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"Caulfield, James"
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Calcium regulation in the human myocardium affected by dilated cardiomyopathy: A structural basis for impaired Ca2+-sensitivity
Calcium regulation in the human heart is impaired during idiopathic dilated cardiomyopathy (IDC). Here, we analyze the structural basis for impairment in the regulatory mechanism. Regulation of contractility was monitored by MgATPase and Ca2+-binding assays as a function of calcium. Myofibrillar proteolysis and expression of troponin T isoforms were established by gel electrophoresis and by Western blots. Myofibrillar ATPase assays in low salt however, revealed a drastic lowering of calcium sensitivity in IDC myofibrils as indicated by reductions in both activation by high calcium and in EGTA-mediated inhibition of MgATPase. Structural changes in myofilament proteins were found in most IDC hearts, specifically proteolysis of myosin light chain 2 (LC2), troponin T and I (TnT and TnI), and sometimes a large isoform shift in TnT. IDC did not induce mutations in LC2 and troponin C (TnC), as established by cDNA sequence data from IDC cases, thus, calcium binding to IDC myofibrils was unaffected. Reassociation of IDC myofibrils with native LC2 raised MgATPase activation at high Ca2+ to control levels, while repletion with intact, canine TnI/TnT restored inhibition at low Ca2+. A model, identifying possible steps in the steric blocking mechanism of regulation, is proposed to explain IDC-induced changes in Ca2+-regulation. Moreover, shifts in TnT isoforms may imply either a genetic or a compensatory factor in the development and pathogenesis of some forms of IDC.
Journal Article
Myofibrillar protein structure and assembly during idiopathic dilated cardiomyopathy
A neutral protease, mekratin, active in human hearts at end stage idiopathic dilated cardiomyopathy (IDC), mediates the breakdown of cardiac myosin LC2. Myosin purified from IDC heart tissue forms unusually short synthetic thick filaments. Therefore, determination of filament length and mekratin distribution in IDC heart muscle were initiated. Native thick filaments were prepared directly from control and IDC tissues and analyzed. Also, paraffin-embedded tissue sections were stained with a fluorescently-labeled anti-protease antibody to establish its distribution in myocardial tissues. Control sections had only very weak, background levels of fluorescence whereas IDC sections stained intensely throughout, indicating a wide ranging distribution of the protease within the myocyte cytoplasm. SDS-PAGE revealed LC2 to be present in stoichiometric amounts in control but greatly reduced in IDC heart muscle. Native thick filaments from control myocardium were structurally stable. They had a median length of 1.65 microm with well-defined bare zones and displayed the 43 nm helical periodicity typical of the relaxed arrangement of myosin heads close to the filaments' shafts. In contrast, native IDC filaments were less stable, and had a median length of 0.9 microm. These filaments were highly disordered: they had no surface periodicity and myosin heads were positioned away from the filaments' shafts. The shorter, less stable, aperiodic thick filaments from IDC hearts appear to result from depletion of LC2 caused by increased activity of mekratin in the IDC myocardium.
Journal Article
Cloning of the cDNA and nucleotide sequence of a skeletal muscle protease from myopathic hamsters
A neutral protease with an estimated Mr of about 26 kD and responsible for cleavage ofmyosin LC2 was isolated from hamster skeletal muscle. Complementary DNAs were generated by RT-PCR using total hamster muscle RNA and degenerate oligonucleotide primers based on the sequences of two internal peptides. The nucleotide sequences of the resultant cDNAs were subsequently determined and the complete amino acid sequence of the protease deduced. Although the hamster protein shared 63-85% identity in nucleotide and amino acid sequences with rat and mouse mast cell proteases, it had a higher degree of specificity for myosin LC2 than mast cell proteases which also digested myosin LC1 and myosin heavy chains. As a result, the hamster protease was designated mekratin because of its unique enzymatic specificities to distinguish it from other mast cell proteases. A polyclonal antibody was raised specific to the hamster muscle and human cardiac muscle mekratins without apparent cross-reaction with rat mast cell proteases. We have earlier demonstrated the presence in excess of a neutral protease that specifically cleaves LC2 in human hearts obtained at end stage idiopathic dilated cardiomyopathy (IDC). Western analyses revealed that heart tissue from patients with IDC contained 5-10 fold more mekratin than control samples. Furthermore, the level of the protease in human IDC tissues was similar to that seen in myopathic hamster skeletal muscle. No bands were recognized by the antibody when IDC myofibrils were probed due to the removal of soluble proteins during sample preparation. Thus, these results strongly suggest that the anti-mekratin antibody will provide positive identification of IDC in many cases and diagnosis by exclusion may be replaced.
Journal Article
Proteolytic Degradation of Fas Ligand by Yersinia pestis
Yersinia pestis, the causative agent of the disease plague, produces the plasminogen activator Pla, a surface-exposed protease that is critical for the progression of the pneumonic form of the disease. Pla is required for bacterial outgrowth in the lungs and to elicit the massive host inflammatory response observed during the later stages of infection. However, the specific host substrates cleaved by Pla to modulate these innate immune responses have not yet been reported. As a means of substrate discovery, a proteomic-scale approach utilizing a peptide microarray was used to identify additional host factors targeted by this protease. By this method, we discovered that Pla directly inactivates the host apoptotic signaling molecule Fas ligand (FasL). Pla is sufficient for FasL cleavage as demonstrated utilizing E. coli inducibly expressing pla, while the catalytic activity of Pla was confirmed as necessary using active site point mutants. FasL glycosylation is required for recognition by Pla, which likely cleaves FasL at multiple sites for its complete inactivation. Functionally, degradation of FasL by Pla prevents the induction of Fas-dependent caspase-3/7 activation and apoptotic signaling cascades, which have been implicated as otherwise protective to the host during bacterial pneumonias. In a murine model of pneumonic plague, the loss of active FasL leads to enhanced outgrowth of Y. pestis in the lungs, reduced cytokine secretion, and elimination of caspase-3/7 activation. In the absence of Pla, increased caspase-3/7 activation is observed in neutrophils recruited to inflammatory lesions within the lungs. Specific inhibition of caspase-3/7 with the peptide inhibitor DEVD recapitulates the loss of FasL by enhancing bacterial outgrowth and reducing cytokine secretion, indicating that the cleavage of FasL by Pla overcomes these caspase-3/7-dependent host defenses. This study provides the first example of a bacterium targeting the Fas-FasL signaling pathway through the specific degradation of FasL, and presents a previously unidentified pathogenic mechanism by which Y. pestis controls host cell death and inflammation during pneumonia.
Dissertation
Influence of the Cardiac Myosin Hinge Region on Contractile Activity
The participation of cardiac myosin hinge in contractility was investigated by in vitro motility and ATPase assays and by measurements of sarcomere shortening. The effect on contractile activity was analyzed using an antibody directed against a 20-amino acid peptide within the hinge region of myosin. This antibody bound specifically at the hinge at a distance of 55 nm from the S1/S2 junction, was specific to human, dog, and rat cardiac myosins, did not crossreact with gizzard or skeletal myosin, and had no effect on ATPase activity of purified S1 and myofibrils. However, it completely suppressed the movement of actin filaments in in vitro motility assays and reduced active shortening of sarcomeres of skinned cardiac myocytes by half. Suppression of motion by the antihinge antibody may reflect a mechanical constraint imposed by the antibody upon the mobility of the S2 region of myosin. The results suggest that the steps in the mechanochemical energy transduction can be separately influenced through S2.
Journal Article