The two mannose 6-phosphate (M6P) receptors were identified because of their ability to bind M6P-containing soluble acid hydrolases in the Golgi and transport them to the endosomal-lysosomal system. During the past decade, we have started to understand the structural features of these receptors that allow them to do this job, and how the receptors themselves are sorted as they pass through various membrane-bound compartments. But trafficking of acid hydrolases is only part of the story. Evidence is emerging that one of the receptors can regulate cell growth and motility, and that it functions as a tumour suppressor.

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Targeting of newly synthesized lysosomal hydrolases to the lysosome is mediated by the cation-dependent mannose 6-phosphate receptor (CD-MPR) and the insulin-like growth factor II/cation-independent mannose 6-phosphate receptor (IGF-II/CI-MPR). The two receptors, which share sequence similarities, constitute the P-type family of animal lectins. We now report the three-dimensional structure of a glycosylation-deficient, yet fully functional form of the extracytoplasmic domain of the bovine CD-MPR (residues 3-154) complexed with mannose 6-phosphate at 1.8 A resolution. The extracytoplasmic domain of the CD-MPR crystallizes as a dimer, and each monomer folds into a nine-stranded flattened beta barrel, which bears a striking resemblance to avidin. The distance of 40 A between the two ligand-binding sites of the dimer provides a structural basis for the observed differences in binding affinity exhibited by the CD-MPR toward various lysosomal enzymes.

The cation-independent mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) undergoes constitutive endocytosis, mediating the internalization of two unrelated classes of ligands, mannose 6-phosphate (Man-6-P)-containing acid hydrolases and insulin-like growth factor II (IGF-II). To determine the role of ligand valency in M6P/IGF-II receptor-mediated endocytosis, we measured the internalization rates of two ligands, ␤-glucuronidase (a homotetramer bearing multiple Man-6-P moieties) and IGF-II. We found that ␤-glucuronidase entered the cell ϳ3-4-fold faster than IGF-II. Unlabeled ␤-glucuronidase stimulated the rate of internalization of 125 I-IGF-II to equal that of 125 I-␤-glucuronidase, but a bivalent synthetic tripeptide capable of occupying both Man-6-P-binding sites on the M6P/IGF-II receptor simultaneously did not. A mutant receptor with one of the two Man-6-P-binding sites inactivated retained the ability to internalize ␤-glucuronidase faster than IGF-II. Thus, the increased rate of internalization required a multivalent ligand and a single Man-6-P-binding site on the receptor. M6P/IGF-II receptor solubilized and purified in Triton X-100 was present as a monomer, but association with ␤-glucuronidase generated a complex composed of two receptors and one ␤-glucuronidase. Neither IGF-II nor the synthetic peptide induced receptor dimerization. These results indicate that intermolecular cross-linking of the M6P/IGF-II receptor occurs upon binding of a multivalent ligand, resulting in an increased rate of internalization.The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF-II receptor) 1 is a type I transmembrane glycoprotein that cycles through the Golgi, endosomes, and the plasma membrane to carry out its role in the biogenesis of lysosomes and in the clearance of the polypeptide insulin-like growth factor II (IGF-II) (1, 2). In the Golgi, the receptor binds newly synthesized acid hydrolases modified with mannose 6-phosphate (Man-6-P) residues on their asparagine-linked oligosaccharides and transports them to endosomes via clathrincoated vesicles (3-5). The acid hydrolases are released in the acidified endosome and then packaged into lysosomes while the receptor either returns to the Golgi to bind another ligand or moves to the plasma membrane (6, 7). At the plasma membrane, the M6P/IGF-II receptor mediates internalization of Man-6-P-containing ligands and IGF-II (3,5,8).The interactions of IGF-II and Man-6-P-containing ligands with the M6P/IGF-II receptor have been characterized in several studies (8 -12). The extracellular portion of the M6P/ IGF-II receptor contains 15 homologous repeating domains of ϳ147 amino acids each (13). Domains 3 and 9 (numbering from the amino terminus) each bind 1 mol of Man-6-P, and the single IGF-II-binding site has been mapped to domain 11 in the extracellular region (14 -16). Man-6-P residues do not inhibit binding of IGF-II to the receptor, verifying that the two ligandbinding sites are distinct. However, proteins containing Man-6...

Background: Acid α-glucosidase, an enzyme replacement therapy for Pompe disease, is poorly targeted to lysosomes when relying on phosphomannose residues.Results: Fusing IGF-II to acid α-glucosidase resulted in more efficient uptake and glycogen clearance from muscle of Pompe mice.Conclusion: Enhanced binding to the cation-independent mannose 6-phosphate receptor (CI-MPR) enabled improved glycogen clearance in Pompe mice.Significance: BMN 701 is now being tested for Pompe disease in human clinical studies.

Fabry disease, the most common lysosomal storage disease, affects multiple organs and results in a shortened life span. This disease is caused by a deficiency of the lysosomal enzyme α-galactosidase A, which leads to glycosphingolipid accumulation in many cell types. Neuropathic pain is an early and severely debilitating symptom in patients with Fabry disease, but the cellular and molecular mechanisms that cause the pain are unknown. We generated a rat model of Fabry disease, the first nonmouse model to our knowledge. Fabry rats had substantial serum and tissue accumulation of α-galactosyl glycosphingolipids and had pronounced mechanical pain behavior. Additionally, Fabry rat dorsal root ganglia displayed global N-glycan alterations, sensory neurons were laden with inclusions, and sensory neuron somata exhibited prominent sensitization to mechanical force. We found that the cation channel transient receptor potential ankyrin 1 (TRPA1) is sensitized in Fabry rat sensory neurons and that TRPA1 antagonism reversed the behavioral mechanical sensitization. This study points toward TRPA1 as a potentially novel target to treat the pain experienced by patients with Fabry disease.

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.

The 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR) mediates the intracellular transport of newly synthesized lysosomal enzymes containing mannose 6-phosphate on their N-linked oligosaccharides. In addition to its role in lysosome biogenesis, the CI-MPR interacts with a number of different extracellular ligands at the cell surface, including latent transforming growth factor-b, insulin-like growth factor-II, plasminogen, and urokinase-type plasminogen activator receptor (uPAR), to regulate cell growth and motility. We have solved the crystal structure of the N-terminal 432 residues of the CI-MPR at 1.8 Å resolution, which encompass three out of the 15 repetitive domains of its extracytoplasmic region. The three domains, which exhibit similar topology to each other and to the 46 kDa cation-dependent mannose 6-phosphate receptor, assemble into a compact structure with the uPAR/plasminogen and the carbohydrate-binding sites situated on opposite faces of the molecule. Knowledge of the arrangement of these three domains has allowed us to propose a model of the entire extracytoplasmic region of the CI-MPR that provides a context with which to envision the numerous binding interactions carried out by this multi-faceted receptor.