The therapeutic potentials of neurotrophic factors for diseases of the nervous system

Nerve growth factor from Vipera lebetina venom

Nerve growth factor was isolated from the Vipera lebetina venom by a four-step procedure including gel filtration, ion exchange, heparin and hydrophobic chromatography. The purified protein is a glycosylated non-covalently bound homodimer with monomeric molecular mass of 14,380 Da. The cDNA encoding NGF is cloned and sequenced. The amino acid sequence translated from the cDNA comprises 117 or 119 amino acids depending on the N-terminus (truncated or not). The recombinant NGF (expressed in Escherichia coli) was used to prepare the anti-NGF antiserum. The antiserum interacted with the wild-type NGF and enabled to localize NGF during the purification procedure in parallel with MALDI-TOF analysis of tryptic peptides. The isolated NGF caused neurite outgrowth from PC12 cells in concentrations beginning from 2.5 ng/ml.

Characterization of nerve growth factors (NGFs) from snake venoms by use of a novel, quantitative bioassay utilizing pheochromocytoma (PC12) cells overexpressing human trkA receptors

Snake venoms are a very abundant source of nerve growth factors (NGF). NGFs of Elapidae showing 65% sequence homology with mouse or human NGF, while the Viperidae NGF shows N-glycosylation (Asn-21) typical of these mammalian NGFs. Snake NGF-induced neurite outgrowth (neurotropic activity) was measured in the past by using PC12 cell or dorsal root ganglion bioassays. The present study was aimed at comparing, by dose-response experiments, the neurotropic activity of cobra and vipera versus mammalian NGFs, by using a novel bioassay involving PC12 cells genetically engineered to overexpress NGF-trkA receptors of human origin. These cells respond to NGF by differentiation (morphologically expressed as neurite outgrowth) by a process mediated by NGF-trkA receptors. This process was evaluated by two different criteria: (1) elongation of neurites (E), and (2) Percentage of responsive cells (PRC) determined by digital acquisition of data and computer analysis. We found that snake venom NGFs were less potent than mouse NGF, and that cobra NGF was more potent than vipera NGF. These data indicate the following order of NGF activity towards recombinant human trkA receptors: recombinant human NGF>mouse NGF>cobra NGF>vipera NGF. The neurotropic efficacy of these NGFs was found to be similar, reaching 80-90% of maximal activity obtained with all NGF forms. Interestingly, cobra (but not vipera) NGF demonstrated prolonged neurotropic activity compared with mouse NGF. The results of the present study indicate that cobra and vipera venom NGFs represent natural agonists of human trkA-receptor of a lower potency, but of similar efficacy, compared with mammalian NGFs. These compounds are important pharmacological tools to characterize the trkA receptor structure-function relationship, and to develop novel neurotropic drugs.

GDNF acutely modulates excitability and A-type K(+) channels in midbrain dopaminergic neurons

Glial cell line-derived neurotrophic factor (GDNF) prevents lesion-induced death of midbrain dopaminergic neurons, but its function in normal brain remains uncertain. Here we show that GDNF acutely and reversibly potentiated the excitability of cultured midbrain neurons by inhibiting transient A-type K(+) channels. The effects of GDNF were limited to large, tyrosine hydroxylase (TH)-positive dopaminergic neurons, and were mediated by mitogen associated protein (MAP) kinase. Application of GDNF also elicited a MAP kinase-dependent enhancement of the excitability in dopaminergic neurons in midbrain slice. These results demonstrate an acute regulation of GDNF on ion channels and its underlying signaling mechanism, and reveal an unexpected role of GDNF in normal midbrain dopaminergic neurons.

Partial prevention of cisplatin-induced neuropathy by electroporation-mediated nonviral gene transfer

Cisplatin-induced sensory peripheral neuropathy is the dose-limiting factor for cisplatin chemotherapy. We describe the preventive effect of NT-3 delivery, using direct gene transfer into muscle by in vivo electroporation in a mouse model of cisplatin-induced neuropathy. Cisplatin-induced neuropathy was produced by weekly injections of cisplatin (five injections). Two doses of plasmid DNA encoding murine NT-3 (pCMVNT-3) were tested (5 and 50 microg/animal/injection). Cisplatin-treated mice were given two intramuscular injections. The first injection of pCMVNT-3 was given 2 days before the first injection of cisplatin and the second injection 2 weeks later. Six weeks after the start of the experiment, measurement of NT-3 levels (ELISA) demonstrated significant levels both in muscle and plasma. We observed a smaller cisplatin-related increase in the latency of the sensory nerve action potential of the caudal nerve in pCMVNT-3-treated mice than in controls (p < 0.0001). Mean sensory distal latencies were not different between the 5- and 50- microg/animal/injection groups. Treatment with gene therapy induced only a slight muscle toxicity and no general side effects. Therefore, neurotrophic factor delivery by direct gene transfer into muscle by electroporation is of potential benefit in the prevention of cisplatin-induced neuropathy and of peripheral neuropathies in general.

Dimeric beta-turn peptidomimetics as ligands for the neurotrophin receptor TrkC

Twelve dimeric peptidomimetics 1 were prepared via a divergent-convergent strategy. These peptidomimetics incorporated the same amino acids as i +1 and i + 2 residues in key beta-turns of the neurotrophin NT-3. Cytosensor microphysiometry was used to gauge the effects of the dimers 1 on cells that overexpress the NT-3 receptor, TrkC. Increases in extracellular acidification rates were observed for some monomers 3, but the active dimers gave greater effects.

Role of neurotrophins and lectins in prevention of ototoxicity

Degeneration of hair cells (HC) and/or spiral ganglion neurons (SGN) is a major cause of hearing loss. Postnatal rat cochlear explant cultures are used to study the toxic actions of different classes of ototoxins and to identify molecules that can protect SGN and HC from ototoxic damage. Various ototoxins induce differential damage to HC and/or SGN. While gentamicin preferentially causes HC death, sodium salicylate selectively induces degeneration of SGN. In contrast, cisplatin results in destruction of both SGN and HC. Specific neurotrophins, including NT-4/5, BDNF, and NT-3, greatly protect SGN from all three types of ototoxins. In contrast, NGF and other growth factors have no effect. Of the 51 compounds examined, only concanavalin A (Con A), a lectin molecule, significantly protects HC from gentamicin. A dose-dependent study of Con A shows that maximal protection occurred at 100 nM. Further experiments indicates that preincubation of Con A with gentamicin does not form a complex, and coaddition of Con A and gentamicin to bacterial cultures, such as E. Coli cultures, does not interfere with the antibiotic activity of gentamicin. When the other 21 lectins are examined, Erythrina cristagalli lectin and Detura stramonium lectin also show activity similar to Con A. These findings may help elucidate the mechanisms of ototoxins and suggest that specific neurotrophins and lectins may be of therapeutic value in the prevention of ototoxin-induced hearing loss.

Clenbuterol induces growth factor mRNA, activates astrocytes, and protects rat brain tissue against ischemic damage

The induction of growth factor synthesis in brain tissue by beta2-adrenoceptor agonists, such as clenbuterol, is a promising approach to protect brain tissue from ischemic damage. Clenbuterol (0.01-0.5 mg/kg) reduced the cortical infarct volume in Long-Evans rats as measured 7 days after permanent occlusion of the middle cerebral artery. Dosages of clenbuterol higher than 1 mg/kg showed no cerebroprotective effect due to a decrease in blood pressure and an increase in plasma glucose level. The increase in the mRNA level of nerve growth factor (NGF), basic fibroblast growth factor (basic FGF), and transforming growth factor-beta1 (TGF-beta1) mRNA in cortical and hippocampal tissue occurred earlier after middle cerebral artery occlusion and was more pronounced in animals treated with clenbuterol than in controls. In addition, glial fibrillary acidic protein (GFAP) mRNA expression was enhanced in astrocytes 6 h after ischemia in clenbuterol-treated animals. The results suggest that growth factor synthesis is enhanced in activated astrocytes and that this could be the mechanism of clenbuterol-induced cerebroprotection after ischemia.

Differential signaling of glial cell line-derived neurothrophic factor and brain-derived neurotrophic factor in cultured ventral mesencephalic neurons

In the ventral mesencephalon, two neurotrophic factors, brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor, have been shown previously to have similar effects on the survival of dopaminergic neurons. Here, we compared the signaling mechanisms for brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor, focusing on the mitogen-associated protein kinase and the transcription factor cyclic-AMP responsive element-binding protein. Double-staining experiments indicated that many neurons co-expressed the receptors for glial cell line-derived neurotrophic factor and brain-derived neurotrophic factor, c-RET and TrkB, suggesting that they are responsive to both brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor. Although both brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor induced a rapid phosphorylation of mitogen-associated protein kinase and cyclic-AMP, responsive element-binding protein, there were significant differences in the kinetics and pharmacology of the phosphorylation. The phosphorylation of mitogen-associated protein kinase by glial cell line-derived neurotrophic factor was transient; within 2 h, the level of mitogen-associated protein kinase phosphorylation returned to baseline. In contrast, the effect of brain-derived neurotrophic factor was long lasting; the mitogen-associated protein kinase remained phosphorylated for up to 4 h after brain-derived neurotrophic factor treatment. PD098059, a specific inhibitor for mitogen-associated protein kinase kinase, completely blocked the glial cell line-derived neurotrophic factor signaling through mitogen-associated protein kinase, but had no effect on brain-derived neurotrophic factor-induced mitogen-associated protein kinase phosphorylation. Both brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor induced the phosphorylation of cyclic-AMP responsive element-binding protein in the nuclei of ventral mesencephalon neurons. However, PD098059 blocked the cyclic-AMP responsive element-binding protein phosphorylation induced by glial cell line-derived neurotrophic factor, but not that by brain-derived neurotrophic factor. These results indicate that, although both brain-derived neurotrophic factor and glial cell line-derived neurotrophic factor act on ventral mesencephalon neurons, the two factors have different signaling mechanisms, which may mediate their distinctive biological functions.

NGF mediates the neuroprotective effect of the beta2-adrenoceptor agonist clenbuterol in vitro and in vivo: evidence from an NGF-antisense study

Previous studies in our laboratory suggested that neuroprotective effects of the beta2-adrenoceptor agonist clenbuterol in vitro and in vivo occurred due to enhanced synthesis of nerve growth factor. The aim of the present study was to evaluate the effects of a phosphothioated NGF oligodeoxynucleotide on neuroprotection by clenbuterol in vitro and in vivo. After clenbuterol treatment (1-100 microM) an increase in nerve growth factor mRNA and protein levels (200-300% of control) was observed in primary cultures of rat cortical astrocytes. Nerve growth factor antisense oligonucleotide (0.3-1 microM for 3 days) reduced the content of nerve growth factor protein in the medium of the astrocytes concentration-dependently to 20% of control level. Nerve growth factor content in the medium of mixed hippocampal cells was reduced to 55% of sister cultures receiving the vehicle or a random control oligonucleotide. In mixed hippocampal cultures pretreated with random oligonucleotide (1 microM, 30 h), clenbuterol (10 microM) reduced the percentage of damaged neurons after glutamate exposure (0.5 mM, 1 h) to 17%. Pretreatment with nerve growth factor antisense oligonucleotide (1 microM) for 30 h before glutamate incubation blocked the protective effect of clenbuterol. In vivo, clenbuterol (0.01-0.1 mg/kg) reduced the infarct volume in a rat model of permanent focal cerebral ischemia dose-dependently. Nerve growth factor antisense oligonucleotides injected into the cortical tissue before ischemia abolished the cerebroprotective effect of clenbuterol. Our results indicate that the nerve growth factor antisense oligonucleotide presented in this study is a useful tool to investigate the effects of nerve growth factor knock down. By using the nerve growth factor antisense oligonucleotide we could demonstrate that nerve growth factor mediated the neuroprotective effects of the beta2-adrenoceptor agonist clenbuterol in vitro and in vivo.

Reduction of ischemic damage in NGF-transgenic mice: correlation with enhancement of antioxidant enzyme activities

If permanent focal ischemia is induced by middle cerebral artery occlusion (MCAO), neurons within the infarcted territory die by necrosis and apoptosis (or programmed cell death). We have previously shown, using a mouse strain transgenic (tg) for the nerve growth factor (NGF) gene, that tg mice have consistently smaller infarcted areas than wild-type (wt) animals, correlated with upregulated NGF synthesis and impaired apoptotic cell death. We studied, in wt and tg mice subjected to MCAO, the activities of several antioxidant enzymes and the synthesis of the proteins of the Bcl-2 family. Our results show that the antiapoptotic Bcl-2 protein and glutathione peroxidase are recruited after MCAO. NGF-tg mice also had an intrinsic resistance to oxidative stress because their basal copper zinc superoxide dismutase (SOD) and glutathione transferase activities were high. Additionally, manganese SOD activity increased in NGF-tg mice after MCAO, correlating strongly with the resistance of these mice to apoptosis.

Differential effects of GDNF and BDNF on cultured ventral mesencephalic neurons

Previous studies have shown that brain derived neurotrophic factor (BDNF) and glial cell line-derived neurotrophic factor (GDNF) can enhance the survival of dopaminergic neurons in the ventral mesencephalon (VM). Here we compared several non-survival functions of the two factors in VM neurons in culture. We found that both BDNF and GDNF elicited an increase in the depolarization-induced release of dopamine, but had no effect on GABA release, in the VM cultures. BDNF, but not GDNF, significantly enhanced the expression of the calcium binding protein calbindin and synaptic protein SNAP25. In contrast, treatment of the cultures with GDNF, but not BDNF, elicited a marked fasciculation of the processes of the VM neurons. Thus, although both act on VM neurons, BDNF and GDNF have distinct functions.

Cloning and characterization of glial cell line-derived neurotrophic factor receptor-B: a novel receptor for members of glial cell line-derived neurotrophic factor family of neurotrophic factors

Glial cell line-derived neurotrophic factor (GDNF) is a potent neurotrophic factor with diverse biological functions. Signal transduction of GDNF is mediated by binding to a glycosyl-phosphatidylinositol (GPI)-linked receptor GDNFR-alpha and activation of c-RET tyrosine kinase. The recent discovery of a new GDNF homolog neurturin raises the possibility that multiple receptors exist for the members in the GDNF family. Here we report isolation of the gene encoding a new receptor called GDNFR-beta. Sequence analysis indicated that GDNFR-beta is also a GPI-linked protein, with 47% identity to GDNFR-alpha. The GDNFR-beta transcript was preferentially expressed in the brain, spleen and lung, but moderate levels of GDNFR-beta mRNA were also found in kidney and the entire gastrointestinal track. In situ hybridization revealed high expression levels in the entorhinal cortex and olfactory bulb, followed by cortex, septum, inferior and superior colliculus, and zona inserta. A laminar pattern of expression was detected in layer III of the cortex. Treatment with GDNF of PC12 cells transfected with the GDNFR-beta gene activated mitogen-activated protein kinase (MAPK) and elicited neurite outgrowth. GDNFR-alpha and GDNFR-beta together form a new family of GPI-linked receptors for GDNF-like molecules.