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

The role of peripheral blood microRNAs in the pathogenesis and treatment response of age-related macular degeneration

Age-related macular degeneration is a leading cause of vision loss in aging populations, driven by complex interactions between genetic, environmental, and molecular factors. MicroRNAs have emerged as crucial regulators of cellular processes such as oxidative stress, inflammation, and angiogenesis, all of which contribute to AMD pathogenesis. This narrative review aims to summarize the involvement of peripheral blood microRNAs in the pathogenesis of AMD, focusing on key pathways such as oxidative stress, inflammation, and angiogenesis. Additionally, it explores their potential as biomarkers for predicting treatment response, particularly to anti-VEGF therapies. The potential of miRNAs as noninvasive biomarkers for early diagnosis and personalized treatment strategies is also explored, highlighting future directions for research.

Decoding molecular mechanisms: brain aging and Alzheimer's disease

The complex morphological, anatomical, physiological, and chemical mechanisms within the aging brain have been the hot topic of research for centuries. The aging process alters the brain structure that affects functions and cognitions, but the worsening of such processes contributes to the pathogenesis of neurodegenerative disorders, such as Alzheimer's disease. Beyond these observable, mild morphological shifts, significant functional modifications in neurotransmission and neuronal activity critically influence the aging brain. Understanding these changes is important for maintaining cognitive health, especially given the increasing prevalence of age-related conditions that affect cognition. This review aims to explore the age-induced changes in brain plasticity and molecular processes, differentiating normal aging from the pathogenesis of Alzheimer's disease, thereby providing insights into predicting the risk of dementia, particularly Alzheimer's disease.

microRNAs as molecular tools for brain health: Neuroprotective potential in neurodegenerative disorders

As research on microRNAs (miRNAs) advances, it is becoming increasingly clear that these small molecules play crucial roles in the central nervous system (CNS). They are involved in various essential neuronal functions, with specific miRNAs preferentially expressed in different cell types within the nervous system. Notably, certain miRNAs are found at higher levels in the brain and spinal cord compared to other tissues, suggesting they may have specialized functions in the CNS. miRNAs associated with long-term neurodegenerative changes could serve as valuable tools for early treatment decisions and disease monitoring. The significance of miRNAs such as miR-320, miR-146 and miR-29 in the early diagnosis of neurodegenerative disorders becomes evident, especially considering that many neurological and physical symptoms manifest only after substantial degeneration of specific neurons. Interestingly, serum miRNA levels such as miR-92 and miR-486 may correlate with various MRI parameters in multiple sclerosis. Targeting miRNAs using antisense strategies, such as antisense miR-146 and miR-485, may provide advantages over targeting mRNAs, as a single anti-miRNA can regulate multiple disease-related genes. In the future, anti-miRNA-based therapeutic approaches could be integrated into the clinical management of neurological diseases. Certain miRNAs, including miR-223, miR-106, miR-181, and miR-146, contribute to the pathogenesis of various neurodegenerative diseases and thus warrant greater attention. This knowledge could pave the way for the identification of new diagnostic, prognostic, and theranostic biomarkers, and potentially guiding the development of RNA-based therapeutic strategies. This review highlights recent research on the roles of miRNAs in the nervous system, particularly their protective functions in neurodegenerative disorders.

Systematic review of bidirectional interaction between gut microbiome, miRNAs, and human pathologies

MicroRNAs (miRNAs) and the gut microbiome are key regulators of human health, with emerging evidence highlighting their complex, bidirectional interactions in chronic diseases. miRNAs, influence gene expression and can modulate the composition and function of the gut microbiome, impacting metabolic and immune processes. Conversely, the microbiome can affect host miRNA expression, influencing inflammatory pathways and disease susceptibility. This systematic review examines recent studies (2020-2024) focusing exclusively on human subjects, selected through rigorous inclusion and exclusion criteria. Studies were included if they investigated the interaction between miRNAs and the gut microbiome in the context of gastrointestinal diseases, obesity, autoimmune diseases, cognitive and neurodegenerative disorders, and autism. In vitro, in vivo and in silico analyses were excluded to ensure a strong translational focus on human pathophysiology. Notably, miRNAs, stable and abundant in patients, are emerging as promising biomarkers of microbiome-driven inflammation. This systematic review provides an overview of miRNAs, their regulatory effects on bacterial strains, and their associations with specific diseases. It also explores therapeutic advances and the potential of miRNA-based therapies to restore microbial balance and reduce inflammation.

SCRN: Single-Cell Gene Regulatory Network Identification in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disease, and it consumes considerable medical resources with increasing number of patients every year. Mounting evidence show that the regulatory disruptions altering the intrinsic activity of genes in brain cells contribute to AD pathogenesis. To gain insights into the underlying gene regulation in AD, we proposed a graph learning method, Single-Cell based Regulatory Network (SCRN), to identify the regulatory mechanisms based on single-cell data. SCRN implements the γ-decaying heuristic link prediction based on graph neural networks and can identify reliable gene regulatory networks using locally closed subgraphs. In this work, we first performed UMAP dimension reduction analysis on single-cell RNA sequencing (scRNA-seq) data of AD and normal samples. Then we used SCRN to construct the gene regulatory network based on three well-recognized AD genes (APOE, CX3CR1, and P2RY12). Enrichment analysis of the regulatory network revealed significant pathways including NGF signaling, ERBB2 signaling, and hemostasis. These findings demonstrate the feasibility of using SCRN to uncover potential biomarkers and therapeutic targets related to AD.

Increased Gal-3 Mediates Microglia Activation and Neuroinflammation via the TREM2 Signaling Pathway in Prion Infection

Galectin 3 (Gal-3) is one of the major elements for activating microglia and mediating neuroinflammation in some types of neurodegenerative diseases. However, its role in the pathogenesis of prion disease is seldom addressed. In this study, markedly increased brain Gal-3 was identified in three scrapie-infected rodent models at the terminal stage. The increased Gal-3 was mainly colocalized with the activated microglia. Coincidental with the increased brain Gal-3 in prion-infected animals, the expression of brain trigger receptor expressed in myeloid cell 2 (TREM2), one of the Gal-3 receptors, and some components in the downstream pathway also significantly increased, whereas Toll-like receptor 4 (TLR4), another Gal-3 receptor, and the main components in its downstream signaling were less changed. The increased Gal-3 signals were distributed at the areas with PrPSc deposit but looked not to colocalize directly with PrPSc/PrP signals. Similar changing profiles of Gal-3, the receptors TREM2 and TLR4, as well as the proteins in the downstream pathways were also observed in prion-infected cell line SMB-S15. Removal of PrPSc replication in SMB-S15 cells reversed the upregulation of cellular Gal-3, TREM2, and the relevant proteins. Moreover, we presented data for interactions of Gal-3 with TREM2 and with TLR4 morphologically and molecularly in the cultured cells. Stimulation of prion-infected cells or their normal partner cells with recombinant mouse Gal-3 in vitro induced obvious responses for activation of TREM2 signaling and TLR4 signaling. Our data here strongly indicate that prion infection or PrPSc deposit induces remarkably upregulated brain Gal-3, which is actively involved in the microglia activation and neuroinflammation mainly via TREM2 signaling.

Targeting epigenetics: A novel promise for Alzheimer's disease treatment

So far, the search for a cure for Alzheimer Disease (AD) has been unsuccessful. The only approved drugs attenuate some symptoms, but do not halt the progress of this disease, which affects 50 million people worldwide and will increase its incidence in the coming decades. Such scenario demands new therapeutic approaches to fight against this devastating dementia. In recent years, multi-omics research and the analysis of differential epigenetic marks in AD subjects have contributed to our understanding of AD; however, the impact of epigenetic research is yet to be seen. This review integrates the most recent data on pathological processes and epigenetic changes relevant for aging and AD, as well as current therapies targeting epigenetic machinery in clinical trials. Evidence shows that epigenetic modifications play a key role in gene expression, which could provide multi-target preventative and therapeutic approaches in AD. Both novel and repurposed drugs are employed in AD clinical trials due to their epigenetic effects, as well as increasing number of natural compounds. Given the reversible nature of epigenetic modifications and the complexity of gene-environment interactions, the combination of epigenetic-based therapies with environmental strategies and drugs with multiple targets might be needed to properly help AD patients.

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.

Exosomal MicroRNA-Based Predictive Model for Preclinical Alzheimer's Disease: A Multicenter Study

BACKGROUND

Exosomal microRNAs (miRNAs) have been demonstrated to be biomarkers of Alzheimer's disease (AD). However, whether exosomal miRNAs can predict AD at the asymptomatic stage remains unclear.

METHODS

This study is a multicenter study with four independent datasets to verify the capacity of exosomal miRNAs to identify preclinical AD. Subjects were recruited from a Beijing center in the pilot study (dataset 1: subjects with AD, n = 20; control subjects, n = 20), from other centers across China (dataset 2: subjects with AD, n = 95; control subjects, n = 93), a longitudinal cohort (dataset 3: subjects with preclinical AD, n = 101; control subjects, n = 102), and a confirmation study on familial AD (dataset 4: mutation carriers, n = 56; nonmutation carriers, n = 57).

RESULTS

A panel of miRNAs was changed in subjects with AD and can detect preclinical AD 5 to 7 years before the onset of cognitive impairment (areas under the curve = 0.85-0.88).

CONCLUSIONS

Exosomal miRNAs can be effective biomarkers for predicting AD 5 to 7 years prior to cognitive impairment onset.

Extracellular Vesicles, Stem Cells and the Role of miRNAs in Neurodegeneration

There are different modalities of intercellular communication governed by cellular homeostasis. In this review, we will explore one of these forms of communication called extracellular vesicles (EVs). These vesicles are released by all cells in the body and are heterogeneous in nature. The primary function of EVs is to share information through their cargo consisting of proteins, lipids and nucleic acids (mRNA, miRNA, dsDNA etc.) with other cells, which have a direct consequence on their microenvironment. We will focus on the role of EVs of mesenchymal stem cells (MSCs) in the nervous system and how these participate in intercellular communication to maintain physiological function and provide neuroprotection. However, deregulation of this same communication system could play a role in several neurodegenerative diseases, such as Alzheimer's disease, Parkinson's disease, Amyotrophic lateral sclerosis, multiple sclerosis, prion disease and Huntington's disease. The release of EVs from a cell provides crucial information to what is happening inside the cell and thus could be used in diagnostics and therapy. We will discuss and explore new avenues for the clinical applications of using engineered MSC-EVs and their potential therapeutic benefit in treating neurodegenerative diseases.

Prediction of P-tau/Aβ42 in the cerebrospinal fluid with blood microRNAs in Alzheimer's disease

BACKGROUND

The most common biomarkers of Alzheimer's disease (AD) are amyloid β (Aβ) and tau, detected in cerebrospinal fluid (CSF) or with positron emission tomography imaging. However, these procedures are invasive and expensive, which hamper their availability to the general population. Here, we report a panel of microRNAs (miRNAs) in serum that can predict P-tau/Aβ42 in CSF and readily differentiate AD from other dementias, including vascular dementia (VaD), Parkinson disease dementia (PDD), behavioral variant frontotemporal dementia (bvFTD), and dementia with Lewy body (DLB).

METHODS

RNA samples were extracted from the participant's blood. P-tau/Aβ42 of CSF was examined for diagnostic purposes. A pilot study (controls, 21; AD, 23), followed by second (controls, 216; AD, 190) and third groups (controls, 153; AD, 151), is used to establish and verify a predictive model of P-tau/Aβ42 in CSF. The test is then applied to a fourth group of patients with different dementias (controls, 139; AD,155; amnestic mild cognitive impairment [aMCI], 55; VaD, 51; PDD, 53; bvFTD, 53; DLB, 52) to assess its diagnostic capacity.

RESULTS

In the pilot study, 29 upregulated and 31 downregulated miRNAs in the AD group were found. In Dataset 2, these miRNAs were then included as independent variables in the linear regression model. A seven-microRNA panel (miR-139-3p, miR-143-3p, miR-146a-5p, miR-485-5p, miR-10a-5P, miR-26b-5p, and miR-451a-5p) accurately predicted values of P-tau/Aβ42 of CSF. In Datasets 3 and 4, by applying the predicted P-tau/Aβ42, the predictive model successfully differentiates AD from controls and VaD, PDD, bvFTD, and DLB.

CONCLUSIONS

This study suggests that the panel of microRNAs is a promising substitute for traditional measurement of P-tau/Aβ42 in CSF as an effective biomarker of AD.

Amyloid peptide exerts a rapid induction of Dicer1 protein in neuron via reducing phosphorylation

A growing number of evidence suggests that altered microRNA network in the brain contributes to the risk of Alzheimer's disease(AD). Dicer1 is a type III riboendonuclease which cleaves pre-microRNA into functional microRNA. Reduction of Dicer1 or Dicer1 mutation has been involved in cancer, aging or age-related macular degeneration. Recently, we found a possible link between Dicer1 and AD. In particular, Dicer1 protein and Dicer1 mRNA is reduced in the hippocampus and the cortex of an animal model of AD and exposure to Aβ42 oligomer(AβO) longer than 6 h reduces the transcription of Dicer1 gene in neuron, via depletion of NF-E2-related factor-2. In this study, exposure to AβO at shorter time increased Dicer1 protein in neuron in a dose-dependent mode; but the mRNA level remained unaltered. Under this treatment regime,AβO reduced phosphorylation level of Dicer1 and of its binding partner, transactivation response element RNA-binding protein(TRBP). Addition of a JNK inhibitor,SP600125, or an ERK inhibitor,U0126, further increased Dicer1 protein compared to Aβo treatment alone, with simultaneaous reduction of phospho-Dicer1, but with different effects on phospho-TRBP. Finally, an inhibitor of calcineurin,FK506, further increased Dicer1 protein compared to Aβo treatment alone. Thus, phosphorylation of Dicer1 and TRBP was determined by mitogen activated protein kinases JNK,ERK, and protein phosphatase 2B(calcineurin) which together determined Dicer1 stability. In summary, reduced phosphorylation of Dicer1 accounted for the rapid induction of Dicer1 by AβO. This study highlights a novel way by which AβO regulates Dicer1.

RNA Dynamics in Alzheimer's Disease

Alzheimer's disease (AD) is the most common age-related neurodegenerative disorder that heavily burdens healthcare systems worldwide. There is a significant requirement to understand the still unknown molecular mechanisms underlying AD. Current evidence shows that two of the major features of AD are transcriptome dysregulation and altered function of RNA binding proteins (RBPs), both of which lead to changes in the expression of different RNA species, including microRNAs (miRNAs), circular RNAs (circRNAs), long non-coding RNAs (lncRNAs), and messenger RNAs (mRNAs). In this review, we will conduct a comprehensive overview of how RNA dynamics are altered in AD and how this leads to the differential expression of both short and long RNA species. We will describe how RBP expression and function are altered in AD and how this impacts the expression of different RNA species. Furthermore, we will also show how changes in the abundance of specific RNA species are linked to the pathology of AD.

Gut microbial involvement in Alzheimer's disease pathogenesis

Alzheimer's disease (AD) is a chronic, progressive neurodegenerative disease characterized by memory loss, inability to carry out everyday daily life, and noticeable behavioral changes. The essential neuropathologic criteria for an AD diagnosis are extracellular β-amyloid deposition and intracellular accumulation of hyperphosphorylated tau. However, the exact pathogenic mechanisms underlying AD remain elusive, and current treatment options show only limited success. New research indicates that the gut microbiota contributes to AD development and progression by accelerating neuroinflammation, promoting senile plaque formation, and modifying neurotransmitter production. This review highlights laboratory and clinical evidence for the pathogenic role of gut dysbiosis on AD and provides potential cues for improved AD diagnostic criteria and therapeutic interventions based on the gut microbiota.

Alzheimer's disease and gut microbiota: does trimethylamine N-oxide (TMAO) play a role?

Alzheimer's disease (AD) is a neurodegenerative disease that affects memory and cognitive function. Clinical evidence has put into question our current understanding of AD development, propelling researchers to look into further avenues. Gut microbiota has emerged as a potential player in AD pathophysiology. Lifestyle factors, such as diet, can have negative effects on the gut microbiota and thus host health. A Western-type diet has been highlighted as a risk factor for both gut microbiota alteration as well as AD development. The gut-derived trimethylamine N-oxide (TMAO) has been previously implied in the development of cardiovascular diseases with recent evidence suggesting a plausible role of TMAO in AD development. Therefore, the main goal of the present review is to provide the reader with potential mechanisms of action through which consumption of a Western-type diet could increase AD risk, by acting through microbiota-produced TMAO. Although a link between TMAO and AD is far from definitive, this review will serve as a call for research into this new area of research.

Unravelling the potential of gut microbiota in sustaining brain health and their current prospective towards development of neurotherapeutics

Increasing incidences of neurological disorders, such as Parkinson's disease (PD), multiple sclerosis (MS), Alzheimer's disease (AD) and amyotrophic lateral sclerosis (ALS) are being reported, but an insight into their pathology remains elusive. Findings have suggested that gut microbiota play a major role in regulating brain functions through the gut-brain axis. A unique bidirectional communication between gut microbiota and maintenance of brain health could play a pivotal role in regulating incidences of neurodegenerative diseases. Contrarily, the present life style with changing food habits and disturbed circadian rhythm may contribute to gut homeostatic imbalance and dysbiosis leading to progression of several neurological disorders. Therefore, dysbiosis, as a primary factor behind intestinal disorders, may also augment inflammation, intestinal and blood-brain barrier permeability through microbiota-gut-brain axis. This review primarily focuses on the gut-brain axis functions, specific gut microbial population, metabolites produced by gut microbiota, their role in regulating various metabolic processes and role of gut microbiota towards development of neurodegenerative diseases. However, several studies have reported a decrease in abundance of a specific gut microbial population and a corresponding increase in other microbial family, with few findings revealing some contradictions. Reports also showed that colonization of gut microbiota isolated from patients suffering from neurodegenerative disease leads to the development of enhance pathological outcomes in animal models. Hence, a systematic understanding of the dominant role of specific gut microbiome towards development of different neurodegenerative diseases could possibly provide novel insight into the use of probiotics and microbial transplantation as a substitute approach for treating/preventing such health maladies.

Gastrointestinal (GI)-Tract Microbiome Derived Neurotoxins and their Potential Contribution to Inflammatory Neurodegeneration in Alzheimer's Disease (AD)

The human gastrointestinal (GI)-tract microbiome is a rich, complex and dynamic source of microorganisms that possess a staggering diversity and complexity. Importantly there is a significant variability in microbial complexity even amongst healthy individuals-this has made it difficult to link specific microbial abundance patterns with age-related neurological disease. GI-tract commensal microorganisms are generally beneficial to human metabolism and immunity, however enterotoxigenic forms of microbes possess significant potential to secrete what are amongst the most neurotoxic and pro-inflammatory biopolymers known. These include toxic glycolipids such as lipopolysaccharide (LPS), enterotoxins, microbial-derived amyloids and small non-coding RNA. One major microbial species of the GI-tract microbiome, about ~100-fold more abundant than Escherichia coli in deep GI-tract regions is Bacteroides fragilis, an anaerobic, rod-shaped Gram-negative bacterium. B. fragilis can secrete: (i) a particularly potent, pro-inflammatory and unique LPS subtype (BF-LPS); and (ii) a zinc-metalloproteinase known as B. fragilis-toxin (BFT) or fragilysin. Ongoing studies indicate that BF-LPS and/or BFT disrupt paracellular-and transcellular-barriers by cleavage of intercellular-proteins resulting in 'leaky' barriers. These barriers: (i) become defective and more penetrable with aging and disease; and (ii) permit entry of microbiome-derived neurotoxins into the systemic-circulation from which they next transit the blood-brain barrier and gain access to the CNS. Here LPS accumulates and significantly alters homeostatic patterns of gene expression. The affinity of LPS for neuronal nuclei is significantly enhanced in the presence of amyloid beta 42 (Aβ42) peptides. Recent research on the appearance of the brain thanatomicrobiome at the time of death and the increasing likelihood of a complex brain microbiome are reviewed and discussed. This paper will also highlight some recent advances in this extraordinary research area that links the pro-inflammatory exudates of the GI-tract microbiome with innate-immune disturbances and inflammatory-signaling within the CNS with reference to Alzheimer's disease (AD) wherever possible.

RNA and Oxidative Stress in Alzheimer's Disease: Focus on microRNAs

Oxidative stress (OS) is one of the major pathomechanisms of Alzheimer's disease (AD), which is closely associated with other key events in neurodegeneration such as mitochondrial dysfunction, inflammation, metal dysregulation, and protein misfolding. Oxidized RNAs are identified in brains of AD patients at the prodromal stage. Indeed, oxidized mRNA, rRNA, and tRNA lead to retarded or aberrant protein synthesis. OS interferes with not only these translational machineries but also regulatory mechanisms of noncoding RNAs, especially microRNAs (miRNAs). MiRNAs can be oxidized, which causes misrecognizing target mRNAs. Moreover, OS affects the expression of multiple miRNAs, and conversely, miRNAs regulate many genes involved in the OS response. Intriguingly, several miRNAs embedded in upstream regulators or downstream targets of OS are involved also in neurodegenerative pathways in AD. Specifically, seven upregulated miRNAs (miR-125b, miR-146a, miR-200c, miR-26b, miR-30e, miR-34a, miR-34c) and three downregulated miRNAs (miR-107, miR-210, miR-485), all of which are associated with OS, are found in vulnerable brain regions of AD at the prodromal stage. Growing evidence suggests that altered miRNAs may serve as targets for developing diagnostic or therapeutic tools for early-stage AD. Focusing on a neuroprotective transcriptional repressor, REST, and the concept of hormesis that are relevant to the OS response may provide clues to help us understand the role of the miRNA system in cellular and organismal adaptive mechanisms to OS.

Biomarkers for Alzheimer's Disease (AD) and the Application of Precision Medicine

An accurate diagnosis of Alzheimer's disease (AD) currently stands as one of the most difficult and challenging in all of clinical neurology. AD is typically diagnosed using an integrated knowledge and assessment of multiple biomarkers and interrelated factors. These include the patient's age, gender and lifestyle, medical and genetic history (both clinical- and family-derived), cognitive, physical, behavioral and geriatric assessment, laboratory examination of multiple AD patient biofluids, especially within the systemic circulation (blood serum) and cerebrospinal fluid (CSF), multiple neuroimaging-modalities of the brain's limbic system and/or retina, followed up in many cases by post-mortem neuropathological examination to finally corroborate the diagnosis. More often than not, prospective AD cases are accompanied by other progressive, age-related dementing neuropathologies including, predominantly, a neurovascular and/or cardiovascular component, multiple-infarct dementia (MID), frontotemporal dementia (FTD) and/or strokes or 'mini-strokes' often integrated with other age-related neurological and non-neurological disorders including cardiovascular disease and cancer. Especially over the last 40 years, enormous research efforts have been undertaken to discover, characterize, and quantify more effectual and reliable biological markers for AD, especially during the pre-clinical or prodromal stages of AD so that pre-emptive therapeutic treatment strategies may be initiated. While a wealth of genetic, neurobiological, neurochemical, neuropathological, neuroimaging and other diagnostic information obtainable for a single AD patient can be immense: (i) it is currently challenging to integrate and formulate a definitive diagnosis for AD from this multifaceted and multidimensional information; and (ii) these data are unfortunately not directly comparable with the etiopathological patterns of other AD patients even when carefully matched for age, gender, familial genetics, and drug history. Four decades of AD research have repeatedly indicated that diagnostic profiles for AD are reflective of an extremely heterogeneous neurological disorder. This commentary will illuminate the heterogeneity of biomarkers for AD, comment on emerging investigative approaches and discuss why 'precision medicine' is emerging as our best paradigm yet for the most accurate and definitive prediction, diagnosis, and prognosis of this insidious and lethal brain disorder.

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.

MicroRNA-146a Is a Wide-Reaching Neuroinflammatory Regulator and Potential Treatment Target in Neurological Diseases

Progressive functional deterioration and loss of neurons underlies neurological diseases and constitutes an important cause of disability and death worldwide. The causes of various types of neurological diseases often share several critical nerve-related cellular mechanisms and pathological features, particularly the neuroinflammatory response in the nervous system. A rapidly growing body of evidence indicates that various microRNAs play pivotal roles in these processes in neurological diseases and might be viable therapeutic targets. Among these microRNAs, microRNA-146a (miR-146a) stands out due to the rapid increase in recent literature on its mechanistic involvement in neurological diseases. In this review, we summarize and highlight the critical role of miR-146a in neurological diseases. MiR-146a polymorphisms are associated with the risk of neurological disease. Alterations in miR-146a expression levels are crucial events in the pathogenesis of numerous neurological diseases that are spatially and temporally diverse. Additionally, the target genes of miR-146a are involved in the regulation of pathophysiological processes in neurological diseases, particularly the neuroinflammatory response. In summary, miR-146a mainly plays a critical role in neuroinflammation during the progression of neurological diseases and might be a prospective biomarker and therapeutic target. Understanding the mechanisms by which miR-146a affects the neuroinflammatory response in different neurological injuries, different cell types, and even different stages of certain neurological diseases will pave the way for its use as a therapeutic target in neurodegenerative diseases.

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-34a Acutely Regulates Synaptic Efficacy in the Adult Dentate Gyrus In Vivo

Activity-dependent synaptic plasticity involves rapid regulation of neuronal protein synthesis on a time-scale of minutes. miRNA function in synaptic plasticity and memory formation has been elucidated by stable experimental manipulation of miRNA expression and activity using transgenic approaches and viral vectors. However, the impact of rapid miRNA modulation on synaptic efficacy is unknown. Here, we examined the effect of acute (12 min), intrahippocampal infusion of a miR-34a antagonist (antimiR) on medial perforant path-evoked synaptic transmission in the dentate gyrus of adult anesthetised rats. AntimiR-34a infusion acutely depressed medial perforant path-evoked field excitatory post-synaptic potentials (fEPSPs). The fEPSP decrease was detected within 9 min of infusion, lasted for hours, and was associated with knockdown of antimiR-34a levels. AntimiR-34a-induced synaptic depression was sequence-specific; no changes were elicited by infusion of scrambled or mismatch control. The rapid modulation suggests that a target, or set of targets, is regulated by miR-34a. Western blot analysis of dentate gyrus lysates revealed enhanced expression of Arc, a known miR-34a target, and four novel predicted targets (Ctip2, PKI-1α, TCF4 and Ube2g1). Remarkably, antimiR-34a had no effect when infused during the maintenance phase of long-term potentiation. We conclude that miR-34a regulates basal synaptic efficacy in the adult dentate gyrus in vivo. To our knowledge, these in vivo findings are the first to demonstrate acute (< 9 min) regulation of synaptic efficacy in the adult brain by a miRNA.

Microbiome Influence in the Pathogenesis of Prion and Alzheimer's Diseases

Misfolded and abnormal β-sheets forms of wild-type proteins, such as cellular prion protein (PrPC) and amyloid beta (Aβ), are believed to be the vectors of neurodegenerative diseases, prion and Alzheimer's disease (AD), respectively. Increasing evidence highlights the "prion-like" seeding of protein aggregates as a mechanism for pathological spread in AD, tauopathy, as well as in other neurodegenerative diseases, such as Parkinson's. Mutations in both PrPC and Aβ precursor protein (APP), have been associated with the pathogenesis of these fatal disorders with clear evidence for their pathogenic significance. In addition, a critical role for the gut microbiota is emerging; indeed, as a consequence of gut-brain axis alterations, the gut microbiota has been involved in the regulation of Aβ production in AD and, through the microglial inflammation, in the amyloid fibril formation, in prion diseases. Here, we aim to review the role of microbiome ("the other human genome") alterations in AD and prion disease pathogenesis.

Curcumin restores innate immune Alzheimer's disease risk gene expression to ameliorate Alzheimer pathogenesis

Alzheimer's disease (AD) genetics implies a causal role for innate immune genes, TREM2 and CD33, products that oppose each other in the downstream Syk tyrosine kinase pathway, activating microglial phagocytosis of amyloid (Aβ). We report effects of low (Curc-lo) and high (Curc-hi) doses of curcumin on neuroinflammation in APPsw transgenic mice. Results showed that Curc-lo decreased CD33 and increased TREM2 expression (predicted to decrease AD risk) and also increased TyroBP, which controls a neuroinflammatory gene network implicated in AD as well as phagocytosis markers CD68 and Arg1. Curc-lo coordinately restored tightly correlated relationships between these genes' expression levels, and decreased expression of genes characteristic of toxic pro-inflammatory M1 microglia (CD11b, iNOS, COX-2, IL1β). In contrast, very high dose curcumin did not show these effects, failed to clear amyloid plaques, and dysregulated gene expression relationships. Curc-lo stimulated microglial migration to and phagocytosis of amyloid plaques both in vivo and in ex vivo assays of sections of human AD brain and of mouse brain. Curcumin also reduced levels of miR-155, a micro-RNA reported to drive a neurodegenerative microglial phenotype. In conditions without amyloid (human microglial cells in vitro, aged wild-type mice), Curc-lo similarly decreased CD33 and increased TREM2. Like curcumin, anti-Aβ antibody (also reported to engage the Syk pathway, increase CD68, and decrease amyloid burden in human and mouse brain) increased TREM2 in APPsw mice and decreased amyloid in human AD sections ex vivo. We conclude that curcumin is an immunomodulatory treatment capable of emulating anti-Aβ vaccine in stimulating phagocytic clearance of amyloid by reducing CD33 and increasing TREM2 and TyroBP, while restoring neuroinflammatory networks implicated in neurodegenerative diseases.

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.

Imbalance of Microglial TLR4/TREM2 in LPS-Treated APP/PS1 Transgenic Mice: A Potential Link Between Alzheimer's Disease and Systemic Inflammation

Clinically, superimposed systemic inflammation generally has significant deleterious effects on the Alzheimer's disease (AD) progression. However, the related molecular mechanisms remain poorly understood. Microglial toll-like receptor 4 (TLR4) and triggering receptor expressed on myeloid cells 2 (TREM2) are two key regulators of inflammation that may play an essential role in this complex pathophysiological process. In this study, intraperitoneal injection of lipopolysaccharide (LPS) into APP/PS1 transgenic AD model was used to mimic systemic inflammation in the development of AD. Initial results from the cortex showed that compared with wild-type mice, APP/PS1 mice exhibited elevated gene and protein expression levels of both TLR4 and TREM2 with different degree. Interestingly, after LPS treatment, TLR4 expression was persistently up-regulated, while TREM2 expression was significantly down-regulated in APP/PS1 mice, suggesting that the negative regulatory effect of TREM2 on inflammation might be suppressed by LPS-induced hyperactive TLR4. This imbalance of TLR4/TREM2 contributed to microglial over-activation, followed by increased neuronal apoptosis in the cortex of APP/PS1 mice; these changes did not alter the expression level of Aβ_1-42. Similar alterations were observed in our in vitro experiment with β-amyloid_1-42 (Aβ_1-42)-treated N9 microglia. Further, Morris water maze (MWM) testing data indicated that LPS administration acutely aggravated cognitive impairment in APP/PS1 mice, suggesting that the addition of systemic inflammation can potentially accelerate the progression of AD. Collectively, we conclude that an imbalance of TLR4/TREM2 may be a potential link between AD and systemic inflammation. TREM2 can serve as a potential therapeutic target for treating systemic inflammation in AD progression.

Emerging Role of microRNAs in Dementia

MicroRNAs are small non-coding RNAs regulating mRNA translation. They play a crucial role in regulating homeostasis in neurons, especially in regulating local and stimulation dependent protein synthesis. Since activity-mediated protein synthesis in neurons is critical for memory and cognition, microRNAs have become key players in modulating these processes. Dementia is a broad term used for symptoms involving decline of memory and cognition. Several studies have implicated the dysregulation of microRNAs in many brain diseases like neurodegenerative diseases, neurodevelopmental disorders, brain injuries and dementia. In this review, we give an overview of microRNA-mediated regulation of proteins and cellular processes affected in dementia pathology, hence illustrating the importance of microRNAs in normal functioning. We also focus on a relatively less explored area in dementia pathology-the importance of activity-mediated protein synthesis at the synapse and the role of microRNAs in modulating this. Overall, this review will be helpful in looking at the significance of microRNAs in dementia from the perspective of defective regulation of protein synthesis and synaptic dysfunction.

The cell type resolved mouse transcriptome in neuron-enriched brain tissues from the hippocampus and cerebellum during prion disease

Multiple cell types and complex connection networks are an intrinsic feature of brain tissue. In this study we used expression profiling of specific microscopic regions of heterogeneous tissue sections isolated by laser capture microdissection (LCM) to determine insights into the molecular basis of brain pathology in prion disease. Temporal profiles in two mouse models of prion disease, bovine spongiform encephalopathy (BSE) and a mouse-adapted strain of scrapie (RML) were performed in microdissected regions of the CA1 hippocampus and granular layer of the cerebellum which are both enriched in neuronal cell bodies. We noted that during clinical disease the number of activated microglia and astrocytes that occur in these areas are increased, thereby likely diluting the neuronal gene expression signature. We performed a comparative analysis with gene expression profiles determined from isolated populations of neurons, microglia and astrocytes to identify transcripts that are enriched in each of these cell types. Although the incubation periods of these two models are quite different, over 300 days for BSE and ~160 days for RML scrapie, these regional microdissections revealed broadly similar profiles. Microglial and astrocyte-enriched genes contributed a profound inflammatory profile consisting of inflammatory cytokines, genes related to phagocytosis, proteolysis and genes coding for extracellular matrix proteins. CA1 pyramidal neurons displayed a net upregulation of transcription factors and stress induced genes at pre-clinical stages of disease while all tissues showed profound decrease of overlapping genes related to neuronal function, in particular transcripts related to neuronal communication including glutamate receptors, phosphatase subunits and numerous synapse-related markers. Of note, we found a small number of genes expressed in neurons that were upregulated during clinical disease including, COX6A2, FZD9, RXRG and SOX11, that may be biomarkers of neurodegeneration.

Unhealthy gut, unhealthy brain: The role of the intestinal microbiota in neurodegenerative diseases

The number of bacterial cells living within the human body is approximately equal to, or greater than, the total number of human cells. This dynamic population of microorganisms, termed the human microbiota, resides mainly within the gastrointestinal tract. It is widely accepted that highly diverse and stable microbiota promote overall human health. Colonization of the gut with maladaptive and pathogenic microbiota, a state also known as dysbiosis, is associated with a variety of peripheral diseases ranging from type 2 diabetes mellitus to cardiovascular and inflammatory bowel disease. More recently, microbial dysbiosis has been associated with a number of brain pathologies, including autism spectrum disorder, Alzheimer's disease (AD), Parkinson's disease (PD), and amyotrophic lateral sclerosis (ALS), suggesting a direct or indirect communication between intestinal bacteria and the central nervous system (CNS). In this review, we illustrate two pathways implicated in the crosstalk between gut microbiota and CNS involving 1) the vagus nerve and 2) transmission of signaling molecules through the circulatory system and across the blood-brain barrier (BBB). We summarize the available evidence of the specific changes in the intestinal microbiota, as well as microorganism-induced modifications to intestinal and BBB permeability, which have been linked to several neurodegenerative disorders including ALS, AD, and PD. Even though each of these diseases arises from unique pathogenetic mechanisms, all are characterized, at least in part, by chronic neuroinflammation. We provide an interpretation for the substantial evidence that healthy intestinal microbiota have the ability to positively regulate the neuroimmune responses in the CNS. Even though the evidence is mainly associative, it has been suggested that bacterial dysbiosis could contribute to an adverse neuroinflammatory state leading to increased risk of neurodegenerative diseases. Thus, developing strategies for regulating and maintaining healthy intestinal microbiota could be a valid approach for lowering individual risk and prevalence of neurodegenerative diseases.

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.

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.

The Gut Microbiota and Alzheimer's Disease

The gut microbiota comprises a complex community of microorganism species that resides in our gastrointestinal ecosystem and whose alterations influence not only various gut disorders but also central nervous system disorders such as Alzheimer's disease (AD). AD, the most common form of dementia, is a neurodegenerative disorder associated with impaired cognition and cerebral accumulation of amyloid-β peptides (Aβ). Most notably, the microbiota-gut-brain axis is a bidirectional communication system that is not fully understood, but includes neural, immune, endocrine, and metabolic pathways. Studies in germ-free animals and in animals exposed to pathogenic microbial infections, antibiotics, probiotics, or fecal microbiota transplantation suggest a role for the gut microbiota in host cognition or AD-related pathogenesis. The increased permeability of the gut and blood-brain barrier induced by microbiota dysbiosis may mediate or affect AD pathogenesis and other neurodegenerative disorders, especially those associated with aging. In addition, bacteria populating the gut microbiota can secrete large amounts of amyloids and lipopolysaccharides, which might contribute to the modulation of signaling pathways and the production of proinflammatory cytokines associated with the pathogenesis of AD. Moreover, imbalances in the gut microbiota can induce inflammation that is associated with the pathogenesis of obesity, type 2 diabetes mellitus, and AD. The purpose of this review is to summarize and discuss the current findings that may elucidate the role of the gut microbiota in the development of AD. Understanding the underlying mechanisms may provide new insights into novel therapeutic strategies for AD.

Deficits in the Proline-Rich Synapse-Associated Shank3 Protein in Multiple Neuropsychiatric Disorders

Signaling between neurons in the human central nervous system (CNS) is accomplished through a highly interconnected network of presynaptic and postsynaptic elements essential in the conveyance of electrical and neurochemical information. One recently characterized core postsynaptic element essential to the efficient operation of this complex network is a relatively abundant ~184.7 kDa proline-rich synapse-associated cytoskeletal protein known as Shank3 (SH3-ankyrin repeat domain; encoded at human chr 22q13.33). In this “Perspectives” article, we review and comment on current advances in Shank3 research and include some original data that show common Shank3 deficits in a number of seemingly unrelated human neurological disorders that include sporadic Alzheimer’s disease (AD), autism spectrum disorder (ASD), bipolar disorder (BD), Phelan–McDermid syndrome (PMS; 22q13.3 deletion syndrome), and schizophrenia (SZ). Shank3 was also found to be downregulated in the CNS of the transgenic AD (TgAD) 5x familial Alzheimer’s disease murine model engineered to overexpress the 42 amino acid amyloid-beta (Aβ42) peptide. Interestingly, the application of known pro-inflammatory stressors, such as the Aβ42 peptide and the metal-neurotoxin aluminum sulfate, to human neuronal–glial cells in primary culture resulted in a significant decrease in the expression of Shank3. These data indicate that deficits in Shank3-expression may be one common denominator linking a wide-range of human neurological disorders that exhibit a progressive or developmental synaptic disorganization that is temporally associated with cognitive decline.

TREM2 in Neurodegenerative Diseases

TREM2 variants have been identified as risk factors for Alzheimer's disease (AD) and other neurodegenerative diseases (NDDs). Because TREM2 encodes a receptor exclusively expressed on immune cells, identification of these variants conclusively demonstrates that the immune response can play an active role in the pathogenesis of NDDs. These TREM2 variants also confer the highest risk for developing Alzheimer's disease of any risk factor identified in nearly two decades, suggesting that understanding more about TREM2 function could provide key insights into NDD pathology and provide avenues for novel immune-related NDD biomarkers and therapeutics. The expression, signaling and function of TREM2 in NDDs have been extensively investigated in an effort to understand the role of immune function in disease pathogenesis and progression. We provide a comprehensive review of our current understanding of TREM2 biology, including new insights into the regulation of TREM2 expression, and TREM2 signaling and function across NDDs. While many open questions remain, the current body of literature provides clarity on several issues. While it is still often cited that TREM2 expression is decreased by pro-inflammatory stimuli, it is now clear that this is true in vitro, but inflammatory stimuli in vivo almost universally increase TREM2 expression. Likewise, while TREM2 function is classically described as promoting an anti-inflammatory phenotype, more than half of published studies demonstrate a pro-inflammatory role for TREM2, suggesting that its role in inflammation is much more complex. Finally, these components of TREM2 biology are applied to a discussion of how TREM2 impacts NDD pathologies and the latest assessment of how these findings might be applied to immune-directed clinical biomarkers and therapeutics.

Secretory Products of the Human GI Tract Microbiome and Their Potential Impact on Alzheimer's Disease (AD): Detection of Lipopolysaccharide (LPS) in AD Hippocampus

Although the potential contribution of the human gastrointestinal (GI) tract microbiome to human health, aging, and disease is becoming increasingly acknowledged, the molecular mechanics and signaling pathways of just how this is accomplished is not well-understood. Major bacterial species of the GI tract, such as the abundant Gram-negative bacilli Bacteroides fragilis (B. fragilis) and Escherichia coli (E. coli), secrete a remarkably complex array of pro-inflammatory neurotoxins which, when released from the confines of the healthy GI tract, are pathogenic and highly detrimental to the homeostatic function of neurons in the central nervous system (CNS). For the first time here we report the presence of bacterial lipopolysaccharide (LPS) in brain lysates from the hippocampus and superior temporal lobe neocortex of Alzheimer's disease (AD) brains. Mean LPS levels varied from two-fold increases in the neocortex to three-fold increases in the hippocampus, AD over age-matched controls, however some samples from advanced AD hippocampal cases exhibited up to a 26-fold increase in LPS over age-matched controls. This “Perspectives” paper will further highlight some very recent research on GI tract microbiome signaling to the human CNS, and will update current findings that implicate GI tract microbiome-derived LPS as an important internal contributor to inflammatory degeneration in the CNS.

The Role of Microglia in Prion Diseases: A Paradigm of Functional Diversity

Inflammation is a major component of neurodegenerative diseases. Microglia are the innate immune cells in the central nervous system (CNS). In the healthy brain, microglia contribute to tissue homeostasis and regulation of synaptic plasticity. Under disease conditions, they play a key role in the development and maintenance of the neuroinflammatory response, by showing enhanced proliferation and activation. Prion diseases are progressive chronic neurodegenerative disorders associated with the accumulation of the scrapie prion protein PrP^Sc, a misfolded conformer of the cellular prion protein PrP^C. This review article provides the current knowledge on the role of microglia in the pathogenesis of prion disease. A large body of evidence shows that microglia can trigger neurotoxic pathways contributing to progressive degeneration. Yet, microglia are also crucial for controlling inflammatory, repair and regenerative processes. This dual role of microglia is regulated by multiple pathways and evidences the ability of these cells to polarize into distinct phenotypes with characteristic functions. The awareness that the neuroinflammatory response is inextricably involved in producing tissue damage as well as repair in neurodegenerative disorders, opens new perspectives for the modulation of the immune system. A better understanding of this complex process will be essential for developing effective therapies for neurodegenerative diseases, in order to improve the quality of life of patients and mitigating the personal, economic and social consequences derived from these diseases.

Genetics of Aggression in Alzheimer's Disease (AD)

Alzheimer's disease (AD) is a terminal, age-related neurological syndrome exhibiting progressive cognitive and memory decline, however AD patients in addition exhibit ancillary neuropsychiatric symptoms (NPSs) and these include aggression. In this communication we provide recent evidence for the mis-regulation of a small family of genes expressed in the human hippocampus that appear to be significantly involved in expression patterns common to both AD and aggression. DNA array- and mRNA transcriptome-based gene expression analysis and candidate gene association and/or genome-wide association studies (CGAS, GWAS) of aggressive attributes in humans have revealed a surprisingly small subset of six brain genes that are also strongly associated with altered gene expression patterns in AD. These genes encoded on five different chromosomes (chr) include the androgen receptor (AR; chrXq12), brain-derived neurotrophic factor (BDNF; chr11p14.1), catechol-O-methyl transferase (COMT; chr22q11.21), neuronal specific nitric oxide synthase (NOS1; chr12q24.22), dopamine beta-hydroxylase (DBH chr9q34.2) and tryptophan hydroxylase (TPH1, chr11p15.1 and TPH2, chr12q21.1). Interestingly, (i) the expression of three of these six genes (COMT, DBH, NOS1) are highly variable; (ii) three of these six genes (COMT, DBH, TPH1) are involved in DA or serotonin metabolism, biosynthesis and/or neurotransmission; and (iii) five of these six genes (AR, BDNF, COMT, DBH, NOS1) have been implicated in the development, onset and/or propagation of schizophrenia. The magnitude of the expression of genes implicated in aggressive behavior appears to be more pronounced in the later stages of AD when compared to MCI. These recent genetic data further indicate that the extent of cognitive impairment may have some bearing on the degree of aggression which accompanies the AD phenotype.

Alterations in micro RNA-messenger RNA (miRNA-mRNA) Coupled Signaling Networks in Sporadic Alzheimer's Disease (AD) Hippocampal CA1

RNA sequencing, DNA microfluidic array, LED-Northern, Western immunoassay and bioinformatics analysis have uncovered a small family of up-regulated human brain enriched microRNAs (miRNAs) and down-regulated messenger RNAs (mRNAs) in short post-mortem interval (PMI) sporadic Alzheimer's disease (AD) brain. At the mRNA level, a large majority of the expression of human brain genes found to be down-regulated in sporadic AD appears to be a consequence of an up-regulation of a specific group of NF-kB-inducible microRNAs (miRNAs). This group of up-regulated miRNAs - including miRNA-34a and miRNA-146a - has strong, energetically favorable, complimentary RNA sequences in the 3' untranslated regions (3'-UTR) of their target mRNAs which ultimately drive the down-regulation in the expression of certain essential brain genes. Interestingly, just 2 significantly up-regulated miRNAs - miRNA-34a and miRNA-146a - appear to down-regulate mRNA targets involved in synaptogenesis (SHANK3), phagocytosis deficits and tau pathology (TREM2), inflammation (CFH; complement factor H) and amyloidogenesis (TSPAN12), all of which are distinguishing pathological features characteristic of middle-to-late stage AD neuropathology. This paper reports the novel finding of parallel miRNA-34a and miRNA-146a up-regulation in sporadic AD hippocampal CA1 RNA pools and proposes an altered miRNA-mRNA coupled signaling network in AD, much of which is supported by current experimental findings in the recent literature.

TREMs in Alzheimer's disease: Genetic and clinical investigations

Triggering receptor expressed on myeloid cells (TREMs) receptors constitute a family modulators in human innate immunity system that encode by a gene cluster. Rare variants in TREM2 were reported to be associated with significant Alzheimer's disease (AD) risk. However, inconsistent results were also reported in some studies of Non-European descents. Recently, the other TREM family members are also considered to involve in AD and cerebrospinal fluid (CSF) soluble form of TREM2 (sTREM2) levels has also been associated with respond to progression of disease. In this review, we converged the data of genetic and clinical investigations to identify the clearer role of TREMs in AD. Here, comprehensively analyze of multidisciplinary fields highlights the contribution of TREMs locus to AD development.

Bacteroides fragilis Lipopolysaccharide and Inflammatory Signaling in Alzheimer's Disease

The human microbiome consists of ~3.8 × 10¹³ symbiotic microorganisms that form a highly complex and dynamic ecosystem: the gastrointestinal (GI) tract constitutes the largest repository of the human microbiome by far, and its impact on human neurological health and disease is becoming increasingly appreciated. Bacteroidetes, the largest phylum of Gram-negative bacteria in the GI tract microbiome, while generally beneficial to the host when confined to the GI tract, have potential to secrete a remarkably complex array of pro-inflammatory neurotoxins that include surface lipopolysaccharides (LPSs) and toxic proteolytic peptides. The deleterious effects of these bacterial exudates appear to become more important as GI tract and blood-brain barriers alter or increase their permeability with aging and disease. For example, presence of the unique LPSs of the abundant Bacteroidetes species Bacteroides fragilis (BF-LPS) in the serum represents a major contributing factor to systemic inflammation. BF-LPS is further recognized by TLR2, TLR4, and/or CD14 microglial cell receptors as are the pro-inflammatory 42 amino acid amyloid-beta (Aβ42) peptides that characterize Alzheimer's disease (AD) brain. Here we provide the first evidence that BF-LPS exposure to human primary brain cells is an exceptionally potent inducer of the pro-inflammatory transcription factor NF-kB (p50/p65) complex, a known trigger in the expression of pathogenic pathways involved in inflammatory neurodegeneration. This 'Perspectives communication' will in addition highlight work from recent studies that advance novel and emerging concepts on the potential contribution of microbiome-generated factors, such as BF-LPS, in driving pro-inflammatory degenerative neuropathology in the AD brain.