Uptake of Shiga-toxigenic Escherichia coli SubAB by HeLa cells requires an actin- and lipid raft-dependent pathway

Bacterial Membrane Vesicles: Physiological Roles, Infection Immunology, and Applications

Bacterial or fungal membrane vesicles, traditionally considered as microbial metabolic wastes, are secreted mainly from the outer membrane or cell membrane of microorganisms. However, recent studies have shown that these vesicles play essential roles in direct or indirect communications among microorganisms and between microorganisms and hosts. This review aims to provide an updated understanding of the physiological functions and emerging applications of bacterial membrane vesicles, with a focus on their biogenesis, mechanisms of adsorption and invasion into host cells, immune stimulatory effects, and roles in the much-concerned problem of bacterial resistance. Additionally, the potential applications of these vesicles as biomarkers, vaccine candidates, and drug delivery platforms are discussed.

Controlled Delivery of an Anti-Inflammatory Toxin to Macrophages by Mutagenesis and Nanoparticle Modification

Advances in drug delivery systems (DDSs) have enabled the specific delivery of drugs to target cells. Subtilase cytotoxin (SubAB) produced by certain enterohemorrhagic Escherichia coli strains induces endoplasmic reticulum (ER) stress and suppresses nitric oxide generation in macrophages. We previously reported that modification of SubAB with poly(D,L-lactide-co-glycolic) acid (PLGA) nanoparticles (SubAB-PLGA NPs) increased intracellular uptake of SubAB and had an anti-inflammatory effect on macrophages. However, specific delivery of SubAB to macrophages could not be achieved because its effects on other cell types were not negligible. Therefore, to suppress non-specific SubAB binding, we used low-binding mutant SubAB_S35A (S35A) in which the 35th serine of the B subunit was mutated to alanine. In a macrophage cell line, PLGA NPs modified with S35A (S35A-PLGA NPs) induced ER stress and had anti-inflammatory effects similar to WT-PLGA NPs. However, in an epithelial cell line, S35A-PLGA NPs induced lower ER stress than WT-PLGA NPs. These results suggest that S35A is selectively delivered to macrophages rather than epithelial cells by modification with PLGA NPs and exerts anti-inflammatory effects. Our findings provide a useful technique for protein delivery to macrophages and encourage medical applications of DDSs for the treatment of inflammatory diseases.

Novel Aspects of the SubA Subunit of the Subtilase Cytotoxin

The subtilase cytotoxin (SubAB) belongs to the family of AB₅ toxins and is produced together with Shiga toxin (Stx) by certain Stx-producing E. coli strains (STEC). For most AB-type toxins, it is assumed that cytotoxic effects can only be induced by a complete holotoxin complex consisting of SubA and SubB. However, it has been shown for SubAB that the enzymatically active subunit SubA, without its transport and binding domain SubB, induces cell death in different eukaryotic cell lines. Interestingly, the molecular structure of SubA resembles that of the SubAB complex. SubA alone is capable of binding to cells and then being taken up autonomously. Once inside the host cell, SubA is transported, similar to the SubAB holotoxin, via a retrograde transport into the endoplasmatic reticulum (ER). In the ER, it exhibits its enzymatic activity by cleaving the chaperone BiP/GRP78 and thereby triggering cell death. Therefore, the existence of toxic single SubA subunits that have not found a B-pentamer for holotoxin assembly might improve the pathogenic potential of subtilase-producing strains. Moreover, from a pharmacological aspect, SubA might be an interesting molecule for the targeted transport of therapeutic molecules into the ER, in order to investigate and specifically modulate processes in the context of ER stress-associated diseases. Since recent studies on bacterial AB₅ toxins contributed mainly to the understanding of the biology of AB-type holotoxins, this mini-review specifically focus on that recently observed single A-effect of the subtilase cytotoxin and addresses whether a fundamental shift of the traditional AB₅ paradigm might be required.

A novel endoplasmic stress mediator, Kelch domain containing 7B (KLHDC7B), increased Harakiri (HRK) in the SubAB-induced apoptosis signaling pathway

Locus for Enterocyte Effacement (LEE)-positive Shiga-toxigenic Escherichia coli (STEC) contributes to many global foodborne diseases, with infection characterized by severe gastrointestinal symptoms, including bloody diarrhea. The incidence of LEE-negative STEC-mediated disease is also increasing globally. Subtilase cytotoxin (SubAB) is released by some LEE-negative STEC strains. It cleaves BiP, which is a chaperone protein located in the endoplasmic reticulum (ER), thereby causing apoptosis induced by ER stress. To date, the apoptotic signaling pathway mediated by SubAB has not been identified. In the current study, RNA-seq analysis showed that SubAB significantly induced the expression of Kelch domain containing 7B (KLHDC7B). We explored the role of KLHDC7B in the SubAB-induced apoptotic pathway. SubAB-induced KLHDC7B mRNA expression was increased after 12 h of incubation of toxin with HeLa cells. KLHDC7B expression was downregulated by knockdown of PKR-like endoplasmic reticulum kinase (PERK), CEBP homologous protein (CHOP), activating transcription factor 4 (ATF4), and CEBP β (CEBPB). KLHDC7B knockdown suppressed SubAB-stimulated CHOP expression, poly(ADP-ribose) polymerase (PARP) cleavage, and cytotoxicity. The over-expressed KLHDC7B was localized to the nucleus and cytosolic fractions. Next, we used RNA-seq to analyze the effect of KLHDC7B knockdown on apoptosis induced by SubAB, and found that the gene encoding for the pro-apoptotic Bcl-2 family protein, Harakiri (HRK), was upregulated in SubAB-treated control cells. However, this effect was not observed in SubAB-treated KLHDC7B-knockdown cells. Therefore, we identified the pathway through which SubAB-induced KLHDC7B regulates HRK expression, which is essential for apoptosis in toxin-mediated ER stress.

Subtilase cytotoxin induces a novel form of Lipocalin 2, which promotes Shiga-toxigenic Escherichia coli survival

Shiga-toxigenic Escherichia coli (STEC) infection causes severe bloody diarrhea, renal failure, and hemolytic uremic syndrome. Recent studies showed global increases in Locus for Enterocyte Effacement (LEE)-negative STEC infection. Some LEE-negative STEC produce Subtilase cytotoxin (SubAB), which cleaves endoplasmic reticulum (ER) chaperone protein BiP, inducing ER stress and apoptotic cell death. In this study, we report that SubAB induces expression of a novel form of Lipocalin-2 (LCN2), and describe its biological activity and effects on apoptotic cell death. SubAB induced expression of a novel LCN2, which was regulated by PRKR-like endoplasmic reticulum kinase via the C/EBP homologous protein pathway. SubAB-induced novel-sized LCN2 was not secreted into the culture supernatant. Increased intracellular iron level by addition of holo-transferrin or FeCl₃ suppressed SubAB-induced PARP cleavage. Normal-sized FLAG-tagged LCN2 suppressed STEC growth, but this effect was not seen in the presence of SubAB- or tunicamycin-induced unglycosylated FLAG-tagged LCN2. Our study demonstrates that SubAB-induced novel-sized LCN2 does not have anti-STEC activity, suggesting that SubAB plays a crucial role in the survival of LEE-negative STEC as well as inducing apoptosis of the host cells.

Host response to the subtilase cytotoxin produced by locus of enterocyte effacement-negative Shiga-toxigenic Escherichia coli

Shiga-toxigenic Escherichia coli (STEC) is a major bacterium responsible for disease resulting from foodborne infection, including bloody diarrhea and hemolytic uremic syndrome. STEC produces important virulence factors such as Shiga toxin (Stx) 1 and/or 2. In the STEC family, some locus of enterocyte effacement-negative STEC produce two different types of cytotoxins, namely, Stx2 and subtilase cytotoxin (SubAB). The Stx2 and SubAB cytotoxins are structurally similar and composed of one A subunit and pentamer of B subunits. The catalytically active A subunit of SubAB is a subtilase-like serine protease and specifically cleaves an endoplasmic reticulum (ER) chaperone 78-kDa glucose-regulated protein (GRP78/BiP), a monomeric ATPase that is crucial in protein folding and quality control. The B subunit binds to cell surface receptors. SubAB recognizes sialic carbohydrate-modified cell surface proteins as a receptor. After translocation into cells, SubAB is delivered to the ER, where it cleaves GRP78/BiP. SubAB-catalyzed BiP cleavage induces ER stress, which causes various cell events including inhibition of protein synthesis, suppression of nuclear factor-kappa B activation, apoptotic cell death, and stress granules formation. In this review, we describe SubAB, the SubAB receptor, and the mechanism of cell response to the toxin.

Preparation of Biodegradable PLGA-Nanoparticles Used for pH-Sensitive Intracellular Delivery of an Anti-inflammatory Bacterial Toxin to Macrophages

Poly(D,L-lactide-co-glycolic) acid (PLGA) is a synthetic copolymer that has been used to design micro/nanoparticles as a carrier for macromolecules, such as protein and nucleic acids, that can be internalized by the endocytosis pathway. However, it is difficult to control the intracellular delivery to target organelles. Here we report an intracellular delivery system of nanoparticles modified with bacterial cytotoxins to the endoplasmic reticulum (ER) and anti-inflammatory activity of the nanoparticles. Subtilase cytotoxin (SubAB) is a bacterial toxin in certain enterohemorrhagic Escherichia coli (EHEC) strains that cleaves the host ER chaperone BiP and suppresses nuclear factor-kappaB (NF-κB) activation and nitric oxide (NO) generation in macrophages at sub-lethal concentration. PLGA-nanoparticles were modified with oligo histidine-tagged (6 × His-tagged) recombinant SubAB (SubAB-PLGA) through a pH-sensitive linkage, and their translocation to the ER in macrophage cell line J774.1 cells, effects on inducible NO synthase (iNOS), and levels of tumor necrosis factor (TNF)-α cytokine induced by lipopolysaccharide (LPS) were examined. Compared with free SubAB, SubAB-PLGA was significantly effective in BiP cleavage and the induction of the ER stress marker C/EBP homologous protein (CHOP) in J774.1 cells. Furthermore, SubAB-PLGA attenuated LPS-stimulated induction of iNOS and TNF-α. Our findings provide useful information for protein delivery to macrophages and may encourage therapeutic applications of nanoparticles to the treatment of inflammatory diseases.

Endocytosis and Lack of Cytotoxicity of Alkyl-Capped Silicon Quantum Dots Prepared from Porous Silicon

Freely-dissolved silicon quantum dots were prepared by thermal hydrosilation of 1-undecene at high-porosity porous silicon under reflux in toluene. This reaction produces a suspension of alkyl-capped silicon quantum dots (alkyl SiQDs) with bright orange luminescence, a core Si nanocrystal diameter of about 2.5 nm and a total particle diameter of about 5 nm. Previous work has shown that these particles are rapidly endocytosed by malignant cell lines and have little or no acute toxicity as judged by the standard 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay for viability and the Terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) assay for apoptosis. We have extended this work to the CACO-2 cell line, an established model for the human small intestinal mucosa, and demonstrate that neither acute nor chronic (14 days) toxicity is observed as judged by cell morphology, viability, ATP production, ROS production and DNA damage (single cell gel electrophoresis) at doses of 50–200 μg mL−1. Quantitative assessment of the extent of uptake of alkyl SiQDs by CACO-2, HeLa, HepG2, and Huh7 cell lines by flow cytometry showed a wide variation. The liver cell lines (HepG2 and Huh7) were the most active and HeLa and CACO-2 showed comparable activity. Previous work has reported a cholesterol-sensitivity of the endocytosis (HeLa), which suggests a caveolin-mediated pathway. However, gene expression analysis by quantitative real–time polymerase chain reaction (RT-PCR) indicates very low levels of caveolins 1 and 2 in HepG2 and much higher levels in HeLa. The data suggest that the mechanism of endocytosis of the alkyl SiQDs is cell-line dependent.

Aggregated SOD1 causes selective death of cultured human motor neurons

Most human neurodegenerative diseases share a phenotype of neuronal protein aggregation. In Amyotrophic Lateral Sclerosis (ALS), the abundant protein superoxide dismutase (SOD1) or the TAR-DNA binding protein TDP-43 can aggregate in motor neurons. Recently, numerous studies have highlighted the ability of aggregates to spread from neuron to neuron in a prion-like fashion. These studies have typically focused on the use of neuron-like cell lines or neurons that are not normally affected by the specific aggregated protein being studied. Here, we have investigated the uptake of pre-formed SOD1 aggregates by cultures containing pluripotent stem cell-derived human motor neurons. We found that all cells take up aggregates by a process resembling fluid-phase endocytosis, just as found in earlier studies. However, motor neurons, despite taking up smaller amounts of SOD1, were much more vulnerable to the accumulating aggregates. Thus, the propagation of disease pathology depends less on selective uptake than on selective response to intracellular aggregates. We further demonstrate that anti-SOD1 antibodies, being considered as ALS therapeutics, can act by blocking the uptake of SOD1, but also by blocking the toxic effects of intracellular SOD1. This work demonstrates the importance of using disease relevant cells even in studying phenomena such as aggregate propagation.

Intracellular Delivery via Noncharged Sequence-Defined Cell-Penetrating Oligomers

Intracellular drug delivery systems are often limited by their poor serum stability and delivery efficiency. Cell-penetrating peptides (CPPs), particularly those derived from basic protein subunits, have been studied extensively in this regard and used for the delivery of a variety of cargoes in vitro. Although promising, traditional cationic CPPs have some drawbacks that hinder their therapeutic application such as rapid proteolytic degradation and undesired interactions with the biological milieu. To overcome these limitations, this article details the discovery of a new class of noncharged cell-penetrating oligoTEAs (CPOTs) that undergo extensive and rapid cellular entry across different cell lines with low cytotoxicity. CPOTs outperform a widely used CPP, R9 peptide. This new class of highly efficient noncharged macromolecular transporters are distinct from their cationic counterparts and show strong promise for the intracellular delivery of hydrophilic small-molecule therapeutics.

Protein Toxins That Utilize Gangliosides as Host Receptors

Subsets of protein toxins utilize gangliosides as host receptors. Gangliosides are preferred receptors due to their extracellular localization on the eukaryotic cell and due to their essential nature in host physiology. Glycosphingolipids, including gangliosides, are mediators of signal transduction within and between eukaryotic cells. Protein toxins possess AB structure-function organization, where the A domain encodes a catalytic function for the posttranslational modification of a host macromolecule, including proteins and nucleic acids, and a B domain, which encodes host receptor recognition, including proteins and glycosphingolipids, alone or in combination. Protein toxins use similar strategies to bind glycans by pockets and loops, generally employing hydrogen bonding and aromatic stacking to stabilize interactions with sugars. In some cases, glycan binding facilitates uptake, while in other cases, cross-linking or a second receptor is necessary to stimulate entry. The affinity that protein toxins have for host glycans is necessary for tissue targeting, but not always sufficient to cause disease. In addition to affinity for binding the glycan, the lipid moiety also plays an important role in productive uptake and tissue tropism. Upon endocytosis, the protein toxin must escape to another intracellular compartment or into cytosol to modify a host substrate, modulating host signaling, often resulting in cytotoxic or apoptotic events in the cell, and a unique morbidity for the organism. The study of protein toxins that utilize gangliosides as host receptors has illuminated numerous eukaryotic cellular processes, identified the basis for developing interventions to prevent disease through vaccines and control bacterial diseases through therapies. In addition, subsets of these protein toxins have been utilized as therapeutic agents to treat numerous human inflictions.

Mechanism of inhibition of Shiga-toxigenic Escherichia coli SubAB cytotoxicity by steroids and diacylglycerol analogues

Shiga toxigenic Escherichia coli (STEC) are responsible for a worldwide foodborne disease, which is characterized by severe bloody diarrhea and hemolytic uremic syndrome (HUS). Subtilase cytotoxin (SubAB) is a novel AB₅ toxin, which is produced by Locus for Enterocyte Effacement (LEE)-negative STEC. Cleavage of the BiP protein by SubAB induces endoplasmic reticulum (ER) stress, followed by induction of cytotoxicity in vitro or lethal severe hemorrhagic inflammation in mice. Here we found that steroids and diacylglycerol (DAG) analogues (e.g., bryostatin 1, Ingenol-3-angelate) inhibited SubAB cytotoxicity. In addition, steroid-induced Bcl-xL expression was a key step in the inhibition of SubAB cytotoxicity. Bcl-xL knockdown increased SubAB-induced apoptosis in steroid-treated HeLa cells, whereas SubAB-induced cytotoxicity was suppressed in Bcl-xL overexpressing cells. In contrast, DAG analogues suppressed SubAB activity independent of Bcl-xL expression at early time points. Addition of Shiga toxin 2 (Stx2) with SubAB to cells enhanced cytotoxicity even in the presence of steroids. In contrast, DAG analogues suppressed cytotoxicity seen in the presence of both toxins. Here, we show the mechanism by which steroids and DAG analogues protect cells against SubAB toxin produced by LEE-negative STEC.

The bile salt sodium taurocholate induces Campylobacter jejuni outer membrane vesicle production and increases OMV-associated proteolytic activity

Campylobacter jejuni, the leading cause of bacterial acute gastroenteritis worldwide, secretes an arsenal of virulence-associated proteins within outer membrane vesicles (OMVs). C. jejuni OMVs contain three serine proteases (HtrA, Cj0511, and Cj1365c) that cleave the intestinal epithelial cell (IEC) tight and adherens junction proteins occludin and E-cadherin, promoting enhanced C. jejuni adhesion to and invasion of IECs. C. jejuni OMVs also induce IECs innate immune responses. The bile salt sodium taurocholate (ST) is sensed as a host signal to coordinate the activation of virulence-associated genes in the enteric pathogen Vibrio cholerae. In this study, the effect of ST on C. jejuni OMVs was investigated. Physiological concentrations of ST do not have an inhibitory effect on C. jejuni growth until the early stationary phase. Coculture of C. jejuni with 0.1% or 0.2% (w/v) ST stimulates OMV production, increasing both lipid and protein concentrations. C. jejuni ST-OMVs possess increased proteolytic activity and exhibit a different protein profile compared to OMVs isolated in the absence of ST. ST-OMVs exhibit enhanced cytotoxicity and immunogenicity to T84 IECs and enhanced killing of Galleria mellonella larvae. ST increases the level of mRNA transcripts of the OMVs-associated serine protease genes and the cdtABC operon that encodes the cytolethal distending toxin. Coculture with ST significantly enhances the OMVs-induced cleavage of E-cadherin and occludin. C. jejuni OMVs also cleave the major endoplasmic reticulum chaperone protein BiP/GRP78 and this activity is associated with the Cj1365c protease. These data suggest that C. jejuni responds to the presence of physiological concentrations of the bile salt ST that increases OMV production and the synthesis of virulence-associated factors that are secreted within the OMVs. We propose that these events contribute to pathogenesis.

Lipopolysaccharide structure impacts the entry kinetics of bacterial outer membrane vesicles into host cells

Outer membrane vesicles are nano-sized microvesicles shed from the outer membrane of Gram-negative bacteria and play important roles in immune priming and disease pathogenesis. However, our current mechanistic understanding of vesicle-host cell interactions is limited by a lack of methods to study the rapid kinetics of vesicle entry and cargo delivery to host cells. Here, we describe a highly sensitive method to study the kinetics of vesicle entry into host cells in real-time using a genetically encoded, vesicle-targeted probe. We found that the route of vesicular uptake, and thus entry kinetics and efficiency, are shaped by bacterial cell wall composition. The presence of lipopolysaccharide O antigen enables vesicles to bypass clathrin-mediated endocytosis, which enhances both their entry rate and efficiency into host cells. Collectively, our findings highlight the composition of the bacterial cell wall as a major determinant of secretion-independent delivery of virulence factors during Gram-negative infections.

Transmembrane Pathways and Mechanisms of Rod-like Paclitaxel Nanocrystals through MDCK Polarized Monolayer

Drug nanocrystals (NCs) appear to be favorable to improving oral bioavailability of poorly water-soluble drugs as evidenced by the great success they have had in the market. However, the pathway and mechanism of drug NCs through epithelial membrane are still unclear. In an attempt to clarify their transport features, paclitaxel nanocrystals (PTX-NCs), and paclitaxel hybrid NCs with lipophilic carbocyanine dyes, were prepared and characterized as the models. The endocytosis, intracellular trafficking, paracellular transport, and transcytosis of PTX-NCs were carefully investigated with Förster resonance energy transfer (FRET) analysis, as well as a colocalization assay, flow cytometry, gene silencing, Western-blot, transepithelial electrical resistance (TEER) study and other approaches on MDCK cells. It was found that rod-like PTX-NCs could transport through the monolayer intact, and the process of endocytosis proved to be time and energy dependent. Endoplasmic reticulum (ER) and Golgi complexes were colocalized with PTX-NCs in cells, so the ER-Golgi complexes/Golgi complexes-basolateral membrane pathway may be involved in the intracellular trafficking and transcytosis of PTX-NCs. It was demonstrated here that cav-1, dynamin, and actin filament modulated the endocytosis process, and Cdc 42 regulated the transcytosis process. In addition, no paracellular transport of PTX-NCs was observed. These findings demonstrated that the rod-like nanocrystals not only enhanced the transcytosis of PTX compared with microparticles of raw drug materials but also changed the pathways of drug delivery. This study certainly provides insight for the oral absorption mechanism of nanocrystals of poorly soluble drugs.

Subtilase cytotoxin produced by locus of enterocyte effacement-negative Shiga-toxigenic Escherichia coli induces stress granule formation

Subtilase cytotoxin (SubAB) is mainly produced by locus of enterocyte effacement (LEE)-negative strains of Shiga-toxigenic Escherichia coli (STEC). SubAB cleaves an endoplasmic reticulum (ER) chaperone, BiP/Grp78, leading to induction of ER stress. This stress causes activation of ER stress sensor proteins and induction of caspase-dependent apoptosis. We found that SubAB induces stress granules (SG) in various cells. Aim of this study was to explore the mechanism by which SubAB induced SG formation. Here, we show that SubAB-induced SG formation is regulated by activation of double-stranded RNA-activated protein kinase (PKR)-like endoplasmic reticulum kinase (PERK). The culture supernatant of STEC O113:H21 dramatically induced SG in Caco2 cells, although subAB knockout STEC O113:H21 culture supernatant did not. Treatment with phorbol 12-myristate 13-acetate (PMA), a protein kinase C (PKC) activator, and lysosomal inhibitors, NH4 Cl and chloroquine, suppressed SubAB-induced SG formation, which was enhanced by PKC and PKD inhibitors. SubAB attenuated the level of PKD1 phosphorylation. Depletion of PKCδ and PKD1 by siRNA promoted SG formation in response to SubAB. Furthermore, death-associated protein 1 (DAP1) knockdown increased basal phospho-PKD1(S916) and suppressed SG formation by SubAB. However, SG formation by an ER stress inducer, Thapsigargin, was not inhibited in PMA-treated cells. Our findings show that SubAB-induced SG formation is regulated by the PERK/DAP1 signalling pathway, which may be modulated by PKCδ/PKD1, and different from the signal transduction pathway that results in Thapsigargin-induced SG formation.

Dynasore - not just a dynamin inhibitor

Dynamin is a GTPase protein that is essential for membrane fission during clathrin-mediated endocytosis in eukaryotic cells. Dynasore is a GTPase inhibitor that rapidly and reversibly inhibits dynamin activity, which prevents endocytosis. However, comparison between cells treated with dynasore and RNA interference of genes encoding dynamin, reveals evidence that dynasore reduces labile cholesterol in the plasma membrane, and disrupts lipid raft organization, in a dynamin-independent manner. To explore the role of dynamin it is important to use multiple dynamin inhibitors, alongside the use of dynamin mutants and RNA interference targeting genes encoding dynamin. On the other hand, dynasore provides an interesting tool to explore the regulation of cholesterol in plasma membranes.

Compounds with anti-influenza activity: present and future of strategies for the optimal treatment and management of influenza. Part I: Influenza life-cycle and currently available drugs

Influenza is a contagious respiratory acute viral disease characterized by a short incubation period, high fever and respiratory and systemic symptoms. The burden of influenza is very heavy. Indeed, the World Health Organization (WHO) estimates that annual epidemics affect 5-15% of the world's population, causing up to 4-5 million severe cases and from 250,000 to 500,000 deaths. In order to design anti-influenza molecules and compounds, it is important to understand the complex replication cycle of the influenza virus. Replication is achieved through various stages. First, the virus must engage the sialic acid receptors present on the free surface of the cells of the respiratory tract. The virus can then enter the cells by different routes (clathrin-mediated endocytosis or CME, caveolae-dependent endocytosis or CDE, clathrin-caveolae-independent endocytosis, or macropinocytosis). CME is the most usual pathway; the virus is internalized into an endosomal compartment, from which it must emerge in order to release its nucleic acid into the cytosol. The ribonucleoprotein must then reach the nucleus in order to begin the process of translation of its genes and to transcribe and replicate its nucleic acid. Subsequently, the RNA segments, surrounded by the nucleoproteins, must migrate to the cell membrane in order to enable viral assembly. Finally, the virus must be freed to invade other cells of the respiratory tract. All this is achieved through a synchronized action of molecules that perform multiple enzymatic and catalytic reactions, currently known only in part, and for which many inhibitory or competitive molecules have been studied. Some of these studies have led to the development of drugs that have been approved, such as Amantadine, Rimantadine, Oseltamivir, Zanamivir, Peramivir, Laninamivir, Ribavirin and Arbidol. This review focuses on the influenza life-cycle and on the currently available drugs, while potential antiviral compounds for the prevention and treatment of influenza are considered in the subsequent review.