Local protective effects of nerve growth factor-secreting fibroblasts against excitotoxic lesions in the rat striatum

Dopamine-glutamate interactions in the striatum: behaviourally relevant modification of excitotoxicity by dopamine receptor-mediated mechanisms

The two most important afferent projections to the striatum contain glutamate and dopamine, respectively. Excitotoxic damage resulting from excessive stimulation of the N-methyl-D-aspartate subtype of glutamate receptor has been implicated in pathophysiology of ischaemic stroke, hypoglycaemic brain damage and Huntington's disease. We studied the ability of the dopamine system to modify the anatomical, neurochemical and behavioural consequences of glutamatergic toxicity in the striatum. In a first set of experiments, the specific N-methyl-D-aspartate receptor agonist quinolinate was injected unilaterally into the striatum of rats pretreated with one of (i) intraperitoneal (i.p.) saline (controls); (ii) i.p. haloperidol, a D2 dopamine receptor agonist; or (iii) 6-hydroxydopamine lesion of the ipsilateral nigrostriatal tract. Quinolinate-induced striatal damage, as assessed by morphometric and neurochemical criteria, was significantly attenuated in the animals with 6-hydroxydopamine lesions and in those pretreated with haloperidol, compared with saline-pretreated controls. There were no significant differences between the 6-OHDA and haloperidol groups. In a second set of experiments, animals received (i) bilateral intrastriatal quinolinate plus perioperative i.p. saline; (ii) bilateral intrastriatal quinolinate plus i.p. haloperidol; or (iii) bilateral intrastriatal saline. Again, the quinolinate-lesioned animals treated with perioperative haloperidol had significantly less striatal damage than the bilateral quinolinate rats. Behavioural assessment in the Morris Water Maze showed the bilateral quinolinate+haloperidol group to be significantly less impaired on a spatial acquisition task than the bilateral quinolinate animals. Measures of spontaneous daytime motor activity showed significant differences in average speed and rest time between the bilateral quinolinate+haloperidol rats and the bilateral quinolinate group. The performance of the bilateral quinolinate+haloperidol group was not significantly different from that of controls on any of the behavioural tasks. These results indicate an important role for D2 dopamine receptor-mediated mechanisms in striatal excitotoxicity. Since the excitotoxic process involves the same fundamental signalling mechanism that is involved in normal glutamatergic transmission, these findings imply an ability of D2 receptor blockade to modify glutamate signalling in the striatum. These results may have implications for treatment strategies in ischaemic stroke, hypoglycaemic brain damage and schizophrenia.

High levels of cerebrospinal fluid glutamate in Rett syndrome

Rett syndrome is a neurodevelopmental disease affecting girls. The cause is not known. Roles for trophic factors and excitatory neurotransmitters have been postulated. To study the significance of excitatory amino acids in Rett syndrome, we determined glutamate and aspartate concentrations in the cerebrospinal fluid from 11 girls with Rett syndrome (age 8 years 4 months +/- 5.7 years, mean +/- SD) and 11 controls (age 7 +/- 4.2 years). In the patients with Rett syndrome, the mean of cerebrospinal fluid glutamate concentration was 355.2 nmol/L (SD +/- 109.2 nmol/L). In the controls it was 203.9 nmol/L (SD +/- 55.5 nmol/L). In Rett syndrome cases, cerebrospinal fluid glutamate concentrations were significantly higher (P = 0.0006) than in the controls. In the Rett syndrome group, the mean cerebrospinal fluid aspartate concentration was 119.4 nmol/L (SD +/- 43.5 nmol/L). In the control group, it was 90.9 nmol/L (SD +/- 20.9 nmol/L). The difference between the cerebrospinal fluid aspartate values was not significant. Glutamate may therefore play an important role in the primary pathogenesis in Rett syndrome. Further investigations are needed, with recognition of possible actions of neuronal growth factors and excitatory neurotransmitters in the damage mechanisms of Rett syndrome.

Low levels of nerve growth factor in cerebrospinal fluid of children with Rett syndrome

A role of neurotrophic factors has been postulated in some human neurodegenerative disorders such as Alzheimer's and Parkinson's disease. The known developmental effects of these substances suggested that, in some neurologic diseases affecting children, neurotrophic factors might be inadequate. Using a sensitive, two-site enzyme-linked immunoassay, we examined the content of nerve growth factor in the cerebrospinal fluid of 11 children with Rett syndrome and of 24 control patients with various neurologic diagnoses or suffering from other diseases. Nerve growth factor levels were significantly lower in the cerebrospinal fluid of the patients with Rett syndrome than in control patients. The lower level of cerebrospinal fluid nerve growth factor in Rett syndrome suggests that lack of nerve growth may be involved in the pathogenesis of this disease or reflect the underlying brain damage present.

TRK and p75 neurotrophin receptor systems in the developing human brain

The prenatal development of the neurons immunoreactive for high-affinity tropomycin-related kinase (trk) receptor (pan trk which recognizes trkA, trkB, and trkC) and low-affinity p75 neurotrophin receptor (p75NTR) was examined in the human brain from embryonic weeks 10 to 34 of gestation. In the embryonic week 10 specimen in which only brainstem regions were available for evaluation, trk immunoreactivity (trk-ir) was observed in the ventral cochlear, solitary, raphe, spinal trigeminal, and hypoglossal nuclei, as well as the vestibular complex and medullary reticular formation. At this time point of gestation, p75ntr-immunoreactive (p75NTR-ir) staining was observed within these same regions plus the inferior olivary and ambiguus nuclei. At embryonic week 14, trk-ir neurons were seen within the subplate zone of the entorhinal cortex, basal forebrain, caudate nucleus, putamen, external segment of the globus pallidus, specific thalamic nuclei, lateral mammillary nucleus, habenula nucleus, select brainstem nuclei, and the dentate nucleus of cerebellum. At this gestational time point, p75NTR-ir neurons were observed in each of these structures, with the exception of the caudate nucleus, specific thalamic nuclei, lateral mammillary nucleus, and habenula nucleus. Additionally, p75NTR-ir neurons were observed within the corpus callosum. The staining pattern for both trk and p75NTR remained unchanged at embryonic weeks 15 to 16 except for the addition of trk-ir and p75NTR-ir within the cortical subplate zone, hippocampus, and subthalamic nucleus. By embryonic week 18, trk-ir neurons were widely expressed within mostly all thalamic nuclei. In contrast, trk-ir was no longer seen within the hypoglossal, cuneate, and gracile nuclei at this time point. This staining pattern for trk and p75NTR remained virtually unchanged from embryonic weeks 19 to 20 and embryonic weeks 16 to 20, respectively. From embryonic weeks 22 to 34, the distribution of both trk-ir and p75NTR-ir neurons changed gradually. During this period, neurons in most thalamic and some brainstem nuclei became progressively immunonegative for trk, whereas neurons in the neocortical subplate zone, corpus callosum, and hilar region of dentate gyrus gradually lost immunoreactivity for p75NTR. These data demonstrate an important and complex role for both the high-(trk) and low- (p75) affinity neurotrophin receptors during the development of multiple neuronal systems in the human brain.

Intrastriatal implants of polymer encapsulated cells genetically modified to secrete human nerve growth factor: trophic effects upon cholinergic and noncholinergic striatal neurons

Nerve growth factor selectively prevents the degeneration of cholinergic neurons following intrastriatal infusion but rescues both cholinergic and noncholinergic striatal neurons if the nerve growth factor is secreted from grafts of genetically modified fibroblasts. The present study evaluated whether grafted fibroblasts genetically modified to secrete human nerve growth factor could provide trophic influences upon intact cholinergic and noncholinergic striatal neurons. Unilateral striatal grafts of polymer-encapsulated cells genetically modified to secrete human nerve growth factor induced hypertrophy and significantly increased the optical density of choline acetyltransferase-immunoreactive striatal neurons one, two, and four weeks post-transplantation relative to rats receiving identical grafts missing only the human nerve growth factor construct. Nerve growth factor secreting grafts also induced a hypertrophy of noncholinergic neuropeptide Y-immunoreactive striatal neurons one, two, and four weeks post-transplantation. Glutamic acid decarboxylase-immunoreactive neurons were unaffected by the human nerve growth factors secreting grafts. The effects upon choline acetyltransferase-immunoreactive and neuropeptide Y-immunoreactive striatal neurons dissipated following retrieval of the implants. Immunocytochemistry for nerve growth factor revealed intense graft-derived immunoreactivity for up to 1000 microns from the capsule extending along the dorsoventral axis of the striatum. Nerve growth factor-immunoreactivity was also observed within a subpopulation of striatal neurons and may represent nerve growth factor consumer neurons which retrogradely transported graft-derived nerve growth factor. When explanted, grafts produced 2-4 ng human nerve growth factor/24 h over the time course of this study indicating that this level of continuous human nerve growth factor secretion was sufficient to mediate the effects presently observed.

Clenbuterol protects mouse cerebral cortex and rat hippocampus from ischemic damage and attenuates glutamate neurotoxicity in cultured hippocampal neurons by induction of NGF

It has been shown previously that clenbuterol, a beta 2-adrenergic receptor agonist, enhances NGF synthesis in adult rat brain. Since NGF is able to protect neurons against damage, we tried to find out whether clenbuterol can rescue cultured hippocampal neurons from excitotoxic damage by induction of NGF. The neuroprotective activity of clenbuterol on neurons in the vulnerable CA1 subfield of the hippocampus was tested in a rat model of transient forebrain ischemia. Additionally, in the mouse model of focal cerebral ischemia the ability of clenbuterol to reduce the infarct size was examined. Exposure of mixed neuronal/glial hippocampal cultures to clenbuterol (1 to 100 microM) enhanced significantly the content of NGF measured in the culture medium by two-site ELISA. The excitotoxic injury was induced in the same type of cells after 14 days in vitro by exposure to 1 mM L-glutamate for 1 h in serum-free medium. NGF itself (0.15 to 100 ng/ml) added to the growth medium 4 h before until 18 h after induction of injury (the point of glutamate-toxicity measurement), protected hippocampal neurons from excitotoxic damage. Clenbuterol (1 to 100 microM) provided similar neuroprotection as NGF under the same experimental conditions. The neuroprotective activity of clenbuterol (100 microM) against glutamate-induced damage in hippocampal cultures was blocked by anti-NGF monoclonal antibodies (0.5 microgram/ml) added to the medium during the clenbuterol exposure, demonstrating that the neuronal rescue is mediated by NGF. Propranolol, a beta-adrenergic receptor antagonist (10 microM) added 20 min before and kept in the medium during exposure of the cultures to clenbuterol (1 microM) reversed the neuroprotective activity, suggesting that the induction of NGF and neuroprotection caused by clenbuterol are mediated via beta-adrenergic receptor activation. The capacity of clenbuterol to protect hippocampal neurons was also demonstrated in vivo in a rat model of transient forebrain ischemia. Clenbuterol (4 x 1 mg/kg) administered intraperitoneally increased the number of viable neurons in CA1 subfield of the rat hippocampus. Furthermore, clenbuterol (0.3 and 1 mg/kg, i.p. and 1 mg/kg, s.c.) reduced significantly the infarct area on the mouse brain surface after occlusion of the middle cerebral artery. The present data demonstrate that clenbuterol induces NGF synthesis in cultured hippocampal cells and protects hippocampal neurons from excitotoxic damage. The neuroprotective activity of clenbuterol is also demonstrated in vivo in two rodent models of cerebral ischemia. The results offer strong evidence that the neuroprotective activity of clenbuterol is caused by activation of beta-adrenergic receptors and the subsequent increased expression of NGF.

Nerve growth factor protects the neonatal brain against hypoxic-ischemic injury

Nerve growth factor (NGF) has been shown to protect specific neurons that express its signaling receptor, trkA, from a variety of insults. There are some data, in particular in the developing brain, indicating that NGF has neuroprotective actions that extend beyond cells expressing trkA. In this study, we asked whether NGF would protect against brain injury in a neonatal model of hypoxia-ischemia. Postnatal day (PD) 7 rat pups received a right carotid ligation and were then exposed to hypoxic conditions. Prior to carotid ligation and 48 hours later, pups received an intracerebroventricular injection of NGF or denatured NGF dissolved in vehicle or vehicle alone. Brains were then assessed at PD21. In vehicle- and denatured NGF-treated animals, there was significant damage (30-40% volume loss) to both the striatum and cortex ipsilateral to the carotid ligation. In contrast, little damage (10% volume loss) was observed in most NGF-treated animals. NGF injection studies revealed that NGF stimulated tyrosine phosphorylation of trkA in multiple brain regions. These results show that NGF appears globally neuroprotective to the developing brain in a neonatal model of hypoxia-ischemia and that there may be novel mechanisms in vivo through which NGF exerts its protective actions.

Astrocytes retrovirally transduced with BDNF elicit behavioral improvement in a rat model of Parkinson's disease

Neurotrophic factors that improve the survival of specific neuronal types during development and after exposure to various neuronal insults hold potential for treatment of neurodegenerative diseases. In particular, brain-derived neurotrophic factor (BDNF) has been shown to exert trophic and protective effects on dopaminergic neurons, the cell type known to degenerate in Parkinson's disease. To determine whether increased levels of biologically produced BDNF affect the function or regeneration of damaged dopaminergic neurons, the effects of grafting astrocytes transduced with the human BDNF gene into the striatum of the partially lesioned hemiparkinsonian rat were examined. Replication deficient retroviruses carrying either human prepro-BDNF or human alkaline phosphatase (AP) cDNA were used to transduce primary type 1 astrocytes purified from neonatal rat cortex. In vitro, BDNF mRNA was expressed by BDNF transduced astrocytes (BDNF astrocytes), but not control AP transduced astrocytes (AP astrocytes), as determined by reverse transcription polymerase chain reaction (RT-PCR). The modified astrocytes were injected into the right striatum 15 days after partial lesioning of the right substantia nigra with 6-hydroxydopamine. Transplantation of BDNF astrocytes, but not AP astrocytes, significantly attenuated amphetamine-induced rotation by 45% 32 days after grafting. Apomorphine-induced rotation increased over time in both groups, but was not significantly different in the BDNF-treated group. The modified BDNF astrocytes survived well with non-invasive growth in the brain for up to 42 days. Although BDNF mRNA positive cells were not detected within the graft site using in situ hybridization, alkaline phosphatase immunoreactive (IR) cells were present in control graft sites suggesting that the retroviral construct continued to be expressed at 42 days. Analysis of the density of tyrosine hydroxylase (TH)-IR fibers showed no effect of BDNF on TH-IR fiber density in the striatum on the lesioned side. These findings suggest that ex vivo gene therapy with BDNF ameliorates parkinsonian symptoms through a mechanism(s) other than one involving an effect of BDNF on regeneration or sprouting from dopaminergic neurons.

Mast cells in neuroimmune function: neurotoxicological and neuropharmacological perspectives

Mast cells are located in close proximity to neurons in the peripheral and central nervous systems, suggesting a functional role in normal and aberrant neurodegenerative states. They also possess many of the features of neurons, in terms of monoaminergic systems, responsiveness to neurotrophins and neuropeptides and the ability to synthesise and release bioactive neurotrophic factors. Mast cells are able to secrete an array of potent mediators which may orchestrate neuroinflammation and affect the integrity of the blood-brain barrier. The 'cross-talk' between mast cells, lymphocytes, neurons and glia constitutes a neuroimmune axis which is implicated in a range of neurodegenerative diseases with an inflammatory and/or autoimmune component, such as multiple sclerosis and Alzheimer's disease. Mast cells appear to make an important contribution to developing, mature and degenerating nervous systems and this should now be recognised when assessing the neurotoxic potential of xenobiotics.

Interleukin-1 beta increases basic fibroblast growth factor mRNA expression in adult rat brain and organotypic hippocampal cultures

In situ hybridization was used to study the effect of IL-1 beta on acidic fibroblast growth factor (aFGF) and basic fibroblast growth factor (bFGF) mRNA expression in rat brain. Intraventricular injection of recombinant human IL-1 beta did not affect hybridization to aFGF mRNA but did induce significant and widespread increases in hybridization to bFGF mRNA. IL-1 beta induced increases in bFGF mRNA were bilaterally distributed and appeared to correspond with the distribution of non-neuronal cells. Thus, hybridization was increased in regions of both gray and white matter (e.g., corpus callosum), the ependymal lining of the third ventricle, and the pia matter. In hippocampus of IL-1 beta injected rats, hybridization was markedly increased in the molecular layers but not significantly increased in the neuronal cell layers. Elevations in bFGF mRNA were transient, peaking at 8 h postinjection in most areas. To determine if IL-1 beta effects were independent of activation of the hypothalamo-pituitary-adrenal axis, and to compare the cellular localization of increases in bFGF mRNA expression induced by IL-1 beta and bFGF, the regulation of bFGF expression was also studied in organotypic hippocampal slice cultures. Treatment of cultures with either IL-1 beta or bFGF stimulated the same general distribution of increases in bFGF mRNA as seen after IL-1 beta treatment in vivo with an additional effect on immature neurons within the hilar side of stratum granulosum; hybridization of bFGF mRNA was not increased in association with the more mature neurons of stratum pyramidale or stratum granulosum. Colocalization of bFGF cRNA hybridization with immunostaining for glial fibrillary acidic protein demonstrated that increases in bFGF mRNA induced both by IL-1 beta in vivo and in vitro and by bFGF in vitro were largely associated with astroglial cells. These findings suggest that IL-1 beta induction of bFGF contributes to the coactivation of these substances following various forms of insult to the CNS and initiates a cascade of trophic interactions that regulates processes of glial proliferation, neurotrophic factor expression, and neuroprotection.

Clinical application of cell transplantation and neurotrophic factors in CNS disorders

Cell transplantation and administration of neurotrophic factors are now being explored as new therapeutic strategies to restore and preserve function in the diseased human central nervous system. Neural grafts show long-term survival and restore function in patients with Parkinson's disease, but the symptomatic relief needs to be increased. Cell transplantation also seems justified in patients with Huntington's disease and, possibly, in demyelinating disorders. Clinical trials with neurotrophic factors have been initiated in amyotrophic lateral sclerosis, dementia and Huntington's disease, and may later be started in Parkinson's disease and after acute brain insults. However, it remains to be shown if neurotrophic factors can rescue damaged cells in the brain and spinal cord of patients with these disorders.

Human neural transplantation

Great advances in neurobiology have resulted from 100 years of neural transplantation research. In the last 20 years, there has been a focus on using neural transplantation to repair the damaged central nervous system (CNS) utilising experimental animal models of various human neurodegenerative disease and CNS injury. Since 1985, there has been a rapid proliferation of adrenal medullary autograft transplantation to the caudate nucleus of humans with Parkinson's disease. However, this operation proved to be unsuccessful and was associated with unacceptable morbidity. Implantation of human fetal mesencephalon into patients with severe parkinsonism has supplanted the adrenal operation and has produced promising results, with some patients reported to improve markedly and some evidence of graft survival noted on positron emission tomography (PET). Host tissue recovery appears to be an important mechanism for this clinical improvement. The optimal technique is to use three to four fetuses from induced abortions of 6.5 to 8 weeks gestation, with multiple stereotactic implants into the putamen and caudate nucleus. Many biological questions still remain and the community remains troubled by the ethical problems of using fetal tissue obtained from abortions. This procedure is still experimental and should be restricted to a few centres with excellence in cell and molecular biology. A multicentre study is needed to more carefully evaluate CNS transplantation. Cloned neural precursor cells or immortalized embryonic cell lines genetically modified to manufacture selected growth factors or neurotransmitters may offer an alternative to the use of human fetal tissue. Much more experimental animal research is necessary before transplantation can be used to treat other CNS maladies.

Nerve growth factor released by transgenic astrocytes enhances the function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease

Previous studies have demonstrated that astrocytes genetically modified to express recombinant nerve growth factor (NGF) support the survival and neuronal transdifferentiation of intrastriatal adrenal chromaffin cell grafts at 2 weeks post-transplantation [15]. The present study was performed to determine whether these effects would be maintained at longer times post-transplantation and, if so, whether the co-grafts would reduce rotational behavior in the unilateral 6-hydroxydopamine-lesioned rat. In the present study, we have demonstrated that primary type I rat astrocytes infected with a replication-defective retrovirus conferring expression of a mouse beta-NGF cDNA sequence secrete NGF at a rate that is approximately 40-fold higher than that of controls (i.e., 8.0 vs. 0.2 pg NGF/h/10(5) cells, respectively). The genetically modified astrocytes were also found to express recombinant NGF following intrastriatal transplantation, as indicated by a 23% increase in striatal NGF content compared with controls, measured at 4 weeks post-transplantation. When NGF-producing astrocytes and adrenal chromaffin cells were co-grafted into the dopamine-denervated striatum of the unilateral 6-hydroxydopamine-lesioned rat, the chromaffin cells displayed extensive neurite outgrowth and a 5-12-fold increase in survival compared to controls at 10 weeks post-grafting. These effects were paralleled by a 60% reduction of apomorphine-induced rotational behavior, suggesting a partial normalization of striatal function. These results suggest that genetically modified astrocytes promote the prolonged survival and function of adrenal chromaffin cell grafts in a rat model of Parkinson's disease.

Implanted fibroblasts genetically engineered to produce brain-derived neurotrophic factor prevent 1-methyl-4-phenylpyridinium toxicity to dopaminergic neurons in the rat

The trophism of brain-derived neurotrophic factor (BDNF) for dopaminergic cells in culture has led to significant interest in the role of BDNF in the etiology and potential treatment of Parkinson disease. Previous in vivo investigation of BDNF delivery to axotomized substantia nigra dopaminergic neurons in the adult rat has shown no protective effect. In this study, we produced nigral degeneration by infusing 1-methyl-4-phenylpyridinium (MPP+), a mitochondrial complex I inhibitor and the active metabolite of 1-methyl-4-phenyl-1,2,3,6- tetrahydropyridine (MPTP), into the rat striatum. The subsequent loss of nigral neurons was presumably due to mitochondrial toxicity after MPP+ uptake and retrograde transport to the substantia nigra. We engineered immortalized rat fibroblasts to secrete human BDNF and implanted these cells near the substantia nigra 7 days before striatal MPP+ infusion. We found that BDNF-secreting fibroblasts markedly increased nigral dopaminergic neuronal survival when compared to control fibroblast implants. The observation that BDNF prevents MPTP-induced dopaminergic neuronal degeneration in the adult brain has significance for the treatment of neurodegenerative disorders, which may involve mitochondrial dysfunction, such as Parkinson disease.

Secreted forms of beta-amyloid precursor protein modulate dendrite outgrowth and calcium responses to glutamate in cultured embryonic hippocampal neurons

In addition to being the major excitatory neurotransmitter in the mammalian brain, glutamate is believed to play a key role in the regulation of neurite outgrowth and synaptogenesis during development. In cultured embryonic hippocampal pyramidal neurons, glutamate inhibits dendrite outgrowth by a mechanism involving elevation of intracellular-free calcium levels ([Ca2+]i). In the present study, secreted forms of the beta-amyloid precursor protein (APPss) counteracted the inhibitory effect of glutamate on dendrite outgrowth in cultured embryonic hippocampal neurons. The prolonged elevation of [Ca2+]i normally induced by glutamate was significantly attenuated in neurons that had been pretreated with 2-10 nM of APPs695 or APPs751. Immunocytochemistry with beta-amyloid precursor protein antibodies showed that immunoreactivity was concentrated in axons and, particularly, in their growth cones. Because beta-amyloid precursor proteins are axonally transported, and APPss can be released from axon terminals/growth cones in response to electrical activity, the present findings suggest that APPss may play a role in developmental and synaptic plasticity by modulating dendritic responses to glutamate.

NT-3 and BDNF protect CNS neurons against metabolic/excitotoxic insults

Neurotrophin-3 (NT-3) and brain-derived neurotrophic factor (BDNF) were recently shown to have biological activity in central neurons. In the present study, NT-3 and BDNF attenuated glucose deprivation-induced neuronal damage dose-dependently in rat hippocampal, septal and cortical cultures. Direct measurements of intraneuronal free calcium levels ([Ca2+]i) and manipulations of calcium influx demonstrated that NT-3 and BDNF each prevented the elevation of [Ca2+]i that mediated glucose deprivation-induced injury. Studies in cultures depleted of glia indicated a direct action of NT-3 and BDNF on neurons. Neurons pretreated with NT-3 or BDNF for 24 hr were more resistant to glutamate neurotoxicity, and showed attenuated [Ca2+]i responses to glutamate. TrkB (BDNF receptor) and trkC (NT-3 receptor) proteins were present in hippocampal, cortical and septal cultures where they were localized to neuronal cell bodies and neurites. The data demonstrate that NT-3 and BDNF can protect neurons against metabolic and excitotoxic insults, and suggest that these neurotrophins may serve [Ca2+]i-stabilizing and neuroprotective functions in the brain.

Increased levels of nerve growth factor (NGF) protein and mRNA and reactive gliosis following kainic acid injection into the rat striatum

After excitotoxic lesion of the rat striatum, the time courses of local nerve growth factor (NGF) and NGF mRNA contents were investigated using a sensitive immunoassay (ELISA) and reverse transcription coupled to polymerase chain reaction (RT/PCR). To investigate a possible correlation of increased NGF expression and excitotoxin-induced reactive gliosis, striata were also analysed by immunohistochemistry with glial markers. We found elevated striatal NGF protein after lesion over the whole observation period. NGF mRNA showed a biphasic increase 10 h and 10 days after lesion, the latter co-inciding with an increased astrogliosis. These results indicate that NGF accumulation after excitotoxin-induced neurodegeneration is partly due to local reactive astrocytes.

Endogenous neuroprotection factors and traumatic brain injury: mechanisms of action and implications for therapy

Throughout evolution the brain has acquired elegant strategies to protect itself against a variety of environmental insults. Prominent among these are signals released from injured cells that are capable of initiating a cascade of events in neurons and glia designed to prevent further damage. Recent research has identified a remarkably large number of neuroprotection factors (NPFs), whose expression is increased in response to brain injury. Examples include the neurotrophins (NGF, NT-3, NT-5, and BDNF), bFGF, IGFs, TGFs, TNFs and secreted forms of the beta-amyloid precursor protein. Animal and cell culture studies have shown that NPFs can attenuate neuronal injury initiated by insults believed to be relevant to the pathophysiology of traumatic brain injury (TBI) including excitotoxins, ischemia, and free radicals. Studies of the mechanism of action of these NPFs indicate that they enhance cellular systems involved in maintenance of Ca2+ homeostasis and free radical metabolism. Recent work has identified several low-molecular-weight lipophilic compounds that appear to mimic the action of NPFs by activating signal transduction cascades involving tyrosine phosphorylation. Such compounds, alone or in combination with antioxidants and calcium-stabilizing agents, have proved beneficial in animal studies of ischemic brain injury and provide opportunities for development of preventative/therapeutic approaches for TBI.

Glutamate, beta-amyloid precursor proteins, and calcium mediated neurofibrillary degeneration

In this article we present evidence supporting the interaction between excitotoxicity, beta APP mismetabolism, metabolic compromise and intracellular calcium destabilization in the process of neurodegeneration associated with Alzheimer's disease (AD). AD is characterized by the presence of neurofibrillary tangles and amyloid-containing plaques in specific regions of the brain. There appear to be several processes which contribute to the neurodegeneration associated with AD. Although AD has been linked to genetic mutations on chromosomes 21, 19 and 14, there are sporadic forms of AD that have no known genetic mutation involved. Aging is the major risk factor for AD. During the course of normal aging several metabolic compromises may occur in the brain. Both decreased glucose transport and utilization, and increased glucocorticoid levels are known to occur with aging and may lead to decreased energy supplies, ATP depletion, failure of Ca2+ buffering systems, excess glutamate release and activation of glutamate receptors. In addition, a reduction in antioxidant enzymes and consequently an increase in free radicals has also been associated with aging. Each of the preceeding alterations would lead to an increase in neuronal [Ca2+]i. Elevated calcium could then activate calcium-dependent proteases which degrade particular cytoskeletal proteins, and lipases which generate free radicals resulting in membrane damage and possible cell death. In this article we provide evidence that amyloid beta-peptide (A beta), the substance which accumulates in AD plaques, exacerbates excitotoxic and metabolic compromises to neurons resulting in changes in the cytoskeleton which resemble those seen in the neurofibrillary tangles of AD. We also provide evidence that secreted forms of beta-amyloid precursor protein (beta APP) are neuroprotective against excitotoxic insults. Recent findings concerning the normal function of beta APP and the mechanism of A beta toxicity place beta APP at the center of changes leading to neuronal degeneration in AD.

Herpesvirus-mediated gene delivery into the rat brain: specificity and efficiency of the neuron-specific enolase promoter

1. Herpesvirus infection with genetically engineered vectors is a way to deliver foreign gene products to various cell populations in culture and in vivo. Selective neuronal gene expression can be achieved using the neuron-specific enolase (NSE) promoter regulating expression of a transgene placed in and delivered by a herpesvirus vector. 2. We sought to determine the anatomical specificity and efficiency of herpesvirus-mediated gene transfer into the rat brain following placement of virus particles carrying a transgene (lacZ) under control of the NSE promoter. The virus utilized was thymidine kinase (TK) deficient and therefore replication deficient in the brain. 3. Infusion of 10(6) plaque-forming units of virus into the striatum caused a limited number of striatal neurons to express the lacZ transgene mRNA and protein product 7 days postinfection. In addition, small numbers of neurons expressing the transgene mRNA and protein were found ipsilateral to the viral injection in the frontal cortex, substantia nigra pars compacta, and thalamus. Neurons at these anatomic loci project directly to the striatal injection site. No other cells within the brains of injected animals expressed the lacZ gene. 4. While this herpesvirus NSE vector was capable of introducing novel functional genetic information into postmitotic neurons within defined neuroanatomic constraints, the numbers of neurons expressing detectable levels of beta-galactosidase was minimal. The calculated efficiency of delivery and transgene expression at 7 days postinfection was 1 transgenic neuron per 10(4) virus particles infused. 5. We conclude that NSE probably is not an optimal promoter for use in gene delivery to CNS neurons in herpesvirus vectors and that the efficacy of gene delivery using other neuron-specific promoters placed at various sites in the herpes viral genome needs to be explored.

Postnatal gene transfer into the central nervous system

Substantial progress has been made in the development of techniques for the expression of foreign genes in the central nervous system of postnatal animals. Fetal and adult brain cells and other cells, including fibroblasts and muscle cells, have been successfully employed as vehicles for foreign gene expression in the central nervous system. Direct gene transfer strategies, such as those using herpes and adenoviral vectors, are presently under intense and fruitful investigation.

On neuronal health

Many recent studies of the degeneration, neuroprotection and regeneration of CNS neurons have departed from previous dichotomous descriptions of neurons as either dead or alive. In this brief article aspects of neuronal health are examined by outlining ways to assess both neuronal resilience and vulnerability to common forms of structural brain insults. According to this theory of neuronal health, neurons exist in a dynamic equilibrium that spans a spectrum of cellular existence, constantly influenced by both extracellular physiological changes and intracellular mechanisms designed to react to external stimuli while maintaining structural integrity. The spectrum between particularly resilient and vulnerable neuronal states is illustrated by experiments in vivo that examine trophic and metabolic fluctuations influencing the likelihood of neuronal death after neuronal insults. Studies show that adult CNS neurons can be protected in vivo by trophic agents or other pharmacological interventions against structural and toxic damage. Conversely, low-level neuronal impairment due to genetic or physiological perturbations can predispose neurons to demise by insults that normally would not cause cell death. The experimental approaches described may help in the study of neuronal pathophysiology, and in investigations towards new treatments for the neurodegenerative diseases like Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and Huntington's disease.

Striatal degeneration induced by mitochondrial blockade is prevented by biologically delivered NGF

Consistent with the notion that a defect in cellular energy metabolism is a cause of human neurodegenerative disease, systemic treatment with the mitochondrial complex II inhibitor 3-nitropropionic acid (3-NPA) can model the striatal neurodegeneration seen in Huntington's disease. Previously, we have found that nerve growth factor (NGF), delivered biologically by the implantation of a genetically altered fibroblast cell-line, can protect locally against striatal degeneration induced by infusions of high doses of glutamate receptor agonists. We now report that implantation of NGF-secreting fibroblasts reduces the size of adjacent striatal 3-NPA lesions by an average of 64%. We conclude that biologically delivered NGF protects neurons against excitotoxicity and mitochondrial blockade--both energy-depleting processes--implying that appropriate neurotrophic support in the adult brain could protect against neurodegenerative diseases caused in part by energy depletion.