Glucosylceramide depletion disrupts endolysosomal function in GBA-linked Parkinsons fibroblasts?
In Gaucher and Niemann-Pick C diseases, the glucosylceramide (GlcCer) depletion hypothesis states that depletion of non-lysosomal sphingolipid pools can lead to dysfunction in the secretory and lysosomal system. The hypothesis suggests: 1) lysosomal dysfunction can be separated from lysosomal storage, 2) Lysosomal/secretory dysfunction/vATPase activity is corrected by increasing non-lysosomal GlcCer pools, and 3) Changes in higher glycosphingolipid synthesis due to changes in Golgi pH and/or GlcCer non-vesicular transport. Evidence for this mechanism includes 1) Successful treatment of cells and animals by imino sugar inhibition of the non-lysosomal neutral pH GlcCer hydrolase GBA2, 2) Increasing ER/cytosol GlcCer increases in vATPase regulatory V0a1 subunit expression. Heterozygous mutations in GBA1, a lysosomal glucocerebrosidase (GCase), cause GCase misfolding and mislocalisation in the ER/cytoplasm which is linked to Parkinsons disease (GBA-PD). Unexpectedly, similar to previous results in storing fibroblasts, N370S and L444P fibroblasts revealed increased endolysosomal pH and size despite the absence of glucolipid storage. Induction of storage by reducing residual lysosomal GCase activity in the N370S/L444P fibroblasts by the addition conduritol B-epoxide had no further effect on lysosomal function. In contrast, the addition of a soluble GlcCer analogue (adaGlcCer) reverses increased endolysosomal pH and volume in N370S mutant fibroblasts. The results are consistent with ER/cytosolic glucolipid depletion in GBA-PD fibroblasts. We discuss the potential for toxic/ectopic GBA1 hydrolysis and disrupted vATPase activity may lead to defective dopamine packaging and synaptic vesicle endocytosis as a new hypothesis in GBA-PD.