The colon contains tryptophan derivatives such as indole, which is a microbiota-derived signaling molecule, and the host-derived serotonin neurotransmitter that is primarily synthesized in the GI tract. Indole is also known to be absorbed by host cells and helps strengthen the integrity of the intestinal barrier, being regarded as a beneficial chemical cue within microbial/host interactions. Indole is synthesized by tryptophanase, which is encoded by the tnaA gene. We have shown that the concentration of indole is significantly higher in the lumen of the colon (the compartment where the microbiota resides) compared to colonic tissues (where indole is absorbed by intestinal epithelial cells). Serotonin is synthesized in enterochromaffin cells by the enzyme tryptophan hydroxylase (TpH1). Upon its synthesis, serotonin is released into the lamina propria and is secreted into the lumen. Serotonin signaling in the intestinal mucosa is terminated by removal of serotonin by the serotonin selective reuptake transporter (SERT), which is expressed by epithelial cells. We showed that both serotonin and indole converge to decrease virulence gene expression from enterohemorrhagic E. coli (EHEC) and Citrobacter rodentium, a murine pathogen employed as a surrogate animal model for EHEC. We also identified the bacterial receptor for these signals as CpxA. Upon sensing serotonin and/or indole, CpxA functions primarily as a phosphatase, dephosphorylating itself and CpxR, that activates virulence in its phosphorylated state. Through transcriptome studies we also identified the Indole Sequestering Receptor (Isr), which in the absence of indole directly activates virulence expression. However, in the presence of indole, Isr is no longer able to activate transcription of virulence genes. Using TpH1 pharmacological inhibitors (decrease the levels of serotonin in the gut) and SERT knockout mice (have increased levels of luminal serotonin), we showed that the presence of higher levels of serotonin in the intestine of mice decreased virulence in C. rodentium, while decreased levels of serotonin are conducive to increased pathogenesis. Moreover, we synthetically altered the concentration of indole in the GI tract of mice. This allowed us to assess the role of self-produced versus microbiota-produced indole, and show that decreased indole concentrations promote bacterial pathogenesis, while increased levels of indole decreases bacterial virulence gene expression during murine infection. Altogether, both serotonin and indole decrease virulence of C. rodentium during murine infection. Our studies show that fluctuations in the levels of indole and the serotonin neurotransmitter significantly impact disease prognosis. However several questions regarding this exquisite signaling regulation of bacterial virulence remain unanswered. Consequently the specific aims of this grant are:
Aim 1. Define the CpxA/CpxR and Isr serotonin/indole signaling cascade.
Aim 2. Investigate the intersection of serotonin with endogenous and exogenous indole signaling in bacterial pathogenesis during mammalian infection.
Pathogens have evolved elaborate mechanisms to sense their environments and, in particular, host neurotransmitters; the sensing of these neurotransmitters is the basis of a novel mode of regulating virulence gene expression. For example, serotonin is a neurotransmitter that is primarily synthesized in the gastrointestinal (GI) tract, which is sensed by pathogens to regulate virulence expression. The related microbiota-derived compound indole is also sensed.
