Italo Mocchetti

GM1 Ganglioside Activates the High‐Affinity Nerve Growth Factor Receptor trkA

Abstract: The monosialoganglioside GM1 has been shown to possess neurotrophic activity in vitro and in vivo and is now used as an experimental treatment for a variety of neurological disorders and trauma. Little is known about the mechanism of action used by GM1. Because GM1 appears to enhance nerve growth factor (NGF) activity, we have used C6trk+ cells, a derivative of C6‐2B glioma cells that express the high‐affinity receptor for NGF trkA, to determine whether the neurotrophic effects of GM1 occurs through induction of trkA activity. Exposure of C6trk+ cells to NGF (10–50 ng/ml) resulted in a five‐ to 10‐fold increase in trkA tyrosine phosphorylation within 5 min. Incubation of cells with GM1 resulted in a threefold increase in trkA phosphorylation beginning within 1 h and peaking between 3 and 6 h. Optimal responses to GM1 were obtained using 80–100 µM concentrations. Moreover, tyrosine phosphorylation of known trkA target proteins, such as extracellular signal‐regulated kinases, and suc‐associated neurotrophic factor‐induced tyrosine‐phosphorylated target, were activated upon stimulation of C6trk+ cells with GM1. In addition, GM1 potentiated the NGF‐mediated activation of tyrosine phosphorylation of trkA. GM1 failed to induce phosphorylation of trkA and target proteins in mock transfected cells. Thus, our data demonstrate that GM1 mimics some of the effects of NGF and suggest that the neurotrophic properties of GM1 may be attributed to its activation of trkA signal transduction.

Basic fibroblast growth factor mRNA increases in specific brain regions following convulsive seizures

Basic fibroblast growth factor (bFGF) is a trophic factor synthesized in the central nervous system (CNS), where it is believed to play a role in neuronal maintenance and repair. Little is known about the regulation of this growth factor in the CNS. To determine whether the expression of the bFGF gene in the brain of adult animals changes in response to alterations of neuronal activity, we examined bFGF mRNA levels in several brain regions of rats experiencing focally-evoked convulsive seizures. Seizures were induced by microinjecting bicuculline unilaterally into an epileptogenic site within the deep prepiriform cortex, area tempestas (AT). By 5 h after initiation of brief limbic motor seizures from AT, there was a four fold increase in the levels of bFGF mRNA in the entorhinal cortex, hippocampus and olfactory bulb, but not in the caudate-putamen. The maximal expression of bFGF mRNA was reached by 10 h after seizure onset. In the same animals, the mRNA encoding nerve growth factor (NGF) was increased in entorhinal cortex and hippocampus, but not in the olfactory bulb. Our results demonstrate that neuronal activity can influence bFGF expression in an anatomically selective fashion and that acute changes in bFGF can occur in the uninjured mature brain. The increase in bFGF expression in response to excessive activation of specific neuronal circuitry may represent an adaptive response to protect against potential injury in those circuits.

Basic and acidic fibroblast growth factors protect spinal motor neurones in vivo after experimental spinal cord injury.

We studied the effect of a single focal injection of recombinant basic (FGF2) or acidic (FGF1) fibroblast growth factor on the survival of spinal motor neurones at 24 h after a standardized spinal cord contusion injury (SCI) in the rat. Both FGF2 and FGF1 (3 μg), microinjected into the injury site at 5 min post‐injury (p.i.), protected at least two functionally important classes of spinal motor neurones, autonomic preganglionic neurones in the intermediolateral (IML) column and somatic motor neurones in the ventral horn (VH). Moreover, there was enhanced choline acetyltransferase (ChAT) immunoreactivity in surviving VH and IML neurones, suggesting an improved functional status. Thus, neurotrophic factors such as FGF2 and FGF1 may contribute to an overall strategy to treat acute SCI and improve recovery of function.

Regional and Temporal Pattern of Expression of Nerve Growth Factor and Basic Fibroblast Growth Factor mRNA in Rat Brain Following Electroconvulsive Shock

We have previously reported that focally evoked limbic motor seizures rapidly increase levels of mRNA encoding nerve growth factor (NGF) and basic fibroblast growth factor (bFGF) in specific limbic system areas of the adult rat brain. The present studies examined the effect of both minimal and maximal electroconvulsive shock, applied via corneal electrodes, on NGF and bFGF mRNA levels in several limbic (entorhinal cortex, hippocampus, olfactory bulb) and extralimbic (striatum and cerebellum) brain regions. By 5 h following limbic motor seizures induced by low-intensity (minimal) electroshock (LES) (0.2 s, 50-70 mA; three times over a 1-h period), bFGF mRNA was significantly increased in entorhinal cortex and hippocampus, but not in the other regions examined. In contrast, tonic extensor seizures evoked by maximal electroshock (MES) (0.2 s, 150 mA; three times over a 1-h period) were associated with a significant increase in bFGF mRNA in all limbic and extralimbic regions examined. In the same animals, increases in NGF mRNA were limited to entorhinal cortex and hippocampus. Adrenal steroids were not required for the seizure-induced increase in NGF or bFGF mRNAs, based on the finding that adrenalectomized rats exhibited electroshock-induced increases in both NGF and bFGF mRNAs equivalent to the increase observed in sham-operated rats. It is suggested that the increase in mRNA levels for the neurotrophic factors occurs selectively in those regions which are especially activated by the specific seizure model, and represents an adaptive response to repeated noninjurious neuronal stimulation.

Chronic vagus nerve stimulation induces neuronal plasticity in the rat hippocampus

Vagus nerve stimulation (VNS) is used to treat pharmacotherapy-resistant epilepsy and depression. However, the mechanisms underlying the therapeutic efficacy of VNS remain unclear. We examined the effects of VNS on hippocampal neuronal plasticity and behaviour in rats. Cell proliferation in the hippocampus of rats subjected to acute (3 h) or chronic (1 month) VNS was examined by injection of bromodeoxyuridine (BrdU) and immunohistochemistry. Expression of doublecortin (DCX) and brain-derived neurotrophic factor (BDNF) was evaluated by immunofluorescence staining. The dendritic morphology of DCX+ neurons was measured by Sholl analysis. Our results show that acute VNS induced an increase in the number of BrdU+ cells in the dentate gyrus that was apparent 24 h and 3 wk after treatment. It also induced long-lasting increases in the amount of DCX immunoreactivity and in the number of DCX+ neurons. Neither the number of BrdU+ cells nor the amount of DCX immunoreactivity was increased 3 wk after the cessation of chronic VNS. Chronic VNS induced long-lasting increases in the amount of BDNF immunoreactivity and the number of BDNF+ cells as well as in the dendritic complexity of DCX+ neurons in the hippocampus. In contrast to chronic imipramine treatment, chronic VNS had no effect on the behaviour of rats in the forced swim or elevated plus-maze tests. Both chronic and acute VNS induced persistent changes in hippocampal neurons that may play a key role in the therapeutic efficacy of VNS. However, these changes were not associated with evident behavioural alterations characteristic of an antidepressant or anxiolytic action.

Developmental expression of the basic fibroblast growth factor gene in rat brain.

Basic fibroblast growth factor (bFGF) is a trophic factor for a variety of neuronal/glial cell populations. The RNase protection assay, with a cRNA complementary to the coding region of bFGF mRNA, was used to investigate the brain distribution and developmental regulation of bFGF mRNA expression. In adult rats bFGF mRNA is distributed throughout the brain, the highest levels being observed in cerebral cortex, hippocampus and spinal cord. The levels of bFGF mRNA in all the brain structures are low in newborn rats, increase thereafter to reach a peak of expression around postnatal day 21. bFGF mRNA levels are significantly different between various brain structures during the first and second postnatal week. Adult and aged rats (Fisher 344) express the same levels of bFGF mRNA in the various brain regions. The onset of bFGF mRNA expression suggests that this growth factor is important for the maturation as well as for the maintenance of different cell populations of the central nervous system.

Increased basic fibroblast growth factor expression following contusive spinal cord injury.

Neurotrophic factors appear to be crucial for the survival and potential regeneration of injured neurons. We have previously demonstrated that contusive spinal cord injury (SCI) increases the levels of mRNA for basic fibroblast growth factor (FGF2). To determine whether FGF2 protein levels also increase, Western blot analysis was performed on extracts of spinal cord tissue after a standardized SCI and compared to laminectomy controls. In spinal cord extracts, a monoclonal antibody to FGF2 recognized various molecular forms of FGF2 (18-24 kDa) and some characteristic proteolytic fragments. Extracts of spinal cords 1 day after SCI showed a slight increase in the levels of these polypeptides. By 4 days, a significant increase (two-fold) was detected in the levels of the 18-kDa and higher molecular weight forms as well as the proteolytic fragments. Immunohistochemical analyses on spinal cord tissue sections confirmed an increased cellular (glial) FGF2 as well as interstitial immunoreactivity surrounding neurons and along blood vessels. Heparinpurified spinal cord extracts from tissue 4 days after SCI showed increased biological activity as indicated by their ability to (i) increase [3H]thymidine incorporation in cultures of Balb/c 3T3 cells and (ii) induce phosphorylation of suc-associated neurotrophic factor-induced tyrosine-phosphorylated target, a FGF2 target protein. These data suggest that SCI induces increased FGF2 expression and support the hypothesis that FGF2 may play a role in the partial recovery of function seen following SCI.

Chronic unpredictable stress promotes neuronal apoptosis in the cerebral cortex.

Stress-mediated loss of synaptogenesis in the hippocampus appears to play a role in depressive and mood disorders. However, little is known about the effect of stress/depression on the plasticity and survival of cortical neurons. In this report, we have examined whether chronic stress increases the vulnerability of neurons in the rat cortex. We have used a chronic unpredictable mild stress (CMS) as a rat model of depression. CMS (5 weeks treatment) produced anedonia and increased corticosterone levels. These effects were accompanied by a detectable increase in caspase-3 positive neurons in the cerebral cortex, suggesting apoptosis. Desipramine (DMI), a well known antidepressant, reversed the pro-apoptotic effect of CMS. These results suggest that antidepressants may reduce the pathological changes seen in stress-induced depressive disorders.

Human immunodeficiency virus type 1 glycoprotein gp120 reduces the levels of brain-derived neurotrophic factor in vivo: potential implication for neuronal cell death.

Neuronal loss has been observed in post mortem brains of patients with human immunodeficiency virus type 1 (HIV‐1). Experimental evidence has implicated HIV‐1‐derived envelope glycoprotein 120 (gp120) in the neuronal cell death observed in these patients. However, the intrinsic mechanisms by which gp120 causes neurotoxicity are still unknown. We have recently shown that the neurotoxic effect of gp120 in vitro is reduced by brain‐derived neurotrophic factor (BDNF). We therefore tested the hypothesis that low levels of BDNF render neurons more sensitive to gp120. Gp120 was injected acutely into the striatum of BDNF heterozygous mice and wild‐type littermates. BDNF heterozygous mice exhibited more apoptotic neurons in the striatum than wild‐type mice, suggesting that BDNF is neuroprotective also in vivo. Because several neurodegenerative disorders are characterized by lack of trophic support, we tested the hypothesis that gp120 may cause apoptosis by reducing BDNF expression. Gp120 was injected acutely in the rat striatum and BDNF levels determined by a two‐site immunoassay at various times after the injection. Gp120 elicited a dramatic decrease in BDNF protein levels by 24 h. Reduced BDNF levels were still present at 4 days. Cellular localization of BDNF immunoreactivity revealed that gp120 decreases BDNF immunoreactivity mainly in neuronal processes. This effect of gp120 precedes the peak of caspase‐3 activation and neuronal cell death. We propose that one of the mechanisms whereby gp120 causes neurotoxicity is a reduction of the neurotrophic factor environment crucial for cell survival.

Gangliosides activate Trk receptors by inducing the release of neurotrophins.

We used NIH-3T3 fibroblasts expressing the different Trk receptors to examine whether GM1 ganglioside and its semisynthetic derivative LIGA20 activate various neurotrophin receptors. GM1 induced autophosphorylation of TrkC more potently than TrkA or TrkB receptors. In contrast, LIGA20 activated TrkB tyrosine phosphorylation only. Therefore, Scatchard analysis was performed to determine whether GM1 binds to TrkC. GM1 failed to displace neurotrophin-3 binding, suggesting that this ganglioside does not act as a ligand for Trk receptors. In addition, GM1 failed to induce autophosphorylation of a chimeric receptor consisting of the extracellular domain of the tumor necrosis factor receptor and the intracellular domain of TrkA, suggesting that GM1 does not affect the tyrosine kinase domain. We next determined whether GM1 induces the release of neurotrophins from fibroblast cells. GM1 induced a rapid and significant increase in the amount of neurotrophin-3, but not other neurotrophins. This effect was independent of the presence of Trk because K252a did not prevent GM1-mediated release of neurotrophin-3. Moreover, GM1-mediated TrkC autophosphorylation was blocked by TrkC-IgG (but not TrkB-IgG) receptor bodies, further suggesting that GM1 activates TrkC by inducing the release of neurotrophin-3. This hypothesis was also tested in cultured cerebellar granule cells. GM1 induced neurotrophin-3 (but not brain-derived neurotrophic factor or nerve growth factor) release. In contrast, LIGA20 increased the secretion of brain-derived neurotrophic factor. Our data show that gangliosides may activate different Trk receptors by differentially affecting the release of neurotrophins.