P2X7 signaling promotes microsphere embolism-triggered microglia activation by maintaining elevation of Fas ligand
© han et al. 2012
Received: 2 April 2012
Accepted: 20 June 2012
Published: 12 July 2012
The cerebral microvascular occlusion elicits microvascular injury which mimics the different degrees of stroke severity observed in patients, but the mechanisms underlying these embolic injuries are far from understood. The Fas ligand (FasL)-Fas system has been implicated in a number of pathogenic states. Here, we examined the contribution of microglia-derived FasL to brain inflammatory injury, with a focus on the potential to suppress the FasL increase by inhibition of the P2X7-FasL signaling with pharmacological or genetic approaches during ischemia.
The cerebral microvascular occlusion was induced by microsphere injection in experimental animals. Morphological changes in microglial cells were studied immunohistochemically. The biochemical analyses were used to examine the intracellular changes of P2X7/FasL signaling. The BV-2 cells and primary microglia from mice genetically deficient in P2X7 were used to further establish a linkage between microglia activation and FasL overproduction.
The FasL expression was continuously elevated and was spatiotemporally related to microglia activation following microsphere embolism. Notably, P2X7 expression concomitantly increased in microglia and presented a distribution pattern that was similar to that of FasL in ED1-positive cells at pathological process of microsphere embolism. Interestingly, FasL generation in cultured microglia cells subjected to oxygen-glucose deprivation-treated neuron-conditioned medium was prevented by the silencing of P2X7. Furthermore, FasL induced the migration of BV-2 microglia, whereas the neutralization of FasL with a blocking antibody was highly effective in inhibiting ischemia-induced microglial mobility. Similar results were observed in primary microglia from wild-type mice or mice genetically deficient in P2X7. Finally, the degrees of FasL overproduction and neuronal death were consistently reduced in P2X7 −/− mice compared with wild-type littermates following microsphere embolism insult.
FasL functions as a key component of an immunoreactive response loop by recruiting microglia to the lesion sites through a P2X7-dependent mechanism. The specific modulation of P2X7/FasL signaling and aberrant microglial activation could provide therapeutic benefits in acute and subacute phase of cerebral microembolic injury.
KeywordsMicrosphere embolism Microglia P2X7 receptor Fas ligand Neuronal cell death
Epidemiological and clinical evidence has indicated that cerebral microvascular occlusion is a common phenomenon throughout life, which might require better understanding as a mechanism of ischemic insult and subsequent neuronal damage [1–3]. Unlike the lethal larger artery occlusion, microsphere embolism elicits microvascular injury which mimics the different degrees of stroke severity observed in patients [2, 4–6], but the mechanisms underlying these embolic injuries are far from understood. Cerebral ischemia is followed by destruction of microvasculature and subsequent inflammatory events during acute and subacute stages of ischemia [3, 7, 8]. It is noteworthy that there is a growing body of evidence that uncontrolled inflammatory response in the brain can contribute to exacerbate the neuronal loss in cerebral ischemia [3, 7, 8]. The spatiotemporal changes in microglial dynamics are dramatically affected by capillary blood flow around the microglial somata in intact ischemic mouse neocortex . However, no link between brain inflammatory response and cerebral microvascular occlusion has been studied thus far.
The Fas ligand (FasL)-Fas system may play a role in both physiological and pathological situations in the brain [10, 11], whether and how FasL-Fas system influences the pathogenesis of microembolic injuries remains unknown. In pathological conditions, dysregulated FasL-Fas system leads to activation of Fas-associated death domain (FADD), caspase 8, and subsequently neuronal death [12, 13]. By contrast, depending on the cell type and its state, FasL-Fas system has also been shown to be instructive signal for ongoing and injury-induced brain regeneration . Despite the autocrine action of FasL that has been shown in vascular smooth muscle cells , epithelial cells , lymphocytes , and intestinal cells , little is known about its effects on microglia or its possible role in brain microembolic injury. Due to the dual and insufficient knowledge about FasL-Fas system in the brain, discrete targets amenable to ischemic processes need to be identified and functionally validated.
Microglial cell activation is a rapidly occurring cellular response to cerebral ischemia, which is a strictly controlled event [18–22]. Activated microglia may exert a cytotoxic effector function by stimulating morphological changes and the production of a wide array of inflammatory cytokines, which may cause bystander damage following ischemia [18, 21]. The mechanisms by which microglia are activated during the acute and subacute phase of brain ischemia, or by which microglia continue to synthase or secrete FasL remain unknown. The exposure of microglia to H2O2 or paraquat rapidly triggers FasL mRNA transcription and protein expression and also activates NF-kappa B [23, 24]. However, activated microglia under certain circumstances may act on different steps in the formation, maturation, and functional integration of the new neurons of both intact and pathological brain . The opposing neurotoxic and neuroprotective properties of microglia during ischemic injury provide a rich and currently unexplored set of research problems.
The present study investigated the pathophysiological relevance of cerebral microembolic injury-induced microglial activation, FasL intracellular signaling, and the neuronal death cascade. We examined the influence of ischemic injury on microglia cells as well as the functional outcome of aberrant FasL signaling in vitro and in vivo. Herein, we report that the activation of P2X7 in microglia results in a gradual accumulation and release of FasL along with the subsequent activation of downstream cascades in ischemic context.
Materials and methods
All chemicals were purchased from Sigma-Aldrich Chemical Co. (St Louis, MO, USA) unless otherwise noted.
Homozygous P2X7 −/− mice (B6.129P2-P2rx7 tm1Gab /J, Jackson Laboratories, Bar Harbor, ME, USA), gld/gld mice (C3H/HeJ-Jms, FasL-deficient, SLC, Japan), and wild-type (WT) littermates of both backgrounds were used for the experiments. All of the animal procedures conformed to appropriate guidelines for the care and use of laboratory animals and were approved by the Committees for Animal Experiments of Zhejiang University in China and Tohoku University in Japan.
Cerebral microemboli model
The microsphere embolism model was prepared as previously described . Briefly, male Wistar rats weighing 220 g to 270 g were subjected to general anesthesia, after which 1,000 non-radioactive microspheres (48.4 ± 0.7 μm in diameter) were injected into the left carotid artery. The rats were decapitated 2, 6, 12, 24, 72, or 168 h after microsphere injection, and the ipsilateral cerebral hemisphere was dissected for further analysis. For P2X7 −/− mice, gld/gld mice, and their respective WT littermates, 5,000 non-radioactive microspheres (15 μm in diameter) were injected into the left carotid artery. Neurological scores were assessed 168 h after microsphere embolism as previously described .
2-[18F-fluoro-2-deoxy-D-glucose (18F-FDG) injections and microPET scans were processed as previously reported . The data of 18F-FDG microPET imaging were acquired from rats 168 h after microsphere embolism by using a microPET R4 scanner (Concorde Microsystems, Knoxville, TN, USA).
Drug administration began after a 1-day recovery period following microsphere embolism in rats. Minocycline hydrochloride (Sigma-Aldrich) (45 mg/kg, dissolved in distilled water) was intraperitoneally administered daily for 6 consecutive days. The ipsilateral hemispheres were then dissected for western blot analysis.
Oxygen-glucose deprivation and neuron-conditioned medium
Cortical neurons were prepared from E18 C57BL/6 mouse embryo cortices as previously described . The mice were decapitated to minimize pain and distress. Dissection and dissociation were carried out in 1× Hank’s balanced salt solution (HBSS) and 0.25% trypsin (Gibco) for 10 min before being terminated with 1× HBSS and 10 mg/mL trypsin inhibitor. Neurons were also dissociated by trituration in HBSS containing DNase I with a flame-narrowed pasteur pipette to fully dissociate the cells. Neurons were seeded at an approximate density of 2 × 107 on 6-cm dishes (430166, Corning) and maintained in 5 mL of neurobasal medium containing B27 supplement (2%, Invitrogen), penicillin-streptomycin (100 U/mL penicillin, 100 μg/mL streptomycin, Gibco) and 0.25% GlutaMAXTM-1 (Gibco). Neurons were grown in vitro for 7 days.
To create an in vitro ischemia-like injury model, the primary cortical neurons were subjected to oxygen-glucose deprivation (OGD). The procedure for OGD challenge was performed as previously reported . The medium was removed from the cultures, and the cells were rinsed twice with glucose-free HBSS prior to being subjected to OGD at defined time points. The OGD-treated cell-free conditioned medium (OCM) was collected to perform related analyses.
BV-2 murine microglial cells culture and migration assay
BV-2 murine microglial cells, which exhibit similar morphological and functional properties comparable to primary microglial cells [28, 29], were grown in Dulbecco’s modified Eagle’s medium supplemented with 10% fetal bovine serum at a density that did not exceed 5 × 105 cells/mL. To mimic ischemia-like injury, the BV-2 immortalized mouse microglial cell line was subjected to OCM. Migration was assessed with a transwell migration chamber as described in a previous report . Briefly, BV-2 murine microglial cells were placed in a transwell migration chamber and cultured in DMEM medium supplemented with 10% fetal bovine serum (FBS) at a density of 2 × 105 cells/mL. The cells were allowed to migrate through a polycarbonate mesh (8.0 μm pore size, polycarbonate membrane, Corning Costar Corp., Cambridge, MA, USA) at 37°C. For the experiments that used neutralizing antibody, the cells were preincubated with anti-mouse FasL neutralizing antibody (2 μg/mL, 8 μg/mL, or 16 μg/mL; Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA, Catalog no: sc-19988) or control preimmune serum for 30 min before ATP or recombinant FasL treatment. Recombinant FasL (100 ng/mL, Sigma-Aldrich, Catalog no: F0552) was placed in the lower chamber to induce migration. The chamber was kept in 5% CO2 at 37°C for 24 h. Microglia that migrated to the lower surface were fixed with 4% paraformaldehyde and stained with the blue-fluorescent DAPI. The rate of microglial migration was calculated by counting the cells of each well using a 10× bright-field objective. The number of cells in each well was normalized to the average number of cells in the control condition (100%). The experiments were repeated at least three times.
To construct the P2X7 short hairpin RNA (shRNA) expression plasmid, two complementary P2X7 DNA oligonucleotides (see below) were synthesized, annealed, and inserted into the pMagic 4.1 plasmid. Reduction of P2X7 expression was achieved through lentiviral infection of BV-2 microglia with the P2X7 cDNA coding sequence 5’-GCAAGTTGTCAAAGGCCAA-3’, and the resulting culture supernatant was used for the lentiviral infection. As a control, microglia were infected in a similar manner with the shRNA sequence 5’-TTCTCCGAACGTGTCACGT-3’. The cells were collected for further experiments at 72 h postinfection.
RNA interference of FasL expression
FasL siRNAs (Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA) were introduced into BV-2 murine microglial cells with transfection medium according to the manufacturer’s instructions. The control set of BV-2 cells was transfected with non-targeted siRNAs. The cells were collected for experiments 24 h after transfection. Knockdown was confirmed with western blotting using whole cell lysates.
ELISAs for extracellular Fas ligand
BV-2 murine microglial cells were either infected with lentivirus directing the expression of non-targeting control or P2X7-specific shRNAs or transfected with siRNAs against FasL. Supernatants from the OGD-treated cortical neurons were applied to BV-2 cultures for the measurement of the FasL level. Target cells were then cultured with OCM for 6 h, and Fas ligand assays were performed with a mouse Fas ligand/TNFSF6 immunoassay kit (R&D Systems, Minneapolis, MN, USA) following the manufacturer’s protocol.
Primary microglial cell culture and chemotaxis assays
Primary mixed glial cultures were prepared according to a previously described method with slight adjustments . Briefly, mixed glial cultures were prepared from the cerebral cortices of 1-day-old mice using 0.25% trypsin, and the cells were cultured in minimal essential medium (MEM) containing 10% FBS(Gibco) at 37°C in a 5% CO2 humidified incubator. After 10 to 14 days, the cultures were shaken by hand for 15 min, the floating cells were collected and centrifuged for 5 min at 1000 × g, and microglial cells were seeded on glass-bottom dishes (Matsunami, Osaka, Japan) at a density of 2 × 104 cells per dish.
For the chemotaxis assays, the cells were exposed to a chemoattractant gradient that was generated by slowly releasing ATP from a micropipette tip that was placed at the center of the imaging field. Cell migration toward the pipette tip was monitored over a 30-min period by phase-contrast time-lapse microscopy. Acquisition was performed using an Olympus microscope with a × 20 air objective (LCACH, 0.4 NA), and the images were processed and analyzed using Fluoview software (FV10-ASW 5.0, Japan) .
Proteins were extracted, and the soluble cytosolic fraction was isolated according to our previous study . Samples containing equivalent amounts of protein were loaded onto 10% to 15% acrylamide denaturing gels (sodium dodecyl sulfate polyacrylamide gel electrophoresis; SDS-PAGE). Membranes were probed overnight at 4°C with antibodies against Fas (sc-736), FasL (sc-6237), P2X7 (sc-25698), High mobility group box 1 (HMGB1, sc-26351), caspase-8 (sc-7890) (Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA); CD3 (Abcam Cambridge, UK, Catalog no: ab5690); FADD (Epitomics, Inc., Burlingame, CA, USA, Catalog no: 2988–1); and β-actin (Sigma Chemical, St Louis, MO, USA, Catalog no:A2228). Immunoreactive proteins were visualized using an enhanced chemiluminescence detection system (Amersham Life Science, Buckinghamshire, UK).
Fluoro-jade C staining
Fluoro-Jade C is a fluorescein derivative that has been shown to specifically stain degenerating neurons. In accordance with previous studies , free-floating sections were immersed in a 0.06% potassium permanganate solution for 10 min and then rinsed in distilled water for 1 min. The slides were then transferred to Fluoro-Jade C (Millipore) staining solution (0.001%) for 10 min. After staining, the sections were rinsed three times for 1 min each in distilled water.
Immunohistochemical labeling, image acquisition, and analysis
Animals were anesthetized and transcardially perfusion-fixed with 4% paraformaldehyde in phosphate-buffered saline (PBS). Whole brains were immediately removed and postfixed overnight at 4°C in the same fixative. To identify distribution of microglial nodule in the brain, paraffin sections (4 μm) were stained with hematoxylineosin (H&E). For immunolabeling, coronal brain sections at the coordinates of anterior or posterior to bregma (35 μm in thickness) were prepared using a microslicer system (Vibratome, St Louis, MO, USA). Then, brain sections were incubated at room temperature with 0.01% Triton-X-100 in PBS for 30 min, followed by incubation in 3% bovine serum albumin (BSA) in PBS for 1 h. For immunolabeling, sections were incubated with antibodies to FasL (sc-6237) and P2X7 (sc-25698) (Santa Cruz Biotechnology Inc., Santa Cruz, CA, USA); GFAP (Sigma Chemical, St Louis, MO, USA, Catalog no: G3893); ED1 (MAB1435) and NeuN (MAB377) (Chemicon, Temecula, CA, USA); and Iba1 (Abcam Cambridge, UK, Catalog no: ab5076) overnight at 4°C, followed by exposure to fluorescent secondary antibodies according to a standard protocol from PerkinElmer Life Sciences, Inc. (Boston, MA, USA). Immunofluorescent images were taken with a confocal laser scanning microscope, and fluorescent density was analyzed using Image J software (NIH, Bethesda, MD, USA).
The data are represented as the mean ± SEM. Statistical significance was determined using one-way analysis of variance (ANOVA) followed by Dunnett’s test for multigroup comparisons. A non-parametric Mann–Whitney U-test was used for analysis of the neurologic deficit data. P <0.05 indicated statistically significant differences.
Microsphere embolism-induced damage to microvessels caused a focal response in neighboring microglial cells
Temporal changes in FasL levels in rats following microsphere embolism
FasL production is associated with microglial activation following microsphere embolism
To identify which cell types could potentially mediate the observed FasL signaling, we used double immunostaining for cell-specific markers to examine the cellular localization of FasL 168 h after the injection of microspheres. As shown in Figure 2C, the distribution patterns of FasL deposits and NeuN-positive staining were significantly different. In sham rats, the injection of 20% dextran did not induce an elevation in FasL expression (Figure 2C). A significant number of FasL-expressing cells, which were diffusely labeled with a round morphology, were found to be restricted to the occluding region 168 h after microsphere embolism (Figure 2C, d). In addition, astrocyte activation is known to be a significant component of postischemic pathophysiology . A comparison of GFAP and FasL immunoreactivities demonstrated a near-complete dissociation of the staining patterns for these proteins in the ipsilateral hemisphere in ME rats (Figure 2D, d, e, f). Nearly all FasL-positive cells co-localized with ED1 (Figure 2E, f), whereas sham rats did not show significant positive staining for FasL or ED1 (Figure 2E, c).
The co-localization of fluorescent signals was analyzed using Image J software. NeuN and GFAP weakly co-localized with FasL-positive cells (FasL/NeuN, 16.4 ± 9.9%; FasL/GFAP, 5.5 ± 3.6%), whereas FasL strongly co-localized with ED1 (the mean overlapping fraction, 76.2 ± 12.3%; Figure 2F). Both Pearson’s r index and an intensity correlation coefficient (ICQ) analysis indicated similar results (Figure 2F). The present data indicate that robust FasL expression is predominantly associated with activated microglia after microsphere embolism in rats.
Immunohistochemical localization of ED1- and Iba1-positive cells in the microembolic brain
We also confirmed the specific activation of microglia during cerebral microemboli by immunostaining for Iba1, which is ubiquitously expressed in all microglial populations . Double-labeling studies revealed that the number of Iba1/ED1-positive cells and the corresponding fluorescence intensity increased significantly in ME animals, including both ramified (12 h after microsphere embolism) (Figure 3D, d-f) and typical ameboid microglia (168 h after microsphere embolism (Figure 3D, g-i)). In addition, Iba1 and ED1 were present in the same cell populations in the ischemic region (Figure 3D, f and i). The observed irregularities in microglial cell shape support the possibility that the degree of morphological changes in Iba1/ED1-immunoreactive microglial cells is proportional to the duration of ischemic injury. The Ki-67 antigen is preferentially activated during all of the active phases of the cell cycle, but it is absent in resting cells. We found co-localization between the Ki-67 antigen and ED1-immunoreactive microglial cells in the ipsilateral brain region (Figure 3E).
FasL elevation is associated with the migration of BV-2 microglia cells
P2X7 signal contributes to OGD-triggered immune responses by maintaining the overproduction of FasL in microglia cells
To ascertain whether OCM-induced increase in FasL protein required new mRNA and/or protein synthesis in microglial cells, we examined FasL levels in BV-2 cells treated with either the protein synthesis inhibitor anisomycin or the RNA polymerase inhibitor actinomycin D. We pretreated BV-2 cells with anisomycin or actinomycin D prior to treating them with OCM (Figure 5D). We found that the synthesis of FasL in microglia was sensitive to anisomycin treatment (Figure 5D). Concomitantly, actinomycin D pretreatment also decreased OCM-induced FasL elevation in BV-2 cells, indicating that OCM-induced FasL protein synthesis might dependent on new mRNA expression (Figure 5D). Thus, the present data suggest that OCM activates the local translation of FasL protein in a transcription-dependent manner. These results rule out the possibility that P2X7 knockdown might affect the general microglia protein translation machinery and suggest that P2X7 signaling is selectively involved in ischemia-induced microglial activation.
FasL is required for microglial migration
In light of the proposed roles for purinergic receptors in regulating microglial motility, we further investigated whether P2X7 −/− mice showed deficits in cellular responses to exogenous ATP. Upon application of Adenosine 5’-[γ-thio] triphosphate tetralithium salt (ATPγS, 1 mM), the vast majority of microglia from WT mice showed robust migration on time-lapse recordings (Figure 6B). In contrast, microglia cells from the P2X7 −/− mice did not show significant chemotactic response to ATP. Notably, preincubation with FasL antibody (8 μg/mL) significantly diminished the ATP-evoked chemotactic response of WT microglia in vitro (Figure 6B; see (Additional file 1: Movie 1, Additional file 2: Movie 2 and Additional file 3: Movie 3). Therefore, these results further suggest the critical functional roles for P2X7-FasL signaling in mediating microglial chemotactic inflammatory responses.
P2X7 is up-regulated in microglia after microsphere embolism
Microglia-induced neuronal injury in conjunction with P2X7 purinoceptor-FasL pathway activation
To conclusively demonstrate the significance of FasL in ischemic injury in vivo, we also tested the neural sensitivity to cerebral microemboli insult in gld/gld mice. An increase in the number of Fluoro-Jade C-positive cells was observed in WT mice following microsphere embolism, whereas few Fluoro-Jade C-positive cells were present in FasL-deficient mice (Figure 8E). In addition, microsphere embolism significantly induced microglia activation and increased the number of FasL-positive cells in the brain ipsilateral hemisphere in WT mice, whereas both ED1- and FasL-positive staining were sparse in gld/gld mice 168 h after microsphere injection (Figure 8F).
ME-induced FasL activation and the apoptosis cascade are inhibited by minocycline treatment
The present study addressed the pathophysiological relevance of cerebral microembolic injury-induced microglial activation, FasL intracellular signaling, and the neuronal death cascade. The primary findings of this study are three-fold. First, during the pathological process of cerebral microembolic injury, FasL signaling associated with ischemia-initiated microglial activation. As a key microenvironmental signal, overproduction of FasL further stimulated an extensive microglial response in an autocrine manner. Second, the effective inhibition of microglial activation was observed with P2X7 shRNA infection in cultured microglia or P2X7 −/− mice in vivo, and the inhibition of microglial activation was paralleled by inhibition of ischemia-induced FasL overgeneration. Third, permanent cerebral microemboli induced morphological and biochemical reactions in microglia that were consistent with neuronal apoptotic cascades. Based on the present findings, we hypothesize that modulation of microglial neurotoxicity may prove to be a potential therapeutic target in neurovascular diseases.
Several prior studies have revealed that microglia play a crucial role in mediating inflammatory responses following the initial insult [21, 42–44]. Consistently, the present study demonstrates the rapid activation of microglia in response to microembolic injury, which is apparent as early as 2 h after the event. The robust FasL expression is predominantly associated with activated microglia in subacute phase of microsphere embolism. Several recent studies reported that purinergic receptors are involved in brain ischemia [39, 45]. However, the relationship between microglial activation, FasL generation and neuronal death is not well understood and may involve multiple intracellular components [46–48]. In the present study, a rapid increase in P2X7 purinoceptor expression occurred in parallel with the early microglial activation in response to permanent microemboli injury, and P2X7 were predominantly expressed in ED1-expressing microglial cells. The P2X7 purinoceptor-mediated Ca2+ influx and activation of phospholipase C and the A2 cytoplasmic signaling pathways may be involved in proinflammatory cytokine release [36, 49]. Here, the BV-2 cells with RNA interference and P2X7 −/− mice were used to further address the linkage between P2X7 and FasL overproduction. Our data revealed that FasL production was increased in the OCM-treated BV-2 cells, whereas P2X7 interference partially but significantly prevented this OCM-induced FasL overproduction. Similar results were also observed in P2X7 −/− mice with microsphere embolism in vivo. Monif et al.  reported that the overexpression of P2X7 purinoceptors was sufficient to drive microglial activation and morphological transformation. Therefore, we suggested that P2X7 purinoceptor may be an important upstream component of FasL generation, the activation of P2X7 in microglia contributes to a gradual accumulation of FasL along with the subsequent activation of downstream cascades.
The autocrine action of FasL-Fas system has been shown in vascular smooth muscle cells , lymphocytes , and leukemia K562 cells . In the present study, the BV-2 cells and primary microglia from mice genetically deficient in P2X7 were used to further establish a linkage between microglia activation and FasL overproduction. Of particular relevance was an examination of microglial cultures with recombinant FasL treatment, which revealed that recombinant FasL significantly triggered microglia migration whereas was blocked by FasL antibody. In functional terms, we found that the levels of FasL generation agreed with the kinetics of migration of BV-2 cells. The microglial-mediated inflammatory autocrine responses involving cytokines are one important consideration in the pathology of neurovascular damage [52–54]. We suggested that the overproduction of FasL itself is sufficient functions as a key component of immunoreactive response loop by recruiting microglia to the lesion sites. Moreover, preincubation with FasL antibody significantly diminished the ATP-evoked chemotactic response of primary microglia in vitro. Therefore, we suggested that OGD-induced, microglia-derived FasL might be a key microenvironmental signal, which stimulated an extensive microglial response in ischemic context.
Tight regulation of microglial activation might be exploited to avoid unexpected and excessive cell death in the compromised brain . We found that the deletion of P2X7 purinoceptors in vivo using P2X7 −/− mice significantly reduced the expression of FasL and concomitantly neuronal cell death in the embolic hemisphere of brain, which implicated that a constitutive elevation of FasL might act in concert with P2X7 to induce neurotoxicity. Mutations in Fas ligand are responsible for the single gene autoimmune gld phenotypes in mice . Here, our data further indicate that gld/gld mice deficient in FasL were highly resistant to ischemia-induced neuronal damage. In addition, we found that minocycline treatment inhibited FasL signaling and subsequently reduced neuronal cell death in animals after microsphere embolism, which further suggested that microglia-derived FasL might contributes to a vicious cycle of neuronal cell death. Together, in vivo data also support the idea that P2X7 is involved in regulating the pathological immune response of microglia to microsphere-induced embolism via maintaining the activation of FasL.
The astrocyte-microglia collaboration forms a first-line innate immune mechanism in the parenchyma to detect mediators released from injured cells [56–58]. Whether FasL generation by activated microglia contributed to the activation of astrocytes in the present study, however, is a topic for further investigation. Additionally, when activated microglia change into a chronic profile following an injury, the cells either maintain their acute phenotype or divert into another activation state, may be chronically beneficial for neuronal survival and differentiation [10, 59]. Therefore, to elucidate the role of P2X7/FasL signaling during chronic phase of ischemia should enable further progress to be made in the therapeutic translation of experimental observations to the bedside.
Bovine serum albumin
Fas-associated death domain
Glial fibrillary acidic protein
High mobility group box 1
Sodium dodecyl sulfate polyacrylamide gel electrophoresis.
This work was supported in part by National Natural Science Foundations of China (30973521 to FH); Projects of International Cooperation and Exchanges NSFC (81120108023 to FH); Zhejiang Provincial Natural Science Foundation of China (R2100281 to FH); The Science Research Foundation of Chinese Ministry of Health (WKJ2009-2-029 to FH); The Cooperative Research Foundation of Japan China Medical Association, Japan (2009 to KF).
- del Zoppo GJ: Microvascular changes during cerebral ischemia and reperfusion. Cerebrovasc Brain Metab Rev 1994, 6:47–96.PubMedGoogle Scholar
- Lam CK, Yoo T, Hiner B, Liu Z, Grutzendler J: Embolus extravasation is an alternative mechanism for cerebral microvascular recanalization. Nature 2010, 465:478–482.View ArticlePubMedPubMed CentralGoogle Scholar
- Iadecola C, Anrather J: The immunology of stroke: from mechanisms to translation. Nat Med 2011, 17:796–808.View ArticlePubMedPubMed CentralGoogle Scholar
- Han F, Shioda N, Moriguchi S, Qin ZH, Fukunaga K: Downregulation of glutamate transporters is associated with elevation in extracellular glutamate concentration following rat microsphere embolism. Neurosci Lett 2008, 430:275–280.View ArticlePubMedGoogle Scholar
- Han F, Shirasaki Y, Fukunaga K: Microsphere embolism-induced endothelial nitric oxide synthase expression mediates disruption of the blood–brain barrier in rat brain. J Neurochem 2006, 99:97–106.View ArticlePubMedGoogle Scholar
- Han F, Tao RR, Zhang GS, Lu YM, Liu LL, Chen YX, Lou YJ, Fukunaga K, Hong ZH: Melatonin ameliorates ischaemic-like injury-evoked nitrosative stress: Involvement of HtrA2/PED pathways in endothelial cells. J Pineal Res 2011, 50:281–291.View ArticlePubMedGoogle Scholar
- del Zoppo GJ, Milner R, Mabuchi T, Hung S, Wang X, Berg GI, Koziol JA: Microglial activation and matrix protease generation during focal cerebral ischemia. Stroke 2007,38(Suppl 2):646–651.View ArticlePubMedGoogle Scholar
- Macrez R, Ali C, Toutirais O, Le Mauff B, Defer G, Dirnagl U, Vivien D: Stroke and the immune system: from pathophysiology to new therapeutic strategies. Lancet Neurol 2011, 10:471–480.View ArticlePubMedGoogle Scholar
- Masuda T, Croom D, Hida H, Kirov SA: Capillary blood flow around microglial somata determines dynamics of microglial processes in ischaemic conditions. Glia 2011, 59:1744–1753.View ArticlePubMedPubMed CentralGoogle Scholar
- Lambert C, Landau AM, Desbarats J: Fas-beyond death: a regenerative role for Fas in the nervous system. Apoptosis 2003, 8:551–562.View ArticlePubMedGoogle Scholar
- Fukunaga K, Ishigami T, Kawano T: Transcriptional regulation of neuronal genes and its effect on neural functions: expression and function of forkhead transcription factors in neurons. J Pharmacol Sci 2005, 98:205–211.View ArticlePubMedGoogle Scholar
- Martin-Villalba A, Hahne M, Kleber S, Vogel J, Falk W, Schenkel J, Krammer PH: Therapeutic neutralization of CD95-ligand and TNF attenuates brain damage in stroke. Cell Death Differ 2001, 8:679–686.View ArticlePubMedGoogle Scholar
- Choi C, Benveniste EN: Fas ligand/Fas system in the brain: regulator of immune and apoptotic responses. Brain Res Brain Res Rev 2004, 44:65–81.View ArticlePubMedGoogle Scholar
- Apenberg S, Freyberg MA, Friedl P: Shear stress induces apoptosis in vascular smooth muscle cells via an autocrine Fas/FasL pathway. Biochem Biophys Res Commun 2003, 310:355–359.View ArticlePubMedGoogle Scholar
- Tan KH, Hunziker W: Compartmentalization of Fas and Fas ligand may prevent auto- or paracrine apoptosis in epithelial cells. Exp Cell Res 2003, 284:283–290.View ArticlePubMedGoogle Scholar
- Hallermalm K, De Geer A, Kiessling R, Levitsky V, Levitskaya J: Autocrine secretion of Fas ligand shields tumor cells from Fas-mediated killing by cytotoxic lymphocytes. Cancer Res 2004, 64:6775–6782.View ArticlePubMedGoogle Scholar
- Giovannini C, Matarrese P, Scazzocchio B, Vari R, D’Archivio M, Straface E, Masella R, Malorni W, De Vicenzi M: Wheat gliadin induces apoptosis of intestinal cells via an autocrine mechanism involving Fas-Fas ligand pathway. FEBS Lett 2003, 540:117–124.View ArticlePubMedGoogle Scholar
- Iadecola C, Sugimoto K, Niwa K, Kazama K, Ross ME: Increased susceptibility to ischaemic brain injury in cyclooxygenase-1-deficient mice. J Cereb Blood Flow Metab 2001, 21:1436–1441.View ArticlePubMedGoogle Scholar
- Jin G, Hayashi T, Kawagoe J, Takizawa T, Nagata T, Nagano I, Syoji M, Abe K: Deficiency of PAR-2 gene increases acute focal ischaemic brain injury. J Cereb Blood Flow Metab 2005, 25:302–313.View ArticlePubMedGoogle Scholar
- Kawano T, Anrather J, Zhou P, Park L, Wang G, Frys KA, Kunz A, Cho S, Orio M, Iadecola C: Prostaglandin E2 EP1 receptors: downstream effectors of COX-2 neurotoxicity. Nat Med 2006, 12:225–229.View ArticlePubMedGoogle Scholar
- Yenari MA, Xu L, Tang XN, Qiao Y, Giffard RG: Microglia potentiate damage to blood–brain barrier constituents: improvement by minocycline in vivo and in vitro. Stroke 2006, 37:1087–1093.View ArticlePubMedGoogle Scholar
- Orio M, Kunz A, Kawano T, Anrather J, Zhou P, Iadecola C: Lipopolysaccharide induces early tolerance to excitotoxicity via nitric oxide and cGMP. Stroke 2007, 38:2812–2817.View ArticlePubMedGoogle Scholar
- Vogt M, Bauer MK, Ferrari D, Schulze-Osthoff K: Oxidative stress and hypoxia/reoxygenation trigger CD95 (APO-1/Fas) ligand expression in microglial cells. FEBS Lett 1998, 429:67–72.View ArticlePubMedGoogle Scholar
- Facchinetti F, Furegato S, Terrazzino S, Leon A: H(2)O(2) induces upregulation of Fas and Fas ligand expression in NGF-differentiated PC12 cells: modulation by cAMP. J Neurosci Res 2002, 69:178–188.View ArticlePubMedGoogle Scholar
- Ekdahl CT, Kokaia Z, Lindvall O: Brain inflammation and adult neurogenesis: the dual role of microglia. Neuroscience 2009, 158:1021–1029.View ArticlePubMedGoogle Scholar
- Gao F, Wang S, Guo Y, Wang J, Lou M, Wu J, Ding M, Tian M, Zhang H: Protective effects of repetitive transcranial magnetic stimulation in a rat model of transient cerebral ischaemia: a microPET study. Eur J Nucl Med Mol Imaging 2010, 37:954–961.View ArticlePubMedGoogle Scholar
- Kim TK, Hemberg M, Gray JM, Costa AM, Bear DM, Wu J, Harmin DA, Laptewicz M, Barbara-Haley K, Kuersten S, Markenscoff-Papadimitriou E, Kuhl D, Bito H, Worley PF, Kreiman G, Greenberg ME: Widespread transcription at neuronal activity-regulated enhancers. Nature 2010, 465:182–187.View ArticlePubMedPubMed CentralGoogle Scholar
- Ullrich O, Diestel A, Bechmann I, Homberg M, Grune T, Hass R, Nitsch R: Turnover of oxidatively damaged nuclear proteins in BV-2 microglial cells is linked to their activation state by poly-ADP-ribose polymerase. FASEB J 2001, 15:1460–1462.PubMedGoogle Scholar
- Jang S, Kelley KW, Johnson RW: Luteolin reduces IL-6 production in microglia by inhibiting JNK phosphorylation and activation of AP-1. Proc Natl Acad Sci USA 2008, 105:7534–7539.View ArticlePubMedPubMed CentralGoogle Scholar
- Honda S, Sasaki Y, Ohsawa K, Imai Y, Nakamura Y, Inoue K, Kohsaka S: Extracellular ATP or ADP induce chemotaxis of cultured microglia through Gi/o-coupled P2Y receptors. J Neurosci 2001, 21:1975–1982.PubMedGoogle Scholar
- Giulian D, Baker TJ: Characterization of ameboid microglia isolated from developing mammalian brain. J Neurosci 1986, 6:2163–2178.PubMedGoogle Scholar
- Dou Y, Wu HJ, Li HQ, Qin S, Wang YE, Li J, Lou HF, Chen Z, Li XM, Luo QM, Duan S: Microglial migration mediated by ATP-induced ATP release from lysosomes. Cell Res 2012, 22:1022–1033.View ArticlePubMedPubMed CentralGoogle Scholar
- Schmued LC, Stowers CC, Scallet AC, Xu L: Fluoro-Jade C results in ultra high resolution and contrast labeling of degenerating neurons. Brain Res 2005, 1035:24–31.View ArticlePubMedGoogle Scholar
- Glezer I, Simard AR, Rivest S: Neuroprotective role of the innate immune system by microglia. Neuroscience 2007, 147:867–883.View ArticlePubMedGoogle Scholar
- Ito D, Imai Y, Ohsawa K, Nakajima K, Fukuuchi Y, Kohsaka S: Microglia-specific localisation of a novel calcium binding protein, Iba1. Brain Res Mol Brain Res 1998, 57:1–9.View ArticlePubMedGoogle Scholar
- Witting A, Walter L, Wacker J, Möller T, Stella N: P2X7 receptors control 2-arachidonoylglycerol production by microglial cells. Proc Natl Acad Sci USA 2004, 101:3214–3219.View ArticlePubMedPubMed CentralGoogle Scholar
- Sharp AJ, Polak PE, Simonini V, Lin SX, Richardson JC, Bongarzone ER, Feinstein DL: P2x7 deficiency suppresses development of experimental autoimmune encephalomyelitis. J Neuroinflammation 2008, 5:33.View ArticlePubMedPubMed CentralGoogle Scholar
- Kim JE, Ryu HJ, Yeo SI, Kang TC: P2X7 receptor regulates leukocyte infiltrations in rat frontoparietal cortex following status epilepticus. J Neuroinflammation 2010, 7:65.View ArticlePubMedPubMed CentralGoogle Scholar
- Melani A, Amadio S, Gianfriddo M, Vannucchi MG, Volontè C, Bernardi G, Pedata F, Sancesario G: P2X7 receptor modulation on microglial cells and reduction of brain infarct caused by middle cerebral artery occlusion in rat. J Cereb Blood Flow Metab 2006, 26:974–982.View ArticlePubMedGoogle Scholar
- Tang M, Alexander H, Clark RS, Kochanek PM, Kagan VE, Bayir H: Minocycline reduces neuronal death and attenuates microglial response after pediatric asphyxial cardiac arrest. J Cereb Blood Flow Metab 2010, 30:119–129.View ArticlePubMedGoogle Scholar
- Henry CJ, Huang Y, Wynne A, Hanke M, Himler J, Bailey MT, Sheridan JF, Godbout JP: Minocycline attenuates lipopolysaccharide (LPS)-induced neuroinflammation, sickness behavior, and anhedonia. J Neuroinflammation 2008, 5:15.View ArticlePubMedPubMed CentralGoogle Scholar
- Streit WJ, Mrak RE, Griffin WS: Microglia and neuroinflammation: a pathological perspective. J Neuroinflammation 2004, 1:14.View ArticlePubMedPubMed CentralGoogle Scholar
- Griffin WS: Inflammation and neurodegenerative diseases. Am J Clin Nutr 2006, 83:470S-474S.PubMedGoogle Scholar
- Mrak RE, Griffin WS: Glia and their cytokines in progression of neurodegeneration. Neurobiol Aging 2005, 26:349–354.View ArticlePubMedGoogle Scholar
- Davalos D, Grutzendler J, Yang G, Kim JV, Zuo Y, Jung S, Littman DR, Dustin ML, Gan WB: ATP mediates rapid microglial response to local brain injury in vivo. Nat Neurosci 2005, 8:752–758.View ArticlePubMedGoogle Scholar
- Bechmann I, Mor G, Nilsen J, Eliza M, Nitsch R, Naftolin F: FasL (CD95L, Apo1L) is expressed in the normal rat and human brain: evidence for the existence of an immunological brain barrier. Glia 1999, 27:62–74.View ArticlePubMedGoogle Scholar
- Beer R, Franz G, Schöpf M, Reindl M, Zelger B, Schmutzhard E, Poewe W, Kampfl A: Expression of Fas and Fas ligand after experimental traumatic brain injury in the rat. J Cereb Blood Flow Metab 2000, 20:669–677.View ArticlePubMedGoogle Scholar
- Terrazzino S, Bauleo A, Baldan A, Leon A: Peripheral LPS administrations up-regulate Fas and FasL on brain microglial cells: a brain protective or pathogenic event? J Neuroimmunol 2002, 124:45–53.View ArticlePubMedGoogle Scholar
- Clark AK, Wodarski R, Guida F, Sasso O, Malcangio M: Cathepsin S release from primary cultured microglia is regulated by the P2X7 receptor. Glia 2010, 58:1710–1726.View ArticlePubMedGoogle Scholar
- Monif M, Reid CA, Powell KL, Smart ML, Williams DA: The P2X7 receptor drives microglial activation and proliferation: a trophic role for P2X7R pore. J Neurosci 2009, 29:3781–3791.View ArticlePubMedGoogle Scholar
- Chen KC, Liu WH, Chang LS: Suppression of ERK signaling evokes autocrine Fas-mediated death in arachidonic acid-treated human chronic myeloid leukemia K562 cells. J Cell Physiol 2010, 222:625–634.View ArticlePubMedGoogle Scholar
- Becher B, Dodelet V, Fedorowicz V, Antel JP: Soluble tumor necrosis factor receptor inhibits interleukin 12 production by stimulated human adult microglial cells in vitro. J Clin Invest 1996, 98:1539–1543.View ArticlePubMedPubMed CentralGoogle Scholar
- Bezzi P, Domercq M, Brambilla L, Galli R, Schols D, De Clercq E, Vescovi A, Bagetta G, Kollias G, Meldolesi J, Volterra A: CXCR4-activated astrocyte glutamate release via TNFalpha: amplification by microglia triggers neurotoxicity. Nat Neurosci 2001, 4:702–710.View ArticlePubMedGoogle Scholar
- Meucci O, Fatatis A, Simen AA, Miller RJ: Expression of CX3CR1 chemokine receptors on neurons and their role in neuronal survival. Proc Natl Acad Sci USA 2000, 97:8075–8080.View ArticlePubMedPubMed CentralGoogle Scholar
- Ramsdell F, Seaman MS, Miller RE, Tough TW, Alderson MR, Lynch DH: gld/gld mice are unable to express a functional ligand for Fas. Eur J Immunol 1994, 24:928–933.View ArticlePubMedGoogle Scholar
- O’Connell J, Houston A, Bennett MW, O’Sullivan GC, Shanahan F: Immune privilege or inflammation? Insights into the Fas ligand enigma. Nat Med 2001, 7:271–274.View ArticlePubMedGoogle Scholar
- Kim JV, Dustin ML: Innate response to focal necrotic injury inside the blood–brain barrier. J Immunol 2006, 177:5269–5277.View ArticlePubMedGoogle Scholar
- Saijo K, Winner B, Carson CT, Collier JG, Boyer L, Rosenfeld MG, Gage FH, Glass CK: A Nurr1/CoREST pathway in microglia and astrocytes protects dopaminergic neurons from inflammation-induced death. Cell 2009, 137:47–59.View ArticlePubMedPubMed CentralGoogle Scholar
- Bonde S, Ekdahl CT, Lindvall O: Long-term neuronal replacement in adult rat hippocampus after status epilepticus despite chronic inflammation. Eur J Neurosci 2006, 23:965–974.View ArticlePubMedGoogle Scholar
This article is published under license to BioMed Central Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.