Classical cadherins are a family of membrane receptors that mediate cell-cell adhesion in mammalian organisms. During embryogenesis cadherins act as key determinants of tissue organisation and patterning as well as being responsible for normal tissue architecture in the adult. It is becoming increasingly clear that cadherin receptor function, and ensuing cell-cell interactions, are extremely dynamic processes coordinated by localised membrane signalling. A key role of such cortical signalling is to regulate the complex relationship between the cadherin and the actin cytoskeleton. In contrast the potential link between cadherins and microtubules has been less extensively investigated. This is the subject of this thesis.
I now identify a pool of microtubules that extend radially into cell-cell contacts and are inhibited by manoeuvres that block the dynamic activity of microtubule plus-ends. Blocking dynamic microtubules perturbed the ability of cells to concentrate and accumulate E-cadherin at cell-cell contacts, as assessed both by quantitative immunofluorescence microscopy and FRAP analysis, but did not affect either transport of E-cadherin to the plasma membrane, nor the amount of E-cadherin expressed at the cell surface. Importantly, dynamic microtubules were necessary for Myosin IIa to accumulate and be activated at cadherin-adhesive contacts, a localised mechanism that supports the focal accumulation of E-cadherin.
Importantly, I found evidence that dynamic plus-ends may mediate this effect on Myosin IIa by regulating the GTPase, RhoA. Both RhoA localisation and activity were dependent on plusend dynamics; RhoA delocalised from cell-cell contacts upon plus-end inhibition, accompanied by a concomitant decrease in the biochemical activity of RhoA. In addition, I established a requirement for dynamic microtubule plus-ends in the cortical localisation of the RhoA GEF, Ect2.
I propose that this population of microtubules represents a novel form of cadherin-microtubule cooperativity, where cadherin adhesions recruit dynamic microtubules that, in turn, support the local concentration of cadherin molecules by regulating Myosin IIa activity at cell-cell contacts.