Bcl-xL mediates RIPK3-dependent necrosis in M. tuberculosis-infected macrophages

Abstract

Virulent Mycobacterium tuberculosis (Mtb) triggers necrosis in host Mφ, which is essential for successful pathogenesis. Here we demonstrate that necrosis of Mtb-infected Mφ is dependent on the action of the cytosolic kinase Receptor Interacting Protein 3 (RIPK3) and the mitochondrial Bcl-2 family member protein B-cell lymphoma - extra large (Bcl-xL). RIPK3-deficient Mφ are able to better control bacterial growth in vitro and in vivo. Cytosolic RIPK3 translocates to the mitochondria where it promotes necrosis and blocks caspase 8-activation and apoptosis via Bcl-xL. Furthermore, necrosis is associated with stabilization of hexokinase II on the mitochondria as well as cyclophilin D-dependent mitochondrial permeability transition (MPT). These events up-regulate the level of reactive oxygen species (ROS) to induce necrosis. Thus, in Mtb-infected Mφ mitochondria are an essential platform for induction of necrosis by activating RIPK3 function and preventing caspase 8 - activation.

NAD+ Depletion Triggers Macrophage Necroptosis, a Cell Death Pathway Exploited by Mycobacterium tuberculosis

“…These experiments conclusively demonstrated that TNT triggers necroptosis, a form of programmed cell death with molecular features similar to necrosis, preventing viruses from replication in infected cells (Dondelinger et al, 2016; Weinlich et al, 2017). These findings are consistent with recent results published by Zhao et al (2017), who showed that cell death of macrophages infected with virulent Mtb is dependent on RIPK3 and MLKL and that Ripk3 −/− mice were more resistant to Mtb infection than WT mice. Our study established that TNT is the main necroptosis-inducing factor in macrophages infected with Mtb.…”

“…TNF-α-dependent cell death of macrophages is associated with apoptosis (Riendeau and Kornfeld, 2003) and was observed with avirulent Mtb strains and/or low bacillary loads (Lee et al, 2006). Our observation that virulent Mtb bypasses the TNF-α receptor and RIPK1 activation and directly activates RIPK3 is consistent with previous studies (Riendeau and Kornfeld, 2003; Zhao et al, 2017). Roca and Ramakrishnan (2013) showed in zebrafish that Mycobacterium marinum induces RIPK1/RIPK3-dependent necroptosis in the presence of excess TNF-α.…”

“…In addition, RIPK3 also migrates, together with Bcl-x L , to mitochondria (pathway 2). The RIPK3-Bcl-x L complex associates with pro-caspase-8 and prevents caspase-8-mediated apoptosis (Zhao et al, 2017). Furthermore, activated RIPK3 triggers formation of the mitochondrial permeability transition pore, depolarizing mitochondria (Zhao et al, 2017).…”

“…The RIPK3-Bcl-x L complex associates with pro-caspase-8 and prevents caspase-8-mediated apoptosis (Zhao et al, 2017). Furthermore, activated RIPK3 triggers formation of the mitochondrial permeability transition pore, depolarizing mitochondria (Zhao et al, 2017). The third pathway is based on connection of the cytosolic and mitochondrial NAD + pools.…”

“…RIPK63 senses lower NAD+ levels and/or increased levels of its degradation products and phosphorylates MLKL (pathway 1), which then forms oligomeric pores in host cell membranes and induces necrotic death (Wang et al, 2014). RIPK3 also migrates, together with Bcl-XL, to mitochondria (pathway 2), prevents caspase-8 activation, and induces formation of the mitochondrial permeability transition (MPT) pore, depolarizing mitochondria (Zhao et al, 2017). The third pathway is a consequence of the connection of the cytosolic and mitochondrial NAD+ pools, as shown previously (Cambronne et al, 2016).…”