Binding of adenovirus and its external proteins to Triton X-114. Dependence on pH

Study of Oncolytic Virus Preservation and Formulation

In recent years, oncolytic viruses (OVs) have emerged as an effective means of treating cancer. OVs have multiple oncotherapeutic functions including specifically infecting and lysing tumor cells, initiating immune cell death, attacking and destroying tumor angiogenesis and triggering a broad bystander effect. Oncolytic viruses have been used in clinical trials and clinical treatment as drugs for cancer therapy, and as a result, oncolytic viruses are required to have long-term storage stability for clinical use. In the clinical application of oncolytic viruses, formulation design plays a decisive role in the stability of the virus. Therefore, this paper reviews the degradation factors and their degradation mechanisms (pH, thermal stress, freeze-thaw damage, surface adsorption, oxidation, etc.) faced by oncolytic viruses during storage, and it discusses how to rationally add excipients for the degradation mechanisms to achieve the purpose of maintaining the long-term stability of oncolytic viral activity. Finally, the formulation strategies for the long-term formulation stability of oncolytic viruses are discussed in terms of buffers, permeation agents, cryoprotectants, surfactants, free radical scavengers, and bulking agent based on virus degradation mechanisms.

The amphipathic helix of adenovirus capsid protein VI contributes to penton release and postentry sorting

Nuclear delivery of the adenoviral genome requires that the capsid cross the limiting membrane of the endocytic compartment and traverse the cytosol to reach the nucleus. This endosomal escape is initiated upon internalization and involves a highly coordinated process of partial disassembly of the entering capsid to release the membrane lytic internal capsid protein VI. Using wild-type and protein VI-mutated human adenovirus serotype 5 (HAdV-C5), we show that capsid stability and membrane rupture are major determinants of entry-related sorting of incoming adenovirus virions. Furthermore, by using electron cryomicroscopy, as well as penton- and protein VI-specific antibodies, we show that the amphipathic helix of protein VI contributes to capsid stability by preventing premature disassembly and deployment of pentons and protein VI. Thus, the helix has a dual function in maintaining the metastable state of the capsid by preventing premature disassembly and mediating efficient membrane lysis to evade lysosomal targeting. Based on these findings and structural data from cryo-electron microscopy, we suggest a refined disassembly mechanism upon entry.

IMPORTANCE

In this study, we show the intricate connection of adenovirus particle stability and the entry-dependent release of the membrane-lytic capsid protein VI required for endosomal escape. We show that the amphipathic helix of the adenovirus internal protein VI is required to stabilize pentons in the particle while coinciding with penton release upon entry and that release of protein VI mediates membrane lysis, thereby preventing lysosomal sorting. We suggest that this dual functionality of protein VI ensures an optimal disassembly process by balancing the metastable state of the mature adenovirus particle.

Conformational changes in the adenovirus hexon subunit responsible for regulating cytoplasmic dynein recruitment

Virus capsids provide genome protection from environmental challenges but are also poised to execute a program of compositional and conformational changes to facilitate virion entry and infection. The most abundant adenovirus serotype 5 (AdV5) capsid protein, hexon, directly recruits the motor protein cytoplasmic dynein following virion entry. Dynein recruitment is crucial for capsid transport to the nucleus and requires the transient exposure of AdV5 hexon to low pH, presumably mimicking passage through the endosomal compartment. These results suggest a pH-dependent capsid modification during early infection. The changes to hexon structure controlling this behavior have not been explored. We report that hexon remains trimeric at low pH but undergoes more subtle conformational changes. These changes are indicated by increased sensitivities to SDS-mediated dissociation and dispase proteolysis. Both effects are reversed at neutral pH, as is dynein binding by low-pH-treated hexon. Dispase cleavage, which we find maps to a specific site within hypervariable region 1 (HVR1) of AdV5 hexon, has no apparent effect on virion entry but completely inhibits its transport to the nucleus. In addition, an AdV5 mutant containing HVR1 of AdV48 is unable to bind dynein and is strongly inhibited in the postentry transport step. These results reveal that conformational changes involving hexon HVR1 are the basis for a novel viral mechanism controlling capsid transport to the nucleus.

IMPORTANCE

The adenovirus serotype 5 (AdV5) capsid protein hexon recruits the molecular motor protein cytoplasmic dynein in a pH-dependent manner, a function critical for efficient transport toward the nucleus and AdV5 infectivity. In this work, we describe how low-pH exposure induces reversible structural changes in AdV5 hexon and how these changes affect dynein binding. In addition, we identified a pH-sensitive dispase cleavage site in hexon HVR1, which depends on the same structural changes and furthermore regulates dynein recruitment and capsid redistribution in infected cells. These data provide the first evidence relating long-known but subtle pH-dependent structural changes in hexon to a more recently established essential but poorly understood role in virus transport. These results have broad implications for understanding virus infectivity in general, and our ability to block the recruitment mechanism has potential therapeutic implications as well.

Synergistic gene and drug tumor therapy using a chimeric peptide

Co-delivery of gene and drug for synergistic therapy has provided a promising strategy to cure devastating diseases. Here, an amphiphilic chimeric peptide (Fmoc)2KH7-TAT with pH-responsibility for gene and drug delivery was designed and fabricated. As a drug carrier, the micelles self-assembled from the peptide exhibited a much faster doxorubicin (DOX) release rate at pH 5.0 than that at pH 7.4. As a non-viral gene vector, (Fmoc)(2)KH(7)-TAT peptide could satisfactorily mediate transfection of pGL-3 reporter plasmid with or without the existence of serum in both 293T and HeLa cell-lines. Besides, the endosome escape capability of peptide/DNA complexes was investigated by confocal laser scanning microscopy (CLSM). To evaluate the co-delivery efficiency and the synergistic anti-tumor effect of gene and drug, p53 plasmid and DOX were simultaneously loaded in the peptide micelles to form micelleplexes during the self-assembly of the peptide. Cellular uptake and intracellular delivery of gene and drug were studied by CLSM and flow cytometry respectively. And p53 protein expression was determined via Western blot analysis. The in vitro cytotoxicity and in vivo tumor inhibition effect were also studied. Results suggest that the co-delivery of gene and drug from peptide micelles resulted in effective cell growth inhibition in vitro and significant tumor growth restraining in vivo. The chimeric peptide-based gene and drug co-delivery system will find great potential for tumor therapy.

Development of adenovirus capsid proteins for targeted therapeutic delivery

The outer shell of the adenovirus capsid comprises three major types of protein (hexon, penton base and fiber) that perform the majority of functions facilitating the early stages of adenovirus infection. These stages include initial cell-surface binding followed by receptor-mediated endocytosis, endosomal penetration and cytosolic entry, and intracellular trafficking toward the nucleus. Numerous studies have shown that the penton base contributes to several of these steps and have supported the development of this protein into a delivery agent for therapeutic molecules. Studies revealing that the fiber and hexon bear unexpected properties of cell entry and/or nuclear homing have supported the development of these capsid proteins, as well into potential delivery vehicles. This review summarizes the findings to date of the protein-cell activities of these capsid proteins in the absence of the whole virus and their potential for therapeutic application with regard to the delivery of foreign molecules.

An N-terminal domain of adenovirus protein VI fragments membranes by inducing positive membrane curvature

Adenovirus (Ad) membrane penetration during cell entry is poorly understood. Here we show that antibodies which neutralize the membrane lytic activity of the Ad capsid protein VI interfere with Ad endosomal membrane penetration. In vitro studies using a peptide corresponding to an N-terminal amphipathic alpha-helix of protein VI (VI-Phi), as well as other truncated forms of protein VI suggest that VI-Phi is largely responsible for protein VI binding to and lysing of membranes. Additional studies suggest that VI-Phi lies nearly parallel to the membrane surface. Protein VI fragments membranes and induces highly curved structures. Further studies suggest that protein VI induces positive membrane curvature. These data support a model in which protein VI binds membranes, inducing positive curvature strain which ultimately leads to membrane fragmentation. These results agree with previous observations of Ad membrane permeabilization during cell entry and provide an initial mechanistic description of a nonenveloped virus membrane lytic protein.

Adenovirus

Of the 53 different human adenovirus (HAdV) serotypes belonging to species A-G, a significant number are associated with acute respiratory, gastrointestinal and ocular infections. Replication-defective HAdV-5-based vectors also continue to play a significant role in gene transfer trials and in vaccine delivery efforts in the clinic. Although significant progress has been made from studies of AdV biology, we still have an incomplete understanding of AdV's structure as well as its multifactorial interactions with the host. Continuing efforts to improve knowledge in these areas, as discussed in this chapter, will be crucial for revealing the mechanisms of AdV pathogenesis and for allowing optimal use of AdV vectors for biomedical applications.

Conformational maturation and post-ER multisubunit assembly of gap junction proteins

For all previously well-characterized oligomeric integral membrane proteins, folding, multisubunit assembly, and recognition of conformationally immature molecules for degradation occurs at their organelle of synthesis. This cannot, however, be the case for the gap junction-forming protein connexin43 (Cx43), which when endogenously expressed undergoes multisubunit assembly into connexons only after its transport to the trans-Golgi network. We have developed two novel assays to assess Cx43 folding and assembly: acquisition of resistance of disulfide bonds to reduction by extracellularly added DTT and Triton X-114 detergent phase partitioning. We show that Cx43 synthesized at physiologically relevant levels undergoes a multistep conformational maturation process in which folding of connexin monomers within the ER is a prerequisite for multisubunit assembly in the TGN. Similar results were obtained with Cx32, disproving the widely reported contention that the site of endogenous beta connexin assembly is the ER. Exogenous overexpression of Cx43, Cx32, or Cx26 allows these events to take place within the ER, the first example of the TGN and ER as alternative sites for oligomeric assembly. Our findings also constitute the first biochemical evidence that defective connexin folding is a cause of the human disorder X-linked Charcot-Marie-Tooth disease.

Typical and atypical trafficking pathways of Ad5 penton base recombinant protein: implications for gene transfer

The adenovirus (Ad) penton base protein facilitates viral infection by binding cell surface integrins, triggering receptor-mediated endocytosis and mediating endosomal penetration. Given these multiple functions, recombinant penton base proteins have been utilized as non-viral vehicles for gene transfer by our lab and others. Although we have previously demonstrated that penton base-derived vectors undergo integrin-specific binding and cell entry, less than desirable levels of gene expression have led us to re-evaluate the recombinant penton base as an agent for gene delivery. To do so, we have examined here the intracellular trafficking of an Ad serotype 5 (Ad5) recombinant penton base protein (PB). Here, we not only observed that PB utilizes a similar, typical trafficking pathway of whole Ad, but also found that PB entered HeLa cells through pathways not yet identified as contributing to cell entry by the whole virus. We show by high-resolution confocal microscopy and biochemical methods that binding to alphav-integrins is a requirement for cell entry, but that early internalization stages did not substantially pass through clathrin-positive and early endosomal compartments. Moreover, a subpopulation of internalized protein localized with caveolin-positive compartments and Golgi markers, suggesting that a certain percentage of proteins pass through non-clathrin-mediated pathways. Similar to the virus, trafficking toward the nucleus was affected by disruption of microtubules and dynein. The majority of penton base molecules avoided the lysosome while facilitating early vesicle release of low molecular weight dextran molecules. In further support of a vesicle escape capacity, a subpopulation of internalized penton base appeared to enter the nucleus, as observed by high-resolution confocal microscopy and cell fractionation. As a confirmation of these findings, we demonstrate that a recombinant penton base facilitated cytosolic entry of an siRNA molecule as observed by RNA interference of a marker gene. Based on our findings here, we suggest that whereas soluble penton base proteins may enter cells through clathrin- and non-clathrin-mediated pathways, vesicle escape and nuclear delivery appear to be supported by a clathrin-mediated pathway. As our previous efforts have focused on utilizing recombinant penton base proteins as delivery agents for therapeutics, these findings allow us to evaluate the use of the penton base as a cell entry and intracellular trafficking agent, and may be of interest concerning the development of vectors for efficient delivery of therapeutics to cells.

Intracellular trafficking of nonviral vectors

Nonviral vectors continue to be attractive alternatives to viruses due to their low toxicity and immunogenicity, lack of pathogenicity, and ease of pharmacologic production. However, nonviral vectors also continue to suffer from relatively low levels of gene transfer compared to viruses, thus the drive to improve these vectors continues. Many studies on vector-cell interactions have reported that nonviral vectors bind and enter cells efficiently, but yield low gene expression, thus directing our attention to the intracellular trafficking of these vectors to understand where the obstacles occur. Here, we will review nonviral vector trafficking pathways, which will be considered here as the steps from cell binding to nuclear delivery. Studies on the intracellular trafficking of nonviral vectors has given us valuable insights into the barriers these vectors must overcome to mediate efficient gene transfer. Importantly, we will highlight the different approaches used by researchers to overcome certain trafficking barriers to gene transfer, many of which incorporate components from biological systems that have naturally evolved the capacity to overcome such obstacles. The tools used to study trafficking pathways will also be discussed.

Adenovirus protein VI mediates membrane disruption following capsid disassembly

In contrast to enveloped viruses, the mechanisms involved in membrane penetration by nonenveloped viruses are not as well understood. In these studies, we determined the relationship between adenovirus (Ad) capsid disassembly and the development of membrane lytic activity. Exposure to low pH or heating induced conformational changes in wild-type Ad but not in temperature-sensitive Ad (ts1) particles that fail to escape the early endosome. Wild-type Ad but not ts1 particles permeabilized model membranes (liposomes) and facilitated the cytosolic delivery of a ribotoxin. Alterations in wild-type Ad capsids were associated with the exposure of a pH-independent membrane lytic factor. Unexpectedly, this factor was identified as protein VI, a 22-kDa cement protein located beneath the peripentonal hexons in the viral capsid. Recombinant protein VI and preprotein VI, but not a deletion mutant lacking an N-terminal amphipathic alpha-helix, possessed membrane lytic activity similar to partially disassembled virions. A new model of Ad entry is proposed based on our present observations of capsid disassembly and membrane penetration.

The structure of the human adenovirus 2 penton

The adenovirus penton, a noncovalent complex of the pentameric penton base and trimeric fiber proteins, comprises the vertices of the adenovirus capsid and contains all necessary components for viral attachment and internalization. The 3.3 A resolution crystal structure of human adenovirus 2 (hAd2) penton base shows that the monomer has a basal jellyroll domain and a distal irregular domain formed by two long insertions, a similar topology to the adenovirus hexon. The Arg-Gly-Asp (RGD) motif, required for interactions with cellular integrins, occurs on a flexible surface loop. The complex of penton base with bound N-terminal fiber peptide, determined at 3.5 A resolution, shows that the universal fiber motif FNPVYPY binds at the interface of adjacent penton base monomers and results in a localized structural rearrangement in the insertion domain of the penton base. These results give insight into the structure and assembly of the adenovirus capsid and will be of use for gene-therapy applications.

Release of canine parvovirus from endocytic vesicles

Canine parvovirus (CPV) is a small nonenveloped virus with a single-stranded DNA genome. CPV enters cells by clathrin-mediated endocytosis and requires an acidic endosomal step for productive infection. Virion contains a potential nuclear localization signal as well as a phospholipase A(2) like domain in N-terminus of VP1. In this study we characterized the role of PLA(2) activity on CPV entry process. PLA(2) activity of CPV capsids was triggered in vitro by heat or acidic pH. PLA(2) inhibitors inhibited the viral proliferation suggesting that PLA(2) activity is needed for productive infection. The N-terminus of VP1 was exposed during the entry, suggesting that PLA(2) activity might have a role during endocytic entry. The presence of drugs modifying endocytosis (amiloride, bafilomycin A(1), brefeldin A, and monensin) caused viral proteins to remain in endosomal/lysosomal vesicles, even though the drugs were not able to inhibit the exposure of VP1 N-terminal end. These results indicate that the exposure of N-terminus of VP1 alone is not sufficient to allow CPV to proliferate. Some other pH-dependent changes are needed for productive infection. In addition to blocking endocytic entry, amiloride was able to block some postendocytic steps. The ability of CPV to permeabilize endosomal membranes was demonstrated by feeding cells with differently sized rhodamine-conjugated dextrans together with the CPV in the presence or in the absence of amiloride, bafilomycin A(1), brefeldin A, or monensin. Dextran with a molecular weight of 3000 was released from vesicles after 8 h of infection, while dextran with a molecular weight of 10,000 was mainly retained in vesicles. The results suggest that CPV infection does not cause disruption of endosomal vesicles. However, the permeability of endosomal membranes apparently changes during CPV infection, probably due to the PLA(2) activity of the virus. These results suggest that parvoviral PLA(2) activity is essential for productive infection and presumably utilized in membrane penetration process of the virus, but CPV also needs other pH-dependent changes or factors to be released to the cytoplasm from endocytic vesicles.

Endocytosis of adenovirus and adenovirus capsid proteins

Key proteins of the icosahedral-shaped adenovirus (Ad) capsid mediate infection, and interact with cellular proteins to coordinate stepwise events of cell entry that produce successful gene transfer. Infection is mediated predominantly by the penton and fiber capsid proteins. The fiber initiates cell binding while the penton binds integrin coreceptors, triggering integrin-mediated endocytosis. Penton integrin signaling precedes viral escape from the endosomal vesicle. After cell binding, the virus undergoes stepwise disassembly of the capsid, shedding proteins during cell entry. Intracellular trafficking of the remaining capsid shell is mediated by the interaction of naked particles with the cytoskeleton. The capsid translocates toward the nucleus, with the majority of capsid proteins accumulating at the nuclear periphery, while viral DNA and associated protein VII are extruded through the nuclear pore. This discussion will encompass the current knowledge on Ad cell entry and trafficking, with an emphasis on the contribution of Ad capsid proteins to these processes. A greater understanding of the highly effective Ad cell entry pathway may lend itself to the development of safer drug and gene delivery alternatives utilizing similar pathways.

Polycation gene delivery systems: escape from endosomes to cytosol

Clinical success of gene therapy based on oligonucleotides (ODNs), ribozymes, RNA and DNA will be greatly dependent on the availability of effective delivery systems. Polycations have gained increasing attention as a non-viral gene delivery vector in the past decades. Significant progress has been made in understanding complex formation between polycations and nucleic acids, entry of the complex into the cells and subsequent entry into the nucleus. Sophisticated molecular architectures of cationic polymers have made the vectors more stable and less susceptible to binding by enzymes or proteins. Incorporation of specific ligands to polycations has resulted in more cell-specific uptake by receptor-mediated mechanisms. However, there are still other barriers limiting the transfection efficiency of polycation gene delivery systems. There is a consensus that polycation-DNA complexes (polyplexes) enter cells via the endocytotic pathway. It is not clearly understood, however, how the polyplexes escape (if they do) from endosomes, how DNA is released from the polyplexes or how the released DNA is expressed. The primary focus of this article is to review various polycation gene delivery systems, which are designed to translocate DNA from endosomes into cytosol. Many polycation gene delivery systems have tried to mimic the mechanisms that viruses use for the endosomal escape. Polycation gene delivery systems are usually coupled with synthetic amphipathic peptides mimicking viral fusogenic peptides, histidine-based gene delivery systems for pH-responsive endosomal escape, polycations with intrinsic endosomolytic activity by the proton sponge mechanism and polyanions to mimic the anionic amphiphilic peptides.

Strategy for nonenveloped virus entry: a hydrophobic conformer of the reovirus membrane penetration protein micro 1 mediates membrane disruption

The mechanisms employed by nonenveloped animal viruses to penetrate the membranes of their host cells remain enigmatic. Membrane penetration by the nonenveloped mammalian reoviruses is believed to deliver a partially uncoated, but still large ( approximately 70-nm), particle with active transcriptases for viral mRNA synthesis directly into the cytoplasm. This process is likely initiated by a particle form that resembles infectious subvirion particles (ISVPs), disassembly intermediates produced from virions by proteolytic uncoating. Consistent with that idea, ISVPs, but not virions, can induce disruption of membranes in vitro. Both activities ascribed to ISVP-like particles, membrane disruption in vitro and membrane penetration within cells, are linked to N-myristoylated outer-capsid protein micro 1, present in 600 copies at the surfaces of ISVPs. To understand how micro 1 fulfills its role as the reovirus penetration protein, we monitored changes in ISVPs during the permeabilization of red blood cells induced by these particles. Hemolysis was preceded by a major structural transition in ISVPs, characterized by conformational change in micro 1 and elution of fibrous attachment protein sigma 1. The altered conformer of micro 1 was required for hemolysis and was markedly hydrophobic. The structural transition in ISVPs was further accompanied by derepression of genome-dependent mRNA synthesis by the particle-associated transcriptases. We propose a model for reovirus entry in which (i) primed and triggered conformational changes, analogous to those in enveloped-virus fusion proteins, generate a hydrophobic micro 1 conformer capable of inserting into and disrupting cell membranes and (ii) activation of the viral particles for membrane interaction and mRNA synthesis are concurrent events. Reoviruses provide an opportune system for defining the molecular details of membrane penetration by a large nonenveloped animal virus.

Role of alpha(v) integrins in adenovirus cell entry and gene delivery

Adenoviruses (Ad) are a significant cause of acute infections in humans; however, replication-defective forms of this virus are currently under investigation for human gene therapy. Approximately 20 to 25% of all the gene therapy trials (phases I to III) conducted over the past 10 years involve the use of Ad gene delivery for treatment inherited or acquired diseases. At present, the most promising applications involve the use of Ad vectors to irradicate certain nonmetastatic tumors and to promote angiogenesis in order to alleviate cardiovascular disease. While specific problems of using Ad vectors remain to be overcome (as is true for almost all viral and nonviral delivery methods), a distinct advantage of Ad is the extensive knowledge of its macromolecular structure, genome organization, sequence, and mode of replication. Moreover, significant information has also been acquired on the interaction of Ad particles with distinct host cell receptors, events which strongly affect virus tropism. This review provides an overview of the structure and function of Ad attachment (coxsackievirus and Ad receptor [CAR]) and internalization (alpha(v) integrins) receptors and discusses their precise role in virus infection and gene delivery. Recent structure studies of integrin-Ad complexes by cryoelectron microscopy are also highlighted. Finally, unanswered questions arising from the current state of knowledge of Ad-receptor interactions are presented in the context of improving Ad vectors for future human gene therapy applications.

Targeted adenoviral vectors for cancer gene therapy

The delineation of the molecular basis of cancer allows for the possibility of specific intervention at the molecular level for therapeutic purposes. To this end, viral and nonviral vectors have been designed for delivery and expression of genes into target malignant and non-malignant cells. Gene transfer by available vectors, applied both in the ex vivo and in in vivo contexts, has resulted frequently in the desired cellular phenotypical changes. In this regard, recombinant adenoviruses have been particularly efficient for in vivo gene transfer. Importantly, numerous human clinical protocols using adenoviruses have rapidly entered Phase I clinical trials. However, major vector-related problems remain to be solved before the transfer of therapeutic genes by adenoviruses can become an effective and common place strategy for cancer. An overriding obstacle is the basic ability to deliver therapeutic genes specifically into tumor cells. Here we review a diverse number of advances in adenoviral vectors being developed for overcoming this obstacle. As vector technology fulfills these requirements for obtaining the "targetable-injectable" vector, it is anticipated that promising results already observed in pre-clinical studies will translate quickly into the clinic.

Adenovirus uncoating and nuclear establishment are not affected by weak base amines

We have used four established lysosomotropic agents, ammonium chloride, amantadine, chloroquine, and methylamine, to monitor the possible interference with an early low-pH-dependent step during adenovirus replication. Two concentrations of each of the different agents were selected; one was essentially nontoxic to uninfected HeLa cells, and the other resulted in some toxicity as measured by trypan blue staining and by interference with cell monolayer establishment, cell proliferation, and radioisotope labelling. It was separately determined that these concentrations displayed pH-raising effects of the same magnitude as higher concentrations previously used in similar studies. Adenovirus uncoating in vivo, normally reaching its maximum within 1 h after infection, was not affected by any of the agents. The subsequent levels of successful nuclear entry events by the parental genomes were monitored by measuring the extent of transcription of an mRNA species coding for the early 72-kDa DNA-binding protein at 10 to 12 h postinfection. In HeLa, KB, HEp-2, and A549 cells, none of the agents were able to affect the levels of early transcription after administration at the point of infection or at 3 h after infection. The cumulative synthesis of the hexon antigen was assessed late in infection, and inhibitory effects were revealed upon administration of 10, 20, and 40 mM ammonium chloride, 10 mM methylamine, and 0.5 mM amantadine, irrespective of the time point of addition. Ammonium chloride at 5 mM reduced the hexon yield by 20% at the most when added within 50 min after infection. Chloroquine at concentrations of 2.5 and 5 microM specifically reduced the hexon yields by 30 to 40% when administered within the first 50 min of infection. On the basis of the lack of effects of nontoxic concentrations of the four agents on the early virus-cell interactive event of uncoating and the early virus-specified transcription, we conclude that a low-pH-dependent step early in the adenovirus replication cycle is not mandatory for a successful infection.

CD5-mediated specific delivery of DNA to T lymphocytes: compartmentalization augmented by adenovirus

Specific DNA delivery has been achieved via interactions between an asialoorosomucoid-polylysine conjugate and the asialoglycoprotein receptor. We have now extended this technology to another cell type. In order to achieve DNA delivery uniquely to T cells, we have employed an antibody-polylysine conjugate which binds and is internalized via CD5. Binding analyses of the T101 monoclonal antibody to Jurkat cells and freshly isolated human peripheral T lymphocytes were performed and Scatchard plots revealed Kd values of 1.4 and 1.2 pM, respectively. To introduce DNA into the T cell, a complex of T101-polylysine and the luciferase plasmid was formed (T101-PL-DNA). 125I-labeled antibody alone or T101-PL-DNA complexes were both shown to internalize. Subcellular fractionation indicated that the complex remained in the endosomal compartment of the cell for up to 90 min. However, with the addition of adenovirus particles, there was a decrease of labeled complex in the endosomal fraction over time suggesting it was no longer 'tethered' to the endosome vesicle. In vitro transfections confirmed this result showing the addition of adenovirus particles during incubation resulted in increased expression of the luciferase protein. Without adenovirus, there was limited expression of the transduced gene. These data revealed that T101 can deliver DNA via an antibody-PL conjugate. The addition of adenovirus allowed the DNA to escape the endosome enabling expression of the reporter gene.

Mechanism of enhancement of DNA expression consequent to cointernalization of a replication-deficient adenovirus and unmodified plasmid DNA

Given the knowledge that replication-deficient adenoviruses can mediate the delivery of unlinked plasmid DNA into eukaryotic cells (K. Yoshimura, M. A. Rosenfeld, P. Seth, and R. G. Crystal, J. Biol. Chem. 268:2300-2303, 1993), this study focuses on the role of receptor-mediated endocytosis in this process. AdCFTR (an E1- E3- adenovirus type 5-based replication-deficient adenovirus containing the 4.5-kb human cystic fibrosis transmembrane conductance regulator cDNA) was added to Cos-7 cells together with plasmid pRSVL (containing the Rous sarcoma virus long terminal repeat promoter followed by the luciferase cDNA), and luciferase activity was quantified as a measure of the expression of the plasmid DNA. When AdCFTR was bound to Cos-7 cells at 4 degrees C and the cells were subsequently incubated at 37 degrees C in the presence of pRSVL, the expression of luciferase activity was increased in proportion to the amount of AdCFTR added, reaching > 10(4)-fold at 3,000 PFU per cell. AdCFTR-mediated increase in pRSVL was inhibited by addition of purified adenovirus fiber but not hexon, suggesting cell surface adenovirus receptors were involved in the cointernalization process. Cell lines with a high number of adenovirus receptors (Cos-7 and HeLa) showed significant AdCFTR-dependent pRSVL expression, while cell lines with low numbers of adenovirus receptors (NIH 3T3 and U-937) showed little. AdCFTR-mediated increase in the expression of pRSVL was prevented when AdCFTR was heat treated and exposed to antibody against adenovirus or when the cointernalization process was evaluated in the presence of chloroquine, conditions all known to prevent adenovirus-mediated disruption of endocytic vesicles. In contrast, the uptake of AdCFTR into Cos-7 cells was not affected by any of these conditions. When AdCFTR was exposed to UV light, its ability to grow in 293 cells was obviated, but AdCFTR-dependent increase in pRSVL expression was minimally reduced. Finally, empty capsids of AdCFTR were able to enhance the delivery and expression of plasmid pRSVL into Cos-7 cells, suggesting that the adenovirus genome is not required for AdCFTR-mediated plasmid cointernalization. Together, these observations suggest that the ability of a replication-deficient recombinantly adenovirus to mediate the cointernalization and expression of plasmids is mediated by the receptor-mediated endocytosis pathway.

Adenovirus-dependent release of choline from plasma membrane vesicles at an acidic pH is mediated by the penton base protein

It has been suggested that during receptor-mediated endocytosis of human adenovirus (Ad) type 2 into cells, Ad disrupts the membrane of endocytic vesicles to enter the cytosol. To understand the mechanism of Ad-mediated disruption of the endocytic vesicles, I exposed Ad to plasma membrane vesicles derived from KB cells. Ad caused release of choline from the plasma membrane vesicles preloaded with [3H]choline. The efflux of choline was dependent on (i) the concentration of Ad, with a half-maximal effect at 0.5 microgram/ml; (ii) the pH of the buffer, with the optimum pH of the reaction ranging from 5.5 to 6.0; (iii) the length of the incubation, with a half-maximal release at 2 min; and (iv) the temperature of the incubation, with the optimum temperature being 37 degrees C. The Ad-dependent release of choline was inhibited by anti-penton base, while antihexon did not block the effect. These results suggest roles for a low-pH environment and the penton base protein in the Ad-dependent efflux of choline from plasma membrane vesicles.

Phase separation of biomolecules in polyoxyethylene glycol nonionic detergents

The advantage of aqueous two-phase systems based on polyoxyethylene detergents over other liquid-liquid two-phase systems lies in their capacity to fractionate membrane proteins simply by heating the solution over a biocompatible range of temperatures (20 to 37 degrees C). This permits the peripheral membrane proteins to be effectively separated from the integral membrane proteins, which remain in the detergent-rich phase due to the interaction of their hydrophobic domains with detergent micelles. Since the first reports of this special characteristic of polyoxyethylene glycol detergents in 1981, numerous reports have consolidated this procedure as a fundamental technique in membrane biochemistry and molecular biology. As examples of their use in these two fields, this review summarizes the studies carried out on the topology, diversity, and anomalous behavior of transmembrane proteins on the distribution of glycosyl-phosphatidylinositol-anchored membrane proteins, and on a mechanism to describe the pH-induced translocation of viruses, bacterial endotoxins, and soluble cytoplasmic proteins related to membrane fusion. In addition, the phase separation capacity of these polyoxyethylene glycol detergents has been used to develop quick fractionation methods with high recoveries, on both a micro- and macroscale, and to speed up or increase the efficiency of bioanalytical assays.

Chicken adenovirus (CELO virus) particles augment receptor-mediated DNA delivery to mammalian cells and yield exceptional levels of stable transformants

Delivery of genes via receptor-mediated endocytosis is severely limited by the poor exit of endocytosed DNA from the endosome. A large enhancement in delivery efficiency has been obtained by including human adenovirus particles in the delivery system. This enhancement is probably a function of the natural adenovirus entry mechanism, which must include passage through or disruption of the endosomal membrane. In an effort to identify safer virus particles useful in this application, we have tested the chicken adenovirus CELO virus for its ability to augment receptor-mediated gene delivery. We report here that CELO virus possesses pH-dependent, liposome disruption activity similar to that of human adenovirus type 5. Furthermore, the chicken adenovirus can be used to augment receptor-mediated gene delivery to levels comparable to those found for the human adenovirus when it is physically linked to polylysine ligand-condensed DNA particles. The chicken adenovirus has the advantage of being produced inexpensively in embryonated eggs, and the virus is naturally replication defective in mammalian cells, even in the presence of wild-type human adenovirus.

Differential insertion of insulin receptor complexes into Triton X-114 bilayer membranes. Evidence for a differential accessibility of the membrane-exposed receptor domain

In the present study, the Triton X-114 phase-separation system has been used to characterize molecular properties of the membrane-exposed domain of an integral-membrane hormone receptor. This approach provides novel details of the structure/function relationship of insulin receptors. Upon raising the temperature of a micellar Triton X-114 solution above the cloud-point, a detergent enriched phase pellets and coprecipitates 95% of the purified insulin-free (alpha beta)2 receptors. In contrast, 83% of the hormone bound (alpha beta)2 receptor complexes prefer the detergent-depleted phase, exhibiting prominent properties of non-membraneous proteins. Kinetic studies show that, following insulin binding, the amphiphilicity of the receptor complexes is immediately altered. Only monodisperse (alpha beta)2 complexes were detected when receptor/insulin complexes of the detergent-depleted phase were analyzed by detergent-free sucrose density centrifugation in the presence of 10 nM insulin. These results can be explained in the light of the lipid-bilayer-like organization of the precipitating Triton X-114; hormone-induced intramolecular alterations of (alpha beta)2 receptors appear to fundamentally restrict access to the membrane-exposed receptor domain. Basically, different molecular properties are found for alpha beta receptors. Only 67% of the insulin-free receptors coprecipitate with the Triton-X-114-enriched phase; following insulin binding the coprecipitation is only decreased to 42%. In contrast to (alpha beta)2 receptors, formation of noncovalently aggregated receptor complexes, which are detected by sucrose density centrifugation, could account for the exclusion of alpha beta receptor species from Triton X-114 membranes.

Infectious entry pathway for canine parvovirus

We have investigated whether canine parvovirus (CPV) infection involves a low pH-dependent entry pathway. The effects of two lysosomotropic bases, NH4Cl and chloroquine, on CPV entry were studied by immunofluorescence and ultrastructural and biochemical methods. In the presence of these reagents, input virions appear to accumulate in large vacuoles. Ultrastructural studies indicated that uptake of virions takes place predominantly in small uncoated vesicles that appear to fuse with larger vesicles. In the presence of NH4Cl, virions accumulate in the latter structures and their uncoating appears to be prevented. Viral DNA as well as antigen synthesis were found to be significantly inhibited in the presence of these reagents. In addition, inhibition of viral DNA and antigen synthesis appeared to be most extensive when NH4Cl was present from 30 min preinfection, whereas no significant inhibition was observed when the cells were treated after 2 hr postinfection. Thus, the results indicate that CPV requires exposure to low pH in an endosomal compartment to initiate a productive infection.

Low pH-induced changes in Pseudomonas exotoxin and its domains: increased binding of Triton X-114

Pseudomonas exotoxin (PE), which is composed of three structural domains, is a 66-kilodalton protein secreted by P. aeruginosa that is cytotoxic for mammalian cells. After binding to cell surface receptors and internalization into low-pH endocytic vesicles, PE or an active fragment kills mammalian cells by translocating across an intracellular membrane to the cytoplasm and shutting down protein synthesis. To investigate possible conformational changes associated with the translocation process, full-length PE or recombinant proteins containing the PE cell recognition domain, translocation domain, enzymatic domain, or translocation plus enzymatic domains were incubated with Triton X-114 at pH values ranging from 3.0 to 7.0. The truncated forms used were intact domains that had been expressed in Escherichia coli and subsequently purified. Previous studies (K. Sandvig and J. O. Moskaug, Biochem. J. 245:899-901, 1987) had shown that full-length PE bound more Triton X-114 at a low pH than at a physiologic pH. Therefore, we investigated whether this increased binding was due to a global change in PE or a change within a particular domain. Results showed that all the truncated toxin proteins displayed a similar pH-dependent entry into the detergent phase as native PE, with a transition point of 4.2 for PE and 4.4 to 4.5 for the truncated toxins. The isoelectric points of the recombinant proteins were measured and indicate that, at a low pH (5.0), the cell recognition domain bears a net positive charge, the translocation domain bears a net negative charge, and the enzymatic domain bears no charge. The results suggest that upon acidification in the endosome, PE becomes globally hydrophobic and is converted into a translocation-competent form.

Effect of lysosomotropic compounds on early events in foot-and-mouth disease virus replication

The effect of three lysosomotropic compounds, chloroquine, monensin and NH4Cl, on the replication of foot-and-mouth disease virus (FMDV) type A12 was studied. Viral replication was almost totally inhibited by 0.5 mM chloroquine, 50 microM monensin, or 25 mM NH4Cl. Monensin and NH4Cl affected replication when added either before or within the first hour of infection. Chloroquine, however, still inhibited viral replication when added up to 2.5 h after infection. Assays of binding of radiolabeled virus to cells showed that these compounds had no effect on viral adsorption. Neither monensin nor NH4Cl had any significant effect on cellular protein synthesis, but there was no evidence of viral protein synthesis in cells infected in the presence of these compounds. In contrast, chloroquine inhibited both cellular and viral protein synthesis. Eclipse assays, performed in the presence of the compounds, showed that while chloroquine and NH4Cl had little effect on cell-induced degradation of incoming virions to 12 S protein subunits, monensin inhibited this reaction. The replication of representative members of all seven serotypes of FMDV was inhibited by monensin although some types were less sensitive to the compound than others. These results are consistent with a model which postulates that viral eclipse is the result of acidification of endocytic vesicles which degrade entrapped virions to 12 S protein subunits resulting in the release of genome RNA.