Mechanisms of HIV-associated neurotoxicity: Tat-induced synapse loss and recovery

Abstract

HIV infection is a worldwide pandemic. A debilitating neurological consequence of HIV infection is progressive cognitive decline, known as HIV-associated neurocognitive disorders (HAND). HAND afflicts up to 50% of all HIV patients to varying degrees, and as survival of HIV patients improves with current antiretroviral therapies, the prevalence of HAND is also increasing. This, coupled with the lack of current effective HAND therapies, creates a dire need to understand the mechanisms underlying the cognitive decline associated with HIV. HAND symptoms correlate closely with processes of neuronal injury, which are early events that precede overt neuronal death. One such injurious process is synapse loss. The HIV protein transactivator of transcription (Tat) is a neurotoxic viral protein released from infected cells into the central nervous system. Tat contributes to the pathologies seen in HAND patients, and induces loss of excitatory synapses between rat hippocampal neurons in culture. Using an innovative live cell imaging assay, our laboratory has previously shown that Tat induces reversible synapse loss via a pathway that is distinct from cell death. In this dissertation, I outline three studies that stem from the current knowledge involving Tat-induced synapse loss. These studies elucidated important information regarding the mechanisms by which HIV Tat exerts its neurotoxic effects, emphasizing the importance of subunit composition when determining toxic or beneficial effects of NMDA receptor activation as well as unmasking the importance of the postsynaptic density as the central target of Tat's effects. Furthermore, these studies highlight the reversibility of synapse loss and uncover a new role for the canonical NO/cGMP/PKG pathway in modulating synapse recovery downstream of GluN2B-containing NMDA receptors. Tat-induced synapse loss and subsequent recovery can correlate to symptoms of cognitive decline seen in HAND. Targeting these mechanisms can shed new light on therapeutic strategies to treat HAND patients.