Microbial metabolites in the gut‐brain axis of an amyotrophic lateral sclerosis model

Abstract

Backgound Amyotrophic Lateral Sclerosis (ALS) is a neurodegenerative disorder characterized by progressive atrophy of motor neurons. Recent studies have demonstrated gut microbiome-associated contributions to human ALS and ALS experimental mice SOD1­G93A. Treatment with dietary butyrate, a natural bacterial product, rectified intestinal abnormalities and reduced disease progression in ALS mice. The emergence of ALS as a multifactorial disorder is further emphasized by understanding gut-brain mechanisms such as microbial metabolites. Methods Here, we hypothesize that butyrate treatment increases the concentration of anti-inflammatory and neuroprotective metabolites. SOD1­G93A (G93A) mice were used as a model of human ALS. G93A and WT mice classified as treated and untreated groups. Treated groups were given 2% dietary butyrate in their drinking water while untreated were given water ad libitum. Fecal samples were collected at 4, 8 and 14 weeks of age for treated groups while samples for untreated groups were obtained at 4, 7,13, and 17 weeks of age. Metabolites in fecalsamples were prepared using the automated MicroLab STAR® system (Hamilton Company) and measured by Metabolon. Raw data were extracted, peak-identified and QC processed using Metabolon's hardware and software. Statistical comparisons between metabolite concentrations in untreated and treated groups were conducted using two-way ANOVA via R. Results In untreated G93A mice, there was an increase in indole derived metabolites at 13 weeks of age, at onset of ALS symptoms. Specifically, concentration of indole acetate and indole acrylate, which are known to be aryl hydrocarbon receptor (AhR) ligands which indirectly aid in neuroprotection, increased at 13 weeks. Additionally, endocannabinoid concentrations, such as behenoyl ethanolamide (BEA) increased at 13 weeks while linoleoyl ethanolamide (LigEA) increased at 17 weeks. Our results suggest that the increased production of anti-inflammatory and neuroprotective metabolites onset and at 17 weeks could be a compensatory mechanism by the gut to alleviate ALS pathology. We also identify specific microbial metabolites associated with ALS as well as investigate the effect of butyrate on metabolites over the course of disease. Conclusion This study profiled fecal metabolites in G93A mice at different stages of ALS and with butyrate treatment. Four metabolites emerged as metabolic markers with diagnostic potential. Our findings contribute to the notion that the gut microbiome may play an essential role in the pathogenesis and pathophysiology of ALS and have significant translational implications for identifying new biomarkers for ALS. Further investigation is needed to explore whether related genes or enzymes could elucidate biological mechanisms of how ALS develops at the systemic level.