FBXL21 regulates diurnal proteostasis in skeletal muscle by targeting DNAJB6 and client proteins

Circadian regulation of proteostasis, a key determinant of muscle health, remains poorly understood. Here, we identify DNAJB6, an Hsp40 (DnaJ) co-chaperone, as a substrate of the circadian E3 ligase FBXL21. FBXL21 mediates the ubiquitination-dependent proteasomal degradation of both DNAJB6 and its client proteins, including Desmin. In contrast, myopathy-causing mutations of DNAJB6 confer resistance to FBXL21-directed degradation. Fbxl21 KO C2C12 cells display aberrant Desmin accumulation, and show aggravated cytoplasmic accumulation of TDP-43, another DNAJB6 client protein, in response to heat shock. Under timed exercise as a physiological stressor, WT mice display robust diurnal rhythms in the levels of stress granule markers (G3BP1 and FUS) and TDP-43 as a function of exercise timing. In contrast, the Fbxl21 hypomorph Psttm mutant mice show elevated expression of these proteins without exercise, which is exacerbated under exercise-induced stress conditions. Importantly, these abnormalities are rescued by skeletal muscle-specific FBXL21 expression. Our study elucidates a novel diurnal regulatory mechanism of skeletal muscle proteostasis via FBXL21 as a chaperone-linked E3 ligase, highlighting the FBXL21-DNAJB6 axis as a potential therapeutic target for myopathies.