The Structural Basis of Necroptotic Cell Death Signaling Petrie, Emma J.; Czabotar, Peter E.; Murphy, James M.
Trends in biochemical sciences (Amsterdam. Regular ed.),
January 2019, 2019-01-00, 20190101, Volume:
44, Issue:
1
Journal Article
Peer reviewed
The recent implication of the cell death pathway, necroptosis, in innate immunity and a range of human pathologies has led to intense interest in the underlying molecular mechanism. Unlike the ...better-understood apoptosis pathway, necroptosis is a caspase-independent pathway that leads to cell lysis and release of immunogens downstream of death receptor and Toll-like receptor (TLR) ligation. Here we review the role of recent structural studies of the core machinery of the pathway, the protein kinases receptor-interacting protein kinase (RIPK)1 and RIPK3, and the terminal effector, the pseudokinase mixed lineage kinase domain-like protein (MLKL), in shaping our mechanistic understanding of necroptotic signaling. Structural studies have played a key role in establishing models that describe MLKL’s transition from a dormant monomer to a killer oligomer and revealing important interspecies differences.
Structures of component signaling modules in all known necroptosis effectors – RIPK1, RIPK3, and MLKL – have been reported over the past 5 years.
Recent NMR studies have provided insights into how RIPK1 and RIPK3 form high-molecular-weight fibrils via their RHIM motifs.
Recent structural and biophysical data highlight interspecies differences in the terminal effector MLKL. MLKL assembles into higher-order complexes: mouse MLKL forms trimers while the human protein is tetrameric. Structural differences in the executioner 4HB domain and the regulatory pseudokinase domain between mouse and human MLKL may reflect mechanistic differences.
Several models have been proposed to describe the mechanism by which MLKL permeabilizes the plasma membrane, leading to debate about the nature of the membrane aperture. Recent studies suggest a role for necroptotic coeffectors in regulating the susceptibility of membranes to MLKL-mediated permeabilization.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Mixed lineage kinase domain-like (MLKL) is the terminal protein in the pro-inflammatory necroptotic cell death program. RIPK3-mediated phosphorylation is thought to initiate MLKL oligomerization, ...membrane translocation and membrane disruption, although the precise choreography of events is incompletely understood. Here, we use single-cell imaging approaches to map the chronology of endogenous human MLKL activation during necroptosis. During the effector phase of necroptosis, we observe that phosphorylated MLKL assembles into higher order species on presumed cytoplasmic necrosomes. Subsequently, MLKL co-traffics with tight junction proteins to the cell periphery via Golgi-microtubule-actin-dependent mechanisms. MLKL and tight junction proteins then steadily co-accumulate at the plasma membrane as heterogeneous micron-sized hotspots. Our studies identify MLKL trafficking and plasma membrane accumulation as crucial necroptosis checkpoints. Furthermore, the accumulation of phosphorylated MLKL at intercellular junctions accelerates necroptosis between neighbouring cells, which may be relevant to inflammatory bowel disease and other necroptosis-mediated enteropathies.
Necroptotic cell death is mediated by the most terminal known effector of the pathway, MLKL. Precisely how phosphorylation of the MLKL pseudokinase domain activation loop by the upstream kinase, ...RIPK3, induces unmasking of the N-terminal executioner four-helix bundle (4HB) domain of MLKL, higher-order assemblies, and permeabilization of plasma membranes remains poorly understood. Here, we reveal the existence of a basal monomeric MLKL conformer present in human cells prior to exposure to a necroptotic stimulus. Following activation, toggling within the MLKL pseudokinase domain promotes 4HB domain disengagement from the pseudokinase domain αC helix and pseudocatalytic loop, to enable formation of a necroptosis-inducing tetramer. In contrast to mouse MLKL, substitution of RIPK3 substrate sites in the human MLKL pseudokinase domain completely abrogated necroptotic signaling. Therefore, while the pseudokinase domains of mouse and human MLKL function as molecular switches to control MLKL activation, the underlying mechanism differs between species.
Phosphorylation of the MLKL pseudokinase by the RIPK3 kinase leads to MLKL oligomerization, translocation to, and permeabilization of, the plasma membrane to induce necroptotic cell death. The ...precise choreography of MLKL activation remains incompletely understood. Here, we report Monobodies, synthetic binding proteins, that bind the pseudokinase domain of MLKL within human cells and their crystal structures in complex with the human MLKL pseudokinase domain. While Monobody-32 constitutively binds the MLKL hinge region, Monobody-27 binds MLKL via an epitope that overlaps the RIPK3 binding site and is only exposed after phosphorylated MLKL disengages from RIPK3 following necroptotic stimulation. The crystal structures identified two distinct conformations of the MLKL pseudokinase domain, supporting the idea that a conformational transition accompanies MLKL disengagement from RIPK3. These studies provide further evidence that MLKL undergoes a large conformational change upon activation, and identify MLKL disengagement from RIPK3 as a key regulatory step in the necroptosis pathway.
The necroptosis cell death pathway has been implicated in host defense and in the pathology of inflammatory diseases. While phosphorylation of the necroptotic effector pseudokinase Mixed Lineage ...Kinase Domain-Like (MLKL) by the upstream protein kinase RIPK3 is a hallmark of pathway activation, the precise checkpoints in necroptosis signaling are still unclear. Here we have developed monobodies, synthetic binding proteins, that bind the N-terminal four-helix bundle (4HB) “killer” domain and neighboring first brace helix of human MLKL with nanomolar affinity. When expressed as genetically encoded reagents in cells, these monobodies potently block necroptotic cell death. However, they did not prevent MLKL recruitment to the “necrosome” and phosphorylation by RIPK3, nor the assembly of MLKL into oligomers, but did block MLKL translocation tomembranes where activatedMLKL normally disrupts membranes to kill cells. An X-ray crystal structure revealed a monobodybinding site centered on the α4 helix of the MLKL 4HB domain, which mutational analyses showed was crucial for reconstitution of necroptosis signaling. These data implicate the α4 helix of its 4HB domain as a crucial site for recruitment of adaptor proteins that mediatemembrane translocation, distinct from known phospholipid binding sites.
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BFBNIB, NMLJ, NUK, PNG, SAZU, UL, UM, UPUK
The programmed cell death pathway, necroptosis, relies on the pseudokinase, Mixed Lineage Kinase domain-Like (MLKL), for cellular execution downstream of death receptor or Toll-like receptor ...ligation. Receptor-interacting protein kinase-3 (RIPK3)-mediated phosphorylation of MLKL's pseudokinase domain leads to MLKL switching from an inert to activated state, where exposure of the N-terminal four-helix bundle (4HB) 'executioner' domain leads to cell death. The precise molecular details of MLKL activation, including the stoichiometry of oligomer assemblies, mechanisms of membrane translocation and permeabilisation, remain a matter of debate. Here, we dissect the function of the two 'brace' helices that connect the 4HB to the pseudokinase domain of MLKL. In addition to establishing that the integrity of the second brace helix is crucial for the assembly of mouse MLKL homotrimers and cell death, we implicate the brace helices as a device to communicate pseudokinase domain phosphorylation event(s) to the N-terminal executioner 4HB domain. Using mouse:human MLKL chimeras, we defined the first brace helix and adjacent loop as key elements of the molecular switch mechanism that relay pseudokinase domain phosphorylation to the activation of the 4HB domain killing activity. In addition, our chimera data revealed the importance of the pseudokinase domain in conferring host specificity on MLKL killing function, where fusion of the mouse pseudokinase domain converted the human 4HB + brace from inactive to a constitutive killer of mouse fibroblasts. These findings illustrate that the brace helices play an active role in MLKL regulation, rather than simply acting as a tether between the 4HB and pseudokinase domains.
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EMUNI, FIS, FZAB, GEOZS, GIS, IJS, IMTLJ, KILJ, KISLJ, MFDPS, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, SBMB, SBNM, UKNU, UL, UM, UPUK, VKSCE, ZAGLJ
Necroptotic cell death has been implicated in many human pathologies and is thought to have evolved as an innate immunity mechanism. The pathway relies on two key effectors: the kinase ...receptor-interacting protein kinase 3 (RIPK3) and the terminal effector, the pseudokinase mixed-lineage kinase-domain-like (MLKL). We identify proteins with high sequence similarity to the pseudokinase domain of MLKL in poxvirus genomes. Expression of these proteins from the BeAn 58058 and Cotia poxviruses, but not swinepox, in human and mouse cells blocks cellular MLKL activation and necroptotic cell death. We show that viral MLKL-like proteins function as dominant-negative mimics of host MLKL, which inhibit necroptosis by sequestering RIPK3 via its kinase domain to thwart MLKL engagement and phosphorylation. These data support an ancestral role for necroptosis in defense against pathogens. Furthermore, mimicry of a cellular pseudokinase by a pathogen adds to the growing repertoire of functions performed by pseudokinases in signal transduction.
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•Some poxviruses encode proteins with homology to mammalian MLKL pseudokinase domains•BAV and Cotia viral MLKL proteins inhibit necroptotic death of human and mouse cells•These viral MLKL proteins target RIPK3 to block activation of cellular MLKL•Viral MLKL proteins inhibit RIPK3 via species-specific mechanisms
Petrie et al. identify proteins encoded by some poxviruses with homology to the pseudokinase domain of the terminal necroptosis effector, MLKL. Via species-specific mechanisms, viral MLKL proteins block necroptotic death in human and mouse cells by sequestering RIPK3 to prevent phosphorylation and activation of MLKL.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
The MLKL pseudokinase is the terminal effector in the necroptosis cell death pathway. Phosphorylation by its upstream regulator, RIPK3, triggers MLKL's conversion from a dormant cytoplasmic protein ...into oligomers that translocate to, and permeabilize, the plasma membrane to kill cells. The precise mechanisms underlying these processes are incompletely understood, and were proposed to differ between mouse and human cells. Here, we examine the divergence of activation mechanisms among nine vertebrate MLKL orthologues, revealing remarkable specificity of mouse and human RIPK3 for MLKL orthologues. Pig MLKL can restore necroptotic signaling in human cells; while horse and pig, but not rat, MLKL can reconstitute the mouse pathway. This selectivity can be rationalized from the distinct conformations observed in the crystal structures of horse and rat MLKL pseudokinase domains. These studies identify important differences in necroptotic signaling between species, and suggest that, more broadly, divergent regulatory mechanisms may exist among orthologous pseudoenzymes.
Necroptosis (or ‘programmed necrosis’) is a caspase‐independent cell death pathway that operates downstream of death receptors, including Tumour Necrosis Factor Receptor‐1 (TNFR1), and the Toll‐like ...receptors, TLR3 and TLR4. Owing to its immunogenicity, necroptosis has been attributed roles in the pathogenesis of several diseases, including inflammatory bowel disease and the tissue damage arising from ischaemic‐reperfusion injuries. Only over the past 7 years has the core machinery of this pathway, the receptor‐interacting protein kinase‐3 (RIPK3) and the pseudokinase, Mixed Lineage Kinase domain‐Like (MLKL), been defined. Our current understanding of the pathway is that RIPK3‐mediated phosphorylation activates cytoplasmic MLKL, which is the most terminal known effector in the pathway, leading to MLKL's oligomerisation, translocation to, and permeabilisation of, the plasma membrane. Here, we discuss the insights gleaned from structural and biophysical studies of MLKL and highlight the known unknowns surrounding MLKL's mechanism of action and activation.
The February 2017 issue contains a Special Feature on Necroptotic death signalling: evolution, mechanisms and disease relevance. In recent years, research into a genetically encoded cell death program termed necroptosis has accelerated into vogue. Many laboratories are now racing to answer key questions such as: How does it occur? When does it occur? What does it do? What is it good (or not so good) for? Answers to these will ultimately guide efforts aimed at manipulating this new pathway for therapeutic benefit. In the six articles in this ICB Special Feature, the current state of play in necroptotic cell death research is dissected in considerable detail. The articles provide timely updates on what we have learnt so far and, importantly, where we might be going. Immunology & Cell Biology thanks the coordinator of this Special Feature ‐ James Vince ‐ for his planning and input.
Although necrosis and necroinflammation are central features of many liver diseases, the role of programmed necrosis in the context of inflammation-dependent hepatocellular death remains to be fully ...determined. Here, we have demonstrated that the pseudokinase mixed lineage kinase domain-like protein (MLKL), which plays a key role in the execution of receptor-interacting protein (RIP) kinase-dependent necroptosis, is upregulated and activated in human autoimmune hepatitis and in a murine model of inflammation-dependent hepatitis. Using genetic and pharmacologic approaches, we determined that hepatocellular necrosis in experimental hepatitis is driven by an MLKL-dependent pathway that occurs independently of RIPK3. Moreover, we have provided evidence that the cytotoxic activity of the proinflammatory cytokine IFN-γ in hepatic inflammation is strongly connected to induction of MLKL expression via activation of the transcription factor STAT1. In summary, our results reveal a pathway for MLKL-dependent programmed necrosis that is executed in the absence of RIPK3 and potentially drives the pathogenesis of severe liver diseases.
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