Endoderm detachment (A,B; black arrow) and abnormal basement membrane (C,D) in Kindlin-2 deficient embryoid bodies. LN: laminin.

Image courtesy of Dr Reinhard Fässler, Max Planck Institute of Biochemistry, Martinsried, Germany.

Transmembrane receptor integrins are essential for establishing and maintaining cell-extracellular matrix (ECM) interactions. Although integrins have been extensively studied, the mechanisms that underlie their activation remain controversial. As reported in Genes and Development, Reinhard Fässler and colleagues now show that Kindlin-2 functions as an integrin co-activator that is crucial in the early stages of mouse development.

The Kindlin family of adaptor proteins is comprised of three members that are characterised by a FERM (Band 4.1/Ezrin/Radixin/Moesin) domain, which is known to interact with integrin 1 subunits. Mutations in the human epithelial Kindlin-1 gene lead to Kindler syndrome — a condition characterised by severe skin blistering. Kindlin-3 is found only in haematopoietic cells and is required for platelet aggregation. The ubiquitously expressed Kindlin-2 affects cell-ECM adhesion and spreading of mammalian cells.

To assess its role in vivo, Reinhard Fässler and colleagues knocked down Kindlin-2 in mice. Disruption of the gene resulted in embryo lethality at the peri-implantation stage. Furthermore, embryoid bodies generated from Kindlin-2-/- embryonic stem cells (ESCs) degenerated and showed severe cell-ECM adhesion defects. Instead of developing a central cavity surrounded by an outer endoderm and inner epithelial cell layer that are separated by a continuous basement membrane, mutant embryoid bodies formed a compact aggregate of ESCs covered by a discontinuous basement membrane and a detached endoderm.

But how does Kindlin-2 affect adhesion, basement membrane deposition and cavity formation? The authors found that the strongly reduced ESC adhesion to ECM proteins was due to reduced integrin activation. Kindlin-2 was shown to associate with the distal NxxY motifs of 1 and 3 integrins. Thus, although Kindlin-2 and talin — a well-established integrin activator — have similar FERM domains, these proteins bind to distinct sites within integrin tails. Furthermore, expression of Kindlin-2 together with talin, but not alone, resulted in a synergistic effect, showing that these two proteins function together to activate integrin.

Integrin-linked kinase (ILK) is a cytoplasmic integrin effector that regulates F-actin rearrangements. Endoderm and epithelial embryoid body cells showed impaired F-actin polarization and abnormally accumulated actin. Isolated mutant embryoid body cells formed fewer focal adhesions lacking ILK and failed to spread when plated on fibronectin, indicating that Kindlin-2 is required for bidirectional integrin signalling.

This work shows that loss of Kindlin-2 causes early embryo lethality due to impaired integrin activation. In the Journal of Cell Biology, Edward Plow's group also reported that Kindlin-2 functions as an integrin co-activator. Together these findings add to the increasing evidence that talin is necessary but not sufficient for integrin activation. However, it remains to be elucidated whether all cell types require Kindlin, as well as the extent to which Kindlins act redundantly and the specific roles of talin and Kindlin.

Author: Kim Baumann