Analysis of brain and blood single-cell transcriptomics in acute and subacute phases after experimental stroke

Cerebral ischemia triggers a powerful inflammatory reaction involving peripheral leukocytes and brain resident cells that contribute to both tissue injury and repair. However, their dynamics and diversity remain poorly understood. To address these limitations, we performed a single-cell transcriptomic study of brain and blood cells 2 or 14 days after ischemic stroke in mice. We observed a strong divergence of post-ischemic microglia, monocyte-derived macrophages and neutrophils over time, while endothelial cells and brain-associated macrophages showed altered transcriptomic signatures at 2 days poststroke. Trajectory inference predicted the in situ trans-differentiation of macrophages from blood monocytes into day 2 and day 14 phenotypes, while neutrophils were projected to be continuously de novo recruited from the blood. Brain single-cell transcriptomes from both female and male aged mice were similar to that of young male mice, but aged and young brains differed in their immune cell composition. Although blood leukocyte analysis also revealed altered transcriptomes after stroke, brain-infiltrating leukocytes displayed higher transcriptomic divergence than their circulating counterparts, indicating that phenotypic diversification occurs within the brain in the early and recovery phases of ischemic stroke. A portal ( https://anratherlab.shinyapps.io/strokevis/ ) is provided to allow user-friendly access to our data.

Meningeal interleukin-17-producing T cells mediate cognitive impairment in a mouse model of salt-sensitive hypertension

Hypertension (HTN), a disease afflicting over one billion individuals worldwide, is a leading cause of cognitive impairment, the mechanisms of which remain poorly understood. In the present study, in a mouse model of HTN, we find that the neurovascular and cognitive dysfunction depends on interleukin (IL)-17, a cytokine elevated in individuals with HTN. However, neither circulating IL-17 nor brain angiotensin signaling can account for the dysfunction. Rather, IL-17 produced by T cells in the dura mater is the mediator released in the cerebrospinal fluid and activating IL-17 receptors on border-associated macrophages (BAMs). Accordingly, depleting BAMs, deleting IL-17 receptor A in brain macrophages or suppressing meningeal T cells rescues cognitive function without attenuating blood pressure elevation, circulating IL-17 or brain angiotensin signaling. Our data unveil a critical role of meningeal T cells and macrophage IL-17 signaling in the neurovascular and cognitive dysfunction in a mouse model of HTN.

Border-associated macrophages promote cerebral amyloid angiopathy and cognitive impairment through vascular oxidative stress

BACKGROUND

Cerebral amyloid angiopathy (CAA) is a devastating condition common in patients with Alzheimer's disease but also observed in the general population. Vascular oxidative stress and neurovascular dysfunction have been implicated in CAA but the cellular source of reactive oxygen species (ROS) and related signaling mechanisms remain unclear. We tested the hypothesis that brain border-associated macrophages (BAM), yolk sac-derived myeloid cells closely apposed to parenchymal and leptomeningeal blood vessels, are the source of radicals through the Aβ-binding innate immunity receptor CD36, leading to neurovascular dysfunction, CAA, and cognitive impairment.

METHODS

Tg2576 mice and WT littermates were transplanted with CD36-/- or CD36+/+ bone marrow at 12-month of age and tested at 15 months. This approach enables the repopulation of perivascular and leptomeningeal compartments with CD36-/- BAM. Neurovascular function was tested in anesthetized mice equipped with a cranial window in which cerebral blood flow was monitored by laser-Doppler flowmetry. Amyloid pathology and cognitive function were also examined.

RESULTS

The increase in blood flow evoked by whisker stimulation (functional hyperemia) or by endothelial and smooth muscle vasoactivity was markedly attenuated in WT → Tg2576 chimeras but was fully restored in CD36-/- → Tg2576 chimeras, in which BAM ROS production was suppressed. CAA-associated Aβ1-40, but not Aβ1-42, was reduced in CD36-/- → Tg2576 chimeras. Similarly, CAA, but not parenchymal plaques, was reduced in CD36-/- → Tg2576 chimeras. These beneficial vascular effects were associated with cognitive improvement. Finally, CD36-/- mice were able to more efficiently clear exogenous Aβ1-40 injected into the neocortex or the striatum.

CONCLUSIONS

CD36 deletion in BAM suppresses ROS production and rescues the neurovascular dysfunction and damage induced by Aβ. CD36 deletion in BAM also reduced brain Aβ1-40 and ameliorated CAA without affecting parenchyma plaques. Lack of CD36 enhanced the vascular clearance of exogenous Aβ. Restoration of neurovascular function and attenuation of CAA resulted in a near complete rescue of cognitive function. Collectively, these data implicate brain BAM in the pathogenesis of CAA and raise the possibility that targeting BAM CD36 is beneficial in CAA and other conditions associated with vascular Aβ deposition and damage.

Comparative multi-omic analyses of cardiac mitochondrial stress in three mouse models of frataxin deficiency

Cardiomyopathy is often fatal in Friedreich ataxia (FA). However, FA hearts maintain adequate function until advanced disease stages, suggesting initial adaptation to the loss of frataxin (FXN). Conditional cardiac knockout mouse models of FXN show transcriptional and metabolic profiles of the mitochondrial integrated stress response (ISRmt), which could play an adaptive role. However, the ISRmt has not been investigated in models with disease-relevant, partial decrease in FXN. We characterized the heart transcriptomes and metabolomes of three mouse models with varying degrees of FXN depletion: YG8-800, KIKO-700 and FXNG127V. Few metabolites were changed in YG8-800 mice, which did not provide a signature of cardiomyopathy or ISRmt; several metabolites were altered in FXNG127V and KIKO-700 hearts. Transcriptional changes were found in all models, but differentially expressed genes consistent with cardiomyopathy and ISRmt were only identified in FXNG127V hearts. However, these changes were surprisingly mild even at advanced age (18 months), despite a severe decrease in FXN levels to 1% of those of wild type. These findings indicate that the mouse heart has low reliance on FXN, highlighting the difficulty in modeling genetically relevant FA cardiomyopathy.

Calcium transients in nNOS neurons underlie distinct phases of the neurovascular response to barrel cortex activation in awake mice

Neuronal nitric oxide (NO) synthase (nNOS), a Ca2+ dependent enzyme, is expressed by distinct populations of neocortical neurons. Although neuronal NO is well known to contribute to the blood flow increase evoked by neural activity, the relationships between nNOS neurons activity and vascular responses in the awake state remain unclear. We imaged the barrel cortex in awake, head-fixed mice through a chronically implanted cranial window. The Ca2+ indicator GCaMP7f was expressed selectively in nNOS neurons using adenoviral gene transfer in nNOScre mice. Air-puffs directed at the contralateral whiskers or spontaneous motion induced Ca2+ transients in 30.2 ± 2.2% or 51.6 ± 3.3% of nNOS neurons, respectively, and evoked local arteriolar dilation. The greatest dilatation (14.8 ± 1.1%) occurred when whisking and motion occurred simultaneously. Ca2+ transients in individual nNOS neurons and local arteriolar dilation showed various degrees of correlation, which was strongest when the activity of whole nNOS neuron ensemble was examined. We also found that some nNOS neurons became active immediately prior to arteriolar dilation, while others were activated gradually after arteriolar dilatation. Discrete nNOS neuron subsets may contribute either to the initiation or to the maintenance of the vascular response, suggesting a previously unappreciated temporal specificity to the role of NO in neurovascular coupling.

Cell autonomous role of border associated macrophages in ApoE4 neurovascular dysfunction and susceptibility to white matter injury

Apolipoprotein-E4 (ApoE4), the strongest genetic risk factor for sporadic Alzheimer's disease, is also a risk factor for microvascular pathologies leading to cognitive impairment, particularly subcortical white matter injury. These effects have been attributed to alterations in the regulation of the brain blood supply, but the cellular source of ApoE4 and the underlying mechanisms remain unclear. In mice expressing human ApoE3 or ApoE4 we report that border associated macrophages (BAM), myeloid cells closely apposed to neocortical microvessels, are both the source and the target of the ApoE4 mediating the neurovascular dysfunction through reactive oxygen species. ApoE4 in BAM is solely responsible for the increased susceptibility to oligemic white matter damage in ApoE4 mice and is sufficient to enhance damage in ApoE3 mice. The data unveil a new aspect of BAM pathobiology and highlight a previously unrecognized cell autonomous role of BAM in the neurovascular dysfunction of ApoE4 with potential therapeutic implications.

The Neurovasculome: Key Roles in Brain Health and Cognitive Impairment: A Scientific Statement From the American Heart Association/American Stroke Association

BACKGROUND

Preservation of brain health has emerged as a leading public health priority for the aging world population. Advances in neurovascular biology have revealed an intricate relationship among brain cells, meninges, and the hematic and lymphatic vasculature (the neurovasculome) that is highly relevant to the maintenance of cognitive function. In this scientific statement, a multidisciplinary team of experts examines these advances, assesses their relevance to brain health and disease, identifies knowledge gaps, and provides future directions.

METHODS

Authors with relevant expertise were selected in accordance with the American Heart Association conflict-of-interest management policy. They were assigned topics pertaining to their areas of expertise, reviewed the literature, and summarized the available data.

RESULTS

The neurovasculome, composed of extracranial, intracranial, and meningeal vessels, as well as lymphatics and associated cells, subserves critical homeostatic functions vital for brain health. These include delivering O2 and nutrients through blood flow and regulating immune trafficking, as well as clearing pathogenic proteins through perivascular spaces and dural lymphatics. Single-cell omics technologies have unveiled an unprecedented molecular heterogeneity in the cellular components of the neurovasculome and have identified novel reciprocal interactions with brain cells. The evidence suggests a previously unappreciated diversity of the pathogenic mechanisms by which disruption of the neurovasculome contributes to cognitive dysfunction in neurovascular and neurodegenerative diseases, providing new opportunities for the prevention, recognition, and treatment of these conditions.

CONCLUSIONS

These advances shed new light on the symbiotic relationship between the brain and its vessels and promise to provide new diagnostic and therapeutic approaches for brain disorders associated with cognitive dysfunction.

Immune compartments at the brain's borders in health and neurovascular diseases

Recent evidence implicates cranial border immune compartments in the meninges, choroid plexus, circumventricular organs, and skull bone marrow in several neuroinflammatory and neoplastic diseases. Their pathogenic importance has also been described for cardiovascular diseases such as hypertension and stroke. In this review, we will examine the cellular composition of these cranial border immune niches, the potential pathways through which they might interact, and the evidence linking them to cardiovascular disease.

Border-associated macrophages promote cerebral amyloid angiopathy and cognitive impairment through vascular oxidative stress

Background: Cerebral amyloid angiopathy (CAA) is a devastating condition common in patients with Alzheimer's disease but also observed in the general population. Vascular oxidative stress and neurovascular dysfunction have been implicated in CAA but the cellular source of reactive oxygen species (ROS) and related signaling mechanisms remain unclear. We tested the hypothesis that brain border-associated macrophages (BAM), yolk sac-derived myeloid cells closely apposed to parenchymal and leptomeningeal blood vessels, are the source of radicals through the Aβ-binding innate immunity receptor CD36, leading to neurovascular dysfunction, CAA, and cognitive impairment. Methods: Tg2576 mice and WT littermates were transplanted with CD36 -/- or CD36 +/+ bone marrow at 12-month of age and tested at 15 months. This approach enables the repopulation of perivascular and leptomeningeal compartments with CD36 -/- BAM. Neurovascular function was tested in anesthetized mice equipped with a cranial window in which cerebral blood flow was monitored by laser-Doppler flowmetry. Amyloid pathology and cognitive function were also examined. Results: The increase in blood flow evoked by whisker stimulation (functional hyperemia) or by endothelial and smooth muscle vasoactivity was markedly attenuated in WT®Tg2576 chimeras but was fully restored in CD36 -/- ®Tg2576 chimeras, in which BAM ROS production was suppressed. CAA-associated Aβ 1-40 , but not Aβ 1-42 , was reduced in CD36 -/- ®Tg2576 chimeras. Similarly, CAA, but not parenchymal plaques, was reduced in CD36 -/- ®Tg2576 chimeras. These beneficial vascular effects were associated with cognitive improvement. Finally, CD36 -/- mice were able to more efficiently clear exogenous Aβ 1-40 injected into the neocortex or the striatum. Conclusions: CD36 deletion in BAM suppresses ROS production and rescues the neurovascular dysfunction and damage induced by Aβ. CD36 deletion in BAM also reduced brain Aβ 1-40 and ameliorated CAA without affecting parenchyma plaques. Lack of CD36 enhanced the vascular clearance of exogenous Aβ. Restoration of neurovascular function and attenuation of CAA resulted in a near complete rescue of cognitive function. Collectively, these data implicate CNS BAM in the pathogenesis of CAA and raise the possibility that targeting BAM CD36 is beneficial in CAA and other conditions associated with vascular Aβ deposition and damage.

Brain and blood single-cell transcriptomics in acute and subacute phases after experimental stroke

Cerebral ischemia triggers a powerful inflammatory reaction involving both peripheral leukocytes and brain resident cells. Recent evidence indicates that their differentiation into a variety of functional phenotypes contributes to both tissue injury and repair. However, the temporal dynamics and diversity of post-stroke immune cell subsets remain poorly understood. To address these limitations, we performed a longitudinal single-cell transcriptomic study of both brain and mouse blood to obtain a composite picture of brain-infiltrating leukocytes, circulating leukocytes, microglia and endothelium diversity over the ischemic/reperfusion time. Brain cells and blood leukocytes isolated from mice 2 or 14 days after transient middle cerebral artery occlusion or sham surgery were purified by FACS sorting and processed for droplet-based single-cell transcriptomics. The analysis revealed a strong divergence of post-ischemic microglia, macrophages, and neutrophils over time, while such diversity was less evident in dendritic cells, B, T and NK cells. Conversely, brain endothelial cells and brain associated-macrophages showed altered transcriptomic signatures at 2 days post-stroke, but low divergence from sham at day 14. Pseudotime trajectory inference predicted the in-situ longitudinal progression of monocyte-derived macrophages from their blood precursors into day 2 and day 14 phenotypes, while microglia phenotypes at these two time points were not connected. In contrast to monocyte-derived macrophages, neutrophils were predicted to be continuously de-novo recruited from the blood. Brain single-cell transcriptomics from both female and male aged mice did not show major changes in respect to young mice, but aged and young brains differed in their immune cell composition. Furthermore, blood leukocyte analysis also revealed altered transcriptomes after stroke. However, brain-infiltrating leukocytes displayed higher transcriptomic divergence than their circulating counterparts, indicating that phenotypic diversification into cellular subsets occurs within the brain in the early and the recovery phase of ischemic stroke. In addition, this resource report contains a searchable database https://anratherlab.shinyapps.io/strokevis/ to allow user-friendly access to our data. The StrokeVis tool constitutes a comprehensive gene expression atlas that can be interrogated at the gene and cell type level to explore the transcriptional changes of endothelial and immune cell subsets from mouse brain and blood after stroke.

Abstract 18: A Novel Neutrophil Subpopulation Mediates Neuroprotection In The Late Phases Of Cerebral Ischemia

Cerebral ischemia triggers a powerful inflammatory reaction involving infiltration of peripheral leukocytes. Neutrophils, the first immune cells entering the ischemic brain, contribute to brain injury by limiting tissue perfusion and destabilizing the blood-brain barrier. However, recent evidence indicates that neutrophils can also participate to the repairing process. Nevertheless, the dynamics and complexity of neutrophil subsets are poorly understood. Here, we performed single-cell transcriptomics to obtain a picture of infiltrating neutrophil diversity in the mouse brain 2 (D02) and 14 days (D14) after transient middle cerebral artery occlusion (MCAo) or sham surgery (Sham). Clustering analysis of the transcripts revealed 5 neutrophil populations. In Sham, most neutrophils fell into clusters 1 and 4, characterized by expression of genes associated with neutrophil development such Retnlg and Mmp8 (cluster 1) and Camp, Ltf, Chil3 and Lcn2 (cluster 4). After MCAo, clusters 1 and 4 gradually decreased, whereas cluster 2, identified by type I response genes ( Isg15, Ifit3, Rsad2, Ifit1, Ifi204 ) and cluster 3, characterized by upregulation of chemokines/cytokines ( Ccl3 , Ccl4 , Csf1 ), increased progressively from D02 to D14. Interestingly, we found that the number of neutrophils in the ischemic brain was 5 times higher at D14 than at D02 (Sham, 0.2 ± 0.1 x10 3 ; D02: 3 ± 1.3 x10 3 ; D14: 14.5 ± 3.5x10 3 cells; p=0.02 D02 vs D14; n=10-14/group). Therefore, we next investigated if neutrophil infiltration at D14 has a beneficial or adverse effect on stroke recovery. To this end, 7 days after MCAo, neutrophils depletion was induced by systemic injection of both rat anti-Ly6G (50μg/mouse, i.p) and CXCR2-antagonist SB225002 (2 mg/kg, i.p), once per day, for 7 consecutive days. We found that neutrophil depletion increased brain tissue loss (-18.7 ± 2.9 vs -8 ± 2.6% IgG/vehicle mice; p=0.01; n=9/group). In addition, neutrophil depletion decreased ambulatory distance (p<0.05 vs vehicle; n=11/group) and stereotypic behavior (p<0.05 vs vehicle; n=11/group) without altering open field performance in sham mice. Our study unveils an unrecognized neutrophil subpopulation protecting the brain in the subacute stroke phase, opening new avenues for stroke treatment.

A preclinical randomized controlled multi-centre trial of anti-interleukin-17A treatment for acute ischaemic stroke

Multiple consensus statements have called for preclinical randomized controlled trials to improve translation in stroke research. We investigated the efficacy of an interleukin-17A neutralizing antibody in a multi-centre preclinical randomized controlled trial using a murine ischaemia reperfusion stroke model. Twelve-week-old male C57BL/6 mice were subjected to 45 min of transient middle cerebral artery occlusion in four centres. Mice were randomly assigned (1:1) to receive either an anti-interleukin-17A (500 µg) or isotype antibody (500 µg) intravenously 1 h after reperfusion. The primary endpoint was infarct volume measured by magnetic resonance imaging three days after transient middle cerebral artery occlusion. Secondary analysis included mortality, neurological score, neutrophil infiltration and the impact of the gut microbiome on treatment effects. Out of 136 mice, 109 mice were included in the analysis of the primary endpoint. Mixed model analysis revealed that interleukin-17A neutralization significantly reduced infarct sizes (anti-interleukin-17A: 61.77 ± 31.04 mm³; IgG control: 75.66 ± 34.79 mm³; P = 0.01). Secondary outcome measures showed a decrease in mortality (hazard ratio = 3.43, 95% confidence interval = 1.157-10.18; P = 0.04) and neutrophil invasion into ischaemic cortices (anti-interleukin-17A: 7222 ± 6108 cells; IgG control: 28 153 ± 23 206 cells; P < 0.01). There was no difference in Bederson score. The analysis of the gut microbiome showed significant heterogeneity between centres (R = 0.78, P < 0.001, n = 40). Taken together, neutralization of interleukin-17A in a therapeutic time window resulted in a significant reduction of infarct sizes and mortality compared with isotype control. It suggests interleukin-17A neutralization as a potential therapeutic target in stroke.

Abstract 007: Hypertension-induced Neurovascular Dysfunction At Single-cell Resolution

Hypertension (HTN) disrupts vital neurovascular control mechanisms, thereby increasing the brain’s susceptibility to vascular insufficiency, white matter lesions, and cognitive impairment. Yet, the distinct vascular, neuronal, and glial cell types targeted by HTN, as well as the ensuing cellular network disruption driving the neurovascular and cognitive deficits remain undefined. Here we sought to uncover transcriptomic changes in neurons and vascular cells using unbiased, single-cell RNA sequencing on the neocortex of 10-week old C57BL/6 male mice with angiotensin II (AngII) HTN. Vehicle or AngII (600ng/kg/min s.c.) were administered for 3 days, when blood-brain barrier (BBB) permeability start to increase, or 42 days, when neurovascular and cognitive dysfunction are fully developed (n=3/group). We analyzed 39,451 single-cell transcriptomes comprising 26 cell types. Surprisingly, 3 days of AngII induced significantly greater transcriptional changes in venular ECs compared to arteriolar ECs (EdgeR; pval< 0.05, logFC> 2), supporting the notion that venular ECs are uniquely sensitive to the early effects of HTN (Hypertension 76:795, 2020). Gene ontology analysis of differentially expressed genes prominently implicated altered venular immune signaling, BBB dysfunction, and, notably, a secretory phenotype characteristic of senescence (pval <0.05). Moreover, unbiased ligand-receptor interaction analysis (CellChat) demonstrated that senescent venular ECs strongly communicate with oligodendrocyte precursors, and NPY-expressing interneurons, pointing to a previously unrecognized early disruption in the oligo-vascular niche, essential for maintaining white matter integrity, and neuronal network stability. Furthermore, at 42 days of AngII we observed an overrepresentation of aging and neurodegeneration-linked genes in oligos and NPY interneurons, relating to myelin disruption, synaptic dysfunction, and metabolic dysregulation. The data reveal a novel endothelial-oligo-interneuron crosstalk and transcriptomic alterations underlying the impact of HTN on the brain. Future studies will establish how these transcriptomic changes are linked to neurovascular dysfunction, white matter damage and cognitive impairment.

DNA damage independent inhibition of NF-κB transcription by anthracyclines

Anthracyclines are among the most used and effective anticancer drugs. Their activity has been attributed to DNA double-strand breaks resulting from topoisomerase II poisoning and to eviction of histones from select sites in the genome. Here, we show that the extensively used anthracyclines Doxorubicin, Daunorubicin, and Epirubicin decrease the transcription of nuclear factor kappa B (NF-κB)-dependent gene targets, but not interferon-responsive genes in primary mouse (Mus musculus) macrophages. Using an NMR-based structural approach, we demonstrate that anthracyclines disturb the complexes formed between the NF-κB subunit RelA and its DNA-binding sites. The anthracycline variants Aclarubicin, Doxorubicinone, and the newly developed Dimethyl-doxorubicin, which share anticancer properties with the other anthracyclines but do not induce DNA damage, also suppressed inflammation, thus uncoupling DNA damage from the effects on inflammation. These findings have implications for anticancer therapy and for the development of novel anti-inflammatory drugs with limited side effects for life-threatening conditions such as sepsis.

Stroke affects intestinal immune cell trafficking to the central nervous system

Stroke is an acute neurological disease with a strong inflammatory component that can be regulated by the intestinal microbiota and intestinal immune cells. Although stroke has been shown to alter immune cell populations in the gut, the dynamics of cell trafficking have not been elucidated. To study the trafficking of gut-derived immune cells after stroke, we used mice expressing the photoconvertible protein Kikume Green-Red, which turns form green to red when exposed to violet light. Mice underwent laparotomy and the small intestine was exposed to violet laser light. Immune cells were isolated from the small intestine immediately after photoconversion and 2 days later. Percentage of immune cells (CD45⁺KikR⁺) that expressed the red variant of the protein (KikR) was higher immediately after photoconversion than 2 days later, indicating cell egress from the small intestine. To investigate whether intestinal immune cells traffic to the periphery and/or the central nervous system (CNS) after stroke, we analyzed KikR⁺ immune cells (2 days after photoconversion) in peripheral lymphoid organs, meninges and brain, 3 and 14 days after transient occlusion of the middle cerebral artery (tMCAo) or sham-surgery. Although migration was observed in naïve and sham animals, stroke induced a higher mobilization of gut KikR⁺ immune cells, especially at 3 days after stroke, to all the organs analyzed. Notably, we detected a significant migration of CD45^hi immune cells from the gut to the brain and meninges at 3 days after stroke. Comparison of cell trafficking between organs revealed a significant preference of intestinal CD11c⁺ cells to migrate from the small intestine to brain and meninges after stroke. We conclude that stroke increases immune cell trafficking from the small intestine to peripheral lymphoid organs and the CNS where they might contribute to post-stroke inflammation.

Role of microglial and endothelial CD36 in post-ischemic inflammasome activation and interleukin-1β-induced endothelial activation

Cerebral ischemia is associated with an acute inflammatory response that contributes to the resulting injury. The innate immunity receptor CD36, expressed in microglia and endothelium, and the pro-inflammatory cytokine interleukin-1β (IL-1β) are involved in the mechanisms of ischemic injury. Since CD36 has been implicated in activation of the inflammasome, the main source of IL-1β, we investigated whether CD36 mediates brain injury through the inflammasome and IL-1β. We found that active caspase-1, a key inflammasome component, is decreased in microglia of CD36-deficient mice subjected to transient middle cerebral artery occlusion, an effect associated with a reduction in brain IL-1β. Conditional deletion of CD36 either in microglia or endothelium reduced ischemic injury in mice, attesting to the pathogenic involvement of CD36 in both cell types. Application of an ischemic brain extract to primary brain endothelial cell cultures from wild type (WT) mice induced IL-1β-dependent endothelial activation, reflected by increases in the cytokine colony stimulating factor-3, a response markedly attenuated in CD36-deficient endothelia. Similarly, the increase in colony stimulating factor-3 induced by recombinant IL-1β was attenuated in CD36-deficient compared to WT endothelia. We conclude that microglial CD36 is a key determinant of post-ischemic IL-1β production by regulating caspase-1 activity, whereas endothelial CD36 is required for the full expression of the endothelial activation induced by IL-1β. The data identify microglial and endothelial CD36 as critical upstream components of the acute inflammatory response to cerebral ischemia and viable putative therapeutic targets.

Effects of COVID-19 on the Nervous System

Neurological complications have emerged as a significant cause of morbidity and mortality in the ongoing COVID-19 pandemic. Beside respiratory insufficiency, many hospitalized patients exhibit neurological manifestations ranging from headache and loss of smell, to confusion and disabling strokes. COVID-19 is also anticipated to take a toll on the nervous system in the long term. Here, we will provide a critical appraisal of the potential for neurotropism and mechanisms of neuropathogenesis of SARS-CoV-2 as they relate to the acute and chronic neurological consequences of the infection. Finally, we will examine potential avenues for future research and therapeutic development.

Endothelium-Macrophage Crosstalk Mediates Blood-Brain Barrier Dysfunction in Hypertension

Hypertension is a leading cause of stroke and dementia, effects attributed to disrupting delivery of blood flow to the brain. Hypertension also alters the blood-brain barrier (BBB), a critical component of brain health. Although endothelial cells are ultimately responsible for the BBB, the development and maintenance of the barrier properties depend on the interaction with other vascular-associated cells. However, it remains unclear if BBB disruption in hypertension requires cooperative interaction with other cells. Perivascular macrophages (PVM), innate immune cells closely associated with cerebral microvessels, have emerged as major contributors to neurovascular dysfunction. Using 2-photon microscopy in vivo and electron microscopy in a mouse model of Ang II (angiotensin II) hypertension, we found that the vascular segments most susceptible to increased BBB permeability are arterioles and venules >10 µm and not capillaries. Brain macrophage depletion with clodronate attenuates, but does not abolish, the increased BBB permeability in these arterioles where PVM are located. Deletion of AT1R (Ang II type-1 receptors) in PVM using bone marrow chimeras partially attenuated the BBB dysfunction through the free radical-producing enzyme Nox2. In contrast, downregulation of AT1R in cerebral endothelial cells using a viral gene transfer-based approach prevented the BBB disruption completely. The results indicate that while endothelial AT1R, mainly in arterioles and venules, initiate the BBB disruption in hypertension, PVM are required for the full expression of the dysfunction. The findings unveil a previously unappreciated contribution of resident brain macrophages to increased BBB permeability of hypertension and identify PVM as a putative therapeutic target in diseases associated with BBB dysfunction.

Immune responses to stroke: mechanisms, modulation, and therapeutic potential

Stroke is the second leading cause of death worldwide and a leading cause of disability. Most strokes are caused by occlusion of a major cerebral artery, and substantial advances have been made in elucidating how ischemia damages the brain. In particular, increasing evidence points to a double-edged role of the immune system in stroke pathophysiology. In the acute phase, innate immune cells invade brain and meninges and contribute to ischemic damage, but may also be protective. At the same time, danger signals released into the circulation by damaged brain cells lead to activation of systemic immunity, followed by profound immunodepression that promotes life-threatening infections. In the chronic phase, antigen presentation initiates an adaptive immune response targeted to the brain, which may underlie neuropsychiatric sequelae, a considerable cause of poststroke morbidity. Here, we briefly review these pathogenic processes and assess the potential therapeutic value of targeting immunity in human stroke.

Distinct Commensal Bacterial Signature in the Gut Is Associated With Acute and Long-Term Protection From Ischemic Stroke

Background and Purpose- Commensal gut bacteria have a profound impact on stroke pathophysiology. Here, we investigated whether modification of the microbiota influences acute and long-term outcome in mice subjected to stroke. Methods- C57BL/6 male mice received a cocktail of antibiotics or single antibiotic. After 4 weeks, fecal bacterial density of the 16S rRNA gene was quantitated by qPCR, and phylogenetic classification was obtained by 16S rRNA gene sequencing. Infarct volume and hemispheric volume loss were measured 3 days and 5 weeks after middle cerebral artery occlusion, respectively. Neurological deficits were tested by the Tape Test and the open field test. Results- Mice treated with a cocktail of antibiotics displayed a significant reduction of the infarct volume in the acute phase of stroke. The neuroprotective effect was abolished in mice recolonized with a wild-type microbiota. Single antibiotic treatment with either ampicillin or vancomycin, but not neomycin, was sufficient to reduce the infarct volume and improved motorsensory function 3 days after stroke. This neuroprotective effect was correlated with a specific microbial population rather than the total bacterial density. In particular, random forest analysis trained for the severity of the brain damage revealed that Bacteroidetes S24.7 and the enzymatic pathway for aromatic metabolism discriminate between large versus small infarct size. Additionally, the microbiota signature in the ampicillin-treated mice was associated with a reduced gut inflammation, long-term favorable outcome shown by an amelioration of the stereotypic behavior, and a reduction of brain tissue loss in comparison to control and was predictive of a regulation of short-chain fatty acids and tryptophan pathways. Conclusions- The findings highlight the importance of the intestinal microbiota in short- and long-term outcomes of ischemic stroke and raises the possibility that targeted modification of the microbiome associated with specific microbial enzymatic pathways may provide a preventive strategy in patients at high risk for stroke. Visual Overview- An online visual overview is available for this article.

Th17 and Cognitive Impairment: Possible Mechanisms of Action

T helper 17 (Th17) cells represent a distinct population of immune cells, important in the defense of the organism against extracellular infectious agents. Because of their cytokine profile and ability to recruit other immune cell types, they are highly pro-inflammatory and are involved in the induction of several autoimmune disorders. Recent studies show that Th17 cells and their signature cytokine IL-17 have also a role in a wide variety of neurological diseases. This review article will briefly summarize the evidence linking Th17 cells to brain diseases associated with cognitive impairment, including multiple sclerosis (MS), ischemic brain injury and Alzheimer’s disease (AD). We will also investigate the mechanisms by which these cells enter the brain and induce brain damage, including direct effects of IL-17 on brain cells and indirect effects mediated through disruption of the blood-brain barrier (BBB), neurovascular dysfunction and gut-brain axis. Finally, therapeutic prospects targeting Th17 cells and IL-17 will be discussed.

Abstract 050: Essential Role of Cerebral Endothelial AT1 Receptors in the Blood-Brain Barrier Disruption Induced by Angiotensin-II Hypertension

The blood-brain barrier (BBB) controls the molecular exchange between blood and brain and is critical for maintaining brain health. Although hypertension (HTN) is well known to disrupt the BBB, the underlying cellular bases and the susceptible cerebrovascular segments(s) remain unclear. Here, we addressed these open questions in male mice in which HTN was induced by angiotensin II (Ang II; 600ng/kg/min s.c.; n=9-14/group) for 14 days. HTN increased BBB permeability to 3000 MW FITC-dextran, measured spectrophotometrically in brain extracts (30 ± 1 vs 18 ± 0.5 ng/g in controls; p<0.01). In vivo 2-photon microscopy revealed that dextran leaks out more from microvessels >10ϒm (+38%; p<0.05) than cerebral capillaries (+7%; p>0.05). Electron microscopy showed that HTN induces tight junction remodeling (length -25%; complexity: -11%), and increases endothelial transcytosis (capillaries 1.9 and arterioles 3.5 folds; p<0.05), which is consistent with the downregulation of tight junction proteins (occludin: -17%; claudin-5: -30%) and of the transcytosis inhibitor Msfd2a (-43%). Since AT1 receptors (AT1R) on brain perivascular macrophages (PVM) mediate the deleterious cerebrovascular effects of Ang II HTN (JCI 126:4674), we tested if PVM also mediate the BBB dysfunction. However, PVM depletion (icv clodronate) or deletion of AT1R on PVM (bone marrow chimeras), attenuated the BBB dysfunction only partially (-60% and -49%, respectively; p<0.05), whereas AT1R-/- mice harboring AT1R+ PVM were completely protected (19 ± 1 ng/g; p>0.05 vs control), pointing to a key role of endothelial AT1R. Consistent with this prediction, cerebral endothelial AT1R deletion using a cerebral endothelial specific AAV-BR1-iCre in AT1R floxed mice prevented the BBB disruption completely (21 ± 1 ng/g; p>0.05 vs control). We conclude that AT1 signaling in cerebral endothelial cells initiates the BBB opening induced by Ang II HTN by increasing both paracellular and vesicular transport mainly in arterioles and venules, and that AT1 signaling in PVM, presumably from circulating Ang II crossing the BBB, amplifies the dysfunction. Such increase in BBB permeability to circulating agents may contribute to the cerebrovascular and cognitive dysfunction associated with HTN.

AGO CLIP Reveals an Activated Network for Acute Regulation of Brain Glutamate Homeostasis in Ischemic Stroke

Post-transcriptional regulation by microRNAs (miRNAs) is essential for complex molecular responses to physiological insult and disease. Although many disease-associated miRNAs are known, their global targets and culminating network effects on pathophysiology remain poorly understood. We applied Argonaute (AGO) crosslinking immunoprecipitation (CLIP) to systematically elucidate altered miRNA-target interactions in brain following ischemia and reperfusion (I/R) injury. Among 1,190 interactions identified, the most prominent was the cumulative loss of target regulation by miR-29 family members. Integration of translational and time-course RNA profiles revealed a dynamic mode of miR-29 target de-regulation, led by acute translational activation and a later increase in RNA levels, allowing rapid proteomic changes to take effect. These functional regulatory events rely on canonical and non-canonical miR-29 binding and engage glutamate reuptake signals, such as glial glutamate transporter (GLT-1), to control local glutamate levels. These results uncover a miRNA target network that acts acutely to maintain brain homeostasis after ischemic stroke.

Abstract 025: Cellular Mechanisms of Blood-Brain Barrier Disruption in Ang II-Induced Hypertension

The blood-brain barrier (BBB) is critically important for brain health by regulating molecular exchanges between blood and brain. Hypertension (HTN) induces breakdown of the BBB which may contribute to its deleterious effect on the brain, but the cellular bases of the BBB opening remain to be established. We used a model of angiotensin II (AngII) HTN (600ng/kg/min x 2 weeks; n>5/group) to investigate the mechanisms of the BBB opening. BBB permeability was assessed spectrophotometrically in C57BL/6J mice using 3000 MW FITC-dextran as a tracer. HTN increased BBB permeability in Ang II HTN (29.8 ± 0.9 vs 18.1 ± 0.5 ng/g in control, p<0.01), an effect partially ameliorated by the angiotensin type 1 receptor (AT1R) antagonist losartan in the drinking water (22.9 ± 0.6 ng/g) but not by hydralazine + hydrochlorothiazide (27.4 ± 0.7 ng/g), suggesting involvement of AT1R and not elevated blood pressure. Next, we sought to identify the mechanisms of the breakdown of the BBB in HTN. First, we used electron microscopy to examine the ultrastructure of endothelial tight junctions (TJ) and assess vesicular transport, key components of the BBB. HTN reduced the length (-25%) and complexity (-11%) of TJ, and increased the number of endothelial vesicles (2.22 vs 1.42 vesicles/endothelial area, p<0.05). The TJ remodeling was associated with a reduction in occludin (-17%, p<0.01) and claudin-5 (-30%, p<0.05) mRNA in microvascular preparations. Additionally, the expression of Mfsd2a, a lipid transporter that also suppresses vesicular transcytosis, was markedly attenuated (-43%, p<0.01). Taken together, these data suggest a strong effect of Ang II on cerebral endothelial cells to induce BBB opening. Since perivascular macrophages (PVM) mediate cerebral endothelial dysfunction in HTN (J Clin Invest 2016;126:4674), we tested their involvement in the BBB opening. PVM depletion with icv clodronate or deletion of AT1 receptors in PVM partially attenuated the BBB opening in Ang II HTN (p<0.05). Thus, we conclude that AT1R in cerebral endothelial cells and PVM mediate the BBB opening by targeting both paracellular and vesicular transport. Such increase in BBB permeability to circulating agents may contribute to the cerebrovascular and cognitive dysfunction associated with HTN.

Diverse Inflammatory Response After Cerebral Microbleeds Includes Coordinated Microglial Migration and Proliferation

Supplemental Digital Content is available in the text.

Background and Purpose—

Cerebral microbleeds are linked to cognitive decline, but it remains unclear how they impair neuronal function. Infarction is not typically observed near microbleeds, suggesting more subtle mechanisms, such as inflammation, may play a role. Because of their small size and largely asymptomatic nature, real-time detection and study of spontaneous cerebral microbleeds in humans and animal models are difficult.

Methods—

We used in vivo 2-photon microscopy through a chronic cranial window in adult mice to follow the inflammatory response after a cortical microhemorrhage of ≈100 µm diameter, induced by rupturing a targeted cortical arteriole with a laser.

Results—

The inflammatory response included the invasion of blood-borne leukocytes, the migration and proliferation of brain-resident microglia, and the activation of astrocytes. Nearly all inflammatory cells responding to the microhemorrhage were brain-resident microglia, but a small number of CX3CR1⁺ and CCR2⁺ macrophages, ultimately originating from the invasion of blood-borne monocytes, were also found near the lesion. We found a coordinated pattern of microglia migration and proliferation, where microglia within 200 µm of the microhemorrhage migrated toward the lesion over hours to days. In contrast, microglia proliferation was not observed until ≈40 hours after the lesion and occurred primarily in a shell-shaped region where the migration of microglia decreased their local density. These data suggest that local microglia density changes may trigger proliferation. Astrocytes activated in a similar region as microglia but delayed by a few days. By 2 weeks, this inflammatory response had largely resolved.

Conclusions—

Although microhemorrhages are small in size, the brain responds to a single bleed with an inflammatory response that involves brain-resident and blood-derived cells, persists for weeks, and may impact the adjacent brain microenvironment.

Abstract 149: CD36 in Perivascular Macrophages Contributes to Neurovascular and Cognitive Dysfunction and Amyloid Angiopathy in Mice Overexpressing the Alzheimer Aβ Peptide

Amyloid-β (Aβ) exerts deleterious effects on the cerebral microcirculation, which may play a role in Alzheimer’s disease and mixed dementias. In mouse models of amyloid precursor protein (APP) overexpression, the vascular effects of Aβ require the innate immunity receptor CD36 in perivascular macrophages (PVM), immune cells located in the Virchow-Robin space surrounding penetrating cerebral vessels. PVM, in turn, induce vascular oxidative stress and dysfunction via NADPH oxidase (Park et al., Circ Res 2017). However, whether CD36 in PVM is involved in the cognitive impairment induced by Aβ remains unknown. We transplanted CD36 -/- bone marrow (BM) into irradiated male APP-overexpressing mice (Tg2576) (age 12 months) to repopulate their perivascular space with CD36 -/- PVM. Three months later, cerebral blood flow (CBF) was measured by laser-Doppler flowmetry in the somatosensory cortex of anesthetized mice with a cranial window (n=5/group). Cognition was also tested (n=10/group). In Tg2576 mice transplanted with WT BM (Wt→Tg), CBF responses induced by whisker stimulation (WS) or neocortical superfusion of the endothelium-dependent vasodilator acetylcholine (ACh) were attenuated compared to WT mice receiving WT BM (Wt→Wt) (WS, -35%; ACh, -36%; p<0.05). CD36 -/- BM transplantation in Tg2576 mice (CD36 -/- →Tg) prevented the cerebrovascular dysfunction (CBF increases: WS: Wt→Tg, 9±1% vs. CD36 -/- →Tg, 21±2%; ACh: Wt→Tg, 9±1% vs CD36 -/- →Tg, 17±2%; p<0.05), and reduced cerebrovascular amyloid deposition (thioflavin-S-based vascular amyloid load: -58%; p<0.05 from Wt→Tg). Wt→Tg mice spent more time finding the escape hole at the Barnes maze test, indicating impairment in spatial memory (Wt→Wt, 965±166 sec; Wt→Tg, 1807±166 sec; p<0.05), and showed impaired nest building ability (nesting score: Wt→Wt, 4.6±0.2; Wt→Tg, 1.8±0.2; p<0.05). These cognitive deficits were ameliorated in CD36 -/- →Tg mice (Barnes maze: 1217±136 sec; nesting score: 2.7±0.3; p<0.05 from Wt→Tg). We conclude that CD36 in PVM is critically involved in the cerebrovascular and cognitive dysfunctions induced by Aβ, and in cerebral amyloid angiopathy. CD36 in PVM may be a new therapeutic target to counteract the detrimental neurovascular and cognitive effects of amyloid pathology.

Abstract 72: The Innate Immunity Receptor CD36 Contributes to Ischemic Brain Injury by Regulating Post-ischemic Caspase-1 Activity and Interleukin-1b Production in Microglia

CD36, an innate immunity receptor enriched in microglia, and IL-1β, a cytokine produced by the inflammasome, play a key role in ischemic brain injury by mediating the inflammatory component of the damage (J Neurosci, 35:4674, 2015; Nat Rev Immunol, 5:629, 2005). However, a link between post-ischemic CD36 activation and IL-1β has not been established. Microglial CD36 could contribute to ischemic brain injury by increasing the post-ischemic production of IL-1β. To test this hypothesis, male wild type (WT) and CD36 –/– mice were subjected to transient middle cerebral artery occlusion (tMCAO), and infarct volume was assessed in cresyl violet-stained brain sections 72 hrs later. At 18 hrs after tMCAO, the expression of IL-1β mRNA in microglia, isolated by cell sorting, increased equally in WT (3.4±0.9) and CD36 –/– mice (3.6±0.8, fold increase; p>0.05; N=3 pools, 3 mice/pool). However, the levels of mature IL-1β protein, measured by cytometric bead arrays in the ischemic hemisphere, were higher in WT mice (104±30 pg/g) than in CD36 –/– mice (59±14 pg/g; p<0.05 from WT; n=5/group). Since caspase-1 regulates active IL-1β levels by cleaving the mature form from its pro-peptide, we next analyzed microglial caspase-1 activity using a flow cytometry-based assay. The percentage of active caspase-1 was greater in WT (4.0±0.2%) than in CD36 –/– microglia (2±0.2 %; p= 0.001; n=5/group). Therefore, the reduction in infarct volume in CD36 –/– mice could be due to reduced caspase-1 and attendant IL1β signaling. Consistent with this hypothesis, pretreatment with recombinant IL-1β receptor antagonist (2 μg into the cerebral ventricles) decreased infarct volume in WT (47±5 vs. 32±5 mm 3 ; p<0.5; n=8-5), but not in CD36 –/– mice (23±6 vs. 21±5 mm 3 ; p> 0.5; n=5). We conclude that CD36 localized to microglia is a key determinant of post-ischemic IL-1β production by regulating caspase-1 activity. Although the CD36 ligand(s) and the molecular mechanisms linking CD36 to caspase-1 activation remain to be established, this study identifies microglial CD36 signaling as a previously unrecognized pathway for inflammasome activation and IL-1β production after cerebral ischemia, and a viable target to mitigate the damaging effects of post-ischemic inflammation.

Abstract TMP94: Dietary Salt Impairs Cognitive Function Through Suppression of Endothelial Nitric Oxide Synthesis and Hippocampal BDNF Signaling

High sodium diet (HSD) is a risk factor for stroke and dementia independent of hypertension, but it remains unclear if dietary salt impairs cognition. To address this question, we fed HSD (8% NaCl) or normal diet (ND; 0.5% NaCl) to males C57BL/6 mice for 12 weeks (n=10/group) and assessed cognitive function. At a novel object recognition task (non-spatial memory) HSD mice failed to identify a new object from a familiar one (ND: 72.4±2%, HSD: 56.4±2%, p<0.05). At the Barnes maze test, HSD mice took longer to locate the escape hole (ND: 26±5sec, HSD: 64±12sec, p<0.05), suggesting impaired spatial memory. Nesting behavior, akin to activities of daily living in humans, was also impaired (ND: 4.6±0.1, HSD: 4.1±0.1, Deacon scale, p<0.05). Since HSD alters endothelial function (Compr Physiol, 6:215, 2015) and endothelial nitric oxide (NO) plays a role in cognition (J Neurosc, 26:11513, 2006), we assessed NO production in isolated brain microvessels using DAF-FM as a marker. In HSD mice, NO levels were markedly reduced (ND: 0.33±0.04, HSD: 0.19±0.02 RFU/μm2, p<0.05) and the NO increase produced by the endothelium-dependent agonist acetylcholine (ACh) was suppressed (ND, Veh: 0.33±0.04, ACh: 0.54±0.05 RFU, p<0.05; HSD, Veh: 0.19±0.02, ACh: 0.25±0.05 RFU/μm2, p>0.05). Endothelial NO participates in synaptic plasticity by contributing to maintain normal levels of the neurotrophic factor BDNF (Eur J Neurosc, 44:2226, 2016). Hippocampal BDNF levels were reduced in HSD mice (-52±3%, p<0.05). Consistent with attenuated BDNF signaling, we also observed reductions in the phosphorylation of the BDNF receptor TrkB (-31±0.1%), and of related signaling molecules (ERK1/2: -36±1%; CREB: -45±1%; p<0.05). Arc, a protein involved in memory formation and dependent on BDNF, was also reduced (-41±1%; p<0.05). The data suggest that HSD-induced endothelial dysfunction leads to reduced NO, which, in turn, suppresses hippocampal BDNF and attendant signaling pathways, resulting in impaired synaptic plasticity and cognitive deficits. The finding that dietary salt has a profound impact of on cognition highlights the harmful effects of endothelial dysfunction on brain health and supports public health efforts to curb salt intake especially in individuals at high vascular risk.

Regulation of Monocyte Tissue Factor Activity by Allogeneic and Xenogeneic Endothelial Cells

The regulation of tissue factor (TF) activity by the cell associated tissue factor pathway inhibitor (TFPI) during monocyte (Mo) and endothelial cell (EC) interactions is not fully understood. This report describes co-ordinate induction of TF antigen (TF-Ag) and membrane-associated TFPI-Ag on human Mo following coculture with human aortic (HAEC) or porcine aortic EC (PAEC) or after stimulation with TNFα. We show that both allo- and xenogeneic EC induce human Mo-TF antigen in short-term culture. However, the TF activity of TNFα-primed Mo is suppressed when these cells are cocultured with HAEC [by 40.3 ± 6.3% (p <0.02)] or PAEC [by 50.5 ± 10.6% (p <0.001)]. Antibody (Ab) blocking studies confirm that TFPI is the principal anticoagulant associated with this suppression of TF-activity. Our data indicate that anti-TF activity originates, at least in part, from the activated human Mo in the coculture; additionally, specific generation of TFPI by Mo is observed under the xenogeneic culture conditions. As Mo associated TF, induced by allo- or xenogeneic EC interactions, is regulated by cell-associated TFPI, we propose that infiltrating Mo may modulate the thrombotic process at sites of vascular injury in association with both allo- and xenograft rejection.

Aspects of the work were presented at the American Society for Hematology meeting in Orlando, FL, December 1996.

Microbiota differences between commercial breeders impacts the post-stroke immune response

Experimental reproducibility between laboratories is a major translational obstacle worldwide, particularly in studies investigating immunomodulatory therapies in relation to brain disease. In recent years increasing attention has been drawn towards the gut microbiota as a key factor in immune cell polarization. Moreover, manipulation of the gut microbiota has been found effective in a diverse range of brain disorders. Within this study we aimed to test the impact of microbiota differences between mice from different sources on the post-stroke neuroinflammatory response. With this rationale, we have investigated the correlation between microbiota differences and the immune response in mice from three commercial breeders with the same genetic background (C57BL/6). While overall bacterial load was comparable, we detected substantial differences in species diversity and microbiota composition on lower taxonomic levels. Specifically, we investigated segmented filamentous bacteria (SFB)-which have been shown to promote T cell polarization-and found that they were absent in mice from one breeder but abundant in others. Our experiments revealed a breeder specific correlation between SFB presence and the ratio of Treg to Th17 cells. Moreover, recolonization of SFB-negative mice with SFB resulted in a T cell shift which mimicked the ratios found in SFB-positive mice. We then investigated the response to a known experimental immunotherapeutic approach, CD28 superagonist (CD28SA), which has been previously shown to expand the Treg population. CD28SA treatment had differing effects between mice from different breeders and was found to be ineffective at inducing Treg expansion in SFB-free mice. These changes directly corresponded to stroke outcome as mice lacking SFB had significantly larger brain infarcts. This study demonstrates the major impact of microbiota differences on T cell polarization in mice during ischemic stroke conditions, and following immunomodulatory therapies.

Abstract 104: Cerebrovascular and Cognitive Dysfunction in DOCA-Salt Hypertension is Mediated by Perivascular Macrophages

Hypertension (HTN) and high-salt diets are important risk factors for stroke and dementia. DOCA-salt is a recognized model of HTN driven by sodium retention and brain renin-angiotensin system (RAS) activation. However, it is unknown whether essential mechanisms regulating the cerebral circulation are altered in DOCA-salt mice, and, if so, whether these alterations are associated with cognitive impairment. To this end, C57BL/6 mice were implanted with 50mg DOCA pellets SQ and received 0.9% NaCl drinking water for 3 weeks. Cerebral blood flow (CBF) was measured in the somatosensory cortex by laser-Doppler flowmetry through a cranial window. DOCA-salt increased systolic blood pressure (BP; 148±3 vs 112±3 mmHg in controls; p<0.01), and attenuated the CBF increase induced by whisker stimulation (WS; 16.0±1.1 vs 22.4±0.6 %; p<0.01) or by cortical application of acetylcholine (ACh; 13.5±0.9 vs 22.8±1.1 %; p<0.01), without affecting the response to the smooth muscle relaxant adenosine. Cerebrovascular dysfunction was associated with cognitive impairment as assessed by Novel Object Recognition and Barnes Maze tasks (p<0.01). Perivascular macrophages (PVM) express AT1R and Nox2, and, as such, may be a key source of radicals mediating the cerebrovascular effects of brain RAS overactivity. To test this hypothesis, brain PVM were depleted by icv administration of clodronate (CLO) liposomes. BP was not affected by CLO in either control or DOCA mice (p>0.05). PVM depletion improved novel object exploration (p<0.01) and time spent in the target quadrant of Barnes Maze (p<0.05), while also restoring the CBF responses to both WS (DOCA-CLO 19.6±0.9%; p<0.05) and ACh (DOCA-CLO 20.0±1.7%; p<0.05). Next, we tested whether reactive oxygen species (ROS) are involved in the cerebrovascular dysfunction. We observed a 45% upregulation in gp91 mRNA in cerebral vessels from DOCA mice, which was prevented by PVM depletion (p<0.05). Application of the ROS scavenger MnTBAP rescued CBF responses to both WS (20.3±0.9%; p<0.05) and ACh (19.0±0.8%; p<0.05) in DOCA-salt HTN. We conclude that PVM play a previously unrecognized role in the cerebrovascular and cognitive dysfunction of DOCA-salt HTN and may represent a new therapeutic target to alleviate the neurocognitive effects of HTN.

Brain perivascular macrophages: characterization and functional roles in health and disease

Perivascular macrophages (PVM) are a distinct population of resident brain macrophages characterized by a close association with the cerebral vasculature. PVM migrate from the yolk sac into the brain early in development and, like microglia, are likely to be a self-renewing cell population that, in the normal state, is not replenished by circulating monocytes. Increasing evidence implicates PVM in several disease processes, ranging from brain infections and immune activation to regulation of the hypothalamic-adrenal axis and neurovascular-neurocognitive dysfunction in the setting of hypertension, Alzheimer disease pathology, or obesity. These effects involve crosstalk between PVM and cerebral endothelial cells, interaction with circulating immune cells, and/or production of reactive oxygen species. Overall, the available evidence supports the idea that PVM are a key component of the brain-resident immune system with broad implications for the pathogenesis of major brain diseases. A better understanding of the biology and pathobiology of PVM may lead to new insights and therapeutic strategies for a wide variety of brain diseases.

Brain Perivascular Macrophages Initiate the Neurovascular Dysfunction of Alzheimer Aβ Peptides

RATIONALE

Increasing evidence indicates that alterations of the cerebral microcirculation may play a role in Alzheimer disease, the leading cause of late-life dementia. The amyloid-β peptide (Aβ), a key pathogenic factor in Alzheimer disease, induces profound alterations in neurovascular regulation through the innate immunity receptor CD36 (cluster of differentiation 36), which, in turn, activates a Nox2-containing NADPH oxidase, leading to cerebrovascular oxidative stress. Brain perivascular macrophages (PVM) located in the perivascular space, a major site of brain Aβ collection and clearance, are juxtaposed to the wall of intracerebral resistance vessels and are a powerful source of reactive oxygen species.

OBJECTIVE

We tested the hypothesis that PVM are the main source of reactive oxygen species responsible for the cerebrovascular actions of Aβ and that CD36 and Nox2 in PVM are the molecular substrates of the effect.

METHODS AND RESULTS

Selective depletion of PVM using intracerebroventricular injection of clodronate abrogates the reactive oxygen species production and cerebrovascular dysfunction induced by Aβ applied directly to the cerebral cortex, administered intravascularly, or overproduced in the brain of transgenic mice expressing mutated forms of the amyloid precursor protein (Tg2576 mice). In addition, using bone marrow chimeras, we demonstrate that PVM are the cells expressing CD36 and Nox2 responsible for the dysfunction. Thus, deletion of CD36 or Nox2 from PVM abrogates the deleterious vascular effects of Aβ, whereas wild-type PVM reconstitute the vascular dysfunction in CD36-null mice.

CONCLUSIONS

The data identify PVM as a previously unrecognized effector of the damaging neurovascular actions of Aβ and unveil a new mechanism by which brain-resident innate immune cells and their receptors may contribute to the pathobiology of Alzheimer disease.

Abstract 220: High Sodium Diet Increases Gut Th17 Differentiation and Induces Neurovascular and Cognitive Dysfunction Through Il-17

High sodium diet (HSD) is a risk factor for stroke and dementia, independent of its effect on blood pressure (BP), but the mechanisms remain unclear. To investigate the cerebrovascular and cognitive impact of HSD, we fed HSD (4-8% NaCl) or normal diet (ND; 0.5% NaCl) to C57BL/6 mice for 12 weeks and assessed cerebral blood flow (CBF) in the somatosensory cortex by laser-Doppler flowmetry. HSD did not increase BP, but attenuated the CBF increase induced by neocortical application of acetylcholine (ACh)(-30.6±0.5%; p<0.05; n=8), a response mediated by endothelial nitric oxide synthase (eNOS), without affecting the response to the smooth muscle relaxant adenosine. These vascular changes were associated with a deficit in novel object exploration (-24±5%; p<0.05; n=15) and spatial learning (Barnes maze) (-31±20%; p<0.05; n=10), suggesting cognitive impairment. The ACh response suppression was associated with increased inhibitory eNOS phosphorylation in pial microvessels (≈1.6 fold increase vs ND; p<0.05; n=7). Since HSD promotes expansion of intestinal helper T lymphocytes producing the vasotoxic cytokine IL17 (Th17) (Nature, 496:518, 2013), we investigated whether IL-17 contributes to the effects of HSD. HSD increased intestinal Th17 cells (+69±20%; p<0,05; n=8), as well as IL-17 plasma levels (ND 0.9±0.2pg/ml; HSD 6.5±1.4pg/ml; p<0.05; n=5). Furthermore, genetic deletion of IL-17 or systemic administration of IL-17 blocking antibodies counteracted the vascular and cognitive effects of HSD, whereas injection of IL-17 (1μg;q.d.x7days;i.p.) reproduced them in full. In brain endothelial cell cultures, IL-17 (10ng/ml) suppressed ACh-induced NO production (-31±2%; p<0,05; n=3), by increasing Rho-kinase-dependent inhibitory eNOS phosphorylation. We conclude that HSD induces profound alterations in neurovascular regulation and cognitive function. The effect is mediated by expansion of intestinal Th17 cells and increased circulating IL-17, which, in turn, leads to inhibitory eNOS phosphorylation and reduced NO production through Rho kinase. The data suggest a neurovascular mechanism for the increased risk of stroke and dementia with HSD, and identifies IL17 as a putative therapeutic target for the deleterious effects of high salt on the brain.

Spatio-temporal profile, phenotypic diversity, and fate of recruited monocytes into the post-ischemic brain

Background

A key feature of the inflammatory response after cerebral ischemia is the brain infiltration of blood monocytes. There are two main monocyte subsets in the mouse blood: CCR2⁺Ly6C^hi “inflammatory” monocytes involved in acute inflammation, and CX3CR1⁺Ly6C^lo “patrolling” monocytes, which may play a role in repair processes. We hypothesized that CCR2⁺Ly6C^hi inflammatory monocytes are recruited in the early phase after ischemia and transdifferentiate into CX3CR1⁺Ly6C^lo “repair” macrophages in the brain.

Methods

CX3CR1^GFP/+CCR2^RFP/+ bone marrow (BM) chimeric mice underwent transient middle cerebral artery occlusion (MCAo). Mice were sacrificed from 1 to 28 days later to phenotype and map subsets of infiltrating monocytes/macrophages (Mo/MΦ) in the brain over time. Flow cytometry analysis 3 and 14 days after MCAo in CCR2^−/− mice, which exhibit deficient monocyte recruitment after inflammation, and NR4A1^−/− BM chimeric mice, which lack circulating CX3CR1⁺Ly6C^lo monocytes, was also performed.

Results

Brain mapping of CX3CR1^GFP/+ and CCR2^RFP/+ cells 3 days after MCAo showed absence of CX3CR1^GFP/+ Mo/MΦ but accumulation of CCR2^RFP/+ Mo/MΦ throughout the ischemic territory. On the other hand, CX3CR1⁺ cells accumulated 14 days after MCAo at the border of the infarct core where CCR2^RFP/+ accrued. Whereas the amoeboid morphology of CCR2^RFP/+ Mo/MΦ remained unchanged over time, CX3CR1^GFP/+ cells exhibited three distinct phenotypes: amoeboid cells with retracted processes, ramified cells, and perivascular elongated cells. CX3CR1^GFP/+ cells were positive for the Mo/MΦ marker Iba1 and phenotypically distinct from endothelial cells, smooth muscle cells, pericytes, neurons, astrocytes, or oligodendrocytes. Because accumulation of CX3CR1⁺Ly6C^lo Mo/MΦ was absent in the brains of CCR2 deficient mice, which exhibit deficiency in CCR2⁺Ly6C^hi Mo/MΦ recruitment, but not in NR4A1^−/− chimeric mice, which lack of circulating CX3CR1⁺Ly6C^lo monocytes, our data suggest a local transition of CCR2⁺Ly6C^hi Mo/MΦ into CX3CR1⁺Ly6C^lo Mo/MΦ phenotype.

Conclusions

CX3CR1⁺Ly6C^lo arise in the brain parenchyma from CCR2⁺Ly6C^hi Mo/MΦ rather than being de novo recruited from the blood. These findings provide new insights into the trafficking and phenotypic diversity of monocyte subtypes in the post-ischemic brain.

Electronic supplementary material

The online version of this article (doi:10.1186/s12974-016-0750-0) contains supplementary material, which is available to authorized users.

Inflammation and Stroke: An Overview

The immune response to acute cerebral ischemia is a major factor in stroke pathobiology and outcome. While the immune response starts locally in occluded and hypoperfused vessels and the ischemic brain parenchyma, inflammatory mediators generated in situ propagate through the organism as a whole. This "spillover" leads to a systemic inflammatory response first, followed by immunosuppression aimed at dampening the potentially harmful proinflammatory milieu. In this overview we will outline the inflammatory cascade from its starting point in the vasculature of the ischemic brain to the systemic immune response elicited by brain ischemia. Potential immunomodulatory therapeutic approaches, including preconditioning and immune cell therapy will also be discussed.

The ubiquitin ligase HERC3 attenuates NF-κB-dependent transcription independently of its enzymatic activity by delivering the RelA subunit for degradation

Activation of NF-κB-dependent transcription represents an important hallmark of inflammation. While the acute inflammatory response is per se beneficial, it can become deleterious if its spatial and temporal profile is not tightly controlled. Classically, NF-κB activity is limited by cytoplasmic retention of the NF-κB dimer through binding to inhibitory IκB proteins. However, increasing evidence suggests that NF-κB activity can also be efficiently contained by direct ubiquitination of NF-κB subunits. Here, we identify the HECT-domain ubiquitin ligase HERC3 as novel negative regulator of NF-κB activity. We find that HERC3 restricts NF-κB nuclear import and DNA binding without affecting IκBα degradation. Instead HERC3 indirectly binds to the NF-κB RelA subunit after liberation from IκBα inhibitor leading to its ubiquitination and protein destabilization. Remarkably, the regulation of RelA activity by HERC3 is independent of its inherent ubiquitin ligase activity. Rather, we show that HERC3 and RelA are part of a multi-protein complex containing the proteasome as well as the ubiquitin-like protein ubiquilin-1 (UBQLN1). We present evidence that HERC3 and UBQLN1 provide a link between NF-κB RelA and the 26S proteasome, thereby facilitating RelA protein degradation. Our findings establish HERC3 as novel candidate regulating the inflammatory response initiated by NF-κB.

Immune interventions in stroke

Approaches for the effective management of acute stroke are sparse, and many measures for brain protection fail. However, our ability to modulate the immune system and modify the progression of multiple sclerosis is increasing. As a result, immune interventions are currently being explored as therapeutic interventions in acute stroke. In this Review, we compare the immunological features of acute stroke with those of multiple sclerosis, identify unique immunological features of stroke, and consider the evidence for immune interventions. In patients with acute stroke, microglial activation and cell death products trigger an inflammatory cascade that damages vessels and the parenchyma within minutes to hours of the ischaemia or haemorrhage. Immune interventions that restrict brain inflammation, vascular permeability and tissue oedema must be administered rapidly to reduce acute immune-mediated destruction and to avoid subsequent immunosuppression. Preliminary results suggest that the use of drugs that modify disease in multiple sclerosis might accomplish these goals in ischaemic and haemorrhagic stroke. Further elucidation of the immune mechanisms involved in stroke is likely to lead to successful immune interventions.

The Myelin and Lymphocyte Protein MAL Is Required for Binding and Activity of Clostridium perfringens ε-Toxin

Clostridium perfringens ε-toxin (ETX) is a potent pore-forming toxin responsible for a central nervous system (CNS) disease in ruminant animals with characteristics of blood-brain barrier (BBB) dysfunction and white matter injury. ETX has been proposed as a potential causative agent for Multiple Sclerosis (MS), a human disease that begins with BBB breakdown and injury to myelin forming cells of the CNS. The receptor for ETX is unknown. Here we show that both binding of ETX to mammalian cells and cytotoxicity requires the tetraspan proteolipid Myelin and Lymphocyte protein (MAL). While native Chinese Hamster Ovary (CHO) cells are resistant to ETX, exogenous expression of MAL in CHO cells confers both ETX binding and susceptibility to ETX-mediated cell death. Cells expressing rat MAL are ~100 times more sensitive to ETX than cells expressing similar levels of human MAL. Insertion of the FLAG sequence into the second extracellular loop of MAL abolishes ETX binding and cytotoxicity. ETX is known to bind specifically and with high affinity to intestinal epithelium, renal tubules, brain endothelial cells and myelin. We identify specific binding of ETX to these structures and additionally show binding to retinal microvasculature and the squamous epithelial cells of the sclera in wild-type mice. In contrast, there is a complete absence of ETX binding to tissues from MAL knockout (MAL-/-) mice. Furthermore, MAL-/- mice exhibit complete resistance to ETX at doses in excess of 1000 times the symptomatic dose for wild-type mice. We conclude that MAL is required for both ETX binding and cytotoxicity.

The role of microglia and myeloid immune cells in acute cerebral ischemia

The immune response to acute cerebral ischemia is a major contributor to stroke pathobiology. The inflammatory response is characterized by the participation of brain resident cells and peripheral leukocytes. Microglia in the brain and monocytes/neutrophils in the periphery have a prominent role in initiating, sustaining and resolving post-ischemic inflammation. In this review we aim to summarize recent literature concerning the origins, fate and role of microglia, monocytes and neutrophils in models of cerebral ischemia and to discuss their relevance for human stroke.

SUMO2/3 is associated with ubiquitinated protein aggregates in the mouse neocortex after middle cerebral artery occlusion

Protein modifications cooperatively act to protect the proteome from cellular stress. Focal cerebral ischemia increases protein ubiquitination, resulting in formation of ubiquitin-rich aggregates. A concurrent elevation in small ubiquitin-related modifier (SUMO)-conjugated proteins has also been reported, but a potential connection to ubiquitin remains unexplored. Here we show that SUMO2/3 conjugates are present in postischemic ubiquitin-rich aggregates, physically associated with ubiquitin. The coaggregation of SUMO2/3 and ubiquitin is induced rapidly after ischemia, depends on reperfusion, and is also observed in the absence of ischemic damage. The association between SUMO and ubiquitin suggests overlapping functional roles after ischemia/reperfusion.

The key role of transient receptor potential melastatin-2 channels in amyloid-β-induced neurovascular dysfunction

Alzheimer's dementia is a devastating and incurable disease afflicting over 35 million people worldwide. Amyloid-β (Aβ), a key pathogenic factor in this disease, has potent cerebrovascular effects that contribute to brain dysfunction underlying dementia by limiting the delivery of oxygen and glucose to the working brain. However, the downstream pathways responsible for the vascular alterations remain unclear. Here we report that the cerebrovascular dysfunction induced by Aβ is mediated by DNA damage caused by vascular oxidative-nitrosative stress in cerebral endothelial cells, which, in turn, activates the DNA repair enzyme poly(ADP)-ribose polymerase. The resulting increase in ADP ribose opens transient receptor potential melastatin-2 (TRPM2) channels in endothelial cells leading to intracellular Ca(2+) overload and endothelial dysfunction. The findings provide evidence for a previously unrecognized mechanism by which Aβ impairs neurovascular regulation and suggest that TRPM2 channels are a potential therapeutic target to counteract cerebrovascular dysfunction in Alzheimer's dementia and related pathologies.