Maintaining a healthy intestinal barrier, the primary physical barrier between intestinal microbiota and the underlying lamina propria, is critical for optimal health. Epithelial integrity is essential for prevention of the entrance of luminal contents, such as bacteria and their products, through the large intestinal barrier. In this study, we investigated the protective functions of biosynthetic, specific sized, hyaluronan around 35 kDa (HA35) on intestinal epithelium in healthy mice, as well as mice infected Citrobacter rodentium, an established model that mimics infection with a serious human pathogen, enteropathogenic E. coli (EPEC). Our results reveal that treatment with HA35 protects mice from Citrobacter infection and enhances the epithelial barrier function. In particular, we have found that HA35 induces the expression of tight junction protein zonula occludens (ZO)-1 in both healthy and Citrobacter infected mice, as demonstrated by immunoflurorescence and Western blot analyses. Furthermore, we determined that HA35 treatment enhances ZO-1 expression and reduces intestinal permeability at the early stages of dextran sulfate sodium (DSS)-induced colitis in mice. Together, our data demonstrate that the expression and functionality of tight junctions, is increased by HA35 treatment, suggesting a novel mechanism for the protection from Citrobacter infection.

Hyaluronan (HA) overproduction is a hallmark of multiple inflammatory diseases, including inflammatory bowel disease (IBD). Hyaluronan can act as a leukocyte recruitment molecule and in the most common mouse model of intestinal inflammation, the chemically induced dextran sodium sulfate (DSS) experimental colitis model, we previously determined that changes in colon distribution of HA occur before inflammation. Therefore, we hypothesized that, during a pathologic challenge, HA promotes inflammation. In this study, we tested the progression of inflammation in mice null for the hyaluronan synthase genes (HAS1, HAS3, or both HAS1 and HAS3) in the DSS-colitis model. Our data demonstrate that both the HAS1/HAS3 double and the HAS3 null mice are protected from colitis, compared to wild-type and HAS1 null mice, as determined by measurement of weight loss, disease activity, serum IL-6 levels, histologic scoring, and immunohistochemistry. Most notable is the dramatic increase in submucosal microvasculature, hyaluronan deposition, and leukocyte infiltration in the inflamed colon tissue of wild-type and HAS1 null mice. Our data suggest, HAS3 plays a crucial role in driving gut inflammation. Developing a temporary targeted therapeutic intervention of HAS3 expression or function in the microcirculation may emerge as a desirable strategy toward tempering colitis in patients undergoing flares of IBD.

Intestinal epithelium plays a critical role in host defense against orally acquired pathogens. Dysregulation of this protective barrier is a primary driver of inflammatory bowel diseases (Crohn's and ulcerative colitis) and also infant gastrointestinal infections. Previously, our lab reported that hyaluronan (HA) isolated from human milk induces the expression of the antimicrobial peptide β-defensin in vivo and protects against Salmonella Typhimurium infection of epithelial cells in vitro. In addition, we demonstrated that commercially available 35 kDa size HA induces the expression of β-defensin, upregulates the expression of tight junction protein zonula occludens-1 (ZO-1), and attenuates murine Citrobacter rodentium infection in vivo. In this current study, we report that HA35 remains largely intact and biologically active during transit through the digestive tract where it directly induces β-defensin expression upon epithelial cell contact. We also demonstrate HA35 abrogation of murine Salmonella Typhimurium infection as well as downregulation of leaky tight junction protein claudin-2 expression. Taken together, we propose a dual role for HA in host innate immune defense at the epithelial cell surface, acting to induce antimicrobial peptide production and also block pathogen-induced leaky gut. HA35 is therefore a promising therapeutic in the defense against bacterially induced colitis in compromised adults and vulnerable newborns.

Hyaluronan is the predominant glycosaminoglycan component of the extracellular matrix with an emerging role in hematopoiesis. Modulation of hyaluronan polymer size is responsible for its control over cellular functions, and the balance of hyaluronan synthesis and degradation determines its molecular size. Although two active somatic hyaluronidases are expressed in mammals, only deficiency in hyaluronidase-2 (Hyal-2) results in thrombocytopenia of unknown mechanism. Our results reveal that Hyal-2 knockout mice accumulate hyaluronan within their bone marrow and within megakaryocytes, the cells responsible for platelet generation. Proplatelet formation by Hyal-2 knockout megakaryocytes was disrupted because of abnormal formation of the demarcation membrane system, which was dilated and poorly developed. Importantly, peptide-mediated delivery of exogenous hyaluronidase rescued deficient proplatelet formation in murine and human megakaryocytes lacking Hyal-2. Together, our data uncover a previously unsuspected mechanism of how hyaluronan and Hyal-2 control platelet generation.

Platelets are specialized cells essential for hemostasis that also function as crucial effectors capable of mediating inflammatory and immune responses. These sentinels continually survey their environment and discriminate between homeostatic and danger signals such as modified components of the extracellular matrix. The glycosaminoglycan hyaluronan (HA) is a major extracellular matrix component that coats the vascular lumen and, under normal conditions, restricts access of inflammatory cells. In response to tissue damage, the endothelial HA matrix enhances leukocyte recruitment and regulates the early stages of the inflammatory response. We have shown that platelets can degrade HA from the surface of activated endothelial cells via the enzyme hyaluronidase-2 (HYAL2) and that HYAL2 is deficient in platelets isolated from patients with inflammatory bowel disease (IBD). Platelets are known to be involved in the pathogenesis of several chronic disease states, including IBD, but they have been largely overlooked in the context of intestinal inflammation. We therefore wanted to define the mechanism by which platelet HYAL2 regulates the inflammatory response during colitis. In this study, we provide evidence that HA catabolism is disrupted in human intestinal microvascular endothelial cells isolated from patients with IBD. Furthermore, mice deficient in HYAL2 are more susceptible to an acute model of colitis, and this increased susceptibility is abrogated by transfusion of HYAL2-competent platelets. Finally, we show that platelets, via HYAL2-dependent degradation of endothelial HA, regulate the early stages of inflammation in colitis by limiting leukocyte extravasation.