IL-6 signaling drives self-renewal and alternative activation of adipose tissue macrophages

Introduction

Obesity is associated with chronic low-grade inflammation of adipose tissue (AT) and an increase of AT macrophages (ATMs) that is linked to the onset of type 2 diabetes. We have recently shown that neutralization of interleukin (IL)-6 in obese AT organ cultures inhibits proliferation of ATMs, which occurs preferentially in alternatively activated macrophage phenotype.

Methods

In this study, we investigated AT biology and the metabolic phenotype of mice with myeloid cell-specific IL-6Rα deficiency (Il6ra Δmyel) after normal chow and 20 weeks of high-fat diet focusing on AT inflammation, ATM polarization and proliferation. Using organotypical AT culture and bone marrow derived macrophages (BMDMs) of IL-4Rα knockout mice (Il4ra -/-) we studied IL-6 signaling.

Results

Obese Il6ra Δmyel mice exhibited no differences in insulin sensitivity or histological markers of AT inflammation. Notably, we found a reduction of ATMs expressing the mannose receptor 1 (CD206), as well as a decrease of the proliferation marker Ki67 in ATMs of Il6ra Δmyel mice. Importantly, organotypical AT culture and BMDM data of Il4ra -/- mice revealed that IL-6 mediates a shift towards the M2 phenotype independent from the IL-6/IL-4Rα axis.

Discussion

Our results demonstrate IL-4Rα-independent anti-inflammatory effects of IL-6 on macrophages and the ability of IL-6 to maintain proliferation rates in obese AT.

Increasing reproducibility in preclinical stroke research: the correlation of immunofluorescence intensity measurements and Western blot analyses strongly depends on antibody clonality and tissue pre-treatment in a mouse model of focal cerebral ischemia

In the setting of stroke, ischemia not only impairs neuronal function, but also detrimentally affects the different components of the neurovascular unit, which are shown to be involved in the transition from reversible to long-lasting tissue damage. In this context, the glial proteins myelin basic protein (MBP) and the 2',3'-cyclic-nucleotide 3'-phosphodiesterase (CNP) as well as the vasculature-associated basement membrane proteins laminin and collagen IV have been identified as ischemia-sensitive elements. However, available data from immunofluorescence and Western blot analyses are often found to be contradictory, which renders interpretation of the respective data rather difficult. Therefore, the present study investigates the impact of tissue pre-treatment and antibody clonality on immunofluorescence measurements of the mentioned proteins in a highly reproducible model of permanent middle cerebral artery occlusion. Here, immunofluorescence labeling using polyclonal antibodies revealed an increased immunofluorescence intensity of MBP, CNP, laminin and collagen IV in ischemic areas, although Western blot analyses did not reveal increased protein levels. Importantly, contrary to polyclonal antibodies, monoclonal ones did not provide increased fluorescence intensities in ischemic areas. Further, we were able to demonstrate that different ways of tissue pre-treatment including paraformaldehyde fixation and antigen retrieval may not only impact on fluorescence intensity measurements in general, but rather one-sidedly affect either ischemic or unaffected tissue. Therefore, immunofluorescence intensity measurements do not necessarily correlate with the actual protein levels, especially in ischemia-affected tissue and should always be complemented by different techniques to enhance reproducibility and to hopefully overcome the translational roadblock from bench to bedside.

Myeloid Cell-Specific IL-4 Receptor Knockout Partially Protects from Adipose Tissue Inflammation

IL-4 receptor signaling is supposed to play a major role in anti-inflammatory polarization and proliferation of adipose tissue macrophages. In this study, we examined the metabolic and inflammatory phenotype of C57BL/6J mice (IIl4ra) with LysM-dependent knockout (IIl4ra ^Δmyel) of the IL-4 receptor α-chain (IL-4Rα), the mandatory signaling component of IL-4 and IL-13, on chow and high-fat diet. Lean IIl4ra ^Δmyel mice showed decreased insulin sensitivity, no divergent adipose tissue macrophage polarization, but an increased percentage of CD8⁺ T cells in visceral adipose tissue. After 20 wk of a high-fat diet, IIl4ra ^Δmyel mice exhibited higher glucose tolerance, no changes in the lymphocyte compartment and fewer M1 macrophages in visceral adipose tissue. In vivo adipose tissue macrophage proliferation measured by BrdU incorporation was unaffected by Il4ra knockout. Interestingly, we show that IL-4Rα signaling directly augmented Itgax (Cd11c) gene expression in bone marrow-derived macrophages and increased the amount of CD11c⁺ macrophages in adipose tissue explants. Myeloid cell-specific knockout of Il4ra deteriorated insulin sensitivity in lean mice but improved parameters of glucose homeostasis and partially protected from adipose tissue inflammation in obese mice. Hence, IL-4Rα signaling probably plays a minor role in maintaining the macrophage M2 population and proliferation rates in vivo. Moreover, our data indicate that IL-4 signaling plays a proinflammatory role in adipose tissue inflammation by directly upregulating CD11c on adipose tissue macrophages.

The Cytoskeletal Elements MAP2 and NF-L Show Substantial Alterations in Different Stroke Models While Elevated Serum Levels Highlight Especially MAP2 as a Sensitive Biomarker in Stroke Patients

In the setting of ischemic stroke, the neurofilament subunit NF-L and the microtubule-associated protein MAP2 have proven to be exceptionally ischemia-sensitive elements of the neuronal cytoskeleton. Since alterations of the cytoskeleton have been linked to the transition from reversible to irreversible tissue damage, the present study investigates underlying time- and region-specific alterations of NF-L and MAP2 in different animal models of focal cerebral ischemia. Although NF-L is increasingly established as a clinical stroke biomarker, MAP2 serum measurements after stroke are still lacking. Therefore, the present study further compares serum levels of MAP2 with NF-L in stroke patients. In the applied animal models, MAP2-related immunofluorescence intensities were decreased in ischemic areas, whereas the abundance of NF-L degradation products accounted for an increase of NF-L-related immunofluorescence intensity. Accordingly, Western blot analyses of ischemic areas revealed decreased protein levels of both MAP2 and NF-L. The cytoskeletal alterations are further reflected at an ultrastructural level as indicated by a significant reduction of detectable neurofilaments in cortical axons of ischemia-affected areas. Moreover, atomic force microscopy measurements confirmed altered mechanical properties as indicated by a decreased elastic strength in ischemia-affected tissue. In addition to the results from the animal models, stroke patients exhibited significantly elevated serum levels of MAP2, which increased with infarct size, whereas serum levels of NF-L did not differ significantly. Thus, MAP2 appears to be a more sensitive stroke biomarker than NF-L, especially for early neuronal damage. This perspective is strengthened by the results from the animal models, showing MAP2-related alterations at earlier time points compared to NF-L. The profound ischemia-induced alterations further qualify both cytoskeletal elements as promising targets for neuroprotective therapies.

CD4+ T cells regulate glucose homeostasis independent of adipose tissue dysfunction in mice

Obesity is frequently associated with a chronic low-grade inflammation in the adipose tissue (AT) and impaired glucose homeostasis. Adipose tissue macrophages (ATMs) have been shown to accumulate in the inflamed AT either by means of recruitment from the blood or local proliferation. ATM proliferation and activation can be stimulated by TH2 cytokines, such as IL-4 and IL-13, suggesting involvement of CD4-positive T cells in ATM proliferation and activation. Furthermore, several studies have associated T cells to alterations in glucose metabolism. Therefore, we sought to examine a direct impact of CD4-positive T cells on ATM activation, ATM proliferation and glucose homeostasis using an in vivo depletion model. Surprisingly, CD4 depletion did not affect ATM activation, ATM proliferation, or insulin sensitivity. However, CD4 depletion led to a significant improvement of glucose tolerance. In line with this, we found moderate disturbances in pancreatic endocrine function following CD4 depletion. Hence, our data suggest that the effect on glucose metabolism observed after CD4 depletion might be mediated by organs other than AT and independent of AT inflammation.

Multinucleated Giant Cells in Adipose Tissue Are Specialized in Adipocyte Degradation

Obesity is associated with chronic low-grade inflammation of visceral adipose tissue (AT) characterized by an increasing number of AT macrophages (ATMs) and linked to type 2 diabetes. AT inflammation is histologically indicated by the formation of so-called crown-like structures, as ATMs accumulate around dying adipocytes, and the occurrence of multinucleated giant cells (MGCs). However, to date, the function of MGCs in obesity is unknown. Therefore, the aim of this study was to characterize MGCs in AT and unravel the function of these cells. We demonstrated that MGCs occurred in obese patients and after 24 weeks of a high-fat diet in mice, accompanying signs of AT inflammation and then representing ∼3% of ATMs in mice. Mechanistically, we found evidence that adipocyte death triggered MGC formation. Most importantly, MGCs in obese AT had a higher capacity to phagocytize oversized particles, such as adipocytes, as shown by live imaging of AT, 45-µm bead uptake ex vivo, and higher lipid content in vivo. Finally, we showed that interleukin-4 treatment was sufficient to increase the number of MGCs in AT, whereas other factors may be more important for endogenous MGC formation in vivo. Most importantly, our data suggest that MGCs are specialized for clearance of dead adipocytes in obesity.

Transcriptional Response and Morphological Features of the Neurovascular Unit and Associated Extracellular Matrix After Experimental Stroke in Mice

Experimental stroke studies yielded insights into single reactions of the neurovascular unit (NVU) and associated extracellular matrix (ECM). However, the extent of simultaneous processes caused by ischemia and their underlying transcriptional changes are still poorly understood. Strictly following the NVU and ECM concept, this study explored transcriptional responses of cellular and non-cellular components as well as their morphological characteristics following ischemia. Mice were subjected to 4 or 24 h of unilateral middle cerebral artery occlusion. In the neocortex and the striatum, cytoskeletal and glial elements as well as blood-brain barrier and ECM components were analyzed using real-time PCR. Western blot analyses allowed characterization of protein levels and multiple immunofluorescence labeling enabled morphological assessment. Out of 37 genes analyzed, the majority exhibited decreased mRNA levels in ischemic areas, while changes occurred as early as 4 h after ischemia. Down-regulated mRNA levels were predominantly localized in the neocortex, such as the structural elements α-catenin 2, N-cadherin, β-catenin 1, and βIII-tubulin, consistently decreasing 4 and 24 h after ischemia. However, a few genes, e.g., claudin-5 and Pcam1, exhibited increased mRNA levels after ischemia. For several components such as βIII-tubulin, N-cadherin, and β-catenin 1, matching transcriptional and immunofluorescence signals were obtained, whereas a few markers including neurofilaments exhibited opposite directions. In conclusion, the variety in gene regulation emphasizes the complexity of interactions within the ischemia-affected NVU and ECM. These data might help to focus future research on a set of highly sensitive elements, which might prospectively facilitate neuroprotective strategies beyond the traditional single target perspective.

Endothelial edema precedes blood-brain barrier breakdown in early time points after experimental focal cerebral ischemia

In the setting of stroke, ischemia-related blood-brain barrier (BBB) dysfunction aggravates the cerebral edema, which critically impacts on the clinical outcome. Further, an impaired vascular integrity is associated with the risk of intracranial bleeding, especially after therapeutic recanalization. Therefore, the present study was aimed to investigate early vascular alterations from 30 min to 4 h after experimental middle cerebral artery occlusion (MCAO) in mice. Here, an extravasation of the permeability marker FITC-albumin was detectable in animals 2 and 4 h after MCAO. Thereby, BBB breakdown correlated with alterations of the endothelial surface, indicated by a discontinuous isolectin-B4 staining, while tight junction strands remained detectable using electron and immunofluorescence microscopy. Noteworthy, already 30 min after MCAO, up to 60% of the ischemia-affected vessels showed an endothelial edema, paralleled by edematous astrocytic endfeet, clearly preceding FITC-albumin extravasation. With increasing ischemic periods, scores of vascular damage significantly increased with up to 60% of the striatal vessels showing loss of endothelial integrity. Remarkably, comparison of permanent and transient ischemia did not provide significant differences 4 h after ischemia induction. As these degenerations also involved penumbral areas of potentially salvageable tissue, adjuvant approaches of endothelial protection may help to reduce the vasogenic edema after ischemic stroke.

Impaired Neurofilament Integrity and Neuronal Morphology in Different Models of Focal Cerebral Ischemia and Human Stroke Tissue

As part of the neuronal cytoskeleton, neurofilaments are involved in maintaining cellular integrity. In the setting of ischemic stroke, the affection of the neurofilament network is considered to mediate the transition towards long-lasting tissue damage. Although peripheral levels of distinct neurofilament subunits are shown to correlate with the clinically observed severity of cerebral ischemia, neurofilaments have so far not been considered for neuroprotective approaches. Therefore, the present study systematically addresses ischemia-induced alterations of the neurofilament light (NF-L), medium (NF-M), and heavy (NF-H) subunits as well as of α-internexin (INA). For this purpose, we applied a multi-parametric approach including immunofluorescence labeling, western blotting, qRT-PCR and electron microscopy. Analyses comprised ischemia-affected tissue from three stroke models of middle cerebral artery occlusion (MCAO), including approaches of filament-based MCAO in mice, thromboembolic MCAO in rats, and electrosurgical MCAO in sheep, as well as human autoptic stroke tissue. As indicated by altered immunosignals, impairment of neurofilament subunits was consistently observed throughout the applied stroke models and in human tissue. Thereby, altered NF-L immunoreactivity was also found to reach penumbral areas, while protein analysis revealed consistent reductions for NF-L and INA in the ischemia-affected neocortex in mice. At the mRNA level, the ischemic neocortex and striatum exhibited reduced expressions of NF-L- and NF-H-associated genes, whereas an upregulation for Ina appeared in the striatum. Further, multiple fluorescence labeling of neurofilament proteins revealed spheroid and bead-like structural alterations in human and rodent tissue, correlating with a cellular edema and lost cytoskeletal order at the ultrastructural level. Thus, the consistent ischemia-induced affection of neurofilament subunits in animals and human tissue, as well as the involvement of potentially salvageable tissue qualify neurofilaments as promising targets for neuroprotective strategies. During ischemia formation, such approaches may focus on the maintenance of neurofilament integrity, and appear applicable as co-treatment to modern recanalizing strategies.

Hedgehog signalling in myeloid cells impacts on body weight, adipose tissue inflammation and glucose metabolism

AIMS/HYPOTHESIS

Recently, hedgehog (Hh) was identified as a crucial player in adipose tissue development and energy expenditure. Therefore, we tested whether Hh ligands are regulated in obesity. Further, we aimed at identifying potential target cells of Hh signalling and studied the functional impact of Hh signalling on adipose tissue inflammation and glucose metabolism.

METHODS

Hh ligands and receptors were analysed in adipose tissue or serum from lean and obese mice as well as in humans. To study the impact on adipose tissue inflammation and glucose metabolism, Hh signalling was specifically blocked in myeloid cells using a conditional knockout approach (Lys-Smo ^-/-).

RESULTS

Desert Hh (DHH) and Indian Hh (IHH) are local Hh ligands, whereas Sonic Hh is not expressed in adipose tissue from mice or humans. In mice, obesity leads to a preferential upregulation of Hh ligands (Dhh) and signalling components (Ptch1, Smo and Gli1) in subcutaneous adipose tissue. Further, adipose tissue macrophages are Hh target cells owing to the expression of Hh receptors, such as Patched1 and 2. Conditional knockout of Smo (which encodes Smoothened, a mandatory Hh signalling component) in myeloid cells increases body weight and adipose tissue inflammation and attenuates glucose tolerance, suggesting an anti-inflammatory effect of Hh signalling. In humans, adipose tissue expression of DHH and serum IHH decrease with obesity and type 2 diabetes, which might be explained by the intake of metformin. Interestingly, metformin reduced Dhh and Ihh expression in mouse adipose tissue explants.

CONCLUSIONS/INTERPRETATION

Hh signalling in myeloid cells affects adipose tissue inflammation and glucose metabolism and may be a potential target to treat type 2 diabetes.

IL-6 Regulates M2 Polarization and Local Proliferation of Adipose Tissue Macrophages in Obesity

Obesity is associated with chronic low-grade inflammation of adipose tissue (AT) and an increase of AT macrophages (ATMs) that is linked to the onset of type 2 diabetes. We have recently shown that focal sites of inflammation around dying adipocytes, so-called crown-like structures, exhibit a unique microenvironment for macrophage proliferation. Interestingly, locally proliferating macrophages were not classically activated (M1), but they exhibited a rather alternatively activated (M2) immune phenotype. In this study, we established organotypic cell cultures of AT explants to study the impact of cytokine treatment on local ATM proliferation, without the bias of early monocyte recruitment. We show that exposure of AT to Th2 cytokines, such as IL-4, IL-13, and GM-CSF, stimulates ATM proliferation, whereas Th1 cytokines, such as TNF-α, inhibit local ATM proliferation. Furthermore, AT from obese mice exhibits an increased sensitivity to IL-4 stimulation, indicated by an increased phosphorylation of STAT6. In line with this, gene expression of the IL-4 receptor (Il4ra) and its ligand IL-13 are elevated in AT of obese C57BL/6 mice. Most importantly, Il4ra expression and susceptibility to IL-4 or IL-13 treatment depend on IL-6 signaling, which seems to be the underlying mechanism of local ATM proliferation in obesity. We conclude that IL-6 acts as a Th2 cytokine in obesity by stimulating M2 polarization and local ATM proliferation, presumably due to upregulation of the IL-4 receptor α.

Intrinsic up-regulation of 2-AG favors an area specific neuronal survival in different in vitro models of neuronal damage

BACKGROUND

The endocannabinoid 2-arachidonoyl glycerol (2-AG) acts as a retrograde messenger and modulates synaptic signaling e. g. in the hippocampus. 2-AG also exerts neuroprotective effects under pathological situations. To better understand the mechanism beyond physiological signaling we used Organotypic Entorhino-Hippocampal Slice Cultures (OHSC) and investigated the temporal regulation of 2-AG in different cell subsets during excitotoxic lesion and dendritic lesion of long range projections in the enthorhinal cortex (EC), dentate gyrus (DG) and the cornu ammonis region 1 (CA1).

RESULTS

2-AG levels were elevated 24 h after excitotoxic lesion in CA1 and DG (but not EC) and 24 h after perforant pathway transection (PPT) in the DG only. After PPT diacylglycerol lipase alpha (DAGL) protein, the synthesizing enzyme of 2-AG was decreased when Dagl mRNA expression and 2-AG levels were enhanced. In contrast to DAGL, the 2-AG hydrolyzing enzyme monoacylglycerol lipase (MAGL) showed no alterations in total protein and mRNA expression after PPT in OHSC. MAGL immunoreaction underwent a redistribution after PPT and excitotoxic lesion since MAGL IR disappeared in astrocytes of lesioned OHSC. DAGL and MAGL immunoreactions were not detectable in microglia at all investigated time points. Thus, induction of the neuroprotective endocannabinoid 2-AG might be generally accomplished by down-regulation of MAGL in astrocytes after neuronal lesions.

CONCLUSION

Increase in 2-AG levels during secondary neuronal damage reflects a general neuroprotective mechanism since it occurred independently in both different lesion models. This intrinsic up-regulation of 2-AG is synergistically controlled by DAGL and MAGL in neurons and astrocytes and thus represents a protective system for neurons that is involved in dendritic reorganisation.

Time dependent neuroprotection of mycophenolate mofetil: effects on temporal dynamics in glial proliferation, apoptosis, and scar formation

Background

Immunosuppressants such as mycophenolate mofetil (MMF) have the capacity to inhibit microglial and astrocytic activation and to reduce the extent of cell death after neuronal injury. This study was designed to determine the effective neuroprotective time frame in which MMF elicits its beneficial effects, by analyzing glial cell proliferation, migration, and apoptosis.

Methods

Using organotypic hippocampal slice cultures (OHSCs), temporal dynamics of proliferation and apoptosis after N-methyl-D-aspartate (NMDA)-mediated excitotoxicity were analyzed by quantitative morphometry of Ki-67 or cleaved caspase-3 immunoreactive glial cells. Treatment on NMDA-lesioned OHSCs with mycophenolate mofetil (MMF)100 μg/mL was started at different time points after injury or performed within specific time frames, and the numbers of propidium iodide (PI)⁺ degenerating neurons and isolectin (I)B₄⁺ microglial cells were determined. Pre-treatment with guanosine 100 μmol/l was performed to counteract MMF-induced effects. The effects of MMF on reactive astrocytic scar formation were investigated in the scratch-wound model of astrocyte monolayers.

Results

Excitotoxic lesion induction led to significant increases in glial proliferation rates between 12 and 36 hours after injury and to increased levels of apoptotic cells between 24 and 72 hours after injury. MMF treatment significantly reduced glial proliferation rates without affecting apoptosis. Continuous MMF treatment potently reduced the extent of neuronal cell demise when started within the first 12 hours after injury. A crucial time-frame of significant neuroprotection was identified between 12 and 36 hours after injury. Pre-treatment with the neuroprotective nucleoside guanosine reversed MMF-induced antiproliferative effects on glial cells. In the scratch-wound model, gap closure was reached within 48 hours in controls, and was potently inhibited by MMF.

Conclusions

Our data indicate that immunosuppression by MMF significantly attenuates the extent of neuronal cell death when administered within a crucial time frame after injury. Moreover, long-lasting immunosuppression, as required after solid-organ transplantation, does not seem to be necessary. Targeting inosine 5-monophosphate dehydrogenase, the rate-limiting enzyme of purine synthesis, is an effective strategy to modulate the temporal dynamics of proliferation and migration of microglia and astrocytes, and thus to reduce the extent of secondary neuronal damage and scar formation.

Site-specific and time-dependent activation of the endocannabinoid system after transection of long-range projections

Background

After focal neuronal injury the endocannabinioid system becomes activated and protects or harms neurons depending on cannabinoid derivates and receptor subtypes. Endocannabinoids (eCBs) play a central role in controlling local responses and influencing neural plasticity and survival. However, little is known about the functional relevance of eCBs in long-range projection damage as observed in stroke or spinal cord injury (SCI).

Methods

In rat organotypic entorhino-hippocampal slice cultures (OHSC) as a relevant and suitable model for investigating projection fibers in the CNS we performed perforant pathway transection (PPT) and subsequently analyzed the spatial and temporal dynamics of eCB levels. This approach allows proper distinction of responses in originating neurons (entorhinal cortex), areas of deafferentiation/anterograde axonal degeneration (dentate gyrus) and putative changes in more distant but synaptically connected subfields (cornu ammonis (CA) 1 region).

Results

Using LC-MS/MS, we measured a strong increase in arachidonoylethanolamide (AEA), oleoylethanolamide (OEA) and palmitoylethanolamide (PEA) levels in the denervation zone (dentate gyrus) 24 hours post lesion (hpl), whereas entorhinal cortex and CA1 region exhibited little if any changes. NAPE-PLD, responsible for biosynthesis of eCBs, was increased early, whereas FAAH, a catabolizing enzyme, was up-regulated 48hpl.

Conclusion

Neuronal damage as assessed by transection of long-range projections apparently provides a strong time-dependent and area-confined signal for de novo synthesis of eCB, presumably to restrict neuronal damage. The present data underlines the importance of activation of the eCB system in CNS pathologies and identifies a novel site-specific intrinsic regulation of eCBs after long-range projection damage.