Optimal inhibition of X4 HIV isolates by the CXC chemokine stromal cell-derived factor 1 alpha requires interaction with cell surface heparan sulfateproteoglycans

The chemokine stromal cell-derived factor 1 (SDF-1) is the natural ligand f or CXC chemokine receptor 4 (CXCR4), SDF-1 inhibits infection of CD4+ cells by X4 (CXCR4-dependent) human immunodeficiency virus (HIV) strains. We pre viously showed that SDF-1 alpha interacts specifically with heparin or hepa ran sulfates (HSs), Herein, we delimited the boundaries of the MS-binding d omain located in the first beta -strand of SDF-1 alpha as the critical resi dues. We also provide evidence that binding to cell surface heparan sulfate proteoglycans (MSPGs) determines the capacity of SDF-1 alpha to prevent th e fusogenic activity of HIV-1 X4 isolates in leukocytes. Indeed, SDF-1 alph a mutants lacking the capacity to interact with HSPGs showed a substantiall y reduced capacity to prevent cell-to-cell fusion mediated by X4 HIV envelo pe glycoproteins. Moreover, the enzymatic removal of cell surface MS dimini shes the HIV-inhibitory capacity of the chemokine to the levels shown by th e HS-binding-disabled mutant counterparts, The mechanisms underlying the op timal HIV-inhibitory activity of SDF-1 alpha when attached to MSPGs were in vestigated, Combining fluorescence resonance energy transfer and laser conf ocal microscopy, we demonstrate the concomitant binding of SDF-1 alpha to C XCR4 and MSPGs at the cell membrane. Using FRET between a Texas Red-labeled SDF-1 alpha and an enhanced green fluorescent protein-tagged CXCR4, we sho w that binding of SDF-1 alpha to cell surface HSPGs modifies neither the ki netics of occupancy nor activation in real time of CXCR4 by the chemokine. Moreover, attachment to HSPGs does not modify the potency of the chemokine to promote internalization of CXCR4, Attachment to cellular MSPGs may co-op erate in the optimal anti-HIV activity of SDF-1 alpha by increasing the loc al concentration of the chemokine in the surrounding environment of CXCR4, thus facilitating sustained occupancy and down-regulation of the HIV corece ptor.