Purification of native myosin filaments from muscle

Analysis of the structure and function of native thick (myosin-containing) filaments of muscle has been hampered in the past by the difficulty of obtaining a pure preparation. We have developed a simple method for purifying native myosin filaments from muscle filament suspensions. The method involves severing thin (actin-containing) filaments into short segments using a Ca(2+)-insensitive fragment of gelsolin, followed by differential centrifugation to purify the thick filaments. By gel electrophoresis, the purified thick filaments show myosin heavy and light chains together with nonmyosin thick filament components. Contamination with actin is below 3.5%. Electron microscopy demonstrates intact thick filaments, with helical cross-bridge order preserved, and essentially complete removal of thin filaments. The method has been developed for striated muscles but can also be used in a modified form to remove contaminating thin filaments from native smooth muscle myofibrils. Such preparations should be useful for thick filament structural and biochemical studies.

Rational design of diflunisal analogues with reduced affinity for human serum albumin

Many lead compounds bind to serum albumin and exhibit markedly reduced efficacy in vivo as compared to their potency in vitro. To aid in the design of compounds with reduced albumin binding, we performed nuclear magnetic resonance (NMR) structural and binding studies on the complex between domain III of human serum albumin (HSA-III) and diflunisal, a cyclooxygenase inhibitor with antiinflammatory activity. The structural studies indicate that the aromatic rings of diflunisal are involved in extensive and specific interactions with hydrophobic residues that comprise the binding pocket in subdomain IIIA. The carboxylic acid of diflunisal forms electrostatic interactions with the protein similar to those observed in the X-ray structure of HSA complexed to myristic acid. In addition to the structural studies, NMR-derived binding constants were obtained for diflunisal and closely related analogues to develop a structure-affinity relationship for binding to subdomain IIIA. On the basis of the structural and binding data, compounds were synthesized that exhibit more than a 100-fold reduction in binding to domain III of HSA, and nearly a 10-fold reduction in affinity for full length albumin. Significantly, several of these compounds maintain activity against cyclooxygenase-2. These results suggest a rational strategy for designing out albumin binding in potential drug molecules by using structure-based design in conjunction with NMR-based screening.

The cytoprotective effects of the glycoprotein 130 receptor-coupled cytokine, cardiotrophin-1, require activation of NF-kappa B

Many cell types mount elaborate, compensatory responses to stress that enhance survival; however, the intracellular signals that govern these responses are poorly understood. Cardiotrophin-1 (CT-1), a stress-induced cytokine, belongs to the interleukin-6/glycoprotein 130 receptor-coupled cytokine family. CT-1 is released from the heart in response to hypoxic stress, and it protects cardiac myocytes from hypoxia-induced apoptosis, thus establishing a central role for this cytokine in the cardiac stress response. In the present study, CT-1 activated p38 and ERK MAPKs as well as Akt in cultured cardiac myocytes; these three pathways were activated in a parallel manner. CT-1 also induced the degradation of the NF-kappa B cytosolic anchor, I kappa B, as well as the translocation of the p65 subunit of NF-kappa B to the nucleus and increased expression of an NF-kappa B-dependent reporter gene. Inhibitors of the p38, ERK, or Akt pathways each partially reduced CT-1-mediated NF-kappa B activation, as well as the cytoprotective effects of CT-1 against hypoxic stress. Together, the inhibitors completely blocked CT-1-dependent NF-kappa B activation and cytoprotection. A cell-permeable peptide that selectively disrupted NF-kappa B activation also completely inhibited the cytoprotective effects of CT-1. These results indicate that CT-1 signals through p38, ERK, and Akt in a parallel manner to activate NF-kappa B and that NF-kappa B is required for CT-1 to mediate its full cytoprotective effects in cardiac myocytes.

Crossbridge and tropomyosin positions observed in native, interacting thick and thin filaments

Tropomyosin movements on thin filaments are thought to sterically regulate muscle contraction, but have not been visualized during active filament sliding. In addition, although 3-D visualization of myosin crossbridges has been possible in rigor, it has been difficult for thick filaments actively interacting with thin filaments. In the current study, using three-dimensional reconstruction of electron micrographs of interacting filaments, we have been able to resolve not only tropomyosin, but also the docking sites for weak and strongly bound crossbridges on thin filaments. In relaxing conditions, tropomyosin was observed on the outer domain of actin, and thin filament interactions with thick filaments were rare. In contracting conditions, tropomyosin had moved to the inner domain of actin, and extra density, reflecting weakly bound, cycling myosin heads, was also detected, on the extreme periphery of actin. In rigor conditions, tropomyosin had moved further on to the inner domain of actin, and strongly bound myosin heads were now observed over the junction of the inner and outer domains. We conclude (1) that tropomyosin movements consistent with the steric model of muscle contraction occur in interacting thick and thin filaments, (2) that myosin-induced movement of tropomyosin in activated filaments requires strongly bound crossbridges, and (3) that crossbridges are bound to the periphery of actin, at a site distinct from the strong myosin binding site, at an early stage of the crossbridge cycle.

The tip of the coiled-coil rod determines the filament formation of smooth muscle and nonmuscle myosin

Myosin II self-assembles to form thick filaments that are attributed to its long coiled-coil tail domain. The present study has determined a region critical for filament formation of vertebrate smooth muscle and nonmuscle myosin II. A monoclonal antibody recognizing the 28 residues from the C-terminal end of the coiled-coil domain of smooth muscle myosin II completely inhibited filament formation, whereas other antibodies recognizing other parts of the coiled-coil did not. To determine the importance of this region in the filament assembly in vivo, green fluorescent protein (GFP)-tagged smooth muscle myosin was expressed in COS-7 cells, and the filamentous localization of the GFP signal was monitored by fluorescence microscopy. Wild type GFP-tagged smooth muscle myosin colocalized with F-actin during interphase and was also recruited into the contractile ring during cytokinesis. Myosin with the nonhelical tail piece deleted showed similar behavior, whereas deletion of the 28 residues at the C-terminal end of the coiled-coil domain abolished this localization. Deletion of the corresponding region of GFP-tagged nonmuscle myosin IIA also abolished this localization. We conclude that the C-terminal end of the coiled-coil domain, but not the nonhelical tail piece, of myosin II is critical for myosin filament formation both in vitro and in vivo.

Myosin light chain kinase binding to a unique site on F-actin revealed by three-dimensional image reconstruction

Ca2+-calmodulin-dependent phosphorylation of myosin regulatory light chains by the catalytic COOH-terminal half of myosin light chain kinase (MLCK) activates myosin II in smooth and nonmuscle cells. In addition, MLCK binds to thin filaments in situ and F-actin in vitro via a specific repeat motif in its NH2 terminus at a stoichiometry of one MLCK per three actin monomers. We have investigated the structural basis of MLCK-actin interactions by negative staining and helical reconstruction. F-actin was decorated with a peptide containing the NH2-terminal 147 residues of MLCK (MLCK-147) that binds to F-actin with high affinity. MLCK-147 caused formation of F-actin rafts, and single filaments within rafts were used for structural analysis. Three-dimensional reconstructions showed MLCK density on the extreme periphery of subdomain-1 of each actin monomer forming a bridge to the periphery of subdomain-4 of the azimuthally adjacent actin. Fitting the reconstruction to the atomic model of F-actin revealed interaction of MLCK-147 close to the COOH terminus of the first actin and near residues 228-232 of the second. This unique location enables MLCK to bind to actin without interfering with the binding of any other key actin-binding proteins, including myosin, tropomyosin, caldesmon, and calponin.

Effects of a cardiomyopathy-causing troponin t mutation on thin filament function and structure

Familial hypertrophic cardiomyopathy (FHC) is caused by missense or premature truncation mutations in proteins of the cardiac contractile apparatus. Mutant proteins are incorporated into the thin filament or thick filament and eventually produce cardiomyopathy. However, it has been unclear how the several, genetically identified defects in protein structure translate into impaired protein and muscle function. We have studied the basis of FHC caused by premature truncation of the most frequently implicated thin filament target, troponin T. Electron microscope observations showed that the thin filament undergoes normal structural changes in response to Ca(2+) binding. On the other hand, solution studies showed that the mutation alters and destabilizes troponin binding to the thin filament to different extents in different regulatory states, thereby affecting the transitions among states that regulate myosin binding and muscle contraction. Development of hypertrophic cardiomyopathy can thus be traced to a defect in the primary mechanism controlling cardiac contraction, switching between different conformations of the thin filament.

Troponin organization on relaxed and activated thin filaments revealed by electron microscopy and three-dimensional reconstruction

The steric model of muscle regulation holds that at low Ca(2+) concentration, tropomyosin strands, running along thin filaments, are constrained by troponin in an inhibitory position that blocks myosin-binding sites on actin. Ca(2+) activation, releasing this constraint, allows tropomyosin movement, initiating actin-myosin interaction and contraction. Although the different positions of tropomyosin on the thin filament are well documented, corresponding information on troponin has been lacking and it has therefore not been possible to test the model structurally. Here, we show that troponin can be detected on thin filaments and demonstrate how its changing association with actin can control tropomyosin position in response to Ca(2+). To accomplish this, thin filaments were reconstituted with an engineered short tropomyosin, creating a favorable troponin stoichiometry and symmetry for three-dimensional analysis. We demonstrate that in the absence of Ca(2+), troponin bound to both tropomyosin and actin can act as a latch to constrain tropomyosin in a position on actin that inhibits actomyosin ATPase. In addition, we find that on Ca(2+) activation the actin-troponin connection is broken, allowing tropomyosin to assume a second position, initiating actomyosin ATPase and thus permitting contraction to proceed.

Calcium-dependent structural changes in scallop heavy meromyosin

The mechanism of calcium regulation of scallop myosin is not understood, although it is known that both myosin heads are required. We have explored possible interactions between the heads of heavy meromyosin (HMM) in the presence and absence of calcium and nucleotides by sedimentation and electron microscope studies. The ATPase activity of the HMM preparation was activated over tenfold by calcium, indicating that the preparation contained mostly regulated molecules. In the presence of ADP or ATP analogs, calcium increased the asymmetry of the HMM molecule as judged by its slower sedimentation velocity compared with that in EGTA. In the absence of nucleotide the asymmetry was high even in EGTA. The shift in sedimentation occurred with a sharp midpoint at a calcium level of about 0.5 microM. Sedimentation of subfragment 1 was not dependent on calcium or on nucleotides. Modeling accounted for the observed sedimentation behavior by assuming that both HMM heads bent toward the tail in the absence of calcium, while in its presence the heads had random positions. The sedimentation pattern showed a single peak at all calcium concentrations, indicating equilibration between the two forms with a t(1/2) less than 70 seconds. Electron micrographs of crosslinked, rotary shadowed specimens indicated that 81 % of HMM molecules in the presence of nucleotide had both heads pointing back towards the tail in the absence of calcium, as compared with 41 % in its presence. This is consistent with the sedimentation data. We conclude that in the "off" state, scallop myosin heads interact with each other, forming a rigid structure with low ATPase activity. When molecules are switched "on" by binding of calcium, communication between the heads is lost, allowing them to flex randomly about the junction with the tail; this could facilitate their interaction with actin in contracting muscle.

A fission yeast homolog of Int-6, the mammalian oncoprotein and eIF3 subunit, induces drug resistance when overexpressed

Through a screen to identify genes that induce multi-drug resistance when overexpressed, we have identified a fission yeast homolog of Int-6, a component of the human translation initiation factor eIF3. Disruption of the murine Int-6 gene by mouse mammary tumor virus (MMTV) has been implicated previously in tumorigenesis, although the underlying mechanism is not yet understood. Fission yeast Int6 was shown to interact with other presumptive components of eIF3 in vivo, and was present in size fractions consistent with its incorporation into a 43S translation preinitiation complex. Drug resistance induced by Int6 overexpression was dependent on the AP-1 transcription factor Pap1, and was associated with increased abundance of Pap1-responsive mRNAs, but not with Pap1 relocalization. Fission yeast cells lacking the int6 gene grew slowly. This growth retardation could be corrected by the expression of full length Int6 of fission yeast or human origin, or by a C-terminal fragment of the fission yeast protein that also conferred drug resistance, but not by truncated human Int-6 proteins corresponding to the predicted products of MMTV-disrupted murine alleles. Studies in fission yeast may therefore help to explain the ways in which Int-6 function can be perturbed during MMTV-induced mammary tumorigenesis.

Measuring how acute care nurse practitioners affect outcomes

Nurse-leaders need a way to measure acute care nurse practitioners' (ACNPs') performance and justify their positions. An objective documentation tool can help to evaluate ACNPs' work activities and measure performance and outcomes.

Tropomyosin and actin isoforms modulate the localization of tropomyosin strands on actin filaments

Tropomyosin is present in virtually all eucaryotic cells, where it functions to modulate actin-myosin interaction and to stabilize actin filament structure. In striated muscle, tropomyosin regulates contractility by sterically blocking myosin-binding sites on actin in the relaxed state. On activation, tropomyosin moves away from these sites in two steps, one induced by Ca(2+) binding to troponin and a second by the binding of myosin to actin. In smooth muscle and non-muscle cells, where troponin is absent, the precise role and structural dynamics of tropomyosin on actin are poorly understood. Here, the location of tropomyosin on F-actin filaments free of troponin and other actin-binding proteins was determined to better understand the structural basis of its functioning in muscle and non-muscle cells. Using electron microscopy and three-dimensional image reconstruction, the association of a diverse set of wild-type and mutant actin and tropomyosin isoforms, from both muscle and non-muscle sources, was investigated. Tropomyosin position on actin appeared to be defined by two sets of binding interactions and tropomyosin localized on either the inner or the outer domain of actin, depending on the specific actin or tropomyosin isoform examined. Since these equilibrium positions depended on minor amino acid sequence differences among isoforms, we conclude that the energy barrier between thin filament states is small. Our results imply that, in striated muscles, troponin and myosin serve to stabilize tropomyosin in inhibitory and activating states, respectively. In addition, they are consistent with tropomyosin-dependent cooperative switching on and off of actomyosin-based motility. Finally, the locations of tropomyosin that we have determined suggest the possibility of significant competition between tropomyosin and other cellular actin-binding proteins. Based on these results, we present a general framework for tropomyosin modulation of motility and cytoskeletal modelling.

p38 MAPK and NF-kappa B collaborate to induce interleukin-6 gene expression and release. Evidence for a cytoprotective autocrine signaling pathway in a cardiac myocyte model system

In cardiac myocytes, the stimulation of p38 MAPK by the MAPKK, MKK6, activates the transcription factor, NF-kappaB, and protects cells from apoptosis. In the present study in primary neonatal rat cardiac myocytes, constitutively active MKK6, MKK6(Glu), bound to IkappaB kinase (IKK)-beta and stimulated its abilities to phosphorylate IkappaB and to activate NF-kappaB. MKK6(Glu) induced NF-kappaB-dependent interleukin (IL)-6 transcription and IL-6 release in a p38-dependent manner. IL-6 protected myocardial cells against apoptosis. Like IL-6, TNF-alpha, which activates both NF-kappaB and p38, also induced p38-dependent IL-6 expression and release and protected myocytes from apoptotis. While TNF-alpha was relatively ineffective, IL-6 activated myocardial cell STAT3 by about 8-fold, indicating a probable role for this transcription factor in IL-6-mediated protection from apoptosis. TNF-alpha-mediated IL-6 induction was inhibited by a kinase-inactive form of the MAPKKK, TGF-beta activated protein kinase (Tak1), which is known to activate p38 and NF-kappaB in other cell types. Thus, by stimulating both p38 and NF-kappaB, Tak1-activating cytokines, like TNF-alpha, can induce IL-6 expression and release. Moreover, the myocyte-derived IL-6 may then function in an autocrine and/or paracrine fashion to augment myocardial cell survival during stresses that activate p38.

An actin subdomain 2 mutation that impairs thin filament regulation by troponin and tropomyosin

Striated muscle thin filaments adopt different quaternary structures, depending upon calcium binding to troponin and myosin binding to actin. Modification of actin subdomain 2 alters troponin-tropomyosin-mediated regulation, suggesting that this region of actin may contain important protein-protein interaction sites. We used yeast actin mutant D56A/E57A to examine this issue. The mutation increased the affinity of tropomyosin for actin 3-fold. The addition of Ca(2+) to mutant actin filaments containing troponin-tropomyosin produced little increase in the thin filament-myosin S1 MgATPase rate. Despite this, three-dimensional reconstruction of electron microscope images of filaments in the presence of troponin and Ca(2+) showed tropomyosin to be in a position similar to that found for muscle actin filaments, where most of the myosin binding site is exposed. Troponin-tropomyosin bound with comparable affinity to mutant and wild type actin in the absence and presence of calcium, and in the presence of myosin S1, tropomyosin bound very tightly to both types of actin. The mutation decreased actin-myosin S1 affinity 13-fold in the presence of troponin-tropomyosin and 2.6-fold in the absence of the regulatory proteins. The results suggest the importance of negatively charged actin subdomain 2 residues 56 and 57 for myosin binding to actin, for tropomyosin-actin interactions, and for regulatory conformational changes in the actin-troponin-tropomyosin complex.

Children's acceptance of xylitol-based foods

Daily consumption of xylitol (5-10 g/day) added to chewing gum and confectionary foods has been previously shown to prevent dental caries in children.

METHODS

Snack foods containing xylitol were developed and tested for acceptability in a convenience sample of 31 children ages 3 to 6 years. In order to mimic an after-meal snack, all children were tested during mid-morning, approximately 1 h after eating. Preference testing was based on the methodology of Birch et al. (J Nutr Educ, 1979; 11: 77-80). In the first phase, each child was presented with a tray of six xylitol-based foods (popsicles, pudding, gum drops, gelatin dessert, cookies, popcorn) and asked to taste each item in any desired order. Immediately after tasting a food, the child was asked to place it in front of one of three cartoon faces (smile, frown or neutral) representing the child's response to the taste of that particular food. In the second phase, the child was asked to rank order the foods in each face category (smile, frown or neutral). Ranks within categories were then combined to obtain a rank ordering for all of the foods.

RESULTS

Non-parametric data analysis indicated significant differences in ranking between the foods when they were compared to each other (Friedman ANOVA by ranks, P<0.01). Pudding was significantly less preferred than the other foods (sign tests, P<0.04). At least 84% of the children found five of the six foods very good or satisfactory, when considered individually.

CONCLUSIONS

These results suggest that snack foods developed with xylitol are generally well accepted by children.

Measure how ACNPs impact outcomes

Nurse leaders need a way to measure acute care nurse practitioners' (ACNPs') performance and justify their positions. An objective documentation tool can help to evaluate ACNPs' work activities and measure performance and outcomes.

Three-dimensional reconstruction of thin filaments containing mutant tropomyosin

Interactions of the components of reconstituted thin filaments were investigated using a tropomyosin internal deletion mutant, D234, in which actin-binding pseudo-repeats 2, 3, and 4 are missing. D234 retains regions of tropomyosin that bind troponin and form end-to-end tropomyosin bonds, but has a length to span only four instead of seven actin monomers. It inhibits acto-myosin subfragment 1 ATPase (acto-S-1 ATPase) and filament sliding in vitro in both the presence and absence of Ca(2+) (, J. Biol. Chem. 272:14051-14056) and lowers the affinity of S-1.ADP for actin while increasing its cooperative binding. Electron microscopy and three-dimensional reconstruction of reconstituted thin filaments containing actin, troponin, and wild-type or D234 tropomyosin were carried out to determine if Ca(2+)-induced movement of D234 occurred in the filaments. In the presence and absence of Ca(2+), the D234 position was indistinguishable from that of the wild-type tropomyosin, demonstrating that the mutation did not affect normal tropomyosin movement induced by Ca(2+) and troponin. These results suggested that, in the presence of Ca(2+) and troponin, D234 tropomyosin was trapped on filaments in the Ca(2+)-induced position and was unable to undergo a transition to a completely activated position. By adding small amounts of rigor-bonded N-ethyl-maleimide-treated S-1 to mutant thin filaments, thus mimicking the myosin-induced "open" state, inhibition could be overcome and full activation restored. This myosin requirement for full activation provides support for the existence of three functionally distinct thin filament states (off, Ca(2+)-induced, myosin-induced; cf.;, J. Mol. Biol. 266:8-14). We propose a further refinement of the three-state model in which the binding of myosin to actin causes allosteric changes in actin that promote the binding of tropomyosin in an otherwise energetically unfavorable "open" state.

Tropomyosin positions in regulated thin filaments revealed by cryoelectron microscopy

Past attempts to detect tropomyosin in electron micrograph images of frozen-hydrated troponin-regulated thin filaments under relaxing conditions have not been successful. This raised the possibility that tropomyosin may be disordered on filaments in the off-state, a possibility at odds with the steric blocking model of muscle regulation. By using cryoelectron microscopy and helical image reconstruction we have now resolved the location of tropomyosin in both relaxing and activating conditions. In the off-state, tropomyosin adopts a position on the outer domain of actin with a binding site virtually identical to that determined previously by negative staining, although at a radius of 3.8 nm, slightly higher than found in stained filaments. Molecular fitting to the atomic model of F-actin shows that tropomyosin is localized over sites on actin subdomain 1 required for myosin binding. Restricting access to these sites would inhibit the myosin-cross-bridge cycle, and hence contraction. Under high Ca(2+) activating conditions, tropomyosin moved azimuthally, away from its blocking position to the same site on the inner domain of actin previously determined by negative staining, also at 3.8 nm radius. These results provide strong support for operation of the steric mechanism of muscle regulation under near-native solution conditions and also validate the use of negative staining in investigations of muscle thin filament structure.

Low bioavailability of cyclosporine microemulsion and tacrolimus in a small bowel transplant recipient: possible relationship to intestinal P-glycoprotein activity

With intestine transplants the allograft is dependent on itself for maintenance of adequate immunosuppression. We evaluated an intestinal transplant recipient who required very large doses of either tacrolimus or cyclosporine emulsion to achieve acceptable blood concentrations. Pharmacokinetic studies revealed bioavailabilities of 2% and 6% respectively, while D-xylose and B12 absorption were found to be within normal limits and fecal fat was only slightly increased, suggesting that there was a selective absorptive defect for these drugs. Biopsies of the allograft ileum revealed a high P-glycoprotein activity compared to the jejunum or to intestinal biopsies from other normal subjects. This may be a contributing factor to poor immunosuppressive drug absorption in this patient and others.

On scene thoracotomy: a case report

We report a case of on scene resuscitative thoracotomy performed by an anaesthetist on a patient in cardiac arrest following a stab wound to the chest. The patient made a good recovery and was discharged from hospital within 2 weeks. The rationale for performing resuscitative thoracotomy and who should perform this procedure are discussed.