Neuroglia and the microbiota-gut-brain axis

Glial cells are key players in the regulation of nervous system functioning in both the central and enteric nervous systems. Glial cells are dynamic and respond to environmental cues to modulate their activity. Increasing evidence suggests that these signals include those originating from the gut microbiota, the community of microorganisms, including bacteria, viruses, archaea, and protozoa, that inhabit the gut. The gut microbiota and the brain communicate in a bidirectional manner across multiple signaling pathways and interfaces that together comprise the microbiota-gut-brain axis. Here, we detail the role of glial cells, including astrocytes, microglia, and oligodendrocytes in the central nervous system, and glial cells in the enteric nervous system along this gut-brain axis. We review what is known regarding the modulation of glia by microbial signals, in particular by microbial metabolites which signal to the brain through systemic circulation and via the vagus nerve. In addition, we highlight what is yet to be discovered regarding the role of other gut microbiota signaling pathways in glial cell modulation and the challenges of research in this area.