Activation of the Wnt Pathway by Mycobacterium tuberculosis: A Wnt-Wnt Situation

Abstract

Mycobacterium tuberculosis (M. tuberculosis), an intracellular pathogenic Gram-positive bacterium, is the cause of tuberculosis (TB), a major worldwide human infectious disease. The innate immune system is the first host defense against M. tuberculosis. The recognition of this pathogen is mediated by several classes of pattern recognition receptors expressed on the host innate immune cells, including Toll-like receptors, Nod-like receptors, and C-type lectin receptors like Dectin-1, the Mannose receptor, and DC-SIGN. M. tuberculosis interaction with any of these receptors activates multiple signaling pathways among which the protein kinase C, the MAPK, and the NFκB pathways have been widely studied. These pathways have been implicated in macrophage invasion, M. tuberculosis survival, and impaired immune response, thus promoting a successful infection and disease. Interestingly, the Wnt signaling pathway, classically regarded as a pathway involved in the control of cell proliferation, migration, and differentiation in embryonic development, has recently been involved in immunoregulatory mechanisms in infectious and inflammatory diseases, such as TB, sepsis, psoriasis, rheumatoid arthritis, and atherosclerosis. In this review, we present the current knowledge supporting a role for the Wnt signaling pathway during macrophage infection by M. tuberculosis and the regulation of the immune response against M. tuberculosis. Understanding the cross talk between different signaling pathways activated by M. tuberculosis will impact on the search for new therapeutic targets to fuel the rational design of drugs aimed to restore the immunological response against M. tuberculosis.

Keywords: Wnt signaling; immune response; inflammation; macrophage infection; microRNAs; tuberculosis.

Figure 1

Mycobacterium tuberculosis (M. tuberculosis) recognition by immune receptors. During the infection, several classes of PPRs such as the Fcγ receptor, the Scavengers, and Mannose receptors recognize M. tuberculosis. Mycobacterium interaction with these receptors activates the protein kinase C pathway. This pathway is implicated in the cytoskeletal arrangements, macrophage invasion, and M. tuberculosis survival. Upon invasion of the macrophage by M. tuberculosis, coronin-1a is recruited to the mycobacterial phagosome leading to the induction of calcium fluxes, thus resulting in calcineurin activation, which blocks the phagosome–lysosome fusion by an unknown mechanism.

Figure 2

The pro-inflammatory and anti-inflammatory cytokines’ genes activated by Mycobacterium tuberculosis (M. tuberculosis). Pro-inflammatory (left). Interaction of M. tuberculosis with TLR2, Dectin-1, and Fcγ receptors activates NFκB and nuclear factor of activated T cells, which promotes transcription of genes such as tumor necrosis factor, IL-6, and IL-1β. Anti-inflammatory (right). Interaction of M. tuberculosis to TLR-2 and Dectin-1 through different mechanisms induces the activation of the transcription factors CREB, AP-1, Sp1, and STAT3, which initiates the transcription of genes such as IL-10, TGF-β, and IL-22.

Figure 3

The canonical Wnt signaling pathway. In the absence of Wnt ligands, β-catenin interacts with a degradation complex, which is formed by Axin, APC, GSK3, and CK1. In this complex, β-catenin is phosphorylated by GSK3 and by CK1 leading to ubiquitylation and subsequently degradation of β-catenin in the proteasome. When β-catenin is degraded, Groucho, which is a co-repressor protein, interacts with LEF1 and TCF. The Groucho/LEF1/TCF complex represses the transcription of target genes (left). Wnt ligands interact with the Frizzled receptor, a G protein-coupled receptor, and with the co-receptor LRP. GSK3 and CK1 induce the phosphorylation of LRP leading to the recruitment of axin and disheveled to the LRP/G protein-coupled receptor complex releasing β-catenin. Then, β-catenin is accumulated in the nucleus where it binds to LEF1 and TCF to induce the transcription of specific genes (right).

Figure 4

The non-canonical Wnt signaling pathway. Two β-catenin independent Wnt signaling pathways have been reported. The planar cell polarity (PCP) pathway is triggered by Wnt5 and Wnt11. Wnt5/11 binds Frizzled leading to the activation of a trimeric G protein that induces the activation of disheveled and DAAM. Together these proteins trigger the activation of the small GTPases RHOA and RAC-1 that leads to the activation of the kinases JUN kinase (JNK) and ROCK. These signaling pathways are involved in cell adhesion, migration, and cell cytoskeleton organization. The stress response pathway involves the activation of JNK, which phosphorylates the transcriptional factors AP-1 and JUN leading to their translocation to the nucleus to regulate gene expression (left). The Wnt/Ca²⁺ signaling pathway is triggered by Wnt5–FZD-2. The activation of this pathway is mediated through G proteins, which induces the phospholipase C activation, leading to the hydrolysis of PIP2 into DAG and IP3. The IP3 induces the release of intracellular Ca⁺⁺, activation of calcineurin, and CAMKII. The active calcineurin induces the nuclear factor of activated T cells (NFAT) activation. Intracellular calcium as well as DAG activates protein kinase C (PKC) increasing the activity of calcineurin triggering the translocation of the transcriptional factor NFAT to the nucleus. Similarly, PKC induces the activation and translocation of NFκB to the nucleus (right).

Figure 5

Mycobacterium tuberculosis (M. tuberculosis) inhibits tumor necrosis factor (TNF) through the Wnt pathway in a paracrine and autocrine manner. Wnt5, Wnt6, and pro-inflammatory cytokines are induced in macrophages in response to mycobacterial infection. Secreted Wnt5 and Wnt6 are recognized by FZD receptors expressed on the cell surface of neighboring macrophages, impairing TNF and IL-12 expression. Additionally, TNF released by the M. tuberculosis-infected macrophages promotes FZD1 expression in neighboring macrophages, rendering them susceptible to Wnt3 actions, which inhibits TNF expression.

Figure 6

Wnt3 promotes apoptosis in response to Mycobacterium tuberculosis (M. tuberculosis) infection. Wnt3 plays a protective role in the M. tuberculosis infection by preventing bacterial dissemination. On one hand, Wnt3 promotes apoptosis in a caspase-dependent manner, while on the other hand inhibits necrosis by promoting glutathione synthase expression (GS) and thus increasing the levels of glutathione (GSH) that in turn results in reduced reactive oxygen species and by inhibiting the PARP/AIF pathway.

Figure 7

Nucleotide-binding oligomerization domain 2 (NOD2) and the activation of the Wnt pathway in response to Mycobacterium tuberculosis infection. Muramyl dipeptide, derived from the Mycobacterial cell wall at early phases of the macrophages’ infection, triggers NOD2 translocation to the membrane where it interacts with the Ly6/PLAUR domain-containing protein 6, which has been previously shown to be recruited to the Wnt receptor complex and promote Wnt signaling resulting in β-catenin nuclear translocation and the expression of the X chromosome-linked inhibitor of apoptosis, that in turn activates the NLRP3 inflammasome resulting in IL-1β maturation.

Figure 8

Mutual regulation between miRNAs and the Wnt pathway determine the survival of Mycobacterium tuberculosis (M. tuberculosis)-infected macrophage and the profile of the inflammatory response. Upon interaction of M. tuberculosis with TLR2 and TLR4, NFκB activation leads to increased miR155 levels which, besides its negative effect on autophagy and apoptosis, through the negative modulation of key inhibitors of the Wnt pathway (APC and HMG-box transcription factor 1) leads to β-catenin/TCF-mediated expression of miR2, miR125b, miR146b (a target of the Wnt5a signaling pathway), and miR29, thus wiping off the pro-inflammatory response. Additionally, miR29 further enhances Wnt signaling by targeting distinct set of Wnt negative regulators (Dikkopf-1, Kremen2, and sFRP2).

Figure 9

The Beijing Mycobacterium tuberculosis (M. tuberculosis) strains regulate the Wnt pathway during macrophage infection of healthy individuals or that of macrophages from patients with active tuberculosis by enhancing miR485 and reducing miR150 levels, respectively. An enrichment pathway analysis performed in the WebGestalt platform using the Kyoto Encyclopedia of Genes and Genomes indicates that the miRNA485 potentiates the canonical Wnt pathway through negatively regulating protein levels involved in the inhibition of these pathways, while by controlling the protein levels of key signaling molecules might inhibit the non-canonical Wnt pathway. In contrast, miR150 has clear inhibitory effect on both the canonical and non-canonical Wnt pathway. Thus, through differentially regulating the expression of these two miRNAs, the Beijing M. tuberculosis strains promote the Wnt canonical pathway.