Connectin, an elastic filamentous protein of striated muscle

Changes in the molecular types of connectin and nebulin during development of chicken skeletal muscle

Changes in the molecular types of connectin and nebulin during development of chicken breast and leg muscles were determined by an improved SDS-polyacrylamide gel electrophoresis (PAGE) using 2% polyacrylamide slab gel. The adult leg-type alpha-connectin (alpha L-connectin) and nebulin (L-nebulin) appeared in embryonic breast muscle, and changed into the adult breast-type ones (alpha B-connectin, B-nebulin) specific for adult breast muscle after hatching. In leg muscle, alpha L-connectin and L-nebulin appeared in an embryonic stage, and remained unchanged in molecular types throughout the entire process of development. alpha-Connectin and nebulin seemed to be regulated by a similar mechanism during development. On the other hand, beta-connectin appeared in an earlier stage of development of the embryonic breast muscle, independently of alpha-connectin.

Passive tension in cardiac muscle: contribution of collagen, titin, microtubules, and intermediate filaments

The passive tension-sarcomere length relation of rat cardiac muscle was investigated by studying passive (or not activated) single myocytes and trabeculae. The contribution of collagen, titin, microtubules, and intermediate filaments to tension and stiffness was investigated by measuring (1) the effects of KCl/KI extraction on both trabeculae and single myocytes, (2) the effect of trypsin digestion on single myocytes, and (3) the effect of colchicine on single myocytes. It was found that over the working range of sarcomeres in the heart (lengths approximately 1.9-2.2 microns), collagen and titin are the most important contributors to passive tension with titin dominating at the shorter end of the working range and collagen at longer lengths. Microtubules made a modest contribution to passive tension in some cells, but on average their contribution was not significant. Finally, intermediate filaments contributed about 10% to passive tension of trabeculae at sarcomere lengths from approximately 1.9 to 2.1 microns, and their contribution dropped to only a few percent at longer lengths. At physiological sarcomere lengths of the heart, cardiac titin developed much higher tensions (> 20-fold) than did skeletal muscle titin at comparable lengths. This might be related to the finding that cardiac titin has a molecular mass of 2.5 MDa, 0.3-0.5 MDa smaller than titin of mammalian skeletal muscle, which is predicted to result in a much shorter extensible titin segment in the I-band of cardiac muscle. Passive stress plotted versus the strain of the extensible titin segment showed that the stress-strain relationships are similar in cardiac and skeletal muscle. The difference in passive stress between cardiac and skeletal muscle at the sarcomere level predominantly resulted from much higher strains of the I-segment of cardiac titin at a given sarcomere length. By expressing a smaller titin isoform, without changing the properties of the molecule itself, cardiac muscle is able to develop significant levels of passive tension at physiological sarcomere lengths.

Myofibrillogenesis in rodent skeletal muscle in vitro: two pathways involving thick filament aggregates

Thick filament aggregates play an important role in myofibrillogenesis in rodent skeletal muscle in vitro. This ultrastructural study describes these aggregates, shows their involvement in the process of myofibril formation, and correlates their appearance and function with current models of myofibrillogenesis. Initially, following myoblast fusion in normal mouse skeletal muscle in vitro, abundant stress fiber-like structures (SFLS) are found near the periphery of early myotubes. These undergo internal rearrangements, forming subcortical sarcomeres and early myofibrils. However, additional thick filaments are synthesized, and some join appositionally to the nascent myofibrils, increasing their diameter. More interiorly, this thick filament synthesis accelerates, with filaments aligning into aggregates resembling discrete A-bands, usually with M-lines and M-regions. The ends of these 'A-band' aggregates are infiltrated with ribosomes and capped by flocculent material. Ultimately, aggregates are incorporated into preexisting myofibrils or associate end-to-end to form new, parallel myofibrils, the flocculent material forming putative I-bands with diminished Z-lines and few thin filaments. As differentiation continues, Z-lines and thin filaments appear, forming true myofibrils. Dysgenic mouse skeletal muscle develops similarly, but when this non-contractile cell matures (i.e., generates action potentials), filaments and their organization break down. Cloned myogenic rat L5/A10 cells also follow this developmental pattern, but in mature, contracting myotubes, Z-lines remain irregular and thin filaments are reduced. In all three types of muscle developing in vitro, thick filament aggregates are a common and predominant feature and as such appear to constitute an additional or alternate pathway to previously described models of myofibrillogenesis.

Microscopic analysis of the elastic properties of nebulin in skeletal myofibrils

The elastic properties of nebulin were studied by measuring the elasticity of single skeletal myofibrils, from which the portion of the thin filament located at the I band had been selectively removed by treatment with plasma gelsolin under rigor conditions. In this myofibril model, a portion of each nebulin molecule at the I band was expected to be free of actin filaments and exposed. The length of the exposed portion of the nebulin molecule was controlled by performing the gelsolin treatment at various sarcomere lengths. The relation between the passive tension and extension of the exposed portion of the nebulin showed a convex curve starting from a slack length, apparently in a fashion similar to that of wool. The slack sarcomere length shifted depending on the length of the exposed portion of the nebulin, however, the relation being represented by a single master curve. The elastic modulus of nebulin was estimated to be two to three orders of magnitude smaller than that of an actin filament. Based on these results, we conclude that nebulin attaches to an actin filament in a side-by-side fashion and that it does not significantly contribute to the elastic modulus of thin filaments. The relation between the passive tension and extension of connectin (titin) was obtained for a myofibril from which thin filaments had been completely removed with gelsolin under contracting conditions; this showed a concave curve, consistent with the previous results obtained in single fibers.

Immunofluorescent studies on titin and myosin in developing hearts of normal and cardiac mutant axolotls

Homozygous recessive cardiac mutant gene c in the axolotl, Ambystoma mexicanum, results in a failure of the embryonic heart to initiate beating. Previous studies show that mutant axolotl hearts fail to form sarcomeric myofibrils even though hearts from their normal siblings exhibit organized myofibrils beginning at stage 34-35. In the present study, the proteins titin and myosin are studied using normal (+/+) axolotl embryonic hearts at stages 26-35. Additionally, titin is examined in normal (+/c) and cardiac mutant (c/c) embryonic axolotl hearts using immunofluorescent microscopy at stages 35-42. At tailbud stage 26, the ventromedially migrating sheets of precardiac mesoderm appear as two-cell-layers. Myosin shows periodic staining at the cell peripheries of the presumptive heart cells at this stage, whereas titin is not yet detectable by immunofluorescent microscopy. At preheartbeat stages 32-33, a myocardial tube begins to form around the endocardial tube. In some areas, periodic myosin staining is found to be separated from the titin staining; other areas in the heart at this stage show a co-localization of the two proteins. Both titin and myosin begin to incorporate into myofibrils at stage 35, when normal hearts initiate beating. Additionally, areas with amorphous staining for both proteins are observed at this stage. These observations indicate that titin and myosin accumulate independently at very early premyofibril stages; the two proteins then appear to associate closely just before assembly into myofibrils. Staining for titin in freshly frozen and paraffin-embedded tissues of normal embryonic hearts at stages 35, 39, and 41 reveals an increased organization of the protein into sarcomeres as development progresses. The mutant siblings, however, first show titin staining only limited to the peripheries of yolk platelets. Although substantial quantities of titin accumulate in mutant hearts at later stages of development (39 and 41), it does not become organized into myofibrils as in normal cells at these stages.

Cellular titin localization in stress fibers and interaction with myosin II filaments in vitro

We previously discovered a cellular isoform of titin (originally named T-protein) colocalized with myosin II in the terminal web domain of the chicken intestinal epithelial cell brush border cytoskeleton (Eilertsen, K.J., and T.C.S. Keller. 1992. J. Cell Biol. 119:549-557). Here, we demonstrate that cellular titin also colocalizes with myosin II filaments in stress fibers and organizes a similar array of myosin II filaments in vitro. To investigate interactions between cellular titin and myosin in vitro, we purified both proteins from isolated intestinal epithelial cell brush borders by a combination of gel filtration and hydroxyapatite column chromatography. Electron microscopy of brush border myosin bipolar filaments assembled in the presence and absence of cellular titin revealed a cellular titin-dependent side-by-side and end-to-end alignment of the filaments into highly ordered arrays. Immunogold labeling confirmed cellular titin association with the filament arrays. Under similar assembly conditions, purified chicken pectoralis muscle titin formed much less regular aggregates of muscle myosin bipolar filaments. Sucrose density gradient analyses of both cellular and muscle titin-myosin supramolecular arrays demonstrated that the cellular titin and myosin isoforms coassembled with a myosin/titin ratio of approximately 25:1, whereas the muscle isoforms coassembled with a myosin:titin ratio of approximately 38:1. No coassembly aggregates were found when cellular myosin was assembled in the presence of muscle titin or when muscle myosin was assembled in the presence of cellular titin. Our results demonstrate that cellular titin can organize an isoform-specific association of myosin II bipolar filaments and support the possibility that cellular titin is a key organizing component of the brush border and other myosin II-containing cytoskeletal structures including stress fibers.

Passive and active tension in single cardiac myofibrils

Single myofibrils were isolated from chemically skinned rabbit heart and mounted in an apparatus described previously (Fearn et al., 1993; Linke et al., 1993). We measured the passive length-tension relation and active isometric force, both normalized to cross sectional area. Myofibrillar cross sectional area was calculated based on measurements of myofibril diameter from both phase-contrast images and electron micrographs. Passive tension values up to sarcomere lengths of approximately 2.2 microns were similar to those reported in larger cardiac muscle specimens. Thus, the element responsible for most, if not all, passive force of cardiac muscle at physiological sarcomere lengths appears to reside within the myofibrils. Above 2.2 microns, passive tension continued to rise, but not as steeply as reported in multicellular preparations. Apparently, structures other than the myofibrils become increasingly important in determining the magnitude of passive tension at these stretched lengths. Knowing the myofibrillar component of passive tension allowed us to infer the stress-strain relation of titin, the polypeptide thought to support passive force in the sarcomere. The elastic modulus of titin is 3.5 x 10(6) dyn cm-2, a value similar to that reported for elastin. Maximum active isometric tension in the single myofibril at sarcomere lengths of 2.1-2.3 microns was 145 +/- 35 mN/mm2 (mean +/- SD; n = 15). This value is comparable with that measured in fixed-end contractions of larger cardiac specimens, when the amount of nonmyofibrillar space in those preparations is considered. However, it is about 4 times lower than the maximum active tension previously measured in single skeletal myofibrils under similar conditions (Bartoo et al., 1993).

Connectin, an elastic protein of striated muscle

Connectin, also called titin, a giant elastic protein of striated muscle (approximately 3000 kDa) mainly consists of fibronectin type III and immunoglobulin C2 globular domains, the beta-sheets of which are parallel to the main axis of the molecule. One connectin molecule runs through the I band and binds onto the myosin filament up to the M line starting from the Z line. It positions the myosin filament at the center of a sarcomere. Connectin is also responsible for resting tension generation. Biodiversity of the connectin family exists in invertebrate muscle.

Sequential appearance of muscle-specific proteins in myoblasts as a function of time after cell division: evidence for a conserved myoblast differentiation program in skeletal muscle

Based on the assumption that a conserved differentiation program governs the assembly of sarcomeres in skeletal muscle in a manner analogous to programs for viral capsid assembly, we have defined the temporal and spatial distribution of 10 muscle-specific proteins in mononucleated myoblasts as a function of the time after terminal cell division. Single cells in mitosis were identified in monolayer cultures of embryonic chicken pectoralis, followed for selected time points (0-24 h postmitosis) by video time-lapse microscopy, and then fixed for immunofluorescence staining. For convenience, the myoblasts were termed x-h-old to define their age relative to their mitotic "birthdate." All 6 h myoblasts that emerged in a mitogen-rich medium were desmin+ but only 50% were positive for a alpha-actin, troponin-I, alpha-actinin, MyHC, zeugmatin, titin, or nebulin. By 15 h postmitosis, approximately 80% were positive for all of the above proteins. The up-regulation of these 7 myofibrillar proteins appears to be stochastic, in that many myoblasts were alpha-actinin+ or zeugmatin+ but MyHC- or titin- whereas others were troponin-I+ or MyHC+ but alpha-actinin- or alpha-actin-. In 15-h-old myoblasts, these contractile proteins were organized into nonstriated myofibrils (NSMFs). In contrast to striated myofibrils (SMFs), the NSMFs exhibited variable stoichiometries of the sarcomeric proteins and these were not organized into any consistent pattern. In this phase of maturation, two other changes occurred: (1) the microtubule network was reorganized into parallel bundles, driving the myoblasts into polarized, needle-shaped cells; and (2) the sarcolemma became fusion-competent. A transition from NSMFs to SMFs took place between 15 and 24 h (or later) postmitosis and was correlated with the late appearance of myomesin, and particularly, MyBP-C (C protein). The emergence of one, or a string of approximately 2 mu long sarcomeres, was invariably characterized by the localization of myomesin and MyBP-C to their mature positions in the developing A-bands. The latter group of A-band proteins may be rate-limiting in the assembly program. The great majority of myoblasts stained positively for desmin and myofibrillar proteins prior to, rather than after, fusing to form myotubes. This sequential appearance of muscle-specific proteins in vitro fully recapitulates myofibrillar assembly steps in myoblasts of the myotome and limb bud in vivo, as well as in nonmuscle cells converted to myoblasts by MyoD. We suggest that this cell-autonomous myoblast differentiation program may be blocked at different control points in immortalized myogenic cell lines.

Maximum velocity of shortening in relation to myosin isoform composition in single fibres from human skeletal muscles

1. Maximum velocity of shortening (Vmax) and compositions of myosin heavy chain (MHC) and myosin light chain (MLC) isoforms were determined in single fibres from the soleus or the lateral region of the quadriceps (vastus lateralis) muscles in man. Muscle samples were obtained by percutaneous biopsy, and membranes were permeabilized by glycerol treatment (chemical skinning) or by freeze-drying. 2. Types I, IIA and IIB MHCs were resolved from single fibre segments by 6% sodium dodecyl sulphate-polyacrylamide gel electrophoresis (SDS-PAGE) and five different fibre types were identified: fibres containing type I MHC, types I and IIA MHCs, type IIA MHC, types IIA and IIB MHCs, and type IIB MHC. Only a few fibres co-expressed types I and IIA MHCs but 28% of all quadriceps fibres expressed both IIA and IIB MHCs in variable proportions. Fibres co-expressing types I and IIB MHCs were not found. 3. Alkali (MLC1 and MLC3) and dithio nitrobenzoic acid (DTNB) (MLC2) myosin light chains were observed in all type II fibres in variable proportions. MLC (MLC1s and MLC2s) isoforms from type I fibres had lower migration rates than the corresponding isoforms from type II fibres (MLC1f and MLC2f). More than half of type I fibres in both soleus (65%) and quadriceps (68%) muscles also expressed 'fast' MLC3 and 36% of the type II fibres from quadriceps muscle expressed the slow isoform of MLC2. 4. Differences were observed in some mechanical characteristics of freeze-dried versus chemically skinned fibres. Maximum tension (P0) and specific tension were lower in freeze-dried types I and IIA fibres than in chemically skinned, while no differences were observed in the IIA/B fibres. The numbers of types I/IIA and IIB fibres were too low to allow statistical comparisons. In chemically skinned fibres, mean specific tension (0.20 +/- 0.01 N/mm2) did not vary with fibre type. In freeze-dried fibres, on the other hand, specific tensions varied according to MHC type: higher (P < 0.01) specific tensions were observed in types IIB (0.19 +/- 0.01 N/mm2) and type IIA/B fibres (0.18 +/- 0.04 N/mm2) than in type I fibres (0.12 +/- 0.02 N/mm2). The specific tension of type IIA fibres (0.12 +/- 0.05 N/mm2) did not differ significantly from the other fibre types. Cross-sectional areas and mean Vmax did not differ between freeze-dried and chemically skinned fibres, either when all fibres were pooled or within respective fibre types. Vmax data from all fibres of a given type, irrespective of membrane permeabilization technique, have therefore been pooled.(ABSTRACT TRUNCATED AT 400 WORDS)

Passive tension and stiffness of vertebrate skeletal and insect flight muscles: the contribution of weak cross-bridges and elastic filaments

Tension and dynamic stiffness of passive rabbit psoas, rabbit semitendinosus, and waterbug indirect flight muscles were investigated to study the contribution of weak-binding cross-bridges and elastic filaments (titin and minititin) to the passive mechanical behavior of these muscles. Experimentally, a functional dissection of the relative contribution of actomyosin cross-bridges and titin and minititin was achieved by 1) comparing mechanically skinned muscle fibers before and after selective removal of actin filaments with a noncalcium-requiring gelsolin fragment (FX-45), and 2) studying passive tension and stiffness as a function of sarcomere length, ionic strength, temperature, and the inhibitory effect of a carboxyl-terminal fragment of smooth muscle caldesmon. Our data show that weak bridges exist in both rabbit skeletal muscle and insect flight muscle at physiological ionic strength and room temperature. In rabbit psoas fibers, weak bridge stiffness appears to vary with both thin-thick filament overlap and with the magnitude of passive tension. Plots of passive tension versus passive stiffness are multiphasic and strikingly similar for these three muscles of distinct sarcomere proportions and elastic proteins. The tension-stiffness plot appears to be a powerful tool in discerning changes in the mechanical behavior of the elastic filaments. The stress-strain and stiffness-strain curves of all three muscles can be merged into one, by normalizing strain rate and strain amplitude of the extensible segment of titin and minititin, further supporting the segmental extension model of resting tension development.

Characterization of beta-connectin (titin 2) from striated muscle by dynamic light scattering

Connectin (titin) is a large filamentous protein (single peptide) with a molecular mass of approximately 3 MDa, contour length approximately 900 nm, and diameter approximately 4 nm, and resides in striated muscle. Connectin links the thick filaments to the Z-lines in a sarcomere and produces a passive elastic force when muscle fiber is stretched. The aim of this study is to elucidate some aspects of physical properties of isolated beta-connectin (titin 2), a proteolytic fragment of connectin, by means of dynamic light-scattering (DLS) spectroscopy. The analysis of DLS spectra for beta-connectin gave the translational diffusion coefficient of 3.60 x 10(-8) cm2/s at 10 degrees C (or the hydrodynamic radius of 44.1 nm), molecular mass little smaller than 3.0 MDa (for a literature value of sedimentation coefficient), the root-mean-square end-to-end distance of 163 nm (or the radius of gyration of 66.6 nm), and the Kuhn segment number of 30 and segment length of 30 nm (or the persistence length of 15 nm). These results permitted to estimate the flexural rigidity of 6.0 x 10(-20) dyn x cm2 for filament bending, and the elastic constant of 7 dyn/cm for extension of one persistence length. Based on a simple model, implications of the present results in muscle physiology are discussed.

Distribution of connectin (titin), nebulin and alpha-actinin at myotendinous junctions of chicken pectoralis muscles: an immunofluorescence and immunoelectron microscopic study

The distribution of connectin (titin), nebulin and alpha-actinin in the areas of myotendinous junctions of chicken pectoralis muscles was examined by immunocytochemical methods. Staining with antibodies against connectin (4C9, SM1 and P1200) and nebulin formed 'doublets' flanking nonterminal Z-bands; near the end of muscle fibres 'singlets' were seen within the terminal sarcomere on the side adjacent to the terminal Z-bands. The apical regions of muscle processes, where no myosin filaments are present although actin filaments exist, were reactive with anti-nebulin but not with anti-connectin. Antibodies against pectoralis (skeletal muscle type) alpha-actinin stained non terminal Z-bands and that against gizzard (smooth muscle type) the sarcolemma. Terminal Z-bands were unreactive with both of these antibodies. These findings indicate that, although terminal and nonterminal Z-bands differ in their molecular composition, connectin and nebulin filaments appear to link myosin and actin filaments, respectively, to both Z-band types.

The globular head domain of titin extends into the center of the sarcomeric M band. cDNA cloning, epitope mapping and immunoelectron microscopy of two titin-associated proteins

Immunoelectron microscopical results have shown that the Z and M bands of the sarcomere are interconnected by the long titin molecules. Here we have characterized by monoclonal antibodies, cDNA cloning and immunoelectron microscopy the two titin-associated proteins (190 and 165 kDa proteins), which seem responsible for the formation of a head structure on one end of the 0.9 micron long titin string. The human 165 kDa (1465 residues) and 190 kDa (1451 residues) proteins have unique N-terminal domains some 110 residues in length. Both proteins show 12 repeat domains with strong homology to either fibronectin type III (motif I) or immunoglobulin C2 (motif II) domains, which are arranged in the order II-II-I-I-I-I-I-II-II-II-II-II. Over these repeat domains the two proteins share 50% sequence identity (70% similarity). Epitopes situated in the C-terminal 138 or in the preceding 206 residues of the 165 kDa protein locate in immunoelectron microscopy to stripes situated 18 or 15 nm from the center of the M band. An epitope situated 277 to 129 residues prior to the C-terminus of the 190 kDa protein (i.e. repeats 10 and 11) locates to the center of the M band. Thus the head structure of the titin molecule extends into the center of the M band. Microsequence data on peptides from the titin-associated bovine 165 kDa protein and from conventionally purified bovine M-protein argue together with the reactivity of the antibodies that 165 kDa protein and M-protein are identical. The integrating structure of the sarcomere, which is based on titin and its side-on (C-protein and 86 kDa protein) or end-on (190 kDa protein and 165 kDa protein) associated proteins arises from muscle-specific members of the superfamily of immunoglobulin-like proteins.

Viscoelasticity of the sarcomere matrix of skeletal muscles. The titin-myosin composite filament is a dual-stage molecular spring

The mechanical roles of sarcomere-associated cytoskeletal lattices were investigated by studying the resting tension-sarcomere length curves of mechanically skinned rabbit psoas muscle fibers over a wide range of sarcomere strain. Correlative immunoelectron microscopy of the elastic titin filaments of the endosarcomeric lattice revealed biphasic extensibility behaviors and provided a structural interpretation of the multiphasic tension-length curves. We propose that the reversible change of contour length of the extensible segment of titin between the Z line and the end of thick filaments underlies the exponential rise of resting tension. At and beyond an elastic limit near 3.8 microns, a portion of the anchored titin segment that adheres to thick filaments is released from the distal ends of thick filament. This increase in extensible length of titin results in a net length increase in the unstrained extensible segment, thereby lowering the stiffness of the fiber, lengthening the slack sarcomere length, and shifting the yield point in postyield sarcomeres. Thus, the titin-myosin composite filament behaves as a dual-stage molecular spring, consisting of an elastic connector segment for normal response and a longer latent segment that is recruited at and beyond the elastic limit of the sarcomere. Exosarcomeric intermediate filaments contribute to resting tension only above 4.5 microns. We conclude that the interlinked endo- and exosarcomeric lattices are both viscoelastic force-bearing elements. These distinct cytoskeletal lattices appear to operate over two ranges of sarcomere strains and collectively enable myofibrils to respond viscoelastically over a broad range of sarcomere and fiber lengths.

Interplay between passive tension and strong and weak binding cross-bridges in insect indirect flight muscle. A functional dissection by gelsolin-mediated thin filament removal

The interplay between passive and active mechanical properties of indirect flight muscle of the waterbug (Lethocerus) was investigated. A functional dissection of the relative contribution of cross-bridges, actin filaments, and C filaments to tension and stiffness of passive, activated, and rigor fibers was carried out by comparing mechanical properties at different ionic strengths of sarcomeres with and without thin filaments. Selective thin filament removal was accomplished by treatment with the actin-severving protein gelsolin. Thin filament, removal had no effect on passive tension, indicating that the C filament and the actin filament are mechanically independent and that passive tension is developed by the C filament in response to sarcomere stretch. Passive tension increased steeply with sarcomere length until an elastic limit was reached at only 6-7% sarcomere extension, which corresponds to an extension of 350% of the C filament. The passive tension-length relation of insect flight muscle was analyzed using a segmental extension model of passive tension development (Wang, K, R. McCarter, J. Wright, B. Jennate, and R Ramirez-Mitchell. 1991. Proc. Natl. Acad. Sci. USA. 88:7101-7109). Thin filament removal greatly depressed high frequency passive stiffness (2.2 kHz) and eliminated the ionic strength sensitivity of passive stiffness. It is likely that the passive stiffness component that is removed by gelsolin is derived from weak-binding cross-bridges, while the component that remains is derived from the C filament. Our results indicate that a significant number of weak-binding cross-bridges exist in passive insect muscle at room temperature and at an ionic strength of 195 mM. Analysis of rigor muscle indicated that while rigor tension is entirely actin based, rigor stiffness contains a component that resists gelsolin treatment and is therefore likely to be C filament based. Active tension and active stiffness of unextracted fibers were directly proportional to passive tension before activation. Similarly, passive stiffness due to weak bridges also increased linearly with passive tension, up to a limit. These correlations lead us to propose a stress-activation model for insect flight muscle in which passive tension is a prerequisite for the formation of both weak-binding and strong-binding cross-bridges.

Elastic filaments in situ in cardiac muscle: deep-etch replica analysis in combination with selective removal of actin and myosin filaments

To clarify the full picture of the connectin (titin) filament network in situ, we selectively removed actin and myosin filaments from cardiac muscle fibers by gelsolin and potassium acetate treatment, respectively, and observed the residual elastic filament network by deep-etch replica electron microscopy. In the A bands, elastic filaments of uniform diameter (6-7 nm) projecting from the M line ran parallel, and extended into the I bands. At the junction line in the I bands, which may correspond to the N2 line in skeletal muscle, individual elastic filaments branched into two or more thinner strands, which repeatedly joined and branched to reach the Z line. Considering that cardiac muscle lacks nebulin, it is very likely that these elastic filaments were composed predominantly of connectin molecules; indeed, anti-connectin monoclonal antibody specifically stained these elastic filaments. Further, striations of approximately 4 nm, characteristic of isolated connectin molecules, were also observed in the elastic filaments. Taking recent analyses of the structure of isolated connectin molecules into consideration, we concluded that individual connectin molecules stretched between the M and Z lines and that each elastic filament consisted of laterally-associated connectin molecules. Close comparison of these images with the replica images of intact and S1-decorated sarcomeres led us to conclude that, in intact sarcomeres, the elastic filaments were laterally associated with myosin and actin filaments in the A and I bands, respectively. Interestingly, it was shown that the elastic property of connectin filaments was not restricted by their lateral association with actin filaments in intact sarcomeres. Finally, we have proposed a new structural model of the cardiac muscle sarcomere that includes connectin filaments.

Elastic properties of connecting filaments along the sarcomere

The elasticity of the connecting filament--the filament that anchors the thick filament to the Z-line--has been investigated using rigor release, freeze-break and immunolabelling techniques. When relaxed insect flight muscle was stretched and then allowed to go into rigor, then released, the recoil forces of the connecting filaments caused sarcomeres to shorten. Thin filaments, prevented from sliding by rigor links, were found crumpled against the Z-line. Thus, rigor release experiments demonstrate the spring-like nature of the connecting filaments in insect flight muscle. In vertebrate skeletal muscle, however, the same protocol did not result in sarcomere shortening. Absence of shortening was due to either smaller stiffness of connecting filaments and/or higher stiffness of the thin filaments relative to insect flight muscle. The spring-like nature of the connecting filament was confirmed with the freeze break technique. When the frozen sarcomeres were broken along the A-I junction, the broken connecting filaments retracted to the N1-line level, independently of the thin filaments, demonstrating the basic elastic nature of these filaments. To study the elastic properties of the connecting filaments along the sarcomere, the muscle was labelled with monoclonal antibodies against a titin epitope near the N1-line, and another very near the A-I junction in the I-band. Before labelling, fibers were pre-stretched to varying extents.(ABSTRACT TRUNCATED AT 250 WORDS)

Elastic properties of titin filaments demonstrated using a "freeze-break" technique

A "freeze-break" technique (Trombitás, K.: Acta Biochim. Biophys. Hung. 6:419-427, 1971) and immunoelectron microscopy were used to study the elastic properties of titin filaments. Small bundles of freshly prepared rabbit psoas muscle fibers were quickly frozen and broken under liquid nitrogen to fracture sarcomeres in planes perpendicular to the filament axis, in each of various regions along the sarcomere. The still-frozen specimens were thawed during fixation to allow elastic filaments to retract. The broken specimens were then labelled with monoclonal anti-titin antibodies against an unique epitope in the I-band. The titin epitopes were normally positioned symmetrically about the Z-line. However, in sarcomeres broken at the A-I junction, the epitopes no longer remained symmetrical: the titin filaments in the broken half-sarcomere retracted, independently of the thin filaments, forming a dense band just near the Z-line. The retracted density apparently did not reach the Z-line; retraction stopped at the level of the so-called N1-line. In sarcomeres broken at the Z-line level, the titin filaments retracted in the opposite direction. In this case the titin epitope retracted all the way to the ends of the thick filaments. It appears then that titin molecules form elastic filaments that are independent of thin filaments in most of the I-band. Near the Z-line, however, the titin filaments either have an inelastic domain or associate firmly with the thin filaments at the N1-line level.

Unloaded shortening after a quick release of a contracting, single fibre from crayfish slow muscle

The time course of shortening at zero load was studied by the slack test method during tetanic contractions in isolated, single, slow muscle fibres of the crayfish. In 28 of 32 shortenings (from 14 different fibres) a biphasic shortening was seen, which consisted of an initial high-velocity phase lasting 3.3-20.8 ms and a following slow-velocity phase lasting for the entire time examined (up to 89.2 ms). Provided that the shortening occurred uniformly along the fibre length, velocity in the initial phase, V1, of the biphasic shortening was 14.4 +/- 3.4 (mean +/- SD, n = 10) microns s-1 per half sarcomere at Lo, the slack length, at 20 degrees C, while that in the second phase, V2, was 7.4 +/- 1.4 microns s-1 per half sarcomere. Lowering temperature decreased both V1 and V2 with Q10 = 1.4 for V1 and 2.0 for V2. Lowering the external Ca concentration from 15 mM, the standard, to 2 mM resulted in a tetanic tension below one-third of that at 15 mM Ca and decreased both V1 (t test; p < 0.01) and V2 (p < 0.001). Prestretching the fibre to 1.5 Lo had no significant effect on V2 (p < 0.3) but increased V1 (p < 0.001). The distance shortened during the initial high-velocity phase, LV1, was 4.0 +/- 1.8% Lo (mean +/- SD, n = 10) at 20 degrees C or about 0.14 microns per half sarcomere on average. LV1 was independent of the tetanic tension level when it was changed by lowering the external Ca concentration or temperature in the same fibre. Prestretching the fibre to 1.5 Lo, at which the sum of the active and the resting tension was lower than Po at Lo in two of three fibres, increased LV1 significantly (p < 0.001). The independency of LV1 from the tension level indicates that the initial high-velocity phase was not from shortening of some inert components in the fibre. One possibility is that the initial high-velocity phase was brought about by an acceleration of shortening by a compressive force, the origin of which has been discussed. The slow-velocity phase seemed to result from the crossbridge turnover with little exogeneous stress on myofilaments. Four different fibres exhibited an unloaded shortening with a constant velocity during the entire time examined (29.9-61.8 ms). This type of shortening had a velocity between the usual V1 and V2 values, suggesting that a compressive force accelerated the shortening during the entire time.

Structural analysis of muscle development: transverse tubules, sarcoplasmic reticulum, and the triad

Increased interest in the mechanism of excitation-contraction (E-C) coupling over the last few years has been accompanied by numerous investigations into the development of the underlying cellular structures. Areas of particular interest include: (1) the compartmentalization and specialization of an external and an internal membrane system, the T-tubules, and the sarcoplasmic reticulum, respectively; (2) interactions between the membrane proteins of both systems upon the formation of a junction, the triad; and (3) membrane-cytoskeletal interactions leading to the orderly arrangement of the triads with respect to the myofibrils. Structural studies using newly available specific molecular probes and a variety of in vivo and in vitro model systems have provided new insights into the cellular and molecular mechanisms involved in the development of the E-C coupling apparatus in skeletal muscle.

Chicken leg muscle alpha-connectin as studied by a monoclonal antibody to the 1200 kDa fragment

Chicken leg gracilis muscle contained only alpha-connectin (ca 3000 kDa) without beta-connectin. When myofibrils were kept standing for 20 hr at 4 degrees C, alpha-connectin was degraded to beta-connectin (ca 2000 kDa) and 1200 kDa peptide. The latter was prepared from myofibrils and purified by gel filtration in the presence of SDS. A monoclonal antibody, alpha 7, to this 1200 kDa fragment was prepared. The antibody reacted with the 1200 kDa fragment and its mother molecule alpha-connectin, but not with beta-connectin. Immunoelectron microscopy using alpha 7, as well as other antibodies to chicken breast muscle beta-connectin, revealed that the 1200 kDa peptide covered the portion of alpha-connectin from the Z line to the N2 line region in the I band of chicken leg gracilis muscle sarcomeres. The results were in good agreement with those observed in rabbit skeletal muscle.

Identification and characterization of two huge protein components of the brush border cytoskeleton: evidence for a cellular isoform of titin

Two extremely high molecular weight proteins were found to be components of the intestinal epithelial cell brush border cytoskeleton. The largest brush border protein, designated T-protein, migrated on SDS gels as a doublet of polypeptides with molecular weights similar to muscle titin T I and T II. The other large brush border protein, designated N-protein, was found to have a polypeptide molecular weight similar to muscle nebulin. In Western analysis, a polyclonal antibody raised against brush border T-protein reacted specifically with T-protein in isolated brush borders and cross-reacted with titin in pectoralis and cardiac muscle samples. T-protein was distinguished from the muscle titins by an anti-cardiac titin mAb. A polyclonal antibody raised against N-protein was specific for N-protein in brush borders and cross-reacted with nothing in pectoralis muscle. Immunolocalization in cryosections of intestinal epithelia and SDS-PAGE analysis of fractionated brush borders revealed that both T-protein and N-protein are concentrated distinctly in the brush border terminal web region subjacent to the microvilli, but absent from the microvilli. EM of rotary-replicated T-protein samples revealed many of the molecules to be long (912 +/- 40 nm) and fibrous with a globular head on one end. In some of the molecules, the head domain appeared to be extended in a fibrous conformation yielding T-protein up to 1,700-nm long. The brush border N-protein was found as long polymers with a repeating structural unit of approximately 450 nm. Our findings indicate that brush border T-protein is a cellular isoform of titin and suggest that both T-protein and N-protein play structural roles in the brush border terminal web.

Localization and elasticity of connectin (titin) filaments in skinned frog muscle fibres subjected to partial depolymerization of thick filaments

The localization and elasticity of connectin (titin) filaments in skinned fibres of frog skeletal muscle were examined for changes in the localization of connectin and in resting tension during partial depolymerization of thick filaments with a relaxing solution containing increased KCl concentrations. Immunoelectron microscopic studies revealed that deposites of antibodies against connectin at a sarcomere length of 3.0 microns remained at about 0.8 microns from the M-line, until the thick filament was depolymerized to the length of approximately 0.4 microns. On further depolymerization, the bound antibodies were found to move towards the Z-line and, on complete depolymerization, were observed to be within 0.3 microns of the Z-line; a marked decrease in resting tension accompanied this further depolymerization. These results suggest that connectin filament starts from the Z-line, extends to the M-line, and contributes to resting tension. After partial depolymerization of thick filaments, the distances between the anti-connectin deposits and the Z-line and between anti-connectin deposits and the M-line increased with sarcomere length, suggesting that connectin filaments are elastic along their entire length.

Regulation of tension development by MgADP and Pi without Ca2+. Role in spontaneous tension oscillation of skeletal muscle

The length of sarcomeres in isolated myofibrils fixed at both ends spontaneously oscillates when MgADP and Pi coexist with MgATP in the absence of Ca2+ (Okamura, N., and S. Ishiwata, 1988. J. Muscle Res. Cell. Motil. 9:111-119). Here, we report that MgADP and Pi function as an activator and an inhibitor, respectively, of tension development of single skeletal muscle fibers in the absence of Ca2+ and the coexistence of MgADP and Pi with MgATP induces spontaneous tension oscillation. First, the isometric tension sharply increased when the concentration of MgADP became higher than approximately 3x that of MgATP and saturated at approximately 90% of the tension obtained under full Ca2+ activation; in parallel with this sigmoidal increase of tension, MgATPase activity appeared. The inhibition of contraction by the regulatory system seems to be desuppressed by the allosteric effect of actomyosin-ADP complex, similarly to so-called rigor complex. The ADP-induced tension was decreased along a reversed sigmoidal curve by the addition of Pi; actomyosin-ADP-Pi complex, which has no desuppression function, may be formed by exogenous Pi; accompanying the decline of tension, spontaneous oscillations of tension and sarcomere length appeared. It is suggested that the length oscillation of each (half) sarcomere would occur through the transition of cross-bridges between force-generating (on) and non-force-generating (off) states, which may be regulated by the mechanical states (strain) of cross-bridges and/or thin filaments.

Spontaneous oscillation of tension and sarcomere length in skeletal myofibrils. Microscopic measurement and analysis

We have devised a simple method for measuring tension development of single myofibrils by micromanipulation with a pair of glass micro-needles. The tension was estimated from the deflection of a flexible needle under an inverted phase-contrast microscope equipped with an image processor, so that the tension development is always accompanied by the shortening of the myofibril (auxotonic condition) in the present setup. The advantage of this method is that the measurement of tension (1/30 s for time resolution and about 0.05 micrograms for accuracy of tension measurement; 0.05 microns as a spatial resolution for displacement of the micro-needle) and the observation of sarcomere structure are possible at the same time, and the technique to hold myofibrils, even single myofibrils, is very simple. This method has been applied to study the tension development of glycerinated skeletal myofibrils under the condition where spontaneous oscillation of sarcomeres is induced, i.e., the coexistence of MgATP, MgADP and inorganic phosphate without free Ca2+. Under this condition, we found that the tension of myofibrils spontaneously oscillates accompanied by the oscillation of sarcomere length with a main period of a few seconds; the period was lengthened and shortened with stretch and release of myofibrils. A possible mechanism of the oscillation is discussed.

Characterization and localization of alpha-connectin (titin 1): an elastic protein isolated from rabbit skeletal muscle

A simplified procedure to isolate alpha-connectin (titin 1, TI), a gigantic elastic protein, from rabbit skeletal muscle is described. A rapid column chromatography step to concentrate alpha-connectin is introduced. Separation of alpha-connectin from beta-connectin is introduced. Separation of alpha-connectin from beta-connectin (titin 2, TII) in the presence of 4 M urea at pH 7.0 did not cause any change in the secondary structure of alpha-connectin as judged by circular dichroic spectra. Ultraviolet absorption spectra and the amino acid composition of alpha-connectin (MW, approximately 3 x 10(6)) were similar to those of its proteolytic product, beta-connectin (MW, approximately 2 x 10(6)). Circular dichroic spectra suggested that both alpha- and beta-connectin consist of 60% beta-sheet and 30% beta-turn. It thus appears that the whole elastic filament of connectin has a folded beta-strand structure. Proteolysis of alpha-connectin by calpain resulted in formation of beta-connectin and smaller peptides. The alpha-connectin interacted with both myosin and actin filaments similarly to beta-connectin. Polyclonal antibodies raised against 1200 kDa peptides obtained from aged rabbit skeletal myofibrils reacted with alpha-connectin (titin 1, TI) but only weakly with beta-connectin (titin 2, TII) in rabbit skeletal muscle. Immunoelectron microscopy and indirect immunofluorescence microscopy revealed that the antibodies bound at the Z-line and at the epitope regions in the I-band near the binding site of a monoclonal antibody SM1 whose position depends on sarcomere length. It thus appears that beta-connectin extends from the edge of M-line to the above epitope region in the I-band.

Immunoelectron microscopic epitope locations of titin in rabbit heart muscle

The location of cardiac titin epitopes in the sarcomere of rabbit cardiac, atrial and ventricular muscle was studied by using polyclonal antibodies against skeletal muscle titin. The results show that incubation with the antibody leads to the appearance of four electron-dense stripes in the A band of both atrial and ventricular cardiac muscle. The location and intensity of these stripes were identical to those observed in skeletal muscle. In conclusion we demonstrate that titins from skeletal and cardiac muscles share some common antigenic determinants.

The Pf332 gene of Plasmodium falciparum codes for a giant protein that is translocated from the parasite to the membrane of infected erythrocytes

We studied the gene structure of the Plasmodium falciparum antigen 332 (Ag332). The gene size was estimated to be approx. 20 kb based on the large size of both the transcript found in mature asexual blood stage parasites and mung bean nuclease fragment generated from genomic DNA. Sequence analysis of genomic and cDNA clones representing different regions of the Pf332 locus showed that the gene product contains a large number of highly degenerated glutamic acid (Glu)-rich repeats (32% Glu). The gene shows dramatic restriction fragment length polymorphism in various P. falciparum isolates and was mapped to the subtelomeric region of chromosome 11. The recombinant 332 fusion protein reacts strongly with the human monoclonal antibody (mAb) 33G2, which is able to inhibit the cytoadherence of parasitized red blood cells on the melanoma cell line C32 and merozoite invasion in in vitro assays. The epitope recognized by this mAb is found frequently in the reported sequence. Ag332 monospecific antibodies were obtained by immunization of mice with a recombinant fusion protein. These antibodies react with a large parasite molecule with an apparent molecular size of 2500 kDa of trophozoite and schizont-infected erythrocytes on Western blot and by immunoprecipitation analysis. Immunofluorescence studies using a confocal microscope showed that Ag332 is exported from the parasite to the infected red blood cell membrane within large vesicle-like structures of about 1 micron diameter.

Structural properties of connectin studied by ultraviolet resonance Raman spectroscopy and infrared dichroism

Ultraviolet resonance Raman spectra of solubilized connectin indicated the presence of beta-sheets and hydrogen-bonded irregular structures. Some Trp and Tyr sidechains are located in hydrophobic environments and some NHs of mainchain amides and Trp indoles are not easily reached by solvent water, suggesting the presence of folded structures constructed of the beta- and irregular parts. Infrared spectra showed an abundance of beta-sheets in a connectin fiber, some of which were aligned with their mainchain axes parallel to the fiber axis. Thus, the beta-spiral structure proposed for elastin is improbable in connectin. This conclusion is also supported by their different amide III frequencies in the visible Raman spectra. A possible filamentous structure of repeated domains, consisting of beta-sheets and irregular parts, is discussed.

Nebulin as a length regulator of thin filaments of vertebrate skeletal muscles: correlation of thin filament length, nebulin size, and epitope profile

Nebulin, a family of giant proteins with size-variants from 600 to 900 kD in various skeletal muscles, have been proposed to constitute a set of inextensible filaments anchored at the Z line (Wang, K., and J. Wright. 1988. J. Cell Biol. 107:2199-2212). This newly discovered filament of the skeletal muscle sarcomere is an attractive candidate for a length-regulating template of thin filaments. To evaluate this hypothesis, we address the question of coextensiveness of nebulin and the thin filament by searching for a correlation between the size of nebulin variants and the length distribution of the thin filaments in several skeletal muscles. A positive linear correlation indeed exists for a group of six skeletal muscles that display narrow thin filament length distributions. To examine the molecular and architectural differences of nebulin size-variants, we carried out immunoelectron microscopic studies to map out epitope profiles of nebulin variants in these muscles. For this purpose, a panel of mAbs to distinct nebulin epitopes was produced against rabbit nebulin purified by an improved protocol. Epitope profiles of nebulin variants in three skeletal muscles revealed that (a) nebulin is inextensible since nebulin epitopes maintain a fixed distance to the Z line irrespective of the degree of sarcomere stretch; (b) a single nebulin polypeptide spans a minimal distance of 0.9 microns from the Z line; (c) nebulin contains repeating epitopes that are spaced at 40 nm or its multiples; (d) nebulin repeats coincide with thin filament periodicity; (e) nebulin variants differ mainly at either or both ends; and (f) nebulin remains in the sarcomere in actin-free sarcomeres produced by gelsolin treatment. Together, these data suggest that nebulin is an inextensible full-length molecular filament that is coextensive with thin filaments in skeletal muscles. We propose that nebulin acts as a length-regulating template that determines thin filament length by matching its large number of 40-nm repeating domains with an equal number of helical repeats of the actin filaments.

The role of troponin C in the length dependence of Ca(2+)-sensitive force of mammalian skeletal and cardiac muscles

1. Skinned fibre preparations of right ventricular trabeculae, psoas and soleus muscles from hamster and rabbit were activated by Ca2+ and the length dependencies of their pCa (-log [Ca2+])-force relationships were compared. 2. Ca2+ sensitivity of the myocardium was higher at 2.2-2.4 microns than that at 1.7-1.9 microns. The length dependence was at least twofold greater in cardiac muscle than in fast skeletal fibres at identical temperatures and salt concentrations. Slow-twitch fibres gave a response similar to that in the myocardium. 3. The effect of the troponin C (TnC) phenotype on the length dependence of Ca2+ sensitivity was measured on both fast skeletal fibres and cardiac muscle with TnC exchange in situ. The length-induced increase in Ca2+ sensitivity was found to be greater in the presence of cardiac TnC than with fast skeletal TnC. Thus the results indicate that a certain domain of TnC is specialized in this length function, and that this domain is different in the two phenotypes. 4. The possibility that the enhanced length dependence of Ca2+ sensitivity after cardiac TnC reconstitution was attributable to reduced TnC binding was excluded when the length dependence of partially extracted fast fibres was reduced to one-half the normal value after a 50% deletion of the native TnC. 5. Two recombinant forms of cardiac TnC (kindly provided by Dr John Putkey, Houston, TX, USA) were used next, to investigate the roles of two specific domains in TnC in the control of length dependence of Ca2+ sensitivity and in the contraction-relaxation switching of cardiac muscle: 6. Using mutant CBM1 [corrected], in which site 1 was modified such as to bind the 4th Ca2+ ion, as in skeletal TnC, the length-induced Ca2+ sensitivity in cardiac muscle was suppressed. The effect was intermediate between cardiac and skeletal TnCs under the same conditions. The pSr (-log [Sr2+])-force relationship of cardiac muscle was also measured. In the presence of the mutant, skinned trabeculae manifest pSr-activation curves identical to those of fast fibres. This indicates that the metal ion binding properties of site 1 in TnC modulate the regulatory action of site 2. 7. Using mutant CBM2A, in which site 2 was inactivated, the activation of cardiac muscle by both Ca2+ and Sr2+ ions was completely blocked. This is the expected result, since both regulatory sites were now inactive, regulatory site 1 being normally inactive in cardiac muscle.(ABSTRACT TRUNCATED AT 400 WORDS)

Regulation of skeletal muscle stiffness and elasticity by titin isoforms: a test of the segmental extension model of resting tension

To explore the role of titin filaments in muscle elasticity, we measured the resting tension-sarcomere length curves of six rabbit skeletal muscles that express three size classes of titin isoform. The stress-strain curves of the split fibers of these muscles displayed a similar multiphasic shape, with an exponential increase in tension at low sarcomere strain followed by a leveling of tension and a decrease in stiffness at and beyond an elastic limit (yield point) at higher sarcomere strain. Significantly, positive correlations exist between the size of the expressed titin isoform, the sarcomere length at the onset of exponential resting tension, and the yield point of each muscle. Immunoelectron microscopic studies of an epitope in the extensible segment of titin revealed a transition in the elastic behavior of the titin filaments near the yield point sarcomere length of these muscles, providing direct evidence of titin's involvement in the genesis of resting tension. Our data led to the formulation of a segmental extension model of resting tension that recognizes the interplay of three major factors in shaping the stress-strain curves: the net contour length of an extensible segment of titin filaments (between the Z line and the ends of the thick filaments), the intrinsic molecular elasticity of titin, and the strength of titin thick filament anchorage. Our data further suggest that skeletal muscle cells may control and modulate stiffness and elastic limit coordinately by selective expression of specific titin isoforms.

Some observations on variations in filament overlap in tetanized muscle fibres and fibres stretched during a tetanus, detected in the electron microscope after rapid fixation

Anomalous tension development which would not be predicted from the descending limb of the length-tension curve occurs during prolonged tetani and after stretch during a tetanus. Variations in filament overlap might account for all or part of the tension enhancement. Fibres isolated from frog skeletal muscle were rapidly fixed during a tetanus with mercuric chloride in ethanol and chloroform so that the correct alignment of the filaments in the overlap zones was preserved. The fibres were examined in polarized light with compensation, and in the electron microscope. There were variations in striation spacing along the length of the fibres, and severe shortening with contraction bands near the tendon insertions, confirming observations made by others on live fibres. Many variations in filament overlap which would not be detectable by light microscopy or laser diffraction were seen in the electron microscope. In a pilot study we measured differences in the width of the overlap zones between half-sarcomeres in a small area and within individual half-sarcomeres. In the latter case the variations were greater in a fibre which developed creep of tension and one which did not. Even greater variations were seen in three fibres stretched during a tetanus, and, in two of these, there were some grossly elongated half-sarcomeres in which the filaments had pulled out of the overlap zones, leaving gaps.

Elastic filaments and giant proteins in muscle

Striated muscle is now known to contain a third major class of filaments, additional to the thick and thin filaments. The presence of such extra filaments has seemed likely for many years, but details of their location, structure, and composition are only now becoming clear. They are composed of massively large proteins and, in contrast to thick and thin filaments, they are elastic.

Projectin is an invertebrate connectin (titin): isolation from crayfish claw muscle and localization in crayfish claw muscle and insect flight muscle

A filamentous protein was isolated from crayfish claw muscle. This protein had physiochemical properties very similar to vertebrate skeletal muscle connectin (titin), although its apparent molecular mass (approximately 1200 kDa) was considerably lower than that of connectin (approximately 3000 kDa). Polyclonal as well as monoclonal antibodies against chicken skeletal muscle connectin reacted with the 1200 kDa protein from crayfish claw muscle. Conversely, polyclonal antibodies against crayfish 1200 kDa protein cross-reacted with chicken connectin. Circular dichroic spectra indicated the abundance of beta-sheet structure (approximately 60%). Low-angle shadowed images showed filamentous structures (0.2-0.5 microns) by electron microscopy. Proteolysis of the 1200 kDa protein by alpha-chymotrypsin or V8 protease rapidly resulted in formation of 1000 kDa or 1100 and 800 kDa peptides. The amino acid composition was very similar to those of vertebrate connectins and of honeybee flight muscle projectin. Based on the molecular weight and amino acid composition, the 1200 kDa protein is regarded to be crayfish projectin. Immunofluorescence and immunoelectron microscopy revealed that crayfish projectin was localized in the A/I junction area and A-band except for its centre region in crayfish claw muscles. Polyclonal antibodies against crayfish claw muscle projectin reacted with 1200 kDa projectin of honeybee and beetle flight muscle. A monoclonal antibody against chicken skeletal muscle connectin also reacted with honeybee and beetle projectin. Immunoelectron microscopic observations revealed that anti-crayfish projectin antibodies bound the connecting filaments linking the Z-line and the thick filaments up to the M-line of honeybee muscle sarcomere. Anti-crayfish projectin antibodies bound the I-band region near the Z-line of beetle flight muscle. It is concluded that the 1200 kDa projectin from crayfish claw muscle is an invertebrate connectin (titin). Recent work with locust flight muscle mini-titin (Nave & Weber, 1990) is in good agreement with the present study, except that the isolated mini-titin estimated as 600 kDa appears to be a proteolytic product (approximately 1100 kDa) of the parent molecule (approximately 1200 kDa).

The cytoskeletal and contractile apparatus of smooth muscle: contraction bands and segmentation of the contractile elements

Confocal laser scanning microscopy of isolated and antibody-labeled avian gizzard smooth muscle cells has revealed the global organization of the contractile and cytoskeletal elements. The cytoskeleton, marked by antibodies to desmin and filamin is composed of a mainly longitudinal, meandering and branched system of fibrils that contrasts with the plait-like, interdigitating arrangement of linear fibrils of the contractile apparatus, labeled with antibodies to myosin and tropomyosin. Although desmin and filamin were colocalized in the body of the cell, filamin antibodies labeled additionally the vinculin-containing surface plaques. In confocal optical sections the contractile fibrils showed a continuous label for myosin for at least 5 microns along their length: there was no obvious or regular interruption of label as might be expected for registered myosin filaments. The cytoplasmic dense bodies, labeled with antibodies to alpha-actinin exhibited a regular, diagonal arrangement in both extended cells and in cells shortened in solution to one-fifth of their extended length: after the same shortening, the fibrils of the cytoskeleton that showed colocalization with the dense bodies in extended cells became crumpled and disordered. It is concluded that the dense bodies serve as coupling elements between the cytoskeletal and contractile systems. After extraction with Triton X-100, isolated cells bound so firmly to a glass substrate that they were unable to shorten as a whole when exposed to exogenous Mg ATP. Instead, they contracted internally, producing integral of 10 regularly spaced contraction nodes along their length. On the basis of differences of actin distribution two types of nodes could be distinguished: actin-positive nodes, in which actin straddled the node, and actin-negative nodes, characterized by an actin-free center flanked by actin fringes of 4.5 microns minimum length on either side. Myosin was concentrated in the center of the node in both cases. The differences in node morphology could be correlated with different degrees of coupling of the contractile with the cytoskeletal elements, effected by a preparation-dependent variability of proteolysis of the cells. The nodes were shown to be closely related to the supercontracted cell fragments shown in the accompanying paper (Small et al., 1990) and furnished further evidence for long actin filaments in smooth muscle. Further, the segmentation of the contractile elements pointed to a hierarchial organization of the myofilaments governed by as yet undetected elements.

Assembly of connectin (titin) in relation to myosin and alpha-actinin in cultured cardiac myocytes

By using polyclonal and monoclonal antibodies against connectin (titin) which stain the A-I junctional area and the A-band domain (polyclonal anti-connectin and monoclonal 4C9) and the I-band domain (monoclonal SM1), the developmental relationship of this elastic protein with sarcomeric proteins, especially and alpha-actin, was examined in embryonic chick cardiac myocytes in vitro under fluorescence microscopy. During premyofibril stages, I-Z-I proteins were detected first (alpha-actinin dots and diffuse actin [phalloidin and anti-troponin C] staining), and later in these areas connectin and myosin dots appeared with nearly identical distribution. Somewhat later, phalloidin-positive nonstriated fibrils were observed in a straight course. They were always reactive with antibodies against alpha-actinin and troponin C, but unreactive or only weakly reactive with anticonnectin and anti-myosin. Initially, alpha-actinin dots were aligned along these fibrils but did not form striations. As they aggregated to form Z-bands, connectin and myosin started to exhibit typical striation ('doublets' and A-bands, respectively). No difference in the staining pattern was observed with two kinds of monoclonal antibodies against different domains of connectin filaments (4C9 and SM1) at early phases. As myosin staining began to show clear A-bands, connectin epitopes became arranged in polarized positions. We conclude that primitive I-Z-I complexes appear prior to the assembly of connectin and myosin filaments and then connectin filaments, developing intimately and coordinately with myosin, become associated with the alpha-actinin lines. Thus it appears that the putative elastic protein connectin plays some role in integrating myosin filaments with the preexisting I-Z-I brushes. The occasional absence of connectin and A-bands between two Z-bands, beyond both of which clear sarcomeres have been formed, indicates that connectin is not a preformed scaffold of myofibrils on which sarcomeric proteins accumulate.

Degradation of connectin (titin) in Fukuyama type congenital muscular dystrophy: immunochemical study with monoclonal antibodies

Connectin (also called titin) is a myofibrillar elastic filament which links a thick filament to a neighbouring Z line in a sarcomere and thus contributes significantly to the elasticity of myofibrils. In a previous study, we demonstrated by Western blot analysis of the biopsied skeletal muscles using an anti-connectin monoclonal antibody that connectin was degraded extensively after 5 years of age in Duchenne muscular dystrophy (DMD), while it was degraded mildly in Becker muscular dystrophy and only minimally in myotonic dystrophy, limb girdle dystrophy, amyotrophic lateral sclerosis and Charcot-Marie-Tooth disease. In the present study, we investigated the degradation state of connectin in Fukuyama type congenital muscular dystrophy (FCMD) by a similar method using 2 distinct anti-connectin monoclonal antibodies. In FCMD, connectin degradation began much earlier than in DMD: Definite degradation was already observed in 5-8-month-old patients. It was presumed that connectin degradation would play an important role in the myofibrillar degeneration in the early stage of FCMD.

Isolation, characterization, and immunochemical properties of a giant protein from sea urchin egg cytomatrix

A giant protein of apparent molecular weight (Mr) 2000 kDa, as determined by SDS-PAGE, was isolated and partially purified, under denaturing conditions, from the detergent-resistant cytomatrix of unfertilized sea urchin egg. Immunoblot analysis and indirect immunofluorescence microscopy observations indicated that this high-molecular-weight protein cross-reacted with the immunospecific serum raised against chicken breast muscle beta-connectin. However, rotary-shadowing electron microscopy images of the protein revealed short threadlike structures which appear morphologically different from beta-connectin structure. Indirect immunofluorescence localization of the protein with anti-beta-connectin serum showed a distribution throughout the whole unfertilized egg cytomatrix. This immunofluorescence pattern seems to change upon egg fertilization, since at metaphase the fluorescence stain appears to be excluded from the mitotic apparatus region as revealed by the double immunolabeling with anti-beta-connectin serum and monoclonal anti-alpha-tubulin antibody. Moreover, when egg cortical fragments were double-labeled with anti-beta-connectin serum and rhodamin-conjugated phalloidin, it was observed that the microfilaments assembled after fertilization seem to be in close association with the protein at the cleavage furrow and other locations. The possible significance of this sea urchin egg connectin(titin)-like protein is discussed.

Morphological and functional characterization of the endosarcomeric elastic filament

The elastic filament was studied in chemically skinned fibers from rabbit psoas muscle by electron microscopy and resting tension measurements. Extraction of skinned fibers with 40 mM sodium pyrophosphate caused a selective removal of about two-thirds of the thick filaments and formed a gap between the remaining portion of the A band and the I band. Very thin filaments were seen in the gap and were decorated by anti-titin antibody. The resting tension of these fibers was comparable to that of unextracted control fibers. When the M band was completely extracted by a solution containing 0.6 M NaCl, the resting tension completely disappeared at sarcomere lengths from 2.8 to approximately 3.4 microns. These results suggest that the elastic force of short sarcomeres is endowed in the titin filaments and that these filaments are anchored to some structures of the Z and M lines. Other filaments were found in the gap between the two I bands of NaCl-extracted sarcomeres. These filaments differed from titin filaments by a larger diameter and the anchoring points. They may represent the sarcomeric structures responsible for the resting tension of extracted fibers stretched at sarcomere lengths longer than 3.4 microns.

A regular pattern of two types of 100-residue motif in the sequence of titin

Titin is the largest polypeptide yet described (relative molecular mass approximately 3 x 10(6); refs 1, 2) and an abundant protein of striated muscle. Its molecules are string-like and in vivo span from the M to Z-lines. I-band regions of titin are thought to make elastic connections between the thick filament and the Z-line, thereby forming a third type of sarcomere filament. These would centre the A-band in the sarcomere and provide structural continuity in relaxed myofibrils. The A-band region of titin seems to be bound to the thick filament, where it has been proposed to act as a 'molecular ruler' regulating filament length and assembly. Here, we show that partial titin complementary DNAs encode a regular pattern of two types of 100-residue motif, each of which probably folds into a separate domain type. Such motifs are present in several evolutionarily divergent muscle proteins, all of which are likely to interact with myosin. One or both of the domain types is therefore likely to bind to myosin.

Myopathy with respiratory failure and typical myofibrillar lesions

16 patients representing 7 different pedigrees exhibited an unusual, adult onset limb-girdle myopathy with typical clinical hallmarks. In a majority of cases there was evidence of an autosomal dominant inheritance. A prominent early finding in all cases was respiratory muscle weakness, and in many of these an acute respiratory incapacity was the reason for the first neurological examination. Neck flexor and sometimes foot extensor weakness were other early symptoms. The clinical picture seems to be at variance with that of the more well known hereditary myopathies. Electrophysiological analysis confirmed a myopathy and serum muscle enzyme concentrations were normal or slightly elevated. Muscle biopsy findings revealed myofibrillar changes which, at the light microscopy level, included plaques that stained strongly with rhodamine-conjugated phalloidin, a specific marker for F-actin. At the ultrastructural level, these plaques were observed to be composed of moderately dense, thin filaments and were related to splitting of Z-discs or formed extensions from Z-discs. We believe that the muscle biopsy changes revealed by cytochemical and ultrastructural observations indicate defective myofibrillogenesis, and the possibility of defective actin polymerization is discussed. A conclusive answer requires further immunocytochemical and immunoelectrophoretic studies and possibly the application of molecular genetics.