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

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. First, I assessed how subunit selectivity influenced N-methyl-D-aspartate (NMDA) receptor activity on the toxic effects of Tat. A pharmacological study showed that GluN2A-containing NMDA receptors had distinct and opposing effects as compared to GluN2B-containing NMDA receptors when modulating Tat-induced cell death, as well as Tat-induced synapse loss and subsequent synapse recovery. Intriguingly, inhibition of GluN2B-containing NMDA receptors induced synapse recovery after Tat-induced synapse loss, indicating a role for tonic suppression of recovery by GluN2B-containing receptors.;Second, I determined the effects of Tat exposure on presynaptic terminals. Tat induced loss of presynaptic terminals concurrent with loss of postsynaptic densities. This loss of presynaptic terminals was dependent on postsynaptic mechanisms, and the integrity of postsynaptic densities was required to maintain the presence of presynaptic terminals after Tat exposure. Tat-induced loss of presynaptic terminals was reversible, and this reversal was dependent on postsynaptic activity as well.;Finally, I determined the molecular mechanisms of synapse recovery downstream of GluN2B-containing NMDA receptors. After Tat-induced synapse loss, GluN2B-containing NMDA receptors were persistently activated, suppressing synapse recovery by activation of neuronal nitric oxide synthase. The subsequent production of nitric oxide stimulated cGMP production by soluble guanylyl cyclase. Protein kinase G was activated and phosphorylated an actin-associated protein during persistent Tat exposure.;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.