Background:  We reported that the heparan sulfate proteoglycan syndecans greatly promote HIV infection. Heparinase treatment, which removes heparan sulfates from syndecans, dramatically reduces HIV-1 infection (70 to 90% decrease in infectivity) in cells that express low CD4 levels, such as macrophages or CD4/CCR5⁺ HeLa cells. More recently, we found that high CD4 levels obviate the need for syndecans, suggesting that artificially high expression of CD4 masks the requirement for syndecans in HIV-1 infectivity. Surprisingly, spinoculation does not rescue HIV-1 infectivity in heparinase-treated cells, suggesting that syndecans do not simply promote virus adsorption, but that they actively participate in the entry/fusion process.

Methods:  To understand how syndecans boost HIV-1 infectivity, we searched for the viral ligand responsible for HIV-1-syndecan interactions.

Results:  We found that a single highly conserved arginine in the V3 loop of gp120 is essential for HIV-1-syndecan interactions. Surprisingly, this arginine is also critical for gp120 binding to CCR5. This led us to postulate that HIV-1 recognizes similar motifs in syndecans and CCR5. Supporting this hypothesis, we found that HIV-1 binds to syndecans via sulfated disaccharide motifs and to CCR5 via sulfated tyrosine motifs. Given that it is thought that gp120 initially binds to the tyrosine sulfated N-terminus and subsequently to an extracellular loop of CCR5, one can envision that syndecans, by mimicking the sulfated N-terminus of CCR5, mediate the initial docking of HIV-1 and promote subsequent interactions with CCR5. We are testing this hypothesis by asking if syndecans rescue HIV-1 infectivity in cells expressing CCR5, which lacks sulfated tyrosines. We recently found that syndecans and CCR5 co-localize on the cell surface. Given that syndecans and CCR5 can be co-immunoprecipitated, this suggests that they preexist as complexes on the cell surface. We are investigating whether there is a direct interaction between these 2 receptors and whether heparinase treatment dramatically modifies the subcellular localization of CCR5, explaining our observation that HIV-1 poorly infects heparinase-treated cells.

Conclusions:  HIV-1 uses a single highly conserved arginine in gp120 to bind both syndecans and CCR5 via common sulfated motifs. Thus, HIV-1 exploits host sulfations to ensure successful infection of cells that express low CD4 levels.

Keywords: HIV ; CCR5; Sulfations