Roles of Rac and cytosolic phospholipase A2 in the intracellular signalling in response to titanium particles

GSK-3β suppression upregulates Gli1 to alleviate osteogenesis inhibition in titanium nanoparticle-induced osteolysis

Wear particle-induced periprosthetic osteolysis (PPO) have become a major reason of joint arthroplasty failure and secondary surgery following joint arthroplasty and thus pose a severe threat to global public health. Therefore, determining how to effectively suppress particle-induced PPO has become an urgent problem. The pathological mechanism involved in the PPO signaling cascade is still unclear. Recently, the interaction between osteogenic inhibition and wear particles at the implant biological interface, which has received increasing attention, has been revealed as an important factor in pathological process. Additionally, Hedgehog (Hh)-Gli1 is a crucial signaling cascade which was regulated by multiple factors in numerous physiological and pathological process. It was revealed to exert a crucial part during embryonic bone development and metabolism. However, whether Hh-Gli1 is involved in wear particle-induced osteogenic inhibition in PPO remains unknown. Our present study explored the mechanism by which the Hh-Gli1 signaling cascade regulates titanium (Ti) nanoparticle-induced osteolysis. We found that Hh-Gli1 signaling was dramatically downregulated upon Ti particle treatment. Mechanistically, glycogen synthesis kinase 3β (GSK-3β) activation was significantly increased in Ti particle-induced osteogenic inhibition via changes in GSK-3β phosphorylation level and was found to participate in the posttranslational modification and degradation of the key transcription factor Gli1, thus decreasing the accumulation of Gli1 and its translocation from the cytoplasm to the nucleus. Collectively, these findings suggest that the Hh-Gli1 signaling cascade utilizes a GSK3β-mediated mechanism and may serve as a rational new therapeutic target against nanoparticle-induced PPO.

MiR-25 overexpression inhibits titanium particle-induced osteoclast differentiation via down-regulation of mitochondrial calcium uniporter in vitro

Background

Mitochondrial calcium uniporter (MCU) is an important ion channel regulating calcium transport across the mitochondrial membrane. Calcium signaling, particularly via the Ca²⁺/NFATc1 pathway, has been identified as an important mediator of the osteoclast differentiation that leads to osteolysis around implants. The present study aimed to investigate whether down-regulation of MCU using microRNA-25 (miR-25) mimics could reduce osteoclast differentiation induced upon exposure to titanium (Ti) particles.

Methods

Ti particles were prepared. Osteoclast differentiation of RAW264.7 cells was induced by adding Ti particles and determined by TRAP staining. Calcium oscillation was determined using a dual-wavelength technique. After exposure of the cells in each group to Ti particles or control medium for 5 days, relative MCU and NFATc1 mRNA expression levels were determined by RT-qPCR. MCU and NFATc1 protein expression was determined by western blotting. NFATc1 activation was determined by immunofluorescence staining. Comparisons among multiple groups were conducted using one-way analysis of variance followed by Tukey test, and differences were considered significant if p < 0.05.

Results

MCU expression was reduced in response to miR-25 overexpression during the process of RAW 264.7 cell differentiation induced by Ti particles. Furthermore, osteoclast formation was inhibited, as evidenced by the low amplitude of calcium ion oscillation, reduced NFATc1 activation, and decreased mRNA and protein expression levels of nuclear factor-κB p65 and calmodulin kinases II/IV.

Conclusions

Regulation of MCU expression can impact osteoclast differentiation, and the underlying mechanism likely involves the Ca²⁺/NFATc1 signal pathway. Therefore, MCU may be a promising target in the development of new strategies to prevent and treat periprosthetic osteolysis.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13018-022-03030-7.

Inhibition of protein phosphatase 2A attenuates titanium-particle induced suppression of bone formation

Peri-prosthetic osteolysis (PPO) often generates after total joint arthroplasty, which can bring implant failure and following revision surgery. Wear debris shed from prostheses strongly enhances bone resorption and attenuates bone formation in osteolytic process. We previously proved that suppression of protein phosphatase 2A (PP2A), a major serine-threonine phosphatase, inhibited wear-debris-induced osteoclastogenesis and alleviated local osteolysis. Whether PP2A inhibition facilitates osteoblastogenesis and bone formation in the osteolytic sites remains unclear. Here, we observed that PP2A inhibition with a selective inhibitor attenuated particle-induced bone destruction by accelerating osteoblast differentiation and promoting bone regeneration. Meanwhile, we proved inhibition of PP2A alleviated the inhibition of osteogenic differentiation by titanium particles in MC3T3-E1 cells. In addition, PP2A inhibition increased β-catenin expression and enhanced β-catenin nuclear translocation, compared with that in the vehicle group. ICG-001, a specific inhibitor of β-catenin, was further applied and was found to weaken the effect of PP2A inhibition on β-catenin expression and nuclear translocation. Therefore, we demonstrated PP2A inhibition exerts protective effects on osteogenic differentiation mainly by activating Wnt/β-catenin signaling pathway. Thus, all the results further revealed PP2A could be a promising target for treating PPO and other bone related diseases.

Targeting Anion Exchange of Osteoclast, a New Strategy for Preventing Wear Particles Induced- Osteolysis

Joint replacement is essential for the treatment of serious joint disease. However, prosthetic failure remains an important clinical issue, with periprosthesis osteolysis (PO), caused by osteoclastic bone resorption induced by wear particles, being the leading cause of failure. Nuclear factor of activated T cells c1 (NFATc1) appears to play an important role in wear particle-induced osteoclastogenesis, with bicarbonate/chloride exchanger, solute carrier family 4, anion exchanger, member 2, (SLC4A2) being upregulated during osteoclastogenesis in an NFATc1-dependent manner. Anion exchange mediated by SLC4A2 in osteoclasts could affect the bone resorption activity by regulating pHi. This study investigated the role and mechanism of SLC4A2 in wear particle-induced osteoclast differentiation and function in vitro. The use of 4, 4'-diisothiocyano-2,2'-stilbenedisulfonic acid (DIDS), an anion exchange inhibitor, suppressed wear particle-induced PO in vivo. Furthermore, controlled release of DIDS from chitosan microspheres can strengthen the PO therapy effect. Therefore, anion exchange mediated by osteoclastic SLC4A2 may be a potential therapeutic target for the treatment of aseptic loosening of artificial joints.

Titanium particle‑mediated osteoclastogenesis may be attenuated via bidirectional ephrin‑B2/eph‑B4 signaling in vitro

The present study investigated the role of bidirectional ephrin-B2/erythropoietin-producing human hepatocellular receptor 4 (ephB4) signaling in the regulation of wear particle-mediated osteoclastogenesis in vitro. Mouse bone marrow macrophages (BMMs) were induced into osteoclasts by receptor activator of nuclear factor-κB ligand (RANKL, 50 ng/ml). EphB4-Fc, an osteoblast membrane surface receptor (4 µg/ml), was used to stimulate the ephrin-B2 ligand of osteoclasts in the presence and absence of titanium (Ti). Tartrate-resistant acid phosphatase (TRAP) staining was used to detect the number of osteoclasts, and phalloidin staining was used to examine the cytoskeletons of the osteoclasts. A bone pit absorption experiment was used to measure osteoclast function. Reverse transcription quantitative polymerase chain reaction and western blot analysis were used to examine osteoclastogenesis. ELISAs were used to detect the production of inflammatory factors. The data demonstrated that Ti significantly promoted the differentiation of BMMs into mature osteoclasts in the presence of RANKL and significantly promoted expression of the ephrin-B2, nuclear factor of activated T-cells 1 (NFATc1), TRAP, Fos proto-oncogene, AP-1 transcription factor subunit (C-FOS), and matrix metalloproteinase 9 (MMP9) genes. Phalloidin and TRAP staining revealed that following the addition of ephB4-Fc, the number, size and cytoskeletal elements of Key words: osteoclasts, osteoblasts, remodeling, ephrin-B2, osteoclastogenesis osteoclasts were significantly decreased compared with those in the titanium particle group without ephB4-Fc. Compared with the titanium particle group, the bone pit absorption experiment revealed significantly decreased absorption pit areas in the titanium particle+ephB4-Fc group. The expression of the NFATc1, TRAP, C-FOS and MMP9 genes was markedly decreased in the ephB4-Fc group; however, the expression of the ephrin-B2 gene was increased compared with the Ti particle group without ephB4-Fc after 5 days. Production of inflammatory cytokines was inhibited by Ti particles through bidirectional signals. Addition of ephB4-Fc inhibited the osteoclast-mediated formation of Ti particles via bidirectional ephrin-B2/ephB4 signaling. Activation of this bidirectional signaling pathway may be a potential clinical treatment for osteolysis surrounding prostheses.

Inhibition of osteoclastogenesis for periprosthetic osteolysis therapy through the suppression of p38 signaling by fraxetin

Periprosthetic osteolysis belongs to osteolytic diseases, which often occur due to an imbalance between osteoclast and osteoblast number or activity. Fraxetin, a natural plant extract, inhibits osteoblast apoptosis and has therapeutic potential for treating osteolytic diseases. However, data pertaining to the effects of fraxetin on osteoclasts are limited. In the present study, it was demonstrated that the inhibition of osteoclastogenesis by fraxetin had an important role on the therapy of titanium particle-induced osteolysis in vivo. In addition, fraxetin was demonstrated to suppress receptor activator of nuclear factor-κB ligand (RANKL)-mediated osteoclast differentiation and bone resorption in vitro in a dose-dependent manner. Fraxetin inhibited osteoclast differentiation and function through the suppression of p38 signaling and subsequently, the suppression of osteoclast-specific gene expression, including tartrate-resistant acid phosphatase, nuclear factor of activated T-cells, cytoplasmic 1, and cathepsin K. In conclusion, fraxetin administration may have potential as a treatment method for periprosthetic osteolysis and other osteolytic diseases.

Melatonin attenuates titanium particle-induced osteolysis via activation of Wnt/β-catenin signaling pathway

Wear debris-induced inhibition of bone regeneration and extensive bone resorption were common features in peri-prosthetic osteolysis (PPO). Here, we investigated the effect of melatonin on titanium particle-stimulated osteolysis in a murine calvariae model and mouse-mesenchymal-stem cells (mMSCs) culture system. Melatonin inhibited titanium particle-induced osteolysis and increased bone formation at osteolytic sites, confirmed by radiological and histomorphometric data. Furthermore, osteoclast numbers decreased dramatically in the low- and high-melatonin administration mice, as respectively, compared with the untreated animals. Melatonin alleviated titanium particle-induced depression of osteoblastic differentiation and mineralization in mMSCs. Mechanistically, melatonin was found to reduce the degradation of β-catenin, levels of which were decreased in presence of titanium particles both in vivo and in vitro. To further ensure whether the protective effect of melatonin was mediated by the Wnt/β-catenin signaling pathway, ICG-001, a selective β-catenin inhibitor, was added to the melatonin-treated groups and was found to attenuate the effect of melatonin on mMSC mineralization. We also demonstrated that melatonin modulated the balance between receptor activator of nuclear factor kappa-B ligand and osteoprotegerin via activation of Wnt/β-catenin signaling pathway. These findings strongly suggest that melatonin represents a promising candidate in the treatment of PPO.

STATEMENT OF SIGNIFICANCE

Peri-prosthetic osteolysis, initiated by wear debris-induced inhibition of bone regeneration and extensive bone resorption, is the leading cause for implant failure and reason for revision surgery. In the current study, we demonstrated for the first time that melatonin can induce bone regeneration and reduce bone resorption at osteolytic sites caused by titanium-particle stimulation. These effects might be mediated by activating Wnt/β-catenin signaling pathway and enhancing osteogenic differentiation. Meanwhile, the ability of melatonin to modulate the balance between receptor activator of nuclear factor kappa-B ligand and osteoprotegerin mediated by Wnt/β-catenin signaling pathway, thereby suppressing osteoclastogenesis, may be implicated in the protective effects of melatonin on titanium-particle-induced bone resorption. These results suggested that melatonin can be considered as a promising therapeutic agent for the prevention and treatment of peri-prosthetic osteolysis.

A comparative evaluation of a novel vaccine in APP/PS1 mouse models of Alzheimer's disease

Immunization against amyloid-beta-peptide (Aβ) has been widely investigated as a potential immunotherapeutic approach for Alzheimer's disease (AD). With the aim of developing an active immunogenic vaccine without need of coadjuvant modification for human trials and therefore avoiding such side effects, we designed the Aβ_1–42 vaccine (EB101), delivered in a liposomal matrix, that based on our previous studies significantly prevents and reverses the AD neuropathology, clearing Aβ plaques while markedly reducing neuronal degeneration, behavioral deficits, and minimizing neuroinflammation in APP/PS1 transgenic mice. Here, the efficacy of our immunogenic vaccine EB101 was compared with the original immunization vaccine cocktail Aβ₄₂ + CFA/IFA (Freund's adjuvant), in order to characterize the effect of sphingosine-1-phosphate (S1P) in the immunotherapeutic response. Quantitative analysis of amyloid burden showed a notable decrease in the neuroinflammation reaction against Aβ plaques when S1P was compared with other treatments, suggesting that S1P plays a key role as a neuroprotective agent. Moreover, EB101 immunized mice presented a protective immunogenic reaction resulting in the increase of Aβ-specific antibody response and decrease of reactive glia in the affected brain areas, leading to a Th2 immunological reaction.

Calcineurin/NFAT pathway mediates wear particle-induced TNF-α release and osteoclastogenesis from mice bone marrow macrophages in vitro

AIM

To investigate the roles of the calcineurin/nuclear factor of activated T cells (NFAT) pathway in regulation of wear particles-induced cytokine release and osteoclastogenesis from mouse bone marrow macrophages in vitro.

METHODS

Osteoclasts were induced from mouse bone marrow macrophages (BMMs) in the presence of 100 ng/mL receptor activator of NF-κB ligand (RANKL). Acridine orange staining and MTT assay were used to detect the cell viability. Osteoclastogenesis was determined using TRAP staining and RT-PCR. Bone pit resorption assay was used to examine osteoclast phenotype. The expression and cellular localization of NFATc1 were examined using RT-PCR and immunofluorescent staining. The production of TNFα was analyzed with ELISA.

RESULTS

Titanium (Ti) or polymethylmethacrylate (PMMA) particles (0.1 mg/mL) did not significantly change the viability of BMMs, but twice increased the differentiation of BMMs into mature osteoclasts, and markedly increased TNF-α production. The TNF-α level in the PMMA group was significantly higher than in the Ti group (96 h). The expression of NFATc1 was found in BMMs in the presence of the wear particles and RANKL. In bone pit resorption assay, the wear particles significantly increased the resorption area and total number of resorption pits in BMMs-seeded ivory slices. Addition of 11R-VIVIT peptide (a specific inhibitor of calcineurin-mediated NFAT activation, 2.0 μmol/L) did not significantly affect the viability of BMMs, but abolished almost all the wear particle-induced alterations in BMMs. Furthermore, VIVIT reduced TNF-α production much more efficiently in the PMMA group than in the Ti group (96 h).

CONCLUSION

Calcineurin/NFAT pathway mediates wear particles-induced TNF-α release and osteoclastogenesis from BMMs. Blockade of this signaling pathway with VIVIT may provide a promising therapeutic modality for the treatment of periprosthetic osteolysis.

Adenovirus-mediated small interfering RNA targeting tumor necrosis factor-α inhibits titanium particle-induced osteoclastogenesis and bone resorption

Wear particles are phagocytosed by macrophages, resulting in cellular activation and the release of pro-inflammatory factors, which cause periprosthetic osteolysis and subsequent aseptic loosening, the most common causes of total joint arthroplasty (TJA) failure. During this pathological process, tumor necrosis factor (TNF)-α plays an important role in wear particle-induced osteolysis. Therefore, in this study, we used adenovirus-mediated small interfering RNA (siRNA) targeting TNF-α to suppress the TNF-α release from activated macrophages in response to titanium particles. Our results showed that recombinant adenovirus (Ad-TNF-α-siRNA) suppressed the TNF-α release from activated macrophages in response to titanium particles, and reduced titanium particle-induced osteoclastogenesis and bone resorption in the presence of receptor activator of nuclear factor-κB ligand (RANKL). In addition, the conditioned medium of macrophages challenged with titanium particles (Ti CM) stimulated osteoprogenitor RANKL expression. The conditioned medium of macrophages challenged with titanium particles and Ad-TNF-α-siRNA (Ti-Ad CM) reduced the mRNA expression in MC3T3-E1 cells compared to Ti CM. Based on these data, TNF-α strongly synergizes with RANKL to promote osteoclast differentiation. Furthermore, TNF-α promoted osteoclast differentiation by stimulating osteoprogenitor RANKL expression. Ad-TNF-α-siRNA effectively suppressed osteoclast differentiation and bone resorption following exposure to titanium particles in the presence of RANKL. In addition, recombinant adenovirus (Ad-TNF-α-siRNA) does not have a toxic effect on the murine macrophage cell line, RAW264.7. Consequently, it can be concluded that recombinant adenovirus-mediated siRNA targeting TNF-α (Ad-TNF-α-siRNA) may provide a novel therapeutic approach for the treatment of periprosthetic osteolysis.

Inhibition of titanium particle-induced inflammation by the proteasome inhibitor bortezomib in murine macrophage-like RAW 264.7 cells

Wear particle-induced inflammatory osteolysis is the major cause of aseptic loosening after total joint replacement. The predominant cell type within periprosthetic tissues is macrophages. We investigate the anti-inflammatory effects of the proteasome inhibitor bortezomib (Bzb) on murine macrophage-like RAW 264.7 cells stimulated with titanium (Ti) particles. RAW 264.7 cells were cultured with 1 nM Bzb and 0.1 mg/ml Ti particles for 48 h; cells without Ti and Bzb or without Bzb were used as negative and loading controls. Results showed that the expression of inflammatory cytokines (TNF-α, IL-1β, IL-6, and IL-10), chemokines [monocyte chemoattractant protein-1 (MCP-1), macrophage inflammatory protein-1 alpha (MIP-1α)], and inflammatory enzymes [inducible nitric oxide synthase (iNOS), cyclooxygenase-2 (COX-2)] increased in RAW 264.7 cells cultured with Ti. Bzb treatment significantly reduced the expression of TNF-α, IL-1β, IL-6, MCP-1, MIP-1α, iNOS, and COX-2 and induced the expression of IL-10 in a time-dependent manner. These results suggest that Bzb inhibits Ti-induced inflammation in macrophages, and provide a promising therapeutic target for treating or preventing aseptic loosening.

Vaccine Development to Treat Alzheimer's Disease Neuropathology in APP/PS1 Transgenic Mice

A novel vaccine addressing the major hallmarks of Alzheimer's disease (AD), senile plaque-like deposits of amyloid beta-protein (Aβ), neurofibrillary tangle-like structures, and glial proinflammatory cytokines, has been developed. The present vaccine takes a new approach to circumvent failures of previous ones tested in mice and humans, including the Elan-Wyeth vaccine (AN1792), which caused massive T-cell activation, resulting in a meningoencephalitis-like reaction. The EB101 vaccine consists of Aβ_1-42 delivered in a novel immunogen-adjuvant composed of liposomes-containing sphingosine-1-phosphate (S1P). EB101 was administered to APPswe/PS1dE9 transgenic mice before and after AD-like pathological symptoms were detectable. Treatment with EB101 results in a marked reduction of Aβ plaque burden, decrease of neurofibrillary tangle-like structure density, and attenuation of astrocytosis. In this transgenic mouse model, EB101 reduces the basal immunological interaction between the T cells and immune activation markers in the affected hippocampal/cortical areas, consistent with decreased amyloidosis-induced inflammation. Therefore, immunization with EB101 prevents and reverses AD-like neuropathology in a significant manner by halting disease progression without developing behavioral spatial deficits in transgenic mice.

Comparison of the cytotoxic and inflammatory responses of titanium particles with different methods for endotoxin removal in RAW264.7 macrophages

It is generally accepted that periprosthetic bone resorption is initiated through aseptic inflammation aggravated by wear particles that are generated from artificial joint. However, some studies have demonstrated that "endotoxin-free" wear particles are almost completely unable to stimulate the macrophage-mediated production of proinflammatory cytokines. Here, we compare the titanium particles with different methods of endotoxin removal. The results indicated that different titanium particle preparation dosages did not significantly change particle size, morphology, and chemical composition. But it could cause variations in the endotoxin concentration of titanium particles and inflammatory responses in RAW264.7 macrophages. The particles with higher endotoxin levels correlated with more extensive inflammatory responses. When testing endotoxins using the supernatant of particle suspensions, it would lead to false negative results compared with testing the particle themselves. And when using the particles themselves, all the particles should be removed by centrifugation to avoid particle interference before the absorbance value was determined. Therefore, we suggest that research concerning wear particles should completely describe the endotoxin testing process, including endotoxin removal from particles and the details of endotoxin testing. Moreover, future research should focus on the surface of wear particles (the potential role of adherent endotoxin) rather than the particles themselves.

Enhancement of osteoblast differentiation that is inhibited by titanium particles through inactivation of NFATc1 by VIVIT peptide

Bone formation, which is inhibited by particulate wear debris, is a pathological factor that contributes to periprosthetic osteolysis. Although the nuclear factor of activated T cells c1 (NFATc1) is known to be involved in osteoblast differentiation, and its effect on osteoblasts in response to wear particles remains unclear. In this study, we investigated the role of NFATc1 in the regulation of osteoblastic differentiation of rat calvaria (RC) cells (a cell-culture model comprising many osteoprogenitors) that were challenged with titanium (Ti) particles. The results showed that the Ti particles inhibited osteoblastic differentiation and mineralization of RC cells. NFATc1 plays a critical role in the Ti-particle inhibition process of the osteoblastic differentiation in RC cells. Inactivation of NFATc1 by the 11R-VIVIT peptide potently enhanced osteoblast differentiation and mineralization inhibition by the Ti particles. The 11R-VIVIT peptide does not have a toxic effect on the RC cells. On the basis of these data, we conclude that inactivation of NFATc1 by the 11R-VIVIT peptide may provide a promising therapeutic target for the treatment of periprosthetic osteolysis by increasing bone formation.

Intracellular phospholipase A2 expression and location in human macrophages: influence of synthetic material surface chemistry

Phospholipase A(2) (PLA(2)) enzymes participate in a potent inflammatory pathway through the liberation of arachidonic acid upon hydrolysis of membrane glycerophospholipids. The presence of implanted polycarbonate-urethane (PCNU) materials, used in several medical applications, has the ability to influence inflammatory responses of human macrophages that are recruited to a tissue-material interface; however, the specific inflammatory pathways that are activated upon macrophage attachment to PCNU are largely unknown. Previous studies suggested the participation of PLA(2) pathways in material degradation with the use of chemical inhibitors, such as aristolochic acid (ARIST), however not accurately defining the specific PLA(2) enzymes involved. The current study aimed to establish specific groups of PLA(2) involved in the macrophage foreign body response to PCNU. ARIST was assessed for specific effects on secretory PLA(2) (sPLA(2)) protein expression and non-specific effects on key proteins, beta-actin and monocyte-specific esterase, implicated in the macrophage attack on PCNU materials. Macrophage attachment to PCNU materials induced increased intracellular expression of cytosolic PLA(2) (cPLA(2)), but not sPLA(2), relative to tissue culture polystyrene (TCPS) as detected by immunoblot analysis, demonstrating an early and delayed stimulation during the time course of increased cPLA(2) protein expression. Laser scanning confocal microscopy images indicated a change in location of cPLA(2) in macrophages adherent to PCNU surfaces compared to TCPS. This study has illustrated changes in macrophage cPLA(2) expression in response to cell-attachment to PCNU surfaces, demonstrating that the macrophage foreign body response to biomaterials induces a potent inflammatory pathway, which may lead to tissue damage near the site of material implantation.

Rac1-NADPH oxidase-regulated generation of reactive oxygen species mediates glutamate-induced apoptosis in SH-SY5Y human neuroblastoma cells

Reactive oxygen species (ROS) are known to play an important role in glutamate-induced neuronal cell death. In the present study, we examined whether NADPH oxidase serves as a source of ROS production and plays a role in glutamate-induced cell death in SH-SY5Y human neuroblastoma cells. Stimulation of the cells with glutamate (100 mM) induced apoptotic cell death and increase in the level of ROS, and these effects of glutamate were significantly suppressed by the inhibitors of the NADPH oxidase, diphenylene iodonium, apocynin, and neopterine. In addition, RT-PCR revealed that SH-SY5Y cells expressed mRNA of gp91phox, p22phox and cytosolic p47phox, p67phox and p40phox, the components of the plasma membrane NADPH oxidase. Treatment with glutamate also resulted in activation and translocation of Rac1 to the plasma membrane. Moreover, the expression of Rac1N17, a dominant negative mutant of Rac1, significantly blocked the glutamate-induced ROS generation and cell death. Collectively, these results suggest that the plasma membrane-bound NADPH oxidase complex may play an essential role in the glutamate-induced apoptotic cell death through increased production of ROS.