Rho GTPases are molecular switches that control a wide variety of signal transduction pathways in all eukaryotic cells. They are known principally for their pivotal role in regulating the actin cytoskeleton, but their ability to influence cell polarity, microtubule dynamics, membrane transport pathways and transcription factor activity is probably just as significant. Underlying this biological complexity is a simple biochemical idea, namely that by switching on a single GTPase, several distinct signalling pathways can be coordinately activated. With spatial and temporal activation of multiple switches factored in, it is not surprising to find Rho GTPases having such a prominent role in eukaryotic cell biology.

Rho GTPases in cell biology.

This review summarizes theoretical progress in the field of active matter, placing it in the context of recent experiments. This approach offers a unified framework for the mechanical and statistical properties of living matter: biofilaments and molecular motors in vitro or in vivo, collections of motile microorganisms, animal flocks, and chemical or mechanical imitations. A major goal of this review is to integrate several approaches proposed in the literature, from semimicroscopic to phenomenological. In particular, first considered are ``dry'' systems, defined as those where momentum is not conserved due to friction with a substrate or an embedding porous medium. The differences and similarities between two types of orientationally ordered states, the nematic and the polar, are clarified. Next, the active hydrodynamics of suspensions or ``wet'' systems is discussed and the relation with and difference from the dry case, as well as various large-scale instabilities of these nonequilibrium states of matter, are highlighted. Further highlighted are various large-scale instabilities of these nonequilibrium states of matter. Various semimicroscopic derivations of the continuum theory are discussed and connected, highlighting the unifying and generic nature of the continuum model. Throughout the review, the experimental relevance of these theories for describing bacterial swarms and suspensions, the cytoskeleton of living cells, and vibrated granular material is discussed. Promising extensions toward greater realism in specific contexts from cell biology to animal behavior are suggested, and remarks are given on some exotic active-matter analogs. Last, the outlook for a quantitative understanding of active matter, through the interplay of detailed theory with controlled experiments on simplified systems, with living or artificial constituents, is summarized.

Hydrodynamics of soft active matter

Recent progress in the design and application of artificial cellular microenvironments and nanoenvironments has revealed the extraordinary ability of cells to adjust their cytoskeletal organization, and hence their shape and motility, to minute changes in their immediate surroundings. Integrin-based adhesion complexes, which are tightly associated with the actin cytoskeleton, comprise the cellular machinery that recognizes not only the biochemical diversity of the extracellular neighbourhood, but also its physical and topographical characteristics, such as pliability, dimensionality and ligand spacing. Here, we discuss the mechanisms of such environmental sensing, based on the finely tuned crosstalk between the assembly of one type of integrin-based adhesion complex, namely focal adhesions, and the forces that are at work in the associated cytoskeletal network owing to actin polymerization and actomyosin contraction.

Environmental sensing through focal adhesions

Cell migration affects all morphogenetic processes and contributes to numerous diseases, including cancer and cardiovascular disease. For most cells in most environments, movement begins with protrusion of the cell membrane followed by the formation of new adhesions at the cell front that link the actin cytoskeleton to the substratum, generation of traction forces that move the cell forwards and disassembly of adhesions at the cell rear. Adhesion formation and disassembly drive the migration cycle by activating Rho GTPases, which in turn regulate actin polymerization and myosin II activity, and therefore adhesion dynamics.

Cell adhesion: integrating cytoskeletal dynamics and cellular tension

Endothelial cell migration is essential to angiogenesis. This motile process is directionally regulated by chemotactic, haptotactic, and mechanotactic stimuli and further involves degradation of the extracellular matrix to enable progression of the migrating cells. It requires the activation of several signaling pathways that converge on cytoskeletal remodeling. Then, it follows a series of events in which the endothelial cells extend, contract, and throw their rear toward the front and progress forward. The aim of this review is to give an integrative view of the signaling mechanisms that govern endothelial cell migration in the context of angiogenesis.

Endothelial Cell Migration During Angiogenesis

https://ressources.unisciel.fr/biocell/chap4/res/04_04_cell_migration_review_rottner.pdf

The lamellipodium: where motility begins

Phorbol ester induces actin cytoskeleton rearrangements in cultured vascular smooth muscle cells. Calponin and SM22 alpha are major components of differentiated smooth muscle and potential regulators of actin cytoskeleton interactions. Here we show that actin fibers decorated with h1 CaP remain stable, whereas SM22 alpha-decorated actin bundles undergo rapid reorganization into podosomes within 30 min of PDBu exposure. Ectopic expression of GFP alpha-actinin had no effect on the stability of the actin cytoskeleton and alpha-actinin was transported rapidly into PDBu-induced podosomes. Our results demonstrate the involvement of CaP and SM22 alpha in coordinating the balance between stabilization and dynamics of the actin cytoskeleton in mammalian smooth muscle. We provide evidence for the existence of two functionally distinct actin filament populations and introduce a molecular mechanism for the stabilization of the actin cytoskeleton by the unique actin-binding interface formed by calponin family-specific CLIK23 repeats.

Calponin repeats regulate actin filament stability and formation of podosomes in smooth muscle cells.

In a living organism, many cell types undergo extensive migration relative to their surrounding tissue to fulfill their biological function. Examples of this include fibroblast locomotion during wound healing, macrophage migration in response to a pathogen, and the extension of growth cones during neuronal organization, as well as in some important pathogenic states, for example, in the migration of metastatic cells (1). The crawling movement of metazoan cells results from coordinated changes in cell shape, orchestrated by a continuous remodeling of the actin cytoskeleton (2,3). Typically, migrating cells display a characteristic polarization with one or more broad, flat, ruffling lamellipodia extending in the direction of migration (the leading edge) and a retracting tail at the rear of the cell. Establishment of this cell shape, characteristic for a motile phenotype, requires a dynamic reorganization of the actin cytoskeleton, leading to the generation of forces necessary for cell body translocation (4) (Movie 1; Table 1). Extension of the leading edge is the first and most important step of cell motility. Abercrombie and colleagues (5,6) described the lamellipodium as the primary organelle of cell motility using time-lapse cinematography and electron microscopy. The lamellipodium is the motor where new actin filaments are generated. This was demonstrated in experiments performed by microinjecting fluorescent actin probes into fibroblasts and observing their dynamics during motility. Such experiments showed clearly that lamellipodia were the major “filament factory” of the cell (Movie 1; Table 1). The discovery of numerous regulatory proteins together with actin in the lamellipodium indicates that this organelle should not be considered solely as a polymerization machine but as a complex structure that is able to integrate signals (7). MICROSCOPY OF THE LAMELLIPODIUM

Shedding light and electrons on the lamellipodium: imaging the motor of crawling cells.

Publisher Summary
This chapter presents a list of various websites from where researchers can collect useful information related to their research work. Cell biologists can tap into the website of the World Federation for Culture Collections (WFCC). The WFCC is a federation concerned with the collection, authentication, maintenance, and distribution of cultures of microorganisms and cultured cells. The RIKEN cell bank has been organized as a unique, nonprofit public collection for deposit, isolation, preservation, and distribution of cultured animal cell lines produced by the life science research community. A website for the Antibody Resource Page provides a broad collection of links to companies that sell antibodies or antibody-related products on the internet. KabatMan site provides information on antibody structure and sequence as well as other useful links. Flow Cytometry on the Website is a collection of links for those interested in flow cytometry. A good starting point if one is looking for software protocols, instrumentation protocols, or any flow cytometry-related subject.

Emmanuel Vignal

Papers

The lamellipodium: where motility begins

Calponin repeats regulate actin filament stability and formation of podosomes in smooth muscle cells.

Shedding light and electrons on the lamellipodium: imaging the motor of crawling cells.

Chapter 54 – Resources