About this Research Topic
The myoplasm of muscle fibers contains two major components, myofibrils and their associated membranes. Myofibrillar precursors, premyofibrils, begin to form as sufficient concentrations of contractile proteins accumulate in the myoplasm. These contain longitudinally oriented arrays of I-Z-I complexes, consisting of thin actin filaments occupying primitive I-bands and attached to either side of Z-bodies, composed of α-actinin. The primitive I-Z-I complexes are connected to miniature A-bands consisting of non-muscle myosin-II. Premyofibrils develop into nascent myofibrils with the integration of at least three additional components: nebulin, titin, and T-cap. Nebulin (~800kDa), a giant protein of striated muscles, associates with actin filaments present in I-Z-I complexes, driving their assembly and determining their length. As this occurs, muscle myosin-II replaces non-muscle myosin-II in maturing thick filaments. Concurrently, titin (3-4MDa), another giant myofibrillar protein, associates with the maturing I-Z-I complexes and with myosin filaments, guiding the assembly of thick filaments into A-bands and facilitating the coordinated integration of thin and thick filaments into sarcomeres. With the appearance of titin, M-line proteins, including myomesin and obscurin (720-900kDa), the third member of muscle giants, are also expressed. These assume a periodic distribution midway of A-bands as soon as they are synthesized, and contribute to the stabilization of myosin thick filaments.
As myofibrillogenesis proceeds, the internal membrane systems also develop. Initially, the endoplasmic reticulum (ER) differentiates into sarcoplasmic reticulum (SR) by the gradual displacement of ER proteins by SR-specific proteins, including the Ca2+-pump SERCA, the Ca2+-release channel ryanodine receptor (RyR), and the Ca2+-binding protein, calsequestrin. Docking of a patch of SR to a corresponding patch of surface membrane follows, as premyofibrils develop into nascent myofibrils. This event triggers the SR proteins to segregate into two compartments: the network SR, which contains higher amounts of SERCA, and the junctional SR (jSR), harboring RyR and calsequestrin. In parallel, primitive t-tubular elements begin to form in small vesicles, as voltage-gated Ca2+-channels, or dihydropyridine receptors (DHPR), accumulate. As nascent myofibrils develop into mature myofibrils, junctions between the jSR and t-tubules form and start to couple depolarization and Ca2+ influx to Ca2+ release. As myotubes transform to myofibers, t-tubules acquire their transverse orientation in the myoplasm, once myofibrils and SR membranes become aligned transversely.
During the last fifty years, a multitude of important advances have occurred in the field of muscle physiology and pathophysiology providing comprehensive answers concerning muscle assembly, structure and function. This was achieved with the use of highly sophisticated and innovative technologies, ranging from systems biology to single cell measurements. The wealth of this knowledge emphasizes the complexity of myofibers, and prompts researchers to interrogate their mechanics and regulation with greater scrutiny. In this Research Topic, we welcome authors to present original research and review articles that will stimulate our continuing efforts to understand the intricacies of striated muscle cells.
Potential topics include:
• Assembly and organization of the sarcomeric and extrasarcomeric cytoskeleton
• Internal membranes and excitation/contraction coupling
• Apoptosis and Ca2+ microdomains
• Muscle biomechanics
• Stem cells and muscle regeneration
• Models of muscle disease
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