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

Microglial TNFR2 signaling regulates the inflammatory response after CNS injury in a sex-specific fashion

Microglia, the resident immune cells of the central nervous system (CNS), play a major role in damage progression and tissue remodeling after acute CNS injury, including ischemic stroke (IS) and spinal cord injury (SCI). Understanding the molecular mechanisms regulating microglial responses to injury may thus reveal novel therapeutic targets to promote CNS repair. Here, we investigated the role of microglial tumor necrosis factor receptor 2 (TNFR2), a transmembrane receptor previously associated with pro-survival and neuroprotective responses, in shaping the neuroinflammatory environment after CNS injury. By inducing experimental IS and SCI in Cx3cr1CreER:Tnfrsf1bfl/fl mice, selectively lacking TNFR2 in microglia, and corresponding Tnfrsf1bfl/fl littermate controls, we found that ablation of microglial TNFR2 significantly reduces lesion size and pro-inflammatory cytokine levels, and favors infiltration of leukocytes after injury. Interestingly, these effects were paralleled by opposite sex-specific modifications of microglial reactivity, which was found to be limited in female TNFR2-ablated mice compared to controls, whereas it was enhanced in males. In addition, we show that TNFR2 protein levels in the cerebrospinal fluid (CSF) of human subjects affected by IS and SCI, as well as healthy donors, significantly correlate with disease stage and severity, representing a valuable tool to monitor the inflammatory response after acute CNS injury. Hence, these results advance our understanding of the mechanisms regulating microglia reactivity after acute CNS injury, aiding the development of sex- and microglia-specific, personalized neuroregenerative strategies.

A Comparison of PKD2L1-Expressing Cerebrospinal Fluid Contacting Neurons in Spinal Cords of Rodents, Carnivores, and Primates

Cerebrospinal fluid contacting neurons (CSF-cNs) are a specific type of neurons located around the ventricles in the brain and the central canal in the spinal cord and have been demonstrated to be intrinsic sensory neurons in the central nervous system. One of the important channels responsible for the sensory function is the polycystic kidney disease 2-like 1 (PKD2L1) channel. Most of the studies concerning the distribution and function of the PKD2L1-expressing CSF-cNs in the spinal cord have previously been performed in non-mammalian vertebrates. In the present study immunohistochemistry was performed to determine the distribution of PKD2L1-immunoreactive (IR) CSF-cNs in the spinal cords of four mammalian species: mouse, rat, cat, and macaque monkey. Here, we found that PKD2L1-expressing CSF-cNs were present at all levels of the spinal cord in these animal species. Although the distribution pattern was similar across these species, differences existed. Mice and rats presented a clear PKD2L1-IR cell body labeling, whereas in cats and macaques the PKD2L1-IR cell bodies were more weakly labeled. Ectopic PKD2L1-IR neurons away from the ependymal layer were observed in all the animal species although the abundance and the detailed locations varied. The apical dendritic protrusions with ciliated fibers were clearly seen in the lumen of the central canal in all the animal species, but the sizes of protrusion bulbs were different among the species. PKD2L1-IR cell bodies/dendrites were co-expressed with doublecortin, MAP2 (microtubule-associated protein 2), and aromatic L-amino acid decarboxylase, but not with NeuN (neuronal nuclear protein), indicating their immature properties and ability to synthesize monoamine transmitters. In addition, in situ hybridization performed in rats revealed PKD2L1 mRNA expression in the cells around the central canal. Our results indicate that the intrinsic sensory neurons are conserved across non-mammalian and mammalian vertebrates. The similar morphology of the dendritic bulbs with ciliated fibers (probably representing stereocilia and kinocilia) protruding into the central canal across different animal species supports the notion that PKD2L1 is a chemo- and mechanical sensory channel that responds to mechanical stimulations and maintains homeostasis of the spinal cord. However, the differences of PKD2L1 distribution and expression between the species suggest that PKD2L1-expressing neurons may receive and process sensory signals differently in different animal species.

Leukocyte TNFR1 and TNFR2 Expression Contributes to the Peripheral Immune Response in Cases with Ischemic Stroke

Tumor necrosis factor receptor 1 and 2 (TNFR1 and TNFR2) have been found in brain parenchyma of stroke patients, and plasma levels are increased in the acute phase of stroke. We evaluated associations between TNFR1 and TNFR2 plasma levels and stroke severity, infarct size, and functional outcome. Furthermore, we examined cellular expression of TNFR1 and TNFR2 on leukocyte subpopulations to explore the origin of the increased receptor levels. Blood samples were taken from 33 acute ischemic stroke patients and 10 healthy controls. TNFR1 and TNFR2 plasma concentrations were measured and correlated against the Scandinavian Stroke Scale at admission, infarct volume, and the modified Rankin Scale score three months after stroke onset. Classical, intermediate, and non-classical monocytes as well as neutrophils were purified, and cellular expression of TNFR1 and TNFR2 was examined using flow cytometry. TNFR1 and TNFR2 plasma levels were both increased after ischemic stroke, but we found no correlation with patient outcome measurements. Compared to healthy controls, ischemic stroke patients had decreased non-classical monocyte and neutrophil populations expressing TNFR1 and increased neutrophils expressing TNFR2, and decreased non-classical populations co-expressing both TNFR1 and TNFR2. This study supports the hypothesis of an acute immunological response orchestrated by the peripheral immune system following an ischemic stroke. However, the origin of the increased TNFR1 and TNFR2 plasma levels could not be clearly linked to peripheral monocytes or neutrophils. Future studies are needed and will help clarify the potential role as treatment target.

Topical Administration of a Soluble TNF Inhibitor Reduces Infarct Volume After Focal Cerebral Ischemia in Mice

Background

Tumor necrosis factor, which exists both as a soluble (solTNF) and a transmembrane (tmTNF) protein, plays an important role in post-stroke inflammation. The objective of the present study was to test the effect of topical versus intracerebroventricular administration of XPro1595 (a solTNF inhibitor) and etanercept (a solTNF and tmTNF inhibitor) compared to saline on output measures such as infarct volume and post-stroke inflammation in mice.

Methods

Adult male C57BL/6 mice were treated topically (2.5 mg/ml/1μl/h for 3 consecutive days) or intracerebroventricularly (1.25 mg/kg/0.5 ml, once) with saline, XPro1595, or etanercept immediately after permanent middle cerebral artery occlusion (pMCAO). Mice were allowed to survive 1 or 3 days. Infarct volume, microglial and leukocyte profiles, and inflammatory markers were evaluated.

Results

We found that topical, and not intracerebroventricular, administration of XPro1595 reduced infarct volume at both 1 and 3 days after pMCAO. Etanercept showed no effect. We observed no changes in microglial or leukocyte populations. XPro1595 increased gene expression of P2ry12 at 1 day and Trem2 at 1 and 3 days, while decreasing Cx3cr1 expression at 1 and 3 days after pMCAO, suggesting a change in microglial activation toward a phagocytic phenotype.

Conclusion

Our data demonstrate that topical administration of XPro1595 for 3 consecutive days decreases infarct volumes after ischemic stroke, while modifying microglial activation and the inflammatory response post-stroke. This suggests that inhibitors of solTNF hold great promise for future neuroprotective treatment in ischemic stroke.

Cortical Morphogenesis during Embryonic Development Is Regulated by miR-34c and miR-204

The porcine brain closely resembles the human brain in aspects such as development and morphology. Temporal miRNA profiling in the developing embryonic porcine cortex revealed a distinct set of miRNAs, including miR-34c and miR-204, which exhibited a highly specific expression profile across the time of cortical folding. These miRNAs were found to target Doublecortin (DCX), known to be involved in neuron migration during cortical folding of gyrencephalic brains. In vivo modulation of miRNA expression in mouse embryos confirmed that miR-34c and miR-204 can control neuronal migration and cortical morphogenesis, presumably by posttranscriptional regulation of DCX.

Genetic ablation of soluble tumor necrosis factor with preservation of membrane tumor necrosis factor is associated with neuroprotection after focal cerebral ischemia

Microglia respond to focal cerebral ischemia by increasing their production of the neuromodulatory cytokine tumor necrosis factor, which exists both as membrane-anchored tumor necrosis factor and as cleaved soluble tumor necrosis factor forms. We previously demonstrated that tumor necrosis factor knockout mice display increased lesion volume after focal cerebral ischemia, suggesting that tumor necrosis factor is neuroprotective in experimental stroke. Here, we extend our studies to show that mice with intact membrane-anchored tumor necrosis factor, but no soluble tumor necrosis factor, display reduced infarct volumes at one and five days after stroke. This was associated with improved functional outcome after experimental stroke. No changes were found in the mRNA levels of tumor necrosis factor and tumor necrosis factor-related genes (TNFR1, TNFR2, TACE), pro-inflammatory cytokines (IL-1β, IL-6) or chemokines (CXCL1, CXCL10, CCL2); however, protein expression of TNF, IL-1β, IL-6 and CXCL1 was reduced in membrane-anchored tumor necrosis factor(Δ/Δ) compared to membrane-anchored tumor necrosis factor(wt/wt) mice one day after experimental stroke. This was paralleled by reduced MHCII expression and a reduction in macrophage infiltration in the ipsilateral cortex of membrane-anchored tumor necrosis factor(Δ/Δ) mice. Collectively, these findings indicate that membrane-anchored tumor necrosis factor mediates the protective effects of tumor necrosis factor signaling in experimental stroke, and therapeutic strategies specifically targeting soluble tumor necrosis factor could be beneficial in clinical stroke therapy.

Conditional ablation of myeloid TNF increases lesion volume after experimental stroke in mice, possibly via altered ERK1/2 signaling

Microglia are activated following cerebral ischemia and increase their production of the neuro- and immunomodulatory cytokine tumor necrosis factor (TNF). To address the function of TNF from this cellular source in focal cerebral ischemia we used TNF conditional knock out mice (LysMcreTNF^fl/fl) in which the TNF gene was deleted in cells of the myeloid lineage, including microglia. The deletion reduced secreted TNF levels in lipopolysaccharide-stimulated cultured primary microglia by ~93%. Furthermore, phosphorylated-ERK/ERK ratios were significantly decreased in naïve LysMcreTNF^fl/fl mice demonstrating altered ERK signal transduction. Micro-PET using ¹⁸[F]-fluorodeoxyglucose immediately after focal cerebral ischemia showed increased glucose uptake in LysMcreTNF^fl/fl mice, representing significant metabolic changes, that translated into increased infarct volumes at 24 hours and 5 days compared to littermates (TNFfl/fl). In naïve LysMcreTNF^fl/fl mice cytokine levels were low and comparable to littermates. At 6 hours, TNF producing microglia were reduced by 56% in the ischemic cortex in LysMcreTNF^fl/fl mice compared to littermate mice, whereas no TNF⁺ leukocytes were detected. At 24 hours, pro-inflammatory cytokine (TNF, IL-1β, IL-6, IL-5 and CXCL1) levels were significantly lower in LysMcreTNF^fl/fl mice, despite comparable infiltrating leukocyte populations. Our results identify microglial TNF as beneficial and neuroprotective in the acute phase and as a modulator of neuroinflammation at later time points after experimental ischemia, which may contribute to regenerative recovery.

Cell therapy centered on IL-1Ra is neuroprotective in experimental stroke

Cell-based therapies are emerging as new promising treatments in stroke. However, their functional mechanism and therapeutic potential during early infarct maturation has so far received little attention. Here, we asked if cell-based delivery of the interleukin-1 receptor antagonist (IL-1Ra), a known neuroprotectant in stroke, can promote neuroprotection, by modulating the detrimental inflammatory response in the tissue at risk. We show by the use of IL-1Ra-overexpressing and IL-1Ra-deficient mice that IL-1Ra is neuroprotective in stroke. Characterization of the cellular and spatiotemporal production of IL-1Ra and IL-1α/β identifies microglia, not infiltrating leukocytes, as the major sources of IL-1Ra after experimental stroke, and shows IL-1Ra and IL-1β to be produced by segregated subsets of microglia with a small proportion of these cells co-expressing IL-1α. Reconstitution of whole body irradiated mice with IL-1Ra-producing bone marrow cells is associated with neuroprotection and recruitment of IL-1Ra-producing leukocytes after stroke. Neuroprotection is also achieved by therapeutic injection of IL-1Ra-producing bone marrow cells 30 min after stroke onset, additionally improving the functional outcome in two different stroke models. The IL-1Ra-producing bone marrow cells increase the number of IL-1Ra-producing microglia, reduce the availability of IL-1β, and modulate mitogen-activated protein kinase (MAPK) signaling in the ischemic cortex. The importance of these results is underlined by demonstration of IL-1Ra-producing cells in the human cortex early after ischemic stroke. Taken together, our results attribute distinct neuroprotective or neurotoxic functions to segregated subsets of microglia and suggest that treatment strategies increasing the production of IL-1Ra by infiltrating leukocytes or microglia may also be neuroprotective if applied early after stroke onset in patients.

Identification of proteins in hyperglycemia and stroke animal models

BACKGROUND

Stroke is a major cause of disability and death in adults. Diabetes mellitus is a metabolic disorder that strongly increases the risk of severe vascular diseases. This study compared changes in proteins of the cerebral cortex during ischemic brain injury between nondiabetic and diabetic animals.

METHODS

Adult male rats were injected with streptozotocin (40 mg/kg) via the intraperitoneal route to induce diabetes and underwent surgical middle cerebral artery occlusion (MCAO) 4 wk after streptozotocin treatment. Cerebral cortex tissues were collected 24 h after MCAO and cerebral cortex proteins were analyzed by two-dimensional gel electrophoresis and mass spectrometry.

RESULTS

Several proteins were identified as differentially expressed between nondiabetic and diabetic animals. Among the identified proteins, we focused on the following metabolism-related enzymes: isocitrate dehydrogenase, glyceraldehyde-3-phosphate dehydrogenase, adenosylhomocysteinase, pyruvate kinase, and glucose-6-phosphate isomerase (neuroleukin). Expression of these proteins was decreased in animals that underwent MCAO. Moreover, protein expression was reduced to a greater extent in diabetic animals than in nondiabetic animals. Reverse transcription-polymerase chain reaction analysis confirmed that the diabetic condition exacerbates the decrease in expression of metabolism-related proteins after MCAO.

CONCLUSIONS

These results suggest that the diabetic condition may exacerbate brain damage during focal cerebral ischemia through the downregulation of metabolism-related proteins.

Natural RNA circles function as efficient microRNA sponges

MicroRNAs (miRNAs) are important post-transcriptional regulators of gene expression that act by direct base pairing to target sites within untranslated regions of messenger RNAs. Recently, miRNA activity has been shown to be affected by the presence of miRNA sponge transcripts, the so-called competing endogenous RNA in humans and target mimicry in plants. We previously identified a highly expressed circular RNA (circRNA) in human and mouse brain. Here we show that this circRNA acts as a miR-7 sponge; we term this circular transcript ciRS-7 (circular RNA sponge for miR-7). ciRS-7 contains more than 70 selectively conserved miRNA target sites, and it is highly and widely associated with Argonaute (AGO) proteins in a miR-7-dependent manner. Although the circRNA is completely resistant to miRNA-mediated target destabilization, it strongly suppresses miR-7 activity, resulting in increased levels of miR-7 targets. In the mouse brain, we observe overlapping co-expression of ciRS-7 and miR-7, particularly in neocortical and hippocampal neurons, suggesting a high degree of endogenous interaction. We further show that the testis-specific circRNA, sex-determining region Y (Sry), serves as a miR-138 sponge, suggesting that miRNA sponge effects achieved by circRNA formation are a general phenomenon. This study serves as the first, to our knowledge, functional analysis of a naturally expressed circRNA.

In situ hybridization of cytokine mRNA using alkaline phosphatase-labelled oligodeoxynucleotide probes

In situ hybridization is a powerful tool for visualizing cellular gene expression in morphologically preserved brain tissue giving precise information on the regional expression of specific mRNA sequences in cells of diverse phenotype. Here, we describe a sensitive, simple, and robust method using alkaline phosphatase (AP)-labelled oligodeoxynucleotide probes to detect cytokine mRNA in the acutely injured or inflamed mouse CNS.

CSF transthyretin neuroprotection in a mouse model of brain ischemia

Brain injury caused by ischemia is a major cause of human mortality and physical/cognitive disability worldwide. Experimentally, brain ischemia can be induced surgically by permanent middle cerebral artery occlusion. Using this model, we studied the influence of transthyretin in ischemic stroke. Transthyretin (TTR) is normally responsible for the transport of thyroid hormones and retinol in the blood and CSF. We found that TTR null mice (TTR(-/-) ) did not show significant differences in cortical infarction 24 h after permanent middle cerebral artery occlusion compared with TTR(+/+) control littermates. However, TTR null mice, heterozygous for the heat-shock transcription factor 1 (TTR(-/-) HSF1(+/-) mice), which compromised the stress response, showed a significant increase in cortical infarction, cerebral edema and the microglial-leukocyte response compared with TTR(+/+) HSF1(+/-) mice. Unexpectedly, we observed novel TTR distribution throughout the infarct, localized to disintegrated β-tubulin III(+) neurons and cell debris. Specific elimination of TTR synthesis in the liver by RNAi had no effect on TTR distribution in the infarct, indicating that the observed TTR infiltration derived from CSF and not from the serum. This finding is corroborated by results from 'in situ' hybridization and real time PCR that excluded the presence of transthyretin mRNA in the infarct and peri-infarct areas. Our data suggest that in conditions of a compromised heat-shock response, CSF TTR contributes to control neuronal cell death, edema and inflammation, thereby influencing the survival of endangered neurons in cerebral ischemia.