Gut microbiota-derived short chain fatty acids act as mediators of the gut-brain axis targeting age-related neurodegenerative disorders: a narrative review

Higher dietary butyrate intake is associated with better cognitive function in older adults: evidence from a cross-sectional study

Background

Studies indicate that butyrate can enhance memory and cognitive functions in mice by inhibiting neuroinflammation and neuronal apoptosis. Elevated fecal butyrate levels in older individuals with mild cognitive impairment correlate with reduced levels of Aβ-42, an Alzheimer's disease biomarker. This study investigated the relationship between butyrate consumption and cognitive performance in older adults, which remains to be elucidated.

Methods

This study employed a cross-sectional, observational design to analyze data gathered from 2,078 participants enrolled in the 2011-2014 US National Health and Nutrition Examination Survey (NHANES). Butyrate intake was determined based on two 24-h dietary assessments. To evaluate cognitive function, three tests were administered: the Animal Fluency Test (AFT) to assess executive function, the Digit Symbol Substitution Test (DSST) for measuring processing speed, and the Consortium to Establish a Registry for Alzheimer's disease (CERAD) subtest for assessing memory. Z scores were computed for each test and overall cognitive performance. Multivariate linear regression models and a generalized additive model (GAM) were used to examine the correlation between butyrate consumption and mental functions. Finally, subgroup analyses and interaction tests were used to verify the robustness of the associations.

Results

The NHANES study encompasses two surveys conducted between 2011 and 2014 that involved 2,078 participants aged 60 years or older. Higher dietary butyrate consumption had a positive correlation between superior performance on DSST, AFT, CERAD-Immediate Recall Test, and Z scores. The participants in the upper quartile of butyrate intake had significantly higher DSST (β = 1.60, 95% CI: 0.04-3.17), AFT scores (β = 0.99, 95% CI: 0.37-1.60), and Z scores (β = 0.09, 95% CI: 0.01-0.17) than individuals in the lowest quartile even after adjusting for potential confounders. Finally, no notable interactions were observed within the groupings. Finally, in subgroup analyses, BMI was found to influence the positive association between butyrate and DSST with Z score, and hypertension also influenced the association between butyrate and DSST.

Conclusion

Higher butyrate intake in individuals aged ≥60 years was linked to better cognitive functioning. This could potentially contribute to maintaining brain function during aging.

Can microbiota gut-brain axis reverse neurodegenerative disorders in human?

The trillions of microbial populations residing in the gut have recently shown that they can be used as a remedy for various diseases. The gut microbiota-brain-axis interface is one unique pathway that the microbiota demonstrates its medicinal value. This medicinal value is further seen when there is a decline in gut microbial diversity (dysbiosis). Dysbiosis leads to neurodegenerative disorders (NDDs). The objective of this review is to ascertain the clinical significance of gut microbiota induced therapeutic strategies. While navigating this important area of interest, we will elucidate the research gaps, the prospects and the potential reverse interventions of the studied NDDs. In addition to our previous work, relevant literature published in English were searched and retrieved from the PubMed database. The 'gut microbiota and Neurodegenerative disorders' were used as keywords during the search period. The Filters applied are: Abstract, Full text, Meta-Analysis, Randomized Controlled Trial, Reviews, in the last 5 years. The articles were analyzed in our unrelenting quest to make sense of the prospects and research gap in gut microbiota-brain-axis. This chapter is a result of this meticulous work. More convincing data from researches on gut microbiota-brain-axis are required to provide clinical significance including neuroimaging studies. Addressing the structural (pathological footprints) and the functional changes (diseases manifestation) involving gut microbiota-brain-axis require a holistic approach. While the pharmacological therapies such as chemotherapeutic and chemobiotic treatment approaches come with low success rates, non-pharmacological interventions are found to be more useful in reversing NDDs. The inability to detect NDDs at an early stage in their clinical history, makes preventive medicinal approaches the must needed and best intervention strategy. Gut-driven treatments have a lot to offer in the management of refractory neurologic diseases.

Euglena gracilis polysaccharide modulated gut dysbiosis of obese individuals via acetic acid in an in vitro fermentation model

Gut dysbiosis is a characteristic feature of obesity and targeting gut microbiota presents a promising approach to attenuate obesity. Euglena gracilis polysaccharide (EGP) has emerged as a potential prebiotic capable of promoting health-beneficial bacteria. However, its effects on the gut dysbiosis of obese individuals remain unclear. This study investigated the impacts of EGP on gut microbiota from both non-obese and obese individuals using an in vitro fermentation model. Results showed that EGP significantly altered the gut microbiota composition and metabolism. Specifically, EGP improved the relative abundance of Paeniclostridium, Clostridium_sensu_stricto_1 and Paraclostridium of the non-obese individuals and Providencia, Enterococcus and Bacteroides of the obese individuals. Metabolomics results showed EGP significantly altered the lipid metabolism especially in the obese group with enriched bile secretion and cholesterol metabolism pathways. Noting that acetic acid was significantly increased in both groups, these acetic acid favorable microbiota from non-obese individuals was collected with acetic acid supplementation. Transplantation of these acetic acid-induced microbiota (AAiM) notably improved the richness and diversity of fecal microbiota of the obese individuals, enhancing the growth of probiotics like Bacteroides and Bifidobacterium. Consequently, AAiM significantly restructured macronutrients (including amino acids, carbohydrates and lipids) metabolism of the gut microbiota from obese individuals. Altogether, this study underscores the potential of EGP and acetic acid favorable microbiota in manipulating obesity-associated gut dysbiosis via acetic acid production.

Microbiota-Gut-Brain Axis in Age-Related Neurodegenerative Diseases

BACKGROUND

Age-related neurodegenerative diseases (NDs) pose a formidable challenge to healthcare systems worldwide due to their complex pathogenesis, significant morbidity, and mortality. Scope and Approach: This comprehensive review aims to elucidate the central role of the microbiotagut- brain axis (MGBA) in ND pathogenesis. Specifically, it delves into the perturbations within the gut microbiota and its metabolomic landscape, as well as the structural and functional transformations of the gastrointestinal and blood-brain barrier interfaces in ND patients. Additionally, it provides a comprehensive overview of the recent advancements in medicinal and dietary interventions tailored to modulate the MGBA for ND therapy.

CONCLUSION

Accumulating evidence underscores the pivotal role of the gut microbiota in ND pathogenesis through the MGBA. Dysbiosis of the gut microbiota and associated metabolites instigate structural modifications and augmented permeability of both the gastrointestinal barrier and the blood-brain barrier (BBB). These alterations facilitate the transit of microbial molecules from the gut to the brain via neural, endocrine, and immune pathways, potentially contributing to the etiology of NDs. Numerous investigational strategies, encompassing prebiotic and probiotic interventions, pharmaceutical trials, and dietary adaptations, are actively explored to harness the microbiota for ND treatment. This work endeavors to enhance our comprehension of the intricate mechanisms underpinning ND pathogenesis, offering valuable insights for the development of innovative therapeutic modalities targeting these debilitating disorders.

The role of gut-derived short-chain fatty acids in Parkinson's disease

The emerging function of short-chain fatty acids (SCFAs) in Parkinson's disease (PD) has been investigated in this article. SCFAs, which are generated via the fermentation of dietary fiber by gut microbiota, have been associated with dysfunction of the gut-brain axis and, neuroinflammation. These processes are integral to the development of PD. This article examines the potential therapeutic implications of SCFAs in the management of PD, encompassing their capacity to modulate gastrointestinal permeability, neuroinflammation, and neuronal survival, by conducting an extensive literature review. As a whole, this article emphasizes the potential therapeutic utility of SCFAs as targets for the management and treatment of PD.

The role of HLA-DR on plasmacytoid dendritic cells in mediating the effects of Butyrivibrio gut microbiota on Parkinson's disease

BACKGROUND

Parkinson's disease (PD) is viewed as a progressively deteriorating neurodegenerative disorder, the exact etiology of which remains not fully deciphered to this date. The gut microbiota could play a crucial role in PD development by modulating the human immune system.

OBJECTIVE

This study aims to explore the relationship between gut microbiota and PD, focusing on how immune characteristics may both directly and indirectly influence their interaction.

METHODS

Utilizing cumulative data from genome-wide association studies (GWAS), our research conducted a two-sample Mendelian randomization (MR) analysis to clarify the association between the gut microbiome and PD. Additionally, by employing a two-step MR approach, we assessed the impact of gut microbiota on PD development via immune characteristics and quantified HLA-DR mediation effect on plasmacytoid dendritic cells (pDCs).

RESULTS

We discovered significant associations between PD and microbiota, comprising one class, one order, two families, and two genera. Furthermore, we explored the extent to which HLA-DR on pDCs mediates the effect of Butyrivibrio gut microbiota on PD.

CONCLUSION

Our study emphasizes the complex interactions between the gut microbiota, immune characteristics, and PD. The relationships and intermediary roles identified in our research provide important insights for developing potential therapies that target the gut microbiome to alleviate symptoms in PD patients.

Insulin resistance as the molecular link between diabetes and Alzheimer's disease

Diabetes mellitus (DM) and Alzheimer's disease (AD) are two major health concerns that have seen a rising prevalence worldwide. Recent studies have indicated a possible link between DM and an increased risk of developing AD. Insulin, while primarily known for its role in regulating blood sugar, also plays a vital role in protecting brain functions. Insulin resistance (IR), especially prevalent in type 2 diabetes, is believed to play a significant role in AD's development. When insulin signalling becomes dysfunctional, it can negatively affect various brain functions, making individuals more susceptible to AD's defining features, such as the buildup of beta-amyloid plaques and tau protein tangles. Emerging research suggests that addressing insulin-related issues might help reduce or even reverse the brain changes linked to AD. This review aims to explore the rela-tionship between DM and AD, with a focus on the role of IR. It also explores the molecular mechanisms by which IR might lead to brain changes and assesses current treatments that target IR. Understanding IR's role in the connection between DM and AD offers new possibilities for treatments and highlights the importance of continued research in this interdisciplinary field.

Butyrate as a potential therapeutic agent for neurodegenerative disorders

Maintaining an optimum microbial community within the gastrointestinal tract is intricately linked to human metabolic, immune and brain health. Disturbance to these microbial populations perturbs the production of vital bioactive compounds synthesised by the gut microbiome, such as short-chain fatty acids (SCFAs). Of the SCFAs, butyrate is known to be a major source of energy for colonocytes and has valuable effects on the maintenance of intestinal epithelium and blood brain barrier integrity, gut motility and transit, anti-inflammatory effects, and autophagy induction. Inducing endogenous butyrate production is likely to be beneficial for gut-brain homeostasis and for optimal neuronal function. For these reasons, butyrate has gained interest as a potential therapy for not only metabolic and immunological disorders, but also conditions related to the brain, including neurodegenerative diseases. While direct and indirect sources of butyrate, including prebiotics, probiotics, butyrate pro-drugs and glucosidase inhibitors, offer a promising therapeutic avenue, their efficacy and dosage in neurodegenerative conditions remain largely unknown. Here, we review current literature on effects of butyrate relevant to neuronal function, the impact of butyrate in a range of neurodegenerative diseases and related treatments that may have potential for the treatment of neurodegenerative diseases.