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Integrin adhesions are a structurally and functionally diverse family of transmembrane, multi-protein complexes that link the intracellular cytoskeleton to the extracellular matrix (ECM). The different members of this family, including focal adhesions (FAs), focal complexes, fibrillar adhesions, podosomes and invadopodia, contain many shared scaffolding and signaling 'adhesome' components, as well as distinct molecules that perform specific functions, unique to each adhesion form. In this Hypothesis, we address the pivotal roles of mechanical forces, generated by local actin polymerization or actomyosin-based contractility, in the formation, maturation and functionality of two members of the integrin adhesions family, namely FAs and invadopodia, which display distinct structures and functional properties. FAs are robust and stable ECM contacts, associated with contractile stress fibers, while invadopodia are invasive adhesions that degrade the underlying matrix and penetrate into it. We discuss here the mechanisms, whereby these two types of adhesion utilize a similar molecular machinery to drive very different - often opposing cellular activities, and hypothesize that early stages of FAs and invadopodia assembly use similar biomechanical principles, whereas maturation of the two structures, and their 'adhesive' and 'invasive' functionalities require distinct sources of biomechanical reinforcement., Competing Interests: Competing interestsThe authors declare no competing or financial interests., (© 2020. Published by The Company of Biologists Ltd.)

The invasive phenotype of metastatic cancer cells is accompanied by the formation of actin-rich invadopodia, which adhere to the extracellular matrix and degrade it. In this study, we explored the role of the tyrosine kinome in the formation of invadopodia in metastatic melanoma cells. Using a microscopy-based siRNA screen, we identified a series of regulators, the knockdown of which either suppresses (e.g., TYK2, IGFR1, ERBB3, TYRO3, FES, ALK, PTK7) or enhances (e.g., ABL2, AXL, CSK) invadopodia formation and function. Notably, the receptor tyrosine kinase AXL displayed a dual regulatory function, where both depletion or overexpression enhanced invadopodia formation and activity. This apparent contradiction was attributed to the capacity of AXL to directly stimulate invadopodia, yet its suppression upregulates the ERBB3 signaling pathway, which can also activate core invadopodia regulators and enhance invadopodia function. Bioinformatic analysis of multiple melanoma cell lines points to an inverse expression pattern of AXL and ERBB3. High expression of AXL in melanoma cells is associated with high expression of invadopodia components and an invasive phenotype. These results provide new insights into the complexity of metastasis-promoting mechanisms and suggest that targeting of multiple invadopodia signaling networks may serve as a potential anti-invasion therapy in melanoma. SIGNIFICANCE: These findings uncover a unique interplay between AXL and ERBB3 in invadopodia regulation that points to the need for combined therapy in order to prevent invadopodia-mediated metastasis in melanoma., (©2019 American Association for Cancer Research.)

In this article, we discuss the complex involvement of a Rho-family GTPase, Rac1, in cell migration and in invadopodia-mediated matrix degradation. We discuss the involvement of invadopodia in invasive cell migration, and their capacity to promote cancer metastasis. Considering the regulation of invadopodia formation, we describe studies that demonstrate the role of Rac1 in the metastatic process, and the suggestion that this effect is attributable to the capacity of Rac1 to promote invadopodia formation. This notion is demonstrated here by showing that knockdown of Rac1 in melanoma cells expressing a wild-type form of this GTPase, reduces invadopodia-dependent matrix degradation. Interestingly, we also show that excessive activity of Rac1, displayed by the P29S, hyperactive, "fast cycling" mutant of Rac1, which is present in 5-10% of melanoma tumors, inhibits invadopodia function. Moreover, knockdown of this hyperactive mutant enhanced matrix degradation, indicating that excessive Rac1 activity by this mutant can negatively regulate invadopodia formation and function., (Copyright © 2016 Elsevier Inc. All rights reserved.)

Invadopodia are actin-rich membrane protrusions through which cells adhere to the extracellular matrix and degrade it. In this study, we explored the mechanical interactions of invadopodia in melanoma cells, using a combination of correlative light and electron microscopy. We show here that the core actin bundle of most invadopodia interacts with integrin-containing matrix adhesions at its basal end, extends through a microtubule-rich cytoplasm, and at its apical end, interacts with the nuclear envelope and indents it. Abolishment of invadopodia by microtubules or src inhibitors leads to the disappearance of these nuclear indentations. Based on the indentation profile and the viscoelastic properties of the nucleus, the force applied by invadopodia is estimated to be in the nanoNewton range. We further show that knockdown of the LINC complex components nesprin 2 or SUN1 leads to a substantial increase in the prominence of the adhesion domains at the opposite end of the invadopodia. We discuss this unexpected, long-range mechanical interplay between the apical and basal domains of invadopodia, and its possible involvement in the penetration of invadopodia into the matrix.