Medicinal chemistry strategies towards the development of non-covalent SARS-CoV-2 Mpro inhibitors

The main protease(M^(pro))of SARS-CoV-2 is an attractive target in anti-COVID-19 therapy for its high conservation and major role in the virus life cycle.The covalent M^(pro)inhibitor nirmatrelvir(in combination with ritonavir,a pharmacokinetic enhancer)and the non-covalent inhibitor ensitrelvir have shown efficacy in clinical trials and have been approved for therapeutic use.Effective antiviral drugs are needed to fight the pandemic,while non-covalent M^(pro)inhibitors could be promising alternatives due to their high selectivity and favorable druggability.Numerous non-covalent M^(pro)inhibitors with desirable properties have been developed based on available crystal structures of M^(pro).In this article,we describe medicinal chemistry strategies applied for the discovery and optimization of non-covalent M^(pro)inhibitors,followed by a general overview and critical analysis of the available information.Prospective viewpoints and insights into current strategies for the development of non-covalent M^(pro)inhibitors are also discussed.

Peptide targeting the interaction of S protein cysteine-rich domain with Ezrin restricts pan-coronavirus infection

Dear Editor,To date,seven human coronaviruses(HCoVs)have been identified,among which the highly pathogenic severe acute respiratory syndrome-associated coronavirus(SARS-CoV),Middle East respiratory syndrome coronavirus(MERS-CoV),and severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)have caused public health disasters worldwide.

The secretory Candida effector Sce1 licenses fungal virulence by masking the immunogenicβ-1,3-glucan and promoting apoptosis of the host cells

Candida albicans deploys a variety of mechanisms such as morphological switch and elicitor release to promote virulence.However,the intricate interactions between the fungus and the host remain poorly understood,and a comprehensive inventory of fungal virulence factors has yet to be established.In this study,we identified a C.albicans secretory effector protein Sce1,whose induction and secretion are associated with vagina‐simulative conditions and chlamydospore formation.Sequence alignment showed that Sce1 belongs to a Pir family in C.albicans,which is conserved across several fungi and primarily characterized as aβ‐glucan binding protein in the Saccharomyces cerevisiae.Mechanically,Sce1 is primarily localized to the cell wall in a cleaved form as an alkali‐labileβ‐1,3‐glucan binding protein and plays a role in maskingβ‐glucan in acidic environments and chlamydospores,a feature that might underline C.albicans'ability to evade host immunity.Further,a cleaved short form of Sce1 protein could be released into extracellular compartments and presented in bone marrow‐derived macrophages infected with chlamydospores.This cleaved short form of Sce1 also demonstrated a unique ability to trigger the caspases‐8/9‐dependent apoptosis in various host cells.Correspondingly,genetic deletion of SCE1 led to dampened vaginal colonization of C.albicans and diminished fungal virulence during systemic infection.The discovery of Sce1 as a versatile virulence effector that executes at various compartments sheds light on the fungus–host interactions and C.albicans pathogenesis.

SARS-CoV-2 NSP5 and N protein counteract the RIG-I signaling pathway by suppressing the formation of stress granules

As a highly pathogenic human coronavirus,SARS-CoV-2 has to counteract an intricate network of antiviral host responses to establish infection and spread.The nucleic acid-induced stress response is an essential component of antiviral defense and is closely related to antiviral innate immunity.However,whether SARS-CoV-2 regulates the stress response pathway to achieve immune evasion remains elusive.In this study,SARS-CoV-2 NSP5 and N protein were found to attenuate antiviral stress granule(avSG)formation.Moreover,NSP5 and N suppressed IFN expression induced by infection of Sendai virus or transfection of a synthetic mimic of dsRNA,poly(I:C),inhibiting TBK1 and IRF3 phosphorylation,and restraining the nuclear translocalization of IRF3.Furthermore,HEK293T cells with ectopic expression of NSP5 or N protein were less resistant to vesicular stomatitis virus infection.Mechanistically,NSP5 suppressed avSG formation and disrupted RIG-I–MAVS complex to attenuate the RIG-I–mediated antiviral immunity.In contrast to the multiple targets of NSP5,the N protein specifically targeted cofactors upstream of RIG-I.The N protein interacted with G3BP1 to prevent avSG formation and to keep the cofactors G3BP1 and PACT from activating RIG-I.Additionally,the N protein also affected the recognition of dsRNA by RIG-I.This study revealed the intimate correlation between SARS-CoV-2,the stress response,and innate antiviral immunity,shedding light on the pathogenic mechanism of COVID-19.

Ribosomal protein S26 serves as a checkpoint of T-cell survival and homeostasis in a p53-dependent manner

Ribosomal proteins(RPs),which play critical roles in ribosome assembly and protein translation,have been proven to be associated with important physiological and pathological processes,including the regulation of T-cell development and immune-related diseases.1,2,3 A recent study reported that ribosomal protein S26(Rps26),which is a Diamond–Blackfan anemia-associated RP located in the 40S subunit that controls oocyte growth,4,5 may influence multiple different immune phenotypes.6 Moreover,a SNP in the 5′UTR of the Rps26 gene in CD4^(+)and CD8+T cells was implicated in several autoimmune diseases.7 However,the function of Rps26 in T lymphocytes remains unknown.Here,we found that Rps26 was highly expressed in T lymphocytes.We examined a T-cell-specific Rps26 knockout mouse model and reported for the first time that ablation of Rps26 in T cells not only impairs peripheral T-cell homeostasis but also leads to T-cell developmental arrest in the thymus.Mechanistically,Rps26 critically regulates T-cell survival in a p53-dependent manner.These findings reveal the indispensable role of the Rps26-p53 axis in T-cell development and homeostasis.

CUL4B facilitates HBV replication by promoting HBx stabilization

Objective:Hepatitis B virus(HBV)infection is a major public health problem worldwide.However,the regulatory mechanisms underlying HBV replication remain unclear.Cullin 4 B-RING ubiquitin E3 ligase(CRL4 B)is involved in regulating diverse physiological and pathophysiological processes.In our study,we aimed to explain the role of CUL4 B in HBV infection.Methods:Cul4 b transgenic mice or conditional knockout mice,as well as liver cell lines with CUL4 B overexpression or knockdown,were used to assess the role of CUL4 B in HBV replication.Immunoprecipitation assays and immunofluorescence staining were performed to study the interaction between CUL4 B and HBx.Cycloheximide chase assays and in vivo ubiquitination assays were performed to evaluate the half-life and the ubiquitination status of HBx.Results:The hydrodynamics-based hepatitis B model in Cul4 b transgenic or conditional knockout mice indicated that CUL4 B promoted HBV replication(P<0.05).Moreover,the overexpression or knockdown system in human liver cell lines validated that CUL4 B increased HBV replication in an HBx-dependent manner.Importantly,immunoprecipitation assays and immunofluorescence staining showed an interaction between CUL4 B and HBx.Furthermore,CUL4 B upregulated HBx protein levels by inhibiting HBx ubiquitination and proteasomal degradation(P<0.05).Finally,a positive correlation between CUL4 B expression and HBV pg RNA level was observed in liver tissues from HBV-positive patients and HBV transgenic mice.Conclusions:CUL4 B enhances HBV replication by interacting with HBx and disrupting its ubiquitin-dependent proteasomal degradation.CUL4 B may therefore be a potential target for anti-HBV therapy.

Role of programmed cell death 4 in diseases: a double-edged sword

The molecule of programmed cell death 4(PDCD4)is named based on its upregulation during apoptosis.It is now known that PDCD4 is highly conserved during evolution and widely expressed in the immune cells and non-immune cells of humans and other animals.PDCD4,as an inhibitor of gene transcription and translation,is very important in maintaining the normal function of cells.1,2 PDCD4 protein contains an N-terminal domain and two MA-3 domains in the central and C-terminal regions.The MA-3 domains can competitively bind to the translation initiation factor eIF4A with eIF4G to inhibit eIF4A activity and to impact the translation of specific mRNAs containing structured 5′-untranslated regions(5′UTRs).

Palmitoylation of SARS-CoV-2 S protein is essential for viral infectivity

Dear Editor,Severe acute respiratory syndrome coronavirus 2(SARS-CoV-2)is the causative agent of the unprecedented coronavirus disease 2019(COVID-19).SARS-CoV-2 entry into host cells is mediated by the viral transmembrane spike(S)glycoprotein that forms homotrimers protruding from the viral surface.

Intrinsic PD-L1 promotes antitumor activity of CD8^(+)cytotoxic T lymphocytes via in cis interaction with CD80

Dear Editor,Programmed cell death-ligand 1(PD-L1)on tumor cells can inhibit CD8+cytotoxic T lymphocyte(CTL)-mediated antitumor response by trans-engagement with programmed death protein 1(PD-1).Besides tumor cells,PD-L1 is expressed on T cells.However,the intrinsic role of PD-L1 in T cells has not been widely studied.PD-L1 expression is essential for the survival of activated CD8+T cells,and PD-L1 blockade at the contraction stage reduced the number of effector T cells[1].CD8+T cell responses to influenza virus infection were also impaired in PD-L1-deficient mice[2].

Severe acute respiratory syndrome coronavirus 2(SARSCoV-2)membrane(M)protein inhibits type I and III interferon production by targeting RIG-I/MDA-5 signaling

Coronavirus disease 2019(COVID-19),caused by severe acute respiratory syndrome coronavirus 2(SARS-CoV-2),has quickly spread worldwide and has affected more than 10 million individuals.A typical feature of COVID-19 is the suppression of type I and III interferon(IFN)-mediated antiviral immunity.However,the molecular mechanism by which SARS-CoV-2 evades antiviral immunity remains elusive.Here,we reported that the SARS-CoV-2 membrane(M)protein inhibits the production of type I and III IFNs induced by the cytosolic dsRNA-sensing pathway mediated by RIG-I/MDA-5–MAVS signaling.In addition,the SARS-CoV-2 M protein suppresses type I and III IFN induction stimulated by SeV infection or poly(I:C)transfection.Mechanistically,the SARS-CoV-2 M protein interacts with RIG-I,MAVS,and TBK1,thus preventing the formation of the multiprotein complex containing RIG-I,MAVS,TRAF3,and TBK1 and subsequently impeding the phosphorylation,nuclear translocation,and activation of IRF3.Consequently,ectopic expression of the SARS-CoV-2 M protein facilitates the replication of vesicular stomatitis virus.Taken together,these results indicate that the SARS-CoV-2 M protein antagonizes type I and III IFN production by targeting RIG-I/MDA-5 signaling,which subsequently attenuates antiviral immunity and enhances viral replication.This study provides insight into the interpretation of SARS-CoV-2-induced antiviral immune suppression and illuminates the pathogenic mechanism of COVID-19.

Severe acute respiratory syndrome coronavirus 2 (SARSCoV-2) membrane (M) protein inhibits type I and IIIinterferon production by targeting RIG-I/MDA-5 signaling

Coronavirus disease 2019(COVID-19),caused by severe acute respiratory syndrome coronavirus 2(SARS-CoV-2),has quickly spread worldwide and has affected more than 10 million individuals.A typical feature of COVID-19 is the suppression of type I and III interferon(IFN)-mediated antiviral immunity.However,the molecular mechanism by which SARS-CoV-2 evades antiviral immunity remains elusive.Here,we reported that the SARS-CoV-2 membrane(M)protein inhibits the production of type I and III IFNs induced by the cytosolic dsRNA-sensing pathway mediated by RIG-I/MDA-5–MAVS signaling.In addition,the SARS-CoV-2 M protein suppresses type I and III IFN induction stimulated by SeV infection or poly(I:C)transfection.Mechanistically,the SARS-CoV-2 M protein interacts with RIG-I,MAVS,and TBK1,thus preventing the formation of the multiprotein complex containing RIG-I,MAVS,TRAF3,and TBK1 and subsequently impeding the phosphorylation,nuclear translocation,and activation of IRF3.Consequently,ectopic expression of the SARS-CoV-2 M protein facilitates the replication of vesicular stomatitis virus.Taken together,these results indicate that the SARS-CoV-2 M protein antagonizes type I and III IFN production by targeting RIG-I/MDA-5 signaling,which subsequently attenuates antiviral immunity and enhances viral replication.This study provides insight into the interpretation of SARS-CoV-2-induced antiviral immune suppression and illuminates the pathogenic mechanism of COVID-19.

PI4P STimulatING innate immune activation:beyond the Golgi

The cGAS–STING pathway is pivotal for sensing cytosolic DNA[2].cGAS,a cytoplasmic DNA sensor,can be stimulated by DNA,and it consumes ATP and GTP to synthesize a second messenger,2'3'-cGAMP,which binds to STING,resulting in STING conformational change,oligomerization,and activation;ultimately,this process leads to the production of cytokines such as type I interferon to mediate antiviral and antitumor immune responses(Fig.1).Phosphoinositides,derived from phosphatidylinositol(PI),are phospholipids in membranes and are also crucial signaling molecules[3].The D-3,D-4,or D-5 positions in the inositol head group can be phosphorylated;therefore,a total of seven different PIP molecules exist in eukaryotes.Phosphatidylinositol 4-phosphate(PI4P)is distributed in various membrane components,with the highest amount in the trans-Golgi network(TGN)[4].The PI4P level in cells is regulated by its PI4K synthetases(PI4KA,PI4KB,PI4K2A,and PI4K2B)and the lipid phosphatase SAC1.In addition to being a precursor for PIP2 and PIP3,PI4P is a crucial signaling molecule involved in vesicular transport,lipid transport,and organelle morphology maintenance.

Increased Tim-3 expression alleviates liver injury by regulating macrophage activation in MCD-induced NASH mice

As an immune checkpoint,Tim-3 plays roles in the regulation of both adaptive and innate immune cells including macrophages and is greatly involved in chronic liver diseases.However,the precise roles of Tim-3 in nonalcoholic steatohepatitis(NASH)remain unstated.In the current study,we analyzed Tim-3 expression on different subpopulations of liver macrophages and further investigated the potential roles of Tim-3 on hepatic macrophages in methionine and choline-deficient diet(MCD)-induced NASH mice.The results of flow cytometry demonstrated the significantly increased expression of Tim-3 on all detected liver macrophage subsets in MCD mice,including F4/80^(+)CD11b^(+),F4/80^(+)CD68^(+),and F4/80^(+)CD169^(+)macrophages.Remarkably,Tim-3 knockout(KO)significantly accelerated MCD-induced liver steatosis,displaying higher serum ALT,larger hepatic vacuolation,more liver lipid deposition,and more severe liver fibrosis.Moreover,compared with wild-type C57BL/6 mice,Tim-3 KO MCD mice demonstrated an enhanced expression of NOX2,NLRP3,and caspase-1 p20 together with increased generation of IL-1βand IL-18 in livers.In vitro studies demonstrated that Tim-3 negatively regulated the production of reactive oxygen species(ROS)and related downstream pro-inflammatory cytokine secretion of IL-1βand IL-18 in macrophages.Exogenous administration of N-Acetyl-L-cysteine(NAC),a small molecular inhibitor of ROS,remarkably suppressed caspase-1 p20 expression and IL-1βand IL-18 production in livers of Tim-3 KO mice,thus significantly reducing the severity of steatohepatitis induced by MCD.In conclusion,Tim-3 is a promising protector in MCD-induced steatohepatitis by controlling ROS and the associated pro-inflammatory cytokine production in macrophages.

First small-molecule PROTACs for G protein-coupled receptors:inducing α1A-adrenergic receptor degradation

Proteolysis targeting chimeras(PROTACs)are dual-functional hybrid molecules that can selectively recruit an E3 ubiquitin ligase to a target protein to direct the protein into the ubiquitinproteasome system(UPS),thereby selectively reducing the target protein level by the ubiquitinproteasome pathway.Nowadays,small-molecule PROTACs are gaining popularity as tools to desrade pathogenic protein.Herein,we present the first small-molecule PROTACs that can induce the alA-adrenergic receptor(α1 A-AR)degradation,which is also the first small-molecule PROTACs for G proteincoupled receptors(GPCRs)to our knowledge.These degradation inducers were developed through conjugation of knownα1-adrenergic receptors(α1-ARs)inhibitor prazosin and cereblon(CRBN)ligand pomalidomide through the different linkers.The representative compound 9 c is proved to inhibit the proliferation of PC-3 cells and result in tumor growth regression,which highlighted the potential of our study as a new therapeutic strategy for prostate cancer.

Hepatitis B virus evades immune recognition via RNA adenosine deaminase ADAR1-mediated viral RNA editing in hepatocytes

HBV is considered as a“stealth”virus that does not invoke interferon(IFN)responses;however,the mechanisms by which HBV bypasses innate immune recognition are poorly understood.In this study,we identified adenosine deaminases acting on RNA 1(ADAR1),which is a key factor in HBV evasion from IFN responses in hepatocytes.Mechanically,ADAR1 interacted with HBV RNAs and deaminated adenosine(A)to generate inosine(I),which disrupted host immune recognition and thus promoted HBV replication.Loss of ADAR1 or its deficient deaminase activity promoted IFN responses and inhibited HBV replication in hepatocytes,and blocking the IFN signaling pathways released the inhibition of HBV replication caused by ADAR1 deficiency.Notably,the HBV X protein(HBx)transcriptionally promoted ADAR1 expression to increase the threshold required to trigger intrinsic immune activation,which in turn enhanced HBV escape from immune recognition,leading to persistent infection.Supplementation with 8-azaadenosine,an ADAR1 inhibitor,efficiently enhanced liver immune activation to promote HBV clearance in vivo and in vitro.Taken together,our results delineate a molecular mechanism by which HBx promotes ADAR1-derived HBV immune escape and suggest a targeted therapeutic intervention for HBV infection.

The hepatic macrophage pool in NASH

Nonalcoholic fatty liver disease(NAFLD)and its inflammatory subtype nonalcoholic steatohepatitis(NASH)are currently the most common chronic liver diseases worldwide and emerging risk factors for hepatocellular carcinoma.1 Numerous studies have demonstrated the critical involvement of hepatic macrophages,including resident Kupffer cells(KCs)and recruited monocyte-derived macrophages(MoMFs).