N-glycan processing in ER quality control

Recruitment of Ahsa1 to Hsp90 is regulated by a conserved peptide that inhibits ATPase stimulation

Hsp90 is a molecular chaperone that acts on its clients through an ATP-dependent and conformationally dynamic functional cycle. The cochaperone Accelerator of Hsp90 ATPase, or Ahsa1, is the most potent stimulator of Hsp90 ATPase activity. Ahsa1 stimulates the rate of Hsp90 ATPase activity through a conserved motif, NxNNWHW. Metazoan Ahsa1, but not yeast, possesses an additional 20 amino acid peptide preceding the NxNNWHW motif that we have called the intrinsic chaperone domain (ICD). The ICD of Ahsa1 diminishes Hsp90 ATPase stimulation by interfering with the function of the NxNNWHW motif. Furthermore, the NxNNWHW modulates Hsp90's apparent affinity to Ahsa1 and ATP. Lastly, the ICD controls the regulated recruitment of Hsp90 in cells and its deletion results in the loss of interaction with Hsp90 and the glucocorticoid receptor. This work provides clues to how Ahsa1 conserved regions modulate Hsp90 kinetics and how they may be coupled to client folding status.

Endoplasmic reticulum stress and quality control in relation to cisplatin resistance in tumor cells

The endoplasmic reticulum (ER) is a crucial organelle that orchestrates key cellular functions like protein folding and lipid biosynthesis. However, it is highly sensitive to disturbances that lead to ER stress. In response, the unfolded protein response (UPR) activates to restore ER homeostasis, primarily through three sensors: IRE1, ATF6, and PERK. ERAD and autophagy are crucial in mitigating ER stress, yet their dysregulation can lead to the accumulation of misfolded proteins. Cisplatin, a commonly used chemotherapy drug, induces ER stress in tumor cells, activating complex signaling pathways. Resistance to cisplatin stems from reduced drug accumulation, activation of DNA repair, and anti-apoptotic mechanisms. Notably, cisplatin-induced ER stress can dualistically affect tumor cells, promoting either survival or apoptosis, depending on the context. ERAD is crucial for degrading misfolded proteins, whereas autophagy can protect cells from apoptosis or enhance ER stress-induced apoptosis. The complex interaction between ER stress, cisplatin resistance, ERAD, and autophagy opens new avenues for cancer treatment. Understanding these processes could lead to innovative strategies that overcome chemoresistance, potentially improving outcomes of cisplatin-based cancer treatments. This comprehensive review provides a multifaceted perspective on the complex mechanisms of ER stress, cisplatin resistance, and their implications in cancer therapy.

High-Throughput Mass Spectrometry Analysis of N-Glycans and Protein Markers after FUT8 Knockdown in the Syngeneic SW480/SW620 Colorectal Cancer Cell Model

Disruption of the glycosylation machinery is a common feature in many types of cancer, and colorectal cancer (CRC) is no exception. Core fucosylation is mediated by the enzyme fucosyltransferase 8 (FucT-8), which catalyzes the addition of α1,6-l-fucose to the innermost GlcNAc residue of N-glycans. We and others have documented the involvement of FucT-8 and core-fucosylated proteins in CRC progression, in which we addressed core fucosylation in the syngeneic CRC model formed by SW480 and SW620 tumor cell lines from the perspective of alterations in their N-glycosylation profile and protein expression as an effect of the knockdown of the FUT8 gene that encodes FucT-8. Using label-free, semiquantitative mass spectrometry (MS) analysis, we found noticeable differences in N-glycosylation patterns in FUT8-knockdown cells, affecting core fucosylation and sialylation, the Hex/HexNAc ratio, and antennarity. Furthermore, stable isotopic labeling of amino acids in cell culture (SILAC)-based proteomic screening detected the alteration of species involved in protein folding, endoplasmic reticulum (ER) and Golgi post-translational stabilization, epithelial polarity, and cellular response to damage and therapy. This data is available via ProteomeXchange with identifier PXD050012. Overall, the results obtained merit further investigation to validate their feasibility as biomarkers of progression and malignization in CRC, as well as their potential usefulness in clinical practice.

Navigating PRKCSH's impact on cancer: from N-linked glycosylation to death pathway and anti-tumor immunity

PRKCSH, also known as Glucosidase II beta subunit (GluIIβ), is a crucial component of the endoplasmic reticulum (ER) quality control system for N-linked glycosylation, essential for identifying and eliminating misfolded proteins. Glucosidase II consists of the catalytic alpha subunit (GluIIα) and the regulatory beta subunit (GluIIβ), ensuring proper protein folding and release from the ER. The induction of PRKCSH in cancer and its interaction with various cellular components suggest broader roles beyond its previously known functions. Mutations in the PRKCSH gene are linked to autosomal dominant polycystic liver disease (ADPLD). Alternative splicing generates distinct PRKCSH isoforms, which can influence processes like epithelial-mesenchymal transition (EMT) and the proliferation of lung cancer cells. PRKCSH's involvement in cancer is multifaceted, impacting cell growth, metastasis, and response to growth factors. Additionally, PRKCSH orchestrates cell death programs, affecting both autophagy and apoptosis. Its role in facilitating N-linked glycoprotein release from the ER is hypothesized to assist cancer cells in managing increased demand and ER stress. Moreover, PRKCSH modulates anti-tumor immunity, with its suppression augmenting NK cell and T cell activity, promising enhanced cancer therapy. PRKCSH's diverse functions, including regulation of IGF1R and IRE1α, implicate it as a therapeutic target and biomarker in cancer immunotherapy. However, targeting its glucosidase II activity alone may not fully counteract its effects, suggesting broader mechanisms in cancer development. Further investigations are needed to elucidate PRKCSH's precise role and validate its therapeutic potential in cancer treatment.

Autophagy-dependent regulation of MHC-I molecule presentation

The major histocompatibility complex (MHC) class I molecules present peptide antigens to MHC class I-restricted CD8+ T lymphocytes to elicit an effective immune response. The conventional antigen-processing pathway for MHC-I presentation depends on proteasome-mediated peptide generation and peptide loading in the endoplasmic reticulum by members of the peptide loading complex. Recent discoveries in this field highlight the role of alternative MHC-I peptide loading and presentation pathways, one of them being autophagy. Autophagy is a cell-intrinsic degradative pathway that ensures cellular homoeostasis and plays critical roles in cellular immunity. In this review article, we discuss the role of autophagy in MHC class I-restricted antigen presentation, elucidating new findings on the crosstalk of autophagy and ER-mediated MHC-I peptide presentation, dendritic cell-mediated cross-presentation and also mechanisms governing immune evasion. A detailed molecular understanding of the key drivers of autophagy-mediated MHC-I modulation holds promising targets to devise effective measures to improve T cell immunotherapies.

Analysis of Selenoprotein F Binding to UDP-Glucose:Glycoprotein Glucosyltransferase (UGGT) by a Photoreactive Crosslinker

In the endoplasmic reticulum glycoprotein quality control system, UDP-glucose : glycoprotein glucosyltransferase (UGGT) functions as a folding sensor. Although it is known to form a heterodimer with selenoprotein F (SelenoF), the details of the complex formation remain obscure. A pulldown assay using co-transfected SelenoF and truncated mutants of human UGGT1 (HUGT1) revealed that SelenoF binds to the TRXL2 domain of HUGT1. Additionally, a newly developed photoaffinity crosslinker was selectively introduced into cysteine residues of recombinant SelenoF to determine the spatial orientation of SelenoF to HUGT1. The crosslinking experiments showed that SelenoF formed a covalent bond with amino acids in the TRXL3 region and the interdomain between βS2 and GT24 of HUGT1 via the synthetic crosslinker. SelenoF might play a role in assessing and refining the disulfide bonds of misfolded glycoproteins in the hydrophobic cavity of HUGT1 as it binds to the highly flexible region of HUGT1 to reach its long hydrophobic cavity. Clarification of the SelenoF-binding domain of UGGT and its relative position will help predict and reveal the function of SelenoF from a structural perspective.

Structure-Based Design of Potent Iminosugar Inhibitors of Endoplasmic Reticulum α-Glucosidase I with Anti-SARS-CoV-2 Activity

Enveloped viruses depend on the host endoplasmic reticulum (ER) quality control (QC) machinery for proper glycoprotein folding. The endoplasmic reticulum quality control (ERQC) enzyme α-glucosidase I (α-GluI) is an attractive target for developing broad-spectrum antivirals. We synthesized 28 inhibitors designed to interact with all four subsites of the α-GluI active site. These inhibitors are derivatives of the iminosugars 1-deoxynojirimycin (1-DNJ) and valiolamine. Crystal structures of ER α-GluI bound to 25 1-DNJ and three valiolamine derivatives revealed the basis for inhibitory potency. We established the structure-activity relationship (SAR) and used the Site Identification by Ligand Competitive Saturation (SILCS) method to develop a model for predicting α-GluI inhibition. We screened the compounds against SARS-CoV-2 in vitro to identify those with greater antiviral activity than the benchmark α-glucosidase inhibitor UV-4. These host-targeting compounds are candidates for investigation in animal models of SARS-CoV-2 and for testing against other viruses that rely on ERQC for correct glycoprotein folding.

The HIV Restriction Factor Profile in the Brain Is Associated with the Clinical Status and Viral Quantities

HIV-encoded DNA, RNA and proteins persist in the brain despite effective antiretroviral therapy (ART), with undetectable plasma and cerebrospinal fluid viral RNA levels, often in association with neurocognitive impairments. Although the determinants of HIV persistence have garnered attention, the expression and regulation of antiretroviral host restriction factors (RFs) in the brain for HIV and SIV remain unknown. We investigated the transcriptomic profile of antiretroviral RF genes by RNA-sequencing with confirmation by qRT-PCR in the cerebral cortex of people who are uninfected (HIV[-]), those who are HIV-infected without pre-mortem brain disease (HIV[+]), those who are HIV-infected with neurocognitive disorders (HIV[+]/HAND) and those with neurocognitive disorders with encephalitis (HIV[+]/HIVE). We observed significant increases in RF expression in the brains of HIV[+]/HIVE in association with the brain viral load. Machine learning techniques identified MAN1B1 as a key gene that distinguished the HIV[+] group from the HIV[+] groups with HAND. Analyses of SIV-associated RFs in brains from SIV-infected Chinese rhesus macaques with different ART regimens revealed diminished RF expression among ART-exposed SIV-infected animals, although ART interruption resulted in an induced expression of several RF genes including OAS3, RNASEL, MX2 and MAN1B1. Thus, the brain displays a distinct expression profile of RFs that is associated with the neurological status as well as the brain viral burden. Moreover, ART interruption can influence the brain's RF profile, which might contribute to disease outcomes.

Synthesis, Processing, and Function of N-Glycans in N-Glycoproteins

Many membrane-resident and secreted proteins, including growth factors and their receptors are N-glycosylated. The initial N-glycan structure is synthesized in the endoplasmic reticulum (ER) as a branched structure on a lipid anchor (dolicholpyrophosphate) and then co-translationally, "en bloc" transferred and linked via N-acetylglucosamine to asparagine within a specific N-glycosylation acceptor sequence of the nascent recipient protein. In the ER and then the Golgi apparatus, the N-linked glycan structure is modified by hydrolytic removal of sugar residues ("trimming") followed by re-glycosylation with additional sugar residues ("processing") such as galactose, fucose or sialic acid to form complex N-glycoproteins. While the sequence of the reactions leading to biosynthesis, "en bloc" transfer and processing of N-glycans is well investigated, it is still not completely understood how N-glycans affect the biological fate and function of N-glycoproteins. This review will discuss the biology of N-glycoprotein synthesis, processing and function with specific reference to the physiology and pathophysiology of the immune and nervous system, as well as infectious diseases such as Covid-19.

Systems-Wide Site-Specific Analysis of Glycoproteins

Glycosylation is one of the most common and complex post-translation modifications that influence the structural and functional properties of proteins. Glycoproteins are highly heterogeneous and exhibit site- and protein-specific expression differences. Mass spectrometry in combination with liquid chromatography has emerged as the most powerful tool for the comprehensive characterization of glycosylation. The analysis of intact glycopeptides has emerged as a promising strategy to analyze glycoproteins for their glycan heterogeneity at both protein- and site-specific levels. Nevertheless, intact glycopeptide characterization is challenging as elucidation of the glycan and peptide moieties requires specific sample preparation workflows that, combined with the tandem mass spectrometry approach, enable the identification of single glycopeptide species. In this chapter, we provide a detailed description of the methods that include procedures for (i) proteolytic digestion using specific proteases, (ii) optional glycopeptide enrichment using hydrophilic interaction liquid chromatography, (iii) nano-LC-MS/MS analysis of glycopeptides, and (iv) data analysis for identification of glycopeptides. Together, our workflow provides a framework for the system-wide site-specific analysis of N- and O-glycopeptides derived from complex biological or clinical samples.

Severe pathogenic variants of intestinal sucrase-isomaltase interact avidly with the wild type enzyme and negatively impact its function and trafficking

Sucrase-isomaltase (SI) is the major disaccharidase of the small intestine, exhibiting a broad α-glucosidase activity profile. The importance of SI in gut health is typified by the development of sucrose and starch maldigestion in individuals carrying mutations in the SI gene, like in congenital sucrase-isomaltase deficiency (CSID). Common and rare defective SI gene variants (SIGVs) have also been shown to increase the risk of irritable bowel syndrome (IBS) with symptoms and clinical features similar to CSID and also in symptomatic heterozygote carriers. Here, we investigate the impact of the most abundant and highly pathogenic SIGVs that occur in heterozygotes on wild type SI (SI^WT) by adapting an in vitro system that recapitulates SI gene heterozygosity. Our results demonstrate that pathogenic SI mutants interact avidly with SI^WT, negatively impact its enzymatic function, alter the biosynthetic pattern and impair the trafficking behavior of the heterodimer. The in vitro recapitulation of a heterozygous state demonstrates potential for SIGVs to act in a semi-dominant fashion, by further reducing disaccharidase activity via sequestration of the SI^WT copy into an inactive form of the enzymatic heterodimer. This study provides novel insights into the potential role of heterozygosity in the pathophysiology of CSID and IBS.

Pro-Survival Factor EDEM3 Confers Therapy Resistance in Prostate Cancer

Prostate cancer is the most common cancer in men, and it is primarily driven by androgen steroid hormones. The glycosylation enzyme EDEM3 is controlled by androgen signalling and is important for prostate cancer viability. EDEM3 is a mannosidase that trims mannose from mis-folded glycoproteins, tagging them for degradation through endoplasmic reticulum-associated degradation. Here, we find that EDEM3 is upregulated in prostate cancer, and this is linked to poorer disease-free survival. Depletion of EDEM3 from prostate cancer cells induces an ER stress transcriptomic signature, and EDEM3 overexpression is cyto-protective against ER stressors. EDEM3 expression also positively correlates with genes involved in the unfolded protein response in prostate cancer patients, and its expression can be induced through exposure to radiation. Importantly, the overexpression of EDEM3 promotes radio-resistance in prostate cancer cells and radio-resistance can be reduced through depletion of EDEM3. Our data thus implicate increased levels of EDEM3 with a role in prostate cancer pathology and reveal a new therapeutic opportunity to sensitise prostate tumours to radiotherapy.

Introduction of C-alkyl branches to L-iminosugars changes their active site binding orientation

L-ido-Deoxynojirimycin (L-ido-DNJ) itself showed no affinity for human lysosomal acid α-glucosidase (GAA), whereas 5-C-methyl-L-ido-DNJ showed a strong affinity for GAA, comparable to the glucose analog DNJ, with a K_i value of 0.060 μM. This excellent affinity for GAA and enzyme stabilization was observed only when methyl and ethyl groups were introduced. Docking simulation analysis revealed that the alkyl chains of 5-C-alkyl-L-ido-DNJs were stored in three different pockets, depending on their length, thereby the molecular orientation was changed. Comparison of the binding poses of DNJ and 5-C-methyl-L-ido-DNJ showed that they formed a common ionic interaction with Asp404, Asp518, and Asp616, but both the binding orientation and the distance between the ligand and each amino acid residue were different. 5-C-Methyl-L-ido-DNJ dose-dependently increased intracellular GAA activity in Pompe patient fibroblasts with the M519V mutation and also promoted enzyme transport to lysosomes. This study provides the first example of a strategy to design high-affinity ligands by introducing alkyl branches into rare sugars and L-sugar-type iminosugars to change the orientation of binding.

Introducing Thioredoxin-Related Transmembrane Proteins: Emerging Roles of Human TMX and Clinical Implications

Significance: The presence of a large number of thioredoxin superfamily members suggests a complex mechanism of redox-based regulation in mammalian cells. However, whether these members are functionally redundant or play separate and distinct roles in each cellular compartment remains to be elucidated. Recent Advances: In the mammalian endoplasmic reticulum (ER), ∼20 thioredoxin-like proteins have been identified. Most ER oxidoreductases are soluble proteins located in the luminal compartment, whereas a small family of five thioredoxin-related transmembrane proteins (TMX) also reside in the ER membrane and play crucial roles with specialized functions. Critical Issues: In addition to the predicted function of ER protein quality control, several independent studies have suggested the diverse roles of TMX family proteins in the regulation of cellular processes, including calcium homeostasis, bioenergetics, and thiol-disulfide exchange in the extracellular space. Moreover, recent studies have provided evidence of their involvement in the pathogenesis of various diseases. Future Directions: Extensive research is required to unravel the physiological roles of TMX family proteins. Given that membrane-associated proteins are prime targets for drug discovery in a variety of human diseases, expanding our knowledge on the mechanistic details of TMX action on the cell membrane will provide the molecular basis for developing novel diagnostic and therapeutic approaches as a potent molecular target in a clinical setting. Antioxid. Redox Signal. 36, 984-1000.

Identification of Endoplasmic Reticulum α-Glucosidase I from a Thermophilic Fungus as a Platform for Structure-Guided Antiviral Drug Design

All viruses depend on host cell proteins for replication. Denying viruses' access to the function of critical host proteins can result in antiviral activity against multiple virus families. In particular, small-molecule drug candidates which inhibit the α-glucosidase enzymes of the endoplasmic reticulum (ER) translation quality control (QC) pathway have demonstrated broad-spectrum antiviral activities and low risk for development of viral resistance. However, antiviral drug discovery focused on the ERQC enzyme α-glucosidase I (α-GluI) has been hampered by difficulties in obtaining crystal structures of complexes with inhibitors. We report here the identification of an orthologous enzyme from a thermophilic fungus, Chaetomium thermophilum (Ct), as a robust surrogate for mammalian ER α-GluI and a platform for inhibitor design. Previously annotated only as a hypothetical protein, the Ct protein was validated as a bona fide α-glucosidase by comparing its crystal structure to that of mammalian α-GluI, by demonstrating enzymatic activity on the unusual α-d-Glcp-(1 → 2)-α-d-Glcp-(1 → 3) substrate glycan, and by showing that well-known inhibitors of mammalian α-GluI (1-DNJ, UV-4, UV-5) also inhibit Ct α-GluI. Crystal structures of Ct α-GluI in complex with three such inhibitors (UV-4, UV-5, EB-0159) revealed extensive interactions with all four enzyme subsites and provided insights into the catalytic mechanism. Identification of ER Ct α-GluI as a surrogate for mammalian α-GluI will accelerate the structure-guided discovery of broad-spectrum antivirals. This study also highlights Ct as a source of thermostable eukaryotic proteins, such as ER α-Glu I, that lack orthologs in bacterial or archaeal thermophiles.

N-Glycosylation at Asn291 Stabilizes TIM-4 and Promotes the Metastasis of NSCLC

T-cell immunoglobulin domain and mucin domain 4 (TIM-4) is a transmembrane protein that promotes epithelial-mesenchymal transition (EMT), migration and invasion of non-small cell lung cancer (NSCLC) cells. Most transmembrane proteins are modified by N-glycosylation and the importance of protein N-glycosylation in cancer cell metastasis has been well appreciated. However, whether TIM-4 is modified by N-glycosylation and the role of TIM-4 N-glycosylation in NSCLC remains largely unknown. In the current study, we reported that TIM-4 was extensively N-glycosylated at Asn291. After the removal of N-glycosylation, the stability of TIM-4 protein was decreased and TIM-4 was more susceptible to degradation by ER-localized ubiquitin ligase-mediated ERAD. Thus, the expression of TIM-4 on the cell surface was decreased, which suppressed TIM-4-mediated metastasis in NSCLC. In summary, the present study identifies TIM-4 N-glycosylation and its role in NSCLS migration, which would provide a valuable biomarker for developing drugs targeting N-glycosylation at Asn291 on TIM-4.

Unlocking the Hydrolytic Mechanism of GH92 α‐1,2‐Mannosidases: Computation Inspires the use of C‐Glycosides as Michaelis Complex Mimics

The conformational changes in a sugar moiety along the hydrolytic pathway are key to understand the mechanism of glycoside hydrolases (GHs) and to design new inhibitors. The two predominant itineraries for mannosidases go via ^OS₂→B_2,5→¹S₅ and ³S₁→³H₄→¹C₄. For the CAZy family 92, the conformational itinerary was unknown. Published complexes of Bacteroides thetaiotaomicron GH92 catalyst with a S‐glycoside and mannoimidazole indicate a ⁴C₁→⁴H₅/¹S₅→¹S₅ mechanism. However, as observed with the GH125 family, S‐glycosides may not act always as good mimics of GH's natural substrate. Here we present a cooperative study between computations and experiments where our results predict the E₅→B_2,5/¹S₅→¹S₅ pathway for GH92 enzymes. Furthermore, we demonstrate the Michaelis complex mimicry of a new kind of C‐disaccharides, whose biochemical applicability was still a chimera.

Interspecific complementation-restoration of phenotype in Arabidopsis cuc2cuc3 mutant by sugarcane CUC2 gene

BACKGROUND

In plants, a critical balance between differentiation and proliferation of stem cells at the shoot apical meristem zone is essential for proper growth. The spatiotemporal regulation of some crucial genes dictates the formation of a boundary within and around budding organs. The boundary plays a pivotal role in distinguishing one tissue type from another and provides a defined shape to the organs at their developed stage. NAM/CUC subfamily of the NAC transcription factors control the boundary formation during meristematic development.

RESULTS

Here, we have identified the CUP-SHAPED COTYLEDON (CUC) genes in sugarcane and named SsCUC2 (for the orthologous gene of CUC1 and CUC2) and SsCUC3. The phylogenetic reconstruction showed that SsCUCs occupy the CUC2 and CUC3 clade together with monocots, whereas eudicot CUC2 and CUC3 settled separately in the different clade. The structural analysis of CUC genes showed that most of the CUC3 genes were accompanied by an intron gain during eudicot divergence. Besides, the study of SsCUCs expression in the RNA-seq obtained during different stages of ovule development revealed that SsCUCs express in developing young tissues, and the expression of SsCUC2 is regulated by miR164. We also demonstrate that SsCUC2 (a monocot) could complement the cuc2cuc3 mutant phenotype of Arabidopsis (eudicot).

CONCLUSIONS

This study further supports that CUC2 has diverged in CUC1 and CUC2 during the evolution of monocots and eudicots from ancestral plants. The functional analysis of CUC expression patterns during sugarcane ovule development and ectopic expression of SsCUC2 in Arabidopsis showed that SsCUC2 has a conserved role in boundary formation. Overall, these findings improve our understanding of the functions of sugarcane CUC genes. Our results reveal the crucial functional role of CUC genes in sugarcane.

Golgi Alpha1,2-Mannosidase IA Promotes Efficient Endoplasmic Reticulum-Associated Degradation of NKCC2

Mutations in the apically located kidney Na-K-2Cl cotransporter NKCC2 cause type I Bartter syndrome, a life-threatening kidney disorder. We previously showed that transport from the ER represents the limiting phase in NKCC2 journey to the cell surface. Yet very little is known about the ER quality control components specific to NKCC2 and its disease-causing mutants. Here, we report the identification of Golgi alpha1, 2-mannosidase IA (ManIA) as a novel binding partner of the immature form of NKCC2. ManIA interaction with NKCC2 takes place mainly at the cis-Golgi network. ManIA coexpression decreased total NKCC2 protein abundance whereas ManIA knock-down produced the opposite effect. Importantly, ManIA coexpression had a more profound effect on NKCC2 folding mutants. Cycloheximide chase assay showed that in cells overexpressing ManIA, NKCC2 stability and maturation are heavily hampered. Deleting the cytoplasmic region of ManIA attenuated its interaction with NKCC2 and inhibited its effect on the maturation of the cotransporter. ManIA-induced reductions in NKCC2 expression were offset by the proteasome inhibitor MG132. Likewise, kifunensine treatment greatly reduced ManIA effect, strongly suggesting that mannose trimming is involved in the enhanced ERAD of the cotransporter. Moreover, depriving ManIA of its catalytic domain fully abolished its effect on NKCC2. In summary, our data demonstrate the presence of a ManIA-mediated ERAD pathway in renal cells promoting retention and degradation of misfolded NKCC2 proteins. They suggest a model whereby Golgi ManIA contributes to ERAD of NKCC2, by promoting the retention, recycling, and ERAD of misfolded proteins that initially escape protein quality control surveillance within the ER.

N-Substituted Valiolamine Derivatives as Potent Inhibitors of Endoplasmic Reticulum α-Glucosidases I and II with Antiviral Activity

Most enveloped viruses rely on the host cell endoplasmic reticulum (ER) quality control (QC) machinery for proper folding of glycoproteins. The key ER α-glucosidases (α-Glu) I and II of the ERQC machinery are attractive targets for developing broad-spectrum antivirals. Iminosugars based on deoxynojirimycin have been extensively studied as ER α-glucosidase inhibitors; however, other glycomimetic compounds are less established. Accordingly, we synthesized a series of N-substituted derivatives of valiolamine, the iminosugar scaffold of type 2 diabetes drug voglibose. To understand the basis for up to 100,000-fold improved inhibitory potency, we determined high-resolution crystal structures of mouse ER α-GluII in complex with valiolamine and 10 derivatives. The structures revealed extensive interactions with all four α-GluII subsites. We further showed that N-substituted valiolamines were active against dengue virus and SARS-CoV-2 in vitro. This study introduces valiolamine-based inhibitors of the ERQC machinery as candidates for developing potential broad-spectrum therapeutics against the existing and emerging viruses.

Towards understanding the extensive diversity of protein N-glycan structures in eukaryotes

N‐glycosylation is an important post‐translational modification of proteins that has been highly conserved during evolution and is found in Eukaryota, Bacteria and Archaea. In eukaryotes, N‐glycan processing is sequential, involving multiple specific steps within the secretory pathway as proteins travel through the endoplasmic reticulum and the Golgi apparatus. In this review, we first summarize the different steps of the N‐glycan processing and further describe recent findings regarding the diversity of N‐glycan structures in eukaryotic clades. This comparison allows us to explore the different regulation mechanisms of N‐glycan processing among eukaryotic clades. Recent findings regarding the regulation of protein N‐glycosylation are highlighted, especially the regulation of the biosynthesis of complex‐type N‐glycans through manganese and calcium homeostasis and the specific role of transmembrane protein 165 (TMEM165) for which homologous sequences have been identified in several eukaryotic clades. Further research will be required to characterize the function of TMEM165 homologous sequences in different eukaryotic clades.

N-Glycosylation and enzymatic activity of the rHuPH20 expressed in Chinese hamster ovary cells

rHuPH20, a neutral pH-active hyaluronidase that degrades glycosaminoglycans under physiologic conditions, has six potential N-glycosylation sites. In this report, the rHuPH20 expressed in Chinese hamster ovary (CHO) cells was analyzed and characterized using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). Removal of the N-linked glycans from rHuPH20 with PNGase F shifted the molecular weight from 66 kDa to approximately 52 kDa, its deduced molecular weight based on sequence analysis, suggesting that most, if not all, of the potential N-glycosylation sites are linked to oligosaccharides. Then the N-linked glycans released from the rHuPH20 by PNGase F were characterized by UPLC-FLR-MS, and the six N-glycosylation sites of the rHuPH20 were identified and characterized by UPLC-MS/MS at peptide levels. Subsequently, we found that the rHuPH20 increased the dispersion of locally subcutaneous injected drugs and the in vitro and in vivo bioactivity were decreased significantly after PNGase F treatment. In particular, rHuPH20 significantly augmented the absolute bioavailability of locally subcutaneous injected large protein therapeutics, while the bioavailability decreased after being digested by PNGase F. These results demonstrated that N-glycosylation is important for the bioactivity of the rHuPH20.

Structural insights reveal the second base catalyst of isomaltose glucohydrolase

Glycoside hydrolase family 15 (GH15) inverting enzymes contain two glutamate residues functioning as a general acid catalyst and a general base catalyst, for isomaltose glucohydrolase (IGHase), Glu178 and Glu335, respectively. Generally, a two-catalytic residue-mediated reaction exhibits a typical bell-shaped pH-activity curve. However, IGHase is found to display atypical non-bell-shaped pH-k_cat and pH-k_cat /K_m profiles, theoretically better-fitted to a three-catalytic residue-associated pH-activity curve. We determined the crystal structure of IGHase by the single-wavelength anomalous dispersion method using sulfur atoms and the cocrystal structure of a catalytic base mutant E335A with isomaltose. Although the activity of E335A was undetectable, the electron density observed in its active site pocket did not correspond to an isomaltose but a glycerol and a β-glucose, cryoprotectant, and hydrolysis product. Our structural and biochemical analyses of several mutant enzymes suggest that Tyr48 acts as a second catalytic base catalyst. Y48F mutant displayed almost equivalent specific activity to a catalytic acid mutant E178A. Tyr48, highly conserved in all GH15 members, is fixed by another Tyr residue in many GH15 enzymes; the latter Tyr is replaced by Phe290 in IGHase. The pH profile of F290Y mutant changed to a bell-shaped curve, suggesting that Phe290 is a key residue distinguishing Tyr48 of IGHase from other GH15 members. Furthermore, F290Y is found to accelerate the condensation of isomaltose from glucose by modifying a hydrogen-bonding network between Tyr290-Tyr48-Glu335. The present study indicates that the atypical Phe290 makes Tyr48 of IGHase unique among GH15 enzymes.

[Reticular and Golgi glycosylation: Advances and associated diseases]

Glycosylation is one of the essential modifications of proteins and lipids. It is carried out mainly in the endoplasmic reticulum and Golgi apparatus, and requires a specific molecular machinery associating several hundreds of glycosyltransferases, glycosidases, transporters and regulating proteins. Modifications of glycosylation are found in numerous diseases, notably in cancers. All types of glycosylation can be affected and this leads to dysfunctions of cellular metabolism. In this review, we present the current knowledge on the regulation of glycosylation mechanisms and illustrate how the alteration of these regulatory mechanisms can lead to abnormal protein and lipid glycosylation, and take part in the development of cancers.

Scrutinizing Coronaviruses Using Publicly Available Bioinformatic Tools: The Viral Structural Proteins as a Case Study

Since early 2020, the world suffers from a new beta-coronavirus, called SARS-CoV-2, that has devastating effects globally due to its associated disease, Covid-19. Until today, Covid-19, which not only causes life-threatening lung infections but also impairs various other organs and tissues, has killed hundreds of thousands of people and caused irreparable damage to many others. Since the very onset of the pandemic, huge efforts were made worldwide to fully understand this virus and numerous studies were, and still are, published. Many of these deal with structural analyses of the viral spike glycoprotein and with vaccine development, antibodies and antiviral molecules or immunomodulators that are assumed to become essential tools in the struggle against the virus. This paper summarizes knowledge on the properties of the four structural proteins (spike protein S, membrane protein M, envelope protein E and nucleocapsid protein N) of the SARS-CoV-2 virus and its relatives, SARS-CoV and MERS-CoV, that emerged few years earlier. Moreover, attention is paid to ways to analyze such proteins using freely available bioinformatic tools and, more importantly, to bring these proteins alive by looking at them on a computer/laptop screen with the easy-to-use but highly performant and interactive molecular graphics program DeepView. It is hoped that this paper will stimulate non-bioinformaticians and non-specialists in structural biology to scrutinize these and other macromolecules and as such will contribute to establishing procedures to fight these and maybe other forthcoming viruses.

Integrated Structural Analysis of N-Glycans and Free Oligosaccharides Allows for a Quantitative Evaluation of ER Stress

Endoplasmic reticulum (ER) stress has been reported in a variety of diseases. Although ER stress can be detected using specific markers, it is still difficult to quantitatively evaluate the degree of stress and to identify the cause of the stress. The ER is the primary site for folding of secretory or transmembrane proteins as well as the site where glycosylation is initiated. This study therefore postulates that tracing the biosynthetic pathway of asparagine-linked glycans (N-glycans) would be a reporter for reflecting the state of the ER and serve as a quantitative descriptor of ER stress. Glycoblotting-assisted mass spectrometric analysis of the HeLa cell line enabled quantitative determination of the changes in the structures of N-glycans and degraded free oligosaccharides (fOSs) in response to tunicamycin- or thapsigargin-induced ER stress. The integrated analysis of neutral and sialylated N-glycans and fOSs showed the potential to elucidate the cause of ER stress, which cannot be readily done by protein markers alone. Changes in the total amount of glycans, increase in the ratio of high-mannose type N-glycans, increase in fOSs, and changes in the ratio of sialylated N-glycans in response to ER stress were shown to be potential descriptors of ER stress. Additionally, drastic clearance of accumulated N-glycans was observed in thapsigargin-treated cells, which may suggest the observation of ER stress-mediated autophagy or ER-phagy in terms of glycomics. Quantitative analysis of N-glycoforms composed of N-glycans and fOSs provides the dynamic indicators reflecting the ER status and the promising strategies for quantitative evaluation of ER stress.

Ca2+ Regulates ERp57-Calnexin Complex Formation

ERp57, a member of the protein disulfide isomerase family, is a ubiquitous disulfide catalyst that functions in the oxidative folding of various clients in the mammalian endoplasmic reticulum (ER). In concert with ER lectin-like chaperones calnexin and calreticulin (CNX/CRT), ERp57 functions in virtually all folding stages from co-translation to post-translation, and thus plays a critical role in maintaining protein homeostasis, with direct implication for pathology. Here, we present mechanisms by which Ca²⁺ regulates the formation of the ERp57-calnexin complex. Biochemical and isothermal titration calorimetry analyses revealed that ERp57 strongly interacts with CNX via a non-covalent bond in the absence of Ca²⁺. The ERp57-CNX complex not only promoted the oxidative folding of human leukocyte antigen heavy chains, but also inhibited client aggregation. These results suggest that this complex performs both enzymatic and chaperoning functions under abnormal physiological conditions, such as Ca²⁺ depletion, to effectively guide proper oxidative protein folding. The findings shed light on the molecular mechanisms underpinning crosstalk between the chaperone network and Ca²⁺.

N-Glycan Biosynthesis: Basic Principles and Factors Affecting Its Outcome

Carbohydrate chains are the most abundant and diverse of nature's biopolymers and represent one of the four fundamental macromolecular building blocks of life together with proteins, nucleic acids, and lipids. Indicative of their essential roles in cells and in multicellular organisms, genes encoding proteins associated with glycosylation account for approximately 2% of the human genome. It has been estimated that 50-80% of all human proteins carry carbohydrate chains-glycans-as part of their structure. Despite cells utilize only nine different monosaccharides for making their glycans, their order and conformational variation in glycan chains together with chain branching differences and frequent post-synthetic modifications can give rise to an enormous repertoire of different glycan structures of which few thousand is estimated to carry important structural or functional information for a cell. Thus, glycans are immensely versatile encoders of multicellular life. Yet, glycans do not represent a random collection of unpredictable structures but rather, a collection of predetermined but still dynamic entities that are present at defined quantities in each glycosylation site of a given protein in a cell, tissue, or organism.In this chapter, we will give an overview of what is currently known about N-glycan synthesis in higher eukaryotes, focusing not only on the processes themselves but also on factors that will affect or can affect the final outcome-the dynamicity and heterogeneity of the N-glycome. We hope that this review will help understand the molecular details underneath this diversity, and in addition, be helpful for those who plan to produce optimally glycosylated antibody-based therapeutics.

PDI Family Members as Guides for Client Folding and Assembly

Complicated and sophisticated protein homeostasis (proteostasis) networks in the endoplasmic reticulum (ER), comprising disulfide catalysts, molecular chaperones, and their regulators, help to maintain cell viability. Newly synthesized proteins inserted into the ER need to fold and assemble into unique native structures to fulfill their physiological functions, and this is assisted by protein disulfide isomerase (PDI) family. Herein, we focus on recent advances in understanding the detailed mechanisms of PDI family members as guides for client folding and assembly to ensure the efficient production of secretory proteins.

Purification and characterization of non-enzymatic glycoprotein (NEGp) from flax seed buffer extract that exhibits anticoagulant and antiplatelet activity

The current study deals with the purification and characterization of non-enzymatic glycoprotein (NEGp) from flax seed buffer extract. Sephadex G-100 and DEAE-A25 column chromatography techniques were employed to isolate NEGp. NEGp showed single sharp band at 29 kDa region on 10% SDS-PAGE, and under reduced and non-reduced conditions revealed its monomeric nature. Besides, NEGp taken up the PAS stain at 29 kDa region reveals the presence of carbohydrate moiety. Purity of NEGp was adjudged by RP-HPLC, as it revealed a single sharp peak at the retention time of 3.4 min. The exact molecular mass of NEGp was found to be 26 kDa which was confirmed by MALDI-TOF. Circular di-chromism spectra of NEGp showed 12.0% α-helix, 24.3% α-helix turn and 63.7% random coils without beta pleated sheets. NEGp was found to exhibit anticoagulant activity by extending clotting time of both platelet rich plasma and platelet poor plasma from control 240 s to 1800 s and 280 s to 2100 s respectively at the concentration of 8 μg. NEGp inhibited the agonists such as ADP, epinephrine and arachidonic acid induced platelet aggregation in washed platelets. The percentage of inhibition was found to be 70%, 80% and 60% respectively. While, it did not interfere in thrombin, PAF and collagen induced platelet aggregation. NEGp did not hydrolyse RBC membrane, devoid of haemorrhagic and edema inducing properties in experimental mice.

The cytoplasmic tail of human mannosidase Man1b1 contributes to catalysis-independent quality control of misfolded alpha1-antitrypsin

The failure of polypeptides to achieve conformational maturation following biosynthesis can result in the formation of protein aggregates capable of disrupting essential cellular functions. In the secretory pathway, misfolded asparagine (N)-linked glycoproteins are selectively sorted for endoplasmic reticulum-associated degradation (ERAD) in response to the catalytic removal of terminal alpha-linked mannose units. Remarkably, ER mannosidase I/Man1b1, the first alpha-mannosidase implicated in this conventional N-glycan-mediated process, can also contribute to ERAD in an unconventional, catalysis-independent manner. To interrogate this functional dichotomy, the intracellular fates of two naturally occurring misfolded N-glycosylated variants of human alpha1-antitrypsin (AAT), Null Hong Kong (NHK), and Z (ATZ), in Man1b1 knockout HEK293T cells were monitored in response to mutated or truncated forms of transfected Man1b1. As expected, the conventional catalytic system requires an intact active site in the Man1b1 luminal domain. In contrast, the unconventional system is under the control of an evolutionarily extended N-terminal cytoplasmic tail. Also, N-glycans attached to misfolded AAT are not required for accelerated degradation mediated by the unconventional system, further demonstrating its catalysis-independent nature. We also established that both systems accelerate the proteasomal degradation of NHK in metabolic pulse-chase labeling studies. Taken together, these results have identified the previously unrecognized regulatory capacity of the Man1b1 cytoplasmic tail and provided insight into the functional dichotomy of Man1b1 as a component in the mammalian proteostasis network.

Global Reprogramming of Gene Transcription in Trichoderma reesei by Overexpressing an Artificial Transcription Factor for Improved Cellulase Production and Identification of Ypr1 as an Associated Regulator

Synthetic biology studies on filamentous fungi are providing unprecedented opportunities for optimizing this important category of microbial cell factory. Artificial transcription factor can be designed and used to offer novel modes of regulation on gene transcription network. Trichoderma reesei is commonly used for cellulase production. In our previous studies, a plasmid library harboring genes encoding artificial zinc finger proteins (AZFPs) was constructed for engineering T. reesei, and the mutant strains with improved cellulase production were selected. However, the underlying mechanism by which AZFP function remain unclear. In this study, a T. reesei Rut-C30 mutant strain T. reesei U5 bearing an AZFP named as AZFP-U5 was focused, which secretes high level protein and shows significantly improved cellulase and xylanase production comparing with its parental strain. In addition, enhanced sugar release was achieved from lignocellulosic biomass using the crude cellulase from T. reesei U5. Comparative transcriptome analysis was further performed, which showed reprogramming of global gene transcription and elevated transcription of genes encoding glycoside hydrolases by overexpressing AZFP-U5. Furthermore, 15 candidate regulatory genes which showed remarkable higher transcription levels by AZFP-U5 insertion were overexpressed in T. reesei Rut-C30 to examine their effects on cellulase biosynthesis. Among these genes, TrC30_93861 (ypr1) and TrC30_74374 showed stimulating effects on filter paper activity (FPase), but deletion of these two genes did not affect cellulase activity. In addition, increased yellow pigment production in T. reesei Rut-C30 by overexpression of gene ypr1 was observed, and changes of cellulase gene transcription were revealed in the ypr1 deletion mutant, suggesting possible interaction between pigment production and cellulase gene transcription. The results in this study revealed novel aspects in regulation of cellulase gene expression by the artificial regulators. In addition, the candidate genes and processes identified in the transcriptome data can be further explored for synthetic biology design and metabolic engineering of T. reesei to enhance cellulase production.

α-glucosidase inhibitors as host-directed antiviral agents with potential for the treatment of COVID-19

The ongoing COVID-19 pandemic, caused by SARS-CoV-2, has pushed the health systems of many countries to breaking point and precipitated social distancing measures that have crippled economic activities across the globe. A return to normality is unlikely until effective therapeutics and a vaccine are available. The immediacy of this problem suggests that drug strategies should focus on repurposing approved drugs or late-stage clinical candidates, as these have the shortest path to use in the clinic. Here, we review and discuss the role of host cell N-glycosylation pathways to virus replication and the drugs available to disrupt these pathways. In particular, we make a case for evaluation of the well-tolerated drugs miglitol, celgosivir and especially miglustat for the treatment of COVID-19.

Assorted dysfunctions of endosomal alkali cation/proton exchanger SLC9A6 variants linked to Christianson syndrome

Genetic screening has identified numerous variants of the endosomal solute carrier family 9 member A6 (SLC9A6)/(Na+,K+)/H+ exchanger 6 (NHE6) gene that cause Christianson syndrome, a debilitating X-linked developmental disorder associated with a range of neurological, somatic, and behavioral symptoms. Many of these variants cause complete loss of NHE6 expression, but how subtler missense substitutions or nonsense mutations that partially truncate its C-terminal cytoplasmic regulatory domain impair NHE6 activity and endosomal function are poorly understood. Here, we describe the molecular and cellular consequences of six unique mutations located in the N-terminal cytoplasmic segment (A9S), the membrane ion translocation domain (L188P and G383D), and the C-terminal regulatory domain (E547*, R568Q, and W570*) of human NHE6 that purportedly cause disease. Using a heterologous NHE6-deficient cell expression system, we show that the biochemical, catalytic, and cellular properties of the A9S and R568Q variants were largely indistinguishable from those of the WT transporter, which obscured their disease significance. By contrast, the L188P, G383D, E547*, and W570* mutants exhibited variable deficiencies in biosynthetic post-translational maturation, membrane sorting, pH homeostasis in recycling endosomes, and cargo trafficking, and they also triggered apoptosis. These findings broaden our understanding of the molecular dysfunctions of distinct NHE6 variants associated with Christianson syndrome.

The involvement of wheat U-box E3 ubiquitin ligase TaPUB1 in salt stress tolerance

U-box E3 ubiquitin ligases play important roles in the ubiquitin/26S proteasome machinery and in abiotic stress responses. TaPUB1-overexpressing wheat (Triticum aestivum L.) were generated to evaluate its function in salt tolerance. These plants had more salt stress tolerance during seedling and flowering stages, whereas the TaPUB1-RNA interference (RNAi)-mediated knock-down transgenic wheat showed more salt stress sensitivity than the wild type (WT). TaPUB1 overexpression upregulated the expression of genes related to ion channels and increased the net root Na⁺ efflux, but decreased the net K⁺ efflux and H⁺ influx, thereby maintaining a low cytosolic Na⁺ /K⁺ ratio, compared with the WT. However, RNAi-mediated knock-down plants showed the opposite response to salt stress. TaPUB1 could induce the expression of some genes that improved the antioxidant capacity of plants under salt stress. TaPUB1 also interacted with TaMP (Triticum aestivum α-mannosidase protein), a regulator playing an important role in salt response in yeast and in plants. Thus, low cytosolic Na⁺ /K⁺ ratios and better antioxidant enzyme activities could be maintained in wheat with overexpression of TaPUB1 under salt stress. Therefore, we conclude that the U-box E3 ubiquitin ligase TaPUB1 positively regulates salt stress tolerance in wheat.

Hepatic metabolomics and transcriptomics to study susceptibility to ketosis in response to prepartal nutritional management

Background

Ketosis in dairy cows is associated with body fat mobilization during the peripartal period. Sub-clinical and clinical ketosis arise more frequently in cows that are overfed energy during the entire dry (last 50 to 45 days prior to parturition) or close-up period (last ~ 28 days prepartum).

Methods

A retrospective analysis was performed on 12 cows from a larger cohort that were fed a higher-energy diet [1.54 Mcal/kg of dry matter (DM); 35.9% of DM corn silage and 13% of DM ground corn] during the close-up dry period, of which 6 did not develop clinical ketosis (OVE, 0.83 mmol/L plasma hydroxybutyrate; BHB) and 6 were diagnosed with clinical ketosis (KET, 1.4 mmol/L BHB) during the first week postpartum. A whole-transcriptome bovine microarray (Agilent Technologies) and metabolomics (GC-MS, LC-MS; Metabolon® Inc.) were used to perform transcript and metabolite profiling of liver tissue harvested at − 10 days relative to parturition which allowed to establish potential associations between prepartal transcriptome/metabolome profiles and susceptibility to clinical ketosis postpartum.

Results

Cows in KET had greater (P = 0.01) overall body weight between − 2 and 1 week around parturition, but similar body condition score than OVE. Although dry matter intake (DMI) did not differ prepartum, KET cows had lower (P < 0.01) DMI and similar milk yield as OVE cows during the first week postpartum. Transcriptome analysis revealed a total of 3065 differentially expressed genes (DEG; P ≤ 0.05) in KET. Metabolomics identified 15 out of 313 total biochemical compounds significantly affected (P ≤ 0.10) in KET. Among those, greater concentrations (P ≤ 0.06, + 2.3-fold) of glycochenodeoxycholate in KET cows also have been detected in humans developing non-alcoholic fatty liver disease. Bioinformatics analysis using the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway database and the DEG revealed that, among the top 20 most-impacted metabolic pathway categories in KET, 65% were overall downregulated. Those included ‘Metabolism of cofactors and vitamins’, ‘Biosynthesis of other secondary metabolites’, ‘Lipid’, ‘Carbohydrate’, and ‘Glycan biosynthesis and metabolism’. The lower relative concentration of glucose-6-phosphate and marked downregulation of fructose-1,6-bisphosphatase 2 and pyruvate dehydrogenase kinase 4 support a strong impairment in gluconeogenesis in prepartal liver of cows developing KET postpartum. Among the top 20 most-impacted non-metabolic pathways, 85% were downregulated. Pathways such as ‘mTOR signalling’ and ‘Insulin signalling’ were among those. ‘Ribosome’, ‘Nucleotide excision repair’, and ‘Adherens junctions’ were the only upregulated pathways in cows with KET.

Conclusions

The combined data analyses revealed more extensive alterations of the prepartal liver transcriptome than metabolome in cows overfed energy and developing ketosis postpartum. The causative link between these tissue-level adaptations and onset of clinical ketosis needs to be studied further.

Manganese-induced cellular disturbance in the baker's yeast, Saccharomyces cerevisiae with putative implications in neuronal dysfunction

Manganese (Mn) is an essential element, but in humans, chronic and/or acute exposure to this metal can lead to neurotoxicity and neurodegenerative disorders including Parkinsonism and Parkinson’s Disease by unclear mechanisms. To better understand the effects that exposure to Mn²⁺ exert on eukaryotic cell biology, we exposed a non-essential deletion library of the yeast Saccharomyces cerevisiae to a sub-inhibitory concentration of Mn²⁺ followed by targeted functional analyses of the positive hits. This screen produced a set of 43 sensitive deletion mutants that were enriched for genes associated with protein biosynthesis. Our follow-up investigations demonstrated that Mn reduced total rRNA levels in a dose-dependent manner and decreased expression of a β-galactosidase reporter gene. This was subsequently supported by analysis of ribosome profiles that suggested Mn-induced toxicity was associated with a reduction in formation of active ribosomes on the mRNAs. Altogether, these findings contribute to the current understanding of the mechanism of Mn-triggered cytotoxicity. Lastly, using the Comparative Toxicogenomic Database, we revealed that Mn shared certain similarities in toxicological mechanisms with neurodegenerative disorders including amyotrophic lateral sclerosis, Alzheimer’s, Parkinson’s and Huntington’s diseases.

The Protein Disulfide Isomerase Family: from proteostasis to pathogenesis

In mammalian cells, nearly one-third of proteins are inserted into the endoplasmic reticulum (ER), where they undergo oxidative folding and chaperoning assisted by approximately 20 members of the protein disulfide isomerase family (PDIs). PDIs consist of multiple thioredoxin-like domains and recognize a wide variety of proteins via highly conserved interdomain flexibility. Although PDIs have been studied intensely for almost 50 years, exactly how they maintain protein homeostasis in the ER remains unknown, and is important not only for fundamental biological understanding but also for protein misfolding- and aggregation-related pathophysiology. Herein, we review recent advances in structural biology and biophysical approaches that explore the underlying mechanism by which PDIs fulfil their distinct functions to promote productive protein folding and scavenge misfolded proteins in the ER, the primary factory for efficient production of the secretome.

The UDP-Glycosyltransferase (UGT) Superfamily: New Members, New Functions, and Novel Paradigms

UDP-glycosyltransferases (UGTs) catalyze the covalent addition of sugars to a broad range of lipophilic molecules. This biotransformation plays a critical role in elimination of a broad range of exogenous chemicals and by-products of endogenous metabolism, and also controls the levels and distribution of many endogenous signaling molecules. In mammals, the superfamily comprises four families: UGT1, UGT2, UGT3, and UGT8. UGT1 and UGT2 enzymes have important roles in pharmacology and toxicology including contributing to interindividual differences in drug disposition as well as to cancer risk. These UGTs are highly expressed in organs of detoxification (e.g., liver, kidney, intestine) and can be induced by pathways that sense demand for detoxification and for modulation of endobiotic signaling molecules. The functions of the UGT3 and UGT8 family enzymes have only been characterized relatively recently; these enzymes show different UDP-sugar preferences to that of UGT1 and UGT2 enzymes, and to date, their contributions to drug metabolism appear to be relatively minor. This review summarizes and provides critical analysis of the current state of research into all four families of UGT enzymes. Key areas discussed include the roles of UGTs in drug metabolism, cancer risk, and regulation of signaling, as well as the transcriptional and posttranscriptional control of UGT expression and function. The latter part of this review provides an in-depth analysis of the known and predicted functions of UGT3 and UGT8 enzymes, focused on their likely roles in modulation of levels of endogenous signaling pathways.

Comparative detection method of early onset cytokine-induced stress in β-cells (INS-1E)

β-Cells contain a prominent endoplasmic reticulum (ER), disrupting ER homeostasis and function, activating the unfolded protein response (UPR). Currently, no direct protocols measure the UPR initiation. Current methods to measure ER stress include the quantification of nitric oxide (NO) (indirect method), Western blotting, and qRT-PCR of downstream components. However, these methods do not account for the overlap with mitochondrial dysfunction. In this study, INS-1E cells were exposed to proinflammatory cytokines to induce ER stress, as determined using NO, thioflavin T (ThT) binding, and β-cell functionality (insulin production). ER stress was confirmed through the upregulation of CHOP. Cell viability was monitored using MTT, sulforhodamine B, and the xCELLigence system. Morphological changes were monitored using electron microscopy. IL-1β exposure-induced β-cell stress after 4 H, decreased insulin levels, and increased thioflavin binding were noted. Increased NO production was only detected after 10 H, highlighting its lack of sensitivity, and the need for a continuous, selective, rapid, convenient, and economical detection method for early onset of ER stress. Standard methods (MTT and NO) failed to detect early ER stress. The xCELLigence coupled with a functional assay such as the detection of insulin levels or ThT are better predictors of ER stress in INS-1E cells.

Paraquat-Mediated Oxidative Stress in Anopheles gambiae Mosquitoes Is Regulated by An Endoplasmic Reticulum (ER) Stress Response

Paraquat is a potent superoxide (O₂⁻)-inducing agent that is capable of inducing an oxidative imbalance in the mosquito midgut. This oxidative imbalance can super-stress the malaria parasite, leading to arrested development in the mosquito midgut and reduced transmission. While several studies have explored the effect of paraquat on malaria parasites, a fundamental understanding of the mosquito response to this compound remains unknown. Here, we quantified the mosquito midgut proteomic response to a paraquat-laced sugar meal, and found that An. gambiae midguts were enriched in proteins that are indicative of cells under endoplasmic reticulum (ER) stress. We also carried out qRT-PCR analyses for nine prominent thioredoxin (Trx) and glutathione (GSH)-dependent genes in mosquito midguts post P. falciparum blood meal ingestion to evaluate the concordance between transcripts and proteins under different oxidative stress conditions. Our data revealed an absence of significant upregulation in the Trx and GSH-dependent genes following infected blood meal ingestion. These data suggest that the intrinsic tolerance of the mosquito midgut to paraquat-mediated oxidative stress is through an ER stress response. These data indicate that mosquitoes have at least two divergent pathways of managing the oxidative stress that is induced by exogenous compounds, and outline the potential application of paraquat-like drugs to act selectively against malaria parasite development in mosquito midguts, thereby blocking mosquito-to-human transmission.

Role of Selenoprotein F in Protein Folding and Secretion: Potential Involvement in Human Disease

Selenoproteins form a group of proteins of which its members contain at least one selenocysteine, and most of them serve oxidoreductase functions. Selenoprotein F (SELENOF), one of the 25 currently identified selenoproteins, is located in the endoplasmic reticulum (ER) organelle and is abundantly expressed in many tissues. It is regulated according to its selenium status, as well as by cell stress conditions. SELENOF may be functionally linked to protein folding and the secretion process in the ER. Several studies have reported positive associations between SELENOF genetic variations and several types of cancer. Also, altered expression levels of SELENOF have been found in cancer cases and neurodegenerative diseases. In this review, we summarize the current understanding of the structure, expression, and potential function of SELENOF and discuss its possible relation with various pathological processes.

Non-C-mannosylable mucin CYS domains hindered proper folding and secretion of mucin

The CYS domain occurs in multiple copies in many gel-forming mucins. It is believed that CYS domains can interact with each other in a reversible manner, suggesting a key role of the domain in gel formation. This domain always contains in its amino-terminal sequence the C-mannosylation motif WXXW, but whether the CYS domain is C-mannosylated is debated, and the putative role of C-mannosylation of the domain is unclear. We prepared recombinant CYS domains of the human mucin MUC5B with (WXXW→AXXW) and without a single amino acid mutation and mini-5B mucins made of a large Ser/Thr/Pro region flanked by two CYS domains with the WXXW motif or with the mutated AXXW motif on the first, second or both CYS domains. We found that the single CYS domain and the two CYS domains of mini-5B mucin must be C-mannosylable for the efficient maturation and secretion of the recombinant molecules; otherwise, they are retained in the cell and co-localized with a resident enzyme of the endoplasmic reticulum.

Mutual interaction between oxidative stress and endoplasmic reticulum stress in the pathogenesis of diseases specifically focusing on non-alcoholic fatty liver disease

Reactive oxygen species (ROS) are produced during normal physiologic processes with the consumption of oxygen. While ROS play signaling roles, when they are produced in excess beyond normal antioxidative capacity this can cause pathogenic damage to cells. The majority of such oxidation occurs in polyunsaturated fatty acids and sulfhydryl group in proteins, resulting in lipid peroxidation and protein misfolding, respectively. The accumulation of misfolded proteins in the endoplasmic reticulum (ER) is enhanced under conditions of oxidative stress and results in ER stress, which, together, leads to the malfunction of cellular homeostasis. Multiple types of defensive machinery are activated in unfolded protein response under ER stress to resolve this unfavorable situation. ER stress triggers the malfunction of protein secretion and is associated with a variety of pathogenic conditions including defective insulin secretion from pancreatic β-cells and accelerated lipid droplet formation in hepatocytes. Herein we use nonalcoholic fatty liver disease (NAFLD) as an illustration of such pathological liver conditions that result from ER stress in association with oxidative stress. Protecting the ER by eliminating excessive ROS via the administration of antioxidants or by enhancing lipid-metabolizing capacity via the activation of peroxisome proliferator-activated receptors represent promising therapeutics for NAFLD.

Endoplasmic Reticulum redox pathways: in sickness and in health

The Endoplasmic Reticulum (ER) is the major site for secretory protein production in eukaryotic cells and like an efficient factory, it has the capacity to expand or contract its output depending on the demand for its services. A primary function of the ER is to co-ordinate the quality control of proteins as they enter this folding factory at the base of the secretory pathway. Reduction-oxidation (redox) reactions have an important role to play in the quality control process, through the provision of disulphide bonds and by maintaining a favourable redox environment for oxidative protein folding. The ER is also a major contributor to calcium homeostasis and is a key site for lipid biosynthesis, two processes that additionally impact upon, and are influenced by, redox in the ER compartment.

Plant G proteins interact with endoplasmic reticulum luminal protein receptors to regulate endoplasmic reticulum retrieval

Maintaining endoplasmic reticulum (ER) homeostasis is essential for the production of biomolecules. ER retrieval, i.e., the retrograde transport of compounds from the Golgi to the ER, is one of the pathways that ensures ER homeostasis. However, the mechanisms underlying the regulation of ER retrieval in plants remain largely unknown. Plant ERD2-like proteins (ERD2s) were recently suggested to function as ER luminal protein receptors that mediate ER retrieval. Here, we demonstrate that heterotrimeric G protein signaling is involved in ERD2-mediated ER retrieval. We show that ERD2s interact with the heterotrimeric G protein Gα and Gγ subunits at the Golgi. Silencing of Gα, Gβ, or Gγ increased the retention of ER luminal proteins. Furthermore, overexpression of Gα, Gβ, or Gγ caused ER luminal proteins to escape from the ER, as did the co-silencing of ERD2a and ERD2b. These results suggest that G proteins interact with ER luminal protein receptors to regulate ER retrieval.

Trimming of N-Glycans by the Golgi-Localized α-1,2-Mannosidases, MNS1 and MNS2, Is Crucial for Maintaining RSW2 Protein Abundance during Salt Stress in Arabidopsis

Asparagine (Asn/N)-linked glycans are important for protein folding, trafficking, and endoplasmic reticulum-associated degradation in eukaryotes. The maturation of glycoproteins involves the trimming of mannosyl residues by mannosidases and addition of other sugar molecules to three-branched N-glycans in the Golgi. However, the biological importance of Golgi-mediated mannose trimming is not fully understood. Here, we show that abolishment of two functionally redundant mannosidases, MNS1 and MNS2, responsible for α-1,2-mannose trimming on the A and C branches of plant N-glycans lead to severe root growth inhibition under salt stress conditions in Arabidopsis. In contrast, mutants with defects in the biosynthesis of the oligosaccharide precursor displayed enhanced salt tolerance in the absence of mannose trimming. However, mutation in EBS3, which is required for the formation of the branched N-glycan precursor, suppressed the salt-sensitive phenotype of mns1 mns2 double mutant. Interestingly, we observed that cellulose biosynthesis was compromised in mns1 mns2 roots under high salinity. Consistently, abundance of a membrane anchored endo-β-1,4-endoglucanase (RSW2/KOR) that plays a key role in cellulose biosynthesis and its mutant variant rsw2-1 were modulated by α-1,2-mannose trimming under salt stress. Overexpression of RSW2 could partially rescue the salt-sensitive phenotype of mns1 mns2. Taken together, these results suggest that MNS1/2-mediated mannose trimming of N-glycans is crucial in modulating glycoprotein abundance to withstand salt stress in plants.