Same hit, different gut punches

Disorders of gut–brain interaction (DGBI) afflict large numbers of people and are associated with gut dysfunctions including gastrointestinal dysmotility and altered stool frequency (Schmulson & Drossman, 2017). More women than men suffer from DGBI, and women also present with more severe symptoms (So & Savidge, 2021). However, understanding what causes DGBIs and their sex-biased prevalence has been difficult, particularly because these disorders do not have a singular pathogenic mechanism. In the last decade, our understanding has evolved to suggest that a confluence of various factors increases the risk of DGBI. Apart from female sex, these include interventions or infections that cause significant intestinal microbial dysbiosis (So & Savidge, 2021). In addition, early life adversity or stress increases the risk of developing gastrointestinal dysfunction later in adulthood (Garcia et al., 2021). Taken together, these studies would lead to the hypothesis that stressors that cause intestinal microbial dysbiosis during the early life period would increase the risk of developing DGBI in a sex-specific manner. Dysbiosis during the early life period has previously been studied in the context of changes to gastrointestinal epithelial cells and to the neurochemistry and functions of enteric neurons (Garcia et al., 2021; Hung et al., 2020). However, these studies do not go far enough in understanding whether these dysbiosis-driven physiological changes are sex-biased. Simply put, we need to understandwhy and how similar early life insults lead to different sex-biased outcomes. A study by Poon et al. (2022) in this issue of The Journal of Physiology is important in this context, as it asks the question of whether and how antibiotic-mediated intestinal dysbiosis during early post-natal life alters the biology of the intestinal epithelium and the enteric nervous system (ENS), two important gut tissues that regulate significant gastrointestinal biology, in a sex-specific manner to cause dysfunction observed in DGBI patients. By dosing neonatal mice with the antibiotic vancomycin, Poon et al. studied whether this specific early life stress caused sex-specific persistent changes in intestinal microbiota and in the host’s biology to cause gastrointestinal dysfunction later in adult life. Interestingly, while dysbiosis during early life showed persistent dysbiosis and changes in colonic mucosal serotonin biology, independent of sex, the deleterious effects of these changes on various aspects of adult hosts’ ENS biology were found to be remarkably sex-specific. Vancomycin treatment specifically altered the structure of the myenteric plexus – the larger of the two ENS networks, by causing a significant reduction in their neuronal and glial density only in male mice. By contrast, neuronal numbers in the submucosal ganglia were found to be reduced only in female vancomycin-treated mice. While the loss of myenteric neurons in male mice was driven by a loss of serotonin-producing and neurofilament M-expressing neurons, a deeper dive into the neurochemistry of submucosal neurons showed an even more interesting change. Vancomycin-treated female mice (which showed loss of submucosal neurons) showed a loss of cholinergic neurons, and vancomycin-treated male mice (which showed no loss of submucosal neurons) showed an increase in the proportions of cholinergic and a decrease in the proportions of VIPergic neurons in their submucosal ganglia. Yet, male submucosal VIPergic neurons showed increased Ca2+ transients when stimulated with a single pulse, while female submucosal cholinergic neurons showed decreased Ca2+ transients when stimulated with a train of 20 electric pulses. While these sex-specific changes in ENS caused a similar increase in faecal water content (wetter stools) regardless of sex, intestinal motility as measured by the whole gut transit time of an orally gavaged dye was significantly delayed only in female mice. The sex-specific nature of alterations in ENS and in the female-specific nature of dysmotility simulates an important feature of DGBI, where women seem to be more prone to developing gastrointestinal dysmotility disorders. These results show that a specific clinically relevant early life insult, which chronically alters intestinal microbiota, causes a significant change to mucosal serotonin biology and to the structure, neurochemistry and behaviour of ENS tissue later in adulthood. Of these, only the ENS showed sex-specific changes, suggesting that the way in which the ENS plexuses responded to the same early life stress was sex-biased. Briefly, in response to the insult, specific changes in the structure and neuronal behaviour of submucosal plexus were best correlated with functional deficits in intestinal motility in female mice. As a caveat, dye-based transit time experiments do not capture the entire range of diverse motility patterns of the gut. Thus, it is plausible that while significant gastrointestinal dysmotility exists in vancomycin-treated male mice, as they show deficits in their myenteric plexus, these functional alterations may not be captured by the dye-transit experiments. This study underscores the importance of carefully cataloguing tissue-specific changes in response to early life stressors in both sexes – especially since these experimental models simulate human diseases that show a significant sex bias. This study opens the doors to many important questions – whether these observations in the colonic tissue extend to other gastrointestinal regions, and what the molecular mechanisms are by which dysbiosis initiated during early life causes sex-biased gastrointestinal dysfunctions in adults. Importantly, the nature of neuronal changes in tissue, and whether normalization of microbiota in this model leads to amelioration of these observed defects would be of immense diagnostic and therapeutic value. While inferring causality between dysbiosis and gastrointestinal dysfunction requires further mechanistic studies, Poon et al.’s study convincingly shows that the various gut tissues