An ovine model for investigation of the microenvironment of the male mammary gland

The specific biology of the male breast remains relatively unexplored in spite of the increasing global prevalence of male breast cancer. Delineation of the microenvironment of the male breast is restricted by the low availability of human samples and a lack of characterisation of appropriate animal models. Unlike the mouse, the male ovine gland persists postnatally. We suggest that the male ovine mammary gland constitutes a promising adjunctive model for the male breast. In this study, we evaluate the male ovine mammary gland microenvironment, comparing intact and neutered males. Assessment of the glandular histo-anatomy highlights the resemblance of the male gland to that of neonatal female sheep and confirms the presence of rudimentary terminal duct lobular units. Irrespective of neutered status, cell proliferation in epithelial and stromal compartments is similarly low in males, and cell proliferation in epithelial cells and in the intralobular stroma is significantly lower than in pubertal female sheep. Between 42% and 72% of the luminal mammary epithelial cells in the male gland express the androgen receptor and expression is significantly reduced by neutering. Luminal epithelial cells within the intact and neutered male gland also express oestrogen receptor alpha, but minimal progesterone receptor expression is observed. The distribution of leukocytes within the ducts and stroma is similar to the mammary gland of female sheep and females of other species. Both macrophages and T lymphocytes are intercalated in the epithelial bilayer and are more abundant in the intralobular stroma than the interlobular stroma, suggesting that they may have a protective immunological function within the vestigial glandular tissue of the male sheep. Mast cells are also observed within the stroma. These cells cluster near the glandular tissue and are frequently located adjacent to blood vessels. The abundance of mast cells is significantly higher in intact males compared to neutered males, suggesting that hormone signalling may impact mast cell recruitment. In this study, we demonstrate the utility of the male ovine mammary gland as a model for furthering our knowledge of postnatal male mammary biology.

In the Murine and Bovine Maternal Mammary Gland Signal Transducer and Activator of Transcription 3 is Activated in Clusters of Epithelial Cells around the Day of Birth

Signal transducers and activators of transcription (STAT) proteins regulate mammary development. Here we investigate the expression of phosphorylated STAT3 (pSTAT3) in the mouse and cow around the day of birth. We present localised colocation analysis, applicable to other mammary studies requiring identification of spatially congregated events. We demonstrate that pSTAT3-positive events are multifocally clustered in a non-random and statistically significant fashion. Arginase-1 expressing cells, consistent with macrophages, exhibit distinct clustering within the periparturient mammary gland. These findings represent a new facet of mammary STAT3 biology, and point to the presence of mammary sub-microenvironments.

Ames test study designs for nitrosamine mutagenicity testing: qualitative and quantitative analysis of key assay parameters

The robust control of genotoxic N-nitrosamine (NA) impurities is an important safety consideration for the pharmaceutical industry, especially considering recent drug product withdrawals. NAs belong to the 'cohort of concern' list of genotoxic impurities (ICH M7) because of the mutagenic and carcinogenic potency of this chemical class. In addition, regulatory concerns exist regarding the capacity of the Ames test to predict the carcinogenic potential of NAs because of historically discordant results. The reasons postulated to explain these discordant data generally point to aspects of Ames test study design. These include vehicle solvent choice, liver S9 species, bacterial strain, compound concentration, and use of pre-incubation versus plate incorporation methods. Many of these concerns have their roots in historical data generated prior to the harmonization of Ames test guidelines. Therefore, we investigated various Ames test assay parameters and used qualitative analysis and quantitative benchmark dose modelling to identify which combinations provided the most sensitive conditions in terms of mutagenic potency. Two alkyl-nitrosamines, N-nitrosodimethylamine (NDMA) and N-nitrosodiethylamine (NDEA) were studied. NDMA and NDEA mutagenicity was readily detected in the Ames test and key assay parameters were identified that contributed to assay sensitivity rankings. The pre-incubation method (30-min incubation), appropriate vehicle (water or methanol), and hamster-induced liver S9, alongside Salmonella typhimurium strains TA100 and TA1535 and Escherichia coli strain WP2uvrA(pKM101) provide the most sensitive combination of assay parameters in terms of NDMA and NDEA mutagenic potency in the Ames test. Using these parameters and further quantitative benchmark dose modelling, we show that N-nitrosomethylethylamine (NMEA) is positive in Ames test and therefore should no longer be considered a historically discordant NA. The results presented herein define a sensitive Ames test design that can be deployed for the assessment of NAs to support robust impurity qualifications.

N-Nitrosodimethylamine investigations in Muta™Mouse define point-of-departure values and demonstrate less-than-additive somatic mutant frequency accumulations

The N-nitrosamine, N-nitrosodimethylamine (NDMA), is an environmental mutagen and rodent carcinogen. Small levels of NDMA have been identified as an impurity in some commonly used drugs, resulting in several product recalls. In this study, NDMA was evaluated in an OECD TG-488 compliant Muta™Mouse gene mutation assay (28-day oral dosing across seven daily doses of 0.02-4 mg/kg/day) using an integrated design that assessed mutation at the transgenic lacZ locus in various tissues and at the endogenous Pig-a gene-locus, along with micronucleus frequencies in peripheral blood. Liver pathology was determined together with NDMA exposure in blood and liver. The additivity of mutation induction was assessed by including two acute single-dose treatment groups (i.e. 5 and 10 mg/kg dose on Day 1), which represented the same total dose as two of the repeat dose treatment groups. NDMA did not induce statistically significant increases in mean lacZ mutant frequency (MF) in bone marrow, spleen, bladder, or stomach, nor in peripheral blood (Pig-a mutation or micronucleus induction) when tested up to 4 mg/kg/day. There were dose-dependent increases in mean lacZ MF in the liver, lung, and kidney following 28-day repeat dosing or in the liver and kidney after a single dose (10 mg/kg). No observed genotoxic effect levels (NOGEL) were determined for the positive repeat dose-response relationships. Mutagenicity did not exhibit simple additivity in the liver since there was a reduction in MF following NDMA repeat dosing compared with acute dosing for the same total dose. Benchmark dose modelling was used to estimate point of departure doses for NDMA mutagenicity in Muta™Mouse and rank order target organ tissue sensitivity (liver > kidney or lung). The BMD50 value for liver was 0.32 mg/kg/day following repeat dosing (confidence interval 0.21-0.46 mg/kg/day). In addition, liver toxicity was observed at doses of ≥ 1.1 mg/kg/day NDMA and correlated with systemic and target organ exposure. The integration of these results and their implications for risk assessment are discussed.

Label-free cell segmentation of diverse lymphoid tissues in 2D and 3D

Unlocking and quantifying fundamental biological processes through tissue microscopy requires accurate, in situ segmentation of all cells imaged. Currently, achieving this is complex and requires exogenous fluorescent labels that occupy significant spectral bandwidth, increasing the duration and complexity of imaging experiments while limiting the number of channels remaining to address the study's objectives. We demonstrate that the excitation light reflected during routine confocal microscopy contains sufficient information to achieve accurate, label-free cell segmentation in 2D and 3D. This is achieved using a simple convolutional neural network trained to predict the probability that reflected light pixels belong to either nucleus, cytoskeleton, or background classifications. We demonstrate the approach across diverse lymphoid tissues and provide video tutorials demonstrating deployment in Python and MATLAB or via standalone software for Windows.

Spatial statistics is a comprehensive tool for quantifying cell neighbor relationships and biological processes via tissue image analysis

Automated microscopy and computational image analysis has transformed cell biology, providing quantitative, spatially resolved information on cells and their constituent molecules from the sub-micron to the whole-organ scale. Here we explore the application of spatial statistics to the cellular relationships within tissue microscopy data and discuss how spatial statistics offers cytometry a powerful yet underused mathematical tool set for which the required data are readily captured using standard protocols and microscopy equipment. We also highlight the often-overlooked need to carefully consider the structural heterogeneity of tissues in terms of the applicability of different statistical measures and their accuracy and demonstrate how spatial analyses offer a great deal more than just basic quantification of biological variance. Ultimately, we highlight how statistical modeling can help reveal the hierarchical spatial processes that connect the properties of individual cells to the establishment of biological function.

Insulin expression in β cells is reduced within islets before islet loss in diabetic cats

OBJECTIVES

Diabetes mellitus is a common condition that requires intensive treatment and markedly impacts the welfare of affected cats. The aim of this study was to identify diabetes mellitus-associated perturbations in the feline pancreatic islet microenvironment. The utility of "clear, unobstructed brain/body imaging cocktails and computational analysis" (CUBIC) for three-dimensional pancreatic analysis was investigated.

METHODS

Formalin-fixed paraffin-embedded tissues from cats with diabetes mellitus, or control cats without pancreatic pathology, were retrospectively identified. Immunohistochemistry for synaptophysin and ionised calcium binding adaptor molecule 1, and immunofluorescence for insulin and synaptophysin, were used to assess changes in islets. An image analysis pipeline was developed to analyse images acquired from two-dimensional immunofluorescence. CUBIC was used to optically clear selected pancreas samples before immunofluorescence and deep three-dimensional confocal microscopy.

RESULTS

Diabetic cats have a significant reduction in synaptophysin-positive islet area. Whilst islets from diabetic patients have similar numbers of β cells to islets from control cats, significantly lower intensity of insulin expression can be observed in the former. CUBIC facilitates clear visualisation of pancreatic islets in three dimensions.

CLINICAL SIGNIFICANCE

The data presented support the theory that there is a decrease in function of β cells before their destruction, suggesting a potentially significant step in the pathogenesis of feline diabetes mellitus. In parallel, we demonstrate CUBIC as a valuable new tool to visualise the shape of feline pancreatic islets and to interrogate pathology occurring in the islets of diabetic pets.

Formulation of Metal-Organic Framework-Based Drug Carriers by Controlled Coordination of Methoxy PEG Phosphate: Boosting Colloidal Stability and Redispersibility

Metal-organic framework nanoparticles (nanoMOFs) have been widely studied in biomedical applications. Although substantial efforts have been devoted to the development of biocompatible approaches, the requirement of tedious synthetic steps, toxic reagents, and limitations on the shelf life of nanoparticles in solution are still significant barriers to their translation to clinical use. In this work, we propose a new postsynthetic modification of nanoMOFs with phosphate-functionalized methoxy polyethylene glycol (mPEG-PO3) groups which, when combined with lyophilization, leads to the formation of redispersible solid materials. This approach can serve as a facile and general formulation method for the storage of bare or drug-loaded nanoMOFs. The obtained PEGylated nanoMOFs show stable hydrodynamic diameters, improved colloidal stability, and delayed drug-release kinetics compared to their parent nanoMOFs. Ex situ characterization and computational studies reveal that PEGylation of PCN-222 proceeds in a two-step fashion. Most importantly, the lyophilized, PEGylated nanoMOFs can be completely redispersed in water, avoiding common aggregation issues that have limited the use of MOFs in the biomedical field to the wet form-a critical limitation for their translation to clinical use as these materials can now be stored as dried samples. The in vitro performance of the addition of mPEG-PO3 was confirmed by the improved intracellular stability and delayed drug-release capability, including lower cytotoxicity compared with that of the bare nanoMOFs. Furthermore, z-stack confocal microscopy images reveal the colocalization of bare and PEGylated nanoMOFs. This research highlights a facile PEGylation method with mPEG-PO3, providing new insights into the design of promising nanocarriers for drug delivery.

Developing ovine mammary terminal duct lobular units have a dynamic mucosal and stromal immune microenvironment

The human breast and ovine mammary gland undergo striking levels of postnatal development, leading to formation of terminal duct lobular units (TDLUs). Here we interrogate aspects of sheep TDLU growth as a model of breast development and to increase understanding of ovine mammogenesis. The distributions of epithelial nuclear Ki67 positivity differ significantly between younger and older lambs. Ki67 expression is polarised to the leading edge of the developing TDLUs. Intraepithelial ductal macrophages exhibit periodicity and considerably increased density in lambs approaching puberty. Stromal macrophages are more abundant centrally than peripherally. Intraepithelial T lymphocytes are more numerous in older lambs. Stromal hotspots of Ki67 expression colocalize with immune cell aggregates that exhibit distinct organisation consistent with tertiary lymphoid structures. The lamb mammary gland thus exhibits a dynamic mucosal and stromal immune microenvironment and constitutes a valuable model system that provides new insights into postnatal breast development.

Empirical Comparison of Genotoxic Potency Estimations: The In Vitro DNA-Damage ToxTracker Endpoints versus the In Vivo Micronucleus Assay

Genetic toxicology is an essential component of compound safety assessment. In the face of a barrage of new compounds, higher throughput, less ethically divisive in vitro approaches capable of effective, human-relevant hazard identification and prioritisation are increasingly important. One such approach is the ToxTracker assay, which utilises murine stem cell lines equipped with green fluorescent protein (GFP)-reporter gene constructs that each inform on distinct aspects of cellular perturbation. Encouragingly, ToxTracker has shown improved sensitivity and specificity for the detection of known in vivo genotoxicants when compared to existing ‘standard battery’ in vitro tests. At the current time however, quantitative genotoxic potency correlations between ToxTracker and well-recognised in vivo tests are not yet available. Here we use dose–response data from the three DNA-damage-focused ToxTracker endpoints and from the in vivo micronucleus assay to carry out quantitative, genotoxic potency estimations for a range of aromatic amine and alkylating agents using the benchmark dose (BMD) approach. This strategy, using both the exponential and the Hill BMD model families, was found to produce robust, visually intuitive and similarly ordered genotoxic potency rankings for 17 compounds across the BSCL2-GFP, RTKN-GFP and BTG2-GFP ToxTracker endpoints. Eleven compounds were similarly assessed using data from the in vivo micronucleus assay. Cross-systems genotoxic potency correlations for the eight matched compounds demonstrated in vitro–in vivo correlation, albeit with marked scatter across compounds. No evidence for distinct differences in the sensitivity of the three ToxTracker endpoints was found. The presented analyses show that quantitative potency determinations from in vitro data enable more than just qualitative screening and hazard identification in genetic toxicology.

Image-Based Cell Profiling Enables Quantitative Tissue Microscopy in Gastroenterology

Immunofluorescence microscopy is an essential tool for tissue-based research, yet data reporting is almost always qualitative. Quantification of images, at the per-cell level, enables "flow cytometry-type" analyses with intact locational data but achieving this is complex. Gastrointestinal tissue, for example, is highly diverse: from mixed-cell epithelial layers through to discrete lymphoid patches. Moreover, different species (e.g., rat, mouse, and humans) and tissue preparations (paraffin/frozen) are all commonly studied. Here, using field-relevant examples, we develop open, user-friendly methodology that can encompass these variables to provide quantitative tissue microscopy for the field. Antibody-independent cell labeling approaches, compatible across preparation types and species, were optimized. Per-cell data were extracted from routine confocal micrographs, with semantic machine learning employed to tackle densely packed lymphoid tissues. Data analysis was achieved by flow cytometry-type analyses alongside visualization and statistical definition of cell locations, interactions and established microenvironments. First, quantification of Escherichia coli passage into human small bowel tissue, following Ussing chamber incubations exemplified objective quantification of rare events in the context of lumen-tissue crosstalk. Second, in rat jejenum, precise histological context revealed distinct populations of intraepithelial lymphocytes between and directly below enterocytes enabling quantification in context of total epithelial cell numbers. Finally, mouse mononuclear phagocyte-T cell interactions, cell expression and significant spatial cell congregations were mapped to shed light on cell-cell communication in lymphoid Peyer's patch. Accessible, quantitative tissue microscopy provides a new window-of-insight to diverse questions in gastroenterology. It can also help combat some of the data reproducibility crisis associated with antibody technologies and over-reliance on qualitative microscopy. © 2020 The Authors. Cytometry Part A published by Wiley Periodicals LLC. on behalf of International Society for Advancement of Cytometry.

A Murine Oral-Exposure Model for Nano- and Micro-Particulates: Demonstrating Human Relevance with Food-Grade Titanium Dioxide

Human exposure to persistent, nonbiological nanoparticles and microparticles via the oral route is continuous and large scale (10¹² -10¹³ particles per day per adult in Europe). Whether this matters or not is unknown but confirmed health risks with airborne particle exposure warns against complacency. Murine models of oral exposure will help to identify risk but, to date, lack validation or relevance to humans. This work addresses that gap. It reports i) on a murine diet, modified with differing concentrations of the common dietary particle, food grade titanium dioxide (fgTiO₂ ), an additive of polydisperse form that contains micro- and nano-particles, ii) that these diets deliver particles to basal cells of intestinal lymphoid follicles, exactly as is reported as a "normal occurrence" in humans, iii) that confocal reflectance microscopy is the method of analytical choice to determine this, and iv) that food intake, weight gain, and Peyer's patch immune cell profiles, up to 18 weeks of feeding, do not differ between fgTiO₂ -fed groups or controls. These findings afford a human-relevant and validated oral dosing protocol for fgTiO₂ risk assessment as well as provide a generalized platform for application to oral exposure studies with nano- and micro-particles.

The origin of heterogeneous nanoparticle uptake by cells

Understanding nanoparticle uptake by biological cells is fundamentally important to wide-ranging fields from nanotoxicology to drug delivery. It is now accepted that the arrival of nanoparticles at the cell is an extremely complicated process, shaped by many factors including unique nanoparticle physico-chemical characteristics, protein-particle interactions and subsequent agglomeration, diffusion and sedimentation. Sequentially, the nanoparticle internalisation process itself is also complex, and controlled by multiple aspects of a cell's state. Despite this multitude of factors, here we demonstrate that the statistical distribution of the nanoparticle dose per endosome is independent of the initial administered dose and exposure duration. Rather, it is the number of nanoparticle containing endosomes that are dependent on these initial dosing conditions. These observations explain the heterogeneity of nanoparticle delivery at the cellular level and allow the derivation of simple, yet powerful probabilistic distributions that accurately predict the nanoparticle dose delivered to individual cells across a population.

In vitro detection of in vitro secondary mechanisms of genotoxicity induced by engineered nanomaterials

BACKGROUND

It is well established that toxicological evaluation of engineered nanomaterials (NMs) is vital to ensure the health and safety of those exposed to them. Further, there is a distinct need for the development of advanced physiologically relevant in vitro techniques for NM hazard prediction due to the limited predictive power of current in vitro models and the unsustainability of conducting nano-safety evaluations in vivo. Thus, the purpose of this study was to develop alternative in vitro approaches to assess the potential of NMs to induce genotoxicity by secondary mechanisms.

RESULTS

This was first undertaken by a conditioned media-based technique, whereby cell culture media was transferred from differentiated THP-1 (dTHP-1) macrophages treated with γ-Fe₂O₃ or Fe₃O₄ superparamagnetic iron oxide nanoparticles (SPIONs) to the bronchial cell line 16HBE14o^-. Secondly construction and SPION treatment of a co-culture model comprising of 16HBE14o^- cells and dTHP-1 macrophages. For both of these approaches no cytotoxicity was detected and chromosomal damage was evaluated by the in vitro micronucleus assay. Genotoxicity assessment was also performed using 16HBE14o^- monocultures, which demonstrated only γ-Fe₂O₃ nanoparticles to be capable of inducing chromosomal damage. In contrast, immune cell conditioned media and dual cell co-culture SPION treatments showed both SPION types to be genotoxic to 16HBE14o^- cells due to secondary genotoxicity promoted by SPION-immune cell interaction.

CONCLUSIONS

The findings of the present study demonstrate that the approach of using single in vitro cell test systems precludes the ability to consider secondary genotoxic mechanisms. Consequently, the use of multi-cell type models is preferable as they better mimic the in vivo environment and thus offer the potential to enhance understanding and detection of a wider breadth of potential damage induced by NMs.

Investigating FlowSight® imaging flow cytometry as a platform to assess chemically induced micronuclei using human lymphoblastoid cells in vitro

Use of imaging flow cytometry to assess induced DNA damage via the cytokinesis block micronucleus (CBMN) assay has thus far been limited to radiation dosimetry in human lymphocytes using high end, 'ImageStream X' series imaging cytometers. Its potential to enumerate chemically induced DNA damage using in vitro cell lines remains unexplored. In the present manuscript, we investigate the more affordable FlowSight® imaging cytometry platform to assess in vitro micronucleus (MN) induction in the human lymphoblastoid TK6 and metabolically competent MCL-5 cells treated with Methyl Methane Sulfonate (MMS) (0-5 µg/ml), Carbendazim (0-1.6 µg/ml), and Benzo[a]Pyrene (B[a]P) (0-6.3 µg/ml) for a period of 1.5-2 cell-cycles. Cells were fixed, and nuclei and MN were stained using the fluorescent nuclear dye DRAQ5™. Image acquisition was carried out using a 20X objective on a FlowSight® imaging cytometer (Amnis, part of Merck Millipore) equipped with a 488 nm laser. Populations of ∼20000 brightfield cell images, alongside DRAQ5™ stained nuclei/MN were rapidly collected (≤10 min). Single, in-focus cells suitable for scoring were then isolated using the IDEAS® software. An overlay of the brightfield cell outlines and the DRAQ5 nuclear fluorescence was used to facilitate scoring of mono-, bi-, tri-, and tetra-nucleated cells with or without MN events and in context of the cytoplasmic boundary of the parent cell.To establish the potential of the FlowSight® platform, and to establish 'ground truth' cell classification for the supervised machine learning based scoring algorithm that represents the next stage of our project, the captured images were scored manually. Alongside, MN frequencies were also derived using the 'gold standard' light microscopy and manual scoring. A minimum of 3000 bi-nucleated cells were assessed using both approaches. Using the benchmark dose approach, the comparability of genotoxic potency estimations for the different compounds and cell lines was assessed across the two scoring platforms as highly similar. This study therefore provides essential proof-of-concept that FlowSight® imaging cytometry is capable of reproducing the results of 'gold standard' manual scoring by light microscopy. We conclude that, with the right automated scoring algorithm, imaging flow cytometry could revolutionise the reportability and scoring throughput of the CBMN assay.

Glycoprotein D of HSV-1 is dependent on tegument protein UL16 for packaging and contains a motif that is differentially required for syncytia formation

Glycoprotein D (gD) of herpes simplex virus type 1 (HSV-1) plays a key role in multiple events during infection including virus entry, cell-to-cell spread, and virus-induced syncytia formation. Here, we provide evidence that an arginine/lysine cluster located at the transmembrane-cytoplasm interface of gD critically contributes to viral spread and cell-cell fusion. Our studies began with the discovery that packaging of gD into virions is almost completely blocked in the absence of tegument protein UL16. We subsequently identified a novel, direct, and regulated interaction between UL16 and gD, but this was not important for syncytia formation. However, a mutational analysis of the membrane-proximal basic residues of gD revealed that they are needed for the gBsyn phenotype, salubrinal-induced fusion of HSV-infected cells, and cell-to-cell spread. Finally, we found that these same gD tail basic residues are not required for cell fusion induced by a gKsyn variant.

The HSV-1 mechanisms of cell-to-cell spread and fusion are critically dependent on host PTP1B

All herpesviruses have mechanisms for passing through cell junctions, which exclude neutralizing antibodies and offer a clear path to neighboring, uninfected cells. In the case of herpes simplex virus type 1 (HSV-1), direct cell-to-cell transmission takes place between epithelial cells and sensory neurons, where latency is established. The spreading mechanism is poorly understood, but mutations in four different HSV-1 genes can dysregulate it, causing neighboring cells to fuse to produce syncytia. Because the host proteins involved are largely unknown (other than the virus entry receptor), we were intrigued by an earlier discovery that cells infected with wild-type HSV-1 will form syncytia when treated with salubrinal. A biotinylated derivative of this drug was used to pull down cellular complexes, which were analyzed by mass spectrometry. One candidate was a protein tyrosine phosphatase (PTP1B), and although it ultimately proved not to be the target of salubrinal, it was found to be critical for the mechanism of cell-to-cell spread. In particular, a highly specific inhibitor of PTP1B (CAS 765317-72-4) blocked salubrinal-induced fusion, and by itself resulted in a dramatic reduction in the ability of HSV-1 to spread in the presence of neutralizing antibodies. The importance of this phosphatase was confirmed in the absence of drugs by using PTP1B^-/- cells. Importantly, replication assays showed that virus titers were unaffected when PTP1B was inhibited or absent. Only cell-to-cell spread was altered. We also examined the effects of salubrinal and the PTP1B inhibitor on the four Syn mutants of HSV-1, and strikingly different responses were found. That is, both drugs individually enhanced fusion for some mutants and reduced fusion for others. PTP1B is the first host factor identified to be specifically required for cell-to-cell spread, and it may be a therapeutic target for preventing HSV-1 reactivation disease.

It is estimated that 67% of the global population is infected with herpes simplex virus type 1 (HSV-1). This virus resides in sensory neurons in a quiescent state but periodically reactivates, producing virus particles that travel down the axon to infect epithelial cells of the skin, where it can be transmitted to additional people. To avoid neutralizing antibodies, herpesviruses have evolved mechanisms for moving directly from one cell to another through their sites of intimate contact; however, the mechanism of cell-to-cell spread is poorly understood. Studies of HSV-1 mutants have implicated numerous viral proteins, but the necessary cellular factors are unknown except for the one that the virus uses to enter cells. Our experiments have identified a cellular enzyme (PTP1B, a tyrosine phosphatase) that is dispensable for the production of infectious virions but is critically important for the cell-to-cell spreading mechanism. Promising drugs targeting PTP1B have already been tested in early clinical trials for possible treatment of obesity and type-2 diabetes, and thus, our study may have immediate utility for attenuating HSV-1 reactivation disease in immunocompromised patients.

Benchmark dose analyses of multiple genetic toxicity endpoints permit robust, cross-tissue comparisons of MutaMouse responses to orally delivered benzo[a]pyrene

Genetic damage is a key event in tumorigenesis, and chemically induced genotoxic effects are a human health concern. Although genetic toxicity data have historically been interpreted using a qualitative screen-and-bin approach, there is increasing interest in quantitative analysis of genetic toxicity dose-response data. We demonstrate an emerging use of the benchmark dose (BMD)-approach for empirically ranking cross-tissue sensitivity. Using a model environmental carcinogen, we quantitatively examined responses for four genetic damage endpoints over an extended dose range, and conducted cross-tissue sensitivity rankings using BMD100 values and their 90% confidence intervals (CIs). MutaMouse specimens were orally exposed to 11 doses of benzo[a]pyrene. DNA adduct frequency and lacZ mutant frequency (MF) were measured in up to 8 tissues, and Pig-a MF and micronuclei (MN) were assessed in immature (RETs) and mature red blood cells (RBCs). The cross-tissue BMD pattern for lacZ MF is similar to that observed for DNA adducts, and is consistent with an oral route-of-exposure and differences in tissue-specific metabolism and proliferation. The lacZ MF BMDs were significantly correlated with the tissue-matched adduct BMDs, demonstrating a consistent adduct conversion rate across tissues. The BMD CIs, for both the Pig-a and the MN endpoints, overlapped for RETs and RBCs, suggesting comparable utility of both cell populations for protracted exposures. Examination of endpoint-specific response maxima illustrates the difficulty of comparing BMD values for a fixed benchmark response across endpoints. Overall, the BMD-approach permitted robust comparisons of responses across tissues/endpoints, which is valuable to our mechanistic understanding of how benzo[a]pyrene induces genetic damage.

Characterizing Nanoparticles in Biological Matrices: Tipping Points in Agglomeration State and Cellular Delivery In Vitro

Understanding the delivered cellular dose of nanoparticles is imperative in nanomedicine and nanosafety, yet is known to be extremely complex because of multiple interactions between nanoparticles, their environment, and the cells. Here, we use 3-D reconstruction of agglomerates preserved by cryogenic snapshot sampling and imaged by electron microscopy to quantify the "bioavailable dose" that is presented at the cell surface and formed by the process of individual nanoparticle sequestration into agglomerates in the exposure media. Critically, using 20 and 40 nm carboxylated polystyrene-latex and 16 and 85 nm silicon dioxide nanoparticles, we show that abrupt, dose-dependent "tipping points" in agglomeration state can arise, subsequently affecting cellular delivery and increasing toxicity. These changes are triggered by shifts in the ratio of the total nanoparticle surface area to biomolecule abundance, with the switch to a highly agglomerated state effectively changing the test article midassay, challenging the dose-response paradigm for nanosafety experiments. By characterizing nanoparticle numbers per agglomerate, we show these tipping points can lead to the formation of extreme agglomeration states whereby 90% of an administered dose is contained and delivered to the cells by just the top 2% of the largest agglomerates. We thus demonstrate precise definition, description, and comparison of the nanoparticle dose formed in different experimental environments and show that this description is critical to understanding cellular delivery and toxicity. We further empirically "stress-test" the commonly used dynamic light scattering approach, establishing its limitations to present an analysis strategy that significantly improves the usefulness of this popular nanoparticle characterization technique.

Comparing BMD-derived genotoxic potency estimations across variants of the transgenic rodent gene mutation assay

There is growing interest in quantitative analysis of in vivo genetic toxicity dose-response data, and use of point-of-departure (PoD) metrics such as the benchmark dose (BMD) for human health risk assessment (HHRA). Currently, multiple transgenic rodent (TGR) assay variants, employing different rodent strains and reporter transgenes, are used for the assessment of chemically-induced genotoxic effects in vivo. However, regulatory issues arise when different PoD values (e.g., lower BMD confidence intervals or BMDLs) are obtained for the same compound across different TGR assay variants. This study therefore employed the BMD approach to examine the ability of different TGR variants to yield comparable genotoxic potency estimates. Review of over 2000 dose-response datasets identified suitably-matched dose-response data for three compounds (ethyl methanesulfonate or EMS, N-ethyl-N-nitrosourea or ENU, and dimethylnitrosamine or DMN) across four commonly-used murine TGR variants (Muta™Mouse lacZ, Muta™Mouse cII, gpt delta and BigBlue® lacI). Dose-response analyses provided no conclusive evidence that TGR variant choice significantly influences the derived genotoxic potency estimate. This conclusion was reliant upon taking into account the importance of comparing BMD confidence intervals as opposed to directly comparing PoD values (e.g., comparing BMDLs). Comparisons with earlier works suggested that with respect to potency determination, tissue choice is potentially more important than choice of TGR assay variant. Scoring multiple tissues selected on the basis of supporting toxicokinetic information is therefore recommended. Finally, we used typical within-group variances to estimate preliminary endpoint-specific benchmark response (BMR) values across several TGR variants/tissues. We discuss why such values are required for routine use of genetic toxicity PoDs for HHRA. Environ. Mol. Mutagen. 58:632-643, 2017. © 2017 Her Majesty the Queen in Right of Canada. Environmental and Molecular Mutagenesis Published by Wiley Periodicals, Inc.

An Analysis of the Practicalities of Multi-Color Nanoparticle Cellular Bar-Coding

Many applications in biomedical research require the long-term identification and tracking of cells over time. In previous work we have demonstrated that by sequentially dosing a cell population with different emission wavelength nanoparticles it is possible to use the random number of nanoparticle loaded vesicles generated by the cells as a barcode for individual cells within the population. In this paper we develop a simple model to describe the number of codes that can be generated using this sequential loading protocol. The methodology is validated by comparison with experiment and subsequently used to predict the effect of varying the number of colors used to encode the cells and also to assess the effect of misreading the cellular code due to errors in imaging the vesicles.

Critical review of the current and future challenges associated with advanced in vitro systems towards the study of nanoparticle (secondary) genotoxicity

With the need to understand the potential biological impact of the plethora of nanoparticles (NPs) being manufactured for a wide range of potential human applications, due to their inevitable human exposure, research activities in the field of NP toxicology has grown exponentially over the last decade. Whilst such increased research efforts have elucidated an increasingly significant knowledge base pertaining to the potential human health hazard posed by NPs, understanding regarding the possibility for NPs to elicit genotoxicity is limited. In vivo models are unable to adequately discriminate between the specific modes of action associated with the onset of genotoxicity. Additionally, in line with the recent European directives, there is an inherent need to move away from invasive animal testing strategies. Thus, in vitro systems are an important tool for expanding our mechanistic insight into NP genotoxicity. Yet uncertainty remains concerning their validity and specificity for this purpose due to the unique challenges presented when correlating NP behaviour in vitro and in vivo This review therefore highlights the current state of the art in advanced in vitro systems and their specific advantages and disadvantages from a NP genotoxicity testing perspective. Key indicators will be given related to how these systems might be used or improved to enhance understanding of NP genotoxicity.

Genetic toxicity assessment of engineered nanoparticles using a 3D in vitro skin model (EpiDerm™)

Background

The rapid production and incorporation of engineered nanomaterials into consumer products alongside research suggesting nanomaterials can cause cell death and DNA damage (genotoxicity) makes in vitro assays desirable for nanosafety screening. However, conflicting outcomes are often observed when in vitro and in vivo study results are compared, suggesting more physiologically representative in vitro models are required to minimise reliance on animal testing.

Method

BASF Levasil® silica nanoparticles (16 and 85 nm) were used to adapt the 3D reconstructed skin micronucleus (RSMN) assay for nanomaterials administered topically or into the growth medium. 3D dose-responses were compared to a 2D micronucleus assay using monocultured human B cells (TK6) after standardising dose between 2D / 3D assays by total nanoparticle mass to cell number. Cryogenic vitrification, scanning electron microscopy and dynamic light scattering techniques were applied to characterise in-medium and air-liquid interface exposures. Advanced transmission electron microscopy imaging modes (high angle annular dark field) and X-ray spectrometry were used to define nanoparticle penetration / cellular uptake in the intact 3D models and 2D monocultured cells.

Results

For all 2D exposures, significant (p < 0.002) increases in genotoxicity were observed (≥100 μg/mL) alongside cell viability decreases (p < 0.015) at doses ≥200 μg/mL (16 nm-SiO₂) and ≥100 μg/mL (85 nm-SiO₂). In contrast, 2D-equivalent exposures to the 3D models (≤300 μg/mL) caused no significant DNA damage or impact on cell viability. Further increasing dose to the 3D models led to probable air-liquid interface suffocation. Nanoparticle penetration / cell uptake analysis revealed no exposure to the live cells of the 3D model occurred due to the protective nature of the skin model’s 3D cellular microarchitecture (topical exposures) and confounding barrier effects of the collagen cell attachment layer (in-medium exposures). 2D monocultured cells meanwhile showed extensive internalisation of both silica particles causing (geno)toxicity.

Conclusions

The results establish the importance of tissue microarchitecture in defining nanomaterial exposure, and suggest 3D in vitro models could play a role in bridging the gap between in vitro and in vivo outcomes in nanotoxicology. Robust exposure characterisation and uptake assessment methods (as demonstrated) are essential to interpret nano(geno)toxicity studies successfully.

Electronic supplementary material

The online version of this article (doi:10.1186/s12989-016-0161-5) contains supplementary material, which is available to authorized users.

Empirical analysis of BMD metrics in genetic toxicology part II: in vivo potency comparisons to promote reductions in the use of experimental animals for genetic toxicity assessment

Genotoxicity tests have traditionally been used only for hazard identification, with qualitative dichotomous groupings being used to identify compounds that have the capacity to induce mutations and/or cytogenetic alterations. However, there is an increasing interest in employing quantitative analysis of in vivo dose-response data to derive point of departure (PoD) metrics that can be used to establish human exposure limits or margins of exposure (MOEs), thereby supporting human health risk assessments and regulatory decisions. This work is an extension of our companion article on in vitro dose-response analyses and outlines how the combined benchmark dose (BMD) approach across included covariates can be used to improve the analyses and interpretation of in vivo genetic toxicity dose-response data. Using the BMD-covariate approach, we show that empirical comparisons of micronucleus frequency dose-response data across multiple studies justifies dataset merging, with subsequent analyses improving the precision of BMD estimates and permitting attendant potency ranking of seven clastogens. Similarly, empirical comparisons of Pig-a mutant phenotype frequency data collected in males and females justified dataset merging across sex. This permitted more effective scrutiny regarding the effect of post-exposure sampling time on the mutagenicity of N-ethyl-N-nitrosourea observed in reticulocytes and erythrocytes in the Pig-a assay. The BMD-covariate approach revealed tissue-specific differences in the induction of lacZ transgene mutations in Muta™Mouse specimens exposed to benzo[a]pyrene (BaP), with the results permitting the formulation of mechanistic hypotheses regarding the observed potency ranking. Lastly, we illustrate how historical dose-response data for assessments that examined numerous doses (i.e. induced lacZ mutant frequency (MF) across 10 doses of BaP) can be used to improve the precision of BMDs derived from datasets with far fewer doses (i.e. lacZ MF for 3 doses of dibenz[a,h]anthracene). Collectively, the presented examples illustrate how innovative use of the BMD approach can permit refinement of the use of in vivo data; improving the efficacy of experimental animal use in genetic toxicology without sacrificing PoD precision.

Empirical analysis of BMD metrics in genetic toxicology part I: in vitro analyses to provide robust potency rankings and support MOA determinations

Genetic toxicity testing has traditionally been used for hazard identification, with dichotomous classification of test results serving to identify genotoxic agents. However, the utility of genotoxicity data can be augmented by employing dose-response analysis and point of departure determination. Via interpolation from a fitted dose-response model, the benchmark dose (BMD) approach estimates the dose that elicits a specified (small) effect size. BMD metrics and their confidence intervals can be used for compound potency ranking within an endpoint, as well as potency comparisons across other factors such as cell line or exposure duration. A recently developed computational method, the BMD covariate approach, permits combined analysis of multiple dose-response data sets that are differentiated by covariates such as compound, cell type or exposure regime. The approach provides increased BMD precision for effective potency rankings across compounds and other covariates that pertain to a hypothesised mode of action (MOA). To illustrate these applications, the covariate approach was applied to the analysis of published in vitro micronucleus frequency dose-response data for ionising radiations, a set of aneugens, two mutagenic azo compounds and a topoisomerase II inhibitor. The ionising radiation results show that the precision of BMD estimates can be improved by employing the covariate method. The aneugen analysis provided potency groupings based on the BMD confidence intervals, and analyses of azo compound data from cells lines with differing metabolic capacity confirmed the influence of endogenous metabolism on genotoxic potency. This work, which is the first of a two-part series, shows that BMD-derived potency rankings can be employed to support MOA evaluations as well as facilitate read across to expedite chemical evaluations and regulatory decision-making. The follow-up (Part II) employs the combined covariate approach to analyse in vivo genetic toxicity dose-response data focussing on how improvements in BMD precision can impact the reduction and refinement of animal use in toxicological research.

Comparison of in vitro and in vivo clastogenic potency based on benchmark dose analysis of flow cytometric micronucleus data

The application of flow cytometry as a scoring platform for both in vivo and in vitro micronucleus (MN) studies has enabled the efficient generation of high quality datasets suitable for comprehensive assessment of dose-response. Using this information, it is possible to obtain precise estimates of the clastogenic potency of chemicals. We illustrate this by estimating the in vivo and the in vitro potencies of seven model clastogenic agents (melphalan, chlorambucil, thiotepa, 1,3-propane sultone, hydroxyurea, azathioprine and methyl methanesulfonate) by deriving BMDs using freely available BMD software (PROAST). After exposing male rats for 3 days with up to nine dose levels of each individual chemical, peripheral blood samples were collected on Day 4. These chemicals were also evaluated for in vitro MN induction by treating TK6 cells with up to 20 concentrations in quadruplicate. In vitro MN frequencies were determined via flow cytometry using a 96-well plate autosampler. The estimated in vitro and in vivo BMDs were found to correlate to each other. The correlation showed considerable scatter, as may be expected given the complexity of the whole animal model versus the simplicity of the cell culture system. Even so, the existence of the correlation suggests that information on the clastogenic potency of a compound can be derived from either whole animal studies or cell culture-based models of chromosomal damage. We also show that the choice of the benchmark response, i.e. the effect size associated with the BMD, is not essential in establishing the correlation between both systems. Our results support the concept that datasets derived from comprehensive genotoxicity studies can provide quantitative dose-response metrics. Such investigational studies, when supported by additional data, might then contribute directly to product safety investigations, regulatory decision-making and human risk assessment.

Poisson-event-based analysis of cell proliferation

A protocol for the assessment of cell proliferation dynamics is presented. This is based on the measurement of cell division events and their subsequent analysis using Poisson probability statistics. Detailed analysis of proliferation dynamics in heterogeneous populations requires single cell resolution within a time series analysis and so is technically demanding to implement. Here, we show that by focusing on the events during which cells undergo division rather than directly on the cells themselves a simplified image acquisition and analysis protocol can be followed, which maintains single cell resolution and reports on the key metrics of cell proliferation. The technique is demonstrated using a microscope with 1.3 μm spatial resolution to track mitotic events within A549 and BEAS‐2B cell lines, over a period of up to 48 h. Automated image processing of the bright field images using standard algorithms within the ImageJ software toolkit yielded 87% accurate recording of the manually identified, temporal, and spatial positions of the mitotic event series. Analysis of the statistics of the interevent times (i.e., times between observed mitoses in a field of view) showed that cell division conformed to a nonhomogeneous Poisson process in which the rate of occurrence of mitotic events, λ exponentially increased over time and provided values of the mean inter mitotic time of 21.1 ± 1.2 hours for the A549 cells and 25.0 ± 1.1 h for the BEAS‐2B cells. Comparison of the mitotic event series for the BEAS‐2B cell line to that predicted by random Poisson statistics indicated that temporal synchronisation of the cell division process was occurring within 70% of the population and that this could be increased to 85% through serum starvation of the cell culture. © 2015 The Authors. Published by Wiley Periodicals, Inc.

New approaches to advance the use of genetic toxicology analyses for human health risk assessment

Genetic toxicology testing has a crucial role in the safety assessment of substances of societal value by reducing human exposure to potential somatic and germ cell mutagens.

Nanoparticle vesicle encoding for imaging and tracking cell populations

For phenotypic behavior to be understood in the context of cell lineage and local environment, properties of individual cells must be measured relative to population-wide traits. However, the inability to accurately identify, track and measure thousands of single cells via high-throughput microscopy has impeded dynamic studies of cell populations. We demonstrate unique labeling of cells, driven by the heterogeneous random uptake of fluorescent nanoparticles of different emission colors. By sequentially exposing a cell population to different particles, we generated a large number of unique digital codes, which corresponded to the cell-specific number of nanoparticle-loaded vesicles and were visible within a given fluorescence channel. When three colors are used, the assay can self-generate over 17,000 individual codes identifiable using a typical fluorescence microscope. The color-codes provided immediate visualization of cell identity and allowed us to track human cells with a success rate of 78% across image frames separated by 8 h.

Automation and validation of micronucleus detection in the 3D EpiDerm™ human reconstructed skin assay and correlation with 2D dose responses

Recent restrictions on the testing of cosmetic ingredients in animals have resulted in the need to test the genotoxic potential of chemicals exclusively in vitro prior to licensing. However, as current in vitro tests produce some misleading positive results, sole reliance on such tests could prevent some chemicals with safe or beneficial exposure levels from being marketed. The 3D human reconstructed skin micronucleus (RSMN) assay is a promising new in vitro approach designed to assess genotoxicity of dermally applied compounds. The assay utilises a highly differentiated in vitro model of the human epidermis. For the first time, we have applied automated micronucleus detection to this assay using MetaSystems Metafer Slide Scanning Platform (Metafer), demonstrating concordance with manual scoring. The RSMN assay’s fixation protocol was found to be compatible with the Metafer, providing a considerably shorter alternative to the recommended Metafer protocol. Lowest observed genotoxic effect levels (LOGELs) were observed for mitomycin-C at 4.8 µg/ml and methyl methanesulfonate (MMS) at 1750 µg/ml when applied topically to the skin surface. In-medium dosing with MMS produced a LOGEL of 20 µg/ml, which was very similar to the topical LOGEL when considering the total mass of MMS added. Comparisons between 3D medium and 2D LOGELs resulted in a 7-fold difference in total mass of MMS applied to each system, suggesting a protective function of the 3D microarchitecture. Interestingly, hydrogen peroxide (H₂O₂), a positive clastogen in 2D systems, tested negative in this assay. A non-genotoxic carcinogen, methyl carbamate, produced negative results, as expected. We also demonstrated expression of the DNA repair protein N-methylpurine-DNA glycosylase in EpiDerm™. Our preliminary validation here demonstrates that the RSMN assay may be a valuable follow-up to the current in vitro test battery, and together with its automation, could contribute to minimising unnecessary in vivo tests by reducing in vitro misleading positives.

Myristylation and palmitylation of HSV-1 UL11 are not essential for its function

All herpesviruses encode a homolog of the herpes simplex virus type-1 UL11 tegument protein. Deletion of UL11 disrupts virus envelopment, causes capsid accumulation within the cytoplasm, and reduces virus release. UL11 requires acylation with myristate and palmitate for membrane binding, lipid raft trafficking, and accumulation at the site of virus envelopment. Thus, it was predicted that acylation of UL11 would be necessary for efficient virion production, similar to HIV-1 Gag which requires myristylation for virus production. Accordingly, recombinant viruses were created to express UL11 derivatives that are not acylated, are partially acylated, or contain foreign acylation signals. Unexpectedly, the non-acylated UL11 rescued some growth defects of a UL11-null mutant, even though the unmodified protein was unstable. Furthermore, a myristylated and palmitylated chimera did not fully rescue the null virus. These results suggest that UL11 maintains some function(s) when not membrane-bound, and the sequence context of the acylations is important for UL11 function.

Sequences in the UL11 tegument protein of herpes simplex virus that control association with detergent-resistant membranes

The product of the UL11 gene of HSV-1 is a small, membrane-bound tegument protein with features that are conserved among all herpesviruses. For all viruses examined, mutants lacking this protein (or its homolog) have budding defects and accumulate capsids in the cytoplasm of the infected cell. UL11 binds to the cytoplasmic faces of host membranes via N-terminal myristate and nearby palmitate moieties. These fatty-acid modifications are typical of proteins that localize to detergent-resistant membranes (DRMs), and the experiments described here revealed that a small amount (approximately 10%) of UL11 retains the ability to float in sucrose gradients following treatment of cells with Triton X-100. However, mutants lacking sequences previously shown to be involved in the trafficking of UL11 from the plasma membrane (LI and acidic cluster motifs) were found to have a dramatically increased association with DRMs. These findings emphasize the dynamic properties of this poorly-understood but conserved tegument protein.

Incorporation of the herpes simplex virus type 1 tegument protein VP22 into the virus particle is independent of interaction with VP16

Herpes simplex virus type 1 (HSV-1) virions contain a proteinaceous layer termed the tegument that lies between the nucleocapsid and viral envelope. The mechanisms underlying tegumentation remain largely undefined for all herpesviruses. Using glutathione S-transferase (GST) pulldowns and coimmunoprecipitation studies, we have identified a domain of the tegument protein VP22 that facilitates interaction with VP16. This region of VP22 (residues 165-225) overlaps the glycoprotein E (gE) binding domain of VP22 (residues 165-270), which is sufficient to mediate VP22 packaging into assembling virus particles. To ascertain the contribution of the VP16 and gE binding activities of VP22 to its virion incorporation, a transfection/infection based virion incorporation assay, using point mutants that discern between the two binding activities, was utilized. Our results suggest that interaction with VP16 is not required for incorporation of VP22 into virus particles and that binding to the cytoplasmic tail of gE is sufficient to facilitate packaging.

Dynamic interactions of the UL16 tegument protein with the capsid of herpes simplex virus

The UL16 tegument protein of herpes simplex virus is conserved throughout the herpesvirus family. It has been reported to be capsid associated and may be involved in budding by providing an interaction with the membrane-bound UL11 protein. UL16 has been shown to be present in all the major locations that capsids are found (i.e., the nucleus, cytoplasm, and virions), but whether it is actually capsid associated in each of these has not been reported. Therefore, capsids were purified from each compartment, and it was found that UL16 was present on cytoplasmic but not nuclear capsids. In extracellular virions, the majority of UL16 (87%) was once again not capsid associated, which suggests that the interaction is transient during egress. Because herpes simplex virus (HSV) buds into the acidic compartment of the trans-Golgi network (TGN), the effect of pH on the interaction was examined. The amount of capsid-associated UL16 dramatically increased when extracellular virions were exposed to mildly acidic medium (pH 5.0 to 5.5), and this association was fully reversible. After budding into the TGN, capsid and tegument proteins also encounter an oxidizing environment, which is conducive to disulfide bond formation. UL16 contains 20 cysteines, including five that are conserved within a putative zinc finger. Any free cysteines that are involved in the capsid interaction or release mechanism of UL16 would be expected to be modified by N-ethylmaleimide, and, consistent with this, the amount of capsid-associated UL16 dramatically increased when virions were incubated with this compound. Taken together, these data suggest a transient interaction between UL16 and capsids, possibly modified in the acidic compartment of secretory vesicles and requiring a release mechanism that involves cysteines.

Genetic Studies of the beta-hairpin loop of Rous sarcoma virus capsid protein

The first few residues of the Rous sarcoma virus (RSV) CA protein comprise a structurally dynamic region that forms part of a Gag-Gag interface in immature virus particles. Dissociation of this interaction during maturation allows refolding and formation of a beta-hairpin structure important for assembly of CA monomers into the mature capsid shell. A consensus binding site for the cellular Ubc9 protein was previously identified within this region, suggesting that binding of Ubc9 and subsequent small ubiquitin-like modifier protein 1 (SUMO-1) modification of CA may play a role either in regulating the assembly activity of CA in immature particles or mature cores or in controlling postentry function(s) during the establishment of infection. In the present study, mutations designed to eliminate the consensus binding site were used to dissect the potentially overlapping functions of these residues. The resulting replication defects could not be traced to a failure to form particles of normal composition but, rather, to a deficit in genome replication. Genetic suppressors of two detrimental beta-hairpin mutations improved infectivity without restoring the consensus site or creating a novel one elsewhere. Optimal restoration of infectivity to a Lys-to-Arg mutant required a combination of secondary changes, one on the surface of each domain of CA. Rather than arguing for a critical role of Ubc9 and SUMO in RSV replication, these findings provide strong support for a structural role of the N-terminal residues and a particularly striking example of long-range interactions between regions of CA in achieving a functional core competent for genome replication.

Herpes simplex virus type 1 tegument proteins VP1/2 and UL37 are associated with intranuclear capsids

The assembly of the tegument of herpes simplex virus type 1 (HSV-1) is a complex process that involves a number of events at various sites within virus-infected cells. Our studies focused on determining whether tegument proteins, VP1/2 and UL37, are added to capsids located within the nucleus. Capsids were isolated from the nuclear fraction of HSV-1-infected cells and purified by rate-zonal centrifugation to separate B capsids (containing the scaffold proteins and no viral DNA) and C capsids (containing DNA and no scaffold proteins). Western blot analyses of these capsids indicated that VP1/2 associated primarily with C capsids and UL37 associated with B and C capsids. The results demonstrate that at least two of the tegument proteins of HSV-1 are associated with capsids isolated from the nuclear fraction, and these capsid-tegument protein interactions may represent initial events of the tegumentation process.

A conserved region of the herpes simplex virus type 1 tegument protein VP22 facilitates interaction with the cytoplasmic tail of glycoprotein E (gE)

Herpes simplex virus type 1 (HSV-1) virions, contain a proteinaceous layer termed the tegument that lies between the nucleocapsid and viral envelope. Current evidence suggests that viral glycoprotein tails play a role in the recruitment of tegument-coated capsids to the site of final envelopment; vesicles derived from the trans-Golgi network. We have identified an interaction between VP22, an abundant tegument protein and the cytoplasmic tail of glycoprotein E (gE). This interaction was identified by coimmunoprecipitation studies and confirmed by a glutathione-S-transferase (GST) pulldown from infected cell lysates. Truncation mutagenesis suggests that residues 165-270 of VP22 facilitate the interaction with the cytoplasmic tail of gE. In fact, this region of VP22 is sufficient to bind to gE in the absence of additional viral proteins. Using a transfection/infection-based virion incorporation assay, residues 165-270 of VP22 fused to GFP competed efficiently with wild-type VP22 for packaging into assembling virus particles.

Packaging determinants in the UL11 tegument protein of herpes simplex virus type 1

The UL11 gene of herpes simplex virus type 1 encodes a 96-amino-acid tegument protein that is myristylated, palmitylated, and phosphorylated and is found on the cytoplasmic faces of nuclear, Golgi apparatus-derived, and plasma membranes of infected cells. Although this protein is thought to play a role in virus budding, its specific function is unknown. Purified virions were found to contain approximately 700 copies of the UL11 protein per particle, making it an abundant component of the tegument. Moreover, comparisons of cell-associated and virion-associated UL11 showed that packaging is selective for underphosphorylated forms, as has been reported for several other tegument proteins. Although the mechanism by which UL11 is packaged is unknown, previous studies have identified several sequence motifs in the protein that are important for membrane binding, intracellular trafficking, and interaction with UL16, another tegument protein. To ascertain whether any of these motifs are needed for packaging, a transfection/infection-based assay was used in which mutant forms of the protein must compete with the wild type. In this assay, the entire C-terminal half of UL11 was found to be dispensable. In the N-terminal half, the sites of myristylation and palmitylation, which enable membrane-binding and Golgi apparatus-specific targeting, were found to be essential for efficient packaging. The acidic cluster motif, which is not needed for Golgi apparatus-specific targeting but is involved in recycling the protein from the plasma membrane and for the interaction with UL16, was found to be essential, too. Thus, something other than mere localization of UL11 to Golgi apparatus-derived membranes is needed for packaging. The critical factor is unlikely to be the interaction with UL16 because other mutants that fail to bind this protein (due to removal of the dileucine-like motif or substitutions with foreign acidic clusters) were efficiently packaged. Collectively, these results suggest that UL11 packaging is not driven by a passive mechanism but instead requires trafficking through a specific pathway.

The C-terminal half of TSG101 blocks Rous sarcoma virus budding and sequesters Gag into unique nonendosomal structures

Retroviral late domains (L domains) are short amino acid sequences in the Gag protein that facilitate the process of budding. L domains act by recruiting the ESCRT complexes, which normally function in the formation of multivesicular bodies. The PTAP late domain of human immunodeficiency virus (HIV) is believed to specifically recruit this machinery by binding the ESCRT protein TSG101. It was recently demonstrated that expression of a C-terminal fragment of TSG101 (TSG-3') blocked the budding of both PTAP-dependent and PPPY-dependent retroviruses. We show here that TSG-3' expression leads to the formation of large spherical entities that we call TICS (TSG-3'-induced cellular structures) in the cytoplasm. Rous sarcoma virus (RSV) and murine leukemia virus (MLV) Gag proteins are selectively recruited to these structures, but HIV type 1 Gag is completely excluded. Experiments with various HIV and RSV vector constructs as well as HIV and RSV chimeras suggest that recruitment to the TICS is late domain independent and does not involve recognition of any single amino acid sequence. TICS appear to have no limiting membrane and do not colocalize with markers for any membranous cellular compartment. Wild-type TSG101 is also recruited to TICS, but most other ESCRT proteins are excluded. These structures are similar in nature to aggresomes, colocalize with the aggresome marker GFP-250, and are highly enriched in ubiquitin but in other ways do not fully meet the description of aggresomes. We conclude that the block to retroviral budding by TSG-3' may be the result of its sequestration of Gag, depletion of free TSG101, or depletion of free ubiquitin.

Lysines close to the Rous sarcoma virus late domain critical for budding

The release of retroviruses from the plasma membrane requires host factors that are believed to be recruited to the site of budding by the late (L) domain of the virus-encoded Gag protein. The L domain of Rous sarcoma virus (RSV) has been shown to interact with a ubiquitin (Ub) ligase, and budding of this virus is dependent on Ub. RSV is similar to other retroviruses in that it contains approximately 100 molecules of Ub, but it is unique in that none of these molecules has been found to be conjugated to Gag. If transient ubiquitination of RSV Gag is required for budding, then replacement of the target lysine(s) with arginine should prevent the addition of Ub and reduce budding. Based on known sites of ubiquitination in other viruses, the important lysines would likely reside near the L domain. In RSV, there are five lysines located just upstream of the L domain in a region of the matrix (MA) protein that is dispensable for membrane binding, and replacement of these with arginine (mutant 1-5KR) reduced budding 80 to 90%. The block to budding was found to be on the plasma membrane; however, the few virions that were released had normal size, morphology, and infectivity. Budding was restored when any one of the residues was changed back to lysine or when lysines were inserted in novel positions, either within this region of MA or within the downstream p10 sequence. Moreover, the 1-5KR mutant could be rescued into particles by coexpression of budding-competent Gag molecules. These data argue that the phenotype of mutant 1-5KR is not due to a conformational defect. Consistent with the idea that efficient budding requires a specific role for lysines, human T-cell leukemia virus type 1, which does not bud well compared to RSV and lacks lysines close to its L domain, was found to be released at a higher level upon introduction of lysines near its L domain. This report strongly supports the hypothesis that ubiquitination of the RSV Gag protein (and perhaps those of other retroviruses) is needed for efficient budding.

Binding partners for the UL11 tegument protein of herpes simplex virus type 1

The product of the U(L)11 gene of herpes simplex virus type 1 (HSV-1) is a 96-amino-acid tegument protein that accumulates on the cytoplasmic face of internal membranes. Although it is thought to be important for nucleocapsid envelopment and egress, the actual function of this protein is unknown. Previous studies focused on the characterization of sequence elements within the UL11 protein that function in membrane binding and trafficking to the Golgi apparatus. Binding was found to be mediated by two fatty acyl groups (myristate and palmitate), while an acidic cluster and a dileucine motif were identified as being important for the recycling of UL11 from the plasma membrane to the Golgi apparatus. The goal of the experiments described here was to identify and characterize binding partners (viral or cellular) of UL11. Using both immunoprecipitation and glutathione S-transferase (GST) pull-down assays, we identified a 40-kDa protein that specifically associates with UL11 from infected Vero cells. Mutational analyses revealed that the acidic cluster and the dileucine motif are required for this association, whereas the entire second half of UL11 is not. In addition, UL11 homologs from pseudorabies and Marek's disease herpesviruses were also found to be capable of binding to the 40-kDa protein from HSV-1-infected cells, suggesting that the interaction is conserved among alphaherpesviruses. Purification and analysis of the 40-kDa protein by mass spectrometry revealed that it is the product of the U(L)16 gene, a virion protein reported to be involved in nucleocapsid assembly. Cells transfected with a UL16-green fluorescent protein expression vector produced a protein that was of the expected size, could be pulled down with GST-UL11, and accumulated in a Golgi-like compartment only when coexpressed with UL11, indicating that the interaction does not require any other viral products. These data represent the first steps toward elucidating the network of tegument proteins that UL11 links to membranes.

Link between genome packaging and rate of budding for Rous sarcoma virus

The subcellular location at which genomic RNA is packaged by Gag proteins during retrovirus assembly remains unknown. Since the membrane-binding (M) domain is most critical for targeting Gag to the plasma membrane, changes to this determinant might alter the path taken through the cell and reduce the efficiency of genome packaging. In this report, a Rous sarcoma virus (RSV) mutant having two acidic-to-basic substitutions in the M domain is described. This mutant, designated Super M, produced particles much faster than the wild type, but the mutant virions were noninfectious and contained only 1/10 the amount of genomic RNA found in wild-type particles. To identify the cause(s) of these defects, we considered data that suggest that RSV Gag traffics through the nucleus to package the viral genome. Although inhibition of the CRM-1 pathway of nuclear export caused the accumulation of wild-type Gag in the nucleus, nuclear accumulation did not occur with Super M. The importance of the nucleocapsid (NC) domain in membrane targeting was also determined, and, importantly, deletion of the NC sequence prevented plasma membrane localization by wild-type Gag but not by Super M Gag. Based on these results, we reasoned that the enhanced membrane-targeting properties of Super M inhibit genome packaging. Consistent with this interpretation, substitutions that reestablished the wild-type number of basic and acidic residues in the Super M Gag M domain reduced the budding efficiency and restored genome packaging and infectivity. Therefore, these data suggest that Gag targeting and genome packaging are normally linked to ensure that RSV particles contain viral RNA.

Membrane association of VP22, a herpes simplex virus type 1 tegument protein

Tegument proteins of herpes simplex virus type 1 (HSV-1) are hypothesized to contain the functional information required for the budding or envelopment process proposed to occur at cytoplasmic compartments of the host cell. One of the most abundant tegument proteins of HSV-1 is the U(L)49 gene product, VP22, a 38-kDa protein of unknown function. To study its subcellular localization, a VP22-green fluorescent protein chimera was expressed in transfected human melanoma (A7) cells. In the absence of other HSV-1 proteins, VP22 localizes to acidic compartments of the cell that may include the trans-Golgi network (TGN), suggesting that this protein is membrane associated. Membrane pelleting and membrane flotation assays confirmed that VP22 partitions with the cellular membrane fraction. Through truncation mutagenesis, we determined that the membrane association of VP22 is a property attributed to amino acids 120 to 225 of this 301-amino-acid protein. The above results demonstrate that VP22 contains specific information required for targeting to membranes of acidic compartments of the cell which may be derived from the TGN, suggesting a potential role for VP22 during tegumentation and/or final envelopment.

In vivo interference of Rous sarcoma virus budding by cis expression of a WW domain

For all enveloped viruses, the actual mechanism by which nascent virus particles separate or "pinch off" from the cell surface is largely unknown. In the case of retroviruses, the Gag protein drives the budding process, and the virus release step is directed by the late (L) assembly domain within Gag. A PPPPY motif within the L domain of Rous sarcoma virus (RSV) was previously characterized as being critical for the release of virions and shown to interact in vitro with the WW domain of Yes-associated protein (Yap). To determine whether WW domain-L domain interactions can occur in vivo, we attempted to interfere with the host cell machinery normally recruited to the site of budding by inserting this WW domain in different locations within Gag. At a C-terminal location, the WW(Yap) domain had no effect on budding, suggesting that the intervening I domains (which provide the major region of Gag-Gag interaction) prevent its access to the L domain. When positioned on the other side of the I domains closer to the L domain, the WW(Yap) domain resulted in a dramatic interference of particle release, and confocal microscopy revealed a block to budding on the plasma membrane. Budding was restored by attachment of the heterologous L domain of human immunodeficiency virus type 1 Gag, which does not bind WW(Yap). These findings suggest that cis expression of WW domains can interfere with RSV particle release in vivo via specific, high-affinity interactions at the site of assembly on the plasma membrane, thus preventing host factor accessibility to the L domain and subsequent virus-cell separation. In addition, they suggest that L domain-specific host factors function after Gag proteins begin to interact.

Budding of equine infectious anemia virus is insensitive to proteasome inhibitors

The only retrovirus protein required for the budding of virus-like particles is the Gag protein; however, recent studies of Rous sarcoma virus (RSV) and human immunodeficiency virus have suggested that modification of Gag with ubiquitin (Ub) is also required. As a consequence, the release of these viruses is reduced in the presence of proteasome inhibitors, which indirectly reduce the levels of free Ub within the cell. Here we show that the budding of equine infectious anemia virus (EIAV) from infected equine cells is largely unaffected by these drugs, although use of one inhibitor (MG-132) resulted in a dramatic block to proteolytic processing of Gag. This lack of sensitivity was also observed in transiently transfected avian cells under conditions that greatly reduce RSV budding. Moreover, insensitivity was observed when the EIAV Gag protein was expressed in the absence of all the other virus products, indicating that they are not required for this phenotype. An activity that enables EIAV to tolerate exposure to proteasome inhibitors was mapped to the C-terminal p9 sequence, as demonstrated by the ability of an RSV Gag-p9 chimera to bud in the presence of the drugs. Intriguingly, the p9 sequence contains a short sequence motif that is similar to a surface-exposed helix of Ub, suggesting that EIAV Gag may have captured a function that allows it to bypass the need for ubiquitination. Thus, the mechanism of EIAV budding may not be substantially different from that of other retroviruses, even though it behaves differently in the presence of proteasome inhibitors.

Intracellular trafficking of the UL11 tegument protein of herpes simplex virus type 1

Growing evidence indicates that herpes simplex virus type 1 (HSV-1) acquires its final envelope in the trans-Golgi network (TGN). During the envelopment process, the viral nucleocapsid as well as the envelope and tegument proteins must arrive at this site in order to be incorporated into assembling virions. To gain a better understanding of how these proteins associate with cellular membranes and target to the correct compartment, we have been studying the intracellular trafficking properties of the small tegument protein encoded by the U(L)11 gene of HSV-1. This 96-amino-acid, myristylated protein accumulates on the cytoplasmic face of internal membranes, where it is thought to play a role in nucleocapsid envelopment and egress. When expressed in the absence of other HSV-1 proteins, the UL11 protein localizes to the Golgi apparatus, and previous deletion analyses have revealed that the membrane-trafficking information is contained within the first 49 amino acids. The goal of this study was to map the functional domains required for proper Golgi membrane localization. In addition to N-terminal myristylation, which allows for weak membrane binding, UL11 appears to be palmitylated on one or more of three consecutive N-terminal cysteines. Using membrane-pelleting experiments and confocal microscopy, we show that palmitylation of UL11 is required for both Golgi targeting specificity and strong membrane binding. Furthermore, we found that a conserved acidic cluster within the first half of UL11 is required for the recycling of this tegument protein from the plasma membrane to the Golgi apparatus. Taken together, our results demonstrate that UL11 has highly dynamic membrane-trafficking properties, which suggests that it may play multiple roles on the plasma membrane as well as on the nuclear and TGN membranes.

Second-site suppressors of Rous sarcoma virus Ca mutations: evidence for interdomain interactions

The capsid (CA) protein, the major structural component of retroviruses, forms a shell that encases the ribonucleoprotein complex in the virion core. The most conserved region of CA, approximately 20 amino acids of the major homology region (MHR), lies within the carboxy-terminal domain of the protein. Structural and sequence similarities among CA proteins of retroviruses and the CA-like proteins of hepatitis B virus and various retrotransposons suggest that the MHR is involved in an aspect of replication common to these reverse-transcribing elements. Conservative substitutions in this region of the Rous sarcoma virus protein were lethal due to a severe deficiency in reverse transcription, in spite of the presence of an intact genome and active reverse transcriptase in the particles. This finding suggests that the mutations interfered with normal interactions among these constituents. A total of four genetic suppressors of three lethal MHR mutations have now been identified. All four map to the sequence encoding the CA-spacer peptide (SP) region of Gag. The F167Y mutation in the MHR was fully suppressed by a single amino acid change in the alpha helix immediately downstream of the MHR, a region that forms the major dimer interface in human immunodeficiency virus CA. This finding suggests that the F167Y mutation indirectly interfered with dimerization. The F167Y defect could also be repaired by a second, independent suppressor in the C-terminal SP that was removed from CA during maturation. This single residue change, which increased the rate of SP cleavage, apparently corrected the F167Y defect by modifying the maturation pathway. More surprising was the isolation of suppressors of the R170Q and L171V MHR mutations, which mapped to the N-terminal domain of the CA protein. This finding suggests that the two domains, which in the monomeric protein are separated by a flexible linker, must communicate with each other at some unidentified point in the viral replication cycle.

Insertion of capsid proteins from nonenveloped viruses into the retroviral budding pathway

Retroviral Gag proteins direct the assembly and release of virus particles from the plasma membrane. The budding machinery consists of three small domains, the M (membrane-binding), I (interaction), and L (late or "pinching-off") domains. In addition, Gag proteins contain sequences that control particle size. For Rous sarcoma virus (RSV), the size determinant maps to the capsid (CA)-spacer peptide (SP) sequence, but it functions only when I domains are present to enable particles of normal density to be produced. Small deletions throughout the CA-SP sequence result in the release of particles that are very large and heterogeneous, even when I domains are present. In this report, we show that particles of relatively uniform size and normal density are released by budding when the size determinant and I domains in RSV Gag are replaced with capsid proteins from two unrelated, nonenveloped viruses: simian virus 40 and satellite tobacco mosaic virus. These results indicate that capsid proteins of nonenveloped viruses can interact among themselves within the context of Gag and be inserted into the retroviral budding pathway merely by attaching the M and L domains to their amino termini. Thus, the differences in the assembly pathways of enveloped and nonenveloped viruses may be far simpler than previously thought.

Repositioning basic residues in the M domain of the Rous sarcoma virus gag protein

The first 86 residues of the Rous sarcoma virus (RSV) Gag protein form a membrane-binding (M) domain that directs Gag to the plasma membrane during budding. Unlike other retroviral Gag proteins, RSV Gag is not myristylated; however, the RSV M domain does contain 11 basic residues that could potentially interact with acidic phospholipids in the plasma membrane. To investigate this possibility, we analyzed mutants in which basic residues in the M domain were replaced with asparagines or glutamines. The data show that neutralizing as few as two basic residues in the M domain blocked particle release and prevented Gag from localizing to the plasma membrane. Though not as severe, single neutralizations also diminished budding and, when expressed in the context of proviral clones, reduced the ability of RSV to spread in cell cultures. To further explore the role of basic residues in particle production, we added lysines to new positions in the M domain. Using this approach, we found that the budding efficiency of RSV Gag can be improved by adding pairs of lysines and that the basic residues in the M domain can be repositioned without affecting particle release. These data provide the first gain-of-function evidence for the importance of basic residues in a retroviral M domain and support a model in which RSV Gag binds to the plasma membrane via electrostatic interactions.

Ubiquitin is part of the retrovirus budding machinery

Retroviruses contain relatively large amounts of ubiquitin, but the significance of this finding has been unknown. Here, we show that drugs that are known to reduce the level of free ubiquitin in the cell dramatically reduced the release of Rous sarcoma virus, an avian retrovirus. This effect was suppressed by overexpressing ubiquitin and also by directly fusing ubiquitin to the C terminus of Gag, the viral protein that directs budding and particle release. The block to budding was found to be at the plasma membrane, and electron microscopy revealed that the reduced level of ubiquitin results in a failure of mature virus particles to separate from each other and from the plasma membrane during budding. These data indicate that ubiquitin is actually part of the budding machinery.