Slit/Robo-mediated chemorepulsion of vagal sensory axons in the fetal gut

Development and regeneration of the vagus nerve

The vagus nerve, with its myriad constituent axon branches and innervation targets, has long been a model of anatomical complexity in the nervous system. The branched architecture of the vagus nerve is now appreciated to be highly organized around the topographic and/or molecular identities of the neurons that innervate each target tissue. However, we are only just beginning to understand the developmental mechanisms by which heterogeneous vagus neuron identity is specified, patterned, and used to guide the axons of particular neurons to particular targets. Here, we summarize our current understanding of the complex topographic and molecular organization of the vagus nerve, the developmental basis of neuron specification and patterned axon guidance that supports this organization, and the regenerative mechanisms that promote, or inhibit, the restoration of vagus nerve organization after nerve damage. Finally, we highlight key unanswered questions in these areas and discuss potential strategies to address these questions.

Innervation in organogenesis

Proper innervation of peripheral organs helps to maintain physiological homeostasis and elicit responses to external stimuli. Disruptions to normal function can result in pathophysiological consequences. The establishment of connections and communication between the central nervous system and the peripheral organs is accomplished through the peripheral nervous system. Neuronal connections with target tissues arise from ganglia partitioned throughout the body. Organ innervation is initiated during development with stimuli being conducted through several types of neurons including sympathetic, parasympathetic, and sensory. While the physiological modulation of mature organs by these nerves is largely understood, their role in mammalian development is only beginning to be uncovered. Interactions with cells in target tissues can affect the development and eventual function of several organs, highlighting their significance. This chapter will cover the origin of peripheral neurons, factors mediating organ innervation, and the composition and function of organ-specific nerves during development. This emerging field aims to identify the functional contribution of innervation to development which will inform future investigations of normal and abnormal mammalian organogenesis, as well as contribute to regenerative and organ replacement efforts where nerve-derived signals may have significant implications for the advancement of such studies.

A chemorepellent inhibits local Ras activation to inhibit pseudopod formation to bias cell movement away from the chemorepellent

The ability of cells to sense chemical gradients is essential during development, morphogenesis, and immune responses. Although much is known about chemoattraction, chemorepulsion remains poorly understood. Proliferating Dictyostelium cells secrete a chemorepellent protein called AprA. AprA prevents pseudopod formation at the region of the cell closest to the source of AprA, causing the random movement of cells to be biased away from the AprA. Activation of Ras proteins in a localized sector of a cell cortex helps to induce pseudopod formation, and Ras proteins are needed for AprA chemorepulsion. Here we show that AprA locally inhibits Ras cortical activation through the G protein–coupled receptor GrlH, the G protein subunits Gβ and Gα8, Ras protein RasG, protein kinase B, the p21-activated kinase PakD, and the extracellular signal–regulated kinase Erk1. Diffusion calculations and experiments indicate that in a colony of cells, high extracellular concentrations of AprA in the center can globally inhibit Ras activation, while a gradient of AprA that naturally forms at the edge of the colony allows cells to activate Ras at sectors of the cell other than the sector of the cell closest to the center of the colony, effectively inducing both repulsion from the colony and cell differentiation. Together, these results suggest that a pathway that inhibits local Ras activation can mediate chemorepulsion.

Microbial influences on gut development and gut-brain communication

The developmental programs that build and sustain animal forms also encode the capacity to sense and adapt to the microbial world within which they evolved. This is abundantly apparent in the development of the digestive tract, which typically harbors the densest microbial communities of the body. Here, we review studies in human, mouse, zebrafish and Drosophila that are revealing how the microbiota impacts the development of the gut and its communication with the nervous system, highlighting important implications for human and animal health.

Gut innervation and enteric nervous system development: a spatial, temporal and molecular tour de force

During embryonic development, the gut is innervated by intrinsic (enteric) and extrinsic nerves. Focusing on mammalian ENS development, in this Review we highlight how important the different compartments of this innervation are to assure proper gut function. We specifically address the three-dimensional architecture of the innervation, paying special attention to the differences in development along the longitudinal and circumferential axes of the gut. We review recent information about the formation of both intrinsic innervation, which is fairly well-known, as well as the establishment of the extrinsic innervation, which, despite its importance in gut-brain signaling, has received much less attention. We further discuss how external microbial and nutritional cues or neuroimmune interactions may influence development of gut innervation. Finally, we provide summary tables, describing the location and function of several well-known molecules, along with some newer factors that have more recently been implicated in the development of gut innervation.

Development of the Autonomic Nervous System: Clinical Implications

Investigations of the cellular and molecular mechanisms that mediate the development of the autonomic nervous system have identified critical genes and signaling pathways that, when disrupted, cause disorders of the autonomic nervous system. This review summarizes our current understanding of how the autonomic nervous system emerges from the organized spatial and temporal patterning of precursor cell migration, proliferation, communication, and differentiation, and discusses potential clinical implications for developmental disorders of the autonomic nervous system, including familial dysautonomia, Hirschsprung disease, Rett syndrome, and congenital central hypoventilation syndrome.

Mapping of Extrinsic Innervation of the Gastrointestinal Tract in the Mouse Embryo

Precise extrinsic afferent (visceral sensory) and efferent (sympathetic and parasympathetic) innervation of the gut is fundamental for gut-brain cross talk. Owing to the limitation of intrinsic markers to distinctively visualize the three classes of extrinsic axons, which intimately associate within the gut mesentery, detailed information on the development of extrinsic gut-innervating axons remains relatively sparse. Here, we mapped extrinsic innervation of the gut and explored the relationships among various types of extrinsic axons during embryonic development in mice. Visualization with characterized intrinsic markers revealed that visceral sensory, sympathetic, and parasympathetic axons arise from different anatomic locations, project in close association via the gut mesentery, and form distinctive innervation patterns within the gut from embryonic day (E)10.5 to E16.5. Genetic ablation of visceral sensory trajectories results in the erratic extension of both sympathetic and parasympathetic axons, implicating that afferent axons provide an axonal scaffold to route efferent axons. Coculture assay further confirmed the attractive effect of sensory axons on sympathetic axons. Taken together, our study provides key information regarding the development of extrinsic gut-innervating axons occurring through heterotypic axonal interactions and provides an anatomic basis to uncover neural circuit assembly in the gut-brain axis (GBA).SIGNIFICANCE STATEMENT Understanding the development of extrinsic innervation of the gut is essential to unravel the bidirectional neural communication between the brain and the gut. Here, with characterized intrinsic markers targeting vagal sensory, spinal sensory, sympathetic, and parasympathetic axons, respectively, we comprehensively traced the spatiotemporal development of extrinsic axons to the gut during embryonic development in mice. Moreover, in line with the somatic nervous system, pretarget sorting via heterotypic axonal interactions is revealed to play critical roles in patterning extrinsic efferent trajectories to the gut. These findings provide basic anatomic information to explore the mechanisms underlying the process of assembling neural circuitry in the gut-brain axis (GBA).

You Are What You (First) Eat

As far back as we can remember, we eat. In fact, we eat before we can remember. Our first meal is amniotic fluid. We swallow it during the first trimester of gestation, and with that, we expose our gut to a universe of molecules. These early molecules have a profound influence on gut and brain function. For example, the taste of the amniotic fluid changes based on the mother's diet. Indeed, recent findings suggest that food preferences begin in utero. Likewise, a baby's first exposure to bacteria, previously thought to be during birth, appears to be in utero as well. And just as postnatal food and microbiota are implicated in brain function and dysfunction, prenatal nutrients and microbes may have a long-lasting impact on the development of the gut-brain neural circuits processing food, especially considering their plasticity during this vulnerable period. Here, we use current literature to put forward concepts needed to understand how the gut first meets the brain, and how this encounter may help us remember food.

Kif1bp loss in mice leads to defects in the peripheral and central nervous system and perinatal death

Goldberg-Shprintzen syndrome is a poorly understood condition characterized by learning difficulties, facial dysmorphism, microcephaly, and Hirschsprung disease. GOSHS is due to recessive mutations in KIAA1279, which encodes kinesin family member 1 binding protein (KIF1BP, also known as KBP). We examined the effects of inactivation of Kif1bp in mice. Mice lacking Kif1bp died shortly after birth, and exhibited smaller brains, olfactory bulbs and anterior commissures, and defects in the vagal and sympathetic innervation of the gut. Kif1bp was found to interact with Ret to regulate the development of the vagal innervation of the stomach. Although newborn Kif1bp^−/− mice had neurons along the entire bowel, the colonization of the gut by neural crest-derived cells was delayed. The data show an essential in vivo role for KIF1BP in axon extension from some neurons, and the reduced size of the olfactory bulb also suggests additional roles for KIF1BP. Our mouse model provides a valuable resource to understand GOSHS.

Development of the intrinsic and extrinsic innervation of the gut

The gastrointestinal (GI) tract is innervated by intrinsic enteric neurons and by extrinsic efferent and afferent nerves. The enteric (intrinsic) nervous system (ENS) in most regions of the gut consists of two main ganglionated layers; myenteric and submucosal ganglia, containing numerous types of enteric neurons and glial cells. Axons arising from the ENS and from extrinsic neurons innervate most layers of the gut wall and regulate many gut functions. The majority of ENS cells are derived from vagal neural crest cells (NCCs), which proliferate, colonize the entire gut, and first populate the myenteric region. After gut colonization by vagal NCCs, the extrinsic nerve fibers reach the GI tract, and Schwann cell precursors (SCPs) enter the gut along the extrinsic nerves. Furthermore, a subpopulation of cells in myenteric ganglia undergoes a radial (inward) migration to form the submucosal plexus, and the intrinsic and extrinsic innervation to the mucosal region develops. Here, we focus on recent progress in understanding the developmental processes that occur after the gut is colonized by vagal ENS precursors, and provide an up-to-date overview of molecular mechanisms regulating the development of the intrinsic and extrinsic innervation of the GI tract.

Are Molecules Involved in Neuritogenesis and Axon Guidance Related to Autism Pathogenesis?

Autism spectrum disorder is a heterogeneous disease, and numerous alterations of gene expression come into play to attempt to explain potential molecular and pathophysiological causes. Abnormalities of brain development and connectivity associated with alterations in cytoskeletal rearrangement, neuritogenesis and elongation of axons and dendrites might represent or contribute to the structural basis of autism pathology. Slit/Robo signaling regulates cytoskeletal remodeling related to axonal and dendritic branching. Components of its signaling pathway (ABL and Cdc42) are suspected to be molecular bases of alterations of normal development. The present review describes the most important mechanisms underlying neuritogenesis, axon pathfinding and the role of GTPases in neurite outgrowth, with special emphasis on alterations associated with autism spectrum disorders. On the basis of analysis of publicly available microarray data, potential biomarkers of autism are discussed.

Slit2 expression and its correlation with subcellular localization of β-catenin in gastric cancer

Gastric cancer is the fourth most common cancer worldwide. Several signaling pathways are involved in gastric cancer development and progression. Slit2 was recently found to be involved in cancer; however, its expression pattern in gastric cancer has not been discovered yet. In the present study, we investigated the expression of Slit2 in human gastric cancer and its correlation with the expression and subcellular localization of β-catenin. Immunohistochemistry (IHC) staining revealed that Slit2 was highly expressed in human gastric cancer tissues, while it was low or weakly expressed in normal gastric tissues. The differences in clinicopathological features between different groups were determined using Pearson's χ2 test. Slit2 levels were significantly associated with differentiation, Lauren's classification, lymph node metastasis and TNM staging. Slit2 levels were positively correlated with β-catenin level in gastric cancer tissues and cell lines. High levels of Slit2 were correlated with the membrane localization of β-catenin, and low levels of Slit2 were correlated with nuclear translocation of β-catenin in both gastric cancer tissues and cell lines assayed by IHC and immunofluorescence staining, respectively. Our data suggest that Slit2 was highly expressed in gastric cancer patients with less advanced clinicopathological features. Slit2 levels were correlated with β-catenin level and subcellular localization.