Evidence for dispensability of protein kinase R in host control of tuberculosis

Filarial nematode phenotypic screening cascade to identify compounds with anti-parasitic activity for drug discovery optimization

Filarial diseases, including lymphatic filariasis and onchocerciasis, are considered among the most devastating of all tropical diseases, affecting over 86 million people worldwide. To control and more rapidly eliminate onchocerciasis requires treatments that target the adult stage of the parasite. Drug discovery efforts are challenged by the lack of preclinical animal models using the human-pathogenic filariae, requiring the use of surrogate parasites for Onchocerca volvulus for both ex vivo and in vivo evaluation. Herein, we describe a platform utilizing phenotypic ex vivo assays consisting of the free-living nematode Caenorhabditis elegans, microfilariae and adult filariae of the bovine filariae Onchocerca lienalis and Onchocerca gutturosa, respectively, as well as microfilariae and adult filariae of the feline filariae Brugia pahangi, the rodent filariae Litomosoides sigmodontis and the human-pathogenic filariae Brugia malayi to assess activity across various surrogate parasites. Utilization of those surrogate nematodes for phenotypic ex vivo assays in order to assess activity across various parasites led to the successful establishment of a screening cascade and identification of multiple compounds with potential macrofilaricidal activity and desirable physicochemical, MW = 200-400 and low lipophilicity, logP <4, and pharmacokinetic properties, rat and human liver S9 stability of ≥70% remaining at 60 min, and AUC exposures above 3 μM h. This platform demonstrated the successful establishment of a screening cascade which resulted in the discovery of potential novel macrofilaricidal compounds for futher drug discovery lead optimization efforts. This screening cascade identified two distinct chemical series wherein one compound produced a significant 68% reduction of adult Litomosoides sigmodontis in the mouse model. Successful demonstration of efficacy prompted lead optimization medicinal chemistry efforts for this novel series.

Protein Kinase R in Bacterial Infections: Friend or Foe?

The global antimicrobial resistance crisis poses a significant threat to humankind in the coming decades. Challenges associated with the development of novel antibiotics underscore the urgent need to develop alternative treatment strategies to combat bacterial infections. Host-directed therapy is a promising new therapeutic strategy that aims to boost the host immune response to bacteria rather than target the pathogen itself, thereby circumventing the development of antibiotic resistance. However, host-directed therapy depends on the identification of druggable host targets or proteins with key functions in antibacterial defense. Protein Kinase R (PKR) is a well-characterized human kinase with established roles in cancer, metabolic disorders, neurodegeneration, and antiviral defense. However, its role in antibacterial defense has been surprisingly underappreciated. Although the canonical role of PKR is to inhibit protein translation during viral infection, this kinase senses and responds to multiple types of cellular stress by regulating cell-signaling pathways involved in inflammation, cell death, and autophagy – mechanisms that are all critical for a protective host response against bacterial pathogens. Indeed, there is accumulating evidence to demonstrate that PKR contributes significantly to the immune response to a variety of bacterial pathogens. Importantly, there are existing pharmacological modulators of PKR that are well-tolerated in animals, indicating that PKR is a feasible target for host-directed therapy. In this review, we provide an overview of immune cell functions regulated by PKR and summarize the current knowledge on the role and functions of PKR in bacterial infections. We also review the non-canonical activators of PKR and speculate on the potential mechanisms that trigger activation of PKR during bacterial infection. Finally, we provide an overview of existing pharmacological modulators of PKR that could be explored as novel treatment strategies for bacterial infections.

Salvia plebeia R. Br. polysaccharides (SPP) against RSV (respiratory syncytial virus) infection: Antiviral effect and mechanisms of action

OBJECTIVE

To investigate the antiviral effect of Salvia plebeia R. Br. polysaccharides (SPP) against RSV and underlying mechanisms.

METHODS

SPP was extracted via alcohol-precipitation method and extract was separated into various fractions using ultrafiltration method. The polysaccharide content was determined using UV-Vis. Antiviral effect of SPP and fractions was measured using MTT method and Reed-Muench method. Sixty Balb/c mice were randomly divided into 6 groups, and received either Ribavirin or SPP. Their body weight and food intake were recorded every day throughout the experiment period. The lung index inhibition ratio and pulmonary virus titer were determined followed by the histological analysis of lungs. Furthermore, time-of-addition and effective stage analysis were carried out to determine the mechanism of action. The TLR-3 and TLR-4 levels in the lungs were determined using qRT-PCR. The levels of IFN-γ, IL-2 and TNF-α in serum were determined using ELISA.

RESULTS

The SPP content is 4.396%. SPP has shown a good anti-RSV effect both in vitro (TI = 123.041) and in vivo models. The antiviral activity of fractions with molecular weight ≥ 10,000 is found to possess more potent antiviral activity than other fractions. SPP inhibits the RSV proliferation and reduces the lung lesions induced by RSV. The mechanism of action involves the inhibition of TLR-3 and TLR-4 in lungs, up-regulation of IFN-γ and IL-2, and down-regulation of TNF-α in serum. It is also shown to improve the body's immune function.

CONCLUSION

SPP has a potential to treat diseases caused by RSV.

Protein Kinase R Restricts the Intracellular Survival of Mycobacterium tuberculosis by Promoting Selective Autophagy

Tuberculosis (TB) is a deadly infectious lung disease caused by the pathogenic bacterium Mycobacterium tuberculosis (Mtb). The identification of macrophage signaling proteins exploited by Mtb during infection will enable the development of alternative host-directed therapies (HDT) for TB. HDT strategies will boost host immunity to restrict the intracellular replication of Mtb and therefore hold promise to overcome antimicrobial resistance, a growing crisis in TB therapy. Protein Kinase R (PKR) is a key host sensor that functions in the cellular antiviral response. However, its role in defense against intracellular bacterial pathogens is not clearly defined. Herein, we demonstrate that expression and activation of PKR is upregulated in macrophages infected with Mtb. Immunological profiling of human THP-1 macrophages that overexpress PKR (THP-PKR) showed increased production of IP-10 and reduced production of IL-6, two cytokines that are reported to activate and inhibit IFNγ-dependent autophagy, respectively. Indeed, sustained expression and activation of PKR reduced the intracellular survival of Mtb, an effect that could be enhanced by IFNγ treatment. We further demonstrate that the enhanced anti-mycobacterial activity of THP-PKR macrophages is mediated by a mechanism dependent on selective autophagy, as indicated by increased levels of LC3B-II that colocalize with intracellular Mtb. Consistent with this mechanism, inhibition of autophagolysosome maturation with bafilomycin A1 abrogated the ability of THP-PKR macrophages to limit replication of Mtb, whereas pharmacological activation of autophagy enhanced the anti-mycobacterial effect of PKR overexpression. As such, PKR represents a novel and attractive host target for development of HDT for TB, and our data suggest value in the design of more specific and potent activators of PKR.

New insights into the evasion of host innate immunity by Mycobacterium tuberculosis

Mycobacterium tuberculosis (Mtb) is an extremely successful intracellular pathogen that causes tuberculosis (TB), which remains the leading infectious cause of human death. The early interactions between Mtb and the host innate immune system largely determine the establishment of TB infection and disease development. Upon infection, host cells detect Mtb through a set of innate immune receptors and launch a range of cellular innate immune events. However, these innate defense mechanisms are extensively modulated by Mtb to avoid host immune clearance. In this review, we describe the emerging role of cytosolic nucleic acid-sensing pathways at the host-Mtb interface and summarize recently revealed mechanisms by which Mtb circumvents host cellular innate immune strategies such as membrane trafficking and integrity, cell death and autophagy. In addition, we discuss the newly elucidated strategies by which Mtb manipulates the host molecular regulatory machinery of innate immunity, including the intranuclear regulatory machinery, the ubiquitin system, and cellular intrinsic immune components. A better understanding of innate immune evasion mechanisms adopted by Mtb will provide new insights into TB pathogenesis and contribute to the development of more effective TB vaccines and therapies.

MxA suppresses TAK1-IKKα/β-NF-κB mediated inflammatory cytokine production to facilitate Mycobacterium tuberculosis infection

OBJECTIVES

Interferons (IFNs) play multifunctional roles in host defense against infectious diseases by inducing IFN-stimulated genes (ISGs). However, little is known about how ISGs regulate host immune response to Mycobacterium tuberculosis (Mtb) infection, the major cause of tuberculosis (TB).

METHODS

We thus profiled the potential effects and mechanisms of eight Mtb-induced ISGs on Mtb infection by RNA interference in human macrophages (Mφs) derived from peripheral blood monocytes (hMDMs) and THP-1 cell line derived Mφs (THP-1-Mφs).

RESULTS

MxA silencing significantly decreased intracellular Mtb infection in Mφs. Mechanistically, MxA silencing promoted inflammatory cytokines IL-1β, IL-6 and TNF-α production, and induced NF-κB p65 activation. Pharmacological inhibition of NF-κB p65 activation or gene silencing of NF-κB p65 blocked the increased production of IL-1β, IL-6 and TNF-α and restored Mtb infection by MxA silencing. Furthermore, pharmacological inhibition of TAK1 and IKKα/β blocked NF-κB p65 activation and subsequent production of pro-inflammatory cytokines by MxA silencing. Isoniazid (INH) treatment and MxA silencing could promote TAK1-IKKα/β-NF-κB signaling pathway activation and combat Mtb infection independently.

CONCLUSIONS

Our results reveal a novel role of MxA in regulating TAK1-IKKα/β-NF-κB signaling activation and production of antimicrobial inflammatory cytokines upon Mtb infection, providing a potential target for clinical treatment of TB.

RNA Sensing of Mycobacterium tuberculosis and Its Impact on TB Vaccination Strategies

Tuberculosis (TB) is still an important global threat and although the causing organism has been discovered long ago, effective prevention strategies are lacking. Mycobacterium tuberculosis (MTB) is a unique pathogen with a complex host interaction. Understanding the immune responses upon infection with MTB is crucial for the development of new vaccination strategies and therapeutic targets for TB. Recently, it has been proposed that sensing bacterial nucleic acid in antigen-presenting cells via intracellular pattern recognition receptors (PRRs) is a central mechanism for initiating an effective host immune response. Here, we summarize key findings of the impact of mycobacterial RNA sensing for innate and adaptive host immunity after MTB infection, with emphasis on endosomal toll-like receptors (TLRs) and cytosolic sensors such as NLRP3 and RLRs, modulating T-cell differentiation through IL-12, IL-21, and type I interferons. Ultimately, these immunological pathways may impact immune memory and TB vaccine efficacy. The novel findings described here may change our current understanding of the host response to MTB and potentially impact clinical research, as well as future vaccination design. In this review, the current state of the art is summarized, and an outlook is given on how progress can be made.

Cytoplasmic RNA Sensor Pathways and Nitazoxanide Broadly Inhibit Intracellular Mycobacterium tuberculosis Growth

To establish stable infection, Mycobacterium tuberculosis (MTb) must overcome host innate immune mechanisms, including those that sense pathogen-derived nucleic acids. Here, we show that the host cytosolic RNA sensing molecules RIG-I-like receptor (RLR) signaling proteins RIG-I and MDA5, their common adaptor protein MAVS, and the RNA-dependent kinase PKR each independently inhibit MTb growth in human cells. Furthermore, we show that MTb broadly stimulates RIG-I, MDA5, MAVS, and PKR gene expression and their biological activities. We also show that the oral FDA-approved drug nitazoxanide (NTZ) significantly inhibits intracellular MTb growth and amplifies MTb-stimulated RNA sensor gene expression and activity. This study establishes prototypic cytoplasmic RNA sensors as innate restriction factors for MTb growth in human cells and it shows that targeting this pathway is a potential host-directed approach to treat tuberculosis disease.

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MTb infection induces RNA sensor (RIG-I, MDA5, PKR) mRNA levels and activities

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RIG-I, MDA5, MAVS, and PKR restrict intracellular MTb growth in human cells

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NTZ enhances MTb-driven RNA sensor mRNA levels and RLR activities

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NTZ and NTZ derivatives inhibit intracellular MTb growth in primary human cells

Correction: Improved Control of Tuberculosis and Activation of Macrophages in Mice Lacking Protein Kinase R

[This corrects the article DOI: 10.1371/journal.pone.0030512.].