Related Journal Article: The gut microbiota-brain axis in behaviour and brain disorders. Nat Rev Microbiol. 2020 Oct 22. doi: 10.1038/s41579-020-00460-0.
The impact of gut microbiota-brain axis in neurological disorders
All animals have developed a strict association with microbial communities during the evolution. This collection of microorganisms, termed the microbiota, is particularly abundant in the gastrointestinal tract and can regulate the function of our metabolism, immune system and even the development of the nervous system. Gut microbiota can influence the brain through the production and modification of various metabolites, immunological and chemical factors, and for this reason, its function is correlated to neuropsychiatric disorders associated with development (ASD), mood (e.g. anxiety and depression) and neurodegenerative diseases (e.g. Parkinson or Alzheimer), in which it was demonstrated that an alteration of gut microbiota could lead to more severe symptoms.
In this review, Morais and colleagues have explored the crucial connections between gut microbiota and the brain, discussing emerging evidences and possible contributions by the gut bacteria to neurological disorders.
The gut microbiota-brain axis represents a network of connections with a bidirectional communication between gut bacteria and the brain. This communication includes both direct and indirect signaling, such as the production of hormones, fatty acids or neurotransmitters which can regulate the host behaviour. Gut microbiota also interact with brain thanks to circulating cytokines and chemokines, which pass the Blood Brain Barrier (BBB) through direct transport. Mice lacking an endogenous microbiota (GF mice) showed an increased BBB permeability, and this is a typical feature of many neuropathological conditions.
Several pre-clinical models of neurological diseases have demonstrated an active role of gut bacteria in these disorders: patients with Autism Spectrum Disorder frequently showed intestinal inflammation and an altered gut permeability, and an alteration of gut microbiota composition lead GF mice to repetitive behaviours and social isolation.
In patients with neurodegenerative disorders such as Parkinson Disease (PD), Alzheimer Disease (AD) and Multiple Sclerosis, gastrointestinal issues like constipation, gut inflammation and increased intestinal permeability frequently occur before the symptoms onset; moreover, an altered gut microbiota could reduce the efficacy of anti-PD or anti-AD drugs. In GF mice, a microbiota transplant could ameliorate some of the symptoms of PD and AD.
Patients with depression disorder showed an altered gut microbiota composition in which some species are more present than others. This suggest that there may be conserved changes into the gut microbiota across individuals with different demographic backgrounds, and that some specific bacteria can exacerbate or alleviate depressive-like behaviours. Concerning the anxiety disorder, GF mice showed altered responses to fear and inability to overcome negative experiences. Recent studies in patients showed that the use of probiotics and prebiotics appear to ameliorate anxiety symptoms.
Even if the proportion of effects induced by gut microbiota is not clear yet because of complexity of human diseases and limitations of animal systems, these pre-clinical studies have been robust and reproducible, offering a modern view of brain diseases as whole-body conditions and bringing up the role of the gut microbiota, which can be targeted using new, safe and efficient therapeutic options for neuropsychiatric and neurodegenerative disorders in the coming years.