Heterogeneous activation of a slow myosin gene in proliferating myoblasts and differentiated single myofibers

Abstract

Each skeletal muscle contains a fixed ratio of fast and slow myofibers that are distributed in a stereotyped pattern to achieve a specific motor function. How myofibers are specified during development and regeneration is poorly understood. Here we address this question using transgenic reporter mice that indelibly mark the myofiber lineages based on activation of fast or slow myosin. Lineage tracing indicates that during development all muscles have activated the fast myosin gene Myl1, but not the slow myosin gene Myh7, which is activated in all slow but a subset of fast myofibers. Similarly, most nascent myofibers do not activate Myh7 during fast muscle regeneration, but the ratio and pattern of fast and slow myofibers are restored at the completion of regeneration. At the single myofiber level, most mature fast myofibers are heterogeneous in nuclear composition, manifested by mosaic activation of Myh7. Strikingly, Myh7 is activated in a subpopulation of proliferating myoblasts that co-express the myogenic progenitor marker Pax7. When induced to differentiate, the Myh7-activated myoblasts differentiate more readily than the non-activated myoblasts, and have a higher tendency, but not restricted, to become slow myotubes. Together, our data reveal significant nuclear heterogeneity within a single myofiber, and challenge the conventional view that myosin genes are only expressed after myogenic differentiation. These results provide novel insights into the regulation of muscle fiber type specification.

Single-nucleus RNA-seq and FISH identify coordinated transcriptional activity in mammalian myofibers

“…In Drosophila, one founder myonucleus was shown to reprogram newly fused myonuclei, inducing coordination of expression between nuclei of the same fiber 44 . Alternatively, distinct populations of preprogrammed myogenic stem cells with distinct potential [45][46][47][48][49] could fuse (homotypic fusion 50 ), activating a single Myh gene along the myofiber. Moreover, extracellular signals from surrounding cells may also intervene 51 .…”

Satellite-like cells contribute to pax7-dependent skeletal muscle repair in adult zebrafish

“…Does the preferential association of satellite cells with slow muscle fibers have implications for regenerative capacity of slow versus fast fibers? Do resident satellite cells give rise to slow and fast muscle equivalently, or is there a hierarchical process (Wang et al, 2015)? Finally, do pax7a and/or pax3a -expressing satellite-like cell populations differentially contribute to fast and slow muscle?…”

High-throughput data-driven analysis of myofiber composition reveals muscle-specific disease and age-associated patterns

“…Here we define myofibers using a data-driven and hypothesis-free analysis, we show that the majority of myofibers express more than one MyHC isoform and the pattern of myofiber composition is distinguished between muscles. Application of semi-automatic image quantification can be combined with data-driven analysis of tens of thousands of myofibers (17,28,29). We considered the full range of MFI levels without binning of the data and show that myofiber composition is altered between muscles, wild type and LMGD models and with age.…”