TrkB expression and early sensory neuron survival are independent of endogenous BDNF

NGF-Dependent and BDNF-Dependent DRG Sensory Neurons Deploy Distinct Degenerative Signaling Mechanisms

The nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) are trophic factors required by distinct population of sensory neurons during development of the nervous system. Neurons that fail to receive appropriate trophic support are lost during this period of naturally occurring cell death. In the last decade, our understanding of the signaling pathways regulating neuronal death following NGF deprivation has advanced substantially. However, the signaling mechanisms promoting BDNF deprivation-induced sensory neuron degeneration are largely unknown. Using a well-established in vitro culture model of dorsal root ganglion (DRG), we have examined degeneration mechanisms triggered on BDNF withdrawal in sensory neurons. Our results indicate differences and similarities between the molecular signaling pathways behind NGF and BDNF deprivation-induced death. For instance, we observed that the inhibition of Trk receptors (K252a), PKC (Gö6976), protein translation (cycloheximide; CHX), or caspases (zVAD-fmk) provides protection from NGF deprivation-induced death but not from degeneration evoked by BDNF-withdrawal. Interestingly, degeneration of BDNF-dependent sensory neurons requires BAX and appears to rely on reactive oxygen species (ROS) generation rather than caspases to induce degeneration. These results highlight the complexity and divergence of mechanisms regulating developmental sensory neuron death.

Loss of neurotrophin-3 from smooth muscle disrupts vagal gastrointestinal afferent signaling and satiation

A large proportion of vagal afferents are dependent on neurotrophin-3 (NT-3) for survival. NT-3 is expressed in developing gastrointestinal (GI) smooth muscle, a tissue densely innervated by vagal mechanoreceptors, and thus could regulate their survival. We genetically ablated NT-3 from developing GI smooth muscle and examined the pattern of loss of NT-3 expression in the GI tract and whether this loss altered vagal afferent signaling or feeding behavior. Meal-induced c-Fos activation was reduced in the solitary tract nucleus and area postrema in mice with a smooth muscle-specific NT-3 knockout (SM-NT-3(KO)) compared with controls, suggesting a decrease in vagal afferent signaling. Daily food intake and body weight of SM-NT-3(KO) mice and controls were similar. Meal pattern analysis revealed that mutants, however, had increases in average and total daily meal duration compared with controls. Mutants maintained normal meal size by decreasing eating rate compared with controls. Although microstructural analysis did not reveal a decrease in the rate of decay of eating in SM-NT-3(KO) mice, they ate continuously during the 30-min meal, whereas controls terminated feeding after 22 min. This led to a 74% increase in first daily meal size of SM-NT-3(KO) mice compared with controls. The increases in meal duration and first meal size of SM-NT-3(KO) mice are consistent with reduced satiation signaling by vagal afferents. This is the first demonstration of a role for GI NT-3 in short-term controls of feeding, most likely involving effects on development of vagal GI afferents that regulate satiation.

Prevention and intervention studies with telmisartan, ramipril and their combination in different rat stroke models

Objectives

The effects of AT1 receptor blocker, telmisartan, and the ACE inhibitor, ramipril, were tested head-to head and in combination on stroke prevention in hypertensive rats and on potential neuroprotection in acute cerebral ischemia in normotensive rats.

Methods

Prevention study: Stroke-prone spontaneously hypertensive rats (SHR-SP) were subjected to high salt and randomly assigned to 4 groups: (1) untreated (NaCl, n = 24), (2) telmisartan (T; n = 27), (3) ramipril (R; n = 27) and (4) telmisartan +ramipril (T+R; n = 26). Drug doses were selected to keep blood pressure (BP) at 150 mmHg in all groups. Neurological signs and stroke incidence at 50% mortality of untreated SHR-SP were investigated.

Intervention study: Normotensive Wistar rats were treated s.c. 5 days prior to middle cerebral artery occlusion (MCAO) for 90 min with reperfusion. Groups (n = 10 each): (1) sham, (2) vehicle (V; 0,9% NaCl), (3) T (0,5 mg/kg once daily), (4) R (0,01 mg/kg twice daily), (5) R (0,1 mg/kg twice daily) or (6) T (0,5 mg/kg once daily) plus R (0,01 mg/kg twice daily). Twenty-four and 48 h after MCAO, neurological outcome (NO) was determined. Forty-eight h after MCAO, infarct volume by MRI, neuronal survival, inflammation factors and neurotrophin receptor (TrkB) were analysed.

Results

Stroke incidence was reduced, survival was prolonged and neurological outcome was improved in all treated SHR-SP with no differences between treated groups. In the acute intervention study, T and T+R, but not R alone, improved NO, reduced infarct volume, inflammation (TNFα), and induced TrkB receptor and neuronal survival in comparison to V.

Conclusions

T, R or T+R had similar beneficial effects on stroke incidence and NO in hypertensive rats, confirming BP reduction as determinant factor in stroke prevention. In contrast, T and T+R provided superior neuroprotection in comparison to R alone in normotensive rats with induced cerebral ischemia.

Dynamic changes of TrkB gene expression in Streptococcus pneumoniae meningitis after treatment with antibiotics and dexamethasone

BACKGROUND

Although more and more new potent antibiotics have been used, the incidence of neurological sequelae of Streptococcus pneumoniae meningitis has not improved in children over the last decade. The expression of TrkB mRNA, a receptor of brain-derived neurotrophic factor, is associated with the incidence of neurological sequelae of Streptococcus pneumoniae meningitis.

METHODS

Rats of 3 weeks old were used to construct a model of Streptococcus pneumoniae meningitis and served as normal controls. They were administered with antibiotics or antibiotics plus dexamethasone, respectively. The expression of the TrkB gene was detected in the brain by in situ hybridization.

RESULTS

In the brains of Streptococcus pneumoniae inoculated rats, TrkB mRNA was significantly up-regulated after inoculation for 24 hours, and then down-regulated in a dose-dependent manner after treatment with antibiotics. This up-regulation was seen after treatment with antibiotics plus dexamethasone. TrkB mRNA expression was also observed in some infiltrating inflammatory cells.

CONCLUSIONS

The results of the study support the hypothesis that TrkB signal transduction pathways might play an important role in Streptococcus pneumoniae meningitis, probably by protecting the brain from damage. The role of TrkB might be weakened after the treatment with antibiotics. Our findings suggest that targeting TrkB receptors might be a rational strategy for prevention of neurological sequelae caused by Streptococcus pneumoniae meningitis.

Brain-derived neurotrophic factor mediates non-cell-autonomous regulation of sensory neuron position and identity

During development, neurons migrate considerable distances to reside in locations that enable their individual functional roles. Whereas migration mechanisms have been extensively studied, much less is known about how neurons remain in their ideal locations. We sought to identify factors that maintain the position of postmigratory dorsal root ganglion neurons, neural crest derivatives for which migration and final position play an important developmental role. We found that an early developing population of sensory neurons maintains the position of later born dorsal root ganglia neurons in an activity-dependent manner. Further, inhibiting or increasing the function of brain-derived neurotrophic factor induces or prevents, respectively, migration of dorsal root ganglia neurons out of the ganglion to locations where they acquire a new identity. Overall, the results demonstrate that neurotrophins mediate non-cell-autonomous maintenance of position and thereby the identity of differentiated neurons.

Mice deficient in brain-derived neurotrophic factor have altered development of gastric vagal sensory innervation

Vagal sensory neurons are dependent on neurotrophins for survival during development. Here, the contribution of brain-derived neurotrophic factor (BDNF) to survival and other aspects of gastric vagal afferent development was investigated. Post-mortem anterograde tracing with 1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbo-cyanine perchlorate (DiI) was used to label selectively vagal projections to the stomach on postnatal days (P) 0, 3, 4, and 6 in wild types and heterozygous or homozygous BDNF mutants. Sampling sites distributed throughout the ventral stomach wall were scanned with a confocal microscope, and vagal axon bundles, single axons, putative mechanoreceptor precursors (intraganglionic laminar endings, IGLEs; intramuscular arrays, IMAs), and efferent terminals were quantified. Also, myenteric neurons, which are innervated by IGLEs, were stained with cuprolinic blue and counted. Quantitative comparisons across wild-type stomach compartments demonstrated that the adult distribution of IMAs was not present at P0 but began to form by P3-6. Among all the quantified elements, at P0, only IGLE density was significantly different in homozygous mutants compared with wild types, exhibiting a 50% reduction. Also, antrum innervation appeared disorganized, and some putative IMA precursors had truncated telodendria. At P3-6, the effect on IGLEs had recovered, the disorganization of antrum innervation had partially recovered, and some IMA telodendria were still truncated. The present results suggest that gastric IGLEs are among the vagal sensory neurons dependent on BDNF for survival or axon guidance. Alternatively, BDNF deficiency may delay gastric IGLE development. Also, BDNF may contribute to IMA differentiation and patterning of antral vagal innervation.

Smooth-muscle-specific expression of neurotrophin-3 in mouse embryonic and neonatal gastrointestinal tract

Vagal gastrointestinal (GI) afferents are essential for the regulation of eating, body weight, and digestion. However, their functional organization and the way that this develops are poorly understood. Neurotrophin-3 (NT-3) is crucial for the survival of vagal sensory neurons and is expressed in the developing GI tract, possibly contributing to their survival and to other aspects of vagal afferent development. The identification of the functions of this peripheral NT-3 thus requires a detailed understanding of the localization and timing of its expression in the developing GI tract. We have studied embryos and neonates expressing the lacZ reporter gene from the NT-3 locus and found that NT-3 is expressed predominantly in the smooth muscle of the outer GI wall of the stomach, intestines, and associated blood vessels and in the stomach lamina propria and esophageal epithelium. NT-3 expression has been detected in the mesenchyme of the GI wall by embryonic day 12.5 (E12.5) and becomes restricted to smooth muscle and lamina propria by E15.5, whereas its expression in blood vessels and esophageal epithelium is first observed at E15.5. Expression in most tissues is maintained at least until postnatal day 4. The lack of colocalization of beta-galactosidase and markers for myenteric ganglion cell types suggests that NT-3 is not expressed in these ganglia. Therefore, NT-3 expression in the GI tract is largely restricted to smooth muscle at ages when vagal axons grow into the GI tract, and when vagal mechanoreceptors form in smooth muscle, consistent with its role in these processes and in vagal sensory neuron survival.

A positive change in energy balance modulates TrkB expression in the hypothalamus and nodose ganglia of rats

Brain-derived neurotrophic factor (BDNF) and its TrkB receptor play critical roles in the synaptic activity and plasticity of mature neurons and enhance adult neurogenesis. Furthermore, treatment with BDNF has been found to attenuate weight gain or even cause weight loss and appetite suppression in rats. The aim of this study was to look at the effect of nutrient intake on BDNF concentrations and cellular proliferation in the brain. Adult male Wistar rats were given one of three diets for 6 weeks: high-carbohydrate, high-fat or high-fat pair-fed diets. Rats were sacrificed at the end of the feeding period and BDNF concentrations in the dorsal vagal complex (DVC), hypothalamus and plasma were measured by ELISA on protein extracts of these samples. Cellular proliferation in the DVC was quantified by Ki-67 immunohistochemistry. Neither BDNF levels nor proliferation were modified by the diet. Secondly, using rats that received the same diets, real-time PCR was performed in the DVC, hypothalamus and nodose ganglia in order to compare TrkB receptor levels. The results showed significantly lower TrkB levels in the hypothalamus and nodose ganglia of fasted rats receiving the high-fat diet when compared to the other groups. These two complementary methodological approaches suggest that there is a relationship between long-term dietary intake and BDNF. More precisely, TrkB expression is more responsive to energy states than to diet composition. An increment in energy stores thus triggers decreased BDNF anorexigenic signaling at the receptor level in the hypothalamus and nodose ganglia, but not in the DVC.

Effects of dietary Na+ deprivation on epithelial Na+ channel (ENaC), BDNF, and TrkB mRNA expression in the rat tongue

BACKGROUND

In rodents, dietary Na+ deprivation reduces gustatory responses of primary taste fibers and central taste neurons to lingual Na+ stimulation. However, in the rat taste bud cells Na+ deprivation increases the number of amiloride sensitive epithelial Na+ channels (ENaC), which are considered as the "receptor" of the Na+ component of salt taste. To explore the mechanisms, the expression of the three ENaC subunits (alpha, beta and gamma) in taste buds were observed from rats fed with diets containing either 0.03% (Na+ deprivation) or 1% (control) NaCl for 15 days, by using in situ hybridization and real-time quantitative RT-PCR (qRT-PCR). Since BDNF/TrkB signaling is involved in the neural innervation of taste buds, the effects of Na+ deprivation on BDNF and its receptor TrkB expression in the rat taste buds were also examined.

RESULTS

In situ hybridization analysis showed that all three ENaC subunit mRNAs were found in the rat fungiform taste buds and lingual epithelia, but in the vallate and foliate taste buds, only alpha ENaC mRNA was easily detected, while beta and gamma ENaC mRNAs were much less than those in the fungiform taste buds. Between control and low Na+ fed animals, the numbers of taste bud cells expressing alpha, beta and gamma ENaC subunits were not significantly different in the fungiform, vallate and foliate taste buds, respectively. Similarly, qRT-PCR also indicated that Na+ deprivation had no effect on any ENaC subunit expression in the three types of taste buds. However, Na+ deprivation reduced BDNF mRNA expression by 50% in the fungiform taste buds, but not in the vallate and foliate taste buds. The expression of TrkB was not different between control and Na+ deprived rats, irrespective of the taste papillae type.

CONCLUSION

The findings demonstrate that dietary Na+ deprivation does not change ENaC mRNA expression in rat taste buds, but reduces BDNF mRNA expression in the fungiform taste buds. Given the roles of BDNF in survival of cells and target innervation, our results suggest that dietary Na+ deprivation might lead to a loss of gustatory innervation in the mouse fungiform taste buds.

Neurotrophin-4/5-depletion induces a delay in maturation of the periodontal Ruffini endings in mice

Neurotrophin-4/5 (NT-4/5) - a member of the neurotrophic factors - is a ligand for TrkB, which has been reported to be expressed in the mechanoreceptive Ruffini endings of the periodontal ligament. The present study examined developmental changes in the terminal morphology and neural density in homozygous mice with a targeted disruption of the nt-4/5 gene and wild-type mice by immunohistochemistry for protein gene product 9.5 (PGP 9.5), a general neuronal marker, and by quantitative analysis using an image analyzer. Postnatal development of terminal Schwann cells was also investigated by enzymatic histochemistry for non-specific cholinesterase activity (ChE). Furthermore, the immuno-expression of TrkB and low affinity nerve growth factor receptor (p75-NGFR) was surveyed in the periodontal Ruffini endings as well as trigeminal ganglion. At postnatal 1 week, the lingual periodontal ligament of both types of mice contained PGP 9.5-positive nerve fibers showing a tree-like ramification with axonal swellings in their course. In both types of mice at 2 weeks of age, comparatively thick nerve fibers with a smooth outline increased in number, and frequently ramified to form nerve terminals with dendritic profiles. However, no typical Ruffini endings with irregular outlines observed in the adult wild-type mice were found in the periodontal ligament at this stage. At postnatal 3 weeks, typical Ruffini endings with irregular outlines were discernable in the periodontal ligament of the wild-type mice while the dendritic endings showing smooth outlines were restricted to the homozygous mice. After postnatal 8 weeks, both types of mice showed an increase in the number of Ruffini endings, but the morphology differed between the wild-type and NT-4/5 homozygous mice. In the wild-type mice, a major population of the Ruffini endings expanded their axonal branches and developed many microprojections, resulting in a reduction of endings with smooth outlines. In contrast, we failed to find such typical Ruffini endings in the periodontal ligament of the homozygous mice: A majority of the periodontal Ruffini endings continued to show smooth outlines at postnatal 12 weeks. Quantitative analysis on neural density demonstrated a reduction in the homozygous mice with a significant difference by postnatal 8 weeks. Enzymatic histochemistry for non-specific ChE did not exhibit a distinct difference in the distribution and density of terminal Schwann cells between wild-type and homozygous mice. Furthermore, TrkB and p75-NGFR mRNA and proteins did not differ in the trigeminal ganglion between the two types. The periodontal Ruffini endings also displayed immunoreactivities for TrkB and p75- NGFR in both phenotypes. These findings suggest that the nt-4/5 gene depletion caused a delay in the formation and maturation of the periodontal Ruffini endings in the mice by inhibiting the growth of the periodontal nerves at an early stage, and indicate that multiple neurotrophins such as NT- 4/5 and BDNF might play roles in the development and/or maturation of the periodontal Ruffini endings.

Lovastatin effect in rat neuroblasts of the CNS: inhibition of cap-dependent translation

Mevalonate biosynthesis pathway is important in cell growth and survival and its blockade by 3-hydroxy-3-methylglutaryl CoA reductase inhibitors, statins, arrest brain neuroblasts growth and induce apoptosis. Translation is among the main biochemical mechanisms that controls gene expression and therefore cell growth or apoptosis. In the CNS, translation regulates synaptic plasticity. Thus, our aim was to investigate the effect of lovastatin in protein translation in rat neuroblasts of the CNS and the biochemical pathways involved. Lovastatin treatment in rat brain neuroblasts causes a significant time- and concentration-inhibition of protein synthesis, which is partially mediated by phosphatydilinositol 3-kinase/mammalian target of rapamycin (mTOR) pathway inhibition. Lovastatin treatment decreases the phosphorylation state of mTOR substrates, p70S6K and eukaryotic translation initiation factor (eIF) 4E-binding protein 1 and simultaneously increases eIF4E-binding protein 1 in a time-dependent manner. Concomitantly, lovastatin causes a decrease in eIF4G cellular amount, which is partially mediated by caspase(s) activity excluding caspase 3. These biochemical pathways affected by lovastatin might explain the protein translation inhibition observed in neuroblasts. Cycloheximide treatment, which blocked protein synthesis, does not induce neuroblasts apoptosis. Therefore, we suggest that lovastatin-induced protein synthesis inhibition might not contribute to the concomitant neuroblasts apoptosis previously observed.

Candesartan but not ramipril pretreatment improves outcome after stroke and stimulates neurotrophin BNDF/TrkB system in rats

OBJECTIVES

Drugs interfering with the renin-angiotensin system (RAS) have been shown to reduce the incidence of stroke in patients at risk and to afford neuroprotection in experimental brain ischemia. This study aimed to compare potential neuroprotective effects of systemic pretreatment with the angiotensin receptor blocker, candesartan, and the angiotensin-converting enzyme (ACE)-inhibitor, ramipril, in normotensive Wistar rats after focal cerebral ischemia, with special emphasis on the regulation of neurotrophins.

METHODS

Equipotent subcutaneous doses of candesartan and ramipril were determined via inhibition of pressor responses to intravenously injected angiotensin II (Ang II) or angiotensin I (Ang I), respectively. Accordingly, animals were treated with candesartan (0.1 mg/kg body weight, twice daily), ramipril (0.01 and 0.1 mg/kg body weight, twice daily) or vehicle (0.9% saline, twice daily), respectively, 5 days prior to middle cerebral artery occlusion (MCAO) with reperfusion. Severity of stroke was estimated via infarct size [magnetic resonance imaging (MRI) 48 h after MCAO] and neurological outcome (24 h, 48 h after MCAO). Measurements of neurotrophins/receptors in brain tissue were performed 48 h after MCAO.

RESULTS

Pretreatment with candesartan and ramipril (low dose) did not reduce blood pressure during MCAO, whereas ramipril high dose did. Candesartan, but not ramipril at any dose, significantly reduced stroke volume and improved neurological outcome. Poststroke mRNA and protein of the neurotrophin receptor, TrkB, were significantly elevated in animals treated with candesartan, but not ramipril.

CONCLUSIONS

Systemic pretreatment with a sub-hypotensive, RAS-blocking dose of candesartan affords neuroprotection after focal ischemia, associated with increased activity of the neurotrophin BDNF/TrkB system. Ramipril at sub-hypotensive and hypotensive, RAS-blocking doses showed no significant neuroprotective effects.

Analysis of gene expression during neurite outgrowth and regeneration

Background

The ability of a neuron to regenerate functional connections after injury is influenced by both its intrinsic state and also by extrinsic cues in its surroundings. Investigations of the transcriptional changes undergone by neurons during in vivo models of injury and regeneration have revealed many transcripts associated with these processes. Because of the complex milieu of interactions in vivo, these results include not only expression changes directly related to regenerative outgrowth and but also unrelated responses to surrounding cells and signals. In vitro models of neurite outgrowth provide a means to study the intrinsic transcriptional patterns of neurite outgrowth in the absence of extensive extrinsic cues from nearby cells and tissues.

Results

We have undertaken a genome-wide study of transcriptional activity in embryonic superior cervical ganglia (SCG) and dorsal root ganglia (DRG) during a time course of neurite outgrowth in vitro. Gene expression observed in these models likely includes both developmental gene expression patterns and regenerative responses to axotomy, which occurs as the result of tissue dissection. Comparison across both models revealed many genes with similar gene expression patterns during neurite outgrowth. These patterns were minimally affected by exposure to the potent inhibitory cue Semaphorin3A, indicating that this extrinsic cue does not exert major effects at the level of nuclear transcription. We also compared our data to several published studies of DRG and SCG gene expression in animal models of regeneration, and found the expression of a large number of genes in common between neurite outgrowth in vitro and regeneration in vivo.

Conclusion

Many gene expression changes undergone by SCG and DRG during in vitro outgrowth are shared between these two tissue types and in common with in vivo regeneration models. This suggests that the genes identified in this in vitro study may represent new candidates worthy of further study for potential roles in the therapeutic regrowth of neuronal connections.

Regulation of neuronal survival and death by extracellular signals during development

Cell death is a prominent feature of the developing vertebrate nervous system, affecting neurons, glial cells and their progenitors. The most extensively studied and best understood phase of cell death occurs in populations of neurons shortly after they begin establishing connections with other neurons and/or non-neural tissues. This phase of cell death makes appropriate adjustments to the relative sizes of interconnected groups of neurons and matches the size of neuronal populations that innervate non-neural tissues to the optimal requirements of these tissues. The fate of neurons during this period of development is regulated by a variety of secreted proteins that either promote survival or bring about cell death after binding to receptors expressed on the neurons. These proteins may be derived from the targets the neurons innervate, the afferents they receive or from associated glial cells, or they may be secreted by the neurons themselves. In this review, I will outline the established and emerging principles that modulate neuronal number in the developing nervous system.