Potential of dietary polyphenols for protection from age-related decline and neurodegeneration: a role for gut microbiota?

Many epidemiological studies have shown the beneficial effects of a largely plant-based diet, and the strong association between the consumption of a Mediterranean-type diet with healthy aging including a lower risk of cognitive decline. The Mediterranean diet is characterized by a high intake of olive oil, fruits and vegetables and is rich in dietary fiber and polyphenols - both of which have been postulated to act as important mediators of these benefits. Polyphenols are large molecules produced by plants to protect them from environmental threats and injury. When ingested by humans, as little as 5% of these molecules are absorbed in the small intestine with the majority metabolized by the gut microbiota into absorbable simple phenolic compounds. Flavan-3-ols, a type of flavonoid, contained in grapes, berries, pome fruits, tea, and cocoa have been associated with many beneficial effects on several risk factors for cardiovascular disease, cognitive function and brain regions involved in memory formation. Both preclinical and clinical studies suggest that these brain and heart benefits can be attributed to endothelial vascular effects and anti-inflammatory properties among others. More recently the gut microbiota has emerged as a potential modulator of the aging brain and intriguingly polyphenols have been shown to alter microbiota composition and be metabolized by different microbial species. However, there is a need for well controlled studies in large populations to identify predictors of response, particularly given the vast inter-individual variation of human gut microbiota.

Disruption of the microbiota-gut-brain axis is a defining characteristic of the α-Gal A (-/0) mouse model of Fabry disease.

Fabry disease (FD) is an X-linked metabolic disease caused by a deficiency in α-galactosidase A (α-Gal A) activity. This causes accumulation of glycosphingolipids, especially globotriaosylceramide (Gb3), in different cells and organs. Neuropathic pain and gastrointestinal (GI) symptoms, such as abdominal pain, nausea, diarrhea, constipation, and early satiety, are the most frequent symptoms reported by FD patients and severely affect their quality of life. It is generally accepted that Gb3 and lyso-Gb3 are involved in the symptoms; nevertheless, the origin of these symptoms is complex and multifactorial, and the exact mechanisms of pathogenesis are still poorly understood. Here, we used a murine model of FD, the male α-Gal A (-/0) mouse, to characterize functionality, behavior, and microbiota in an attempt to elucidate the microbiota-gut-brain axis at three different ages. We provided evidence of a diarrhea-like phenotype and visceral hypersensitivity in our FD model together with reduced locomotor activity and anxiety-like behavior. We also showed for the first time that symptomology was associated with early compositional and functional dysbiosis of the gut microbiota, paralleled by alterations in fecal short-chain fatty acid levels, which partly persisted with advancing age. Interestingly, most of the dysbiotic features suggested a disruption of gut homeostasis, possibly contributing to accelerated intestinal transit, visceral hypersensitivity, and impaired communication along the gut-brain axis.

Gamma-aminobutyric acid-producing lactobacilli positively affect metabolism and depressive-like behaviour in a mouse model of metabolic syndrome.

Metabolic and neuroactive metabolite production represents one of the mechanisms through which the gut microbiota can impact health. One such metabolite, gamma-aminobutyric acid (GABA), can modulate glucose homeostasis and alter behavioural patterns in the host. We previously demonstrated that oral administration of GABA-producing Lactobacillus brevis DPC6108 has the potential to increase levels of circulating insulin in healthy rats. Therefore, the objective of this study was to assess the efficacy of endogenous microbial GABA production in improving metabolic and behavioural outcomes in a mouse model of metabolic dysfunction. Diet-induced obese and metabolically dysfunctional mice received one of two GABA-producing strains, L. brevis DPC6108 or L. brevis DSM32386, daily for 12 weeks. After 8 and 10 weeks of intervention, the behavioural and metabolic profiles of the mice were respectively assessed. Intervention with both L. brevis strains attenuated several abnormalities associated with metabolic dysfunction, causing a reduction in the accumulation of mesenteric adipose tissue, increased insulin secretion following glucose challenge, improved plasma cholesterol clearance and reduced despair-like behaviour and basal corticosterone production during the forced swim test. Taken together, this exploratory dataset indicates that intervention with GABA-producing lactobacilli has the potential to improve metabolic and depressive- like behavioural abnormalities associated with metabolic syndrome in mice.

The antimicrobial capacity of embalming solutions: a comparative study.

AIMS: Infectious health risks are associated with handling human cadavers and to decrease such risks, cadavers are embalmed using different chemicals. The aim of this study is to quantify the amount of micro-organisms present in different regions of human cadavers before embalming, after embalming and over a period of 8 months. METHODS AND RESULTS: Human cadavers were embalmed using Thiel, formalin, Genelyn and the Imperial College London soft-preservation (ICL-SP) solution with two cadavers per technique. Sterile swabs were used to collect samples from different regions. Samples were collected every 2 months. All cadavers had a high number of microbial colonies before embalming. While no colonies were detected on formalin and Genelyn embalmed cadavers post-embalming, the number of colonies decreased significantly in Thiel-embalmed cadavers and stayed relatively the same in ICL-SP-embalmed cadavers. CONCLUSIONS: Formalin-embalmed cadavers showed the strongest disinfecting abilities followed by Thiel-embalmed cadavers, then Genelyn-embalmed cadavers and finally by ICL-SP cadavers. SIGNIFICANCE AND IMPACT OF THE STUDY: This study highlights how under researched this area is and the evident variation in the antimicrobial abilities of different embalming solutions on the cadaver as a whole and within different regions of the same cadaver.

Dimensional thinking in psychiatry in the era of the Research Domain Criteria (RDoC).

The biological mechanisms underlying psychiatric diagnoses are not well defined. Clinical diagnosis based on categorical systems exhibit high levels of heterogeneity and co-morbidity. The Research Domain Criteria (RDoC) attempts to reconceptualize psychiatric disorders into transdiagnostic functional dimensional constructs based on neurobiological measures and observable behaviour. By understanding the underlying neurobiology and pathophysiology of the relevant processes, the RDoC aims to advance biomarker development for disease prediction and treatment response. This important evolving dimensional framework must also consider environmental factors. Emerging evidence suggests that gut microbes (microbiome) play a physiological role in brain diseases by modulating neuroimmune, neuroendocrine and neural signalling pathways between the gut and the brain. The integration of the gut microbiome signature as an additional dimensional component of the RDoC may enhance precision psychiatry.

Kynurenine pathway metabolism and the neurobiology of treatment-resistant depression: Comparison of multiple ketamine infusions and electroconvulsive therapy.

Current first-line antidepressants can take weeks or months to decrease depressive symptoms. Low dose ketamine, an N-methyl-D-aspartate (NMDA) receptor antagonist, shows potential for a more rapid antidepressant effect, with efficacy also evident in previously treatment-resistant populations. However, a greater understanding of the physiological mechanisms underlying such effects is required. We assessed the potential impact of ketamine infusion on neurobiological drivers of kynurenine pathway metabolism in major depression (HPA axis hyperactivity, inflammation) in patients with treatment-resistant depression compared to gender-matched healthy controls. Furthermore, we assessed these biomarkers before and after electroconvulsive therapy (ECT), which is currently the gold standard for management of treatment-resistant depression. As previously demonstrated, treatment with ketamine and ECT was associated with improved depressive symptoms in patients. At baseline, waking cortisol output was greater in the ECT cohort, kynurenine was greater in the ketamine cohort, and kynurenic acid was lower in patients compared to healthy controls, although inflammatory markers (IL-6, IL-8, IL-10 or IFN-γ) were similar in patients and controls. Furthermore, in patients who responded to ECT, the cortisol awakening response was decreased following treatment. Despite a trend towards reduced kynurenine concentrations in those who responded to ketamine, ketamine was not associated with significant alterations in any of the biomarkers assessed.

A Survey of tooth morphology teaching methods employed in the United Kingdom and Ireland.

BACKGROUND: Tooth morphology is a central component of the dental curriculum and is applicable to all dental specialities. Traditional teaching methods are being supplemented with innovative strategies to tailor teaching and accommodate the learning styles of the recent generation of students. METHODS: An online survey was compiled and distributed to the staff involved in teaching tooth morphology in the United Kingdom and Ireland to assess the importance of tooth morphology in the dentistry curriculum and the methodologies employed in teaching. RESULTS: The results of the survey show that tooth morphology constitutes a small module in the dental curriculum. It is taught in the first 2 years of the dental curriculum but is applicable in the clinical years and throughout the dental career. Traditional teaching methods, lecture and practical, are being augmented with innovative teaching including e-learning via virtual learning environment, tooth atlas and e-books leading to blended learning. The majority of the schools teach both normal dental anatomy and morphologic variations of dental anatomy and utilise plastic teeth for practical and examination purposes. Learning the 3D aspects of tooth morphology was deemed important by most of the respondents who also agreed that tooth morphology is a difficult topic for the students. CONCLUSION: Despite being core to the dental curriculum, overall minimal time is dedicated to the delivery of tooth morphology, creating a reliance on the student to learn the material. New forms of delivery including computer-assisted learning tools should help sustain learning and previously acquired knowledge.

Evaluation of an animation tool developed to supplement dental student study of the cranial nerves.

INTRODUCTION: The structure/function of the cranial nerves is a core topic for dental students. However, due to the perceived complexity of the subject, it is often difficult for students to develop a comprehensive understanding of key concepts using textbooks and models. It is accepted that the acquisition of anatomical knowledge can be facilitated by visualisation of structures. This study aimed to develop and assess a novel cranial nerve animation as a supplemental learning aid for dental students. MATERIALS AND METHODS: A multidisciplinary team of anatomists, neuroscientists and a computer scientist developed a novel animation depicting the cranial nerves. The animation was viewed by newly enrolled first-year dental students, graduate entry dental students (year 1) and dental hygiene students (year 1). A simple life scenario employing the use of the cranial nerves was developed using a cartoon-type animation with a viewing time of 3.58 minutes. The animation was developed with emphasis on a life scenario. The animation was placed online for 2 weeks with open access or viewed once in a controlled laboratory setting. Questionnaires were designed to assess the participants' attitude towards the animation and their knowledge of the cranial nerves before and after visualisation. This study was performed before the delivery of core lectures on the cranial nerves. RESULTS: Our findings indicate that the use of the animation can act as a supplemental tool to improve student knowledge of the cranial nerves. Indeed, data indicate that a single viewing of the animation, in addition to 2-week access to the animation, can act as a supplemental learning tool to assist student understanding of the structure and function of cranial nerves. The animation significantly enhanced the student's opinion that their cranial nerve knowledge had improved. From a qualitative point of view, the students described the animation as an enjoyable and useful supplement to reading material/lectures and indicated that the animation was a useful tool in understanding the cranial nerves. CONCLUSION: Overall, these findings indicate that an animation demonstrating the cranial nerves in a simple, everyday functional scenario may act as a learning aid in the study of cranial nerves.

Faecal microRNAs: indicators of imbalance at the host-microbe interface?

The enteric microbiota is characterised by a balance and composition that is unique to the host. It is important to understand the mechanisms through which the host can maintain the composition of the gut microbiota. MicroRNAs (miRNA) are implicated in intercellular communication and have been isolated from bodily fluids including stool. Recent findings suggest that miRNA produced by the host's intestinal epithelial cells (IECs) participate in shaping the microbiota. To investigate whether miRNA expression was influenced by the gut microbiota we measured the expression of miRNAs expressed by intestinal epithelial cells in faeces. Specifically, we measured miRNA expression in faeces from germ-free (GF) and conventional mice and similarly in a rat model of antibiotic-mediated depletion of the gut microbiota control rats. In adult male GF and conventional mice and adult Sprague Dawley (SD) rats were treated with a combination of antibiotics for 8 weeks; total RNA was extracted from faecal pellets taken at week 0, 2, 4, 6 week 8 and the expression of let-7b-3p, miR-141-3p, miR-200a-3p and miR-1224-5p (miRNAs known to be expressed in IECs) were measured relative to U6 at each time point using qRT-PCR. In GF animals the expression of let-7b, miR-141 and miR-200a in faeces was lower compared to conventional mice. Following antibiotic-mediated depletion of gut microbiota, rats showed two divergent profiles of miRNA expression. Following two weeks of antibiotic treatment, the expression of let-7b and miR-1224 dropped significantly and remained low for the remainder of the study. The expression of miR-200a and miR-141 was significantly higher at week 2 than before antibiotic treatment commenced. Subsequently, the expression of miR-200a and miR-141 decreased at week 4 and continued to decrease at week 6. This data demonstrates that miRNAs can be used as an independent, non-invasive marker of microbial fluctuations along with gut pathology in the intestine.

The microbiome regulates amygdala-dependent fear recall.

The amygdala is a key brain region that is critically involved in the processing and expression of anxiety and fear-related signals. In parallel, a growing number of preclinical and human studies have implicated the microbiome-gut-brain in regulating anxiety and stress-related responses. However, the role of the microbiome in fear-related behaviours is unclear. To this end we investigated the importance of the host microbiome on amygdala-dependent behavioural readouts using the cued fear conditioning paradigm. We also assessed changes in neuronal transcription and post-transcriptional regulation in the amygdala of naive and stimulated germ-free (GF) mice, using a genome-wide transcriptome profiling approach. Our results reveal that GF mice display reduced freezing during the cued memory retention test. Moreover, we demonstrate that under baseline conditions, GF mice display altered transcriptional profile with a marked increase in immediate-early genes (for example, Fos, Egr2, Fosb, Arc) as well as genes implicated in neural activity, synaptic transmission and nervous system development. We also found a predicted interaction between mRNA and specific microRNAs that are differentially regulated in GF mice. Interestingly, colonized GF mice (ex-GF) were behaviourally comparable to conventionally raised (CON) mice. Together, our data demonstrates a unique transcriptional response in GF animals, likely because of already elevated levels of immediate-early gene expression and the potentially underlying neuronal hyperactivity that in turn primes the amygdala for a different transcriptional response. Thus, we demonstrate for what is to our knowledge the first time that the presence of the host microbiome is crucial for the appropriate behavioural response during amygdala-dependent memory retention.

Regulation of behaviour by the nuclear receptor TLX.

The orphan nuclear receptor Tlx (Nr2e1) is a key regulator of both embryonic and adult hippocampal neurogenesis. Several different mouse models have been developed which target Tlx in vivo including spontaneous deletion models (from birth) and targeted and conditional knockouts. Although some conflicting findings have been reported, for the most part studies have demonstrated that Tlx is important in regulating processes that underlie neurogenesis, spatial learning, anxiety-like behaviour and interestingly, aggression. More recent data have demonstrated that disrupting Tlx during early life induces hyperactivity and that Tlx plays a role in emotional regulation. Moreover, there are sex- and age-related differences in some behaviours in Tlx knockout mice during adolescence and adulthood. Here, we discuss the role of Tlx in motor-, cognitive-, aggressive- and anxiety-related behaviours during adolescence and adulthood. We examine current evidence which provides insight into Tlx during neurodevelopment, and offer our thoughts on the function of Tlx in brain and behaviour. We further hypothesize that Tlx is a key target in understanding the emergence of neurobiological disorders during adolescence and early adulthood.

The microbiota-gut-brain axis as a key regulator of neural function and the stress response: Implications for human and animal health.

The brain-gut-microbiota axis comprises an extensive communication network between the brain, the gut, and the microbiota residing there. Development of a diverse gut microbiota is vital for multiple features of behavior and physiology, as well as many fundamental aspects of brain structure and function. Appropriate early-life assembly of the gut microbiota is also believed to play a role in subsequent emotional and cognitive development. If the composition, diversity, or assembly of the gut microbiota is impaired, this impairment can have a negative impact on host health and lead to disorders such as obesity, diabetes, inflammatory diseases, and even potentially neuropsychiatric illnesses, including anxiety and depression. Therefore, much research effort in recent years has focused on understanding the potential of targeting the intestinal microbiota to prevent and treat such disorders. This review aims to explore the influence of the gut microbiota on host neural function and behavior, particularly those of relevance to stress-related disorders. The involvement of microbiota in diverse neural functions such as myelination, microglia function, neuronal morphology, and blood-brain barrier integrity across the life span, from early life to adolescence to old age, will also be discussed. Nurturing an optimal gut microbiome may also prove beneficial in animal science as a means to manage stressful situations and to increase productivity of farm animals. The implications of these observations are manifold, and researchers are hopeful that this promising body of preclinical work can be successfully translated to the clinic and beyond.

Stress and adolescent hippocampal neurogenesis: diet and exercise as cognitive modulators.

Adolescence is a critical period for brain maturation. Deciphering how disturbances to the central nervous system at this time affect structure, function and behavioural outputs is important to better understand any long-lasting effects. Hippocampal neurogenesis occurs during development and continues throughout life. In adulthood, integration of these new cells into the hippocampus is important for emotional behaviour, cognitive function and neural plasticity. During the adolescent period, maturation of the hippocampus and heightened levels of hippocampal neurogenesis are observed, making alterations to neurogenesis at this time particularly consequential. As stress negatively affects hippocampal neurogenesis, and adolescence is a particularly stressful time of life, it is important to investigate the impact of stressor exposure at this time on hippocampal neurogenesis and cognitive function. Adolescence may represent not only a time for which stress can have long-lasting effects, but is also a critical period during which interventions, such as exercise and diet, could ameliorate stress-induced changes to hippocampal function. In addition, intervention at this time may also promote life-long behavioural changes that would aid in fostering increased hippocampal neurogenesis and cognitive function. This review addresses both the acute and long-term stress-induced alterations to hippocampal neurogenesis and cognition during the adolescent period, as well as changes to the stress response and pubertal hormones at this time which may result in differential effects than are observed in adulthood. We hypothesise that adolescence may represent an optimal time for healthy lifestyle changes to have a positive and long-lasting impact on hippocampal neurogenesis, and to protect against stress-induced deficits. We conclude that future research into the mechanisms underlying the susceptibility of the adolescent hippocampus to stress, exercise and diet and the consequent effect on cognition may provide insight into why adolescence may be a vital period for correct conditioning of future hippocampal function.

Modelling depression in animals: at the interface of reward and stress pathways.

RATIONALE: Despite substantial research efforts the aetiology of major depressive disorder (MDD) remains poorly understood, which is due in part to the heterogeneity of the disorder and the complexity of designing appropriate animal models. However, in the last few decades, a focus on the development of novel stress-based paradigms and a focus on using hedonic/anhedonic behaviour have led to renewed optimism in the use of animal models to assess aspects of MDD. OBJECTIVES: Therefore, in this review article, dedicated to Athina Markou, we summarise the use of stress-based animal models for studying MDD in rodents and how reward-related readouts can be used to validate/assess the model and/or treatment. RESULTS: We reveal the use and limitations of chronic stress paradigms, which we split into non-social (i.e. chronic mild stress), social (i.e. chronic social defeat) and drug-withdrawal paradigms for studying MDD and detail numerous reward-related readouts that are employed in preclinical research. Finally, we finish with a section regarding important factors to consider when using animal models. CONCLUSIONS: One of the most consistent findings following chronic stress exposure in rodents is a disruption of the brain reward system, which can be easily assessed using sucrose, social interaction, food, drug of abuse or intracranial self-stimulation as a readout. Probing the underlying causes of such alterations is providing a greater understanding of the potential systems and processes that are disrupted in MDD.

Early-life adversity and brain development: Is the microbiome a missing piece of the puzzle?

The prenatal and postnatal early-life periods are both dynamic and vulnerable windows for brain development. During these important neurodevelopmental phases, essential processes and structures are established. Exposure to adverse events that interfere with this critical sequence of events confers a high risk for the subsequent emergence of mental illness later in life. It is increasingly accepted that the gastrointestinal microbiota contributes substantially to shaping the development of the central nervous system. Conversely, several studies have shown that early-life events can also impact on this gut community. Due to the bidirectional communication between the gut and the brain, it is possible that aberrant situations affecting either organ in early life can impact on the other. Studies have now shown that deviations from the gold standard trajectory of gut microbiota establishment and development in early life can lead not only to disorders of the gastrointestinal tract but also complex metabolic and immune disorders. These are being extended to disorders of the central nervous system and understanding how the gut microbiome shapes brain and behavior during early life is an important new frontier in neuroscience.

Kynurenine pathway metabolism and the microbiota-gut-brain axis.

It has become increasingly clear that the gut microbiota influences not only gastrointestinal physiology but also central nervous system (CNS) function by modulating signalling pathways of the microbiota-gut-brain axis. Understanding the neurobiological mechanisms underpinning the influence exerted by the gut microbiota on brain function and behaviour has become a key research priority. Microbial regulation of tryptophan metabolism has become a focal point in this regard, with dual emphasis on the regulation of serotonin synthesis and the control of kynurenine pathway metabolism. Here, we focus in detail on the latter pathway and begin by outlining the structural and functional dynamics of the gut microbiota and the signalling pathways of the brain-gut axis. We summarise preclinical and clinical investigations demonstrating that the gut microbiota influences CNS physiology, anxiety, depression, social behaviour, cognition and visceral pain. Pertinent studies are drawn from neurogastroenterology demonstrating the importance of tryptophan and its metabolites in CNS and gastrointestinal function. We outline how kynurenine pathway metabolism may be regulated by microbial control of neuroendocrine function and components of the immune system. Finally, preclinical evidence demonstrating direct and indirect mechanisms by which the gut microbiota can regulate tryptophan availability for kynurenine pathway metabolism, with downstream effects on CNS function, is reviewed. Targeting the gut microbiota represents a tractable target to modulate kynurenine pathway metabolism. Efforts to develop this approach will markedly increase our understanding of how the gut microbiota shapes brain and behaviour and provide new insights towards successful translation of microbiota-gut-brain axis research from bench to bedside. This article is part of the Special Issue entitled 'The Kynurenine Pathway in Health and Disease'.

Bifidobacterium longum 1714 as a translational psychobiotic: modulation of stress, electrophysiology and neurocognition in healthy volunteers.

The emerging concept of psychobiotics-live microorganisms with a potential mental health benefit-represents a novel approach for the management of stress-related conditions. The majority of studies have focused on animal models. Recent preclinical studies have identified the B. longum 1714 strain as a putative psychobiotic with an impact on stress-related behaviors, physiology and cognitive performance. Whether such preclinical effects could be translated to healthy human volunteers remains unknown. We tested whether psychobiotic consumption could affect the stress response, cognition and brain activity patterns. In a within-participants design, healthy volunteers (N=22) completed cognitive assessments, resting electroencephalography and were exposed to a socially evaluated cold pressor test at baseline, post-placebo and post-psychobiotic. Increases in cortisol output and subjective anxiety in response to the socially evaluated cold pressor test were attenuated. Furthermore, daily reported stress was reduced by psychobiotic consumption. We also observed subtle improvements in hippocampus-dependent visuospatial memory performance, as well as enhanced frontal midline electroencephalographic mobility following psychobiotic consumption. These subtle but clear benefits are in line with the predicted impact from preclinical screening platforms. Our results indicate that consumption of B. longum 1714 is associated with reduced stress and improved memory. Further studies are warranted to evaluate the benefits of this putative psychobiotic in relevant stress-related conditions and to unravel the mechanisms underlying such effects.

Behavioural and neurochemical consequences of chronic gut microbiota depletion during adulthood in the rat.

Gut microbiota colonization is a key event for host physiology that occurs early in life. Disruption of this process leads to altered brain development which ultimately manifests as changes in brain function and behaviour in adulthood. Studies using germ-free (GF) mice highlight the extreme impact on brain health that results from life without commensal microbes. However, the impact of microbiota disturbances occurring in adulthood is less studied. To this end, we depleted the gut microbiota of 10-week-old male SpragueDawley rats via chronic antibiotic treatment. Following this marked, sustained depletion of the gut bacteria, we investigated behavioural and molecular hallmarks of gut-brain communication. Our results reveal that depletion of the gut microbiota during adulthood results in deficits in spatial memory as tested by Morris water maze, decreased visceral sensitivity and a greater display of depressive-like behaviours in the forced swim test. In tandem with these clear behavioural alterations we found changes in altered CNS serotonin concentration along with changes in the mRNA levels of corticotrophin releasing hormone receptor 1 and glucocorticoid receptor. Additionally, we found changes in the expression of brain derived neurotrophic factor (BDNF), a hallmark of altered microbiota-gut-brain axis signalling. In summary, this model of antibiotic-induced depletion of the gut microbiota can be used for future studies interested in the impact of the gut microbiota on host health without the confounding developmental influence of early-life microbial alterations.