BAG3 Myofibrillar Myopathy

Résumé du livre

"BAG3 myofibrillar myopathy (MFM) is a rare and severe form of childhood muscular dystrophy. Typically caused by the pathogenic Pro209Leu substitution in the BAG3 protein, this myopathy results in progressive muscle weakness, spinal rigidity and peripheral neuropathy. Cardiomyopathy, and respiratory failure typically occur in the second decade of life and this is often the cause of death in these patients. BAG3 is a multifunctional protein, perhaps best known for its role as a co-chaperone to HSP70 as a part of the chaperone assisted selective autophagy (CASA) complex. CASA is of particular importance to skeletal and cardiac muscle due to the high rate of protein turnover in these tissues owing to the mechanical stress it experiences. The Pro209Leu variant stalls this complex, inducing toxic aggregate formation and preventing cargo processing, making it a toxic gain of function leading to loss of function. We have developed a murine model of this disease, the first with both whole body expression of the protein and an appreciable phenotype that recapitulates that seen in human patients. This model has one knock-in allele with an analogous varniant to the human variant, Pro215Leu, and one knock-out allele. This was chosen due to the lack of phenotype in the homozygous knock-in, and the presumed loss of functional BAG3 due to aggregation. The skeletal muscle of this model shows pathological changes characteristic of BAG3 MFM, including aggregates, inflammation, moth-eaten fibers, ragged red fibers and Z-disk streaming. Their phenotype is relatively mild, displaying only a lack of voluntary movement. Changes to the nuclei of these mice were also noted, such as abnormal envelopes, intranuclear accumulations which contained BAG3 and lamin A/C and increased heterochromatin. This led us to investigate the role of the nucleus and mechanosensing in the pathophysiology of this disease. Using fibroblasts from a patient bearing the P209L variant, we showed defects in both the nuclear envelope and cytoskeleton. The nucleus, which is significantly larger than that of controls but smaller than expected for their large cell body size, fails to change in size in response to extracellular matrix stiffness changes and does not change in size with the cell body at the same rate as control. These cells show disorganization of the cytoskeleton and nuclear damage which worsens with increases in tension. The accumulation of cytoskeletal components appears to be affecting the cells' ability to migrate as shown in a wound healing assay, and this defect was rescued by the inhibition of myosin through blebbistatin. Indeed, we observed poor actin turnover in fluorescence recovery after photobleaching (FRAP) and an increase in active myosin light chain in the space surrounding the nucleus. An apparent consequence of this was the lack of activation of the mechanosensitive transcriptional co-activator YAP, with its lower nuclear entry in cells. This suggests that the changes in morphology are in relation to changes in mechanotransduction within the cell, and reducing this connection reduces the aberrant signaling. This thesis presents both an original model of BAG3 myofibrillar myopathy, as well as proposes a novel aspect of the pathophysiology of ths MFM, a defect in mechanosensing caused by defects to and uncoupling of the cytoskeleton and nuclear envelope"--