ACTIN FROM HEART MUSCLE: ISOLATION, PURIFICATION, AND PHYSICOCHEMICAL PROPERTIES

Effective top-down LC/MS+ method for assessing actin isoforms as a potential cardiac disease marker

Actin is the major component of the cytoskeleton, playing an essential role in the structure and motility of both muscle and nonmuscle cells. It is highly conserved and encoded by a multigene family. α-Cardiac actin (αCAA) and α-skeletal actin (αSKA), encoded by two different genes, are the primary actin isoforms expressed in striated muscles. The relative expression levels of αSKA and αCAA have been shown to vary between species and under pathological conditions. In particular, an increased αSKA expression is believed to be a programmed response of a diseased heart. Therefore, it is essential to quantify the relative expression of αSKA and αCAA, which remains challenging due to the high degree of sequence similarity between these isoforms (98.9%). Herein, we developed a top-down liquid chromatography/mass spectrometry-based ("LC/MS+") method for the rapid purification and comprehensive analysis of α-actin extracted from muscle tissues. We thoroughly investigated all of the actin isoforms in healthy human cardiac and skeletal muscles. We found that αSKA is the only isoform expressed in skeletal muscle, whereas αCAA and αSKA are coexpressed in cardiac muscle. We then applied our method to quantify the α-actin isoforms in human healthy hearts and failing hearts with dilated cardiomyopathy (DCM). We found that αSKA is augmented in DCM compared with healthy controls, 43.1 ± 0.9% versus 23.7 ± 1.7%, respectively. As demonstrated, top-down LC/MS+ provides an effective and comprehensive method for the purification, quantification, and characterization of α-actin isoforms, enabling assessment of their clinical potential as cardiac disease markers.

Experimental myocarditis induced in Swiss mice by homologous heart immunization resembles chronic experimental Chagas' heart disease

The Swiss mouse is considered a satisfactory model for experimental chronic chagasic myocarditis and there is some evidence of an immunopathologic mechanism in the development of this disease. To further support this conjecture, 45-day-old albino Swiss mice (40 animals) were immunized with homologous heart in complete Freund's adjuvant. As controls, 20 animals were likewise inoculated with allogeneic testis, as "non-related" antigen. Three mice from the former group died suddenly at 19-21 days postinoculation while the survivors were sacrificed at 60 days for serum samples, and histologic analysis of the heart and skeletal muscle. Electrocardiographic records were taken at Days 0, 30, and 60 postinoculation. Of myocardium-inoculated animals and testis-inoculated mice 33/37 (89%) and 1/20 (5%), respectively, exhibited myocarditis (P less than 0.001). Histologic lesions were highly reminiscent of those observed in chronic experimental Chagas' disease of Swiss mice. Antimuscle antibodies were seen, by indirect immunofluorescence employing cryostat sections, in 30/33 (91%) of the former group and in 3/20 (15%) of the latter (P less than 0.001), some of which recognized a surface antigen of primary cultured fetal rat myocardiocytes. Mice inoculated with myocardium also exhibited electrocardiographic abnormalities consisting in QRS interval widening. Results show that following an autoimmune experimental design the main features of chronic chagasic myocarditis may be reproduced in the Swiss mouse. This agrees with the likely role of an immunopathologic mechanism in heart damage due to Trypanosoma cruzi infection.

Purification of actin from cardiac muscle

Actin from cardiac acetone and ether powders is compared to actin from skeletal acetone powder using a modification of an established extraction procedure. The yield of actin from cardiac ether powder is nearly the same as the yield from skeletal acetone powder whereas significantly less actin is obtained from cardiac acetone powder. Sodium dodecyl sulfate polyacrylamide gel electrophoresis shows the actins from each of the sources to be virtually identical in terms of purity and mobility. Molecular sieve chromatography of G-actin demonstrates cardiac actin from ether powder to have identical polymerization and mobility properties as skeletal actin from acetone powder. A simplified procedure, developed for highly purified actin from muscle powder and prepared in a single day is presented. The storage of F-actin at -80 degrees C is also discussed.

p-NN'-phenylenebismaleimide, a specific cross-linking agent for F-actin

Covalent cross-links can be inserted between the subunits of F-actin by using p-NN'-phenylenebismaleimide. Cross-linking reaches its maximum value when one molecule of reagent has reacted with each actin subunit. p-NN'-Phenylenebismaleimide reacts initially with a cysteine residue on one subunit, the slower cross-linking reaction involving a lysine residue on a neighbouring subunit. Hydrolysis of the actin-bound reagent limits the extent of cross-linking. Quantitative analysis of the amounts of cross-linked oligomers seen on polyacrylamide gels containing sodium dodecyl sulphate suggests that neither the binding of the reagent to actin nor the formation of cross-links introduces strain into the structure. The cross-links do not join together different F-actin filaments, and evidence is presented that suggests that the cross-links join subunits of the same long-pitched helix.

Heart contractile proteins

That several proteins of the sarcomere differ from one muscle to the next is well documented, and it is becoming evident that homogeneous muscles, like the heart, are also species specific. 1) Clear-cut evidence is available concerning myosin, and, to date, several types of molecules have been described. a) The myosins of white skeletal, heart, and smooth muscle differ in the activity of their Ca2+ and K+ATPases, as also in the structure of their light subunits. b) The Ca2+ATPases of the various cardiac myosins have been shown to exhibit species differences and correlate with the speed of shortening of the muscle. 2) The structures of tropomyosin, some troponin components, and alpha actinin (but not actin) appear to be unlike in the different types of muscle. 3) These phylogenic modifications may be related to the changes characteristic of the particular muscles under pathological conditions, which are accompanied by substantial increase in protein synthesis.

Leucocyte myosin and its location in the cell

The intracellular location of the binding site of antibody against purified myosin prepared from equine leucocytes was investigated in neutrophils and lymphocytes by electron microscopy using peroxidase-labelled antibody method. The myosin extracted from equine leucocytes could bind skeletal muscle F-actin and the formed complex showed the biophysical and biochemical properties and electron microscopic appearance of actomyosin. On immunodiffusion, the leucocyte myosin formed a single precipitin line with its antibody prepared in rabbits. The antibody also formed single precipitin lines with myosins from lymphocytes and thrombocytes, fusing with each other. The antibody against the leucocyte myosin did not react with myosins from skeletal or arterial smooth muscle. The specificity of the antibody was further established by determination of K+-EDTA-activated ATPase activity remained in the supernate of antigen-antibody mixture. Under electron microscope, the intracellular immunoreactive products of peroxidase labelled antibody were found in cytoplasm of neutrophils and lymphocytes incubated with antibody against leucocyte myosin, but not in neutrophils or lymphocytes treated with IgG from normal rabbits.

Evidence from electron microscope studies on actin paracrystals concerning the origin of the cross-striation in the thin filaments of vertebrate skeletal muscle

When purified F-actin is precipitated by Mg 2+ , it forms paracrystals consisting of regularly packed filaments that retain the double-helical structure characteristic of actin polymers. In negatively stained preparations the only axial periodicity observed is that of the actin polymers. In sections, the polymer structure is not resolved and the paracrystals do not appear cross-striated. The paracrystals formed from unpurified actin preparations likewise contain regularly packed filaments in which the F-actin structure is observed but, in addition, the assembly of filaments is crossed at fairly regular intervals (approximately 37 nm) by bands of unstained amorphous material. Sections of these paracrystals show a corresponding cross-striation (mean spacing 38 nm). Other proteins known to be present in unpurified preparations of actin were prepared separately and added in solution to purified F-actin; paracrystals were formed on Mg 2+ precipitation. The cross-striation was reproduced when a mixture of tropomyosin and troponin had been added to the actin. The addition of purified tropomyosin alone resulted in paracrystals that lacked the cross-striation. Comparison of the cross-striation in the paracrystals with that in the thin filament assembly of the myofibril (as seen in sections and in negatively stained I-segments) supports the conclusion that filaments resembling the natural ones have been synthesized from actin, tropomyosin and the troponin complex. It follows that the cross-striation in the thin myo­filaments can be attributed to the location of part or all of the troponin complex at sites spaced at regular intervals along the filaments.

Regulation of cardiac muscle contractility

The heart's physiological performance, unlike that of skeletal muscle, is regulated primarily by variations in the contractile force developed by the individual myocardial fibers. In an attempt to identify the basis for the characteristic properties of myocardial contraction, the individual cardiac contractile proteins and their behavior in contractile models in vitro have been examined. The low shortening velocity of heart muscle appears to reflect the weak ATPase activity of cardiac myosin, but this enzymatic activity probably does not determine active state intensity. Quantification of the effects of Ca(++) upon cardiac actomyosin supports the view that myocardial contractility can be modified by changes in the amount of calcium released during excitation-contraction coupling. Exchange of intracellular K(+) with Na(+) derived from the extracellular space also could enhance myocardial contractility directly, as highly purified cardiac actomyosin is stimulated when K(+) is replaced by an equimolar amount of Na(+). On the other hand, cardiac glycosides and catecholamines, agents which greatly increase the contractility of the intact heart, were found to be without significant actions upon highly purified reconstituted cardiac actomyosin.