Tissue plasminogen activator is a ligand of cation-independent mannose 6-phosphate receptor and consists of glycoforms that contain mannose 6-phosphate.

Plasmin is the key enzyme in fibrinolysis. Upon interaction with plasminogen activators, the zymogen plasminogen is converted to active plasmin. Some studies indicate plasminogen activation is regulated by cation-independent mannose 6-phosphate receptor (CI-MPR), a protein that facilitates lysosomal enzyme trafficking and insulin-like growth factor 2 downregulation. Plasminogen regulation may be accomplished by CI-MPR binding to plasminogen or urokinase plasminogen activator receptor. We asked whether other members of the plasminogen activation system, such as tissue plasminogen activator (tPA), also interact with CI-MPR. Because tPA is a glycoprotein with three N-linked glycosylation sites, we hypothesized that tPA contains mannose 6-phosphate (M6P) and binds CI-MPR in a M6P-dependent manner. Using surface plasmon resonance, we found that two sources of tPA bound the extracellular region of human and bovine CI-MPR with low-mid nanomolar affinities. Binding was partially inhibited with phosphatase treatment or M6P. Subsequent studies revealed that the five N-terminal domains of CI-MPR were sufficient for tPA binding, and this interaction was also partially mediated by M6P. The three glycosylation sites of tPA were analyzed by mass spectrometry, and glycoforms containing M6P and M6P-N-acetylglucosamine were identified at position N448 of tPA. In summary, we found that tPA contains M6P and is a CI-MPR ligand.

Targeting transdifferentiated hepatic stellate cells and monitoring the hepatic fibrogenic process by means of IGF2R-specific peptides designed in silico.

The lack of accurate and easily applicable methods for the diagnosis of liver fibrosis, a disease characterized by an accumulation of the extracellular matrix released by activated hepatic stellate cells (HSCs), has been a major limitation for the clinical management of liver diseases. The identification of biomarkers specific to liver microstructure alterations, combined with a non-invasive optical imaging modality, could guide clinicians towards a therapeutic strategy. In this study, structural information of the insulin-like growth factor 2 receptor (IGF2R), an overexpressed protein on activated HSCs, was used for in silico screening of novel IGF2R-specific peptide ligands. Molecular dynamics simulations, followed by computational alanine scanning of the IGF2R/IGF2 complex, led to the identification of a putative peptide sequence containing the most relevant amino acids for the receptor-ligand interaction (IGF2 E12-C21). The Residue Scan tool, implemented in the MOE software, was then used to optimize the binding affinity of this sequence by amino acid mutations. The designed peptides and their associated scrambled sequences were fluorescently labelled and their binding affinity to LX-2 cells (model for activated human HSCs) was tested using flow cytometry and confocal microscopy. In vitro binding was verified for all sequences (KD ≤ 13.2 µM). With respect to the putative binding sequence, most mutations led to an increased affinity. All sequences have shown superior binding compared to their associated scrambled sequences. Using HPLC, all peptides were tested in vitro for their proteolytic resistance and showed a stability of ≥60% intact after 24 h at 37 °C in 50% v/v FBS. In view of their prospective diagnostic application, a comparison of binding affinity was performed in perpetuated and quiescent-like LX-2 cells. Furthermore, the IGF2R expression for different cell phenotypes was analysed by a quantitative mass spectrometric approach. Our peptides showed increased binding to the perpetuated cell state, indicating their good selectivity for the diagnostically relevant phenotype. In summary, the increased binding affinity of our peptides towards perpetuated LX-2 cells, as well as the satisfactory proteolytic stability, proves that the in silico designed sequences offer a new potential strategy for the targeting of hepatic fibrosis.

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.

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.

Glycan Recognition and Application of P-Type Lectins.

Cation-dependent mannose 6-phosphate receptor (CD-MPR) and cation-independent MPR (CI-MPR) belong to the P-type lectin family. Both intracellular and cell surface MPRs can recognize and bind with the terminal mannose 6-phospahte (M6P) residues of N-glycans attached to the mammalian lysosomal enzymes and the related co-factors. Domain9 (Dom9), which is one of the extracytoplasmic region of the CI-MPR, has relatively higher affinity for M6P residues. Here we describe the production of recombinant Dom9-His protein by Pichia pastris, purification, and application as a probe for lectin blotting.

Mechanism of cargo recognition by retromer-linked SNX-BAR proteins.

Endocytic recycling of internalized transmembrane proteins is essential for many important physiological processes. Recent studies have revealed that retromer-related Sorting Nexin family (SNX)-Bin/Amphiphysin/Rvs (BAR) proteins can directly recognize cargoes like cation-independent mannose 6-phosphate receptor (CI-MPR) and Insulin-like growth factor 1 receptor (IGF1R); however, it remains poorly understood how SNX-BARs select specific cargo proteins and whether they recognize additional ligands. Here, we discovered that the binding between SNX-BARs and CI-MPR or IGF1R is mediated by the phox-homology (PX) domain of SNX5 or SNX6 and a bipartite motif, termed SNX-BAR-binding motif (SBM), in the cargoes. Using this motif, we identified over 70 putative SNX-BAR ligands, many of which play critical roles in apoptosis, cell adhesion, signal transduction, or metabolite homeostasis. Remarkably, SNX-BARs could cooperate with both SNX27 and retromer in the recycling of ligands encompassing the SBM, PDZ-binding motif, or both motifs. Overall, our studies establish that SNX-BARs function as a direct cargo-selecting module for a large set of transmembrane proteins transiting the endosome, in addition to their roles in phospholipid recognition and biogenesis of tubular structures.

Marked structural rearrangement of mannose 6-phosphate/IGF2 receptor at different pH environments.

Many cell surface receptors internalize their ligands and deliver them to endosomes, where the acidic pH causes the ligand to dissociate. The liberated receptor returns to the cell surface in a process called receptor cycling. The structural basis for pH-dependent ligand dissociation is not well understood. In some receptors, the ligand binding domain is composed of multiple repeated sequences. The insulin-like growth factor 2 receptor (IGF2R) contains 15 ß strand-rich repeat domains. The overall structure and the mechanism by which IGF2R binds IGF2 and releases it are unknown. We used cryo-EM to determine the structures of the IGF2R at pH 7.4 with IGF2 bound and at pH 4.5 in the ligand-dissociated state. The results reveal different arrangements of the receptor in different pH environments mediated by changes in the interactions between the repeated sequences. These results have implications for our understanding of ligand release from receptors in endocytic compartments.

Molecular identification of a BAR domain-containing coat complex for endosomal recycling of transmembrane proteins.

Protein trafficking requires coat complexes that couple recognition of sorting motifs in transmembrane cargoes with biogenesis of transport carriers. The mechanisms of cargo transport through the endosomal network are poorly understood. Here, we identify a sorting motif for endosomal recycling of cargoes, including the cation-independent mannose-6-phosphate receptor and semaphorin 4C, by the membrane tubulating BAR domain-containing sorting nexins SNX5 and SNX6. Crystal structures establish that this motif folds into a ß-hairpin, which binds a site in the SNX5/SNX6 phox homology domains. Over sixty cargoes share this motif and require SNX5/SNX6 for their recycling. These include cargoes involved in neuronal migration and a Drosophila snx6 mutant displays defects in axonal guidance. These studies identify a sorting motif and provide molecular insight into an evolutionary conserved coat complex, the 'Endosomal SNX-BAR sorting complex for promoting exit 1' (ESCPE-1), which couples sorting motif recognition to the BAR-domain-mediated biogenesis of cargo-enriched tubulo-vesicular transport carriers.

Topological Requirements for CI-M6PR-Mediated Cell Uptake.

The 300 kDa cation-independent M6P receptor (CI-MPR) mediates ligand internalization and trafficking to the endolysosomal compartments. Because of its endocytotic nature, it has been recognized as a promising class of receptors for target component delivery. Its cellular uptake involves the simultaneous binding of two protein units resulting in the formation of receptor dimers. While many multivalent glycoconjugates have been reported to date, little is known about the topological requests to induce an effective recruitment of CI-MPRs. We herein describe the synthesis and cell uptake ability of a set of highly organized glycoclusters bearing one to three saccharide units. The spatial arrangement of carbohydrate ligands is ensured by a heterocyclic γ-peptide central core.

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.

Takifugu rubripes cation independent mannose 6-phosphate receptor: Cloning, expression and functional characterization of the IGF-II binding domain.

Mannose 6-phosphate/IGF-II receptor mediated lysosomal clearance of insulin-like growth factor-II is significantly associated with the evolution of placental mammals. The protein is also referred to as the IGF-II receptor. Earlier studies suggested relatively low binding affinity between the receptor and ligand in prototherian and metatherian mammals. In the present study, we cloned the IGF-II binding domain of the early vertebrate fugu fish and expressed it in bacteria. A 72000Da truncated receptor containing the IGF-II binding domain was obtained. Analysis of this protein (covering domains 11-13 of the CIMPR) for its affinity to fish and human IGF-II by ligand blot assays and ELISA showed that the expressed receptor can specifically bind to both fish and human IGF-II. Additionally, a peptide-specific antibody raised against the region of the IGF-II binding domain also was able to recognize the IGF-II binding regions of mammalian and non-mammalian cation independent MPR protein. These interactions were further characterized by Surface Plasma resonance support that the receptor binds to fish IGF-II, with a dissociation constant of 548nM. Preliminary analysis suggests that the binding mechanism as well as the affinity of the fish and human receptor for IGF-II may have varied according to different evolutionary pressures.

Development of a peptide-modified siRNA nanocomplex for hepatic stellate cells.

Insulin-like growth factor 2 receptor (IGF2R) is overexpressed in activated hepatic stellate cells (HSCs) and therefore can be utilized for HSC-specific drug delivery. We recently discovered an IGF2R-specific peptide using a novel biopanning. Here, we adopted biotin-conjugated IGF2R-specific peptide, cholesterol, and vitamin A as the targeting ligands for the neutravidin-based siRNA nanocomplex to deliver PCBP2 siRNA, a potentially antifibrotic agent, to HSCs. Compared to vitamin A and cholesterol, the IGF2R-specific peptide exhibited the highest targeting effect to human LX-2 HSC, rat HSC-T6 cell line, and activated primary rat HSCs. Accordingly, the IGF2R-specific peptide coupled nanocomplex demonstrated higher silencing activity of PCBP2 and better inhibition on the migration of activated HSCs. Compared to free siRNA and the nanocomplexes coupled with vitamin A and cholesterol, the IGF2R-specific peptide coupled nanocomplex showed the highest uptake in the liver and lowest uptake in the lung and kidney of the rats with CCl4-induced liver fibrosis.

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.

Sequence-dependent cargo recognition by SNX-BARs mediates retromer-independent transport of CI-MPR.

Endosomal recycling of transmembrane proteins requires sequence-dependent recognition of motifs present within their intracellular cytosolic domains. In this study, we have reexamined the role of retromer in the sequence-dependent endosome-to-trans-Golgi network (TGN) transport of the cation-independent mannose 6-phosphate receptor (CI-MPR). Although the knockdown or knockout of retromer does not perturb CI-MPR transport, the targeting of the retromer-linked sorting nexin (SNX)-Bin, Amphiphysin, and Rvs (BAR) proteins leads to a pronounced defect in CI-MPR endosome-to-TGN transport. The retromer-linked SNX-BAR proteins comprise heterodimeric combinations of SNX1 or SNX2 with SNX5 or SNX6 and serve to regulate the biogenesis of tubular endosomal sorting profiles. We establish that SNX5 and SNX6 associate with the CI-MPR through recognition of a specific WLM endosome-to-TGN sorting motif. From validating the CI-MPR dependency of SNX1/2-SNX5/6 tubular profile formation, we provide a mechanism for coupling sequence-dependent cargo recognition with the biogenesis of tubular profiles required for endosome-to-TGN transport. Therefore, the data presented in this study reappraise retromer's role in CI-MPR transport.

Cargo-selective SNX-BAR proteins mediate retromer trimer independent retrograde transport.

The retromer complex, which recycles the cation-independent mannose 6-phosphate receptor (CI-MPR) from endosomes to the trans-Golgi network (TGN), is thought to consist of a cargo-selective VPS26-VPS29-VPS35 trimer and a membrane-deforming subunit of sorting nexin (SNX)-Bin, Amphyphysin, and Rvs (BAR; SNX-BAR) proteins. In this study, we demonstrate that heterodimers of the SNX-BAR proteins, SNX1, SNX2, SNX5, and SNX6, are the cargo-selective elements that mediate the retrograde transport of CI-MPR from endosomes to the TGN independently of the core retromer trimer. Using quantitative proteomics, we also identify the IGF1R, among more potential cargo, as another SNX5 and SNX6 binding receptor that recycles through SNX-BAR heterodimers, but not via the retromer trimer, in a ligand- and activation-dependent manner. Overall, our data redefine the mechanics of retromer-based sorting and call into question whether retromer indeed functions as a complex of SNX-BAR proteins and the VPS26-VPS29-VPS35 trimer.

General Linker Diversification Approach to Bivalent Ligand Assembly: Generation of an Array of Ligands for the Cation-Independent Mannose 6-Phosphate Receptor.

A generalized strategy is presented for the rapid assembly of a set of bivalent ligands with a variety of linking functionalities from a common monomer. Herein, an array of phosphatase-inert mannose-6-phosphonate-presenting ligands for the cation-independent-mannose 6-phosphate receptor (CI-MPR) is constructed. Receptor binding affinity varies with linking functionality-the simple amide and 1,5-triazole(tetrazole) being preferred over the 1,4-triazole. This approach is expected to find application across chemical biology, particularly in glycoscience, wherein multivalency often governs molecular recognition.

Chlamydia interfere with an interaction between the mannose-6-phosphate receptor and sorting nexins to counteract host restriction.

Chlamydia trachomatis is an obligate intracellular pathogen that resides in a membrane-bound compartment, the inclusion. The bacteria secrete a unique class of proteins, Incs, which insert into the inclusion membrane and modulate the host-bacterium interface. We previously reported that IncE binds specifically to the Sorting Nexin 5 Phox domain (SNX5-PX) and disrupts retromer trafficking. Here, we present the crystal structure of the SNX5-PX:IncE complex, showing IncE bound to a unique and highly conserved hydrophobic groove on SNX5. Mutagenesis of the SNX5-PX:IncE binding surface disrupts a previously unsuspected interaction between SNX5 and the cation-independent mannose-6-phosphate receptor (CI-MPR). Addition of IncE peptide inhibits the interaction of CI-MPR with SNX5. Finally, C. trachomatis infection interferes with the SNX5:CI-MPR interaction, suggesting that IncE and CI-MPR are dependent on the same binding surface on SNX5. Our results provide new insights into retromer assembly and underscore the power of using pathogens to discover disease-related cell biology.

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 ß-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.

Elucidating the Activation Mechanism of the Insulin-Family Proteins with Molecular Dynamics Simulations.

The insulin-family proteins bind to their own receptors, but insulin-like growth factor II (IGF-II) can also bind to the A isoform of the insulin receptor (IR-A), activating unique and alternative signaling pathways from those of insulin. Although extensive studies of insulin have revealed that its activation is associated with the opening of the B chain-C terminal (BC-CT), the activation mechanism of the insulin-like growth factors (IGFs) still remains unknown. Here, we present the first comprehensive study of the insulin-family proteins comparing their activation process and mechanism using molecular dynamics simulations to reveal new insights into their specificity to the insulin receptor. We have found that all the proteins appear to exhibit similar stochastic dynamics in their conformational change to an active state. For the IGFs, our simulations show that activation involves two opening locations: the opening of the BC-CT section away from the core, similar to insulin; and the additional opening of the BC-CT section away from the C domain. Furthermore, we have found that these two openings occur simultaneously in IGF-I, but not in IGF-II, where they can occur independently. This suggests that the BC-CT section and the C domain behave as a unified domain in IGF-I, but as two independent domains in IGF-II during the activation process, implying that the IGFs undergo different activation mechanisms for receptor binding. The probabilities of the active and inactive states of the proteins suggest that IGF-II is hyperactive compared to IGF-I. The hinge residue and the hydrophobic interactions in the core are found to play a critical role in the stability and activity of IGFs. Overall, our simulations have elucidated the crucial differences and similarities in the activation mechanisms of the insulin-family proteins, providing new insights into the molecular mechanisms responsible for the observed differences between IGF-I and IGF-II in receptor binding.

Chemoenzymatic Synthesis and Receptor Binding of Mannose-6-Phosphate (M6P)-Containing Glycoprotein Ligands Reveal Unusual Structural Requirements for M6P Receptor Recognition.

Mannose-6-phosphate (M6P)-terminated oligosaccharides are important signals for M6P-receptor-mediated targeting of newly synthesized hydrolases from Golgi to lysosomes, but the precise structural requirement for the M6P ligand-receptor recognition has not been fully understood due to the difficulties in obtaining homogeneous M6P-containing glycoproteins. We describe here a chemoenzymatic synthesis of homogeneous phosphoglycoproteins carrying natural M6P-containing N-glycans. The method includes the chemical synthesis of glycan oxazolines with varied number and location of the M6P moieties and their transfer to the GlcNAc-protein by an endoglycosynthase to provide homogeneous M6P-containing glycoproteins. Simultaneous attachment of two M6P-oligosaccahrides to a cyclic polypeptide was also accomplished to yield bivalent M6P-glycopeptides. Surface plasmon resonance binding studies reveal that a single M6P moiety located at the low α-1,3-branch of the oligomannose context is sufficient for a high-affinity binding to receptor CI-MPR, while the presence of a M6P moiety at the α-1,6-branch is dispensable. In addition, a binding study with the bivalent cyclic and linear polypeptides reveals that a close proximity of two M6P-oligosaccharide ligands is critical to achieve high affinity for the CI-MPR receptor. Taken together, the present study indicates that the location and valency of the M6P moieties and the right oligosaccharide context are all critical for high-affinity binding with the major M6P receptor. The chemoenzymatic method described here provides a new avenue for glycosylation remodeling of recombinant enzymes to enhance the uptake and delivery of enzymes to lysosomes in enzyme replacement therapy for the treatment of lysosomal storage diseases.