Intermediate filaments and the regulation of focal adhesion Rudolf E Leube, Marcin Moch and Reinhard Windoffer Focal adhesions are localized actin filament-anchoring signalling centres at the cell–extracellular matrix interface. The currently emerging view is that they fulfil an all-embracing coordinating function for the entire cytoskeleton. This review highlights the tight relationship between focal adhesions and the intermediate filament cytoskeleton. We summarize the accumulating evidence for direct binding of intermediate filaments to focal adhesion components and their mutual cross-talk through signalling molecules. Examples are presented to emphasize the high degree of complexity of these interactions equipping cells with a precisely controlled machinery for context-dependent adjustment of their biomechanical properties. Addresses Institute of Molecular and Cellular Anatomy, RWTH Aachen University, Wendlingweg 2, 52074 Aachen, Germany Corresponding author: Leube, Rudolf E (rleube@ukaachen.de) Current Opinion in Cell Biology 2015, 32:13–20 This review comes from a themed issue on Cell architecture Edited by Sandrine Etienne-Manneville and Elly M Hol http://dx.doi.org/10.1016/j.ceb.2014.09.011 0955-0674/# 2014 Elsevier Ltd. All right reserved. Introduction The metazoan cytoskeleton fulfils two seemingly incom- patible tasks. It provides both cellular rigidity and flexi- bility. This challenge is met in part by biomechanically unique filamentous components that are expressed in cell type-specific admixtures thereby supporting a broad spec- trum of cellular properties ranging from static/resilient to dynamic/fragile for engagement in various tissue functions. Situations requiring rapid changes in cellular mechanics, however, require additional mechanisms. One such situ- ation is the conversion of a sessile, tightly integrated epi- thelial cell into a highly motile migratory cell upon wounding or malignant transformation. The change is brought about by activation and formation of cell surface receptors, which cluster together with cytoplasmic proteins to build focal adhesions (FAs) at the cell–extracellular matrix (ECM) interface. FAs induce major alterations in cytoskeletal organization: they promote localized actin stress fibres to provide pulling and pushing forces in con- junction with myosin motors and they redirect microtubules to facilitate polarized trafficking [1,2]. Comparatively little is known, however, in which way and how FA formation affects the intermediate filament (IF) cytoskeleton and vice versa. We will present examples for the growing evidence of direct molecular links and for signalling molecule-mediated cross talk between FA components and IFs in different functional settings supporting context-dependent dynamic and static interactions. Functional and structural diversity of focal adhesion–cytoskeletal interactions FAs consist of localized membrane-associated multicom- ponent complexes, which couple the actin cytoskeleton to the ECM and serve as major mechanosensory signal- ling nodes to control multiple cell functions [1–3]. The dynamics of FAs have been well characterized in cultured cells. They appear as nascent adhesions of less than 0.25 mm diameter in active lamellipodia, enlarge into focal complexes at the lamellipodial-lamellar transition zone and give rise to bona fide focal adhesions, which further mature into fibrillar adhesions [1,2,4  ]. At the core of these different FA types are clustered integrin recep- tors in the plasma membrane linking the ECM to the cell interior. A variable ensemble of proteins regulates cyto- skeletal filament attachment and dynamics and also gen- erates signals to modulate other cell functions [1,2,4  ]. The wealth of knowledge on these prominent structures in cultured cells is contrasted by rather limited insights into the functional diversity of related structures in vivo (Figure 1). For example, they have been described in podosomes and invadopodia, together referred to as inva- dosomes, supporting migration of fibroblasts, immune cells or epithelial cells through ECM and have been noted in neuronal growth cones guiding axonal out- growth. Compositionally similar structures function in other cells as stabilizing architectural devices, for example, costameres in striated muscle, pedicle base- ment membrane attachments in renal podocytes and the sealing zones of osteoclasts. Only few reports are available on the IF organization in these different scenarios (e.g., [5–8,9  ,10]). But an emerging theme is that IFs are backbones of the respective structures ser- ving as counter bearings for local actomyosin-related forces. It is obvious that the structural diversity and functionality of the different FA types must be reflected by specialized molecular and organizational principles involving the cytoskeletal filament networks in a context-dependent fashion. The isotype-specific and cell Available online at www.sciencedirect.com ScienceDirect www.sciencedirect.com Current Opinion in Cell Biology 2015, 32:13–20