Antiretroviral therapy (ART) has increased dramatically the life span of HIV-1 infected individuals, but damaging effects of HIV-1 persist throughout the nervous system, and the prevalence of HIV-1 associated neurocognitive disorder (HAND) is greater than 50% of HIV-1 infected people in the USA. Our long-term goal is to understand the pathogenesis of HAND and develop therapeutic interventions. The proposed studies are focused to determine novel endolysosome-dependent calcium regulatory mechanisms whereby ART drugs and HIV-1 transactivator of transcription protein (HIV-1 Tat) contribute to the development of HAND. Our central hypothesis is that elevation of endolysosome pH centrally connects ART drugs and HIV-1 Tat to the development of HAND pathology, and that ART drugs (those that elevate endolysosome pH) mimic and potentiate HIV-1 Tat-induced synaptic disruption and neuronal injury by elevating endolysosome pH and activating a novel endolysosome-dependent calcium regulatory mechanism. Guided by our preliminary findings, this novel hypothesis will be tested by pursuing three specific aims. (1) Determine, in vitro, the extent to which and mechanisms by which ART drugs and HIV-1 Tat activate a novel endolysosome-dependent calcium regulatory mechanism. (2) Determine, in vitro, the underlying mechanisms and interplay between ART drugs and HIV-1 Tat on synaptic integrity and neuronal injury. (3) Determine, in vivo, the underlying mechanisms and interplay between ART drugs and HIV-1 Tat on synaptic integrity and neuronal injury. We expect to demonstrate that endolysosomes, through novel endolysosome-dependent calcium regulatory mechanisms that is upstream of, for example, ER and mitochondrial effects of HIV-1 Tat and ART drugs, play a critical role in synaptic disruptions and neuronal injury. Such expected findings will provide novel insights into the pathogenesis of HAND, and may lead to the discovery of new effective therapeutic strategies against HAND.