Real time kinetics of insulin-like growth factor II (IGF-II) interaction with the IGF-II/mannose 6-phosphate receptor: the effects of domain 13 and pH

Two Distinct Lysosomal Targeting Strategies Afford Trojan Horse Antibodies With Pan-Filovirus Activity

Multiple agents in the family Filoviridae (filoviruses) are associated with sporadic human outbreaks of highly lethal disease, while others, including several recently identified agents, possess strong zoonotic potential. Although viral glycoprotein (GP)-specific monoclonal antibodies have demonstrated therapeutic utility against filovirus disease, currently FDA-approved molecules lack antiviral breadth. The development of broadly neutralizing antibodies has been challenged by the high sequence divergence among filovirus GPs and the complex GP proteolytic cleavage cascade that accompanies filovirus entry. Despite this variability in the antigenic surface of GP, all filoviruses share a site of vulnerability-the binding site for the universal filovirus entry receptor, Niemann-Pick C1 (NPC1). Unfortunately, this site is shielded in extracellular GP and only uncovered by proteolytic cleavage by host proteases in late endosomes and lysosomes, which are generally inaccessible to antibodies. To overcome this obstacle, we previously developed a 'Trojan horse' therapeutic approach in which engineered bispecific antibodies (bsAbs) coopt viral particles to deliver GP:NPC1 interaction-blocking antibodies to their endo/lysosomal sites of action. This approach afforded broad protection against members of the genus Ebolavirus but could not neutralize more divergent filoviruses. Here, we describe next-generation Trojan horse bsAbs that target the endo/lysosomal GP:NPC1 interface with pan-filovirus breadth by exploiting the conserved and widely expressed host cation-independent mannose-6-phosphate receptor for intracellular delivery. Our work highlights a new avenue for the development of single therapeutics protecting against all known and newly emerging filoviruses.

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.

A radioligand binding assay for the insulin-like growth factor 2 receptor

Insulin-like growth factors 2 and 1 (IGF2 and IGF1) and insulin are closely related hormones that are responsible for the regulation of metabolic homeostasis, development and growth of the organism. Physiological functions of insulin and IGF1 are relatively well-studied, but information about the role of IGF2 in the body is still sparse. Recent discoveries called attention to emerging functions of IGF2 in the brain, where it could be involved in processes of learning and memory consolidation. It was also proposed that these functions could be mediated by the receptor for IGF2 (IGF2R). Nevertheless, little is known about the mechanism of signal transduction through this receptor. Here we produced His-tagged domain 11 (D11), an IGF2-binding element of IGF2R; we immobilized it on the solid support through a well-defined sandwich, consisting of neutravidin, biotin and synthetic anti-His-tag antibodies. Next, we prepared specifically radiolabeled [125I]-monoiodotyrosyl-Tyr2-IGF2 and optimized a sensitive and robust competitive radioligand binding assay for determination of the nanomolar binding affinities of hormones for D11 of IGF2. The assay will be helpful for the characterization of new IGF2 mutants to study the functions of IGF2R and the development of new compounds for the treatment of neurological disorders.

Diabetes and hepatocellular carcinoma: A pathophysiological link and pharmacological management

Both diabetes mellitus (DM) and cancer are multifarious, dissimilar, and long-lasting, fatal diseases with a remarkable influence on health worldwide. DM is not only related to cardiovascular diseases, neuropathy, nephropathy, and retinopathy, but also related to a number of liver diseases such as nonalcoholic fatty liver disease, steatohepatitis, and liver cirrhosis. Recently, it is hypothesized that DM has a greater risk for many forms of cancer, such as breast, colorectal, endometrial, pancreatic, gallbladder, renal, and liver cancer including hepatocellular carcinoma (HCC). Both DM and cancer have many common risk factors, but the association between these two is poorly stated. Several epidemiologic studies have revealed the association between pathogenic and prognostic characteristics of DM and a higher incidence of HCC, thus representing DM as an independent risk factor for HCC development. The etiological and pathophysiological relationship between DM and HCC has been presented in this review by linking hyperglycemia, hyperinsulinemia, insulin resistance, and activation of insulin-like growth factor signaling pathways and pharmacological management of HCC associated with DM.

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.

Functional evolution of IGF2:IGF2R domain 11 binding generates novel structural interactions and a specific IGF2 antagonist

Among the 15 extracellular domains of the mannose 6-phosphate/insulin-like growth factor-2 receptor (M6P/IGF2R), domain 11 has evolved a binding site for IGF2 to negatively regulate ligand bioavailability and mammalian growth. Despite the highly evolved structural loops of the IGF2:domain 11 binding site, affinity-enhancing AB loop mutations suggest that binding is modifiable. Here we examine the extent to which IGF2:domain 11 affinity, and its specificity over IGF1, can be enhanced, and we examine the structural basis of the mechanistic and functional consequences. Domain 11 binding loop mutants were selected by yeast surface display combined with high-resolution structure-based predictions, and validated by surface plasmon resonance. We discovered previously unidentified mutations in the ligand-interacting surface binding loops (AB, CD, FG, and HI). Five combined mutations increased rigidity of the AB loop, as confirmed by NMR. When added to three independently identified CD and FG loop mutations that reduced the koff value by twofold, these mutations resulted in an overall selective 100-fold improvement in affinity. The structural basis of the evolved affinity was improved shape complementarity established by interloop (AB-CD) and intraloop (FG-FG) side chain interactions. The high affinity of the combinatorial domain 11 Fc fusion proteins functioned as ligand-soluble antagonists or traps that depleted pathological IGF2 isoforms from serum and abrogated IGF2-dependent signaling in vivo. An evolved and reengineered high-specificity M6P/IGF2R domain 11 binding site for IGF2 may improve therapeutic targeting of the frequent IGF2 gain of function observed in human cancer.

Different Pathways to the Lysosome: Sorting out Alternatives

Considerable research supports a model in which hydrolytic enzymes of mammalian lysosomes are sorted to their destinations in a receptor-dependent mechanism. The ligand for the mammalian sorting receptors is mannose 6-phosphate (M6P). Two M6P receptors have been defined in mammals. Here, we review the foundational evidence supporting this mechanism and highlight the remaining gaps in our understanding of the mammalian mechanism, including evidence for M6P-independent sorting, and its relevance to lysosomal enzyme sorting in metazoa.

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.

Targeting tumor micro-environment for design and development of novel anti-angiogenic agents arresting tumor growth

Angiogenesis: a process of generation of new blood vessels has been proved to be necessary for sustained tumor growth and cancer progression. Inhibiting angiogenesis pathway has long been remained a significant hope for the development of novel, effective and target orientated antitumor agents arresting the tumor proliferation and metastasis. The process of neoangiogenesis as a biological process is regulated by several pro- and anti-angiogenic factors, especially vascular endothelial growth factor, fibroblast growth factor, epidermal growth factor, hypoxia inducible factor 1 and transforming growth factor. Every endothelial cell destined for vessel formation is equipped with receptors for these angiogenic peptides. Moreover, numerous other angiogenic cytokines such as platelet derived growth factor (PGDF), placenta growth factor (PGF), nerve growth factor (NGF), stem-cell factor (SCF), and interleukins-2, 4, 6 etc. These molecular players performs critical role in regulating the angiogenic switch. Couple of decade's research in molecular aspects of tumor biology has unraveled numerous structural and functional mysteries of these angiogenic peptides. In present article, a detailed update on the functional and structural peculiarities of the various angiogenic peptides is described focusing on structural opportunities made available that has potential to be used to modulate function of these angiogenic peptides in developing therapeutic agents targeting neoplastic angiogenesis. The data may be useful in the mainstream of developing novel anticancer agents targeting tumor angiogenesis. We also discuss major therapeutic agents that are currently used in angiogenesis associated therapies as well as those are subject of active research or are in clinical trials.

The mannose 6-phosphate-binding sites of M6P/IGF2R determine its capacity to suppress matrix invasion by squamous cell carcinoma cells

The M6P (mannose 6-phosphate)/IGF2R (insulin-like growth factor II receptor) interacts with a variety of factors that impinge on tumour invasion and metastasis. It has been shown that expression of wild-type M6P/IGF2R reduces the tumorigenic and invasive properties of receptor-deficient SCC-VII squamous cell carcinoma cells. We have now used mutant forms of M6P/IGF2R to assess the relevance of the different ligand-binding sites of the receptor for its biological activities in this cellular system. The results of the present study demonstrate that M6P/IGF2R does not require a functional binding site for insulin-like growth factor II for inhibition of anchorage-independent growth and matrix invasion by SCC-VII cells. In contrast, the simultaneous mutation of both M6P-binding sites is sufficient to impair all cellular functions of the receptor tested. These findings highlight that the interaction between M6P/IGF2R and M6P-modified ligands is not only important for intracellular accumulation of lysosomal enzymes and formation of dense lysosomes, but is also crucial for the ability of the receptor to suppress SCC-VII growth and invasion. The present study also shows that some of the biological activities of M6P/IGF2R in SCC-VII cells strongly depend on a functional M6P-binding site within domain 3, thus providing further evidence for the non-redundant cellular functions of the individual carbohydrate-binding domains of the receptor.

Glycosylation-independent lysosomal targeting of acid α-glucosidase enhances muscle glycogen clearance in pompe mice

We have used a peptide-based targeting system to improve lysosomal delivery of acid α-glucosidase (GAA), the enzyme deficient in patients with Pompe disease. Human GAA was fused to the glycosylation-independent lysosomal targeting (GILT) tag, which contains a portion of insulin-like growth factor II, to create an active, chimeric enzyme with high affinity for the cation-independent mannose 6-phosphate receptor. GILT-tagged GAA was taken up by L6 myoblasts about 25-fold more efficiently than was recombinant human GAA (rhGAA). Once delivered to the lysosome, the mature form of GILT-tagged GAA was indistinguishable from rhGAA and persisted with a half-life indistinguishable from rhGAA. GILT-tagged GAA was significantly more effective than rhGAA in clearing glycogen from numerous skeletal muscle tissues in the Pompe mouse model. The GILT-tagged GAA enzyme may provide an improved enzyme replacement therapy for Pompe disease patients.

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.

Circulating levels of insulin-like growth factor-II/mannose-6-phosphate receptor in obesity and type 2 diabetes

OBJECTIVE

The extracellular domain of the insulin-like growth factor II/mannose-6-phosphate receptor (IGF-II/M6P-R) is present in the circulation, but its relationship with plasma IGF-II is largely unknown. As IGF-II appears to be nutritionally regulated, we studied the impact of obesity, type 2 diabetes (T2D) and weight loss on circulating levels of IGF-II and its soluble receptor.

METHODS

Twenty-three morbidly obese non-diabetic subjects were studied before and after gastric banding (GB), reducing their BMI from 59.3+/-1.8 to 52.7+/-1.6 kg/m(2). Lean controls (n=10, BMI 24.2+/-0.5 kg/m(2)), moderately obese controls (n=21, BMI 31.8+/-1.0 kg/m(2)) and obese T2D patients (n=20, BMI 32.3+/-0.8 kg/m(2)) were studied before and after a hyperinsulinaemic euglycaemic clamp.

RESULTS

Morbidly obese subjects had elevated IGF-II/M6P-R and IGF-II levels, which both decreased following GB (IGF-II/M6P-R: from 0.97+/-0.038 to 0.87+/-0.030 nmol/l, P=0.001; IGF-II: from 134+/-7 to 125+/-6 nmol/l, P=0.01), as did fasting plasma glucose and insulin (P<0.05). However, the metabolic parameters correlated with neither IGF-II nor IGF-II/M6P-R. Obese diabetics had increased IGF-II/M6P-R as compared with lean and obese controls (0.82+/-0.031 vs. 0.70+/-0.033 vs. 0.74+/-0.026 nmol/l; P<0.03) and levels were unaffected by clamp. In the latter cohort, IGF-II/M6P-R but not IGF-II correlated with HbA1c, and fasting plasma C-peptide, insulin and glucose (0.34<r<0.45; P<0.05). In all subjects, BMI correlated with IGF-II/M6P-R (r=0.57; P<0.001) and IGF-II (r=0.39; P<0.005). IGF-II/M6P-R and IGF-II were not associated.

CONCLUSION

Serum IGF-II/M6P-R is up-regulated in morbid obesity, down-regulated by weight loss and elevated in moderately obese T2D. However, although plasma IGF-II was also reduced following GB, the two peptides were not statistically correlated. No acute effect of insulin was seen. These findings indicate that the IGF-II/M6P-R is nutritionally regulated, independently of IGF-II.

Residues essential for plasminogen binding by the cation-independent mannose 6-phosphate receptor

The 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR) is a multifunctional protein that binds diverse intracellular and extracellular ligands with high affinity. The CI-MPR is a receptor for plasminogen, and this interaction can be inhibited by lysine analogues. To characterize the molecular basis for this interaction, surface plasmon resonance (SPR) analyses were performed using truncated forms of the CI-MPR and plasminogen. The results show that the N-terminal region of the CI-MPR containing domains 1 and 2, but not domain 1 alone, of the receptor's 15-domain extracytoplasmic region binds plasminogen (K(d) = 5 +/- 1 nM) with an affinity similar to that of the full-length receptor (K(d) = 20 +/- 6 nM). In addition to its C-terminal serine protease domain, plasminogen contains lysine binding sites (LBS), which are located within each of its five kringle domains, except kringle 3. We show that kringles 1-4, but not kringles 1-3, bind the CI-MPR, indicating an essential role for the LBS in kringle 4 of plasminogen. To identify the lysine residue(s) of the CI-MPR that serve(s) as an essential determinant for recognition by the LBS of plasminogen, site-directed mutagenesis studies were carried out using a construct encoding the N-terminal three domains of the CI-MPR (Dom1-3His) which contains both a mannose 6-phosphate (Man-6-P) and plasminogen binding site. The results demonstrate two lysine residues (Lys53 located in domain 1 and Lys125 located in the loop connecting domains 1 and 2) of the CI-MPR are key determinants for plasminogen binding but are not required for Man-6-P binding.

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.

Mannose-6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) in carcinogenesis

The cation-independent mannose-6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) is a multifunctional receptor. It is involved in a variety of cellular processes which become dysregulated in cancer. Its tumor suppressor role was recognized a long time ago. However, due to its multifunctionality, it is not easy to understand the extent of its relevance to normal cellular physiology. Accordingly, it is even more difficult understanding its role in carcinogenesis. This review presents critical and focused highlights of data relating to M6P/IGF2R, obtained during more than 25 years of cancer research.

Interactions of IGF-II with the IGF2R/cation-independent mannose-6-phosphate receptor mechanism and biological outcomes

The cation-independent mannose-6-phosphate/insulin-like growth factor-II receptor (IGF2R) is a membrane-bound glycoprotein consisting of 15 homologous extracellular repeat domains. The major function of this receptor is trafficking of lysosomal enzymes from the trans-Golgi network to the endosomes and their subsequent transfer to lysosomes. The IGF2R also plays a major role in binding and regulating the circulating and tissue levels of IGF-II. As this ligand is important for cell growth, survival, and migration, the maintenance of correct IGF-II levels influences its actions in normal growth and development. Deregulation of IGF2R expression has therefore been associated with growth related disease and cancer. This review highlights recent advances in understanding the IGF2R structure and mechanism of interaction with its ligands, in particular IGF-II. Recent mutagenesis studies combined with the crystal structure of domains 11-14 in complex with IGF-II have mapped the sites of interaction and explain how the IGF2R specificity for IGF-II is achieved. The role of domain 13 in high-affinity IGF-II binding is also revealed. Characterization of ligand:IGF2R interactions is vital for the understanding of the mechanism of IGF2R actions and will allow the development of specific cancer therapies in the future.

Structure and function of the human Gly1619Arg polymorphism of M6P/IGF2R domain 11 implicated in IGF2 dependent growth

The mannose 6-phosphate/IGF 2 receptor (IGF2R) is comprised of 15 extra-cellular domains that bind IGF2 and mannose 6-phosphate ligands. IGF2R transports ligands from the Golgi to the pre-lysosomal compartment and thereafter to and from the cell surface. IGF2R regulates growth, placental development, tumour suppression and signalling. The ligand IGF2 is implicated in the growth phenotype, where IGF2R normally limits bioavailability, such that loss and gain of IGF2R results in increased and reduced growth respectively. The IGF2R exon 34 (5002A>G) polymorphism (rs629849) of the IGF2 specific binding domain has been correlated with impaired childhood growth (A/A homozygotes). We evaluated the function of the Gly1619Arg non-synonymous amino acid modification of domain 11. NMR and X-ray crystallography structures located 1619 remote from the ligand binding region of domain 11. Arg1619 was located close to the fibronectin type II (FnII) domain of domain 13, previously implicated as a modifier of IGF2 ligand binding through indirect interaction with the AB loop of the binding cleft. However, comparison of binding kinetics of IGF2R, Gly1619 and Arg1619 to either IGF2 or mannose 6-phosphate revealed no differences in 'on' and 'off' rates. Quantitative PCR, (35)S pulse chase and flow cytometry failed to demonstrate altered gene expression, protein half-life and cell membrane distribution, suggesting the polymorphism had no direct effect on receptor function. Intronic polymorphisms were identified which may be in linkage disequilibrium with rs629849 in certain populations. Other potential IGF2R polymorphisms may account for the correlation with childhood growth, warranting further functional evaluation.

High-affinity ligand binding by wild-type/mutant heteromeric complexes of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor has diverse ligand-binding properties contributing to its roles in lysosome biogenesis and growth suppression. Optimal receptor binding and internalization of mannose 6-phosphate (Man-6-P)-bearing ligands requires a dimeric structure leading to bivalent high-affinity binding, presumably mediated by cooperation between sites on both subunits. Insulin-like growth factor II (IGF-II) binds to a single site on each monomer. It is hypothesized that IGF-II binding to cognate sites on each monomer occurs independently, but bivalent Man-6-P ligand binding requires cooperative contributions from sites on both monomers. To test this hypothesis, we co-immunoprecipitated differentially epitope-tagged soluble mini-receptors and assessed ligand binding. Pairing of wild-type and point-mutated IGF-II binding sites between two dimerized mini-receptors had no effect on the function of the contralateral binding site, indicating IGF-II binding to each side of the dimer is independent and manifests no intersubunit effects. As expected, heterodimeric receptors composed of a wild-type monomer and a mutant bearing two Man-6-P-binding knockout mutations form functional IGF-II binding sites. By contrast to prediction, such heterodimeric receptors also bind Man-6-P-based ligands with high affinity, and the amount of binding can be attributed entirely to the immunoprecipitated wild-type receptors. Anchoring of both C-terminal ends of the heterodimer produces optimal binding of both IGF-II and Man-6-P ligands. Thus, IGF-II binds independently to both subunits of the dimeric mannose 6-phosphate/insulin-like growth factor II receptor. Although wild-type/mutant hetero-oligomers form readily when mixed, it appears that multivalent Man-6-P ligands bind preferentially to wild-type sites, possibly by cross-bridging receptors within clusters of immobilized receptors.

Isolated compared to membrane-bound receptors exhibit altered insulin/IGF interaction

Insulin and insulin-like growth factors (IGFs) bind to their cognate receptors with high affinities, but due to their homology they may cross-react with each other's receptors. We performed a series of binding studies to reanalyze the cross-reactivity of insulin, IGF-I, and IGF-II to affinity-purified insulin (IR) and type 2 IGF receptors (IGF-2R) from human placental membranes. IR and IGF-2R were purified using insulin- and mannose-6-phosphate affinity chromatography (I-AC and M6P-AC). Binding studies were performed with (125)I-labeled and unlabeled ligands. According to immunoblotting, the only receptor species isolated by I-AC was IR, whereas the only receptor isolated by M6P-AC was IGF-2R. Isolated IR reacted to similar extent with (125)I-labeled insulin and (125)I-labeled IGF-II and significantly less with (125)I-labeled IGF-I, implicating predominance of IR-A. The affinity of IR towards heterologous ligands increased after its separation from other membrane proteins. Affinity-purified IGF-2R was almost unable to bind ligands under experimental conditions used in this work, but when incubated with (125)I-labeled ligands prior to affinity chromatography, IGF-2R interacted not only with IGF-II, but to a certain extent with the other two ligands. In the competitive M6P-AC, the binding of labeled ligands was inhibited with either homologous or heterologous ligands, in a dose dependent manner. In competitive ligand-blotting, specific interactions between (125)I-labeled insulin and IR, and (125)I-labeled IGF-II and IGF-2R were also inhibited with all unlabeled ligands, although to a different extent. The results presented in this work imply that isolation of IR an IGF-2R from their membrane milieu increases their reactivity towards all members of the insulin/IGF ligand family.

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.

Proteomics Discovery of Disease Biomarkers

Recent technological developments in proteomics have shown promising initiatives in identifying novel biomarkers of various diseases. Such technologies are capable of investigating multiple samples and generating large amount of data end-points. Examples of two promising proteomics technologies are mass spectrometry, including an instrument based on surface enhanced laser desorption/ionization, and protein microarrays. Proteomics data must, however, undergo analytical processing using bioinformatics. Due to limitations in proteomics tools including shortcomings in bioinformatics analysis, predictive bioinformatics can be utilized as an alternative strategy prior to performing elaborate, high-throughput proteomics procedures. This review describes mass spectrometry, protein microarrays, and bioinformatics and their roles in biomarker discovery, and highlights the significance of integration between proteomics and bioinformatics.

Heterotrimeric G proteins and the single-transmembrane domain IGF-II/M6P receptor: functional interaction and relevance to cell signaling

The G protein-coupled receptor (GPCR) family represents the largest and most versatile group of cell surface receptors. Classical GPCR signaling constitutes ligand binding to a seven-transmembrane domain receptor, receptor interaction with a heterotrimeric G protein, and the subsequent activation or inhibition of downstream intracellular effectors to mediate a cellular response. However, recent reports on direct, receptor-independent G protein activation, G protein-independent signaling by GPCRs, and signaling of nonheptahelical receptors via trimeric G proteins have highlighted the intrinsic complexities of G protein signaling mechanisms. The insulin-like growth factor-II/mannose-6 phosphate (IGF-II/M6P) receptor is a single-transmembrane glycoprotein whose principal function is the intracellular transport of lysosomal enzymes. In addition, the receptor also mediates some biological effects in response to IGF-II binding in both neuronal and nonneuronal systems. Multidisciplinary efforts to elucidate the intracellular signaling pathways that underlie these effects have generated data to suggest that the IGF-II/M6P receptor might mediate transmembrane signaling via a G protein-coupled mechanism. The purpose of this review is to outline the characteristics of traditional and nontraditional GPCRs, to relate the IGF-II/M6P receptor's structure with its role in G protein-coupled signaling and to summarize evidence gathered over the years regarding the putative signaling of the IGF-II/M6P receptor mediated by a G protein.

Structure and functional analysis of the IGF-II/IGF2R interaction

Embryonic development and normal growth require exquisite control of insulin-like growth factors (IGFs). In mammals the extracellular region of the cation-independent mannose-6-phosphate receptor has gained an IGF-II-binding function and is termed type II IGF receptor (IGF2R). IGF2R sequesters IGF-II; imbalances occur in cancers and IGF2R is implicated in tumour suppression. We report crystal structures of IGF2R domains 11-12, 11-12-13-14 and domains 11-12-13/IGF-II complex. A distinctive juxtaposition of these domains provides the IGF-II-binding unit, with domain 11 directly interacting with IGF-II and domain 13 modulating binding site flexibility. Our complex shows that Phe19 and Leu53 of IGF-II lock into a hydrophobic pocket unique to domain 11 of mammalian IGF2Rs. Mutagenesis analyses confirm this IGF-II 'binding-hotspot', revealing that IGF-binding proteins and IGF2R have converged on the same high-affinity site.

Structural insights into the interaction of insulin-like growth factor 2 with IGF2R domain 11

The insulin-like growth factor II/mannose-6-phosphate receptor (IGF2R) mediates trafficking of mannose-6-phosphate (M6P)-containing proteins and the mitogenic hormone IGF2. IGF2R also plays an important role as a tumor suppressor, as mutation is frequently associated with human carcinogenesis. IGF2 binds to domain 11, one of 15 extracellular domains on IGF2R. The crystal structure of domain 11 and the solution structure of IGF2 have been reported, but, to date, there has been limited success when using crystallography to study the interaction of IGFs with their binding partners. As an approach to investigate the interaction between IGF2 and IGF2R, we have used heteronuclear NMR in combination with existing mutagenesis data to derive models of the domain 11-IGF2 complex by using the program HADDOCK. The models reveal that the molecular interaction is driven by critical hydrophobic residues on IGF2 and IGF2R, while a ring of flexible, charged residues on IGF2R may modulate binding.

A Novel Binding Site for the Human Insulin-like Growth Factor-II (IGF-II)/Mannose 6-Phosphate Receptor on IGF-II

The mammalian insulin-like growth factor (IGF)-II/cation-independent mannose 6-phosphate receptor (IGF2R) binds IGF-II with high affinity. By targeting IGF-II to lysosomal degradation, it plays a role in the maintenance of correct IGF-II levels in the circulation and in target tissues. Loss of IGF2R function is associated with tumor progression; therefore, the IGF2R is often referred to as a tumor suppressor. The interaction between IGF2R and IGF-II involves domains 11 and 13 of the 15 extracellular domains of the receptor. Recently, a hydrophobic binding region was identified on domain 11 of the IGF2R. In contrast, relatively little is known about the residues of IGF-II that are involved in IGF2R binding and the determinants of IGF2R specificity for IGF-II over the structurally related IGF-I. Using a series of novel IGF-II analogues and surface plasmon resonance assays, this study revealed a novel binding surface on IGF-II critical for IGF2R binding. The hydrophobic residues Phe(19) and Leu(53) are critical for IGF2R binding, as are residues Thr(16) and Asp(52). Furthermore, Thr(16) was identified as playing a major role in determining why IGF-II, but not IGF-I, binds with high affinity to the IGF2R.

Functional evaluation of novel soluble insulin-like growth factor (IGF)-II–specific ligand traps based on modified domain 11 of the human IGF2 receptor

Ligands transported by the mannose 6-phosphate/insulin-like growth factor (IGF)-II receptor (IGF2R) include IGF-II- and mannose 6-phosphate-modified proteins. Increased extracellular supply of IGF-II, either secondary to loss of the clearance function of IGF2R, loss of IGF binding protein function, or increased IGF2 gene expression, can lead to embryonic overgrowth and cancer promotion. Reduced supply of IGF-II is detrimental to tumor growth, and this suggests that gain of function of IGF-II is a molecular target for human cancer therapy. Domain 11 of IGF2R binds IGF-II with high specificity and affinity. Mutagenesis studies have shown that substitution of glutamic acid for lysine at residue 1554 results in a 6-fold higher affinity for IGF-II (20.5 nmol/L) than native domain 11 (119 nmol/L). Here, we generate a novel high-affinity IGF-II ligand trap by fusion of mutated human 11(E1554K) to a COOH-terminal human IgG1 Fc domain (11(E1554K)-Fc). The resulting homodimer has a significantly increased affinity for IGF-II (1.79 nmol/L) when measured by surface plasmon resonance. IGF-II signaling via the IGF-I receptor and the proliferative effect of IGF-II were specifically inhibited by 11(E1554K)-Fc in both HaCaT and Igf2(-/-) mouse embryonic fibroblast cells. These data confirm that a novel engineered and soluble IGF2R-11(E1554K)-Fc protein functions as an IGF-II-specific and high-affinity ligand trap in vitro and that this protein has potential application as an IGF-II antagonist for cancer therapy following in vivo experimental evaluation.

Mannose 6-phosphate receptor (MPR 300) proteins from goat and chicken bind human IGF-II

Mannose 6-phosphate receptor proteins (MPR 300 and 46) in mammals have been shown to mediate transport of lysosomal enzymes to lysosomes intracellularly. Both receptors are also expressed on the plasma membrane. Only MPR 300 protein on the plasma membrane has been shown to be a multifunctional protein which in addition to binding mannose 6-phosphate containing proteins also binds human insulin-like growth factor-II (IGF-II) causing its internalization [Hille-Rehfeld, A. (1995) Mannose 6-phosphate receptors in sorting and transport of lysosomal enzymes. Biochim. Biophys. Acta. 1241: 177-194]. This property has been shown to be exhibited by other mammalian receptors but not by the chicken and frog receptors. In a recent study however it was shown that the fish embryo MPR 300 binds human IGF-II. [Mendez, E., Planas, J.V., Castillo, J., Navarro, I. and Gutierrez, J. (2001) Identification of a type II insulin-like growth factor receptor in fish embryos. Endocrinology, 142: 1090-1097]. In the present study, we demonstrate that the purified goat and chicken liver receptors bind human IGF-II by employing cross-linking experiments (purified receptors and radiolabeled IGF-II) and by ligand blotting (using purified receptors and biotinylated IGF-II). Further CEF cells (chicken embryonic fibroblasts) that are known to contain the putative MPR 300 protein were employed to demonstrate that the CEF cell receptor binds human IGF-II.

Kinetics of Insulin-like Growth Factor II (IGF-II) Interaction with Domain 11 of the Human IGF-II/Mannose 6-phosphate Receptor: Function of CD and AB Loop Solvent-exposed Residues

Ligands of the IGF-II/mannose 6-phosphate receptor (IGF2R) include IGF-II and mannose 6-phosphate modified proteins. Disruption of the negative regulatory effects of IGF2R on IGF-II-induced growth can lead to embryonic lethality and cancer promotion. Of the 15 IGF2R extracellular domains, domains 1-3 and 11 are known to have a conserved beta-barrel structure similar to that of avidin and the cation-dependent mannose 6-phosphate receptor, yet only domain 11 binds IGF-II with high specificity and affinity. In order to define the functional basis of this critical biological interaction, we performed alanine mutagenesis of structurally determined solvent-exposed loop residues of the IGF-II-binding site of human domain 11, expressed these mutant forms in Pichia pastoris, and determined binding kinetics with human IGF-II using isothermal calorimetry and surface plasmon resonance with transition state thermodynamics. Two hydrophobic residues in the CD loop (F1567 and I1572) were essential for binding, with a further non-hydrophobic residue (T1570) that slows the dissociation rate. Aside from alanine mutations of AB loop residues that decrease affinity by modifying dissociation rates (e.g. Y1542), a novel mutation (E1544A) of the AB loop enhanced affinity by threefold compared to wild-type. Conversion from an acidic to a basic residue at this site (E1544K) results in a sixfold enhancement of affinity via modification principally of the association rate, with enhanced salt-dependence, decreased entropic barrier and retained specificity. These data suggest that a functional hydrophobic binding site core is formed by I1572 and F1567 located in the CD loop, which initially anchors IGF-II. Within the AB loop, residues normally act to either stabilise or function as negative regulators of the interaction. These findings have implications for the molecular architecture and evolution of the domain 11 IGF-II-binding site, and the potential interactions with other domains of IGF2R.

Computational model for the IGF-II/IGF2r complex that is predictive of mutational and surface plasmon resonance data

Insulin-like growth factors (IGFs) are key regulators of cell proliferation, differentiation, and transformation, and are thus pivotal in cancer, especially breast, prostate, and colon neoplasm. Their potent mitogenic and anti-apoptotic actions depend primarily on their availability to bind to the signaling IGF cell surface receptors. One mechanism by which IGF-II availability is thought to be modulated is by binding to the nonsignaling IGF-II receptor (IGF2R). This binding is essentially mediated by domain 11 in the multidomain IGF2R extracellular region. The crystal structure of domain 11 of the human IGF-II receptor (IGF2R-d11) has identified a putative IGF-II binding site, and a nuclear magnetic resonance (NMR) solution structure for the IGF-II ligand has also been characterized. These structures have now been used to model in silico the protein-protein interaction between IGF-II and IGF2R-d11 using the program 3D-Dock. Because the IGF-II data comprise an ensemble of 20 structures, all of which satisfy the NMR constraints, the docking procedure was applied to each member of the ensemble. Only those models in which residue Ile1572 of IGF2R-d11, known to be essential for the binding of IGF-II, was at the interface were considered further. These plausible complexes were then critically assessed using an array of analysis techniques including consideration of additional mutagenesis data. One model was strongly supported by these analyses and is discussed here in detail. Furthermore, we demonstrate in vitro experimental support for this model by studying the binding of chimeras of IGF-I and IGF-II to IGF2R fragments.

Domain interactions of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) forms oligomeric structures important for optimal function in binding and internalization of Man-6-P-bearing extracellular ligands as well as lysosomal biogenesis and growth regulation. However, neither the mechanism of inter-receptor interaction nor the dimerization domain has yet been identified. We hypothesized that areas near the ligand binding domains of the receptor would contribute preferentially to oligomerization. Two panels of minireceptors were constructed that involved truncations of either the N- or C-terminal regions of the M6P/IGF2R encompassing deletions of various ligand binding domains. alpha-FLAG or alpha-Myc-based immunoprecipitation assays showed that all of the minireceptors tested were able to associate with a full-length, Myc-tagged M6P/IGF2R (WT-M). In the alpha-FLAG but not alpha-Myc immunoprecipitation assays, the degree of association of a series of C-terminally truncated minireceptors with WT-M showed a positive trend with length of the minireceptor. In contrast, length did not seem to affect the association of the N-terminally truncated minireceptors with WT-M, except that the 12th extracytoplasmic repeat appeared exceptionally important in dimerization in the alpha-FLAG assays. The presence of mutations in the ligand-binding sites of the minireceptors had no effect on their ability to associate with WT-M. Thus, association within the heterodimers was not dependent on the presence of functional ligand binding domains. Heterodimers formed between WT-M and the minireceptors demonstrated high affinity IGF-II and Man-6-P-ligand binding, suggesting a functional association. We conclude that there is no finite M6P/IGF2R dimerization domain, but rather that interactions between dimer partners occur all along the extracytoplasmic region of the receptor.

Insulin-like growth factor ligands, receptors, and binding proteins in cancer

This review aims to summarize experimental evidence supporting the role of the insulin-like growth factor (IGF) signalling system in the progression, maintenance, and treatment of cancer. These data implicate the IGF system as an important modifier of cancer cell proliferation, survival, growth, and treatment sensitivity. The role of the IGF system in cancer should be examined in the context of the extra-cellular and intra-cellular signalling networks, in particular: phosphatidylinositol 3-kinase (PI3K), protein kinase B (Akt/PKB), mammalian target of rapamycin (mTOR), and forkhead transcription factors (FOXO). This review highlights evidence derived from molecular structure and functional genetics with respect to how the extra-cellular components of the IGF system function normally, and their subsequent modifications in cancer. The therapeutic relevance of the research evidence described is also addressed, as the challenge is to apply this knowledge to human health.

Identification of a low affinity mannose 6-phosphate-binding site in domain 5 of the cation-independent mannose 6-phosphate receptor

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR) and the 46-kDa cation-dependent MPR (CD-MPR) are type I integral membrane glycoproteins that play a critical role in the intracellular delivery of newly synthesized mannose 6-phosphate (Man-6-P)-containing acid hydrolases to the lysosome. The extracytoplasmic region of the CI-MPR contains 15 contiguous domains, and the two high affinity ( approximately 1 nm) Man-6-P-binding sites have been mapped to domains 1-3 and 9, with essential residues localized to domains 3 and 9. Domain 5 of the CI-MPR exhibits significant sequence homology to domains 3 and 9 as well as to the CD-MPR. A structure-based sequence alignment was performed that predicts that domain 5 contains the four conserved key residues (Gln, Arg, Glu, and Tyr) identified as essential for carbohydrate recognition by the CD-MPR and domains 3 and 9 of the CI-MPR, but lacks two cysteine residues predicted to form a disulfide bond within the binding pocket. To determine whether domain 5 harbors a carbohydrate-binding site, a construct that encodes domain 5 alone (Dom5His) was expressed in Pichia pastoris. Microarray analysis using 30 different oligosaccharides demonstrated that Dom5His bound specifically to a Man-6-P-containing oligosaccharide (pentamannosyl 6-phosphate). Frontal affinity chromatography showed that the affinity of Dom5His for Man-6-P was approximately 300-fold lower (K(i) = 5.3 mm) than that observed for domains 1-3 and 9. The interaction affinity for the lysosomal enzyme beta-glucuronidase was also much lower (K(d) = 54 microm) as determined by surface plasmon resonance analysis. Taken together, these results demonstrate that the CI-MPR contains a third Man-6-P recognition site that is located in domain 5 and that exhibits lower affinity than the carbohydrate-binding sites present in domains 1-3 and 9.

The insulin-like growth factor-II/mannose-6-phosphate receptor: structure, distribution and function in the central nervous system

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein which, along with the cation-dependent M6P (CD-M6P) receptor, mediates the trafficking of M6P-containing lysosomal enzymes from the trans-Golgi network (TGN) to lysosomes. Cell surface IGF-II/M6P receptors also function in the degradation of the non-glycosylated IGF-II polypeptide hormone, as well as in the capture and activation/degradation of extracellular M6P-bearing ligands. In recent years, the multifaceted role of the receptor has become apparent, as several lines of evidence have indicated that in addition to its role in lysosomal enzyme trafficking, clearance and/or activation of a variety of growth factors and endocytosis-mediated degradation of IGF-II, the IGF-II/M6P receptor may also mediate transmembrane signal transduction in response to IGF-II binding under certain conditions. However, very little is known about the physiological significance of the receptor in the function of the central nervous system (CNS). This review aims to delineate what is currently known about IGF-II/M6P receptor structure, its ligand binding properties and role in lysosomal enzyme transport. It also summarizes the recent data regarding the role of the receptor in the CNS, including its distribution, possible importance for normal and activity-dependent functioning as well as its implications in neurodegenerative disorders such as Alzheimer's disease (AD).

Structure of uPAR, plasminogen, and sugar-binding sites of the 300 kDa mannose 6-phosphate receptor

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-beta, 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 A 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.

Binding of urokinase-type plasminogen activator receptor (uPAR) to the mannose 6-phosphate/insulin-like growth factor II receptor: contrasting interactions of full-length and soluble forms of uPAR

Urokinase-type plasminogen activator receptor (uPAR) binding by the mannose 6-phosphate/insulin-like growth factor II receptor (Man-6-P/IGF2R) is considered important to Man-6-P/IGF2R tumor suppressor function via regulation of cell surface proteolytic activity. Our goal was to map the uPAR binding site of the Man-6-P/IGF2R by analyzing the uPAR binding characteristics of a panel of minireceptors containing different regions of the Man-6-P/IGF2R extracytoplasmic domain. Coimmunoprecipitation assays revealed that soluble recombinant uPAR (suPAR) bound the Man-6-P/IGF2R at two distinct sites, one localized to the amino-terminal end of the Man-6-P/IGF2R extracytoplasmic domain (repeats 1-3) and the other to the more carboxyl-terminal end (repeats 7-9). These sites correspond with the positions of the two Man-6-P binding domains of Man-6-P/IGF2R. Indeed, the suPAR-Man-6-P/IGF2R interaction was inhibited by Man-6-P, and binding-competent su-PAR species represented a minor percentage (8-30%) of the suPAR present. In contrast, Man-6-P/IGF2R binding of endogenous, full-length uPAR solubilized from plasma membranes of the prostate cancer cell line, PC-3, was not inhibited by Man-6-P. Further studies showed that very little (<5%) endogenous uPAR was Man-6-P/IGF2R binding-competent. We conclude that, contrary to previous reports, the interaction between uPAR and Man-6-P/IGF2R is a low percentage binding event and that suPAR and full-length uPAR bind the Man-6-P/IGF2R by different mechanisms.

Mannose 6-phosphate receptors: new twists in the tale

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.

Localization of the carbohydrate recognition sites of the insulin-like growth factor II/mannose 6-phosphate receptor to domains 3 and 9 of the extracytoplasmic region

The insulin-like growth factor II/mannose 6-phosphate receptor is a multifunctional receptor that binds to a diverse array of mannose 6-phosphate (Man-6-P) modified proteins as well as nonglycosylated ligands. Previous studies have mapped its two Man-6-P binding sites to a minimum of three domains, 1-3 and 7-9, within its 15-domain extracytoplasmic region. Since the primary amino acid determinants of carbohydrate recognition by the insulin-like growth factor II/mannose 6-phosphate receptor are predicted by sequence alignment to the cation-dependent mannose 6-phosphate receptor to reside within domains 3 and 9, constructs encoding either domain 3 alone or domain 9 alone were expressed in a Pichia pastoris expression system and tested for their ability to bind several carbohydrate ligands, including Man-6-P, pentamannosyl phosphate, the lysosomal enzyme, beta-glucuronidase, and the carbohydrate modifications (mannose 6-sulfate and Man-6-P methyl ester) found on Dictyostelium discoideum lysosomal enzymes. Although both constructs were functional in ligand binding and dissociation, these studies demonstrate the ability of domain 9 alone to fold into a high affinity (K(d) = 0.3 +/- 0.1 nm) carbohydrate-recognition domain whereas the domain 3 alone construct is capable of only low affinity binding (K(d) approximately 500 nm) toward beta-glucuronidase, suggesting that residues in adjacent domains (domains 1 and/or 2) are important, either directly or indirectly, for optimal binding by domain 3.

Survey of the year 2001 commercial optical biosensor literature

We have assembled references of 700 articles published in 2001 that describe work performed using commercially available optical biosensors. To illustrate the technology's diversity, the citation list is divided into reviews, methods and specific applications, as well as instrument type. We noted marked improvements in the utilization of biosensors and the presentation of kinetic data over previous years. These advances reflect a maturing of the technology, which has become a standard method for characterizing biomolecular interactions.

Structure of a functional IGF2R fragment determined from the anomalous scattering of sulfur

Insulin-like growth factor II receptor (IGF2R) is a multifunctional cell surface receptor implicated in tumour suppression. Its growth inhibitory activity has been associated with an ability to bind IGF-II. IGF2R contains 15 homologous extracellular domains, with domain 11 primarily responsible for IGF-II binding. We report a 1.4 A resolution crystal structure of domain 11, solved using the anomalous scattering signal of sulfur. The structure consists of two crossed beta-sheets forming a flattened beta-barrel. Structural analysis identifies the putative IGF-II binding site at one end of the beta-barrel whilst crystal lattice contacts suggest a model for the full-length IGF2R extracellular region. The structure factors and coordinates of IGF2R domain 11 have been deposited in the Protein Data Bank (accession codes 1GP0 and 1GP3).

The insulin-like growth factor system as a therapeutic target in colorectal cancer

The purpose of this review is to examine recent evidence that investigates the role of the insulin-like growth factor (IGF) system in colorectal cancer. We concentrate on the evidence that makes the case for the investigation of strategies that might be used to disrupt the IGF system in prevention and treatment. Even though the weight of evidence suggests that components of the IGF system may be appropriate targets, there are a lack of studies that make a systematic characterisation of all the system components in human colorectal cancer. It is anticipated that this information, and the new therapeutic molecules which follow, will impact on the prevention and treatment of patients with this disease.