Gas1 is a receptor for sonic hedgehog to repel enteric axons

GAS1RR, an immune-related enhancer RNA, is related to biochemical recurrence-free survival in prostate cancer

Prostate cancer (PCa) is one of the malignant tumors of urinary system with a high morbidity. Enhancer RNA is a subclass of long non-coding RNA transcribed from active enhancer regions, which plays a critical role in gene transcriptional regulation. However, the role of enhancer RNA (eRNA) in PCa remains extremely mysterious. This study is aimed at exploring key prognostic eRNAs in PCa. First, we downloaded gene expression data and clinical data of 33 cancer types from UCSC Xena platform. Second, we selected reported putative eRNA-target pairs and performed the Kaplan-Meier survival and correlation analysis to determine the crucial eRNAs most related to biochemical recurrence (BCR)-free survival. Third, we explored the clinical characteristics with the key eRNA GAS1 adjacent regulatory RNA (GAS1RR) and performed a computational difference algorithm and the Cox regression analysis. Next, Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses were performed to explore the underlying mechanisms. Finally, we used the pan-cancer data from The Cancer Genome Atlas (TCGA) and performed reverse transcription-quantitative polymerase chain reaction (RT-qPCR) of 18 pairs of specimens to prove the results we acquired. Among all 2695 putative eRNAs, 6 pairs of eRNA-target genes were prominently related to BCR-free survival. Growth arrest-specific protein 1 (GAS1) was a target gene of GAS1RR (r = 0.86, P < 0.001). Patients with low GAS1RR expression were likely to have unfavorable clinical characteristics. The result of computational Cox regression analysis demonstrated that GAS1RR may predict the prognosis of PCa independently. RT-qPCR results illuminated that GAS1RR and GAS1 were both downregulated in PCa tissues, and they show a strong positive correlation. GO and KEGG analyses revealed biological processes that GAS1RR was mainly associated with. Immune infiltration analysis indicated that GAS1RR expression is correlated with the infiltration level of six kinds of immune cells. Our results suggest that GAS1RR may be clinically useful in the prediction of PCa prognosis. Moreover, it may also be a prognostic predictor and theoretic target with great promise in PCa.

Embryology and anatomy of Hirschsprung disease

Bowel has its own elegant nervous system - the enteric nervous system (ENS) which is a complex network of neurons and glial clones. Derived from neural crest cells (NCCs), this little brain controls muscle contraction, motility, and bowel activities in response to stimuli. Failure of developing enteric ganglia at the distal bowel results in intestinal obstruction and Hirschsprung disease (HSCR). This Review summarises the important embryological development of the ENS including proliferation, migration, and differentiation of NCCs. We address the signalling pathways which determine NCC cell fate and discuss how they are altered in the context of HSCR. Finally, we outline the anatomical defects and the mechanisms underlying gut motility in HSCR.

Hedgehog Signaling Pathway Orchestrates Human Lung Branching Morphogenesis

The Hedgehog (HH) signaling pathway plays an essential role in mouse lung development. We hypothesize that the HH pathway is necessary for branching during human lung development and is impaired in pulmonary hypoplasia. Single-cell, bulk RNA-sequencing data, and human fetal lung tissues were analyzed to determine the spatiotemporal localization of HH pathway actors. Distal human lung segments were cultured in an air-liquid interface and treated with an SHH inhibitor (5E1) to determine the effect of HH inhibition on human lung branching, epithelial-mesenchymal markers, and associated signaling pathways in vitro. Our results showed an early and regulated expression of HH pathway components during human lung development. Inhibiting HH signaling caused a reduction in branching during development and dysregulated epithelial (SOX2, SOX9) and mesenchymal (ACTA2) progenitor markers. FGF and Wnt pathways were also disrupted upon HH inhibition. Finally, we demonstrated that HH signaling elements were downregulated in lung tissues of patients with a congenital diaphragmatic hernia (CDH). In this study, we show for the first time that HH signaling inhibition alters important genes and proteins required for proper branching of the human developing lung. Understanding the role of the HH pathway on human lung development could lead to the identification of novel therapeutic targets for childhood pulmonary diseases.

TALPID3/KIAA0586 Regulates Multiple Aspects of Neuromuscular Patterning During Gastrointestinal Development in Animal Models and Human

TALPID3/KIAA0586 is an evolutionary conserved protein, which plays an essential role in protein trafficking. Its role during gastrointestinal (GI) and enteric nervous system (ENS) development has not been studied previously. Here, we analyzed chicken, mouse and human embryonic GI tissues with TALPID3 mutations. The GI tract of TALPID3 chicken embryos was shortened and malformed. Histologically, the gut smooth muscle was mispatterned and enteric neural crest cells were scattered throughout the gut wall. Analysis of the Hedgehog pathway and gut extracellular matrix provided causative reasons for these defects. Interestingly, chicken intra-species grafting experiments and a conditional knockout mouse model showed that ENS formation did not require TALPID3, but was dependent on correct environmental cues. Surprisingly, the lack of TALPID3 in enteric neural crest cells (ENCC) affected smooth muscle and epithelial development in a non-cell-autonomous manner. Analysis of human gut fetal tissues with a KIAA0586 mutation showed strikingly similar findings compared to the animal models demonstrating conservation of TALPID3 and its necessary role in human GI tract development and patterning.

Concomitant variants in NF1, LZTR1 and GNAZ genes probably contribute to the aggressiveness of plexiform neurofibroma and warrant treatment with MEK inhibitor

Neurofibromatosis 1 (NF1) is caused by germline mutations in the NF1 gene and manifests as proliferation of various tissues, including plexiform neurofibromas. The plexiform neurofibroma phenotype varies from indolent to locally aggressive, suggesting contributions of other modifiers in addition to somatic loss of NF1. In this study, we investigated a life-threatening plexiform neurofibroma in a 9-month-old female infant with NF1. Germline mutations in two RASopathy-associated genes were identified using whole-exome sequencing-a de novo pathogenic variant in the NF1 gene, and a known pathogenic variant in the LZTR1 gene. Somatic analysis of the plexiform neurofibroma revealed NF1 loss of heterozygosity and a variant in GNAZ, a gene encoding a G protein-coupled receptor. Cells expressing mutant GNAZ exhibited increased ERK 1/2 activation compared to those expressing wild-type GNAZ. Taken together, we suggest the variants in NF1, LZRT1 and GNAZ act synergistically in our patient, leading to MAPK pathway activation and contributing to the severity of the patient's plexiform neurofibromatosis. After treatment with the MEK inhibitor, trametinib, a prominent clinical improvement was observed in this patient. This case study contributes to the knowledge of germline and somatic non-NF1 variants affecting the NF1 clinical phenotype and supports use of personalized, targeted therapy.

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.

Hedgehog Signaling in Intestinal Development and Homeostasis

The hedgehog (Hh) signaling pathway plays several diverse regulatory and patterning roles during organogenesis of the intestine and in the regulation of adult intestinal homeostasis. In the embryo, fetus, and adult, intestinal Hh signaling is paracrine: Hh ligands are expressed in the endodermally derived epithelium, while signal transduction is confined to the mesenchymal compartment, where at least a dozen distinct cell types are capable of responding to Hh signals. Epithelial Hh ligands not only regulate a variety of mesenchymal cell behaviors, but they also direct these mesenchymal cells to secrete additional soluble factors (e.g., Wnts, Bmps, inflammatory mediators) that feed back to regulate the epithelial cells themselves. Evolutionary conservation of the core Hh signaling pathway, as well as conservation of epithelial/mesenchymal cross talk in the intestine, has meant that work in many diverse model systems has contributed to our current understanding of the role of this pathway in intestinal organogenesis, which is reviewed here.

Desert Hedgehog-Driven Endothelium Integrity Is Enhanced by Gas1 (Growth Arrest-Specific 1) but Negatively Regulated by Cdon (Cell Adhesion Molecule-Related/Downregulated by Oncogenes)

OBJECTIVE

Evidences accumulated within the past decades identified hedgehog signaling as a new regulator of endothelium integrity. More specifically, we recently identified Dhh (desert hedgehog) as a downstream effector of Klf2 (Kruppel-like factor 2) in endothelial cells (ECs). The purpose of this study is to investigate whether hedgehog coreceptors Gas1 (growth arrest-specific 1) and Cdon (cell adhesion molecule-related/downregulated by oncogenes) may be used as therapeutic targets to modulate Dhh signaling in ECs. Approach and Results: We demonstrated that both Gas1 and Cdon are expressed in adult ECs and relied on either siRNAs- or EC-specific conditional knockout mice to investigate their role. We found that Gas1 deficiency mainly phenocopies Dhh deficiency especially by inducing VCAM-1 (vascular cell adhesion molecule 1) and ICAM-1 (intercellular adhesion molecule 1) overexpression while Cdon deficiency has opposite effects by promoting endothelial junction integrity. At a molecular level, Cdon prevents Dhh binding to Ptch1 (patched-1) and thus acts as a decoy receptor for Dhh, while Gas1 promotes Dhh binding to Smo (smoothened) and as a result potentiates Dhh effects. Since Cdon is upregulated in ECs treated by inflammatory cytokines, including TNF (tumor necrosis factor)-α and Il (interleukin)-1β, we then tested whether Cdon inhibition would promote endothelium integrity in acute inflammatory conditions and found that both fibrinogen and IgG extravasation were decreased in association with an increased Cdh5 (cadherin-5) expression in the brain cortex of EC-specific Cdon knockout mice administered locally with Il-1β.

CONCLUSIONS

Altogether, these results demonstrate that Gas1 is a positive regulator of Dhh in ECs while Cdon is a negative regulator. Interestingly, Cdon blocking molecules may then be used to promote endothelium integrity, at least in inflammatory conditions.

PAI-1-Dependent Inactivation of SMAD4-Modulated Junction and Adhesion Complex in Obese Endometrial Cancer

While plasminogen activator inhibitor-1 (PAI-1) is known to potentiate cellular migration via proteolytic regulation, this adipokine is implicated as an oncogenic ligand in the tumor microenvironment. To understand the underlying paracrine mechanism, here, we conduct transcriptomic analysis of 1,898 endometrial epithelial cells (EECs) exposed and unexposed to PAI-1-secreting adipose stromal cells. The PAI-1-dependent action deregulates crosstalk among tumor-promoting and tumor-repressing pathways, including transforming growth factor β (TGF-β). When PAI-1 is tethered to lipoprotein receptor-related protein 1 (LRP1), the internalized signaling causes downregulation of SMAD4 at the transcriptional and post-translational levels that attenuates TGF-β-related transcription programs. Repression of genes encoding the junction and adhesion complex preferentially occurs in SMAD4-underexpressed EECs of persons with obesity. The findings highlight a role of PAI-1 signaling that renders ineffective intercellular communication for the development of adiposity-associated endometrial cancer.

Lin et al. demonstrate that PAI-1 secreted by adipose stromal cells interacts with LRP1 to repress TGF-β/SMAD4-regulated genes linked to cellular junction and adhesion complexes. This action disrupts cell-cell communication and facilitates the development of obesity-driven endometrial cancer.

Semaphorin 3A controls enteric neuron connectivity and is inversely associated with synapsin 1 expression in Hirschsprung disease

Most of the gut functions are controlled by the enteric nervous system (ENS), a complex network of enteric neurons located throughout the wall of the gastrointestinal tract. The formation of ENS connectivity during the perinatal period critically underlies the establishment of gastrointestinal motility, but the factors involved in this maturation process remain poorly characterized. Here, we examined the role of Semaphorin 3A (Sema3A) on ENS maturation and its potential implication in Hirschsprung disease (HSCR), a developmental disorder of the ENS with impaired colonic motility. We found that Sema3A and its receptor Neuropilin 1 (NRP1) are expressed in the rat gut during the early postnatal period. At the cellular level, NRP1 is expressed by enteric neurons, where it is particularly enriched at growth areas of developing axons. Treatment of primary ENS cultures and gut explants with Sema3A restricts axon elongation and synapse formation. Comparison of the ganglionic colon of HSCR patients to the colon of patients with anorectal malformation shows reduced expression of the synaptic molecule synapsin 1 in HSCR, which is inversely correlated with Sema3A expression. Our study identifies Sema3A as a critical regulator of ENS connectivity and provides a link between altered ENS connectivity and HSCR.

Ptch2/Gas1 and Ptch1/Boc differentially regulate Hedgehog signalling in murine primordial germ cell migration

Gas1 and Boc/Cdon act as co-receptors in the vertebrate Hedgehog signalling pathway, but the nature of their interaction with the primary Ptch1/2 receptors remains unclear. Here we demonstrate, using primordial germ cell migration in mouse as a developmental model, that specific hetero-complexes of Ptch2/Gas1 and Ptch1/Boc mediate the process of Smo de-repression with different kinetics, through distinct modes of Hedgehog ligand reception. Moreover, Ptch2-mediated Hedgehog signalling induces the phosphorylation of Creb and Src proteins in parallel to Gli induction, identifying a previously unknown Ptch2-specific signal pathway. We propose that although Ptch1 and Ptch2 functionally overlap in the sequestration of Smo, the spatiotemporal expression of Boc and Gas1 may determine the outcome of Hedgehog signalling through compartmentalisation and modulation of Smo-downstream signalling. Our study identifies the existence of a divergent Hedgehog signal pathway mediated by Ptch2 and provides a mechanism for differential interpretation of Hedgehog signalling in the germ cell niche.

How co-receptors Gas1 and Boc interact with Ptch1/2 receptors and regulate Hh signalling is unclear. Here, the authors demonstrate that the spatiotemporal expression of Gas1 and Boc determines how Hh signalling affects the dynamic migration of murine primordial germ cells.

Development of the intrinsic innervation of the small bowel mucosa and villi

Detection of nutritional and noxious food components in the gut is a crucial component of gastrointestinal function. Contents in the gut lumen interact with enteroendocrine cells dispersed throughout the gut epithelium. Enteroendocrine cells release many different hormones, neuropeptides, and neurotransmitters that communicate either directly or indirectly with the central nervous system and the enteric nervous system, a network of neurons and glia located within the gut wall. Several populations of enteric neurons extend processes that innervate the gastrointestinal lamina propria; however, how these processes develop and begin to transmit information from the mucosa is not fully understood. In this study, we found that Tuj1-immunoreactive neurites begin to project out of the myenteric plexus at embryonic day (E)13.5 in the mouse small intestine, even before the formation of villi. Using live calcium imaging, we discovered that neurites were capable of transmitting electrical information from stimulated villi to the plexus by E15.5. In unpeeled gut preparations where all layers were left intact, we also mimicked the basolateral release of 5-HT from enteroendocrine cells, which triggered responses in myenteric cell bodies at postnatal day (P)0. Altogether, our results show that enteric neurons extend neurites out of the myenteric plexus early during mouse enteric nervous system development, innervating the gastrointestinal mucosa, even before villus formation in mice of either sex. Neurites are already able to conduct electrical information at E15.5, and responses to 5-HT develop postnatally.NEW & NOTEWORTHY How enteric neurons project into the gut mucosa and begin to communicate with the epithelium during development is not known. Our study shows that enteric neurites project into the lamina propria as early as E13.5 in the mouse, before development of the submucous plexus and before formation of intestinal villi. These neurites are capable of transmitting electrical signals back to their cell bodies by E15.5 and respond to serotonin applied to neurite terminals by birth.

Airway epithelial cell differentiation relies on deficient Hedgehog signalling in COPD

BACKGROUND

Hedgehog (HH) pathway is constantly under scrutiny in the context of organ development. Lung morphogenesis requires HH signalling which participates thereafter to the pulmonary homeostasis by regulating epithelial cell quiescence and repair. Since epithelial remodelling is a hallmark of Chronic Obstructive Pulmonary Disease (COPD), we investigated whether the main molecular actors of HH pathway participate to airway epithelial cell differentiation and we analysed their alterations in COPD patients.

METHODS

Sonic HH (Shh) secretion was assessed by ELISA in airway epithelial cell (AEC) air-liquid interface culture supernatants. HH pathway activation was evaluated by RT-qPCR, western blot and immunostaining. Inhibition of HH signalling was achieved upon Shh chelation during epithelial cell differentiation. HH pathway core components localization was investigated in lung tissues from non-COPD and COPD patients.

FINDINGS

We demonstrate that progenitors of AEC produced Shh responsible for the activation of HH signalling during the process of differentiation. Preventing the ligand-induced HH activation led to the establishment of a remodelled epithelium with increased number of basal cells and reduced ciliogenesis. Gli2 activating transcription factor was demonstrated as a key-element in the regulation of AEC differentiation. More importantly, Gli2 and Smo were lost in AEC from COPD patients.

INTERPRETATION

Our data suggest that HH pathway is crucial for airway epithelial cell differentiation and highlight its role in COPD-associated epithelial remodelling.

Muscle stem cell renewal suppressed by Gas1 can be reversed by GDNF in mice

Muscle undergoes progressive weakening and regenerative dysfunction with age due in part to the functional decline of skeletal muscle stem cells (MuSCs). MuSCs are heterogeneous but whether their gene expression changes with age and the implication of such changes are unclear. Here we show that in mice, Growth arrest-specific gene 1 (Gas1) is expressed in a small subset of young MuSCs with its expression progressively increasing in larger fractions of MuSCs later in life. Over-expression of Gas1 in young MuSCs and inactivation of Gas1 in aged MuSCs support that Gas1 reduces the quiescence and self-renewal capacity of MuSCs. Gas1 reduces Ret signaling, which is required for MuSC quiescence and self-renewal. Indeed, we show that the Ret ligand, Glial Cell-Derived Neurotrophic Factor (GDNF), can counteract Gas1 by stimulating Ret signaling and enhancing MuSC self-renewal and regeneration, thus improving muscle function. We propose that strategies aimed to target this pathway can be exploited to improve the regenerative decline of muscle stem cells.

Pax3 cooperates with Ldb1 to direct local chromosome architecture during myogenic lineage specification

Chromatin looping allows enhancer-bound regulatory factors to influence transcription. Large domains, referred to as topologically associated domains, participate in genome organization. However, the mechanisms underlining interactions within these domains, which control gene expression, are not fully understood. Here we report that activation of embryonic myogenesis is associated with establishment of long-range chromatin interactions centered on Pax3-bound loci. Using mass spectrometry and genomic studies, we identify the ubiquitously expressed LIM-domain binding protein 1 (Ldb1) as the mediator of looping interactions at a subset of Pax3 binding sites. Ldb1 is recruited to Pax3-bound elements independently of CTCF-Cohesin, and is necessary for efficient deposition of H3K4me1 at these sites and chromatin looping. When Ldb1 is deleted in Pax3-expressing cells in vivo, specification of migratory myogenic progenitors is severely impaired. These results highlight Ldb1 requirement for Pax3 myogenic activity and demonstrate how transcription factors can promote formation of sub-topologically associated domain interactions involved in lineage specification.

Bipotent stem cells support the cyclical regeneration of endometrial epithelium of the murine uterus

The endometrial epithelium of the uterus regenerates periodically. The cellular source of newly regenerated endometrial epithelia during a mouse estrous cycle or a human menstrual cycle is presently unknown. Here, I have used single-cell lineage tracing in the whole mouse uterus to demonstrate that epithelial stem cells exist in the mouse uterus. These uterine epithelial stem cells provide a resident cellular supply that fuels endometrial epithelial regeneration. They are able to survive cyclical uterine tissue loss and persistently generate all endometrial epithelial lineages, including the functionally distinct luminal and glandular epithelia, to maintain uterine cycling. The uterine epithelial stem cell population also supports the regeneration of uterine endometrial epithelium post parturition. The 5-ethynyl-2'-deoxyuridine pulse-chase experiments further reveal that this stem cell population may reside in the intersection zone between luminal and glandular epithelial compartments. This tissue distribution allows these bipotent uterine epithelial stem cells to bidirectionally differentiate to maintain homeostasis and regeneration of mouse endometrial epithelium under physiological conditions. Thus, uterine function over the reproductive lifespan of a mouse relies on stem cell-maintained rhythmic endometrial regeneration.

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.

Non-endocannabinoid N-acylethanolamines and 2-monoacylglycerols in the intestine

This review focuses on recent findings of the physiological and pharmacological role of non-endocannabinoid N-acylethanolamines (NAEs) and 2-monoacylglycerols (2-MAGs) in the intestine and their involvement in the gut-brain signalling. Dietary fat suppresses food intake, and much research concerns the known gut peptides, for example, glucagon-like peptide-1 (GLP-1) and cholecystokinin (CCK). NAEs and 2-MAGs represent another class of local gut signals most probably involved in the regulation of food intake. We discuss the putative biosynthetic pathways and targets of NAEs in the intestine as well as their anorectic role and changes in intestinal levels depending on the dietary status. NAEs can activate the transcription factor PPARα, but studies to evaluate the role of endogenous NAEs are generally lacking. Finally, we review the role of diet-derived 2-MAGs in the secretion of anorectic gut peptides via activation of GPR119. Both PPARα and GPR119 have potential as pharmacological targets for the treatment of obesity and the former for treatment of intestinal inflammation. LINKED ARTICLES: This article is part of a themed section on 8^th European Workshop on Cannabinoid Research. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v176.10/issuetoc.

Genetic interactions between the hedgehog co-receptors Gas1 and Boc regulate cell proliferation during murine palatogenesis

Abnormal regulation of Sonic hedgehog (Shh) signaling has been described in a variety of human cancers and developmental anomalies, which highlights the essential role of this signaling molecule in cell cycle regulation and embryonic development. Gas1 and Boc are membrane co-receptors for Shh, which demonstrate overlapping domains of expression in the early face. This study aims to investigate potential interactions between these co-receptors during formation of the secondary palate. Mice with targeted mutation in Gas1 and Boc were used to generate Gas1; Boc compound mutants. The expression of key Hedgehog signaling family members was examined in detail during palatogenesis via radioactive in situ hybridization. Morphometric analysis involved computational quantification of BrdU-labeling and cell packing; whilst TUNEL staining was used to assay cell death. Ablation of Boc in a Gas1 mutant background leads to reduced Shh activity in the palatal shelves and an increase in the penetrance and severity of cleft palate, associated with failed elevation, increased proliferation and reduced cell death. Our findings suggest a dual requirement for Boc and Gas1 during early development of the palate, mediating cell cycle regulation during growth and subsequent fusion of the palatal shelves.

Contributions of Noncanonical Smoothened Signaling During Embryonic Development

The Sonic Hedgehog (Shh) signaling pathway is active during embryonic development in metazoans, and provides instructional cues necessary for proper tissue patterning. The pathway signal transducing component, Smoothened (Smo), is a G protein-coupled receptor (GPCR) that has been demonstrated to signal through at least two effector routes. The first is a G protein–independent canonical route that signals to Gli transcriptional effectors to establish transcriptional programs specifying cell fate during early embryonic development. The second, commonly referred to as the noncanonical Smo signal, induces rapid, transcription-independent responses that are essential for establishing and maintaining distinct cell behaviors during development. Herein, we discuss contributions of this noncanonical route during embryonic development. We also highlight important open questions regarding noncanonical Smo signal route selection during development, and consider implications of noncanonical signal corruption in disease.

Molecular interplays in hepatic stellate cells: apoptosis, senescence, and phenotype reversion as cellular connections that modulate liver fibrosis

Liver fibrosis is a pathophysiological process correlated with intense repair and cicatrization mechanisms in injured liver, and over the past few years, the characterization of the fine-tuning of molecular interconnections that support the development of liver fibrosis has been investigated. In this cellular process, the hepatic stellate cells (HSCs) support the organ fibrogenesis. The HSCs are found in two distinct morpho-physiological states: quiescent and activated. In normal liver, most HSCs are found in quiescent state, presenting a considerable amount of lipid droplets in the cytoplasm, while in injured liver, the activated phenotype of HSCs is a myofibroblast, that secrete extracellular matrix elements and contribute to the establishment of the fibrotic process. Studies on the molecular mechanisms by which HSCs try to restore their quiescent state have been performed; however, no effective treatment to reverse fibrosis has been so far prescribed. Therefore, the elucidation of the cellular and molecular mechanisms of apoptosis, senescence, and the cell reversion phenotype process from activate to quiescent state will certainly contribute to the development of effective therapies to treat hepatic fibrosis. In this context, this review aimed to address central elements of apoptosis, senescence, and reversal of HSC phenotype in the control of hepatic fibrogenesis, as a guide to future development of therapeutic strategies.

Sonic -'Jack-of-All-Trades' in Neural Circuit Formation

As reflected by the term morphogen, molecules such as Shh and Wnts were identified based on their role in early development when they instruct precursor cells to adopt a specific cell fate. Only much later were they implicated in neural circuit formation. Both in vitro and in vivo studies indicated that morphogens direct axons during their navigation through the developing nervous system. Today, the best understood role of Shh and Wnt in axon guidance is their effect on commissural axons in the spinal cord. Shh was shown to affect commissural axons both directly and indirectly via its effect on Wnt signaling. In fact, throughout neural circuit formation there is cross-talk and collaboration of Shh and Wnt signaling. Thus, although the focus of this review is on the role of Shh in neural circuit formation, a separation from Wnt signaling is not possible.

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.

The Many Hats of Sonic Hedgehog Signaling in Nervous System Development and Disease

Sonic hedgehog (Shh) signaling occurs concurrently with the many processes that constitute nervous system development. Although Shh is mostly known for its proliferative and morphogenic action through its effects on neural stem cells and progenitors, it also contributes to neuronal differentiation, axonal pathfinding and synapse formation and function. To participate in these diverse events, Shh signaling manifests differently depending on the maturational state of the responsive cell, on the other signaling pathways regulating neural cell function and the environmental cues that surround target cells. Shh signaling is particularly dynamic in the nervous system, ranging from canonical transcription-dependent, to non-canonical and localized to axonal growth cones. Here, we review the variety of Shh functions in the developing nervous system and their consequences for neurodevelopmental diseases and neural regeneration, with particular emphasis on the signaling mechanisms underlying Shh action.

Structural Perspectives on Axon Guidance

Axon guidance relies on a combinatorial code of receptor and ligand interactions that direct adhesive/attractive and repulsive cellular responses. Recent structural data have revealed many of the molecular mechanisms that govern these interactions and enabled the design of sophisticated mutant tools to dissect their biological functions. Here, we discuss the structure/function relationships of four major classes of guidance cues (ephrins, semaphorins, slits, netrins) and examples of morphogens (Wnt, Shh) and of cell adhesion molecules (FLRT). These cell signaling systems rely on specific modes of receptor-ligand binding that are determined by selective binding sites; however, defined structure-encoded receptor promiscuity also enables cross talk between different receptor/ligand families and can also involve extracellular matrix components. A picture emerges in which a multitude of highly context-dependent structural assemblies determines the finely tuned cellular behavior required for nervous system development.

Arl13b regulates Shh signaling from both inside and outside the cilium

The regulatory GTPase Arl13b localizes to primary cilia, where it regulates Sonic hedgehog (Shh) signaling. Missense mutations in ARL13B can cause the ciliopathy Joubert syndrome, while the mouse null allele is embryonic lethal. We used mouse embryonic fibroblasts as a system to determine the effects of Arl13b mutations on Shh signaling. We tested a total of seven different mutants, three JS-causing variants, two point mutants predicted to alter guanine nucleotide handling, one that disrupts cilia localization, and one that prevents palmitoylation and thus membrane binding, in assays of transcriptional and non-transcriptional Shh signaling. We found that mutations disrupting Arl13b's palmitoylation site, cilia localization signal, or GTPase handling altered the Shh response in distinct assays of transcriptional or non-transcriptional signaling. In contrast, JS-causing mutations in Arl13b did not affect Shh signaling in these same assays, suggesting these mutations result in more subtle defects, likely affecting only a subset of signaling outputs. Finally, we show that restricting Arl13b from cilia interferes with its ability to regulate Shh-stimulated chemotaxis, despite previous evidence that cilia themselves are not required for this non-transcriptional Shh response. This points to a more complex relationship between the ciliary and non-ciliary roles of this regulatory GTPase than previously envisioned.

Gas1 up-regulation is inducible and contributes to cell apoptosis in reactive astrocytes in the substantia nigra of LPS and MPTP models

Background

Reactive astrogliosis is a remarkable pathogenetic hallmark of the brains of Parkinson’s disease (PD) patients, but its progressive fate and regulation mechanisms are poorly understood. In this study, growth arrest specific 1 (Gas1), a tumor growth suppressor oncogene, was identified as a novel modulator of the cell apoptosis of reactive astrocytes in primary culture and the injured substantia nigra.

Methods

Animal models and cell cultures were utilized in the present study. Lipopolysaccharide (LPS)- and 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated animal models were used to detect Gas1 expression in the brain via immunohistochemistry and western blot. Cell cultures were performed to analyze Gas1 functions in the viability and apoptosis of reactive astrocytes and SH-SY5Y cells by double labeling, CCK-8, LDH, TUNEL, flow cytometry, and siRNA knockdown methods.

Results

Gas1 expressions were significantly elevated in the majority of the reactive astrocytes of the brains with LPS or MPTP insults. In the injured substantia nigras, GFAP-positive astrocytes exhibited higher levels of cleaved caspase-3. In cell culture, the up-regulated Gas1 expression induced apoptosis of reactive astrocytes that were insulted by LPS in combination with interferon-γ and tumor necrosis factor-a. This effect was confirmed through siRNA knockdown of Gas1 gene expression. Finally and interestingly, the potential underlying signaling pathways were evidently related to an increase in the Bax/Bcl-2 ratio, the abundant generation of reactive oxygen species and the activation of cleaved caspase-3.

Conclusions

This study demonstrated that the up-regulation of inducible Gas1 contributed to the apoptosis of reactive astrocytes in the injured nigra. Gas1 signaling may function as a novel regulator of astrogliosis and is thus a potential intervention target for inflammatory events in PD conditions.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0643-2) contains supplementary material, which is available to authorized users.

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.

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.

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.

Growth Arrest Specific-1 (GAS1) Is a C/EBP Target Gene That Functions in Ovulation and Corpus Luteum Formation in Mice

Ovulation and luteinization are initiated in preovulatory follicles by the luteinizing hormone (LH) surge; however, the signaling events that mediate LH actions in these follicles remain incompletely defined. Two key transcription factors that are targets of LH surge are C/EBPalpha and C/EBPbeta, and their depletion in granulosa cells results in complete infertility. Microarray analyses of these mutant mice revealed altered expression of a number of genes, including growth arrest specific-1 (Gas1). To investigate functions of Gas1 in ovulation- and luteinization-related processes, we crossed Cyp19a1-Cre and Gas1^flox/flox mice to conditionally delete Gas1 in granulosa and cumulus cells. While expression of Gas1 is dramatically increased in granulosa and cumulus cells around 12–16 h post-human chorionic gonadotropin (hCG) stimulation in wild-type mice, this increase is abolished in Cebpa/b double mutant and in Gas1 mutant mice. GAS1 is also dynamically expressed in stromal cells of the ovary independent of C/EBPalpha/beta. Female Gas1 mutant mice are fertile, exhibit enhanced rates of ovulation, increased fertility, and higher levels of Areg and Lhcgr mRNA in granulosa cells. The morphological appearance and vascularization of corpora lutea appeared normal in these mutant females. Interestingly, levels of mRNA for a number of genes (Cyp11a1, Star, Wnt4, Prlr, Cd52, and Sema3a) associated with luteinization are decreased in corpora lutea of Gas1 mutant mice as compared with controls at 24 h post-hCG; these differences were no longer detectable by 48 h post-hCG. The C/EBP target Gas1 is induced in granulosa cells and is associated with ovulation and luteinization.

Sonic hedgehog controls enteric nervous system development by patterning the extracellular matrix

The enteric nervous system (ENS) develops from neural crest cells that migrate along the intestine, differentiate into neurons and glia, and pattern into two plexuses within the gut wall. Inductive interactions between epithelium and mesenchyme regulate gut development, but the influence of these interactions on ENS development is unknown. Epithelial-mesenchymal recombinations were constructed using avian hindgut mesenchyme and non-intestinal epithelium from the bursa of Fabricius. These recombinations led to abnormally large and ectopically positioned ganglia. We hypothesized that sonic hedgehog (Shh), a secreted intestinal epithelial protein not expressed in the bursa, mediates this effect. Inhibition of Shh signaling, by addition of cyclopamine or a function-blocking antibody, resulted in large, ectopic ganglia adjacent to the epithelium. Shh overexpression, achieved in ovo using Shh-encoding retrovirus and in organ culture using recombinant protein, led to intestinal aganglionosis. Shh strongly induced the expression of versican and collagen type IX, whereas cyclopamine reduced expression of these chondroitin sulfate proteoglycans that are known to be inhibitory to neural crest cell migration. Shh also inhibited enteric neural crest-derived cell (ENCC) proliferation, promoted neuronal differentiation, and reduced expression of Gdnf, a key regulator of ENS formation. Ptc1 and Ptc2 were not expressed by ENCCs, and migration of isolated ENCCs was not inhibited by Shh protein. These results suggest that epithelial-derived Shh acts indirectly on the developing ENS by regulating the composition of the intestinal microenvironment.

Identification of GLI Mutations in Patients With Hirschsprung Disease That Disrupt Enteric Nervous System Development in Mice

BACKGROUND & AIMS

Hirschsprung disease is characterized by a deficit in enteric neurons, which are derived from neural crest cells (NCCs). Aberrant hedgehog signaling disrupts NCC differentiation and might cause Hirschsprung disease. We performed genetic analyses to determine whether hedgehog signaling is involved in pathogenesis.

METHODS

We performed deep-target sequencing of DNA from 20 patients with Hirschsprung disease (16 men, 4 women), and 20 individuals without (controls), and searched for mutation(s) in GLI1, GLI2, GLI3, SUFU, and SOX10. Biological effects of GLI mutations were tested in luciferase reporter assays using HeLa or neuroblastoma cell lines. Development of the enteric nervous system was studied in Sufu(f/f), Gli3(Δ699), Wnt1-Cre, and Sox10(NGFP) mice using immunohistochemical and whole-mount staining procedures to quantify enteric neurons and glia and analyze axon fasciculation, respectively. NCC migration was studied using time-lapse imaging.

RESULTS

We identified 3 mutations in GLI in 5 patients with Hirschsprung disease but no controls; all lead to increased transcription of SOX10 in cell lines. SUFU, GLI, and SOX10 form a regulatory loop that controls the neuronal vs glial lineages and migration of NCCs. Sufu mutants mice had high Gli activity, due to loss of Sufu, disrupting the regulatory loop and migration of enteric NCCs, leading to defective axonal fasciculation, delayed gut colonization, or intestinal hypoganglionosis. The ratio of enteric neurons to glia correlated inversely with Gli activity.

CONCLUSIONS

We identified mutations that increase GLI activity in patients with Hirschsprung disease. Disruption of the SUFU-GLI-SOX10 regulatory loop disrupts migration of NCCs and development of the enteric nervous system in mice.

Induction of autophagy and apoptosis by miR-148a through the sonic hedgehog signaling pathway in hepatic stellate cells

Autophagy is an evolutionarily conserved biological process that is activated in response to stress. Increasing evidence indicate that dysregulated miRNAs significantly contribute to autophagy and are thus implicated in various pathological conditions, including hepatic fibrosis. MiR-148a, a member of the miR-148/152 family, has been found to be downregulated in hepatic fibrosis and human hepatocellular carcinoma. However, the role of miR-148a in the development of hepatic fibrosis remains largely unknown. In this study, we describe the epigenetic regulation of miR-148a and its impact on autophagy in hepatic stellate cells (HSCs), exploring new targets of miR-148a. We found that miR-148a expression was significantly increased under starvation-induced conditions in LX-2 and T-6 cells. In addition, dual-luciferase reporter assays showed that miR-148a suppressed target gene expression by directly interacting with the 3'-untranslated regions (3'-UTRs) of growth arrest-specific gene 1 (Gas1) transcripts. Intriguingly, Gas1, which encodes a Hedgehog surface binding receptor and facilitates the Hedgehog (Hh) signaling pathway, inhibited autophagosome synthesis. Furthermore, we demonstrated a novel function for miR-148a as a potent inducer of autophagy in HSCs. Overexpressing of miR-148a increased autophagic activity, which inhibited proliferation and promoted apoptosis in HSCs. In conclusion, these data support a novel role for miR-148a as a key regulator of autophagy through the Hh signaling pathway, making miR-148a a potential candidate for the development of novel therapeutic strategies.

Crossing the embryonic midline: molecular mechanisms regulating axon responsiveness at an intermediate target

In bilaterally symmetric animals, the precise assembly of neural circuitry at the midline is essential for coordination of the left and right sides of the body. Commissural axons must first be directed across the midline and then be prevented from re-crossing in order to ensure proper midline connectivity. Here, we review the attractants and repellents that direct axonal navigation at the ventral midline and the receptors on commissural neurons through which they signal. In addition, we discuss the mechanisms that commissural axons use to switch their responsiveness to midline-derived cues, so that they are initially responsive to midline attractants and subsequently responsive to midline repellents.