Interactions between actin, myosin, and an actin-binding protein from rabbit alveolar macrophages. Alveolar macrophage myosin Mg-2+-adenosine triphosphatase requires a cofactor for activation by actin

Mechanism Study on Radiosensitization Effect of Curcumin in Bladder Cancer Cells Regulated by Filamin A

Objective

To study the radiosensitization effect of curcumin, a natural product with anti-inflammatory and anti-cancer properties, in bladder cancer cells and identify the specific role of FLNA gene in that process.

Methods

CCK-8 method was initially adopted to identify the proper interventional concentration of curcumin. T24 bladder cancer cells were subjected to CCK-8, flow cytometry, and colony formation assay to study the cell biological behaviors under different interventions. γ-H2AX test was performed to test the level of damage in T24 cells. RT-qPCR and Western blot were conducted to measure FLNA mRNA and protein levels.

Results

Low-dose curcumin (10, 20 μM) following X-ray exposure resulted in increased DNA damage, augmented apoptosis, and reduced proliferation of T24 cells. Certain radiosensitization was demonstrated when curcumin was applied at 10 μM. Additionally, elevation of FLNA gene and protein levels was also indicated upon combination treatment.

Conclusion

Low-dose curcumin has certain radiosensitization effect in bladder cancer, where FLNA plays a certain regulatory role.

Understanding the molecular basis of cardiomyopathy

Inherited cardiomyopathies are a major cause of mortality and morbidity worldwide and can be caused by mutations in a wide range of proteins located in different cellular compartments. The present review is based on Dr. Ju Chen's 2021 Robert M. Berne Distinguished Lectureship of the American Physiological Society Cardiovascular Section, in which he provided an overview of the current knowledge on the cardiomyopathy-associated proteins that have been studied in his laboratory. The review provides a general summary of the proteins in different compartments of cardiomyocytes associated with cardiomyopathies, with specific focus on the proteins that have been studied in Dr. Chen's laboratory.

Filamin, a synaptic organizer in Drosophila, determines glutamate receptor composition and membrane growth

Filamin is a scaffolding protein that functions in many cells as an actin-crosslinker. FLN90, an isoform of the Drosophila ortholog Filamin/cheerio that lacks the actin-binding domain, is here shown to govern the growth of postsynaptic membrane folds and the composition of glutamate receptor clusters at the larval neuromuscular junction. Genetic and biochemical analyses revealed that FLN90 is present surrounding synaptic boutons. FLN90 is required in the muscle for localization of the kinase dPak and, downstream of dPak, for localization of the GTPase Ral and the exocyst complex to this region. Consequently, Filamin is needed for growth of the subsynaptic reticulum. In addition, in the absence of filamin, type-A glutamate receptor subunits are lacking at the postsynapse, while type-B subunits cluster correctly. Receptor composition is dependent on dPak, but independent of the Ral pathway. Thus two major aspects of synapse formation, morphological plasticity and subtype-specific receptor clustering, require postsynaptic Filamin.

DOI:http://dx.doi.org/10.7554/eLife.19991.001

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.

Filamin C plays an essential role in the maintenance of the structural integrity of cardiac and skeletal muscles, revealed by the medaka mutant zacro

Filamin C is an actin-crosslinking protein that is specifically expressed in cardiac and skeletal muscles. Although mutations in the filamin C gene cause human myopathy with cardiac involvement, the function of filamin C in vivo is not yet fully understood. Here we report a medaka mutant, zacro (zac), that displayed an enlarged heart, caused by rupture of the myocardiac wall, and progressive skeletal muscle degeneration in late embryonic stages. We identified zac to be a homozygous nonsense mutation in the filamin C (flnc) gene. The medaka filamin C protein was found to be localized at myotendinous junctions, sarcolemma, and Z-disks in skeletal muscle, and at intercalated disks in the heart. zac embryos showed prominent myofibrillar degeneration at myotendinous junctions, detachment of myofibrils from sarcolemma and intercalated disks, and focal Z-disk destruction. Importantly, the expression of γ-actin, which we observed to have a strong subcellular localization at myotendinous junctions, was specifically reduced in zac mutant myotomes. Inhibition of muscle contraction by anesthesia alleviated muscle degeneration in the zac mutant. These results suggest that filamin C plays an indispensable role in the maintenance of the structural integrity of cardiac and skeletal muscles for support against mechanical stress.

Loss of FilaminC (FLNc) results in severe defects in myogenesis and myotube structure

FilaminC (FLNc) is the muscle-specific member of a family of actin binding proteins. Although it interacts with many proteins involved in muscular dystrophies, its unique role in muscle is poorly understood. To address this, two models were developed. First, FLNc expression was stably reduced in C2C12 myoblasts by RNA interference. While these cells start differentiation normally, they display defects in differentiation and fusion ability and ultimately form multinucleated "myoballs" rather than maintain elongated morphology. Second, a mouse model carrying a deletion of last 8 exons of Flnc was developed. FLNc-deficient mice die shortly after birth, due to respiratory failure, and have severely reduced birth weights, with fewer muscle fibers and primary myotubes, indicating defects in primary myogenesis. They exhibit variation in fiber size, fibers with centrally located nuclei, and some rounded fibers resembling the in vitro phenotype. The similarity of the phenotype of FLNc-deficient mice to the filamin-interacting TRIO null mice was further confirmed by comparing FLNc-deficient C2C12 cells to TRIO-deficient cells. These data provide the first evidence that FLNc has a crucial role in muscle development and maintenance of muscle structural integrity and suggest the presence of a TRIO-FLNc-dependent pathway in maintaining proper myotube structure.

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

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

Myosin Va bound to phagosomes binds to F-actin and delays microtubule-dependent motility

We established a light microscopy-based assay that reconstitutes the binding of phagosomes purified from mouse macrophages to preassembled F-actin in vitro. Both endogenous myosin Va from mouse macrophages and exogenous myosin Va from chicken brain stimulated the phagosome-F-actin interaction. Myosin Va association with phagosomes correlated with their ability to bind F-actin in an ATP-regulated manner and antibodies to myosin Va specifically blocked the ATP-sensitive phagosome binding to F-actin. The uptake and retrograde transport of phagosomes from the periphery to the center of cells in bone marrow macrophages was observed in both normal mice and mice homozygous for the dilute-lethal spontaneous mutation (myosin Va null). However, in dilute-lethal macrophages the accumulation of phagosomes in the perinuclear region occurred twofold faster than in normal macrophages. Motion analysis revealed saltatory phagosome movement with temporarily reversed direction in normal macrophages, whereas almost no reversals in direction were observed in dilute-lethal macrophages. These observations demonstrate that myosin Va mediates phagosome binding to F-actin, resulting in a delay in microtubule-dependent retrograde phagosome movement toward the cell center. We propose an "antagonistic/cooperative mechanism" to explain the saltatory phagosome movement toward the cell center in normal macrophages.

Delivery systems for penetration enhancement of peptide and protein drugs: design considerations

This paper discusses the challenges to be met in designing delivery systems that maximize the absorption of peptide and protein drugs from the gastrointestinal and respiratory tracts. The ideal delivery system for either route of administration is one that will release its contents only at a favorable region of absorption, where the delivery system attaches by virtue of specific interaction with surface determinants unique to that region and where the delivery system travels at a rate independent of the transitory constraints inherent of the route of administration. Such a delivery system, which is as yet unavailable, will benefit not only peptide and protein drugs, but other poorly absorbed drugs.

Filamins as integrators of cell mechanics and signalling

Filamins are large actin-binding proteins that stabilize delicate three-dimensional actin webs and link them to cellular membranes. They integrate cellular architectural and signalling functions and are essential for fetal development and cell locomotion. Here, we describe the history, structure and function of this group of proteins.

Filamin 2 (FLN2): A muscle-specific sarcoglycan interacting protein

Mutations in genes encoding for the sarcoglycans, a subset of proteins within the dystrophin-glycoprotein complex, produce a limb-girdle muscular dystrophy phenotype; however, the precise role of this group of proteins in the skeletal muscle is not known. To understand the role of the sarcoglycan complex, we looked for sarcoglycan interacting proteins with the hope of finding novel members of the dystrophin-glycoprotein complex. Using the yeast two-hybrid method, we have identified a skeletal muscle-specific form of filamin, which we term filamin 2 (FLN2), as a gamma- and delta-sarcoglycan interacting protein. In addition, we demonstrate that FLN2 protein localization in limb-girdle muscular dystrophy and Duchenne muscular dystrophy patients and mice is altered when compared with unaffected individuals. Previous studies of filamin family members have determined that these proteins are involved in actin reorganization and signal transduction cascades associated with cell migration, adhesion, differentiation, force transduction, and survival. Specifically, filamin proteins have been found essential in maintaining membrane integrity during force application. The finding that FLN2 interacts with the sarcoglycans introduces new implications for the pathogenesis of muscular dystrophy.

Phagosomal proteins of Dictyostelium discoideum

In recognizing food particles. Dictyostelium cell-surface molecules initiate cytoskeletal rearrangements that result in phagosome formation. After feeding D. discoideum cells latex beads, early phagosomes were isolated on sucrose step gradients. Protein analyses of these vesicles showed that they contained glycoproteins and surface-labeled species corresponding to integral plasma membrane proteins. Cytoskeletal proteins also were associated with phagosomes, including myosin II, actin and a 30 kDa-actin bundling protein. As seen by the acridine orange fluorescence of vesicles containing bacteria, phagosomes were acidified rapidly by a vacuolar H(+)-ATPase that was detected by immunoblotting. Except for the loss of cytoskeletal proteins, few other changes over time were noted in the protein profiles of phagosomes, suggesting that phagosome maturation was incomplete. The indigestibility of the beads possibly inhibited further endocytic processing, which has been observed by others. Since nascent phagosomes contained molecules of both the cytoskeleton and plasma membrane, they will be useful in studies aimed at identifying specific protein associations occurring between membrane proteins and the cytoskeleton during phagocytosis.

A novel protein accumulated during maturation of the pods of the plant Impatiens balsamina

A high molecular weight protein has been isolated as a major polypeptide comprising 85% of the total extractable proteins in the fruit pericarp of the plant Impatiens balsamina. The protein has been purified to homogeneity following fractionation of the crude cell supernatant with ammonium sulphate, chromatography on Sepharose 4B and Sephacryl-S-300. This protein appeared to be a homo-tetramer consisting of subunits, Mr 75 K. Amino acid analysis showed the presence of more acidic amino acids, with an isoelectric point 5.8. The interaction of this protein with filamentous actin, both from rabbit muscle and Mung bean embryo was monitored by transmission electron microscopy, light scattering analysis, viscometry and sedimentation assay. This protein inhibited appreciably the polymerization of G-actin to F-actin.

Proteolytic fragmentation of brain myosin and localisation of the heavy-chain phosphorylation site

The heavy chains and the 19-kDa and 20-kDa light chains of bovine brain myosin can by phosphorylated. To localise the site of heavy-chain phosphorylation, the myosin was initially subjected to digestion with chymotrypsin and papain under a variety of conditions and the fragments thus produced were identified. Irrespective of the ionic strength, i.e. whether the myosin was monomeric or filamentous, chymotryptic digestion produced two major fragments of 68 kDa and 140 kDa; the 140-kDa fragment was further digested by papain to yield a 120-kDa and a 23-kDa fragment. These fragments were characterised by (a) a gel overlay technique using 125I-labelled light chains, which showed that the 140-kDa and 23-kDa polypeptides contain the light-chain-binding sites; (b) using myosin photoaffinity labelled at the active site with [3H]UTP, which showed that the 68-kDa fragment contained the catalytic site, and (c) electron microscopy, using rotary shadowing and negative-staining techniques, which demonstrated that after chymotryptic digestion the myosin head remains attached to the tail whereas on papain digestion isolated heads and tails were observed. Thus the 120-kDa polypeptide derived from the 140-kDa fragment is the tail of the myosin, and the 68-kDa fragment containing the catalytic site and the 23-kDa fragment, with the light-chain-binding sites, form the head (S1) portion of the myosin. When [32P]-phosphorylated brain myosin was digested with chymotrypsin and papain it was shown that the heavy-chain phosphorylation site is located in a 5-kDa peptide at the C-terminal end of the heavy chain, i.e. the end of the myosin tail. Using hydrodynamic and electron microscopic techniques, no significant effect of either light-chain or heavy-chain phosphorylation on the stability of brain myosin filaments was observed, even in the presence of MgATP. Brain myosin filaments appear to be more stable than those of other non-muscle myosins. Light-chain phosphorylation did, however, have an effect on the conformation of brain myosin, for example in the presence of MgATP non-phosphorylated myosin molecules were induced to fold into a very compact folded state.

Filamin inhibits actomyosin ATPase activity in platelet

Filamin, an actin cross-linker protein, has been shown to exist in platelet. The role of this protein in the platelet has remained unclear. In this report, we show that filamin inhibits the actin-activated Mg2+ -ATPase activity of platelet myosin. The activation caused by platelet actin is inhibited by 50% at the molar ratio of filamin to actin of 1/50. Platelet tropomyosin, which we showed to enhance the ATPase activity, does not abolish the effect of filamin. The results support the view that filamin stabilizes the actin network in the resting platelet.

Crosslinking of actin filaments is caused by caldesmon aggregates, but not by its dimers

A recent report by Bretscher [(1984) J. Biol. Chem. 259, 12873-12880] showed that caldesmon prepared by his method crosslinks actin filaments to form thick bundles. This is in contrast to the results of previous work that caldesmon binds to F-actin but does not cause any gelation [(1981) Proc. Natl. Acad. Sci. USA 78, 5652-5655]. The present work clearly showed that caldesmon purified according to Bretscher does not cause any gelation of F-actin. However, caldesmon aggregates formed by concentration or by freeze-thawing gelated F-actin to form bundles.

Angiotensin II stimulation of granuloma macrophage phagocytesis and actin polymerization in murine schistosomiasis mansoni

In murine schistosomiasis, components of the renin-angiotensin system are present in isolated liver granulomas. Furthermore, granuloma macrophages have receptors for angiotensin II (AII). These observations with other evidence suggest that angiotensins function in immunoregulation. Since AII can stimulate actin-myosin interactions and since macrophages have contractile proteins, it was determined whether AII could alter the phagocytic activity of granuloma macrophages. AII, at concentrations which saturate AII receptors, enhanced phagocytic activity for both unopsonized and opsonized sheep red blood cells (RBC). Using a DNase inhibitory assay, it was also shown that similar concentrations of AII stimulated intracellular actin polymerization. Cytochalasin B, which impedes actin filament interactions, inhibited AII-stimulated phagocytosis. AII analogs, with little-known agonist activity, blocked AII-stimulated actin polymerization and phagocytosis of RBC and RBC opsonized with IgG. These data support the contention that AII binds to AII receptors on the macrophage and stimulates phagocytosis possibly through polymerization of intracellular actin.

Activation of macrophage complement receptors for phagocytosis

Macrophage complement receptors, while innately incapable of promoting phagocytosis, can be activated to do so by a number of inflammatory stimuli and by several immunologic mechanisms. Studies with a complement receptor-activating lymphokine reveal that activation occurs as a result of mobilization of innately immobile receptors and suggest that receptor mobility is a prerequisite for phagocytosis. Since Fc receptors are susceptible to blockade by immune complexes at inflammatory sites, phagocytosis mediated by macrophage complement receptors may be of prime importance in vivo.

Fine structural aspects of phagocytotic activity in inverted follicle cells of cultured porcine thyroids

Inversion of thyroid follicles took place when they were isolated by collagenase and trypsin and cultured in suspension in Eagle's medium supplemented with 10% fetal calf serum without TSH. The apical surface facing the culture medium contained numerous microvilli and a central cilium, while the luminal surface became flattened. Phagocytotic activity by pseudopods was promoted after addition of TSH to the culture medium. When the inverted follicles were incubated in culture medium containing TSH (50 mU/ml) and human red blood cells, or TSH and polystyrene latex beads (2.02 micron in diameter) for 1-3 h, numerous red blood cells or latex beads respectively were observed to be taken up by the epithelial follicle cells by scanning electron microscopy, as well as conventional thin-section electron microscopy. These results show that the apical surface (culture medium side) of the epithelial cell of the cultured thyroid follicle whose polarity is reversed phagocytoses red blood cells and latex beads.

The identification and localization of actin and actin-like filaments in lactating guinea pig mammary gland alveolar cells

Cytochalasin B, a microfilament-altering drug, inhibits lactose synthesis in lactating guinea pig mammary gland [Biochim. Biophys. Acta 392:20, 1975] but not primary by inhibiting glucose transport [Eur. J. Cell Biol. 20:150, 1979]. In order to study the possible role of microfilaments in lactose synthesis and secretion, we isolated both the alveolar (milk-secreting) and myoepithelial (contractile) cells from lactating mammary gland. Light microscopy shows that the alveolar cell fraction (viability approximately 71%) is homogenous and that the cells retain strong polarity of secretory structures in the apical region. Two proteins were extracted from the alveolar cell fraction. One (mol wt 42,000) comigrates with skeletal muscle actin on SDS-PAGE gels. The other, a high-molecular-weight (180,000) protein (HMWP) may be analogous to actin-binding protein or clathrin. An extract from the myoepithelial cell fraction also contains a protein that comigrates with actin but no HMWP. Whole tissue extract contains the 42K protein, and a 185K HMWP. Examination of the alveolar cell extract by electron microscopic (EM) negative staining revealed meshworks of multistranded, interconnecting filaments, with attached globular structures (100-200 A) (possibly the HMWP) and single filaments (40-60 A diameter) branching off. To localize these filamentous structures in situ, whole tissue was glycerinated and incubated with rabbit skeletal muscle heavy meromyosin (HMM). Masses of filaments in myoepithelial cells served as convenient standards for HMM decoration. Decorated filaments have cross-arms or projections, unlike the narrow, smooth filaments of control tissue. Decorated filaments in alveolar cells are located beneath the plasma membrane, in close association with secretory vacuoles, and near the Golgi apparatus; filaments near the latter two are often oriented perpendicular to the plasma membrane. Microvesicles are embedded in meshworks under the plasmalemma and near the Golgi apparatus. Intermediate-sized (85-115 A diameter), non-decorated filaments diverge from the meshworks of decorated filaments. Micro-vesicles are associated with intermediate-sized filaments as well. The association of actin-like filaments with secretory vacuoles and microvesicles and their location in areas of the cell concerned with biosynthetic activities suggest a possible function in the intracellular transport of secretory products.

Interaction of alpha-actinin, filamin and tropomyosin with F-actin

The abilities of alpha-actinin, filamin and tropomyosin to bind F-actin were examined by cosedimentation experiments. Results indicated that smooth muscle alpha-actinin and filamin can bind to actin filaments simultaneously with little evidence of competition. In contrast, tropomyosin exhibits marked competition with either filamin or alpha-actinin for sites on actin filaments.

The detection of a contractile apparatus in murine multinucleate giant cells

Cinemicrographic investigations have shown multinucleate giant cells to be motile and contractile while immunofluorescent and biochemical studies have demonstrated the presence of actin-like proteins. These contractile proteins are concentrated in the centrospheric and perinuclear regions, while a more streaky distribution is present in the more peripheral parts of the polykaryon. This distribution corresponds with that of cytoplasmic filaments measuring 6-7 nm as seen by electron microscopy. The presence of similar proteins in macrophages, the precursor cells of multinucleate giant cells, indicates that these contractile proteins continue to be produced after macrophage fusion and that they are reorganised within the structural arrangements of the syncytium.