Domain 5 of the Cation-Independent Mannose 6-Phosphate Receptor Preferentially Binds Phosphodiesters (Mannose 6-Phosphate N-Acetylglucosamine Ester)

A universal GlycoDesign for lysosomal replacement enzymes to improve circulation time and biodistribution

Currently available enzyme replacement therapies for lysosomal storage diseases are limited in their effectiveness due in part to short circulation times and suboptimal biodistribution of the therapeutic enzymes. We previously engineered Chinese hamster ovary (CHO) cells to produce α-galactosidase A (GLA) with various N-glycan structures and demonstrated that elimination of mannose-6-phosphate (M6P) and conversion to homogeneous sialylated N-glycans prolonged circulation time and improved biodistribution of the enzyme following a single-dose infusion into Fabry mice. Here, we confirmed these findings using repeated infusions of the glycoengineered GLA into Fabry mice and further tested whether this glycoengineering approach, Long-Acting-GlycoDesign (LAGD), could be implemented on other lysosomal enzymes. LAGD-engineered CHO cells stably expressing a panel of lysosomal enzymes [aspartylglucosamine (AGA), beta-glucuronidase (GUSB), cathepsin D (CTSD), tripeptidyl peptidase (TPP1), alpha-glucosidase (GAA) or iduronate 2-sulfatase (IDS)] successfully converted all M6P-containing N-glycans to complex sialylated N-glycans. The resulting homogenous glycodesigns enabled glycoprotein profiling by native mass spectrometry. Notably, LAGD extended the plasma half-life of all three enzymes tested (GLA, GUSB, AGA) in wildtype mice. LAGD may be widely applicable to lysosomal replacement enzymes to improve their circulatory stability and therapeutic efficacy.

Allosteric regulation of lysosomal enzyme recognition by the cation-independent mannose 6-phosphate receptor

The cation-independent mannose 6-phosphate receptor (CI-MPR, IGF2 receptor or CD222), is a multifunctional glycoprotein required for normal development. Through the receptor's ability to bind unrelated extracellular and intracellular ligands, it participates in numerous functions including protein trafficking, lysosomal biogenesis, and regulation of cell growth. Clinically, endogenous CI-MPR delivers infused recombinant enzymes to lysosomes in the treatment of lysosomal storage diseases. Although four of the 15 domains comprising CI-MPR's extracellular region bind phosphorylated glycans on lysosomal enzymes, knowledge of how CI-MPR interacts with ~60 different lysosomal enzymes is limited. Here, we show by electron microscopy and hydroxyl radical protein footprinting that the N-terminal region of CI-MPR undergoes dynamic conformational changes as a consequence of ligand binding and different pH conditions. These data, coupled with X-ray crystallography, surface plasmon resonance and molecular modeling, allow us to propose a model explaining how high-affinity carbohydrate binding is achieved through allosteric domain cooperativity.

Structure of the Human Cation-Independent Mannose 6-Phosphate/IGF2 Receptor Domains 7-11 Uncovers the Mannose 6-Phosphate Binding Site of Domain 9

The cation-independent mannose 6-phosphate (M6P)/Insulin-like growth factor-2 receptor (CI-MPR/IGF2R) is an ∼300 kDa transmembrane protein responsible for trafficking M6P-tagged lysosomal hydrolases and internalizing IGF2. The extracellular region of the CI-MPR has 15 homologous domains, including M6P-binding domains (D) 3, 5, 9, and 15 and IGF2-binding domain 11. We have focused on solving the first structures of human D7-10 within two multi-domain constructs, D9-10 and D7-11, and provide the first high-resolution description of the high-affinity M6P-binding D9. Moreover, D9 stabilizes a well-defined hub formed by D7-11 whereby two penta-domains intertwine to form a dimeric helical-type coil via an N-glycan bridge on D9. Remarkably the D7-11 structure matches an IGF2-bound state of the receptor, suggesting this may be an intrinsically stable conformation at neutral pH. Interdomain clusters of histidine and proline residues may impart receptor rigidity and play a role in structural transitions at low pH.

The glycosylation design space for recombinant lysosomal replacement enzymes produced in CHO cells

Lysosomal replacement enzymes are essential therapeutic options for rare congenital lysosomal enzyme deficiencies, but enzymes in clinical use are only partially effective due to short circulatory half-life and inefficient biodistribution. Replacement enzymes are primarily taken up by cell surface glycan receptors, and glycan structures influence uptake, biodistribution, and circulation time. It has not been possible to design and systematically study effects of different glycan features. Here we present a comprehensive gene engineering screen in Chinese hamster ovary cells that enables production of lysosomal enzymes with N-glycans custom designed to affect key glycan features guiding cellular uptake and circulation. We demonstrate distinct circulation time and organ distribution of selected glycoforms of α-galactosidase A in a Fabry disease mouse model, and find that an α2-3 sialylated glycoform designed to eliminate uptake by the mannose 6-phosphate and mannose receptors exhibits improved circulation time and targeting to hard-to-reach organs such as heart. The developed design matrix and engineered CHO cell lines enables systematic studies towards improving enzyme replacement therapeutics.

Lysosomal replacement enzymes are taken up by cell surface receptors that recognize glycans, the effects of different glycan features are unknown. Here the authors present a gene engineering screen in CHO cells that allows custom N-glycan-decorated enzymes with improved circulation time and organ distribution.

Lysosome biogenesis in health and disease

This review focuses on the pathways that regulate lysosome biogenesis and that are implicated in numerous degenerative storage diseases, including lysosomal storage disorders and late-onset neurodegenerative diseases. Lysosomal proteins are synthesized in the endoplasmic reticulum and trafficked to the endolysosomal system through the secretory route. Several receptors have been characterized that execute post-Golgi trafficking of lysosomal proteins. Some of them recognize their cargo proteins based on specific amino acid signatures, others based on a particular glycan modification that is exclusively found on lysosomal proteins. Nearly all receptors serving lysosome biogenesis are under the transcriptional control of transcription factor EB (TFEB), a master regulator of the lysosomal system. TFEB coordinates the expression of lysosomal hydrolases, lysosomal membrane proteins, and autophagy proteins in response to pathways sensing lysosomal stress and the nutritional conditions of the cell among other stimuli. TFEB is primed for activation in lysosomal storage disorders but surprisingly its function is impaired in some late-onset neurodegenerative storage diseases like Alzheimer's and Parkinson's, because of specific detrimental interactions that limit TFEB expression or activation. Thus, disrupted TFEB function presumably plays a role in the pathogenesis of these diseases. Multiple studies in animal models of degenerative storage diseases have shown that exogenous expression of TFEB and pharmacological activation of endogenous TFEB attenuate disease phenotypes. These results highlight TFEB-mediated enhancement of lysosomal biogenesis and function as a candidate strategy to counteract the progression of these diseases. This article is part of the Special Issue "Lysosomal Storage Disorders".

Quiescin sulfhydryl oxidase 1 (QSOX1) glycosite mutation perturbs secretion but not Golgi localization

Quiescin sulfhydryl oxidase 1 (QSOX1) catalyzes the formation of disulfide bonds in protein substrates. Unlike other enzymes with related activities, which are commonly found in the endoplasmic reticulum, QSOX1 is localized to the Golgi apparatus or secreted. QSOX1 is upregulated in quiescent fibroblast cells and secreted into the extracellular environment, where it contributes to extracellular matrix assembly. QSOX1 is also upregulated in adenocarcinomas, though the extent to which it is secreted in this context is currently unknown. To achieve a better understanding of factors that dictate QSOX1 localization and function, we aimed to determine how post-translational modifications affect QSOX1 trafficking and activity. We found a highly conserved N-linked glycosylation site to be required for QSOX1 secretion from fibroblasts and other cell types. Notably, QSOX1 lacking a glycan at this site arrives at the Golgi, suggesting that it passes endoplasmic reticulum quality control but is not further transported to the cell surface for secretion. The QSOX1 transmembrane segment is dispensable for Golgi localization and secretion, as fully luminal and transmembrane variants displayed the same trafficking behavior. This study provides a key example of the effect of glycosylation on Golgi exit and contributes to an understanding of late secretory sorting and quality control.

Mannose 6-phosphate-dependent lysosomal enzyme targeting in hydra: a biochemical, immunological and structural elucidation

Mannose 6-phosphate (M6P)-dependent lysosomal enzyme targeting to endosome/lysosome complex is poorly understood among lower invertebrates. So far, only a M6P-independent lysosomal enzyme sorting protein, named LERP, has been described in Drosophila. Here, we have identified mannose 6-phosphate receptor (MPR) homologues in Hydra vulgaris, a basal Cnidarian, at genome level and further purified a cation-dependent MPR-like protein from hydra using affinity chromatography. Structural comparisons of hydra MPRs with mammalian MPRs confirm that the residues important for interacting with the M6P ligand are conserved. Based on our results, we report for the first time the occurrence of MPR-related proteins and M6P-dependent lysosomal enzyme targeting in H. vulgaris.

Inhibition of insulin-like growth factor II (IGF-II)-dependent cell growth by multidentate pentamannosyl 6-phosphate-based ligands targeting the mannose 6-phosphate/IGF-II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) binds M6P-capped ligands and IGF-II at different binding sites within the ectodomain and mediates ligand internalization and trafficking to the lysosome. Multivalent M6P-based ligands can cross-bridge the M6P/IGF2R, which increases the rate of receptor internalization, permitting IGF-II binding as a passenger ligand and subsequent trafficking to the lysosome, where the IGF-II is degraded. This unique feature of the receptor may be exploited to design novel therapeutic agents against IGF-II-dependent cancers that will lead to decreased bioavailable IGF-II within the tumor microenvironment. We have designed a panel of M6P-based ligands that bind to the M6P/IGF2R with high affinity in a bivalent manner and cause decreased cell viability. We present evidence that our ligands bind through the M6P-binding sites of the receptor and facilitate internalization and degradation of IGF-II from conditioned medium to mediate this cellular response. To our knowledge, this is the first panel of synthetic bivalent ligands for the M6P/IGF2R that can take advantage of the ligand-receptor interactions of the M6P/IGF2R to provide proof-of-principle evidence for the feasibility of novel chemotherapeutic agents that decrease IGF-II-dependent growth of cancer cells.

Cloning, Expression, and Purification of the Glycosylated Transmembrane Protein, Cation-Dependent Mannose 6-Phosphate Receptor, from Sf9 Cells Using the Baculovirus System

The cation-dependent mannose 6-phosphate receptor (CD-MPR) is a single-pass type I membrane protein. This protein functions to transport lysosomal enzymes displaying phosphomannosyl residues from the Golgi complex and the cell surface to the lysosome. This glycosylated protein contains three disulfide bridges in its 159-residue extracytoplasmic domain. One of the problems with studying eukaryotic membrane proteins is isolating sufficient quantities. Structural studies typically require several hundred milligrams of highly purified protein. Here we present a method to isolate milligram quantities of CD-MPR/Asn81 suitable for structural studies.

Insulin-Like Growth Factor-II/Cation-Independent Mannose 6-Phosphate Receptor in Neurodegenerative Diseases

The insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein. Recent studies have advanced our understanding of the structure, ligand-binding properties, and trafficking of the IGF-II/M6P receptor. This receptor has been implicated in a variety of important cellular processes including growth and development, clearance of IGF-II, proteolytic activation of enzymes, and growth factor precursors, in addition to its well-known role in the delivery of lysosomal enzymes. The IGF-II/M6P receptor, distributed widely in the central nervous system, has additional roles in mediating neurotransmitter release and memory enhancement/consolidation, possibly through activating IGF-II-related intracellular signaling pathways. Recent studies suggest that overexpression of the IGF-II/M6P receptor may have an important role in regulating the levels of transcripts and proteins involved in the development of Alzheimer's disease (AD)-the prevalent cause of dementia affecting the elderly population in our society. It is reported that IGF-II/M6P receptor overexpression can increase the levels/processing of amyloid precursor protein leading to the generation of β-amyloid peptide, which is associated with degeneration of neurons and subsequent development of AD pathology. Given the significance of the receptor in mediating the transport and functioning of the lysosomal enzymes, it is being considered for therapeutic delivery of enzymes to the lysosomes to treat lysosomal storage disorders. Notwithstanding these results, additional studies are required to validate and fully characterize the function of the IGF-II/M6P receptor in the normal brain and its involvement in various neurodegenerative disorders including AD. It is also critical to understand the interaction between the IGF-II/M6P receptor and lysosomal enzymes in neurodegenerative processes, which may shed some light on developing approaches to detect and prevent neurodegeneration through the dysfunction of the receptor and the endosomal-lysosomal system.

Glyco-engineering strategies for the development of therapeutic enzymes with improved efficacy for the treatment of lysosomal storage diseases

Lysosomal storage diseases (LSDs) are a group of inherent diseases characterized by massive accumulation of undigested compounds in lysosomes, which is caused by genetic defects resulting in the deficiency of a lysosomal hydrolase. Currently, enzyme replacement therapy has been successfully used for treatment of 7 LSDs with 10 approved therapeutic enzymes whereas new approaches such as pharmacological chaperones and gene therapy still await evaluation in clinical trials. While therapeutic enzymes for Gaucher disease have N-glycans with terminal mannose residues for targeting to macrophages, the others require N-glycans containing mannose-6-phosphates that are recognized by mannose-6-phosphate receptors on the plasma membrane for cellular uptake and targeting to lysosomes. Due to the fact that efficient lysosomal delivery of therapeutic enzymes is essential for the clearance of accumulated compounds, the suitable glycan structure and its high content are key factors for efficient therapeutic efficacy. Therefore, glycan remodeling strategies to improve lysosomal targeting and tissue distribution have been highlighted. This review describes the glycan structures that are important for lysosomal targeting and provides information on recent glyco-engineering technologies for the development of therapeutic enzymes with improved efficacy. [BMB Reports 2015; 48(8): 438-444]

Single-chain antibody-fragment M6P-1 possesses a mannose 6-phosphate monosaccharide-specific binding pocket that distinguishes N-glycan phosphorylation in a branch-specific manner†

The acquisition of mannose 6-phosphate (Man6P) on N-linked glycans of lysosomal enzymes is a structural requirement for their transport from the Golgi apparatus to lysosomes mediated by the mannose 6-phosphate receptors, 300 kDa cation-independent mannose 6-phosphate receptor (MPR300) and 46 kDa cation-dependent mannose 6-phosphate receptor (MPR46). Here we report that the single-chain variable domain (scFv) M6P-1 is a unique antibody fragment with specificity for Man6P monosaccharide that, through an array-screening approach against a number of phosphorylated N-glycans, is shown to bind mono- and diphosphorylated Man6 and Man7 glycans that contain terminal αMan6P(1 → 2)αMan(1 → 3)αMan. In contrast to MPR300, scFv M6P-1 does not bind phosphodiesters, monophosphorylated Man8 or mono- or diphosphorylated Man9 structures. Single crystal X-ray diffraction analysis to 2.7 Å resolution of Fv M6P-1 in complex with Man6P reveals that specificity and affinity is achieved via multiple hydrogen bonds to the mannose ring and two salt bridges to the phosphate moiety. In common with both MPRs, loss of binding was observed for scFv M6P-1 at pH values below the second pKa of Man6P (pKa = 6.1). The structures of Fv M6P-1 and the MPRs suggest that the change of the ionization state of Man6P is the main driving force for the loss of binding at acidic lysosomal pH (e.g. lysosome pH ∼ 4.6), which provides justification for the evolution of a lysosomal enzyme transport pathway based on Man6P recognition.

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.

Structure of the lectin mannose 6-phosphate receptor homology (MRH) domain of glucosidase II, an enzyme that regulates glycoprotein folding quality control in the endoplasmic reticulum

Here we report for the first time the three-dimensional structure of a mannose 6-phosphate receptor homology (MRH) domain present in a protein with enzymatic activity, glucosidase II (GII). GII is involved in glycoprotein folding in the endoplasmic reticulum. GII removes the two innermost glucose residues from the Glc3Man9GlcNAc2 transferred to nascent proteins and the glucose added by UDP-Glc:glycoprotein glucosyltransferase. GII is composed of a catalytic GIIα subunit and a regulatory GIIβ subunit. GIIβ participates in the endoplasmic reticulum localization of GIIα and mediates in vivo enhancement of N-glycan trimming by GII through its C-terminal MRH domain. We determined the structure of a functional GIIβ MRH domain by NMR spectroscopy. It adopts a β-barrel fold similar to that of other MRH domains, but its binding pocket is the most shallow known to date as it accommodates a single mannose residue. In addition, we identified a conserved residue outside the binding pocket (Trp-409) present in GIIβ but not in other MRHs that influences GII glucose trimming activity.

Synthesis of novel bivalent mimetic ligands for mannose-6-phosphate receptors

Mannose-6-phosphate (M6P)-containing N-linked glycans are essential signaling molecules for sorting hydrolases in eukaryotic cells. Their receptors, especially the cation-independent M6P receptors (CI-MPRs), have emerged as promising protein targets for targeted drug delivery for the treatment of lysosomal storage disease and liver fibrosis. In this Letter, we describe the design and synthesis of novel bivalent mimetic ligands for CI-MPRs. We report that for the first time, a newly-discovered binding motif, GlcNAc-M6P, has been incorporated in mimetic ligands. M6P- and GlcNAc-M6P-containing building blocks, equipped with NH2 and CO2H handles, have been prepared and assembled with an ornithine linker through amide coupling reactions. Efficient global deprotection protocols have also been developed which have been showcased in the synthesis of our novel bivalent mimetic ligands.

Rab31 expression levels modulate tumor-relevant characteristics of breast cancer cells

Background

Rab proteins constitute a large family of monomeric GTP-binding proteins that regulate intracellular vesicle transport. Several Rab proteins, including rab31, have been shown to affect cancer progression and are related with prognosis in various types of cancer including breast cancer. Recently, the gene encoding rab31 was found to be overexpressed in estrogen receptor-positive breast cancer tissue. In a previous study we found a significant association of high rab31 mRNA expression with poor prognosis in node-negative breast cancer patients. In the present study, we aimed to investigate the impact of rab31 (over)-expression on important aspects of tumor progression in vitro and in vivo.

Methods

Breast cancer cells displaying low (MDA-MB-231) or no (CAMA-1) endogenous rab31 expression were stably transfected with a rab31 expression plasmid. Batch-transfected cells as well as selected cell clones, expressing different levels of rab31 protein, were analyzed with regard to proliferation, cell adhesion, the invasive capacity of tumor cells, and in vivo in a xenograft tumor model. Polyclonal antibodies directed to recombinantly expressed rab31 were generated and protein expression analyzed by immunohistochemistry, Western blot analysis, and a newly developed sensitive ELISA.

Results

Elevated rab31 protein levels were associated with enhanced proliferation of breast cancer cells. Interestingly, weak to moderate overexpression of rab31 in cell lines with no detectable endogenous rab31 expression was already sufficient to elicit distinct effects on cell proliferation. By contrast, increased expression of rab31 in breast cancer cells led to reduced adhesion towards several extracellular matrix proteins and decreased invasive capacity through Matrigel^TM. Again, the rab31-mediated effects on cell adhesion and invasion were dose-dependent. Finally, in a xenograft mouse model, we observed a significantly impaired metastatic dissemination of rab31 overexpressing MDA-MB-231 breast cancer cells to the lung.

Conclusions

Overexpression of rab31 in breast cancer cells leads to a switch from an invasive to a proliferative phenotype as indicated by an increased cell proliferation, reduced adhesion and invasion in vitro, and a reduced capacity to form lung metastases in vivo.

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.

P-Type Lectins: Cation-Dependent Mannose-6-Phosphate Receptor

In eukaryotic cells, post-translational modification of secreted proteins and intracellular protein transport between organelles are ubiquitous features. One of the most studied systems is the N-linked glycosylation pathway in the synthesis of secreted glycoproteins (Schrag et al. 2003). The N-linked glycoproteins are subjected to diverse modifications and are transported through ER and Golgi apparatus to their final destinations in- and outside the cell. Incorporation of cargo glycoproteins into transport vesicles is mediated by transmembrane cargo receptors, which have been identified as intracellular lectins. For example, mannose 6-phosphate receptors (Ghosh et al. 2003) function as a cargo receptor for lysosomal proteins in the trans-Golgi network, whereas ERGIC-53 (Zhang et al. 2003) and its yeast orthologs Emp46/47p (Sato and Nakano 2002) are transport lectins for glycoproteins that are transported out of ER.

The glycan-binding properties of the cation-independent mannose 6-phosphate receptor are evolutionary conserved in vertebrates

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR) plays an essential role in the biogenesis of lysosomes by delivering newly synthesized lysosomal enzymes from the trans Golgi network to the endosomal system. The CI-MPR is expressed in most eukaryotes, with Saccharomyces cerevisiae and Caenorhabditis elegans being notable exceptions. Although the repertoire of glycans recognized by the bovine receptor has been studied extensively, little is known concerning the ligand-binding properties of the CI-MPR from non-mammalian species. To assess the evolutionary conservation of the CI-MPR, surface plasmon resonance analyses using lysosomal enzymes with defined N-glycans were carried out to probe the glycan-binding specificity of the Danio rerio CI-MPR. The results demonstrate that the D. rerio CI-MPR harbors three glycan-binding sites that, like the bovine CI-MPR, map to domains 3, 5 and 9 of its 15-domain-containing extracytoplasmic region. Analyses on a phosphorylated glycan microarray further demonstrated the unique binding properties of each of the three sites and showed that, similar to the bovine CI-MPR, only domain 5 of the D. rerio CI-MPR is capable of recognizing Man-P-GlcNAc-containing glycans.

Preparation of a mannose-6-phosphate glycan microarray through fluorescent derivatization, phosphorylation, and immobilization of natural high-mannose N-glycans and application in ligand identification of P-type lectins

Glycan microarrays prepared by immobilization of amino-functionalized glycans on NHS-activated glass slides have been successfully used to study protein-glycan interactions. Fluorescently tagged glycans with an amino functional group can be prepared from natural glycans released from glycoproteins. These tagged glycans can be enzymatically modified with various glycosyltransferases, phosphotransferases, sulfotransferases, etc., to quickly expand the size and diversity of the tagged glycan libraries (TGLs). The TGLs, presented in the format of microarrays, provide a convenient platform for identifying the glycan ligands of glycan-binding proteins (GBPs). The chapter provides the background to prepare a defined glycan microarray and uses as an example glycans generated as phosphodiesters and phosphomonoesters of high-mannose type N-glycans. The method describes the preparation of high-mannose type glycan-AEAB conjugates (GAEABs), the purification of their phosphodiesters, and the subsequent mild acid hydrolysis to obtain corresponding phosphomonoesters. These GAEABs are covalently printed as a phosphorylated glycan microarray and used for analysis of the glycan ligand specificities of P-type lectins, such as the mannose-6-phosphate receptors (Man-6-P receptors or MPRs).

Chemical synthesis of N-linked glycans carrying both mannose-6-phosphate and GlcNAc-mannose-6-phosphate motifs

Mannose-6-phosphate (M6P) containing N-linked glycans are the essential targeting signals for hydrolases sorting in eukaryotic cells. To facilitate their structural and binding analyses, a highly efficient and convergent method has been developed to prepare complex N-linked glycans with well-defined M6P and N-acetylglucosamine (GlcNAc)-M6P motifs, a newly identified binding element for M6P receptors. The GlcNAc-M6P motif was stereoselectively installed at the late stage of the synthesis. Sequential deprotection of benzyl and acetate groups provided the fully deprotected N-glycans in excellent yield.

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.

Extensive mannose phosphorylation on leukemia inhibitory factor (LIF) controls its extracellular levels by multiple mechanisms

In addition to soluble acid hydrolases, many nonlysosomal proteins have been shown to bear mannose 6-phosphate (Man-6-P) residues. Quantification of the extent of mannose phosphorylation and the relevance to physiological function, however, remain poorly defined. In this study, we investigated the mannose phosphorylation status of leukemia inhibitory factor (LIF), a previously identified high affinity ligand for the cation-independent mannose 6-phosphate receptor (CI-MPR), and we analyzed the effects of this modification on its secretion and uptake in cultured cells. When media from LIF-overexpressing cells were fractionated using a CI-MPR affinity column, 35-45% of the total LIF molecules were bound and specifically eluted with free Man-6-P thus confirming LIF as a bona fide Man-6-P-modified protein. Surprisingly, mass spectrometric analysis of LIF glycopeptides enriched on the CI-MPR column revealed that all six N-glycan sites could be Man-6-P-modified. The relative utilization of these sites, however, was not uniform. Analysis of glycan-deleted LIF mutants demonstrated that loss of glycans bearing the majority of Man-6-P residues leads to higher steady-state levels of secreted LIF. Using mouse embryonic stem cells, we showed that the mannose phosphorylation of LIF mediates its internalization thereby reducing extracellular levels and stimulating embryonic stem cell differentiation. Finally, immunofluorescence experiments indicate that LIF is targeted directly to lysosomes following its biosynthesis, providing another mechanism whereby mannose phosphorylation serves to control extracellular levels of LIF. Failure to modify LIF in the context of mucolipidosis II and its subsequent accumulation in the extracellular space may have important implications for disease pathogenesis.

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.

Cation-independent mannose 6-phosphate receptor: a composite of distinct phosphomannosyl binding sites

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR), which contains multiple mannose 6-phosphate (Man-6-P) binding sites that map to domains 3, 5, and 9 within its 15-domain extracytoplasmic region, functions as an efficient carrier of Man-6-P-containing lysosomal enzymes. To determine the types of phosphorylated N-glycans recognized by each of the three carbohydrate binding sites of the CI-MPR, a phosphorylated glycan microarray was probed with truncated forms of the CI-MPR. Surface plasmon resonance analyses using lysosomal enzymes with defined N-glycans were performed to evaluate whether multiple domains are needed to form a stable, high affinity carbohydrate binding pocket. Like domain 3, adjacent domains increase the affinity of domain 5 for phosphomannosyl residues, with domain 5 exhibiting approximately 60-fold higher affinity for lysosomal enzymes containing the phosphodiester Man-P-GlcNAc when in the context of a construct encoding domains 5-9. In contrast, domain 9 does not require additional domains for high affinity binding. The three sites differ in their glycan specificity, with only domain 5 being capable of recognizing Man-P-GlcNAc. In addition, domain 9, unlike domains 1-3, interacts with Man(8)GlcNAc(2) and Man(9)GlcNAc(2) oligosaccharides containing a single phosphomonoester. Together, these data indicate that the assembly of three unique carbohydrate binding sites allows the CI-MPR to interact with the structurally diverse phosphorylated N-glycans it encounters on newly synthesized lysosomal enzymes.

Carbohydrate recognition by the mannose-6-phosphate receptors

The two P-type lectins, the 46kDa cation-dependent mannose-6-phosphate (Man-6-P) receptor (CD-MPR), and the 300kDa cation-independent Man-6-P receptor (CI-MPR), are the founding members of the growing family of mannose-6-phosphate receptor homology (MRH) proteins. A major cellular function of the MPRs is to transport Man-6-P-containing acid hydrolases from the Golgi to endosomal/lysosomal compartments. Recent advances in the structural analyses of both CD-MPR and CI-MPR have revealed the structural basis for phosphomannosyl recognition by these receptors and provided insights into how the receptors load and unload their cargo. A surprising finding is that the CD-MPR is dynamic, with at least two stable quaternary states, the open (ligand-bound) and closed (ligand-free) conformations, similar to those of hemoglobin. Ligand binding stabilizes the open conformation; changes in the pH of the environment at the cell surface and in endosomal compartments weaken the ligand-receptor interaction and/or weaken the electrostatic interactions at the subunit interface, resulting in the closed conformation.

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.

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

The mannose 6-phosphate (Man-6-P) lysosomal targeting signal on acid hydrolases is synthesized by the sequential action of uridine 5'-diphosphate-N-acetylglucosamine: lysosomal enzyme N-acetylglucosamine-1-phosphotransferase (GlcNAc-1-phosphotransferase) and GlcNAc-1-phosphodiester alpha-N-acetylglucosaminidase ("uncovering enzyme" or UCE). Mutations in the two genes that encode GlcNAc-1-phosphotransferase give rise to lysosomal storage diseases (mucolipidosis type II and III), whereas no pathological conditions have been associated with the loss of UCE activity. To analyze the consequences of UCE deficiency, the UCE gene was inactivated via insertional mutagenesis in mice. The UCE -/- mice were viable, grew normally and lacked detectable histologic abnormalities. However, the plasma levels of six acid hydrolases were elevated 1.6- to 5.4-fold over wild-type levels. These values underestimate the degree of hydrolase hypersecretion as these enzymes were rapidly cleared from the plasma by the mannose receptor. The secreted hydrolases contained GlcNAc-P-Man diesters, exhibited a decreased affinity for the cation-independent mannose 6-phosphate receptor and failed to bind to the cation-dependent mannose 6-phosphate receptor. These data demonstrate that UCE accounts for all the uncovering activity in the Golgi. We propose that in the absence of UCE, the weak binding of the acid hydrolases to the cation-independent mannose 6-phosphate receptor allows sufficient sorting to lysosomes to prevent the tissue abnormalities seen with GlcNAc-1-phosphotranferase deficiency.

Structure and function of Pseudomonas aeruginosa protein PA1324 (21-170)

Pseudomonas aeruginosa is the prototypical biofilm-forming gram-negative opportunistic human pathogen. P. aeruginosa is causatively associated with nosocomial infections and with cystic fibrosis. Antibiotic resistance in some strains adds to the inherent difficulties that result from biofilm formation when treating P. aeruginosa infections. Transcriptional profiling studies suggest widespread changes in the proteome during quorum sensing and biofilm development. Many of the proteins found to be upregulated during these processes are poorly characterized from a functional standpoint. Here, we report the solution NMR structure of PA1324, a protein of unknown function identified in these studies, and provide a putative biological functional assignment based on the observed prealbumin-like fold and FAST-NMR ligand screening studies. PA1324 is postulated to be involved in the binding and transport of sugars or polysaccharides associated with the peptidoglycan matrix during biofilm formation.

The repertoire of glycan determinants in the human glycome

The number of glycan determinants that comprise the human glycome is not known. This uncertainty arises from limited knowledge of the total number of distinct glycans and glycan structures in the human glycome, as well as limited information about the glycan determinants recognized by glycan-binding proteins (GBPs), which include lectins, receptors, toxins, microbial adhesins, antibodies, and enzymes. Available evidence indicates that GBP binding sites may accommodate glycan determinants made up of 2 to 6 linear monosaccharides, together with their potential side chains containing other sugars and modifications, such as sulfation, phosphorylation, and acetylation. Glycosaminoglycans, including heparin and heparan sulfate, comprise repeating disaccharide motifs, where a linear sequence of 5 to 6 monosaccharides may be required for recognition. Based on our current knowledge of the composition of the glycome and the size of GBP binding sites, glycoproteins and glycolipids may contain approximately 3000 glycan determinants with an additional approximately 4000 theoretical pentasaccharide sequences in glycosaminoglycans. These numbers provide an achievable target for new chemical and/or enzymatic syntheses, and raise new challenges for defining the total glycome and the determinants recognized by GBPs.

The mannose 6-phosphate/insulin-like growth factor II receptor restricts the tumourigenicity and invasiveness of squamous cell carcinoma cells

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) mediates biosynthetic sorting and endocytosis of various factors that impinge on the proliferation, migration and invasiveness of tumour cells. The gene encoding M6P/IGF2R is frequently lost or mutated in a wide range of malignant tumours including squamous cell carcinomas. We have previously shown that M6P/IGF2R-deficient SCC-VII murine squamous cell carcinoma cells secrete large amounts of pro-invasive lysosomal proteinases. Furthermore, the formation of mature lysosomes is impaired in SCC-VII cells. To assess the link between M6P/IGF2R status and tumour invasion, we have now generated SCC-VII lines stably transfected with human M6P/IGF2R cDNA. Reconstitution of functional M6P/IGF2R expression in SCC-VII cells strongly improves the intracellular retention of lysosomal proteinases and restores the formation of mature lysosomes. In addition, the presence of heterologous M6P/IGF2R compromises the growth of SCC-VII cells both in vitro and in vivo. Remarkably, M6P/IGF2R expression also reduces the invasive capacity of SCC-VII cells in response to various chemoattractants. These results indicate that the M6P/IGF2R status influences the metastatic propensity of squamous cell carcinomas.

Biochemical and pharmacological characterization of different recombinant acid alpha-glucosidase preparations evaluated for the treatment of Pompe disease

Pompe disease results in the accumulation of lysosomal glycogen in multiple tissues due to a deficiency of acid alpha-glucosidase (GAA). Enzyme replacement therapy for Pompe disease was recently approved in Europe, the U.S., Canada, and Japan using a recombinant human GAA (Myozyme, alglucosidase alfa) produced in CHO cells (CHO-GAA). During the development of alglucosidase alfa, we examined the in vitro and in vivo properties of CHO cell-derived rhGAA, an rhGAA purified from the milk of transgenic rabbits, as well as an experimental version of rhGAA containing additional mannose-6-phosphate intended to facilitate muscle targeting. Biochemical analyses identified differences in rhGAA N-termini, glycosylation types and binding properties to several carbohydrate receptors. In a mouse model of Pompe disease, glycogen was more efficiently removed from the heart than from skeletal muscle for all enzymes, and overall, the CHO cell-derived rhGAA reduced glycogen to a greater extent than that observed with the other enzymes. The results of these preclinical studies, combined with biochemical characterization data for the three molecules described within, led to the selection of the CHO-GAA for clinical development and registration as the first approved therapy for Pompe disease.

Strategies for carbohydrate recognition by the mannose 6-phosphate receptors

The two members of the P-type lectin family, the 46 kDa cation-dependent mannose 6-phosphate receptor (CD-MPR) and the 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR), are ubiquitously expressed throughout the animal kingdom and are distinguished from all other lectins by their ability to recognize phosphorylated mannose residues. The best-characterized function of the MPRs is their ability to direct the delivery of approximately 60 different newly synthesized soluble lysosomal enzymes bearing mannose 6-phosphate (Man-6-P) on their N-linked oligosaccharides to the lysosome. In addition to its intracellular role in lysosome biogenesis, the CI-MPR, but not the CD-MPR, participates in a number of other biological processes by interacting with various molecules at the cell surface. The list of extracellular ligands recognized by this multifunctional receptor has grown to include a diverse spectrum of Man-6-P-containing proteins as well as several non-Man-6-P-containing ligands. Recent structural studies have given us a clearer view of how these two receptors use related, but yet distinct, approaches in the recognition of phosphomannosyl residues.

Structural insights into the mechanism of pH-dependent ligand binding and release by the cation-dependent mannose 6-phosphate receptor

The cation-dependent mannose 6-phosphate receptor (CD-MPR) is a key component of the lysosomal enzyme targeting system that binds newly synthesized mannose 6-phosphate (Man-6-P)-containing acid hydrolases and transports them to endosomal compartments. The interaction between the MPRs and its ligands is pH-dependent; the homodimeric CD-MPR binds lysosomal enzymes optimally in the pH environment of the trans Golgi network (pH approximately 6.5) and releases its cargo in acidic endosomal compartments (<pH 5.5) and at the cell surface. In addition, CD-MPR binding affinities are modulated by divalent cations. Our previous crystallographic studies have shown that at pH 6.5, the CD-MPR bound to Man-6-P adopts a significantly different quaternary conformation than the CD-MPR in a ligand-unbound state, a feature unique among known lectin structures. To determine whether different pH conditions elicit conformational changes in the receptor that alters ligand binding affinities, we have obtained additional crystal structures representative of the various environments encountered by the receptor including: 1) the CD-MPR bound at pH 6.5 (i.e. trans Golgi network) to a high affinity ligand (the terminally phosphorylated trisaccharide P-Man(alpha1,2)Man(alpha1,2)Man-O-(CH(2))(8)COOMe), 2) the CD-MPR at pH 4.8 in an unbound state (i.e. endosome), and 3) the CD-MPR at pH 7.4 (i.e. cell surface). A detailed comparison of the available CD-MPR structures reveals the positional invariability of specific binding pocket residues and implicates intermonomer contact(s), as well as the protonation state of Man-6-P, as regulators of pH-dependent carbohydrate binding.