A cytosolic heat shock protein 90 and cochaperone CDC37 complex is required for RIP3 activation during necroptosis. Proc Natl Acad Sci U S A. 2015;112:5017-5022. [PMID: 25852146 DOI: 10.1073/pnas.1505244112]

Research trends, hot spots and prospects for necroptosis in the field of neuroscience

There are two types of cell death-apoptosis and necrosis. Apoptosis is cell death regulated by cell signaling pathways, while necrosis has until recently been considered a passive mechanism of cell death caused by environmental pressures. However, recent studies show that necrosis can also be regulated by specific cell signaling pathways. This mode of death, termed necroptosis, has been found to be related to the occurrence and development of many diseases. We used bibliometrics to analyze the global output of literature on necroptosis in the field of neuroscience published in the period 2007–2019 to identify research hotspots and prospects. We included 145 necroptosis-related publications and 2239 references published in the Web of Science during 2007–2019. Visualization analysis revealed that the number of publications related to necroptosis has increased year by year, reaching a peak in 2019. China is the country with the largest number of publications. Key word and literature analyses demonstrated that mitochondrial function change, stroke, ischemia/reperfusion and neuroinflammation are likely the research hotspots and future directions of necroptosis research in the nervous system. The relationship between immune response-related factors, damage-associated molecular patterns, pathogen-associated molecular patterns and necroptosis may become a potential research hotspot in the future. Taken together, our findings suggest that although the inherent limitations of bibliometrics may affect the accuracy of the literature-based prediction of research hotspots, the results obtained from the included publications can provide a reference for the study of necroptosis in the field of neuroscience.

Necroptosis, the Other Main Caspase-Independent Cell Death

The past decades witnessed the discovery of novel modes of cell death, such as ferroptosis, pyroptosis and necroptosis, all of them presenting common necrotic traits. In this chapter, we revisit the early discoveries that unveiled necroptosis as a distinct cell death mechanism. We describe necroptosis, its main regulators and their role in maintaining cellular homeostasis and in the disease state. We conclude by discussing its phenotypic similarities with ferroptosis and the possible crosstalk between these pathways.

Casein kinase 1G2 suppresses necroptosis-promoted testis aging by inhibiting receptor-interacting kinase 3

Casein kinases are a large family of intracellular serine/threonine kinases that control a variety of cellular signaling functions. Here we report that a member of casein kinase 1 family, casein kinase 1G2, CSNK1G2, binds and inhibits the activation of receptor-interacting kinase 3, RIPK3, thereby attenuating RIPK3-mediated necroptosis. The binding of CSNK1G2 to RIPK3 is triggered by auto-phosphorylation at serine 211/threonine 215 sites in its C-terminal domain. CSNK1G2-knockout mice showed significantly enhanced necroptosis response and premature aging of their testis, a phenotype that was rescued by either double knockout of the Ripk3 gene or feeding the animal with a RIPK1 kinase inhibitor-containing diet. Moreover, CSNK1G2 is also co-expressed with RIPK3 in human testis, and the necroptosis activation marker phospho-MLKL was observed in the testis of old (>80) but not young men, indicating that the testis-aging program carried out by the RIPK3-mediated and CSNK1G2-attenuated necroptosis is evolutionarily conserved between mice and men.

The molecular mechanisms of MLKL-dependent and MLKL-independent necrosis

Necrosis, a type of unwanted and passive cell demise, usually occurs under the excessive external stress and is considered to be unregulated. However, under some special conditions such as caspase inhibition, necrosis is regulable in a well-orchestrated way. The term 'regulated necrosis' has been proposed to describe such programed necrosis. Recently, several forms of necrosis, including necroptosis, pyroptosis, ferroptosis, parthanatos, oxytosis, NETosis, and Na+/K+-ATPase-mediated necrosis, have been identified, and some crucial regulators governing regulated necrosis have also been discovered. Mixed lineage kinase domain-like pseudokinase (MLKL), a core regulator in necroptosis, acts as an executioner in response to ligands of death receptor family. Its activation requires the receptor-interacting protein kinases, RIP1 and RIP3. However, MLKL is only involved in necroptosis, i.e. MLKL is dispensable for necrosis. Therefore, this review is aimed at summarizing the molecular mechanisms of MLKL-dependent and MLKL-independent necrosis.

Targeting necroptosis in anticancer therapy: mechanisms and modulators

Necroptosis, a genetically programmed form of necrotic cell death, serves as an important pathway in human diseases. As a critical cell-killing mechanism, necroptosis is associated with cancer progression, metastasis, and immunosurveillance. Targeting necroptosis pathway by small molecule modulators is emerging as an effective approach in cancer therapy, which has the advantage to bypass the apoptosis-resistance and maintain antitumor immunity. Therefore, a better understanding of the mechanism of necroptosis and necroptosis modulators is necessary to develop novel strategies for cancer therapy. This review will summarize recent progress of the mechanisms and detecting methods of necroptosis. In particular, the relationship between necroptosis and cancer therapy and medicinal chemistry of necroptosis modulators will be focused on.

Anticancer Strategy Targeting Cell Death Regulators: Switching the Mechanism of Anticancer Floxuridine-Induced Cell Death from Necrosis to Apoptosis

Cell death can be broadly characterized as either necrosis or apoptosis, depending on the morphological and biochemical features of the cell itself. We have previously reported that the treatment of mouse mammary carcinoma FM3A cells with the anticancer drug floxuridine (FUdR) induces necrosis in the original clone F28-7 but apoptosis in the variant F28-7-A. We have identified regulators, including heat shock protein 90, lamin-B1, cytokeratin-19, and activating transcription factor 3, of cell death mechanisms by using comprehensive gene and protein expression analyses and a phenotype-screening approach. We also observed that the individual inhibition or knockdown of the identified regulators in F28-7 results in a shift from necrotic to apoptotic morphology. Furthermore, we investigated microRNA (miRNA, miR) expression profiles in sister cell strains F28-7 and F28-7-A using miRNA microarray analyses. We found that several unique miRNAs, miR-351-5p and miR-743a-3p, were expressed at higher levels in F28-7-A than in F28-7. Higher expression of these miRNAs in F28-7 induced by transfecting miR mimics resulted in a switch in the mode of cell death from necrosis to apoptosis. Our findings suggest that the identified cell death regulators may play key roles in the decision of cell death mechanism: necrosis or apoptosis.

Inhibiting Heat Shock Protein 90 Protects Nucleus Pulposus-Derived Stem/Progenitor Cells From Compression-Induced Necroptosis and Apoptosis

Nucleus pulposus-derived stem/progenitor cells (NPSCs) provide novel prospects for the regeneration of degenerated intervertebral disc (IVD). Nevertheless, with aging and degeneration of IVD, the frequency of NPSCs markedly decreases. Excessive cell death could be the main reason for declined frequency of NPSCs, however, the exact mechanisms remain elusive. Thus, the present study was undertaken to explore the mechanisms of compression-induced NPSCs death, and the effects of heat shock protein 90 (HSP90) on NPSCs survival. Here, we found that compression could trigger receptor-interacting protein kinase 1 (RIPK1)/receptor-interacting protein kinase 3 (RIPK3)/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis of NPSCs. Furthermore, we found that elevated expression of HSP90 was involved in compression-induced NPSCs death, and inhibiting HSP90 could dramatically attenuate compression-induced necroptosis of NPSCs via regulating the expression and activity of RIPK1/RIPK3/MLKL, and alleviating the mitochondrial dysfunction (mitochondrial membrane potential loss and ATP depletion) and oxidative stress [production of mitochondrial reactive oxygen species (ROS), cellular total ROS and malondialdehyde, and downregulation of superoxide dismutase 2]. Besides necroptosis, compression-induced apoptosis of NPSCs was also attenuated by HSP90 inhibition. In addition, we found that enhanced expression of HSP70 contributed to the cytoprotective effects of inhibiting HSP90. More encouragingly, our results demonstrated that inhibiting HSP90 could also mitigate the exhaustion of NPSCs in vivo. In conclusion, RIPK1/RIPK3/MLKL-mediated necroptosis participates in compression-induced NPSCs death. Furthermore, targeting HSP90 to simultaneously inhibit necroptosis and apoptosis of NPSCs might be an efficient strategy for preventing the death of NPSCs, thus rescuing the endogenous repair capacity of NP tissue.

RSK3 mediates necroptosis by regulating phosphorylation of RIP3 in rat retinal ganglion cells

Receptor-interacting protein 3 (RIP3) plays an important role in the necroptosis signaling pathway. Our previous studies have shown that the RIP3/mixed lineage kinase domain-like protein (MLKL)-mediated necroptosis occurs in retinal ganglion cell line 5 (RGC-5) following oxygen-glucose deprivation (OGD). However, upstream regulatory pathways of RIP3 are yet to be uncovered. The purpose of the present study was to investigate the role of p90 ribosomal protein S6 kinase 3 (RSK3) in the phosphorylation of RIP3 in RGC-5 cell necroptosis following OGD. Our results showed that expression of RSK3, RIP3, and MLKL was upregulated in necroptosis of RGC-5 after OGD. A computer simulation based on our preliminary results indicated that RSK3 might interact with RIP3, which was subsequently confirmed by co-immunoprecipitation. Further, we found that the application of a specific RSK inhibitor, LJH685, or rsk3 small interfering RNA (siRNA), downregulated the phosphorylation of RIP3. However, the overexpression of rip3 did not affect the expression of RSK3, thereby indicating that RSK3 could be a possible upstream regulator of RIP3 phosphorylation in OGD-induced necroptosis of RGC-5 cells. Moreover, our in vivo results showed that pretreatment with LJH685 before acute high intraocular pressure episodes could reduce the necroptosis of retinal neurons and improve recovery of impaired visual function. Taken together, our findings suggested that RSK3 might work as an upstream regulator of RIP3 phosphorylation during RGC-5 necroptosis.

Regulatory mechanisms of RIPK1 in cell death and inflammation

Receptor Interacting Protein Kinase 1 (RIPK1) and RIPK3 are key adaptors that play critical roles in inflammatory and cell death signaling. Work in recent years have shown that their activities are tightly regulated by ubiquitination, phosphorylation and proteolysis. In addition to these post-translational modifications, the expression and activities of these kinases can further be tuned by environmental changes in pH and oxygen content. Proper control of these regulatory processes is crucial for the RIP kinases to execute their functions in immune responses and tissue homeostasis. In this review, we discuss recent advance in our understanding of the molecular mechanisms that regulate the activities of the RIP kinases. We will also discuss how the different regulatory mechanisms contribute to the functions of RIPK1 and RIPK3 in different pathophysiological settings.

MLKL trafficking and accumulation at the plasma membrane control the kinetics and threshold for necroptosis

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.

Mixed lineage kinase domain-like (MLKL) is the terminal protein in the pro-inflammatory necroptotic cell death program. Here the authors show that MLKL trafficking and plasma membrane accumulation are crucial necroptosis checkpoints, and that accumulation of phosphorylated MLKL at intercellular junctions promotes necroptosis.

Cross electro-nape-acupuncture ameliorates cerebral hemorrhage-induced brain damage by inhibiting necroptosis

BACKGROUND

Receptor interacting protein kinase 1 (RIPK1)-mediated cell death, including apoptosis and necroptosis, belongs to programmed cell death. It has been reported that RIPK1-mediated necroptosis exists in lesions of cerebral hemorrhage (CH). Electroacupuncture, a treatment derived from traditional Chinese medicine, could improve neurological impairment in patients with brain injury.

AIM

To investigate the protective role of cross electro-nape acupuncture (CENA) in CH, and clarify the potential mechanism.

METHODS

CH rat models were established, and CENA was applied to the experimental rats. Neurological functions and encephaledema were then measured. Necrotic cells in the brain of rats with CH were evaluated by propidium iodide staining. Necroptosis was assessed by immunofluorescence. Activation of the necroptosis-related pathway was detected by western blot. Extraction of brain tissue, cerebrospinal fluid and serum samples was conducted to measure the expression and secretion of inflammatory cytokines by quantitative real-time polymerase chain reaction and enzyme-linked immunosorbent assay.

RESULTS

The necroptotic marker p-MLKL was detectable in the brains of rats with CH. Next, we found that CENA could ameliorate neurological functions in rat models of CH. Moreover, the upregulation of RIPK1-mediated necroptosis-related molecules in the brains of rats with CH were inhibited by CENA. Further investigation revealed that CENA partially blocked the interaction between RIPK1 and RIPK3. Finally, in vivo assays showed that CENA decreased the expression of the inflammatory cytokines tumor necrosis factor-α, interleukin-6 and interleukin-8 in CH rat models.

CONCLUSION

These findings revealed that CENA exerts a protective role in CH models by inhibiting RIPK1-mediated necroptosis.

Dissecting Programmed Cell Death with Small Molecules

Programmed cell death (PCD) is fundamentally an indispensable process in all cellular activities, including cell development, wound healing, and immune surveillance of tumors (Galluzzi, L. et al. Cell Death Differ. 2018, 25, 486-541). Malfunctioning of PCD has been shown to be closely related to human diseases such as acute pancreatitis, neurodegenerative diseases, and diverse types of cancers. To date, multiple PCD processes have been discovered and the corresponding regulatory pathways have been elucidated. For example, apoptosis and autophagy are two PCD mechanisms that have been well studied by sophisticated models and probe toolkits. However, limited genetic and chemical tools for other types of PCD hamper the elucidation of their molecular mechanisms. Our group has been studying PCD using both function-oriented synthesis and chemical biology strategies, including the development of diverse chemical probes based on novel PCD modulators. For instance, in the development of downstream programmed necrosis (or necroptosis) inhibitor necrosulfonamide, we used a chemical probe to unveil a functional protein that was not previously implicated in necroptosis, mixed lineage kinase domain-like protein (MLKL). In addition, high throughput screening and medicinal chemistry enabled the discovery of bioymifi, a small molecule agonist which selectively causes oligomerization of the death receptor 5 (DR5), to induce extrinsic apoptosis. Furthermore, we developed a biomimetic synthetic strategy based on diverse Diels-Alder reactions in the total syntheses of ainsliadimers A and B, ainsliatrimers A and B, and gonchnatiolides A-C, which are natural product inhibitors or activators for PCD. Using synthetic ainsliadimer A probe, we elucidated that ainsliadimer A inhibits the NF-κB pathway by covalently binding to Cys46 of IKKβ and triggers apoptosis of cancer cells. We have also revealed that IKKβ is allosterically inhibited by ainsliadimer A. In addition to total synthesis, we have developed a bioorthogonal click hetero-Diels-Alder cycloaddition of vinyl thioether and o-quinolinone quinone methide (TQ-ligation) to facilitate small molecule target identification. The combination of total synthesis and TQ-ligation enables subcellular imaging and identification of the cellular target of ainsliatrimer A to be PPARγ. In addition, TQ-ligation has been applied in the discovery of heat shock protein 90 (HSP90) as one of the functional target proteins for kongensin A. We also confirmed that kongensin A covalently attaches to Cys420 within HSP90 and demonstrated that kongensin A blocks the interaction between HSP90 and CDC37 and subsequently inhibits necroptosis. Our development of these diverse PCD modulators provides not only effective chemical tools for fundamental biomedical research, but also the foundation for drug discovery targeting important human diseases such as cancers and inflammation caused by malfunction of PCD.

CK1δ as a potential therapeutic target to treat bladder cancer

Bladder cancer is the second most common genitourinary malignancy in the world. However, only immune-checkpoint inhibitors and erdafitinib are available to treat advanced bladder cancer. Our previous study reported that 4-((4-(4-ethylpiperazin-1-yl) phenyl)amino)-N-(3,4,5-trichlorophenyl)-7H-pyrrolo-[2, 3-d]pyrimidine-7-carboxamide hydrochloride (13i HCl) is a potent CK1δ inhibitor showing significant anti-bladder cancer activity. In this study, we elucidated the pharmacological mechanisms underlying 13i HCl’s inhibition of human bladder cancer. Our results demonstrate that expression of the CSNK1D gene, which codes for CK1δ, is upregulated in superficial and infiltrating bladder cancer patients in two independent datasets. CK1δ knockdown decreased β-catenin expression in bladder cancer cells and inhibited their growth. Additionally, 13i HCl suppressed bladder cancer cell proliferation and increased apoptosis. We also observed that inhibition of CK1δ using 13i HCl or PF-670462 triggers necroptosis in bladder cancer cells. Finally, 13i HCl inhibited bladder cancer cell migration and reversed their mesenchymal characteristics. These findings suggest further development of 13i HCl as a potential therapeutic agent to treat bladder cancer is warranted.

Necroptosis-blocking compound NBC1 targets heat shock protein 70 to inhibit MLKL polymerization and necroptosis

Necroptosis is a regulated necrotic cell death pathway involved in development and disease. Its signaling cascade results in the formation of disulfide bond-dependent amyloid-like polymers of mixed lineage kinase domain-like protein (MLKL), which mediate proinflammatory cell membrane disruption. We screened compound libraries provided by the National Cancer Institute and identified a small-molecule inhibitor of necroptosis named necroptosis-blocking compound 1 (NBC1). Biotin-labeled NBC1 specifically conjugates to heat shock protein Hsp70. NBC1 and PES-Cl, a known Hsp70 substrate-binding inhibitor, block the formation of MLKL polymers, but not MLKL tetramers in necroptosis-induced cells. In vitro, recombinant Hsp70 interacts with the N-terminal domain (NTD) of MLKL and promotes NTD polymerization, which has been shown to mediate the cell killing activity. Furthermore, the substrate-binding domain (SBD) of Hsp70 is sufficient to promote MLKL polymerization. NBC1 covalently conjugates cysteine 574 and cysteine 603 of the SBD to block its function. In addition, an SBD mutant with both cysteines mutated to serines loses its ability to promote MLKL polymerization. Interestingly, knockdown of Hsp70 in cells leads to MLKL destabilization, suggesting that MLKL might also be a client protein of Hsp70. In summary, using NBC1, an inhibitor of necroptosis, we identified Hsp70 as a molecular chaperone performing dual functions in necroptosis. It stabilizes MLKL protein under normal condition and promotes MLKL polymerization through its substrate-binding domain during necroptosis.

Necroptosis, ADAM proteases and intestinal (dys)function

Recently, an unexpected connection between necroptosis and members of the a disintegrin and metalloproteinase (ADAM) protease family has been reported. Necroptosis represents an important cell death routine which helps to protect from viral, bacterial, fungal and parasitic infections, maintains adult T cell homeostasis and contributes to the elimination of potentially defective organisms before parturition. Equally important for organismal homeostasis, ADAM proteases control cellular processes such as development and differentiation, immune responses or tissue regeneration. Notably, necroptosis as well as ADAM proteases have been implicated in the control of inflammatory responses in the intestine. In this review, we therefore provide an overview of the physiology and pathophysiology of necroptosis, ADAM proteases and intestinal (dys)function, discuss the contribution of necroptosis and ADAMs to intestinal (dys)function, and review the current knowledge on the role of ADAMs in necroptotic signaling.

A cytosolic heat shock protein 90 and co-chaperone p23 complex activates RIPK3/MLKL during necroptosis of endothelial cells in acute respiratory distress syndrome

Necrosis with inflammation plays a crucial role in acute respiratory distress syndrome (ARDS). Receptor-interacting protein 3 (RIPK3) regulates a newly discovered programmed form of necrosis called necroptosis. However, the underlying mechanism of necroptosis in ARDS remains unknown. Thus, the purpose of this study was to examine the possible involvement of RIPK3 in ARDS-associated necroptosis. RIPK3 protein levels were found to be significantly elevated in the plasma and bronchoalveolar lavage fluid of ARDS patients. Next, we utilised a mouse model of severe ARDS induced with high-dose lipopolysaccharide and found that lung injury was mainly due to RIPK3-mixed lineage kinase domain-like pseudokinase (MLKL)-mediated necroptosis and endothelial dysfunction. The activation of RIPK3-MLKL by tumour necrosis factor receptor 1 (TNFR1) and TNFR1-associated death domain protein (TRADD) required catalytically active RIPK1 and the inhibition of Fas-associated protein with death domain (FADD)/caspase-8 catalytic activity. We further showed that the molecular chaperone heat shock protein 90 (Hsp90)/p23, as a novel RIPK3- and MLKL-interacting complex, played an important role in RIP-MLKL-mediated necroptosis, inflammation and endothelial dysfunction in the pulmonary vasculature, which resulted in ARDS. Collectively, the results of our study indicate that necroptosis is an important mechanism of cell death in ARDS and the inhibition of necroptosis may be a therapeutic intervention for ARDS. KEY MESSAGES: Lung injury in high-dose LPS-induced severe ARDS is mainly due to RIP3-MLKL-mediated necroptosis and endothelial dysfunction. Chaperone HSP90/p23 is a novel RIP3- and MLKL-interacting complex in HPAECs. HSP90/p23 is a novel RIP3- and MLKL-interacting complex in RIP-MLKL-mediated necroptosis, inflammation and endothelial dysfunction.

Molecular mechanisms of necroptosis and relevance for neurodegenerative diseases

Necroptosis is a regulated cell death pathway morphologically similar to necrosis that depends on the kinase activity of receptor interacting protein 3 (RIP3) and the subsequent activation of the pseudokinase mixed lineage kinase domain-like protein (MLKL), being also generally dependent on RIP1 kinase activity. Necroptosis can be recruited during pathological conditions, usually following the activation of death receptors under specific cellular contexts. In this regard, necroptosis has been implicated in the pathogenesis of multiple disorders, including acute and chronic neurodegenerative diseases, such as Parkinson's and Alzheimer's diseases, and multiple sclerosis. Here, we summarize the molecular mechanisms regulating the induction of necroptosis and downstream effectors of this form of cell death, besides exploring non-necroptotic roles for necroptosis-related proteins that may impact on alternative cell death pathways and inflammatory mechanisms in disease. Finally, we outline the recent evidence implicating necroptosis in neurodegenerative conditions and the emerging therapeutic perspectives targeting necroptosis in these diseases.

CK1α, CK1δ, and CK1ε are necrosome components which phosphorylate serine 227 of human RIPK3 to activate necroptosis

Necroptosis is a regulated necrotic cell death pathway, mediated by a supermolecular complex called the necrosome, which contains receptor-interacting protein kinase 1 and 3 (RIPK1, RIPK3) and mixed-lineage kinase domain-like protein (MLKL). Phosphorylation of human RIPK3 at serine 227 (S227) has been shown to be required for downstream MLKL binding and necroptosis progression. Tandem immunoprecipitation of RIPK3 reveals that casein kinase 1 (CK1) family proteins associate with the necrosome upon necroptosis induction, and this interaction depends on the kinase activity of RIPK3. In addition, CK1 proteins colocalize with RIPK3 puncta during necroptosis. Importantly, CK1 proteins directly phosphorylate RIPK3 at S227 in vitro and in vivo. Loss of CK1 proteins abolishes S227 phosphorylation and blocks necroptosis. Furthermore, a RIPK3 mutant with mutations in the CK1 recognition motif fails to be phosphorylated at S227, does not bind or phosphorylate MLKL, and is unable to activate necroptosis. These results strongly suggest that CK1 proteins are necrosome components which are responsible for RIPK3-S227 phosphorylation.

HSP70 promotes MLKL polymerization and necroptosis

Mixed lineage kinase domain-like protein (MLKL) is the proposed executioner of necroptosis. Our recent findings identify a novel inhibitor necroptosis-blocking compound 1 (NBC1) which specifically conjugates to two cysteines of heat shock protein 70 (HSP70) to block its function. Importantly, HSP70 promotes MLKL polymerization to activate necroptosis.

Therapeutic Potential of the Hsp90/Cdc37 Interaction in Neurodegenerative Diseases

Alzheimer's, Huntington's, and Parkinson's are devastating neurodegenerative diseases that are prevalent in the aging population. Patient care costs continue to rise each year, because there is currently no cure or disease modifying treatments for these diseases. Numerous efforts have been made to understand the molecular interactions governing the disease development. These efforts have revealed that the phosphorylation of proteins by kinases may play a critical role in the aggregation of disease-associated proteins, which is thought to contribute to neurodegeneration. Interestingly, a molecular chaperone complex consisting of the 90 kDa heat shock protein (Hsp90) and Cell Division Cycle 37 (Cdc37) has been shown to regulate the maturation of many of these kinases as well as regulate some disease-associated proteins directly. Thus, the Hsp90/Cdc37 complex may represent a potential drug target for regulating proteins linked to neurodegenerative diseases, through both direct and indirect interactions. Herein, we discuss the broad understanding of many Hsp90/Cdc37 pathways and how this protein complex may be a useful target to regulate the progression of neurodegenerative disease.

Current translational potential and underlying molecular mechanisms of necroptosis

Cell death has a fundamental impact on the evolution of degenerative disorders, autoimmune processes, inflammatory diseases, tumor formation and immune surveillance. Over the past couple of decades extensive studies have uncovered novel cell death pathways, which are independent of apoptosis. Among these is necroptosis, a tightly regulated, inflammatory form of cell death. Necroptosis contribute to the pathogenesis of many diseases and in this review, we will focus exclusively on necroptosis in humans. Necroptosis is considered a backup mechanism of apoptosis, but the in vivo appearance of necroptosis indicates that both caspase-mediated and caspase-independent mechanisms control necroptosis. Necroptosis is regulated on multiple levels, from the transcription, to the stability and posttranslational modifications of the necrosome components, to the availability of molecular interaction partners and the localization of receptor-interacting serine/threonine-protein kinase 1 (RIPK1), receptor-interacting serine/threonine-protein kinase 3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). Accordingly, we classified the role of more than seventy molecules in necroptotic signaling based on consistent in vitro or in vivo evidence to understand the molecular background of necroptosis and to find opportunities where regulating the intensity and the modality of cell death could be exploited in clinical interventions. Necroptosis specific inhibitors are under development, but >20 drugs, already used in the treatment of various diseases, have the potential to regulate necroptosis. By listing necroptosis-modulated human diseases and cataloging the currently available drug-repertoire to modify necroptosis intensity, we hope to kick-start approaches with immediate translational potential. We also indicate where necroptosis regulating capacity should be considered in the current applications of these drugs.

Fundamental Mechanisms of Regulated Cell Death and Implications for Heart Disease

Twelve regulated cell death programs have been described. We review in detail the basic biology of nine including death receptor-mediated apoptosis, death receptor-mediated necrosis (necroptosis), mitochondrial-mediated apoptosis, mitochondrial-mediated necrosis, autophagy-dependent cell death, ferroptosis, pyroptosis, parthanatos, and immunogenic cell death. This is followed by a dissection of the roles of these cell death programs in the major cardiac syndromes: myocardial infarction and heart failure. The most important conclusion relevant to heart disease is that regulated forms of cardiomyocyte death play important roles in both myocardial infarction with reperfusion (ischemia/reperfusion) and heart failure. While a role for apoptosis in ischemia/reperfusion cannot be excluded, regulated forms of necrosis, through both death receptor and mitochondrial pathways, are critical. Ferroptosis and parthanatos are also likely important in ischemia/reperfusion, although it is unclear if these entities are functioning as independent death programs or as amplification mechanisms for necrotic cell death. Pyroptosis may also contribute to ischemia/reperfusion injury, but potentially through effects in non-cardiomyocytes. Cardiomyocyte loss through apoptosis and necrosis is also an important component in the pathogenesis of heart failure and is mediated by both death receptor and mitochondrial signaling. Roles for immunogenic cell death in cardiac disease remain to be defined but merit study in this era of immune checkpoint cancer therapy. Biology-based approaches to inhibit cell death in the various cardiac syndromes are also discussed.

cdc37 is essential for JNK pathway activation and wound closure in Drosophila

Wound closure in the Drosophila larval epidermis mainly involves nonproliferative, endocyling epithelial cells. Consequently, it is largely mediated by cell growth and migration. We discovered that both cell growth and migration in Drosophila require the cochaperone-encoding gene cdc37. Larvae lacking cdc37 in the epidermis failed to close wounds, and the cells of the epidermis failed to change cell shape and polarize. Likewise, wound-induced cell growth was significantly reduced, and correlated with a reduction in the size of the cell nucleus. The c-Jun N-terminal kinase (JNK) pathway, which is essential for wound closure, was not typically activated in injured cdc37 knockdown larvae. In addition, JNK, Hep, Mkk4, and Tak1 protein levels were reduced, consistent with previous reports showing that Cdc37 is important for the stability of various client kinases. Protein levels of the integrin β subunit and its wound-induced protein expression were also reduced, reflecting the disruption of JNK activation, which is crucial for expression of integrin β during wound closure. These results are consistent with a role of Cdc37 in maintaining the stability of the JNK pathway kinases, thus mediating cell growth and migration during Drosophila wound healing.

RIP1/RIP3-regulated necroptosis as a target for multifaceted disease therapy (Review)

Necroptosis is a type of programmed cell death with necrotic morphology, occurring in a variety of biological processes, including inflammation, immune response, embryonic development and metabolic abnormalities. The current nomenclature defines necroptosis as cell death mediated by signal transduction from receptor‑interacting serine/threonine kinase (RIP) 1 to RIP3 (hereafter called RIP1/RIP3). However, RIP3‑dependent cell death would be a more precise definition of necroptosis. RIP3 is indispensable for necroptosis, while RIP1 is not consistently involved in the signal transduction. Notably, deletion of RIP1 even promotes RIP3‑mediated necroptosis under certain conditions. Necroptosis was previously thought as an alternate process of cell death in case of apoptosis inhibition. Currently, necroptosis is recognized to serve a pivotal role in regulating various physiological processes. Of note, it mediates a variety of human diseases, such as ischemic brain injury, immune system disorders and cancer. Targeting and inhibiting necroptosis, therefore, has the potential to be used for therapeutic purposes. To date, research has elucidated the suppression of RIP1/RIP3 via effective inhibitors and highlighted their potential application in disease therapy. The present review focused on the molecular mechanisms of RIP1/RIP3‑mediated necroptosis, explored the functions of RIP1/RIP3 in necroptosis, discussed their potential as a novel therapeutic target for disease therapy, and provided valuable suggestions for further study in this field.

Ferroptosis is a type of autophagy-dependent cell death

Macroautophagy (hereafter referred to as autophagy) involves an intracellular degradation and recycling system that, in a context-dependent manner, can either promote cell survival or accelerate cellular demise. Ferroptosis was originally defined in 2012 as an iron-dependent form of cancer cell death different from apoptosis, necrosis, and autophagy. However, this latter assumption came into question because, in response to ferroptosis activators (e.g., erastin and RSL3), autophagosomes accumulate, and because components of the autophagy machinery (e.g., ATG3, ATG5, ATG4B, ATG7, ATG13, and BECN1) contribute to ferroptotic cell death. In particular, NCOA4-facilitated ferritinophagy, RAB7A-dependent lipophagy, BECN1-mediated system x_c^- inhibition, STAT3-induced lysosomal membrane permeabilization, and HSP90-associated chaperone-mediated autophagy can promote ferroptosis. In this review, we summarize current knowledge on the signaling pathways involved in ferroptosis, while focusing on the regulation of autophagy-dependent ferroptotic cell death. The molecular comprehension of these phenomena may lead to the development of novel anticancer therapies.

Chaperone-mediated autophagy is involved in the execution of ferroptosis

Necroptosis and ferroptosis are two distinct necrotic cell death modalities with no known common molecular mechanisms. Necroptosis is activated by ligands of death receptors such as tumor necrosis factor-α (TNF-α) under caspase-deficient conditions, whereas ferroptosis is mediated by the accumulation of lipid peroxides upon the depletion/or inhibition of glutathione peroxidase 4 (GPX4). The molecular mechanism that mediates the execution of ferroptosis remains unclear. In this study, we identified 2-amino-5-chloro-N,3-dimethylbenzamide (CDDO), a compound known to inhibit heat shock protein 90 (HSP90), as an inhibitor of necroptosis that could also inhibit ferroptosis. We found that HSP90 defined a common regulatory nodal between necroptosis and ferroptosis. We showed that inhibition of HSP90 by CDDO blocked necroptosis by inhibiting the activation of RIPK1 kinase. Furthermore, we showed that the activation of ferroptosis by erastin increased the levels of lysosome-associated membrane protein 2a to promote chaperone-mediated autophagy (CMA), which, in turn, promoted the degradation of GPX4. Importantly, inhibition of CMA stabilized GPX4 and reduced ferroptosis. Our results suggest that activation of CMA is involved in the execution of ferroptosis.

The bromodomain protein BRD4 positively regulates necroptosis via modulating MLKL expression

Necroptosis is a programmed form of necrotic cell death, which is tightly regulated by the necroptotic signaling pathway containing receptor-interacting protein (RIP)1, RIP3, and mixed-lineage kinase domain-like (MLKL) protein. In addition to the RIP1-RIP3-MLKL axis, other factors regulating necroptosis are still largely unknown. Here a cell-based small-molecule screening led to the finding that BET inhibitors protected cells from necroptosis in the TNFα/Smac-mimetic/Z-VAD-FMK (TSZ)-induced cell necroptosis model. Mechanistic studies revealed that BET inhibitors acted by downregulating MLKL expression. Further research demonstrated that BRD4, IRF1, P-TEFb, and RNA polymerase II formed a transcription complex to regulate the expression of MLKL, and BET inhibitors interfered with the transcription complex formation. In necroptosis-related disease model, the BET inhibitor JQ-1 showed promising therapeutic effects. Collectively, our studies establish, for the first time, BRD4 as a new epigenetic factor regulating necroptosis, and highlight the potential of BET inhibitors in the treatment of necroptosis-related diseases.

The Structural Basis of Necroptotic Cell Death Signaling

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.

Cell Death in the Lung: The Apoptosis-Necroptosis Axis

Regulated cell death is a major mechanism to eliminate damaged, infected, or superfluous cells. Previously, apoptosis was thought to be the only regulated cell death mechanism; however, new modalities of caspase-independent regulated cell death have been identified, including necroptosis, pyroptosis, and autophagic cell death. As an understanding of the cellular mechanisms that mediate regulated cell death continues to grow, there is increasing evidence that these pathways are implicated in the pathogenesis of many pulmonary disorders. This review summarizes our understanding of regulated cell death as it pertains to the pathogenesis of chronic obstructive pulmonary disease, asthma, idiopathic pulmonary fibrosis, acute respiratory distress syndrome, and pulmonary arterial hypertension.

c-Jun N-terminal kinases differentially regulate TNF- and TLRs-mediated necroptosis through their kinase-dependent and -independent activities

Tumor necrosis factor (TNF) and Toll-like receptor (TLR)3/TLR4 activation trigger necroptotic cell death through downstream signaling complex containing receptor-interacting protein kinase 1 (RIPK1), RIPK3, and pseudokinase mixed lineage kinase-domain-like (MLKL). However, the regulation of necroptotic signaling pathway is far less investigated. Here we showed that c-Jun N-terminal kinases (JNK1 and JNK2) displayed kinase-dependent and -independent functions in regulating TNF- and TLRs-mediated necroptosis. We found that RIPK1 and RIPK3 promoted cell-death-independent JNK activation in macrophages, which contributed to pro-inflammatory cytokines production. Meanwhile, blocking the kinase activity of JNK dramatically reduced TNF and TLRs-induced necroptotic cell death. Consistently, inhibition of JNK activity protected mice from TNF-induced death and Staphylococcus aureus-mediated lung damage. However, depletion of JNK protein using siRNA sensitized macrophages to necroptosis that was triggered by LPS or poly I:C but still inhibited TNF-induced necroptosis. Mechanistic studies revealed that RIPK1 recruited JNK to the necrosome complex and their kinase activity was required for necrosome formation and the phosphorylation of MLKL in TNF- and TLRs-induced necroptosis. Loss of JNK protein consistently suppressed the phosphorylation of MLKL and necrosome formation in TNF-triggered necroptosis, but differentially promoted the phosphorylation of MLKL and necrosome formation in poly I:C-triggered necroptosis by promoting the oligomeration of TRIF. In conclusion, our findings define a differential role for JNK in regulating TNF- and TLRs-mediated necroptosis by their kinase or scaffolding activities.

Origin and Consequences of Necroinflammation

When cells undergo necrotic cell death in either physiological or pathophysiological settings in vivo, they release highly immunogenic intracellular molecules and organelles into the interstitium and thereby represent the strongest known trigger of the immune system. With our increasing understanding of necrosis as a regulated and genetically determined process (RN, regulated necrosis), necrosis and necroinflammation can be pharmacologically prevented. This review discusses our current knowledge about signaling pathways of necrotic cell death as the origin of necroinflammation. Multiple pathways of RN such as necroptosis, ferroptosis, and pyroptosis have been evolutionary conserved most likely because of their differences in immunogenicity. As the consequence of necrosis, however, all necrotic cells release damage associated molecular patterns (DAMPs) that have been extensively investigated over the last two decades. Analysis of necroinflammation allows characterizing specific signatures for each particular pathway of cell death. While all RN-pathways share the release of DAMPs in general, most of them actively regulate the immune system by the additional expression and/or maturation of either pro- or anti-inflammatory cytokines/chemokines. In addition, DAMPs have been demonstrated to modulate the process of regeneration. For the purpose of better understanding of necroinflammation, we introduce a novel classification of DAMPs in this review to help detect the relative contribution of each RN-pathway to certain physiological and pathophysiological conditions.

Necrosis and necroptosis in germ cell depletion from mammalian ovary

The maximum number of germ cells is present during the fetal life in mammals. Follicular atresia results in rapid depletion of germ cells from the cohort of the ovary. At the time of puberty, only a few hundred (<1%) germ cells are either culminated into oocytes or further get eliminated during the reproductive life. Although apoptosis plays a major role, necrosis as well as necroptosis, might also be involved in germ cell elimination from the mammalian ovary. Both necrosis and necroptosis show similar morphological features and are characterized by an increase in cell volume, cell membrane permeabilization, and rupture that lead to cellular demise. Necroptosis is initiated by tumor necrosis factor and operated through receptor interacting protein kinase as well as mixed lineage kinase domain-like protein. The acetylcholinesterase, cytokines, starvation, and oxidative stress play important roles in necroptosis-mediated granulosa cell death. The granulosa cell necroptosis directly or indirectly induces susceptibility toward necroptotic or apoptotic cell death in oocytes. Indeed, prevention of necrosis and necroptosis pathways using their specific inhibitors could enhance growth/differentiation factor-9 expression, improve survivability as well as the meiotic competency of oocytes, and prevent decline of reproductive potential in several mammalian species and early onset of menopause in women. This study updates the information and focuses on the possible involvement of necrosis and necroptosis in germ cell depletion from the mammalian ovary.

Progress in studies of necroptosis and its relationship to disease processes

This review briefly introduces the mechanism and detection methods of necroptosis in recent years. The most significant points of this review focus on the involvement of necroptotic proteins in disease progression. The following aspects are summarized: 1) RIPs, MLKL, and the upstream and downstream molecules that mediate necroptosis; 2) The development of detection methods for necroptosis; 3) The involvement of related necroptotic proteins in diverse diseases etiology; and 4) The application of necroptotic proteins in disease diagnosis.

SARS-Coronavirus Open Reading Frame-3a drives multimodal necrotic cell death

The molecular mechanisms underlying the severe lung pathology that occurs during SARS-CoV infections remain incompletely understood. The largest of the SARS-CoV accessory protein open reading frames (SARS 3a) oligomerizes, dynamically inserting into late endosomal, lysosomal, and trans-Golgi-network membranes. While previously implicated in a non-inflammatory apoptotic cell death pathway, here we extend the range of SARS 3a pathophysiologic targets by examining its effects on necrotic cell death pathways. We show that SARS 3a interacts with Receptor Interacting Protein 3 (Rip3), which augments the oligomerization of SARS 3a helping drive necrotic cell death. In addition, by inserting into lysosomal membranes SARS 3a triggers lysosomal damage and dysfunction. Consequently, Transcription Factor EB (TFEB) translocates to the nucleus increasing the transcription of autophagy- and lysosome-related genes. Finally, SARS 3a activates caspase-1 either directly or via an enhanced potassium efflux, which triggers NLRP3 inflammasome assembly. In summary, Rip3-mediated oligomerization of SARS 3a causes necrotic cell death, lysosomal damage, and caspase-1 activation-all likely contributing to the clinical manifestations of SARS-CoV infection.

SARS-Coronavirus Open Reading Frame-3a drives multimodal necrotic cell death

The molecular mechanisms underlying the severe lung pathology that occurs during SARS-CoV infections remain incompletely understood. The largest of the SARS-CoV accessory protein open reading frames (SARS 3a) oligomerizes, dynamically inserting into late endosomal, lysosomal, and trans-Golgi-network membranes. While previously implicated in a non-inflammatory apoptotic cell death pathway, here we extend the range of SARS 3a pathophysiologic targets by examining its effects on necrotic cell death pathways. We show that SARS 3a interacts with Receptor Interacting Protein 3 (Rip3), which augments the oligomerization of SARS 3a helping drive necrotic cell death. In addition, by inserting into lysosomal membranes SARS 3a triggers lysosomal damage and dysfunction. Consequently, Transcription Factor EB (TFEB) translocates to the nucleus increasing the transcription of autophagy- and lysosome-related genes. Finally, SARS 3a activates caspase-1 either directly or via an enhanced potassium efflux, which triggers NLRP3 inflammasome assembly. In summary, Rip3-mediated oligomerization of SARS 3a causes necrotic cell death, lysosomal damage, and caspase-1 activation—all likely contributing to the clinical manifestations of SARS-CoV infection.

Simvastatin functions as a heat shock protein 90 inhibitor against triple-negative breast cancer

Acetylation plays an important role in regulating the chaperone activity of heat shock protein 90 (Hsp90) during malignant transformation through the stabilization and conformational maturation of oncogenic proteins. However, the functional acetylation sites, potential anticancer drug targets, are still emerging. We found that acetylation at K292 in Hsp90α is critical for the development and treatment of breast cancer. Acetylation at K292 not only augments the affinity of Hsp90 to ATP, cochaperones, and client proteins but it also promotes cancer cell colony formation, migration, and invasion in vitro as well as tumor growth in vivo. Importantly, K292‐acetylated Hsp90 has been validated as an exciting anticancer drug target by interfering with the complex formation between K292‐acetylated Hsp90 and cochaperone Cdc37, leading to diminishment of kinase client maturation and proteasome‐dependent degradation of kinase substrates. Furthermore, we showed that simvastatin prevented, whereas LBH589 promoted, the progression of Hsp90 chaperone cycling and client maturation, resulting in an increment of cell apoptosis by the combination of simvastatin and LBH589 in a mouse xenograft model. These data suggest that simvastatin is a novel Hsp90 inhibitor to disrupt the formation of the K292‐acetylated Hsp90/Cdc37 complex in triple‐negative breast cancer cells. The combination of simvastatin with LBH589 could be used as a novel therapeutic strategy for triple‐negative breast cancer.

Inhibition of HSP90α protects cultured neurons from oxygen-glucose deprivation induced necroptosis by decreasing RIP3 expression

Heat shock protein 90α (HSP90α) maintains cell stabilization and regulates cell death, respectively. Recent studies have shown that HSP90α is involved in receptor interacting protein 3 (RIP3)-mediated necroptosis in HT29 cells. It is known that oxygen and glucose deprivation (OGD) can induce necroptosis, which is regulated by RIP3 in neurons. However, it is still unclear whether HSP90α participates in the process of OGD-induced necroptosis in cultured neurons via the regulation of RIP3. Our study found that necroptosis occurs in primary cultured cortical neurons and PC-12 cells following exposure to OGD insult. Additionally, the expression of RIP3/p-RIP3, MLKL/p-MLKL, and the RIP1/RIP3 complex (necrosome) significantly increased following OGD, as measured through immunofluorescence (IF) staining, Western blotting (WB), and immunoprecipitation (IP) assay. Additionally, data from computer simulations and IP assays showed that HSP90α interacts with RIP3. In addition, HSP90α was overexpressed following OGD in cultured neurons, as measured through WB and IF staining. Inhibition of HSP90α in cultured neurons, using the specific inhibitor, geldanamycin (GA), and siRNA/shRNA of HSP90α, protected cultured neurons from necrosis. Our study showed that the inhibitor of HSP90α, GA, rescued cultured neurons not only by decreasing the expression of total RIP3/MLKL, but also by decreasing the expression of p-RIP3/p-MLKL and the RIP1/RIP3 necrosome. In this study, we reveal that inhibition of HSP90α protects primary cultured cortical neurons and PC-12 cells from OGD-induced necroptosis through the modulation of RIP3 expression.

Hsp72 protects against liver injury via attenuation of hepatocellular death, oxidative stress, and JNK signaling

BACKGROUND & AIMS

Heat shock protein (Hsp) 72 is a molecular chaperone that has broad cytoprotective functions and is upregulated in response to stress. To determine its hepatic functions, we studied its expression in human liver disorders and its biological significance in newly generated transgenic animals.

METHODS

Double transgenic mice overexpressing Hsp72 (gene Hspa1a) under the control of a tissue-specific tetracycline-inducible system (Hsp72-LAP mice) were produced. Acute liver injury was induced by a single injection of acetaminophen (APAP). Feeding with either a methionine choline-deficient (MCD; 8 weeks) or a 3,5-diethoxycarbonyl-1,4-dihydrocollidine-supplemented diet (DDC; 12 weeks) was used to induce lipotoxic injury and Mallory-Denk body (MDB) formation, respectively. Primary hepatocytes were treated with palmitic acid.

RESULTS

Patients with non-alcoholic steatohepatitis and chronic hepatitis C infection displayed elevated HSP72 levels. These levels increased with the extent of hepatic inflammation and HSP72 expression was induced after treatment with either interleukin (IL)-1β or IL-6. Hsp72-LAP mice exhibited robust, hepatocyte-specific Hsp72 overexpression. Primary hepatocytes from these animals were more resistant to isolation-induced stress and Hsp72-LAP mice displayed lower levels of hepatic injury in vivo. Mice overexpressing Hsp72 had fewer APAP protein adducts and were protected from oxidative stress and APAP-/MCD-induced cell death. Hsp72-LAP mice and/or hepatocytes displayed significantly attenuated Jnk activation. Overexpression of Hsp72 did not affect steatosis or the extent of MDB formation.

CONCLUSIONS

Our results demonstrate that HSP72 induction occurs in human liver disease, thus, HSP72 represents an attractive therapeutic target owing to its broad hepatoprotective functions.

LAY SUMMARY

HSP72 constitutes a stress-inducible, protective protein. Our data demonstrate that it is upregulated in patients with chronic hepatitis C and non-alcoholic steatohepatitis. Moreover, Hsp72-overexpressing mice are protected from various forms of liver stress.

Targeting the Hsp90-Cdc37-client protein interaction to disrupt Hsp90 chaperone machinery

Heat shock protein 90 (Hsp90) is a critical molecular chaperone protein that regulates the folding, maturation, and stability of a wide variety of proteins. In recent years, the development of Hsp90-directed inhibitors has grown rapidly, and many of these inhibitors have entered clinical trials. In parallel, the functional dissection of the Hsp90 chaperone machinery has highlighted the activity disruption of Hsp90 co-chaperone as a potential target. With the roles of Hsp90 co-chaperones being elucidated, cell division cycle 37 (Cdc37), a ubiquitous co-chaperone of Hsp90 that directs the selective client proteins into the Hsp90 chaperone cycle, shows great promise. Moreover, the Hsp90-Cdc37-client interaction contributes to the regulation of cellular response and cellular growth and is more essential to tumor tissues than normal tissues. Herein, we discuss the current understanding of the clients of Hsp90-Cdc37, the interaction of Hsp90-Cdc37-client protein, and the therapeutic possibilities of targeting Hsp90-Cdc37-client protein interaction as a strategy to inhibit Hsp90 chaperone machinery to present new insights on alternative ways of inhibiting Hsp90 chaperone machinery.

Molecular mechanisms of cell death: recommendations of the Nomenclature Committee on Cell Death 2018

Over the past decade, the Nomenclature Committee on Cell Death (NCCD) has formulated guidelines for the definition and interpretation of cell death from morphological, biochemical, and functional perspectives. Since the field continues to expand and novel mechanisms that orchestrate multiple cell death pathways are unveiled, we propose an updated classification of cell death subroutines focusing on mechanistic and essential (as opposed to correlative and dispensable) aspects of the process. As we provide molecularly oriented definitions of terms including intrinsic apoptosis, extrinsic apoptosis, mitochondrial permeability transition (MPT)-driven necrosis, necroptosis, ferroptosis, pyroptosis, parthanatos, entotic cell death, NETotic cell death, lysosome-dependent cell death, autophagy-dependent cell death, immunogenic cell death, cellular senescence, and mitotic catastrophe, we discuss the utility of neologisms that refer to highly specialized instances of these processes. The mission of the NCCD is to provide a widely accepted nomenclature on cell death in support of the continued development of the field.

Molecular mechanism and therapy application of necrosis during myocardial injury

Necrosis is an ancient topic which gains new attraction in the research area these years. There is no doubt that some necrosis can be regulated by genetic manipulation other than an accidental cell death resulting from physical or chemical stimuli. Recent advances in the molecular mechanism underlying the programmed necrosis show a fine regulation network which indicates new therapy targets in human diseases. Heart diseases seriously endanger our health and have high fatality rates in the patients. Cell death of cardiac myocytes is believed to be critical in the pathogenesis of heart diseases. Although necrosis is likely to play a more important role in cardiac cell death than apoptosis, apoptosis has been paid much attention in the past 30 years because it used to be considered as the only form of programmed cell death. However, recent findings of programmed necrosis and the related signalling pathways have broadened our horizon in the field of programmed cell death and promote new pharmacological application in the treatment of heart diseases. In this review, we summarize the advanced progress in these signalling pathways and discuss the pathos-physiological relevance and therapeutic implication of targeting necrosis in heart diseases treatment.

TNF induced cleavage of HSP90 by cathepsin D potentiates apoptotic cell death

During apoptosis induction by TNF, the extrinsic and intrinsic apoptosis pathways converge at the lysosomal-mitochondrial interface. Earlier studies showed that the lysosomal aspartic protease Cathepsin D (CtsD) cleaves Bid to tBid, resulting in the amplification of the initial apoptotic cascade via mitochondrial outer membrane permeabilization (MOMP).

The goal of this study was to identify further targets for CtsD that might be involved in activation upon death receptor ligation. Using a proteomics screen, we identified the heat shock protein 90 (HSP90) to be cleaved by CtsD after stimulation of U937 or other cell lines with TNF, FasL and TRAIL. HSP90 cleavage corresponded to apoptosis sensitivity of the cell lines to the different stimuli. After mutation of the cleavage site, HSP90 partially prevented apoptosis induction in U937 and Jurkat cells. Overexpression of the cleavage fragments in U937 and Jurkat cells showed no effect on apoptosis, excluding a direct pro-apoptotic function of these fragments. Pharmacological inhibition of HSP90 with 17AAG boosted ligand mediated apoptosis by enhancing Bid cleavage and caspase-9 activation. Together, we demonstrated that HSP90 plays an anti-apoptotic role in death receptor signalling and that CtsD-mediated cleavage of HSP90 sensitizes cells for apoptosis. These findings identify HSP90 as a potential target for cancer therapy in combination with death ligands (e.g. TNF or TRAIL).

Immunological consequences of kidney cell death

Death of renal cells is central to the pathophysiology of acute tubular necrosis, autoimmunity, necrotizing glomerulonephritis, cystic kidney disease, urosepsis, delayed graft function and transplant rejection. By means of regulated necrosis, immunogenic damage-associated molecular patterns (DAMPs) and highly reactive organelles such as lysosomes, peroxisomes and mitochondria are released from the dying cells, thereby causing an overwhelming immunologic response. The rupture of the plasma membrane exhibits the "point of no return" for the immunogenicity of regulated cell death, explaining why apoptosis, a highly organized cell death subroutine with long-lasting plasma membrane integrity, elicits hardly any immune response. Ferroptosis, an iron-dependent necrotic type cell death, results in the release of DAMPs and large amounts of lipid peroxides. In contrast, anti-inflammatory cytokines are actively released from cells that die by necroptosis, limiting the DAMP-induced immune response to a surrounding microenvironment, whereas at the same time, inflammasome-associated caspases drive maturation of intracellularly expressed interleukin-1β (IL-1β). In a distinct setting, additionally interleukin-18 (IL-18) is expressed during pyroptosis, initiated by gasdermin-mediated plasma membrane rupture. As all of these pathways are druggable, we provide an overview of regulated necrosis in kidney diseases with a focus on immunogenicity and potential therapeutic interventions.

Necroptosis and neutrophil-associated disorders

Necroptosis is a form of regulated necrosis and is dependent on a signaling pathway involving receptor interacting protein kinase-3 (RIPK3) and mixed lineage kinase domain-like protein (MLKL). Necroptosis is considered to have important functions in inflammation and, based on studies with animal disease models, is believed likely to be involved in the pathogenesis of many human inflammatory diseases. In neutrophils, necroptosis has recently been reported to be triggered by tumor necrosis factor (TNF) stimulation, ligation of adhesion receptors, exposure to monosodium urate (MSU) crystals, or phagocytosis of Staphylococcus aureus (S. aureus). Because neutrophils are involved in many kinds of tissue inflammation and disease, neutrophil necroptosis probably plays a vital role in such processes. Dissecting the signaling pathway of neutrophil necroptotic death may help to identify novel drug targets for inflammatory or autoimmune diseases. In this review, we discuss different mechanisms which regulate neutrophil necroptosis and are thus potentially important in neutrophil-associated disorders.

Regulated Cell Death

In this chapter, the various subroutines of regulated cell death are neatly described by highlighting apoptosis and subforms of regulated necrosis such as necroptosis, ferroptosis, pyroptosis, and NETosis. Typically, all forms of regulated necrosis are defined by finite rupture of the plasma cell membrane. Apoptosis is characterized by an enzymatic machinery that consists of caspases which cause the morphologic features of this type of cell death. Mechanistically, apoptosis can be instigated by two major cellular signalling pathways: an intrinsic pathway that is initiated inside cells by mitochondrial release of pro-apoptotic factors or an extrinsic pathway that is initiated at the cell surface by various death receptors. In necroptosis, the biochemical processes are distinct from those found in apoptosis; in particular, there is no caspase activation. As such, necroptosis is a kinase-mediated cell death that relies on “receptor-interacting protein kinase 3” which mediates phosphorylation of the pseudokinase “mixed lineage kinase domain-like protein.” While ferroptosis is an iron-dependent, oxidative form of regulated necrosis that is biochemically characterized by accumulation of ROS from iron metabolism, oxidase activity, and lipid peroxidation products, pyroptosis is defined as a form of cell death (predominantly of phagocytes) that develops during inflammasome activation and is executed by caspase-mediated cleavage of the pore-forming protein gasdermin D. Finally, NETosis refers to a regulated death of neutrophils that is characterized by the release of chromatin-derived weblike structures released into the extracellular space. The chapter ends up with a discussion on the characteristic feature of regulated necrosis: the passive release of large amounts of constitutive DAMPs as a consequence of final plasma membrane rupture as well as the active secretion of inducible DAMPs earlier during the dying process. Notably, per cell death subroutine, the active secretion of inducible DAMPs varies, thereby determining different immunogenicity of dying cells.

Diverse ubiquitin linkages regulate RIP kinases-mediated inflammatory and cell death signaling

Members of the RIP kinase family are key regulators of inflammation and cell death signaling implicated in maintaining immune responses and proper tissue homeostasis. Increasing evidence points to post-translational modifications of RIP1, RIP2 and RIP3 as being critical for regulating their function. Ubiquitination and the E3 ligases, such as inhibitors of apoptosis (IAP) proteins and LUBAC, that direct substrate selectivity as well as the deubiquitinating enzymes, such as A20 and OTULIN, that reverse these modifications dictate the outcome of RIP kinase signaling. Perturbation of the tightly regulated RIP1, RIP2 and RIP3 ubiquitination can lead to signaling disbalance in TNF, TLR and NOD1/2-controlled pathways and result in severe human pathologies. In this review, we focus on the biological function of ubiquitin-modifying enzymes in the context of RIP1, RIP2 and RIP3 signaling. We also discuss the impact of deregulated ubiquitin networks in RIP1, RIP2 and RIP3 signaling pathways on human health.