Differential regulation of protein kinase C isoforms of macrophages by pathogenic and non-pathogenic mycobacteria

Coronin-1 is phosphorylated at Thr-412 by protein kinase Cα in human phagocytic cells

Coronin-1, a hematopoietic cell-specific actin-binding protein, is thought to be involved in the phagocytic process through its interaction with actin filaments. The dissociation of coronin-1 from phagosomes after its transient accumulation on the phagosome surface is associated with lysosomal fusion. We previously reported that 1) coronin-1 is phosphorylated by protein kinase C (PKC), 2) coronin-1 has two phosphorylation sites, Ser-2 and Thr-412, and 3) Thr-412 of coronin-1 is phosphorylated during phagocytosis. In this study, we examined which PKC isoform is responsible for the phosphorylation of coronin-1 at Thr-412 by using isotype-specific PKC inhibitors and small interfering RNAs (siRNAs). Thr-412 phosphorylation of coronin-1 was suppressed by Gö6976, an inhibitor of PKCα and PKCβI. This phosphorylation was attenuated by siRNA for PKCα, but not by siRNA for PKCβ. Furthermore, Thr-412 of coronin-1 was phosphorylated by recombinant PKCα in vitro, but not by recombinant PKCβ. We next examined the effects of Gö6976 on the intracellular distribution of coronin-1 in HL60 cells during phagocytosis. The confocal fluorescence microscopic observation showed that coronin-1 was not dissociated from phagosomes in Gö6976-treated cells. These results indicate that phosphorylation of coronin-1 at Thr-412 by PKCα regulates intracellular distribution during phagocytosis.

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Phosphorylation of coronin-1 at Thr-412 is suppressed by PKCα/β inhibitor.

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PKCα not PKCβ phosphorylates coronin-1 at Thr-412 in vitro.

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Dissociation of coronin-1 from phagosome is regulated by PKCα.

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Phosphorylation of coronin-1 at Thr-412 may trigger phagosome maturation.

Comparative proteomics identified immune response proteins involved in response to vaccination with heat-inactivated Mycobacterium bovis and mycobacterial challenge in cattle

There is an imperative need for effective control of bovine tuberculosis (BTB) on a global scale and vaccination of cattle may prove to be pivotal in achieving this. The oral and parenteral use of a heat-inactivated Mycobacterium bovis (M. bovis) vaccine has previously been found to confer partial protection against BTB in several species. A role for complement factor C3 has been suggested in wild boar, but the exact mechanism by which this vaccine provides protection remains unclear. In the present study, a quantitative proteomics approach was used to analyze the white blood cell proteome of vaccinated cattle in comparison to unvaccinated controls, prior (T0) and in response to vaccination, skin test and challenge (T9 and T12). The Fisher's exact test was used to compare the proportion of positive reactors to standard immunological assays for BTB (the BOVIGAM^® assay, IDEXX TB ELISA and skin test) between the vaccinated and control groups. Using reverse-phase liquid-chromatography tandem mass spectrometry (RP-LC-MS/MS), a total of 12,346 proteins were identified with at least two peptides per protein and the Chi²-test (P = 0.05) determined 1,222 to be differentially represented at the key time point comparisons. Gene ontology (GO) analysis was performed in order to determine the biological processes (BPs), molecular functions (MFs) and cell components (CCs) the proteins formed part of. The analysis was focused on immune system BPs, specifically. GO analysis revealed that the most overrepresented proteins in immune system BPs, were kinase activity and receptor activity molecular functions and extracellular, Golgi apparatus and endosome cell components and included complement factor C8α and C8β as well as toll-like receptors 4 (TLR4) and 9 (TLR9). Proteins of the Janus kinase (JAK)-signal transducer and activator of transcription (STAT) (JAK-STAT) and protein kinase C (PKC) signaling pathways were furthermore found to potentially be involved in the immune response elicited by the inactivated vaccine. In conclusion, this study provides a first indication of the role of several immune system pathways in response to the heat-inactivated M. bovis vaccine and mycobacterial challenge.

Tuberculosis: Smart manipulation of a lethal host

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a global threat to human health. Development of drug resistance and co-infection with HIV has increased the morbidity and mortality caused by TB. Macrophages serve as primary defense against microbial infections, including TB. Upon recognition and uptake of mycobacteria, macrophages initiate a series of events designed to lead to generation of effective immune responses and clearance of infection. However, pathogenic mycobacteria utilize multiple mechanisms for manipulating macrophage responses to protect itself from being killed and to survive within these cells that are designed to kill them. The outcomes of mycobacterial infection are determined by several host- and pathogen-related factors. Significant advancements in understanding mycobacterial pathogenesis have been made in recent years. In this review, some of the important factors/mechanisms regulating mycobacterial survival inside macrophages are discussed.

PknG supports mycobacterial adaptation in acidic environment

Mycobacterium tuberculosis (Mtb), causative agent of human tuberculosis (TB), has the remarkable ability to adapt to the hostile environment inside host cells. Eleven eukaryotic like serine-threonine protein kinases (STPKs) are present in Mtb. Protein kinase G (PknG) has been shown to promote mycobacterial survival inside host cells. A homolog of PknG is also present in Mycobacterium smegmatis (MS), a fast grower, non-pathogenic mycobacterium. In the present study, we have analyzed the role of PknG in mycobacteria during exposure to acidic environment. Expression of pknG in MS was decreased in acidic medium. Recombinant MS ectopically expressing pknG (MS-G) showed higher growth in acidic medium compared to wild type counterpart. MS-G also showed higher resistance upon exposure to 3.0 pH and better adaptability to acidic pH. Western blot analysis showed differential threonine but not serine phosphorylation of cellular proteins in MS at acidic pH which was restored by ectopic expression of pknG in MS. In Mtb H37Ra (Mtb-Ra), expression of pknG was increased at acidic pH. We also observed decreased expression of pknG in MS during infection in macrophages while the expression of pknG in Mtb-Ra was increased in similar conditions. Taken together, our data strongly suggests that pknG regulates growth of mycobacteria in acidic environment and is differentially transcribed in MS and Mtb under these conditions.

Interaction of the CD43 Sialomucin with the Mycobacterium tuberculosis Cpn60.2 Chaperonin Leads to Tumor Necrosis Factor Alpha Production

Mycobacterium tuberculosis is the causal agent of tuberculosis. Tumor necrosis factor alpha (TNF-α), transforming growth factor β (TGF-β), and gamma interferon (IFN-γ) secreted by activated macrophages and lymphocytes are considered essential to contain Mycobacterium tuberculosis infection. The CD43 sialomucin has been reported to act as a receptor for bacilli through its interaction with the chaperonin Cpn60.2, facilitating mycobacterium-macrophage contact. We report here that Cpn60.2 induces both human THP-1 cells and mouse-derived bone marrow-derived macrophages (BMMs) to produce TNF-α and that this production is CD43 dependent. In addition, we present evidence that the signaling pathway leading to TNF-α production upon interaction with Cpn60.2 requires active Src family kinases, phospholipase C-γ (PLC-γ), phosphatidylinositol 3-kinase (PI3K), p38, and Jun N-terminal protein kinase (JNK), both in BMMs and in THP-1 cells. Our data highlight the role of CD43 and Cpn60.2 in TNF-α production and underscore an important role for CD43 in the host-mycobacterium interaction.

Downregulation of protein kinase C-alpha enhances intracellular survival of Mycobacteria: role of PknG

Background

Intracellular trafficking of mycobacteria is comprehensively dependent on the unusual regulation of host proteins. Recently, we have reported that infection of macrophages by Mycobacterium tuberculosis H37Rv (Rv) selectively downregulates the expression of PKCα while infection by Mycobacterium smegmatis (MS) does not.

Results

Based on our earlier study, we have extrapolated for the first time that knockdown of PKCα, impairs phagocytosis of mycobacteria by macrophages while their intracellular survival is drastically increased. Mycobacterium bovis BCG (BCG) and Mycobacterium tuberculosis H37Ra (Ra) have also been shown to downregulate the expression of PKCα during the infection. Since PknG is uniquely expressed in BCG, Ra, Rv but not in MS and has been reported to promote intracellular survival of mycobacteria, led us to believe that PknG may be involved in such downregulation of PKCα. THP-1 cells infected with recombinant MS expressing PknG (MS-G), showed significant reduction in PKCα expression. In normal THP-1 cells survival of MS-G was enhanced as compared to MS, while their behavior in PKCα deficient cells could not be distinguished. The results strongly demonstrate that pathogenic mycobacteria recognize and then inhibit PKCα to circumvent phagocytosis and the hostile environment of macrophages. We emphasize that, this inhibition is controlled by PknG.

Conclusions

All together, our data reveal a mechanism that shows substantial interdependence of PKCα with PknG, in sustaining mycobacterial infection.