The Gut Microbiota and Developmental Programming of the Testis in Mice

Nutrients and environmental chemicals, including endocrine disruptors, have been incriminated in the current increase in male reproductive dysfunction, but the underlying mechanisms remain unknown. The gastrointestinal tract represents the largest surface area exposed to our environment and thereby plays a key role in connection with exposure of internal organs to exogenous factors. In this context the gut microbiome (all bacteria and their metabolites) have been shown to be important contributors to body physiology including metabolism, cognitive functions and immunity. Pivotal to male reproduction is a proper development of the testis, including the formation of the blood-testis barrier (BTB) that encapsulates and protects germ cells from stress induced environmental cues, e.g. pathogenic organisms and xenobiotics. Here we used specific pathogen free (SPF) mice and germ-free (GF) mice to explore whether gut microbiota and/or their metabolites can influence testis development and regulation of BTB. Lumen formation in the seminiferous tubules, which coincides with the development of the BTB was delayed in the testes of GF mice at 16 days postpartum. In addition, perfusion experiments (Evans blue) demonstrated increased BTB permeability in these same mice. Reduced expressions of occludin, ZO-2 and E-cadherin in GF testis suggested that the microbiota modulated BTB permeability by regulation of cell-cell adhesion. Interestingly, exposure of GF mice to Clostridium Tyrobutyricum (CBUT), which secrete high levels of butyrate, restored the integrity of the BTB and normalized the levels of cell adhesion proteins. Moreover, the GF mice exhibited lower serum levels of gonadotropins (LH and FSH) than the SPF group. In addition, the intratesticular content of testosterone was lower in GF compared to SPF or CBUT animals. Thus, the gut microbiome can modulate the permeability of the BTB and might play a role in the regulation of endocrine functions of the testis.

营养物质与环境化学物（包括内分泌干扰物（endocrine disruptors））被认为与当前男性生殖功能障碍患病率上升有关，但其潜在机制仍未明确。胃肠道是人体与外界环境接触面积最大的器官，因此在内脏器官暴露于外源性因素的过程中发挥关键作用。在此背景下，肠道菌群（gut microbiome，即所有细菌及其代谢产物）已被证实对机体生理功能具有重要调控作用，涉及代谢、认知功能与免疫等多个方面。男性生殖功能的核心环节之一是睾丸的正常发育，其中包括血-睾屏障（blood-testis barrier, BTB）的形成——该屏障可包裹并保护生殖细胞，使其免受环境应激因素（如病原微生物与外源性化学物）的损伤。本研究选用无特定病原体（specific pathogen free, SPF）小鼠与无菌（germ-free, GF）小鼠，旨在探究肠道菌群及其代谢产物是否可影响睾丸发育与血-睾屏障的调控功能。产后16日龄的无菌小鼠睾丸中，与生后血-睾屏障发育同步的生精小管管腔形成过程出现延迟。此外，伊文思蓝（Evans blue）灌注实验结果显示，该类小鼠的血-睾屏障通透性升高。无菌小鼠睾丸中闭合蛋白（occludin）、紧密连接蛋白ZO-2以及上皮型钙黏蛋白E-钙黏蛋白的表达水平均出现下调，提示肠道菌群可通过调控细胞间黏附来调节血-睾屏障的通透性。有趣的是，将无菌小鼠暴露于高产丁酸的酪丁酸梭菌（Clostridium Tyrobutyricum, CBUT）后，其血-睾屏障完整性得以恢复，细胞黏附蛋白的表达水平也恢复至正常水平。此外，与无特定病原体小鼠组相比，无菌小鼠的血清促性腺激素（gonadotropins，包括黄体生成素LH与卵泡刺激素FSH）水平更低。与此同时，无菌小鼠的睾丸内睾酮含量也低于无特定病原体小鼠组与酪丁酸梭菌处理组小鼠。综上，肠道菌群可调控血-睾屏障的通透性，或可参与睾丸内分泌功能的调控。