AIDS can produce substantial dysfunction in the central nervous system, including subcortical areas, cortex and retina. These adverse effects may occur in the absence of superinfection with opportunistic organisms or productive HIV-1 infection of neurons. Therefore, one possibility is that HIV-1 somehow produces a toxic agent(s) that injures neurons, such as retinal ganglion cells and cortical neurons. Along these lines, HIV-1 coat protein gp120 has been found by this laboratory to increase intracellular free calcium ([Ca2+]) and subsequently to injure cultured primary central neurons, including retinal ganglion cells, in mixed neuronal/glial cultures. Studies from our laboratory indicated that the N-methyl-D- aspartate (NMDA) subtype of glutamate receptor is involved in gp120-induced toxicity. Furthermore, from work in our laboratory and in others, it has become evident recently that at least one pathway to neuronal injury is mediated by toxic factors that are secreted by either HIV-infected or gp120-stimulated macrophages. These putative toxic factors include cysteine, platelet-activating factor (PAF), and arachidonic acid and its metabolites. In this proposal, we plan to characterize further these putative neurotoxic factors. First, we will characterize the release of the NMDA-like neurotoxin, cysteine, from gp120-stimulated human macrophages; gp 120-induced release of cysteine has been documented in the preliminary experiments. Second, we will perform patch-clamp recordings and digital calcium imaging to determine if the putative macrophage- released toxic factors cysteine and PAF directly or indirectly activate NMDA receptor channels (or other membrane ion channels). Cysteine apparently acts as an NMDA agonist while PAF is thought to increase glutamate release from neurons. We will also determine if gp120-induced arachidonic acid production from macrophages affects glutamate uptake/efflux in both astrocytes and synaptosomes, thereby increasing the local concentration of glutamate and hence neurotoxicity. Next, with conventional neurotoxicity assays, drugs that have been shown to down- regulate NMDA receptor activity by oxidizing the receptor's redox modulatory site, a site discovered in this laboratory, will be tested to prevent the neuronal injury induced by these toxic factors. These drugs include nitroso-compounds that produce redox congeners of nitric oxide, such as nitroglycerin (NTG) and sodium nitroprusside (SNP). We will also test other clinically tolerated NMDA antagonists whose mechanism of action has been studies by this laboratory. These drugs include the adamantanes, such as memantine and novel congeners, which produce open-channel block of the NMDA receptor-operated ion channel. Finally, PAF antagonists will be used to try to abrogate neurotoxicity in culture to implicate PAF in at least one HIV-related pathway to neuronal injury. These in vitro studies will use rat retinal ganglion cells and cortical neurons as well as human fetal cortical neurons. The long-term objectives of this study are to better understand the pathways and mechanisms to HIV-induced neuronal injury in order to lead to possible treatments of the neurological manifestations of AIDS.
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