A Key Enzyme in the Biogenesis of Lysosomes Is a Protease That Regulates Cholesterol Metabolism

Department of Biochemistry, Children's Hospital, University Medical Center Hamburg-Eppendorf, Hamburg, Germany.
Science (Impact Factor: 33.61). 07/2011; 333(6038):87-90. DOI: 10.1126/science.1205677
Source: PubMed


Mucolipidosis II is a severe lysosomal storage disorder caused by defects in the α and β subunits of the hexameric N-acetylglucosamine-1-phosphotransferase complex essential for the formation of the mannose 6-phosphate targeting signal on
lysosomal enzymes. Cleavage of the membrane-bound α/β-subunit precursor by an unknown protease is required for catalytic activity.
Here we found that the α/β-subunit precursor is cleaved by the site-1 protease (S1P) that activates sterol regulatory element–binding
proteins in response to cholesterol deprivation. S1P-deficient cells failed to activate the α/β-subunit precursor and exhibited
a mucolipidosis II–like phenotype. Thus, S1P functions in the biogenesis of lysosomes, and lipid-independent phenotypes of
S1P deficiency may be caused by lysosomal dysfunction.

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Available from: Sandra Pohl, Sep 17, 2014
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    • "Phosphotransferase is a hexameric enzyme (2 α, 2 β, and 2 γ subunits) that mediates the first step of the synthesis of Man-6-P, but there are other genes involved in this pathway, such as the NAGPA gene which encodes " uncovering enzyme " (EC, (reviewed in [1]) and the MBTPS gene that encodes the S1P enzyme. S1P or " site-1 protease " (EC acts post-translationally to cleave the inactive α/β precursor to generate the α and β subunits of mature phosphotransferase [2]. At least theoretically, allelic mutations in the NAGPA or MBTPS genes could cause ML II/III, even though they have never been reported. "
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    ABSTRACT: Mucolipidosis II and III alpha/beta (ML II/III alpha/beta) are rare autosomal recessive lysosomal storage diseases that are caused by a deficiency of UDP-GlcNAc:lysosomal enzyme N-acetylglucosamine-1-phosphotransferase, the enzyme responsible for the synthesis of the mannose 6-phosphate targeting signal on lysosomal hydrolases. A Brazilian patient suspected of having a very mild ML III was investigated using whole next-generation sequencing (NGS). Two mutations in the GNPTAB gene were detected and confirmed to be in trans status by parental analysis: c.1208T>C (p.Ile403Thr), previously reported as being pathogenic, and the novel mutation c.1723G>A (p.Gly575Arg). This study demonstrates the effectiveness of using whole NGS for the molecular diagnosis of very mild ML III alpha/beta patients.
    Full-text · Article · Mar 2015 · Molecular Genetics and Metabolism Reports
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    • "In this study we investigated all current known functions of S1P: induction of ER stress response, induction of cholesterol and fatty acid biosynthesis, processing of BBF2H7 to effect Sec23a expression, and the processing of the α/β subunit precursor of N-acetylglucosamine-1-phosphotransferase complex (GNPTAB). Recently it was demonstrated that GNPTAB, involved in the addition of mannose 6-phosphate residues on lysosomal enzymes, is a direct S1P substrate [55]. In our preliminary experiments, there is at least equivalent GNPTAB α/β precursor cleavage activity in S1Pcko chondrocytes as in chondrocytes from its wild type littermates (not shown). "
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    ABSTRACT: The proprotein convertase site-1 protease (S1P) converts latent ER-membrane bound transcription factors SREBPs and ATF6 to their active forms. SREBPs are involved in cholesterol and fatty acid homeostasis whereas ATF6 is involved in unfolded protein response pathways (UPR). Cartilage-specific ablation of S1P in mice (S1Pcko) results in abnormal cartilage devoid of type II collagen protein (Col II). S1Pcko mice also lack endochondral bone development. To analyze S1Pcko cartilage we performed double-labeled immunofluorescence studies for matrix proteins that demonstrated that type IIB procollagen is trapped inside the ER in S1Pcko chondrocytes. This retention is specific to type IIB procollagen; other cartilage proteins such as type IIA procollagen, cartilage oligomeric matrix protein (COMP) and aggrecan are not affected. The S1Pcko cartilage thus exhibits COMP-, aggrecan-, and type IIA procollagen-derived matrices but is characterized by the absence of a type IIB procollagen-derived matrix. To understand the molecular reason behind S1Pcko phenotypes we performed genome-wide transcriptional profiling of cartilage isolated from S1Pcko and wild type littermates. While the UPR pathways are unaffected, the SREBPs-directed cholesterol and fatty acid pathways are significantly down-regulated in S1Pcko chondrocytes, with maximal down-regulation of the stearoyl-CoA desaturase-1 (Scd1) gene. However, mouse models that lack Scd1 or exhibit reduction in lipid homeostasis do not suffer from the ER retention of Col II or lack endochondral bone. These studies indicate an indispensable role for S1P in type IIB procollagen trafficking from the ER. This role appears not to be related to lipid pathways or other current known functions of S1P and is likely dependent on additional, yet unknown, S1P substrates in chondrocytes.
    Full-text · Article · Aug 2014 · PLoS ONE
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    • "Targeting of soluble lysosomal hydrolases requires mannose 6‐ phosphate (M6P) residues formed by the sequential action of the Golgi‐resident N‐acetylglucosamine (GlcNAc)‐1‐phosphotrans- ferase complex consisting of three subunits (a 2 b 2 g 2 ), and the GlcNAc‐1‐phosphodiester a‐N‐acetylglucosaminidase ('uncovering enzyme') (Bao et al, 1996; Braulke et al, 2013; Kollmann et al, 2010). The a and b subunits encoded by GNPTAB (Kudo et al, 2005; Tiede et al, 2005) are synthesized as a type III membrane precursor protein that is activated proteolytically by site‐1 protease (Marschner et al, 2011). The function of the soluble g subunit, encoded by the GNPTG gene (Raas‐Rothschild et al, 2000), is unknown. "
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    ABSTRACT: Mucolipidosis type II (MLII) is a severe multi-systemic genetic disorder caused by missorting of lysosomal proteins and the subsequent lysosomal storage of undegraded macromolecules. Although affected children develop disabling skeletal abnormalities, their pathogenesis is not understood. Here we report that MLII knock-in mice, recapitulating the human storage disease, are runted with accompanying growth plate widening, low trabecular bone mass and cortical porosity. Intralysosomal deficiency of numerous acid hydrolases results in accumulation of storage material in chondrocytes and osteoblasts, and impaired bone formation. In osteoclasts, no morphological or functional abnormalities are detected whereas osteoclastogenesis is dramatically increased in MLII mice. The high number of osteoclasts in MLII is associated with enhanced osteoblastic expression of the pro-osteoclastogenic cytokine interleukin-6, and pharmacological inhibition of bone resorption prevented the osteoporotic phenotype of MLII mice. Our findings show that progressive bone loss in MLII is due to the presence of dysfunctional osteoblasts combined with excessive osteoclastogenesis. They further underscore the importance of a deep skeletal phenotyping approach for other lysosomal diseases in which bone loss is a prominent feature.
    Full-text · Article · Dec 2013 · EMBO Molecular Medicine
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