Enteric nervous system development: migration, differentiation, and disease

Abstract

The enteric nervous system (ENS) provides the intrinsic innervation of the bowel and is the most neurochemically diverse branch of the peripheral nervous system, consisting of two layers of ganglia and fibers encircling the gastrointestinal tract. The ENS is vital for life and is capable of autonomous regulation of motility and secretion. Developmental studies in model organisms and genetic studies of the most common congenital disease of the ENS, Hirschsprung disease, have provided a detailed understanding of ENS development. The ENS originates in the neural crest, mostly from the vagal levels of the neuraxis, which invades, proliferates, and migrates within the intestinal wall until the entire bowel is colonized with enteric neural crest-derived cells (ENCDCs). After initial migration, the ENS develops further by responding to guidance factors and morphogens that pattern the bowel concentrically, differentiating into glia and neuronal subtypes and wiring together to form a functional nervous system. Molecules controlling this process, including glial cell line-derived neurotrophic factor and its receptor RET, endothelin (ET)-3 and its receptor endothelin receptor type B, and transcription factors such as SOX10 and PHOX2B, are required for ENS development in humans. Important areas of active investigation include mechanisms that guide ENCDC migration, the role and signals downstream of endothelin receptor type B, and control of differentiation, neurochemical coding, and axonal targeting. Recent work also focuses on disease treatment by exploring the natural role of ENS stem cells and investigating potential therapeutic uses. Disease prevention may also be possible by modifying the fetal microenvironment to reduce the penetrance of Hirschsprung disease-causing mutations.

Keywords: Hirschsprung disease; axonal targeting; cell migration; chain migration; development; enteric nervous system; gene-environment interactions; genetic interactions; neural crest; neural crest-derived stem cells; neurochemical coding; pseudoobstruction.

Fig. 1.

Initial colonization of the mouse gastrointestinal tract by enteric neural crest (NC)-derived cells (ENCDCs). A: during neural tube closure, NC cells (black) delaminate from the vagal region of the dorsal neural tube and migrate (arrows denote direction) in the ventral stream to the region adjacent to the foregut, which expresses glial cell line-derived neurotrophic factor (GDNF). B–E: after these pre-ENCDCs invade the foregut, they migrate rostrocaudally, proliferate, and differentiate first into neurons (green) and later into glia (purple: earliest glial marker brain fatty acid-binding protein). As this process proceeds, the bowel lengthens and changes shape, from a straight line (B) to a single bend with midgut and hindgut closely apposed (C); then the cecal appendage grows, and the entire bowel lengthens further (D and E). At embryonic days 11 and 12, ENCDCs invade the colon by crossing the mesentery and transiting the cecum (C). Cecal and transmesenteric populations then fuse to form the enteric nervous system (ENS) in the rostral colon (D), and the transmesenteric population populates the terminal colon as the smaller sacral ENCDC population enters the bowel and migrates caudorostrally (E). Regions of peak Gdnf (red) and endothelin 3 (Edn3) (blue) production are shown (A–E). Peaks of Gdnf expression partially, but imperfectly, mirror the extent of ENCDC migration, while peak Edn3 expression is centered at the cecum. A smaller domain of Gdnf expression in the antimesenteric side of the terminal colon may attract ENCDCs across the mesentery (C). Human ENS development proceeds through a similar process.

Fig. 2.

Primary and secondary migration of mouse ENCDCs. While the wave front of ENCDCs in the bowel moves steadily rostrocaudally, individual ENCDCs have complex and unpredictable behaviors. At and immediately behind the wave front (A, middle and right), ENCDCs migrate in chains and are often closely associated with the caudally projecting neurites of immature neurons, which extend up to the wave front. ENCDC connections are transient, and cells often swap neighbors within a chain or detach to switch chains or divide. Onset of neuronal lineage differentiation occurs very close to the wave front (A, left), and these cells retain some of their motility as they begin to extend neurites. In colonized regions in mice (B, cross-sectional illustration), a secondary centripetal migration of ENCDCs is triggered by trophic factors and the morphogens that control the patterning of the bowel wall. Netrin 1 and Netrin 3 are attractive to ENCDCs and are expressed in the epithelium, outer mesenchyme, and pancreatic buds, triggering the secondary migration of ENCDCs toward these structures. This broad attractive signal is probably refined by repulsive signals from sonic hedgehog (SHH) in the epithelium and later bone morphogenetic protein 4 (BMP4) expression in the inner mesenchyme, which SHH induces. A layer of BMP antagonist Noggin-expressing cells is located just inside the primary ENCDC migration layer, which may protect that region from the influence of BMP4. Precise timing of these signals in relation to each other and the secondary migration process have not been established. A similar secondary migration occurs in humans, but this process appears to proceed differently in birds.

Fig. 3.

Molecules and pathways implicated in ENS development. Roles of molecules and pathways discussed in this review are shown in the contexts of ENCDC migration (top), neuronal differentiation (bottom left), and glial differentiation (bottom right). Markers used to distinguish these developmental stages are listed outside the cells. Intracellular signaling molecules with important activating or inhibitory roles in RET signaling within ENCDCs are boxed (inactivating in red and activating in green). Transcription factors with known (color) or likely (gray) roles in ENS development are shown in nuclei. Important mechanisms that remain unresolved, including the mechanism and targets of endothelin-3 (ET-3)/endothelin receptor type B (EDNRB) signaling in ENCDCs, the conditions that specify each subtype of neuron, the factors other than GDNF that control axonal targeting and circuit formation, and the role of neurogenesis in adults, are highlighted with black question marks. RA, retinoic acid; PSA-NCAM, polysialic acid-neural cell adhesion molecule; ECE, endothelin-converting enzyme; PP1, protein phosphatase 1; PTEN, phosphatase and tensin homolog; ENCDC, enteric neural crest-derived cell.