One challenge in Human Immunodeficiency Virus (HIV) research involves the discovery and characterization of compounds that prevent or limit the neurodegeneration that follows HIV infection of the brain. The chemokine CCL5 inhibits entry of M-tropic HIV strains into macrophages/microglia by affecting the binding of the envelop protein gp120 to the co-receptor CCR5. Interestingly, CCL5 also prevents neuronal cell death mediated by the T-tropic gp120 and the viral protein Tat, which have no affinity for CCR5. Therefore, the mechanism of action of this chemokine remains to be fully characterized. Recent studies and our preliminary data have shown that CCL5 activates a G protein coupled-receptor 75 (GPR75) which encodes for a 540 amino-acid orphan receptor of the Gq? family. This receptor is more abundant in the brain than in the immune organs. Moreover, CCL5 activates various pro-survival signaling molecules, including inositol triphosphate, phosphatidylinositol 3-kinase and its downstream targets protein kinase B and extracellular signal-regulated kinases, in SH-SY5Y cells. These cells do not express CCR5, CCR3 and CCR1, receptors known to bind to CCL5. Moreover, CCL4, CCL7 and CCL3, other chemokines that bind to CCR5, CCR3 and CCR1, failed to activate these signaling molecules in these cells. Therefore, GPR75 could be the missing link to explain the neuroprotective activity of CCL5. In this exploratory proposal we will test the innovative hypothesis that GPR75 activation by CCL5 promotes neuroprotection against viral proteins (e.g. gp120 and Tat). To test this hypothesis, we propose to identify and characterize CCL5 binding activity and affinity for GPR75 by Scatchard analysis, cross-linking, receptor internalization, mass spectrometry and surface plasmon resonance (Biacore) studies (AIM 1) in both human as well as rodent neurons. These experiments will be accompanied by studies examining the role of GPR75 in CCL5-mediated neuroprotection against T-tropic gp120 and Tat in neurons in vitro (AIM 2). We expect to provide new significant data on the ability of CCL5 to bind to GPR75 and its role in preventing the neurotoxicity of HIV proteins. These data will help in the design of small molecule GPR75 agonists as an adjunct therapy against synaptic simplification caused by HIV.