Detection of Necroptosis by Phospho-RIPK3 Immunohistochemical Labeling

An immunohistochemical atlas of necroptotic pathway expression

Necroptosis is a lytic form of regulated cell death reported to contribute to inflammatory diseases of the gut, skin and lung, as well as ischemic-reperfusion injuries of the kidney, heart and brain. However, precise identification of the cells and tissues that undergo necroptotic cell death in vivo has proven challenging in the absence of robust protocols for immunohistochemical detection. Here, we provide automated immunohistochemistry protocols to detect core necroptosis regulators - Caspase-8, RIPK1, RIPK3 and MLKL - in formalin-fixed mouse and human tissues. We observed surprising heterogeneity in protein expression within tissues, whereby short-lived immune barrier cells were replete with necroptotic effectors, whereas long-lived cells lacked RIPK3 or MLKL expression. Local changes in the expression of necroptotic effectors occurred in response to insults such as inflammation, dysbiosis or immune challenge, consistent with necroptosis being dysregulated in disease contexts. These methods will facilitate the precise localisation and evaluation of necroptotic signaling in vivo.

ZBP1 and TRIF trigger lethal necroptosis in mice lacking caspase-8 and TNFR1

Necroptosis is a lytic form of cell death that is mediated by the kinase RIPK3 and the pseudokinase MLKL when caspase-8 is inhibited downstream of death receptors, toll-like receptor 3 (TLR3), TLR4, and the intracellular Z-form nucleic acid sensor ZBP1. Oligomerization and activation of RIPK3 is driven by interactions with the kinase RIPK1, the TLR adaptor TRIF, or ZBP1. In this study, we use immunohistochemistry (IHC) and in situ hybridization (ISH) assays to generate a tissue atlas characterizing RIPK1, RIPK3, Mlkl, and ZBP1 expression in mouse tissues. RIPK1, RIPK3, and Mlkl were co-expressed in most immune cell populations, endothelial cells, and many barrier epithelia. ZBP1 was expressed in many immune populations, but had more variable expression in epithelia compared to RIPK1, RIPK3, and Mlkl. Intriguingly, expression of ZBP1 was elevated in Casp8-/- Tnfr1-/- embryos prior to their succumbing to aberrant necroptosis around embryonic day 15 (E15). ZBP1 contributed to this embryonic lethality because rare Casp8-/- Tnfr1-/- Zbp1-/- mice survived until after birth. Necroptosis mediated by TRIF contributed to the demise of Casp8-/- Tnfr1-/- Zbp1-/- pups in the perinatal period. Of note, Casp8-/- Tnfr1-/- Trif-/- Zbp1-/- mice exhibited autoinflammation and morbidity, typically within 5-7 weeks of being born, which is not seen in Casp8-/- Ripk1-/- Trif-/- Zbp1-/-, Casp8-/- Ripk3-/-, or Casp8-/- Mlkl-/- mice. Therefore, after birth, loss of caspase-8 probably unleashes RIPK1-dependent necroptosis driven by death receptors other than TNFR1.

A systematic review of the cell death mechanisms in retinal pigment epithelium cells and photoreceptors after subretinal hemorrhage - Implications for treatment options

Humans rely on vision as their most important sense. This is accomplished by photoreceptors (PRs) in the retina that detect light but cannot function without the support and maintenance of the retinal pigment epithelium (RPE). In subretinal hemorrhage (SRH), blood accumulates between the neurosensory retina and the RPE or between the RPE and the choroid. Blood breakdown products subsequently damage PRs and the RPE and lead to poor vision and blindness. Hence, there is a high need for options to preserve the retina and visual functions. We conducted a systematic review of the literature in accordance with the PRISMA guidelines to identify the cell death mechanisms in RPE and PRs after SRH to deepen our understanding of the pathways involved. After screening 736 publications published until November 8, 2022, we identified 19 records that assessed cell death in PRs and/or RPE in experimental models of SRH. Among the different cell death mechanisms, apoptosis was the most widely investigated mechanism (11 records), followed by ferroptosis (4), whereas necroptosis, pyroptosis, and lysosome-dependent cell death were only assessed in one study each. We discuss different therapeutic options that were assessed in these studies, including the removal of the hematoma/iron chelation, cytoprotection, anti-inflammatory agents, and antioxidants. Further systematic investigations will be necessary to determine the exact cell death mechanisms after SRH with respect to different blood breakdown components, cell types, and time courses. This will form the basis for the development of novel treatment options for SRH.

TNF-α induced extracellular release of keratinocyte high-mobility group box 1 in Stevens-Johnson syndrome/toxic epidermal necrolysis: Biomarker and putative mechanism of pathogenesis

Decreased epidermal high-mobility group box 1 (HMGB1) expression is an early marker of epidermal injury in Stevens-Johnson syndrome/toxic epidermal necrolysis (SJS/TEN). Etanercept, an anti-tumor necrosis factor therapeutic, is effective in the treatment of SJS/TEN. The objective was to characterize antitumor necrosis factor-alpha (TNF-α)-mediated HMGB1 keratinocyte/epidermal release and etanercept modulation. HMGB1 release from TNF-α treated (± etanercept), or doxycycline-inducible RIPK3 or Bak-expressing human keratinocyte cells (HaCaTs) was determined by western blot/ELISA. Healthy skin explants were treated with TNF-α or serum (1:10 dilution) from immune checkpoint inhibitor-tolerant, lichenoid dermatitis or SJS/TEN patients ± etanercept. Histological and immunohistochemical analysis of HMGB1 was undertaken. TNF-α induced HMGB1 release in vitro via both necroptosis and apoptosis. Exposure of skin explants to TNF-α or SJS/TEN serum resulted in significant epidermal toxicity/detachment with substantial HMGB1 release which was attenuated by etanercept. Whole-slide image analysis of biopsies demonstrated significantly lower epidermal HMGB1 in pre-blistered SJS/TEN versus control (P < 0.05). Keratinocyte HMGB1 release, predominantly caused by necroptosis, can be attenuated by etanercept. Although TNF-α is a key mediator of epidermal HMGB1 release, other cytokines/cytotoxic proteins also contribute. Skin explant models represent a potential model of SJS/TEN that could be utilized for further mechanistic studies and targeted therapy screening.

RIPK3 promoter hypermethylation in hepatocytes protects from bile acid-induced inflammation and necroptosis

Necroptosis facilitates cell death in a controlled manner and is employed by many cell types following injury. It plays a significant role in various liver diseases, albeit the cell-type-specific regulation of necroptosis in the liver and especially hepatocytes, has not yet been conceptualized. We demonstrate that DNA methylation suppresses RIPK3 expression in human hepatocytes and HepG2 cells. In diseases leading to cholestasis, the RIPK3 expression is induced in mice and humans in a cell-type-specific manner. Overexpression of RIPK3 in HepG2 cells leads to RIPK3 activation by phosphorylation and cell death, further modulated by different bile acids. Additionally, bile acids and RIPK3 activation further facilitate JNK phosphorylation, IL-8 expression, and its release. This suggests that hepatocytes suppress RIPK3 expression to protect themselves from necroptosis and cytokine release induced by bile acid and RIPK3. In chronic liver diseases associated with cholestasis, induction of RIPK3 expression may be an early event signaling danger and repair through releasing IL-8.

Beneficial effects of aloperine on inflammation and oxidative stress by suppressing necroptosis in lipopolysaccharide-induced acute lung injury mouse model

RATIONALE

Alveolar epithelial cell death, inflammation, and oxidative stress are typical features of acute lung injury (ALI). Aloperine (Alo), an alkaloid isolated from Sophora alopecuroides, has been reported to display various biological effects, such as anti-inflammatory, immunoregulatory, and anti-oxidant properties. In this study, we investigated the effects and mechanisms of Alo in treating a lipopolysaccharide (LPS)-induced ALI in a murine model.

METHODS

The effects of Alo in LPS-induced ALI were investigated in C57BL/6 mice. The RIPK1 inhibitor (Nec-1) and the RIPK3 inhibitor (GSK'872) were used to evaluate the relationship of necroptosis, NF-κB activation, and PDC subunits in LPS-treated mouse alveolar epithelial cells (MLE-12). Then the effects of Alo on necroptosis, inflammation, and oxidative stress of LPS-stimulated MLE-12 cells were evaluated.

RESULTS

Alo significantly attenuated histopathological lung injuries and reduced lung wet/dry ratio in LPS-induced ALI mice. Alo also remarkedly reduced total protein and neutrophils recruitment in bronchoalveolar lavage fluid of ALI mice. Meanwhile, Alo ameliorated the LPS-induced necroptosis in the lungs of ALI mice. The RIPK3 inhibitor GSK'872, but not the RIPK1 inhibitor Nec-1, reversed LPS-induced p65 phosphorylation and translocation to the nucleus in MLE-12 cells. GSK'872 also reversed the LPS-induced increase in ROS and binding of RIPK3 and PDC subunits in MLE-12 cells. Moreover, Alo down-regulated the levels of p-RIPK1, p-RIPK3, p-MLKL, p-p65, the translocation of p65 to the nucleus, and reduced the expression of IL-6 and IL-8 in LPS-stimulated MLE-12 cells. Alo also inhibited the binding of RIPK3 and PDC-E1α, PDC-E1β, PDC-E2, and PDC-E3 and the ROS production in LPS-treated MLE-12 cells.

CONCLUSION

The present study validated the beneficial effects of Alo on LPS-induced ALI , suggesting Alo may be a new drug candidate against ALI.

The expression of RIPK3 is associated with cell turnover of gastric mucosa in the mouse and human stomach

Necroptosis is a novel manner of programmed cell death and important for tissue development, homeostasis, damage, and repair. Activation of receptor-interacting protein kinase 3 (RIPK3), a key member of receptor-interacting protein family in contributing significantly to necroptosis, in tissues is a hallmark of cells dying by necroptosis. However, there are few studies that examine the expression of RIPK3 in the glandular cells of stomachs under physiological condition. We have therefore conducted this study to immunohistochemically characterize the key element of necroptosis, RIPK3, in the mouse and human stomach. Results showed that RIPK3 positive cells could be observed in the surface mucosal cells, granular cells, and lamina propria cells in both mouse and human stomach tissues. Ratios of PCNA/RIPK3 positive cells in the glandular cells were ~ 2.1 in mouse and ~ 4.15 in human sections respectively. Morphological and double immunofluorescence analysis confirmed that these RIPK3 positive cells were mucous, parietal and lamina propria cells. Our results indicate that the expression of RIPK3 in different cell types might contribute to cell turnover of gastric mucosa in the mouse and human stomach under physiological condition.

Mechanisms of Regulated Cell Death: Current Perspectives

Balancing cell survival and cell death is fundamental to development and homeostasis. Cell death is regulated by multiple interconnected signaling pathways and molecular mechanisms. Regulated cell death (RCD) is implicated in fundamental processes such as organogenesis and tissue remodeling, removal of unnecessary structures or cells, and regulation of cell numbers. RCD can also be triggered by exogenous perturbations of the intracellular or extracellular microenvironment when the adaptive processes that respond to stress fail. During the past few years, many novel forms of non-apoptotic RCD have been identified, and the characterization of RCD mechanisms at a molecular level has deepened our understanding of diseases encountered in human and veterinary medicine. Given the complexity of these processes, it has become clear that the identification of RCD cannot be based simply on morphologic characteristics and that descriptive and diagnostic terms presently used by pathologists-such as individual cell apoptosis or necrosis-appear inadequate and possibly misleading. In this review, the current understanding of the molecular machinery of each type of non-apoptotic RCD mechanisms is outlined. Due to the continuous discovery of new mechanisms or nuances of previously described processes, the limitations of the terms apoptosis and necrosis to indicate microscopic findings are also reported. In addition, the need for a standard panel of biomarkers and functional tests to adequately characterize the underlying RCD and its role as a mechanism of disease is considered.

Validating Immunohistochemistry Assay Specificity in Investigative Studies: Considerations for a Weight of Evidence Approach

Immunohistochemistry (IHC) is a fundamental molecular technique that provides information on protein expression in the context of spatial localization and tissue morphology. IHC is used in all facets of pathology from identifying infectious agents or characterizing tumors in diagnostics, to characterizing cellular and molecular processes in investigative and experimental studies. Confidence in an IHC assay is primarily driven by the degree to which it is validated. There are many approaches to validate an IHC assay's specificity including bioinformatics approaches using published protein sequences, careful design of positive and negative tissue controls, use of cell pellets with known target protein expression, corroboration of IHC findings with western blots and other analytical methods, and replacement of the primary antibody with an appropriate negative control reagent. Each approach has inherent strengths and weaknesses, and the thoughtful use of these approaches provides cumulative evidence, or a weight of evidence, to support the IHC assay's specificity and build confidence in a study's conclusions. Although it is difficult to be 100% confident in the specificity of any IHC assay, it is important to consider how validation approaches provide evidence to support or to question the specificity of labeling, and how that evidence affects the overall interpretation of a study's results. In this review, we discuss different approaches for IHC antibody validation, with an emphasis on the characterization of antibody specificity in investigative studies. While this review is not prescriptive, it is hoped that it will be thought provoking when considering the interpretation of IHC results.

Ubiquitin Ligases cIAP1 and cIAP2 Limit Cell Death to Prevent Inflammation

Cellular inhibitor of apoptosis proteins cIAP1 and cIAP2 ubiquitinate nuclear factor κB (NF-κB)-inducing kinase (NIK) to suppress non-canonical NF-κB signaling and substrates such as receptor interacting protein kinase 1 (RIPK1) to promote cell survival. We investigate how these functions contribute to homeostasis by eliminating cIap2 from adult cIap1-deficient mice. cIAP1 and cIAP2 (cIAP1/2) deficiency causes rapid weight loss and inflammation, with aberrant cell death, indicated by cleaved caspases-3 and -8, prevalent in intestine and liver. Deletion of Casp8 and Ripk3 prevents this aberrant cell death, reduces the inflammation, and prolongs mouse survival, whereas Ripk3 loss alone offers little benefit. Residual inflammation in mice lacking cIap1/2, Casp8, and Ripk3 is reduced by inhibition of NIK. Loss of Casp8 and Mlkl (mixed lineage kinase domain-like), but not Mlkl loss alone, also prevents cIAP1/2-deficient mice from dying around embryonic day 11. Therefore, a major function of cIAP1/2 in vivo is to suppress caspase-8-dependent cell death.

Identification of the hallmarks of necroptosis and ferroptosis by transmission electron microscopy

Apoptosis is the prototype for a regulated form of cell death, but recent studies have revealed other types of regulated forms of cell death, including necroptosis and ferroptosis. The molecular mechanisms underlying the execution of these processes have been intensively investigated, yet the hallmarks of their morphology are not fully understood. Here, we report that electron lucent cytoplasm was a common feature of both necroptosis and ferroptosis, which was consistent with cytoplasmic vacuolization due to a defect in the cytoplasmic membrane integrity. Notably, the perinuclear space was dilated in necroptosis, but such dilation did not occur in ferroptosis. Cells undergoing ferroptosis, but not necroptosis, exhibited an electron lucent nucleus. We previously reported that one of the nuclear danger-associated molecular patterns (DAMPs), high mobility group box (HMGB)1, is rapidly released from the nucleus to the extracellular spaces of cells undergoing necroptosis through the ruptured nuclear and cytoplasmic membrane. Via time-lapse imaging of cells stably expressing HMGB1 fused to a fluorescence protein, we found that HMGB1 was also released from the nucleus to the cytosol, and then eventually released into the extracellular spaces in cells undergoing ferroptosis. Thus, nuclear membrane damage was induced prior to cytoplasmic membrane rupture in ferroptosis. Thus, dilation of the perinuclear space and an electron lucent nucleus may be the hallmarks of necroptosis and ferroptosis, respectively.

Cell death mechanisms in eukaryotes

Like the organism they constitute, the cells also die in different ways. The death can be predetermined, programmed, and cleanly executed, as in the case of apoptosis, or it can be traumatic, inflammatory, and sudden as many types of necrosis exemplify. Nevertheless, there are a number of cell deaths-some of them bearing a resemblance to apoptosis and/or necrosis, and many, distinct from each-that serve a multitude of roles in either supporting or disrupting the homoeostasis. Apoptosis is coordinated by death ligands, caspases, b-cell lymphoma-2 (Bcl-2) family proteins, and their downstream effectors. Events that can lead to apoptosis include mitotic catastrophe and anoikis. Necrosis, although it has been considered an abrupt and uncoordinated cell death, has many molecular events associated with it. There are cell death mechanisms that share some standard features with necrosis. These include methuosis, necroptosis, NETosis, pyronecrosis, and pyroptosis. Autophagy, generally a catabolic pathway that operates to ensure cell survival, can also kill the cell through mechanisms such as autosis. Other cell-death mechanisms include entosis, ferroptosis, lysosome-dependent cell death, and parthanatos.

The impact of endothelial cell death in the brain and its role after stroke: A systematic review

The supply of oxygen and nutrients to the brain is vital for its function and requires a complex vascular network that, when disturbed, results in profound neurological dysfunction. As part of the pathology in stroke, endothelial cells die. As endothelial cell death affects the surrounding cellular environment and is a potential target for the treatment and prevention of neurological disorders, we have systematically reviewed important aspects of endothelial cell death with a particular focus on stroke. After screening 2876 publications published between January 1, 2010 and August 7, 2019, we identified 154 records to be included. We found that endothelial cell death occurs rapidly as well as later after the onset of stroke conditions. Among the different cell death mechanisms, apoptosis was the most widely investigated (92 records), followed by autophagy (20 records), while other, more recently defined mechanisms received less attention, such as lysosome-dependent cell death (2 records) and necroptosis (2 records). We also discuss the differential vulnerability of brain cells to injury after stroke and the role of endothelial cell death in the no-reflow phenomenon with a special focus on the microvasculature. Further investigation of the different cell death mechanisms using novel tools and biomarkers will greatly enhance our understanding of endothelial cell death. For this task, at least two markers/criteria are desirable to determine cell death subroutines according to the recommendations of the Nomenclature Committee on Cell Death.

Necroptosis of Intestinal Epithelial Cells Induces Type 3 Innate Lymphoid Cell-Dependent Lethal Ileitis

A short form of cellular FLICE-inhibitory protein encoded by CFLARs promotes necroptosis. Although necroptosis is involved in various pathological conditions, the detailed mechanisms are not fully understood. Here we generated transgenic mice wherein CFLARs was integrated onto the X chromosome. All male CFLARs Tg mice died perinatally due to severe ileitis. Although necroptosis was observed in various tissues of CFLARs Tg mice, large numbers of intestinal epithelial cells (IECs) died by apoptosis. Deletion of Ripk3 or Mlkl, essential genes of necroptosis, prevented both necroptosis and apoptosis, and rescued lethality of CFLARs Tg mice. Type 3 innate lymphoid cells (ILC3s) were activated and recruited to the small intestine along with upregulation of interleukin-22 (Il22) in CFLARs Tg mice. Deletion of ILC3s or Il22 rescued lethality of CFLARs Tg mice by preventing apoptosis, but not necroptosis of IECs. Together, necroptosis-dependent activation of ILC3s induces lethal ileitis in an IL-22-dependent manner.

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CFLARs Tg mice develop severe ileitis in utero

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Intestinal epithelial cells die by apoptosis and necroptosis in CFLARs Tg mice

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Blockade of necroptosis rescues lethality of CFLARs Tg mice

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Necroptosis activates type 3 innate lymphoid cells, resulting in severe ileitis

The pathological features of regulated necrosis

Necrosis of a cell is defined by the loss of its plasma membrane integrity. Morphologically, necrosis occurs in several forms such as coagulative necrosis, colliquative necrosis, caseating necrosis, fibrinoid necrosis, and others. Biochemically, necrosis was demonstrated to represent a number of genetically determined signalling pathways. These include (i) kinase-mediated necroptosis, which depends on receptor interacting protein kinase 3 (RIPK3)-mediated phosphorylation of the pseudokinase mixed lineage kinase domain like (MLKL); (ii) gasdermin-mediated necrosis downstream of inflammasomes, also referred to as pyroptosis; and (iii) an iron-catalysed mechanism of highly specific lipid peroxidation named ferroptosis. Given the molecular understanding of the nature of these pathways, specific antibodies may allow direct detection of regulated necrosis and correlation with morphological features. Necroptosis can be specifically detected by immunohistochemistry and immunofluorescence employing antibodies to phosphorylated MLKL. Likewise, it is possible to generate cleavage-specific antibodies against epitopes in gasdermin protein family members. In ferroptosis, however, specific detection requires quantification of oxidative lipids by mass spectrometry (oxylipidomics). Together with classical cell death markers, such as TUNEL staining and detection of cleaved caspase-3 in apoptotic cells, the extension of the arsenal of necrosis markers will allow pathological detection of specific molecular pathways rather than isolated morphological descriptions. These novel pieces of information will be extraordinarily helpful for clinicians as inhibitors of necroptosis (necrostatins), ferroptosis (ferrostatins), and inflammasomes have emerged in clinical trials. Anatomical pathologists should embrace these novel ancillary tests and the concepts behind them and test their impact on diagnostic precision, prognostication, and the prediction of response to the upcoming anti-necrotic therapies. Copyright © 2019 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.