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 M) 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.Glutamate, the endogenous neurotransmitter required for normal physiological excitation, is also involved in the pathophysiology of hypoxic/ischemic neuronal injury (1-3). Although this neuropathological process can be mediated by any of the excitatory amino acid receptors, the N-methyl-D-aspartate (NMDA) 1 glutamate receptor subtype plays a major role. In addition, NMDA receptors mediate adaptive responses important for synaptic plasticity during development (4, 5). The molecular mechanism(s) by which NMDA receptors mediate such opposing effects are not clear.The role of NMDA receptors in glutamate-mediated excitotoxicity has been studied in rat cerebellar granule cells in vitro, in which overactivation of NMDA receptors results in neuronal cell death (6 -8). Paradoxically, in these cells subtoxic concentrations of NMDA have been shown to protect vulnerable neurons against an excitotoxic concentration of glutamate (9). Moreover, recent studies have shown that the neuroprotective effect of NMDA is blocked by coincubation with either the RNA synthesis inhibitor actinomycin D or the protein synthesis inhibitor cycloheximide (9). Thus, the neuroprotective effect of NMDA may involve a polypeptide(s) with neuroprotective properties.Cerebellar granule cells are responsive to various neurotrophic factors and in particular to the neurotrophins (10, 11), a family of trophic factors related by primary amino acid sequence homology, whose members include brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NT-3), and NT-4/5 (12-17). The biological activity of neurotrophins depends upon the activation of high-affinity receptors (Trk...

The molecular mechanisms leading to synaptic simplification and neuronal apoptosis in human immunodeficiency virus type 1 (HIV) 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 cognitive 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 prior to 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 anti-apoptotic and pro-apoptotic neurotrophins. Our studies identify a new mechanism that may explain how HIV promotes neuronal injury.

Interleukin-10 (IL-10) has been shown to reduce neuronal degeneration after CNS injury. However, the molecular mechanisms underlying the neuroprotective properties of this cytokine are still under investigation. Glutamate exacerbates secondary injury caused by trauma. Thus, we examined whether IL-10 prevents glutamate-mediated cell death. We used rat cerebellar granule cells in culture because these neurons undergo apoptosis upon exposure to toxic concentrations of glutamate (100-500 microm) or NMDA (300 microm). Pretreatment of cerebellar granule cells with IL-10 (1-50 ng/ml) elicited a dose- and time-dependent reduction of glutamate-induced excitotoxicity. Most importantly, IL-10 reduced the number of apoptotic cells when added to the cultures together or 1 hr after glutamate. Using patch-clamping and fluorescence Ca(2+) imaging techniques, we examined whether IL-10 prevents glutamate toxicity by blocking the function of NMDA channel. IL-10 failed to affect NMDA channel properties and to reduce NMDA-mediated rise in intracellular Ca(2+). Thus, this cytokine appears to prevent glutamate toxicity by a mechanism unrelated to a blockade of NMDA receptor function. Various proteases, such as caspase-3, and transcription factors, such as nuclear factor kappaB (NF-kappaB), have been proposed to participate in glutamate-mediated apoptosis. Thus, we examined whether IL-10 modulates the activity of these apoptotic markers. IL-10 blocked both the glutamate-mediated induction of caspase-3 as well as NF-kappaB DNA binding activity, suggesting that the neuroprotective properties of IL-10 may rely on its ability to block the activity of proapoptotic proteins.