Mannose 6-phosphate receptors in sorting and transport of lysosomal enzymes

Structure of a truncated human GlcNAc-1-phosphotransferase variant reveals the basis for its hyperactivity

Mutations that cause loss of function of GlcNAc-1-phosphotransferase (PTase) lead to the lysosomal storage disorder mucolipidosis II. PTase is the key enzyme of the mannose 6-phosphate (M6P) targeting system that is responsible for tagging lysosomal hydrolases with the M6P moiety for their delivery to the lysosome. We had previously generated a truncated hyperactive form of PTase termed S1S3 which was shown to notably increase the phosphorylation level of secreted lysosomal enzymes and enhance their uptake by cells. Here, we report the 3.4 Å cryo-EM structure of soluble S1S3 lacking both transmembrane domains and cytosolic tails. The structure reveals a high degree of conservation of the catalytic core to full-length PTase. In this dimeric structure, the EF-hand of one protomer is observed interacting with the conserved region four of the other. In addition, we present a high-quality EM 3D map of the UDP-GlcNAc bound form of the full-length soluble protein showing the key molecular interactions between the nucleotide sugar donor and side chain amino acids of the protein. Finally, although the domain organization of S1S3 is very similar to that of the Drosophila melanogaster (fruit fly) PTase homolog, we establish that the latter does not act on lysosomal hydrolases.

Loss of the HOPS complex disrupts early-to-late endosome transition, impairs endosomal recycling and induces accumulation of amphisomes

The multisubunit HOPS tethering complex is a well-established regulator of lysosome fusion with late endosomes and autophagosomes. However, the role of the HOPS complex in other stages of endo-lysosomal trafficking is not well understood. To address this, we made HeLa cells knocked out for the HOPS-specific subunits Vps39 or Vps41, or the HOPS-CORVET-core subunits Vps18 or Vps11. In all four knockout cells, we found that endocytosed cargos were trapped in enlarged endosomes that clustered in the perinuclear area. By correlative light-electron microscopy, these endosomes showed a complex ultrastructure and hybrid molecular composition, displaying markers for early (Rab5, PtdIns3P, EEA1) as well as late (Rab7, CD63, LAMP1) endosomes. These "HOPS bodies" were not acidified, contained enzymatically inactive cathepsins and accumulated endocytosed cargo and cation-independent mannose-6-phosphate receptor (CI-MPR). Consequently, CI-MPR was depleted from the TGN, and secretion of lysosomal enzymes to the extracellular space was enhanced. Strikingly, HOPS bodies also contained the autophagy proteins p62 and LC3, defining them as amphisomes. Together, these findings show that depletion of the lysosomal HOPS complex has a profound impact on the functional organization of the entire endosomal system and suggest the existence of a HOPS-independent mechanism for amphisome formation.

Characterization of a HIR-Fab-IDS, Novel Iduronate 2-Sulfatase Fusion Protein for the Treatment of Neuropathic Mucopolysaccharidosis Type II (Hunter Syndrome)

BACKGROUND

Mucopolysaccharidosis type II is a severe lysosomal storage disease caused by deficient activity of the enzyme iduronate-2-sulfatase. The only medicinal product approved by the US Food and Drug Administration for enzyme replacement therapy, recombinant iduronate-2-sulfatase (idursulfase, Elaprase^®), is a large molecule that is not able to cross the blood-brain barrier and neutralize progressive damage of the central nervous system caused by the accumulation of glycosaminoglycans. Novel chimeric protein HIR-Fab-IDS is an anti-human insulin receptor Fab fragment fused to recombinant modified iduronate-2-sulfatase. This modification provides a highly selective interaction with the human insulin receptor, which leads to the HIR-Fab-IDS crossing the blood-brain barrier owing to internalization of the hybrid molecule by transcytosis into endothelial cells adjacent to the nervous system by the principle of a 'molecular Trojan horse'.

OBJECTIVES

In this work, the physicochemical and biological characterization of a blood-brain barrier-penetrating fusion protein, HIR-Fab-IDS, is carried out. HIR-Fab-IDS consists of an anti-human insulin receptor Fab fragment fused to recombinant iduronate-2-sulfatase.

METHODS

Comprehensive analytical characterization utilizing modern techniques (including surface plasmon resonance and mass spectrometry) was performed using preclinical and clinical batches of HIR-Fab-IDS. Critical quality parameters that determine the therapeutic effect of iduronate-2-sulfatase, as well as IDS enzymatic activity and in vitro cell uptake activity were evaluated in comparison with the marketed IDS product Elaprase^® (IDS RP). In vivo efficiency of HIR-Fab-IDS in reversing mucopolysaccharidosis type II pathology in IDS-deficient mice was also investigated. The affinity of the chimeric molecule for the INSR was also determined by both an enzyme-linked immunosorbent assay and surface plasmon resonance. We also compared the distribution of ¹²⁵I-radiolabeled HIR-Fab-IDS and IDS RP in the tissues and brain of cynomolgus monkeys after intravenous administration.

RESULTS

The HIR-Fab-IDS primary structure investigation showed no significant post-translational modifications that could affect IDS activity, except for the formylglycine content, which was significantly higher for HIR-Fab-IDS compared with that for IDS RP (~ 76.5 vs ~ 67.7%). Because of this fact, the specific enzyme activity of HIR-Fab-IDS was slightly higher than that of IDS RP (~ 2.73 × 10⁶ U/μmol vs ~ 2.16 × 10⁶ U/μmol). However, differences were found in the glycosylation patterns of the compared IDS products, causing a minor reduced in vitro cellular uptake of HIR-Fab-IDS by mucopolysaccharidosis type II fibroblasts compared with IDS RP (half-maximal effective concentration ~ 26.0 vs ~ 23.0 nM). The efficacy of HIR-Fab-IDS in IDS-deficient mice has demonstrated a statistically significant reduction in the level of glycosaminoglycans in the urine and tissues of the main organs to the level of healthy animals. The HIR-Fab-IDS has revealed high in vitro affinity for human and monkey insulin receptors, and the radioactively labeled product has been shown to penetrate to all parts of the brain and peripheral tissues after intravenous administration to cynomolgus monkeys.

CONCLUSIONS

These findings indicate that HIR-Fab-IDS, a novel iduronate-2-sulfatase fusion protein, is a promising candidate for the treatment of central nervous system manifestations in neurological mucopolysaccharidosis type II.

Sortilin knock-down alters the expression and distribution of cathepsin D and prosaposin and up-regulates the cation-dependent mannose-6-phosphate receptor in rat epididymal cells

The selective transport to lysosomes can be mediated by either mannose-6-phosphate receptors (CD-MPR and CI-MPR) or sortilin. In mammalian epididymis, some lysosomal proteins are secreted into the lumen through unknown mechanisms. To investigate the underlying mechanisms of lysosomal protein transport in epididymal cells we studied the expression and distribution of cathepsin D (CatD) and prosaposin (PSAP) in a sortilin knocked down RCE-1 epididymal cell line (RCE-1 KD) in comparison with non-transfected RCE-1 cells. In RCE-1 cells, CatD was found in the perinuclear zone and co-localize with sortilin, whereas in RCE-1 KD cells, the expression, distribution and processing of the enzyme were altered. In turn, PSAP accumulated intracellularly upon sortilin knock-down and redistributed from LAMP-1-positive compartment to a perinuclear location, remaining co-localized with CatD. Interestingly, the sortilin knock-down induced CD-MPR overexpression and a redistribution of the receptor from the perinuclear zone to a dispersed cytoplasmic location, accompanied by an increased co-localization with CatD. The increase in CD-MPR could result from a compensatory response for the proper delivery of CatD to lysosomes in epididymal cells. The intracellular pathway taken by lysosomal proteins could be an approach for addressing further studies to understand the mechanism of exocytosis and therefore the role of these proteins in the epididymis.

Invariant chain regulates endosomal fusion and maturation through an interaction with the SNARE Vti1b

The invariant chain (Ii, also known as CD74) is a multifunctional regulator of adaptive immune responses and is responsible for sorting major histocompatibility complex class I and class II (MHCI and MHCII, respectively) molecules, as well as other Ii-associated molecules, to a specific endosomal pathway. When Ii is expressed, endosomal maturation and proteolytic degradation of proteins are delayed and, in non-antigen presenting cells, the endosomal size increases, but the molecular mechanisms underlying this are not known. We identified that a SNARE, Vti1b, is essential for regulating these Ii-induced effects. Vti1b binds to Ii and is localized at the contact sites of fusing Ii-positive endosomes. Furthermore, truncated Ii lacking the cytoplasmic tail, which is not internalized from the plasma membrane, relocates Vti1b to the plasma membrane. Knockout of Ii in an antigen-presenting cell line was found to speed up endosomal maturation, whereas silencing of Vti1b inhibits the Ii-induced maturation delay. Our results suggest that Ii, by interacting with the SNARE Vti1b in antigen-presenting cells, directs specific Ii-associated SNARE-mediated fusion in the early part of the endosomal pathway that leads to a slower endosomal maturation for efficient antigen processing and MHC antigen loading.

Identification of immune-related proteins of Dreissena polymorpha hemocytes and plasma involved in host-microbe interactions by differential proteomics

Biological responses of zebra mussel Dreissena polymorpha are investigated to assess the impact of contaminants on aquatic organisms and ecosystems. In addition to concentrate chemical contaminants in their tissues, zebra mussels accumulate several microorganisms such as viruses, protozoa and bacteria. In order to understand the molecular mechanisms involved in the defence against microorganisms this study aims at identifying immune proteins from D. polymorpha hemolymph involved in defence against protozoa and viruses. For this purpose, hemolymph were exposed ex vivo to Cryptosporidium parvum and RNA poly I:C. Differential proteomics on both hemocytes and plasma revealed immune proteins modulated under exposures. Different patterns of response were observed after C. parvum and RNA poly I:C exposures. The number of modulated proteins per hemolymphatic compartments suggest that C. parvum is managed in cells while RNA poly I:C is managed in plasma after 4 h exposure. BLAST annotation and GO terms enrichment analysis revealed further characteristics of immune mechanisms. Results showed that many proteins involved in the recognition and destruction of microorganisms were modulated in both exposure conditions, while proteins related to phagocytosis and apoptosis were exclusively modulated by C. parvum. This differential proteomic analysis highlights in zebra mussels modulated proteins involved in the response to microorganisms, which reflect a broad range of immune mechanisms such as recognition, internalization and destruction of microorganisms. This study paves the way for the identification of new markers of immune processes that can be used to assess the impact of both chemical and biological contaminations on the health status of aquatic organisms.

Distinct anterograde trafficking pathways of BACE1 and amyloid precursor protein from the TGN and the regulation of amyloid-β production

Processing of amyloid precursor protein (APP) by the β-secretase BACE1 is the initial step of the amyloidogenic pathway to generate amyloid-β (Aβ). Although newly synthesized BACE1 and APP are transported along the secretory pathway, it is not known whether BACE1 and APP share the same post-Golgi trafficking pathways or are partitioned into different transport routes. Here we demonstrate that BACE1 exits the Golgi in HeLa cells and primary neurons by a pathway distinct from the trafficking pathway for APP. By using the Retention Using Selective Hooks system, we show that BACE1 is transported from the trans-Golgi network to the plasma membrane in an AP-1- and Arf1/4-dependent manner. Subsequently, BACE1 is endocytosed to early and recycling endosomes. Perturbation of BACE1 post-Golgi trafficking results in an increase in BACE1 cleavage of APP and increased production of both Aβ40 and Aβ42. These findings reveal that Golgi exit of BACE1 and APP in primary neurons is tightly regulated, resulting in their segregation along different transport routes, which limits APP processing.

Genome-wide Association of Endophenotypes for Schizophrenia From the Consortium on the Genetics of Schizophrenia (COGS) Study

IMPORTANCE

The Consortium on the Genetics of Schizophrenia (COGS) uses quantitative neurophysiological and neurocognitive endophenotypes with demonstrated deficits in schizophrenia as a platform from which to explore the underlying neural circuitry and genetic architecture. Many of these endophenotypes are associated with poor functional outcome in schizophrenia. Some are also endorsed as potential treatment targets by the US Food and Drug Administration.

OBJECTIVE

To build on prior assessments of heritability, association, and linkage in the COGS phase 1 (COGS-1) families by reporting a genome-wide association study (GWAS) of 11 schizophrenia-related endophenotypes in the independent phase 2 (COGS-2) cohort of patients with schizophrenia and healthy comparison participants (HCPs).

DESIGN, SETTING, AND PARTICIPANTS

A total of 1789 patients with schizophrenia and HCPs of self-reported European or Latino ancestry were recruited through a collaborative effort across the COGS sites and genotyped using the PsychChip. Standard quality control filters were applied, and more than 6.2 million variants with a genotyping call rate of greater than 0.99 were available after imputation. Association was performed for data sets stratified by diagnosis and ancestry using linear regression and adjusting for age, sex, and 5 principal components, with results combined through weighted meta-analysis. Data for COGS-1 were collected from January 6, 2003, to August 6, 2008; data for COGS-2, from June 30, 2010, to February 14, 2014. Data were analyzed from October 28, 2016, to May 4, 2018.

MAIN OUTCOMES AND MEASURES

A genome-wide association study was performed to evaluate association for 11 neurophysiological and neurocognitive endophenotypes targeting key domains of schizophrenia related to inhibition, attention, vigilance, learning, working memory, executive function, episodic memory, and social cognition.

RESULTS

The final sample of 1533 participants included 861 male participants (56.2%), and the mean (SD) age was 41.8 (13.6) years. In total, 7 genome-wide significant regions (P < 5 × 10-8) and 2 nearly significant regions (P < 9 × 10-8) containing several genes of interest, including NRG3 and HCN1, were identified for 7 endophenotypes. For each of the 11 endophenotypes, enrichment analyses performed at the level of P < 10-4 compared favorably with previous association results in the COGS-1 families and showed extensive overlap with regions identified for schizophrenia diagnosis.

CONCLUSIONS AND RELEVANCE

These analyses identified several genomic regions of interest that require further exploration and validation. These data seem to demonstrate the utility of endophenotypes for resolving the genetic architecture of schizophrenia and characterizing the underlying biological dysfunctions. Understanding the molecular basis of these endophenotypes may help to identify novel treatment targets and pave the way for precision-based medicine in schizophrenia and related psychotic disorders.

Syndecan-1 Mediates Sorting of Soluble Lipoprotein Lipase with Sphingomyelin-Rich Membrane in the Golgi Apparatus

In the secretory pathway, budding of vesicular transport carriers from the trans-Golgi network (TGN) must coordinate specification of lipid composition with selection of secreted proteins. We elucidate a mechanism of soluble protein cargo sorting into secretory vesicles with a sphingomyelin-rich membrane; the integral membrane proteoglycan Syndecan-1 (SDC1) acts as a sorting receptor, capturing the soluble enzyme lipoprotein lipase (LPL) during export from the TGN. Sorting of LPL requires bivalent interactions between LPL and SDC1-linked heparan sulfate chains and between LPL and the Golgi membrane. Physical features of the SDC1 transmembrane domain, rather than a specific sequence, confer targeting of SDC1 and bound LPL into the sphingomyelin secretion pathway. This study establishes that physicochemical properties of a protein transmembrane domain that drive lateral heterogeneity of the plasma membrane also operate at the TGN to confer sorting of an integral membrane protein and its ligand within the biosynthetic secretory pathway.

Cargo Sorting at the trans-Golgi Network for Shunting into Specific Transport Routes: Role of Arf Small G Proteins and Adaptor Complexes

The trans-Golgi network (TGN) is responsible for selectively recruiting newly synthesized cargo into transport carriers for delivery to their appropriate destination. In addition, the TGN is responsible for receiving and recycling cargo from endosomes. The membrane organization of the TGN facilitates the sorting of cargoes into distinct populations of transport vesicles. There have been significant advances in defining the molecular mechanism involved in the recognition of membrane cargoes for recruitment into different populations of transport carriers. This machinery includes cargo adaptors of the adaptor protein (AP) complex family, and monomeric Golgi-localized γ ear-containing Arf-binding protein (GGA) family, small G proteins, coat proteins, as well as accessory factors to promote budding and fission of transport vesicles. Here, we review this literature with a particular focus on the transport pathway(s) mediated by the individual cargo adaptors and the cargo motifs recognized by these adaptors. Defects in these cargo adaptors lead to a wide variety of diseases.

Insulin-Like Growth Factor 2 Receptor Expression Is Promoted by Human Herpesvirus 8-Encoded Interleukin-6 and Contributes to Viral Latency and Productive Replication

Human herpesvirus 8 (HHV-8) viral interleukin-6 (vIL-6) localizes largely to the endoplasmic reticulum (ER) and here associates functionally with both the gp130 signal transducer and the novel ER membrane protein vitamin K epoxide reductase complex subunit 1 variant-2 (VKORC1v2). The latter interaction contributes to the viability of latently infected primary effusion lymphoma (PEL) cells and to HHV-8 productive replication, in part via promotion of ER-associated degradation (ERAD) of nascent pro-cathepsin D (pCatD) and consequent suppression of lysosome-localized proapoptotic mature CatD. Here we report that VKORC1v2 associates with insulin-like growth factor 2 receptor (IGF2R), also known as cation-independent mannose-6-phosphate receptor, which is involved in trafficking of mannose-6-phosphate-conjugated glycoproteins to lysosomes. VKORC1v2 effected reduced IGF2R expression in a manner dependent on VKORC1v2-IGF2R interaction, while vIL-6, which could inhibit VKORC1v2-IGF2R interaction, effected increased expression of IGF2R. These effects were independent of changes in IGF2R mRNA levels, indicating likely posttranslational mechanisms. In kinetic analyses involving labeling of either newly synthesized or preexisting IGF2R, vIL-6 promoted accumulation of the former while having no detectable effect on the latter. Furthermore, vIL-6 led to decreased K48-linked ubiquitination of IGF2R and suppression of ERAD proteins effected increased IGF2R expression and loss of IGF2R regulation by vIL-6. Depletion-based experiments identified IGF2R as a promoter of PEL cell viability and virus yields from lytically reactivated cultures. Our findings identify ER-transiting nascent IGF2R as an interaction partner of VKORC1v2 and target of vIL-6 regulation and IGF2R as a positive contributor to HHV-8 biology, thereby extending understanding of the mechanisms of VKORC1v2-associated vIL-6 function.IMPORTANCE HHV-8 vIL-6 promotes productive replication in the context of reactivated lytic replication in primary effusion lymphoma (PEL) and endothelial cells and sustains latently infected PEL cell viability. Viral IL-6 is also considered to contribute significantly to HHV-8-associated pathogenesis, since vIL-6 can promote cell proliferation, cell survival, and angiogenesis that are characteristic of HHV-8-associated Kaposi's sarcoma, PEL and multicentric Castleman's disease (MCD), in addition to proinflammatory activities observed in MCD-like "Kaposi's sarcoma-associated herpesvirus-induced cytokine syndrome." We show in the present study that vIL-6 can promote productive replication and latent PEL cell viability through upregulation of the mannose-6-phosphate- and peptide hormone-interacting receptor IGF2R, which is a positive factor in HHV-8 biology via these activities. VKORC1v2-enhanced ER-associated degradation of IGF2R and vIL-6 promotion of IGF2R expression through prevention of its interaction with VKORC1v2 and consequent rescue from degradation represent newly recognized activities of VKOCR1v2 and vIL-6.

The Structure and Biological Function of CREG

The cellular repressor of E1A-stimulated genes (CREG) is a 220 amino acid glycoprotein structurally similar to oxidoreductases. However, CREG does not have enzymatic activities because it cannot bind to the cofactor flavin mononucleotide. Although CREG can be secreted, it is mainly an intracellular protein localized in the endocytic-lysosomal compartment. It undergoes proteolytic maturation mediated by lysosomal cysteine proteases. Biochemical studies have demonstrated that CREG interacts with mannose-6-phosphate/insulin-like growth factor-2 receptor (M6P/IGF2R) and exocyst Sec8. CREG inhibits proliferation and induces differentiation and senescence when overexpressed in cultured cells. In Drosophila, RNAi-mediated knockdown of CREG causes developmental lethality at the pupal stage. In mice, global deletion of the CREG1 gene leads to early embryonic death. These findings establish an essential role for CREG in development. CREG1 haploinsufficient and liver-specific knockout mice are susceptible to high fat diet-induced obesity, hepatic steatosis and insulin resistance. The purpose of this review is to provide an overview of what we know about the biochemistry and biology of CREG and to discuss the important questions that remain to be addressed in the future.

Rescue of Pompe disease in mice by AAV-mediated liver delivery of secretable acid α-glucosidase

Glycogen storage disease type II or Pompe disease is a severe neuromuscular disorder caused by mutations in the lysosomal enzyme, acid α-glucosidase (GAA), which result in pathological accumulation of glycogen throughout the body. Enzyme replacement therapy is available for Pompe disease; however, it has limited efficacy, has high immunogenicity, and fails to correct pathological glycogen accumulation in nervous tissue and skeletal muscle. Using bioinformatics analysis and protein engineering, we developed transgenes encoding GAA that could be expressed and secreted by hepatocytes. Then, we used adeno-associated virus (AAV) vectors optimized for hepatic expression to deliver the GAA transgenes to Gaa knockout (Gaa^-/-) mice, a model of Pompe disease. Therapeutic gene transfer to the liver rescued glycogen accumulation in muscle and the central nervous system, and ameliorated cardiac hypertrophy as well as muscle and respiratory dysfunction in the Gaa^-/- mice; mouse survival was also increased. Secretable GAA showed improved therapeutic efficacy and lower immunogenicity compared to nonengineered GAA. Scale-up to nonhuman primates, and modeling of GAA expression in primary human hepatocytes using hepatotropic AAV vectors, demonstrated the therapeutic potential of AAV vector-mediated liver expression of secretable GAA for treating pathological glycogen accumulation in multiple tissues in Pompe disease.

Selective induction of apoptosis in MCF7 cancer-cell by targeted liposomes functionalised with mannose-6-phosphate

Liposomes are versatile platforms to carry anticancer drugs in targeted drug delivery; they can be surface modified by different strategies and, when coupled with targeting ligands, are able to increase cellular internalisation and organelle-specific drug delivery. An interesting strategy of antitumoral therapy could involve the use of lysosomotropic ligand-targeted liposomes loaded with molecules, which can induce lysosomal membrane permeabilization (LMP), leakage of cathepsins into the cytoplasm and subsequent apoptosis. We have previously demonstrated the ability of liposomes functionalised with a mannose-6-phosphate to reach lysosomes; in this research we compare the behaviour of M6P-modified and non-functionalised liposomes in MCF7 tumour cell and in HDF normal cells. With this aim, we first demonstrated by Western blotting the overexpression of mannose-6-phosphate/insulin-like growth factor (M6P/IGF-II) receptor in MCF7. Then, we prepared calcein-loaded liposomes and we revealed the increased uptake of M6P-functionalised liposomes in MCF7 cells respect to HDF cells by flow cytometry analysis. Finally, we loaded functionalised and not functionalised liposomes with N-hexanoyl-d-erythro-sphingosine (C6Cer), able to initiate LMP-induced apoptosis; after having studied the stability of both vesicles in the presence of serum by Dynamic Light Scattering and Spectrophotometric turbidity measurements, we showed that ceramide-loaded M6P-liposomes significantly increased apoptosis in MCF7 with respect to HDF cells.

Development of idursulfase therapy for mucopolysaccharidosis type II (Hunter syndrome): the past, the present and the future

Mucopolysaccharidosis type II (MPS II; Hunter syndrome; OMIM 309900) is a rare, multisystemic, progressive lysosomal storage disease caused by deficient activity of the iduronate-2-sulfatase (I2S) enzyme. Accumulation of the glycosaminoglycans dermatan sulfate and heparan sulfate results in a broad range of disease manifestations that are highly variable in presentation and severity; notably, approximately two-thirds of individuals are affected by progressive central nervous system involvement. Historically, management of this disease was palliative; however, during the 1990s, I2S was purified to homogeneity for the first time, leading to cloning of the corresponding gene and offering a means of addressing the underlying cause of MPS II using enzyme replacement therapy (ERT). Recombinant I2S (idursulfase) was produced for ERT using a human cell line and was shown to be indistinguishable from endogenous I2S. Preclinical studies utilizing the intravenous route of administration provided valuable insights that informed the design of the subsequent clinical studies. The pivotal Phase II/III clinical trial of intravenous idursulfase (Elaprase^®; Shire, Lexington, MA, USA) demonstrated improvements in a range of clinical parameters; based on these findings, intravenous idursulfase was approved for use in patients with MPS II in the USA in 2006 and in Europe and Japan in 2007. Evidence gained from post-approval programs has helped to improve our knowledge and understanding of management of patients with the disease; as a result, idursulfase is now available to young pediatric patients, and in some countries patients have the option to receive their infusions at home. Although ERT with idursulfase has been shown to improve somatic signs and symptoms of MPS II, the drug does not cross the blood–brain barrier and so treatment of neurological aspects of the disease remains challenging. A number of novel approaches are being investigated, and these may help to improve the care of patients with MPS II in the future.

Unravelling novel functions of the endosomal transporter mannose 6-phosphate/insulin-like growth factor receptor (CD222) in health and disease: An emerging regulator of the immune system

Properly balanced cellular responses require both the mutual interactions of soluble factors with cell surface receptors and the crosstalk of intracellular molecules. In particular, immune cells exposed unceasingly to an array of positive and negative stimuli must distinguish between what has to be tolerated and attacked. Protein trafficking is one of crucial pathways involved in this labour. The approximately >270-kDa protein transporter called mannose 6- phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222) is a type I transmembrane glycoprotein present largely intracellularly in the Golgi apparatus and endosomal compartments, but also at the cell surface. It is expressed ubiquitously in a vast majority of higher eukaryotic cell types. Through binding and trafficking multiple unrelated extracellular and intracellular ligands, CD222 is involved in the regulation of a plethora of functions, and thus implicated in many physiological but also pathophysiological conditions. This review describes, first, general features of CD222, such as its evolution, genomic structure and regulation, protein structure and ligands; and second, its specific functions with a special focus on the immune system.

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.

mTORC1 regulates mannose-6-phosphate receptor transport and T-cell vulnerability to regulatory T cells by controlling kinesin KIF13A

Mannose-6-phosphate receptor (M6PR) that facilitates cellular uptake of M6P-bearing proteins, including serine-protease granzyme-B (Gzm-B) has an important role in T-cell activation, migration and contraction. However, molecular mechanisms controlling M6PR expression in T cells remain poorly understood. Here, we show that M6PR expression on T cells is distinctively controlled by two common γ-chain cytokines interleukin-2 (IL-2) and IL-7, and the differential M6PR expression is not caused by an altered synthesis of M6PR protein, but is a result of distinct regulation of kinesin-3 motor-protein KIF13A that transport M6PR onto cell surfaces. Using signaling pathway-specific inhibitors, we determine that IL-2 and IL-7 distinctly regulate KIF13A and β1-adaptin and cell-surface M6PR by controlling a kinase mammalian target of rapamycin complex-1 (mTORC1). Inflammatory cytokine IL-2 and prosurvival cytokine IL-7 induce strong and weak activation of mTORC1, leading to up- and downregulation of motor-protein KIF13A and KIF13A-motorized M6PR on T cells, and formation of IL-2 and IL-7 effectors with M6PR^high and M6PR^low cell-surface expression, respectively. Inhibition of mTORC1 by rapamycin reduces T-cell expression of KIF13A and cell-surface M6PR, and increases T-cell survival in Listeria monocytogenes-infected mice. Using regulatory T (T_reg)-cell-enriched mouse tumor model, we determine that M6PR^high IL-2 effectors but not M6PR^low IL-7 effectors adoptively transferred into tumors are vulnerable to T_reg Gzm-B-mediated cell apoptosis. Inhibition of mTORC1 or small interfering RNA-mediated knockdown of KIF13A or M6PR renders IL-2 effectors refractory to T_reg Gzm-B lethal hit. Overall, our data offer novel mechanistic insights into T-cell M6PR regulation, and T_reg-resistant/T_reg-susceptible phenomenon. Furthermore, regulation of T-cell fate vis-à-vis T_reg suppression via the mTORC1-KIF13A-M6PR axis provides a proof of concept for therapeutic strategies to target cancer, infectious and autoimmune diseases.

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.

Changes in lysosomal enzymes and mannose-6-phosphate receptors related to sexual maturation in bull epididymis

One of the most striking features of the mammalian epididymis is the secretion of lysosomal enzymes (LE). These LE may play a role in sperm maturation. In the present study we investigated the activity and distribution of four LE (?-galactosidase (?-Gal), N-acetyl-?-D-glucosaminidase (?-NAG), ?-mannosidase (?-Man) and ?-glucuronidase (?-Glu)) in bull epididymis at two different ages (6 months and 4 years) to determine whether these enzymes vary with sexual maturity. In young, sexually immature (SI) bulls we found high LE activity in the epididymal tissue that accounts for a developed and active lysosomal apparatus. In contrast, low LE activity was measured in sexually mature (SM) bulls, and ?-NAG and ?-Gal were mostly secreted into the lumen. We also attempted to correlate LE distribution with the expression and functionality of mannose-6-phosphate receptors (MPRs), which are thought to be involved in proper delivery of LE to lysosomes. The cation-dependent MPR was highly expressed in SI bulls, with expression decreasing during adulthood, whereas the expression of the cation-independent MPR was higher in SM than SI bulls. In addition, the four enzymes recovered from the epididymal lumen interact with both MPRs at each age. We conclude that the activity and distribution of LE in bull epididymis varies with sexual maturity and that the distribution is regulated differently by the two types of MPR. These findings could provide some molecular basis for male infertility.

Overexpression of the IGF-II/M6P receptor in mouse fibroblast cell lines differentially alters expression profiles of genes involved in Alzheimer's disease-related pathology

Alzheimer’s disease (AD) is the most common type of senile dementia affecting elderly people. The processing of amyloid precursor protein (APP) leading to the generation of β-amyloid (Aβ) peptide contributes to neurodegeneration and development of AD pathology. The endocytic trafficking pathway, which comprises of the endosomes and lysosomes, acts as an important site for Aβ generation, and endocytic dysfunction has been linked to increased Aβ production and loss of neurons in AD brains. Since insulin-like growth factor-II (IGF-II) receptor plays a critical role in the transport of lysosomal enzymes from the trans-Golgi network to endosomes, it is likely that the receptor may have a role in regulating Aβ metabolism in AD pathology. However, very little is known on how altered levels of the IGF-II receptor can influence the expression/function of various molecules involved in AD pathology. To address this issue, we evaluated the expression profiles of 87 selected genes related to AD pathology in mouse fibroblast MS cells that are deficient in murine IGF-II receptor and corresponding MS9II cells overexpressing ∼500 times the human IGF-II receptors. Our results reveal that an elevation in IGF-II receptor levels alters the expression profiles of a number of genes including APP as well as enzymes regulating Aβ production, degradation and clearance mechanisms. Additionally, it influences the expression of various lysosomal enzymes and protein kinases that are involved in Aβ toxicity. IGF-II receptor overexpression also alters expression of several genes involved in intracellular signalling as well as cholesterol metabolism, which play a critical role in AD pathology. The altered gene profiles observed in this study closely match with the corresponding protein levels, with a few exceptions. These results, taken together, suggest that an elevation in IGF-II receptor levels can influence the expression profiles of transcripts as well as proteins that are involved in AD pathogenesis.

Insulin-like growth factor II peptide fusion enables uptake and lysosomal delivery of α-N-acetylglucosaminidase to mucopolysaccharidosis type IIIB fibroblasts

Enzyme replacement therapy for MPS IIIB (mucopolysaccharidosis type IIIB; also known as Sanfilippo B syndrome) has been hindered by inadequate mannose 6 phosphorylation and cellular uptake of rhNAGLU (recombinant human α-N-acetylglucosaminidase). We expressed and characterized a modified rhNAGLU fused to the receptor-binding motif of IGF-II (insulin-like growth factor 2) (rhNAGLU-IGF-II) to enhance its ability to enter cells using the cation-independent mannose 6-phosphate receptor, which is also the receptor for IGF-II (at a different binding site). RhNAGLU-IGF-II was stably expressed in CHO (Chinese-hamster ovary) cells, secreted and purified to apparent homogeneity. The Km and pH optimum of the fusion enzyme was similar to those reported for rhNAGLU. Both intracellular uptake and confocal microscopy suggested that MPS IIIB fibroblasts readily take up the fusion enzyme via receptor-mediated endocytosis that was inhibited significantly (P<0.001) by the monomeric IGF-II peptide. Glycosaminoglycan storage was reduced by 60% (P<0.001) to near background levels in MPS IIIB cells after treatment with rhNAGLU-IGF-II, with half-maximal correction at concentrations of 3-12 pM. A similar cellular uptake mechanism via the IGF-II receptor was also demonstrated in two different brain tumour-derived cell lines. Fusion of rhNAGLU to IGF-II enhanced its cellular uptake while maintaining enzymatic activity, supporting its potential as a therapeutic candidate for treating MPS IIIB.

First-in-human study with new recombinant agalsidase beta (ISU303) in healthy subjects

ISU303 is a new recombinant agalsidase beta (Agal) enzyme replacement therapy under investigation for Fabry disease, caused by a deficiency in α-galactosidase A activity that leads to fatty deposits in tissues. We evaluated the pharmacokinetic (PK) parameters, safety and tolerability of ISU303 in healthy adult volunteers. The study was a dose block-randomized, double-blinded, placebo-controlled, single-dosing, and dose escalation phase 1 clinical trial. A total of 18 healthy subjects were enrolled (0.3 mg/kg, n = 6; 1.0 mg/kg, n = 6; placebo, n = 6). Blood samples for PK analysis were collected according to planned time. The PK parameters in each 0.3 and 1.0 mg/kg Agal group were as follows: Cmax (mU/mL) 43.19 ± 5.9 and 195.86 ± 32.3; AUClast (h·mU/mL) 207.91 ± 25.1 and 939.96 ± 158.3; t1/2 (hours) 1.13 ± 0.3 and 1.46 ± 0.2; Cl (mL/min/kg) 1.79 ± 0.2 and 1.34 ± 0.2, respectively. There were seven adverse events (AE) overall. All AEs were resolved without any complications. None were related to the study drug. There were no immunogenicity or any significant infusion-related reactions. The new Agal product exhibited a dose-dependent PK and was well tolerated with no significant AEs in healthy adult volunteers.

Long-term correction of biochemical and neurological abnormalities in MLD mice model by neonatal systemic injection of an AAV serotype 9 vector

As both the immune system and the blood-brain barrier (BBB) are likely to be developmentally immature in the perinatal period, neonatal gene transfer may be useful for the treatment of lysosomal storage disease (LSD) with neurological involvements such as metachromatic leukodystrophy (MLD). In this experiment, we examined the feasibility of single-strand adeno-associated viral serotype-9 (ssAAV9)-mediated systemic neonatal gene therapy of MLD mice. ssAAV9 vector expressing human arylsulfatase A (ASA) and green fluorescent protein (GFP) (ssAAV9/ASA) was injected into the jugular vein of newborn MLD mice. High levels of ASA expression were observed in the muscle and heart for at least 15 months. ASA was continuously secreted into plasma without development of antibodies against ASA. Global gene transfer into the brain and spinal cord (SC), across the BBB, and long-term ASA expression in the central nervous system were detected in treated mice. Significant inhibition of the accumulation of sulfatide (Sulf) in the brain and cervical SC was confirmed by Alcian blue staining and biochemical analysis of the Sulf content. In a behavior test, treated mice showed a greater ability to traverse narrow balance beams than untreated mice. These data clearly demonstrate that MLD mice model can be effectively treated through neonatal systemic injection of ssAAV9/ASA.

Growth factor signaling and memory formation: temporal and spatial integration of a molecular network

Growth factor (GF) signaling is critically important for developmental plasticity. It also plays a crucial role in adult plasticity, such as that required for memory formation. Although different GFs interact with receptors containing distinct types of kinase domains, they typically signal through converging intracellular cascades (e.g., Ras–MEK–MAPK) to mediate overlapping functional endpoints. Several GFs have been implicated in memory formation, but due to a high level of convergent signaling, the unique contributions of individual GFs as well as the interactions between GF signaling cascades during the induction of memory is not well known. In this review, we highlight the unique roles of specific GFs in dendritic plasticity, and discuss the spatial and temporal profiles of different GFs during memory formation. Collectively, the data suggest that the roles of GF signaling in long-lasting behavioral and structural plasticity may be best viewed as interactive components in a complex molecular network.

The AP-1 complex regulates intracellular localization of insulin receptor substrate 1, which is required for insulin-like growth factor I-dependent cell proliferation

The activation of the insulin/insulin-like growth factor I (IGF-I) receptor and the subsequent tyrosine phosphorylation of insulin receptor substrates (IRSs) are key initial events in a variety of insulin/IGF bioactivities, including mitogenesis. It has been reported that IRS-1 associates with intracellular membrane compartments, and this localization is believed to be important for insulin/IGF signal transduction. However, the molecular mechanisms underlying IRS-1 localization remain unclear. Here we show that in L6 myoblasts, IRS-1 associates with μ1A of the ubiquitously expressed AP-1 complex, which packages cargo proteins into clathrin-coated vesicles derived from intracellular membranes. While wild-type IRS-1 was predominantly localized to vesicular structures, IRS-1 mutants lacking three YXXΦ motifs responsible for binding to μ1A were mislocalized to the mannose-6-phosphate receptor-positive structures, suggesting that AP-1-dependent transport to peripheral vesicles is inhibited in these mutants. Furthermore, deletion of AP-1 binding sites in IRS-1 impaired IGF-I-induced cell proliferation, accompanied by reduced tyrosine phosphorylation of IRS-1 and its association with phosphoinositide (PI) 3-kinase. These data demonstrate the importance of AP-1-dependent localization of IRS-1 in mediating IGF-I-stimulated signaling and maximum mitogenic response.

Testosterone influences the expression and distribution of the cation-dependent mannose-6-phosphate receptor in rat epididymis. Implications in the distribution of enzymes

The mammalian epididymis plays a role in sperm maturation through its secretory activity. Among the proteins secreted by the epithelium, there are significant amounts of acid hydrolases. In most cell types, the normal distribution of lysosomal enzymes is mediated by mannose-6-phosphate receptors (MPRs). In this study, we analysed the expression and distribution of the cation-dependent MPR (CD-MPR) in epididymis from control, castrated or castrated rats with testosterone replacement. It was observed that expression of CD-MPR increased due to castration in all regions of the epididymis, which was reversed by injection of testosterone. We also measured the activity of α-mannosidase and observed that the castration tends to increase the retention of this enzyme in the tissue, which is reversed by the hormone replacement. In corpus, this resulted in a reduced secretion of the enzyme. Immunohistochemistry showed that CD-MPR has a supranuclear location (different from the cation-independent MPR), most likely in principal cells, and low reactivity in other cell types. The signal in castrated animals was more intense and tended to redistribute towards the apical cytoplasm. Thus, we concluded that expression and distribution of CD-MPR is affected by decrease of testosterone in rat epididymis, and this could change the distribution of lysosomal enzymes.

A bacterial glycosidase enables mannose-6-phosphate modification and improved cellular uptake of yeast-produced recombinant human lysosomal enzymes

Lysosomal storage diseases are treated with human lysosomal enzymes produced in mammalian cells. Such enzyme therapeutics contain relatively low levels of mannose-6-phosphate, which is required to target them to the lysosomes of patient cells. Here we describe a method for increasing mannose-6-phosphate modification of lysosomal enzymes produced in yeast. We identified a glycosidase from C. cellulans that 'uncaps' N-glycans modified by yeast-type mannose-Pi-6-mannose to generate mammalian-type N-glycans with a mannose-6-phosphate substitution. Determination of the crystal structure of this glycosidase provided insight into its substrate specificity. We used this uncapping enzyme together with α-mannosidase to produce in yeast a form of the Pompe disease enzyme α-glucosidase rich in mannose-6-phosphate. Compared with the currently used therapeutic version, this form of α-glucosidase was more efficiently taken up by fibroblasts from Pompe disease patients, and it more effectively reduced cardiac muscular glycogen storage in a mouse model of the disease.

Single-transmembrane domain IGF-II/M6P receptor: potential interaction with G protein and its association with cholesterol-rich membrane domains

The IGF-II/mannose 6-phosphate (M6P) receptor is a single-transmembrane domain glycoprotein that plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of IGF-II. The receptor may also mediate certain biological effects in response to IGF-II binding by interacting with G proteins. However, the nature of the IGF-II/M6P receptor's interaction with the G protein or with G protein-coupled receptor (GPCR) interacting proteins such as β-arrestin remains unclear. Here we report that [(125)I]IGF-II receptor binding in the rat hippocampal formation is sensitive to guanosine-5'-[γ-thio]triphosphate, mastoparan, and Mas-7, which are known to interfere with the coupling of the classical GPCR with G protein. Monovalent and divalent cations also influenced [(125)I]IGF-II receptor binding. The IGF-II/M6P receptor, as observed for several GPCRs, was found to be associated with β-arrestin 2, which exhibits sustained ubiquitination after stimulation with Leu(27)IGF-II, an IGF-II analog that binds rather selectively to the IGF-II/M6P receptor. Activation of the receptor by Leu(27)IGF-II induced stimulation of extracellular signal-related kinase 1/2 via a pertussis toxin-dependent pathway. Additionally, we have shown that IGF-II/M6P receptors under normal conditions are associated mostly with detergent-resistant membrane domains, but after stimulation with Leu(27)IGF-II, are translocated to the detergent-soluble fraction along with a portion of β-arrestin 2. Collectively these results suggest that the IGF-II/M6P receptor may interact either directly or indirectly with G protein as well as β-arrestin 2, and activation of the receptor by an agonist can lead to alteration in its subcellular distribution along with stimulation of an intracellular signaling cascade.

Artificial ubiquitylation is sufficient for sorting of a plasma membrane ATPase to the vacuolar lumen of Arabidopsis cells

Sorting of transmembrane proteins into the inner vesicles of multivesicular bodies for subsequent delivery to the vacuole/lysosome can be induced by attachment of a single ubiquitin or K63-linked ubiquitin chains to the cytosolic portion of the cargo in yeast and mammals. In plants, large efforts have been undertaken to elucidate the mechanisms of vacuolar trafficking of soluble proteins. Sorting of transmembrane proteins, by contrast, is still largely unexplored. As a proof of principle, that ubiquitin is involved in vacuolar sorting in plants we show that a translational fusion of a single ubiquitin to the Arabidopsis plasma membrane ATPase PMA-EGFP is sufficient to induce its endocytosis and sorting into the vacuolar lumen. Sorting of the artificial reporter is not dependent on ubiquitin chain formation, but involves ubiquitin's hydrophobic patch and can be inhibited by coexpression of a dominant-negative version of the ESCRT (endosomal sorting complex required for transport) related protein AtSKD1 (SUPPRESSOR OF K+ TRANSPORT GROWTH DEFECT1). Our results suggest that ubiquitin can in principle act as vacuolar sorting signal in plants.

(Pro)renin and its receptors: pathophysiological implications

Tissue angiotensin generation depends on the uptake of circulating (kidney-derived) renin and/or its precursor prorenin [together denoted as (pro)renin]. Since tissue renin levels are usually somewhat higher than expected based upon the amount of (renin-containing) blood in tissue, an active uptake mechanism has been proposed. Several candidates have been evaluated in the past three decades, including a renin-binding protein, the mannose 6-phosphate/insulin-like growth factor II receptor and the (pro)renin receptor. Although the latter seemed the most promising, its nanomolar affinity for renin and prorenin is several orders of magnitude above their actual (picomolar) levels in blood, raising doubt on whether (pro)renin–(pro)renin receptor interaction will ever occur in vivo. A wide range of in vitro studies have now demonstrated (pro)renin-receptor-induced effects at nanomolar renin and prorenin concentrations, resulting in a profibrotic phenotype. In addition, beneficial in vivo effects of the putative (pro)renin receptor blocker HRP (handle region peptide) have been observed, particularly in diabetic animal models. Despite these encouraging results, many other studies have reported either no or even contrasting effects of HRP, and (pro)renin-receptor-knockout studies revealed lethal consequences that are (pro)renin-independent, most probably due to the fact that the (pro)renin receptor co-localizes with vacuolar H+-ATPase and possibly determines the stability of this vital enzyme. The present review summarizes all of the recent findings on the (pro)renin receptor and its blockade, and critically compares it with the other candidates that have been proposed to mediate (pro)renin uptake from blood. It ends with the conclusion that the (pro)renin–(pro)renin receptor interaction, if it occurs in vivo, is limited to (pro)renin-synthesizing organs such as the kidney.

The distribution of mannose-6-phosphate receptors changes from newborns to adults in rat liver

The co-existence of two types of mannose-6-phosphate receptors (CD-MPR and CI-MPR) in most cell types is still not well explained. Some evidence suggests that the CI-MPR could be actively involved in the regulation of growth factors in the early stages of mammalian organ development. In this study, it was demonstrated that both receptors are distributed in a non-overlapping fashion in rat liver, and that the distribution of CI-MPR changes over a percoll gradient between newborn and adult animals. By using marker proteins it was observed that in newborns the CI-MPR is located both in intracellular fractions and in fractions that coincide with a plasma membrane marker, whereas in adults it is only detected in intracellular fractions. It was also noted that N-acetyl-β-D-glucosaminidase distribution is closer to CI-MPR than to CD-MPR and that acid phosphatase did not match with any receptor. This evidence may also suggest that both receptors have different functions, mainly at early stages in the development of organs.

Role of miR-204 in the regulation of apoptosis, endoplasmic reticulum stress response, and inflammation in human trabecular meshwork cells

PURPOSE

To investigate the biological functions of miR-204 in human trabecular meshwork (HTM) cells.

METHODS

Changes in gene expression induced by miR-204 in HTM cells were evaluated by gene array analysis using arrays and confirmed by quantitative-PCR (Q-PCR). Direct targeting of miR-204 to 12 potential novel targets was confirmed using a luciferase system, and five of them were verified by Western blot analysis. Effects of miR-204 on apoptosis, cell viability, and accumulation of carbonylated proteins were evaluated in HTM cells treated with H(2)O(2). Induction of endoplasmic reticulum (ER) stress markers by tunicamycin was analyzed by Q-PCR, and expression of IL-8 and IL-11 was analyzed by ELISA.

RESULTS

MiR-204 decreased the expression of multiple genes in HTM cells. Twelve genes (AP1S2, Bcl2l2, BIRC2, EDEM1, EZR, FZD1, M6PR, RAB22A, RAB40B, SERP1, TCF12, and TCF4) were validated as direct targets of miR-204. Downregulation of expressions at protein levels of Bcl2l2, BIRC2, EZR, M6PR, and SERP1 were confirmed by Western blot analysis. HTM cells transfected with miR-204 showed increased levels of apoptosis, decreased viability, increased accumulation of oxidized proteins after H(2)O(2) treatment, decreased induction of ER stress response markers, and reduced expression of inflammatory mediators IL-8 and IL-11.

CONCLUSIONS

MiR-204 potentially plays an important role in the regulation of multiple functions in HTM cells including apoptosis, accumulation of damaged proteins, ER stress response, and expression of inflammatory mediators.

Whole-blood alpha-D-galactosidase A activity for the identification of Fabry's patients

OBJECTIVES

ERT application to Fabry's disease patients needs sensitive assay method of the missing enzyme (α-d-galactosidase A) to achieve early diagnosis.

DESIGN AND METHODS

A new fluorimetric assay method of α-d-galactosidase A was developed, using whole blood (WB) from 30 healthy individuals, 7 hemizygous males and 7 heterozygous females with Fabry's disease. This method was compared with the traditional dried blood spot (DBS) method.

RESULTS

WB method analytical characteristics are: linearity up to 2000mU/L; detection limit: 4mU/L; linearity versus time: 6h; enzyme stability: 7 days at 4°C; total analytical imprecision: from 3.27% to 5.72%. Sensitivity was higher in WB than DBS method. All hemizygous Fabry's patients were identified by both the WB and DBS methods. With regards to the seven heterozygous carriers five could be identified by the WB methods and three by the DBS method.

CONCLUSION

The WB assay method for α-D-galactosidase A appears to be reliable and proposable as a routine method for prompt diagnosis of Fabry disease in selected at-risk populations.

Direct gene transfer to the CNS prevents emergence of neurologic disease in a murine model of mucopolysaccharidosis type I

The mucopolysaccharidoses (MPSs) are a group of 11 storage diseases caused by disruptions in glycosaminoglycan (GAG) catabolism, leading to their accumulation in lysosomes. Resultant multisystemic disease is manifested by growth delay, hepatosplenomegaly, skeletal dysplasias, cardiopulmonary obstruction, and, in severe MPS I, II, III, and VII, progressive neurocognitive decline. Some MPSs are treated by allogeneic hematopoietic stem cell transplantation (HSCT) and/or recombinant enzyme replacement therapy (ERT), but effectiveness is limited by central nervous system (CNS) access across the blood-brain barrier. To provide a high level of gene product to the CNS, we tested neonatal intracerebroventricular (ICV) infusion of an adeno-associated virus (AAV) serotype 8 vector transducing the human α-L-iduronidase gene in MPS I mice. Supranormal levels of iduronidase activity in the brain (including 40× normal levels in the hippocampus) were associated with transduction of neurons in motor and limbic areas identifiable by immunofluorescence staining. The treatment prevented accumulation of GAG and GM3 ganglioside storage materials and emergence of neurocognitive dysfunction in a modified Morris water maze test. The results suggest the potential of improved outcome for MPSs and other neurological diseases when a high level of gene expression can be achieved by direct, early administration of vector to the CNS.

Glycoproteomic analysis of embryonic stem cells: identification of potential glycobiomarkers using lectin affinity chromatography of glycopeptides

Numerous studies have recently focused on the identification of specific glycan biomarkers, given the important roles that protein linked glycans play, for example, during development and disease progression. The identification of protein glycobiomarkers, which are part of a very complex proteome, has involved the use of fractionation techniques such as lectin affinity chromatography. In this study, the glycoproteomic characterization of pluripotent murine embryonic stem cells (ES) and from ES cells that were differentiated into embroid bodies (EB) was performed using immobilized Concanavalin A (ConA). This procedure allowed the isolation of glycopeptides that express biantennary and hybrid N-linked structures (ConA2 fraction) as well as high mannose glycans (ConA3 fraction) that were abundant in both ES and EB stages. A total of 293 unique N-linked glycopeptide sequences (from 180 glycoproteins) were identified in the combined data sets from ES and EB cells. Of these glycopeptides, a total of 119 sequences were identified exclusively in only one of the lectin-bound fractions (24 in the ES-ConA2, 15 in the ES-ConA3, 16 in the EB-ConA2, and 64 in the EB-ConA3). Results from this study allowed the identification of individual N-glycosylation sites of proteins that express specific glycan types. The absence of some of these lectin-bound glycopeptides in a cell stage suggested that they were derived from proteins that were either expressed exclusively on a defined developmental stage or were expressed in both cell stages but carried the lectin-bound oligosaccharides in only one of them. Therefore, these lectin-bound glycopeptides can be considered as stage-specific glycobiomarkers.

Leu27 insulin-like growth factor-II, an insulin-like growth factor-II analog, attenuates depolarization-evoked GABA release from adult rat hippocampal and cortical slices

Accumulated evidence suggests that the single transmembrane domain insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/M6P or IGF-II receptor) plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of insulin like growth factor (IGF-II). However, the role of this receptor in signal transduction following IGF-II binding remains controversial. In the present study, we revealed that Leu(27)IGF-II, an analog which binds preferentially to the IGF-II receptor, can attenuate K(+)-as well as veratridine-evoked GABA release from the adult rat hippocampal formation. Tetrodotoxin failed to alter the effects of Leu(27)IGF-II on GABA release, thus suggesting the lack of involvement of voltage-dependent Na(+) channels. Interestingly, the effect is found to be sensitive to pertussis toxin (PTX), indicating the possible involvement of a Gi/o protein-dependent pathway in mediating the release of GABA from the hippocampal slices. Additionally, Leu(27)IGF-II was found to attenuate GABA release from frontal cortex but not from striatum. These results, together with the evidence that IGF-II receptors are localized on GABAergic neurons, raised the possibility that this receptor, apart from mediating intracellular trafficking, may also be involved in the regulation of endogenous GABA release by acting directly on GABAergic terminals.

Lignin Peroxidase Isozymes from Phanerochaete chrysosporium Can Be Enzymatically Dephosphorylated

The extracellular lignin peroxidase (LIP) protein profile of the fungus Phanerochaete chrysosporium, grown in nonimmersed liquid culture under conditions of excess nitrogen, changed markedly with culture age. At peak LIP activity (day 4), the heme-protein profile in the extracellular fluid, analyzed by anion-exchange high-pressure liquid chromatography, was characterized by a predominance of the LIP isozymes H1 and H2, small amounts of H6 and H8, and other minor peaks, designated Ha and Hb. On day 5, the level of H1 increased and it became the dominant isozyme, with a corresponding decrease in the level of H2. Moreover, the relative levels of H6 and H8 decreased with corresponding increases in Ha and Hb levels. This change in LIP profile occurred extracellularly and resulted from the enzymatic dephosphorylation of LIP isozymes. An enzymatic fraction responsible for LIP isozyme dephosphorylation, termed LIP dephosphorylating (LpD) fraction, was partially purified from the culture fluid. Incubation of the LpD fraction with (sup32)P-labeled H2, H6, H8, and H10 isozymes separated from nitrogen-limited cultures resulted in the formation of the dephosphorylated isozymes H1, Ha, Hb, and Hc, respectively. Dephosphorylation did not significantly change the catalytic properties of the LIP isozymes with veratryl alcohol as a substrate. LIP dephosphorylation is therefore suggested to be a posttranslational modification process catalyzed extracellularly by the LpD activity.

Successful treatment of metachromatic leukodystrophy using bone marrow transplantation of HoxB4 overexpressing cells

To evaluate the contribution of bone marrow (BM) cells to treat neurological disorders, we examined the effectiveness of BM cells expressing the homeobox B4 (HoxB4) gene to cure mice with metachromatic leukodystrophy (MLD) through transplantation. Increased number of donor cells was observed in brains of the MLD mice transplanted with HoxB4-transduced BM cells (B4MLD) in contrast to those transplanted with control green fluorescent protein (GFP)-transduced BM cells (MIGMLD). Immunohistochemical staining showed that most of the GFP(+) cells were Iba1(+) microglia. In addition, O4(+) oligodendrocytes were identified only in the B4MLD brains but not in the MIGMLD brain. Alcian blue staining showed that accumulation of sulfatide was dramatically reduced in brain tissue from B4MLD mice, and there was a corresponding improvement in the animals' ability to walk a balance beam 8 months after transplantation. Thus transplantation of BM cells overexpressing HoxB4 appears to effectively prevent the progression of MLD in this mouse model. These findings support the idea that hematopoietic stem cells (HSCs) transduced with a HoxB4 expression vector could be the useful carriers of therapeutic proteins into the brain for regeneration of oligodendrocytes to treat such demyelinating disorders as MLD, Krabbe disease, and multiple sclerosis.

A membrane-proximal region in the C-terminal tail of NHE7 is required for its distribution in the trans-Golgi network, distinct from NHE6 localization at endosomes

Mammalian Na(+)/H(+) exchanger (NHE) isoform NHE6 is localized in sorting/recycling endosomes, whereas NHE7 is localized in the trans-Golgi network (TGN) and mid-trans-Golgi stacks. The mechanism targeting each NHE to a specific organelle is largely unknown, although the targeting is thought to be important for pH control in the lumen of various organelles. NHE6 and NHE7 exhibit distinct localization despite conserved amino acid sequences. To specify the intramolecular region involved in the specific localization, we examined the intracellular localization of chimeric NHE6 and NHE7 constructs. NHEs are composed of an N-terminal transmembrane domain (TM) and a C-terminal hydrophilic tail domain (Ct). Exchange of the Ct between the isoforms suggested that the Ct is required for the specific localization. We further split the Ct into three regions, and chimeras with various combinations of these small regions indicated that the most membrane-proximal region among the three contributes to the specific localization. Mutant forms of NHE7 with sequential alanine substitutions in the most membrane-proximal region, between residues 530 and 589, showed that two regions (residues 553-559 and 563-568) are required for NHE7-like localization. However, NHE6 with alanine substitutions in the membrane-proximal region exhibited no apparent change in localization. These results suggest that two membrane proximal regions (residues 533-559 and 563-568) play an important role in targeting NHE7 to the TGN.

The clavesin family, neuron-specific lipid- and clathrin-binding Sec14 proteins regulating lysosomal morphology

Clathrin-coated vesicles (CCVs) originating from the trans-Golgi network (TGN) provide a major transport pathway from the secretory system to endosomes/lysosomes. Herein we describe paralogous Sec14 domain-bearing proteins, clavesin 1/CRALBPL and clavesin 2, identified through a proteomic analysis of CCVs. Clavesins are enriched on CCVs and form a complex with clathrin heavy chain (CHC) and adaptor protein-1, major coat components of TGN-derived CCVs. The proteins co-localize with markers of endosomes and the TGN as well as with CHC and adaptor protein-1. A membrane mimic assay using the Sec14 domain of clavesin 1 reveals phosphatidylinositol 3,5-bisphosphate as a specific lipid partner. Phosphatidylinositol 3,5-bisphosphate is localized to late endosomes/lysosomes, and interestingly, isoform-specific knockdown of clavesins in neurons using lentiviral delivery of interfering RNA leads to enlargement of a lysosome-associated membrane protein 1-positive membrane compartment with no obvious influence on the CCV machinery at the TGN. Since clavesins are expressed exclusively in neurons, this new protein family appears to provide a unique neuron-specific regulation of late endosome/lysosome morphology.

Glycan microarray analysis of P-type lectins reveals distinct phosphomannose glycan recognition

The specificity of the cation-independent and -dependent mannose 6-phosphate receptors (CI-MPR and CD-MPR) for high mannose-type N-glycans of defined structure containing zero, one, or two Man-P-GlcNAc phosphodiester or Man-6-P phosphomonoester residues was determined by analysis on a phosphorylated glycan microarray. Amine-activated glycans were covalently printed on N-hydroxysuccinimide-activated glass slides and interrogated with different concentrations of recombinant CD-MPR or soluble CI-MPR. Neither receptor bound to non-phosphorylated glycans. The CD-MPR bound weakly or undetectably to the phosphodiester derivatives, but strongly to the phosphomonoester-containing glycans with the exception of a single Man7GlcNAc2-R isomer that contained a single Man-6-P residue. By contrast, the CI-MPR bound with high affinity to glycans containing either phospho-mono- or -diesters although, like the CD-MPR, it differentially recognized isomers of phosphorylated Man7GlcNAc2-R. This differential recognition of phosphorylated glycans by the CI- and CD-MPRs has implications for understanding the biosynthesis and targeting of lysosomal hydrolases.

The inactivation of the sortilin gene leads to a partial disruption of prosaposin trafficking to the lysosomes

Lysosomes are intracellular organelles which contain enzymes and activator proteins involved in the digestion and recycling of a variety of cellular and extracellular substances. We have identified a novel sorting receptor, sortilin, which is involved in the lysosomal trafficking of the sphingolipid activator proteins, prosaposin and GM(2)AP, and the soluble hydrolases cathepsin D, cathepsin H, and acid sphingomyelinase. Sortilin belongs to a growing family of receptors with homology to the yeast Vps10 protein, which acts as a lysosomal sorting receptor for carboxypeptidase Y. In this study we examined the effects of the sortilin gene inactivation in mice. The inactivation of this gene did not yield any noticeable lysosomal pathology. To determine the existence of an alternative receptor complementing the sorting function of sortilin, we quantified the concentration of prosaposin in the lysosomes of the nonciliated epithelial cells lining the efferent ducts. These cells were chosen because they express sortilin and have a large number of lysosomes containing prosaposin. In addition, the nonciliated cells are known to endocytose luminal prosaposin that is synthesized and secreted by Sertoli cells into the seminiferous luminal fluids. Consequently, the nonciliated cells are capable of targeting both exogenous and endogenous prosaposin to the lysosomes. Using electron microscope immunogold labeling and quantitative analysis, our results demonstrate that inactivation of the sortilin gene produces a significant decrease of prosaposin in the lysosomes. When luminal prosaposin was excluded from the efferent ducts, the level of prosaposin in lysosomes was even lower in the mutant mice. Nonetheless, a significant amount of prosaposin continues to reach the lysosomal compartment. These results strongly suggest the existence of an alternative receptor that complements the function of sortilin and explains the lack of lysosomal storage disorders in the sortilin-deficient mice.