Targeting and transport: how microtubules control focal adhesion dynamics

Abstract

Directional cell migration requires force generation that relies on the coordinated remodeling of interactions with the extracellular matrix (ECM), which is mediated by integrin-based focal adhesions (FAs). Normal FA turnover requires dynamic microtubules, and three members of the diverse group of microtubule plus-end-tracking proteins are principally involved in mediating microtubule interactions with FAs. Microtubules also alter the assembly state of FAs by modulating Rho GTPase signaling, and recent evidence suggests that microtubule-mediated clathrin-dependent and -independent endocytosis regulates FA dynamics. In addition, FA-associated microtubules may provide a polarized microtubule track for localized secretion of matrix metalloproteases (MMPs). Thus, different aspects of the molecular mechanisms by which microtubules control FA turnover in migrating cells are beginning to emerge.

Figure 1.

+TIP-mediated microtubule–FA interactions in the front of a migrating cell. The FA life cycle consists of nascent FA assembly near the cell’s leading edge, actomyosin-mediated maturation, and subsequent disassembly as the cell migrates forward. FA interactions with the microtubule cytoskeleton are important for the regulation of FA dynamics, and three types of +TIPs have been implicated in mediating microtubule–FA interactions. It is unclear how the functions of these proteins overlap, but based on published biochemical interactions and RNAi depletion experiments, we propose a hierarchy of APC, the spectraplakin MACF1/ACF7, and CLASPs, which is indicated by the arrows with broken lines. (A) APC is transported along microtubules to the cell edge and directly interacts with polarity signals such as the Wnt signaling pathway. APC may be involved in stabilizing nascent FAs. It is important to note, however, that only a subset of FAs associate with APC clusters, so other mechanisms must exist. (B) MACF1/ACF7 mediates microtubule interactions with F-actin stress fibers, and is required to guide microtubule growth toward FAs. (C) CLASPs stabilize microtubules in a domain around mature FAs. CLASP accumulation near FAs depends on interactions with the PIP3-binding protein LL5β. Different cell polarity pathways are thought to result in local GSK3β inactivation that in turn stimulates microtubule and/or FA interactions of APC, MACF1/ACF7, and CLASPs. FA disassembly in the retracting rear of the cells differs mechanistically from FA turnover in the front, and it is not known to what extent the same molecules are involved. Disassembling FAs are symbolized by red dots. (D) Contrast-inverted image of CLASP2-decorated microtubules around FAs (labeled with Paxillin-mCherry) near the leading edge of a migrating epithelial cell.

Figure 2.

Microtubule-mediated vesicle trafficking pathways involved in FA dynamics. (A) ILK participates in microtubule stabilization near the leading cell edge, and is required for caveolin transport toward FAs. Caveolae may provide an alternative integrin endocytosis pathway. (B) Exocytosis of MT1-MMP is involved in ECM degradation around FAs. Transport of MT1-MMP vesicles requires microtubules, and ECM proteolysis may initiate FA disassembly. (C) Integrins are internalized by different endocytosis pathways and at least partially recycled back to the cell front. Clathrin-mediated endocytosis is thought to be involved in uptake of FA-associated, activated integrin molecules, whereas clathrin-independent pathways may be more important for endocytosis of inactive integrins. Arrows with broken lines indicate integrin transport and recycling pathways.