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The LMNA gene encodes the nuclear envelope proteins Lamins A and C, which comprise a major part of the nuclear lamina, provide mechanical support to the nucleus, and participate in diverse-intracellular signaling. LMNA mutations give rise to a collection of diseases called laminopathies, including dilated cardiomyopathy ( LMNA-DCM) and muscular dystrophies. Although nuclear deformities are a hallmark of LMNA-DCM, the role of nuclear abnormalities in the pathogenesis of -DCM remains incompletely understood. Using induced-pluripotent stem cell-derived cardiomyocytes (iPSC-CMs) from LMNA-mutant patients and healthy controls, we show that LMNA mutant iPSC-CM nuclei have altered shape or increased size compared with healthy control iPSC-CM nuclei. The LMNA mutation exhibiting the most severe nuclear deformities, R249Q, additionally caused reduced nuclear stiffness and increased nuclear fragility. Importantly, for all cell lines, the degree of nuclear abnormalities corresponded to the degree of Lamin A/C and Lamin B1 mislocalization from the nuclear envelope. The mislocalization was likely due to altered assembly of Lamin A/C. Collectively, these results point to the importance of correct lamin assembly at the nuclear envelope in providing mechanical stability to the nucleus and suggests that defects in nuclear lamina organization may contribute to the nuclear and cellular dysfunction in LMNA-DCM. 

The nuclei of multinucleated skeletal muscles experience substantial external force during development and muscle contraction. Protection from such forces is partly provided by lamins, intermediate filaments that form a scaffold lining the inner nuclear membrane. Lamins play a myriad of roles, including maintenance of nuclear shape and stability, mediation of nuclear mechanoresponses, and nucleo-cytoskeletal coupling. Herein, we investigate how disease-causing mutant lamins alter myonuclear properties in response to mechanical force. This was accomplished via a novel application of a micropipette harpooning assay applied to larval body wall muscles of Drosophila models of lamin-associated muscular dystrophy. The assay enables the measurement of both nuclear deformability and intracellular force transmission between the cytoskeleton and nuclear interior in intact muscle fibers. Our studies revealed that specific mutant lamins increase nuclear deformability while other mutant lamins cause nucleo-cytoskeletal coupling defects, which were associated with loss of microtubular nuclear caging. We found that microtubule caging of the nucleus depended on Msp300, a KASH domain protein that is a component of the linker of nucleoskeleton and cytoskeleton (LINC) complex. Taken together, these findings identified residues in lamins required for connecting the nucleus to the cytoskeleton and suggest that not all muscle disease-causing mutant lamins produce similar defects in subcellular mechanics.