Human Skeletal Muscle Fiber Heterogeneity Beyond Myosin Heavy Chains

Abstract

AbstractSkeletal muscle is an inherently heterogenous tissue comprised primarily of myofibers, which are historically classified into three distinct fiber types in humans: one “slow” (type 1) and two “fast” (type 2A and type 2X), delineated by the expression of myosin heavy chain isoforms (MYHs)1. However, whether discrete fiber types exist or whether fiber heterogeneity reflects a continuum remains unclear. Furthermore, whether MYHs are the main classifiers of skeletal muscle fibers has not been examined in an unbiased manner. Through the development and application of novel transcriptomic and proteomic workflows, applied to 1050 and 1038 single muscle fibers from humanvastus lateralis, respectively, we show that MYHs are not the principal drivers of skeletal muscle fiber heterogeneity. Instead, ribosomal heterogeneity and cell junction proteins drive the majority of variance between skeletal muscle fibers in a continual fashion, independent of slow/fast fiber type. Furthermore, whilst slow and fast fiber clusters can be identified, described by their contractile and metabolic profiles, our data challenge the concept that type 2X are phenotypically distinct to other fast fibers at an omics level. Moreover, MYH-based classifications do not adequately describe the phenotype of skeletal muscle fibers in one of the most common genetic muscle diseases, nemaline myopathy, with fibers shifting towards a non-oxidative phenotype independently of MYH-based fiber type. Instead, we characterize novel transcriptomic and proteomic features of slow and fast skeletal muscle fibers, including identifying several muscle fiber type-specific polypeptides, termed microproteins, encoded by transcripts annotated as non-coding RNA. Overall, our data challenges the currently accepted model of multiple distinct fiber types defined by the expression of specific MYHs, and proposes new avenues for skeletal muscle research.Graphical Abstract