Cytoskeleton remodeling Fibronectin-binding integrins in cell motility
Fibronectin-binding integrins in cell motility Fibronectin -binding integrins such as alpha-5/beta-1 andalpha-V/beta-3 can promote different aspects of cell migration. Adhesion tofibronectin by alpha-V/beta-3 integrins supports persistent migration,whereas alpha-5/beta-1 integrins promote random migration . Integrin clustering induces focal adhesion kinase ( FAK1 )autophosphorylation, which creates a binding site for the proto-oncogene tyrosine-proteinkinase Src ( c-Src ). c-Src -mediated phosphorylation of FAK1promotes its maximal catalytic activity. The integrin -binding proteintalin recruits FAK1 and vinculin to focal contacts.Alpha-actinin is a cytoskeletal protein that binds to vinculin andcrosslinks actin in actomyosin stress fibres and tethers them to the focalcontacts. Phosphorylation of alpha-actinin by FAK1 reduces thecrosslinking of stress fibres and prevents the maturation of focal contacts . Vinculin transiently recruits the actin-related protein complexArp2/3 to new sites of integrin aggregation . Arp2/3complex nucleates new actin filaments from the sides of preexisting filaments.This interaction requires phosphorylation of Arp2/3 complex by PAK1(p21-activated kinase 1), which promotes actin polymerization ,. FAK1 plays a key role in the control of focal adhesion dynamics and cellmigration under the regulation of small GTPases of Rho family ( Rac1,Cdc42 and Rho -A ) , . RhoAdownstream effector ROCK (protein Rho-associated kinase) directly phosphorylatesLIMK1 and LIMK2 (LIM-kinases), which in turn phosphorylate cofilin(actin-associated protein). Cofilin exhibits actin -depolymerizing activityfollowed by reorganization of the actin cytoskeleton . Activity of LIMK1 is also regulated by PAK1, the downstream effector ofRac1 and Cdc42 . Cdc42 effector N-WASP (ahomolog to the Wiskott-Aldrich syndrome protein)  regulates actinpolymerization by stimulating the actin -nucleating activity of the Arp2/3complex . The Arp2/3 complex and cofilin are involved in the generation ofpropulsive force at the leading edge: the severing activity of cofilin and thebranching activity of Arp2/3 act in synergy to drive the extension oflamellipodia. Cofilin is also required for the maintenance of a polarizedcytoskeleton and thus for directional cell migration . It was shown that the persistent mode of migration of cells bound tofibronectin by alpha-V/beta-3 integrins is associated with relatively highlevels of cofilin activity and low levels of RhoA activity. Adhesion byalpha-5/beta-1 instead stimulates an increase in RhoA -mediatedphosphorylation of cofilin and supports random cell migration . Integrin signaling is mediated by distinct and separable interactions of theintegrin beta tails. c-Src was shown to bind constitutively andselectively to beta3 integrins . c-Src could, in turn,phosphorylate and stimulate GTPase-activating protein (GAP) p190 RhoGAP. The GAPactivity of RhoGAP is specific for RhoA inhibition . Vav1, a guanine nucleotide exchange factor for Rac1, Cdc42 andRho-A, that stimulates the exchange of bound GDP for GTP, was shown to stimulatepreferentially Rac1 and RhoA under integrin alpha-v/beta-3 -mediatedadhesion of hematopoietic cells, but this effect was investigated on vitronectinsubstrate . Tyrosine kinase Fyn could phosphorylate and activate Vav1 . Caveolin-1 was shown to function as a membrane adaptor to link theintegrin alpha-5 or alpha-v subunit to the Fyn . Vav1 activity is directly controlled by substrates and products ofphosphatidylinositol 3-kinase ( PI3K ). Phosphatidylinositol-4,5-bisphosphate (PI(4,5)P2 ) inhibits activation of Vav1, whereas the productphosphatidylinositol-3,4,5-trisphosphate ( PI(3,4,5)P3 ) enhances phosphorylationand activation of Vav1 . FAK1 activation may lead to itsassociation with the regulatory subunit of PI3K ( PI3K reg p85 ), which cansubsequently activate PI3K during cell adhesion , .PI3K reg p85 could also associate with Vav1 in some types of cells . The protein Nischarin was found to bind preferentially to the cytoplasmic tailof the integrin alpha-5 subunit and to inhibit cell migration .Nischarin was not found in focal adhesion sites, suggesting that it binds tointegrins when they are not ligated by the proteins of the extracellular matrix.Once the integrins enter into adhesion sites, Nischarin is released,allowing it to bind to the activated PAK1. This binding is enhanced by activeRac1. Interaction with Nischarin strongly inhibits the ability ofPAK1 to phosphorylate substrates and this effect closely parallelsNischarin's ability to inhibit cell motility , .