mRNA targets of these miRNAs revealed transcripts enriched in biological processes relevant to the post-mortem posterior cingulate cortex transcriptome in YOAD from a previously published microarray study, including those related to neuron projections, synaptic signaling, metabolism, apoptosis, and the immune system. Hence, these miRNAs represent novel targets for uncovering disease mechanisms and for biomarker development in both YOAD and LOAD.

Microbiome-Mediated Upregulation of MicroRNA-146a in Sporadic Alzheimer's Disease

The first indication of a potential mechanistic link between the pathobiology of the human gastrointestinal (GI)-tract microbiome and its contribution to the pathogenetic mechanisms of sporadic Alzheimer's disease (AD) came a scant 4 years ago (1). Ongoing research continues to strengthen the hypothesis that neurotoxic microbial-derived components of the GI tract microbiome can cross aging GI tract and blood-brain barriers and contribute to progressive proinflammatory neurodegeneration, as exemplified by the AD-process. Of central interest in these recent investigations are the pathological roles played by human GI tract resident Gram-negative anaerobic bacteria and neurotropic viruses-two prominent divisions of GI tract microbiome-derived microbiota-which harbor considerable pathogenic potential. It is noteworthy that the first two well-studied microbiota-the GI tract abundant Gram-negative bacteria Bacteroides fragilis and the neurotropic herpes simplex virus-1 both share a final common pathway of NF-κB (p50/p65) activation and microRNA-146a induction with ensuing pathogenic stimulation of innate-immune and neuroinflammatory pathways. These appear to strongly contribute to the inflammation-mediated amyloidogenic neuropathology of AD. This communication: (i) will review recent research contributions that have expanded our understanding of the nature of the translocation of microbiome-derived neurotoxins-across biophysiological barriers; (ii) will assess multiple-recent investigations of the induction of the proinflammatory pathogenic microRNA-146a by these two prominent classes of human microbiota; and (iii) will discuss the role of molecular neurobiology and mechanistic contribution of polymicrobial infections to AD-type neuropathological change.

Up-regulated Pro-inflammatory MicroRNAs (miRNAs) in Alzheimer's disease (AD) and Age-Related Macular Degeneration (AMD)

Alzheimer's disease (AD) of the brain neocortex and age-related macular degeneration (AMD) of the retina are two complex neurodegenerative disorders, which (i) involve the progressive dysregulation and deterioration of multiple neurobiological signaling pathways, (ii) exhibit the temporal accumulation of pro-inflammatory lesions including the amyloid beta (Aβ) peptide-containing senile plaques of AD and the drusen of AMD, and (iii) culminate in an insidious inflammatory neurodegeneration ending, respectively, in neural cell atrophy and death and progressive loss of cognition and central visual function. Recent independent research studies have indicated that AD and AMD share common, pathological signaling defects and disease mechanisms at the molecular genetic level. Using high-integrity total RNA samples pooled from AD brain and AMD retina, microfluidic hybridization miRNA arrays, and bioinformatics, the current study was undertaken to quantify microRNA (miRNA) speciation and complexity common to both AD and AMD. These small non-coding (sncRNAs) are known to post-transcriptionally regulate multiple neurobiological pathways and an abundance of research information has already been generated on the roles of these miRNAs in pathological situations involving inflammatory neuropathology and neural cell decline. Here, for the first time, we report the sequence and abundance of a septet of sncRNAs including miRNA-7, miRNA-9-1, miRNA-23a/miRNA-27a, miRNA-34a, miRNA-125b-1, miRNA-146a, and miRNA-155 that are significantly increased in abundance and common to both AD-affected superior temporal lobe neocortex (Brodmann A22) and the AMD-affected macular region of the retina. Bioinformatics, miRNA-mRNA complementarity, next-gen RNA sequencing, and feature alignment analysis further indicate that these 7 up-regulated miRNAs have the potential to interact with and down-regulate ~ 9460 target messenger RNAs (mRNAs; about 3.5% of the genome) involved in the synchronization of amyloid production and clearance, phagocytosis, innate-immune, pro-inflammatory, and neurotrophic signaling and/or synaptogenesis in diseased tissues.

Circulating brain-enriched microRNAs as novel biomarkers for detection and differentiation of neurodegenerative diseases

Background

Minimally invasive specific biomarkers of neurodegenerative diseases (NDs) would facilitate patient selection and disease progression monitoring. We describe the assessment of circulating brain-enriched microRNAs as potential biomarkers for Alzheimer’s disease (AD), frontotemporal dementia (FTD), Parkinson’s disease (PD), and amyotrophic lateral sclerosis (ALS).

Methods

In this case-control study, the plasma samples were collected from 250 research participants with a clinical diagnosis of AD, FTD, PD, and ALS, as well as from age- and sex-matched control subjects (n = 50 for each group), recruited from 2003 to 2015 at the University of Pennsylvania Health System, including the Alzheimer’s Disease Center, the Parkinson’s Disease and Movement Disorders Center, the Frontotemporal Degeneration Center, and the Amyotrophic Lateral Sclerosis Clinic. Each group was randomly divided into training and confirmation sets of equal size. To evaluate the potential of circulating microRNAs enriched in specific brain regions affected by NDs and present in synapses as biomarkers of NDs, the levels of 37 brain-enriched and inflammation-associated microRNAs in the plasma of all participants were measured using individual qRT-PCR. A “microRNA pair” approach was used for data normalization.

Results

MicroRNA pairs and their combinations (classifiers) capable of differentiating NDs from control and from each other were defined using independently and jointly analyzed training and confirmation datasets. AD, PD, FTD, and ALS are differentiated from control with accuracy of 0.89, 0.90, 0.88, and 0.83 (AUCs, 0.96, 0.96, 0.94, and 0.93), respectively; NDs are differentiated from each other with accuracy ranging from 0.77 (AUC, 0.87) for AD vs. FTD to 0.93 (AUC, 0.98) for AD vs. ALS. The data further indicate sex dependence of some microRNA markers. The average increase in accuracy in distinguishing ND from control for all and male/female groups is 0.06; the largest increase is for ALS, from 0.83 for all participants to 0.92/0.98 for male/female participants.

Conclusions

The work presented here suggests the possibility of developing microRNA-based diagnostics for detection and differentiation of NDs. Larger multicenter clinical studies are needed to further evaluate circulating brain-enriched microRNAs as biomarkers for NDs and to investigate their association with other ND biomarkers in clinical trial settings.

Electronic supplementary material

The online version of this article (doi:10.1186/s13195-017-0316-0) contains supplementary material, which is available to authorized users.

MicroRNAs in Hearing Disorders: Their Regulation by Oxidative Stress, Inflammation and Antioxidants

MicroRNAs (miRs) are small non-coding single-stranded RNAs that bind to their complimentary sequences in the 3′-untranslated regions (3′-UTRs) of the target mRNAs that prevent their translation into the corresponding proteins. Since miRs are strongly expressed in cells of inner ear and play a role in regulating their differentiation, survival and function, alterations in their expression may be involved in the pathogenesis of hearing disorders. Although increased oxidative stress and inflammation are involved in initiation and progression of hearing disorders, it is unknown whether the mechanisms of damage produced by these biochemical events on inner ear cells are mediated by altering the expression of miRs. In neurons and non-neuronal cells, reactive oxygen species (ROS) and pro-inflammatory cytokines mediate their damaging effects by altering the expression of miRs. Preliminary data indicate that a similar mechanism of damage on hair cells produced by oxidative stress may exist in this disease. Antioxidants protect against hearing disorders induced by ototoxic agents or adverse health conditions; however, it is unknown whether the protective effects of antioxidants in hearing disorders are mediated by changing the expression of miRs. Antioxidants protect mammalian cells against oxidative damage by changing the expression of miRs. Therefore, it is proposed that a similar mechanism of protection by antioxidants against stress may be found in hearing disorders. This review article discusses novel concepts: (a) alterations in the expression of miRs may be involved in the pathogenesis of hearing disorders; (b) presents evidence from neurons and glia cells to show that oxidative stress and pro-inflammatory cytokines mediate their damaging effects by altering the expression of miRs; and proposes that a similar mechanism of damage by these biochemical events may be found in hearing loss; and (c) present data to show that antioxidants protect mammalian cells against oxidative by altering the expression of miRs. A similar role of antioxidants in protecting against hearing disorders is put forward. New studies are proposed to fill the gaps in the areas listed above.

A microRNA cluster (let-7c, miRNA-99a, miRNA-125b, miRNA-155 and miRNA-802) encoded at chr21q21.1-chr21q21.3 and the phenotypic diversity of Down's syndrome (DS; trisomy 21)

Down's syndrome (DS) is the most common genetic cause of intellectual disability and cognitive deficit attributable to a naturally-occurring abnormality of gene dosage. DS is caused by a triplication of all or part of human chromosome 21 (chr21) and currently there are no effective treatments for this incapacitating disorder of neurodevelopment. First described by the English physician John Langdon Down in 1862, propelled by the invention of karyotype analytical techniques in the early 1950s and the discovery in 1959 by the French geneticist Jerome Lejune that DS resulted from an extra copy of chr21, DS was the first neurological disorder linking a chromosome dosage imbalance to a defect in intellectual development with ensuing cognitive disruption. Especially over the last 60 years, it has been repeatedly demonstrated that DS is not an easily defined disease entity but rather possesses a remarkably wide variability in the 'phenotypic spectrum' associated with this trisomic disorder. This commentary describes the presence of a 5 member cluster of chr21-encoded microRNAs (miRNAs) that includes let-7c, miRNA-99a, miRNA-125b, miRNA-155 and miRNA-802 located on the long arm of human chr21, spanning the chr21q21.1-chr21q21.3 region and flanking the beta amyloid precursor (βAPP) gene, and reviews the potential contribution of these 5 miRNAs to the remarkably diverse DS phenotype.

Chromosome 21-Encoded microRNAs (mRNAs): Impact on Down's Syndrome and Trisomy-21 Linked Disease

Down's syndrome (DS; also known as trisomy 21; T21) is caused by a triplication of all or part of human chromosome 21 (chr21). DS is the most common genetic cause of intellectual disability attributable to a naturally-occurring imbalance in gene dosage. DS incurs huge medical, healthcare, and socioeconomic costs, and there are as yet no effective treatments for this incapacitating human neurogenetic disorder. There is a remarkably wide variability in the 'phenotypic spectrum' associated with DS; the progression of symptoms and the age of DS onset fluctuate, and there is further variability in the biophysical nature of the chr21 duplication. Besides the cognitive disruptions and dementia in DS patients other serious health problems such as atherosclerosis, altered lipogenesis, Alzheimer's disease, amyotrophic lateral sclerosis (Lou Gehrig's disease), autoimmune disease, various cancers including lymphoma, leukemia, glioma and glioblastoma, status epilepticus, congenital heart disease, hypotonia, manic depression, prostate cancer, Usher syndrome, motor disorders, Hirschsprung disease, and various physical anomalies such as early aging occur at elevated frequencies, and all are part of the DS 'phenotypic spectrum.' This communication will review the genetic link between these fore-mentioned diseases and a small group of just five stress-associated microRNAs (miRNAs)-that include let-7c, miRNA-99a, miRNA-125b, miRNA-155, and miRNA-802-encoded and clustered on the long arm of human chr21 and spanning the chr21q21.1-chr21q21.3 region.

Serum microRNAs are non-invasive biomarkers for the presence and progression of subarachnoid haemorrhage

miRNAs are important regulators of translation and have been associated with the pathogenesis of a number of cardiovascular diseases including stroke and may be possible prognostic biomarkers. The purpose of the present study was to determine the expression levels of miRNAs in the sera of subarachnoid haemorrhage (SAH) patients and to evaluate their relationships with the severity and clinical outcome of SAH. Serum samples on day 3 after the onset of SAH were subjected to microarray analysis with Exqion miRCURY^TM LNA array and quantitative PCR analysis. Serum samples from SAH patients (n=60) and healthy controls (n=10) were subjected to quantitative PCR analysis. The severities and clinical outcomes of the SAH patients were evaluated with the WFNS grade and the Modified Rankin Scale (mRS). Three miRNAs, miR-502-5p, miR-1297 and miR-4320 were significantly up-regulated in the sera of SAH patients when compared with the healthy controls. The serum miR-502-5p and miR-1297 levels were significantly higher in the patients with severe SAH and a poor outcome than in those with mild SAH and a good outcome (P<0.05). The areas under the receiver operating characteristic (ROC) curves (AUCs) of miR-502-5p, miR-1297 and miR-4320 to distinguish the SAH patients from the healthy controls were 0.958 (P<0.001), 0.950 (P<0.001) and 0.843 (P<0.001) respectively. Taken together, these results indicate that miR-502-5p and miR-1297 are potentially valuable indicators of the diagnosis, severity and prognosis of SAH, and miR-4320 was a potentially valuable indicator of the diagnosis of SAH.

Deficiency in the Ubiquitin Conjugating Enzyme UBE2A in Alzheimer's Disease (AD) is Linked to Deficits in a Natural Circular miRNA-7 Sponge (circRNA; ciRS-7)

Our understanding of the highly specialized functions for small non-coding single-stranded RNA (ssRNA) in the transcriptome of the human central nervous system (CNS) continues to evolve. Circular RNAs (circRNAs), a recently discovered class of ssRNA enriched in the brain and retina, are extremely stable and intrinsically resilient to degradation by exonuclease. Conventional methods of ssRNA, microRNA (miRNA), or messenger RNA (mRNA) detection and quantitation requiring free ribonucleotide ends may have considerably underestimated the quantity and significance of CNS circRNA in the CNS. Highly-specific small ssRNAs, such as the ~23 nucleotide (nt) Homo sapien microRNA-7 (hsa-miRNA-7; chr 9q21.32), are not only abundant in the human limbic system but are, in addition, associated with a ~1400 nt circRNA for miRNA-7 (ciRS-7) in the same anatomical region. Structurally, ciRS-7 contains about ~70 tandem anti-miRNA-7 sequences and acts as an endogenous, anti-complementary miRNA-7 “sponge” that attracts, binds, and, hence, quenches, natural miRNA-7 functions. Using a combination of DNA and miRNA array technologies, enhanced LED-Northern and Western blot hybridization, and the magnesium-dependent exoribonuclease and circRNA-sensitive probe RNaseR, here we provide evidence of a significantly misregulated ciRS-7-miRNA-7-UBE2A circuit in sporadic Alzheimer’s disease (AD) neocortex (Brodmann A22) and hippocampal CA1. Deficits in ciRS-7-mediated “sponging events”, resulting in excess ambient miRNA-7 appear to drive the selective down-regulation in the expression of miRNA-7-sensitive mRNA targets, such as that encoding the ubiquitin conjugating enzyme E2A (UBE2A; chr Xq24). UBE2A, which normally serves as a central effector in the ubiquitin-26S proteasome system, coordinates the clearance of amyloid peptides via proteolysis, is known to be depleted in sporadic AD brain and, hence, contributes to amyloid accumulation and the formation of senile plaque deposits. Dysfunction of circRNA-miRNA-mRNA regulatory systems appears to represent another important layer of epigenetic control over pathogenic gene expression programs in the human CNS that are targeted by the sporadic AD process.

Over-Expressed Pathogenic miRNAs in Alzheimer's Disease (AD) and Prion Disease (PrD) Drive Deficits in TREM2-Mediated Aβ42 Peptide Clearance

One prominent and distinguishing feature of progressive, age-related neurological diseases such as Alzheimer’s disease (AD) and prion disease (PrD) is the gradual accumulation of amyloids into dense, insoluble end-stage protein aggregates. These polymorphic proteolipid lesions are known to contribute to immunogenic and inflammatory pathology in these insidious and fatal disorders of the human central nervous system (CNS). For example, the evolution of self-aggregating amyloid-beta (Aβ) peptides, such as the 42 amino acid Aβ42 peptide monomer into higher order aggregates are largely due to: (1) the inability of natural processes to clear them from the cellular environment; and/or (2) the overproduction of these amyloid monomers which rapidly mature into higher order oligomers, fibrils and insoluble, end-stage senile plaques. Cells of the CNS such as microglial (MG) cells have evolved essential homeostatic mechanisms to clear Aβ peptides to avoid their accumulation, however, when defective, these clearance mechanisms become overwhelmed and excessive deposition and aggregation of these amyloids result. This ‘Perspectives’ paper will highlight some emerging concepts on the up-regulation of an inducible microRNA-34a in AD and PrD that drives the down-regulation of the amyloid sensing- and clearance receptor protein TREM2 (the triggering receptor expressed in myeloid/microglial cells). The impairment of this inducible, miRNA-34a-regulated TREM2- and MG-cell based amyloid clearance mechanism may thereby contribute to the age-related amyloidogenesis associated with both AD and PrD.

MicroRNAs: Key Regulators in the Central Nervous System and Their Implication in Neurological Diseases

MicroRNAs (miRNAs) are a class of small, well-conserved noncoding RNAs that regulate gene expression post-transcriptionally. They have been demonstrated to regulate a lot of biological pathways and cellular functions. Many miRNAs are dynamically regulated during central nervous system (CNS) development and are spatially expressed in adult brain indicating their essential roles in neural development and function. In addition, accumulating evidence strongly suggests that dysfunction of miRNAs contributes to neurological diseases. These observations, together with their gene regulation property, implicated miRNAs to be the key regulators in the complex genetic network of the CNS. In this review, we first focus on the ways through which miRNAs exert the regulatory function and how miRNAs are regulated in the CNS. We then summarize recent findings that highlight the versatile roles of miRNAs in normal CNS physiology and their association with several types of neurological diseases. Subsequently we discuss the limitations of miRNAs research based on current studies as well as the potential therapeutic applications and challenges of miRNAs in neurological disorders. We endeavor to provide an updated description of the regulatory roles of miRNAs in normal CNS functions and pathogenesis of neurological diseases.