Neurotrophins and extracellular matrix molecules modulate sensory axon outgrowth

Neurotrophins have been known to play a pivotal role in axonal guidance. Recent research has implicated the role of extracelluar matrix molecules in co-ordinating axonal movement. In this study, we examined the influence of neurotrophins (nerve growth factor (NGF) and neurotrophin-3 (NT-3)) and extracellular matrix molecules (laminin, fibronectin, and poly-l-lysin) on sensory neurite outgrowth in thoracic dorsal root ganglia (DRG) dissected from rats at embryonic day 13. Adjacent DRG were embedded in a collagen gel matrix and supplemented with NGF or NT-3. Under NT-3 conditions, DRG axons extended towards each other and intermingled, while neurites from NGF-treated DRG demonstrated a strong repellent effect, resulting in turning responses and growth cone collapse. This effect was not observed on a collagen culture surface. Interestingly, the composition of the extracellular matrix strongly influenced the observed repellent effect. Sensory neurites from NGF-stimulated DRG again demonstrated a repellent effect when plated on a laminin surface, but showed intermingling behavior when plated on poly-l-lysin or fibronectin. This observation suggests that a factor secreted by NGF-treated DRG axons interacts with laminin, enabling repulsion. This factor and its interaction with the extracellular matrix play an important role in the mechanism of sensory axonal pathfinding.

Expression and function of neurotrophins and their receptors in cultured human keratinocytes

Whereas nerve growth factor has been extensively studied in human keratinocytes, little is known on the role of other members of the neurotrophin family. We investigated the expression and function of neurotrophins and neurotrophin receptors in cultured human keratinocytes. We demonstrated by reverse transcription-polymerase chain reaction that keratinocytes synthesize neurotrophin-3, brain-derived neurotrophic factor, and neurotrophin-4/5. These cells also express tyrosinase kinase A and C, the nerve growth factor and neuro-trophin-3 high-affinity receptors, respectively. On the other hand, only the truncated extracellular isoform of tyrosinase kinase B, the high-affinity brain-derived neurotrophic factor and neurotrophin-4/5 receptor, is detected in keratinocytes. Moreover, neurotrophin-3, brain-derived neurotrophic factor, and neurotrophin-4/5 proteins are secreted by human keratinocytes at low levels. Keratinocyte stem cells synthesize the highest amounts of nerve growth factor, while they secrete higher levels of nerve growth factor as compared with transit amplifying cells. Neurotrophin-3 stimulates keratinocyte proliferation, where brain-derived neurotrophic factor or neurotrophin-4/5 does not exert any effect on keratinocyte proliferation. Addition of neurotrophin-3 slightly upregulates the secretion of nerve growth factor, whereas nerve growth factor strongly augments neurotrophin-3 release. Ultraviolet B irradiation downregulates nerve growth factor, whereas it augments neurotrophin-3 and neurotrophin-4/5 protein levels. Ultraviolet A irradiation increases the level of neurotrophin-3, whereas it does not exert any effect on the other neurotrophins. Finally, neurotrophins other than nerve growth factor fail to protect human keratinocytes from ultraviolet B-induced apoptosis. This work delineates a functional neurotrophin network, which may contribute to epidermal homeostasis.

Neural crest stem cell and cardiac endothelium defects in the TrkC null mouse

TrkC null mice have multiple cardiac malformations. Since neural crest cells participate in cardiac outflow tract septation, the aim of this study was to determine at the cellular level the putative neural crest defect. We have identified three types of progenitor cells: stem cells that undergo self-renewal and can generate many cell types, cells that are restricted in their developmental potentials, and cells that are committed to the smooth muscle cell lineage. In TrkC null mice, there is a greater than 50% decrease in stem cell numbers and an equivalent increase in fate-restricted cells. The outflow tract wall is thickened and the endothelial tube is disorganized. We conclude that deletion of the TrkC gene causes precocious fate restrictions of the neural crest stem cell and a defect of the outflow tract endothelium, both of which may contribute to the outflow tract malformations that occur in TrkC null mice.

[The neurotrophic effect of endogenous NT-3 from adult cat spared dorsal root ganglion on ganglionic neurons]

OBJECTIVE

To investigate the neurotrophic effect of endogenous NT-3 from adult cat dorsal root ganglion (DRG) on ganglionic neurons.

METHODS

Rhizotomy of bilateral L1, L3, L5 and L7 dorsal roots of cats was performed, leaving L2, L4 and L6 DRG as spared DRGs. The separate neurons of normal (control) DRG, spared DRG and anti-NT-3 antibody blocking DRG were cultured in vitro respectively. The number of survival neurons and the length of neurites were measured and used for comparison in the control, spared DRG, and block groups.

RESULTS

There were survival neurons and cell clusters in every group. The number of survival neurons and cell clusters of spared DRG group were much larger than those of the control and block groups. The neurite length of neurons, the neurite number and the length of cell clusters of spared DRG group were much greater than those of control and block groups.

CONCLUSION

Endogenous NT-3 from spared DRG may act on ganglionic neurons to maintain survival of neuron and stimulate growth of neurite.

Chronic exposure to ethanol alters neurotrophin content in the basal forebrain-cortex system in the mature rat: Effects on autocrine-paracrine mechanisms

Neurotrophins are broadly expressed in the mammalian forebrain: notably in cerebral cortex and the basal forebrain (e.g., the septal and basal nuclei). These factors promote neuronal survival and plasticity, and have been implicated as key players in learning and memory. Chronic exposure to ethanol causes learning and memory deficits. We tested the hypothesis that ethanol affects neurotrophin expression and predicted that these changes would be consistent with alterations in retrograde or autocrine/paracrine systems. Mature rats were fed a liquid diet containing ethanol daily for 8 or 24 weeks. Weight-matched controls were pair-fed an isocaloric, isonutritive diet. Proteins from five structures (parietal and entorhinal cortices, hippocampus, and the basal and septal nuclei) were studied. ELISAs were used to determine the concentration of nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3). All three neurotrophins were detected in each structure examined. Ethanol treatment significantly (p < 0.05) affected neurotrophin expression in time- and space-dependent manners. NGF content was generally depressed by ethanol exposure, whereas NT-3 content increased. BDNF concentration was differentially affected by ethanol: it increased in the parietal cortex and the basal forebrain and decreased in the hippocampus. With the exception of NGF in the septohippocampal system, the ethanol-induced changes in connected structures were inconsistent with changes that would be predicted from a retrograde model. Thus, the present data (a) support the concept that neurotrophins act through a nonretrograde system (i.e., a local autocrine/paracrine system), and (b) that chronic exposure to ethanol disrupts these regulatory mechanisms.

Changes in retinal expression of neurotrophins and neurotrophin receptors induced by ocular hypertension

Open angle glaucoma is defined as a progressive and time-dependent death of retinal ganglion cells concomitant with high intraocular pressure, leading to loss of visual field. Because neurotrophins are a family of growth factors that support neuronal survival, we hypothesized that quantitative and qualitative changes in neurotrophins or their receptors may take place early in ocular hypertension, preceding extensive cell death and clinical features of glaucoma. We present molecular, biochemical, and phenotypic evidence that significant neurotrophic changes occur in retina, which correlate temporally with retinal ganglion cell death. After 7 days of ocular hypertension there is a transient up-regulation of retinal NGF, while its receptor TrkA is up-regulated in a sustained fashion in retinal neurons. After 28 days of ocular hypertension there is sustained up-regulation of retinal BDNF, but its receptor TrkB remains unchanged. Throughout, NT-3 levels remain unchanged but there is an early and sustained increase of its receptor TrkC in Müller cells but not in retinal ganglion cells. These newly synthesized glial TrkC receptors are truncated, kinase-dead isoforms. Expression of retinal p75 also increases late at day 28. Asymmetric up-regulation of neurotrophins and neurotrophin receptors may preclude efficient neurotrophic rescue of RGCs from apoptosis. A possible rationale for therapeutic intervention with Trk receptor agonists and p75 receptor antagonists is proposed.

Implantation of dendritic cells in injured adult spinal cord results in activation of endogenous neural stem/progenitor cells leading to de novo neurogenesis and functional recovery

We report a treatment for spinal cord injury involving implantation of dendritic cells (DCs), which act as antigen-presenting cells in the immune system. The novel mechanisms underlying this treatment produce functional recovery. Among the immune cells tested, DCs showed the strongest activity inducing proliferation and survival of neural stem/progenitor cells (NSPCs) in vitro. Furthermore, in DC-implanted adult mice, endogenous NSPCs in the injured spinal cord were activated for mitotic de novo neurogenesis. These DCs produced neurotrophin-3 and activated endogenous microglia in the injured spinal cord. Behavioral analysis revealed the locomotor functions of DC-implanted mice to have recovered significantly as compared to those of control mice. Our results suggest that DC-implantation exerts trophic effects, including activation of endogenous NSPCs, leading to repair of the injured adult spinal cord.

Oleic acid induces GAP-43 expression through a protein kinase C-mediated mechanism that is independent of NGF but synergistic with NT-3 and NT-4/5

We have recently shown that the presence of albumin in astrocytes triggers the synthesis and release of oleic acid, which behaves as a neurotrophic factor for neurons. Thus, oleic acid promotes axonal growth, neuronal clustering, and the expression of the axonal growth-associated protein, GAP-43. In this work we show that oleic acid upregulates GAP-43 expression by a protein kinase C (PKC)-dependent mechanism. Since GAP-43 expression has been shown to be upregulated by several neurotrophins, we investigated the relationship between the effect of oleic acid and that of NGF, neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5) on GAP-43 expression. Our results indicate that NGF is not involved in the neurotrophic effect of oleic acid because the addition of NGF did not modify the effect of oleic acid on GAP-43 expression. Neither NT-3 nor NT-4/5 alone modified GAP-43 expression. However, NT-3 and NT-4/5 acted synergistically with oleic acid to increase GAP-43 expression. The lack of effect of NGF as compared to other neurotrophins is not unexpected since we have not found TrkA expression under our experimental conditions. The effect of oleic acid on GAP-43 expression must be independent of autocrine factors synthesized by neurons because this effect was also observed at low cellular densities. In conclusion, our results indicate that oleic acid behaves as a neurotrophic factor, inducing GAP-43 expression through a PKC-mediated mechanism that is not mediated by other neurotrophic factors but that is strongly synergized by NT-3 and NT-4/5.

Changes in spinal GDNF, BDNF, and NT-3 expression after transient spinal cord ischemia in the rat

Previous studies have demonstrated that the expression of several growth factors including glial cell-derived neurotrophic factor (GDNF), brain-derived growth factor (BDNF), and neurotrophin-3 (NT-3) play an important role in defining neuronal survival after brain ischemia. In the present study, using a well-defined model of transient spinal ischemia in rat, we characterized the changes in spinal GDNF, BDNF, and NT-3 expression as defined by enzyme-linked immunosorbent assay (ELISA) and immunofluorescence coupled with deconvolution microscopy. In control animals, baseline levels of GDNF, BDNF, and NT-3 (74 +/- 22, 3,600 +/- 270, 593 +/- 176 pg/g tissue, respectively) were measured. In the ischemic group, 6 min of spinal ischemia resulted in a biphasic response with increases in tissue GDNF and BDNF concentrations at the 2-hr and 72-hr points after ischemia. No significant differences in NT-3 concentration were detected. Deconvolution analysis revealed that the initial increase in tissue GDNF concentration corresponded to a neuronal upregulation whereas the late peak seen at 72 hr corresponded with increased astrocyte-derived GDNF synthesis. Increased expression of BDNF was seen in neurons, astrocytes, and oligodendrocytes. These data suggest that the early increase in neuronal GDNF/BDNF expression likely modulates neuronal resistance/recovery during the initial period of postischemic reflow. Increased astrocyte-derived BDNF/GDNF expression corresponds with transient activation of astrocytes and may play an active role in neuronal plasticity after non-injurious intervals of spinal ischemia.

Secretion of intrinsic cues controls repulsion of nociceptive neurons

The longstanding question of how the pattern of skin sensory innervation arises led us to investigate the behavior of rat DRG sensory axonal outgrowth. Outgrowing neurites from NGF-stimulated DRGs placed in close vicinity demonstrated repulsive behavior in the form of turning responses. In contrast, NT3-dependent neurites intermingled, as did neurites cultured without collagen embedding. These observations raise the possibility that secretion and not contact repulsion is the dermatome-building mechanism of nociceptive territories. Further experiments with functional antibodies against known secreted guidance molecules had no blocking effect. Our data provide evidence that the segmented pattern of skin nociceptive sensory maps is supported by unknown intrinsic cues released from the sensory axons themselves.

Reverse transcription-PCR analysis of laser-captured cells points to potential paracrine and autocrine actions of neurotrophins in pancreatic cancer

PURPOSE

Neurotrophins (NTs) can stimulate cell proliferation and differentiation in various cell types, and play a role in certain human cancers. In this study we analyzed the expression and localization of NTs and their receptors in microdissected pancreatic cancer tissue samples, and studied their ability to stimulate cell growth.

EXPERIMENTAL DESIGN

The expression of nerve growth factor, brain-derived neurotrophic factor, NT-3, and NT-4/5, and the receptors tropomyosin receptor kinase A, B, and C, and P75 was studied in pancreatic cancer cell lines, and normal and pancreatic ductal adenocarcinoma (PDAC) tissue samples by quantitative reverse transcription-PCR. Laser capture microdissection was performed in 21 PDAC samples, and mRNA levels were determined in cancer cells, acinar cells, desmoplastic stroma areas, and nerve fibers. Cell growth assays with NTs and in coculture with dorsal root ganglia were performed.

RESULTS

NT receptors were differentially expressed in the cancer cell lines, whereas tropomyosin receptor kinase C was not detectable. NTs modulated pancreatic cancer cell growth. Analysis of nonmicrodissected samples revealed that all of the receptors and tested ligands were overexpressed in PDAC when compared with normal pancreas. Analysis of laser captured samples revealed that NTs and their receptors were expressed in the cancer cells but were especially abundant in the intratumoral nerves. Coculture of dorsal root ganglia with T3M4 cells significantly enhanced the proliferation of this cell line.

CONCLUSION

The abundance of NTs in the intratumoral nerves in PDAC and the presence of NT receptors in the cancer cells, in conjunction with the ability of NTs to modulate pancreatic cancer cell growth, point to potential paracrine and autocrine effects of NTs in PDAC. Thus, our findings provide additional evidence that blocking NT actions may have a therapeutic potential in PDAC.

Oestrogen regulates sympathetic neurite outgrowth by modulating brain derived neurotrophic factor synthesis and release by the rodent uterus

Sympathetic innervation of the adult rodent uterus undergoes cyclic remodelling. Terminal sympathetic axons degenerate when oestrogen levels rise and regenerate when oestrogen levels decline. This study examined the role of neurotrophins in oestrogen-mediated uterine sympathetic nerve remodelling. Oestrogen injection of ovariectomized female rats did not affect uterine NT-3 levels 24 h postinjection, and increased endometrial NGF protein, indicating that reduced NGF or NT-3 is not responsible for the oestrogen-induced denervation. Oestrogen also raised BDNF protein and mRNA in myometrium and endometrium. To assess whether increased BDNF affects uterine receptivity to sympathetic outgrowth, sympathetic ganglion explants were co-cultured with myometrium. Myometrium from ovariectomized rats induced neuritogenesis in oestrogen-free conditions, and this was abolished when BDNF was added to the medium. Neuritogenesis induced by ovariectomized myometrium was suppressed by oestrogen, and restored by a BDNF function-blocking antibody. To determine if target BDNF synthesis is required for oestrogen to suppress sympathetic neurite outgrowth, uteri from wild-type mice and mice homozygous or heterozygous for recombinant mutations of the BDNF gene were cultured with rat sympathetic ganglia. Neuritogenesis induced by wild-type uteri was diminished by oestrogen. Neurite formation in the presence of homozygous BDNF mutant uteri was not affected by oestrogen, but was lower than that of wild-type mice. Uteri from mice heterozygous for the BDNF mutation, who have reduced BDNF synthesis, showed normal neuritogenic properties, but were not affected by oestrogen. These findings suggest that oestrogen alters neuritogenic properties of the rodent uterus by regulating BDNF synthesis, which inhibits sympathetic neurite outgrowth.

The outgrowth response of the axons of developing and regenerating rat retinal ganglion cells in vitro to neurotrophin treatment

BDNF and NT-4 (but not NT-3 or CNTF) significantly enhanced the outgrowth of early embryonic and adult regenerating RGC axons when provided with a supportive substrate in vitro. BDNF and NT-4 treatment transiently increased RGC axon outgrowth from E15 rat retinas but not from retinas at older embryonic ages. The transient effect of BDNF and NT-4 and the inability of the neurotrophins to promote outgrowth from older embryonic retinal explants suggests a time frame of neurotrophin action and that other chemical factors (target-derived or otherwise) may be necessary for the continued maintenance of developing RGC axons. BDNF and NT-4 also enhanced the outgrowth of regenerating axons from adult retinal explants, but appeared to have a more subtle effect on axon outgrowth, in that the growth-promoting effects of BDNF and NT-4 appeared continuous throughout the incubation period. The suppression of RGC axon outgrowth from embryonic and adult retinae cultured in trkB-IgG-containing medium suggests that the response of developing and regenerating axons, to BDNF and NT-4 are likely to occur through trkB signalling.

Early postnatal corticosterone administration regulates neurotrophins and their receptors in septum and hippocampus of the rat

The principal glucocorticoid in rats, corticosterone, interacts with neurons in the limbic system and leads to morphological and behavioral changes. Putative corticosterone-triggered mediators are neurotrophins. In the present study we investigated the effects of early postnatal corticosterone treatment in rats on neurotrophic factors of the nerve growth factor (NGF) family and their receptors. Newborn rats were treated with corticosterone-containing polymers until postnatal day 12. The mRNA and protein levels of the neurotrophins of the NGF family (NGF, BDNF, NT-3 and NT-4/5) and their receptors (trkA, trkB, trkC and p75) were quantified in septum and hippocampus using RT-PCR. In the septal region, we found an unchanged mRNA expression after corticosterone treatment, whereas in the hippocampus there was a general increase in mRNA. Particularly, the gene expression of NGF, NT-3, and the high affinity receptors trkA, trkB and trkC increased significantly. Quantification of the neurotrophin protein levels using an ELISA revealed significant treatment effects for NGF and NT-4/5 in the hippocampus. The present study of corticosterone treatment in young rats demonstrates interactions of steroid hormones with neurotrophic factors and their receptors in the septo-hippocampal system during the first two postnatal weeks.

Hippocampal neurotrophin levels in a kainate model of temporal lobe epilepsy: a lack of correlation between brain-derived neurotrophic factor content and progression of aberrant dentate mossy fiber sprouting

A significant upregulation of neurotrophins particularly brain-derived neurotrophic factor (BDNF) is believed to be involved in the initiation of epileptogenic changes such as the aberrant axonal sprouting and synaptic reorganization in the injured hippocampus. However, it is unknown which of the neurotrophins are upregulated during the peak period of aberrant mossy fiber sprouting in the chronically injured hippocampus. We measured chronic changes in the levels of BDNF, nerve growth factor (NGF) and neurotrophin-3 (NT-3) in the adult hippocampus using enzyme-linked immunosorbent assay (ELISA) after a unilateral intracerebroventricular administration of kainic acid (KA), a model of temporal lobe epilepsy. For comparison, neurotrophins were also measured from the control intact hippocampus. Further, to see the association between changes in neurotrophin levels and the progression of mossy fiber sprouting, chronic changes in the mossy fiber distribution within the dentate supragranular layer (DSGL) were quantified. In the KA-lesioned hippocampus, the neurotrophins BDNF and NGF were upregulated at 4 days post-lesion, in comparison to their levels in the intact hippocampus. However, the concentration of BDNF reached the baseline level at 45 days post-lesion and dramatically diminished at 120 days post-lesion. In contrast, the upregulation of NGF observed at 4 days post-lesion was sustained at both 45 days and 120 days post-lesion. The concentration of NT-3 was upregulated at 45 days post-lesion but remained comparable to baseline levels at 4 days and 120 days post-lesion. Interestingly, analysis of mossy fiber sprouting revealed that most of the aberrant sprouting in the lesioned hippocampus occurs between 45 days and 120 days post-lesion. Taken together, these results suggest that the period of robust mossy fiber sprouting does not correlate with the phase of post-lesion BDNF upregulation. Rather, it shows a relationship with the time of upregulation of neurotrophins NGF and NT-3.

Voluntary exercise increases neurotrophin-3 and its receptor TrkC in the spinal cord

We have evaluated changes in the expression of neurotrophin-3 (NT-3) and its tyrosine kinase C (TrkC) receptor in the neuromuscular system as a result of voluntary physical activity. We assessed changes in the mRNAs and proteins for NT-3 and TrkC in the lumbar spinal cord and associated soleus muscle following 3 and 7 days of voluntary wheel running. We used quantitative Taqman RT-PCR to measure mRNA and ELISA to assess protein levels. NT-3 mRNA and protein levels increased in the spinal cord to reach statistical significance after 7 days of exercise compared to sedentary control rats. Immunohistochemical analyses localized the elevated NT-3 to the substantia gelatinosa (SG) and nucleus of the dorsal horn. TrkC mRNA levels were significantly elevated in the spinal cord after 3 and 7 days of running. In the soleus muscle, NT-3 mRNA levels and its receptor TrkC were elevated after 3 days, while NT-3 protein levels remained unaffected. The results demonstrate that voluntary exercise has a differential effect on NT-3 as well as its receptor TrkC in the neural and muscular components of the neuromuscular system, and emphasize the role of voluntary activity on the spinal cord and muscle.

Lack of Bdnf and TrkB signalling in the postnatal cochlea leads to a spatial reshaping of innervation along the tonotopic axis and hearing loss

Members of the neurotrophin gene family and their high-affinity Trk receptors control innervation of the cochlea during embryonic development. Lack of neurotrophin signalling in the cochlea has been well documented for early postnatal animals, resulting in a loss of cochlear sensory neurones and a region-specific reduction of target innervation along the tonotopic axis. However, how reduced neurotrophin signalling affects the innervation of the mature cochlea is currently unknown. Here, we have analysed the consequences of a lack of the TrkB receptor and its ligand, the neurotrophin brain-derived neurotrophic factor (Bdnf), in the late postnatal or adult cochlea using mouse mutants. During early postnatal development, mutant animals show a lack of afferent innervation of outer hair cells in the apical part of the cochlea, whereas nerve fibres in the basal part are maintained. Strikingly, this phenotype is reversed during subsequent maturation of the cochlea, which results in a normal pattern of outer hair cell innervation in the apex and loss of nerve fibres at the base in adult mutants. Measurements of auditory brain stem responses of these mice revealed a significant hearing loss. The observed innervation patterns correlate with opposing gradients of Bdnf and Nt3 expression in cochlear neurones along the tonotopic axis. Thus, the reshaping of innervation may be controlled by autocrine signalling between neurotrophins and their receptors in cochlear neurones. Our results indicate a substantial potential for re-innervation processes in the mature cochlea, which may also be of relevance for treatment of hearing loss in humans.

Chronic neurotrophin-3 strengthens synaptic connections to motoneurons in the neonatal rat

We report that neurotrophin-3 (NT-3), delivered chronically via fibroblasts implanted intrathecally into neonatal rats, can facilitate synaptic transmission in the spinal cord. A small collagen plug containing NT-3-secreting fibroblasts was placed on the exposed dorsal surface of the spinal cord (L1) of 2-d-old rats; controls received beta-galactosidase-secreting fibroblasts. After 6 hr to 12 d of survival, synaptic potentials (EPSP) elicited by two synaptic inputs, L5 dorsal root and ventrolateral funiculus (VLF), were recorded intracellularly in L5 motoneurons in vitro. Preparations treated with NT-3 implants exhibited enhanced monosynaptic synaptic transmission from both inputs, which persisted over the entire testing period. Unlike acute enhancement of transmission by NT-3 (Arvanian and Mendell, 2001a), the chronic effect could occur at connections not normally eliciting an NMDA receptor-mediated response at the time of NT-3 exposure. Using susceptibility to blockade of the NMDA receptor by Mg2+ and APV, we confirmed that chronic treatment with NT-3 did not enhance NMDA receptor activity at these connections. Cords treated with chronic NT-3 also transiently displayed polysynaptic components activated by VLF that were blocked by the NMDA receptor antagonist APV. These novel NMDA receptor-mediated potentials may reflect changes in interneurons near the site of fibroblast implantation. We conclude that chronic NT-3 enhances the potency of segmental and descending projections via mechanisms different from those underlying acute changes.

Possible involvement of induction of brain-derived neurotrophic factor in the neuroprotective effect of a 5-phenylpyrimidine derivative

When primary cortical neurons prepared from the brains of rat embryos (E18) were cultured in the absence of serum, most of the neurons died after 3 days in vitro. We used this model to discover compounds which support neuronal survival, and found that a new 5-phenylpyrimidine derivative named FU248 (2-amino-5-(2,4-dichlorophenyl) pyrimidine) inhibited the neuronal cell death in a dose-dependent manner up to 1 microg/mL. Semiquantitative RT-PCR analysis revealed that an exposure of the primary cortical neurons to 1 microg/mL of FU248 transiently and significantly enhanced the expression of genes including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), and neurotrophin-3 (NT-3). The enhancement of the gene expression was maximal 6 hr after the addition of FU248, and the expression returned to the basal level after 24 hr. Expression of neurotrophin-4 was not detectable throughout the experimental period. The amount of the transcript for BDNF was approximately nine times and sixteen times more abundant than those for NT-3 and NGF, respectively (t=6 hr). Moreover, an anti-BDNF antibody suppressed the effect of FU248, whereas the control antibody did not show any effects on the neuronal survival. These findings strongly suggest that FU248 exerts its neuroprotective effect, at least in part, through induction of BDNF.

Functional expression of the TrkC gene, encoding a high affinity receptor for NT-3, in antigen-specific T helper type 2 (Th2) cells

Neurotrophins, including nerve growth factor, brain-derived neurotrophic factor and neurotrophin-3 (NT-3) are essential factors for the development of the nervous system. In this report, we demonstrate gene expression of neurotrophins and their receptors in T helper 1 (Th1) and T helper 2 (Th2) cells induced from naïve CD4+ CD45RB+ T cells of ovalbumin-specific DO11.10 T cell receptor transgenic mice. Interestingly, the TrkC gene, which encodes a high affinity receptor for NT-3, was expressed in Th2 cells, but not in Th1 and naïve CD4+ T cells. Expression of the TrkC gene was markedly augmented by addition of anti-IFN-gamma monoclonal antibody (mAb) into the culture, whereas it was blocked by anti-IL-4 mAb. Moreover, NT-3 synergistically enhanced anti-CD3 mAb-induced IL-4 production by Th2 cells, but did not affect IFN-gamma production by Th1 cells. These data suggest that NT-3, through its receptor TrkC, plays a critical role in regulating the Th1/Th2 balance.

Exercise-induced changes in hippocampal brain-derived neurotrophic factor and neurotrophin-3: effects of rat strain

We tested the hypothesis that exercise-induced changes in hippocampal brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) differ among rat strains exhibiting a range of voluntary wheel running activity. Four strains (Sprague-Dawley, Brown Norway, Dark Agouti and PVG) were given access to running wheels (1 or 7 nights). Over 7 nights, the average distance run per night was higher in PVG versus other strains, and higher in Brown Norway versus Sprague-Dawley rats. Hippocampal BDNF concentrations in sedentary rats were higher in PVG versus Sprague-Dawley rats. When data from all strains were combined, BDNF levels increased with 7 nights of wheel running and were positively correlated to the previous night distance run. Sedentary hippocampal NT-3 levels were not different between rat strains, but decreased with 7 nights of wheel access; NT-3 was negatively correlated with previous night distance run. There were no differences between strains in the correlation between distance run and BDNF or NT-3 levels. Although exercise decreases hippocampal NT-3, strain does not alter NT-3 levels. In contrast, BDNF levels increase with exercise and basal levels differ between strains, possibly due to strain differences in spontaneous activity.

Neurotrophin-3 prevents the proximal accumulation of neurofilament proteins in sensory neurons of streptozocin-induced diabetic rats

The relation between neurofilament expression and/or phosphorylation in the proximal versus distal components of the sensory peripheral neuraxis was studied and related to disorders in structure and function of the distal axon of streptozocin (STZ)-induced diabetic rats studied for 14 weeks. The ability of neurotrophin-3 (NT-3) to prevent abnormalities in neurofilament biology was also investigated. Compared with age-matched controls, neurofilament heavy (NF-H) (3.3-fold) and neurofilament medium (NF-M) (2.5-fold), but not neurofilament light (NF-L), subunits accumulated in the proximal axon of sensory neurons of the lumbar dorsal root ganglia (DRG) in untreated diabetic rats. Neurofilament accumulation was prevented by NT-3. Small- and large-diameter sensory neurons exhibited elevated levels of NF-H protein accumulation and phosphorylation in the DRG of untreated diabetic rats, levels that were ameliorated by NT-3. The sural nerve of untreated diabetic rats showed a 50% decrease in the levels of NF-H and NF-M, but not NF-L, subunits; NT-3 only partially normalized the defect in NF-M expression. These observations were associated with significant lowering of motor and sensory nerve conduction velocity but no alteration in the mean axonal diameter of myelinated axons in the sural nerve in untreated diabetic rats. It is proposed that the accumulation of NF-H and NF-M subunits in the proximal axon is an etiologic factor in the distal axon degeneration observed in diabetes.

Neurotrophin-3 down-regulates trkA mRNA, NGF high-affinity binding sites, and associated phenotype in adult DRG neurons

Neurotrophin-3 (NT-3) binds to multiple trks, not only its initially identified receptor trkC. Recent studies in our laboratory show that NT-3 negatively regulates nociceptive phenotype associated with the trkA subpopulation. Due to the extensive overlap in trkA and trkC expression it is uncertain whether there is a direct influence of NT-3 on trkA in adult sensory neurons. Thus, the aim of this study was to examine whether NT-3 might alter trkA and associated neuronal phenotype outside of the trkC subpopulation. The effect of a seven-day intrathecal infusion of NT-3 on intact, uninjured adult rat dorsal root ganglion neurons was investigated. Serial sections were processed for receptor radioautography or in situ hybridization to identify and colocalize neurons expressing high-affinity nerve growth factor (NGF) binding sites, substance P (SP), trkC, or trkA mRNAs and to examine the influence of NT-3 on these populations. NT-3 does not appear to alter trkC expression, but evokes a notable reduction in trkA, high-affinity NGF binding sites, and SP levels. It is unlikely that signalling by trkC greatly influences this response because the down-regulation of SP occurs most notably in trkA neurons that lack trkC. Moreover, we have shown here that message levels of two trkA isoforms are differentially modulated by NT-3; infusion results in greater down-regulation of the noninsert containing isoform. These findings suggest a clinically relevant role for NT-3 as an antagonist to NGF, but also raise the caution that not just trkC-positive neurons are influenced following exposure to the neurotrophin.

Neurotrophin-3 and TrkC in the frog visual system: changes after axotomy

Neurotrophins are potent regulators of the survival of different neuronal populations in the CNS. Little is known of the immunodistribution of neurotrophin-3 (NT-3) and tyrosine kinase C (TrkC) receptor in the frog visual system, which can successfully regenerate and recover vision after injury. In this study we show that both NT-3 and TrkC are present in the frog retina and tectum, and that their distribution changes after optic nerve transection. Both NT-3 and TrkC are present in the ganglion cell layer, inner nuclear layer, nerve fiber layer and outer plexiform layer, and in Müller cells of control retinas. Quantification of identified RGCs shows that there are only small changes in the proportion, or intensity, of NT-3 immunostained cells surviving after axotomy and regeneration. Müller cell staining, however, is increased. TrkC staining in the retina does not change after axotomy. In the tectum, NT-3 immunoreactivity is present in the retinorecipient layer 9, and in radial processes of neurons and ependymoglia. TrkC is present in ependymoglia and in tectal neurons. After axotomy or colchicine treatment fewer NT-3-immunoreactive processes are present in layer 9 and there is decreased staining of tectal neurons. These data are consistent with the hypothesis that NT-3 is synthesized in the retina and anterogradely transported to the tectum. TrkC immunostaining, on the other hand, increases in tectal cells after optic nerve transection, suggesting that it may be regulated by the supply of NT-3 from the retina.

Embryonic geniculate ganglion neurons in culture have neurotrophin-specific electrophysiological properties

Geniculate ganglion neurons provide a major source of innervation to mammalian taste organs, including taste buds in the soft palate and in fungiform papillae on the anterior two thirds of the tongue. In and around the fungiform papillae, before taste buds form, neurotrophin mRNAs are expressed in selective spatial and temporal patterns. We hypothesized that neurotrophins would affect electrophysiological properties in embryonic geniculate neurons. Ganglia were explanted from rats at gestational day 16, when growing neurites have entered the papilla core, and maintained in culture with added brain-derived neurotrophic factor (BDNF), neurotrophin 4 (NT4), nerve growth factor (NGF) or neurotrophin 3 (NT3). Neuron survival with BDNF or NT4 was about 80%, whereas with NGF or NT3 less than 15% of neurons survived over 6 days in culture. Whole cell recordings from neurons in ganglion explants with each neurotrophin condition demonstrated distinctive neurophysiological properties related to specific neurotrophins. Geniculate neurons cultured with either BDNF or NT4 had similar passive-membrane and action potential properties, but these characteristics were significantly different from those of neurons cultured with NGF or NT3. NGF-maintained neurons had features of increased excitability including a higher resting membrane potential and a lower current threshold for the action potential. About 70% of neurons produced repetitive action potentials at threshold. Furthermore, compared with neurons cultured with other neurotrophins, a decreased proportion had an inflection on the falling phase of the action potential. NT3-maintained neurons had action potentials that were of relatively large amplitude and short duration, with steep rising and falling slopes. In addition, about 20% responded with a repetitive train of action potentials at threshold. In contrast, with BDNF or NT4 repetitive action potential trains were not observed. The data demonstrate different neurophysiological properties in developing geniculate ganglion neurons maintained with specific neurotrophins. Therefore, we suggest that neurotrophins might influence acquisition of distinctive neurophysiological properties in embryonic geniculate neurons that are fundamental to the formation of peripheral taste circuits and a functioning taste system.