Signal transduction through cell surface receptors

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Although adhesion can be regulated by the expression of integrin and its igand, adhesion can be also mediated by integrin without any change of the expression level. This regulation of adhesion is called avidity modulation. Bone-marrow derived mast cells adhere to fibronectin via VLA-5 only stimulated with PMA,steel factor, or crosslinking with FceRI without any change of expression levels of VLA-5. Avidity modulation of integrin can be brought by either spatial redistribution of integrin molecules or a change of affinity for a ligand.First we analyzed morphological changes and subcellulare localization of integrin VLA-5 with or without these stimuli. Without stimuli, VLA-5 were distributed as a cluster on the cell body. Stimulation with SLF and PMA,but not FceRI induced ruffle-like membrane structures and diffused distribution of VLA-5, which was located on both the cell body and ruffle-like structures. The affinity of VLA-5 against a 80 kD fibronectin fragment is beyond detectable levels in unstimulated mast cells. Stimulation with crosslinking of FceRI,but not with SLF or PMA increased the affinity of VLA-5. Therefore, SLF and PMA modulate mainly a spatial organization of VLA-5 molecules, while FceRI modulates the affinity of VLA-5 against fibronectin. These results suggest that spatial organization of VLA-5 on mast cells were differentially regulated with distinct stimuli.