Mannose phosphorylation in health and disease

doi:10.1016/j.ejcb.2009.10.008

European Journal of Cell Biology

Volume 89, Issue 1, January 2010, Pages 117-123

https://doi.org/10.1016/j.ejcb.2009.10.008 Get rights and content

Abstract

Lysosomal hydrolases catalyze the degradation of a variety of macromolecules including proteins, carbohydrates, nucleic acids and lipids. The biogenesis of lysosomes or lysosome-related organelles requires a continuous substitution of soluble acid hydrolases and lysosomal membrane proteins. The targeting of lysosomal hydrolases depends on mannose 6-phosphate residues (M6P) that are recognized by specific receptors mediating their transport to an endosomal/prelysosomal compartment. The key role in the formation of M6P residues plays the GlcNAc-1-phosphotransferase localized in the Golgi apparatus. Two genes have been identified recently encoding the type III α/β-subunit precursor membrane protein and the soluble γ-subunit of GlcNAc-1-phosphotransferase. Mutations in these genes result in two severe diseases, mucolipidosis type II (MLII) and III (MLIII), biochemically characterized by the missorting of multiple lysosomal hydrolases due to impaired formation of the M6P recognition marker, and general lysosomal dysfunction. This review gives an update on structural properties, localization and functions of the GlcNAc-1-phosphotransferase subunits and improvements of pre- and postnatal diagnosis of ML patients. Further, the generation of recombinant single-chain antibody fragments against M6P residues and of new mouse models of MLII and MLIII will have considerable impact to provide deeper insight into the cell biology of lysosomal dysfunctions and the pathomechanisms underlying these lysosomal disorders.

Introduction

In 1967, Leroy and DeMars described an autosomal recessive inherited disorder which was called I-cell disease due to the presence of large, phase-dense inclusions in patient fibroblasts (Leroy and DeMars, 1967). Later it was found that these cells were deficient for multiple acid hydrolases many of which were present in abnormal high levels in culture medium (Lightbody et al., 1971; Tondeur et al., 1971). Hickman and Neufeld (1972) observed that I-cell fibroblasts were capable to endocytose acid hydrolases secreted by fibroblasts of healthy individuals whereas enzymes secreted by I-cell fibroblasts could not be internalized by normal cells. These findings have opened the field of signal-dependent transport of lysosomal enzymes and suggested the existence of a common recognition marker of lysosomal enzymes for uptake and transport to lysosomes. This recognition marker is lacking in enzymes secreted by I-cell fibroblasts which was subsequently confirmed by the identification of the mannose 6-phosphate (M6P) marker by Sly and colleagues (Kaplan et al., 1977; Natowicz et al., 1979).

The M6P marker is formed on newly synthesized lysosomal enzymes in the Golgi apparatus in a process that requires the sequential action of two separate enzymes. The first enzyme catalyzes the addition of an N-acetylglucosamine-1-phosphate (GlcNAc-1-phosphate) residue to high-mannose-type oligosaccharides, and the second enzyme removes the remaining GlcNAc residue to expose the linked M6P residue (Reitman and Kornfeld, 1981; Varki and Kornfeld, 1983; Waheed et al., 1981). Lysosomal enzymes modified with M6P bind in the trans-Golgi network to two types of M6P receptors (MPR), namely the MPR46 and MPR300, and are subsequently translocated via endosomes to lysosomes (Braulke and Bonifacino, 2009). The MPR300 also mediates the endocytosis of extracellular M6P-containing enzymes and their delivery to lysosomes. This is the rationale for enzyme replacement therapies which have been clinically established for some lysosomal storage diseases caused by a single enzyme deficiency (Platt and Lachmann, 2009). Several cell types and organs of I-cell disease patients have nearly normal levels of lysosomal enzymes suggesting the existence of alternative M6P-independent pathways (Braulke and Bonifacino, 2009; Pohl et al., 2009a).

This review summarizes our current knowledge on the molecular basis of the M6P-dependent pathways of lysosomal enzyme transport and provides new insights into the molecular heterogeneity of I-cell and related diseases.

Section snippets

Structural properties of the GlcNAc-1-phosphotransferase complex

The bovine GlcNAc-1-phosphotransferase was proposed to exist as a hexameric complex with a molecular mass of 540 kDa composed of two α-, two β-, and two γ-subunits (Bao et al., 1996a). The genes of the α/β-precursor (GNPTAB; MIM607840) and the γ-subunit (GNPTG; MIM607838) of the human GlcNAc-1-phosphotransferase have been cloned recently (Kudo et al., 2005; Raas-Rothschild et al., 2000; Tiede et al., 2005a).

The α/β-precursor is synthesized as an N-glycosylated 190-kDa type III membrane protein

Functions of the GlcNAc-1-phosphotransferase subunits

Retroviral expression of the α/β-subunit precursor in GlcNAc-1-phosphotransferase-deficient human fibroblasts resulted in the correction of lysosomal enzyme missorting and a 4.5-fold increase in GlcNAc-1-phosphotransferase activity in comparison to healthy controls (Tiede et al., 2005a) (Fig. 1B). These data suggest that the α/β-subunit precursor contains the catalytic center of the GlcNAc-1-phosphotransferase. Additionally, it has been reported that the proteolytic cleavage of the

Subcellular localization of GlcNAc-1-phosphotransferase

Subcellular fractionation studies in the early 80s have identified GlcNAc-1-phosphotransferase activity to be associated with the Golgi complex decreasing from cis to trans (Goldberg and Kornfeld, 1983; Waheed et al., 1981). Immunofluorescence studies supported these data demonstrating that α- and β-subunits of the GlcNAc-1-phosphotransferase are localized in the cis-Golgi apparatus of human fibroblasts (Tiede et al., 2005a) and in transfected COS7-cells (Fig. 2A). Treatment of COS7-cells with

Mutations in phosphotransferase genes cause mucolipidosis II and III

GlcNAc-1-phosphotransferase activity is lacking or reduced in two distinct autosomal recessive human diseases impairing lysosomal enzyme trafficking, mucolipidosis (ML) II and MLIII, respectively. Although rare diseases, they have been the subject of extensive studies in the last 40 years since their first description (Leroy and DeMars, 1967).

MLII patients are characterized by dwarfism, skeletal abnormalities, facial dysmorphism, stiff skin, delayed development and cardiomegaly leading to death

Animal models for mucolipidosis II alpha/beta and mucolipidosis III gamma

In 1996, Bosshard and colleagues described a colony of domestic short-hair cats that displayed several clinical features similar to the human disease MLII alpha/beta such as facial dysmorphism, failure to thrive, behavioral dullness and ataxia, elevated activities of acid hydrolases in serum as well as inclusions in fibroblasts (Bosshard et al., 1996; Hubler et al., 1996; Mazrier et al., 2003). However, the affected cats developed progressive retinal degeneration resulting in blindness by a few

Perspective

The identification of the genes encoding the α/β-precursor and the γ-subunit of GlcNAc-1-phosphotransferase has improved the pre- and postnatal diagnosis of MLII and MLIII patients in the last few years. Many important issues, however, remain to be solved, e.g. the detailed analysis of the modular structure of the α/β-precursor for the binding of lysosomal enzymes, high-mannose-type oligosaccharides, and UDP-GlcNAc as well as subunit oligomerization. Furthermore, the identification of the

Acknowledgements

The studies were supported by the Deutsche Forschungsgemeinschaft (Sonderforschungsbereich 470/C6; 470/C1; GRK1459). We thank S. Alves and L. Lacerda (Porto, Portugal) and A. Raas-Rothschild (Jerusalem, Israel) for providing cells from MLIII gamma patients.

Note added in proof

Recently a morpholino-based phosphotransferase knockdown zebrafish has been described exhibiting multiple developmental defects (Flanagan-Steet et al., 2009).

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Mannose phosphorylation in health and disease

Abstract

Introduction

Section snippets

Structural properties of the GlcNAc-1-phosphotransferase complex

Functions of the GlcNAc-1-phosphotransferase subunits

Subcellular localization of GlcNAc-1-phosphotransferase

Mutations in phosphotransferase genes cause mucolipidosis II and III

Animal models for mucolipidosis II alpha/beta and mucolipidosis III gamma

Perspective

Acknowledgements

J. Biol. Chem.

J. Biol. Chem.

Mol. Genet. Metab.

Cell

Biochim. Biophys. Acta

J. Biol. Chem.

Biochem. Biophys. Res. Commun.

Biochem. Biophys. Res. Commun.

J. Biol. Chem.

J. Biol. Chem.

Am. J. Hum. Genet.

J. Biol. Chem.

Biochim. Biophys. Acta

Biochim. Biophys. Acta

J. Biol. Chem.

Mol. Genet. Metab.

J. Pediatr.

J. Neurol. Sci.

J. Biol. Chem.

J. Biol. Chem.

Spontaneous mucolipidosis in a cat: an animal model of human I-cell disease

Vet. Pathol.

Molecular order in mucolipidosis II and III nomenclature

Am. J. Med. Genet. A

Alternative mechanisms for trafficking of lysosomal enzymes in mannose 6-phosphate receptor-deficient mice are cell type-specific

J. Cell Sci.

Molecular analysis of the GNPTAB and GNPTG genes in 13 patients with mucolipidosis type II or type III – identification of eight novel mutations

Clin. Genet.

Mice lacking alpha/beta subunits of GlcNAc-1-phosphotransferase exhibit growth retardation, retinal degeneration, and secretory cell lesions

Invest. Ophthalmol. Vis. Sci.