Neurotrophin-4/5 protects hippocampal and cortical neurons against energy deprivation- and excitatory amino acid-induced injury

The Roles of Neurotrophins in Traumatic Brain Injury

Neurotrophins are a collection of structurally and functionally related proteins. They play important roles in many aspects of neural development, survival, and plasticity. Traumatic brain injury (TBI) leads to different levels of central nervous tissue destruction and cellular repair through various compensatory mechanisms promoted by the injured brain. Many studies have shown that neurotrophins are key modulators of neuroinflammation, apoptosis, blood–brain barrier permeability, memory capacity, and neurite regeneration. The expression of neurotrophins following TBI is affected by the severity of injury, genetic polymorphism, and different post-traumatic time points. Emerging research is focused on the potential therapeutic applications of neurotrophins in managing TBI. We conducted a comprehensive review by organizing the studies that demonstrate the role of neurotrophins in the management of TBI.

Phytochemicals in Ischemic Stroke

Stroke is the second foremost cause of mortality worldwide and a major cause of long-term disability. Due to changes in lifestyle and an aging population, the incidence of stroke continues to increase and stroke mortality predicted to exceed 12 % by the year 2030. However, the development of pharmacological treatments for stroke has failed to progress much in over 20 years since the introduction of the thrombolytic drug, recombinant tissue plasminogen activator. These alarming circumstances caused many research groups to search for alternative treatments in the form of neuroprotectants. Here, we consider the potential use of phytochemicals in the treatment of stroke. Their historical use in traditional medicine and their excellent safety profile make phytochemicals attractive for the development of therapeutics in human diseases. Emerging findings suggest that some phytochemicals have the ability to target multiple pathophysiological processes involved in stroke including oxidative stress, inflammation and apoptotic cell death. Furthermore, epidemiological studies suggest that the consumption of plant sources rich in phytochemicals may reduce stroke risk, and so reinforce the possibility of developing preventative or neuroprotectant therapies for stroke. In this review, we describe results of preclinical studies that demonstrate beneficial effects of phytochemicals in experimental models relevant to stroke pathogenesis, and we consider their possible mechanisms of action.

Calbindin-D28K is increased in the ventral horn of spinal cord by neuroprotective factors for motor neurons

Slow glutamate-mediated neuronal degeneration is implicated in the pathophysiology of motor neuron diseases such as amyotrophic lateral sclerosis (ALS). The calcium-binding proteins calbindin-D28K and parvalbumin have been reported to protect neurons against excitotoxic insults. Expression of calbindin-D28K is low in adult human motor neurons, and vulnerable motor neurons additionally may lack parvalbumin. Thus, it has been speculated that the lack of calcium-binding proteins may, in part, be responsible for early degeneration of the population of motor neurons most vulnerable in ALS. Using a rat organotypic spinal cord slice system, we examined whether the most potent neuroprotective factors for motor neurons can increase the expression of calbindin-D28K or parvalbumin proteins in the postnatal spinal cord. After 4 weeks of incubation of spinal cord slices with 1) glial cell line-derived neurotrophic factor (GDNF), 2) neurturin, 3) insulin-like growth factor I (IGF-I), or 4) pigment epithelium-derived factor (PEDF), the number of calbindin-D28K -immunopositive large neurons (>20 μm) in the ventral horn was higher under the first three conditions, but not after PEDF, compared with untreated controls. Under the same conditions, parvalbumin was not upregulated by any neuroprotective factor. The same calbindin increase was true of IGF-I and GDNF in a parallel glutamate toxicity model of motor neuron degeneration. Taken together with our previous reports from the same model, which showed that all these neurotrophic factors can potently protect motor neurons from slow glutamate injury, the data here suggest that upregulation of calbindin-D28K by some of these factors may be one mechanism by which motor neurons can be protected from glutamate-induced, calcium-mediated excitotoxicity.

Elevated neurotrophin-3 and neurotrophin 4/5 levels in unmedicated bipolar depression and the effects of lithium

BACKGROUND

Bipolar disorder (BD) has been associated with diverse abnormalities in neural plasticity and cellular resilience. Neurotrophin-3 (NT-3) and neurotrophin-4/5 (NT-4/5) support synaptic neuronal survival and differentiation. NT-3 and NT-4/5 levels were found to be altered in BD, potentially representing a physiological response against cellular stress. However, the use of psychopharmacological agents and heterogeneous mood states may constitute important biases in such studies. Thus, we aimed to assess NT-3 and NT-4/5 levels in medication-free BD type I or II individuals in a current depressive episode, before and after 6 weeks of lithium monotherapy and matched with healthy controls.

METHODS

Twenty-three patients with BD type I or II during a depressive episode and 28 healthy controls were studied. Patients were required to have a 21-item Hamilton Depression Rating Scale score ≥18 and had not undergone any psychopharmacological treatment for at least 6 weeks prior to study entry. Patients were treated with lithium for 6 weeks and plasma NT-3 and NT-4/5 levels were determined at baseline and endpoint using ELISA method.

RESULTS

Baseline plasma levels of both NT-3 and NT-4/5 were significantly increased in acutely depressed BD subjects in comparison to healthy controls (p=0.040 and 0.039, respectively). The NT-3 and NT-4/5 levels did not significantly change after lithium treatment. NT-3 and NT-4/5 levels were positively correlated to illness duration in BD (p=0.032 and 0.034, respectively).

CONCLUSION

Our findings suggest that NT-3 and NT-4/5 levels are increased in the depressive phase of BD, which seems directly associated with illness duration. The increased levels of NT-3 and NT-4/5 may underlie a biological response to cellular stress associated with the course of BD.

Intervention effects of ganoderma lucidum spores on epileptiform discharge hippocampal neurons and expression of neurotrophin-4 and N-cadherin

Epilepsy can cause cerebral transient dysfunctions. Ganoderma lucidum spores (GLS), a traditional Chinese medicinal herb, has shown some antiepileptic effects in our previous studies. This was the first study of the effects of GLS on cultured primary hippocampal neurons, treated with Mg²⁺ free medium. This in vitro model of epileptiform discharge hippocampal neurons allowed us to investigate the anti-epileptic effects and mechanism of GLS activity. Primary hippocampal neurons from <1 day old rats were cultured and their morphologies observed under fluorescence microscope. Neurons were confirmed by immunofluorescent staining of neuron specific enolase (NSE). Sterile method for GLS generation was investigated and serial dilutions of GLS were used to test the maximum non-toxic concentration of GLS on hippocampal neurons. The optimized concentration of GLS of 0.122 mg/ml was identified and used for subsequent analysis. Using the in vitro model, hippocampal neurons were divided into 4 groups for subsequent treatment i) control, ii) model (incubated with Mg²⁺ free medium for 3 hours), iii) GLS group I (incubated with Mg²⁺ free medium containing GLS for 3 hours and replaced with normal medium and incubated for 6 hours) and iv) GLS group II (neurons incubated with Mg²⁺ free medium for 3 hours then replaced with a normal medium containing GLS for 6 hours). Neurotrophin-4 and N-Cadherin protein expression were detected using Western blot. The results showed that the number of normal hippocampal neurons increased and the morphologies of hippocampal neurons were well preserved after GLS treatment. Furthermore, the expression of neurotrophin-4 was significantly increased while the expression of N-Cadherin was decreased in the GLS treated group compared with the model group. This data indicates that GLS may protect hippocampal neurons by promoting neurotrophin-4 expression and inhibiting N-Cadherin expression.

Brain derived neurotrophic factor and neurotrophic factor 3 modulate neurotransmitter receptor expressions on developing spiral ganglion neurons

Cochlear spiral ganglion neurons (SGN) provide the only pathway for transmitting sound evoked activity from the hair cells to the central auditory system. Neurotrophic factor 3 (NT-3) and brain derived neurotrophic factor (BDNF) released from hair cells and supporting cells exert a profound effect on SGN survival and neural firing patterns; however, it is unclear what the effects NT-3 and BDNF have on the type of neurotransmitter receptors expressed on SGN. To address this question, the whole-cell patch clamp recording technique was used to determine what effect NT-3 and BDNF had on the function and expression of glutamate, GABA and glycine receptors (GlyR) on SGN of cochlea from postnatal C57 mouse. Receptor currents induced by the agonist of each receptor were recorded from SGN cultured with or without BDNF or NT-3. NT-3 and BDNF exerted different effects. NT-3, and to a lesser extent BDNF, enhanced the expression of GABA receptors and had comparatively little effect on glutamate receptors. Absence of BDNF and NT-3 resulted in the emergence of glycine-induced currents; however, GlyR currents were absent from the short term cultured SGN. In contrast, NT-3 and BDNF suppressed GlyR expression on SGN. These results indicate that NT-3 and BDNF exert a profound effect on the types of neurotransmitter receptors expressed on postnatal SGN, results that may have important implications for neural development and plasticity.

The brain uncoupling protein UCP4 attenuates mitochondrial toxin-induced cell death: role of extracellular signal-regulated kinases in bioenergetics adaptation and cell survival

Increased bioenergetics demand can stimulate compensatory increases in glucose metabolism. We previously reported that neural cells expressing the brain uncoupling protein UCP4 exhibit enhanced dependency on glucose for support of cellular bioenergetics and survival. The switch from oxidative toward glycolytic metabolism reduces the production of toxic reactive oxygen species (ROS) and increases cellular resistance to toxicity induced by 3-nitropropionic acid, a mitochondrial complex II inhibitor that compromises cellular bioenergetics. In this study we elucidate the underlying mechanism whereby expression of UCP4 promotes bioenergetics adaptation and cell survival. We found that activation of extracellular signal-regulated kinases (ERKs) is necessary and sufficient for the increased dependency on glucose utilization. Pharmacological inhibition of ERKs not only abrogated bioenergetics adaptation but also reduced the activation of cAMP-responsive element-binding (CREB) protein suggesting that CREB protein signaling contributes in part to UCP4-dependent cell death rescue from 3-nitropropionic acid-induced toxicity. We also demonstrated that activation of ERKs by growth factors ameliorated the bioenergetics compromise and reduced cellular toxicity induced by 3-nitropropionic acid. Collectively, our results support the involvement of ERKs in UCP4 dependent bioenergetics adaptation and cell survival.

SIV infection decreases sympathetic innervation of primate lymph nodes: the role of neurotrophins

The sympathetic nervous system regulates immune responses in part through direct innervation of lymphoid organs. Recent data indicate that viral infections can alter the structure of lymph node innervation. To determine the molecular mechanisms underlying sympathetic denervation during Simian Immunodeficiency Virus (SIV) infection, we assessed the expression of neurotrophic factors and neuromodulatory cytokines within lymph nodes from experimentally infected rhesus macaques. Transcription of nerve growth factor (NGF), brain-derived neurotropic factor (BDNF) and neurotrophin-4 (NT4) decreased significantly in vivo during chronic SIV infection, whereas expression of the neuro-inhibitory cytokine interferon-gamma (IFN gamma) was up-regulated. Acute SIV infection of macaque leukocytes in vitro induced similar changes in the expression of neurotrophic and neuro-inhibitory factors, indicative of an innate immune response. Statistical mediation analyses of data from in vivo lymph node gene expression suggested that coordinated changes in expression of multiple neuromodulatory factors may contribute to SIV-induced depletion of catecholaminergic varicosities within lymphoid tissue. Given previous evidence that lymph node catecholaminergic varicosities can enhance SIV replication in vivo, these results are consistent with the hypothesis that reduced expression of neurotrophic factors during infection could constitute a neurobiological component of the innate immune response to viral infection.

Restoration of calbindin after fetal hippocampal CA3 cell grafting into the injured hippocampus in a rat model of temporal lobe epilepsy

Degeneration of the CA3 pyramidal and dentate hilar neurons in the adult rat hippocampus after an intracerebroventricular kainic acid (KA) administration, a model of temporal lobe epilepsy, leads to permanent loss of the calcium binding protein calbindin in major fractions of dentate granule cells and CA1 pyramidal neurons. We hypothesize that the enduring loss of calbindin in the dentate gyrus and the CA1 subfield after CA3-lesion is due to disruption of the hippocampal circuitry leading to hyperexcitability in these regions; therefore, specific cell grafts that are capable of both reconstructing the disrupted circuitry and suppressing hyperexcitability in the injured hippocampus can restore calbindin. We compared the effects of fetal CA3 or CA1 cell grafting into the injured CA3 region of adult rats at 45 days after KA-induced injury on the hippocampal calbindin. The calbindin immunoreactivity in the dentate granule cells and the CA1 pyramidal neurons of grafted animals was evaluated at 6 months after injury (i.e. at 4.5 months post-grafting). Compared with the intact hippocampus, the calbindin in "lesion-only" hippocampus was dramatically reduced at 6 months post-lesion. However, calbindin expression was restored in the lesioned hippocampus receiving CA3 cell grafts. In contrast, in the lesioned hippocampus receiving CA1 cell grafts, calbindin expression remained less than the intact hippocampus. Thus, specific cell grafting restores the injury-induced loss of calbindin in the adult hippocampus, likely via restitution of the disrupted circuitry. Since loss of calbindin after hippocampal injury is linked to hyperexcitability, re-expression of calbindin in both dentate gyrus and CA1 subfield following CA3 cell grafting may suggest that specific cell grafting is efficacious for ameliorating injury-induced hyperexcitability in the adult hippocampus. However, electrophysiological studies of KA-lesioned hippocampus receiving CA3 cell grafts are required in future to validate this possibility.

Specific AAV serotypes stably transduce primary hippocampal and cortical cultures with high efficiency and low toxicity

Most current methods of gene delivery for primary cultured hippocampal neurons are limited by toxicity, transient expression, the use of immature neurons and/or low efficiency. We performed a direct comparison of seven serotypes of adeno-associated virus (AAV) vectors for genetic manipulation of primary cultured neurons in vitro. Serotypes 1, 2, 7, 8 and 9 mediated highly efficient, nontoxic, stable long-term gene expression in cultured cortical and hippocampal neurons aged 0-4 weeks in vitro; serotypes 5 and 6 were associated with toxicity at high doses. AAV1 transduced over 90% of all cells with approximately 80% of the transduced cells being neurons. The method was readily adapted to a high-throughput format to demonstrate neurotrophin-mediated neuroprotection from glutamate toxicity in cultured neurons at 2 weeks in vitro. These vectors should prove highly useful for efficient overexpression or downregulation of genes in primary neuronal cultures at any developmental stage.

Neurotrophins in the dentate gyrus

Since the discovery of nerve growth factor (NGF) in the 1950s and brain-derived neurotrophic factor (BDNF) in the 1980s, a great deal of evidence has mounted for the roles of neurotrophins (NGF; BDNF; neurotrophin-3, NT-3; and neurotrophin-4/5, NT-4/5) in development, physiology, and pathology. BDNF in particular has important roles in neural development and cell survival, as well as appearing essential to molecular mechanisms of synaptic plasticity and larger scale structural rearrangements of axons and dendrites. Basic activity-related changes in the central nervous system (CNS) are thought to depend on BDNF modulation of synaptic transmission. Pathologic levels of BDNF-dependent synaptic plasticity may contribute to conditions such as epilepsy and chronic pain sensitization, whereas application of the trophic properties of BDNF may lead to novel therapeutic options in neurodegenerative diseases and perhaps even in neuropsychiatric disorders. In this chapter, I review neurotrophin structure, signal transduction mechanisms, localization and regulation within the nervous system, and various potential roles in disease. Modulation of neurotrophin action holds significant potential for novel therapies for a variety of neurological and psychiatric disorders.

Reductions in neurotrophin receptor mRNAs in the prefrontal cortex of patients with schizophrenia

Patients with schizophrenia have reduced neurotrophin levels in their dorsolateral prefrontal cortex (DLPFC) compared to normal unaffected individuals. The tyrosine kinase-containing receptors, trkB and trkC, mediate the growth-promoting effects of neurotrophins and respond to changes in growth factor availability. We hypothesized that trkB and/or trkC expression would be altered in the DLPFC of patients with schizophrenia. We measured mRNA encoding the tyrosine kinase domain (TK+)-containing form of trkB and measured pan trkC mRNA in schizophrenics (N=14) and controls (N=15) using in situ hybridization. TrkB and trkC mRNAs were detected in large and small neurons in multiple cortical layers of the human DLPFC. We found significantly diminished expression of trkB(TK+) mRNA in large neurons in multiple cortical layers of patients as compared to controls, while small neurons also showed reductions in trkB(TK+) mRNA that did not reach statistical significance. In normals, strong positive correlations were found between trkB(TK+) mRNA levels and brain-derived neurotrophic factor (BDNF) mRNA levels among various neurons, while no correlation between BDNF and trkB(TK+) was found in patients with schizophrenia. TrkC mRNA was also reduced in the DLPFC of schizophrenics in large neurons in layers II, III, V and VI and in small neurons in layer IV. Since neurons in the DLPFC integrate and communicate signals to various cortical and subcortical regions, these reductions in growth factor receptors may compromise the function and plasticity of the DLPFC in schizophrenia.

Ethanol effects on neonatal rat cortex: comparative analyses of neurotrophic factors, apoptosis-related proteins, and oxidative processes during vulnerable and resistant periods

The developing central nervous system (CNS) is highly susceptible to ethanol, with acute or chronic exposure producing an array of anomalies and cell loss. Certain periods of vulnerability have been defined for various CNS regions, and are often followed by periods of relative ethanol resistance. In the present study, neonatal rats were acutely exposed to ethanol during a time when peak cell death is found in developing cerebral cortex (postnatal day 7; P7), and during a later neonatal period of ethanol resistance (P21). Comparisons at the two ages were made of basal levels of neurotrophic factors (NTFs), and in addition, ethanol-mediated changes in NTFs, apoptosis-related proteins, antioxidant activities, and generation of reactive oxygen species (ROS) were quantified at 0, 2, and 12 h following termination of exposure. It was found that at P21, basal levels of NTF nerve growth factor (NGF) and brain-derived neurotrophic factor (BDNF) were considerably higher than at P7, possibly affording protection against ethanol neurotoxicity at this age. Following ethanol treatment at P7, approximately equal numbers of pro-apoptotic and pro-survival changes were produced, although most of the pro-apoptotic alterations occurred rapidly following termination of treatment, a critical period for initiation of apoptosis. At P21, however, the large majority of ethanol-mediated changes were adaptive, favoring survival. We speculate that the capacity of the older CNS to upregulate a number of protective elements within the cellular milieu serves to greatly mitigate ethanol neurotoxicity, while in younger animals, such adjustments are minimal, thus enhancing ethanol vulnerability within this developing region.

Excitotoxic and excitoprotective mechanisms: abundant targets for the prevention and treatment of neurodegenerative disorders

Activation of glutamate receptors can trigger the death of neurons and some types of glial cells, particularly when the cells are coincidentally subjected to adverse conditions such as reduced levels of oxygen or glucose, increased levels of oxidative stress, exposure to toxins or other pathogenic agents, or a disease-causing genetic mutation. Such excitotoxic cell death involves excessive calcium influx and release from internal organelles, oxyradical production, and engagement of programmed cell death (apoptosis) cascades. Apoptotic proteins such as p53, Bax, and Par-4 induce mitochondrial membrane permeability changes resulting in the release of cytochrome c and the activation of proteases, such as caspase-3. Events occurring at several subcellular sites, including the plasma membrane, endoplasmic reticulum, mitochondria and nucleus play important roles in excitotoxicity. Excitotoxic cascades are initiated in postsynaptic dendrites and may either cause local degeneration or plasticity of those synapses, or may propagate the signals to the cell body resulting in cell death. Cells possess an array of antiexcitotoxic mechanisms including neurotrophic signaling pathways, intrinsic stress-response pathways, and survival proteins such as protein chaperones, calcium-binding proteins, and inhibitor of apoptosis proteins. Considerable evidence supports roles for excitotoxicity in acute disorders such as epileptic seizures, stroke and traumatic brain and spinal cord injury, as well as in chronic age-related disorders such as Alzheimer's, Parkinson's, and Huntington's disease and amyotrophic lateral sclerosis. A better understanding of the excitotoxic process is not only leading to the development of novel therapeutic approaches for neurodegenerative disorders, but also to unexpected insight into mechanisms of synaptic plasticity.

Reduced brain-derived neurotrophic factor in prefrontal cortex of patients with schizophrenia

Anatomical and molecular abnormalities of excitatory neurons in the dorsolateral prefrontal cortex (DLPFC) are found in schizophrenia. We hypothesized that brain-derived neurotrophic factor (BDNF), a protein capable of increasing pyramidal neuron spine density and augmenting synaptic efficacy of glutamate, may be abnormally expressed in the DLPFC of patients with schizophrenia. Using an RNase protection assay and Western blotting, we detected a significant reduction in BDNF mRNA (mean=23%) and protein (mean=40%) in the DLPFC of patients with schizophrenia compared to normal individuals. At the cellular level, BDNF mRNA was expressed at varying intensities in pyramidal neurons throughout layers II, III, V, and VI of DLPFC. In patients with schizophrenia; neuronal BDNF expression was decreased in layers III, V and VI. Our study demonstrates a reduction in BDNF production and availability in the DLPFC of schizophrenics, and suggests that intrinsic cortical neurons, afferent neurons, and target neurons may receive less trophic support in this disorder.

Neurotrophic factor effects on oxidative stress-induced neuronal death

Neurotrophic factors have been shown to potentiate necrotic neuronal death in cortical cultures. In this study we characterized the death induced by various oxidative insults and tested the effects of neurotrophic factors on that death. Treatment with fibroblast growth factor-2, neurotrophin-4, or insulin-like growth factor-1 potentiated neuronal cell death induced by iron-citrate (Fe) or buthionine sulfoximine (BSO), but not ethacrynic acid (EA). Neuronal death induced by each insult was blocked by the free radical scavenger, trolox. An analysis of the death indicated that Fe and BSO induced necrotic cell death, while EA induced apoptotic cell death. BSO and EA caused decreased cellular glutathione levels, whereas Fe had no effect on glutathione levels. Neurotrophic factors had no effect on the changes in glutathione. The results indicate that oxidative insults can induce either apoptotic or necrotic death and that the effects of neurotrophic factors are dependent on the type of cell death.

Neurotrophin potentiation of iron-induced spinal cord injury

Previous studies have shown that pretreatment with neurotrophins can potentiate the vulnerability of cultured neurons to excitotoxic and free radical-induced necrosis, in contrast to their well known neuroprotective effects against apoptosis. Here we tested the hypothesis that this unexpected injury-potentiating effect of neurotrophins would also take place in the adult rat spinal cord. Fe(3+)-citrate was injected stereotaxically into spinal cord gray matter in adult rats in amounts sufficient to produce minimal tissue injury 24 h later. Twenty-four-hour pretreatment with brain-derived neurotrophic factor, neurotrophin-3, or neurotrophin-4/5, but not nerve growth factor, markedly enhanced tissue injury in the gray matter as evidenced by an increase in the damaged area, as well as the loss of neurons and oligodendrocytes. Consistent with maintained free radical mediation, the neurotrophin-potentiated iron-induced spinal cord damage was blocked by co-application of the antioxidant N-tert-butyl-(2-sulfophenyl)-nitrone. These data support the hypothesis that the overall neuroprotective properties of neurotrophins in models of acute injury to the spinal cord may be limited by an underlying potentiation of free radical-mediated necrosis.

Protection and reversal of excitotoxic neuronal damage by glucagon-like peptide-1 and exendin-4

Glucagon-like peptide-1 (7-36)-amide (GLP-1) is an endogenous insulinotropic peptide that is secreted from the L cells of the gastrointestinal tract in response to food. It has potent effects on glucose-dependent insulin secretion, insulin gene expression, and pancreatic islet cell formation. In type 2 diabetes, GLP-1, by continuous infusion, can normalize blood glucose and is presently being tested in clinical trials as a therapy for this disease. More recently, GLP-1 has been found to have central nervous system (CNS) effects and to stimulate neurite outgrowth in cultured cells. We now report that GLP-1, and its longer-acting analog exendin-4, can completely protect cultured rat hippocampal neurons against glutamate-induced apoptosis. Extrapolating these effects to a well defined rodent model of neurodegeneration, GLP-1 and exendin-4 greatly reduced ibotenic acid-induced depletion of choline acetyltransferase immunoreactivity in basal forebrain cholinergic neurons. These findings identify a novel neuroprotective/neurotrophic function of GLP-1 and suggest that such peptides may have potential for halting or reversing neurodegenerative processes in CNS disorders, such as Alzheimer's disease, and in neuropathies associated with type 2 diabetes mellitus.

Brain-derived neurotrophic factor in the control human brain, and in Alzheimer's disease and Parkinson's disease

Brain-derived neurotrophic factor (BDNF) is a small dimeric protein, structurally related to nerve growth factor, which is abundantly and widely expressed in the adult mammalian brain. BDNF has been found to promote survival of all major neuronal types affected in Alzheimer's disease and Parkinson's disease, like hippocampal and neocortical neurons, cholinergic septal and basal forebrain neurons, and nigral dopaminergic neurons. In this article, we summarize recent work on the molecular and cellular biology of BDNF, including current ideas about its intracellular trafficking, regulated synthesis and release, and actions at the synaptic level, which have considerably expanded our conception of BDNF actions in the central nervous system. But our primary aim is to review the literature regarding BDNF distribution in the human brain, and the modifications of BDNF expression which occur in the brain of individuals with Alzheimer's disease and Parkinson's disease. Our knowledge concerning BDNF actions on the neuronal populations affected in these pathological states is also reviewed, with an aim at understanding its pathogenic and pathophysiological relevance.

Estrogens: trophic and protective factors in the adult brain

Our appreciation that estrogens are important neurotrophic and neuroprotective factors has grown rapidly. Although a thorough understanding of the molecular and cellular mechanisms that underlie this effect requires further investigation, significant progress has been made due to the availability of animal models in which we can test potential candidates. It appears that estradiol can act via mechanisms that require classical intracellular receptors (estrogen receptor alpha or beta) that affect transcription, via mechanisms that include cross-talk between estrogen receptors and second messenger pathways, and/or via mechanisms that may involve membrane receptors or channels. This area of research demands attention since estradiol may be an important therapeutic agent in the maintenance of normal neural function during aging and after injury.

NT-4/5 exacerbates free radical-induced neuronal necrosis in vitro and in vivo

Neurotrophins render neurons highly vulnerable to certain injuries. We examined the possibility that NT-4/5 would enhance free radical neurotoxicity in vivo as well as in vitro. Striatal neurons exposed to 10 microM Fe(2+) or 1 mM l-buthionine-[S, R]-sulfoximine (BSO) underwent mild degeneration within 24 h. With concurrent addition of 10-100 ng/ml NT-4/5, neuronal death following exposure to Fe(2+) or BSO was significantly increased and suppressed by addition of 100 microM trolox, an antioxidant. In the adult brain, the intrastriatal injections of 20 nmol Fe(2+) revealed features of neuronal necrosis such as swelling cell body and mitochondria, fenestration of plasma membrane prior to nuclear membrane, and scattering condensation of nuclear chromatin. Cotreatment with 1.8 microg NT-4/5 augmented the striatal damage 24 h following the injections of Fe(2+). This study implies that free radicals produce necrotic degeneration in vivo as well as in vitro that becomes more sensitive in the presence of neurotrophins.

Glutamate slows axonal transport of neurofilaments in transfected neurons

Neurofilaments are transported through axons by slow axonal transport. Abnormal accumulations of neurofilaments are seen in several neurodegenerative diseases, and this suggests that neurofilament transport is defective. Excitotoxic mechanisms involving glutamate are believed to be part of the pathogenic process in some neurodegenerative diseases, but there is currently little evidence to link glutamate with neurofilament transport. We have used a novel technique involving transfection of the green fluorescent protein-tagged neurofilament middle chain to measure neurofilament transport in cultured neurons. Treatment of the cells with glutamate induces a slowing of neurofilament transport. Phosphorylation of the side-arm domains of neurofilaments has been associated with a slowing of neurofilament transport, and we show that glutamate causes increased phosphorylation of these domains in cell bodies. We also show that glutamate activates members of the mitogen-activated protein kinase family, and that these kinases will phosphorylate neurofilament side-arm domains. These results provide a molecular framework to link glutamate excitotoxicity with neurofilament accumulation seen in some neurodegenerative diseases.

Phosphorylation of neurofilament heavy chain side-arms by stress activated protein kinase-1b/Jun N-terminal kinase-3

Neurofilaments comprise three subunit proteins; neurofilament light, middle and heavy chains (NF-L, NF-M and NF-H). The carboxy-terminal domains of NF-M and NF-H form side-arms that project from the filament and that of NF-H contains multiple repeats of the motif lys-ser-pro, the serines of which are targets for phosphorylation. The level of phosphorylation on the lys-ser-pro repeats varies topographically within the cell; in cell bodies and proximal axons, the side-arms are largely non-phosphorylated whereas in more distal regions of axons, the side-arms are heavily phosphorylated. Here we show that stress activated protein kinase 1b (SAPK1b), a major SAPK in neurones will phosphorylate NF-H side-arms both in vitro and in transfected cells. These studies suggest that SAPK1b targets multiple phosphorylation sites within NF-H side-arms. Additionally, we show that glutamate treatment induces activation of SAPK1b in primary cortical neurones and increased phosphorylation of NF-H in cell bodies. This suggests that glutamate causes increased NF-H phosphorylation at least in part by activation of stress activated protein kinases.

Preclinical testing of neuroprotective neurotrophic factors in a model of chronic motor neuron degeneration

Many neurotrophic factors have been shown to enhance survival of embryonic motor neurons or affect their response to injury. Few studies have investigated the potential effects of neurotrophic factors on more mature motor neurons that might be relevant for neurodegenerative diseases. Using organotypic spinal cord cultures from postnatal rats, we have demonstrated that insulin-like growth factor-I (IGF-I) and glial-derived neurotrophic factor (GDNF) significantly increase choline acetyltransferase (ChAT) activity, but brain-derived neurotrophic factor (BDNF), neurotrophin-4 (NT-4/5), and neurotrophin-3 (NT-3) do not. Surprisingly, ciliary neurotrophic factor (CNTF) actually reduces ChAT activity compared to age-matched control cultures. Neurotrophic factors have also been shown to alter the sensitivity of some neurons to glutamate neurotoxicity, a postulated mechanism of injury in the neurodegenerative disease, amyotrophic lateral sclerosis (ALS). Incubation of organotypic spinal cord cultures in the presence of the glutamate transport inhibitor threo-hydroxyaspartate (THA) reproducibly causes death of motor neurons which is glutamate-mediated. In this model of motor neuron degeneration, IGF-I, GDNF, and NT-4/5 are potently neuroprotective, but BDNF, CNTF, and NT-3 are not. The organotypic glutamate toxicity model appears to be the best preclinical predictor to date of success in human clinical trials in ALS.

Early BDNF, NT-3, and NT-4 signaling events

Much more is known about nerve growth factor (NGF) signaling than that initiated by brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3), or NT-4. We sought to study early BDNF, NT-3, and NT-4 signaling events. Using TrkB-expressing cells, we found that BDNF and NT-4 individually induced tyrosine phosphorylation of TrkB in a dose-dependent fashion. At maximally effective concentrations, BDNF or NT-4 induced robust TrkB tyrosine phosphorylation at 5 min; this progressively declined at 15, 30, and 60 min. Using immunoprecipitation, PI3-kinase and tyrosine phosphorylated PLC-gamma1 and SHC were shown to be associated with tyrosine phosphorylated TrkB in response to both BDNF and NT-4. BDNF and NT-4 induced similar intensities of phosphorylation of TrkB and signaling intermediates at equivalent doses. NT-3 treatment of TrkC-expressing cells induced very similar patterns for induction of TrkC tyrosine phosphorylation and recruitment of signaling intermediates. BDNF, NT-3, and NT-4 caused rapid tyrosine phosphorylation of ERK and SNT. These data suggest that the earliest signaling events for BDNF, NT-3, and NT-4 are very similar to those for NGF.

Neurotrophin-mediated potentiation of neuronal injury

The neurotrophins are a diverse family of peptides which activate specific tyrosine kinase-linked receptors. Over the past five decades, since the pioneering work of Levi-Montalcini and colleagues, the critical role that neurotrophins play in shaping the developing nervous system has become increasingly established. These molecules, which include the nerve growth factor (NGF)-related peptides, NGF, brain-derived neurotrophic factor (BDNF), NT-4/5 and NT-3, promote differentiation and survival in the developing nervous system, and to a lesser extent in the adult nervous system. As survival-promoting molecules, neurotrophins have been studied as potential neuroprotective agents, and have shown beneficial effects in many model systems. However, a surprising "dark side" to neurotrophin behavior has emerged from some of these studies implying that, under certain pathological conditions, neurotrophins may exacerbate, rather than alleviate, injury. How neurotrophins cause these deleterious consequences is a question which is only beginning to be answered, but initial work supports altered free radical handling or modification of glutamate receptor expression as possible mechanisms underlying these effects. This review will focus on evidence suggesting that neurotrophins may enhance injury under certain circumstances and on the mechanisms behind these injury-promoting aspects.

The mitogen-activated protein kinase pathway mediates estrogen neuroprotection after glutamate toxicity in primary cortical neurons

Pharmacological and biochemical approaches were used to elucidate the involvement of growth factor signaling pathways mediating estrogen neuroprotection in primary cortical neurons after glutamate excitotoxicity. We addressed the activation of mitogen-activated protein kinase (MAPK) signaling pathways, which are activated by growth factors such as nerve growth factor (NGF). Inhibition of MAPK signaling with the MAPK kinase inhibitor PD98059 blocks both NGF and estrogen neuroprotection in these neurons. These results correlate with a rapid and sustained increase in MAPK activity within 30 min of estrogen exposure. The involvement of signaling molecules upstream from MAPK was also examined to determine whether activation of MAPK by estrogen is mediated by tyrosine kinase activity. Estrogen produces a rapid, transient activation of src-family tyrosine kinases and tyrosine phosphorylation of p21(ras)-guanine nucleotide activating protein. Effects of estrogen on neuroprotection, as well as rapid activation of tyrosine kinase and MAPK activity, are blocked by the anti-estrogen ICI 182,780. This provides evidence that activation of the MAPK pathway by estrogen participates in mediating neuroprotection via an estrogen receptor. These results describe a novel mechanism by which cytoplasmic actions of the estrogen receptor may activate the MAPK pathway, thus broadening the understanding of effects of estrogen in neurons.

Regulation of pituitary adenylate cyclase activating polypeptide and its receptor type 1 after traumatic brain injury: comparison with brain-derived neurotrophic factor and the induction of neuronal cell death

Neurotrophic factors are known to promote neuronal survival during development and after acute brain injury. Recent data suggest that some neuropeptides also exhibit neurotrophic activities, as shown for the pituitary adenylate cyclase activating polypeptide, which increases the survival of various neuronal populations in culture. Employing in situ hybridization techniques, we have studied the regulation of messenger RNA for pituitary adenylate cyclase activating polypeptide and its receptor type 1 after a moderate traumatic brain injury to rat brain cortex. We have further compared their messenger RNA expression to that of brain-derived neurotrophic factor and to the amount of cell death occurring in the brain at various times after the brain injury. Levels of brain-derived neurotrophic factor messenger RNA increased rapidly within 2 h after trauma in cortex and hippocampus, and returned to control levels thereafter. The levels of messenger RNA for pituitary adenylate cyclase activating polypeptide also increased with time in the injured brains and reached maximal expression at 72 h, i.e. the end of the observation period. The alterations in pituitary adenylate cyclase activating polypeptide messenger RNA levels were particularly pronounced in the perifocal region and in the ipsilateral dentate gyrus of the brain injury. In contrast, the messenger RNA levels encoding pituitary adenylate cyclase activating polypeptide receptor type 1 first decreased after trauma and were then normalized in the dentate gyrus. There was a large increase in the number of cells labelled for DNA breaks at 12 h post-trauma, indicative of enhanced cell death. The number of labelled cells, however, decreased at later stages concomitant with an increase in the expression of pituitary adenylate cyclase activating polypeptide messenger RNA. Pituitary adenylate cyclase activating polypeptide rescued cortical neurons in cultures against ionomycin-induced cell death, supporting the concept of a neuroprotective effect for the peptide. These results demonstrate a differential regulation of messenger RNA for brain-derived neurotrophic factor and the pituitary adenylate cyclase activating polypeptide and its receptor after brain trauma. The data also suggest that pituitary adenylate cyclase activating polypeptide might have a beneficial effect in brain injury by counteracting neuronal cell death.

GDNF induces the calretinin phenotype in cultures of embryonic striatal neurons

Glial cell line-derived neurotrophic factor (GDNF), a member of the transforming growth factor-beta (TGF-beta) superfamily, is a potent neurotrophic factor for several neuron populations in the central and peripheral nervous system. Members of the neurotrophin, neurokine, and TGF-beta families of growth factors can affect neurons beyond their capacity to promote survival. They can play instructive roles including the determination of a particular transmitter phenotype. Here, we show that GDNF enhances the number of calretinin (CaR)-positive neurons in serum-free cultures of striatal cells isolated from embryonic rats. The effect is dose-dependent, can be elicited with concentrations as low as 0.1 ng/ml, and is not accompanied by increased incorporation of 5-bromo-2'-desoxyuridine and appearance of glial fibrillary acidic protein-positive cells. Similar, but weaker effects can be elicited by brain-derived neurotrophic factor, neurotrophin-3 and -4, fibroblast growth factor-2. Ciliary neurotrophic factor, nerve growth factor, and TGF-beta 1 do not affect striatal CaR expression. GDNF can augment CaR-positive cells at any time point and with a minimal exposure of 18 hr, suggesting induction of the phenotype rather than increased survival. By reverse transcription polymerase chain reaction (RT-PCR), we show that GDNF is expressed in the E16 striatum and in cultures derived from this tissue. GDNF also protected striatal CaR-positive neurons against glutamate toxicity. We conclude that striatal GDNF, in addition to its retrograde trophic role for nigrostriatal dopaminergic neurons, may also act locally within the striatum (e.g., by inducing the CaR phenotype and protecting these cells against toxic insult).

Cell death of adult pyramidal CA1 neurons after intraventricular injection of a novel peptide derived from trkA

Members of the nerve growth factor (NGF) family of neurotrophins bind to the second leucine-rich motif (LRM2) within the extracellular domains of their respective receptors (trkA, trkB, trkC). Small LRM2 peptides have been recently demonstrated to selectively bind the neurotrophins revealing similar complex binding characteristics as full-length receptors. We extend our recent findings, showing that the peptides (A and C) do not block nerve fiber outgrowth through high affinity trk receptors in a ganglia bioassay. Since the highest concentration of neurotrophins [NGF, brain-derived neurotrophic factor (BDNF), neurotrophin-3 (NT-3)] is found in the hippocampus, the peptides were injected into the 3rd ventricle of anesthetized adult rats. The (NGF binding) LRM2-A peptide, but not the (BDNF binding) LRM2-B or the (NT-3 binding) LRM2-C peptides, caused severe apoptotic neurodegeneration of hippocampal pyramidal CA1 neurons as revealed by cresyl violet staining and the TUNEL reaction. The degeneration was protected by intrahippocampal injection of NGF-beta and by the non-N-methyl-D-aspartate (NMDA) antagonist CNQX (6-cyano-7-nitroquinoxaline-2,3-dione), indicating a glutamatergic mechanism. In situ hybridization revealed that pyramidal CA1 neurons did not express trkA and p75 receptor mRNA in sham and LRM2-A-lesioned animals. It is concluded that the LRM2-A peptide represents a novel peptide with properties to induce apoptotic cell death of pyramidal CA1 neurons and may be useful as an experimental agent.

Aging in the hippocampus: interrelated actions of neurotrophins and glucocorticoids

Over the past two decades, evidence has been accumulating that diffusible molecules, such as growth factors and steroids hormones, play an important part in neural senescence, particularly in the hippocampus. There is also evidence that these molecules do not act as independent signals, but show interrelated regulation and cooperative control over the aging process. Here, we review some of the changes that occur in the hippocampus with age, and the influence of two classes of signaling substances: glucocorticoids and neurotrophins. We also examine the interactions between these substances and how this could influence the aging process.

Differential susceptibility to neurotoxicity mediated by neurotrophins and neuronal nitric oxide synthase

NMDA neurotoxicity, which is mediated, in part, by formation of nitric oxide (NO) via activation of neuronal NO synthase (nNOS), is modulated by neurotrophins. nNOS expression in rat and mouse primary neuronal cultures grown on a glial feeder layer is significantly less than that of neurons grown on a polyornithine (Poly-O) matrix. Neurotrophins markedly increase the number of nNOS neurons, nNOS protein, and NOS catalytic activity and enhance NMDA neurotoxicity via NO-dependent mechanisms when neurons are grown on glial feeder layers. In contrast, when rat or mouse primary cortical neurons are grown on a Poly-O matrix, neurotrophins have no influence on nNOS neuronal number or NOS catalytic activity and reduce NMDA neurotoxicity. Primary neuronal cultures from mice lacking nNOS grown on a glial feeder layer fail to respond to neurotrophin-mediated enhancement of neurotoxicity. Together, these results indicate that nNOS expression and NMDA NO-mediated neurotoxicity are dependent, in part, on the culture paradigm, and neurotrophins regulate the susceptibility to NMDA neurotoxicity via modulation of nNOS. Furthermore, these results support the idea that NMDA neurotoxicity in culture is critically dependent on the developmental state of the neurons being assessed and suggest that, when cortical neurons are cultured on a glial feeder layer, they do not reach nearly as mature a phenotype as when grown on a Poly-O matrix.

Erythropoietin receptor is expressed in rat hippocampal and cerebral cortical neurons, and erythropoietin prevents in vitro glutamate-induced neuronal death

Recently, erythropoietin has been shown to be produced by astrocytes and its production is hypoxia-inducible. In the present study, we demonstrated, using a reverse transcription-polymerase chain reaction assay and immunostaining of the cells, that the erythropoietin receptor was expressed in cultured hippocampal and cerebral cortical neurons of day 19 rat embryo. Erythropoietin protected the cultured neurons from glutamate neurotoxicity. Neurons cultured for seven to 10 days were exposed to glutamate for 15 min and after culture for a further 24 h in the absence of glutamate the neuron survival was assayed. Significant protection was observed with erythropoietin from 3 pM (c. 100 pg/ml) in a dose-dependent manner. The protection was completely reversed by co-application of a soluble erythropoietin receptor, an extracellular domain capable of binding with erythropoietin. For exhibition of the neuroprotective effect, exposure of neurons to erythropoietin approximately 8 h prior to exposure to glutamate was required. Experiments with the inhibitors indicated that RNA and protein syntheses were necessary for the protection. However, exposure to erythropoietin for a short period (5 min or less) was sufficient to elicit the protective effect. The protective effect of erythropoietin was blocked by the simultaneous addition of EGTA. These findings and the previous finding that erythropoietin induces a rapid and transient increase in intracellular Ca2+ concentration in neuronal cells suggest that erythropoietin plays a neuroprotective role in brain injury caused by hypoxia or ischemia and that erythropoietin-induced Ca2+ influx from outside of the cells is a critical initial event yielding an enhanced resistance of the neurons to glutamate toxicity.

Selective acid vulnerability of dopaminergic neurons and its recovery by brain-derived neurotrophic factor

Among the pathogenetic phenomena of Parkinson's disease, the character of the selective degeneration of nigrostriatal system with severe gliosis is not fully understood. Here, we have shown that dopaminergic neurons may be exclusively sensitive to elevated acidity elicited after the addition of glial mitogenic factors such as epidermal growth factor and basic fibroblast growth factor or after the direct treatment with hydrochloric acid. The acid sensitivity was specific to dopaminergic neurons. The neurons other than dopaminergic neurons in culture from the ventral mesencephalon were not sensitive to acidity and the neurons from several brain areas were the same as above, except for the hippocampal neurons which had slight acid vulnerability. Choline acetyltransferase assay studies demonstrated that the cholinergic neuronal population in the septum and corpus striatum had no acid sensitivity. The vulnerability of dopaminergic neurons either elicited by glial mitogenic factor or derived from the direct acid exposure was inhibited by the addition of brain-derived neurotrophic factor (BDNF), but not by neurotrophin-3 or nerve growth factor. These findings suggest that dopaminergic neurons have selective acid vulnerability on which BDNF has a pronounced protective effect.