Macrophages play an essential role in defending against invading pathogens by migrating to the sites of infection, removing apoptotic cells, and secreting inflammatory cytokines. Macrophage migration is a critical step in the regulation of immune response and is also associated with many chronic inflammatory diseases such as rheumatoid arthritis, multiple sclerosis, atherosclerosis and cancer. Thus, understanding the mechanisms controlling macrophage migration within different environments is of paramount importance. Although it is clear that adhesion signaling via integrin receptors and the surrounding ECM play a significant role in regulating migration of macrophages to site of inflammation, the underlying cellular and molecular mechanisms responsible for these process is still not fully characterized. Defining the molecular circuits through which integrins regulate macrophage motility is therefore important for gaining a better understanding of macrophage function. Glia maturation factor gamma (GMFG), a novel ADF/cofilin superfamily protein that is predominantly expressed in inflammatory cells, has been implicated in regulating actin reorganization. We have previously demonstrated that GMFG plays a role in regulating neutrophil chemotaxis and migration. We now examine whether GMFG has similar effect on macrophage and the cellular mechanism for this effect by using small-interfering RNA technic to knockdown GMFG in human macrophage like THP-1cells. Knockdown of endogenous GMFG results in significantly reduced chemotactic migration toward fMLP and enhanced adhesion on fibronectin compared with control siRNA transfected cells. Flow cytometry analysis shows that GMFG-knockdown cells expresses significant amount of α5β1 integrin on the surface compared with control cells. Confocal microscopy analysis exhibited that GMFG was distributed throughout the cytosol and colocalized with the multiple endocytic compartment such as early endosome marker EEA1, Rab5, Rab4 and Rab11. Importantly, we found that most of the α5β1 integrins accumulated in clusters at the rear of the GMFG-knockdown cells, along with colocalization with endosome marker EEA1, rather than localization toward the leading edge in response to fMLP stimulation in control cells. These results suggest that impaired efficient α5β1 integrin recycling cause to increased adhesion, which is responsible for reduced macrophage migration on fibronectin in GMFG depletion macrophages. Accordingly, GMFG is required for proper trafficking of endosomal α5β1 integrin to the plasma membrane and is fundamental for integrin-mediated macrophage motility.
