The multifaceted proprotein convertases: their unique, redundant, complementary, and opposite functions

Pro-fibrotic effect of the susceptible gene PCSK5 in vascular fibrosis of Takayasu arteritis via TGF-β and SMAD3 signaling pathway activation

BACKGROUND

Vascular fibrosis directly causes vascular thickening in Takayasu arteritis (TAK), in which sustained transforming growth factor beta (TGF-β) activation is critical. Understanding TGF-β activation regulation and blocking it might yield a therapeutic effect in TAK. Proprotein convertase subtilisin/kexin type 5 (PCSK5) rs6560480 (T/C) is associated with TAK development. In this study, we assessed the association between the PCSK5 rs6560480 genotype and PCSK5 expression in TAK and explored its molecular role in TGF-β activation and vascular fibrosis development.

METHODS

In TAK patients, PCSK5 and TGF-β expression in plasma and aortic tissue was examined by ELISA and immunohistochemical staining, and PCSK5 rs6560480 was genotyped. The correlation between PCSK5 and extracellular matrix (ECM) expression was examined by Western blotting (WB) and immunohistochemistry staining. Detection by co-immunoprecipitation was performed to detect the interaction between PCSK5 and TGF-β in adventitial fibroblasts (AAFs). Downstream signaling pathways were detected by WB and validated with appropriate inhibitors. Potential immunosuppressive agents to inhibit the effects of PCSK5 were explored in cell culture and TAK patients.

RESULTS

Patients with PCSK5 rs6560480 TT patients had significantly higher PCSK5 levels and more thickened vascular lesions than patients with PCSK5 rs6560480 CT. PCSK5 expression was significantly increased in alpha smooth muscle actin (α-SMA)-positive myofibroblasts in TAK vascular lesions. Overexpressing PCSK5 facilitated TGF-β and downstream SMAD2/3 activation and ECM expression in AAFs and aorta in in-vitro culture. The mechanistic study supported that PCSK5 activated precursor TGF-β (pro-TGF-β) to the mature form by binding the pro-TGF-β cleavage site. Leflunomide inhibited PCSK5 and pro-TGF-β binding, decreasing TGF-β activation and ECM expression, which was also partially validated in leflunomide-treated patients.

CONCLUSION

The findings revealed a novel pro-fibrotic mechanism of PCSK5 in TAK vascular fibrosis via TGF-β and downstream SMAD2/3 pathway activation. Leflunomide might be anti-fibrotic by disrupting PCSK5 and pro-TGF-β binding, presenting a new TAK treatment approach.

Development, validation and analysis of a human profurin 3D model using comparative modeling and molecular dynamics simulations

The emergence of new viruses can lead to the outbreak of pandemics as occurred at the end of 2019 with the coronavirus disease (or COVID-19). The fastest way to effectively control viral infections is to develop broad-spectrum antivirals that can fight at least an entire class of viruses. Profurin, the furin precursor propeptide, is responsible for the autoactivation step which is crucial for the maturation of several viral substrates. This role makes the study of furin and profurin interactions interesting for the development of new potential broad-spectrum antivirals for the treatment against several human viral diseases. Since there is no 3D model of profurin published in the literature or deposited in a database, this work reports the development, validation and analysis of a profurin 3D model using comparative modeling and molecular dynamics. The model is available in ModelArchive at https://www.modelarchive.org/doi/10.5452/ma-ct8l7. The usage of this model will make possible further studies of molecular docking and MD simulations of the profurin-furin system, in the design of new potential broad-spectrum antivirals for the treatment against several human viral diseases.Communicated by Ramaswamy H. Sarma.

Epithelial Na + Channels Function as Extracellular Sensors

The epithelial Na + channel (ENaC) resides on the apical surfaces of specific epithelia in vertebrates and plays a critical role in extracellular fluid homeostasis. Evidence that ENaC senses the external environment emerged well before the molecular identity of the channel was reported three decades ago. This article discusses progress toward elucidating the mechanisms through which specific external factors regulate ENaC function, highlighting insights gained from structural studies of ENaC and related family members. It also reviews our understanding of the role of ENaC regulation by the extracellular environment in physiology and disease. After familiarizing the reader with the channel's physiological roles and structure, we describe the central role protein allostery plays in ENaC's sensitivity to the external environment. We then discuss each of the extracellular factors that directly regulate the channel: proteases, cations and anions, shear stress, and other regulators specific to particular extracellular compartments. For each regulator, we discuss the initial observations that led to discovery, studies investigating molecular mechanism, and the physiological and pathophysiological implications of regulation. © 2024 American Physiological Society. Compr Physiol 14:5407-5447, 2024.

Prodomain-driven enzyme dimerization: a pH-dependent autoinhibition mechanism that controls Plasmodium Sub1 activity before merozoite egress

Malaria symptoms are associated with the asexual multiplication of Plasmodium falciparum within human red blood cells (RBCs) and fever peaks coincide with the egress of daughter merozoites following the rupture of the parasitophorous vacuole (PV) and the RBC membranes. Over the last two decades, it has emerged that the release of competent merozoites is tightly regulated by a complex cascade of events, including the unusual multi-step activation mechanism of the pivotal subtilisin-like protease 1 (Sub1) that takes place in three different cellular compartments and remains poorly understood. Following an initial auto-maturation in the endoplasmic reticulum (ER) between its pro- and catalytic domains, the Sub1 prodomain (PD) undergoes further cleavages by the parasite aspartic protease plasmepsin X (PmX) within acidic secretory organelles that ultimately lead to full Sub1 activation upon discharge into the PV. Here, we report the crystal structure of full-length P. falciparum Sub1 (PfS1FL) and demonstrate, through structural, biochemical, and biophysical studies, that the atypical Plasmodium-specific Sub1 PD directly promotes the assembly of inactive enzyme homodimers at acidic pH, whereas Sub1 is primarily monomeric at neutral pH. Our results shed new light into the finely tuned Sub1 spatiotemporal activation during secretion, explaining how PmX processing and full activation of Sub1 can occur in different cellular compartments, and uncover a robust mechanism of pH-dependent subtilisin autoinhibition that plays a key role in P. falciparum merozoites egress from infected host cells.IMPORTANCEMalaria fever spikes are due to the rupture of infected erythrocytes, allowing the egress of Plasmodium sp. merozoites and further parasite propagation. This fleeting tightly regulated event involves a cascade of enzymes, culminating with the complex activation of the subtilisin-like protease 1, Sub1. Differently than other subtilisins, Sub1 activation strictly depends upon the processing by a parasite aspartic protease within acidic merozoite secretory organelles. However, Sub1 biological activity is required in the pH neutral parasitophorous vacuole, to prime effectors involved in the rupture of the vacuole and erythrocytic membranes. Here, we show that the unusual, parasite-specific Sub1 prodomain is directly responsible for its acidic-dependent dimerization and autoinhibition, required for protein secretion, before its full activation at neutral pH in a monomeric form. pH-dependent Sub1 dimerization defines a novel, essential regulatory element involved in the finely tuned spatiotemporal activation of the egress of competent Plasmodium merozoites.

A bibliometric analysis of PCSK9 inhibitors from 2007 to 2022

Background

Since the approval of the proprotein convertase subtilisin/kexin type 9 (PCSK9) monoclonal antibodies for marketing in 2015, PCSK9 inhibitors have attracted significant interest in the field of cardiovascular endocrinology. A large number of clinical trials have confirmed the efficacy and safety of PCSK9 inhibitors in reducing cholesterol and the risk of cardiovascular events. No bibliometric analysis of PCSK9 inhibitors has been performed as of yet. This study aims to analyze the research trends and hotspots of PCSK9 inhibitors through bibliometric analysis.

Methods

We searched the Web of Science Core Collection (WoSCC) database for PCSK9 inhibitor-related publications from 2007 to 2022. Data visualization analysis was performed using CiteSpace software. Microsoft Excel and Graphpad software were used for the drawing of some tables and figures.

Results

A total of 1072 pieces of literature were retrieved between 2007 and 2022. The number of publications concerning PCSK9 inhibitors is growing annually. The top five countries with the most articles published were the United States, England, Canada, Italy, and France. Harvard University, Amgen, Brigham & Women's Hospital, Harvard Medical School, and Imperial College London are the five institutions with the highest output. The Journal of Clinical Lipidology is the most popular journal in this field. The most frequently cited journal is the New England Journal of Medicine. As for authors, Sabatine MS and Giugliano RP from Brigham & Women's Hospital have the highest number of published articles. Amgen is the funding agency for most of the research. According to keyword analysis, "low density lipoprotein", "familial hypercholesterolemia", "PCSK9 inhibitor", "PCSK9", and "efficacy" are the five keywords with the highest frequency of co-occurrence.

Conclusion

The past 15 years have witnessed a rapid and fruitful development of PCSK9 inhibitors. The research trend and focus for PCSK9 inhibitors are from the mechanism of reducing low-density lipoprotein cholesterol to related clinical trials. Developed countries such as the United States have contributed prominently in this area. Coronary artery and inflammation are currently at the forefront of research in the field and are in an explosion period.

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Enhancing the Drug-Like Profile of a Potent Peptide PACE4 Inhibitor by the Formation of a Host-Guest Inclusion Complex with β-Cyclodextrin

The enzyme PACE4 has been validated as a promising therapeutic target to expand the range of prostate cancer (PCa) treatments. In recent years, we have developed a potent peptidomimetic inhibitor, namely, compound C23 (Ac-(DLeu)LLLRVK-4-amidinobenzylamide). Like many peptides, C23 suffers from an unfavorable drug-like profile which, despite our efforts, has not yet benefited from the usual SAR studies. Hence, we turned our attention toward a novel formulation strategy, i.e., the use of cyclodextrins (CDs). CDs can benefit compounds through the formation of "host-guest" complexes, shielding the guest from degradation and enhancing biological survival. In this study, a series of βCD-C23 complexes have been generated and their properties evaluated, including potency toward the enzyme in vitro, a cell-based proliferation assay, and stability in plasma. As a result, a new βCD-formulated lead compound has been identified, which, in addition to being more soluble and more potent, also showed an improved stability profile.

Cleavage of the V-ATPase associated prorenin receptor is mediated by PACE4 and is essential for growth of prostate cancer cells

Phosphatase and tensin homolog (PTEN) mutation is common in prostate cancer during progression to metastatic and castration resistant forms. We previously reported that loss of PTEN function in prostate cancer leads to increased expression and secretion of the Prorenin Receptor (PRR) and its soluble processed form, the soluble Prorenin Receptor (sPRR). PRR is an essential factor required for proper assembly and activity of the vacuolar-ATPase (V-ATPase). The V-ATPase is a rotary proton pump required for the acidification of intracellular vesicles including endosomes and lysosomes. Acidic vesicles are involved in a wide range of cancer related pathways such as receptor mediated endocytosis, autophagy, and cell signalling. Full-length PRR is cleaved at a conserved consensus motif (R-X-X-R↓) by a member of the proprotein convertase family to generate sPRR, and a smaller C-terminal fragment, designated M8.9. It is unclear which convertase processes PRR in prostate cancer cells and how processing affects V-ATPase activity. In the current study we show that PRR is predominantly cleaved by PACE4, a proprotein convertase that has been previously implicated in prostate cancer. We further demonstrate that PTEN controls PRR processing in mouse tissue and controls PACE4 expression in prostate cancer cells. Furthermore, we demonstrate that PACE4 cleavage of PRR is needed for efficient V-ATPase activity and prostate cancer cell growth. Overall, our data highlight the importance of PACE4-mediated PRR processing in normal physiology and prostate cancer tumorigenesis.

ILC2s control obesity by regulating energy homeostasis and browning of white fat

Innate lymphoid cells (ILCs) have been a hot topic in recent research, they are widely distributed in vivo and play an important role in different tissues. The important role of group 2 innate lymphoid cells (ILC2s) in the conversion of white fat into beige fat has attracted widespread attention. Studies have shown that ILC2s regulate adipocyte differentiation and lipid metabolism. This article reviews the types and functions of ILCs, focusing on the relationship between differentiation, development and function of ILC2s, and elaborates on the relationship between peripheral ILC2s and browning of white fat and body energy homeostasis. This has important implications for the future treatment of obesity and related metabolic diseases.

The Subtilisin-Like Protease Furin Regulates Hemin-Dependent Ectodomain Shedding of Glycoprotein VI

INTRODUCTION

 Hemolysis results in release of free hemoglobin and hemin liberation from erythrocytes. Hemin has been described to induce platelet activation and to trigger thrombosis.

METHODS

 We evaluated the effect of hemin on platelet function and surface expression of the platelet collagen receptor glycoprotein VI (GPVI). Isolated platelets were stimulated with increasing concentrations of hemin.

RESULTS

 We found that hemin strongly enhanced platelet activation, aggregation, and aggregate formation on immobilized collagen under flow. In contrast, we found that surface expression of GPVI was significantly reduced upon hemin stimulation with high hemin concentrations indicating that hemin-induced loss of surface GPVI does not hinder platelet aggregation. Loss of hemin-induced surface expression of GPVI was caused by shedding of the ectodomain of GPVI as verified by immunoblotting and is independent of the GPVI or CLEC-2 mediated ITAM (immunoreceptor-tyrosine-based-activation-motif) signaling pathway as inhibitor studies revealed. Hemin-induced GPVI shedding was independent of metalloproteinases such as ADAM10 or ADAM17, which were previously described to regulate GPVI degradation. Similarly, concentration-dependent shedding of CD62P was also induced by hemin. Unexpectedly, we found that the subtilisin-like proprotein convertase furin controls hemin-dependent GPVI shedding as shown by inhibitor studies using the specific furin inhibitors SSM3 and Hexa-D-arginine. In the presence of SSM3 and Hexa-D-arginine, hemin-associated GPVI degradation was substantially reduced. Further, SSM3 inhibited hemin-induced but not CRP-XL-induced platelet aggregation and thrombus formation, indicating that furin controls specifically hemin-associated platelet functions.

CONCLUSION

 In summary, we describe a novel mechanism of hemin-dependent GPVI shedding and platelet function mediated by furin.

The impact of the PCSK-9/VLDL-Receptor axis on inflammatory cell polarization

BACKGROUND

Studies have shown that lipoproteins, such as LDL and VLDL, as well as its major protein component ApoE2 impact on macrophage polarization important in atherosclerosis. Proprotein convertase subtilisin/kexin 9 (PCSK9) is a key regulator of lipoprotein receptor expression. The present study investigated the effect of the VLDL/VLDL-receptor (VLDL-R) axis on mononuclear cell polarization, as well as the role of PCSK9 and PCSK9 inhibitors (PCSK9i) within this network.

METHODS

Human monocytic THP-1 cells and human monocyte-derived macrophages isolated from peripheral blood mononuclear cells (PBMC) were treated with either LPS/IFN-γ to induce a pro-inflammatory phenotype, or with IL-4/IL-13 to induce an anti-inflammatory phenotype. Cells were then subjected to further treatments by lipoproteins, PCSK9, PCSK9i and lipoprotein receptor blockers.

RESULTS

LPS/IFN-γ treatment promoted a pro-inflammatory state with an increased expression of pro-inflammatory mediators such as TNF-α, CD80 and IL-1β. VLDL co-treatment induced a switch of this pro-inflammatory phenotype to an anti-inflammatory phenotype. In pro-inflammatory cells, VLDL significantly decreased the expression of pro-inflammatory markers e.g., TNF-α, CD80, and IL-1β. These effects were eliminated by PCSK9 and restored by co-incubation with a specific anti-PCSK9 monoclonal antibody (PCSK9i). Migration assays demonstrated that pro-inflammatory cells displayed a significantly higher invasive capacity when compared to untreated cells or anti-inflammatory cells. Moreover, pro-inflammatory cell chemotaxis was significantly decreased by VLDL-mediated acquisition of the anti-inflammatory phenotype. PCSK9 significantly lessened this VLDL-mediated migration inhibition, which was reversed by the PCSK9i.

CONCLUSION

VLDL promotes mononuclear cell differentiation towards an anti-inflammatory phenotype. PCSK9, via its capacity to inhibit VLDL-R expression, reverses the VLDL-mediated anti-inflammatory action, thereby promoting a pro-inflammatory phenotype. Thus, PCSK9 targeting therapies may exert anti-inflammatory properties within the vessel wall.

What Are the Roles of Proprotein Convertases in the Immune Escape of Tumors?

Protein convertases (PCs) play a significant role in post-translational procedures by transforming inactive precursor proteins into their active forms. The role of PCs is crucial for cellular homeostasis because they are involved in cell signaling. They have also been described in many diseases such as Alzheimer's and cancer. Cancer cells are secretory cells that send signals to the tumor microenvironment (TME), remodeling the surrounding space for their own benefits. One of the most important components of the TME is the immune system of the tumor. In this review, we describe recent discoveries that link PCs to the immune escape of tumors. Among PCs, many findings have determined the role of Furin (PC3) as a paramount enzyme causing the TME to induce tumor immune evasion. The overexpression of various cytokines and proteins, for instance, IL10 and TGF-B, moves the TME towards the presence of Tregs and, consequently, immune tolerance. Furthermore, Furin is implicated in the regulation of macrophage activity that contributes to the increased impairment of DCs (dendritic cells) and T effector cells. Moreover, Furin interferes in the MHC Class_1 proteolytic cleavage in the trans-Golgi network. In tumors, the T cytotoxic lymphocytes (CTLs) response is impeded by the PD1 receptor (PD1-R) located on CTLs and its ligand, PDL1, located on cancer cells. The inhibition of Furin is a subtle means of enhancing the antitumor response by repressing PD-1 expression in tumors or macrophage cells. The impacts of other PCs in tumor immune escape have not yet been clarified to the extent that Furin has. Accordingly, the influence of other types of PCs in tumor immune escape is a promising topic for further consideration.

Evolocumab, a PCSK9 inhibitor, protects human endothelial cells against H2O2-induced oxidative stress

CONTEXT

Recent surveys have shown an association between proprotein convertase subtilisin/kexin type 9 (PCSK9) and oxidative stress.

OBJECTIVE

In this investigation, the effect of evolocumab an anti-PCSK9 antibody was assessed against oxidative damage caused by hydrogen peroxide (H₂O₂) in human umbilical vein endothelial cells (HUVEC).

MATERIAL AND METHODS

Viability of HUVEC was measured by MTT assay. Hydroperoxides and malondialdehyde (MDA) levels, and ferric reducing antioxidant power (FRAP) were detected in HUVEC that pre-treated with evolocumab and, then exposed to H₂O₂.

RESULTS

Evolocumab significantly prevented the cytotoxicity induced by H₂O₂ at the concentrations of 5-100 µg/ml. Pre-treatment of HUVEC with evolocumab reduced hydroperoxides and MDA levels and also increased FRAP value in intra- and extra-cellular mediums compared with H₂O₂ stimulated cells at different concentration ranges.

CONCLUSION

This study displayed anti-oxidative and cytoprotective activities of evolocumab against oxidative damage caused by H₂O₂ in endothelial cells.

Pathogenic gain-of-function mutations in the prodomain and C-terminal domain of PCSK9 inhibit LDL binding

Proprotein convertase subtilisin/kexin type-9 (PCSK9) is a secreted protein that binds and mediates endo-lysosomal degradation of low-density lipoprotein receptor (LDLR), limiting plasma clearance of cholesterol-rich LDL particles in liver. Gain-of-function (GOF) point mutations in PCSK9 are associated with familial hypercholesterolemia (FH). Approximately 30%–40% of PCSK9 in normolipidemic human plasma is bound to LDL particles. We previously reported that an R496W GOF mutation in a region of PCSK9 known as cysteine-histidine–rich domain module 1 (CM1) prevents LDL binding in vitro [Sarkar et al., J. Biol. Chem. 295 (8), 2285–2298 (2020)]. Herein, we identify additional GOF mutations that inhibit LDL association, localized either within CM1 or a surface-exposed region in the PCSK9 prodomain. Notably, LDL binding was nearly abolished by a prodomain S127R GOF mutation, one of the first PCSK9 mutations identified in FH patients. PCSK9 containing alanine or proline substitutions at amino acid position 127 were also defective for LDL binding. LDL inhibited cell surface LDLR binding and degradation induced by exogenous PCSK9-D374Y but had no effect on an S127R-D374Y double mutant form of PCSK9. These studies reveal that multiple FH-associated GOF mutations in two distinct regions of PCSK9 inhibit LDL binding, and that the Ser-127 residue in PCSK9 plays a critical role.

Proprotein Convertases and the Complement System

Proteins destined for secretion - after removal of the signal sequence - often undergo further proteolytic processing by proprotein convertases (PCs). Prohormones are typically processed in the regulated secretory pathway, while most plasma proteins travel though the constitutive pathway. The complement system is a major proteolytic cascade in the blood, serving as a first line of defense against microbes and also contributing to the immune homeostasis. Several complement components, namely C3, C4, C5 and factor I (FI), are multi-chain proteins that are apparently processed by PCs intracellularly. Cleavage occurs at consecutive basic residues and probably also involves the action of carboxypeptidases. The most likely candidate for the intracellular processing of complement proteins is furin, however, because of the overlapping specificities of basic amino acid residue-specific proprotein convertases, other PCs might be involved. To our surprise, we have recently discovered that processing of another complement protein, mannan-binding lectin-associated serine protease-3 (MASP-3) occurs in the blood by PCSK6 (PACE4). A similar mechanism had been described for the membrane protease corin, which is also activated extracellularly by PCSK6. In this review we intend to point out that the proper functioning of the complement system intimately depends on the action of proprotein convertases. In addition to the non-enzymatic components (C3, C4, C5), two constitutively active complement proteases are directly activated by PCs either intracellularly (FI), or extracellularly (MASP-3), moreover indirectly, through the constitutive activation of pro-factor D by MASP-3, the activity of the alternative pathway also depends on a PC present in the blood.

The Role of Proprotein Convertases in Upper Airway Remodeling

Chronic rhinosinusitis (CRS) is a multifactorial, heterogeneous disease characterized by persistent inflammation of the sinonasal mucosa and tissue remodeling, which can include basal/progenitor cell hyperplasia, goblet cell hyperplasia, squamous cell metaplasia, loss or dysfunction of ciliated cells, and increased matrix deposition. Repeated injuries can stimulate airway epithelial cells to produce inflammatory mediators that activate epithelial cells, immune cells, or the epithelial-mesenchymal trophic unit. This persistent inflammation can consequently induce aberrant tissue remodeling. However, the molecular mechanisms driving disease within the different molecular CRS subtypes remain inadequately characterized. Numerous secreted and cell surface proteins relevant to airway inflammation and remodeling are initially synthesized as inactive precursor proteins, including growth/differentiation factors and their associated receptors, enzymes, adhesion molecules, neuropeptides, and peptide hormones. Therefore, these precursor proteins require post-translational cleavage by proprotein convertases (PCs) to become fully functional. In this review, we summarize the roles of PCs in CRS-associated tissue remodeling and discuss the therapeutic potential of targeting PCs for CRS treatment.

Post-Transcriptional Effects of miRNAs on PCSK7 Expression and Function: miR-125a-5p, miR-143-3p, and miR-409-3p as Negative Regulators

The regulatory mechanism of PCSK7 gene is still unknown, although its encoded protein PC7 is the most ancient and highly conserved of all proprotein convertases and exhibits enzymatic and non-enzymatic functions in liver triglyceride regulation. Bioinformatics algorithms were used to predict regulatory microRNAs (miRNAs) of PCSK7 expression. This led to the identification of four miRNAs, namely miR-125a-5p, miR-143-3p, miR-409-3p, and miR-320a-3p, with potential binding sites on the 3′-untranslated region (3′-UTR) of human PCSK7 mRNA. The expression patterns of these miRNAs and PCSK7 mRNA were assessed in three different cell lines with quantitative polymerase chain reaction (qPCR), which revealed reciprocal expression patterns between the expression levels of the four selected miRNAs and PCSK7. Next, the interactions and effects of these miRNAs on PCSK7 expression levels were investigated via cell-based expression analysis, dual-luciferase assay, and Western blot analysis. The data revealed that PCSK7 mRNA levels decreased in cells transfected with vectors overexpressing miR-125a-5p, miR-143-3p, and miR-409-3p, but not miR-320a-3p. The dual-luciferase assay demonstrated that the above three miRNAs could directly interact with putative target sites in PCSK7 3′-UTR and regulate its expression, whereas miR-320-3p exhibited no interaction. Western blot analysis further revealed that the overexpression of miR-125a-5p in Huh7 cells inhibits the expression and ability of PC7 to cleave human transferrin receptor 1. Our results support a regulatory role of these miRNAs on PCSK7 expression and function and open the way to assess their roles in the regulation of PC7 activity in vivo in the development of hepatic steatosis.

The Multifaceted Biology of PCSK9

This article reviews the discovery of PCSK9, its structure-function characteristics, and its presently known and proposed novel biological functions. The major critical function of PCSK9 deduced from human and mouse studies, as well as cellular and structural analyses, is its role in increasing the levels of circulating low-density lipoprotein (LDL)-cholesterol (LDLc), via its ability to enhance the sorting and escort of the cell surface LDL receptor (LDLR) to lysosomes. This implicates the binding of the catalytic domain of PCSK9 to the EGF-A domain of the LDLR. This also requires the presence of the C-terminal Cys/His-rich domain, its binding to the secreted cytosolic cyclase associated protein 1, and possibly another membrane-bound "protein X". Curiously, in PCSK9-deficient mice, an alternative to the downregulation of the surface levels of the LDLR by PCSK9 is taking place in the liver of female mice in a 17β-estradiol-dependent manner by still an unknown mechanism. Recent studies have extended our understanding of the biological functions of PCSK9, namely its implication in septic shock, vascular inflammation, viral infections (Dengue; SARS-CoV-2) or immune checkpoint modulation in cancer via the regulation of the cell surface levels of the T-cell receptor and MHC-I, which govern the antitumoral activity of CD8+ T cells. Because PCSK9 inhibition may be advantageous in these processes, the availability of injectable safe PCSK9 inhibitors that reduces by 50% to 60% LDLc above the effect of statins is highly valuable. Indeed, injectable PCSK9 monoclonal antibody or small interfering RNA could be added to current immunotherapies in cancer/metastasis.

Pcsk6 Deficiency Promotes Cardiomyocyte Senescence by Modulating Ddit3-Mediated ER Stress

Cardiac aging is a critical determinant of cardiac dysfunction, which contributes to cardiovascular disease in the elderly. Proprotein convertase subtilisin/kexin 6 (PCSK6) is a proteolytic enzyme important for the maintenance of cardiac function and vascular homeostasis. To date, the involvement of PCSK6 in cardiac aging remains unknown. Here we report that PCSK6 expression decreased in the hearts of aged mice, where high levels cyclin dependent kinase inhibitor 2A (P16) and cyclin dependent kinase inhibitor 1A (P21) (senescence markers) were observed. Moreover, PCSK6 protein expression was significantly reduced in senescent rat embryonic cardiomyocytes (H9c2) induced by D-galactose. Pcsk6 knockdown in H9c2 cells increased P16 and P21 expression levels and senescence-associated beta-galactosidase activity. Pcsk6 knockdown also impaired cardiomyocyte function, as indicated by increased advanced glycation end products, reactive oxygen species level, and apoptosis. Overexpression of PCSK6 blunted the senescence phenotype and cellular dysfunction. Furthermore, RNA sequencing analysis in Pcsk6-knockdown H9c2 cells identified the up-regulated DNA-damage inducible transcript 3 (Ddit3) gene involved in endoplasmic reticulum (ER) protein processing. Additionally, DDIT3 protein levels were remarkably increased in aged mouse hearts. In the presence of tunicamycin, an ER stress inducer, DDIT3 expression increased in Pcsk6-deficient H9c2 cells but reduced in PCSK6-overexpressing cells. In conclusion, our findings indicate that PCSK6 modulates cardiomyocyte senescence possibly via DDIT3-mediated ER stress.

Why All the Fury over Furin?

Analysis of the SARS-CoV-2 sequence revealed a multibasic furin cleavage site at the S1/S2 boundary of the spike protein distinguishing this virus from SARS-CoV. Furin, the best-characterized member of the mammalian proprotein convertases, is an ubiquitously expressed single pass type 1 transmembrane protein. Cleavage of SARS-CoV-2 spike protein by furin promotes viral entry into lung cells. While furin knockout is embryonically lethal, its knockout in differentiated somatic cells is not, thus furin provides an exciting therapeutic target for viral pathogens including SARS-CoV-2 and bacterial infections. Several peptide-based and small-molecule inhibitors of furin have been recently reported, and select cocrystal structures have been solved, paving the way for further optimization and selection of clinical candidates. This perspective highlights furin structure, substrates, recent inhibitors, and crystal structures with emphasis on furin's role in SARS-CoV-2 infection, where the current data strongly suggest its inhibition as a promising therapeutic intervention for SARS-CoV-2.

Distinct Dibasic Cleavage Specificities of Neuropeptide-Producing Cathepsin L and Cathepsin V Cysteine Proteases Compared to PC1/3 and PC2 Serine Proteases

Neuropeptides, functioning as peptide neurotransmitters and hormones, are generated from proneuropeptide precursors by proteolytic processing at dibasic residue sites (i.e., KR, RK, KK, RR). The cysteine proteases cathepsin L and cathepsin V, combined with the serine proteases proprotein convertases 1 and 2 (PC1/3 and PC2), participate in proneuropeptide processing to generate active neuropeptides. To compare the dibasic cleavage properties of these proteases, this study conducted global, unbiased substrate profiling of these processing proteases using a diverse peptide library in multiplex substrate profiling by mass spectrometry (MSP-MS) assays. MSP-MS utilizes a library of 228 14-mer peptides designed to contain all possible protease cleavage sites, including the dibasic residue sites of KR, RK, KK, and RR. The comprehensive MSP-MS analyses demonstrated that cathepsin L and cathepsin V cleave at the N-terminal side and between the dibasic residues (e.g., ↓K↓R, ↓R↓K, and K↓K), with a preference for hydrophobic residues at the P2 position of the cleavage site. In contrast, the serine proteases PC1/3 and PC2 displayed cleavage at the C-terminal side of dibasic residues of a few peptide substrates. Further analyses with a series of dipeptide-AMC and tripeptide-AMC substrates containing variant dibasic sites with hydrophobic P2 residues indicated the preferences of cathepsin L and cathepsin V to cleave between dibasic residue sites with preferences for flanking hydrophobic residues at the P2 position consisting of Leu, Trp, Phe, and Tyr. Such hydrophobic amino acids reside in numerous proneuropeptides such as pro-NPY and proenkephalin that are known to be processed by cathepsin L. Notably, cathepsin L displayed the highest specific activity that was 10-, 64-, and 1268-fold greater than cathepsin V, PC1/3, and PC2, respectively. Peptide-AMC substrates with dibasic residues confirmed that PC1/3 and P2 cleaved almost exclusively at the C-terminal side of dibasic residues. These data demonstrate distinct dibasic cleavage site properties and a broad range of proteolytic activities of cathepsin L and cathepsin V, compared to PC1/3 and PC2, which participate in producing neuropeptides for cell-cell communication.

Loss-of-function mutation in Pcsk1 increases serum APOA1 level and LCAT activity in mice

Background

The convertase subtilisin/kexin family 1 gene (PCSK1) has been associated in various human genetics studies with a wide spectrum of metabolic phenotypes, including early-onset obesity, hyperphagia, diabetes insipidus, and others. Despite the evident influence of PCSK1 on obesity and the known functions of other PCSKs in lipid metabolism, the role of PCSK1 specifically in lipid and cholesterol metabolism remains unclear. This study evaluated the effect of loss of PCSK1 function on high-density lipoprotein (HDL) metabolism in mice.

Results

HDL cholesterol, apolipoprotein A1 (APOA1) levels in serum and liver, and the activities of two enzymes (lecithin-cholesterol acyltransferase, LCAT and phospholipid transfer protein, PLTP) were evaluated in 8-week-old mice with a non-synonymous single nucleotide mutation leading to an amino acid substitution in PCSK1, which results in a loss of protein’s function. Mutant mice had similar serum HDL cholesterol concentration but increased levels of serum total and mature APOA1, and LCAT activity in comparison to controls.

Conclusions

This study presents the first evaluation of the role of PCSK1 in HDL metabolism using a loss-of-function mutant mouse model. Further investigations will be needed to determine the underlying molecular mechanism.

Supply chain logistics - the role of the Golgi complex in extracellular matrix production and maintenance

The biomechanical and biochemical properties of connective tissues are determined by the composition and quality of their extracellular matrix. This, in turn, is highly dependent on the function and organisation of the secretory pathway. The Golgi complex plays a vital role in directing matrix output by co-ordinating the post-translational modification and proteolytic processing of matrix components prior to their secretion. These modifications have broad impacts on the secretion and subsequent assembly of matrix components, as well as their function in the extracellular environment. In this Review, we highlight the role of the Golgi in the formation of an adaptable, healthy matrix, with a focus on proteoglycan and procollagen secretion as example cargoes. We then discuss the impact of Golgi dysfunction on connective tissue in the context of human disease and ageing.

Novel Roles of Small Extracellular Vesicles in Regulating the Quiescence and Proliferation of Neural Stem Cells

Neural stem cell (NSC) quiescence plays pivotal roles in avoiding exhaustion of NSCs and securing sustainable neurogenesis in the adult brain. The maintenance of quiescence and transition between proliferation and quiescence are complex processes associated with multiple niche signals and environmental stimuli. Exosomes are small extracellular vesicles (sEVs) containing functional cargos such as proteins, microRNAs, and mRNAs. The role of sEVs in NSC quiescence has not been fully investigated. Here, we applied proteomics to analyze the protein cargos of sEVs derived from proliferating, quiescent, and reactivating NSCs. Our findings revealed fluctuation of expression levels and functional clusters of gene ontology annotations of differentially expressed proteins especially in protein translation and vesicular transport among three sources of exosomes. Moreover, the use of exosome inhibitors revealed exosome contribution to entrance into as well as maintenance of quiescence. Exosome inhibition delayed entrance into quiescence, induced quiescent NSCs to exit from the G0 phase of the cell cycle, and significantly upregulated protein translation in quiescent NSCs. Our results suggest that NSC exosomes are involved in attenuating protein synthesis and thereby regulating the quiescence of NSCs.

Distribution of Cardiac and Renal Corin and Proprotein Convertase Subtilisin/Kexin-6 in the Experimental Model of Cardio-Renal Syndrome of Various Severities

Congestive heart failure (CHF) often leads to progressive cardiac hypertrophy and salt/water retention. However, its pathogenesis remains largely unclarified. Corin, a cardiac serine protease, is responsible for converting proANP and proBNP to biologically active peptides. Although the involvement of corin in cardiac hypertrophy and heart failure was extensively studied, the alterations in corin and proprotein convertase subtilisin/kexin-6 (PCSK6), a key enzyme in the conversion of procorin to corin, has not been studied simultaneously in the cardiac and renal tissues in cardiorenal syndrome. Thus, this study aims to examine the status of PCSK6/corin in the cardiac and renal tissues of rats with CHF induced by the creation of aorto-caval fistula (ACF). We divided rats with ACF into two subgroups based on the pattern of their urinary sodium excretion, namely, compensated and decompensated. Placement of ACF led to cardiac hypertrophy, pulmonary congestion, and renal dysfunction, which were more profound in the decompensated subgroup. Corin immunoreactive peptides were detected in all heart chambers at the myocyte membranal and cytosolic localization and in the renal tissue, especially in the apical membrane of the proximal tubule, mTAL, and the collecting duct. Interestingly, the expression and abundance of corin in both the cardiac ventricles and renal tissues were significantly increased in compensated animals as compared with the decompensated state. Noteworthy, the abundance of PCSK6 in these tissues followed a similar pattern as corin. In contrast, furin expression was upregulated in the cardiac and renal tissues in correlation with CHF severity. We hypothesize that the obtained upregulation of cardiac and renal PCSK6/corin in rats with compensated CHF may represent a compensatory response aiming at maintaining normal Na⁺ balance, whereas the decline in these two enzymes may contribute to the pathogenesis of avid sodium retention, cardiac hypertrophy, and blunted atrial natriuretic peptide/brain natriuretic peptide actions in decompensated CHF.

The PCSK9 discovery, an inactive protease with varied functions in hypercholesterolemia, viral infections, and cancer

In 2003, the sequences of mammalian proprotein convertase subtilisin/kexin type 9 (PCSK9) were reported. Radiolabeling pulse-chase analyses demonstrated that PCSK9 was synthesized as a precursor (proPCSK9) that undergoes autocatalytic cleavage in the endoplasmic reticulum into PCSK9, which is then secreted as an inactive enzyme in complex with its inhibitory prodomain. Its high mRNA expression in liver hepatocytes and its gene localization on chromosome 1p32, a third locus associated with familial hypercholesterolemia, other than LDLR or APOB, led us to identify three patient families expressing the PCSK9 variants S127R or F216L. Although Pcsk9 and Ldlr were downregulated in mice that were fed a cholesterol-rich diet, PCSK9 overexpression led to the degradation of the LDLR. This led to the demonstration that gain-of-function and loss-of-function variations in PCSK9 modulate its bioactivity, whereby PCSK9 binds the LDLR in a nonenzymatic fashion to induce its degradation in endosomes/lysosomes. PCSK9 was also shown to play major roles in targeting other receptors for degradation, thereby regulating various processes, including hypercholesterolemia and associated atherosclerosis, vascular inflammation, viral infections, and immune checkpoint regulation in cancer. Injectable PCSK9 monoclonal antibody or siRNA is currently used in clinics worldwide to treat hypercholesterolemia and could be combined with current therapies in cancer/metastasis. In this review, we present the critical information that led to the discovery of PCSK9 and its implication in LDL-C metabolism. We further analyze the underlying functional mechanism(s) in the regulation of LDL-C, as well as the evolving novel roles of PCSK9 in both health and disease states.

α-Synuclein inhibits Snx3-retromer retrograde trafficking of the conserved membrane-bound proprotein convertase Kex2 in the secretory pathway of Saccharomyces cerevisiae

We tested the ability of alpha-synuclein (α-syn) to inhibit Snx3-retromer mediated retrograde trafficking of Kex2 and Ste13 between late endosomes and the trans-Golgi (TGN) using a Saccharomyces cerevisiae model of Parkinson's disease (PD). Kex2 and Ste13 are a conserved, membrane-bound proprotein convertase and dipeptidyl aminopeptidase, respectively, that process pro-α-factor and pro-killer toxin. Each of these proteins contains a cytosolic tail that binds to sorting nexin Snx3. Using a combination of techniques, including fluorescence microscopy, western blotting and a yeast mating assay, we found that α-syn disrupts Snx3-retromer trafficking of Kex2-GFP and GFP-Ste13 from the late endosome to the TGN, resulting in these two proteins transiting to the vacuole by default. Using three α-syn variants (A53T, A30P, and α-synΔC, which lacks residues 101-140), we further found that A53T and α-synΔC, but not A30P, reduce Snx3-retromer trafficking of Kex2-GFP, which is likely to be due to weaker binding of A30P to membranes. Degradation of Kex2 and Ste13 in the vacuole should result in the secretion of unprocessed, inactive forms of α-factor, which will reduce mating efficiency between MATa and MATα cells. We found that wild-type α-syn but not A30P significantly inhibited the secretion of α-factor. Collectively, our results support a model in which the membrane-binding ability of α-syn is necessary to disrupt Snx3-retromer retrograde recycling of these two conserved endopeptidases.

Inclisiran in lipid management: A Literature overview and future perspectives

Primary and secondary prevention protocols aim at reducing the plasma levels of lipids - with particular reference to low-density lipoprotein cholesterol (LDL-C) plasma concentrations - in order to improve the overall survival and reduce the occurrence of major adverse cardiovascular events. The use of statins has been widely considered as the first-line approach in lipids management as they can dramatically impact on the cardiovascular risk profile of individuals. The introduction of ezetimibe and proprotein convertase subtilisin-kexin type 9 (PCSK9) inhibitors overcame the adverse effects of statins and ameliorate the achievement of the target lipids levels. Indeed, advances in therapies promote the use of specific molecules - i.e. short strands of RNA named small-interfering RNAs (siRNAs) - to suppress the transcription of genes related to lipids metabolism. Recently, the inclisiran has been developed: this is a siRNA able to block the mRNA of the PCSK9 gene. About 50% reduction in low-density lipoprotein cholesterol levels have been observed in randomized controlled trials with inclisiran. The aim of this review was to summarize the literature regarding inclisiran and its possible role in the general management of patients with lipid disorders and/or in primary/secondary prevention protocols.

OFF-State-Specific Inhibition of the Proprotein Convertase Furin

The pro-protein convertase furin is a highly specific serine protease involved in the proteolytic maturation of many proteins in the secretory pathway. It also activates surface proteins of many viruses including the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). Furin inhibitors effectively suppress viral replication and thus are promising antiviral therapeutics with broad application potential. Polybasic substrate-like ligands typically trigger conformational changes shifting furin’s active site cleft from the OFF-state to the ON-state. Here, we solved the X-ray structures of furin in complex with four different arginine mimetic compounds with reduced basicity. These guanylhydrazone-based inhibitor complexes showed for the first time an active site-directed binding mode to furin’s OFF-state conformation. The compounds undergo unique interactions within the S1 pocket, largely different compared to substrate-like ligands. A second binding site was identified at the S4/S5 pocket of furin. Crystallography-based titration experiments confirmed the S1 site as the primary binding pocket. We also tested the proprotein convertases PC5/6 and PC7 for inhibition by guanylhydrazones and found an up to 7-fold lower potency for PC7. Interestingly, the observed differences in the K_i values correlated with the sequence conservation of the PCs at the allosteric sodium binding site. Therefore, OFF-state-specific targeting of furin can serve as a valuable strategy for structure-based development of PC-selective small-molecule inhibitors.

Evaluating Targeted Therapies in Ovarian Cancer Metabolism: Novel Role for PCSK9 and Second Generation mTOR Inhibitors

BACKGROUND

Dysregulated lipid metabolism is emerging as a hallmark in several malignancies, including ovarian cancer (OC). Specifically, metastatic OC is highly dependent on lipid-rich omentum. We aimed to investigate the therapeutic value of targeting lipid metabolism in OC. For this purpose, we studied the role of PCSK9, a cholesterol-regulating enzyme, in OC cell survival and its downstream signaling. We also investigated the cytotoxic efficacy of a small library of metabolic (n = 11) and mTOR (n = 10) inhibitors using OC cell lines (n = 8) and ex vivo patient-derived cell cultures (PDCs, n = 5) to identify clinically suitable drug vulnerabilities. Targeting PCSK9 expression with siRNA or PCSK9 specific inhibitor (PF-06446846) impaired OC cell survival. In addition, overexpression of PCSK9 induced robust AKT phosphorylation along with increased expression of ERK1/2 and MEK1/2, suggesting a pro-survival role of PCSK9 in OC cells. Moreover, our drug testing revealed marked differences in cytotoxic responses to drugs targeting metabolic pathways of high-grade serous ovarian cancer (HGSOC) and low-grade serous ovarian cancer (LGSOC) PDCs. Our results show that targeting PCSK9 expression could impair OC cell survival, which warrants further investigation to address the dependency of this cancer on lipogenesis and omental metastasis. Moreover, the differences in metabolic gene expression and drug responses of OC PDCs indicate the existence of a metabolic heterogeneity within OC subtypes, which should be further explored for therapeutic improvements.

GPR37 is processed in the N-terminal ectodomain by ADAM10 and furin

GPR37 is an orphan G protein-coupled receptor (GPCR) implicated in several neurological diseases and important physiological pathways in the brain. We previously reported that its long N-terminal ectodomain undergoes constitutive metalloprotease-mediated cleavage and shedding, which have been rarely described for class A GPCRs. Here, we demonstrate that the protease that cleaves GPR37 at Glu167↓Gln168 is a disintegrin and metalloprotease 10 (ADAM10). This was achieved by employing selective inhibition, RNAi-mediated downregulation, and genetic depletion of ADAM10 in cultured cells as well as in vitro cleavage of the purified receptor with recombinant ADAM10. In addition, the cleavage was restored in ADAM10 knockout cells by overexpression of the wild type but not the inactive mutant ADAM10. Finally, postnatal conditional depletion of ADAM10 in mouse neuronal cells was found to reduce cleavage of the endogenous receptor in the brain cortex and hippocampus, confirming the physiological relevance of ADAM10 as a GPR37 sheddase. Additionally, we discovered that the receptor is subject to another cleavage step in cultured cells. Using site-directed mutagenesis, the site (Arg54↓Asp55) was localized to a highly conserved region at the distal end of the ectodomain that contains a recognition site for the proprotein convertase furin. The cleavage by furin was confirmed by using furin-deficient human colon carcinoma LoVo cells and proprotein convertase inhibitors. GPR37 is thus the first multispanning membrane protein that has been validated as an ADAM10 substrate and the first GPCR that is processed by both furin and ADAM10. The unconventional N-terminal processing may represent an important regulatory element for GPR37.

Differential Neuropeptidomes of Dense Core Secretory Vesicles (DCSV) Produced at Intravesicular and Extracellular pH Conditions by Proteolytic Processing

Neuropeptides mediate cell–cell signaling in the nervous and endocrine systems. The neuropeptidome is the spectrum of peptides generated from precursors by proteolysis within dense core secretory vesicles (DCSV). DCSV neuropeptides and contents are released to the extracellular environment where further processing for neuropeptide formation may occur. To assess the DCSV proteolytic capacity for production of neuropeptidomes at intravesicular pH 5.5 and extracellular pH 7.2, neuropeptidomics, proteomics, and protease assays were conducted using chromaffin granules (CG) purified from adrenal medulla. CG are an established model of DCSV. The CG neuropeptidome consisted of 1239 unique peptides derived from 15 proneuropeptides that were colocalized with 64 proteases. Distinct CG neuropeptidomes were generated at the internal DCSV pH of 5.5 compared to the extracellular pH of 7.2. Class-specific protease inhibitors differentially regulated neuropeptidome production involving aspartic, cysteine, serine, and metallo proteases. The substrate cleavage properties of CG proteases were assessed by multiplex substrate profiling by mass spectrometry (MSP-MS) that uses a synthetic peptide library containing diverse cleavage sites for endopeptidases and exopeptidases. Parallel inhibitor-sensitive cleavages for neuropeptidome production and peptide library proteolysis led to elucidation of six CG proteases involved in neuropeptidome production, represented by cathepsins A, B, C, D, and L and carboxypeptidase E (CPE). The MSP-MS profiles of these six enzymes represented the majority of CG proteolytic cleavages utilized for neuropeptidome production. These findings provide new insight into the DCSV proteolytic system for production of distinct neuropeptidomes at the internal CG pH of 5.5 and at the extracellular pH of 7.2.

Cell-specific expression and individual function of prohormone convertase PC1/3 in Tribolium larval growth highlights major evolutionary changes between beetle and fly neuroendocrine systems

Background

The insect neuroendocrine system acts in the regulation of physiology, development and growth. Molecular evolution of this system hence has the potential to allow for major biological differences between insect groups. Two prohormone convertases, PC1/3 and PC2, are found in animals and both function in the processing of neuropeptide precursors in the vertebrate neurosecretory pathway. Whereas PC2-function is conserved between the fly Drosophila and vertebrates, ancestral PC1/3 was lost in the fly lineage and has not been functionally studied in any protostome.

Results

In order to understand its original functions and the changes accompanying the gene loss in the fly, we investigated PC1/3 and PC2 expression and function in the beetle Tribolium castaneum. We found that PC2 is broadly expressed in the nervous system, whereas surprisingly, PC1/3 expression is restricted to specific cell groups in the posterior brain and suboesophageal ganglion. Both proteases have parallel but non-redundant functions in adult beetles’ viability and fertility. Female infertility following RNAi is caused by a failure to deposit sufficient yolk to the developing oocytes. Larval RNAi against PC2 produced moulting defects where the larvae were not able to shed their old cuticle. This ecdysis phenotype was also observed in a small subset of PC1/3 knockdown larvae and was strongest in a double knockdown. Unexpectedly, most PC1/3-RNAi larvae showed strongly reduced growth, but went through larval moults despite minimal to zero weight gain.

Conclusions

The cell type-specific expression of PC1/3 and its essential requirement for larval growth highlight the important role of this gene within the insect neuroendocrine system. Genomic conservation in most insect groups suggests that it has a comparable individual function in other insects as well, which has been replaced by alternative mechanisms in flies.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13227-021-00179-w.

The rs508487, rs236911, and rs236918 Genetic Variants of the Proprotein Convertase Subtilisin-Kexin Type 7 (PCSK7) Gene Are Associated with Acute Coronary Syndrome and with Plasma Concentrations of HDL-Cholesterol and Triglycerides

Dyslipidemia has a substantial role in the development of acute coronary syndrome (ACS). Previous reports, including genome-wide associations studies (GWAS), have shown that some genetic variants of the proprotein convertase subtilisin–kexin type 7 (PCSK7) gene are associated with plasma lipid levels. In the present study, we evaluated whether PCSK7 gene polymorphisms are significantly associated with the plasma lipid profile and ACS. Three PCSK7 gene polymorphisms (rs508487 T/C, rs236911 C/A, and rs236918 C/G) were determined using TaqMan genotyping assays in a group of 603 ACS patients and 622 healthy controls. The plasma lipid profile was determined in the study groups by enzymatic/colorimetric assays. Under the recessive model, the rs236918 C allele was associated with a high risk of ACS (OR = 2.11, pC = 0.039). In the same way, under the recessive and additive models, the rs236911 C allele was associated with a high risk of ACS (OR = 1.95, pC = 0.037, and OR = 1.28, pC = 0.037, respectively). In addition, under the co-dominant model, the rs508487 T allele was associated with a higher risk of ACS (OR = 1.78, pC = 0.010). The CCC and TCC haplotypes were associated with a high risk of ACS (OR = 1.21, pC = 0.047, and OR = 1.80, pC = 0.001, respectively). The rs236911 CC and rs236918 CC genotypes were associated with lower high-density lipoproteins-cholesterol (HDL-C) plasma concentrations, whereas the rs236911 CC genotype was associated with a higher concentration of triglycerides, as demonstrated in the control individuals who were not receiving antidyslipidemic drugs. Our data suggest that the PCSK7 rs508487 T/C, rs236911 C/A, and rs236918 C/G polymorphisms are associated with the risk of developing ACS, and with plasma concentrations of HDL-C and triglycerides.

Mouse Models of Human Proprotein Convertase Insufficiency

The kexin-like proprotein convertases perform the initial proteolytic cleavages that ultimately generate a variety of different mature peptide and proteins, ranging from brain neuropeptides to endocrine peptide hormones, to structural proteins, among others. In this review, we present a general introduction to proprotein convertase structure and biochemistry, followed by a comprehensive discussion of each member of the kexin-like subfamily of proprotein convertases. We summarize current knowledge of human proprotein convertase insufficiency syndromes, including genome-wide analyses of convertase polymorphisms, and compare these to convertase null and mutant mouse models. These mouse models have illuminated our understanding of the roles specific convertases play in human disease and have led to the identification of convertase-specific substrates; for example, the identification of procorin as a specific PACE4 substrate in the heart. We also discuss the limitations of mouse null models in interpreting human disease, such as differential precursor cleavage due to species-specific sequence differences, and the challenges presented by functional redundancy among convertases in attempting to assign specific cleavages and/or physiological roles. However, in most cases, knockout mouse models have added substantively both to our knowledge of diseases caused by human proprotein convertase insufficiency and to our appreciation of their normal physiological roles, as clearly seen in the case of the furin, proprotein convertase 1/3, and proprotein convertase 5/6 mouse models. The creation of more sophisticated mouse models with tissue- or temporally-restricted expression of specific convertases will improve our understanding of human proprotein convertase insufficiency and potentially provide support for the emerging concept of therapeutic inhibition of convertases.

Exploring the landscape of ectodomain shedding by quantitative protein terminomics

Ectodomain shedding is a proteolytic process that regulates the levels and functions of membrane proteins. Dysregulated shedding is linked to severe diseases, including cancer and Alzheimer's disease. However, the exact cleavage sites of shedding substrates remain largely unknown. Here, we explore the landscape of ectodomain shedding by generating large-scale, cell-type-specific maps of shedding cleavage sites. By means of N- and C-terminal peptide enrichment and quantitative mass spectrometry, we quantified protein termini in the culture media of 10 human cell lines and identified 489 cleavage sites on 163 membrane proteins whose proteolytic terminal fragments are downregulated in the presence of a broad-spectrum metalloprotease inhibitor. A major fraction of the presented cleavage sites was identified in a cell-type-specific manner and mapped onto receptors, cell adhesion molecules, and protein kinases and phosphatases. We confidently identified 86 cleavage sites as metalloprotease substrates by means of knowledge-based scoring.

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Secretomes across 10 human cell lines were investigated by protein terminomics

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Cell-type-specific maps of shedding cleavage sites were generated

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Most of the cleavage sites were identified in a cell-type-specific manner

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Knowledge-based scoring enabled prediction of responsible sheddases

Identification of Novel Genetic Regulatory Region for Proprotein Convertase FURIN and Interferon Gamma in T Cells

The proprotein convertase enzyme FURIN promotes the proteolytic maturation of pro-proteins and thereby it serves as an important factor for maintaining cellular homeostasis. In T cells, FURIN is critical for maintaining the T regulatory cell dependent peripheral immune tolerance and intact T helper cell polarization. The enzymatic activity of FURIN is directly associated with its expression levels, but genetic determinants for cell-type specific Furin gene regulation have remained elusive. By exploring the histone acetyltransferase p300 binding patterns in T helper cells, a putative regulatory region at ca. 20kB upstream of Furin gene was identified. When this region was deleted with CRISPR/Cas9 the production of Furin mRNA was significantly reduced in activated mouse T cells. Genome-wide RNA profiling by sequencing revealed that the novel Furin regulator region also impacted the expression of several genes that have previously been associated with the Th1 type hall mark cytokine IFNγ regulation or function. Finally, Furin genetic regulatory region was found to specifically promote the secretion of IFNγ by activated T cells. In sum, our data unravels the presence of Furin expression regulatory region in T cells that has characteristics of a super-enhancer for Th1 cell fate.

Physiology and role of PCSK9 in vascular disease: Potential impact of localized PCSK9 in vascular wall

Proprotein convertase subtilisin/kexin type-9 (PCSK9), a member of the proprotein convertase family, is an important drug target because of its crucial role in lipid metabolism. Emerging evidence suggests a direct role of localized PCSK9 in the pathogenesis of vascular diseases. With this in our consideration, we reviewed PCSK9 physiology with respect to recent development and major studies (clinical and experimental) on PCSK9 functionality in vascular disease. PCSK9 upregulates low-density lipoprotein (LDL)-cholesterol levels by binding to the LDL-receptor (LDLR) and facilitating its lysosomal degradation. PCSK9 gain-of-function mutations have been confirmed as a novel genetic mechanism for familial hypercholesterolemia. Elevated serum PCSK9 levels in patients with vascular diseases may contribute to coronary artery disease, atherosclerosis, cerebrovascular diseases, vasculitis, aortic diseases, and arterial aging pathogenesis. Experimental models of atherosclerosis, arterial aneurysm, and coronary or carotid artery ligation also support PCSK9 contribution to inflammatory response and disease progression, through LDLR-dependent or -independent mechanisms. More recently, several clinical trials have confirmed that anti-PCSK9 monoclonal antibodies can reduce systemic LDL levels, total nonfatal cardiovascular events, and all-cause mortality. Interaction of PCSK9 with other receptor proteins (LDLR-related proteins, cluster of differentiation family members, epithelial Na⁺ channels, and sortilin) may underlie its roles in vascular disease. Improved understanding of PCSK9 roles and molecular mechanisms in various vascular diseases will facilitate advances in lipid-lowering therapy and disease prevention.

Increased FURIN expression in rheumatoid arthritis patients and its anti-inflammatory effect

Background

FURIN belongs to the proprotein convertase family that processes proproteins and is involved in many diseases. However, the role of FURIN in rheumatoid arthritis (RA) remains unknown. In this study, we investigated the association between circulating FURIN and disease activity in patients with RA and the effect of FURIN in THP‐1‐derived macrophages.

Methods

A total of 108 RA patients and 39 healthy controls participants were included in this study. RA patients were divided into four disease activity groups determined by the Disease Activity Score of 28 joints (DAS28). FURIN expression in peripheral blood mononuclear cells (PBMCs) and serum was detected by using quantitative real‐time polymerase chain reaction (qRT‐PCR) and enzyme‐linked immunosorbent assay (ELISA), respectively. Western blotting and qRT‐PCR were used to detect cytokines level after interfering FURIN expressed in THP‐1‐derived macrophages.

Results

Both FURIN mRNA and protein levels were significantly higher in RA patients than in healthy controls participants (P < .001). No significant difference in FURIN expression was observed among the four RA groups (P > .05). Spearman correlation revealed that FURIN positively correlated with transforming growth factor‐β1(TGF‐β1), rheumatoid factor (RF), and anti‐cyclic citrullinated peptide (anti‐CCP). Moreover, the inhibition of FURIN in THP‐1‐derived macrophages promoted the caspase‐1 and IL‐1β expression (P < .05).

Conclusion

FURIN levels were significantly increased in the peripheral blood of RA patients and were not associated with disease activity. The inhibition of FURIN in THP‐1‐derived macrophages with elevated IL‐1β levels shows that FURIN may have an anti‐inflammatory effect.

Shedding of cancer susceptibility candidate 4 by the convertases PC7/furin unravels a novel secretory protein implicated in cancer progression

The proprotein convertases (PCs) are responsible for the maturation of precursor proteins, and are involved in multiple and critical biological processes. Over the past 30 years, the PCs have had great translational applications, but the physiological roles of PC7, the seventh member of the family, are still obscure. Searching for new substrates of PC7, a quantitative proteomics screen for selective enrichment of N-glycosylated polypeptides secreted from hepatic HuH7 cells identified two human type-II transmembrane proteins of unknown function(s): Cancer Susceptibility Candidate 4 (CASC4) and Golgi Phosphoprotein of 130 kDa (GPP130/GOLIM4). Concentrating on CASC4, its mutagenesis characterized the PC7/Furin-shedding site to occur at KR₆₆↓NS, in HEK293 cells. We defined PC7 and Furin trafficking and activity, and demonstrated that CASC4 shedding occurs in acidic endosomes and/or in the trans-Golgi Network. Our data unraveled a cancer-protective role for CASC4, because siRNA silencing of endogenous CASC4 expression in the invasive triple-negative breast cancer human cell line MDA-MB-231 resulted in a significantly increased cellular migration and invasion. Conversely, MDA-MB-231 cells stably expressing CASC4 exhibited reduced migration and invasion, which can be explained by an increased number of paxillin-positive focal adhesions. This phenotypic cancer-protective role of CASC4 is reversed in cells overexpressing an optimally PC7/Furin-cleaved CASC4 mutant, or upon overexpression of the N-terminally convertase-generated membrane-bound segment. This phenotype was associated with increased formation of podosome-like structures, especially evident in cells overexpressing the N-terminal fragment. In accord, breast cancer patients’ data sets show that high CASC4 and PCSK7 expression levels predict a significantly worse prognosis compared to high CASC4 but low PCSK7 levels. In conclusion, CASC4 shedding not only disrupts its anti-migratory/invasive role, but also generates a membrane-bound fragment that drastically modifies the actin cytoskeleton, resulting in an enhanced cellular migration and invasion. This phenotype might be clinically relevant in the prognosis of breast cancer patients.

Design, Engineering and Discovery of Novel α-Helical and β-Boomerang Antimicrobial Peptides against Drug Resistant Bacteria

In an era where the pipeline of new antibiotic development is drying up, the continuous rise of multi-drug resistant (MDR) and extensively drug resistant (XDR) bacteria are genuine threats to human health. Although antimicrobial peptides (AMPs) may serve as promising leads against drug resistant bacteria, only a few AMPs are in advanced clinical trials. The limitations of AMPs, namely their low in vivo activity, toxicity, and poor bioavailability, need to be addressed. Here, we review engineering of frog derived short α-helical AMPs (aurein, temporins) and lipopolysaccharide (LPS) binding designed β-boomerang AMPs for further development. The discovery of novel cell selective AMPs from the human proprotein convertase furin is also discussed.

Growth differentiation factor 15 in adverse cardiac remodelling: from biomarker to causal player

Heart failure is a growing health issue as a negative consequence of improved survival upon myocardial infarction, unhealthy lifestyle, and the ageing of our population. The large and complex pathology underlying heart failure makes diagnosis and especially treatment very difficult. There is an urgent demand for discriminative biomarkers to aid disease management of heart failure. Studying cellular pathways and pathophysiological mechanisms contributing to disease initiation and progression is crucial for understanding the disease process and will aid to identification of novel biomarkers and potential therapeutic targets. Growth differentiation factor 15 (GDF15) is a proven valuable biomarker for different pathologies, including cancer, type 2 diabetes, and cardiovascular diseases. Although the prognostic value of GDF15 in heart failure is robust, the biological function of GDF15 in adverse cardiac remodelling is not fully understood. GDF15 is a distant member of the transforming growth factor-β family and involved in various biological processes including inflammation, cell cycle, and apoptosis. However, more research is suggesting a role in fibrosis, hypertrophy, and endothelial dysfunction. As GDF15 is a pleiotropic protein, elucidating the exact role of GDF15 in complex disease processes has proven to be a challenge. In this review, we provide an overview of the role GDF15 plays in various intracellular and extracellular processes underlying heart failure, and we touch upon crucial points that need consideration before GDF15 can be integrated as a biomarker in standard care or when considering GDF15 for therapeutic intervention.

Involvement of Spike Protein, Furin, and ACE2 in SARS-CoV-2-Related Cardiovascular Complications

The novel coronavirus disease 2019 (COVID-19) is a global epidemic caused by severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2). SARS-CoV-2 has a similar structure to severe acute respiratory syndrome coronavirus-1(SARS-CoV-1). The S protein on the surface of the virus is cleaved by host proprotein convertases (PCs) to expose the active N-terminal S1 extracellular domain. Its receptors are angiotensin-converting enzyme 2 (ACE2), and the C-terminal S2 membrane anchoring protein is responsible for translocating the virus into the cell. Among patients with COVID-19, there is a higher prevalence of cardiovascular disease, and more than 7% of patients have suffered myocardial damage due to the infection, but the internal mechanism is still poorly understood. There is currently no specific and effective targeted treatment. Reduction of the patient’s morbidity and mortality is an urgent problem that needs to be solved clinically. By exploring the theoretical analysis of PCs and ACE2 in COVID-19 cardiovascular susceptibility, some insights on how to prevent and alleviate adverse cardiovascular prognosis have been provided in this study.

Phylogenetic Analysis and Structural Modeling of SARS-CoV-2 Spike Protein Reveals an Evolutionary Distinct and Proteolytically Sensitive Activation Loop

The 2019 novel coronavirus (2019-nCoV/SARS-CoV-2) originally arose as part of a major outbreak of respiratory disease centered on Hubei province, China. It is now a global pandemic and is a major public health concern. Taxonomically, SARS-CoV-2 was shown to be a Betacoronavirus (lineage B) closely related to SARS-CoV and SARS-related bat coronaviruses, and it has been reported to share a common receptor with SARS-CoV (ACE-2). Subsequently, betacoronaviruses from pangolins were identified as close relatives to SARS-CoV-2. Here, we perform structural modeling of the SARS-CoV-2 spike glycoprotein. Our data provide support for the similar receptor utilization between SARS-CoV-2 and SARS-CoV, despite a relatively low amino acid similarity in the receptor binding module. Compared to SARS-CoV and all other coronaviruses in Betacoronavirus lineage B, we identify an extended structural loop containing basic amino acids at the interface of the receptor binding (S1) and fusion (S2) domains. We suggest this loop confers fusion activation and entry properties more in line with betacoronaviruses in lineages A and C, and be a key component in the evolution of SARS-CoV-2 with this structural loop affecting virus stability and transmission.

Protease propeptide structures, mechanisms of activation, and functions

Proteases are a diverse group of hydrolytic enzymes, ranging from single-domain catalytic molecules to sophisticated multi-functional macromolecules. Human proteases are divided into five mechanistic classes: aspartate, cysteine, metallo, serine and threonine proteases, based on the catalytic mechanism of hydrolysis. As a protective mechanism against uncontrolled proteolysis, proteases are often produced and secreted as inactive precursors, called zymogens, containing inhibitory N-terminal propeptides. Protease propeptide structures vary considerably in length, ranging from dipeptides and propeptides of about 10 amino acids to complex multifunctional prodomains with hundreds of residues. Interestingly, sequence analysis of the different protease domains has demonstrated that propeptide sequences present higher heterogeneity compared with their catalytic domains. Therefore, we suggest that protease inhibition targeting propeptides might be more specific and have less off-target effects than classical inhibitors. The roles of propeptides, besides keeping protease latency, include correct folding of proteases, compartmentalization, liganding, and functional modulation. Changes in the propeptide sequence, thus, have a tremendous impact on the cognate enzymes. Small modifications of the propeptide sequences modulate the activity of the enzymes, which may be useful as a therapeutic strategy. This review provides an overview of known human proteases, with a focus on the role of their propeptides. We review propeptide functions, activation mechanisms, and possible therapeutic applications.

Evaluation of the Potential Role of Proprotein Convertase Subtilisin/Kexin Type 9 (PCSK9) in Niemann-Pick Disease, Type C1

Niemann–Pick disease, type C1, is a cholesterol storage disease where unesterified cholesterol accumulates intracellularly. In the cerebellum this causes neurodegeneration of the Purkinje neurons that die in an anterior-to-posterior and time-dependent manner. This results in cerebellar ataxia as one of the major outcomes of the disease. Proprotein convertase subtilisin/kexin type 9 (PCSK9) plays a significant role in the regulation of serum cholesterol levels by modulating LDL receptor levels on peripheral tissues. In the central nervous system, PCSK9 may have a similar effect on the closely related VLDL and ApoE2 receptors to regulate brain cholesterol. In addition, regulation of VLDLR and ApoER2 by PCSK9 may contribute to neuronal apoptotic pathways through Reelin, the primary ligand of VLDLR and ApoER2. Defects in reelin signaling results in cerebellar dysfunction leading to ataxia as seen in the Reeler mouse. Our recent findings that Pcsk9 is expressed ~8-fold higher in the anterior lobules of the cerebellum compared to the posterior lobule X, which is resistant to neurodegeneration, prompted us to ask whether PCSK9 could play a role in NPC1 disease progression. We addressed this question genetically, by characterizing NPC1 disease in the presence or absence of PCSK9. Analysis of double mutant Pcsk9^-/-/Npc1^-/- mice by disease severity scoring, motor assessments, lifespan, and cerebellar Purkinje cell staining, showed no obvious difference in NPC1 disease progression with that of Npc1^-/- mice. This suggests that PCSK9 does not play an apparent role in NPC1 disease progression.

Structural modeling of 2019-novel coronavirus (nCoV) spike protein reveals a proteolytically-sensitive activation loop as a distinguishing feature compared to SARS-CoV and related SARS-like coronaviruses

The 2019 novel coronavirus (2019-nCoV) is currently causing a widespread outbreak centered on Hubei province, China and is a major public health concern. Taxonomically 2019-nCoV is closely related to SARS-CoV and SARS-related bat coronaviruses, and it appears to share a common receptor with SARS-CoV (ACE-2). Here, we perform structural modeling of the 2019-nCoV spike glycoprotein. Our data provide support for the similar receptor utilization between 2019-nCoV and SARS-CoV, despite a relatively low amino acid similarity in the receptor binding module. Compared to SARS-CoV, we identify an extended structural loop containing basic amino acids at the interface of the receptor binding (S1) and fusion (S2) domains, which we predict to be proteolytically-sensitive. We suggest this loop confers fusion activation and entry properties more in line with MERS-CoV and other coronaviruses, and that the presence of this structural loop in 2019-nCoV may affect virus stability and transmission.

A transient amphipathic helix in the prodomain of PCSK9 facilitates binding to low-density lipoprotein particles

Proprotein convertase subtilisin/kexin type-9 (PCSK9) is a ligand of low-density lipoprotein (LDL) receptor (LDLR) that promotes LDLR degradation in late endosomes/lysosomes. In human plasma, 30–40% of PCSK9 is bound to LDL particles; however, the physiological significance of this interaction remains unknown. LDL binding in vitro requires a disordered N-terminal region in PCSK9's prodomain. Here, we report that peptides corresponding to a predicted amphipathic α-helix in the prodomain N terminus adopt helical structure in a membrane-mimetic environment. This effect was greatly enhanced by an R46L substitution representing an atheroprotective PCSK9 loss-of-function mutation. A helix-disrupting proline substitution within the putative α-helical motif in full-length PCSK9 lowered LDL binding affinity >5-fold. Modeling studies suggested that the transient α-helix aligns multiple polar residues to interact with positively charged residues in the C-terminal domain. Gain-of-function PCSK9 mutations associated with familial hypercholesterolemia (FH) and clustered at the predicted interdomain interface (R469W, R496W, and F515L) inhibited LDL binding, which was completely abolished in the case of the R496W variant. These findings shed light on allosteric conformational changes in PCSK9 required for high-affinity binding to LDL particles. Moreover, the initial identification of FH-associated mutations that diminish PCSK9's ability to bind LDL reported here supports the notion that PCSK9-LDL association in the circulation inhibits PCSK9 activity.

The motif EXEXXXL in the cytosolic tail of the secretory human proprotein convertase PC7 regulates its trafficking and cleavage activity

Many secretory proteins are activated by cleavage at specific sites. The proprotein convertases (PCs) form a family of nine secretory subtilisin-like serine proteases, seven of which cleave at specific basic residues within the trans-Golgi network, granules, or at the cell surface/endosomes. The seventh member, PC7, is a type-I transmembrane (TM) protein with a 97-residue-long cytosolic tail (CT). PC7 sheds human transferrin receptor 1 (hTfR1) into soluble shTfR1 in endosomes. To better understand the physiological roles of PC7, here we focused on the relationship between the CT-regulated trafficking of PC7 and its ability to shed hTfR1. Deletion of the TMCT resulted in soluble PC7 and loss of its hTfR1 shedding activity. Extensive CT deletions and mutagenesis analyses helped us zoom in on three residues in the CT, namely Glu-719, Glu-721, and Leu-725, that are part of a novel motif, EXEXXXL⁷²⁵, critical for PC7 activity on hTfR1. NMR studies of two 14-mer peptides mimicking this region of the CT and its Ala variants revealed that the three exposed residues are on the same side of the molecule. This led to the identification of adaptor protein 2 (AP-2) as a protein that recognizes the EXEXXXL⁷²⁵ motif, thus representing a potentially new regulator of PC7 trafficking and cleavage activity. Immunocytochemistry of the subcellular localization of PC7 and its Ala variants of Leu-725 and Glu-719 and Glu-721 revealed that Leu-725 enhances PC7 localization to early endosomes and that, together with Glu-719 and Glu-721, it increases the endosomal activity of PC7 on hTfR1.