Cdc42 and beta1 integrin in cell migration

Cell migration plays a central role in the development and
maintenance of multicellular organisms. It involves regulated cell
adhesion, mediated by integrins, and polarized changes of the
cytoskeleton, controlled by Rho GTPases such as Cdc42. Aim of this
study was to investigate the role of integrins and Cdc42 in cell
migration and in particular the cross-talk between these molecules.
In addition, the structure–function relationship of beta1 integrin
in mediating migration associated events was studied. To test
whether Cdc42 is essential for directed cell migration in mammalian
cells and to investigate the cross-talks between integrin and Cdc42
mediated signalling, fibroblastoid cell lines lacking a functional
Cdc42 gene were established and analyzed in wound closure assays.
Contrary to the expectations, we could show that Cdc42 is neither
required for integrin activation nor for integrin mediated
protrusion formation. Moreover, Cdc42 has no significant influence
on the speed of directed migration. However, it contributes to the
directionality of migration and to the re-orientation of the Golgi
apparatus into the direction of migration by a mechanism
independent of Gsk3beta phosphorylation. Furthermore, we
demonstrated that Cdc42 controls cell morphology, quite likely by
regulating Rac1 activity. Expression of dominant negative Cdc42
(dnCdc42) in Cdc42-null cells revealed that dnCdc42
non-specifically inhibits other Rho GTPases besides Cdc42, since it
aggravates the impairments observed in Cdc42-null cells, resulting
in strongly reduced directed migration, severely reduced single
cell directionality, and complete loss of Golgi polarization and of
directionality of protrusion formation towards the wound. Beta1
integrins were previously shown to activate Cdc42 in response to
wounding and thus to regulate the directionality of migration. We
demonstrated now, that fourfold reduction of beta1 integrin
expression in keratinocytes in vivo did not impair wound healing.
However, keratinocyte stem cells with normal levels of beta1
integrin had a competitive advantage over the hypomorphic cells and
expanded steadily in the skin of mice harbouring both cell types in
the epidermis. Finally, we analysed the importance of specific
amino acids of the intracellular domain of beta1 integrin in
keratinocytes in vivo by generating 8 mice strains which in skin
express only point or deletion mutants of beta1 integrin. Our data
are for the most part strikingly different from previous results
obtained in vitro and significantly revise proposed models for the
role of serine and tyrosine phosphorylation and the function of a
salt bridge between the integrin beta subunits and the integrin
alpha tails.