College of Arts & Sciences

Yuan Liu

Research Description:

A variety of human diseases, in their active stages, are characterized by migration of large numbers of neutrophils (polymorphonuclear leukocytes, PMN) through vascular endothelium and across mucosal epithelial linings. For example, in inflammatory pulmonary diseases, such as bronchitis, cystic fibrosis, bronchiectasis and exacerbations of bronchial asthma, acute inflammation of the airway is characterized by infiltration of bronchial epithelium with neutrophils. In the urinary system, migration of neutrophils across tubular and transitional epithelium is associated with cystitis and pyelonephritis. In the gastrointestinal tract, active inflammatory disease characterized by migration of neutrophils across the epithelial lining is a hallmark of chronic and self-limited diseases such as ulcerative colitis, Crohn's disease and bacterial enterocolitis. Actually, inflammatory intestinal disease is a major health problem. In the United States alone, there are over 3 million reports of Crohn's and ulcerative colitis and an estimated annual 20 million cases of food derived cases of intestinal bacterial colitis. Not only does this result in significant suffering, but chronic intestinal inflammation is also associated with an increased risk of colon cancer.

My research is focused on defining the cellular and molecular mechanisms that regulate neutrophil transmigration across

epithelial barrier during acute and chronic inflammatory diseases. The process of neutrophil migration from the blood stream across mucosal barrier is composed of a complex set of events that are incompletely defined. To initiate extravasation, circulating neutrophils are activated by inflammatory mediators, leading to their sequestration and adhesion to the endothelium. Subsequent neutrophil migration across endothelium requires sequential interactions of neutrophil integrins and endothelial cell surface adhesion molecules (ICAM, PECAM, etc). Further neutrophil migration in the extravascular space depends on the presence of chemotactic stimuli and interactions between extracellular matrix proteins and adhesion molecules. Neutrophil migration across the epithelium is initiated after neutrophil adhere to the basolateral surface of the epithelial cells, which in turn signals the opening of tight junctions between epithelial cells. Subsequent neutrophil migration across the epithelial paracellular space and tight junctions is then directed towards tissue or bacterial-derived chemoattractants (e.g. fMLP). The entire transmigration process requires multiple steps of cell surface adhesive interactions and cascades of intracellular signal transduction regulations.

While many of the molecular steps involved in this complex transmigration process are still undefined, we and others have demonstrated that a transmembrane protein termed CD47 plays an important role in regulating neutrophil migration across endothelium, extracellular matrix and epithelium. In addition, CD47-dependent events in transepithelial migration occur after ß2 integrin-mediated neutrophil adhesion. CD47 is a cell surface glycoprotein widely expressed on all hematopoietic cells and most other cell types. In polarized intestinal epithelial cells, CD47 is richly expressed on the basolateral surface of the monolayers. During our studies of CD47 in neutrophil transmigration, we have further elucidated that CD47 facilitates neutrophil transmigration by positively regulating the migration rate (Liu, etc, 2000). The crucial role of CD47 suggested by these in vitro studies is consistent with studies with CD47-deficient mice (Lindberg, etc.1996). It has been shown that CD47 deficiency results in decreased host defense responses that evidently associate with an early defect in neutrophil accumulation at the infection site, further demonstrating the key role of CD47-mediated regulation in timely arrival of neutrophils at inflammatory sites in vivo . During studies of the signaling pathways that regulate neutrophil transmigration, we have further demonstrated that specific tyrosine phosphorylation events serve as the down stream signal(s) in CD47-mediated regulation (Liu, etc, 2000). In addition, we have observed differential roles of tyrosine kinases, MAP kinase and PI3-kinase in neutrophil transmigration and migration-related functions, suggesting these signaling molecules play important roles during neutrophil migration.

As a cell surface Ig superfamily adhesive protein, we demonstrated that neutrophil CD47 directly binds to SIRP a and that these interactions regulate neutrophil transmigration (Liu, etc. 2002). Signal regulatory proteins (SIRP a and SIRP b ) are a family of transmembrane glycoproteins richly expressed in neutrophils, macrophages, and certain other hematopoietic cell types. In addition to binding to CD47, protein structures of SIRPs also suggest functional roles as cell signaling molecules. In particular, SIRP a has an intracellular domain that contains immunoreceptor tyrosine-based inhibitory motifs (ITIM), while SIRP b comprises sequence elements that bind to immunoreceptor tyrosine-based activation motif (ITAM) containing protein DAP12. CD47 selectively binds to SIRP a and role of these interactions, as well as SIRPs in neutrophil functions are focuses of our research.

Significance :
Directed movement of neutrophil along a chemotactic gradient plays an extremely important role in host defense and inflammation. While all human cells can migrate, neutrophil chemotactic migration is at least 10 times faster than any other cell type. The process of PMN migration is complicated, involving continuous cell adhesion, lamellar protrusion, de-adhesion and contraction. This integration requires multiple cell surface protein-protein interactions and regulated cell signaling cascades. The detailed molecular mechanism(s) that are involved in neutrophil transmigration, however, are far from understood. My research aims to further understand these mechanisms and shed new insights into cellular events that regulate neutrophil migration, which may provide new ideas for anti-inflammatory therapy.