Caco-2 cells labeled for tight junction molecule cingulin (green), actin (red), vinculin (pink) and DNA (blue).
Epithelial cells growing on a patterned adhesive surface with the shape of the Weizmann Institute tree.
Desmosomes in mouse tongue epithelium (by transmission electron microscope).
Porcine aortic endothelial cell, double-labeled for actin (green) and phospho-tyrosine (red).
“Molecular composition map” of focal adhesions and stress fibers.
Myeloma cancer cell responding to shear flow (by scanning electron microscope).
Dr. Tova Volberg
The roles of integrin-linked kinase (ILK) in adhesion maturation and molecular architecture
ILK, a scaffold molecule that links the cytoplasmic domains of β integrin receptors to the actin cytoskeleton, is considered to be a key regulator of integrin adhesions.
We show that ILK participates in three distinct stages of integrin adhesion organization: (1) nucleation of FA formation under the lamellipodium; (2) organization of FAs and their associated stress fibers; and (3) induction of FN fibrillogenesis and formation of FBs. In ILK-/- cells FXs are absence and FAs assemble inefficiently, leading not only to slow cell spreading, but also to limited polarization and poor actin alignment. Interestingly, ILK also appears to be crucial for FA turnover.
Another notable effect of ILK knock-out is the inhibition of FN fibrillogenesis and abolishment of tensin-rich FB. It was previously shown that FN fibrillogenesis is driven by mechanical forces, applied to extracellular FN through integrin adhesion. It was further demonstrated that this process was accompanied by the formation of a special type of integrin adhesion, called fibrillar adhesions (FBs), which extend centripetally from FAs and are transported towards the cell center. The relationships between FBs and FN fibrils are well-established; yet the functional hierarchy between them is not known: are FBs formed first and then organize FN, or is it the other way around? Here, we show that in the absence of ILK, FN fibrils are not formed, and consequently tensin, the hallmark of FBs, is confined to and enriched in FAs, while knock-down of tensin has no apparent effect on FN fibril formation, nor on formation of ILK-rich FBs along their lengths. These results indicate that ILK is an “upstream regulator”, inducing FN fibrillogenesis that, in turn, drives FB formation and the recruitment of molecules such as tensin to the adhesion sites.