Carotid Body-Mediated Chemoreflex Function in Aging and the Role of Receptor-Interacting Protein Kinase.

Ventilatory impairment during aging has been linked to carotid body (CB) dysfunction. Anatomical/morphological studies evidenced CB degeneration and reductions in the number of CB chemoreceptor cells during aging. The mechanism(s) related to CB degeneration in aging remains elusive. Programmed cell death encompasses both apoptosis and necroptosis. Interestingly, necroptosis can be driven by molecular pathways related to low-grade inflammation, one hallmark of the aging process. Accordingly, we hypothesized that necrotic cell death dependent on receptor-interacting protein kinase-3 (RIPK3) may contribute, at least in part, to impair CB function during aging. Adult (3 months) and aged (24 months) wild type (WT) and RIPK3-/- mice were used to study chemoreflex function. Aging results in significant reductions in both the hypoxic (HVR) and hypercapnic ventilatory responses (HCVR). Adult RIPK3-/- mice showed normal HVR and HCVR compared to adult WT mice. Remarkable, aged RIPK3-/- mice displayed no reductions in HVR nor in HCVR. Indeed, chemoreflex responses obtained in aged RIPK3-/- KO mice were undistinguishable from the ones obtained in adult WT mice. Lastly, we found high prevalence of breathing disorders during aging and this was absent in aged RIPK3-/- mice. Together our results support a role for RIPK3-mediated necroptosis in CB dysfunction during aging.

SARS-CoV-2 immune complex triggers human monocyte necroptosis.

We analyzed the ability of severe acute respiratory syndrome coronavirus-2 (SARS-CoV-2) itself and SARS-CoV-2-IgG immune complexes to trigger human monocyte necroptosis. SARS-CoV-2 was able to induce monocyte necroptosis dependently of MLKL activation. Necroptosis-associated proteins (RIPK1, RIPK3 and MLKL) were involved in SARS-CoV-2N1 gene expression in monocytes. SARS-CoV-2 immune complexes promoted monocyte necroptosis in a RIPK3- and MLKL-dependent manner, and Syk tyrosine kinase was necessary for SARS-CoV-2 immune complex-induced monocyte necroptosis, indicating the involvement of Fcγ receptors on necroptosis. Finally, we provide evidence that elevated LDH levels as a marker of lytic cell death are associated with COVID-19 pathogenesis.

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.

RIPK3 is a novel prognostic marker for lower grade glioma and further enriches IDH mutational status subgrouping.

PURPOSE: Necroptosis is a necrotic-like cell death pathway in which Receptor-interacting serine/threonine-protein kinase 3 (RIPK3) plays a central role and may induce inflammation and immunity. Lower RIPK3 levels have been correlated with a poor prognosis in breast and colorectal cancer patients. Instead, in gliomas, the most prevalent among central nervous system cancers, necrosis concurs with a more aggressive and lethal outcome, suggesting that, in these cases, necrotic-like pathways may be linked to worse prognoses. Lower-grade gliomas (LGG) exhibit highly diverse clinical behaviors, ranging from slow-paced growth to fast progression to glioblastoma yet patient outcomes cannot be fully predicted through the available markers. To date, IDH mutational status is the most broadly used prognostic marker, albeit several candidates have been proposed to refine LGG subgrouping. Here, we aimed to assess RIPK3 role as a prognostic marker for LGG patients, independently of or in combination with IDH. METHODS: Using publicly available discovery (513 patients) and validation (134 patients) cohorts, we performed Kaplan Meier survival analysis and uni- and multivariate Cox regression models. RESULTS: RIPK3 is an independent prognostic marker in LGG patients, even when controlled by age and molecular or histological diagnostic criteria. Contrary to what was previously reported for other cancers, high RIPK3 expression levels correlates with an increased risk of death. Importantly, RIPK3 expression levels further split both the mutant and wild-type IDH patients into distinct risk groups. CONCLUSION: RIPK3 expression levels can be used in combination with IDH mutational status to better subgroup LGG patients regarding overall survival.

The NOD2 signaling in peripheral macrophages contributes to neuropathic pain development.

Neuropathic pain is one of the most important types of chronic pain. It is caused by neuronal damage. Clinical and experimental studies suggest a critical role for neuroimmune interactions in the development of neuropathic pain. In this article, we have shown that the cytoplasmic receptor Nod-like receptor-2, NOD2, and its adaptor-signaling molecule RIPK2 participate in the development of neuropathic pain after peripheral nerve injury (spared nerve injury model). The activation of NOD2 signaling in peripheral macrophage mediates the development of neuropathic pain through the production of pronociceptive cytokines (tumor necrosis factor and IL-1ß). This study found that peripheral nerve injury promoted a systemic increase in the NOD2 ligand. These results highlight a previously undetermined role for NOD2 signaling in the development of neuropathic pain, suggesting a new potential target for preventing neuropathic pain.

RIPK1-dependent apoptosis bypasses pathogen blockade of innate signaling to promote immune defense.

Many pathogens deliver virulence factors or effectors into host cells in order to evade host defenses and establish infection. Although such effector proteins disrupt critical cellular signaling pathways, they also trigger specific antipathogen responses, a process termed "effector-triggered immunity." The Gram-negative bacterial pathogen Yersinia inactivates critical proteins of the NF-κB and MAPK signaling cascade, thereby blocking inflammatory cytokine production but also inducing apoptosis. Yersinia-induced apoptosis requires the kinase activity of receptor-interacting protein kinase 1 (RIPK1), a key regulator of cell death, NF-κB, and MAPK signaling. Through the targeted disruption of RIPK1 kinase activity, which selectively disrupts RIPK1-dependent cell death, we now reveal that Yersinia-induced apoptosis is critical for host survival, containment of bacteria in granulomas, and control of bacterial burdens in vivo. We demonstrate that this apoptotic response provides a cell-extrinsic signal that promotes optimal innate immune cytokine production and antibacterial defense, demonstrating a novel role for RIPK1 kinase-induced apoptosis in mediating effector-triggered immunity to circumvent pathogen inhibition of immune signaling.

Necroptosis in development, inflammation and disease.

In the early 2000s, receptor-interacting serine/threonine protein kinase 1 (RIPK1), a molecule already recognized as an important regulator of cell survival, inflammation and disease, was attributed an additional function: the regulation of a novel cell death pathway that came to be known as necroptosis. Subsequently, the related kinase RIPK3 and its substrate mixed-lineage kinase domain-like protein (MLKL) were also implicated in the necroptotic pathway, and links between this pathway and apoptosis were established. In this Timeline article, we outline the discoveries that have helped to identify the roles of RIPK1, RIPK3, MLKL and other regulators of necroptosis, and how they interact to determine cell fate.

Role of NOD2 and RIP2 in host-microbe interactions with Gram-negative bacteria: insights from the periodontal disease model.

NOD2 is a member of the NLR family of proteins that participate in the activation of the innate immune response. RIP2 is a downstream kinase activated by both NOD1 and NOD2. There is scarcity of information regarding the relevance of NOD2 in periodontitis, a chronic inflammatory condition characterized by inflammatory bone resorption. We used NOD2-KO and RIP2-KO mice in a model of microbial-induced periodontitis. Heat-killed Aggregatibacter actinomycetemcomitans was injected in the gingival tissues three times/wk for 4 wk. Bone resorption was assessed by µCT analysis; osteoclasts were identified by immunohistochemical staining for TRAP and inflammation was assessed using a severity score system in H/E-stained sections. In vitro studies using primary macrophages assessed the response macrophages using qPCR-based array and multi-ligand ELISA. Bone resorption and osteoclastogenesis were significantly reduced in NOD2-KO mice. Severity of inflammation was not affected. qPCR-focused arrays and multi-ligand ELISA showed that expression of pro-inflammatory mediators was reduced in NOD2- and RIP2-deficient cells. RANKL-induced osteoclastogenesis was impaired in NOD2- and RIP2-deficient macrophages. We conclude that NOD2 is important for osteoclast differentiation and inflammatory bone resorption in vivo and also for the macrophage response to Gram-negative bacteria.

Necroptotic Cell Death Signaling and Execution Pathway: Lessons from Knockout Mice.

Under stress conditions, cells in living tissue die by apoptosis or necrosis depending on the activation of the key molecules within a dying cell that either transduce cell survival or death signals that actively destroy the sentenced cell. Multiple extracellular (pH, heat, oxidants, and detergents) or intracellular (DNA damage and Ca(2+) overload) stress conditions trigger various types of the nuclear, endoplasmic reticulum (ER), cytoplasmatic, and mitochondrion-centered signaling events that allow cells to preserve the DNA integrity, protein folding, energetic, ionic and redox homeostasis, thus escaping from injury. Along the transition from reversible to irreversible injury, death signaling is highly heterogeneous and damaged cells may engage autophagy, apoptotic, or necrotic cell death programs. Studies on multiple double- and triple- knockout mice identified caspase-8, flip, and fadd genes as key regulators of embryonic lethality and inflammation. Caspase-8 has a critical role in pro- and antinecrotic signaling pathways leading to the activation of receptor interacting protein kinase 1 (RIPK1), RIPK3, and the mixed kinase domain-like (MLKL) for a convergent execution pathway of necroptosis or regulated necrosis. Here we outline the recent discoveries into how the necrotic cell death execution pathway is engaged in many physiological and pathological outcome based on genetic analysis of knockout mice.

MyD88-, but not Nod1- and/or Nod2-deficient mice, show increased susceptibility to polymicrobial sepsis due to impaired local inflammatory response.

Pathogen recognition and triggering of the inflammatory response following infection in mammals depend mainly on Toll-like and Nod-like receptors. Here, we evaluated the role of Nod1, Nod2 and MyD88-dependent signaling in the chemokine production and neutrophil recruitment to the infectious site during sepsis induced by cecal ligation and puncture (CLP) in C57Bl/6 mice. We demonstrate that Nod1 and Nod2 are not involved in the release of chemokines and recruitment of neutrophils to the infectious site during CLP-induced septic peritonitis because these events were similar in wild-type, Nod1-, Nod2-, Nod1/Nod2- and Rip2-deficient mice. Consequently, the local and systemic bacterial loads were not altered. Accordingly, neither Nod1 nor Nod2 was involved in the production of the circulating cytokines and in the accumulation of leukocytes in the lungs. By contrast, we showed that MyD88-dependent signaling is crucial for the establishment of the local inflammatory response during CLP-induced sepsis. MyD88-deficient mice were susceptible to sepsis because of an impaired local production of chemokines and defective neutrophil recruitment to the infection site. Altogether, these data show that Nod1, Nod2 and Rip2 are not required for local chemokine production and neutrophil recruitment during CLP-induced sepsis, and they reinforce the importance of MyD88-dependent signaling for initiation of a protective host response.

Heme induces programmed necrosis on macrophages through autocrine TNF and ROS production.

Diseases that cause hemolysis or myonecrosis lead to the leakage of large amounts of heme proteins. Free heme has proinflammatory and cytotoxic effects. Heme induces TLR4-dependent production of tumor necrosis factor (TNF), whereas heme cytotoxicity has been attributed to its ability to intercalate into cell membranes and cause oxidative stress. We show that heme caused early macrophage death characterized by the loss of plasma membrane integrity and morphologic features resembling necrosis. Heme-induced cell death required TNFR1 and TLR4/MyD88-dependent TNF production. Addition of TNF to Tlr4(-/-) or to Myd88(-/-) macrophages restored heme-induced cell death. The use of necrostatin-1, a selective inhibitor of receptor-interacting protein 1 (RIP1, also known as RIPK1), or cells deficient in Rip1 or Rip3 revealed a critical role for RIP proteins in heme-induced cell death. Serum, antioxidants, iron chelation, or inhibition of c-Jun N-terminal kinase (JNK) ameliorated heme-induced oxidative burst and blocked macrophage cell death. Macrophages from heme oxygenase-1 deficient mice (Hmox1(-/-)) had increased oxidative stress and were more sensitive to heme. Taken together, these results revealed that heme induces macrophage necrosis through 2 synergistic mechanisms: TLR4/Myd88-dependent expression of TNF and TLR4-independent generation of ROS.

Nucleotide-binding oligomerization domain-1 and -2 play no role in controlling Brucella abortus infection in mice.

Nucleotide-binding oligomerization domain proteins (NODs) are modular cytoplasmic proteins implicated in the recognition of peptidoglycan-derived molecules. Further, several in vivo studies have demonstrated a role for Nod1 and Nod2 in host defense against bacterial pathogens. Here, we demonstrated that macrophages from NOD1-, NOD2-, and Rip2-deficient mice produced lower levels of TNF-α following infection with live Brucella abortus compared to wild-type mice. Similar reduction on cytokine synthesis was not observed for IL-12 and IL-6. However, NOD1, NOD2, and Rip2 knockout mice were no more susceptible to infection with virulent B. abortus than wild-type mice. Additionally, spleen cells from NOD1-, NOD2-, and Rip2-deficient mice showed unaltered production of IFN-γ compared to C57BL/6 mice. Taken together, this study demonstrates that NOD1, NOD2 and Rip2 are dispensable for the control of B. abortus during in vivo infection.