[(18)F]-fluoro-L-thymidine PET and advanced MRI for preoperative grading of gliomas.

PURPOSE: Conventional MRI based on contrast enhancement is often not sufficient in differentiating grade II from grade III and grade III from grade IV diffuse gliomas. We assessed advanced MRI, MR spectroscopy and [(18)F]-fluoro-l-thymidine ([(18)F]-FLT) PET as tools to overcome these limitations. METHODS: In this prospective study, thirty-nine patients with diffuse gliomas of grades II, III or IV underwent conventional MRI, perfusion, diffusion, proton MR spectroscopy ((1)H-MRS) and [(18)F]-FLT-PET imaging before surgery. Relative cerebral blood volume (rCBV), apparent diffusion coefficient (ADC), Cho/Cr, NAA/Cr, Cho/NAA and FLT-SUV were compared between grades. RESULTS: Cho/Cr showed significant differences between grade II and grade III gliomas (p = 0.03). To discriminate grade II from grade IV and grade III from grade IV gliomas, the most relevant parameter was the maximum value of [(18)F]-FLT uptake FLTmax (respectively, p < 0.001 and p < 0.0001). The parameter showing the best correlation with the grade was the mean value of [(18)F]-FLT uptake FLTmean (R(2) = 0.36, p < 0.0001) and FLTmax (R(2) = 0.5, p < 0.0001). CONCLUSION: Whereas advanced MRI parameters give indications for the grading of gliomas, the addition of [(18)F]-FLT-PET could be of interest for the accurate preoperative classification of diffuse gliomas, particularly for identification of doubtful grade III and IV gliomas.

D-JNKi, a peptide inhibitor of c-Jun N-terminal kinase, promotes functional recovery after transient focal cerebral ischemia in rats.

The c-Jun-N-terminal kinase (JNK) pathway has been shown to play an important role in excitotoxic neuronal death and several studies have demonstrated a neuroprotective effect of D-JNKi, a peptide inhibitor of JNK, in various models of cerebral ischemia. We have now investigated the effect of D-JNKi in a model of transient focal cerebral ischemia (90 min) induced by middle cerebral artery occlusion (MCAo) in adult male rats. D-JNKi (0.1 mg/kg), significantly decreased the volume of infarct, 3 days after cerebral ischemia. Sensorimotor and cognitive deficits were then evaluated over a period of 6 or 10 days after ischemia and infarct volumes were measured after behavioral testing. In behavioral studies, D-JNKi improved the general state of the animals as demonstrated by the attenuation of body weight loss and improvement in neurological score, as compared with animals receiving the vehicle. Moreover, D-JNKi decreased sensorimotor deficits in the adhesive removal test and improved cognitive function in the object recognition test. In contrast, D-JNKi did not significantly affect the infarct volume at day 6 and at day 10. This study shows that D-JNKi can improve functional recovery after transient focal cerebral ischemia in the rat and therefore supports the use of this molecule as a potential therapy for stroke.

Expression of the gene encoding the pro-apoptotic BNIP3 protein and stimulation of hypoxia-inducible factor-1alpha (HIF-1alpha) protein following focal cerebral ischemia in rats.

Hypoxia is a common cause of cell death and is implicated in many disease processes including stroke and chronic degenerative disorders. In response to hypoxia, cells express a variety of genes which allow adaptation to altered metabolic demands, decreased oxygen demands, and the removal of irreversibly damaged cells. Hypoxia-inducible factor-1 (HIF-1) is a transcription factor that regulates the adaptive response to hypoxia in cells. In this study, we reported an early, time-related, gradual up-regulation of HIF-1alpha, and a moderate increase in vascular endothelial growth factor (VEGF)- and erythropoietin (Epo)-levels following transient focal ischemia. Moreover, we demonstrated, for the first time a specific localization of the pro-apoptotic regulator BNIP3 in striatal and cortical neurons after transient focal ischemia in rats. Prolonged intranuclear BNIP3 immunoreactivity was associated with delayed neuronal death. Experiments showed protein increases on Western blots of brain tissue with peaks at 48h after ischemia. Epo responds to ischemia in an early stage, whereas VEGF and BNIP3 accumulate in cells at later times after ischemia. This suggests the possibility that BH3-only proteins might be one of the major downstream effectors of HIF-1alpha in hypoxic cell death. These findings open the possibility that the hypoxia-regulated pro-apoptotic protein BNIP3 enters the nucleus and could interact with other proteins involved in DNA structure, transcription or mRNA splicing after focal brain ischemia.

Angiopoietin-1-induced PI3-kinase activation prevents neuronal apoptosis.

Although angiopoietin-1 (Ang-1) is recognized as an endothelial growth factor, its presence in brain following an ischemic event suggests a role in the evolution of neuronal damage. Using primary neuronal cultures, we showed that neurons express Ang-1 and possess the functional angiopoietin-receptor Tie-2, which is phosphorylated in the presence of Ang-1. We further investigated in vitro whether Ang-1 could protect neurons against either excitotoxic necrosis or apoptosis induced by serum deprivation (SD). A neuroprotective effect for Ang-1 was detected exclusively in the apoptotic paradigm. Treatment of cells with the phosphatidyl-inositol 3-kinase (PI3-K) inhibitor, LY294002, inhibited Ang-1-induced phosphorylation of Akt, restored the cleavage of the effector caspase-3, and reduced the protective effect of Ang-1 against SD-induced toxicity. These findings suggest that Ang-1 has a neuroprotective effect against apoptotic stress and that this effect is dependent on the PI3-K/Akt pathway and inhibition of caspase-3 cleavage. This study provides evidence that Ang-1 is not just angiogenic but also neuroprotective. The understanding of neuroprotective mechanisms induced by Ang-1 may promote strategies based on the pleiotropic effects of angiogenic factors. Such approaches could be useful for the treatment of brain diseases in which both neuronal death and angiogenesis are involved.

Psychosis: pathological activation of limbic thalamocortical circuits by psychomimetics and schizophrenia?

Non-competitive NMDA receptor antagonists, such as phencyclidine, ketamine and MK801, produce psychosis in humans. These drugs also produce injury to cingulate-retrosplenial cortex in adult rodents that can be prevented by GABA-receptor agonists and antipsychotics such as haloperidol and clozapine. MK801 injections into anterior thalamus reproduce limbic cortex injury, and GABA-receptor agonist injections into anterior thalamus prevent injury produced by systemic MK801. Inhibition of NMDA receptors on GABAergic thalamic reticular nucleus neurons might activate thalamocortical 'injury' circuits in animals. Pathological activation of thalamocortical circuits might also mediate the psychosis produced by NMDA-receptor antagonists in humans, and might contribute to psychosis in schizophrenia.

Hypoxia-inducible factor in brain.

HIF-1 is composed of HIF-1alpha and HIF-1beta protein subunits. HIF-1 is induced by hypoxia and binds to promoter/enhancer elements and stimulates the transcription of hypoxia-inducible target genes. Because HIF-1 activation might promote cell survival in hypoxic tissues, we studied the effect of stroke on the expression of HIF-1alpha, HIF-1beta and several HIF-1 target genes in adult rat brain. After focal cerebral ischemia, mRNAs encoding HIF-1alpha, glucose transporter-1 and several glycolytic enzymes including lactate dehydrogenase were up-regulated in the areas around the infarction. HIF and its target genes were induced by 7.5 hours after the onset of ischemia and increased further at 19 and 24 hours. Since hypoxia induces HIF in other tissues, systemic hypoxia (6% O2 for 4.5 h) was also shown to increase HIF-1alpha protein expression in the adult rat brain. It is proposed that decreased blood flow to the penumbra decreases the supply of oxygen and that this induces HIF-1 and its target genes. Because HIF-1 activation may promote cell survival in hypoxic tissues, we studied the effect of hypoxic preconditioning on HIF-1 expression in neonatal rat brain. Hypoxic preconditioning (8% O2/3 hrs), a treatment known to protect the newborn rat brain against hypoxic-ischemic injury, markedly increased HIF-1alpha and HIF-1beta expression. We also studied the effect of two other known HIF-1 inducers, cobalt chloride (CoCl2) and desferrioxamine (DFX), on HIF-1 expression and neuroprotection in newborn brain. HIF-1alpha and HIF-1beta protein levels were markedly increased after i.p. injection of CoCl2 and DFX. Preconditioning with CoCl2 or DFX 24 hours before the stroke decreased infarction by 75% and 56% respectively, compared with vehicle-injected, littermate controls. Thus, HIF-1 activation could contribute to protective brain preconditioning.

Glial responses, clusterin, and complement in permanent focal cerebral ischemia in the mouse.

There is considerable evidence that complement activation occurs within the CNS in inflammatory and degenerative disorders, but little is known about its involvement in the pathophysiology of cerebral ischemia. Our study sought to characterize the glial response and the expression of complement factors after permanent focal cerebral ischemia in the mouse, using semiquantitative reverse transcription-polymerase chain reaction (RT-PCR), in situ hybridization, and immunohistochemistry. mRNA expression of glial fibrillary acidic protein (GFAP) increased at day 1 and peaked 3 days after middle cerebral artery (MCA) occlusion in the perifocal area. Immunohistochemical staining for GFAP indicated that astroglia were activated the day after MCA occlusion. Microglial activation, as assessed by lectin-binding experiments, increased by 1 day after MCA occlusion in the perifocal area and peaked at 3 days postocclusion. RT-PCR experiments demonstrated an increased expression of clusterin, C1qB, and C4 mRNA in the ischemic cortex, with a peak level at 3 days after MCA occlusion. Clusterin, C1qB, and C4 mRNA were located in the perifocal area, as assessed by in situ hybridization. Reactive astrocytes within the cortex medial to the ischemic lesion were found to be strongly immunoreactive for clusterin. In addition, we observed C1q-positive macrophage-like cells within the infarcted core at 3 days postocclusion. At 7 days after the onset of ischemia, increased C4 immunostaining was restricted to perifocal neurons. We conclude that local expression of complement components may contribute to the inflammation observed in this model, thereby representing an important process in secondary injury mechanisms after focal cerebral ischemia.

Neurons and astrocytes express EPO mRNA: oxygen-sensing mechanisms that involve the redox-state of the brain.

Erythropoietin (Epo), the major hormone controlling the hypoxia-induced increase in the number of erythrocytes, has also a functional role in the brain. However, few data exist as to the cellular source of brain-derived Epo as well as to the molecular mechanisms that control Epo expression in the central nervous system. Using patch-clamp and RT-PCR methods, we provide direct evidence that, besides astrocytes, neurons are a source of Epo in the brain. Both the astrocytic and neuronal expression of Epo mRNA are induced not only by hypoxia, but also by desferrioxamine (DFX) and cobalt chloride (CoCl(2)), two agents known to mimic the hypoxic induction of Epo in hepatoma cells. This induction is blocked by cycloheximide suggesting that de novo protein synthesis is required. Furthermore, the addition of H(2)O(2) decreases the hypoxia-induced Epo mRNA levels. These data indicate that, following hypoxia, a common oxygen sensing and signaling pathway leads to increased Epo gene expression in both nervous and hepatoma cells; this pathway would be dependent on the redox-state of the brain. Furthermore, we show that the in vivo administration of CoCl(2) and DFX to mice induces an increased Epo mRNA level in the neocortex. As Epo protects the brain against ischemia, our in vivo experiments suggest that the use of molecules such as CoCl(2) or DFX, that provoke an increased Epo gene expression in the brain, could be useful in the development of potential therapeutic strategies for the treatment of hypoxic or ischemic brain injury.

Hypoxia-induced vascular endothelial growth factor expression precedes neovascularization after cerebral ischemia.

We investigated the hypothesis that hypoxia induces angiogenesis and thereby may counteract the detrimental neurological effects associated with stroke. Forty-eight to seventy-two hours after permanent middle cerebral artery occlusion we found a strong increase in the number of newly formed vessels at the border of the infarction. Using the hypoxia marker nitroimidazole EF5, we detected hypoxic cells in the ischemic border of the neocortex. Expression of vascular endothelial growth factor (VEGF), which is the main regulator of angiogenesis and is inducible by hypoxia, was strongly up-regulated in the ischemic border, at times between 6 and 24 hours after occlusion. In addition, both VEGF receptors (VEGFRs) were up-regulated at the border after 48 hours and later in the ischemic core. Finally, the two transcription factors, hypoxia-inducible factor-1 (HIF-1) and HIF-2, known to be involved in the regulation of VEGF and VEGFR gene expression, were increased in the ischemic border after 72 hours, suggesting a regulatory function for these factors. These results strongly suggest that the VEGF/VEGFR system, induced by hypoxia, leads to the growth of new vessels after cerebral ischemia. Exogenous support of this natural protective mechanism might lead to enhanced survival after stroke.

Expression of receptors for complement anaphylatoxins C3a and C5a following permanent focal cerebral ischemia in the mouse.

In the present study, we have examined the expression of anaphylatoxin C3a and C5a receptors (C3aR and C5aR) at the mRNA and protein levels in ischemic brain tissues following permanent middle cerebral artery (MCA) occlusion in the mouse. C3aR and C5aR mRNAs were both detected by semiquantitative reverse transcription and polymerase chain reaction (RT-PCR) and the cellular distribution of each receptor was analyzed by immunohistochemistry. Significant increases in the expression of C3aR and C5aR mRNAs in the ischemic cortex were observed; the expression of both reached a peak at 2 days after MCA occlusion (4.3- and 3.4-fold increases, respectively, compared with nonoperated control cortical samples; P < 0.00625 with Bonferroni's correction, n = 3). C3aR and C5aR stainings were found constitutively on neurons and astrocytes. In ischemic tissues, we observed that C3aR and C5aR were expressed de novo on endothelial cells of blood vessels, at 6 h and 2 days after MCA occlusion, respectively. C3aR and C5aR immunostaining was increased in macrophage-like cells and reactive astrocytes 7 days postocclusion. C3a and C5a may play an important role in promoting inflammatory and/or repair processes in the ischemic brain by regulating glial cell activation and chemotaxis.

A potential role for erythropoietin in focal permanent cerebral ischemia in mice.

The present study describes, for the first time, a temporal and spatial cellular expression of erythropoietin (Epo) and Epo receptor (Epo-R) with the evolution of a cerebral infarct after focal permanent ischemia in mice. In addition to a basal expression of Epo in neurons and astrocytes, a postischemic Epo expression has been localized specifically to endothelial cells (1 day), microglia/macrophage-like cells (3 days), and reactive astrocytes (7 days after occlusion). Under these conditions, the Epo-R expression always precedes that of Epo for each cell type. These results support the hypothesis that there is a continuous formation of Epo, with its corresponding receptor, during the active evolution of a focal cerebral infarct and that the Epo/Epo-R system might be implicated in the processes of neuroprotection and restructuring (such as angiogenesis and gliosis) after ischemia. To support this hypothesis, a significant reduction in infarct volume (47%; P < 0.0002) was found in mice treated with recombinant Epo 24 hours before induction of cerebral ischemia. Based on the above, we propose that the Epo/Epo-R system is an endogenous mechanism that protects the brain against damages consequent to a reduction in blood flow, a mechanism that can be amplified by the intracerebroventricular application of exogenous recombinant Epo.

Evidence of type I and type II transforming growth factor-beta receptors in central nervous tissues: changes induced by focal cerebral ischemia.

The peptides of the transforming growth factor-beta (TGF-beta) family transduce their signal through ligand-induced heteromeric complexes that consist of type I and type II serine/threonine kinases. Both TGF-beta receptors are abundant in many peripheral tissues, but clear evidence of their expression in cortical astrocytes and neurons has not been published so far. In this study, we investigated the expression of type I and type II TGF-beta receptors and their potential ligands (TGF-beta1, TGF-beta2, and TGF-beta3) in the CNS by using RT-PCR and immunohistochemistry. Moreover, to further the study of those cell types that exhibit TGF-beta isoforms and related receptors, we examined through the use of RT-PCR whether cortical neurons and astrocytes in culture express the mRNAs for TGF-betas and their receptors. We show that the three TGF-beta isoform mRNAs are present in the CNS. However, although astrocytes in culture display all three isoforms, neurons in culture express only TGF-beta2. We have demonstrated that both type I and type II TGF-beta receptor mRNAs and proteins are present in the CNS and in cultures of cortical neurons and astrocytes. Thus, TGF-betas may act as autocrine and paracrine signals in the CNS between both neurons and astrocytes via the same receptor systems as those found in peripheral tissues. TGF-beta1 has been shown to be induced following hypoxic-ischemic brain injury and may play a critical role in the pathophysiology of degenerative processes in the CNS. In the present investigation, we confirmed that the expression of TGF-beta1 was increased markedly up until 24 h and thereafter was stable over the first 3 days following permanent occlusion of the middle cerebral artery in mice. However, whereas the expression of the type I TGF-beta receptor was not altered by the ischemic insult, the pattern of the type II TGF-beta receptors was modified dramatically in the ischemic area 3 days after the occlusion. These data show that, even if ligands are present, they may not be able to transduce their signal. Finally, the present study clearly demonstrates that a knowledge of the expression of ligand-specific receptors following brain injury is a fundamental step in clarifying the involvement of cytokines in neurodegenerative diseases.

Selective neuronal vulnerability and specific glial reactions in hippocampal and neocortical organotypic cultures submitted to ischemia.

Neurons from cerebral neocortex and hippocampus exhibit a striking difference in vulnerability to transient global ischemia. In order to study the contribution of neuronal connections and neuron-glia interactions to this variation in neuronal vulnerability, we used hippocampal and neocortical cultures submitted to various periods of histotoxic ischemia. Organotypic cultures were exposed at 37 degrees C for 0, 7, 30 and 60 min to a glucose-free NaCN-containing medium. Histological analysis using thionin staining and MAP2 immunostaining showed differences in the temporal profile of neuronal damage in hippocampal and neocortical structures, i.e., in decreasing order, CA1 (7 min) > CA3 and neocortical layers II, III, V, VI (30 min) > DG and neocortical layer IV (60 min). In parallel to the neurodegeneration study, the time course and the regional pattern of microglial and astroglial changes were also examined using GS-B4 isolectin and GFAP as immunohistochemical markers, respectively. The GS-B4 isolectin staining revealed an early (at 7 min for the hippocampus) and a specific microglial activation located in areas undergoing neuronal damage. For both organotypic cultures, astrogliosis occurred later (after 30 min of stress) with no specific regional distribution. Both hippocampal and neocortical cultures submitted to histotoxic ischemia allowed the replication of many of the cellular events observed in response to global ischemia in vivo. These findings support the hypothesis that neuron-neuron connections as well as interactions between neurons and glial cells are essential to reproduce in vitro the selective neuronal vulnerability described in vivo.