We have studied the involvement of endothelial heparan sulfate in inflammation by inactivating heparan sulfate biosynthetic enzymes in endothelial cells and leukocytes. Mutant mice developed normally but showed altered neutrophil infiltration in various inflammation models. These effects were due to changes in heparan sulfate specifically in endothelial cells. Neutrophil infiltration was either reduced or enhanced due to altered rolling velocity correlated with differential binding of L-selectin to endothelial cells. Chemokine binding and transcytosis across endothelial cells and presentation on the cell surface also differed, resulting in altered neutrophil firm adhesion and migration. Thus, endothelial heparan sulfate has three functions in inflammation: by acting as a ligand for L-selectin during neutrophil rolling; in chemokine transcytosis; and by binding and presenting chemokines at the lumenal surface of the endothelium. We are currently examining how the pattern of sulfation along the chains affects neutrophil infiltration and whether specific proetoglycans mediate binding and transport.
Another area of interest concerns the function of heparan sulfate proteoglycans in receptor activation. In one set of studies we searched for ligands that bind heparan sulfate and uncovered a number of intracellular proteins that serve as damage associated molecule patterns. One of these proteins, high mobility group protein-1 (HMGB1), signals via the receptor for advanced glycation end products (RAGE). RAGE signaling shows an absolute dependence on heparan sulfate, and the requirement for heparan sulfate is manifested at the level of the receptor, not the ligand. Structure-function studies are underway to map the heparin binding site in RAGE and to understand how docking to heparan sulfate activates the receptor.
Finally, we have an ongoing interest in the role of proteoglycans in microbial infection. Collaborative studies are underway with Victor Nizetís lab that focus on bacterial meningitis, a devastating disease of infants with a high mortality rate. At least two-thirds of the deaths are caused by Group B Streptococcus (GBS) and Escherichia coli but many other bacteria are known to cause meningitis as well. We recently showed that heparan sulfate proteoglycans play a role in passage of GBS across the blood-brain barrier. Current studies focus on the importance of particular proteoglycans or modifications of the heparan sulfate for bacterial adherence, invasion, and translocation across the blood-brain barrier. A better understanding of the role of proteoglycans involved in bacterial meningitis might define molecular targets for therapeutic intervention.