The enteric nervous system (ENS) is involved in regulating gastrointestinal tract homeostasis through crosstalk between the brain, the gut microbiota, the endocrine and the immune system. Although in mice the ENS is mostly developed during embryogenesis and early postnatal life, recent research has found that it undergoes a dynamic renewal during adulthood. However, mechanistic studies on gut microbiota’s role in the maturation of colonic enteric neurons are missing.
A new study, led by Prof. Fredrik Bäckhed from the Department of Molecular and Clinical Medicine at University of Gothenburg (Sweden), has found that the gut microbiota is involved in regulating maturation and plasticity of the adult enteric nervous system via serotonin signaling in mice.
Based on background findings regarding the role of gut commensal bacteria in regulating the production of serotonin (5-HT) in the gut and its neurogenerative and neuroprotective actions through activation of the 5-HT4 receptor, the researchers sought to determine whether the gut microbiota could drive maturation of the adult ENS in a 5-HT-dependent way.
Colonization of germ-free (GF) mice with the gut microbiota from conventionally raised mice led to a normalization of transit time after 15 days and increased the density of the glial network. However, depletion of the gut microbiota by antibiotics in conventionally raised donors reduced innervation of the colon mucosa and was associated with a decrease in the glial network, which suggests that the gut microbiota is crucial for the maintenance of the glial network. Besides this, colonization of GF mice induced maturation of neuronal precursors in the myenteric plexus of the colon, which was reported by a reduced proportion of neurons with the neuronal precursor marker Nestin.
By using GF mice and colonized mice lacking in tryptophan hydroxylase 1 (Tph1)—a rate-limiting enzyme for 5-HT synthesis—it was found that mucosal 5-HT was neuroprotective in the early phases of colonization. Whereas no significant changes were detected in the neuroanatomy of the ENS in conventionally raised mice and GF mice, GF Tph1-knockout mice colonized with the gut microbiota from a conventionally raised donor had a decreased number of myenteric neurons together with a reduced proportion of Nestin+ neurons. These findings suggest that mucosal 5-HT production is required to maintain the neuroanatomy of the ENS.
When studying whether the gut microbiota could affect neuronal 5-HT production, the researchers found that serotonergic neuronal networks were almost absent in GF mice but were gradually restored by colonization with a gut microbiota. Next, the researchers studied the role of mucosal and neuronal 5-HT in the maturation of neuronal precursors through a selective and irreversible inhibition of the rate-limiting synthetic enzymes TPH1 and TPH2 with parachlorophenylalanine (PCPA) or depletion of neuronal 5-HT with reserpine. Blocking 5-HT production and depleting endogenous 5-HT resulted in a reduced density of myenteric neurons similar to that observed in GF Tph1-knockout mice colonized with the gut microbiota from a conventionally raised donor together with an inhibition of neuronal differentiation from Nestin+ progenitors. As reserpine inhibits the vesicular monoamine transporter that is also involved in transporting dopamine and noradrenaline, the authors highlight in the paper that these neurotransmitters could also contribute to the maturation of the ENS after colonization.
Finally, using pharmacological modulation of the 5-HT4 receptor expressed in the intestine, a mechanistic link was established between the gut microbiota and neuronal activation of 5-HT4 receptors. While blocking 5-HT4R prevented maturation of the ENS, GF mice had restored innervation of the gut when treated with 5-HT4R agonists. These results show that maturation of the adult ENS also requires 5-HT4R-specific signaling.
Taken together, these findings show that in adult mice the gut microbiota is able to regulate the maturation and plasticity of the adult ENS by changing gut motility and modulating 5-HT signaling pathways at different levels, including 5-HT4R-specific signaling and both endogenous mucosal and neuronal 5-HT.
De Vadder F, Grasset E, Manneras Holm L, et al. Gut microbiota regulates maturation of the adult enteric nervous system via enteric serotonin networks. Proc Natl Acad Sci U S A. 2018; doi: 10.1073/pnas.1720017115.