Axonal outgrowth from adult mouse nodose ganglia in vitro is stimulated by neurotrophin-4 in a Trk receptor and mitogen-activated protein kinase-dependent way

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

BACKGROUND

The vagus nerve is the major neural connection between the gastrointestinal tract and the central nervous system. During fetal development, axons from the cell bodies of the nodose ganglia and the dorsal motor nucleus grow into the gut to find their enteric targets, providing the vagal sensory and motor innervations respectively. Vagal sensory and motor axons innervate selective targets, suggesting a role for guidance cues in the establishment of the normal pattern of enteric vagal innervation.

PURPOSE

This review explores known molecular mechanisms that guide vagal innervation in the gastrointestinal tract. Guidance and growth factors, such as netrin-1 and its receptor, deleted in colorectal cancer, extracellular matrix molecules, such as laminin-111, and members of the neurotrophin family of molecules, such as brain-derived neurotrophic factor have been identified as mediating the guidance of vagal axons to the fetal mouse gut. In addition to increasing our understanding of the development of enteric innervation, studies of vagal development may also reveal clinically relevant insights into the underlying mechanisms of vago-vagal communication with the gastrointestinal tract.

Vagal activities are involved in antigen-specific immune inflammation in the intestine

BACKGROUND AND AIMS

The mechanism of intestinal immune inflammation, such as food allergy, remains to be further understood. The present study aims to investigate the role of the vagal nerve in the pathogenesis of skewed T-helper 2 (Th2) responses in the intestine.

METHODS

The expression of the immunoglobulin E (IgE) receptor on the vagus nerve in the mouse intestine was observed by immunohistochemistry. Vagus ganglion neurons (VGN) were isolated from mice and cultured in vitro. The IgE receptor/IgE complex on vagus neurons was examined by immune precipitation assay. A food allergy mouse model was developed; the effect of the partial removal of the vagal nerve (PRVn) via surgery or administration with anticholinergic agents on the suppression of Th2 inflammation was evaluated.

RESULTS

The high-affinity IgE receptor was detected on the intestinal vagus nerve. An increase in the expression of the IgE receptor on the vagus nerve was observed in the intestines of mice with intestinal immune inflammation. Isolated mouse VGN express IgE receptor I, which could form complexes with IgE. Re-exposure to specific antigens activated the sensitized VGN, manifesting the release of transmitter glutamate that could activate dendritic cells by increasing the expression of CD80 and major compatibility complex class II and suppressing interleukin-12. The PRVn suppressed Th2 inflammation in the intestine.

CONCLUSIONS

The intestinal vagus nerve in mice expresses a high-affinity IgE receptor. An antigen-specific immune response can activate the vagus nerve in the intestine and induces the release of transmitters to modulate dendritic cell phenotypes that facilitate the development of skewed Th2 polarization in the intestine.

Impaired axonal regeneration by isolectin B4-binding dorsal root ganglion neurons in vitro

The subpopulation of dorsal root ganglion (DRG) neurons recognized by Griffonia simplicifolia isolectin B4 (IB4) differ from other neurons by expressing receptors for glial cell line-derived neurotrophic factor (GDNF) rather than neurotrophins. Additionally, IB4-labeled neurons do not express the laminin receptor, alpha7-integrin (Gardiner et al., 2005), necessary for optimal axonal regeneration in the peripheral nervous system. In cultures of dissociated DRG neurons of adult mice on laminin, robust spontaneous neurite outgrowth from IB4-negative neurons occurs and is strongly enhanced by previous axotomy. In contrast, IB4-labeled neurons show little neurite outgrowth and do not express GAP 43, even after axotomy or culture with GDNF. Moreover, growth of their axons through collagen gels is impaired compared with other DRG neurons. To determine whether the sparse neurite outgrowth of IB4-labeled neurons is attributable to lack of integrin expression, DRG cultures were infected with a herpes simplex 1 vector encoding alpha7-integrin, but its forced expression failed to promote neurite outgrowth in either IB4-labeled or other DRG neurons or in cultured adult retinal ganglion cells. Forced coexpression of both alpha7-integrin and GAP 43 also failed to promote neurite outgrowth in IB4-labeled neurons. In addition, cultured sciatic nerve segments were found to release much lower levels of GDNF, demonstrated by ELISA, than nerve growth factor. These findings together with their impaired intrinsic axonal regeneration capacity may contribute to the known vulnerability of the IB4-labeled population of DRG neurons to peripheral nerve injury.

cAMP regulates axon outgrowth and guidance during optic nerve regeneration in goldfish

Increased cAMP improves neuronal survival and axon regeneration in mammals. Here, we assess cAMP levels and identify activated pathways in a spontaneously regenerating central nervous system. Following optic nerve crush in goldfish, almost all retinal ganglion cells (RGC) survive and regenerate retinotectal topography. Goldfish received injections of a cAMP analogue (CPT-cAMP), a protein kinase A (PKA) inhibitor (KT5720), both compounds combined, or PBS (control). RGC survival in experimental groups was unaffected at any stage. The rate of axon regeneration was accelerated by the activator and decelerated both by the inhibitor and by combined injections, suggesting a PKA-dependent pathway. In addition, errors in regenerate retinotectal topography were observed when agents were applied in vivo and RGC response to the guidance cue ephrin-A5 in vitro was altered by the inhibitor. Our results highlight that therapeutic manipulation of cAMP levels to enhance axonal regeneration in mammals must ensure that topography, and consequently function, is not disrupted.

Musings on the wanderer: what's new in our understanding of vago-vagal reflexes? V. Remodeling of vagus and enteric neural circuitry after vagal injury

The vago-vagal reflexes mediate a wide range of digestive functions such as motility, secretion, and feeding behavior. Previous articles in this series have discussed the organization and functions of this important neural pathway. The focus of this review will be on some of the events responsible for the adaptive changes of the vagus and the enteric neutral circuitry that occur after vagal injury. The extraordinary plasticity of the neural systems to regain functions when challenged with neural injury will be discussed. In general, neuropeptides and transmitter-related enzymes in the vagal sensory neurons are downregulated after vagal injury to protect against further injury. Conversely, molecules previously absent or present at low levels begin to appear or are upregulated and are available to participate in the survival-regeneration process. Neurotrophins and other related proteins made at the site of the lesion and then retrogradely transported to the soma may play an important role in the regulation of neuropeptide phenotype expression and axonal growth. Vagal injury also triggers adaptive changes within the enteric nervous system to minimize the loss of gastrointestinal functions resulting from the interruption of the vago-vagal pathways. These may include rearrangement of the enteric neural circuitry, changes in the electrophysiological properties of sensory receptors in the intramural neural networks, an increase in receptor numbers, and changes in the affinity states of receptors on enteric neurons.

L- and D-threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) inhibit neurite outgrowth from SH-SY5Y cells

Gangliosides and extracellular matrix molecules influence neurite outgrowth, but the combinatorial effects of these endogenous agents on outgrowth are unclear. Exogenous gangliosides inhibit neurite outgrowth from SH-SY5Y cells stimulated with platelet-derived growth factor-BB, and different isoforms of the ceramide analog threo-1-phenyl-2-decanoylamino-3-morpholino-1-propanol (PDMP) stimulate (L-PDMP) or inhibit (D-PDMP) glycosphingolipid biosynthesis. In this study, we determined whether altering the endogenous ganglioside levels with PDMP in SH-SY5Y cells regulates neurite outgrowth on the outgrowth-supporting extracellular matrix molecule, laminin. In cells stimulated with 20 ng/ml platelet-derived growth factor-BB to promote outgrowth, we used image analysis to evaluate neurite outgrowth from SH-SY5Y cells grown on endogenous matrix or laminin and exposed to L- or D-PDMP. Both L- and D-PDMP decreased neurite initiation (the number of neurites/cell, the percent of neurite-bearing cells), elongation (the length of the longest neurite/cell, the total neurite length/cell), and branching (the number of branch points/neurite) from SH-SY5Y cells on endogenous matrix or laminin in a dose-dependent manner in serum-free or serum-containing medium. The inhibitory effects of each PDMP isoform were reversible. Inhibition of neurite outgrowth by L-PDMP could be mimicked by addition of exogenous gangliosides or C2-ceramide. Our analyses of neurite outgrowth in SH-SY5Y cells, a model of developing or regenerating noradrenergic neurons, demonstrate that increasing or decreasing endogenous ganglioside levels decreases neurite outgrowth. These results may indicate that SH-SY5Y cells undergo tight regulation by gangliosides, possibly through modulation of growth/trophic factor- and/or extracellular matrix-activated signaling cascades.

Climbing fiber development: do neurotrophins have a part to play?

The climbing fiber input to the cerebellum is crucial for its normal function but those factors which control the development of this precisely organized pathway are not fully elucidated. The neurotrophins are a family of peptides, which have many roles during development of the nervous system, including the cerebellum. Since the cerebellum and inferior olive express neurotrophins and their receptors, we propose that neurotrophins are involved in the regulation of climbing fiber development. Here we review the temporo-spatial expression of neurotrophins and their receptors at key ages during climbing fiber development and then examine evidence linking neurotrophins to climbing fiber development, including some of the intracellular pathways involved. During prenatal development the expression of neurotrophins in the hindbrain coupled with their function in neurogenesis and migration, is consistent with a role of NT3 in inferior olivary genesis. Subsequently, cerebellar expression of two neurotrophins, NT3 and NT4, is concurrent with olivary receptor expression and the time of olivary axonal outgrowth and this continues postnatally during early climbing fiber synaptogenesis on Purkinje cells. The expression-pattern of neurotrophins changes with age, with falling NGF, NT3 and NT4 but increasing granule cell BDNF. Importantly, olivary expression of neurotrophin receptors, and therefore climbing fiber responsiveness to neurotrophins, falls specifically during maturation of the climbing fiber-Purkinje cell synapse. The function of BDNF is less certain, but experimental studies indicate that it has a role in climbing fiber innervation of Purkinje cells, particularly synaptogenesis and synaptic plasticity. Its importance is highlighted by the overlap of BDNF signalling with several cellular pathways, which regulate climbing fiber maturation. From the data presented, we propose not only that neurotrophins are involved in climbing fiber development, but also that several act in a specific temporal order.

Trachea enhances neurite regeneration from adult rat nodose ganglia in vitro

Trachea is intensely innervated with vagal afferent nerve fibers, and may play an important role in vagus nerve regeneration after axonal injury caused by trauma and surgical operation. We investigated the effects of tracheal tissue on neuronal cell survival and neurite regeneration in adult rat nodose ganglia (NG) in vitro. Co-culture with trachea significantly increased the average number of neurites regenerated from transected nerve terminals of NG explants, from 73.7 to 154.2 after 3 days, from 68 to 186.7 after 5 days, and from 31 to 101.5 after 7 days in culture. Dissociated NG neurons could continue to survive and extend neurites only in the co-existence with satellite cells in collagen gel. Co-cultured trachea improved the ratios of survival and neurite-bearing cells of NG neurons, from 56.7% and 11.1% to 72.3% and 37.6% after 4 days, and from 41.1% and 20.3% to 56.4% and 47.2% after 7 days in culture, respectively. These results imply that tracheal tissue secretes a factor, which could enhance neuronal cell survival and neurite regeneration in NG in the presence of satellite cells in vitro.

Neurotrophin-4 deficient mice have a loss of vagal intraganglionic mechanoreceptors from the small intestine and a disruption of short-term satiety

Intraganglionic laminar endings (IGLEs) and intramuscular arrays (IMAs) are the two putative mechanoreceptors that the vagus nerve supplies to gastrointestinal smooth muscle. To examine whether neurotrophin-4 (NT-4)-deficient mice, which have only 45% of the normal number of nodose ganglion neurons, exhibit selective losses of these endings and potentially provide a model for assessing their functional roles, we inventoried IGLEs and IMAs in the gut wall. Vagal afferents were labeled by nodose ganglion injections of wheat germ agglutinin-horseradish peroxidase, and a standardized sampling protocol was used to map the terminals in the stomach, duodenum, and ileum. NT-4 mutants had a substantial organ-specific reduction of IGLEs; whereas the morphologies and densities of both IGLEs and IMAs in the stomach were similar to wild-type patterns, IGLEs were largely absent in the small intestine (90 and 81% losses in duodenum and ileum, respectively). Meal pattern analyses revealed that NT-4 mutants had increased meal durations with solid food and increased meal sizes with liquid food. However, daily total food intake and body weight remained normal because of compensatory changes in other meal parameters. These findings indicate that NT-4 knock-out mice have a selective vagal afferent loss and suggest that intestinal IGLEs (1) may participate in short-term satiety, probably by conveying feedback about intestinal distension or transit to the brain, (2) are not essential for long-term control of feeding and body weight, and (3) play different roles in regulation of solid and liquid diet intake.