Mice lacking mannose 6-phosphate uncovering enzyme activity have a milder phenotype than mice deficient for N-acetylglucosamine-1-phosphotransferase activity

Lysosomal enzyme trafficking factor LYSET enables nutritional usage of extracellular proteins

Mammalian cells can generate amino acids through macropinocytosis and lysosomal breakdown of extracellular proteins, which is exploited by cancer cells to grow in nutrient-poor tumors. Here, through genetic screens in defined nutrient conditions we characterized LYSET, a transmembrane protein (TMEM251) selectively required when cells consume extracellular proteins. LYSET was found to associate in the Golgi with GlcNAc-1-phosphotransferase, which targets catabolic enzymes to lysosomes through mannose-6-phosphate modification. Without LYSET, GlcNAc-1-phosphotransferase was unstable owing to a hydrophilic transmembrane domain. Consequently, LYSET-deficient cells were depleted of lysosomal enzymes and impaired in turnover of macropinocytic and autophagic cargoes. Thus, LYSET represents a core component of the lysosomal enzyme trafficking pathway, underlies the pathomechanism for hereditary lysosomal storage disorders, and may represent a target to suppress metabolic adaptations in cancer.

Mechanisms regulating the sorting of soluble lysosomal proteins

Lysosomes are key regulators of many fundamental cellular processes such as metabolism, autophagy, immune response, cell signalling and plasma membrane repair. These highly dynamic organelles are composed of various membrane and soluble proteins, which are essential for their proper functioning. The soluble proteins include numerous proteases, glycosidases and other hydrolases, along with activators, required for catabolism. The correct sorting of soluble lysosomal proteins is crucial to ensure the proper functioning of lysosomes and is achieved through the coordinated effort of many sorting receptors, resident ER and Golgi proteins, and several cytosolic components. Mutations in a number of proteins involved in sorting soluble proteins to lysosomes result in human disease. These can range from rare diseases such as lysosome storage disorders, to more prevalent ones, such as Alzheimer’s disease, Parkinson’s disease and others, including rare neurodegenerative diseases that affect children. In this review, we discuss the mechanisms that regulate the sorting of soluble proteins to lysosomes and highlight the effects of mutations in this pathway that cause human disease. More precisely, we will review the route taken by soluble lysosomal proteins from their translation into the ER, their maturation along the Golgi apparatus, and sorting at the trans-Golgi network. We will also highlight the effects of mutations in this pathway that cause human disease.

Identification of a shared genetic risk locus for Kawasaki disease and immunoglobulin A vasculitis by a cross-phenotype meta-analysis

OBJECTIVES

Combining of genomic data of different pathologies as a single phenotype has emerged as a useful strategy to identify genetic risk loci shared among immune-mediated diseases. Our study aimed to increase our knowledge of the genetic contribution to Kawasaki disease (KD) and IgA vasculitis (IgAV) by performing the first comprehensive large-scale analysis on the genetic overlap between them.

METHODS

A total of 1190 vasculitis patients and 11 302 healthy controls were analysed. First, in the discovery phase, genome-wide data of 405 KD patients and 6252 controls and 215 IgAV patients and 1324 controls, all of European origin, were combined using an inverse variance meta-analysis. Second, the top associated polymorphisms were selected for replication in additional independent cohorts (570 cases and 3726 controls). Polymorphisms with P-values ≤5 × 10-8 in the global IgAV-KD meta-analysis were considered as shared genetic risk loci.

RESULTS

A genetic variant, rs3743841, located in an intron of the NAGPA gene, reached genome-wide significance in the cross-disease meta-analysis (P = 8.06 × 10-10). Additionally, when IgAV was individually analysed, a strong association between rs3743841 and this vasculitis was also evident [P = 1.25 × 10-7; odds ratio = 1.47 (95% CI 1.27, 1.69)]. In silico functional annotation showed that this polymorphism acts as a regulatory variant modulating the expression levels of the NAGPA and SEC14L5 genes.

CONCLUSION

We identified a new risk locus with pleiotropic effects on the two childhood vasculitides analysed. This locus represents the strongest non-HLA signal described for IgAV to date.

N-Glycosylation

N-glycosylation is a highly conserved glycan modification, and more than 7000 proteins are N-glycosylated in humans. N-glycosylation has many biological functions such as protein folding, trafficking, and signal transduction. Thus, glycan modification to proteins is profoundly involved in numerous physiological and pathological processes. The N-glycan precursor is biosynthesized in the endoplasmic reticulum (ER) from dolichol phosphate by sequential enzymatic reactions to generate the dolichol-linked oligosaccharide composed of 14 sugar residues, Glc₃Man₉GlcNAc₂. The oligosaccharide is then en bloc transferred to the consensus sequence N-X-S/T (X represents any amino acid except proline) of nascent proteins. Subsequently, the N-glycosylated nascent proteins enter the folding step, in which N-glycans contribute largely to attaining the correct protein fold by recruiting the lectin-like chaperones, calnexin, and calreticulin. Despite the N-glycan-dependent folding process, some glycoproteins do not fold correctly, and these misfolded glycoproteins are destined to degradation by proteasomes in the cytosol. Properly folded proteins are transported to the Golgi, and N-glycans undergo maturation by the sequential reactions of glycosidases and glycosyltransferases, generating complex-type N-glycans. N-Acetylglucosaminyltransferases (GnT-III, GnT-IV, and GnT-V) produce branched N-glycan structures, affording a higher complexity to N-glycans. In this chapter, we provide an overview of the biosynthetic pathway of N-glycans in the ER and Golgi.

β-Strand-mediated interactions of protein domains

Protein domains exist by themselves or in combination with other domains to form complex multidomain proteins. Defining domain boundaries in proteins is essential for understanding their evolution and function but is not trivial. More specifically, partitioning domains that interact by forming a single β-sheet is known to be particularly troublesome for automatic structure-based domain decomposition pipelines. Here, we study edge-to-edge β-strand interactions between domains in a protein chain, to help define the boundaries for some more difficult cases where a single β-sheet spanning over two domains gives an appearance of one. We give a number of examples where β-strands belonging to a single β-sheet do not belong to a single domain and highlight the difficulties of automatic domain parsers on these examples. This work can be used as a baseline for defining domain boundaries in homologous proteins or proteins with similar domain interactions in the future.

Mucolipidoses Overview: Past, Present, and Future

Mucolipidosis II and III (ML II/III) are caused by a deficiency of uridine-diphosphate N-acetylglucosamine: lysosomal-enzyme-N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase, EC2.7.8.17), which tags lysosomal enzymes with a mannose 6-phosphate (M6P) marker for transport to the lysosome. The process is performed by a sequential two-step process: first, GlcNAc-1-phosphotransferase catalyzes the transfer of GlcNAc-1-phosphate to the selected mannose residues on lysosomal enzymes in the cis-Golgi network. The second step removes GlcNAc from lysosomal enzymes by N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase (uncovering enzyme) and exposes the mannose 6-phosphate (M6P) residues in the trans-Golgi network, in which the enzymes are targeted to the lysosomes by M6Preceptors. A deficiency of GlcNAc-1-phosphotransferase causes the hypersecretion of lysosomal enzymes out of cells, resulting in a shortage of multiple lysosomal enzymes within lysosomes. Due to a lack of GlcNAc-1-phosphotransferase, the accumulation of cholesterol, phospholipids, glycosaminoglycans (GAGs), and other undegraded substrates occurs in the lysosomes. Clinically, ML II and ML III exhibit quite similar manifestations to mucopolysaccharidoses (MPSs), including specific skeletal deformities known as dysostosis multiplex and gingival hyperplasia. The life expectancy is less than 10 years in the severe type, and there is no definitive treatment for this disease. In this review, we have described the updated diagnosis and therapy on ML II/III.

Human GNPTAB stuttering mutations engineered into mice cause vocalization deficits and astrocyte pathology in the corpus callosum

Stuttering is a common neurodevelopmental disorder that has been associated with mutations in genes involved in intracellular trafficking. However, the cellular mechanisms leading to stuttering remain unknown. Engineering a mutation in N-acetylglucosamine-1-phosphate transferase subunits α and β (GNPTAB) found in humans who stutter into the mouse Gnptab gene resulted in deficits in the flow of ultrasonic vocalizations similar to speech deficits of humans who stutter. Here we show that other human stuttering mutations introduced into this mouse gene, Gnptab Ser321Gly and Ala455Ser, produce the same vocalization deficit in 8-day-old pup isolation calls and do not affect other nonvocal behaviors. Immunohistochemistry showed a marked decrease in staining of astrocytes, particularly in the corpus callosum of the Gnptab Ser321Gly homozygote mice compared to wild-type littermates, while the staining of cerebellar Purkinje cells, oligodendrocytes, microglial cells, and dopaminergic neurons was not significantly different. Diffusion tensor imaging also detected deficits in the corpus callosum of the Gnptab Ser321Gly mice. Using a range of cell type-specific Cre-drivers and a Gnptab conditional knockout line, we found that only astrocyte-specific Gnptab-deficient mice displayed a similar vocalization deficit. These data suggest that vocalization defects in mice carrying human stuttering mutations in Gnptab derive from abnormalities in astrocytes, particularly in the corpus callosum, and provide support for hypotheses that focus on deficits in interhemispheric communication in stuttering.

A Lifetime of Adventures in Glycobiology

My initial research experience involved studying how bacteria synthesize nucleotide sugars, the donors for the formation of cell wall polysaccharides. During this time, I became aware that mammalian cells also have a surface coat of sugars and was intrigued as to whether these sugars might be arranged in specific sequences that function as information molecules in biologic processes. Thus began a long journey that has taken me from glycan structural analysis and determination of plant lectin-binding preferences to the biosynthesis of Asn-linked oligosaccharides and the mannose 6-phosphate (Man-6-P) lysosomal enzyme targeting pathway. The Man-6-P system represents an early example of a glycan serving as an information molecule in a fundamental cellular function. The remarkable advances in the field of glycobiology since I entered have uncovered scores of additional examples of oligosaccharide-lectin interactions mediating critical biologic processes. It has been a rewarding experience to participate in the efforts that have established a central role for glycans in biology.

Cathepsin D and its newly identified transport receptor SEZ6L2 can modulate neurite outgrowth

How, in the absence of a functional mannose 6-phosphate (Man-6-P)-signal-dependent transport pathway, some acid hydrolases remain sorted to endolysosomes in the brain is poorly understood. We demonstrate that cathepsin D binds to mouse SEZ6L2, a type 1 transmembrane protein predominantly expressed in the brain. Studies of the subcellular trafficking of SEZ6L2, and its silencing in a mouse neuroblastoma cell line reveal that SEZ6L2 is involved in the trafficking of cathepsin D to endosomes. Moreover, SEZ6L2 can partially correct the cathepsin D hypersecretion resulting from the knockdown of UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase in HeLa cells (i.e. in cells that are unable to synthesize Man-6-P signals). Interestingly, cleavage of SEZ6L2 by cathepsin D generates an N-terminal soluble fragment that induces neurite outgrowth, whereas its membrane counterpart prevents this. Taken together, our findings highlight that SEZ6L2 can serve as receptor to mediate the sorting of cathepsin D to endosomes, and suggest that proteolytic cleavage of SEZ6L2 by cathepsin D modulates neuronal differentiation.

Glucosidase II and MRH-domain containing proteins in the secretory pathway

N-glycosylation in the endoplasmic reticulum (ER) consists of the transfer of a preassembled glycan conserved among species (Glc3Man9GlcNAc2) from a lipid donor to a consensus sequence within a nascent protein that is entering the ER. The protein-linked glycans are then processed by glycosidases and glycosyltransferases in the ER producing specific structures that serve as signalling molecules for the fate of the folding glycoprotein: to stay in the ER during the folding process, to be retrotranslocated to the cytosol for proteasomal degradation if irreversibly misfolded, or to pursue transit through the secretory pathway as a mature glycoprotein. In the ER, each glycan signalling structure is recognized by a specific lectin. A domain similar to that of the mannose 6-phosphate receptors (MPRs) has been identified in several proteins of the secretory pathway. These include the beta subunit of glucosidase II (GII), a key enzyme in the early processing of the transferred glycan that removes middle and innermost glucoses and is involved in quality control of glycoprotein folding in the ER (QC), the lectins OS-9 and XTP3-B, proteins involved in the delivery of ER misfolded proteins to degradation (ERAD), the gamma subunit of the Golgi GlcNAc-1-phosphotransferase, an enzyme involved in generating the mannose 6-phosphate (M6P) signal for sorting acidic hydrolases to lysosomes, and finally the MPRs that deliver those hydrolytic enzymes to the lysosome. Each of the MRH-containing proteins recognizes a different signalling N-glycan structure. Three-dimensional structures of some of the MRH domains have been solved, providing the basis to understand recognition mechanisms.

Bacterial expression of the phosphodiester-binding site of the cation-independent mannose 6-phosphate receptor for crystallographic and NMR studies

The cation-independent mannose 6-phosphate receptor (CI-MPR) is a multifunctional protein that interacts with diverse ligands and plays central roles in autophagy, development, and tumor suppression. By delivering newly synthesized phosphomannosyl-containing acid hydrolases from the Golgi to endosomal compartments, CI-MPR is an essential component in the generation of lysosomes that are critical for the maintenance of cellular homeostasis. The ability of CI-MPR to interact with ∼60 different acid hydrolases is facilitated by its large extracellular region, with four out of its 15 domains binding phosphomannosyl residues. Although the glycan specificity of CI-MPR has been elucidated, the molecular basis of carbohydrate binding has not been determined for two out of these four carbohydrate recognition domains (CRD). Here we report expression of CI-MPR's CRD located in domain 5 that preferentially binds phosphodiester-containing glycans. Domain 5 of CI-MPR was expressed in Escherichia coli BL21 (DE3) cells as a fusion protein containing an N-terminal histidine tag and the small ubiquitin-like modifier (SUMO) protein. The His6-SUMO-CRD construct was recovered from inclusion bodies, refolded in buffer to facilitate disulfide bond formation, and subjected to Ni-NTA affinity chromatography and size exclusion chromatography. Surface plasmon resonance analyses demonstrated that the purified protein was active and bound phosphorylated glycans. Characterization by NMR spectroscopy revealed high quality (1)H-(15)N HSQC spectra. Additionally, crystallization conditions were identified and a crystallographic data set of the CRD was collected to 1.8Å resolution. Together, these studies demonstrate the feasibility of producing CI-MPR's CRD suitable for three-dimensional structure determination by NMR spectroscopic and X-ray crystallographic approaches.

Identification of a fourth mannose 6-phosphate binding site in the cation-independent mannose 6-phosphate receptor

The 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR) plays an essential role in lysosome biogenesis by targeting ∼ 60 different phosphomannosyl-containing acid hydrolases to the lysosome. This type I membrane glycoprotein has a large extracellular region comprised of 15 homologous domains. Two mannose 6-phosphate (M6P) binding sites have been mapped to domains 3 and 9, whereas domain 5 binds preferentially to the phosphodiester, M6P-N-acetylglucosamine (GlcNAc). A structure-based sequence alignment predicts that the C-terminal domain 15 contains three out of the four conserved residues identified as essential for carbohydrate recognition by domains 3, 5 and 9 of the CI-MPR, but lacks two cysteine residues that are predicted to form a disulfide bond. To determine whether domain 15 of the CI-MPR has lectin activity and to probe its carbohydrate-binding specificity, truncated forms of the CI-MPR were tested for binding to acid hydrolases with defined N-glycans in surface plasmon resonance analyses, and used to interrogate a phosphorylated glycan microarray. The results show that a construct encoding domains 14-15 binds both M6P and M6P-GlcNAc with similar affinity (Kd = 13 and 17 μM, respectively). Site-directed mutagenesis studies demonstrate the essential role of the conserved Tyr residue in domain 15 for phosphomannosyl binding. A structural model of domain 15 was generated that predicted an Arg residue to be in the binding pocket and mutagenesis studies confirmed its important role in carbohydrate binding. Together, these results show that the CI-MPR contains a fourth carbohydrate-recognition site capable of binding both phosphomonoesters and phosphodiesters.

Oleanolic acid has similar effects as retinoic acid in inducing mouse embryonic stem cell 1B10 to differentiate towards germ cells

This study investigated the effects of the compound oleanolic acid (OA) in inducing mouse embryonic stem cells (MESC) to differentiate towards germ cells (GC). MESC 1B10 was used as the model cell. 1B10 was cultured, and embryoid bodies (EBs) were produced from 1B10. The EBs were allowed to attach to the bottoms of culturing disks and grow. OA was added into the medium to induce the EBs to differentiate. Retinoic acid (RA) was used as the positive drug. After 72 h, total RNA was extracted, cDNA was synthesized, and real-time fluorescence quantitative PCR was performed to measure the transcriptional expression profiles of 11 reproduction-related genes affected by OA and RA, respectively. When the data were compared, it was found that OA up-regulated the transcriptional levels of Oct-4, GDF-9, Stra8, Mvh, ZP2, ZP3, Itga6, and TP2, and down-regulated transcriptional levels of SCP3, ZP1, and Itgb1; RA up-regulated the transcriptional levels of GDF-9, Stra8, Mvh, ZP2, ZP3, Itga6, Itgb1, and TP2, and down-regulated the transcriptional levels of Oct-4, SCP3, and ZP1. The data showed that OA and RA had similar effects in inducing differentiation of MESC towards GC.

Structure and function of the DUF2233 domain in bacteria and in the human mannose 6-phosphate uncovering enzyme

DUF2233, a domain of unknown function (DUF), is present in many bacterial and several viral proteins and was also identified in the mammalian transmembrane glycoprotein N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase ("uncovering enzyme" (UCE)). We report the crystal structure of BACOVA_00430, a 315-residue protein from the human gut bacterium Bacteroides ovatus that is the first structural representative of the DUF2233 protein family. A notable feature of this structure is the presence of a surface cavity that is populated by residues that are highly conserved across the entire family. The crystal structure was used to model the luminal portion of human UCE (hUCE), which is involved in targeting of lysosomal enzymes. Mutational analysis of several residues in a highly conserved surface cavity of hUCE revealed that they are essential for function. The bacterial enzyme (BACOVA_00430) has ∼1% of the catalytic activity of hUCE toward the substrate GlcNAc-P-mannose, the precursor of the Man-6-P lysosomal targeting signal. GlcNAc-1-P is a poor substrate for both enzymes. We conclude that, for at least a subset of proteins in this family, DUF2233 functions as a phosphodiester glycosidase.

A role for inherited metabolic deficits in persistent developmental stuttering

Stuttering is a common but poorly understood speech disorder. Consistent evidence for the involvement of genetic factors in stuttering has motivated studies aimed at identifying causative genetic variants that could shed light on the underlying molecular and cellular deficits in this disorder. Such studies have begun to identify causative genes. The purpose of this review is to summarize the gene discoveries to date, and to cover the subsequent functional studies that are beginning to provide insights into how these gene mutations might cause stuttering. Surprisingly, the first variant genes to be associated with stuttering are those encoding the lysosomal targeting system, GNPTAB, GNPTG, and NAGPA. Although mutations in NAGPA have not been associated with a disorder in humans, mutations in GNPTAB and GNPTG cause mucolipidosis types II and III, which are rare autosomal recessive lysosomal storage disorders, associated with pathology of bone, connective tissue, liver, spleen, and brain. Analysis of mutations in these genes has so far identified predominantly missense mutations in stuttering, in contrast to the truncating and other mutations that result in very low GNPTAB/G enzyme activity and are historically associated with mucolipidosis. Genetic evidence for the role of lysosomal targeting mutations in stuttering has now been buttressed by biochemical studies of the mutant enzymes found in this disorder. While data on the GlcNAc-phosphotransferase encoded by GNPTAB/G remains limited and only suggestive, a study of the enzyme encoded by NAGPA has shown that the mutations found in stuttering reduce the overall cellular activity of this enzyme by about half, and that they result in deficits in intracellular processing and trafficking that lead to a reduced cellular half life. How these deficits result in the presumed speech-specific neuropathology associated with stuttering is not yet known. However these findings have opened several new lines of inquiry, including studies in mice carrying human stuttering mutations, that represent promising approaches to this disorder.

Mannose-6-phosphate pathway: a review on its role in lysosomal function and dysfunction

Lysosomal hydrolases are synthesized in the rough endoplasmic reticulum and specifically transported through the Golgi apparatus to the trans-Golgi network, from which transport vesicles bud to deliver them to the endosomal/lysosomal compartment. The explanation of how are the lysosomal enzymes accurately recognized and selected over many other proteins in the trans-Golgi network relies on being tagged with a unique marker: the mannose-6-phosphate (M6P) group, which is added exclusively to the N-linked oligosaccharides of lysosomal soluble hydrolases, as they pass through the cis-Golgi network. Generation of the M6P recognition marker depends on a reaction involving two different enzymes: UDP-N-acetylglucosamine 1-phosphotransferase and α-N-acetylglucosamine-1-phosphodiester α-N-acetylglucosaminidase. The M6P groups are then recognized by two independent transmembrane M6P receptors, present in the trans-Golgi network: the cation-independent M6P receptor and/or the cation-dependent M6P receptor. These proteins bind to lysosomal hydrolases on the lumenal side of the membrane and to adaptins in assembling clathrin coats on the cytosolic side. In this way, the M6P receptors help package the hydrolases into vesicles that bud from the trans-Golgi network to deliver their contents to endosomes that ultimately will develop into mature lysosomes, where recently-delivered hydrolases may start digesting the endocyted material. The above described process is known as the M6P-dependent pathway and is responsible for transporting most lysosomal enzymes. This review synthesizes the current knowledge on each of the major proteins involved in the M6P-dependent pathway. Impairments in this pathway will also be addressed, highlighting the lysosomal storage disorders associated to GlcNAc-1-phosphotransferase loss of function: mucolipidosis type II and III.

Disruption of the Man-6-P targeting pathway in mice impairs osteoclast secretory lysosome biogenesis

Osteoclasts are specialized cells that secrete lysosomal acid hydrolases at the site of bone resorption, a process critical for skeletal formation and remodeling. However, the cellular mechanism underlying this secretion and the organization of the endo-lysosomal system of osteoclasts have remained unclear. We report that osteoclasts differentiated in vitro from murine bone marrow macrophages contain two types of lysosomes. The major species is a secretory lysosome containing cathepsin K and tartrate-resistant acid phosphatase (TRAP), two hydrolases critical for bone resorption. These secretory lysosomes are shown to fuse with the plasma membrane, allowing the regulated release of acid hydrolases at the site of bone resorption. The other type of lysosome contains cathepsin D, but little cathepsin K or TRAP. Osteoclasts from Gnptab(-/-) (gene encoding GlcNAc-1-phosphotransferase α, β-subunits) mice, which lack a functional mannose 6-phosphate (Man-6-P) targeting pathway, show increased secretion of cathepsin K and TRAP and impaired secretory lysosome formation. However, cathepsin D targeting was intact, showing that osteoclasts have a Man-6-P-independent pathway for selected acid hydrolases.

Mannose 6-phosphate receptor homology (MRH) domain-containing lectins in the secretory pathway

BACKGROUND

The mannose 6-phosphate receptor homology (MRH) domain-containing family of proteins, which include recycling receptors (mannose 6-phosphate receptors, MPRs), resident endoplasmic reticulum (ER) proteins (glucosidase II β-subunit, XTP3-B, OS-9), and a Golgi glycosyltransferase (GlcNAc-phosphotransferase γ-subunit), are characterized by the presence of one or more MRH domains. Many MRH domains act as lectins and bind specific phosphorylated (MPRs) or non-phosphorylated (glucosidase II β-subunit, XTP3-B and OS-9) high mannose-type N-glycans. The MPRs are the only proteins known to bind mannose 6-phosphate (Man-6-P) residues via their MRH domains.

SCOPE OF REVIEW

Recent biochemical and structural studies that have provided valuable insight into the glycan specificity and mechanisms of carbohydrate recognition by this diverse group of MRH domain-containing proteins are highlighted.

MAJOR CONCLUSIONS

Currently, three-dimensional structures are known for ten MRH domains, revealing the conservation of a similar fold. OS-9 and the MPRs use the same four residues (Gln, Arg, Glu, and Tyr) to bind mannose.

GENERAL SIGNIFICANCE

The MRH domain-containing proteins play key roles in the secretory pathway: glucosidase II, XTP3-B, and OS-9 are involved in the recognition of nascent glycoproteins, whereas the MPRs play an essential role in lysosome biogenesis by targeting Man-6-P-containing lysosomal enzymes to the lysosome.

Vacuolization of mucolipidosis type II mouse exocrine gland cells represents accumulation of autolysosomes

We previously reported that mice deficient in UDP-GlcNAc:lysosomal enzyme GlcNAc-1-phosphotransferase (mucolipidosis type II or Gnptab -/- mice), the enzyme that initiates the addition of the mannose 6-phosphate lysosomal sorting signal on acid hydrolases, exhibited extensive vacuolization of their exocrine gland cells, while the liver, brain, and muscle appeared grossly unaffected. Similar pathological findings were observed in several exocrine glands of patients with mucolipidosis II. To understand the basis for this cell type-specific abnormality, we analyzed these tissues in Gnptab -/- mice using a combined immunoelectron microscopy and biochemical approach. We demonstrate that the vacuoles in the exocrine glands are enlarged autolysosomes containing undigested cytoplasmic material that accumulate secondary to deficient lysosomal function. Surprisingly, the acid hydrolase levels in these tissues ranged from normal to modestly decreased, in contrast to skin fibroblasts, which accumulate enlarged lysosomes and/or autolysosomes also but exhibit very low levels of acid hydrolases. We propose that the lysosomal defect in the exocrine cells is caused by the combination of increased secretion of the acid hydrolases via the constitutive pathway along with their entrapment in secretory granules. Taken together, our results provide new insights into the mechanisms of the tissue-specific abnormalities seen in mucolipidosis type II.

Structural basis for recognition of phosphodiester-containing lysosomal enzymes by the cation-independent mannose 6-phosphate receptor

Mannose 6-phosphate (Man-6-P)-dependent trafficking is vital for normal development. The biogenesis of lysosomes, a major cellular site of protein, carbohydrate, and lipid catabolism, depends on the 300-kDa cation-independent Man-6-P receptor (CI-MPR) that transports newly synthesized acid hydrolases from the Golgi. The CI-MPR recognizes lysosomal enzymes bearing the Man-6-P modification, which arises by the addition of GlcNAc-1-phosphate to mannose residues and subsequent removal of GlcNAc by the uncovering enzyme (UCE). The CI-MPR also recognizes lysosomal enzymes that elude UCE maturation and instead display the Man-P-GlcNAc phosphodiester. This ability of the CI-MPR to target phosphodiester-containing enzymes ensures lysosomal delivery when UCE activity is deficient. The extracellular region of the CI-MPR is comprised of 15 repetitive domains and contains three distinct Man-6-P binding sites located in domains 3, 5, and 9, with only domain 5 exhibiting a marked preference for phosphodiester-containing lysosomal enzymes. To determine how the CI-MPR recognizes phosphodiesters, the structure of domain 5 was determined by NMR spectroscopy. Although domain 5 contains only three of the four disulfide bonds found in the other seven domains whose structures have been determined to date, it adopts the same fold consisting of a flattened beta-barrel. Structure determination of domain 5 bound to N-acetylglucosaminyl 6-phosphomethylmannoside, along with mutagenesis studies, revealed the residues involved in diester recognition, including Y679. These results show the mechanism by which the CI-MPR recognizes Man-P-GlcNAc-containing ligands and provides new avenues to investigate the role of phosphodiester-containing lysosomal enzymes in the biogenesis of lysosomes.

Glycan microarray analysis of P-type lectins reveals distinct phosphomannose glycan recognition

The specificity of the cation-independent and -dependent mannose 6-phosphate receptors (CI-MPR and CD-MPR) for high mannose-type N-glycans of defined structure containing zero, one, or two Man-P-GlcNAc phosphodiester or Man-6-P phosphomonoester residues was determined by analysis on a phosphorylated glycan microarray. Amine-activated glycans were covalently printed on N-hydroxysuccinimide-activated glass slides and interrogated with different concentrations of recombinant CD-MPR or soluble CI-MPR. Neither receptor bound to non-phosphorylated glycans. The CD-MPR bound weakly or undetectably to the phosphodiester derivatives, but strongly to the phosphomonoester-containing glycans with the exception of a single Man7GlcNAc2-R isomer that contained a single Man-6-P residue. By contrast, the CI-MPR bound with high affinity to glycans containing either phospho-mono- or -diesters although, like the CD-MPR, it differentially recognized isomers of phosphorylated Man7GlcNAc2-R. This differential recognition of phosphorylated glycans by the CI- and CD-MPRs has implications for understanding the biosynthesis and targeting of lysosomal hydrolases.