Integrin traffic - the update

Abstract

Integrins are a family of transmembrane cell surface molecules that constitute the principal adhesion receptors for the extracellular matrix (ECM) and are indispensable for the existence of multicellular organisms. In vertebrates, 24 different integrin heterodimers exist with differing substrate specificity and tissue expression. Integrin-extracellular-ligand interaction provides a physical anchor for the cell and triggers a vast array of intracellular signalling events that determine cell fate. Dynamic remodelling of adhesions, through rapid endocytic and exocytic trafficking of integrin receptors, is an important mechanism employed by cells to regulate integrin-ECM interactions, and thus cellular signalling, during processes such as cell migration, invasion and cytokinesis. The initial concept of integrin traffic as a means to translocate adhesion receptors within the cell has now been expanded with the growing appreciation that traffic is intimately linked to the cell signalling apparatus. Furthermore, endosomal pathways are emerging as crucial regulators of integrin stability and expression in cells. Thus, integrin traffic is relevant in a number of pathological conditions, especially in cancer. Nearly a decade ago we wrote a Commentary in Journal of Cell Science entitled 'Integrin traffic'. With the advances in the field, we felt it would be appropriate to provide the growing number of researchers interested in integrin traffic with an update.

Keywords: Integrin; Invasion; Migration; Rab GTPases; Signalling crosstalk; Trafficking.

Fig. 1.

Role of cytoskeletal proteins in integrin traffic. Internalisation: clathrin-dependent endocytosis of integrins from FAs is promoted by acetylated microtubules and adaptors, such as Dab2 and ARH. The microtubule motor kinesin KIF15 promotes endocytosis of inactive integrins by delivering Dab2 to these receptors, whereas Myo6, an actin motor, mediates internalisation of α5β1 integrins from fibrillar adhesions. Recycling: in the early endosome compartment, WASH-mediated recruitment of Arp2/3 and subsequent Arp2/3-mediated actin reorganisation drives integrin traffic to recycling endosomes. Here, several mechanisms support integrin recycling back to the plasma membrane. For example, KIF1C promotes α5β1 recycling towards the rear of migrating cells for FA maturation, and Myo10 traffics integrins to filopodia tips. Degradation: in the absence of WASH, integrins are rerouted towards degradation.

Fig. 2.

Trafficking of active and inactive integrin heterodimers. (A) Stepwise model of integrin activation through a conformational switch. (B) Schematic representation of the trafficking routes of active and inactive integrin heterodimers. Internalisation: at the plasma membrane, both active and inactive integrins are endocytosed to early endosomes in a Rab5- or Rab21-dependent fashion. Dab2 acts as an adaptor for endocytosis of inactive (unengaged) integrins, whereas an Nrp1–GIPC1–Myo6–APPL module mediates endocytosis of active α5β1 integrin from fibrillar adhesions. Recycling: inactive β1 integrins are rapidly recycled to Arf6-positive protrusions in a Rab4-dependent manner, whereas active receptors are trafficked through the Rab11 long-loop pathway (the red asterisk indicates recycling from PNRC – omitted here for simplicity). Degradation: in early endosomes, SNX17 binding to the cytoplasmic domain of inactive β-integrin promotes recycling of the receptor over degradation. Similarly, in the late endosome and lysosome compartment, CLIC3-mediated recycling prevents degradation of the active integrin receptor.

Fig. 3.

Role of small GTPases in integrin traffic. Rab and Arf GTPase family members and their GTPase regulators are intimately involved in different steps of trafficking pathways. Internalization: integrin endocytosis to early endosomes can be triggered by both Rab5 and Rab21, regardless of integrin activation status. Other reported pathways of integrin internalization involve the activation of Arf5 facilitated by a BRAG2–clathrin–AP2 complex at clathrin-coated pits (dependent on PtdIns4,5P₂), or the activation of R-Ras and the formation of an R-Ras–RIN2–Rab5 complex at the plasma membrane downstream of integrin–ECM signalling. In early endosomes, this complex also activates Rac1, through the Rac1 GEF Tiam1, ultimately promoting Rac1 translocation to the plasma membrane and the assembly of new nascent adhesions at lamellipodia (Rac1 activation and translocation is represented by red arrows). Recycling: in EEA1-containing early endosomes, RASA1 outcompetes Rab21 for binding to the β-integrin subunit and drives β1 integrin recycling. Rab25 interacts directly with α5β1 on endosomes to facilitate receptor recycling. Arf6-dependent β1 integrin recycling is regulated by the Arf6 GAPs ARAP2 and ACAP1 which localise to different Arf6-positive endosome compartments. ARAP2 promotes the transition of the integrin receptor from APPL-containing endosomes to EEA1-containing early endosomes and towards recycling compartments and/or degradation (blue arrows). In addition, ARAP2 might prevent the direct recycling of integrins from the early endosomes to the tubular recycling compartments or to the membrane (grey arrows). ACAP1 has been implicated in rapid β1 integrin recycling from Arf6-positive tubular endosomes (green arrow). The red asterisk indicates integrin traffic from recycling endosomes.

Fig. 4.

Effect of reciprocal recycling of α5β1 and αvβ3 integrins on cell invasion. Top panel: increased αvβ3 recycling. In many cell types, including cancer cells, αvβ3 recycling (e.g. following PDGF stimulation) suppresses the recycling of α5β1 to promote Rac-mediated cell migration in 2D and invasion into low-fibronectin ECM. Bottom panel: increased α5β1 recycling. Manipulating αvβ3 recycling, either by blocking adhesive function (e.g. treatment with cilengitide or cRGD) or indirectly through expression of mutant p53 (found in cancer cells and associated with increased Myo10 expression and filopodia formation) or by manipulating syndecan-4 phosphorylation and/or engagement (important for regulation of Arf6 activity), promotes α5β1 integrin traffic. Increased α5β1 recycling then triggers a RhoA–ROCK-dependent mode of 2D random cell migration, and cell invasion into fibronectin-rich ECM. Red arrows delineate signalling events, whereas black and grey arrows indicate active and inactive endocytic trafficking pathways, respectively. EE, early endosome; FN, fibronectin; LE, late endosome; RE, recycling endosome; RTK, receptor tyrosine kinase.