The gut microbiome shapes local and systemic immunity. The liver is presumed to be a protected sterile site. As such, a hepatic microbiome has not been examined. Here, we showed a liver microbiome in mice and humans that is distinct from that of the gut and is enriched in Proteobacteria. It undergoes dynamic alterations with age and is influenced by the environment and host physiology. Fecal microbial transfer experiments revealed that the liver microbiome is populated from the gut in a highly selective manner. Hepatic immunity is dependent on the microbiome, specifically the bacteroidetes species. Targeting bacteroidetes with oral antibiotics reduced hepatic immune cells by approximately 90%, prevented antigen-presenting cell (APC) maturation, and mitigated adaptive immunity. Mechanistically, our findings are consistent with presentation of bacteroidetes-derived glycosphingolipids to NKT cells promoting CCL5 signaling, which drives hepatic leukocyte expansion and activation, among other possible host-microbe interactions. Collectively, we reveal a microbial/glycosphingolipid/NKT/CCL5 axis that underlies hepatic immunity.

There is currently no criterion to select appropriate bioinformatics tools and reference databases for analysis of 16S rRNA amplicon data in the human oral microbiome. Our study aims to determine the influence of multiple tools and reference databases on α-diversity measurements and β-diversity comparisons analyzing the human oral microbiome. We compared the results of taxonomical classification by Greengenes, the Human Oral Microbiome Database (HOMD), National Center for Biotechnology Information (NCBI) 16S, SILVA, and the Ribosomal Database Project (RDP) using Quantitative Insights Into Microbial Ecology (QIIME) and the Divisive Amplicon Denoising Algorithm (DADA2). There were 15 phyla present in all of the analyses, four phyla exclusive to certain databases, and different numbers of genera were identified in each database. Common genera found in the oral microbiome, such as Veillonella, Rothia, and Prevotella, are annotated by all databases; however, less common genera, such as Bulleidia and Paludibacter, are only annotated by large databases, such as Greengenes. Our results indicate that using different reference databases in 16S rRNA amplicon data analysis could lead to different taxonomic compositions, especially at genus level. There are a variety of databases available, but there are no defined criteria for data curation and validation of annotations, which can affect the accuracy and reproducibility of results, making it difficult to compare data across studies.

Since the identification of succinate's receptor in 2004, studies supporting the involvement of succinate signaling through its receptor in various diseases have accumulated and most of these investigations have highlighted succinate's pro-inflammatory role. Taken with the fact that succinate is an intermediate metabolite in the center of mitochondrial activity, and considering its potential regulation of protein succinylation through succinyl-coenzyme A, a review on the overall multifaceted actions of succinate to discuss whether and how these actions relate to the cellular locations of succinate is much warranted. Mechanistically, it is important to consider the sources of succinate, which include somatic cellular released succinate and those produced by the microbiome, especially the gut microbiota, which is an equivalent, if not greater contributor of succinate levels in the body. Continue learning the critical roles of succinate signaling, known and unknown, in many pathophysiological conditions is important. Furthermore, studies to delineate the regulation of succinate levels and to determine how succinate elicits various types of signaling in a temporal and spatial manner are also required.

Periodontal disease (PD) is one of the most common inflammatory diseases in humans and is initiated by an oral microbial dysbiosis that stimulates inflammation and bone loss. Here, we report an abnormal elevation of succinate in the subgingival plaque of subjects with severe PD. Succinate activates succinate receptor-1 (SUCNR1) and stimulates inflammation. We detected SUCNR1 expression in the human and mouse periodontium and hypothesize that succinate activates SUCNR1 to accelerate periodontitis through the inflammatory response. Administration of exogenous succinate enhanced periodontal disease, whereas SUCNR1 knockout mice were protected from inflammation, oral dysbiosis, and subsequent periodontal bone loss in two different models of periodontitis. Therapeutic studies demonstrated that a SUCNR1 antagonist inhibited inflammatory events and osteoclastogenesis in vitro and reduced periodontal bone loss in vivo. Our study reveals succinate’s effect on periodontitis pathogenesis and provides a topical treatment for this disease.

Succinate is a metabolite in the tricarboxylic acid cycle (TCA) which plays a central role in mitochondrial activity. Excess succinate is known to be transported out of the cytosol, where it activates a succinate receptor (SUCNR1) to enhance inflammation through macrophages in various contexts. In addition, the intracellular role of succinate beyond an intermediate metabolite and prior to its extracellular release is also important to the polarization of macrophages. However, the role of succinate in microglial cells has not been characterized. Lipopolysaccharide (LPS) stimulates the elevation of intracellular succinate levels. To reveal the function of intracellular succinate associated with LPS-stimulated inflammatory response in microglial cells, we assessed the levels of ROS, cytokine production and mitochondrial fission in the primary microglia pretreated with cell-permeable diethyl succinate mimicking increased intracellular succinate. Our results suggest that elevated intracellular succinate exerts a protective role in the primary microglia by preventing their conversion into the pro-inflammatory M1 phenotype induced by LPS. This protective effect is SUCNR1-independent and mediated by reduced mitochondrial fission and cellular ROS production.