Structural states in the Z band of skeletal muscle correlate with states of active and passive tension

The effect of neuromuscular electrical stimulation applied at different muscle lengths on muscle architecture and sarcomere morphology in rats

Neuromuscular electrical stimulation (NMES) is often used to increase muscle strength and functionality. Muscle architecture is important for the skeletal muscle functionality. The aim of this study was to investigate the effects of NMES applied at different muscle lengths on skeletal muscle architecture. Twenty-four rats were randomly assigned to four groups (two NMES groups and two control groups). NMES was applied on the extensor digitorum longus muscle at long muscle length, which is the longest and stretched position of the muscle at 170° plantar flexion, and at medium muscle length, which is the length of the muscle at 90° plantar flexion. A control group was created for each NMES group. NMES was applied for 8 weeks, 10 min/day, 3 days/week. After 8 weeks, muscle samples were removed at the NMES intervention lengths and examined macroscopically, and microscopically using a transmission electron microscope and streo-microscope. Muscle damage, and architectural properties of the muscle including pennation angle, fibre length, muscle length, muscle mass, physiological cross-sectional area, fibre length/muscle length, sarcomere length, sarcomere number were then evaluated. There was an increase in fibre length and sarcomere number, and a decrease in pennation angle at both lengths. In the long muscle length group, muscle length was increased, but widespread muscle damage was observed. These results suggest that the intervention of NMES at long muscle length can increase the muscle length but also causes muscle damage. In addition, the greater longitudinal increase in muscle length may be a result of the continuous degeneration-regeneration cycle.

Ultrastructural and kinetic evidence support that thick filaments slide through the Z-disc

How myofilaments operate at short mammalian skeletal muscle lengths is unknown. A common assumption is that thick (myosin-containing) filaments get compressed at the Z-disc. We provide ultrastructural evidence of sarcomeres contracting down to 0.44 µm-approximately a quarter of thick filament resting length-in long-lasting contractions while apparently keeping a regular, parallel thick filament arrangement. Sarcomeres produced force at such extremely short lengths. Furthermore, sarcomeres adopted a bimodal length distribution with both modes below lengths where sarcomeres are expected to generate force in classic force-length measurements. Mammalian fibres did not restore resting length but remained short after deactivation, as previously reported for amphibian fibres, and showed increased forces during passive re-elongation. These findings are incompatible with viscoelastic thick filament compression but agree with predictions of a model incorporating thick filament sliding through the Z-disc. This more coherent picture of mechanical mammalian skeletal fibre functioning opens new perspectives on muscle physiology.

Striated muscle proteins are regulated both by mechanical deformation and by chemical post-translational modification

All cells sense force and build their cytoskeleton to optimize function. How is this achieved? Two major systems are involved. The first is that load deforms specific protein structures in a proportional and orientation-dependent manner. The second is post-translational modification of proteins as a consequence of signaling pathway activation. These two processes work together in a complex way so that local subcellular assembly as well as overall cell function are controlled. This review discusses many cell types but focuses on striated muscle. Detailed information is provided on how load deforms the structure of proteins in the focal adhesions and filaments, using α-actinin, vinculin, talin, focal adhesion kinase, LIM domain-containing proteins, filamin, myosin, titin, and telethonin as examples. Second messenger signals arising from external triggers are distributed throughout the cell causing post-translational or chemical modifications of protein structures, with the actin capping protein CapZ and troponin as examples. There are numerous unanswered questions of how mechanical and chemical signals are integrated by muscle proteins to regulate sarcomere structure and function yet to be studied. Therefore, more research is needed to see how external triggers are integrated with local tension generated within the cell. Nonetheless, maintenance of tension in the sarcomere is the essential and dominant mechanism, leading to the well-known phrase in exercise physiology: "use it or lose it."

Cryo-electron tomography of cardiac myofibrils reveals a 3D lattice spring within the Z-discs

The Z-disc forms a boundary between sarcomeres, which constitute structural and functional units of striated muscle tissue. Actin filaments from adjacent sarcomeres are cross-bridged by α-actinin in the Z-disc, allowing transmission of tension across the myofibril. Despite decades of studies, the 3D structure of Z-disc has remained elusive due to the limited resolution of conventional electron microscopy. Here, we observed porcine cardiac myofibrils using cryo-electron tomography and reconstructed the 3D structures of the actin-actinin cross-bridging complexes within the Z-discs in relaxed and activated states. We found that the α-actinin dimers showed contraction-dependent swinging and sliding motions in response to a global twist in the F-actin lattice. Our observation suggests that the actin-actinin complex constitutes a molecular lattice spring, which maintains the integrity of the Z-disc during the muscle contraction cycle.

Nanoscopic changes in the lattice structure of striated muscle sarcomeres involved in the mechanism of spontaneous oscillatory contraction (SPOC)

Muscles perform a wide range of motile functions in animals. Among various types are skeletal and cardiac muscles, which exhibit a steady auto-oscillation of force and length when they are activated at an intermediate level of contraction. This phenomenon, termed spontaneous oscillatory contraction or SPOC, occurs devoid of cell membranes and at fixed concentrations of chemical substances, and is thus the property of the contractile system per se. We have previously developed a theoretical model of SPOC and proposed that the oscillation emerges from a dynamic force balance along both the longitudinal and lateral axes of sarcomeres, the contractile units of the striated muscle. Here, we experimentally tested this hypothesis by developing an imaging-based analysis that facilitates detection of the structural changes of single sarcomeres at unprecedented spatial resolution. We found that the sarcomere width oscillates anti-phase with the sarcomere length in SPOC. We also found that the oscillatory dynamics can be altered by osmotic compression of the myofilament lattice structure of sarcomeres, but they are unchanged by a proteolytic digestion of titin/connectin—the spring-like protein that provides passive elasticity to sarcomeres. Our data thus reveal the three-dimensional mechanical dynamics of oscillating sarcomeres and suggest a structural requirement of steady auto-oscillation.

Three-dimensional structure of the basketweave Z-band in midshipman fish sonic muscle

Striated muscle enables movement in all animals by the contraction of myriads of sarcomeres joined end to end by the Z-bands. The contraction is due to tension generated in each sarcomere between overlapping arrays of actin and myosin filaments. At the Z-band, actin filaments from adjoining sarcomeres overlap and are cross-linked in a regular pattern mainly by the protein α-actinin. The Z-band is dynamic, reflected by the 2 regular patterns seen in transverse section electron micrographs; the so-called small-square and basketweave forms. Although these forms are attributed, respectively, to relaxed and actively contracting muscles, the basketweave form occurs in certain relaxed muscles as in the muscle studied here. We used electron tomography and subtomogram averaging to derive the 3D structure of the Z-band in the swimbladder sonic muscle of type I male plainfin midshipman fish (Porichthys notatus), into which we docked the crystallographic structures of actin and α-actinin. The α-actinin links run diagonally between connected pairs of antiparallel actin filaments and are oriented at an angle of about 25° away from the actin filament axes. The slightly curved and flattened structure of the α-actinin rod has a distinct fit into the map. The Z-band model provides a detailed understanding of the role of α-actinin in transmitting tension between actin filaments in adjoining sarcomeres.

Myosin filament sliding through the Z-disc relates striated muscle fibre structure to function

Striated muscle contraction requires intricate interactions of microstructures. The classic textbook assumption that myosin filaments are compressed at the meshed Z-disc during striated muscle fibre contraction conflicts with experimental evidence. For example, myosin filaments are too stiff to be compressed sufficiently by the muscular force, and, unlike compressed springs, the muscle fibres do not restore their resting length after contractions to short lengths. Further, the dependence of a fibre's maximum contraction velocity on sarcomere length is unexplained to date. In this paper, we present a structurally consistent model of sarcomere contraction that reconciles these findings with the well-accepted sliding filament and crossbridge theories. The few required model parameters are taken from the literature or obtained from reasoning based on structural arguments. In our model, the transition from hexagonal to tetragonal actin filament arrangement near the Z-disc together with a thoughtful titin arrangement enables myosin filament sliding through the Z-disc. This sliding leads to swivelled crossbridges in the adjacent half-sarcomere that dampen contraction. With no fitting of parameters required, the model predicts straightforwardly the fibre's entire force-length behaviour and the dependence of the maximum contraction velocity on sarcomere length. Our model enables a structurally and functionally consistent view of the contractile machinery of the striated fibre with possible implications for muscle diseases and evolution.

Mechanical Regulation of Cardiac Aging in Model Systems

Unlike diet and exercise, which individuals can modulate according to their lifestyle, aging is unavoidable. With normal or healthy aging, the heart undergoes extensive vascular, cellular, and interstitial molecular changes that result in stiffer less compliant hearts that experience a general decline in organ function. Although these molecular changes deemed cardiac remodeling were once thought to be concomitant with advanced cardiovascular disease, they can be found in patients without manifestation of clinical disease. It is now mostly acknowledged that these age-related mechanical changes confer vulnerability of the heart to cardiovascular stresses associated with disease, such as hypertension and atherosclerosis. However, recent studies have aimed at differentiating the initial compensatory changes that occur within the heart with age to maintain contractile function from the maladaptive responses associated with disease. This work has identified new targets to improve cardiac function during aging. Spanning invertebrate to vertebrate models, we use this review to delineate some hallmarks of physiological versus pathological remodeling that occur in the cardiomyocyte and its microenvironment, focusing especially on the mechanical changes that occur within the sarcomere, intercalated disc, costamere, and extracellular matrix.

Three-Dimensional Structure of Vertebrate Muscle Z-Band: The Small-Square Lattice Z-Band in Rat Cardiac Muscle

The Z-band in vertebrate striated muscle crosslinks actin filaments of opposite polarity from adjoining sarcomeres and transmits tension along myofibrils during muscular contraction. It is also the location of a number of proteins involved in signalling and myofibrillogenesis; mutations in these proteins lead to myopathies. Understanding the high-resolution structure of the Z-band will help us understand its role in muscle contraction and the role of these proteins in the function of muscle. The appearance of the Z-band in transverse-section electron micrographs typically resembles a small-square lattice or a basketweave appearance. In longitudinal sections, the Z-band width varies more with muscle type than species: slow skeletal and cardiac muscles have wider Z-bands than fast skeletal muscles. As the Z-band is periodic, Fourier methods have previously been used for three-dimensional structural analysis. To cope with variations in the periodic structure of the Z-band, we have used subtomogram averaging of tomograms of rat cardiac muscle in which subtomograms are extracted and compared and similar ones are averaged. We show that the Z-band comprises four to six layers of links, presumably α-actinin, linking antiparallel overlapping ends of the actin filaments from the adjoining sarcomeres. The reconstruction shows that the terminal 5-7nm of the actin filaments within the Z-band is devoid of any α-actinin links and is likely to be the location of capping protein CapZ.

The structure and regulation of human muscle α-actinin

The spectrin superfamily of proteins plays key roles in assembling the actin cytoskeleton in various cell types, crosslinks actin filaments, and acts as scaffolds for the assembly of large protein complexes involved in structural integrity and mechanosensation, as well as cell signaling. α-actinins in particular are the major actin crosslinkers in muscle Z-disks, focal adhesions, and actin stress fibers. We report a complete high-resolution structure of the 200 kDa α-actinin-2 dimer from striated muscle and explore its functional implications on the biochemical and cellular level. The structure provides insight into the phosphoinositide-based mechanism controlling its interaction with sarcomeric proteins such as titin, lays a foundation for studying the impact of pathogenic mutations at molecular resolution, and is likely to be broadly relevant for the regulation of spectrin-like proteins.

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Structure of human α-actinin-2 in an autoinhibited closed conformation

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Facilitation of PIP2-induced allosteric modulation for opening and titin binding

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Essentiality of structural flexibility for crosslinking antiparallel F-actin

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Relevance for the intramolecular pseudoligand regulation mechanism of the spectrin family

The sarcomeric Z-disc and Z-discopathies

The sarcomeric Z-disc defines the lateral borders of the sarcomere and has primarily been seen as a structure important for mechanical stability. This view has changed dramatically within the last one or two decades. A multitude of novel Z-disc proteins and their interacting partners have been identified, which has led to the identification of additional functions and which have now been assigned to this structure. This includes its importance for intracellular signalling, for mechanosensation and mechanotransduction in particular, an emerging importance for protein turnover and autophagy, as well as its molecular links to the t-tubular system and the sarcoplasmic reticulum. Moreover, the discovery of mutations in a wide variety of Z-disc proteins, which lead to perturbations of several of the above-mentioned systems, gives rise to a diverse group of diseases which can be termed Z-discopathies. This paper provides a brief overview of these novel aspects as well as points to future research directions.

Effect of aging on cellular mechanotransduction

Aging is becoming a critical heath care issue and a burgeoning economic burden on society. Mechanotransduction is the ability of the cell to sense, process, and respond to mechanical stimuli and is an important regulator of physiologic function that has been found to play a role in regulating gene expression, protein synthesis, cell differentiation, tissue growth, and most recently, the pathophysiology of disease. Here we will review some of the recent findings of this field and attempt, where possible, to present changes in mechanotransduction that are associated with the aging process in several selected physiological systems, including musculoskeletal, cardiovascular, neuronal, respiratory systems and skin.

The sarcomeric cytoskeleton: who picks up the strain?

In striated muscle sarcomeres, the contractile actin and myosin filaments are organised by a subset of specialised cytoskeletal proteins, the sarcomeric cytoskeleton. They include α-actinin, myomesin, and the giant proteins titin, obscurin and nebulin, which combine architectural, mechanical and signalling functions. Mechanics and signalling in the sarcomere appear tightly interdependent, but the exact contributions of the various sarcomeric cytoskeleton proteins to strain handling or signalling are only just emerging. General mechanisms of cytoskeletal mechanics and signalling may be gleaned from the sarcomere as a specialised actomyosin system. Recent work has led to insight into the interactions, structure, and mechanical stability of sarcomeric protein complexes that fulfil both structural and signalling roles.

The vertebrate muscle Z-disc: sarcomere anchor for structure and signalling

The Z-disc, appearing as a fine dense line forming sarcomere boundaries in striated muscles, when studied in detail reveals crosslinked filament arrays that transmit tension and house myriads of proteins with diverse functions. At the Z-disc the barbed ends of the antiparallel actin filaments from adjoining sarcomeres interdigitate and are crosslinked primarily by layers of α-actinin. The Z-disc is therefore the site of polarity reversal of the actin filaments, as needed to interact with the bipolar myosin filaments in successive sarcomeres. The layers of α-actinin determine the Z-disc width: fast fibres have narrow (~30–50 nm) Z-discs and slow and cardiac fibres have wide (~100 nm) Z-discs. Comprehensive reviews on the roles of the numerous proteins located at the Z-disc in signalling and disease have been published; the aim here is different, namely to review the advances in structural aspects of the Z-disc.

Inter-sarcomere coordination in muscle revealed through individual sarcomere response to quick stretch

The force generation and motion of muscle are produced by the collective work of thousands of sarcomeres, the basic structural units of striated muscle. Based on their series connection to form a myofibril, it is expected that sarcomeres are mechanically and/or structurally coupled to each other. However, the behavior of individual sarcomeres and the coupling dynamics between sarcomeres remain elusive, because muscle mechanics has so far been investigated mainly by analyzing the averaged behavior of thousands of sarcomeres in muscle fibers. In this study, we directly measured the length-responses of individual sarcomeres to quick stretch at partial activation, using micromanipulation of skeletal myofibrils under a phase-contrast microscope. The experiments were performed at ADP-activation (1 mM MgATP and 2 mM MgADP in the absence of Ca(2+)) and also at Ca(2+)-activation (1 mM MgATP at pCa 6.3) conditions. We show that under these activation conditions, sarcomeres exhibit 2 distinct types of responses, either "resisting" or "yielding," which are clearly distinguished by the lengthening distance of single sarcomeres in response to stretch. These 2 types of sarcomeres tended to coexist within the myofibril, and the sarcomere "yielding" occurred in clusters composed of several adjacent sarcomeres. The labeling of Z-line with anti-alpha-actinin antibody significantly suppressed the clustered sarcomere "yielding." These results strongly suggest that the contractile system of muscle possesses the mechanism of structure-based inter-sarcomere coordination.

Flies deficient in Muscleblind protein model features of myotonic dystrophy with altered splice forms of Z-band associated transcripts

Myotonic dystrophy (DM) is a dominantly inherited neuromuscular disorder characterised by muscle weakness and wasting. There are two forms of DM; both of which are caused by the expansion of repeated DNA sequences. DM1 is associated with a CTG repeat located in the 3' untranslated region of a gene, DMPK and DM2 with a tetranucleotide repeat expansion, CCTG, located in the first intron of a different gene, ZNF9. Recent data suggest a dominant RNA gain-of-function mechanism underlying DM, as transcripts containing either CUG or CCUG repeat expansions accumulate as foci in the nuclei of DM1 and DM2 cells respectively, where they exert a toxic effect, sequestering specific RNA binding proteins such as Muscleblind, which leads to splicing defects and the disruption of normal cellular functions. Z-band disruption is a well-known histological feature of DM1 muscle, which has also been reported in Muscleblind deficient flies. In order to determine whether there is a common molecular basis for this abnormality we have examined the alternative splicing pattern of transcripts that encode proteins associated with the Z-band in both organisms. Our results demonstrate that the missplicing of ZASP/LDB3 leads to the expression of an isoform in DM1 patient muscle, which is not present in normal controls, nor in other myopathies. Furthermore the Drosophila homologue, CG30084, is also misspliced, in Muscleblind deficient flies. Another Z-band transcript, alpha actinin, is misspliced in mbl mutant flies, but not in DM1 patient samples. These results point to similarities but subtle differences in the molecular breakdown of Z-band structures in flies and DM patients and emphasise the relevance of Muscleblind proteins in DM pathophysiology.

At the crossroads of myocardial signaling: the role of Z-discs in intracellular signaling and cardiac function

Understanding the molecular interactions among components of cardiac Z-discs and their role in signaling has become pivotal in explaining long- and short-term regulation of cardiac function. In striated muscle, the ends of the thin filaments from opposing sarcomeres overlap and are cross-linked by an elaborate array of proteins to form a highly ordered, yet dynamic network that is the Z-disc. We review here a current picture of the function and structure of the Z-disc of mammalian cardiac myocytes. We emphasize provocative findings that advance new theories about the place of cardiac Z-discs in myocardial intra- and intercellular signaling in myocardial physiology and pathology. Relatively new approaches, especially yeast two-hybrid screens, immunoprecipitation, and pull down assays, as well as immunohistochemical analysis have significantly altered previous views of the protein content of the Z-disc. These studies have generally defined domain structure and binding partners for Z-disc proteins, but the functional significance of the binding network and of the domains in cardiac cell biology remains an unfolding story. Yet, even at the present level of understanding, perceptions of potential functions of the Z-disc proteins are expanding greatly and leading to new and exciting experimental approaches toward mechanistic understanding. The theme of the following discussion of these Z-disc proteins centers on their potential to function not only as a physical anchor for myofilament and cytoskeletal proteins, but also as a pivot for reception, transduction, and transmission of mechanical and biochemical signals.

The three-dimensional structure of a vertebrate wide (slow muscle) Z-band: lessons on Z-band assembly

The vertebrate muscle Z-band organizes and tethers antiparallel actin filaments in adjacent sarcomeres and hence propagates the tension generated by the actomyosin interaction during muscular contraction. The axial width of the Z-band varies with fibre and muscle type: fast twitch muscles have narrow (approximately 30-50 nm) Z-bands, while slow-twitch and cardiac muscles have wide (approximately 100-140 nm) Z-bands. In electron micrographs of longitudinal sections of fast fibres like those found in fish body white muscle, the Z-band appears as a characteristic zigzag layer of density connecting the mutually offset actin filament arrays in adjacent sarcomeres. Wide Z-bands in slow fibres such as the one studied here (bovine neck muscle) show a stack of three or four zigzag layers. The variable Z-band width incorporating variable numbers of zigzag layers presumably relates to the different mechanical properties of the respective muscles. Three-dimensional reconstructions of Z-bands reveal that individual zigzag layers are often composed of more than one set of protein bridges, called Z-links, probably alpha-actinin, between oppositely oriented actin filaments. Fast muscle Z-bands comprise two or three layers of Z-links. Here we have applied Fourier reconstruction methods to obtain clear three-dimensional density maps of the Z-bands in beef muscle. The bovine slow muscle investigated here reveals a Z-band comprising six sets of Z-links, which, due to their shape and the way their projected densities overlap, appear in longitudinal sections as either three or four zigzag layers, depending on the lattice view. There has been great interest recently in the suggestion that Z-band variability with fibre type may be due to differences in the repetitive region (tandem Z-repeats) in the Z-band part of titin (also called connectin). We discuss this in the context of our results and present a systematic classification of Z-band types according to the numbers of Z-links and titin Z-repeats.

Three-dimensional structure of a vertebrate muscle Z-band: implications for titin and alpha-actinin binding

The Z-band in vertebrate striated muscles, mainly comprising actin filaments, alpha-actinin, and titin, serves to organise the antiparallel actin filament arrays in adjacent sarcomeres and to transmit tension between sarcomeres during activation. Different Z-band thicknesses, formed from different numbers of zigzag crosslinking layers and found in different fibre types, are thought to be associated with the number of repetitive N-terminal sequence domains of titin. In order to understand myofibril formation it is necessary to correlate the ultrastructures and sequences of the actin filaments, titin, and alpha-actinin in characteristic Z-bands. Here electron micrographs of the intermediate width, basketweave Z-band of plaice fin muscle have been subject to a novel 3D reconstruction process. The reconstruction shows that antiparallel actin filaments overlap in the Z-band by about 22-25 nm. There are three levels of Z-links (probably alpha-actinin) in which at each level two nearly diametrically opposed links join an actin filament to two of its antiparallel neighbours. One set of links is centrally located in the Z-band and there are flanking levels orthogonal to this. A 3D model of the observed structure shows how Z-bands of different widths may be formed and it provides insights into the structural arrangements of titin and alpha-actinin in the Z-band. The model shows that the two observed symmetries in different Z-bands, c2 and p12(1), may be attributed respectively to whether the number of Z-link levels is odd or even.

Exchange of alpha-actinin in isolated rigor myofibrils

In the current study, the process of alpha-actinin binding to the myofibrillar Z-line was investigated to determine its mechanism. Pretreatment of rigor myofibrils with unlabeled alpha-actinin did not prevent or slow the incorporation of fluorescein skeletal alpha-actinin into myofibrils suggesting that incorporation was not the filling of empty binding sites but rather an exchange reaction. Further support for this was obtained using quantitative measures of labeled alpha-actinin incorporation and measures of total myofibrillar alpha-actinin. These results showed that there was no change in myofibrillar alpha-actinin content when up to 15% of the total alpha-actinin was the labeled protein. Measurement of the time-course of fluorescein alpha-actinin incorporation by quantitative fluorescence microscopy showed that the increase in Z-line fluorescence was well described by a rapid (unresolved) incorporation of fluorescence followed by a much slower phase. The slower phase was independent of fluorescein alpha-actinin concentration (2.5-160 nM) and had an apparent rate of 0.008-0.016 min(-1). Pretreatment of myofibrils with fluorescein alpha-actinin followed by incubation with unlabeled alpha-actinin resulted in a decrease in Z-line fluorescence with an apparent rate of 0.021 min(-1). The slow phase was interpreted as representing the dissociation rate of intrinsic Z-line alpha-actinin. Thus, the dissociation rate for the in situ interaction of alpha-actinin with actin appears to be three orders of magnitude slower than that determined from solution studies.

Z-line structural diversity in frog single muscle fiber in the passive state

The structural changes of the Z-line between small square net (ss) and basket weave (bw) cross-sectional patterns were examined using intact single fibers and mechanically skinned fibers in the passive state to determine if the pattern is related to the sarcomere length (SL) and if the pattern undergoes a reversible transition in low- and high-osmotic medium. Frog single fibers were isolated from the anterior tibial muscle in Ringer's solution. Entirely or partially skinned single fibers were prepared in relaxing solution (also called low-osmotic medium). The high osmotic medium contained 10% polyvinylpyrrolidone (PVP) in relaxing solution. The sarcomere length (SL) of each fiber was measured directly by use of a laser beam or indirectly from electron micrographs with use of a correction factor. The ss and bw forms in cross sections were quantified by analysis of electron micrographs. The results show that the structural change of Z-line occurs around bw << 2.3-2.4 microns << ss (n = 25) and bw << 3.1-3.2 microns << ss (n = 13) in intact single fibers and skinned fibers, respectively. With the quick freeze-freeze substitution method, an intact single fiber with a SL of 2.35 microns showed almost 100% of ss form. The structural transition in cross section was also confirmed in four partially skinned fibers, where patterns went from mostly ss form (intact portion) to mostly bw form (skinned portion) at the SL between 2.40 to 3.20 microns. The reversibility of the change between ss and bw was proved by using low- and high-osmotic medium. The transition and reversion of cross-sectional patterns both occur in the passive state.

Synchronous behavior of spontaneous oscillations of sarcomeres in skeletal myofibrils under isotonic conditions

An isotonic control system for studying dynamic properties of single myofibrils was developed to evaluate the change of sarcomere lengths in glycerinated skeletal myofibrils under conditions of spontaneous oscillatory contraction (SPOC) in the presence of inorganic phosphate and a high ADP-to-ATP ratio. Sarcomere length oscillated spontaneously with a peak-to-peak amplitude of about 0.5 microns under isotonic conditions in which the external loads were maintained constant at values between 1.5 x 10(4) and 3.5 x 10(4) N/m2. The shortening and yielding of sarcomeres occurred in concert, in contrast to the previously reported conditions (isomeric or auxotonic) under which the myofibrillar tension is allowed to oscillate. This synchronous SPOC appears to be at a higher level of synchrony than in the organized state of SPOC previously observed under auxotonic conditions. The period of sarcomere length oscillation did not largely depend on external load. The active tension under SPOC conditions increased as the sarcomere length increased from 2.1 to 3.2 microns, although it was still smaller than the tension under normal Ca2+ contraction (which is on the order of 10(5) N/m2). The synchronous SPOC implies that there is a mechanism for transmitting information between sarcomeres such that the state of activation of sarcomeres is affected by the state of adjacent sarcomeres. We conclude that the change of myofibrillar tension is not responsible for the SPOC of each sarcomere but that it affects the level of synchrony of sarcomere oscillations.

Volume changes during contraction of isolated frog muscle fibers

A microscope objective and electronic imaging system were used to determine how isolated frog skeletal muscle fibers adjust their volume during an isometric tetanus. Cross-sectional area and volume of the middle third of a fiber increased rapidly with the development of active tension, which indicates that contraction produced components of force perpendicular to the long axis. The extreme ends are known to shorten whether or not the middle of a fiber is isometric or stretched. Shortening of the ends may shift water towards the middle, which could account for the volume changes we observed. The cytoskeletal matrices of muscle evidently adjust rapidly during contraction to maintain a dynamic equilibrium between the axial and radial forces that stabilize the whole cell. The Z disks have been shown to expand during active, but not passive, tension development. Z disks might be the elastic elements of the muscle cytoskeleton primarily involved in rapid balancing of the radial components of active force.

Vertebrate muscle Z-line structure: an electron microscopic study of negatively-stained myofibrils

Structural features of the Z-lines of rabbit psoas muscle myofibrils have been studied in the electron microscope with a negative staining technique. The results obtained suggest the presence of about 20 nm periodicity in the structural organization of the Z-line region: a band pattern of five bands of extra density spaced about 20 nm apart was revealed in the Z-region and the Z-filaments connecting actin filaments from neighbouring sarcomeres often appeared to be positioned at intervals of 17-20 nm. An electron microscopic investigation of the interaction in vitro of two major Z-line proteins, alpha-actinin and F-actin, indicated that the positions of alpha-actinin bridges between actin filaments are defined by relative azimuthal positions of actin subunits. A possible arrangement of actin-linking macromolecular bridges in the Z-region is considered. It is supposed that the arrangement of the Z-filaments is related to the helical symmetry of actin-containing filaments. Also, the banded appearance of the Z-region is interpreted as arising from the arrangement of crossbridges connecting thin filaments of the same sarcomeres.

Similar features in Z bands of both skeletal and cardiac muscle revealed by image enhancement

We have shown previously that the small square (ss) and basket weave (bw) states of the Z band lattice in cardiac and skeletal muscle are related to the contractile state of the muscle. We have used two-dimensional image processing techniques on digitized electron micrographs to enhance the structural features of each projected lattice form in cardiac and skeletal muscle. Four different processing techniques were employed to assess the effect of enhancement artifacts on the resulting Z band images. We observed only slight differences between enhanced images of a particular Z band form produced by the four different techniques. Every enhanced image showed an approximate four-fold symmetry independent of muscle type or Z band lattice form. Each enhanced image showed four cross-connecting Z-filaments which appeared to connect each axial filament to the four nearest axial filaments. In bw images from both cardiac and skeletal muscle, axial filaments had a greater apparent diameter and a greater interaxial filament spacing than in the ss images. In both muscle types, the cross-connecting Z-filaments appeared to overlap half-way between axial filaments in the ss images while the bw images showed no such overlap. These structural features are consistent with a dynamic Z band lattice that participates in muscle contraction.

Nonuniform volume changes during muscle contraction

We measured dynamic changes in volume during contraction of live, intact frog skeletal muscle fibers through a high-speed, intensified, digital-imaging microscope. Optical cross-sections along the axis of resting cells were scanned and compared with sections during the plateau of isometric tetanic contractions. Contraction caused an increase in volume of the central third of a cell when axial force was maximum and constant and the central segment was stationary or lengthened slightly. But changes were unequal along a cell and not predicted by a cell's resting area or shape (circularity). Rapid local adjustments in the cytoskeletal evidently keep forces in equilibrium during contraction of living skeletal muscle. These results also show that optical signals may be distorted by nonuniform volume changes during contraction.

Two structural states of the vertebrate Z band

Ultrastructural analysis of the vertebrate Z band suggests that two reversible states of a single intricate lattice are essential for the contractile process. The two structural states of the Z band lattice (ss and bw) have been described in cross section in skeletal and cardiac muscle in different physiological states. The lattice responds to active tension but resists passive deformation. Changes in Z band form and dimension are correlated with cross-bridge binding. Two-dimensional image processing techniques show enhanced structural features that vary with the observed changes in lattice dimension. All projected images from all lattices show an approximate four-fold symmetry. Each image reveals differences in the appearance of axial filaments which enter from opposite sides of the Z band and cross-connecting filaments of similar curvature which appear to connect each axial filament to four nearest axial filaments. In the ss images, the apparent diameter of cross-cut axial filaments and the Z band interaxial filament spacing are smaller than in bw images. Cross-connecting filaments appear to overlap in the region half-way between axial filaments in ss images. We conclude that the Z band is an essential and dynamic part of the sarcomere, uniquely suited to transmit tension while maintaining dimensions appropriate for cross-bridge interaction.

The Z-band lattice in skeletal muscle in rigor

Previous work with tetanized and relaxed muscle has shown a correlation between active tension and the structure of the Z-band. This suggests that there is a correlation between the cross-bridge binding in the A-band and the structure of the Z-band. Using electron microscopy and optical diffraction we have examined this correlation in glycerinated muscle in rigor and in unstimulated intact muscle. We have found that the Z-bands of muscles in rigor always show the basketweave form, while those of the unstimulated muscles always show the small square form. The basketweave form found in rigor muscles is similar in form and dimension to that found in tetanized muscle. Thus it appears that the small square form of the Z-band is found in physiological states with little cross-bridge binding and the basketweave form is found in states with a high degree of cross-bridge binding.