Transcriptome profiling and protease inhibition experiments identify proteases that activate H3N2 influenza A and influenza B viruses in murine airways

Development of ketobenzothiazole-based peptidomimetic TMPRSS13 inhibitors with low nanomolar potency

TMPRSS13, a member of the Type II Transmembrane Serine Proteases (TTSP) family, is involved in cancer progression and in respiratory virus cell entry. To date, no inhibitors have been specifically developed for this protease. In this study, a chemical library of 65 ketobenzothiazole-based peptidomimetic molecules was screened against a proteolytically active form of recombinant TMPRSS13 to identify novel inhibitors. Following an initial round of screening, subsequent synthesis of additional derivatives supported by molecular modelling revealed important molecular determinants involved in TMPRSS13 inhibition. One inhibitor, N-0430, achieved low nanomolar affinity towards TMPRSS13 activity in a cellular context. Using a SARS-CoV-2 pseudovirus cell entry model, we further demonstrated the ability of N-0430 to block TMPRSS13-dependent entry of the pseudovirus. The identified peptidomimetic inhibitors and the molecular insights into their potency gained from this study will aid in the development of specific TMPRSS13 inhibitors.

Exploring TMPRSS2 Drug Target to Combat Influenza and Coronavirus Infection

Respiratory viral infections, including influenza and coronaviruses, present significant health risks worldwide. The recent COVID-19 pandemic highlights the urgent need for novel and effective antiviral agents. The host cell protease, transmembrane serine protease 2 (TMPRSS2), facilitates viral pathogenesis by playing a critical role in viral invasion and disease progression. This protease is coexpressed with the viral receptors of angiotensin-converting enzyme 2 (ACE2) for SARS-CoV-2 in the human respiratory tract and plays a significant role in activating viral proteins and spreading. TMPRSS2 activates the coronavirus spike (S) protein and permits membrane fusion and viral entry by cleaving the virus surface glycoproteins. It also activates the hemagglutinin (HA) protein, an enzyme necessary for the spread of influenza virus. TMPRSS2 inhibitors can reduce viral propagation and morbidity by blocking viral entry into respiratory cells and reducing viral spread, inflammation, and disease severity. This review examines the role of TMPRSS2 in viral replication and pathogenicity. It also offers potential avenues to develop targeted antivirals to inhibit TMPRSS2 function, suggesting a possible focus on targeted antiviral development. Ultimately, the review seeks to contribute to improving public health outcomes related to these viral infections.

Unveiling the Role of TMPRSS2 in the Proteolytic Activation of Pandemic and Zoonotic Influenza Viruses and Coronaviruses in Human Airway Cells

The zoonotic transmission of influenza A viruses (IAVs) and coronaviruses (CoVs) may result in severe disease. Cleavage of the surface glycoproteins hemagglutinin (HA) and spike protein (S), respectively, is essential for viral infectivity. The transmembrane serine protease 2 (TMPRSS2) is crucial for cleaving IAV HAs containing monobasic cleavage sites and severe acute respiratory syndrome (SARS)-CoV-2 S in human airway cells. Here, we analysed and compared the TMPRSS2-dependency of SARS-CoV, Middle East respiratory syndrome (MERS)-CoV, the 1918 pandemic H1N1 IAV and IAV H12, H13 and H17 subtypes in human airway cells. We used the peptide-conjugated morpholino oligomer (PPMO) T-ex5 to knockdown the expression of active TMPRSS2 and determine the impact on virus activation and replication in Calu-3 cells. The activation of H1N1/1918 and H13 relied on TMPRSS2, whereas recombinant IAVs carrying H12 or H17 were not affected by TMPRSS2 knockdown. MERS-CoV replication was strongly suppressed in T-ex5 treated cells, while SARS-CoV was less dependent on TMPRSS2. Our data underline the importance of TMPRSS2 for certain (potentially) pandemic respiratory viruses, including H1N1/1918 and MERS-CoV, in human airways, further suggesting a promising drug target. However, our findings also highlight that IAVs and CoVs differ in TMPRSS2 dependency and that other proteases are involved in virus activation.

Human parainfluenza virus 3 field strains undergo extracellular fusion protein cleavage to activate entry

Human parainfluenza virus 3 (HPIV3) infection is driven by the coordinated action of viral surface glycoproteins hemagglutinin-neuraminidase (HN) and fusion protein (F). Receptor-engaged HN activates F to insert into the target cell membrane and drive virion-cell membrane fusion. For F to mediate entry, its precursor (F0) must first be cleaved by host proteases. F0 cleavage has been thought to be executed during viral glycoprotein transit through the trans-Golgi network by the ubiquitously expressed furin because F0 proteins of laboratory-adapted viruses contain a furin recognition dibasic cleavage motif RXKR around residue 108. Here, we show that the F proteins of field strains have a different cleavage motif from laboratory-adapted strains and are cleaved by unidentified proteases expressed in only a narrow subset of cell types. We demonstrate that extracellular serine protease inhibitors block HPIV3 F0 cleavage for field strains, suggesting F0 cleavage occurs at the cell surface facilitated by transmembrane proteases. Candidate proteases that may process HPIV3 F in vivo were identified by a genome-wide CRISPRa screen in HEK293/dCas9-VP64 + MPH cells. The lung-expressed extracellular serine proteases TMPRSS2 and TMPRSS13 are both sufficient to cleave HPIV3 F and enable infectious virus release by otherwise non-permissive cells. Our findings support an alternative mechanism of F activation in vivo, reliant on extracellular membrane-bound serine proteases expressed in a narrow subset of cells. The proportion of HPIV3 F proteins cleaved and infectious virus release is determined by host cell expression of requisite proteases, allowing just-in-time activation of F and positioning F cleavage as another key regulator of HPIV3 spread.

IMPORTANCE

Enveloped viruses cause a wide range of diseases in humans. At the first step of infection, these viruses must fuse their envelope with a cell membrane to initiate infection. This fusion is mediated by viral proteins that require a critical activating cleavage event. It was previously thought that for parainfluenza virus 3, an important cause of respiratory disease and a representative of a group of important pathogens, this cleavage event was mediated by furin in the cell secretory pathways prior to formation of the virions. We show that this is only true for laboratory strain viruses, and that clinical viruses that infect humans utilize extracellular proteases that are only made by a small subset of cells. These results highlight the importance of studying authentic clinical viruses that infect human tissues for understanding natural infection.

The non-canonical bivalent gene Wfdc15a controls spermatogenic protease and immune homeostasis

Male infertility can be caused by chromosomal abnormalities, mutations and epigenetic defects. Epigenetic modifiers pre-program hundreds of spermatogenic genes in spermatogonial stem cells (SSCs) for expression later in spermatids, but it remains mostly unclear whether and how those genes are involved in fertility. Here, we report that Wfdc15a, a WFDC family protease inhibitor pre-programmed by KMT2B, is essential for spermatogenesis. We found that Wfdc15a is a non-canonical bivalent gene carrying both H3K4me3 and facultative H3K9me3 in SSCs, but is later activated along with the loss of H3K9me3 and acquisition of H3K27ac during meiosis. We show that WFDC15A deficiency causes defective spermiogenesis at the beginning of spermatid elongation. Notably, depletion of WFDC15A causes substantial disturbance of the testicular protease-antiprotease network and leads to an orchitis-like inflammatory response associated with TNFα expression in round spermatids. Together, our results reveal a unique epigenetic program regulating innate immunity crucial for fertility.

Development of ketobenzothiazole-based peptidomimetic TMPRSS13 inhibitors with low nanomolar potency

TMPRSS13, a member of the Type II Transmembrane Serine Proteases (TTSP) family, is involved in cancer progression and in cell entry of respiratory viruses. To date, no inhibitors have been specifically developed toward this protease. In this study, a chemical library of 65 ketobenzothiazole-based peptidomimetic molecules was screened against a proteolytically active form of recombinant TMPRSS13 to identify novel inhibitors. Following an initial round of screening, subsequent synthesis of additional derivatives supported by molecular modelling, uncovered important molecular determinants involved in TMPRSS13 inhibition. One inhibitor, N-0430, achieved low nanomolar affinity towards TMPRSS13 activity in a cellular context. Using a SARS-CoV-2 pseudovirus cell entry model, we further show the ability of N-0430 to block TMPRSS13-dependent entry of the pseudovirus. The identified peptidomimetic inhibitors and the molecular insights of their potency gained from this study will aid in the development of specific TMPRSS13 inhibitors.

PK/PD investigation of antiviral host matriptase/TMPRSS2 inhibitors in cell models

Certain corona- and influenza viruses utilize type II transmembrane serine proteases for cell entry, making these enzymes potential drug targets for the treatment of viral respiratory infections. In this study, the cytotoxicity and inhibitory effects of seven matriptase/TMPRSS2 inhibitors (MI-21, MI-463, MI-472, MI-485, MI-1900, MI-1903, and MI-1904) on cytochrome P450 enzymes were evaluated using fluorometric assays. Additionally, their antiviral activity against influenza A virus subtypes H1N1 and H9N2 was assessed. The metabolic depletion rates of these inhibitors in human primary hepatocytes were determined over a 120-min period by LC-MS/MS, and PK parameters were calculated. The tested compounds, with the exception of MI-21, displayed potent inhibition of CYP3A4, while all compounds lacked inhibitory effects on CYP1A2, CYP2C9, CYP2C19, and CYP2D6. The differences between the CYP3A4 activity within the series were rationalized by ligand docking. Elucidation of PK parameters showed that inhibitors MI-463, MI-472, MI-485, MI-1900 and MI-1904 were more stable compounds than MI-21 and MI-1903. Anti-H1N1 properties of inhibitors MI-463 and MI-1900 and anti-H9N2 effects of MI-463 were shown at 20 and 50 µM after 24 h incubation with the inhibitors, suggesting that these inhibitors can be applied to block entry of these viruses by suppressing host matriptase/TMPRSS2-mediated cleavage.

Analysis of alpha-1-antitrypsin (AAT)-regulated, glucocorticoid receptor-dependent genes in macrophages reveals a novel host defense function of AAT

Alpha-1-antitrypsin (AAT) plays a homeostatic role in attenuating excessive inflammation and augmenting host defense against microbes. We demonstrated previously that AAT binds to the glucocorticoid receptor (GR) resulting in significant anti-inflammatory and antimycobacterial consequences in macrophages. Our current investigation aims to uncover AAT-regulated genes that rely on GR in macrophages. We incubated control THP-1 cells (THP-1control) and THP-1 cells knocked down for GR (THP-1GR-KD) with AAT, performed bulk RNA sequencing, and analyzed the findings. In THP-1control cells, AAT significantly upregulated 408 genes and downregulated 376 genes. Comparing THP-1control and THP-1GR-KD cells, 125 (30.6%) of the AAT-upregulated genes and 154 (41.0%) of the AAT-downregulated genes were significantly dependent on GR. Among the AAT-upregulated, GR-dependent genes, CSF-2 that encodes for granulocyte-monocyte colony-stimulating factor (GM-CSF), known to be host-protective against nontuberculous mycobacteria, was strongly upregulated by AAT and dependent on GR. We further quantified the mRNA and protein of several AAT-upregulated, GR-dependent genes in macrophages and the mRNA of several AAT-downregulated, GR-dependent genes. We also discussed the function(s) of selected AAT-regulated, GR-dependent gene products largely in the context of mycobacterial infections. In conclusion, AAT regulated several genes that are dependent on GR and play roles in host immunity against mycobacteria.

Restriction of Viral Glycoprotein Maturation by Cellular Protease Inhibitors

The human genome is estimated to encode more than 500 proteases performing a wide range of important physiological functions. They digest proteins in our food, determine the activity of hormones, induce cell death and regulate blood clotting, for example. During viral infection, however, some proteases can switch sides and activate viral glycoproteins, allowing the entry of virions into new target cells and the spread of infection. To reduce unwanted effects, multiple protease inhibitors regulate the proteolytic processing of self and non-self proteins. This review summarizes our current knowledge of endogenous protease inhibitors, which are known to limit viral replication by interfering with the proteolytic activation of viral glycoproteins. We describe the underlying molecular mechanisms and highlight the diverse strategies by which protease inhibitors reduce virion infectivity. We also provide examples of how viruses evade the restriction imposed by protease inhibitors. Finally, we briefly outline how cellular protease inhibitors can be modified and exploited for therapeutic purposes. In summary, this review aims to summarize our current understanding of cellular protease inhibitors as components of our immune response to a variety of viral pathogens.

Palmitoylation of the hemagglutinin of influenza B virus by ER-localized DHHC enzymes 1, 2, 4, and 6 is required for efficient virus replication

IMPORTANCE

Influenza viruses are a public health concern since they cause seasonal outbreaks and occasionally pandemics. Our study investigates the importance of a protein modification called "palmitoylation" in the replication of influenza B virus. Palmitoylation involves attaching fatty acids to the viral protein hemagglutinin and has previously been studied for influenza A virus. We found that this modification is important for the influenza B virus to replicate, as mutating the sites where palmitate is attached prevented the virus from generating viable particles. Our experiments also showed that this modification occurs in the endoplasmic reticulum. We identified the specific enzymes responsible for this modification, which are different from those involved in palmitoylation of HA of influenza A virus. Overall, our research illuminates the similarities and differences in fatty acid attachment to HA of influenza A and B viruses and identifies the responsible enzymes, which might be promising targets for anti-viral therapy.

Existing Evidence for Influenza B Virus Adaptations to Drive Replication in Humans as the Primary Host

Influenza B virus (IBV) is one of the two major types of influenza viruses that circulate each year. Unlike influenza A viruses, IBV does not harbor pandemic potential due to its lack of historical circulation in non-human hosts. Many studies and reviews have highlighted important factors for host determination of influenza A viruses. However, much less is known about the factors driving IBV replication in humans. We hypothesize that similar factors influence the host restriction of IBV. Here, we compile and review the current understanding of host factors crucial for the various stages of the IBV viral replication cycle. While we discovered the research in this area of IBV is limited, we review known host factors that may indicate possible host restriction of IBV to humans. These factors include the IBV hemagglutinin (HA) protein, host nuclear factors, and viral immune evasion proteins. Our review frames the current understanding of IBV adaptations to replication in humans. However, this review is limited by the amount of research previously completed on IBV host determinants and would benefit from additional future research in this area.

Expression of TMPRSS2 is up-regulated by bacterial flagellin, LPS, and Pam3Cys in human airway cells

Many viruses require proteolytic activation of their envelope proteins for infectivity, and relevant host proteases provide promising drug targets. The transmembrane serine protease 2 (TMPRSS2) has been identified as a major activating protease of influenza A virus (IAV) and various coronaviruses (CoV). Increased TMPRSS2 expression has been associated with a higher risk of severe influenza infection and enhanced susceptibility to SARS-CoV-2. Here, we found that Legionella pneumophila stimulates the increased expression of TMPRSS2-mRNA in Calu-3 human airway cells. We identified flagellin as the dominant structural component inducing TMPRSS2 expression. The flagellin-induced increase was not observed at this magnitude for other virus-activating host proteases. TMPRSS2-mRNA expression was also significantly increased by LPS, Pam3Cys, and Streptococcus pneumoniae, although less pronounced. Multicycle replication of H1N1pdm and H3N2 IAV but not SARS-CoV-2 and SARS-CoV was enhanced by flagellin treatment. Our data suggest that bacteria, particularly flagellated bacteria, up-regulate the expression of TMPRSS2 in human airway cells and, thereby, may support enhanced activation and replication of IAV upon co-infections. In addition, our data indicate a physiological role of TMPRSS2 in antimicrobial host response.

Alpha-1-antitrypsin antagonizes COVID-19: a review of the epidemiology, molecular mechanisms, and clinical evidence

Alpha-1-antitrypsin (AAT), a serine protease inhibitor (serpin), is increasingly recognized to inhibit SARS-CoV-2 infection and counter many of the pathogenic mechanisms of COVID-19. Herein, we reviewed the epidemiologic evidence, the molecular mechanisms, and the clinical evidence that support this paradigm. As background to our discussion, we first examined the basic mechanism of SARS-CoV-2 infection and contend that despite the availability of vaccines and anti-viral agents, COVID-19 remains problematic due to viral evolution. We next underscored that measures to prevent severe COVID-19 currently exists but teeters on a balance and that current treatment for severe COVID-19 remains grossly suboptimal. We then reviewed the epidemiologic and clinical evidence that AAT deficiency increases risk of COVID-19 infection and of more severe disease, and the experimental evidence that AAT inhibits cell surface transmembrane protease 2 (TMPRSS2) - a host serine protease required for SARS-CoV-2 entry into cells - and that this inhibition may be augmented by heparin. We also elaborated on the panoply of other activities of AAT (and heparin) that could mitigate severity of COVID-19. Finally, we evaluated the available clinical evidence for AAT treatment of COVID-19.

Type II Transmembrane Serine Proteases as Modulators in Adipose Tissue Phenotype and Function

Adipose tissue is a crucial organ in energy metabolism and thermoregulation. Adipose tissue phenotype is controlled by various signaling mechanisms under pathophysiological conditions. Type II transmembrane serine proteases (TTSPs) are a group of trypsin-like enzymes anchoring on the cell surface. These proteases act in diverse tissues to regulate physiological processes, such as food digestion, salt-water balance, iron metabolism, epithelial integrity, and auditory nerve development. More recently, several members of the TTSP family, namely, hepsin, matriptase-2, and corin, have been shown to play a role in regulating lipid metabolism, adipose tissue phenotype, and thermogenesis, via direct growth factor activation or indirect hormonal mechanisms. In mice, hepsin deficiency increases adipose browning and protects from high-fat diet-induced hyperglycemia, hyperlipidemia, and obesity. Similarly, matriptase-2 deficiency increases fat lipolysis and reduces obesity and hepatic steatosis in high-fat diet-fed mice. In contrast, corin deficiency increases white adipose weights and cell sizes, suppresses adipocyte browning and thermogenic responses, and causes cold intolerance in mice. These findings highlight an important role of TTSPs in modifying cellular phenotype and function in adipose tissue. In this review, we provide a brief description about TTSPs and discuss recent findings regarding the role of hepsin, matriptase-2, and corin in regulating adipose tissue phenotype, energy metabolism, and thermogenic responses.

Circulating oxidative stress and acute phase protein levels in horses infested with ticks

Ticks have saliva rich in immunoregulatory molecules that interfere with the host's physiology in order to feed. This study aimed to evaluate the concentration of acute phase proteins and circulating oxidative stress in response to infestation by Amblyomma sculptum and Dermacentor nitens in two breed horses, Mangalarga Marchador and Breton Postier, to define resistance or susceptibility to ticks. Among the oxidative stress markers, we observed lower malondialdehyde and nitric oxide in horses with tick infestation, consequently not altering the antioxidant enzymes. Breton Postier with tick infestation showed a reduction in the ferric reducing ability of plasma (FRAP), which may be due to lower feeding of the host due to the stress caused by the infestation or even to sequestration of components induced by the tick during blood feeding. The alpha-1-antitrypsin, an acute phase protein, showed an increase in Mangalarga Marchador with tick infestation; curiously it is related to a protective action against tissue damage, pathogens and parasites. We could assume that Mangalarga Marchador showed a better response to ticks when compared to Breton Postier. However, it is still early to define the resistance or susceptibility to ticks, as we did not observe significant changes in most of the analyzed variables. Further studies are needed to understand the compounds and mechanisms of action of the tick saliva in the acute phase proteins and the possible relationships of oxidative stress in the host and the tick during blood feeding.

The role of influenza-A virus and coronavirus viral glycoprotein cleavage in host adaptation

While receptor binding is well recognized as a factor in influenza-A virus (IAV) and coronavirus (CoV) host adaptation, the role of viral glycoprotein cleavage has not been studied in detail so far. Interestingly, recent studies suggest that host species may differ in their protease repertoire available for cleavage. Furthermore, it was shown for certain bat-derived CoVs that proteolytic activation provides a critical barrier to infect human cells. Understanding the role of glycoprotein cleavage in different species and how IAV and CoVs adapt to a new protease repertoire may allow evaluating the zoonotic potential and risk posed by these viruses. Here, we summarize the current knowledge on the emergence of a multibasic cleavage site (CS) in the glycoproteins of IAVs and CoVs in different host species. Additionally, we discuss the role of transmembrane serine protease 2 (TMPRSS2) in virus activation and entry and a role of neuropilin-1 in acquisition of a multibasic CS in different hosts.

α1-Antitrypsin Binds to the Glucocorticoid Receptor with Anti-Inflammatory and Antimycobacterial Significance in Macrophages

α1-Antitrypsin (AAT), a serine protease inhibitor, is the third most abundant protein in plasma. Although the best-known function of AAT is irreversible inhibition of elastase, AAT is an acute-phase reactant and is increasingly recognized to have a panoply of other functions, including as an anti-inflammatory mediator and a host-protective molecule against various pathogens. Although a canonical receptor for AAT has not been identified, AAT can be internalized into the cytoplasm and is known to affect gene regulation. Because AAT has anti-inflammatory properties, we examined whether AAT binds the cytoplasmic glucocorticoid receptor (GR) in human macrophages. We report the finding that AAT binds to GR using several approaches, including coimmunoprecipitation, mass spectrometry, and microscale thermophoresis. We also performed in silico molecular modeling and found that binding between AAT and GR has a plausible stereochemical basis. The significance of this interaction in macrophages is evinced by AAT inhibition of LPS-induced NF-κB activation and IL-8 production as well as AAT induction of angiopoietin-like 4 protein, which are, in part, dependent on GR. Furthermore, this AAT-GR interaction contributes to a host-protective role against mycobacteria in macrophages. In summary, this study identifies a new mechanism for the gene regulation, anti-inflammatory, and host-defense properties of AAT.

Transmembrane serine protease TMPRSS2 implicated in SARS-CoV-2 infection is autoactivated intracellularly and requires N-glycosylation for regulation

Transmembrane protease serine 2 (TMPRSS2) is a membrane-bound protease expressed in many human epithelial tissues, including the airway and lung. TMPRSS2-mediated cleavage of viral spike protein is a key mechanism in severe acute respiratory syndrome coronavirus 2 activation and host cell entry. To date, the cellular mechanisms that regulate TMPRSS2 activity and cell surface expression are not fully characterized. In this study, we examined two major post-translational events, zymogen activation and N-glycosylation, in human TMPRSS2. In experiments with human embryonic kidney 293, bronchial epithelial 16HBE, and lung alveolar epithelial A549 cells, we found that TMPRSS2 was activated via intracellular autocatalysis and that this process was blocked in the presence of hepatocyte growth factor activator inhibitors 1 and 2. By glycosidase digestion and site-directed mutagenesis, we showed that human TMPRSS2 was N-glycosylated. N-glycosylation at an evolutionarily conserved site in the scavenger receptor cysteine-rich domain was required for calnexin-assisted protein folding in the endoplasmic reticulum and subsequent intracellular trafficking, zymogen activation, and cell surface expression. Moreover, we showed that TMPRSS2 cleaved severe acute respiratory syndrome coronavirus 2 spike protein intracellularly in human embryonic kidney 293 cells. These results provide new insights into the cellular mechanism in regulating TMPRSS2 biosynthesis and function. Our findings should help to understand the role of TMPRSS2 in major respiratory viral diseases.

TMPRSS13 zymogen activation, surface localization, and shedding is regulated by proteolytic cleavage within the non-catalytic stem region

TMPRSS13 is a member of the type II transmembrane serine protease (TTSP) family. Here we characterize a novel post-translational mechanism important for TMPRSS13 function: proteolytic cleavage within the extracellular TMPRSS13 stem region located between the transmembrane domain and the first site of N-linked glycosylation at asparagine (N)-250 in the scavenger receptor cysteine rich (SRCR) domain. Importantly, the catalytic competence of TMPRSS13 is essential for stem region cleavage, suggesting an autonomous mechanism of action. Site-directed mutagenesis of the 10 basic amino acids (four arginine and six lysine residues) in this region abrogated zymogen activation and catalytic activity of TMPRSS13, as well as phosphorylation, cell surface expression, and shedding. Mutation analysis of individual arginine residues identified R223, a residue located between the low-density lipoprotein receptor class A domain and the SRCR domain, as important for stem region cleavage. Mutation of R223 causes a reduction in the aforementioned functional processing steps of TMPRSS13. These data provide further insight into the roles of different post-translational modifications as regulators of the function and localization of TMPRSS13. Additionally, the data suggest the presence of complex interconnected regulatory mechanisms that may serve to ensure the proper levels of cell-surface and pericellular TMPRSS13-mediated proteolysis under homeostatic conditions.

Investigational antiviral drugs for the treatment of COVID-19 patients

In the current pandemic of coronavirus disease 2019 (COVID-19), antiviral drugs are at the center of attention because of their critical role against severe acute respiratory disease syndrome coronavirus 2 (SARS-CoV-2). In addition to designing new antivirals against SARS-COV-2, a drug repurposing strategy is a practical approach for treating COVID-19. A brief insight about antivirals would help clinicians to choose the best medication for the treatment of COVID-19. In this review, we discuss both novel and repurposed investigational antivirals, focusing on in vitro, in vivo, and clinical trial studies.

Posttranslational modifications of serine protease TMPRSS13 regulate zymogen activation, proteolytic activity, and cell surface localization

TMPRSS13, a member of the type II transmembrane serine protease (TTSP) family, harbors four N-linked glycosylation sites in its extracellular domain. Two of the glycosylated residues are located in the scavenger receptor cysteine-rich (SRCR) protein domain, while the remaining two sites are in the catalytic serine protease (SP) domain. In this study, we examined the role of N-linked glycosylation in the proteolytic activity, autoactivation, and cellular localization of TMPRSS13. Individual and combinatory site-directed mutagenesis of the glycosylated asparagine residues indicated that glycosylation of the SP domain is critical for TMPRSS13 autoactivation and catalytic activity toward one of its protein substrates, the prostasin zymogen. Additionally, SP domain glycosylation-deficient TMPRSS13 displayed impaired trafficking of TMPRSS13 to the cell surface, which correlated with increased retention in the endoplasmic reticulum. Importantly, we showed that N-linked glycosylation was a critical determinant for subsequent phosphorylation of endogenous TMPRSS13. Taken together, we conclude that glycosylation plays an important role in regulating TMPRSS13 activation and activity, phosphorylation, and cell surface localization.

Correlation between α1-Antitrypsin Deficiency and SARS-CoV-2 Infection: Epidemiological Data and Pathogenetic Hypotheses

The most common hereditary disorder in adults, α1-antitrypsin deficiency (AATD), is characterized by reduced plasma levels or the abnormal functioning of α1-antitrypsin (AAT), a major human blood serine protease inhibitor, which is encoded by the SERine Protein INhibitor-A1 (SERPINA1) gene and produced in the liver. Recently, it has been hypothesized that the geographic differences in COVID-19 infection and fatality rates may be partially explained by ethnic differences in SERPINA1 allele frequencies. In our review, we examined epidemiological data on the correlation between the distribution of AATD, SARS-CoV-2 infection, and COVID-19 mortality rates. Moreover, we described shared pathogenetic pathways that may provide a theoretical basis for our epidemiological findings. We also considered the potential use of AAT augmentation therapy in patients with COVID-19.

Hemagglutinins of avian influenza viruses are proteolytically activated by TMPRSS2 in human and murine airway cells

Cleavage of the influenza A virus (IAV) hemagglutinin (HA) by host proteases is indispensable for virus replication. Most IAVs possess a monobasic HA cleavage site cleaved by trypsin-like proteases. Previously, the transmembrane protease TMPRSS2 was shown to be essential for proteolytic activation of IAV HA subtypes H1, H2, H7 and H10 in mice. In contrast, additional proteases are involved in activation of certain H3 IAVs, indicating that HAs with monobasic cleavage site can differ in their sensitivity to host proteases. Here, we investigated the role of TMPRSS2 in proteolytic activation of avian HA subtypes H1 to H11 and H14 to H16 in human and mouse airway cell cultures. Using reassortant viruses carrying representative HAs, we analysed HA cleavage and multicycle replication in (i) lung cells of TMPRSS2-deficient mice and (ii) Calu-3 cells and primary human bronchial cells subjected to morpholino oligomer-mediated knockdown of TMPRSS2 activity. TMPRSS2 was found to be crucial for activation of H1 to H11, H14 and H15 in airway cells of human and mouse. Only H9 with an R-S-S-R cleavage site and H16 were proteolytically activated in the absence of TMPRSS2 activity, albeit with reduced efficiency. Moreover, a TMPRSS2-orthologous protease from duck supported activation of H1 to H11, H15 and H16 in MDCK cells. Together, our data demonstrate that in human and murine respiratory cells, TMPRSS2 is the major activating protease of almost all IAV HA subtypes with monobasic cleavage site. Furthermore, our results suggest that TMPRSS2 supports activation of IAV with monobasic cleavage site in ducks. Importance Human infections with avian influenza A viruses upon exposure to infected birds are frequently reported and have received attention as a potential pandemic threat. Cleavage of the envelope glycoprotein hemagglutinin (HA) by host proteases is a prerequisite for membrane fusion and essential for virus infectivity. In this study, we identify the transmembrane protease TMPRSS2 as the major activating protease of avian influenza virus HAs of subtypes H1 to H11, H14 and H15 in human and murine airway cells. Our data demonstrate that inhibition of TMPRSS2 activity may provide a useful approach for the treatment of human infections with avian influenza viruses that should be considered for pandemic preparedness as well. Additionally, we show that a TMPRSS2-orthologous protease from duck can activate avian influenza virus HAs with a monobasic cleavage site and thus represents a potential virus-activating protease in waterfowl, the primary reservoir for influenza A viruses.

Trypsin-Like Proteases and Their Role in Muco-Obstructive Lung Diseases

Trypsin-like proteases (TLPs) belong to a family of serine enzymes with primary substrate specificities for the basic residues, lysine and arginine, in the P1 position. Whilst initially perceived as soluble enzymes that are extracellularly secreted, a number of novel TLPs that are anchored in the cell membrane have since been discovered. Muco-obstructive lung diseases (MucOLDs) are characterised by the accumulation of hyper-concentrated mucus in the small airways, leading to persistent inflammation, infection and dysregulated protease activity. Although neutrophilic serine proteases, particularly neutrophil elastase, have been implicated in the propagation of inflammation and local tissue destruction, it is likely that the serine TLPs also contribute to various disease-relevant processes given the roles that a number of these enzymes play in the activation of both the epithelial sodium channel (ENaC) and protease-activated receptor 2 (PAR2). More recently, significant attention has focused on the activation of viruses such as SARS-CoV-2 by host TLPs. The purpose of this review was to highlight key TLPs linked to the activation of ENaC and PAR2 and their association with airway dehydration and inflammatory signalling pathways, respectively. The role of TLPs in viral infectivity will also be discussed in the context of the inhibition of TLP activities and the potential of these proteases as therapeutic targets.

Alpha 1 Antitrypsin is an Inhibitor of the SARS-CoV-2-Priming Protease TMPRSS2

BACKGROUND

Host proteases have been suggested to be crucial for dissemination of MERS, SARS-CoV, and SARS-CoV-2 coronaviruses, but the relative contribution of membrane versus intracellular proteases remains controversial. Transmembrane serine protease 2 (TMPRSS2) is regarded as one of the main proteases implicated in the coronavirus S protein priming, an important step for binding of the S protein to the angiotensin-converting enzyme 2 (ACE2) receptor before cell entry.

METHODS

We developed a cell-based assay to identify TMPRSS2 inhibitors. Inhibitory activity was established in SARS-CoV-2 viral load systems.

RESULTS

We identified the human extracellular serine protease inhibitor (serpin) alpha 1 anti-trypsin (A1AT) as a novel TMPRSS2 inhibitor. Structural modeling revealed that A1AT docked to an extracellular domain of TMPRSS2 in a conformation that is suitable for catalysis, resembling similar serine protease inhibitor complexes. Inhibitory activity of A1AT was established in a SARS-CoV-2 viral load system. Notably, plasma A1AT levels were associated with COVID-19 disease severity.

CONCLUSIONS

Our data support the key role of extracellular serine proteases in SARS CoV-2 infections and indicate that treatment with serpins, particularly the FDA-approved drug A1AT, may be effective in limiting SARS-CoV-2 dissemination by affecting the surface of the host cells.

Exposure of human intestinal epithelial cells and primary human hepatocytes to trypsin-like serine protease inhibitors with potential antiviral effect

Human intestinal epithelial cell line-6 (HIEC-6) cells and primary human hepatocytes (PHHs) were treated with 3-amidinophenylalanine-derived inhibitors of trypsin-like serine proteases for 24 hours. It was proven that treatment with MI-1900 and MI-1907 was tolerated up to 50 μM in HIEC-6. These inhibitors did not cause elevations in extracellular H₂O₂ levels and in the concentrations of interleukin (IL)-6 and IL-8 and did not alter occludin distribution in HIEC-6. It was also found that MI-1900 and MI-1907 up to 50 μM did not affect cell viability, IL-6 and IL-8 and occludin levels of PHH. Based on our findings, these inhibitors could be safely applicable at 50 μM in HIEC-6 and in PHH; however, redox status was disturbed in case of PHH. Moreover, it has recently been demonstrated that MI-1900 prevents the replication and spread of the new SARS-CoV-2 in infected Calu-3 cells, most-likely via an inhibition of the membrane-bound host protease TMPRSS2.

Camostat mesylate inhibits SARS-CoV-2 activation by TMPRSS2-related proteases and its metabolite GBPA exerts antiviral activity

BACKGROUND

Antivirals are needed to combat the COVID-19 pandemic, which is caused by SARS-CoV-2. The clinically-proven protease inhibitor Camostat mesylate inhibits SARS-CoV-2 infection by blocking the virus-activating host cell protease TMPRSS2. However, antiviral activity of Camostat mesylate metabolites and potential viral resistance have not been analyzed. Moreover, antiviral activity of Camostat mesylate in human lung tissue remains to be demonstrated.

METHODS

We used recombinant TMPRSS2, reporter particles bearing the spike protein of SARS-CoV-2 or authentic SARS-CoV-2 to assess inhibition of TMPRSS2 and viral entry, respectively, by Camostat mesylate and its metabolite GBPA.

FINDINGS

We show that several TMPRSS2-related proteases activate SARS-CoV-2 and that two, TMPRSS11D and TMPRSS13, are robustly expressed in the upper respiratory tract. However, entry mediated by these proteases was blocked by Camostat mesylate. The Camostat metabolite GBPA inhibited recombinant TMPRSS2 with reduced efficiency as compared to Camostat mesylate. In contrast, both inhibitors exhibited similar antiviral activity and this correlated with the rapid conversion of Camostat mesylate into GBPA in the presence of serum. Finally, Camostat mesylate and GBPA blocked SARS-CoV-2 spread in human lung tissue ex vivo and the related protease inhibitor Nafamostat mesylate exerted augmented antiviral activity.

INTERPRETATION

Our results suggest that SARS-CoV-2 can use TMPRSS2 and closely related proteases for spread in the upper respiratory tract and that spread in the human lung can be blocked by Camostat mesylate and its metabolite GBPA.

FUNDING

NIH, Damon Runyon Foundation, ACS, NYCT, DFG, EU, Berlin Mathematics center MATH+, BMBF, Lower Saxony, Lundbeck Foundation, Novo Nordisk Foundation.

Finding proteases that make cells go viral

The activation of influenza virus hemagglutinin (HA) glycoprotein via cleavage by host cell proteases is essential for viral infectivity, and understanding the mechanisms for HA protein cleavage and how they may differ depending on the biological context is important for the development of flu treatments. However, the HA proteases involved in the activation of many viral strains remain unidentified. In this issue, Harbig et al. identify a repertoire of proteases that cleave HA and determine the proteases' functionality against specific HA glycoproteins.

Hypothesis: Alpha-1-antitrypsin is a promising treatment option for COVID-19

No definitive treatment for COVID-19 exists although promising results have been reported with remdesivir and glucocorticoids. Short of a truly effective preventive or curative vaccine against SARS-CoV-2, it is becoming increasingly clear that multiple pathophysiologic processes seen with COVID-19 as well as SARS-CoV-2 itself should be targeted. Because alpha-1-antitrypsin (AAT) embraces a panoply of biologic activities that may antagonize several pathophysiologic mechanisms induced by SARS-CoV-2, we hypothesize that this naturally occurring molecule is a promising agent to ameliorate COVID-19. We posit at least seven different mechanisms by which AAT may alleviate COVID-19. First, AAT is a serine protease inhibitor (SERPIN) shown to inhibit TMPRSS-2, the host serine protease that cleaves the spike protein of SARS-CoV-2, a necessary preparatory step for the virus to bind its cell surface receptor ACE2 to gain intracellular entry. Second, AAT has anti-viral activity against other RNA viruses HIV and influenza as well as induces autophagy, a known host effector mechanism against MERS-CoV, a related coronavirus that causes the Middle East Respiratory Syndrome. Third, AAT has potent anti-inflammatory properties, in part through inhibiting both nuclear factor-kappa B (NFκB) activation and ADAM17 (also known as tumor necrosis factor-alpha converting enzyme), and thus may dampen the hyper-inflammatory response of COVID-19. Fourth, AAT inhibits neutrophil elastase, a serine protease that helps recruit potentially injurious neutrophils and implicated in acute lung injury. AAT inhibition of ADAM17 also prevents shedding of ACE2 and hence may preserve ACE2 inhibition of bradykinin, reducing the ability of bradykinin to cause a capillary leak in COVID-19. Fifth, AAT inhibits thrombin, and venous thromboembolism and in situ microthrombi and macrothrombi are increasingly implicated in COVID-19. Sixth, AAT inhibition of elastase can antagonize the formation of neutrophil extracellular traps (NETs), a complex extracellular structure comprised of neutrophil-derived DNA, histones, and proteases, and implicated in the immunothrombosis of COVID-19; indeed, AAT has been shown to change the shape and adherence of non-COVID-19-related NETs. Seventh, AAT inhibition of endothelial cell apoptosis may limit the endothelial injury linked to severe COVID-19-associated acute lung injury, multi-organ dysfunction, and pre-eclampsia-like syndrome seen in gravid women. Furthermore, because both NETs formation and the presence of anti-phospholipid antibodies are increased in both COVID-19 and non-COVID pre-eclampsia, it suggests a similar vascular pathogenesis in both disorders. As a final point, AAT has an excellent safety profile when administered to patients with AAT deficiency and is dosed intravenously once weekly but also comes in an inhaled preparation. Thus, AAT is an appealing drug candidate to treat COVID-19 and should be studied.

Priming of SARS-CoV-2 S protein by several membrane-bound serine proteinases could explain enhanced viral infectivity and systemic COVID-19 infection

The ongoing COVID-19 pandemic has already caused over a million deaths worldwide, and this death toll will be much higher before effective treatments and vaccines are available. The causative agent of the disease, the coronavirus SARS-CoV-2, shows important similarities with the previously emerged SARS-CoV-1, but also striking differences. First, SARS-CoV-2 possesses a significantly higher transmission rate and infectivity than SARS-CoV-1 and has infected in a few months over 60 million people. Moreover, COVID-19 has a systemic character, as in addition to the lungs, it also affects the heart, liver, and kidneys among other organs of the patients and causes frequent thrombotic and neurological complications. In fact, the term "viral sepsis" has been recently coined to describe the clinical observations. Here I review current structure-function information on the viral spike proteins and the membrane fusion process to provide plausible explanations for these observations. I hypothesize that several membrane-associated serine proteinases (MASPs), in synergy with or in place of TMPRSS2, contribute to activate the SARS-CoV-2 spike protein. Relative concentrations of the attachment receptor, ACE2, MASPs, their endogenous inhibitors (the Kunitz-type transmembrane inhibitors, HAI-1/SPINT1 and HAI-2/SPINT2, as well as major circulating serpins) would determine the infection rate of host cells. The exclusive or predominant expression of major MASPs in specific human organs suggests a direct role of these proteinases in e.g., heart infection and myocardial injury, liver dysfunction, kidney damage, as well as neurological complications. Thorough consideration of these factors could have a positive impact on the control of the current COVID-19 pandemic.

Smoking and COVID-19: Similar bronchial ACE2 and TMPRSS2 expression and higher TMPRSS4 expression in current versus never smokers

Uncertainties remain concerning the pathophysiology, epidemiology, and potential therapeutics for COVID-19. Among unsettled controversies is whether tobacco smoking increases or protects from severe COVID-19. Several epidemiological studies reported reduced COVID-19 hospitalizations among smokers, while other studies reported the opposite trend. Some authors assumed that smokers have elevated airway expression of ACE2, the cell recognition site of the SARS-Cov-2 spike protein, but this suggestion remains unverified. We therefore performed data mining of two independent NCBI GEO genome-wide RNA expression files (GSE7894 and GSE994) and report that in both data sets, current smokers and never smokers have, on average, closely similar bronchial epithelial cell mRNA levels of ACE2, as well as TMPRSS2, coding for a serine protease priming SARS-Cov-2 for cell entry, and ADAM17, coding for a protease implicated in ACE2 membrane shedding. In contrast, the expression levels of TMPRSS4, coding for a protease that primes SARS-CoV-2 for cell entry similarly to TMPRSS2, were elevated in bronchial epithelial cells from current smokers compared with never smokers, suggesting that higher bronchial TMPRSS4 levels in smokers might put them at higher SARS-Cov-2 infection risk. The effects of smoking on COVID-19 severity need clarification with larger studies. Additionally, the postulated protective effects of nicotine and nitric oxide, which may presumably reduce the risk of a "cytokine storm" in infected individuals, deserve assessment by controlled clinical trials.

Alpha 1 Antitrypsin is an Inhibitor of the SARS-CoV-2-Priming Protease TMPRSS2

Host proteases have been suggested to be crucial for dissemination of MERS, SARS-CoV, and SARS-CoV-2 coronaviruses, but the relative contribution of membrane versus intracellular proteases remains controversial. Transmembrane serine protease 2 (TMPRSS2) is regarded as one of the main proteases implicated in the coronavirus S protein priming, an important step for binding of the S protein to the angiotensin-converting enzyme 2 (ACE2) receptor before cell entry. The main cellular location where the SARS-CoV-2 S protein priming occurs remains debatable, therefore hampering the development of targeted treatments. Herein, we identified the human extracellular serine protease inhibitor (serpin) alpha 1 antitrypsin (A1AT) as a novel TMPRSS2 inhibitor. Structural modeling revealed that A1AT docked to an extracellular domain of TMPRSS2 in a conformation that is suitable for catalysis, resembling similar serine protease-inhibitor complexes. Inhibitory activity of A1AT was established in a SARS-CoV-2 viral load system. Notably, plasma A1AT levels were associated with COVID-19 disease severity. Our data support the key role of extracellular serine proteases in SARS-CoV-2 infections and indicate that treatment with serpins, particularly the FDA-approved drug A1AT, may be effective in limiting SARS-CoV-2 dissemination by affecting the surface of the host cells.

SUMMARY

Delivery of extracellular serine protease inhibitors (serpins) such as A1AT has the capacity to reduce SARS-CoV-2 dissemination by binding and inhibiting extracellular proteases on the host cells, thus, inhibiting the first step in SARS-CoV-2 cell cycle (i.e. cell entry).

Camostat mesylate inhibits SARS-CoV-2 activation by TMPRSS2-related proteases and its metabolite GBPA exerts antiviral activity

Antiviral therapy is urgently needed to combat the coronavirus disease 2019 (COVID-19) pandemic, which is caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2). The protease inhibitor camostat mesylate inhibits SARS-CoV-2 infection of lung cells by blocking the virus-activating host cell protease TMPRSS2. Camostat mesylate has been approved for treatment of pancreatitis in Japan and is currently being repurposed for COVID-19 treatment. However, potential mechanisms of viral resistance as well as camostat mesylate metabolization and antiviral activity of metabolites are unclear. Here, we show that SARS-CoV-2 can employ TMPRSS2-related host cell proteases for activation and that several of them are expressed in viral target cells. However, entry mediated by these proteases was blocked by camostat mesylate. The camostat metabolite GBPA inhibited the activity of recombinant TMPRSS2 with reduced efficiency as compared to camostat mesylate and was rapidly generated in the presence of serum. Importantly, the infection experiments in which camostat mesylate was identified as a SARS-CoV-2 inhibitor involved preincubation of target cells with camostat mesylate in the presence of serum for 2 h and thus allowed conversion of camostat mesylate into GBPA. Indeed, when the antiviral activities of GBPA and camostat mesylate were compared in this setting, no major differences were identified. Our results indicate that use of TMPRSS2-related proteases for entry into target cells will not render SARS-CoV-2 camostat mesylate resistant. Moreover, the present and previous findings suggest that the peak concentrations of GBPA established after the clinically approved camostat mesylate dose (600 mg/day) will result in antiviral activity.