Bacterially produced GABA protects neurons from degeneration

Caenorhabditis elegans and its cognate bacterial diet comprise a reliable, widespread model to study diet and microbiota effects on host physiology. Nonetheless, how diet influences the rate at which neurons die remains largely unknown. A number of models have been used in C. elegans as surrogates for neurodegeneration. One of these is a C. elegans strain expressing a neurotoxic allele of the MEC-4(d) DEG/ENaC channel which causes the progressive degeneration of the touch receptor neurons (TRNs). Using such model, this study evaluated the effect of various dietary bacteria on neurodegeneration dynamics. While degeneration of TRNs was steadily carried and completed at adulthood in the strain routinely used for C. elegans maintenance Escherichia coli OP50, it was significantly reduced in environmental and other laboratory bacterial strains. Strikingly, neuroprotection reached more than 40% in the E. coli HT115 strain. HT115 protection was long lasting well into old age of animals and not restricted to the TRNs. Small amounts of HT115 on OP50 bacteria as well as UV-killed HT115 were still sufficient to produce neuroprotection. Early growth of worms in HT115 protected neurons from degeneration during later growth in OP50. HT115 diet promoted the nuclear translocation of the DAF-16/FOXO transcription factor, a phenomenon previously reported to underlie neuroprotection caused by downregulation of the insulin receptor in this system. Moreover, a daf-16 loss of function mutation abolishes HT115-driven neuroprotection. Comparative genomics, transcriptomics and metabolomics approaches pinpointed the neurotransmitter γ-aminobutyric acid (GABA) as a metabolite differentially produced between E. coli HT115 and OP50. HT115 mutant lacking glutamate decarboxylase enzyme genes (gad), which catalyze the conversion of GABA from glutamate, lost the ability to produce GABA and also to stop neurodegeneration. Moreover, in situ GABA supplementation or heterologous expression of glutamate decarboxylase in E. coli OP50 conferred neuroprotective activity to this strain. Specific C. elegans GABA transporters and receptors were required for full HT115-mediated neuroprotection. Together, these results demonstrate that bacterially produced GABA exerts an effect of neuroprotection in the host, highlighting the role of neuroactive compounds of the diet in nervous system homeostasis.