Mechanically knocking out titin reveals protein tension loss as a trigger of muscle disease.

Titin, the elastic protein scaffold of muscle sarcomeres, has multifunctional roles in mechanosignalling and is implicated in muscle disease. However, the consequences of disrupting titin's mechanical function in vivo remain incompletely understood. Here, by leveraging site-directed polypeptide severing as a 'mechanical knock-out' method for abolishing force transmission across titin, we show that the loss of titin tension in homozygous mechanically knocked-out muscles reduces force generation and induces severe atrophy and widespread transcriptional dysregulation. Although mechanically knocked-out myofibres persist, they shrink and undergo progressive sarcomere depletion, which correlates with the rapid upregulation of muscle-specific RING finger protein 1 (MuRF1) and with altered levels of other titin-associated atrophy regulators. The affected fibres also exhibit mitochondrial aggregation and myonuclei internalization, preceded by desmin mislocalization. Heterozygous mechanically knocked-out muscles show milder phenotypes that closely resemble titin-related human myopathy. Our findings suggest that slack titin molecules drive muscle disease, potentially through mechanisms shared with other mechanical proteins.