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E-cadherin is a calcium dependent cell adhesion molecule that is key to the organization of cells in the epithelial tissue. It is a multidomain, trans-membrane protein in which the extracellular domain forms the homotypic, adhesive interaction while the intracellular domain interacts with the actin cytoskeleton through the catenin family of adaptor proteins. A number of recent studies have provided novel insights into the mechanism of adhesion formation by this class of adhesion proteins. Here, we describe an updated view of the process of E-cadherin adhesion formation with an emphasis on the role of molecular mobility, clustering, and active cellular processes., (Copyright © 2017 Elsevier Inc. All rights reserved.)

Although myosin II filaments are known to exist in non-muscle cells, their dynamics and organization are incompletely understood. Here, we combined structured illumination microscopy with pharmacological and genetic perturbations, to study the process of actomyosin cytoskeleton self-organization into arcs and stress fibres. A striking feature of the myosin II filament organization was their 'registered' alignment into stacks, spanning up to several micrometres in the direction orthogonal to the parallel actin bundles. While turnover of individual myosin II filaments was fast (characteristic half-life time 60 s) and independent of actin filament turnover, the process of stack formation lasted a longer time (in the range of several minutes) and required myosin II contractility, as well as actin filament assembly/disassembly and crosslinking (dependent on formin Fmnl3, cofilin1 and α-actinin-4). Furthermore, myosin filament stack formation involved long-range movements of individual myosin filaments towards each other suggesting the existence of attractive forces between myosin II filaments. These forces, possibly transmitted via mechanical deformations of the intervening actin filament network, may in turn remodel the actomyosin cytoskeleton and drive its self-organization.

The ability to mechanically interact with the extracellular matrix is a fundamental feature of adherent eukaryotic cells. Cell-matrix adhesion in many cell types is mediated by protein complexes called focal adhesions (FAs). Recent progress in super resolution microscopy revealed FAs possess an internal organization, yet such methods do not enable observation of the formation and dynamics of their internal structure in living cells. Here, we combine structured illumination microscopy (SIM) with total internal reflection fluorescence microscopy (TIRF) to show that the proteins inside FA patches are distributed along elongated subunits, typically 300 ± 100 nm wide, separated by 400 ± 100 nm, and individually connected to actin cables. We further show that the formation and dynamics of these linear subunits are intimately linked to radial actin fiber formation and actomyosin contractility. We found FA growth to be the result of nucleation of new linear subunits and their coordinated elongation. Taken together, this study reveals that the basic units of mature focal adhesion are 300-nm-wide elongated, dynamic structures. We anticipate this ultrastructure to be relevant to investigation of the function of FAs and their behavior in response to mechanical stress., (© 2015 Wiley Periodicals, Inc.)

Epithelial (E)-cadherin-mediated cell-cell junctions play important roles in the development and maintenance of tissue structure in multicellular organisms. E-cadherin adhesion is thus a key element of the cellular microenvironment that provides both mechanical and biochemical signaling inputs. Here, we report in vitro reconstitution of junction-like structures between native E-cadherin in living cells and the extracellular domain of E-cadherin (E-cad-ECD) in a supported membrane. Junction formation in this hybrid live cell-supported membrane configuration requires both active processes within the living cell and a supported membrane with low E-cad-ECD mobility. The hybrid junctions recruit α-catenin and exhibit remodeled cortical actin. Observations suggest that the initial stages of junction formation in this hybrid system depend on the trans but not the cis interactions between E-cadherin molecules, and proceed via a nucleation process in which protrusion and retraction of filopodia play a key role.

Actin dynamics play an important role in tissue morphogenesis, yet the control of actin filament growth takes place at the molecular level. A challenge in the field is to link the molecular function of actin regulators with their physiological function. Here, we report an in vivo role of the actin-capping protein CAP-1 in the Caenorhabditis elegans germline. We show that CAP-1 is associated with actomyosin structures in the cortex and rachis, and its depletion or overexpression led to severe structural defects in the syncytial germline and oocytes. A 60% reduction in the level of CAP-1 caused a twofold increase in F-actin and non-muscle myosin II activity, and laser incision experiments revealed an increase in rachis contractility. Cytosim simulations pointed to increased myosin as the main driver of increased contractility following loss of actin-capping protein. Double depletion of CAP-1 and myosin or Rho kinase demonstrated that the rachis architecture defects associated with CAP-1 depletion require contractility of the rachis actomyosin corset. Thus, we uncovered a physiological role for actin-capping protein in regulating actomyosin contractility to maintain reproductive tissue architecture. [ABSTRACT FROM AUTHOR]

Abstract Animal cells exhibit a wide range of shapes that reflect their diverse functions. Cell shape is determined by a balance between internal and external forces and therefore involves the cytoskeleton and its associated adhesion structures. Cell shape dynamics during development and homeostasis are tightly regulated and closely coordinated with cell fate determination. Defects in cell shape are a hallmark of many pathological conditions including cancer and skin diseases. This review highlights the links between cell shape and cell fate in the epidermis, which have been studied for over 40 years both in vitro and in vivo. Briefly discussing seminal experiments showing the strong coupling between keratinocyte cell shape and their fate we primarily focus on recent studies uncovering novel cellular and molecular mechanisms linking epidermal cell shape with cell growth, differentiation, asymmetric division, and delamination. [ABSTRACT FROM AUTHOR]

The actomyosin cytoskeleton is responsible for most force-driven processes in cells and tissues. How it assembles into the necessary structures at the right time and place is an important question. Here, we focus on molecular mechanisms of actomyosin regulation recently elucidated in animal models, and highlight several common principles that emerge. The architecture of the actomyosin network – an important determinant of its function – results from actin polymerization, crosslinking and turnover, localized myosin activation, and contractility-driven self-organization. Spatiotemporal regulation is achieved by tissue-specific expression and subcellular localization of Rho GTPase regulators. Subcellular anchor points of actomyosin structures control the outcome of their contraction, and molecular feedback mechanisms dictate whether they are transient, cyclic, or persistent. Highlights Actomyosin contractility, which shapes cells and tissues during development, wound repair, and physiology, is regulated by the actin network architecture, connectivity and turnover, as well as myosin-II dependent self-organization. Spatiotemporal regulation of contractility results from localized and timely activation of Rho signaling downstream of cellular polarity regulators, or membrane receptors and tissue-specific transcription factors. Anchoring of actomyosin structures at cell–cell and cell–extracellular matrix adhesions is important for force transmission, as well as regulation of contractility. Mechanical feedback from adhesions on actomyosin regulates developmental processes. Feedback loops generating cycles of Rho activation and inactivation are responsible for pulsatile contractile behaviors that are prevalent in animal development and physiology. [ABSTRACT FROM AUTHOR]

The article offers information on the adherens junctions (AJ) cadherins located on apposing cells from outside and anchored through adaptor proteins to the actomyosin cytoskeleton from inside. It mentions that AJ are formed by adhesion between the cells. It discusses that this adhesion is mediated by a twofold symmetric interaction between the first extracellular (EC1) domains of two cadherins on juxtapositioned cells.

Cell biologists studying cell adhesion have already figured out that cell–extracellular matrix connections, mediated by integrin receptors, are diverse and extremely complex structures. Dozens of adaptors—linking integrins with the cytoskeleton, and numerous enzymes and signaling proteins—regulating adhesion site dynamics, collectively referred to as the integrin adhesome, cooperate in mediating adhesion and activating specific signaling networks. Recent proteomic studies indicate that the known adhesome complexity is just the tip of the iceberg. In each existing category of molecular function the number of candidate components more than double the known components and several new categories are suggested. Proteomic analysis of different integrin heterodimers points to integrin-specific variations in composition and analysis of adhesion complexes under varying tension regimes highlights the force-dependent recruitment of different components, most notably LIM domain proteins. [ABSTRACT FROM AUTHOR]

Adherens junctions connect the actin cytoskeleton of neighboring cells through transmembrane cadherin receptors and a network of adaptor proteins. The interactions between these adaptors and cadherin as well as the activity of actin regulators localized to adherens junctions are tightly controlled to facilitate cell junction assembly or disassembly in response to changes in external or internal forces and/or signaling. Phosphorylation of tyrosine, serine, or threonine residues acts as a switch on the majority of adherens junction proteins, turning "on" or "off" their interactions with other proteins and/or their enzymatic activity. Here, we provide an overview of the kinases and phosphatases regulating phosphorylation of adherens junction proteins and bring examples of phosphorylation events leading to the assembly or disassembly of adherens junctions, highlighting the important role of phosphorylation switches in regulating their dynamics. [ABSTRACT FROM AUTHOR]

Proper distribution of organelles can play an important role in a moving cell's performance. During C. elegans gonad morphogenesis, the nucleus of the leading distal tip cell (DTC) is always found at the front, yet the significance of this localization is unknown. Here, we identified the molecular mechanism that keeps the nucleus at the front, despite a frictional force that pushes it backward. The Klarsicht/ANC-1/Syne homology (KASH) domain protein UNC-83 links the nucleus to the motor protein kinesin-1 that moves along a polarized acentrosomal microtubule network. Interestingly, disrupting nuclear positioning on its own did not affect gonad morphogenesis. However, reducing actomyosin contractility on top of nuclear mispositioning led to a dramatic phenotype: DTC splitting and gonad bifurcation. Long-term live imaging of the double knockdown revealed that, while the gonad attempted to perform a planned U-turn, the DTC was stretched due to the lagging nucleus until it fragmented into a nucleated cell and an enucleated cytoplast, each leading an independent gonadal arm. Remarkably, the enucleated cytoplast had polarity and invaded, but it could only temporarily support germ cell proliferation. Based on a qualitative biophysical model, we conclude that the leader cell employs two complementary mechanical approaches to preserve its integrity and ensure proper organ morphogenesis while navigating through a complex 3D environment: active nuclear positioning by microtubule motors and actomyosin-driven cortical contractility. [Display omitted] • LINC complex protein UNC-83 and kinesin motor position nucleus at the DTC forefront • Misplaced nucleus and low contractility cause DTC deformation during gonadal turn • Excessive strain during the turn leads to DTC fragmentation and gonad bifurcation • Enucleated DTC can invade but does not retain stem cell niche function for long Agarwal et al. combine cell-specific knockdown, long-term imaging, and physical modeling to show that the leader distal tip cell (DTC) employs two complementary mechanical strategies, active nuclear positioning by microtubule motors and actomyosin-driven cortical contractility, to prevent its deformation and ensure normal C. elegans gonadogenesis. [ABSTRACT FROM AUTHOR]

Adherens junctions, broadly defined as attachment sites in which cadherin adhesion receptors connect the actin cytoskeletons of neighboring animal cells, are multi-tasking by nature. In addition to mediating cell–cell adhesion and providing the tissue with mechanical continuity and barrier function, they maintain polarity, are sites of mechanosensing and signaling, and they regulate actomyosin dynamics and can thus generate forces to drive morphogenesis. Here we propose that the key to performing such diverse tasks is the integration within the cadherin adhesome of functional modules that evolved independently to perform other duties within the cell, and we discuss three such functional modules: force transmission, actin dynamics regulation, and contractile force generation. We compare each module to a more ancient cellular structure with similar function, identify shared components, and speculate on how the module was integrated into the cadherin adhesome. [ABSTRACT FROM AUTHOR]