A quantitative study of microglial-macrophage synthesis of tumor necrosis factor during acute and late focal cerebral ischemia in mice

Toll-like receptor 4 is involved in subacute stress-induced neuroinflammation and in the worsening of experimental stroke

BACKGROUND AND PURPOSE

Psychological stress causes an inflammatory response in the brain and is able to exacerbate brain damage caused by experimental stroke. We previously reported that subacute immobilization stress in mice worsens stroke outcome through mechanisms that involve inflammatory mechanisms, such as accumulation of oxidative/nitrosative mediators and expression of inducible nitric oxide synthase and cyclooxygenase-2 in the brain. Some of these inflammatory mediators could be regulated by innate immunity, the activation of which takes place in the brain and produces an inflammatory response mediated by toll-like receptors (TLRs). Recently, we described the implications of TLR4 in ischemic injury, but the role of TLR4 in stress has not yet been examined. We therefore investigated whether inflammation produced by immobilization stress differs in mice that lack a functional TLR4 signaling pathway.

METHODS

We used an experimental paradigm consisting of the exposure of mice to repeated immobilization sessions (1 hour daily for 7 days) before permanent middle cerebral artery occlusion.

RESULTS

We found that TLR4-deficient mice subjected to subacute stress had a better behavioral condition compared with normal mice (C3H/HeN) and that this effect was associated with a minor inflammatory response (cyclooxygenase-2 and inducible nitric oxide synthase expression) and lipid peroxidation (malondialdehyde levels) in brain tissue. Furthermore, previous exposure to stress was followed by a smaller infarct volume after permanent middle cerebral artery occlusion in TLR4-deficient mice than in mice that express TLR4 normally.

CONCLUSIONS

Our results indicate that TLR4 is involved in the inflammatory response after subacute stress and its exacerbating effect on stroke. These data implicate the effects of innate immunity on inflammation and damage in the brain after stroke.

Pattern of time-dependent reduction of histologically determined infarct volume after focal ischaemia in mice

The mouse model of transcranial permanent occlusion of the middle cerebral artery (tpMCAO) is widely used in stroke research. Here we quantified infarct size using a conventional histological method at several post-ischaemic times, going beyond the commonly analysed period of up to 2 days, following artery occlusion. Two different mouse strains, which are widely used for pharmacological studies of neuroprotection and for genetic engineering, were used. A drill whole was made into the skull of anaesthetised mice and ischaemia was induced by electrocoagulation of the middle cerebral artery. In both mouse strains tested (C57Black/6 and NMRI), the measured infarct volumes decreased significantly during the first days after tpMCAO. Notably, 13 days after surgery, ischaemic and sham-operated animals had indistinguishably small lesions, which where in the range of only 5% of the infarct size on day 2 post-ischaemia. The standard method of calculating oedema and shrinkage correction provided no sufficient explanation for this significant decrease in infarct volume. There was, however, evidence that structural changes in the residual ipsilateral hemisphere may compromise the significance of results arising from the method of calculating oedema and shrinkage correction. In conclusion, our study indicates that the pronounced and fast, time-dependent decrease in histologically defined infarct volume can compromise results when studying the lasting neuroprotective effects of potential drugs.

Changes in brain levels of N-acylethanolamines and 2-arachidonoylglycerol in focal cerebral ischemia in mice

The N-acylethanolamines (NAEs) and 2-arachidonoylglycerol (2-AG) are bioactive lipids that can modulate inflammatory responses and protect neurons against glutamatergic excitotoxicity. We have used a model of focal cerebral ischemia in young adult mice to investigate the relationship between focal cerebral ischemia and endogenous NAEs. Over the first 24 h after induction of permanent middle cerebral artery occlusion, we observed a time-dependent increase in all the investigated NAEs, except for anandamide. Moreover, we found an accumulation of 2-AG at 4 h that returned to basal level 12 h after induction of ischemia. Accumulation of NAEs did not depend on regulation of N-acylphosphatidylethanolamine-hydrolyzing phospholipase D or fatty acid amide hydrolase. Treatment with the fatty acid amide hydrolase inhibitor URB597 (cyclohexyl carbamic acid 3'-carbamoyl-biphenyl-3-yl ester; 1 mg/kg; i.p.) 1.5 h before arterial occlusion decreased the infarct volume in our model system. Our results suggest that NAEs and 2-AG may be involved in regulation of neuroprotection during focal cerebral ischemia in mice.

Toll-like receptor 4 is involved in brain damage and inflammation after experimental stroke

BACKGROUND

Stroke is the second to third leading cause of death. Toll-like receptor 4 (TLR4) is a signaling receptor in innate immunity that is a specific immunologic response to systemic bacterial infection and cerebral injury. The role of TLR4 in brain ischemia has not been examined yet. We have therefore investigated whether cerebral ischemia and inflammation produced by permanent occlusion of the middle cerebral artery differ in mice that lack a functional TLR4 signaling pathway.

METHODS AND RESULTS

Permanent occlusion of the middle cerebral artery was performed on 2 strains of TLR4-deficient mice (C3H/HeJ and C57BL/10ScNJ) and respective controls (C3H/HeN and C57BL/10ScSn). Stroke outcome was evaluated by determination of infarct volume and assessment of neurological scores. Brains were collected 24 hours and 7 days after stroke. When compared with control mice, TLR4-deficient mice had lower infarct volumes and better outcomes in neurological and behavioral tests. Mice that lacked TLR4 had minor expression of stroke-induced interferon regulatory factor-1, inducible nitric oxide synthase, and cyclooxygenase-2, mediators implicated in brain damage. The levels of interferon-beta and of the lipid peroxidation marker malondialdehyde were also lower in brains from TLR4-deficient mice than in those from control mice. In addition, the expression of matrix metalloproteinase-9, which is induced and mediates brain damage, was also reduced in TLR4-deficient mice after experimental stroke.

CONCLUSIONS

TLR4-deficient mice have minor infarctions and less inflammatory response after an ischemic insult. These data demonstrate that TLR4 signaling and innate immunity are involved in brain damage and in inflammation triggered by ischemic injury.

Microglia and macrophages express tumor necrosis factor receptor p75 following middle cerebral artery occlusion in mice

The proinflammatory and potential neurotoxic cytokine tumor necrosis factor (TNF) is produced by activated CNS resident microglia and infiltrating blood-borne macrophages in infarct and peri-infarct areas following induction of focal cerebral ischemia. Here, we investigated the expression of the TNF receptors, TNF-p55R and TNF-p75R, from 1 to 10 days following permanent occlusion of the middle cerebral artery in mice. Using quantitative polymerase chain reaction (PCR), we observed that the relative level of TNF-p55R mRNA was significantly increased at 1-2 days and TNF-p75R mRNA was significantly increased at 1-10 days following arterial occlusion, reaching peak values at 5 days, when microglial-macrophage CD11b mRNA expression was also increased. In comparison, the relative level of TNF mRNA was significantly increased from 1 to 5 days, with peak levels 1 day after arterial occlusion. In situ hybridization revealed mRNA expression of both receptors in predominantly microglial- and macrophage-like cells in the peri-infarct and subsequently in the infarct, and being most marked from 1 to 5 days. Using green fluorescent protein-bone marrow chimeric mice, we confirmed that TNF-p75R was expressed in resident microglia and blood-borne macrophages located in the peri-infarct and infarct 1 and 5 days after arterial occlusion, which was supported by Western blotting. The data show that increased expression of the TNF-p75 receptor following induction of focal cerebral ischemia in mice can be attributed to expression in activated microglial cells and blood-borne macrophages.

Gene expression profiling in the human middle cerebral artery after cerebral ischemia

We have investigated the gene expression in human middle cerebral artery (MCA) after ischemia. Ischemic stroke affects the perfusion in the affected area and experimental cerebral ischemia results in upregulation of vasopressor receptors in the MCA leading to the ischemic area. We obtained human MCA samples distributing to the ischemic area, 7-10 days post-stroke. The gene expression was examined with real-time polymerase chain reaction (PCR) and microarray, proteins were studied with immunohistochemistry. We investigated genes previously shown to be upregulated in animal models of cerebral ischemia (e.g. ET(A), ET(B), AT1, AT2, and 5-HT(2A/1B/1D)). Their mRNA expression was increased compared with controls, consistent with findings in experimental stroke. Immunohistochemistry showed upregulation of the receptors localized on the smooth muscle cells. The gene expression was profiled with microarray and seven genes chosen for further investigation with real-time PCR; ELK3, LY64, Metallothionin IG, POU3F4, Actin alpha2, RhoA and smoothelin. Six of these were regulated the same way when confirming array expression with real-time PCR. Gene expression studies in the human MCA leading to the ischemic region is similar to that seen after MCA occlusion in rats. We found new genes that support the dynamic changes that occur in the MCA distributing to the ischemic region.

Secretory phospholipase A2 IIA is up-regulated by TNF-alpha and IL-1alpha/beta after transient focal cerebral ischemia in rat

Cerebral ischemia initiates an inflammatory response in the brain that is associated with the induction of a variety of cytokines, including tumor necrosis factor-alpha (TNF-alpha) and interleukin-1alpha/beta (IL-1alpha/beta) that contributes to stroke injury. Transient middle cerebral artery occlusion (tMCAO) in spontaneously hypertensive rat (SHR) resulted in significant increases in TNF-alpha and IL-1beta levels. We have previously demonstrated up-regulation of secretory phospholipase A2 IIA (sPLA2 IIA) mRNA and protein expression, increased PLA2 activity, and loss of phosphatidylcholine after 1-h tMCAO and 24-h reperfusion in SHR. Treatment with TNF-alpha antibody or IL-1 receptor antagonist significantly attenuated infarction volume, sPLA2 IIA protein expression, PLA2 activity and significantly restored phosphatidylcholine levels after tMCAO. This suggests that cytokine induction up-regulates sPLA2 IIA protein expression, resulting in altered lipid metabolism that contributes to stroke injury.

Tumor necrosis factor and its p55 and p75 receptors are not required for axonal lesion-induced microgliosis in mouse fascia dentata

Tumor necrosis factor (TNF) is a potent pro-inflammatory and neuromodulatory cytokine. In the CNS it is produced primarily by microglia and considered to regulate microglial activation. On the basis of previous observations of increased microglial TNF mRNA synthesis in areas of anterograde axonal and terminal degeneration in mice, we studied the effect of TNF and its p55 and p75 receptors on axonal lesion-induced microglial activation in fascia dentata following transection of the perforant path (PP) projection. Unexpectedly, cell counting showed that the axonal lesion-induced microglial response in TNF and TNF-p55p75 receptor knock out mice and C57BL/6 mice was similar 5 days after the lesion. In addition, the microglial expression of the lysosomal-associated antigen CD68, and the clearance of MBP(+) myelin debris appeared similar in TNF and TNF-p55p75 receptor knock out mice compared to C57BL/6 mice. Quantitative PCR and in situ hybridization showed the expression of TNF mRNA to be maximally upregulated 6 h after the lesion, and confirmed that TNF mRNA was still upregulated 5 days after lesion when microglial numbers, CD11b mRNA level, and cellular TNF-p55 and -p75 receptor mRNA level reached maximum. However, in spite of the induction of TNF mRNA, TNF protein level remained at base-line in fascia dentata using immunohistochemistry and ELISA. In conclusion, the results showed a lower than expected lesion-induced increase in TNF protein, and that neither TNF nor its receptors were required for the axonal lesion-induced microglial morphological transformation and proliferation or for the initial clearance of degenerated myelin in the PP-deafferented fascia dentata.

Anti-inflammatory effects of PJ34, a poly(ADP-ribose) polymerase inhibitor, in transient focal cerebral ischemia in mice

BACKGROUND AND PURPOSE

Activation of poly(ADP-ribose) polymerase (PARP) is deleterious during cerebral ischemia. We assessed the influence of PARP activation induced by cerebral ischemia on the synthesis of proinflammatory mediators including the cytokines, tumour necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6) and the adhesion molecules, E-selectin and intercellular adhesion molecule-1 (ICAM-1).

EXPERIMENTAL APPROACH

Ischemia was induced by intravascular occlusion of the left middle cerebral artery for 1 h in male Swiss mice anaesthetized with ketamine and xylazine. The PARP inhibitor PJ34 (1.25-25 mg kg(-1)) was administered intraperitoneally 15 min before and 4 hours after, the onset of ischemia. Animals were killed 6 h or 24 h after ischemia and cerebral tissue removed for analysis.

KEY RESULTS

Ischemia increased TNF-alpha protein in cerebral tissue at 6 and 24 h after ischemia. All doses of PJ34 blocked the increase in TNF-alpha at 6 h and 25 mg kg(-1) PJ34 had a sustained effect for up to 24 h. Quantitative real time polymerase chain reaction showed that PJ34 (25 mg kg(-1)) reduced the increase in TNF-alpha mRNA by 70% at 6 h. PJ34 also prevented the increase in mRNAs encoding IL-6 (-41%), E-selectin (-81%) and ICAM-1 (-54%). PJ34 (25 mg kg(-1)) reduced the infarct volume (-26%) and improved neurological deficit, 24 h after ischemia.

CONCLUSIONS AND IMPLICATIONS

PJ34 inhibited the increase in the mRNAs of four inflammatory mediators, caused by cerebral ischemia. The contribution of this effect of PJ34 to neuroprotection remains to be clarified.

Glyceraldehyde-3-phosphate dehydrogenase versus toluidine blue as a marker for infarct volume estimation following permanent middle cerebral artery occlusion in mice

Infarct size is a good predictor of the neurological outcome following stroke. Estimation of infarct size in the early phase following experimental stroke depends on the availability of reliable techniques that can distinguish ischemic from nonischemic tissue. The objective of this study was to provide a simple and robust method for reliable delineation of the ischemic infarct area in fresh frozen cryosections from mice subjected to focal cerebral ischemia. Mice were subjected to permanent middle cerebral artery (MCA) occlusion and euthanised after 30 min, 1, 2, 4, 6, 12 and 24 h. The size of the developing infarct was compared in parallel series of sections in situ hybridized for mRNA encoding the enzyme glyceraldehyde-3-phosphate dehydrogenase (GAPDH) or stained with toluidine blue (TB). The infarct was clearly delineated in GAPDH mRNA in situ hybridized sections as soon as 4 h after MCA occlusion. Infarct size was similar at 4 and 6 h in GAPDH mRNA in situ hybridized sections. Sections hybridized for GAPDH mRNA showed significantly larger infarcts than sections stained with TB after 6 h but not after 24 h of ischemia. Analysis of in situ hybridized sections revealed changes in neuronal GAPDH mRNA in areas prone to undergo degeneration 30 min to 1 h after MCA occlusion, thereby preceding visible pycnosis in TB-stained sections. The results showed that in situ hybridization for GAPDH mRNA was a reliable method and superior to TB staining for precise infarct delineation prior to 6 h of permanent MCA occlusion.

Differential role of tumor necrosis factor receptors in mouse brain inflammatory responses in cryolesion brain injury

Tumor necrosis factor-alpha (TNF-alpha) is one of the mediators dramatically increased after traumatic brain injury that leads to the activation, proliferation, and hypertrophy of mononuclear, phagocytic cells and gliosis. Eventually, TNF-alpha can induce both apoptosis and necrosis via intracellular signaling. This cytokine exerts its functions via interaction with two receptors: type-1 receptor (TNFR1) and type-2 receptor (TNFR2). In this work, the inflammatory response after a freeze injury (cryolesion) in the cortex was studied in wild-type (WT) animals and in mice lacking TNFR1 (TNFR1 KO) or TNFR2 (TNFR2 KO). Lack of TNFR1, but not of TNFR2, significantly decreased the inflammatory response and tissue damage elicited by the cryolesion at both 3 and 7 days postlesion, with decreased gliosis, lower IL-1beta immunostaining, and a reduction of apoptosis markers. Cryolesion produced a clear induction of the proinflammatory cytokines interleukin (IL)-1alpha, IL-1beta, IL-6, and TNF-alpha; this induction was significantly lower in the TNFR1 KO mice. Host response genes (ICAM-1, A20, EB22/5, and GFAP) were also induced by the cryolesion, but to a lesser extent in TNFR1 KO mice. Lack of TNFR1 signaling also affected the expression of apoptosis/cell death-related genes (Fas, Rip, p53), matrix metalloproteinases (MMP3, MMP9, MMP12), and their inhibitors (TIMP1), suggesting a role of TNFR1 in extracellular matrix remodeling after injury. However, GDNF, NGF, and BDNF expression were not affected by TNFR1 deficiency. Overall, these results suggest that TNFR1 is involved in the early establishment of the inflammatory response and that its deficiency causes a decreased inflammatory response and tissue damage following brain injury.

Validation of two reference genes for mRNA level studies of murine disease models in neurobiology

Reverse transcription of extracted cellular RNA combined with real-time PCR is now an established method for sensitive detection and quantification of specific mRNA level changes in experimental models of neurological diseases. To neutralize the impact of experimental error and make quantification more precise, normalization of test gene data using data from a constantly expressed gene, a reference gene that is tested along with the test gene, is required. There is no single gene constantly expressed under all experimental conditions. For a given set of conditions or a given disease model, identification of an unaffected reference gene is necessary. In this report, we present our findings from evaluation and validation of the genes encoding hypoxanthine guanine phosphoribosyl transferase 1 (HPRT1) and glyceraldehyde phosphate dehydrogenase (GAPDH) as individual reference genes in mRNA level studies involving four murine neurological disease models. We find both genes are suitable as a reference gene with these four models, provided quantification of subtle changes are avoided. We furthermore demonstrate that above a certain threshold of test mRNA level changes and given high quality RNA processing, normalization to total RNA alone provides for equally reliable quantitative mRNA level results.

A quantitative in situ hybridization and polymerase chain reaction study of microglial-macrophage expression of interleukin-1beta mRNA following permanent middle cerebral artery occlusion in mice

Interleukin-1beta (IL-1beta) is known to play a central role in ischemia-induced brain damage in rodents. In comparison to the rat, however, the available data on the cellular synthesis of IL-1beta mRNA and protein in the mouse are very limited. Here, we report on the time profile, the topography and the quantitative, cellular expression of IL-1beta mRNA in mice subjected to permanent occlusion of the distal middle cerebral artery (MCA). The in situ hybridization analysis showed that IL-1beta mRNA was expressed during the first post-surgical hour in a small number of high-expressing macrophage-like cells, located in cortical layers I and II of the future infarct. At 2 h, a significant number of faintly labeled IL-1beta mRNA-expressing cells had appeared in the developing peri-infarct, and the number remained constant at 4 h and 6 h, when the hybridization signal began to distribute to the cellular processes. Quantitative PCR performed on whole hemispheres showed a significant 20-fold increase in the relative level of IL-1beta mRNA at 12 h and a highly significant 42-fold increase at 24 h, at which time single IL-1beta mRNA-expressing cells were supplemented by aggregates and perivascular infiltrates of intensely labeled IL-1beta mRNA-expressing cells. Immunohistochemistry and double immunohistochemical stainings in addition to combined in situ hybridization, confirmed that the intensely labeled IL-1beta mRNA-expressing and IL-1beta protein synthesizing cells predominantly were glial fibrillary acidic protein-immunonegative, macrophage associated antigen-1-immunopositive microglia-macrophages. By day 5 there was a dramatic decline in the relative level of IL-1beta mRNA in the ischemic hemisphere. In summary, the data provide evidence that permanent occlusion of the distal MCA in mice results in expression of IL-1beta mRNA and IL-1beta synthesis in spatially and temporally segregated subpopulations of microglia and macrophages.

Increased expression of macrophage receptor with collagenous structure (MARCO) in mouse cortex following middle cerebral artery occlusion

Ischaemia induces activation of resident microglia and infiltration of peripheral monocyte/macrophage cells into the central nervous system. The role of scavenger receptors, receptors critical to the recognition and clearance of cell debris, has not been investigated during cerebral ischaemia. MARCO is an inducible member of the scavenger receptor family unique to cells of monocytic lineage and is a cell surface marker that plays a critical role in the differentiation of monocytes to dendritic cells. To understand the role of MARCO in cerebral ischaemia, we investigated its expression in mice following middle cerebral artery (MCA) occlusion. No MARCO mRNA expression was observed in naive mouse brain. There was no significant increase in expression of MARCO mRNA following transient occlusion (60min) of the MCA at any time point up to 24 h. However, a significant, marked increase in MARCO mRNA expression was observed at 24 h in the cortex of mouse brains after a permanent occlusion of the MCA. The increased expression of MARCO mRNA at 24 h after prolonged ischaemia is consistent with its putative role in the clearance of debris and/or degenerating cells after severe ischaemia and supports previous publications showing the presence of dendritic cells around permanently occluded lesions.