Myofibrillogenesis and the avian precardiac explant system

Abstract

Cardiac muscle contraction is critically dependent upon the extensive level of organization of cytoskeletal proteins found in the repeating sarcomeric units of individual myofibrils. Within these units, thick and thin filament systems are assembled and aligned to the precision of single molecules. For years, scientists have been challenged to uncover the mechanisms by which this is accomplished. To date, however, these mechanisms remain relatively unclear due in large part to the lack of suitable in vitro models that faithfully recapitulate the events of myofibril assembly observed in vivo. Several years ago, an avian embryo explant system was developed to investigate other aspects of heart development. Within this system, premyocardial cells differentiate in culture and commence beating in a temporal pattern that corresponds with cardiomyocyte differentiation in vivo. We hypothesized that premyocardial explants could also serve as a particularly advantageous system for investigating myofibrillogenesis. To test this, in Chapter 2, we characterized the temporal/spatial relationships between sarcomeric components during assembly using immunofluorescence microscopy. Our results indicated that events of myofibril assembly in explants mirrored those observed in vivo. Furthermore, these cells are accessible to experimental manipulation (Chapter 5). In Chapter 3, we utilized the precardiac explant system to investigate events of actin (thin) filament assembly during development. Immunofluorescence and ultrastructural analyses revealed that thin filament and sarcomere lengths increase gradually as cardiomyocytes mature. FRAP analyses also demonstrated that the thin filament pointed-end capping activity of E-Tmod is more dynamic during early assembly stages, a property that could dramatically affect the rate of actin monomer exchange/addition during myofibrillogenesis. Research continues in an attempt to identify potential mechanisms regulating E-Tmod dynamics. Finally, in Chapter 4, we investigated the function of a unique elastic region of I-band titin called titin-N2B. In this study, GFP-tagged constructs of titin-N2B were overexpressed in cardiomyocytes in an attempt to disrupt the potential interaction of endogenous N2B with an intracellular ligand. Our results suggested that the NH2-terminal domains of N2B are directly or indirectly critical for stabilizing thin filament structure; thus, N2B emerges as a unique region of titin that is critical for the maintenance of cardiac myofibrils.