The major goal of this R21 proposal is to understand how Listeria monocytogenes (Lm) subverts
syndecan-1 (Sdc1) functions to promote its pathogenesis. Host-pathogen interactions largely dictate the onset,
progression, and outcome of infectious diseases. Many pathogens, including viruses, bacteria and parasites,
bind to the heparan sulfate (HS) moiety of HS proteoglycans (HSPGs) in vitro, and this activity is thought to be
critical in the attachment and invasion of host cells. However, there are surprisingly very few examples
supporting the importance of HSPG-pathogen interactions in microbial pathogenesis in vivo. Furthermore
HSPGs are expressed in different cells and tissues at different times and levels, and they are thought to
function specifically, but the identity of HSPGs that facilitate attachment and invasion is unknown for most
heparin-binding pathogens. In addition, although HSPGs are multi-functional molecules, most studies have so
far focused on their role as attachment/internalization receptors, and have largely overlooked the possibility
that HSPGs may have other functions in microbial pathogenesis. Our proposal is designed to address these
gaps in our understanding of HSPG biology in bacterial infections, using Lm as a model heparin-binding
bacterial pathogen. Lm is a food-borne Gram-positive bacterial pathogen that causes listeriosis, a rare but
significant infectious disease associated with a high rate of hospitalization and mortality. Preliminary studies
showed that Sdc1 null (Sdc1-/-) mice are significantly less susceptible to both intragastric and intravenous Lm
infection compared to wild type (Wt) mice. This phenotype is not seen in Sdc3-/- or Sdc4-/- mice, indicating
that ablation of Sdc1 causes a specific gain of function that enables mice to resist listeriosis. However, Sdc1
does not support Lm attachment or invasion of host cells, indicating that Sdc1 does not promote pathogenesis
as a cell surface Lm receptor. Instead, Sdc1 inhibits the clearance of Lm before the bacterium gains access to
its intracellular niche. Large intravascular aggregates of neutrophils and neutrophil extracellular traps (NETs)
embedded with antimicrobial compounds are formed in Sdc1-/- livers, which trap and kill Lm. Lm infection
induces Sdc1 shedding in hepatic tissues of Wt mice, which is directly associated with the decrease in size of
intravascular aggregated NETs. Furthermore, administration of purified Sdc1 ectodomains or DNase inhibits
the formation of intravascular aggregated neutrophils and NETs and significantly increases the liver bacterial
burden in Sdc1-/- mice. Based on these preliminary data, this proposal will explore key pathways of how Lm
induces Sdc1 shedding (Aim 1) and how Sdc1 ectodomains inhibit NETs in listeriosis (Aim 2).