Human Subtilisin Kexin Isozyme-1 (SKI-1)/Site-1 Protease (S1P) regulates cytoplasmic lipid droplet abundance: A potential target for indirect-acting anti-dengue virus agents

Lipid Droplets: Formation, Degradation, and Their Role in Cellular Responses to Flavivirus Infections

Lipid droplets (LDs) are cellular organelles derived from the endoplasmic reticulum (ER), serving as lipid storage sites crucial for maintaining cellular lipid homeostasis. Recent attention has been drawn to their roles in viral replication and their interactions with viruses. However, the precise biological functions of LDs in viral replication and pathogenesis remain incompletely understood. To elucidate the interaction between LDs and viruses, it is imperative to comprehend the biogenesis of LDs and their dynamic interactions with other organelles. In this review, we explore the intricate pathways involved in LD biogenies within the cytoplasm, encompassing the uptake of fatty acid from nutrients facilitated by CD36-mediated membranous protein (FABP/FATP)-FA complexes, and FA synthesis via glycolysis in the cytoplasm and the TCL cycle in mitochondria. While LD biogenesis primarily occurs in the ER, matured LDs are intricately linked to multiple organelles. Viral infections can lead to diverse consequences in terms of LD status within cells post-infection, potentially involving the breakdown of LDs through the activation of lipophagy. However, the exact mechanisms underlying LD destruction or accumulation by viruses remain elusive. The significance of LDs in viral replication renders them effective targets for developing broad-spectrum antivirals. Moreover, considering that reducing neutral lipids in LDs is a strategy for anti-obesity treatment, LD depletion may not pose harm to cells. This presents LDs as promising antiviral targets for developing therapeutics that are minimally or non-toxic to the host.

Targeting ABCG1 and SREBP-2 mediated cholesterol homeostasis ameliorates Zika virus-induced ocular pathology

Zika virus (ZIKV) infection during pregnancy causes severe neurological and ocular abnormalities in infants, yet no vaccine or antivirals are available. Our transcriptomic analysis of ZIKV-infected retinal pigment epithelial (RPE) cells revealed alterations in the cholesterol pathway. Thus, we investigated the functional roles of ATP binding cassette transporter G1 (ABCG1) and sterol response element binding protein 2 (SREPB-2), two key players in cholesterol metabolism, during ocular ZIKV infection. Our in vitro data showed that increased ABCG1 activity via liver X receptors (LXRs), reduced ZIKV replication, while ABCG1 knockdown increased replication with elevated intracellular cholesterol. Conversely, inhibiting SREBP-2 or its knockdown reduced ZIKV replication by lowering cholesterol levels. In vivo, LXR agonist or SREBP-2 inhibitor treatment mitigated ZIKV-induced chorioretinal lesions in mice, concomitant with decreased expression of inflammatory mediators and increased activation of antiviral response genes. In summary, our study identifies ABCG1's antiviral role and SREBP-2's proviral effects in ocular ZIKV infection, offering cholesterol metabolism as a potential target to develop antiviral therapies.

Lipid droplets in Zika neuroinfection: Potential targets for intervention?

Lipid droplets (LD) are evolutionarily conserved lipid-enriched organelles with a diverse array of cell- and stimulus-regulated proteins. Accumulating evidence demonstrates that intracellular pathogens exploit LD as energy sources, replication sites, and part of the mechanisms of immune evasion. Nevertheless, LD can also favor the host as part of the immune and inflammatory response to pathogens. The functions of LD in the central nervous system have gained great interest due to their presence in various cell types in the brain and for their suggested involvement in neurodevelopment and neurodegenerative diseases. Only recently have the roles of LD in neuroinfections begun to be explored. Recent findings reveal that lipid remodelling and increased LD biogenesis play important roles for Zika virus (ZIKV) replication and pathogenesis in neural cells. Moreover, blocking LD formation by targeting DGAT-1 in vivo inhibited virus replication and inflammation in the brain. Therefore, targeting lipid metabolism and LD biogenesis may represent potential strategies for anti-ZIKV treatment development. Here, we review the progress in understanding LD functions in the central nervous system in the context of the host response to Zika infection.

The interplay between lipid droplets and virus infection

As an intracellular parasite, the virus usurps cellular machinery and modulates cellular metabolism pathways to replicate itself in cells. Lipid droplets (LDs) are universally conserved energy storage organelles that not only play vital roles in maintaining lipid homeostasis but are also involved in viral replication. Increasing evidence has demonstrated that viruses take advantage of cellular lipid metabolism by targeting the biogenesis, hydrolysis, and lipophagy of LD during viral infection. In this review, we summarize the current knowledge about the modulation of cellular LD by different viruses, with a special emphasis on the Hepatitis C virus, Dengue virus, and SARS-CoV-2.

The proprotein convertase SKI-1/S1P is a critical host factor for Nairobi sheep disease virus infectivity

Nairobi sheep disease virus (NSDV) belongs to the Orthonairovirus genus in the Bunyavirales order and is genetically related to human-pathogenic Crimean-Congo hemorrhagic fever virus (CCHFV). NSDV is a zoonotic pathogen transmitted by ticks and primarily affects naïve small ruminants in which infection leads to severe and often fatal hemorrhagic gastroenteritis. Despite its veterinary importance and the striking similarities in the clinical picture between NSDV-infected ruminants and CCHFV patients, the molecular pathogenesis of NSDV and its interactions with the host cell are largely unknown. Here, we identify the membrane-bound proprotein convertase site-1 protease (S1P), also known as subtilisin/kexin-isozyme-1 (SKI-1), as a host factor affecting NSDV infectivity. Absence of S1P in SRD-12B cells, a clonal CHO-K1 cell variant with a genetic defect in the S1P gene (MBTPS1), results in significantly decreased NSDV infectivity while transient complementation of SKI-1/S1P rescues NSDV infection. SKI-1/S1P is dispensable for virus uptake but critically required for production of infectious virus progeny. Moreover, we provide evidence that SKI-1/S1P is involved in the posttranslational processing of the NSDV glycoprotein precursor. Our results demonstrate the role of SKI-1/S1P in the virus life cycle of NSDV and suggest that this protease is a common host factor for orthonairoviruses and may thus represent a promising broadly-effective, indirect antiviral target.

Proprotein convertases regulate trafficking and maturation of key proteins within the secretory pathway

Proprotein Convertases (PCs) are serine endoproteases that regulate the homeostasis of protein substrates in the cell. The PCs family counts 9 members-PC1/3, PC2, PC4, PACE4, PC5/6, PC7, Furin, SKI-1/S1P, and PCSK9. The first seven PCs are known as Basic Proprotein Convertases due to their propensity to cleave after polybasic clusters. SKI-1/S1P requires the additional presence of hydrophobic residues for processing, whereas PCSK9 is catalytically dead after autoactivation and exerts its functions using mechanisms alternative to direct cleavage. All PCs traffic through the canonical secretory pathway, reaching different compartments where the various substrates reside. Despite PCs members do not share the same subcellular localization, most of the cellular organelles count one or more Proprotein Convertases, including ER, Golgi stack, endosomes, secretory granules, and plasma membranes. The widespread expression of these enzymes at the systemic level speaks for their importance in the homeostasis of a large number of biological functions. Among others, PCs cleave precursors of hormones and growth factors and activate receptors and transcription factors. Notably, dysregulation of the enzymatic activity of Proprotein Convertases is associated to major human pathologies, such as cardiovascular diseases, cancer, diabetes, infections, inflammation, autoimmunity diseases, and Parkinson. In the current COVID-19 pandemic, Furin has further attracted the attention as a key player for conferring high pathogenicity to SARS-CoV-2. Here, we review the Proprotein Convertases family and their most important substrates along the secretory pathway. Knowledge about the complex functions of PCs is important to identify potential drug strategies targeting this class of enzymes.

Murine diabetic models for dengue virus infection

Dengue virus (DENV) is a critical public health concern in tropical and subtropical regions worldwide. Thus, immunocompetent murine models of DENV infection with robust viremia are required for vaccine studies. Diabetes is highly prevalent worldwide, making it frequent comorbidity in patients with dengue fever. Therefore, murine models are needed to understand viral pathogenesis and disease progression. Acquired-induced and inherently diabetic C57BL/6 and db/db mice were inoculated with DENV-3 via the tail vein. After infection, both the diabetic C57BL/6 and db/db mice showed obvious weight loss with clinical manifestations. Quantitative reverse-transcription polymerase chain reaction revealed robust and replicable viremia in the two types of diabetic mice. Immunohistochemical detection showed persistent DENV-3 infection in the liver. Enzyme-linked immunosorbent assay for cytokine detection revealed that diabetic mice showed more severe inflammatory responses than did nondiabetic mice, and significant histological alterations were observed in diabetic mice. Thus, the diabetic mice were more susceptible to DENV infection than the nondiabetic mice. Taken together, we established two types of immunocompetent diabetic mice for DENV infection, which can be used to further study the mechanisms of dengue pathogenesis in diabetes and to develop antiviral pharmaceuticals and treatments.

Lipid Droplets and Their Participation in Zika Virus Infection

Lipid droplets (LDs) are highly conserved and dynamic intracellular organelles. Their functions are not limited to serving as neutral lipid reservoirs; they also participate in non-energy storage functions, such as cell lipid metabolism, protection from cell stresses, maintaining protein homeostasis, and regulating nuclear function. During a Zika virus (ZIKV) infection, the viruses hijack the LDs to provide energy and lipid sources for viral replication. The co-localization of ZIKV capsid (C) protein with LDs supports its role as a virus replication platform and a key compartment for promoting the generation of progeny virus particles. However, in view of the multiple functions of LDs, their role in ZIKV infection needs further elucidation. Here, we review the basic mechanism of LD biogenesis and biological functions and discuss how ZIKV infection utilizes these effects of LDs to facilitate virus replication, along with the future application strategy of developing new antiviral drugs based on the interaction of ZIKV with LDs.

Targeting lipid biosynthesis pathways for hepatitis B virus cure

Chronic hepatitis B virus (HBV) infection is characterized by the presence of high circulating levels of non-infectious lipoprotein-like HBV surface antigen (HBsAg) particles thought to contribute to chronic immune dysfunction in patients. Lipid and metabolomic analysis of humanized livers from immunodeficient chimeric mice (uPA/SCID) revealed that HBV infection dysregulates several lipid metabolic pathways. Small molecule inhibitors of lipid biosynthetic pathway enzymes acetyl-CoA carboxylase (ACC), fatty acid synthase, and subtilisin kexin isozyme-1/site-1 protease in HBV-infected HepG2-NTCP cells demonstrated potent and selective reduction of extracellular HBsAg. However, a liver-targeted ACC inhibitor did not show antiviral activity in HBV-infected liver chimeric mice, despite evidence of on-target engagement. Our study suggests that while HBsAg production may be dependent on hepatic de novo lipogenesis in vitro, this may be overcome by extrahepatic sources (such as lipolysis or diet) in vivo. Thus, a combination of agents targeting more than one lipid metabolic pathway may be necessary to reduce HBsAg levels in patients with chronic HBV infection.

Rotavirus exploits SREBP pathway for hyper lipid biogenesis during replication

Species A rotavirus (RVA) is one of the pathogens causing severe acute gastroenteritis in young children and animals worldwide. RVA replicates and assembles its immature particle within electron dense compartments known as viroplasm. Despite the importance of lipid droplet (LD) formation in the RVA viroplasm, the upstream molecules modulating LD formation have remained elusive. Here, we demonstrate that RVA infection reprogrammes sterol regulatory element binding proteins (SREBPs)-dependent lipogenic pathways in virus-infected cells. Interestingly, silencing of SREBPs significantly reduced RVA protein synthesis, genome replication and progeny virus production. Moreover, knockout of SREBP-1c gene conferred resistance to RVA-induced diarrhoea, reduction of RVA replication, and mitigation of small intestinal pathology in mice. This study identifies SREBPs-mediated lipogenic reprogramming in RVA-infected host cells for facilitating virus replication and SREBPs as a potential target for developing therapeutics against RVA infection.

Subtilisin of Leishmania amazonensis as Potential Druggable Target: Subcellular Localization, In Vitro Leishmanicidal Activity and Molecular Docking of PF-429242, a Subtilisin Inhibitor

Subtilisin proteases, found in all organisms, are enzymes important in the post-translational steps of protein processing. In Leishmania major and L. donovani, this enzyme has been described as essential to their survival; however, few compounds that target subtilisin have been investigated for their potential as an antileishmanial drug. In this study, we first show, by electron microscopy and flow cytometry, that subtilisin has broad localization throughout the cytoplasm and membrane of the parasite in the promastigote form with foci in the flagellar pocket. Through in silico analysis, the similarity between subtilisin of different Leishmania species and that of humans were determined, and based on molecular docking, we evaluated the interaction capacity of a serine protease inhibitor against both life cycle forms of Leishmania. The selected inhibitor, known as PF-429242, has already been used against the dengue virus, arenaviruses, and the hepatitis C virus. Moreover, it proved to have antilipogenic activity in a mouse model and caused hypolipidemia in human cells in vitro. Here, PF-429242 significantly inhibited the growth of L. amazonensis promastigotes of four different strains (IC50 values = 3.07 ± 0.20; 0.83 ± 0.12; 2.02 ± 0.27 and 5.83 ± 1.2 µM against LTB0016, PH8, Josefa and LV78 strains) whilst having low toxicity in the host macrophages (CC50 = 170.30 µM). We detected by flow cytometry that there is a greater expression of subtilisin in the amastigote form; however, PF-429242 had a low effect against this intracellular form with an IC50 of >100 µM for intracellular amastigotes, as well as against axenic amastigotes (94.12 ± 2.8 µM for the LV78 strain). In conclusion, even though PF-429242 does not affect the intracellular forms, this drug will serve as a tool to explore pharmacological and potentially leishmanicidal targets.

Biogenesis and Breakdown of Lipid Droplets in Pathological Conditions

Lipid droplets (LD) have long been considered as mere fat drops; however, LD have lately been revealed to be ubiquitous, dynamic and to be present in diverse organelles in which they have a wide range of key functions. Although incompletely understood, the biogenesis of eukaryotic LD initiates with the synthesis of neutral lipids (NL) by enzymes located in the endoplasmic reticulum (ER). The accumulation of NL leads to their segregation into nanometric nuclei which then grow into lenses between the ER leaflets as they are further filled with NL. The lipid composition and interfacial tensions of both ER and the lenses modulate their shape which, together with specific ER proteins, determine the proneness of LD to bud from the ER toward the cytoplasm. The most important function of LD is the buffering of energy. But far beyond this, LD are actively integrated into physiological processes, such as lipid metabolism, control of protein homeostasis, sequestration of toxic lipid metabolic intermediates, protection from stress, and proliferation of tumours. Besides, LD may serve as platforms for pathogen replication and defense. To accomplish these functions, from biogenesis to breakdown, eukaryotic LD have developed mechanisms to travel within the cytoplasm and to establish contact with other organelles. When nutrient deprivation occurs, LD undergo breakdown (lipolysis), which begins with the LD-associated members of the perilipins family PLIN2 and PLIN3 chaperone-mediated autophagy degradation (CMA), a specific type of autophagy that selectively degrades a subset of cytosolic proteins in lysosomes. Indeed, PLINs CMA degradation is a prerequisite for further true lipolysis, which occurs via cytosolic lipases or by lysosome luminal lipases when autophagosomes engulf portions of LD and target them to lysosomes. LD play a crucial role in several pathophysiological processes. Increased accumulation of LD in non-adipose cells is commonly observed in numerous infectious diseases caused by intracellular pathogens including viral, bacterial, and parasite infections, and is gradually recognized as a prominent characteristic in a variety of cancers. This review discusses current evidence related to the modulation of LD biogenesis and breakdown caused by intracellular pathogens and cancer.

How Do Enveloped Viruses Exploit the Secretory Proprotein Convertases to Regulate Infectivity and Spread?

Inhibition of the binding of enveloped viruses surface glycoproteins to host cell receptor(s) is a major target of vaccines and constitutes an efficient strategy to block viral entry and infection of various host cells and tissues. Cellular entry usually requires the fusion of the viral envelope with host plasma membranes. Such entry mechanism is often preceded by “priming” and/or “activation” steps requiring limited proteolysis of the viral surface glycoprotein to expose a fusogenic domain for efficient membrane juxtapositions. The 9-membered family of Proprotein Convertases related to Subtilisin/Kexin (PCSK) serine proteases (PC1, PC2, Furin, PC4, PC5, PACE4, PC7, SKI-1/S1P, and PCSK9) participate in post-translational cleavages and/or regulation of multiple secretory proteins. The type-I membrane-bound Furin and SKI-1/S1P are the major convertases responsible for the processing of surface glycoproteins of enveloped viruses. Stefan Kunz has considerably contributed to define the role of SKI-1/S1P in the activation of arenaviruses causing hemorrhagic fever. Furin was recently implicated in the activation of the spike S-protein of SARS-CoV-2 and Furin-inhibitors are being tested as antivirals in COVID-19. Other members of the PCSK-family are also implicated in some viral infections, such as PCSK9 in Dengue. Herein, we summarize the various functions of the PCSKs and present arguments whereby their inhibition could represent a powerful arsenal to limit viral infections causing the present and future pandemics.

The novel therapeutic target and inhibitory effects of PF-429242 against Zika virus infection

Zika virus (ZIKV) is a re-emerging mosquito-borne flavivirus of African origin that is transmitted by Aedes mosquitoes. ZIKV was historically limited to Africa and Asia, where mild cases were reported. However, ZIKV has recently been responsible for major global outbreaks associated with a wide range of neurological complications. Since no antiviral therapy exists for ZIKV, drug discovery research for ZIKV is crucial. Intracellular lipids regulated by sterol regulatory element-binding proteins (SREBPs) are important in flavivirus pathogenesis. PF-429242 has been reported to inhibit the activity of site-1 protease (S1P), which regulates the expression of SREBP target genes. Our primary objective in this study is to elucidate the mechanism of the antiviral activity of PF-429242 against the African genotype (ZIKV_MR-766) and Asian genotypes (ZIKV _{H/PF 2013} and ZIKV _PRVABC59) using several primate-derived cell lines. The virus titer was determined via a focus-forming assay; we used flow cytometry to quantify intracellular lipids in ZIKV-infected and mock-treated cells. The PF-429242 molecule effectively suppressed ZIKV infection in neuronal cell lines; T98G, U-87MG, SK-N-SH and primary monocytes cell, indicating that PF-429242 molecule can be used therapeutically. A strong reduction in ZIKV replication was observed at 12 μM and 30 μM in in neuronal cell lines and primary monocytes, respectively. Interestingly, the inhibitory effects of the PF-429242 molecule were observed when it was tested on various ZIKV-lineage infections. Lipid quantification reveals that ZIKV increases lipogenesis in infected cells, while the exogenous addition of cholesterol effectively blocks ZIKV replication. Furthermore, the supplementation of oleic acid increases the ZIKV titer. Fenofibrate, an inhibitor of lipid droplet formation, reduces the ZIKV titer. Collectively, our results demonstrate that the development of antiviral drugs against ZIKV could be based on key regulators of lipid metabolism. In addition, this study reveals that the mechanism of the PF-429242-mediated suppression among flavivirus infections is not entirely identical. Our results warrant further evaluation of PF-429242 as a prospective antiviral drug, given the multiple advantageous properties of this compound, such as its limited toxicity, neuroprotective properties, and broad spectrum of capabilities.

Manipulation of Host Cell Organelles by Intracellular Pathogens

Pathogenic intracellular bacteria, parasites and viruses have evolved sophisticated mechanisms to manipulate mammalian host cells to serve as niches for persistence and proliferation. The intracellular lifestyles of pathogens involve the manipulation of membrane-bound organellar compartments of host cells. In this review, we described how normal structural organization and cellular functions of endosomes, endoplasmic reticulum, Golgi apparatus, mitochondria, or lipid droplets are targeted by microbial virulence mechanisms. We focus on the specific interactions of Salmonella, Legionella pneumophila, Rickettsia rickettsii, Chlamydia spp. and Mycobacterium tuberculosis representing intracellular bacterial pathogens, and of Plasmodium spp. and Toxoplasma gondii representing intracellular parasites. The replication strategies of various viruses, i.e., Influenza A virus, Poliovirus, Brome mosaic virus, Epstein-Barr Virus, Hepatitis C virus, severe acute respiratory syndrome virus (SARS), Dengue virus, Zika virus, and others are presented with focus on the specific manipulation of the organelle compartments. We compare the specific features of intracellular lifestyle and replication cycles, and highlight the communalities in mechanisms of manipulation deployed.

Molecular Events Occurring in Lipophagy and Its Regulation in Flaviviridae Infection

Diseases caused by Flaviviridae have a wide global and economic impact due to high morbidity and mortality. Flaviviridae infection usually leads to severe, acute or chronic diseases, such as liver injury and liver cancer resulting from hepatitis C virus (HCV) infection, dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS) caused by dengue virus (DENV). Given the highly complex pathogenesis of Flaviviridae infections, they are still not fully understood at present. Accumulating evidence suggests that host autophagy is disrupted to regulate the life cycle of Flaviviridae. Organelle-specific autophagy is able to selectively target different organelles for quality control, which is essential for regulating cellular homeostasis. As an important sub process of autophagy, lipophagy regulates lipid metabolism by targeting lipid droplets (LDs) and is also closely related to the infection of a variety of pathogenic microorganisms. In this review, we briefly understand the LDs interaction relationship with Flaviviridae infection, outline the molecular events of how lipophagy occurs and the related research progress on the regulatory mechanisms of lipophagy in Flaviviridae infection. Exploring the crosstalk between viral infection and lipophagy induced molecular events may provide new avenues for antiviral therapy.

PF-429242, a Subtilisin Inhibitor, Is Effective in vitro Against Leishmania infantum

PF-429242 is an inhibitor of subtilisin, an important protease found in Leishmania. However, studies regarding the effect of PF-429242 on Leishmania are scarce. In this work we evaluated the antileishmanial effect of PF-429242 against Leishmania infantum and the mechanism involved in the death of the parasite. PF-429242 had low toxicity against mammalian cells (peritoneal macrophages) (CC₅₀ = 189.07 μM) and presented activity against L. infantum promastigotes (IC₅₀ = 2.78 μM) and intracellular amastigotes (IC₅₀ = 14.07 μM), indicating selectivity toward the parasite. Transmission electron microscopy (TEM), as well as staining of L. infantum promastigotes with MitoTracker^® Red, rhodamine 123 and MitoSOX, revealed that the mitochondria was a potential target of PF-429242. In addition, PF-429242 caused an accumulation of neutral lipids in promastigotes, which was demonstrated by Nile Red staining and TEM, and induced oxidative stress (H₂DCFDA staining). Furthermore the formation of autophagic vacuoles in L. infantum promastigotes was observed by MDC staining and TEM. However, the killing induced by PF-429242 in L. infantum promastigotes appeared to be unrelated to apoptosis and/or necrosis as there was no phosphatidylserine externalization, DNA fragmentation or alterations in the permeability of the parasite plasma membrane, as assessed by annexin V-FITC, TUNEL and propidium iodide staining, respectively. The morphological and ultrastructural evaluation of the promastigotes by optical microscopy, scanning electron microscopy (SEM) and TEM, revealed the presence of parasites with flagellar defects. TEM analysis of the intracellular amastigotes indicated that mitochondrial damage and autophagy could also be involved in the death of these forms after treatment with PF-429242. In addition, PF-429242 treatment stimulated NO production from infected macrophage, but only at a high concentration (100 μM), as well as an increase of TNF levels after treatment with 10 μM of PF-429242. The compound did not stimulate ROS or IL-10 production. Together, these data highlight the antileishmanial potential of PF-429242, inducing several cellular alterations in the parasite, such as mitochondrial damage, neutral lipids accumulation, oxidative stress and autophagy which culminate in the death of L. infantum, as well as modulating host cellular responses that favor the development of an immune response against the parasite.

Hijacking of Lipid Droplets by Hepatitis C, Dengue and Zika Viruses-From Viral Protein Moonlighting to Extracellular Release

Hijacking and manipulation of host cell biosynthetic pathways by human enveloped viruses are essential for the viral lifecycle. Flaviviridae members, including hepatitis C, dengue and Zika viruses, extensively manipulate host lipid metabolism, underlining the importance of lipid droplets (LDs) in viral infection. LDs are dynamic cytoplasmic organelles that can act as sequestration platforms for a unique subset of host and viral proteins. Transient recruitment and mobilization of proteins to LDs during viral infection impacts host-cell biological properties, LD functionality and canonical protein functions. Notably, recent studies identified LDs in the nucleus and also identified that LDs are transported extracellularly via an autophagy-mediated mechanism, indicating a novel role for autophagy in Flaviviridae infections. These developments underline an unsuspected diversity and localization of LDs and potential moonlighting functions of LD-associated proteins during infection. This review summarizes recent breakthroughs concerning the LD hijacking activities of hepatitis C, dengue and Zika viruses and potential roles of cytoplasmic, nuclear and extracellular LD-associated viral proteins during infection.

Microbes as Master Immunomodulators: Immunopathology, Cancer and Personalized Immunotherapies

The intricate interplay between the immune system and microbes is an essential part of the physiological homeostasis in health and disease. Immunological recognition of commensal microbes, such as bacterial species resident in the gut or lung as well as dormant viral species, i.e., cytomegalovirus (CMV) or Epstein-Barr virus (EBV), in combination with a balanced immune regulation, is central to achieve immune-protection. Emerging evidence suggests that immune responses primed to guard against commensal microbes may cause unexpected pathological outcomes, e.g., chronic inflammation and/or malignant transformation. Furthermore, translocation of immune cells from one anatomical compartment to another, i.e., the gut-lung axis via the lymphatics or blood has been identified as an important factor in perpetrating systemic inflammation, tissue destruction, as well as modulating host-protective immune responses. We present in this review immune response patterns to pathogenic as well as non-pathogenic microbes and how these immune-recognition profiles affect local immune responses or malignant transformation. We discuss personalized immunological therapies which, directly or indirectly, target host biological pathways modulated by antimicrobial immune responses.

Contemporary Strategies and Current Trends in Designing Antiviral Drugs against Dengue Fever via Targeting Host-Based Approaches

Dengue virus (DENV) is an arboviral human pathogen transmitted through mosquito bite that infects an estimated ~400 million humans (~5% of the global population) annually. To date, no specific therapeutics have been developed that can prevent or treat infections resulting from this pathogen. DENV utilizes numerous host molecules and factors for transcribing the single-stranded ~11 kb positive-sense RNA genome. For example, the glycosylation machinery of the host is required for viral particles to assemble in the endoplasmic reticulum. Since a variety of host factors seem to be utilized by the pathogens, targeting these factors may result in DENV inhibitors, and will play an important role in attenuating the rapid emergence of other flaviviruses. Many experimental studies have yielded findings indicating that host factors facilitate infection, indicating that the focus should be given to targeting the processes contributing to pathogenesis along with many other immune responses. Here, we provide an extensive literature review in order to elucidate the progress made in the development of host-based approaches for DENV viral infections, focusing on host cellular mechanisms and factors responsible for viral replication, aiming to aid the potential development of host-dependent antiviral therapeutics.

Development of Novel DNA-Encoded PCSK9 Monoclonal Antibodies as Lipid-Lowering Therapeutics

Elevated low-density lipoprotein cholesterol (LDL-C) is one of the major contributors to cardiovascular heart disease (CHD), the leading cause of death worldwide. Due to severe side effects of statins, alternative treatment strategies are required for statin-intolerant patients. Monoclonal antibodies (mAbs) targeting proprotein convertase subtilisin/kexin type 9 (PCSK9) have shown great efficacy in LDL-C reduction. Limitations for this approach include the need for multiple injections as well as increased costs associated with patient management. Here, we engineered a DNA-encoded mAb (DMAb) targeting PCSK9 (daPCSK9), as an alternative approach to protein-based lipid-lowering therapeutics, and we characterized its expression and activity. A single intramuscular administration of mouse daPCSK9 generated expression in vivo for over 42 days that corresponded with a substantial decrease of 28.6% in non-high-density lipoprotein cholesterol (non-HDL-C) and 10.3% in total cholesterol by day 7 in wild-type mice. Repeated administrations of the DMAb plasmid led to increasing expression, with DMAb levels of 7.5 μg/mL at day 62. daPCSK9 therapeutics may provide a novel, simple, less frequent, cost-effective approach to reducing LDL-C, either as a stand-alone therapy or in combination with other LDL-lowering therapeutics for synergistic effect.

Pharmacologic inhibition of S1P attenuates ATF6 expression, causes ER stress and contributes to apoptotic cell death

Mammalian cells express unique transcription factors embedded in the endoplasmic reticulum (ER) membrane, such as the sterol regulatory element-binding proteins (SREBPs), that promote de novo lipogenesis. Upon their release from the ER, the SREBPs require proteolytic activation in the Golgi by site-1-protease (S1P). As such, inhibition of S1P, using compounds such as PF-429242 (PF), reduces cholesterol synthesis and may represent a new strategy for the management of dyslipidemia. In addition to the SREBPs, the unfolded protein response (UPR) transducer, known as the activating transcription factor 6 (ATF6), is another ER membrane-bound transcription factor that requires S1P-mediated activation. ATF6 regulates ER protein folding capacity by promoting the expression of ER chaperones such as the 78-kDa glucose-regulated protein (GRP78). ER-resident chaperones like GRP78 prevent and/or resolve ER polypeptide accumulation and subsequent ER stress-induced UPR activation by folding nascent polypeptides. Here we report that pharmacological inhibition of S1P reduced the expression of ATF6 and GRP78 and induced the activation of UPR transducers inositol-requiring enzyme-1α (IRE1α) and protein kinase RNA-like ER kinase (PERK). As a consequence, S1P inhibition also increased the susceptibility of cells to ER stress-induced cell death. Our findings suggest that S1P plays a crucial role in the regulation of ER folding capacity and also identifies a compensatory cross-talk between UPR transducers in order to maintain adequate ER chaperone expression and activity.

Inhibition of dengue virus infection by mannoside glycolipid conjugates

Dengue virus (DENV), a mosquito-borne flavivirus, causes severe and potentially fatal symptoms in millions of infected individuals each year. Although dengue fever represents a major global public health problem, the vaccines or antiviral drugs proposed so far have not shown sufficient efficacy and safety, calling for new antiviral developments. Here we have shown that a mannoside glycolipid conjugate (MGC) bearing a trimannose head with a saturated lipid chain inhibited DENV productive infection. It showed remarkable cell promiscuity, being active in human skin dendritic cells, hepatoma cell lines and Vero cells, and was active against all four DENV serotypes, with an IC₅₀ in the low micromolar range. Time-of-addition experiments and structure-activity analyses revealed the importance of the lipid chain to interfere with an early viral infection step. This, together with a correlation between antiviral activity and membrane polarization by the lipid moiety indicated that the inhibitor functions by blocking viral envelope fusion with the endosome membrane. These finding establish MGCs as a novel class of antivirals against the DENV.

Dengue Virus and Its Inhibitors: A Brief Review

The global occurrence of viral infectious diseases poses a significant threat to human health. Dengue virus (DENV) infection is one of the most noteworthy of these infections. According to a WHO survey, approximately 400 million people are infected annually; symptoms deteriorate in approximately one percent of cases. Numerous foundational and clinical investigations on viral epidemiology, structure and function analysis, infection source and route, therapeutic targets, vaccines, and therapeutic drugs have been conducted by both academic and industrial researchers. At present, CYD-TDV or Dengvaxia^® is the only approved vaccine, but potent inhibitors are currently under development. In this review, an overview of the viral life circle and the history of DENVs is presented, and the most recently reported antiviral candidates and newly discovered promising targets are focused and summarized. We believe that these successes and failures have enabled progress in anti-DENV drug discovery and hope that our review will stimulate further innovation in this area.