M-band structure, M-bridge interactions and contraction speed in vertebrate cardiac muscles

Abstract

Cardiac muscle M-band structures in several mammals (guinea pig, rabbit, rat and cow) and also from three teleosts (plaice, carp and roach), have been studied using electron microscopy and image processing. Axial structure seen in negatively stained isolated myofibrils or negatively stained cryo-sections shows the presence of five strong M-bridge lines (M6, M4, M1, M4' and M6') except in the case of the teleost M-bands in which the central M-line (M1) is absent, giving a four-line M-band. The M4 (M4') lines are consistently strong in all muscles, supporting the suggestion that bridges at this position are important for the structural integrity of the A-band myosin filament lattice. Across the vertebrate kingdom, cardiac M-band ultrastructure appears to correlate roughly with heartbeat frequency, just as in skeletal muscles it correlates with contraction speed, reinforcing the suggestion that some M-band components may have a significant physiological role. Apart from rat heart, which is relatively fast and has a conventional five-line M-band with M1 and M4 approximately equal, the rabbit, guinea pig and beef heart M-bands from a new 1 + 4 class; M1 is relatively very much stronger than M4. Transverse sections of the teleost (roach) cardiac A-band show a simple lattice arrangement of myosin filaments, just as teleost skeletal muscles. Almost all other vertebrate striated muscles, including mammalian heart muscles, have a statistical superlattice structure. The high degree of filament lattice order in teleost cardiac muscles indicates their potential usefulness for ultrastructural studies. It is shown that, in four-line M-bands in which the central (M1) M-bridges are missing, interactions at M4 (M4') are sufficient to define the different myosin filament orientations in simple lattice and superlattice A-bands. However the presence of M1 bridges may improve the axial order of the A-band.

The M-band: an elastic web that crosslinks thick filaments in the center of the sarcomere

“…The approximate positions of titin, myomesin and M-protein in the sarcomere were determined by biochemical assays and EM epitope localization; these data led to a molecular model of the M-band [11,18,19]. In this model, M-protein molecules bridge the myosin filaments at the level of the central M1 line (see Box 1), which is consistent with previous EM observations [20,21]. However, the model cannot explain how this connection is established in muscles devoid of M-protein, such as the embryonic heart [22] and slow-twitch fibers of skeletal muscle [23,24].…”

“…Furthermore, the binding of the muscle isoform of creatine kinase (MM-CK) to myomesin domains My7-My8 [69] correlates with the M4 0 and M4 lines (Box 1). According to EM studies, these lines are of the same intensity in all muscles studied [21], which is consistent with the prominent expression of myomesin in all muscle types [26]. The M1 line correlates with the expression of M-protein in several muscles [20,21], but the mode of its incorporation into the M-band remains The N-terminal parts of both proteins, as well as the alternatively spliced embryonic heart (EH) fragment are predicted to be in an intrinsically disordered state (Box 2).…”

“…The thick filaments in vertebrate striated muscle are arranged either in a simple lattice or in a superlattice (Box 1) determined by the M4 bridges, as suggested by previous investigators [21,32,33]. According to EM data, myomesin is the main candidate for the role of M4/M4 0 bridges [11].…”

“…In addition, the M-band contains two principal candidates for the role of M-bridges, originally named myomesin [12,13] (myomesin-1 in RefSeq database) and M-protein [14] lines missing), whereas the slowest fibers have a four-line pattern (M1 line missing), and fibers of intermediate speed have variations of a fiveline pattern [23,65,66]. Accordingly, the M-band appearance changes during differentiation of slow fibers of the rat [23] and correlates with the beating rate in the heart of different species [21]. These observations indicate that the M-band structure depends on contractile characteristics.…”

Tissue-specific Isoforms of Chicken Myomesin Are Generated by Alternative Splicing

“…Correlation between the absence of Mprotein from slow muscle fibers (type I) of rat skeletal muscle (musculus tibialis anterior and musculus soleus) and the absence of subline M1 from M-bands of the same fibers indicate that M-protein might be the main component of the M-bridges in M1 (Carlsson and Thornell, 1987). This idea is supported by the observation that M-bridges in M4/M4Ј rather than in M1 seem to be essential for the maintenance of the thick filament lattice in both skeletal and heart muscle (Luther et al, 1981;Luther and Squire, 1980;Pask et al, 1994). If M-protein is indeed the main component of the M-bridge in M1, then myomesin could contribute to the stabilization of the distal region of the M-band where the M6/M6Ј sublines occur.…”