Glia Maturation Factor-γ Regulates Monocyte Migration through Modulation of β1-Integrin

Abstract

Monocyte migration requires the dynamic redistribution of integrins through a regulated endo-exocytosis cycle, but the complex molecular mechanisms underlying this process have not been fully elucidated. Glia maturation factor-␥ (GMFG), a novel regulator of the Arp2/3 complex, has been shown to regulate directional migration of neutrophils and T-lymphocytes. In this study, we explored the important role of GMFG in monocyte chemotaxis, adhesion, and ␤1-integrin turnover. We found that knockdown of GMFG in monocytes resulted in impaired chemotactic migration toward formyl-Met-Leu-Phe (fMLP) and stromal cell-derived factor 1␣ (SDF-1␣) as well as decreased ␣5␤1-integrin-mediated chemoattractant-stimulated adhesion. These GMFG knockdown impaired effects could be reversed by cotransfection of GFP-tagged full-length GMFG. GMFG knockdown cells reduced the cell surface and total protein levels of ␣5␤1-integrin and increased its degradation. Importantly, we demonstrate that GMFG mediates the ubiquitination of ␤1-integrin through knockdown or overexpression of GMFG. Moreover, GMFG knockdown retarded the efficient recycling of ␤1-integrin back to the plasma membrane following normal endocytosis of ␣5␤1-integrin, suggesting that the involvement of GMFG in maintaining ␣5␤1-integrin stability may occur in part by preventing ubiquitin-mediated degradation and promoting ␤1-integrin recycling. Furthermore, we observed that GMFG interacted with syntaxin 4 (STX4) and syntaxinbinding protein 4 (STXBP4); however, only knockdown of STXBP4, but not STX4, reduced monocyte migration and decreased ␤1-integrin cell surface expression. Knockdown of STXBP4 also substantially inhibited ␤1-integrin recycling in human monocytes. These results indicate that the effects of GMFG on monocyte migration and adhesion probably occur through preventing ubiquitin-mediated proteasome degradation of ␣5␤1-integrin and facilitating effective ␤1-integrin recycling back to the plasma membrane.The migration of monocytes from the circulation toward the surrounding extravascular space is a crucial event in physiological and pathological immune and inflammatory responses. The extravasation of monocytes is a highly orchestrated multistep adhesion cascade that is mediated by dynamic regulation of adhesion molecules expressed on both monocytes and endothelial cells, including integrins (1, 2). Integrins are a large family of heterodimeric transmembrane proteins consisting of an ␣-and ␤-subunit that bind extracellular matrix components to provide sites of adhesion and signals for the dynamic rearrangement of cytoskeletal elements in various stages of cell migration (3, 4). Monocytes, like other leukocytes, express distinct integrins, but those most relevant to monocyte migration belong to the ␤1-and ␤2-integrin subfamilies. Although the important role of ␤1/␤2-integrins in regulating monocyte transmigration across the endothelium to sites of inflammation has been relatively well studied (5-7), the molecular mechanisms underpinning the dynamic regulation of endo-exocyt...

Contractile Skeletal Muscle Cells Cultured with a Conducting Soft Wire for Effective, Selective Stimulation

“…The average migration speed of the monocytes toward the contracting muscle bundle was calculated to be 5.1 ± 1.5 µm min −1 (n = 10) from the total migration distance divided by migration time. This migration speed was similar to that of THP-1 monocytes migrating toward a linear gradient of chemoattractant evaluated using a commercially-available chemotaxis assay system 28 . To our best knowledge, this is the first study in quantifying the migration speed of THP-1 monocytes toward co-cultured contracting skeletal muscle cells in real-time, which impossible in the Boyden chamber assay.…”

GMF as an Actin Network Remodeling Factor

“…In addition to its roles in cell migration, GMF regulates actin-dependent endocytosis in yeast and mammals [13, 14, 51], and promotes the turnover of focal adhesions in immune cells [43, 52]. Vinculin, a central component of focal adhesions that anchors cells to their substrate, binds to the Arp2/3 complex at focal adhesions [53].…”

Function and regulation of the Arp2/3 complex during cell migration in diverse environments

“…In addition to NPFs and upstream signaling factors, a number of proteins function in the regulation of Arp2/3 activity to control cell migration [10,67–77]. Recent studies have centered on a newly-discovered negative regulator of Arp2/3, Arpin [78].…”