Update on current models of HIV-related neuronal injury: Platelet-activating factor, arachidonic acid and nitric oxide

This review aims to summarize recent work related to the pathogenesis and possible treatment of neuronal injury in the acquired immunodeficiency syndrome (AIDS), especially with reference to potential neurotoxic substances released by HIV-infected or gp120-stimulated macrophages/microglia. Approximately a third of adults and half of children with AIDS eventually suffer from neurological manifestations, including dysfunction of cognition, movement, and sensation. Among the various pathologies reported in brains of patients with AIDS is neuronal injury and loss. A paradox arises, however, because neurons themselves are for all intents and purposes not infected by human immunodeficiency virus type 1 (HIV-1). This paper reviews recent evidence suggesting that at least part of the neuronal injury observed in the brains of AIDS patients is related to excessive influx of Ca²⁺ after the release of potentially noxious substances from HIV-infected or gp120-stimulated macrophages/microglia.

There is growing support for the existence of HIV- or immune-related toxins that lead indirectly to the injury or demise of neurons via a potentially complex web of interactions between macrophages (or microglia), astrocytes, and neurons. HIV-infected monocytoid cells (macrophages, microglia or monocytes), especially after interacting with astrocytes, secrete substances that potentially contribute to neurotoxicity. Not all of these substances are yet known, but they may include eicosanoids, i.e. arachidonic acid and its metabolites, as well as platelet-activating factor. Other candidate toxins include nitric oxide (NO·), superoxide anion (02·⁻), and the N-methyl-scd-aspartate (NMDA) agonist, cysteine. Similarly, macrophages activated by HIV-1 envelope protein gp120 also appear to release arachidonic acid and its metabolites, and cysteine. These factors can lead to increased glutamate release or decreased glutamate re-uptake. In addition, interferon-γ (IFN-γ) stimulation of macrophages induces release of the NMDA-like agonist, quinolinate. HIV-infected or gp120-stimulated macrophages also produce cytokines, including TNF-α and ILI-β, which contribute to astrocytosis. A final common pathway for neuronal susceptibility appears to be operative, similar to that observed in stroke, trauma, epilepsy, neuropathic pain and several neurodegenerative diseases, possibly including Huntington's disease, Parkinson's disease, and amyotrophic lateral sclerosis. This mechanism involves the activation of voltage-dependent Ca²⁺ channels and NMDA receptor-operated channels with resultant generation of free radicals, and therefore offers hope for future pharmacological intervention.