Therapeutic potential of modulating the microbiota-gut-immune-brain axis for stress and neurodevelopmental disorders

Abstract

There is a growing recognition of the involvement of the gastrointestinal microbiota in the regulation of physiology and behaviour. This is in tandem with increasing demands for novel microbiota-targeted interventions affecting gut-brain signalling and host behaviour. Moreover, a better mechanistic understanding of signalling mediators across the microbiota-gut-immune-brain axis is required. In this thesis, we explore the potential of two such microbiota-targeted interventions: the fermented milk drink kefir and the gut microbial-derived metabolites short-chain fatty acids (SCFAs), to modulate host physiology and behaviour. Kefir has previously been reported to modulate the gut microbiota composition of mice, but no research has yet been done into whether kefir influences gut-brain signalling and host behaviour. Thus, we sought to investigate the effects of preparations derived from two different kefir grains (UK4 and Fr1) on the microbiota-gut-immune-brain axis. Both kefirs differentially impacted gut microbiota composition and function on a strain level, as well as the fecal metabolome. Also at a physiological level both kefirs induced different actions on host with Fr1 affecting colonic serotonin levels and systemic innate immunity, whereas UK4 impacted the adaptive immune system. Behaviourally, kefir UK4 decreased repetitive behaviour in the marble burying test, reward-seeking behaviour in the female urine sniffing test and affected cognition and learning in the fear conditioning test and appetitive Y-maze. Alternatively, kefir Fr1 ameliorated kefir gavage-induced deficits in reward-seeking behaviour in the saccharin preference test. We subsequently tested whether the kefir that decreased repetitive behaviour could also do so in the mouse model of autism spectrum disorder, the BTBR mouse. As expected, kefir decreased repetitive behaviour in the marble burying test and additionally induced social recognition, which was not present in mice given a milk control. Cumulatively, these data reveal that kefir can modulate the microbiota-immune-gut-brain axis and host behaviour, with particular implications in autism spectrum disorder. As for SCFAs, these microbial metabolites have primarily been implicated in host gut, metabolic and immune function, even though their role in microbiota-gut-immune-brain axis signalling and host behaviour has remained largely unexplored in both healthy and stressful conditions. Host gut, metabolic and immune function have previously been demonstrated to be adversely affected by stress, thus we hypothesized that supplementation of the three principal SCFAs could ameliorate specific psychosocial stress-induced deficits. We show that SCFA can alleviate psychosocial stress-induced HPA-axis hyper-reactivity as measured by stress-induced hyperthermia and corticosterone response. Similarly, psychosocial stress-induced alterations in reward-seeking behaviour in the female urine sniffing test and increases in in vivo intestinal permeability were ameliorated by SCFA supplementation. Considering the crucial role for monocyte trafficking in chronic stress-induced deficits in brain and behaviour, we hypothesised that SCFAs could downregulate trafficking markers in human monocytes. Surprisingly, the SCFA butyrate increased the expression of the trafficking receptor CCR2 receptor on human classical monocytes as measured by flow cytometry, emphasising the complexity in which SCFAs can influence host physiology. Overall, these results provide novel strategies for manipulation of the microbiota-gut-immune-brain axis and host behaviour, allowing for a more efficient development of microbiota-based (nutritional) therapeutics for treating autism spectrum disorder, as well as stress-related disorders.