Structure of three tandem filamin domains reveals auto-inhibition of ligand binding

Mechanotransduction through protein stretching

Cells sense and respond to subtle changes in their physicality, and via a myriad of different mechanosensitive processes, convert these physical cues into chemical and biochemical signals. This process, called mechanotransduction, is possible due to a highly sophisticated machinery within cells. One mechanism by which this can occur is via the stretching of mechanosensitive proteins. Stretching proteins that contain force-dependent regions results in altered geometry and dimensions of the connections, as well as differential spatial organization of signals bound to the stretched protein. The purpose of this mini-review is to discuss some of the intense recent activity in this area of mechanobiology that strives to understand how protein stretching can influence signaling outputs and cellular responses.

PACSIN2 regulates platelet integrin β1 hemostatic function

BACKGROUND

Upon vessel injury, platelets adhere to exposed matrix constituents via specific membrane receptors, including the von Willebrand factor receptor glycoprotein (GP)Ib-IX-V complex and integrins β1 and β3. In platelets, the Fes/CIP4-homology Bin-Amphiphysin-Rvs protein PACSIN2 associates with the cytoskeletal and scaffolding protein filamin A (FlnA), linking GPIbα and integrins to the cytoskeleton.

OBJECTIVES

Here we investigated the role of PACSIN2 in platelet function.

METHODS

Platelet parameters were evaluated in mice lacking PACSIN2 and platelet integrin β1.

RESULTS

Pacsin2-/- mice displayed mild thrombocytopenia, prolonged bleeding time, and delayed thrombus formation in a ferric chloride-mediated carotid artery injury model, which was normalized by injection of control platelets. Pacsin2-/- platelets formed unstable thrombi that embolized abruptly in a laser-induced cremaster muscle injury model. Pacsin2-/- platelets had hyperactive integrin β1, as evidenced by increased spreading onto surfaces coated with the collagen receptor α2β1-specific peptide GFOGER and increased binding of the antibody 9EG7 directed against active integrin β1. By contrast, Pacsin2-/- platelets had normal integrin αIIbβ3 function and expressed P-selectin normally following stimulation through the collagen receptor GPVI or with thrombin. Deletion of platelet integrin β1 in Pacsin2-/- mice normalized platelet count, hemostasis, and thrombus formation. A PACSIN2 peptide mimicking the FlnA-binding site mediated the pull-down of a FlnA rod 2 construct by integrin β7, a model for integrin β-subunits.

CONCLUSIONS

Pacsin2-/- mice displayed severe thrombus formation defects due to hyperactive platelet integrin β1. The data suggest that PACSIN2 binding to FlnA negatively regulates platelet integrin β1 hemostatic function.

Upregulated Cytoskeletal Proteins Promote Pathological Angiogenesis in Moyamoya Disease

BACKGROUND

Moyamoya disease (MMD) is a rare progressive vascular disease that leads to intracranial internal carotid artery stenosis and eventual occlusion. However, its pathogenesis remains unclear. The purpose of this study is to explore the role of abnormally expressed proteins in the pathogenesis of MMD.

METHODS

Data-independent acquisition mass spectrometry identifies the differentially expressed proteins in MMD serum by detecting the serum from 60 patients with MMD and 20 health controls. The differentially expressed proteins were validated using enzyme linked immunosorbent assays. Immunofluorescence for superficial temporal artery and middle cerebral artery specimens was used to explore the morphological changes of vascular wall in MMD. In vitro experiments were used to explore the changes and mechanisms of differentially expressed proteins on endothelial cells.

RESULTS

Proteomic analysis showed that a total of 14 726 peptides and 1555 proteins were quantified by mass spectrometry data. FLNA (filamin A) and ZYX (zyxin) proteins were significantly higher in MMD serum compared with those in health controls (Log2FC >2.9 and >2.8, respectively). Immunofluorescence revealed an intimal hyperplasia in superficial temporal artery and middle cerebral artery specimens of MMD. FLNA and ZYX proteins increased the proportion of endothelial cells in S phase and promoted their proliferation, angiogenesis, and cytoskeleton enlargement. Mechanistic studies revealed that AKT (serine/threonine kinase)/GSK-3β (glycogen synthase kinase 3β)/β-catenin signaling pathway plays a major role in these FLNA- and ZYX-induced changes in endothelial cells.

CONCLUSIONS

This study provides proteomic data on a large sample size of MMD. The differential expression of FLNA and ZYX in patient with MMD and following in vitro experiments suggest that these upregulated proteins are related to the pathology of cerebrovascular intimal hyperplasia in MMD and are involved in MMD pathogenesis, with diagnostic and therapeutic ramifications.

Identification of Filamin A Mechanobinding Partner III: SAV1 Specifically Interacts with Filamin A Mechanosensitive Domain 21

Filamin A (FLNA) cross-links actin filaments and mediates mechanotransduction by force-induced conformational changes of its domains. FLNA's mechanosensitive immunoglobulin-like repeats (R) interact with each other to create cryptic binding sites, which can be exposed by physiologically relevant mechanical forces. Using the FLNA mechanosensing domains as an affinity ligand followed by stable isotope labeling by amino acids in cell culture (SILAC)-based proteomics, we recently identified smoothelin and fimbacin as FLNA mechanobinding proteins. Here, using the mechanosensing domain as an affinity ligand and two labeled amino acids, we identify salvador homologue 1 (SAV1), a component of the Hippo pathway kinase cascade, as a new FLNA mechanobinding partner. We demonstrate that SAV1 specifically interacts with the cryptic C-D cleft of FLNA R21 and map the FLNA-binding site on SAV1. We show that point mutations on the R21 C strand block the SAV1 interaction and find that SAV1 contains a FLNA-binding motif in the central region (¹¹⁶Phe-¹²⁴Val). Point mutations F116A and T118A (FT/AA) disrupt the interaction. A proximity ligation assay reveals that their interaction occurs in the cytosol in an actin polymerization-dependent manner. Although SAV1 is typically found in the cytosol, disrupting the interaction between SAV1 and FLNA causes SAV1 to diffuse to the nucleus and YAP1 to diffuse to the cytosol in an inverse relationship. These results suggest that FLNA mediates regulation of the Hippo pathway through actin polymerization-dependent interaction with SAV1.

The force-dependent filamin A-G3BP1 interaction regulates phase-separated stress granule formation

Filamin A (FLNA) is an actin crosslinking protein that mediates mechanotransduction. External and internal mechanical forces, through the actin cytoskeleton, can induce conformational changes of the FLNA molecule to expose cryptic binding sites for its binding partners. Here, we identified Ras GTPase-activating protein SH3 domain-binding protein 1 (G3BP1) as a new FLNA mechanobinding partner. Unlike other FLNA binding partners to the mechanosensing domain repeat 21 (R21), G3BP1 requires an additional neighboring repeat R22 to interact. We demonstrated that their interaction occurs in the cytosol of living cells in an actin polymerization-dependent manner. We also mapped the FLNA-binding site on G3BP1 and found that a F360A point mutation in the RNA recognition motif disrupts the interaction. RNA interfered with the FLNA-G3BP1 interaction, and FLNA did not localize in RNA-rich stress granules (SGs). Disruption of the interaction was sufficient to promote phase-separated SG formation, and arsenite treatment further stimulated the formation of SGs. Taken together, these data identify G3BP1 as a new mechanobinding protein that interacts with the FLNA mechanosensing domain R21 and suggest that SG formation is partially regulated by mechanical force.

RNA epitranscriptomics dysregulation: A major determinant for significantly increased risk of ASD pathogenesis

Autism spectrum disorders (ASDs) are perhaps the most severe, intractable and challenging child psychiatric disorders. They are complex, pervasive and highly heterogeneous and depend on multifactorial neurodevelopmental conditions. Although the pathogenesis of autism remains unclear, it revolves around altered neurodevelopmental patterns and their implications for brain function, although these cannot be specifically linked to symptoms. While these affect neuronal migration and connectivity, little is known about the processes that lead to the disruption of specific laminar excitatory and inhibitory cortical circuits, a key feature of ASD. It is evident that ASD has multiple underlying causes and this multigenic condition has been considered to also dependent on epigenetic effects, although the exact nature of the factors that could be involved remains unclear. However, besides the possibility for differential epigenetic markings directly affecting the relative expression levels of individual genes or groups of genes, there are at least three mRNA epitranscriptomic mechanisms, which function cooperatively and could, in association with both genotypes and environmental conditions, alter spatiotemporal proteins expression patterns during brain development, at both quantitative and qualitative levels, in a tissue-specific, and context-dependent manner. As we have already postulated, sudden changes in environmental conditions, such as those conferred by maternal inflammation/immune activation, influence RNA epitranscriptomic mechanisms, with the combination of these processes altering fetal brain development. Herein, we explore the postulate whereby, in ASD pathogenesis, RNA epitranscriptomics might take precedence over epigenetic modifications. RNA epitranscriptomics affects real-time differential expression of receptor and channel proteins isoforms, playing a prominent role in central nervous system (CNS) development and functions, but also RNAi which, in turn, impact the spatiotemporal expression of receptors, channels and regulatory proteins irrespective of isoforms. Slight dysregulations in few early components of brain development, could, depending upon their extent, snowball into a huge variety of pathological cerebral alterations a few years after birth. This may very well explain the enormous genetic, neuropathological and symptomatic heterogeneities that are systematically associated with ASD and psychiatric disorders at large.

Interaction of LARP4 to filamin A mechanosensing domain regulates cell migrations

Filamin A (FLNA) is an actin cross-linking protein that mediates mechanotransduction. Force-dependent conformational changes of FLNA molecule expose cryptic binding site of FLNA, allowing interaction with partners such as integrin, smoothelin, and fimbacin. Here, we identified La-related protein 4 (LARP4) as a new FLNA mechanobinding partner. LARP4 specifically interacts with the cleft formed by C and D strands of immunoglobulin-like repeat 21 (R21) which is blocked by A strand of R20 without force. We validated the interaction between LARP4 and FLNA R21 both in vivo and in vitro. We also determined the critical amino acid that is responsible for the interaction and generated the non-FLNA-binding mutant LARP4 (F277A in human: F273A in mouse Larp4) that disrupts the interaction. Fluorescence recovery after photobleaching (FRAP) of GFP-labeled LARP4 in living cells demonstrated that mutant LARP4 diffuses faster than WT LARP4. Proximity ligation assay (PLA) also confirmed their interaction and disruption of actin polymerization diminishes the interaction. Data mining of RNAseq analysis of LARP4 knockdown (KD) HEK293T cells suggested that LARP4 is involved in morphogenesis and cell motility. Consistent with this prediction, we found that KD of LARP4 increases cell migration speed and expression of the F277A mutant LARP4 in LARP4-KD cells also leads to a higher cell migration speed compared to WT LARP4. These results demonstrated that the LARP4 interaction with FLNA regulates cell migration.

Cytoskeletal and Cytoskeleton-Associated Proteins: Key Regulators of Cancer Stem Cell Properties

Cancer stem cells (CSCs) are a subpopulation of cancer cells possessing stemness characteristics that are closely associated with tumor proliferation, recurrence and resistance to therapy. Recent studies have shown that different cytoskeletal components and remodeling processes have a profound impact on the behavior of CSCs. In this review, we outline the different cytoskeletal components regulating the properties of CSCs and discuss current and ongoing therapeutic strategies targeting the cytoskeleton. Given the many challenges currently faced in targeted cancer therapy, a deeper comprehension of the molecular events involved in the interaction of the cytoskeleton and CSCs will help us identify more effective therapeutic strategies to eliminate CSCs and ultimately improve patient survival.

Rare clinical phenotype of filaminopathy presenting as restrictive cardiomyopathy and myopathy in childhood

Background

FLNC is one of the few genes associated with all types of cardiomyopathies, but it also underlies neuromuscular phenotype. The combination of concomitant neuromuscular and cardiac involvement is not often observed in filaminopathies and the impact of this on the disease prognosis has hitherto not been analyzed.

Results

Here we provide a detailed clinical, genetic, and structural prediction analysis of distinct FLNC-associated phenotypes based on twelve pediatric cases. They include early-onset restrictive cardiomyopathy (RCM) in association with congenital myopathy. In all patients the initial diagnosis was established during the first year of life and in five out of twelve (41.7%) patients the first symptoms were observed at birth. RCM was present in all patients, often in combination with septal defects. No ventricular arrhythmias were noted in any of the patients presented here. Myopathy was confirmed by neurological examination, electromyography, and morphological studies. Arthrogryposes was diagnosed in six patients and remained clinically meaningful with increasing age in three of them. One patient underwent successful heart transplantation at the age of 18 years and two patients are currently included in the waiting list for heart transplantation. Two died due to congestive heart failure. One patient had ICD instally as primary prevention of SCD. In ten out of twelve patients the disease was associated with missense variants and only in two cases loss of function variants were detected. In half of the described cases, an amino acid substitution A1186V, altering the structure of IgFLNc10, was found.

Conclusions

The present description of twelve cases of early-onset restrictive cardiomyopathy with congenital myopathy and FLNC mutation, underlines a distinct unique phenotype that can be suggested as a separate clinical form of filaminopathies. Amino acid substitution A1186V, which was observed in half of the cases, defines a mutational hotspot for the reported combination of myopathy and cardiomyopathy. Several independent molecular mechanisms of FLNC mutations linked to filamin structure and function can explain the broad spectrum of FLNC-associated phenotypes. Early disease presentation and unfavorable prognosis of heart failure demanding heart transplantation make awareness of this clinical form of filaminopathy of great clinical importance.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13023-022-02477-5.

A protective role for <i>Drosophila</i> Filamin in nephrocytes via Yorkie mediated hypertrophy

Podocytes are specialized epithelial cells of the kidney glomerulus and are an essential part of the filtration barrier. Because of their position, they are exposed to constant biomechanical forces such as shear stress and hydrostatic pressure. These forces increase during disease, resulting in podocyte injury. It is likely podocytes have adaptative responses to help buffer against deleterious mechanical force and thus reduce injury. However, these responses remain largely unknown. Here, using the <i>Drosophila</i> model, we show the mechanosensor Cheerio (dFilamin) provides a key protective role in nephrocytes. We found expression of an activated mechanosensitive variant of Cheerio rescued filtration function and induced compensatory and hypertrophic growth in nephrocytes depleted of the nephrocyte diaphragm proteins Sns or Duf. Delineating the protective pathway downstream of Cheerio we found repression of the Hippo pathway induces nephrocyte hypertrophy, whereas Hippo activation reversed the Cheerio-mediated hypertrophy. Furthermore, we find Yorkie was activated upon expression of active Cheerio. Taken together, our data suggest that Cheerio acts via the Hippo pathway to induce hypertrophic growth, as a protective response in abnormal nephrocytes.

Computational analysis of missense filamin-A variants, including the novel p.Arg484Gln variant of two brothers with periventricular nodular heterotopia

Background

Periventricular nodular heterotopia (PNH) is a cell migration disorder associated with mutations in Filamin-A (FLNA) gene on chromosome X. Majority of the individuals with PNH-associated FLNA mutations are female whereas liveborn males with FLNA mutations are very rare. Fetal viability of the males seems to depend on the severity of the variant. Splicing or severe truncations presumed loss of function of the protein product, lead to male lethality and only partial-loss-of-function variants are reported in surviving males. Those variants mostly manifest milder clinical phenotypes in females and thus avoid detection of the disease in females.

Methods

We describe a novel p.Arg484Gln variant in the FLNA gene by performing whole exome analysis on the index case, his one affected brother and his healthy non-consanguineous parents. The transmission of PNH from a clinically asymptomatic mother to two sons is reported in a fully penetrant classical X-linked dominant mode. The variant was verified via Sanger sequencing. Additionally, we investigated the impact of missense mutations reported in affected males on the FLNa protein structure, dynamics and interactions by performing molecular dynamics (MD) simulations to examine the disease etiology and possible compensative mechanisms allowing survival of the males.

Results

We observed that p.Arg484Gln disrupts the FLNa by altering its structural and dynamical properties including the flexibility of certain regions, interactions within the protein, and conformational landscape of FLNa. However, these impacts existed for only a part the MD trajectories and highly similar patterns observed in the other 12 mutations reported in the liveborn males validated this mechanism.

Conclusion

It is concluded that the variants seen in the liveborn males result in transient pathogenic effects, rather than persistent impairments. By this way, the protein could retain its function occasionally and results in the survival of the males besides causing the disease.

Crystal Structure Analysis and IgE Epitope Mapping of Allergic Predominant Region in Scylla paramamosain Filamin C, Scy p 9

Filamin C (FLN c) is a novel allergen in shellfish. In this study, FLN c from Scylla paramamosain was divided into three regions for recombinant expression based on the number of domains and amino acids. Using dot blot and basophil activation tests, the allergic predominant region of FLN c was determined to be 336-531 amino acid positions (named FLN c-M). It was confirmed that by X-ray diffraction, the crystal structure of FLN c-M with immunoglobulin-like folding at a resolution of 1.7 Å was obtained. The monomer was a barrel structure composed of 16 β-strands and 2 α-helices. Three conformational epitopes were predicted, six linear epitopes were verified by serological test, and they were positioned on the crystal structure of FLN c-M. For the first time, the crystal structure of the allergic predominant region of FLN c was determined, and it provided an accurate template for the localization of IgE epitopes.

Maintaining proteostasis under mechanical stress

Cell survival, tissue integrity and organismal health depend on the ability to maintain functional protein networks even under conditions that threaten protein integrity. Protection against such stress conditions involves the adaptation of folding and degradation machineries, which help to preserve the protein network by facilitating the refolding or disposal of damaged proteins. In multicellular organisms, cells are permanently exposed to stress resulting from mechanical forces. Yet, for long time mechanical stress was not recognized as a primary stressor that perturbs protein structure and threatens proteome integrity. The identification and characterization of protein folding and degradation systems, which handle force-unfolded proteins, marks a turning point in this regard. It has become apparent that mechanical stress protection operates during cell differentiation, adhesion and migration and is essential for maintaining tissues such as skeletal muscle, heart and kidney as well as the immune system. Here, we provide an overview of recent advances in our understanding of mechanical stress protection.

Protein nanomechanics in biological context

How proteins respond to pulling forces, or protein nanomechanics, is a key contributor to the form and function of biological systems. Indeed, the conventional view that proteins are able to diffuse in solution does not apply to the many polypeptides that are anchored to rigid supramolecular structures. These tethered proteins typically have important mechanical roles that enable cells to generate, sense, and transduce mechanical forces. To fully comprehend the interplay between mechanical forces and biology, we must understand how protein nanomechanics emerge in living matter. This endeavor is definitely challenging and only recently has it started to appear tractable. Here, I introduce the main in vitro single-molecule biophysics methods that have been instrumental to investigate protein nanomechanics over the last 2 decades. Then, I present the contemporary view on how mechanical force shapes the free energy of tethered proteins, as well as the effect of biological factors such as post-translational modifications and mutations. To illustrate the contribution of protein nanomechanics to biological function, I review current knowledge on the mechanobiology of selected muscle and cell adhesion proteins including titin, talin, and bacterial pilins. Finally, I discuss emerging methods to modulate protein nanomechanics in living matter, for instance by inducing specific mechanical loss-of-function (mLOF). By interrogating biological systems in a causative manner, these new tools can contribute to further place protein nanomechanics in a biological context.

The role of filamins in mechanically stressed podocytes

Glomerular hypertension induces mechanical load to podocytes, often resulting in podocyte detachment and the development of glomerulosclerosis. Although it is well known that podocytes are mechanosensitive, the mechanosensors and mechanotransducers are still unknown. Since filamin A, an actin-binding protein, is already described to be a mechanosensor and mechanotransducer, we hypothesized that filamins could be important for the outside-in signaling as well as the actin cytoskeleton of podocytes under mechanical stress. In this study, we demonstrate that filamin A is the main isoform of the filamin family that is expressed in cultured podocytes. Together with filamin B, filamin A was significantly up-regulated during mechanical stretch (3 days, 0.5 Hz, and 5% extension). To study the role of filamin A in cultured podocytes under mechanical stress, filamin A was knocked down (Flna KD) by specific siRNA. Additionally, we established a filamin A knockout podocyte cell line (Flna KO) by CRISPR/Cas9. Knockdown and knockout of filamin A influenced the expression of synaptopodin, a podocyte-specific protein, focal adhesions as well as the morphology of the actin cytoskeleton. Moreover, the cell motility of Flna KO podocytes was significantly increased. Since the knockout of filamin A has had no effect on cell adhesion of podocytes during mechanical stress, we simultaneously knocked down the expression of filamin A and B. Thereby, we observed a significant loss of podocytes during mechanical stress indicating a compensatory mechanism. Analyzing hypertensive mice kidneys as well as biopsies of patients suffering from diabetic nephropathy, we found an up-regulation of filamin A in podocytes in contrast to the control. In summary, filamin A and B mediate matrix-actin cytoskeleton interactions which are essential for the adaptation of cultured podocyte to mechanical stress.

SlyD Accelerates trans-to-cis Prolyl Isomerization in a Mechanosignaling Protein under Load

Prolyl isomerization is recognized as one of the key regulatory mechanisms, which plays a crucial role in cell signaling, ion channel gating, phage virus infection, and molecular timing. This isomerization is usually slow but often accelerated by an enzyme, called peptidyl-prolyl isomerase (PPIase). In the current project, we investigate using single-molecule force spectroscopy (SMFS) the impact of a bacterial PPIase, SlyD, on the cis-trans isomerization of the proline 2225 (P2225) in an isolated 20th domain of a cytoskeletal mechanosensing protein filamin-A (FlnA20). To explore the FlnA20-PPIase interaction, we have used multiple SMFS modes, like constant velocity, constant distance, and jumping trap experiments. In our previous study, we reported the unique nature of the P2225, which is conserved in all naturally occurring filamins and can slowly (minutes) interconvert between cis-trans isomers, in absence of any PPIase. Our current results show a staggering 25-fold acceleration of the trans-to-cis isomerization rate in the presence of saturating SlyD concentration (7.25 μM) compared to the unenzymatic condition. A SlyD concentration-dependent depletion of the trans isomeric lifetime was also observed. Additionally, we observed that SlyD stabilizes the cis-isomer in the native state of FlnA20 by ∼2 k_BT. This is the first single-molecule observation of the cis-trans isomerization catalysis by a PPIase in a mechanosensing protein.

Exon skip-inducing variants in FLNA in an attenuated form of frontometaphyseal dysplasia

Pathogenic variation in the X-linked gene FLNA causes a wide range of human developmental phenotypes. Loss-of-function is usually male embryonic-lethal, and most commonly results in a neuronal migration disorder in affected females. Gain-of-function variants cause a spectrum of skeletal dysplasias that present with variable additional, often distinctive, soft-tissue anomalies in males and females. Here we present two, unrelated, male individuals with novel, intronic variants in FLNA that are predicted to be pathogenic. Their phenotypes are reminiscent of the gain-of-function spectrum without the skeletal manifestations. Most strikingly, they manifest urethral anomalies, cardiac malformations, and keloid scarring, all commonly encountered features of frontometaphyseal dysplasia. Both variants prevent inclusion of exon 40 into the FLNA transcript, predicting the in-frame deletion of 42 amino acids, however the abundance of FLNA protein was equivalent to that observed in healthy individuals. Loss of these 42 amino acids removes sites that mediate key FLNA functions, including binding of some ligands and phosphorylation. This phenotype further expands the spectrum of the FLNA filaminopathies.

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.

Homozygous expression of the myofibrillar myopathy-associated p.W2710X filamin C variant reveals major pathomechanisms of sarcomeric lesion formation

Filamin C (FLNc) is mainly expressed in striated muscle cells where it localizes to Z-discs, myotendinous junctions and intercalated discs. Recent studies have revealed numerous mutations in the FLNC gene causing familial and sporadic myopathies and cardiomyopathies with marked clinical variability. The most frequent myopathic mutation, p.W2710X, which is associated with myofibrillar myopathy, deletes the carboxy-terminal 16 amino acids from FLNc and abolishes the dimerization property of Ig-like domain 24. We previously characterized "knock-in" mice heterozygous for this mutation (p.W2711X), and have now investigated homozygous mice using protein and mRNA expression analyses, mass spectrometry, and extensive immunolocalization and ultrastructural studies. Although the latter mice display a relatively mild myopathy under normal conditions, our analyses identified major mechanisms causing the pathophysiology of this disease: in comparison to wildtype animals (i) the expression level of FLNc protein is drastically reduced; (ii) mutant FLNc is relocalized from Z-discs to particularly mechanically strained parts of muscle cells, i.e. myotendinous junctions and myofibrillar lesions; (iii) the number of lesions is greatly increased and these lesions lack Bcl2-associated athanogene 3 (BAG3) protein; (iv) the expression of heat shock protein beta-7 (HSPB7) is almost completely abolished. These findings indicate grave disturbances of BAG3-dependent and -independent autophagy pathways that are required for efficient lesion repair. In addition, our studies reveal general mechanisms of lesion formation and demonstrate that defective FLNc dimerization via its carboxy-terminal domain does not disturb assembly and basic function of myofibrils. An alternative, more amino-terminally located dimerization site might compensate for that loss. Since filamins function as stress sensors, our data further substantiate that FLNc is important for mechanosensing in the context of Z-disc stabilization and maintenance.

Drosophila OTK Is a Glycosaminoglycan-Binding Protein with High Conformational Flexibility

The transmembrane protein OTK plays an essential role in plexin and Wnt signaling during Drosophila development. We have determined a crystal structure of the last three domains of the OTK ectodomain and found that OTK shows high conformational flexibility resulting from mobility at the interdomain interfaces. We failed to detect direct binding between Drosophila Plexin A (PlexA) and OTK, which was suggested previously. We found that, instead of PlexA, OTK directly binds semaphorin 1a. Our binding analyses further revealed that glycosaminoglycans, heparin and heparan sulfate, are ligands for OTK and thus may play a role in the Sema1a-PlexA axon guidance system.

Chapter 22: Structural and signaling functions of integrins

The integrin family of transmembrane adhesion receptors is essential for sensing and adhering to the extracellular environment. Integrins are heterodimers composed of non-covalently associated α and β subunits that engage extracellular matrix proteins and couple to intracellular signaling and cytoskeletal complexes. Humans have 24 different integrin heterodimers with differing ligand binding specificities and non-redundant functions. Complex structural rearrangements control the ability of integrins to engage ligands and to activate diverse downstream signaling networks, modulating cell adhesion and dynamics, processes which are crucial for metazoan life and development. Here we review the structural and signaling functions of integrins focusing on recent advances which have enhanced our understanding of how integrins are activated and regulated, and the cytoplasmic signaling networks downstream of integrins.

Structure and Function of Filamin C in the Muscle Z-Disc

Filamin C (FLNC) is one of three filamin proteins (Filamin A (FLNA), Filamin B (FLNB), and FLNC) that cross-link actin filaments and interact with numerous binding partners. FLNC consists of a N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats with two intervening calpain-sensitive hinges separating R15 and R16 (hinge 1) and R23 and R24 (hinge-2). The FLNC subunit is dimerized through R24 and calpain cleaves off the dimerization domain to regulate mobility of the FLNC subunit. FLNC is localized in the Z-disc due to the unique insertion of 82 amino acid residues in repeat 20 and necessary for normal Z-disc formation that connect sarcomeres. Since phosphorylation of FLNC by PKC diminishes the calpain sensitivity, assembly, and disassembly of the Z-disc may be regulated by phosphorylation of FLNC. Mutations of FLNC result in cardiomyopathy and muscle weakness. Although this review will focus on the current understanding of FLNC structure and functions in muscle, we will also discuss other filamins because they share high sequence similarity and are better characterized. We will also discuss a possible role of FLNC as a mechanosensor during muscle contraction.

Prune belly syndrome in surviving males can be caused by Hemizygous missense mutations in the X-linked Filamin A gene

Background

Prune belly syndrome (PBS) is a rare, multi-system congenital myopathy primarily affecting males that is poorly described genetically. Phenotypically, its morbidity spans from mild to lethal, however, all isolated PBS cases manifest three cardinal pathological features: 1) wrinkled flaccid ventral abdominal wall with skeletal muscle deficiency, 2) urinary tract dilation with poorly contractile smooth muscle, and 3) intra-abdominal undescended testes. Despite evidence for a genetic basis, previously reported PBS autosomal candidate genes only account for one consanguineous family and single cases.

Methods

We performed whole exome sequencing (WES) of two maternal adult half-brothers with syndromic PBS (PBS + Otopalatodigital spectrum disorder [OPDSD]) and two unrelated sporadic individuals with isolated PBS and further functionally validated the identified mutations.

Results

We identified three unreported hemizygous missense point mutations in the X-chromosome gene Filamin A (FLNA) (c.4952 C > T (p.A1448V), c.6727C > T (p.C2160R), c.5966 G > A (p.G2236E)) in two related cases and two unrelated sporadic individuals. Two of the three PBS mutations map to the highly regulatory, stretch-sensing Ig19–21 region of FLNA and enhance binding to intracellular tails of the transmembrane receptor β-integrin 1 (ITGβ1).

Conclusions

FLNA is a regulatory actin-crosslinking protein that functions in smooth muscle cells as a mechanosensing molecular scaffold, transmitting force signals from the actin-myosin motor units and cytoskeleton via binding partners to the extracellular matrix. This is the first evidence for an X-linked cause of PBS in multiple unrelated individuals and expands the phenotypic spectrum associated with FLNA in males surviving even into adulthood.

Filamin A: key actor in platelet biology

Filamins (FLNs) are large dimeric actin-binding proteins that regulate actin cytoskeleton remodeling. In addition, FLNs serve as scaffolds for signaling proteins, such as tyrosine kinases, GTPases, or phosphatases, as well as for adhesive receptors, such as integrins. Thus, they connect adhesive receptors to signaling pathways and to cytoskeleton. There are 3 isoforms of FLN (filamin a [FLNa], FLNb, FLNc) that originate from 3 homologous genes. FLNa has been the recent focus of attention because its mutations are responsible for a wide spectrum of defects called filaminopathies A, affecting brain (peri-ventricular nodular heterotopia), heart (valve defect), skeleton, gastrointestinal tract, and, more recently, the megakaryocytic lineage. This review will focus on the physiological and pathological roles of FLNa in platelets. Indeed, FLNa mutations alter platelet production from their bone marrow precursors, the megakaryocytes, yielding giant platelets in reduced numbers (macrothrombocytopenia). In platelets per se, FLNa mutations may lead to impaired αIIbβ3 integrin activation or in contrast, increased αIIbβ3 activation, potentially enhancing the risk of thrombosis. Experimental work delineating the interaction of FLNa with its platelet partners, including αIIbβ3, the von Willebrand factor receptor GPIb-IX-V, the tyrosine kinase Syk, and the signaling pathway of the collagen receptor GPVI, will also be reviewed.

Clinical exome sequencing revealed that FLNC variants contribute to the early diagnosis of cardiomyopathies in infant patients

BACKGROUND

FLNC encodes actin-binding protein and is mainly concentrated in skeletal and cardiac muscle. Mutations in FLNC were found in cardiomyopathies. To date, studies on FLNC-cardiomyopathies have mainly been reported in adults. There are limited studies that have investigated FLNC variants in pediatric patients with cardiomyopathies.

METHODS

We summarized the patients who carried rare variants of FLNC from May 2016 to May 2019 in the Center for Molecular Medicine, Children's Hospital of Fudan University, from clinical exome sequencing data.

RESULTS

A total of 5 patients with FLNC rare variants were included. Of them, 3 were male and 2 were female. The median age was 3 months (range from 19 days to 30 months). A1186V was a known pathogenic variant reported in pediatric patients with cardiomyopathy (PMID: 29858533), and the other four variants were novel. In the four novel variants, there are one splicing (c.2265+4del) and three missense (p.R441I, p.C1639Y, and p.A2648S). Two patients (patients 1 and 3) were diagnosed with restrictive cardiomyopathy, two patients (patients 2 and 5) were diagnosed with dilated cardiomyopathy, and one patient (patient 4) was diagnosed with arrhythmia.

CONCLUSIONS

All five patients have survived to date. In summary, FLNC rare variants identified by clinical exome sequencing provide genetic evidence to make early diagnosis of cardiomyopathy in infant patients.

Adhesions Assemble!-Autoinhibition as a Major Regulatory Mechanism of Integrin-Mediated Adhesion

The advent of cell-cell and cell-extracellular adhesion enabled cells to interact in a coherent manner, forming larger structures and giving rise to the development of tissues, organs and complex multicellular life forms. The development of such organisms required tight regulation of dynamic adhesive structures by signaling pathways that coordinate cell attachment. Integrin-mediated adhesion to the extracellular matrix provides cells with support, survival signals and context-dependent cues that enable cells to run different cellular programs. One mysterious aspect of the process is how hundreds of proteins assemble seemingly spontaneously onto the activated integrin. An emerging concept is that adhesion assembly is regulated by autoinhibition of key proteins, a highly dynamic event that is modulated by a variety of signaling events. By enabling precise control of the activation state of proteins, autoinhibition enables localization of inactive proteins and the formation of pre-complexes. In response to the correct signals, these proteins become active and interact with other proteins, ultimately leading to development of cell-matrix junctions. Autoinhibition of key components of such adhesion complexes—including core components integrin, talin, vinculin, and FAK and important peripheral regulators such as RIAM, Src, and DLC1—leads to a view that the majority of proteins involved in complex assembly might be regulated by intramolecular interactions. Autoinhibition is relieved via multiple different signals including post-translation modification and proteolysis. More recently, mechanical forces have been shown to stabilize and increase the lifetimes of active conformations, identifying autoinhibition as a means of encoding mechanosensitivity. The complexity and scope for nuanced adhesion dynamics facilitated via autoinhibition provides numerous points of regulation. In this review, we discuss what is known about this mode of regulation and how it leads to rapid and tightly controlled assembly and disassembly of cell-matrix adhesion.

Filamin A mediates isotropic distribution of applied force across the actin network

In this work, Kumar et al. use their previously developed talin tension sensor to study the immediate response of cells to uniaxial stretch. Tension measurements together with high-resolution electron microscopy reveal a novel role for the actin cross-linking protein filamin A in mediating tensional symmetry within the F-actin network.

Cell sensing of externally applied mechanical strain through integrin-mediated adhesions is critical in development and physiology of muscle, lung, tendon, and arteries, among others. We examined the effects of strain on force transmission through the essential cytoskeletal linker talin. Using a fluorescence-based talin tension sensor (TS), we found that uniaxial stretch of cells on elastic substrates increased tension on talin, which was unexpectedly independent of the orientation of the focal adhesions relative to the direction of strain. High-resolution electron microscopy of the actin cytoskeleton revealed that stress fibers (SFs) are integrated into an isotropic network of cortical actin filaments in which filamin A (FlnA) localizes preferentially to points of intersection between SFs and cortical actin. Knockdown (KD) of FlnA resulted in more isolated, less integrated SFs. After FlnA KD, tension on talin was polarized in the direction of stretch, while FlnA reexpression restored tensional symmetry. These data demonstrate that a FlnA-dependent cortical actin network distributes applied forces over the entire cytoskeleton–matrix interface.

Septo-optic dysplasia caused by a novel FLNA splice site mutation: a case report

Background

Septo-optic dysplasia (SOD), also known as de-Morsier syndrome, is a rare disorder characterized by any combination of optic nerve hypoplasia, pituitary gland hypoplasia, and midline abnormalities of the brain including absence of the septum pellucidum and corpus callosum dysgenesis. The variable presentation of SOD includes visual, neurologic, and/or hypothalamic-pituitary endocrine defects. The unclear aetiology of a large proportion of SOD cases underscores the importance of identifying novel SOD-associated genes.

Case presentation

To identify the disease-causing gene in a male infant with neonatal hypoglycaemia, dysmorphic features, and hypoplasia of the optic nerve and corpus callosum, we designed a targeted next-generation sequencing panel for brain morphogenesis defects. We identified a novel hemizygous deletion, c.6355 + 4_6355 + 5delAG, in intron 38 of the FLNA gene that the patient had inherited from his mother. cDNA studies showed that this variant results in the production of 3 aberrant FLNA transcripts, the most abundant of which results in retention of intron 38 of FLNA.

Conclusions

We report for the first time a case of early-onset SOD associated with a mutation in the FLNA gene. This finding broadens the spectrum of genetic causes of this rare disorder and expands the phenotypic spectrum of the FLNA gene.

The Host Scaffolding Protein Filamin A and the Exocyst Complex Control Exocytosis during InlB-Mediated Entry of Listeria monocytogenes

Listeria monocytogenes is a foodborne bacterium that causes gastroenteritis, meningitis, or abortion. Listeria induces its internalization (entry) into some human cells through interaction of the bacterial surface protein InlB with its host receptor, the Met tyrosine kinase. InlB and Met promote entry, in part, through stimulation of localized exocytosis. How exocytosis is upregulated during entry is not understood. Here, we show that the human signaling proteins mTOR, protein kinase C-α (PKC-α), and RalA promote exocytosis during entry by controlling the scaffolding protein Filamin A (FlnA). InlB-mediated uptake was accompanied by PKC-α-dependent phosphorylation of serine 2152 in FlnA. Depletion of FlnA by RNA interference (RNAi) or expression of a mutated FlnA protein defective in phosphorylation impaired InlB-dependent internalization. These findings indicate that phosphorylation of FlnA by PKC-α contributes to entry. mTOR and RalA were found to mediate the recruitment of FlnA to sites of InlB-mediated entry. Depletion of PKC-α, mTOR, or FlnA each reduced exocytosis during InlB-mediated uptake. Because the exocyst complex is known to mediate polarized exocytosis, we examined if PKC-α, mTOR, RalA, or FlnA affects this complex. Depletion of PKC-α, mTOR, RalA, or FlnA impaired recruitment of the exocyst component Exo70 to sites of InlB-mediated entry. Experiments involving knockdown of Exo70 or other exocyst proteins demonstrated an important role for the exocyst complex in uptake of Listeria Collectively, our results indicate that PKC-α, mTOR, RalA, and FlnA comprise a signaling pathway that mobilizes the exocyst complex to promote infection by Listeria.

A comprehensive analysis of the allergenicity and IgE epitopes of myosinogen allergens in Scylla paramamosain

BACKGROUND

Scylla paramamosain is one of the most common and serious food allergens in Asia. Therefore, research on its prevalence, accurate diagnosis, and IgE-binding pattern of the allergens is crucial.

OBJECTIVE

To identify the IgE epitopes of the myosinogen allergens in S. paramamosain using phage peptide library.

METHODS

The prevalence of allergy to crabs (AC) and of sensitization was analysed using a questionnaire and a serological assay. BAT was performed by flow cytometry, and its diagnostic performance was evaluated in relation to allergens purified from crab myosinogen. IgE-binding epitopes were identified by phage display using the IgE from patients with AC. Sequence- and structure-based bioinformatics analyses were performed to identify allergenic epitopes.

RESULTS

Crab was the most common cause of food allergies in this study. Subjects with AC (n = 30) with clear clinical symptoms were identified by immunoblotting and BAT. All of the myosinogen allergens triggered basophil activation; surface expression of CD63 and CD203c was higher in patients allergic to AK and FLN c than in patients allergic to SCP and TIM. In addition to six conformational epitopes of SCP, six linear epitopes and eight conformational epitopes of AK were identified. Five linear epitopes and three conformational epitopes of TIM, nine linear and ten conformational epitopes of FLN c were also identified, and the sequence VH(I/T) L was appeared in epitopes of both TIM and FLN c. The number of epitopes showed consistency with the value of BAT.

CONCLUSIONS AND CLINICAL RELEVANCE

BAT can be used for accurate diagnosis of AC. Identification of particular allergenic motifs could be a valuable tool for prevention, diagnosis, and treatment of food allergies.

Structural basis of the filamin A actin-binding domain interaction with F-actin

Actin-cross-linking proteins assemble actin filaments into higher-order structures essential for orchestrating cell shape, adhesion, and motility. Missense mutations in the tandem calponin homology domains of their actin-binding domains (ABDs) underlie numerous genetic diseases, but a molecular understanding of these pathologies is hampered by the lack of high-resolution structures of any actin-cross-linking protein bound to F-actin. Here, taking advantage of a high-affinity, disease-associated mutant of the human filamin A (FLNa) ABD, we combine cryo-electron microscopy and functional studies to reveal at near-atomic resolution how the first calponin homology domain (CH1) and residues immediately N-terminal to it engage actin. We further show that reorientation of CH2 relative to CH1 is required to avoid clashes with actin and to expose F-actin-binding residues on CH1. Our data explain localization of disease-associated loss-of-function mutations to FLNaCH1 and gain-of-function mutations to the regulatory FLNaCH2. Sequence conservation argues that this provides a general model for ABD-F-actin binding.

cAMP/PKA-induced filamin A (FLNA) phosphorylation inhibits SST2 signal transduction in GH-secreting pituitary tumor cells

An efficient intracellular response to somatostatin analogs (SSA) in pituitary tumors requires filamin A (FLNA). Since cAMP pathway plays an important role in GH-secreting pituitary tumors pathogenesis and FLNA is phosphorylated by PKA on S2152, aim of this study was to investigate in tumoral somatotrophs the impact of cAMP pathway activation and SSA stimulation on FLNA phosphorylation and the consequences on SST2 function. We found a PKA-mediated increase (2-fold) and SST2 agonist-induced decrease (-50%) of FLNA phosphorylation in GH3, GH4C1 and primary somatotroph tumor cells. This modification regulates FLNA function. Indeed, phosphomimetic S2152D FLNA mutant, but not phosphodeficient S2152A, abolished the known SSA antitumoral effects, namely: 1) inhibition of cell proliferation, reduction of cyclin D3 and increase of p27; 2) increase of cell apoptosis; 3) inhibition of cell migration via RhoA activation and cofilin phosphorylation. Coimmunoprecipitation and immunofluorescence assays showed that S2152A FLNA was recruited to activated SST2, whereas S2152D FLNA constitutively bound SST2 on the plasma membrane, but prevented G_αi proteins recruitment to SST2. In conclusion, we demonstrated that FLNA phosphorylation, promoted by cAMP pathway activation and inhibited by SSA, prevented SST2 signaling in GH-secreting tumoral pituitary cells.

Comparative analysis of the two extremes of FLNB-mutated autosomal dominant disease spectrum: from clinical phenotypes to cellular and molecular findings

Non-randomly distributed missense mutations of Filamin B (FLNB) can lead to a spectrum of autosomal dominant-inherited skeletal malformations caused by bone hypoplasia, including Larsen syndrome (LS), atelosteogenesi-I (AO-I), atelosteogenesi-I (AO-III) and boomerang dysplasia (BD). Among this spectrum of diseases, LS causes a milder hypoplasia of the skeletal system, compared to BD's much more severe symptoms. Previous studies revealed limited molecular mechanisms of FLNB-related diseases but most of them were carried out with HEK293 cells from the kidney which could not reproduce FLNB's specificity to skeletal tissues. Instead, we elected to use ATDC5, a chondrogenic stem cell line widely used to study endochondral osteogenesis. In this study, we established FLNB-transfected ATDC5 cell model. We reported a pedigree of LS with mutation of FLNB^G1586R and reviewed a case of BD with mutation of FLNB^L171R . Using the ATDC5 cell model above, we compared cellular and molecular phenotypes of BD-associated FLNB^L171R and LS-associated FLNB^G1586R . We found that while both phenotypes had an increased expression of Runx2, FLNB^L171R-expressing ATDC5 cells presented globular aggregation of FLNB protein and increased cellular apoptosis rate while FLNB^G1586R-expressing ATDC5 cells presented evenly distributed FLNB protein and decreased cellular migration. These findings support our explanation for the cause of differences in clinical phenotypes between LS and BD. Our study makes a comparative analysis of two extremes of the FLNB-mutated autosomal dominant spectrum, relating known clinical phenotypes to our new cellular and molecular findings. These results indicated next steps for future research on the role of FLNB in the physiological process of endochondral osteogenesis.

Differential regulation of two FLNA transcripts explains some of the phenotypic heterogeneity in the loss-of-function filaminopathies

Loss-of-function mutations in the X-linked gene FLNA can lead to abnormal neuronal migration, vascular and cardiac defects, and congenital intestinal pseudo-obstruction (CIPO), the latter characterized by anomalous intestinal smooth muscle layering. Survival in male hemizygotes for such mutations is dependent on retention of residual FLNA function but it is unclear why a subgroup of males with mutations in the 5' end of the gene can present with CIPO alone. Here, we demonstrate evidence for the presence of two FLNA isoforms differing by 28 residues at the N-terminus initiated at ATG⁺¹ and ATG⁺⁸² . A male with CIPO (c.18_19del) exclusively expressed FLNA ATG⁺⁸² , implicating the longer protein isoform (ATG⁺¹ ) in smooth muscle development. In contrast, mutations leading to reduction of both isoforms are associated with compound phenotypes affecting the brain, heart, and intestine. RNA-seq data revealed three distinct transcription start sites, two of which produce a protein isoform utilizing ATG⁺¹ while the third utilizes ATG⁺⁸² . Transcripts sponsoring translational initiation at ATG⁺¹ predominate in intestinal smooth muscle, and are more abundant compared with the level measured in fibroblasts. Together these observations describe a new mechanism of tissue-specific regulation of FLNA that could reflect the differing mechanical requirements of these cell types during development.

Disruption of TACE-filamin interaction can inhibit TACE-mediated ectodomain shedding

Ectodomain shedding regulates functions of many membrane proteins through the cleavage of their juxtamembrane region mainly by a disintegrin and metalloproteinase family proteinases. Tumor necrosis factor-alpha converting enzyme (TACE) is known to be responsible for phorbol myristate acetate (PMA)-induced shedding of various membrane proteins. How PMA regulates TACE-dependent shedding and how TACE exhibits substrate specificity without proteolysis of other membrane proteins are questionable. Here, we show that TACE can interact with an actin-binding protein, filamin, through 20th filamin repeat. We found that the interaction between TACE and filamin was increased by PMA treatment. In addition, loss of filamin or specific disruption of TACE-filamin interaction inhibited ectodomain shedding of representative TACE substrates, CD44 and amyloid protein precursor. From these data, we suggest that filamin may work as a scaffold that can recruit TACE and its substrates in a PMA-dependent manner to achieve substrate specificity for TACE.

Skeletal Dysplasia Mutations Effect on Human Filamins' Structure and Mechanosensing

Cells' ability to sense mechanical cues in their environment is crucial for fundamental cellular processes, leading defects in mechanosensing to be linked to many diseases. The actin cross-linking protein Filamin has an important role in the conversion of mechanical forces into biochemical signals. Here, we reveal how mutations in Filamin genes known to cause Larsen syndrome and Frontometaphyseal dysplasia can affect the structure and therefore function of Filamin domains 16 and 17. Employing X-ray crystallography, the structure of these domains was first solved for the human Filamin B. The interaction seen between domains 16 and 17 is broken by shear force as revealed by steered molecular dynamics simulations. The effects of skeletal dysplasia associated mutations of the structure and mechanosensing properties of Filamin were studied by combining various experimental and theoretical techniques. The results showed that Larsen syndrome associated mutations destabilize or even unfold domain 17. Interestingly, those Filamin functions that are mediated via domain 17 interactions with other proteins are not necessarily affected as strongly interacting peptide binding to mutated domain 17 induces at least partial domain folding. Mutation associated to Frontometaphyseal dysplasia, in turn, transforms 16-17 fragment from compact to an elongated form destroying the force-regulated domain pair.

Conformational plasticity and evolutionary analysis of the myotilin tandem Ig domains

Myotilin is a component of the sarcomere where it plays an important role in organisation and maintenance of Z-disk integrity. This involves direct binding to F-actin and filamin C, a function mediated by its Ig domain pair. While the structures of these two individual domains are known, information about their relative orientation and flexibility remains limited. We set on to characterise the Ig domain pair of myotilin with emphasis on its molecular structure, dynamics and phylogeny. First, sequence conservation analysis of myotilin shed light on the molecular basis of myotilinopathies and revealed several motifs in Ig domains found also in I-band proteins. In particular, a highly conserved Glu344 mapping to Ig domain linker, was identified as a critical component of the inter-domain hinge mechanism. Next, SAXS and molecular dynamics revealed that Ig domain pair exists as a multi-conformation species with dynamic exchange between extended and compact orientations. Mutation of AKE motif to AAA further confirmed its impact on inter-domain flexibility. We hypothesise that the conformational plasticity of the Ig domain pair in its unbound form is part of the binding partner recognition mechanism.

Structural and thermodynamic basis of a frontometaphyseal dysplasia mutation in filamin A

Filamin-mediated linkages between transmembrane receptors (TR) and the actin cytoskeleton are crucial for regulating many cytoskeleton-dependent cellular processes such as cell shape change and migration. A major TR binding site in the immunoglobulin repeat 21 (Ig21) of filamin is masked by the adjacent repeat Ig20, resulting in autoinhibition. The TR binding to this site triggers the relief of Ig20 and protein kinase A (PKA)-mediated phosphorylation of Ser-2152, thereby dynamically regulating the TR-actin linkages. A P2204L mutation in Ig20 reportedly cause frontometaphyseal dysplasia, a skeletal disorder with unknown pathogenesis. We show here that the P2204L mutation impairs a hydrophobic core of Ig20, generating a conformationally fluctuating molten globule-like state. Consequently, unlike in WT filamin, where PKA-mediated Ser-2152 phosphorylation is ligand-dependent, the P2204L mutant is readily accessible to PKA, promoting ligand-independent phosphorylation on Ser-2152. Strong TR peptide ligands from platelet GP1bα and G-protein-coupled receptor MAS effectively bound Ig21 by displacing Ig20 from autoinhibited WT filamin, but surprisingly, the capacity of these ligands to bind the P2204L mutant was much reduced despite the mutation-induced destabilization of the Ig20 structure that supposedly weakens the autoinhibition. Thermodynamic analysis indicated that compared with WT filamin, the conformationally fluctuating state of the Ig20 mutant makes Ig21 enthalpically favorable to bind ligand but with substantial entropic penalty, resulting in total higher free energy and reduced ligand affinity. Overall, our results reveal an unusual structural and thermodynamic basis for the P2204L-induced dysfunction of filamin and frontometaphyseal dysplasia disease.

Evidence for the mechanosensor function of filamin in tissue development

Cells integrate mechanical properties of their surroundings to form multicellular, three-dimensional tissues of appropriate size and spatial organisation. Actin cytoskeleton-linked proteins such as talin, vinculin and filamin function as mechanosensors in cells, but it has yet to be tested whether the mechanosensitivity is important for their function in intact tissues. Here we tested, how filamin mechanosensing contributes to oogenesis in Drosophila. Mutations that require more or less force to open the mechanosensor region demonstrate that filamin mechanosensitivity is important for the maturation of actin-rich ring canals that are essential for Drosophila egg development. The open mutant was more tightly bound to the ring canal structure while the closed mutant dissociated more frequently. Thus, our results show that an appropriate level of mechanical sensitivity is required for filamins’ function and dynamics during Drosophila egg growth and support the structure-based model in which the opening and closing of the mechanosensor region regulates filamin binding to cellular components.

Induction and adaptation of chaperone-assisted selective autophagy CASA in response to resistance exercise in human skeletal muscle

Chaperone-assisted selective autophagy (CASA) is a tension-induced degradation pathway essential for muscle maintenance. Impairment of CASA causes childhood muscle dystrophy and cardiomyopathy. However, the importance of CASA for muscle function in healthy individuals has remained elusive so far. Here we describe the impact of strength training on CASA in a group of healthy and moderately trained men. We show that strenuous resistance exercise causes an acute induction of CASA in affected muscles to degrade mechanically damaged cytoskeleton proteins. Moreover, repeated resistance exercise during 4 wk of training led to an increased expression of CASA components. In human skeletal muscle, CASA apparently acts as a central adaptation mechanism that responds to acute physical exercise and to repeated mechanical stimulation.

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.

An adventitious interaction of filamin A with RhoGDI2(Tyr153Glu)

Filamin A (FLNA) is an actin filament crosslinking protein with multiple intracellular binding partners. Mechanical force exposes cryptic FLNA binding sites for some of these ligands. To identify new force-dependent binding interactions, we used a fusion construct composed of two FLNA domains, one of which was previously identified as containing a force-dependent binding site as a bait in a yeast two-hybrid system and identified the Rho dissociation inhibitor 2 (RhoGDI2) as a potential interacting partner. A RhoGDI2 truncate with 81 N-terminal amino acid residues and a phosphomimetic mutant, RhoGDI(Tyr153Glu) interacted with the FLNA construct. However, neither wild-type or full-length RhoGDI2 phosphorylated at Y153 interacted with FLNA. Our interpretation of these contradictions is that truncation and/or mutation of RhoGDI2 perturbs its conformation to expose a site that adventitiously binds FLNA and is not a bona-fide interaction. Therefore, previous studies reporting that a RhoGDI(Y153E) mutant suppresses the metastasis of human bladder cancer cells must be reinvestigated in light of artificial interaction of this point mutant with FLNA.

G protein-coupled receptors directly bind filamin A with high affinity and promote filamin phosphorylation

Although interaction of a few G protein-coupled receptors (GPCRs) with Filamin A, a key actin cross-linking and biomechanical signal transducer protein, has been observed, a comprehensive structure-function analysis of this interaction is lacking. Through a systematic sequence-based analysis, we found that a conserved filamin binding motif is present in the cytoplasmic domains of >20% of the 824 GPCRs encoded in the human genome. Direct high-affinity interaction of filamin binding motif peptides of select GPCRs with the Ig domain of Filamin A was confirmed by nuclear magnetic resonance spectroscopy and isothermal titration calorimetric experiments. Engagement of the filamin binding motif with the Filamin A Ig domain induced the phosphorylation of filamin by protein kinase A in vitro. In transfected cells, agonist activation as well as constitutive activation of representative GPCRs dramatically elicited recruitment and phosphorylation of cellular Filamin A, a phenomenon long known to be crucial for regulating the structure and dynamics of the cytoskeleton. Our data suggest a molecular mechanism for direct GPCR-cytoskeleton coupling via filamin. Until now, GPCR signaling to the cytoskeleton was predominantly thought to be indirect, through canonical G protein-mediated signaling cascades involving GTPases, adenylyl cyclases, phospholipases, ion channels, and protein kinases. We propose that the GPCR-induced filamin phosphorylation pathway is a conserved, novel biochemical signaling paradigm.

Flexible Structure of Peptide-Bound Filamin A Mechanosensor Domain Pair 20-21

Filamins (FLNs) are large, multidomain actin cross-linking proteins with diverse functions. Besides regulating the actin cytoskeleton, they serve as important links between the extracellular matrix and the cytoskeleton by binding cell surface receptors, functioning as scaffolds for signaling proteins, and binding several other cytoskeletal proteins that regulate cell adhesion dynamics. Structurally, FLNs are formed of an amino terminal actin-binding domain followed by 24 immunoglobulin-like domains (IgFLNs). Recent studies have demonstrated that myosin-mediated contractile forces can reveal hidden protein binding sites in the domain pairs IgFLNa18–19 and 20–21, enabling FLNs to transduce mechanical signals in cells. The atomic structures of these mechanosensor domain pairs in the resting state are known, as well as the structures of individual IgFLN21 with ligand peptides. However, little experimental data is available on how interacting protein binding deforms the domain pair structures. Here, using small-angle x-ray scattering-based modelling, x-ray crystallography, and NMR, we show that the adaptor protein migfilin-derived peptide-bound structure of IgFLNa20–21 is flexible and adopts distinctive conformations depending on the presence or absence of the interacting peptide. The conformational changes reported here may be common for all peptides and may play a role in the mechanosensor function of the site.

Study of the influence of actin-binding proteins using linear analyses of cell deformability

The actin cytoskeleton plays a key role in the deformability of the cell and in mechanosensing. Here we analyze the contributions of three major actin cross-linking proteins, myosin II, α-actinin and filamin, to cell deformability, by using micropipette aspiration of Dictyostelium cells. We examine the applicability of three simple mechanical models: for small deformation, linear viscoelasticity and drop of liquid with a tense cortex; and for large deformation, a Newtonian viscous fluid. For these models, we have derived linearized equations and we provide a novel, straightforward methodology to analyze the experiments. This methodology allowed us to differentiate the effects of the cross-linking proteins in the different regimes of deformation. Our results confirm some previous observations and suggest important relations between the molecular characteristics of the actin-binding proteins and the cell behavior: the effect of myosin is explained in terms of the relation between the lifetime of the bond to actin and the resistive force; the presence of α-actinin obstructs the deformation of the cytoskeleton, presumably mainly due to the higher molecular stiffness and to the lower dissociation rate constants; and filamin contributes critically to the global connectivity of the network, possibly by rapidly turning over cross-links during the remodeling of the cytoskeletal network, thanks to the higher rate constants, flexibility and larger size. The results suggest a sophisticated relationship between the expression levels of actin-binding proteins, deformability and mechanosensing.

Regulation of integrin-mediated adhesions

Integrins are heterodimeric transmembrane adhesion receptors that couple the actin cytoskeleton to the extracellular environment and bidirectionally relay signals across the cell membrane. These processes are critical for cell attachment, migration, differentiation, and survival, and therefore play essential roles in metazoan development, physiology, and pathology. Integrin-mediated adhesions are regulated by diverse factors, including the conformation-specific affinities of integrin receptors for their extracellular ligands, the clustering of integrins and their intracellular binding partners into discrete adhesive structures, mechanical forces exerted on the adhesion, and the intracellular trafficking of integrins themselves. Recent advances shed light onto how the interaction of specific intracellular proteins with the short cytoplasmic tails of integrins controls each of these activities.

Exon skipping causes atypical phenotypes associated with a loss-of-function mutation in FLNA by restoring its protein function

Loss-of-function mutations in filamin A (FLNA) cause an X-linked dominant disorder with multiple organ involvement. Affected females present with periventricular nodular heterotopia (PVNH), cardiovascular complications, thrombocytopenia and Ehlers-Danlos syndrome. These mutations are typically lethal to males, and rare male survivors suffer from failure to thrive, PVNH, and severe cardiovascular and gastrointestinal complications. Here we report two surviving male siblings with a loss-of-function mutation in FLNA. They presented with multiple complications, including valvulopathy, intestinal malrotation and chronic intestinal pseudo-obstruction (CIPO). However, these siblings had atypical clinical courses, such as a lack of PVNH and a spontaneous improvement of CIPO. Trio-based whole-exome sequencing revealed a 4-bp deletion in exon 40 that was predicted to cause a lethal premature protein truncation. However, molecular investigations revealed that the mutation induced in-frame skipping of the mutated exon, which led to the translation of a mutant FLNA missing an internal region of 41 amino acids. Functional analyses of the mutant protein suggested that its binding affinity to integrin, as well as its capacity to induce focal adhesions, were comparable to those of the wild-type protein. These results suggested that exon skipping of FLNA partially restored its protein function, which could contribute to amelioration of the siblings' clinical courses. This study expands the diversity of the phenotypes associated with loss-of-function mutations in FLNA.

Role of EGFR expression levels in the regulation of integrin function by EGF

Activation of β1 integrins in dormant tumor cells has been linked to metastatic progression, suggesting that therapies designed to maintain β1 integrins in an inactive state may be useful in the prevention of metastatic disease. Our earlier studies have demonstrated that EGF regulates the activation state of the α5β1 integrin in EGFR overexpressing tumor cells through an ERK/p90RSK signaling pathway. Activation of this pathway by EGF resulted in the filamin A dependent inactivation of the α5β1 integrin receptor for fibronectin. The current study was designed to address the role of EGFR overexpression in the regulation of α5β1 integrin activation state by EGF. Lentiviral knockdown of EGFR coupled with limited dilution cloning was used to develop A431 squamous carcinoma cell lines expressing high, moderate, and low levels of EGFR. Inactivation of α5β1 integrin by EGF was shown to correlate with both the level of EGFR expression and the extent of p90RSK phosphorylation, but not with the level of ERK phosphorylation, suggesting that high levels of EGFR promote α5β1 integrin inactivation through sustained activation of p90RSK. Treatment of cells with EGFR kinase inhibitor resulted in a reactivation of the integrin which could be reversed with the phosphatase inhibitor, menadione. Taken together, these findings indicate that p90RSK may function to maintain dormancy in tumor cells expressing high levels of EGFR. © 2015 Wiley Periodicals, Inc.