Summary Mechanotransduction, a key determinant of tissue homeostasis and tumor progression, is driven by intercellular adhesions, cell contractility and forces generated with the microenvironment, dependent on extracellular matrix composition, organization and compliance. Caveolin-1 (Cav1) favors cell elongation in 3D cultures and promotes Rho-and force-dependent contraction, matrix alignment and microenvironment stiffening through regulation of p190RhoGAP. In turn, microenvironment remodeling by Cav1-fibroblasts forces cell elongation. Cav1-deficient mice have disorganized stromal tissue architecture. Stroma associated with human carcinomas and melanoma metastases is enriched in Cav1-expressing carcinoma-associated fibroblasts (CAFs). Cav1 expression in breast CAFs correlates with low survival, and Cav1 depletion in CAFs decreases CAF contractility. Consistently, fibroblast expression of Cav1, through p190RhoGAP regulation, favors directional migration and invasiveness of carcinoma cells in vitro. In vivo, stromal Cav1 remodels peri- and intratumoral microenvironments to facilitate tumor invasion, correlating with increased metastatic potency. Thus, Cav1 modulates tissue responses through force-dependent architectural regulation of the microenvironment.

Development, angiogenesis, wound healing, and metastasis all involve the movement of cells in response to changes in the extracellular environment. To determine whether caveolin-1 plays a role in cell migration, we have used fibroblasts from knockout mice. Caveolin-1–deficient cells lose normal cell polarity, exhibit impaired wound healing, and have decreased Rho and increased Rac and Cdc42 GTPase activities. Directional persistency of migration is lost, and the cells show an impaired response to external directional stimuli. Both Src inactivation and p190RhoGAP knockdown restore the wild-type phenotype to caveolin-1–deficient cells, suggesting that caveolin-1 stimulates normal Rho GTP loading through inactivation of the Src–p190RhoGAP pathway. These findings highlight the importance of caveolin-1 in the establishment of cell polarity during directional migration through coordination of the signaling of Src kinase and Rho GTPases.

An essential property of the plasma membrane of mammalian cells is its plasticity, which is required for sensing and transmitting of signals, and for accommodating the tensional changes imposed by its environment or its own biomechanics. Caveolae are unique invaginated membrane nanodomains that play a major role in organizing signaling, lipid homeostasis and adaptation to membrane tension. Caveolae are frequently associated with stress fibers, a major regulator of membrane tension and cell shape. In this Commentary, we discuss recent studies that have provided new insights into the function of caveolae and have shown that trafficking and organization of caveolae are tightly regulated by stress-fiber regulators, providing a functional link between caveolae and stress fibers. Furthermore, the tension in the plasma membrane determines the curvature of caveolae because they flatten at high tension and invaginate at low tension, thus providing a tension-buffering system. Caveolae also regulate multiple cellular pathways, including RhoA-driven actomyosin contractility and other mechanosensitive pathways, suggesting that caveolae could couple mechanotransduction pathways to actin-controlled changes in tension through their association with stress fibers. Therefore, we argue here that the association of caveolae with stress fibers could provide an important strategy for cells to deal with mechanical stress.

The Abl-interactor (Abi) family of adaptor proteins has been linked to signaling pathways involving the Abl tyrosine kinases and the Rac GTPase. Abi proteins localize to sites of actin polymerization in protrusive membrane structures and regulate actin dynamics in vitro. Here we demonstrate that Abi2 modulates cell morphogenesis and migration in vivo. Homozygous deletion of murine abi2 produced abnormal phenotypes in the eye and brain, the tissues with the highest Abi2 expression. In the absence of Abi2, secondary lens fiber orientation and migration were defective in the eye, without detectable defects in proliferation, differentiation, or apoptosis. These phenotypes were consistent with the localization of Abi2 at adherens junctions in the developing lens and at nascent epithelial cell adherens junctions in vitro. Downregulation of Abi expression by RNA interference impaired adherens junction formation and correlated with downregulation of the Wave actin-nucleation promoting factor. Loss of Abi2 also resulted in cell migration defects in the neocortex and hippocampus, abnormal dendritic spine morphology and density, and severe deficits in short-and long-term memory. These findings support a role for Abi2 in the regulation of cytoskeletal dynamics at adherens junctions and dendritic spines, which is critical for intercellular connectivity, cell morphogenesis, and cognitive functions.

SummaryCaveolae are relatively stable membrane invaginations that compartmentalize signaling, regulate lipid metabolism and mediate viral entry. Caveolae are closely associated with actin fibers and internalize in response to diverse stimuli. Loss of cell adhesion is known to induce rapid and robust caveolae internalization and trafficking toward a Rab11-positive recycling endosome; however, pathways governing this process are poorly understood. Here, we report that filamin A is required to maintain the F-actin-dependent linear distribution of caveolin-1. High spatiotemporal resolution particle tracking of caveolin-1-GFP vesicles by total internal reflection fluorescence (TIRF) microscopy revealed that FLNa is required for the F-actin-dependent arrest of caveolin-1 vesicles in a confined area and their stable anchorage to the plasma membrane. The linear distribution and anchorage of caveolin-1 vesicles are both required for proper caveolin-1 inwards trafficking. De-adhesion-triggered caveolae inward trafficking towards a recycling endosome is impaired in FLNa-depleted HeLa and FLNa-deficient M2-melanoma cells. Inwards trafficking of caveolin-1 requires both the ability of FLNa to bind actin and cycling PKC-dependent phosphorylation of FLNa on Ser2152 after cell detachment.

Actin polymerization provides the driving force for the formation of cell-cell junctions and is mediated by two types of actin nucleators, Arp2/3 and formins. Proteins required for coordinately linking cadherin-mediated adhesion to Arp2/3-dependent versus formin-dependent nucleation have yet to be defined. Here we show a role for Abi, the Abi-binding partner Nap1, and the Nap1-binding protein Sra1 in the regulation of cadherindependent adhesion. We found that Abi, which is known to interact with Wave, leading to activation of the Arp2/3 complex, is also capable of interacting with the Diaphanous (Dia)-related formins in the absence of Wave. Knockdown of Abi, Nap1, Sra1, or Dia markedly inhibited cell-cell junctions, whereas knockdown of Wave or Arp2/3 produced mild and transient phenotypes. Dia and Abi colocalized with ␤-catenin at cell-cell junctions. Further, Dia and Wave bound to overlapping sites on Abi1, and Wave competed with Dia for Abi1 binding. Notably, an active Dia1 C-terminal fragment that localizes to cell-cell junctions rescued the abnormal junctions induced by depletion of Abi or Nap1 in epithelial cells. These findings uncover a novel link between cadherin-mediated adhesion and the regulation of actin dynamics through the requirement for an Abi/Dia complex for the formation and stability of cell-cell junctions.

The Abl interactor 1 (Abi-1) protein has been implicated in the regulation of actin dynamics and localizes to the tips of lamellipodia and filopodia. Here, we show that Abi-1 binds the actin nucleator protein Wave-1 through an amino-terminal Wave-binding (WAB) domain and that disruption of the Abi-1-Wave-1 interaction prevents Abi-1 from reaching the tip of the lamellipodium. Abi-1 binds to the Wave homology domain of Wave-1, a region that is required for translocation of Wave-1 to the lamellipodium. Mouse embryo fibroblasts that lack one allele of Abi-1 and are homozygous null for the related Abi-2 protein exhibit decreased Wave-1 protein levels. This phenotype is rescued by Abi-1 proteins that retain Wave-1 binding but not by Abi-1 mutants that cannot bind to Wave-1. Moreover, we uncovered an overlapping SNARE domain in the amino terminus of Abi-1 that interacts with Syntaxin-1, a SNARE family member. Further, we demonstrated that Abi-1 shuttles in and out of the nucleus in a leptomycin B (LMB)-dependent manner and that complete nuclear translocation of Abi-1 in the absence of LMB requires the combined inactivation of the SNARE, WAB, and SH3 domains of Abi-1. Thus, Abi-1 undergoes nucleocytoplasmic shuttling and functions at the leading edge to regulate Wave-1 localization and protein levels.

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