Generation of a humanized mAce2 and a conditional hACE2 mouse models permissive to SARS-COV-2 infection

The Severe Acute Respiratory Syndrome Coronavirus-2 (SARS-CoV-2) remains a public health concern and a subject of active research effort. Development of pre-clinical animal models is critical to study viral-host interaction, tissue tropism, disease mechanisms, therapeutic approaches, and long-term sequelae of infection. Here, we report two mouse models for studying SARS-CoV-2: A knock-in mAce2F83Y,H353K mouse that expresses a mouse-human hybrid form of the angiotensin-converting enzyme 2 (ACE2) receptor under the endogenous mouse Ace2 promoter, and a Rosa26 conditional knock-in mouse carrying the human ACE2 allele (Rosa26hACE2). Although the mAce2F83Y,H353K mice were susceptible to intranasal inoculation with SARS-CoV-2, they did not show gross phenotypic abnormalities. Next, we generated a Rosa26hACE2;CMV-Cre mouse line that ubiquitously expresses the human ACE2 receptor. By day 3 post infection with SARS-CoV-2, Rosa26hACE2;CMV-Cre mice showed significant weight loss, a variable degree of alveolar wall thickening and reduced survival rates. Viral load measurements confirmed inoculation in lung and brain tissues of infected Rosa26hACE2;CMV-Cre mice. The phenotypic spectrum displayed by our different mouse models translates to the broad range of clinical symptoms seen in the human patients and can serve as a resource for the community to model and explore both treatment strategies and long-term consequences of SARS-CoV-2 infection.

Strong immunogenicity & protection in mice with PlaCCine: A COVID-19 DNA vaccine formulated with a functional polymer

DNA- based vaccines have demonstrated the potential as a safe and effective modality. PlaCCine, a DNA-based vaccine approach described subsequently relies on a synthetic DNA delivery system and is independent of virus or device. The synthetic functionalized polymer combined with DNA demonstrated stability over 12 months at 4C and for one month at 25C. Transfection efficiency compared to naked DNA increased by 5-15-fold in murine skeletal muscle. Studies of DNA vaccines expressing spike proteins from variants D614G (pVAC15), Delta (pVAC16), or a D614G + Delta combination (pVAC17) were conducted. Mice immunized intramuscular injection (IM) with pVAC15, pVAC16 or pVAC17 formulated with functionalized polymer and adjuvant resulted in induction of spike-specific humoral and cellular responses. Antibody responses were observed after one immunization. And endpoint IgG titers increased to greater than 1x 105 two weeks after the second injection. Neutralizing antibodies as determined by a pseudovirus competition assay were observed following vaccination with pVAC15, pVAC16 or pVAC17. Spike specific T cell immune responses were also observed following vaccination and flow cytometry analysis demonstrated the cellular immune responses included both CD4 and CD8 spike specific T cells. The immune responses in vaccinated mice were maintained for up to 14 months after vaccination. In an immunization and challenge study of K18 hACE2 transgenic mice pVAC15, pVAC16 and pVAC17 induced immune responses lead to decreased lung viral loads by greater than 90 % along with improved clinical score. These findings suggest that PlaCCine DNA vaccines are effective and stable and further development against emerging SARS-CoV-2 variants is warranted.

The DNA glycosylase NEIL2 plays a vital role in combating SARS-CoV-2 infection

Compromised DNA repair capacity of individuals could play a critical role in the severity of SARS-CoV-2 infection-induced COVID-19. We therefore analyzed the expression of DNA repair genes in publicly available transcriptomic datasets of COVID-19 patients and found that the level of NEIL2, an oxidized base specific mammalian DNA glycosylase, is particularly low in the lungs of COVID-19 patients displaying severe symptoms. Downregulation of pulmonary NEIL2 in CoV-2-permissive animals and postmortem COVID-19 patients validated these results. To investigate the potential roles of NEIL2 in CoV-2 pathogenesis, we infected Neil2-null (Neil2-/-) mice with a mouse-adapted CoV-2 strain and found that Neil2-/- mice suffered more severe viral infection concomitant with increased expression of proinflammatory genes, which resulted in an enhanced mortality rate of 80%, up from 20% for the age matched Neil2+/+ cohorts. We also found that infected animals accumulated a significant amount of damage in their lung DNA. Surprisingly, recombinant NEIL2 delivered into permissive A549-ACE2 cells significantly decreased viral replication. Toward better understanding the mechanistic basis of how NEIL2 plays such a protective role against CoV-2 infection, we determined that NEIL2 specifically binds to the 5'-UTR of SARS-CoV-2 genomic RNA and blocks protein synthesis. Together, our data suggest that NEIL2 plays a previously unidentified role in regulating CoV-2-induced pathogenesis, via inhibiting viral replication and preventing exacerbated proinflammatory responses, and also via its well-established role of repairing host genome damage.

Efficacy and self-similarity of SARS-CoV-2 thermal decontamination

Dry heat decontamination has been shown to effectively inactivate viruses without compromising the integrity of delicate personal protective equipment (PPE), allowing safe reuse and helping to alleviate shortages of PPE that have arisen due to COVID-19. Unfortunately, current thermal decontamination guidelines rely on empirical data which are often sparse, limited to a specific virus, and unable to provide fundamental insight into the underlying inactivation reaction. In this work, we experimentally quantified dry heat decontamination of SARS-CoV-2 on disposable masks and validated a model that treats the inactivation reaction as thermal degradation of macromolecules. Furthermore, upon nondimensionalization, all of the experimental data collapse onto a unified curve, revealing that the thermally driven decontamination process exhibits self-similar behavior. Our results show that heating surgical masks to 70 °C for 5 min inactivates over 99.9% of SARS-CoV-2. We also characterized the chemical and physical properties of disposable masks after heat treatment and did not observe degradation. The model presented in this work enables extrapolation of results beyond specific temperatures to provide guidelines for safe PPE decontamination. The modeling framework and self-similar behavior are expected to extend to most viruses-including yet-unencountered novel viruses-while accounting for a range of environmental conditions.

SARS-CoV-2 induced oxidative stress promotes HMGB1 secretion to induce inflammation

The coronavirus disease (COVID-19) caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection is a major ongoing pandemic, which results in life-threatening pneumonia and multiple-organ failure. Although the molecular mechanisms by which SARS-CoV-2 lead to various disease sequalae remain elusive, exaggerated inflammatory and immune responses can lead to the debilitating and lethal outcomes of this disease. Understanding how immune mechanisms contribute to the development of severe symptoms is essential for designing effective countermeasures against SARS-CoV-2. Our studies show that SARS-CoV-2 induces increased extracellular secretion of high-mobility group box 1 (HMGB1), a prominent damage-associated molecular pattern family member that alerts the immune system to potential damage and triggers immediate response. The goal of this study is to dissect the molecular mechanisms that promotes the release of HMGB1 and its associated cellular signaling in response to SARS-CoV-2 infection. Studies were conducted in human lung epithelial cells including A549-ACE2 and Calu-3/2B4 cells, mice, and COVID-19 patient samples. HMGB1 levels and oxidative stress markers were measured by an immune assay, Western blot, and qRT-PCR. SARS-CoV-2 infection induced severe oxidative stress, leading to enhanced extracellular release of HMGB1 and subsequent activation of signaling cascades to promote inflammation. Therefore, modulation of HMGB1’s proinflammatory signaling may reduce SARS-CoV-2-induced inflammation by controlling oxidative and inflammatory stress in COVID-19 disease.

This study was supported by a Young Clinical Scientist Award from the Flight Attendant Medical Research Institute (FAMRI) to YMH (grant I.D. number 123385), NIH/NIAID R21 AI35619 to YMH, and Rapid Response Pilot grant from NIEHS P30 Center Grant ES030285 to YMH. Additional support was from the Institute for Translational Science at UTMB, supported by a Clinical and Translational Science Award Mentored Career Development (KL2 Award; KL2TR001441), and Pilot grant (85393) from Institute for Human Infections & Immunity at UTMB, supported by Sealy & Smith Foundation to YMH.

Respiratory syncytial virus infection induces the release of transglutaminase 2 from human airway epithelial cells

Respiratory syncytial virus (RSV) is an important human pathogen that causes severe lower respiratory tract infections in young children, the elderly, and the immunocompromised, yet no effective treatments or vaccines are available. The precise mechanism underlying RSV-induced acute airway disease and associated sequelae are not fully understood; however, early lung inflammatory and immune events are thought to play a major role in the outcome of the disease. Moreover, oxidative stress responses in the airways play a key role in the pathogenesis of RSV. Oxidative stress has been shown to elevate cytosolic calcium (Ca²⁺) levels, which in turn activate Ca²⁺-dependent enzymes, including transglutaminase 2 (TG2). Transglutaminase 2 is a multifunctional cross-linking enzyme implicated in various physiological and pathological conditions; however, its involvement in respiratory virus-induced airway inflammation is largely unknown. In this study, we demonstrated that RSV-induced oxidative stress promotes enhanced activation and release of TG2 from human lung epithelial cells as a result of its translocation from the cytoplasm and subsequent release into the extracellular space, which was mediated by Toll-like receptor (TLR)-4 and NF-κB pathways. Antioxidant treatment significantly inhibited RSV-induced TG2 extracellular release and activation via blocking viral replication. Also, treatment of RSV-infected lung epithelial cells with TG2 inhibitor significantly reduced RSV-induced matrix metalloprotease activities. These results suggested that RSV-induced oxidative stress activates innate immune receptors in the airways, such as TLRs, that can activate TG2 via the NF-κB pathway to promote cross-linking of extracellular matrix proteins, resulting in enhanced inflammation.

RBFOX2 is critical for maintaining alternative polyadenylation patterns and mitochondrial health in rat myoblasts

RBFOX2, which has a well-established role in alternative splicing, is linked to heart diseases. However, it is unclear whether RBFOX2 has other roles in RNA processing that can influence gene expression in muscle cells, contributing to heart disease. Here, we employ both 3'-end and nanopore cDNA sequencing to reveal a previously unrecognized role for RBFOX2 in maintaining alternative polyadenylation (APA) signatures in myoblasts. RBFOX2-mediated APA modulates mRNA levels and/or isoform expression of a collection of genes, including contractile and mitochondrial genes. Depletion of RBFOX2 adversely affects mitochondrial health in myoblasts, correlating with disrupted APA of mitochondrial gene Slc25a4. Mechanistically, RBFOX2 regulation of Slc25a4 APA is mediated through consensus RBFOX2 binding motifs near the distal polyadenylation site, enforcing the use of the proximal polyadenylation site. In sum, our results unveil a role for RBFOX2 in fine-tuning expression of mitochondrial and contractile genes via APA in myoblasts relevant to heart diseases.

Abstract 321: RBFOX2 is critical for maintaining alternative polyadenylation patterns in cardiomyoblasts

Background: The RNA binding protein RBFOX2 is implicated in human heart diseases. However, RBFOX2-regulated RNA networks are not well defined. RBFOX2 has a well-characterized role in alternative splicing (AS) while accumulating evidence suggests that RBFOX2 may also have a role in alternative polyadenylation (APA). Recent studies showed that RBFOX2 binds to regions close to poly(A) sites in the 3’UTR of pre-mRNAs. In addition, RBFOX2 binds to the cleavage and polyadenylation specificity factors. Therefore, we aimed to determine whether RBFOX2 has a role in regulating APA.

Method: We employed poly(A)click sequencing (PAC-seq) and DPAC (Differential Poly(A) Cluster analysis) computational pipeline to identify differential poly(A) usage and mRNA abundance. We also used nanopore sequencing to identify different spliced variants and the coordinated AS and APA events.

Results: We report that knockdown of RBFOX2 in embryonic rat heart derived cells leads to altered alternative polyadenylation (APA) of hundreds of genes. RBFOX2-mediated APA changes impacted both mRNA levels and generation of different gene isoforms. Nanopore sequencing identified full-length transcripts regulated by RBFOX2 and revealed RBFOX2-mediated isoform switches via both APA and AS in cardiac cells. Notably, RBFOX2-regulated APA networks affect genes such as Tpm1 and Tnnt1 involved in cardiac contractility. Identification of RBFOX2-regulated RNA networks provides novel insights into the pathogenesis of heart diseases in which RBFOX2 is involved and pave the way for designing therapeutics.

Conclusions: RBFOX2 regulates alternative polyadenylation via splicing dependent and independent mechanisms. RBFOX2-mediated APA affects mRNA levels of contractile genes.

Release of transglutaminase 2 from respiratory syncytial virus-infected airway epithelial cells and its role in disease pathogenesis

Respiratory syncytial virus (RSV) is the common cause of severe lower respiratory tract infections in young children, the elderly and the immunocompromised, for which no effective treatment or vaccine is currently available. RSV infections have also been linked to both the development and severity of asthma. Oxidative stress plays a major role in the pathogenesis of many lung inflammatory diseases. Our studies have shown that oxidative stress responses in the airways play a major role in the pathogenesis of RSV. Oxidative stress has been shown to elevate cytosolic calcium levels, which in turn activate calcium dependent enzymes, including transglutaminase 2 (TG2). TG2 is a transamidating acyltransferase that catalyzes Ca2+-dependent protein modifications and has been implicated in the pathogenesis of many diseases. This study’s objective was to investigate the release of TG2 in airway epithelial cells in response to RSV infection and its potential mechanism. Studies were conducted in airway epithelial cells, a human lung carcinoma cell line and normal small alveolar epithelial cells. The changes in the TG2 levels were measured by Western blot and RT-PCR after cells were pretreated with antioxidants and infected with RSV. TG2 levels were significantly increased in the secretome with a concomitant decrease in the cell lysates, cytoplasmic and nuclear extracts in response to RSV infection. Antioxidants treatment significantly inhibited RSV-induced TG2 release. TG2 release was induced only by RSV but not RSV-induced cytokines or chemokines. These results suggest that modulation of oxidative stress represents a potential novel pharmacological approach to ameliorate virus-induced acute airway disease through TG2 release blockade.

Proinflammatory Effects of Respiratory Syncytial Virus-Induced Epithelial HMGB1 on Human Innate Immune Cell Activation

High mobility group box 1 (HMGB1) is a multifunctional nuclear protein that translocates to the cytoplasm and is subsequently released to the extracellular space during infection and injury. Once released, it acts as a damage-associated molecular pattern and regulates immune and inflammatory responses. Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract infections in infants and elderly, for which no effective treatment or vaccine is currently available. This study investigated the effects of HMGB1 on cytokine secretion, as well as the involvement of NF-κB and TLR4 pathways in RSV-induced HMGB1 release in human airway epithelial cells (AECs) and its proinflammatory effects on several human primary immune cells. Purified HMGB1 was incubated with AECs (A549 and small alveolar epithelial cells) and various immune cells and measured the release of proinflammatory mediators and the activation of NF-κB and P38 MAPK. HMGB1 treatment significantly increased the phosphorylation of NF-κB and P38 MAPK but did not induce the release of cytokines/chemokines from AECs. However, addition of HMGB1 to immune cells did significantly induce the release of cytokines/chemokines and activated the NF-κB and P38 MAPK pathways. We found that activation of NF-κB accounted for RSV-induced HMGB1 secretion in AECs in a TLR4-dependent manner. These results indicated that HMGB1 secreted from AECs can facilitate the secretion of proinflammatory mediators from immune cells in a paracrine mechanism, thus promoting the inflammatory response that contributes to RSV pathogenesis. Therefore, blocking the proinflammatory function of HMGB1 may be an effective approach for developing novel therapeutics.

Antioxidant treatment ameliorates cigarette smoke and respiratory syncytial virus-induced inflammatory response by blocking HMGB1 release

Respiratory syncytial virus (RSV) is a major cause of acute lower respiratory tract infections in infants, and causes severe respiratory morbidity and mortality in the elderly. Second hand tobacco smoke is the major environmental risk factor for the development and severity of lower respiratory tract infections, however, the molecular mechanism of tobacco smoke-induced RSV severity is not fully understood. High mobility group box 1 protein (HMGB1) has been shown to functions in the regulation of cellular transcription and the activation of proinflammatory responses in many pathogenic conditions, but little is known about its molecular mechanism of action during its release in respiratory viral infections and tobacco smoke-induced lung inflammation. In this study we investigated the mechanism of HMGB1 release and associated cellular signaling in RSV and cigarette smoke (CS)-induced airway inflammation. A549 cells, a human lung carcinoma cell line and normal small alveolar epithelial (SAE) cells were pretreated with antioxidants and exposed to CS followed by RSV infection. CS exposure in combination with RSV infection synergistically enhanced HMGB1 secretion with concomitant decrease in the total cell lysates when compared to uninfected as well as each stimulus alone. NFκB and MAPK were significantly activated in CS exposed and RSV-infected cells. Treatment with antioxidant significantly inhibited RSV and CS-induced HMGB1 secretion, cytokine release, and NFκB and MAPK activation. The results of these studies will help to design new therapeutic strategies not only for RSV infection, but also for other respiratory diseases that have been shown to be caused or exacerbated by environmental toxicants such as tobacco smoke.

Nuclear Countermeasure Activity of TP508 Linked to Restoration of Endothelial Function and Acceleration of DNA Repair

There is increasing evidence that radiation-induced damage to endothelial cells and loss of endothelial function may contribute to both acute radiation syndromes and long-term effects of whole-body nuclear irradiation. Therefore, several drugs are being developed to mitigate the effects of nuclear radiation, most of these drugs will target and protect or regenerate leukocytes and platelets. Our laboratory has demonstrated that TP508, a 23-amino acid thrombin peptide, activates endothelial cells and stem cells to revascularize and regenerate tissues. We now show that TP508 can mitigate radiation-induced damage to endothelial cells in vitro and in vivo. Our in vitro results demonstrate that human endothelial cells irradiation attenuates nitric oxide (NO) signaling, disrupts tube formation and induces DNA double-strand breaks (DSB). TP508 treatment reverses radiation effects on NO signaling, restores tube formation and accelerates the repair of radiation-induced DSB. The radiation-mitigating effects of TP508 on endothelial cells were also seen in CD-1 mice where systemic injection of TP508 stimulated endothelial cell sprouting from aortic explants after 8 Gy irradiation. Systemic doses of TP508 that mitigated radiation-induced endothelial cell damage, also significantly increased survival of CD-1 mice when injected 24 h after 8.5 Gy exposure. These data suggest that increased survival observed with TP508 treatment may be due to its effects on vascular and microvascular endothelial cells. Our study supports the usage of a regenerative drug such as TP508 to activate endothelial cells as a countermeasure for mitigating the effects of nuclear radiation.

A kinetic fluorescence assay reveals unusual features of Ca⁺⁺ uptake in Plasmodium falciparum-infected erythrocytes

Background

To facilitate development within erythrocytes, malaria parasites increase their host cell uptake of diverse solutes including Ca⁺⁺. The mechanism and molecular basis of increased Ca⁺⁺ permeability remains less well studied than that of other solutes.

Methods

Based on an appropriate Ca⁺⁺ affinity and its greater brightness than related fluorophores, Fluo-8 was selected and used to develop a robust fluorescence-based assay for Ca⁺⁺ uptake by human erythrocytes infected with Plasmodium falciparum.

Results

Both uninfected and infected cells exhibited a large Ca⁺⁺-dependent fluorescence signal after loading with the Fluo-8 dye. Probenecid, an inhibitor of erythrocyte organic anion transporters, abolished the fluorescence signal in uninfected cells; in infected cells, this agent increased fluorescence via mechanisms that depend on parasite genotype. Kinetic fluorescence measurements in 384-well microplates revealed that the infected cell Ca⁺⁺ uptake is not mediated by the plasmodial surface anion channel (PSAC), a parasite nutrient channel at the host membrane; it also appears to be distinct from mammalian Ca⁺⁺ channels. Imaging studies confirmed a low intracellular Ca⁺⁺ in uninfected cells and higher levels in both the host and parasite compartments of infected cells. Parasite growth inhibition studies revealed a conserved requirement for extracellular Ca⁺⁺.

Conclusions

Nondestructive loading of Fluo-8 into human erythrocytes permits measurement of Ca⁺⁺ uptake kinetics. The greater Ca⁺⁺ permeability of cells infected with malaria parasites is apparent when probenecid is used to inhibit Fluo-8 efflux at the host membrane. This permeability is mediated by a distinct pathway and may be essential for intracellular parasite development. The miniaturized assay presented here should help clarify the precise transport mechanism and may identify inhibitors suitable for antimalarial drug development.

Proteolysis at a specific extracellular residue implicates integral membrane CLAG3 in malaria parasite nutrient channels

The plasmodial surface anion channel mediates uptake of nutrients and other solutes into erythrocytes infected with malaria parasites. The clag3 genes of P. falciparum determine this channel’s activity in human malaria, but how the encoded proteins contribute to transport is unknown. Here, we used proteases to examine the channel’s composition and function. While proteases with distinct specificities all cleaved within an extracellular domain of CLAG3, they produced differing degrees of transport inhibition. Chymotrypsin-induced inhibition depended on parasite genotype, with channels induced by the HB3 parasite affected to a greater extent than those of the Dd2 clone. Inheritance of functional proteolysis in the HB3×Dd2 genetic cross, DNA transfection, and gene silencing experiments all pointed to the clag3 genes, providing independent evidence for a role of these genes. Protease protection assays with a Dd2-specific inhibitor and site-directed mutagenesis revealed that a variant L1115F residue on a CLAG3 extracellular loop contributes to inhibitor binding and accounts for differences in functional proteolysis. These findings indicate that surface-exposed CLAG3 is the relevant pool of this protein for channel function. They also suggest structural models for how exposed CLAG3 domains contribute to pore formation and parasite nutrient uptake.

PfSETvs methylation of histone H3K36 represses virulence genes in Plasmodium falciparum

The variant antigen Plasmodium falciparum erythrocyte membrane protein 1 (PfEMP1), which is expressed on the surface of P. falciparum-infected red blood cells, is a critical virulence factor for malaria. Each parasite has 60 antigenically distinct var genes that each code for a different PfEMP1 protein. During infection the clonal parasite population expresses only one gene at a time before switching to the expression of a new variant antigen as an immune-evasion mechanism to avoid the host antibody response. The mechanism by which 59 of the 60 var genes are silenced remains largely unknown. Here we show that knocking out the P. falciparum variant-silencing SET gene (here termed PfSETvs), which encodes an orthologue of Drosophila melanogaster ASH1 and controls histone H3 lysine 36 trimethylation (H3K36me3) on var genes, results in the transcription of virtually all var genes in the single parasite nuclei and their expression as proteins on the surface of individual infected red blood cells. PfSETvs-dependent H3K36me3 is present along the entire gene body, including the transcription start site, to silence var genes. With low occupancy of PfSETvs at both the transcription start site of var genes and the intronic promoter, expression of var genes coincides with transcription of their corresponding antisense long noncoding RNA. These results uncover a previously unknown role of PfSETvs-dependent H3K36me3 in silencing var genes in P. falciparum that might provide a general mechanism by which orthologues of PfSETvs repress gene expression in other eukaryotes. PfSETvs knockout parasites expressing all PfEMP1 proteins may also be applied to the development of a malaria vaccine.

A complex of three related membrane proteins is conserved on malarial merozoites

Invasion of human red blood cells by the malaria parasite Plasmodium falciparum is a coordinated, multi-step process. Here, we describe three novel integral membrane proteins that colocalize on the inner membrane complex immediately beneath the merozoite plasma membrane. Each has six predicted transmembrane domains and is conserved in diverse apicomplexan parasites. Immunoprecipitation studies using specific antibodies reveal that these proteins assemble into a heteromeric complex. Each protein was also expressed on insect cells using the baculovirus vector system with a truncated SUMO tag that facilitates maximal expression and protein purification while permitting cleavage with SUMO protease to release unmodified parasite protein. The expressed proteins were successfully reconstituted into artificial liposomes, but were not recognized by human immune sera. Because all three genes are highly conserved in apicomplexan parasites, the complex formed by their encoded proteins likely serves an essential role for invasive merozoites.

Electrophysiological studies of malaria parasite-infected erythrocytes: current status

The altered permeability characteristics of erythrocytes infected with malaria parasites have been a source of interest for over 30 years. Recent electrophysiological studies have provided strong evidence that these changes reflect transmembrane transport through ion channels in the host erythrocyte plasma membrane. However, conflicting results and differing interpretations of the data have led to confusion in this field. In an effort to unravel these issues, the groups involved recently came together for a week of discussion and experimentation. In this article, the various models for altered transport are reviewed, together with the areas of consensus in the field and those that require a better understanding.