characterization-mlkl-mediated-plasma-membrane-rupture

Characterization of MLKL-mediated Plasma Membrane Rupture in Necroptosis

Necroptosis is a programmed cell death pathway triggered by activation of receptor interacting protein kinase 3 (RIPK3), which phosphorylates and ...

St. Jude Children's Research Hospital

Autophagy is a lysosomal degradation pathway that converts macromolecules into substrates for energy production during nutrient-scarce conditions such as those encountered in tumor microenvironments. Constitutive mitochondrial uptake of endoplasmic reticulum (ER) Ca²⁺ mediated by inositol triphosphate receptors (IP₃Rs) maintains cellular bioenergetics, thus suppressing autophagy. We show that the ER membrane protein Bax inhibitor-1 (BI-1) promotes autophagy in an IP₃R-dependent manner. By reducing steady-state levels of ER Ca²⁺ via IP₃Rs, BI-1 influences mitochondrial bioenergetics, reducing oxygen consumption, impacting cellular ATP levels, and stimulating autophagy. Furthermore, BI-1-deficient mice show reduced basal autophagy, and experimentally reducing BI-1 expression impairs tumor xenograft growth in vivo. BI-1's ability to promote autophagy could be dissociated from its known function as a modulator of IRE1 signaling in the context of ER stress. The results reveal BI-1 as a novel autophagy regulator that bridges Ca²⁺ signaling between ER and mitochondria, reducing cellular oxygen consumption and contributing to cellular resilience in the face of metabolic stress.

Endoplasmic reticulum protein BI-1 regulates Ca²⁺-mediated bioenergetics to promote autophagy.

Programmed necrosis (or necroptosis) is a form of cell death triggered by the activation of receptor interacting protein kinase-3 (RIPK3). Several reports have implicated mitochondria and mitochondrial reactive oxygen species (ROS) generation as effectors of RIPK3-dependent cell death. Here, we directly test this idea by employing a method for the specific removal of mitochondria via mitophagy. Mitochondria-deficient cells were resistant to the mitochondrial pathway of apoptosis, but efficiently died via tumor necrosis factor (TNF)-induced, RIPK3-dependent programmed necrosis or as a result of direct oligomerization of RIPK3. Although the ROS scavenger butylated hydroxyanisole (BHA) delayed TNF-induced necroptosis, it had no effect on necroptosis induced by RIPK3 oligomerization. Furthermore, although TNF-induced ROS production was dependent on mitochondria, the inhibition of TNF-induced necroptosis by BHA was observed in mitochondria-depleted cells. Our data indicate that mitochondrial ROS production accompanies, but does not cause, RIPK3-dependent necroptotic cell death.

Widespread mitochondrial depletion via mitophagy does not compromise necroptosis.

Receptor-interacting protein kinase (RIPK)-1 is involved in RIPK3-dependent and -independent signaling pathways leading to cell death and/or inflammation. Genetic ablation of ripk1 causes postnatal lethality, which was not prevented by deletion of ripk3, caspase-8, or fadd. However, animals that lack RIPK1, RIPK3, and either caspase-8 or FADD survived weaning and matured normally. RIPK1 functions in vitro to limit caspase-8-dependent, TNFR-induced apoptosis, and animals lacking RIPK1, RIPK3, and TNFR1 survive to adulthood. The role of RIPK3 in promoting lethality in ripk1(-/-) mice suggests that RIPK3 activation is inhibited by RIPK1 postbirth. Whereas TNFR-induced RIPK3-dependent necroptosis requires RIPK1, cells lacking RIPK1 were sensitized to necroptosis triggered by poly I:C or interferons. Disruption of TLR (TRIF) or type I interferon (IFNAR) signaling delayed lethality in ripk1(-/-)tnfr1(-/-) mice. These results clarify the complex roles for RIPK1 in postnatal life and provide insights into the regulation of FADD-caspase-8 and RIPK3-MLKL signaling by RIPK1.

RIPK1 blocks early postnatal lethality mediated by caspase-8 and RIPK3.

The role of apoptosis inducing factor (AIF) in promoting cell death versus survival remains controversial. We report that the loss of AIF in fibroblasts led to mitochondrial electron transport chain defects and loss of proliferation that could be restored by ectopic expression of the yeast NADH dehydrogenase Ndi1. Aif-deficiency in T cells led to decreased peripheral T cell numbers and defective homeostatic proliferation, but thymic T cell development was unaffected. In contrast, Aif-deficient B cells developed and functioned normally. The difference in the dependency of T cells versus B cells on AIF for function and survival correlated with their metabolic requirements. Ectopic Ndi1 expression rescued homeostatic proliferation of Aif-deficient T cells. Despite its reported roles in cell death, fibroblasts, thymocytes and B cells lacking AIF underwent normal death. These studies suggest that the primary role of AIF relates to complex I function, with differential effects on T and B cells.

Apoptosis-Inducing-Factor-Dependent Mitochondrial Function Is Required for T Cell but Not B Cell Function.

Cell senescence is an important tumour suppressor mechanism and driver of ageing. Both functions are dependent on the development of the senescent phenotype, which involves an overproduction of pro-inflammatory and pro-oxidant signals. However, the exact mechanisms regulating these phenotypes remain poorly understood. Here, we show the critical role of mitochondria in cellular senescence. In multiple models of senescence, absence of mitochondria reduced a spectrum of senescence effectors and phenotypes while preserving ATP production via enhanced glycolysis. Global transcriptomic analysis by RNA sequencing revealed that a vast number of senescent-associated changes are dependent on mitochondria, particularly the pro-inflammatory phenotype. Mechanistically, we show that the ATM, Akt and mTORC1 phosphorylation cascade integrates signals from the DNA damage response (DDR) towards PGC-1β-dependent mitochondrial biogenesis, contributing to aROS-mediated activation of the DDR and cell cycle arrest. Finally, we demonstrate that the reduction in mitochondrial content in vivo, by either mTORC1 inhibition or PGC-1β deletion, prevents senescence in the ageing mouse liver. Our results suggest that mitochondria are a candidate target for interventions to reduce the deleterious impact of senescence in ageing tissues.

Mitochondria are required for pro-ageing features of the senescent phenotype.

Necroptosis is a cell death pathway regulated by the receptor interacting protein kinase 3 (RIPK3) and the mixed lineage kinase domain-like (MLKL) pseudokinase. How MLKL executes plasma membrane rupture upon phosphorylation by RIPK3 remains controversial. Here, we characterize the hierarchical transduction of structural changes in MLKL that culminate in necroptosis. The MLKL brace, proximal to the N-terminal helix bundle (NB), is involved in oligomerization to facilitate plasma membrane targeting through the low-affinity binding of NB to phosphorylated inositol polar head groups of phosphatidylinositol phosphate (PIP) phospholipids. At the membrane, the NB undergoes a "rolling over" mechanism to expose additional higher-affinity PIP-binding sites responsible for robust association to the membrane and displacement of the brace from the NB. PI(4,5)P2 is the preferred PIP-binding partner. We investigate the specific association of MLKL with PIPs and subsequent structural changes during necroptosis. 

Sequential Engagement of Distinct MLKL Phosphatidylinositol-Binding Sites Executes Necroptosis.

During apoptosis, pro-apoptotic BAX and BAK are activated, causing mitochondrial outer membrane permeabilisation (MOMP), caspase activation and cell death. However, even in the absence of caspase activity, cells usually die following MOMP Such caspase-independent cell death is accompanied by inflammation that requires mitochondrial DNA (mtDNA) activation of cGAS-STING signalling. Because the mitochondrial inner membrane is thought to remain intact during apoptosis, we sought to address how matrix mtDNA could activate the cytosolic cGAS-STING signalling pathway. Using super-resolution imaging, we show that mtDNA is efficiently released from mitochondria following MOMP In a temporal manner, we find that following MOMP, BAX/BAK-mediated mitochondrial outer membrane pores gradually widen. This allows extrusion of the mitochondrial inner membrane into the cytosol whereupon it permeablises allowing mtDNA release. Our data demonstrate that mitochondrial inner membrane permeabilisation (MIMP) can occur during cell death following BAX/BAK-dependent MOMP Importantly, by enabling the cytosolic release of mtDNA, inner membrane permeabilisation underpins the immunogenic effects of caspase-independent cell death.

Article	Year
Endoplasmic reticulum protein BI-1 regulates Ca²⁺-mediated bioenergetics to promote autophagy. Genes & development\| PubMed ID: 22588718	2012
Widespread mitochondrial depletion via mitophagy does not compromise necroptosis. Cell reports\| PubMed ID: 24268776	2013
RIPK1 blocks early postnatal lethality mediated by caspase-8 and RIPK3. Cell\| PubMed ID: 24813850	2014
Apoptosis-Inducing-Factor-Dependent Mitochondrial Function Is Required for T Cell but Not B Cell Function. Immunity\| PubMed ID: 26795252	2016
Mitochondria are required for pro-ageing features of the senescent phenotype. The EMBO journal\| PubMed ID: 26848154	2016
Sequential Engagement of Distinct MLKL Phosphatidylinositol-Binding Sites Executes Necroptosis. Molecular cell\| PubMed ID: 26853145	2016
Mitochondrial inner membrane permeabilisation enables mtDNA release during apoptosis. The EMBO journal\| PubMed ID: 30049712	2018

Giovanni Quarato

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