Filamins in cardiovascular development

Role of Filamin A in Growth and Migration of Breast Cancer-Review

Despite ongoing research in the field of breast cancer, the morbidity rates indicate that the disease remains a significant challenge. While patients with primary tumors have relatively high survival rates, these chances significantly decrease once metastasis begins. Thus, exploring alternative approaches, such as targeting proteins overexpressed in malignancies, remains significant. Filamin A (FLNa), an actin-binding protein (ABP), is involved in various cellular processes, including cell migration, adhesion, proliferation, and DNA repair. Overexpression of the protein was confirmed in samples from patients with numerous oncological diseases such as prostate, lung, gastric, colorectal, and pancreatic cancer, as well as breast cancer. Although most researchers concur on its role in promoting breast cancer progression and aggressiveness, discrepancies exist among studies. Moreover, the precise mechanisms through which FLNa affects cell migration, invasion, and even cancer progression remain unclear, highlighting the need for further research. To evaluate FLNa's potential as a therapeutic target, we have summarized its roles in breast cancer.

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

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

Structural and signaling proteins in the Z-disk and their role in cardiomyopathies

The sarcomere is the smallest functional unit of muscle contraction. It is delineated by a protein-rich structure known as the Z-disk, alternating with M-bands. The Z-disk anchors the actin-rich thin filaments and plays a crucial role in maintaining the mechanical stability of the cardiac muscle. A multitude of proteins interact with each other at the Z-disk and they regulate the mechanical properties of the thin filaments. Over the past 2 decades, the role of the Z-disk in cardiac muscle contraction has been assessed widely, however, the impact of genetic variants in Z-disk proteins has still not been fully elucidated. This review discusses the various Z-disk proteins (alpha-actinin, filamin C, titin, muscle LIM protein, telethonin, myopalladin, nebulette, and nexilin) and Z-disk-associated proteins (desmin, and obscurin) and their role in cardiac structural stability and intracellular signaling. This review further explores how genetic variants of Z-disk proteins are linked to inherited cardiac conditions termed cardiomyopathies.

Interaction of LARP4 to filamin A mechanosensing domain regulates cell migrations

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

Systematic identification of cancer-associated-fibroblast-derived genes in patients with colorectal cancer based on single-cell sequencing and transcriptomics

Colorectal cancer (CRC) has a high incidence rate and poor prognosis, and the available treatment approaches have limited therapeutic benefits. Therefore, understanding the underlying mechanisms of occurrence and development is particularly crucial. Increasing attention has been paid to the pathophysiological role of cancer-associated fibroblasts (CAFs) in the heterogeneous tumour microenvironment. CAFs play a crucial role in tumorigenesis, tumour progression and treatment response. However, routine tissue sequencing cannot adequately reflect the heterogeneity of tumours. In this study, single-cell sequencing was used to examine the fibroblast population in CRC. After cluster analysis, the fibroblast population was divided into four subgroups. The distribution and role of these four subgroups in CRC were found to be different. Based on differential gene expression and lasso regression analysis of the main marker genes in these subgroups, four representative genes were obtained, namely, TCF7L1, FLNA, GPX3 and MMP11. Patients with CRC were divided into the low- and high-risk groups using the prognostic risk model established based on the expression of these four genes. The prognosis of patients in different risk groups varied significantly; patients with low-risk scores had a greater response to PDL1 inhibitors, significant clinical benefits and significantly prolonged overall survival. These effects may be attributed to inhibition of the function of T cells in the immune microenvironment and promotion of the function of tumour-associated macrophages.

A-to-I RNA editing of Filamin A regulates cellular adhesion, migration and mechanical properties

A-to-I RNA editing by ADARs is an abundant epitranscriptomic RNA-modification in metazoa. In mammals, Flna pre-mRNA harbours a single conserved A-to-I RNA editing site that introduces a Q-to-R amino acid change in Ig repeat 22 of the encoded protein. Previously, we showed that FLNA editing regulates smooth muscle contraction in the cardiovascular system and affects cardiac health. The present study investigates how ADAR2-mediated A-to-I RNA editing of Flna affects actin crosslinking, cell mechanics, cellular adhesion and cell migration. Cellular assays and AFM measurements demonstrate that the edited version of FLNA increases cellular stiffness and adhesion but impairs cell migration in both, mouse fibroblasts and human tumour cells. In vitro, edited FLNA leads to increased actin crosslinking, forming actin gels of higher stress resistance. Our study shows that Flna RNA editing is a novel regulator of cytoskeletal organisation, affecting the mechanical property and mechanotransduction of cells.

Cardiovascular, Brain, and Lung Involvement in a Newborn With a Novel FLNA Mutation: A Case Report and Literature Review

BACKGROUND

Filamin A (FLNA) is an intracellular actin-binding protein, encoded by the FLNA gene, with a wide tissue expression. It is involved in several cellular functions, and extracellular matrix structuring. FLNA gene alterations lead to diseases with a wide phenotypic spectrum, such as brain periventricular nodular heterotopia (PVNH), cardiovascular abnormalities, skeletal dysplasia, and lung involvement.

CLINICAL FINDINGS

We present the case of a female infant who showed at birth aortic valve stenosis and PVNH, and subsequently developed interstitial lung disease with severe pulmonary hypertension.

PRIMARY DIAGNOSIS

The association of aortic valve dysplasia, left ventricular outflow obstruction, persistent patent ductus arteriosus, and brain heterotopic gray matter suggested a possible FLNA gene alteration. A novel heterozygous intronic variant in the FLNA gene (NM_001110556.1), c.4304-1G >A, was detected.

INTERVENTIONS

In consideration of valve morphology and severity of stenosis, the neonate was scheduled for a transcatheter aortic valvuloplasty. At 3 months of life, she developed hypoxemic respiratory failure with evidence of severe pulmonary hypertension. Inhaled nitric oxide (iNO) and milrinone on continuous infusion were started. Because of a partial response to iNO, an intravenous continuous infusion of sildenafil was introduced.

OUTCOMES

In consideration of severe clinical course and fatal outcome, the new FLNA gene mutation described in our patient seems to be associated with a loss of function of FLNA.

PRACTICE RECOMMENDATIONS

Lung and brain involvement, in association with left ventricular outflow obstruction and persistent patency of ductus arteriosus, should be considered highly suggestive of FLNA gene alterations, in a female newborn.

Downregulation of Filamin a Expression in the Aorta Is Correlated With Aortic Dissection

Filamins (FLNs) are actin cross-linking proteins, and as scaffolding proteins, FLNs are closely associated with the stabilization of the cytoskeleton. Nevertheless, the biological importance of FLNs in aortic dissection (AD) has not been well-elucidated. In this study, we first reanalyzed datasets downloaded from the Gene Expression Omnibus (GEO) database, and we found that in addition to the extracellular matrix, the actin cytoskeleton is a key structure associated with AD. Given that FLNs are involved in remodeling the cytoskeleton to affect cellular functions, we measured their expression levels in the aortas of patients with Stanford type A AD (TAAD). Our results showed that the mRNA and protein levels of FLNA were consistently decreased in dissected aortas of both humans and mice, while the FLNB protein level was upregulated despite decreased FLNB mRNA levels, and comparable expression levels of FLNC were observed between groups. Furthermore, the immunohistochemistry results demonstrated that FLNA was highly expressed in smooth muscle cells (SMCs) of aorta in non-AD samples, and downregulated in the medial layer of the dissected aortas of humans and mice. Moreover, we revealed that FOS and JUN, forming a dimeric transcription factor called AP-1 (activating protein-1), were positively correlated with the expression of FLNA in aorta. Either overexpression of FOS or JUN alone, or overexpression of FOS and JUN together, facilitated the expression of FLNA in primary cultured human aortic SMCs. In the present study, we not only detected the expression pattern of FLNs in aortas of humans and mice with or without AD, but we also found that the expression of FLNA in the AD samples was significantly reduced and that AP-1 might regulate the expression of FLNA. Our findings will contribute to the elucidation of the pathological mechanisms of AD and provide potential therapeutic targets for AD.

The variants at FLNA and FLNB contribute to the susceptibility of hypertension and stroke with differentially expressed mRNA

BACKGROUND

Filamin A and filamin B were involved in vascular development and remodeling. Herein, it is important to explore the associations of FLNA and FLNB variants with hypertension and stroke.

METHODS

The associations of two single-nucleotide polymorphisms (SNPs) at FLNA and five SNPs at FLNB with hypertension and stroke were examined in two case-control studies and a cohort study in Chinese Han population. Risks were estimated as odds ratio (OR) and hazard ratio (HR) by Logistic and Cox regression analysis respectively. In addition, filamin B, FLNA and FLNB mRNA expression were measured.

RESULTS

In the case-control study of hypertension, FLNA rs2070816 (CT + TT vs. CC) and rs2070829 (CG + GG vs. CC) were significantly associated with hypertension in <55 years group (OR = 1.338, P = 0.018; OR = 1.615, P = 0.005) and FLNB rs839240 (AG + GG vs. AA) was significantly associated with hypertension in females (OR = 0.828, P = 0.041) and nonsmokers (OR = 0.829, P = 0.020). In the follow-up study, rs2070829 GG genotype carriers presented a higher risk of hypertension than CC/CG in males (HR = 1.737, P = 0.014) and smokers (HR = 1.949, P = 0.012). In the case-control study of stroke, FLNB rs1131356 variation was significantly associated with ischemic stroke (IS) and intracerebral hemorrhage (ICH), ORs of additive model were 1.342 and 1.451, with P values of 0.001 and 0.007. The FLNA transcript 2, FLNB transcript 3, transcript 4 mRNA, and filamin B expression levels were significantly different between IS cases and hypertension controls and among the genotypes of rs839240 in hypertensive individuals (P < 0.05).

CONCLUSIONS

Our findings support the genetic contribution of FLNA and FLNB to hypertension, and stroke with differentially mRNA expression.

Filamin A Regulates Cardiovascular Remodeling

Filamin A (FLNA) is a large actin-binding cytoskeletal protein that is important for cell motility by stabilizing actin networks and integrating them with cell membranes. Interestingly, a C-terminal fragment of FLNA can be cleaved off by calpain to stimulate adaptive angiogenesis by transporting multiple transcription factors into the nucleus. Recently, increasing evidence suggests that FLNA participates in the pathogenesis of cardiovascular and respiratory diseases, in which the interaction of FLNA with transcription factors and/or cell signaling molecules dictate the function of vascular cells. Localized FLNA mutations associate with cardiovascular malformations in humans. A lack of FLNA in experimental animal models disrupts cell migration during embryogenesis and causes anomalies, including heart and vessels, similar to human malformations. More recently, it was shown that FLNA mediates the progression of myocardial infarction and atherosclerosis. Thus, these latest findings identify FLNA as an important novel mediator of cardiovascular development and remodeling, and thus a potential target for therapy. In this update, we summarized the literature on filamin biology with regard to cardiovascular cell function.

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

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

Screening and analysis of agouti signaling protein interaction partners in Pelodiscus sinensis suggests a role in lipid metabolism

Agouti signaling protein (ASP) is a secreted paracrine protein that has been widely reported to function in melanogenesis and obesity and could potentially be a core protein that regulates the color and fatty phenotype of P. sinensis. In this study, we screened out interacting proteins of ASP by combined co-immunoprecipitation mass spectrometry (CoIP-MS), yeast two hybrid (Y2H) analysis, and computational predictions. We performed docking of ASP with its well-known receptor melanocortin receptor 4 (MC4R) to predict the binding capacity and to screen out actual ASP interacting proteins, CoIP-MS was performed where identified 32 proteins that could bind with ASP and Y2H confirmed seven proteins binding with ASP directly. CoIP-MS and Y2H screening results including PPI prediction revealed that vitronectin (VTN), apolipoprotein A1 (APOA1), apolipoprotein B (APOB), and filamin B (FLNB) were the key interacting proteins of ASP. VTN, APOA1, and APOB are functional proteins in lipid metabolism and various skin disorders, suggesting ASP may function in lipid metabolism through these partners. This study provided protein-protein interaction information of ASP, and the results will promote further research into the diverse roles of ASP, as well as its binding partners, and their function in different strains of P. sinensis.

Proteomic analyses reveal divergent ubiquitylation patterns in hepatocellula carcinoma cell lines with different metastasis potential

Hepatocellular carcinoma (HCC) is one of the most common malignant tumours, metastasis and recurrence remain the primary reasons for poor prognosis. Ubiquitination serves as a degradation mechanism of proteins, but it is involved in additional cellular processes including metastasis. Here, by using label-free quantification, double-glycine (di-Gly) antibody affinity purification and high-resolution liquid chromatography tandem mass spectrometry (LC-MS/MS), we investigated quantitative proteome, ubiquitylome, and the crosstalk between the two datasets in HCC cell lines with different metastasis potential to identify biomarkers associated with HCC metastasis. In total, 83 ubiquitinated proteins significantly and steadily changed their abundance according to their metastatic potential, and the participated biological processes of these ubiquitinated proteins were tightly associated with tumour metastasis. Further signaling pathway analysis revealed that the ribosome and proteasome were significantly over-activated in the highly metastatic cells. Furthermore, we analyzed the crosstalk between the whole proteome and the ubiquitylome, and further discussed the mechanism that how ubiquitination events affect HCC metastasis. Eventually, the ubiquitination of Ku80 was validated to be significantly down-regulated in the high-metastatic cells comparing with the low-metastatic cells. We believe that these findings will help us better understand the underlying molecular mechanisms of the metastasis of HCC. SIGNIFICANCE: In this manuscript, we used label free based proteomics combined with diglycine antibody (di-Gly) affinity purification approach to identify biomarkers associated with HCC recurrence/metastasis in in a serial HCC cell lines with increasing invasion and metastasis potential. And then, we analyzed the crosstalk between the whole proteome and the ubiquitylome. Eventually, the ubiquitination of Ku80 was confirm to be closely associated with invasion and migration of HCC cells. As far as we know, this is the first time to use quantitative proteomic approach to study the ubiquitylomics in HCC cell lines with increasing metastasis ability.

Modulation of ADAR mRNA expression in patients with congenital heart defects

Adenosine (A) to inosine (I) RNA editing is a hydrolytic deamination reaction catalyzed by the adenosine deaminase (ADAR) enzyme acting on double-stranded RNA. This posttranscriptional process diversifies a plethora of transcripts, including coding and noncoding RNAs. Interestingly, few studies have been carried out to determine the role of RNA editing in vascular disease. The aim of this study was to determine the potential role of ADARs in congenital heart disease. Strong downregulation of ADAR2 and increase in ADAR1 expression was observed in blood samples from congenital heart disease (CHD) patients. The decrease in expression of ADAR2 was in line with its downregulation in ventricular tissues of dilated cardiomyopathy patients. To further decipher the plausible regulatory pathway of ADAR2 with respect to heart physiology, miRNA profiling of ADAR2 was performed on tissues from ADAR2-/- mouse hearts. Downregulation of miRNAs (miR-29b, miR-405, and miR-19) associated with cardiomyopathy and cardiac fibrosis was observed. Moreover, the upregulation of miR-29b targets COL1A2 and IGF1, indicated that ADAR2 might be involved in cardiac myopathy. The ADAR2 target vascular development associated protein-coding gene filamin B (FLNB) was selected. The editing levels of FLNB were dramatically reduced in ADAR2^-/- mice; however, no observable changes in FLNB expression were noted in ADAR2^-/- mice compared to wild-type mice. This study proposes that sufficient ADAR2 enzyme activity might play a vital role in preventing cardiovascular defects.

Thoracic aortic aneurysm in patients with loss of function Filamin A mutations: Clinical characterization, genetics, and recommendations

The frequency and gender distribution of thoracic aortic aneurysm as a cardiovascular manifestation of loss-of-function (LOF) X-linked FilaminA (FLNA) mutations are not known. Furthermore, there is very limited cardiovascular morbidity or mortality data in children and adults. We analyzed cardiac data on the largest series of 114 patients with LOF FLNA mutations, both children and adults, with periventricular nodular heterotopia (PVNH), including 48 study patients and 66 literature patients, median age of 22.0 years (88 F, 26 M, range: 0-71 years), with 75 FLNA mutations observed in 80 families. Most (64.9%) subjects had a cardiac anomaly or vascular abnormality (80.8% of males and 60.2% of females). Thoracic aortic aneurysms or dilatation (TAA) were found in 18.4% (n = 21), and were associated with other structural cardiac malformations in 57.1% of patients, most commonly patent ductus arteriosus (PDA) and valvular abnormalities. TAA most frequently involved the aortic root and ascending aorta, and sinus of Valsalva aneurysms were present in one third of TAA patients. Six TAA patients (28.5%) required surgery (median age 37 yrs, range 13-41 yrs). TAA with its associated complications was also the only recorded cause of premature, non-accidental mortality in adults (2 M, 2 F). Two adult patients (1 F, 1 M, median 38.5 yrs), died of spontaneous aortic rupture at aortic dimensions smaller than current recommendations for surgery for other aortopathies. Data from this largest series of LOF FLNA mutation patients underscore the importance of serial follow-up to identify and manage these potentially devastating cardiovascular complications.

Periostin overexpression in collecting ducts accelerates renal cyst growth and fibrosis in polycystic kidney disease

In polycystic kidney disease (PKD), persistent activation of cell proliferation and matrix production contributes to cyst growth and fibrosis, leading to progressive deterioration of renal function. Previously, we showed that periostin, a matricellular protein involved in tissue repair, is overexpressed by cystic epithelial cells of PKD kidneys. Periostin binds α_Vβ₃-integrins and activates integrin-linked kinase (ILK), leading to Akt/mammalian target of rapamycin (mTOR)-mediated proliferation of human PKD cells. By contrast, periostin does not stimulate the proliferation of normal human kidney cells. This difference in the response to periostin is due to elevated expression of α_Vβ₃-integrins by cystic cells. To determine whether periostin accelerates cyst growth and fibrosis, we generated mice with conditional overexpression of periostin in the collecting ducts (CDs). Ectopic CD expression of periostin was not sufficient to induce cyst formation or fibrosis in wild-type mice. However, periostin overexpression in pcy/pcy ( pcy) kidneys significantly increased mTOR activity, cell proliferation, cyst growth, and interstitial fibrosis; and accelerated the decline in renal function. Moreover, CD-specific overexpression of periostin caused a decrease in the survival of pcy mice. These pathological changes were accompanied by increased renal expression of vimentin, α-smooth muscle actin, and type I collagen. We also found that periostin increased gene expression of pathways involved in repair, including integrin and growth factor signaling and ECM production, and it stimulated focal adhesion kinase, Rho GTPase, cytoskeletal reorganization, and migration of PKD cells. These results suggest that periostin stimulates signaling pathways involved in an abnormal tissue repair process that contributes to cyst growth and fibrosis in PKD.

Filamin-A as a Balance between Erk/Smad Activities During Cardiac Valve Development

Mitral valve prolapse (MVP) affects 2.4% of the population and has poorly understood etiology. Recent genetic studies have begun to unravel the complexities of MVP and through these efforts, mutations in the FLNA (Filamin-A) gene were identified as disease causing. Our in vivo and in vitro studies have validated these genetic findings and have revealed FLNA as a central regulator of valve morphogenesis. The mechanisms by which FLNA mutations result in myxomatous mitral valve disease are currently unknown, but may involve proteins previously associated with mutated regions of the FLNA protein, such as the small GTPase signaling protein, R-Ras. Herein, we report that Filamin-A is required for R-Ras expression and activation of the Ras-Mek-Erk pathway. Loss of the Ras/Erk pathway correlated with hyperactivation of pSmad2/3, increased extracellular matrix (ECM) production and enlarged mitral valves. Analyses of integrin receptors in the mitral valve revealed that Filamin-A was required for β1-integrin expression and provided a potential mechanism for impaired ECM compaction and valve enlargement. Our data support Filamin-A as a protein that regulates the balance between Erk and Smad activation and an inability of Filamin-A deficient valve interstitial cells to effectively remodel the increased ECM production through a β1-integrin mechanism. As a consequence, loss of Filamin-A function results in increased ECM production and generation of a myxomatous phenotype characterized by improperly compacted mitral valve tissue. Anat Rec, 302:117-124, 2019. © 2018 Wiley Periodicals, Inc.

Overexpression of filamin c in chronic intermittent hypoxia-induced cardiomyocyte apoptosis is a potential cardioprotective target for obstructive sleep apnea

PURPOSE

Chronic intermittent hypoxia (CIH) is key pathological mechanism of obstructive sleep apnea (OSA), which induced cardiac dysfunction. Filamin c (FLNC) is a muscle-restricted isoform and predominantly expressed in muscle tissue. In this study, we utilized a recently developed CIH rat model to mimic OSA, investigated the expression of FLNC in cardiomyocytes, and examined the correlations of FLNC with active caspase-3 to ascertain whether FLNC regulates the survival of cardiomyocytes.

METHODS

Forty Sprague-Dawley rats were randomly divided into normoxia and CIH groups. All rats were exposed either to normoxia or CIH 8 h daily for 6 weeks. Echocardiogram and HE staining were used to examine cardiac pathology, structure, and function. Body weight, heart weight, and blood gas values were recorded, respectively. The FLNC, Bax, Bcl-2, BNIP 3, and active caspase-3 proteins were detected by western blot; FLNC was examined by immunohistochemistry and immunofluorescence. Association of FLNC with cardiomyocyte apoptosis was detected by immunofluorescence.

RESULTS

CIH induced cardiac injuries and caused arterial blood gas disorder. FLNC significantly increased in CIH-induced cardiomyocytes than that in normoxia tissues. Pro-apoptotic BNIP 3 and Bax proteins were significantly increased in CIH, whereas anti-apoptotic member Bcl-2 was decreased. Active caspase-3, a universal marker of apoptosis, was significantly increased in CIH group. Co-localizations of FLNC and active caspase-3 were observed in CIH group.

CONCLUSIONS

These results suggested FLNC is implicated in the pathogenesis of CIH-induced cardiomyocyte apoptosis, and FLNC may serve as a novel cardioprotective target for OSA patients.

Filamin A expression predicts early recurrence of hepatocellular carcinoma after hepatectomy

BACKGROUND & AIMS

Recurrence of hepatocellular carcinoma (HCC) after hepatectomy is very high. A predictive marker of early recurrence (ER) capable of personalizing follow-up and developing a new target therapy would be beneficial. The overexpression of Filamin-A (FLNA), a cytoskeleton protein with scaffolding properties, has recently been associated with progression in tumours. The aim of this study was to test the expression of FLNA in a cohort of patients operated for HCC.

METHODS

A retrospective cohort of patients who underwent hepatic resection at Humanitas Clinical and Research Center between January 2004 and December 2014 was analysed. FLNA was tested, using a tissue microarray, in the HCC and in the surrounding tissues. The endpoint was the role of FLNA expression in predicting ER of HCC after hepatectomy. Analyses were performed following the REMARK guidelines.

RESULTS

A total of 113 patients were considered. FLNA was expressed only in the tumoral tissue. Several variables, including T stage, tumour number, tumour size, type of viral hepatitis, type of hepatectomy and intra and peritumoral immune-reactivity to FLNA were significantly associated with ER by univariate analysis. With multivariate analysis, only T stage (HR=2.108; P=.002), tumour number (HR=1.586; P=.023), intra-tumoral (HR=2.672; P<.001) and peritumoral immune-reactivity to FLNA (HR=2.569; P<.001), significantly correlated with ER. The logistic regression analysis revealed that advanced T stage (OR=2.985; P=.001), HCV-infection (OR=1.219; P=.008) and advanced tumour grading (OR=2.781; P=.002) were associated with intratumoral FLNA immune-reactivity.

CONCLUSIONS

FLNA expression predicts recurrence of HCC after hepatectomy. This finding provides important insights that would help physicians to personalize follow-up strategies.

When signalling goes wrong: pathogenic variants in structural and signalling proteins causing cardiomyopathies

Cardiomyopathies are a diverse group of cardiac disorders with distinct phenotypes, depending on the proteins and pathways affected. A substantial proportion of cardiomyopathies are inherited and those will be the focus of this review article. With the wide application of high-throughput sequencing in the practice of clinical genetics, the roles of novel genes in cardiomyopathies are recognised. Here, we focus on a subgroup of cardiomyopathy genes [TTN, FHL1, CSRP3, FLNC and PLN, coding for Titin, Four and a Half LIM domain 1, Muscle LIM Protein, Filamin C and Phospholamban, respectively], which, despite their diverse biological functions, all have important signalling functions in the heart, suggesting that disturbances in signalling networks can contribute to cardiomyopathies.

Overview of the Muscle Cytoskeleton

Cardiac and skeletal striated muscles are intricately designed machines responsible for muscle contraction. Coordination of the basic contractile unit, the sarcomere, and the complex cytoskeletal networks are critical for contractile activity. The sarcomere is comprised of precisely organized individual filament systems that include thin (actin), thick (myosin), titin, and nebulin. Connecting the sarcomere to other organelles (e.g., mitochondria and nucleus) and serving as the scaffold to maintain cellular integrity are the intermediate filaments. The costamere, on the other hand, tethers the sarcomere to the cell membrane. Unique structures like the intercalated disc in cardiac muscle and the myotendinous junction in skeletal muscle help synchronize and transmit force. Intense investigation has been done on many of the proteins that make up these cytoskeletal assemblies. Yet the details of their function and how they interconnect have just started to be elucidated. A vast number of human myopathies are contributed to mutations in muscle proteins; thus understanding their basic function provides a mechanistic understanding of muscle disorders. In this review, we highlight the components of striated muscle with respect to their interactions, signaling pathways, functions, and connections to disease. © 2017 American Physiological Society. Compr Physiol 7:891-944, 2017.

Ehlers-Danlos syndrome with lethal cardiac valvular dystrophy in males carrying a novel splice mutation in FLNA

Filamin A is an X-linked, ubiquitous actin-binding protein whose mutations are associated to multiple disorders with limited genotype-phenotype correlations. While gain-of-function mutations cause various bone dysplasias, loss-of-function variants are the most common cause of periventricular nodular heterotopias with variable soft connective tissue involvement, as well as X-linked cardiac valvular dystrophy (XCVD). The term "Ehlers-Danlos syndrome (EDS) with periventricular heterotopias" has been used in females with neurological, cardiovascular, integument and joint manifestations, but this nosology is still a matter of debate. We report the clinical and molecular update of an Italian family with an X-linked recessive soft connective tissue disorder and which was described, in 1975, as the first example of EDS type V of the Berlin nosology. The cutaneous phenotype of the index patient was close to classical EDS and all males died for a lethal cardiac valvular dystrophy. Whole exome sequencing identified the novel c.1829-1G>C splice variation in FLNA in two affected cousins. The nucleotide change was predicted to abolish the canonical splice acceptor site of exon 13 and to activate a cryptic acceptor site 15 bp downstream, leading to in frame deletion of five amino acid residues (p.Phe611_Gly615del). The predicted in frame deletion clusters with all the mutations previously identified in XCVD and falls within the N-terminus rod 1 domain of filamin A. Our findings expand the male-specific phenotype of FLNA mutations that now includes classical-like EDS with lethal cardiac valvular dystrophy, and offer further insights for the genotype-phenotype correlations within this spectrum. © 2016 Wiley Periodicals, Inc.

Filamin C is a highly dynamic protein associated with fast repair of myofibrillar microdamage

Filamin c (FLNc) is a large dimeric actin-binding protein located at premyofibrils, myofibrillar Z-discs and myofibrillar attachment sites of striated muscle cells, where it is involved in mechanical stabilization, mechanosensation and intracellular signaling. Mutations in the gene encoding FLNc give rise to skeletal muscle diseases and cardiomyopathies. Here, we demonstrate by fluorescence recovery after photobleaching that a large fraction of FLNc is highly mobile in cultured neonatal mouse cardiomyocytes and in cardiac and skeletal muscles of live transgenic zebrafish embryos. Analysis of cardiomyocytes from Xirp1 and Xirp2 deficient animals indicates that both Xin actin-binding repeat-containing proteins stabilize FLNc selectively in premyofibrils. Using a novel assay to analyze myofibrillar microdamage and subsequent repair in cultured contracting cardiomyocytes by live cell imaging, we demonstrate that repair of damaged myofibrils is achieved within only 4 h, even in the absence of de novo protein synthesis. FLNc is immediately recruited to these sarcomeric lesions together with its binding partner aciculin and precedes detectable assembly of filamentous actin and recruitment of other myofibrillar proteins. These data disclose an unprecedented degree of flexibility of the almost crystalline contractile machinery and imply FLNc as a dynamic signaling hub, rather than a primarily structural protein. Our myofibrillar damage/repair model illustrates how (cardio)myocytes are kept functional in their mechanically and metabolically strained environment. Our results help to better understand the pathomechanisms and pathophysiology of early stages of FLNc-related myofibrillar myopathy and skeletal and cardiac diseases preceding pathological protein aggregation.

Smooth muscle filamin A is a major determinant of conduit artery structure and function at the adult stage

Human mutations in the X-linked FLNA gene are associated with a remarkably diverse phenotype, including severe arterial morphological anomalies. However, the role for filamin A (FlnA) in vascular cells remains partially understood. We used a smooth muscle (sm)-specific conditional mouse model to delete FlnA at the adult stage, thus avoiding the developmental effects of the knock-out. Inactivation of smFlnA in adult mice significantly lowered blood pressure, together with a decrease in pulse pressure. However, both the aorta and carotid arteries showed a major outward hypertrophic remodeling, resistant to losartan, and normally occurring in hypertensive conditions. Notably, arterial compliance was significantly enhanced in the absence of smFlnA. Moreover, reactivity of thoracic aorta rings to a variety of vasoconstrictors was elevated, while basal contractility in response to KCl depolarization was reduced. Enhanced reactivity to the thromboxane A2 receptor agonist U46619 was fully reversed by the ROCK inhibitor Y27632. We discuss the possibility that a reduction in arterial stiffness upon smFlnA inactivation might cause a compensatory increase in conduit artery diameter for normalization of parietal tension, independently of the ROCK pathway. In conclusion, deletion of smFlnA in adult mice recapitulates the vascular phenotype of human bilateral periventricular nodular heterotopia, culminating in aortic dilatation.

Arterial Myogenic Activation through Smooth Muscle Filamin A

Mutations in the filamin A (FlnA) gene are frequently associated with severe arterial abnormalities, although the physiological role for this cytoskeletal element remains poorly understood in vascular cells. We used a conditional mouse model to selectively delete FlnA in smooth muscle (sm) cells at the adult stage, thus avoiding the developmental effects of the knockout. Basal blood pressure was significantly reduced in conscious smFlnA knockout mice. Remarkably, pressure-dependent tone of the resistance caudal artery was lost, whereas reactivity to vasoconstrictors was preserved. Impairment of the myogenic behavior was correlated with a lack of calcium influx in arterial myocytes upon an increase in intraluminal pressure. Notably, the stretch activation of CaV1.2 was blunted in the absence of smFlnA. In conclusion, FlnA is a critical upstream element of the signaling cascade underlying the myogenic tone. These findings allow a better understanding of the molecular basis of arterial autoregulation and associated disease states.

An adventitious interaction of filamin A with RhoGDI2(Tyr153Glu)

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

Piezo1 in Smooth Muscle Cells Is Involved in Hypertension-Dependent Arterial Remodeling

The mechanically activated non-selective cation channel Piezo1 is a determinant of vascular architecture during early development. Piezo1-deficient embryos die at midgestation with disorganized blood vessels. However, the role of stretch-activated ion channels (SACs) in arterial smooth muscle cells in the adult remains unknown. Here, we show that Piezo1 is highly expressed in myocytes of small-diameter arteries and that smooth-muscle-specific Piezo1 deletion fully impairs SAC activity. While Piezo1 is dispensable for the arterial myogenic tone, it is involved in the structural remodeling of small arteries. Increased Piezo1 opening has a trophic effect on resistance arteries, influencing both diameter and wall thickness in hypertension. Piezo1 mediates a rise in cytosolic calcium and stimulates activity of transglutaminases, cross-linking enzymes required for the remodeling of small arteries. In conclusion, we have established the connection between an early mechanosensitive process, involving Piezo1 in smooth muscle cells, and a clinically relevant arterial remodeling.

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

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

Protein assemblies of sodium and inward rectifier potassium channels control cardiac excitability and arrhythmogenesis

The understanding of how cardiac ion channels function in the normal and the diseased heart has greatly increased over the last four decades thanks to the advent of patch-clamp technology and, more recently, the emergence of genetics, as well as cellular and molecular cardiology. However, our knowledge of how these membrane-embedded proteins physically interact with each other within macromolecular complexes remains incomplete. This review focuses on how the main cardiac inward sodium channel (NaV1.5) and the strong inward rectifier potassium channel (Kir2.1) function within macromolecular complexes to control cardiac excitability. It has become increasingly clear that these two important ion channel proteins physically interact with multiple other protein partners and with each other from early stages of protein trafficking and targeting through membrane anchoring, recycling, and degradation. Recent findings include compartmentalized regulation of NaV1.5 channel expression and function through a PDZ (postsynaptic density protein, Drosophila disc large tumor suppressor, and zonula occludens-1 protein) domain-binding motif, and interaction of caveolin-3 with Kir2.1 and ankyrin-G as a molecular platform for NaV1.5 signaling. At the cardiomyocyte membrane, NaV1.5 and Kir2.1 interact through at least two distinct PDZ domain-scaffolding proteins (synapse-associated protein-97 and α1-syntrophin), thus modulating reciprocally their cell-surface expression at two different microdomains. Emerging evidence also shows that inheritable mutations in plakophilin-2, ankyrin-G, dystrophin, syntrophin, synapse-associated protein-97, and caveolin-3, among others, modify functional expression and/or localization in the cardiac cell of NaV1.5, Kir2.1 or both to give rise to arrhythmogenic diseases. Unveiling the mechanistic underpinnings of macromolecular interactions should increase our understanding of inherited and acquired arrhythmogenic cardiac diseases and may lead to advances in therapy.

Systems Chemo-Biology and Transcriptomic Meta-Analysis Reveal the Molecular Roles of Bioactive Lipids in Cardiomyocyte Differentiation

Lipids, which are essential constituents of biological membranes, play structural and functional roles in the cell. In recent years, certain lipids have been identified as regulatory signaling molecules and have been termed "bioactive lipids". Subsequently, the importance of bioactive lipids in stem cell differentiation and cardiogenesis has gained increasing recognition. Therefore, the aim of this study was to identify the biological processes underlying murine cardiac differentiation and the mechanisms by which bioactive lipids affect these processes. For this purpose, a transcriptomic meta-analysis of microarray and RNA-seq data from murine stem cells undergoing cardiogenic differentiation was performed. The differentially expressed genes identified via this meta-analysis, as well as bioactive lipids, were evaluated using systems chemo-biology tools. These data indicated that bioactive lipids are associated with the regulation of cell motility, cell adhesion, cytoskeletal rearrangement, and gene expression. Moreover, bioactive lipids integrate the signaling pathways involved in cell migration, the secretion and remodeling of extracellular matrix components, and the establishment of the cardiac phenotype. In conclusion, this study provides new insights into the contribution of bioactive lipids to the induction of cellular responses to various stimuli, which may originate from the extracellular environment and morphogens, and the manner in which this contribution directly affects murine heart morphogenesis.

Targeting filamin B induces tumor growth and metastasis via enhanced activity of matrix metalloproteinase-9 and secretion of VEGF-A

Filamins regulate cell locomotion and associate with diverse signaling molecules. We have recently found that targeting filamin A (FLNA) reduces RAS-induced lung adenocarcinomas. In this study, we explored the role of another major filamin isoform, filamin B (FLNB), in tumor development. In contrast to FLNA, we report that targeting FLNB enhances RAS-induced tumor growth and metastasis which is associated with higher matrix metallopeptidase-9 (MMP-9) and extracellular signal-regulated kinase (ERK) activity. Flnb deficiency in mouse embryonic fibroblasts results in increased proteolytic activity of MMP-9 and cell invasion mediated by the RAS/ERK pathway. Similarly, silencing FLNB in multiple human cancer cells increases the proteolytic activity of MMP-9 and tumor cell invasion. Furthermore, we observed that Flnb-deficient RAS-induced tumors display more capillary structures that is correlated with increased vascular endothelial growth factor-A (VEGF-A) secretion. Inhibition of ERK activation blocks phorbol myristate acetate-induced MMP-9 activity and VEGF-A secretion in vitro. In addition, silencing FLNB in human ovarian cancer cells increases secretion of VEGF-A that induces endothelial cells to form more vascular structures in vitro. We conclude that FLNB suppresses tumor growth and metastasis by regulating the activity of MMP-9 and secretion of VEGF-A which is mediated by the RAS/ERK pathway.

Filamin A regulates MMP-9 expression and suppresses prostate cancer cell migration and invasion

This study aims to analyze the expression and clinical significance of Filamin A (FLNA) in prostate carcinoma and the biological effect in its cell line by FLNA overexpression. Immunohistochemistry and Western blot were used to analyze FLNA protein expression in 68 cases of prostate cancer and 37 cases of normal tissues to study the influence of the upregulated expression of FLNA that might be found on PC-3 cell biological effect. In the immunohistochemical analysis, the level of FLNA protein expression was found to be significantly lower in prostate cancer tissue than in normal tissues (P < 0.05). In the Western blot analysis, the relative amount of FLNA protein in prostate cancer tissue was found to be significantly lower than in normal tissues (P < 0.05). The level of FLNA protein expression was not correlated with age and PSA concentration (P > 0.05), but it was correlated with T stages, lymph node metastasis, clinic stage, and Gleason score (P < 0.05). The result of biological function showed that PC-3 cell transfected FLNA had a lower survival fraction, a significant decrease in migration and invasion, and a lower matrix metallopeptidase 9 (MMP-9) protein expression compared with PC-3 cell untransfected FLNA (P < 0.05). FLNA expression decreased in prostate cancer and correlated significantly with T stages, lymph node metastasis, clinic stage, and Gleason score, suggesting that FLNA may play important roles as a negative regulator to prostate cancer PC-3 cell by promoting the degradation of MMP-9.

The dual role of filamin A in cancer: can't live with (too much of) it, can't live without it

Filamin A (FlnA) has been associated with actin as cytoskeleton regulator. Recently its role in the cell has come under scrutiny for FlnA's involvement in cancer development. FlnA was originally revealed as a cancer-promoting protein, involved in invasion and metastasis. However, recent studies have also found that under certain conditions, it prevented tumor formation or progression, confusing the precise function of FlnA in cancer development. Here, we try to decipher the role of FlnA in cancer and the implications for its dual role. We propose that differences in subcellular localization of FlnA dictate its role in cancer development. In the cytoplasm, FlnA functions in various growth signaling pathways, such as vascular endothelial growth factor, in addition to being involved in cell migration and adhesion pathways, such as R-Ras and integrin signaling. Involvement in these pathways and various others has shown a correlation between high cytoplasmic FlnA levels and invasive cancers. However, an active cleaved form of FlnA can localize to the nucleus rather than the cytoplasm and its interaction with transcription factors has been linked to a decrease in invasiveness of cancers. Therefore, overexpression of FlnA has a tumor-promoting effect, only when it is localized to the cytoplasm, whereas if FlnA undergoes proteolysis and the resulting C-terminal fragment localizes to the nucleus, it acts to suppress tumor growth and inhibit metastasis. Development of drugs to target FlnA and cause cleavage and subsequent localization to the nucleus could be a new and potent field of research in treating cancer.

Absent expression of FLNA is correlated with poor prognosis of nasopharyngeal cancer

This study aimed to analyze the expression, clinical significance of filamin A (FLNA) in nasopharyngeal carcinoma, and the biological effect in its cell line by FLNA overexpression. Immunohistochemistry and western blot were used to analyze FLNA protein expression in 63 cases of nasopharyngeal cancer and 21 cases of normal tissues to study the relationship between FLNA expression and clinical factors. FLNA lentiviral vector and empty vector were respectively transfected into nasopharyngeal cancer CNE2 cell line. Reverse transcription-polymerase chain reaction (RT-PCR) and western blot were used to detect the mRNA level and protein of FLNA. 3-[4,5-Dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide, migration, and invasion assays were also conducted as to the influence of the upregulated expression of FLNA that might be found on CNE2 cell biological effect. Immunohistochemistry: the level of FLNA protein expression was found to be significantly lower in nasopharyngeal cancer tissue than normal tissues (P < 0.05). Western blot: the relative amount of FLNA protein in nasopharyngeal cancer tissue was found to be significantly lower than in normal tissues (P < 0.05). The level of FLNA protein expression was correlated with T stages, lymph node metastasis, clinic stage, and histological grade (P < 0.05). Loss of FLNA expression correlated significantly with poor overall survival time by Kaplan-Meier analysis (P < 0.05). The result of biological function has shown that CNE2 cell-transfected FLNA had a lower survival fraction, significant decrease in migration and invasion, and lower matrix metallopeptidase 9 (MMP-9) protein expression compared with CNE2 cell-untransfected FLNA (P < 0.05). FLNA expression decreased in nasopharyngeal cancer and correlated significantly lymph node metastasis, clinic stage, histological grade, and poor overall survival, suggesting that FLNA may play important roles as a negative regulator to nasopharyngeal cancer CNE2 cell by promoting degradation of MMP-9.

Valvular dystrophy associated filamin A mutations reveal a new role of its first repeats in small-GTPase regulation

Filamin A (FlnA) is a ubiquitous actin binding protein which anchors various transmembrane proteins to the cell cytoskeleton and provides a scaffold to many cytoplasmic signaling proteins involved in actin cytoskeleton remodeling in response to mechanical stress and cytokines stimulation. Although the vast majority of FlnA binding partners interact with the carboxy-terminal immunoglobulin like (Igl) repeats of FlnA, little is known on the role of the amino-N-terminal repeats. Here, using cardiac mitral valvular dystrophy associated FlnA-G288R and P637Q mutations located in the N-terminal Igl repeat 1 and 4 respectively as a model, we identified a new role of FlnA N-terminal repeats in small Rho-GTPases regulation. Using FlnA-deficient melanoma and HT1080 cell lines as expression systems we showed that FlnA mutations reduce cell spreading and migration capacities. Furthermore, we defined a signaling network in which FlnA mutations alter the balance between RhoA and Rac1 GTPases activities in favor of RhoA and provided evidences for a role of the Rac1 specific GTPase activating protein FilGAP in this process. Together our work ascribed a new role to the N-terminal repeats of FlnA in Small GTPases regulation and supports a conceptual framework for the role of FlnA mutations in cardiac valve diseases centered around signaling molecules regulating cellular actin cytoskeleton in response to mechanical stress.

ASB2α, an E3 ubiquitin ligase specificity subunit, regulates cell spreading and triggers proteasomal degradation of filamins by targeting the filamin calponin homology 1 domain

Filamins are actin-binding and cross-linking proteins that organize the actin cytoskeleton and anchor transmembrane proteins to the cytoskeleton and scaffold signaling pathways. During hematopoietic cell differentiation, transient expression of ASB2α, the specificity subunit of an E3-ubiquitin ligase complex, triggers acute proteasomal degradation of filamins. This led to the proposal that ASB2α regulates hematopoietic cell differentiation by modulating cell adhesion, spreading, and actin remodeling through targeted degradation of filamins. Here, we show that the calponin homology domain 1 (CH1), within the filamin A (FLNa) actin-binding domain, is the minimal fragment sufficient for ASB2α-mediated degradation. Combining an in-depth flow cytometry analysis with mutagenesis of lysine residues within CH1, we find that arginine substitution at each of a cluster of three lysines (Lys-42, Lys-43, and Lys-135) renders FLNa resistant to ASB2α-mediated degradation without altering ASB2α binding. These lysines lie within previously predicted actin-binding sites, and the ASB2α-resistant filamin mutant is defective in targeting to F-actin-rich structures in cells. However, by swapping CH1 with that of α-actinin1, which is resistant to ASB2α-mediated degradation, we generated an ASB2α-resistant chimeric FLNa with normal subcellular localization. Notably, this chimera fully rescues the impaired cell spreading induced by ASB2α expression. Our data therefore reveal ubiquitin acceptor sites in FLNa and establish that ASB2α-mediated effects on cell spreading are due to loss of filamins.

Differential expression of filamin A and its clinical significance in breast cancer

Changes in filamin A (FLNa) expression contribute to the development and progression of numerous malignancies. However, in vitro studies of breast cancer have shown conflicting results. Thus, the present study aimed to detect the expression of FLNa in breast cancer tissue samples and the association with clinicopathological data, in order to provide insightful ex vivo data. A total of 96 breast cancer and distant normal breast tissues and 20 benign tumor tissue specimens were subjected to immunohistochemistry or reverse transcription polymerase chain reaction (RT-PCR) analysis of FLNa expression. Clinicopathological data were collected to analyze the association with FLNa expression. The FLNa protein was overexpressed in breast cancer tissues compared with distant normal mammary gland and benign breast tissues. The FLNa protein was expressed in 63.5% of breast cancer, with positive rates of 36, 66.7 and 84.6%, respectively, in stage I, II and III breast cancer patients (P<0.05). Overexpression of the FLNa protein was associated with advanced stage, lymph node metastasis, vascular or neural invasion, menstruation state and other risk stratifications for breast cancer. The overexpression of FLNa in breast cancer was validated by RT-PCR, indicating transcriptional regulation of FLNa overexpression in breast cancer. FLNa mRNA and protein were overexpressed in breast cancer tissues, which was associated with advanced stage, lymph node metastasis and vascular or neural invasion of breast cancer, suggesting that FLNa contributes to breast cancer development and progression.

Autoimmunoreactive IgGs from patients with postural orthostatic tachycardia syndrome

PURPOSE

Autoantibodies are implicated in the pathogenesis of cardiovascular diseases and cardiac arrhythmias. In this pilot study, we tested the hypothesis that autoantibodies are present in patients with postural orthostatic tachycardia syndrome (POTS).

EXPERIMENTAL DESIGN

Seven control subjects (6 F:1 M, average age 36.1 years) and ten patients with the diagnosis of POTS (7 F: 3 M, average age 35.1 years) provided informed consent and 30 mL of venous blood. Human heart membrane proteins were resolved by 2DE and immunoblotted against purified IgGs from controls and patients.

RESULTS

Eighteen protein spots immunoreactive specifically against patient IgGs were detected and they were excised from gels, trypsin-digested, and analyzed by nanoLC-electrospray MS/MS. Forty unique proteins were identified and these include proteins that are associated with cardiac hypertrophy (mimecan, myozenin), cardiac remodeling (periostin), cardiomyopathy (desmin, desmoplakin), cell survival (laminin), structural integrity (filamin), chaperone proteins (crystalline, HSP70), mitochondrial enzymes, and channel proteins. Ingenuity Pathway Analysis showed multiple pathways were involved including those that regulate energy metabolism, redox, fibrosis, cardiac hypertrophy, and degeneration.

CONCLUSIONS AND CLINICAL RELEVANCE

Autoantibodies are present in patients with POTS. These autoantibodies cross-react with a wide range of cardiac proteins and may induce alterations in cardiac function. Autoimmune pathogenetic mechanisms should be further explored in these patients.

Filamin A controls matrix metalloproteinase activity and regulates cell invasion in human fibrosarcoma cells

Filamins are an important family of actin-binding proteins that, in addition to bundling actin filaments, link cell surface adhesion proteins, signaling receptors and channels to the actin cytoskeleton, and serve as scaffolds for an array of intracellular signaling proteins. Filamins are known to regulate the actin cytoskeleton, act as mechanosensors that modulate tissue responses to matrix density, control cell motility and inhibit activation of integrin adhesion receptors. In this study, we extend the repertoire of filamin activities to include control of extracellular matrix (ECM) degradation. We show that knockdown of filamin increases matrix metalloproteinase (MMP) activity and induces MMP2 activation, enhancing the ability of cells to remodel the ECM and increasing their invasive potential, without significantly altering two-dimensional random cell migration. We further show that within filamin A, the actin-binding domain is necessary, but not sufficient, to suppress the ECM degradation seen in filamin-A-knockdown cells and that dimerization and integrin binding are not required. Filamin mutations are associated with neuronal migration disorders and a range of congenital malformations characterized by skeletal dysplasia and various combinations of cardiac, craniofacial and intestinal anomalies. Furthermore, in breast cancers loss of filamin A has been correlated with increased metastatic potential. Our data suggest that effects on ECM remodeling and cell invasion should be considered when attempting to provide cellular explanations for the physiological and pathological effects of altered filamin expression or filamin mutations.

Filamins in mechanosensing and signaling

Filamins are essential, evolutionarily conserved, modular, multidomain, actin-binding proteins that organize the actin cytoskeleton and maintain extracellular matrix connections by anchoring actin filaments to transmembrane receptors. By cross-linking and anchoring actin filaments, filamins stabilize the plasma membrane, provide cellular cortical rigidity, and contribute to the mechanical stability of the plasma membrane and the cell cortex. In addition to binding actin, filamins interact with more than 90 other binding partners including intracellular signaling molecules, receptors, ion channels, transcription factors, and cytoskeletal and adhesion proteins. Thus, filamins scaffold a wide range of signaling pathways and are implicated in the regulation of a diverse array of cellular functions including motility, maintenance of cell shape, and differentiation. Here, we review emerging structural and functional evidence that filamins are mechanosensors and/or mechanotransducers playing essential roles in helping cells detect and respond to physical forces in their local environment.

R-Ras interacts with filamin a to maintain endothelial barrier function

The molecular mechanisms regulating vascular barrier integrity remain incompletely elucidated. We have previously reported an association between the GTPase R-Ras and repeat 3 of Filamin A (FLNa). Loss of FLNa has been linked to increased vascular permeability. We sought to determine whether FLNa's association with R-Ras affects endothelial barrier function. We report that in endothelial cells endogenous R-Ras interacts with endogenous FLNa as determined by co-immunoprecipitations and pulldowns with the FLNa-GST fusion protein repeats 1-10. Deletion of FLNa repeat 3 (FLNaΔ3) abrogated this interaction. In these cells FLNa and R-Ras co-localize at the plasma membrane. Knockdown of R-Ras and/or FLNa by siRNA promotes vascular permeability, as determined by TransEndothelial Electrical Resistance and FITC-dextran transwell assays. Re-expression of FLNa restored endothelial barrier function in cells lacking FLNa whereas re-expression of FLNaΔ3 did not. Immunostaining for VE-Cadherin in cells with knocked down R-Ras and FLNa demonstrated a disorganization of VE-Cadherin at adherens junctions. Loss of R-Ras and FLNa or blocking R-Ras function via GGTI-2133, a selective R-Ras inhibitor, induced vascular permeability and increased phosphorylation of VE-Cadherin (Y731) and Src (Y416). Expression of dominant negative R-Ras promoted vascular permeability that was blocked by the Src inhibitor PP2. These findings demonstrate that maintaining endothelial barrier function is dependent upon active R-Ras and association between R-Ras and FLNa and that loss of this interaction promotes VE-Cadherin phosphorylation and changes in downstream effectors that lead to endothelial leakiness.

The E3 ubiquitin ligase specificity subunit ASB2α targets filamins for proteasomal degradation by interacting with the filamin actin-binding domain

Filamins are an important family of actin-binding and crosslinking proteins that mediate remodeling of the actin cytoskeleton and maintain extracellular matrix connections by anchoring transmembrane proteins to actin filaments and linking them to intracellular signaling cascades. We recently found that filamins are targeted for proteasomal degradation by the E3 ubiquitin ligase specificity subunit ASBα and that acute degradation of filamins through this ubiquitin-proteasome pathway correlates with cell differentiation. Specifically, in myeloid leukemia cells retinoic-acid-induced expression of ASB2α triggers filamin degradation and recapitulates early events crucial for cell differentiation. ASB2α is thought to link substrates to the ubiquitin transferase machinery; however, the mechanism by which ASB2α interacts with filamin to induce degradation remained unknown. Here, we use cell-based and biochemical assays to show that the subcellular localization of ASB2α to actin-rich structures is dependent on filamin and that the actin-binding domain (ABD) of filamin mediates the interaction with ASB2α. Furthermore, we show that the ABD is necessary and sufficient for ASB2α-mediated filamin degradation. We propose that ASB2α exerts its effect by binding the ABD and mediating its polyubiquitylation, so targeting filamins for degradation. These studies provide the molecular basis for ASB2α-mediated filamin degradation and unravel an important mechanism by which filamin levels can be acutely regulated.

Podosomal proteins as causes of human syndromes: a role in craniofacial development?

Podosomes and invadopodia are actin-rich protrusions of the plasma membrane important for matrix degradation and cell migration. Most of the information in this field has been obtained in cancer cells, where the presence of invadopodia has been related to increased invasiveness and metastatic potential. The importance of the related podosome structure in other pathological or physiological processes that require cell invasion is relatively unexplored. Recent evidence indicates that essential components of podosomes are responsible for several human syndromes, some of which are characterized by serious developmental defects involving the craniofacial area, skeleton and heart, and very poor prognosis. Here we will review them and discuss the possible role of podosomes as a player in correct embryo development.

FLNA and PGK1 are two potential markers for progression in hepatocellular carcinoma

BACKGROUND/AIMS

Hepatocellular carcinoma (HCC) is one of the most deadly diseases; metastasis and recurrence are the most important factors that affect the therapy of HCC chronically. Until now, the prognosis for the metastasis of HCC had not improved. Recently, several proteins that are related to metastasis and invasion of HCC were identified, but the effective markers still remain to be elucidated.

METHODS

In this study, comparative proteomics was used to study the differentially expressed proteins in two HCC cell lines MHCC97L and HCCLM9, which have low and high metastatic potentials, respectively.

RESULTS

Our findings indicated that filamin A (FLNA) and phosphoglycerate kinase 1 (PGK1) were two significantly differentially expressed proteins, with high expression in HCCLM9 cells, and may influence the metastasis of HCC cells.

CONCLUSION

Taken together with the confirmation of expression on the mRNA level, we propose the use of FLNA and PGK1 as potential markers for the progression of HCC.

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.

Filamin a mediates HGF/c-MET signaling in tumor cell migration

Deregulated hepatocyte growth factor (HGF)/c-MET axis has been correlated with poor clinical outcome and drug resistance in many human cancers. Identification of novel regulatory mechanisms influencing HGF/c-MET signaling may therefore be necessary to develop more effective cancer therapies. In our study, we show that multiple human cancer tissues and cells express filamin A (FLNA), a large cytoskeletal actin-binding protein, and expression of c-MET is significantly reduced in human tumor cells deficient for FLNA. The FLNA-deficient tumor cells exhibited poor migrative and invasive ability in response to HGF. On the other hand, the anchorage-dependent and independent tumor cell proliferation was not altered by HGF. The FLNA-deficiency specifically attenuated the activation of the c-MET downstream signaling molecule AKT in response to HGF stimulation. Furthermore, FLNA enhanced c-MET promoter activity by its binding to SMAD2. The impact of FLNA deficiency on c-MET expression and HGF-mediated cell migration in human tumor cells was confirmed in primary mouse embryonic fibroblasts deficient for Flna. These data suggest that FLNA is one of the important regulators of c-MET signaling and HGF-induced tumor cell migration.