Inhibitors of signal peptide peptidase (SPP) affect HSV-1 infectivity in vitro and in vivo

BmSPP is a virus resistance gene in Bombyx mori

Introduction

Signal peptide peptidase (SPP) is an intramembrane protease involved in a variety of biological processes, it participates in the processing of signal peptides after the release of the nascent protein to regulate the endoplasmic reticulum associated degradation (ERAD) pathway, binds misfolded membrane proteins, and aids in their clearance process. Additionally, it regulates normal immune surveillance and assists in the processing of viral proteins. Although SPP is essential for many viral infections, its role in silkworms remains unclear. Studying its role in the silkworm, Bombyx mori , may be helpful in breeding virus-resistant silkworms.

Methods

First, we performed RT-qPCR to analyze the expression pattern of BmSPP. Subsequently, we inhibited BmSPP using the SPP inhibitor 1,3-di-(N-carboxybenzoyl-L-leucyl-L-leucylaminopropanone ((Z-LL)2-ketone) and downregulated the expression of BmSPP using CRISPR/Cas9 gene editing. Furthermore, we assessed the impact of these interventions on the proliferation of Bombyx mori nucleopolyhedrovirus (BmNPV).

Results

We observed a decreased in the expression of BmSPP during viral proliferation. It was found that higher concentration of the inhibitor resulted in greater inhibition of BmNPV proliferation. The down-regulation of BmSPP in both in vivo and in vitro was found to affect the proliferation of BmNPV. In comparison to wild type silkworm, BmSPPKO silkworms exhibited a 12.4% reduction in mortality rate.

Discussion

Collectively, this work demonstrates that BmSPP plays a negative regulatory role in silkworm resistance to BmNPV infection and is involved in virus proliferation and replication processes. This finding suggests that BmSPP servers as a target gene for BmNPV virus resistance in silkworms and can be utilized in resistance breeding programs.

Knockout of signal peptide peptidase in the eye reduces HSV-1 replication and eye disease in ocularly infected mice

We previously reported that knocking out signal peptide peptidase (SPP), a glycoprotein K (gK) binding partner, in mouse peripheral sensory neurons reduced latency-reactivation in infected mice without affecting primary virus replication or eye disease. Since virus replication in the eye plays an essential role in eye disease, we generated a conditional knockout mouse lacking SPP expression in the eye by crossing Pax6 (paired box 6)-Cre mice that have intact Pax6 expression with SPPflox/flox mice. Significantly less SPP protein expression was detected in the eyes of Pax6-SPP-/- mice than in WT control mice. HSV-1 replication in the eyes of Pax6-SPP-/- mice was significantly lower than in WT control mice. Levels of gB, gK, and ICP0 transcripts in corneas, but not trigeminal ganglia (TG), of Pax6-SPP-/- infected mice were also significantly lower than in WT mice. Corneal scarring and angiogenesis were significantly lower in Pax6-SPP-/- mice than in WT control mice, while corneal sensitivity was significantly higher in Pax6-SPP-/- mice compared with WT control mice. During acute viral infection, absence of SPP in the eye did not affect CD4 expression but did affect CD8α and IFNγ expression in the eye. However, in the absence of SPP, latency-reactivation was similar in Pax6-SPP-/- and WT control groups. Overall, our results showed that deleting SPP expression in the eyes reduced primary virus replication in the eyes, reduced CD8α and IFNγ mRNA expression, reduced eye disease and reduced angiogenesis but did not alter corneal sensitivity or latency reactivation to HSV-1 infection. Thus, blocking gK binding to SPP in the eye may have therapeutic potential by reducing both virus replication in the eye and eye disease associated with virus replication.

Signal peptide peptidase: A potential therapeutic target for parasitic and viral infections

INTRODUCTION

Signal peptide peptidase (SPP) is a GxGD-type intramembrane-cleaving aspartyl protease responsible for clearing accumulating signal peptides in the endoplasmic reticulum. SPP is conserved among all kingdoms and is essential for maintaining cell homeostasis. Inhibition of SPP with selective inhibitors and the structurally similar HIV protease inhibitors results in signal peptide accumulation and subsequent cell death. Identification of SPP homologues in major human parasitic infections has opened a new therapeutic opportunity. Moreover, the essentiality of mammalian SPP-mediated viral protein processing during infection is emerging.

AREAS COVERED

This review introduces the discovery and biological function of human SPP enzymes and identify parasitic homologues as pharmacological targets of both SPP and HIV protease inhibitors. Later, the role of mammalian SPP during viral infection and how disruption of host SPP can be employed as a novel antiviral therapy are examined and discussed.

EXPERT OPINION

Parasitic and viral infections cause severe health and economic burden, exacerbated by the lack of new therapeutics in the pipeline. SPP has been shown to be essential for malaria parasite growth and encouraging evidence in other parasites demonstrates broad essentiality of these proteases as therapeutic targets. As drug resistant parasite and viruses emerge, SPP inhibition will provide a new generation of compounds to counter the growing threat of antimicrobial resistance.

Absence of signal peptide peptidase in peripheral sensory neurons affects latency-reactivation in HSV-1 ocularly infected mice

We previously reported that HSV-1 infectivity in vitro and in vivo requires HSV glycoprotein K (gK) binding to the ER signal peptide peptidase (SPP). Anterograde-retrograde transport via peripheral nerves between the site of infection (i.e., eye) and the site of latency (neurons) is a critical process to establish latency and subsequent viral reactivation. Given the essential role of neurons in HSV-1 latency-reactivation, we generated mice lacking SPP specifically in peripheral sensory neurons by crossing Advillin-Cre mice with SPP^fl/fl mice. Expression of SPP mRNA and protein were significantly lower in neurons of Avil-SPP^-/- mice than in control mice despite similar levels of HSV-1 replication in the eyes of Avil-SPP^-/- mice and control mice. Viral transcript levels in isolated neurons of infected mice on days 2 and 5 post infection were lower than in control mice. Significantly less LAT, gB, and PD-1 expression was seen during latency in isolated neurons and total trigeminal ganglia (TG) of Avil-SPP^-/- mice than in control mice. Finally, reduced latency and reduced T cell exhaustion in infected Avil-SPP^-/- mice correlated with slower and no reactivation. Overall, our results suggest that blocking SPP expression in peripheral sensory neurons does not affect primary virus replication or eye disease but does reduce latency-reactivation. Thus, blocking of gK binding to SPP may be a useful tool to reduce latency-reactivation.

HSV-1 gK and the ER protein SPP are both essential and highly conserved proteins. Their interaction is important for virus infectivity in vitro and in vivo. To evaluate the importance of gK binding to SPP in the peripheral nervous system, we generated SPP conditional knockout mice in peripheral nervous system using Advillin-Cre mice. The absence of SPP in peripheral nervous system significantly reduced latency-reactivation as well as T cell exhaustion.

Blocking HSV-1 glycoprotein K binding to signal peptide peptidase reduces virus infectivity in vitro and in vivo

HSV glycoprotein K (gK) is an essential herpes protein that contributes to enhancement of eye disease. We previously reported that gK binds to signal peptide peptidase (SPP) and that depletion of SPP reduces HSV-1 infectivity in vivo. To determine the therapeutic potential of blocking gK binding to SPP on virus infectivity and pathogenicity, we mapped the gK binding site for SPP to a 15mer peptide within the amino-terminus of gK. This 15mer peptide reduced infectivity of three different virus strains in vitro as determined by plaque assay, FACS, and RT-PCR. Similarly, the 15mer peptide reduced ocular virus replication in both BALB/c and C57BL/6 mice and also reduced levels of latency and exhaustion markers in infected mice when compared with control treated mice. Addition of the gK-15mer peptide also increased the survival of infected mice when compared with control mice. These results suggest that blocking gK binding to SPP using gK peptide may have therapeutic potential in treating HSV-1-associated infection.

Signal peptide peptidase (SPP) and HSV-1 glycoprotein K (gK) are essential genes in the host and virus, respectively. SPP and gK genes are both highly conserved. Previously we reported that gK binding to SPP is important for virus infectivity in vitro and in vivo. In this study we have identified the gK binding site to SPP and have shown that a gK peptide that blocks gK binding to SPP can block HSV-1 infectivity in vitro and in vivo using different strains of virus and mice. Thus, the ability of this peptide to block gK binding to SPP may be a useful tool to control HSV-1-induced eye disease in patients with herpes stromal keratitis (HSK).

Safe-in-Man Broad Spectrum Antiviral Agents

Emerging and re-emerging viral diseases occur with regularity within the human population. The conventional 'one drug, one virus' paradigm for antivirals does not adequately allow for proper preparedness in the face of unknown future epidemics. In addition, drug developers lack the financial incentives to work on antiviral drug discovery, with most pharmaceutical companies choosing to focus on more profitable disease areas. Safe-in-man broad spectrum antiviral agents (BSAAs) can help meet the need for antiviral development by already having passed phase I clinical trials, requiring less time and money to develop, and having the capacity to work against many viruses, allowing for a speedy response when unforeseen epidemics arise. In this chapter, we discuss the benefits of repurposing existing drugs as BSAAs, describe the major steps in safe-in-man BSAA drug development from discovery through clinical trials, and list several database resources that are useful tools for antiviral drug repositioning.

Physiological functions of SPP/SPPL intramembrane proteases

Intramembrane proteolysis describes the cleavage of substrate proteins within their hydrophobic transmembrane segments. Several families of intramembrane proteases have been identified including the aspartyl proteases Signal peptide peptidase (SPP) and its homologues, the SPP-like (SPPL) proteases SPPL2a, SPPL2b, SPPL2c and SPPL3. As presenilin homologues, they employ a similar catalytic mechanism as the well-studied γ-secretase. However, SPP/SPPL proteases cleave transmembrane proteins with a type II topology. The characterisation of SPP/SPPL-deficient mouse models has highlighted a still growing spectrum of biological functions and also promoted the substrate discovery of these proteases. In this review, we will summarise the current hypotheses how phenotypes of these mouse models are linked to the molecular function of the enzymes. At the cellular level, SPP/SPPL-mediated cleavage events rather provide specific regulatory switches than unspecific bulk proteolysis. By this means, a plethora of different cell biological pathways is influenced including signal transduction, membrane trafficking and protein glycosylation.

Ibuprofen, a traditional drug that may impact the course of COVID-19 new effective formulation in nebulizable solution

The traditional formulation of ibuprofen is poorly soluble in water, so the administered dose must be 10 times higher than the dose required for a therapeutic effect. The development of a hydrosoluble form of ibuprofen can be a strategy to reach a high concentration in the lungs by using modern inhalation devices. Therefore, the development of an inhalable formulation with high bioavailability in the lungs was the leitmotiv of our investigation. The hypertonic ibuprofen solution to be nebulized (NIH) presents great relevant characteristics: bactericidal, virucidal, mucolytic and has a known anti-inflammatory property. Bactericidal and virucidal effects are related to the physico-chemical properties of Na-ibuprofenate as an amphipathic molecule. It has the capability to insert into the bilayer membranes destabilizing the structure, altering its biological properties and avoiding the duplication or infection. Our preliminary results indicate that the presence of this high ionic strength solution reduces 10 times the amount of ibuprofen necessary to kill bacteria, but also the time to kill 1x10⁶ bacteria, from 4 h (in its absence) to only three minutes (in its presence). That was observed using Pseudomona aeruginosa, methicillin-resistant Staphylococcus aureus and Burkholderia cepacia. Also, "in vitro'' ibuprofen demonstrated virucidal activity against the so-called enveloped virus, a family that includes coronavirus strain (2019-nCoV). We observed too, the markedly reduced local inflammation in the airways after administering NIH lays on its ability to inhibit the enzyme cyclooxygenase and to markedly diminish reactive oxygen species (ROS). Other investigators also showed the importance of actin in the rapid spread of virus infection. Furthermore, reorganization of the actin filaments is a key step in lung inflammation induced by systemic inflammatory responses caused by SARS-CoV-2. These findings suggest that the interaction between actin proteins and S1 is involved in the 2019-nCoV infection and pathogenesis. Consequently, the possibility of interfering in this interaction could represent a valid hypothesis for the development of promising therapeutic and prevention strategies. In conclusion, we consider that treating people with COVID-19 with NIH may be beneficial and an opportunity to contribute for the current global health emergency.

Absence of Signal Peptide Peptidase, an Essential Herpes Simplex Virus 1 Glycoprotein K Binding Partner, Reduces Virus Infectivity In Vivo

We previously reported that herpes simplex virus (HSV) glycoprotein K (gK) binds to signal peptide peptidase (SPP), also known as minor histocompatibility antigen H13. Binding of gK to SPP is required for HSV-1 infectivity in vitro SPP is a member of the γ-secretase family, and mice lacking SPP are embryonic lethal. To determine how SPP affects HSV-1 infectivity in vivo, the SPP gene was deleted using a tamoxifen-inducible Cre recombinase driven by the ubiquitously expressed ROSA26 promoter. SPP mRNA was reduced by more than 93% in the cornea and trigeminal ganglia (TG) and by 99% in the liver of tamoxifen-injected mice, while SPP protein expression was reduced by 90% compared to the level in control mice. Mice lacking SPP had significantly less HSV-1 replication in the eye as well as reduced gK, UL20, ICP0, and gB transcripts in the cornea and TG compared to levels in control mice. In addition, reduced infiltration of CD45⁺, CD4⁺, CD8⁺, F4/80⁺, CD11c⁺, and NK1.1⁺ T cells was observed in the cornea and TG of SPP-inducible knockout mice compared to that in control mice. Finally, in the absence of SPP, latency was significantly reduced in SPP-inducible knockout mice compared to that in control mice. Thus, in this study we have generated SPP-inducible knockout mice and shown that the absence of SPP affects virus replication in the eye of ocularly infected mice and that this reduction is correlated with the interaction of gK and SPP. These results suggest that blocking this interaction may have therapeutic potential in treating HSV-1-associated eye disease.IMPORTANCE Glycoprotein K (gK) is an essential and highly conserved HSV-1 protein. Previously, we reported that gK binds to SPP, an endoplasmic reticulum (ER) protein, and blocking this binding reduces virus infectivity in vitro and also affects gK and UL20 subcellular localization. To evaluate the function of gK binding to SPP in vivo, we generated SPP-inducible knockout mice and observed the following in the absence of SPP: (i) that significantly less HSV-1 replication was seen in ocularly infected mice than in control mice; (ii) that expression of various HSV-1 genes and cellular infiltrates in the eye and trigeminal ganglia of infected mice was less than that in control mice; and (iii) that latency was significantly reduced in infected mice. Thus, blocking of gK binding to SPP may be a useful tool to control HSV-1-induced eye disease in patients with herpes stromal keratitis (HSK).

Role of Herpes Simplex Virus Type 1 (HSV-1) Glycoprotein K (gK) Pathogenic CD8+ T Cells in Exacerbation of Eye Disease

HSV-1-induced corneal scarring (CS), also broadly referred to as Herpes Stromal Keratitis (HSK), is the leading cause of infectious blindness in developed countries. It is well-established that HSK is in fact an immunopathological disease. The contribution of the potentially harmful T cell effectors that lead to CS remains an area of intense study. Although the HSV-1 gene(s) involved in eye disease is not yet known, we have demonstrated that gK, which is one of the 12 known HSV-1 glycoproteins, has a crucial role in CS. Immunization of HSV-1 infected mice with gK, but not with any other known HSV-1 glycoprotein, significantly exacerbates CS, and dermatitis. The gK-induced eye disease occurs independently of the strain of the virus or mouse. HSV-1 mutants that lack gK are unable to efficiently infect and establish latency in neurons. HSV-1 recombinant viruses expressing two additional copies of the gK (total of three gK genes) exacerbated CS as compared with wild type HSV-1 strain McKrae that contains one copy of gK. Furthermore, we have shown that an 8mer (ITAYGLVL) within the signal sequence of gK enhanced CS in ocularly infected BALB/c mice, C57BL/6 mice, and NZW rabbits. In HSV-infected “humanized” HLA-A^*0201 transgenic mice, this gK 8mer induced strong IFN-γ-producing cytotoxic CD8⁺ T cell responses. gK induced CS is dependent on gK binding to signal peptide peptidase (SPP). gK also binds to HSV-1 UL20, while UL20 binds GODZ (DHHC3) and these quadruple interactions are required for gK induced pathology. Thus, potential therapies might include blocking of gK-SPP, gK-UL20, UL20-GODZ interactions, or a combination of these strategies.

The Absence of DHHC3 Affects Primary and Latent Herpes Simplex Virus 1 Infection

UL20, an essential herpes simplex virus 1 (HSV-1) protein, is involved in cytoplasmic envelopment of virions and virus egress. We reported recently that UL20 can bind to a host protein encoded by the zinc finger DHHC-type containing 3 (ZDHHC3) gene (also known as Golgi-specific DHHC zinc finger protein [GODZ]). Here, we show for the first time that HSV-1 replication is compromised in murine embryonic fibroblasts (MEFs) isolated from GODZ^-/- mice. The absence of GODZ resulted in blocking palmitoylation of UL20 and altered localization and expression of UL20 and glycoprotein K (gK); the expression of gB and gC; and the localization and expression of tegument and capsid proteins within HSV-1-infected MEFs. Electron microscopy revealed that the absence of GODZ limited the maturation of virions at multiple steps and affected the localization of virus and endoplasmic reticulum morphology. Virus replication in the eyes of ocularly HSV-1-infected GODZ^-/- mice was significantly lower than in HSV-1-infected wild-type (WT) mice. The levels of UL20, gK, and gB transcripts in the corneas of HSV-1-infected GODZ^-/- mice on day 5 postinfection were markedly lower than in WT mice, whereas only UL20 transcripts were reduced in trigeminal ganglia (TG). In addition, HSV-1-infected GODZ^-/- mice showed notably lower levels of corneal scarring, and HSV-1 latency reactivation was also reduced. Thus, normal HSV-1 infectivity and viral pathogenesis are critically dependent on GODZ-mediated palmitoylation of viral UL20.IMPORTANCE HSV-1 infection is widespread. Ocular infection can cause corneal blindness; however, approximately 70 to 90% of American adults exposed to the virus show no clinical symptoms. In this study, we show for the first time that the absence of a zinc finger protein called GODZ affects primary and latent infection, as well as reactivation, in ocularly infected mice. The reduced virus infectivity is due to the absence of the GODZ interaction with HSV-1 UL20. These results strongly suggest that binding of UL20 to GODZ promotes virus infectivity in vitro and viral pathogenesis in vivo.

Insights into the mechanism of isoenzyme-specific signal peptide peptidase-mediated translocation of heme oxygenase

It has recently been shown that signal peptide peptidase (SPP) can catalyze the intramembrane cleavage of heme oxygenase-1 (HO-1) that leads to translocation of HO-1 into the cytosol and nucleus. While there is consensus that translocated HO-1 promotes tumor progression and drug resistance, the physiological signals leading to SPP-mediated intramembrane cleavage of HO-1 and the specificity of the process remain unclear. In this study, we used co-immunoprecipitation and confocal laser scanning microscopy to investigate the translocation mechanism of HO-1 and its regulation by SPP. We show that HO-1 and the closely related HO-2 isoenzyme bind to SPP under normoxic conditions. Under hypoxic conditions SPP mediates intramembrane cleavage of HO-1, but not HO-2. In experiments with an inactive HO-1 mutant (H25A) we show that translocation is independent of the catalytic activity of HO-1. Studies with HO-1 / HO-2 chimeras indicate that the membrane anchor, the PEST-domain and the nuclear shuttle sequence of HO-1 are necessary for full cleavage and subsequent translocation under hypoxic conditions. In the presence of co-expressed exogenous SPP, the anchor and the PEST-domain are sufficient for translocation. Taken together, we identified the domains involved in HO-1 translocation and showed that SPP-mediated cleavage is isoform-specific and independent of HO-activity. A closer understanding of the translocation mechanism of HO-1 is of particular importance because nuclear HO-1 seems to lead to tumor progression and drug resistance.

Binding of Herpes Simplex Virus 1 UL20 to GODZ (DHHC3) Affects Its Palmitoylation and Is Essential for Infectivity and Proper Targeting and Localization of UL20 and Glycoprotein K

Herpes simplex virus 1 (HSV-1) UL20 plays a crucial role in the envelopment of the cytoplasmic virion and its egress. It is a nonglycosylated envelope protein that is regulated as a γ1 gene. Two-hybrid and pulldown assays demonstrated that UL20, but no other HSV-1 gene-encoded proteins, binds specifically to GODZ (also known as DHHC3), a cellular Golgi apparatus-specific Asp-His-His-Cys (DHHC) zinc finger protein. A catalytically inactive dominant-negative GODZ construct significantly reduced HSV-1 replication in vitro and affected the localization of UL20 and glycoprotein K (gK) and their interactions but not glycoprotein C (gC). GODZ is involved in palmitoylation, and we found that UL20 is palmitoylated by GODZ using a GODZ dominant-negative plasmid. Blocking of palmitoylation using 2-bromopalmitate (2-BP) affected the virus titer and the interaction of UL20 and gK but did not affect the levels of these proteins. In conclusion, we have shown that binding of UL20 to GODZ in the Golgi apparatus regulates trafficking of UL20 and its subsequent effects on gK localization and virus replication. We also have demonstrated that GODZ-mediated UL20 palmitoylation is critical for UL20 membrane targeting and thus gK cell surface expression, providing new mechanistic insights into how UL20 palmitoylation regulates HSV-1 infectivity.IMPORTANCE HSV-1 UL20 is a nonglycosylated essential envelope protein that is highly conserved among herpesviruses. In this study, we show that (i) HSV-1 UL20 binds to GODZ (also known as DHHC3), a Golgi apparatus-specific Asp-His-His-Cys (DHHC) zinc finger protein; (ii) a GODZ dominant-negative mutant and an inhibitor of palmitoylation reduced HSV-1 titers and altered the localization of UL20 and glycoprotein K; and (iii) UL20 is palmitoylated by GODZ, and this UL20 palmitoylation is required for HSV-1 infectivity. Thus, blocking of the interaction of UL20 with GODZ, using a GODZ dominant-negative mutant or possibly GODZ shRNA, should be considered a potential alternative therapy in not only HSV-1 but also other conditions in which GODZ processing is an integral component of pathogenesis.

Signal peptide peptidase and SPP-like proteases - Possible therapeutic targets?

Signal peptide peptidase (SPP) and the four homologous SPP-like proteases SPPL2a, SPPL2b, SPPL2c and SPPL3 are GxGD-type intramembrane-cleaving proteases (I-CLIPs). In addition to divergent subcellular localisations, distinct differences in the mechanistic properties and substrate requirements of individual family members have been unravelled. SPP/SPPL proteases employ a catalytic mechanism related to that of the γ-secretase complex. Nevertheless, differential targeting of SPP/SPPL proteases and γ-secretase by inhibitors has been demonstrated. Furthermore, also within the SPP/SPPL family significant differences in the sensitivity to currently available inhibitory compounds have been reported. Though far from complete, our knowledge on pathophysiological functions of SPP/SPPL proteases, in particular based on studies in mice, has been significantly increased over the last years. Based on this, inhibition of distinct SPP/SPPL proteases has been proposed as a novel therapeutic concept e.g. for the treatment of autoimmunity and viral or protozoal infections, as we will discuss in this review. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.