Emerging roles of gut microbiota and the immune system in the development of the enteric nervous system

Introduction The enteric nervous system (ENS) encompasses the intrinsic neuroglial networks of the gut, which are organized into two layers of interconnected ganglia, the outer myenteric and the inner submucosal plexus, which control virtually all aspects of gastrointestinal physiology (1, 2). Enteric neurons are classified into distinct subtypes according to morphological characteristics, intrinsic electrophysiological properties, or the combinatorial expression of neurotransmitters and neuropeptides such as 5-hydroxytryptamine (5-HT), calcitonin gene–related peptide (CGRP), or neuronal NOS (nNOS) (3–6). Unlike enteric neurons, which are located exclusively within enteric ganglia, enteric glial cells (EGCs) are also found within interganglionic tracts, the smooth muscle layers, and the lamina propria of the mucosa (7–9). Both lineages of the ENS are derived from neuroectodermal progenitors, which delaminate from the neural crest and colonize the gut during embryogenesis (10–14). Enteric neurogenesis commences as soon as neural crest cells invade the gut (in the mouse at around embryonic day 9.0–9.5), peaks at mid-gestation, and continues, albeit at a decreasing pace, for the remaining fetal period and early postnatal life, until it terminates after weaning (15–17). Nucleotide analog (such as BrdU) incorporation during embryogenesis in conjunction with marker analysis in adult mice has demonstrated that enteric neuron subtypes are generated during specific but overlapping periods of development. Thus, 5-HT+ enteric neurons are among the first to be born during embryogenesis, while CGRP+ neurons are generated mostly postnatally (18, 19). Unlike many parts of the CNS, the period of enteric gliogenesis overlaps extensively with neurogenesis: it commences at around midgestation (when neurogenesis is at its peak) and starts to decline postnatally (17). Low levels of gliogenesis are detected in the gut throughout life, but the significance of this observation is unclear (20). Human genetics and gene knockout studies in rodents have identified a number of transcription factors and signaling pathways that play key roles at various stages of ENS development (21). Among them are the nuclear factors SOX10 (an SRY-related HMG-box transcription factor) and FOXD3 (a member of the forkhead protein family), which are expressed in early neural crest cells (22–26). Together with the homeodomain transcription factor PHOX2B (27, 28), they are required for expression of the receptor tyrosine kinase RET, which along with the glial cell line– derived neurotrophic factor (GDNF) receptors α 1–3 (GFRα1-3) and their cognate ligands of the GDNF family of ligands (GFL), constitute a signaling pathway that is essential for ENS development (29–36). Another signaling cascade that is critical for enteric ganglia formation in the colon is the peptide endothelin-3 (ET-3) and its GPCR endothelin receptor type B (EDNRB) (37–39). Mutant forms of genes that encode critical regulators of ENS development in animal models have also been identified in familial cases of Hirschsprung disease (HSCR), a congenital neurodevelopmental abnormality that is characterized by failure of enteric ganglia formation in the distal colon and functional obstruction of the gut (40). Although most anatomical constituents of the adult ENS form during embryogenesis, newly born enteric neurons and glia continue to be integrated into pre-existing functional neural circuits for several weeks after birth (17, 18). Postnatal neurogenesis and gliogenesis coincide with and almost certainly contribute to the maturation of the intrinsic neural circuits of the gut and the acquisition of spontaneous and induced motility patterns that are characteristic of adult animals (41). These observations suggest that changes in gastrointestinal physiology and environment that are associated with nutrition or the establishment of luminal microflora and the maturation of the mucosal immune system are likely to affect the postnatal phase of ENS development, particularly the late-born groups of enteric neurons. Such factors may also affect the activity of neuroglial networks and the level of constiThe enteric nervous system (ENS) consists of neurons and glial cells that differentiate from neural crest progenitors. During embryogenesis, development of the ENS is controlled by the interplay of neural crest cell–intrinsic factors and instructive cues from the surrounding gut mesenchyme. However, postnatal ENS development occurs in a different context, which is characterized by the presence of microbiota and an extensive immune system, suggesting an important role of these factors on enteric neural circuit formation and function. Initial reports confirm this idea while further studies in this area promise new insights into ENS physiology and pathophysiology. Emerging roles of gut microbiota and the immune system in the development of the enteric nervous system