Biomedical subjects

Masaru Shibata

Publications and source records attributed to Masaru Shibata.

12 recordsLinked to original sources

PubMed2006

Binary neutron stars: Equilibrium models beyond spatial conformal flatness.

Equilibria of binary neutron stars in close circular orbits are computed numerically in a waveless formulation: the full Einstein-relativistic-Euler system is solved on an initial hypersurface to obtain an asymptotically flat form of the 4-metric and an extrinsic curvature whose time derivative vanishes in a comoving frame. Two independent numerical codes are developed, and solution sequences that model inspiraling binary neutron stars during the final several orbits are successfully computed. The binding energy of the system near its final orbit deviates from earlier results of third post-Newtonian and of spatially conformally flat calculations. The new solutions may serve as initial data for merger simulations and as members of quasiequilibrium sequences to generate gravitational-wave templates, and may improve estimates of the gravitational-wave cutoff frequency set by the last inspiral orbit.

Journal Article↗

PubMed2006

Collapse of magnetized hypermassive neutron stars in general relativity.

Hypermassive neutron stars (HMNSs)--equilibrium configurations supported against collapse by rapid differential rotation--are possible transient remnants of binary neutron-star mergers. Using newly developed codes for magnetohydrodynamic simulations in dynamical spacetimes, we are able to track the evolution of a magnetized HMNS in full general relativity for the first time. We find that secular angular momentum transport due to magnetic braking and the magnetorotational instability results in the collapse of an HMNS to a rotating black hole, accompanied by a gravitational wave burst. The nascent black hole is surrounded by a hot, massive torus undergoing quasistationary accretion and a collimated magnetic field. This scenario suggests that HMNS collapse is a possible candidate for the central engine of short gamma-ray bursts.

Journal Article↗

PubMed2006

Magnetized hypermassive neutron-star collapse: a central engine for short gamma-ray bursts.

A hypermassive neutron star (HMNS) is a possible transient formed after the merger of a neutron-star binary. In the latest axisymmetric magnetohydrodynamic simulations in full general relativity, we find that a magnetized HMNS undergoes "delayed" collapse to a rotating black hole (BH) as a result of angular momentum transport via magnetic braking and the magnetorotational instability. The outcome is a BH surrounded by a massive, hot torus with a collimated magnetic field. The torus accretes onto the BH at a quasisteady accretion rate [FORMULA: SEE TEXT]; the lifetime of the torus is approximately 10 ms. The torus has a temperature [FORMULA: SEE TEXT], leading to copious ([FORMULA: SEE TEXT]) thermal radiation that could trigger a fireball. Therefore, the collapse of a HMNS is a promising scenario for generating short-duration gamma-ray bursts and an accompanying burst of gravitational waves and neutrinos.

Journal Article↗

PubMed2005

Constraining nuclear equations of state using gravitational waves from hypermassive neutron stars.

Latest general relativistic simulations for the merger of binary neutron stars with realistic equations of states (EOSs) show that a hypermassive neutron star of an ellipsoidal figure is formed after the merger if the total mass is smaller than a threshold value which depends on the EOSs. The effective amplitude of quasiperiodic gravitational waves from such hypermassive neutron stars is approximately 6-7 x 10(-21) at a distance of 50 Mpc, which may be large enough for detection by advanced laser interferometric gravitational wave detectors although the frequency is high, approximately 3 kHz. We point out that the detection of such signals may lead to constraining the EOSs for neutron stars.

Journal Article↗

PubMed2005

Inactivation and deficiency of core proteins of photosystems I and II caused by genetical phylloquinone and plastoquinone deficiency but retained lamellar structure in a T-DNA mutant of Arabidopsis.

Phylloquinone, a substituted 1,4-naphthoquinone with an 18-carbon-saturated phytyl tail, functions as a bound one-electron carrier cofactor at the A1 site of photosystem I (PSI). A Feldmann tag line mutant, no. 2755 (designated as abc4 hereafter), showed pale-green young leaves and white old leaves. The mutated nuclear gene encoded 1,4-dihydroxy-2-naphtoic acid phytyltransferase, an enzyme of phylloquinone biosynthesis, and high-performance liquid chromatography analysis revealed that the abc4 mutant contained no phylloquinone, and only about 3% plastoquinone. Photooxidation of P700 of PSI in the abc4 mutant was not observed, and reduced-versus-oxidized difference spectroscopy indicated that the abc4 mutant had no P700. The maximum quantum yield of photosystem II (PSII) in the abc4 mutant was much decreased, and the electron transfer from PSII to PSI in the abc4 mutant did not occur. For the pale-green leaves of the abc4 mutant plant, the ultrastructure of the chloroplasts was almost the same as that of the wild-type plant. However, the chloroplasts in the albino leaves of the mutant were smaller and had a lot of grana thylakoids and few stroma thylakoids. The amounts of PSI and PSII core subunits in the abc4 mutant were significantly decreased compared with those in the wild type. These results suggested that a deficiency of phylloquinone in PSI caused the abolishment of PSI and a partial defect of PSII due to a significant decrease of plastoquinone, but did not influence the ultrastructure of the chloroplasts in young leaves.

Alkyl and Aryl Transferases↗

PubMed2004

Accumulation of menaquinones with incompletely reduced side chains and loss of alpha-tocopherol in rice mutants with alternations in the chlorophyll moiety.

The rice mutants M249 and M134 accumulate chlorophyllides a and b which are esterified with incompletely reduced alcohols such as geranylgeraniol, dihydrogeranylgeraniol, and tetrahydrogeranylgeraniol. Quantities of alpha-tocopherol, phylloquinone, and menaquinones in leaves of these mutants were determined by high performance liquid chromatography (HPLC) with a fluorescence detector after post-column chemical reduction to convert quinones to fluorescent quinols. Methylnaphthoquinones, varying in the reduction state of the side chain (menaquinones), were detected in leaf segments of the rice mutants on HPLC analyses with both high selectivity and sensitivity to plant quinones. Mutant M249 preferentially accumulated menaquinone, which contains tetrahydrogeranylgeraniol as its side chain. However, mutant M134 exhibited preferential accumulation of menaquinone with a geranylgeraniol side chain. In both mutants, the accumulation patterns of menaquinones with different prenyl side chains were similar to those of chlorophyll with the corresponding prenyl side chains. The content of P700, the photosystem I primary electron donor, in the wild type was greater than that of either mutant, on both a chlorophyll and a fresh weight basis. However, the ratios of total methylnaphthoquinones to P700 were similar in both the wild type and the mutants. Since no comparative large differences in photosynthetic activity exist between the wild type and the mutants, these results suggest that the hydrogenation of the methylnaphthoquinone side chain to phytol is not an essential requirement for it to function as an electron acceptor in photosystem I. On the other hand, alpha-tocopherol was detected in fully developed leaves of the wild type, but not in those of the mutants. Accumulation of menaquinones and the loss of alpha-tocopherol in mutant leaves suggest that the reduction of chlorophyll-geranylgeraniol to phytol and that of geranylgeranyl pyrophosphate to phytyl pyrophosphate are catalysed by the same enzyme.

Chlorophyll↗

PubMed2004

Actin filaments play a permissive role in the inhibition of store-operated Ca2+ entry by extracellular ATP in rat brown adipocytes.

Stimulation of P2 receptors with micromolar concentration of ATP evokes a transient increase in [Ca2+]i (intracellular free Ca2+ concentration), primarily due to release of Ca2+ from intracellular stores; such stimulation also triggers almost complete suppression of thapsigargin-evoked sustained [Ca2+]i increase mediated through a store-operated Ca2+ entry pathway in rat brown adipocytes. We investigated the role of cytoskeletal actin in the inhibitory effect of the extracellular ATP on store-operated Ca2+ entry, using fura 2 fluorescence for continuous measurement of [Ca2+]i, and using Alexa fluor 488-phalloidin staining of actin. Disassembly of actin networks by cytochalasin D (1 microM) or latrunculin A (3 microM) prevented the inhibitory effect of ATP (10 microM) on the thapsigargin (100 nM)-evoked store-operated Ca2+ entry, without changing the effect of ATP in increasing [Ca2+]i. In normal cells, bath application of ATP induced a transient [Ca2+]i increase, consisting of a rapid increase (the rising phase) and the subsequent decrease (the declining phase) to a lower steady level despite the continued presence of the agonist. Disruption of actin assemblies did not significantly affect the rising phase, but prevented the declining phase. Cells incubated with 10 microM ATP for 4 min demonstrated marked accumulations of actin filaments at the cell periphery, showing protrusions at the cell surface; this actin-assembly process is mediated through P2 receptors. In cells treated with cytochalasin D or latrunculin A, extracellular ATP did not induce actin redistribution. These results suggest that the actin reorganization plays a role in ATP-induced inhibition of store-operated Ca2+ entry in rat brown adipocytes.

Actin Cytoskeleton↗

PubMed2004

Analytical and computational techniques for exogenous depolymerization of xenobiotic polymers.

We analyze a mathematical model for exogenous depolymerization of xenobiotic polymers. We derive a sufficient condition for the solvability of its inverse problem to determine degradation rates. We also introduce a numerical technique to solve the inverse problem with an example to show how our numerical technique is applied to gel permeation chromatography data of polyethylene wax obtained before and after three-week cultivation of a fungus Aspergillus sp. AK-3. In addition, we present a numerical result which we obtained by applying the degradation rate based on the three week cultivation to simulate the transition of the weight distribution after cultivation of the fungus for five weeks.

Algorithms↗

PubMed2004

An Analysis of the Mechanism of the Low-wave Phenomenon of Chlorophyll Fluorescence.

The low-wave phenomenon, i.e., the transient drop of yield of modulated chlorophyll fluorescence shortly after application of a pulse of saturating light, was investigated in intact leaves of tobacco and Camellia by measuring fluorescence, CO(2) assimilation and absorption at 830 nm simultaneously. Limitations on linear electron flow, due to low electron acceptor levels that were induced by low CO(2), induced the low waves of chlorophyll fluorescence. Low-wave amplitudes obtained under different CO(2) concentrations and photon-flux densities yielded single-peak curves when plotted as functions of fluorescence parameters such as PhiPS II (quantum yield of Photosystem II) and qN (coefficient of non-photochemical quenching), suggesting that low-wave formation depends on the redox state of the electron transport chain. Low waves paralleled redox changes of P700, the reaction center of Photosystem I (PS I), and an additional electron flow through PS I was detected during the application of saturating pulses that induced low-waves. It is suggested that low waves of chlorophyll fluorescence are induced by increased non-photochemical quenching, as a result of the formation of a trans-thylakoid proton gradient due to cyclic electron flow around PS I.

Journal Article↗

PubMed2004

Inversions of the factor VIII gene in Japanese patients with severe hemophilia A.

Hemophilia A is genetically very heterogeneous because disease-causing mutations involving deletions, point mutations, insertions, and inversions are scattered throughout the factor VIII gene. Of these mutations, inversions, which are intrachromosomal recombinations between int22h-1 (intron 22 homologous region 1) and 1 of 2 other extragenic copies located 500 kilobases upstream, are the more frequently found defects, especially in patients with severe hemophilia A. Reportedly, approximately half of all severe hemophilia A patients have inversions in intron 22. A group of unrelated patients from the middle of Japan with severe hemophilia A were screened by Southern blot analysis for gene inversions. Forty-two of 100 severely affected patients presented factor VIII gene rearrangements. Of these patients, 36 exhibited the distal type of inversion, and 6 exhibited the proximal type. No other variant type of recombination was observed. In this study, neither the prevalence of inhibitor development against factor VIII nor the frequency of sporadic cases in the group presenting gene inversions was significantly different from that in the group without chromosomal inversions. Southern blot analysis successfully detected a carrier in a hemophilia family for which no patient was available. Genetic counseling of patients with severe hemophilia A and their families will be considerably improved, because the inversions occur in 42% of the Japanese patients with severe hemophilia.

Blotting, Southern↗

PubMed2003

Leaf factors affecting the relationship between chlorophyll fluorescence and the rate of photosynthetic electron transport as determined from CO2 uptake.

CO2 uptake and chlorophyll fluorescence were measured under non-photorespiratory conditions in leaves from 14 plant species. The rate of CO2-dependent electron transport (JCO2) was calculated as four times rate of gross photosynthesis. The quantum yield of electron transport in photosystem II was estimated from the ratio delta F/Fm', where delta F is the difference between steady-state and maximal fluorescence in the light. As photon flux density (PFD) increased, JCO2 increased linearly first, and then reached saturation. The product (delta F/Fm')PFD, which is a function of electron transport rate, showed a similar response. Therefore, the relationship between (delta F/Fm') PFD versus JCO2 was proportional. However, under high light, a linear correlation was not always maintained. Factors affecting the linear correlation were analyzed by measuring CO2 uptake and chlorophyll fluorescence under illumination from either the upper (adaxial) or lower (abaxial) leaf surface, and by using plants with anatomically symmetric leaves having palisade tissues on both sides. Consequently, it was shown that the parameter delta F/Fm' is based on chlorophyll fluorescence emitted from chloroplasts present near the illuminated surface. Further, it was suggested that this restriction of the origin of fluorescence actually measured is significant in a leaf with high chlorophyll content, resulting in the deviation from linearity in the relationship between JCO2 and (delta F/Fm')PFD.

Carbon Dioxide↗

PubMed2002

A novel mechanism of factor VIII protection by von Willebrand factor from activated protein C-catalyzed inactivation.

The protective effect of von Willebrand factor (VWF) toward activated protein C (APC)-catalyzed inactivation of factor VIII (FVIII) has been attributed mainly to inhibition of FVIII binding to phospholipid. In the present study, we demonstrated that VWF-mediated FVIII protection from APC also results from direct inhibition of FVIII binding to APC. Inhibition of FVIII binding to anhydro-APC by VWF would be consistent with partial or complete overlap of the FVIII binding sites for APC and VWF. We examined, therefore, the inhibitory effects of 6 synthetic peptides spanning residues 1996 to 2028 around the previously localized APC binding region (FVIII residues 2009-2018). Peptide 2009 to 2018 inhibited FVIII binding to anhydro-APC by 83% (50% inhibition, 55 microM). Similarly, peptide 2013 to 2022 inhibited FVIII binding to VWF by 84% (50% inhibition, 25 microM). It was also found that peptides 2009 to 2018 and 2013 to 2022 optimally bound to anhydro-APC and VWF, respectively. A rabbit antipeptide IgG, raised against peptide 2009 to 2022, blocked the binding of both anhydro-APC and VWF to FVIII. This immunoglobulin G inhibited proteolytic cleavage of FVIII by APC. Our results indicate that the essential regions for the binding of APC and VWF to FVIII overlap and that the protective effect of VWF on APC-catalyzed FVIII inactivation includes competitive inhibition of APC binding to FVIII by VWF.

Amino Acid Sequence↗