Ca2+ homeostasis modulation enhances the amenability of L444P glucosylcerebrosidase to proteostasis regulation in patient-derived fibroblasts.

Gaucher's disease is caused by deficiency of lysosomal glucocerebrosidase (GC) activity and accumulation of GC substrate, glucosylceramide. A number of point mutations in GC encoding gene have been reported to destabilize the enzyme native structure, resulting in protein misfolding and degradation. Particularly, the L444P GC variant, often associated with neuropathic manifestations of the disease, is severely destabilized and immediately degraded, resulting in complete loss of enzymatic activity. In addition, glucosylceramide accumulation causes Ca(2+) efflux from the endoplasmic reticulum (ER) through ryanodine receptors (RyRs) in the neurons of Gaucher's disease patients. We hypothesized that excessive Ca(2+) efflux impairs ER folding and studied how modulation of Ca(2+) affects folding of L444P GC in patient-derived fibroblasts. We report that RyRs blockers mediated [Ca(2+)] modulation, recreating a "wild type-like" folding environment in the ER, more amenable to rescuing the folding of mutated L444P GC through proteostasis regulation. Treating patient-derived fibroblasts with a RyRs blocker and a proteostasis modulator, MG-132, results in enhanced folding, trafficking, and activity of the severely destabilized L444P GC variant. Global gene expression profiling and mechanistic studies were conducted to investigate the folding quality control expression pattern conducive to native folding of mutated L444P GC and revealed that the ER-lumenal BiP/GRP78 plays a key role in the biogenesis of this GC variant.