Investigating molecular mechanisms of glutamate toxicity in HIV-1 Tat-induced cognitive impairment

About This Grant

Project Summary HIV-1 associated neurocognitive disorders (HAND) affect 15 to 40% of people living with HIV (PWH), despite the use of anti-retroviral therapies (ART). Persistence of HAND in the presence of ART suggests that factors outside of viral replication contribute to neurocognitive impairment. The HIV-1 transactivator of transcription (Tat) is a neurotoxic viral protein that recapitulates cognitive impairment in the absence of viral replication and persists in virally suppressed PWH, likely generated from cellular reservoirs including microglia and astrocytes. Prior studies have separately shown that in the presence of Tat, the microglial transporter xCT is upregulated – increasing extracellular glutamate – and astrocytic EAAT2 is downregulated. Nonetheless, these mechanisms have been demonstrated with varying techniques and agnostic to how Tat expression within cellular reservoirs affects Tat-mediated toxicity and cognitive symptoms of HAND. Thus, we hypothesize that Tat expressed independently from microglia and astrocytes promotes aberrant glutamatergic neurotransmission causing NMDAR dependent excitotoxicity in the prefrontal cortex and cognitive impairment in HAND. In this proposal, we will use lentiviral transduction to model Tat expression from microglia and astrocytes, as the HIV-1 reservoirs of the CNS. We will then assess glutamate toxicity in this model in vitro and in vivo, by evaluating 1) EAAT2 or xCT expression and markers of gliosis [GFAP, Iba1], 2) extracellular glutamate levels in culture supernatants, 3) neuronal NMDA receptor expression and signaling via calcium levels. These outcomes will provide an understanding of how microglia and astrocytes respond to Tat and influence NMDAR- mediated neurotoxicity. To understand how these cell types drive toxicity and cognitive impairment in turn, lentivirus will be injected intracerebrally to prefrontal cortex of Sprague-Dawley rats. Two weeks after surgery, we will assess behavioral and molecular outcomes; or calcium levels in neurons and astrocytes. Animals will undergo testing in novel object recognition, spatial object recognition, and attentional set-shifting tasks, to assess learning and memory and cognitive flexibility. Brain tissue will then be assessed by immunoblot, RT- PCR, and calcium imaging to correlate cognitive impairments with molecular mechanisms; further paralleling the in vitro results to contextualize the contribution of Tat-mediated mechanisms to cognitive impairment. This study will elucidate the role of microglia and astrocytes as separate sources of Tat for their effects on glutamatergic neurotransmission and PFC-mediated cognitive functions. The proposal addresses a significant gap in the literature on microglia as the primary viral reservoir generating Tat, while accounting for the distinct impacts of each cellular reservoir on Tat-mediated glutamate toxicity and cognitive impairment. This will prompt future study into the microglial reservoir, and glutamatergic disease mechanisms that could be refined as therapeutic targets that are clinically relevant to neuropathology in PWH.