Skip to main content
. 2018 Jul 26;3(14):e121596. doi: 10.1172/jci.insight.121596

Figure 2. S1P deficiency causes a partially defective M6P-dependent Golgi-to-lysosome transport of lysosomal enzymes.

Figure 2

(A) Representative immunofluorescence images of fibroblasts from the patient and her parents. ML-II and ML-IIIa cells are positive controls derived from patients with these conditions. Cathepsin B, lysosome enzyme; LAMP1, lysosome marker; TO-PRO, nuclear counterstaining. Scale bar: 10 μm. The number of N indicates the exact number of samples. (B) M6P-dependent lysosomal enzyme β-hexosaminidase activity from Saos2 lysates or supernatants, as indicated. Data represent mean ± SEM; n = 3. *P < 0.05, 1-way ANOVA followed by Student’s t test. (C) Inter-organelle trafficking of cathepsin D. Without M6P modification, cathepsin D precursor is directly secreted into extracellular space. Ionomycin triggers the release of lysosomal contents through inducing the lysosomal exocytosis of matured cathepsin D. (D) Western blotting of precursor (p), intermediate (i), and mature (m) forms of cathepsin D from Saos2 lysates or supernatants, with or without ionomycin treatment. The precursor form of cathepsin D was detected in supernatant from GPT-KO and S1P-KO cells. However, upon ionomycin treatment, the mature form of cathepsin D was detected in supernatant from S1P-KO cells, indicating correct targeting of cathepsin D in lysosome in S1P-KO cells. (E) Sera from the patient (pat) and her mother (mom) were analyzed by Western blotting (top). Lysates from Saos2 cells were used as a control. The same membrane was stained with Ponceau S to confirm equivalent loading of serum proteins (bottom). (F) Immunofluorescence images of WT and mutant Saos2 cells. Insets show high-magnification images of lysosomes. Scale bars: 5 μm; 125 nm (insets). Enlarged lysosomes were found in GPT-KO and S1P-KO cells. Unlike GPT-KO cells in which cathepsin D is absent in lysosomes, cathepsin D was detected in lysosomes in S1P-KO cells. (G) Phase-contrast images of WT and mutant Saos2 cells. Arrowheads indicate inclusion bodies. Scale bar: 10 μm.