Significant metabolic interactions exist between the gut microbiota and the mammalian host. These interactions, mediated by the exchange of metabolites under environmental influences, are generally symbiotic in nature, but can become detrimental in certain pathological states. One prime example is the delivery of urea, a waste product from host nitrogen metabolism, into the colon and subsequent hydrolysis into carbon dioxide and ammonia by the bacterial enzyme urease. The host can reabsorb the ammonia for additional nitrogen metabolism, or the bacteria can use it for amino acid biosynthesis and contribute to host amino acid homeostasis. In pathological conditions of hyperammonemia such as hepatic encephalopathy (HE) in liver disease and urea cycle disorders, this process results in worsened morbidity and mortality. The pathogenesis of HE is complex and not fully elucidated, and current treatments remain inadequate. We show that fecal microbiota transplantation (FMT) using Altered Schaedler Flora (ASF), a defined consortium of eight murine bacteria with minimal urease gene content, leads to improved neurobehavioral deficits and survival in the thioacetamide (TAA) murine model of hepatic injury. The objective of this proposal is to characterize the mechanism(s) by which ASF exerts its beneficial effects. The central hypothesis is that a urease-deficient bacterial consortium such as ASF modulates nitrogen balance between the gut microbiota and its host leading to an alteration in amino acid metabolism through urease-dependent mechanisms. This hypothesis will be tested through three inter-related specific aims that will systematically evaluate the role of bacterial urease in amino acid homeostasis in mice fed a normal protein diet (Aim 1), a low protein diet (Aim 2), and in the setting of TAA-induced hepatic injury (Aim 3). The experiments will use various innovative approaches including inoculation of a defined consortium composed of human bacterial strains that lack urease, heavy isotope 15N- label studies, 16S rRNA gene microbiota sequencing, and murine metabolic phenotyping to address these physiologically-relevant mechanisms. The proposed research is significant because it will provide new information about the interaction between the gut microbiota and its host in the pathogenesis of HE, with the rationale of improving treatment of HE through potential therapeutic FMT. University of Pennsylvania provides the perfect research environment to conduct this investigation given local expertise in the inter-related fields of gut microbiome and metabolism. The candidate will gain experience in compositional and functional analysis of the microbiome, acquire fundamental skills in metabolomics and microbial isolation and culturing techniques, and broaden his knowledge base of metabolic physiology. These skill sets will greatly enhance the candidate's career development into an independent scientific investigator, with the long-term goal of examining the broader impact of the gut microbiome in human metabolic diseases for the advancement of medical treatment.
Hepatic encephalopathy is one of the most frequent and debilitating complications of liver disease. I propose to study the role of urease, an enzyme produced by certain bacteria residing in the human gut, in amino acid metabolism in order to understand the role of gut bacteria in the pathogenesis of hepatic encephalopathy. The results of these studies will help to identify mechanisms that can be exploited to treat hepatic encephalopathy by altering the gut microbiota.
