Muscle specific stress fibers give rise to sarcomeres and are mechanistically distinct from stress fibers in non-muscle cells

Abstract

The sarcomere is the basic contractile unit within cardiomyocytes driving heart muscle contraction. We sought to test the mechanisms regulating thin (i.e., actin) and thick (i.e., myosin) filament assembly during sarcomere formation. Thus, we developed an assay using human cardiomyocytes to test de novo sarcomere assembly. Using this assay, we report a population of muscle-specific stress fibers are essential sarcomere precursors. We show sarcomeric actin filaments arise directly from these muscle stress fibers. This process requires formin-mediated but not Arp2/3-mediated actin polymerization and nonmuscle myosin IIB but not non-muscle myosin IIA. Furthermore, we show a short species of β cardiac myosin II filaments grows to form ~1.5 long filaments that then "stitch" together to form the stack of filaments at the core of the sarcomere (i.e., A-band). Interestingly, these are different from mechanisms that have previously been reported during stress fiber assembly in non-muscle cells. Thus, we provide a new model of cardiac sarcomere assembly based on distinct mechanisms of stress fiber regulation between non-muscle and muscle cells.

Focal adhesion kinase regulates early steps of myofibrillogenesis in cardiomyocytes

“…The model we are proposing in Figure 5F is an extension of a model we recently proposed based on data that clearly shows that myofibrils arise from muscle-specific stress fiber precursors (Fenix et al, 2017). These myofibril precursors are assembled parallel to the edge of the cell and move away from the cell through retrograde flow while sarcomeres elaborate along their lengths.…”

“…These myofibril precursors are assembled parallel to the edge of the cell and move away from the cell through retrograde flow while sarcomeres elaborate along their lengths. In particular, Z-lines of the new assembling sarcomeres are perpendicular to the edge (Fenix et al, 2017) and parallel to the long axis of mature FAs ( Figure 5F). However, a recent study has proposed an alternative model of myofibrillogenesis, where myofibrils nucleate directly from FAs and have their Z-lines positioned with the opposite orientation as our data shows (Chopra et al, 2018).…”

“…In this study, we utilized induced pluripotent stem cell (iPSC)-derived human cardiomyocytes (hCMs) as a model system for de novo myofibril and adhesion assembly (Fenix et al, 2017). iPSC hCMs have robust survival and are amenable to genetic manipulation and exogenous protein expression, allowing for a unique opportunity to study myofibrillogenesis with unprecedented temporal and spatial resolution (Peter et al, 2016).…”