Large MAF transcription factors reawaken evolutionarily dormant fast-glycolytic type IIb myofibers in human skeletal muscle

Sadaki, S, Tsuji, R, Hayashi, T, Watanabe, M, Iwai, R, Wenchao, G, Semenova, EA, Sultanov, RI, Zhelankin, AV, Generozov, EV, Ahmetov, II, Sakakibara, I, Ojima, K, Sakurai, H, Muratani, M, Kudo, T, Takahashi, S and Fujita, R (2025) Large MAF transcription factors reawaken evolutionarily dormant fast-glycolytic type IIb myofibers in human skeletal muscle. Skeletal Muscle, 15 (1).

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Abstract

Background Small mammals such as mice rely on type IIb myofibers, which express the fast-contracting myosin heavy chain isoform Myh4, to achieve rapid movements. In contrast, larger mammals, including humans, have lost MYH4 expression. Thus, they favor slower-contracting myofiber types. However, the mechanisms underlying this evolutionary shift remain unclear. We recently identified the large Maf transcription factor family (Mafa, Mafb, and Maf) as key regulators of type IIb myofiber specification in mice. In this study, we investigate whether large MAFs play a conserved role in the induction of MYH4 expression and glycolytic metabolism in human and bovine skeletal muscle. Methods We performed adenovirus-mediated overexpression of large MAFs in iPSC-derived human myotubes and primary bovine myotubes. We subsequently quantified MYH4 expression using RT-qPCR, RNA sequencing (RNA-seq), and LC-MS/MS analysis. Glycolytic capacity was assessed using a flux analyzer and metabolic gene expression profiling. Additionally, RNA-seq analysis of human muscle biopsy samples was conducted to determine the correlations between large MAFs and the expression of MYH4 and other myosin genes, as well as their association with fast fiber composition and athletic training. Results Overexpression of large MAFs in human and bovine myotubes robustly induced MYH4 expression, with mRNA levels increasing by 100- to 1000-fold. LC-MS/MS analysis provided clear evidence of MYH4 protein expression in human myotubes, where it was previously undetectable. RNA-seq and flux analyzer data revealed that large MAFs significantly enhanced glycolytic capacity by upregulating the expression of key genes involved in glucose metabolism. Moreover, RNA-seq analysis of human muscle biopsy samples revealed a positive correlation between MAFA, MAF, and MYH4 expression. Furthermore, MAFA and MAF expression levels were elevated in power-trained individuals, accompanied by increased expression of MYH4 and other fast myosin genes. Conclusions Our findings establish large MAF transcription factors as key regulators of MYH4 expression and glycolytic metabolism in human skeletal muscle. This discovery provides novel insights into the evolutionary loss of type IIb myofibers in larger mammals and suggests potential strategies for enhancing muscle performance and mitigating fast-twitch fiber loss associated with aging and muscle degeneration.

Item Type:	Article
Uncontrolled Keywords:	3105 Genetics; 3208 Medical physiology
Subjects:	R Medicine > RC Internal medicine > RC1200 Sports Medicine
Divisions:	Sport and Exercise Sciences
Publisher:	Springer
Date of acceptance:	16 July 2025
Date of first compliant Open Access:	31 July 2025
Date Deposited:	31 Jul 2025 08:52
Last Modified:	31 Jul 2025 09:00
DOI or ID number:	10.1186/s13395-025-00391-5
URI:	https://researchonline.ljmu.ac.uk/id/eprint/26868

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