Exposing Toxoplasma gondii hiding inside the vacuole: a role for GBPs, autophagy and host cell death

Abstract

The intracellular parasite Toxoplasma gondii resides inside a vacuole, which shields it from the host's intracellular defense mechanisms. The cytokine interferon gamma (IFNγ) upregulates host cell effector pathways that are able to destroy the vacuole, restrict parasite growth and induce host cell death. Interferon-inducible GTPases such as the Guanylate Binding Proteins (GBPs), autophagy proteins and ubiquitin-driven mechanisms play important roles in Toxoplasma control in mice and partly also in humans. The host inflammasome is regulated by GBPs in response to bacterial infection in murine cells and may also respond to Toxoplasma infection. Elucidation of murine Toxoplasma defense mechanisms are guiding studies on human cells, while inevitably leading to the discovery of human-specific pathways that often function in a cell type-dependent manner.

Figure 1

GBP-mediated restriction of Toxoplasma in murine cells. In murine cells, type II and III Toxoplasma vacuoles are attacked by a range of host proteins leading to the disruption of the vacuolar membrane. The ‘GMS’ IRGs (not shown) block ‘GKS’ IRG activation. Once activated, ‘GKS’ IRGs accumulate on the vacuole and recruit an unknown seeding E3 ubiquitin ligase, as well as the p62-interacting E3 ubiquitin ligases TRAF6 and TRIM21. GBPs target to the vacuole via p62-dependent and independent mechanisms. Ubiquitination of the vacuole is of the K48 and K63 linkage type on substrate proteins that potentially include IRGs and GBPs themselves. Rupture of the vacuole is dependent on IRGs, GBPs and p62.

Figure 2

Autophagy-mediated restriction of Toxoplasma in human and murine cells. In mice, the Atg proteins Atg7, Atg3 and the Atg12-Atg5-Atg16L1 complex, all involved in delivery and conjugation of LC3 to the autophagosomal membrane, are necessary to target the IRGs and GBPs to the Toxoplasma PVM. GATE-16 is the only LC3-like protein essential for controlling Toxoplasma infection in vivo, by activating the Golgi-localized membrane trafficking regulator Arf1 and keeping GBPs in a non-aggregated form in the cytoplasm of cells. GBPs and IRGs disrupt the PVM and LC3-driven autophagosomes either clear the parasite itself or the membrane remnants that remain. In humans, Atg7/16L1 (not pictured) target ubiquitin to the Toxoplasma PVM. This leads to the recruitment of p62 and NDP52 and subsequently LC3, without acidification of the PV and disruption of the PVM. The parasite is eventually enveloped in the autophagic double membrane where it fails to grow and replicate further.

Figure 3

Inflammasome activation driven by GBPs. GBPs can mediate inflammasome activation by lysing the vacuole of pathogens and/or direct lysing cytosolic bacteria leading to the exposure of PAMPS such as LPS and DNA which can activate Casp11 and AIM2, respectively. Certain GBPs can also tetramerize and bind to Casp11 or Nlrp3 thereby lowering the threshold for their activation. For more details see main text.