Structural & functional consequences of chronic psychosocial stress on the microbiome & host

Highlights

• Chronic psychosocial stress induces complex structural changes in the gut microbiota.

• Certain microbial groups are sensitive to the state of the host.

• Shifts in the microbial community are associated with social behavioral deficits.

• Defeated mice exhibit sustained immunoregulatory changes.

• Predicted abundance of microbe derived short chain fatty acids & tryptophan synthesis are lower following stress.

Abstract

Introduction

Given the lasting impact of psychological distress on behavior, along with the role of the microbiome in neurobehavioral development, we sought to examine the relationship between the microbiota and stress-induced behavioral deficits.

Methods

Male C57BL/6 mice exposed to chronic social defeat were subjected to behavioral analysis and profiling of the intestinal microbiome. Mice were also analyzed for phenotypic and functional immune changes. A computational approach on 16S rRNA marker gene sequences was used to predict functional changes in the metagenome as a consequence of structural shifts in the microbiota.

Results

Chronic social defeat induced behavioral changes that were associated with reduced richness and diversity of the gut microbial community, along with distinct shifts at the level of operational taxonomic units (OTU) across phyla. The degree of deficits in social, but not exploratory behavior was correlated with group differences between the microbial community profile. In silico analysis predicted a shift in the functional profile of the microbiome: defeated mice exhibited reduced functional diversity and a lower prevalence of pathways involved in the synthesis and metabolism of neurotransmitter precursors and short-chain fatty acids. Defeated mice also exhibited sustained alterations in dendritic cell activation, and transiently elevated levels of IL-10+ T regulatory cells that were suppressed over time.

Conclusions

This study indicates that stress-induced disruptions in neurologic function are associated with altered immunoregulatory responses and complex OTU-level shifts in the microbiota. It is thus suggested that a dysbiotic state, along with specific changes in microbial markers, may predict the onset of adverse neurocognitive deficits commonly observed following exposure to severe stressors. The data also predict novel pathways that might underlie microbiota-mediated effects on brain and behavior, thus presenting targets for investigations into mechanisms and potential therapy.

Introduction

There is abundant evidence demonstrating the adverse impact of stress on physiology and neurocognitive correlates during development and adulthood: trauma or abuse during early life increase the risk of psychiatric conditions and can impair the development of the stress response (Heim et al., 2008), while inadequate coping behavior contributes to the etiology of diseases such as gastrointestinal disorders and increased risk of depression (Dinan, 2005, Mayer, 2000). Amidst the efforts to elucidate the mechanisms underpinning this association, there has been a growing recognition of the importance of the microbiota to normal development and function of several physiological processes, including metabolism, immunity, and behavior. Indeed, so integral are these symbionts to host function that it has been suggested that almost all animals, including humans, should be viewed as multi-species organisms or “holobionts” (Gilbert et al., 2012).

The body of work demonstrating the systemic role of the microbiome, especially in neural development and function, is extensive. Disruption or absence of the microbiome impairs behavior and its development, leading to increased exploration, decreased apprehension, and impaired social behavior (Bercik et al., 2011, Desbonnet et al., 2014). Conversely, chronic administration of Lactobacillus rhamnosus (JB-1) alters GABAR expression in the brain, and reduces anxiety-like and depressive behavior (Bravo et al., 2011). Particularly, there is compelling evidence of bidirectional interaction between stress and the microbiome. Exposure to stress alters the structural composition of the intestinal microbiota (Bailey et al., 2011, O’Mahony et al., 2009), while germ-free (GF) status and intestinal colonization alter the developmental trajectory of the stress response (Neufeld et al., 2011, Sudo et al., 2004). Within the context of the holobiont paradigm, the influence exerted by these microorganisms on brain development and behavior is a consequence of the evolution of a multi-species organism. To provide clearer insight into the implications of the concept of the collective “self” for health and disease, and understand the functional relationship between the microbiota and stress-induced alterations, we need greater insight into the mechanisms, pathways, and consequences of communication along the microbiota-gut-brain axis.

Here, an anthropomorphic model is used to examine the impact of psychosocial stress on host-microbiota interactions, and the relationship between the microbiome and stress-induced behavioral deficits. We profile the community structure and species-level shifts in the intestinal microbiota, including in the relative abundance of Akkermancia muciniphilia and Coriobacteriaceae. These specific taxa have been previously reported to be associated with healthy and stress-exposed microbiome communities, respectively (Bendtsen et al., 2012, Everard et al., 2013). Accordingly, in an effort to investigate the use of specific microbial community markers to predict adverse consequences on the host, we examined whether such alterations are retained, amidst broader shifts in the microbiota, across experimental studies. We also examine the nature of the stress-induced dysbiosis—specifically, the Firmicutes/Bacteroidetes ratio, which signals the status of the human gut microbiota in models of obesity and antibiotic-induced dysbiosis (Mariat et al., 2009, Sanderson et al., 2006, Thompson et al., 2015).

Given evidence of immune-mediated signaling along the microbiota-gut-brain axis in the literature (Desbonnet et al., 2010, Forsythe et al., 2010), we profile the immunoregulatory and innate immune phenotype, as well as the function of the peripheral immune system. Moreover, using a computational approach, we address the biological pathways that may be driving the effects of the microbiome on brain and behavior by profiling the predicted functional implications of structural shifts in the microbiome.