MLKL post-translational modifications: road signs to infection, inflammation and unknown destinations

The importance of murine phospho-MLKL-S345 in situ detection for necroptosis assessment in vivo

Necroptosis is a caspase-independent modality of cell death implicated in many inflammatory pathologies. The execution of this pathway requires the formation of a cytosolic platform that comprises RIPK1 and RIPK3 which, in turn, mediates the phosphorylation of the pseudokinase MLKL (S345 in mouse). The activation of this executioner is followed by its oligomerisation and accumulation at the plasma-membrane where it leads to cell death via plasma-membrane destabilisation and consequent permeabilisation. While the biochemical and cellular characterisation of these events have been amply investigated, the study of necroptosis involvement in vivo in animal models is currently limited to the use of Mlkl-/- or Ripk3-/- mice. Yet, even in many of the models in which the involvement of necroptosis in disease aetiology has been genetically demonstrated, the fundamental in vivo characterisation regarding the question as to which tissue(s) and specific cell type(s) therein is/are affected by the pathogenic necroptotic death are missing. Here, we describe and validate an immunohistochemistry and immunofluorescence-based method to reliably detect the phosphorylation of mouse MLKL at serine 345 (pMLKL-S345). We first validate the method using tissues derived from mice in which Caspase-8 (Casp8) or FADD are specifically deleted from keratinocytes, or intestinal epithelial cells, respectively. We next demonstrate the presence of necroptotic activation in the lungs of SARS-CoV-infected mice and in the skin and spleen of mice bearing a Sharpin inactivating mutation. Finally, we exclude necroptosis occurrence in the intestines of mice subjected to TNF-induced septic shock. Importantly, by directly comparing the staining of pMLKL-345 with that of cleaved Caspase-3 staining in some of these models, we identify spatio-temporal and functional differences between necroptosis and apoptosis supporting a role of RIPK3 in inflammation independently of MLKL versus the role of RIPK3 in activation of necroptosis.

Erythronecroptosis: an overview of necroptosis or programmed necrosis in red blood cells

Necroptosis is considered a programmed necrosis that requires receptor-interacting protein kinase 1 (RIPK1), receptor-interacting protein kinase 3 (RIPK3), and pore-forming mixed lineage kinase domain-like protein (MLKL) to trigger a regulated cell membrane lysis. Membrane rupture in necroptosis has been shown to fuel innate immune response due to release of damage-associated molecular patterns (DAMPs). Recently published studies indicate that mature erythrocytes can undergo necroptosis as well. In this review, we provide an outline of multiple cell death modes occurring in erythrocytes, discuss possible immunological aspects of diverse erythrocyte cell deaths, summarize available evidence related to the ability of erythrocytes to undergo necroptosis, outline key involved molecular mechanisms, and discuss the potential implication of erythrocyte necroptosis in the physiology and pathophysiology. Furthermore, we aim to highlight the interplay between necroptosis and eryptosis signaling in erythrocytes, emphasizing specific characteristics of these pathways distinct from their counterparts in nucleated cells. Thus, our review provides a comprehensive summary of the current knowledge of necroptosis in erythrocytes. To reflect critical differences between necroptosis of nucleated cells and necroptosis of erythrocytes, we suggest a term erythronecroptosis for necroptosis of enucleated cells.

12th Tuscany Retreat on Cancer Research and Apoptosis: Genetic profiling, resistance mechanisms and novel treatment concepts in cancer and neurodegeneration

The 12th Tuscany Retreat on Cancer Research and Apoptosis was held on August 19-26, 2023. The biennial retreat aims to bring together scientists who advance research in cancer, cell death, and neurodegenerative diseases. Topics covered ranged from drug resistance in cancer to insights into novel molecular cell signaling mechanisms and targets, all related to the pathways and molecules that regulate programmed cell death and the diseases that result from the dysregulation of programmed cell death. In this meeting review, we summarize the content of the most recent retreat.

Paralichthys olivaceus MLKL-mediated necroptosis is activated by RIPK1/3 and involved in anti-microbial immunity

Necroptosis is a type of proinflammatory programmed necrosis essential for innate immunity. The receptor interacting protein kinases 1/3 (RIPK1/3) and the substrate mixed lineage kinase domain-like protein (MLKL) are core components of the necroptotic axis. The activation and immunological function of necroptosis in fish remain elusive. Herein, we studied the function and activation of RIPK1/3 (PoRIPK1/3) and MLKL (PoMLKL) in teleost Paralichthys olivaceus. Bacterial infection increased the expression of RIPK1/3 and MLKL. The N-terminal four-helix bundle (4HB) domain of PoMLKL exhibited necroptosis-inducing activity, and the C-terminal pseudokinase domain exerted auto-inhibitory effect on the 4HB domain. PoRIPK3 was capable of phosphorylating the T360/S361 residues in the PoMLKL C-terminal domain and initiated necroptosis, and this necroptosis-inducing activity was enhanced by PoRIPK1. PoRIPK1/3 interacted with PoMLKL in a manner that depended on the RIP homotypic interaction motif (RHIM), and deletion of RHIM from PoRIPK1/3 led to the dissociation of PoRIPK1/3 with PoMLKL. Inhibition of PoMLKL-mediated necroptosis increased Edwardsiella tarda infection in fish cells and tissues, and led to significantly enhanced lethality of the host. Taken together, these results revealed the activation mechanism of PoRIPK1/3-PoMLKL signaling pathway and the immunological function of necroptosis in the immune defense of teleost.

SARS-CoV-2 envelope protein induces necroptosis and mediates inflammatory response in lung and colon cells through receptor interacting protein kinase 1

SARS-CoV-2 Envelope protein (E) is one of the crucial components in virus assembly and pathogenesis. The current study investigated its role in the SARS-CoV-2-mediated cell death and inflammation in lung and gastrointestinal epithelium and its effect on the gastrointestinal-lung axis. We observed that transfection of E protein increases the lysosomal pH and induces inflammation in the cell. The study utilizing Ethidium bromide/Acridine orange and Hoechst/Propidium iodide staining demonstrated necrotic cell death in E protein transfected cells. Our study revealed the role of the necroptotic marker RIPK1 in cell death. Additionally, inhibition of RIPK1 by its specific inhibitor Nec-1s exhibits recovery from cell death and inflammation manifested by reduced phosphorylation of NFκB. The E-transfected cells' conditioned media induced inflammation with differential expression of inflammatory markers compared to direct transfection in the gastrointestinal-lung axis. In conclusion, SARS-CoV-2 E mediates inflammation and necroptosis through RIPK1, and the E-expressing cells' secretion can modulate the gastrointestinal-lung axis. Based on the data of the present study, we believe that during severe COVID-19, necroptosis is an alternate mechanism of cell death besides ferroptosis, especially when the disease is not associated with drastic increase in serum ferritin.

Interaction among inflammasome, PANoptosise, and innate immune cells in infection of influenza virus: Updated review

BACKGROUND

Influenza virus (IV) is a leading cause of respiratory tract infections, eliciting responses from key innate immune cells such as Macrophages (MQs), Neutrophils, and Dendritic Cells (DCs). These cells employ diverse mechanisms to combat IV, with Inflammasomes playing a pivotal role in viral infection control. Cellular death mechanisms, including Pyroptosis, Apoptosis, and Necroptosis (collectively called PANoptosis), significantly contribute to the innate immune response.

METHODS

In this updated review, we delve into the intricate relationship between PANoptosis and Inflammasomes within innate immune cells (MQs, Neutrophils, and DCs) during IV infections. We explore the strategies employed by IV to evade these immune defenses and the consequences of unchecked PANoptosis and inflammasome activation, including the potential development of severe complications such as cytokine storms and tissue damage.

RESULTS

Our analysis underscores the interplay between PANoptosis and Inflammasomes as a critical aspect of the innate immune response against IV. We provide insights into IV's various mechanisms to subvert these immune pathways and highlight the importance of understanding these interactions to develop effective antiviral medications.

CONCLUSION

A comprehensive understanding of the dynamic interactions between PANoptosis, Inflammasomes, and IV is essential for advancing our knowledge of innate immune responses to viral infections. This knowledge will be invaluable in developing targeted antiviral therapies to combat IV and mitigate potential complications, including cytokine storms and tissue damage.

Cleavage of cFLIP restrains cell death during viral infection and tissue injury and favors tissue repair

Cell death coordinates repair programs following pathogen attack and tissue injury. However, aberrant cell death can interfere with such programs and cause organ failure. Cellular FLICE-like inhibitory protein (cFLIP) is a crucial regulator of cell death and a substrate of Caspase-8. However, the physiological role of cFLIP cleavage by Caspase-8 remains elusive. Here, we found an essential role for cFLIP cleavage in restraining cell death in different pathophysiological scenarios. Mice expressing a cleavage-resistant cFLIP mutant, CflipD377A, exhibited increased sensitivity to severe acute respiratory syndrome coronavirus (SARS-CoV)-induced lethality, impaired skin wound healing, and increased tissue damage caused by Sharpin deficiency. In vitro, abrogation of cFLIP cleavage sensitizes cells to tumor necrosis factor(TNF)-induced necroptosis and apoptosis by favoring complex-II formation. Mechanistically, the cell death-sensitizing effect of the D377A mutation depends on glutamine-469. These results reveal a crucial role for cFLIP cleavage in controlling the amplitude of cell death responses occurring upon tissue stress to ensure the execution of repair programs.

The Many Faces of MLKL, the Executor of Necroptosis

Necroptosis is a recently discovered form of regulated cell death characterized by the disruption of plasma membrane integrity and the release of intracellular content. Mixed lineage kinase domain-like (MLKL) protein is the main player of this cell death pathway as it mediates the final step of plasma membrane permeabilization. Despite the significant progress in our knowledge of the necroptotic pathway and MLKL biology, the precise mechanism of how MLKL functions remain unclear. To understand in what way MLKL executes necroptosis, it is crucial to decipher how the molecular machinery of regulated cell death is activated in response to different stimuli or stressors. It is also indispensable to unveiling the structural elements of MLKL and the cellular players that are required for its regulation. In this review, we discuss the key steps that lead to MLKL activation, possible models that explain how it becomes the death executor in necroptosis, and its emerging alternative functions. We also summarize the current knowledge about the role of MLKL in human disease and provide an overview of existing strategies aimed at developing new inhibitors that target MLKL for necroptosis intervention.