In-vivo detection of inflammation and neurodegeneration in the chronic phase after permanent embolic stroke in rats

Neopterin, kynurenine metabolites, and indexes related to vitamin B6 are associated with post-stroke cognitive impairment: The Nor-COAST study

BACKGROUND AND AIMS

We have previously shown that systemic inflammation was associated with post-stroke cognitive impairment (PSCI). Because neopterin, kynurenine pathway (KP) metabolites, and B6 vitamers are linked to inflammation, in our study we investigated whether those biomarkers were associated with PSCI.

MATERIAL AND METHODS

The Norwegian Cognitive Impairment After Stroke study is a prospective multicenter cohort study of patients with acute stroke recruited from May 2015 through March 2017. Plasma samples of 422 participants (59 % male) with ischemic stroke from the index hospital stay and 3 months post-stroke were available for analyses of neopterin, KP metabolites, and B6 vitamers using liquid chromatography-tandem mass spectrometry. Mixed linear regression analyses adjusted for age, sex, and creatinine, were used to assess whether there were associations between those biomarkers and cognitive outcomes, measured by the Montreal Cognitive Assessment scale (MoCA) at 3-, 18-, and 36-month follow-up.

RESULTS

Participants had a mean (SD) age of 72 (12) years, with a mean (SD) National Institutes of HealthStroke Scale score of 2.7 (3.6) at Day 1. Higher baseline values of quinolinic acid, PAr (i.e., an inflammatory marker based on vitamin B6 metabolites), and HKr (i.e., a marker of functional vitamin B6 status based on selected KP metabolites) were associated with lower MoCA score at 3, 18, and 36 months post-stroke (p < 0.01). Higher baseline concentrations of neopterin and 3-hydroxykynurenine were associated with lower MoCA scores at 18 and 36 months, and higher concentrations of xanthurenic acid were associated with higher MoCA score at 36 months (p < 0.01). At 3 months post-stroke, higher concentrations of neopterin and lower values of pyridoxal 5́-phosphate were associated with lower MoCA scores at 18- and 36-month follow-up, while lower concentrations of picolinic acid were associated with a lower MoCA score at 36 months (p < 0.01).

CONCLUSION

Biomarkers and metabolites of systemic inflammation, including biomarkers of cellular immune activation, indexes of vitamin B6 homeostasis, and several neuroactive metabolites of the KP pathway, were associated with PSCI.

TRIAL REGISTRATION

ClinicalTrials.gov: NCT02650531.

Myeloid cells in vascular dementia and Alzheimer's disease: Possible therapeutic targets?

Growing evidence supports the suggestion that the peripheral immune system plays a role in different pathologies associated with cognitive impairment, such as vascular dementia (VD) or Alzheimer's disease (AD). The aim of this review is to summarize, within the peripheral immune system, the implications of different types of myeloid cells in AD and VD, with a special focus on post-stroke cognitive impairment and dementia (PSCID). We will review the contributions of the myeloid lineage, from peripheral cells (neutrophils, platelets, monocytes and monocyte-derived macrophages) to central nervous system (CNS)-associated cells (perivascular macrophages and microglia). Finally, we will evaluate different potential strategies for pharmacological modulation of pathological processes mediated by myeloid cell subsets, with an emphasis on neutrophils, their interaction with platelets and the process of immunothrombosis that triggers neutrophil-dependent capillary stall and hypoperfusion, as possible effector mechanisms that may pave the way to novel therapeutic avenues to stop dementia, the epidemic of our time. LINKED ARTICLES: This article is part of a themed issue From Alzheimer's Disease to Vascular Dementia: Different Roads Leading to Cognitive Decline. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v181.6/issuetoc.

Drivers of Chronic Pathology Following Ischemic Stroke: A Descriptive Review

Stroke is the third leading cause of death and long-term disability in the world. Considered largely a disease of aging, its global economic and healthcare burden is expected to rise as more people survive into advanced age. With recent advances in acute stroke management, including the expansion of time windows for treatment with intravenous thrombolysis and mechanical thrombectomy, we are likely to see an increase in survival rates. It is therefore critically important to understand the complete pathophysiology of ischemic stroke, both in the acute and subacute stages and during the chronic phase in the months and years following an ischemic event. One of the most clinically relevant aspects of the chronic sequelae of stroke is its extended negative effect on cognition. Cognitive impairment may be related to the deterioration and dysfunctional reorganization of white matter seen at later timepoints after stroke, as well as ongoing progressive neurodegeneration. The vasculature of the brain also undergoes significant insult and remodeling following stroke, undergoing changes which may further contribute to chronic stroke pathology. While inflammation and the immune response are well established drivers of acute stroke pathology, the chronicity and functional role of innate and adaptive immune responses in the post-ischemic brain and in the peripheral environment remain largely uncharacterized. In this review, we summarize the current literature on post-stroke injury progression, its chronic pathological features, and the putative secondary injury mechanisms underlying the development of cognitive impairment and dementia. We present findings from clinical and experimental studies and discuss the long-term effects of ischemic stroke on both brain anatomy and functional outcome. Identifying mechanisms that occur months to years after injury could lead to treatment strategies in the chronic phase of stroke to help mitigate stroke-associated cognitive decline in patients.

The Implications of Microglial Regulation in Neuroplasticity-Dependent Stroke Recovery

Stroke causes varying degrees of neurological deficits, leading to corresponding dysfunctions. There are different therapeutic principles for each stage of pathological development. Neuroprotection is the main treatment in the acute phase, and functional recovery becomes primary in the subacute and chronic phases. Neuroplasticity is considered the basis of functional restoration and neurological rehabilitation after stroke, including the remodeling of dendrites and dendritic spines, axonal sprouting, myelin regeneration, synapse shaping, and neurogenesis. Spatiotemporal development affects the spontaneous rewiring of neural circuits and brain networks. Microglia are resident immune cells in the brain that contribute to homeostasis under physiological conditions. Microglia are activated immediately after stroke, and phenotypic polarization changes and phagocytic function are crucial for regulating focal and global brain inflammation and neurological recovery. We have previously shown that the development of neuroplasticity is spatiotemporally consistent with microglial activation, suggesting that microglia may have a profound impact on neuroplasticity after stroke and may be a key therapeutic target for post-stroke rehabilitation. In this review, we explore the impact of neuroplasticity on post-stroke restoration as well as the functions and mechanisms of microglial activation, polarization, and phagocytosis. This is followed by a summary of microglia-targeted rehabilitative interventions that influence neuroplasticity and promote stroke recovery.

Plasma derived extracellular vesicle biomarkers of microglia activation in an experimental stroke model

Chronic microglia activation post-stroke is associated with worse neurological and cognitive outcomes. However, measurement of microglia activation in vivo is currently limited. Plasma derived extracellular vesicles (EVs) are cell-specific indicators that may allow for non-invasive measurement of microglia phenotype. The aim of this study was to identify activation-state specific microglia EVs (MEVs) in vitro followed by validation in an experimental stroke model. Following pro-inflammatory activation, MEVs contain the microglia protein TMEM119 alongside increased expression of the Toll-like receptor 4 co-receptor CD14. Immunoprecipitation followed by fluorescent nanoparticle tracking analysis (ONI Nanoimager) was used to confirm the isolation of TMEM119⁺/CD14⁺ EVs from rat plasma. Electron microscopy confirmed that TMEM119 and CD14 localize to the MEV membrane. To model ischemia, plasma was collected from 3-month wildtype Fischer344 rats prior to, 7 and 28 days after endothelin-1 or saline injection into the dorsal right striatum. Fluorescently labelled MEVs were directly measured in the plasma using nanoflow cytometry (Apogee A60 Microplus). We report a significant increase in circulating TMEM119⁺/CD14⁺ EVs 28-days post-stroke in comparison to baseline levels and saline-injected rats, which correlated weakly with stroke volume. TMEM119⁺/MHC-II⁺ EVs were also increased post-stroke in comparison to baseline and saline-injected animals. This study is the first to describe an EV biomarker of activated microglia detected directly in plasma following stroke and represents a future tool for the measurement of microglia activity in vivo.

Microglia-mediated neuroinflammation and neuroplasticity after stroke

Stroke remains a major cause of long-term disability and mortality worldwide. The immune system plays an important role in determining the condition of the brain following stroke. As the resident innate immune cells of the central nervous system, microglia are the primary responders in a defense network covering the entire brain parenchyma, and exert various functions depending on dynamic communications with neurons, astrocytes, and other neighboring cells under both physiological or pathological conditions. Microglia activation and polarization is crucial for brain damage and repair following ischemic stroke, and is considered a double-edged sword for neurological recovery. Microglia can exist in pro-inflammatory states and promote secondary brain damage, but they can also secrete anti-inflammatory cytokines and neurotrophic factors and facilitate recovery following stroke. In this review, we focus on the role and mechanisms of microglia-mediated neuroinflammation and neuroplasticity after ischemia and relevant potential microglia-based interventions for stroke therapy.

Neuroinflammation as a Key Driver of Secondary Neurodegeneration Following Stroke?

Ischaemic stroke involves the rapid onset of focal neurological dysfunction, most commonly due to an arterial blockage in a specific region of the brain. Stroke is a leading cause of death and common cause of disability, with over 17 million people worldwide suffering from a stroke each year. It is now well-documented that neuroinflammation and immune mediators play a key role in acute and long-term neuronal tissue damage and healing, not only in the infarct core but also in distal regions. Importantly, in these distal regions, termed sites of secondary neurodegeneration (SND), spikes in neuroinflammation may be seen sometime after the initial stroke onset, but prior to the presence of the neuronal tissue damage within these regions. However, it is key to acknowledge that, despite the mounting information describing neuroinflammation following ischaemic stroke, the exact mechanisms whereby inflammatory cells and their mediators drive stroke-induced neuroinflammation are still not fully understood. As a result, current anti-inflammatory treatments have failed to show efficacy in clinical trials. In this review we discuss the complexities of post-stroke neuroinflammation, specifically how it affects neuronal tissue and post-stroke outcome acutely, chronically, and in sites of SND. We then discuss current and previously assessed anti-inflammatory therapies, with a particular focus on how failed anti-inflammatories may be repurposed to target SND-associated neuroinflammation.

A pilot [11C]PBR28 PET/MRI study of neuroinflammation and neurodegeneration in chronic stroke patients

Neuroinflammation occurs in response to acute ischemic stroke, and has been speculated to underlie secondary poststroke pathologies, such as depression, that often develop over time poststroke. However, no study has examined whether neuroinflammation is present in chronic stroke patients (e.g., ​≥ ​1 year poststroke). This study tested whether neuroinflammation is present in chronic stroke patients, and is associated with neurodegeneration, using [¹¹C]PBR28 PET and diffusion MRI.

Eight patients with middle cerebral artery (MCA) ischemic stroke incurred 1–3 years prior and 16 healthy controls underwent [¹¹C]PBR28 PET to measure glial activation and diffusion MRI to measure microstructural integrity by mean diffusivity (MD) and fractional anisotropy (FA) using an integrated PET/MRI scanner. Group differences in [¹¹C]PBR28 binding, MD and FA were analyzed voxelwise across the whole brain excluding the infarct zone defined as voxels containing the infarct in any patient.

Compared to controls, patients showed elevations in [¹¹C]PBR28 binding in several brain regions outside the infarct zone, including regions with presumed direct neuroanatomical connections to the infarct (e.g., ipsilesional internal capsule and thalamus) and those without known direct connections (e.g., contralesional thalamus and cingulate gyrus). Patients also showed widespread elevations in MD, with a subset of these regions having reduced FA. In patients, MD was more elevated in regions with co-localized elevations in [¹¹C]PBR28 binding than in contralateral regions without elevations in [¹¹C]PBR28 binding.

This pilot study supports the presence of extensive glial activation along with widespread loss in microstructural integrity in non-infarcted tissue in a cohort of patients with chronic MCA stroke. The loss in microstructural integrity was greater in regions with co-localized glial activation. It is possible that stroke risk factors (e.g., hypertension) contributed to these tissue changes in patients.

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Chronic neuroinflammation speculated to underlie secondary poststroke pathologies such as depression.

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Measured neuroinflammation in chronic stroke patients using [¹¹C]PBR28 PET.

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First study showing extensive neuroinflammation in non-infarcted tissue in chronic stroke patients.

Substrate Elasticity Exerts Functional Effects on Primary Microglia

Microglia-the brain's primary immune cells-exert a tightly regulated cascade of pro- and anti-inflammatory effects upon brain pathology, either promoting regeneration or neurodegeneration. Therefore, harnessing microglia emerges as a potential therapeutic concept in neurological research. Recent studies suggest that-besides being affected by chemokines and cytokines-various cell entities in the brain relevantly respond to the mechanical properties of their microenvironment. For example, we lately reported considerable effects of elasticity on neural stem cells, regarding quiescence and differentiation potential. However, the effects of elasticity on microglia remain to be explored.Under the hypothesis that the elasticity of the microenvironment affects key characteristics and functions of microglia, we established an in vitro model of primary rat microglia grown in a polydimethylsiloxane (PDMS) elastomer-based cell culture system. This way, we simulated the brain's physiological elasticity range and compared it to supraphysiological stiffer PDMS controls. We assessed functional parameters of microglia under "resting" conditions, as well as when polarized towards a pro-inflammatory phenotype (M1) by lipopolysaccharide (LPS), or an anti-inflammatory phenotype (M2) by interleukin-4 (IL-4). Microglia viability was unimpaired on soft substrates, but we found various significant effects with a more than two-fold increase in microglia proliferation on soft substrate elasticities mimicking the brain (relative to PDMS controls). Furthermore, soft substrates promoted the expression of the activation marker vimentin in microglia. Moreover, the M2-marker CD206 was upregulated in parallel to an increase in the secretion of Insulin-Like Growth Factor-1 (IGF-1). The upregulation of CD206 was abolished by blockage of stretch-dependent chloride channels. Our data suggest that the cultivation of microglia on substrates of brain-like elasticity promotes a basic anti-inflammatory activation state via stretch-dependent chloride channels. The results highlight the significance of the omnipresent but mostly overlooked mechanobiological effects exerted on microglia and contribute to a better understanding of the complex spatial and temporal interactions between microglia, neural stem cells, and glia, in health and disease.

The impact of hyperpolarization-activated cyclic nucleotide-gated (HCN) and voltage-gated potassium KCNQ/Kv7 channels on primary microglia function

Background

Microglia are essential to maintain cell homeostasis in the healthy brain and are activated after brain injury. Upon activation, microglia polarize towards different phenotypes. The course of microglia activation is complex and depends on signals in the surrounding milieu. Recently, it has been suggested that microglia respond to ion currents, as a way of regulating their activity and function.

Methods and results

Under the hypothesis that HCN and KCNQ/Kv7 channels impact on microglia, we studied primary rat microglia in the presence or absence of specific pharmacological blockade or RNA silencing. Primary microglia expressed the subunits HCN1-4, Kv7.2, Kv7.3, and Kv7.5. The expression of HCN2, as well as Kv7.2 and Kv7.3, varied among different microglia phenotypes. The pharmacological blockade of HCN channels by ZD7288 resulted in cell depolarization with slowly rising intracellular calcium levels, leading to enhanced survival and reduced proliferation rates of resting microglia. Furthermore, ZD7288 treatment, as well as knockdown of HCN2 RNA by small interfering RNA, resulted in an attenuation of later microglia activation—both towards the anti- and pro-inflammatory phenotype. However, HCN channel inhibition enhanced the phagocytic capacity of IL4-stimulated microglia. Blockade of Kv7/KCNQ channel by XE-991 exclusively inhibited the migratory capacity of resting microglia.

Conclusion

These observations suggest that the HCN current contributes to various microglia functions and impacts on the course of microglia activation, while the Kv7/KCNQ channels affect microglia migration. Characterizing the role of HCN channels in microglial functioning may offer new therapeutic approaches for targeted modulation of neuroinflammation as a hallmark of various neurological disorders.

Extracellular vesicles from mesenchymal stem cells reduce microglial-mediated neuroinflammation after cortical injury in aged Rhesus monkeys

Cortical injury, such as injuries after stroke or age-related ischemic events, triggers a cascade of degeneration accompanied by inflammatory responses that mediate neurological deficits. Therapeutics that modulate such neuroinflammatory responses in the aging brain have the potential to reduce neurological dysfunction and promote recovery. Extracellular vesicles (EVs) from mesenchymal stem cells (MSCs) are lipid-bound, nanoscale vesicles that can modulate inflammation and enhance recovery in rodent stroke models. We recently assessed the efficacy of intravenous infusions of MSC-EVs (24-h and 14-days post-injury) as a treatment in aged rhesus monkeys (Macaca mulatta) with cortical injury that induced impairment of fine motor function of the hand. Aged monkeys treated with EVs after injury recovered motor function more rapidly and more fully than aged monkeys given a vehicle control. Here, we describe EV-mediated inflammatory changes using histological assays to quantify differences in markers of neuroinflammation in brain tissue between EV and vehicle-treated aged monkeys. The activation status of microglia, the innate macrophages of the brain, is critical to cell fate after injury. Our findings demonstrate that EV treatment after injury is associated with greater densities of ramified, homeostatic microglia, along with reduced pro-inflammatory microglial markers. These findings are consistent with a phenotypic switch of inflammatory hypertrophic microglia towards anti-inflammatory, homeostatic functions, which was correlated with enhanced functional recovery. Overall, our data suggest that EVs reduce neuroinflammation and shift microglia towards restorative functions. These findings demonstrate the therapeutic potential of MSC-derived EVs for reducing neuroinflammation after cortical injury in the aged brain.

Persistent Effects on Cardiorespiratory and Nervous Systems Induced by Long-Term Lead Exposure: Results from a Longitudinal Study

Long-term lead (Pb) exposure alters the normal development of the nervous system and physiology. It affects multiple organ systems, causing hypertension, cardiorespiratory dysfunction, being a well-known neurotoxin, inducing changes in neurogenesis, neurodegeneration, and glial cells. However, studies of the developmental effects of lead and its outcomes throughout life are lacking. Determine morphofunctional, behavioral, and cognitive developmental effects of long-term lead exposure at three different ages. Wistar rats were exposed to a Pb-acetate solution from fetal period until adulthood and compared to a non-exposed control group. General behavior and cognitive skills were evaluated by behavioral tests and physiological data and cardiorespiratory reflexes measured. Neurodegeneration, neuroinflammation, and synaptic activity were assessed by immunohistochemistry. Lead exposure caused long-lasting anxiety-like behavior and strong long-term memory impairment without changes in locomotor and exploratory activity. Hypertension was observed at all time points, concomitant with baroreflex impairment and increased chemoreflex sensitivity. Persistent neuroinflammation, transient synaptic overexcitation without neurodegeneration was observed. Long-term Pb exposure, since fetal period, causes long-lasting anxiety-like behavior, concomitant with hypertension, without general motor skills impairment. Synaptic overexcitation, reactive astrogliosis, and microgliosis could underlie behavioral and long-term memory changes, which might have been caused during developmental phases and consolidated during adulthood. Also, alterations observed in the cardiorespiratory reflexes can explain persistent hypertension. This longitudinal study identifies and characterizes lead toxicity nature and magnitude, important to devise and test potential interventions to attenuate the long-term harmful effects of lead on the nervous and cardiovascular systems.

Assessing the Effects of Cytoprotectants on Selective Neuronal Loss, Sensorimotor Deficit and Microglial Activation after Temporary Middle Cerebral Occlusion

Although early reperfusion after stroke salvages the still-viable ischemic tissue, peri-infarct selective neuronal loss (SNL) can cause sensorimotor deficits (SMD). We designed a longitudinal protocol to assess the effects of cytoprotectants on SMD, microglial activation (MA) and SNL, and specifically tested whether the KCa3.1-blocker TRAM-34 would prevent SNL. Spontaneously hypertensive rats underwent 15 min middle-cerebral artery occlusion and were randomized into control or treatment group, which received TRAM-34 intraperitoneally for 4 weeks starting 12 h after reperfusion. SMD was assessed longitudinally using the sticky-label test. MA was quantified at day 14 using in vivo [¹¹C]-PK111195 positron emission tomography (PET), and again across the same regions-of-interest template by immunofluorescence together with SNL at day 28. SMD recovered significantly faster in the treated group (p = 0.004). On PET, MA was present in 5/6 rats in each group, with no significant between-group difference. On immunofluorescence, both SNL and MA were present in 5/6 control rats and 4/6 TRAM-34 rats, with a non-significantly lower degree of MA but a significantly (p = 0.009) lower degree of SNL in the treated group. These findings document the utility of our longitudinal protocol and suggest that TRAM-34 reduces SNL and hastens behavioural recovery without marked MA blocking at the assessed time-points.

Anti-Neuroinflammatory Effect of Alantolactone through the Suppression of the NF-κB and MAPK Signaling Pathways

Neuroinflammation is a major cause of central nervous system (CNS) damage and can result in long-term disability and mortality. Therefore, the development of effective anti-neuroinflammatory agents for neuroprotection is vital. To our surprise, the naturally occurring molecule alantolactone (Ala) was reported to significantly inhibit tumor growth and metastasis as a result of its excellent anti-inflammatory effects. Thus, we proposed that it could also act as an anti-neuroinflammatory agent. Thus, in this study, a coculture system of BV2 cells and PC12 cells were used as an in vitro neuroinflammatory model to investigate the anti-neuroinflammatory mechanism of Ala. The results indicated that Ala downregulated the expression of proinflammatory factors by suppressing the nuclear factor kappa light-chain enhancer of activated B cells (NF-κB) and mitogen-activated protein kinase (MAPK) signaling pathways. Further evaluation using a middle cerebral artery occlusion and reperfusion (MCAO/R) rat model supported the conclusion that Ala could (1) alleviate cerebral ischemia-reperfusion injury; (2) reduce neurological deficits, cerebral infarct volume, and brain edema; and (3) attenuate the apoptosis and necrosis of neurons. In sum, Ala demonstrates anti-neuroinflammatory properties that contribute to the amelioration of CNS damage, and it could be a promising candidate for future applications in CNS injury treatment.

Global brain inflammation in stroke

Stroke, including acute ischaemic stroke and intracerebral haemorrhage, results in neuronal cell death and the release of factors such as damage-associated molecular patterns (DAMPs) that elicit localised inflammation in the injured brain region. Such focal brain inflammation aggravates secondary brain injury by exacerbating blood-brain barrier damage, microvascular failure, brain oedema, oxidative stress, and by directly inducing neuronal cell death. In addition to inflammation localised to the injured brain region, a growing body of evidence suggests that inflammatory responses after a stroke occur and persist throughout the entire brain. Global brain inflammation might continuously shape the evolving pathology after a stroke and affect the patients' long-term neurological outcome. Future efforts towards understanding the mechanisms governing the emergence of so-called global brain inflammation would facilitate modulation of this inflammation as a potential therapeutic strategy for stroke.

PET Evaluation of Microglial Activation in Non-neurodegenerative Brain Diseases

PURPOSE OF THE REVIEW

Microglial cell activation is an important component of neuroinflammation, and it is generally well accepted that chronic microglial activation is indicative of accumulating tissue damage in neurodegenerative conditions, particularly in the earlier stages of disease. Until recently, there has been less focus on the role of neuroinflammation in other forms of neurological and neuropsychiatric conditions. Through this review, we hope to demonstrate the important role TSPO PET imaging has played in illuminating the pivotal role of neuroinflammation and microglial activation underpinning these conditions.

RECENT FINDINGS

TSPO is an 18 kDa protein found on the outer membrane of mitochondria and can act as a marker of microglial activation using nuclear imaging. Through the development of radiopharmaceuticals targeting TSPO, researchers have been able to better characterise the spatial-temporal evolution of chronic neurological conditions, ranging from the focal autoimmune reactions seen in multiple sclerosis to the Wallerian degeneration at remote parts of the brain months following acute cerebral infarction. Development of novel techniques to investigate neuroinflammation within the central nervous system, for the purposes of diagnosis and therapeutics, has flourished over the past few decades. TSPO has proven itself a robust and sensitive biomarker of microglial activation and neuroimaging affords a minimally invasive technique to characterise neuroinflammatory processes in vivo.

Ongoing Secondary Degeneration of the Limbic System in Patients With Ischemic Stroke: A Longitudinal MRI Study

Purpose: Ongoing post-stroke structural degeneration and neuronal loss preceding neuropsychological symptoms such as cognitive decline and depression are poorly understood. Various substructures of the limbic system have been linked to cognitive impairment. In this longitudinal study, we investigated the post-stroke macro- and micro-structural integrity of the limbic system using structural and diffusion tensor magnetic resonance imaging.

Materials and Methods: Nineteen ischemic stroke patients (11 men, 8 women, average age 53.4 ± 12.3, range 18–75 years), with lesions remote from the limbic system, were serially imaged three times over 1 year. Structural and diffusion-tensor images (DTI) were obtained on a 3.0 T MRI system. The cortical thickness, subcortical volume, mean diffusivity (MD), and fractional anisotropy (FA) were measured in eight different regions of the limbic system. The National Institutes of Health Stroke Scale (NIHSS) was used for clinical assessment. A mixed model for multiple factors was used for statistical analysis, and p-values <0.05 was considered significant.

Results: All patients demonstrated improved NIHSS values over time. The ipsilesional subcortical volumes of the thalamus, hippocampus, and amygdala significantly decreased (p < 0.05) and MD significantly increased (p < 0.05). The ipsilesional cortical thickness of the entorhinal and perirhinal cortices was significantly smaller than the contralesional hemisphere at 12 months (p < 0.05). The cortical thickness of the cingulate gyrus at 12 months was significantly decreased at the caudal and isthmus regions as compared to the 1 month assessment (p < 0.05). The cingulum fibers had elevated MD at the ipsilesional caudal-anterior and posterior regions compared to the corresponding contralesional regions.

Conclusion: Despite the decreasing NIHSS scores, we found ongoing unilateral neuronal loss/secondary degeneration in the limbic system, irrespective of the lesion location. These results suggest a possible anatomical basis for post stroke psychiatric complications.

PET Imaging of Crossed Cerebellar Diaschisis after Long-Term Cerebral Ischemia in Rats

Crossed cerebellar diaschisis (CCD) is a decrease of regional blood flow and metabolism in the cerebellar hemisphere contralateral to the injured brain hemisphere as a common consequence of stroke. Despite CCD has been detected in patients with stroke using neuroimaging modalities, the evaluation of this phenomenon in rodent models of cerebral ischemia has been scarcely evaluated so far. Here, we report the in vivo evaluation of CCD after long-term cerebral ischemia in rats using positron emission tomography (PET) imaging with 2-deoxy-2-[¹⁸F]fluoro-D-glucose ([¹⁸F]FDG). Imaging studies were combined with neurological evaluation to assess functional recovery. In the ischemic territory, imaging studies showed a significant decrease in glucose metabolism followed by a progressive recovery later on. Conversely, the cerebellum showed a contralateral hypometabolism from days 7 to 14 after reperfusion. Neurological behavior showed major impaired outcome at day 1 after ischemia followed by a significant recovery of the sensorimotor function from days 7 to 28 after experimental stroke. Taken together, these results suggest that the degree of CCD after cerebral ischemia might be predictive of neurological recovery.

The plasticity of primary microglia and their multifaceted effects on endogenous neural stem cells in vitro and in vivo

Background

Microglia—the resident immune cells of the brain—are activated after brain lesions, e.g., cerebral ischemia, and polarize towards a classic “M1” pro-inflammatory or an alternative “M2” anti-inflammatory phenotype following characteristic temporo-spatial patterns, contributing either to secondary tissue damage or to regenerative responses. They closely interact with endogenous neural stem cells (NSCs) residing in distinct niches of the adult brain. The current study aimed at elucidating the dynamics of microglia polarization and their differential effects on NSC function.

Results

Primary rat microglia in vitro were polarized towards a M1 phenotype by LPS, or to a M2 phenotype by IL4, while simultaneous exposure to LPS plus IL4 resulted in a hybrid phenotype expressing both M1- and M2-characteristic markers. M2 microglia migrated less but exhibit higher phagocytic activity than M1 microglia. Defined mediators switched microglia from one polarization state to the other, a process more effective when transforming M2 microglia towards M1 than vice versa. Polarized microglia had differential effects on the differentiation potential of NSCs in vitro and in vivo, with M1 microglia promoting astrocytogenesis, while M2 microglia supported neurogenesis. Regardless of their polarization, microglia inhibited NSC proliferation, increased NSC migration, and accelerated NSC differentiation.

Conclusion

Overall, this study shed light on the complex conditions governing microglia polarization and the effects of differentially polarized microglia on critical functions of NSCs in vitro and in vivo. Refining the understanding of microglia activation and their modulatory effects on NSCs is likely to facilitate the development of innovative therapeutic concepts supporting the innate regenerative capacity of the brain.

Electronic supplementary material

The online version of this article (10.1186/s12974-018-1261-y) contains supplementary material, which is available to authorized users.

11C-DPA-713 Versus 18F-GE-180: A Preclinical Comparison of Translocator Protein 18 kDa PET Tracers to Visualize Acute and Chronic Neuroinflammation in a Mouse Model of Ischemic Stroke

Neuroinflammation plays a key role in neuronal injury after ischemic stroke. PET imaging of translocator protein 18 kDa (TSPO) permits longitudinal, noninvasive visualization of neuroinflammation in both preclinical and clinical settings. Many TSPO tracers have been developed, however, it is unclear which tracer is the most sensitive and accurate for monitoring the in vivo spatiotemporal dynamics of neuroinflammation across applications. Hence, there is a need for head-to-head comparisons of promising TSPO PET tracers across different disease states. Accordingly, the aim of this study was to directly compare 2 promising second-generation TSPO tracers, ¹¹C-DPA-713 and ¹⁸F-GE-180, for the first time at acute and chronic time points after ischemic stroke. Methods: After distal middle cerebral artery occlusion or sham surgery, mice underwent consecutive PET/CT imaging with ¹¹C-DPA-713 and ¹⁸F-GE-180 at 2, 6, and 28 d after stroke. T2-weighted MR images were acquired to enable delineation of ipsilateral (infarct) and contralateral brain regions of interest (ROIs). PET/CT images were analyzed by calculating percentage injected dose per gram in MR-guided ROIs. SUV ratios were determined using the contralateral thalamus (SUV_Th) as a pseudoreference region. Ex vivo autoradiography and immunohistochemistry were performed to verify in vivo findings. Results: Significantly increased tracer uptake was observed in the ipsilateral compared with contralateral ROI (SUV_Th, 50-60 min summed data) at acute and chronic time points using ¹¹C-DPA-713 and ¹⁸F-GE-180. Ex vivo autoradiography confirmed in vivo findings demonstrating increased TSPO tracer uptake in infarcted versus contralateral brain tissue. Importantly, a significant correlation was identified between microglial/macrophage activation (cluster of differentiation 68 immunostaining) and ¹¹C-DPA-713- PET signal, which was not evident with ¹⁸F-GE-180. No significant correlations were observed between TSPO PET and activated astrocytes (glial fibrillary acidic protein immunostaining). Conclusion: ¹¹C-DPA-713 and ¹⁸F-GE-180 PET enable detection of neuroinflammation at acute and chronic time points after cerebral ischemia in mice. ¹¹C-DPA-713 PET reflects the extent of microglial activation in infarcted distal middle cerebral artery occlusion mouse brain tissue more accurately than ¹⁸F-GE-180 and appears to be slightly more sensitive. These results highlight the potential of ¹¹C-DPA-713 for tracking microglial activation in vivo after stroke and warrant further investigation in both preclinical and clinical settings.

Multi-modal imaging of long-term recovery post-stroke by positron emission tomography and matrix-assisted laser desorption/ionisation mass spectrometry

RATIONALE

Stroke is a leading cause of disability worldwide. Understanding the recovery process post-stroke is essential; however, longer-term recovery studies are lacking. In vivo positron emission tomography (PET) can image biological recovery processes, but is limited by spatial resolution and its targeted nature. Untargeted mass spectrometry imaging offers high spatial resolution, providing an ideal ex vivo tool for brain recovery imaging.

METHODS

Magnetic resonance imaging (MRI) was used to image a rat brain 48 h after ischaemic stroke to locate the infarcted regions of the brain. PET was carried out 3 months post-stroke using the tracers [¹⁸ F]DPA-714 for TSPO and [¹⁸ F]IAM6067 for sigma-1 receptors to image neuroinflammation and neurodegeneration, respectively. The rat brain was flash-frozen immediately after PET scanning, and sectioned for matrix-assisted laser desorption/ionisation mass spectrometry (MALDI-MS) imaging.

RESULTS

Three months post-stroke, PET imaging shows minimal detection of neurodegeneration and neuroinflammation, indicating that the brain has stabilised. However, MALDI-MS images reveal distinct differences in lipid distributions (e.g. phosphatidylcholine and sphingomyelin) between the scar and the healthy brain, suggesting that recovery processes are still in play. It is currently not known if the altered lipids in the scar will change on a longer time scale, or if they are stabilised products of the brain post-stroke.

CONCLUSIONS

The data demonstrates the ability to combine MALD-MS with in vivo PET to image different aspects of stroke recovery.

In vivo imaging biomarkers of neuroinflammation in the development and assessment of stroke therapies - towards clinical translation

Modulation of the inflammatory microenvironment after stroke opens a new avenue for the development of novel neurorestorative therapies in stroke. Understanding the spatio-temporal profile of (neuro-)inflammatory imaging biomarkers in detail thereby represents a crucial factor in the development and application of immunomodulatory therapies. The early integration of quantitative molecular imaging biomarkers in stroke drug development may provide key information about (i) early diagnosis and follow-up, (ii) spatio-temporal drug-target engagement (pharmacodynamic biomarker), (iii) differentiation of responders and non-responders in the patient cohort (inclusion/exclusion criteria; predictive biomarkers), and (iv) the mechanism of action. The use of targeted imaging biomarkers for may thus allow clinicians to decipher the profile of patient-specific inflammatory activity and the development of patient-tailored strategies for immunomodulatory and neuro-restorative therapies in stroke. Here, we highlight the recent developments in preclinical and clinical molecular imaging biomarkers of neuroinflammation (endothelial markers, microglia, MMPs, cell labeling, future developments) in stroke and outline how imaging biomarkers can be used in overcoming current translational roadblocks and attrition in order to advance new immunomodulatory compounds within the clinical pipeline.

Exosomes: a novel therapeutic target for Alzheimer's disease?

Extracellular exosomes are formed inside the cytoplasm of cells in compartments known as multivesicular bodies. Thus, exosomes contain cytoplasmic content. Multivesicular bodies fuse with the plasma membrane and release exosomes into the extracellular environment. Comprehensive research suggests that exosomes act as both inflammatory intermediaries and critical inducers of oxidative stress to drive progression of Alzheimer's disease. An important role of exosomes in Alzheimer's disease includes the formation of neurofibrillary tangles and beta-amyloid production, clearance, and accumulation. In addition, exosomes are involved in neuroinflammation and oxidative stress, which both act as triggers for beta-amyloid pathogenesis and tau hyperphosphorylation. Further, it has been shown that exosomes are strongly associated with beta-amyloid clearance. Thus, effective measures for regulating exosome metabolism may be novel drug targets for Alzheimer's disease.

Low intensity transcranial electric stimulation: Safety, ethical, legal regulatory and application guidelines

Low intensity transcranial electrical stimulation (TES) in humans, encompassing transcranial direct current (tDCS), transcutaneous spinal Direct Current Stimulation (tsDCS), transcranial alternating current (tACS), and transcranial random noise (tRNS) stimulation or their combinations, appears to be safe. No serious adverse events (SAEs) have been reported so far in over 18,000 sessions administered to healthy subjects, neurological and psychiatric patients, as summarized here. Moderate adverse events (AEs), as defined by the necessity to intervene, are rare, and include skin burns with tDCS due to suboptimal electrode-skin contact. Very rarely mania or hypomania was induced in patients with depression (11 documented cases), yet a causal relationship is difficult to prove because of the low incidence rate and limited numbers of subjects in controlled trials. Mild AEs (MAEs) include headache and fatigue following stimulation as well as prickling and burning sensations occurring during tDCS at peak-to-baseline intensities of 1-2mA and during tACS at higher peak-to-peak intensities above 2mA. The prevalence of published AEs is different in studies specifically assessing AEs vs. those not assessing them, being higher in the former. AEs are frequently reported by individuals receiving placebo stimulation. The profile of AEs in terms of frequency, magnitude and type is comparable in healthy and clinical populations, and this is also the case for more vulnerable populations, such as children, elderly persons, or pregnant women. Combined interventions (e.g., co-application of drugs, electrophysiological measurements, neuroimaging) were not associated with further safety issues. Safety is established for low-intensity 'conventional' TES defined as <4mA, up to 60min duration per day. Animal studies and modeling evidence indicate that brain injury could occur at predicted current densities in the brain of 6.3-13A/m2 that are over an order of magnitude above those produced by tDCS in humans. Using AC stimulation fewer AEs were reported compared to DC. In specific paradigms with amplitudes of up to 10mA, frequencies in the kHz range appear to be safe. In this paper we provide structured interviews and recommend their use in future controlled studies, in particular when trying to extend the parameters applied. We also discuss recent regulatory issues, reporting practices and ethical issues. These recommendations achieved consensus in a meeting, which took place in Göttingen, Germany, on September 6-7, 2016 and were refined thereafter by email correspondence.

Thalamic alterations remote to infarct appear as focal iron accumulation and impact clinical outcome

See Duering and Schmidt (doi:10.1093/awx135) for a scientific commentary on this article.Thalamic alterations have been observed in infarcts initially sparing the thalamus but interrupting thalamo-cortical or cortico-thalamic projections. We aimed at extending this knowledge by demonstrating with in vivo imaging sensitive to iron accumulation, one marker of neurodegeneration, that (i) secondary thalamic alterations are focally located in specific thalamic nuclei depending on the initial infarct location; and (ii) such secondary alterations can contribute independently to the long-term outcome. To tackle this issue, 172 patients with an infarct initially sparing the thalamus were prospectively evaluated clinically and with magnetic resonance imaging to quantify iron through R2* map at 24-72 h and at 1-year follow-up. An asymmetry index was used to compare R2* within the thalamus ipsilateral versus contralateral to infarct and we focused on the 95th percentile of R2* as a metric of high iron content. Spatial distribution within the thalamus was analysed on an average R2* map from the entire cohort. The asymmetry index of the 95th percentile within individual nuclei (medio-dorsal, pulvinar, lateral group) were compared according to the initial infarct location in simple and multiple regression analyses and using voxel-based lesion-symptom mapping. Associations between the asymmetry index of the 95th percentile and functional, cognitive and emotional outcome were calculated in multiple regression models. We showed that R2* was not modified at 24-72 h but showed heterogeneous increase at 1 year mainly within the medio-dorsal and pulvinar nuclei. The asymmetry index of the 95th percentile within the medio-dorsal nucleus was significantly associated with infarcts involving anterior areas (frontal P = 0.05, temporal P = 0.02, lenticular P = 0.01) while the asymmetry index of the 95th percentile within the pulvinar nucleus was significantly associated with infarcts involving posterior areas (parietal P = 0.046, temporal P < 0.001) independently of age, gender and infarct volume, which was confirmed by voxel-based lesion-symptom mapping. The asymmetry index of the 95th percentile within the entire thalamus at 1 year was independently associated with poor functional outcome (P = 0.04), poor cognitive outcome (P = 0.03), post-stroke anxiety (P = 0.04) and post-stroke depression (P = 0.02). We have therefore identified that iron accumulates within the thalamus ipsilateral to infarct after a delay with a focal distribution that is strongly linked to the initial infarct location (in relation with the pattern of connectivity between thalamic nuclei and cortical areas or deep nuclei), which independently contributes to functional, cognitive and emotional outcome.

Effect of Cigarette Smoking on a Marker for Neuroinflammation: A [11C]DAA1106 Positron Emission Tomography Study

In the brain, microglia continuously scan the surrounding extracellular space in order to respond to damage or infection by becoming activated and participating in neuroinflammation. When activated, microglia increase the expression of translocator protein (TSPO) 18 kDa, thereby making the TSPO expression a marker for neuroinflammation. We used the radiotracer [¹¹C]DAA1106 (a ligand for TSPO) and positron emission tomography (PET) to determine the effect of smoking on availability of this marker for neuroinflammation. Forty-five participants (30 smokers and 15 non-smokers) completed the study and had usable data. Participants underwent a dynamic PET scanning session with bolus injection of [¹¹C]DAA1106 (with smokers in the satiated state) and blood draws during PET scanning to determine TSPO affinity genotype and plasma nicotine levels. Whole-brain standardized uptake values (SUVs) were determined, and analysis of variance was performed, with group (smoker vs non-smoker) and genotype as factors, thereby controlling for genotype. Smokers and non-smokers differed in whole-brain SUVs (P=0.006) owing to smokers having 16.8% lower values than non-smokers. The groups did not differ in injected radiotracer dose or body weight, which were used to calculate SUV. An inverse association was found between whole-brain SUV and reported cigarettes per day (P<0.05), but no significant relationship was found for plasma nicotine. Thus, smokers have less [¹¹C]DAA1106 binding globally than non-smokers, indicating less microglial activation. Study findings are consistent with much prior research demonstrating that smokers have impaired inflammatory functioning compared with non-smokers and that constituents of tobacco smoke other than nicotine affect inflammatory processes.

Stroke and Risks of Development and Progression of Kidney Diseases and End-Stage Renal Disease: A Nationwide Population-Based Cohort Study

BACKGROUND

There is little information about the association between stroke and kidney diseases. We aimed to investigate the impact of stroke on long-term renal outcomes.

METHODS

In this large population-based retrospective cohort study, we identified 100,353 subjects registered in the National Health Insurance Research Database of Taiwan from January 1, 2000, through December 31, 2012, including 33,451 stroke patients and 66,902 age-, sex- and Charlson's comorbidity index score-matched controls.

RESULTS

The incidence rate of chronic kidney disease (CKD) was higher in the stroke than in the control cohort (17.5 vs. 9.06 per 1000 person-years). After multivariate adjustment, the risk of developing CKD was significantly higher in patients with stroke (adjusted hazard ratio [aHR] 1.43, 95% confidence interval [CI] 1.36-1.50, P<0.001). Subgroup analysis showed that stroke patients <50 years (aHR 1.61, P<0.001) and those with concomitant diabetes mellitus (aHR 2.12, P<0.001), hyperlipidemia (aHR 1.53, P<0.001) or gout (aHR 1.84, P<0.001) were at higher risk of incident CKD. Additionally, the risks of progression to advanced CKD and end-stage renal disease (ESRD) were significantly higher for stroke patients (aHRs, 1.22 and 1.30; P = 0.04 and P = 0.008, respectively), independent of age, sex, comorbidities and long-term medications.

CONCLUSIONS

Stroke is associated with higher risks for incident CKD, decline in renal function and ESRD. Younger stroke patients, as well as those with concomitant diabetes mellitus, hyperlipidemia or gout are at greater risk for kidney diseases.

TIGAR contributes to ischemic tolerance induced by cerebral preconditioning through scavenging of reactive oxygen species and inhibition of apoptosis

Previous study showed that TIGAR (TP53-induced glycolysis and apoptosis regulator) protected ischemic brain injury via enhancing pentose phosphate pathway (PPP) flux and preserving mitochondria function. This study was aimed to study the role of TIGAR in cerebral preconditioning. The ischemic preconditioning (IPC) and isoflurane preconditioning (ISO) models were established in primary cultured cortical neurons and in mice. Both IPC and ISO increased TIGAR expression in cortical neurons. Preconditioning might upregulate TIGAR through SP1 transcription factor. Lentivirus mediated knockdown of TIGAR significantly abolished the ischemic tolerance induced by IPC and ISO. ISO also increased TIGAR in mouse cortex and hippocampus and alleviated subsequent brain ischemia-reperfusion injury, while the ischemic tolerance induced by ISO was eliminated with TIGAR knockdown in mouse brain. ISO increased the production of NADPH and glutathione (GSH), and scavenged reactive oxygen species (ROS), while TIGAR knockdown decreased GSH and NADPH production and increased the level of ROS. Supplementation of ROS scavenger NAC and PPP product NADPH effectively rescue the neuronal injury caused by TIGAR deficiency. Notably, TIGAR knockdown inhibited ISO-induced anti-apoptotic effects in cortical neurons. These results suggest that TIGAR participates in the cerebral preconditioning through reduction of ROS and subsequent cell apoptosis.

Concise Review: Cell Therapies for Stroke and Traumatic Brain Injury: Targeting Microglia

We present a model hypothesis of how several types of cell therapies may target microglia as one of the principal cell types contributing to the inflammatory response after brain injury and discuss how imaging of brain inflammation could potentially be applied to develop biomarkers in patients with stroke and TBI enrolled into stem cell clinical trials.

Imaging of neuroinflammation in Alzheimer's disease, multiple sclerosis and stroke: Recent developments in positron emission tomography

Neuroinflammation is thought to play a pivotal role in many diseases affecting the brain, including Alzheimer's disease, multiple sclerosis and stroke. Neuroinflammation is characterised predominantly by microglial activation, which can be visualised using positron emission tomography (PET). Traditionally, translocator protein 18kDa (TSPO) is the target for imaging of neuroinflammation using PET. In this review, recent preclinical and clinical research using PET in Alzheimer's disease, multiple sclerosis and stroke is summarised. In addition, new molecular targets for imaging of neuroinflammation, such as monoamine oxidases, adenosine receptors and cannabinoid receptor type 2, are discussed. This article is part of a Special Issue entitled: Neuro Inflammation edited by Helga E. de Vries and Markus Schwaninger.

The macrosphere model-an embolic stroke model for studying the pathophysiology of focal cerebral ischemia in a translational approach

The main challenge of stroke research is to translate promising experimental findings from the bench to the bedside. Many suggestions have been made how to achieve this goal, identifying the need for appropriate experimental animal models as one key issue. We here discuss the macrosphere model of focal cerebral ischemia in the rat, which closely resembles the pathophysiology of human stroke both in its acute and chronic phase. Key pathophysiological processes such as brain edema, cortical spreading depolarizations (CSD), neuroinflammation, and stem cell-mediated regeneration are observed in this stroke model, following characteristic temporo-spatial patterns. Non-invasive in vivo imaging allows studying the macrosphere model from the very onset of ischemia up to late remodeling processes in an intraindividual and longitudinal fashion. Such a design of pre-clinical stroke studies provides the basis for a successful translation into the clinic.

Characterization of Behaviour and Remote Degeneration Following Thalamic Stroke in the Rat

Subcortical ischemic strokes are among the leading causes of cognitive impairment. Selective atrophy of remote brain regions connected to the infarct is thought to contribute to deterioration of cognitive functions. The mechanisms underlying this secondary degenerative process are incompletely understood, but are thought to include inflammation. We induce ischemia by unilateral injection of endothelin-I into the rat dorsomedial thalamic nucleus, which has defined reciprocal connections to the frontal cortex. We use a comprehensive test battery to probe for changes in behaviour, including executive functions. After a four-week recovery period, brain sections are stained with markers for degeneration, microglia, astrocytes and myelin. Degenerative processes are localized within the stroke core and along the full thalamocortical projection, which does not translate into measurable behavioural deficits. Significant microglia recruitment, astrogliosis or myelin loss along the axonal projection or within the frontal cortex cannot be detected. These findings indicate that critical effects of stroke-induced axonal degeneration may only be measurable beyond a threshold of stroke severity and/or follow a different time course. Further investigations are needed to clarify the impact of inflammation accompanying axonal degeneration on delayed remote atrophy after stroke.