Yersinia infection induces glucose depletion and AMPK-dependent inhibition of pyroptosis in mice.

Nutritional status and pyroptosis are important for host defence against infections. However, the molecular link that integrates nutrient sensing into pyroptosis during microbial infection is unclear. Here, using metabolic profiling, we found that Yersinia pseudotuberculosis infection results in a significant decrease in intracellular glucose levels in macrophages. This leads to activation of the glucose and energy sensor AMPK, which phosphorylates the essential kinase RIPK1 at S321 during caspase-8-mediated pyroptosis. This phosphorylation inhibits RIPK1 activation and thereby restrains pyroptosis. Boosting the AMPK-RIPK1 cascade by glucose deprivation, AMPK agonists, or RIPK1-S321E knockin suppresses pyroptosis, leading to increased susceptibility to Y. pseudotuberculosis infection in mice. Ablation of AMPK in macrophages or glucose supplementation in mice is protective against infection. Thus, we reveal a molecular link between glucose sensing and pyroptosis, and unveil a mechanism by which Y. pseudotuberculosis reduces glucose levels to impact host AMPK activation and limit host pyroptosis to facilitate infection.

ZBP1-mediated apoptosis and inflammation exacerbate steatotic liver ischemia/reperfusion injury.

Steatotic donor livers are becoming more and more common in liver transplantation. However, the current use of steatotic grafts is less acceptable than normal grafts due to their higher susceptibility to ischemia/reperfusion (I/R) injury. To investigate the mechanism underlying the susceptibility of steatotic liver to I/R injury, we detected cell death markers and inflammation in clinical donor livers and animal models. We found that caspase-8-mediated hepatic apoptosis is activated in steatotic liver I/R injury. However, ablation of caspase-8 only slightly mitigated steatotic liver I/R injury without affecting inflammation. We further demonstrated that RIPK1 kinase induces both caspase-8-mediated apoptosis and cell death-independent inflammation. Inhibition of RIPK1 kinase significantly protects against steatotic liver I/R injury by alleviating both hepatic apoptosis and inflammation. Additionally, we found that RIPK1 activation is induced by Z-DNA binding protein 1 (ZBP1) but not the canonical TNF-α pathway during steatotic liver I/R injury. Deletion of ZBP1 substantially decreases the steatotic liver I/R injury. Mechanistically, ZBP1 is amplified by palmitic acid-activated JNK pathway in steatotic livers. Upon I/R injury, excessive reactive oxygen species trigger ZBP1 activation by inducing its aggregation independent of the Z-nucleic acids sensing action in steatotic livers, leading to the kinase activation of RIPK1 and the subsequent aggravation of liver injury. Thus, ZBP1-mediated RIPK1-driven apoptosis and inflammation exacerbate steatotic liver I/R injury, which could be targeted to protect steatotic donor livers during transplantation.

Meet the authors: Shouqiao Hou and Daichao Xu.

Molecular Cell speaks with first author Shouqiao Hou and corresponding author Daichao Xu about the scientific journey toward their paper, "PARP5A and RNF146 phase separation restrains RIPK1-dependent necroptosis" (in this issue of Molecular Cell), the challenges they faced, and interesting remaining questions for the field.

A palmitoylation-depalmitoylation relay spatiotemporally controls GSDMD activation in pyroptosis.

Gasdermin D (GSDMD) is the executor of pyroptosis, which is important for host defence against pathogen infection. Following activation, caspase-mediated cleavage of GSDMD releases an amino-terminal fragment (GSDMD-NT), which oligomerizes and forms pores in the plasma membrane, leading to cell death and release of proinflammatory cytokines. The spatial and temporal regulation of this process in cells remains unclear. Here we identify GSDMD as a substrate for reversible S-palmitoylation on C192 during pyroptosis. The palmitoyl acyltransferase DHHC7 palmitoylates GSDMD to direct its cleavage by caspases. Subsequently, palmitoylation of GSDMD-NT promotes its translocation to the plasma membrane, where APT2 depalmitoylates GSDMD-NT to unmask the C192 residue and promote GSDMD-NT oligomerization. Perturbation of either palmitoylation or depalmitoylation suppresses pyroptosis, leading to increased survival of mice with lipopolysaccharide-induced lethal septic shock and increased sensitivity to bacterial infection. Our findings reveal a model through which a palmitoylation-depalmitoylation relay spatiotemporally controls GSDMD activation during pyroptosis.

Defective prelamin A processing promotes unconventional necroptosis driven by nuclear RIPK1.

Defects in the prelamin A processing enzyme caused by loss-of-function mutations in the ZMPSTE24 gene are responsible for a spectrum of progeroid disorders characterized by the accumulation of farnesylated prelamin A. Here we report that defective prelamin A processing triggers nuclear RIPK1-dependent signalling that leads to necroptosis and inflammation. We show that accumulated prelamin A recruits RIPK1 to the nucleus to facilitate its activation upon tumour necrosis factor stimulation in ZMPSTE24-deficient cells. Kinase-activated RIPK1 then promotes RIPK3-mediated MLKL activation in the nucleus, leading to nuclear envelope disruption and necroptosis. This signalling relies on prelamin A farnesylation, which anchors prelamin A to nuclear envelope to serve as a nucleation platform for necroptosis. Genetic inactivation of necroptosis ameliorates the progeroid phenotypes in Zmpste24-/- mice. Our findings identify an unconventional nuclear necroptosis pathway resulting from ZMPSTE24 deficiency with pathogenic consequences in progeroid disorder and suggest RIPK1 as a feasible target for prelamin A-associated progeroid disorders.

Reduction of DHHC5-mediated beclin 1 S-palmitoylation underlies autophagy decline in aging.

Autophagy is a lysosome-dependent degradation pathway essential for cellular homeostasis, which decreases with age. However, it is unclear how aging induces autophagy decline. Here we show the role of protein S-palmitoylation in autophagy. We identify the palmitoyl acyltransferase DHHC5 as a regulator of autophagy by mediating the palmitoylation of beclin 1, which in turn promotes the formation of ATG14L-containing class III phosphatidylinositol-3-kinase complex I and its lipid kinase activity by promoting the hydrophobic interactions between beclin 1 and adapter proteins ATG14L and VPS15. In aging brains of human and nonhuman primate, the levels of DHHC5 exhibit a marked decrease in expression. We show that DHHC5 deficiency in neurons leads to reduced cellular protein homeostasis in two established murine models of Alzheimer's disease, which exaggerates neurodegeneration in an autophagy-dependent manner. These findings identify reduction of DHHC5-mediated beclin 1 S-palmitoylation as an underlying mechanism by which aging induces autophagy decline.

PARP5A and RNF146 phase separation restrains RIPK1-dependent necroptosis.

Phase separation is a vital mechanism that mediates the formation of biomolecular condensates and their functions. Necroptosis is a lytic form of programmed cell death mediated by RIPK1, RIPK3, and MLKL downstream of TNFR1 and has been implicated in mediating many human diseases. However, whether necroptosis is regulated by phase separation is not yet known. Here, we show that upon the induction of necroptosis and recruitment by the adaptor protein TAX1BP1, PARP5A and its binding partner RNF146 form liquid-like condensates by multivalent interactions to perform poly ADP-ribosylation (PARylation) and PARylation-dependent ubiquitination (PARdU) of activated RIPK1 in mouse embryonic fibroblasts. We show that PARdU predominantly occurs on the K376 residue of mouse RIPK1, which promotes proteasomal degradation of kinase-activated RIPK1 to restrain necroptosis. Our data demonstrate that PARdU on K376 of mouse RIPK1 provides an alternative cell death checkpoint mediated by phase separation-dependent control of necroptosis by PARP5A and RNF146.

Elevated peripheral levels of receptor-interacting protein kinase 1 (RIPK1) and IL-8 as biomarkers of human amyotrophic lateral sclerosis.

Amyotrophic lateral sclerosis (ALS) is a devastating fatal neurodegenerative disease with no cure. Receptor-interacting protein kinase 1 (RIPK1) has been proposed to mediate pathogenesis of ALS. Primidone has been identified as an old drug that can also inhibit RIPK1 kinase. We conducted a drug-repurposing biomarker study of primidone as a RIPK1 inhibitor using SOD1G93A mice and ALS patients. SOD1G93A mice treated with primidone showed significant delay of symptomatic onset and improved motor performance. One-hundred-sixty-two ALS participants dosed daily with primidone (62.5 mg) completed 24-week follow-up. A significant reduction was showed in serum levels of RIPK1 and IL-8, which were significantly higher in ALS patients than that of healthy controls (P < 0.0001). Serum RIPK1 levels were correlated positively with the severity of bulbar symptoms (P < 0.05). Our study suggests that serum levels of RIPK1 and IL-8 in peripheral can be used as clinical biomarkers for the activation of RIPK1 in central nervous system in human ALS patients. Repurposing primidone may provide a promising therapeutic strategy for ALS. The effect of primidone for the treatment of other inflammatory diseases may also be considered, since the activation of RIPK1 has been implicated in mediating a variety of inflammatory diseases including COVID-19-associated cytokine release syndrome (CRS). (ChiCTR2200060149).

Prolonged hypoxia alleviates prolyl hydroxylation-mediated suppression of RIPK1 to promote necroptosis and inflammation.

The prolyl hydroxylation of hypoxia-inducible factor 1α (HIF-1α) mediated by the EGLN-pVHL pathway represents a classic signalling mechanism that mediates cellular adaptation under hypoxia. Here we identify RIPK1, a known regulator of cell death mediated by tumour necrosis factor receptor 1 (TNFR1), as a target of EGLN1-pVHL. Prolyl hydroxylation of RIPK1 mediated by EGLN1 promotes the binding of RIPK1 with pVHL to suppress its activation under normoxic conditions. Prolonged hypoxia promotes the activation of RIPK1 kinase by modulating its proline hydroxylation, independent of the TNFα-TNFR1 pathway. As such, inhibiting proline hydroxylation of RIPK1 promotes RIPK1 activation to trigger cell death and inflammation. Hepatocyte-specific Vhl deficiency promoted RIPK1-dependent apoptosis to mediate liver pathology. Our findings illustrate a key role of the EGLN-pVHL pathway in suppressing RIPK1 activation under normoxic conditions to promote cell survival and a model by which hypoxia promotes RIPK1 activation through modulating its proline hydroxylation to mediate cell death and inflammation in human diseases, independent of TNFR1.

Metabolic orchestration of cell death by AMPK-mediated phosphorylation of RIPK1.

Adenosine monophosphate-activated protein kinase (AMPK) activity is stimulated to promote metabolic adaptation upon energy stress. However, sustained metabolic stress may cause cell death. The mechanisms by which AMPK dictates cell death are not fully understood. We report that metabolic stress promoted receptor-interacting protein kinase 1 (RIPK1) activation mediated by TRAIL receptors, whereas AMPK inhibited RIPK1 by phosphorylation at Ser415 to suppress energy stress-induced cell death. Inhibiting pS415-RIPK1 by Ampk deficiency or RIPK1 S415A mutation promoted RIPK1 activation. Furthermore, genetic inactivation of RIPK1 protected against ischemic injury in myeloid Ampkα1-deficient mice. Our studies reveal that AMPK phosphorylation of RIPK1 represents a crucial metabolic checkpoint, which dictates cell fate response to metabolic stress, and highlight a previously unappreciated role for the AMPK-RIPK1 axis in integrating metabolism, cell death, and inflammation.

UDP-glucuronate metabolism controls RIPK1-driven liver damage in nonalcoholic steatohepatitis.

Hepatocyte apoptosis plays an essential role in the progression of nonalcoholic steatohepatitis (NASH). However, the molecular mechanisms underlying hepatocyte apoptosis remain unclear. Here, we identify UDP-glucose 6-dehydrogenase (UGDH) as a suppressor of NASH-associated liver damage by inhibiting RIPK1 kinase-dependent hepatocyte apoptosis. UGDH is progressively reduced in proportion to NASH severity. UGDH absence from hepatocytes hastens the development of liver damage in male mice with NASH, which is suppressed by RIPK1 kinase-dead knockin mutation. Mechanistically, UGDH suppresses RIPK1 by converting UDP-glucose to UDP-glucuronate, the latter directly binds to the kinase domain of RIPK1 and inhibits its activation. Recovering UDP-glucuronate levels, even after the onset of NASH, improved liver damage. Our findings reveal a role for UGDH and UDP-glucuronate in NASH pathogenesis and uncover a mechanism by which UDP-glucuronate controls hepatocyte apoptosis by targeting RIPK1 kinase, and suggest UDP-glucuronate metabolism as a feasible target for more specific treatment of NASH-associated liver damage.

SENP1 prevents steatohepatitis by suppressing RIPK1-driven apoptosis and inflammation.

Activation of RIPK1-driven cell death and inflammation play important roles in the progression of nonalcoholic steatohepatitis (NASH). However, the mechanism underlying RIPK1 activation in NASH remains unclear. Here we identified SENP1, a SUMO-specific protease, as a key endogenous inhibitor of RIPK1. SENP1 is progressively reduced in proportion to NASH severity in patients. Hepatocyte-specific SENP1-knockout mice develop spontaneous NASH-related phenotypes in a RIPK1 kinase-dependent manner. We demonstrate that SENP1 deficiency sensitizes cells to RIPK1 kinase-dependent apoptosis by promoting RIPK1 activation following TNFα stimulation. Mechanistically, SENP1 deSUMOylates RIPK1 in TNF-R1 signaling complex (TNF-RSC), keeping RIPK1 in check. Loss of SENP1 leads to SUMOylation of RIPK1, which re-orchestrates TNF-RSC and modulates the ubiquitination patterns and activity of RIPK1. Notably, genetic inhibition of RIPK1 effectively reverses disease progression in hepatocyte-specific SENP1-knockout male mice with high-fat-diet-induced nonalcoholic fatty liver. We propose that deSUMOylation of RIPK1 by SENP1 provides a pathophysiologically relevant cell death-restricting checkpoint that modulates RIPK1 activation in the pathogenesis of nonalcoholic steatohepatitis.

Nuclear RIPK1 promotes chromatin remodeling to mediate inflammatory response.

RIPK1 is a master regulator of multiple cell death pathways, including apoptosis and necroptosis, and inflammation. Importantly, activation of RIPK1 has also been shown to promote the transcriptional induction of proinflammatory cytokines in cells undergoing necroptosis, in animal models of amyotrophic lateral sclerosis (ALS) and Alzheimer's disease (AD), and in human ALS and AD. Rare human genetic carriers of non-cleavable RIPK1 variants (D324V and D324H) exhibit distinct symptoms of recurrent fevers and increased transcription of proinflammatory cytokines. Multiple RIPK1 inhibitors have been advanced into human clinical trials as new therapeutics for human inflammatory and neurodegenerative diseases, such as ALS and AD. However, it is unclear whether and how RIPK1 kinase activity directly mediates inflammation independent of cell death as the nuclear function of RIPK1 has not yet been explored. Here we show that nuclear RIPK1 is physically associated with the BAF complex. Upon RIPK1 activation, the RIPK1/BAF complex is recruited by specific transcription factors to active enhancers and promoters marked by H3K4me1 and H3K27ac. Activated nuclear RIPK1 mediates the phosphorylation of SMARCC2, a key component of the BAF complex, to promote chromatin remodeling and the transcription of specific proinflammatory genes. Increased nuclear RIPK1 activation and RIPK1/BAF-mediated chromatin-remodeling activity were found in cells expressing non-cleavable RIPK1, and increased enrichment of activated RIPK1 on active enhancers and promoters was found in an animal model and human pathological samples of ALS. Our results suggest that RIPK1 kinase serves as a transcriptional coregulator in nucleus that can transmit extracellular stimuli to the BAF complex to modulate chromatin accessibility and directly regulate the transcription of specific genes involved in mediating inflammatory responses.

Genetic Regulation of RIPK1 and Necroptosis.

The receptor-interacting protein kinase 1 (RIPK1) is recognized as a master upstream regulator that controls cell survival and inflammatory signaling as well as multiple cell death pathways, including apoptosis and necroptosis. The activation of RIPK1 kinase is extensively modulated by ubiquitination and phosphorylation, which are mediated by multiple factors that also control the activation of the NF-κB pathway. We discuss current findings regarding the genetic modulation of RIPK1 that controls its activation and interaction with downstream mediators, such as caspase-8 and RIPK3, to promote apoptosis and necroptosis. We also address genetic autoinflammatory human conditions that involve abnormal activation of RIPK1. Leveraging these new genetic and mechanistic insights, we postulate how an improved understanding of RIPK1 biology may support the development of therapeutics that target RIPK1 for the treatment of human inflammatory and neurodegenerative diseases.

RIPK1 regulates starvation resistance by modulating aspartate catabolism.

RIPK1 is a crucial regulator of cell death and survival. Ripk1 deficiency promotes mouse survival in the prenatal period while inhibits survival in the early postnatal period without a clear mechanism. Metabolism regulation and autophagy are critical to neonatal survival from severe starvation at birth. However, the mechanism by which RIPK1 regulates starvation resistance and survival remains unclear. Here, we address this question by discovering the metabolic regulatory role of RIPK1. First, metabolomics analysis reveals that Ripk1 deficiency specifically increases aspartate levels in both mouse neonates and mammalian cells under starvation conditions. Increased aspartate in Ripk1-/- cells enhances the TCA  flux and ATP production. The energy imbalance causes defective autophagy induction by inhibiting the AMPK/ULK1 pathway. Transcriptional analyses demonstrate that Ripk1-/- deficiency downregulates gene expression in aspartate catabolism by inactivating SP1. To summarize, this study reveals that RIPK1 serves as a metabolic regulator responsible for starvation resistance.

Modulating TRADD to restore cellular homeostasis and inhibit apoptosis.

Cell death in human diseases is often a consequence of disrupted cellular homeostasis. If cell death is prevented without restoring cellular homeostasis, it may lead to a persistent dysfunctional and pathological state. Although mechanisms of cell death have been thoroughly investigated1-3, it remains unclear how homeostasis can be restored after inhibition of cell death. Here we identify TRADD4-6, an adaptor protein, as a direct regulator of both cellular homeostasis and apoptosis. TRADD modulates cellular homeostasis by inhibiting K63-linked ubiquitination of beclin 1 mediated by TRAF2, cIAP1 and cIAP2, thereby reducing autophagy. TRADD deficiency inhibits RIPK1-dependent extrinsic apoptosis and proteasomal stress-induced intrinsic apoptosis. We also show that the small molecules ICCB-19 and Apt-1 bind to a pocket on the N-terminal TRAF2-binding domain of TRADD (TRADD-N), which interacts with the C-terminal domain (TRADD-C) and TRAF2 to modulate the ubiquitination of RIPK1 and beclin 1. Inhibition of TRADD by ICCB-19 or Apt-1 blocks apoptosis and restores cellular homeostasis by activating autophagy in cells with accumulated mutant tau, α-synuclein, or huntingtin. Treatment with Apt-1 restored proteostasis and inhibited cell death in a mouse model of proteinopathy induced by mutant tau(P301S). We conclude that pharmacological targeting of TRADD may represent a promising strategy for inhibiting cell death and restoring homeostasis to treat human diseases.

Sequential activation of necroptosis and apoptosis cooperates to mediate vascular and neural pathology in stroke.

Apoptosis and necroptosis are two regulated cell death mechanisms; however, the interaction between these cell death pathways in vivo is unclear. Here we used cerebral ischemia/reperfusion as a model to investigate the interaction between apoptosis and necroptosis. We show that the activation of RIPK1 sequentially promotes necroptosis followed by apoptosis in a temporally specific manner. Cerebral ischemia/reperfusion insult rapidly activates necroptosis to promote cerebral hemorrhage and neuroinflammation. Ripk3 deficiency reduces cerebral hemorrhage and delays the onset of neural damage mediated by inflammation. Reduced cerebral perfusion resulting from arterial occlusion promotes the degradation of TAK1, a suppressor of RIPK1, and the transition from necroptosis to apoptosis. Conditional knockout of TAK1 in microglial/infiltrated macrophages and neuronal lineages sensitizes to ischemic infarction by promoting apoptosis. Taken together, our results demonstrate the critical role of necroptosis in mediating neurovascular damage and hypoperfusion-induced TAK1 loss, which subsequently promotes apoptosis and cerebral pathology in stroke and neurodegeneration.

TBK1 Suppresses RIPK1-Driven Apoptosis and Inflammation during Development and in Aging.

Aging is a major risk factor for both genetic and sporadic neurodegenerative disorders. However, it is unclear how aging interacts with genetic predispositions to promote neurodegeneration. Here, we investigate how partial loss of function of TBK1, a major genetic cause for amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) comorbidity, leads to age-dependent neurodegeneration. We show that TBK1 is an endogenous inhibitor of RIPK1 and the embryonic lethality of Tbk1-/- mice is dependent on RIPK1 kinase activity. In aging human brains, another endogenous RIPK1 inhibitor, TAK1, exhibits a marked decrease in expression. We show that in Tbk1+/- mice, the reduced myeloid TAK1 expression promotes all the key hallmarks of ALS/FTD, including neuroinflammation, TDP-43 aggregation, axonal degeneration, neuronal loss, and behavior deficits, which are blocked upon inhibition of RIPK1. Thus, aging facilitates RIPK1 activation by reducing TAK1 expression, which cooperates with genetic risk factors to promote the onset of ALS/FTD.

Necroptosis promotes cell-autonomous activation of proinflammatory cytokine gene expression.

Necroptosis, a form of regulated necrotic cell death, is mediated by receptor interacting protein 1 (RIPK1), RIPK3, and mixed lineage kinase domain-like protein (MLKL). However, the mechanism by which necroptosis promotes inflammation is still unclear. Here we report that the expression of cytokines is robustly upregulated in a cell-autonomous manner during necroptosis induced by tumor necrosis factor alpha (TNFα). We demonstrate that TNFα-induced necroptosis leads to two waves of cytokine production. The first wave, more transient and weaker than the second, is in response to TNFα alone; whereas the second wave depends upon the necroptotic signaling. We show that necroptosis promotes the transcription of TNFα-target genes in a cell-intrinsic manner. The activation of both NF-κB and p38 by the necroptotic machinery, RIPK1, RIPK3, and MLKL, is involved in mediating the robust induction of cytokine expression in the second wave. In contrast, necroptosis induced by direct oligomerization of MLKL promotes cytokine production at much lower levels than that of necroptosis induced with TNFα. Thus, we conclude that TNFα-induced necroptosis signaling events mediated by RIPK1 and RIPK3 activation, in addition to the MLKL oligomerization, promotes the expression of cytokines involving multiple intracellular signaling mechanisms including NF-κB pathway and p38. These findings reveal that the necroptotic cell death machinery mounts an immune response by promoting cell-autonomous production of cytokines. Our study provides insights into the mechanism by which necroptosis promotes inflammation in human diseases.

Structural insights into the ubiquitin recognition by OPTN (optineurin) and its regulation by TBK1-mediated phosphorylation.

OPTN (optineurin), a ubiquitin-binding scaffold protein, functions as an important macroautophagy/autophagy receptor in selective autophagy processes. Mutations in OPTN have been linked with human neurodegenerative diseases including ALS and glaucoma. However, the mechanistic basis underlying the recognition of ubiquitin by OPTN and its regulation by TBK1-mediated phosphorylation are still elusive. Here, we demonstrate that the UBAN domain of OPTN preferentially recognizes linear ubiquitin chain and forms an asymmetric 2:1 stoichiometry complex with the linear diubiquitin. In addition, our results provide new mechanistic insights into how phosphorylation of UBAN would regulate the ubiquitin-binding ability of OPTN and how disease-associated mutations in the OPTN UBAN domain disrupt its interaction with ubiquitin. Finally, we show that defects in ubiquitin-binding may affect the recruitment of OPTN to linear ubiquitin-decorated mutant Huntington protein aggregates. Taken together, our findings clarify the interaction mode between UBAN and linear ubiquitin chain in general, and expand our knowledge of the molecular mechanism of ubiquitin-decorated substrates recognition by OPTN as well as the pathogenesis of neurodegenerative diseases caused by OPTN mutations.