Expression of neurotrophins and their receptors in peripheral lung cells of mice

Neurotrophins play an essential role in nerve systems. Recent reports indicated that neurotrophins [nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5)] have numerous effects on non-neural cells, especially on immune cells. However, whether lung cells express neurotrophins and/or their receptors (TrkA for NGF, TrkB for BDNF and NT-4/5, and TrkC for NT-3) has never been systematically investigated. We investigated constitutive expression of neurotrophin family and their Trk receptor family in alveolar macrophages and other peripheral lung cells of mice. New findings were: (1) RT-PCR for neurotrophins and their receptors detected NT-3 and NT-4/5 in alveolar macrophages, BDNF, NT-4/5, trkA, the truncated form of trkB, and trkC in lung homogenate, but no trks in alveolar macrophages, (2) immunohistochemistry for neurotrophin receptors detected TrkA in capillary cells, the truncated form of TrkB, and TrkC in interstitial macrophages, (3) immunoelectron microscopy for TrkC revealed expression of TrkC on the surface of interstitial macrophages, and (4) in situ hybridization for neurotrophins detected BDNF in interstitial macrophages and alveolar type I cells, NT-3 in alveolar macrophages, and NT-4/5 in alveolar and interstitial macrophages. These findings indicate that a previously unknown signal trafficking occurs through neurotrophins in peripheral lung.     

Developmental changes in the protective effect of exogenous brain-derived neurotrophic factor and neurotrophin-3 against ototoxic drugs in cultured rat vestibular ganglion neurons

We examine developmental changes in the responsiveness of rat vestibular ganglion neurons (VGNs) to two neurotrophic factors (NTFs), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) and investigate the protective effects of these NTFs against ototoxic drugs during postnatal development in dissociated cultures. VGNs were obtained from rats on postnatal days (P) 1, 3, 7 and 14. BDNF facilitated neuronal survival as well as neurite sprouting of VGNs obtained from younger rats (P1 and P3), whereas these effects were not observed in older rats (P7 and P14). BDNF was also effective in facilitating neurite extension in VGNs at each of the postnatal ages. NT-3 also facilitated neuronal survival and neurite extension of VGNs from younger rats but these effects were significantly smaller than those of BDNF (p?<?0.05). The protective effects of BDNF and NT-3 against ototoxic drugs, gentamicin and cisplatin, were also age-dependent: they were effective for neuronal survival, neurite sprouting and neurite extension in VGNs from younger rats, whereas these effects tended to disappear in VGNs from older rats. Analysis of the changes in the expression of the receptors of NTFs revealed that expression of TrkB and TrkC proteins and their mRNA did not change during the developmental period, whereas expression of p75^NTR protein was down-regulated together with that of p75^NTR mRNA during the developmental period. Developmental changes in the responsiveness to exogenous NTFs in VGNs, which is not caused by the changes of their receptors but probably caused by changes in the intracellular signaling pathways, should be taken into consideration in the prevention of neuronal degeneration caused by ototoxic drugs.     

Characterization of the Responses of Purkinje Cells to Neurotrophin Treatment

Abstract: The ability of the neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4/5 (NT-4/5) to promote neuronal survival and phenotypic differentiation was examined in dissociated cultures from embryonic day 16 rat cerebellum. BDNF treatment increased the survival of neuron-specific enolase-immunopositive cells by 250 and 400% after 8 and 10 days in culture, respectively. A subpopulation of these neurons, the Purkinje cells, identified by calbindin staining, was increased to an equivalent extent, ∼200%, following BDNF, NT-4/5, or NT-3 treatment. The number of GABAergic neurons, identified by GABA immunoreactivity, was greatly increased by treatment with BDNF (470%) and moderately by NT-4/5 (46%), whereas NT-3 was without effect. NGF failed to increase the number of either Purkinje cells or GABAergic neurons. Addition of BDNF within 48 h of cell plating was required to obtain a maximal increase in Purkinje cell number after 8 days. In contrast, the NT-3 responses were nearly equivalent even if treatment was delayed for 96 h after plating. BDNF, NT-4/5, and NT-3, but not NGF, induced the rapid expression of the immediate early gene c- fos . Immunocytochemical double-labeling with antibodies to c-fos and calbindin was used to identify Purkinje cells that responded to neurotrophin treatment by induction of c-fos. After 4 days in vitro, both BDNF and NT-3 induced the formation of c-fos protein in calbindin-immunopositive neurons, whereas NT-4/5 did not. The latter results suggest that although BDNF and NT-4/5 have been shown to act through a common receptor, TrkB, it appears that the effects of BDNF and NT-4/5 are not identical.     

The neurotrophins BDNF, NT-3, and NGF display distinct patterns of retrograde axonal transport in peripheral and central neurons.

The pattern of retrograde axonal transport of the target-derived neurotrophic molecule, nerve growth factor (NGF), correlates with its trophic actions in adult neurons. We have determined that the NGF-related neurotrophins, brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3), are also retrogradely transported by distinct populations of peripheral and central nervous system neurons in the adult. All three 125I-labeled neurotrophins are retrogradely transported to sites previously shown to contain neurotrophin-responsive neurons as assessed in vitro, such as dorsal root ganglion and basal forebrain neurons. The patterns of transport also indicate the existence of neuronal populations that selectively transport NT-3 and/or BDNF, but not NGF, such as spinal cord motor neurons, neurons in the entorhinal cortex, thalamus, and neurons within the hippocampus itself. Our observations suggest that neurotrophins are transported by overlapping as well as distinct populations of neurons when injected into a given target field. Retrograde transport may thus be predictive of neuronal types selectively responsive to either BDNF or NT-3 in the adult, as first demonstrated for NGF.     

Developing vestibular ganglion neurons switch trophic sensitivity from BDNF to GDNF after target innervation

Recent evidence showing a distinctive cell loss in vestibular and cochlear ganglia of brain-derived neurotrophic factor (BDNF) versus neurotrophin-3 (NT-3) null mutant mice demonstrates that these neurotrophins play a critical role in inner ear development. In this study, biological functions of BDNF and NT-3 in the chick vestibular and cochlear ganglion development was assessed in vitro and compared to those of other neurotrophic factors. The embryonic day (E)8-12 vestibular ganglion neurons showed an extensive outgrowth in response to BDNF with less outgrowth to NT-3. In contrast, NT-3 had stronger neurotrophic effects on the E12 cochlear ganglion neurons compared to BDNF. These results support previous evidence that neurotrophins play important roles in the vestibular and cochlear ganglion neuron development. However, the responsiveness to the neurotrophins declined and became undetectable by E16. Unexpectedly, glial cell line-derived neurotrophic factor (GDNF) promoted neurite outgrowth from vestibular ganglia at E12-16, later than the stages at which BDNF had neurotrophic effects. The time of switching sensitivity of the vestibular ganglion neurons from BDNF to GDNF correlated with the time of completion of synaptogenesis on their peripheral and central targets. Furthermore, a factor released from E12 inner ears exerted neurotrophic effects on late-stage vestibular ganglion neurons that were not responsive to the E4 otocyst-derived factor. These results raise the possibility that the vestibular ganglion neurons become responsive to GDNF upon target innervation and that the changes in sensitivity are regulated by changes in available factors released from their peripheral targets, the inner ear epithelia.     

Brain-derived neurotrophic factor is an anterograde survival factor in the rat visual system

BACKGROUND: The neurotrophins, which include nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), NT-4/5 and NT-6, are a family of proteins that play fundamental roles in the differentiation, survival and maintenance of peripheral and central neurons. Much research has focused on the role of neurotrophins as target-derived, retrogradely transported trophic molecules. Although there is recent evidence that BDNF and NT-3 can be transported in an anterograde direction along peripheral and central axons, there is as yet no conclusive evidence that these anterograde factors have direct post-synaptic actions. RESULTS: We report that BDNF travels in an anterograde direction along the optic nerve. The anterogradely transported BDNF had rapid effects on retinal target neurons in the superior colliculus and lateral geniculate nucleus of the brain. When endogenous BDNF within the developing superior colliculus was neutralised, the rate of programmed neuronal death increased. Conversely, provision of an afferent supply of BDNF prevented the degeneration of geniculate neurons after removal of their cortical target. CONCLUSIONS: BDNF released from retinal ganglion cells acts as a survival factor for post-synaptic neurons in retinal target fields.     

Immunohistochemical distribution of NGF, BDNF, NT-3, and NT-4 in adult rhesus monkey brains.

Immunohistochemical distribution and cellular localization of neurotrophins was investigated in adult monkey brains using antisera against nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Western blot analysis showed that each antibody specifically recognized appropriate bands of approximately 14.7 kDa, 14.2 kDa, 13.6 kDa, and 14.5 kDa, for NGF, BDNF, NT-3, and NT-4, respectively. These positions coincided with the molecular masses of the neurotrophins studied. Furthermore, sections exposed to primary antiserum preadsorbed with full-length NGF, BDNF, NT-3, and NT-4 exhibited no detectable immunoreactivity, demonstrating specificities of the antibodies against the tissues prepared from rhesus monkeys. The study provided a systematic report on the distribution of NGF, BDNF, NT-3, and NT-4 in the monkey brain. Varying intensity of immunostaining was observed in the somata and processes of a wide variety of neurons and glial cells in the cerebrum, cerebellum, hippocampus, and other regions of the brain. Neurons in some regions such as the cerebral cortex and the hippocampus, which stained for neurotrophins, also expressed neurotrophic factor mRNA. In some other brain regions, there was discrepancy of protein distribution and mRNA expression reported previously, indicating a retrograde or anterograde action mode of neurotrophins. Results of this study provide a morphological basis for the elucidation of the roles of NGF, BDNF, NT-3, and NT-4 in adult primate brains.     

Neurotrophins and Neurotrophin Receptors in Proliferative Diabetic Retinopathy

Neurotrophins (NTs) are emerging as important mediators of angiogenesis and fibrosis. We investigated the expression of the NTs nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) and their receptors TrkA, TrkB, and TrkC in proliferative diabetic retinopathy (PDR). As a comparison, we examined the expression of NTs and their receptors in the retinas of diabetic rats. Vitreous samples from 16 PDR and 15 nondiabetic patients were studied by Western blot analysis and enzyme-linked immunosorbent assay (ELISA). Epiretinal membranes from 17 patients with PDR were studied by immunohistochemistry. Rats were made diabetic with a single high dose of streptozotocin and retinas of rats were examined by Western blot analysis. Western blot analysis revealed a significant increase in the expression of NT-3 and NT-4 and the shedding of receptors TrkA and TrkB in vitreous samples from PDR patients compared to nondiabetic controls, whereas NGF and BDNF and the receptor TrkC were not detected with the use of Western blot analysis and ELISA. In epiretinal membranes, vascular endothelial cells and myofibroblasts expressed NT-3 and the receptors TrkA, TrkB and TrkC in situ, whereas NT-4 was not detected. The expression levels of NT-3 and NT-4 and the receptors TrkA and TrkB, both in intact and solubilized forms, were upregulated in the retinas of diabetic rats, whereas the receptor TrkC was not detected. Co-immunoprecipitation studies revealed binding between NT-3 and the receptors TrkA and TrkB in the retinas of diabetic rats. Our findings in diabetic eyes from humans and rats suggest that the increased expression levels within the NT-3 and NT-4/Trk axis are associated with the progression of PDR.     

Neurotrophins and neurotrophin receptors in pulmonary sarcoidosis - granulomas as a source of expression

Background

Pulmonary sarcoidosis is an inflammatory disease, characterized by an accumulation of CD4⁺ lymphocytes and the formation of non-caseating epithelioid cell granulomas in the lungs. The disease either resolves spontaneously or develops into a chronic disease with fibrosis. The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF) and neurotrophin-3 (NT-3) have been suggested to be important mediators of inflammation and mediate tissue remodelling. In support of this, we have recently reported enhanced NGF levels in the airways of patients with pulmonary sarcoidosis. However, less is known about levels of BDNF and NT-3, and moreover, knowledge in the cellular sources of neurotrophins and the distribution of the corresponding neurotrophin receptors in airway tissue in sarcoidosis is lacking.

Methods

The concentrations of NGF, BDNF and NT-3 in bronchoalveolar lavage fluid (BALF) of 41 patients with newly diagnosed pulmonary sarcoidosis and 27 healthy controls were determined with ELISA. The localization of neurotrophins and neurotrophin receptors were examined by immunohistochemistry on transbronchial lung biopsies from sarcoidosis patients.

Results

The sarcoidosis patients showed significantly enhanced NT-3 and NGF levels in BALF, whereas BDNF was undetectable in both patients and controls. NT-3 levels in BALF were found higher in patients with non-Löfgren sarcoidosis as compared to patients with Löfgren''s syndrome, and in more advanced disease stage. Epithelioid cells and multinucleated giant cells within the sarcoid granulomas showed marked immunoreactivity for NGF, BDNF and NT-3. Also, immunoreactivity for the neurotrophin receptor TrkA, TrkB and TrkC, was found within the granulomas. In addition, alveolar macrophages showed positive immunoreactivity for NGF, BDNF and NT-3 as well as for TrkA, TrkB and TrkC.

Conclusions

This study provides evidence of enhanced neurotrophin levels locally within the airways of patients with sarcoidosis. Findings suggest that sarcoid granuloma cells and alveolar macrophages are possible cellular sources of, as well as targets for, neurotrophins in the airways of these patients.     

An immunoglobulin-like domain determines the specificity of neurotrophin receptors.

The neurotrophins influence survival and maintenance of vertebrate neurons in the embryonic, early post-natal and post-developmental stages of the nervous system. Binding of neurotrophins to receptors encoded by the gene family trk initiates signal transduction into the cell. trkA interacts preferably with nerve growth factor (NGF), trkB with brain-derived neurotrophic factor (BDNF) and neurotrophin-4/5 (NT-4/5) and trkC with neurotrophin-3 (NT-3). By constructing 17 different chimeras and domain deletions of the human trk receptors and analyzing their binding affinities to the neurotrophins we have shown that an immunoglobulin-like domain located adjacent to the transmembrane domain is the structural element that determines the interaction of neurotrophins with their receptors. Chimeras of trkC where this domain was exchanged for the homologous sequences from trkB or trkA gained high affinity binding to BDNF or NGF respectively, while deletion of this domain in trkC or trkA abolished binding to NT-3 or NGF respectively. This domain alone retained affinities to neurotrophins similar to the full-length receptors and when expressed on NIH 3T3 cells in fusion with the kinase domain showed neurotrophin-dependent activation.     

BDNF and NT4/5 promote survival and neurite outgrowth of pontocerebellar mossy fiber neurons.

The neurotrophins nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT3), and NT4/5 are all found in the developing cerebellum. Granule cells, the major target neurons of mossy fibers, express BDNF during mossy fiber synaptogenesis. To determine whether neurotrophins contribute to the development of cerebellar afferent axons, we characterized the effects of neurotrophins on the growth of mossy fiber neurons from mice and rats in vitro. For a mossy fiber source, we used the basilar pontine nuclei (BPN), the major source of cerebellar mossy fibers in mammals. BDNF and NT4/5 increased BPN neuron survival, neurite outgrowth, growth cone size, and elongation rate, while neither NT3 nor NGF increased survival or outgrowth. In addition, BDNF and NT4/5 reduced the size of neurite bundles. Consistent with these effects, in situ hybridization on cultured basilar pontine neurons revealed the presence of mRNA encoding the TrkB receptor which binds both BDNF and NT4/5 with high affinity. We detected little or no message encoding the TrkC receptor which preferentially binds NT3. BDNF and NT4/5 also increased TrkB mRNA levels in BPN neurons. In addition to previously established functions as an autocrine/paracrine trophic factor for granule cells, the present results indicate that cerebellar BDNF may also act as a target-derived trophic factor for basilar pontine mossy fibers.     

TrkA receptor "hot spots" for binding of NT-3 as a heterologous ligand

Neurotrophins signal via Trk tyrosine kinase receptors. Nerve growth factor (NGF) is the cognate ligand for TrkA, the brain-derived neurotrophic factor for TrkB, and NT-3 for TrkC. NT-3 also binds TrkA as a lower affinity heterologous ligand. Because neurotrophin-3 (NT-3) interactions with TrkA are biologically relevant, we aimed to define the TrkA "hot spot" functional docking sites of NT-3. The Trk extracellular domain consists of two cysteine-rich subdomains (D1 and D3), flanking a leucine-rich subdomain (D2), and two immunoglobulin-like subdomains IgC1(D4) and IgC2(D5). Previously, the D5 subdomain was defined as the primary ligand-binding site of neurotrophins for their cognate receptors (e.g. NGF binds and activates through TRKA-D5 hot spots). Here binding studies with truncated and chimeric extracellular subdomains show that TRKA-D5 also includes an NT-3 docking and activation hot spot (site 1), and competition studies show that the NGF and NT-3 hot spots on TRKA-D5 are distinct but partially overlapping. In addition, ligand binding studies provide evidence for an NT-3-binding/allosteric site on TRKA-D4 (site 2). NT-3 docking on sites 1 and/or 2 partially blocks NGF binding. Functional survival studies showed that sites 1 and 2 regulate TrkA activation. NT-3 docking on both sites 1 and 2 affords full agonism, which can be additive with NGF activation of Trk. However, NT-3 docking solely on site 1 is partially agonistic but noncompetitively antagonizes NGF binding and activation of Trk. This study demonstrates that Trk signaling is more complex than previously thought because it involves several receptor subdomains and hot spots.     

Selective dependence of mammalian dorsal root ganglion neurons on nerve growth factor during embryonic development.

We have investigated the NGF dependence of dorsal root ganglion (DRG) neurons in mammals using a paradigm of multiple in utero injections of a high titer anti-NGF antiserum. We have determined the specificity of our antiserum in relation to other members of the NGF neurotrophin family and found no cross-reactivity with brain-derived neurotrophic factor (BDNF) or neurotrophin-3 (NT-3). To identify various classes of DRG neurons, we have stained their characteristic central projections with Dil. We show here that the NGF dependence of DRG neurons is strikingly selective. Although a majority of DRG neurons are lost after NGF deprivation during embryonic life, these are almost exclusively small diameter neurons that project to laminae I and II of the dorsal horn and presumably subserve nociception and thermoreception. Larger neurons that project to more ventral spinal laminae and subserve other sensory modalities do not require NGF for survival. These NGF-independent DRG neurons likely require one of the more recently identified neurotrophins, BDNF or NT-3.     

Effects of castration on the immunoreactivity to NGF, BDNF and their receptors in the pelvic ganglia of the male rat

Nerve growth factor (NGF) and brain derived neurotrophic factor (BDNF) and are members of the neurotrophin family, a family of neurotrophic factors that also includes neurotrophin (NT) 3 and NT4/5. Neurotrophins have essential roles in the survival, development and differentiation of neurons in the central and peripheral nervous systems. Neurotrophins exert their effects by binding to corresponding receptors which are formed by the tyrosine protein kinases TrkA, TrkB and TrkC, and the low affinity neurotrophic receptor (p75NTR). In the present study, using immunohistochemistry and quantitative analysis, we have investigated immunoreactivity to BDNF, NGF, TrkB, p75NTR and TrkA in the pelvic ganglia of normal and castrated rats. Neurons of the pelvic ganglia expressed both these neurotrophins and their receptors. After castration the immunoreactivity persisted. However, the number of BDNF- and p75NTR-IR cells statistically significant decreased after castration. These results suggest that castration modulates the expression of neurotrophins and their receptors in pelvic autonomic neurons.     

Differential Trk expression in explant and dissociated trigeminal ganglion cell cultures

During embryonic development, expression of neurotrophin receptor tyrosine kinases (Trks) by sensory ganglia is continuously and dynamically regulated. Neurotrophin signaling promotes selective survival and axonal differentiation of sensory neurons. In embryonic day (E) 15 rat trigeminal ganglion (TG), NGF receptor TrkA is expressed by small diameter neurons, NT-3 receptor TrkC and BDNF receptor TrkB are expressed by large diameter neurons. Organotypic explant and dissociated cell cultures of the TG (and dorsal root ganglia) are commonly used to assay neurotrophin effects on developing sensory neurons. In this study, we compared Trk expression in E15 rat TG explant and dissociated cell cultures with or without neurotrophin treatment. Only a subset of TG cells express each of the three Trk receptors in wholemount explant cultures as in vivo conditions. In contrast, all TG neurons co-express all three Trk receptors upon dissociation, regardless of neurotrophin treatment. Neurons cultured in low concentrations of one neurotrophin first, and switched to higher concentrations of another after 1 day, survive and display morphological characteristics of neurons cultured in a mixture of both neurotrophins for 3 days. Our results indicate that wholemount explant cultures of sensory ganglia represent in vivo conditions in terms of Trk expression patterns; whereas dissociation dramatically alters Trk expression by primary sensory neurons.     

Target specificity and size of avian sensory neurons supported in vitro by nerve growth factor,brain-derived neurotrophic factor,and neurotrophin-3

To obtain insight into which subpopulations of sensory neurons in dorsal root ganglia are supported by different neurotrophins, we retrogradely labeled cutaneous and muscle afferents in embryonic day 9 chick embryos and followed their survival in neuron-enriched cultures supplemented with either nerve growth factor (NGF), brain-derived neurotrophic factor (BDNF), or neurotrophin-3 (NT-3). We found that NGF is a wide survival factor for subpopulations of both cutaneous and muscle afferents, whereas the survival effects of BDNF and NT-3 are restricted primarily to muscle afferents. We also measured soma size in each neurotrophic factor. These new data show that BDNF- and NT-3–dependent cells appear to be a mixture of two populations of neurons: one small diameter and the other large diameter. In contrast, based on size alone, NGF-dependent cells appear to be a single population of only small-diameter neurons. Thus, BDNF and NT-3 may have some new, previously unreported effects on small-diameter afferent neurons. © 1994 John Wiley & Sons, Inc. 1994 John Wiley & Sons, Inc.     

Expression of nerve growth factors in pancreatic neural tissue and pancreatic cancer.

One of the characteristics of pancreatic cancer is its tendency to invade neural tissue. We hypothesized that the affinity of cancer cells for nerve tissue is related to the presence of growth factors in neural tissue and their receptors in cancer cells. Sections of pancreatic cancer and normal pancreatic tissue were examined by immunohistochemistry for the expression of the neurotrophins NGF, BDNF, NT-3, NT-4, and their receptors TrkA, TrkB, and TrkC, as well as the low-affinity receptor, p75NTR. TrkA expression was found in duct, islet, and cancer cells; TrkB was found in the alpha-cells of the islet only. The anti-pan-Trk antibody (TrkB3), which is presumed to recognize all three receptors, immunoreacted with duct and acinar cells in normal tissue and with cancer cells. The staining with TrkC was similar to that of TrkA. The low-affinity receptor p75NTR was expressed in the neural tissue and in scattered duct cells of the normal tissue only. Duct and acinar cells, as well as neural tissue and cancer cells, showed weak to strong immunoreactivity with NGF. NT-3 expression was noted in capillary endothelia and erythrocytes. NT-4 showed specific staining for ductule cells. The expression and distribution of neurotrophins and their receptors suggest their role in the potential of pancreatic cancer cells for neural invasion.     

Differential expression of mRNAs for neurotrophins and their receptors after axotomy of the sciatic nerve

The neurotrophin family includes NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4). Previous studies have demonstrated that expression of NGF and its low-affinity receptor is induced in nonneuronal cells of the distal segment of the transected sciatic nerve suggesting a role for NGF during axonal regeneration (Johnson, E. M., M. Taniuchi, and P. S. DeStefano. 1988. Trends Neurosci. 11:299-304). To assess the role of the other neurotrophins and the members of the family of Trk signaling neurotrophin receptors, we have here quantified the levels of mRNAs for BDNF, NT-3, and NT-4 as well as mRNAs for trkA, trkB, and trkC at different times after transection of the sciatic nerve in adult rats. A marked increase of BDNF and NT-4 mRNAs in the distal segment of the sciatic nerve was seen 2 wk after the lesion. The increase in BDNF mRNA was mediated by a selective activation of the BDNF exon IV promoter and adrenalectomy attenuated this increase by 50%. NT-3 mRNA, on the other hand, decreased shortly after the transection but returned to control levels 2 wk later. In Schwann cells ensheathing the sciatic nerve, only trkB mRNA encoding truncated TrkB receptors was detected with reduced levels in the distal part of the lesioned nerve. Similar results were seen using a probe that detects all forms of trkC mRNA. In the denervated gastrocnemius muscle, the level of BDNF mRNA increased, NT-3 mRNA did not change, while NT-4 mRNA decreased. In the spinal cord, only small changes were seen in the levels of neutrophin and trk mRNAs. These results show that expression of mRNAs for neurotrophins and their Trk receptors is differentially regulated after a peripheral nerve injury. Based on these results a model is presented for how the different neurotrophins could cooperate to promote regeneration of injured peripheral nerves.     

Regulation of neuropeptide expression in the brain by neurotrophins

Neurotrophins, which are structurally related to nerve growth factor, have been shown to promote survival of various neurons. Recently, we found a novel activity of a neurotrophin in the brain: Brain-derived neurotrophic factor (BDNF) enhances expression of various neuropeptides. The neuropeptide differentiation activity was then compared among neurotrophins both in vivo and in vitro. In cultured neocortical neurons, BDNF and neurotrophin-5 (NT-5) remarkably increased levels of neuropeptide Y and somatostatin, and neurotrophin-3 (NT-3) also increased these peptides but required higher concentrations. At elevating substance P, however, NT-3 was as potent as BDNF. In contrast, NGF had negligible or no effect. Neurotrophins administered into neonatal brain exhibited slightly different potencies for increasing these neuropeptides: The most marked increase in neuropeptide Y levels was obtained in the neocortex by NT-5, whereas in the striatum and hippocampus by BDNF, although all three neurotrophins increased somatostatin similarly in all the brain regions examined. Overall spatial patterns of the neuropeptide induction were similar among the neurotrophins. Neurons in adult rat brain can also react with the neurotrophins and alter neuropeptide expression in a slightly different fashion. Excitatory neuronal activity and hormones are known to change expression of neurotrophins. Therefore, neurotrophins, neuronal activity, and hormones influence each other and all regulate neurotransmitter/peptide expression in developing and mature brain. Physiological implication of the neurotransmitter/peptide differentiation activities is also discussed.