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PubMed2006

Assembling the myofibril: coordinating contractile cable construction with calcium.

Over the last half century, major theoretical and experimental advances have been made in understanding the molecular architecture (e.g., sarcomeric organization) and biophysics (e.g., excitation-contraction coupling) of striated muscle. Studies of how the contractile apparatus is assembled have a shorter history, but our understanding has deepened considerably over the last decade. This review focuses on spontaneous intracellular calcium (Ca2+) signals and their role in skeletal muscle myofibrillogenesis. In embryonic skeletal muscle, several classes of spontaneous Ca2+ signal occur both in vivo and in culture, and blocking their production prevents de novo sarcomere assembly. This review includes a brief overview of myofibrillogenesis, discussion of spontaneous Ca2+ signals produced in embryonic skeletal muscle, the Xenopus model system, the role of Ca2+ signals in regulating assembly of the three major filament systems (actin, titin, and myosin), integration of physiological and biochemical approaches to the problem, and the clinical relevance of basic research in this area. Interspersed throughout are suggestions for future directions and citations for reviews in closely related areas not covered herein.

Animals↗
PubMed1994

Oscillatory contraction of single sarcomere in single myofibril of glycerinated, striated adductor muscle of scallop.

Unloaded contraction of a single sarcomere was studied on fiber fragments (1-micron thick, 4- to 6-microns wide, and less than 50 microns long) from glycerinated scallop striated muscle. The fragment was activated from rigor by a rapid solution exchange method. Sarcomere length (SL) was measured in the intensity profile caused by scanning the phase-contrast image of the striation of the fragment every 5.4 ms using an optical scanner-photomultiplier system with an accuracy of 0.04 micron. The sarcomere started to contract with a latency of 44 +/- 11 ms (mean +/- SD, n = 21) after the solution change. Averaging a few SLs in a fragment clearly revealed a phase, in which shortening was close to zero, preceded and followed by a rapid shortening phase. The contraction of a single sarcomere appeared to occur in an oscillatory manner with one or more of the zero-velocity phases during the observed period (up to 250 ms) when the data points were approximated with a polynominal curve. The distance between the near zero-velocity phases was 0.17 +/- 0.09 micron (n = 18). As the contraction was approximated with a staircase pattern consisting of a rapid shortening and a zero-velocity phase, the duration of the zero-velocity phase was 44 +/- 18 ms (n = 23). The velocity and the extent of the rapid shortening phase were 19.0 +/- 7.9 microns/sarcomere (n = 29) and 0.16 +/- 0.06 micron (20 degrees C), respectively, both of which were independent of the SL examined (1.7-2.6 microns). The velocity in the shortening phase was independent of the extent of the shortening but decreased with increasing duration of the shortening phase. Probably because of the loss of light chains relating to the Ca-regulation from some myosins during preparation, an increase in the concentration of ATP in the absence of Ca2+ resulted in a single shortening step of the sarcomere with a latency of 82 +/- 23 ms, followed by a phase of little shortening; the velocity in and the extent of the shortening step were 12.8 +/- 8.4 microns/s/sarcomere (n = 5) and 0.12 +/- 0.04 micron, respectively. The mechanism underlying the observed oscillatory contraction was considered in terms of the cross-bridge mechanism.

Animals↗
PubMed1999

Plakoglobin is essential for myocardial compliance but dispensable for myofibril insertion into adherens junctions.

Plakoglobin (gamma-catenin), a member of the armadillo family of proteins, is a constituent of the cytoplasmic plaque of cardiac junctions and is involved in anchorage of cytoskeletal filaments to specific cadherins. Its genetic inactivation leads to an embryonic lethal phenotype due to heart dysfunction related to an impairment in the architecture of intercalated discs and in the stability of the heart tissue. To elucidate the functional consequences of the loss of plakoglobin for myofibrillar function, we monitored passive stress-strain relationship and contractility parameters of demembranated embryonic fibers. Heart fibers obtained from plakoglobin-deficient embryonic mice were significantly less compliant than were fibers from wild-type embryos. This difference was especially pronounced at lower fiber extension levels: at 120% of slack length, compliance was 2.5-fold lower in plakoglobin-deficient mice than in the corresponding wild-type group. Contractile paramenters (force per cross-section; Ca2+ sensitivity of isometric force and shortening velocity at near-zero load) were comparable in all experimental groups. Therefore, we suggest that plakoglobin is important for cardiac compliance but not necessary for the attachment of the myofibrillar apparatus to adherens junctions. Thus, we conclude that the loss of function of desmosomes and the profound disarrangement of junctional components in plakoglobin null embryos is associated with a decreased passive compliance, which may explain the ventricular rupture and consequent pericardial tamponade in embryos lacking plakoglobin.

Animals↗
PubMed1999

Creatine kinase reaction in skinned rat psoas muscle fibers and their myofibrils.

The aim of this study was to evaluate myofibrillar creatine kinase (EC 2.7.3.2) activity on the background of the effect of substrate channeling by myosin ATPase and to compare it with creatine kinase (CK) activity of whole skinned fibers. In order to assess CK activity, skinned fibers were prepared from the rat psoas major muscles defined by light microscopy. The activity in permeabilized fibers after treatment with saponin, Triton X-100 and Ca(2+)-free medium reached 2.80, 6.97 and 3.32 micromol ATP min(-1) mg(-1) protein, respectively, when a coupled enzyme assay system with external hexokinase and glucose-6-phosphate dehydrogenase was used. Transmission electron microscopy (TEM) revealed a possible interference among activities of sarcolemmal, sarcoplasmic, myofibrillar and mitochondrial CK from persisting structures. For evaluation of the myofibrillar CK itself, a pure myofibrillar fraction was prepared. Fraction purity was confirmed by TEM and by enzymatic assays for marker enzymes. Two procedures, i.e. the coupled enzyme assay and the evaluation of phosphocreatine (PCr) concentration before and after the CK reaction, were used for measurement of CK activity in this fraction. The procedures resulted in 3.2 nmol ATP min(-1) mg(-1) protein and 7.6 nmol PCr min(-1) mg(-1) protein, respectively. These alternative approaches revealed a discrepancy between the reacting portions of PCr by more than 50 %, which provides information about the size of the effect, generally described as substrate channeling.

Adenosine Triphosphate↗
PubMed1979

[Myofibril breakdown during spreading damage. II. A calcium-free medium].

Peculiarities of Zenker's degeneration (ZD) have been investigated in fast muscle fibres of the frog incubated in a Ringer solution free of Ca++ (R--Ca) with a normal or increased (by 100 mM) concentration of KCl. ZD in these solutions is distinguished by a 10--90 minutes delay of the appearance of the primary contraction knot and cessation of ZD development in the majority of fibres after formation of several (1--5) contraction knots. In the presence of 0.5 mM EDTA in R--Ca, after a few typical contraction knots are formed, fibres commonly fall into large fragments that retain cross-striation. Contracted or super-contracted state of sarcomeres in detached contraction knots and at the necrosis boundary, as well as an increasing lysis of contactile material and proliferation of fibre membrane structures in the region of ZD arrested boundary, are characteristic of ultrastructural changes during ZD in calcium-free solutions.

Animals↗