Dysbiosis and Alzheimer's Disease: Cause or Treatment Opportunity?

Western diet components that increase intestinal permeability with implications on health

Intestinal permeability is a physiological property that allows necessary molecules to enter the organism. This property is regulated by tight junction proteins located between intestinal epithelial cells. However, various factors can increase intestinal permeability (IIP), including diet. Specific components in the Western diet (WD), such as monosaccharides, fat, gluten, salt, alcohol, and additives, can affect the tight junctions between enterocytes, leading to increased permeability. This review explains how these components promote IIP and outlines their potential implications for health. In addition, we describe how a reduction in WD consumption may help improve dietary treatment of diseases associated with IIP. Research has shown that some of these components can cause changes in the gut microbiota, leading to dysbiosis, which can promote greater intestinal permeability and displacement of endotoxins into the bloodstream. These endotoxins include lipopolysaccharides derived from gram-negative bacteria, and their presence has been associated with various diseases, such as autoimmune, neurological, and metabolic diseases like diabetes and cardiovascular disease. Therefore, nutrition professionals should promote the reduction of WD consumption and consider the inclusion of healthy diet components as part of the nutritional treatment for diseases associated with increased intestinal permeability.

Systematic characterization of multi-omics landscape between gut microbial metabolites and GPCRome in Alzheimer's disease

Shifts in the magnitude and nature of gut microbial metabolites have been implicated in Alzheimer's disease (AD), but the host receptors that sense and respond to these metabolites are largely unknown. Here, we develop a systems biology framework that integrates machine learning and multi-omics to identify molecular relationships of gut microbial metabolites with non-olfactory G-protein-coupled receptors (termed the "GPCRome"). We evaluate 1.09 million metabolite-protein pairs connecting 408 human GPCRs and 335 gut microbial metabolites. Using genetics-derived Mendelian randomization and integrative analyses of human brain transcriptomic and proteomic profiles, we identify orphan GPCRs (i.e., GPR84) as potential drug targets in AD and that triacanthine experimentally activates GPR84. We demonstrate that phenethylamine and agmatine significantly reduce tau hyperphosphorylation (p-tau181 and p-tau205) in AD patient induced pluripotent stem cell-derived neurons. This study demonstrates a systems biology framework to uncover the GPCR targets of human gut microbiota in AD and other complex diseases if broadly applied.

[Ginkgolide B: mechanisms of neurobiological effects, prospects for use in the therapy of Alzheimer's disease]

The review presents an analysis of experimental data on the study of neurobiological effects of ginkgolide B, which may find application in the therapy of Alzheimer's disease (AD). Ginkgolide B is a diterpene trilactone isolated from the leaves of the relict woody plant Ginkgo biloba L., which has been used for thousands of years in traditional Chinese medicine as a neuroprotective agent. In recent years, this compound has attracted attention because of its wide range of neurobiological effects. The neuroprotective effect of ginkgolide B on brain neurons when exposed to various neurotoxins has been established. This compound has also been shown to effectively protect neurons from the effects of beta-amyloid. Studies have revealed the ability of ginkgolide B to reduce microglia activity and regulate neurotransmitter release. In vivo experiments have shown that this substance significantly increases the expression of brain-derived neurotrophic factor (BDNF) and improves cognitive functions, including memory and learning. It is concluded that ginkgolide B, apparently, may find application in the future as a multi-targeted agent of complex therapy of AD.

Active Compounds of Panax ginseng in the Improvement of Alzheimer's Disease and Application of Spatial Metabolomics

Panax ginseng C.A. Meyer (P. ginseng) is one of the more common traditional Chinese medicines (TCMs). It contains numerous chemical components and exhibits a range of pharmacological effects. An enormous burden is placed on people's health and life by Alzheimer's disease (AD), a neurodegenerative condition. Recent research has shown that P. ginseng's chemical constituents, particularly ginsenosides, have a significant beneficial impact on the prevention and management of neurological disorders. To understand the current status of research on P. ginseng to improve AD, this paper discusses the composition of P. ginseng, its mechanism of action, and its clinical application. The pathogenesis of AD includes amyloid beta protein (Aβ) generation and aggregation, tau protein hyperphosphorylation, oxidant stress, neuroinflammation, mitochondrial damage, and neurotransmitter and gut microbiota disorders. This review presents the key molecular mechanisms and signaling pathways of the active ingredients in P. ginseng involved in improving AD from the perspective of AD pathogenesis. A P. ginseng-related signaling pathway network was constructed to provide effective targets for the treatment of AD. In addition, the application of spatial metabolomics techniques in studying P. ginseng and AD is discussed. In summary, this paper discusses research perspectives for the study of P. ginseng in the treatment of AD, including a systematic and in-depth review of the mechanisms of action of the active substances in P. ginseng, and evaluates the feasibility of applying spatial metabolomics in the study of AD pathogenesis and pharmacological treatment.

Advances in the study of the effects of gut microflora on microglia in Alzheimer's disease

Alzheimer's disease (AD) is a central nervous system (CNS) degenerative disorder, is caused by various factors including β-amyloid toxicity, hyperphosphorylation of tau protein, oxidative stress, and others. The dysfunction of microglia has been associated with the onset and advancement of different neurodevelopmental and neurodegenerative disorders, such as AD. The gut of mammals harbors a vast and complex population of microorganisms, commonly referred to as the microbiota. There's a growing recognition that these gut microbes are intrinsically intertwined with mammalian physiology. Through the circulation of metabolites, they establish metabolic symbiosis, enhance immune function, and establish communication with different remote cells, including those in the brain. The gut microbiome plays a crucial part in influencing the development and performance of microglia, as indicated by recent preclinical studies. Dysbiosis of the intestinal flora leads to alterations in the microglia transcriptome that regulate the interconversion of microglia subtypes. This conversation explores recent research that clarifies how gut bacteria, their byproducts, and harmful elements affect the activation and characteristics of microglia. This understanding opens doors to innovative microbial-based therapeutic strategies for early identification and treatment goals in AD.

Microbiome Alterations and Alzheimer's Disease: Modeling Strategies with Transgenic Mice

In the last decade, the role of the microbiota-gut-brain axis has been gaining momentum in the context of many neurodegenerative and metabolic disorders, including Alzheimer's disease (AD) and diabetes, respectively. Notably, a balanced gut microbiota contributes to the epithelial intestinal barrier maintenance, modulates the host immune system, and releases neurotransmitters and/or neuroprotective short-chain fatty acids. However, dysbiosis may provoke immune dysregulation, impacting neuroinflammation through peripheral-central immune communication. Moreover, lipopolysaccharide or detrimental microbial end-products can cross the blood-brain barrier and induce or at least potentiate the neuropathological progression of AD. Thus, after repeated failure to find a cure for this dementia, a necessary paradigmatic shift towards considering AD as a systemic disorder has occurred. Here, we present an overview of the use of germ-free and/or transgenic animal models as valid tools to unravel the connection between dysbiosis, metabolic diseases, and AD, and to investigate novel therapeutical targets. Given the high impact of dietary habits, not only on the microbiota but also on other well-established AD risk factors such as diabetes or obesity, consistent changes of lifestyle along with microbiome-based therapies should be considered as complementary approaches.

Gut Microbiota is an Impact Factor based on the Brain-Gut Axis to Alzheimer's Disease: A Systematic Review

Alzheimer's disease (AD) is a degenerative disease of the central nervous system. The pathogenesis of AD has been explained using cholinergic, β-amyloid toxicity, tau protein hyperphosphorylation, and oxidative stress theories. However, an effective treatment method has not been developed. In recent years, with the discovery of the brain-gut axis (BGA) and breakthroughs made in Parkinson's disease, depression, autism, and other diseases, BGA has become a hotspot in AD research. Several studies have shown that gut microbiota can affect the brain and behavior of patients with AD, especially their cognitive function. Animal models, fecal microbiota transplantation, and probiotic intervention also provide evidence regarding the correlation between gut microbiota and AD. This article discusses the relationship and related mechanisms between gut microbiota and AD based on BGA to provide possible strategies for preventing or alleviating AD symptoms by regulating gut microbiota.

The Association between Dysbiosis and Neurological Conditions Often Manifesting with Chronic Pain

The prevalence of neurological conditions which manifest with chronic pain is increasing globally, where the World Health Organisation has now classified chronic pain as a risk factor for death by suicide. While many chronic pain conditions have a definitive underlying aetiology, non-somatic conditions represent difficult-to-diagnose and difficult-to-treat public health issues. The interaction of the immune system and nervous system has become an important area in understanding the occurrence of neuroinflammation, nociception, peripheral and central sensitisation seen in chronic pain. More recently, however, the role of the resident microbial species in the human gastrointestinal tract has become evident. Dysbiosis, an alteration in the microbial species present in favour of non-beneficial and pathogenic species has emerged as important in many chronic pain conditions, including functional somatic syndromes, autoimmune disease and neurological diseases. In particular, a decreased abundance of small chain fatty acid, e.g., butyrate-producing bacteria, including Faecalibacterium, Firmicutes and some Bacteroides spp., is frequently evident in morbidities associated with long-term pain. Microbes involved in the production of neurotransmitters serotonin, GABA, glutamate and dopamine, which mediate the gut-brain, axis are also important. This review outlines the dysbiosis present in many disease states manifesting with chronic pain, where an overlap in morbidities is also frequently present in patients.

Gut Microbiota and Alzheimer's Disease: How to Study and Apply Their Relationship

Gut microbiota (GM), the microorganisms in the gastrointestinal tract, contribute to the regulation of brain homeostasis through bidirectional communication between the gut and the brain. GM disturbance has been discovered to be related to various neurological disorders, including Alzheimer's disease (AD). Recently, the microbiota-gut-brain axis (MGBA) has emerged as an enticing subject not only to understand AD pathology but also to provide novel therapeutic strategies for AD. In this review, the general concept of the MGBA and its impacts on the development and progression of AD are described. Then, diverse experimental approaches for studying the roles of GM in AD pathogenesis are presented. Finally, the MGBA-based therapeutic strategies for AD are discussed. This review provides concise guidance for those who wish to obtain a conceptual and methodological understanding of the GM and AD relationship with an emphasis on its practical application.

Estrogen signaling via estrogen receptor alpha and its implications for neurodegeneration associated with Alzheimer's disease in aging women

Estrogen receptor alpha (ERα) is a transcription factor activated by estrogenic hormones to regulate gene expression in certain organs, including the brain. In the brain, estrogen signaling pathways are central for maintaining cognitive functions. Herein, we review the neuroprotective effects of estrogens mediated by ERα. The estrogen/ERα pathways are affected by the reduction of estrogens in menopause, and this event may be a risk factor for neurodegeneration associated with Alzheimer's disease in women. Thus, developing a better understanding of estrogen/ERα signaling may be critical for defining new biomarkers and potential therapeutic targets for Alzheimer's disease in women.

A Recent Update on Pathophysiology and Therapeutic Interventions of Alzheimer's Disease

AIM

Alzheimer's disease (AD) has been identified as a progressive brain disorder associated with memory dysfunction and the accumulation of β-amyloid plaques and neurofibrillary tangles of τ protein. Mitochondria is crucial in maintaining cell survival, cell death, calcium regulation, and ATP synthesis. Mitochondrial dysfunction and linked calcium overload have been involved in the pathogenesis of AD. CRM2 (Collapsin response mediator protein-2) is involved in endosomal lysosomal trafficking as well as autophagy, and their reduced level is also a primary culprit in the progression of AD. In addition, Cholinergic neurotransmission and neuroinflammation are two other mechanisms implicated in AD onset and might be protective targets to attenuate disease progression. The microbiota-gut-brain axis (MGBA) is another crucial target for AD treatment. Crosstalk between gut microbiota and brain mutually benefitted each other, dysbiosis in gut microbiota affects the brain functions and leads to AD progression with increased AD-causing biomarkers. Despite the complexity of AD, treatment is only limited to symptomatic management. Therefore, there is an urgent demand for novel therapeutics that target associated pathways responsible for AD pathology. This review explores the role of different mechanisms involved in AD and possible therapeutic targets to protect against disease progression.

BACKGROUND

Amidst various age-related diseases, AD is the most deleterious neurodegenerative disorder that affects more than 24 million people globally. Every year, approximately 7.7 million new cases of dementia have been reported. However, to date, no novel disease-modifying therapies are available to treat AD.

OBJECTIVE

The aim of writing this review is to highlight the role of key biomarker proteins and possible therapeutic interventions that could play a crucial role in mitigating the ongoing prognosis of Alzheimer's disease.

MATERIALS AND METHODS

The available information about the disease was collected through multiple search engines, including PubMed, Science Direct, Clinical Trials, and Google Scholar.

RESULTS

Accumulated pieces of evidence reveal that extracellular aggregation of β-amyloid plaques and intracellular tangles of τ protein are peculiar features of perpetuated Alzheimer's disease (AD). Further, the significant role of mitochondria, calcium, and cholinergic pathways in the pathogenesis of AD makes the respiratory cell organelle a crucial therapeutic target in this neurodegenerative disease. All currently available drugs either delay the clinical damage to cells or temporarily attenuate some symptoms of Alzheimer's disease.

CONCLUSION

The pathological features of AD are extracellular deposition of β-amyloid, acetylcholinesterase deregulation, and intracellular tangles of τ protein. The multifactorial heterogeneity of disease demands more research work in this field to find new therapeutic biological targets.

Gut-Brain Axis, Neurodegeneration and Mental Health: A Personalized Medicine Perspective

Neurological conditions such as neurodegenerative diseases and mental health disorders are a result of multifactorial underpinnings, leading to individual-based complex phenotypes. Demystification of these multifactorial connections will promote disease diagnosis and treatment. Personalized treatment rather than a one-size-fits-all approach would enable us to cater to the unmet healthcare needs based on protein-protein and gene-environment interactions. Gut-brain axis, as the name suggests, is a two-way biochemical communication pathway between the central nervous system (CNS) and enteric nervous system (ENS), enabling a mutual influence between brain and peripheral intestinal functions. The gut microbiota is a major component of this bidirectional communication, the composition of which is varied depending on the age, and disease conditions, among other factors. Gut microbiota profile is typically unique and personalized therapeutic intervention can aid in treating or delaying neurodegeneration and mental health conditions. Besides, research on the gut microbial influence on these conditions is gaining attention, and a better understanding of this concept can lead to identification of novel targeted therapies.

Graphical Abstract

Human Gut Microbiota in Coronary Artery Disease: A Systematic Review and Meta-Analysis

In recent years, the importance of the gut microbiome in human health and disease has increased. Growing evidence suggests that gut dysbiosis might be a crucial risk factor for coronary artery disease (CAD). Therefore, we conducted a systematic review and meta-analysis to determine whether or not CAD is associated with specific changes in the gut microbiome. The V3-V4 regions of the 16S rDNA from fecal samples were analyzed to compare the gut microbiome composition between CAD patients and controls. Our search yielded 1181 articles, of which 21 met inclusion criteria for systematic review and 7 for meta-analysis. The alpha-diversity, including observed OTUs, Shannon and Simpson indices, was significantly decreased in CAD, indicating the reduced richness of the gut microbiome. The most consistent results in a systematic review and meta-analysis pointed out the reduced abundance of Bacteroidetes and Lachnospiraceae in CAD patients. Moreover, Enterobacteriaceae, Lactobacillus, and Streptococcus taxa demonstrated an increased trend in CAD patients. The alterations in the gut microbiota composition are associated with qualitative and quantitative changes in bacterial metabolites, many of which have pro-atherogenic effects on endothelial cells, increasing the risk of developing and progressing CAD.

Gut Microbiota Shifting in Irritable Bowel Syndrome: The Mysterious Role of Blastocystis sp

Irritable bowel syndrome (IBS) is a chronic disorder, which its causative agent is not completely clear; however, the interaction between microorganisms and gastrointestinal (GI) epithelial cells plays a critical role in the development of IBS and presenting symptoms. During recent decades, many studies have highlighted the high prevalence of Blastocystis sp. in patients with IBS and suggested a probable role for this protist in this disease. Recent studies have documented changes in the gut microbiota composition in patients with IBS regarding the presence of Blastocystis sp., but it is not clear that either disturbance of the gut during GI disorders is a favorable condition for Blastocystis sp. colonization or the presence of this protist may lead to alteration in the gut microbiota in IBS patients. In this review, we comprehensively gather and discuss scientific findings covering the role of Blastocystis sp. in IBS via gut microbiota shifting.

Neuroprotective Effects of Probiotic-Supplemented Diet on Cognitive Behavior of 3xTg-AD Mice

Alzheimer's disease (AD) is recognized as one of the most common types of senile dementia. AD patients first suffer memory loss for recent events (short-term memory impairment). As the disease progresses, they are deprived of self-awareness. This study aims to explore the effects of a probiotic-supplemented diet on the cognitive behaviors and pathological features of mouse models of Alzheimer's disease (AD). Mice in the control group and the 3xTg-AD group were fed a regular diet and a probiotic-supplemented diet, respectively, for 20 weeks. Behavioral experiments like Morris's water maze and Y maze were conducted. Then, feces of mice were collected for 16S sRNA gene sequencing for microorganisms. In the end, soluble and insoluble Aβ40 and Aβ42 in the hippocampus and cortex of mice in each group were quantitatively analyzed with a double-antibody Sandwich ELISA. The expression levels of tau protein and gliocyte in the hippocampus and cortex were detected using the Western Blot method. The result of the Morris water maze experiment indicated that, in the place navigation test, the mice in the 3xTg-AD group experienced a significant decline in the learning ability and a longer escape latency and in the space exploration test, the swimming time of mice in the 3xTg-AD group in the target quadrant decreased and after being treated with the probiotic diet, mice in the 3xTg-AD group had improved learning and memory ability. The result of Y maze showed that the probiotic diet can improve the spontaneous alternation accuracy of mice in the 3xTg-AD group. The result of 16s rRNA gene sequencing showed that, compared with mice in the WT group, those in the 3xTg-AD group experienced a change in the intestinal flora. The Western Blot result displayed a decreased expression level of tau (pS202) (P < 0.05) and decreased expression levels of Iba-1 and GFAP (P < 0.05). The result of the ELISA experiment showed decreased levels of soluble and insoluble Aβ40 and Aβ42 in 3xTg-AD mice (P < 0.05). In conclusion, a probiotic diet can prevent and treat AD by improving the intestinal flora of 3xTg-AD.

A Tale of Two Diseases: Exploring Mechanisms Linking Diabetes Mellitus with Alzheimer's Disease

Dementias, including the type associated with Alzheimer's disease (AD), are on the rise worldwide. Similarly, type 2 diabetes mellitus (T2DM) is one of the most prevalent chronic diseases globally. Although mechanisms and treatments are well-established for T2DM, there remains much to be discovered. Recent research efforts have further investigated factors involved in the etiology of AD. Previously perceived to be unrelated diseases, commonalities between T2DM and AD have more recently been observed. As a result, AD has been labeled as "type 3 diabetes". In this review, we detail the shared processes that contribute to these two diseases. Insulin resistance, the main component of the pathogenesis of T2DM, is also present in AD, causing impaired brain glucose metabolism, neurodegeneration, and cognitive impairment. Dysregulation of insulin receptors and components of the insulin signaling pathway, including protein kinase B, glycogen synthase kinase 3β, and mammalian target of rapamycin are reported in both diseases. T2DM and AD also show evidence of inflammation, oxidative stress, mitochondrial dysfunction, advanced glycation end products, and amyloid deposition. The impact that changes in neurovascular structure and genetics have on the development of these conditions is also being examined. With the discovery of factors contributing to AD, innovative treatment approaches are being explored. Investigators are evaluating the efficacy of various T2DM medications for possible use in AD, including but not limited to glucagon-like peptide-1 receptor agonists, and peroxisome proliferator-activated receptor-gamma agonists. Furthermore, there are 136 active trials involving 121 therapeutic agents targeting novel AD biomarkers. With these efforts, we are one step closer to alleviating the ravaging impact of AD on our communities.

Introduction to the Special Issue "The Brain-Gut Axis"

This special Issue presents comprehensive and state-of-the-art advances in supporting the crucial role of the bidirectional interactions between the Brain-Gut Axis in health and diseases with an emphasis on the microbiome-gut-brain axis and its implications in variety of neurological disorders. There are intimate connections between the brain and the digestive system. Gut microbiota dysbiosis activates the intestinal immune system, enhances intestinal permeability and bacterial translocation, leading to neuroinflammation, epigenetic changes, cerebrovascular alterations, amyloid β formation and α-synuclein protein aggregates. These alterations may participate in the development of hypertension, Alzheimer, Parkinson, stroke, epilepsy and autism. Brainstem nuclei such as the nucleus tractus solitarius (NTS) and the dorsal motor nucleus of the vagus (DMV) regulate gastric motor function by way of bidirectional inputs through the vagus nerve.

Fecal Microbiota Transplantation: A Microbiome Modulation Technique for Alzheimer's Disease

Alzheimer’s disease (AD) is the most common form of dementia and the fifth leading cause of death among the elderly. AD involves parts of the brain that can lead to progressive memory loss and impaired language skills and cognitive thinking, affecting one’s ability to carry out daily activities. Aging, bad dietary habits, family history, as well as altered gut microbiota composition may play a role in the pathogenesis of AD. Although the association between the imbalance of gut microbiota and AD is still difficult to determine, it has been suggested that dysbiosis can lead to the increased secretion of lipopolysaccharides and amyloid, which may impair the permeability of the intestine and the blood-brain barrier. Moreover, it can progress the process of neuroinflammation, amyloid-beta formation, and ultimately neuronal death. Microbiota-targeted interventions such as personalized diet, probiotics, or fecal microbiota transplantation (FMT) might represent a potential therapeutic option for AD. This review article discusses the procedure of FMT and its possible side effects on the recipient’s body. In addition, we review the role of FMT in the context of its application in various nervous system-related disorders (AD, Parkinson’s disease, multiple sclerosis).

Roles and Mechanisms of Gut Microbiota in Patients With Alzheimer's Disease

Alzheimer’s disease (AD) is the most common age-related progressive neurodegenerative disease, characterized by a decline in cognitive function and neuronal loss, and is caused by several factors. Numerous clinical and experimental studies have suggested the involvement of gut microbiota dysbiosis in patients with AD. The altered gut microbiota can influence brain function and behavior through the microbiota–gut–brain axis via various pathways such as increased amyloid-β deposits and tau phosphorylation, neuroinflammation, metabolic dysfunctions, and chronic oxidative stress. With no current effective therapy to cure AD, gut microbiota modulation may be a promising therapeutic option to prevent or delay the onset of AD or counteract its progression. Our present review summarizes the alterations in the gut microbiota in patients with AD, the pathogenetic roles and mechanisms of gut microbiota in AD, and gut microbiota–targeted therapies for AD. Understanding the roles and mechanisms between gut microbiota and AD will help decipher the pathogenesis of AD from novel perspectives and shed light on novel therapeutic strategies for AD.