Microbial metabolism of host androgens regulates enteric nervous system function.

Abstract

Gastrointestinal (GI) motility is crucial for organismal health and homeostasis. Crosstalk between neural circuits embedded in the gut wall and luminal factors, including microbial metabolites, can modulate GI motility of the host. Recently, our lab established that androgen signaling in peripheral neurons is necessary for normal GI motility in male mice by selectively stimulating colonic motility. It remains unclear whether androgen signaling is required for normal motility in females, the molecular mechanism through which androgens modulate GI motility, and why this signaling pathway is specific to the colon. Microorganisms inhabiting the intestinal lumen can alter the host androgen axis, but it remains unclear to what extent microbial modulation of androgens can affect the function of the peripheral nervous system, including GI motility. We hypothesized that microbes alter levels of androgens that can signal to the enteric nervous system to stimulate GI motility. To first address whether androgen signaling is important for motility in female mice, androgen receptor (AR) expression was characterized across the female colon, and while transcript levels were comparable to those of males, AR protein levels were lower in females. Moreover, enteric neurons were rarely AR-immunoreactive in female mice. Using pharmacological and mouse genetic tools, we found that androgen signaling has minimal effects on GI motility in the female murine colon. To examine the molecular mechanism of how androgens regulate motility in male mice, we examined AR expression in the colon of male mice and found that 95% of AR+ enteric neurons are inhibitory motor neurons labeled by NOS1 immunoreactivity. Using mouse genetic tools, we learned that androgen signaling to NOS1+ enteric neurons is necessary for normal GI motility in male mice. Given that microbe-depleted mice have fewer colonic NOS1+ neurons, reduced levels of bioavailable androgens in the gut, and slowed GI transit compared to mice with intact flora, we hypothesized that bacteria are necessary to generate androgens that signal to NOS1+ enteric neurons to regulate GI motility. Male mice exposed to broad-spectrum antibiotics (ABX) for 7 days to cause microbial depletion had slowed GI transit times, reduced levels of systemic androgens, and loss of AR expression in enteric neurons. Androgen supplementation was sufficient to rescue all of these deficits. Steroid hormones are normally glucuronidated in the liver and partially excreted through the GI tract. Bacteria that express beta-glucuronidase (GUS) enzymes can cleave the glucuronide moiety to regenerate bioactive steroids in the intestinal lumen. Shotgun metagenomics of stool revealed functional differences between pre- and post-puberty mice, which can be accounted for by differences in gus gene presence. Finally, rectal infusion of a single GUS enzyme in microbe-depleted mice was sufficient to restore androgen signaling to NOS1+ enteric neurons. Together, these findings revealed a critical role for gut bacteria in modulating host androgen levels to affect enteric nervous system function.