Selective deletion of interleukin-1 alpha in microglia regulates neuronal responses and neurorepair processes after experimental ischemic stroke

Inflammation is a key contributor to stroke pathogenesis and drives exacerbated brain damage leading to poor outcome. Interleukin-1 (IL-1) is an important regulator of post-stroke inflammation, and blocking its actions is beneficial in pre-clinical stroke models and safe in the clinical setting. IL-1α and IL-1β are the two major IL-1 type 1 receptor (IL-1R1) agonists from the IL-1 family. The distinct roles of both isoforms, and particularly that of IL-1α, remain largely unknown. Here we show that IL-1α and IL-1β have different spatio-temporal expression profiles in the brain after experimental stroke, with early microglial IL-1α expression (4 h) and delayed IL-1β expression in infiltrated neutrophils and a small microglial subset (24-72 h). We examined the specific contribution of microglial-derived IL-1α in experimental permanent and transient ischemic stroke through cell-specific tamoxifen-inducible Cre-loxP-mediated recombination. Microglial IL-1α deletion did not influence acute brain damage, cerebral blood flow, IL-1β expression, neutrophil infiltration, microglial nor endothelial activation after ischemic stroke. However, microglial IL-1α knock out (KO) mice showed reduced peri-infarct vessel density and reactive astrogliosis at 14 days post-stroke, alongside a worse functional recovery. RNA sequencing analysis and subsequent pathway analysis on ipsilateral/contralateral cortex 4 h after stroke revealed a downregulation of the neuronal CREB signaling pathway in microglial IL-1α KO compared to WT mice. Our study identifies for the first time a critical role for microglial IL-1α on neuronal activity, neurorepair and functional recovery after stroke, highlighting the importance of targeting specific IL-1 mechanisms in brain injury to develop more effective therapies.

Itaconate and fumarate derivatives inhibit priming and activation of the canonical NLRP3 inflammasome in macrophages

The NLRP3 inflammasome is a multiprotein complex that regulates caspase-1 activation and subsequent interleukin (IL)-1β and IL-18 release from innate immune cells in response to infection or injury. Derivatives of the metabolites itaconate and fumarate, dimethyl itaconate (DMI), 4-octyl itaconate (4OI) and dimethyl fumarate (DMF) limit both expression and release of IL-1β following NLRP3 inflammasome activation. However, the direct effects of these metabolite derivatives on NLRP3 inflammasome responses require further investigation. Using murine bone marrow-derived macrophages, mixed glia and organotypic hippocampal slice cultures (OHSCs), we demonstrate that DMI, 4OI and DMF pretreatments inhibit pro-inflammatory cytokine production in response to lipopolysaccharide (LPS), as well as inhibit subsequent NLRP3 inflammasome activation induced by nigericin. DMI, 4OI, DMF and monomethyl fumarate (MMF), another fumarate derivative, also directly inhibited biochemical markers of NLRP3 activation in LPS-primed macrophages, mixed glia, OHSCs and human macrophages in response to nigericin and imiquimod, including ASC speck formation, caspase-1 activation, gasdermin D cleavage and IL-1β release. DMF, an approved treatment of multiple sclerosis, as well as DMI, 4OI and MMF, inhibited NLRP3 activation in macrophages in response to lysophosphatidylcholine, which is used to induce demyelination, suggesting a possible mechanism for DMF in multiple sclerosis through NLRP3 inhibition. The derivatives also reduced pro-IL-1α cleavage in response to the calcium ionophore ionomycin. Together, these findings reveal the immunometabolic regulation of both the priming and activation steps of NLRP3 activation in macrophages. Furthermore, we highlight itaconate and fumarate derivatives as potential therapeutic options in NLRP3- and IL-1α-driven diseases, including in the brain.

Robust thrombolytic and anti-inflammatory action of a constitutively active ADAMTS13 variant in murine stroke models

Advances in our understanding of ADAMTS13 structure, and the conformation changes required for full activity, have rejuvenated the possibility of its use as a thrombolytic therapy. We have tested a novel Ala1144Val ADAMTS13 variant (constitutively active [ca] ADAMTS13) that exhibits constitutive activity, characterized using in vitro assays of ADAMTS13 activity, and greatly enhanced thrombolytic activity in 2 murine models of ischemic stroke, the distal FeCl3 middle cerebral artery occlusion (MCAo) model and transient middle cerebral artery occlusion (tMCAO) with systemic inflammation and ischemia/reperfusion injury. The primary measure of efficacy in both models was restoration of regional cerebral blood flow (rCBF) to the MCA territory, which was determined using laser speckle contrast imaging. The caADAMTS13 variant exhibited a constitutively active conformation and a fivefold enhanced activity against fluorescence resonance energy transfer substrate von Willebrand factor 73 (FRETS-VWF73) compared with wild-type (wt) ADAMTS13. Moreover, caADAMTS13 inhibited VWF-mediated platelet capture at subphysiological concentrations and enhanced t-PA/plasmin lysis of fibrin(ogen), neither of which were observed with wtADAMTS13. Significant restoration of rCBF and reduced lesion volume was observed in animals treated with caADAMTS13. When administered 1 hour after FeCl3 MCAo, the caADAMTS13 variant significantly reduced residual VWF and fibrin deposits in the MCA, platelet aggregate formation, and neutrophil recruitment. When administered 4 hours after reperfusion in the tMCAo model, the caADAMTS13 variant induced a significant dissolution of platelet aggregates and a reduction in the resulting tissue hypoperfusion. The caADAMTS13 variant represents a potentially viable therapeutic option for the treatment of acute ischemic stroke, among other thrombotic indications, due to its enhanced in vitro and in vivo activities that result from its constitutively active conformation.

LRRC8A is dispensable for a variety of microglial functions and response to acute stroke

Microglia, resident brain immune cells, are critical in orchestrating responses to central nervous system (CNS) injury. Many microglial functions, such as phagocytosis, motility and chemotaxis, are suggested to rely on chloride channels, including the volume‐regulated anion channel (VRAC), but studies to date have relied on the use of pharmacological tools with limited specificity. VRAC has also been proposed as a drug target for acute CNS injury, and its role in microglial function is of considerable interest for developing CNS therapeutics. This study aimed to definitively confirm the contribution of VRAC in microglia function by using conditional LRRC8A‐knockout mice, which lacked the essential VRAC subunit LRRC8A in microglia. We demonstrated that while VRAC contributed to cell volume regulation, it had no effect on phagocytic activity, cell migration or P2YR12‐dependent chemotaxis. Moreover, loss of microglial VRAC did not affect microglial morphology or the extent of ischemic damage following stroke. We conclude that VRAC does not critically regulate microglial responses to brain injury and could be targetable in other CNS cell types (e.g., astrocytes) without impeding microglial function. Our results also demonstrate a role for VRAC in cell volume regulation but show that VRAC is not involved in several major cellular functions that it was previously thought to regulate, and point to other, alternative mechanisms of chloride transport in innate immunity.

Abstract TP21: The Role Of Interleukin-1α Cytokine In Poststroke Inflammation

Stroke remains the second leading cause of death globally, with a total of 5.5 million deaths in 2016 and an estimated global lifetime risk of stroke for those aged 25 years or older at 24.9%. A key feature of stroke and poststroke injury is the inflammatory response, which is driven by a variety of pro-inflammatory cytokines to include interleukin-1 (IL-1). Evidence from pre-clinical studies have demonstrated a deleterious effect of IL-1 poststroke with a beneficial effect observed after blocking IL-1 in both pre-clinical and clinical settings. The IL-1 family has both IL-1β and IL-1α forms, and although IL-1β has been most closely studied, the distinct roles of both IL-1 isoforms during poststroke inflammation is largely unknown. In this study, we examined the contribution of IL-1α in ischemic stroke. We characterized the spatio-temporal effect of IL-1 in the brain after stroke using an in vivo model of middle cerebral artery thrombosis in an inducible IL-1α knockout mouse through topical application of FeCl3. We found that IL-1α precedes IL-1β expression after ischemic stroke and is restricted to microglia whereas IL-1β is expressed by both neutrophils and microglia. Intriguingly, microglial IL-1α deletion does not influence brain damage after ischemic stroke, and furthermore does not influence cerebral blood flow or neutrophil infiltration and IL-1β expression up to 24h after ischemic stroke. Pathway analysis following RNAseq suggests that microglial IL-1α regulates neuronal activity through CREB signaling. In vitro studies in Neuro2a (N2a) murine cell line additionally show neuronal activity regulation through CREB signaling via microglial IL-1α after oxygen glucose deprivation and reperfusion (OGD/R) model of stroke. As the role of IL-1α in stroke has yet to be fully elucidated, future experiments using both murine and in vitro models will clarify the role that IL-1α has in the pathogenesis of stroke and its relevance to inflammatory features posttroke, which may lead to future therapeutic insight.

Thrombolysis by PLAT/tPA increases serum free IGF1 leading to a decrease of deleterious autophagy following brain ischemia

Cerebral ischemia is a pathology involving a cascade of cellular mechanisms, leading to the deregulation of proteostasis, including macroautophagy/autophagy, and finally to neuronal death. If it is now accepted that cerebral ischemia induces autophagy, the effect of thrombolysis/energy recovery on proteostasis remains unknown. Here, we investigated the effect of thrombolysis by PLAT/tPA (plasminogen activator, tissue) on autophagy and neuronal death. In two in vitro models of hypoxia reperfusion and an in vivo model of thromboembolic stroke with thrombolysis by PLAT/tPA, we found that ischemia enhances neuronal deleterious autophagy. Interestingly, PLAT/tPA decreases autophagy to mediate neuroprotection by modulating the PI3K-AKT-MTOR pathways both in vitro and in vivo. We identified IGF1R (insulin-like growth factor I receptor; a tyrosine kinase receptor) as the effective receptor and showed in vitro, in vivo and in human stroke patients and that PLAT/tPA is able to degrade IGFBP3 (insulin-like growth factor binding protein 3) to increase IGF1 (insulin-like growth factor 1) bioavailability and thus IGF1R activation.Abbreviations: AKT/protein kinase B: thymoma viral proto-oncogene 1; EGFR: epidermal growth factor receptor; Hx: hypoxia; IGF1: insulin-like growth factor 1; IGF1R: insulin-like growth factor I receptor; IGFBP3: insulin-like growth factor binding protein 3; Ka: Kainate; MAP1LC3/LC3: microtubule-associated protein 1 light chain 3; MAPK/ERK: mitogen-activated protein kinase; MTOR: mechanistic target of rapamycin kinase; MTORC1: MTOR complex 1; OGD: oxygen and glucose deprivation; OGD_reox: oxygen and glucose deprivation + reoxygentation; PepA: pepstatin A1; PI3K: phosphoinositide 3-kinase; PLAT/tPA: plasminogen activator, tissue; PPP: picropodophyllin; SCH77: SCH772984; ULK1: unc-51 like kinase 1; Wort: wortmannin.

Abstract P767: Understanding the Role of Microglial Il-1α After Stroke

The role of inflammation driven by the pro-inflammatory cytokine interleukin-1 (IL-1) during poststroke injury has been the focus of intense research. Indeed, pre-clinical studies have demonstrated the deleterious actions of IL-1 after stroke, whilst blocking its actions is beneficial in pre-clinical and clinical settings. Whilst most studies have focused on the role of IL-1β, the role of IL-1α during poststroke inflammation has been largely overlooked and very little has been done to examine the selective contribution of each IL-1 isoform in ischaemic stroke. In this study, we have investigated the contribution of IL-1α to ischaemic and haemorrhagic stroke. Methods : in vivo model of middle cerebral artery thrombosis through topical application of FeCl3 (40%) (n=4/group (4, 24, 72h) histology, n=6 /group (4, 24, 72 ) qPCR) in vivo model of haemorrhagic stroke induced by using the collagenase model (intra-striatal injection of collagenase VII-S, 0.045U) (n=4/group (4, 24h) histology). We first investigated the spatio-temporal expression of IL-1 in the brain after ischaemic stroke using an in vivo model of middle cerebral artery thrombosis through topical application of FeCl3 (n=4-6/group). We observed IL-1α positive microglia as early as 4 hours after ischaemic stroke. At 24 hours we observed IL-1α and also IL-1β positive microglia, moreover, IL-1β was also expressed by neutrophils and monocytes. These results have been confirmed by qPCR. Interestingly, we observed similar results after haemorrhagic stroke, induced by using the collagenase model, whereby IL-1α expression in microglia precedes IL-1β expression (n=4/group). The early expression of IL-1α in microglia in both models suggest a critical role for the microglial response during sterile inflammation. To further investigate the role of microglial IL-1α, we have generated a conditional IL-1α mouse mutant crossed with CX3CR1 Cre-ERT2 mice to induce a specific deletion of IL-1α in microglia. Future experiments using this genetic model will clarify the role of IL-1α during the acute phase of strokes.

Hallmarks of NLRP3 inflammasome activation are observed in organotypic hippocampal slice culture

Microglial inflammation driven by the NACHT, LRR and PYD domain-containing protein 3 (NLRP3) inflammasome contributes to brain disease and is a therapeutic target. Most mechanistic studies on NLRP3 activation use two-dimensional pure microglial cell culture systems. Here we studied the activation of the NLRP3 inflammasome in organotypic hippocampal slices, which allowed us to investigate microglial NLRP3 activation in a three-dimensional, complex tissue architecture. Toll-like receptor 2 and 4 activation primed microglial inflammasome responses in hippocampal slices by increasing NLRP3 and interleukin-1β expression. Nigericin-induced NLRP3 inflammasome activation was dynamically visualized in microglia through ASC speck formation. Downstream caspase-1 activation, gasdermin D cleavage, pyroptotic cell death and interleukin-1β release were also detected, and these findings were consistent when using different NLRP3 stimuli such as ATP and imiquimod. NLRP3 inflammasome pathway inhibitors were effective in organotypic hippocampal slices. Hence, we have highlighted organotypic hippocampal slice culture as a valuable ex vivo tool to allow the future study of NLRP3 inflammasomes in a representative tissue section, aiding the discovery of further mechanistic insights and drug development.

Extent of Ischemic Brain Injury After Thrombotic Stroke Is Independent of the NLRP3 (NACHT, LRR and PYD Domains-Containing Protein 3) Inflammasome

Supplemental Digital Content is available in the text.

Background and Purpose—

A major process contributing to cell death in the ischemic brain is inflammation. Inflammasomes are multimolecular protein complexes that drive inflammation through activation of proinflammatory cytokines, such as IL (interleukin)-1β. Preclinical evidence suggests that IL-1β contributes to a worsening of ischemic brain injury.

Methods—

Using a mouse middle cerebral artery thrombosis model, we examined the inflammatory response after stroke and the contribution of the NLRP3 (NACHT, LRR and PYD domains-containing protein 3) inflammasome to ischemic injury.

Results—

There was a marked inflammatory response after stroke characterized by increased expression of proinflammatory cytokines and NLRP3 and by recruitment of leukocytes to the injured tissue. Targeting NLRP3 with the inhibitor MCC950, or using mice in which NLRP3 was knocked out, had no effect on the extent of injury caused by stroke.

Conclusions—

These data suggest that the NLRP3 pathway does not contribute to the inflammation exacerbating ischemic brain damage, contradicting several recent reports to the contrary.

Pentraxin 3 regulates neutrophil infiltration to the brain during neuroinflammation

Introduction: The acute phase protein pentraxin 3 (PTX3) is known for its anti-inflammatory effects through downregulating neutrophil transmigration during peripheral inflammation. Furthermore, we have previously demonstrated a neuroprotective and neuroreparative effect of PTX3 after cerebral ischaemia. Here we investigated, to our knowledge for the first time, the role of PTX3 in neutrophil transmigration and neurotoxicity following lipopolysaccharide (LPS)-induced cerebral inflammation and cerebral ischaemia.

Methods: Neutrophil transmigration through interleukin-1β (IL-1β) activated brain endothelium and neurotoxicity of neutrophils isolated from wild-type (WT) or PTX3 knock-out (KO) mice was assessed in vitro. Primary cortical neuronal death after treatment with transmigrated neutrophils was quantified by lactate dehydrogenase (LDH) assay. Cerebral inflammation or ischemia was induced in WT and PTX3 KO mice via intrastriatal LPS injection or by transient middle cerebral artery occlusion (MCAo) respectively. Subsequent neutrophil infiltration in the brain was assessed by immunohistochemistry and the expression of pro-inflammatory cytokines interleukin-6 (IL-6) and IL-1β by enzyme-linked immunosorbent assay (ELISA).

Results: Neutrophils isolated from WT mice after intrastriatal LPS injection transmigrated significantly more through IL-1β activated brain endothelium compared to neutrophils from PTX3 KO mice. Transmigrated WT and PTX3 KO neutrophils were significantly more neurotoxic than corresponding non-transmigrated neutrophils; however, no significant differences in neurotoxicity between genotypes were observed. PTX3 reduced the number of transmigrated neutrophils to the brain after intrastriatal LPS injection. Furthermore, PTX3 KO mice showed significantly increased levels of neutrophils in the brain after LPS administration or in the ischaemic hemisphere after MCAo, compared to WT mice.

Conclusion: Our study shows that PTX3 regulates neutrophil transmigration in the CNS during neuroinflammation, demonstrating the potential of PTX3 as an effective therapeutic target in neuroinflammatory conditions.

Ligature-induced periodontitis induces systemic inflammation but does not alter acute outcome after stroke in mice

Background

Stroke is a major cause of disability and mortality. Poorer outcome after stroke is associated with concomitant inflammatory and infectious disease. Periodontitis is a chronic inflammatory disease of the dental supporting structures and is a prominent risk factor for many systemic disorders, including cardiovascular disease and stroke. While epidemiological studies suggest that periodontitis increases the likelihood of stroke, its impact on stroke severity is poorly understood. Here, we sought to determine the contribution of periodontitis to acute stroke pathology.

Methods

We characterized a murine ligature model of periodontitis for inflammatory responses that could potentially impact stroke outcome. We applied this model and then subjected mice to either transient or permanent middle cerebral artery occlusion. We also enhanced the periodontitis model with repeated intravenous administration of a periodontal-specific lipopolysaccharide to better mimic the clinical condition.

Results

Ligature-induced periodontitis caused bone loss, bacterial growth, and increased local inflammatory cell trafficking. Systemically, periodontitis increased circulating levels of pro-inflammatory cytokines, and primed bone marrow monocytes to produce elevated tumour necrosis factor-alpha (TNFα). Despite these changes, periodontitis alone or in tandem with repeated lipopolysaccharide challenge did not alter infarct volume, blood–brain barrier breakdown, or systemic inflammation after experimental stroke.

Conclusions

Our data show that despite elevated systemic inflammation in periodontitis, oral inflammatory disease does not impact acute stroke pathology in terms of severity, determined primarily by infarct volume. This indicates that, at least in this experimental paradigm, periodontitis alone does not alter acute outcome after cerebral ischemia.

Pentraxin 3 promotes long-term cerebral blood flow recovery, angiogenesis, and neuronal survival after stroke

Restoration of cerebral blood flow (CBF) and upregulation of angiogenesis are crucial for brain repair and functional recovery after cerebral ischaemia. Pentraxin 3 (PTX3) is a key regulator of angiogenesis and is emerging as a promising target for cerebrovascular repair after stroke. Here, we investigated for the first time the role of PTX3 in long-term CBF, angiogenesis, and neuronal viability after ischaemic stroke induced by transient middle cerebral artery occlusion (MCAo). Lack of PTX3 had no effect on early brain damage, but significantly impaired restoration of CBF, 14 and 28 days after MCAo, compared to wild-type (WT) mice. Immunohistochemical analysis revealed that PTX3 KO mice have significantly greater neuronal loss, significantly decreased vessel diameter, vessel proliferation, vascular density, and reactive astrocytes and decreased expression of vascular endothelial growth factor receptor 2 (VEGR2), vascular extracellular matrix (ECM)-proteins (collagen IV, laminin), and integrin-β, in the ipsilateral (stroke) hemisphere compared to WT mice, 28 days after MCAo. Therefore, PTX3 promotes sustained long-term recovery of CBF, angiogenesis, and neuronal viability after cerebral ischaemia. Collectively, these findings demonstrate the potential and clinical relevance of PTX3 as a promising therapeutic target, providing sustained long-term post-stroke neurovascular repair and reducing the loss of neurons. KEY MESSAGES: Pentraxin 3 (PTX3) is a key regulator of angiogenesis and is emerging as a promising target for cerebrovascular repair after stroke. Restoration of cerebral blood flow (CBF) and angiogenesis are crucial for brain repair and functional recovery after cerebral ischaemia. PTX3 promotes sustained long-term recovery of CBF, angiogenesis, and neuronal viability after cerebral ischaemia.

A brain in flame; do inflammasomes and pyroptosis influence stroke pathology?

Stroke is one of the leading causes of death and disability worldwide. Inflammation plays a key role across the time course of stroke, from onset to the post-injury reparative phase days to months later. Several regulatory molecules are implicated in inflammation, but the most established inflammatory mediator of acute brain injury is the cytokine interleukin-1. Interleukin-1 is regulated by large, macromolecular complexes called inflammasomes, which play a central role in cytokine release and cell death. In this review we highlight recent advances in inflammasome research and propose key roles for inflammasome components in the progression of stroke damage.

Stressed neurons protect themselves by a tissue-type plasminogen activator-mediated EGFR-dependent mechanism

In the central nervous system, tissue-type plasminogen activator (tPA) has been associated with both pro-death and prosurvival actions on neurons. In most cases, this has been related to exogenous tPA. In the present study, we addressed the influence of endogenous tPA. We first observed an increased transcription of tPA following either in vivo global brain ischemia in rats or in vitro oxygen glucose deprivation (OGD) on mice and rats hippocampal slices. Hippocampal slices from tPA-deficient mice were more sensitive to OGD than wild-type slices. Pharmacological approaches targeting the known receptors of tPA revealed that only the inhibition of phosphorylation of epidermal growth factor receptors (EGFRs) prevented the neuroprotective effect of endogenous tPA. This study shows that ischemic hippocampal neurons overproduce endogenous tPA as an intend to protect themselves from ischemic death, by a mechanism involving an activation of EGFRs. Thus, strategies contributing to promote either endogenous production of tPA or its associated EGFR-linked signaling pathway may have beneficial effects following brain injuries such as stroke.

An Uncommon Electrocardiographic Change in Subarachnoid Hemorrhage

In this paper we are reporting a case of electrical left ventricular hypertrophy with increased Sokolow's index following subarachnoid hemorrhage. Two-dimensional echocardiography ruled out anatomical left ventricular hypertrophy, and the Sokolow's index eventually reverted to normal. This electrocardiographic abnormality has rarely been associated with subarachnoid hemorrhage and does not appear to be related to the neurologic grade of the patient because he presented with headaches only and was conscious throughout (Hunt and Hess grade I). The possibility that such electrocardiographic changes should reflect anatomical changes in the heart should always be ruled out by proper investigations (i.e., echocardiography and coronarography), as the presence of cardiac disorders would greatly influence medical and surgical management. In terms of electrocardiography, this observation suggests that the Sokolow's index is a poor indicator of left ventricular hypertrophy, and tends to reflect a more comprehensive catecholaminergic process.