Emergence of Caenorhabditis elegans as a Model Organism for Dissecting the Gut-Brain Axis.

Accumulating evidence links the gut microbiome to neuronal functions in the brain. Given the increasing prevalence of brain disorders, there is a critical need to understand how gut microbes impact neuronal functions so that targeted therapeutic interventions can be developed. In this commentary, we discuss what makes the nematode Caenorhabditis elegans a valuable model for dissecting the molecular basis of gut microbiome-brain interactions. With a fully mapped neuronal circuitry, C. elegans is an effective model for studying signaling of the nervous system in a context that bears translational relevance to human disease. We highlight C. elegans as a potent but underexploited tool to interrogate the influence of the bacterial variable on the complex equation of the nervous system. We envision that routine use of gnotobiotic C. elegans to examine the gut-brain axis will be an enabling technology for the development of novel therapeutic interventions for brain diseases

Discovery of a Gut Bacterial Metabolic Pathway that Drives α‑Synuclein Aggregation

Parkinson's disease (PD) etiology is associated with aggregation and accumulation of α-synuclein (α-syn) proteins in midbrain dopaminergic neurons. Emerging evidence suggests that in certain subtypes of PD, α-syn aggregates originate in the gut and subsequently spread to the brain. However, mechanisms that instigate α-syn aggregation in the gut have remained elusive. In the brain, the aggregation of α-syn is induced by oxidized dopamine. Such a mechanism has not been explored in the context of the gastrointestinal tract, a niche harboring 46% of the body's dopamine reservoirs. Here, we report that Enterobacteriaceae, a bacterial family prevalent in human gut microbiotas, induce α-syn aggregation. More specifically, our in vitro data indicate that respiration of nitrate by Escherichia coli K-12, which results in production of nitrite that mediates oxidation of Fe2+ to Fe3+, creates an oxidizing redox potential. These oxidizing conditions enabled the formation of dopamine-derived quinones and α-syn aggregates. Exposing nitrite, but not nitrate, to enteroendocrine STC-1 cells induced aggregation of α-syn that is natively expressed in these cells, which line the intestinal tract. Taken together, our findings indicate that bacterial nitrate reduction may be critical for initiating intestinal α-syn aggregation

Alternative pathway for dopamine production by acetogenic gut bacteria that O-Demethylate 3-Methoxytyramine, a metabolite of catechol O-Methyltransferase.

AimsThe gut microbiota modulates dopamine levels in vivo, but the bacteria and biochemical processes responsible remain incompletely characterized. A potential precursor of bacterial dopamine production is 3-methoxytyramine (3MT); 3MT is produced when dopamine is O-methylated by host catechol O-methyltransferase (COMT), thereby attenuating dopamine levels. This study aimed to identify whether gut bacteria are capable of reverting 3MT to dopamine.Methods and resultsHuman faecal bacterial communities O-demethylated 3MT and yielded dopamine. Gut bacteria that mediate this transformation were identified as acetogens Eubacterium limosum and Blautia producta. Upon exposing these acetogens to propyl iodide, a known inhibitor of cobalamin-dependent O-demethylases, 3MT O-demethylation was inhibited. Culturing E. limosum and B. producta with 3MT afforded increased acetate levels as compared with vehicle controls.ConclusionsGut bacterial acetogens E. limosum and B. producta synthesized dopamine from 3MT. This O-demethylation of 3MT was likely performed by cobalamin-dependent O-demethylases implicated in reductive acetogenesis.Significance and impact of the studyThis is the first report that gut bacteria can synthesize dopamine by O-demethylation of 3MT. Owing to 3MT being the product of host COMT attenuating dopamine levels, gut bacteria that reverse this transformation-converting 3MT to dopamine-may act as a counterbalance for dopamine regulation by COMT

Decoding Biomass-Sensing Regulons of <i>Clostridium thermocellum</i> Alternative Sigma-I Factors in a Heterologous <i>Bacillus subtilis</i> Host System

<div><p>The Gram-positive, anaerobic, cellulolytic, thermophile <i>Clostridium</i> (<i>Ruminiclostridium</i>) <i>thermocellum</i> secretes a multi-enzyme system called the cellulosome to solubilize plant cell wall polysaccharides. During the saccharolytic process, the enzymatic composition of the cellulosome is modulated according to the type of polysaccharide(s) present in the environment. <i>C</i>. <i>thermocellum</i> has a set of eight alternative RNA polymerase sigma (σ) factors that are activated in response to extracellular polysaccharides and share sequence similarity to the <i>Bacillus subtilis</i> σ^I factor. The aim of the present work was to demonstrate whether individual <i>C</i>. <i>thermocellum</i> σ^I-like factors regulate specific cellulosomal genes, focusing on <i>C</i>. <i>thermocellum</i> σ^I6 and σ^I3 factors. To search for putative σ^I6- and σ^I3-dependent promoters, bioinformatic analysis of the upstream regions of the cellulosomal genes was performed. Because of the limited genetic tools available for <i>C</i>. <i>thermocellum</i>, the functionality of the predicted σ^I6- and σ^I3-dependent promoters was studied in <i>B</i>. <i>subtilis</i> as a heterologous host. This system enabled observation of the activation of 10 predicted σ^I6-dependent promoters associated with the <i>C</i>. <i>thermocellum</i> genes: <i>sigI6</i> (itself, Clo1313_2778), <i>xyn11B</i> (Clo1313_0522), <i>xyn10D</i> (Clo1313_0177), <i>xyn10Z</i> (Clo1313_2635), <i>xyn10Y</i> (Clo1313_1305), <i>cel9V</i> (Clo1313_0349), <i>cseP</i> (Clo1313_2188), <i>sigI1</i> (Clo1313_2174), <i>cipA</i> (Clo1313_0627), and <i>rsgI5</i> (Clo1313_0985). Additionally, we observed the activation of 4 predicted σ^I3-dependent promoters associated with the <i>C</i>. <i>thermocellum</i> genes: <i>sigI3</i> (itself, Clo1313_1911), <i>pl11</i> (Clo1313_1983), <i>ce12</i> (Clo1313_0693) and <i>cipA</i>. Our results suggest possible regulons of σ^I6 and σ^I3 in <i>C</i>. <i>thermocellum</i>, as well as the σ^I6 and σ^I3 promoter consensus sequences. The proposed -35 and -10 promoter consensus elements of σ^I6 are CNNAAA and CGAA, respectively. Additionally, a less conserved CGA sequence next to the C in the -35 element and a highly conserved AT sequence three bases downstream of the -10 element were also identified as important nucleotides for promoter recognition. Regarding σ^I3, the proposed -35 and -10 promoter consensus elements are CCCYYAAA and CGWA, respectively. The present study provides new clues for understanding these recently discovered alternative σ^I factors.</p></div

Identification of conserved elements of σ^I-dependent promoter sequences.

<p>(A) WebLogo generated with the <i>Bacillales sigI</i> promoters shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146316#pone.0146316.s006" target="_blank">S4 Table</a>. (B) WebLogo generated with the <i>C</i>. <i>thermocellum</i> and <i>C</i>. <i>straminisolvens sigI</i> promoters shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146316#pone.0146316.t001" target="_blank">Table 1</a>, and the <i>C</i>. <i>clariflavum</i>, <i>A</i>. <i>cellulolyticus and Pseudobacteroides cellulosolvens sigI</i> promoters shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0146316#pone.0146316.s007" target="_blank">S5 Table</a>.</p

Evaluation of σ^I6 promoter sequence validity by mutagenesis.

<p>The activities are shown as relative activities, with the control promoter <i>xyn10Zshort</i> without mutations set to 100%. ND means not detected.</p

Identification of conserved elements of σ^I6-dependent promoter sequences.

<p>WebLogo generated with σ^I6-dependent promoter sequences of <i>C</i>. <i>thermocellum</i> and orthologous promoter sequences of <i>C</i>. <i>straminisolvens</i>.</p

Identification of conserved elements of σ^I3-dependent promoter sequences.

<p>WebLogo generated with σ^I3-dependent promoter sequences of <i>C</i>. <i>thermocellum</i> and orthologous promoter sequences of <i>C</i>. <i>straminisolvens</i>.</p