Chemogenetic Disconnection between the Orbitofrontal Cortex and the Rostromedial Caudate Nucleus Disrupts Motivational Control of Goal-Directed Action.

The orbitofrontal cortex (OFC) and its major downstream target within the basal ganglia-the rostromedial caudate nucleus (rmCD)-are involved in reward-value processing and goal-directed behavior. However, a causal contribution of the pathway linking these two structures to goal-directed behavior has not been established. Using the chemogenetic technology of designer receptors exclusively activated by designer drugs with a crossed inactivation design, we functionally and reversibly disrupted interactions between the OFC and rmCD in two male macaque monkeys. We injected an adeno-associated virus vector expressing an inhibitory designer receptor, hM4Di, into the OFC and contralateral rmCD, the expression of which was visualized in vivo by positron emission tomography and confirmed by postmortem immunohistochemistry. Functional disconnection of the OFC and rmCD resulted in a significant and reproducible loss of sensitivity to the cued reward value for goal-directed action. This decreased sensitivity was most prominent when monkeys had accumulated a certain amount of reward. These results provide causal evidence that the interaction between the OFC and the rmCD is needed for motivational control of action on the basis of the relative reward value and internal drive. This finding extends the current understanding of the physiological basis of psychiatric disorders in which goal-directed behavior is affected, such as obsessive-compulsive disorder.SIGNIFICANCE STATEMENT In daily life, we routinely adjust the speed and accuracy of our actions on the basis of the value of expected reward. Abnormalities in these kinds of motivational adjustments might be related to behaviors seen in psychiatric disorders such as obsessive-compulsive disorder. In the current study, we show that the connection from the orbitofrontal cortex to the rostromedial caudate nucleus is essential for motivational control of action in monkeys. This finding expands our knowledge about how the primate brain controls motivation and behavior and provides a particular insight into disorders like obsessive-compulsive disorder in which altered connectivity between the orbitofrontal cortex and the striatum has been implicated.

Chemogenetic activation of nigrostriatal dopamine neurons in freely moving common marmosets.

To interrogate particular neuronal pathways in nonhuman primates under natural and stress-free conditions, we applied designer receptors exclusively activated by designer drugs (DREADDs) technology to common marmosets. We injected adeno-associated virus vectors expressing the excitatory DREADD hM3Dq into the unilateral substantia nigra (SN) in four marmosets. Using multi-tracer positron emission tomography imaging, we detected DREADD expression in vivo, which was confirmed in nigrostriatal dopamine neurons by immunohistochemistry, as well as by assessed activation of the SN following agonist administration. The marmosets rotated in a contralateral direction relative to the activated side 30-90 min after consuming food containing the highly potent DREADD agonist deschloroclozapine (DCZ) but not on the following days without DCZ. These results indicate that non-invasive and reversible DREADD manipulation will extend the utility of marmosets as a primate model for linking neuronal activity and natural behavior in various contexts.

Pharmacological and Genetic Inhibition of Translocator Protein 18 kDa Ameliorated Neuroinflammation in Murine Endotoxemia Model.

ABSTRACT: Sepsis-associated encephalopathy (SAE) is a diffuse brain dysfunction associated with sepsis. The development of an effective strategy for early diagnosis and therapeutic intervention is essential for the prevention of poor prognosis of SAE. Translocator protein 18 kDa (TSPO) is a mitochondrial protein implicated in steroidogenesis and inflammatory responses. Despite accumulating evidence that implicates TSPO in the neuroinflammatory response of the central nervous system, the possible role of TSPO in SAE remains unclear. The aim of this study is to address a role of TSPO in neuroinflammation using mice 24 h after systemic injection of LPS, which consistently demonstrated microglial activation and behavioral inhibition. Quantitative polymerase chain reaction analysis revealed that hippocampal TSPO expression was induced following the systemic LPS injection, associated with an increase in pro-inflammatory cytokines such as tumor necrosis factor-α and interleukin-1ß. Interestingly, pretreatment with the TSPO antagonist, ONO-2952, or germ-line deletion of the TSPO gene exhibited an anti-inflammatory effect with significant suppression of LPS-induced production of those cytokines. These effects demonstrated by the ONO-2952 or TSPO knockout were associated with significant recovery from behavioral inhibition, as shown by improved locomotor activity in the open field analysis. Histological analysis revealed that ONO-2952 pretreatment suppressed the LPS-induced activation of TSPO-expressing microglia in the hippocampus of mice. Collectively, these results suggest that TSPO plays a critical role in the SAE mouse model. Based on this finding, monitoring TSPO activity, as well as the progress of endotoxemia and its sequelae in the animal model, would deepen our understanding of the underlying molecular mechanism of SAE.

Steroidogenic abnormalities in translocator protein knockout mice and significance in the aging male.

The translocator protein (TSPO) has been proposed to act as a key component in a complex important for mitochondrial cholesterol importation, which is the rate-limiting step in steroid hormone synthesis. However, TSPO function in steroidogenesis has recently been challenged by the development of TSPO knockout (TSPO-KO) mice, as they exhibit normal baseline gonadal testosterone and adrenal corticosteroid production. Here, we demonstrate that despite normal androgen levels in young male TSPO-KO mice, TSPO deficiency alters steroidogenic flux and results in reduced total steroidogenic output. Specific reductions in the levels of progesterone and corticosterone as well as age-dependent androgen deficiency were observed in both young and aged male TSPO-KO mice. Collectively, these findings indicate that while TSPO is not critical for achieving baseline testicular and adrenal steroidogenesis, either indirect effects of TSPO on steroidogenic processes, or compensatory mechanisms and functional redundancy, lead to subtle steroidogenic abnormalities which become exacerbated with aging.

Affinity States of Striatal Dopamine D2 Receptors in Antipsychotic-Free Patients with Schizophrenia.

Background: Dopamine D2 receptors are reported to have high-affinity (D2High) and low-affinity (D2Low) states. Although an increased proportion of D2High has been demonstrated in animal models of schizophrenia, few clinical studies have investigated this alteration of D2High in schizophrenia in vivo. Methods: Eleven patients with schizophrenia, including 10 antipsychotic-naive and 1 antipsychotic-free individuals, and 17 healthy controls were investigated. Psychopathology was assessed by Positive and Negative Syndrome Scale, and a 5-factor model was used. Two radioligands, [11C]raclopride and [11C]MNPA, were employed to quantify total dopamine D2 receptor and D2High, respectively, in the striatum by measuring their binding potentials. Binding potential values of [11C]raclopride and [11C]MNPA and the binding potential ratio of [11C]MNPA to [11C]raclopride in the striatal subregions were statistically compared between the 2 diagnostic groups using multivariate analysis of covariance controlling for age, gender, and smoking. Correlations between binding potential and Positive and Negative Syndrome Scale scores were also examined. Results: Multivariate analysis of covariance demonstrated a significant effect of diagnosis (schizophrenia and control) on the binding potential ratio (P=.018), although the effects of diagnosis on binding potential values obtained with either [11C]raclopride or [11C]MNPA were nonsignificant. Posthoc test showed that the binding potential ratio was significantly higher in the putamen of patients (P=.017). The Positive and Negative Syndrome Scale "depressed" factor in patients was positively correlated with binding potential values of both ligands in the caudate. Conclusions: The present study indicates the possibilities of: (1) a higher proportion of D2High in the putamen despite unaltered amounts of total dopamine D2 receptors; and (2) associations between depressive symptoms and amounts of caudate dopamine D2 receptors in patients with schizophrenia.

Multimodal Imaging for DREADD-Expressing Neurons in Living Brain and Their Application to Implantation of iPSC-Derived Neural Progenitors.

Chemogenetic manipulation of neuronal activities has been enabled by a designer receptor (designer receptor exclusively activated by designer drugs, DREADD) that is activated exclusively by clozapine-N-oxide (CNO). Here, we applied CNO as a functional reporter probe to positron emission tomography (PET) of DREADD in living brains. Mutant human M4 DREADD (hM4Di) expressed in transgenic (Tg) mouse neurons was visualized by PET with microdose [11C]CNO. Deactivation of DREADD-expressing neurons in these mice by nonradioactive CNO at a pharmacological dose could also be captured by arterial spin labeling MRI (ASL-MRI). Neural progenitors derived from hM4Di Tg-induced pluripotent stem cells were then implanted into WT mouse brains and neuronal differentiation of the grafts could be imaged by [11C]CNO-PET. Finally, ASL-MRI captured chemogenetic functional manipulation of the graft neurons. Our data provide the first demonstration of multimodal molecular/functional imaging of cells expressing a functional gene reporter in the brain, which would be translatable to humans for therapeutic gene transfers and cell replacements. SIGNIFICANCE STATEMENT: The present work provides the first successful demonstration of in vivo positron emission tomographic (PET) visualization of a chemogenetic designer receptor (designer receptor exclusively activated by designer drugs, DREADD) expressed in living brains. This technology has been applied to longitudinal PET reporter imaging of neuronal grafts differentiated from induced pluripotent stem cells. Differentiated from currently used reporter genes for neuroimaging, DREADD has also been available for functional manipulation of target cells, which could be visualized by functional magnetic resonance imaging (fMRI) in a real-time manner. Multimodal imaging with PET/fMRI enables the visualization of the differentiation of iPSC-derived neural progenitors into mature neurons and DREADD-mediated functional manipulation along the time course of the graft and is accordingly capable of fortifying the utility of stem cells in cell replacement therapies.

PET imaging-guided chemogenetic silencing reveals a critical role of primate rostromedial caudate in reward evaluation.

The rostromedial caudate (rmCD) of primates is thought to contribute to reward value processing, but a causal relationship has not been established. Here we use an inhibitory DREADD (Designer Receptor Exclusively Activated by Designer Drug) to repeatedly and non-invasively inactivate rmCD of macaque monkeys. We inject an adeno-associated viral vector expressing the inhibitory DREADD, hM4Di, into the rmCD bilaterally. To visualize DREADD expression in vivo, we develop a non-invasive imaging method using positron emission tomography (PET). PET imaging provides information critical for successful chemogenetic silencing during experiments, in this case the location and level of hM4Di expression, and the relationship between agonist dose and hM4Di receptor occupancy. Here we demonstrate that inactivating bilateral rmCD through activation of hM4Di produces a significant and reproducible loss of sensitivity to reward value in monkeys. Thus, the rmCD is involved in making normal judgments about the value of reward.

Assessment of neuroinflammation in a mouse model of obesity and ß-amyloidosis using PET.

BACKGROUND: Obesity has been identified as a risk factor for cognitive decline and Alzheimer's disease (AD). The aim of this study was to investigate the effect of obesity on neuroinflammation and cerebral glucose metabolism using PET in a mouse model of ß-amyloidosis and determine the relationship between these PET imaging biomarkers, pathogenic changes, and functional outcomes. METHODS: Three-month-old C57BL/J6 mice were fed either a standard (control group) or high-fat diet (obese group) for 3 months and intracerebroventricularly infused with vehicle or human beta amyloid 1-42 (Aß42). We assessed obesity-induced abnormalities in peripheral metabolic indices including adiposity, fasting glucose, and glucose tolerance. Brain glucose metabolism was assessed by (18)F-FDG PET, and glial activation was assessed using the translocator protein (TSPO) ligand (11)C-PBR-28. TSPO expression was confirmed by immunohistochemistry of brain sections obtained from scanned mice. The association between inflammatory state and (11)C-PBR-28 PET signals was characterized by examination of the cytokine expression profile in both the serum and hippocampus by antibody array. Learning and memory performance was assessed in the object recognition task, and anxiety-related behavior was assessed in the elevated plus maze. RESULTS: Obesity combined with Aß infusion promoted neuroinflammation and cerebral hypermetabolism, and these signals were significant predictors of learning and memory performance in the object recognition task. In vivo TSPO signals were associated with inflammatory markers including CXCL1, CXCL2, CXCL12, CCL3, CCL5, TIMP-1, G-CSF, sICAM-1, and IL-1ra. CONCLUSIONS: In vivo cerebral metabolism and TSPO signals indicate that obesity can accelerate amyloid-induced inflammation and associated cognitive decline.

[(11)C]TASP457, a novel PET ligand for histamine H3 receptors in human brain.

PURPOSE: The histamine H3 receptors are presynaptic neuroreceptors that inhibit the release of histamine and other neurotransmitters. The receptors are considered a drug target for sleep disorders and neuropsychiatric disorders with cognitive decline. We developed a novel PET ligand for the H3 receptors, [(11)C]TASP0410457 ([(11)C]TASP457), with high affinity, selectivity and favorable kinetic properties in the monkey, and evaluated its kinetics and radiation safety profile for quantifying the H3 receptors in human brain. METHODS: Ten healthy men were scanned for 120 min with a PET scanner for brain quantification and three healthy men were scanned for radiation dosimetry after injection of 386 ± 6.2 MBq and 190 ± 7.5 MBq of [(11)C]TASP457, respectively. For brain quantification, arterial blood sampling and metabolite analysis were performed using high-performance liquid chromatography. Distribution volumes (V T) in brain regions were determined by compartment and graphical analyses using the Logan plot and Ichise multilinear analysis (MA1). For dosimetry, radiation absorbed doses were estimated using the Medical Internal Radiation Dose scheme. RESULTS: [(11)C]TASP457 PET showed high uptake (standardized uptake values in the range of about 3 - 6) in the brain and fast washout in cortical regions and slow washout in the pallidum. The two-tissue compartment model and graphical analyses estimated V T with excellent identification using 60-min scan data (about 16 mL/cm(3) in the pallidum, 9 - 14 in the basal ganglia, 6 - 9 in cortical regions, and 5 in the pons), which represents the known distribution of histamine H3 receptors. For parametric imaging, MA1 is recommended because of minimal underestimation with small intersubject variability. The organs with the highest radiation doses were the pancreas, kidneys, and liver. The effective dose delivered by [(11)C]TASP457 was 6.9 µSv/MBq. CONCLUSION: [(11)C]TASP457 is a useful novel PET ligand for the investigation of the density of histamine H3 receptors in human brain.

Development of 1-N-(11)C-Methyl-L- and -D-Tryptophan for pharmacokinetic imaging of the immune checkpoint inhibitor 1-Methyl-Tryptophan.

1-Methyl-tryptophan (1MTrp) is known as a specific inhibitor targeting the immune-checkpoint protein indoleamine-2,3-dioxygenase, in two stereoisomers of levorotary (L) and dextrorotary (D). A long-standing debate exists in immunology and oncology: which stereoisomer has the potential of antitumor immunotherapy. Herein, we developed two novel radioprobes, 1-N-(11)C-methyl-L- and -D-tryptophan ((11)C-L-1MTrp and (11)C-D-1MTrp), without modifying the chemical structures of the two isomers, and investigated their utility for pharmacokinetic imaging of the whole body. (11)C-L-1MTrp and (11)C-D-1MTrp were synthesized rapidly with radiochemical yields of 47 ± 6.3% (decay-corrected, based on (11)C-CO2), a radiochemical purity of >98%, specific activity of 47-130 GBq/µmol, and high enantiomeric purity. PET/CT imaging in rats revealed that for (11)C-L-1MTrp, the highest distribution of radioactivity was observed in the pancreas, while for (11)C-D-1MTrp, it was observed in the kidney. Ex vivo biodistribution confirmed the PET/CT results, indicating the differences in pharmacokinetics between the two isomers. Both (11)C-L-1MTrp and (11)C-D-1MTrp are therefore useful PET probes for delineating the distribution and action of the checkpoint inhibitor 1MTrp in vivo. This study represents the first step toward using whole-body and real-time insight to disentangle the antitumor potential of the two stereoisomers of 1MTrp, and it can facilitate the development of 1MTrp immunotherapy.

Evaluation of [(11)C]oseltamivir uptake into the brain during immune activation by systemic polyinosine-polycytidylic acid injection: a quantitative PET study using juvenile monkey models of viral infection.

BACKGROUND: Abnormal behaviors of young patients after taking the anti-influenza agent oseltamivir (Tamiflu®, F. Hoffmann-La Roche, Ltd., Basel, Switzerland) have been suspected as neuropsychiatric adverse events (NPAEs). Immune response to viral infection is suspected to cause elevation of drug concentration in the brain of adolescents. In the present study, the effect of innate immune activation on the brain uptake of [(11)C]oseltamivir was quantitatively evaluated in juvenile monkeys. METHODS: Three 2-year-old monkeys underwent positron emission tomography (PET) scans at baseline and immune-activated conditions. Both scans were conducted under pre-dosing of clinically relevant oseltamivir. The immune activation condition was induced by the intravenous administration of polyinosine-polycytidylic acid (poly I:C). Dynamic [(11)C]oseltamivir PET scan and serial arterial blood sampling were performed to obtain [(11)C]oseltamivir kinetics. Brain uptake of [(11)C]oseltamivr was evaluated by its normalized brain concentration, brain-to-plasma concentration ratio, and plasma-to-brain transfer rate. Plasma pro-inflammatory cytokine levels were also measured. RESULTS: Plasma interleukin-6 was elevated after intravenous administration of poly I:C in all monkeys. Brain radioactivity was uniform both at baseline and under poly I:C treatment. The mean brain concentrations of [(11)C]oseltamivir were 0.0033 and 0.0035% ID/cm(3) × kg, the mean brain-to-plasma concentration ratios were 0.58 and 0.65, and the plasma-to-brain transfer rates were 0.0047 and 0.0051 mL/min/cm(3) for baseline and poly I:C treatment, respectively. Although these parameters were slightly changed by immune activation, the change was not notable. CONCLUSIONS: The brain uptake of [(11)C]oseltamivir was unchanged by poly I:C treatment in juvenile monkeys. This study demonstrated that the innate immune response similar to the immune activation of influenza would not notably change the brain concentration of oseltamivir in juvenile monkeys.

Biodistribution and radiation dosimetry in humans of [¹¹C]FLB 457, a positron emission tomography ligand for the extrastriatal dopamine D2 receptor.

PURPOSE: [(11)C]FLB 457, a radioligand with very high affinity and selectivity for dopamine D2/3 receptors, is used to measure receptor binding in extrastriatal regions showing low density of the receptors. The purpose of this study was to estimate the whole-body biodistribution of radioactivity and the radiation absorbed doses to organs after intravenous injection of [(11)C]FLB 457 in healthy human subjects. METHODS: Whole-body images were acquired for 2 h after an injection of [(11)C]FLB 457 in six healthy humans. Radiation absorbed doses were estimated by the MIRD scheme implemented in OLINDA/EXM 1.1 software. RESULTS: Organs with the longest residence time were the liver, lungs, and brain. The organs with the highest radiation doses were the kidneys, liver, and pancreas. The effective dose delivered by [(11)C]FLB 457 is 5.9 µSv/MBq, similar to those of other (11)C-labeled tracers. CONCLUSIONS: This effective dose would allow multiple scans in the same individual based on prevailing maximum recommended-dose guidelines in the USA and Europe.

[Imaging blood-brain barrier function by using positron emission tomography to evaluate drug penetration into the brain].

In drug development, the extent to which a target drug penetrates the blood-brain barrier (BBB) is important. Positron emission tomography (PET) has enabled imaging of BBB functions in vivo, directly or indirectly, based on the characteristics of PET probes. Recent studies have demonstrated that drug penetration into the brain is limited by drug efflux transporters such as P-glycoprotein (P-gp) and breast cancer resistance protein , which are expressed at the BBB. [11C]verapamil is the first probe used to evaluate P-gp function at the BBB because verapamil is a substrate of P-gp and is usually extruded from the brain via active transport by P-gp. However, many studies have shown that [11C]verapamil uptake is altered on administration of P-gp inhibitors and in neuropsychiatric diseases. Further, various PET probes evaluating these efflux transporters are currently under development. We used [11C]FLB 457 to evaluate dopamine D2 receptor occupancy by several antipsychotic drugs in the pituitary gland located outside the BBB and the temporal cortex located in the brain. We compared the receptor occupancy in both regions and found that high occupancy in the pituitary gland meant low penetration of the drug into the brain, as in the case of sulpiride, whereas low occupancy in the pituitary gland meant high penetration, as in the case of olanzapine. This indicated that sulpiride penetrated less into the brain and caused hyperprolactinemia due to the high pituitary occupancy. Furthermore, we developed a tracer, [11C]sulpiride, to directly demonstrate the low penetration of sulpiride into the brain.

Correlation between decreased motor activity and dopaminergic degeneration in the ventrolateral putamen in monkeys receiving repeated MPTP administrations: a positron emission tomography study.

Parkinson's disease (PD) patients have remarkably reduced levels of dopaminergic biomarkers in the caudal putamen. However, the relationship between motor impairments and the localization of intrastriatal dopaminergic degeneration in monkey PD models remains unclear. To identify the striatal areas with dopaminergic dysfunction responsible for motor impairments, we measured changes in both general motor activity and in vivo dopaminergic biomarkers in three cynomolgus monkeys that repeatedly received 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), starting in the normal state and continuing until after tremor appearance. Binding of dopamine transporters (DAT) and D(2) receptors were measured by positron emission tomography (PET) using [(11)C]PE2I and [(11)C]raclopride, respectively. Region-of-interest-based regression analysis demonstrated the degree of general motor activity reduction to be explained by striatal DAT binding but not by D(2) receptor binding. Furthermore, voxel-based analysis revealed a significant correlation between reduced general motor activity and decreased DAT binding, specifically in the ventrolateral putamen, which corresponds to the area receiving upper body motor inputs from the primary motor cortex. These results suggest that specific functional deficits in PD models are closely related to the degeneration of dopaminergic terminals in the striatal subregion responsible for these functions and that the level of deficit is dependent on the degree of degeneration.

An alteration in the lateral geniculate nucleus of experimental glaucoma monkeys: in vivo positron emission tomography imaging of glial activation.

We examined lateral geniculate nucleus (LGN) degeneration as an indicator for possible diagnosis of glaucoma in experimental glaucoma monkeys using positron emission tomography (PET). Chronic intraocular pressure (IOP) elevation was induced by laser trabeculoplasty in the left eyes of 5 cynomolgus monkeys. Glial cell activation was detected by PET imaging with [(11)C]PK11195, a PET ligand for peripheral-type benzodiazepine receptor (PBR), before and at 4 weeks after laser treatment (moderate glaucoma stage). At mild, moderate, and advanced experimental glaucoma stages (classified by histological changes based on the extent of axonal loss), brains were stained with cresyl violet, or antibodies against PBR, Iba-1 (a microglial marker), and GFAP (an activated astrocyte marker). In laser-treated eyes, IOP was persistently elevated throughout all observation periods. PET imaging showed increased [(11)C]PK11195 binding potential in the bilateral LGN at 4 weeks after laser treatment; the increase in the ipsilateral LGN was statistically significant (P<0.05, n = 4). Immunostaining showed bilateral activations of microglia and astrocytes in LGN layers receiving input from the laser-treated eye. PBR-positive cells were observed in LGN layers receiving input from laser-treated eye at all experimental glaucoma stages including the mild glaucoma stage and their localization coincided with Iba-1 positive microglia and GFAP-positive astrocytes. These data suggest that glial activation occurs in the LGN at a mild glaucoma stage, and that the LGN degeneration could be detected by a PET imaging with [(11)C]PK11195 during the moderate experimental glaucoma stage after unilateral ocular hypertension. Therefore, activated glial markers such as PBR in the LGN may be useful in noninvasive molecular imaging for diagnosis of glaucoma.

Quantitative assessment of central nervous system disorder induced by prenatal X-ray exposure using diffusion and manganese-enhanced MRI.

Prenatal radiation exposure induces various central nervous system (CNS) disorders depending on the dose, affected region and gestation period. The goal of this study was to assess noninvasively a CNS development disorder induced by prenatal X-ray exposure using quantitative manganese-enhanced MRI (MEMRI) as well as apparent diffusion coefficient (ADC) and transverse relaxation time (T(2)) maps in comparison with immunohistological staining. The changes in ΔR(1) (increase in the longitudinal relaxation rate (R(1)) from before and after MnCl(2) administration.) induced by the Mn(2+) contrast agent were evaluated in the CNS of normal and prenatally irradiated rats. ADC and T(2) were also compared with the histological results obtained using hematoxylin and eosin (to estimate cell density), activated caspase-3 (apoptotic cells) and glial fibrillary acidic protein (proliferation of astrocytes/astroglia). We found the following: (i) the decreased Mn(2+) uptake (indicated by a smaller ΔR(1)) for radiation-exposed rats was predominantly correlated with a decrease in cell viability (apoptotic cytopathogenicity) and CNS cell density after prenatal radiation exposure; (ii) the longer T(2) and ADC were associated with a decrease in CNS cell density and apoptotic alteration after radiation exposure. In addition to the slight proliferation of astroglia (+58%), there was a substantial decrease in cell density (-78%) and an excessive increase in apoptotic cells (+613%) in our prenatal radiation exposure model. The results suggest that MEMRI in the prenatal X-ray exposure model predominantly reflected the decrease in cell density and viability rather than the proliferation of astroglia. In conclusion, quantitative MEMRI with ADC/T(2) mapping provides objective information for the in vivo assessment of cellular level alterations by prenatal radiation exposure, and has the potential to be used as a standard approach for the evaluation of the cellular damage of radiotherapy.

Mechanistic involvement of the calpain-calpastatin system in Alzheimer neuropathology.

The mechanism by which amyloid-ß peptide (Aß) accumulation causes neurodegeneration in Alzheimer's disease (AD) remains unresolved. Given that Aß perturbs calcium homeostasis in neurons, we investigated the possible involvement of calpain, a calcium-activated neutral protease. We first demonstrated close postsynaptic association of calpain activation with Aß plaque formation in brains from both patients with AD and transgenic (Tg) mice overexpressing amyloid precursor protein (APP). Using a viral vector-based tracer, we then showed that axonal termini were dynamically misdirected to calpain activation-positive Aß plaques. Consistently, cerebrospinal fluid from patients with AD contained a higher level of calpain-cleaved spectrin than that of controls. Genetic deficiency of calpastatin (CS), a calpain-specific inhibitor protein, augmented Aß amyloidosis, tau phosphorylation, microgliosis, and somatodendritic dystrophy, and increased mortality in APP-Tg mice. In contrast, brain-specific CS overexpression had the opposite effect. These findings implicate that calpain activation plays a pivotal role in the Aß-triggered pathological cascade, highlighting a target for pharmacological intervention in the treatment of AD.

In vivo positron emission tomographic imaging of glial responses to amyloid-beta and tau pathologies in mouse models of Alzheimer's disease and related disorders.

Core pathologies of Alzheimer's disease (AD) are aggregated amyloid-ß peptides (Aß) and tau, and the latter is also characteristic of diverse neurodegenerative tauopathies. These amyloid lesions provoke microglial activation, and recent neuroimaging technologies have enabled visualization of this response in living brains using radioligands for the peripheral benzodiazepine receptor also known as the 18 kDa translocator protein (TSPO). Here, we elucidated contributions of Aß and tau deposits to in vivo TSPO signals in pursuit of mechanistic and diagnostic significance of TSPO imaging in AD and other tauopathies. A new antibody to human TSPO revealed induction of TSPO-positive microgliosis by tau fibrils in tauopathy brains. Emergence of TSPO signals before occurrence of brain atrophy and thioflavin-S-positive tau amyloidosis was also demonstrated in living mice transgenic for mutant tau by positron emission tomography (PET) with two classes of TSPO radioligands, [(11)C]AC-5216 and [(18)F]fluoroethoxy-DAA1106. Meanwhile, only modest TSPO elevation was observed in aged mice modeling Aß plaque deposition, despite the notably enhanced in vivo binding of amyloid radiotracer, [(11)C]Pittsburgh Compound-B, to plaques. In these animals, [(11)C]AC-5216 yielded better TSPO contrasts than [(18)F]fluoroethoxy-DAA1106, supporting the possibility of capturing early neurotoxicity with high-performance TSPO probes. Furthermore, an additional line of mice modeling intraneuronal Aß accumulation displayed elevated TSPO signals following noticeable neuronal loss, unlike TSPO upregulation heralding massive neuronal death in tauopathy model mice. Our data corroborate the utility of TSPO-PET imaging as a biomarker for tau-triggered toxicity, and as a complement to amyloid scans for diagnostic assessment of tauopathies with and without Aß pathologies.