Mll4 in skeletal muscle fibers maintains muscle stem cells

Background: Muscle stem cells (MuSCs) undergo numerous state transitions throughout life, which are critical for supporting normal muscle growth and regeneration. Epigenetic modifications in skeletal muscle play a significant role in influencing the niche and cellular states of MuSCs. Mixed-lineage leukemia 4 (Mll4) is a histone methyltransferase critical for activating the transcription of various target genes and is highly expressed in skeletal muscle. This raises the question of whether Mll4 has a regulatory function in modulating the state transitions of MuSCs, warranting further investigation. Methods: To assess if myofiber-expressed Mll4, a histone methyltransferase, contributes to the maintenance of MuSCs, we crossed MCK^Cre/+ or HSA^MerCreMer/+ mice to Mll4^f/f mice to generate myofiber-specific Mll4-deleted mice. Investigations were conducted using 8-week-old and 4-week-old MCK^Cre/+;Mll4^f/f mice, and adult HSA^MerCreMer/+;Mll4^f/f mice between the ages of 3 months and 6 months. Results: During postnatal myogenesis, Mll4 deleted muscles were observed with increased number of cycling MuSCs that proceeded to a differentiation state, leading to MuSC deprivation. This phenomenon occurred independently of gender. When Mll4 was ablated in adult muscles using the inducible method, adult MuSCs lost their quiescence and differentiated into myoblasts, also causing the depletion of MuSCs. Such roles of Mll4 in myofibers coincided with decreased expression levels of distinct Notch ligands: Jag1 and Dll1 in pubertal and Jag2 and Dll4 in adult muscles. Conclusions: Our study suggests that Mll4 is crucial for maintaining MuSCs in both pubertal and adult muscles, which may be accomplished through the modulation of distinct Notch ligand expressions in myofibers. These findings offer new insights into the role of myofiber-expressed Mll4 as a master regulator of MuSCs, highlighting its significance not only in developmental myogenesis but also in adult muscle, irrespective of sex.