Although aberrations in the number and function of glutamate AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid) receptors are thought to underlie neuropsychiatric disorders, no methods are ...currently available for visualizing AMPA receptors in the living human brain. Here we developed a positron emission tomography (PET) tracer for AMPA receptors. A derivative of 4-2-(phenylsulfonylamino)ethylthio-2,6-difluoro-phenoxyacetamide radiolabeled with
C (
CK-2) showed specific binding to AMPA receptors. Our clinical trial with healthy human participants confirmed reversible binding of
CK-2 in the brain according to Logan graphical analysis (UMIN000020975; study design: non-randomized, single arm; primary outcome: dynamics and distribution volumes of
CK-2 in the brain; secondary outcome: adverse events of
CK-2 during the 4-10 d following dosing; this trial met prespecified endpoints). In an exploratory clinical study including patients with epilepsy, we detected increased
CK-2 uptake in the epileptogenic focus of patients with mesial temporal lobe epilepsy, which was closely correlated with the local AMPA receptor protein distribution in surgical specimens from the same individuals (UMIN000025090; study design: non-randomized, single arm; primary outcome: correlation between
CK-2 uptake measured with PET before surgery and AMPA receptor protein density examined by biochemical study after surgery; secondary outcome: adverse events during the 7 d following PET scan; this trial met prespecified endpoints). Thus,
CK-2 is a potent PET tracer for AMPA receptors, potentially providing a tool to examine the involvement of AMPA receptors in neuropsychiatric disorders.
Synaptic dysfunction provoking dysregulated cortical neural circuits is currently hypothesized as a key pathophysiological process underlying clinical manifestations in Alzheimer's disease and ...related neurodegenerative tauopathies. Here, we conducted PET along with postmortem assays to investigate time course changes of excitatory and inhibitory synaptic constituents in an rTg4510 mouse model of tauopathy, which develops tau pathologies leading to noticeable brain atrophy at 5-6 months of age. Both male and female mice were analyzed in this study. We observed that radiosignals derived from
Cflumazenil, a tracer for benzodiazepine receptor, in rTg4510 mice were significantly lower than the levels in nontransgenic littermates at 2-3 months of age. In contrast, retentions of (E)-
CABP688, a tracer for mGluR5, were unaltered relative to controls at 2 months of age but then gradually declined with aging in parallel with progressive brain atrophy. Biochemical and immunohistochemical assessment of postmortem brain tissues demonstrated that inhibitory, but not excitatory, synaptic constituents selectively diminished without overt loss of somas of GABAergic interneurons in the neocortex and hippocampus of rTg4510 mice at 2 months of age, which was concurrent with enhanced immunoreactivity of cFos, a well-characterized immediate early gene, suggesting that impaired inhibitory neurotransmission may cause hyperexcitability of cortical circuits. Our findings indicate that tau-induced disruption of the inhibitory synapse may be a critical trigger of progressive neurodegeneration, resulting in massive neuronal loss, and PET assessments of inhibitory versus excitatory synapses potentially offer
indices for hyperexcitability and excitotoxicity early in the etiologic pathway of neurodegenerative tauopathies.
In this study, we examined the in vivo status of excitatory and inhibitory synapses in the brain of the rTg4510 tauopathy mouse model by PET imaging with (E)-
CABP688 and
Cflumazenil, respectively. We identified inhibitory synapse as being significantly dysregulated before brain atrophy at 2 months of age, while excitatory synapse stayed relatively intact at this stage. In line with this observation, postmortem assessment of brain tissues demonstrated selective attenuation of inhibitory synaptic constituents accompanied by the upregulation of cFos before the formation of tau pathology in the forebrain at young ages. Our findings indicate that selective degeneration of inhibitory synapse with hyperexcitability in the cortical circuit constitutes the critical early pathophysiology of tauopathy.
Positron emission tomography (PET) allows biomolecular tracking but PET monitoring of brain networks has been hampered by a lack of suitable reporters. Here, we take advantage of bacterial ...dihydrofolate reductase, ecDHFR, and its unique antagonist, TMP, to facilitate in vivo imaging in the brain. Peripheral administration of radiofluorinated and fluorescent TMP analogs enabled PET and intravital microscopy, respectively, of neuronal ecDHFR expression in mice. This technique can be used to the visualize neuronal circuit activity elicited by chemogenetic manipulation in the mouse hippocampus. Notably, ecDHFR‐PET allows mapping of neuronal projections in non‐human primate brains, demonstrating the applicability of ecDHFR‐based tracking technologies for network monitoring. Finally, we demonstrate the utility of TMP analogs for PET studies of turnover and self‐assembly of proteins tagged with ecDHFR mutants. These results establish opportunities for a broad spectrum of previously unattainable PET analyses of mammalian brain circuits at the molecular level.
SYNOPSIS
ecDHFR‐based reporter system can be utilized for bimodal fluorescence and Positron emission tomography (PET) imaging of expression and dynamics of its fused protein of interest in living animal brains, offering broad‐spectrum analyses of a mammalian deep brain circuit at molecular levels.
We established a genetically encoded ecDHFR‐based reporter system applicable for bimodal optical and PET imaging in living animal brains.
The reporter gene expression driven by an activity‐dependent promoter illuminates neuronal ensemble activities elicited by chemogenetic manipulation in the mouse hippocampal circuit.
ecDHFR/TMP systems enable visualization of neuronal tracts in deep brain regions of non‐human primates.
The utility of TMP analogs for PET monitoring of aggregation and turnover of proteins tagged with mutant forms of ecDHFR.
Application of bacterial dihydrofolate reductase ecDHFR and its unique antagonist TMP achieves a broad spectrum of previously unattainable in vivo PET analyses of mammalian brain circuits at the molecular level.
Tauopathy is characterized by the fibrillar tau accumulation in neurons and glial cells. In order to advance our understanding of the causative mechanisms of tauopathy, neuroinflammation, which has ...been suggested to play important roles in disease progression, will require particular attention. Neuroinflammation is characterized predominantly by microglial activation. At present, it is still under debate whether microglial activation is a cause or a result of neurodegeneration. To search for a temporal relationship between neurodegeneration and neuroinflammation, our group demonstrated that in vivo imaging (e.g., tau-PET, TSPO-PET, and volumetric MRI) of tauopathy mice strongly supports the evidence of microglial activation along with both pathological tau accumulation and brain atrophy. Both in vivo imaging and histochemical analysis confirmed that microglial TSPO accumulation was the late event during the pathogenesis of tauopathy. On the other hand, it is known that purinergic receptor P2Y12 as a marker of homeostatic microglia cells was reduced at an early stage of disease progression. In this review, we will introduce a phenotypic change of microglia in a mouse model of tauopathy and propose novel approaches to the establishment of imaging biomarkers, thereby targeting the early diagnosis of tauopathy.
Tau imaging using PET is a promising tool for the diagnosis and evaluation of tau-related neurodegenerative disorders, but the relationship among PET-detectable tau, neuroinflammation, and ...neurodegeneration is not yet fully understood.
We aimed to elucidate sequential changes in tau accumulation, neuroinflammation, and brain atrophy by PET and MRI in a tauopathy mouse model.
rTg4510 transgenic (tg) mice expressing P301L mutated tau and non-tg mice were examined with brain MRI and PET imaging (analyzed numbers: tg = 17, non-tg = 13; age 2.5∼14 months). As PET probes, 11CPBB3 (Pyridinyl-Butadienyl-Benzothiazole 3) and 11CAC-5216 were used to visualize tau pathology and 18-kDa translocator protein (TSPO) neuroinflammation. Tau pathology and microglia activation were subsequently analyzed by histochemistry.
PET studies revealed age-dependent increases in 11CPBB3 and 11CAC-5216 signals, which were correlated with age-dependent volume reduction in the forebrain on MRI. However, the increase in 11CPBB3 signals reached a plateau at age 7 months, and therefore its significant correlation with 11CAC-5216 disappeared after age 7 months. In contrast, 11CAC-5216 showed a strong correlation with both age and volume reduction until age 14 months. Histochemical analyses confirmed the relevance of pathological tau accumulation and elevated TSPO immunoreactivity in putative microglia.
Our results showed that tau accumulation is associated with neuroinflammation and brain atrophy in a tauopathy mouse model. The time-course of the 11CPBB3- and TSPO-PET finding suggests that tau deposition triggers progressive neuroinflammation, and the sequential changes can be evaluated in vivo in mouse brains.
Fluctuations of neuronal activities in the brain may underlie relatively slow components of neurofunctional alterations, which can be modulated by food intake and related systemic metabolic statuses. ...Glutamatergic neurotransmission plays a major role in the regulation of excitatory tones in the central nervous system, although just how dietary elements contribute to the tuning of this system remains elusive. Here, we provide the first demonstration by bimodal positron emission tomography (PET) and magnetic resonance spectroscopy (MRS) that metabotropic glutamate receptor subtype 5 (mGluR5) ligand binding and glutamate levels in human brains are dynamically altered in a manner dependent on food intake and consequent changes in plasma glucose levels. The brain-wide modulations of central mGluR5 ligand binding and glutamate levels and profound neuronal activations following systemic glucose administration were further proven by PET, MRS, and intravital two-photon microscopy, respectively, in living rodents. The present findings consistently support the notion that food-associated glucose intake is mechanistically linked to glutamatergic tones in the brain, which are translationally accessible in vivo by bimodal PET and MRS measurements in both clinical and non-clinical settings.
Background
Several clinical studies indicate that subclinical epileptiform discharge is observed in patients with Alzheimer’s disease (AD), and neuronal hyperexcitability accelerates their cognitive ...decline. Previous studies indicated that the expression level of Nav1.1, which is coded by the Scn1a gene and a voltage‐gated sodium channel abundantly expressed in parvalbumin‐positive inhibitory neurons, decreased in AD patients. Because it is reported that excitatory neural activity stimulates tau release from neurons, we hypothesized that suppressing neuronal hyperexcitability by enhancing inhibitory neural activities could ameliorate disease progression in a mouse model of tauopathy. In this study, we investigated if our in‐house Nav1.1 potentiator, compound‐X, prevents the progression of tau accumulation and neurodegeneration in a mouse model of tauopathy, rTg4510.
Method
We performed electrophysiological recording to determine the specificity and potency of compound‐X in cells stably expressing various Nav subtypes, and cortical slices of Scn1a<+/‐> and rTg4510 mice. We subsequently tested the in‐vivo effects of compound‐X on Nav1.1 by examining if compound‐X can prevent hyperthermia‐induced seizure in Scn1a<+/‐> mice. To assess the effect of compound‐X on tau‐induced neuropathologies in rTg4510 mice, we orally treated rTg4510 mice with this compound from 4 months of age, and evaluated tau accumulation and brain atrophy of these animals by longitudinal tau PET with 18FPM‐PBB3 and MRI, respectively, at 6 and 8 months of age.
Result
The auto‐patch‐clamp system using Nav stably expressing cells and current‐clamp recoding using cortical slices demonstrated that compound‐X could potentiate Nav1.1 current in a concentration‐dependent manner, leading to activation of inhibitory neurons. Scn1a<+/‐> mice administered with compound‐X showed an increased threshold of the rectal temperature of hyperthermia‐induced seizure, and prolonged latency to seizure, indicating that compound‐X can potentiate Nav1.1 in vivo. Significantly, tau accumulation and neuronal loss assessed by cerebral atrophy and lateral ventricular enlargement were profoundly suppressed in the brains of rTg4510 mice relative to vehicle‐treated controls.
Conclusion
Pharmacological enhancement of inhibitory neuronal function can be a beneficial approach to the prevention of hyperexcitability‐mediated tau pathologies and neuronal death in a mouse model of tauopathy.
Orexin receptors (OXRs) in the brain have been implicated in diverse physiological and neuropsychiatric conditions. Here we describe the design, synthesis, and evaluation of OXR ligands related to ...(1R,2S)-2-(((2-methyl-4-methoxymethylpyrimidin-5-yl)oxy)methyl)-N-(5-fluoropyridin-2-yl)-2-(3-fluorophenyl)cyclopropanecarboxamide (9a) applicable to positron emission tomography (PET) imaging. Structural features were incorporated to increase binding affinity for OXRs, to enable carbon-11 radiolabeling, and to adjust lipophilicity considered optimal for brain penetration and low nonspecific binding. 9a displayed nanomolar affinity for OXRs, and autoradiography using rat brain sections showed that specific binding of 11C9a was distributed primarily to neocortical layer VI and hypothalamus, consistent with reported localizations of orexin-2 receptors (OX2Rs). In vivo PET study of 11C9a demonstrated moderate uptake of radioactivity into rat and monkey brains under deficiency or blockade of P-glycoprotein, and distribution of PET signals in the brain was in agreement with autoradiographic data. Our approach and findings have provided significant information for development of OX2R PET tracers.
Background
α-Amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptor is a primary mediator of fast glutamatergic excitatory signaling in the brain and has been implicated in diverse ...neuropsychiatric diseases. We recently developed a novel positron emission tomography (PET) ligand, 2-(1-(3-(
11
Cmethylamino)phenyl)-2-oxo-5-(pyrimidin-2-yl)-1,2-dihydropyridin-3-yl) benzonitrile (
11
CHMS011). This compound is a radiolabelled derivative of perampanel, an antiepileptic drug acting on AMPA receptors, and was demonstrated to have promising in vivo properties in the rat and monkey brains. In the current study, we performed a human PET study using
11
CHMS011 to evaluate its safety and kinetics.
Four healthy male subjects underwent a 120-min PET scan after injection of
11
CHMS011. Arterial blood sampling and metabolite analysis were performed to obtain parent input functions for three of the subjects using high-performance liquid chromatography. Regional distribution volumes (
V
T
s) were calculated based on kinetic models with and without considering radiometabolite in the brain. The binding was also quantified using a reference tissue model with white matter as reference.
Results
Brain uptake of
11
CHMS011 was observed quickly after the injection, followed by a rapid clearance. Three hydrophilic and one lipophilic radiometabolites appeared in the plasma, with notable individual variability. The kinetics in the brain with apparent radioactivity retention suggested that the lipophilic radiometabolite could enter the brain. A dual-input graphical model, an analytical model designed in consideration of a radiometabolite entering the brain, well described the kinetics of
11
CHMS011. A reference tissue model showed small radioligand binding potential (
BP
*
ND
) values in the cortical regions (
BP
*
ND
= 0–0.15). These data suggested specific binding component of
11
CHMS011 in the brain.
Conclusions
Kinetic analyses support some specific binding of
11
CHMS011 in the human cortex. However, this ligand may not be suitable for practical AMPA receptor PET imaging due to the small dynamic range and metabolite in the brain.
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![A human PET study of [11C]H...](http://api.summon.serialssolutions.com/2.0.0/image/issn/XR4PS5YY4K/2191-219X/small "A human PET study of [11C]HMS011, a potential radioligand for AMPA receptors")