Plasticity and structural alterations of mitochondria and sarcoplasmic organelles in muscles of mice deficient in α-dystrobrevin, a component of the dystrophin-glycoprotein complex.

The dystrophin-glycoprotein complex (DGC) plays a crucial role in maintaining the structural integrity of the plasma membrane and the neuromuscular junction. In this study, we investigated the impact of the deficiency of α-dystrobrevin (αdbn), a component of the DGC, on the homeostasis of intracellular organelles, specifically mitochondria and the sarcoplasmic reticulum (SR). In αdbn deficient muscles, we observed a significant increase in the membrane-bound ATP synthase complex levels, a marker for mitochondria in oxidative muscle fiber types compared to wild-type. Furthermore, examination of muscle fibers deficient in αdbn using electron microscopy revealed profound alterations in the organization of mitochondria and the SR within certain myofibrils of muscle fibers. This included the formation of hyper-branched intermyofibrillar mitochondria with extended connections, an extensive network spanning several myofibrils, and a substantial increase in the number/density of subsarcolemmal mitochondria. Concurrently, in some cases, we observed significant structural alterations in mitochondria, such as cristae loss, fragmentation, swelling, and the formation of vacuoles and inclusions within the mitochondrial matrix cristae. Muscles deficient in αdbn also displayed notable alterations in the morphology of the SR, along with the formation of distinct anomalous concentric SR structures known as whorls. These whorls were prevalent in αdbn-deficient mice but were absent in wild-type muscles. These results suggest a crucial role of the DGC αdbn in regulating intracellular organelles, particularly mitochondria and the SR, within muscle cells. The remodeling of the SR and the formation of whorls may represent a novel mechanism of the unfolded protein response (UPR) in muscle cells.

GRASP55 Regulates Sorting and Maturation of the Lysosomal Enzyme ß-Hexosaminidase A.

The Golgi apparatus plays a crucial role in the delivery of lysosomal enzymes. Golgi Reassembly Stacking Proteins, GRASP55 and GRASP65, are vital for maintaining Golgi structure and function. GRASP55 depletion results in the missorting and secretion of the lysosomal enzyme Cathepsin D (Xiang et al ., 2013), though the mechanisms remain unclear. In this study, we conducted secretomic analyses of GRASP55 knockout (KO) cells and found a significant increase in lysosome-associated proteins in the extracellular medium. Using the lysosomal beta-hexosaminidase subunit alpha (HEXA) as a model, we found that GRASP55 depletion disrupted normal trafficking and processing of HEXA, resulting in increased secretion of the immature (pro-form) HEXA into the extracellular milieu, along with decreased levels of the mature form and enzymatic activity within the cell. GRASP55 depletion significantly reduced the complex formation between HEXA and mannose-6-phosphate (M6P) receptors (MPR), despite no overall change in MPR expression. And finally, we found there was a notable reduction in the expression of GNPTAB, leading to a reduction in M6P modification of HEXA, hindering its efficient targeting to lysosomes. These findings reveal the role of GRASP55 in regulating lysosomal enzyme dynamics, emphasizing its role in the sorting and trafficking of lysosomal proteins.