Mitochondrial Trafficking of MLKL, Bak/Bax, and Drp1 Is Mediated by RIP1 and ROS which Leads to Decreased Mitochondrial Membrane Integrity during the Hyperglycemic Shift to Necroptosis

Apoptosis and necroptosis overlap in their initial signaling but diverge to produce non-inflammatory and pro-inflammatory outcomes, respectively. High glucose pushes signaling in favor of necroptosis producing a hyperglycemic shift from apoptosis to necroptosis. This shift depends on receptor-interacting protein 1 (RIP1) and mitochondrial reactive oxygen species (ROS). Here, we show that RIP1, mixed lineage kinase domain-like (MLKL) protein, Bcl-2 agonist/killer (Bak), Bcl-2 associated x (Bax) protein, and dynamin-related protein 1 (Drp1) traffic to the mitochondria in high glucose. RIP1 and MLKL appear in the mitochondria in their activated, phosphorylated states while Drp1 appears in its activated, dephosphorylated state in high glucose. Mitochondrial trafficking is prevented in rip1 KO cells and upon treatment with N-acetylcysteine. Induction of ROS replicated the mitochondrial trafficking seen in high glucose. MLKL forms high MW oligomers in the outer and inner mitochondrial membranes while Bak and Bax form high MW oligomers in the outer mitochondrial membrane in high glucose, suggesting pore formation. MLKL, Bax, and Drp1 promoted cytochrome c release from the mitochondria as well as a decrease in mitochondrial membrane potential in high glucose. These results indicate that mitochondrial trafficking of RIP1, MLKL, Bak, Bax, and Drp1 are key events in the hyperglycemic shift from apoptosis to necroptosis. This is also the first report to show oligomerization of MLKL in the inner and outer mitochondrial membranes and dependence of mitochondrial permeability on MLKL.

Necroptosis of Lung Epithelial Cells Triggered by Ricin Toxin and Bystander Inflammation

BACKGROUND/AIMS

The ribosome-inactivating proteins include the biothreat agent, ricin toxin (RT). When inhaled, RT causes near complete destruction of the lung epithelium coincident with a proinflammatory response that includes TNF family cytokines, which are death-inducing ligands. We previously demonstrated that the combination of RT and TNF-related apoptosis inducing ligand (TRAIL) induces caspase-dependent apoptosis, while RT and TNF-α or RT and Fas ligand (FasL) induces cathepsin-dependent cell death in lung epithelial cells. We hypothesize that airway macrophages constitute a major source of cytokines that drive lung epithelial cell death.

METHODS

Here, we show that RT-induced apoptosis of the monocytic cell line, U937, leads to the bystander killing of the lung epithelial cell line, A549. U937 cells were treated with ricin. Following this, A549 cells were treated with supernatants from U937 cells and death was measured by WST-1 viability assay.

RESULTS

Upon RT-induced U937 cell death, released RT and FasL contributed to A549 cell death. U937 cells also released nuclear protein HMGB1. The release of RT, FasL, and HMGB1 triggered A549 cell necroptosis, rather than cathepsin-dependent killing observed previously with RT and FasL. Reactive oxygen species (ROS) were produced in A549 cells due to HMGB1 ligation of the receptor for advanced glycation end products (RAGE).

CONCLUSION

These findings demonstrate the potential for bystander necroptosis of lung epithelial cells during RT toxicosis which may perpetuate or increase the proinflammatory response.

Sensitization of Airway Epithelial Cells to Toxin-Induced Death by TNF Superfamily Cytokines

The TNF superfamily of proinflammatory and proapoptotic cytokines influence tissue-wide responses to molecular insults such as small molecules, toxins, and viral infections that perturb cellular homeostasis at the level of DNA replication, transcription, and translation. In the context of acute lung injury, for example, TNF superfamily members like TNF-α and TRAIL can severely exacerbate disease pathophysiology. This chapter describes a systematic approach to optimization of mammalian cell viability assays and transcriptional profiling through nCounter^® Technology to permit a detailed examination of how TNF-α and TRAIL modulate programmed cell death pathways in concert with ricin toxin, a ribosome-inactivating protein (RIP) and a potent inducer of acute respiratory distress. We compare two widely used luciferase- and colorimetric-based cell viability assays and provide optimization protocols for adherent and non-adherent cell lines. We provide a computational workflow to facilitate downstream analysis of datasets generated from nCounter^® gene expression panels. While combined treatment with ricin toxin and TRAIL serves as the exemplar, the methodologies are applicable to any TNF superfamily member in combination with any biological agent of interest.

Mitochondrial ROS prime the hyperglycemic shift from apoptosis to necroptosis

We have previously identified a shift from TNF-α-induced apoptosis to necroptosis that occurs under hyperglycemic conditions. This shift involves the downregulation or silencing of caspases and concurrent upregulation of necroptotic proteins leading to activation of the necrosome. In addition, under hyperglycemic conditions in vivo, this shift in cell death mechanisms exacerbates neonatal hypoxia-ischemia (HI) brain injury. Here, we identify two major factors that drive the hyperglycemic shift to necroptosis: (1) reactive oxygen species (ROS) and (2) receptor-interacting protein kinase 1 (RIP1). ROS, including mitochondrial superoxide, led to the oxidation of RIP1, as well as formation and activation of the necrosome. Concurrently, ROS mediate a decrease in the levels and activation of executioner caspases-3, -6, and -7. Importantly, hyperglycemia and mitochondrial ROS result in the oxidation of RIP1 and loss of executioner caspases prior to death receptor engagement by TNF-α. Moreover, RIP1 partially controlled levels of mitochondrial ROS in the context of hyperglycemia. As a result of its regulation of ROS, RIP1 also regulated necrosome activation and caspase loss. Mitochondrial ROS exacerbated neonatal HI-brain injury in hyperglycemic mice, as a result of the shift from apoptosis to necroptosis.

Cell Fractionation of U937 Cells in the Absence of High-speed Centrifugation

In this protocol we detail a method to obtain subcellular fractions of U937 cells without the use of ultracentrifugation or indiscriminate detergents. This method utilizes hypotonic buffers, digitonin, mechanical lysis and differential centrifugation to isolate the cytoplasm, mitochondria and plasma membrane. The process can be scaled to accommodate the needs of researchers, is inexpensive and straightforward. This method will allow researchers to determine protein localization in cells without specialized centrifuges and without the use of commercial kits, both of which can be prohibitively expensive. We have successfully used this method to separate cytosolic, plasma membrane and mitochondrial proteins in the human monocyte cell line U937.

Storage Primes Erythrocytes for Necroptosis and Clearance

BACKGROUND/AIMS

Like nucleated cells, erythrocytes (red blood cells, RBCs) are capable of executing programmed cell death pathways. RBCs undergo necroptosis in response to CD59-specific pore-forming toxins (PFTs). The relationship between blood bank storage and RBC necroptosis was explored in this study.

METHODS

Human RBCs were stored in standard blood bank additive solutions (AS-1, AS-3, or AS-5) for 1 week and hemolysis was evaluated in the context of necroptosis inhibitors and reactive oxygen species (ROS) scavengers. Activation of key factors including RIP1, RIP3, and MLKL was determined using immunoprecipitations and western blot. RBC vesiculation and formation of echinocytes was determined using phase-contrast microscopy. The effect of necroptosis and storage on RBC clearance was determined using a murine transfusion model.

RESULTS

Necroptosis is associated with increased RBC clearance post-transfusion. Moreover, storage in AS-1, AS-3, or AS-5 sensitizes RBCs for necroptosis. Importantly, storage-sensitized RBCs undergo necroptosis in response to multiple PFTs, regardless of specificity for CD59. Storage-sensitized RBCs undergo necroptosis via NADPH oxidase-generated ROS. RBC storage led to RIP1 phosphorylation and necrosome formation in an NADPH oxidase-dependent manner suggesting the basis for this sensitization. In addition, storage led to increased RBC clearance post-transfusion. Clearance of these RBCs was due to Syk-dependent echinocyte formation.

CONCLUSION

Storage-induced sensitization to RBC necroptosis and clearance is important as it may be relevant to hemolytic transfusion reactions.

Environmental pH modulates inerolysin activity via post-binding blockade

The cholesterol dependent cytolysins (CDCs) are a family of pore-forming toxins produced by a wide range of bacteria. Some CDCs are important virulence factors for their cognate organisms, but their activity must be tightly regulated to ensure they operate at appropriate times and within the appropriate subcellular compartments. pH-dependent activity has been described for several CDCs, but the mechanism of such regulation has been studied in depth only for listeriolysin O (LLO), which senses environmental pH through a triad of acidic residues that mediate protein unfolding. Here we present data supporting a distinct mechanism for pH-dependence for inerolysin (INY), the CDC produced by Lactobacillus iners. Inerolysin (INY) has an acidic pH optimum with loss of activity at neutral pH. INY pH-dependence is characterized by reversible loss of pore formation with preservation of membrane binding. Fluorescent membrane probe assays indicated that INY insertion into host cell membranes, but not oligomerization, was defective at neutral pH. These data support the existence of a newly appreciated form of CDC pH-dependence functioning at a late stage of pore formation.

Hyperglycemic Conditions Prime Cells for RIP1-dependent Necroptosis

Necroptosis is a RIP1-dependent programmed cell death (PCD) pathway that is distinct from apoptosis. Downstream effector pathways of necroptosis include formation of advanced glycation end products (AGEs) and reactive oxygen species (ROS), both of which depend on glycolysis. This suggests that increased cellular glucose may prime necroptosis. Here we show that exposure to hyperglycemic levels of glucose enhances necroptosis in primary red blood cells (RBCs), Jurkat T cells, and U937 monocytes. Pharmacologic or siRNA inhibition of RIP1 prevented the enhanced death, confirming it as RIP1-dependent necroptosis. Hyperglycemic enhancement of necroptosis depends upon glycolysis with AGEs and ROS playing a role. Total levels of RIP1, RIP3, and mixed lineage kinase domain-like (MLKL) proteins were increased following treatment with high levels of glucose in Jurkat and U937 cells and was not due to transcriptional regulation. The observed increase in RIP1, RIP3, and MLKL protein levels suggests a potential positive feedback mechanism in nucleated cell types. Enhanced PCD due to hyperglycemia was specific to necroptosis as extrinsic apoptosis was inhibited by exposure to high levels of glucose. Hyperglycemia resulted in increased infarct size in a mouse model of brain hypoxia-ischemia injury. The increased infarct size was prevented by treatment with nec-1s, strongly suggesting that increased necroptosis accounts for exacerbation of this injury in conditions of hyperglycemia. This work reveals that hyperglycemia represents a condition in which cells are extraordinarily susceptible to necroptosis, that local glucose levels alter the balance of PCD pathways, and that clinically relevant outcomes may depend on glucose-mediated effects on PCD.

Proving lipid rafts exist: membrane domains in the prokaryote Borrelia burgdorferi have the same properties as eukaryotic lipid rafts

Lipid rafts in eukaryotic cells are sphingolipid and cholesterol-rich, ordered membrane regions that have been postulated to play roles in many membrane functions, including infection. We previously demonstrated the existence of cholesterol-lipid-rich domains in membranes of the prokaryote, B. burgdorferi, the causative agent of Lyme disease [LaRocca et al. (2010) Cell Host & Microbe 8, 331–342]. Here, we show that these prokaryote membrane domains have the hallmarks of eukaryotic lipid rafts, despite lacking sphingolipids. Substitution experiments replacing cholesterol lipids with a set of sterols, ranging from strongly raft-promoting to raft-inhibiting when mixed with eukaryotic sphingolipids, showed that sterols that can support ordered domain formation are both necessary and sufficient for formation of B. burgdorferi membrane domains that can be detected by transmission electron microscopy or in living organisms by Förster resonance energy transfer (FRET). Raft-supporting sterols were also necessary and sufficient for formation of high amounts of detergent resistant membranes from B. burgdorferi. Furthermore, having saturated acyl chains was required for a biotinylated lipid to associate with the cholesterol-lipid-rich domains in B. burgdorferi, another characteristic identical to that of eukaryotic lipid rafts. Sterols supporting ordered domain formation were also necessary and sufficient to maintain B. burgdorferi membrane integrity, and thus critical to the life of the organism. These findings provide compelling evidence for the existence of lipid rafts and show that the same principles of lipid raft formation apply to prokaryotes and eukaryotes despite marked differences in their lipid compositions.

Specialized domains (“lipid rafts”) rich in specific membrane lipids (sphingolipids and cholesterol) have been proposed to form in the cell membranes of higher organisms, and to be of functional importance. We recently found that domains can be detected in the membranes of the bacterium that causes Lyme disease, Borrelia burgdorferi. In this report it is shown that, despite a lack of sphingolipids in B. burgdorferi, these domains have all the characteristic properties of lipid rafts, and can be detected in living B. burgdorferi. This shows that true lipid rafts can form in bacteria. In addition, it is shown that sterols having a structure that promotes lipid raft formation are necessary and sufficient for those sterols to maintain B. burgdorferi membrane integrity. This is suggestive of a role for membrane domains in B. burgdorferi membrane integrity. Therefore, interfering with lipid raft formation may have biomedical applications in combatting B. burgdorferi infections.

Cholesterol lipids of Borrelia burgdorferi form lipid rafts and are required for the bactericidal activity of a complement-independent antibody

Borrelia burgdorferi, the agent of Lyme disease, is unusual as it contains free cholesterol and cholesterol glycolipids. It is also susceptible to complement-independent bactericidal antibodies, such as CB2, a monoclonal IgG1 against outer surface protein B (OspB). We find that the bactericidal action of CB2 requires the presence of cholesterol glycolipids and cholesterol. Ultrastructural, biochemical, and biophysical analysis revealed that the bacterial cholesterol glycolipids exist as lipid raft-like microdomains in the outer membrane of cultured and mouse-derived B. burgdorferi and in model membranes from B. burgdorferi lipids. The order and size of the microdomains are temperature sensitive and correlate with the bactericidal activity of CB2. This study demonstrates the existence of cholesterol-containing lipid raft-like microdomains in a prokaryote, and we suggest that the temperature dependence of B. burgdorferi lipid raft organization may have significant implications in the transmission cycle of the spirochetes which are exposed to a range of temperatures.

Bactericidal action of a complement-independent antibody against relapsing fever Borrelia resides in its variable region

A single chain variable fragment (scFv) of CB515, a complement-independent bactericidal monoclonal IgM against a relapsing fever Borrelia, was constructed to investigate the region wherein the unique bactericidal function resides. Monomeric CB515 scFv (26 kDa) was capable of binding its Ag on whole organisms and by immunoblot. This binding was shown to be species and serotype-specific to the 19 kDa variable small protein, recognized by its parent monoclonal IgM. A dose-dependent bactericidal effect of the CB515 scFv was detected by direct enumeration of spirochetes. Spirochetes incubated with the CB515 scFv before inoculation into mice grew into escape mutants, whereas spirochetes incubated with an irrelevant scFv developed as the original infecting serotype. This bactericidal effect, as seen at the ultrastructural level, was due to disruption of the outer membrane and to severe membrane blebbing eventually progressing to lysis. These results indicate that the variable region of CB515 is responsible for this bactericidal activity and that the constant region of the Ab is dispensable.