Bidirectional control of the inner dynamics of focal adhesions promotes cell migration

Abstract

Focal adhesions (FA) are bidirectional mechanical biosensors that allow cells to integrate intracellular and extracellular cues. Their function is tightly regulated by changes in molecular composition and also by variation in the spatio-temporal dynamics of FA components within this structure. A closely regulated turnover of FA proteins within FA sites allows cells to respond appropriately to their environment, thereby impacting on cell shape and function. FA protein dynamics are linked to FA maturation and rates of assembly and disassembly, and have a significant influence on tumor cell migration. Using the FRAP technique to investigate the hidden internal dynamics of FA, we identified two new regulators of FA dynamics and cell migration: the Mgat5/galectin-3 lattice and tyrosine phosphorylated caveolin-1 (pY14Cav1). In this short review we first introduce FA and their complex dynamic behavior. We then present the Mgat5/galectin-3 lattice and caveolin-1 and discuss their concerted role in FA dynamics, which defines previously unknown, interdependent roles in tumor cell migration. We conclude with a discussion of interesting unexplored avenues that might lead to a better understanding of the complex mechanism of FA dynamics.

Figure 1

Increased FAK exchange at FA sites correlates with FA maturation and directional cell migration. Adapted from ref . (A) Mgat5^-/- and Mgat5-Rescue cells transfected with FAK-GFP were imaged before laser bleaching of one FA-containing region of interest (ROI; red square). A time-lapse sequence (in seconds) shows the corresponding ROI before photobleaching (prebleach), immediately after photobleaching (bleach) and during recovery (recovery). Quantification of FAK-GFP fluorescence over time in FA region of interest (ROI) is presented for Mgat5^-/- and Rescue cells. Percentage of recovery (boxes) shows the extent of FAK-GFP mobile fraction. *p < 0.05. Bar, 20 µm. (B) Mgat5^-/- and Rescue cells were immunofluorescently labeled with Hoechst (blue) and with Alexa568-phalloidin (red) and anti-vinculin (green) antibodies. The diagram depicts the differential dynamics of FA components, and their impact on FA maturation, highlighted by the FRAP technique.

Figure 2

The Mgat5/Galectin-3 lattice and pY14Cav1 work in concert to stabilize FAK exchange at FA sites. Adapted from. The Mgat5/galectin lattice mediates integrin clustering and activation and can induce the formation of FA and cell spreading, but does not favor FAK stabilization in FA domains. Expression of pY14Cav1 results in the formation of an ordered membrane domain and activation of RhoGTPase activity that results in the stabilization within the FA of integrin, FAK and paxillin as well as, potentially, other FA components. Stable association within FA of FAK, its Src-dependent phosphorylation and the associated recruitment of signaling partners within FA lead to disassembly and turnover of FA and directed cell migration.