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Hum Mol Genet. 2012 Dec 1;21(23):5159-73. doi: 10.1093/hmg/dds367. Epub 2012 Sep 4.

Reduced cathepsins B and D cause impaired autophagic degradation that can be almost completely restored by overexpression of these two proteases in Sap C-deficient fibroblasts.

Author information

1
Department of Haematology, Oncology and Molecular Medicine, Istituto Superiore di Sanità, Viale Regina Elena, 299, 00161 Rome, Italy.

Abstract

Saposin (Sap) C deficiency, a rare variant form of Gaucher disease, is due to mutations in the Sap C coding region of the prosaposin (PSAP) gene. Sap C is required as an activator of the lysosomal enzyme glucosylceramidase (GCase), which catalyzes glucosylceramide (GC) degradation. Deficit of either GCase or Sap C leads to the accumulation of undegraded GC and other lipids in lysosomes of monocyte/macrophage lineage. Recently, we reported that Sap C mutations affecting a cysteine residue result in increased autophagy. Here, we characterized the basis for the autophagic dysfunction. We analyzed Sap C-deficient and GCase-deficient fibroblasts and observed that autophagic disturbance was only associated with lack of Sap C. By a combined fluorescence microscopy and biochemical studies, we demonstrated that the accumulation of autophagosomes in Sap C-deficient fibroblasts is not due to enhanced autophagosome formation but to delayed degradation of autolysosomes caused, in part, to decreased amount and reduced enzymatic activity of cathepsins B and D. On the contrary, in GCase-deficient fibroblasts, the protein level and enzymatic activity of cathepsin D were comparable with control fibroblasts, whereas those of cathepsin B were almost doubled. Moreover, the enhanced expression of both these lysosomal proteases in Sap C-deficient fibroblasts resulted in close to functional autophagic degradation. Our data provide a novel example of altered autophagy as secondary event resulting from insufficient lysosomal function.

PMID:
22949512
DOI:
10.1093/hmg/dds367
[Indexed for MEDLINE]

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