Structural and functional aspects of filamins

Combination of a pesticide exposure and a bacterial challenge: in vivo effects on immune response of Pacific oyster, Crassostrea gigas (Thunberg)

To assess the impact of pollution induced by pesticides on Pacific oyster, Crassostrea gigas, health in France, in vivo effects of combined pesticide exposure and bacterial challenge on cell activities and gene expression in hemocytes were tested using flow cytometry and real-time PCR. As a first step, an in vivo model of experimental contamination was developed. Pacific oysters were exposed to a mixture of eight pesticides (atrazine, glyphosate, alachlor, metolachlor, fosetyl-alumimium, terbuthylazine, diuron and carbaryl) at environmentally relevant concentrations over a 7-day period. Hemocyte parameters (cell mortality, enzyme activities and phagocytosis) were monitored using flow cytometry and gene expression was evaluated by real-time PCR (RT-PCR). The expression of 19 genes involved in C. gigas hemocyte functions was characterized using RT-PCR. After 7 days of exposure, phagocytosis was significantly reduced and the 19 selected genes were down-regulated in treated animals. As a second step, the experimental contamination method previously developed was used to study interactions between pesticide exposure and bacterial challenge by intramuscular injection of two Vibrio splendidus-related pathogenic strains. Oyster mortality and expression of 10 of the 19 selected genes were followed 4 and 24h post-injection. Oyster mortality was higher in pesticide-treated oysters compared to untreated oysters after the bacterial challenge. Gene expression was up-regulated in pesticide-treated oysters compared to untreated oysters after the bacterial challenge. We hypothesize that gene over-expression due to an interaction between pesticides and bacteria could lead to an injury of host tissues, resulting in higher mortality rates. In conclusion, this study is the first to show effects of pesticides at environmentally relevant concentrations on C. gigas hemocytes and to hypothesize that pesticides modulate the immune response to a bacterial challenge in oysters.

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

Human filamins are large actin-crosslinking proteins composed of an N-terminal actin-binding domain followed by 24 Ig-like domains (IgFLNs), which interact with numerous transmembrane receptors and cytosolic signaling proteins. Here we report the 2.5 A resolution structure of a three-domain fragment of human filamin A (IgFLNa19-21). The structure reveals an unexpected domain arrangement, with IgFLNa20 partially unfolded bringing IgFLNa21 into close proximity to IgFLNa19. Notably the N-terminus of IgFLNa20 forms a beta-strand that associates with the CD face of IgFLNa21 and occupies the binding site for integrin adhesion receptors. Disruption of this IgFLNa20-IgFLNa21 interaction enhances filamin binding to integrin beta-tails. Structural and functional analysis of other IgFLN domains suggests that auto-inhibition by adjacent IgFLN domains may be a general mechanism controlling filamin-ligand interactions. This can explain the increased integrin binding of filamin splice variants and provides a mechanism by which ligand binding might impact filamin structure.

Disruption of the Flnb gene in mice phenocopies the human disease spondylocarpotarsal synostosis syndrome

Spondylocarpotarsal synostosis syndrome (SCT) is an autosomal recessive disease that is characterized by short stature, and fusions of the vertebrae and carpal and tarsal bones. SCT results from homozygosity or compound heterozygosity for nonsense mutations in FLNB. FLNB encodes filamin B, a multifunctional cytoplasmic protein that plays a critical role in skeletal development. Protein extracts derived from cells of SCT patients with nonsense mutations in FLNB did not contain filamin B, demonstrating that SCT results from absence of filamin B. To understand the role of filamin B in skeletal development, an Flnb-/- mouse model was generated. The Flnb-/- mice were phenotypically similar to individuals with SCT as they exhibited short stature and similar skeletal abnormalities. Newborn Flnb-/- mice had fusions between the neural arches of the vertebrae in the cervical and thoracic spine. At postnatal day 60, the vertebral fusions were more widespread and involved the vertebral bodies as well as the neural arches. In addition, fusions were seen in sternum and carpal bones. Analysis of the Flnb-/- mice phenotype showed that an absence of filamin B causes progressive vertebral fusions, which is contrary to the previous hypothesis that SCT results from failure of normal spinal segmentation. These findings suggest that spinal segmentation can occur normally in the absence of filamin B, but the protein is required for maintenance of intervertebral, carpal and sternal joints, and the joint fusion process commences antenatally.

A 90 kDa fragment of filamin A promotes Casodex-induced growth inhibition in Casodex-resistant androgen receptor positive C4-2 prostate cancer cells

Prostate tumors are initially dependent on androgens for growth, but the majority of patients treated with anti-androgen therapy progress to androgen-independence characterized by resistance to such treatment. This study investigates a novel role for filamin A (FlnA), a 280 kDa cytoskeletal protein (consisting of an actin-binding domain (ABD) followed by 24 sequential repeats), in androgen-independent (AI) growth. Full-length FlnA is cleaved to 170 kDa (ABD+FlnA1-15) and 110 kDa fragments (FlnA16-24); the latter is further cleaved to a 90 kDa fragment (repeats 16-23) capable of nuclear translocation and androgen receptor (AR) binding. Here, we demonstrate that in androgen-dependent LNCaP prostate cancer cells, the cleaved 90 kDa fragment is localized to the nucleus, whereas in its AI subline C4-2, FlnA failed to cleave and remained cytoplasmic. Transfection of FlnA16-24 cDNA in C4-2 cells restored expression and nuclear localization of 90 kDa FlnA. Unlike LNCaP, C4-2 cells proliferate in androgen-reduced medium and in the presence of the AR-antagonist Casodex. They also exhibit increased Akt phosphorylation compared to LNCaP, which may contribute to their AI phenotype. Nuclear expression of 90 kDa FlnA in C4-2 cells decreased Akt phosphorylation, prevented proliferation in androgen-reduced medium and restored Casodex sensitivity. This effect was inhibited by constitutive activation of Akt indicating that FlnA restored Casodex sensitivity in C4-2 cells by decreasing Akt phosphorylation. In addition, FlnA-specific siRNA which depleted FlnA levels, but not control siRNA, induced resistance to Casodex in LNCaP cells. Our results demonstrate that expression of nuclear FlnA is necessary for androgen dependence in these cells.

The pathomechanism of filaminopathy: altered biochemical properties explain the cellular phenotype of a protein aggregation myopathy

Myofibrillar myopathy (MFM) is a pathologically defined group of hereditary human muscle diseases, characterized by focal myofibrillar destruction and cytoplasmic aggregates that contain several Z-disc-related proteins. The previously reported MFM-associated mutation (8130G --> A; W2710X) in the filamin C gene (FLNC) leads to a partial disturbance of the secondary structure of the dimerization domain of filamin C, resulting in massive protein aggregation in skeletal muscle fibers of the patients. Here, we provide a thorough characterization of the biochemical, biophysical and cellular properties of the mutated filamin C polypeptide. Our experiments revealed that the mutant dimerization domain is less stable and more susceptible to proteolysis. As a consequence, it does not dimerize properly and forms aggregates in vitro. Furthermore, the expression of mutant filamin in cultured cells results in the formation of protein aggregates. The mutant filamin does not associate with wild type filamin. These findings are of great importance to explain the pathomechanism of this disease.

Expression, crystallization and preliminary crystallographic data analysis of filamin A repeats 14-16

Human filamin A is a 280 kDa protein involved in actin-filament cross-linking. It is structurally divided into an actin-binding headpiece (ABD) and a rod domain containing 24 immunoglobulin-like (Ig) repeats. A fragment of human filamin A (Ig repeats 14-16) was cloned and expressed in Escherichia coli and the purified protein was crystallized in 1.6 M ammonium sulfate, 2% PEG 1000 and 100 mM HEPES pH 7.5. The crystals diffracted to 1.95 A and belong to space group P2(1)2(1)2(1), with unit-cell parameters a = 50.63, b = 52.10, c = 98.46 A, alpha = beta = gamma = 90 degrees.

Cyclin B1/Cdk1 binds and phosphorylates Filamin A and regulates its ability to cross-link actin

Substantial actin remodelling occurs prior to mitosis as cells alter their shape in preparation for cytokinesis. In mammalian cells, mitosis is initiated by a heterodimer of cyclin B1 and the cyclin dependent kinase 1 (Cdk1) serine/threonine kinase. In this report. we show that human cyclin B1 binds the actin cross-linking protein Filamin-A (FLNa). The proteins co-immunoprecipitate and co-localize in mitotic human cells. We find that cyclin B1/Cdk1 can phosphorylate FLNa in vitro and reduce its ability to gelate actin. We have also identified serine 1436 as one FLNa residue phosphorylated by cyclin B1/Cdk1 in vitro. Our results suggest a role for cyclin B1/Cdk1 in FLNa-dependent actin remodelling.

Crystal structure of human filamin C domain 23 and small angle scattering model for filamin C 23-24 dimer

Filamin C is a dimeric, actin-binding protein involved in organization of cortical cytoskeleton and of the sarcomere. We performed crystallographic, small-angle X-ray scattering and analytical ultracentrifugation experiments on the constructs containing carboxy-terminal domains of the protein (domains 23-24 and 19-21). The crystal structure of domain 23 of filamin C showed that the protein adopts the expected immunoglobulin (Ig)-like fold. Small-angle X-ray scattering experiments performed on filamin C tandem Ig-like domains 23 and 24 reveal a dimer that is formed by domain 24 and that domain 23 has little interactions with itself or with domain 24, while the analytical ultracentrifugation experiments showed that the filamin C domains 19-21 form elongated monomers in diluted solutions.

HIV infection changes glomerular podocyte cytoskeletal composition and results in distinct cellular mechanical properties

In addition to forming the selective filtration barrier for the renal glomerulus, podocytes maintain glomerular capillary architecture by opposing distending hemodynamic forces. To understand the relationship of cytoskeletal properties and the mechanical characteristics of podocytes, we studied filamin expression and distribution and measured cell membrane deformability in conditionally immortalized wild-type (WT) mouse podocytes, and in podocytes derived from a mouse model of HIV-associated nephropathy (HIVAN). In the WT cells, filamin and F-actin were localized at the periphery and in prominent stress fibers. In the HIVAN cells, filamin expression was reduced, and stress fibers were sparse. In a microaspiration assay, HIVAN cells ruptured under minimal negative pressure. Atomic force microscopy demonstrated that the WT cells had a stiffness of 17 kPa, whereas the value for the HIVAN cells was 4 kPa. These results demonstrate that the mechanical properties of WT and HIVAN podocytes are markedly different in a manner that is consistent with differences in the composition and arrangement of their cytoskeletons. The mechanical properties of the WT podocytes suggest that these cells can better maintain capillary integrity than the HIVAN podocytes and implicate pathological assembly of the cytoskeleton as a mechanism of HIVAN.

Mutations in the gene encoding filamin A as a cause for familial cardiac valvular dystrophy

BACKGROUND

Myxomatous dystrophy of the cardiac valves affects approximately 3% of the population and remains one of the most common indications for valvular surgery. Familial inheritance has been demonstrated with autosomal and X-linked transmission, but no specific molecular abnormalities have been documented in isolated nonsyndromic forms. We have investigated the genetic causes of X-linked myxomatous valvular dystrophy (XMVD) previously mapped to chromosome Xq28.

METHODS AND RESULTS

A familial and genealogical survey led us to expand the size of a large, previously identified family affected by XMVD and to refine the XMVD locus to a 2.5-Mb region. A standard positional cloning approach identified a P637Q mutation in the filamin A (FLNA) gene in all affected members. Two other missense mutations (G288R and V711D) and a 1944-bp genomic deletion coding for exons 16 to 19 in the FLNA gene were identified in 3 additional, smaller, unrelated families affected by valvular dystrophy, which demonstrates the responsibility of FLNA as a cause of XMVD. Among carriers of FLNA mutation, the penetrance of the disease was complete in men and incomplete in women. Female carriers could be mildly affected, and the severity of the disease was highly variable among mutation carriers.

CONCLUSIONS

Our data demonstrate that FLNA is the first gene known to cause isolated nonsyndromic MVD. This is the first step to understanding the pathophysiological mechanisms of the disease and to defining pathways that may lead to valvular dystrophy. Screening for FLNA mutations could be important for families affected by XMVD to provide adequate follow-up and genetic counseling.

Structural determinants of LL5beta subcellular localisation and association with filamin C

PI3K signalling pathways link cell surface receptors to the control of several intracellular functions including cell growth, survival and movement. Filamins are important regulators of cortical actin structure and function. LL5beta is a filamin binding protein that is an effector of the PI3K signalling pathway. We define an N-terminal region of LL5beta that is responsible for binding to the C-terminus of filamins. Under conditions of very low PI3K activity, we show that this region, together with an additional domain of the protein, is responsible for localising the complex to punctate structures that are also decorated by L-FILIP (a protein previously characterised to bind filamin and accelerate its destruction). Under conditions of significant PI3K activity, PtdIns(3,4,5)P(3) binding to the C-terminal PH domain in LL5beta prevents localisation to these structures. These observations start to define the basis for PI3K regulation of filamin through LL5beta.

Interaction of zonula occludens-1 (ZO-1) with alpha-actinin-4: application of functional proteomics for identification of PDZ domain-associated proteins

The use of recombinant "bait" proteins to capture protein-binding partners, followed by identification of protein interaction networks by mass spectrometry (MS), has gained popularity and widespread acceptance. We have developed an approach using recombinant PDZ protein interaction modules of the membrane-associated guanylate kinase (MAGUK) protein zonula occludens-1 (ZO-1) to pull-down and screen for proteins that interact with these modules via their PDZ domain binding motifs. Identification of proteins by MS of pull-down material was achieved using a vacuum-based chromatography sample preparation device designed for matrix-assisted laser desorption/ionization (MALDI) MS. MS analysis of tryptic fragments in pull-down material revealed a number of potential ZO-1 interacting candidates, including the presence of peptides corresponding to the cortical membrane scaffolding protein alpha-actinin-4. Interaction of alpha-actinin-4 with ZO-1 was confirmed by coimmunoprecipitation of these two proteins from cultured cells, as well as from brain, liver, and heart, and by immunoblot detection of alpha-actinin-4 after pull-down with the first PDZ domain of ZO-1. In contrast, the highly homologous alpha-actinin family member, alpha-actinin-1, displayed no association with ZO-1. Immunofluorescence showed colocalization of alpha-actinin-4 with ZO-1 in cultured HeLa and C6 glioma cells, as well as in a variety of tissues in vivo, including brain, heart, liver, and lung. This study demonstrates the utility of MS-based functional proteomics for identifying cellular components of the ZO-1 scaffolding network. Our finding of the interaction of ZO-1 with alpha-actinin-4 provides a mechanism for linking the known protein recruitment and signaling activities of ZO-1 with alpha-actinin-4-associated plasma membrane proteins that have regulatory activities at cell-cell and cell-extracellular matrix contacts.

Sensitization to the conditioned rewarding effects of morphine modulates gene expression in rat hippocampus

Opiates addiction is characterized by its long-term persistence. In order to study the enduring changes in long-term memory in hippocampus, a pivotal region for this process, we used suppression subtractive hybridization to compare hippocampal gene expression in morphine and saline-treated rats. Animals were subjected to an extended place preference paradigm consisting of four conditioning phases. Sensitization to the reinforcing effects of the drug occurred after three conditioning phases. After 25 days of treatment rats were euthanized and the complementary DNA (cDNA) from the hippocampus of morphine-dependent and saline-treated animals were then screened for differentially expressed cDNAs. The selected 177 clones were then subjected to a microarray procedure and 20 clones were found differentially regulated. The pattern of regulated genes suggests impairments in neurotransmitter release and the activation of neuroprotective pathways.

Identification of a filamin docking site on PTP-PEST

PTP-PEST is a cytoplasmic protein-tyrosine phosphatase (PTP) implicated in the regulation of biological processes such as cell motility, cytokinesis, focal adhesion disassembly, and lymphocyte activation. Using a proteomics approach, filamin-A was identified as a novel interacting protein that bound to GST-PTP-PEST. This interaction was confirmed in vitro and in cells by coimmunoprecipitation. The site of filamin interaction on PTP-PEST was mapped to the fourth proline-rich region (Pro4). PTP-PEST has previously been implicated in the regulation of cytokinesis. In further support of this finding, expression of PTP-PEST in HeLa cells resulted in the formation of multinucleated cells. A PTP-PEST mutant lacking Pro4 and unable to bind filamin-A failed to induce the multinucleated phenotype. Further, depletion of filamin-A in HeLa cells was found to reduce the PTP-PEST-dependent multinucleation phenotype. Hence, we conclude that the interaction of PTP-PEST with filamin-A may function in the control of cytokinesis in mammalian cells.

Filamin A is required for T cell activation mediated by protein kinase C-theta

Induction of T cell responses following engagement of the Ag-specific TCR depends on TCR-initiated rearrangements of the cellular actin cytoskeleton and highly coordinated and tightly regulated interactions and of diverse intracellular signaling proteins. In this study, we show that filamin A (FLNa), an actin-binding and signal mediator scaffolding protein, is required for T cell activation. Following Ag stimulation, FLNa was recruited to the T cell-APC contact area, where it colocalized with protein kinase C-theta (PKCtheta). Depletion of FLNa by RNA interference did not affect TCR-induced early tyrosine phosphorylation or actin polymerization but, nevertheless, resulted in impaired IL-2 expression by human primary T cells and reduced activation of NF-kappaB, AP-1, and NFAT reporter genes in transfected T cells. TCR stimulation induced stable physical association of FLNa with PKCtheta. Furthermore, the TCR/CD28-induced membrane translocation of PKCtheta was inhibited in FLNa-depleted T cells. These results reveal novel role for FLNa in the TCR/CD28 signaling pathway leading to transcription factor activation and IL-2 production, and suggest that this role is mediated, in part, through the inducible interaction of FLNa with PKCtheta.

Mutations in two regions of FLNB result in atelosteogenesis I and III

The filamins are a family of cytoplasmic proteins that bind to and organize actin filaments, link membrane proteins to the cytoskeleton, and provide a scaffold for signaling molecules. Mutations in the gene encoding filamin B (FLNB) cause a spectrum of osteochondrodysplasias, including atelosteogenesis type I (AOI) and atelosteogenesis type III (AOIII). AOI and AOIII are autosomal dominant lethal skeletal dysplasias characterized by overlapping clinical findings that include vertebral abnormalities, disharmonious skeletal maturation, hypoplastic long bones, and joint dislocations. Previous studies have shown that heterozygosity for missense mutations that alter the CH2 domain and repeat 6 region of filamin B produce AOI and AOIII. In this study, 14 novel missense mutations in FLNB were found in 15 unrelated patients with AOI and AOIII. The majority of the mutations resided in exon 2 and exon 3, which encode the CH2 domain of the actin-binding region of filamin B. The remaining mutations were found in exon 28 and exon 29, which encode repeats 14 and 15 of filamin B. These results show that clustering of mutations in two regions of FLNB produce AOI/AOIII, and highlight the important role of this cytoskeletal protein in normal skeletogenesis.

Filamins: promiscuous organizers of the cytoskeleton

Filamins are elongated homodimeric proteins that crosslink F-actin. Each monomer chain of filamin comprises an actin-binding domain, and a rod segment consisting of six (Dictyostelium filamin) up to 24 (human filamin) highly homologous repeats of approximately 96 amino acid residues, which adopt an immunoglobulin-like fold. Two hinges in the rod segment, together with the reversible unfolding of single repeats, might be the structural basis for the intrinsic flexibility of the actin networks generated by filamins. There are numerous filamin-binding proteins that associate, in most cases, along the repeats of the rod repeats. This rather promiscuous behaviour renders filamin a versatile scaffold between the actin network and finely tuned molecular cascades from the membrane to the cytoskeleton.

Unusual splicing events result in distinct Xin isoforms that associate differentially with filamin c and Mena/VASP

Filamin c is the predominantly expressed filamin isoform in striated muscles. It is localized in myofibrillar Z-discs, where it binds FATZ and myotilin, and in myotendinous junctions and intercalated discs. Here, we identify Xin, the protein encoded by the human gene 'cardiomyopathy associated 1' (CMYA1) as filamin c binding partner at these specialized structures where the ends of myofibrils are attached to the sarcolemma. Xin directly binds the EVH1 domain proteins Mena and VASP. In the adult heart, Xin and Mena/VASP colocalize with filamin c in intercalated discs. In cultured cardiomyocytes, the proteins also localize in the nonstriated part of myofibrils, where sarcomeres are assembled and an extensive reorganization of the actin cytoskeleton occurs. Unusual intraexonic splicing events result in the existence of three Xin isoforms that associate differentially with its ligands. The identification of the complex filamin c-Xin-Mena/VASP provides a first glance on the role of Xin in the molecular mechanisms involved in developmental and adaptive remodeling of the actin cytoskeleton during cardiac morphogenesis and sarcomere assembly.

The molecular basis of filamin binding to integrins and competition with talin

The ability of adhesion receptors to transmit biochemical signals and mechanical force across cell membranes depends on interactions with the actin cytoskeleton. Filamins are large, actin-crosslinking proteins that connect multiple transmembrane and signaling proteins to the cytoskeleton. Here, we describe the high-resolution structure of an interface between filamin A and an integrin adhesion receptor. When bound, the integrin beta cytoplasmic tail forms an extended beta strand that interacts with beta strands C and D of the filamin immunoglobulin-like domain (IgFLN) 21. This interface is common to many integrins, and we suggest it is a prototype for other IgFLN domain interactions. Notably, the structurally defined filamin binding site overlaps with that of the integrin-regulator talin, and these proteins compete for binding to integrin tails, allowing integrin-filamin interactions to impact talin-dependent integrin activation. Phosphothreonine-mimicking mutations inhibit filamin, but not talin, binding, indicating that kinases may modulate this competition and provide additional means to control integrin functions.

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.

Silencing of filamin A gene expression inhibits Ca2+-sensing receptor signaling

Filamin plays an important role in actin cytoskeleton organization, membrane stabilization, and anchoring of transmembrane proteins. Using short interfering RNA (siRNA) to selectively target the filamin A gene and silence its expression, we studied the role of filamin A in G protein coupled receptor (GPCR) signaling. Silencing of filamin A protein expression was determined by immunoblotting and immunofluorescence. Functional consequences of filamin A gene silencing were measured by studying its role in MAPK signaling pathways activated by the Ca2+ -sensing receptor. This work defines filamin A involvement in GPCR signaling pathways and describes an additional method for studying its function.

Prestressed F-actin networks cross-linked by hinged filamins replicate mechanical properties of cells

We show that actin filaments, shortened to physiological lengths by gelsolin and cross-linked with recombinant human filamins (FLNs), exhibit dynamic elastic properties similar to those reported for live cells. To achieve elasticity values of comparable magnitude to those of cells, the in vitro network must be subjected to external prestress, which directly controls network elasticity. A molecular requirement for the strain-related behavior at physiological conditions is a flexible hinge found in FLNa and some FLNb molecules. Basic physical properties of the in vitro filamin-F-actin network replicate the essential mechanical properties of living cells. This physical behavior could accommodate passive deformation and internal organelle trafficking at low strains yet resist externally or internally generated high shear forces.

Four and half lim protein 2 (FHL2) stimulates osteoblast differentiation

FHL2, a molecule that interacts with many integrins and transcription factors, was found to play an important role in osteoblast differentiation. Overexpression of FHL2 increases the accumulation of osteoblast differentiation markers and matrix mineralization, whereas FHL2 deficiency results in inhibition of osteoblast differentiation and decreased bone formation.

INTRODUCTION

Integrin-matrix interaction plays a critical role in osteoblast function. It has been shown that the cytoplasmic domains of integrin beta subunits mediate signal transduction induced by integrin-matrix interaction. We reasoned that the identification of proteins interacting with beta-cytoplasmic tails followed by analysis of the function of these proteins would enhance our understanding on integrin signaling and the roles of these proteins in osteoblast activities.

MATERIALS AND METHODS

Yeast two hybrid assay was used to identify proteins interacting with the cytoplasmic domain of integrin beta5 subunit. The association of these proteins with integrin alphavbeta5 was confirmed by confocal analysis and co-immunoprecipitation. A stable MC3T3-E1 cells line overexpressing Four and Half Lim Protein 2 (FHL2) and mouse osteoblasts deficient in FHL2 were used to study the roles of FHL2 in osteoblast differentiation and bone formation. Matrix protein expression was determined by mRNA analysis and Western blotting. Matrix mineralization was detected by Alizarin red staining. Alkaline phosphatase activity was also measured. muCT was used to determine bone histomorphometry.

RESULTS AND CONCLUSIONS

FHL2 and actin-binding proteins, palladin and filamin A, were identified as proteins interacting with beta5 cytoplasmic domain. FHL2 co-localized with alphavbeta5 at the focal adhesion sites in association with palladin and filamin A. FHL2 was also present in nuclei. Osteoblasts overexpressing FHL2 exhibited increased adhesion to and migration on matrix proteins. Conversely, FHL2 stimulation of CREB activity was dependent on integrin function because it was inhibited by Gly-Arg-Gly-Asp-Ser (GRGDS) peptide. The expression of osteoblast differentiation markers and Msx2 was upregulated, and bone matrix mineralization was increased in FHL2 overexpressing cells. In contrast, FHL2-deficient bone marrow cells and osteoblasts displayed decreased osteoblast colony formation and differentiation, respectively, compared with wildtype cells. Moreover, FHL2-deficient female mice exhibited greater bone loss than the wildtype littermates after ovariectomy. Thus, FHL2 plays an important role in osteoblast differentiation and bone formation.

The new bone biology: Pathologic, molecular, and clinical correlates

Bone and cartilage and their disorders are addressed under the following headings: functions of bone; normal and abnormal bone remodeling; osteopetrosis and osteoporosis; epithelial-mesenchymal interaction, condensation and differentiation; osteoblasts, markers of bone formation, osteoclasts, components of bone, and pathology of bone; chondroblasts, markers of cartilage formation, secondary cartilage, components of cartilage, and pathology of cartilage; intramembranous and endochondral bone formation; RUNX genes and cleidocranial dysplasia (CCD); osterix; histone deacetylase 4 and Runx2; Ligand to receptor activator of NFkappaB (RANKL), RANK, osteoprotegerin, and osteoimmunology; WNT signaling, LRP5 mutations, and beta-catenin; the role of leptin in bone remodeling; collagens, collagenopathies, and osteogenesis imperfecta; FGFs/FGFRs, FGFR3 skeletal dysplasias, craniosynostosis, and other disorders; short limb chondrodysplasias; molecular control of the growth plate in endochondral bone formation and genetic disorders of IHH and PTHR1; ANKH, craniometaphyseal dysplasia, and chondrocalcinosis; transforming growth factor beta, Camurati-Engelmann disease (CED), and Marfan syndrome, types I and II; an ACVR1 mutation and fibrodysplasia ossificans progressiva; MSX1 and MSX2: biology, mutations, and associated disorders; G protein, activation of adenylyl cyclase, GNAS1 mutations, McCune-Albright syndrome, fibrous dysplasia, and Albright hereditary osteodystrophy; FLNA and associated disorders; and morphological development of teeth and their genetic mutations.

Filamin-a and rheological properties of cultured melanoma cells

Here we report the rheological properties of cultured hsFLNa (filamin-A)-expressing (FIL+) and hsFLNa-deficient (FIL-) melanoma cells. Using magnetic twisting cytometry over a wide range of probing frequencies, and targeting either cortical or deeper cytoskeletal structures, we found that differences in stiffness of FIL+ versus FIL- cells were remarkably small. When probed through deep cytoskeletal structures, FIL+ cells were, at most, 30% stiffer than FIL- cells, whereas when probed through more peripheral cytoskeletal structures FIL- cells were not different except at very high frequencies. The loss tangent, expressed as an effective cytoskeletal temperature, was systematically greater in FIL- than FIL+ cells, but these differences were small and showed that the FIL+ cells were only slightly closer to a solidlike state. To quantify cytoskeletal remodeling, we measured spontaneous motions of beads bound to cortical cytoskeletal structures and found no difference in FIL+ versus FIL- cells. Although mechanical differences between FIL+ and FIL- cells were evident both in cortical and deeper structures, these differences were far smaller than expected based on measurements of the rheology of purified actin-filamin solutions. These findings do not rule out an important contribution of filamin to the mechanical properties of the cortical cytoskeleton, but suggest that effects of filamin in the cortex are not exerted on the length scale of the probe used here. These findings would appear to rule out any important contribution of filamin to the bulk mechanical properties of the cytoplasm, however. Although filamin is present in the cytoplasm, it may be inactive, its mechanical effects may be small compared with other crosslinkers, or mechanical properties of the matrix may be dominated by an overriding role of cytoskeletal prestress.

The ABP-120 C-end region from Entamoeba histolytica interacts with sulfatide, a new lipid target

EhABP-120 is the first filamin identified in the parasitic protozoan Entamoeba histolytica. Filamins are a family of cross-linking actin-binding proteins that promote a dynamic orthogonal web. They have been reported to interact directly with more than 30 cellular proteins and some phosphoinositides. The biochemical consequences of these interactions may have either positive or negative effects on the cross-linking function and also form a link between the cytoskeleton and plasma membrane. In this study, the EhABP-120 carboxy-terminal domain (END) was biochemically characterized. This domain was able to associate to 3-sulfate galactosyl ceramide, a new lipid target for a member of the filamin family. Also, the END domain was able to dimerize "in vitro." Molecular modeling analysis showed that the dimeric region is stabilized by a disulfide bond. Electrostatic and docking studies suggest that an electropositive concave pocket at the dimeric END domain interacts simultaneously with several sulfogalactose moieties of the sulfatide.

The structure of the GPIb-filamin A complex

Filamin A (FLNa), a dimeric actin cross-linking and scaffold protein with numerous intracellular binding partners, anchors the platelet adhesion glycoprotein (GP) Ib-IX-V receptor to actin cytoskeleton. We mapped the GPIbalpha binding site to a single domain of FLNa and resolved the structure of this domain and its interaction complex with the corresponding GPIbalpha cytoplasmic domain. This is the first atomic structure of this class of membrane glycoprotein-cytoskeleton connection. GPIbalpha binds in a groove formed between the C and D beta strands of FLNa domain 17. The interaction is strikingly similar to that between the beta7 integrin tail and a different FLNa domain, potentially defining a conserved motif for FLNa binding. Nevertheless, the structures also reveal specificity of the interfaces, which explains different regulatory mechanisms. To verify the topology of GPIb-FLNa interaction we also purified the native complex from platelets and showed that GPIb interacts with the C-terminus of FLNa, which is in accordance with our biochemical and structural data.

CEACAM1 functionally interacts with filamin A and exerts a dual role in the regulation of cell migration

The carcinoembryonic antigen-related cell adhesion molecule CEACAM1 (CD66a) and the scaffolding protein filamin A have both been implicated in tumor cell migration. In the present study we identified filamin A as a novel binding partner for the CEACAM1-L cytoplasmic domain in a yeast two-hybrid screen. Direct binding was shown by surface plasmon resonance analysis and by affinity precipitation assays. The association was shown for human and rodent CEACAM1-L in endogenous CEACAM1-L expressing cells. To address functional aspects of the interaction, we used a well-established melanoma cell system. We found in different migration studies that the interaction of CEACAM1-L and filamin A drastically reduced migration and cell scattering, whereas each of these proteins when expressed alone, acted promigratory. CEACAM1-L binding to filamin A reduced the interaction of the latter with RalA, a member of the Ras-family of GTPases. Furthermore, co-expression of CEACAM1-L and filamin A led to a reduced focal adhesion turnover. Independent of the presence of filamin A, the expression of CEACAM1-L led to an increased phosphorylation of focal adhesions and to altered cytoskeletal rearrangements during monolayer wound healing assays. Together, our data demonstrate a novel mechanism for how CEACAM1-L regulates cell migration via its interaction with filamin A.

Molecular physiology of cardiac repolarization

The heart is a rhythmic electromechanical pump, the functioning of which depends on action potential generation and propagation, followed by relaxation and a period of refractoriness until the next impulse is generated. Myocardial action potentials reflect the sequential activation and inactivation of inward (Na(+) and Ca(2+)) and outward (K(+)) current carrying ion channels. In different regions of the heart, action potential waveforms are distinct, owing to differences in Na(+), Ca(2+), and K(+) channel expression, and these differences contribute to the normal, unidirectional propagation of activity and to the generation of normal cardiac rhythms. Changes in channel functioning, resulting from inherited or acquired disease, affect action potential repolarization and can lead to the generation of life-threatening arrhythmias. There is, therefore, considerable interest in understanding the mechanisms that control cardiac repolarization and rhythm generation. Electrophysiological studies have detailed the properties of the Na(+), Ca(2+), and K(+) currents that generate cardiac action potentials, and molecular cloning has revealed a large number of pore forming (alpha) and accessory (beta, delta, and gamma) subunits thought to contribute to the formation of these channels. Considerable progress has been made in defining the functional roles of the various channels and in identifying the alpha-subunits encoding these channels. Much less is known, however, about the functioning of channel accessory subunits and/or posttranslational processing of the channel proteins. It has also become clear that cardiac ion channels function as components of macromolecular complexes, comprising the alpha-subunits, one or more accessory subunit, and a variety of other regulatory proteins. In addition, these macromolecular channel protein complexes appear to interact with the actin cytoskeleton and/or the extracellular matrix, suggesting important functional links between channel complexes, as well as between cardiac structure and electrical functioning. Important areas of future research will be the identification of (all of) the molecular components of functional cardiac ion channels and delineation of the molecular mechanisms involved in regulating the expression and the functioning of these channels in the normal and the diseased myocardium.

Heptaspanning membrane receptors and cytoskeletal/scaffolding proteins: focus on adenosine, dopamine, and metabotropic glutamate receptor function

Most cellular functions are mediated by multiprotein complexes. In neurons, these complexes are directly involved in the proper neuronal transmission, which is responsible for phenomena like learning, memory, and development. In recent years studies based on two-hybrid screens and proteomic, biochemical, and cell biology approaches have shown that intracellular domains of G protein-coupled receptors (GPCRs) or heptaspanning membrane receptors (HSMRs) interact with intracellular proteins. These interactions are the basis of a protein network associated with these receptors, which includes scaffolding proteins containing one or several PDZ (postsynaptic-density-95/discs-large/zona occludens-1) domains, signaling proteins, and proteins of the cytoskeleton. The present article is focused on the emerging evidence for interactions of adenosine, dopamine, and metabotropic glutamate receptors, with scaffolding and cytoskeletal proteins that play a role in the targeting and anchoring of these receptors to the plasma membrane, thus contributing to neuronal development and plasticity. Finally, given the complexity of neurological disorders such as ischemic stroke, Alzheimer's disease, and epilepsy, exploitation of these HSMR-associated interactions might prove to be efficient in the treatment of such disorders.

Inhibition of the phosphatidylinositol 3-kinase/Akt/mammalian target of rapamycin pathway but not the MEK/ERK pathway attenuates laminin-mediated small cell lung cancer cellular survival and resistance to imatinib mesylate or chemotherapy

The fact that small cell lung cancer (SCLC) is commonly incurable despite being initially responsive to chemotherapy, combined with disappointing results from a recent SCLC clinical trial with imatinib, has intensified efforts to identify mechanisms of SCLC resistance. Adhesion to extracellular matrix (ECM) is one mechanism that can increase therapeutic resistance in SCLC cells. To address whether adhesion to ECM increases resistance through modulation of signaling pathways, a series of SCLC cell lines were plated on various ECM components, and activation of two signaling pathways that promote cellular survival, the phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway and the mitogen-activated protein kinase kinase/extracellular signal-regulated kinase (MEK/ERK) pathway, was assessed. Although differential activation was observed, adhesion to laminin increased Akt activation, increased cellular survival after serum starvation, and caused the cells to assume a flattened, epithelial morphology. Inhibitors of the PI3K/Akt/mTOR pathway (LY294002, rapamycin) but not the MEK/ERK pathway (U0126) abrogated laminin-mediated survival. SCLC cells plated on laminin were not only resistant to serum starvation-induced apoptosis but were also resistant to apoptosis caused by imatinib. Combining imatinib with LY294002 or rapamycin but not U0126 caused greater than additive increases in apoptosis compared with apoptosis caused by the inhibitor or imatinib alone. Similar results were observed when adenoviruses expressing mutant Akt were combined with imatinib, or when LY294002 was combined with cisplatin or etoposide. These studies identify laminin-mediated activation of the PI3K/Akt/mTOR pathway as a mechanism of cellular survival and therapeutic resistance in SCLC cells and suggest that inhibition of the PI3K/Akt/mTOR pathway is one strategy to overcome SCLC resistance mediated by ECM.

The Z-disc proteins myotilin and FATZ-1 interact with each other and are connected to the sarcolemma via muscle-specific filamins

Myotilin and the calsarcin family member FATZ-1 (also called calsarcin-2 or myozenin-1) are recently discovered sarcomeric proteins implicated in the assembly and stabilization of the Z-discs in skeletal muscle. The essential role of myotilin in skeletal muscle is attested by the observation that certain forms of myofibrillar myopathy and limb girdle muscular dystrophy are caused by mutations in the human myotilin gene. Here we show by transfection, biochemical and/or yeast two-hybrid assay that: (1) myotilin is able to interact with the C-terminal region of FATZ-1 and that the N- or C-terminal truncations of myotilin abrogate binding; (2) myotilin can also interact with another calsarcin member, FATZ-2 (calsarcin-1, myozenin-2); (3) myotilin and FATZ-1 bind not only to the C-terminal region of filamin-C containing the Ig repeats 19-24, but also to the other two filamins, filamin-A and filamin-B, as well as the newly identified filamin-Bvar-1variant; (4) the binding of myotilin to filamin-C involves binding sites in its N-terminal region, whereas FATZ-1 associates with filamin-C via sequences within either its N- or C-terminal region; and finally, (5) the C-terminal region of filamin-C like filamin-B and filamin-Bvar-1, shows binding activity with the β1A integrin subunit. Our findings further dissect the molecular interactions within the Z-disc that are essential for its organization, and provide evidence for a novel connection between Z-disc proteins and the sarcolemma via filamins and β1 integrins. These data shed new light on the complex organization of the Z-disc that is highly relevant to understanding muscular dystrophies.

A mutation in the dimerization domain of filamin c causes a novel type of autosomal dominant myofibrillar myopathy

Myofibrillar myopathy (MFM) is a human disease that is characterized by focal myofibrillar destruction and pathological cytoplasmic protein aggregations. In an extended German pedigree with a novel form of MFM characterized by clinical features of a limb-girdle myopathy and morphological features of MFM, we identified a co-segregating, heterozygous nonsense mutation (8130G-->A; W2710X) in the filamin c gene (FLNC) on chromosome 7q32.1. The mutation is the first found in FLNC and is localized in the dimerization domain of filamin c. Functional studies showed that, in the truncated mutant protein, this domain has a disturbed secondary structure that leads to the inability to dimerize properly. As a consequence of this malfunction, the muscle fibers of our patients display massive cytoplasmic aggregates containing filamin c and several Z-disk-associated and sarcolemmal proteins.

Ehlers-Danlos syndrome and periventricular nodular heterotopia in a Spanish family with a single FLNA mutation

BACKGROUND

The Ehlers-Danlos syndrome (EDS) comprises a group of hereditary connective tissue disorders. Periventricular nodular heterotopia (PNH) is a human neuronal migration disorder characterised by seizures and conglomerates of neural cells around the lateral ventricles of the brain, caused by FLNA mutations. FLNA encodes filamin A, an actin binding protein involved in cytoskeletal organisation. The amino-terminal actin binding domain (ABD) of filamins contains two tandem calponin homology domains, CHD1 and CHD2.

OBJECTIVE

To report clinical and genetic analyses in a Spanish family affected by a connective tissue disorder suggestive of EDS type III and PNH.

METHODS

A clinical and molecular study was undertaken in the three affected women. Clinical histories, physical and neurological examinations, brain magnetic resonance imaging studies, and skin biopsies were done. Genetic analysis of the FLNA gene was undertaken by direct sequencing and restriction fragment length polymorphism analysis.

RESULTS

Mutation analysis of the FLNA gene resulted in the identification of a novel mutation in exon 3 (c.383C-->T) segregating with the combination of both syndromes. This mutation results in a substitution of an alanine residue (A128V) in CHD1.

CONCLUSIONS

The findings suggest that the Ala128Val mutation causes the dual EDS-PNH phenotype. This association constitutes a new variant within the EDS spectrum. This is the first description of a familial EDS-PNH association with a mutation in FLNA.

Identification of a unique filamin A binding region within the cytoplasmic domain of glycoprotein Ibalpha

Binding of the platelet GPIb/V/IX (glycoprotein Ib/V/IX) receptor to von Willebrand factor is critical for platelet adhesion and aggregation under conditions of rapid blood flow. The adhesive function of GPIbalpha is regulated by its anchorage to the membrane skeleton through a specific interaction with filamin A. In the present study, we examined the amino acid residues within the cytoplasmic tail of GPIbalpha, which are critical for association with filamin A, using a series of 25-mer synthetic peptides that mimic the cytoplasmic tail sequences of wild-type and mutant forms of GPIbalpha. Peptide binding studies of purified human filamin A have demonstrated a major role for the conserved hydrophobic stretch L567FLWV571 in mediating this interaction. Progressive alanine substitutions of triple, double and single amino acid residues within the Pro561-Arg572 region suggested an important role for Trp570 and Phe568 in promoting GPIbalpha binding to filamin A. The importance of these two residues in promoting filamin A binding to GPIbalpha in vivo was confirmed from the study of Chinese-hamster ovary cells expressing GPIbalpha Trp570-->Ala and Phe568-->Ala substitutions. Phenotypic analysis of these cell lines in flow-based adhesion studies revealed a critical role for these residues in maintaining receptor anchorage to the membrane skeleton and in maintaining cell adhesion to a von Willebrand factor matrix under high-shear conditions. These studies demonstrate a novel filamin A binding motif in the cytoplasmic tail of GPIbalpha, which is critically dependent on both Trp570 and Phe568.

Cytoskeletal proteins talin and vinculin in integrin-mediated adhesion

The cytoskeletal proteins talin and vinculin form part of a macromolecular complex on the cytoplasmic face of integrin-mediated cellular junctions with the extracellular matrix. Recent genetic, biochemical and structural data show that talin is essential for the assembly of such junctions, whereas vinculin appears to be important in regulating adhesion dynamics and cell migration.

Evolutionarily conserved human targets of adenosine to inosine RNA editing

A-to-I RNA editing by ADARs is a post-transcriptional mechanism for expanding the proteomic repertoire. Genetic recoding by editing was so far observed for only a few mammalian RNAs that are predominantly expressed in nervous tissues. However, as these editing targets fail to explain the broad and severe phenotypes of ADAR1 knockout mice, additional targets for editing by ADARs were always expected. Using comparative genomics and expressed sequence analysis, we identified and experimentally verified four additional candidate human substrates for ADAR-mediated editing: FLNA, BLCAP, CYFIP2 and IGFBP7. Additionally, editing of three of these substrates was verified in the mouse while two of them were validated in chicken. Interestingly, none of these substrates encodes a receptor protein but two of them are strongly expressed in the CNS and seem important for proper nervous system function. The editing pattern observed suggests that some of the affected proteins might have altered physiological properties leaving the possibility that they can be related to the phenotypes of ADAR1 knockout mice.

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.

G protein-coupled receptor kinase regulates dopamine D3 receptor signaling by modulating the stability of a receptor-filamin-beta-arrestin complex. A case of autoreceptor regulation

In addition to its postsynaptic role, the dopamine D3 receptor (D3R) serves as a presynaptic autoreceptor, where it provides continuous feedback regulation of dopamine release at nerve terminals for processes as diverse as emotional tone and locomotion. D3R signaling ability is supported by an association with filamin (actin-binding protein 280), which localizes the receptor with G proteins in plasma membrane lipid rafts but is not appreciably antagonized in a classical sense by the ligand-mediated activation of G protein-coupled receptor kinases (GRKs) and beta-arrestins. In this study, we investigate GRK-mediated regulation of D3R.filamin complex stability and its effect on D3R.G protein signaling potential. Studies in HEK-293 cells show that in the absence of agonist the D3R immunoprecipitates in a complex containing both filamin A and beta-arrestin2. Moreover, the filamin directly interacts with beta-arrestin2 as assessed by immunoprecipitation and yeast two-hybrid studies. With reductions in basal GRK2/3 activity, an increase in the basal association of filamin A and beta-arrestin2 with D3R is observed. Conversely, increases in the basal GRK2/3 activity result in a reduction in the interaction between the D3R and filamin but a relative increase in the agonist-mediated interaction between beta-arrestin2 and the D3R. Our data suggest that the D3R, filamin A, and beta-arrestin form a signaling complex that is destabilized by agonist- or expression-mediated increases in GRK2/3 activity. These findings provide a novel GRK-based mechanism for regulating D3R signaling potential and provide insight for interpreting D3R autoreceptor behavior.

A novel 9 bp deletion in the filamin a gene causes an otopalatodigital-spectrum disorder with a variable, intermediate phenotype

We report a four-generation pedigree with six affected females with cranial hyperostosis and various skeletal abnormalities. The phenotype is similar to frontometaphyseal dysplasia, which is part of the otopalatodigital (OPD) spectrum. We identified a novel in-frame deletion in exon 29 of the Filamin A gene (c.4904_4912del, p.R1635_V1637del) encoding rod domain repeat 14 of the protein. The disorder resulted in early lethality in male children. The phenotype of female individuals in this family is variable and rather mild, and bridges the phenotypes of various OPD-spectrum disorders.

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.

SH2-containing 5'-inositol phosphatase, SHIP2, regulates cytoskeleton organization and ligand-dependent down-regulation of the epidermal growth factor receptor

Phosphoinositide lipid second messengers are integral components of signaling pathways mediated by insulin, growth factors, and integrins. SHIP2 dephosphorylates phosphatidylinositol 3,4,5-trisphosphate generated by the activated phosphatidylinositol 3'-kinase. SHIP2 down-regulates insulin signaling and is present at higher levels in diabetes and obesity. SHIP2 associates with p130Cas and filamin, regulators of cell adhesion/migration and cytoskeleton, influencing cell adhesion/spreading. Type I collagen specifically induces Src-mediated tyrosine phosphorylation of SHIP2. To better understand SHIP2 function, we employed RNA interference (RNAi) approach to silence the expression of the endogenous SHIP2 in HeLa cells. Suppression of SHIP2 levels caused severe F-actin deformities characterized by weak cortical actin and peripheral actin spikes. SHIP2 RNAi cells displayed cell-spreading defects involving a notable absence of focal contact structures and the formation of multiple slender membrane protrusions capped by actin spikes. Furthermore, decreased SHIP2 levels altered distribution of early endocytic antigen 1 (EEA1)-positive endocytic vesicles and of vesicles containing internalized epidermal growth factor (EGF) and transferrin. EGF treatment of SHIP2 RNAi cells led to the following: enhanced EGF receptor (EGFR) degradation; increased EGFR ubiquitination; and increased association of EGFR with c-Cbl ubiquitin ligase. Taken together, these experiments demonstrate that SHIP2 functions in the maintenance and dynamic remodeling of actin structures as well as in endocytosis, having a major impact on ligand-induced EGFR internalization and degradation. Accordingly, we suggest that, in HeLa cells, SHIP2 plays a distinct role in signaling pathways mediated by integrins and growth factor receptors.

Fascin protrusions in cell interactions

Cell protrusions are outward extensions of the plasma membrane of individual cells that function in sensing the cell environment and in making initial, dynamic adhesions to extracellular matrix and other cells. Cell protrusions can be grouped into two major categories on the basis of morphology: localized, finger-like structures of highly defined shape and various lengths; or broad, irregular extensions of the plasma membrane. A key requirement of all cell protrusions is the need for a rigid cytoskeleton to support the localized extension of the plasma membrane. This is achieved either by a core unipolar bundle of actin microfilaments in finger-like protrusions, or by a combination of radial, rib-like, actin bundles integrated with a dendritic meshwork of microfilaments in the broad, lamellipodial protrusions. From studies of multiple cell types in vertebrates and invertebrates, fascin-1 has emerged as an actin-bundling protein of general importance for a diverse set of cell protrusions with functions in cell adhesion, cell interactions, and cell migration. This review discusses current knowledge of the molecular and cellular properties and functions of fascin, the roles of fascin-based protrusions in the cardiovascular system in health and disease, and areas of future interest.

Chicken gizzard filamin, retina filamin and cgABP260 are respectively, smooth muscle-, non-muscle- and pan-muscle-type isoforms: Distribution and localization in muscles

We determined the full cDNA sequences of chicken gizzard filamin and cgABP260 (chicken gizzard actin-binding protein 260). The primary and secondary structures predicted by these sequences were similar to those of chicken retina filamin and human filamins. Like mammals, chickens have 3 filamin isoforms. Comparison of their amino acid sequences indicated that gizzard filamin, retina filamin, and cgABP260 were the counterparts of human FLNa (filamin a), b, and c, respectively. Antibodies against the actin-binding domain (ABD) of these 3 filamin isoforms were raised in rabbits. Using immunoabsorption and affinity chromatography, we prepared the monospecific antibody against the ABD of each filamin. In immunoblotting, the antibody against the gizzard filamin ABD detected a single band in gizzard, but not in striated muscles or brain. In brain, only the antibody against the retina filamin ABD produced a strong single band. The antibody against the cgABP260 ABD detected a single peptide band in smooth, skeletal, and cardiac muscle. In immunofluorescence microscopy of muscular tissues using these antibodies, the antibody against the gizzard filamin ABD only stained smooth muscle cells, and the antibody against the retina filamin ABD strongly stained endothelial cells of blood vessels and weakly stained cells in connective tissue. The antibody against the cgABP260 ABD stained the Z-lines and myotendinous junctions of breast muscle, the Z-lines and intercalated disks of cardiac muscle, and dense plaques of smooth muscle. These findings indicate that chicken gizzard filamin, retina filamin, and cgABP260 are, respectively, smooth muscle-type, non-muscle-type, and pan-muscle-type filamin isoforms.

Structural Basis for Vertebrate Filamin Dimerization

Filamins are essential in cell motility and many developmental processes. They are large actin cross linking proteins that contain actin binding domains in their N termini and a long rod region constructed from 24 tandem Ig domains. Dimerization is crucial for the actin crosslinking function of filamins and requires the most C-terminal Ig domain. We describe here the crystal structure of this 24th Ig domain (Ig24) of human filamin C and show how it mediates dimerization. The dimer interface is novel and quite different to that seen in the Dictyostelium discoideum filamin analog. The sequence signature of the dimerization interface suggests that the C-terminal domains of all vertebrate filamins share the same dimerization mechanism. Furthermore, we show that point mutations in the dimerization interface disrupt the dimer and that the dissociation constant for recombinant Ig24 is in the micromolar range.

A novel filamin A D203Y mutation in a female patient with otopalatodigital type 1 syndrome and extremely skewed X chromosome inactivation

Otopalatodigital syndrome type 1 (OPD1) [OMIM 311300] is an X-linked dominant multiple congenital anomalies disease mainly characterized by a generalized skeletal dysplasia, mild mental retardation, hearing loss, cleft palate, and typical facial anomalies. OPD1 belongs to a group of X-linked skeletal dysplasias known as oto-palato-digital syndrome spectrum disorders that also include OPD2, Melnick-Needles syndrome (MNS), and frontometaphyseal dysplasia (FMD). Recently, it has been demonstrated that mutations in the gene encoding the cytoskeletal protein Filamin A (FLNA) are responsible for this group of clinically overlapping human syndromes. We present the phenotypic and molecular data of a sporadic female patient clinically diagnosed with an OPD1 syndrome who carried a novel FLNA point mutation resulting in an Asp203Tyr substitution in the actin-binding domain of the protein. X-inactivation analyses demonstrated an extremely skewed pattern towards her maternal chromosome. Our results add to the molecular spectrum of the oto-palato-digital related syndromes and contribute to the delineation of phenotype-genotype correlation in this group of X-linked skeletal disorders.

X-linked mental retardation (XLMR): from clinical conditions to cloned genes

X-linked mental retardation (XLMR) is a heterogenous set of conditions responsible for a large proportion of inherited mental retardation. Approximately 200 XLMR conditions and 45 cloned genes are now listed in our catalogue on the Internet at http://xlmr.interfree.it/home.htm. Traditionally, XLMR conditions were subdivided into specific (MRXS) and nonspecific (MRX) forms, depending on their clinical presentation. Now that a growing number of candidate genes have become available for screening XLMR families and patients, this distinction is becoming less useful and similar conditions that had been previously listed as separate can now be grouped together because different mutations in the same gene have been identified. Furthermore, different mutations in the same XLMR gene may account for diseases of increasing severity, but can also cause different phenotypes. As the functions of proteins corresponding to these genes are characterized, biological networks involved in causing mental retardation and conversely in supporting normal intellectual functioning will be discovered. Molecular biologists and neurobiologists will need to cooperate in order to verify the effects of XLMR gene mutations in the context of neuronal circuitry. Eventually, DNA and protein microarray technologies will assist researchers and physicians in reaching a diagnosis even in small families or in individual patients with XLMR.

The many faces of filamin: a versatile molecular scaffold for cell motility and signalling

Filamins were discovered as the first family of non-muscle actin-binding protein. They are lage cytoplasmic proteins that cross-link cortical actin into a dynamic three-dimensional structure. Filamins have also been reported to interact with a large number of cellular proteins of great functional diversity, suggesting that they are unusually versatile signalling scaffolds. More recently, genetic mutations in filamin A and B have been reported to cause a wide range of human diseases, suggesting that different diseases highlight distinct filamin interactions.

Leech filamin and Tractin: markers for muscle development and nerve formation

The Lan3-14 and Laz10-1 monoclonal antibodies recognize a 400 kDa antigen that is specifically expressed by all muscle cells in leech. We show that the antigen recognized by both antibodies is a member of the filamin family of actin binding proteins. Leech filamin has two calponin homology domains and 35 filamin/ABP-repeat domains. In addition, we used the Laz10-1 antibody to characterize the development of the segmentally iterated dorsoventral flattener muscles. We demonstrate that the dorsoventral flattener muscle develops as three discrete bundles of myofibers and that CNS axons pioneering the DP nerve extend only along the middle bundle. Interestingly, the middle dorsoventral muscle anlage is associated with only non-neuronal expression of the L1-family cell adhesion molecule Tractin. This expression is transient and occurs at the precise developmental stages when DP nerve formation takes place. Based on these findings we propose that the middle dorsoventral muscle anlagen provides a substrate for early axonal outgrowth and nerve formation and that this function may be associated with differential expression of distinct cell adhesion molecules.