Filamin-actin interaction. Dissociation of binding from gelation by Ca2+-activated proteolysis

Filamin A Regulates Cardiovascular Remodeling

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

Myofibrillar Z-discs Are a Protein Phosphorylation Hot Spot with Protein Kinase C (PKCα) Modulating Protein Dynamics

The Z-disc is a protein-rich structure critically important for the development and integrity of myofibrils, which are the contractile organelles of cross-striated muscle cells. We here used mouse C2C12 myoblast, which were differentiated into myotubes, followed by electrical pulse stimulation (EPS) to generate contracting myotubes comprising mature Z-discs. Using a quantitative proteomics approach, we found significant changes in the relative abundance of 387 proteins in myoblasts versus differentiated myotubes, reflecting the drastic phenotypic conversion of these cells during myogenesis. Interestingly, EPS of differentiated myotubes to induce Z-disc assembly and maturation resulted in increased levels of proteins involved in ATP synthesis, presumably to fulfill the higher energy demand of contracting myotubes. Because an important role of the Z-disc for signal integration and transduction was recently suggested, its precise phosphorylation landscape further warranted in-depth analysis. We therefore established, by global phosphoproteomics of EPS-treated contracting myotubes, a comprehensive site-resolved protein phosphorylation map of the Z-disc and found that it is a phosphorylation hotspot in skeletal myocytes, underscoring its functions in signaling and disease-related processes. In an illustrative fashion, we analyzed the actin-binding multiadaptor protein filamin C (FLNc), which is essential for Z-disc assembly and maintenance, and found that PKCα phosphorylation at distinct serine residues in its hinge 2 region prevents its cleavage at an adjacent tyrosine residue by calpain 1. Fluorescence recovery after photobleaching experiments indicated that this phosphorylation modulates FLNc dynamics. Moreover, FLNc lacking the cleaved Ig-like domain 24 exhibited remarkably fast kinetics and exceedingly high mobility. Our data set provides research community resource for further identification of kinase-mediated changes in myofibrillar protein interactions, kinetics, and mobility that will greatly advance our understanding of Z-disc dynamics and signaling.

The Molecular Mechanisms of Calpains Action on Skeletal Muscle Atrophy

Skeletal muscle atrophy is associated with a loss of muscle protein which may result from both increased proteolysis and decreased protein synthesis. Investigations on cell signaling pathways that regulate muscle atrophy have promoted our understanding of this complicated process. Emerging evidence implicates that calpains play key roles in dysregulation of proteolysis seen in muscle atrophy. Moreover, studies have also shown that abnormally activated calpain results muscle atrophy via its downstream effects on ubiquitin-proteasome pathway (UPP) and Akt phosphorylation. This review will discuss the role of calpains in regulation of skeletal muscle atrophy mainly focusing on its collaboration with either UPP or Akt in atrophy conditions in hope to stimulate the interest in development of novel therapeutic interventions for skeletal muscle atrophy.

Actin crosslinking protein filamin A during early pregnancy in the rat uterus

During early pregnancy the endometrium undergoes a major transformation in order for it to become receptive to blastocyst implantation. The actin cytoskeleton and plasma membrane of luminal uterine epithelial cells (UECs) and the underlying stromal cells undergo dramatic remodelling to facilitate these changes. Filamin A (FLNA), a protein that crosslinks actin filaments and also mediates the anchorage of membrane proteins to the actin cytoskeleton, was investigated in the rat uterus at fertilisation (Day 1) and implantation (Day 6) to determine the role of FLNA in actin cytoskeletal remodelling of UECs and decidua during early pregnancy. Localisation of FLNA in UECs at the time of fertilisation was cytoplasmic, whilst at implantation it was distributed apically; its localisation is under the influence of progesterone. FLNA was also concentrated to the first two to three stromal cell layers at the time of fertilisation and shifted to the primary decidualisation zone at the time of implantation. This shift in localisation was found to be dependent on the decidualisation reaction. Protein abundance of the FLNA 280-kDa monomer and calpain-cleaved fragment (240 kDa) did not change during early pregnancy in UECs. Since major actin cytoskeletal remodelling occurs during early pregnancy in UECs and in decidual cells, the changing localisation of FLNA suggests that it may be an important regulator of cytoskeletal remodelling of these cells to allow uterine receptivity and decidualisation necessary for implantation in the rat.

An updated patent review of calpain inhibitors (2012 - 2014)

INTRODUCTION

Calpain is a family of cysteine proteases found in eukaryotes and a few bacteria. There is considerable interest in the search for calpain inhibitors because the enzyme has been implicated in several diseases including ocular disorders, neurodegenerative disorders, metabolic disorders and cancer.

AREAS COVERED

An overview of calpain inhibitors disclosed between 2012 and 2014 is presented. Among these are epoxysuccinates, dipeptide imaging agents, macrocyclic inhibitors, α-helical peptidomimetic inhibitors, carboxamides, 5-azolones and α-mercaptoacrylates. Additionally, preclinical studies of calpain inhibitors in pathologies such blood disorders, ocular disorders, neurological disorders and muscle disorders are discussed.

EXPERT OPINION

Major advances made in calpain inhibitor research between 2012 and 2014 include: i) the discovery of cytosolic-stable carboxamide calpain inhibitors; ii) synthesis of epoxysuccinates with excellent bioavailability; iii) disclosure of the X-ray crystal structures of novel α-mercaptoacrylates bound to the pentaEF hand region from human calpain; and iv) disclosure of calpain inhibitors as anti-sickling agents. Several calpain inhibitors were reported but limited effort was directed towards the discovery of calpain isoform selective agents, which continues to dampen the therapeutic potential of calpain inhibitors.

Predicting protein-protein interactions in the post synaptic density

The post synaptic density (PSD) is a specialization of the cytoskeleton at the synaptic junction, composed of hundreds of different proteins. Characterizing the protein components of the PSD and their interactions can help elucidate the mechanism of long-term changes in synaptic plasticity, which underlie learning and memory. Unfortunately, our knowledge of the proteome and interactome of the PSD is still partial and noisy. In this study we describe a computational framework to improve the reconstruction of the PSD network. The approach is based on learning the characteristics of PSD protein interactions from a set of trusted interactions, expanding this set with data collected from large scale repositories, and then predicting novel interaction with proteins that are suspected to reside in the PSD. Using this method we obtained thirty predicted interactions, with more than half of which having supporting evidence in the literature. We discuss in details two of these new interactions, Lrrtm1 with PSD-95 and Src with Capg. The first may take part in a mechanism underlying glutamatergic dysfunction in schizophrenia. The second suggests an alternative mechanism to regulate dendritic spines maturation.

Calpastatin overexpression in the skeletal muscle of mice prevents clenbuterol-induced muscle hypertrophy and phenotypic shift

Accumulating evidence suggests that the calpain/calpastatin system is involved in skeletal muscle remodelling induced by β(2) -adrenoceptor agonist treatment. In addition to other pathways, the Akt/mammalian target of rapamycin (mTOR) pathway, controlling protein synthesis, and the calcium/calmodulin-dependent protein kinase 2 (CamK2) and AMP-activated protein kinase (AMPK) pathways, recently identified as calpain substrates, could be relevant in β(2) -adrenoceptor agonist-induced skeletal muscle remodelling. In the present study we investigated muscle hypertrophy and phenotypic shifts, as well as the molecular response of components of the Akt/mTOR pathway (i.e. Akt, eukaryotic initiation factor 4E-binding protein 1 (4E-BP1), ribosomal protein S6 (rpS6), CamK2 and AMPK), in response to calpastatin overexpression in the skeletal muscle of mice treated with 1 mg/kg per day clenbuterol for 21 days. Using gene electrotransfer of a calpastatin expression vector into the tibialis anterior of adult mice, we found that calpastatin overexpression attenuates muscle hypertrophy and phenotypic shifts induced by clenbuterol treatment. At the molecular level, calpastatin overexpression markedly decreased calpain activity, but was ineffective in altering the phosphorylation of Akt, 4E-BP1 and rpS6. In contrast, calpastatin overexpression increased the protein expression of both total AMPK and total CamK2. In conclusion, the results support the contention that the calpain/calpastatin system plays a crucial role in skeletal muscle hypertrophy and phenotypic shifts under chronic clenbuterol treatment, with AMPK and CamK2 probably playing a minor role. Moreover, the calpastatin-induced inhibition of hypertrophy under clenbuterol treatment was not related to a decreased mTOR-dependent initiation of protein translation.

Time course in calpain activity and autolysis in slow and fast skeletal muscle during clenbuterol treatment

Calpains are Ca2+ cysteine proteases that have been proposed to be involved in the cytoskeletal remodeling and wasting of skeletal muscle. Cumulative evidence also suggests that β2-agonists can lead to skeletal muscle hypertrophy through a mechanism probably related to calcium-dependent proteolytic enzyme. The aim of our study was to monitor calpain activity as a function of clenbuterol treatment in both slow and fast phenotype rat muscles. For this purpose, for 21 days we followed the time course of the calpain activity and of the ubiquitous calpain 1 and 2 autolysis, as well as muscle remodeling in the extensor digitorum longus (EDL) and soleus muscles of male Wistar rats treated daily with clenbuterol (4 mg·kg–1). A slow to fast fiber shift was observed in both the EDL and soleus muscles after 9 days of treatment, while hypertrophy was observed only in EDL after 9 days of treatment. Soleus muscle but not EDL muscle underwent an early apoptonecrosis phase characterized by hematoxylin and eosin staining. Total calpain activity was increased in both the EDL and soleus muscles of rats treated with clenbuterol. Moreover, calpain 1 autolysis increased significantly after 14 days in the EDL, but not in the soleus. Calpain 2 autolysis increased significantly in both muscles 6 hours after the first clenbuterol injection, indicating that clenbuterol-induced calpain 2 autolysis occurred earlier than calpain 1 autolysis. Together, these data suggest a preferential involvement of calpain 2 autolysis compared with calpain 1 autolysis in the mechanisms underlying the clenbuterol-induced skeletal muscle remodeling.

Structural Basis for Vertebrate Filamin Dimerization

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

The calpain system

The calpain system originally comprised three molecules: two Ca2+-dependent proteases, mu-calpain and m-calpain, and a third polypeptide, calpastatin, whose only known function is to inhibit the two calpains. Both mu- and m-calpain are heterodimers containing an identical 28-kDa subunit and an 80-kDa subunit that shares 55-65% sequence homology between the two proteases. The crystallographic structure of m-calpain reveals six "domains" in the 80-kDa subunit: 1). a 19-amino acid NH2-terminal sequence; 2). and 3). two domains that constitute the active site, IIa and IIb; 4). domain III; 5). an 18-amino acid extended sequence linking domain III to domain IV; and 6). domain IV, which resembles the penta EF-hand family of polypeptides. The single calpastatin gene can produce eight or more calpastatin polypeptides ranging from 17 to 85 kDa by use of different promoters and alternative splicing events. The physiological significance of these different calpastatins is unclear, although all bind to three different places on the calpain molecule; binding to at least two of the sites is Ca2+ dependent. Since 1989, cDNA cloning has identified 12 additional mRNAs in mammals that encode polypeptides homologous to domains IIa and IIb of the 80-kDa subunit of mu- and m-calpain, and calpain-like mRNAs have been identified in other organisms. The molecules encoded by these mRNAs have not been isolated, so little is known about their properties. How calpain activity is regulated in cells is still unclear, but the calpains ostensibly participate in a variety of cellular processes including remodeling of cytoskeletal/membrane attachments, different signal transduction pathways, and apoptosis. Deregulated calpain activity following loss of Ca2+ homeostasis results in tissue damage in response to events such as myocardial infarcts, stroke, and brain trauma.

Molecular mechanisms involved in development of cerebral vasospasm

Object

Although the agents responsible for production of vasospasm have not yet been clearly identified, the author reviews the molecular mechanisms involved in development of vasospasm mainly based on the experimental data in a canine two-hemorrhage model.

Methods

The blood products after subarachnoid hemorrhage most likely stimulate many cell membrane receptors, such as G protein–coupled receptors and receptor tyrosine kinases, to activate the tyrosine kinase pathway of the vascular smooth muscle cells. The activation of the tyrosine kinase pathway is associated with continuous elevation of intracellular Ca++levels and activation of μ-calpain; the former may result mainly not from Ca++release but from Ca++influx from outside the cells. The increased intracellular Ca++concentrations stimulate Ca++/calmodulin (CaM)–dependent myosin light chain kinase to phosphorylate myosin light chain continuously during vasospasm. A topical application of genistein, ethylene-glycol-bis(β-aminoethylether) N,N'-tetraacetic acid, or various L-type Ca++channel blockers likely induces reversal of vasospasm as a result of a decrease in intracellular Ca++levels. The blood products also activate the rho/rho-associated kinase pathway during vasospasm most likely via G protein–coupled receptors, and the activated rho-associated kinase inhibits myosin phosphatase through phosphorylation at its myosin-binding subunit to induce Ca++-independent development of vasospasm. The enhanced generation of arachidonic acid during vasospasm may also contribute to inhibition of myosin phosphatase, at least in part, through the rho/rho-associated kinase pathway. The activity of myosin phosphatase in vasospam can also be inhibited by activated protein kinase C independently of the rho/rho-associated kinase pathway, but the inhibition may play a minor and transient role in contractile regulation. The protein levels of thin filament–associated proteins, calponin and caldesmon, are progressively decreased in vasospasm, whereas their phosphorylation levels are increased. Both changes probably contribute to the enhancement of smooth muscle contractility. Contractile and cytoskeletal proteins appear to be degraded in vasospasm by proteolysis with activated μ-calpain, suggesting that the intracellular devices responsible for smooth-muscle contraction are severely degraded in vasospasm.

Conclusions

It remains to be determined the extent to which Ca++-dependent and -independent contractile regulations, proteolysis and phosphorylation of thin filament–associated proteins, and degradation of contractile and cytoskeletal proteins are involved in the development of vasospasm.

Structural and functional aspects of filamins

Filamins are a family of high molecular mass cytoskeletal proteins that organize filamentous actin in networks and stress fibers. Over the past few years it has become clear that filamins anchor various transmembrane proteins to the actin cytoskeleton and provide a scaffold for a wide range of cytoplasmic signaling proteins. The recent cloning of three human filamins and studies on filamin orthologues from chicken and Drosophila revealed unexpected complexity of the filamin family, the biological implications of which have just started to be addressed. Expression of dysfunctional filamin-A leads to the genetic disorder of ventricular heterotopia and gives reason to expect that abnormalities in the other isogenes may also be connected with human disease. In this review aspects of filamin structure, its splice variants, binding partners and biological function will be discussed.

Human beta-filamin is a new protein that interacts with the cytoplasmic tail of glycoprotein Ibalpha

We have cloned and sequenced a 9.4-kilobase cDNA specifying a new 280-kDa protein interacting with the cytoplasmic tail of glycoprotein (Gp) Ibalpha and showing considerable homology to actin-binding protein 280 (ABP-280) and chicken retinal filamin. We term this protein human beta-filamin. The gene for beta-filamin localizes to chromosome 3p14.3-p21.1. beta-Filamin mRNA expression was observed in many tissues and in cultured human umbilical vein endothelial cells (HUVECs); only minimal expression was detected in platelets and the megakaryocytic cell line CHRF-288. Like ABP-280, beta-filamin contains an NH2-terminal actin-binding domain, a backbone of 24 tandem repeats, and two "hinge" regions. A polyclonal antibody to the unique beta-filamin first hinge sequence identifies a strong 280-kDa band in HUVECs but only a weak band in platelets, and stains normal human endothelial cells in culture and in situ. We have confirmed the interaction of beta-filamin and GpIbalpha in platelet and HUVEC lysates. In addition, using two-hybrid analysis with deletion mutants, we have localized the binding domain for GpIbalpha in beta-filamin to residues 1862-2148, an area homologous to the GpIbalpha binding domain in ABP-280. beta-Filamin is a new member of the filamin family that may have significance for GpIbalpha function in endothelial cells and platelets.

Properties of the novel intermediate filament protein synemin and its identification in mammalian muscle

We examined specific properties of highly purified synemin (230 kDa), recently identified as a novel intermediate filament (IF) protein, from avian smooth muscle. Soluble synemin in 10 mM Tris-HCl, pH 8.5, appears as approximately 11-nm-diameter globular structures by negative-stain and low-angle shadow electron microscopy. Chemical crosslinking and SDS-PAGE analysis indicate that soluble synemin molecules contain two 230-kDa subunits. The pH- and ionic strength-dependent solubility properties of synemin are similar to those of the type III IF protein desmin, but under physiological-like conditions in which desmin self-assembles into long approximately 10-nm-diameter IFs, synemin self-associates into complex, approx 15- to 25-nm-diameter globular structures. Calpain digestion demonstrated that synemin is extremely proteolytically labile. Western blot analysis, with monospecific polyclonal antibodies against avian synemin, shows the presence of the reactive 230-kDa synemin band in samples of adult avian skeletal, cardiac, and smooth muscle and of two reactive bands at approximately 225 kDa (major) and approximately 195 kDa in adult porcine skeletal, cardiac, and smooth muscle. Partial purification of synemin from porcine smooth muscle also resulted in fractions highly enriched in the approximately 225- and approximately 195-kDa polypeptides. Conventional immunofluorescence and immunoconfocal microscopy of isolated myofibrils and of frozen sections also demonstrated, for the first time, that synemin is present in all three adult porcine muscle cell types and is colocalized with desmin in skeletal and cardiac muscle cells at the myofibrillar Z-lines.

Expression and subcellular distribution of filamin isotypes in endothelial cells and pericytes

Two principal forms of the actin binding protein, filamin, are expressed in mammalian cells: nonmuscle and muscle isotypes (FLN-1 and FLN-2). A protein that copurifies with an alpha-naphthyl acetate hydrolyzing esterase from human omentum microvessel endothelial cells (EC) is isolated by nondenaturing electrophoresis, sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis and electroblotting. The purified protein is subjected to in situ trypsin cleavage, reversed-phase high performance liquid chromatography (HPLC) and automated Edman degradation. Six peptide fragments from the protein are identified to have 60-66% identity with nonmuscle filamin (ABP-280). Two of these peptides are 100% identical to a previously sequenced human muscle filamin fragment. Polyclonal antibody is produced using a 16-residue synthetic peptide corresponding to a structural beta-sheet region of muscle filamin. Compared with a variety of vascular cells evaluated, retinal pericytes express an abundance of both muscle and non-muscle filamin isotypes. Pericytes contain at least 10 times more muscle filamin than human umbilical vein EC and at least three times the amount expressed in human omentum microvessel and bovine pulmonary artery EC. Differential detergent fractionation indicates that both filamin isotypes are primarily localized in the cytosol and membrane/organelle fractions of pericytes. Another actin crosslinking protein, alpha-actinin, is primarily found in the cytosol and cytoskeletal fractions. The dynamic regulation of actin microfilament organization in pericytes may be controlled in part by the two filamin isotypes, which in turn may contribute to pericyte contractility.

Actin filament cross-linking by chicken gizzard filamin is regulated by phosphorylation in vitro

Filamin is a dimeric muscle phosphoprotein that cross-links actin filaments. We have found that purified chicken gizzard filamin is phosphorylated in vitro at serine residues by the Ca2+/calmodulin-dependent protein kinase II (CaM kinase II). Up to 0.9 mol of phosphate can be incorporated into 1 mol of filamin dimer. Phosphorylation by CaM kinase II increases filamin's critical actin filament gelling concentration and diminishes the amount of actin sedimented by filamin at low G-force. The modulation of filamin function by CaM kinase II requires ATP, Ca2+, and calmodulin, and it is abolished when CaM kinase II is inactivated with heat. Protein phosphatase 2A removed the phosphate added by CaM kinase II and restored filamin's actin filament cross-linking activity to the untreated basal level. In cosedimentation experiments, phosphorylation reduces the binding of filamin to actin filaments. The Kd for binding of filamin to actin filaments increases approximately 2-fold, from 3.2 to 6.9 microM, following CaM kinase II-mediated phosphorylation. Phosphorylation by CaM kinase II, therefore, regulates the binding of filamin to actin filaments.

Filamin and gelsolin influence Ca(2+)-sensitivity of smooth muscle thin filaments

Sheep aorta thin filaments were prepared by ultracentrifugation of an ATP-containing extract in the presence of different concentrations of ethanediol. Thin filaments prepared without ethanediol contained small quantities of tropomyosin (0.027 Tm:actin) and caldesmon (0.017 CD:actin) and activated the MgATPase of skeletal myosin independently of Ca2+. Ultracentrifugation in the presence of 10-20% ethanediol resulted in preparation of thin filaments with increased content of tropomyosin (0.17 Tm:actin) and caldesmon (0.04 CD:actin). These thin filaments possessed high Ca(2+)-sensitivity in activation of skeletal muscle myosin ATPase. Besides actin, tropomyosin and caldesmon, thin filaments contained gelsolin and filamin. Gelsolin content (0.007 gelsolin:actin) was independent of the presence of ethanediol. The filamin content decreased from 0.015 to 0.007 mol:mol actin when the ethanediol concentration was increased from 0 to 20%, and was negatively correlated with the Ca2+ sensitivity of thin filaments. In a reconstituted system, pure filamin or gelsolin affected caldesmon regulation of actomyosin ATPase. Gelsolin (0.01:actin) reduced the inhibition of actomyosin ATPase caused by caldesmon and increased the potency of Ca(2+)-calmodulin in reversing this inhibition. Filamin (0.007:actin) also decreased the inhibitory action of caldesmon on actin-activated myosin ATPase and also potentiated the reversal of this inhibition by calmodulin. We conclude that minor components of smooth muscle thin filaments (gelsolin and filamin) significantly modify caldesmon mediated regulation of actomyosin ATPase. We suggest a tropomyosin-mediated mechanism by which filamin or gelsolin could exert similar effects.

Different temporal patterns of expression result in the same type, amount, and distribution of filamin (ABP) in cardiac and skeletal myofibrils

The morphogenesis of functional myofibrils in chick skeletal and cardiac muscle occurs in greatly different time spans, in about 7 and 2 days, respectively. In chick skeletal myogenic cells, one isoform of the 250 kD actin-binding protein (ABP) filamin is associated with stress fiber-like structures of myoblasts and early myotubes, then disappears for approximately 4 days, whereupon a second filamin isoform reappears at the Z-disc periphery. We sought to determine if cardiac myogenesis involves this sequence of appearance, disappearance, and reappearance of a new filamin isoform in a compressed time scale. It was known that in mature heart, filamin is localized at the Z-disc periphery as in mature (fast) skeletal muscle, and is also associated with intercalated discs. We find that myocardial filamin has an apparent molecular weight similar to that of adult skeletal muscle filamin and lower than that of smooth muscle filamin, and that both skeletal and cardiac muscle contain roughly 200 filamin monomers per sarcomere. Two-dimensional peptide mapping shows that myocardial filamin is very similar to skeletal muscle filamin. Myocardial, slow skeletal, and fast skeletal muscle filamins are all phosphorylated, as previously shown for filamin of non-striated muscle. Using immunofluorescence, we found that filamin could not be detected in the developing heart until the 14-somite stage, when functional myofibrils exist and the heart has been beating for 3 to 4 hours. We conclude that in cardiac and skeletal myogenesis, different sequences of filamin gene expression result in myofibrils with similar filamin distributions and isoforms.

Immunoblotting of contractile and cytoskeletal proteins of canine basilar artery in vasospasm

Vasospasm was produced in the canine basilar arteries by a two-hemorrhage method, and voltage- and receptor-dependent contractions of the normal canine basilar arteries were induced by local applications of potassium chloride (KCI) and serotonin, respectively, after transclival exposure. Actin, myosin, desmin, filamin, talin, vinculin, and alpha-actinin in the basilar artery were studied by immunoblotting. The immunoblots showed a decrease or loss in immunoreactivity of some native proteins and generation of protein fragments, smaller in size than native proteins, in spastic, KCI, and serotonin groups, indicating a proteolytic degradation. In the spastic group on Day 2, actin, desmin, and filamin were usually degraded slightly; myosin moderately; and talin and alpha-actinin substantially. Vinculin and metavinculin remained intact. In the spastic group on Day 7, actin and desmin were usually decomposed slightly; myosin, filamin, and vinculin substantially; and talin, metavinculin, and alpha-actinin markedly. In the KCI and serotonin groups, slight degradation was usually observed in filamin, often in alpha-actinin, and occasionally in actin, whereas desmin, vinculin, and metavinculin were not degraded. In addition, myosin was usually degraded moderately in the KCI group and slightly in the serotonin group, and talin was generally decomposed slightly in the KCI group and moderately in the serotonin group. The degraded fragments, although variable in number and immunoreactivity, were similar in size in the three groups. We suggest that the intracellular devices responsible for contraction of the basilar arteries are degraded more severely in the spastic group than in the KCI or serotonin group, probably by similar proteolytic mechanism and progressively with the passage of time after subarachnoid hemorrhage in vasospasm.

Calpain-calpastatin system of canine basilar artery in vasospasm

Vasospasm was produced in the canine basilar artery by a two-hemorrhage method, while contraction was induced in the normal canine basilar artery by a local application of KCl or serotonin after transclival exposure. The control animals were injected with saline instead of fresh blood. The activation of mu-calpain, a Ca(++)-dependent neutral protease, in the basilar artery was studied by evaluating the conversion from its inactivated into its activated form on immunoblots. In addition, the activity of calpastatin, an intrinsic inhibitor of calpain, in the basilar artery was determined by assay. The majority of the mu-calpain was inactivated in the control group. In the spastic group, mu-calpain was generally activated markedly in the early stage of vasospasm and moderately thereafter. The contraction induced by KCl or serotonin application was classified into the early phasic and the later tonic stages; mu-calpain was usually activated in the phasic stage and inactivated in the tonic stage. Calpastatin activity was significantly decreased during vasospasm, whereas it was not significantly changed in KCl- or serotonin-induced contraction. The final activity of mu-calpain results from the balance of mu-calpain and calpastatin. This suggests that mu-calpain activity was enhanced continuously in the spastic group and transiently in the KCl or serotonin group, and that the continuous activation of mu-calpain during vasospasm probably induced more proteolytic changes compared to those in the KCl or serotonin group.

Localization of two myosin-subfragment-1 binding contacts in the 96-132 region of actin subdomain-1

Many direct observations and indirect experimental approaches have pin-pointed two segments (sequences 1-28 and 360-372) in actin subdomain-1 which bind to myosin subfragment-1. In a previous investigation [Labbé, J. P., Méjean, C., Benyamin, Y. & Roustan, C. (1990) Biochem. J. 271, 407-413], we have observed competition between myosin subfragment-1 and anti-actin antibodies specific to epitopes including Thr103. A multisite interface model has also been proposed to take into account myosin-head binding to the N-terminal and C-terminal regions and to more central 40-113 sequence of actin. In the present study, two limited actin segments encompassing residues 96-103 and 112-125 were identified as myosin-head-binding sites. Myosin subfragment-1 competed for monomeric actin with the antibodies directed against sequences 96-105 and 114-120 and its binding to the tryptic 96-113 and synthetic 112-125 actin peptides was prevented by magnesium pyrophosphate but not by calcium pyrophosphate. In the presence of ATP-Mg2+, myosin subfragment-1 was dissociated by filamin from its complex with monomeric actin or with peptide 105-120. Contact points of filamin on actin were previously located in the 105-120 and 360-372 actin sequences [Méjean, C., Lebart, M. C., Boyer, M., Roustan, C. & Benyamin, Y. (1992) Eur. J. Biochem. 209, 555-562]. The in vitro inhibitory effect of filamin on actin-activated Mg2+-ATPase would thus be explained by this competition. Furthermore, the (27-kDa-50-kDa-20-kDa) trypsin-split myosin subfragment-1 which could no longer be activated by actin, did not bind at all to the two sites located in the 96-125 region, but it still interacted with the 360-372 segment. Our results regarding the position of the myosin head on actin monomers in rigor conditions provide evidence on the presence of two topologically independent contact points in the myosin-head/actin interface. One group exposed residues in the 1-7, 21-29, 77-95 and 96-103 actin segment, another, on the opposite side of subdomain-1, included residues from 112-125 and 360-372 sequences.

Cell-penetrating inhibitors of calpain block both membrane fusion and filamin cleavage in chick embryonic myoblasts

Benzyloxycarbonyl(Z)-Leu-nLeu-H (calpeptin) and Z-Leu-Met-H, cell-penetrating inhibitors of calpain, were found to block myoblast fusion without any effect on cell proliferation and alignment along their bipolar axis. They also inhibited the accumulation of creatine kinase during myogenesis. These effects were dose-dependent, and could be reversed upon removal of the drug from the culture medium. Furthermore, treatment of the inhibitors prevented the hydrolysis of filamin, which is sensitive to cleavage by calpain in vitro and interferes with actin-myosin filament formation by cross-linking F-actin molecules. On the other hand, leupeptin, which can also inhibit calpain in vitro but can not penetrate into cells, showed little or no effect on both myoblast fusion and filamin clevage. These results suggest that calpain may play an important role in cytoskeletal reorganization that is requisite for myoblast fusion. The role of calpain on the expression of muscle-specific proteins remains unknown.

Translocation of spin-labeled phospholipids through plasma membrane during thrombin- and ionophore A23187-induced platelet activation

After incorporation of spin-labeled phosphatidylcholine, phosphatidylserine, and phosphatidylethanolamine analogues in the outer leaflet of the plasma membrane in resting platelets, more than 90% amino-head analogues accumulated within 30 min in the inner leaflet by aminophospholipid translocase activity, while choline analogues mostly remained on the outer leaflet. Platelets were then activated by thrombin or Ca2+ ionophore A23187. No outward movement of internally located spin-labeled aminophospholipids was observed during thrombin-induced activation, whereas the influx of externally located probes increased slightly. During A23187-mediated activation, similar slightly increased influx was observed, while 40-50% of the initially internally located aminophospholipids could then be extracted from the outer leaflet. This sudden exposure on the outer face was dependent on an increase in intracellular Ca2+ and achieved in less than 2 min at 37 degrees C. Inhibition of translocase activity by N-ethylmaleimide did not induce any aminophospholipid outflux. When probes were incorporated on the outer face of the plasma membrane in resting platelets, they were still fully accessible from the extracellular medium after A23187-induced activation. Moreover, they were distributed between the vesicles and remnant platelets in proportion to the external membrane phospholipidic content in each structure. This suggested that no scrambling of plasma membrane leaflets occurred during the vesicle blebbing. Moreover, the spin-labeled aminophospholipids exposure rate and amplitude were unchanged when vesicle formation was inhibited by the calpain inhibitor calpeptin. These results indicate that loss of asymmetry thus inducing generation of a catalytic surface is not the consequence of vesicle formation. Conversely, we propose that vesicle shedding is an effect of PL transverse redistribution and calpain-mediated proteolysis during activation.

Increase in the level of m-calpain correlates with the elevated cleavage of filamin during myogenic differentiation of embryonic muscle cells

The activity of Ca(2+)-activated proteinase requiring millimolar Ca2+ (m-calpain) was found to increase dramatically in cultured chick embryonic myoblasts during the early period of myogenic differentiation. Furthermore, the protein level of m-calpain also markedly increased in parallel with the rise in its activity, and both remained elevated thereafter. On the other hand, the activity level of calpastatin, an endogenous inhibitor of the proteinase, remained similar during the entire period of the culture. In addition, the activity of Ca(2+)-activated proteinase requiring micromolar Ca2+ (mu-calpain) was not detected in either proliferating or differentiated myoblasts. Thus, the overall capacity of Ca(2+)-dependent proteolysis is likely to increase in differentiating myoblasts and should be contributed by m-calpain. Filamin (250 kDa), that is known to facilitate actin microfilament assembly and interfere with actin-myosin filament formation, was found to be cleaved in cultured myoblasts to 240 kDa products. This filamin-cleavage occurred in a manner similar to the in vitro cleavage of the cytoskeletal protein by the purified m-calpain. Moreover, the filamin-cleavage was most evident at the period of the cell fusion. Thus, it seems likely that the in vivo cleavage of filamin is mediated by m-calpain. These results suggest that m-calpain may play an important role in cytoskeletal reorganization that is requisite for myoblast fusion.

Localization and identification of actin structures involved in the filamin-actin interaction

The interface between gizzard filamin and skeletal muscle actin was located on the actin monomer. Conserved sequences 105-120 and 360-372, in the actin subdomain 1 near the myosin binding sites, were involved in this interaction. The corresponding peptides for these sequences were each found to bind filamin and compete in the actin-filamin interaction. When these two peptides were used together in the presence of filamin and filamentous actin, they dissociated sedimentable complexes formed by these two proteins.

Inositol phospholipid-induced suppression of F-actin-gelating activity of smooth muscle filamin

Filamin, a high molecular weight actin-binding protein, cross-links actin filaments and produces a gel composed of F-actin. The effects of polyphosphoinositides on the gelating activity of smooth muscle filamin were examined by measuring the low shear viscosity of the F-actin solutions containing filamin incubated with phosphatidylinositol (PI), phosphatidylinositol 4-monophosphate (PIP), or phosphatidylinositol 4,5-bisphosphate (PIP2). Micelles of these inositol phospholipids bound to filamin inhibited the ability to form a gel of F-actin. The inhibiting activity of each phospholipid was in the following order, PIP2 greater than PIP greater than PI. The F-actin binding assay of filamin revealed that the inhibition of F-actin-gelation resulted in the loss of the F-actin-binding activity of filamin. Thus, polyphosphoinositides may play important roles in regulating the gelating activity of filamin.

Ultrastructural and immunocytochemical analysis of the circumferential microfilament bundle in avian retinal pigmented epithelial cells in vitro

The dedifferentiated phenotype of pigmented epithelial cells in vitro is bipotential and is effected by environmental alterations mediated by the cell surface and associated cytoskeleton. We have begun an investigation into the role that contractile microfilaments play in maintaining cell contact and cell shape in retinal pigmented epithelial cells in vitro. In this paper, we report a structural analysis of the intersection of the circumferential microfilament bundle with the cell membrane of cultured pigmented epithelial cells from chick retina. Techniques of electron microscopy, including freeze-fracturing and deep-etching, reveal that microfilaments of this bundle associate with a junctional complex in the apical cell compartment and with membrane domains which are not components of the junction. Microfilaments link with the cell membrane either at their termini or along the membrane-apposed surface of the circumferential bundle. Furthermore, we report the immunocytochemical localization of filamin (a high molecular weight actin-binding protein, which forms fiber bundles and sheet-like structures when bound with F-actin in solution) in the circumferential/microfilament bundle.

Polarity and length of actin filaments at the fascia adherens of the cardiac intercalated disk

Digestion of canine and bovine intercalated disks with a calcium-activated protease (CAF) removes the electron-dense material similar to that found at the Z-line and presumably consisting primarily of alpha-actinin. The major filaments exposed by CAF are actin, and the polarity is away from the intercalated disk, as was confirmed by decoration with heavy meromyosin. The length of actin filaments associated with the fascia adherens region at the concave region is 1.2- to 2.2-fold that of actin filaments (I-filaments) in the sarcomere and varies depending on the interdigitation of the membrane at the cell junction. Actin filaments at the intercalated disk seem to be attached (or very close) to the membrane in a direct, rather than looping, manner.

Calpain II involvement in mitosis

Mitotic spindle disassembly requires major structural alterations in the associated cytoskeletal proteins and mitosis is known to be associated with Ca2+-sequestering phenomena and calcium transients. To examine the possible involvement of a ubiquitous Ca2+-activated protease, calpain II, in the mitotic process, synchronized PtK1 cells were monitored by immunofluorescence for the relocation of calpain II. The plasma membrane was the predominant location of calpain II in interphase. However, as mitosis progressed, calpain II relocated to (i) an association with mitotic chromosomes, (ii) a perinuclear location in anaphase, and (iii) a mid-body location in telophase. Microinjection of calpain II near the nucleus of a PtK1 cell promoted the onset of metaphase. Injection of calpain II at late metaphase promoted a precocious disassembly of the mitotic spindle and the onset of anaphase. These data suggest that calpain II is involved in mitosis.

Actin-binding and dimerization domains of HeLa cell filamin

HeLa cell filamin is a linear, bivalent, homodimer of high molecular weight subunits (Mr 250,000 that may cross-link actin filaments in vivo into supramolecular structures such as networks and bundles. We used millimolar Ca protease from chicken breast muscle to cleave the subunit into smaller fragments that we mapped with respect to the overall structure of the dimer. The enzyme cleaves HeLa filamin into a larger (Mr 192,000) and a smaller (Mr 104,000) fragment; the smaller fragment is the precursor of a still smaller (Mr 92,000) fragment. Only the larger fragment bound to actin in a cosedimentation test, suggesting that it contains the actin-binding region of the subunit. Digestion of HeLa filamin that had been cross-linked with dimethyl adipimidate produced a good yield of the Mr 192,000 fragment but a poor yield of the Mr 104,000/92,000 fragments. Since native filamins are head-to-head dimers, it was expected that cross-linking would proceed most readily at the dimerization site and, therefore, it appears that the Mr 192,000 fragment is cleaved from cross-linked filamin because it is distal to the dimerization region, while the Mr 104,000/92,000 fragments are absent because they lie at the dimerization region and were cross-linked to a form that was not identifiable by sodium dodecyl sulfate electrophoresis.

Binding and distribution of fluorescently labeled filamin in permeabilized and living cells

This study reports the first development of a fluorescently labeled filamin. Smooth muscle filamin was labeled with fluorescent dyes in order to study its interaction with stress fibers and myofibrils, both in living cells and in permeabilized cells. The labeled filamin bound to the Z bands of isolated cross-striated myofibrils and to the Z bands and intercalated discs in both permeabilized embryonic cardiac myocytes and in frozen sections of adult rat ventricle. In permeabilized embryonic chick myotubes, filamin bound to early myotubes but was absent at later stages. In living embryonic chick myotubes, the fluorescently labeled filamin was incorporated into the Z bands of myofibrils during early and late stages of development but was absent during an intermediate stage. In living cardiac myocytes, filamin-IAR was incorporated into nascent as well as fully formed sarcomeres throughout development. In permeabilized nonmuscle cells, labeled filamin bound to attachment plaques and foci of polygonal networks and to the dense bodies in stress fibers. The periodic bands of filamin in stress fibers had a longer spacing in fibroblasts than in epithelial cells. When injected into living cells, filamin was readily incorporated into stress fibers in a striated pattern. The fluorescent filamin bands were broader in injected cells, however, than they were in permeabilized cells. We have interpreted these results from living and permeabilized cells to mean that native filamin is distributed along the full length of the actin filaments in the stress fibers, with a higher concentration present in the dense bodies. A sarcomeric model is presented indicating the position of filamin with respect to other proteins in the stress fiber.

Possible role of calpain I and calpain II in differentiating muscle

The variable distribution of the 80-kD subunit of two calcium-activated proteases, calpain I and calpain II, has been examined in L8 and L6 myoblasts, and their non-fusing variants, fu-1 and M3A using non-cross-reacting monoclonal antibodies to both subunits. Immunofluorescence results have shown that while the 80-kD subunit of calpain I is localized in the cytoplasm of all the myoblasts, the 80-kD subunit of calpain II appears to be predominantly associated with the plasma membranes of L8 and L6 myoblasts. The distribution of the 80-kD subunit of calpain II in non-fusing myoblasts, fu-1 and M3A, is generally cytoplasmic and diffuse. Immunoblot analysis of membrane and cytosol fractions of all the myoblasts using the monoclonal antibodies described above essentially confirms the immunofluorescence findings. Because calpain II exhibits a peripheral distribution in cells which are fusion-competent, L6 and L8 myoblasts, but not in fu-1 and M3A myoblasts, we suggest that calpain II may play a role in the Ca2+-mediated fusion events of differentiating (prefusion) myoblasts.

Role of Ca2+ and Ca2+-activated protease in myoblast fusion

In this report, we have examined the effects of a calcium chelator, EGTA, and a calcium ionophore, A23187, on fusion of a cloned muscle cell line, L6. Our results confirm that EGTA essentially blocks all myoblast fusion because the lateral alignment of presumptive myoblasts cannot occur in the absence of extracellular calcium. A23187, however, promotes the precocious fusion of myoblasts, apparently by facilitating Ca2+ transport into myoblasts. We have also demonstrated that a Ca2+-activated protease, CAF (mM), appears to relocate in response to the Ca2+ flux, changing from a random, dispersed distribution in proliferative myoblasts to a predominantly peripheral distribution in prefusion myoblasts. Coincident with the mM CAF relocation is an altered distribution of a surface glycoprotein, fibronectin. Extracellular fibronectin is seen in abundance in proliferating myoblasts, but is essentially absent from the surface of fusing myoblasts. We suggest that mM CAF when activated by Ca2+ influx may act to promote the release of fibronectin from the myoblast cell surface, thus providing a mechanism by which the membrane of the fusing myoblast may be rearranged to accommodate fusion.

The thin filaments of smooth muscles

Contraction in vertebrate smooth and striated muscles results from the interaction of the actin filaments with crossbridges arising from the myosin filaments. The functions of the actin based thin filaments are (1) interaction with myosin to produce force; (2) regulation of force generation in response to Ca2+ concentration; and (3) transmission of the force to the ends of the cell. The major protein components of smooth muscle thin filaments are actin, tropomyosin and caldesmon, present in molar ratios of 28:4:1 respectively. Other smooth muscle proteins which may be associated with the thin filaments in the cell are filamin, vinculin, alpha-actinin, myosin light chain kinase and calmodulin. We have reviewed the structural and functional properties of these proteins and where possible we have suggested what their function and mechanism of action may be. We propose that actin and tropomyosin are involved in the force producing interaction with myosin, and that this interaction is controlled by a Ca2+-dependent mechanism involving caldesmon, tropomyosin and calmodulin. Vinculin, alpha-actinin and filamin appear to be involved in the attachment of the thin filaments to the cell membrane and their spatial organization within the cell. We conclude that the filaments of smooth muscles share many common properties with those from skeletal muscle, but that they are also quite distinct in terms of both their caldesmon based regulatory mechanism and their mode of organization into a contractile apparatus.

The effects of a calcium dependent protease on the ultrastructure and contractile mechanics of skinned uterine smooth muscle

In situ substrates for a vascular smooth muscle calcium-dependent protease (CDP) were investigated using a chemically skinned uterine smooth muscle preparation. Treatment of skinned smooth muscles with CDP had no effect on the total content of actin and myosin. Electron microscopical observations demonstrated that membrane plaques, cytoplasmic dense bodies, and intermediate filaments were all degraded by CDP. In addition, CDP reduced both isometric force and isotonic shortening velocity of contracted muscles in a concentration and time-dependent manner. Treatment of contracting muscles with CDP resulted in a condensation of myofilaments away from the plasma membrane concurrent with the loss of contractility. The condensation of myofilaments was ATP-dependent and could be inhibited by removal of ATP prior to proteolysis. The effects of proteolysis on smooth muscle ultrastructure and contractility support previously proposed models which assign a role to cytoskeletal elements in coordinating the molecular interaction of actomyosin to produce muscle contraction. The loss of cytoskeletal structures following protease treatment suggests that one of the functions of CDP in smooth muscle may be the disassembly of the cell cytoskeleton.

Reduction of filamin in late passage human diploid fibroblasts (IMR-90)

Progressive subcultivation of IMR-90 cells results in non-proliferative, heterogeneous cultures which may reflect aging of the diploid line (Hayflick, Exp. Cell Res., 37 (1965) 614). We have observed that late passage cells exhibit different rates of spreading and morphogenesis when compared to early passage groups, phenomena which we attribute to altered reassembly of the cytoskeleton in senescent cells (Kelley et al. Mech. Ageing Dev., 13 (1980) 127). To determine whether potential differences in cytoskeletal proteins develop with progressive subcultivation, early and late passage cultures were extracted with 0.5% Triton X-100 for 1 min followed by 1.0% sodium dodecyl sulfate (SDS) prior to separation and characterization of extracted proteins by electrophoresis on 7.5-15% gradient SDS gels. Extractions were made of both culture groups 3, 6 and 24 h after reseeding. Cytoskeletal ultrastructure at each stage of spreading was examined either in replicas of extracted cells or directly by scanning electron microscopy. Although considerable variation in cytoskeletal organization was observed, qualitative differences in gel banding patterns of actin, myosin and tubulin were not apparent at selected time points. However, late passage cells at 6 h and 24 h did not exhibit filamin associated with the Triton insoluble fraction as did early passage cells. Since it has been demonstrated that filamin is capable of cross-linking actin microfilaments into bundles or sheets, we suggest that it is a principal element for the variant cell shape and cytoskeletal morphology observed during altered spreading behavior of late passage human diploid fibroblasts.

Dual effect of filamin on actomyosin ATPase activity

Filamin binds to F-actin and influences the myosin-actin interaction. At relatively low concentrations, filamin activates actomyosin Mg2+-ATPase, whereas higher concentrations of filamin exert an inhibitory effect. Activation of ATPase activity occurs under conditions where a loose meshwork of actin filaments is present and inhibition is associated with the appearance of closely apposed bundles of actin filaments. Maximum activation (about fourfold) of actomyosin ATPase activity by filamin occurs between 30 and 65 mM KCl, at pH 6.5, and at temperatures not less than 30 degrees C. ATPase activation requires higher concentrations of filamin in the presence than in the absence of tropomyosin. Filamin does not activate Mg2+-ATPase activity of acto-subfragment-1 and has only a slight effect on the Mg2+-ATPase of acto-heavy meromyosin, but it inhibits the activity of both these systems under conditions similar to those that inhibit actomyosin ATPase activity.

Evidence for membrane-associated calpain I in human erythrocytes. Detection by an immunoelectrophoretic blotting method using monospecific antibody

Low and high Ca2+-requiring forms of Ca2+-dependent cysteine proteinase are known as calpain I and calpain II, respectively. We have obtained, for the first time, monospecific antibodies for calpain I and for calpain II. Using these antibodies and an electrophoretic blotting method, we have found that a small, but reproducible, amount of calpain I was associated with human erythrocyte membranes while the bulk of the protease was contained in the cytosol. Most of membrane-associated calpain I was extractable with 1% Triton X-100, but not with 0.1% detergent. In the presence of 0.1 mM Ca2+ and 5 mM cysteine, membrane-associated calpain I degraded the membrane protein band 4.1 preferentially and band 3 protein only slowly. The Ca2+-induced autodigestion of the membrane preparation was inhibited by leupeptin but not by a cytosolic calpain inhibitor, calpastatin, added to the incubation medium. No calpain II was detected in either erythrocyte cytosol or membranes when anti-calpain II antibody was used under the same conditions as those for the detection of calpain I.