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Neurotrophic factors produced by enteric glia in response to microbiota and alarmin cues regulate IL-22 production by group 3 innate lymphoid cells in the gut; disruption of this pathway leads to impaired clearance of Citrobacter rodentium and defects in epithelial integrity in a model of intestinal inflammation. A novel defence mechanism in the gut Henrique Veiga-Fernandes and colleagues show that neurotrophic factors produced by enteric glial cells in response to microbiota-derived cues contribute to the interleukin-22 production and regulation of group 3 innate lymphoid cells in the gut. Disruption of this pathway leads to impaired clearance of Citrobacter rodentium and defects in epithelial integrity in a model of intestinal inflammation. Group 3 innate lymphoid cells (ILC3) are major regulators of inflammation and infection at mucosal barriers 1 . ILC3 development is thought to be programmed 1 , but how ILC3 perceive, integrate and respond to local environmental signals remains unclear. Here we show that ILC3 in mice sense their environment and control gut defence as part of a glial–ILC3–epithelial cell unit orchestrated by neurotrophic factors. We found that enteric ILC3 express the neuroregulatory receptor RET. ILC3-autonomous Ret ablation led to decreased innate interleukin-22 (IL-22), impaired epithelial reactivity, dysbiosis and increased susceptibility to bowel inflammation and infection. Neurotrophic factors directly controlled innate Il22 downstream of the p38 MAPK/ERK-AKT cascade and STAT3 activation. Notably, ILC3 were adjacent to neurotrophic-factor-expressing glial cells that exhibited stellate-shaped projections into ILC3 aggregates. Glial cells sensed microenvironmental cues in a MYD88-dependent manner to control neurotrophic factors and innate IL-22. Accordingly, glial-intrinsic Myd88 deletion led to impaired production of ILC3-derived IL-22 and a pronounced propensity towards gut inflammation and infection. Our work sheds light on a novel multi-tissue defence unit, revealing that glial cells are central hubs of neuron and innate immune regulation by neurotrophic factor signals.

Haematopoietic stem cells are direct targets for neurotrophic factors, indicating that haematopoietic stem cells and neurons are regulated by similar signals. RET proto-oncogene aids stem-cell survival Henrique Veiga-Fernandes and colleagues have found that neuronal growth factors are important for survival, expansion and function of haematopoietic stem cells (HSCs). This is achieved though the neurotrophic factor receptor RET, which also provides the surviving cues Bcl2 and Bcl2l1 . Positive modulation of RET signalling drives mouse and human HSC expansion and transplantation, without compromising steady-state haematopoiesis. Haematopoiesis is a developmental cascade that generates all blood cell lineages in health and disease. This process relies on quiescent haematopoietic stem cells capable of differentiating, self renewing and expanding upon physiological demand 1 , 2 . However, the mechanisms that regulate haematopoietic stem cell homeostasis and function remain largely unknown. Here we show that the neurotrophic factor receptor RET (rearranged during transfection) drives haematopoietic stem cell survival, expansion and function. We find that haematopoietic stem cells express RET and that its neurotrophic factor partners are produced in the haematopoietic stem cell environment. Ablation of Ret leads to impaired survival and reduced numbers of haematopoietic stem cells with normal differentiation potential, but loss of cell-autonomous stress response and reconstitution potential. Strikingly, RET signals provide haematopoietic stem cells with critical Bcl2 and Bcl2l1 surviving cues, downstream of p38 mitogen-activated protein (MAP) kinase and cyclic-AMP-response element binding protein (CREB) activation. Accordingly, enforced expression of RET downstream targets, Bcl2 or Bcl2l1 , is sufficient to restore the activity of Ret null progenitors in vivo . Activation of RET results in improved haematopoietic stem cell survival, expansion and in vivo transplantation efficiency. Remarkably, human cord-blood progenitor expansion and transplantation is also improved by neurotrophic factors, opening the way for exploration of RET agonists in human haematopoietic stem cell transplantation. Our work shows that neurotrophic factors are novel components of the haematopoietic stem cell microenvironment, revealing that haematopoietic stem cells and neurons are regulated by similar signals.

Parkin and the glial cell line-derived neurotrophic factor (GDNF) receptor RET have both been independently linked to the dopaminergic neuron degeneration that underlies Parkinson's disease (PD). In the present study, we demonstrate that there is genetic crosstalk between parkin and the receptor tyrosine kinase RET in two different mouse models of PD. Mice lacking both parkin and RET exhibited accelerated dopaminergic cell and axonal loss compared with parkin-deficient animals, which showed none, and RET-deficient mice, in which we found moderate degeneration. Transgenic expression of parkin protected the dopaminergic systems of aged RET-deficient mice. Downregulation of either parkin or RET in neuronal cells impaired mitochondrial function and morphology. Parkin expression restored mitochondrial function in GDNF/RET-deficient cells, while GDNF stimulation rescued mitochondrial defects in parkin-deficient cells. In both cases, improved mitochondrial function was the result of activation of the prosurvival NF-κB pathway, which was mediated by RET through the phosphoinositide-3-kinase (PI3K) pathway. Taken together, these observations indicate that parkin and the RET signaling cascade converge to control mitochondrial integrity and thereby properly maintain substantia nigra pars compacta dopaminergic neurons and their innervation in the striatum. The demonstration of crosstalk between parkin and RET highlights the interplay in the protein network that is altered in PD and suggests potential therapeutic targets and strategies to treat PD.

Glial cell line-derived neurotrophic factor (GDNF) is a potent survival and regeneration-promoting factor for dopaminergic neurons in cell and animal models of Parkinson disease (PD). GDNF is currently tested in clinical trials on PD patients with so far inconclusive results. The receptor tyrosine kinase Ret is the canonical GDNF receptor, but several alternative GDNF receptors have been proposed, raising the question of which signaling receptor mediates here the beneficial GDNF effects. To address this question we overexpressed GDNF in the striatum of mice deficient for Ret in dopaminergic neurons and subsequently challenged these mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP). Strikingly, in this established PD mouse model, the absence of Ret completely abolished GDNF’s neuroprotective and regenerative effect on the midbrain dopaminergic system. This establishes Ret signaling as absolutely required for GDNF’s effects to prevent and compensate dopaminergic system degeneration and suggests Ret activation as the primary target of GDNF therapy in PD.

RET can be activated in cis or trans by its co-receptors and ligands in vitro, but the physiological roles of trans signaling are unclear. Rapidly adapting (RA) mechanoreceptors in dorsal root ganglia (DRGs) express Ret and the co-receptor Gfrα2 and depend on Ret for survival and central projection growth. Here, we show that Ret and Gfrα2 null mice display comparable early central projection deficits, but Gfrα2 null RA mechanoreceptors recover later. Loss of Gfrα1, the co-receptor implicated in activating RET in trans, causes no significant central projection or cell survival deficit, but Gfrα1;Gfrα2 double nulls phenocopy Ret nulls. Finally, we demonstrate that GFRα1 produced by neighboring DRG neurons activates RET in RA mechanoreceptors. Taken together, our results suggest that trans and cis RET signaling could function in the same developmental process and that the availability of both forms of activation likely enhances but not diversifies outcomes of RET signaling. During development, cells send and receive numerous signaling molecules. In order to trigger a biological response, such signaling molecules must first bind to a specific receptor protein, often located on the cell surface. These receptor proteins can either work alone or with partner proteins called co-receptors. When the co-receptor is produced by the same cell as the receptor, it is called cis signaling. When the co-receptor is produced by other cells, it is called trans signaling. RET is one such receptor that is important for the development of the nervous system and many other biological processes. It interacts with a particular family of signaling molecules, the glial cell line-derived neurotrophic factor (GDNF) family ligands, which first bind to a co-receptor, GFRα, before binding to RET. These co-receptors can come from the same cell as RET, or from a different cell. Previous studies have indicated that RET can receive both cis and trans signals using cultured cells, but it was not clear whether both types of signal occur during normal development and contribute to the same biological processes. Fleming, Vysochan et al. investigated this question by analyzing the roles of RET signaling in a type of mouse neuron that is involved in sensing touch. RET is important for the survival and development of these neurons, which express both RET and its co-receptor GFRa2. Another RET co-receptor, GFRa1, is produced by other cells that are next to the cell bodies and projections of these touch-sensing neurons. To investigate the roles of different GFRa co-receptors further, Fleming, Vysochan et al. generated a variety of mouse mutants, including mice with mutations in one or both types of co-receptor. The neurons in mice lacking both co-receptors shared the same defects as the neurons in the mice lacking RET. Loss of either co-receptor alone did not produce these abnormalities. This indicates that both co-receptors can mediate the normal development of these neurons, with GFRa2 signaling in cis and GFRa1 signaling in trans. Fleming, Vysochan et al. propose that cis and trans RET signaling can lead to the same biological outcomes in these neurons. Future experiments should reveal if cis and trans RET signaling contribute towards common biological processes in other cell types inside the body as well. Such findings might also be important for understanding the role of RET signaling in cancer and other human diseases.

Enteric neurons and blood vessels form intricate networks throughout the gastrointestinal tract. To support the hypothesis of a possible interaction of both networks, we investigated whether primary mesenteric vascular cells (MVCs) and enteric nervous system (ENS)-derived cells (ENSc) depend on each other using two- and three-dimensional in vitro assays. In a confrontation assay, both cell types migrated in a target-oriented manner towards each other. The migration of MVCs was significantly increased when cultured in ENSc-conditioned medium. Co-cultures of ENSc with MVCs resulted in an improved ENSc proliferation and differentiation. Moreover, we analysed the formation of the vascular and nervous system in developing mice guts. It was found that the patterning of newly formed microvessels and neural stem cells, as confirmed by nestin and SOX2 stainings, is highly correlated in all parts of the developing gut. In particular in the distal colon, nestin/SOX2-positive cells were found in the tissues adjacent to the capillaries and in the capillaries themselves. Finally, in order to provide evidences for a mutual interaction between endothelial and neural cells, the vascular patterns of a RET (−/−) knockout mouse model as well as human Hirschsprung’s cases were analysed. In the distal colon of postnatal RET (−/−) knockout mice, the vascular and neural networks were similarly disrupted. In aganglionic zones of Hirschsprung’s patients, the microvascular density was significantly increased compared with the ganglionic zone within the submucosa. Taken together, these findings indicate a strong interaction between the enteric nervous and vascular system.

The receptor tyrosine kinase RET is expressed in a number of neuroblastoma tissues and cell lines, but its role in neuroblastoma remains to be determined. In this study, we examined the roles of RET protein in neuroblastoma by the RNA interference technique using the NB‐39‐nu neuroblastoma cell line. NB‐39‐nu neuroblastoma cells show high expression and elevated tyrosine phosphorylation of RET, although short interfering RNA against RET (RET siRNA) did not significantly inhibit cell proliferation or suppression of basal levels of phosphorylation of extracellular regulated kinase (ERK)1/2 or protein kinase B (AKT). By the addition of glial cell line‐derived neurotrophic factor (GDNF), both the expression and phosphorylation of RET and the phosphorylation of ERK1/2 and AKT were further increased, whereas cell proliferation was not stimulated under normal culture conditions. However, proliferation of cells cultured under non‐adherent conditions was significantly increased by GDNF. The increased proliferation was suppressed by RET siRNA, which also caused inhibition of the phosphorylation of ERK1/2 and AKT. These results suggest that RET signaling plays an important role in GDNF‐induced enhancement of non‐adherent proliferation of NB‐39‐nu cells, which might contribute to the metastasis of neuroblastoma. (Cancer Sci 2009; 100: 1034–1039)

The establishment of synaptic connections requires precise alignment of pre- and postsynaptic terminals. The glial cell line-derived neurotrophic factor (GDNF) receptor GFRalpha1 is enriched at pre- and postsynaptic compartments in hippocampal neurons, suggesting that it has a function in synapse formation. GDNF triggered trans-homophilic binding between GFRalpha1 molecules and cell adhesion between GFRalpha1-expressing cells. This represents the first example of a cell-cell interaction being mediated by a ligand-induced cell adhesion molecule (LICAM). In the presence of GDNF, ectopic GFRalpha1 induced localized presynaptic differentiation in hippocampal neurons, as visualized by clustering of vesicular proteins and neurotransmitter transporters, and by activity-dependent vesicle recycling. Presynaptic differentiation induced by GDNF was markedly reduced in neurons lacking GFRalpha1. Gdnf mutant mice showed reduced synaptic localization of presynaptic proteins and a marked decrease in the density of presynaptic puncta, indicating a role for GDNF signaling in hippocampal synaptogenesis in vivo. We propose that GFRalpha1 functions as a LICAM to establish precise synaptic contacts and induce presynaptic differentiation.

Congenital anomalies of the kidney and urinary tract (CAKUT) constitute the most common underlying cause of chronic kidney disease in pediatric populations. Maternal hypovitaminosis D links to mesoderm-related birth defects, leading to our hypothesis that maternal vitamin D deficiency (VDD) impairs renal development (a mesoderm-derived process) and induces offspring CAKUT. To investigate whether a low-vitamin D level can cause CAKUT, we used vitamin D-free diets to induce a maternal vitamin D deficiency mice model. The maternal vitamin D deficiency (VDD) mice models and normal vitamin D status (CON) were successfully established by administering a vitamin D-free or vitamin D-sufficient diet for 4 weeks prior to pregnancy. The overall incidence of CAKUT was significantly increased in VDD neonatal mice (19.4% vs. 2.44%; p = 0.0006), with a higher incidence of early duplicated budding in E11.5. E11.5 ureteric bud tissue revealed significantly increased activity of Gdnf-Ret-p-Erk1/2 signaling in the VDD group. In vivo intervention with the p-Erk1/2 antagonist U0126 in the pregnant VDD mice model at E10.5 improved CAKUT occurrence in offspring with p-Erk1/2 expression decreasing toward normal levels. Early metanephric ureteric bud H3K4me3 CUT&TAG analysis at E12.5 revealed chromatin activation patterns, which revealed that the downregulation of Hnf1β promoter region peaks was accompanied by reduced Hnf1β expression, and Robo2 promoter region peak was upregulated with increased Robo2 expression in the VDD group. Maternal vitamin D deficiency in mice significantly increased offspring CAKUT incidence. This phenotype was mediated by enhanced Gdnf-Ret-p-Erk1/2 signaling and reversed by p-Erk1/2 inhibition, with VDD inducing epigenetic remodeling of Hnf1β and Robo2 promoters.