Integrin-matrix clusters form podosome-like adhesions in the absence of traction forces.

[wiskott-aldrich syndrome]

Matrix-activated integrins can form different adhesion structures . We report that nontransformed fibroblasts develop podosome-like adhesions when spread on fluid Arg-Gly-Asp peptide (RGD )-lipid surfaces , whereas they habitually form focal adhesions on rigid RGD glass surfaces . Similar to classic macrophage podosomes , the podosome-like adhesions are protrusive and characterized by doughnut-shaped RGD rings that surround characteristic core components including F-actin , N-WASP , and Arp 2 /Arp 3 . Furthermore , there are 18 podosome markers in these adhesions , though they lack matrix metalloproteinases that characterize invadopodia and podosomes of Src-transformed cells . When nontransformed cells develop force on integrin-RGD clusters by pulling RGD lipids to prefabricated rigid barriers (metal lines spaced by 1 -2 Â Î¼m ), these podosomes fail to form and instead form focal adhesions . The formation of podosomes on fluid surfaces is mediated by local activation of phosphoinositide 3 -kinase (PI 3 K ) and the production of phosphatidylinositol-(3 ,4 ,5 )-triphosphate (PIP 3 ) in a FAK /PYK 2 -dependent manner . Enrichment of PIP 3 precedes N-WASP activation and the recruitment of RhoA-GAP ARAP 3 . We propose that adhesion structures can be modulated by traction force development and that production of PIP 3 stimulates podosome formation and subsequent RhoA downregulation in the absence of traction force .