Inflammasome signaling affects anxiety- and depressive-like behavior and gut microbiome composition

Abstract

The inflammasome is hypothesized to be a key mediator of the response to physiological and psychological stressors, and its dysregulation may be implicated in major depressive disorder. Inflammasome activation causes the maturation of caspase-1 and activation of interleukin (IL)-1β and IL-18, two proinflammatory cytokines involved in neuroimmunomodulation, neuroinflammation and neurodegeneration. In this study, C57BL/6 mice with genetic deficiency or pharmacological inhibition of caspase-1 were screened for anxiety- and depressive-like behaviors, and locomotion at baseline and after chronic stress. We found that genetic deficiency of caspase-1 decreased depressive- and anxiety-like behaviors, and conversely increased locomotor activity and skills. Caspase-1 deficiency also prevented the exacerbation of depressive-like behaviors following chronic stress. Furthermore, pharmacological caspase-1 antagonism with minocycline ameliorated stress-induced depressive-like behavior in wild-type mice. Interestingly, chronic stress or pharmacological inhibition of caspase-1 per se altered the fecal microbiome in a very similar manner. When stressed mice were treated with minocycline, the observed gut microbiota changes included increase in relative abundance of Akkermansia spp. and Blautia spp., which are compatible with beneficial effects of attenuated inflammation and rebalance of gut microbiota, respectively, and the increment in Lachnospiracea abundance was consistent with microbiota changes of caspase-1 deficiency. Our results suggest that the protective effect of caspase-1 inhibition involves the modulation of the relationship between stress and gut microbiota composition, and establishes the basis for a gut microbiota-inflammasome-brain axis, whereby the gut microbiota via inflammasome signaling modulate pathways that will alter brain function, and affect depressive- and anxiety-like behaviors. Our data also suggest that further elucidation of the gut microbiota-inflammasome-brain axis may offer novel therapeutic targets for psychiatric disorders.

Figure 1

Caspase-1 (casp1) deficiency decreases anxiety-like and depressive like behavior and affects chronic restraint stress response. (a) Casp1 knockout (casp1^−/−) mice displayed decreased floating time in the forced swim test in comparison with wild-type (wt) mice and (b) displayed decreased anxiety-like behavior as measured by the open/closed arms time ratio in the elevated plus maze. (c) In the novelty suppressed feeding test, casp1^−/− mice showed significantly decreased latency to feed following 16 h of fasting but not water deprivation. (d) Moreover, casp1 deficiency resulted in less marbles buried in the marble burying test. (e) In the open field test, we observed a decreased number of fecal boli as a result of casp1 deficiency as well as a different response to chronic restraint stress. Data are presented as mean±s.e.m. Genotype effect: *P<0.05, **P<0.01, ****P<0.0001; stress effect: ⁺P<0.05, ⁺⁺P<0.01, ⁺⁺⁺⁺P<0.0001; genotype × stress effect: ^####P<0.0001. BL, baseline; STR, after chronic restraint stress paradigm; wt, wild type.

Figure 2

Caspase-1 (casp1) deficiency increases spontaneous locomotion and locomotory skills. (a) Casp1 knockout (casp1^−/−) mice had increased locomotor activity in the open field test when compared with wild-type (wt) mice and (b) acquired quicker the skills to perform the rotarod test. Data are mean±s.e.m. Genotype effect: P<0.05; **P<0.01; ****P<0.0001. BL, baseline; STR, after chronic restraint stress paradigm.

Figure 3

Caspase-1 antagonism affects chronic restraint stress response. (a) Minocycline treatment (mino) in wild-type (wt) animals during chronic restraint stress (STR) prevented stress-induced increased floating time in the forced swim test. (b) Respirometry measurement for volume of CO₂ exhaled revealed a significant effect of stress as well as treatment. Data are mean±s.e.m. Treatment effect: *P<0.05; stress effect: ⁺⁺P<0.01; treatment × stress effect: ^#P<0.05. BL, baseline.

Figure 4

Minocycline treatment and chronic restraint stress affect the gut microbiome and chronic restraint stress changes the gut Firmicutes/Bacteroidetes (F/B) ratio. (a) Box and whiskers plot displayed the analysis of the differences of the main composition of the microbiota (Firmicutes to Bacteroidetes). Upper and lower quartiles defined the box with median midline, and the whiskers were assessed using Tukey's method. (b) Microbiota distribution at species level of taxon contributing to 97.5% of sample variations. Heatmap shows square root-transformed read counts for the 20 taxa determined by similarity percentage analysis. The dendrogram shows the clustering of genera based on Ward's hierarchical clustering method. Phyla are abbreviated as follows: Actinobacteria (A), Bacteroidetes (B), Firmicutes (F), Proteobacteria (P) and Verrocomicrobia (V).

Figure 5

The effect of minocycline treatment, chronic restraint stress and their combination assessed at the level of individual taxa. Individual minocycline effect on the (a) Turicibacter and (b) Bifidobacterium populations; synergistic effect of minocycline and chronic restraint stress on the (c) Akkermansia, (d) Blautia and (e) Lachnospiraceae populations; and antagonistic effect of minocycline and chronic restraint stress on the (f) Lactobacillus and (g) Anaerovorax populations. Significant difference between treatment groups: *P<0.05, **P<0.01, ***P<0.001.