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PubMedDate not supplied

Neurotrophins and their receptors in nerve injury and repair.

Cytokines are a heterogenous group of polypeptide mediators that have been associated with activation of numerous functions, including the immune system and inflammatory responses. The cytokine families include, but are not limited to, interleukins (IL-I alpha, IL-I beta, ILIra and IL-2-IL-15), chemokines (IL-8/ NAP-I, NAP-2, MIP-I alpha and beta, MCAF/MCP-1, MGSA and RANTES), tumor necrosis factors (TNF-alpha and TNF-beta), interferons (INF-alpha, beta and gamma), colony stimulating factors (G-CSF, M-CSF, GM-CSF, IL-3 and some of the other ILs), growth factors (EGF, FGF, PDGF, TGF alpha, TGF beta and ECGF), neuropoietins (LIF, CNTF, OM and IL-6), and neurotrophins (BDNF, NGF, NT-3-NT-6 and GDNF). The neurotrophins represent a family of survival and differentiation factors that exert profound effects in the central and peripheral nervous system (PNS). The neurotrophins are currently under investigation as therapeutic agents for the treatment of neurodegenerative disorders and nerve injury either individually or in combination with other trophic factors such as ciliary neurotrophic factor (CNTF) or fibroblast growth factor (FGF). Responsiveness of neurons to a given neurotrophin is governed by the expression of two classes of cell surface receptor. For nerve growth factor (NGF), these are p75NTR (p75) and p140trk (referred to as trk or trkA), which binds both BDNF and neurotrophin (NT)-4/5, and trkC receptor, which binds only NT-3. After binding ligand, the neurotrophin-receptor complex is internalized and retrogradely transported in the axon to the soma. Both receptors undergo ligand-induced dimerization, which activates multiple signal transduction pathways. These include the ras-dependent pathway utilized by trk to mediate neurotrophin effects such as survival and differentiation. Indeed, cellular diversity in the nervous system evolves from the concerted processes of cell proliferation, differentiation, migration, survival, and synapse formation. Neural adhesion and extracellular matrix molecules have been shown to play crucial roles in axonal migration, guidance, and growth cone targeting. Proinflammatory cytokines, released by activated macrophages and monocytes during infection, can act on neural targets that control thermogenesis, behavior, and mood. In addition to induction of fever, cytokines induce other biological functions associated with the acute phase response, including hypophagia and sleep. Cytokine production has been detected within the central nervous system as a result of brain injury, following stab wound to the brain, during viral and bacterial infections (AIDS and meningitis), and in neurodegenerative processes (multiple sclerosis and Alzheimer's disease). Novel cytokine therapies, such as anticytokine antibodies or specific receptor antagonists acting on the cytokine network may provide an optimistic feature for treatment of multiple sclerosis and other diseases in which cytokines have been implicated.

Animals↗

PubMed2004

Correlations among neural cell adhesion molecule, nerve growth factor, and its receptors, TrkA, TrkB, TrkC, and p75, in perineural invasion by basal cell and cutaneous squamous cell carcinomas.

BACKGROUND: Neural cell adhesion molecule; nerve growth factor; and its receptors TrkA, TrkB, TrkC, and p75(NGFR) regulate unique survival pathways in the cells of different noncutaneous malignancies exhibiting a perineural invasive phenotype. Their expression is currently unknown in basal cell carcinoma and cutaneous squamous cell carcinoma exhibiting perineural invasion. OBJECTIVE: The objective was to evaluate neural cell adhesion molecule, nerve growth factor, TrkA, TrkB, TrkC, and p75(NGFR) expression when perineural invasion occurs in basal cell carcinoma and cutaneous squamous cell carcinoma. METHODS: Four basal cell carcinomas and five cutaneous squamous cell carcinomas that were perineural invasion-positive and four basal cell carcinomas and three cutaneous squamous cell carcinomas that were perineural invasion-negative were evaluated for the expression of neural cell adhesion molecule, nerve growth factor, TrkA, TrkB, TrkC, and p75(NGFR) by immunohistochemical staining. RESULTS: For neural cell adhesion molecule, six of eight basal cell carcinomas and two of eight cutaneous squamous cell carcinomas stained positive. For nerve growth factor, seven of nine perineural invasion-positive and six of seven perineural invasion-negative tumors stained moderately or greater. For TrkA, B, and C, the perineural invasion-positive cutaneous squamous cell carcinomas generally stained darker than the perineural invasion-negative cutaneous squamous cell carcinomas. For p75(NGFR), four of five perineural invasion-positive cutaneous squamous cell carcinoma stain patterns indicated that p75(NGFR) expression was higher perineurally than elsewhere in the tumor. CONCLUSION: In basal cell carcinoma and cutaneous squamous cell carcinoma, the presence of neural cell adhesion molecule may determine tumor aggression and increased levels of nerve growth factor and TrkA, B, and C may reflect unique survival pathways. Higher levels of expression of Trk receptors in cutaneous squamous cell carcinoma cells may predict perineural invasion and the increased p75(NGFR) expression found in cutaneous squamous cell carcinoma perineurally may allow p75(NGFR) immunohistochemical staining to be used for detecting sites of perineural invasion.

Aged↗

PubMed2000

Autocrine expression and ontogenetic functions of the PACAP ligand/receptor system during sympathetic development.

The superior cervical ganglion (SCG) is a well-characterized model of neural development, in which several regulatory signals have been identified. Vasoactive intestinal peptide (VIP) has been found to regulate diverse ontogenetic processes in sympathetics, though functional requirements for high peptide concentrations suggest that other ligands are involved. We now describe expression and functions of pituitary adenylate cyclase-activating polypeptide (PACAP) during SCG ontogeny, suggesting that the peptide plays critical roles in neurogenesis. PACAP and PACAP receptor (PAC(1)) mRNA's were detected at embryonic days 14.5 (E14.5) through E17.5 in vivo and virtually all precursors exhibited ligand and receptor, indicating that the system is expressed as neuroblasts proliferate. Exposure of cultured precursors to PACAP peptides, containing 27 or 38 residues, increased mitogenic activity 4-fold. Significantly, PACAP was 1000-fold more potent than VIP and a highly potent and selective antagonist entirely blocked effects of micromolar VIP, consistent with both peptides acting via PAC(1) receptors. Moreover, PACAP potently enhanced precursor survival more than 2-fold, suggesting that previously defined VIP effects were mediated via PAC(1) receptors and that PACAP is the more significant developmental signal. In addition to neurogenesis, PACAP promoted neuronal differentiation, increasing neurite outgrowth 4-fold and enhancing expression of neurotrophin receptors trkC and trkA. Since PACAP potently activated cAMP and PI pathways and increased intracellular Ca(2+), the peptide may interact with other developmental signals. PACAP stimulation of precursor mitosis, survival, and trk receptor expression suggests that the signaling system plays a critical autocrine role during sympathetic neurogenesis.

Animals↗

PubMed1998

Neurotrophins prevent death and differentially affect tyrosine hydroxylase of adult rat nigrostriatal neurons in vivo.

Brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), and neurotrophin-4 (NT-4) promote survival of mesencephalic dopaminergic neurons in vitro and affect normal and damaged ones in vivo. Here, these neurotrophins had markedly different potencies to prevent the death of axotomized nigrostriatal dopaminergic neurons when infused close to the rostral end of the nigral nucleus of adult rats (NT-4 > BDNF > NT-3; nerve growth factor or NGF without effect). With a high dose of BDNF (30 micrograms/day) complete protection was achieved in the rostral but not caudal nigral regions, consistent with its poor diffusion characteristics in brain tissue. Measurements of tyrosine hydroxylase immunoreactivity suggest that BDNF and NT-4 (presumably through their TrkB receptor) reduce the synthesis of this rate-limiting enzyme for dopamine synthesis in rescued as well as in normal neurons. In sharp contrast, survival-promoting doses of NT-3 (presumably through its TrkC receptor) maintained normal levels of tyrosine hydroxylase immunoreactivity in the rescued nigrostriatal neurons. These results suggest that for these adult central nervous system neurons, some neurotrophic factors are predominantly involved in facilitating cell survival, whereas others are more involved in regulating neurotransmitter function.

Animals↗

PubMed2001

Association of Trk neurotrophin receptors with components of the cytoplasmic dynein motor.

Nerve growth factor (NGF) initiates its trophic effects by long-range signaling through binding, internalization, and transport of a ligand-receptor complex from the axon terminal to the cell body. However, the mechanism by which retrograde transport of NGF takes place has not been elucidated. Here we describe an interaction between the Trk receptor tyrosine kinase and a 14 kDa light chain of cytoplasmic dynein. After transfection in human embryonic kidney 293 cells, this 14 kDa dynein light chain was found to bind to TrkA, TrkB, and TrkC receptors. Mapping experiments indicated that the 14 kDa dynein light chain binds to the distal region of the TrkA juxtamembrane domain. Coimmunoprecipitation experiments in vivo indicate that Trk receptors are in a complex with the 14 kDa light chain and 74 kDa intermediate chain of dynein. Confirming the physiological relevance of this association, a marked accumulation of Trk with the 14 kDa and the 74 kDa dynein components was observed after ligation of the sciatic nerve. The association of Trk receptors with components of cytoplasmic dynein suggests that transport of neurotrophins during vesicular trafficking may occur through a direct interaction of the Trk receptor with the dynein motor machinery.

Animals↗

PubMed2000

Differential regulation of trophic factor receptor mRNAs in spinal motoneurons after sciatic nerve transection and ventral root avulsion in the rat.

After sciatic nerve lesion in the adult rat, motoneurons survive and regenerate, whereas the same lesion in the neonatal animal or an avulsion of ventral roots from the spinal cord in adults induces extensive cell death among lesioned motoneurons with limited or no axon regeneration. A number of substances with neurotrophic effects have been shown to increase survival of motoneurons in vivo and in vitro. Here we have used semiquantitative in situ hybridization histochemistry to detect the regulation in motoneurons of mRNAs for receptors to ciliary neurotrophic factor (CNTF), leukemia inhibitory factor (LIF), glial cell line-derived neurotrophic factor (GDNF), brain-derived neurotrophic factor (BDNF), and neurotrophin-3 (NT-3) 1-42 days after the described three types of axon injury. After all types of injury, the mRNAs for GDNF receptors (GFRalpha-1 and c-RET) and the LIF receptor LIFR were distinctly (up to 300%) up-regulated in motoneurons. The CNTF receptor CNTFRalpha mRNA displayed only small changes, whereas the mRNA for membrane glycoprotein 130 (gp130), which is a critical receptor component for LIF and CNTF transduction, was profoundly down-regulated in motoneurons after ventral root avulsion. The BDNF full-length receptor trkB mRNA was up-regulated acutely after adult sciatic nerve lesion, whereas after ventral root avulsion trkB was down-regulated. The NT-3 receptor trkC mRNA was strongly down-regulated after ventral root avulsion. The results demonstrate that removal of peripheral nerve tissue from proximally lesioned motor axons induces profound down-regulations of mRNAs for critical components of receptors for CNTF, LIF, and NT-3 in affected motoneurons, but GDNF receptor mRNAs are up-regulated in the same situation. These results should be considered in relation to the extensive cell death among motoneurons after ventral root avulsion and should also be important for the design of therapeutical approaches in cases of motoneuron death.

Aging↗

PubMed1997

Nerve growth factor induces apoptosis in human medulloblastoma cell lines that express TrkA receptors.

Neurotrophins act through their cognate receptors to promote the differentiation and/or survival of neuronal progenitor cells, immature neurons, and other cells. Here, we examined the effects of nerve growth factor (NGF) and its cognate receptor (Trk or TrkA) on the survival of a common childhood brain tumor, i.e., medulloblastoma, a tumor that resembles CNS neuroepithelial progenitor cells. To do this, we engineered two human medulloblastoma cell lines (i.e., D283MED and DAOY cells) to express human TrkA using a retroviral expression vector. Surprisingly, NGF-treated medulloblastoma cells expressing the TrkA receptor (D283trk and DAOYtrk cells) grown in the presence or absence of serum underwent massive apoptosis, but similar treatment did not induce apoptosis in wild-type uninfected cells, cells expressing an empty vector, or cells expressing the TrkC receptor. Furthermore, D283MED cells engineered to express the human p75 NGF receptor (D283p75) also did not undergo apoptosis. Significantly, NGF-induced apoptosis in D283trk and DAOYtrk cells can be inhibited by anti-NGF antibodies and by K-252a, an inhibitor of TrkA tyrosine phosphorylation and mimicked by high concentrations of NT3. Because NGF treatment primarily eliminated D283trk cells from the S phase of the cell cycle, this form of NGF-mediated apoptosis is cell cycle-dependent. These findings suggest that a NGF/TrkA signal transduction pathway could activate apoptotic cell death programs in CNS neuroepithelial progenitor cells and in childhood brain tumors.

3T3 Cells↗

PubMed1997

Effects of X-irradiation on nerve growth factor in the developing mouse brain.

The involvement of neurotrophins after radiation injury during brain development were studied in pregnant mice (C 57/B1) exposed on gestation day 15 to X-ray doses of 0.02-2 Gy. Nerve growth factor protein (NGF) and different cholinergic markers were investigated on postnatal day 1 (P1) and day 21 (P21); in situ hybridization with brain-derived neurotrophic factor (BDNF) and trkC (receptor serving to bind neurotrophin-3) probes was investigated on P21 in cortex, hippocampus, septum and cerebellum. The level of NGF protein was increased in irradiated forebrain on P1 in a dose-related manner. However, on P21 the NGF protein dropped down below the control levels in irradiated hippocampus and cerebellum. The response of acetylcholine esterase (AChE) activity in cerebellum at P21 was correlated with the changes in the amount of NGF. The intensity of cell labelling with trkC probe decreased after irradiation in the region of the hippocampus at P21, especially in dentate gyrus. The expression of BDNF mRNA was increased at P21 by low doses of irradiation (0.02-1 Gy) but was decreased by a high dose (2 Gy) in the same area. Thus, the radiation induced an alteration of neurotrophins, and the changes varied depending on the dose or time after irradiation. Such alterations in the pattern of growth factor production may modulate the response of cells to radiation. Furthermore, NGF protein levels and the expression of BDNF and trkC mRNA were affected by radiation doses as low as 0.02 Gy, indicating that during development the neurotrophins and their receptors are very sensitive to radiation.

Acetylcholinesterase↗

PubMed2000

TrkB receptor ligands promote activity-dependent inhibitory synaptogenesis.

Organotypic cerebellar cultures derived from newborn mice were simultaneously exposed to activity-blocking agents and neurotrophins for 2 weeks. Activity-blocked explants treated with the TrkB receptor ligands BDNF and neurotrophin-4 (NT-4) developed a full complement of Purkinje cell inhibitory axosomatic synapses, as defined ultrastructurally, and displayed control spontaneous cortical discharge rates after recovery from activity blockade. Otherwise untreated activity-blocked cultures and activity-blocked cultures exposed to the TrkC receptor ligand NT-3 had reduced inhibitory synapse development and persistent cortical hyperactivity after recovery. The added TrkB receptor ligands did not induce axonal sprouting to account for increased inhibitory synaptogenesis. Addition of neurotrophins to untreated cerebellar cultures did not increase the complement of Purkinje cell axosomatic synapses. Exposure of cerebellar cultures to a combination of antibodies to BDNF and NT-4 resulted in reduced inhibitory synapse formation, similar to the effects of activity blockade, indicating the necessity for endogenous neurotrophins for development of the full complement of inhibitory synapses in the presence of neuronal activity. Application of antibodies to BDNF and NT-4 to cerebellar explants exposed to picrotoxin to increase neuronal activity prevented the hyperinnervation of Purkinje cell somata by inhibitory terminals characteristic of cultures exposed to picrotoxin alone. These results are consistent with the concept that TrkB receptor ligands promote inhibitory synaptogenesis. The ability of neurotrophins to substitute for neuronal activity in encouraging development of inhibitory synapses may have therapeutic implications.

Animals↗

PubMed2001

TrkA and TrkB neurotrophin receptor immunoreactivity in the teleost (Scorpaena porcus) endocrine pancreas.

Mammalian-like and specific neurotrophins, as well as their cognate tyrosine kinase Trk-like receptors have been identified in teleosts. They are mainly distributed in neuronal tissues, but evidence suggests that some non-neuronal tissues also express Trks. In this study we used immunohistochemistry to investigate the occurrence and cell distribution of Trks in the pancreas of teleosts (Scorpaena porcus). Immunoreactivity for TrkA and TrkB, but not for TrkC receptors, was found in the scorpionfish pancreas. TrkA-like positive cells were exclusively observed within pancreatic islets, among insulin- and glucagon-containing cells, but apparently did not co-localize with these hormones. TrkB immunostaining was found in islet cells, presumably colocalized with glucagon, as well as in some cells of the exocrine portion. These data provide a morphological basis for a role of the TrkA and TrkB ligands in the endocrine pancreas of teleosts.

Animals↗

PubMed2005

Neurotrophin system activation in bronchoalveolar lavage fluid immune cells in pulmonary sarcoidosis.

BACKGROUND AND AIM OF THE STUDY: Pulmonary sarcoidosis is a chronic granulomatous disease characterized by macrophage and CD4+ T-cell accumulation at the site of inflammation. Analysis of the cytokine network has substantially improved knowledge on immunopathogenesis of sarcoidosis. We hypothesize that neurotrophins (NTs), nerve growth factor (NGF), brain derived neurotrophic factor (BDNF) and NT-3, besides their importance in immune system activities, participate in chronic inflammatory disorders and in repair processes. METHODS: The expression of NTs and NT receptors was assessed in broncho alveolar lavage (BAL) macrophages, CD4 and CD8 T-cells, from 10 patients with pulmonary sarcoidosis, using molecular biology, Western blotting and immunocytochemistry. RESULTS: Increased levels of NTs and of high affinity NT receptor (Trks) transcripts and proteins in BAL macrophages, CD4+ and CD8+ T-cells from pulmonary sarcoidosis patients were demonstrated in comparison with healthy controls. Contrarily to healthy controls, in pulmonary sarcoidosis the expression of NGF was increased in alveolar macrophages as well as NGF and BDNF in CD4+ and CD8+ T-cells. An increased expression of TrkA, TrkB and TrkC receptors was also noticeable. Furthermore, BDNF expression in alveolar macrophages and NT-3 expression in the three different BAL immune cell populations investigated were induced during sarcoidosis. A significant correlation was observed between CD4:CD8 ratio, lymphocytosis, radiological stage and CD4 and CD8 NT expression. CONCLUSIONS: These findings suggest that NTs are exaggeratedly expressed in BAL immune cells in pulmonary sarcoidosis and may participate in the progression of disease modulating immune cell functions.

Adult↗

PubMed1997

Neurotrophins in cerebellar granule cell development and medulloblastoma.

Medulloblastomas may be derived from granule cells of the developing cerebellum. Children with tumors expressing high levels of the neurotrophin-3 receptor, TrkC, have a more favorable outcome. During development, TrkC is expressed in the most mature granule cells. Favorable medulloblastomas may be derived from more highly differentiated granule cells.

Animals↗

PubMed2001

Brn3a is a transcriptional regulator of soma size, target field innervation and axon pathfinding of inner ear sensory neurons.

The POU domain transcription factors Brn3a, Brn3b and Brn3c are required for the proper development of sensory ganglia, retinal ganglion cells, and inner ear hair cells, respectively. We have investigated the roles of Brn3a in neuronal differentiation and target innervation in the facial-stato-acoustic ganglion. We show that absence of Brn3a results in a substantial reduction in neuronal size, abnormal neuronal migration and downregulation of gene expression, including that of the neurotrophin receptor TrkC, parvalbumin and Brn3b. Selective loss of TrkC neurons in the spiral ganglion of Brn3a(-/-) cochlea leads to an innervation defect similar to that of TrkC(-/-) mice. Most remarkably, our results uncover a novel role for Brn3a in regulating axon pathfinding and target field innervation by spiral and vestibular ganglion neurons. Loss of Brn3a results in severe retardation in development of the axon projections to the cochlea and the posterior vertical canal as early as E13.5. In addition, efferent axons that use the afferent fibers as a scaffold during pathfinding also show severe misrouting. Interestingly, despite the well-established roles of ephrins and EphB receptors in axon pathfinding, expression of these molecules does not appear to be affected in Brn3a(-/-) mice. Thus, Brn3a must control additional downstream genes that are required for axon pathfinding.

Animals↗

PubMed2000

Neurotrophin-3 (NT-3) diminishes susceptibility of the oligodendroglial lineage to AMPA glutamate receptor-mediated excitotoxicity.

Prior reports demonstrated that cells of the oligodendroglial lineage are susceptible to excitotoxic necrosis mediated by alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid glutamate receptors (AMPA-GluR), and also showed that these cells express the high affinity neurotrophin receptors, TrkC and TrkA. We now report that: a) oligodendroglial progenitors (OP) and immature oligodendroglia are more vulnerable to AMPA-GluR-mediated excitotoxicity than are mature oligodendroglia; b) TrkC expression falls substantially during differentiation of cultured OP to mature oligodendroglia, whereas TrkA expression increases markedly; and c) neurotrophin-3, and to a lesser extent, nerve growth factor, protect the oligodendroglial lineage against AMPA-GluR-mediated excitotoxicity.

Animals↗

PubMed1996

Developmental and denervation changes in c-ret proto-oncogene expression in chick motoneurons.

Receptor tyrosine kinases (RTKs) play important roles in cellular differentiation, survival, and proliferation. To search for genes involved in the survival of motoneurons, we isolated the RTKs specifically expressed on them. We used RT-PCR, by which RNA was obtained from purified embryonic day 5 (E5) chick motoneurons, and screened by in situ hybridization. Of 17 RTK cDNA clones, c-ret expression gradually increased in the motoneurons of the spinal cord during development. Compared with the expression of the neurotrophin receptor, TrkC, that of c-ret was very high in motoneurons from around E17 to adulthood. The level of c-ret expression on the motoneurons was unaffected by deafferentation at E2, but changed after denervation at post-hatching day 2. The in situ hybridization signal for c-ret mRNA increased slightly at day 1, then decreased progressively up to day 8, and increased again 2 weeks after sciatic nerve denervation in the motoneurons of the lumbar spinal cord. There were also changes in the signal of the lesioned sciatic nerve. In the most distal part from the lesioned site, the signal for c-ret mRNA significantly increased from day 3 on after denervation. These results suggested that the c-ret gene may encode the receptor for a factor involved in motoneuron differentiation and the promotion of regeneration of injured peripheral nerves.

Amino Acid Sequence↗

PubMed2002

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.

Animals↗

PubMed1999

Neurotrophin-3 (NT-3) modulates early differentiation of oligodendrocytes in rat brain cortical cultures.

Multiple extracellular signals are required for oligodendroglia survival, proliferation and differentiation, and increasing evidence has accumulated that also neurotrophins regulate glial cell development in the central nervous system (CNS). In the present study we have investigated the influence of neurotrophin-3 (NT-3) on the in vitro differentiation and proliferation of oligodendrocytes prepared from the brains of newborn rats. Cells were grown in chemically defined growth medium, in the absence of fetal calf serum (FCS). RT-PCR analysis confirmed the expression of mRNA encoding the NT-3 receptor trkC in oligodendrocytes throughout in vitro development. Cell morphology was observed by phase contrast microscopy and indirect immunofluorescence staining using anti-galactocerebroside (GalC) antibodies. An increase in process formation and arborization was observed 8-24 h after the treatment with NT-3 (5-50 ng/ml). Concomitantly, NT-3 caused an increase in the appearance of GalC-positive cells. Long-term treatment with NT-3 (up to seven days) did not yield any further improvement of process formation. To elucidate the molecular mechanisms and signal transduction pathways underlying the effect of NT-3 in oligodendrocytes, the time- and concentration-dependent effect of NT-3 on c-Fos protein expression was studied by Western blot analysis. The data show that NT-3 stimulated the appearance of two c-Fos immunoreactive polypeptides with apparent molecular weights of 62 and 55 kDa, respectively. This effect was maximal at a concentration of 50 ng/ml of NT-3 after 8-24 h. NT-3-modulated morphological differentiation and c-Fos protein expression was regulated by protein kinases. Whereas the protein kinase C (PKC) inhibitors staurosporine and chelerythrine chloride had a stimulatory effect on NT-3-promoted process formation, the tyrosine kinase inhibitor genistein had an inhibitory effect and mainly cells with a bipolar and immature morphology were observable. The inhibition of tyrosine kinase activity prevented NT-3-promoted induction of c-Fos protein. Thus, in addition to its mitogenic effects, NT-3 during early time points influences the in vitro differentiation of oligodendrocytes. This process involves the induction of c-Fos protein and is mediated by PKC and trosine kinase activities.

Animals↗

PubMed1999

Neurotrophins and their receptors in the tench retina during optic nerve regeneration.

To understand the role of neurotrophins in the visual system, we investigated the distribution of both neurotrophins and their receptors within the retina of a fish that has the capacity to spontaneously regenerate its optic nerve axons after lesion. Intact retinas and retinas from tench, whose optic nerve had been crushed, were analyzed by immunohistochemistry and in situ hybridization. Trk receptors were mainly immunolocalized in cells of the inner nuclear and ganglion cell layers, a distribution coincident with that of their mRNAs. Nerve growth factor (NGF) immunoreactivity was detected exclusively in Müller cell processes, and brain-derived neurotrophic factor (BDNF) was found in both neuronal bodies and Müller cell processes. Neurotrophin-3 (NT-3) was detected in most of the cell nuclei, and neurotrophin-4/5 (NT-4/5) was localized in fibers and in a few cells in the inner retina. An increase in both TrkA protein and mRNA was detected during axonal regeneration within the retinal ganglion cell layer, reaching a maximum 30 days postcrush and returning to normal levels by day 90, when optic nerve regeneration is almost completed in this fish. None of the other neurotrophins and receptors showed appreciable changes. The heterogeneous distribution patterns of neurotrophins and their receptors in fish retina, their differences from the distribution observed in other species, and the TrkA changes after optic nerve crush suggest an important role for these molecules in the normal physiology of the fish retina and during the regeneration process.

Animals↗