ALBERT

Description: Abstract 1282 Ezrin is a member of the ERM (ezrin, moesin and radixin) protein family that links plasma membrane proteins to the actin cytoskeleton. Ezrin in other in vitro cell systems has been hypothesized to participate in cell-cell contact and could have a role in stem/ progenitor cell mobilization and adhesion. To test this hypothesis, we crossed ezrinflox/flox mice with Mx1 cre transgenic mice to generate an inducible ezrin knock out mouse model. Inducible disruption of the ezrin gene in hematopoietic cells was achieved by the administration of polyIC. Ezrin knock out HSPCs exhibited a 30–40% decrease in baseline and chemokine stromal cell-derived factor-1 (SDF-1) stimulated motility in transwell migration assays in vitro. In addition, loss of ezrin led to a 60% decrease in the homing capacity of HSPCs in lethally irradiated recipient mice following transplantation. There was a 40–55% decrease in colony forming cells in peripheral blood and spleen of the mice following ezrin knock out, suggesting that ezrin knock out HSPCs may be deficient in egressing out of the bone marrow. To further understand the cause of the impaired motility of ezrin knock out HSPCs, we examined F-actin level of HSPCs at baseline and in response to SDF-1. Ezrin knock out HSPCs displayed 1.5 to 2 fold higher level of F-actin at baseline when compared with wild type cells. Following stimulation with SDF-1, wild type HSPCs that migrated to the bottom compartment of the transwell demonstrated a 2 time greater decrease in F-actin level when compared with ezrin knock out cells, suggesting that ezrin may participate in the regulation of F-actin depolymerization in HSPCs. In summary, we demonstrate that ezrin modulates HSPC migration and homing likely through its regulation on F-actin organization. Disclosures: No relevant conflicts of interest to declare.

Description: Abstract 18 Eosinophils are increasingly recognized as important myeloid effector cells in the inflammatory environment of many human diseases. Although eosinophils critically contribute to chronic asthmatic inflammation, few therapies directly target these cells. Eosinophils rapidly migrate to eotaxin elicited by allergic sensitization and challenge, a chemokine that ligates the CCR3 receptor. Eotaxin:CCR3 signaling critically regulates allergen-induced eosinophil infiltration in murine models by activating the Rho-family proteins. In several cell systems, the Rho proteins Rac and CDC42 activate p21-activated kinase 1 (PAK1), which we have previously shown to regulate F-actin dynamics and histamine release in the degranulating mast cell. In these studies, we examined eotaxin-induced eosinophil migration using genetic and hematopoietic ablation of Pak1 (Pak1−/−) in a murine asthma model. Using an in vitro transwell migration assay system, we evaluated the migration of bone marrow derived eosinophils of both genotypes to eotaxin (N=10). Pak1−/− eosinophils exhibited profoundly diminished eotaxin-induced chemotaxis in vitro relative to wild-type (Pak1+/+) eosinophils (p 〈 0.0001) with a 30% overall decrease in migrating Pak1−/− compared to Pak1+/+ eosinophils. Furthermore, we compared the eotaxin-induced localization and arrangement of F-actin in eosinophils of both genotypes by fluorescence cytometry and deconvolution confocal microscopy of fluorescently-tagged phalloidin in seeking to explain this migration defect. Preliminary findings suggest decreased F-actin polymerization in eotaxin-treated Pak1−/− eosinophils. In an independent line of experiments designed to compare eotaxin-mediated eosinophil recruitment in vivo we injected mice of both genotypes with an intraperitoneal dose of eotaxin or saline. Pak1+/+ mice showed an 8 fold eotaxin-mediated increase in eosinophil recruitment over control whereas Pak1−/− mice demonstrated only a modest 3–4 fold increase (p〈 0.05). Finally we pursued PAK1's function in an experimental disease model in which the eosinophil's key role in pathogenesis is well documented. In 3 cohorts of 7 age, gender and strain matched Pak1+/+ and Pak1−/− ova albumin (OVA)-sensitized and challenged mice, we scored lung eosinophilic inflammation by histology and compared eosinophil counts and eotaxin concentrations in broncho-alveolar lavage fluid (BALF) by fluorescence cytometry and ELISA respectively. We also assessed OVA-specific T-cell subset cytokine secretion in our asthma mice by ELISA. Lung-parenchymal eosinophilic inflammation was diminished in Pak1−/− ova-sensitized mice versus Pak1+/+'s (p