Search

Morphine has been shown to increase the expression of brain-derived neurotrophic factor (BDNF) in the brain. However, little is known about the effect of morphine withdrawal on BDNF and its precursor protein, or proBDNF, which induces neuronal apoptosis. In this work, we examined whether BDNF and proBDNF levels change in rats chronically injected with escalating doses of morphine and those who undergo spontaneous withdrawal for 60 h. We observed, in the frontal cortex and striatum, that the ratio of BDNF to proBDNF changed depending upon the experimental paradigm. Morphine treatment and morphine withdrawal increased both BDNF and proBDNF levels. However, the increase in proBDNF immunoreactivity in withdrawal rats was more robust than that observed in morphine-treated rats. proBDNF is processed either intracellularly by furin or extracellularly by the tissue plasminogen activator (tPA)/plasminogen system or matrix metalloproteases (MMPs). To examine the mechanisms whereby chronic morphine treatment and morphine withdrawal differentially affects BDNF/proBDNF, the levels MMP-3 and MMP-7, furin, and tPA were analyzed. We found that morphine increases tPA levels, whereas withdrawal causes a decrease. To confirm the involvement of tPA in the morphine-mediated effect on BDNF/proBDNF, we exposed cortical neurons to morphine in the presence of the tPA inhibitor plasminogen activator inhibitor-1 (PAI-1). This inhibitor reversed the morphine-mediated decrease in proBDNF, supporting the hypothesis that morphine increases the availability of BDNF by promoting the extracellular processing of proBDNF by tPA. Because proBDNF could negatively influence synaptic repair, preventing withdrawal is crucial for reducing neurotoxic mechanisms associated with opioid abuse.

Many people infected with the human immunodeficiency virus type-1 (HIV) exhibit mild or severe neurological problems, termed HIV-associated neurocognitive disorder (HAND), even when receiving antiretroviral therapy. Thus, novel adjunctive therapies must be developed to overcome the neurotoxic effect of HIV. New therapies require a better understanding of the molecular and cellular mechanisms of HIV-induced neurotoxicity and the risk factors that, besides inflammation and T-cell depletion and drugs of abuse, render the central nervous system (CNS) a target of HIV-induced neurotoxicity. HIV appears to impair neuronal plasticity, which refers to the innate ability of the CNS respond to injury and promote recovery of function. The availability of brain-derived neurotrophic factor (BDNF), a potent neurotrophic factor that is present in abundance in the adult brain, is essential for neuronal plasticity. BDNF acts through a receptor system composed of Trk and p75NTR. Here, we present experimental evidence that some of the clinical features of HIV-mediated neurological impairment could result from altered BDNF/TrkB/p75NTR regulation and function., (© 2013 The Association for the Publication of the Journal of Internal Medicine.)

The molecular mechanisms leading to synaptic simplification and neuronal apoptosis in human immunodeficiency virus type 1 (HIV-1)-positive subjects are unknown. The HIV protein gp120 reduced the length of neuronal processes similarly to the proneurotrophin pro-brain-derived neurotrophic factor (proBDNF). Intriguingly, the effects of both proBDNF and gp120 were blocked by inhibitors of the p75 neurotrophin receptor, suggesting that proBDNF and gp120 share a similar mechanism of neurotoxicity. Therefore, we tested the hypothesis that gp120 affects the release of proBDNF. Using rat primary neurons, we observed that gp120 promotes a time-dependent intracellular and extracellular accumulation of proBDNF concomitantly with a decrease in mature BDNF. A similar imbalance in the ratio proBDNF/mature BDNF was confirmed in postmortem brains of HIV-positive subjects cognitively impaired and motor impaired. Therefore, it is conceivable to formulate the hypothesis that HIV neurotoxicity includes a gp120-mediated alteration of BDNF processing. To determine the cellular mechanism whereby gp120 produces an accumulation of proBDNF, we examined the levels of intracellular and extracellular enzymes that proteolytically cleave proBDNF furin and tissue plasminogen, respectively. In rat neurons exposed to gp120, intracellular furin levels decreased before cell death, whereas tissue plasminogen changed only during apoptosis. Our data suggest that HIV, through gp120, reduces proBDNF processing by affecting furin levels, and therefore causes an altered balance between antiapoptotic and proapoptotic neurotrophins. Our studies identify a new mechanism that may explain how HIV promotes neuronal injury.

Gangliosides are lipophilic compounds found in cell plasma membranes throughout the brain that play a role in neuronal plasticity and regeneration. Indeed, absence or abnormal accumulation of gangliosides has been shown to lead to neurological disorders. Experimental data have shown that exogenous gangliosides exhibit properties similar to the neurotrophins, a family of neurotrophic factors that are important in the survival and maintenance of neurons and prevention of neurological diseases. Brain-derived neurotrophic factor (BDNF) is the most abundant of the neurotrophins. This work was done to reveal the neurotrophic mechanism of exogenous gangliosides. In particular, we examined whether gangliosides promote the release of BDNF. Rat hippocampal neurons or human neuroblastoma cells were transduced with a recombinant adenovirus expressing BDNF-flag to facilitate detection of BDNF. Release of BDNF was then determined by Western blot analysis and a two-site immunoassay of culture medium. The depolarizing agent KCl was used as a comparison. In hippocampal neurons, both GM1 ganglioside and KCl evoked within minutes the release of mature BDNF. In human cells, GM1 and other gangliosides released both mature BDNF and pro-BDNF. The effect of gangliosides was structure-dependent. In fact, GT1b preferentially released mature BDNF whereas GM1 released both mature and pro-BDNF. Ceramide and sphingosine did not modify the release of BDNF. This work provides additional experimental evidence that exogenous gangliosides can be used to enhance the neurotrophic factor environment and promote neuronal survival in neurological diseases. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'., (Copyright © 2010 Elsevier Ltd. All rights reserved.)

Neurotrophins control cell survival. Therefore, we examined whether HIV-1 reduces neurotrophin levels. Serum of HIV-positive individuals exhibited lower concentrations of brain-derived neurotrophic factor (BDNF), but not of other neurotrophins, than HIV-negative individuals. In addition, R5 and X4 strains of HIV-1 decreased BDNF expression in T cells. Our results support the hypothesis that reduced levels of BDNF may be a risk factor for T-cell apoptosis and for neurological complications associated with HIV-1 infection.

Chemokine receptors, and in particular CXCR4 and CCR5 play a key role in the neuropathogenesis of Human Immunodeficiency Virus-1 (HIV)4 associated dementia (HAD). Thus, new insight into the expression of CXCR4 in the central nervous system may help develop therapeutic compounds against HAD. Brain-derived neurotrophic factor (BDNF) is neuroprotective in vitro against two strains of the HIV envelope protein gp120 that binds to CXCR4 or CCR5. Therefore, we examined whether BDNF modulates chemokine receptor expression in vivo. The content of CXCR4 mRNA and proteins was determined in the cerebral cortex and hippocampus of 6-month-old BDNF heterozygous mice and wild type littermates by using polymerase chain reaction and immunohistochemistry, respectively. BDNF heterozygous mice exhibited an increase in CXCR4 mRNA compared to wild type. Histological analyses revealed an up-regulation of CXCR4 immunoreactivity mainly in neurons. Most of these neurons were positive for TrkB, the BDNF receptor with a tyrosine kinase activity. Increases in CXCR4 mRNA levels were observed in 18-month-old BDNF heterozygous mice but not in 7-day-old mice, suggesting that the modulatory role of BDNF occurs only in mature animals. To determine whether BDNF affects also CXCR4 internalization, SH-SY5Y neuroblastoma cells were exposed to BDNF and cell surface CXCR4 levels were measured at various times. BDNF induced CXCR4 internalization within minutes. Lastly, BDNF heterozygous mice showed higher levels of CCR5 and CXCR3 mRNA than wild type in the cerebral cortex, hippocampus and striatum. Our data indicate that BDNF may modulate the availability of chemokine receptors implicated in HIV infection.

Human immunodeficiency virus type 1 (HIV-1) causes neuronal degeneration and, at a late stage, creates HIV-associated dementia (HAD) and other neurological abnormalities. Therefore, the need for neuroprotective agents is great. However, therapeutic agents that reduce HIV neurotoxicity are difficult to characterize and develop because rodents are not infected by HIV. This study was undertaken to develop an animal model of HIV neurotoxicity by using the HIV-1 envelope glycoprotein 120 (gp120). Vehicle or gp120 was injected acutely in the striatum of adult rats. gp120 produced loss of nigrostriatal neurons, as shown both by histochemical analysis of brain sections for apoptosis and biochemical determination of dopamine. The neurotrophin brain-derived neurotrophic factor (BDNF) delivered by a recombinant adeno-associated viral vector prevented gp120 toxicity. This study's results support the notion that gp120 produces a widespread neurotoxicity similar to that observed in HIV-positive individuals and that BDNF may be a suitable neuroprotective agent for HAD.

Progressive and irreversible loss of specific neuronal cell populations is commonly seen in chronic neurodegenerative diseases such as Parkinson's disease (PD). Evidence is accumulating that apoptosis is a crucial cellular event responsible for the dysfunction and death of neurons in this disease. Thus, limiting apoptosis may prevent disease pathogenesis. Key to reducing apoptosis is the discovery of neuroprotective compounds that can be given to patients to minimize neuronal damage. In this manuscript, we reviewed the rationale of using an experimental strategy to provide neurotrophic support to injured neurons. Such rationale includes the increase of endogenous production of brain-derived neurotrophic factor (BDNF). BDNF is a potent inhibitor of apoptosis-mediated cell death and neurotoxin-induced degeneration of dopaminergic neurons. However, availability of BDNF may be reduced when dopaminergic neurons degenerate. Therefore, in this work, we have used several well-established neurotoxins for dopaminergic neurons, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, 6-OH-dopamine (6-OHDA), and the HIV protein gp120, to examine whether degeneration of nigrostriatal fibers alters BDNF expression. Our data show that these neurotoxins do not decrease the levels of BDNF in the substantia nigra, suggesting that up-regulation of BDNF synthesis by pharmacological means may be a viable therapy to slow down the progress of PD and other neurodegenerative diseases.

Glycoprotein 120 (gp120) from the T-tropic strain of the human immunodeficiency virus type 1 has been shown to cause neuronal apoptosis through activation of the chemokine receptor CXCR4. Therefore, reducing CXCR4 expression may prevent gp120-mediated apoptosis. Brain-derived neurotrophic factor (BDNF) is known to reduce both gp120 neurotoxicity and CXCR4 expression in vitro. The scope of this work is to establish whether BDNF is neuroprotective against gp120 in vivo and, if so, whether this effect correlates with its ability to down-regulate CXCR4. Serotype 2 adeno-associated viral vector encoding for BDNF (rAAV-BDNF) or control vector was microinjected into the striata of adult rats. Two weeks later gp120 was injected into the same striatum, and apoptosis determined. Pretreatment with rAAV-BDNF prior to gp120 microinjection prevented caspase-3 activation as well as in situ terminal deoxynucleotidyl transferase biotin-dUTP nick end labelling in the striatum and substantia nigra. In addition, rAAV-BDNF reversed the loss of tyrosine hydroxylase immunoreactivity induced by gp120 in both areas. CXCR4 expression was then determined by immunohistochemistry and RT-PCR, and found to be decreased in striata of rAAV-BDNF-treated rats. Conversely, BDNF heterozygous mice exhibited an increase in CXCR4 mRNA levels compared to wild-type littermates. Our data suggest that down-regulation of CXCR4 expression may contribute to the neuroprotective activity of BDNF against gp120 toxicity in the basal ganglia.

Uninfected neurons of the substantia nigra (SN) degenerate in human immunodeficiency virus (HIV)-positive patients through an unknown etiology. The HIV envelope glycoprotein 120 (gp120) causes apoptotic neuronal cell death in the rodent striatum, but its primary neurotoxic mechanism is still under investigation. Previous studies have shown that gp120 causes neurotoxicity in the rat striatum by reducing brain-derived neurotrophic factor (BDNF). Because glial cell line-derived neurotrophic factor (GDNF) and BDNF are neurotrophic factors crucial for the survival of dopaminergic neurons of the SN, we investigated whether gp120 reduces GDNF and BDNF levels concomitantly to induce apoptosis. Rats received a microinjection of gp120 or vehicle into the striatum and were sacrificed at various time intervals. GDNF but not BDNF immunoreactivity was decreased in the SN by 4 days in gp120-treated rats. In these animals, a significant increase in the number of caspase-3- positive neurons, both tyrosine hydroxylase (TH)-positive and -negative, was observed. Analysis of TH immunoreactivity revealed fewer TH-positive neurons and fibers in a medial and lateral portion of cell group A9 of the SN, an area that projects to the striatum, suggesting that gp120 induces retrograde degeneration of nigrostriatal neurons. We propose that dysfunction of the nigrostriatal dopaminergic system associated with HIV may be caused by a reduction of neurotrophic factor expression by gp120., ((c) 2006 Wiley Periodicals, Inc.)

Patients with human immunodeficiency virus type 1 (HIV-1) infection develop a broad spectrum of motor impairments and cognitive deficits, which follow or parallel cellular loss and atrophy in their brains. The viral envelope glycoprotein 120 (gp120) has been suggested to be a causal agent of neuronal loss. Therefore, reducing gp120 neurotoxicity may prevent neuronal degeneration seen in these patients. Here, we describe in vitro and in vivo experimental evidence that gp120 toxicity can be reduced by brain-derived neurotrophic factor (BDNF), a naturally occurring peptide that has been shown to block neurotoxin and trauma-induced neuronal injury. Moreover, we review the survival promoting properties of BDNF and the issues concerning its delivery into the brain, in an attempt to explain the rationale for exploring BDNF as a prototype trophic factor for a therapy to reduce neuronal cell death in HIV-1 infected patients.

Human immunodeficiency virus type 1 (HIV-1)-positive patients in the late phase of infection develop AIDS dementia complex, an array of neurological complications that include extrapyramidal symptoms, cognitive impairments, and psychiatric disturbances. Brains of these patients exhibit brain injury. The HIV-1 envelope glycoprotein 120 (gp120) has been suggested to be a causal agent of neuronal loss; however, several strains of gp120 exist during the infection and the relative neurotoxic potential of each strain is presently unknown. Using cultured cerebellar granule neurons, we determined whether two strains of gp120, gp120IIIB and gp120BaL, which bind to CXCR4 and CCR5 chemokine receptors, respectively, induce cell death. Apoptotic cell death and activated caspase-3 were evident within a few hours in neurons exposed to low nanomolar concentrations of either gp120IIIB or gp120BaL. However, the neurotoxic effect of gp120IIIB was more rapid and occurred at lower concentrations than that of gp120BaL, suggesting that cerebellar granule cells may be more sensitive to apoptotic signals activated by the CXCR4 receptor. The neurotrophin brain-derived neurotrophic factor (BDNF) has been shown to block neuronal apoptosis. Therefore, we examined whether BDNF protects against both strains of gp120. Preexposure of cerebellar granule cells to BDNF prior to both gp120s decreased apoptosis and consequently enhanced their survival. These findings underlie the rationale for exploring the ability of BDNF to reduce HIV-1-mediated neuronal cell death in vivo.

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.

Patients with the human immunodeficiency virus type 1 (HIV-1) develop in the late phase of infection a complex of neurological signs termed Acquired Immune Deficiency Syndrome-Related Dementia (ADC). These patients exhibit cortical and subcortical atrophy. Considerable experimental data indicate that the HIV-1 envelope glycoprotein gp120 may be one of the agents causing neuronal cell death. Gp120 causes neuronal cell death both in vitro and in vivo by activating a caspase-dependent apoptotic pathway, and in particular caspase-3. The neurotrophin brain-derived neurotrophic factor (BDNF) has been shown to prevent gp120-mediated apoptosis of cerebellar granule cells by inhibiting caspase-3 activation. However, the signal transduction pathway that contributes to the neuroprotective effects of BDNF has not been determined. BDNF binds with high affinity to the tyrosine kinase receptor TrkB and activates different intracellular signaling cascade including the extracellular signal-related kinases (ERK) and the phosphatidylinositol 3-kinase (PI3-K). Pharmacological inhibition of TrkB or ERK1/2, but not PI3-K, greatly reduced the ability of BDNF to block gp120-mediated apoptosis of cerebellar granule cells. These findings suggest that TrkB-mediated activation of ERK1/2 is the main signaling pathway that contributes to neuroprotection against gp120.

The human immunodeficiency virus type 1 (HIV-1) envelope protein gp120 has been implicated in the pathogenesis of HIV-1 dementia. Thus, inhibition of gp120 activity could reduce HIV toxicity in the brain. We have used primary cultures of rat cerebellar granule cells to examine mechanisms whereby gp120 causes cell death and to characterize neuroprotective agents. gp120 induced a time- and concentration-dependent apoptotic cell death, which was caspase-3-mediated but caspase-1 independent, and was totally blocked by the irreversible caspase-3-like protease inhibitor N-acetyl-Asp-Glu-Val-Asp-chloromethylketone. Caspase-3 activation was observed only in neurons that internalize gp120, indicating that internalization is key to gp120 toxicity. Because brain-derived neurotrophic factor (BDNF) prevents caspase-3-mediated neuronal cell death, we examined whether BDNF could prevent gp120-mediated apoptosis. Preincubation of neurons with BDNF before the addition of gp120 reduced caspase-3 activation, and consequently rescued 80% of neurons from apoptosis. Most importantly, BDNF reduced the levels of CXC chemokine receptor-4 (CXCR4), a receptor that mediates HIV-1 gp120-induced apoptosis. This effect correlated with the ability of BDNF to reduce gp120 internalization and apoptosis. Moreover, BDNF blocked the neurotoxic effect of stromal-derived factor-1alpha, a natural ligand for CXCR4, further establishing a correlation between neuroprotection and downregulation of CXCR4. We propose that BDNF may be a valid therapy to slow down the progression of HIV/gp120-mediated neurotoxicity.

The molecular mechanism(s) of N-methyl-D-aspartate (NMDA) neuroprotective properties were investigated in primary cultures of cerebellar granule cell neurons. Granule cells express the neurotrophin receptor TrkB but not TrkA or TrkC. In these cells, the TrkB ligand brain-derived neurotrophic factor (BDNF) prevents glutamate toxicity. Therefore, we have tested the hypothesis that NMDA activates synthesis and release of BDNF, which may prevent glutamate toxicity by an autocrine loop. Exposure of granule cells for 2 and 5 min to a subtoxic concentration of NMDA (100 microM) evoked an accumulation of BDNF in the medium without concomitant changes in the intracellular levels of BDNF protein or mRNA. The increase in BDNF in the medium is followed by enhanced TrkB tyrosine phosphorylation, suggesting that NMDA increases the release of BDNF and therefore the activity of TrkB receptors. To examine whether BDNF and TrkB signaling play a role in the NMDA-mediated neuroprotective properties, neurons were exposed to soluble trkB receptor-IgG fusion protein, which is known to inhibit the activity of extracellular BDNF, and to K252a, a tyrosine kinase inhibitor. Both compounds blocked the NMDA-mediated TrkB tyrosine phosphorylation and subsequently its neuroprotective properties. We suggest that NMDA activates the TrkB receptor via a BDNF autocrine loop, resulting in neuronal survival.

Evidence has accumulated to suggest that the NMDA glutamate receptor subtype plays an important role in neuronal degeneration evoked by hypoxia, ischemia, or trauma. Cerebellar granule cells in culture are vulnerable to NMDA-induced neuronal excitotoxicity. In these cells, brain-derived neurotrophic factor (BDNF) and basic fibroblast growth factor (FGF2) prevent the excitotoxic effect of NMDA. However, little is known about the molecular mechanisms underlying the protective properties of these trophic factors. Using cultured rat cerebellar granule cells, we investigated whether BDNF and FGF2 prevent NMDA toxicity by downregulating NMDA receptor function. Western blot and RNase protection analyses were used to determine the expression of the various NMDA receptor subunits (NR1, NR2A, NR2B, and NR2C) after BDNF or FGF2 treatment. FGF2 and BDNF elicited a time-dependent decrease in the expression of NR2A and NR2C subunits. Because NMDA receptor activation leads to increased intracellular Ca2+ concentration ([Ca2+]i), we studied the effect of the BDNF- and FGF2-induced reduction in NR2A and NR2C synthesis on the NMDA-evoked Ca2+ responses by single-cell fura-2 fluorescence ratio imaging. BDNF and FGF2 reduced the NMDA-mediated [Ca2+]i increase with a time dependency that correlates with their ability to decrease NR2A and NR2C subunit expression, suggesting that these trophic factors also induce a functional downregulation of the NMDA receptor. Because sustained [Ca2+]i is believed to be causally related to neuronal injury, we suggest that BDNF and FGF2 may protect cerebellar granule cells against excitotoxicity by altering the NMDA receptor-Ca2+ signaling via a downregulation of NMDA receptor subunit expression.