Does Severity of Alzheimer's Disease Contribute to Its Responsiveness to Modifying Gut Microbiota? A Double Blind Clinical Trial

Interplay of Good Bacteria and Central Nervous System: Cognitive Aspects and Mechanistic Considerations

The human gastrointestinal tract hosts trillions of microorganisms that is called “gut microbiota.” The gut microbiota is involved in a wide variety of physiological features and functions of the body. Thus, it is not surprising that any damage to the gut microbiota is associated with disorders in different body systems. Probiotics, defined as living microorganisms with health benefits for the host, can support or restore the composition of the gut microbiota. Numerous investigations have proved a relationship between the gut microbiota with normal brain function as well as many brain diseases, in which cognitive dysfunction is a common clinical problem. On the other hand, increasing evidence suggests that the existence of a healthy gut microbiota is crucial for normal cognitive processing. In this regard, interplay of the gut microbiota and cognition has been under focus of recent researches. In the present paper, I review findings of the studies considering beneficial effects of either gut microbiota or probiotic bacteria on the brain cognitive function in the healthy and disease statuses.

The impact of the microbiota-gut-brain axis on Alzheimer's disease pathophysiology

The gut microbiota is a complex ecosystem that comprises of more than 100 trillion symbiotic microbial cells. The microbiota, the gut, and the brain form an association, 'the microbiota-gut-brain axis,' and synchronize the gut with the central nervous system and modify the behavior and brain immune homeostasis. The bidirectional communication between gut and brain occurs via the immune system, the vagus nerve, the enteric nervous system, and microbial metabolites, including short-chain fatty acids (SCFAs), proteins, and tryptophan metabolites. Recent studies have implicated the gut microbiota in many neurodegenerative diseases, including Alzheimer's disease (AD). In this review, we present an overview of gut microbiota, including Firmicutes, Bacteroidetes, SCFA, tryptophan, bacterial composition, besides age-related changes in gut microbiota composition, the microbiota-gut-brain axis pathways, the role of gut metabolites in amyloid-beta clearance, and gut microbiota modulation from experimental and clinical AD models. Understanding the role of the microbiota may provide new targets for treatment to delay the onset, progression, or reverse AD, and may help in reducing the prevalence of AD.

Roles of Gut Microbiota in the Regulation of Hippocampal Plasticity, Inflammation, and Hippocampus-Dependent Behaviors

The study of the gut microbiota-brain axis has become an intriguing field, attracting attention from both gastroenterologists and neurobiologists. The hippocampus is the center of learning and memory, and plays a pivotal role in neurodegenerative diseases, such as Alzheimer’s disease (AD). Previous studies using diet administration, antibiotics, probiotics, prebiotics, germ-free mice, and fecal analysis of normal and specific pathogen-free animals have shown that the structure and function of the hippocampus are affected by the gut microbiota. Furthermore, hippocampal pathologies in AD are positively correlated with changes in specific microbiota. Genomic and neurochemical analyses revealed significant alterations in genes and amino acids in the hippocampus of AD subjects following a remarkable shift in the gut microbiota. In a recent study, when young animals were transplanted with fecal microbiota derived from AD patients, the recipients showed significant impairment of cognitive behaviors, AD pathologies, and changes in neuronal plasticity and cytokines. Other studies have demonstrated the side effects of antibiotic administration along with the beneficial effects of probiotics, prebiotics, and specific diets on the composition of the gut microbiota and hippocampal functions, but these have been mostly preliminary with unclear mechanisms. Since some specific gut bacteria are positively or negatively correlated to the structure and function of the hippocampus, it is expected that specific gut bacteria administration and other microbiota-based interventions could be potentially applied to prevent or treat hippocampus-based memory impairment and neuropsychiatric disorders such as AD.

The Immunopathogenesis of Alzheimer's Disease Is Related to the Composition of Gut Microbiota

The microbiota-gut-brain axis plays an important role in the development of neurodegenerative diseases. Commensal and pathogenic enteric bacteria can influence brain and immune system function by the production of lipopolysaccharides and amyloid. Dysbiosis of the intestinal microbiome induces local and consecutively systemic immune-mediated inflammation. Proinflammatory cytokines then trigger neuroinflammation and finally neurodegeneration. Immune-mediated oxidative stress can lead to a deficiency of vitamins and essential micronutrients. Furthermore, the wrong composition of gut microbiota might impair the intake and metabolization of nutrients. In patients with Alzheimer's disease (AD) significant alterations of the gut microbiota have been demonstrated. Standard Western diet, infections, decreased physical activity and chronic stress impact the composition and diversity of gut microbiota. A higher abundancy of "pro-inflammatory" gut microbiota goes along with enhanced systemic inflammation and neuroinflammatory processes. Thus, AD beginning in the gut is closely related to the imbalance of gut microbiota. Modulation of gut microbiota by Mediterranean diet, probiotics and curcumin can slow down cognitive decline and alter the gut microbiome significantly. A multi-domain intervention approach addressing underlying causes of AD (inflammation, infections, metabolic alterations like insulin resistance and nutrient deficiency, stress) appears very promising to reduce or even reverse cognitive decline by exerting positive effects on the gut microbiota.

Environmental Nanoparticles, SARS-CoV-2 Brain Involvement, and Potential Acceleration of Alzheimer's and Parkinson's Diseases in Young Urbanites Exposed to Air Pollution

Alzheimer's and Parkinson's diseases (AD, PD) have a pediatric and young adult onset in Metropolitan Mexico City (MMC). The SARS-CoV-2 neurotropic RNA virus is triggering neurological complications and deep concern regarding acceleration of neuroinflammatory and neurodegenerative processes already in progress. This review, based on our MMC experience, will discuss two major issues: 1) why residents chronically exposed to air pollution are likely to be more susceptible to SARS-CoV-2 systemic and brain effects and 2) why young people with AD and PD already in progress will accelerate neurodegenerative processes. Secondary mental consequences of social distancing and isolation, fear, financial insecurity, violence, poor health support, and lack of understanding of the complex crisis are expected in MMC residents infected or free of SARS-CoV-2. MMC residents with pre-SARS-CoV-2 accumulation of misfolded proteins diagnostic of AD and PD and metal-rich, magnetic nanoparticles damaging key neural organelles are an ideal host for neurotropic SARS-CoV-2 RNA virus invading the body through the same portals damaged by nanoparticles: nasal olfactory epithelium, the gastrointestinal tract, and the alveolar-capillary portal. We urgently need MMC multicenter retrospective-prospective neurological and psychiatric population follow-up and intervention strategies in place in case of acceleration of neurodegenerative processes, increased risk of suicide, and mental disease worsening. Identification of vulnerable populations and continuous effort to lower air pollution ought to be critical steps.

Probiotics treatment improves cognitive impairment in patients and animals: A systematic review and meta-analysis

The gut-brain axis has received considerable attention in recent years, and the "psychobiotics" concept indicates that probiotics have a potential positive effect on cognitive function. Therefore, the aim of this study was to quantitatively evaluate the influence of probiotics on cognition. We conducted a random-eff ;ects meta-analysis of 7 controlled clinical trials and 11 animals studies to evaluate the eff ;ects of probiotics on cognitive function. Probiotics supplementation enhanced cognitive function in both human (0.24 [0.05-0.42]; I² = 0 %) and animal studies (0.90 [0.47-1.34]; I² = 74 %). Subgroup analyses indicated that the effects of probiotics on cognitively impaired individuals (0.25 [0.05-0.45]; I² = 0 %) were greater than those on healthy ones (0.15 [-0.30 to 0.60]; I² = 0 %). Furthermore, compared with a multiple-probiotic supplement, a single strain of probiotics was more effective in humans. The meta-analysis provided some suggestions for probiotics intervention and tended to support a customized approach for different individuals to ameliorate cognitive disorders. Future additional clinical trials are necessary to evaluate therapeutic effect and influencing factors.

Functional Foods: An Approach to Modulate Molecular Mechanisms of Alzheimer's Disease

A new epoch is emerging with intense research on nutraceuticals, i.e., "food or food product that provides medical or health benefits including the prevention and treatment of diseases", such as Alzheimer's disease. Nutraceuticals act at different biochemical and metabolic levels and much evidence shows their neuroprotective effects; in particular, they are able to provide protection against mitochondrial damage, oxidative stress, toxicity of β-amyloid and Tau and cell death. They have been shown to influence the composition of the intestinal microbiota significantly contributing to the discovery that differential microorganisms composition is associated with the formation and aggregation of cerebral toxic proteins. Further, the routes of interaction between epigenetic mechanisms and the microbiota-gut-brain axis have been elucidated, thus establishing a modulatory role of diet-induced epigenetic changes of gut microbiota in shaping the brain. This review examines recent scientific literature addressing the beneficial effects of some natural products for which mechanistic evidence to prevent or slowdown AD are available. Even if the road is still long, the results are already exceptional.

Role of gut-brain axis, gut microbial composition, and probiotic intervention in Alzheimer's disease

Gut microbiota represents a diverse and dynamic population of microorganisms harboring the gastrointestinal tract, which influences the host health and disease. Gut microbiota communicates with the brain and vice versa through complex bidirectional communication systems - the gut-brain axis, which integrates the peripheral intestinal function with emotional and cognitive brain centers via neuro-immuno-endocrine mediators. Aging alters the gut microbial population, which not only leads to gastrointestinal disturbances but also causes central nervous system (CNS) disorders such as dementia. Alzheimer's disease (AD) is the most common form of dementia affecting the older person, characterized by beta-amyloid (Aβ) plaques and neurofibrillary tangles leading to the cognitive deficit and memory impairment. Multiple experimental and clinical studies revealed the role of gut microbiota in host cognition, and its dysbiosis associated with aging leads to neurodegeneration. Gut microbial dysbiosis leads to the secretion of amyloid and lipopolysaccharides (LPS), which disturbs the gastrointestinal permeability and blood-brain barrier. Thereby modulates the inflammatory signaling pathway promoting neuroinflammation, neuronal injury, and ultimately leading to neuronal death in AD. A recent study revealed the antimicrobial property of Aβ peptide as an innate immune response against pathogenic microbes. Another study showed that bacterial amyloid shares molecular mimicry with Aβ peptide, which elicits misfolding and aggregation of Aβ peptide, it's seeding, and propagation through the gut-brain axis followed by microglial cell activation. As aging together with poor diet and gut-derived inflammatory response due to dysbiosis contributes to the pathogenesis of AD, modification of gut microbial composition by uptake of probiotic-rich food can act as a preventive/therapeutic option for AD. The objective of the present review is to summarize the recent findings on the role of gut microbiota in the development of AD. Understanding the relationship between gut microbiota and CNS will help identify novel therapeutic strategies, especially probiotic-based supplementation, for the treatment of AD.

Target Dysbiosis of Gut Microbes as a Future Therapeutic Manipulation in Alzheimer's Disease

Alzheimer’s disease (AD) is commonly an age-associated dementia with neurodegeneration. The pathogenesis of AD is complex and still remains unclear. The inflammation, amyloid β (Aβ), and neurofibrillary tangles as well misfolded tau protein in the brain may contribute to the occurrence and development of AD. Compared with tau protein, Aβ is less toxic. So far, all efforts made in the treatments of AD with targeting these pathogenic factors were unsuccessful over the past decades. Recently, many studies demonstrated that changes of the intestinal environment and gut microbiota via gut–brain axis pathway can cause neurological disorders, such as AD, which may be involved in the pathogenesis of AD. Thus, remodeling the gut microbiota by various ways to maintain their balance might be a novel therapeutic strategy for AD. In the review article, we analyzed the characteristics of gut microbiota and its dysbiosis in AD and its animal models and investigated the possibility of targeting the gut microbiota in the treatment of the patients with AD in the future.

The effects of probiotic/synbiotic supplementation compared to placebo on biomarkers of oxidative stress in adults: a systematic review and meta-analysis of randomized controlled trials

BACKGROUND AND AIMS

During the last decades, there has been a burst of scientific literature hypothesizing the antioxidant effect of probiotics. However, the results of these studies are inconsistent and a final conclusion has yet to be reached. Thus, the aim of this study was to assess the effects of probiotic/synbiotic supplementation on serum total antioxidant capacity (TAC), glutathione (GSH), malondialdehyde (MDA) and nitric oxide (NO) levels in adults.

METHODS AND RESULTS

The following online databases were searched until August 26th 2020: PubMed/Medline, Scopus, Clarivate Analytics Web of Science, Cochrane Central Register of Controlled Trials, Science Direct, Google Scholar and Igaku Chuo Zasshi. The effect sizes were expressed as the weighted mean difference (WMD) with 95% confidence intervals (CI). A total of 31 eligible trials with 1681 participants (839 cases and 842 controls) were included in this meta-analysis. The results revealed that the supplementation with probiotics/synbiotics, significantly increased serum TAC (WMD: 54.14 mmol/L, 95% CI: 27.87, 80.40, P < 0.001), GSH (WMD: 40.38 μmol/L, 95% CI: 20.72, 60.03, P < 0.001) and NO (WMD: 3.54 μmol/L, 95% CI: 1.73, 5.34, P < 0.001) levels. In addition, MDA levels were significantly reduced (WMD: -0.45 μmol/L, 95% CI: -0.58,-0.32, P < 0.001) following probiotic/synbiotic supplementation. None of the variables showed a significant change in the sensitivity analysis.

CONCLUSION

Available evidence suggests that probiotic/synbiotic supplementation can significantly increase serum TAC, GSH and NO, as well as reduce MDA levels in adults. Therefore, probiotic/synbiotic supplementation may play a role in improving antioxidant indices and reducing oxidative stress in the body.

Gut Microbiota: Implications in Alzheimer's Disease

Alzheimer’s disease (AD), the most common cause of dementia, is a neurodegenerative disease that seriously threatens human health and life quality. The main pathological features of AD include the widespread deposition of amyloid-beta and neurofibrillary tangles in the brain. So far, the pathogenesis of AD remains elusive, and no radical treatment has been developed. In recent years, mounting evidence has shown that there is a bidirectional interaction between the gut and brain, known as the brain–gut axis, and that the intestinal microbiota are closely related to the occurrence and development of neurodegenerative diseases. In this review, we will summarize the laboratory and clinical evidence of the correlation between intestinal flora and AD, discuss its possible role in the pathogenesis, and prospect its applications in the diagnosis and treatment of AD.

Associations Between Gut Microbiota and Alzheimer's Disease: Current Evidences and Future Therapeutic and Diagnostic Perspectives

Developing novel agents for unexplored targets to combat Alzheimer's disease (AD) represents an urgent task due to its increasing prevalence worldwide. The present paper summarizes the latest studies emerged in the past few years investigating the associations between the "forgotten organ" gut microbiota and AD from the following two aspects: 1) the associations between gut microbiota and AD development by animal models and human studies; and 2) the effects of gut microbiota modulation-based intervention for AD. Then, we propose future perspectives in two promising research areas: 1) developing gut microbiota modulation-based intervention; and 2) developing gut microbiota-associated diagnostic biomarkers for AD. Knowledge gaps and potential barriers to overcome towards these two goals are also discussed.

Probiotic treatment differentially affects the behavioral and electrophysiological aspects in ethanol exposed animals

OBJECTIVES

Harmful effects of alcohol on brain function including cognitive phenomena are well known. Damage to gut microbiota is linked to neurological disorders. Evidence indicates that intestinal flora can be strengthened by probiotic bacteria. In this study, we evaluated the effect of probiotics administration on LTP induction in rats receiving ethanol.

MATERIALS AND METHODS

To assess if probiotic treatment influences toxic effect of ethanol, vehicle (CON) and probiotic treated (CON+PRO) control rats, and chronic ethanol (CE) exposed and CE probiotic treated (CE+PRO) animals were entered into the experiments. Shuttle box test and in vivo electrophysiological recordings were accomplished to evaluate memory and hippocampal baseline filed excitatory postsynaptic potentials (fEPSPs) and long term potentiation (LTP), respectively.

RESULTS

Ethanol impaired memory in the CE rats. It also diminished the slope size of fEPSPs and prevented LTP induction. While the probiotic supplementation improved memory in the CE+PRO rats, it did not influence synaptic transmission in these animals.

CONCLUSION

Conclusively, behavioral but not electrophysiological aspect of cognition is sensitive to probiotic treatment in the ethanol exposed animals.

Adapted MMSE and TYM cognitive tests: how much powerful in screening for Alzheimer's disease in Iranian people

Alzheimer's disease (AD) is a major global health priority and providing an efficient way for early diagnosis of people developing dementia is important. The Mini-Mental State Examination (MMSE, total score = 30) and Test Your Memory (TYM, total score = 50) are widely used as screening tests for cognitive function. In the present study 174 subjects including healthy people (CON group) and those having Alzheimer's disease (AD group) were introduced to MMSE and TYM cognitive tests adjusted to Iranian population. Sensitivities and specificities with optimal cut-off scores, area under curve (AUC), positive predictive value (PPV) and negative predictive value (NPV) were measured for both tests. The MMSE scores of the CON and AD groups were 23.77 ± 0.327 and 10.88 ± 0.762, respectively. The TYM scores were 44.32 ± 0.389 and 14.37 ± 1.368 in the CON and AD participants, respectively. Findings in the MMSE test were: AUC = 0.962, optimal cut-off score = 18.5, sensitivity = 0.90 and specificity = 0.96. Values in the TYM test were: AUC = 0.991, optimal cut-off score = 31, sensitivity = 0.90 and specificity = 1. We found no correlation between the cognitive performance and age in the CON group but a positive correlation in the AD patients. On the other hand, t-test analysis indicated that achievement of the test scores are significantly sex dependent, with more scores attained by the females. Taken together, in regard to correct classification rate (CCR); the TYM test seems to be more appropriate for cognitive screening in our study. However, considering an analogous AUC, both tests are comparable and have high sensitivity and specificity for discriminating between people with and without AD.

Clinical Trials of Probiotic Strains in Selected Disease Entities

Probiotics are live microorganisms which when administered in adequate amounts confer a health benefit on the host. Although their mechanism of action is not clearly explained, it is known that they positively modulate the immune system, which leads to immunity potentiation. A number of studies prove that probiotics strengthen cognitive functions, reduce anxiety, and regulate the lipid metabolism in the human body. Probiotics used in humans are most often of the Lactobacillus and Bifidobacterium species. However, as more research is conducted, new species with beneficial, probiotic properties are being discovered. This paper provides a review of available information about the influence of probiotics on human health. It summarizes the current knowledge on the mechanism of action of probiotics as well as clinical trial results proving their efficacy in allergic, neurodegenerative, and cardiac diseases. This review also discusses the data concerning the safety of probiotics in clinical treatment.

Role of gut microbiota on intestinal barrier function in acute pancreatitis

Acute pancreatitis (AP) is a common gastrointestinal disorder. Approximately 15%-20% of patients develop severe AP. Systemic inflammatory response syndrome and multiple organ dysfunction syndrome may be caused by the massive release of inflammatory cytokines in the early stage of severe AP, followed by intestinal dysfunction and pancreatic necrosis in the later stage. A study showed that 59% of AP patients had associated intestinal barrier injury, with increased intestinal mucosal permeability, leading to intestinal bacterial translocation, pancreatic tissue necrosis and infection, and the occurrence of multiple organ dysfunction syndrome. However, the real effect of the gut microbiota and its metabolites on intestinal barrier function in AP remains unclear. This review summarizes the alterations in the intestinal flora and its metabolites during AP development and progression to unveil the mechanism of gut failure in AP.

The effects of probiotic supplementation on mental health, biomarkers of inflammation and oxidative stress in patients with psychiatric disorders: A systematic review and meta-analysis of randomized controlled trials

BACKGROUND AND OBJECTIVE

In the current meta-analysis of randomized controlled trials (RCTs), the effects of probiotic supplementation on mental health, biomarkers of inflammation and oxidative stress in patients with psychiatric disorders were assessed.

METHODS

The following databases were search up to February 2019: PubMed, Scopus, Web of Science, Google scholar and Cochrane Central Register of Controlled Trials.

RESULTS

Twelve studies were included in the current meta-analysis. The findings demonstrated that probiotic supplementation resulted in a significant reduction in Hamilton Depression Rating Scale (HAMD) [Weighted Mean Difference (WMD): -9.60; 95 % CI: -10.08, -9.11]. In addition, a significant reduction in C-reactive protein (CRP) (WMD: -1.59; 95 % CI: -2.22, -0.97), interleukin 10 (IL-10) (WMD: -0.29; 95 % CI: -0.48, -0.11) and malondialdehyde (MDA) levels (WMD: -0.38; 95 % CI: -0.63, -0.13) was found after probiotics supplementation. No significant change was seen in Beck Depression Inventory (BDI) score (WMD: -11.17; 95 % CI: -24.99, 2.65), tumor necrosis factor-α (TNF-α) (WMD: -0.12; 95 % CI: -0.20, -0.05), IL-1B (WMD: -0.34; 95 % CI: -1.43, 0.74), IL-6 (WMD: 0.03; 95 % CI: -0.32, 0.38), nitric oxide (NO) (WMD: -0.54; 95 % CI: -2.16, 1.08), glutathione (GSH) (WMD: 46.79; 95 % CI: -17.25, 110.83) and total antioxidant capacity (TAC) levels (WMD: 15.21; 95 % CI: -59.96, 90.37) after probiotics supplementation.

CONCLUSION

Overall, the current meta-analysis demonstrated that taking probiotic by patients with psychiatric disorders had beneficial effects on HAMD, CRP, IL-10 and MDA levels, but it did not affect BDI score, other markers of inflammation and oxidative stress.

Efficacy of probiotics on cognition, and biomarkers of inflammation and oxidative stress in adults with Alzheimer's disease or mild cognitive impairment - a meta-analysis of randomized controlled trials

Probiotics are live microbes that confer health benefits to the host. Preliminary animal evidence supports the potential role of probiotics in ameliorating cognitive health, however, findings from clinical trials in Alzheimer’s disease (AD) or mild cognitive impairment (MCI) subjects are controversial. Thus, a meta-analysis is needed to clarify the efficacy of probiotics on cognition in AD or MCI patients. EMBASE, PubMed, Web of Science and Cochrane library were systematically searched and manually screened for relevant published randomized controlled trials (RCTs). Among the 890 citations identified, 5 studies involving 297 subjects met eligibility. There was a significant improvement in cognition (SMD = 0.37; 95% CI, 0.14, 0.61; P = 0.002; I² = 24%), while a significant reduction in malondialdehyde (SMD = −0.60; 95% CI, −0.91, −0.28; P = 0.000; I² = 0.0%) and high-sensitivity C-reactive protein (SMD = −0.57; 95% CI, −0.95, −0.20; P = 0.003; I² = 0.0%) post-intervention levels between the probiotics and control group. This meta-analysis indicated that probiotics improved cognitive performance in AD or MCI patients, possibly through decreasing levels of inflammatory and oxidative biomarkers. However, current evidence is insufficient, and more reliable evidence from large-scale, long-period, RCT is needed.

Neuroinflammation and Neurogenesis in Alzheimer's Disease and Potential Therapeutic Approaches

In adult brain, new neurons are generated throughout adulthood in the subventricular zone and the dentate gyrus; this process is commonly known as adult neurogenesis. The regulation or modulation of adult neurogenesis includes various intrinsic pathways (signal transduction pathway and epigenetic or genetic modulation pathways) or extrinsic pathways (metabolic growth factor modulation, vascular, and immune system pathways). Altered neurogenesis has been identified in Alzheimer's disease (AD), in both human AD brains and AD rodent models. The exact mechanism of the dysregulation of adult neurogenesis in AD has not been completely elucidated. However, neuroinflammation has been demonstrated to alter adult neurogenesis. The presence of various inflammatory components, such as immune cells, cytokines, or chemokines, plays a role in regulating the survival, proliferation, and maturation of neural stem cells. Neuroinflammation has also been considered as a hallmark neuropathological feature of AD. In this review, we summarize current, state-of-the art perspectives on adult neurogenesis, neuroinflammation, and the relationship between these two phenomena in AD. Furthermore, we discuss the potential therapeutic approaches, focusing on the anti-inflammatory and proneurogenic interventions that have been reported in this field.

Calorie restriction slows age-related microbiota changes in an Alzheimer's disease model in female mice

Alzheimer's disease (AD) affects an estimated 5.8 million Americans, and advanced age is the greatest risk factor. AD patients have altered intestinal microbiota. Accordingly, depleting intestinal microbiota in AD animal models reduces amyloid-beta (Aβ) plaque deposition. Age-related changes in the microbiota contribute to immunologic and physiologic decline. Translationally relevant dietary manipulations may be an effective approach to slow microbiota changes during aging. We previously showed that calorie restriction (CR) reduced brain Aβ deposition in the well-established Tg2576 mouse model of AD. Presently, we investigated whether CR alters the microbiome during aging. We found that female Tg2576 mice have more substantial age-related microbiome changes compared to wildtype (WT) mice, including an increase in Bacteroides, which were normalized by CR. Specific gut microbiota changes were linked to Aβ levels, with greater effects in females than in males. In the gut, Tg2576 female mice had an enhanced intestinal inflammatory transcriptional profile, which was reversed by CR. Furthermore, we demonstrate that Bacteroides colonization exacerbates Aβ deposition, which may be a mechanism whereby the gut impacts AD pathogenesis. These results suggest that long-term CR may alter the gut environment and prevent the expansion of microbes that contribute to age-related cognitive decline.

Prebiotics and probiotics as potential therapy for cognitive impairment

Cognitive functions, such as learning and memory, may be impaired during aging. Age-related cognitive impairment is associated with selective neuronal loss, oxidative changes that lead to microglia activation and neuroinflammation. In addition, it is associated to alteration reduction in trophic factors affecting neurogenesis and synaptic plasticity. In recent years, attention has been paid to the relationship between gut microbiota and brain. In aging, there is an alteration in microbiota, gut microbiota diversity is perturbed with an increase in pathogenic bacteria at the expense of beneficial ones. Dysbiosis may lead to chronic inflammation, and a decrease in bacteria metabolites such as short-chain fatty acids which have been related to an upregulation of neurotrophic factors. Supplementation with prebiotics and probiotics can modulate gut microbiota, returning it to a more physiological state; thus, they may be considered as a possible treatment for age-related cognitive impairment.

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.

Gut Microbiota and Microbiota-Related Metabolites as Possible Biomarkers of Cognitive Aging

Gut microbiota composition and functionality can influence the pathophysiology of age-related cognitive impairment and dementia, according to a large number of animal studies. The translation of this concept to humans is still uncertain, due to the relatively low number of clinical studies focused on fecal microbiota and large number of environmental factors that influence the microbiota composition. However, the fecal microbiota composition of older patients with dementia is deeply different from that of healthy active controls, conditioning a different metabolic profile. The possible use of fecal microbiota-related parameters and microbiota-derived metabolites as biomarkers of cognitive performance and dementia is critically reviewed in this paper, focusing on the most promising areas of research for the future.

Microbiota-Gut-Brain Axis: New Therapeutic Opportunities

The traditional fields of pharmacology and toxicology are beginning to consider the substantial impact our gut microbiota has on host physiology. The microbiota-gut-brain axis is emerging as a particular area of interest and a potential new therapeutic target for effective treatment of central nervous system disorders, in addition to being a potential cause of drug side effects. Microbiota-gut-brain axis signaling can occur via several pathways, including via the immune system, recruitment of host neurochemical signaling, direct enteric nervous system routes and the vagus nerve, and the production of bacterial metabolites. Altered gut microbial profiles have been described in several psychiatric and neurological disorders. Psychobiotics, live biotherapeutics or substances whose beneficial effects on the brain are bacterially mediated, are currently being investigated as direct and/or adjunctive therapies for psychiatric and neurodevelopmental disorders and possibly for neurodegenerative disease, and they may emerge as new therapeutic options in the clinical management of brain disorders.

Probiotic treatment improves the impaired spatial cognitive performance and restores synaptic plasticity in an animal model of Alzheimer's disease

Studies demonstrate that damage to gut microbiota is associated with some brain disorders including neurodegenerative diseases such as Alzheimer's disease (AD). Accordingly, supporting gut microbiota has been considered as a possible strategy for AD treatment. We evaluated behavioral and electrophysiological aspects of the brain function in an animal model of AD made by intracerebroventricular injection of β-amyloid. Two groups of control rats recieved either water as vehicle (Con) or probitics (Pro + Con). Also two groups of Alzheimeric animals were treated by either vehicle (Alz) or probiotics (Pro + Alz). Sham group was only subjected to surgical procedure and received the vehicle. Spatial learning and memory was assessed in Morris water maze. Also, basic synaptic transmission and long-term potentiation (LTP) were assessed by recording field excitatory postsynaptic potentials (fEPSPs) in hippocampus. Change in anti-oxidant/oxidant factors was assessed via measuring plasma level of total anti-oxidant capacity (TAC) and malondealdehyde (MDA). Brain staining was done to confirm β-amyloid accumulation. Fecal bacteria quantification was accomplished to find how probiotic supplement affected gut microbiota. We found that while the Alz animals displayed a weak spatial performance, probiotic treatment improved the maze navigation in the Pro + Alz rats. Whereas basic synaptic transmission remained unchanged in the Alz rats, LTP was suppressed in this group. Probiotic treatment significantly restored LTP in the Pro + Alz group and further enhanced it in the Pro + Con rats. The intervention also showed a favorable effect on balance of the anti-oxidant/oxidant biomarkers in the Pro + Alz rats. This study provides the first proof on positive effect of probiotics on synaptic plasticity in an animal model of AD.

Mild cognitive impairment has similar alterations as Alzheimer's disease in gut microbiota

OBJECTIVE

Gut microbiota changes before the onset of Alzheimer's disease (AD) and the alterations could be detected in the stage of mild cognitive impairment (MCI). The findings might offer diagnostic biomarkers before the onset of dementia.

BACKGROUND

AD is the most common cause of dementia, and MCI is the predementia state. Recent studies suggest the alterations in the gut microbial communities associated with AD, whereas the microbiota in MCI before the onset of dementia has not been discovered and characterized in humans.

NEW/UPDATED HYPOTHESIS

We hypothesize that the dysbiosis happens in the MCI stage. Patients with AD and MCI have decreased microbial diversity, and changes in gut microbiota could be detected for early detection of AD. In our preliminary study, we identified differences between AD and normal controls in 11 genera from the feces and 11 genera from the blood. No difference in genera between AD and MCI was detected. Using the diagnostic model from fecal samples with all different genera input, 93% (28 in 30) of patients with MCI could be identified correctly.

MAJOR CHALLENGES FOR THE HYPOTHESIS

The diagnosis of MCI and AD in the study was based on symptoms and neuroimaging, and AD biomarkers should be included for precise diagnosis in further validating studies. Besides, as the microbiota changes longitudinally, their relationship with the progress of dementia needs to be studied in the prospective studies.

LINKAGE TO OTHER MAJOR THEORIES

Escherichia was observed increased at genus level in both fecal and blood samples from AD and MCI. For AD biomarker, postmortem brain tissue from patients with AD showed lipopolysaccharides and gram-negative Escherichia coli fragments colocalize with amyloid plaque. In this way, the amyloid pathogenesis for AD would be triggered during MCI by gut microbiota shifting. Besides, systemic inflammatory reactions caused by compounds secreted by bacteria may impair the blood-brain barrier and promote neuroinflammation and/or neurodegeneration. Furthermore, abnormal metabolites caused by microbial gene functions have an impact on neurodegeneration.

Diet and Alzheimer's dementia - Nutritional approach to modulate inflammation

BACKGROUND

Alzheimer's disease (AD) is the most common neurodegenerative disease causing dementia in the elderly population. Due to the fact that there is still no cure for Alzheimer's dementia and available treatment strategies bring only symptomatic benefits, there is a pressing demand for other effective strategies such as diet. Since the inflammation hypothesis gained considerable significance in the AD pathogenesis, elucidating the modulatory role of dietary factors on inflammation may help to prevent, delay the onset and slow the progression of AD. Current evidence clearly shows that synergistic action of combined supplementation and complex dietary patterns provides stronger benefits than any single component considered separately. Recent studies reveal the growing importance of novel factors such as dietary advanced glycation end products (d-AGE), gut microbiota, butyrate and vitamin D₃ on inflammatory processes in AD.

CONCLUSION

This paper summarizes the available evidence of pro- and anti-inflammatory activity of some dietary components including fatty acids, vitamins, flavonoids, polyphenols, probiotics and d-AGE, and their potential for AD prevention and treatment.

Psychobiotics in mental health, neurodegenerative and neurodevelopmental disorders

Psychobiotics are a group of probiotics that affect the central nervous system (CNS) related functions and behaviors mediated by the gut-brain-axis (GBA) via immune, humoral, neural, and metabolic pathways to improve not only the gastrointestinal (GI) function but also the antidepressant and anxiolytic capacity. As a novel class of probiotics, the application of psychobiotics has led researchers to focus on a new area in neuroscience. In the past five years, some psychobiotics strains were reported to inhibit inflammation and decreased cortisol levels, resulting in an amelioration of the symptoms of anxiety and depression. Psychobiotics are efficacious in improving neurodegenerative and neurodevelopmental disorders, including autism spectrum disorder (ASD), Parkinson’s disease (PD) and Alzheimer’s disease (AD). Use of psychobiotics can improve GI function, ASD symptoms, motor functions of patients with PD and cognition in patients with AD. However, the evidence for the effects of psychobiotics on mental and neurological conditions/ disorders remains limited. Further studies of psychobiotics are needed in order to determine into their effectiveness and mechanism as treatments for various psychiatric disorders in the future.

Effect of probiotic supplementation on seizure activity and cognitive performance in PTZ-induced chemical kindling

Epilepsy is one of the most common neurological disorders that severely affect life quality of many people worldwide. Ion transport in the neuronal membrane, inhibitory-excitatory mechanisms, and regulatory modulator systems have been implicated in the pathogenesis of epilepsy. A bidirectional communication is proposed between brain and gut where the brain modulates the gastrointestinal tract, and the gut can affect brain function and behavior. The gut microbiome takes an important role in health and disease where dysbiosis is involved in several neurological disorders. Probiotics as living microorganisms are beneficial to humans and animals when adequately administered. In the present work, we evaluated the effect of a probiotic bacteria mixture on seizure activity, cognitive function, and gamma-aminobutyric acid (GABA), nitric oxide (NO), malondealdehyde (MDA), and total antioxidant capacity (TAC) level of the brain tissue in the pentylenetetrazole (PTZ)-induced kindled rats. The Racine score and performance in water maze were considered as indices of the epileptic severity and the spatial learning and memory, respectively. We found that the probiotic supplementation substantially reduces seizure severity so that almost no probiotic-treated animals showed full kindling. The oral bacteriotherapy partially improved the spatial learning and memory in the kindled rats. The intervention decreased NO and MDA and increased TAC concentration of the brain. The probiotic treatment also increased the inhibitory neurotransmitter GABA. Our findings are the first preclinical report to show positive effect of probiotic bacteria on seizure-induced neurological disorders. Further investigation is required to answer the questions raised about the probable mechanisms involved.

Microbiome-The Missing Link in the Gut-Brain Axis: Focus on Its Role in Gastrointestinal and Mental Health

The central nervous system (CNS) and the human gastrointestinal (GI) tract communicate through the gut-brain axis (GBA). Such communication is bi-directional and involves neuronal, endocrine, and immunological mechanisms. There is mounting data that gut microbiota is the source of a number of neuroactive and immunocompetent substances, which shape the structure and function of brain regions involved in the control of emotions, cognition, and physical activity. Most GI diseases are associated with altered transmission within the GBA that are influenced by both genetic and environmental factors. Current treatment protocols for GI and non-GI disorders may positively or adversely affect the composition of intestinal microbiota with a diverse impact on therapeutic outcome(s). Alterations of gut microbiota have been associated with mood and depressive disorders. Moreover, mental health is frequently affected in GI and non-GI diseases. Deregulation of the GBA may constitute a grip point for the development of diagnostic tools and personalized microbiota-based therapy. For example, next generation sequencing (NGS) offers detailed analysis of microbiome footprints in patients with mental and GI disorders. Elucidating the role of stem cell⁻host microbiome cross talks in tissues in GBA disorders might lead to the development of next generation diagnostics and therapeutics. Psychobiotics are a new class of beneficial bacteria with documented efficacy for the treatment of GBA disorders. Novel therapies interfering with small molecules involved in adult stem cell trafficking are on the horizon.

Exercise and probiotics attenuate the development of Alzheimer's disease in transgenic mice: Role of microbiome

It has been suggested that exercise training and probiotic supplementation could decelerate the progress of functional and biochemical deterioration in APP/PS1 transgenic mice (APP/PS1^TG). APP/PS1^TG mice were subjected to exercise training and probiotic treatments and functional, biochemical and microbiome markers were analyzed. Under these conditions the mice significantly outperformed controls on The Morris Maze Test, and the number of beta-amyloid plaques decreased in the hippocampus. B. thetaiotaomicron levels correlated highly with the results of the Morris Maze Test (p < 0.05), and this group of bacteria was significantly elevated in the microbiome of the APP/PS1^TG mice compared to the wild type. L. johnsonii levels positively correlated with the beta amyloid content and area. Data revealed that exercise and probiotic treatment can decrease the progress of Alzheimer's Disease and the beneficial effects could be partly mediated by alteration of the microbiome.