Histol Histopathol (1998) 13: 283-291 Histology and 001: 10.14670/HH-13.283 Histopathology http://www.hh.um.es From Cell Biology to Tissue Engineering

Invited Review

The cytoskeleton in skeletal, cardiac and smooth muscle cells

M.H. Stromer Department of Animal Science, Iowa State University, Ames, lA, USA

Summary. The muscle cell cytoskeleton consists of cell that maintain the overall structural order of proteins or structures whose primary function is to link, components inside the muscle cell but that do not anchor or tether structural components inside the cell. actually participate in contraction per se. Unfortunately Two important attributes of the cytoskeleton are strength this has sometimes evoked the concept that the of the various attachments and flexibility to accommo­ cytoskeleton of muscle cells is relatively static and less date the changes in cell geometry that occur during interesting than the contractile machinery. The well­ contraction. In striated muscle cells, extramyofibrillar documented observations that skeletal muscle maintains and intramyofibrillar domains of the cytoskeleton have a remarkably constant cell volume during contraction by been identified, Evidence of the extramyofibrillar simultaneously increasing cell diameter as cell length cytoskeleton is seen at the cytoplasmic face of the decreases and that smooth muscle cells can contract to sarcolemma in striated muscle where vinculin- and 60% of rest length and can return to rest length with dystrophin-rich costameres adjacent to sarcomeric Z their internal structure relatively intact suggest that the lines anchor intermediate filaments that span from cytoskeleton must be highly adaptable to cell shape peripheral myofibrils to the sarcolemma. Intermediate changes. How this adaptation occurs is generally filaments also link Z lines of adjacent myofibrils and unknown. For this review, a muscle cell cytoskeletal may, in some muscles, link successive Z lines within a component is defined as a protein or structure whose myofibril at the surface of the myofibril. The intramyo­ primary role is usually considered to be that of a linker, fibrillar cytoskeletal domain includes elastic titin a n anchor or tether that connects two structural filaments from adjacent sarcomeres that are anchored in components. In striated muscle, two compartments of the Z line and continue through the M line at the center the cytoskeleton will be discussed, the extramyofibrillar of the sarcomere; inelastic nebulin filaments also and the intramyofibrillar. The principal proteins that anchored in the Z line and co-extensible with thin constitute the thick and the thin sarcomeric filaments filaments; the Z line, which also anchors thin filaments will not be considered as part of the muscle cell from adjacent sarcomeres; and the M line, which forms cytoskeleton. This definition is consistent with that bridges between the centers of adjacent thick filaments, proposed by Walsh (1997). For reviews on the muscle In smooth muscle, the cytoskeleton includes adherens cell cytoskeleton, see Small et al. (1992) and Stromer junctions at the cytoplasmic face of the sarcolemma, (1995). The existence in muscle of multiple isoforms of which a nchor B-actin filaments and intermediate actin, however, complicate the classification system. filaments of the cytoskeleton, and dense bodies in the Early embryonic and some cultured skeletal muscle cells cytoplasm, which also anchor actin filaments and express mainly cardiac a-sarcomeric actin and intermediate filaments and which may be the interface cytoplasmic or nonmuscle 13- and y-actin isoforms between cytoskeletal and contractile elements. (Vandekerckhove et aI., 1986; Otey et aI., 1988). In adult skeletal muscle, the skeletal a-sarcomeric isoform Key words: Cytoskeleton, Muscle, Skeletal, Cardiac, predominates but the cytoplasmic 13- and y-isoforms are Smooth also present and apparently are not completely segregated from the a-isoform. Otey et al. (1988) used immunofluorescence to demonstrate that isolated Introduction myofibrils from mature skeletal muscle contained mainly a-actin but also contained lesser amounts of y­ The muscle cell cytoskeleton has frequently been cytoplasmic actin, Immunogold labeling of L6 cells considered to include those components of the muscle showed that y-cytoplasmic actin predominated in cortical actin filaments, skeletal a-actin predominated in nascent Offprint requests to: Dr. Marvin H. Stromer, 3116 Molecular Biology myofibrils and both actin isoforms existed at each of Building, Iowa State University, Ames, IA 50011 -3260, USA these locations (Otey et aI., 1988). Smooth muscle cells 284 The muscle cell cytoskeleton

in th e adult ve rtebrate vascul ar and di gestive systems th e sarcolemm a by f3-d ys troglyca n, a 43 kDa trans­ contain up to 30% of the total ac tin in th e 13 - and y­ membrane subunit of dystroglycan, a glyco pro tein th at cy topl asmi c isoform s and th e remain der in a - and y­ also contain s a -d ys troglycan, a 156 kD a extracellul ar contrac til e isoform s (Small , 1995). A furthe r laminin-binding subunit (Henry and Camp be ll , 1996). complicating factor is that there is a possibility th at some The biological fun cti ons of the dystroglyca n compl ex in protein s such as titin, nebulin ~ n d myosin binding stri ated muscl e and in other ti ss ues have been rev iewed protein C (MyBP-C), usuall y considered as cy toskeletal by Matsumura et al. ( 1997). The amino-terminal domain pro teins, may modul ate th e acti on of contrac til e protein s of dys trophin contains an ac tin binding domain and is in thi c k and in thin fil ame nt s. A n example is th e th ought to link y-actin fil ament s to th e sarcolemm a. inhibition of in vitro motility of F-actin or reconstituted Although the first three domains of dystrophin ex hi bit thin fil aments when titin was bound to th ese fil ament s sequence homology with actin cross-linking proteins (Kell erm ayer and Granzier, 1996). These exampl es point such as a -actinin and spec trin , Rybakova et al. (1996) out both the complex ity of the muscle ce ll cy toskeleton found that a dystrophin-glycoprotein complex could not and why thi s is an exciting area of research. cross-link F-actin fil ament s. Instea d, Rybakova et al. (1996) identified an additional actin binding sit e near th e Striated muscle ce nter of th e dystrophin rod domain and observed th at when the dystro phin-glycoprotein compl ex was bound to The extramyofibrillar cytoskeleton: Connections between F- actin, th e depolymeri zati on of F- ac tin was slowed. the sarcolemma and peripheral myofibrils These observa tions suggest th at dystrophin may interact with up to 24 actin monomers and may bind along th e The locali za ti on, by immunoflu o rescence, o f side of actin fil aments instead of, or in addition to, the vin culin, a 116 kDa protein , in rib-like bands call ed end . In ra t cardi ac myocytes, dys trophin is not prese nt in costameres th at are located opposit e Z lines at th e a costameri c pattern , but in stead is uni fo rml y distributed cy topl asmi c face of the sarcolemm a in ca rdiac mu scl e at the cytopl asmic fa ce of non-interca lated di sk regions (Pardo et aI. , 1983a) and in skeletal muscl e (Pardo et aI. , of the sarcolemma (Stevenso n et aI. , 1997). Su bce llular 1983b) and at intercalated di sks of ca rdiac mu scle fractionati on of rabbit ventricles demonstra ted th at about (Pardo et aI. , 1983a) suggested th at fil ament attachm ent 55 % of dystrophin was recovered with the sa rcolemm a sit es ex isted at th e sarcole mma. This provided an and 35 % of dystrophin was associa te d with the ex pl anati on for how the fil aments obse rved by Pi erobon­ myo fibrill ar fracti on (Meng et aI. , 1996). Immu nogold Bormi oli ( 198 1) in th e s pace betwee n Z lines of labeling of th ese myofibrils showed that Z lin es were peripheral myofibrils and th e sarcolemm a of skeletal preferentiall y labeled. Additional expe riments will be mu scl e could be anchored at th e membrane. Myo­ needed to dete rmine th e s ig ni f icance o f th e two tendinous and neuromuscular junctions in av ian anteri or subsa rcolemm al distributio ns of dystrophin in skeletal lati ssimus dorsi (ALD) and posteri or lati ss imus dorsi and ca rdiac mu scle and of the presence of dystrophin at (PLD) twitch fibers, and subsa rcoJemm al de nse patches the Z line. over th e I bands in tonic ALD fi be rs, all contain vinculin The inte rcala ted di sk in ca rdi ac mu scle is a (S hea r and Bl ock, 1985). The colocali zati on of y-actin, speciali zed cell-cell cont act th at consists of three intermedi ate filament pro teins and spectrin with vinculin morphologica ll y identifiable regions. The nex us or ga p at costa me res (Craig a nd Pardo, 1983) prov id ed juncti on is para ll el to th e myofib ril ax is, th e mac ul a additional ev id ence that costameres are sit es where adh erens or desmosome is import ant for ce ll-ce ll cytoskeletal fil aments are attached to the sarcolemm a. adh esion and also serv es as an anchor for desmi n­ Both Pierobon-Bormioli (198 1) and Pardo et al. (1983a) cont aining intermedi ate filam ent s ( IFs) of th e found th at the spacing between fil ament attachm ent sites cy toskeleton (Gree n and Jones, 1996) and th e fascia a nd th e costa meres, respecti ve ly, coin c id ed with adh erens is perp endicul ar to th e myofibril ax is and sa rcomere length and that, as sarcomeres shorte ned, the anchors actin fil ament s. In chi cken ca rdiac mu scl e, sarcolemm a protruded outward betw ee n attachm ent immunogold labeling demonstrated th at vin cu lin was sites. The possibility that additional prote ins are present associated with th e cytopl asmic side of th e fasc ia in the costamere, understanding th e arrangement of adh erens and with intracellul ar pl aqu es th at we re not proteins in th e costamere and a detail ed compariso n of part of th e inte rca lated di sk but we re adj ace nt to costameres with adherens j un cti ons in cardi ac mu scle coll agen or other conn ecti ve ti ssue fib ers (Yolk and and dense pl aqu es in smooth mu scl e will add to our Geiger, 1986). The relati onship, if any, between th ese understanding of costameres. immunogold labeled pl aqu es and th e chi cken cardi ac Dystrophin, a 427 kD a four domain protein , is costameres described by Pardo et al. ( 1983a) is unclear. localized at costameres in rat skeletal mu scle (Porter et Labeling fo r a -actinin was extensive on cardi ac Z lines aI. , 1992) wh ere it is an import ant component of th e and was also observed along the cytopl as mi c face of th e subsarcolemm al cytoskeleton in muscl e cell s (for rev iew fasc ia adherens (Yolk and Geige r, 1986). Cultured see Erv as ti and Campbell , 1993). The absence of or cardiac myocytes fo rm intercalated disks, whi ch co nsist ab norm ality in dys trophin is th e cause of Du chenn e/ of fascia ad herens and mac ul a adherens where ce ll-ce ll Becker muscul ar dystro ph y. Dystro phin is anchored to contact occurs (Lu et aI. , 1992). The fascia adherens in

------285 The muscle cell cytoskeleton

these cells stains positively for vinculin, sarcomeric a­ Ramirez-Mitchell (1983). Skelemin, a 195 kDa protein actinin and a-actin. In regions of cell-substrate contact, located at the periphery of the M line in striated muscle, subsarcolemmal adhesion plaques (SAPs) were seen that contains intermediate filament-like motifs that may stained positively for vinculin, sarcomeric a-actinin, a­ facilitate the attachment of a small number of actin, talin, integrin and titin. Either a fascia adherens or longitudinally-oriented desmin filaments to the M line a SAP caps each striated myofibril and suggests that (Price and Gomer, 1993). For a review on intermediate these two sites are myofibril nucleation sites in cultured filaments in muscle, see Stromer (1990). Bard and cardiac cells. The presence of vinculin, a-actinin, talin Franzini-Armstrong (1991) used the binding of S-1 and titin in SAPs further suggests that cytoskeletal fragments of myosin to identify a population of actin components and/or linking proteins are important in filaments attached to Z lines at the surface of isolated myofibril formation. skeletal muscle myofibrils. The actin isoform present in Talin, a 270 kDa actin binding protein, has been these filaments and whether or not these filaments are localized at costameres in cardiac and skeletal muscle also present between the peripheral myofibrils and the and in intercalated disks of cardiac muscle (Belkin et aI., sarcolemma is unknown. 1986). In vitro, purified talin can crosslink F-actin filaments into networks and bundles in a pH- and ionic The intramyofibrillar cytoskeleton: connections within strength-dependent manner (Zhang et aI. , 1996). Calpain myofibrils II (m-calpain) can cleave talin into 47 kDa and 190 kDa fragments. A model proposed by Isenberg and The Z line, sometimes called the Z disk, is an Goldmann (1992) shows the N-terminal 47 kDa domain electron dense structure, perpendicular to the myofibril of talin partially inserted into the sarcolemma and the axis, that delimits the boundaries of the sarcomeres in 190 kDa domain interacting with both vinculin and an vertebrate skeletal and cardiac muscle. It is widely actin filament. Talin can nucleate actin filament accepted that sarcomeric a-actin-containing thin assembly via binding to G-actin but does not limit the filaments are inserted into and tethered by the Z line in a addition of actin monomers because talin is not a square array with thin filaments from the adjacent capping protein (Isenberg and Goldmann, 1992). The sarcomere centered in each square of the array. The binding of tal in and actin to vinculin is regulated by extent of overlap of actin filaments dictates the Z line phosphatidylinositol-4, 5-bisphosphate which dissociates width. The simplest vertebrate Z line such as that in the vinculin's head-tail interaction and exposes the talin- and guppy may be only 20 nm wide. Fast twitch glycolytic actin-binding sites (Gilmore and Burridge, 1996). skeletal muscles have 55 nm wide Z lines; fast twitch Additional models for plasma membrane-cytoskeleton oxidative/glycolytic and some slow twitch oxidative interactions in several cell types including muscle are skeletal muscles have 93 nm wide Z lines; other slow included in a review by Luna and Hitt (1992). twitch oxidative skeletal muscles and cardiac muscle The existence of filaments between myofibrils was have 131 nm wide Z lines. The significance of thin noted by Garamvolgyi (1965) who described bridges filament overlap in the Z line was realized when low between Z lines in bee flight muscles and by Gregory et ionic strength solutions were utilized to selectively a!. (1968) who saw filamentous structures in the extract Z lines (Stromer et aI., 1967) and rod bodies in cytoplasm adjacent to M and Z lines in developing muscle from nemaline myopathy patients. This blowfly flight muscle. Transverse filaments are also controlled extraction removed the dense material from located between Z lines in adjacent myofibrils and the rod bodies and revealed that the rod body backbone between the sarcolemma and both M and Z lines in consists of overlapping actin filaments linked at regular mammalian skeletal muscle (Pierobon-Bormioli, 1981). intervals by cross-connecting filaments (Stromer et aI., Immunofluorescence labeling (Campbell et aI., 1979; 1976; Yamaguchi et aI., 1978). The rod body is really an Lazarides, 1980; Thornell et aI. , 1980) and immuno­ expanded Z line or a Z line polymer that may grow to electron microscopy (Richardson et aI., 1981; Tokuyasu lengths of 3 to 5 ,urn. The replacement of functional et aI., 1983a,b) have identified transverse filaments in sarcomeres with noncontractile rod bodies probably several muscle types as desmin intermediate filaments. contributes to the muscle weakness that is a symptom of The question of how these desmin filaments are linked nemaline myopathy. to Z lines has not been answered. Proteins hypothesized A detailed model of vertebrate muscle Z lines has to be involved with linking desmin filaments to Z lines demonstrated that overlapping antipolar actin filaments include ankyrin, spectrin and, in chicken skeletal muscle, are linked every 38 nm by rod-shaped a-actinin synemin and in chicken cardiac muscle, paranemin molecules that constitute the cross-connecting filaments (Thornell and Price, 1991). In addition, plectin has been (Yamaguchi et a!., 1985). The width of Z lines in localized with desmin filaments at the Z line and nanometers can be readily defined as W = n(38) + 17 sarcolemma in skeletal muscle (Foisner and Wiche, where n is the number of 38 nm intervals between cross­ 1991) and has the potential to link desmin filaments at connecting filaments and 17 is the offset axial distance these two sites. Intermediate filaments oriented parallel from the end of a thin filament to the point on an to the myofibril axis that could link adjacent Z lines in adjacent thin filament where a connecting filament is the same myofibrils have been identified by Wang and linked. The model relates the 11 nm small square pattern 286 The muscle cell cytoskeleton

seen in cross sections to cross-connecting Z filaments that may also interact with the C-terminus of nebulin that are bent at a near 90% angle and linked at their and/or the N-terminus of titin. Nebulin, an ~ 800 kDa centers to an adjoining Z filament. Partial straightening protein, has its C-terminus in the Z line (Pfuhl et aI., of the Z filaments, which could be caused by weakening 1996). The N-terminal 209 kDa segment of titin is inside the central linkage and/or by a decrease in thin filament the Z line (Labeit and Kolmerer, 1995), but the ends of overlap, would cause the appearance in cross section of a titin molecules from adjacent sarcomeres exhibit little basket weave pattern which, in turn, would become a overlap (Gautel et aI., 1996). Other than actin and a­ diagonal square pattern with additional Z filament actinin, there is no information about which structural straightening and/or an additional decrease in thin component(s) of the Z line may be contributed by the filament overlap. Goldstein et al. (1991) have related other Z line proteins. changes in internal Z line structure to the development A single nebulin molecule forms an inextensible of active tension in skeletal muscle and have suggested filament that extends from its C-terminal anchor point in that the Z line is a dynamic structure that participates in the Z line to the free end of a thin filament at the determining some of the mechanical properties of proximal edge of the H zone (Kruger et aI., 1991). The muscle. The structure of the Z line has recently been size of nebulin isoforms is proportional to the length of investigated by image analysis technology (Luther, thin filaments in skeletal muscle and implies that nebulin 1995; Schroeter et aI., 1996) to gain additional insights may be a protein ruler that regulates thin filament length into the structural elements of the Z line. (Kruger eta I., 1991). N ebu I in has so fa r not been In addition to actin and a-actinin, two well­ detected in cardiac muscle, which may explain the established components of the Z line, an extensive list of heterogeneous length of cardiac thin filaments. The other proteins has at one time or another been classified sequence of nebulin has suggested that about 200 actin as Z line proteins (Vigoreaux, 1994). Some have been binding domains exist along the nebulin molecule that determined to not be Z line components; others have could permit nebulin to associate laterally with an actin been identified as proteolytic fragments of more recently filament in a zipper-like fashion (Chen et aI., 1993). A characterized proteins. I will include in this list only model for how nebulin could interact with a native thin those proteins for which the current evidence seems filament has been presented by Wang et al. (1996). compelling. Vinculin seems to be associated with Z lines Native thin filaments isolated from rabbit skeletal in both cardiac and skeletal muscle but there is some muscle contained both nebulin and a-actinin (Meng et debate if vinculin is located at the periphery of an aI., 1995), which suggested a strong association between individual Z line (Gomer and Lazarides, 1981) or in the nebulin and thin filaments. The strength of the binding interior of Z lines (Terracio et aI., 1990). A barbed-end of nebulin to native thin filaments has, however, been actin filament capping protein, CapZ, is a Z line questioned by Cuneo et al. (1996) who isolated thin and component (Cassella et aI., 1987; Schafer et aI., 1993) thick filaments at high and at low ionic strength from

Z-d~c half I-band I/g.modu,e myosin soleus tandem Ig-variant 9 oFN·moduie I C·prolein ---==~ Bunique sequence I p94 (pro/ease) ..., ... t - .... [1 PEVK-segmenl I tisslJ8ospeci~c p p94 U V SplIce SIte I

halfA-bBnd An/unction 0-_ C-zon. p-_ .,.,Ine

Fig. 1. Domain architecture and sarcomeric layout of the titin filament. The domain structure of the human soleus titin, as predicted by its 100-kb mRNA, is shown. The 3.7-MD soleus titin peptide contains 297 copies of lOa-residue repeats . which are members of the Ig and FN3 superfamilies. Each of these domains folds into a 10- to 12-kD small globular subunit. Specific for the I-band segment of titin are strings of tandemly repeated Ig domains (tandem-Ig titin) and the "PEVK domain ," rich in proline, glutamate, valine, and lysine residues. The tandem-Ig and the PEVK region of titin represent those parts of the titin filament that extend during physiological amounts of stretch. Specific for the A-band titin are regular patterns of Ig and FN3 domains, referred to as "super repeats." These super repeats provide multiple and structurally ordered binding sites for myosin and C protein. In addition to the Ig/FN3 repeats and the PEVK region of titin, 8% to 10% of titin's mass is formed by unique sequence insertions. Among the encoded peptides are phosphorylation motifs (Pi) and a serine/threonine kinase. The mapped calpain p94-binding sites are shown. Arrows above the domain pattern indicate the sites at which muscle type-specific alternative splicing occurs. Reproduced from Labeit et aI. , Circ. Res., copyright 1997 American Heart Association, with permission.

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287 The muscle cell cytoskeleton

rabbit and bovine skeletal muscle and found nebulin was Ig domains specific for the C zone of the myosin always absent from the thin filament fraction and present filaments. The M line at the center of the sarcomere is in the thick filament fraction. electron dense and is the site of a group of proteins, A single - 3000 kDa titin molecule (also referred to some of which form bridges between adjacent thick as connectin) has its N-terminus anchored in the Z line filaments. Three to five major sets of bridges may be and a 210 kDa portion of the C-terminus in the M line at present and are numbered with Ml at the center of the M the center of the sarcomere (Labeit and Kolmerer, 1995). line, with M4 and M4' on the right and left of Ml, and A diagram of the sarcomeric position of the various titin with M6 and M6' on the right and left of the M4-M4' domains is shown in Fig. 1. A titin filament formed by a sets. These sets of bridges are precisely aligned and single titin molecule is > 1 .urn long. The length of a titin produce a pattern of transverse lines or periodicities molecule depends on the muscle from which it is within the M line that is dependent on species, isolated and is due to differences in the number of chronological age, specific muscle and fiber type tandem immunoglobulin (Ig) domains in the I band near (Carlsson and Thornell, 1987). A three-line pattern (M4, the Z line and in the size of the PEVK-region in the I Ml and M4') and a five-line pattern (M6, M4, Ml, M4' band (Tskhovrebova and Trinick, 1997). Predictions and M6') occur most frequently, but a four-line pattern from the titin sequence suggest a length of 1.07 Ilm for (M6, M4, M4' and M6') or no detectable M line are also human cardiac titin to 1.37 .urn for titin from human seen. M-protein (165 kDa) is located in the Ml line in soleus and diaphragm. The length of titin from the M fast skeletal and cardiac fibers and MM-creatine kinase line to the PEVK-region is predicted to be a constant (2 x 43 kDa) is located in M4 and M4'. The third known 0.89Ilm. Direct visualization of the differential elasticity M line protein, myomesin (185 kDa in skeletal muscle) of titin in skeletal muscle (Wang et aI., 1993) and in may have the N-terminus attached to the thick filament cardiac myocytes (Granzier et aI., 1996) has been and the remainder of the bent molecule oriented parallel obtained by labeling titin epitopes in the A band and in to both the thick filament and to the titin filament so that the I band at different degrees of stretch. Epitopes in the myomesin crosses the M1 line (Obermann et aI., 1996). A band retain their position independent of sarcomere The identity of the protein constituting the M6 and M6' length. The amount of movement of I band epitopes lines is unknown. Although some have suggested that it during stretch depends on epitope location. Gautel and is myomesin (Nave et aI., 1989), recent studies suggest Goulding (1996) have proposed that epitope movement that myomesin may be part of the M filaments that are in the Ig domains in the I band precedes movement in parallel to thick filaments between M4 and M4' lines the PEVK region and is limited by the ability of the (Obermann et aI., 1996). For a proposed model of the folded Ig domains to straighten. Epitopes in the PEVK locations of M-protein and myomesin, see Obermann et region, approximately midway between the Z line and al. (1996). This model also includes the concept, derived the edge of the A band, move away from both the Z line from antibody localization, that an individual titin and the edge of the A band during stretch (Granzier et molecule extends through the M line and about 60 nm aI. , 1996). The elastic properties of titin have been into the opposite half of the sarcomere. Both M-protein rel ated to the development of passive tension in skeletal and myomesin associate strongly with isolated titin muscle (Gautel and Goulding, 1996) and in cardiac molecules (Nave et aI., 1989; Vinkemeier et aI., 1993). It muscle (Granzier et aI., 1996). Gautel et al. (1996) have has recently been determined that one titin Ig domain, identified unique 45 residue repeats that are m4, interacts with myomesin domains My4-6 and that differentially spliced into the Z-line end of titin and have this interaction is blocked if Ser 482 between My4 and proposed that the insertion of different numbers of these My5 is phosphorylated (Obermann et aI., 1997). repeats could affect both the width and mechanical Obermann et al. (1997) have proposed that interactions strength of Z lines. between two sarcomeric cytoskeletal components, e.g., Although titin seems inelastic in the A band, the titin and myomesin, may have a role in assembly and/or relationships between titin and proteins associated with turnover of sarcomeres. In summary, titin molecules the thick filament have stimulated much interest. Bennett extend from the center of the Z line, where they may and Gautel (1996) have suggested that the pattern of have a role in determining Z-line width, through the M groups of fibronectin-3 titin domains that are inside the line. The elastic properties of the I band part of titin edge of the A band is related to the myosin molecules seems related to passive tension in muscle, and the A that are missing near the end of the thick filament and band part of titin may have both a template function for that titin could specify the length of thick filaments. The thick filament assembly and a role in sarcomere assem­ central third of each half of the A band has been named bly. For a recent review on titin, see Labeit et al. (1997). the C zone because myosin-binding proteins C (MyBP­ C) and H are attached to thick filaments in 11 stripes Microtubules in striated muscle spaced 43 nm apart. Most, and in some instances all, of these stripes contain MyBP-C. Isolated radioiodinated Microtubules are one-third of the troika (with MyBP-C binds directly to isolated titin (Furst et aI., intermediate filaments and actin filaments) of cyto­ 1992). Freiburg and Gautel (1996) found that the skeletal elements in non-muscle cells but have received interaction between titin and MyBP-C is directed by titin comparatively little attention in muscle, particularly in 288 The muscle cell cytoskeleton

skeletal muscle. Adult rat skeletal muscle cells contained skeletal muscle and, together with the presence of a­ microtubules with various orientations that were present actinin in the dense body, has caused the dense body to in greater density in the subsarcolemmal space and were be referred to as the functional analog of the Z line. organized in a dense perinuclear network (Boudriau et Supercontraction of isolated smooth muscle cells has aI., 1993). Adult rat slow twitch soleus muscle had a 1.7 indicated that actin filaments average - 4.5 m long, times greater content of a-tubulin than fast twitch which is 4-4.5 times longer than if! skeletal muscle lateralus muscle (Boudriau et aI., 1993). The number of (Small et aI., 1990). microtubules in postnatal and adult slow skeletal and Lehman et al. (1987) used anti-caldesmon and anti­ cardiac muscle were similar at corresponding ages and filamin to separate isolated smooth muscle thin filaments were relatively stable in the adult (Cartwright and into two populations. Lehman (1991) again used anti­ Goldstein, 1985). In normal rat hearts, Watkins et al. caldesmon and anti-filamin to separate isolated thin (1987) found that microtubules were oriented mainly filaments from smooth muscle and found that thin longitudinally, perpendicular to the intermyofibrillar filaments precipitated by anti-filamin contained actin, desmin filaments. Cardiac hypertrophy induced by aortic tropomyosin, filamin and calponin but nearly no stenosis caused experimental rats to increase the number caldesmon. Thin filaments precipitated by anti­ of microtubules near nuclei and between myofibrils. For caldesmon contained actin, tropomyosin and caldesmon a brief review on microtubules and cardiac hypertrophy, but nearly no calponin. The existence of two types of see Walsh (1997). A more extensive review on smooth muscle thin filaments led to the hypothesis that microtubules in cardiac muscle has been written by the filamin-calponin filaments were cytoskeletal thin Rappaport and Samuel (1988). filaments and the caldesmon-tropomyosin filaments were in the contractile domain. North et a!. (1994), Smooth muscle however, found that calponin was localized in both the cytoskeletal and the contractile domains of smooth Although smooth muscle does not have an intra­ muscle and that dense bodies and AJs were heavily cellular sarcomeric arrangement, some of the cyto­ labeled with anti-calponin. These data are consistent skeletal elements in the smooth muscle cell have been with an observation by Winder and Walsh (1993) that identified. For reviews on smooth muscle structure, see calponin is a more effective in vitro regulator of Bagby (1990), Somlyo and Somlyo (1992), Small (1995) actomyosin ATPase than caldesmon and, therefore, is and Stromer (1995), and for a review on smooth muscle interacting in some way with the contractile domain. contraction, see Horowitz et al. (1996). Three domains Intermediate filaments are also associated with dense exist at the cytoplasmic face of the sarcolemma. One bodies, but there is no agreement about the nature of the domain consists of the contacts between the sarcolemma attachment. Bond and Somlyo (1982) have shown lateral and the junctional sarcoplasmic reticulum. Another attachments between intermediate filaments and dense contains caveolae and is enriched in dystrophin (North et bodies. Chou et al. (1994) have demonstrated that, in a!., 1993). The third contains adherens junctions (AJ) developing smooth muscle cells, side-by-side parallel that are also referred to as dense plaques, membrane­ alignment of segments of intermediate filaments is an associated dense bodies or attachment plaques and that early step in dense body formation and that both are filament anchoring sites. Cytoplasmic or f3-actin intermediate filament and actin filament profiles exist filaments associated with AJs bind S-l fragments of inside these dense bodies. This suggests that the myosin to form arrowhead structures that point away backbone of dense bodies, at least in developing cells, from the sarcolemma. In addition to the cytoplasmic may consist of an overlapping array of intermediate, and actin filaments that seem to enter the AJ, other proteins perhaps actin, filaments that are linked together by associated with AJs include a-actinin, integrin, vinculin, a-actinin. Bond and Somlyo (1982) have also described metavinculin, calponin, filamin, talin, paxillin, tensin actin filament profiles inside dense bodies. The and plectin (Small, 1995). Intermediate filaments of the movement of dense bodies seen in cross-sections toward desmin and/or vimentin type are often associated with the center of stretched smooth muscle cells compared AJs and usually are assumed to be anchored there with a more uniform distribution in normal length cells (Bagby, 1983). (Cooke and Fay, 1972; Cooke, 1976) or in cells treated A prominent filament anchoring site in the interior with EDTA to remove thick and thin filaments showed of smooth muscle cells is the dense body, also referred to that dense bodies were connected to each other and to as the cytoplasmic dense body. Dense bodies label the sarcolemma by intermediate filaments. Dense bodies strongly with antibodies to cytoplasmic actin (North et labeled with anti-a-actinin in contracting isolated aI., 1994) and with antibodies to a-actinin (Schollmeyer smooth muscle cells were either members of a group that et a!., 1976; Kargacin et aI. , 1989). Actin filaments moved little or were members of a group that moved emanate from the two ends of dense bodies and bind S-1 rapidly toward each other axially (Kargacin et aI., 1989). myosin fragments to form arrowheads that point away The interpretation of this result is unclear. An axial from the dense body (Bond and Somlyo, 1982). Actin intermediate filament bundle can readily be seen in polarity with respect to the dense body in smooth muscle chicken gizzard smooth muscle cells in close association is analogous to actin polarity with respect to the Z line in with the centrally located nucleus and with mitochondria

- -.•. ------.-. ------_._._------_.

289 The muscle cell cytoskeleton

at th e end s of th e nucleus (Stromer and Bendayan, In: Biochemistry of smooth muscle. Vol. 1. Stephens N. L. (ed). CRC 1988). Individual filaments from this bundle extend Press, Inc. Boca Raton, FL. pp 1-84. toward th e nucl ear env elope and toward mitochondria Bagby R.M. (1990). Ultrastructure, cytochemistry and organization of where they seem to make end-on contact with th e myofilaments in vertebrate smooth muscle. In: Ultrastructu re of organell es (Stromer and Bendayan, 1990). Both th e smooth muscle. Motta P.M. (ed) . Kluwer Acad. Pub. Norwell, MA. pp individual filaments and the filament bundle label with 23-61. anti-desmin. Fo r a model that shows how this axial Bard F. and Franzini-Armstrong C. (1991). Extra actin filaments at the interm ediate filament bundle could provide a ce ntral periphery of skeletal muscle myofibrils. Tissue Cell 23, 191-197. anchoring structure in smooth mu scle cells, see Stromer Belkin A.M., Zhidkova N.1. and Koteliansky V.E. (1986). Localization of ( 1995). talin in skeletal and cardiac muscles. FEBS Lett. 200, 32-36. An important unanswered question is how these Bennett P.M . and Gautel M. (1996) . Titin domain patterns correlate with cytoskeletal e lement s in smooth mu scle cells are the axial disposition of myosin at the end of the thi ck fi lament. J. linked to ge th er, and what is their relationship both Mol. BioI. 259, 896-903. structurally and functionally to the contractile apparatus? Bond M. and Somlyo A.V. (1982) . Dense bodies and actin polarity in Immunoflu orescence and immunoelectron microscope vertebrate smooth muscle. J. Cell BioI. 95, 403-413. label ing of g izzard smooth muscle cell s have Boudriau S. , Vincent M" Cote C.H. and Rog ers P.A. (1993) . demonstrated that 13 -cytopl as mic actin is located near or Cyloskeletal structure of skeletal muscle: Identifi cation of an intricate with intermediate filaments and th at y-smoot h mu scle exosarcomeric microtubule lattice in slow- and fast-twitch muscle actin is ne ar myos in filaments (North et aI., 1994). These fibers. J. Histochem. Cytochem. 41, 1013-1021. observati ons reinforced the view summari zed in Small Campbell G.R., Cham ley-Campbell J., Groschel-Stewart U .. Small J.V. (1995), th at smooth mu scle contains separate cyto­ and Anderson P. (1979) . Antibody staining of 1O-nm (100 A) skeletal and contractile domains. There are several filaments in cultured smoolh, ca rdiac and skeletal muscle. J. Ce ll questions related to th e two-domain hypothesis: does the Sci. 37, 303-322. two-domai n pattern apply to all smooth muscle types or Carlsson E. and Thornell L.-E . (1987). Diversificati on of the myofibrillar is it limited to ce ll s from certain ti ssue types?; if dense M-band in rat skeletal muscle during postnatal development. Cell bod ies co nt ain , or attach to, onl y B-cytoplasmic actin and Tissue Res . 248, 169-180. no u- or y- mu scle actin , where are the actin filaments Cartwright J. and Goldstein M.A. (1985) . Microtubules in the heart anch ored that participate in contraction?; and are the two muscle of the postnatal and adult rat. J. Mol. Cell. Cardiol. 17, 1-7. types of moveme nt of dense bodies observ ed by Cassella J.F., Craig S.W. , Maack D.J. and Brown A.E. (1987). Cap Kargaci n et a!. (1989) indica tive of dense bodies in both ZbS/32). a barbed end actin-capping protein is a component of the Z­ th e cytoskeletal and contractile domains or is thi s line of skeletal muscle. J. Cell BioI. 105, 371-379. observa tion a function of th e re la tiv e position Chen M.-J.G. , Shih C.-L. and Wang K. (1993). Nebulin as an actin (perpendicul ar vs. ob lique to th e direction of sight) of zipper. J. BioI. Chem. 268 , 20327-20334. dense bodies in th e cell at th e time of observation? If Chou R.-G.R., Stromer M.H., Robson R.M. and Huiatt T.w. (1994). there is onl y a single domain, how are the contractile and Substru cture of cytoplasmic dense bodies and changes in cytoskeletal filaments arranged and linked so they can distribution of desmin and rt-actinin in developing smooth muscle accomplish the contractile and tension-ge nerating cells. Cell Motil. Cytoskeleton 29, 204-214. req uirements of smooth muscle and also maintain the Cooke P.H. (1976) . A fi lamentous cytoskeleton in vertebrate smooth typica l arrangemen t of cell component s? Will new muscle fibers. J. Cell BioI. 68, 539-556. proteins be discovered that wi ll help explain the unique Cooke P.H. and Fay F.S. (1972) . Correlation between fiber length , structure-functi on relationships in smooth muscle or will ultrastructure, and the length-tension relationship of mammali an some of the known proteins have dual functions in the smooth muscle. J. Cell BioI. 52, 105-116. cy toskeleton and in contraction? Answers to these and Craig S.W . and Pardo J .V. (1983). Gamma actin, spectrin , and ot her questions abou t the arrangement and function of intermediate filament proteins colocalize with vinculin at costameres , cytoskeletal and contractile components in smooth myofibril-to-sarcolemma attachment sites. Cell Motil. 3. 449-462. mu scle ce ll s will be helpful in und erstanding thi s unique Cuneo P. , Trombetta G., Adami R. and Grazi E. (1996). Is nebulin truly muscle type. a component of the th in filam ent? FEBS Lett. 397,136-138. Ervasti J.M. and Campbell K.P. (1993) . Dystrophin and the membrane Acknowledgements. I thank Dr. R.M. Robson for his comments on the skeleton. CurroOpin . Cell BioI. 5, 82-87. manuscript and Lynn Newbold for typing the manuscript. Studies cited Foisner R. and Wiche G. (1991). Intermediate filament-associated from the au th or's laboratory were supported by the American Heart proteins. CurroOp in. Cell BioI. 3, 75-81. Association - Iowa Affiliate. This is journal paper J-17350 of the Iowa Freiburg A. and Gautel M. (1996) . A molecular map of the interactions Agriculture and Home Economics Experiment Station, Ames, IA 50011 between titin and myosin-binding protein C. Eur. J. Biochem. 235 , (Project 3349) , that was also supported by Hatch Act and State of Iowa 317-323 . funds. Furst D.O. , Vinkemeier U. and Weber K. (1992) . Mammalian skeletal muscle C-protein: purification from bovine muscle, binding to titin References and the characterization of a fu ll-length human cDNA. J. Cell Sci . 102,769-778. Bagby R.M. (1983) . Organization of contracti le/cytoskeletal elements. Garamvolgyi N. (1965) . Inter-Z bridges in the flight muscle of the bee. J. 290 The muscle cell cytoskeleton

Ultrastruct. Res. 13, 435-443. 1007-1022. Gautel M. and Goulding D. (1996) . A molecular map of titin/connectin Luna E.J. and Hilt A.L. (1992). Cytoskeleton-plasma membrane elasticity reveals two different mechanisms acting in series. FEBS interactions. Science 258, 955-964. Lett. 385,11-14. Luther P.K. (1995) . Symmetry of a vertebrate muscle basketweave Z­ Gautel M., Goulding D., Bullard B., Weber K. and Furst D.O. (1996). band. J. Struct. BioI. 115, 275-282. The central Z-disk region of titin is assembled from a novel repeat in Matsumura K., Yamada H., Saito F. , Sunada Y. and Shimizu T. (1997). variable copy numbers. J. Cell Sci. 109, 2747-2754. The role of dystroglycan, a novel receptor of laminin and agrin, in Gilmore A.P. and Burridge K. (1996) . Regulation of vinculin binding to cell differentiation. Histol. Histopathol. 12, 195-203. talin and actin by phosphatidylinositol-4 ,5-bisphosphate. Nature 381, Meng H. , Leddy J.J ., Frank J., Holland P. and Tuana B.S. (1996) . The 53 1-535. association of cardiac dystrophin with myofibrils/Z-disc regions in Goldstein MA, Schroeter J.P. and Michael L.H. (1991 ). Role of the Z cardiac muscle suggests a novel role in the contractile apparatus. J. band in the mechanical properties of the heart. FASEB J. 5, 2167- BioI. Chem. 271 , 12364-12371. 2174. Meng Y., Yasunaga T. and Wakabayashi T. (1995). Determination of Gomer R. and Lazarides E. (1981). The synthesis and deployment of the protein components of native thin filaments isolated from natural fi lamin in chicken skeletal muscle. Cell 23, 524-532. actomyosin: Nebulin and a-actinin are associated with actin Granzier H., Helmes M. and Trombitas K. (1996). Nonuniform elasticity filaments. J. Biochem. 118,422-427. of titin in cardiac myocytes: A study using immunoelectron Nave R., Furst D.O. and Weber K. (1989). Visualization of the polarity of microscopy and cellular mechanics. Biophys. J. 70, 430-442. isolated titin molecules: A single globular head on a long thin rod as Green K.J. and Jones J .C.R. (1996). Desmosomes and hemi­ the M band anchoring domain? J. Cell BioI. 109,2177-2187. desmosomes: structure and function of molecular components. North A.J. , Galazkiewicz B. , Byers T.J., Glenney J.R. and Small J.v. FASEB J. 10, 871-881 . (1993). Complementary distributions of vinculin and dystrophin Gregory D.W., Lennie R.W. and Birt L.M. (1968). An electron define two distinct sarcolemma domains in smooth muscle. J. Cell microscopic study of flight muscle development in the blowfly Lucilia BioI. 120, 1159-1167. cuprina . J. Roy. Microsc. Soc. 88 , 151 -175. North A.J ., Gimona M., Lando Z. and Small J.v. (1994) . Actin isoform Henry M.D. and Campbell K.P. (1996). Dystroglycan: an extracellular compartments in chicken gizzard smooth muscle cells. J. Cell Sci. matrix receptor linked to the cytoskeleton. Curro Opin. Cell BioI. 8, 107, 445-455. 625-631 . Obermann W.M.J. , Gautel M., Steiner F., van der Ven P.F.M. , Weber K. Horowitz A., Menice C .B., Laporte R. and Morgan K.G. (1996). and Furst D.O. (1996) . The structure of the sarcomeric M band: Mechanisms of smooth muscle contraction. Physiol. Rev. 76, 967- Localization of defined domains of myomesin, M-protein, and the 1003. 250-kD carboxy-terminal region of titin by immunoelectron Isenberg G. and Goldmann W.H . (1992). Actin-membrane coupling: a microscopy. J. Cell BioI. 134, 1441-1453. role for talin . J. Muscle Res. Cell Motil. 13, 587-589. Obermann W.M .J ., Gautel M., Weber K. and Furst D.O. (1997). Kargacin G.J. , Cooke P.H., Abramson S.B. and Fay F.S. (1989). Molecular structure of the sarcomeric M band: mapping of titin and Periodic organization of the contractile apparatus in smooth muscle myosin binding domains in myomesin and the id entification of a revealed by the motion of dense bodies in single cells. J. Cell BioI. potential regulatory phosphorylation site in myomesin. EMBO J. 16, 108, 1465-1 475. 211-220. Kellermayer M.S.Z. and Granzier H.L. (1996). Calcium-dependent Otey CA, Kalnoski M.H . and Bulinski J.C. (1988). Immunolocalization inhibition of in vitro thin-filament motility by native titin. FEBS Lett. of muscle and non muscle isoforms of actin in myogenic cells and 380, 281 -286. adult skeletal muscle. Cell Motil. Cytoskeleton 9, 337-348. Kruger M. , Wright J. and Wang K. (1991). Nebulin as a length regulator Pardo J.V. , Siliciano J.D. and Craig S.W. (1983a). Vinculin is a of thin filaments of vertebrate skeletal muscles: Correlation of th in component of an extensive network of myofibril-sarcolemma filament length; nebulin size and epitope profile. J. Cell BioI. 115,97- attachment regions in cardiac muscle fibers. J. Cell BioI. 97, 1081- 107. 1088. Labeit S. and Kolmerer B. (1995). The complete primary structure of Pardo J.V., Siliciano J.D. and Craig S.W. (1983b). A vinculin-containing human nebulin and its correlation to muscle structure. J. Mol. BioI. cortical lattice in skeletal muscle: Transverse lattice elements 248, 308-315. ("costa meres"} mark sites of attachment between myofibrils and Labeit S., Kolmerer B. and Linke W.A. (1997). The giant protein titin. sarcolemma. ProC. Natl. Acad. Sci . USA 80, 1008-1012. Emerging roles in physiology and pathophysiology. Circ. Res. 80, Pierobon-Bormioli S. (1981). Transverse sarcomere filamentous 290-294. systems: Z- and M-cables. J. Muscle Res. Cell Motil. 2. 401 -4 13. Lazarides E. (1980). Intermediate filaments as mechanical integrators of Pfuhl M., Winder S.J., Castiglione Morelli M.A., Labeit S. and Pastore A. cellular space. Nature 283, 249-256. (1996). Correlation between conformational and binding properties Lehman W. (1991). Calponin and the composition of smooth muscle thin of nebulin repeats. J. Mol. BioI. 257, 367-384. filaments. J. Muscle Res. Cell Motil. 12, 221-224. Porter GA, Dmytrenko G.M., Winkelmann J.C. and Bloch R.J. (1992). Lehman w., Sheldon A. and Madonia W. (1987) . Diversity of smooth Dystrophin colocalizes with B-spectrin in distinct subsarcolemmal muscle thin filament composition. Biochim. Biophys. Acta 914, 35- domains in mammalian skeletal muscle. J. Cell BioI. 117. 997-1005. 39. Price M.G. and Gomer R.H . (1993) . Skelemin, a cytoskeletal M-disc Lu M.-H., DiLullo C., Schultheiss T. , Holtzer S., Murray J.M., Choi J., periphery protein, contains motifs of adhesion/recogn ition and Fischman D.A. and Holtzer H. (1992). The vinculin/sarcomeric-a­ intermediate filament proteins. J. BioI. Chem. 268, 21800-21810. actinin/a-actin nexus in cultured cardiac myocytes. J. Cell BioI. 117, Rappaport L. and Samuel J.L. (1988). Microtubules in cardiac myocytes.

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291 The muscle cell cytoskeleton

Int. Rev. Cytol. 113. 101 -143. K.A. (1980) . The distribution of intermediate filament protein Richardson F.L.. Stromer M.H ., Huiatt T.w. and Robson A.M . (1981). (skeletin) in normal and diseased human skeletal muscle, an Immunoelectron and immunofluorescence localization of desmin in immunohistochemical and electron-microscopic study. J. Neurol. mature avian muscles. Eur. J. Cell BioI. 26, 91 -101 . Sci. 47, 153-170. Rybakova I.N., Amann K.J . and Ervasti J.M. (1996) . A new model for Thornell L.-E. and Price M.G . (1991). The Cy1oskeleton in muscle cells the interaction of dystrophin with F-actin. J. Cell BioI. 135, 661-672. in relation to function . Biochem. Soc. Trans. 19, 1116-1120. Schafer DA, Waddle J.A. and Cooper J.A. (1993) . Localization of CapZ Tokuyasu K.T.. Dutton A.H. and Singer S.J. (1983a). Immunoelectron during myofibrillogenesis in cultured chicken muscle. Cell Motil. microscopic studies of desmin (skeletin) localization and Cytoskeleton 25, 317-335. intermediate filament organization in chicken skeletal muscle. J. Cell Schollmeyer J.E., Furcht L.T., Goll D.E. , Robson R.M . and Stromer M.H. BioI. 96, 1727-1735. (1976). Local ization of contractile proteins in smooth muscle cells Tokuyasu K.T .. Dutton A.H. and Singer S.J. (1983b). Immunoelectron and in normal and transformed fibroblasts. In : Cell motility. Book A. microscopic studies of desmin (skeletin) localization and Goldman R .. Pol lard T. and Rosenbaum J. (eds). Cold Spring intermediate fil ament organization in chicken cardiac muscle. J. Cell Harbor Laboratory. Cold Spring Harbor, NY. pp 361-388. BioI. 96, 1736-1742. Schroeter JP., Bretaudiere J.-P., Sass R.L. and Goldstein M.A. (1996). TSkhovrebova L. and Trin ick J . (1997). Direc t visualization of Th ree dimensional structure of the Z band in a normal mammalian extensibility in isolated titin molecules. J. Mol. BioI. 265.100-106. skelelal muscle. J. Cell BioI. 133, 571-583. Vandekerckhove J ., Bugaisky G. and Buckingham M. (1986). Shear C.R. and Bloch R.J. (1985). Vinculin in subsarcolemmal densities Simultaneous expression of skeletal and heart actin proteins in in chicken skeletal muscle: localization and relationship to various striated muscle tissues and cells. J. BioI. Chem. 261 , 1838- intracellular and extracellular structures. J. Cell BioI. 101 , 240-256. 1843. Small J.v. (1995) . Structure-function relationships in smooth muscle: the Vigoreaux J.O. (1994) . The muscle Z band: lessons in stress missing links. BioEssays 17, 785-792. management. J. Muscle Res. Cell Motil. 15, 237-255. Small J.V .. Furst D.O. and Thornell L.-E . (1992) . The cytoskeletallattice Vinkemeier U., Obermann W., Weber K. and Furst D.O. (1993) . The of muscle cells. Eur. J. Biochem. 208, 559-572. globular head domain of titin extends into the center of the Small J.V ., Herzog M. , Barth M. and Draeger A. (1990) . Super­ sarcomeric M band . J. Cell Sci. 106,319-330. contracted state of vertebrate smooth muscle cell fragments reveals Volk T. and Geiger B. (1986). A-CAM: A 135-kD receptor of intercell ular myofilament lengths. J. Cell BioI. 111,2451-2461. adherens junctions I. Immunoelectron microscopic localization and Somlyo A.P. and Somlyo A.V. (1992). Smooth muscle structure and biochemical studies. J. Cell BioI. 103, 1441 -1450. function. In: The heart and cardiovascular system. Vol. 2, 2nd edn . Walsh A.A. (1997). Microtubules and pressure-overload hypertrophy. Fozzard HA, Haber E., Jennings R.B., Katz A.M . and Morgan H.E. Circ. Res. 80, 295-296. (eds) . Raven Press, Ltd. New York. pp 1295-1324. Wang K. and Ram irez-Mitchell R. (1983) . A network of transverse and Stevenson S., Rothery S., Cullen M.J . and Severs N.J. (1997). longitudinal intermediate filaments is associated with sarcomeres of Dystrophin is not a specific component of the cardiac costamere. adult vertebrate skeletal muscle. J. Cell BioI. 96, 562-570. Circ. Res . 80, 269-280. Wang K., McCarter R., Wright J., Beverly J. and Ramirez-Mitchell R. Stromer M.H. (1990) . Intermediate (10-nm) filaments in muscle. In : (1993). Viscoelasticity of the sarcomere matrix of skeletal muscles. Cellular and molecular biology of intermediate filaments. Goldman The titin-myosin composite filament is a dual-stage molecular spring. R.D. and Steinert P.M. (eds). Plenum Pub. Corp. New York. pp 19- Biophys. J. 64 , 1161 -1177. 36. Wang K .. Kn ipfer M., Huang 0 .- 0. , van Heerden A., Hsu L.C.-L., Stromer M.H . (1995). Immunocytochemistry of the muscle cell Gutierrez G. , Ouian X.-L. and Stedman H. (1996) . Human skeletal Cy1oskeleton. Microsc. Res . Tech. 31 , 95-105. muscle nebulin sequence encodes a blueprint for thin filament Stromer M.H . and Bendayan M. (1988). Arrangement of desmin architecture. J. BioI. Chem. 271,4304-4314. intermediate filaments in smooth muscle cells as shown by high Watkins S.C .. Samuel J.L. , Marotte F .. Bertier-Savalle B. and Rappaport resolution immunocytochemistry. Cell Moti! . Cy1oskeleton 11 , 117- L. (1987). Microtubules and desmin filaments during onset of heart 125. hypertrophy in rat: A double immunoelectron microscope study. Circ. Stromer M. H. and Bendayan M. (1990). Immunocytochemical Res. 60, 327-336. identification of cytoskeletal linkages to smooth muscle cell nuclei Winder S.J. and Walsh M.P. (1993) . Calponin: thin filament-linked and mitochondria. Cell Moti! Cy1oskeleton 17, 11 -18. regulation of smooth muscle contraction. Cell. Signal. 5, 677- Stromer M.H., Hartshorne D.J. and Rice A.V. (1967) . Removal and 686. re constitutio n of Z-li ne material in striated muscle. J. Cell BioI. 35, Yamaguchi M., Robson R.M. , Stromer M.H. , Dahl D.S. and Oda T. C23-C28. (1978) . Actin filaments form the backbone of nemaline myopathy Stromer M.H. , Tabatabai L.B., Robson A.M .. Goll D.E. and Zeece M.G. rods. Nature 271 , 265-267. (1976). Nemalinemyopathy, an integrated study: selective Yamaguchi M., Izumimoto M., Robson A.M . and Stromer M.H. (1985). extraction. Exp. Neurol. 50, 402-421 . Fine structure of wide and narrow vertebrate muscle Z-lines. A Terracio L., Simpson D.G. , Hilenski L. , Carver W. , Decker R.S. , Vinson proposed model and computer simulation of Z-l ine architecture. J. N. and Borg T.K. (1990) . Distribution of vinculin in the Z-disk of Mol. BioI. 184,621-644. striated muscle: analysis by laser scanning confocal microscopy. J. Zhang J., Robson A.M ., Schmidt J.M. and Stromer M.H. (1996) . Talin Cell. Physiol. 145, 78-87. can crosslink actin filaments into both networks and bundles. Thornell L. -E., Edstrom L., Eriksson A., Hendriksson K.-G. and Angqvist Biochem. Biophys. Res. Comm . 218, 530-537.