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.
Display omitted
•MLKL activation is governed by a hierarchical transduction of structural changes•The brace undergoes oligomerization prior to plasma membrane recruitment•Low-affinity NB binding to PIP mediates plasma membrane targeting•High-affinity PIP binding to NB displaces the brace for robust membrane association
Quarato et al. show the consequences of MLKL activation for plasma membrane rupture in necroptosis. Upon activation, MLKL undergoes oligomerization mediated by the brace domain, being recruited to the plasma membrane through avidity of NB for PIP. Robust membrane association follows the initial recruitment through PIP-induced displacement of the brace.
The activation of mixed lineage kinase-like (MLKL) by receptor-interacting protein kinase-3 (RIPK3) results in plasma membrane (PM) disruption and a form of regulated necrosis, called necroptosis. ...Here, we show that, during necroptosis, MLKL-dependent calcium (Ca2+) influx and phosphatidylserine (PS) exposure on the outer leaflet of the plasma membrane preceded loss of PM integrity. Activation of MLKL results in the generation of broken, PM “bubbles” with exposed PS that are released from the surface of the otherwise intact cell. The ESCRT-III machinery is required for formation of these bubbles and acts to sustain survival of the cell when MLKL activation is limited or reversed. Under conditions of necroptotic cell death, ESCRT-III controls the duration of plasma membrane integrity. As a consequence of the action of ESCRT-III, cells undergoing necroptosis can express chemokines and other regulatory molecules and promote antigenic cross-priming of CD8+ T cells.
Display omitted
•Phosphatidylserine is exposed prior to loss of cell integrity during necroptosis•ESCRT-III facilitates the shedding of MLKL-damaged plasma membrane from intact cells•ESCRT-III supports cell survival and functions downstream of active MLKL•ESCRT-III allows necroptotic cells to signal surrounding cells
Cells undergoing necroptosis are not always headed towards death; ESCRT-III helps preserve the plasma membrane in these cells, contributing to survival.
Asymmetric cell division, the partitioning of cellular components in response to polarizing cues during mitosis, has roles in differentiation and development. It is important for the self-renewal of ...fertilized zygotes in Caenorhabditis elegans and neuroblasts in Drosophila, and in the development of mammalian nervous and digestive systems. T lymphocytes, upon activation by antigen-presenting cells (APCs), can undergo asymmetric cell division, wherein the daughter cell proximal to the APC is more likely to differentiate into an effector-like T cell and the distal daughter is more likely to differentiate into a memory-like T cell. Upon activation and before cell division, expression of the transcription factor c-Myc drives metabolic reprogramming, necessary for the subsequent proliferative burst. Here we find that during the first division of an activated T cell in mice, c-Myc can sort asymmetrically. Asymmetric distribution of amino acid transporters, amino acid content, and activity of mammalian target of rapamycin complex 1 (mTORC1) is correlated with c-Myc expression, and both amino acids and mTORC1 activity sustain the differences in c-Myc expression in one daughter cell compared to the other. Asymmetric c-Myc levels in daughter T cells affect proliferation, metabolism, and differentiation, and these effects are altered by experimental manipulation of mTORC1 activity or c-Myc expression. Therefore, metabolic signalling pathways cooperate with transcription programs to maintain differential cell fates following asymmetric T-cell division.
To fulfill bioenergetic demands of activation, T cells perform aerobic glycolysis, a process common to highly proliferative cells in which glucose is fermented into lactate rather than oxidized in ...mitochondria. However, the signaling events that initiate aerobic glycolysis in T cells remain unclear. We show T cell activation rapidly induces glycolysis independent of transcription, translation, CD28, and Akt and not involving increased glucose uptake or activity of glycolytic enzymes. Rather, TCR signaling promotes activation of pyruvate dehydrogenase kinase 1 (PDHK1), inhibiting mitochondrial import of pyruvate and facilitating breakdown into lactate. Inhibition of PDHK1 reveals this switch is required acutely for cytokine synthesis but dispensable for cytotoxicity. Functionally, cytokine synthesis is modulated via lactate dehydrogenase, which represses cytokine mRNA translation when aerobic glycolysis is disengaged. Our data provide mechanistic insight to metabolic contribution to effector T cell function and suggest that T cell function may be finely tuned through modulation of glycolytic activity.
Display omitted
•Naive and effector CD8+ T cells induce aerobic glycolysis minutes after activation•TCR signaling is directly tied to glycolysis via pyruvate dehydrogenase kinase 1 (PDHK1)•PDHK1-initiated aerobic glycolysis is required for optimal cytokine production•Cytokine synthesis is controlled via repression of mRNA binding by lactate dehydrogenase
Menk et al. show rapid induction of aerobic glycolysis after activation of effector T cells that is required for acute cytokine production. These data provide mechanistic insight into the regulation of T cell function through nutrient availability.
Necroptosis is a form of programmed necrotic cell death mediated by the kinase RIPK3 and its substrate MLKL. MLKL, which displays plasma membrane (PM) pore-forming activity upon phosphorylation, ...functions as the executioner during necroptosis. Thus, it was previously assumed that MLKL phosphorylation is the endpoint of the necroptotic signaling pathway. Here, we summarize several events that characterize the dying necroptotic cells after MLKL phosphorylation, including Ca
2+
influx, phosphatidylserine (PS) externalization, PM repair by ESCRT-III activation, and the final compromise of PM integrity. These processes add several unexpected regulatory events downstream of MLKL signaling. We have also observed that CoCl
2
, which may mimic hypoxia, can induce necroptosis, which suggests that in vivo triggers of necroptosis might include a transient lack of O
2
.
The ability of mitochondria to buffer a rapid rise in cytosolic Ca2+ is a hallmark of proper cell homeostasis. Here, we employed m-3M3FBS, a putative phospholipase C (PLC) agonist, to explore the ...relationships between intracellular Ca2+ imbalance, mitochondrial physiology, and cell death. m-3M3FBS induced a potent dose-dependent Ca2+ release from the endoplasmic reticulum (ER), followed by a rise in intra-mitochondrial Ca2+. When the latter exceeded the organelle buffering capacity, an abrupt mitochondrial inner membrane permeabilization (MIMP) occurred, releasing matrix contents into the cytosol. MIMP was followed by cell death that was independent of Bcl-2 family members and inhibitable by the intracellular Ca2+ chelator BAPTA-AM. Cyclosporin A (CsA), capable of blocking the mitochondrial permeability transition (MPT), completely prevented cell death induced by m-3M3FBS. However, CsA acted upstream of mitochondria by preventing Ca2+ release from ER stores. Therefore, loss of Ca2+ intracellular balance and mitochondrial Ca2+ overload followed by MIMP induced a cell death process that is distinct from Bcl-2 family-regulated mitochondrial outer membrane permeabilization (MOMP). Further, the inhibition of cell death by CsA or its analogues can be independent of effects on the MPT.
Systemic lupus erythematosus (SLE) is a complex autoimmune disease with genetic and environmental contributions. Hallmarks of the disease are the appearance of immune complexes (IC) containing ...autoreactive Abs and TLR-activating nucleic acids, whose deposition in kidney glomeruli is suspected to promote tissue injury and glomerulonephritis (GN). Here, using a mouse model based on the human SLE susceptibility locus TNFAIP3-interacting protein 1 (TNIP1, also known as ABIN1), we investigated the pathogenesis of GN. We found that GN was driven by TLRs but, remarkably, proceeded independently of ICs. Rather, disease in 3 different mouse models and patients with SLE was characterized by glomerular accumulation of patrolling monocytes (PMos), a cell type with an emerging key function in vascular inflammation. Consistent with such function in GN, monocyte-specific deletion of ABIN1 promoted kidney disease, whereas selective elimination of PMos provided protection. In contrast to GN, PMo elimination did not protect from reduced survival or disease symptoms such as IC generation and splenomegaly, suggesting that GN and other inflammatory processes are governed by distinct pathogenic mechanisms. These data identify TLR-activated PMos as the principal component of an intravascular process that contributes to glomerular inflammation and kidney injury.
Myelopoiesis is necessary for the generation of mature myeloid cells during homeostatic turnover and immunological insults; however, the metabolic requirements for this process remain poorly defined. ...Here, we demonstrate that myelopoiesis, including monocyte and macrophage differentiation, requires mechanistic target of rapamycin complex 1 (mTORC1) signaling and anabolic metabolism. Loss of mTORC1 impaired myelopoiesis under steady state and dampened innate immune responses against
infection. Stimulation of hematopoietic progenitors with macrophage colony-stimulating factor (M-CSF) resulted in mTORC1-dependent anabolic metabolism, which in turn promoted expression of M-CSF receptor and transcription factors PU.1 and IRF8, thereby constituting a feed-forward loop for myelopoiesis. Mechanistically, mTORC1 engaged glucose metabolism and initiated a transcriptional program involving Myc activation and sterol biosynthesis after M-CSF stimulation. Perturbation of glucose metabolism or disruption of Myc function or sterol biosynthesis impaired myeloid differentiation. Integrative metabolomic and genomic profiling further identified one-carbon metabolism as a central node in mTORC1-dependent myelopoiesis. Therefore, the interplay between mTORC1 signaling and metabolic reprogramming underlies M-CSF-induced myelopoiesis.
The precise molecular mechanism underlying the regulation of early B cell lymphopoiesis is unclear. The PLCγ signaling pathway is critical for antigen receptor-mediated lymphocyte activation, but its ...function in cytokine signaling is unknown. Here we show that PLCγ1/PLCγ2 double deficiency in mice blocks early B cell development at the pre-pro-B cell stage and renders B cell progenitors unresponsive to IL-7. PLCγ pathway inhibition blocks IL-7-induced activation of mTOR, but not Stat5. The PLCγ pathway activates mTOR through the DAG/PKC signaling branch, independent of the conventional Akt/TSC/Rheb signaling axis. Inhibition of PLCγ/PKC-induced mTOR activation impairs IL-7-mediated B cell development. PLCγ1/PLCγ2 double-deficient B cell progenitors have reduced expression of genes related to B cell lineage, IL-7 signaling, and cell cycle. Thus, IL-7 receptor controls early B lymphopoiesis through activation of mTOR via PLCγ/DAG/PKC signaling, not via Akt/Rheb signaling.
Degradation of defective mitochondria is an essential process to maintain cellular homeostasis and it is strictly regulated by the ubiquitin-proteasome system (UPS) and lysosomal activities. Here, ...using genome-wide CRISPR and small interference RNA screens, we identified a critical contribution of the lysosomal system in controlling aberrant induction of apoptosis following mitochondrial damage. After treatment with mitochondrial toxins, activation of the PINK1-Parkin axis triggered a BAX- and BAK-independent process of cytochrome c release from mitochondria followed by APAF1 and caspase 9-dependent apoptosis. This phenomenon was mediated by UPS-dependent outer mitochondrial membrane (OMM) degradation and was reversed using proteasome inhibitors. We found that the subsequent recruitment of the autophagy machinery to the OMM protected cells from apoptosis, mediating the lysosomal degradation of dysfunctional mitochondria. Our results underscore a major role of the autophagy machinery in counteracting aberrant noncanonical apoptosis and identified autophagy receptors as key elements in the regulation of this process.
PDF
