Development of the vagal innervation of the gut: steering the wandering nerve

Gut-Brain Axis Modulation of Metabolic Disorders: Exploring the Intertwined Neurohumoral Pathways and Therapeutic Prospects

A significant rise in metabolic disorders, frequently brought on by lifestyle choices, is alarming. A wide range of preliminary studies indicates the significance of the gut-brain axis, which regulates bidirectional signaling between the gastrointestinal tract and the cognitive system, and is crucial for regulating host metabolism and cognition. Intimate connections between the brain and the gastrointestinal tract provide a network of neurohumoral transmission that can transmit in both directions. The gut-brain axis successfully establishes that the wellness of the brain is always correlated with the extent to which the gut operates. Research on the gut-brain axis has historically concentrated on how psychological health affects how well the gastrointestinal system works. The latest studies, however, revealed that the gut microbiota interacts with the brain via the gut-brain axis to control phenotypic changes in the brain and in behavior. This study addresses the significance of the gut microbiota, the role of the gut-brain axis in management of various metabolic disorders, the hormonal and neural signaling pathways and the therapeutic treatments available. Its objective is to establish the significance of the gut-brain axis in metabolic disorders accurately and examine the link between the two while evaluating the therapeutic strategies to be incorporated in the future.

Major depressive disorder and chronic gastritis: A bidirectional two-sample Mendelian randomization study

OBJECTIVE

Observational studies have posited a strong correlation between chronic gastritis (CG) and major depressive disorder (MDD), but the nature of this association remains uncertain, owing to the challenges of establishing the temporal sequence. The present study sought to elucidate the elusive relationship between CG and MDD by employing a bidirectional two-sample Mendelian randomization (MR) approach.

METHODS

We extracted instrumental variants for MDD and CG from published genome-wide association study data, focusing on individuals of primarily European descent. A comprehensive suite of MR estimations and sensitivity analyses was performed to ensure the robustness of the findings. Each outcome database was analyzed separately in both directions.

RESULTS

For MDD and CG, 221 and 5 genetic variants, respectively, were selectively extracted as instrumental variants. The results suggest that MDD is causally associated with an elevated risk of CG (IVW: 23andMe, OR = 1.33; 95% CI = 1.15-1.54; p = 1.06 × 10-4); conversely, no strong evidence was found to corroborate that CG exerts a causal effect on the incidence of MDD (IVW: OR = 1.01; 95% CI = 0.95-1.07; p = 0.68).

CONCLUSIONS

These findings provide novel insights into the causal relationship between CG and MDD, which may have implications for clinical decision-making in patients with MDD and CG.

Ontogeny and Trophic Factor Sensitivity of Gastrointestinal Projecting Vagal Sensory Cell Types

Vagal sensory neurons (VSNs) located in the nodose ganglion provide information, such as stomach stretch or the presence of ingested nutrients, to the caudal medulla via specialized cell types expressing unique marker genes. Here, we leverage VSN marker genes identified in adult mice to determine when specialized vagal subtypes arise developmentally and the trophic factors that shape their growth. Experiments to screen for trophic factor sensitivity revealed that brain-derived neurotrophic factor (BDNF) and glial cell-derived neurotrophic factor (GDNF) robustly stimulate neurite outgrowth from VSNs in vitro Perinatally, BDNF was expressed by neurons of the nodose ganglion itself, while GDNF was expressed by intestinal smooth muscle cells. Thus, BDNF may support VSNs locally, whereas GDNF may act as a target-derived trophic factor supporting the growth of processes at distal innervation sites in the gut. Consistent with this, expression of the GDNF receptor was enriched in VSN cell types that project to the gastrointestinal tract. Last, the mapping of genetic markers in the nodose ganglion demonstrates that defined vagal cell types begin to emerge as early as embryonic day 13, even as VSNs continue to grow to reach gastrointestinal targets. Despite the early onset of expression for some marker genes, the expression patterns of many cell type markers appear immature in prenatal life and mature considerably by the end of the first postnatal week. Together, the data support location-specific roles for BDNF and GDNF in stimulating VSN growth, and a prolonged perinatal timeline for VSN maturation in male and female mice.

Linking α-synuclein-induced synaptopathy and neural network dysfunction in early Parkinson’s disease

The prodromal phase of Parkinson’s disease is characterized by aggregation of the misfolded pathogenic protein α-synuclein in select neural centres, co-occurring with non-motor symptoms including sensory and cognitive loss, and emotional disturbances. It is unclear whether neuronal loss is significant during the prodrome. Underlying these symptoms are synaptic impairments and aberrant neural network activity. However, the relationships between synaptic defects and network-level perturbations are not established. In experimental models, pathological α-synuclein not only impacts neurotransmission at the synaptic level, but also leads to changes in brain network-level oscillatory dynamics—both of which likely contribute to non-motor deficits observed in Parkinson’s disease. Here we draw upon research from both human subjects and experimental models to propose a ‘synapse to network prodrome cascade’ wherein before overt cell death, pathological α-synuclein induces synaptic loss and contributes to aberrant network activity, which then gives rise to prodromal symptomology. As the disease progresses, abnormal patterns of neural activity ultimately lead to neuronal loss and clinical progression of disease. Finally, we outline goals and research needed to unravel the basis of functional impairments in Parkinson’s disease and other α-synucleinopathies.

Microbial influences on gut development and gut-brain communication

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

Development of the infant gut microbiome predicts temperament across the first year of life

Perturbations to the gut microbiome are implicated in altered neurodevelopmental trajectories that may shape life span risk for emotion dysregulation and affective disorders. However, the sensitive periods during which the microbiome may influence neurodevelopment remain understudied. We investigated relationships between gut microbiome composition across infancy and temperament at 12 months of age. In 67 infants, we examined if gut microbiome composition assessed at 1-3 weeks, 2, 6, and 12 months of age was associated with temperament at age 12 months. Stool samples were sequenced using the 16S Illumina MiSeq platform. Temperament was assessed using the Infant Behavior Questionnaire-Revised (IBQ-R). Beta diversity at age 1-3 weeks was associated with surgency/extraversion at age 12 months. Bifidobacterium and Lachnospiraceae abundance at 1-3 weeks of age was positively associated with surgency/extraversion at age 12 months. Klebsiella abundance at 1-3 weeks was negatively associated with surgency/extraversion at 12 months. Concurrent composition was associated with negative affectivity at 12 months, including a positive association with Ruminococcus-1 and a negative association with Lactobacillus. Our findings support a relationship between gut microbiome composition and infant temperament. While exploratory due to the small sample size, these results point to early and late infancy as sensitive periods during which the gut microbiome may exert effects on neurodevelopment.

Neurotrophin-4 is essential for survival of the majority of vagal afferents to the mucosa of the small intestine, but not the stomach

Vagal afferents form the primary gut-to-brain neural axis, communicating signals that regulate gastrointestinal (GI) function and promote satiation, appetition and reward. Neurotrophin-4 (NT-4) is essential for the survival of vagal smooth muscle afferents of the small intestine, but not the stomach. Here we took advantage of near-complete labeling of GI vagal mucosal afferents in Nav1.8cre-Rosa26tdTomato transgenic mice to determine whether these afferents depend on NT-4 for survival. We quantified the density and distribution of vagal afferent terminals in the stomach and small intestine mucosa and their central terminals in the solitary tract nucleus (NTS) and area postrema in NT-4 knockout (KO) and control mice. NT-4KO mice exhibited a 75% reduction in vagal afferent terminals in proximal duodenal villi and a 55% decrease in the distal ileum, whereas, those in the stomach glands remained intact. Vagal crypt afferents were also reduced in some regions of the small intestine, but to a lesser degree. Surprisingly, NT-4KO mice exhibited an increase in labeled terminals in the medial NTS. These findings, combined with previous results, suggest NT-4 is essential for survival of a large proportion of all classes of vagal afferents that innervate the small intestine, but not those that supply the stomach. Thus, NT-4KO mice could be valuable for distinguishing gastric and intestinal vagal afferent regulation of GI function and feeding. The apparent plasticity of central vagal afferent terminals - an increase in their density - could have compensated for loss of peripheral terminals by maintaining near-normal levels of satiety signaling.

Development of the Autonomic Nervous System: Clinical Implications

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

High-Fat Diet During the Perinatal Period Induces Loss of Myenteric Nitrergic Neurons and Increases Enteric Glial Density, Prior to the Development of Obesity

Diet-induced obesity induces peripheral inflammation accompanied by a loss of myenteric neurons. Few studies, however, have investigated the effects of a high-fat diet (HFD) on either the development of myenteric neurons or prior to the occurrence of obesity. The present study assessed the effects of maternal HFD on the density and neurochemical phenotype of myenteric ganglia in the upper gastrointestinal tract. Sprague-Dawley rats were fed either a control or HFD (14% or 60% kcal from fat, respectively) from embryonic day 13; the fundus, corpus and duodenum were fixed thereafter at postnatal 2, 4, 6 and 12 weeks of age for subsequent immunohistochemical studies. While myenteric ganglion size did not differ throughout the study, HFD exposure decreased the number of nitrergic neurons by 6 weeks of age in all regions. This decrease was accompanied by a loss of PGP-immunoreactive neurons, suggesting a decline in myenteric neuronal number. HFD also increased myenteric plexus glial cell density in all regions by 4 weeks of age. These changes occurred in the absence of an increase in serum or gastric inflammatory markers. The present study suggests that exposure to a HFD during the perinatal time period results in glial proliferation and loss of inhibitory nitrergic neurons prior to the onset of obesity, suggesting that dietary alterations may affect gastrointestinal functions independently of increased adiposity or glycemic dysregulation.

Netrin-1-like-immunoreactivity Coexpresses With DCC and Has a Differential Level in the Myenteric Cholinergic and Nitrergic Neurons of the Adult Mouse Colon

Netrin-1 is a potent axonal and neuronal guidance cue in the developing nervous system. Netrin-1 functions are mediated by its receptors, such as deleted in colorectal cancer (DCC) present on axons and neurons. Localization of DCC and Netrin-1 on various types of enteric neurons and their role in the mature enteric nervous system is unknown. The results of our study revealed that almost all enteric neurons and processes express DCC and Netrin-1 in the adult mice. Netrin-1-like-immunoreactivity (IR) was detected in the cytoplasm of neurons with some showing strong or weak staining. The majority of Netrin-1-like-immunoreactive enteric neurons were choline acetyltransferase (ChAT)-positive. However, ~19% of neurons were strongly Netrin-1-like-positive but ChAT-negative while ~8% of neurons were Netrin-1-like-negative but strongly ChAT-positive. In contrast, almost all nitric oxide synthase (nNOS)-positive enteric neurons displayed strong Netrin-1-like-IR. This differential intensity of Netrin-1 expression in the myenteric neurons might determine major neuronal subtypes regulating intestinal motility, ChAT-IR excitatory, and nNOS-IR inhibitory muscle motor and interneurons. This is the first study demonstrating the localization of DCC and Netrin-1 in the colonic myenteric plexus of the adult mice and their expression level determining two major neuronal subtypes regulating intestinal motility.

You Are What You (First) Eat

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

Molecular Analysis of Sensory Axon Branching Unraveled a cGMP-Dependent Signaling Cascade

Axonal branching is a key process in the establishment of circuit connectivity within the nervous system. Molecular-genetic studies have shown that a specific form of axonal branching—the bifurcation of sensory neurons at the transition zone between the peripheral and the central nervous system—is regulated by a cyclic guanosine monophosphate (cGMP)-dependent signaling cascade which is composed of C-type natriuretic peptide (CNP), the receptor guanylyl cyclase Npr2, and cGMP-dependent protein kinase Iα (cGKIα). In the absence of any one of these components, neurons in dorsal root ganglia (DRG) and cranial sensory ganglia no longer bifurcate, and instead turn in either an ascending or a descending direction. In contrast, collateral axonal branch formation which represents a second type of axonal branch formation is not affected by inactivation of CNP, Npr2, or cGKI. Whereas axon bifurcation was lost in mouse mutants deficient for components of CNP-induced cGMP formation; the absence of the cGMP-degrading enzyme phosphodiesterase 2A had no effect on axon bifurcation. Adult mice that lack sensory axon bifurcation due to the conditional inactivation of Npr2-mediated cGMP signaling in DRG neurons demonstrated an altered shape of sensory axon terminal fields in the spinal cord, indicating that elaborate compensatory mechanisms reorganize neuronal circuits in the absence of bifurcation. On a functional level, these mice showed impaired heat sensation and nociception induced by chemical irritants, whereas responses to cold sensation, mechanical stimulation, and motor coordination are normal. These data point to a critical role of axon bifurcation for the processing of acute pain perception.

Increased expression of Semaphorin 3A in the endothelin receptor-B null mouse model of Hirschsprung disease

PURPOSE

Semaphorins are guidance cues for developing neurons, implicated in the determination of the migratory pathway of neural crest-derived neural precursors during enteric nervous system development. Recently, it has been reported that Semaphorin 3A (SEMA3A) expression is up-regulated in the aganglionic colon in Hirschsprung disease (HD) patients, suggesting that increased SEMA3A expression may be a risk factor for HD. Thus, the aim of our study was to determine the expression of SEMA3A using Sox10-Venus mice gut.

METHODS

We harvested the gut on postnatal day 2 (P2). SOX10-Venus+/EDNRB^-/- mice were compared with SOX10-Venus+/EDNRB^+/+ mice as controls. QRT-PCR was performed to determine gene expression of SEMA3A (n=8). Fluorescent immunohistochemistry was performed to assess protein distribution.

RESULTS

On P2, gene expression levels of SEMA3A were significantly increased in the HD group compared to controls in the proximal and distal colon (p<0.05). Laser scanning microscopy revealed SEMA3A expression was localized within the submucosa and muscle layer of the gut in both HD and controls. In HD, SEMA3A was highly expressed in the proximal and distal colon.

CONCLUSIONS

In the present study, we demonstrated that SEMA3A expression is increased in the EDNRB-/- HD model on P2, suggesting that SEMA3A may interfere with ENCC migration, resulting in an absence of enteric neurons.

The microbiota-gut-brain axis as a key regulator of neural function and the stress response: Implications for human and animal health

The brain-gut-microbiota axis comprises an extensive communication network between the brain, the gut, and the microbiota residing there. Development of a diverse gut microbiota is vital for multiple features of behavior and physiology, as well as many fundamental aspects of brain structure and function. Appropriate early-life assembly of the gut microbiota is also believed to play a role in subsequent emotional and cognitive development. If the composition, diversity, or assembly of the gut microbiota is impaired, this impairment can have a negative impact on host health and lead to disorders such as obesity, diabetes, inflammatory diseases, and even potentially neuropsychiatric illnesses, including anxiety and depression. Therefore, much research effort in recent years has focused on understanding the potential of targeting the intestinal microbiota to prevent and treat such disorders. This review aims to explore the influence of the gut microbiota on host neural function and behavior, particularly those of relevance to stress-related disorders. The involvement of microbiota in diverse neural functions such as myelination, microglia function, neuronal morphology, and blood-brain barrier integrity across the life span, from early life to adolescence to old age, will also be discussed. Nurturing an optimal gut microbiome may also prove beneficial in animal science as a means to manage stressful situations and to increase productivity of farm animals. The implications of these observations are manifold, and researchers are hopeful that this promising body of preclinical work can be successfully translated to the clinic and beyond.

Irritable Bowel Syndrome and Stress-Related Psychiatric Co-morbidities: Focus on Early Life Stress

Irritable bowel syndrome is a functional gastrointestinal disorder, with stress playing a major role in onset and exacerbation of symptoms such as abdominal pain and altered bowel movements. Stress-related disorders including anxiety and depression often precede the development of irritable bowel syndrome and vice versa. Stressor exposure during early life has the potential to increase an individual's susceptibility to both irritable bowel syndrome and psychiatric disease indicating that there may be a common origin for these disorders. Moreover, adverse early life events significantly impact upon many of the communication pathways within the brain-gut-microbiota axis, which allows bidirectional interaction between the central nervous system and the gastrointestinal tract. This axis is proposed to be perturbed in irritable bowel syndrome and studies now indicate that dysfunction of this axis is also seen in psychiatric disease. Here we review the co-morbidity of irritable bowel syndrome and psychiatric disease with their common origin in mind in relation to the impact of early life stress on the developing brain-gut-microbiota axis. We also discuss the therapeutic potential of targeting this axis in these diseases.

Calcium, mitochondrial dysfunction and slowing the progression of Parkinson's disease

Parkinson's disease is characterized by progressively distributed Lewy pathology and neurodegeneration. The motor symptoms of clinical Parkinson's disease (cPD) are unequivocally linked to the degeneration of dopaminergic neurons in the substantia nigra pars compacta (SNc). Several features of these neurons appear to make them selectively vulnerable to factors thought to cause cPD, like aging, genetic mutations and environmental toxins. Among these features, Ca²⁺ entry through Cav1 channels is particularly amenable to pharmacotherapy in early stage cPD patients. This review outlines the linkage between these channels, mitochondrial oxidant stress and cPD pathogenesis. It also summarizes considerations that went into the design and execution of the ongoing Phase 3 clinical trial with an inhibitor of these channels - isradipine.

Selective neuronal vulnerability in Parkinson disease

Intracellular α-synuclein (α-syn)-rich protein aggregates called Lewy pathology (LP) and neuronal death are commonly found in the brains of patients with clinical Parkinson disease (cPD). It is widely believed that LP appears early in the disease and spreads in synaptically coupled brain networks, driving neuronal dysfunction and death. However, post-mortem analysis of human brains and connectome-mapping studies show that the pattern of LP in cPD is not consistent with this simple model, arguing that, if LP propagates in cPD, it must be gated by cell- or region-autonomous mechanisms. Moreover, the correlation between LP and neuronal death is weak. In this Review, we briefly discuss the evidence for and against the spreading LP model, as well as evidence that cell-autonomous factors govern both α-syn pathology and neuronal death.

Extracerebral Dysfunction in Animal Models of Autism Spectrum Disorder

Genetic factors might be largely responsible for the development of autism spectrum disorder (ASD) that alone or in combination with specific environmental risk factors trigger the pathology. Multiple mutations identified in ASD patients that impair synaptic function in the central nervous system are well studied in animal models. How these mutations might interact with other risk factors is not fully understood though. Additionally, how systems outside of the brain are altered in the context of ASD is an emerging area of research. Extracerebral influences on the physiology could begin in utero and contribute to changes in the brain and in the development of other body systems and further lead to epigenetic changes. Therefore, multiple recent studies have aimed at elucidating the role of gene-environment interactions in ASD. Here we provide an overview on the extracerebral systems that might play an important associative role in ASD and review evidence regarding the potential roles of inflammation, trace metals, metabolism, genetic susceptibility, enteric nervous system function and the microbiota of the gastrointestinal (GI) tract on the development of endophenotypes in animal models of ASD. By influencing environmental conditions, it might be possible to reduce or limit the severity of ASD pathology.

The Role of the Mesentery in Crohn's Disease: The Contributions of Nerves, Vessels, Lymphatics, and Fat to the Pathogenesis and Disease Course

Crohn's disease (CD) is a complex gastrointestinal disorder involving multiple levels of cross talk between the immunological, neural, vascular, and endocrine systems. The current dominant theory in CD is based on the unidirectional axis of dysbiosis-innate immunity-adaptive immunity-mesentery-body system. Emerging clinical evidence strongly suggests that the axis be bidirectional. The morphologic and/or functional abnormalities in the mesenteric structures likely contribute to the disease progression of CD, to a less extent the disease initiation. In addition to adipocytes, mesentery contains nerves, blood vessels, lymphatics, stromal cells, and fibroblasts. By the secretion of adipokines that have endocrine functions, the mesenteric fat tissue exerts its activity in immunomodulation mainly through response to afferent signals, neuropeptides, and functional cytokines. Mesenteric nerves are involved in the pathogenesis and prognosis of CD mainly through neuropeptides. In addition to angiogenesis observed in CD, lymphatic obstruction, remodeling, and impaired contraction maybe a cause and consequence of CD. Lymphangiogenesis and angiogenesis play a concomitant role in the progress of chronic intestinal inflammation. Finally, the interaction between neuropeptides, adipokines, and vascular and lymphatic endothelia leads to adipose tissue remodeling, which makes the mesentery an active participator, not a bystander, in the disease initiation and precipitation CD. The identification of the role of mesentery, including the structure and function of mesenteric nerves, vessels, lymphatics, and fat, in the intestinal inflammation in CD has important implications in understanding its pathogenesis and clinical management.

Prokaryotes Versus Eukaryotes: Who is Hosting Whom?

Microorganisms represent the largest component of biodiversity in our world. For millions of years, prokaryotic microorganisms have functioned as a major selective force shaping eukaryotic evolution. Microbes that live inside and on animals outnumber the animals' actual somatic and germ cells by an estimated 10-fold. Collectively, the intestinal microbiome represents a "forgotten organ," functioning as an organ inside another that can execute many physiological responsibilities. The nature of primitive eukaryotes was drastically changed due to the association with symbiotic prokaryotes facilitating mutual coevolution of host and microbe. Phytophagous insects have long been used to test theories of evolutionary diversification; moreover, the diversification of a number of phytophagous insect lineages has been linked to mutualisms with microbes. From termites and honey bees to ruminants and mammals, depending on novel biochemistries provided by the prokaryotic microbiome, the association helps to metabolize several nutrients that the host cannot digest and converting these into useful end products (such as short-chain fatty acids), a process, which has huge impact on the biology and homeostasis of metazoans. More importantly, in a direct and/or indirect way, the intestinal microbiota influences the assembly of gut-associated lymphoid tissue, helps to educate immune system, affects the integrity of the intestinal mucosal barrier, modulates proliferation and differentiation of its epithelial lineages, regulates angiogenesis, and modifies the activity of enteric as well as the central nervous system. Despite these important effects, the mechanisms by which the gut microbial community influences the host's biology remain almost entirely unknown. Our aim here is to encourage empirical inquiry into the relationship between mutualism and evolutionary diversification between prokaryotes and eukaryotes, which encourage us to postulate: who is hosting whom?

Spatiotemporal mapping of vascularization and innervation in the fetal murine intestine

BACKGROUND

In mice, the intestinal tube develops from the splanchopleure before embryonic day 9.5. Subsequent patterning of nerves and blood vessels is critical for normal digestive function. A hierarchical branching vascular network allows for efficient nutrient absorption, while the complex enteric nervous system regulates intestinal motility as well as secretion, absorption, and blood flow. Despite the well-recognized significance of these systems, the precise mechanisms by which they develop have not been clearly established in mammals.

RESULTS

Using a novel whole-mount immunohistochemical protocol, we visualize the pattern of intestinal neurovascular development in mice between embryonic day 10.5 and birth. In particular, we focus on the development and remodeling of the enteric vascular plexus, the migration and organization of enteric neural crest-derived cells, and the integration of peripheral sympathetic nerves with the enteric nervous system. These correlative data lead us to hypothesize a functional interaction between migrating neural crest-derived cells and endothelial cells of the primary capillary plexus, as well as a subsequent interaction between developing peripheral autonomic nerves and differentiated neural crest-derived cells.

CONCLUSIONS

These studies provide useful anatomical data for continuing investigations on the functional mechanisms underlying intestinal organogenesis.

Lipopolysaccharides and trophic factors regulate the LPS receptor complex in nodose and trigeminal neurons

Binding of bacterial lipopolysaccharides (LPS) to toll-like receptor 4 (TLR4) triggers an innate immunoresponse associated with pain and inflammation. The expression, and to a greater extent the regulation of TLR4 and its auxiliary proteins (myeloid differentiation protein 1 (MD1), myeloid differentiation protein 2 (MD2) and cluster of differentiation 14 (CD14)), are both poorly understood in trigeminal and nodose neurons. We used a combination of Western blotting, semi-quantitative polymerase chain reaction (PCR), pharmacological manipulation and immunohistochemistry. The expression pattern and regulation by LPS and trophic factors of TLR4/MD2/CD14 and radioprotective protein of 105kDa (RP105)/MD1 were determined in neonatal trigeminal and nodose mice neurons. We found that all these proteins were expressed in both trigeminal and nodose neurons. The trophic factors Artemin and nerve growth factor (NGF) up-regulated MD2 and RP105 mRNA levels in trigeminal neurons. In nodose neurons the trophic factors brain-derived neurotrophic factor (BDNF), ciliary neurotrophic factor (CNTF) and leukemia inhibitory factor (LIF) up-regulated MD1 and RP105 mRNA levels. Also we observed that in both neuronal types LPS acutely (within 20 min) down-regulated CD14 and MD2 mRNAs. In addition, LPS increased significantly the proportion of trigeminal and nodose neurons expressing nociceptin/orphanin FQ in culture probably acting via TLR4/MD2. Although the exact mechanisms underlying the regulation by trophic factors and LPS require further elucidation, the findings of this study indicate that LPS acts through its archetypical receptor in trigeminal and nodose neurons.

Priming for health: gut microbiota acquired in early life regulates physiology, brain and behaviour

The infant gut microbiome is dynamic, and radical shifts in composition occur during the first 3 years of life. Disruption of these developmental patterns, and the impact of the microbial composition of our gut on brain and behaviour, has attracted much recent attention. Integrating these observations is an important new research frontier.

CONCLUSION

Early-life perturbations of the developing gut microbiota can impact on the central nervous system and potentially lead to adverse mental health outcomes.

Serotonin, tryptophan metabolism and the brain-gut-microbiome axis

The brain-gut axis is a bidirectional communication system between the central nervous system and the gastrointestinal tract. Serotonin functions as a key neurotransmitter at both terminals of this network. Accumulating evidence points to a critical role for the gut microbiome in regulating normal functioning of this axis. In particular, it is becoming clear that the microbial influence on tryptophan metabolism and the serotonergic system may be an important node in such regulation. There is also substantial overlap between behaviours influenced by the gut microbiota and those which rely on intact serotonergic neurotransmission. The developing serotonergic system may be vulnerable to differential microbial colonisation patterns prior to the emergence of a stable adult-like gut microbiota. At the other extreme of life, the decreased diversity and stability of the gut microbiota may dictate serotonin-related health problems in the elderly. The mechanisms underpinning this crosstalk require further elaboration but may be related to the ability of the gut microbiota to control host tryptophan metabolism along the kynurenine pathway, thereby simultaneously reducing the fraction available for serotonin synthesis and increasing the production of neuroactive metabolites. The enzymes of this pathway are immune and stress-responsive, both systems which buttress the brain-gut axis. In addition, there are neural processes in the gastrointestinal tract which can be influenced by local alterations in serotonin concentrations with subsequent relay of signals along the scaffolding of the brain-gut axis to influence CNS neurotransmission. Therapeutic targeting of the gut microbiota might be a viable treatment strategy for serotonin-related brain-gut axis disorders.

Vessel-dependent recruitment of sympathetic axons: looking for innervation in all the right places

Autonomic sympathetic axons extend along and innervate resistance arteries to control vascular tone and participate in blood pressure regulation. In this issue of the JCI, Brunet and colleagues reveal that sympathetic innervation of arteries is facilitated by secretion of the axon guidance molecule netrin-1 by arterial VSMCs. Furthermore, disruption of the signaling cascade induced by netrin-1 through its receptor DCC resulted in defective arterial innervation and sympathetic control of vasoconstriction. This comprehensive study represents a major step forward in our understanding of the coordinated wiring of the vascular and nervous systems in various tissues.

Bifurcation of axons from cranial sensory neurons is disabled in the absence of Npr2-induced cGMP signaling

Axonal branching is a prerequisite for the establishment of complex neuronal circuits and their capacity for parallel information processing. Previously, we have identified a cGMP signaling pathway composed of the ligand C-type natriuretic peptide (CNP), its receptor, the guanylyl cyclase natriuretic peptide receptor 2 (Npr2), and the cGMP-dependent kinase Iα (cGKIα) that regulates axon bifurcation of dorsal root ganglion (DRG) neurons in the spinal cord. Now we asked whether this cascade also controls axon bifurcation elsewhere in the nervous system. An Npr2-lacZ reporter mouse line was generated to clarify the pattern of the CNP receptor expression. It was found that during the period of axonal outgrowth, Npr2 and cGKIα were strongly labeled in neurons of all cranial sensory ganglia (gV, gVII, gVIII, gIX, and gX). In addition, strong complementary expression of CNP was detected in the hindbrain at the entry zones of sensory afferents. To analyze axon branching in individual Npr2-positive neurons, we generated a mouse mutant expressing a tamoxifen-inducible variant of Cre recombinase expressed under control of the Npr2-promoter (Npr2-CreER(T2)). After crossing this strain with conditional reporter mouse lines, we revealed that the complete absence of Npr2 activity indeed prohibited the bifurcation of cranial sensory axons in their entrance region. Consequently, axons only turned in either an ascending or descending direction, while collateral formation and growth of the peripheral arm was not affected. These findings indicate that in neurons of the cranial sensory ganglia, as in DRG neurons, cGMP signals are necessary for the execution of an axonal bifurcation program.

Tissue specific somatic mutations and aganglionosis in Hirschsprung's disease

BACKGROUND

RET proto-oncogene intron 1 variations [e.g. SNP1 (rs2506004) and SNP2 (rs 2435357)] have been shown to be etiologically important in the pathogenesis of Hirschsprung's disease (HSCR). Although activating somatic RET rearrangements have been identified in certain tumours, this is the first study to confirm somatic gene variation in HSCR.

METHODS

DNA was extracted from 53 paraffin embedded tissue samples (HSCR patients n=33, multiple levels n=17), and controls (n=3). Patients were grouped into aganglionic (Group 1), ganglionated (group 2), and transitional (group 3). PCR products of RET intron 1 were screened for genetic variation by semi-automated bi-directional sequencing analysis and matched to unaffected controls from the general population. Comparison was by Fishers exact test. P <0.05 was regarded as significant.

RESULTS

HSCR patients included short segment (n=26), long segment colonic [(n=4 (24%)], and total colonic aganglionosis (n=3). RET intronic variations [SNP1 (rs2506004) or SNP2 (rs 2435357)] showed somatic homozygous in affected tissue in 9/12 (75%) Group 1 (aganglionic tissue) compared with 2/5 (40%) and 1/10 (10%) of groups 2 and 3 (P<0.001). Homozygous SNP2 variation was observed in all long segment versus 4/10 short segment. 50% of the short segment cases showing homozygous SNP 1 variation.

CONCLUSION

We report somatic mutations in the RET intron 1 region of affected HSCR tissue, confirming for the first time that somatic mutations are present in aganglionic tissue and may promote local aganglionosis through deregulated receptor activity. Detailed understanding of the somatic genetic events that drive congenital aganglionosis may have bearing on diagnosis and therapy.

Gastritis and mental disorders

Although previous studies have suggested an association between various gastrointestinal disorders and mood and anxiety disorders, no previous study has examined the relationship between a diagnosis of gastritis and mood and anxiety disorders in the community. This work aimed to investigate the association between physician-diagnosed gastritis and DSM-IV mood and anxiety disorders among adults in the general population, and to examine sex differences in these relationships. Data were drawn from a population-based, representative sample of 4181 adults aged 18-79 in the German National Health Interview and Examination Survey. Anxiety disorders (27.0% vs. 15.3%) and affective disorders (20.1% vs. 11.5%) were significantly more common among adults with compared to without a diagnosis of gastritis. Lifetime and current physician diagnosed gastritis were associated with an increased prevalence of panic attacks, social phobia, any mood disorder and major depression, compared to those without gastritis. There were no significant sex differences in these associations. A diagnosis of gastritis appears to be associated with significantly increased odds of mood and anxiety disorders among adults in the general population. Contrary to findings from animal studies, we found the relationship between gastritis and mood/anxiety consistent among both sexes.

Guidance for life, cell death, and colorectal neoplasia by netrin dependence receptors

This review is focusing on a critical mediator of embryonic and postnatal development with multiple implications in inflammation, neoplasia, and other pathological situations in brain and peripheral tissues. These morphogenetic guidance and dependence processes are involved in several malignancies targeting the epithelial and immune systems including the progression of human colorectal cancers. We consider the most important findings and their impact on basic, translational, and clinical cancer research. Expected information can bring new cues for innovative, efficient, and safe strategies of personalized medicine based on molecular markers, protagonists, signaling networks, and effectors inherent to the Netrin axis in pathophysiological states.