Metabotropic glutamate receptor subtype 5 is altered in LPS-induced murine neuroinflammation model and in the brains of AD and ALS patients

An antagonist-based two-photon fluorogenic probe for imaging metabotropic glutamate receptor 5 in neuronal cells

The expression of metabotropic glutamate receptor 5 (mGluR5) is subject to developmental regulation and undergoes significant changes in neuropsychiatric disorders and diseases. Visualizing mGluR5 by fluorescence imaging is a highly desired innovative technology for biomedical applications. Nevertheless, there are substantial problems with the chemical probes that are presently accessible. In this study, we have successfully developed a two-photon fluorogenic probe, mGlu-5-TP, based on the structure of mGluR5 antagonist 6-methyl-2-(phenylethynyl)pyridine (MPEP). Due to this antagonist-based probe selectively recognizes mGluR5, high expression of mGluR5 on living SH-SY5Y human neuroblastoma cells has been detected during intracellular inflammation triggered by lipopolysaccharides (LPS). Of particular significance, the probe can be employed along with two-photon fluorescence microscopy to enable real-time visualization of the mGluR5 in Aβ fiber-treated neuronal cells, thereby establishing a connection to the progression of Alzheimer's disease (AD). These results revealed that the probe can be a valuable imaging tool for studying mGluR5-related diseases in the nervous system.

Potential of neuroimaging as a biomarker in amyotrophic lateral sclerosis: from structure to metabolism

Amyotrophic lateral sclerosis (ALS) is a progressive neurodegenerative disease characterized by motor neuron degeneration. The development of ALS involves metabolite alterations leading to tissue lesions in the nervous system. Recent advances in neuroimaging have significantly improved our understanding of the underlying pathophysiology of ALS, with findings supporting the corticoefferent axonal disease progression theory. Current studies on neuroimaging in ALS have demonstrated inconsistencies, which may be due to small sample sizes, insufficient statistical power, overinterpretation of findings, and the inherent heterogeneity of ALS. Deriving meaningful conclusions solely from individual imaging metrics in ALS studies remains challenging, and integrating multimodal imaging techniques shows promise for detecting valuable ALS biomarkers. In addition to giving an overview of the principles and techniques of different neuroimaging modalities, this review describes the potential of neuroimaging biomarkers in the diagnosis and prognostication of ALS. We provide an insight into the underlying pathology, highlighting the need for standardized protocols and multicenter collaborations to advance ALS research.

Bioinformatics analysis revealed the potential crosstalk genes and molecular mechanisms between intracranial aneurysms and periodontitis

OBJECTIVES

The risk of intracranial aneurysms (IAs) development and rupture is significantly higher in patients with periodontitis (PD), suggesting an association between the two. However, the specific mechanisms of association between these two diseases have not been fully investigated.

MATERIALS AND METHODS

In this study, we downloaded IAs and PD data from the Gene Expression Omnibus. Differentially expressed genes (DEGs) were identified, and functional enrichment analysis was performed. The protein-protein interaction (PPI) network and weighted gene co-expression network analysis (WGCNA) was performed to identified key modules and key crosstalk genes. In addition, the immune cell landscape was assessed and the correlation of key crosstalk genes with each immune cell was calculated. Finally, transcription factors (TFs) regulating key crosstalk genes were explored.

RESULTS

127 overlapping DEGs were identified and functional enrichment analysis highlighted the important role of immune reflection in the pathogenesis of IAs and PD. We identified ITGAX and COL4A2 as key crosstalk genes. In addition, the expression of multiple immune cells was significantly elevated in PDs and IAs compared to controls, and both key crosstalk genes were significantly negatively associated with Macrophages M2. Finally, GATA2 was identified as a potential key transcription factor (TF), which regulates two key crosstalk gene.

CONCLUSIONS

The present study identifies key crosstalk genes and TF in PD and IAs, providing new insights for further study of the co-pathogenesis of PD and IAs from an immune and inflammatory perspective. Also, this is the first study to report the above findings.

The Key Role of Astrocytes in Amyotrophic Lateral Sclerosis and Their Commitment to Glutamate Excitotoxicity

In the last two decades, there has been increasing evidence supporting non-neuronal cells as active contributors to neurodegenerative disorders. Among glial cells, astrocytes play a pivotal role in driving amyotrophic lateral sclerosis (ALS) progression, leading the scientific community to focus on the "astrocytic signature" in ALS. Here, we summarized the main pathological mechanisms characterizing astrocyte contribution to MN damage and ALS progression, such as neuroinflammation, mitochondrial dysfunction, oxidative stress, energy metabolism impairment, miRNAs and extracellular vesicles contribution, autophagy dysfunction, protein misfolding, and altered neurotrophic factor release. Since glutamate excitotoxicity is one of the most relevant ALS features, we focused on the specific contribution of ALS astrocytes in this aspect, highlighting the known or potential molecular mechanisms by which astrocytes participate in increasing the extracellular glutamate level in ALS and, conversely, undergo the toxic effect of the excessive glutamate. In this scenario, astrocytes can behave as "producers" and "targets" of the high extracellular glutamate levels, going through changes that can affect themselves and, in turn, the neuronal and non-neuronal surrounding cells, thus actively impacting the ALS course. Moreover, this review aims to point out knowledge gaps that deserve further investigation.

PET and SPECT Imaging of ALS: An Educational Review

Amyotrophic lateral sclerosis (ALS) is a disease leading to progressive motor degeneration and ultimately death. It is a complex disease that can take a significantly long time to be diagnosed, as other similar pathological conditions must be ruled out for a definite diagnosis of ALS. Noninvasive imaging of ALS has shed light on disease pathology and altered biochemistry in the ALS brain. Other than magnetic resonance imaging (MRI), two types of functional imaging, positron emission tomography (PET) and single photon emission computed tomography (SPECT), have provided valuable data about what happens in the brain of ALS patients compared to healthy controls. PET imaging has revealed a specific pattern of brain metabolism through [18F]FDG, while other radiotracers have uncovered neuroinflammation, changes in neuronal density, and protein aggregation. SPECT imaging has shown a general decrease in regional cerebral blood flow (rCBF) in ALS patients. This educational review summarizes the current state of ALS imaging with various PET and SPECT radiopharmaceuticals to better understand the pathophysiology of ALS.

Metabotropic Glutamate Receptor Subtype 5 Positron-Emission-Tomography Radioligands as a Tool for Central Nervous System Drug Development: Between Progress and Setbacks

The metabotropic glutamate receptor subtype 5 (mGluR5) is a class C G-protein-coupled receptor (GPCR) that has been implicated in various neuronal processes and, consequently, in several neuropsychiatric or neurodevelopmental disorders. Over the past few decades, mGluR5 has become a major focus for pharmaceutical companies, as an attractive target for drug development, particularly through the therapeutic potential of its modulators. In particular, allosteric binding sites have been targeted for better specificity and efficacy. In this context, Positron Emission Tomography (PET) appears as a useful tool for making decisions along a drug candidate's development process, saving time and money. Thus, PET provides quantitative information about a potential drug candidate and its target at the molecular level. However, in this area, particular attention has to be given to the interpretation of the PET signal and its conclusions. Indeed, the complex pharmacology of both mGluR5 and radioligands, allosterism, the influence of endogenous glutamate and the choice of pharmacokinetic model are all factors that may influence the PET signal. This review focuses on mGluR5 PET radioligands used at several stages of central nervous system drug development, highlighting advances and setbacks related to the complex pharmacology of these radiotracers.

Inhibition of mGluR5 ameliorates lipid accumulation and inflammation in HepG2 cells

Nonalcoholic fatty liver disease (NAFLD) is a common chronic liver disease characterized by ectopic lipid accumulation in hepatocytes. To date, no specific drug has been approved for its treatment. Metabotropic glutamate receptor 5 (mGluR5) has been showed expressed in hepatocytes and related to some liver diseases such as alcoholic steatosis. However, the function of mGluR5 in NAFLD is not clear. This work aims to investigate the effect and potential mechanism of mGluR5 in NAFLD. We found that mGluR5 expression was increased in the livers of HFD-fed mice and in palmitate-treated HepG2 cells. Suppression of mGluR5 by the specific antagonist MPEP could ameliorate palmitate-induced lipid accumulation, whereas the mGluR5 agonist CHPG promoted lipid deposition in the cells. Knockdown of mGluR5 by small interfering RNA further demonstrated that inhibition of mGluR5 could reduce lipid accumulation. Furthermore, our results revealed that mGluR5 regulated lipid metabolism by increasing the gene expression of lipogenesis. Inflammatory factors and phosphorylation levels of NF-κB-p65 and JNK were also tested in treated hepatocytes. mGluR5 promoted the inflammatory reaction and JNK phosphorylation. Inhibition of JNK signaling by JNK-IN-8 rescued CHPG-induced lipogenesis and inflammation. This study showed mGluR5 regulated lipid accumulation and inflammation in palmitic acid-treated HepG2 cells via the JNK signaling pathway. mGluR5 might be a potential drug target for NAFLD.

Docosahexaenoic acid protects against lipopolysaccharide-induced fetal growth restriction via inducing the ubiquitination and degradation of NF-κB p65 in placental trophoblasts

Lipopolysaccharide (LPS) could induce adverse birth outcomes by evoking inflammation. We investigated the effect and mechanism of docosahexaenoic acid (DHA) on LPS-induced placental inflammation and fetal growth restriction (FGR). In vivo, pregnant CD-1 mice were divided into four groups: Ctrl, DHA, LPS and DHA+LPS group. We found that DHA pretreatment reduced the incidence of FGR induced by LPS and activated the expression of peroxisome proliferators-activated receptor gamma (PPARγ) in placental tissue. Moreover, the LPS-induced increase of mRNA levels of Tnf-α, Il-6, Il-1β, Mip-2 and Kc in placental tissue was significantly attenuated by DHA pretreatment. A similar effect of DHA was observed in serum of pregnant mice and amniotic fluid. In contrast, the levels of the IL-10 were significantly increased after DHA pretreatment. In vitro, we clarified that DHA antagonized the activation of the NF-κB signaling pathway induced by LPS, which was dependent on PPARγ. Subsequently, CHX (translation inhibitor) was used to indicated that PPARγ significantly increased the degradation rate of p65, an effect that was inhibited by MG132 (proteasome inhibitor) treatment. Finally, it was confirmed that the activation of PPARγ could significantly promote the ubiquitination and degradation of p65. Our results suggested that DHA alleviated LPS-induced inflammatory responses and FGR by activating PPARγ expression, leading to p65 ubiquitination and degradation.

Promoting Endogenous Neurogenesis as a Treatment for Alzheimer's Disease

Alzheimer's disease (AD) is the most universal neurodegenerative disorder characterized by memory loss and cognitive impairment. AD is biologically defined by production and aggregation of misfolded protein including extracellular amyloid β (Aβ) peptide and intracellular microtubule-associated protein tau tangles in neurons, leading to irreversible neuronal loss. At present, regulation of endogenous neurogenesis to supplement lost neurons has been proposed as a promising strategy for treatment of AD. However, the exact underlying mechanisms of impaired neurogenesis in AD have not been fully explained and effective treatments targeting neurogenesis for AD are limited. In this review, we mainly focus on the latest research of impaired neurogenesis in AD. Then we discuss the factors affecting stages of neurogenesis and the interplay between neural stem cells (NSCs) and neurogenic niche under AD pathological conditions. This review aims to explore potential therapeutic strategies that promote endogenous neurogenesis for AD treatments.

Different Aberrant Changes of mGluR5 and Its Downstream Signaling Pathways in the Scrapie-Infected Cell Line and the Brains of Scrapie-Infected Experimental Rodents

Metabotropic glutamate receptor subtype 5 (mGluR5) is a G-protein-coupled receptor found widely in the central nervous system. It has been involved in the development and progression of some neurodegenerative diseases, but its role in prion diseases is rarely described. In this study, the changes of mGluR5 and its downstream signaling pathways in prion-infected cell line SMB-S15 and the brains of scrapie-infected experimental rodents were evaluated by various methodologies. We found the levels of mGluR5 were significantly increased in a prion-infected cell line SMB-S15 and the cultured cells transiently express an abnormal form PrP (Cyto-PrP). Using immunoprecipitation tests and immunofluorescent assays (IFA), molecular interaction and morphological colocalization between PrP and mGluR5 were observed in the cultured cells. We identified that the (GPCRs)-IP3-IP3R-Ca2+ pathway was activated and the levels of the downstream kinases p38, ERK, and JNK were increased in SMB-S15 cells. After treated with mGluR5 antagonist (MTEP) or the removal of prion replication by resveratrol in SMB-S15 cells, the upregulations of mGluR5 and the downstream kinases were restored in a certain degree. Moreover, increased mGluR5 contributes to the cell damage in prion-infected cells. Contrarily, the levels of mGluR5 in the brains of several scrapie-infected rodent models were decreased at terminal stage. IFA of the brain sections of scrapie-infected rodents demonstrated that the signals of mGluR5 were preferentially colocalized with the NeuN-positive cells, accompanying with severe neuron losses in Nissl staining, which might be a reason for the decrease of mGluR5. Our data indicate the different aberrant alterations of mGluR5 and the downstream signaling pathways during prion infection in vivo and in vitro.

Restoration and targeting of aberrant neurotransmitters in Parkinson's disease therapeutics

Neurotransmitters are considered as a fundamental regulator in the process of neuronal growth, differentiation and survival. Parkinson's Disease (PD) occurs due to extensive damage of dopamine-producing neurons; this causes dopamine deficits in the midbrain, followed by the alternation of various other neurotransmitters (glutamate, GABA, serotonin, etc.). It has been observed that fluctuation of neurotransmission in the basal ganglia exhibits a great impact on the pathophysiology of PD. Dopamine replacement therapy, such as the use of L-DOPA, can increase the dopamine level, but it majorly ameliorates the motor symptoms and is also associated with long-term complications (for e.g., LID). While the non-dopaminergic system can efficiently target non-motor symptoms, for instance, the noradrenergic system regulates the synthesis of BDNF via the MAPK pathway, which is important in learning and memory. Herein, we briefly discuss the role of different neurotransmitters, implementation of neurotransmitter receptors in PD. We also illustrate the recent advances of neurotransmitter-based drugs, which are currently under in vivo and clinical studies. Reinstating normal neurotransmitter levels has been believed to be advantageous in the treatment of PD. Thus, there is an increasing demand for drugs that can specifically target the neurotransmission system and reinstate the normal levels of neurotransmitters, which might prevent or delay neurodegeneration in PD.

Positron Emission Tomography in Animal Models of Tauopathies

The microtubule-associated protein tau (MAPT) plays an important role in Alzheimer's disease and primary tauopathy diseases. The abnormal accumulation of tau contributes to the development of neurotoxicity, inflammation, neurodegeneration, and cognitive deficits in tauopathy diseases. Tau synergically interacts with amyloid-beta in Alzheimer's disease leading to detrimental consequence. Thus, tau has been an important target for therapeutics development for Alzheimer's disease and primary tauopathy diseases. Tauopathy animal models recapitulating the tauopathy such as transgenic, knock-in mouse and rat models have been developed and greatly facilitated the understanding of disease mechanisms. The advance in PET and imaging tracers have enabled non-invasive detection of the accumulation and spread of tau, the associated microglia activation, metabolic, and neurotransmitter receptor alterations in disease animal models. In vivo microPET studies on mouse or rat models of tauopathy have provided significant insights into the phenotypes and time course of pathophysiology of these models and allowed the monitoring of treatment targeting at tau. In this study, we discuss the utilities of PET and recently developed tracers for evaluating the pathophysiology in tauopathy animal models. We point out the outstanding challenges and propose future outlook in visualizing tau-related pathophysiological changes in brain of tauopathy disease animal models.

Receptor mapping using methoxy phenyl piperazine derivative: Preclinical PET imaging

This study aimed at assessing 2-methoxyphenyl piperazine derivative for its binding specificity and suitability in mapping metabotropic glutamate receptor subtype 1, which is implicated in several neuropsychiatric disorders. N-(2-(4-(2-Methoxyphenyl)piperazin-1-yl)ethyl)-N-methylpyridin-2-amine was synthesised and evaluated for brain imaging subsequent to radiolabelling with [¹¹C] radioisotope via methylation process in 98.9% purity and 52 ± 6% yield (decay corrected). The specific activity was in the range of 72-93 GBq/µmol. The haemolysis of blood was 2-5% for initial 4 hr and remained < 10% after 24 h of incubation indicating low toxicity. In vitro autoradiograms after coincubation with unlabelled ligand confirmed the high uptake of the PET radioligand in the mGluR1 receptor rich regions. The PET as well as biodistribution studies also showed high activity in the brain with a direct correlation between receptor abundance distribution pattern and tracer activity. The biodistribution analyses revealed initial high brain uptake (4.18 ± 0.48). The highest uptake was found in cerebellum (SUV 4.7 ± 0.2), followed by thalamus (SUV 3.5 ± 0.1), and striatum (SUV 3 ± 0.1). In contrast, pons had negligible tracer activity. The high uptake observed in all the regions with known mGluR1 activity indicates suitability of the ligand for mGluR1 imaging.

A beneficial role for elevated extracellular glutamate in Amyotrophic Lateral Sclerosis and cerebral ischemia

This hypothesis proposes that increased extracellular glutamate in Amyotrophic Lateral Sclerosis (ALS) and cerebral ischemia, currently viewed as a trigger for excitotoxicity, is actually beneficial as it stimulates the utilization of glutamate as metabolic fuel. Renewed appreciation of glutamate oxidation by ischemic neurons has raised questions regarding the role of extracellular glutamate in ischemia. Is it detrimental, as suggested by excitotoxicity in early in vitro studies, or beneficial, as suggested by its oxidation in later in vivo studies? The answer may depend on the activity of N-methyl-D-aspartate (NMDA) glutamate receptors. Early in vitro procedures co-activated NMDA receptors (NMDARs) containing 2A (GluN2A) and 2B (GluN2B) subunits, an event now believed to trigger excitotoxicity; however, during in vivo ischemia D-serine and zinc molecules are released and these ensure only GluN2B receptors are stimulated. This not only prevents excitotoxicity but also initiates signaling cascades that allow ischemic neurons to import and oxidize glutamate.

Noncanonical Metabotropic Glutamate Receptor 5 Signaling in Alzheimer's Disease

Metabotropic glutamate receptor 5 (mGluR5) is ubiquitously expressed in brain regions responsible for memory and learning. It plays a key role in modulating rapid changes in synaptic transmission and plasticity. mGluR5 supports long-term changes in synaptic strength by regulating the transcription and translation of essential synaptic proteins. β-Amyloid 42 (Aβ42) oligomers interact with a mGluR5/cellular prion protein (PrP^C) complex to disrupt physiological mGluR5 signal transduction. Aberrant mGluR5 signaling and associated synaptic failure are considered an emerging pathophysiological mechanism of Alzheimer's disease (AD). Therefore, mGluR5 represents an attractive therapeutic target for AD, and recent studies continue to validate the efficacy of various mGluR5 allosteric modulators in improving memory deficits and mitigating disease pathology. However, sex-specific differences in the pharmacology of mGluR5 and activation of noncanonical signaling downstream of the receptor suggest that its utility as a therapeutic target in female AD patients needs to be reconsidered. Expected final online publication date for the Annual Review of Pharmacology and Toxicology, Volume 62 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Targeting the dysfunction of glutamate receptors for the development of novel antidepressants

Increasing evidence indicates that dysfunction of glutamate receptors is involved in the pathophysiology of major depressive disorder (MDD). Although accumulating efforts have been made to elucidate the applications and mechanisms underlying antidepressant-like effects of ketamine, a non-selective antagonist of N-methyl-d-aspartate receptor (NMDAR), the role of specific glutamate receptor subunit in regulating depression is not completely clear. The current review aims to discuss the relationships between glutamate receptor subunits and depressive-like behaviors. Research literatures were searched from inception to July 2020. We summarized the alterations of glutamate receptor subunits in patients with MDD and animal models of depression. Animal behaviors in response to dysfunction of glutamate receptor subunits were also surveyed. To fully understand mechanisms underlying antidepressant-like effects of modulators targeting glutamate receptors, we discussed effects of each glutamate receptor subunit on serotonin system, synaptic plasticity, neurogenesis and neuroinflammation. Finally, we collected most recent clinical applications of glutamate receptor modulators and pointed out the limitations of these candidates in the treatment of MDD.

Inhibition of lysophosphatidic acid receptor 1 attenuates neuroinflammation via PGE2/EP2/NOX2 signalling and improves the outcome of intracerebral haemorrhage in mice

Lysophosphatidic acid receptor 1 (LPA1) plays a critical role in proinflammatory processes in the central nervous system by modulating microglia activation. The aim of this study was to explore the anti-inflammatory effects and neurological function improvement of LPA1 inhibition after intracerebral haemorrhage (ICH) in mice and to determine whether prostaglandin E2 (PGE2), E-type prostaglandin receptor 2 (EP2), and NADPH oxidase 2 (NOX2) signalling are involved in LPA1-mediated neuroinflammation. ICH was induced in CD1 mice by autologous whole blood injection. AM966, a selective LPA1 antagonist, was administered by oral gavage 1 h and 12 h after ICH. The LPA1 endogenous ligand, LPA was administered to verify the effect of LPA1 activation. To elucidate potential inflammatory mechanisms of LPA1, the selective EP2 activator butaprost was administered by intracerebroventricular injection with either AM966 or LPA1 CRISPR knockout (KO). Water content of the brain, neurobehavior, immunofluorescence staining, and western blot were performed. After ICH, EP2 was expressed in microglia whereas LPA1 was expressed in microglia, neurons, and astrocytes, which peaked after 24 h. AM966 inhibition of LPA1 improved neurologic function, reduced brain oedema, and suppressed perihematomal inflammatory cells after ICH. LPA administration aggravated neurological deficits after ICH. AM966 treatment and LPA1 CRISPR KO both decreased the expressions of PGE2, EP2, NOX2, NF-κB, TNF-α, IL-6, and IL-1β expressions after ICH, which was reversed by butaprost. This study demonstrated that inhibition of LPA1 attenuated neuroinflammation caused by ICH via PGE2/EP2/NOX2 signalling pathway in mice, which consequently improved neurobehavioral functions and alleviated brain oedema. LPA1 may be a promising therapeutic target to attenuate ICH-induced secondary brain injury.

A Review of Molecular Imaging of Glutamate Receptors

Molecular imaging with positron emission tomography (PET) and single photon emission computed tomography (SPECT) is a well-established and important in vivo technique to evaluate fundamental biological processes and unravel the role of neurotransmitter receptors in various neuropsychiatric disorders. Specific ligands are available for PET/SPECT studies of dopamine, serotonin, and opiate receptors, but corresponding development of radiotracers for receptors of glutamate, the main excitatory neurotransmitter in mammalian brain, has lagged behind. This state of affairs has persisted despite the central importance of glutamate neurotransmission in brain physiology and in disorders such as stroke, epilepsy, schizophrenia, and neurodegenerative diseases. Recent years have seen extensive efforts to develop useful ligands for molecular imaging of subtypes of the ionotropic (N-methyl-D-aspartate (NMDA), kainate, and AMPA/quisqualate receptors) and metabotropic glutamate receptors (types I, II, and III mGluRs). We now review the state of development of radioligands for glutamate receptor imaging, placing main emphasis on the suitability of available ligands for reliable in vivo applications. We give a brief account of the radiosynthetic approach for selected molecules. In general, with the exception of ligands for the GluN2B subunit of NMDA receptors, there has been little success in developing radiotracers for imaging ionotropic glutamate receptors; failure of ligands for the PCP/MK801 binding site in vivo doubtless relates their dependence on the open, unblocked state of the ion channel. Many AMPA and kainite receptor ligands with good binding properties in vitro have failed to give measurable specific binding in the living brain. This may reflect the challenge of developing brain-penetrating ligands for amino acid receptors, compounded by conformational differences in vivo. The situation is better with respect to mGluR imaging, particularly for the mGluR5 subtype. Several successful PET ligands serve for investigations of mGluRs in conditions such as schizophrenia, depression, substance abuse and aging. Considering the centrality and diversity of glutamatergic signaling in brain function, we have relatively few selective and sensitive tools for molecular imaging of ionotropic and metabotropic glutamate receptors. Further radiopharmaceutical research targeting specific subtypes and subunits of the glutamate receptors may yet open up new investigational vistas with broad applications in basic and clinical research.

Reduced uptake of [11C]-ABP688, a PET tracer for metabolic glutamate receptor 5 in hippocampus and amygdala in Alzheimer's dementia

INTRODUCTION

Metabotropic glutamate receptors play a critical role in the pathogenesis of Alzheimer's disease due to their involvement in processes of memory formation, neuroplasticity, and synaptotoxity. The objective of the current study was to study mGluR5 availability measured by [¹¹ C]-ABP688 (ABP) in patients with clinically diagnosed Alzheimer's dementia (AD).

METHODS

A bolus-infusion protocol of [¹¹ C]-ABP688 was applied in 9 subjects with AD and 10 cognitively healthy controls (Controls) to derive distribution volume estimates of mGluR5. Furthermore, we also estimated cerebral perfusion by averaging early frame signal of initial ABP bolus injection.

RESULTS

Subjects with Alzheimer's dementia (mean age: 77.3/SD 5.7) were older than controls (mean age: 68.5/SD: 9.6) and scored lower on the MMSE (22.1/SD2.7 vs. 29.0/SD0.8). There were no overall differences in ABP signal. However, distribution volume ratio (DVR) for ABP was reduced in the bilateral hippocampus (AD: 1.34/SD: 0.40 vs. Control: 1.84/SD:0.31, p = .007) and the bilateral amygdala (AD:1.86/SD:0.26 vs. Control:2.33/SD:0.37 p = .006) in AD patients compared to controls. Estimate of cerebral blood flow was reduced in the bilateral hippocampus in AD (AD:0.75/SD:0.10 vs. Control:0.86/SD:0.09 p = .02).

CONCLUSION

Our findings demonstrate reduced mGluR5 binding in the hippocampus and amygdala in Alzheimer's dementia. Whether this is due to synaptic loss and/or consecutive reduction of potential binding sites or reflects disease inherent mechanisms remains to be elucidated in future studies.

PET imaging of mGluR5 in Alzheimer's disease

BACKGROUND

Metabotropic glutamate subtype 5 receptors (mGluR5) modulate synaptic transmission and may constitute an important therapeutic target in Alzheimer's disease (AD) by mediating the synaptotoxic action of amyloid-β oligomers. We utilized the positron emission tomography (PET) radioligand [18F]FPEB to investigate mGluR5 binding in early AD.

METHODS

Sixteen individuals with amnestic mild cognitive impairment (MCI) due to AD or mild AD dementia who were positive for brain amyloid were compared to 15 cognitively normal (CN) participants who were negative for brain amyloid. Diagnostic groups were well balanced for age, sex, and education. Dynamic PET scans were acquired for 60 min, starting at 60 min after the initial administration of up to 185 MBq of [18F]FPEB using a bolus-plus-constant-infusion method (Kbol = 190 min). Equilibrium modeling with a cerebellum reference region was used to estimate [18F]FPEB binding (BPND) to mGluR5. Analyses were performed with and without corrections for gray matter atrophy and partial volume effects.

RESULTS

Linear mixed model analysis demonstrated a significant effect of group (p = 0.011) and the group × region interaction (p = 0.0049) on BPND. Post hoc comparisons revealed a significant reduction (43%) in mGluR5 binding in the hippocampus of AD (BPND = 0.76 ± 0.41) compared to CN (BPND = 1.34 ± 0.58, p = 0.003, unpaired t test) participants, and a nonsignificant trend for a reduction in a composite association cortical region in AD (BPND = 1.57 ± 0.25) compared to CN (BPND = 1.86 ± 0.63, p = 0.093) participants. Exploratory analyses suggested additional mGluR5 reductions in the entorhinal cortex and parahippocampal gyrus in the AD group. In the overall sample, hippocampal mGluR5 binding was associated with episodic memory scores and global function.

CONCLUSIONS

[18F]FPEB-PET revealed reductions in hippocampal mGluR5 binding in early AD. Quantification of mGluR5 binding in AD may expand our understanding of AD pathogenesis and accelerate the development of novel biomarkers and treatments.

Glutamate Chemical Exchange Saturation Transfer (GluCEST) Magnetic Resonance Imaging in Pre-clinical and Clinical Applications for Encephalitis

BACKGROUND

Encephalitis is a common central nervous system inflammatory disease that seriously endangers human health owing to the lack of effective diagnostic methods, which leads to a high rate of misdiagnosis and mortality. Glutamate is implicated closely in microglial activation, and activated microglia are key players in encephalitis. Hence, using glutamate chemical exchange saturation transfer (GluCEST) imaging for the early diagnosis of encephalitis holds promise.

METHODS

The sensitivity of GluCEST imaging with different concentrations of glutamate and other major metabolites in the brain was validated in phantoms. Twenty-seven Sprague-Dawley (SD) rats with encephalitis induced by Staphylococcus aureus infection were used for preclinical research of GluCEST imaging in a 7.0-Tesla scanner. For the clinical study, six patients with encephalitis, six patients with lacunar infarction, and six healthy volunteers underwent GluCEST imaging in a 3.0-Tesla scanner.

RESULTS

The number of amine protons on glutamate that had a chemical shift of 3.0 ppm away from bulk water and the signal intensity of GluCEST were concentration-dependent. Under physiological conditions, glutamate is the main contributor to the GluCEST signal. Compared with normal tissue, in both rats and patients with encephalitis, the encephalitis areas demonstrated a hyper-intense GluCEST signal, while the lacunar infarction had a decreased GluCEST signal intensity. After intravenous immunoglobulin therapy, patients with encephalitis lesions showed a decrease in GluCEST signal, and the results were significantly different from the pre-treatment signal (1.34 ± 0.31 vs 5.0 ± 0.27%, respectively; p = 0.000).

CONCLUSION

Glutamate plays a role in encephalitis, and the GluCEST imaging signal has potential as an in vivo imaging biomarker for the early diagnosis of encephalitis. GluCEST will provide new insight into encephalitis and help improve the differential diagnosis of brain disorders.

Age dependency of mGluR5 availability in 5xFAD mice measured by PET

The major pathologies of Alzheimer's disease (AD) are amyloid plaques and hyperphosphorylated tau. The deposition of amyloid plaques leads to synaptic dysfunction, neuronal cell death, and cognitive impairment. Among the neurotransmitters, glutamate is the most abundant in the mammalian brain and plays an important role in synaptic plasticity. With respect to synaptic transmission, metabotropic glutamate receptor 5 (mGluR5) is highly affected by amyloid pathology. However, the neuropathologic changes in the protein expression of mGluR5 in AD remain unclear. Therefore, to elucidate the alteration in mGluR5 expression with the progression of AD, we performed serial behavioral tests, longitudinal imaging studies, and histopathological immunoassay for both 5xFAD (n = 14) mice and age-matched wild-type mice (n = 14). The 5xFAD mice started showing severe hyperactivity and memory impairment from 7 months of age. In addition, mGluR5 positron emission tomography revealed that while the binding values in the wild-type mice were similar over time, those in 5xFAD mice fluctuated from 5 months of age. Furthermore, the 5xFAD mice presented a 35% decrease in the binding values of their cortical and subcortical areas at 9 months of age compared with those at 3 months of age. Magnetic resonance spectroscopy and histopathological studies showed similar changes. In conclusion, mGluR5 availability changes with age, and mGluR5 positron emission tomography could successfully detect this synaptic change in the 5xFAD mice.