Scientific Report 2005

Immunology

Syndecans and HIV Type 1 Pathogenesis

M. Bobardt, U. Chatterji, A. de Parseval,* J. Elder,* P. Gallay * Department of Molecular Biology, Scripps Research

Syndecans are transmembrane receptors highly expressed on adherent cells (e.g., macrophages, epithelial or endothelial cells) but poorly expressed on suspension cells (e.g., T lymphocytes). The syndecan family has 4 members, syndecan-1 through syndecan-4. The ectodomain in each syndecan has linear heparan sulfate chains, which are composed of a repetition of a sulfated disaccharide motif. The sulfation pattern of the heparan sulfates dictates the ligand specificity of the syndecan.

Syndecans function as receptors for HIV type 1 (HIV-1). Pretreatment of target cells with heparinase that removes heparan sulfates from syndecans dramatically reduces HIV-1 infectivity. We showed that syndecans also serve as in trans (i.e., on the surface of cells opposite each other) receptors for HIV-1. Specifically, HIV-1 binds syndecans richly expressed on the endothelium. HIV-1 bound to syndecans remains infectious for a week, whereas cell-free virus loses its infectivity after a single day. Most importantly, HIV-1 attached to the endothelium via syndecans is an in trans source of infection for circulating T cells. These findings suggest that syndecan-rich endothelium can provide a microenvironment that amplifies HIV-1 replication in T cells. Last, we showed that syndecans on brain microvascular endothelial cells play a significant role in HIV-1 transmigration through the blood-brain barrier. Altogether these observations suggest that syndecans, by acting as in cis (i.e., on the surface of the same cell) and in trans receptors, may profoundly affect HIV-1 pathogenesis.

We recently found that a single conserved arginine at position 298 in the HIV envelope glycoprotein gp120 governs HIV-1 binding to syndecans. An amine group on the side chain of this residue is necessary for syndecan utilization by HIV-1. We showed that HIV-1 binds syndecans via a 6-O sulfation, indicating that this binding is not the result of random interactions between basic residues and negative charges but the result of specific contacts between gp120 and a well-defined sulfation in syndecans. Surprisingly, arginine at position 298 that mediates HIV-1 binding to syndecans also mediates HIV-1 binding to the major HIV-1 entry receptor CCR5.We postulated that HIV-1 recognizes similar motifs on syndecans and CCR5. In support of this hypothesis, we found that the 6-O sulfation recognized by HIV-1 on syndecans mimics the sulfated tyrosines recognized by HIV-1 in the N terminus of CCR5. Our finding that CCR5 and syndecans are exploited by HIV-1 via a single determinant echoes the mechanisms by which chemokines use these 2 disparate receptors and suggests that the gp120-chemokine mimicry may represent a common strategy in microbial pathogenesis.

Publications

de Parseval, A., Bobardt, M.D., Chatterji, A., Chatterji, U., Elder, J.H., David, G., Zolla-Pazner, S., Farzan, M., Lee, T.-H., Gallay, P.A. A highly conserved arginine in gp120 governs HIV-1 binding to both syndecans and CCR5 via sulfated motifs. J. Biol. Chem., in press.

Gallay, P. Syndecans and HIV-1 pathogenesis. Microbes Infect. 6:617, 2004.

Innate Intracellular Immunity and Infection With HIV Type 1

U. Chatterji, M. Bobardt, P. Gallay

Nonhuman primate cells contain intracellular innate factors that inhibit infection by HIV type 1 (HIV-1) by targeting the incoming viral capsid core, which makes up the shell that surrounds the viral genome. These restriction factors block HIV-1 replication at steps before integration but subsequent to entry. The first intracellular primate restriction factor identified, TRIM5α, is a member of the tripartite motif (TRIM) family of proteins. The introduction of TRIM5α derived from species that highly restrict HIV-1 (e.g., Rhesus macaque, African green or owl monkeys) into cells that are normally permissive (e.g., human cells) makes the cells nonpermissive to HIV-1. However, although TRIM5α was identified more than 2 years ago and several laboratories are actively studying it, we still do not have any idea of how this restriction factor blocks HIV-1 infection.

An understanding of the nature of restrictions to HIV-1 infection after cellular entry of the virus in primates is critical for several reasons. First, information on the viral and cellular factors that modulate these processes will shed light on the poorly understood series of events that govern the fate of capsids after entry. Second, species-specific barriers to HIV-1 infection present obstacles to the development of animal models for the study of HIV-1 pathogenesis, treatment, and prophylaxis. Finally, an understanding of this critical part of the HIV-1 life cycle may suggest approaches to intervene in HIV-1 transmission to or spread within the host.

Publications

Chatterji, U., Bobardt, M.D., Stanfield, R., Ptak, R.C., Pallansch, L.A., Ward, P.A., Jones, M.J., Stoddart, C.A., Scalfaro, P., Dumont, J.-M., Besseghir, K., Rosenwirth, B., Gallay, P.A. A naturally occurring capsid motif renders HIV-1 cyclophilin A independent in human cells and TRIM-cyclophilin resistant in owl monkey cells. J. Biol. Chem., in press.

Galigniana, M.D., Morishima, Y., Gallay, P.A., Pratt, W.B. Cyclophilin-A is bound through its peptidylprolyl isomerase domain to the cytoplasmic dynein motor protein complex. J. Biol. Chem. 279:55754, 2004.