Gut dysbiosis, or pathological alterations in the gut microbiota, is observed in a variety of clinical conditions, including autism, alcohol abuse, acute stress disorder and psychiatric disorders comorbid with inflammatory bowel disease. These conditions are all associated with increases in anxiety behavior, along with other psychological disorders. However, specific mechanisms whereby the gut microbiota affect brain function and behavior remain largely unknown. Lipopolysaccharide (LPS, a.k.a. endotoxin) is a ubiquitous component of the gut microbiota that can cause inflammatory responses, predominantly by activating the innate immune receptor toll-like receptor 4 (TLR4). However, increased growth of pathogenic gram-negative bacteria, observed in both autistic and IBD populations, may increase activation of intestinal epithelial TLR4. A vast body of literature demonstrates that direct systemic injections of LPS, yielding high plasma levels of LPS, increase anxiety behavior along with activation of a number of sickness-related behaviors, presumably after activating TLR4. However, these systemic injections of LPS do not accurately model the actions of LPS derived from the gut lumen, which result in substantially lower plasma levels of LPS relative to those observed after direct systemic injections of LPS. Whether gut-derived LPS increases anxiety in a manner similar to systemic injections of LPS remains unknown. Furthermore, if gut-derived LPS activates neural circuits that increase anxiety behavior, then the site and mechanism of action remains unknown. Oral administration of LPS serves as an acute model of gut dysbiosis, and preliminary data indicates that this treatment increases anxiety-like behavior in mice. TLR4s on intestinal epithelial cells are an attractive target, as their activation modulates gut barrier integrity (yielding gut microbiota access to the lamina propria, which houses the intestinal immune system) and produces cytokines that may increase anxiety behavior. Cytokines more closely associated with the TLR4/Myd88 signaling cascade may be responsible for anxiety behavior induced by gut-derived LPS, as these have been shown to increase intestinal permeability. Using TLR4 antagonists that selectively block the TLR4/TRIF or TLR4/Myd88 pathway, I will test whether inhibition of either signaling pathway blocks increases in anxiety behavior induced by oral administration of LPS. In addition, using a line of mice with inducible deletion of Tlr4, selectively in villin-expressing (predominantly intestinal epithelial) cells, I will test whether Tlr4 expression in intestinal epithelial cells is required for increases in anxiety behavior induced by oral administartion of LPS. The central hypothesis is that gut dysbiosis increases anxiety behavior and neuroinflammation via heightened intestinal epithelial TLR4/Myd88-dependent signaling. Exploring this potential mechanism in mice orally administered LPS will lay the groundwork for understanding the chain of events that lead to anxiety in conditions that exhibit gut dysbiosis.
A number of rodent models for neuropsychiatric disorders that exhibit increased anxiety behaviors also exhibit pathological alterations in gut microbiota, or gut dysbiosis, which is often reported to exhibit increased intestinal LPS levels and a 2- or 3-fold increase in serum LPS levels (called ?metabolic endotoxemia?). This project explores whether gut-derived LPS can produce anxiety behavior, and whether blocking immune signaling caused by gut-derived LPS blocks associated expressions of anxiety behavior.
|Fields, Christopher T; Sampson, Timothy R; Bruce-Keller, Annadora J et al. (2018) Defining Dysbiosis in Disorders of Movement and Motivation. J Neurosci 38:9414-9422|
|Fields, Christopher T; Chassaing, Benoit; Castillo-Ruiz, Alexandra et al. (2018) Effects of gut-derived endotoxin on anxiety-like and repetitive behaviors in male and female mice. Biol Sex Differ 9:7|