Neuronal Differentiation in Schwann Cell Lineage Underlies Postnatal Neurogenesis in the Enteric Nervous System. J Neurosci. 2015;35:9879-9888. [PMID: 26156989 DOI: 10.1523/JNEUROSCI.1239-15.2015]

Concise Review: Cellular and Molecular Mechanisms of Postnatal Injury-Induced Enteric Neurogenesis

Although still controversial, there is increasing agreement that postnatal neurogenesis occurs in the enteric nervous system (ENS) in response to injury. Following acute colitis, there is significant cell death of enteric neurons and evidence suggests that subsequent neural regeneration follows. An enteric neural stem/progenitor cell population with neurogenic potential has been identified in culture; in vivo, compensatory neurogenesis is driven by enteric glia and may also include de-differentiated Schwann cells. Recent evidence suggests that changes in the enteric microenvironment due to injury-associated increases in glial cell-derived neurotrophic factor (GDNF), serotonin (5-hydroxytryptamine [HT]), products from the gut microbiome, and possibly endocannabinoids may lead to the transdifferentiation of mature enteric glia and may reprogram recruited Schwann cells. Targeting neurogenic pathways presents a promising avenue toward the development of new and innovative treatments for acquired damage to the ENS. In this review, we discuss potential sources of newly generated adult enteric neurons, the involvement of GDNF, 5-HT, endocannabinoids, and lipopolysaccharide, as well as therapeutic applications of this evolving work. Stem Cells 2019;37:1136-1143.

Enteric Neurogenesis During Life Span Under Physiological and Pathophysiological Conditions

The enteric nervous system (ENS) controls gastrointestinal key functions and is mainly characterized by two ganglionated plexus located in the gut wall: the myenteric plexus and the submucous plexus. The ENS harbors a high number and diversity of enteric neurons and glial cells, which generate neuronal circuitry to regulate intestinal physiology. In the past few years, the pivotal role of enteric neurons in the underlying mechanism of several intestinal diseases was revealed. Intestinal diseases are associated with neuronal death that could in turn compromise intestinal functionality. Enteric neurogenesis and regeneration is therefore a crucial aspect within the ENS and could be revealed not only during embryogenesis and early postnatal periods, but also in the adulthood. Enteric glia and/or enteric neural precursor/progenitor cells differentiate into enteric neurons, both under homeostatic and pathologic conditions beyond the perinatal period. The unique role of the intestinal microbiota and serotonin signaling in postnatal and adult neurogenesis has been shown by several studies in health and disease. In this review article, we will mainly focus on different recent studies, which advanced the concept of postnatal and adult ENS neurogenesis. Moreover, we will discuss the key factors and underlying mechanisms, which promote enteric neurogenesis. Finally, we will shortly describe neurogenesis of transplanted enteric neural progenitor cells. Anat Rec, 302:1345-1353, 2019. © 2019 Wiley Periodicals, Inc.

Immunohistochemical and ultrastructural analysis of the maturing larval zebrafish enteric nervous system reveals the formation of a neuropil pattern

The gastrointestinal tract is constructed with an intrinsic series of interconnected ganglia that span its entire length, called the enteric nervous system (ENS). The ENS exerts critical local reflex control over many essential gut functions; including peristalsis, water balance, hormone secretions and intestinal barrier homeostasis. ENS ganglia exist as a collection of neurons and glia that are arranged in a series of plexuses throughout the gut: the myenteric plexus and submucosal plexus. While it is known that enteric ganglia are derived from a stem cell population called the neural crest, mechanisms that dictate final neuropil plexus organization remain obscure. Recently, the vertebrate animal, zebrafish, has emerged as a useful model to understand ENS development, however knowledge of its developing myenteric plexus architecture was unknown. Here, we examine myenteric plexus of the maturing zebrafish larval fish histologically over time and find that it consists of a series of tight axon layers and long glial cell processes that wrap the circumference of the gut tube to completely encapsulate it, along all levels of the gut. By late larval stages, complexity of the myenteric plexus increases such that a layer of axons is juxtaposed to concentric layers of glial cells. Ultrastructurally, glial cells contain glial filaments and make intimate contacts with one another in long, thread-like projections. Conserved indicators of vesicular axon profiles are readily abundant throughout the larval plexus neuropil. Together, these data extend our understanding of myenteric plexus architecture in maturing zebrafish, thereby enabling functional studies of its formation in the future.

Down syndrome mouse models have an abnormal enteric nervous system

Children with trisomy 21 (Down syndrome [DS]) have a 130-fold increased incidence of Hirschsprung Disease (HSCR), a developmental defect where the enteric nervous system (ENS) is missing from distal bowel (i.e., distal bowel is aganglionic). Treatment for HSCR is surgical resection of aganglionic bowel, but many children have bowel problems after surgery. Post-surgical problems like enterocolitis and soiling are especially common in children with DS. To determine how trisomy 21 affects ENS development, we evaluated the ENS in two DS mouse models, Ts65Dn and Tc1. These mice are trisomic for many chromosome 21 homologous genes, including Dscam and Dyrk1a, which are hypothesized to contribute to HSCR risk. Ts65Dn and Tc1 mice have normal ENS precursor migration at E12.5 and almost normal myenteric plexus structure as adults. However, Ts65Dn and Tc1 mice have markedly reduced submucosal plexus neuron density throughout the bowel. Surprisingly, the submucosal neuron defect in Ts65Dn mice is not due to excess Dscam or Dyrk1a, since normalizing copy number for these genes does not rescue the defect. These findings suggest the possibility that the high frequency of bowel problems in children with DS and HSCR may occur because of additional unrecognized problems with ENS structure.

Addressing Interdisciplinary Difficulties in Developmental Biology/Mathematical Collaborations: A Neural Crest Example

Mathematical modeling can allow insight into the biological processes that can be difficult to access by conventional biological means alone. Such projects are becoming increasingly attractive with the appearance of faster and more powerful quantitative techniques in both biological data acquisition and data storage, manipulation, and presentation. However, as is frequent in interdisciplinary research, the main hurdles are not within the mindset and techniques of each discipline but are usually encountered in attempting to meld the different disciplines together. Based upon our experience in applying mathematical methods to investigate how neural crest cells interact to form the enteric nervous system, we present our views on how to pursue biomathematical modeling projects, what difficulties to expect, and how to overcome, or at least survive, these hurdles. The main advice being: persevere.

Schwann Cell Precursors; Multipotent Glial Cells in Embryonic Nerves

The cells of the neural crest, often referred to as neural crest stem cells, give rise to a number of sub-lineages, one of which is Schwann cells, the glial cells of peripheral nerves. Crest cells transform to adult Schwann cells through the generation of two well defined intermediate stages, the Schwann cell precursors (SCP) in early embryonic nerves, and immature Schwann cells (iSch) in late embryonic and perinatal nerves. SCP are formed when neural crest cells enter nascent nerves and form intimate relationships with axons, a diagnostic feature of glial cells. This involves large-scale changes in gene expression, including the activation of established glial cell markers. Like early glia in the CNS, radial glia, SCP retain developmental multipotency and contribute to other crest-derived lineages during embryonic development. SCP, as well as closely related cells termed boundary cap cells, and later stages of the Schwann cell lineage have all been implicated as the tumor initiating cell in NF1 associated neurofibromas. iSch are formed from SCP in a process that involves the appearance of additional differentiation markers, autocrine survival circuits, cellular elongation, a formation of endoneurial connective tissue and basal lamina. Finally, in peri- and post-natal nerves, iSch are reversibly induced by axon-associated signals to form the myelin and non-myelin Schwann cells of adult nerves. This review article discusses early Schwann cell development in detail and describes a large number of molecular signaling systems that control glial development in embryonic nerves.

The carotid body: a physiologically relevant germinal niche in the adult peripheral nervous system

Oxygen constitutes a vital element for the survival of every single cell in multicellular aerobic organisms like mammals. A complex homeostatic oxygen-sensing system has evolved in these organisms, including detectors and effectors, to guarantee a proper supply of the element to every cell. The carotid body represents the most important peripheral arterial chemoreceptor organ in mammals and informs about hypoxemic situations to the effectors at the brainstem cardiorespiratory centers. To optimize organismal adaptation to maintained hypoxemic situations, the carotid body has evolved containing a niche of adult tissue-specific stem cells with the capacity to differentiate into both neuronal and vascular cell types in response to hypoxia. These neurogenic and angiogenic processes are finely regulated by the niche and by hypoxia itself. Our recent data on the cellular and molecular mechanisms underlying the functioning of this niche might help to comprehend a variety of different diseases coursing with carotid body failure, and might also improve our capacity to use these stem cells for the treatment of neurological disease. Herein, we review those data about the recent characterization of the carotid body niche, focusing on the study of the phenotype and behavior of multipotent stem cells within the organ, comparing them with other well-documented neural stem cells within the adult nervous system.

Schwann Cell Precursors Generate the Majority of Chromaffin Cells in Zuckerkandl Organ and Some Sympathetic Neurons in Paraganglia

In humans, neurosecretory chromaffin cells control a number of important bodily functions, including those related to stress response. Chromaffin cells appear as a distinct cell type at the beginning of midgestation and are the main cellular source of adrenalin and noradrenalin released into the blood stream. In mammals, two different chromaffin organs emerge at a close distance to each other, the adrenal gland and Zuckerkandl organ (ZO). These two structures are found in close proximity to the kidneys and dorsal aorta, in a region where paraganglioma, pheochromocytoma and neuroblastoma originate in the majority of clinical cases. Recent studies showed that the chromaffin cells comprising the adrenal medulla are largely derived from nerve-associated multipotent Schwann cell precursors (SCPs) arriving at the adrenal anlage with the preganglionic nerve fibers, whereas the migratory neural crest cells provide only minor contribution. However, the embryonic origin of the ZO, which differs from the adrenal medulla in a number of aspects, has not been studied in detail. The ZO is composed of chromaffin cells in direct contact with the dorsal aorta and the intraperitoneal cavity and disappears through an autophagy-mediated mechanism after birth. In contrast, the adrenal medulla remains throughout the entire life and furthermore, is covered by the adrenal cortex. Using a combination of lineage tracing strategies with nerve- and cell type-specific ablations, we reveal that the ZO is largely SCP-derived and forms in synchrony with progressively increasing innervation. Moreover, the ZO develops hand-in-hand with the adjacent sympathetic ganglia that coalesce around the dorsal aorta. Finally, we were able to provide evidence for a SCP-contribution to a small but significant proportion of sympathetic neurons of the posterior paraganglia. Thus, this cellular source complements the neural crest, which acts as a main source of sympathetic neurons. Our discovery of a nerve-dependent origin of chromaffin cells and some sympathoblasts may help to understand the origin of pheochromocytoma, paraganglioma and neuroblastoma, all of which are currently thought to be derived from the neural crest or committed sympathoadrenal precursors.

The enteric neural crest progressively loses capacity to form enteric nervous system

Cells of the vagal neural crest (NC) form most of the enteric nervous system (ENS) by a colonising wave in the embryonic gut, with high cell proliferation and differentiation. Enteric neuropathies have an ENS deficit and cell replacement has been suggested as therapy. This would be performed post-natally, which raises the question of whether the ENS cell population retains its initial ENS-forming potential with age. We tested this on the avian model in organ culture in vitro (3 days) using recipient aneural chick midgut/hindgut combined with ENS-donor quail midgut or hindgut of ages QE5 to QE10. ENS cells from young donor tissues (≤ QE6) avidly colonised the aneural recipient, but this capacity dropped rapidly 2-3 days after the transit of the ENS cell wavefront. This loss in capability was autonomous to the ENS population since a similar decline was observed in ENS cells isolated by HNK1 FACS. Using QE5, 6, 8 and 10 midgut donors and extending the time of assay to 8 days in chorio-allantoic membrane grafts did not produce 'catch up' colonisation. NC-derived cells were counted in dissociated quail embryo gut and in transverse sections of chick embryo gut using NC, neuron and glial marker antibodies. This showed that the decline in ENS-forming ability correlated with a decrease in proportion of ENS cells lacking both neuronal and glial differentiation markers, but there were still large numbers of such cells even at stages with low colonisation ability. Moreover, ENS cells in small numbers from young donors were far superior in colonisation ability to larger numbers of apparently undifferentiated cells from older donors. This suggests that the decline of ENS-forming ability has both quantitative and qualitative aspects. In this case, ENS cells for cell therapies should aim to replicate the embryonic ENS stage rather than using post-natal ENS stem/progenitor cells.

DRG-Derived Neural Progenitors Differentiate into Functional Enteric Neurons Following Transplantation in the Postnatal Colon

Cell therapy has great promise for treating gastrointestinal motility disorders caused by intestinal nervous system (ENS) diseases. However, appropriate sources, other than enteric neural stem cells and human embryonic stem cells, are seldom reported. Here, we show that neural progenitors derived from the dorsal root ganglion (DRG) of EGFP mice survived, differentiated into enteric neurons and glia cells, migrated widely from the site of injection, and established neuron-muscle connections following transplantation into the distal colon of postnatal mice. The exogenous EGFP+ neurons were physiologically functional as shown by the activity of calcium imaging. This study shows that that other tissues besides the postnatal bowel harbor neural crest stem cells or neural progenitors that have the potential to differentiate into functional enteric neurons in vivo and can potentially be used for intestinal nerve regeneration. These DRG-derived neural progenitor cells may be a choice for cell therapy of ENS disease as an allograft. The new knowledge provided by our study is important for the development of neural crest stem cell and cell therapy for the treatment of intestinal neuropathy.

Enteric nervous system development: what could possibly go wrong?

The gastrointestinal tract contains its own set of intrinsic neuroglial circuits - the enteric nervous system (ENS) - which detects and responds to diverse signals from the environment. Here, we address recent advances in the understanding of ENS development, including how neural-crest-derived progenitors migrate into and colonize the bowel, the formation of ganglionated plexuses and the molecular mechanisms of enteric neuronal and glial diversification. Modern lineage tracing and transcription-profiling technologies have produced observations that simultaneously challenge and affirm long-held beliefs about ENS development. We review many genetic and environmental factors that can alter ENS development and exert long-lasting effects on gastrointestinal function, and discuss how developmental defects in the ENS might account for some of the large burden of digestive disease.

Role of Schwann cells in cutaneous wound healing

Dermal wound healing is the process of repairing and remodeling skin following injury. Delayed or aberrant cutaneous healing poses a challenge for the health care system. The lack of detailed understanding of cellular and molecular mechanisms involved in this process hampers the development of effective targeted treatments. In a recent study, Parfejevs et al.-using state-of-the-art technologies, including in vivo sophisticated Cre/loxP techniques in combination with a mouse model of excisional cutaneous wounding-reveal that Schwann cells induce adult dermal wound healing. Strikingly, genetic ablation of Schwann cells delays wound contraction and closure, decreases myofibroblast formation, and impairs skin re-epithelization after injury. From a drug development perspective, Schwann cells are a new cellular candidate to be activated to accelerate skin healing. Here, we summarize and evaluate recent advances in the understanding of Schwann cells roles in the skin microenvironment.

Neural crest Notch/Rbpj signaling regulates olfactory gliogenesis and neuronal migration

The neural crest-derived ensheathing glial cells of the olfactory nerve (OECs) are unique in spanning both the peripheral and central nervous systems: they ensheathe bundles of axons projecting from olfactory receptor neurons in the nasal epithelium to their targets in the olfactory bulb. OECs are clinically relevant as a promising autologous cell transplantation therapy for promoting central nervous system repair. They are also important for fertility, being required for the migration of embryonic gonadotropin-releasing hormone (GnRH) neurons from the olfactory placode along terminal nerve axons to the medial forebrain, which they enter caudal to the olfactory bulbs. Like Schwann cell precursors, OEC precursors associated with the developing olfactory nerve express the glial marker myelin protein zero and the key peripheral glial transcription factor Sox10. The transition from Schwann cell precursors to immature Schwann cells is accelerated by canonical Notch signaling via the Rbpj transcription factor. Here, we aimed to test the role of Notch/Rbpj signaling in developing OECs by blocking the pathway in both chicken and mouse. Our results suggest that Notch/Rbpj signaling prevents the cranial neural crest cells that colonize the olfactory nerve from differentiating as neurons, and at later stages contributes to the guidance of GnRH neurons.

Cre-driver lines used for genetic fate mapping of neural crest cells in the mouse: An overview

The neural crest is one of the embryonic structures with the broadest developmental potential in vertebrates. Morphologically, neural crest cells emerge during neurulation in the dorsal folds of the neural tube before undergoing an epithelial‐to‐mesenchymal transition (EMT), delaminating from the neural tube, and migrating to multiple sites in the growing embryo. Neural crest cells generate cell types as diverse as peripheral neurons and glia, melanocytes, and so‐called mesectodermal derivatives that include craniofacial bone and cartilage and smooth muscle cells in cardiovascular structures. In mice, the fate of neural crest cells has been determined mainly by means of transgenesis and genome editing technologies. The most frequently used method relies on the Cre‐loxP system, in which expression of Cre‐recombinase in neural crest cells or their derivatives genetically enables the expression of a Cre‐reporter allele, thus permanently marking neural crest‐derived cells. Here, we provide an overview of the Cre‐driver lines used in the field and discuss to what extent these lines allow precise neural crest stage and lineage‐specific fate mapping.

Germ layers, the neural crest and emergent organization in development and evolution

Discovered in chick embryos by Wilhelm His in 1868 and named the neural crest by Arthur Milnes Marshall in 1879, the neural crest cells that arise from the neural folds have since been shown to differentiate into almost two dozen vertebrate cell types and to have played major roles in the evolution of such vertebrate features as bone, jaws, teeth, visceral (pharyngeal) arches, and sense organs. I discuss the discovery that ectodermal neural crest gave rise to mesenchyme and the controversy generated by that finding; the germ layer theory maintained that only mesoderm could give rise to mesenchyme. A second topic of discussion is germ layers (including the neural crest) as emergent levels of organization in animal development and evolution that facilitated major developmental and evolutionary change. The third topic is gene networks, gene co-option, and the evolution of gene-signaling pathways as key to developmental and evolutionary transitions associated with the origin and evolution of the neural crest and neural crest cells.

Schwann cell precursor: a neural crest cell in disguise?

Schwann cell precursors (SCPs) are multipotent embryonic progenitors covering all developing peripheral nerves. These nerves grow and navigate with unprecedented precision, delivering SCP progenitors to almost all locations in the embryonic body. Within specific developing tissues, SCPs detach from nerves and generate neuroendocrine cells, autonomic neurons, mature Schwann cells, melanocytes and other cell types. These properties of SCPs evoke resemblances between them and their parental population, namely, neural crest cells. Neural crest cells are incredibly multipotent migratory cells that revolutionized the course of evolution in the lineage of early chordate animals. Given this similarity and recent data, it is possible to hypothesize that proto-neural crest cells are similar to SCPs spreading along the nerves. Here, we review the multipotency of SCPs, the signals that govern them, their potential therapeutic value, SCP's embryonic origin and their evolutionary connections. We dedicate this article to the memory of Wilhelm His, the father of the microtome and "Zwischenstrang", currently known as the neural crest.

Multipotent peripheral glial cells generate neuroendocrine cells of the adrenal medulla

Adrenaline is a fundamental circulating hormone for bodily responses to internal and external stressors. Chromaffin cells of the adrenal medulla (AM) represent the main neuroendocrine adrenergic component and are believed to differentiate from neural crest cells. We demonstrate that large numbers of chromaffin cells arise from peripheral glial stem cells, termed Schwann cell precursors (SCPs). SCPs migrate along the visceral motor nerve to the vicinity of the forming adrenal gland, where they detach from the nerve and form postsynaptic neuroendocrine chromaffin cells. An intricate molecular logic drives two sequential phases of gene expression, one unique for a distinct transient cellular state and another for cell type specification. Subsequently, these programs down-regulate SCP-gene and up-regulate chromaffin cell-gene networks. The AM forms through limited cell expansion and requires the recruitment of numerous SCPs. Thus, peripheral nerves serve as a stem cell niche for neuroendocrine system development.

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.

Schwann cell precursors in health and disease

Schwann cell precursors (SCPs) are frequently regarded as neural crest-derived cells (NCDCs) found in contact with axons during nerve formation. Nevertheless, cells with SCPs properties can be found up to the adulthood. They are well characterized with regard to both gene expression profile and cellular behavior -for instance, proliferation, migratory capabilities and survival requirements-. They differ in origin regarding their anatomic location: even though most of them are derived from migratory NCCs, there is also contribution of the boundary cap neural crest cells (bNCCs) to the skin and other tissues. Many functions are known for SCPs in normal development, including nerve fasciculation and target innervation, arterial branching patterning and differentiation, and other morphogenetic processes. In addition, SCPs are now known to be a source of many neural (glia, endoneural fibroblasts, melanocytes, visceral neurons, and chromaffin cells) and non-neural-like (mesenchymal stromal cells, able e.g., to generate dentine-producing odontoblasts) cell types. Until now no reports of endoderm-like derivatives were reported so far. Interestingly, in the Schwann cell lineage only early SCPs are likely able to differentiate into melanocytes and bone marrow mesenchymal stromal cells. We have also herein discussed the literature regarding their role in repair as well as in disease mechanisms, such as in diverse cancers. Moreover, many caveats in our knowledge of SCPs biology are highlighted all through this article. Future research should expand more into the relevance of SCPs in pathologies and in other regenerative mechanisms which might bring new unexpected clinically-relevant knowledge.

Dual origin of enteric neurons in vagal Schwann cell precursors and the sympathetic neural crest

Most of the enteric nervous system derives from the "vagal" neural crest, lying at the level of somites 1-7, which invades the digestive tract rostro-caudally from the foregut to the hindgut. Little is known about the initial phase of this colonization, which brings enteric precursors into the foregut. Here we show that the "vagal crest" subsumes two populations of enteric precursors with contrasted origins, initial modes of migration, and destinations. Crest cells adjacent to somites 1 and 2 produce Schwann cell precursors that colonize the vagus nerve, which in turn guides them into the esophagus and stomach. Crest cells adjacent to somites 3-7 belong to the crest streams contributing to sympathetic chains: they migrate ventrally, seed the sympathetic chains, and colonize the entire digestive tract thence. Accordingly, enteric ganglia, like sympathetic ones, are atrophic when deprived of signaling through the tyrosine kinase receptor ErbB3, while half of the esophageal ganglia require, like parasympathetic ones, the nerve-associated form of the ErbB3 ligand, Neuregulin-1. These dependencies might bear relevance to Hirschsprung disease, with which alleles of Neuregulin-1 are associated.

Bidirectional brain-gut interactions and chronic pathological changes after traumatic brain injury in mice

OBJECTIVES

Traumatic brain injury (TBI) has complex effects on the gastrointestinal tract that are associated with TBI-related morbidity and mortality. We examined changes in mucosal barrier properties and enteric glial cell response in the gut after experimental TBI in mice, as well as effects of the enteric pathogen Citrobacter rodentium (Cr) on both gut and brain after injury.

METHODS

Moderate-level TBI was induced in C57BL/6mice by controlled cortical impact (CCI). Mucosal barrier function was assessed by transepithelial resistance, fluorescent-labelled dextran flux, and quantification of tight junction proteins. Enteric glial cell number and activation were measured by Sox10 expression and GFAP reactivity, respectively. Separate groups of mice were challenged with Cr infection during the chronic phase of TBI, and host immune response, barrier integrity, enteric glial cell reactivity, and progression of brain injury and inflammation were assessed.

RESULTS

Chronic CCI induced changes in colon morphology, including increased mucosal depth and smooth muscle thickening. At day 28 post-CCI, increased paracellular permeability and decreased claudin-1 mRNA and protein expression were observed in the absence of inflammation in the colon. Colonic glial cell GFAP and Sox10 expression were significantly increased 28days after brain injury. Clearance of Cr and upregulation of Th1/Th17 cytokines in the colon were unaffected by CCI; however, colonic paracellular flux and enteric glial cell GFAP expression were significantly increased. Importantly, Cr infection in chronically-injured mice worsened the brain lesion injury and increased astrocyte- and microglial-mediated inflammation.

CONCLUSION

These experimental studies demonstrate chronic and bidirectional brain-gut interactions after TBI, which may negatively impact late outcomes after brain injury.

From proliferation to target innervation: signaling molecules that direct sympathetic nervous system development

The sympathetic division of the autonomic nervous system includes a variety of cells including neurons, endocrine cells and glial cells. A recent study (Furlan et al. 2017) has revised thinking about the developmental origin of these cells. It now appears that sympathetic neurons and chromaffin cells of the adrenal medulla do not have an immediate common ancestor in the form a "sympathoadrenal cell", as has been long believed. Instead, chromaffin cells arise from Schwann cell precursors. This review integrates the new findings with the expanding body of knowledge on the signalling pathways and transcription factors that regulate the origin of cells of the sympathetic division of the autonomic nervous system.

Quo vadis: tracing the fate of neural crest cells

The neural crest is a transient structure in vertebrate embryos that produces migratory cells with an astonishing developmental potential. While neural crest fate maps have originally been established through interspecies transplantation assays, dye labeling, and retroviral infection, more recent methods rely on approaches involving transgenesis and genome editing. These technologies allowed the identification of minor neural crest-derived cell populations in tissues of non-neural crest origin. Furthermore, in vivo multipotency at the single cell level and stage-dependent fate acquisitions were demonstrated using genetic technologies. Finally, recent reports indicate that neural crest-derived cells become activated in response to injury to secrete factors supporting tissue repair. Thus, neural crest-derived cells apparently contribute to tissue formation and regeneration by cell autonomous and non-autonomous mechanisms.

Colitis promotes neuronal differentiation of Sox2+ and PLP1+ enteric cells

Mechanisms mediating adult enteric neurogenesis are largely unknown. Using inflammation-associated neurogenesis models and a transgenic approach, we aimed to understand the cell-source for new neurons in infectious and inflammatory colitis. Dextran sodium sulfate (DSS) and Citrobacter rodentium colitis (CC) was induced in adult mice and colonic neurons were quantified. Sox2GFP and PLP1GFP mice confirmed the cell-type specificity of these markers. Sox2CreER:YFP and PLP1creER:tdT mice were used to determine the fate of these cells after colitis. Sox2 expression was investigated in colonic neurons of human patients with Clostridium difficile or ulcerative colitis. Both DSS and CC led to increased colonic neurons. Following colitis in adult Sox2CreER:YFP mice, YFP initially expressed predominantly by glia becomes expressed by neurons following colitis, without observable DNA replication. Similarly in PLP1CreER:tdT mice, PLP1 cells that co-express S100b but not RET also give rise to neurons following colitis. In human colitis, Sox2-expressing neurons increase from 1–2% to an average 14% in colitis. The new neurons predominantly express calretinin, thus appear to be excitatory. These results suggest that colitis promotes rapid enteric neurogenesis in adult mice and humans through differentiation of Sox2- and PLP1-expressing cells, which represent enteric glia and/or neural progenitors. Further defining neurogenesis will improve understanding and treatment of injury-associated intestinal motility/sensory disorders.

Adult enteric nervous system in health is maintained by a dynamic balance between neuronal apoptosis and neurogenesis

According to current dogma, there is little or no ongoing neurogenesis in the fully developed adult enteric nervous system. This lack of neurogenesis leaves unanswered the question of how enteric neuronal populations are maintained in adult guts, given previous reports of ongoing neuronal death. Here, we confirm that despite ongoing neuronal cell loss because of apoptosis in the myenteric ganglia of the adult small intestine, total myenteric neuronal numbers remain constant. This observed neuronal homeostasis is maintained by new neurons formed in vivo from dividing precursor cells that are located within myenteric ganglia and express both Nestin and p75NTR, but not the pan-glial marker Sox10. Mutation of the phosphatase and tensin homolog gene in this pool of adult precursors leads to an increase in enteric neuronal number, resulting in ganglioneuromatosis, modeling the corresponding disorder in humans. Taken together, our results show significant turnover and neurogenesis of adult enteric neurons and provide a paradigm for understanding the enteric nervous system in health and disease.

Cell therapy for GI motility disorders: comparison of cell sources and proposed steps for treating Hirschsprung disease

Cell therapeutic approaches to treat a range of congenital and degenerative neuropathies are under intense investigation. There have been recent significant advancements in the development of cell therapy to treat disorders of the enteric nervous system (ENS), enteric neuropathies. These advances include the efficient generation of enteric neural progenitors from pluripotent stem cells and the rescue of a Hirschsprung disease model mouse following their transplantation into the bowel. Furthermore, a recent study provides evidence of functional innervation of the bowel muscle by neurons derived from transplanted ENS-derived neural progenitors. This mini-review discusses these recent findings, compares endogenous ENS-derived progenitors and pluripotent stem cell-derived progenitors as a cell source for therapy, and proposes the key steps for cell therapy to treat Hirschsprung disease.

Neural Crest Cells Contribute an Astrocyte-like Glial Population to the Spleen

Neural crest cells (NCC) are multi-potent cells of ectodermal origin that colonize diverse organs, including the gastrointestinal tract to form the enteric nervous system (ENS) and hematopoietic organs (bone marrow, thymus) where they participate in lymphocyte trafficking. Recent studies have implicated the spleen as an anatomic site for integration of inflammatory signals from the intestine with efferent neural inputs. We have previously observed alterations in splenic lymphocyte subsets in animals with defective migration of NCC that model Hirschsprung's disease, leading us to hypothesize that there may be a direct cellular contribution of NCC to the spleen. Here, we demonstrate that NCC colonize the spleen during embryogenesis and persist into adulthood. Splenic NCC display markers indicating a glial lineage and are arranged anatomically adjacent to blood vessels, pericytes and nerves, suggesting an astrocyte-like phenotype. Finally, we identify similar neural-crest derived cells in both the avian and non-human primate spleen, showing evolutionary conservation of these cells.

Ancient evolutionary origin of vertebrate enteric neurons from trunk-derived neural crest

The enteric nervous system (ENS) of jawed vertebrates arises primarily from vagal neural crest cells that migrate to the foregut and subsequently colonize and innervate the entire gastrointestinal tract. To gain insight into its evolutionary origin, we examined ENS development in the basal jawless vertebrate, the sea lamprey. Surprisingly, we found no evidence for the existence of a vagally-derived enteric neural crest population in the lamprey. Rather, DiI labeling showed that late-migrating cells, originating from the trunk neural tube and associated with nerve fibers, differentiated into neurons within the gut wall and typhlosole. We propose that these trunk-derived neural crest cells are homologous to Schwann cell precursors (SCPs), recently shown in mammalian embryos to populate post-embryonic parasympathetic ganglia^1,2, including enteric ganglia³. Our results suggest that neural crest-derived SCPs made an important contribution to the ancient ENS of early jawless vertebrates, a role that was largely subsumed by vagal neural crest cells in early gnathostomes.

Toward a better understanding of enteric gliogenesis

Most of gastrointestinal functions are controlled by the enteric nervous system (ENS), which contains a vast diversity of neurons and glial cells. In accordance with its key role, defective ENS formation is the cause of several diseases that affect quality of life and can even be life-threatening. Treatment of these diseases would greatly benefit from a better understanding of the molecular mechanisms underlying ENS formation. In this regard, although several important discoveries have been made over the years, how the full spectrum of enteric neuronal and glial cell subtypes is generated from neural crest cells during development still remains enigmatic. Because they also have stem cell properties, such knowledge would be especially important for the enteric glial cell lineage. In a recent study, we identified the NR2F1 transcription factor as a new key regulator of enteric gliogenesis. Here we discuss our recent findings and briefly review what is already known about the mechanisms and signaling pathways involved in enteric gliogenesis, with an emphasis on Hedgehog and Notch signaling.

Nerve-associated neural crest: peripheral glial cells generate multiple fates in the body

Recent studies demonstrated that neural crest-derived Schwann cell precursors (SCPs) dwelling in the nerves are multipotent and can be recruited in the local tissue to provide building blocks of neural crest-derived nature. The variety of fates produced by SCPs is widening with every year and currently includes melanocytes/melanophores, parasympathetic and enteric neurons, endoneural fibroblast, mesenchymal stem cells and, of course, mature Schwann cells of different subtypes. However, it is still unclear if SCPs are, in fact, nerve-dwelling population of the neural crest or they are rather a different, more specialized, cell type. This review outlines the field and focuses on the capacity of nerve-associated glial progenitors to contribute to the development and regeneration of numerous tissues in various groups of vertebrates.

Gastrointestinal hormones and the gut connectome

PURPOSE OF REVIEW

Provision of adequate nutrients by the gut is essential for survival and essential behaviors are linked to the proper ingestion and digestion of food. Recently, a new neural connection has been reported between sensory cells of the gut epithelium and the nervous system that mediates signals from the gut to the brain.

RECENT FINDINGS

This review describes how the gut senses its environment, relays those signals to the brain, and how the brain influences the gut.

SUMMARY

This gut-brain connection provides a pathway for how the body handles food.

Intestinal smooth muscle is required for patterning the enteric nervous system

The development of the enteric nervous system (ENS) and intestinal smooth muscle occurs in a spatially and temporally correlated manner, but how they influence each other is unknown. In the developing mid-gut of the chick embryo, we find that α-smooth muscle actin expression, indicating early muscle differentiation, occurs after the arrival of migrating enteric neural crest-derived cells (ENCCs). In contrast, hindgut smooth muscle develops prior to ENCC arrival. Smooth muscle development is normal in experimentally aganglionic hindguts, suggesting that proper development and patterning of the muscle layers does not rely on the ENS. However, inhibiting early smooth muscle development severely disrupts ENS patterning without affecting ENCC proliferation or apoptosis. Our results demonstrate that early intestinal smooth muscle differentiation is required for patterning the developing ENS.

Enteric nervous system development: A crest cell's journey from neural tube to colon

The enteric nervous system (ENS) is comprised of a network of neurons and glial cells that are responsible for coordinating many aspects of gastrointestinal (GI) function. These cells arise from the neural crest, migrate to the gut, and then continue their journey to colonize the entire length of the GI tract. Our understanding of the molecular and cellular events that regulate these processes has advanced significantly over the past several decades, in large part facilitated by the use of rodents, avians, and zebrafish as model systems to dissect the signals and pathways involved. These studies have highlighted the highly dynamic nature of ENS development and the importance of carefully balancing migration, proliferation, and differentiation of enteric neural crest-derived cells (ENCCs). Proliferation, in particular, is critically important as it drives cell density and speed of migration, both of which are important for ensuring complete colonization of the gut. However, proliferation must be tempered by differentiation among cells that have reached their final destination and are ready to send axonal extensions, connect to effector cells, and begin to produce neurotransmitters or other signals. Abnormalities in the normal processes guiding ENCC development can lead to failure of ENS formation, as occurs in Hirschsprung disease, in which the distal intestine remains aganglionic. This review summarizes our current understanding of the factors involved in early development of the ENS and discusses areas in need of further investigation.

Recent advances in regenerative medicine to treat enteric neuropathies: use of human cells

As current options for treating most enteric neuropathies are either non-effective or associated with significant ongoing problems, cell therapy is a potential attractive possibility to treat congenital and acquired neuropathies. Studies using animal models have shown that following transplantation of enteric neural progenitors into the bowel of recipients, the transplanted cells migrate, proliferate, and generate neurons that are electrically active and receive synaptic inputs. Recent studies have transplanted human enteric neural progenitors into the mouse colon and shown engraftment. In this article, we summarize the significance of these recent advances and discuss priorities for future research that might lead to the use of regenerative medicine to treat enteric neuropathies in the clinic.

Epigenomic Regulation of Schwann Cell Reprogramming in Peripheral Nerve Injury

The rapid and dynamic transcriptional changes of Schwann cells in response to injury are critical to peripheral nerve repair, yet the epigenomic reprograming that leads to the induction of injury-activated genes has not been characterized. Polycomb Repressive Complex 2 (PRC2) catalyzes the trimethylation of lysine 27 of histone H3 (H3K27me3), which produces a transcriptionally repressive chromatin environment. We find that many promoters and/or gene bodies of injury-activated genes of mature rat nerves are occupied with H3K27me3. In contrast, the majority of distal enhancers that gain H3K27 acetylation after injury are not repressed by H3K27 methylation before injury, which is normally observed in developmentally poised enhancers. Injury induces demethylation of H3K27 in many genes, such as Sonic hedgehog (Shh), which is silenced throughout Schwann cell development before injury. In addition, experiments using a Schwann cell-specific mouse knock-out of the Eed subunit of PRC2 indicate that demethylation is a rate-limiting step in the activation of such genes. We also show that some transcription start sites of H3K27me3-repressed injury genes of uninjured nerves are bound with a mark of active promoters H3K4me3, for example, Shh and Gdnf, and the reduction of H3K27me3 results in increased trimethylation of H3K4. Our findings identify reversal of polycomb repression as a key step in gene activation after injury.

SIGNIFICANCE STATEMENT

Peripheral nerve regeneration after injury is dependent upon implementation of a novel genetic program in Schwann cells that supports axonal survival and regeneration. Identifying means to enhance Schwann cell reprogramming after nerve injury could be used to foster effective remyelination in the treatment of demyelinating disorders and in identifying pathways involved in regenerative process of myelination. Although recent progress has identified transcriptional determinants of successful reprogramming of the Schwann cell transcriptome after nerve injury, our results have highlighted a novel epigenomic pathway in which reversal of the Polycomb pathway of repressive histone methylation is required for activation of a significant number of injury-induced genes.

Suppression of c-Kit signaling induces adult neurogenesis in the mouse intestine after myenteric plexus ablation with benzalkonium chloride

Adult neurogenesis rarely occurs in the enteric nervous system (ENS). In this study, we demonstrated that, after intestinal myenteric plexus (MP) ablation with benzalkonium chloride (BAC), adult neurogenesis in the ENS was significantly induced in c-kit loss-of-function mutant mice (W/W^v). Almost all neurons and fibers in the MP disappeared after BAC treatment. However, 1 week after ablation, substantial penetration of nerve fibers from the non-damaged area was observed in the MP, longitudinal muscle and subserosal layers in both wildtype and W/W^v mice. Two weeks after BAC treatment, in addition to the penetrating fibers, a substantial number of ectopic neurons appeared in the subserosal and longitudinal muscle layers of W/W^v mice, whereas only a few ectopic neurons appeared in wildtype mice. Such ectopic neurons expressed either excitatory or inhibitory intrinsic motor neuron markers and formed ganglion-like structures, including glial cells, synaptic vesicles and basal lamina. Furthermore, oral administration of imatinib, an inhibitor of c-Kit and an anticancer agent for gastrointestinal stromal tumors, markedly induced appearance of ectopic neurons after BAC treatment, even in wildtype mice. These results suggest that adult neurogenesis in the ENS is negatively regulated by c-Kit signaling in vivo.

Phenotype and distribution pattern of nestin-GFP-expressing cells in murine myenteric plexus

The enteric nervous system has to adapt to altering dietary or environmental conditions and presents an enormous plasticity that is conserved over the whole lifespan. It harbours neural-crest-derived neurons, glial cells and their precursors. Based on a nestin-green fluorescent protein (NGFP) transgenic model, a histological inventory has been performed to deliver an overview of neuronal and glial markers for the various parts of the gastrointestinal tract in newborn (postnatal day 7) and adult mice under homeostatic conditions. Whereas NGFP-positive glial cells can be found in all parts of the gut at any individual age, a specific NGFP population is present with both neuronal morphology and marker expression in the myenteric plexus (nNGFP). These cells appear in variable quantities, depending on age and location. Their overall abundance decreases from newborn to adults and their spatial distribution is also age-dependent. In newborn gut, nNGFP cells are found in similar quantities throughout the gut, with a significantly lower presence in the duodenum. Their expression increases in the adult mouse from the stomach to the colon. All of these nNGFP cells expressed either (but not both) of the glia markers S100 or glial fibrillary acidic protein (GFAP). In the S100-positive glia population, a subset of cells also shows a neuronal morphology (nS100), without expressing nestin. Thus, the presence of premature neurons that express NGFP demonstrates that neurogenesis takes place far beyond birth. In enteric neurons, NGFP acts as a marker for neuronal plasticity showing the differentiation and change in the phenotype of neuronal precursor cells.

Optimizing neurogenic potential of enteric neurospheres for treatment of neurointestinal diseases

BACKGROUND

Enteric neurospheres derived from postnatal intestine represent a promising avenue for cell replacement therapy to treat Hirschsprung disease and other neurointestinal diseases. We describe a simple method to improve the neuronal yield of spontaneously formed gut-derived neurospheres.

MATERIALS AND METHODS

Enteric neurospheres were formed from the small and large intestines of mouse and human subjects. Neurosphere size, neural crest cell content, cell migration, neuronal differentiation, and neuronal proliferation in culture were analyzed. The effect of supplemental neurotrophic factors, including glial cell line-derived neurotrophic factor (GDNF) and endothelin-3, was also assessed.

RESULTS

Mouse small intestine-derived neurospheres contained significantly more P75-expressing neural crest-derived cells (49.9 ± 15.3% versus 21.6 ± 11.9%, P < 0.05) and gave rise to significantly more Tuj1-expressing neurons than colon-derived neurospheres (69.9 ± 8.6% versus 46.2 ± 15.6%, P < 0.05). A similar pattern was seen in neurospheres isolated from human small and large intestine (32.6 ± 17.5% versus 10.2 ± 8.2% neural crest cells, P < 0.05; 29.7 ± 16.4% versus 16.0 ± 13.5% enteric neurons, P < 0.05). The addition of GDNF to the culture media further improved the neurogenic potential of small intestinal neurospheres (75.9 ± 4.0% versus 67.8 ± 5.8%, P < 0.05) whereas endothelin-3 had no effect.

CONCLUSIONS

Enteric neurospheres formed from small intestine and supplemented with GDNF yield an enriched population of neural crest-derived progenitor cells and give rise to a high density of enteric neurons.

Reprogramming of avian neural crest axial identity and cell fate

Neural crest populations along the embryonic body axis of vertebrates differ in developmental potential and fate, so that only the cranial neural crest can contribute to the craniofacial skeleton in vivo. We explored the regulatory program that imbues the cranial crest with its specialized features. Using axial-level specific enhancers to isolate and perform genome-wide profiling of the cranial versus trunk neural crest in chick embryos, we identified and characterized regulatory relationships between a set of cranial-specific transcription factors. Introducing components of this circuit into neural crest cells of the trunk alters their identity and endows these cells with the ability to give rise to chondroblasts in vivo. Our results demonstrate that gene regulatory circuits that support the formation of particular neural crest derivatives may be used to reprogram specific neural crest-derived cell types.

Mouse models of Hirschsprung disease and other developmental disorders of the enteric nervous system: Old and new players

Hirschsprung disease (HSCR, intestinal aganglionosis) is a multigenic disorder with variable penetrance and severity that has a general population incidence of 1/5000 live births. Studies using animal models have contributed to our understanding of the developmental origins of HSCR and the genetic complexity of this disease. This review summarizes recent progress in understanding control of enteric nervous system (ENS) development through analyses in mouse models. An overview of signaling pathways that have long been known to control the migration, proliferation and differentiation of enteric neural progenitors into and along the developing gut is provided as a framework for the latest information on factors that influence enteric ganglia formation and maintenance. Newly identified genes and additional factors beyond discrete genes that contribute to ENS pathology including regulatory sequences, miRNAs and environmental factors are also introduced. Finally, because HSCR has become a paradigm for complex oligogenic diseases with non-Mendelian inheritance, the importance of gene interactions, modifier genes, and initial studies on genetic background effects are outlined.

Simultaneous Detection of Both GDNF and GFRα1 Expression Patterns in the Mouse Central Nervous System

Glial cell line-derived neurotrophic factor (GDNF) is proposed as a therapeutic tool in Parkinson's disease, addiction-related disorders, and neurodegenerative conditions affecting motor neurons (MNs). Despite the high amount of work about GDNF therapeutic application, the neuronal circuits requiring GDNF trophic support in the brain and spinal cord (SC) are poorly characterized. Here, we defined GDNF and GDNF family receptor-α 1 (GFRα1) expression pattern in the brain and SC of newborn and adult mice. We performed systematic and simultaneous detection of EGFP and LacZ expressing alleles in reporter mice and asked whether modifications of this signaling pathway lead to a significant central nervous system (CNS) alteration. GFRα1 was predominantly expressed by neurons but also by an unexpected population of non-neuronal cells. GFRα1 expression pattern was wider in neonatal than in adult CNS and GDNF expression was restricted in comparison with GFRα1 at both developmental time points. The use of confocal microscopy to imaging X-gal deposits and EGFP allowed us to identify regions containing cells that expressed both proteins and to discriminate between auto and non-autotrophic signaling. We also suggested long-range GDNF-GFRα1 circuits taking advantage of the ability of the EGFP genetically encoded reporter to label long distance projecting axons. The complete elimination of either the ligand or the receptor during development did not produce major abnormalities, suggesting a preponderant role for GDNF signaling during adulthood. In the SC, our results pointed to local modulatory interneurons as the main target of GDNF produced by Clarke's column (CC) cells. Our work increases the understanding on how GDNF signals in the CNS and establish a crucial framework for posterior studies addressing either the biological role of GDNF or the optimization of trophic factor-based therapies.

Enteric nervous system assembly: Functional integration within the developing gut

Co-ordinated gastrointestinal function is the result of integrated communication between the enteric nervous system (ENS) and "effector" cells in the gastrointestinal tract. Unlike smooth muscle cells, interstitial cells, and the vast majority of cell types residing in the mucosa, enteric neurons and glia are not generated within the gut. Instead, they arise from neural crest cells that migrate into and colonise the developing gastrointestinal tract. Although they are "later" arrivals into the developing gut, enteric neural crest-derived cells (ENCCs) respond to many of the same secreted signalling molecules as the "resident" epithelial and mesenchymal cells, and several factors that control the development of smooth muscle cells, interstitial cells and epithelial cells also regulate ENCCs. Much progress has been made towards understanding the migration of ENCCs along the gastrointestinal tract and their differentiation into neurons and glia. However, our understanding of how enteric neurons begin to communicate with each other and extend their neurites out of the developing plexus layers to innervate the various cell types lining the concentric layers of the gastrointestinal tract is only beginning. It is critical for postpartum survival that the gastrointestinal tract and its enteric circuitry are sufficiently mature to cope with the influx of nutrients and their absorption that occurs shortly after birth. Subsequently, colonisation of the gut by immune cells and microbiota during postnatal development has an important impact that determines the ultimate outline of the intrinsic neural networks of the gut. In this review, we describe the integrated development of the ENS and its target cells.