Browsing by Subject "Tat"
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Item The mechanism of HIV-1 Tat-induced changes in NMDA receptor function(2014-08) Krogh, Kelly A.Worldwide, more than 35 million people are currently infected with the human immunodeficiency virus (HIV). Approximately half of HIV-infected patients in the U.S. experience cognitive impairment despite effective control of viral load with combination anti-retroviral therapy (cART). The neurological complications that stem from an HIV infection are known as HIV-associated neurocognitive disorders (HAND). HAND ranges in severity from subtle difficulties with day-to-day tasks to severe functional impairment requiring assistance to survive. Although cART has improved patient survival by effectively managing viral load, it is ineffective at treating the majority of HAND. Consequently, the prevalence of HAND remains persistently high. The symptoms of HAND correlate with neuronal damage, such as synapse loss and dendritic beading. Such synaptodendritic damage results from HIV-infected cells within the central nervous system (CNS) shedding neurotoxic agents, such as the HIV-1 protein transactivator of transcription (Tat). Tat potentiates N-methyl D-aspartate (NMDA) receptor function allowing excessive Ca2+ influx leading to neurotoxicity. In this dissertation, two studies are outlined investigating the mechanisms of NMDA receptor (NMDAR) dysfunction following exposure to Tat. The graphical abstract summarizes these studies. First, the effect of Tat on NMDAR function was investigated. This study showed that Tat caused a time-dependent, biphasic change in NMDAR function. Initially, Tat potentiated NMDAR function via the low-density lipoprotein receptor-related protein (LRP) and activation of Src tyrosine kinase. Subsequently, NMDAR function adapted by gradually returning to basal levels following 24 h exposure to Tat and eventually falling below control responses by 48 h. Adaptation resulted from activation of a nitric oxide synthase (NOS), soluble guanylate cyclase (sGC), cGMP-dependent protein kinase (PKG) signaling pathway. Next, effectors downstream of PKG responsible for adaptation of NMDAR function were identified. Tat activated a signaling pathway including the small GTPase RhoA and Rho-associated protein kinase (ROCK). RhoA/ROCK activation caused remodeling of the actin cytoskeleton resulting in reduced NMDAR function. Taken together, these studies indicate that Tat causes a biphasic change in NMDAR function. Potentiation of NMDAR function is mediated by LRP-dependent activation of Src kinase; adaptation of NMDAR function occurs after activation of a NOS/sGC/PKG signaling pathway leading to RhoA/ROCK-mediated remodeling of the actin cytoskeleton. Adaptation of NMDAR function may be a neuroprotective mechanism to reduce excess Ca2+ influx and prevent neurotoxicity. These studies provide molecular and temporal detail of the dynamic changes in NMDAR function following exposure to Tat and offer insight into potential therapeutic targets for the treatment of HAND.Item Mechanisms of HIV-associated neurotoxicity: Tat-induced synapse loss and recovery(2013-05) Brunner, Angela HaijungHIV 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.Item Modulation of the Endocannabinoid System By Hiv Tat Protein(2020-06) Wu, MariahDespite the success of combined antiretroviral therapy (cART) in extending the lifespan and enhancing the quality of life for those infected with HIV, nearly half of the 37.9 million HIV-positive individuals experience some degree of neurocognitive impairment. These impairments are collectively termed HIV-associated neurocognitive disorders (HAND). HAND symptoms range from subtle difficulties in the tasks of daily living to severe functional impairment and dementia. cART effectively suppresses viral load and improves patient survival; however, while the severity is reduced, the prevalence of HAND remains high and may be increasing due to the prolonged lifespan of HIV patients. Currently, there are no effective treatments for HAND. HIV neurotoxicity is mainly mediated by inflammatory and excitotoxic agents released from infected macrophages and microglia. The HIV trans-activator protein Tat is one such agent that plays a major role in the neuropathogenesis of HAND. While cART suppresses viral replication, it does not halt the production of Tat once HIV has integrated into the host genome. Numerous studies have demonstrated that Tat causes excitotoxicity, which leads to a loss of spine density and altered network function; this synaptic dysfunction predicts marked impairments in learning and memory paradigms in various in vitro and in vivo models of HAND. Thus, it is essential to elucidate the mechanisms that cause and exacerbate neurotoxicity, in order to find effective therapeutic targets to ultimately treat HAND symptoms. A growing body of literature indicates that components of the endocannabinoid (eCB) system may be viable therapeutic targets. Across many models of neurological disorders that share a common pathophysiological pathway including excitotoxicity and neuroinflammation, activation of the eCB system attenuates detrimental cellular processes. Recent reports show that activating the eCB system protects against HIV-induced neurotoxicity, indicating that modulation of the eCB system may be a viable therapeutic approach to suppress the neuronal damage that underlies HAND. However, one important determinant of clinical outcome is the degree to which the target biological system is functional. Exposure to excitotoxic stimuli alters the eCB system; whether eCB signaling is altered in the presence of HIV is unknown. In this dissertation, I explore the effects of HIV Tat protein on eCB signaling in vitro. Using an electrophysiological approach, I determined that Tat impaires cannabinoid type 1 receptor (CB1R) function in a subset of neurons without affecting other components of the eCB system. Taken together, these results indicate that Tat specifically impairs CB1R-mediated presynaptic inhibition of glutamate release. This observation suggests that eCB-mediated neuroprotection may be reduced in vivo, possibly exacerbating synaptodendritic injury caused by HIV. Thus, this dissertation provides new insight into processes that further contribute to HIV-induced neurotoxicity and suggests that protecting or enhancing the neuroprotective eCB system may attenuate the symptoms of HAND.Item Neural adaptations following exposure to HIV proteins(2018-07) Green, MatthewHIV associated neurocognitive disorders (HAND) affect nearly half of the population infected with HIV. Symptoms range from subclinical cognitive deficits to severe dementia which can affect daily living. The neurotoxicity produced is likely indirect as the virus does not infect neurons but infects other cells in the brain such as microglia and astrocytes which can release viral proteins and inflammatory cytokines that then affect neurons and lead to synaptodendritic damage. Thus, understanding how these toxic viral factors affect neurons and the mechanisms underlying these changes will aid in the discovery of therapies for HAND. In this dissertation, I report viral proteins altering two types of receptors involved in regulating neuronal excitability and synaptic transmission, the excitatory NMDA receptor (NMDAR) and the inhibitory GABAAR. NMDARs initially become overactivated but then downregulate in hippocampal cultures exposed to the HIV protein transactivator of transcription (Tat). This downregulation was mediated by GluN2A-containing NMDARs signaling to the kinase Akt and the E3 ubiquitin ligase, Mdm2. This novel mechanism delineates a signaling pathway activated by viral proteins that regulates NMDAR-dependent loss of excitatory synapses. Second, I determined how the viral protein gp120 affects GABAergic inhibition. In hippocampal cultures, gp120 caused an increase in tonic GABAAR currents mediated by extrasynaptic GABAARs and an increase in the number of inhibitory synapses. Both increases in inhibition were dependent on microglial activation and release of interleukin-1β which activated interleukin-1 receptors. The increase in inhibitory synapses was dependent on IL-1R-mediated Src activation and subsequent potentiation of GluN2A-containing NMDARs and protein synthesis. On the other hand, the increase in tonic inhibition was dependent on IL-1R-mediated p38 MAPK activation and selective upregulation of α5-containing GABARs. The increases in inhibition likely dampen neuronal excitability and network function and may contribute to the cognitive deficits in HAND. These studies elucidate changes in two types of receptors affected by viral proteins and identify novel signaling pathways that may lead to therapeutic targets for HAND.