Zinc regulates ERp44-dependent protein quality control in the early secretory pathway

Depleting inositol pyrophosphate 5-InsP7 protected the heart against ischaemia-reperfusion injury by elevating plasma adiponectin

AIMS

Adiponectin is an adipocyte-derived circulating protein that exerts cardiovascular and metabolic protection. Due to the futile degradation of endogenous adiponectin and the challenges of exogenous administration, regulatory mechanisms of adiponectin biosynthesis are of significant pharmacological interest.

METHODS AND RESULTS

Here, we report that 5-diphosphoinositol 1,2,3,4,6-pentakisphosphate (5-InsP7) generated by inositol hexakisphosphate kinase 1 (IP6K1) governed circulating adiponectin levels via thiol-mediated protein quality control in the secretory pathway. IP6K1 bound to adiponectin and DsbA-L and generated 5-InsP7 to stabilize adiponectin/ERp44 and DsbA-L/Ero1-Lα interactions, driving adiponectin intracellular degradation. Depleting 5-InsP7 by either IP6K1 deletion or pharmacological inhibition blocked intracellular adiponectin degradation. Whole-body and adipocyte-specific deletion of IP6K1 boosted plasma adiponectin levels, especially its high molecular weight forms, and activated AMPK-mediated protection against myocardial ischaemia-reperfusion injury. Pharmacological inhibition of 5-InsP7 biosynthesis in wild-type but not adiponectin knockout mice attenuated myocardial ischaemia-reperfusion injury.

CONCLUSION

Our findings revealed that 5-InsP7 is a physiological regulator of adiponectin biosynthesis that is amenable to pharmacological intervention for cardioprotection.

Structure of full-length ERGIC-53 in complex with MCFD2 for cargo transport

ERGIC-53 transports certain subsets of newly synthesized secretory proteins and membrane proteins from the endoplasmic reticulum to the Golgi apparatus. Despite numerous structural and functional studies since its identification, the overall architecture and mechanism of action of ERGIC-53 remain unclear. Here we present cryo-EM structures of full-length ERGIC-53 in complex with its functional partner MCFD2. These structures reveal that ERGIC-53 exists as a homotetramer, not a homohexamer as previously suggested, and comprises a four-leaf clover-like head and a long stalk composed of three sets of four-helix coiled-coil followed by a transmembrane domain. 3D variability analysis visualizes the flexible motion of the long stalk and local plasticity of the head region. Notably, MCFD2 is shown to possess a Zn2+-binding site in its N-terminal lid, which appears to modulate cargo binding. Altogether, distinct mechanisms of cargo capture and release by ERGIC- 53 via the stalk bending and metal binding are proposed.

Structures, Mechanisms, and Physiological Functions of Zinc Transporters in Different Biological Kingdoms

Zinc transporters take up/release zinc ions (Zn2+) across biological membranes and maintain intracellular and intra-organellar Zn2+ homeostasis. Since this process requires a series of conformational changes in the transporters, detailed information about the structures of different reaction intermediates is required for a comprehensive understanding of their Zn2+ transport mechanisms. Recently, various Zn2+ transport systems have been identified in bacteria, yeasts, plants, and humans. Based on structural analyses of human ZnT7, human ZnT8, and bacterial YiiP, we propose updated models explaining their mechanisms of action to ensure efficient Zn2+ transport. We place particular focus on the mechanistic roles of the histidine-rich loop shared by several zinc transporters, which facilitates Zn2+ recruitment to the transmembrane Zn2+-binding site. This review provides an extensive overview of the structures, mechanisms, and physiological functions of zinc transporters in different biological kingdoms.

Zinc supplementation with polaprezinc was associated with improvements in albumin, prothrombin time activity, and hemoglobin in chronic liver disease

Zinc deficiency occurs in a variety of diseases, including chronic liver disease (CLD). We investigated the correlation between zinc levels and biochemical and hematological tests in CLD and the effect of zinc supplementation with polaprezinc on these values. The first study (Study 1) was a retrospective observational study of 490 patients with CLD not receiving zinc supplementation, with data available from September 2009 to August 2021. Univariate and multiple regression analysis showed that serum zinc levels correlated most strongly with albumin (Alb) and also significantly with prothrombin time activity (PT%) and hemoglobin (Hb). A subsequent study (Study 2) focused on patients with advanced CLD who used polaprezinc for more than 90 days between January 2005 and August 2021. Using a self-controlled design with the 6-month period prior to polaprezinc as the control period, comparisons showed that Alb (p<0.0001), PT% (p<0.0005), and Hb (p<0.01) were significantly improved in the polaprezinc-treated patients compared to the control group. In conclusion, serum zinc levels were correlated with serum Alb, Hb, and PT% in patients with CLD, and zinc supplementation with polaprezinc was associated with improvements in Alb, Hb, and PT% within at least 6 months.

Zn2+-dependent functional switching of ERp18, an ER-resident thioredoxin-like protein

ERp18 is an endoplasmic reticulum (ER)-resident thioredoxin (Trx) family protein, similar to cytosolic Trx1. The Trx-like domain occupies a major portion of the whole ERp18 structure, which is postulated to be an ER paralog of cytosolic Trx1. Here, we elucidate that zinc ion (Zn2+) binds ERp18 through its catalytic motif, triggering oligomerization of ERp18 from a monomer to a trimer. While the monomeric ERp18 has disulfide oxidoreductase activity, the trimeric ERp18 acquires scavenger activity for hydrogen peroxide (H2O2) in the ER. Depletion of ERp18 thus causes the accumulation of H2O2, which is produced during the oxidative folding of nascent polypeptides in the ER. ERp18 knockdown in C. elegans without Prx4 and GPx7/8, both of which are also known to have H2O2 scavenging activity in the ER, shortened the lifespan, suggesting that ERp18 may form a primitive and essential H2O2 scavenging system for the maintenance of redox homeostasis in the ER.

Exploring the Influence of Zinc Ions on the Conformational Stability and Activity of Protein Disulfide Isomerase

The interplay between metal ion binding and the activity of thiol proteins, particularly within the protein disulfide isomerase family, remains an area of active investigation due to the critical role that these proteins play in many vital processes. This research investigates the interaction between recombinant human PDIA1 and zinc ions, focusing on the subsequent implications for PDIA1's conformational stability and enzymatic activity. Employing isothermal titration calorimetry and differential scanning calorimetry, we systematically compared the zinc binding capabilities of both oxidized and reduced forms of PDIA1 and assessed the structural consequences of this interaction. Our results demonstrate that PDIA1 can bind zinc both in reduced and oxidized states, but with significantly different stoichiometry and more pronounced conformational effects in the reduced form of PDIA1. Furthermore, zinc binding was observed to inhibit the catalytic activity of reduced-PDIA1, likely due to induced alterations in its conformation. These findings unveil a potential regulatory mechanism in PDIA1, wherein metal ion binding under reductive conditions modulates its activity. Our study highlights the potential role of zinc in regulating the catalytic function of PDIA1 through conformational modulation, suggesting a nuanced interplay between metal binding and protein stability in the broader context of cellular redox regulation.

Zn2+ is essential for Ca2+ oscillations in mouse eggs

Changes in the intracellular concentration of free calcium (Ca2+) underpin egg activation and initiation of development in animals and plants. In mammals, the Ca2+ release is periodical, known as Ca2+ oscillations, and mediated by the type 1 inositol 1,4,5-trisphosphate receptor (IP3R1). Another divalent cation, zinc (Zn2+), increases exponentially during oocyte maturation and is vital for meiotic transitions, arrests, and polyspermy prevention. It is unknown if these pivotal cations interplay during fertilization. Here, using mouse eggs, we showed that basal concentrations of labile Zn2+ are indispensable for sperm-initiated Ca2+ oscillations because Zn2+-deficient conditions induced by cell-permeable chelators abrogated Ca2+ responses evoked by fertilization and other physiological and pharmacological agonists. We also found that chemically or genetically generated eggs with lower levels of labile Zn2+ displayed reduced IP3R1 sensitivity and diminished ER Ca2+ leak despite the stable content of the stores and IP3R1 mass. Resupplying Zn2+ restarted Ca2+ oscillations, but excessive Zn2+ prevented and terminated them, hindering IP3R1 responsiveness. The findings suggest that a window of Zn2+ concentrations is required for Ca2+ responses and IP3R1 function in eggs, ensuring optimal response to fertilization and egg activation.

Metalation and activation of Zn2+ enzymes via early secretory pathway-resident ZNT proteins

Zinc (Zn2+), an essential trace element, binds to various proteins, including enzymes, transcription factors, channels, and signaling molecules and their receptors, to regulate their activities in a wide range of physiological functions. Zn2+ proteome analyses have indicated that approximately 10% of the proteins encoded by the human genome have potential Zn2+ binding sites. Zn2+ binding to the functional site of a protein (for enzymes, the active site) is termed Zn2+ metalation. In eukaryotic cells, approximately one-third of proteins are targeted to the endoplasmic reticulum; therefore, a considerable number of proteins mature by Zn2+ metalation in the early secretory pathway compartments. Failure to capture Zn2+ in these compartments results in not only the inactivation of enzymes (apo-Zn2+ enzymes), but also their elimination via degradation. This process deserves attention because many Zn2+ enzymes that mature during the secretory process are associated with disease pathogenesis. However, how Zn2+ is mobilized via Zn2+ transporters, particularly ZNTs, and incorporated in enzymes has not been fully elucidated from the cellular perspective and much less from the biophysical perspective. This review focuses on Zn2+ enzymes that are activated by Zn2+ metalation via Zn2+ transporters during the secretory process. Further, we describe the importance of Zn2+ metalation from the physiopathological perspective, helping to reveal the importance of understanding Zn2+ enzymes from a biophysical perspective.

The organization and function of the Golgi apparatus in dendrite development and neurological disorders

Dendrites are specialized neuronal compartments that sense, integrate and transfer information in the neural network. Their development is tightly controlled and abnormal dendrite morphogenesis is strongly linked to neurological disorders. While dendritic morphology ranges from relatively simple to extremely complex for a specified neuron, either requires a functional secretory pathway to continually replenish proteins and lipids to meet dendritic growth demands. The Golgi apparatus occupies the center of the secretory pathway and is regulating posttranslational modifications, sorting, transport, and signal transduction, as well as acting as a non-centrosomal microtubule organization center. The neuronal Golgi apparatus shares common features with Golgi in other eukaryotic cell types but also forms distinct structures known as Golgi outposts that specifically localize in dendrites. However, the organization and function of Golgi in dendrite development and its impact on neurological disorders is just emerging and so far lacks a systematic summary. We describe the organization of the Golgi apparatus in neurons, review the current understanding of Golgi function in dendritic morphogenesis, and discuss the current challenges and future directions.

Structure of a putative immature form of a Rieske-type iron-sulfur protein in complex with zinc chloride

Iron-sulfur clusters are prosthetic groups of proteins involved in various biological processes. However, details of the immature state of the iron-sulfur cluster into proteins have not yet been elucidated. We report here the first structural analysis of the Zn-containing form of a Rieske-type iron-sulfur protein, PetA, from Thermochromatium tepidum (TtPetA) by X-ray crystallography and small-angle X-ray scattering analysis. The Zn-containing form of TtPetA was indicated to be a dimer in solution. The zinc ion adopts a regular tetra-coordination with two chloride ions and two cysteine residues. Only a histidine residue in the cluster-binding site exhibited a conformational difference from the [2Fe-2S] containing form. The Zn-containing structure indicates that the conformation of the cluster binding site is already constructed and stabilized before insertion of [2Fe-2S]. The binding mode of ZnCl2, similar to the [2Fe-2S] cluster, suggests that the zinc ions might be involved in the insertion of the [2Fe-2S] cluster.

Annexin A1 is a cell-intrinsic metalloregulator of zinc in human ILC2s

Group 2 innate lymphoid cells (ILC2s) produce large amounts of type 2 cytokines including interleukin-5 (IL-5) and IL-13 in response to various stimuli, causing allergic and eosinophilic diseases. However, the cell-intrinsic regulatory mechanisms of human ILC2s remain unclear. Here, we analyze human ILC2s derived from different tissues and pathological conditions and identify ANXA1, encoding annexin A1, as a commonly highly expressed gene in non-activated ILC2s. The expression of ANXA1 decreases when ILC2s activate, but it increases autonomously as the activation subsides. Lentiviral vector-based gene transfer experiments show that ANXA1 suppresses the activation of human ILC2s. Mechanistically, ANXA1 regulates the expression of the metallothionein family genes, including MT2A, which modulate intracellular zinc homeostasis. Furthermore, increased intracellular zinc levels play an essential role in the activation of human ILC2s by promoting the mitogen-activated protein kinase (MAPK) and nuclear factor κB (NF-κB) pathways and GATA3 expression. Thus, the ANXA1/MT2A/zinc pathway is identified as a cell-intrinsic metalloregulatory mechanism for human ILC2s.

M1-aminopeptidase family - beyond antigen-trimming activities

Antigen (Ag)-trimming aminopeptidases belong to the oxytocinase subfamily of M1 metallopeptidases. In humans, this subfamily contains the endoplasmic reticulum aminopeptidases 1 and 2 (ERAP1 and 2) and the insulin-responsive aminopeptidase (IRAP, synonym oxytocinase), an endosomal enzyme. The ability of these enzymes to trim antigenic precursors and to generate major histocompatibility class-I ligands has been demonstrated extensively for ERAP1, less for ERAP2, which is absent in rodents, and exclusively in the context of cross-presentation for IRAP. During 20 years of research on these aminopeptidases, their enzymatic function has been very well characterized and their genetic association with autoimmune diseases, cancers, and infections is well established. The mechanisms by which these proteins are associated to human diseases are not always clear. This review discusses the Ag-trimming-independent functions of the oxytocinase subfamily of M1 aminopeptidases and the new questions raised by recent publications on IRAP and ERAP2.

Zinc homeostasis governed by Golgi-resident ZnT family members regulates ERp44-mediated proteostasis at the ER-Golgi interface

Many secretory enzymes acquire essential zinc ions (Zn²⁺) in the Golgi complex. ERp44, a chaperone operating in the early secretory pathway, also binds Zn²⁺ to regulate its client binding and release for the control of protein traffic and homeostasis. Notably, three membrane transporter complexes, ZnT4, ZnT5/ZnT6 and ZnT7, import Zn²⁺ into the Golgi lumen in exchange with protons. To identify their specific roles, we here perform quantitative Zn²⁺ imaging using super-resolution microscopy and Zn²⁺-probes targeted in specific Golgi subregions. Systematic ZnT-knockdowns reveal that ZnT4, ZnT5/ZnT6 and ZnT7 regulate labile Zn²⁺ concentration at the distal, medial, and proximal Golgi, respectively, consistent with their localization. Time-course imaging of cells undergoing synchronized secretory protein traffic and functional assays demonstrates that ZnT-mediated Zn²⁺ fluxes tune the localization, trafficking, and client-retrieval activity of ERp44. Altogether, this study provides deep mechanistic insights into how ZnTs control Zn²⁺ homeostasis and ERp44-mediated proteostasis along the early secretory pathway.

The differential placental expression of ERp44 and pre-eclampsia; association with placental zinc, the ERAP1 and the renin-angiotensin-system

INTRODUCTION

Endoplasmic reticulum resident protein 44 (ERp44) is a zinc-metalloprotein, regulating Endoplasmic reticulum aminopeptidase 1 (ERAP1) and Angiotensin II (Ang II). We explored placental ERp44 expression and components of the renin-angiotensin-system (RAS) in pre-eclampsia (PE), correlating these to ERAP1 expression and placental zinc concentrations.

METHODS

Placental tissue, taken at time of delivery in normotensive women or women with PE (n = 12/group), were analysed for ERp44, AT1R, AT2R and AT4R by qPCR. Protein ERp44 expression was measured by immunohistochemistry and compared to previously measured ERAP1 expression. Placental zinc was measured by inductively-coupled-mass-spectrometry.

RESULTS

ERp44 gene/protein expression were increased in PE (P < 0.05). AT1R expression was increased (P = 0.02) but AT4R decreased (P = 0.01) in PE, compared to normotensive controls. A positive association between ERp44 and AT2R expression was observed in all groups. ERp44 was negatively correlated with ERAP1 protein expression in all samples. Placental zinc concentrations were lower in women with PE (P = 0.001) and negatively associated with ERp44 gene expression.

DISCUSSION

Increased placental ERp44 could further reduce ERAP1 release in PE, potentially preventing release of Ang IV and thus lowering levels of Ang IV which consequently diminishes the possibility of counterbalancing the activity of vasoconstrictive, Ang II. The lower placental zinc may contribute to dysfunction of the ERp44/ERAP1 complex, exacerbating the hypertension in PE.

SUMOylation of ERp44 enhances Ero1α ER retention contributing to the pathogenesis of obesity and insulin resistance

OBJECTIVE

As the only E2 conjugating enzyme for the SUMO system, Ubc9-mediated SUMOylation has been recognized to regulate diverse biological processes, but its impact on adipocytes relevant to obesity and insulin resistance is yet to be elucidated.

METHODS

We established adipocyte-specific Ubc9 deficient mice to explore the effects of Ubc9 on obesity and metabolic disorders induced by high-fat diet (HFD) in adult mice. The molecular targets of SUMOylation were explored by liquid chromatography-mass spectrometry, and the regulatory mechanism of SUMOylation in T2D was analyzed.

RESULTS

Adipocyte-specific depletion of Ubc9 (AdipoQ-Cre-Ubc9^fl/fl, Ubc9^AKO) protected mice from HFD-induced obesity, insulin resistance, and hepatosteatosis. The Ubc9^AKO mice were featured by the reduced HFD-induced endoplasmic reticulum (ER) stress and inflammatory response. Mechanically, over nutrition rendered adipocytes to undergo a SUMOylation turnover characterized by the change of SUMOylation levels and substrates. ERp44 displayed the highest change in terms of SUMOylation levels of substrates involved in ER-related functions. The lack of ERp44 SUMOylation at lysine 76 (K76) located within the thioredoxin (TRX)-like domain by Ubc9 deficiency enhanced its degradation and suppressed its covalent binding to Ero1α, an oxidase that exists in the ER but lacks ER retention motif, thereby alleviating endoplasmic reticulum stress by promoting Ero1α secretion.

CONCLUSIONS

Our data suggest that modulation of ERp44 SUMOylation in adipocytes could be a feasible strategy against obesity and insulin resistance in clinical settings.

SARS-CoV-2 Envelope-mediated Golgi pH dysregulation interferes with ERAAP retention in cells

Endoplasmic reticulum (ER) aminopeptidase associated with antigen processing (ERAAP) trims peptide precursors in the ER for presentation by major histocompatibility (MHC)-I molecules to surveying CD8+ T-cells. This function allows ERAAP to regulate the nature and quality of the peptide repertoire and, accordingly, the resulting immune responses. We recently showed that infection with murine cytomegalovirus leads to a dramatic loss of ERAAP levels in infected cells. In mice, this loss is associated with the activation of QFL T-cells, a subset of T-cells that monitor ERAAP integrity and eliminate cells experiencing ERAAP dysfunction. In this study, we aimed to identify host factors that regulate ERAAP expression level and determine whether these could be manipulated during viral infections. We performed a CRISPR knockout screen and identified ERp44 as a factor promoting ERAAP retention in the ER. ERp44's interaction with ERAAP is dependent on the pH gradient between the ER and Golgi. We hypothesized that viruses that disrupt the pH of the secretory pathway interfere with ERAAP retention. Here, we demonstrate that expression of the Envelope (E) protein from Severe Acute Respiratory Syndrome Coronavirus 2 (SARS-CoV-2) leads to Golgi pH neutralization and consequently decrease of ERAAP intracellular levels. Furthermore, SARS-CoV-2-induced ERAAP loss correlates with its release into the extracellular environment. ERAAP's reliance on ERp44 and a functioning ER/Golgi pH gradient for proper localization and function led us to propose that ERAAP serves as a sensor of disturbances in the secretory pathway during infection and disease.

Myocardial TRPC6-mediated Zn2+ influx induces beneficial positive inotropy through β-adrenoceptors

Baroreflex control of cardiac contraction (positive inotropy) through sympathetic nerve activation is important for cardiocirculatory homeostasis. Transient receptor potential canonical subfamily (TRPC) channels are responsible for α₁-adrenoceptor (α₁AR)-stimulated cation entry and their upregulation is associated with pathological cardiac remodeling. Whether TRPC channels participate in physiological pump functions remains unclear. We demonstrate that TRPC6-specific Zn²⁺ influx potentiates β-adrenoceptor (βAR)-stimulated positive inotropy in rodent cardiomyocytes. Deletion of trpc6 impairs sympathetic nerve-activated positive inotropy but not chronotropy in mice. TRPC6-mediated Zn²⁺ influx boosts α₁AR-stimulated βAR/G_s-dependent signaling in rat cardiomyocytes by inhibiting β-arrestin-mediated βAR internalization. Replacing two TRPC6-specific amino acids in the pore region with TRPC3 residues diminishes the α₁AR-stimulated Zn²⁺ influx and positive inotropic response. Pharmacological enhancement of TRPC6-mediated Zn²⁺ influx prevents chronic heart failure progression in mice. Our data demonstrate that TRPC6-mediated Zn²⁺ influx with α₁AR stimulation enhances baroreflex-induced positive inotropy, which may be a new therapeutic strategy for chronic heart failure.

Selective Secretion of KDEL-Bearing Proteins: Mechanisms and Functions

In multicellular organisms, cells must continuously exchange messages with the right meaning, intensity, and duration. Most of these messages are delivered through cognate interactions between membrane and secretory proteins. Their conformational maturation is assisted by a vast array of chaperones and enzymes, ensuring the fidelity of intercellular communication. These folding assistants reside in the early secretory compartment (ESC), a functional unit that encompasses endoplasmic reticulum (ER), intermediate compartment and cis-Golgi. Most soluble ESC residents have C-terminal KDEL-like motifs that prevent their transport beyond the Golgi. However, some accumulate in the ER, while others in downstream stations, implying different recycling rates. Moreover, it is now clear that cells can actively secrete certain ESC residents but not others. This essay discusses the physiology of their differential intracellular distribution, and the mechanisms that may ensure selectivity of release.

Molecular physiology of zinc in Drosophila melanogaster

New research in Drosophila melanogaster has revealed the molecular mechanisms of zinc involvement in many biological processes. A newly discovered Metallothionein is predicted to have a higher zinc specificity than the other isoforms. Zinc negatively regulates tyrosine hydroxylase activity by antagonizing iron binding, thus rendering the enzyme ineffective or non-functional. The identification of a new chaperone of the protein disulfide isomerase family provided mechanistic insight into the protein trafficking defects caused by zinc dyshomeostasis in the secretory pathway. Insect models of tumor pathogenesis indicate that zinc regulates the structural stabilization of cells by transcriptionally regulating matrix metalloproteinases while zinc dyshomeostasis in the secretory pathway modulates cell signaling through endoplastic recticulum stress.

Zinc transport via ZNT5-6 and ZNT7 is critical for cell surface glycosylphosphatidylinositol-anchored protein expression

Glycosylphosphatidylinositol (GPI)-anchored proteins play crucial roles in various enzyme activities, cell signaling and adhesion, and immune responses. While the molecular mechanism underlying GPI-anchored protein biosynthesis has been well studied, the role of zinc transport in this process has not yet been elucidated. Zn transporter (ZNT) proteins mobilize cytosolic zinc to the extracellular space and to intracellular compartments. Here, we report that the early secretory pathway ZNTs (ZNT5–ZNT6 heterodimers [ZNT5-6] and ZNT7–ZNT7 homodimers [ZNT7]), which supply zinc to the lumen of the early secretory pathway compartments are essential for GPI-anchored protein expression on the cell surface. We show, using overexpression and gene disruption/re-expression strategies in cultured human cells, that loss of ZNT5-6 and ZNT7 zinc transport functions results in significant reduction in GPI-anchored protein levels similar to that in mutant cells lacking phosphatidylinositol glycan anchor biosynthesis (PIG) genes. Furthermore, medaka fish with disrupted Znt5 and Znt7 genes show touch-insensitive phenotypes similar to zebrafish Pig mutants. These findings provide a previously unappreciated insight into the regulation of GPI-anchored protein expression and protein quality control in the early secretory pathway.

Organelle-Level Labile Zn2+ Mapping Based on Targetable Fluorescent Sensors

Although many Zn²⁺ fluorescent probes have been developed, there remains a lack of consensus on the labile Zn²⁺ concentrations ([Zn²⁺]) in several cellular compartments, as the fluorescence properties and zinc affinity of the fluorescent probes are greatly affected by the pH and redox environments specific to organelles. In this study, we developed two turn-on-type Zn²⁺ fluorescent probes, namely, ZnDA-2H and ZnDA-3H, with low pH sensitivity and suitable affinity (K_d = 5.0 and 0.16 nM) for detecting physiological labile Zn²⁺ in various cellular compartments, such as the cytosol, nucleus, ER, and mitochondria. Due to their sufficient membrane permeability, both probes were precisely localized to the target organelles in HeLa cells using HaloTag labeling technology. Using an in situ standard quantification method, we identified the [Zn²⁺] in the tested organelles, resulting in the subcellular [Zn²⁺] distribution as [Zn²⁺]_ER < [Zn²⁺]_mito < [Zn²⁺]_cyto ∼ [Zn²⁺]_nuc.

Biogenesis of secretory immunoglobulin M requires intermediate non-native disulfide bonds and engagement of the protein disulfide isomerase ERp44

Antibodies of the immunoglobulin M (IgM) class represent the frontline of humoral immune responses. They are secreted as planar polymers in which flanking µ2 L2 "monomeric" subunits are linked by two disulfide bonds, one formed by the penultimate cysteine (C575) in the tailpiece of secretory µ chains (µs tp) and the second by C414 in the Cµ3. The latter bond is not present in membrane IgM. Here, we show that C575 forms a non-native, intra-subunit disulfide bond as a key step in the biogenesis of secretory IgM. The abundance of this unexpected intermediate correlates with the onset and extent of polymerization. The rearrangement of the C-terminal tails into a native quaternary structure is guaranteed by the engagement of protein disulfide isomerase ERp44, which attacks the non-native C575 bonds. The resulting conformational changes promote polymerization and formation of C414 disulfide linkages. This unusual assembly pathway allows secretory polymers to form without the risk of disturbing the role of membrane IgM as part of the B cell antigen receptor.

Profound architectural and functional readjustments of the secretory pathway in decidualization of endometrial stromal cells

Certain cell types must expand their exocytic pathway to guarantee efficiency and fidelity of protein secretion. A spectacular case is offered by decidualizing human endometrial stromal cells (EnSCs). In the midluteal phase of the menstrual cycle, progesterone stimulation induces proliferating EnSCs to differentiate into professional secretors releasing proteins essential for efficient blastocyst implantation. Here, we describe the architectural rearrangements of the secretory pathway of a human EnSC line (TERT-immortalized human endometrial stromal cells (T-HESC)). As in primary cells, decidualization entails proliferation arrest and the coordinated expansion of the entire secretory pathway without detectable activation of unfolded protein response (UPR) pathways. Decidualization proceeds also in the absence of ascorbic acid, an essential cofactor for collagen biogenesis, despite also the secretion of some proteins whose folding does not depend on vitamin C is impaired. However, even in these conditions, no overt UPR induction can be detected. Morphometric analyses reveal that the exocytic pathway does not increase relatively to the volume of the cell. Thus, differently from other cell types, abundant production is guaranteed by a coordinated increase of the cell size following arrest of proliferation.

Regulation and biological function of metal ions in Drosophila

A conceptual framework is offered for critically approaching the formidable ability of insects to segregate metal ions to their multiple destinations in proteins and subcellular compartments. New research in Drosophila melanogaster suggests that nuclear iron regulatory proteins and oxidative stress transcription factors mediate metal-responsive gene expression. Identification of a zinc-regulated chaperone in the endoplasmic reticulum potentially explains membrane protein trafficking defects observed in zinc transporter mutants. Compartmentalized zinc is utilized in fertilization, embryogenesis and for the activation of zinc-finger transcription factors - the latter function demonstrated during muscle development, while dietary zinc is sensed through gating of a chloride channel. Another emerging theme in cellular metal homeostasis is that transporters and related proteins meet at endoplasmic reticulum-mitochondria associated membranes with physiologically relevant consequences during aging.

Post-ER degradation of misfolded GPI-anchored proteins is linked with microautophagy

Glycosylphosphatidylinositol-anchored proteins (GPI-APs) are membrane-conjugated cell-surface proteins with diverse structural, developmental, and signaling functions and clinical relevance. Typically, after biosynthesis and attachment to the preassembled GPI anchor, GPI-APs rapidly leave the endoplasmic reticulum (ER) and rely on post-ER quality control. Terminally misfolded GPI-APs end up inside the vacuole/lysosome for degradation, but their trafficking itinerary to this organelle and the processes linked to their uptake by the vacuole/lysosome remain uncharacterized. In a yeast mutant that is lacking Pep4, a key vacuolar protease, several misfolded model GPI-APs accumulated in the vacuolar membrane. In the same mutant, macroautophagy and the multi-vesicular body (MVB) pathway were intact, hinting at a hitherto-unknown trafficking pathway for the degradation of misfolded GPI-APs. To unravel it, we used a genome-wide screen coupled to high-throughput fluorescence microscopy and followed the fate of the misfolded GPI-AP: Gas1^∗. We found that components of the early secretory and endocytic pathways are involved in its targeting to the vacuole and that vacuolar transporter chaperones (VTCs), with roles in microautophagy, negatively affect the vacuolar uptake of Gas1^∗. In support, we demonstrate that Gas1^∗ internalizes from vacuolar membranes into membrane-bound intravacuolar vesicles prior to degradation. Our data link post-ER degradation with microautophagy.

A virtuous cycle operated by ERp44 and ERGIC-53 guarantees proteostasis in the early secretory compartment

The composition of the secretome depends on the combined action of cargo receptors that facilitate protein transport and sequential checkpoints that restrict it to native conformers. Acting after endoplasmic reticulum (ER)-resident chaperones, ERp44 retrieves its clients from downstream compartments. To guarantee efficient quality control, ERp44 should exit the ER as rapidly as its clients, or more. Here, we show that appending ERp44 to different cargo proteins increases their secretion rates. ERp44 binds the cargo receptor ER-Golgi intermediate compartment (ERGIC)-53 in the ER to negotiate preferential loading into COPII vesicles. Silencing ERGIC-53, or competing for its COPII binding with 4-phenylbutyrate, causes secretion of Prdx4, an enzyme that relies on ERp44 for intracellular localization. In more acidic, zinc-rich downstream compartments, ERGIC-53 releases its clients and ERp44, which can bind and retrieve non-native conformers via KDEL receptors. By coupling the transport of cargoes and inspector proteins, cells ensure efficiency and fidelity of secretion.

In situ detection of protein interactions for recombinant therapeutic enzymes

Despite their therapeutic potential, many protein drugs remain inaccessible to patients since they are difficult to secrete. Each recombinant protein has unique physicochemical properties and requires different machinery for proper folding, assembly, and posttranslational modifications (PTMs). Here we aimed to identify the machinery supporting recombinant protein secretion by measuring the protein-protein interaction (PPI) networks of four different recombinant proteins (SERPINA1, SERPINC1, SERPING1, and SeAP) with various PTMs and structural motifs using the proximity-dependent biotin identification (BioID) method. We identified PPIs associated with specific features of the secreted proteins using a Bayesian statistical model and found proteins involved in protein folding, disulfide bond formation, and N-glycosylation were positively correlated with the corresponding features of the four model proteins. Among others, oxidative folding enzymes showed the strongest association with disulfide bond formation, supporting their critical roles in proper folding and maintaining the ER stability. Knockdown of disulfide-isomerase PDIA4, a measured interactor with significance for SERPINC1 but not SERPINA1, led to the decreased secretion of SERPINC1, which relies on its extensive disulfide bonds, compared to SERPINA1, which has no disulfide bonds. Proximity-dependent labeling successfully identified the transient interactions supporting synthesis of secreted recombinant proteins and refined our understanding of key molecular mechanisms of the secretory pathway during recombinant protein production.

Zinc transporters and their functional integration in mammalian cells

Zinc is a ubiquitous biological metal in all living organisms. The spatiotemporal zinc dynamics in cells provide crucial cellular signaling opportunities, but also challenges for intracellular zinc homeostasis with broad disease implications. Zinc transporters play a central role in regulating cellular zinc balance and subcellular zinc distributions. The discoveries of two complementary families of mammalian zinc transporters (ZnTs and ZIPs) in the mid-1990s spurred much speculation on their metal selectivity and cellular functions. After two decades of research, we have arrived at a biochemical description of zinc transport. However, in vitro functions are fundamentally different from those in living cells, where mammalian zinc transporters are directed to specific subcellular locations, engaged in dedicated macromolecular machineries, and connected with diverse cellular processes. Hence, the molecular functions of individual zinc transporters are reshaped and deeply integrated in cells to promote the utilization of zinc chemistry to perform enzymatic reactions, tune cellular responsiveness to pathophysiologic signals, and safeguard cellular homeostasis. At present, the underlying mechanisms driving the functional integration of mammalian zinc transporters are largely unknown. This knowledge gap has motivated a shift of the research focus from in vitro studies of purified zinc transporters to in cell studies of mammalian zinc transporters in the context of their subcellular locations and protein interactions. In this review, we will outline how knowledge of zinc transporters has been accumulated from in-test-tube to in-cell studies, highlighting new insights and paradigm shifts in our understanding of the molecular and cellular basis of mammalian zinc transporter functions.

The pivotal role of ERp44 in patrolling protein secretion

Interactions between protein ligands and receptors are the main language of intercellular communication; hence, how cells select proteins to be secreted or presented on the plasma membrane is a central concern in cell biology. A series of checkpoints are located along the secretory pathway, which ensure the fidelity of such protein signals (quality control). Proteins that pass the checkpoints operated in the endoplasmic reticulum (ER) by the binding immunoglobulin protein (BiP; also known as HSPA5 and GRP78) and the calnexin-calreticulin systems, must still overcome additional scrutiny in the ER-Golgi intermediate compartment (ERGIC) and the Golgi. One of the main players of this process in all metazoans is the ER-resident protein 44 (ERp44); by cycling between the ER and the Golgi, ERp44 controls the localization of key enzymes designed to act in the ER but that are devoid of suitable localization motifs. ERp44 also patrols the secretion of correctly assembled disulfide-linked oligomeric proteins. Here, we discuss the mechanisms driving ERp44 substrate recognition, with important consequences on the definition of 'thiol-mediated quality control'. We also describe how pH and zinc gradients regulate the functional cycle of ERp44, coupling quality control and membrane trafficking along the early secretory compartment.

Quantitative Imaging of Labile Zn2+ in the Golgi Apparatus Using a Localizable Small-Molecule Fluorescent Probe

Fluorescent Zn²⁺ probes used for the quantitative analysis of labile Zn²⁺ concentration ([Zn²⁺]) in target organelles are crucial for understanding the role of Zn²⁺ in biological processes. Although several fluorescent Zn²⁺ probes have been developed to date, there is still a lack of consensus concerning the [Zn²⁺] in intracellular organelles. In this study, we describe the development of ZnDA-1H, a small-molecule fluorescent probe for Zn²⁺, which exhibits less pH sensitivity, high Zn²⁺ selectivity, and large fluorescence enhancement upon binding to Zn²⁺. Through protein labeling technology, ZnDA-1H was precisely targeted in various intracellular organelles, such as the nucleus, mitochondria, endoplasmic reticulum, and Golgi apparatus. ZnDA-1H exhibited a reversible fluorescence response toward labile Zn²⁺ in these organelles in live cells. Using this probe, the [Zn²⁺] in the Golgi apparatus was estimated to be 25 ± 1 nM, suggesting that labile Zn²⁺ plays a physiological role in the secretory pathway.

Oxidoreductases in Glycoprotein Glycosylation, Folding, and ERAD

N-linked glycosylation and sugar chain processing, as well as disulfide bond formation, are among the most common post-translational protein modifications taking place in the endoplasmic reticulum (ER). They are essential modifications that are required for membrane and secretory proteins to achieve their correct folding and native structure. Several oxidoreductases responsible for disulfide bond formation, isomerization, and reduction have been shown to form stable, functional complexes with enzymes and chaperones that are involved in the initial addition of an N-glycan and in folding and quality control of the glycoproteins. Some of these oxidoreductases are selenoproteins. Recent studies also implicate glycan machinery–oxidoreductase complexes in the recognition and processing of misfolded glycoproteins and their reduction and targeting to ER-associated degradation. This review focuses on the intriguing cooperation between the glycoprotein-specific cell machineries and ER oxidoreductases, and highlights open questions regarding the functions of many members of this large family.

Galvanization of Protein-Protein Interactions in a Dynamic Zinc Interactome

The presence of Zn²⁺ at protein-protein interfaces modulates complex function, stability, and introduces structural flexibility/complexity, chemical selectivity, and reversibility driven in a Zn²⁺-dependent manner. Recent studies have demonstrated that dynamically changing Zn²⁺ affects numerous cellular processes, including protein-protein communication and protein complex assembly. How Zn²⁺-involved protein-protein interactions (ZPPIs) are formed and dissociate and how their stability and reactivity are driven in a zinc interactome remain poorly understood, mostly due to experimental obstacles. Here, we review recent research advances on the role of Zn²⁺ in the formation of interprotein sites, their architecture, function, and stability. Moreover, we underline the importance of zinc networks in intersystemic communication and highlight bioinformatic and experimental challenges required for the identification and investigation of ZPPIs.

Role of the early secretory pathway in SARS-CoV-2 infection

Similar to other RNA viruses, SARS-CoV-2 must (1) enter a target/host cell, (2) reprogram it to ensure its replication, (3) exit the host cell, and (4) repeat this cycle for exponential growth. During the exit step, the virus hijacks the sophisticated machineries that host cells employ to correctly fold, assemble, and transport proteins along the exocytic pathway. Therefore, secretory pathway-mediated assemblage and excretion of infective particles represent appealing targets to reduce the efficacy of virus biogenesis, if not to block it completely. Here, we analyze and discuss the contribution of the molecular machines operating in the early secretory pathway in the biogenesis of SARS-CoV-2 and their relevance for potential antiviral targeting. The fact that these molecular machines are conserved throughout evolution, together with the redundancy and tissue specificity of their components, provides opportunities in the search for unique proteins essential for SARS-CoV-2 biology that could also be targeted with therapeutic objectives. Finally, we provide an overview of recent evidence implicating proteins of the early secretory pathway as potential antiviral targets with effective therapeutic applications.

Peri/Epicellular Thiol Oxidoreductases as Mediators of Extracellular Redox Signaling

Significance: Supracellular redox networks regulating cell-extracellular matrix (ECM) and organ system architecture merge with structural and functional (catalytic or allosteric) properties of disulfide bonds. This review addresses emerging evidence that exported thiol oxidoreductases (TORs), such as thioredoxin, protein disulfide isomerases (PDIs), quiescin sulfhydryl oxidases (QSOX)1, and peroxiredoxins, composing a peri/epicellular (pec)TOR pool, mediate relevant signaling. pecTOR functions depend mainly on kinetic and spatial regulation of thiol-disulfide exchange reactions governed by redox potentials, which are modulated by exported intracellular low-molecular-weight thiols, together conferring signal specificity. Recent Advances: pecTOR redox-modulates several targets including integrins, ECM proteins, surface molecules, and plasma components, although clear-cut documentation of direct effects is lacking in many cases. TOR catalytic pathways, displaying common patterns, culminate in substrate thiol reduction, oxidation, or isomerization. Peroxiredoxins act as redox/peroxide sensors, contrary to PDIs, which are likely substrate-targeted redox modulators. Emerging evidence suggests important pecTOR roles in patho(physio)logical processes, including blood coagulation, vascular remodeling, mechanosensing, endothelial function, immune responses, and inflammation. Critical Issues: Effects of pecPDIs supporting thrombosis/platelet activation have been well documented and reached the clinical arena. Roles of pecPDIA1 in vascular remodeling/mechanosensing are also emerging. Extracellular thioredoxin and pecPDIs redox-regulate immunoinflammation. Routes of TOR externalization remain elusive and appear to involve Golgi-independent routes. pecTORs are particularly accessible drug targets. Future Directions: Further understanding mechanisms of thiol redox reactions and developing assays for assessing pecTOR redox activities remain important research avenues. Also, addressing pecTORs as disease markers and achieving more efficient/specific drugs for pecTOR modulation are major perspectives for diagnostic/therapeutic improvements.

Detailed analyses of the crucial functions of Zn transporter proteins in alkaline phosphatase activation

Numerous zinc ectoenzymes are metalated by zinc and activated in the compartments of the early secretory pathway before reaching their destination. Zn transporter (ZNT) proteins located in these compartments are essential for ectoenzyme activation. We have previously reported that ZNT proteins, specifically ZNT5-ZNT6 heterodimers and ZNT7 homodimers, play critical roles in the activation of zinc ectoenzymes, such as alkaline phosphatases (ALPs), by mobilizing cytosolic zinc into these compartments. However, this process remains incompletely understood. Here, using genetically-engineered chicken DT40 cells, we first determined that Zrt/Irt-like protein (ZIP) transporters that are localized to the compartments of the early secretory pathway play only a minor role in the ALP activation process. These transporters included ZIP7, ZIP9, and ZIP13, performing pivotal functions in maintaining cellular homeostasis by effluxing zinc out of the compartments. Next, using purified ALP proteins, we showed that zinc metalation on ALP produced in DT40 cells lacking ZNT5-ZNT6 heterodimers and ZNT7 homodimers is impaired. Finally, by genetically disrupting both ZNT5 and ZNT7 in human HAP1 cells, we directly demonstrated that the tissue-nonspecific ALP-activating functions of both ZNT complexes are conserved in human cells. Furthermore, using mutant HAP1 cells, we uncovered a previously-unrecognized and unique spatial regulation of ZNT5-ZNT6 heterodimer formation, wherein ZNT5 recruits ZNT6 to the Golgi apparatus to form the heterodimeric complex. These findings fill in major gaps in our understanding of the molecular mechanisms underlying zinc ectoenzyme activation in the compartments of the early secretory pathway.

Paradoxical activation of the protein kinase-transcription factor ERK5 by ERK5 kinase inhibitors

The dual protein kinase-transcription factor, ERK5, is an emerging drug target in cancer and inflammation, and small-molecule ERK5 kinase inhibitors have been developed. However, selective ERK5 kinase inhibitors fail to recapitulate ERK5 genetic ablation phenotypes, suggesting kinase-independent functions for ERK5. Here we show that ERK5 kinase inhibitors cause paradoxical activation of ERK5 transcriptional activity mediated through its unique C-terminal transcriptional activation domain (TAD). Using the ERK5 kinase inhibitor, Compound 26 (ERK5-IN-1), as a paradigm, we have developed kinase-active, drug-resistant mutants of ERK5. With these mutants, we show that induction of ERK5 transcriptional activity requires direct binding of the inhibitor to the kinase domain. This in turn promotes conformational changes in the kinase domain that result in nuclear translocation of ERK5 and stimulation of gene transcription. This shows that both the ERK5 kinase and TAD must be considered when assessing the role of ERK5 and the effectiveness of anti-ERK5 therapeutics.

Zinc homeostasis in the secretory pathway in yeast

It is estimated that up to 10% of proteins in eukaryotes require zinc for their function. Although the majority of these proteins are located in the nucleus and cytosol, a small subset is secreted from cells or is located within an intracellular compartment. As many of these compartmentalized metalloproteins fold to their native state and bind their zinc cofactor inside an organelle, cells require mechanisms to maintain supply of zinc to these compartments even under conditions of zinc deficiency. At the same time, intracellular compartments can also be the site for storing zinc ions, which then can be mobilized when needed. In this review, we highlight insight that has been obtained from yeast models about how zinc homeostasis is maintained in the secretory pathway and vacuole.

Metalation and Maturation of Zinc Ectoenzymes: A Perspective

Numerous zinc ectoenzymes are folded and activated in the compartments of the early secretory pathway, such as the ER and the Golgi apparatus, before reaching their final destination. During this process, zinc must be incorporated into the active site; therefore, metalation of the nascent protein is indispensable for the expression of the active enzyme. However, to date, the molecular mechanism underlying this process has been poorly investigated. This is in sharp contrast to the physiological and pathophysiological roles of zinc ectoenzymes, which have been extensively investigated over the past decades. This manuscript concisely outlines the present understanding of zinc ectoenzyme activation through metalation by zinc and compares this with copper ectoenzyme activation, in which elaborate copper metalation mechanisms are known. Moreover, based on the comparison, several hypotheses are discussed. Approximately 80 years have passed since the first zinc enzyme was identified; therefore, it is necessary to improve our understanding of zinc ectoenzymes from a biochemical perspective, which will further our understanding of their biological roles.

The molecular basis of chaperone-mediated interleukin 23 assembly control

The functionality of most secreted proteins depends on their assembly into a defined quaternary structure. Despite this, it remains unclear how cells discriminate unassembled proteins en route to the native state from misfolded ones that need to be degraded. Here we show how chaperones can regulate and control assembly of heterodimeric proteins, using interleukin 23 (IL-23) as a model. We find that the IL-23 α-subunit remains partially unstructured until assembly with its β-subunit occurs and identify a major site of incomplete folding. Incomplete folding is recognized by different chaperones along the secretory pathway, realizing reliable assembly control by sequential checkpoints. Structural optimization of the chaperone recognition site allows it to bypass quality control checkpoints and provides a secretion-competent IL-23α subunit, which can still form functional heterodimeric IL-23. Thus, locally-restricted incomplete folding within single-domain proteins can be used to regulate and control their assembly.

It is unclear how unassembled secretory pathway proteins are discriminated from misfolded ones. Here the authors combine biophysical and cellular experiments to study the folding of heterodimeric interleukin 23 and describe how ER chaperones recognize unassembled proteins and aid their assembly into protein complexes while preventing the premature degradation of unassembled units.

The role of ERp44 in glucose and lipid metabolism

Endoplasmic Reticulum Protein 44 (ERp44) is a member of the PDI family, named for a molecular weight of 44 kD. White adipose tissue has metabolic and endocrine functions that are important to metabolism. The role of ERp44 in glucose and lipid metabolism is not known yet. The current study was undertaken to investigate the implication of ERp44 in glucose and lipid metabolism. In this study, we generated and characterized ERp44^-/- mice. We used type 2 diabetes models and ERp44 knockout mice to show the implication of ERp44 in glucose and lipid metabolism. Knockout newborns had lower blood glucose compared to wild-type. Adult knockouts had abnormal intraperitoneal, glucose, insulin and pyruvic acid tolerance. Lipocytes were smaller and fewer in knockout mice compared to wild-type. Knockouts resisted to high-fat diet-induced obesity. ERp44 expression in white adipose tissue decreased significantly in type 2 diabetes models. Results suggest that ERp44 is closely associated with glucose and lipid metabolism.

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.