Neurotoxic HIV-1 viral proteins contribute to the development of HIV-associated neurocognitive disorders (HAND), the prevalence of which remains high (30 to 50%) with no effective treatment is available. Estrogen is a known neuroprotective agent; however, the diverse mechanisms of estrogen action on the different types of estrogen receptors is not completely understood. In this study, we determined the extent to which and mechanisms by which 17α-estradiol (17αE2), a natural less-feminizing estrogen, offers neuroprotection against HIV-1 gp120-induced neuronal injury. Endolysosomes are important for neuronal function and endolysosomal dysfunction contributes to HAND and other neurodegenerative disorders. In hippocampal neurons, estrogen receptor alpha (ERα) is localized to endolysosomes and 17αE2 acidifies endolysosomes. ERα knockdown or over-expressing an ERα mutant that is deficient in endolysosome localization prevents 17αE2-induced endolysosome acidification. Furthermore, 17αE2-induced increases in dendr...