Regulation of Host Nitrogen Balance by the Gut Microbiome

描述（由申请人提供）：结肠微生物群是地球上最密集的微生物栖息地之一。最近几份备受瞩目的报告显示，肠道微生物组产生的物质被宿主吸收，然后显著影响代谢功能。因此，肠道微生物生态及其与宿主代谢组相互作用的研究已成为当代营养和代谢研究的重要前沿。最近来自我们实验室和其他实验室的数据明确表明，饮食是肠道微生物群组成的主要决定因素。低蛋白质饮食可用于治疗肝性脑病、先天性代谢异常（如尿素循环异常或有机酸血症）和慢性肾衰竭患者。此外，由于疾病或无法获得食物（如营养不良）而导致的蛋白质摄入不足也会产生有害影响。生理学的经典研究表明，肠道微生物群在宿主氮平衡中起着重要作用。由宿主产生的尿素在结肠中被产脲酶的肠道细菌水解，产生的氨被宿主吸收并用于蛋白质代谢。然而，在这种情况下，还没有研究仔细研究蛋白质限制对肠道微生物组组成的影响或微生物组对宿主氮平衡的影响。在初步数据中，我们表明，喂食低蛋白饮食的小鼠肠道微生物组的组成发生了重大变化，这些小鼠表现出严重的代谢应激，表现为仅仅为了保持体重而惊人地增加（40%）卡路里消耗。此外，由于尿生成减少，这些小鼠出现了严重的节水缺陷，表现为严重的多食和多尿。给这些小鼠口服抗生素可显著逆转这些不良后遗症。这些观察结果表明，低蛋白饮食会以一种引起有害后果的方式改变肠道微生物群。我们假设低蛋白饮食改变了肠道微生物组的组成和功能，导致宿主代谢发生重大变化，特别是在宿主氮平衡方面。为了解决这一假设，我们将通过分析宿主的尿原作用和肠道中产脲菌的尿素水解来关注宿主和肠道微生物组之间的代谢相互作用。我们提出的项目将利用具有代谢疾病、深度基因组测序和计算生物学经验的高级研究人员的专业知识，这些研究人员是最近资助的NIH途径倡议资助的人类微生物组项目。我们还将利用一个独特的机会来验证我们的小鼠研究结果，通过确定低蛋白饮食的人类受试者肠道微生物组的组成和功能，作为一种罕见的遗传疾病与先天性代谢错误的治疗方法，如尿素循环异常和有机酸尿。这些研究的结果将首次确定小鼠和人类肠道微生物组组成（通过16S rDNA系统分型）、细菌基因表达（通过宏基因组学）和代谢功能之间的因果关系。

DESCRIPTION (provided by applicant): The colonic microbiome is remarkable in that it is amongst the most densely populated microbial habitats on Earth. Several recent high profile reports reveal that the gut microbiome produces substances that are absorbed by the host, which then significantly influence metabolic function. For this reason, the study of gut microbial ecology and its interplay with the host metabolome has emerged as a critical frontier in contemporary nutritional and metabolic research. Recent data from our laboratory and others has unequivocally shown that diet is the principal determinant of the composition of the gut microbiome. Diets low in protein are utilized in the treatment of patients with hepatic encephalopathy, in born errors of metabolism such as urea cycle abnormalities or organic acidemias, and chronic renal failure. Furthermore low protein intake due to disease or insufficient access to food, such as kwashiorkor, can have deleterious effects. Classic studies in physiology have shown that the gut microbiome plays an important role in nitrogen balance of the host. Urea, produced by the host, is hydrolyzed in the colon by urease-producing gut bacteria and the resultant ammonia is absorbed by the host and utilized for protein metabolism. However, no studies have yet scrutinized either the effect of protein restriction on the composition of the gut microbiome or the impact of the microbiome on host nitrogen balance in this setting. In Preliminary Data, we show that there are significant alterations in the composition of the gut microbiome in mice fed a low protein diet and that these mice show evidence of severe metabolic stress, manifested as a striking (40%) increase in calorie consumption simply in order to maintain body weight. In addition, due to a reduction in ureagenesis, these mice suffer a profound defect of water conservation manifested as severe polydypsia and polyuria. The administration of oral antibiotics to these mice dramatically reverses these adverse sequelae. These observations suggest that a low protein diet alters the gut microbiome in a manner that evokes deleterious consequences. We hypothesize that a low protein diet alters the composition and function of the gut microbiome leading to major changes in host metabolism, especially in host nitrogen balance. To address this hypothesis, we will focus on the metabolic interaction between the host and gut microbiome through an analysis of ureagenesis by the host and urea hydrolysis by urease-producing bacteria in the gut. Our proposed project will utilize the expertise of senior investigators with experience in metabolic diseases, deep genomic sequencing, and computational biology assembled for a recently funded NIH pathway initiative grant known as the Human Microbiome Project. We will also take advantage of a unique opportunity to validate our murine findings by characterizing the composition and function of the gut microbiome in human subjects on defined low protein diets as therapy for a rare set of genetic disorders with inborn errors of metabolism such as urea cycle abnormalities and organic acidurias. The results of these studies will, for the first time, determine cause-and-effect relationships between the composition of the gut microbiome (via 16S rDNA phylotyping), bacterial gene representation (via metagenomics), and metabolic function in both mice and humans. PUBLIC HEALTH RELEVANCE: Diets low in protein are utilized in the treatment of patients with hepatic encephalopathy, in born errors of metabolism such as urea cycle abnormalities or organic acidurias, and chronic renal failure. Furthermore low protein intake due to disease or insufficient access to food, such as kwashiorkor, can have deleterious effects. Classic studies in physiology have shown that the gut microbiome plays an important role in nitrogen balance of the host. However, no studies have yet scrutinized either the effect of protein restriction on the composition of the gut microbiome or the impact of the microbiome on host nitrogen balance in this setting. The results of the studies in this proposal will, for the first time, determine cause-and-effect relationships between the composition of the gut microbiome, bacterial gene representation, and host metabolic function in both mice and humans on a low protein diet.