Brain-Derived Neurotrophic Factor Inhibits Human Immunodeficiency Virus-1/gp120-Mediated Cerebellar Granule Cell Death by Preventing gp120 Internalization

Abstract

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.

Human immunodeficiency virus gp120-induced apoptosis of human neuroblastoma cells in the absence of CXCR4 internalization

“…Thus, the apparent discrepancy with our findings could be due to various scenarios, including differences in the cell types and experimental approaches, the reported binding of gp120 to other molecules expressed on the neuronal surface, such as glycosphingolipids Long et al, 1994), and/or the expression of unknown CD4-like molecules in granule neurons. Alternatively, the HIV envelope protein could enter the cell via ligand-independent chemokine receptor internalization (Neel et al, 2005;Signoret et al, 2000), which would be consistent with the kinetics of gp120 accumulation in cerebellar granule neurons (Bachis et al, 2003). On the other hand, the dynamics of CXCR4 expression on the membrane of SH-SY5Y cells treated with CXCL12 (Figure 2) is a typical example of ligand-induced receptor endocytosis, a common and reversible mechanism of desensitization of chemokine receptor and GPCRs in general (Ferguson, 2001;Neel et al, 2005).…”

“…The effect of gp120 on CXCR4 internalization/surface expression, however, is not reported in their study. Moreover, gp120 internalization was not observed in glial cells (Bachis et al, 2003), which also express CXCR4, thus questioning whether this is a CXCR4-dependent event (Lazarini et al, 2003). Thus, the apparent discrepancy with our findings could be due to various scenarios, including differences in the cell types and experimental approaches, the reported binding of gp120 to other molecules expressed on the neuronal surface, such as glycosphingolipids Long et al, 1994), and/or the expression of unknown CD4-like molecules in granule neurons.…”

“…This hypothesis is also supported by our previous studies (Khan et al, 2004) showing that pretreatment of CD4 -/CXCR4 + cells with gp120 IIIB did not abolish calcium responses stimulated by the physiological CXCR4 agonist (CXCL12), suggesting that the viral protein is unable to induce proper CXCR4 down-regulation despite that it activates several intracellular responses (Meucci et al, 1998, and this study). However, a recent confocal microscopy study reported a time-dependent internalization of recombinant gp120 in cerebellar granule neurons (Bachis et al, 2003). The authors find that internalization of biotin-conjugated gp120 is blocked by brain-derived neurotrophic factor, which also prevents gp120 neurotoxicity and reduces CXCR4 expression, i.e., CXCR4 total protein levels.…”

Multiple roles of chemokine CXCL12 in the central nervous system: A migration from immunology to neurobiology

“…One line of research suggests that gp120 and CXCL12 could directly induce neuronal degeneration and apoptosis through binding to CXCR4, but that cell death can be reduced through heterologous desensitization of CXCR4 (Hesselgesser J et al, 1998;Meucci O et al, 1998;Corasaniti MT et al, 2001;Bachis A and Mocchetti I, 2003). Another protection mechanism implicates BDNF, which reduced expression of CXCR4, an effect which correlated with the ability of BDNF to reduce gp120 internalization and neuronal apoptosis.…”

HIV-1 coreceptors CCR5 and CXCR4 both mediate neuronal cell death but CCR5 paradoxically can also contribute to protection

“…7,26 These effects can be modeled in mixed neuronal/glial cerebrocortical cultures from rodents or humans in which purified or recombinant gp120 initiates neuronal injury and apoptosis. 3,7,11,17,18,27,28 In order to further dissect the role of chemokine receptors in HIV/gp120-induced neurotoxicity, we used mixed neuronal/ glial cerebrocortical cultures prepared from wild-type (WT) mice or rats, and mutant mice deficient in CXCR4, 29 CCR5 30 or both chemokine receptors. Our findings suggest that, depending on the viral strain, HIV/gp120 can utilize CCR5 or CXCR4 or both receptors for induction of neurotoxicity.…”