Identification of the binding partners for flightless I, A novel protein bridging the leucine-rich repeat and the gelsolin superfamilies

Cytoskeleton-associated gelsolin responds to the midgut distention process in saline meal-fed Aedes aegypti and affects arbovirus dissemination from the midgut

The mosquito, Aedes aegypti, is the principal vector for several arboviruses. The mosquito midgut is the initial tissue that gets infected with an arbovirus acquired along with a blood meal from a vertebrate host. Blood meal ingestion leads to midgut tissue distention thereby increasing the pore size of the surrounding basal lamina. This allows newly synthesized virions to exit the midgut by traversing the distended basal lamina to infect secondary tissues of the mosquito. We conducted a quantitative label-free proteomic time course analysis with saline meal-fed Ae. aegypti females to identify host factors involved in midgut tissue distention. Around 2000 proteins were detected during each of the seven sampling time points and 164 of those were uniquely expressed. Forty-five of 97 differentially expressed proteins were upregulated during the 96-h time course and most of those were involved in cytoskeleton modulation, metabolic activity, and vesicle/vacuole formation. The F-actin-modulating Ae. aegypti (Aa)-gelsolin was selected for further functional studies. Stable knockout of Aa-gelsolin resulted in a mosquito line, which showed distorted actin filaments in midgut-associated tissues likely due to diminished F-actin processing by gelsolin. Zika virus dissemination from the midgut of these mosquitoes was diminished and delayed. The loss of Aa-gelsolin function was associated with an increased induction of apoptosis in midgut tissue indicating an involvement of Aa-gelsolin in apoptotic signaling in mosquitoes. Here, we used proteomics to discover a novel host factor, Aa-gelsolin, which affects the midgut escape barrier for arboviruses in mosquitoes and apoptotic signaling in the midgut.

A Novel Cargo Delivery System-AnCar-ExoLaIMTS Ameliorates Arthritis via Specifically Targeting Pro-Inflammatory Macrophages

Macrophages are heterogenic phagocytic cells that play distinct roles in physiological and pathological processes. Targeting different types of macrophages has shown potent therapeutic effects in many diseases. Although many approaches are developed to target anti-inflammatory macrophages, there are few researches on targeting pro-inflammatory macrophages, which is partially attributed to their non-s pecificity phagocytosis of extracellular substances. In this study, a novel recombinant protein is constructed that can be anchored on an exosome membrane with the purpose of targeting pro-inflammatory macrophages via antigen recognition, which is named AnCar-ExoLaIMTS . The data indicate that the phagocytosis efficiencies of pro-inflammatory macrophages for different AnCar-ExoLaIMTS show obvious differences. The AnCar-ExoLaIMTS3 has the best targeting ability for pro-inflammatory macrophages in vitro and in vivo. Mechanically, AnCar-ExoLaIMTS3 can specifically recognize the leucine-rich repeat domain of the TLR4 receptor, and then enter into pro-inflammatory macrophages via the TLR4-mediated receptor endocytosis pathway. Moreover, AnCar-ExoLaIMTS3 can efficiently deliver therapeutic cargo to pro-inflammatory macrophages and inhibit the synovial inflammatory response via downregulation of HIF-1α level, thus ameliorating the severity of arthritis in vivo. Collectively, the work established a novel gene/drug delivery system that can specifically target pro-inflammatory macrophages, which may be beneficial for the treatments of arthritis and other inflammatory diseases.

FLII Modulates the Myogenic Differentiation of Progenitor Cells via Actin Remodeling-Mediated YAP1 Regulation

The dynamic rearrangement of the actin cytoskeleton plays an essential role in myogenesis, which is regulated by diverse mechanisms, such as mechanotransduction, modulation of the Hippo signaling pathway, control of cell proliferation, and the influence of morphological changes. Despite the recognized importance of actin-binding protein Flightless-1 (FLII) during actin remodeling, the role played by FLII in the differentiation of myogenic progenitor cells has not been explored. Here, we investigated the roles of FLII in the proliferation and differentiation of myoblasts. FLII was found to be enriched in C2C12 myoblasts, and its expression was stable during the early stages of differentiation but down-regulated in fully differentiated myotubes. Knockdown of FLII in C2C12 myoblasts resulted in filamentous actin (F-actin) accumulation and inhibited Yes-associated protein 1 (YAP1) phosphorylation, which triggers its nuclear translocation from the cytoplasm. Consequently, the expressions of YAP1 target genes, including PCNA, CCNB1, and CCND1, were induced, and the cell cycle and proliferation of myoblasts were promoted. Moreover, FLII knockdown significantly inhibited the expression of myogenic regulatory factors, i.e., MyoD and MyoG, thereby impairing myoblast differentiation, fusion, and myotube formation. Thus, our findings demonstrate that FLII is crucial for the differentiation of myoblasts via modulation of the F-actin/YAP1 axis and suggest that FLII is a putative novel therapeutic target for muscle wasting.

A human FLII gene variant alters sarcomeric actin thin filament length and predisposes to cardiomyopathy

To better understand the genetic basis of heart disease, we identified a variant in the Flightless-I homolog (FLII) gene that generates a R1243H missense change and predisposes to cardiac remodeling across multiple previous human genome-wide association studies (GWAS). Since this gene is of unknown function in the mammalian heart we generated gain- and loss-of-function genetically altered mice, as well as knock-in mice with the syntenic R1245H amino acid substitution, which showed that Flii protein binds the sarcomeric actin thin filament and influences its length. Deletion of Flii from the heart, or mice with the R1245H amino acid substitution, show cardiomyopathy due to shortening of the actin thin filaments. Mechanistically, Flii is a known actin binding protein that we show associates with tropomodulin-1 (TMOD1) to regulate sarcomere thin filament length. Indeed, overexpression of leiomodin-2 in the heart, which lengthens the actin-containing thin filaments, partially rescued disease due to heart-specific deletion of Flii. Collectively, the identified FLII human variant likely increases cardiomyopathy risk through an alteration in sarcomere structure and associated contractile dynamics, like other sarcomere gene-based familial cardiomyopathies.

Wound Healing from an Actin Cytoskeletal Perspective

Wound healing requires a complex cascade of highly controlled and conserved cellular and molecular processes. These involve numerous cell types and extracellular matrix molecules regulated by the actin cytoskeleton. This microscopic network of filaments is present within the cytoplasm of all cells and provides the shape and mechanical support required for cell movement and proliferation. Here, an overview of the processes of wound healing are described from the perspective of the cell in relation to the actin cytoskeleton. Key points of discussion include the role of actin, its binding proteins, signaling pathways, and events that play significant roles in the phases of wound healing. The identification of cytoskeletal targets that can be used to manipulate and improve wound healing is included as an emerging area of focus that may inform future therapeutic approaches to improve healing of complex wounds.

Multifunctional Roles of the Actin-Binding Protein Flightless I in Inflammation, Cancer and Wound Healing

Flightless I is an actin-binding member of the gelsolin family of actin-remodeling proteins that inhibits actin polymerization but does not possess actin severing ability. Flightless I functions as a regulator of many cellular processes including proliferation, differentiation, apoptosis, and migration all of which are important for many physiological processes including wound repair, cancer progression and inflammation. More than simply facilitating cytoskeletal rearrangements, Flightless I has other important roles in the regulation of gene transcription within the nucleus where it interacts with nuclear hormone receptors to modulate cellular activities. In conjunction with key binding partners Leucine rich repeat in the Flightless I interaction proteins (LRRFIP)1/2, Flightless I acts both synergistically and competitively to regulate a wide range of cellular signaling including interacting with two of the most important inflammatory pathways, the NLRP3 inflammasome and the MyD88-TLR4 pathways. In this review we outline the current knowledge about this important cytoskeletal protein and describe its many functions across a range of health conditions and pathologies. We provide perspectives for future development of Flightless I as a potential target for clinical translation and insights into potential therapeutic approaches to manipulate Flightless I functions.

The Activities of the Gelsolin Homology Domains of Flightless-I in Actin Dynamics

Flightless-I is a unique member of the gelsolin superfamily alloying six gelsolin homology domains and leucine-rich repeats. Flightless-I is an established regulator of the actin cytoskeleton, however, its biochemical activities in actin dynamics are still largely elusive. To better understand the biological functioning of Flightless-I we studied the actin activities of Drosophila Flightless-I by in vitro bulk fluorescence spectroscopy and single filament fluorescence microscopy, as well as in vivo genetic approaches. Flightless-I was found to interact with actin and affects actin dynamics in a calcium-independent fashion in vitro. Our work identifies the first three gelsolin homology domains (1–3) of Flightless-I as the main actin-binding site; neither the other three gelsolin homology domains (4–6) nor the leucine-rich repeats bind actin. Flightless-I inhibits polymerization by high-affinity (∼nM) filament barbed end capping, moderately facilitates nucleation by low-affinity (∼μM) monomer binding, and does not sever actin filaments. Our work reveals that in the presence of profilin Flightless-I is only able to cap actin filament barbed ends but fails to promote actin assembly. In line with the in vitro data, while gelsolin homology domains 4–6 have no effect on in vivo actin polymerization, overexpression of gelsolin homology domains 1–3 prevents the formation of various types of actin cables in the developing Drosophila egg chambers. We also show that the gelsolin homology domains 4–6 of Flightless-I interact with the C-terminus of Drosophila Disheveled-associated activator of morphogenesis formin and negatively regulates its actin assembly activity.

Flightless anchors IQGAP1 and R-ras to mediate cell extension formation and matrix remodeling

Tractional remodeling of collagen fibrils by fibroblasts requires long cell extensions that mediate fibril alignment. The formation of these cell extensions involves flightless I (FliI), an actin-binding protein that contains a leucine-rich-repeat (LRR), which binds R-ras and may regulate cdc42. We considered that FliI interacts with small GTPases and their regulators to mediate assembly of cell extensions. Mass spectrometry analyses of FliI immunoprecipitates showed abundant Ras GTPase-activating-like protein (IQGAP1), which in immunostained samples colocalized with FliI at cell adhesions. Knockdown of IQGAP1 reduced the numbers of cell extensions and the alignment of collagen fibrils. In experiments using dominant negative mutants, cdc42 activity was required for the formation of short extensions while R-ras was required for the formation of long extensions. Immunoprecipitation of wild-type and mutant constructs showed that IQGAP1 associated with cdc42 and R-ras; this association required the GAP-related domain (1004–1237 aa) of IQGAP1. In cells transfected with FliI mutants, the LRR of FliI, but not its gelsolin-like domains, mediated association with cdc42, R-ras, and IQGAP1. We conclude that FliI interacts with IQGAP1 and co-ordinates with cdc42 and R-ras to control the formation of cell extensions that enable collagen tractional remodeling.

Regulation of signaling mediated by nucleic acid sensors for innate interferon-mediated responses during viral infection

Type I and type III interferons are important anti-viral cytokines that are massively induced during viral infection. This dynamic process is regulated by many executors and regulators for efficient eradication of invading viruses and protection from harmful, excessive responses. An array of innate sensors recognizes virus-derived nucleic acids to activate their downstream signaling to evoke cytokine responses including interferons. In particular, a cytoplasmic RNA sensor RIG-I (retinoic acid-inducible gene I) is involved in the detection of multiple types of not only RNA viruses but also DNA viruses. Accumulating findings have revealed that activation of nucleic acid sensors and the related signaling mediators is regulated on the basis of post-translational modification such as ubiquitination, phosphorylation and ADP-ribosylation. In addition, long non-coding RNAs (lncRNAs) have been implicated as a new class of regulators in innate signaling. A comprehensive understanding of the regulatory mechanisms of innate sensor activation and its signaling in host-virus interaction will provide a better therapeutic strategy to efficiently control viral infection and maintain immune homeostasis.

LRRFIP1 expression triggers platelet agglutination by enhancing αIIbβ3 expression

Platelets primarily participate in hemostasis and antimicrobial host defense. The present study aimed to investigate the effects of leucine-rich repeat flightless-interacting protein-1 (LRRFIP1) on platelet agglutination. The bacterial strain of LRRFIP1 was used to synthesize the recombinant protein and a mouse model of LRRFIP1 gene knockout was established. Platelets were isolated from the mice and divided into the different trial groups according to their treatment with collagen, thrombin receptor SFLLRN, anti-wild-type (w)LRRFIP1monoclonal antibodies and the model of LRRFIP1 gene knockout. The platelets were prepared and platelet agglutination was examined using platelet aggregation apparatus. The active αIIbβ3 integrin was examined by flow cytometry. The results revealed that the combined wLRRFIP1 protein was successfully expressed. wLRRFIP1 treatment significantly triggered platelet agglutination of collagen, thrombin and monoclonal antibody treated platelets. wLRRFIP1 knockout significantly decreased αIIbβ3 levels compared with the wild-type. Platelet agglutination was also significantly inhibited in the LRRFIP1^−/−mouse model compared with the wild-type. LRRFIP1 knockout significantly decreased the αIIbβ3 levels in platelets undergoing convulxin treatment. In conclusion, LRRFIP1 treatment triggered platelet agglutination and LRRFIP1 gene knockout inhibited platelet agglutination. In addition, LRRFIP1 gene knockout significantly decreased the levels of αIIbβ3. This suggests that LRRFIP1 my be applied to patients in a clinical setting to trigger platelet agglutination in inflammatory diseases and atherothrombotic diseases.

Multidisciplinary Roles of LRRFIP1/GCF2 in Human Biological Systems and Diseases

Leucine Rich Repeat of Flightless-1 Interacting Protein 1/GC-binding factor 2 (LRRFIP1/GCF2) cDNA was cloned for a transcriptional repressor GCF2, which bound sequence-specifically to a GC-rich element of epidermal growth factor receptor (EGFR) gene and repressed its promotor. LRRFIP1/GCF2 was also cloned as a double stranded RNA (dsRNA)-binding protein to trans-activation responsive region (TAR) RNA of Human Immunodeficiency Virus-1 (HIV-1), termed as TAR RNA interacting protein (TRIP), and as a binding protein to the Leucine Rich Repeat (LRR) of Flightless-1(Fli-1), termed as Flightless-1 LRR associated protein 1 (FLAP1) and LRR domain of Flightless-1 interacting Protein 1 (LRRFIP1). Subsequent functional studies have revealed that LRRFIP1/GCF2 played multiple roles in the regulation of diverse biological systems and processes, such as in immune response to microorganisms and auto-immunity, remodeling of cytoskeletal system, signal transduction pathways, and transcriptional regulations of genes. Dysregulations of LRRFIP1/GCF2 have been implicated in the causes of several experimental and clinico-pathological states and the responses to them, such as autoimmune diseases, excitotoxicity after stroke, thrombosis formation, inflammation and obesity, the wound healing process, and in cancers. LRRFIP1/GCF2 is a bioregulator in multidisciplinary systems of the human body and its dysregulation can cause diverse human diseases.

Flightless-I mediates the repression of estrogen receptor α target gene expression by the glucocorticoid receptor in MCF-7 cells

The human homologue of flightless-I (FLII) belong to the gelsolin protein family and contain a gelsolin-like domain at the C-terminus and a leucine-rich repeat (LRR) domain at the N-terminus. FLII regulates estrogen receptor alpha (ERα) and glucocorticoid receptor (GR)-mediated transcription by direct interaction through different domains, suggestive of its potential role in the crosstalk between the ERα and GR signaling pathway. Here, we demonstrate that FLII plays a critical role in GR-mediated repression of ERα target gene expression. In FLII-depleted cells, the reduction in 17-β-estradiol (E2)-induced ERα occupancy following treatment with dexamethasone (Dex) at the estrogen responsive element (ERE) site of the ERα target gene was significantly inhibited. The ERE binding of GR by the cotreatment with E2 and Dex was significantly inhibited by FLII depletion, indicating that FLII is required for the recruitment of GR at the ERE sites of ERα target genes. In addition, the recruitment of ERα-induced FLII to ERE sites was significantly reduced by Dex treatment. In protein binding assays, GR inhibited the E2-induced interaction between ERα and FLII, suggesting that GR interferes with the binding of ERα and FLII at the ERα target genes, resulting in the release of ERα and FLII from EREs. Taken together, our data reveal an unknown mechanism by which the transcription coactivator FLII regulates the GR-mediated repression of ERα target gene expression in MCF-7 cells.

Cyclic Stretching Exacerbates Tendinitis by Enhancing NLRP3 Inflammasome Activity via F-Actin Depolymerization

Modern molecular techniques have highlighted the presence of inflammation throughout the spectrum of tendinopathy. Previous studies have suggested that excessive inflammation in the tendon is a major factor leading to poor clinical treatment. Furthermore, the NLRP3 inflammasome, as a new term, is closely associated with the pathogenesis of many diseases. In the present study, we examined whether the NLRP3 inflammasome contributes to the development of tendinitis and whether cyclic stretching plays a prominent role in inflammation in the tendon. In the present study, we showed that hydrogen peroxide (H₂O₂) remarkably enhances the expression and release of IL-1β, TNF-α, and IL-6. The maturation of IL-1β, induced by H₂O₂, depends on the activation of the NLRP3 inflammasome. Cyclic stretching enhances the maturation of IL-1β via promoting H₂O₂-induced NLRP3 inflammasome activation in tenocytes. Furthermore, we also found that the depolymerization of filamentous actin (F-actin) was required for cyclic stretching-enhanced NLRP3 inflammasome activation. The present study suggests that NLRP3 inflammasome plays an important regulatory role in the pathogenesis of tendinitis. Disruption of the cytoskeleton by cyclic stretching exerts a proinflammatory effect via further activating the NLRP3/IL-1β pathway and hence contributes to tendinitis. These results may provide theoretical support for a new treatment strategy for preventing excessive inflammation in the tendon.

The leucine-rich region of Flightless I interacts with R-ras to regulate cell extension formation

Flightless I (FliI) is a calcium-dependent, actin severing and capping protein that localizes to cell matrix adhesions, contributes to the generation of cell extensions, and colocalizes with Ras. Currently, the mechanism by which FliI interacts with Ras to enable assembly of actin-based cell protrusions is not defined. R-Ras, but not K-ras, H-ras, or N-ras, associated with the leucine-rich region (LRR) of FliI. Mutations of the proline-rich region of R-ras (P202A, P203A) prevented this association. Knockdown of Ras GTPase-activating SH3 domain-binding protein (G3BP1) or Rasgap¹²⁰ by small interfering RNA inhibited the formation of cell extensions and prevented interaction of R-ras and G3BP1 in FliI wild-type (WT) cells. Pull-down assays using G3BP1 fusion proteins showed a strong association of R-ras with the C-terminus of G3BP1 (amino acids 236-466), which also required the LRR of FliI. In cells that expressed the truncated N-terminus or C-terminus of G3BP1, the formation of cell extensions was blocked. Endogenous Rasgap¹²⁰ interacted with the N-terminus of G3BP1 (amino acids 1-230). We conclude that in cells plated on collagen FliI-LRR interacts with R-ras to promote cell extension formation and that FliI is required for the interaction of Rasgap¹²⁰ with G3BP1 to regulate R-ras activity and growth of cell extensions.

TRPV4 mediates the Ca2+ influx required for the interaction between flightless-1 and non-muscle myosin, and collagen remodeling

Fibroblasts remodel extracellular matrix collagen, in part, through phagocytosis. This process requires formation of cell extensions, which in turn involves interaction of the actin-binding protein flightless-1 (FliI) with non-muscle myosin IIA (NMMIIA; heavy chain encoded by MYH9) at cell-matrix adhesion sites. As Ca²⁺ plays a central role in controlling actomyosin-dependent functions, we examined how Ca²⁺ controls the generation of cell extensions and collagen remodeling. Ratio fluorimetry demonstrated localized Ca²⁺ influx at the extensions of fibroblasts. Western blotting and quantitative (q)PCR showed high expression levels of the Ca²⁺-permeable transient receptor potential vanilloid-4 (TRPV4) channel, which co-immunoprecipitated with β1 integrin and localized to adhesions. Treatment with α2β1-integrin-blocking antibody or the TRPV4-specific antagonist AB159908, as well as reduction of TRPV4 expression through means of siRNA, blocked Ca²⁺ influx. These treatments also inhibited the interaction of FliI with NMMIIA, reduced the number and length of cell extensions, and blocked collagen remodeling. Pulldown assays showed that Ca²⁺ depletion inhibited the interaction of purified FliI with NMMIIA filaments. Fluorescence resonance energy transfer experiments showed that FliI-NMMIIA interactions require Ca²⁺ influx. We conclude that Ca²⁺ influx through the TRPV4 channel regulates FliI-NMMIIA interaction, which in turn enables generation of the cell extensions essential for collagen remodeling.

The flightless I protein interacts with RNA-binding proteins and is involved in the genome-wide mRNA post-transcriptional regulation in lung carcinoma cells

The flightless I protein (FLII) belongs to the gelsolin family. Its function has been associated with actin remodeling, embryonic development, wound repair, and more recently with cancer. The structure of FLII is characterized by the N-terminal leucine-rich repeats (LRR) and C-terminal gesolin related repeated units that are both protein-protein inter-action domains, suggesting that FLII may exert its function by interaction with other proteins. Therefore, systematic study of protein interactions of FLII in cells is important for the understanding of FLII functions. In this study, we found that FLII was downregulated in lung carcinoma cell lines H1299 and A549 as compared with normal HBE (human bronchial epithelial) cell line. The investigation of FLII interactome in H1299 cells revealed that 74 of the total 132 putative FLII interactors are involved in RNA post-transcriptional modification and trafficking. Furthermore, by using high-throughput transcriptome and translatome sequencing combined with cell fractionation, we showed that the overexpression or knockdown of FLII impacts on the overall nuclear export, and translation of mRNAs. IPA analysis revealed that the majority of these target mRNAs encode the proteins whose functions are reminiscent of those previously reported for FLII, suggesting that the post-transcriptional regulation of mRNA might be a major mechanism of action for FLII.

Flightless-I homolog regulates glucocorticoid receptor-mediated transcription via direct interaction of the leucine-rich repeat domain

Flightless-I homolog (FLII) is a member of the gelsolin family of proteins, and has been identified as a coactivator of estrogen receptor-mediated transcription. Here, we investigate the role of FLII in the glucocorticoid receptor (GR) signaling pathway. Reporter gene assay and real-time quantitative PCR in A549 were performed to investigate the function of FLII in the expression of GR target genes. Co-immunoprecipitation assay and in vitro binding assay were used to identify binding domain of FLII. Chromatin immunoprecipitation assay were carried out with FLII-depleted A549 cells to determine the role of FLII at GR binding sites. We demonstrate that FLII potentiates GR-mediated reporter gene activity synergistically with CARM1 and p300 to enhance GR transcriptional activity in the presence of dexamethasone (Dex) in A549 cells. Depletion of endogenous FLII inhibited the expression of Dex-regulated GR target genes in A549 cells, indicating that FLII is required for GR-mediated transcription. Further, we observed that FLII binds to GR via its N-terminal leucine-rich repeat (LRR) region, suggesting that the enhancement of GR activation may occur through the interaction of GR and FLII. Moreover, chromatin immunoprecipitation analysis demonstrated that FLII is recruited to the GR binding sites. In addition, depletion of endogenous FLII decreased the recruitment of p300, and subsequently RNA polymerase II, to specific sites of GR target genes. Taken together, these studies reveal a functional involvement of FLII in activating transcription of GR target genes, suggesting a physiological role for FLII in the GR signaling pathway.

F-actin dampens NLRP3 inflammasome activity via Flightless-I and LRRFIP2

NLRP3 and ASC are able to form a large multimeric complex called inflammasome in response to a number danger signals. The NLRP3 inflammasome is required for the activation of caspase-1 and subsequent maturation of pro-IL-1β into active IL-1β. Although the mechanisms regulating the formation and activity of NLRP3 inflammasome are yet not fully elucidated, data suggest that the assembly of NLRP3 inflammasome requires microtubules to induce the proximity of ASC and NLRP3. In this study we show that microfilaments (F-actin) inhibit NLRP3 inflammasome activity and interact with NLRP3 and ASC. We demonstrate that the inhibition depends on the actin polymerization state but not on the active polymerization process. In ATP- or nigericin-activated macrophages, our data further indicate that Flightless-I (FliI) and leucine-rich repeat FliI-interaction protein 2 (LRRFIP2) are required for the co-localization of NLRP3, ASC and F-actin. We also established that the ability of Ca²⁺ to accentuate the activity of NLRP3 inflammasome is abrogated in FliI- and LRRFIP2-knockdown macrophages, suggesting that Ca²⁺ signaling requires the presence of FliI and LRRFIP2. Accordingly, we observed that Ca²⁺/FliI-dependent severing of F-actin suppresses F-actin/FliI/LRRFIP2-dependent NLRP3 inflammasome inhibition leading to increase IL-1β production. Altogether, our results unveil a new function of F-actin in the regulation of NLRP3 inflammasome activity strengthening the importance of cytoskeleton in the regulation of inflammation.

Cytoskeletal Regulation of Inflammation and Its Impact on Skin Blistering Disease Epidermolysis Bullosa Acquisita

Actin remodelling proteins regulate cytoskeletal cell responses and are important in both innate and adaptive immunity. These responses play a major role in providing a fine balance in a cascade of biological events that results in either protective acute inflammation or chronic inflammation that leads to a host of diseases including autoimmune inflammation mediated epidermolysis bullosa acquisita (EBA). This review describes the role of the actin cytoskeleton and in particular the actin remodelling protein called Flightless I (Flii) in regulating cellular inflammatory responses and its subsequent effect on the autoimmune skin blistering disease EBA. It also outlines the potential of an antibody based therapy for decreasing Flii expression in vivo to ameliorate the symptoms associated with EBA.

Defining the consequences of genetic variation on a proteome-wide scale

Genetic variation modulates protein expression through both transcriptional and post-transcriptional mechanisms. To characterize the consequences of natural genetic diversity on the proteome, here we combine a multiplexed, mass spectrometry-based method for protein quantification with an emerging outbred mouse model containing extensive genetic variation from eight inbred founder strains. By measuring genome-wide transcript and protein expression in livers from 192 Diversity outbred mice, we identify 2,866 protein quantitative trait loci (pQTL) with twice as many local as distant genetic variants. These data support distinct transcriptional and post-transcriptional models underlying the observed pQTL effects. Using a sensitive approach to mediation analysis, we often identified a second protein or transcript as the causal mediator of distant pQTL. Our analysis reveals an extensive network of direct protein-protein interactions. Finally, we show that local genotype can provide accurate predictions of protein abundance in an independent cohort of collaborative cross mice.

Differential Rac1 signalling by guanine nucleotide exchange factors implicates FLII in regulating Rac1-driven cell migration

The small GTPase Rac1 has been implicated in the formation and dissemination of tumours. Upon activation by guanine nucleotide exchange factors (GEFs), Rac1 associates with a variety of proteins in the cell thereby regulating various functions, including cell migration. However, activation of Rac1 can lead to opposing migratory phenotypes raising the possibility of exacerbating tumour progression when targeting Rac1 in a clinical setting. This calls for the identification of factors that influence Rac1-driven cell motility. Here we show that Tiam1 and P-Rex1, two Rac GEFs, promote Rac1 anti- and pro-migratory signalling cascades, respectively, through regulating the Rac1 interactome. In particular, we demonstrate that P-Rex1 stimulates migration through enhancing the interaction between Rac1 and the actin-remodelling protein flightless-1 homologue, to modulate cell contraction in a RhoA-ROCK-independent manner.

Slit2N Inhibits Transmission of HIV-1 from Dendritic Cells to T-cells by Modulating Novel Cytoskeletal Elements

Dendritic cells are among the first cells to encounter sexually acquired human immunodeficiency virus (HIV-1), in the mucosa, and they can transmit HIV-1 to CD4(+) T-cells via an infectious synapse. Recent studies reveal that actin-rich membrane extensions establish direct contact between cells at this synapse and facilitate virus transmission. Genesis of these contacts involves signaling through c-Src and Cdc42, which modulate actin polymerization and filopodia formation via the Arp2/3 complex and Diaphanous 2 (Diaph2). We found that Slit2N, a ligand for the Roundabout (Robo) receptors, blocked HIV-1-induced signaling through Arp2/3 and Diaph2, decreased filopodial extensions on dendritic cells, and inhibited cell-to-cell transmission of HIV-1 in a Robo1-dependent manner. Employing proteomic analysis, we identified Flightless-1 as a novel, Robo1-interacting protein. Treatment with shRNAs reduced levels of Flightless-1 and demonstrated its role in efficient cell-to-cell transfer of HIV-1. These results suggest a novel strategy to limit viral infection in the host by targeting the Slit/Robo pathway with modulation of cytoskeletal elements previously unrecognized in HIV-1 transmission.

LRRFIP1 Inhibits Hepatitis C Virus Replication by Inducing Type I Interferon in Hepatocytes

BACKGROUND

Hepatitis C virus infection is one of the leading causes of end stage liver diseases. The innate immune response slows down viral replication by activating cytokines such as type I interferon (IFN-α/β), which trigger the synthesis of antiviral proteins and modulate the adaptive immune system. Recently, leucine-rich repeat (in Flightless I) interacting protein-1 (LRRFIP1) was reported contributing to the production of interferon-β in macrophages.

OBJECTIVES

The aim of this study was to assess the role of LRRFIP1 in induction of IFN-β and inhibition of HCV infection in hepatocytes.

MATERIALS AND METHODS

Induction of IFN-β by LRRFIP1 in Huh7 and Huh7.5.1 was determined by real-time PCR and western blotting in vitro. Inhibition of HCV replication by LRRFIP1 overexpression in hepatocytes was assessed.

RESULTS

LRRFIP1 increased the expression of IFN-β in hepatocytes with or without HCV infection. Induction of IFN-β by LRRFIP1 was enhanced with the presence of hepatitis C virus. Overexpression of LRRFIP1 in hepatocytes inhibited HCV replication. However, HCV infection did not regulate intracellular expression of LRRFIP1.

CONCLUSIONS

LRRFIP1 and its mediated production of type I interferon play a role in controlling HCV infection. The findings of this study provide new target for HCV treatment and contribute to development of anti-HCV drugs.

Flightless I interacts with NMMIIA to promote cell extension formation, which enables collagen remodeling

The role of the actin-capping protein flightless I in collagen remodeling by mouse fibroblasts is examined. Flightless and nonmuscle myosin IIA cooperate to enable collagen phagocytosis.

We examined the role of the actin-capping protein flightless I (FliI) in collagen remodeling by mouse fibroblasts. FliI-overexpressing cells exhibited reduced spreading on collagen but formed elongated protrusions that stained for myosin10 and fascin and penetrated pores of collagen-coated membranes. Inhibition of Cdc42 blocked formation of cell protrusions. In FliI-knockdown cells, transfection with constitutively active Cdc42 did not enable protrusion formation. FliI-overexpressing cells displayed increased uptake and degradation of exogenous collagen and strongly compacted collagen fibrils, which was blocked by blebbistatin. Mass spectrometry analysis of FliI immunoprecipitates showed that FliI associated with nonmuscle myosin IIA (NMMIIA), which was confirmed by immunoprecipitation. GFP-FliI colocalized with NMMIIA at cell protrusions. Purified FliI containing gelsolin-like domains (GLDs) 1–6 capped actin filaments efficiently, whereas FliI GLD 2–6 did not. Binding assays showed strong interaction of purified FliI protein (GLD 1–6) with the rod domain of NMMIIA (k_D = 0.146 μM), whereas FliI GLD 2–6 showed lower binding affinity (k_D = 0.8584 μM). Cells expressing FliI GLD 2–6 exhibited fewer cell extensions, did not colocalize with NMMIIA, and showed reduced collagen uptake compared with cells expressing FliI GLD 1–6. We conclude that FliI interacts with NMMIIA to promote cell extension formation, which enables collagen remodeling in fibroblasts.

Transcriptome sequencing and positive selected genes analysis of Bombyx mandarina

The wild silkworm Bombyx mandarina is widely believed to be an ancestor of the domesticated silkworm, Bombyx mori. Silkworms are often used as a model for studying the mechanism of species domestication. Here, we performed transcriptome sequencing of the wild silkworm using an Illumina HiSeq2000 platform. We produced 100,004,078 high-quality reads and assembled them into 50,773 contigs with an N50 length of 1764 bp and a mean length of 941.62 bp. A total of 33,759 unigenes were identified, with 12,805 annotated in the Nr database, 8273 in the Pfam database, and 9093 in the Swiss-Prot database. Expression profile analysis found significant differential expression of 1308 unigenes between the middle silk gland (MSG) and posterior silk gland (PSG). Three sericin genes (sericin 1, sericin 2, and sericin 3) were expressed specifically in the MSG and three fibroin genes (fibroin-H, fibroin-L, and fibroin/P25) were expressed specifically in the PSG. In addition, 32,297 Single-nucleotide polymorphisms (SNPs) and 361 insertion-deletions (INDELs) were detected. Comparison with the domesticated silkworm p50/Dazao identified 5,295 orthologous genes, among which 400 might have experienced or to be experiencing positive selection by Ka/Ks analysis. These data and analyses presented here provide insights into silkworm domestication and an invaluable resource for wild silkworm genomics research.

Dynactin integrity depends upon direct binding of dynamitin to Arp1

Dynactin is an adaptor complex that binds cytoplasmic dynein to cellular cargoes. Its dynamitin (p50) component plays a key role in dynactin stability. Affinity chromatography, direct binding assays, and RNAi demonstrate that the unstructured dynamitin N-terminus binds the dynactin component Arp1 directly.

Dynactin is a multiprotein complex that works with cytoplasmic dynein and other motors to support a wide range of cell functions. It serves as an adaptor that binds both dynein and cargoes and enhances single-motor processivity. The dynactin subunit dynamitin (also known as p50) is believed to be integral to dynactin structure because free dynamitin displaces the dynein-binding p150^Glued subunit from the cargo-binding Arp1 filament. We show here that the intrinsically disordered dynamitin N-terminus binds to Arp1 directly. When expressed in cells, dynamitin amino acids (AA) 1–87 causes complete release of endogenous dynamitin, p150, and p24 from dynactin, leaving behind Arp1 filaments carrying the remaining dynactin subunits (CapZ, p62, Arp11, p27, and p25). Tandem-affinity purification–tagged dynamitin AA 1–87 binds the Arp filament specifically, and binding studies with purified native Arp1 reveal that this fragment binds Arp1 directly. Neither CapZ nor the p27/p25 dimer contributes to interactions between dynamitin and the Arp filament. This work demonstrates for the first time that Arp1 can directly bind any protein besides another Arp and provides important new insight into the underpinnings of dynactin structure.

LRRFIP2 negatively regulates NLRP3 inflammasome activation in macrophages by promoting Flightless-I-mediated caspase-1 inhibition

The NLRP3 inflammasome is the most characterized inflammasome activated by cellular infection or stress, which is responsible for the maturation of proinflammatory cytokines IL-1β and IL-18. The precise molecular mechanism for negative regulation of NLRP3 inflammasome activation needs to be further defined. Here we identify leucine-rich repeat Fli-I-interacting protein 2 (LRRFIP2) as an NLRP3-associated protein and an inhibitor for NLRP3 inflammasome activation. LRRFIP2 binds to NLRP3 via its N terminus upon NLRP3 inflammasome activation, and also interacts with Flightless-I, a pseudosubstrate of caspase-1, via its Coil motif. Knockdown of Flightless-I significantly promotes NLRP3 inflammasome activation. LRRFIP2 enhances the interaction between Flightless-I and caspase-1, facilitating the inhibitory effect of Flightless-I on caspase-1 activation. Furthermore, silencing of Flightless-I abrogates the inhibitory effect of LRRFIP2 on NLRP3 inflammasome. These data demonstrate that LRRFIP2 inhibits NLRP3 inflammasome activation by recruiting the caspase-1 inhibitor Flightless-I, thus outlining a new mechanism for negative regulation of NLRP3 inflammasome.

Inflammasomes promote the maturation of inflammatory cytokines in response to signals associated with damage and infection, but it remains unclear how these signals are attenuated. Here, the authors show that the NLRP3 inflammasome is inhibited by LRRFIP2 through recruitment of the protein Flightless I.

Characterization of Gcf2/Lrrfip1 in experimental cerebral ischemia and its role as a modulator of Akt, mTOR and β-catenin signaling pathways

Leucine-rich repeat in Flightless-1 interaction protein 1 (Lrrfip1) is an up-regulated protein after cerebral ischemia whose precise role in the brain both in healthy and ischemic conditions is unclear. Different Lrrfip1 isoforms with distinct roles have been reported in human and mouse species. The present study aimed to analyze the Lrrfip1 transcriptional variants expressed in rat cortex, to characterize their expression patterns and subcellular location after ischemia, and to define their putative role in the brain. Five transcripts were identified and three of them (Lrrfip1, CRA_g and CRA_a' (Fli-I leucine-rich repeat associated protein 1 - Flap-1)) were analyzed by quantitative real-time polymerase chain reaction (qPCR). All the transcripts were up-regulated and showed differential expression patterns after in vivo and in vitro ischemia models. The main isoform, Lrrfip1, was found to be up-regulated from the acute to the late phases of ischemia in the cytoplasm of neurons and astrocytes of the peri-infarct area. This study demonstrates that Lrrfip1 activates β-catenin, Akt, and mammalian target of rapamycin (mTOR) proteins in astrocytes and positively regulates the expression of the excitatory amino acid transporter subtype 2 (GLT-1). Our findings point to Lrrfip1 as a key brain protein that regulates pro-survival pathways and proteins and encourages further studies to elucidate its role in cerebral ischemia as a potential target to prevent brain damage and promote functional recovery after stroke.

Diet-induced over-expression of flightless-I protein and its relation to flightlessness in Mediterranean fruit fly, Ceratitis capitata

The Mediterranean fruit fly (medfly), Ceratitis capitata is among the most economically important pests worldwide. Understanding nutritional requirement helps rearing healthy medfly for biocontrol of its population in fields. Flight ability is a high priority criterion. Two groups of medfly larvae were reared with two identical component diets except one with fatty acids (diet A) and another without it (diet B). Adults from larvae reared on diet B demonstrated 20±8% of normal flight ability, whereas those from larvae reared on diet A displayed full flight ability of 97±1%. Proteomes were profiled to compare two groups of medfly pupae using shotgun proteomics to study dietary effects on flight ability. When proteins detected in pupae A were compared with those in pupae B, 233 and 239 proteins were, respectively, under- and over-expressed in pupae B, while 167 proteins were overlapped in both pupae A and B. Differential protein profiles indicate that nutritional deficiency induced over-expression of flightless-I protein (fli-I) in medfly. All proteins were subjected to Ingenuity Pathway Analysis (IPA) to create 13 biological networks and 17 pathways of interacting protein clusters in human ortholog. Fli-I, leucine-rich repeat (LRR)-containing G protein-coupled receptor 2, LRR protein soc-2 and protein wings apart-like were over-expressed in pupae B. Inositol-1,4,5-trisphosphate receptor, protocadherin-like wing polarity protein stan and several Wnt pathway proteins were under-expressed in pupae B. These results suggest down-regulation of the Wnt/wingless signaling pathway, which consequently may result in flightlessness in pupae B. The fli-I gene is known to be located within the Smith-Magenis syndrome (SMS) region on chromosome 17, and thus, we speculate that nutritional deficiency might induce over-expression of fli-I (or fli-I gene) and be associated with human SMS. However, more evidence would be needed to confirm our speculation.

The microRNA miR-132 targets Lrrfip1 to block vascular smooth muscle cell proliferation and neointimal hyperplasia

OBJECTIVE

The proliferation and remodeling of vascular smooth muscle cells (VSMCs) is an important pathological event in atherosclerosis and restenosis. Here we report that microRNA-132 (miR-132) blocks vascular smooth muscle cells (VSMC) proliferation by inhibiting the expression of LRRFIP1 [leucine-rich repeat (in Flightless 1) interacting protein-1].

METHODS AND RESULTS

MicroRNA microarray revealed that miR-132 was upregulated in the rat carotid artery after catheter injury, which was further confirmed by quantitative real-time RT-PCR. Transfection of a miR-132 mimic significantly inhibited the proliferation of VSMCs, whereas transfection of a miR-132 antagomir increased it. miR-132 mimic inhibited VSMC migration and induced apoptosis. miR-132 mimic increased the protein amounts of both p27 and smooth muscle (SM) α-actin, whereas it decreased SM α-actin and Bcl2. Bioinformatics showed that LRRFIP1 is a target candidate of miR-132. miR-132 down-regulated luciferase activity driven by a vector containing the 3'-untranslated region of Lrrfip1 in a sequence-specific manner. LRRFIP1 induced VSMC proliferation and increased phosphorylation of ERK. Immunohistochemical analysis revealed that Lrrfip1 was clearly expressed along with the basal laminar area of smooth muscle, and its expression pattern was disrupted 7 days after arterial injury. LRRFIP1 mRNA was decreased 14 days after injury. Delivery of miR-132 to rat carotid artery reduced LRRFIP1 expression and attenuated neointimal proliferation in carotid artery injury models.

CONCLUSIONS

Our results suggest that miR-132 is a novel regulator of VSMC proliferation that represses neointimal formation by inhibiting LRRFIP1 expression.

Innate immune responses to DNA viruses

The innate immune system acts as the first line of defense against pathogens, which is also essential for initiation of adaptive immunity. Innate immune responses are initiated by pattern-recognition receptors (PRRs), which recognize conserved molecular structures of pathogens called pathogen-associated molecular patterns (PAMPs). The infection of virus triggers a series of signaling events leading to transcriptional induction of type I interferons (IFNs) and proinflammatory cytokines. In recent years, the mechanisms of viral RNA recognition and RNA virus-triggered signaling pathways have been well studied. However, it remains unclear on how DNA virus infection is sensed by host cells and triggers the host antiviral defense. Although ten potential viral DNA sensors have been reported, none of them is validated as a generally used sensor for distinct DNA viruses in divergent cell types and animals. Here, we provide a summary and perspective on recent advances in innate immune responses to DNA viruses.

Cytoskeletal regulation of dermal regeneration

Wound healing results in the repair of injured tissues however fibrosis and scar formation are, more often than not the unfortunate consequence of this process. The ability of lower order vertebrates and invertebrates to regenerate limbs and tissues has been all but lost in mammals; however, there are some instances where glimpses of mammalian regenerative capacity do exist. Here we describe the unlocked potential that exists in mammals that may help us understand the process of regeneration post-injury and highlight the potential role of the actin cytoskeleton in this process. The precise function and regulation of the cytoskeleton is critical to the success of the healing process and its manipulation may therefore facilitate regenerative healing. The gelsolin family of actin remodelling proteins in particular has been shown to have important functions in wound healing and family member Flightless I (Flii) is involved in both regeneration and repair. Understanding the interactions between different cytoskeletal proteins and their dynamic control of processes including cellular adhesion, contraction and motility may assist the development of therapeutics that will stimulate regeneration rather than repair.

Immunohistochemical analysis of medullary breast carcinoma autoantigens in different histological types of breast carcinomas

BACKGROUND

On the past decade a plethora of investigations were directed on identification of molecules involved in breast tumorogenesis, which could represent a powerful tool for monitoring, diagnostics and treatment of this disease. In current study we analyzed six previously identified medullary breast carcinoma autoantigens including LGALS3BP, RAD50, FAM50A, RBPJ, PABPC4, LRRFIP1 with cancer restricted serological profile in different histological types of breast cancer.

METHODS

Semi-quantitative immunohistochemical analysis of 20 tissue samples including medullary breast carcinoma, invasive ductal carcinoma, invasive lobular carcinoma and non-cancerous tissues obtained from patients with fibrocystic disease (each of five) was performed using specifically generated polyclonal antibodies. Differences in expression patterns were evaluated considering percent of positively stained cells, insensitivity of staining and subcellular localization in cells of all tissue samples.

RESULTS

All 6 antigens predominantly expressed in the most cells of all histological types of breast tumors and non-cancerous tissues with slight differences in intensity of staining and subcellular localization. The most significant differences in expression pattern were revealed for RAD50 and LGALS3BP in different histological types of breast cancer and for PABPC4 and FAM50A antigens in immune cells infiltrating breast tumors.

CONCLUSIONS

This pilot study made possible to select 4 antigens LGALS3BP, RAD50, PABPC4, and FAM50A as promising candidates for more comprehensive research as potential molecular markers for breast cancer diagnostics and therapy.

VIRTUAL SLIDES

The virtual slides' for this article can be found here: http://www.diagnosticpathology.diagnomx.eu/vs/1860649350796892.

Crystal structure of the dimeric coiled-coil domain of the cytosolic nucleic acid sensor LRRFIP1

LRRFIP1 binds cytoplasmic double-stranded DNA and RNA and interacts with FLI, the mammalian homolog of Drosophila flightless I, through a highly conserved 87-amino acid domain. Upon binding nucleic acid ligands, LRRFIP1 recruits and activates β-catenin, leading to the IRF3-dependent production of type I interferon. However, the molecular mechanism of LRRFIP1 signaling is not well understood. Here we show that the FLI-interacting domain of LRRFIP1 forms a classic parallel, homodimeric coiled coil with 10 heptad repeats and 22 helical turns. The coiled coil domain is also a dimer in solution. However, a longer LRRFIP1 construct spanning the coiled coil and DNA binding domains assembles into higher order oligomers in solution. The structure of LRRFIP1-CC constitutes a valuable tool for probing the mechanism of LRRFIP1 signaling and for structural studies of larger LRRFIP1 constructs.

Flightless I is a focal adhesion-associated actin-capping protein that regulates cell migration

The role of adhesion-associated actin-binding proteins in cell migration is not well defined. In mouse fibroblasts we screened for focal adhesion-associated proteins that were isolated with collagen-coated beads and detected by tandem mass spectrometry. We identified flightless I (FliI) as an actin-binding protein in focal adhesion fractions, which was verified by immunoblotting. By confocal microscopy most FliI was distributed throughout the cytosol and in focal adhesions. By sedimentation assays and in vitro binding assays, we found that FliI associates with actin filaments and actin monomers. Assays using purified proteins showed that FliI inhibits actin polymerization and caps but does not sever actin filaments. Cells with FliI knockdown or cells overexpressing FliI migrated more or less rapidly, respectively, than wild-type controls. Compared with controls, cells with FliI knockdown were less adherent than wild-type cells, exhibited reduced numbers of focal adhesions containing activated β1 integrins and vinculin, and exhibited increased incorporation of actin monomers into nascent filaments at focal adhesions. These data indicate that FliI regulates cell migration through its localization to focal adhesions and its ability to cap actin filaments, which collectively affect focal adhesion maturation.

Flightless, secreted through a late endosome/lysosome pathway, binds LPS and dampens cytokine secretion

Flightless (Flii) is upregulated in response to wounding and has been shown to function in wound closure and scarring. In macrophages intracellular Flii negatively modulates TLR signalling and dampens cytokine production. We now show that Flii is constitutively secreted from macrophages and fibroblasts and is present in human plasma. Secretion from fibroblasts is upregulated in response to scratch wounding and LPS-activated macrophages also temporally upregulate their secretion of Flii. Using siRNA, wild-type and mutant proteins we show that Flii is secreted via a late endosomal/lysosomal pathway that is regulated by Rab7 and Stx11. Flii contains 11 leucine rich repeat (LRR) domains in its N-terminus that have nearly 50% similarity to those in the extracellular pathogen binding portion of Toll-like receptor 4 (TLR4). We show secreted Flii can also bind LPS and has the ability to alter macrophage activation. LPS activation of macrophages in Flii depleted conditioned media leads to enhanced macrophage activation and increased TNF secretion compared to cells activated in the presence of Flii. These results show secreted Flii binds to LPS and in doing so alters macrophage activation and cytokine secretion, suggesting that like the intracellular pool of Flii, secreted Flii also has the ability to alter inflammation.

Mouse strains for the ubiquitous or conditional overexpression of the Flii gene

The gelsolin related actin binding protein, Flii, is able to regulate wound healing; mice with decreased Flii expression show improved wound healing whereas mice with elevated Flii expression exhibit impaired wound healing. In both mice and humans Flii expression increases with age and amelioration of FLII activity represents a possible therapeutic strategy for improved wound healing in humans. Despite analysis of Flii function in a variety of organisms we know little of the molecular mechanisms underlying Flii action. Two new murine alleles of Flii have been produced to drive constitutive or tissue-specific expression of Flii. Each strain is able to rescue the embryonic lethality associated with a Flii null allele and to impair wound healing. These strains provide valuable resources for ongoing investigation of Flii function in a variety of biological processes.

Decreased expression of Flightless I, a gelsolin family member and developmental regulator, in early-gestation fetal wounds improves healing

Up until late in the third trimester of gestation and through to adulthood, the healing response acts more to regenerate than to repair a wound. The mechanisms underlying this "scar-free" healing remain unknown although the actin cytoskeleton has a major role. Flightless I (Flii), an actin-remodelling protein and essential developmental regulator, negatively affects wound repair but its effect on scar-free fetal healing is unknown. Using fetal skin explants from E17 (regenerate) and E19 (repair) rats, the function of Flii in fetal wound repair was determined. Expression of Flii increased between E17 and E19 days of gestation and wounding transiently increased Flii expression in E17 but not E19 wounds. However, both confocal and immunofluorescent analysis showed E17 keratinocytes immediately adjacent to the wounds downregulated Flii. As a nuclear coactivator and inhibitor of proliferation and migration, the absence of Flii in cells at the edge of the wound could be instrumental in allowing these cells to proliferate and migrate into the wound deficit. In contrast, Flii was strongly expressed within the cytoplasm and nucleus of keratinocytes within epidermal cells at the leading edge of E19 wounded fetal skin explants. This increase in Flii expression in E19 wounds could affect the way these cells migrate into the wound space and contribute to impaired wound healing. Neutralising Flii protein improved healing of early- but not late-gestation wounds. Flii did not colocalise with actin cables formed around E17 wounds suggesting an independent mechanism of action distinct from its actin-binding function in scar-free wound repair.

Transcription profiling in human platelets reveals LRRFIP1 as a novel protein regulating platelet function

Within the healthy population, there is substantial, heritable, and interindividual variability in the platelet response. We explored whether a proportion of this variability could be accounted for by interindividual variation in gene expression. Through a correlative analysis of genome-wide platelet RNA expression data from 37 subjects representing the normal range of platelet responsiveness within a cohort of 500 subjects, we identified 63 genes in which transcript levels correlated with variation in the platelet response to adenosine diphosphate and/or the collagen-mimetic peptide, cross-linked collagen-related peptide. Many of these encode proteins with no reported function in platelets. An association study of 6 of the 63 genes in 4235 cases and 6379 controls showed a putative association with myocardial infarction for COMMD7 (COMM domain-containing protein 7) and a major deviation from the null hypo thesis for LRRFIP1 [leucine-rich repeat (in FLII) interacting protein 1]. Morpholino-based silencing in Danio rerio identified a modest role for commd7 and a significant effect for lrrfip1 as positive regulators of thrombus formation. Proteomic analysis of human platelet LRRFIP1-interacting proteins indicated that LRRFIP1 functions as a component of the platelet cytoskeleton, where it interacts with the actin-remodeling proteins Flightless-1 and Drebrin. Taken together, these data reveal novel proteins regulating the platelet response.

The cytosolic nucleic acid sensor LRRFIP1 mediates the production of type I interferon via a beta-catenin-dependent pathway

Intracellular nucleic acid sensors detect microbial RNA and DNA and trigger the production of type I interferon. However, the cytosolic nucleic acid-sensing system remains to be fully identified. Here we show that the cytosolic nucleic acid-binding protein LRRFIP1 contributed to the production of interferon-beta (IFN-beta) induced by vesicular stomatitis virus (VSV) and Listeria monocytogenes in macrophages. LRRFIP1 bound exogenous nucleic acids and increased the expression of IFN-beta induced by both double-stranded RNA and double-stranded DNA. LRRFIP1 interacted with beta-catenin and promoted the activation of beta-catenin, which increased IFN-beta expression by binding to the C-terminal domain of the transcription factor IRF3 and recruiting the acetyltransferase p300 to the IFN-beta enhanceosome via IRF3. Therefore, LRRFIP1 and its downstream partner beta-catenin constitute another coactivator pathway for IRF3-mediated production of type I interferon.

Actin complexes in the cell nucleus: new stones in an old field

Actin is a well-known protein that has shown a myriad of activities in the cytoplasm. However, recent findings of actin involvement in nuclear processes are overwhelming. Actin complexes in the nucleus range from very dynamic chromatin-remodeling complexes to structural elements of the matrix with single partners known as actin-binding proteins (ABPs). This review summarizes the recent findings of actin-containing complexes in the nucleus. Particular attention is given to key processes like chromatin remodeling, transcription, DNA replication, nucleocytoplasmic transport and to actin roles in nuclear architecture. Understanding the mechanisms involving ABPs will definitely lead us to the principles of the regulation of gene expression performed via concerting nuclear and cytoplasmic processes.

Flightless-I regulates proinflammatory caspases by selectively modulating intracellular localization and caspase activity

Caspase-1 and caspase-11 are proinflammatory caspases that regulate cytokine production and leukocyte migration during pathogen infection. In an attempt to identify new intracellular regulators of caspase-11, we found that Flightless-I, a member of the gelsolin superfamily of actin-remodeling proteins, interacts and regulates both caspase-11 and caspase-1. Flightless-I targets caspase-11 to the Triton X-100–insoluble cytoskeleton fraction and the cell leading edge. In addition, Flightless-I inhibits caspase-1 activation and caspase-1–mediated interleukine-1β (IL-1β) maturation. The physiological relevance of these findings is supported by the opposing effects of Flightless-I overexpression and knockdown on caspase-1 activity and IL-1β maturation. Our results suggest that Flightless-I may be a bona fide caspase-1 inhibitor that acts through a mechanism similar to that of cytokine response modifier A, a potent caspase-1 inhibitor from the cowpox virus. Our study provides a new mechanism controlling the localization and activation of proinflammatory caspases.

The Flightless I homolog, fli-1, regulates anterior/posterior polarity, asymmetric cell division and ovulation during Caenorhabditis elegans development

Flightless I (Fli I) is an evolutionarily conserved member of the gelsolin family, containing actin-binding and severing activity in vitro. The physiological function of Fli I during animal development remains largely undefined. In this study, we reveal a key role of the Caenorhabditis elegans Fli I homolog, fli-1, in specifying asymmetric cell division and in establishing anterior-posterior polarity in the zygote. The fli-1 gene also regulates the cytokinesis of somatic cells and the development of germline and interacts with the phosphoinositol-signaling pathway in the regulation of ovulation. The fli-1 reporter gene shows that the localization of FLI-1 coincides with actin-rich regions and that the actin cytoskeleton is impaired in many tissues in the fli-1 mutants. Furthermore, the function of fli-1 in C. elegans can be functionally substituted by the Drosophila Fli I. Our studies demonstrate that fli-1 plays an important role in regulating the actin-dependent events during C. elegans development.

Flightless I deficiency enhances wound repair by increasing cell migration and proliferation

Wound healing disorders are a therapeutic problem of increasing clinical importance involving substantial morbidity, mortality, and rising health costs. Our studies investigating flightless I (FliI), a highly conserved actin-remodelling protein, now reveal that FliI is an important regulator of wound repair whose manipulation may lead to enhanced wound outcomes. We demonstrate that FliI-deficient + /- mice are characterized by improved wound healing with increased epithelial migration and enhanced wound contraction. In contrast, FliI-overexpressing mice have significantly impaired wound healing with larger less contracted wounds and reduced cellular proliferation. We show that FliI is secreted in response to wounding and that topical application of antibodies raised against the leucine-rich repeat domain of the FliI protein (FliL) significantly improves wound repair. These studies reveal that FliI affects wound repair via mechanisms involving cell migration and proliferation and that FliI might represent an effective novel therapeutic factor to improve conditions in which wound healing is impaired.

Mechanism of depolymerization and severing of actin filaments and its significance in cytoskeletal dynamics

The actin cytoskeleton is one of the major structural components of the cell. It often undergoes rapid reorganization and plays crucial roles in a number of dynamic cellular processes, including cell migration, cytokinesis, membrane trafficking, and morphogenesis. Actin monomers are polymerized into filaments under physiological conditions, but spontaneous depolymerization is too slow to maintain the fast actin filament dynamics observed in vivo. Gelsolin, actin-depolymerizing factor (ADF)/cofilin, and several other actin-severing/depolymerizing proteins can enhance disassembly of actin filaments and promote reorganization of the actin cytoskeleton. This review presents advances as well as a historical overview of studies on the biochemical activities and cellular functions of actin-severing/depolymerizing proteins.

Interplay of Fli-I and FLAP1 for regulation of beta-catenin dependent transcription

β-catenin mediates Wnt/wingless signaling and transcriptional activation by lymphocyte enhancer binding factor 1/T cell factor (LEF1/TCF) proteins with the assistance of multiple coregulators, including positive cofactors like p300/CBP and negative cofactors like HDACs. We previously demonstrated that a developmentally essential protein, Flightless-I (Fli-I), serves as a coactivator for nuclear receptor-mediated transcription. To further understand the action mechanism of Fli-I, we investigated the functional roles of Fli-I and Fli-I leucine rich repeat associated protein 1 (FLAP1) in transcriptional activation by β-catenin and LEF1/TCF. β-catenin-dependent transcription was activated by exogenous FLAP1 but inhibited by Fli-I. Reduction of endogenous FLAP1 levels compromised transcriptional activation by LEF1/TCF, β-catenin and the p160 coactivator GRIP1. FLAP1 interacted directly with β-catenin, GRIP1 and p300 and enhanced their activity. Furthermore, FLAP1 was strongly synergistic with p300 in supporting transcriptional activation by β-catenin and LEF1/TCF, but Fli-I disrupted the synergy of FLAP1 with p300 and β-catenin. Thus the opposing effects of Fli-I and FLAP1 may be a key regulatory mechanism for β-catenin and LEF1/TCF-mediated transcription and thus for Wnt signaling, and some mutations of Fli-I may result in developmental defects, such as the flightless phenotype of Drosophila, by causing dysregulation of the Wnt/β-catenin pathway.