Dnajb5 From Antarctic Fish Reveals a Redox-Sensitive Mechanism Coordinating Muscle Regeneration via mTORC1 and HDAC4

Dnajb5, a member of the heat shock protein family, has not been previously reported to play a role in muscle differentiation. We identify Dnajb5 as a negative regulator of myogenesis via mammalian target of rapamycin (mTOR) and histone deacetylase 4 (HDAC4) signalling, functioning as a central controller of muscle growth and metabolism.

mTOR-binding proteins were screened by mass spectrometry in Antarctic fish (Notothenia coriiceps) muscle extracts. Functional roles of Dnajb5 were examined in C2C12 and primary myoblasts using shRNA-mediated knockdown. In vivo effects were analysed using a BaCl₂-induced muscle injury model in mice (n = 6-12 per group).

Mass spectrometry identified Dnajb5 as an mTOR-binding protein. This interaction was conserved in mammalian cells and weakened by hydrogen peroxide (p < 0.05). Dnajb5 knockdown in C2C12 myoblasts selectively increased S6K1 phosphorylation without altering basal Akt phosphorylation. Myogenic differentiation increased, characterized by upregulated Myog, Igf2 and Ckm expression (p < 0.05) and elevated differentiation index (p < 0.05). In vivo, Dnajb5 depletion improved tibialis anterior muscle regeneration, increasing regenerating fibre cross-sectional area by approximately 26% at Day 5 post-injury (p < 0.05). Dnajb5 knockdown increased Ppargc1a expression via release of mTORC1 inhibition and preventing the nuclear retention of HDAC4. Mitochondrial DNA copy number increased 1.3-fold (p < 0.05), and oxidative enzyme activity rose significantly (succinate dehydrogenase-positive fibres: 2.8-fold (p < 0.01), cytochrome c oxidase-positive fibres: 1.4-fold (p < 0.05). Grip strength improved by 14% (p < 0.05), and rotarod endurance increased by 31% (p < 0.01) relative to controls.

Dnajb5 functions as a negative regulator of muscle differentiation and regeneration by coordinating mTORC1 signalling and the HDAC4-MEF2 axis. These findings reveal a redox-sensitive dual-pathway mechanism linking protein synthesis and mitochondrial biogenesis.