Functional expression of the seven-transmembrane HIV-1 co-receptor APJ in neural cells

APJ is a recently described seven-transmembrane (7TM) receptor that is abundantly expressed in the central nervous system (CNS). This suggests an important role for APJ in neural development and/or function, but neither its cellular distribution nor its function have been defined. APJ can also serve as a co-receptor with CD4 for fusion and infection by some strains of human immunodeficiency virus (HIV-1) in vitro, suggesting a role in HIV neuropathogenesis if it were expressed on CD4-positive CNS cells. To address this, we examined APJ expression in cultured neurons, astrocytes, oligodendrocytes, microglia and monocyte-derived macrophages utilizing both immunocytochemical staining with a polyclonal anti-APJ antibody and RT - PCR. We also analyzed the ability of a recently identified APJ peptide ligand, apelin, to induce calcium elevations in cultured neural cells. APJ was expressed at a high level in neurons and oligodendrocytes, and at lower levels in astrocytes. In contrast, APJ was not expressed in either primary microglia or monocyte-derived macrophages. Several forms of the APJ peptide ligand induced calcium elevations in neurons. Thus, APJ is selectively expressed in certain CNS cell types and mediates intracellular signals in neurons, suggesting that APJ may normally play a role in signaling in the CNS. However, the absence of APJ expression in microglia and macrophages, the prinicpal CD4-positive cell types in the brain, indicates that APJ is unlikely to mediate HIV-1 infection in the CNS.