Human immunodeficiency virus type 1 (HIV-1) enters the central nervous system (CNS) early in the course of infection, where it can persist in resident microglia and astrocytes(1, 2). In untreated individuals, unchecked viral replication can rapidly lead to end-stage neurodegeneration. The advent of combination antiretroviral therapy (cART) has helped to reduce the incidence of this extreme clinical outcome(3), but persistent CNS infection even with cART still results in the development of debilitating neurocognitive deficits, known as HIV associated neurocognitive disorders (HANDs), in greater than 50% of people living with HIV-1(4, 5). Thus, there is a growing need for therapies that can help mitigate the neurologic symptoms of persistent HIV infection. To this end, our lab and others have identified mixed lineage kinase 3 (MLK3) as a potential target for neuroprotection in the context of HIV-1 infection. MLK3 is an important upstream regulator of mitogen-activated protein kinases (MAPKs) p38 and c-Jun-N-terminal-kinase (JNK), two kinases that play major roles in neuroinflammation and neuronal apoptosis(6-10), as well as an important inducer of the pro-inflammatory NF-? pathway(11). MLK3 inhibition using the only available MLK3 inhibitor CEP-1347 has already demonstrated a neuroprotective effect in various models of HIV infection(12, 13). Based on these findings, I hypothesize that: Inhibition of MLK3 has a neuroprotective effect in the HIV infected CNS. This neuroprotection is imparted by blockading neurotoxic pathways in microglia and invasive peripheral immune cells, as well as by restoring the normal neuronal supportive function of microglia. I will utilize in viv imaging technologies and novel culture systems to study the pathologic changes in the interaction between neurons and microglia when either or both cell types are exposed to the HIV-1 neurotoxin Tat. Furthermore, an NIH-funded collaboration led by our lab has developed a new brain-penetrant small molecule MLK3 inhibitor. I will investigate the potential neuroprotective properties of this inhibitor in a murine model of HAND, as well as in various in vitro models. These data will help advance our understanding of the role of MLK3 as a therapeutic target in HAND. PUBLIC HEALTH RELEVANCE: HIV associated neurocognitive disorders are a growing cause of disability in the HIV infected population. This project will validate inhibition of the enzyme Mixed Lineage Kinase 3 as a target in the brain for neuroprotection in the context of HIV infection, so that new therapies to decrease the activity of this enzyme can potentially improve the quality of life for millions of HIV infected individuals.