Outside-In Signal Transmission by Conformational Changes in Integrin Mac-1

Abstract

Intracellular signals associated with or triggered by integrin ligation can control cell survival, differentiation, proliferation, and migration. Despite accumulating evidence that conformational changes regulate integrin affinity to its ligands, how integrin structure regulates signal transmission from the outside to the inside of the cell remains elusive. Using fluorescence resonance energy transfer, we addressed whether conformational changes in integrin Mac-1 are sufficient to transmit outside-in signals in human neutrophils. Mac-1 conformational activation induced by ligand occupancy or activating Ab binding, but not integrin clustering, triggered similar patterns of intracellular protein tyrosine phosphorylation, including Akt phosphorylation, and inhibited spontaneous neutrophil apoptosis, indicating that global conformational changes are critical for Mac-1-dependent outside-in signal transduction. In neutrophils and myeloid K562 cells, ligand ICAM-1 or activating Ab binding promoted switchblade-like extension of the Mac-1 extracellular domain and separation of the αM and β2 subunit cytoplasmic tails, two structural hallmarks of integrin activation. These data suggest the primacy of global conformational changes in the generation of Mac-1 outside-in signals.

Protein Kinase C-θ Is Required for Murine Neutrophil Recruitment and Adhesion Strengthening under Flow

“…Furthermore, neutrophil transmigration through the endothelial wall was significantly impaired in the cremaster model. Although initial arrest is LFA-1-dependent, the main b 2 integrin in the context of neutrophil adhesion strengthening seems to be Mac-1 (5,6). Thus, we hypothesized that the expression of Mac-1 might be reduced in Prkcq deficient neutrophils was reduced ∼50% compared with WT neutrophils in a static in vitro assay.…”

“…Whereas the level of LFA-1 expression on neutrophils is unaltered by inflammatory stimuli, the b 2 integrin macrophage-1 Ag (Mac-1) can be rapidly mobilized to the cell surface after exposure to inflammatory stimuli (4). Neutrophil arrest is LFA-1-dependent and Mac-1-independent, whereas Mac-1 is required for many b 2 integrindependent neutrophil functions such as phagocytosis and respiratory burst (5,6).…”

Metalloproteinase-mediated Shedding of Integrin β2 Promotes Macrophage Efflux from Inflammatory Sites

“…Mac-1 deficient macrophages (mice lacking integrin ␣M) did not exit the peritoneal cavity in the accelerated exiting model (17), although we observed more rapid exiting in the same model with mice lacking integrin ␤2. Outside-in signaling (29) and binding to low density lipoprotein receptor-related protein (LRP)-1, which is required for migration (30), are both impaired in the absence of integrin ␣M. Thus, although Mac-1 null mice still express ␣X␤2 that could mediate fibrin binding, their macrophages would be impaired in "outside-in" signaling from fibrin.…”

Structural insight on the recognition of surface-bound opsonins by the integrin I domain of complement receptor 3

“…This may lead to actin remodeling, phagocytosis, degranulation, and changes in leukocyte cytokine production (2,(5)(6)(7). CR3, and to a lesser degree CR4, are essential for the phagocytosis of complement-opsonized particles or complexes (6,8,9).…”