MYLK and colitis: Higher colonic 5-HT levels and disruption of the epithelial barrier function via myosin light chain kinase (MLCK) by AR exposure are associated with perturbation of the gut microbiota in specific pathogen-free (SPF) mice, while AR exposure also induces mild colitis and elevates colonic 5-HT in naïve germ-free (GF) mice.