Ion channel activation by SPC3, a peptide derived from the HIV-1 gp120 V-3loop

SPC3 is a multibranched peptide containing eight identical GPGRAF motifs wh ich are derived from the human immunodeficiency virus (HIV)-1 gp120 V3 loop consensus sequence. This molecule was reported to prevent the infection of CD4(+) cells by various HIV-1 and HIV-2 strains. However, the molecular mo de of action of SPC3 remains unclear. Here, we investigated the possibility that SPC3 could interact with alpha/beta -chemokine receptors following ob servations that, first, the V3 loop is likely to be involved in alpha/beta -chemokine receptor-dependent HIV entry and, second, natural ligands of the se receptors are potent inhibitors of cell infection. To address this point , we examined the effects of SPC3 on Xenopus oocytes either uninjected or e xpressing exogenous human CXCR4 alpha -chemokine receptors. Extracellular a pplications of micromolar concentrations of SPC3 onto Xenopus oocytes trigg er potent inward chloride currents which can be inhibited by increasing ext racellular Ca2+ concentration. This effect can be blocked by chloride chann el antagonists and is highly specific to SPC3 as it is not triggered by str uctural analogs of SPC3. The SPC3-induced chloride conductance in oocytes i s alpha/beta -chemokine receptor dependent because: (i) SPC3 alters the sen sitivity of this channel to external applications of human recombinant MIP- ls a natural ligand of human CCR5 receptor, and (ii) the amplitude of the i nward current could be increased by the expression of exogenous human CXCR4 chemokine receptor. The effect of SPC3 appears to rely on the activation o f a phospholipase Az signaling pathway, but is not affected by changes in c ytosolic Ca2+ concentration, or by alterations in Gi/Go protein, adenylate cyclase, phospholipase C or protein kinase C activity. Altogether, the data indicate that SPC3 is capable of activating a surface alpha/beta -chemokin e-like receptor-mediated signaling pathway in competent cells, thereby trig gering, either directly or indirectly, a Ca2+-inactivated chloride conducta nce.