Partial characterization of zeugmatin indicates that it is part of the Z-band region of titin

STED analysis reveals the organization of nonmuscle muscle II, muscle myosin II, and F-actin in nascent myofibrils

A three-step model has been proposed to describe myofibril assembly in vertebrate cardiac and skeletal muscle cells beginning with premyofibrils, followed by nascent myofibrils, and ending as mature myofibrils (reviewed in Sanger, Wang, et al. (2017). Assembly and maintenance of myofibrils in striated muscle. Handbook of Experimental Pharmacology 235, 39-75; Wang, Fan, (2020). Myofibril assembly and the roles of the ubiquitin proteasome system. Cytoskeleton 77, 456-479). Premyofibrils are composed of minisarcomeres that contain nonmuscle myosin II filaments interdigitating with actin filaments embedded at their barbed ends in muscle-specific alpha-actinin-rich Z-bodies. Sarcomeres in mature myofibrils have filaments of muscle myosin II that interact with actin filaments that are attached to muscle alpha-actinin in Z-bands. Nascent myofibrils, the transitional step between premyofibrils and mature myofibrils, possess two types of myosins II, that is, nonmuscle myosin II and muscle myosin II. The relationship of these two different myosins II in nascent myofibrils, however, is not clear. Stimulated emission depletion (STED) microscopic analyses of nascent myofibrils in both embryonic chick cardiomyocytes, and hiPSC-derived cardiomyocytes revealed that nonmuscle myosin II is in the middle of the nascent myofibril, surrounded by overlapping muscle myosin II filaments at the periphery, and non-striated filamentous actin is present in the nascent myofibril. These findings support the original three-step model of myofibril assembly proposed by Rhee, Sanger, and Sanger, (1994). The premyofibrils: Evidence for its role in myofibrillogenesis. Cell Motility and the Cytoskeleton 28, 1-24.

Inhibitors of the Ubiquitin Proteasome System block myofibril assembly in cardiomyocytes derived from chick embryos and human pluripotent stem cells

Details of sarcomeric protein assembly during de novo myofibril formation closely resemble myofibrillogenesis in skeletal and cardiac myocytes in birds, rodents and zebrafish. The arrangement of proteins during myofibrillogenesis follows a three-step process: beginning with premyofibrils, followed by nascent myofibrils, and concluding with mature myofibrils (reviewed in Sanger et al., 2017). Our aim is to determine if the same pathway is followed in human cardiomyocytes derived from human inducible pluripotent stem cells. We found that the human cardiomyocytes developed patterns of protein organization identical to the three-step series seen in the model organisms cited above. Further experiments showed that myofibril assembly can be blocked at the nascent myofibril by five different inhibitors of the Ubiquitin Proteasome System (UPS) stage in both avian and human cardiomyocytes. With the exception of Carfilzomib, removal of the UPS inhibitors allows nascent myofibrils to proceed to mature myofibrils. Some proteasomal inhibitors, such as Bortezomib and Carfilzomib, used to treat multiple myeloma patients, have off-target effects of damage to hearts in three to six percent of these patients. These cardiovascular adverse events may result from prevention of mature myofibril formation in the cardiomyocytes. In summary, our results support a common three-step model for the formation of myofibrils ranging from avian to human cardiomyocytes. The Ubiquitin Proteasome System is required for progression from nascent myofibrils to mature myofibrils. Our experiments suggest a possible explanation for the cardiac and skeletal muscle off-target effects reported in multiple myeloma patients treated with proteasome inhibitors. This article is protected by copyright. All rights reserved.

Effect of MG-132 on myofibrillogenesis and the ubiquitination of GAPDH in quail myotubes

In the three-step myofibrillogenesis model, mature myofibrils are formed through two intermediate structures: premyofibrils and nascent myofibrils. We have recently reported that several inhibitors of the Ubiquitin Proteosome System, for example, MG-132, and DBeQ, reversibly block progression of nascent myofibrils to mature myofibrils. In this investigation, we studied the effects of MG132 and DBeQ on the expression of various myofibrillar proteins including actin, myosin light and heavy chains, tropomyosin, myomesin, and myosin binding protein-C in cultured embryonic quail myotubes by western blotting using two loading controls-α-tubulin and glyceraldehyde 3-phosphate dehydrogenase (GAPDH). Surprisingly, we found that MG-132 affected the level of expression of GAPDH but DBeQ did not. Reverse transcription polymerase chain reaction (RT-PCR) and quantitative reverse transcription-PCR (qRT-PCR) showed no significant effect of MG-132 on GAPDH transcription. Two-dimensional (2D) western blot analyses with extracts of control and MG-132-treated cells using anti-ubiquitin antibody indicated that MG132-treated myotubes show a stronger emitter-coupled logic signal. However, Spot% and Spot volume calculations for all spots from both western blot film signals and matched Coomassie-stained 2D polyacrylamide gel electrophoresis showed that the intensity of staining in a spot of ~39 kDa protein is 3.5-fold lower in the gel of MG-132-treated extracts. Mass spectrometry analyses identified the ~39 kDa protein as quail GAPDH. Immunohistochemical analysis of fixed MG-132-treated myotubes with anti-GAPDH antibody showed extensive clump formation, which may be analogous to granule formation by stress response factors in MG132-treated cells. This is the first report on in vivo ubiquitination of GAPDH. This may be essential for the moonlighting (Jeffery, 1999) activity of GAPDH for tailoring stress in myotubes.

Arg/Abl-binding protein, a Z-body and Z-band protein, binds sarcomeric, costameric, and signaling molecules

ArgBP2 (Arg/Abl-Binding Protein) is expressed at high levels in the heart and is localized in the Z-bands of mature myofibrils. ArgBP2 is a member of a small family of proteins that also includes vinexin and CAP (c-Cbl-associated protein), all characterized by having one sorbin homology (SOHO) domain and three C-terminal SH3 domains. Antibodies directed against ArgBP2 also react with the Z-bodies of myofibril precursors: premyofibrils and nascent myofibrils. Expression in cardiomyocytes of plasmids encoding Yellow Fluorescent Protein (YFP) fused to either full length ArgBP2, the SOHO, mid-ArgBP or the SH3 domains of ArgBP2 led to Z-band targeting of the fusion proteins, whereas an N-terminal fragment lacking these domains did not target to Z-bands. Although ArgBP2 is not found in skeletal muscle cells, YFP-ArgBP2 did target to Z-bodies and Z-bands in cultured myotubes. GST-ArgBP2-SH3 bound actin, α-actinin and vinculin proteins in blot overlays, cosedimentation assays, and EM negative staining techniques. Over-expression of ArgBP2 and ArgBP2-SH3 domains, but not YFP alone, led to loss of myofibrils in cardiomyocytes. Fluorescence recovery after photobleaching was used to measure the rapid dynamics of both the full length and some truncated versions of ArgBP2. Our results indicate that ArgBP2 may play an important role in the assembly and maintenance of myofibrils in cardiomyocytes.

Contractile smooth muscle cells derived from hair-follicle stem cells

AIMS

We hypothesized that hair-follicle stem cells can differentiate toward smooth contractile muscle cells, providing an autologous cell source for cardiovascular tissue regeneration.

METHODS AND RESULTS

Smooth muscle progenitor cells (SMPCs) were obtained from ovine hair follicles using a tissue-specific promoter and fluorescence-activated cell sorting. Hair-follicle smooth muscle progenitor cells (HF-SMPCs) expressed several markers of vascular smooth muscle including alpha-actin, calponin, myosin heavy chain (MHC), caldesmon, smoothelin, and SM22. HF-SMPCs were highly proliferative and showed high clonogenic potential without any signs of chromosomal abnormalities as evidenced by karyotype analysis. HF-SMPCs compacted fibrin hydrogels to a similar extent as vascular smooth muscle cells from ovine umbilical veins (V-SMCs), indicating the development of the force-generating machinery. In addition, cylindrical tissue equivalents prepared with HF-SMPCs displayed significant contractility in response to vasoactive agonists including KCl and the thromboxane A2 mimetic U46619, suggesting that these cells had developed receptor and non-receptor-mediated pathways of contractility. Finally, transforming growth factor-beta1 promoted differentiation of HF-SMPCs toward a mature SMC phenotype as suggested by increased expression of MHC and enhanced matrix compaction.

CONCLUSION

Our results suggest that hair follicles may be an easily accessible, autologous, and rich source of functional SMPC for cardiovascular tissue engineering and regenerative medicine.

The sarcomere and sarcomerogenesis

Striated muscle owes its name to the microscopic appearance, caused by the longitudinal alignment of thousands of highly ordered contractile units, the sarcomeres. The assembly (and disassembly) of these multiprotein complexes (sarcomere assembly or sarcomerogenesis) follows ordered pathways, which are regulated on the transcriptional, translational and posttranslational level. Furthermore, myofibril assembly involves the participation of transient scaffolds and adaptors, notably the microtubule network. Studies in cell culture and developing embryos have revealed common pathways of sarcomere assembly in heart and skeletal muscle. Disruptions in these pathways are implicated in muscle diseases.

How to build a myofibril

Building a myofibril from its component proteins requires the interactions of many different proteins in a process whose details are not understood. Several models have been proposed to provide a framework for understanding the increasing data on new myofibrillar proteins and their localizations during muscle development. In this article we discuss four current models that seek to explain how the assembly occurs in vertebrate cross-striated muscles. The models hypothesize: (a) stress fiber-like structures as templates for the assembly of myofibrils, (b) assembly in which the actin filaments and Z-bands form subunits independently from A-band subunits, with the two subsequently joined together to form a myofibril, (c) premyofibrils as precursors of myofibrils, or (d) assembly occurring without any intermediary structures. The premyofibril model, proposed by the authors, is discussed in more detail as it could explain myofibrillogenesis under a variety of different conditions: in ovo, in explants, and in tissue culture studies on cardiac and skeletal muscles.

Inhibitors arrest myofibrillogenesis in skeletal muscle cells at early stages of assembly

A three-step model for myofibrillogenesis has been proposed for the formation of myofibrils [Rhee et al., 1994: Cell Motil. Cytoskeleton 28:1-24; Sanger et al., 2002: Adv. Exp. Med. 481:89-105]: premyofibril to nascent myofibril to mature myofibril. We have found two chemically related inhibitors that will arrest development at both the first and second step. Cultured quail embryonic skeletal myoblasts were treated with ethyl methane sulfonate (EMS) or 2-aminoethyl-methanesulfonate (MTSEA+). When the myoblasts fused in the presence of either of these compounds, myosheets rather than myotubes formed. Treated cells were fixed and immunostained against multiple proteins commonly found in muscle cells. Protein expression and localization throughout the myosheet were similar to that of developing myotube tips. Cells treated with high concentrations of EMS (10 mM) stained for non-muscle myosin II, sarcomeric alpha-actinin, and tropomyosin. No zeugmatin (Z-band region of titin) or muscle myosin II antibody staining was detected in fibers in this treatment group. These fibers are comparable to premyofibrils in control myotubes. At lower concentrations of EMS (7.5 to 5 mM), fibers that formed stained for muscle myosin II and titin as well as for non-muscle myosin IIB, sarcomeric alpha-actinin, and tropomyosin. Muscle myosin II was in an unbanded pattern. These fibers are comparable to nascent myofibrils observed during normal myofibrillogenesis. Similar effects to those obtained by treating cells with EMS were obtained when we treated cultured cells with MTSEA+ (5 mM) and stained them with sarcomeric alpha-actinin. MTSEA+ is chemically related to EMS, and is a well-known inhibitor of ryanodine receptors in skeletal muscle cells. Some abnormalities such as nemaline-like rods and other protein aggregates also appear within the myosheet during EMS and MTSEA+ treatment. Removal of these two inhibitors of myofibrillogenesis allows the premyofibrils and nascent myofibrils to form mature myofibrils.

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

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

Assembly of the giant protein projectin during myofibrillogenesis in Drosophila indirect flight muscles

BACKGROUND

Projectin is a giant modular protein of Drosophila muscles and a key component of the elastic connecting filaments (C-filaments), which are involved in stretch activation in insect Indirect Flight Muscles. It is comparable in its structure to titin, which has been implicated as a scaffold during vertebrate myofibrillogenesis.

METHODS

We performed immunofluorescence studies on Drosophila pupal tissue squashes and isolated myofibrils to identify the pattern of appearance and assembly for projectin and several other myofibrillar proteins, using both wild type and mutant fly stocks.

RESULTS AND CONCLUSIONS

In the first step of assembly, projectin immunolocalization appears as random aggregates colocalizing with alpha-actinin, kettin and Z(210), as well as, F-actin. In the second step of assembly, all these proteins become localized within discrete bands, leading ultimately to the regularly spaced I-Z-I regions of myofibrils. This assembly process is not affected in myosin heavy chain mutants, indicating that the anchoring of projectin to the thick filament is not essential for the assembly of projectin into the developing myofibrils. In the actin null mutation, KM88, the early step involving the formation of the aggregates takes place despite the absence of the thin filaments. All tested Z-band proteins including projectin are present and are colocalized over the aggregates. This supports the idea that interactions of projectin with other Z-band associated proteins are sufficient for its initial assembly into the forming myofibrils. In KM88, though, mature Z-bands never form and projectin I-Z-I localization is lost at a later stage during pupal development. In contrast, treatment of adult myofibrils with calpain, which removes the Z-bands, does not lead to the release of projectin. This suggests that after the initial assembly with the Z-bands, projectin also establishes additional anchoring points along the thick and/or thin filaments. In conclusion, during pupation the initial assembly of projectin into the developing myofibril relies on early association with Z-band proteins, but in the mature myofibrils, projectin is also held in position by interactions with the thick and/or the thin filaments.

Myofibrillogenesis in skeletal muscle cells in the presence of taxol

We address the controversy of whether mature myofibrils can form in the presence of taxol, a microtubule-stabilizing compound. Previous electron microscopic studies reported the absence of actin filaments and Z-bands in taxol-treated myocytes [Antin et al., 1981: J Cell Biol 90:300-308; Toyoma et al., 1982: Proc Natl Acad Sci USA 79:6556-6560]. Quail skeletal myoblasts were isolated from 10-day-old embryos and grown in the presence or absence of taxol. Taxol inhibited the formation of multinucleated elongated myotubes. Myocytes cultured in the continual presence of taxol progressed from rounded to stellate shapes. Groups of myocytes that were clustered together after the isolation procedure fused in the presence of taxol but did not form elongated myotubes. Actin filaments and actin-binding proteins were detected with several different fluorescent probes in all myofibrils that formed in the presence of taxol. The Z-bands contained both alpha-actinin and titin, and the typical arrays of A-Bands were always associated with actin filaments in the myofibrils. Myofibril formation was followed by fixing cells each day in culture and staining with probes for actin, muscle-specific alpha-actinin, myosin II, nebulin, troponin, tropomyosin, and non-muscle myosin II. Small linear aggregates of alpha-actinin or Z-bodies, premyofibrils, were detected at the edges of the myocytes and in the arms of the taxol-treated cells and were always associated with actin filaments. Non-muscle myosin II was detected at the edges of the taxol-treated cells. Removal of the taxol drug led to the cells assuming a normal compact elongated shape. During the recovery process, additional myofibrils formed at the spreading edges of these elongated and thicker myotubes. Staining of these taxol-recovering cells with specific fluorescent reagents reveals three different classes of actin fibers. These results are consistent with a model of myofibrillogenesis that involves the transition of premyofibrils to mature myofibrils.

Transient association of titin and myosin with microtubules in nascent myofibrils directed by the MURF2 RING-finger protein

Assembly of muscle sarcomeres is a complex dynamic process and involves a large number of proteins. A growing number of these have regulatory functions and are transiently present in the myofibril. We show here that the novel tubulin-associated RING/B-box protein MURF2 associates transiently with microtubules, myosin and titin during sarcomere assembly. During sarcomere assembly, MURF2 first associates with microtubules at the exclusion of tyrosinated tubulin. Then, MURF2-labelled microtubules associate transiently with sarcomeric myosin and later with A-band titin when non-striated myofibrils differentiate into mature sarcomeres. Finally, MURF2 labelled microtubules disappear from the sarcomere after the incorporation of myosin filaments and the elongation of titin. This suggests that the incorporation of myosin into nascent sarcomeres and the elongation of titin require an active, microtubule-dependent transport process and that MURF2-associated microtubules play a role in the alignment and extension of nascent sarcomeres. MURF2 is expressed in at least four isoforms, of which a 27 kDa isoform is cardiac specific. A C-terminal isoform is generated by alternative reading frame use, a novelty in muscle proteins. In mature cardiac sarcomeres, endogenous MURF2 can associate with the M-band, and is translocated to the nucleus. MURF2 can therefore act as a transient adaptor between microtubules, titin and nascent myosin filaments, as well as being involved in signalling from the sarcomere to the nucleus.

Amorphin is phosphorylase; phosphorylase is an alpha-actinin-binding protein

In a study of myofibrillar proteins, Chowrashi and Pepe [1982: J. Cell Biol. 94:565-573] reported the isolation of a new, 85-kD Z-band protein that they named amorphin. We report that partial sequences of purified amorphin protein indicate that amorphin is identical to phosphorylase, an enzyme important in the metabolism of glycogen. Anti-amorphin antibodies also reacted with purified chicken and rabbit phosphorylase. To explore the basis for phosphorylase's (amorphin's) localization in the Z-bands of skeletal muscles, we reacted biotinylated alpha-actinin with purified amorphin and with purified phosphorylase and found that alpha-actinin bound to each. Radioimmune assays also indicated that phosphorylase (amorphin) bound to alpha-actinin, and, with lower affinity, to F-actin. Negative staining of actin filaments demonstrated that alpha-actinin mediates the binding of phosphorylase to actin filaments. There are several glycolytic enzymes that bind actin (e.g., aldolase, phosphofructokinase, and pyruvate kinase), but phosphorylase is the first one demonstrated to bind alpha-actinin. Localization of phosphorylase in live cells was assessed by transfecting cultures of quail embryonic myotubes with plasmids expressing phosphorylase fused to Green Fluorescent Protein (GFP). This resulted in targeting of the fusion protein to Z-bands accompanied by a diffuse pattern in the cytoplasm.

Myofibrillogenesis in skeletal muscle cells

How are myofibrils assembled in skeletal muscles? The current authors present evidence that myofibrils assemble through a three-step model: premyofibrils to nascent myofibrils to mature myofibrils. This three-step sequence was based initially on studies of living and fixed cultured cells from cardiac muscle. Data from avian primary muscle cells and from a transgenic skeletal mouse cell line indicate that a premyofibril model for myofibrillogenesis also holds for skeletal muscle cells. Premyofibrils are characterized by minisarcomeres bounded by Z-bodies composed of the muscle isoform of alpha-actinin. Actin filaments are connected to these Z-bodies and to the mini-A-bands composed of nonmuscle myosin II filaments. Nascent myofibrils are formed when premyofibrils align and are modified by the addition of titin and muscle myosin II filaments. Mature myofibrils result when nonmuscle myosin II is eliminated from the myofibrils and the alpha-actinin rich Z-bodies fuse as the distance between them increases from 0.5 microm in premyofibrils to 2 to 2.5 microm in the mature myofibrils.

Assembly of myofibrils in cardiac muscle cells

How do myofibrils assemble in cardiac muscle cells? When does titin first assemble into myofibrils? What is the role of titin in the formation of myofibrils in cardiac muscle cells? This chapter reviews when titin is first detected in cultured cardiomyocytes that have been freshly isolated from embryonic avian hearts. Our results support a model for myofibrillogenesis that involves three stages of assembly: premyofibrils, nascent myofibrils and mature myofibrils. Titin and muscle thick filaments were first detected associated with the nascent myofibrils. The Z-band targeting site for titin is localized in the N-terminus of titin. This region of titin binds alpha-actinin and less avidly vinculin. Thus the N-terminus of titin via its binding to alpha-actinin, and vinculin could also help mediate the costameric attachment of the Z-bands of mature myofibrils to the nearest cell surfaces.

D-Titin: a giant protein with dual roles in chromosomes and muscles

Previously, we reported that chromosomes contain a giant filamentous protein, which we identified as titin, a component of muscle sarcomeres. Here, we report the sequence of the entire titin gene in Drosophila melanogaster, D-Titin, and show that it encodes a two-megadalton protein with significant colinear homology to the NH(2)-terminal half of vertebrate titin. Mutations in D-Titin cause chromosome undercondensation, chromosome breakage, loss of diploidy, and premature sister chromatid separation. Additionally, D-Titin mutants have defects in myoblast fusion and muscle organization. The phenotypes of the D-Titin mutants suggest parallel roles for titin in both muscle and chromosome structure and elasticity, and provide new insight into chromosome structure.

Myofibrillogenesis and formation of cell contacts mediate the localization of N-RAP in cultured chick cardiomyocytes

The expression of N-RAP was investigated in immuofluorescently stained embryonic chick cardiomyocyte cultures. After 1 day in culture, the cardiomyocytes were spherical and N-RAP, titin, alpha-actinin, and vinculin were all diffusely distributed. As the cardiomyocytes spread and formed myofibrils and cell contacts, N-RAP became localized to distinct areas in the cells. During myofibrillogenesis, N-RAP was found concentrated in premyofibrils. As the premyofibrils transformed into bundles of mature myofibrils, N-RAP became concentrated at the longitundal ends of the cells, and was not found in the mature sarcomeres. At sites of cell-cell contacts, N-RAP was localized to the cell junction even in cells without any significant myofibril formation. As the cell-cell contacts became more extensive and formed structures resembling the intercalated disks found in hearts, N-RAP became even more specifically concentrated at these junctions. The results show that myofibrillogenesis and cell contact formation can each independently target N-RAP to the longitudinal ends of cardiomyocytes.

Premyofibrils in spreading adult cardiomyocytes in tissue culture: evidence for reexpression of the embryonic program for myofibrillogenesis in adult cells

Do adult cardiomyocytes use the same pathways hypothesized for the formation of myofibrils in embryonic cardiomyocytes in tissue culture. [Rhee, et al., Cell Motil. Cytoskeleton 28:1-24, 1994]? Premyofibrils in embryonic cardiomyocytes are composed of short sarcomeric units of alpha-actinin (Z-bodies) and actin filaments held together by short nonmuscle myosin IIB filaments. Premyofibrils are believed to be transformed into nascent myofibrils by their capture of muscle-specific myosin II filaments aligned in aperiodic arrays. Nascent myofibrils are thought to transform into mature myofibrils by the loss of nonmuscle myosin IIB, the fusion of the Z-bodies into Z-bands, and the periodic alignment of muscle myosin II filaments into A-bands. Freshly isolated cat and rat adult cardiomyocytes placed in tissue culture lack premyofibrils and nascent myofibrils. Adult cardiomyocytes spreading in culture reinitiate the synthesis of nonmuscle myosin IIB. Moreover, patterns similar to the proposed embryonic myofibrillar program first detected in spreading chick embryonic hearts were also detected in these spreading adult mammalian cardiomyocytes. The isolated adult cardiomyocytes begin to spread after 1 day in culture by sending out lamellipodia. When these cells are injected with fluorescently labeled alpha-actinin, linear arrays of short spacings of beaded alpha-actinin bodies are detected in the spreading edges of the adult cardiomyocytes. These dense bodies (Z-bodies) stain positively for the same sarcomeric-specific isoform of alpha-actinin that is in the Z-bands of mature sarcomeres. These linear arrays of alpha-actinin-containing Z-bodies have other characteristics of premyofibrils and are detected only in the spreading regions of the cells. Thus, these premyofibrils at the edges of the spreading adult cardiomyocytes stain positively for nonmuscle myosin IIB but negatively for muscle-specific myosin II. Initially, no vinculin is associated with any parts of the premyofibrils in the spreading regions of the early spreading cardiomyocytes. However, later, vinculin is found to be associated with the ends of the premyofibrils. Fibers that stain solidly for muscle-specific myosin II (i.e., nascent myofibrils) are localized between the peripheral premyofibrils and the centrally positioned, mature myofibrils. It is suggested that the puzzling ability of cardiomyocytes in hypertrophic hearts to reinitiate the synthesis of fetal sarcomeric proteins may be related to the reinitiation of the embryonic premyofibril program for myofibrillogenesis.

A functional knock-out of titin results in defective myofibril assembly

Titin, also called connectin, is a giant muscle protein that spans the distance from the sarcomeric Z-disc to the M-band. Titin is thought to direct the assembly of sarcomeres and to maintain sarcomeric integrity by interacting with numerous sarcomeric proteins and providing a mechanical linkage. Since severe defects of such an important molecule are likely to result in embryonic lethality, a cell culture model should offer the best practicable tool to probe the cellular functions of titin. The myofibroblast cell line BHK-21/C13 was described to assemble myofibrils in culture. We have now characterized the sub-line BHK-21-Bi, which bears a small deletion within the titin gene. RNA analysis revealed that in this mutant cell line only a small internal portion of the titin mRNA is deleted. However, western blots, immunofluorescence microscopy and immunoprecipitation experiments showed that only the N-terminal, approx. 100 kDa central Z-disc portion of the 3 MDa titin protein is expressed, due to the homozygous deletion in the gene. Most importantly, in BHK-21-Bi cells the formation of thick myosin filaments and the assembly of myofibrils are impaired, although sarcomeric proteins are expressed. Lack of thick filament formation and of ordered actin-myosin arrays was confirmed by electron microscopy. Myogenisation induced by transfection with MyoD yielded myofibrils only in myotubes formed from wild type and not from mutant cells, ruling out that a principal failure in myogenic commitment of the BHK-21-Bi cells might cause the observed effects. These experiments provide the first direct evidence for the crucial role of titin in both thick filament formation as a molecular ruler and in the coordination of myofibrillogenesis.

Targeting of cardiac muscle titin fragments to the Z-bands and dense bodies of living muscle and non-muscle cells

A 6.5-kb N-terminal region of embryonic chick cardiac titin, including the region previously reported as part of the protein zeugmatin, has been sequenced, further demonstrating that zeugmatin is part of the N-terminal region of titin, and not a separate Z-band protein. This Z-band region of cardiac titin, from both 7- and 19-day embryos as well as from adult animals, was found to contain six different small motifs, termed z-repeats [Gautel et al., 1996: J. Cell Sci. 109:2747-2754], of approximately 45 amino acids each sandwiched between flanking regions containing Ig domains. Fragments of Z-band titin, linked to GFP, were expressed in cultured cardiomyocytes to determine which regions were responsible for Z-band targeting. Transfections of primary cultures of embryonic chick cardiomyocytes demonstrated that the z-repeats play the major role in targeting titin fragments to the Z-band. Similar transfections of skeletal myotubes and non-muscle cells lead to the localization of these cardiac z-repeats in the Z-bands of the myofibrils and the dense bodies of the stress fibers. Over-expression of these z-repeat constructs in either muscle or non-muscle cells lead to the loss of the myofibrils or stress fibers, respectively. The transfection experiments also indicated that small domains of a protein, 40 to 50 amino acids, can be studied for their localization properties in living cells if a suitable linker is placed between these small domains and the much larger 28 kDa GFP protein.

Muscle assembly: a titanic achievement?

The formation of perfectly aligned myofibrils in striated muscle represents a dramatic example of supramolecular assembly in eukaryotic cells. Recently, considerable progress has been made in deciphering the roles that titin, the third most abundant protein in muscle, has in this process. An increasing number of sarcomeric proteins (ligands) are being identified that bind to specific titin domains. Titin may serve as a molecular blueprint for sarcomere assembly and turnover by specifying the precise position of its ligands within each half-sarcomere in addition to functioning as a molecular spring that maintains the structural integrity of the contracting myofibrils.

Human autoantibodies reveal titin as a chromosomal protein

Assembly of the higher-order structure of mitotic chromosomes is a prerequisite for proper chromosome condensation, segregation and integrity. Understanding the details of this process has been limited because very few proteins involved in the assembly of chromosome structure have been discovered. Using a human autoimmune scleroderma serum that identifies a chromosomal protein in human cells and Drosophila embryos, we cloned the corresponding Drosophila gene that encodes the homologue of vertebrate titin based on protein size, sequence similarity, developmental expression and subcellular localization. Titin is a giant sarcomeric protein responsible for the elasticity of striated muscle that may also function as a molecular scaffold for myofibrillar assembly. Molecular analysis and immunostaining with antibodies to multiple titin epitopes indicates that the chromosomal and muscle forms of titin may vary in their NH2 termini. The identification of titin as a chromosomal component provides a molecular basis for chromosome structure and elasticity.

Molecular structure of the sarcomeric Z-disk: two types of titin interactions lead to an asymmetrical sorting of alpha-actinin

The sarcomeric Z-disk, the anchoring plane of thin (actin) filaments, links titin (also called connectin) and actin filaments from opposing sarcomere halves in a lattice connected by alpha-actinin. We demonstrate by protein interaction analysis that two types of titin interactions are involved in the assembly of alpha-actinin into the Z-disk. Titin interacts via a single binding site with the two central spectrin-like repeats of the outermost pair of alpha-actinin molecules. In the central Z-disk, titin can interact with multiple alpha-actinin molecules via their C-terminal domains. These interactions allow the assembly of a ternary complex of titin, actin and alpha-actinin in vitro, and are expected to constrain the path of titin in the Z-disk. In thick skeletal muscle Z-disks, titin filaments cross over the Z-disk centre by approximately 30 nm, suggesting that their alpha-actinin-binding sites overlap in an antiparallel fashion. The combination of our biochemical and ultrastructural data now allows a molecular model of the sarcomeric Z-disk, where overlapping titin filaments and their interactions with the alpha-actinin rod and C-terminal domain can account for the essential ultrastructural features.

Tissue-specific expression and alpha-actinin binding properties of the Z-disc titin: implications for the nature of vertebrate Z-discs

Titins are giant filamentous proteins which connect Z-discs and M-lines in the sarcomeres of vertebrate striated muscles. Comparison of the N-terminal region of titin (Z-disc region) from different skeletal and cardiac muscles reveals a 900-residue segment which is expressed in different length variants, dependent on tissue type. When searching for ligands of this differentially expressed domain by a yeast-two hybrid approach, we detected binding to alpha-actinin. The isolated alpha-actinin cDNAs were derived from the C-terminal region of the alpha-actinin isoform (alpha-actinin-2) encoded by the ACTN2 gene. Therefore, the two antiparallel subunits of an alpha-actinin-2 homodimer will attach to actin at their respective C termini, whereas they will bind to the Z-disc titin at their N termini. This may thus explain how alpha-actinins can cross-link antiparallel titin and thin filaments from opposing sarcomeres. The alpha-actinin-2 binding site of the Z-disc titin is located within a sequence of 45-residue repeats, referred to as Z-repeat region. Both the N-terminal and C-terminal Z-repeats have alpha-actinin binding properties and are expressed in all striated muscles. By contrast, the more central Z-repeats are expressed in slow and fast skeletal muscles, as well as embryonic and adult cardiac muscles, in different copy numbers. Such alternative splicing of the Z-disc titin appears to be important for the tissue and fibre type diversity of the Z-disc lattice.

The N-terminal Z repeat 5 of connectin/titin binds to the C-terminal region of alpha-actinin

Connectin/titin, a 3000 kDa protein, links the Z line to the myosin filament in striated muscle sarcomeres. Using the yeast two-hybrid system, the present work shows that the N-terminal Z repeat 5 region (amino acids 447-472) of connectin binds to the C-terminal region (amino acids 825-897) of alpha-actinin, the main constituent of the Z line.

An N-terminal fragment of titin coupled to green fluorescent protein localizes to the Z-bands in living muscle cells: overexpression leads to myofibril disassembly

Cultures of nonmuscle cells, skeletal myotubes, and cardiomyocytes were transfected with a fusion construct (Z1.1GFP) consisting of a 1.1-kb cDNA (Z1.1) fragment from the Z-band region of titin linked to the cDNA for green fluorescent protein (GFP). The Z1.1 cDNA encodes only 362 amino acids of the approximately 2000 amino acids that make up the Z-band region of titin; nevertheless, the Z1.1GFP fusion protein targets the alpha-actinin-rich Z-bands of contracting myofibrils in vivo. This fluorescent fusion protein also localizes in the nascent and premyofibrils at the edges of spreading cardiomyocytes. Similarly, in transfected nonmuscle cells, the Z1.1GFP fusion protein localizes to the alpha-actinin-containing dense bodies of the stress fibers in vivo. A dominant negative phenotype was also observed in living cells expressing high levels of this Z1.1GFP fusion protein, with myofibril disassembly occurring as titin-GFP fragments accumulated. These data indicate that the Z-band region of titin plays an important role in maintaining and organizing the structure of the myofibril. The Z1.1 cDNA was derived from a chicken cardiac lambda gt11 expression library, screened with a zeugmatin antibody. Recent work has suggested that zeugmatin is actually part of the N-terminal region of the 81-kb titin cDNA. A reverse transcriptase polymerase chain reaction using a primer from the distal end (5' end) of the Z1.1 zeugmatin cDNA and a primer from the nearest known proximal (3' end) chicken titin (also called connectin) cDNA resulted in a predicted 0.3-kb polymerase chain reaction product linking the two known chicken titin cDNAs to each other. The linking region had a 79% identity at the amino acid level to human cardiac titin. This result and a Southern blot analysis of chicken genomic DNA hybridized with Z1.1 add further support to our original suggestion that zeugmatin is a proteolytic fragment from the N-terminal region of titin.