Developmental expression of the IGF-II/mannose 6-phosphate receptor

Population pharmacokinetic modeling and dosing simulation of avalglucosidase alfa for selecting alternative dosing regimen in pediatric patients with late-onset pompe disease

Avalglucosidase alfa (AVAL) was approved in the United States (2021) for patients with late-onset Pompe disease (LOPD), aged ≥ 1 year. In the present study, pharmacokinetic (PK) simulations were conducted to propose alternative dosing regimens for pediatric LOPD patients based on a bodyweight cut-off. Population PK (PopPK) analysis was performed using nonlinear mixed effect modeling approach on pooled data from three clinical trials with LOPD patients, and a phase 2 study (NCT03019406) with infantile-onset Pompe disease (IOPD: 1-12 years) patients. A total of 2257 concentration-time points from 91 patients (LOPD, n = 75; IOPD, n = 16) were included in the analysis. The model was bodyweight dependent allometric scaling with time varying bodyweight included on clearance and distribution volume. Simulations were performed for two dosing regimens (20 mg/kg or 40 mg/kg) with different bodyweight cut-off (25, 30, 35 and 40 kg) by generating virtual pediatric (1-17 years) and adult patients. Corresponding simulated individual exposures (maximal concentration, Cmax and area under the curve in the 2-week dosing interval, AUC2W), and distributions were calculated. It was found that dosing of 40 mg/kg and 20 mg/kg in pediatric patients < 30 kg and ≥ 30 kg, respectively, achieved similar AVAL exposure (based on AUC2W) to adult patients receiving 20 mg/kg. PK simulations conducted on the basis of this model provided supporting data for the currently approved US labelling for dosing adapted bodyweight in LOPD patients ≥ 1 year by USFDA.

Insulin Receptor Isoforms and Insulin Growth Factor-like Receptors: Implications in Cell Signaling, Carcinogenesis, and Chemoresistance

This comprehensive review thoroughly explores the intricate involvement of insulin receptor (IR) isoforms and insulin-like growth factor receptors (IGFRs) in the context of the insulin and insulin-like growth factor (IGF) signaling (IIS) pathway. This elaborate system encompasses ligands, receptors, and binding proteins, giving rise to a wide array of functions, including aspects such as carcinogenesis and chemoresistance. Detailed genetic analysis of IR and IGFR structures highlights their distinct isoforms, which arise from alternative splicing and exhibit diverse affinities for ligands. Notably, the overexpression of the IR-A isoform is linked to cancer stemness, tumor development, and resistance to targeted therapies. Similarly, elevated IGFR expression accelerates tumor progression and fosters chemoresistance. The review underscores the intricate interplay between IRs and IGFRs, contributing to resistance against anti-IGFR drugs. Consequently, the dual targeting of both receptors could present a more effective strategy for surmounting chemoresistance. To conclude, this review brings to light the pivotal roles played by IRs and IGFRs in cellular signaling, carcinogenesis, and therapy resistance. By precisely modulating these receptors and their complex signaling pathways, the potential emerges for developing enhanced anti-cancer interventions, ultimately leading to improved patient outcomes.

Translational studies of intravenous and intracerebroventricular routes of administration for CNS cellular biodistribution for BMN 250, an enzyme replacement therapy for the treatment of Sanfilippo type B

BMN 250 is being developed as enzyme replacement therapy for Sanfilippo type B, a primarily neurological rare disease, in which patients have deficient lysosomal alpha-N-acetylglucosaminidase (NAGLU) enzyme activity. BMN 250 is taken up in target cells by the cation-independent mannose 6-phosphate receptor (CI-MPR, insulin-like growth factor 2 receptor), which then facilitates transit to the lysosome. BMN 250 is dosed directly into the central nervous system via the intracerebroventricular (ICV) route, and the objective of this work was to compare systemic intravenous (IV) and ICV delivery of BMN 250 to confirm the value of ICV dosing. We first assess the ability of enzyme to cross a potentially compromised blood-brain barrier in the Naglu-/- mouse model and then assess the potential for CI-MPR to be employed for receptor-mediated transport across the blood-brain barrier. In wild-type and Naglu-/- mice, CI-MPR expression in brain vasculature is high during the neonatal period but virtually absent by adolescence. In contrast, CI-MPR remains expressed through adolescence in non-affected non-human primate and human brain vasculature. Combined results from IV administration of BMN 250 in Naglu-/- mice and IV and ICV administration in healthy juvenile non-human primates suggest a limitation to therapeutic benefit from IV administration because enzyme distribution is restricted to brain vascular endothelial cells: enzyme does not reach target neuronal cells following IV administration, and pharmacological response following IV administration is likely restricted to clearance of substrate in endothelial cells. In contrast, ICV administration enables central nervous system enzyme replacement with biodistribution to target cells.

Plasma Pharmacokinetics of Valanafusp Alpha, a Human Insulin Receptor Antibody-Iduronidase Fusion Protein, in Patients with Mucopolysaccharidosis Type I

BACKGROUND

Mucopolysaccharidosis type I (MPSI) is caused by mutations in the gene encoding the α-L-iduronidase (IDUA) lysosomal enzyme and the majority of MPSI patients have severe central nervous system (CNS) involvement. Enzyme replacement therapy (ERT) with recombinant IDUA does not treat the CNS, due to the lack of transport of the enzyme across the blood-brain barrier (BBB). Human IDUA has been re-engineered as an IgG-IDUA fusion protein, valanafusp alpha, where the IgG domain is a monoclonal antibody (MAb) against the human insulin receptor (HIR). The HIRMAb domain binds the endogenous insulin receptor on the human BBB to trigger receptor-mediated transport across the BBB, and acts as a molecular Trojan horse to ferry the fused IDUA into the brain of patients with MPSI.

METHODS

The present investigation describes the initial dosing, plasma pharmacokinetics, and plasma glucose response to the intravenous infusion of doses of valanafusp alpha ranging from 0.3 to 3 mg/kg in five adults and from 1 to 6 mg/kg in 13 pediatric subjects with MPSI.

RESULTS

Valanafusp alpha plasma clearance is increased four-fold in children, and shows a linear pharmacokinetic response over the dose range of 0.3-3 mg/kg with a stable plasma elimination half-life (t_½). The plasma pharmacokinetic parameters for valanafusp alpha overlapped with the same parameters previously reported for recombinant human IDUA (laronidase). The majority of the tested subjects had been receiving laronidase ERT for years, and some showed high levels of anti-drug antibodies (ADAs). However, the presence of these ADAs did not generally alter the rate of plasma clearance of valanafusp alpha in MPSI. The infusion of 0.3-6 mg/kg doses of valanafusp alpha had no effect on plasma glucose for up to 24 h after the drug infusion.

CONCLUSION

The plasma clearance of valanafusp alpha is increased four-fold in children with MPSI compared with adult subjects at a dose of 1-3 mg/kg. The plasma pharmacokinetic profile of valanafusp alpha, at a dose of 1-3 mg/kg, is comparable to that of laronidase in children with MPSI.

Atlas of tissue- and developmental stage specific gene expression for the bovine insulin-like growth factor (IGF) system

The insulin-like growth factor (IGF) axis is fundamental for mammalian growth and development. However, no comprehensive reference data on gene expression across tissues and pre- and postnatal developmental stages are available for any given species. Here we provide systematic promoter- and splice variant specific information on expression of IGF system components in embryonic (Day 48), fetal (Day 153), term (Day 277, placenta) and juvenile (Day 365–396) tissues of domestic cow, a major agricultural species and biomedical model. Analysis of spatiotemporal changes in expression of IGF1, IGF2, IGF1R, IGF2R, IGFBP1-8 and IR genes, as well as lncRNAs H19 and AIRN, by qPCR, indicated an overall increase in expression from embryo to fetal stage, and decrease in expression from fetal to juvenile stage. The stronger decrease in expression of lncRNAs (average ―16-fold) and ligands (average ―12.1-fold) compared to receptors (average ―5.7-fold) and binding proteins (average ―4.3-fold) is consistent with known functions of IGF peptides and supports important roles of lncRNAs in prenatal development. Pronounced overall reduction in postnatal expression of IGF system components in lung (―12.9-fold) and kidney (―13.2-fold) are signatures of major changes in organ function while more similar hepatic expression levels (―2.2-fold) are evidence of the endocrine rather than autocrine/paracrine role of IGFs in postnatal growth regulation. Despite its rapid growth, placenta displayed a more stable expression pattern than other organs during prenatal development. Quantitative analyses of contributions of promoters P0-P4 to global IGF2 transcript in fetal tissues revealed that P4 accounted for the bulk of transcript in all tissues but skeletal muscle. Demonstration of IGF2 expression in fetal muscle and postnatal liver from a promoter orthologous to mouse and human promoter P0 provides further evidence for an evolutionary and developmental shift from placenta-specific P0-expression in rodents and suggests that some aspects of bovine IGF expression may be closer to human than mouse.

Guanidinylated Neomycin Conjugation Enhances Intranasal Enzyme Replacement in the Brain

Iduronidase (IDUA)-deficient mice accumulate glycosaminoglycans in cells and tissues and exhibit many of the same neuropathological symptoms of patients suffering from Mucopolysaccharidosis I. Intravenous enzyme-replacement therapy for Mucopolysaccharidosis I ameliorates glycosaminoglycan storage and many of the somatic aspects of the disease but fails to treat neurological symptoms due to poor transport across the blood-brain barrier. In this study, we examined the delivery of IDUA conjugated to guanidinoneomycin (GNeo), a molecular transporter. GNeo-IDUA and IDUA injected intravenously resulted in reduced hepatic glycosaminoglycan accumulation but had no effect in the brain due to fast clearance from the circulation. In contrast, intranasally administered GNeo-IDUA entered the brain rapidly. Repetitive intranasal treatment with GNeo-IDUA reduced glycosaminoglycan storage, lysosome size and number, and neurodegenerative astrogliosis in the olfactory bulb and primary somatosensory cortex, whereas IDUA was less effective. The enhanced efficacy of GNeo-IDUA was not the result of increased nose-to-brain delivery or enzyme stability, but rather due to more efficient uptake into neurons and astrocytes. GNeo conjugation also enhanced glycosaminoglycan clearance by intranasally delivered sulfamidase to the brain of sulfamidase-deficient mice, a model of Mucopolysaccharidosis IIIA. These findings suggest the general utility of the guanidinoglycoside-based delivery system for restoring missing lysosomal enzymes in the brain.

Hematopoietic Stem cell transplantation and lentiviral vector-based gene therapy for Krabbe's disease: Present convictions and future prospects

Currently, presymtomatic hematopoietic stem and progenitor cell transplantation (HSPCT) is the only therapeutic modality that alleviates Krabbe's disease (KD)‐induced central nervous system damage. However, all HSPCT‐treated patients exhibit severe deterioration in peripheral nervous system function characterized by major motor and expressive language pathologies. We hypothesize that a combination of several mechanisms contribute to this phenomenon, including 1) nonoptimal conditioning protocols with consequent inefficient engraftment and biodistribution of donor‐derived cells and 2) insufficient uptake of donor cell‐secreted galactocerebrosidease (GALC) secondary to a naturally low expression level of the cation‐independent mannose 6‐phosphate‐receptor (CI‐MPR). We have characterized the effects of a busulfan (Bu) based conditioning regimen on the efficacy of HSPCT in prolonging twi mouse average life span. There was no correlation between the efficiency of bone marrow engraftment of donor cells and twi mouse average life span. HSPCT prolonged the average life span of twi mice, which directly correlated with the aggressiveness of the Bu‐mediated conditioning protocols. HSPC transduced with lentiviral vectors carrying the GALC cDNA under control of cell‐specific promoters were efficiently engrafted in twi mouse bone marrow. To facilitate HSPCT‐mediated correction of GALC deficiency in target cells expressing low levels of CI‐MPR, a novel GALC fusion protein including the ApoE1 receptor was developed. Efficient cellular uptake of the novel fusion protein was mediated by a mannose‐6‐phosphate‐independent mechanism. The novel findings described here elucidate some of the cellular mechanisms that impede the cure of KD patients by HSPCT and concomitantly open new directions to enhance the therapeutic efficacy of HSPCT protocols for KD. © 2016 The Authors. Journal of Neuroscience Research Published by Wiley Periodicals, Inc.

Case report of unexpected gastrointestinal involvement in type 1 Gaucher disease: comparison of eliglustat tartrate treatment and enzyme replacement therapy

BACKGROUND

Gastrointestinal involvement in Gaucher disease is very rare, and appears to be unresponsive to enzyme replacement therapy (ERT).

CASE PRESENTATION

Here, we describe identical twin, splenectomized, non-neuronopathic Gaucher patients on long-term ERT for 9 years, who complained of epigastric discomfort due to Gaucher cell infiltration of the gastroduodenal mucosa. Rare compound heterozygous mutations (p.Arg48Trp and p.Arg257Gln) of the GBA gene were found in both. Improvement in the gastroduodenal infiltration and reduced chitotriosidase levels were observed in one who switched to eliglustat tartrate for 1 year, whereas the other one who maintained ERT showed no improvement of chitotriosidase level and persistent duodenal lesions.

CONCLUSION

This shows that eliglustat might be an effective treatment for Gaucher disease patients having lesions resistant to ERT.

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

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

Soluble M6P/IGFIIR in the circulation

Soluble M6P/IGFIIR has the potential to be a significant carrier of IGF-II and mannose 6-P proteins in the circulation and play an important role as an antagonist to the cellular receptor. Evidence suggests that soluble receptor plays a role in fetal and childhood growth by opposing the growth stimulatory effects of IGF-II. Maternal serum levels of M6P/IGFIIR are elevated in late pregnancy and the IGF-II:soluble M6P/IGFIIR ratio in cord blood correlates strongly with weight at birth and placental weight suggesting an important role in fetal growth and development. However, elevated soluble receptor levels may also be indicative of disease in later life, such as liver cirrhosis and some tumor types and may be a useful marker for monitoring treatment and progression of the disease. Further investigation of the regulation of this soluble receptor in health and disease is required to fully elucidate its role in the circulation.

Tumors, IGF-2, and hypoglycemia: insights from the clinic, the laboratory, and the historical archive

Tumors of mesenchymal and epithelial origin produce IGF-2, which activates pathways in the tumors. In a minority of patients, the tumors (hepatomas, fibromas, and fibrosarcomas are the most common among many) release into the circulation enough IGF-2-related peptides to mimic the fasting hypoglycemia characteristic of patients with insulin-producing islet-cell tumors. Rarely, markedly elevated IGF-2 levels produce somatic changes suggestive of acromegaly. Typically, the elevated IGF-2 levels are associated with suppressed plasma levels of insulin, IGF-1, and GH. Complicating the pathophysiology are the IGF binding proteins (IGFBPs) that can bind IGF-2 and IGF-1, modifying hormone metabolism and action. IGFBP concentrations are often altered in the presence of these tumors. At the cellular level, the 3 hormone-related ligands, IGF-2, IGF-1, and insulin, all bind to 4 (or more) types of IGF-1 receptor (IGF-1R) and insulin receptor (IR). Each receptor has its own characteristic affinity for each ligand, a tyrosine kinase, and overlapping profiles of action in the target cells. The IGF-2R, in addition to binding mannose-6-phosphate-containing proteins, provides an IGF-2 degradation pathway. Recent evidence suggests IGF-2R involvement also in signal transduction. Surgery, the treatment of choice, can produce a cure. For patients not cured by surgery, multiple therapies exist, for the tumor and for hypoglycemia. Potential future therapeutic approaches are sketched. From 1910 to 1930, hypoglycemia, insulin, insulinomas, and non-islet-cell tumors were recognized. The latter third of the century witnessed the emergence of the immunoassay for insulin; the IGFs, their binding proteins, and assays to measure them; and receptors for the insulin-related peptides as well as the intracellular pathways beyond the receptor. In closing, we replace non-islet-cell tumor hypoglycemia, an outdated and misleading label, with IGF-2-oma, self-explanatory and consistent with names of other hormone-secreting tumors.

Renin and the IGFII/M6P receptor system in cardiac biology

Nonenzymatic cardiac activities of renin are well described during the last years and contribute either to cardiac-specific effects of the renin-angiotensin-aldosterone-system (RAAS) or to the pharmacological effects of RAAS inhibition. The interaction of renin with insulin-like growth factor II/mannose-6-phosphate (IGFII/M6P) receptors participates in nonclassical renin effects and contributes to cardiac remodelling caused by RAAS activation. The current findings suggest an important role for renin IGFII/M6P receptor interaction in cardiac adaptation to stress and support the idea that excessive accumulation of renin during inhibition of RAAS directly contributes to blood pressure-independent effects of these pharmacological interventions. It becomes a challenge for future studies focussing on chronic hypertension or myocardial infarction to comprise regulatory adaptations of the kidney, the main source of plasma renin and prorenin, because they directly contribute to key steps in regulation of cardiac (mal)adaptation via IGFII/M6P receptors. This receptor system is part of peptide/receptor interactions that modifies and possibly limits adverse remodelling effects caused by angiotensin II. Evaluation of interactions of renin with other pro-hypertrophic agonists is required to decide whether this receptor may become a target of pharmacological intervention.

Elevation of IGF-2 receptor and the possible underlying implications in end-stage heart failure patients before and after heart transplantation

Up-regulation of insulin-like growth factor 2 receptor (IGF-2R) involved in angiotensin II–induced cell apoptosis in cardiomyoblasts, and correlated with cardiomyocyte apoptosis in hypertensive rat hearts. Here, we detected IGF-2R levels and explored the possible underlying implications in end-stage heart failure (HF) patients before and after heart transplantation. Western blot and immunohistochemistry were used to measure cardiac IGF-2R levels. ELISA was used to detect serum IGF-2R and CD8 levels. Labelling of DNA strand breaks and dihydroethidium detection were used to determine cellular apoptosis and reactive oxygen species, respectively. Cardiac IGF-2R levels increased in end-stage HF patients (n = 11) compared with non-failing control subjects. Leu27-IGF-2, an IGF-2 analogue to activate specially the IGF-2R, could induce apoptosis and reactive oxygen species production in neonatal rat ventricular myocytes. The serum IGF-2R levels were significantly higher in HF patients than those in non-failing control subjects. An unexpected observation is that the serum IGF-2R levels further increased after heart transplantation, peaked at the first month, and gradually reduced close to the levels before heart transplantation at the 6th months after heart transplantation. Serum CD8, a marker of acute rejection, had no change after heart transplantation, but IGF-2R and Granzyme B, as a ligand for the IGF-2R and a marker for CD8 T lymphocyte activation, coexisted in the transplanted hearts. Our preliminary studies suggest that elevation of IGF-2R may participate in pathological process of end-stage HF and involved in the acute cellular rejection after heart transplantation.

Biology and significance of signalling pathways activated by IGF-II

Insulin-like growth factor-II (IGF-II) affects many aspects of cellular function through its ability to activate several different receptors and, consequently, numerous intracellular signalling molecules. Thus, IGF-II is a key regulator of normal foetal development and growth. However, abnormalities in IGF-II function are associated with cardiovascular disease and cancer. Here, we review the cellular mechanisms by which IGF-II's physiological and pathophysiological actions are exerted by discussing the involvement of the type 1 and type 2 IGF receptors (IGF1R and IGF2R), the insulin receptor and the downstream MAP kinase, PI-3 kinase and G-protein-coupled signalling pathways in mediating IGF-II stimulated cellular proliferation, survival, differentiation and migration.

Insulin and insulin-like growth factor I signaling pathways in rainbow trout (Oncorhynchus mykiss) during adipogenesis and their implication in glucose uptake

Primary cultures of rainbow trout ( Oncorhynchus mykiss ) adipocytes were used to examine the main signaling pathways of insulin and insulin-like growth factor I (IGF-I) during adipogenesis. We first determined the presence of IGF-I receptors (IGF-IR) and insulin receptors (IR) in trout preadipocytes ( day 5) and adipocytes ( day 14). IGF-IRs were more abundant and appeared to be in higher levels in differentiated cells than in preadipocytes, whereas IRs were detected in lower but constant levels throughout the culture. The cells were immunoreactive against ERK1/2 MAPK, and AKT/PI3K, components of the two main signal transduction pathways for insulin and IGF-I receptors. Stimulation of MAPK phosphorylation by IGF-I was higher in preadipocytes than in adipocytes, while no effects were observed in MAPK phosphorylation after incubation of cells with insulin. AKT phosphorylation increased in the presence of both insulin and IGF-I, with higher levels of stimulation in adipocytes than in preadipocytes. Activation of both pathways was blocked by the use of specific inhibitors of MAPK (PD98059) and AKT (wortmannin). We describe here, for the first time, the effects of IGF-I and insulin on 2-deoxyglucose uptake in primary culture of trout adipocytes. IGF-I was more potent in stimulating glucose uptake than insulin, and PD98059 and wortmannin inhibited the stimulation of glucose uptake by this growth factor, suggesting that IGF-I plays an important metabolic role in trout adipocytes. Our results suggest that differential activation of the MAPK and AKT pathways are involved in the IGF-I- and insulin-induced effects of trout adipocytes during the various stages of adipogenesis.

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.

Mannose-6-phosphate/Insulin-like Growth Factor II Receptor Expression and Tumor Development

The mannose-6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR) is a multi-functional transmembrane glycoprotein whose major function is to bind and transport M6P-bearing glycoproteins from the trans-Golgi network or the cell surface to lysosomes. The cell surface M6P/IGF-IIR also bind and internalizes the insulin-like growth factor II. The receptor gene is considered a « candidate » tumor suppressor gene. The phenotypic consequences of loss of M6P/IGF-IIR through somatic mutation are potentially very complex since M6P/IGF-IIR has a number of roles in cellular physiology. Loss of function mutations in M6P/IGF-IIR gene could contribute to multi-step carcinogenesis. In the light of the multi-functional cellular potential roles of the M6P/IGF-IIR the purpose of this review is to highlight some recent data concerning its normal functions and the potential role of its loss in tumor pathophysiology with the aim to try to clarify the possible underlying mechanisms of its involvement in tumor development.

Changes in phosphomannosyl ligands correlate with cation-dependent mannose-6-phosphate receptors in rat liver during perinatal development

The co-existence of two mannose-6-phosphate receptors (CD-MPR and CI-MPR) in most cell types is still a dilemma to be resolved. In this study, some parameters were measured to explore lysosomal apparatus evolution in rat liver during perinatal development, and establish a possible involvement of CD- and/or CI-MPR in lysosome maturation. Activity of four acid hydrolases was measured in the whole organ at different ages and it was found that N-acetyl-beta-D-glucosaminidase (NAG), beta-galactosidase, and beta-glucuronidase change during development, reaching a peak at the 10th day after birth. These results correlated with the expression and binding properties of CD-MPR previously reported. We also used a method that recognizes phosphomannosylated ligands by using purified biotinylated CI-MPR as a probe, and found that the highest concentrations of ligands also appear around the 10th day. Binding assays were also carried out, incubating endogenous NAG from 10-day-old and adult rats with membranes from their respective ages, and the results indicated that cation-dependent mannose-6-phosphate receptor (CD-MPR) has more impact on trafficking of the enzyme at the 10th day after birth. We concluded that lysosome maturation in the rat liver occurs around the 10th day after birth, and that the CD-MPR may participate in that event.

Developmental differences between cation-independent and cation-dependent mannose-6-phosphate receptors in rat brain at perinatal stages

Mannose-6-phosphate receptors (MPRs) play a role in the selective transport of macromolecules bearing mannose-6-phosphate residue to lysosomes. To date, two types of MPRs have been described in most of cells and tissues: the cation-dependent (CD-MPR) and cation-independent mannose-6-phosphate receptor (CI-MPR). In order to elucidate their possible role in the central nervous system, the expression and binding properties of both MPRs were studied in rat brain along perinatal development. It was observed that the expression of CI-MPR decreases progressively from fetuses to adults, while the CD-MPR increases around the 10th day of birth, and maintains these values up to adulthood. Binding assays showed differences in the Bmax and KD values between the ages studied, and they did not correlate with the expression levels of both MPRs. Variations in lysosomal enzyme activities and expression of phosphomannosylated ligands during development correlated more with CD-MPR than with CI-MPR expression. These results suggest that both receptors play a different role in rat brain during perinatal development, being CD-MPR mostly involved in lysosome maturation.

Short-term, high dose enzyme replacement therapy in sialidosis mice

Given the success of enzyme replacement therapy (ERT) in treating the systemic manifestations in a number of lysosomal storage disorders (LSDs), we evaluated the effect of ERT on the mouse model of sialidosis. This glycoproteinosis, which affects primarily the reticuloendothelial (RE) system, is caused by deficiency of lysosomal neuraminidase (NEU1) and consequent accumulation of sialylated glycoconjugates. NEU1 lacks a functional mannose-6-phosphate recognition marker and is not endocytosed by mammalian cells. However, the enzyme produced in insect cells has features that allow its effective uptake by RE cells and macrophages via the mannose receptor, and therefore represent an alternative method of therapy. In this study we tested the therapeutic efficacy of baculovirus (BV) expressed mouse neuraminidase (Neu1) in sialidosis mice. Four-week-old Neu1-/- mice were first injected intravenously with a single dose of the recombinant enzyme for assessment of the half-life of mannosylated Neu1 in vivo. Afterwards, a short-term ERT with a total of five enzyme injections over a 2-week period was performed for evaluation of phenotype correction. Neu1 infused alone or co-administered with its associated protein, protective protein/cathepsin A (PPCA) was effectively taken up by resident macrophages in many tissues. Restored Neu1 activity persisted for up to 4 days, depending on the tissue, and resulted in a significant reduction of lysosomal storage. However, beyond 2 weeks of treatment, ERT mice developed a severe immune response towards the exogenous Neu1 enzyme. These results may have important implications for ERT in sialidosis patients.

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).

Down-regulation of the M6P/IGF-II receptor increases cell proliferation and reduces apoptosis in neonatal rat cardiac myocytes

BACKGROUND

The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF2R) is a multi-functional protein that has been implicated in regulation of cell growth and apoptosis. Cardiac myocytes express relatively high levels of M6P/IGF2R, and cardiomyocyte apoptosis has been identified in a variety of cardiovascular disorders, such as myocardial infarction and heart failure. However, involvement of M6P/IGF2R in the pathogenesis of these conditions has not been determined. Thus, the objective of this study was to determine the role of M6P/IGF2R in regulation of cardiac myocyte growth and apoptosis.

RESULTS

We down-regulated the expression of M6P/IGF2R in neonatal rat cardiac myocytes and examined the effect on cell proliferation and apoptosis. Infection of neonatal cardiomyocytes with an adenovirus expressing a ribozyme targeted against the M6P/IGF2R significantly reduced the level of M6P/IGF2R mRNA, as determined by RT-PCR and Ribonuclease Protection Assay (RPA). M6P-containing protein binding and endocytosis as well as the M6P/IGF2R-mediated internalization of 125I-IGF-II were lower in the ribozyme-treated cells than the control myocytes, indicating that the number of functional M6P/IGF2R in the ribozyme treated cells was reduced. Accordingly, a marked increase in cell proliferation and a reduced cell susceptibility to hypoxia- and TNF-induced apoptosis were observed in the ribozyme-treated cells.

CONCLUSIONS

These findings suggest that M6P/IGF2R may play a role in regulation of cardiac myocyte growth and apoptosis. Down regulation of this gene in cardiac tissues might be a new approach to prevention of cell death or promotion of mitogenesis for certain heart diseases.

Insulin-like growth factor-II/mannose-6-phosphate receptor: widespread distribution in neurons of the central nervous system including those expressing cholinergic phenotype

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is single transmembrane glycoprotein that plays a critical role in the trafficking of lysosomal enzymes and the internalization of circulating IGF-II. At present, there is little information regarding the cellular distribution of the IGF-II/M6P receptor within the adult rat brain. With the use of immunoblotting and immunocytochemical methods, we found that the IGF-II/M6P receptor is widely but selectively expressed in all major brain areas, including the olfactory bulb, striatum, cortex, hippocampus, thalamus, hypothalamus, cerebellum, brainstem, and spinal cord. Intense IGF-II/M6P receptor immunoreactivity was apparent on neuronal cell bodies within the striatum, deeper layers (layers IV and V) of the cortex, pyramidal and granule cell layers of the hippocampal formation, selected thalamic nuclei, Purkinje cells of the cerebellum, pontine nucleus and motoneurons of the brainstem as well as in the spinal cord. Moderate neuronal labeling was evident in the olfactory bulb, basal forebrain areas, hypothalamus, superior colliculus, midbrain areas, granule cells of the cerebellum and in the intermediate regions of the spinal gray matter. We also observed dense neuropil labeling in many regions, suggesting that this receptor is localized in dendrites and/or axon terminals. Double-labeling studies further indicated that a subset of IGF-II/M6P receptor colocalizes with cholinergic cell bodies and fibers in the septum, striatum, diagonal band complex, nucleus basalis, cortex, hippocampus, and motoneurons of the brainstem and spinal cord. The observed widespread distribution and colocalization of IGF-II/M6P receptor in the adult rat brain provide an anatomic basis to suggest a multifunctional role for the receptor in a wide-spectrum of central nervous system neurons, including those expressing a cholinergic phenotype.

The influence of growth factors on the development of preimplantation mammalian embryos

The development of the preimplantation mammalian embryo from a fertilized egg to a blastocyst capable of implanting in the uterus is a complex process. Cell division must be carefully programmed. The embryonic genome must be activated at the appropriate stage of development, and the pattern of gene expression must be carefully coordinated for the initiation of the correct program of differentiation. Cell fates must be chosen to establish specific cell types such as the inner cell mass and the trophectoderm, which give rise to the embryo proper and the placenta, respectively. This review summarizes recent findings concerning the influence of growth factors on the development of preimplantation mammalian embryos. Maternal factors secreted into the lumen of the female reproductive tract as well as substances synthesized by the developing embryo itself help to regulate this process. Studies of embryos in culture and investigations using homologous recombination to create embryos and animals null for specific genes have enabled the identification of several growth factors that appear essential for preimplantation mammalian embryo development. Some of the factors are required maternal factors; others are embryo-derived autocrine and paracrine factors. Studies using molecular biology are beginning to identify differences in the patterns of genes expressed by naturally derived embryos and those developing in culture. The knowledge gained from studies on growth factors, media, embryonic development, and gene expression should help improve culture conditions for embryos and will provide for safer outcomes from assisted reproductive procedures in human and animal clinics.

TGFbeta2 activation status during cardiac morphogenesis

Transforming growth factor beta (TGFbeta) is secreted as a biologically inactive complex by many cell types in vitro, but little is known of TGFbeta's activation status in vivo. This study examined the in vivo expression of active and total (active + acid-activatable) TGFbeta2 in embryonic chicken hearts during cardiac morphogenesis (Hamburger-Hamilton stage 10-24). The concentration of TGFbeta2 was measured by an enzyme-linked immunoassay that recognized active TGFbeta2. Whole heart homogenates were either left untreated to measure active TGFbeta2 or treated with acid before assay to measure total (active + acid-activatable) TGFbeta2. Total TGFbeta2 concentration increased more than 16-fold between stage 10/11 and stage 24. Active TGFbeta2 concentration was highest at stage 14/15, but overall remained relatively constant varying at most by 2.8-fold. When expressed relative to total TGFbeta2, the amount of active TGFbeta2 progressively declined from 70% in stage 10/11 hearts to 7% in stage 24 hearts. The distribution of active and total TGFbeta2 was examined by immunostaining with an antibody against active TGFbeta2. Before immunostaining, sections were either treated with acid or left untreated to determine the distribution of total and active TGFbeta2, respectively. Active TGFbeta2 immunostaining was first detected in the endothelium, myocardium, and cardiac jelly of stage 14 hearts. Acid treatment had no effect on the distribution or intensity of immunostaining at this stage. Faint, active TGFbeta2 immunostaining was restricted to the ventricular myocardium in stage 18 hearts. Acid treatment resulted in a marked increase in staining intensity in the ventricle, but no staining was observed in the atrium or outflow tract. In stage 24 hearts, faint active TGFbeta2 staining was detected in the ventricle before acid treatment. After acid treatment, patches of intense punctate stain were found in all regions of the embryonic heart. Increases in TGFbeta2 concentration and immunostaining intensity after acidification suggest that a significant amount of TGFbeta2 is in the latent form. Stage-dependent differences in activation status suggest that activation may be a developmentally regulated process in the chick heart and support the notion that activation is an important step in regulating TGFbeta actions in vivo.

High-affinity prorenin binding to cardiac man-6-P/IGF-II receptors precedes proteolytic activation to renin

Mannose-6-phosphate (man-6-P)/insulin-like growth factor-II (man-6-P/IgF-II) receptors are involved in the activation of recombinant human prorenin by cardiomyocytes. To investigate the kinetics of this process, the nature of activation, the existence of other prorenin receptors, and binding of native prorenin, neonatal rat cardiomyocytes were incubated with recombinant, renal, or amniotic fluid prorenin with or without man-6-P. Intact and activated prorenin were measured in cell lysates with prosegment- and renin-specific antibodies, respectively. The dissociation constant (K(d)) and maximum number of binding sites (B(max)) for prorenin binding to man-6-P/IGF-II receptors were 0.6 +/- 0.1 nM and 3,840 +/- 510 receptors/myocyte, respectively. The capacity for prorenin internalization was greater than 10 times B(max). Levels of internalized intact prorenin decreased rapidly (half-life = 5 +/- 3 min) indicating proteolytic prosegment removal. Prorenin subdivision into man-6-P-free and man-6-P-containing fractions revealed that only the latter was bound. Cells also bound and activated renal but not amniotic fluid prorenin. We concluded that cardiomyocytes display high-affinity binding of renal but not extrarenal prorenin exclusively via man-6-P/IGF-II receptors. Binding precedes internalization and proteolytic activation to renin thereby supporting the concept of cardiac angiotensin formation by renal prorenin.

Identification of a type II insulin-like growth factor receptor in fish embryos

To determine whether fish have an insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6-P) receptor similar to that of mammals, we have performed binding, cross-linking, and immunoprecipitation experiments with wheat-germ-agglutinin- and mannose 6-phosphate (M6-P)-affinity-purified receptor preparations from fish embryos. In both receptor preparations, IGF-II binding was specific, because labeled IGF-II could only be completely displaced by cold IGF-II but not by IGF-I or insulin. Labeled IGF-II bound to a protein with a molecular mass of approximately 250 kDa, which could be immunoprecipitated with an antibody against the rat IGF-II receptor. IGF-II stimulated tyrosine kinase activity in wheat germ agglutinin preparations and was more potent than insulin or IGF-I, but neither peptide stimulated tyrosine kinase activity in M6-P preparations. Two fish cell lines (CHSE-214 and EPC) were used to confirm the IGF-II binding data obtained in the receptor preparations, revealing the presence of highly specific IGF-II binding and the absence of insulin binding. Furthermore, a decrease of the IGF-I receptors on the cell surface did not alter IGF-II binding in EPC cells. In conclusion, we have detected the presence of IGF-II/M6-P receptors in fish embryos that are similar in structure and specificity for their ligand to those found in mammals.

Mannose 6-phosphate/insulin-like growth factor 2 receptor, a bona fide tumor suppressor gene or just a promising candidate?

The mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) is considered a "candidate" tumor suppressor gene. This hypothesis has been provoked by the identification of loss of heterozygosity (LOH) at the M6P/IGF2R locus on chromosome 6q26 in breast and liver cancer, accompanied by point mutations in the remaining allele. Somatic mutations in coding region microsatellites have also been described in replication error positive (RER+) tumors of the gastrointestinal tract, endometrium and brain. These genetic data are compelling, but a tumor suppressor gene candidate has to meet functional as well as genetic criteria. This review weighs the evidence and discusses the observations that are necessary to promote M6P/IGF2R from candidate to bona fide tumor suppressor gene.

Regulation of soluble insulin-like growth factor II/mannose 6-phosphate receptor in human serum: measurement by enzyme-linked immunosorbent assay

The soluble form of the insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6-P) receptor has been detected in serum from a variety of mammalian species. We report the development of a highly sensitive quantitative human IGF-II/M6-P receptor immunoassay. Antibodies raised to receptor purified from a human hepatoma cell line by phosphomannan affinity chromatography were used to develop a specific enzyme-linked immunosorbent assay. In this assay, the serum level of soluble receptor for healthy adult subjects was 0.70 +/- 0.23 mg/L. We have shown that soluble receptor is developmentally regulated, with levels in infant (1.12 +/- 0.28 mg/L) and prepubertal (1.18 +/- 0.6 mg/L) subjects dropping by 40% during adolescence (0.73 +/- 0.61 mg/L) and remaining constant throughout adulthood. Further, the receptor is gestationally regulated, with a highly significant association between gestational age and maternal serum receptor levels (r = 0.947; P < 0.0001). Noninsulin-dependent diabetes mellitus (0.98 +/- 0.25 mg/L) and insulin-dependent diabetes mellitus (0.98 +/- 0.25 mg/L) mildly elevated soluble receptor levels, whereas end-stage renal failure (0.75 +/- 0.23 mg/L) and acromegaly (0.79 +/- 0.25 mg/L) did not affect receptor levels. Additionally, we have shown that soluble receptor is present in amniotic fluid, but at a 100-fold lower concentration than serum levels. The ability to quantitate soluble IGF-II/M6-P receptor levels in serum and other fluids provides a valuable tool that will help to further elucidate the role of the receptor in human physiology and disease states.

Mannose-6-phosphate/insulin-like growth factor-II receptor is a receptor for retinoic acid

Retinoic acid (RA) exerts diverse biological effects in the control of cell growth in embryogenesis and oncogenesis. These effects of RA are thought to be mediated by the nuclear retinoid receptors. Mannose-6-phosphate (M6P)/insulin-like growth factor-II (IGF-II) receptor is a multifunctional membrane glycoprotein that is known to bind both M6P and IGF-II and function primarily in the binding and trafficking of lysosomal enzymes, the activation of transforming growth factor-beta, and the degradation of IGF-II. M6P/IGF-II receptor has recently been implicated in fetal development and carcinogenesis. Despite the functional similarities between RA and the M6P/IGF-II receptor, no direct biochemical link has been established. Here, we show that the M6P/IGF-II receptor also binds RA with high affinity at a site that is distinct from those for M6P and IGF-II, as identified by a photoaffinity labeling technique. We also show that the binding of RA to the M6P/IGF-II receptor enhances the primary functions of this receptor. The biological consequence of the interaction appears to be the suppression of cell proliferation and/or induction of apoptosis. These findings suggest that the M6P/IGF-II receptor mediates a RA response pathway that is important in cell growth regulation. This discovery of the interaction of RA with the M6P/IGF-II receptor may have important implications for our understanding of the roles of RA and the M6P/IGF-II receptor in development, carcinogenesis, and lysosomal enzyme-related diseases.

Developmental expression of insulin-like growth factor II receptor (IGF-IIR) in congenic mouse embryonic lungs: correlation between IGF-IIR mRNA and protein levels and heterochronic lung development

Embryonic lung maturation in the H-2 congenic pair, B10.A and B10, proceeds at different rates. The dependence of this heterochronic development on maternal haplotype suggests the involvement of a parentally imprinted gene. Since B10.A (H-2a) and B10 (H-2b) mice are genetically identical except for a 3-18 cM region of chromosome 17 that includes the H-2 complex, we sought a promising candidate gene(s) involved in regulating the rate of lung development from genes encoded in this region. The best candidate is the gene encoding the type II insulin-like growth factor receptor (IGF-IIR), whose ligand is the growth factor IGF-II. Only the maternal copy of this gene is expressed in postimplantation embryos. This receptor does not appear to transduce mitogenic signals; instead, IGF-IIR appears to regulate the levels of its ligand available to the growth-promoting type I IGF receptor (IGF-IR). Using in situ hybridization and indirect immunofluorescence, we demonstrate that IGF-IIR mRNA and protein are localized throughout the pulmonary mesenchyme, as well as in branching epithelia of the pseudoglandular and canalicular stages. We also examined the levels of IGF-IIR mRNA and protein expression by RNase protection assay and ligand blotting during the embryonic period of lung development in B10.A and B10 mice, and found that there is a highly significant positive correlation of IGF-IIR levels with progressive development in both strains. Further, slower-developing B10.A lungs contain significantly higher levels of IGF-IIR mRNA and protein than the more rapidly developing B10 lungs. These results suggest that haplotype-dependent elevation of IGF-IIR levels reduces the available concentration of IGF-II, resulting in a decreased rate of morphogenesis in B10.A mice. Heterochronic lung maturation, then, appears consequent to variable extracellular levels of this important growth factor. These results may be of clinical importance to predicting susceptibility to Respiratory Distress Syndrome in prenatal newborns.

Identification of functional insulin-like growth factor-II/mannose-6-phosphate receptors in isolated bone cells

The role of the IGF-II/cation independent mannose-6-phosphate (IGF-II/M6P) receptor in the transduction of cellular effects evoked by IGF-II has been extensively debated in the literature. Many reports suggest that IGF-II transduces its effects through the IGF-I receptor, while others show that IGF-II utilizes the type II receptor to affect cellular activity. This study 1) verifies the presence of the IGF-II/M6P receptor in rat calvarial osteoblasts, and 2) evaluates the ability of the receptor to initiate intracellular signals. Using conventional receptor binding assays, it was found that osteoblasts bind IGF-II with high affinity. Scatchard analyses indicated that there are 5.08 x 10(4) IGF-II/M6P receptors per osteoblast with a Kd near 2.0 nM). The receptor protein was further identified by cross-linking with 125I-IGF-II. Northern analysis was used to identify an mRNA transcript for the IGF-II/M6P receptor protein. To examine if the IGF-II/M6P receptor can initiate second messenger signals, the ability of IGF-II to evoke Ca2+ transients was evaluated. Osteoblasts pretreated with IGF-I did not lose their ability to respond to IGF-II. Further, a polyclonal antibody against the rat IGF-II/M6P receptor (R-II-PAB1) 1) was able to evoke its own Ca2+ response, and 2) was able to block the generation of Ca2+ transients caused by IGF-II. The data in this report show that the osteoblastic Ca2+ response to IGF-II appears to be caused by an intracellular release of Ca2+ which is mediated by the IGF-II/M6P receptor making it possible to envision how the receptor may be an important modulator of osteoblast mediated osteogenesis.

The insulin-like growth factor-II/mannose-6-phosphate receptor is present in fetal bovine tissues throughout gestation

The insulin-like growth factor-II (IGF-II) and the IGF-II/mannose-6-phosphate (M6P) receptor are thought to play an important role in fetal growth and development. We have studied the expression of the IGF-II/M6P receptor in fetal bovine tissues from 5 through 36 weeks' gestation. Tissues from bovine fetuses were extracted in buffer containing 2% Triton-X-100 and 2% sodium dodecyl sulfate (SDS). Aliquots of the protein extracts were analyzed by SDS polyacrylamide gel electrophoresis and the protein bands were transferred onto nitrocellulose. Immunoblotting was performed with anti-bovine IGF-II/M6P receptor antiserum. In a subset of experiments, ligand blotting was carried out with radiolabeled IGF-II and subsequent autoradiography. IGF-II/M6P receptors were expressed in all tissues examined, with the highest amount of receptor being present in fetal lung and liver. Low amounts of receptors were measured in fetal brain. The amount of receptor was developmentally regulated throughout fetal life. The developmental regulation of receptor expression varied among the different tissues. In conclusion, the IGF-II/M6P receptor is present in all fetal bovine tissues examined. The presence of the IGF-II/M6P receptor seems to be developmentally regulated during bovine fetal life. We hypothesize that this receptor exerts important biologic effects during fetal growth and tissue and organ development.

Disproportionate growth in mice with Igf-2 transgenes

Injection transgenesis was used to study the long-term effects of excess insulin-like growth factor II on mouse growth and differentiation. By using a construct in which the coding region of the mouse insulin like growth factor II gene (Igf-2) was placed under the control of a keratin gene promoter, four transgenic lines were established, all of which displayed overgrowth of the skin as judged by wrinkling. In addition to high levels of expression in the skin, transgene transcripts were also present in the alimentary canal and uterus. At most of the sites of transgene expression the cell number (DNA content) was greatly increased, indicating a local action of the excess insulin-like growth factor II on cell multiplication. Adult total live weight was slightly increased and there was no macroscopic evidence of tumor formation. The characteristics of these transgenic mice indicate distinct local and systemic actions for insulin-like growth factor II.