Recently, a novel form of Congenital Muscular Dystrophies (CMDs) was identified in which the dystrophic muscle is characterized by abnormally distributed mitochondria. This new autosomal recessive CMD is caused by a mutation of choline kinase beta (CHKB), the first enzyme in the de novo phosphatidylcholine (PC) biosynthesis pathway and causes a phospholipid imbalance in cell membranes. While CMDs are normally characterized by neonatal or early infancy onset of muscle weakness, hypotonia, loss of spontaneous motor movement, and progressive muscular atrophy, CHKB CMD patients also experience severe cognitive and intellectual impairments, most never acquire meaningful language and they can die as early as 2 years of age from cardiomyopathy. No progress has been made in developing successful therapeutic treatments to cure or ease the severe morbidity associated with CHKB CMD and many other CMDs. At the tissue level, skeletal muscle from CHKB CMD patients have abnormally enlarged mitochondria that are mislocalized to the periphery and depleted from the center of the muscle fiber. Given that the skeletal muscle fibers with the greatest defects in mitochondrial distribution also display the greatest signs of fibrosis and muscular dystrophy, there is a correlation between CHKB CMD muscle atrophy and defects in mitochondria. Our preliminary data show that mitochondria are not the only organelles affected in CHKB CMD. We also find a disruption of the structure and morphology of the sarcoplasmatic reticulum (SR) that mimics the mitochondrial defects in both a mouse model of CHKB CMD and CHKB patients. Along with SR morphology defects, we also detect a dysfunction of the Ryanodine Receptor (RyR), the Ca2+ channels responsible for initiating muscle contraction. Specifically, we find that muscle fibers from CHKB mice exhibit RyR leakage and elevated levels of cytosolic Ca2+. In fact, RyR leakage is one of the first events that occur upon short-term inhibition of CHKB in C2C12 myotubes. Given that Ca2+ overload of mitochondria due to RyR leakage has also been shown to decrease mitochondrial membrane potential (??), promote mitochondrial ROS production and produce defects in mitochondrial morphology and distribution similar to those observed in CHKB CMD, we propose the hypothesis that inhibition of PC biosynthesis leads to phospholipid imbalance in the SR, which in turn, causes RyR leakage that lead to mitochondrial dysfunction and dystrophy in CHKB CMD. The major goal of the proposed research is to determine whether RyR leakage is the underlying mechanism of muscle dysfunction in CHKB CMD. We propose to test this hypothesis in a mouse model and CHKB patient myofibers by detecting RyR leakage and mitochondrial dysfunction. We will also test whether interventions that promote PC biosynthesis, stabilize RyRs, and/or protect cells from oxidative damage reduce or improve muscular function in cell and mouse models for CHKB CMD
Congenital muscular dystrophy (CMD), one of the most frequent dystrophies of childhood, is characterized by neonatal muscle hypotonia, muscle weakness, stiff or frozen joints and delayed motor milestones. The focal point of these studies is CHKB CMD, a newly identified CMD in which patient's exhibit generalized muscle wasting and weakness from early infancy with ambulatory delays and severe mental retardation. CHKB CMD patients can die as early as 2 years of age from cardiomyopathy. Currently, there is no cure for CHKB CMD. We will study the mechanism underlying CHKB CMD and possible therapeutic interventions.
| Garcia, Enrique J; Vevea, Jason D; Pon, Liza A (2018) Lipid droplet autophagy during energy mobilization, lipid homeostasis and protein quality control. Front Biosci (Landmark Ed) 23:1552-1563 |
