Integrin Traffic

More recently, using an evolutionary biology approach, we discovered a mechanism whereby integrin heterodimers can be selectively endocytosed, thus reducing their bioavailability for ECM engagement at a given time. Specifically, we identified an evolutionary preserved binding motif for the AP2 adaptor in several α-integrin subunits including 2 out of 4 collagen-binding integrins and one fibronectin-binding integrin heterodimer. Mutation of this AP2 motif resulted in defects in cell migration and cell spreading demonstrating that selective integrin turnover has a functional outcome on cell behaviour (see Publications; De Franceschi et al., 2015).

We also revealed, for the first time, that integrin signalling is not restricted to the plasma membrane as previously thought, but can be preserved during integrin endocytosis. These results are extremely exciting, particularly as integrins have been implicated as drivers of cancer cell migration, invasion and metastasis. Interestingly, the integrin-ECM signal initiated at the plasma membrane is prolonged and enhanced upon integrin endocytosis to EEA1-containing early endosomes. Integrin-induced FAK (focal adhesion kinase) activity within endosomes suppresses anoikis, thus supporting anchorage-independent growth of cancer cells and importantly cancer metastasis in vivo (see Publications; Alanko et al., 2015).

Integrin Traffic

Introduction: The endocytic pathway is now recognised as an important mechanism to regulate receptor function, cell migration and tumour cell invasion and has been described for many receptors including RTKs (e.g. VEGF, EGF and PDGF) and other cell surface receptors such as syndecans. Integrin traffic represents a crucial mechanism whereby cells confer tight control over integrin signalling and facilitate differential expression of integrin heterodimers on the cell surface to elicit specific cellular responses.

The cycle of receptor traffic primarily involves three steps: 1) invagination of the plasma membrane surrounding the receptor and formation of intracellular vesicles, 2) delivery of receptors to endosomal compartments for sorting, 3) targeting of receptors to either lysosomes for degradation or to recycling endosomes for redelivery back to the cell membrane and engagement of new ligand. Each of these steps can occur through multiple routes (e.g. clathrin-dependent or clathrin-independent endocytosis) and requires the spatial and temporal coordination of multiple molecules, including the Rho and Rab family of small GTPases, to precisely regulate receptor traffic and orchestrate cellular functions. We aim to understand the nature of the different pathways of integrin endocytosis and the conditions under which each pathway is activated in order to appreciate fully integrin-specific signalling in health and disease.

Our research: We have previously demonstrated a critical role for Rab21 small GTPase and p120RasGAP in the constitutive recycling of β1-integrins (see Publications; Pellinen et al. 2006; Mai et al. 2011) that is important for normal cell cytokinesis (see Publications; Pellinen et al. 2008). We have also identified distinct recycling pathways for integrin heterodimers based on the activation state of the molecule (see Publications; Arjonen et al. 2012).

An example of integrin trafficking pathways. Active integrins (extended ligand-bound receptors) and inactive integrins (bent conformation) are endocytosed via clathrin- and dynamin-dependent routes. The heterodimers are then recycled back to the plasma membrane via distinct mechanisms depending on their conformation. See Arjonen et al,. 2012.

Illustration on the right: Integrin signalling is preserved during endocytosis. Summary of findings presented in Alanko et al., 2015.