Filamin depletion blocks endoplasmic spreading and destabilizes force-bearing adhesions

Mechanobiology of organelles: illuminating their roles in mechanosensing and mechanotransduction

Mechanobiology studies the mechanisms by which cells sense and respond to physical forces, and the role of these forces in shaping cells and tissues themselves. Mechanosensing can occur at the plasma membrane, which is directly exposed to external forces, but also in the cell's interior, for example, through deformation of the nucleus. Less is known on how the function and morphology of organelles are influenced by alterations in their own mechanical properties, or by external forces. Here, we discuss recent advances on the mechanosensing and mechanotransduction of organelles, including the endoplasmic reticulum (ER), the Golgi apparatus, the endo-lysosmal system, and the mitochondria. We highlight open questions that need to be addressed to gain a broader understanding of the role of organelle mechanobiology.

Intracellular remodeling associated with endoplasmic reticulum stress modifies biomechanical compliance of bladder cells

Bladder cells face a challenging biophysical environment: mechanical cues originating from urine flow and regular contraction to enable the filling voiding of the organ. To ensure functional adaption, bladder cells rely on high biomechanical compliance, nevertheless aging or chronic pathological conditions can modify this plasticity. Obviously the cytoskeletal network plays an essential role, however the contribution of other, closely entangled, intracellular organelles is currently underappreciated. The endoplasmic reticulum (ER) lies at a crucial crossroads, connected to both nucleus and cytoskeleton. Yet, its role in the maintenance of cell mechanical stability is less investigated. To start exploring these aspects, T24 bladder cancer cells were treated with the ER stress inducers brefeldin A (10-40nM BFA, 24 h) and thapsigargin (0.1-100nM TG, 24 h). Without impairment of cell motility and viability, BFA and TG triggered a significant subcellular redistribution of the ER; this was associated with a rearrangement of actin cytoskeleton. Additional inhibition of actin polymerization with cytochalasin D (100nM CytD) contributed to the spread of the ER toward cell periphery, and was accompanied by an increase of cellular stiffness (Young´s modulus) in the cytoplasmic compartment. Shrinking of the ER toward the nucleus (100nM TG, 2 h) was related to an increased stiffness in the nuclear and perinuclear areas. A similar short-term response profile was observed also in normal human primary bladder fibroblasts. In sum, the ER and its subcellular rearrangement seem to contribute to the mechanical properties of bladder cells opening new perspectives in the study of the related stress signaling cascades. Video Abstract.

A mechanism of platelet integrin αIIbβ3 outside-in signaling through a novel integrin αIIb subunit-filamin-actin linkage

The communication of talin-activated integrin αIIbβ3 with the cytoskeleton (integrin outside-in signaling) is essential for platelet aggregation, wound healing, and hemostasis. Filamin, a large actin crosslinker and integrin binding partner critical for cell spreading and migration, is implicated as a key regulator of integrin outside-in signaling. However, the current dogma is that filamin, which stabilizes inactive αIIbβ3, is displaced from αIIbβ3 by talin to promote the integrin activation (inside-out signaling), and how filamin further functions remains unresolved. Here, we show that while associating with the inactive αIIbβ3, filamin also associates with the talin-bound active αIIbβ3 to mediate platelet spreading. Fluorescence resonance energy transfer-based analysis reveals that while associating with both αIIb and β3 cytoplasmic tails (CTs) to maintain the inactive αIIbβ3, filamin is spatiotemporally rearranged to associate with αIIb CT alone on activated αIIbβ3. Consistently, confocal cell imaging indicates that integrin α CT-linked filamin gradually delocalizes from the β CT-linked focal adhesion marker-vinculin likely because of the separation of integrin α/β CTs occurring during integrin activation. High-resolution crystal and nuclear magnetic resonance structure determinations unravel that the activated integrin αIIb CT binds to filamin via a striking α-helix→β-strand transition with a strengthened affinity that is dependent on the integrin-activating membrane environment containing enriched phosphatidylinositol 4,5-bisphosphate. These data suggest a novel integrin αIIb CT-filamin-actin linkage that promotes integrin outside-in signaling. Consistently, disruption of such linkage impairs the activation state of αIIbβ3, phosphorylation of focal adhesion kinase/proto-oncogene tyrosine kinase Src, and cell migration. Together, our findings advance the fundamental understanding of integrin outside-in signaling with broad implications in blood physiology and pathology.

Cell-Dependent Pathogenic Roles of Filamin B in Different Skeletal Malformations

Mutations of filamin B (FLNB) gene can lead to a spectrum of autosomal skeletal malformations including spondylocarpotarsal syndrome (SCT), Larsen syndrome (LRS), type I atelosteogenesis (AO1), type III atelosteogenesis (AO3), and boomerang dysplasia (BD). Among them, LRS is milder while BD causes a more severe phenotype. However, the molecular mechanism underlying the differences in clinical phenotypes of different FLNB variants has not been fully determined. Here, we presented two patients suffering from autosomal dominant LRS and autosomal recessive vitamin D-dependent rickets type IA (VDDR-IA). Whole-exome sequencing revealed two novel missense variants in FLNB, c.4846A>G (p.T1616A) and c.7022T>G (p.I2341R), which are located in repeat 15 and 22 of filamin B, respectively. The expression of FLNB^I2341R in the muscle tissue from our LRS patient was remarkably increased. And in vitro studies showed that both variants led to a lack of filopodia and accumulation of the mutants in the perinuclear region in HEK293 cells. We also found that c.4846A>G (p.T1616A) and c.7022T>G (p.I2341R) regulated endochondral osteogenesis in different ways. c.4846A>G (p.T1616A) activated AKT pathways through inhibiting SHIP2, suppressed the Smad3 pathway, and impaired the expression of Runx2 in both Saos-2 and ATDC5 cells. c.7022T>G (p.I2341R) activated both AKT and Smad3 pathways and increased the expression of Runx2 in Saos-2 cells, while in ATDC5 cells it activated AKT pathways through inhibiting SHIP2, suppressed the Smad3 pathway, and reduced the expression of Runx2. Our study demonstrated the pathogenic mechanisms of two novel FLNB variants in two different clinical settings and proved that FLNB variants could not only directly cause skeletal malformations but also worsen skeletal symptoms in the setting of other skeletal diseases. Besides, FLNB variants differentially affect skeletal development which contributes to clinical heterogeneity of FLNB-related disorders.

Intertwined and Finely Balanced: Endoplasmic Reticulum Morphology, Dynamics, Function, and Diseases

The endoplasmic reticulum (ER) is an organelle that is responsible for many essential subcellular processes. Interconnected narrow tubules at the periphery and thicker sheet-like regions in the perinuclear region are linked to the nuclear envelope. It is becoming apparent that the complex morphology and dynamics of the ER are linked to its function. Mutations in the proteins involved in regulating ER structure and movement are implicated in many diseases including neurodegenerative diseases such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis (ALS). The ER is also hijacked by pathogens to promote their replication. Bacteria such as Legionella pneumophila and Chlamydia trachomatis, as well as the Zika virus, bind to ER morphology and dynamics-regulating proteins to exploit the functions of the ER to their advantage. This review covers our understanding of ER morphology, including the functional subdomains and membrane contact sites that the organelle forms. We also focus on ER dynamics and the current efforts to quantify ER motion and discuss the diseases related to ER morphology and dynamics.

Network organisation and the dynamics of tubules in the endoplasmic reticulum

The endoplasmic reticulum (ER) is a eukaryotic subcellular organelle composed of tubules and sheet-like areas of membrane connected at junctions. The tubule network is highly dynamic and undergoes rapid and continual rearrangement. There are currently few tools to evaluate network organisation and dynamics. We quantified ER network organisation in Vero and MRC5 cells, and developed an analysis workflow for dynamics of established tubules in live cells. The persistence length, tubule length, junction coordination number and angles of the network were quantified. Hallmarks of imbalances in ER tension, indications of interactions with microtubules and other subcellular organelles, and active dynamics were observed. Clear differences in dynamic behaviour were observed for established tubules at different positions within the cell using itemset mining. We found that tubules with activity-driven fluctuations were more likely to be located away from the cell periphery and a population of peripheral tubules with no signs of active motion was found.

Probing mechanobiological role of filamin A in migration and invasion of human U87 glioblastoma cells using submicron soft pillars

Filamin A (FLNa) belongs to an actin-binding protein family in binding and cross-linking actin filaments into a three-dimensional structure. However, little attention has been given to its mechanobiological role in cancer cells. Here, we quantitatively investigated the role of FLNa by analyzing the following parameters in negative control (NC) and FLNa-knockdown (KD) U87 glioma cells using submicron pillars (900 nm diameter and 2 μm height): traction force (TF), rigidity sensing ability, cell aspect ratio, migration speed, and invasiveness. During the initial phase of cell adhesion (< 1 h), FLNa-KD cells polarized more slowly than did NC cells, which can be explained by the loss of rigidity sensing in FLNa-KD cells. The higher motility of FLNa-KD cells relative to NC cells can be explained by the high TF exerted by FLNa-KD cells when compared to NC cells, while the higher invasiveness of FLNa-KD cells relative to NC cells can be explained by a greater number of filopodia in FLNa-KD cells than in NC cells. Our results suggest that FLNa plays important roles in suppressing motility and invasiveness of U87 cells.

Endoplasmic Reticulum Adaptation and Autophagic Competence Shape Response to Fluid Shear Stress in T24 Bladder Cancer Cells

Accumulation of xenobiotics and waste metabolites in the urinary bladder is constantly accompanied by shear stress originating from the movement of the luminal fluids. Hence, both chemical and physical cues constantly modulate the cellular response in health and disease. In line, bladder cells have to maintain elevated mechanosensory competence together with chemical stress response adaptation potential. However, much of the molecular mechanisms sustaining this plasticity is currently unknown. Taking this as a starting point, we investigated the response of T24 urinary bladder cancer cells to shear stress comparing morphology to functional performance. T24 cells responded to the shear stress protocol (flow speed of 0.03 ml/min, 3 h) by significantly increasing their surface area. When exposed to deoxynivalenol-3-sulfate (DON-3-Sulf), bladder cells increased this response in a concentration-dependent manner (0.1-1 µM). DON-3-Sulf is a urinary metabolite of a very common food contaminant mycotoxin (deoxynivalenol, DON) and was already described to enhance proliferation of cancer cells. Incubation with DON-3-Sulf also caused the enlargement of the endoplasmic reticulum (ER), decreased the lysosomal movement, and increased the formation of actin stress fibers. Similar remodeling of the endoplasmic reticulum and area spread after shear stress were observed upon incubation with the autophagy activator rapamycin (1-100 nM). Performance of experiments in the presence of chloroquine (chloroquine, 30 μM) further contributed to shed light on the mechanistic link between adaptation to the biomechanical stimulation and ER stress response. At the molecular level, we observed that ER reshaping was linked to actin organization, with the two components mutually regulating each other. Indeed, we identified in the ER stress-cytoskeletal rearrangement an important axis defining the physical/chemical response potential of bladder cells and created a workflow for further investigation of urinary metabolites, food constituents, and contaminants, as well as for pharmacological profiling.

RACK1 plays a critical role in mast cell secretion and Ca2+ mobilization by modulating F-actin dynamics

Although RACK1 is known to act as a signaling hub in immune cells, its presence and role in mast cells (MCs) is undetermined. MC activation via antigen stimulation results in mediator release and is preceded by cytoskeleton reorganization and Ca2+ mobilization. In this study, we found that RACK1 was distributed throughout the MC cytoplasm both in vivo and in vitro. After RACK1 knockdown (KD), MCs were rounded, and the cortical F-actin was fragmented. Following antigen stimulation, in RACK1 KD MCs, there was a reduction in cortical F-actin, an increase in monomeric G-actin and a failure to organize F-actin. RACK1 KD also increased and accelerated degranulation. CD63+ secretory granules were localized in F-actin-free cortical regions in non-stimulated RACK1 KD MCs. Additionally, RACK1 KD increased antigen-stimulated Ca2+ mobilization, but attenuated antigen-stimulated depletion of ER Ca2+ stores and thapsigargin-induced Ca2+ entry. Following MC activation there was also an increase in interaction of RACK1 with Orai1 Ca2+-channels, β-actin and the actin-binding proteins vinculin and MyoVa. These results show that RACK1 is a critical regulator of actin dynamics, affecting mediator secretion and Ca2+ signaling in MCs. This article has an associated First Person interview with the first author of the paper.

Remodelling of membrane tubules by the actin cytoskeleton

Inside living cells, the remodelling of membrane tubules by actomyosin networks is crucial for processes such as intracellular trafficking or organelle reshaping. In this review, we first present various in vivo situations in which actin affects membrane tubule remodelling, then we recall some results on force production by actin dynamics and on membrane tubules physics. Finally, we show that our knowledge of the underlying mechanisms by which actomyosin dynamics affect tubule morphology has recently been moved forward. This is thanks to in vitro experiments that mimic cellular membranes and actin dynamics and allow deciphering the physics of tubule remodelling in biochemically controlled conditions, and shed new light on tubule shape regulation.

Molecular Tuning of Filamin A Activities in the Context of Adhesion and Migration

The dynamic organization of actin cytoskeleton meshworks relies on multiple actin-binding proteins endowed with distinct actin-remodeling activities. Filamin A is a large multi-domain scaffolding protein that cross-links actin filaments with orthogonal orientation in response to various stimuli. As such it plays key roles in the modulation of cell shape, cell motility, and differentiation throughout development and adult life. The essentiality and complexity of Filamin A is highlighted by mutations that lead to a variety of severe human disorders affecting multiple organs. One of the most conserved activity of Filamin A is to bridge the actin cytoskeleton to integrins, thereby maintaining the later in an inactive state. We here review the numerous mechanisms cells have developed to adjust Filamin A content and activity and focus on the function of Filamin A as a gatekeeper to integrin activation and associated adhesion and motility.

Noncanonical function of long myosin light chain kinase in increasing ER-PM junctions and augmentation of SOCE

Increased endothelial permeability leads to excessive exudation of plasma proteins and leukocytes in the interstitium, which characterizes several vascular diseases including acute lung injury. The myosin light chain kinase long (MYLK-L) isoform is canonically known to regulate the endothelial permeability by phosphorylating myosin light chain (MLC-P). Compared to the short MYLK isoform, MYLK-L contains an additional stretch of ~919 amino acid at the N-terminus of unknown function. We show that thapsigargin and thrombin-induced SOCE was markedly reduced in Mylk-L-/- endothelial cells (EC) or MYLK-L-depleted human EC. These agonists also failed to increase endothelial permeability in MYLK-L-depleted EC and Mylk-L-/- lungs, thus demonstrating the novel role of MYLK-L-induced SOCE in increasing vascular permeability. MYLK-L augmented SOCE by increasing endoplasmic reticulum (ER)-plasma membrane (PM) junctions and STIM1 translocation to these junctions. Transduction of N-MYLK domain (amino acids 1-919 devoid of catalytic activity) into Mylk-L-/- EC rescued SOCE to the level seen in control EC in a STIM1-dependent manner. N-MYLK-induced SOCE augmented endothelial permeability without MLC-P via an actin-binding motif, DVRGLL. Liposomal-mediated delivery of N-MYLK mutant but not ∆DVRGLL-N-MYLK mutant in Mylk-L-/- mice rescued vascular permeability increase in response to endotoxin, indicating that targeting of DVRGLL motif within MYLK-L may limit SOCE-induced vascular hyperpermeability.

Loss of tumor susceptibility gene 101 (TSG101) perturbs endoplasmic reticulum structure and function

Tumor susceptibility gene 101 (TSG101), an ESCRT-I protein, is implicated in multiple cellular processes and its functional depletion can lead to blocked lysosomal degradation, cell cycle arrest, demyelination and neurodegeneration. Here, we show that loss of TSG101 results in endoplasmic reticulum (ER) stress and this causes ER membrane remodelling (EMR). This correlates with an expansion of ER, increased vacuolation, altered relative distribution of the rough and smooth ER and disruption of three-way junctions. Blocked lysosomal degradation due to TSG101 depletion leads to ER stress and Ca²⁺ leakage from ER stores, causing destabilization of actin cytoskeleton. Inhibiting Ca²⁺ release from the ER by blocking ryanodine receptors (RYRs) with Dantrolene partially rescues the ER stress phenotypes. Hence, in this study we have identified the involvement of TSG101 in modulating ER stress mediated remodelling by engaging the actin cytoskeleton. This is significant because functional depletion of TSG101 effectuates ER-stress, perturbs the structure, mobility and function of the ER, all aspects closely associated with neurodegenerative diseases. SUMMARY STATEMENT: We show that tumor susceptibility gene (TSG) 101 regulates endoplasmic reticulum (ER) stress and its membrane remodelling. Loss of TSG101 perturbs structure, mobility and function of the ER as a consequence of actin destabilization.

Long-Range and Directional Allostery of Actin Filaments Plays Important Roles in Various Cellular Activities

A wide variety of uniquely localized actin-binding proteins (ABPs) are involved in various cellular activities, such as cytokinesis, migration, adhesion, morphogenesis, and intracellular transport. In a micrometer-scale space such as the inside of cells, protein molecules diffuse throughout the cell interior within seconds. In this condition, how can ABPs selectively bind to particular actin filaments when there is an abundance of actin filaments in the cytoplasm? In recent years, several ABPs have been reported to induce cooperative conformational changes to actin filaments allowing structural changes to propagate along the filament cables uni- or bidirectionally, thereby regulating the subsequent binding of ABPs. Such propagation of ABP-induced cooperative conformational changes in actin filaments may be advantageous for the elaborate regulation of cellular activities driven by actin-based machineries in the intracellular space, which is dominated by diffusion. In this review, we focus on long-range allosteric regulation driven by cooperative conformational changes of actin filaments that are evoked by binding of ABPs, and discuss roles of allostery of actin filaments in narrow intracellular spaces.

Costameres, dense plaques and podosomes: the cell matrix adhesions in cardiovascular mechanosensing

The stiffness of the cardiovascular environment changes during ageing and in disease and contributes to disease incidence and progression. For instance, increased arterial stiffness can lead to atherosclerosis, while stiffening of the heart due to fibrosis can increase the chances of heart failure. Cells can sense the stiffness of the extracellular matrix through integrin adhesions and other mechanosensitive structures and in response to this initiate mechanosignalling pathways that ultimately change the cellular behaviour. Over the past decades, interest in mechanobiology has steadily increased and with this also our understanding of the molecular basis of mechanosensing and transduction. However, much of our knowledge about the mechanisms is derived from studies investigating focal adhesions in non-muscle cells, which are distinct in several regards from the cell-matrix adhesions in cardiomyocytes (costameres) or vascular smooth muscle cells (dense plaques or podosomes). Therefore, we will look here first at the evidence for mechanical sensing in the cardiovascular system, before comparing the different cytoskeletal arrangements and adhesion sites in cardiomyocytes and vascular smooth muscle cells and what is known about mechanical sensing through the various structures.

The matrix environmental and cell mechanical properties regulate cell migration and contribute to the invasive phenotype of cancer cells

The minimal structural unit of a solid tumor is a single cell or a cellular compartment such as the nucleus. A closer look inside the cells reveals that there are functional compartments or even structural domains determining the overall properties of a cell such as the mechanical phenotype. The mechanical interaction of these living cells leads to the complex organization such as compartments, tissues and organs of organisms including mammals. In contrast to passive non-living materials, living cells actively respond to the mechanical perturbations occurring in their microenvironment during diseases such as fibrosis and cancer. The transformation of single cancer cells in highly aggressive and hence malignant cancer cells during malignant cancer progression encompasses the basement membrane crossing, the invasion of connective tissue, the stroma microenvironments and transbarrier migration, which all require the immediate interaction of the aggressive and invasive cancer cells with the surrounding extracellular matrix environment including normal embedded neighboring cells. All these steps of the metastatic pathway seem to involve mechanical interactions between cancer cells and their microenvironment. The pathology of cancer due to a broad heterogeneity of cancer types is still not fully understood. Hence it is necessary to reveal the signaling pathways such as mechanotransduction pathways that seem to be commonly involved in the development and establishment of the metastatic and mechanical phenotype in several carcinoma cells. We still do not know whether there exist distinct metastatic genes regulating the progression of tumors. These metastatic genes may then be activated either during the progression of cancer by themselves on their migration path or in earlier stages of oncogenesis through activated oncogenes or inactivated tumor suppressor genes, both of which promote the metastatic phenotype. In more detail, the adhesion of cancer cells to their surrounding stroma induces the generation of intracellular contraction forces that deform their microenvironments by alignment of fibers. The amplitude of these forces can adapt to the mechanical properties of the microenvironment. Moreover, the adhesion strength of cancer cells seems to determine whether a cancer cell is able to migrate through connective tissue or across barriers such as the basement membrane or endothelial cell linings of blood or lymph vessels in order to metastasize. In turn, exposure of adherent cancer cells to physical forces, such as shear flow in vessels or compression forces around tumors, reinforces cell adhesion, regulates cell contractility and restructures the ordering of the local stroma matrix that leads subsequently to secretion of crosslinking proteins or matrix degrading enzymes. Hence invasive cancer cells alter the mechanical properties of their microenvironment. From a mechanobiological point-of-view, the recognized physical signals are transduced into biochemical signaling events that guide cellular responses such as cancer progression after the malignant transition of cancer cells from an epithelial and non-motile phenotype to a mesenchymal and motile (invasive) phenotype providing cellular motility. This transition can also be described as the physical attempt to relate this cancer cell transitional behavior to a T1 phase transition such as the jamming to unjamming transition. During the invasion of cancer cells, cell adaptation occurs to mechanical alterations of the local stroma, such as enhanced stroma upon fibrosis, and therefore we need to uncover underlying mechano-coupling and mechano-regulating functional processes that reinforce the invasion of cancer cells. Moreover, these mechanisms may also be responsible for the awakening of dormant residual cancer cells within the microenvironment. Physicists were initially tempted to consider the steps of the cancer metastasis cascade as single events caused by a single mechanical alteration of the overall properties of the cancer cell. However, this general and simple view has been challenged by the finding that several mechanical properties of cancer cells and their microenvironment influence each other and continuously contribute to tumor growth and cancer progression. In addition, basement membrane crossing, cell invasion and transbarrier migration during cancer progression is explained in physical terms by applying physical principles on living cells regardless of their complexity and individual differences of cancer types. As a novel approach, the impact of the individual microenvironment surrounding cancer cells is also included. Moreover, new theories and models are still needed to understand why certain cancers are malignant and aggressive, while others stay still benign. However, due to the broad variety of cancer types, there may be various pathways solely suitable for specific cancer types and distinct steps in the process of cancer progression. In this review, physical concepts and hypotheses of cancer initiation and progression including cancer cell basement membrane crossing, invasion and transbarrier migration are presented and discussed from a biophysical point-of-view. In addition, the crosstalk between cancer cells and a chronically altered microenvironment, such as fibrosis, is discussed including the basic physical concepts of fibrosis and the cellular responses to mechanical stress caused by the mechanically altered microenvironment. Here, is highlighted how biophysical approaches, both experimentally and theoretically, have an impact on classical hallmarks of cancer and fibrosis and how they contribute to the understanding of the regulation of cancer and its progression by sensing and responding to the physical environmental properties through mechanotransduction processes. Finally, this review discusses various physical models of cell migration such as blebbing, nuclear piston, protrusive force and unjamming transition migration modes and how they contribute to cancer progression. Moreover, these cellular migration modes are influenced by microenvironmental perturbances such as fibrosis that can induce mechanical alterations in cancer cells, which in turn may impact the environment. Hence, the classical hallmarks of cancer need to be refined by including biomechanical properties of cells, cell clusters and tissues and their microenvironment to understand mechano-regulatory processes within cancer cells and the entire organism.

Unfolded Protein Response-Dependent Communication and Contact among Endoplasmic Reticulum, Mitochondria, and Plasma Membrane

The function of the endoplasmic reticulum (ER) can be impaired by changes to the extra- and intracellular environment, such as disruption of calcium homeostasis, expression of mutated proteins, and oxidative stress. In response to disruptions to ER homeostasis, eukaryotic cells activate canonical branches of signal transduction cascades, collectively termed the unfolded protein response (UPR). The UPR functions to remove or recover the activity of misfolded proteins that accumulated in the ER and to avoid irreversible cellular damage. Additionally, the UPR plays unique physiological roles in the regulation of diverse cellular events, including cell differentiation and development and lipid biosynthesis. Recent studies have shown that these important cellular events are also regulated by contact and communication among organelles. These reports suggest strong involvement among the UPR, organelle communication, and regulation of cellular homeostasis. However, the precise mechanisms for the formation of contact sites and the regulation of ER dynamics by the UPR remain unresolved. In this review, we summarize the current understanding of how the UPR regulates morphological changes to the ER and the formation of contact sites between the ER and other organelles. We also review how UPR-dependent connections between the ER and other organelles affect cellular and physiological functions.

Filamin A Is Required for Optimal T Cell Integrin-Mediated Force Transmission, Flow Adhesion, and T Cell Trafficking

T cells traffic from the bloodstream into tissues to perform their functions in the immune system and are therefore subjected to a range of different mechanical forces. Integrins are essential for T cell trafficking into the tissues, as they mediate firm adhesion between the T cell and the endothelium under shear flow conditions. In addition, integrins are important for the formation of the contact between the T cell and the APC required for T cell activation. The actin-binding protein filamin A (FlnA) provides an important link between the integrin and the actin cytoskeleton. FlnA has been reported to function as an integrin inhibitor by competing with talin. However, its role in regulating integrin-dependent immune functions in vivo is currently poorly understood. In this study, we have investigated the role of FlnA in T cells, using T cell-specific FlnA knockout mice. We report that FlnA is required for the formation of strong integrin-ligand bonds under shear flow and for the generation of integrin-mediated T cell traction forces on ligand-coated hydrogels. Consequently, absence of FlnA leads to a reduction in T cell adhesion to integrin ligands under conditions of shear flow, as well as reduced T cell trafficking into lymph nodes and sites of skin inflammation. In addition, FlnA is not needed for T cell activation in vivo, which occurs in shear-free conditions in lymphoid organs. Our results therefore reveal a role of FlnA in integrin force transmission and T cell trafficking in vivo.

Asb2α-Filamin A Axis Is Essential for Actin Cytoskeleton Remodeling During Heart Development

RATIONALE

Heart development involves differentiation of cardiac progenitors and assembly of the contractile sarcomere apparatus of cardiomyocytes. However, little is known about the mechanisms that regulate actin cytoskeleton remodeling during cardiac cell differentiation.

OBJECTIVE

The Asb2α (Ankyrin repeat-containing protein with a suppressor of cytokine signaling box 2) CRL5 (cullin 5 RING E3 ubiquitin ligase) triggers polyubiquitylation and subsequent degradation by the proteasome of FLNs (filamins). Here, we investigate the role of Asb2α in heart development and its mechanisms of action.

METHODS AND RESULTS

Using Asb2 knockout embryos, we show that Asb2 is an essential gene, critical to heart morphogenesis and function, although its loss does not interfere with the overall patterning of the embryonic heart tube. We show that the Asb2α E3 ubiquitin ligase controls Flna stability in immature cardiomyocytes. Importantly, Asb2α-mediated degradation of the actin-binding protein Flna marks a previously unrecognized intermediate step in cardiac cell differentiation characterized by cell shape changes and actin cytoskeleton remodeling. We further establish that in the absence of Asb2α, myofibrils are disorganized and that heartbeats are inefficient, leading to embryonic lethality in mice.

CONCLUSIONS

These findings identify Asb2α as an unsuspected key regulator of cardiac cell differentiation and shed light on the molecular and cellular mechanisms determining the onset of myocardial cell architecture and its link with early cardiac function. Although Flna is known to play roles in cytoskeleton organization and to be required for heart function, this study now reveals that its degradation mediated by Asb2α ensures essential functions in differentiating cardiac progenitors.

Filamin A Regulates Neutrophil Adhesion, Production of Reactive Oxygen Species, and Neutrophil Extracellular Trap Release

Neutrophils are of fundamental importance in the early immune response and use various mechanisms to neutralize invading pathogens. They kill endocytosed pathogens by releasing reactive oxygen species in the phagosome and release neutrophil extracellular traps (NETs) into their surroundings to immobilize and kill invading micro-organisms. Filamin A (FlnA) is an important actin cross-linking protein that is required for cellular processes involving actin rearrangements, such cell migration. It has also been shown to negatively regulate integrin activation and adhesion. However, its role in the regulation of β₂ integrin-dependent adhesion, as well as in other cellular functions in neutrophils, is poorly understood. Using a transgenic mouse model in which FlnA is selectively depleted in myeloid cells, such as neutrophils, we show that FlnA negatively regulates β₂ integrin adhesion to complement component iC3b and ICAM-1 in shear-free, but not shear-flow, conditions. FlnA deletion does not affect phagocytosis of Escherichia coli or Staphylococcus aureus or their intracellular killing. However, FlnA negatively regulates production of reactive oxygen species upon cell activation. Conversely, neutrophil activation through TLR4, as well as through activation by the Gram-negative bacteria E. coli, results in reduced NET production in FlnA-depleted neutrophils. Thus, FlnA is a negative regulator of β₂ integrin-dependent cell adhesion and reactive oxygen species production but is required for NET production in primary murine neutrophils.

ADP-ribosylation factor-like 4C binding to filamin-A modulates filopodium formation and cell migration

Filamin-A plays a key role in tumorigenesis as well as the metastatic progression of prostate cancer, ovarian cancer, and gastric carcinoma. In this study, we identified filamin-A as a novel effector of Arl4C and showed that binding between Arl4C and FLNa modulates the formation of filopodia and cell migration by promoting activation of Cdc42.

Changes in cell morphology and the physical forces that occur during migration are generated by a dynamic filamentous actin cytoskeleton. The ADP-ribosylation factor–like 4C (Arl4C) small GTPase acts as a molecular switch to regulate morphological changes and cell migration, although the mechanism by which this occurs remains unclear. Here we report that Arl4C functions with the actin regulator filamin-A (FLNa) to modulate filopodium formation and cell migration. We found that Arl4C interacted with FLNa in a GTP-dependent manner and that FLNa IgG repeat 22 is both required and sufficient for this interaction. We also show that interaction between FLNa and Arl4C is essential for Arl4C-induced filopodium formation and increases the association of FLNa with Cdc42-GEF FGD6, promoting cell division cycle 42 (Cdc42) GTPase activation. Thus our study revealed a novel mechanism, whereby filopodium formation and cell migration are regulated through the Arl4C-FLNa–mediated activation of Cdc42.

Filamin A upregulation correlates with Snail-induced epithelial to mesenchymal transition (EMT) and cell adhesion but its inhibition increases the migration of colon adenocarcinoma HT29 cells

Filamin A (FLNA) is actin filament cross-linking protein involved in cancer progression. Its importance in regulating cell motility is directly related to the epithelial to mesenchymal transition (EMT) of tumor cells. However, little is known about the mechanism of action of FLNA at this early stage of cancer invasion. Using immunochemical methods, we evaluated the levels and localization of FLNA, pFLNA[Ser2152], β1 integrin, pβ1 integrin[Thr788/9], FAK, pFAK[Y379], and talin in stably transfected HT29 adenocarcinoma cells overexpressing Snail and looked for the effect of Snail in adhesion and migration assays on fibronectin-coated surfaces before and after FLNA silencing. Our findings indicate that FLNA upregulation correlates with Snail-induced EMT in colorectal carcinoma. FLNA localizes in the cytoplasm and at the sites of focal adhesion (FA) of invasive cells. Silencing of FLNA inhibits Snail-induced cell adhesion, reduces the size of FA sites, induces the relocalization of talin from the cytoplasm to the membrane area and augments cell migratory properties. Our findings suggest that FLNA may not act as a classic integrin inhibitor in invasive carcinoma cells, but is involved in other pro-invasive pathways. FLNA upregulation, which correlates with cell metastatic properties, maybe an additional target for combination therapy in colorectal carcinoma tumor progression.

Filamin B: The next hotspot in skeletal research?

Filamin B (FLNB) is a large dimeric actin-binding protein which crosslinks actin cytoskeleton filaments into a dynamic structure. Up to present, pathogenic mutations in FLNB are solely found to cause skeletal deformities, indicating the important role of FLNB in skeletal development. FLNB-related disorders are classified as spondylocarpotarsal synostosis (SCT), Larsen syndrome (LS), atelosteogenesis (AO), boomerang dysplasia (BD), and isolated congenital talipes equinovarus, presenting with scoliosis, short-limbed dwarfism, clubfoot, joint dislocation and other unique skeletal abnormalities. Several mechanisms of FLNB mutations causing skeletal malformations have been proposed, including delay of ossification in long bone growth plate, reduction of bone mineral density (BMD), dysregulation of muscle differentiation, ossification of intervertebral disc (IVD), disturbance of proliferation, differentiation and apoptosis in chondrocytes, impairment of angiogenesis, and hypomotility of osteoblast, chondrocyte and fibroblast. Interventions on FLNB-related diseases require prenatal surveillance by sonography, gene testing in high-risk carriers, and proper orthosis or orthopedic surgeries to correct malformations including scoliosis, cervical spine instability, large joint dislocation, and clubfoot. Gene and cell therapies for FLNB-related diseases are also promising but require further studies.

Molecular stretching modulates mechanosensing pathways

For individual cells in tissues to create the diverse forms of biological organisms, it is necessary that they must reliably sense and generate the correct forces over the correct distances and directions. There is considerable evidence that the mechanical aspects of the cellular microenvironment provide critical physical parameters to be sensed. How proteins sense forces and cellular geometry to create the correct morphology is not understood in detail but protein unfolding appears to be a major component in force and displacement sensing. Thus, the crystallographic structure of a protein domain provides only a starting point to then analyze what will be the effects of physiological forces through domain unfolding or catch-bond formation. In this review, we will discuss the recent studies of cytoskeletal and adhesion proteins that describe protein domain dynamics. Forces applied to proteins can activate or inhibit enzymes, increase or decrease protein-protein interactions, activate or inhibit protein substrates, induce catch bonds and regulate interactions with membranes or nucleic acids. Further, the dynamics of stretch-relaxation can average forces or movements to reliably regulate morphogenic movements. In the few cases where single molecule mechanics are studied under physiological conditions such as titin and talin, there are rapid cycles of stretch-relaxation that produce mechanosensing signals. Fortunately, the development of new single molecule and super-resolution imaging methods enable the analysis of single molecule mechanics in physiologically relevant conditions. Thus, we feel that stereotypical changes in cell and tissue shape involve mechanosensing that can be analyzed at the nanometer level to determine the molecular mechanisms involved.

Sarcoplasmic reticulum-mitochondria communication in cardiovascular pathophysiology

Repetitive, calcium-mediated contractile activity renders cardiomyocytes critically dependent on a sustained energy supply and adequate calcium buffering, both of which are provided by mitochondria. Moreover, in vascular smooth muscle cells, mitochondrial metabolism modulates cell growth and proliferation, whereas cytosolic calcium levels regulate the arterial vascular tone. Physical and functional communication between mitochondria and sarco/endoplasmic reticulum and balanced mitochondrial dynamics seem to have a critical role for optimal calcium transfer to mitochondria, which is crucial in calcium homeostasis and mitochondrial metabolism in both types of muscle cells. Moreover, mitochondrial dysfunction has been associated with myocardial damage and dysregulation of vascular smooth muscle proliferation. Therefore, sarco/endoplasmic reticulum-mitochondria coupling and mitochondrial dynamics are now viewed as relevant factors in the pathogenesis of cardiac and vascular diseases, including coronary artery disease, heart failure, and pulmonary arterial hypertension. In this Review, we summarize the evidence related to the role of sarco/endoplasmic reticulum-mitochondria communication in cardiac and vascular muscle physiology, with a focus on how perturbations contribute to the pathogenesis of cardiovascular disorders.

The ER Stress Sensor PERK Coordinates ER-Plasma Membrane Contact Site Formation through Interaction with Filamin-A and F-Actin Remodeling

Loss of ER Ca²⁺ homeostasis triggers endoplasmic reticulum (ER) stress and drives ER-PM contact sites formation in order to refill ER-luminal Ca²⁺. Recent studies suggest that the ER stress sensor and mediator of the unfolded protein response (UPR) PERK regulates intracellular Ca²⁺ fluxes, but the mechanisms remain elusive. Here, using proximity-dependent biotin identification (BioID), we identified the actin-binding protein Filamin A (FLNA) as a key PERK interactor. Cells lacking PERK accumulate F-actin at the cell edges and display reduced ER-PM contacts. Following ER-Ca²⁺ store depletion, the PERK-FLNA interaction drives the expansion of ER-PM juxtapositions by regulating F-actin-assisted relocation of the ER-associated tethering proteins Stromal Interaction Molecule 1 (STIM1) and Extended Synaptotagmin-1 (E-Syt1) to the PM. Cytosolic Ca²⁺ elevation elicits rapid and UPR-independent PERK dimerization, which enforces PERK-FLNA-mediated ER-PM juxtapositions. Collectively, our data unravel an unprecedented role of PERK in the regulation of ER-PM appositions through the modulation of the actin cytoskeleton.

Intermediate Filaments and the Plasma Membrane

A variety of intermediate filament (IF) types show intricate association with plasma membrane proteins, including receptors and adhesion molecules. The molecular basis of linkage of IFs to desmosomes at sites of cell-cell interaction and hemidesmosomes at sites of cell-matrix adhesion has been elucidated and involves IF-associated proteins. However, IFs also interact with focal adhesions and cell-surface molecules, including dystroglycan. Through such membrane interactions, it is well accepted that IFs play important roles in the establishment and maintenance of tissue integrity. However, by organizing cell-surface complexes, IFs likely regulate, albeit indirectly, signaling pathways that are key to tissue homeostasis and repair.

Actin binding domain of filamin distinguishes posterior from anterior actin filaments in migrating Dictyostelium cells

Actin filaments in different parts of a cell interact with specific actin binding proteins (ABPs) and perform different functions in a spatially regulated manner. However, the mechanisms of those spatially-defined interactions have not been fully elucidated. If the structures of actin filaments differ in different parts of a cell, as suggested by previous in vitro structural studies, ABPs may distinguish these structural differences and interact with specific actin filaments in the cell. To test this hypothesis, we followed the translocation of the actin binding domain of filamin (ABD_FLN) fused with photoswitchable fluorescent protein (mKikGR) in polarized Dictyostelium cells. When ABD_FLN-mKikGR was photoswitched in the middle of a polarized cell, photoswitched ABD_FLN-mKikGR rapidly translocated to the rear of the cell, even though actin filaments were abundant in the front. The speed of translocation (>3 μm/s) was much faster than that of the retrograde flow of cortical actin filaments. Rapid translocation of ABD_FLN-mKikGR to the rear occurred normally in cells lacking GAPA, the only protein, other than actin, known to bind ABD_FLN. We suggest that ABD_FLN recognizes a certain feature of actin filaments in the rear of the cell and selectively binds to them, contributing to the posterior localization of filamin.

Mechanosensing Controlled Directly by Tyrosine Kinases

To understand how cells form tissues, we need to understand how the tyrosine kinases are involved in controlling cell mechanics, whether they act directly as parts of mechanosensing machines or indirectly. Cells test the critical parameter of matrix rigidity by locally contracting ("pinching") matrices and measuring forces, and the depletion of contractile units causes transformation. We report here that knocking down the receptor tyrosine kinases (RTKs), AXL, and ROR2, alters rigidity sensing and increases the magnitude or duration of local contraction events, respectively. Phospho-AXL and ROR2 localize to contraction units and bind major contractile components, tropomyosin 2.1 (AXL), myosin IIA (AXL), and filamin A (ROR2). At a molecular level, phosphorylated AXL localizes to active myosin filaments and phosphorylates tropomyosin at a tyrosine critical for adhesion formation. ROR2 binding of ligand is unnecessary, but binding filamin A helps function. Thus, AXL and ROR2 alter rigidity sensing and consequently morphogenic processes by directly controlling local mechanosensory contractions without ligands.

Regulation of chemokine receptor CCR2 recycling by filamin a phosphorylation

Proper endosomal trafficking of ligand-activated G protein-coupled receptors (GPCRs) is essential to spatiotemporally tune their physiological responses. For the monocyte chemoattractant receptor 2 (CCR2B), endocytic recycling is important to sustain monocyte migration; while filamin A (FLNa) is essential for CCL2-induced monocyte migration. Here, we analyze the role of FLNa in the trafficking of CCR2B along the endocytic pathway. In FLNa knockdown cells, activated CCR2B accumulated in enlarged EEA-1-positive endosomes, which exhibited slow movement and fast fluorescence recovery, suggesting an imbalance between receptor entry and exit rates. Utilizing super-resolution microscopy, we observed that FLNa-GFP, CCR2B and β2-adrenergic receptor (β2AR) were present in actin-enriched endosomal microdomains. Depletion of FLNa decreased CCR2B association with these microdomains and concomitantly delayed CCR2B endosomal traffic, without apparently affecting the number of microdomains. Interestingly, CCR2B and β2AR signaling induced phosphorylation of FLNa at S2152 and this phosphorylation event was contributes to sustain receptor recycling. Thus, our data strongly suggest that CCR2B and β2AR signals to FLNa to stimulate its endocytosis and recycling to the plasma membrane.

Integrin-beta3 clusters recruit clathrin-mediated endocytic machinery in the absence of traction force

The turnover of integrin receptors is critical for cell migration and adhesion dynamics. Here we find that force development at integrins regulates adaptor protein recruitment and endocytosis. Using mobile RGD (Arg-Gly-Asp) ligands on supported lipid membranes (RGD membranes) and rigid RGD ligands on glass (RGD-glass), we find that matrix force-dependent integrin signals block endocytosis. Dab2, an adaptor protein of clathrin-mediated endocytosis, is not recruited to activated integrin-beta3 clusters on RGD-glass; however, it is recruited to integrin-mediated adhesions on RGD membranes. Further, when force generation is inhibited on RGD-glass, Dab2 binds to integrin-beta3 clusters. Dab2 binding to integrin-beta3 excludes other adhesion-related adaptor proteins, such as talin. The clathrin-mediated endocytic machinery combines with Dab2 to facilitate the endocytosis of RGD-integrin-beta3 clusters. From these observations, we propose that loss of traction force on ligand-bound integrin-beta3 causes recruitment of Dab2/clathrin, resulting in endocytosis of integrins.

Connecting the cytoskeleton to the endoplasmic reticulum and Golgi

A tendency in cell biology is to divide and conquer. For example, decades of painstaking work have led to an understanding of endoplasmic reticulum (ER) and Golgi structure, dynamics, and transport. In parallel, cytoskeletal researchers have revealed a fantastic diversity of structure and cellular function in both actin and microtubules. Increasingly, these areas overlap, necessitating an understanding of both organelle and cytoskeletal biology. This review addresses connections between the actin/microtubule cytoskeletons and organelles in animal cells, focusing on three key areas: ER structure and function; ER-to-Golgi transport; and Golgi structure and function. Making these connections has been challenging for several reasons: the small sizes and dynamic characteristics of some components; the fact that organelle-specific cytoskeletal elements can easily be obscured by more abundant cytoskeletal structures; and the difficulties in imaging membranes and cytoskeleton simultaneously, especially at the ultrastructural level. One major concept is that the cytoskeleton is frequently used to generate force for membrane movement, with two potential consequences: translocation of the organelle, or deformation of the organelle membrane. While initially discussing issues common to metazoan cells in general, we subsequently highlight specific features of neurons, since these highly polarized cells present unique challenges for organellar distribution and dynamics.

Filamin A is required in injured axons for HDAC5 activity and axon regeneration

Microtubule dynamics are important for axon growth during development as well as axon regeneration after injury. We have previously identified HDAC5 as an injury-regulated tubulin deacetylase that functions at the injury site to promote axon regeneration. However, the mechanisms involved in the spatial control of HDAC5 activity remain poorly understood. Here we reveal that HDAC5 interacts with the actin binding protein filamin A via its C-terminal domain. Filamin A plays critical roles in HDAC5-dependent tubulin deacetylation because, in cells lacking filamin A, the levels of acetylated tubulin are elevated markedly. We found that nerve injury increases filamin A axonal expression in a protein synthesis-dependent manner. Reducing filamin A levels or interfering with the interaction between HDAC5 and filamin A prevents injury-induced tubulin deacetylation as well as HDAC5 localization at the injured axon tips. In addition, neurons lacking filamin A display reduced axon regeneration. Our findings suggest a model in which filamin A local translation following axon injury controls localized HDAC5 activity to promote axon regeneration.

Filamin A Mediates Wound Closure by Promoting Elastic Deformation and Maintenance of Tension in the Collagen Matrix

Cell-mediated remodeling and wound closure are critical for efficient wound healing, but the contribution of actin-binding proteins to contraction of the extracellular matrix is not defined. We examined the role of filamin A (FLNa), an actin filament cross-linking protein, in wound contraction and maintenance of matrix tension. Conditional deletion of FLNa in fibroblasts in mice was associated with ~4 day delay of full-thickness skin wound contraction compared with wild-type (WT) mice. We modeled the healing wound matrix using cultured fibroblasts plated on grid-supported collagen gels that create lateral boundaries, which are analogues to wound margins. In contrast to WT cells, FLNa knockdown (KD) cells could not completely maintain tension when matrix compaction was resisted by boundaries, which manifested as relaxed matrix tension. Similarly, WT cells on cross-linked collagen, which requires higher levels of sustained tension, exhibited approximately fivefold larger deformation fields and approximately twofold greater fiber alignment compared with FLNa KD cells. Maintenance of boundary-resisted tension markedly influenced the elongation of cell extensions: in WT cells, the number (~50%) and length (~300%) of cell extensions were greater than FLNa KD cells. We conclude that FLNa is required for wound contraction, in part by enabling elastic deformation and maintenance of tension in the matrix.

Mechanical stimulation induces formin-dependent assembly of a perinuclear actin rim

Cells constantly sense and respond to mechanical signals by reorganizing their actin cytoskeleton. Although a number of studies have explored the effects of mechanical stimuli on actin dynamics, the immediate response of actin after force application has not been studied. We designed a method to monitor the spatiotemporal reorganization of actin after cell stimulation by local force application. We found that force could induce transient actin accumulation in the perinuclear region within ∼ 2 min. This actin reorganization was triggered by an intracellular Ca(2+) burst induced by force application. Treatment with the calcium ionophore A23187 recapitulated the force-induced perinuclear actin remodeling. Blocking of actin polymerization abolished this process. Overexpression of Klarsicht, ANC-1, Syne Homology (KASH) domain to displace nesprins from the nuclear envelope did not abolish Ca(2+)-dependent perinuclear actin assembly. However, the endoplasmic reticulum- and nuclear membrane-associated inverted formin-2 (INF2), a potent actin polymerization activator (mutations of which are associated with several genetic diseases), was found to be important for perinuclear actin assembly. The perinuclear actin rim structure colocalized with INF2 on stimulation, and INF2 depletion resulted in attenuation of the rim formation. Our study suggests that cells can respond rapidly to external force by remodeling perinuclear actin in a unique Ca(2+)- and INF2-dependent manner.

Geometry regulates traction stresses in adherent cells

Cells generate mechanical stresses via the action of myosin motors on the actin cytoskeleton. Although the molecular origin of force generation is well understood, we currently lack an understanding of the regulation of force transmission at cellular length scales. Here, using 3T3 fibroblasts, we experimentally decouple the effects of substrate stiffness, focal adhesion density, and cell morphology to show that the total amount of work a cell does against the substrate to which it is adhered is regulated by the cell spread area alone. Surprisingly, the number of focal adhesions and the substrate stiffness have little effect on regulating the work done on the substrate by the cell. For a given spread area, the local curvature along the cell edge regulates the distribution and magnitude of traction stresses to maintain a constant strain energy. A physical model of the adherent cell as a contractile gel under a uniform boundary tension and mechanically coupled to an elastic substrate quantitatively captures the spatial distribution and magnitude of traction stresses. With a single choice of parameters, this model accurately predicts the cell's mechanical output over a wide range of cell geometries.

Diverse phenotypic consequences of mutations affecting the C-terminus of FLNA

Filamin A, the filamentous protein encoded by the X-linked gene FLNA, cross-links cytoskeletal actin into three-dimensional networks, facilitating its role as a signalling scaffold and a mechanosensor of extrinsic shear forces. Central to these functions is the ability of FLNA to form V-shaped homodimers through its C-terminal located filamin repeat 24. Additionally, many proteins that interact with FLNA have a binding site that includes the C-terminus of the protein. Here, a cohort of patients with mutations affecting this region of the protein is studied, with particular emphasis on the phenotype of male hemizygotes. Seven unrelated families are reported, with five exhibiting a typical female presentation of periventricular heterotopia (PH), a neuronal migration disorder typically caused by loss-of-function mutations in FLNA. One male presents with widespread PH consistent with previous male phenotypes attributable to hypomorphic mutations in FLNA. In stark contrast, two brothers are described with a mild PH presentation, due to a missense mutation (p.Gly2593Glu) inserting a large negatively charged amino acid into the hydrophobic dimerisation interface of FLNA. Co-immunoprecipitation, in vitro cross-linking studies and gel filtration chromatography all demonstrated that homodimerisation of isolated FLNA repeat 24 is abolished by this p.Gly2593Glu substitution but that extended FLNA(Gly2593Glu) repeat 16-24 constructs exhibit dimerisation. These observations imply that other interactions apart from those mediated by the canonical repeat 24 dimerisation interface contribute to FLNA homodimerisation and that mutations affecting this region of the protein can have broad phenotypic effects.

KEY MESSAGES

• Mutations in the X-linked gene FLNA cause a spectrum of syndromes. • Genotype-phenotype correlations are emerging but still remain unclear. • C-term mutations can confer male lethality, survival or connective tissue defects. • Mutations leading to the latter affect filamin dimerisation. • This deficit is compensated for by remotely acting domains elsewhere in FLNA.

Force transmission at cell-cell and cell-matrix adhesions

All cells are subjected to mechanical forces throughout their lifetimes. These forces are sensed by cell surface adhesion receptors and trigger robust actin cytoskeletal rearrangements and growth of the associated adhesion complex to counter the applied force. In this review, we discuss how integrins and cadherins sense force and transmit these forces into the cell interior. We focus on the complement of proteins each adhesion complex recruits to bear the force and the signal transduction pathways activated to allow the cell to tune its contractility. A discussion of the similarities, differences, and crosstalk between cadherin- and integrin-mediated force transmission is also presented.

Intermediate filaments: a dynamic network that controls cell mechanics

In humans the superfamily of intermediate filament (IF) proteins is encoded by more than 70 different genes, which are expressed in a cell- and tissue-specific manner. IFs assemble into approximately 10 nm-wide filaments that account for the principal structural elements at the nuclear periphery, nucleoplasm, and cytoplasm. They are also required for organizing the microtubule and microfilament networks. In this review, we focus on the dynamics of IFs and how modifications regulate it. We also discuss the role of nuclear IF organization in determining nuclear mechanics as well as that of cytoplasmic IFs organization in maintaining cell stiffness, formation of lamellipodia, regulation of cell migration, and permitting cell adhesion.

ER sheet persistence is coupled to myosin 1c-regulated dynamic actin filament arrays

The endoplasmic reticulum (ER) comprises a dynamic three-dimensional (3D) network with diverse structural and functional domains. Proper ER operation requires an intricate balance within and between dynamics, morphology, and functions, but how these processes are coupled in cells has been unclear. Using live-cell imaging and 3D electron microscopy, we identify a specific subset of actin filaments localizing to polygons defined by ER sheets and tubules and describe a role for these actin arrays in ER sheet persistence and, thereby, in maintenance of the characteristic network architecture by showing that actin depolymerization leads to increased sheet fluctuation and transformations and results in small and less abundant sheet remnants and a defective ER network distribution. Furthermore, we identify myosin 1c localizing to the ER-associated actin filament arrays and reveal a novel role for myosin 1c in regulating these actin structures, as myosin 1c manipulations lead to loss of the actin filaments and to similar ER phenotype as observed after actin depolymerization. We propose that ER-associated actin filaments have a role in ER sheet persistence regulation and thus support the maintenance of sheets as a stationary subdomain of the dynamic ER network.

Analysis of the local organization and dynamics of cellular actin networks

Live cell imaging, high-resolution microscopy, and computational modeling show that dynamic formin-filamin-actin asters self-organize into an actomyosin contractile network that may maintain mechanical coherence of cytoplasm.

A ctin filaments, with the aid of multiple accessory proteins, self-assemble into a variety of network patterns. We studied the organization and dynamics of the actin network in nonadhesive regions of cells bridging fibronectin-coated adhesive strips. The network was formed by actin nodes associated with and linked by myosin II and containing the formin disheveled-associated activator of morphogenesis 1 (DAAM1) and the cross-linker filamin A (FlnA). After Latrunculin A (LatA) addition, actin nodes appeared to be more prominent and demonstrated drift-diffusion motion. Superresolution microscopy revealed that, in untreated cells, DAAM1 formed patches with a similar spatial arrangement to the actin nodes. Node movement (diffusion coefficient and velocity) in LatA-treated cells was dependent on the level and activity of myosin IIA, DAAM1, and FlnA. Based on our results, we developed a computational model of the dynamic formin-filamin-actin asters that can self-organize into a contractile actomyosin network. We suggest that such networks are critical for connecting distant parts of the cell to maintain the mechanical coherence of the cytoplasm.

Phosphorylation of serine 323 of ASB2α is pivotal for the targeting of filamin A to degradation

ASB proteins are the specificity subunits of cullin5-RING E3 ubiquitin ligases (CRL5) that play roles in ubiquitin-mediated protein degradation. However, how their activity is regulated remains poorly understood. Here, we unravel a novel mechanism of regulation of a CRL5 through phosphorylation of its specificity subunit ASB2α. Indeed, using mass spectrometry, we showed for the first time that ASB2α is phosphorylated and that phosphorylation of serine-323 (Ser-323) of ASB2α is crucial for the targeting of the actin-binding protein filamin A (FLNa) to degradation. Mutation of ASB2α Ser-323 to Ala had no effect on intrinsic E3 ubiquitin ligase activity of ASB2α but abolished the ability of ASB2α to induce degradation of FLNa. In contrast, the ASB2α Ser-323 to Asp phosphomimetic mutant induced acute degradation of FLNa. Moreover, inhibition of the extracellular signal-regulated kinases 1 and 2 (Erk1/2) activity reduced ASB2α-mediated FLNa degradation. We further showed that the subcellular localization of ASB2α to actin-rich structures is dependent on ASB2α Ser-323 phosphorylation and propose that the interaction with FLNa depends on the electrostatic potential redistribution induced by the Ser-323 phosphate group. Taken together, these data unravel an important mechanism by which ASB2α-mediated FLNa degradation can be regulated.

ASB2α regulates migration of immature dendritic cells

The actin-binding protein filamins (FLNs) are major organizers of the actin cytoskeleton. They control the elasticity and stiffness of the actin network and provide connections with the extracellular microenvironment by anchoring transmembrane receptors to the actin filaments. Although numerous studies have revealed the importance of FLN levels, relatively little is known about the regulation of its stability in physiological relevant settings. Here, we show that the ASB2α cullin 5-ring E3 ubiquitin ligase is highly expressed in immature dendritic cells (DCs) and is down-regulated after DC maturation. We further demonstrate that FLNs are substrates of ASB2α in immature DCs and therefore are not stably expressed in these cells, whereas they exhibit high levels of expression in mature DCs. Using ASB2 conditional knockout mice, we show that ASB2α is a critical regulator of cell spreading and podosome rosette formation in immature DCs. Furthermore, we show that ASB2(-/-) immature DCs exhibit reduced matrix-degrading function leading to defective migration. Altogether, our results point to ASB2α and FLNs as newcomers in DC biology.

Complex roles of filamin-A mediated cytoskeleton network in cancer progression

Filamin-A (FLNA), also called actin-binding protein 280 (ABP-280), was originally identified as a non-muscle actin binding protein, which organizes filamentous actin into orthogonal networks and stress fibers. Filamin-A also anchors various transmembrane proteins to the actin cytoskeleton and provides a scaffold for a wide range of cytoplasmic and nuclear signaling proteins. Intriguingly, several studies have revealed that filamin-A associates with multiple non-cytoskeletal proteins of diverse function and is involved in several unrelated pathways. Mutations and aberrant expression of filamin-A have been reported in human genetic diseases and several types of cancer. In this review, we discuss the implications of filamin-A in cancer progression, including metastasis and DNA damage response.

Dynamic force sensing of filamin revealed in single-molecule experiments

Mechanical forces are important signals for cell response and development, but detailed molecular mechanisms of force sensing are largely unexplored. The cytoskeletal protein filamin is a key connecting element between the cytoskeleton and transmembrane complexes such as integrins or the von Willebrand receptor glycoprotein Ib. Here, we show using single-molecule mechanical measurements that the recently reported Ig domain pair 20-21 of human filamin A acts as an autoinhibited force-activatable mechanosensor. We developed a mechanical single-molecule competition assay that allows online observation of binding events of target peptides in solution to the strained domain pair. We find that filamin force sensing is a highly dynamic process occurring in rapid equilibrium that increases the affinity to the target peptides by up to a factor of 17 between 2 and 5 pN. The equilibrium mechanism we find here can offer a general scheme for cellular force sensing.

Endoplasmic spreading requires coalescence of vimentin intermediate filaments at force-bearing adhesions

Interaction of vimentin filaments (vIFs) and force-bearing adhesions is essential for endoplasm spreading. For adhesions to be connected to a contractile network involved in endoplasm spreading, vIFs are needed. Thus endoplasm spreading and microtubule stabilization in the periphery require a multicomponent actin network anchored at adhesions.

For cells to develop long-range forces and carry materials to the periphery, the microtubule and organelle-rich region at the center of the cell—the endoplasm—needs to extend to near the cell edge. Depletion of the actin cross-linking protein filamin A (FlnA) causes a collapse of the endoplasm into a sphere around the nucleus of fibroblasts and disruption of matrix adhesions, indicating that FlnA is involved in endoplasmic spreading and adhesion growth. Here, we report that treatment with the calpain inhibitor N-[N-(N-acetyl-l-leucyl)-l-leucyl]-l-norleucine (ALLN) restores endoplasmic spreading as well as focal adhesion (FA) growth on fibronectin-coated surfaces in a Fln-depleted background. Addback of calpain-uncleavable talin, not full-length talin, achieves a similar effect in Fln-depleted cells and indicates a crucial role for talin in endoplasmic spreading. Because FA maturation involves the vimentin intermediate filament (vIF) network, we also examined the role of vIFs in endoplasmic spreading. Wild-type cells expressing a vimentin variant incapable of polymerization exhibit deficient endoplasmic spreading as well as defects in FA growth. ALLN treatment restores FA growth despite the lack of vIFs but does not restore endoplasmic spreading, implying that vIFs are essential for endoplasm spreading. Consistent with that hypothesis, vIFs are always displaced from adhesions when the endoplasm does not spread. In Fln-depleted cells, vIFs extend beyond adhesions, nearly to the cell edge. Finally, inhibiting myosin II–mediated contraction blocks endoplasmic spreading and adhesion growth. Thus we propose a model in which myosin II–mediated forces and coalescence of vIFs at mature FAs are required for endoplasmic spreading.