The mechanically induced structural disorder in barium hexaferrite, BaFe12O19, and its impact on magnetism.

The response of the structure of the M-type barium hexaferrite (BaFe12O19) to mechanical action through high-energy milling and its impact on the magnetic behaviour of the ferrite are investigated. Due to the ability of the (57)Fe Mössbauer spectroscopic technique to probe the environment of the Fe nuclei, a valuable insight on a local atomic scale into the mechanically induced changes in the hexagonal structure of the material is obtained. It is revealed that the milling of BaFe12O19 results in the deformation of its constituent polyhedra (FeO6 octahedra, FeO4 tetrahedra and FeO5 triangular bi-pyramids) as well as in the mechanically triggered transition of the Fe(3+) cations from the regular 12k octahedral sites into the interstitial positions provided by the magnetoplumbite structure. The response of the hexaferrite to the mechanical treatment is found to be accompanied by the formation of a non-uniform nanostructure consisting of an ordered crystallite surrounded/separated by a structurally disordered surface shell/interface region. The distorted polyhedra and the non-equilibrium cation distribution are found to be confined to the amorphous near-surface layers of the ferrite nanoparticles with the thickness extending up to about 2 nm. The information on the mechanically induced short-range structural disorder in BaFe12O19 is complemented by an investigation of its magnetic behaviour on a macroscopic scale. It is demonstrated that the milled ferrite nanoparticles exhibit a pure superparamagnetism at room temperature. As a consequence of the far-from-equilibrium structural disorder in the surface shell of the nanoparticles, the mechanically treated BaFe12O19 exhibits a reduced magnetization and an enhanced coercivity.

Thermal changes of the crystal structure and the influence of thermo-chemical annealing on the optical properties of YbAlO3 crystals.

The paper presents experimental results of investigations on the influence of temperature and thermo-chemical annealing on the structural and optical properties of YbAlO(3) crystals. Thermal behaviour of the crystal structure has been studied in the temperature range of 19-1173 K by means of in situ high-resolution x-ray powder diffraction using synchrotron radiation. Lattice expansion of YbAlO(3) displays a strongly anisotropic character: the relative expansion along the b direction is about two times smaller than that along the a and c axes. The influence of thermo-chemical annealing in oxidizing and reducing atmospheres has been studied by means of in situ optical spectroscopy. The increase of the absorption in the range of 300-600 nm during annealing in an oxidizing atmosphere as well as its bleaching during reduction has been observed in the temperature range 700-1000 K. The oxidation and reduction kinetics have been analysed by means of mathematical models taking into account diffusion and the quasi-chemical reaction of the defect centres.

High temperature optical spectroscopy investigations on Zr0.78Y0.18Sm0.04O1.89 and Zr0.78Y0.018Ho0.04O1.89 single crystals.

Results are reported from a temperature dependent optical study of the title compounds performed at temperatures up to 1323 K. Absorption maxima and integrated absorption of rare earth f-f transitions exhibit a considerable temperature dependence, which is quantitatively described in the framework of the optical high temperature spectroscopy of d-ions. A strong, reversible band gap shift of doped and pure YSZ is observed and evaluated according to the Tauc equation. Single crystals of the title compounds were prepared by the skull melting method. Their linear expansion coefficients were determined by high-temperature X-ray diffraction.

Electrical and optical characterization of undoped BaTiO(3) in the quenched state.

On undoped polycrystalline BaTiO(3-delta) (99.9% nominal purity), that had been equilibrated and quenched at 1000 degrees C under different oxygen activities in the range of -19.4 < or = log a(O(2)) < 0.1, were measured optical reflectance spectra at 298 K and ac conductivity at 473, 523 and 573 K, respectively. It was observed that all specimens quenched at log a(O(2)) > -17 are electrically insulating and white in color, once powdered, and show optical absorption only at the absorption edge at approximately 3 eV. In contrast, those quenched at log a(O(2)) < -17 are electronically conducting and dark and exhibit a significant absorption in the IR region. This demarcating oxygen activity for the conductivity transition is ca. 13 orders of magnitude lower than that for the n-to-p type transition (delta = 0) in the equilibrium state, contrary to expectation. It is, thus, suggested that contrary to the textbook knowledge, the hole traps responsible for the conductivity transition in undoped BaTiO(3) may be variable-valent acceptor defects like Mn(Ti). The strong IR absorption in the semiconducting state is ascribed to small-polaron (Ti'(Ti)) hopping processes.

Optical in situ study of the reduction/oxidation processes in YAlO(3):Mn crystals.

The paper presents experimental results on an in situ optical absorption study of the reduction/oxidation processes in YAlO(3):Mn crystals that are visible in the [Formula: see text] recharging of manganese ions. The appearance of the Mn(5+) absorption during annealing in an oxidizing atmosphere as well as its bleaching during annealing of the crystal in a reducing atmosphere have been studied in the temperature range from 800 to 1250 K. The oxidation and reduction kinetics are analyzed in the framework of the vacancies diffusion model and compared with the nominally pure and Nd-doped YAlO(3) crystals studied previously.

Exploring candidate gene associations with neuropsychological performance.

We hypothesize that quantitative phenotypes related to Alzheimer's disease (AD), rather than the dichotomous disease phenotype, will increase the statistical power for identifying genetic risk factors. Neuropsychological test scores, which allow for the measurement of loss of cognitive function over time, are a particularly promising option for this approach. Using data from a cohort study of prodromal AD in 365 community-recruited subjects with and without memory problems with a baseline and often one or more follow-up administrations of a detailed neuropsychological test battery, we performed both cross-sectional and longitudinal analyses using the known AD gene APOE and four other putative AD genes as predictors. APOE and a promoter polymorphism in insulin degrading enzyme (IDE_4U) showed evidence for association with cross-sectional and longitudinal changes in memory (P = 0.016-0.025) and other cognitive functions. APOE and a polymorphism in the alpha-2-macroglobulin gene (A2M18i) also showed evidence for association with cross-sectional and longitudinal changes in executive functioning (P = 0.010-0.042). In some cases, longitudinal analysis offered stronger evidence for association than could be seen cross-sectionally. These preliminary results suggest that this approach has promised the development of a quantitative phenotype related to AD, but more elaborate methods will be required to address multiple comparisons issues in the setting of correlated data.

High temperature optical spectroscopy of cubic holmium doped zirconia, Zr(0.78)Y(0.21)Ho(0.01)O(1.90).

The electronic absorption spectra of a Zr(0.78)Y(0.21)Ho(0.01)O(1.90) single crystal have been measured in in situ conditions between room temperature and 1289 K. The evolution of the spectra with rising temperatures is characterised by an overall increase in intensity. Notably the intensities of the (5)I(8) --> (5)F(2), (5)I(8) --> (5)S(2) and (5)I(8) --> (5)G(6) transitions increase by factors of two to three. The evaluation of the spectral parameters favours the conclusion that the intensity increases are due to vibronic coupling associated with effective frequencies ranging between 1 x 10(13) and 5 x 10(13) s(-1) (330 cm(-1) and 1670 cm(-1)). The changes in the optical spectra are reversible and are not connected with structural changes in the material.

Effect of hole-trapping on mass/charge transport properties in acceptor-doped BaTiO3.

The equilibrium conductivity and chemical diffusivity of Al-doped, single-crystalline BaTiO3 [C. R. Song and H. I. Yoo, J. Am. Ceram. Soc., 2000, 83, 773] have been re-analyzed by taking account of possible hole-trapping by acceptor-dopants (Al) with reference to the data on undoped BaTiO3 [C. R. Song and H. I. Yoo, Phys. Rev. B, 2000, 61, 3975]. The trapping effect is accounted for by the effective mobility of free holes involving the weighted mobility of trapped holes and by the modified thermodynamic factor due to the shift of the stoichiometric composition of the acceptor-doped system. The equilibrium constant of the trapping reaction Al(Ti)x = Al(Ti)' + h* is evaluated as Ka = 1.9 x 10(23) exp(-1.04 eV/kT) cm(-3) and the mobility of trapped holes (Al(Ti)x) as u(x) approximately = 720 exp(-1.04 eV/kT) cm2 V(-1) s(-1) as the upper bound while those of free electrons and holes as un = 0.044 + 0.010 cm2 V(-1) s(-1) and up approximately = 0.024 +/- 0.007 cm2 V(-1) s(-1), respectively, in the temperature range of 800 degrees C to 1100 degres C. The effect of hole-trapping on DC conductivity, chemical diffusivity and oxygen nonstoichiometry is exhaustively analyzed and further experiments are proposed that allow one to determine the trapping energy of acceptor impurities.

Myocardial mechanics and collagen structure in the osteogenesis imperfecta murine (oim).

Because the amount and structure of type I collagen are thought to affect the mechanics of ventricular myocardium, we investigated myocardial collagen structure and passive mechanical function in the osteogenesis imperfecta murine (oim) model of pro-alpha2(I) collagen deficiency, previously shown to have less collagen and impaired biomechanics in tendon and bone. Compared with wild-type littermates, homozygous oim hearts exhibited 35% lower collagen area fraction (P:<0.05), 38% lower collagen fiber number density (P:<0.05), and 42% smaller collagen fiber diameter (P:<0.05). Compared with wild-type, oim left ventricular (LV) collagen concentration was 45% lower (P:<0.0001) and nonreducible pyridinoline cross-link concentration was 22% higher (P:<0.03). Mean LV volume during passive inflation from 0 to 30 mm Hg in isolated hearts was 1.4-fold larger for oim than wild-type (P:=NS). Uniaxial stress-strain relations in resting right ventricular papillary muscles exhibited 60% greater strains (P:<0.01), 90% higher compliance (P:=0.05), and 64% higher nonlinearity (P:<0.05) in oim. Mean opening angle, after relief of residual stresses in resting LV myocardium, was 121+/-9 degrees in oim compared with 45+/-4 degrees in wild-type (P:<0.0001). Mean myofiber angle in oim was 23+/-8 degrees greater than wild-type (P:<0.02). Decreased myocardial collagen diameter and amount in oim is associated with significantly decreased fiber and chamber stiffness despite modestly increased collagen cross-linking. Altered myofiber angles and residual stress may be beneficial adaptations to these mechanical alterations to maintain uniformity of transmural fiber strain. In addition to supporting and organizing myocytes, myocardial collagen contributes directly to ventricular stiffness at high and low loads and can influence stress-free state and myofiber architecture.

Selective requirement of myosin light chain 2v in embryonic heart function.

Two major myosin light chain 2 isoforms are coexpressed in the early stages of murine cardiogenesis, a cardiac ventricular isoform and a cardiac atrial isoform, each of which is tightly regulated in a muscle cell-type-specific manner during embryogenesis (Chien, K. R., Zhu, H., Knowlton, K. U., Miller-Hance, W., van Bilsen, M., O'Brien, T. X., and Evans, S. M. (1993) Annu. Rev. Physiol. 55, 77-95). We have disrupted myosin light chain 2v gene in mice and monitored in vivo cardiac function in living myosin light chain 2v -/- embryos. The mutant embryos die at approximately embryonic day 12.5. In mutant ventricles, the myosin light chain 2a protein level is increased and reaches levels comparable to the myosin light chain 2v in the ventricles of wild type littermates and is appropriately incorporated into the thick filaments of mutant embryonic hearts. However, despite the substitution of myosin light chain 2a, ultrastructural analysis revealed defects in sarcomeric assembly and an embryonic form of dilated cardiomyopathy characterized by a significantly reduced left ventricular ejection fraction in mutant embryos compared with wild type littermates. We conclude that myosin light chain 2v may have a unique function in the maintenance of cardiac contractility and ventricular chamber morphogenesis during mammalian cardiogenesis and that a chamber-specific combinatorial code for sarcomeric assembly may exist that ultimately requires myosin light chain 2v in ventricular muscle cells.

Ras-dependent pathways induce obstructive hypertrophy in echo-selected transgenic mice.

To overcome the genetic and interindividual variability frequently noted in complex phenotypes, we used echocardiographic selection to develop a substrain of myosin light chain (MLC)-Ras (RAS) transgenic mice with an enhanced ventricular hypertrophic phenotype. These echo-selected mice were then compared with wild-type (WT) animals and a pressure overload hypertrophy model (transverse aortic constriction; TAC). Echocardiography demonstrated increased wall thickness in RAS compared with the other groups. We developed novel miniaturized physiological technology to quantitatively identify in vivo intraventricular gradients; increased systolic Doppler velocity was seen in the left ventricle (LV) in 69% of RAS vs. none of WT or TAC. Intracavitary pressure gradients were present in 3 of 10 RAS vs. none of TAC or WT. Passive diastolic LV stiffness was not different among the three groups. Myofibrillar disarray was present in all RAS animals and was significantly more extensive (21.7% area fraction) than in TAC (1.5%) or WT (0.0%). RAS mice had selective induction of natriuretic peptide genes in the LV, a pattern distinct from that induced by pressure overload. Juvenile mortality was significantly increased in the offspring of echo-selected RAS parents. We conclude that adaptation of echocardiography to the mouse permits selection for cardiac phenotypes, and that selectively inbred MLC-Ras transgenic mice faithfully reproduce the molecular, physiological, and pathological features of human hypertrophic cardiomyopathy (HCM). Because previous studies support the concept that hypertrophy in human HCM is secondary to dysfunction created by sarcomeric protein mutations, the current studies suggest that Ras-dependent pathways might play a similar role in forms of human HCM.

Point mutations in human beta cardiac myosin heavy chain have differential effects on sarcomeric structure and assembly: an ATP binding site change disrupts both thick and thin filaments, whereas hypertrophic cardiomyopathy mutations display normal assembly.

Hypertrophic cardiomyopathy is a human heart disease characterized by increased ventricular mass, focal areas of fibrosis, myocyte, and myofibrillar disorganization. This genetically dominant disease can be caused by mutations in any one of several contractile proteins, including beta cardiac myosin heavy chain (beta MHC). To determine whether point mutations in human beta MHC have direct effects on interfering with filament assembly and sarcomeric structure, full-length wild-type and mutant human beta MHC cDNAs were cloned and expressed in primary cultures of neonatal rat ventricular cardiomyocytes (NRC) under conditions that promote myofibrillogenesis. A lysine to arginine change at amino acid 184 in the consensus ATP binding sequence of human beta MHC resulted in abnormal subcellular localization and disrupted both thick and thin filament structure in transfected NRC. Diffuse beta MHC K184R protein appeared to colocalize with actin throughout the myocyte, suggesting a tight interaction of these two proteins. Human beta MHC with S472V mutation assembled normally into thick filaments and did not affect sarcomeric structure. Two mutant myosins previously described as causing human hypertrophic cardiomyopathy, R249Q and R403Q, were competent to assemble into thick filaments producing myofibrils with well defined I bands, A bands, and H zones. Coexpression and detection of wild-type beta MHC and either R249Q or R403Q proteins in the same myocyte showed these proteins are equally able to assemble into the sarcomere and provided no discernible differences in subcellular localization. Thus, human beta MHC R249Q and R403Q mutant proteins were readily incorporated into NRC sarcomeres and did not disrupt myofilament formation. This study indicates that the phenotype of myofibrillar disarray seen in HCM patients which harbor either of these two mutations may not be directly due to the failure of the mutant myosin heavy chain protein to assemble and form normal sarcomeres, but may rather be a secondary effect possibly resulting from the chronic stress of decreased beta MHC function.

The utility of fluorescent in vivo reporter genes in molecular cardiology.

In vivo reporter genes can be used in different ways in molecular cardiology. In this paper studies are presented using the green fluorescent protein and one of its mutants, S65T-GFP, as in vivo reporter genes. With this new molecular tool we studied cell type specificity of the murine ventricular myosin light chain 2 promoter, positive cell identification prior to patch clamp procedures, and the use of fluorescence activated cell sorting of transiently transfected mammalian cells.

Strategies for studying cardiovascular phenotypes in genetically manipulated mice.

Unraveling the pathogenesis of complex cardiovascular diseases, such as hypertension, requires the development of in vivo animal model systems. Although large-animal models have long served as the gold standard, recent advances in transgenic and gene-targeting approaches, mouse genetics, and microsurgical technology are initiating a revolution that has led to the unexpected coupling of in vivo molecular physiology with genetically engineered mice. This article discusses the design of strategies to study complex cardiovascular phenotypes in genetically modified mice, including both transgenic and gene-targeted animals. At this time, a number of strategies are used to address specific molecular or physiological questions, and examples are briefly highlighted. In addition, a number of potential problems in the generation and use of transgenic mice in the study of cardiovascular biology are presented.

Transformation of Drosophila melanogaster with the wild-type myosin heavy-chain gene: rescue of mutant phenotypes and analysis of defects caused by overexpression.

We have transformed Drosophila melanogaster with a genomic construct containing the entire wild-type myosin heavy-chain gene, Mhc, together with approximately 9 kb of flanking DNA on each side. Three independent lines stably express myosin heavy-chain protein (MHC) at approximately wild-type levels. The MHC produced is functional since it rescues the mutant phenotypes of a number of different Mhc alleles: the amorphic allele Mhc1, the indirect flight muscle and jump muscle-specific amorphic allele Mhc10, and the hypomorphic allele Mhc2. We show that the Mhc2 mutation is due to the insertion of a transposable element in an intron of Mhc. Since a reduction in MHC in the indirect flight muscles alters the myosin/actin protein ratio and results in myofibrillar defects, we determined the effects of an increase in the effective copy number of Mhc. The presence of four copies of Mhc results in overabundance of the protein and a flightless phenotype. Electron microscopy reveals concomitant defects in the indirect flight muscles, with excess thick filaments at the periphery of the myofibrils. Further increases in copy number are lethal. These results demonstrate the usefulness and potential of the transgenic system to study myosin function in Drosophila. They also show that overexpression of wild-type protein in muscle may disrupt the function of not only the indirect flight but also other muscles of the organism.

Genetic and transgenic approaches to dissecting muscle development and contractility using the Drosophila model system.

Both genetic and transgenic analyses of Drosophila melanogaster, the common fruit fly, are providing important insights into the mechanisms of muscle cell determination and development, myofibril assembly, and muscle contraction. This model system affords tremendous advantages such as ease of isolating mutants defective in these processes, determining the identity of affected genes, and analyzing protein function by transformation with in vitro mutagenized versions of such genes. These approaches have identified a series of proteins that are critical to mesoderm and muscle determination, many of which are likely to serve similar roles in vertebrates. The effects of mutating structural protein genes upon myofibril assembly and function in Drosophila help to define the differential roles of contractile protein isoforms and the importance of proper protein stoichiometry for physiologic function. These studies may also provide insight into the role of structural proteins in vertebrate contractility.

Analysis of Drosophila paramyosin: identification of a novel isoform which is restricted to a subset of adult muscles.

In this report we show that Drosophila melanogaster muscles contain the standard form of the thick filament protein paramyosin, as well as a novel paramyosin isoform, which we call miniparamyosin. We have isolated Drosophila paramyosin using previously established methods. This protein is approximately 105 kD and cross-reacts with polyclonal antibodies made against Caenorhabditis elegans or Heliocopris dilloni paramyosin. The Heliocopris antibody also cross-reacts with a approximately 55-kD protein which may be miniparamyosin. We have cloned and sequenced cDNA's encoding both Drosophila isoforms. Standard paramyosin has short nonhelical regions at each terminus flanking the expected alpha-helical heptad repeat seen in other paramyosins and in myosin heavy chains. The COOH-terminal 363 amino acids are identical in standard and miniparamyosin. However, the smaller isoform has 114 residues at the NH2 terminus that are unique as compared to the current protein sequence data base. The paramyosin gene is located at chromosome position 66E1. It appears to use two promoters to generate mRNA's that have either of two different 5' coding sequences joined to common 3' exons. Each protein isoform is encoded by two transcripts that differ only in the usage of polyadenylation signals. This results in four size classes of paramyosin mRNA which are expressed in a developmentally regulated pattern consistent with that observed for other muscle-specific RNA's in Drosophila. In situ hybridization to Drosophila tissue sections shows that standard paramyosin is expressed in all larval and adult muscle tissues whereas miniparamyosin is restricted to a subset of the adult musculature. Thus miniparamyosin is a novel muscle-specific protein that likely plays a role in thick filament structure or function in some adult muscles of Drosophila.

Alternative RNA splicing generates transcripts encoding a thorax-specific isoform of Drosophila melanogaster myosin heavy chain.

Genomic and cDNA sequencing studies show that transcripts from the muscle myosin heavy-chain (MHC) gene of Drosophila melanogaster are alternatively spliced, producing RNAs that encode at least two MHC isoforms with different C termini. Transcripts encoding an MHC isoform with 27 unique C-terminal amino acids accumulate during both larval and adult muscle differentiation. Transcripts for the second isoform encode one unique C-terminal amino acid and accumulate almost exclusively in pupal and adult thoracic segments, the location of the indirect flight muscles. The 3' splice acceptor site preceding the thorax-specific exon is unusually purine rich and thus may serve as a thorax-specific splicing signal. We suggest that the alternative C termini of these two MHC isoforms control myofilament assembly and may play a role in generating the distinctive myofilament organizations of flight muscle and other muscle types.