A new member of the LIM protein family binds to filamin B and localizes at stress fibers

Profound Effects of Dexamethasone on the Immunological State, Synthesis and Secretion Capacity of Human Testicular Peritubular Cells

The functions of human testicular peritubular cells (HTPCs), forming a small compartment located between the seminiferous epithelium and the interstitial areas of the testis, are not fully known but go beyond intratesticular sperm transport and include immunological roles. The expression of the glucocorticoid receptor (GR) indicates that they may be regulated by glucocorticoids (GCs). Herein, we studied the consequences of the GC dexamethasone (Dex) in cultured HTPCs, which serves as a unique window into the human testis. We examined changes in cytokines, mainly by qPCR and ELISA. A holistic mass-spectrometry-based proteome analysis of cellular and secreted proteins was also performed. Dex, used in a therapeutic concentration, decreased the transcript level of proinflammatory cytokines, e.g., IL6, IL8 and MCP1. An siRNA-mediated knockdown of GR reduced the actions on IL6. Changes in IL6 were confirmed by ELISA measurements. Of note, Dex also lowered GR levels. The proteomic results revealed strong responses after 24 h (31 significantly altered cellular proteins) and more pronounced ones after 72 h of Dex exposure (30 less abundant and 42 more abundant cellular proteins). Dex also altered the composition of the secretome (33 proteins decreased, 13 increased) after 72 h. Among the regulated proteins were extracellular matrix (ECM) and basement membrane components (e.g., FBLN2, COL1A2 and COL3A1), as well as PTX3 and StAR. These results pinpoint novel, profound effects of Dex in HTPCs. If transferrable to the human testis, changes specifically in ECM and the immunological state of the testis may occur in men upon treatment with Dex for medical reasons.

TK, Ahmad MN, Okashah SS, Kamaraj B, Al-Subaie AM, C. GPD, Zayed H. Deciphering the Role of Filamin B Calponin-Homology Domain in Causing the Larsen Syndrome, Boomerang Dysplasia, and Atelosteogenesis Type I Spectrum Disorders via a Computational Approach

Filamins (FLN) are a family of actin-binding proteins involved in regulating the cytoskeleton and signaling phenomenon by developing a network with F-actin and FLN-binding partners. The FLN family comprises three conserved isoforms in mammals: FLNA, FLNB, and FLNC. FLNB is a multidomain monomer protein with domains containing an actin-binding N-terminal domain (ABD 1-242), encompassing two calponin-homology domains (assigned CH1 and CH2). Primary variants in FLNB mostly occur in the domain (CH2) and surrounding the hinge-1 region. The four autosomal dominant disorders that are associated with FLNB variants are Larsen syndrome, atelosteogenesis type I (AOI), atelosteogenesis type III (AOIII), and boomerang dysplasia (BD). Despite the intense clustering of FLNB variants contributing to the LS-AO-BD disorders, the genotype-phenotype correlation is still enigmatic. In silico prediction tools and molecular dynamics simulation (MDS) approaches have offered the potential for variant classification and pathogenicity predictions. We retrieved 285 FLNB missense variants from the UniProt, ClinVar, and HGMD databases in the current study. Of these, five and 39 variants were located in the CH1 and CH2 domains, respectively. These variants were subjected to various pathogenicity and stability prediction tools, evolutionary and conservation analyses, and biophysical and physicochemical properties analyses. Molecular dynamics simulation (MDS) was performed on the three candidate variants in the CH2 domain (W148R, F161C, and L171R) that were predicted to be the most pathogenic. The MDS analysis results showed that these three variants are highly compact compared to the native protein, suggesting that they could affect the protein on the structural and functional levels. The computational approach demonstrates the differences between the FLNB mutants and the wild type in a structural and functional context. Our findings expand our knowledge on the genotype-phenotype correlation in FLNB-related LS-AO-BD disorders on the molecular level, which may pave the way for optimizing drug therapy by integrating precision medicine.

Regulation of cell adhesion: a collaborative effort of integrins, their ligands, cytoplasmic actors, and phosphorylation

Integrins are large heterodimeric type 1 membrane proteins expressed in all nucleated mammalian cells. Eighteen α-chains and eight β-chains can combine to form 24 different integrins. They are cell adhesion proteins, which bind to a large variety of cellular and extracellular ligands. Integrins are required for cell migration, hemostasis, translocation of cells out from the blood stream and further movement into tissues, but also for the immune response and tissue morphogenesis. Importantly, integrins are not usually active as such, but need activation to become adhesive. Integrins are activated by outside-in activation through integrin ligand binding, or by inside-out activation through intracellular signaling. An important question is how integrin activity is regulated, and this topic has recently drawn much attention. Changes in integrin affinity for ligand binding are due to allosteric structural alterations, but equally important are avidity changes due to integrin clustering in the plane of the plasma membrane. Recent studies have partially solved how integrin cell surface structures change during activation. The integrin cytoplasmic domains are relatively short, but by interacting with a variety of cytoplasmic proteins in a regulated manner, the integrins acquire a number of properties important not only for cell adhesion and movement, but also for cellular signaling. Recent work has shown that specific integrin phosphorylations play pivotal roles in the regulation of integrin activity. Our purpose in this review is to integrate the present knowledge to enable an understanding of how cell adhesion is dynamically regulated.

Identification of Filamin A Mechanobinding Partner I: Smoothelin Specifically Interacts with the Filamin A Mechanosensitive Domain 21

Filamin A (FLNA) is a ubiquitously expressed actin cross-linking protein and a scaffold of numerous binding partners to regulate cell proliferation, migration, and survival. FLNA is a homodimer, and each subunit has an N-terminal actin-binding domain followed by 24 immunoglobulin-like repeats (R). FLNA mediates mechanotransduction by force-induced conformational changes of its cryptic integrin-binding site on R21. Here, we identified two novel FLNA-binding partners, smoothelins (SMTN A and B) and leucine zipper protein 1 (LUZP1), using stable isotope labeling by amino acids in cell culture (SILAC)-based proteomics followed by an in silico screening for proteins having a consensus FLNA-binding domain. We found that, although SMTN does not interact with full-length FLNA, it binds to FLNA variant 1 (FLNAvar-1) that exposes the cryptic CD cleft of R21. Point mutations on the C strand that disrupt the integrin binding also block the SMTN interaction. We identified FLNA-binding domains on SMTN using mutagenesis and used the mutant SMTN to investigate the role of the FLNA-SMTN interaction on the dynamics and localization of SMTN in living cells. Fluorescence recovery after photobleaching (FRAP) of GFP-labeled SMTN in living cells demonstrated that the non-FLNA-binding mutant SMTN diffuses faster than wild-type SMTN. Moreover, inhibition of Rho-kinase using Y27632 also increases the diffusion. These data demonstrated that SMTN specifically interacts with FLNAvar-1 and mechanically activated FLNA in cells. The companion report (Wang and Nakamura, 2019) describes the interactions of FLNA with the transcript of the LUZP1 gene.

Possible involvement of a cell adhesion molecule, Migfilin, in brain development and pathogenesis of autism spectrum disorders

Migfilin, encoded by FBLIM1 at the 1p36 locus, is a multi-domain adaptor protein essential for various cellular processes such as cell morphology and migration. Small deletions and duplications at the 1p36 locus, monosomy of which results in neurodevelopmental disorders and multiple congenital anomalies, have also been identified in patients with autism spectrum disorder (ASD). However, the impact of FBLIM1, the gene within 1p36, on the pathogenesis of ASD is unknown. In this study, we performed morphological analyses of migfilin to elucidate its role in brain development. Migfilin was detected specifically in the embryonic and perinatal stages of the mouse brain. Either silencing or overexpression of migfilin in embryos following in utero electroporation disrupted Neocortical neuronal migration. Additionally, neurite elongation was impaired when migfilin was silenced in cultured mouse hippocampal neurons. We then screened FBLIM1 for rare exonic deletions/duplications in 549 Japanese ASD patients and 824 controls, detecting one case of ASD and intellectual delay that harbored a 26-kb deletion at 1p36.21 that solely included the C-terminal exon of FBLIM1. The FBLIM1 mRNA expression level in this case was reduced compared to levels in individuals without FBLIM1 deletion. Our findings indicate that tightly regulated expression of migfilin is essential for neuronal development and that FBLIM1 disruption may be related to the phenotypes associated with ASD and related neurodevelopmental disorders.

The role of kindlin in neutrophil recruitment to inflammatory sites

PURPOSE OF REVIEW

Since the discovery of the lack of kindlin-3 expression as the reason for the immunopathology leukocyte adhesion deficiency III syndrome, the role of kindlin-3 in inflammatory processes was investigated in a numerous studies. This review gives an overview about recent findings regarding the role of kindlin-3 in neutrophil activation and recruitment.

RECENT FINDINGS

Kindlin-3, together with talin-1, contributes essentially to the activation of β2-integrins in neutrophils. During inside-out signaling, kindlin-3 binds to the β-cytoplasmic integrin tail and is indispensable for the integrin conformational shift into the high-affinity ligand binding conformation, but not for the intermediate (extended) conformation. During outside-in signaling (as a consequence of integrin ligand binding) kindlin-3 interacts with distinct signaling molecules and is required for cell-autonomous functions like migration and spreading.

SUMMARY

Leukocyte adhesion deficiency III syndrome, which is caused by absence of kindlin-3, is a rarely occurring disease. However, the investigation of the clinical symptoms as well as the underlying molecular mechanisms gave rise to a huge amount of new insights into the processes of integrin activation in neutrophils and the consequences of defects in these processes.

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.

Structural Analysis of G1691S Variant in the Human Filamin B Gene Responsible for Larsen Syndrome: A Comparative Computational Approach

Larsen syndrome (LRS) is a rare genetic disease associated with variable manifestations including skeletal malformations, dislocations of the large joints, and notable changes in facial and limb features. Genetic variants in the Filamin B (FLNB) gene are associated with the development of LRS. We searched two literature databases (OMIM and PubMed) and three gene variant databases (HGMD, UniProt, & dbSNP) to capture all the possible variants associated with LRS phenotype, which may have an impact on the FLNB function. Our search yielded 77 variants that might impact the FLNB protein function in patients with LRS. We performed rigorous computational analysis such as conservational, biochemical, pathogenicity, and structural computational analyses to understand the deleterious effect of the G1691S variant. Further, the structural changes of the G1691S variant was compared with a null variant (G1691A) and the native protein through a molecular dynamic simulation study of 50 ns. We found that the variant G1691S was highly deleterious and destabilize the protein when compared to the native and variant G1691A. This might be due to the physicochemical changes in the variant G1691S when compared to the native and variant G1691A. The destabilization was further supported by transformation of bend to coil in variant G1691S whereas bend was retained in native and variant G1691A through molecular dynamics analysis. Our study shed light on the importance of computational methods to understand the molecular nature of genetic variants and structural insights on the function of the FLNB protein. J. Cell. Biochem. 118: 1900-1910, 2017. © 2017 Wiley Periodicals, Inc.

Interactome disassembly during apoptosis occurs independent of caspase cleavage

Protein-protein interaction networks (interactomes) define the functionality of all biological systems. In apoptosis, proteolysis by caspases is thought to initiate disassembly of protein complexes and cell death. Here we used a quantitative proteomics approach, protein correlation profiling (PCP), to explore changes in cytoplasmic and mitochondrial interactomes in response to apoptosis initiation as a function of caspase activity. We measured the response to initiation of Fas-mediated apoptosis in 17,991 interactions among 2,779 proteins, comprising the largest dynamic interactome to date. The majority of interactions were unaffected early in apoptosis, but multiple complexes containing known caspase targets were disassembled. Nonetheless, proteome-wide analysis of proteolytic processing by terminal amine isotopic labeling of substrates (TAILS) revealed little correlation between proteolytic and interactome changes. Our findings show that, in apoptosis, significant interactome alterations occur before and independently of caspase activity. Thus, apoptosis initiation includes a tight program of interactome rearrangement, leading to disassembly of relatively few, select complexes. These early interactome alterations occur independently of cleavage of these protein by caspases.

Supervillin binds the Rac/Rho-GEF Trio and increases Trio-mediated Rac1 activation

We investigated cross-talk between the membrane-associated, myosin II-regulatory protein supervillin and the actin-regulatory small GTPases Rac1, RhoA, and Cdc42. Supervillin knockdown reduced Rac1-GTP loading, but not the GTP loading of RhoA or Cdc42, in HeLa cells with normal levels of the Rac1-activating protein Trio. No reduction in Rac1-GTP loading was observed when supervillin levels were reduced in Trio-depleted cells. Conversely, overexpression of supervillin isoform 1 (SV1) or, especially, isoform 4 (SV4) increased Rac1 activation. Inhibition of the Trio-mediated Rac1 guanine nucleotide exchange activity with ITX3 partially blocked the SV4-mediated increase in Rac1-GTP. Both SV4 and SV1 co-localized with Trio at or near the plasma membrane in ruffles and cell surface projections. Two sequences within supervillin bound directly to Trio spectrin repeats 4-7: SV1-171, which contains N-terminal residues found in both SV1 and SV4 and the SV4-specific differentially spliced coding exons 3, 4, and 5 within SV4 (SV4-E345; SV4 amino acids 276-669). In addition, SV4-E345 interacted with the homologous sequence in rat kalirin (repeats 4-7, amino acids 531-1101). Overexpressed SV1-174 and SV4-E345 affected Rac1-GTP loading, but only in cells with endogenous levels of Trio. Trio residues 771-1057, which contain both supervillin-interaction sites, exerted a dominant-negative effect on cell spreading. Supervillin and Trio knockdowns, separately or together, inhibited cell spreading, suggesting that supervillin regulates the Rac1 guanine nucleotide exchange activity of Trio, and potentially also kalirin, during cell spreading and lamellipodia extension.

Documentation and localization of force-mediated filamin A domain perturbations in moving cells

Endogenously and externally generated mechanical forces influence diverse cellular activities, a phenomenon defined as mechanotransduction. Deformation of protein domains by application of stress, previously documented to alter macromolecular interactions in vitro, could mediate these effects. We engineered a photon-emitting system responsive to unfolding of two repeat domains of the actin filament (F-actin) crosslinker protein filamin A (FLNA) that binds multiple partners involved in cell signalling reactions and validated the system using F-actin networks subjected to myosin-based contraction. Expressed in cultured cells, the sensor-containing FLNA construct reproducibly reported FLNA domain unfolding strikingly localized to dynamic, actively protruding, leading cell edges. The unfolding signal depends upon coherence of F-actin-FLNA networks and is enhanced by stimulating cell contractility. The results establish protein domain distortion as a bona fide mechanism for mechanotransduction in vivo.

TMEM158 and FBLP1 as novel marker genes of cisplatin sensitivity in non-small cell lung cancer cells

Even after development of molecular targeting therapies, platinum-based chemotherapy is still a standard care for treatment of locally advanced non-small cell lung cancer (NSCLC). So far, critical molecular markers capable to predict the therapeutic response in NSCLC patients remain undetermined. We here attempted to identify novel biomarker genes for cisplatin (CDDP) for a tailored therapy. Initial screening to explorer association of IC(50) values of CDDP obtained by MTT assay and gene expression levels measured with oligonucleotide microarray and real-time RT-PCR provided 6 candidate genes, namely, NUBPL, C9orf30, ZNF12, TMEM158, GSK3B, and FBLP1 using 9 lung cancer cells consisting of 3 small and 6 NSCLC cells. These 6 genes together with 5 reported biomarkers, i.e., GSTP1, ERCC1, BRCA1, FRAP1, and RRM1, were subjected to a linear regression analysis using 12 NSCLC cell lines including 6 additional NSCLC cells: only FBLP1 and TMEM158 genes showed positive associations with statistical significances (P = .016 and .026, respectively). The biological significance of these genes was explored by in vitro experiments: Knockdown experiments in PC-9/CDDP cells revealed that the reduced expression of TMEM158 significantly decreased the chemo-resistance against CDDP (P <.0001), while 2 transformants of PC-6 cells stably over-expressing FBLP1 resulted in an enhanced resistance to CDDP (P = .004 and P = .001). Furthermore, a stepwise multiple regression analysis demonstrated the best prediction formula could be fixed when we used expression data of TMEM158 and FBLP1 (R(2) = 0.755, P = .0018). TMEM158 and FBLP1 may be powerful predictive biomarkers for CDDP therapy in NSCLC.

The activation of proteinase-activated receptor-1 (PAR1) promotes gastric cancer cell alteration of cellular morphology related to cell motility and invasion

Cell motility proceeds by cycles of edge protrusion, adhesion and retraction. Whether these functions are coordinated by biochemical or biomechanical processes is unknown. Tumor invasion and metastasis is directly related to cell motility. We showed that stimulation of proteinase-activated receptor-1 (PAR1) can trigger an array of responses that would promote tumor cell growth and invasion. Thus, we examined aspects of PAR1 activation related to cell morphological change that might contribute to cell motility. We established a PAR1 stably transfected MKN45 gastric cancer cell line (MKN45/PAR1). We examined morphological changes, Rho family activation and overexpression of cytoskeletal protein in cells exposed to PAR1 agonists (α-thrombin and TFLLR-NH2). MKN45/PAR1 grows with an elongated and polarized morphology, extending pseudopodia at the leading edge. However, in the presence of PAR1 antagonist, MKN45/PAR1 did not show any changes in cell shape upon addition of either α-thrombin or TFLLR-NH2. Activated PAR1 induced RhoA and Rac1 phosphorylation, and subsequent overexpression of myosin IIA and filamin B which are stress fiber components that were identified by PMF analysis of peptide mass data obtained by MALDI-TOF/MS measurement. Upon stimulation of MKN45/PAR1 for 24 h with either α-thrombin or TFLLR-NH2, the distribution of both myosin IIA and filamin B proteins shifted to being distributed throughout the cytoplasm to the membrane, with more intense luminescence signals than in the absence of stimulation. These results demonstrate that PAR1 activation induces cell morphological change associated with cell motility via Rho family activation and cytoskeletal protein overexpression, and has a critical role in gastric cancer cell invasion and metastasis.

Filamin interacts with epithelial sodium channel and inhibits its channel function

Epithelial sodium channel (ENaC) in the kidneys is critical for Na(+) balance, extracellular volume, and blood pressure. Altered ENaC function is associated with respiratory disorders, pseudohypoaldosteronism type 1, and Liddle syndrome. ENaC is known to interact with components of the cytoskeleton, but the functional roles remain largely unclear. Here, we examined the interaction between ENaC and filamins, important actin filament components. We first discovered by yeast two-hybrid screening that the C termini of ENaC α and β subunits bind filamin A, B, and C, and we then confirmed the binding by in vitro biochemical assays. We demonstrated by co-immunoprecipitation that ENaC, either overexpressed in HEK, HeLa, and melanoma A7 cells or natively expressed in LLC-PK1 and IMCD cells, is in the same complex with native filamin. Furthermore, the biotinylation and co-immunoprecipitation combined assays showed the ENaC-filamin interaction on the cell surface. Using Xenopus oocyte expression and two-electrode voltage clamp electrophysiology, we found that co-expression of an ENaC-binding domain of filamin substantially reduces ENaC channel function. Western blot and immunohistochemistry experiments revealed that the filamin A C terminus (FLNAC) modestly reduces the expression of the ENaC α subunit in oocytes and A7 cells. After normalizing the current by plasma membrane expression, we found that FLNAC results in ~50% reduction in the ENaC channel activity. The inhibitory effect of FLNAC was confirmed by lipid bilayer electrophysiology experiments using purified ENaC and FLNAC proteins, which showed that FLNAC substantially reduces ENaC single channel open probability. Taken together, our study demonstrated that filamin reduces ENaC channel function through direct interaction on the cell surface.

Migfilin sensitizes cisplatin-induced apoptosis in human glioma cells in vitro

AIM

Filamin binding LIM protein 1, also known as migfilin, is a skeleton organization protein that binds to mitogen-inducible gene 2 at cell-extracellular matrix adhesions. The aim of this study was to investigate the role of migfilin in cisplatin-induced apoptosis in human glioma cells, to determine the functional domains of migfilin, and to elucidate the molecular mechanisms underlying the regulation of cisplatin-related chemosensitivity.

METHODS

The human glioma cell lines Hs683, H4, and U-87 MG were transfected with pEGFP-C2-migfilin to elevate the expression level of migfilin. RNA interference was used to reduce the expression of migfilin. To determine the functional domains of migfilin, U-87 MG cells were transfected with plasmids of migfilin deletion mutants. After treatment with cisplatin (40 μmol/L) for 24 h, the cell viability was assessed using the MTS assay, and the cell apoptotic was examined using the DAPI staining assay and TUNEL analysis. Expression levels of apoptosis-related proteins were detected by Western blot analysis.

RESULTS

Overexpression of migfilin significantly enhanced cisplatin-induced apoptosis in Hs683, H4, and U-87 MG cells, whereas downregulation of migfilin expression inhibited the chemosensitivity of these cell lines. The N-terminal region of migfilin alone was able to enhance the cisplatin-induced apoptosis. However, despite the existence of the N-terminal region, mutants of migfilin with any one of three LIM domains deleted led to a function loss. Furthermore, apoptotic proteins (PARP and caspase-3) and the anti-apoptotic protein Bcl-xL were modulated by the expression level of migfilin in combination with cisplatin.

CONCLUSION

The LIM1-3 domains of migfilin play a key role in sensitizing glioma cells to cisplatin-induced apoptosis through regulation of apoptosis-related proteins.

Migfilin and filamin as regulators of integrin activation in endothelial cells and neutrophils

Cell adhesion and migration depend on engagement of extracellular matrix ligands by integrins. Integrin activation is dynamically regulated by interactions of various cytoplasmic proteins, such as filamin and integrin activators, talin and kindlin, with the cytoplasmic tail of the integrin β subunit. Although filamin has been suggested to be an inhibitor of integrin activation, direct functional evidence for the inhibitory role of filamin is limited. Migfilin, a filamin-binding protein enriched at cell-cell and cell-extracellular matrix contact sites, can displace filamin from β1 and β3 integrins and promote integrin activation. However, its role in activation and functions of different β integrins in human vascular cells is unknown. In this study, using flow cytometry, we demonstrate that filamin inhibits β1 and αIIbβ3 integrin activation, and migfilin can overcome its inhibitory effect. Migfilin protein is widely expressed in different adherent and circulating blood cells and can regulate integrin activation in naturally-occurring vascular cells, endothelial cells and neutrophils. Migfilin can activate β1, β2 and β3 integrins and promote integrin mediated responses while migfilin depletion impairs the spreading and migration of endothelial cells. Thus, filamin can act broadly as an inhibitor and migfilin is a promoter of integrin activation.

Cell Adhesion and Transcriptional Activity - Defining the Role of the Novel Protooncogene LPP

Integrating signals from the extracellular matrix through the cell surface into the nucleus is an essential feature of metazoan life. To date, many signal transducers known as shuttle proteins have been identified to act as both a cytoskeletal and a signaling protein. Among them, the most prominent representatives are zyxin and lipoma preferred (translocation) partner (LPP). These proteins belong to the LIM domain protein family and are associated with cell migration, proliferation, and transcription. LPP was first identified in benign human lipomas and was subsequently found to be overexpressed in human malignancies such as lung carcinoma, soft tissue sarcoma, and leukemia. This review portrays LPP in the context of human neoplasia based on a study of the literature to define its important role as a novel protooncogene in carcinogenesis.

The filamins: organizers of cell structure and function

Filamin A (FLNa), the first non-muscle actin filament cross-linking protein, was identified in 1975. Thirty five years of FLNa research has revealed its structure in great detail, discovered its isoforms (FLNb and c), and identified over 90 binding partners including channels, receptors, intracellular signaling molecules, and even transcription factors. Due to this diversity, mutations in human FLN genes result in a wide range of anomalies with moderate to lethal consequences. This review focuses on the structure and functions of FLNa in cell migration and adhesion.

Loss of migfilin expression has no overt consequences on murine development and homeostasis

Migfilin is a LIM-domain-containing protein of the zyxin family of adaptor proteins and is found at cell-matrix and cell-cell adhesion sites and in the nucleus. In vitro studies have suggested that migfilin promotes β1 integrin activity, regulates cell spreading and migration and induces cardiomyocyte differentiation. To test directly the function of migfilin in vivo, we generated a migfilin-null mouse strain. Here, we report that loss of migfilin expression permits normal development and normal postnatal aging. Fibroblasts and keratinocytes from migfilin-null mice display normal spreading and adhesion, and normal integrin expression and activation. The migration velocity and directionality of migfilin-null embryonic fibroblasts were normal, whereas the velocity of migfilin-null keratinocytes in wound scratch assays was slightly but significantly reduced. Our findings indicate that the roles of migfilin are functionally redundant during mouse development and tissue homeostasis.

Biochemical characterization of the cell-biomaterial interface by quantitative proteomics

Surface topography and texture of cell culture substrata can affect the differentiation and growth of adherent cells. The biochemical basis of the transduction of the physical and mechanical signals to cellular responses is not well understood. The lack of a systematic characterization of cell-biomaterial interaction is the major bottleneck. This study demonstrated the use of a novel subcellular fractionation method combined with quantitative MS-based proteomics to enable the robust and high-throughput analysis of proteins at the adherence interface of Madin-Darby canine kidney cells. This method revealed the enrichment of extracellular matrix proteins and membrane and stress fibers proteins at the adherence surface, whereas it shows depletion of extracellular matrix belonging to the cytoplasmic, nucleus, and lateral and apical membranes. The asymmetric distribution of proteins between apical and adherence sides was also profiled. Apart from classical proteins with clear involvement in cell-material interactions, proteins previously not known to be involved in cell attachment were also discovered.

Kindlins in FERM adhesion

The Kindlin family of intracellular proteins has recently emerged as key regulators of cellular functions and cell-matrix interactions. The 3 members of this family, Kindlin-1, -2, and -3, perform an essential role in activation of integrin adhesion receptors, and expression of at least 1 Kindlin paralog is required to enable integrin activation in physiologically relevant settings. In humans, deficiencies in Kindlin-3 lead to a number of abnormalities affecting hemostasis, the immune system, and bone function, whereas the lack of Kindlin-1 causes profound skin defects. The importance of Kindlins is underscored by the results of animal knockout studies, which clearly show the indispensable and nonredundant functions of all 3 Kindlins in development and normal physiology. This review discusses recent progress in the studies of Kindlin protein family, emphasizing newly identified functions and potential mechanisms underlying differential activities of the family members.

Isoform divergence of the filamin family of proteins

The vertebrate filamin family (A, B, and C) is part of the spectrin family of actin cross-linking proteins. Family members share high sequence similarity (>64%) and have both common and isoform-distinct functionalities. To identify the basis for isoform-specific functionality, we perform an evolutionary trace of chordate filamin at the granularity of single residues. Our trace methodology is constrained to focus on neofunctionality by requiring that one isoform remain the ancestral type, whereas at least one isoform has an accepted mutation. We call divergence meeting these characteristics "class-distinctive." To obtain a temporal and spatial context for class-distinctive residues, we derive an all-atom model of full-length filamin A by homology modeling and joining individual domains. We map onto our model both conserved and class-distinctive residues along with the period (Teleostei, Amphibian, and Mammalian) in which they diverged. Our phylogenetic analysis suggests that filamins diverged from a common ancestral gene between urochordate and vertebrate lineages. Filamins also diverged the most just after gene duplication, in the Teleostei period, with filamin C remaining closest to ancestral filamin. At the residue level, domains with well-characterized interfaces, IgFLN 17 and IgFLN 21 (immunoglobulin, Ig), have diverged in potentially critical residues in their adhesion protein-binding interfaces, signifying that isoforms may bind or regulate ligand binding differentially. Similarly, isoform divergence in a region associated with F actin-binding regulation suggests that isoforms differentially regulate F-actin binding. In addition, we observe some class-distinctive residues in the vicinity of missense mutations that cause filamin A and B-associated skeletal disorders. Our analysis, utilizing both spatial and temporal granularity, has identified potentially important residues responsible for vertebrate filamin isoform-specific divergence-significantly in regions where few binding partners have been discovered to date- and suggests yet to be discovered filamin-binding partners and isoform-specific differential regulation with these binding partners.

Identification and characterization of multiple similar ligand-binding repeats in filamin: implication on filamin-mediated receptor clustering and cross-talk

The actin-binding protein filamin links membrane receptors to the underlying cytoskeleton. The cytoplasmic domains of these membrane receptors have been shown to bind to various filamin immunoglobulin repeats. Notably, among 24 human filamin repeats, repeat 17 was reported to specifically bind to platelet receptor glycoprotein Ibalpha and repeat 21 to integrins. However, a complete sequence alignment of all 24 human filamin repeats reveals that repeats 17 and 21 actually belong to a distinct filamin repeat subgroup (containing repeats 4, 9, 12, 17, 19, 21, and 23) that shares a conserved ligand-binding site. Using isothermal calorimetry and NMR analyses, we show that all repeats in this subgroup can actually bind glycoprotein Ibalpha, integrins, and a cytoskeleton regulator migfilin in similar manners. These data provide a new view on the ligand specificity of the filamin repeats. They also suggest a multiple ligand binding mechanism where similar repeats within a filamin monomer may promote receptor clustering or receptor cross-talking for regulation of the cytoskeleton organization and diverse filamin-mediated cellular activities.

Disease-associated substitutions in the filamin B actin binding domain confer enhanced actin binding affinity in the absence of major structural disturbance: Insights from the crystal structures of filamin B actin binding domains

Missense mutations in filamin B (FLNB) are associated with the autosomal dominant atelosteogenesis (AO) and the Larsen group of skeletal malformation disorders. These mutations cluster in particular FLNB protein domains and act in a presumptive gain-of-function mechanism. In contrast the loss-of-function disorder, spondylocarpotarsal synostosis syndrome, is characterised by the complete absence of FLNB. One cluster of AO missense mutations is found within the second of two calponin homology (CH) domains that create a functional actin-binding domain (ABD). This N-terminal ABD is required for filamin F-actin crosslinking activity, a crucial aspect of filamin's role of integrating cell-signalling events with cellular scaffolding and mechanoprotection. This study characterises the wild type FLNB ABD and investigates the effects of two disease-associated mutations on the structure and function of the FLNB ABD that could explain a gain-of-function mechanism for the AO diseases. We have determined high-resolution X-ray crystal structures of the human filamin B wild type ABD, plus W148R and M202V mutants. All three structures display the classic compact monomeric conformation for the ABD with the CH1 and CH2 domains in close contact. The conservation of tertiary structure in the presence of these mutations shows that the compact ABD conformation is stable to the sequence substitutions. In solution the mutant ABDs display reduced melting temperatures (by 6-7 degrees C) as determined by differential scanning fluorimetry. Characterisation of the wild type and mutant ABD F-actin binding activities via co-sedimentation assays shows that the mutant FLNB ABDs have increased F-actin binding affinities, with dissociation constants of 2.0 microM (W148R) and 0.56 microM (M202V), compared to the wild type ABD K(d) of 7.0 microM. The increased F-actin binding affinity of the mutants presents a biochemical mechanism that differentiates the autosomal dominant gain-of-function FLNB disorders from those that arise through the complete loss of FLNB protein.

Migfilin, a molecular switch in regulation of integrin activation

The linkage of heterodimeric (alpha/beta) integrin receptors with their extracellular matrix ligands and intracellular actin cytoskeleton is a fundamental step for controlling cell adhesion and migration. Binding of the actin-linking protein, talin, to integrin beta cytoplasmic tails (CTs) induces high affinity ligand binding (integrin activation), whereas binding of another actin-linking protein, filamin, to the integrin beta CTs negatively regulates this process by blocking the talin-integrin interaction. Here we show structurally that migfilin, a novel cytoskeletal adaptor highly enriched in the integrin adhesion sites, strongly interacts with the same region in filamin where integrin beta CTs bind. We further demonstrate that the migfilin interaction dissociates filamin from integrin and promotes the talin/integrin binding and integrin activation. Migfilin thus acts as a molecular switch to disconnect filamin from integrin for regulating integrin activation and dynamics of extracellular matrix-actin linkage.

Structural basis of the migfilin-filamin interaction and competition with integrin beta tails

A link between sites of cell adhesion and the cytoskeleton is essential for regulation of cell shape, motility, and signaling. Migfilin is a recently identified adaptor protein that localizes at cell-cell and cell-extracellular matrix adhesion sites, where it is thought to provide a link to the cytoskeleton by interacting with the actin cross-linking protein filamin. Here we have used x-ray crystallography, NMR spectroscopy, and protein-protein interaction studies to investigate the molecular basis of migfilin binding to filamin. We report that the N-terminal portion of migfilin can bind all three human filamins (FLNa, -b, or -c) and that there are multiple migfilin-binding sites in FLNa. Human filamins are composed of an N-terminal actin-binding domain followed by 24 immunoglobulin-like (IgFLN) domains and we find that migfilin binds preferentially to IgFLNa21 and more weakly to IgFLNa19 and -22. The filamin-binding site in migfilin is localized between Pro(5) and Pro(19) and binds to the CD face of the IgFLNa21 beta-sandwich. This interaction is similar to the previously characterized beta 7 integrin-IgFLNa21 interaction and migfilin and integrin beta tails can compete with one another for binding to IgFLNa21. This suggests that competition between filamin ligands for common binding sites on IgFLN domains may provide a general means of modulating filamin interactions and signaling. In this specific case, displacement of integrin tails from filamin by migfilin may provide a mechanism for switching between different integrin-cytoskeleton linkages.

Smooth Muscle Expression of Lipoma Preferred Partner Is Mediated by an Alternative Intronic Promoter That Is Regulated by Serum Response Factor/Myocardin

Lipoma preferred partner (LPP) was recently recognized as a smooth muscle marker that plays a role in smooth muscle cell migration. In this report, we focus on the transcriptional regulation of the LPP gene. In particular, we investigate whether LPP is directly regulated by serum response factor (SRF). We show that the LPP gene contains 3 evolutionarily conserved CArG boxes and that 1 of these is part of an alternative promoter in intron 2. Quantitative RT-PCR shows that this alternative promoter directs transcription specifically to smooth muscle containing tissues in vivo. By using chromatin immunoprecipitation, we demonstrate that 2 of the CArG boxes, including the promoter-associated CArG box, bind to endogenous SRF in cultured aortic smooth muscle cells. Electrophoretic mobility-shift assays show that the conserved CArG boxes bind SRF in vitro. In reporter experiments, we show that the alternative promoter has transcriptional capacity that is dependent on SRF/myocardin and that the promoter associated CArG box is required for that activity. Finally, we show by quantitative RT-PCR that the alternative promoter is strongly downregulated in SRF-deficient embryonic stem cells and in smooth muscle tissues derived from conditional SRF knockout mice. Collectively, our data demonstrate that expression of LPP in smooth muscle is mediated by an alternative promoter that is regulated by SRF/myocardin.

Filamins in cardiovascular development

Filamins are classically recognized as large cytoplasmic proteins that cross-link cortical actin into dynamic 3-dimensional structures and transmit extracellular signals through integrin receptors into the cytoplasm. However, recent reports indicate that filamins interact with a large number of other proteins with diverse functions, including transcriptional factors and cellular molecules involved in signaling, adhesion, and cellular motility, and are also present in the cell nucleus. In addition, genetic mutations in filamins have been linked to a wide range of human genetic disorders, including skeletal, central nervous system, and cardiovascular malformations, highlighting distinct filamin interactions. Here, we update the cardiovascular phenotypes of patients with mutations in filamin genes and mice deficient in filamins and filamin-interacting proteins.

Heart development in fibronectin-null mice is governed by a genetic modifier on chromosome four

Absence of the fibronectin (FN) gene leads to early embryonic lethality in both 129S4 and C57BL/6J strains due to severe cardiovascular defects. However, heart development is arrested at different stages in these embryos depending on the genetic background. In the majority of 129S4 FN-null embryos, heart progenitors remain at their anterior bilateral positions and fail to fuse at the midline to form a heart tube. However, on the C57BL/6J genetic background, cardiac development progresses further and results in a centrally positioned and looped heart. To find factor(s) involved in embryonic heart formation and governing the extent of heart development in FN-null embryos in 129S4 and C57BL/6J strains, we performed genetic mapping and haplotype analyses. These analyses lead to identification of a significant linkage to a 1-Mbp interval on chromosome four. Microarray analysis and sequencing identified 21 genes in this region, including five that are differentially expressed between the strains, as potential modifiers. Since none of these genes was previously known to play a role in heart development, one or more of them is likely to be a novel modifier affecting cardiac development. Identification of the modifier would significantly enhance our understanding of the molecular underpinning of heart development and disease.

Filamin B mutations cause chondrocyte defects in skeletal development

Filamin B (FLNB) is a cytoplasmic protein that regulates the cytoskeletal network by cross-linking actin, linking cell membrane to the cytoskeleton and regulating intracellular signaling pathways responsible for skeletal development (Stossel, T.P., Condeelis, J., Cooley, L., Hartwig, J.H., Noegel, A., Schleicher, M. and Shapiro, S.S. (2001) Filamins as integrators of cell mechanics and signalling. Nat. Rev. Mol. Cell Biol., 2, 138-145). Mutations in FLNB cause human skeletal disorders [boomerang dysplasia, spondylocarpotarsal (SCT), Larsen, and atelosteogenesis I/III syndromes], which are characterized by disrupted vertebral segmentation, joint formation and endochondral ossification [Krakow, D., Robertson, S.P., King, L.M., Morgan, T., Sebald, E.T., Bertolotto, C., Wachsmann-Hogiu, S., Acuna, D., Shapiro, S.S., Takafuta, T. et al. (2004) Mutations in the gene encoding filamin B disrupt vertebral segmentation, joint formation and skeletogenesis. Nat. Genet., 36, 405-410; Bicknell, L.S., Morgan, T., Bonafe, L., Wessels, M.W., Bialer, M.G., Willems, P.J., Cohn, D.H., Krakow, D. and Robertson, S.P. (2005) Mutations in FLNB cause boomerang dysplasia. J. Med. Genet., 42, e43]. Here we show that Flnb deficient mice have shortened distal limbs with small body size, and develop fusion of the ribs and vertebrae, abnormal spinal curvatures, and dysmorphic facial/calvarial bones, similar to the human phenotype. Characterization of the mutant mice demonstrated increased apoptosis along the bone periphery of the distal appendages, consistent with reduced bone width. No changes in the initial proliferative rate of chondrocytes were observed, but the progressive differentiation of chondrocyte precursors was impaired, consistent with reduced bone length. The extracellular matrix appeared disrupted and phosphorylated beta1-integrin (a collagen receptor and Flnb binding partner) expression was diminished in the mutant growth plate. Like integrin-deficient chondrocytes, adhesion to the ECM was decreased in Flnb(-/-) chondrocytes, and inhibition of beta1-integrin in these cells led to further impairments in cell spreading. These data suggest that disruption of the ECM-beta1-integrin-Flnb pathway contributes to defects in vertebral and distal limb development, similar to those seen in the human autosomal recessive SCT due to Flnb mutations.

Cloning and functional expression of a rodent brain cDNA encoding a novel protein with agmatinase activity, but not belonging to the arginase family

A rat brain cDNA encoding for a novel protein with agmatinase activity was cloned and functionally expressed. The protein was expressed as a histidine-tagged fusion product with a molecular weight of about 63 kDa. Agmatine hydrolysis was strictly dependent on Mn(2+); K(m) and k(cat) values were 2.5+/-0.2 mM and 0.8+/-0.2 s(-1), respectively. The product putrescine was a linear competitive inhibitor (K(i)=5+/-0.5 mM). The substrate specificity, metal ion requirement and pH optimum (9.5) coincide with those reported for Escherichia coli agmatinase, the best characterized of the agmatinases. However, as indicated by the k(cat)/K(m) (320 M(-1)s(-1)), the recombinant protein was about 290-fold less efficient than the bacterial enzyme. The deduced amino sequence revealed great differences with all known agmatinases, thus excluding the protein from the arginase family. It was, however, highly identical (>85%) to the predicted sequences for fragments of hypothetical or unnamed LIM domain-containing proteins. As a suggestion, the agmatinase activity is adscribed to a protein with an active site that promiscuously catalyze a reaction other than the one it evolved to catalyze.

A molecular and clinical study of Larsen syndrome caused by mutations in FLNB

BACKGROUND

Larsen syndrome is an autosomal dominant osteochondrodysplasia characterised by large-joint dislocations and craniofacial anomalies. Recently, Larsen syndrome was shown to be caused by missense mutations or small inframe deletions in FLNB, encoding the cytoskeletal protein filamin B. To further delineate the molecular causes of Larsen syndrome, 20 probands with Larsen syndrome together with their affected relatives were evaluated for mutations in FLNB and their phenotypes studied.

METHODS

Probands were screened for mutations in FLNB using a combination of denaturing high-performance liquid chromatography, direct sequencing and restriction endonuclease digestion. Clinical and radiographical features of the patients were evaluated.

RESULTS AND DISCUSSION

The clinical signs most frequently associated with a FLNB mutation are the presence of supernumerary carpal and tarsal bones and short, broad, spatulate distal phalanges, particularly of the thumb. All individuals with Larsen syndrome-associated FLNB mutations are heterozygous for either missense or small inframe deletions. Three mutations are recurrent, with one mutation, 5071G-->A, observed in 6 of 20 subjects. The distribution of mutations within the FLNB gene is non-random, with clusters of mutations leading to substitutions in the actin-binding domain and filamin repeats 13-17 being the most common cause of Larsen syndrome. These findings collectively define autosomal dominant Larsen syndrome and demonstrate clustering of causative mutations in FLNB.

Focal adhesions: what's new inside

The cytoplasmic side of focal adhesions is comprised of large molecular complexes that link transmembrane receptors, such as integrins, to the actin cytoskeleton and mediate signals modulating cell attachment, migration, proliferation, differentiation, and gene expression. These complexes are heterogeneous and dynamic structures that are apparent targets of regulatory signals that control the function of focal adhesions. Recent studies using genetic approaches in invertebrate and vertebrate systems have begun to reveal the structure and function of these complexes in vivo.

Migfilin interacts with vasodilator-stimulated phosphoprotein (VASP) and regulates VASP localization to cell-matrix adhesions and migration

Cell migration is a complex process that is coordinately regulated by cell-matrix adhesion and actin cytoskeleton. We report here that migfilin, a recently identified component of cell-matrix adhesions, is a biphasic regulator of cell migration. Loss of migfilin impairs cell migration. Surprisingly, overexpression of migfilin also reduces cell migration. Molecularly, we have identified vasodilator-stimulated phosphoprotein (VASP) as a new migfilin-binding protein. The interaction is mediated by the VASP EVH1 domain and a single L104PPPPP site located within the migfilin proline-rich domain. Migfilin and VASP form a complex in both suspended and adhered cells, and in the latter, they co-localize in cell-matrix adhesions. Functionally, migfilin facilitates VASP localization to cell-matrix adhesions. Using two different approaches (VASP-binding defective migfilin mutants and small interfering RNA-mediated VASP knockdown), we show that the interaction with VASP is crucially involved in migfilin-mediated regulation of cell migration. Our results identify migfilin as an important regulator of cell migration and provide new information on the mechanism by which migfilin regulates this process.

Migfilin and its binding partners: from cell biology to human diseases

Links between the plasma membrane and the actin cytoskeleton are essential for maintaining tissue integrity and for controlling cell morphology and behavior. Studies over the past several decades have identified dozens of components of such junctions. One of the most recently identified is migfilin, a widely expressed protein consisting of an N-terminal filamin-binding domain, a central proline-rich domain and three C-terminal LIM domains. Migfilin is recruited to cell-matrix contacts in response to adhesion and colocalizes with β-catenin at cell-cell junctions in epithelial and endothelial cells. Migfilin also travels from the cytoplasm into the nucleus, a process that is regulated by RNA splicing and calcium signaling. Through interactions with multiple binding partners, including Mig-2, filamin and VASP, migfilin links the cell adhesion structures to the actin cytoskeleton. It regulates actin remodeling, cell morphology and motility. In nuclei, migfilin interacts with the cardiac transcriptional factor CSX/NKX2-5 and promotes cardiomyocyte differentiation. It probably functions as a key regulator both at cell adhesion sites and nuclei, coordinating multiple cellular processes, and is implicated in the pathogenesis of several human diseases.

Physical and functional association of migfilin with cell-cell adhesions

Cell-cell junctions are essential for epithelial and endothelial tissue formation and communication between neighboring cells. We report here that migfilin, a recently identified component of cell-extracellular matrix adhesions, is recruited to cell-cell junctions in response to cadherin-mediated cell-cell adhesions. Migfilin is detected at cell-cell junctions in both epithelial and endothelial cells. It forms detergent-resistant, discrete clusters that associate with actin bundles bridging neighboring cells. Immunoelectron microscopic analyses reveal that migfilin is closely associated with beta-catenin, but not desmosomes, at cell-cell junctions. Furthermore, we show that the C-terminal LIM domains, but not its N-terminal domain, mediates migfilin localization to cell-cell junctions. The site mediating the localization of migfilin to cell-cell junctions at least partially overlaps with that mediating the localization of migfilin to cell-ECM adhesions. Finally, siRNA-mediated depletion of migfilin compromised the organization of adherens junctions and weakened cell-cell association. These results identify migfilin as a component of adherens junctions and suggest an important role for migfilin in the organization of the cell-cell adhesion structure.

Assembly and Signaling of Adhesion Complexes

Cell-extracellular matrix (ECM) adhesion is crucial for control of cell behavior. It connects the ECM to the intracellular cytoskeleton and transduces bidirectional signals between the extracellular and intracellular compartments. The subcellular machinery that mediates cell-ECM adhesion and signaling is complex. It consists of transmembrane proteins (e.g., integrins) and at least several dozens of membrane-proximal proteins that assemble into a network through multiple protein interactions. Furthermore, despite sharing certain common components, cell-ECM adhesions exhibit considerable heterogeneity in different types of cells (e.g., the cell-ECM adhesions in cardiac myocytes are considerably different from those in fibroblasts). Here, we will first briefly describe the general properties of the integrin-mediated cell-ECM adhesion and signal transduction. Next, we will focus on one of the recently discovered cell-ECM adhesion protein complexes consisting of PINCH, integrin-linked kinase (ILK), and Parvin and use it as an example to illustrate the molecular basis underlying the assembly and functions of cell-ECM adhesions. Finally, we will discuss in detail the structure and regulation of cell-ECM adhesion complexes in cardiac myocytes, which illustrate the importance and complexity of the cell-ECM adhesion structures in organogenesis and diseases.

The LIM domain: from the cytoskeleton to the nucleus

First described 15 years ago as a cysteine-rich sequence that was common to a small group of homeodomain transcription factors, the LIM domain is now recognized as a tandem zinc-finger structure that functions as a modular protein-binding interface. LIM domains are present in many proteins that have diverse cellular roles as regulators of gene expression, cytoarchitecture, cell adhesion, cell motility and signal transduction. An emerging theme is that LIM proteins might function as biosensors that mediate communication between the cytosolic and the nuclear compartments.