A consequence of our progressively ageing global population is the increasing prevalence of worldwide age-related cognitive decline and dementia. In the absence of effective therapeutic interventions, identifying risk factors associated with cognitive decline becomes increasingly vital. Novel perspectives suggest that a dynamic bidirectional communication system between the gut, its microbiome, and the central nervous system, commonly referred to as the microbiota-gut-brain axis, may be a contributing factor for cognitive health and disease. However, the exact mechanisms remain undefined. Microbial-derived metabolites produced in the gut can cross the intestinal epithelial barrier, enter systemic circulation and trigger physiological responses both directly and indirectly affecting the central nervous system and its functions. Dysregulation of this system (i.e., dysbiosis) can modulate cytotoxic metabolite production, promote neuroinflammation and negatively impact cognition. In this review, we explore critical connections between microbial-derived metabolites (secondary bile acids, trimethylamine-N-oxide (TMAO), tryptophan derivatives and others) and their influence upon cognitive function and neurodegenerative disorders, with a particular interest in their less-explored role as risk factors of cognitive decline.

The impact of sex and menopausal status in Alzheimer’s disease remains understudied despite increasing evidence of greater female risk, particularly in APOE4 carriers. Utilizing female APOE ‐TR mice maintained on a high‐fat diet background we induced ovarian failure through repeated VCD injections, to mimic human menopause. At 12 months of age, recognition memory and spatial memory were assessed using object recognition, Y‐maze spontaneous alternation, and Barnes maze. A VCD*genotype interaction reduced the recognition memory ( P < .05), with APOE4 VCD‐treated animals unable to distinguish between novel and familiar objects. APOE4 mice displayed an additional 37% and 12% reduction in Barnes ( P < .01) and Y‐maze ( P < .01) performance, indicative of genotype‐specific spatial memory impairment. Molecular analysis indicated both VCD and genotype‐related deficits in synaptic plasticity with BDNF, Akt, mTOR, and ERK signaling compromised. Subsequent reductions in the transcription factors Creb1 and Atf4 were also evident. Furthermore, the VCD*genotype interaction specifically diminished Ephb2 expression, while Fos , and Cnr1 expression reduced as a consequence of APOE4 genotype. Brain DHA levels were 13% lower in VCD‐treated animals independent of genotype. Consistent with this, we detected alterations in the expression of the DHA transporters Acsl6 and Fatp4 . Our results indicate that the combination of ovarian failure and APOE4 leads to an exacerbation of cognitive and neurological deficits.

The gut microbiota-brain axis has been associated with the pathogenesis of numerous disorders, but the mechanism(s) underlying these links are generally poorly understood. Accumulating evidence indicates the involvement of gut microbe-derived metabolites. Circulating levels of the gut microbe/host co-metabolite p-cresol sulfate (pCS) correlate with cerebrovascular event risk in individuals with chronic kidney disease (CKD), but whether this relationship is mechanistic is unclear. We hypothesized that pCS would impair the function of the blood-brain barrier (BBB), the primary brain vasculature interface. We report that pCS exposure impairs BBB integrity in human cells in vitro and both acutely (≤6 hours) and chronically (28 days) in mice, enhancing tracer extravasation, disrupting barrier-regulating tight junction components and ultimately exerting a suppressive effect upon whole-brain transcriptomic activity. In vitro and in vivo mechanistic studies showed that pCS activated epidermal growth factor receptor (EGFR) signaling, sequentially activating the intracellular signaling proteins annexin A1 and STAT3 to induce mobilization of matrix metalloproteinase MMP-2/9 and disruption to the integrity of the BBB. This effect was confirmed as specific to the EGFR through the use of both pharmacological and RNA interference approaches. Confirming the translational relevance of this work, exposure of the cerebromicrovascular endothelia to serum from hemodialysis patients in vitro led to a significant increase in paracellular permeability, with the magnitude of permeabilization closely correlating with serum pCS, but not most other uremic toxin, content. Notably, this damaging effect of hemodialysis patient serum was prevented by pharmacological blockade of the EGFR. Our results define a pathway linking the co-metabolite pCS with BBB damage and suggest that targeting the EGFR may mitigate against cerebrovascular damage in CKD. This work further provides mechanistic evidence indicating the role of gut microbe-derived metabolites in human disease.

As the population ages, the incidence of age-related neurodegenerative diseases is rapidly increasing, and novel approaches to mitigate this soaring prevalence are sorely needed. Recent studies have highlighted the importance of gut microbial homeostasis and its impact on brain functions, commonly referred to as the gut–brain axis, in maintaining overall health and wellbeing. Nonetheless, the mechanisms by which this system acts remains poorly defined. In this review, we will explore how (poly)phenols, a class of natural compounds found in many plant-based foods and beverages, can modulate the gut–brain axis, and thereby promote neural health. While evidence indicates a beneficial role of (poly)phenol consumption as part of a balanced diet, human studies are scarce and mechanistic insight is still lacking. In this regard, we make the case that dietary (poly)phenols should be further explored to establish their therapeutic efficacy on brain health through modulation of the gut–brain axis, with much greater emphasis on carefully designed human interventions.

Anthocyanin-rich foods, such as berries, reportedly ameliorate age-related cognitive deficits in both animals and humans. Despite this, investigation into the mechanisms which underpin anthocyanin-mediated learning and memory benefits remains relatively limited. The present study investigates the effects of anthocyanin intake on a spatial working memory paradigm, assessed via the cross-maze apparatus, and relates behavioural test performance to underlying molecular mechanisms. Six-week supplementation with pure anthocyanins (2% w/w), administered throughout the learning phase of the task, improved both spatial and psychomotor performances in aged rats. Behavioural outputs were accompanied by changes in the expression profile of key proteins integral to synaptic function/maintenance, with upregulation of dystrophin, protein kinase B (PKB/Akt) and tyrosine hydroxylase, and downregulation of apoptotic proteins B-cell lymphoma-extra-large (Bcl-xL) and the phosphorylated rapidly accelerated fibrosarcoma (p-Raf). Separate immunoblot analysis supported these observations, indicating increased activation of extracellular signal-related kinase (ERK1), Akt Ser473, mammalian target of rapamycin (mTOR) Ser2448, activity-regulated cytoskeleton-associated protein (Arc/Arg 3.1) and brain-derived neurotrophic factor (BDNF) in response to anthocyanin treatment, whilst α-E-catenin, c-Jun N-terminal kinase (JNK1) and p38 protein levels decreased. Together, these findings suggest that purified anthocyanin consumption enhances spatial learning and motor coordination in aged animals and can be attributed to the modulation of key synaptic proteins, which support integrity and maintenance of synaptic function.