Lamin A/C coordinates nuclear mechanics, chromatin architecture, and transcriptional homeostasis in skeletal muscle in vivo

The nuclear lamina provides mechanical integrity to the eukaryotic nucleus and organizes lamina-associated chromatin domains that are important for chromatin architecture and gene-expression regulation. Lamin A/C, a major component of the nuclear lamina, is disrupted in hereditary laminopathies and has also been implicated in aging-associated nuclear dysfunction. Although the mechanical role of Lamin A/C has been extensively studied in vitro, particularly in monolayer cell culture and isolated nuclei, its role in maintaining nuclear mechanics and chromatin organization in intact tissues remains incompletely understood. Here, we investigated how partial and complete Lamin A/C disruption affects nuclear shape, chromatin architecture, intranuclear mechanics, and gene expression in vivo. Across multiple murine tissues, Lamin A/C deficiency did not cause a generalized collapse of nuclear shape or gross tissue architecture. Instead, Lamin A/C disruption preferentially altered chromatin architecture in mechanically stiff tissues, including skeletal muscle and heart. Using live in vivo deformation microscopy during controlled hindlimb muscle stimulation, we quantified real-time multiscale deformation of skeletal muscle tissue and nuclei. These measurements revealed reduced effective nuclear stiffness and altered load sharing between euchromatin-rich and heterochromatin-rich domains after Lamin A/C loss. Super-resolution imaging further showed that partial and complete Lamin A/C disruption uncoupled H3K9me3 from DAPI-dense heterochromatin, indicating a spatial disruption of repressive chromatin organization. Exploratory ATAC-seq suggested increased chromatin accessibility in heterozygous muscle, whereas RNA-seq showed that complete Lamin A/C loss caused broad myopathic transcriptional dysregulation while partial loss preserved a near-wild-type transcriptomic state. Integrated analysis identified HDAC2 as a candidate mechanosensitive compensatory node that may help buffer gene expression after partial Lamin A/C disruption. Together, these results establish Lamin A/C as an in vivo coordinator of nuclear mechanics, heterochromatin organization, and transcriptional homeostasis in skeletal muscle.