Shifting from Spreading to Retraction

Abstract

Integrin signaling has been implicated in both cell spreading (mediated through the small GTP-binding proteins Rac and cdc42) and cell retraction (mediated through RhoA). However, the mechanism whereby integrin temporally regulates these opposing processes--which must be coordinated in cell migration and wound repair--has been unclear. Noting that calpain cleavage of the cytoplasmic domain of integrin β 3 is inhibited by tyrosine phosphorylation at Y 759 , Flevaris et al . expressed a phosphorylation-mimicking integrin β 3 mutant in Chinese hamster ovary (CHO) cells, along with the wild-type integrin α IIb subunit, to investigate the role of calpain cleavage in integrin signaling. After determining that this R760E mutant was in fact resistant to calpain cleavage when expressed in CHO cells adherent to fibrinogen, the authors showed that cell-mediated clot retraction was inhibited with the R760E mutant cells, whereas cell spreading on fibrinogen was enhanced. In contrast, a deletion mutant that mimicked calpain cleavage exhibited defective cell spreading. RhoA was activated in cells with cleaved integrin β 3 , and pharmacological analysis indicated that defective spreading in the deletion mutant depended on activation of RhoA and Rho-dependent kinase (ROCK)-mediated retraction. A combination of pharmacological analysis and investigation of a dominant-negative c-Src mutant revealed that integrin β 3 -dependent c-Src signaling inhibited RhoA activation, thereby promoting cell spreading; integrin β 3 cleavage disrupted its binding to c-Src, thereby relieving its inhibition of RhoA and stimulating retraction. Thus, the authors propose that the phosphorylation-regulated cleavage of the integrin β 3 cytoplasmic domain by calpain switches the integrin β 3 signal to promote retraction instead of spreading. P. Flevaris, A. Stojanovic, H. Gong, A. Chishti, E. Welch, X. Du, A molecular switch that controls cell spreading and retraction. J. Cell Biol. 179 , 553-565 (2007). [Abstract] [Full Text]