Clathrin- and caveolae-independent entry of feline infectious peritonitis virus in monocytes depends on dynamin

Endocytosis at the maternal-fetal interface: balancing nutrient transport and pathogen defense

Endocytosis represents a category of regulated active transport mechanisms. These encompass clathrin-dependent and -independent mechanisms, as well as fluid phase micropinocytosis and macropinocytosis, each demonstrating varying degrees of specificity and capacity. Collectively, these mechanisms facilitate the internalization of cargo into cellular vesicles. Pregnancy is one such physiological state during which endocytosis may play critical roles. A successful pregnancy necessitates ongoing communication between maternal and fetal cells at the maternal-fetal interface to ensure immunologic tolerance for the semi-allogenic fetus whilst providing adequate protection against infection from pathogens, such as viruses and bacteria. It also requires transport of nutrients across the maternal-fetal interface, but restriction of potentially harmful chemicals and drugs to allow fetal development. In this context, trogocytosis, a specific form of endocytosis, plays a crucial role in immunological tolerance and infection prevention. Endocytosis is also thought to play a significant role in nutrient and toxin handling at the maternal-fetal interface, though its mechanisms remain less understood. A comprehensive understanding of endocytosis and its mechanisms not only enhances our knowledge of maternal-fetal interactions but is also essential for identifying the pathogenesis of pregnancy pathologies and providing new avenues for therapeutic intervention.

Porcine deltacoronavirus enters ST cells by clathrin-mediated endocytosis and does not require Rab5, Rab7, or Rab11

IMPORTANCE

Porcine deltacoronavirus (PDCoV) is a newly emerged enteric virus threatening pig industries worldwide. Our previous work showed that PDCoV enters porcine kidney (PK-15) cells through a caveolae-dependent pathway, but the entry mechanism for PDCoV into swine testicle (ST) cells remains unclear. Mechanisms of virus entry can be different with different virus isolates and cell types. Here, we determined that PDCoV enters ST cells via clathrin-mediated endocytosis. Additionally, we found that PDCoV entry does not require Rab5, Rab7, or Rab11. These findings provide additional understanding of the entry mechanisms of PDCoV and possible antiviral targets.

Coronavirus and the Cytoskeleton of Virus-Infected Cells

The cytoskeleton, which includes actin filaments, microtubules, and intermediate filaments, is one of the most important networks in the cell and undertakes many fundamental life activities. Among them, actin filaments are mainly responsible for maintaining cell shape and mediating cell movement, microtubules are in charge of coordinating all cargo transport within the cell, and intermediate filaments are mainly thought to guard against external mechanical pressure. In addition to this, cytoskeleton networks are also found to play an essential role in multiple viral infections. Due to the COVID-19 epidemic, including SARS-CoV-2, SARS-CoV and MERS-CoV, so many variants have caused wide public concern, that any virus infection can potentially bring great harm to human beings and society. Therefore, it is of great importance to study coronavirus infection and develop antiviral drugs and vaccines. In this chapter, we summarize in detail how the cytoskeleton responds and participates in coronavirus infection by analyzing the possibility of the cytoskeleton and its related proteins as antiviral targets, thereby providing ideas for finding more effective treatments.

Distinct Uptake Routes Participate in Silver Nanoparticle Engulfment by Earthworm and Human Immune Cells

The consequences of engineered silver nanoparticle (AgNP) exposure and cellular interaction with the immune system are poorly understood. The immunocytes of the Eisenia andrei earthworm are frequently applied in ecotoxicological studies and possess functional similarity to vertebrate macrophages. Hence, we characterized and compared the endocytosis mechanisms for the uptake of 75 nm AgNPs by earthworm coelomocytes, human THP-1 monocytes, and differentiated THP-1 (macrophage-like) cells. Our results indicate that microtubule-dependent, scavenger-receptor, and PI3K signaling-mediated macropinocytosis are utilized during AgNP engulfment by human THP-1 and differentiated THP-1 cells. However, earthworm coelomocytes employ actin-dependent phagocytosis during AgNPs uptake. In both human and earthworm immunocytes, AgNPs were located in the cytoplasm, within the endo-/lysosomes. We detected that the internalization of AgNPs is TLR/MyD88-dependent, also involving the bactericidal/permeability-increasing protein (BPI) in the case of human immunocytes. The exposure led to decreased mitochondrial respiration in human immunocytes; however, in coelomocytes, it enhanced respiratory parameters. Our findings provide more data about NP trafficking as nano-carriers in the nanomedicine field, as well as contribute to an understanding of the ecotoxicological consequences of nanoparticle exposure.

Ionophore Antibiotics Inhibit Type II Feline Coronavirus Proliferation In Vitro

Feline coronaviruses (FCoVs) infect cats worldwide and cause severe systemic diseases, such as feline infectious peritonitis (FIP). FIP has a high mortality rate, and drugs approved by the Food and Drug Administration have been ineffective for the treatment of FIP. Investigating host factors and the functions required for FCoV replication is necessary to develop effective drugs for the treatment of FIP. FCoV utilizes an endosomal trafficking system for cellular entry after binding between the viral spike (S) protein and its receptor. The cellular enzymes that cleave the S protein of FCoV to release the viral genome into the cytosol require an acidic pH optimized in the endosomes by regulating cellular ion concentrations. Ionophore antibiotics are compounds that form complexes with alkali ions to alter the endosomal pH conditions. This study shows that ionophore antibiotics, including valinomycin, salinomycin, and nigericin, inhibit FCoV proliferation in vitro in a dose-dependent manner. These results suggest that ionophore antibiotics should be investigated further as potential broad-spectrum anti-FCoV agents.

Interaction of monodispersed strontium containing bioactive glass nanoparticles with macrophages

The cellular response of murine primary macrophages to monodisperse strontium containing bioactive glass nanoparticles (SrBGNPs), with diameters of 90 ± 10 nm and a composition (mol%) of 88.8 SiO₂-1.8CaO-9.4SrO (9.4% Sr-BGNPs) was investigated for the first time. Macrophage response is critical as applications of bioactive nanoparticles will involve the nanoparticles circulating in the blood stream and macrophages will be the first cells to encounter the particles, as part of inflammatory response mechanisms. Macrophage viability and total DNA measurements were not decreased by particle concentrations of up to 250 μg/mL. The Sr-BGNPs were actively internalised by the macrophages via formation of endosome/lysosome-like vesicles bordered by a membrane inside the cells. The Sr-BGNPs degraded inside the cells, with the Ca and Sr maintained inside the silica network. When RAW264.7 cells were incubated with Sr-BGNPs, the cells were polarised towards the pro-regenerative M2 population rather than the pro-inflammatory M1 population. Sr-BGNPs are potential biocompatible vehicles for therapeutic cation delivery for applications in bone regeneration.

The role of lipids in the pathophysiology of coronavirus infections

Coronaviruses, which have been known to cause diseases in animals since the 1930s, utilize cellular components during their replication cycle. Lipids play important roles in viral infection, as coronaviruses target cellular lipids and lipid metabolism to modify their host cells to become an optimal environment for viral replication. Therefore, lipids can be considered as potential targets for the development of antiviral agents. This review provides an overview of the roles of cellular lipids in different stages of the life cycle of coronaviruses.

Involvement of adaptor proteins in clathrin-mediated endocytosis of virus entry

The first step in the initiation of effective viral infection is breaking through the cytomembrane to enter the cell. Clathrin-mediated endocytosis is a key vesicular trafficking process in which a variety of cargo molecules are transported from the outside to the inside of the cell. This process is hijacked by numerous families of enveloped or non-enveloped viruses, which use it to enter host cells, followed by trafficking to their replicating sites. Various adaptor proteins that assist in cargo selection, coat assembly, and clathrin-coated bud maturation are important in this process. Research data documented on the involvement of adaptor proteins, such as AP-2, Eps-15, Epsin1, and AP180/CALM, in the invasion of viruses via the clathrin-mediated endocytosis have provided novel insights into understanding the viral life cycle and have led to the development of novel therapeutics. Here, we summarize the latest discoveries on the role of these adaptor proteins in clathrin-mediated endocytosis of virus entry and also discuss the future trends in this field.

Bacterial Engulfment Mechanism Is Strongly Conserved in Evolution Between Earthworm and Human Immune Cells

Invertebrates, including earthworms, are applied to study the evolutionarily conserved cellular immune processes. Earthworm immunocytes (so-called coelomocytes) are functionally similar to vertebrate myeloid cells and form the first line of defense against invading pathogens. Hereby, we compared the engulfment mechanisms of THP-1 human monocytic cells, differentiated THP-1 (macrophage-like) cells, and Eisenia andrei coelomocytes towards Escherichia coli and Staphylococcus aureus bacteria applying various endocytosis inhibitors [amantadine, 5-(N-ethyl-N-isopropyl) amiloride, colchicine, cytochalasin B, cytochalasin D, methyl-ß-cyclodextrin, and nystatin]. Subsequently, we investigated the messenger RNA (mRNA) expressions of immune receptor-related molecules (TLR, MyD88, BPI) and the colocalization of lysosomes with engulfed bacteria following uptake inhibition in every cell type. Actin depolymerization by cytochalasin B and D has strongly inhibited the endocytosis of both bacterial strains in the studied cell types, suggesting the conserved role of actin-dependent phagocytosis. Decreased numbers of colocalized lysosomes/bacteria supported these findings. In THP-1 cells TLR expression was increased upon cytochalasin D pretreatment, while this inhibitor caused a dropped LBP/BPI expression in differentiated THP-1 cells and coelomocytes. The obtained data reveal further insights into the evolution of phagocytes in eukaryotes. Earthworm and human phagocytes possess analogous mechanisms for bacterial internalization.

Feline Coronavirus Antivirals: A Review

Feline coronaviruses (FCoV) are common viral pathogens of cats. They usually induce asymptomatic infections but some FCoV strains, named Feline Infectious Peritonitis Viruses (FIPV) lead to a systematic fatal disease, the feline infectious peritonitis (FIP). While no treatments are approved as of yet, numerous studies have been explored with the hope to develop therapeutic compounds. In recent years, two novel molecules (GS-441524 and GC376) have raised hopes given the encouraging results, but some concerns about the use of these molecules persist, such as the fear of the emergence of viral escape mutants or the difficult tissue distribution of these antivirals in certain affected organs. This review will summarize current findings and leads in the development of antiviral therapy against FCoV both in vitro and in vivo, with the description of their mechanisms of action when known. It highlights the molecules, which could have a broader effect on different coronaviruses. In the context of the SARS-CoV-2 pandemic, the development of antivirals is an urgent need and FIP could be a valuable model to help this research area.

Studying SARS-CoV-2 infectivity and therapeutic responses with complex organoids

Clinical management of patients with severe complications of COVID-19 has been hindered by a lack of effective drugs and a failure to capture the extensive heterogeneity of the disease with conventional methods. Here we review the emerging roles of complex organoids in the study of SARS-CoV-2 infection, modelling of COVID-19 disease pathology and in drug, antibody and vaccine development. We discuss opportunities for COVID-19 research and remaining challenges in the application of organoids.

Coronaviruses, cholesterol and statins: Involvement and application for Covid-19

The infectious power of coronaviruses is dependent on cholesterol present in the membranes of their target cells. Indeed, the virus enters the infected cell either by fusion or by endocytosis, in both cases involving cholesterol-enriched membrane microdomains. These membrane domains can be disorganized in-vitro by various cholesterol-altering agents, including statins that inhibit cell cholesterol biosynthesis. As a consequence, numerous cell physiology processes, such as signaling cascades, can be compromised. Also, some examples of anti-bacterial and anti-viral effects of statins have been observed for infectious agents known to be cholesterol dependent. In-vivo, besides their widely-reported hypocholesterolemic effect, statins display various pleiotropic effects mediated, at least partially, by perturbation of membrane microdomains as a consequence of the alteration of endogenous cholesterol synthesis. It should thus be worth considering a high, but clinically well-tolerated, dose of statin to treat Covid-19 patients, in the early phase of infection, to inhibit virus entry into the target cells, in order to control the viral charge and hence avoid severe clinical complications. Based on its efficacy and favorable biodisposition, an option would be considering Atorvastatin, but randomized controlled clinical trials are required to test this hypothesis. This new therapeutic proposal takes benefit from being a drug repurposing, applied to a widely-used drug presenting a high efficiency-to-toxicity ratio. Additionally, this therapeutic strategy avoids any risk of drug resistance by viral mutation since it is host-targeted. Noteworthy, the same pharmacological approach could also be proposed to address different animal coronavirus endemic infections that are responsible for heavy economic losses.

Cytoskeleton-a crucial key in host cell for coronavirus infection

The emerging coronavirus (CoV) pandemic is threatening the public health all over the world. Cytoskeleton is an intricate network involved in controlling cell shape, cargo transport, signal transduction, and cell division. Infection biology studies have illuminated essential roles for cytoskeleton in mediating the outcome of host‒virus interactions. In this review, we discuss the dynamic interactions between actin filaments, microtubules, intermediate filaments, and CoVs. In one round of viral life cycle, CoVs surf along filopodia on the host membrane to the entry sites, utilize specific intermediate filament protein as co-receptor to enter target cells, hijack microtubules for transportation to replication and assembly sites, and promote actin filaments polymerization to provide forces for egress. During CoV infection, disruption of host cytoskeleton homeostasis and modification state is tightly connected to pathological processes, such as defective cytokinesis, demyelinating, cilia loss, and neuron necrosis. There are increasing mechanistic studies on cytoskeleton upon CoV infection, such as viral protein‒cytoskeleton interaction, changes in the expression and post-translation modification, related signaling pathways, and incorporation with other host factors. Collectively, these insights provide new concepts for fundamental virology and the control of CoV infection.

Membrane Rafts: Portals for Viral Entry

Membrane rafts are dynamic, small (10-200 nm) domains enriched with cholesterol and sphingolipids that compartmentalize cellular processes. Rafts participate in roles essential to the lifecycle of different viral families including virus entry, assembly and/or budding events. Rafts seem to participate in virus attachment and recruitment to the cell surface, as well as the endocytic and non-endocytic mechanisms some viruses use to enter host cells. In this review, we will introduce the specific role of rafts in viral entry and define cellular factors implied in the choice of one entry pathway over the others. Finally, we will summarize the most relevant information about raft participation in the entry process of enveloped and non-enveloped viruses.

Kallikrein 13 serves as a priming protease during infection by the human coronavirus HKU1

Unlike SARS-CoV-2, the human coronavirus HKU1 normally causes relatively mild respiratory tract infections; however, it shares with SARS-CoV-2 the mechanism of using its surface spike (S) protein to enter target cells. Because the host receptor for HCoV-HKU1 is unknown, efforts to study the virus in cell culture systems have proved difficult. Milewska et al. found that knockout of the protease kallikrein 13 (KLK13) in human airway epithelial cells blocked their infection by HCoV-HKU1, that overexpression of KLK13 in nonpermissive cells enabled their infection by the virus, and that KLK13 cleaved the viral S protein. Together, these findings suggest that KLK13 is a priming enzyme for viral entry and may help to establish cell lines that can facilitate further investigation of the mechanism of viral pathogenesis.

Human coronavirus HKU1 (HCoV-HKU1) is associated with respiratory disease and is prevalent worldwide, but an in vitro model for viral replication is lacking. An interaction between the coronaviral spike (S) protein and its receptor is the primary determinant of tissue and host specificity; however, viral entry is a complex process requiring the concerted action of multiple cellular elements. Here, we found that the protease kallikrein 13 (KLK13) was required for the infection of human respiratory epithelial cells and was sufficient to mediate the entry of HCoV-HKU1 into nonpermissive RD cells. We also demonstrated the cleavage of the HCoV-HKU1 S protein by KLK13 in the S1/S2 region, suggesting that KLK13 is the priming enzyme for this virus. Together, these data suggest that protease distribution and specificity determine the tissue and cell specificity of the virus and may also regulate interspecies transmission.

Coronavirus Infections in Companion Animals: Virology, Epidemiology, Clinical and Pathologic Features

Coronaviruses are enveloped RNA viruses capable of causing respiratory, enteric, or systemic diseases in a variety of mammalian hosts that vary in clinical severity from subclinical to fatal. The host range and tissue tropism are largely determined by the coronaviral spike protein, which initiates cellular infection by promoting fusion of the viral and host cell membranes. Companion animal coronaviruses responsible for causing enteric infection include feline enteric coronavirus, ferret enteric coronavirus, canine enteric coronavirus, equine coronavirus, and alpaca enteric coronavirus, while canine respiratory coronavirus and alpaca respiratory coronavirus result in respiratory infection. Ferret systemic coronavirus and feline infectious peritonitis virus, a mutated feline enteric coronavirus, can lead to lethal immuno-inflammatory systemic disease. Recent human viral pandemics, including severe acute respiratory syndrome (SARS), Middle East respiratory syndrome (MERS), and most recently, COVID-19, all thought to originate from bat coronaviruses, demonstrate the zoonotic potential of coronaviruses and their potential to have devastating impacts. A better understanding of the coronaviruses of companion animals, their capacity for cross-species transmission, and the sharing of genetic information may facilitate improved prevention and control strategies for future emerging zoonotic coronaviruses. This article reviews the clinical, epidemiologic, virologic, and pathologic characteristics of nine important coronaviruses of companion animals.

Dominant role of splenic marginal zone lipid rafts in the classical complement pathway against S. pneumoniae

Lipid rafts (LRs) play crucial roles in complex physiological processes, modulating innate and acquired immune responses to pathogens. The transmembrane C-type lectins human dendritic cell-specific intercellular adhesion molecule-3-grabbing nonintegrin (DC-SIGN) and its mouse homolog SIGN-R1 are distributed in LRs and expressed on splenic marginal zone (MZ) macrophages. The DC-SIGN-C1q or SIGN-R1-C1q complex could mediate the immunoglobulin (Ig)-independent classical complement pathway against Streptococcus pneumoniae. Precise roles of LRs during this complement pathway are unknown. Here we show that LRs are indispensable for accelerating the DC-SIGN- or SIGN-R1-mediated classical complement pathway against S. pneumoniae, thus facilitating rapid clearance of the pathogen. The trimolecular complex of SIGN-R1-C1q-C4 was exclusively enriched in LRs of splenic MZ macrophages and their localization was essential for activating C3 catabolism and enhancing pneumococcal clearance, which were abolished in SIGN-R1-knockout mice. However, DC-SIGN replacement on splenic MZ macrophage’s LRs of SIGN-R1-depleted mice reversed these defects. Disruption of LRs dramatically reduced pneumococcal uptake and decomposition. Additionally, DC- SIGN, C1q, C4, and C3 were obviously distributed in splenic LRs of cadavers. Therefore, LRs on splenic SIGN-R1⁺ or DC-SIGN⁺ macrophages could provide spatially confined and optimal bidirectional platforms, not only for usual intracellular events, for example recognition and phagocytosis of pathogens, but also an unusual extracellular event such as the complement system. These findings improve our understanding of the orchestrated roles of the spleen, unraveling a new innate immune system initiated from splenic MZ LRs, and yielding answers to several long-standing problems, including the need to understand the profound role of LRs in innate immunity, the need to identify how such a small portion of splenic SIGN-R1⁺ macrophages (<0.05% of splenic macrophages) effectively resist S. pneumoniae, and the need to understand how LRs can promote the protective function of DC-SIGN against S. pneumoniae in the human spleen.

Structural insights into coronavirus entry

Coronaviruses (CoVs) have caused outbreaks of deadly pneumonia in humans since the beginning of the 21st century. The severe acute respiratory syndrome coronavirus (SARS-CoV) emerged in 2002 and was responsible for an epidemic that spread to five continents with a fatality rate of 10% before being contained in 2003 (with additional cases reported in 2004). The Middle-East respiratory syndrome coronavirus (MERS-CoV) emerged in the Arabian Peninsula in 2012 and has caused recurrent outbreaks in humans with a fatality rate of 35%. SARS-CoV and MERS-CoV are zoonotic viruses that crossed the species barrier using bats/palm civets and dromedary camels, respectively. No specific treatments or vaccines have been approved against any of the six human coronaviruses, highlighting the need to investigate the principles governing viral entry and cross-species transmission as well as to prepare for zoonotic outbreaks which are likely to occur due to the large reservoir of CoVs found in mammals and birds. Here, we review our understanding of the infection mechanism used by coronaviruses derived from recent structural and biochemical studies.

Infectious bronchitis virus entry mainly depends on clathrin mediated endocytosis and requires classical endosomal/lysosomal system

Although several reports suggest that the entry of infectious bronchitis virus (IBV) depends on lipid rafts and low pH, the endocytic route and intracellular trafficking are unclear. In this study, we aimed to shed greater light on early steps in IBV infection. By using chemical inhibitors, RNA interference, and dominant negative mutants, we observed that lipid rafts and low pH was indeed required for virus entry; IBV mainly utilized the clathrin mediated endocytosis (CME) for entry; GTPase dynamin 1 was involved in virus containing vesicle scission; and the penetration of IBV into cells led to active cytoskeleton rearrangement. By using R18 labeled virus, we found that virus particles moved along with the classical endosome/lysosome track. Functional inactivation of Rab5 and Rab7 significantly inhibited IBV infection. Finally, by using dual R18/DiOC labeled IBV, we observed that membrane fusion was induced after 1 h.p.i. in late endosome/lysosome.

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Intact lipid rafts is involved in IBV entry.

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Low pH in intracyplasmic vesicles is required for IBV entry.

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IBV penetrates cells via clathrin mediated endocytosis.

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IBV moves along with the classical endosome/lysosome track, finally fuses with late endosome/lysosome.

Porcine sapovirus Cowden strain enters LLC-PK cells via clathrin- and cholesterol-dependent endocytosis with the requirement of dynamin II

Caliciviruses in the genus Sapovirus are a significant cause of viral gastroenteritis in humans and animals. However, the mechanism of their entry into cells is not well characterized. Here, we determined the entry mechanism of porcine sapovirus (PSaV) strain Cowden into permissive LLC-PK cells. The inhibition of clathrin-mediated endocytosis using chlorpromazine, siRNAs, and a dominant negative (DN) mutant blocked entry and infection of PSaV Cowden strain, confirming a role for clathrin-mediated internalization. Entry and infection were also inhibited by the cholesterol-sequestering drug methyl-β-cyclodextrin and was restored by the addition of soluble cholesterol, indicating that cholesterol also contributes to entry and infection of this strain. Furthermore, the inhibition of dynamin GTPase activity by dynasore, siRNA depletion of dynamin II, or overexpression of a DN mutant of dynamin II reduced the entry and infection, suggesting that dynamin mediates the fission and detachment of clathrin- and cholesterol-pits for entry of this strain. In contrast, the inhibition of caveolae-mediated endocytosis using nystatin, siRNAs, or a DN mutant had no inhibitory effect on entry and infection of this strain. It was further determined that cell entry of PSaV Cowden strain required actin rearrangements for vesicle internalization, endosomal trafficking from early to late endosomes through microtubules, and late endosomal acidification for uncoating. We conclude that PSaV strain Cowden is internalized into LLC-PK cells by clathrin- and cholesterol-mediated endocytosis that requires dynamin II and actin rearrangement, and that the uncoating occurs in the acidified late endosomes after trafficking from the early endosomes through microtubules.

Canine respiratory coronavirus employs caveolin-1-mediated pathway for internalization to HRT-18G cells

Canine respiratory coronavirus (CRCoV), identified in 2003, is a member of the Coronaviridae family. The virus is a betacoronavirus and a close relative of human coronavirus OC43 and bovine coronavirus. Here, we examined entry of CRCoV into human rectal tumor cells (HRT-18G cell line) by analyzing co-localization of single virus particles with cellular markers in the presence or absence of chemical inhibitors of pathways potentially involved in virus entry. We also targeted these pathways using siRNA. The results show that the virus hijacks caveolin-dependent endocytosis to enter cells via endocytic internalization.

Nanoparticulate vacuolar ATPase blocker exhibits potent host-targeted antiviral activity against feline coronavirus

Feline infectious peritonitis (FIP), caused by a mutated feline coronavirus, is one of the most serious and fatal viral diseases in cats. The disease remains incurable, and there is no effective vaccine available. In light of the pathogenic mechanism of feline coronavirus that relies on endosomal acidification for cytoplasmic entry, a novel vacuolar ATPase blocker, diphyllin, and its nanoformulation are herein investigated for their antiviral activity against the type II feline infectious peritonitis virus (FIPV). Experimental results show that diphyllin dose-dependently inhibits endosomal acidification in fcwf-4 cells, alters the cellular susceptibility to FIPV, and inhibits the downstream virus replication. In addition, diphyllin delivered by polymeric nanoparticles consisting of poly(ethylene glycol)-block-poly(lactide-co-glycolide) (PEG-PLGA) further demonstrates an improved safety profile and enhanced inhibitory activity against FIPV. In an in vitro model of antibody-dependent enhancement of FIPV infection, diphyllin nanoparticles showed a prominent antiviral effect against the feline coronavirus. In addition, the diphyllin nanoparticles were well tolerated in mice following high-dose intravenous administration. This study highlights the therapeutic potential of diphyllin and its nanoformulation for the treatment of FIP.

The cholesterol transport inhibitor U18666A inhibits type I feline coronavirus infection

Feline infectious peritonitis (FIP) is a feline coronavirus (FCoV)-induced fatal disease in wild and domestic cats. FCoV exists in two serotypes. Type I FCoV is the dominant serotype worldwide. Therefore, it is necessary to develop antiviral drugs against type I FCoV infection. We previously reported that type I FCoV is closely associated with cholesterol throughout the viral life cycle. In this study, we investigated whether U18666A, the cholesterol synthesis and transport inhibitor, shows antiviral effects against type I FCoV. U18666A induced cholesterol accumulation in cells and inhibited type I FCoV replication. Surprisingly, the antiviral activity of U18666A was suppressed by the histone deacetylase inhibitor (HDACi), Vorinostat. HDACi has been reported to revert U18666A-induced dysfunction of Niemann-Pick C1 (NPC1). In conclusion, these findings demonstrate that NPC1 plays an important role in type I FCoV infection. U18666A or other cholesterol transport inhibitor may be considered as the antiviral drug for the treatment of cats with FIP.

Apoptosis transcriptional mechanism of feline infectious peritonitis virus infected cells

Apoptosis has been postulated to play an important role during feline infectious peritonitis virus (FIPV) infection; however, its mechanism is not well characterized. This study is focused on apoptosis and transcriptional profiling of FIPV-infected cells following in vitro infection of CRFK cells with FIPV 79-1146 WSU. Flow cytometry was used to determine mode of cell death in first 42 h post infection (hpi). FIPV infected cells underwent early apoptosis at 9 hpi (p < 0.05) followed by late apoptosis at 12 hpi (p < 0.05) and necrosis from 24 hpi (p < 0.05). Then, next generation sequencing was performed on 9 hpi and control uninfected cells by Illumina analyzer. An aggregate of 4546 genes (2229 down-regulated and 2317 up-regulated) from 17 cellular process, 11 molecular functions and 130 possible biological pathways were affected by FIPV. 131 genes from apoptosis cluster (80 down-regulated and 51 up-regulated) along with increase of apoptosis, p53, p38 MAPK, VEGF and chemokines/cytokines signaling pathways were probably involved in apoptosis process. Six of the de-regulated genes expression (RASSF1, BATF2, MAGEB16, PDCD5, TNFα and TRAF2) and TNFα protein concentration were analyzed by RT-qPCR and ELISA, respectively, at different time-points. Up-regulations of both pro-apoptotic (i.e. PDCD5) and anti-apoptotic (i.e. TRAF2) were detected from first hpi and continuing to deregulate during apoptosis process in the infected cells.

Differential effect of cholesterol on type I and II feline coronavirus infection

Feline infectious peritonitis (FIP) is a fatal disease of domestic and wild felidae that is caused by feline coronavirus (FCoV). FCoV has been classified into types I and II. Since type I FCoV infection is dominant in the field, it is necessary to develop antiviral agents and vaccines against type I FCoV infection. However, few studies have been conducted on type I FCoV. Here, we compare the effects of cholesterol on types I and II FCoV infections. When cells were treated methyl-β-cyclodextrin (MβCD) and inoculated with type I FCoV, the infection rate decreased significantly, and the addition of exogenous cholesterol to MβCD-treated cells resulted in the recovery of the infectivity of type I FCoV. Furthermore, exogenous cholesterol increased the infectivity of type I FCoV. In contrast, the addition of MβCD and exogenous cholesterol had little effect on the efficiency of type II FCoV infection. These results strongly suggest that the dependence of infection by types I and II FCoV on cholesterol differs.

ATP1A1-mediated Src signaling inhibits coronavirus entry into host cells

In addition to transporting ions, the multisubunit Na(+),K(+)-ATPase also functions by relaying cardiotonic steroid (CTS)-binding-induced signals into cells. In this study, we analyzed the role of Na(+),K(+)-ATPase and, in particular, of its ATP1A1 α subunit during coronavirus (CoV) infection. As controls, the vesicular stomatitis virus (VSV) and influenza A virus (IAV) were included. Using gene silencing, the ATP1A1 protein was shown to be critical for infection of cells with murine hepatitis virus (MHV), feline infectious peritonitis virus (FIPV), and VSV but not with IAV. Lack of ATP1A1 did not affect virus binding to host cells but resulted in inhibited entry of MHV and VSV. Consistently, nanomolar concentrations of the cardiotonic steroids ouabain and bufalin, which are known not to affect the transport function of Na(+),K(+)-ATPase, inhibited infection of cells with MHV, FIPV, Middle East respiratory syndrome (MERS)-CoV, and VSV, but not IAV, when the compounds were present during virus inoculation. Cardiotonic steroids were shown to inhibit entry of MHV at an early stage, resulting in accumulation of virions close to the cell surface and, as a consequence, in reduced fusion. In agreement with an early block in infection, the inhibition of VSV by CTSs could be bypassed by low-pH shock. Viral RNA replication was not affected when these compounds were added after virus entry. The antiviral effect of ouabain could be relieved by the addition of different Src kinase inhibitors, indicating that Src signaling mediated via ATP1A1 plays a crucial role in the inhibition of CoV and VSV infections.

IMPORTANCE

Coronaviruses (CoVs) are important pathogens of animals and humans, as demonstrated by the recent emergence of new human CoVs of zoonotic origin. Antiviral drugs targeting CoV infections are lacking. In the present study, we show that the ATP1A1 subunit of Na(+),K(+)-ATPase, an ion transporter and signaling transducer, supports CoV infection. Targeting ATP1A1 either by gene silencing or by low concentrations of the ATP1A1-binding cardiotonic steroids ouabain and bufalin resulted in inhibition of infection with murine, feline, and MERS-CoVs at an early entry stage. Infection with the control virus VSV was also inhibited. Src signaling mediated by ATP1A1 was shown to play a crucial role in the inhibition of virus entry by ouabain and bufalin. These results suggest that targeting the Na(+),K(+)-ATPase using cardiotonic steroids, several of which are FDA-approved compounds, may be an attractive therapeutic approach against CoV and VSV infections.

Coronavirus cell entry occurs through the endo-/lysosomal pathway in a proteolysis-dependent manner

Enveloped viruses need to fuse with a host cell membrane in order to deliver their genome into the host cell. While some viruses fuse with the plasma membrane, many viruses are endocytosed prior to fusion. Specific cues in the endosomal microenvironment induce conformational changes in the viral fusion proteins leading to viral and host membrane fusion. In the present study we investigated the entry of coronaviruses (CoVs). Using siRNA gene silencing, we found that proteins known to be important for late endosomal maturation and endosome-lysosome fusion profoundly promote infection of cells with mouse hepatitis coronavirus (MHV). Using recombinant MHVs expressing reporter genes as well as a novel, replication-independent fusion assay we confirmed the importance of clathrin-mediated endocytosis and demonstrated that trafficking of MHV to lysosomes is required for fusion and productive entry to occur. Nevertheless, MHV was shown to be less sensitive to perturbation of endosomal pH than vesicular stomatitis virus and influenza A virus, which fuse in early and late endosomes, respectively. Our results indicate that entry of MHV depends on proteolytic processing of its fusion protein S by lysosomal proteases. Fusion of MHV was severely inhibited by a pan-lysosomal protease inhibitor, while trafficking of MHV to lysosomes and processing by lysosomal proteases was no longer required when a furin cleavage site was introduced in the S protein immediately upstream of the fusion peptide. Also entry of feline CoV was shown to depend on trafficking to lysosomes and processing by lysosomal proteases. In contrast, MERS-CoV, which contains a minimal furin cleavage site just upstream of the fusion peptide, was negatively affected by inhibition of furin, but not of lysosomal proteases. We conclude that a proteolytic cleavage site in the CoV S protein directly upstream of the fusion peptide is an essential determinant of the intracellular site of fusion.

Enveloped viruses need to fuse with a host cell membrane in order to deliver their genome into the host cell. In the present study we investigated the entry of coronaviruses (CoVs). CoVs are important pathogens of animals and man with high zoonotic potential as demonstrated by the emergence of SARS- and MERS-CoVs. Previous studies resulted in apparently conflicting results with respect to CoV cell entry, particularly regarding the fusion-activating requirements of the CoV S protein. By combining cell-biological, infection, and fusion assays we demonstrated that murine hepatitis virus (MHV), a prototypic member of the CoV family, enters cells via clathrin-mediated endocytosis. Moreover, although MHV does not depend on a low pH for fusion, the virus was shown to rely on trafficking to lysosomes for proteolytic cleavage of its spike (S) protein and membrane fusion to occur. Based on these results we predicted and subsequently demonstrated that MERS- and feline CoV require cleavage by different proteases and escape the endo/lysosomal system from different compartments. In conclusion, we elucidated the MHV entry pathway in detail and demonstrate that a proteolytic cleavage site in the S protein of different CoVs is an essential determinant of the intracellular site of fusion.

An update on feline infectious peritonitis: virology and immunopathogenesis

Feline infectious peritonitis (FIP) continues to be one of the most researched infectious diseases of cats. The relatively high mortality of FIP, especially for younger cats from catteries and shelters, should be reason enough to stimulate such intense interest. However, it is the complexity of the disease and the grudging manner in which it yields its secrets that most fascinate researchers. Feline leukemia virus infection was conquered in less than two decades and the mysteries of feline immunodeficiency virus were largely unraveled in several years. After a half century, FIP remains one of the last important infections of cats for which we have no single diagnostic test, no vaccine and no definitive explanations for how virus and host interact to cause disease. How can a ubiquitous and largely non-pathogenic enteric coronavirus transform into a highly lethal pathogen? What are the interactions between host and virus that determine both disease form (wet or dry) and outcome (death or resistance)? Why is it so difficult, and perhaps impossible, to develop a vaccine for FIP? What role do genetics play in disease susceptibility? This review will explore research conducted over the last 5 years that attempts to answer these and other questions. Although much has been learned about FIP in the last 5 years, the ultimate answers remain for yet more studies.

Clathrin- and serine proteases-dependent uptake of porcine epidemic diarrhea virus into Vero cells

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PEDV enters Vero cells via clathrin-mediated endocytosis.

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PEDV entry in Vero cells is low pH sensitive.

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PEDV requires serine proteolysis during entry.

Porcine epidemic diarrhea virus (PEDV), a member of the genus Alphacoronavirus, is a causative agent of porcine enteric disease characterized by acute watery diarrhea and dehydration in sucking piglet. Similar to other coronaviruses, PEDV spike protein mediates its cell entry by binding to cellular receptors and inducing membrane fusion between viral envelopes and cellular membranes. However, the entry mechanism of PEDV is not studied. Here, we determined the entry mechanism of PEDV into Vero cells. Our data confirmed that PEDV entry followed clathrin-mediated endocytosis independence of caveolae-coated pit assembly. The internalized PEDV was co-localized with the clathrin-mediated endocytic marker, but not with the caveolae-mediated endocytic marker. In addition, cells treated with lysosomotropic agents and serine protease inhibitors were resistant to PEDV. Our data revealed that PEDV entry followed clathrin-mediated endocytosis and was dependent on a low pH and serine proteolysis for successful entry into cells.

Prolonged activity of the pestiviral RNase Erns as an interferon antagonist after uptake by clathrin-mediated endocytosis

The RNase activity of the envelope glycoprotein E(rns) of the pestivirus bovine viral diarrhea virus (BVDV) is required to block type I interferon (IFN) synthesis induced by single-stranded RNA (ssRNA) and double-stranded RNA (dsRNA) in bovine cells. Due to the presence of an unusual membrane anchor at its C terminus, a significant portion of E(rns) is also secreted. In addition, a binding site for cell surface glycosaminoglycans is located within the C-terminal region of E(rns). Here, we show that the activity of soluble E(rns) as an IFN antagonist is not restricted to bovine cells. Extracellularly applied E(rns) protein bound to cell surface glycosaminoglycans and was internalized into the cells within 1 h of incubation by an energy-dependent mechanism that could be blocked by inhibitors of clathrin-dependent endocytosis. E(rns) mutants that lacked the C-terminal membrane anchor retained RNase activity but lost most of their intracellular activity as an IFN antagonist. Surprisingly, once taken up into the cells, E(rns) remained active and blocked dsRNA-induced IFN synthesis for several days. Thus, we propose that E(rns) acts as an enzymatically active decoy receptor that degrades extracellularly added viral RNA mainly in endolysosomal compartments that might otherwise activate intracellular pattern recognition receptors (PRRs) in order to maintain a state of innate immunotolerance.

IMPORTANCE

The pestiviral RNase E(rns) was previously shown to inhibit viral ssRNA- and dsRNA-induced interferon (IFN) synthesis. However, the localization of E(rns) at or inside the cells, its species specificity, and its mechanism of interaction with cell membranes in order to block the host's innate immune response are still largely unknown. Here, we provide strong evidence that the pestiviral RNase E(rns) is taken up within minutes by clathrin-mediated endocytosis and that this uptake is mostly dependent on the glycosaminoglycan binding site located within the C-terminal end of the protein. Remarkably, the inhibitory activity of E(rns) remains for several days, indicating the very potent and prolonged effect of a viral IFN antagonist. This novel mechanism of an enzymatically active decoy receptor that degrades a major viral pathogen-associated molecular pattern (PAMP) might be required to efficiently maintain innate and, thus, also adaptive immunotolerance, and it might well be relevant beyond the bovine species.

Tropism-modified AAV vectors overcome barriers to successful cutaneous therapy

Autologous human keratinocytes (HK) forming sheet grafts are approved as skin substitutes. Genetic engineering of HK represents a promising technique to improve engraftment and survival of transplants. Although efficacious in keratinocyte-directed gene transfer, retro-/lentiviral vectors may raise safety concerns when applied in regenerative medicine. We therefore optimized adeno-associated viral (AAV) vectors of the serotype 2, characterized by an excellent safety profile, but lacking natural tropism for HK, through capsid engineering. Peptides, selected by AAV peptide display, engaged novel receptors that increased cell entry efficiency by up to 2,500-fold. The novel targeting vectors transduced HK with high efficiency and a remarkable specificity even in mixed cultures of HK and feeder cells. Moreover, differentiated keratinocytes in organotypic airlifted three-dimensional cultures were transduced following topical vector application. By exploiting comparative gene analysis we further succeeded in identifying αvβ8 integrin as a target receptor thus solving a major challenge of directed evolution approaches and describing a promising candidate receptor for cutaneous gene therapy.

Monodispersed bioactive glass submicron particles and their effect on bone marrow and adipose tissue-derived stem cells

Spherical monodispersed bioactive particles are potential candidates for nanocomposite synthesis or as injectable particles that could be internalized by cells for the local sustained delivery of inorganic therapeutic ions (e.g., calcium or strontium). Particles are also likely to be released from porous bioactive glass and sol-gel hybrid scaffolds as they degrade; thus, it is vital to investigate their interaction with cells. Spherical monodispersed bioactive glass particles (mono-SMBG), with diameters of 215 ± 20 nm are synthesized using a modified Stöber process. Confocal and transmission electron microscopy demonstrate that mono-SMBGs are internalized by human bone marrow (MSCs) and adipose-derived stem cells (ADSCs) and located within cell vesicles and in the cytoplasm. Particle dissolution inside the cells is observed. Alamar Blue, MTT and Cyquant assays demonstrate that 50 μg mL(-1) of mono-SMBGs did not inhibit significantly MSC or ADSC metabolic activity. However, at higher concentrations (100 and 200 μg mL(-1)) small decrease in metabolic activity and total DNA is observed. Mono-SMBG did not induce ALPase activity, an early marker of osteogenic differentiation, without osteogenic supplements; however, in their presence osteogenic differentiation is achieved. Additionally, large numbers of particles are internalized by the cells but have little effect on cell behavior.

Multiple internalization pathways of polyelectrolyte multilayer capsules into mammalian cells

Polyelectrolyte multilayer (PEM) capsules are carrier vehicles with great potential for biomedical applications. With the future aim of designing biocompatible, effective therapeutic delivery systems (e.g., for cancer), the pathway of internalization (uptake and fate) of PEM capsules was investigated. In particular the following experiments were performed: (i) the study of capsule co-localization with established endocytic markers, (ii) switching-off endocytotic pathways with pharmaceutical/chemical inhibitors, and (iii) characterization and quantification of capsule uptake with confocal and electron microscopy. As result, capsules co-localized with lipid rafts and with phagolysosomes, but not with other endocytic vesicles. Chemical interference of endocytosis with chemical blockers indicated that PEM capsules enter the investigated cell lines through a mechanism slightly sensitive to electrostatic interactions, independent of clathrin and caveolae, and strongly dependent on cholesterol-rich domains and organelle acidification. Microscopic characterization of cells during capsule uptake showed the formation of phagocytic cups (vesicles) to engulf the capsules, an increased number of mitochondria, and a final localization in the perinuclear cytoplasma. Combining all these indicators we conclude that PEM capsule internalization in general occurs as a combination of different sequential mechanisms. Initially, an adsorptive mechanism due to strong electrostatic interactions governs the stabilization of the capsules at the cell surface. Membrane ruffling and filopodia extensions are responsible for capsule engulfing through the formation of a phagocytic cup. Co-localization with lipid raft domains activates the cell to initiate a lipid-raft-mediated macropinocytosis. Internalization vesicles are very acidic and co-localize only with phagolysosome markers, excluding caveolin-mediated pathways and indicating that upon phagocytosis the capsules are sorted to heterophagolysosomes.

Streptococcus pneumoniae invades endothelial host cells via multiple pathways and is killed in a lysosome dependent manner

Streptococcus pneumoniae is one of the major causative agents of pneumonia, sepsis, meningitis and other morbidities. In spite of its heavy disease burden, surprisingly little is known about the mechanisms involved in the switch of life style, from commensal colonizer of the nasopharynx to invasive pathogen. In vitro experiments, and mouse models have shown that S. pneumoniae can be internalized by host cells, which coupled with intracellular vesicle transport through the cells, i.e. transcytosis, is suggested to be the first step of invasive disease. To further dissect the process of S. pneumoniae internalization, we chemically inhibited discrete parts of the cellular uptake system. We show that this invasion of the host cells was facilitated via both clathrin- and caveolae-mediated endocytosis. After internalization we demonstrated that the bulk of the internalized S. pneumoniae was killed in the lysosome. Interestingly, inhibition of the lysosome altered transcytosis dynamics as it resulted in an increase in the transport of the internalized bacteria out of the cells via the basal side. These results show that uptake of S. pneumoniae into host cells occurs via multiple pathways, as opposed to the often proposed view of invasion being dependent on specific, and singular receptor-mediated endocytosis. This indicates that the endothelium not only has a critical role as a physical barrier against S. pneumoniae in the blood stream, but also in degrading S. pneumonia cells that have adhered to, and invaded the endothelial cells.

Endocytic pathways involved in filovirus entry: advances, implications and future directions

Detailed knowledge of the host-virus interactions that accompany filovirus entry into cells is expected to identify determinants of viral virulence and host range, and to yield targets for the development of antiviral therapeutics. While it is generally agreed that filovirus entry into the host cytoplasm requires viral internalization into acidic endosomal compartments and proteolytic cleavage of the envelope glycoprotein by endo/lysosomal cysteine proteases, our understanding of the specific endocytic pathways co-opted by filoviruses remains limited. This review addresses the current knowledge on cellular endocytic pathways implicated in filovirus entry, highlights the consensus as well as controversies, and discusses important remaining questions.

Transcriptome analysis on Chinese shrimp Fenneropenaeus chinensis during WSSV acute infection

Previous studies have discovered a lot of immune-related genes responding to white spot syndrome virus (WSSV) infection in crustacean. However, little information is available in relation to underlying mechanisms of host responses during the WSSV acute infection stage in naturally infected shrimp. In this study, we employed next-generation sequencing and bioinformatic techniques to observe the transcriptome differences of the shrimp between latent infection stage and acute infection stage. A total of 64,188,426 Illumina reads, including 31,685,758 reads from the latent infection group and 32,502,668 reads from the acute infection group, were generated and assembled into 46,676 unigenes (mean length: 676 bp; range: 200-15,094 bp). Approximately 24,000 peptides were predicted and classified based on homology searches, gene ontology, clusters of orthologous groups of proteins, and biological pathway mapping. Among which, 805 differentially expressed genes were identified and categorized into 11 groups based on their possible function. Genes in the Toll and IMD pathways, the Ras-activated endocytosis process, the RNA interference pathway, anti-lipopolysaccharide factors and many other genes, were found to be activated in shrimp from latent infection stage to acute infection stage. The anti-bacterially proPO-activating cascade was firstly uncovered to be probably participated in antiviral process. These genes contain not only members playing function in host defense against WSSV, but also genes utilized by WSSV for its rapid proliferation. In addition, the transcriptome data provides detail information for identifying novel genes in absence of the genome database of shrimp.

Positively charged cholesterol-recombinant human gelatins foster the cellular uptake of proteins and murine immune reactions

Purpose

Recombinant human gelatins with defined molecular weights were modified with cholesterol to make them amphiphilic in nature. We investigated the feasibility of these modified human gelatins acting as a carrier of antigenic proteins for inducing cellular immunity. The aim of this study was to synthesize novel and effective compounds for vaccine delivery in vivo.

Methods

Two types of cholesterol-modified gelatin micelles, anionic cholesterol-modified gelatin (aCMG) and cationic-cholesterol modified gelatin (cCMG), were synthesized using different cholesterol derivatives such as the cholesterol-isocyanate (Ch-I) for aCMG and amino-modified cholesterol for cCMG. One was anionic and the other cationic, and therefore they differed in terms of their zeta potential. The aCMG and cCMG were characterized for their size, zeta potential, and in their ability to form micelles. Cytotoxicity was also evaluated. The modified human gelatins were then investigated as a carrier of antigenic proteins for inducing cellular immunity both in vitro in DC 2.4 cells, a murine dendritic cell line, as well as in vivo. The mechanism of entry of the polymeric micelles into the cells was also evaluated.

Results

It was found that only cCMG successfully complexed with the model antigenic protein, fluorescein-isothiocyanate ovalbumin (OVA) and efficiently delivered and processed proteins in DC 2.4 cells. It was hypothesized that cCMG enter the cells predominantly by a caveolae-mediated pathway that required tyrosine kinase receptors on the cell surface. Animal testing using mice showed that the cationic cholesterol-modified gelatin complexed with OVA produced significantly high antibody titers against OVA: 2580-fold higher than in mice immunized with free OVA.

Conclusion

Conclusively, cCMG has shown to be very effective in stimulating an immune response due to its high efficiency, stability, and negligible cytotoxicity.

From endocytosis to membrane fusion: emerging roles of dynamin in virus entry

Dynamin, a large guanosine triphosphatase (GTPase), has been implicated in virus entry, but its mechanisms of action are controversial. The entry procedure of most enveloped viruses involves endocytosis and membrane fusion. Dynamin has been suggested to act both as a regulatory GTPase by controlling the early stages of clathrin-mediated endocytosis (CME), which is an important endocytic pathway utilized by many viruses, and as a mechanochemical enzyme that induces membrane fission and pinches endocytic vesicles off from the cellular plasma membrane in later stages in several endocytic pathways, including CME. In addition to its involvement in virus endocytosis, dynamin has also been proposed to participate in membrane fusion between the virus and endosomes following endocytosis. Crystal structures and cryo-electron micrography (cryo-EM) have elucidated the structure of dynamin, which led to development of a mechanochemical model of how dynamin-mediated membrane fission occurs. Based on this, we propose a hypothetical model that explains how dynamin facilitates virus membrane fusion and discuss its roles in virus entry.

Ready, set, fuse! The coronavirus spike protein and acquisition of fusion competence

Coronavirus-cell entry programs involve virus-cell membrane fusions mediated by viral spike (S) proteins. Coronavirus S proteins acquire membrane fusion competence by receptor interactions, proteolysis, and acidification in endosomes. This review describes our current understanding of the S proteins, their interactions with and their responses to these entry triggers. We focus on receptors and proteases in prompting entry and highlight the type II transmembrane serine proteases (TTSPs) known to activate several virus fusion proteins. These and other proteases are essential cofactors permitting coronavirus infection, conceivably being in proximity to cell-surface receptors and thus poised to split entering spike proteins into the fragments that refold to mediate membrane fusion. The review concludes by noting how understanding of coronavirus entry informs antiviral therapies.

Function of membrane rafts in viral lifecycles and host cellular response

Membrane rafts are small (10–200 nm) sterol- and sphingolipid-enriched domains that compartmentalize cellular processes. Membrane rafts play an important role in viral infection cycles and viral virulence. Viruses are divided into four main classes, enveloped DNA virus, enveloped RNA virus, nonenveloped DNA virus, and nonenveloped RNA virus. General virus infection cycle is also classified into two sections, the early stage (entry process) and the late stage (assembly, budding, and release processes of virus particles). In the viral cycle, membrane rafts act as a scaffold of many cellular signal transductions, which are associated with symptoms caused by viral infections. In this paper, we describe the functions of membrane rafts in viral lifecycles and host cellular response according to each virus classification, each stage of the virus lifecycle, and each virus-induced signal transduction.

Feline and canine coronaviruses: common genetic and pathobiological features

A new human coronavirus responsible for severe acute respiratory syndrome (SARS) was identified in 2003, which raised concern about coronaviruses as agents of serious infectious disease. Nevertheless, coronaviruses have been known for about 50 years to be major agents of respiratory, enteric, or systemic infections of domestic and companion animals. Feline and canine coronaviruses are widespread among dog and cat populations, sometimes leading to the fatal diseases known as feline infectious peritonitis (FIP) and pantropic canine coronavirus infection in cats and dogs, respectively. In this paper, different aspects of the genetics, host cell tropism, and pathogenesis of the feline and canine coronaviruses (FCoV and CCoV) will be discussed, with a view to illustrating how study of FCoVs and CCoVs can improve our general understanding of the pathobiology of coronaviruses.

Intriguing interplay between feline infectious peritonitis virus and its receptors during entry in primary feline monocytes

Two potential receptors have been described for the feline infectious peritonitis virus (FIPV): feline aminopeptidase N (fAPN) and feline dendritic cell-specific intercellular adhesion molecule grabbing non-integrin (fDC-SIGN). In cell lines, fAPN serves as a receptor for serotype II, but not for serotype I FIPV. The role of fAPN in infection of in vivo target cells, monocytes, is not yet confirmed. Both serotype I and II FIPVs use fDC-SIGN for infection of monocyte-derived cells but how is not known. In this study, the role of fAPN and fDC-SIGN was studied at different stages in FIPV infection of monocytes. First, the effects of blocking the potential receptor(s) were studied for the processes of attachment and infection. Secondly, the level of co-localization of FIPV and the receptors was determined. It was found that FIPV I binding and infection were not affected by blocking fAPN while blocking fDC-SIGN reduced FIPV I binding to 36% and practically completely inhibited infection. Accordingly, 66% of bound FIPV I particles co-localized with fDC-SIGN. Blocking fAPN reduced FIPV II binding by 53% and infection by 80%. Further, 60% of bound FIPV II co-localized with fAPN. fDC-SIGN was not involved in FIPV II binding but infection was reduced with 64% when fDC-SIGN was blocked. In conclusion, FIPV I infection of monocytes depends on fDC-SIGN. Most FIPV I particles already interact with fDC-SIGN at the plasma membrane. For FIPV II, both fAPN and fDC-SIGN are involved in infection with only fAPN playing a receptor role at the plasma membrane.

Gateways for the intracellular access of nanocarriers: a review of receptor-mediated endocytosis mechanisms and of strategies in receptor targeting

IMPORTANCE OF THE FIELD

The last 10 years have seen a dramatic growth in understanding and controlling how complex, drug-loaded (nano)structures, as well as pathogens, or biopharmaceuticals can gather access to the cytoplasm, which is a key step to increasing the effectiveness of their action.

AREAS COVERED IN THIS REVIEW

The review offers an updated overview of the current knowledge of endocytic processes; furthermore, the cell surface receptors most commonly used in drug delivery are here discussed on the basis of their reported internalization mechanisms, with examples of their use as nanocarrier targets taken from the most recent scientific literature.

WHAT THE READER WILL GAIN

Knowledge of molecular biology details is increasingly necessary for a rational design of drug delivery systems. Here, the aim is to provide the reader with an attempt to link a mechanistic knowledge of endocytic mechanisms with the identification of appropriate targets (internalization receptors) for nanocarriers.

TAKE HOME MESSAGE

Much advance is still needed to create a complete and coherent biological picture of endocytosis, but current knowledge already allows individuation of a good number of targetable groups for a predetermined intracellular fate of nanocarriers.

Cellular entry of G3.5 poly (amido amine) dendrimers by clathrin- and dynamin-dependent endocytosis promotes tight junctional opening in intestinal epithelia

PURPOSE

This study investigates the mechanisms of G3.5 poly (amido amine) dendrimer cellular uptake, intracellular trafficking, transepithelial transport and tight junction modulation in Caco-2 cells in the context of oral drug delivery.

METHODS

Chemical inhibitors blocking clathrin-, caveolin- and dynamin-dependent endocytosis pathways were used to investigate the mechanisms of dendrimer cellular uptake and transport across Caco-2 cells using flow cytometry and confocal microscopy.

RESULTS

Dendrimer cellular uptake was found to be dynamin-dependent and was reduced by both clathrin and caveolin endocytosis inhibitors, while transepithelial transport was only dependent on dynamin- and clathrin-mediated endocytosis. Dendrimers were quickly trafficked to the lysosomes after 15 min of incubation and showed increased endosomal accumulation at later time points, suggesting saturation of this pathway. Dendrimers were unable to open tight junctions in cell monolayers treated with dynasore, a selective inhibitor of dynamin, confirming that dendrimer internalization promotes tight junction modulation.

CONCLUSION

G3.5 PAMAM dendrimers take advantage of several receptor-mediated endocytosis pathways for cellular entry in Caco-2 cells. Dendrimer internalization by dynamin-dependent mechanisms promotes tight junction opening, suggesting that dendrimers act on intracellular cytoskeletal proteins to modulate tight junctions, thus catalyzing their own transport via the paracellular route.

A clathrin independent macropinocytosis-like entry mechanism used by bluetongue virus-1 during infection of BHK cells

Acid dependent infection of Hela and Vero cells by BTV-10 occurs from within early-endosomes following virus uptake by clathrin-mediated endocytosis (Forzan et al., 2007: J Virol 81: 4819–4827). Here we report that BTV-1 infection of BHK cells is also dependent on a low endosomal pH; however, virus entry and infection were not inhibited by dominant-negative mutants of Eps15, AP180 or the ‘aa’ splice variant of dynamin-2, which were shown to inhibit clathrin-mediated endocytosis. In addition, infection was not inhibited by depletion of cellular cholesterol, which suggests that virus entry is not mediated by a lipid-raft dependent process such as caveolae-mediated endocytosis. Although virus entry and infection were not inhibited by the dominant-negative dynamin-2 mutant, entry was inhibited by the general dynamin inhibitor, dynasore, indicating that virus entry is dynamin dependent. During entry, BTV-1 co-localised with LAMP-1 but not with transferrin, suggesting that virus is delivered to late-endosomal compartments without first passing through early-endosomes. BTV-1 entry and infection were inhibited by EIPA and cytochalasin-D, known macropinocytosis inhibitors, and during entry virus co-localised with dextran, a known marker for macropinocytosis/fluid-phase uptake. Our results extend earlier observations with BTV-10, and show that BTV-1 can infect BHK cells via an entry mechanism that is clathrin and cholesterol-independent, but requires dynamin, and shares certain characteristics in common with macropinocytosis.

Endocytosis of murine norovirus 1 into murine macrophages is dependent on dynamin II and cholesterol

Although noroviruses cause the vast majority of nonbacterial gastroenteritis in humans, little is known about their life cycle, including viral entry. Murine norovirus (MNV) is the only norovirus to date that efficiently infects cells in culture. To elucidate the productive route of infection for MNV-1 into murine macrophages, we used a neutral red (NR) infectious center assay and pharmacological inhibitors in combination with dominant-negative (DN) and small interfering RNA (siRNA) constructs to show that clathrin- and caveolin-mediated endocytosis did not play a role in entry. In addition, we showed that phagocytosis or macropinocytosis, flotillin-1, and GRAF1 are not required for the major route of MNV-1 uptake. However, MNV-1 genome release occurred within 1 h, and endocytosis was significantly inhibited by the cholesterol-sequestering drugs nystatin and methyl-beta-cyclodextrin, the dynamin-specific inhibitor dynasore, and the dominant-negative dynamin II mutant K44A. Therefore, we conclude that the productive route of MNV-1 entry into murine macrophages is rapid and requires host cholesterol and dynamin II.

Caveolin-1-dependent occludin endocytosis is required for TNF-induced tight junction regulation in vivo

Although tight junction morphology is not obviously affected by TNF, this proinflammatory cytokine promotes internalization of occludin, resulting in disrupted barrier function within the intestine.

Epithelial paracellular barrier function, determined primarily by tight junction permeability, is frequently disrupted in disease. In the intestine, barrier loss can be mediated by tumor necrosis factor (α) (TNF) signaling and epithelial myosin light chain kinase (MLCK) activation. However, TNF induces only limited alteration of tight junction morphology, and the events that couple structural reorganization to barrier regulation have not been defined. We have used in vivo imaging and transgenic mice expressing fluorescent-tagged occludin and ZO-1 fusion proteins to link occludin endocytosis to TNF-induced tight junction regulation. This endocytosis requires caveolin-1 and is essential for structural and functional tight junction regulation. These data demonstrate that MLCK activation triggers caveolin-1–dependent endocytosis of occludin to effect structural and functional tight junction regulation.