ER stress and lipid droplet-dependent proteostasis in response to lipid stress in yeast and a novel congenital muscular dystrophy

Phospholipids are the major components of cell membranes and have a wide variety of structures, shapes and properties. Different ratios of phospholipid species confer different properties to membranes and contribute to the normal function of organelles. We have previously shown that acute phosphatidylcholine (PC) biosynthesis inhibition leads to a severe form of lipid imbalance that disrupts ER morphology and structure. Furthermore, our previous studies also revealed a mechanism for ER proteostasis under conditions of lipid-imbalance-induced ER stress in yeast, whereby unfolded ER proteins are removed by lipid droplets (LDs) and targeted to the vacuole for degradation by microlipophagy. Here, we find that LDs also contribute to ER proteostasis during chemically induced ER stress. Furthermore, we find that ER stress results in an increase in ubiquitinated proteins in LDs as well as recruitment of cytosolic and ER heat shock proteins, as well as ER proteins to LDs. ER stress-induced microlipophagy does not require core ATG genes and can occur in the absence of lipid ordered microdomains (Lo) in the vacuolar membrane. Instead, we find that the ESCRT machinery is up-regulated and localizes to the vacuolar membrane in response to ER stress induced microlipophagy and that ESCRT I, II and III complexes are required for microlipophagy in response to each of these stressors. Similar to yeast, we find that lipid imbalance in skeletal muscle from CHKB CMD, new autosomal recessive CMD (Congenital Muscular Dystrophy) caused by a mutation of choline kinase beta (CHKB), results in abnormal SR/ER morphology. CHKB is the first enzyme in the de novo PC biosynthesis pathway and causes phospholipid imbalance in cell membranes similar to that observed in yeast. Besides the disruption of SR morphology, we also detect a dysfunction of the Ryanodine Receptor (RyR), the Ca2+ channels responsible for initiating muscle contraction. Specifically, we observe abnormal RyR morphology and increased association of RyRs with lipid droplets (LD) in muscle fibers from a CHKB CMD patient. Finally, we detect ER stress and pronounced UPR activation in rmd mice, a mouse model of CHKB CMD. Given these results, we propose that inhibition of PC biosynthesis leads to phospholipid imbalance in the SR, which in turn, causes RyR leakage and ER stress which lead to mitochondrial dysfunction and dystrophy in CHKB CMD.