OTULIN inhibits RIPK1-mediated keratinocyte necroptosis to prevent skin inflammation in mice

Exploring the anti-skin inflammation substances and mechanism of Paeonia lactiflora Pall. Flower via network pharmacology-HPLC integration

BACKGROUND

Paeonia lactiflora Pall. (PL) is widely used in China as a homologous plant of medicine and food. PL flower is rich in bioactive substances with anti-inflammatory effects, while the pathogenesis of skin inflammation is complex and the specific mechanism is not clear, the current treatment of skin inflammation is mainly hormonal drugs, and hormonal drugs have obvious toxic side effects. The research on the treatment of skin inflammation by PL flowers is relatively small, so this study provides a basis for the development and utilisation of PL resources.

OBJECTIVE

Our study was to investigate the interventional effects of PL flower extracts on skin inflammation and thus to understand its functional role in the treatment of skin inflammation and its molecular mechanisms.

METHODS

The major active substances in PL flower extracts were investigated by the HPLC-DAD method, and the potential targets of action were predicted by network pharmacology, which was combined with in vitro experimental validation to explore the mechanism of PL flower extracts on the regulation of skin inflammation. The HPLC-DAD analysis identified seven major active components in PL flower extracts, and in response to the results, combined with the potential mechanism of network pharmacological prediction with skin inflammation, the PL flower extract is closely related to MAPK and NF-κB signaling pathways. In addition, we also investigated the interventional effects of PL flower extract on skin inflammation by western blot detection of MAPK signaling pathway and NF-κB signaling pathway proteins in cells.

RESULT

Seven active components were identified and quantified from the extract of PL flowers, including Gallic acid, 1,2,3,4,6-O-Pentagalloylglucose, Oxypaeoniflorin, Paeoniflorin, Albiflorin, Benzoyloxypeoniflorin, and Rutin. It was predicted targets for the treatment of skin inflammation, with PPI showing associations with targets such as TNF, MAPK1, and IL-2. KEGG enrichment analysis revealed that the main signaling pathways involved included MAPK and T cell receptor signaling pathways. Cell experiments showed that the peony flower extract could inhibit the release of NO and inflammatory factors, as well as reduce ROS levels and inhibit cell apoptosis. Furthermore, the extract was found to inhibit the activation of the MAPK and NF-κB signaling pathways in cells.

CONCLUSIONS

In this study, we found that PL flower extract can inhibit the production of cell inflammatory substances, suppress the release of inflammatory factors, and deactivate inflammatory signaling pathways, further inhibiting the production of cell inflammation. This indicates that PL flower extract has a therapeutic effect on skin inflammation.

Therapeutic potential of the linalool against cadmium-induced testicular tissue damage

Cadmium (Cd) is a heavy metal that has harmful effects and is one of the contaminants found in the environment. Cd exposure causes important pathophysiological processes, such as reproductive toxicity. Linalool (Lnl) is a monoterpene, a component of essential oils known to be produced synthetically. Additionally, Lnl has many important beneficial effects, such as anti-inflammatory and antioxidant effects. The objective of this study is to determine whether Lnl has a healing impact in opposition to testicular tissue damage due to Cd exposure. In the study, 28 male rats were divided at random into four equal groups (n = 7). No treatment was applied to the control group. CdCl2 was applied intraperitoneally to the Cd group at a dose of 3 mg/kg for the first 7 days of the trial. For the Cd + Lnl group, 3 mg/kg CdCl2 was applied intraperitoneally for the first 7 days of the trial, and 100 mg/kg/day Lnl was applied. Upon completion of all applications, the rats were sacrificed and blood samples and testicular tissue were taken. Cd exposure caused histopathological changes, oxidative stress, inflammation, and an increase in apoptotic cells in testicular tissue. However, Cd altered endocrine hormones in the hypothalamic-pituitary-gonad axis. However, Lnl application against Cd exposure was able to regulate the negativity caused by Cd in both testicular tissue and endocrine hormone levels. In conclusion, Lnl may be a potential therapeutic strategy against Cd-induced reproductive toxicity. We believe that Lnl has a high potential for further studies to determine its detailed mechanisms of action and cellular signaling pathways.

Cell death as an architect of adult skin stem cell niches

Our skin provides a physical and immunological barrier against dehydration and environmental insults ranging from microbial attacks, toxins and UV irradiation to wounding. Proper functioning of the skin barrier largely depends on the interplay between keratinocytes- the epithelial cells of the skin- and immune cells. Two spatially distinct populations of keratinocyte stem cells (SCs) maintain the epidermal barrier function and the hair follicle. These SCs are inherently long-lived, but cell death can occur within their niches and impacts their functionality. The default cell death programme in skin is apoptosis, an orderly and non-inflammatory suicide programme. However, recent findings are shedding light on the significance of various modes of regulated necrotic cell death, which are lytic and can provoke inflammation within the local skin environment. While the presence of dying cells was generally regarded as a mere consequence of inflammation, findings in various human dermatological conditions and experimental mouse models of aberrant cell death control demonstrated that cell death programmes in keratinocytes (KCs) can drive skin inflammation and even tumour initiation. When cells die, they need to be removed by phagocytosis and KCs can function as non-professional phagocytes of apoptotic cells with important implications for their SC capacities. It is becoming apparent that in conditions of heightened SC activity, distinct cell death modalities differentially impact the different skin SC populations in their local niches. Here, we describe how regulated cell death modalities functionally affect epidermal SC niches along with their relevance to injury repair, inflammatory skin disorders and cancer.

OTULIN Can Improve Spinal Cord Injury by the NF-κB and Wnt/β-Catenin Signaling Pathways

Spinal cord injury (SCI) is a significant health concern, as it presently has no effective treatment in the clinical setting. Inflammation is a key player in the pathophysiological process of SCI, with a number of studies evidencing that the inhibition of the NF-κB signaling pathway may impede the inflammatory response and improve SCI. OTULIN, as a de-ubiquitination enzyme, the most notable is its anti-inflammatory effect. OTULIN can inhibit the NF-κB signaling pathway to suppress the inflammatory reaction via de-ubiquitination. In addition, OTULIN may promote vascular regeneration through the Wnt/β-catenin pathway in the wake of SCI. In this review, we analyze the structure and physiological function of OTULIN, along with both NF-κB and Wnt/β-catenin signaling pathways. Furthermore, we examine the significant role of OTULIN in SCI through its impairment of the NF-κB signaling pathway, which could open the possibility of it being a novel interventional target for the condition.

OTULIN-related conditions: Report of a new case and review of the literature using GenIA

OTULIN encodes an eponymous linear deubiquitinase (DUB), which through the regulation of M1-Ub dynamics, is essential for controlling inflammation as a negative regulator of the canonical NF-𝛋B signaling pathway. Biallelic loss-of-function (LOF) mutations in OTULIN cause an autosomal recessive condition named Otulin-Related Autoinflammatory Syndrome (ORAS), also known as Otulipenia or AutoInflammation, Panniculitis, and Dermatosis Syndrome (AIPDS). Monoallelic OTULIN LOF, also known as OTULIN Haploinsufficiency (OHI) or Immunodeficiency 107 (IMD107), has been linked to an incompletely penetrant, dominantly inherited susceptibility to invasive Staphylococcal infections. At the same time, a recent novel ORAS-like inflammatory syndrome was described in association with a heterozygous missense mutation that appears to exert dominant negative effects. In this manuscript, we report the identification of a novel homozygous missense mutation, c.595T>A; p.(Trp199Arg), in a Moroccan infant with an ORAS phenotype. We go on to systematically review the literature for OTULIN-related human disease phenotypes by using the GenIA databaseto collect, extract and harmonize all clinical, laboratory and functional data for published patients and variants. Our comprehensive synthesis of genotypic, phenotypic, and mechanistic data enables a more in-depth view of the diverse mechanisms and pathways by which the OTULIN pathogenic variants may lead to human immune disease. This review may help variant classification activities and the drafting of diagnostic and management guidelines; but it also identifies outstanding knowledge gaps and raises additional questions for future investigation.

Transcriptomic analysis of skin immunity genes in the Chinese spiny frog (Quasipaa spinosa) after Proteus mirabilis infection

Recently, populations of Chinese spiny frogs (Quasipaa spinosa), an important amphibian species in China, have decreased, mainly due to a disease caused by the gram-negative bacteria Proteus mirabilis. To elucidate the immune response of the frogs, this study aimed to identify novel candidate genes functionally associated with P. mirabilis infection-induced "rotting skin" disease. Chinese spiny frogs were infected with P. mirabilis, and the skin transcriptome was sequenced using the MGISEQ-2000 platform. A total of 233,965 unigenes were obtained by sequencing, of which 27.23 % were known genes. Screening of differentially expressed genes (DEGs) indicated 210 unigenes differentially expressed after P. mirabilis infection, of which 132 unigenes were up-regulated, and 78 unigenes were down-regulated. Using Kyoto Encyclopedia of Genes and Genomes enrichment analysis, DEGs were identified as enriched in signal pathways, such as oxidative phosphorylation, apoptosis, and the Janus kinase-signal transducer and activator of transcription pathway. Of the DEGs, there was a significant upregulation of the colony stimulating factor 2 receptor beta common subunit, interleukin 2 receptor subunit gamma, cathelicidin antimicrobial peptide, interleukin-17 receptor E, receptor-interacting serine/threonine-protein kinase 3, and pulmonary surfactant-associated protein D immune genes following P. mirabilis infection. Conversely, scavenger receptor cysteine-rich domain-containing group B protein, tumor protein p53 inducible nuclear protein 2, suppressor of cytokine signaling 2, and metalloreductase STEAP3 were significantly downregulated. In conclusion, the first skin transcriptome database of Chinese spiny frogs was established, and several immune genes were identified to elucidate the pathogenic mechanism of "skin rot" in Chinese spiny frogs and other cultured frogs.

Extracellular vesicles derived from mesenchymal stem cells: the wine in Hebe's hands to treat skin aging

Owing to its constant exposure to the external environment and various stimuli, skin ranks among the organs most vulnerable to manifestations of aging. Preventing and delaying skin aging has become one of the prominent research subjects in recent years. Mesenchymal stem cells (MSCs) are multipotent stem cells derived from mesoderm with high self-renewal ability and multilineage differentiation potential. MSC-derived extracellular vesicles (MSC-EVs) are nanoscale biological vesicles that facilitate intercellular communication and regulate biological behavior. Recent studies have shown that MSC-EVs have potential applications in anti-aging therapy due to their anti-inflammatory, anti-oxidative stress, and wound healing promoting abilities. This review presents the latest progress of MSC-EVs in delaying skin aging. It mainly includes the MSC-EVs promoting the proliferation and migration of keratinocytes and fibroblasts, reducing the expression of matrix metalloproteinases, resisting oxidative stress, and regulating inflammation. We then briefly discuss the recently discovered treatment methods of MSC-EVs in the field of skin anti-aging. Moreover, the advantages and limitations of EV-based treatments are also presented.

Generating dual structurally and functionally skin-mimicking hydrogels by crosslinking cell-membrane compartments

The skin is intrinsically a cell-membrane-compartmentalized hydrogel with high mechanical strength, potent antimicrobial ability, and robust immunological competence, which provide multiple protective effects to the body. Methods capable of preparing hydrogels that can simultaneously mimic the structure and function of the skin are highly desirable but have been proven to be a challenge. Here, dual structurally and functionally skin-mimicking hydrogels are generated by crosslinking cell-membrane compartments. The crosslinked network is formed via free radical polymerization using olefinic double bond-functionalized extracellular vesicles as a crosslinker. Due to the dissipation of stretching energy mediated by vesicular deformation, the obtained compartment-crosslinked network shows enhanced mechanical strength compared to hydrogels crosslinked by regular divinyl monomers. Biomimetic hydrogels also exhibit specific antibacterial activity and adequate ability to promote the maturation and activation of dendritic cells given the existence of numerous extracellular vesicle-associated bioactive substances. In addition, the versatility of this approach to tune both the structure and function of the resulting hydrogels is demonstrated through introducing a second network by catalyst-free click reaction-mediated crosslinking between alkyne-double-ended polymers and azido-decorated extracellular vesicles. This study provides a platform to develop dual structure- and function-controllable skin-inspired biomaterials.

Mind bomb 2 limits inflammatory dermatitis in Sharpin mutant mice independently of cell death

Skin inflammation is a complex process implicated in various dermatological disorders. The chronic proliferative dermatitis (cpd) phenotype driven by the cpd mutation (cpdm) in the Sharpin gene is characterized by dermal inflammation and epidermal abnormalities. Tumour necrosis factor (TNF) and caspase-8-driven cell death causes the pathogenesis of Sharpincpdm mice; however, the role of mind bomb 2 (MIB2), a pro-survival E3 ubiquitin ligase involved in TNF signaling, in skin inflammation remains unknown. Here, we demonstrate that MIB2 antagonizes inflammatory dermatitis in the context of the cpd mutation. Surprisingly, the role of MIB2 in limiting skin inflammation is independent of its known pro-survival function and E3 ligase activity. Instead, MIB2 enhances the production of wound-healing molecules, granulocyte colony-stimulating factor, and Eotaxin, within the skin. This discovery advances our comprehension of inflammatory cytokines and chemokines associated with cpdm pathogenesis and highlights the significance of MIB2 in inflammatory skin disease that is independent of its ability to regulate TNF-induced cell death.

N-acetylcysteine combined with insulin attenuates myocardial injury in canines with type 1 diabetes mellitus by modulating TNF-α-mediated apoptotic pathways and affecting linear ubiquitination

The exact pathogenesis of type 1 diabetes mellitus (DM) is still unclear. Numerous organs, including the heart, will suffer damage and malfunction as a result of long-term hyperglycemia. Currently, insulin therapy alone is still not the best treatment for type 1 DM. In order to properly treat and manage patients with type 1 DM, it is vital to seek a combination that includes both insulin and additional medications. This study aims to explore the therapeutic effect and mechanism of N-acetylcysteine (NAC) combined with insulin on type 1 DM. By giving beagle canines injections of streptozotocin (STZ) and alloxan (ALX) (20 mg/kg each), a model of type 1 DM was created. The results showed that this combination could effectively control blood sugar level, improve heart function, avoid the damage of mitochondria and myocardial cells, and prevent the excessive apoptosis of myocardial cells. Importantly, the combination can activate nuclear factor kappa-B (NF-κB) by promoting linear ubiquitination of receptor-interacting protein kinase 1 (RIPK1) and NF-κB-essential modulator (NEMO) and inhibitor of NF-κB (IκB) phosphorylation. The combination can increase the transcription and linear ubiquitination of Cellular FLICE (FADD-like IL-1β-converting enzyme) -inhibitory protein (c-FLIP), diminish the production of cleaved-caspase-8 p18 and cleaved-caspase-3 to reduce apoptosis. This study confirmed that NAC combined with insulin can promote the linear ubiquitination of RIPK1, NEMO and c-FLIP and regulate the apoptosis pathway mediated by TNF-α to attenuate the myocardial injury caused by type 1 DM. Meanwhile, the research served as a resource when choosing a clinical strategy for DM cardiac complications.

Myeloid OTULIN deficiency couples RIPK3-dependent cell death to Nlrp3 inflammasome activation and IL-1β secretion

Loss-of-function mutations in the deubiquitinase OTULIN result in an inflammatory pathology termed "OTULIN-related autoinflammatory syndrome" (ORAS). Genetic mouse models revealed essential roles for OTULIN in inflammatory and cell death signaling, but the mechanisms by which OTULIN deficiency connects cell death to inflammation remain unclear. Here, we identify OTULIN deficiency as a cellular condition that licenses RIPK3-mediated cell death in murine macrophages, leading to Nlrp3 inflammasome activation and subsequent IL-1β secretion. OTULIN deficiency uncoupled Nlrp3 inflammasome activation from gasdermin D-mediated pyroptosis, instead allowing RIPK3-dependent cell death to act as an Nlrp3 inflammasome activator and mechanism for IL-1β release. Accordingly, elevated serum IL-1β levels in myeloid-specific OTULIN-deficient mice were diminished by deleting either Ripk3 or Nlrp3. These findings identify OTULIN as an inhibitor of RIPK3-mediated IL-1β release in mice.

In an experimental myocardial infarction model, L-arginine pre-intervention may exert cardioprotective effects by regulating OTULIN levels and mitochondrial dynamics

The experimental myocardial infarction (MI) model originating from isoproterenol (ISO) is frequently preferred in research due to its similarity to MI-induced damage in humans. Beneficial effects of L-arginine (L-Arg), a semi-essential amino acid, in cardiovascular diseases have been shown in many studies. This study was carried out to determine whether L-Arg pre-intervention has protective effects on heart tissue in the experimental MI model. The 28 rats used in the study were randomly divided into 4 equal groups: control, L-Arg, ISO, and L-Arg+ISO. Upon completion of all applications, cardiac markers in serum and biochemical, histopathological, and immunohistochemical examinations in cardiac tissues were performed. Cardiac markers, histopathological changes, oxidative stress, inflammation, and apoptosis were increased in the experimental MI model. In addition, administration of ISO deregulated OTULIN levels and mitochondrial dynamics in heart tissue. However, L-Arg pre-intervention showed a significant protective effect against changes in ISO-induced MI. L-Arg supplementation with cardioprotective effect may reduce the risks of possible pathophysiological processes in MI.

The mechanism of linear ubiquitination in regulating cell death and correlative diseases

Linear ubiquitination is a specific post-translational modification in which ubiquitin is linked through M1 residue to form multiple types of polyubiquitin chains on substrates in order to regulate cellular processes. LUBAC comprised by HOIP, HOIL-1L, and SHARPIN as a sole E3 ligase catalyzes the generation of linear ubiquitin chains, and it is simultaneously adjusted by deubiquitinases such as OTULIN and CYLD. Several studies have shown that gene mutation of linear ubiquitination in mice accompanied by different modalities of cell death would develop relative diseases. Cell death is a fundamental physiological process and responsible for embryonic development, organ maintenance, and immunity response. Therefore, it is worth speculating that linear ubiquitin mediated signaling pathway would participate in different diseases. The relative literature search was done from core collection of electronic databases such as Web of Science, PubMed, and Google Scholar using keywords about main regulators of linear ubiquitination pathway. Here, we summarize the regulatory mechanism of linear ubiquitination on cellular signaling pathway in cells with apoptosis, necroptosis, autophagy, pyroptosis, and ferroptosis. Intervening generation of linear ubiquitin chains in relative signaling pathway to regulate cell death might provide novel therapeutic insights for various human diseases.

The therapeutic potential of targeting regulated non-apoptotic cell death

Cell death is critical for the development and homeostasis of almost all multicellular organisms. Moreover, its dysregulation leads to diverse disease states. Historically, apoptosis was thought to be the major regulated cell death pathway, whereas necrosis was considered to be an unregulated form of cell death. However, research in recent decades has uncovered several forms of regulated necrosis that are implicated in degenerative diseases, inflammatory conditions and cancer. The growing insight into these regulated, non-apoptotic cell death pathways has opened new avenues for therapeutic targeting. Here, we describe the regulatory pathways of necroptosis, pyroptosis, parthanatos, ferroptosis, cuproptosis, lysozincrosis and disulfidptosis. We discuss small-molecule inhibitors of the pathways and prospects for future drug discovery. Together, the complex mechanisms governing these pathways offer strategies to develop therapeutics that control non-apoptotic cell death.

OTULIN protects the intestinal epithelium from apoptosis during inflammation and infection

The intestinal epithelium is a single cell layer that is constantly renewed and acts as a physical barrier that separates intestinal microbiota from underlying tissues. In inflammatory bowel disease (IBD) in humans, as well as in experimental mouse models of IBD, this barrier is impaired, causing microbial infiltration and inflammation. Deficiency in OTU deubiquitinase with linear linkage specificity (OTULIN) causes OTULIN-related autoinflammatory syndrome (ORAS), a severe inflammatory pathology affecting multiple organs including the intestine. We show that mice with intestinal epithelial cell (IEC)-specific OTULIN deficiency exhibit increased susceptibility to experimental colitis and are highly sensitive to TNF toxicity, due to excessive apoptosis of OTULIN deficient IECs. OTULIN deficiency also increases intestinal pathology in mice genetically engineered to secrete excess TNF, confirming that chronic exposure to TNF promotes epithelial cell death and inflammation in OTULIN deficient mice. Mechanistically we demonstrate that upon TNF stimulation, OTULIN deficiency impairs TNF receptor complex I formation and LUBAC recruitment, and promotes the formation of the cytosolic complex II inducing epithelial cell death. Finally, we show that OTULIN deficiency in IECs increases susceptibility to Salmonella infection, further confirming the importance of OTULIN for intestinal barrier integrity. Together, these results identify OTULIN as a major anti-apoptotic protein in the intestinal epithelium and provide mechanistic insights into how OTULIN deficiency drives gastrointestinal inflammation in ORAS patients.

Repressive Control of Keratinocyte Cytoplasmic Inflammatory Signaling

The overactivity of keratinocyte cytoplasmic signaling contributes to several cutaneous inflammatory and immune pathologies. An important emerging complement to proteins responsible for this overactivity is signal repression brought about by several proteins and protein complexes with the native role of limiting inflammation. The signaling repression by these proteins distinguishes them from transmembrane receptors, kinases, and inflammasomes, which drive inflammation. For these proteins, defects or deficiencies, whether naturally arising or in experimentally engineered skin inflammation models, have clearly linked them to maintaining keratinocytes in a non-activated state or returning cells to a post-inflamed state after a signaling event. Thus, together, these proteins help to resolve acute inflammatory responses or limit the development of chronic cutaneous inflammatory disease. We present here an integrated set of demonstrated or potentially inflammation-repressive proteins or protein complexes (linear ubiquitin chain assembly complex [LUBAC], cylindromatosis lysine 63 deubiquitinase [CYLD], tumor necrosis factor alpha-induced protein 3-interacting protein 1 [TNIP1], A20, and OTULIN) for a comprehensive view of cytoplasmic signaling highlighting protein players repressing inflammation as the needed counterpoints to signal activators and amplifiers. Ebb and flow of players on both sides of this inflammation equation would be of physiological advantage to allow acute response to damage or pathogens and yet guard against chronic inflammatory disease. Further investigation of the players responsible for repressing cytoplasmic signaling would be foundational to developing new chemical-entity pharmacologics to stabilize or enhance their function when clinical intervention is needed to restore balance.

Disulfiram reduces the severity of mouse acute pancreatitis by inhibiting RIPK1-dependent acinar cell necrosis

Acute pancreatitis (AP) is a frequent abdominal inflammatory disease. Despite the high morbidity and mortality, the management of AP remains unsatisfactory. Disulfiram (DSF) is an FDA-proved drug with potential therapeutic effects on inflammatory diseases. In this study, we aim to investigate the effect of DSF on pancreatic acinar cell necrosis, and to explore the underlying mechanisms. Cell necrosis was induced by sodium taurocholate or caerulein, AP mice model was induced by nine hourly injections of caerulein. Network pharmacology, molecular docking, and molecular dynamics simulation were used to explore the potential targets of DSF in protecting against cell necrosis. The results indicated that DSF significantly inhibited acinar cell necrosis as evidenced by a decreased ratio of necrotic cells in the pancreas. Network pharmacology, molecular docking, and molecular dynamics simulation identified RIPK1 as a potent target of DSF in protecting against acinar cell necrosis. qRT-PCR analysis revealed that DSF decreased the mRNA levels of RIPK1 in freshly isolated pancreatic acinar cells and the pancreas of AP mice. Western blot showed that DSF treatment decreased the expressions of RIPK1 and MLKL proteins. Moreover, DSF inhibited NF-κB activation in acini. It also decreased the protein expression of TLR4 and the formation of neutrophils extracellular traps (NETs) induced by damage-associated molecular patterns released by necrotic acinar cells. Collectively, DSF could ameliorate the severity of mouse acute pancreatitis by inhibiting RIPK-dependent acinar cell necrosis and the following formation of NETs.

Targeting RIPK1 kinase for modulating inflammation in human diseases

Receptor-Interacting Serine/Threonine-Protein Kinase 1 (RIPK1) is a master regulator of TNFR1 signaling in controlling cell death and survival. While the scaffold of RIPK1 participates in the canonical NF-κB pathway, the activation of RIPK1 kinase promotes not only necroptosis and apoptosis, but also inflammation by mediating the transcriptional induction of inflammatory cytokines. The nuclear translocation of activated RIPK1 has been shown to interact BAF-complex to promote chromatin remodeling and transcription. This review will highlight the proinflammatory role of RIPK1 kinase with focus on human neurodegenerative diseases. We will discuss the possibility of targeting RIPK1 kinase for the treatment of inflammatory pathology in human diseases.

Death by TNF: a road to inflammation

Tumour necrosis factor (TNF) is a central cytokine in inflammatory reactions, and biologics that neutralize TNF are among the most successful drugs for the treatment of chronic inflammatory and autoimmune pathologies. In recent years, it became clear that TNF drives inflammatory responses not only directly by inducing inflammatory gene expression but also indirectly by inducing cell death, instigating inflammatory immune reactions and disease development. Hence, inhibitors of cell death are being considered as a new therapy for TNF-dependent inflammatory diseases.

HOIP modulates the stability of GPx4 by linear ubiquitination

LUBAC-mediated linear ubiquitination plays a pivotal role in regulation of cell death and inflammatory pathways. Genetic deficiency in LUBAC components leads to severe immune dysfunction or embryonic lethality. LUBAC has been extensively studied for its role in mediating TNF signaling. However, Tnfr1 knockout is not able to fully rescue the embryonic lethality of LUBAC deficiency, suggesting that LUBAC may modify additional key cellular substrates in promoting cell survival. GPx4 is an important selenoprotein involved in regulating cellular redox homeostasis in defense against lipid peroxidation-mediated cell death known as ferroptosis. Here we demonstrate that LUBAC deficiency sensitizes to ferroptosis by promoting GPx4 degradation and downstream lipid peroxidation. LUBAC binds and stabilizes GPx4 by modulating its linear ubiquitination both in normal condition and under oxidative stress. Our findings identify GPx4 as a key substrate of LUBAC and a previously unrecognized role of LUBAC-mediated linear ubiquitination in regulating cellular redox status and cell death.

The regulation of necroptosis by ubiquitylation

Necroptosis is a programmed necrosis that is mediated by receptor-interacting protein kinases RIPK1, RIPK3 and the mixed lineage kinase domain-like protein, MLKL. Necroptosis must be strictly regulated to maintain normal tissue homeostasis, and dysregulation of necroptosis leads to the development of various inflammatory, infectious, and degenerative diseases. Ubiquitylation is a widespread post-translational modification that is essential for balancing numerous physiological processes. Over the past decade, considerable progress has been made in the understanding of the role of ubiquitylation in regulating necroptosis. Here, we will discuss the regulatory functions of ubiquitylation in necroptosis signaling pathway. An enhanced understanding of the ubiquitylation enzymes and regulatory proteins in necroptotic signaling pathway will be exploited for the development of new therapeutic strategies for necroptosis-related diseases.

Post-Translational Modifications in Atopic Dermatitis: Current Research and Clinical Relevance

Atopic dermatitis (AD) is a chronic and relapsing cutaneous disorder characterized by compromised immune system, excessive inflammation, and skin barrier disruption. Post-translational modifications (PTMs) are covalent and enzymatic modifications of proteins after their translation, which have been reported to play roles in inflammatory and allergic diseases. However, less attention has been paid to the effect of PTMs on AD. This review summarized the knowledge of six major classes (including phosphorylation, acetylation, ubiquitination, SUMOylation, glycosylation, o-glycosylation, and glycation) of PTMs in AD pathogenesis and discussed the opportunities for disease management.

Exploring the Role of Staphylococcus aureus in Inflammatory Diseases

Staphylococcus aureus is a very common Gram-positive bacterium, and S. aureus infections play an extremely important role in a variety of diseases. This paper describes the types of virulence factors involved, the inflammatory cells activated, the process of host cell death, and the associated diseases caused by S. aureus. S. aureus can secrete a variety of enterotoxins and other toxins to trigger inflammatory responses and activate inflammatory cells, such as keratinocytes, helper T cells, innate lymphoid cells, macrophages, dendritic cells, mast cells, neutrophils, eosinophils, and basophils. Activated inflammatory cells can express various cytokines and induce an inflammatory response. S. aureus can also induce host cell death through pyroptosis, apoptosis, necroptosis, autophagy, etc. This article discusses S. aureus and MRSA (methicillin-resistant S. aureus) in atopic dermatitis, psoriasis, pulmonary cystic fibrosis, allergic asthma, food poisoning, sarcoidosis, multiple sclerosis, and osteomyelitis. Summarizing the pathogenic mechanism of Staphylococcus aureus provides a basis for the targeted treatment of Staphylococcus aureus infection.

Cell Death-Related Ubiquitin Modifications in Inflammatory Syndromes: From Mice to Men

Aberrant cell death can cause inflammation and inflammation-related diseases. While the link between cell death and inflammation has been widely established in mouse models, evidence supporting a role for cell death in the onset of inflammatory and autoimmune diseases in patients is still missing. In this review, we discuss how the lessons learnt from mouse models can help shed new light on the initiating or contributing events leading to immune-mediated disorders. In addition, we discuss how multiomic approaches can provide new insight on the soluble factors released by dying cells that might contribute to the development of such diseases.

Deubiquitinases in cell death and inflammation

Apoptosis, pyroptosis, and necroptosis are distinct forms of programmed cell death that eliminate infected, damaged, or obsolete cells. Many proteins that regulate or are a part of the cell death machinery undergo ubiquitination, a post-translational modification made by ubiquitin ligases that modulates protein abundance, localization, and/or activity. For example, some ubiquitin chains target proteins for degradation, while others function as scaffolds for the assembly of signaling complexes. Deubiquitinases (DUBs) are the proteases that counteract ubiquitin ligases by cleaving ubiquitin from their protein substrates. Here, we review the DUBs that have been found to suppress or promote apoptosis, pyroptosis, or necroptosis.

RIP1 post-translational modifications

Receptor interacting protein 1 (RIP1) kinase is a critical regulator of inflammation and cell death signaling, and plays a crucial role in maintaining immune responses and proper tissue homeostasis. Mounting evidence argues for the importance of RIP1 post-translational modifications in control of its function. Ubiquitination by E3 ligases, such as inhibitors of apoptosis (IAP) proteins and LUBAC, as well as the reversal of these modifications by deubiquitinating enzymes, such as A20 and CYLD, can greatly influence RIP1 mediated signaling. In addition, cleavage by caspase-8, RIP1 autophosphorylation, and phosphorylation by a number of signaling kinases can greatly impact cellular fate. Disruption of the tightly regulated RIP1 modifications can lead to signaling disbalance in TNF and/or TLR controlled and other inflammatory pathways, and result in severe human pathologies. This review will focus on RIP1 and its many modifications with an emphasis on ubiquitination, phosphorylation, and cleavage, and their functional impact on the RIP1's role in signaling pathways.

Updated Perspectives on Keratinocytes and Psoriasis: Keratinocytes are More Than Innocent Bystanders

Psoriasis is a complex disease triggered by genetic, immunologic, and environmental stimuli. Many genes have been linked to psoriasis, like the psoriasis susceptibility genes, some of which are critical in keratinocyte biology and epidermal barrier function. Still, the exact pathogenesis of psoriasis is unknown. In the disease, the balance between the proliferative and differentiative processes of keratinocytes becomes altered. Multiple studies have highlighted the role of dysregulated immune cells in provoking the inflammatory responses seen in psoriasis. In addition to immune cells, accumulating evidence shows that keratinocytes are involved in psoriasis pathogenesis, as discussed in this review. Although certain immune cell-derived factors stimulate keratinocyte hyperproliferation, activated keratinocytes can also produce anti-microbial peptides, cytokines, and chemokines that can promote their proliferation, as well as recruit immune cells to help initiate and reinforce inflammatory feedback loops. Psoriatic keratinocytes also show intrinsic differences from normal keratinocytes even after removal from the in vivo inflammatory environment; thus, psoriatic keratinocytes have been found to exhibit abnormal calcium metabolism and possible epigenetic changes that contribute to psoriasis. The Koebner phenomenon, in which injury promotes the development of psoriatic lesions, also provides evidence for keratinocytes' contributions to disease pathogenesis. Furthermore, transgenic mouse studies have confirmed the importance of keratinocytes in the etiology of psoriasis. Finally, in addition to immune cells and keratinocytes, data in the literature support roles for other cell types, tissues, and systems in psoriasis development. These other contributors are all potential targets for therapies, suggesting the importance of a holistic approach when treating psoriasis.

The potent inhibitory role of suppressing TBK1 in RIPK1 associated cerebral ischemia-reperfusion injury

The pathological mechanism of cell death features in cerebral ischemia-reperfusion injury (CIRI) was complicated. The occurrence of various cell death pathways during the progression of ischemia/reperfusion injury promoted complex further neuroinflammation. RIPK1, receptor interacting protein kinase 1, was convinced to be involved in both necroptosis and apoptosis, which is a special RIPK1-dependent apoptosis. More evidences indicated the physiological role of RIPK1 in necroptosis, apoptosis and also autophagy. In this study, we elucidated the RIPK1 exhibited characterization in various cell death pathways in time-course dependent feature. The necroptosis occupied dominant neuron death within 24 hours after ischemia/reperfusion injury. However, the neuronal death feature seemed turned to apoptosis 24 hours after reperfusion. In this study, it was also found that TBK1 (TANK binding kinase 1) played as suppressor in the regulation of kinase activity of RIPK1. This result might provide a potential approach in mediating the kinase activity of RIPK1 in clinic.

OTULIN maintains skin homeostasis by controlling keratinocyte death and stem cell identity

OTULIN is a deubiquitinase that specifically cleaves linear ubiquitin chains. Here we demonstrate that the ablation of Otulin selectively in keratinocytes causes inflammatory skin lesions that develop into verrucous carcinomas. Genetic deletion of Tnfr1, knockin expression of kinase-inactive Ripk1 or keratinocyte-specific deletion of Fadd and Mlkl completely rescues mice with OTULIN deficiency from dermatitis and tumorigenesis, thereby identifying keratinocyte cell death as the driving force for inflammation. Single-cell RNA-sequencing comparing non-lesional and lesional skin reveals changes in epidermal stem cell identity in OTULIN-deficient keratinocytes prior to substantial immune cell infiltration. Keratinocytes lacking OTULIN display a type-1 interferon and IL-1β response signature, and genetic or pharmacologic inhibition of these cytokines partially inhibits skin inflammation. Finally, expression of a hypomorphic mutant Otulin allele, previously shown to cause OTULIN-related autoinflammatory syndrome in humans, induces a similar inflammatory phenotype, thus supporting the importance of OTULIN for restraining skin inflammation and maintaining immune homeostasis.