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.

The neurotrophins nerve growth factor, brain-derived neurotrophic factor, neurotrophin-3, and neurotrophin-4 are survival and activation factors for eosinophils in patients with allergic bronchial asthma

Neurotrophins (nerve growth factor [NGF], brain-derived neurotrophic factor [BDNF], neurotrophin [NT]-3, and NT-4) have been observed in elevated concentrations in allergic diseases. Neurotrophin levels are up-regulated endobronchially after allergen challenge. This coincides with an influx of activated eosinophils into the bronchial lumen. These eosinophils have an increased viability and CD69 expression 18 h after segmental allergen provocation (SAP) which is not present in peripheral blood. To investigate whether these observations are related we studied the influence of neurotrophins on eosinophil function in allergic asthma. Incubation with NGF, BDNF, NT-3, or NT-4 caused a significant increase in the viability and CD69 expression of isolated eosinophils from bronchoalveolar lavage fluid (BALF) but not from peripheral blood, suggesting a unique sensitivity of endobronchial eosinophils to neurotrophins. To elucidate the underlying mechanisms expression of the neurotrophin receptors p75NTR, trkA, trkB, and trkC on eosinophils was analyzed by RT-PCR and immunocytology. After SAP expression of all neurotrophin receptors was markedly elevated on eosinophils from BALF. Our findings suggest that neurotrophin-mediated activation of bronchial eosinophils might play a role in the regulation of eosinophilic inflammation in allergic asthma.

Integrins regulate neuronal neurotrophin gene expression through effects on voltage-sensitive calcium channels

Integrin adhesion receptors regulate gene expression during growth and differentiation in various cell types. Recent work, implicating integrins in functional synaptic plasticity, suggest they may have similar activities in adult brain. The present study tested if integrins binding the arginine-glycine-aspartate (RGD) matrix sequence regulate neurotrophin and neurotrophin receptor gene expression in cultured hippocampal slices. The soluble RGD-containing peptide glycine-arginine-glycine-aspartate-serine-proline (GRGDSP) increased neurotrophin mRNA levels in transcript- and subfield-specific fashions. Integrin ligand effects were greatest for brain-derived neurotrophic factor transcripts I and II and barely detectable for transcript III. In accordance with increased nerve growth factor mRNA levels, GRGDSP increased c-fos expression as well. In contrast, growth-associated protein-43, amyloid precursor protein and fibroblast growth factor-1 mRNAs were not elevated. Ligand effects on brain-derived neurotrophic factor transcript II and c-fos mRNA did not depend on the integrity of the actin cytoskeleton, neuronal activity, or various signaling pathways but were blocked by L-type voltage-sensitive calcium-channel blockers. These results indicate that in mature hippocampal neurons integrin engagement regulates expression of a subset of growth-related genes at least in part through effects on calcium influx. Accordingly, these synaptic adhesion receptors may play the same role in maintaining an adult, differentiated state in brain as they do in other tissues and changes in integrin activation and/or engagement may contribute to dynamic changes in neurotrophin expression and to neuronal calcium signaling.

Neurotrophin-like immunoreactivity in the gut of teleost species

By means of immunochemistry and immunohistochemistry, we investigated in the gut of teleostean species the presence and localization of three neurotrophins: nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and neurotrophin (NT)-3. In all studied species both NGF- and NT-3-like immunoreactivity (IR) were present in the enteric nervous system, while BDNF-like IR was never detected. More in particular, both NGF and NT-3-like IR were detected in neurons of small and large intestine, while only NT3-like IR was also observed in stomach plexuses. Furthermore, Western blot analysis revealed the presence of molecules immunoreactive to NGF and NT-3, which weight were very similar to those of mammalian corresponding neurotrophins. These results extend to teleost species the presence and distribution of NGF- and NT-3-like IR in the enteric nervous system, suggesting a well-preserved presence of these substances in the gut during vertebrate phylogenesis.

Neurons and astrocytes secrete factors that cause stem cells to differentiate into neurons and astrocytes, respectively

We examined the role of soluble factors secreted by neurons and astrocytes in the differentiation of CNS stem cells. We showed that the soluble factors from neurons strongly induced multipotent cortical stem cells to acquire neuronal identity, while the factors from astrocytes promoted astrocytic differentiation. Neurons secreted the brain-derived neurotrophic factor and neurotrophin-3 to induce neuron differentiation, while astrocytes secreted ciliary neurotrophic factor for astrocyte differentiation. Both neurons and astrocytes secrete bone morphogenetic proteins (BMPs). Using BMP antagonists it was shown that BMPs were responsible for the neuron-induced neuronal differentiation, as well as the astrocyte-induced astrocytic differentiation. These findings demonstrate the importance of soluble signals in lineage-specific differentiation and provide evidence for the roles of neurons and astrocytes in stem cell differentiation.

Increased expression of neurotrophins and their receptors in the mechanically compressed spinal cord of the spinal hyperostotic mouse (twy/twy)

The purpose of the present study was to identify any compensatory changes at the site of chronic compression of the spinal cord and neighboring segments. For this purpose, serial immunohistochemical and immunoblot analyses were performed for the expression levels of endogenous brain-derived neurotrophic factor (BDNF), neurotrophin (NT)-3, and their receptors, trkB and trkC in 24 tip-toe walking Yoshimura mice (twy/twy) aged 12-24 weeks. The twy mouse exhibits spontaneous calcified deposits posteriorly at the C1-C2 level, compressing the spinal cord. Immunoreactivities for BDNF, NT-3, trkB and trkC were preferentially localized in the gray matter, particularly in the anterior horn cells. In 24-week-old twy mice with severe compression, expression levels of these neurotrophins at the site of maximal compression were significantly lower than at the less- or non-compressed sites. In contrast, the expression levels of BDNF, NT-3, trkB and trkC were significantly higher at the rostral and caudal sites immediately adjacent to the maximal compression site. No such changes were noted in 12-week-old twy mice or in control Institute of Cancer Research mice. Our results suggest that overexpression of BDNF, NT-3, trkB and trkC in motoneuron areas neighboring the site of mechanical compression may represent compensatory changes in response to the compromised neuronal function at the level of compression, and that these proteins possibly contribute to neuronal survival and plasticity.

Neural stem cells constitutively secrete neurotrophic factors and promote extensive host axonal growth after spinal cord injury

Neural stem cells (NSCs) offer the potential to replace lost tissue after nervous system injury. This study investigated whether grafts of NSCs (mouse clone C17.2) could also specifically support host axonal regeneration after spinal cord injury and sought to identify mechanisms underlying such growth. In vitro, prior to grafting, C17.2 NSCs were found for the first time to naturally constitutively secrete significant quantities of several neurotrophic factors by specific ELISA, including nerve growth factor, brain-derived neurotrophic factor, and glial cell line-derived neurotrophic factor. When grafted to cystic dorsal column lesions in the cervical spinal cord of adult rats, C17.2 NSCs supported extensive growth of host axons of known sensitivity to these growth factors when examined 2 weeks later. Quantitative real-time RT-PCR confirmed that grafted stem cells expressed neurotrophic factor genes in vivo. In addition, NSCs were genetically modified to produce neurotrophin-3, which significantly expanded NSC effects on host axons. Notably, overexpression of one growth factor had a reciprocal effect on expression of another factor. Thus, stem cells can promote host neural repair in part by secreting growth factors, and their regeneration-promoting activities can be modified by gene delivery.

Antiepileptic drugs and apoptosis in the developing brain

Epilepsy is the most common neurologic disorder in young humans. Antiepileptic drugs (AEDs), used to treat seizures in children, infants, and pregnant women, cause cognitive impairment, microcephaly, and birth defects by unknown mechanisms. We tested whether common AEDs cause neurodegeneration in the developing rat brain. Rats aged 3-30 days received phenytoin, phenobarbital, diazepam, clonazepam, vigabatrin, or valproic acid. Histologic examination of the brains revealed that these drugs cause widespread and dose-dependent apoptotic neurodegeneration in the developing rat brain during the brain growth spurt period. Apoptotic neurodegeneration was triggered at plasma drug levels relevant for seizure control in humans. Antiepileptic drugs lead to reduced expression of neurotrophins and decreased concentrations of the active forms of ERK1/2, RAF, and AKT. beta-Estradiol, which stimulates pathways that are activated by neurotrophins, ameliorated AEDs-induced apoptotic neurodegeneration. Our findings present one possible mechanism to explain cognitive impairment and reduced brain mass associated with pre- or postnatal exposure of humans to antiepileptic therapy.

BDNF gene replacement reveals multiple mechanisms for establishing neurotrophin specificity during sensory nervous system development

Neurotrophins have multiple functions during peripheral nervous system development such as controlling neuronal survival, target innervation and synaptogenesis. Neurotrophin specificity has been attributed to the selective expression of the Trk tyrosine kinase receptors in different neuronal subpopulations. However, despite overlapping expression of TrkB and TrkC in many sensory ganglia, brain-derived neurotrophic factor (BDNF) and neurotrophin 3 (NT3) null mutant mice display selective losses in neuronal subpopulations. In the present study we have replaced the coding part of the BDNF gene in mice with that of NT3 (BDNF(NT3/NT3)) to analyse the specificity and selective roles of BDNF and NT3 during development. Analysis of BDNF(NT3/NT3) mice showed striking differences in the ability of NT3 to promote survival, short-range innervation and synaptogenesis in different sensory systems. In the cochlea, specificity is achieved by a tightly controlled spatial and temporal ligand expression. In the vestibular system TrkB or TrkC activation is sufficient to promote vestibular ganglion neuron survival, while TrkB activation is required to promote proper innervation and synaptogenesis. In the gustatory system, NT3 is unable to replace the actions of BDNF possibly because of a temporally selective expression of TrkB in taste neurons. We conclude that there is no general mechanism by which neurotrophin specificity is attained and that specificity is achieved by (i) a tightly controlled spatial and temporal expression of ligands, (ii) different Trk receptors playing distinct roles within the same neuronal subpopulation, or (iii) selective receptor expression in sensory neuron subpopulations.

Presynaptic neurotrophin-3 increases the number of tectal synapses, vesicle density, and number of docked vesicles in chick embryos

To determine whether presynaptically derived neurotrophins may contribute to synaptic plasticity, we examined whether neurotrophin-3 (NT-3) changed the number, size, vesicle content, or vesicle distribution of synapses within the retinorecipient layers of the chick optic tectum. In this system, endogenous NT-3 derives presynaptically from retinal ganglion cell axons. Retinotectal synapses comprise the majority of synapses in superficial tectal layers, as demonstrated by destruction of retinotectal input by intraocular application of the drug monensin. To examine the effect of increased or decreased levels of NT-3, either exogenous NT-3 or monoclonal NT-3 blocking antibodies were injected into the optic tectum of 19-day-old chick embryos, spiked with radiolabeled protein to verify the success of injections and estimate effective concentrations. After 48 hours, the ultrastructure of superficial tectal layers was analyzed and compared with samples from control tecta injected with cytochrome C. NT-3 increased the number of synapses, synaptic vesicles/profile, synaptic vesicle densities, the number of docked vesicles, and the length of the synaptic profile. Deprivation of anterogradely transported endogenous NT-3 with NT-3 antibodies resulted in the opposite effect: decreased numbers of synapses, decreased vesicle densities, and decreased numbers of docked vesicles. Brain-derived neurotrophic factor (BDNF) had a largely different effect than NT-3. BDNF increased the density of vesicles and deprivation of endogenous TrkB ligands with TrkB fusion protein reduced the density of vesicles in the synapses, without effects on synapse number or docked vesicles. We conclude that anterogradely transported NT-3 affects synapse strength in a way that differs from that of presumably postsynaptic-derived BDNF.

Glutamate stimulates neurotrophin expression in cultured Müller cells

The uptake of excess extracellular glutamate and the secretion of neurotrophins by glial cells have been suggested to protect CNS neurons from glutamate-induced toxicity. In the retina, perturbation of glutamate transport and decreased retrograde transport of neurotrophic factors such as brain-derived neurotrophic factor (BDNF) may contribute to ganglion cell death in experimental glaucoma. Although many studies show a clear relationship between glutamate and neurotrophic factors, such relationship has not been thoroughly investigated in the retinal environment. In the following study, we determined the effects of glutamate on early passaged rat Müller cells, specifically their expression of neurotrophic factors including BDNF, nerve growth factor (NGF), neurotrophin-3 (NT-3), neurotrophin-4 (NT-4), and glial-cell line derived neurotrophic factor (GDNF); and of glutamate receptors and transporters using immunoblots or enzyme-linked immunosorbent assays. Binding of BDNF to its cognate receptor TrkB was also determined using co-immunoprecipitation studies. Cultured Müller cells grown in the presence of glutamate were also assayed for survival using 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium (MTS). Our study showed that while glutamate treatment did not promote cell death, it upregulated secretion of BDNF, NGF, NT-3, NT-4, and GDNF by Müller cells. While solitary bands at approximately 13-14 kDa were observed for NGF, NT-3, and NT-4; two BDNF-reactive bands were observed in immunoblots: a faster migrating band at the reported size of the BDNF monomer (approximately 13 kDa); and a more intense band at approximately 36 kDa. GDNF-reactive bands were observed at approximately 22, approximately 28, and approximately 55 kDa. Glutamate also induced significant changes in glutamate receptor and transporter proteins, as well maintained the association of BDNF to TrkB in Müller cells. The decreased N-methyl-D-aspartate receptor (NMDAR) levels and sustained activation of TrkB by BDNF could serve as protective mechanisms for Müller cell survival. Moreover, the increased secretion of neurotrophic factors and upregulation of L-glutamate/L-aspartate transporter (GLAST) expression in Müller cells may protect retinal neurons from glutamate toxicity.

Continuous infusion of neurotrophin-3 triggers sprouting, decreases the levels of TrkA and TrkC, and inhibits epileptogenesis and activity-dependent axonal growth in adult rats

Neurotrophin-3 (NT-3), a member of the neurotrophin family of neurotrophic factors, is important for cell survival, axonal growth and neuronal plasticity. Epileptiform activation can regulate the expression of neurotrophins, and increases or decreases in neurotrophins can affect both epileptogenesis and seizure-related axonal growth. Interestingly, the expression of nerve growth factor and brain-derived neurotrophic factor is rapidly up-regulated following seizures, while NT-3 mRNA remains unchanged or undergoes a delayed down-regulation, suggesting that NT-3 might have a different function in epileptogenesis. In the present study, we demonstrate that continuous intraventricular infusion of NT-3 in the absence of kindling triggers mossy fiber sprouting in the inner molecular layer of the dentate gyrus and the stratum oriens of the CA3 region. Furthermore, despite this NT-3-related sprouting effect, continuous infusion of NT-3 retards the development of behavioral seizures and inhibits kindling-induced mossy fiber sprouting in the inner molecular layer of the dentate gyrus. We also show that prolonged infusion of NT-3 leads to a decrease in kindling-induced Trk phosphorylation and a down-regulation of the high-affinity Trk receptors, TrkA and TrkC, suggesting an involvement of both cholinergic nerve growth factor receptors and hippocampal NT-3 receptors in these effects. Our results demonstrate an important inhibitory role for NT-3 in seizure development and seizure-related synaptic reorganization.

Expression of cytokines, neurotrophins, neurotrophin receptors and NOS mRNA in dorsal root ganglion of a rat tourniquet model

We studied temporal changes in mRNA expression patterns for nitric oxide synthase (NOS), cytokines, neurotrophins and neurotrophin receptors in the dorsal root ganglion (DRG) of the rat, after application of a tourniquet to the hind limb. Collapsed myelin and degenerated axons were observed in the tourniquet segment of the sciatic nerve. Gene expression level of inducible nitric oxide synthase (iNOS) and neuronal nitric oxide synthase (nNOS) was significantly increased in ipsilateral DRG samples at 4h after application of the tourniquet but not in the contralateral or control DRG samples. Upregulation of tumor necrosis factor (TNF)-alpha, activating transcription factor (ATF)-3 and neurotrophin-3 (NT3) expressions began at 1h after application of the tourniquet in ipsilateral DRGs. It is likely that transient expression of these molecules triggers secondary events that may be beneficial to wound repair and regeneration.

Expression of PTPRO during mouse development suggests involvement in axonogenesis and differentiation of NT-3 and NGF-dependent neurons

Competition and cooperation between type II and type III receptor protein tyrosine phosphatases (RPTPs) regulate axon extension and pathfinding in Drosophila. The first step to investigate whether RPTPs influence axon growth in the more complex vertebrate nervous system is to identify which neurons express a particular RPTP. We studied the expression of mouse PTPRO, a type III RPTP with an extracellular region containing eight fibronectin type III domains, during embryogenesis and after birth. Mouse PTPRO mRNA is expressed exclusively in two cell types: neurons and kidney podocytes. Maximal expression in the brain was coincident with mid to late gestation and axonogenesis in the brain. We cloned two cDNAs, including a splice variant without sequence coding of 28 amino acids within the juxtamembrane domain that was found mostly in kidney. In situ hybridization detected mPTPRO mRNA in the cerebral cortex, olfactory bulb and nucleus, hippocampus, motor neurons, and the spinal cord midline. In addition, mPTPRO mRNA was found throughout dorsal root, cranial, and sympathetic ganglia and within kidney glomeruli. Mouse PTPRO mRNA was observed in neuron populations expressing TrkA, the high-affinity nerve growth factor receptor, or TrkC, the neurotrophin-3 receptor, and immunoreactive mPTPRO and TrkC colocalized in large dorsal root ganglia proprioceptive neurons. Our results suggest that mPTPRO is involved in the differentiation and axonogenesis of central and peripheral nervous system neurons, where it is in a position to modulate intracellular responses to neurotrophin-3 and/or nerve growth factor.

Neurotrophin-3 expressed in situ induces axonal plasticity in the adult injured spinal cord

The mammalian CNS lacks the ability to effectively compensate for injury by the regeneration of damaged axons or axonal plasticity of intact axons. However, reports suggest that molecular or cellular manipulations can induce compensatory processes that could support regeneration or plasticity after trauma. We tested whether local, sustained release of the neurotrophic factor neurotrophin-3 (NT-3) would support axonal plasticity in the spinal cord distal to the site of injury in rats. The corticospinal tract (CST) was cut unilaterally at the level of the medulla. This avoided excessive inflammation, secondary cell death, vascular disruption, and the release of inhibitory molecules in the lumbar spinal cord. A replication-defective adenoviral vector (Adv) carrying the NT-3 gene (Adv.EFalpha-NT3) was delivered to the spinal motoneurons by retrograde transport through the sciatic nerve. Retrograde transport of the adenoviral vectors avoided the inflammatory response that would be associated with direct injection into the spinal cord. Transduction of spinal motoneurons with Adv.EFalpha-NT3 resulted in a significant increase in the concentration of NT-3 in the L3-L6 region of the spinal cord for up to 3 weeks. In animals with a CST lesion, this local expression of NT-3 induced growth of axons from the intact CST across the midline to the denervated side. If the CST remained intact, overexpression of NT-3 did not lead to an increase in the number of axons crossing the midline. These data demonstrate that local, sustained expression of NT-3 will support axonal plasticity of intact CST axons after trauma-induced denervation.

Decreased potassium channel IK1 and its regulator neurotrophin-3 (NT-3) in inflamed human bowel

Calcium-activated potassium currents of intermediate conductance (IK1) have been described in the rodent enteric nervous system, where they may regulate afterhyperpolarisation of intrinsic primary afferent neurons. Using specific antibodies for immuno-cytochemistry, we now report IK1-like immunoreactivity for the first time in enteric neurons of human colon, and a significant decrease of IK1-positive cells in myenteric plexus in inflamed colon from patients with Crohn's disease and ulcerative colitis (p = 0.031). Neurotrophin-3 (NT-3), which regulates IK1 expression, was also observed in fewer neurons of the myenteric ganglia in Crohn's bowel (p = 0.048), and in inflamed colonic extracts by Western blotting (p = 0.004); the numbers of neurons expressing the NT-3 high affinity receptor trk C were unchanged. Our findings may explain the diarrhoea and colicky abdominal pain produced by inflammatory bowel disease, and by IK1-blocking pyridine drugs prescribed for neuromuscular disorders.

Induction of rapid eye movement sleep by neurotrophin-3 and its co-localization with choline acetyltransferase in mesopontine neurons

Because neurotrophin-3 (NT-3), a neurotrophic factor closely related to nerve growth factor, is capable of modulating neuronal activity [Yamuy et al., Neuroscience 95 (2000a) 1089-1100], we sought to examine if the microinjection of NT-3 into the nucleus reticularis pontis oralis (NPO) of chronically prepared cats also induced changes in behavior. In contrast to vehicle administration, NT-3 injection induced, with a mean latency of 4.7 min, long-duration episodes (mean, 21.6 min) of a state that was polygraphically indistinguishable from naturally occurring REM sleep. If NT-3 plays a physiologic role in the generation of REM sleep, then an endogenous source for this neurotrophin that is capable of controlling the activity of NPO neurons should exist. We therefore determined whether cholinergic neurons in the latero-dorsal and pedunculo-pontine tegmental (LDT and PPT) nuclei, which are involved in the initiation of REM sleep and project to the NPO, contained NT-3. Most, if not all, of the LDT-PPT cholinergic neurons exhibited NT-3 immunoreactivity. A portion (10%) of the NT-3+ neurons in the LDT-PPT were not cholinergic. The present data indicate that NT-3 rapidly modulates the activity of NPO neurons involved in REM sleep and that cholinergic neurons in the LDT and PPT contain NT-3. Taken together, these results support the hypothesis that NT-3 may be involved in the control of naturally occurring REM sleep.

The noncatalytic TrkCNC2 receptor is cleaved by metalloproteases upon neurotrophin-3 stimulation

The trkC locus encodes catalytic and noncatalytic receptors, generated by alternative splicing. These primary high-affinity neurotrophin-3 (NT-3) receptors may act in concert to modulate responsiveness to NT-3. Signal modulation can also be achieved by receptors that are post-translationally processed. We report that the noncatalytic TrkC receptor, TrkCNC2, is cleaved at the membrane-proximal region of its extracellular domain. This generates a soluble ectodomain (gp90(TrkCNC2)) recovered in the cell culture medium and a membrane-bound fragment (p20(TrkCNC2)), which contains the transmembrane and intracellular regions including the juxtamembrane and the NC2-specific cytoplasmic domains. We also show that this processing, which does not occur in the TrkC catalytic counterpart, is upregulated by NT-3 and upon treatment with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate. Moreover, cleavage inhibition after EDTA or 1.10 phenanthroline treatment suggests involvement of a metalloprotease(s). Finally, this post-translational processing was observed not only in TrkCNC2-overexpressing NIH3T3 cells but also in primary cultures of cortical neurons and brain extracts. This study shows that, in addition to alternative splicing, ectodomain shedding represents a novel means of regulating TrkC receptor signaling, and consequently NT-3 biological effects on target cells.

Neurotrophin-like immunoreactivity in the human pre-term newborn, infant, and adult cerebellum

The immunohistochemical occurrence of the neurotrophin (NT) proteins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4), and neurotrophin-3 (NT-3) is shown in the pre-term newborn, infant, and adult human post-mortem cerebellum. The NT-like immunoreactive structures were unevenly distributed and showed regional differences among cerebellar lobules and folia. NGF-, NT-4-, and NT-3-positive neuronal perikarya were observed in all specimens examined. At variance with the other neurotrophins, the BDNF antiserum labelled neuronal cell bodies only in newborn life and infancy, as well as extensive nerve fibre systems, whose density increased with age. The NT-antibodies, tested by Western blot on human cerebellum homogenates, revealed immunoreactive bands corresponding to proteins of heterogenous molecular weight. The results obtained provide a first demonstration of the tissue localization of the NTs in the human cerebellum from perinatal to adult age, thus suggesting their involvement in the development, differentiation and maintenance of the cerebellar connectivity. Codistribution of the four NTs or sets of them was observed in cortical and deep nuclei neurons. Multiple trophic roles for NTs, encompassing the classic target-derived and local mechanisms of support, are envisaged as significant in development, differentiation, and maintenance of the human cerebellar connectivity.

Brn3c null mutant mice show long-term, incomplete retention of some afferent inner ear innervation

BACKGROUND

Ears of Brn3c null mutants develop immature hair cells, identifiable only by certain molecular markers, and undergo apoptosis in neonates. This partial development of hair cells could lead to enough neurotrophin expression to sustain sensory neurons through embryonic development. We have therefore investigated in these mutants the patterns of innervation and of expression of known neurotrophins.

RESULTS

At birth there is a limited expression of BDNF and NT-3 in the mutant sensory epithelia and DiI tracing shows no specific reduction of afferents or efferents that resembles neurotrophin null mutations. At postnatal day 7/8 (P7/8), innervation is severely reduced both qualitatively and quantitatively. 1% of myosin VIIa-positive immature hair cells are present in the mutant cochlea, concentrated in the base. Around 20% of immature hair cells exist in the mutant vestibular sensory epithelia. Despite more severe loss of hair cells (1% compared to 20%), the cochlea retains many more sensory neurons (46% compared to 15%) than vestibular epithelia. Even 6 months old mutant mice have some fibers to all vestibular sensory epithelia and many more to the cochlear apex which lacks MyoVIIa positive hair cells. Topologically organized central cochlea projections exist at least until P8, suggesting that functional hair cells are not required to establish such projections.

CONCLUSION

The limited expression of neurotrophins in the cochlea of Brn3c null mice suffices to support many sensory neurons, particularly in the cochlea, until birth. The molecular nature of the long term survival of apical spiral neurons remains unclear.

Interplay between brain-derived neurotrophic factor and signal transduction modulators in the regulation of the effects of exercise on synaptic-plasticity

This study was designed to identify molecular mechanisms by which exercise affects synaptic-plasticity in the hippocampus, a brain area whose function, learning and memory, depends on this capability. We have focused on the central role that brain-derived neurotrophic factor (BDNF) may play in mediating the effects of exercise on synaptic-plasticity. In fact, this impact of exercise is exemplified by our finding that BDNF regulates the mRNA levels of two end products important for neural function, i.e. cAMP-response-element binding (CREB) protein and synapsin I. CREB and synapsin I have the ability to modify neuronal function by regulating gene-transcription and affecting synaptic transmission, respectively. Furthermore, we show that BDNF is capable of concurrently increasing the mRNA levels of both itself and its tyrosine kinaseB (TrkB) receptor, suggesting that exercise may employ a feedback loop to augment the effects of BDNF on synaptic-plasticity. The use of a novel microbead injection method in our blocking experiments and Taqman reverse transcription polymerase reaction (RT-PCR) for RNA quantification, have enabled us to evaluate the contribution of different pathways to the exercise-induced increases in the mRNA levels of BDNF, TrkB, CREB, and synapsin I. We found that although BDNF mediates exercise-induced hippocampal plasticity, additional molecules, i.e. the N-methyl-D-aspartate receptor, calcium/calmodulin protein kinase II and the mitogen-activated protein kinase cascade, modulate its effects. Since these molecules have a well-described association to BDNF action, our results illustrate a basic mechanism through which exercise may promote synaptic-plasticity in the adult brain.

Molecular developmental neurobiology of formation, guidance and survival of primary vestibular neurons

Untangling the molecular and physiological interactions that generate the proper connections of the primary vestibular neurons in normal gravity requires two parallel approaches. One approach needs to use mutant mice to delineate the molecular basis of developmental mechanisms that govern ear development, including formation and differentiation of neurons and establishment of their peripheral and central connections. Beyond that and in addition to it, we need physiological investigations using microgravity and/or hypergravity, as well as absence of otoconia, to understand the role played by vestibular stimuli to fine tune connections of primary and secondary vestibular neurons. This paper provides an overview of some of the molecular mechanisms uncovered over the last few years that guide development, differentiation and survival of primary vestibular neurons of the mammalian ear. Briefly, several genes that are essential for primary neuron formation have been identified, all genes that govern embryonic survival are known and the first genes and mechanisms that guide formation of proper connections are being revealed. While still incomplete, the progress has been astounding and the completion of the mouse genome project will further accelerate the pace. Such data pave the way to put the research on the influence of altered gravity stimulation within a molecular framework.

Motoneuron-derived neurotrophin-3 is a survival factor for PAX2-expressing spinal interneurons

Rat spinal cord interneurons undergo programmed cell death shortly after birth. We investigated here whether cell death of interneurons could be regulated by trophic factors produced by motoneurons, one of their main targets. To test this hypothesis, we studied the effect of the selective destruction of motoneurons on the survival of interneurons in organotypic cultures of embryonic rat spinal cords. Motoneurons were eliminated by an anti-p75(NTR)-specific immunotoxin (192 IgG-saporin). We then observed a decrease of 28% in the number of ventral spinal interneurons immunoreactive (IR) for the homeoprotein PAX2. This was correlated with an increase in the number of apoptotic nuclei in the same area. Because neurotrophin-3 (NT-3) is specifically produced by motoneurons and because interneurons express the NT-3 high-affinity receptor trkC, we examined the role of NT-3 in the survival of PAX2-IR interneurons. Addition of NT-3 to 192 IgG-saporin-treated explants rescued ventral PAX2-IR interneurons. Depletion of secreted NT-3 by anti-NT-3 antibodies induced 66% loss of ventral PAX2-IR interneurons. We conclude that motoneuron-derived NT-3 is a trophic factor for ventral PAX2-IR interneurons.

Elevated levels of neurotrophins in human biceps brachii tissue of amyotrophic lateral sclerosis

Previous studies suggest that neurotrophins support regeneration and survival of injured motoneurons. Based on these findings, brain-derived neurotrophic factor (BDNF) has been clinically investigated for its therapeutic potential in amyotrophic lateral sclerosis (ALS), a rapidly progressing and fatal motoneuronal disease. We questioned whether imbalances of neurotrophic levels are indeed involved in the pathology of ALS. Therefore the expression of nerve growth factor (NGF), BDNF, neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5) was investigated in postmortem muscle tissue of the biceps from 15 patients with neuropathologically confirmed sporadic ALS and 15 age-matched controls. Using mRNA analysis techniques and quantitative protein measurements, we have demonstrated that both mRNA and protein levels of all four neurotrophins are increased in muscle tissue of ALS patients. The production levels displayed a disease duration dependency and different expression patterns emerged for the four neurotrophins. Whereas the early phase of the disease was characterized by a strong upregulation of BDNF, levels of NGF, NT-3, and NT-4/5 gradually increased in the course of the disorder, peaking at later stages. We conclude that decreased neurotrophic support from muscle tissue is most likely not the cause of motoneuron degeneration in ALS. On the contrary, our results suggest that degenerating motoneurons in ALS are exposed to elevated levels of muscle-derived neurotrophins.