Glycoprotein folding and quality-control mechanisms in protein-folding diseases

Chemical Approaches to Elucidate Enzymatic Profiles of UDP-Glucose: Glycoprotein Glucosyltransferase

In the endoplasmic reticulum (ER), uridine 5'-diphosphate-glucose: glycoprotein glucosyltransferase 1 (UGGT1) recognizes misfolded glycoproteins and transfers a glucose residue to the specific non-reducing end of high-mannose-type glycans. However, precise molecular mechanism by which UGGT1 senses the folding has not been understood clearly. To address this issue, various model substrates for UGGT1 have been prepared using biological approaches. Recently, we introduced chemical approaches using synthetic glycan probes that were designed for studying N-glycan processing in the ER and Golgi apparatus. Our approach can outfit the homogeneous and functionalized glycan probes. In this review, recent results on functional analysis of UGGT1 are summarized.

Glycosylation of voltage-gated calcium channels in health and disease

Voltage-gated calcium channels (VGCCs) are transmembrane proteins that translate electrical activities into intracellular calcium elevations and downstream signaling pathways. They serve essential physiological functions including communication between nerve cells, muscle contraction, cardiac activity, and release of hormones and neurotransmitters. Asparagine-linked glycosylation has emerged as an essential post-translational modification to control the number of channels embedded in the plasma membrane but also their functional gating properties. This review provides a comprehensive overview about the current state of knowledge on the role of glycosylation in the expression and functioning of VGCCs, and discusses how variations in the glycosylation of the channel proteins can contribute to pathological conditions.

N-linked glycosylation at Asn152 on CD147 affects protein folding and stability: promoting tumour metastasis in hepatocellular carcinoma

Cluster of differentiation 147 (CD147), also known as extracellular matrix metalloproteinase inducer, is a transmembrane glycoprotein that mediates oncogenic processes partly through N-glycosylation modifications. N-glycosylation has been demonstrated to be instrumental for the regulation of CD147 function during malignant transformation. However, the role that site-specific glycosylation of CD147 plays in its defective function in hepatocellular carcinomacells needs to be determined. Here, we demonstrate that the modification of N-glycosylation at Asn152 on CD147 strongly promotes hepatocellular carcinoma (HCC) invasion and migration. After the removal of N-glycans at Asn152, CD147 was more susceptible to degradation by ER-localized ubiquitin ligase-mediated endoplasmic reticulum-associated degradation (ERAD). Furthermore, N-linked glycans at Asn152 were required for CD147 to acquire and maintain proper folding in the ER. Moreover, N-linked glycans at Asn152 functioned as a recognition motif that was directly mediated by the CNX quality control system. Two phases in the retention-based ER chaperones system drove ER-localized CD147 trafficking to degradation. Deletion of N-linked glycosylation at Asn152 on CD147 significantly suppressed in situ tumour metastasis. These data could potentially shed light on the molecular regulation of CD147 through glycosylation and provide a valuable means of developing drugs that target N-glycans at Asn152 on CD147.

The Dependence of Carbohydrate-Aromatic Interaction Strengths on the Structure of the Carbohydrate

Interactions between proteins and carbohydrates are ubiquitous in biology. Therefore, understanding the factors that determine their affinity and selectivity are correspondingly important. Herein, we have determined the relative strengths of intramolecular interactions between a series of monosaccharides and an aromatic ring close to the glycosylation site in an N-glycoprotein host. We employed the enhanced aromatic sequon, a structural motif found in the reverse turns of some N-glycoproteins, to facilitate face-to-face monosaccharide-aromatic interactions. A protein host was used because the dependence of the folding energetics on the identity of the monosaccharide can be accurately measured to assess the strength of the carbohydrate-aromatic interaction. Our data demonstrate that the carbohydrate-aromatic interaction strengths are moderately affected by changes in the stereochemistry and identity of the substituents on the pyranose rings of the sugars. Galactose seems to make the weakest and allose the strongest sugar-aromatic interactions, with glucose, N-acetylglucosamine (GlcNAc) and mannose in between. The NMR solution structures of several of the monosaccharide-containing N-glycoproteins were solved to further understand the origins of the similarities and differences between the monosaccharide-aromatic interaction energies. Peracetylation of the monosaccharides substantially increases the strength of the sugar-aromatic interaction in the context of our N-glycoprotein host. Finally, we discuss our results in light of recent literature regarding the contribution of electrostatics to CH-π interactions and speculate on what our observations imply about the absolute conservation of GlcNAc as the monosaccharide through which N-linked glycans are attached to glycoproteins in eukaryotes.

Arsenic Triglutathione [As(GS)3] Transport by Multidrug Resistance Protein 1 (MRP1/ABCC1) Is Selectively Modified by Phosphorylation of Tyr920/Ser921 and Glycosylation of Asn19/Asn23

The ATP-binding cassette (ABC) transporter multidrug resistance protein 1 (MRP1/ABCC1) is responsible for the cellular export of a chemically diverse array of xenobiotics and endogenous compounds. Arsenic, a human carcinogen, is a high-affinity MRP1 substrate as arsenic triglutathione [As(GS)3]. In this study, marked differences in As(GS)3 transport kinetics were observed between MRP1-enriched membrane vesicles prepared from human embryonic kidney 293 (HEK) (Km 3.8 µM and Vmax 307 pmol/mg per minute) and HeLa (Km 0.32 µM and Vmax 42 pmol/mg per minute) cells. Mutant MRP1 lacking N-linked glycosylation [Asn19/23/1006Gln; sugar-free (SF)-MRP1] expressed in either HEK293 or HeLa cells had low Km and Vmax values for As(GS)3, similar to HeLa wild-type (WT) MRP1. When prepared in the presence of phosphatase inhibitors, both WT- and SF-MRP1-enriched membrane vesicles had a high Km value for As(GS)3 (3-6 µM), regardless of the cell line. Kinetic parameters of As(GS)3 for HEK-Asn19/23Gln-MRP1 were similar to those of HeLa/HEK-SF-MRP1 and HeLa-WT-MRP1, whereas those of single glycosylation mutants were like those of HEK-WT-MRP1. Mutation of 19 potential MRP1 phosphorylation sites revealed that HEK-Tyr920Phe/Ser921Ala-MRP1 transported As(GS)3 like HeLa-WT-MRP1, whereas individual HEK-Tyr920Phe- and -Ser921Ala-MRP1 mutants were similar to HEK-WT-MRP1. Together, these results suggest that Asn19/Asn23 glycosylation and Tyr920/Ser921 phosphorylation are responsible for altering the kinetics of MRP1-mediated As(GS)3 transport. The kinetics of As(GS)3 transport by HEK-Asn19/23Gln/Tyr920Glu/Ser921Glu were similar to HEK-WT-MRP1, indicating that the phosphorylation-mimicking substitutions abrogated the influence of Asn19/23Gln glycosylation. Overall, these data suggest that cross-talk between MRP1 glycosylation and phosphorylation occurs and that phosphorylation of Tyr920 and Ser921 can switch MRP1 to a lower-affinity, higher-capacity As(GS)3 transporter, allowing arsenic detoxification over a broad concentration range.

Site-specific characterization of N-linked glycosylation in human urinary glycoproteins and endogenous glycopeptides

Glycosylation is a very important post-translational modification involved in various cellular processes, such as cell adhesion, signal transduction and immune response. Urine is a rich source of glycoproteins and attractive biological fluid for biomarker discovery, owing to its availability, ease of collection, and correlation with pathophysiology of diseases. Although the urinary proteomics have been explored previously, the urinary glycoproteome characterization remains challenging requiring the development and optimization of analytical and bioinformatics methods for protein glycoprofiling. This study describes the high confident identification of 472 unique N-glycosylation sites covering 256 urinary glycoproteins. Besides, 202 unique N-glycosylation sites were identified in low molecular weight endogenous glycopeptides, which belong to 90 glycoproteins. Global site-specific characterization of the N-linked glycan heterogeneity was achieved by intact glycopeptide analysis, revealing 303 unique glycopeptides most of them displaying complex/hybrid glycans composed by sialic acid and fucose. These datasets consist in a valuable resource of glycoproteins and N-glycosylation sites found in healthy human urine that can be further explored in different disorders, in which the N-linked glycosylation may be aberrant.

Effect of brefeldin A and castanospermine on resistant cell lines as supplements in anticancer therapy

In the present study, we analyzed the influence of brefeldin A (BFA) and castanospermine (CAS) on the activity, stability and localization of P-glycoprotein (Pgp) and breast cancer resistance protein (BCRP) in various resistant cell lines. The impact of BFA and CAS on cell viability was assessed using the MTT test. Western blotting (WB) was performed to assess the effect of the inhibitors on the expression of the investigated proteins. Immunofluorescence was employed to assess the effect of BFA and CAS on the cellular localization of the proteins. Flow cytometry was used to verify the functional role of inhibitors on drug uptake and efflux. The MTT test showed that BFA had a significant effect on cell viability in LoVo/Dx and W1PR cell lines. WB analysis demonstrated that BFA partially blocked Pgp N-glycosylation and induced BCRP degradation and CASP 3-dependent apoptosis in W1TR cells; however, the BFA activity was p53-independent. CAS had no effect on the stability of Pgp but increased the level of non-glycosylated BCRP. The expression of p53 protein decreased in all of the cells that were treated with CAS. Immunofluorescence revealed that BFA caused a more granular Pgp signal in W1PR and BCRP in A2780T1 cells. Furthermore, BFA caused morphological changes in LoVo/Dx and W1TR cell lines. CAS also induced a granular signal in all of the cell lines, except W1TR. The flow cytometry showed higher dye accumulation in sensitive cell lines. We observed an increase in the mean fluorescence intensity (MFI) of Rho123 in LoVo/Dx cells treated with BFA and CAS, but no differences were observed in W1PR. BFA had no effect on the MFI of W1TR, but CAS led to an increase in the level of intracellular H33342 in W1TR and A2780T1 cells. These results suggest that these compounds are likely to be useful as supplements in anticancer therapy.

Hydrophobic Tagged Dihydrofolate Reductase for Creating Misfolded Glycoprotein Mimetics

In the endoplasmic reticulum (ER), nascent glycoproteins that have not acquired the native conformation are either repaired or sorted for degradation by specific quality-control systems composed by various proteins. Among them, UDP-glucose:glycoprotein glucosyltransferase (UGGT) serves as a folding sensor in the ER. However, the molecular mechanism of its recognition remains obscure. This study used pseudo-misfolded glycoproteins, comprising a modified dihydrofolate reductase with artificial pyrene-cysteine moiety on the protein surface (pDHFR) and Man9 GlcNAc2 -methotrexate (M9-MTX). All five M9-MTX/pDHFR complexes, with a pyrene group at different positions, were found to be good substrates of UGGT, irrespective of the site of pyrene modification. These results suggest UGGT's mode of substrate recognition is fuzzy, thus allowing various glycoproteins to be accommodated in the folding cycle.

Interactions between N-linked glycosylation and polymerisation of neuroserpin within the endoplasmic reticulum

The neuronal serpin neuroserpin undergoes polymerisation as a consequence of point mutations that alter its conformational stability, leading to a neurodegenerative dementia called familial encephalopathy with neuroserpin inclusion bodies (FENIB). Neuroserpin is a glycoprotein with predicted glycosylation sites at asparagines 157, 321 and 401. We used site-directed mutagenesis, transient transfection, western blot, metabolic labelling and ELISA to probe the relationship between glycosylation, folding, polymerisation and degradation of neuroserpin in validated cell models of health and disease. Our data show that glycosylation at N157 and N321 plays an important role in maintaining the monomeric state of neuroserpin, and we propose this is the result of steric hindrance or effects on local conformational dynamics that can contribute to polymerisation. Asparagine residue 401 is not glycosylated in wild type neuroserpin and in several polymerogenic variants that cause FENIB, but partial glycosylation was observed in the G392E mutant of neuroserpin that causes severe, early-onset dementia. Our findings indicate that N401 glycosylation reports lability of the C-terminal end of neuroserpin in its native state. This C-terminal lability is not required for neuroserpin polymerisation in the endoplasmic reticulum, but the additional glycan facilitates degradation of the mutant protein during proteasomal impairment. In summary, our results indicate how normal and variant-specific N-linked glycosylation events relate to intracellular folding, misfolding, degradation and polymerisation of neuroserpin.

Beyond water homeostasis: Diverse functional roles of mammalian aquaporins

BACKGROUND

Aquaporin (AQP) water channels are best known as passive transporters of water that are vital for water homeostasis.

SCOPE OF REVIEW

AQP knockout studies in whole animals and cultured cells, along with naturally occurring human mutations suggest that the transport of neutral solutes through AQPs has important physiological roles. Emerging biophysical evidence suggests that AQPs may also facilitate gas (CO2) and cation transport. AQPs may be involved in cell signalling for volume regulation and controlling the subcellular localization of other proteins by forming macromolecular complexes. This review examines the evidence for these diverse functions of AQPs as well their physiological relevance.

MAJOR CONCLUSIONS

As well as being crucial for water homeostasis, AQPs are involved in physiologically important transport of molecules other than water, regulation of surface expression of other membrane proteins, cell adhesion, and signalling in cell volume regulation.

GENERAL SIGNIFICANCE

Elucidating the full range of functional roles of AQPs beyond the passive conduction of water will improve our understanding of mammalian physiology in health and disease. The functional variety of AQPs makes them an exciting drug target and could provide routes to a range of novel therapies.

Neutralization function affected by single amino acid replacement in the HIV-1 antibody targets

The viral envelope glycoproteins are essential for entry into their host cells and studied extensively for designing vaccines. We hypothesize that the glycosylation on the HIV-1 viral envelope glycoprotein 41(gp41) at critical residues offers viral escape from the specific immune surveillant neutralizing antibodies Z13, 4E10 and 10E8 targeted to their linear epitopes in the Membrane Proximal External Region (MPER). The glycosylation occurring on the 50th residue (Asparagine) contained in the target (NWFNIT) can mask itself to be inaccessible for these neutralizing antibodies. The glycosylation rate of the epitopes which are shared by the Z13, 4E10 and 10E8 neutralizing antibodies of HIV-1 were predicited in silico. We analyzed the reliable frequency of glycosylation on the HIV-1 envelope gp41 using prediction tools to unravel the plausibility of the glycosylation by a mannose at 50th residue in the 59 amino acid long HIV-gp41 trimer (PDBID: 2M7W and 2LP7). It is evident that the glycosylation by a mannose that masks these targets is possible only when the 50th amino-acid is N (Asparagine, Asn) which is not possible when N is mutated to D (Aspartatic acid, Asp). The additive advantage for the retrovirus is its error-prone reverse transcriptase which can choose to copy these survivable mutants with Asn N-50 that can be glycosylated as explained by the Copy-choice model. So the glycan shields varying in their intensity and patterns have to be essentially studied to understand the viral escape strategies that will give a way forward towards a successful vaccine that can elicit a neutralizing antibody response to confer protection.

Using pharmacological chaperones to restore proteostasis

Normal organismal physiology depends on the maintenance of proteostasis in each cellular compartment to achieve a delicate balance between protein synthesis, folding, trafficking, and degradation while minimizing misfolding and aggregation. Defective proteostasis leads to numerous protein misfolding diseases. Pharmacological chaperones are cell-permeant small molecules that promote the proper folding and trafficking of a protein via direct binding to that protein. They stabilize their target protein in a protein-pharmacological chaperone state, increasing the natively folded protein population that can effectively engage trafficking machinery for transport to the final destination for function. Here, as regards the application of pharmacological chaperones, we focus on their capability to promote the folding and trafficking of lysosomal enzymes, G protein coupled receptors (GPCRs), and ion channels, each of which is presently an important drug target. Pharmacological chaperones hold great promise as potential therapeutics to ameliorate a variety of protein misfolding diseases.