Changes In Microglial Activation Research Articles

Novelties on Neuroinflammation in Alzheimer's Disease-Focus on Gut and Oral Microbiota Involvement.

Recent studies underscore the role of gut and oral microbiota in influencing neuroinflammation through the microbiota-gut-brain axis, including in Alzheimer's disease (AD). This review aims to provide a comprehensive synthesis of recent findings on the involvement of gut and oral microbiota in the neuroinflammatory processes associated with AD, emphasizing novel insights and therapeutic implications. This review reveals that dysbiosis in AD patients' gut and oral microbiota is linked to heightened peripheral and central inflammatory responses. Specific bacterial taxa, such as Bacteroides and Firmicutes in the gut, as well as Porphyromonas gingivalis in the oral cavity, are notably altered in AD, leading to significant changes in microglial activation and cytokine production. Gut microbiota alterations are associated with increased intestinal permeability, facilitating the translocation of endotoxins like lipopolysaccharides (LPS) into the bloodstream and exacerbating neuroinflammation by activating the brain's toll-like receptor 4 (TLR4) pathways. Furthermore, microbiota-derived metabolites, including short-chain fatty acids (SCFAs) and amyloid peptides, can cross the blood-brain barrier and modulate neuroinflammatory responses. While microbial amyloids may contribute to amyloid-beta aggregation in the brain, certain SCFAs like butyrate exhibit anti-inflammatory properties, suggesting a potential therapeutic avenue to mitigate neuroinflammation. This review not only highlights the critical role of microbiota in AD pathology but also offers a ray of hope by suggesting that modulating gut and oral microbiota could represent a novel therapeutic strategy for reducing neuroinflammation and slowing disease progression.

Neuroinflammation following anti-parkinsonian drugs in early Parkinson’s disease: a longitudinal PET study

The progression of neuroinflammation after anti-parkinsonian therapy on the Parkinson’s disease (PD) brain and in vivo evidence of the therapy purporting neuroprotection remain unclear. To elucidate this, we examined changes in microglial activation, nigrostriatal degeneration, and clinical symptoms longitudinally after dopamine replacement therapy in early, optimally-controlled PD patients with and without zonisamide treatment using positron emission tomography (PET). We enrolled sixteen PD patients (Hoehn and Yahr stage 1–2), and age-matched normal subjects. PD patients were randomly divided into two groups: one (zonisamide+) that did and one (zonisamide−) that did not undergo zonisamide therapy. Annual changes in neuroinflammation ([11C]DPA713 PET), dopamine transporter availability ([11C]CFT PET) and clinical severity were examined. Voxelwise differentiations in the binding of [11C]DPA713 (BPND) and [11C]CFT (SUVR) were compared with normal data and between the zonisamide+ and zonisamide− PD groups. The cerebral [11C]DPA713 BPND increased with time predominantly over the parieto-occipital region in PD patients. Comparison of the zonisamide+ group with the zonisamide− group showed lower levels in the cerebral [11C]DPA713 BPND in the zonisamide+ group. While the striatal [11C]CFT SUVR decreased longitudinally, the [11C]CFT SUVR in the nucleus accumbens showed a higher binding in the zonisamide+ group. A significant annual increase in attention score were found in the zonisamide+ group. The current results indicate neuroinflammation proceeds to the whole brain even after anti-parkinsonian therapy, but zonisamide coadministration might have the potential to ameliorate proinflammatory responses, exerting a neuroprotective effect in more damaged nigrostriatal regions with enhanced attention in PD.

Transforming Growth Factor β1 Ameliorates Microglial Activation in Perioperative Neurocognitive Disorders.

Perioperative neurocognitive disorder (PND) is a common complication of surgery and anesthesia, especially among older patients. Microglial activation plays a crucial role in the occurrence and development of PND and transforming growth factor beta 1 (TGF-β1) can regulate microglial homeostasis. In the present study, abdominal surgery was performed on 12-14 months-old C57BL/6 mice to establish a PND model. The expression of TGF-β1, TGF-β receptor 1, TGF-β receptor 2, and phosphor-smad2/smad3 (psmad2/smad3) was assessed after anesthesia and surgery. Additionally, we examined changes in microglial activation, morphology, and polarization, as well as neuroinflammation and dendritic spine density in the hippocampus. Behavioral tests, including the Morris water maze and open field tests, were used to examine cognitive function, exploratory locomotion, and emotions. We observed decreased TGF-β1 expression after surgery and anesthesia. Intranasally administered exogenous TGF-β1 increased psmad2/smad3 colocalization with microglia positive for ionized calcium-binding adaptor molecule 1. TGF-β1 treatment attenuated microglial activation, reduced microglial phagocytosis, and reduced surgery- and anesthesia-induced changes in microglial morphology. Compared with the surgery group, TGF-β1 treatment decreased M1 microglial polarization and increased M2 microglial polarization. Additionally, surgery- and anesthesia-induced increase in interleukin 1 beta and tumor necrosis factor-alpha levels was ameliorated by TGF-β1 treatment at postoperative day 3. TGF-β1 also ameliorated cognitive function after surgery and anesthesia as well as rescue dendritic spine loss. In conclusion, surgery and anesthesia induced decrease in TGF-β1 levels in older mice, which may contribute to PND development; however, TGF-β1 ameliorated microglial activation and cognitive dysfunction in PND mice.

Maternal infection during pregnancy aggravates the behavioral response to an immune challenge during adolescence in female rats

Prenatal and early postnatal infection have been associated with changes in microglial activity and the development of psychiatric disorders. Here, we investigated the effect of prenatal immune activation and postnatal immune challenge, alone and combined, on behavior and microglial cell density in female Wistar rats. Pregnant rats were injected with poly I:C to induce a maternal immune activation (MIA). Their female offspring were subsequently exposed to a lipopolysaccharide (LPS) immune challenge during adolescence. Anhedonia, social behavior, anxiety, locomotion, and working memory were measured with the sucrose preference, social interaction, open field, elevated-plus maze, and Y-maze test, respectively. Microglia cell density was quantified by counting the number of Iba-1 positive cells in the brain cortex. Female MIA offspring were more susceptible to the LPS immune challenge during adolescence than control offspring as demonstrated by a more pronounced reduction in sucrose preference and body weight on the days following the LPS immune challenge. Furthermore, only the rats exposed to both MIA and LPS showed long-lasting changes in social behavior and locomotion. Conversely, the combination MIA and LPS prevented the anxiety induced by MIA alone during adulthood. MIA, LPS, or their combination did not change microglial cell density in the parietal and frontal cortex of adult rats. The results of our study suggest that the maternal immune activation during pregnancy aggravates the response to an immune challenge during adolescence in female rats.

Antibody-mediated inhibition of tissue-type plasminogen activator binding to the low-density lipoprotein receptor-related protein 1 as a potential beneficial modulator for stroke therapy.

The acute ischemic stroke therapy of choice is the application of Alteplase, a drug containing the enzyme tissue-type plasminogen activator (tPa) which rapidly destabilizes blood clots. A central hallmark of stroke pathology is blood-brain barrier (BBB) breakdown associated with tight junction (TJ) protein degradation, which seems to be significantly more severe under therapeutic conditions. The exact mechanisms how tPa facilitates BBB breakdown are not entirely understood. There is evidence that an interaction with the lipoprotein receptor-related protein 1 (LRP1), allowing tPa transport across the BBB into the central nervous system, is necessary for this therapeutic side effect. Whether tPa-mediated disruption of BBB integrity is initiated directly on microvascular endothelial cells or other brain cell types is still elusive. In this study we could not observe any changes of barrier properties in microvascular endothelial cells after tPa incubation. However, we present evidence that tPa causes changes in microglial activation and BBB breakdown after LRP1-mediated transport across the BBB. Using a monoclonal antibody targeting the tPa binding sites of LRP1 decreased tPa transport across an endothelial barrier. Our results indicate that limiting tPa transport from the vascular system into the brain by coapplication of a LRP1-blocking monoclonal antibody might be a novel approach to minimize tPa-related BBB damage during acute stroke therapy.

Partial Depletion of Microglia Attenuates Long-Term Potentiation Deficits following Repeated Blast Traumatic Brain Injury in Organotypic Hippocampal Slice Cultures.

Blast-induced traumatic brain injury (bTBI) has been a health concern in both military and civilian populations due to recent military and geopolitical conflicts. Military service members are frequently exposed to repeated bTBI throughout their training and deployment. Our group has previously reported compounding functional deficits as a result of increased number of blast exposures. In this study, we further characterized the decrease in long-term potentiation (LTP) by varying the blast injury severity and the inter-blast interval between two blast exposures. LTP deficits were attenuated with increasing inter-blast intervals. We also investigated changes in microglial activation; expression of CD68 was increased and expression of CD206 was decreased after multiple blast exposures. Expression of macrophage inflammatory protein (MIP)-1α, interleukin (IL)-1β, monocyte chemoattractant protein (MCP)-1, interferon gamma-inducible protein (IP)-10, and regulated on activation, normal T cell expressed and secreted (RANTES) increased, while expression of IL-10 decreased in the acute period after both single and repeated bTBI. By partially depleting microglia prior to injury, LTP deficits after injury were significantly reduced. Treatment with the novel drug, MW-189, prevented LTP deficits when administered immediately following a repeated bTBI and even when administered only for an acute period (24 h) between two blast injuries. These findings could inform the development of therapeutic strategies to treat the neurological deficits of repeated bTBI suggesting that microglia play a major role in functional neuronal deficits and may be a viable therapeutic target to lessen the neurophysiological deficits after bTBI.

Different Methods for Evaluating Microglial Activation Using Anti-Ionized Calcium-Binding Adaptor Protein-1 Immunohistochemistry in the Cuprizone Model.

Microglia play an important role in the pathology of various central nervous system disorders, including multiple sclerosis (MS). While different methods exist to evaluate the extent of microglia activation, comparative studies investigating the sensitivity of these methods are missing for most models. In this study, we systematically evaluated which of the three commonly used histological methods (id est, quantification of microglia density, densitometrically evaluated staining intensity, or cellular morphology based on the determination of a ramification index, all measured in anti-ionized calcium-binding adaptor protein-1 (IBA1) immunohistochemical stains) is the most sensitive method to detect subtle changes in the microglia activation status in the context of MS. To this end, we used the toxin-induced cuprizone model which allows the experimental induction of a highly reproducible demyelination in several central nervous system regions, paralleled by early microglia activation. In this study, we showed that after 3 weeks of cuprizone intoxication, all methods reveal a significant microglia activation in the white matter corpus callosum. In contrast, in the affected neocortical grey matter, the evaluation of anti-IBA1 cell morphologies was the most sensitive method to detect subtle changes of microglial activation. The results of this study provide a useful guide for future immunohistochemical evaluations in the cuprizone and other neurodegenerative models.

Culture shock: microglial heterogeneity, activation, and disrupted single-cell microglial networks in vitro

BackgroundMicroglia, the resident immune cells of the brain, play a critical role in numerous diseases, but are a minority cell type and difficult to genetically manipulate in vivo with viral vectors and other approaches. Primary cultures allow a more controlled setting to investigate these cells, but morphological and transcriptional changes upon removal from their normal brain environment raise many caveats from in vitro studies.MethodsTo investigate whether cultured microglia recapitulate in vivo microglial signatures, we used single-cell RNA sequencing (scRNAseq) to compare microglia freshly isolated from the brain to primary microglial cultures. We performed cell population discovery, differential expression analysis, and gene co-expression module analysis to compare signatures between in vitro and in vivo microglia. We constructed causal predictive network models of transcriptional regulators from the scRNAseq data and identified a set of potential key drivers of the cultured phenotype. To validate this network analysis, we knocked down two of these key drivers, C1qc and Prdx1, in primary cultured microglia and quantified changes in microglial activation markers.ResultsWe found that, although often assumed to be a relatively homogenous population of cells in culture, in vitro microglia are a highly heterogeneous population consisting of distinct subpopulations of cells with transcriptional profiles reminiscent of macrophages and monocytes, and are marked by transcriptional programs active in neurodegeneration and other disease states. We found that microglia in vitro presented transcriptional activation of a set of “culture shock genes” not found in freshly isolated microglia, characterized by strong upregulation of disease-associated genes including Apoe, Lyz2, and Spp1, and downregulation of homeostatic microglial markers, including Cx3cr1, P2ry12, and Tmem119. Finally, we found that cultured microglia prominently alter their transcriptional machinery modulated by key drivers from the homeostatic to activated phenotype. Knockdown of one of these drivers, C1qc, resulted in downregulation of microglial activation genes Lpl, Lyz2, and Ccl4.ConclusionsOverall, our data suggest that when removed from their in vivo home environment, microglia suffer a severe case of “culture shock”, drastically modulating their transcriptional regulatory network state from homeostatic to activated through upregulation of modules of culture-specific genes. Consequently, cultured microglia behave as a disparate cell type that does not recapitulate the homeostatic signatures of microglia in vivo. Finally, our predictive network model discovered potential key drivers that may convert activated microglia back to their homeostatic state, allowing for more accurate representation of in vivo states in culture. Knockdown of key driver C1qc partially attenuated microglial activation in vitro, despite C1qc being only weakly upregulated in culture. This suggests that even genes that are not strongly differentially expressed across treatments or preparations may drive downstream transcriptional changes in culture.

Electroacupuncture promotes microglial M2 polarization in ischemic stroke via annexin A1.

Neuroinflammation is the leading cause of neurological sequelae in ischemic stroke. Recently, we reported that the anti-inflammatory mediator annexin A1 (ANXA1) favored microglial M2 polarization in brain injury. The purpose of this study was to investigate electroacupuncture (EA) treatment and its potentially ANXA1-mediated anti-inflammatory effects in the middle cerebral artery occlusion/reperfusion (MCAO/R) mouse model of stroke. Treatment with EA consisted of dense-sparse frequencies (alternating 4 Hz sparse waves for 1.5 s and 16 Hz dense waves for 1.5 s) at CV24 and GV26. Intracerebroventricular (ICV) injection of Boc-2 (5 µM) or short hairpin RNA (sh)ANXA1 (2 µL) 3 days before EA was performed to block the effects of ANXA1. EA pretreatment enhanced expression of ANXA1 and its receptor, formyl peptide receptor (FPR), when compared to MCAO/R alone. EA treatment also rescued MCAO/R-induced deficits in neurological performance, and learning and memory, and reduced infarct volume. Double immunofluorescent labeling showed that EA prevented MCAO/R-induced changes in microglial activation and morphology. EA also reduced the release of pro-inflammatory cytokines, such as interleukin (IL)-1β, inducible nitric oxide synthase (iNOS) and tumor necrosis factor (TNF)-α, while increasing the release of anti-inflammatory cytokines, such as arginase-1 (Arg-1) and brain-derived neurotrophic factor (BDNF). All EA-induced effects were either partially or completely prevented by prior administration of FPR antagonist Boc-2 or shANXA1. The current study provides strong evidence that EA treatment has protective effects against ischemic stroke in the MCAO/R mouse model and that the mechanism likely involves the promotion of M2 polarization in microglia via ANXA1.

Cannabinoid receptor 2 is necessary to induce toll-like receptor-mediated microglial activation.

The tight regulation of microglia activity is key for precise responses to potential threats, while uncontrolled and exacerbated microglial activity is neurotoxic. Microglial toll-like receptors (TLRs) are indispensable for sensing different types of assaults and triggering an innate immune response. Cannabinoid receptor 2 (CB2) signaling is a key pathway to control microglial homeostasis and activation, and its activation is connected to changes in microglial activity. We aimed to investigate how CB2 signaling impacts TLR-mediated microglial activation. Here, we demonstrate that deletion of CB2 causes a dampened transcriptional response to prototypic TLR ligands in microglia. Loss of CB2 results in distinct microglial gene expression profiles, morphology, and activation. We show that the CB2-mediated attenuation of TLR-induced microglial activation is mainly p38 MAPK-dependent. Taken together, we demonstrate that CB2 expression and signaling are necessary to fine-tune TLR-induced activation programs in microglia.

Activation of microglial GLP-1R in the trigeminal nucleus caudalis suppresses central sensitization of chronic migraine after recurrent nitroglycerin stimulation

BackgroundCentral sensitization is considered a critical pathogenic mechanism of chronic migraine (CM). Activation of microglia in the trigeminal nucleus caudalis (TNC) contributes to this progression. Microglial glucagon-like peptide-1 receptor (GLP-1R) activation can alleviate pain; however, whether it is involved in the mechanism of CM has not been determined. Thus, this study aims to investigate the precise role of GLP-1R in the central sensitization of CM.MethodsRepeated nitroglycerin injection-treated mice were used as a CM animal model in the experiment. To identify the distribution and cell localization of GLP-1R in the TNC, we performed immunofluorescence staining. Changes in the expression of GLP-1R, Iba-1, PI3K and p-Akt in the TNC were examined by western blotting. To confirm the effect of GLP-1R and PI3K/Akt in CM, a GLP-1R selective agonist (liraglutide) and antagonist (exendin(9–39)) and a PI3K selective antagonist (LY294002) were administered. Mechanical hypersensitivity was measured through von Frey filaments. To investigate the role of GLP-1R in central sensitization, calcitonin gene-related peptide (CGRP) and c-fos were determined using western blotting and immunofluorescence. To determine the changes in microglial activation, IL-1β and TNF-α were examined by western blotting, and the number and morphology of microglia were measured by immunofluorescence. We also confirmed the effect of GLP-1R on microglial activation in lipopolysaccharide-treated BV-2 microglia.ResultsThe protein expression of GLP-1R was increased in the TNC after nitroglycerin injection. GLP-1R was colocalized with microglia and astrocytes in the TNC and was fully expressed in BV-2 microglia. The GLP-1R agonist liraglutide alleviated basal allodynia and suppressed the upregulation of CGRP, c-fos and PI3K/p-Akt in the TNC. Similarly, the PI3K inhibitor LY294002 prevented nitroglycerin-induced hyperalgesia. In addition, activating GLP-1R reduced Iba-1, IL-1β and TNF-α release and inhibited TNC microglial number and morphological changes (process retraction) following nitroglycerin administration. In vitro, the protein levels of IL-1β and TNF-α in lipopolysaccharide-stimulated BV-2 microglia were also decreased by liraglutide.ConclusionsThese findings suggest that microglial GLP-1R activation in the TNC may suppress the central sensitization of CM by regulating TNC microglial activation via the PI3K/Akt pathway.

Microglia activation in postmortem brains with schizophrenia demonstrates distinct morphological changes between brain regions.

Schizophrenia (SCZ) is a psychiatric disorder that can include symptoms of disorganized speech and thoughts with uncertain underlying mechanisms possibly linked to over‐activated microglia. In this study, we used brain samples from sixteen donors with SCZ and thirteen control donors to assess the differential activation of microglia by quantifying density and 3D reconstruction of microglia stained with ionized calcium‐binding adaptor molecule‐1 (Iba1). Our samples consisted of sections from the frontal, temporal, and cingulate cortical gray matter, subcortical white matter regions (SCWM), and included the anterior corpus callosum. In the first series of studies, we performed a density analysis followed by a spatial analysis to ascertain the microglial density, distribution, and soma size in SCZ brains. Second, we performed a series of morphological quantification techniques to investigate the arborization patterns of the microglia in SCZ. The results demonstrated an increase in microglia density in the cortical gray matter regions in SCZ cases, while in the SCWM, there was a significant increase in microglia density in the frontal and temporal, but not in the other brain regions of interest (ROIs). Spatial analysis using the “nearest neighbor” demonstrated that there was no effect in “clustering”, but there were shorter distances between microglia seen in the SCZ cases. The morphological measures showed that there was a region‐dependent increase in the microglia soma size in the SCZ cases while the Sholl analysis revealed a significant decrease in the microglia arborization in the SCZ cases across all the ROI’s studied. An in‐depth 3D reconstruction of microglia in Brodmann area 9 cortical region found that there was a significant association between age and reduced microglial arborization in the SCZ cases. This region‐dependent age association can help determine whether longitudinal changes in microglial activation across age are brain region‐dependent, which may point to potential therapeutic targets.

Pre-therapeutic microglia activation and sex determine therapy effects of chronic immunomodulation.

Modulation of the innate immune system is emerging as a promising therapeutic strategy against Alzheimer's disease (AD). However, determinants of a beneficial therapeutic effect are ill-understood. Thus, we investigated the potential of 18 kDa translocator protein positron-emission-tomography (TSPO-PET) for assessment of microglial activation in mouse brain before and during chronic immunomodulation.Methods: Serial TSPO-PET was performed during five months of chronic microglia modulation by stimulation of the peroxisome proliferator-activated receptor (PPAR)-γ with pioglitazone in two different mouse models of AD (PS2APP, AppNL-G-F). Using mixed statistical models on longitudinal TSPO-PET data, we tested for effects of therapy and sex on treatment response. We tested correlations of baseline with longitudinal measures of TSPO-PET, and correlations between PET results with spatial learning performance and β-amyloid accumulation of individual mice. Immunohistochemistry was used to determine the molecular source of the TSPO-PET signal.Results: Pioglitazone-treated female PS2APP and AppNL-G-F mice showed attenuation of the longitudinal increases in TSPO-PET signal when compared to vehicle controls, whereas treated male AppNL-G-F mice showed the opposite effect. Baseline TSPO-PET strongly predicted changes in microglial activation in treated mice (R = -0.874, p < 0.0001) but not in vehicle controls (R = -0.356, p = 0.081). Reduced TSPO-PET signal upon pharmacological treatment was associated with better spatial learning despite higher fibrillar β-amyloid accumulation. Immunohistochemistry confirmed activated microglia to be the source of the TSPO-PET signal (R = 0.952, p < 0.0001).Conclusion: TSPO-PET represents a sensitive biomarker for monitoring of immunomodulation and closely reflects activated microglia. Sex and pre-therapeutic assessment of baseline microglial activation predict individual immunomodulation effects and may serve for responder stratification.

Tolerance-inducing effect and properties of innate immune stimulation on chronic stress-induced behavioral abnormalities in mice

Over-activation of the innate immune system constitutes a risk factor for the development of nervous system disorders but may reduce the severity of these disorders by inducing tolerance effect. Here, we studied the tolerance-inducing effect and properties of innate immune stimulation on chronic social defeat stress (CSDS)-induced behavioral abnormalities in mice. A single injection of the innate immune enhancer lipopolysaccharide (LPS) one day before stress exposure prevented CSDS-induced impairment in social interaction and increased immobility time in the tail suspension test and forced swimming test. This effect was observed at varying doses (100, 500, and 1000 μg/kg) and peaked at 100 μg/kg. A single LPS injection (100 μg/kg) either one or five but not ten days before stress exposure prevented CSDS-induced behavioral abnormalities. A second LPS injection ten days after the first LPS injection, or a 2 × or 4 × LPS injections ten days before stress exposure also induced tolerance against stress-induced behavioral abnormalities. Our results furthermore showed that a single LPS injection one day before stress exposure skewed the neuroinflammatory response in the hippocampus and prefrontal cortex of CSDS-exposed mice toward an anti-inflammatory phenotype. Inhibiting the central innate immune response by pretreatment with minocycline or PLX3397 abrogated the tolerance-inducing effect of LPS preconditioning on CSDS-induced behavioral abnormalities and neuroinflammatory responses in the brain. These results provide evidence for a prophylactic effect of innate immune stimulation on stress-induced behavioral abnormalities via changes in microglial activation, which may help develop novel strategies for the prevention of stress-induced psychological disorders.

PET imaging shows no changes in TSPO brain density after IFN-\u03b1 immune challenge in healthy human volunteers

Depression is associated with peripheral inflammation, but its link with brain microglial activity remains unclear. In seven healthy males, we used repeated translocator protein-Positron Emission Tomography (TSPO-PET) dynamic scans with [11C]PBR28 to image brain microglial activation before and 24 h after the immune challenge interferon (IFN)-α. We also investigated the association between changes in peripheral inflammation, changes in microglial activity, and changes in mood. IFN-α administration decreased [11C]PBR28 PET tissue volume of distribution (Vt) across the brain (−20 ± 4%; t6 = 4.1, p = 0.01), but after correction for radioligand free-plasma fraction there were no longer any changes (+23 ± 31%; t = 0.1, p = 0.91). IFN-α increased serum IL-6 (1826 ± 513%, t6 = −7.5, p < 0.001), IL-7 (39 ± 12%, t6 = −3.6, p = 0.01), IL-10 (328 ± 48%, t6 = −12.8, p < 0.001), and IFN-γ (272 ± 64%, t6 = −7.0, p < 0.001) at 4–6 h, and increased serum TNF-α (49 ± 7.6%, t6 = −7.5, p < 0.001), IL-8 (39 ± 12%, t6 = −3.5, p = 0.013), and C-reactive protein (1320 ± 459%, t6 = −7.2, p < 0.001) at 24 h. IFN-α induced temporary mood changes and sickness symptoms after 4–6 h, measured as an increase in POMS-2 total mood score, confusion and fatigue, and a decrease in vigor and friendliness (all p ≤ 0.04). No association was found between changes in peripheral inflammation and changes in PET or mood measures. Our work suggests that brain TSPO-PET signal is highly dependent of inflammation-induced changes in ligand binding to plasma proteins. This limits its usefulness as a sensitive marker of neuroinflammation and consequently, data interpretation. Thus, our results can be interpreted as showing either that [11C]PBR28 is not sensitive enough under these conditions, or that there is simply no microglial activation in this model.

Natalizumab treatment reduces microglial activation in the white matter of the MS brain.

ObjectiveTo evaluate whether natalizumab treatment reduces microglial activation in MS.MethodsWe measured microglial activation using the 18-kDa translocator protein (TSPO)-binding radioligand [11C]PK11195 and PET imaging in 10 patients with MS before and after 1 year treatment with natalizumab. Microglial activation was evaluated as the distribution volume ratio (DVR) of the specifically bound radioligand in brain white and gray matter regions of interest. MRI and disability measurements were performed for comparison. Evaluation was performed identically with 11 age- and sex-matched patients with MS who had no MS therapy.ResultsNatalizumab treatment reduced microglial activation in the normal-appearing white matter (NAWM; baseline DVR vs DVR after 1 year of treatment 1.25 vs 1.22, p = 0.014, Wilcoxon) and at the rim of chronic lesions (baseline DVR vs DVR after 1 year of treatment 1.24 vs 1.18, p = 0.014). In patients with MS with no treatment, there was an increase in microglial activation at the rim of chronic lesions (1.23 vs 1.27, p = 0.045). No alteration was observed in microglial activation in gray matter areas. In the untreated patient group, higher microglial activation at baseline was associated with more rapid disability progression during an average of 4 years of follow-up.ConclusionsTSPO-PET imaging can be used as a tool to assess longitudinal changes in microglial activation in the NAWM and in the perilesional areas in the MS brain in vivo. Natalizumab treatment reduces the diffuse compartmentalized CNS inflammation related to brain resident innate immune cells.

Microglia in the RVLM of SHR have reduced P2Y12R and CX3CR1 expression, shorter processes, and lower cell density.

The RVLM of spontaneously hypertensive rats (SHR) contains over-active C1 neurons, which model the pathology of essential hypertension. Hypertension involves chronic low-grade neuroinflammation. Inflammation in the brain is produced and maintained primarily by microglia. We assessed microglial gene expression (P2Y12R and CX3CR1) and morphology in the RVLM of SHR compared to normotensive Wistar-Kyoto rats (WKY). The gene expression of the metabotropic purinergic receptor P2Y12 and the fractalkine receptor CX3CR1 was downregulated in the RVLM of SHR compared to WKY (by 37.3% and 30.9% respectively). P2Y12R and CX3CR1 are required for normal microglial function, and reduced P2Y12R expression is associated with changes in microglial activity. Histological analysis showed a 22.9% reduction in microglial cell density, along with 18.7% shorter microglial processes, a phenotypic indicator of activation, in the RVLM of SHR compared to WKY. These results indicate a subtle loss of function, or a mild state of inflammation, in the RVLM microglia of SHR.

State-associated changes in longitudinal [18F]-PBR111 TSPO PET imaging of psychosis patients: Evidence for the accelerated ageing hypothesis?

ObjectiveTo determine whether state-associated changes in microglial activity, measured with translocator-protein positron emission tomography (TSPO PET), can be identified in psychosis patients through longitudinal evaluation of their regional tracer uptake over the clinical course from acute psychosis to post-treatment follow-up, and comparison to healthy controls. We also evaluated the relation between tracer uptake, clinical symptoms and peripheral immunological markers. MethodSecond-generation radioligand [18F]-PBR111 TSPO PET-CT was used for longitudinal dynamic imaging in 14 male psychosis patients and 17 male age-matched healthy control subjects. Patients were first scanned during an acute psychotic episode followed by a second scan after treatment. Prior genotyping of subjects for the rs6917 polymorphism distinguished high- and mixed-affinity binders. The main outcome was regional volume of distribution (VT), representing TSPO binding. Plasma concentrations of CRP, cytokines and kynurenines were measured at each timepoint. ResultsWe found a significant three-way interaction between time of scan, age and cohort (cortical grey matter F6.50, p.020). Age-dependent differences in VT existed between cohorts during the psychotic state, but not at follow-up. Patients’ relative change in VT over time correlated with age (cortical grey matter Pearson’s r.574). PANSS positive subscale scores correlated with regional VT during psychosis (cortical grey matter r.767). Plasma CRP and quinolinic acid were independently associated with lower VT. ConclusionsWe identified a differential age-dependent pattern of TSPO binding from psychosis to follow-up in our cohort of male psychosis patients. We recommend future TSPO PET studies in psychosis patients to differentiate between clinical states and consider potential age-related effects.

Early Microglial Activation Following Closed-Head Concussive Injury Is Dominated by Pro-Inflammatory M-1 Type.

Microglial activation is a pathological hallmark of traumatic brain injury (TBI). Following brain injury, activated microglia/macrophages adopt different phenotypes, generally categorized as M-1, or classically activated, and M-2, or alternatively activated. While the M-1, or pro-inflammatory phenotype is detrimental to recovery, M-2, or the anti-inflammatory phenotype, aids in brain repair. Recent findings also suggest the existence of mixed phenotype following brain injury, where activated microglia simultaneously express both M-1 and M-2 markers. The present study sought to determine microglial activation states at early time points (6–72 h) following single or repeated concussive injury in rats. Closed-head concussive injury was modeled in rats using projectile concussive impact injury, with either single or repeated impacts (4 impacts, 1 h apart). Brain samples were examined using immunohistochemical staining, inflammatory gene profiling and real-time polymerase chain reaction analyses to detect concussive injury induced changes in microglial activation and phenotype in cortex and hippocampal regions. Our findings demonstrate robust microglial activation following concussive brain injury. Moreover, we show that multiple concussions induced a unique rod-shaped microglial morphology that was also observed in other diffuse brain injury models. Histological studies revealed a predominance of MHC-II positive M-1 phenotype in the post-concussive microglial milieu following multiple impacts. Although there was simultaneous expression of M-1 and M-2 markers, gene expression results indicate a clear dominance in M-1 pro-inflammatory markers following both single and repeated concussions. While the increase in M-1 markers quickly resolved after a single concussion, they persisted following repeated concussions, indicating a pro-inflammatory environment induced by multiple concussions that may delay recovery and contribute to long-lasting consequences of concussion.

An early and late peak in microglial activation in Alzheimer's disease trajectory.

Amyloid-β deposition, neuroinflammation and tau tangle formation all play a significant role in Alzheimer's disease. We hypothesized that there is microglial activation early on in Alzheimer's disease trajectory, where in the initial phase, microglia may be trying to repair the damage, while later on in the disease these microglia could be ineffective and produce proinflammatory cytokines leading to progressive neuronal damage. In this longitudinal study, we have evaluated the temporal profile of microglial activation and its relationship between fibrillar amyloid load at baseline and follow-up in subjects with mild cognitive impairment, and this was compared with subjects with Alzheimer's disease. Thirty subjects (eight mild cognitive impairment, eight Alzheimer's disease and 14 controls) aged between 54 and 77 years underwent 11C-(R)PK11195, 11C-PIB positron emission tomography and magnetic resonance imaging scans. Patients were followed-up after 14 ± 4 months. Region of interest and Statistical Parametric Mapping analysis were used to determine longitudinal alterations. Single subject analysis was performed to evaluate the individualized pathological changes over time. Correlations between levels of microglial activation and amyloid deposition at a voxel level were assessed using Biological Parametric Mapping. We demonstrated that both baseline and follow-up microglial activation in the mild cognitive impairment cohort compared to controls were increased by 41% and 21%, respectively. There was a longitudinal reduction of 18% in microglial activation in mild cognitive impairment cohort over 14 months, which was associated with a mild elevation in fibrillar amyloid load. Cortical clusters of microglial activation and amyloid deposition spatially overlapped in the subjects with mild cognitive impairment. Baseline microglial activation was increased by 36% in Alzheimer's disease subjects compared with controls. Longitudinally, Alzheimer's disease subjects showed an increase in microglial activation. In conclusion, this is one of the first longitudinal positron emission tomography studies evaluating longitudinal changes in microglial activation in mild cognitive impairment and Alzheimer's disease subjects. We found there is an initial longitudinal reduction in microglial activation in subjects with mild cognitive impairment, while subjects with Alzheimer's disease showed an increase in microglial activation. This could reflect that activated microglia in mild cognitive impairment initially may adopt a protective activation phenotype, which later change to a cidal pro-inflammatory phenotype as disease progresses and amyloid clearance fails. Thus, we speculate that there might be two peaks of microglial activation in the Alzheimer's disease trajectory; an early protective peak and a later pro-inflammatory peak. If so, anti-microglial agents targeting the pro-inflammatory phenotype would be most beneficial in the later stages of the disease.